Document [original]

Synthesis, characterization and catalytic testing of metal

tungstates as catalysts for activation of lower alkanes

vorgelegt von

Master of Science

Xuan Li

aus Hunan (China)

Von der Fakultät II – Mathematik und Naturwissenschaften

der Technischen Universität Berlin

Zur Erlangung des akademischen Grades

Doktor der Naturwissenschaften

Dr. rer. nat

genehmigte Dissertation

Promotionsausschuss:

Vorsitzender: Prof. Dr. Peter Hildebrandt

Gutachter: Prof. Dr. Robert Schlögl

Gutachter: Prof. Dr. Reinhard Schomäcker

Gutachter: Prof. Dr. Klaus Rademann

Tag der wissenschaftlichen Aussprache: 28.02.2017

Berlin 2017

Acknowledgement

First and foremost, I want to thank Prof. Schlögl for giving me the opportunity to conduct my research

at the Fritz-Haber-Institute of the Max-Planck-Society. I felt excited every morning on the way to

work. Thanks him for bringing together so many characterization techniques, providing chance to

pursue more comprehensive and deeper understanding in heterogeneous catalysis. His delightful and

humorous way of conveying his feelings and thoughts makes it a pleasure to present him the scientific

result, although there is always a chance to receive his criticism. His broad knowledge and deep

understanding in catalysis greatly helps my PhD project.

I would like to deeply acknowledge my supervisor Dr. Annette Trunschke, for her continuous

encouragement, suggestions and also criticism. In China, there is a saying “Yan shi chu gao tu”,

meaning strict teacher educates good students. And I would also like to thank her for the freedom she

gave me to pursue my ideas. Her curiosity into the scientific truth always stimulates me.

I would like to particularly acknowledge my supervisor Dr. Frank Rosowski, for providing me the

opportunity to conduct my PhD research as collaboration between BasCat UniCat BASF JointLab and

Fritz-Haber-Institute. And I would like to thank him for his patience and freedom to my study, and for

his continuous support.

I would also like to acknowledge the members of the doctoral board: Prof. Reinhard Schomacker at

the Technical University of Berlin for hosting me as an external student, Prof. Klaus Rademann at the

Humboldt University of Berlin for serving as the referee. I also thank Prof. Peter Hildebrandt at the

Technical University of Berlin for taking the chair during the defense.

I would like to thank Pierre Kube and Mateusz Jastak for transferring knowledge about the parallel

reactor. I would also like to thank Dr. Xing Huang for introduction to basic and practical knowledge

about transmission electron microscope. Similarly, I would like to thank Verena Pfeifer for her help in

XPS and NEXAFS analysis, and introduction to fundamental knowledge about these techniques.

I would like to thank Dr. Thomas Lunkenbein for his continuous support in high resolution

transmission electron microscopic analysis. And in a similar manner, I would like to thank Dr. Detre

Teschner for his support in laboratory XPS analysis.

A member of colleagues in the Department of Inorganic Chemistry and BasCat lab contributed in

performing experiments, calculation, technical assistance and discussions. I would like to

acknowledge the following people (in no particular order): Jutta Kröhnert (IR), Dr. Friedrich Seitz (in

situ IR), Dr. Teng Fu (IR and Raman), Dr. Travis Jones (Theory), Prof. Dr. Sophia Klokishner

(Theory), Dr. Frank Girgsdies (XRD), Gisela Weiberg (SEM), Wiebke Frandsen (TEM and SEM),

Maike Hashagen (N2 physisorption), Dr. Johannes Noack (Raman), Christian Schulz (catalytic testing

in ODH of butane), Jasmin Allan (TG-MS and XRD), Dr. Olaf Timper (XRF).

I would like to thank my officemates Hamideh Ahi, Marie Mathilde-Millet and Dr. Teng Fu for the

warm atmosphere and help in many aspects. I also thank members of “Reactivity group” and “BasCat”

for valuable discussions and encouragement.

Berlin International Graduate School of Natural Science and Engineering (BIG-NSE) is gratefully

acknowledged for a scholarship.

Last but certainly not least important, I thank my wife Cen Kuang, my parents and parents-in-law for

their support, encouragement and love. This thesis is dedicated to them!

Abstract

The electronic and structural properties of vanadium-containing phases govern the formation of

isolated active sites at the surface of these catalysts for selective alkane oxidation. This concept is not

restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn the typical

combustion catalyst manganese oxide into a selective catalyst for oxidative dehydrogenation of

propane (ODP). Nanostructured, crystalline MnWO4 serves as the support that stabilizes a defect-rich

MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction

temperature. Terminating MnOx zigzag chains on the (010) crystal planes are supposed to bear

structurally site-isolated oxygen defects that account for the unexpectedly good performance of the

catalyst in propane activation.

The mechanism of C-H activation in selective oxidation reactions of short-chain alkane molecules

over transition metal oxides is critically affected by the balance of acid-base and redox sites at the

surface of the catalyst. Using the example of manganese tungstate we discuss how the relative

abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of

the thermodynamic stable monoclinic MnWO4 phase have been prepared by hydrothermal synthesis.

Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed

of particles with varying aspect ratio. The synthesis products have been analysed by transmission

electron microscopy, X-ray diffraction, infrared and photoelectron spectroscopy. In-situ Raman

spectroscopy was used to investigate the dissolution-re-crystallization processes occurring under

hydrothermal conditions. Ethanol oxidation was applied to probe the surface functionalities in terms

of acid-base and redox properties. Changes in the aspect ratio of the primary catalyst particles are

reflected in the product distribution induced by altering the fraction of acid-base and redox sites

exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth

by re-crystallization during ageing under hydrothermal conditions.

Phase-pure CoWO4 catalysts were successfully prepared by hydrothermal synthesis and tested in ODP.

Higher initial pH value in the synthesis gel gives rise to a higher Co/W ratio in the near surface region

as detected by XPS. Furthermore, there exists a correlation between the specific propane consumption

rate and the Co/W ratio. CoWO4 enriched in tungsten in the near surface region shows low selectivity

to propene, and the selectivity to conversion trajectory is similar to that of mesoporous tungsten oxide.

However, the catalyst with the highest Co/W ratio shows the best selectivity to propene at around 9 %

propane conversion. Moreover, the surface termination by a Co chain structure on the (010) crystal

planes is confirmed by HRTEM. FT-IR spectroscopy exhibits a band at 3387 cm-1 attributed to

surface Co bridging hydroxyl groups. The terminating oxo-hydroxy layer in the catalyst precursor is

III

proposed to develop a Co-rich defective surface responsible for high activity and selectivity of

CoWO4 in ODP.

Zusammenfassung

Elektronische und strukturelle Eigenschaften vanadiumhaltiger Phasen steuern die Entstehung

isolierter aktiver Zentren an der Oberfläche von Katalysatoren für die selektive Oxidation von

Alkanen. Wir zeigen, dass dieses Konzept nicht nur auf Vanadiumoxide beschränkt ist. Durch gezielte

Anwendung hydrothermaler Synthesemethoden konnte aus dem typischen Verbrennungskatalysator

Manganoxid ein selektiver Katalysator für die oxidative Dehydrierung von Propan (ODP) entwickelt

werden. Nanostrukturiertes, kristallines MnWO4 dient als Trägermaterial, das in der Synthese eine

defektreiche MnOx-Oberflächenphase hervorbringt. Sauerstoffdefekte können bei

Reaktionstemperatur reversibel verbraucht und regeneriert werden. Es wird vermutet, dass MnOx-

Zickzack-Ketten an der Oberfläche von (010)-Kristallflächen strukturell isolierte Sauerstoffdefekte

tragen, die für die unerwartet gute Leistung des Katalysators in der Propanaktivierung verantwortlich

sind.

Der Mechanismus der C-H-Aktivierung in selektiven Oxidationsreaktionen von kurzkettigen

Alkanmolekülen gegenüber Übergangsmetalloxiden wird durch die Balance der Säure-Base- und

Redox-Zentren an der Oberfläche von Katalysatoren kritisch beeinflusst. Am Beispiel des

Manganwolframats wird diskutiert, wie die Verteilung dieser Zentren mittels synthetischer Techniken

gesteuert werden kann. Phasenreine Katalysatoren aus der thermodynamisch stabilen monoklinen

MnWO4 Phase wurden durch hydrothermale Synthese hergestellt. Eine Variation des pH-Werts in der

Synthese führte zu nanokristallinen MnWO4 Katalysatoren, die aus stabförmigen Partikeln mit

unterschiedlichem Aspektverhältnis bestehen. Die Syntheseprodukte wurden mittels

Transmissionselektronenmikroskopie, Röntgenbeugung, Infrarot- und Photoelektronenspektroskopie

analysiert. In-situ Ramanspektroskopie wurde verwendet, um die unter hydrothermalen Bedingungen

auftretenden Lösungs- und Rekristallisationsverfahren zu untersuchen. Die Ethanoloxidation diente

als Testreaktion, um die Oberflächenfunktionalitäten hinsichtlich Säure-Base- und Redox-

Eigenschaften zu untersuchen. Eine Änderung des Aspektverhältnisses der primären

Katalysatorteilchen spiegelt sich in der Produktverteilung wider. In Übereinstimmung mit dem

vorgeschlagenen Mechanismus des Teilchenwachstums durch Rekristallisation während des Alterns

unter hydrothermalen Bedingungen bestimmt das Aspektverhältnis den Anteil an Säure-Base- und

Redox-Zentren, die an der Oberfläche der Katalysatoren exponiert sind, und damit die katalytischen

Eigenschaften.

Phasereine CoWO4 Katalysatoren wurden erfolgreich durch hydrothermale Synthese hergestellt und

in der ODP-Reaktion getestet. Ein höherer anfänglicher pH-Wert führt zu einem höheren Co/W

Verhältnis in der oberflächennahen Region der Katalysatoren. Es besteht eine Korrelation zwischen

der spezifischen Propan-Verbrauchsgeschwindigkeit und dem Co/W Verhältnis der Katalysatoren.

Der oberflächlich mit W angereicherte Katalysator zeigt eine viel geringere Selektivität hinsichtlich

Propen. Der Verlauf der Selektivität als Funktion des Umsatzes an diesem Katalysator ähnelt dem von

mesoporösen Wolframoxid. Dagegen zeigt der Katalysator mit dem höchsten Co/W Verhältnis die

beste Selektivität zu Propen bei einem Propanumsatz von 9%. HRTEM bestätigt, das dieser

Katalysator durch eine Co-Kettenstruktur auf den (010) Kristallflächen terminiert ist. Mittels FT-IR

Spektroskopie wird eine Bande bei 3387 cm-1 für oberflächenverbrückende Hydroxylgruppen

beobachtet. Das Vorhandensein einer solchen Oxo-Hydroxy- Schicht an der Oberfläche des

Katalysatorvorläufers könnte für die Entwicklung der hohen Aktivität und Selektivität des

Katalysators in der ODP-Reaktion verantwortlich sein, die auf eine defekt- und Co-reiche Oberfläche

zurückzuführen ist.

List of figures

Figure 1.1 Near surface concentration of Te and V determined by XPS (escape depth 0.8 nm) at

T=298 K, p=30 Pa O2 as a function of the concentration in the crystalline M1 framework measured by

EDX (redrawn from ref. [1]). .................................................................................................................. 4

Figure 1.2 STEM image of one M1 particle along the <001> direction. Red and blue circles highlight

heptagonal and hexagonal open channels, respectively. Green circles highlight the M{M5} structure.

The bright dots represent columns of M-O polyhedra (reproduced from ref. [46]). ............................... 6

Figure 2.1 (S)TEM analysis of the MnWO4 powder catalyst: a) Overview TEM micrograph of the

MnWO4 nanorods; b) HRTEM image of one nanorod particle viewed along [100]. The inset denotes

the power spectra recorded on either side of the defect; c) HAADF-STEM image of condensed [001]

oriented MnWO4 nanorods. The surface termination was identified by phase analysis of

corresponding atomic resolution HAADF-STEM images; d) and e) atomic resolution HAADF-STEM

images of (c); f) Schematic representation of the crystal structure of MnWO4 viewed along [001]. The

Mn atoms are presented in white, W is coloured green and O atoms are displayed in red. The original

images are given in the SI (Figure S2.1, 6c). ........................................................................................ 11

Figure 2.2 Surface termination of the b plane viewed along the growth direction [001] by FFT filtered

atomic resolution STEM images a) HAADF, and b) inverted HAADF; The violet and green circles

denote W and Mn atoms, respectively. The micrographs correspond to magnified images of Figure 1d.

Original images are given in the SI (Figure S2.6). ............................................................................... 12

Figure 2.3 Depth profile of elemental composition of MnWO4 nano-rods at two different depths

represented by the inelastic mean free path (IMFP) of electrons measured by synchrotron-based NAP-

XPS at T=300°C applying a total pressure of 0.25 mbar O2 and He at flows of 2 and 2.2 sccm,

respectively. .......................................................................................................................................... 12

Figure 2.4 In-situ Raman spectra in the 280-1000 cm-1 range measured during hydrothermal synthesis

of MnWO4 nano-rods; Bands denoted with asterisks belong to the sapphire window of the Raman

probe. .................................................................................................................................................... 14

Figure 2.5 Catalytic performance of nano-structured MnWO4 (T=400°C, W/F=1.8-0.9 g s/ml) in

comparison to VOx/SiO2 (T=400-420°C, W/F=1.8 g s/ml) and various manganese oxides (T=330-

395°C, W/F=1.8 g s/ml) in the oxidative dehydrogenation of propane in a feed composed of

C3H8:O2:N2 in a ratio of 10:5:85. The selectivity to propene is shown as a function of propane

conversion. Other C-containing products are mainly CO, and CO2. .................................................... 16

Figure 3.1 In situ Raman spectra recorded during the synthesis of the catalysts a) AR1.5 and c) AR3.9

and the corresponding profiles of temperature and pH during synthesis of b) AR1.5 and d) AR3.9;

The symcbol * in the Raman spectra indicates the bands of the sapphire window of the Raman probe.

.............................................................................................................................................................. 39

VII

Figure 3.2 Electron microscopy images of the as-synthesized (top row) and thermally treated (bottom

row) nanostructured MnWO4 materials AR1.5 (a) and f)), AR1.7 (b) and g)), AR3.2 (c) and h)),

AR3.9 (d) and i)), and AR5.1 (e) and j)); Uncoloured TEM images are presented in the Supporting

Information (Fig. S3.4). ........................................................................................................................ 41

Figure 3.3 HAADF-STEM images of MnWO4 nanoparticles viewed along <001> with different

aspect ratios: a) AR1.5, b) AR5.1 and c) perspective model for a typical faceted nanoparticle. The

original uncoloured images including Fast Fourier transform analysis are given in the Supporting

Information, Fig. S3.6. .......................................................................................................................... 42

Figure 3.4 Schematic representation of the proposed anisotropic mechanism of particle growth. ....... 43

Figure 3.5 Selectivity in ethanol oxidation over the nanostructured MnWO4 catalysts at 10% ethanol

conversion. ............................................................................................................................................ 44

Figure 3.6 a) Specific reaction rates measured at T=310°C and normalized to surface area, and

apparent activation energy as a function of the aspect ratio; Red solid circle: acetaldehyde formation

rate; red open circle: apparent activation energy of acetaldehyde formation; black solid triangle:

ethylene formation rate; black open triangle: apparent activation energy of ethylene formation; b) Rate

of formation of ethylene as a function of Brønsted acid site density at the catalyst surface determined

by ammonia adsorption and specific surface area measurements. ........................................................ 44

Figure 3.7 a) Infrared spectra in the region of OH stretching vibrations after thermal treatment of the

catalyst in the infrared cell in vacuum at 300°C; The measurement was performed in vacuum at 40°C;

W 4f spectra (b)), and O 1s spectra (c)) of the catalysts AR1.5 and AR5.1 measured by synchrotron-

based near ambient pressure X-ray photoemission spectroscopy (NAP-XPS) at an inelastic mean free

path (IMFP) of ca. 0.6 nm in 0.25 mbar in O2/He at a total gas flow of 4.2 sccm at 300°C. ............... 46

Figure 4.1 In situ Raman spectra recorded during the synthesis of the catalysts a) CoW6.2, b) CoW7.6

and c) CoW8.5. ..................................................................................................................................... 60

Figure 4.2 XRD patterns of as-synthesized CoWO4 catalyst precursors. ............................................. 60

Figure 4.3 SEM images of the a) CoW6.2, b) CoW7.6, c) CoW8.1 and d) CoW8.5 catalysts. ............ 62

Figure 4.4 HAADF-STEM images of the CoW8.5 catalyst. ................................................................ 62

Figure 4.5 a) HAADF-STEM image of one particle in the CoW8.5 catalyst viewed along the <001>

direction, b) HR-STEM-HAADF and c) HR-STEM-BF images of the surface of the same catalyst

viewed along the <100> direction. ........................................................................................................ 63

Figure 4.6 Catalytic performance of mesoporous WO3, commercial Co3O4 and nanostructured CoWO4

catalysts (T=400°C, W/F=0.9-2.4 g s mL-1) in the oxidative dehydrogenation of propane in a

C3H8/O2/N2 feed (10:5:85). The selectivity to propene is shown as a function of propane conversion.

Other carbon-containing products are mainly CO and CO2. ................................................................. 64

Figure 4.7 Steady state propane consumption rate as a function of Co/W ratio in the near surface

region determined by XPS. ................................................................................................................... 65

Figure 4.8 FT-IR spectra of CoWO4 catalysts in He flow at a) room temperature and b) 400°C. ....... 66

VIII

Figure 4.9 Influence of partial pressure of a) propane, b) oxygen and c) water on the specific product

formation rate on CoW6.2 catalyst. ...................................................................................................... 67

Figure 4.10 Influence of partial pressure of a) propane, b) oxygen and c) water on the specific product

formation rate on CoW8.5 catalyst. ...................................................................................................... 68

List of tables

Table 2.1. Active site density and reactivity of the catalysts in the oxidative dehydrogenation of

propane at T=400°C, W/F=1.8 g s/ml in a feed composed of C3H8:O2:N2 in a ratio of 10:5:85. ............... 16

Table 3.1 Specific surface area, results of shape analysis based on TEM, and crystallite size calculated

from anisotropic fitting in Rietveld refinement of the XRD patterns ......................................................... 43

Table 4.1 Specific surface area, synthetic parameters and crystallite size calculated from anisotropic

fitting in Rietveld refinement of the XRD patterns. .................................................................................... 61

Table 4.2 Surface-near molar ratios of CoWO4 according to XPS. ............................................................ 63

Table 4.3 Apparent reaction orders with respect to propane, oxygen and water on tungsten containing

catalysts. ...................................................................................................................................................... 68

List of schemes

Scheme 1.1 Reaction network (adapted from ref. [11]) on supported V-, Mo- and W-oxide catalysts. ....... 2

Scheme 1.3 Termination model of the M1 phase (reproduced from ref. [42]). ............................................ 5

Scheme 1.4 Scheme of the wolframite crystal structure. A= Fe, Mn, Co, Ni, Mg and Zn. .......................... 7

List of supporting figures

Figure S 2.2 Rietveld refinement of the powder XRD of the MnWO4 catalyst (ID 19116). .................. 25

Figure S 2.3 Particle size distribution in the thermally treated nano-structured MnWO4 catalyst

(ID 19116) based on the analysis of approximately 146 particles in TEM images (see for

example Figure 2.1a in the main manuscript). ....................................................................................... 26

Figure S 2.4 HRTEM images and fast Fourier transform (FFT) analysis of two MnWO4 nanorods

in the catalyst (ID 19116). ........................................................................................................................ 26

Figure S 2.5 Inverse Fast Fourier transformation (IFFT) of the 110 spots in Figure 2.1b. .................. 27

Figure S 2.6 FFT filtered HR-STEM images of MnWO4 (ID 19116) a) HAADF, b) inverted HAADF

and c) HR-HAADF-STEM images. ............................................................................................................. 28

Figure S 2.7 Raman spectrum of the MnWO4 catalyst (ID 19116) (black line) compared to the

Raman spectrum of commercial MnWO4 (orange line). ......................................................................... 29

Figure S 2.8 HRTEM images and electron diffraction patterns of MnWO4 nano-rods. ........................ 29

Figure S 2.9 Schematic representation of the crystal structure of MnWO4 viewed along a) the

[001] axis and b) the [100] axis. White ball represents Mn2+, green ball represents W6+, red ball

represents O2- (bridging oxygen) and wine ball represents terminal oxygen atoms (tungsten

oxygen double bond). ................................................................................................................................ 30

Figure S 2.10 Recorded workflow during hydrothermal synthesis of nano-structured MnWO4;

blue line: pH value measured in the autoclave, red line: temperature of the synthesis gel. ............... 31

Figure S 2.11 Time on stream plot of propane conversion and selectivity to propene in the

oxidative dehydrogenation of propane at T=400°C, and W/F=1.8 g s/ml over nano-structured

MnWO4 (catalyst ID 19116); The feed was composed of C3H8:O2:N2=10:5:85; The changes in the

conversion (X) of propane and the selectivity (S) to propylene and carbon oxides (COx: CO + CO2)

are shown with time on stream. ............................................................................................................... 32

Figure S 2.12 Mn 2p spectra of nano-structured MnWO4 within different detection depths

represented by the inelastic mean free path (IMFP) of electrons measured by synchrotron-

based NAP-XPS at T=300 °C applying a total pressure of 0.25 mbar O2 and He flows of 2 and 2.2

sccm, respectively. ..................................................................................................................................... 33

Figure S 2.13 Mn 2p spectra of nano-structured MnWO4 within different detection depths

represented by the inelastic mean free path (IMFP) of electrons measured by synchrotron-

based NAP-XPS at T=300 °C applying a total pressure of 0.25 mbar under different reaction

atmospheres; Red lines: O2 and He flows of 2 and 2.2 sccm, respectively; Blue lines: O2, C3H8, and

He flows of 1, 2, and 1.2 sccm, respectively. ............................................................................................ 34

Figure S 2.14 NEXAFS of nano-structured MnWO4 measured at the Mn L2,3-edge in total electron

yield (TEY) in different reaction atmospheres at T=380°C; Red lines: O2 and He flows of 1 and

3.2 sccm, respectively; Blue lines: O2, C3H8, and He flows of 1, 2 and 1.2 sccm, respectively. ............ 35

Figure S 2.15 Temperature-programmed oxidation (TPO) (top), and temperature-programmed

reduction (TPR) (bottom) profiles of nano-structured MnWO4. ........................................................... 36

Figure S 3.1

SEM image of the intermediate formed by reaction of manganese nitrate with

sodium tungstate at room temperature.

.................................................................................................. 51

Figure S 3.2

XRD patterns of the hydrothermal products; The pH of the starting solution is

provided in the legend of the figure. For allocation of the corresponding final catalyst, please

refer to Table 3.1 in the main text.

.......................................................................................................... 52

Figure S 3.3

XRD patterns of the catalysts after activation by thermal treatment of the

hydrothermal products in flowing Ar at 400°C.

................................................................................... 52

Figure S 3.4

TEM micrographs of as-synthesized (top row) and at 400 °C thermally activated

(bottom row) nanostructured MnWO

catalysts with different aspect ratio: 1.5 (a) and (f), 1.7

(b) and (g), 3.2 (c) and (h), 3.9 (d) and (i), 5.1 (e) and (j).

.................................................................. 53

Figure S 3.5

Distribution of A) diameter, B) length and C) aspect ratio of the nanostructured

MnWO

catalysts after thermal treatment; In each plot, a, b, c, d, and e represent the catalysts

AR1.5, AR1.7, AR3.2, AR3.9, and AR5.1, respectively.

.................................................................... 53

Figure S 3.6

HAADF_STEM images of MnWO

nanoparticles viewed along <001> with

different aspect ratios: a) AR1.5, b) AR5.1 and c) perspective model for a typical faceted

nanoparticle.

.............................................................................................................................................. 54

Figure S 3.7

Catalytic performance of top-left) AR1.5, top-right) AR1.7, middle-left) AR3.2,

middle-right) AR3.9, and bottom-left) AR5.1 sample in ethanol oxidation reaction at different

temperatures; For reaction conditions see Experimental in the main text.

........................................ 54

Figure S 3.8

Schematic representation of the formation of W-OH groups at (001) planes during

dissolution-recrystallization under hydrothermal conditions at 180°C.

............................................. 55

Figure S 3.9 FTIR spectra of NH3 adsorbed at the surface of the catalysts AR1.5, AR1.7, and AR3.9

after pretreatment at 300°C for 1h in vacuum; Adsorption of ammonia was performed at 40°C; The

spectra have been recorded in presence of gas phase ammonia (p=6.508-7.042 mbar). ............................ 55

Figure S 3.10 W 4f spectra (a)), and O 1s spectra (b)) of the catalysts AR1.5 and AR5.1 measured by

synchrotron-based near ambient pressure X-ray photoemission spectroscopy (NAP-XPS) at an

inelastic mean free path (IMFP) of ca. 1.6 nm in 0.25 mbar in O2/He at a total gas flow of 4.2 sccm at

300°C. ......................................................................................................................................................... 56

Figure S 3.11 Mn 2p spectra of the catalysts AR1.5 and AR5.1 measured by synchrotron-based near

ambient pressure X-ray photoemission spectroscopy (NAP-XPS) at an inelastic mean free path (IMFP)

of ca. 0.6 nm (a) and 1.6 nm (b) in 0.25 mbar in O2/He at a total gas flow of 4.2 sccm at 300°C. ............ 56

Figure S 4.1 Raman spectra of tungsten containing compounds measured as solids using 633 cm-1

excitation wavelength. ................................................................................................................................ 71

Figure S 4.2 Raman spectra of ammonium paratungstate (APT), ammonium metatungstate (AMT)

and sodium tungstate in aqueous solution at room temperature. Bands denoted with asterisks belong to

the sapphire window of the Raman probe. .................................................................................................. 71

Figure S 4.3 Co2p(3/2), O1s and W4f (and 5p3/2) core level XPS spectra of the CoWO4 samples after

Shirley background subtraction and charging correction. ........................................................................... 72

Figure S 4.4 CO2/CO ratio in the ODP reaction over m-WO3, Co3O4 and CoWO4 catalysts. W/F=1.8-

2.4 g s mL-1 in the oxidative dehydrogenation of propane in a C3H8/O2/N2 feed (10:5:85). ...................... 73

Figure S 4.5 a) Propene formation, b) CO2 formation and c) CO formation rates over m-WO3 and

CoWO4 catalysts. W/F=0.75-2.4 g s mL-1 in the oxidative dehydrogenation of propane in a

C3H8/O2/N2 feed (10:5:85). ......................................................................................................................... 74

Figure S 4.6 FT-IR spectra of CoW6.2 catalyst in He flow. ....................................................................... 75

Figure S 4.7 FT-IR spectra of CoW7.6 catalyst in He flow. ....................................................................... 75

Figure S 4.8 FT-IR spectra of CoW8.1 catalyst in He flow. ....................................................................... 76

Figure S 4.9 FT-IR spectra of CoW8.5 catalyst in He flow. ....................................................................... 76

Figure S 4.10 Influence of partial pressure of a) propane, b) oxygen and c) water on the specific

product formation rate on m-WO3 catalyst. ................................................................................................ 77

List of supporting tables

Table S 2.1 Average crystallite sizes D of the particles in the MnWO4 catalyst (ID 19116) along

the a, b, and c axis calculated from anisotropic fitting in Rietveld refinement based on the XRD

patterns presented in Figure S2.2. ........................................................................................................... 21

Table S 2.2 Intensity ratio of the peaks at 640 and 641.4 eV in the NEXAFS of nano-structured

MnWO4 at the Mn L2,3-edge. The spectra in Total Energy Yield (TEY) are presented in Figure

S2.14. .......................................................................................................................................................... 22

Table S 2.3 Hydrogen consumption during TPR and oxygen consumption during TPO

experiments with nano-structured MnWO4. ........................................................................................... 22

Table S 2.4 Chemical composition of the washing solution after treatment of nano-structured

MnWO4 with nitric acid as determined by XRF. ...................................................................................... 23

XII

Table of contents

Acknowledgement ......................................................................................................................................... I

Abstract ....................................................................................................................................................... III

Zusammenfassung ........................................................................................................................................ V

List of figures ............................................................................................................................................ VII

List of tables ................................................................................................................................................ IX

List of schemes ........................................................................................................................................... IX

List of supporting figures ............................................................................................................................ IX

List of supporting tables ............................................................................................................................ XII

1 Introduction ........................................................................................................................................... 1

1.1 Lower alkane utilization ................................................................................................................ 1

1.2 Reaction networks and mechanisms in ODP reaction on metal oxide catalysts ............................ 1

1.3 Active site in oxidative dehydrogenation of propane .................................................................... 3

1.4 Nano-structuring of metal oxide .................................................................................................... 3

1.5 Surface termination of bulk metal oxides under reaction condition .............................................. 4

1.6 Metal tungstate in the wolframite crystal structure ........................................................................ 6

1.7 Motivation and aim of the thesis.................................................................................................... 7

1.8 Outline of the thesis ....................................................................................................................... 8

2 Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface

of MnWO4 Nanorods .................................................................................................................................... 9

2.1 Introduction: .................................................................................................................................. 9

2.2 Results and discussion: ................................................................................................................ 10

2.3 Conclusion: .................................................................................................................................. 16

2.4 Experimental details: ................................................................................................................... 17

2.4.1 Synthesis of the catalyst ....................................................................................................... 17

2.4.2 Catalyst characterization ...................................................................................................... 18

2.5 Supporting information ................................................................................................................ 21

2.5.1 Supporting tables .................................................................................................................. 21

2.5.2 Supporting figures ................................................................................................................ 24

2.6 Acknowledgements ...................................................................................................................... 36

XIII

3 Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for

Selective Oxidation ..................................................................................................................................... 37

3.1 Introduction: ................................................................................................................................ 37

3.2 Results and discussion ................................................................................................................. 38

3.2.1 Phase formation and particle growth under hydrothermal conditions ................................. 38

3.2.2 Ethanol oxidation ................................................................................................................. 43

3.3 Conclusions ................................................................................................................................. 48

3.4 Experimental details .................................................................................................................... 49

3.4.1 Hydrothermal synthesis ........................................................................................................ 49

3.4.2 Characterization of catalysts ................................................................................................ 50

3.5 Supporting information ................................................................................................................ 51

3.6 Acknowledgement ....................................................................................................................... 57

4 Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for

Oxidative Dehydrogenation of Propane ...................................................................................................... 58

4.1 Introduction: ................................................................................................................................ 58

4.2 Results and discussion ................................................................................................................. 59

4.3 Conclusions ................................................................................................................................. 68

4.4 Experimental details: ................................................................................................................... 69

4.4.1 Synthesis of the catalysts ..................................................................................................... 69

4.4.2 Catalyst characterization ...................................................................................................... 69

4.5 Supporting information ................................................................................................................ 70

5 Conclusions ......................................................................................................................................... 78

6 Bibliography ....................................................................................................................................... 80

Appendix ..................................................................................................................................................... 88

Curriculum vitae ..................................................................................................................................... 88

XIV

Introduction

1 Introduction

1.1 Lower alkane utilization

Efficient utilization of lower alkanes (methane, ethane, propane and n-butane) to produce chemical

intermediates, such as olefins and oxygenates, is economically attractive from a practical perspective

because of their abundance and cheaper prices compared to their olefin counterparts. However,

industrial applications of direct transformation of alkanes are limited due to insufficient selectivity

and activity.[2] Currently, selective oxidation of n-butane to maleic anhydride on the so-called VPO

catalyst is the only industrial process in selective oxidation using saturated alkane as feedstock.[3] In

response to the low price of “shale gas”, the chemical industry shifted from naphtha towards ethane in

the steam cracking process, which led to a lack of propylene and butadiene supplies. However, the

increasing demand for propylene motivates “on purpose” production of it. Direct catalytic

dehydrogenation of propane (DHP) to propylene is one of the solutions, but it requires low price pure

propane resources. The current DHP industrial processes uses Pt-based catalysts[4]. However, the

catalyst suffers from deactivation due to coke formation, and the conversion of the catalytic reaction

is limited by thermodynamics caused by the endothermic nature of the reaction. On the other hand,

oxidative dehydrogenation of propane (ODP) is advantageous since no coke formation and no

thermodynamic limitations occur. Bulk or supported (mixed) metal oxides are the most promising

catalytic material investigated.[5, 6] Vanadium containing oxides[7], molybdates [8, 9], tungstates[10]

and cobalt and nickel containing mixed metal oxides are representative redox-type catalysts. At high

temperatures it is also possible to activate propane by non-redox active materials. Despite the fact that

a large number of catalytic materials has been investigated, the low selectivity to propylene still

leaves the ODP process far away from industrial application. So far it is not comprehensively clear

which factors determine selectivity in the catalytic oxidation of short-chain alkanes. To fully

understand the structure-function relationships, deeper understanding of the reaction network is

required.

1.2 Reaction networks and mechanisms in ODP reaction on metal

oxide catalysts

The reaction network shown in Scheme 1.1 has been proposed for supported vanadium, molybdenum

and tungsten oxide catalysts including direct combustion of propane and consecutive combustion of

propylene as main unselective pathways.[11, 12]

Introduction

Irreversible C-H bond scission via abstraction of the first hydrogen atom in adsorbed propane (equ.

1.1) by nucleophilic oxygen species was suggested to be the rate-determining step (equ. 1.2) of the

entire reaction on a supported vanadium oxide catalysts.[13] After two consecutive C-H bond

abstraction reactions (equs. 1.2 and 1.3), oxygen vacancies can be re-created by recombination of the

hydroxyl groups formed (equ. 1.4). Gas phase oxygen molecules then fill the oxygen vacancies,

completing the catalytic cycles (equ. 1.5).

C3H8 + O* ⇌ C3H8O* (1.1)

C3H8O* + O* → C3H7O* + OH* (1.2)

C3H7O* → C3H6+ OH* (1.3)

OH* + OH* ⇌ H2O + O* + * (1.4)

O2 + * + * → O* + O* (1.5)

Micro-kinetic investigations on supported and bulk vanadium oxide catalysts shows that the apparent

reaction order of propane consumption is close to propane and close to 0 with respect to oxygen. The

macrokinetics have been successfully treated using a following rate expression.[7]

𝑟

𝐶3𝐻8=k𝑃

𝐶3𝐻8

1𝑃

𝑂2

0 (2.1)

However, on Co0.5Ni0.5MoO4 catalysts, the reaction order of propane consumption with respect to

propane partial pressure is close to 1, but the reaction order of carbon oxides formation with respect to

oxygen partial pressure is close to 0.5.[9] It was suggested that the formation of propene follows a

Mars-van Krevelen-type mechanism (Scheme 1.2, step 1) but the formation of carbon oxides from

propene follows a Langmuir-Hinshelwood mechanism (Scheme 1.2, step 4). In other words, it was

suggested that lattice oxygen is responsible for propene formation whereas dissociatively chemisorbed

oxygen species are responsible for total oxidation of propene. A similar dependence of rates for

propane consumption and carbon oxides formation on the partial pressures of propane and oxygen

was reported on a Mg4V2Sb2Ox catalyst.[14, 15]

Scheme

1.1 Reaction network (adapted from ref. [11]) on supported V-, Mo- and W-oxide catalysts

Introduction

Some other types of reaction mechanism were also proposed.[16] In general, limited propene

selectivity on vanadium-based catalysts was attributed to consecutive combustion of propene.[12, 17]

Supported molybdenum and tungsten oxide catalysts also suffer from such problems.[11] However,

the consecutive combustion of propene was less severely on the Co0.5Ni0.5MoO4.[9] Furthermore, in

the catalytic cycle on supported vanadium oxide catalysts, it was generally believed that C-H bond

activation proceeds much more slowly than the re-oxidation of the oxide.[18-21] The rate of the

former step was estimated as 102 to 105 times as that of the latter step, depending on the structure of

the vanadium oxide.[18, 19]

1.3 Active site in oxidative dehydrogenation of propane

The nature of the active site has an impact on the reaction network.[22] The active sites on supported

vanadium oxide catalysts contain V-O moieties, although an unambiguous assignment of

spectroscopic features to V=O, V-O-V or V-O-(Support) species has not been achieved so far.[23] In

a study of Ni-W-O mixed metal oxide catalysts in the oxidative dehydrogenation of ethane, Ni-O sites

were proposed as active sites.[24]

To be noted, site isolation was brought up as a very important concept guiding the improvement of

selectivity for selective oxidation catalysts.[6, 25, 26] The following hypothesis has been proposed by

Grasselli et al.: “Reactive surface lattice oxygens must be spatially isolated from each other in defined

groupings on a catalyst surface to achieve selectivity. The number of oxygens in a given isolated

grouping (domain) would determine the reaction channel through the stoichiometry requirements

imposed on the reaction by the availability of oxygens, or lack thereof, at the reaction site”.

1.4 Nano-structuring of metal oxide

Nano-structuring of metal oxide catalyst is important not only because of higher surface areas

possessed by the nanoparticles. The synthetic techniques developed during the recent “Nano Rush”

can actually produce nanoparticles with tailored morphology and size, these are important in

determining the surface geometric structure, i.e. facets exposure, and electronic structure of the

Scheme

1.2 Reaction network (adapted from ref. [9]) on Co0.5Ni0.5MoO4

Introduction

synthesized solids.[27] Moreover, morphology has been found to have important influences on

catalytic properties of nano-crystalline metal oxides.[28, 29] Different intrinsic reaction rates have

been found over the differently-shaped nanostructured metal oxides exposing various crystal planes,

as exemplified by the research concerning Co3O4, [30, 31] CeO2 [32-34] and Cu2O[35]. It has been

proposed that mono-atomic step sites on the surface of MgO are the active sites for the oxidative

coupling of methane,[36, 37] illustrating the important role of surface defects.

1.5 Surface termination of bulk metal oxides under reaction

condition

A full understanding of the activity-structure relationship is, however, very challenging, since the

complexity of the surface structure of metal oxides regarding surface termination mode (metal

termination or oxygen termination),[38] hydroxylation, defects[39-41] and metal oxidation state is

high although in some cases the prepared catalysts exposed regular facets. For example, the surface

elemental composition of a Mo-V-Te-Nb-O mixed oxide in M1 crystal phase is different from that of

the bulk (Figure 1.1). The difference might be partly attributed to the openness of the 6 or 7-

membered rings at the surface, which then liberates Te originally contained in these closed channels

(Scheme 1.3).

Figure

1.1

Near surface concentration of Te and V determined by XPS (escape depth 0.8 nm) at T=298 K,

p=30 Pa O

2 as a function of the concentration in the crystalline M1 framework measured by EDX (re

drawn

from ref.

[1]).

Introduction

The surface termination mode was also investigated by transmission electron microscopy (Figure 1.2).

More importantly, dynamic rearrangements of the catalyst surface responding to temperature and gas

atmospheres have been observed. In situ synchrotron based X-ray photoelectron spectroscopic

experiments indicate the formation of two-dimensional vanadium oxide layers terminating crystalline

vanadium-containing phases, such as (VO)2P2O7 and Mo-V-Te-Nb-O composed the M1 phase.[1, 42-

45] The surface layer deviates significantly from the bulk crystal structure in terms of elemental

composition and the vanadium oxidation state.

Scheme

1.3 Termination model of the M1 phase (reproduced from ref. [42]).

Introduction

1.6 Metal tungstate in the wolframite crystal structure

Metal tungstates AWO4 (A is bivalent ions with relatively small radius, such as Fe, Mn, Co, Ni, Mg,

and Zn) crystallize in the wolframite-type structure. Metal tungstate materials attract much scientific

interest in various fields such as in photonics and photoelectronics.[47, 48] Especially, optical

(phonon) properties,[49, 50] electronic structure,[51-53] conductivity[54, 55] and magnetic

properties[56, 57] of manganese tungstate have been widely investigated. In the wolframite crystal

structure (Scheme 1.4), distorted WO6 motifs form zigzag chains by edge sharing oxygen atoms along

the c axis and bi-valent transition metal ions fill in the octahedrons by corner sharing oxygen atoms

with WO6 clusters forming also zigzag chains along the same crystallographic axis. Along the a axis

the two types of zigzag chains stack alternately by edge sharing.

Figure

1.2 STEM image of one M1 particle along the <001> direction. Red and blue circles

highlight

heptagonal and hexagonal open channels, respect

ively. Green circles highlight the M{M5}

structure.

The bright dots represent columns of M

-O polyhedra (reproduced from ref. [46]).

Introduction

1.7 Motivation and aim of the thesis

From the viewpoint of searching for new catalytic materials for selective activation of lower alkanes,

a novel strategy is needed. Despite comprehensive investigation, the performance of vanadium-based

materials is still far from industrial application. In the case of ODH of propane, selectivity to propene

at high propane conversions is limited due to consecutive oxidation of propene. On the other hand,

deeper understanding of the reaction network and mechanism of the oxidative dehydrogenation of

propane and the nature of active site would be very beneficial for designing new catalytic materials.

As outlined above, site isolation has been postulated as an important requirement for selectivity in

partial oxidation on oxide catalysts. The surface structure of the vanadium-based multi-metal mixed

oxide catalysts is extremely complex and dynamic as has been introduced above.

The aim of the thesis is, therefore, to gain knowledge about the structure-function relationships in

propane activation over mixed metal oxides by reducing the chemical complexity of the catalyst.

Metal tungstates have been chosen, since these oxide materials contain only of two metals. The

tungstate motifs are supposed to provide a structure-stabilizing matrix that hosts redox-active metals,

such as Mn, Fe, Ni, or Co.

This work focuses in particular on the influence of the morphology and surface termination of the

catalyst particles on the oxidative dehydrogenation of propane over manganese and cobalt tungstate

catalysts. Morphology and surface termination of catalyst particles will be tuned by systematic

variation of the synthetis parameters. The iso-structure shared by the wolframite family renders

comparative studies of the effect of composition in terms of the redox-active element possible.

Besides hydrothermally synthesized tungstates, commercial manganese tungstate, cobalt oxide

Scheme

1.4

Scheme of the wolframite crystal structure. A= Fe, Mn, Co, Ni, Mg and Zn.

Introduction

(Co3O4), manganese oxides (MnO, Mn3O4, Mn2O3, MnO2) and mesoporous tungsten trioxide have

been included as references.

1.8 Outline of the thesis

In Chapter 2, the discovery of unexpected activity and selectivity of rod-shaped nanostructured

manganese tungstate in the oxidative dehydrogenation of propane is discussed. The selectivity to

propene over the mixed metal oxide is much higher than that over the binary manganese or tungsten

oxides, respectively. The performance of the catalyst is attributed to “site isolation” of surface

manganese oxide chains. The unique surface structure was formed under specific hydrothermal

synthesis conditions.

In Chapter 3, hydrothermal synthesis of a series of manganese tungstate is described. The influence of

acidity and basicity in the synthesis gel on the aspect ratio of the synthesized nanostructured

manganese tungstate particles is highlighted. A “dissolution-re-crystallization” mechanism during the

aging process of the hydrothermal synthesis was proposed to be responsible for the anisotropic crystal

growth. The aspect ratio was found to determine the acid-base and redox properties of the catalysts.

In Chapter 4, hydrothermal synthesis of cobalt tungstate is illustrated. Again, a variation of the initial

pH in the autoclave resulted in differently shaped nanostructured cobalt tungstate particles. Surface

termination by cobalt oxide species was found to be beneficial for both activity and selectivity in the

oxidative dehydrogenation of propane.

In Chapter 5, final conclusions and an outlook are given.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

2 Selective Alkane Oxidation by Manganese Oxide: Site

Isolation of MnOx Chains at the Surface of MnWO4

Nanorods1

Abstract: The electronic and structural properties of vanadium-containing phases govern the

formation of isolated active sites at the surface of these catalysts for selective alkane oxidation. This

concept is not restricted to vanadium oxide. The deliberate use of hydrothermal techniques can turn

the typical combustion catalyst manganese oxide into a selective catalyst for oxidative propane

dehydrogenation. Nanostructured, crystalline MnWO4 serves as the support that stabilizes a defect-

rich MnOx surface phase. Oxygen defects can be reversibly replenished and depleted at the reaction

temperature. Terminating MnOx zigzag chains on the (010) crystal planes are suspected to bear

structurally site-isolated oxygen defects that account for the unexpectedly good performance of the

catalyst in propane activation.

Keywords: propane oxidative dehydrogenation •hydrothermal synthesis • in situ Raman • manganese

tungstate • heterogeneous catalysis

2.1 Introduction:

Prospective changes in the raw material basis in chemical industry to alternative feedstock bear new

scientific challenges. This tackles, in particular, the area of oxidation catalysis where small saturated

hydrocarbon molecules are going to be used as building blocks for olefins and aromatics.[58] The

activation of inert C-H bonds in alkanes requires highly active catalysts. Often, high activity entails

low selectivity due to over-oxidation of more reactive intermediates and desired products to CO and

CO2.[59] Vanadium oxide is the most prominent material that has been widely studied in selective

oxidation of hydrocarbons and oxygenates.[7, 13, 60-64] Surface-sensitive in-situ experiments

indicate that some well-known selective catalysts, composed of crystalline V-containing phases, are

terminated by two-dimensional vanadium oxide layers.[1, 42-45, 65-69] These layers deviate in terms

of composition and oxidation state of V significantly from the bulk crystal structure. The layer

accounts for dynamic charge transfer between bulk and surface. This is reflected in the gas-phase-

dependent response of the work function, electron affinity, and surface potential barrier, which was

1The following chapter is the submitted version of [104], the peer reviewed published version

can be found with publisher DOI link: http://dx.doi.org/10.1002/anie.201510201 9

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

not found for the less selective bulk V2O5.[68]

2.2 Results and discussion:

Herein, we conceptually verified that the selectivity of other unselective oxides, like Mn oxide, is also

tunable by applying this extended site-isolation approach. We present the first example of a

vanadium-free analogue that accomplishes efficient activation of propane by establishing a two

dimensional Mn oxide layer in form of MnOx chains at the surface of phase-pure, rod-shaped,

nanostructured MnWO4 (Figure 1, Figure S2.1). The catalyst was prepared by hydrothermal synthesis.

The synthesis procedure reported previously,[70] was modified in the current work (description

provided in the Supporting Information).

Phase purity of the synthesis product was confirmed by Rietveld refinement of powder X-ray

diffraction (Figure S2.2) applying anisotropic fitting. Transmission electron microscopy (TEM)

imaging reveals typical rod-shaped nanoparticles with diameters varying from 13 to 51 nm (Figure 1

a-b, Figure S2.3). Fast Fourier transform (FFT) analysis of bright field TEM images of several

particles (Figure S2.4) indicates in contrast to a former report[70] the preferential growth of the rods

along the [001] direction. In addition, the power spectrum in Figure 1b reveals elongated spots, in

particular for the (011) direction indicating a defective structure. Inverse Fast Fourier transformation

(IFFT) of the 011 spots (Figure S2.5) indicates the occurrence of planar defects within the lattice.

From the basal-area of two condensed nano-rods (Figure 2.1c) surface terminations can be

distinguished that include (010), (110) and (100) planes. The dimensions of the oriented particles in a

and b direction shown in Figure 1c are in reasonable agreement with the average crystallite size

determined by anisotropic Rietveld refinement (Table S2.1). This implies that the particles chosen for

TEM analysis are good representatives of the polycrystalline powder catalyst. A few bigger particles

are also found by TEM (Fig. 2.1b). The atomic resolution high angle annular dark field- scanning

transmission electron microscopy (HAADF-STEM) images (Figure 2.1d and e) viewed along [001]

indicate the presence of two kinds of atomic dumbbells which can be distinguished by their different

contrast. In HAADF-STEM the contrast is due to Rutherford scattering proportional to approximately

Z². Thus, the dumbbells can be attributed to W2O8 (high contrast) and Mn2Oy (less contrast) dimers. In

the schematic representation of the MnWO4 crystal structure they correspond to the orange and white

edge-sharing octahedrons, respectively (Figure 2.1f).

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Atomic resolution HAADF-STEM images of the surface structure of the (010) plane viewed along

[001] are shown in Figure 2.2 and illustrate a preferential surface exposure of Mn ions as unimers or

dimers. The images indicate a slight out of center shift of some Mn ions compared to their bulk

crystallographic position.

Figure

2.1 (S)TEM analysis of the MnWO4 powder catalyst: a) Overview TEM micrograph of the MnWO

nanorods; b) HRTEM image of one nanorod particle viewed along [100]. The inset denotes the power spectra

recorded on either side of the defect; c) HAADF

-STEM image of condensed [001] oriented MnWO4

nanorods.

The surface termination was identified by phase analysis of corresponding atomic resolution HAADF

STEM

images; d) and e) atomic resolution HAADF

STEM images of (c); f) Schematic representation of the crystal

structure of MnWO

4 viewed along [001].

The Mn atoms are presented in white, W is coloured green and O

atoms are displayed in red. The original images are given in the SI (Figure S2.1, 6c).

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

The Raman spectrum (Figure S7) of the nano-structured MnWO4 agrees well with the spectrum of

crystalline MnWO4.[49, 50] However, two additional, previously unreported bands appear at 615 and

665 cm-1. Since phase purity and high crystallinity of the nano-structured material has been confirmed

by TEM (Figure 1 and Figure S2.8) and XRD (Figure S2.2), these two bands are attributed to the

MnOx clusters at the surface of the nano-rods (Figure S2.9) that have been visualized by STEM

(Figure 2.2).[71, 72]

100

IMFP=0.6 nm

IMFP=1.6 nm

Metal composition (atom.%)

0.0

0.5

1.0

1.5

2.0

Mn/W ratio

Mn/W atomic ratio

Figure

2.3 Depth profile of elemental composition of MnWO4 nano-rods a

t two different depths represented by

the inelastic mean free path (IMFP) of electrons measured by synchrotron

-based NAP-

XPS at T=300°C

applying a total pressure of 0.25 mbar O

2 and He at flows of 2 and 2.2 sccm, respectively.

Figure

2.2 Surface termination of the b

plane viewed along the growth direction [001] by FFT filtered atomic

resolution STEM images a) HAADF, and b) inverted HAADF; The violet and green circles denote W and Mn

atoms, respectively. The micrographs correspond to magnified images of Figure 1d. Original images are given in

the SI (Figure S2.6).

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Synchrotron-based near ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) reveals the

enrichment of Mn on the outmost surface (Inelastic mean free path (IMFP) = 0.6 nm), within which

the molar ratio of Mn to W is 1.9 (Figure 2.3). The Mn/W ratio decreases to 0.8 in the sub-surface

(IMFP = 1.6 nm), which is close to bulk stoichiometry of Mn and W.

In summary, an enrichment of the surface of MnWO4 nano-rods with manganese in the phase-pure,

highly crystalline material was indicated by photoelectron spectroscopy. The specific surface

termination is also reflected in the Raman spectrum. In line with these integral methods, the locally

resolved atomic resolution HAADF-STEM images (Figure 2.2) present a partial Mn surface

termination of the (010) planes. Thus, it is possible that the Mn enrichment observed by integral

methods could be primarily attributed to an increased occurrence of the Mn terminated (010) planes

(Figure 2.2) in the nano-structured material.

The specific surface / nano-structure of MnWO4 is generated under hydrothermal conditions. In-situ

Raman spectra taken during the synthesis (Figure 2.4) provide important information on the phase

formation of MnWO4. MnWO4 nano-rods develop at around 125°C while the mixture of the aqueous

solutions of Na2WO4 and Mn(NO3)2 is heated in the autoclave (Figure S2.10). An intermediate (910

cm-1) is transformed to crystalline MnWO4, indicated by the appearance of bands at 884, 325, 397,

510, 544, 672 and 698 cm-1. The strongest band at 884 cm-1 has been assigned to the stretching mode

of W=O in distorted WO6 octahedrons.[49, 50] Interestingly, two new bands at 615 and 665 cm-1

assigned to surface MnOx clusters[71, 72] grow in intensity with time when the synthesis temperature

of 180°C has been reached. These changes in the Raman spectra are attributed to dissolution-

recrystallization processes that most likely lead to the unique rod-shaped nano-structure and the

specific surface modification of the nano-structured material. MnWO4 crystallizes in a monoclinic

structure (wolframite-type, ICSD-67906) in which WO6 clusters form zigzag chains by sharing edges

along the [001] axis (Figure S2.9). In the basic medium under hydrothermal conditions (pH=9), WOx

clusters on the surface of the (010) planes could be dissolved due to the nucleophilic attack of OH-

ions at the bridging W-O-W bonds (Equation 1) leaving behind leached (010) surfaces composed of

MnOx zigzag chains. Subsequently, the bridging oxygen anions at the (001) surface may condense

with the dissolved WO42- monomers (shoulder at 926 cm-1 in Figure 2.4). Hence, the zigzag chains

will propagate in one dimension by forming new O–W-O edge-sharing bridging bonds. Such a

dissolution-recrystallization process may result in the observed anisotropic crystal growth along the

[001] axis resulting in the rod-shaped morphology of the MnWO4 particles.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

MnWO4 + 2OH- ⇋ Mn(OH)2 + WO42- (2.1)

The nano-structured MnWO4 material was studied as a catalyst in oxidative dehydrogenation of

propane. Conventional bulk crystalline MnWO4 is rather inactive in the reaction, which is reflected in

no measurable conversion at 450°C (reaction conditions described in the Supporting Information).

Nano-structuring, however, as achieved by hydrothermal synthesis in the present work, can turn this

material into an active catalyst. Moreover, the unique surface structure of the MnWO4 nanorods

exhibits superior selectivity to the desired product propylene than bulk manganese oxide (Figure 2.5).

Manganese oxide is very active (Table 2.1), but not very selective and transforms propane almost

completely into carbon oxides. Electronic modification and/or geometric separation of the MnOx

zigzag chains on the (010) planes by W2O8 units (Figure 2.2) apparently account for the improved

performance. Propane conversion has been changed by varying the contact time at 400°C (Figue 2.5).

The comparatively low temperature was chosen to avoid the influence of homogeneous gas phase

reactions, which normally contribute to non-negligible conversion at elevated reaction temperatures

above 450°C. The catalytic performance of MnWO4 nano-rods reached steady state after 70 hours on

stream and showed no sign of deactivation within 108 hours (Figure S2.11). Importantly, Mn

terminating MnWO4 nano-rods exhibit much higher apparent turnover frequency (TOF) than

vanadium oxide species supported on silica (Table 2.1) when all Mn atoms at the (010) surfaces and

all V atoms are taken into account as active sites. In reality, the number of active sites at the surface of

the MnWO4 nano-rods is, however, perhaps much lower. The Mn 2p XPS spectra (Figure S2.12)

indicate the predominance of Mn in oxidation state two at the surface and within the subsurface. Tiny

300 400 500 600 700 800 900 1000

926

618

657

552 min, 182°C

323 min, 182°C

143 min, 182°C

139 min, 182°C

93 min, 181°C

65 min, 121°C

63 min, 118°C

61 min, 117°C

5 min, 22°C

Intensity (a.u.)

Raman shift (cm

-1

)

325

397

510

544 698

884

910

672

Figure

2.4 In-situ Raman spectra in the 280-1000 cm-1 range measured during hydrothermal synthesis of

MnWO

4 nano-rods; Bands denoted with asterisks belong to the sapphire window of the Raman probe.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

differences have been observed in the spectra under different reaction atmospheres (Figure S2.13),

however, the interpretation of the Mn 2p spectra is not straightforward due to less distinct variations

in the binding energies of compounds containing Mn in different oxidation states, an elaborate

multiplet splitting, and the appearance of satellites.[73, 74] In contrast, near edge X-ray absorption

fine structure (NEXAFS) is sensitive to detect the surface oxidation of manganese oxides,[75, 76] and

changes in the coordination environment of the Mn ions.[77] The measured line shape of the Mn L2,3-

edge (Figure S2.14) agrees well with those from large single crystals of MnWO4.[57, 78] A

predominance of Mn in oxidation state two is confirmed by the spectra both in oxygen and reaction

atmosphere. Nevertheless, a small increase in the intensity ratio of the peaks at 640.0 and 641.4 eV

has been observed during reaction (Table S2.2). With increasing oxidation state or, in other words,

with increasing coordination of the Mn ions by oxygen atoms, intensity in the Mn L2,3-edge spectrum

occurs at higher energy.[75, 76] Hence, the changes in the intensity ratio of the peaks at 640.0 and

641.4 eV indicate that in reaction feed the concentration of oxygen defects is higher compared to

oxygen atmosphere suggesting a substantial impact of oxygen defects on catalyst performance.

Oxygen defects have been quantified by temperature-programmed oxidation (TPO) and reduction

(TPR) cycles (Figure S15). Before first TPO run, the catalyst was heated in Argon at 400°C for 2

hours. Approximately 3 oxygen atoms per nm2 could be replenished after this treatment (Table S2.3).

This underlines the notion[22, 79] that only a fraction of surface atoms is catalytically active, which,

however, holds great challenges in terms of the identification of active sites. The very similar

hydrogen consumption profiles of the two TPR runs indicate that approximately 5% of the surface

oxygen atoms were reversibly removed by reaction with H2 (Table S2.3). In addition, twice as much

oxygen atoms were replenished during the first TPO, indicating a higher oxygen vacancy density after

Ar treatment, which was also reflected by the higher initial activity in oxidative dehydrogenation of

propane during the first 18 hours on stream (Figure S2.11).

After washing with nitric acid solution and re-calcination under the same condition as that of the

pristine MnWO4 nano-rods, the catalytic activity decreased dramatically (Table 2.1). Only Mn was

detected in the washing solution (Table S2.4). This further corroborates our argument that surface

MnOx chains are the active sites of the nano-structured MnWO4 catalyst in oxidative dehydrogenation

of propane.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

2.3 Conclusion:

In summary, we demonstrated that a catalytically inactive solid, like MnWO4, was converted into a

highly active and selective catalyst by knowledge-based synthesis. Hydrothermal techniques guided

by in-situ spectroscopy[80] have been applied to control the surface termination. In-situ Raman

Figure

2.5 Catalytic performance of nano-structured MnWO4 (T=400°C, W/F=1.8-0.9 g s/ml) in comparison to

x/SiO2 (T=400-420°C, W/F=1.8 g s/ml) and various manganese oxides (T=330-395°C, W/F=1.8 g s/ml) in

the oxidative dehydrogenation of propane

in a feed composed of C3H8:O2:N2 in a ratio of 10:5:85. The

selectivity to propene is shown as a function of propane conversion. Other C

containing products are mainly CO

and CO

Table 2.1. Active site density and reactivity of the catalysts in the oxidative dehydrogenation of propane at

T=400°C, W/F=1.8 g s/ml in a feed composed of C3H8:O2:N2 in a ratio of 10:5:85.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

spectroscopy provided insight into molecular processes of crystallization, surface dissolution and

recrystallization under hydrothermal conditions. The unique self-supported structure of one-

dimensional MnOx clusters at the surface of nano-structured MnWO4 demonstrates the importance of

understanding the synthetic inorganic chemistry from a molecular point of view[81] and the

significance of studying the surface termination[1, 42, 46] of well-defined nano-structured metal

oxides[82]. By identification of the highly active surface MnOx on the MnWO4 nano-rods, the

promotional effect of Mn in many catalytic systems could be better understood. The design of

selective oxidation catalysts will benfit from this knowledge and it will lead to an improvement of

current Mn doped catalytic systems.

2.4 Experimental details:

2.4.1 Synthesis of the catalyst

2.4.1.1 Starting materials

Mn(NO3)2·4H2O (98%, Roth), Na2WO4·2H2O (99%, Sigma Aldrich), NaOH (98%, Alfa Aesar),

were used as received. Ultrapure water was obtained by using the Milli-Q Synthesis System

(MQ). Commercial MnWO4 (ID 18507) used as reference catalyst was purchased from Alfa Aesar

(99.9% metal basis, 200 mesh powder), the powder was then pressed and sieved to a particle

size of 250-355 µm for catalytic testing.

2.4.1.2 Hydrothermal synthesis of nano-structured MnWO4

Nano-structured MnWO4 was synthesized by a method modified from literature.[70] In the first step,

an aqueous 0.2 M solution of Mn(NO3)2 was added to an aqueous 0.2 M solution of Na2WO4 while

stirring at 295 K. In the second step, 5.8 mL of an aqueous 0.1 M NaOH solution was added to adjust

the pH value to 9.9. In the third step, the mixture was transferred to an analytical autoclave HPM-PT-

040 (Premex Reactor GmbH), and the temperature was raised from 295 K to 453 K at a rate of 5

K/min. Hydrothermal synthesis was performed at 453 K at autogenous pressure for 12 h. During

hydrothermal synthesis the pH was recorded (Figure S10) using a pH probe (ZrO2 probe Model A2

and Ag/AgCl reference electrode, both with a 1/2“ outer tubing made from C-276; Corr Instruments).

In the fourth step, the solid product was separated from the mother liquor by centrifugation, and

washed twice with de-ionized water. In the final step, the solid was dried in a muffle furnace in air at

353 K for 12 h. A brownish solid (ID 18942) was collected and thermally treated in argon (flow rate:

50 mL/min) at 673 K for 2 h in a rotating quartz tube to receive the final catalyst (ID 19116).

2.4.1.3 Washing of nano-structured MnWO4 with nitric acid solution

As-prepared MnWO4 (ID 18942) was washed with 2 M solution of nitric acid at 60°C for 1 hour.

The solid was separated from the washing solution by centrifugation. The washing solution was

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

analysed by XRF. After centrifugation, the solid was washed with de-ionized water twice and

dried in a muffle furnace in air at 353 K for 12 h (ID 20640). Then, the material was thermally

treated in argon (flow rate: 50 mL/min) at 673 K for 2 h in a rotating quartz tube to receive the

final acid washed catalyst (ID 20655).

2.4.1.4 Reference catalysts manganese oxides and VOx/SiO2

MnO2 (ID 18625) with SBET=5.1 m2/g was achieved by thermal decomposition of Mn(NO3)2·4H2O

in O2/Ar at 280°C. Mn3O4 (ID 18856) with SBET=11.0 m2/g was synthesized by mixing Na2B4O7

(0.01 mol) and Mn(II)ac2 (0.01 mol) dissolved in 100 mL milipore water and subsequent

addition of a stated amount of aqueous solution of NaOH (0.1 mol) and then vacuum drying of

the mixture. Mn2O3 (ID 19405) with SBET=0.5 m2/g was purchased from Aldrich.

VOx/SiO2 (ID 18341) with SBET=377 m2/g and vanadium loading of 4.1wt.% was prepared by ion

exchange, in which an aqueous NH4VO3 solution was added to dispersed modified-SBA-15 (ID

18026) functionalized by (3-Aminopropyl) trimethoxysilane (APTMS) in de-ionized water as

described before.[83] The catalyst was calcined in a rotating furnace (Xerion Advanced Heating

GmbH) at 550°C for 8 h under O2/Ar (20/80) (500ml/min). For preparation of modified SBA-15,

44.8g of Pluronic P-123 (poly(ethylene glycol)-poly(propylene glycol)- poly(ethylene glycol))

was dissolved in 1.6L of HCl (1.6M), stirred and heated at 35°C in an automated laboratory

reactor (LabMax, Mettler-Toledo). After complete dissolution, 85.1g of TEOS were added. After

12h stirring at 35°C, another 45g of TEOS were added. After stirring, the solution was heated in

autoclaves at 110°C for 24h. Then, the solid was filtered and washed until the filtrate was

neutral. The solid was dried in a furnace at 80°C overnight (ID 18009) and then calcined in two

batches at 550°C for 8h under O2/Ar (20/80) (500ml/min) to obtain the final modified SBA-15

support (ID 18026).

2.4.2 Catalyst characterization

2.4.2.1 Electron microscopy

Transmission electron microscopy (TEM) studies were conducted on a Philips CM200 FEG

transmission electron microscope operating at 200 kV. High resolution TEM (HRTEM) and high

resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM)

were performed on a Cs corrected FEI TITAN 80-300 operated at 300 kV and a double corrected

JEOL JEM-ARM200F equipped with CEOS CESCOR, and CEOS CETCOR hexapole aberration

correctors for probe and image forming lenses, respectively, and a cold field emission gun (CFEG).

The acceleration voltage was set to 200 kV. TEM samples were prepared by drop deposition from

ethanolic suspensions onto lacey-carbon coated Cu grids. Field emission scanning electron

microscopy (FESEM) was carried out with a Hitachi S4800 instrument operating at 5 kV.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

2.4.2.2 X-ray diffraction (XRD) and Rietveld refinement analysis

The X-ray diffraction (XRD) measurement was performed in Bragg-Brentano geometry on a Bruker

AXS D8 Advance theta/theta diffractometer, using Ni filtered Cu Kα radiation and a position

sensitive LynxEye silicon strip detector. The sample powder was filled into the recess of a cup-shaped

sample holder, the surface of the powder bed being flush with the sample holder edge (front loading).

XRD data were evaluated by whole powder pattern fitting according to the Rietveld method as

routine fitting, which uses an isotropic peak width model (i.e. the diffraction profile widths are

described as a smooth function of the diffraction angle, independent of hkl), systematic peak profile

mismatches of varying degree were observed. With the anisotropic crystallite shape observed by

electron microscopy in mind, we developed an appropriate anisotropic (i.e. hkl dependent) peak width

model. A model, which worked well, was obtained by modifying the phenomenological model

published by Stephens.[84] Due to the macro language implemented in TOPAS, user defined peak

models can be implemented easily. The original Stephens model, which was derived to describe

anisotropic strain broadening, did not work well with our data. Since we expected anisotropic

crystallite size to be the predominant peak broadening factor in our case, we replaced the angular

dependent term tan(θ) (representing strain) of the original Stephens model with a cos(θ)-1 (i.e. size)

term, while retaining the hkl dependent expression. In addition to a good overall fit, this modified

model allowed us to obtain the nominal crystallite size for different crystal directions. The largest

dimension was consistently calculated for the 00l direction. Nominal crystallite sizes are reported here

for the principal crystal directions h00, 0k0 and 00l (Table S1). It should be noted that such values

represent volume weighted average lengths of unit cell columns, LVol-IB. This includes averaging

over parallel columns of different lengths within crystallites (shape dependent), as well as averaging

over different crystallites of possibly different size (size distribution dependent). Thus, the reported

LVol-IB values cannot be directly compared to physical dimensions of discrete crystallites as e.g.

observed by electron microscopy. Nevertheless, the XRD derived dimensions may be considered to

represent a (volume weighted) average crystallite morphology. To simplify a comparison with the

results of other methods, “aspect ratio” Da/Db, and Dc/Db, respectively, were calculated from the

principal dimensions in Table S1.

2.4.2.3 Raman spectroscopy

Confocal Raman spectroscopy was performed using a Horiba Jobin LABRAM instrument equipped

with a microscope (Olympus). A He-Ne laser (wavelength 632.8 nm, 1.5 mW at the sample position)

was used for the excitation. A pressed wafer of the sample was mounted on the sample holder for

recording spectrum.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

In situ Raman during hydrothermal synthesis was carried out using a Raman probe (RXN1,

immersion optics 1/4”OD (HC276); Kaiser Optical Systems). The Raman spectra were automatically

recorded every 2 min at a wavelength of 785 nm with an exposure time of 30 s.

2.4.2.4 X-ray fluorescence (XRF)

XRF was performed using a Bruker S4 Pioneer X-ray spectrometer. For sample conditioning, beaker

of 25 mm diameter with 6 µ MYLAR foil was used to contain 5 ml of sample solution without any

pretreatment. Samples were measured under He atmosphere. The solvent (water) was assumed as

matrix and iteratively calculated to sum up the total to 100 %.

2.4.2.5 X-ray photoelectron spectroscopy (XPS) and Near edge X-ray absorption fine structure

(NEXAFS)

XPS and NEXAFS analyses were conducted at the near ambient pressure XPS end station of the

ISISS beamline at HZB/BESSY II (Berlin, Germany). Details of the setup have been published earlier.

[85] For the XPS measurements, the kinetic energy of the photoelectrons was varied between 150 eV

and 750 eV separately for each core level, hence escape depths (63% of the detected signal stems

from this depth) of ca. 0.6 nm (150 eV, denominated as surface) and ca. 1.6 nm (750 eV, denominated

as subsurface) were probed. The experiments have been performed at a total pressure of 0.25 mbar in

O2/He or C3H8/O2/He mixtures with a total gas flow of 4.2 sccm at temperatures between 300 °C and

400 °C. The error bar of the absolute elemental composition can be estimated to be 30% due to

uncertainties in the monochromatic photon flux, cross sections and peak area determination. However,

only the uncertainty in the peak area determination contributes to relative uncertainties in an

experimental series (different conditions with the same catalysts or different catalysts under the same

condition), and therefore the relative error bar in the XPS figures can be estimated to be

approximately 5%. Might be that the roughness of the surface changes as a function of the catalytic

conditions (oxidative or reductive). In this case the depth profile will change and therefore the error

bar increases.

NEXAFS spectra were recorded simultaneously in total electron yield (TEY) and Auger electron yield

(AEY) mode. Due to the low inelastic mean free path of electrons in solids, electron yield X-Ray

absorption spectroscopy (XAS) is more surface sensitive than fluorescence based techniques. The

highest surface sensitivity of XAS is given in the AEY mode in which Auger electrons on an selected

energy interval are analyzed by the spectrometer.[86] In the case of the recorded Mn L2,3- edge,

electrons with a kinetic energy around 50 eV were analyzed.

2.4.2.6 Nitrogen adsorption

The surface area determination was carried out in a volumetric N2 physisorption set-up (Autosorb-6-B,

Quantachrome) at the temperature of liquid nitrogen. The sample was degassed in dynamic vacuum at

a temperature of 573 K for 2 h prior to adsorption. The relative N2 pressure was varied (p/p0=0.05–

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

0.3), and 11 data points were measured. The linear range of the adsorption isotherm was considered to

calculate the specific surface area according to the BET method.

2.4.2.7 Temperature-programmed oxidation and reduction (TPO-TPR)

TPO was performed in a fixed-bed quartz reactor using 300 mg of the sample. Prior to the first TPO

measurement, the sample was pretreated at 400 °C for 2h in Ar (flow rate 50 ml/min, heating rate of

5 °C/min). The TPO measurement was performed up to 400°C in a mixture 0.24% O2/He (flow rate

100 ml/min), applying a heating rate of 5°C/min and a holding time of 60 min. O2 consumption was

monitored with a paramagnetic detector. After the TPO run, TPR was performed up to 400°C in 0.25%

H2/Ar (flow rate 100 ml/min), applying the same heating rate and holding time. H2 consumption was

monitored with a thermal conductivity detector (TCD). The TCD detector was calibrated by reducing

a known amount of CuO. Then the second TPO measurement was done followed by the second TPR

run applying the same procedures as in the first runs.

2.4.2.8 Catalytic testing of oxidative dehydrogenation of propane (ODP)

The catalytic tests were carried out using a setup for partial oxidation (Integrated Lab Solutions) with

8 fixed bed quartz reactors (6 mm inner diameter) in parallel. Each reactor was equipped with a

thermocouple for measuring the temperature inside the catalyst bed containing 300 mg of catalyst

previously sieved to a particle size of 250-355 µm and the catalytic performances were determined at

atmospheric pressure. The reactant feed comprised the C3H8, O2, and N2 as diluent passed each reactor

at a flow rate of 10 mL/min. The composition of the feed was 10 % C3H8, 5 % O2 and 85% N2.

The product (and bypass) gas mixtures were analyzed by an online gas chromatograph (Agilent 7890).

A system of Plot-Q and Plot-molsieve columns connected to a thermal conductivity detector (TCD)

separated the permanent gases CO, CO2, N2, O2, and CH4. A system of a FFAP and a Plot-Q column

connected to a flame ionization detector (FID) allowed the separation of C2-C3 hydrocarbons and

oxygenates.

2.5 Supporting information

2.5.1 Supporting tables

Da /nm

Db /nm

Dc /nm

Ratio Da/Db

Ratio Dc/Db

24.0

17.3

55.4

1.4

3.2

Table S

2.1

Average crystallite sizes D of the particles in the MnWO4 catalyst (ID 19116) along the

and

c axis calculated from anisotropic fitting in Rietveld refinement based on the XRD patterns presented

in Figure S2.2.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Treatment

Mode1

I640

I641.5

Peak ratio2

O2/He, n min

TEY

7.67

3.50

2.2

O2/He, n min

AEY

7.48

4.45

1.7

O2/He, n min

6.59

3.84

1.7

O2/He, n min

6.15

3.54

1.7

O2/He/C3, n min

TEY

8.11

3.59

2.3

O2/He/C3, n min

AEY

6.97

3.59

1.9

O2/He/C3, n min

7.14

3.33

2.1

O2/He/C3, n min

7.32

3.25

2.2

1 mode of measurement

2 peak ratio 640/641.5

TPO-1

TPR-1

TPO-2

TPR-2

O2 or H2 consumption /mmol mol-1

22.1

17.7

7.13

16.6

O2 or H2 consumption /10-6 mol m-2

2.54

2.03

0.82

1.91

Number of oxygen atoms replenished

or reduced

/nm

-2

3.06

1.22

0.99

1.15

Percentage of surface oxygen atoms

replenished or reduced

15.3

6.1

5.0

5.8

1Assuming that oxidation and reduction only occur at the surface;

Assuming a surface oxygen atom density of 20/nm

Table S

2.2

Intensity ratio of the peaks at 640 and 641.4 eV in the NEXAFS of nano-structured MnWO4 at

the Mn L

2,3

-edge. The spectra in Total Energy Yield (TEY) are presented in Figure S2.14.

Table S

2.3

Hydrogen consumption during TPR and oxygen consumption during TPO experiments with

nano-structured MnWO

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

MnO

Na2O

WO3

H2O

Concentration

/wt %

0.39

0.00

99.6

Table S5. Surface area and lattice parameters of the nano-structured MnWO4 materials.

* The catalyst contains small amounts of an unknown phase.

Table S

2.4

Chemical composition of the washing solution after treatment of nano-structured MnWO4

with nitric acid as determined by XRF.

SBET

-1

unit cell parameters

/Ǻ

/ Ǻ

/°

As-synthesized MnWO4

nano-rods (ID 18942)

31.0

4.824(1) 5.761(2) 4.998(7) 91.18(2)

Calcined MnWO4 nano-rods

(ID 19116)

28.7

4.828(7) 5.762(5) 5.000(7) 91.18(9)

Nitric acid washed and re-

calcined MnWO

(ID

20655)*

29.5

4.828(5) 5.758(8) 4.997(7) 91.18(3)

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

2.5.2 Supporting figures

Figure S

2.1

Original images presented in Figure 2.1.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

10 20 30 40 50 60 70 80

Yobserved

Diff

Intensity (a.u.)

2Theta (degree)

I Bragg peaks

Ycalculated

Figure S

2.2

Rietveld refinement of the powder XRD of the MnWO4 catalyst (ID 19116).

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

50 100 150 200 250 300

Count

Length (nm)

20 30 40 50

Diameter (nm)

23456789

Aspect ratio (a.u.)

Figure S

2.3

Particle size distribution in the thermally treated nano-structured MnWO4 catalyst (ID 19116

based on the analysis of

approximately 146 particles in TEM images (see for example Figure 2.1a in the

main manuscript).

Figure S

2.4

HRTEM images and fast Fourier transform (FFT) analysis of two MnWO

nanorods in the

catalyst (ID 19116).

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Figure S

2.5

Inverse Fast Fourier transformation (IFFT) of the 110 spots in Figure 2.1b.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Figure S

2.6

FFT filtered HR-STEM images of MnWO4 (ID 19116) a) HAADF, b) inverted HAADF and c)

HR-HAADF-STEM images.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

200 400 600 800 1000

MnWO

nanorod

commercial MnWO

210

Normalized intensity (a.u.)

Raman shift (cm

-1

)

264

332

402515

549

679

702

777

615

665

Figure S

2.7

Raman spectrum of the MnWO4 catalyst (ID 19116) (black line) compared to the Raman

spectrum of commercial MnWO

(orange line).

Figure S

2.8

HRTEM images and electron diffraction patterns of MnWO4 nano-rods.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Figure S

2.9

Schematic representation of the crystal structure of MnWO

viewed along a) the [001] axis

and b) the [100] axis. White ball represents Mn

2+, green ball represents W6+, red ball represents O2-

(bridging oxygen) and wine ball represents terminal oxygen atoms (tungsten oxygen double bond).

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

0100 200 300 400 500 600 700 800 900

100

120

140

160

180

200

Temperature

Temperature (°C)

Time (min)

Figure S

2.10

Recorded workflow during hydrothermal synthesis of nano-structured MnWO4; blue line:

pH value measured in the autoclave, red line: temperature of the synthesis gel.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Figure S

2.11

Time on stream plot of propane conversion and selectivity to propene in the oxidative

dehydrogenation of propane at T=400°C, and W/F=1.8 g s/ml over nano

-structured MnWO4 (catalyst ID

19116); The feed

was composed of C3H8:O2:N2

=10:5:85; The changes in the conversion (X) of propane and

the selectivity (S) to propylene and carbon oxides (COx: CO + CO

) are shown with time on stream.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

660 650 640

0.0

0.2

0.4

0.6

0.8

1.0

Mn 2p XPS intensity (norm.)/ arb. unit

Binding energy / eV

IMFP:

0.6 nm

1.6 nm

Figure S

2.12

Mn 2p spectra of nano-structured MnWO4 within different detection depths represented by

the inelastic mean free path (IMFP) of electrons measured by synchrotron

-based NAP-XPS at T=300 °C

applying a total pressure of 0.25 mbar O

and He flows of 2 and 2.2 sccm, respectively.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Figure S

2.13

Mn 2p spectra of nano-structured MnWO

within different detection depths represented by

the inelastic mean free path (IMFP) of electrons measured by synchrotron

-based NAP-XPS at T=300 °C

applying a total pressure of 0.25 mbar under different reaction atmospheres; Red lines: O

2 and He flows

of 2 and 2.2 sccm, respectively; Blue lines: O

, C

, and He flows of 1, 2, and 1.2 sccm, respectively.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

Figure S

2.14

NEXAFS of nano-structured MnWO

measured at the Mn L

2,3

-edge in total electron yield

(TEY) in different reaction atmospheres at T=380°C; Red lines: O

2 and He flows of 1 and 3.2 sccm,

respectively; Blue lines: O

, C

, and He flows of 1, 2 and 1.2 sccm, respectively.

Selective Alkane Oxidation by Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4

Nanorods

2.6 Acknowledgements

This work was conducted in the framework of the BasCat collaboration between BASF SE, TU Berlin,

FHI, and the cluster of excellence “Unified Concepts in Catalysis”. X.L. acknowledges the Berlin

International Graduate School of Natural Sciences and Engineering (BIG NSE) as part of UniCat for

financial support. The authors thank Maike Hashagen, Jasmin Allan, Achim Klein-Hoffmann, Dr.

Olaf Timpe, and Caroline Dessal for technical assistance. We thank the HZB staff for their continual

support of the electron spectroscopy activities of the FHI at BESSY II.

0.0

0.2

0.4

0.6

0.8

1.0

1.2

020 40 60 80 100 120

0.0

0.2

0.4

0.6

0.8

1.0

1st TPO

2nd TPO

O2 consumption (µmol/min)

100

150

200

250

300

350

400

450

391

Temperature (°C)

369

192 294

1st TPR

2nd TPR

H2 consumption (µmol/min)

Time (min)

100

150

200

250

300

350

400

450

Temperature

Temperature (°C)

264

336

Figure S

2.15

Temperature-programmed oxidation (TPO) (top), and temperature-programmed reduction

(TPR) (bottom) profiles of nano-structured MnWO

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

3 Hydrothermal synthesis of bi-functional

nanostructured manganese tungstate catalysts for

selective oxidation1

Abstract: The mechanism of C-H activation in selective oxidation reactions of short-chain alkane

molecules over transition metal oxides is affected by the balance of acid-base and redox sites on the

surface of the catalyst. Using the example of manganese tungstate we discuss how the relative

abundance of these sites can be controlled via synthetic techniques. Phase-pure catalysts composed of

the thermodynamic stable monoclinic MnWO4 phase have been prepared by hydrothermal synthesis.

Variation of the initial pH value resulted in rod-shaped nano-crystalline MnWO4 catalysts composed

of particles with varying aspect ratio. The synthesis products have been analysed by transmission

elecron microscopy, X-ray diffraction, infrared, and photoelectron spectroscopy. In-situ Raman

spectroscopy was used to investigate the dissolution-re-crystallization processes occuring under

hydrothermal conditions. Ethanol oxidation reaction was applied to probe the surface functionalities

in terms of acid-base and redox properties. Changes of the aspect ratio of the catalyst particles are

reflected in the product distribution induced by the different fraction of acid-base and redox sites

exposed at the surface of the catalysts in agreement with the proposed mechanism of particle growth

by re-crystallization during ageing under hydrothermal conditions.

3.1 Introduction:

Metal oxides are widely used as heterogeneous catalysts in the synthesis of chemicals, and in

energy conversion and storage applications.[22, 87-89] High performance, selectivity, and

stability of oxides in heterogeneous catalysis are bound to homogeneity of the solid.[90] In

complex reactions, such as selective oxidation of hydrocarbons, multi-functionality is

necessarily required and can be achieved either chemically or by nano-structuring.[58, 91]

Solvothermal techniques have been efficiently used in the preparation of metastable phases or

oxides with particular morphological properties.[92-97] However, the underlying preparation

strategies are often based on experience and parameter variation. Targeted design of metal

oxide catalysts with predictable functionalities needs deeper understanding of the chemistry in

precursor solutions and during nucleation, growth, and ageing.

1 The following chapter is the submitted version of [116], the peer reviewed published version

can be found with publisher DOI link: http://dx.doi.org/10.1039/C5FD00191A 37

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

In-situ Raman spectroscopy has been proven beneficial in the investigation of condensation

reactions of transition metal oxide species occurring at elevated temperature and pressure

inside an autoclave.[98, 99] Our recent study concerning the speciation of molybdates in

aqueous media revealed that the molecular structure of the oxo-anions in the temperature

range between 100 and 200°C differs significantly from room temperature at comparable

concentration and pH values.[99] Implicit understanding of the known condensation chemistry

of molybdates under ambient conditions is, therefore, not beneficial in terms of knowledge-

based planning the hydrothermal synthesis of molybdenum oxide-based catalysts. Likewise,

systematic in-situ spectroscopic studies of the synthesis of complex metastable mixed oxide

phases, like the so-called M1 phase of MoVTeNb oxide, resulted in the development of

modular preparation techniques that yield the desired phase more effectively in shorter

synthesis time with improved catalytic properties.[98]

Herein we present in-situ Raman spectroscopic and transmission electron microscopy studies

that provide insight into the mechanism of particle growth and re-crystallization during the

synthesis of the thermodynamic stable monoclinic manganese tungstate phase in wolframite-

type structure (ICSD 67906).[100] Nano-structuring of manganese tungstate features a feasible

strategy to control bi-functional properties of the mixed metal oxide with implications on the

selectivity in oxidation reactions applied to upgrade alkanes. We demonstrate the viability of

such a strategy by studying rod-shaped nano-crystalline MnWO

catalysts consisting of

particles with varying aspect ratio (AR). The particle morphology was controlled via the

concentration of OH

ions under hydrothermal synthesis conditions. Oxidation of ethanol was

chosen to probe the surface chemistry of the synthesized materials in a catalytic reaction. The

substrate molecule may undergo multiple pathways depending on the nature of the active sites

at the catalyst surface. Generally, the alcohol is oxidized to the aldehyde at redox active

centres whereas dehydration of ethanol to ethylene or the formation of diethyl ether indicate

surface sites that facilitate acid/base reactions.

3.2 Results and discussion

3.2.1 Phase formation and particle growth under hydrothermal conditions

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

The preparation of monoclinic manganese tungstate composed of the wolframite-type structure (ICSD

67906) was performed by hydrothermal synthesis for 12 hours at 180°C in aqueous medium. The

formation of the solid within the autoclave has been monitored by in-situ Raman spectroscopy. Figure

3.1 shows the spectra taken during synthesis of a slightly acidulated synthesis gel (pH of the starting

mixture 6.3, Figure 3.1a)) and a mixture in which the pH has been adjusted to 9.1 by addition of

NaOH solution (Figure 3.1b). Reaction between manganese nitrate and sodium tungstate occurs

immediately after mixing the precursor solutions at room temperature as it becomes evident from the

Raman spectra. The Raman spectrum of the Mn precursor solution shows only the nitrate peak at

1048 cm-1 and no peaks due to Mn-O stretching vibrations, since divalent manganese ions are present

under these conditions as an octahedral aquo complex. In the Raman spectrum of sodium tungstate

solution peaks at 931, 837, and 324 cm-1 occur due to the presence of dissolved [WO4]2- ions. The

peaks belong to the four Raman active fundamental vibrational modes (A,+E +2F2) of an undistorted

tetrahedron (Td symmetry).[101]

By mixing the two precursor solutions, the peak at 837 cm-1 disappears immediately, peaks at 507 and

713 cm-1 emerge, and the peak at 931 cm-1 is shifted to 912 cm-1 suggesting reaction between

Figure

3.1 In situ Raman spectra recorded during the synthesis of the catalysts a) AR1.5 and c) AR3.9 and the

corresponding profiles of temperature and pH during synthesis of b) AR1.5 and d) AR3.9; The symcbol * in the

Raman spect

ra indicates the bands of the sapphire window of the Raman probe.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

manganese nitrate and sodium tungstate solution at room temperature. The bands are attributed to the

formation of a partially crystalline product of unknown crystal structure that exhibits a layered-like

morphology (Fig. S3.1), indicated also by a peak in the XRD patterns at small angles that suggests

regular stacking with a d spacing of 0.8 nm. The intermediate contains Mn and W in a molar ratio

close to 1. During heating, bands at 205, 256, 325, 397, 510, 544, 698, and 884 cm-1 indicate phase

formation of MnWO4 above 100°C that is completed at 120°C.[102, 103] It has to be noted at this

point that the Raman spectra measured by using the immersion probe originate from contributions of

dispersed nanoparticles as well as from molecular species dissolved in the mother liquor. The phase

formation is additionally indicated by a sharp drop of the pH in the temperature range from 110-

120°C. The pH at 180°C rises above 8 during the synthesis of all catalysts. The average value

increases with increasing starting pH and amounts to 8.1 in the synthesis of the catalysts AR1.5 and

AR1.7, to 9.0 in the synthesis of the catalysts AR3.2 and AR3.9, and to 9.3 in the synthesis of catalyst

AR5.1. The regulation of the initial pH involves changes in the ionic strength of the mother liquor

from 1.20 mol/L for catalyst AR1.5 to 1.22 mol/L for catalyst AR5.1. The interpretation of the

evolution of the pH values during hydrothermal synthesis is not straightforward, since the measured

value results from superimposing condensation and hydrolysis reactions. All peaks observed at 180°C

except weak bands at 619 and 665 cm-1 belong to the normal modes of crystalline MnWO4.[102, 103]

The extra peaks are tentatively attributed to manganese hydroxide species as will be discussed below.

Irrespective of the initial pH value, crystallization occurs at 120°C (Fig. 3.1) implying that the

crystallization process is determined by thermodynamics, since the monoclinic wolframite-type P2/c

structure is the thermodynamic stable phase under the applied synthesis conditions.[100] According to

XRD, all synthesis products are phase-pure materials (Fig. S3.2). Thermal treatment in inert gas at

400°C does not change the phase composition of the products (Fig. S3.3).

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

The morphology of nano-sized MnWO4 can be tuned kinetically by adjusting the chemical potential

during synthesis within the autoclave. The structural alterations are induced by different dissolution-

recrystallization rates of the involved manganese tungstate species. These rates are predominantly

controlled by the starting pH value of the synthesis gel. Thus, the shape of the primary MnWO4

nanoparticles in the hydrothermal product crucially depends on the starting pH and varies from cube-

like particles (starting pH=6.3) to anisotropic nano-rods (starting pH=9.9) (Fig. 3.2). These changes

are reflected in the aspect ratio of the MnWO4 nanoparticles, which can be obtained by measuring

length and width of the nanoparticles from the TEM images (Figs. 3.2, S3.4, and S3.5, Tab. 3.1).

Before catalytic testing the as-synthesised materials have to be thermally treated above the reaction

temperature of the catalytic reaction. Annealing in argon at 400°C has no significant influence on

shape and size of the nanoparticles (Fig. 3.2), but the specific surface areas are slightly reduced (Table

3.1). Electron microscopy indicates a defect-rich structure and preferential growth of the rods along

the <001> axis. Normally, crystallization and particle growth are induced by minimization of the free

energy of the system. At the given chemical potential this observation suggests that the (001) surface

is a high-energy surface. Thus, growing along <001> avoids its exposure to the environment. In

addition, the fraction of surfaces with lower energies increases significantly, which improves the

stabilization of the nanoparticle. In fact, analysis of the HAADF-STEM images of AR1.5 and AR5.1

allows for the allocation of specific facets of the particles, including (100), (010) and (110) crystal

planes as illustrated in Fig. 3.3.

Figure

3.2 Electron microscopy images of the as-synthesized (top row) and thermally treated (bottom row)

nanostructured MnWO

4 materials AR1.5 (a) and f)), AR1.7 (b) and g)), AR3.2 (c) and h)), AR3.9 (d) and i)), and

AR5.1 (e) and j)); Uncoloured TEM images are presented in the Supporting Information (Fig. S

3.4).

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

The dissolution-recrystallization processes occurring during ageing under hydrothermal conditions in

the autoclave at 180°C are illustrated in Fig. 3.4. The dissolution of [WO4]2- ions and the hydrolysis of

[Mn(H2O)6]2+ species by deprotonation of water ligands is facilitated in basic environment in the

presence of a high concentration of OH- ions. Anisotropic growth is imaginable when the local

structure of the WO6 chains at different planes at the surface of the MnWO4 crystals is taken into

consideration. At the (010) crystal planes, edge sharing -O-W-O- bonds are exposed that can be easily

attacked by OH- ions leading to dissolution of [WO4]2- species. Hydrolysis is hindered at other planes,

for example at (100) and (110), due to geometric reasons. Only half of the –O-W-O- bridges in a WO6

octahedron are exposed at these planes. Therefore, the WO4 units cannot be dissolved. Free [WO4]2-

species could condense with W-O(H) groups at the high-energy (001) basal planes propagating the

tungsten oxide zigzag chains by forming new edge sharing O–W-O bonds (Fig. 3.4). Divalent

manganese ions may interact with negatively charged tungstate chains and are in this way

incorporated into the structure. With increasing OH- concentration, the dissolution-recrystallization

process is enhanced and nanoparticles with increased aspect ratio are formed as evidenced by particle

size analysis (Fig. S3.5, Table 3.1). The statistical analysis based on the TEM images is in agreement

with the average particle diameter found by XRD. With increasing initial pH value, the (010) peaks in

the XRD patterns (Fig. S3.3) become more broadened, providing additional support for smaller

crystallite sizes along the <010> direction with increasing pH (Table 3.1). During dissolution-

recrystallization, hydrolysis of [Mn(H2O)6]2+ species leads to the formation of Mn(OH)x species at the

surface of the catalysts as indicated by an increasing intensity of the two bands at 619 and 665 cm-1 in

the Raman spectra recorded during hydrothermal synthesis that are tentatively attributed to surface

manganese oxide-hydroxide species.[71] The appearance of the corresponding bands is more distinct

in reaction mixtures that contain a higher concentration of OH- ions (compare Fig. 3.1a and c).

Figure

3.3 HAADF-STEM images of MnWO4 nanoparticles viewed along <001> with different aspect ratios: a)

AR1.5, b) AR5.1 and c) perspective model for a typical faceted nanoparticle. The original uncoloured images

including Fast Fourier transform analysis are given in the Supporting Informatio

n, Fig. S3.6.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

However, the concentration of dissolved tungsten oxide and manganese species is apparently rather

low, since no peaks due to such species in solution are detectable by Raman spectroscopy using the

immersion probe.

a required to identify different batches of catalyst synthesis, catalyst after thermal activation at 400°C for 2 hours

in Argon

b pH value adjusted before the hydrothermal synthesis

c after drying at 80°C

d number average values analysed based on TEM images as shown in Figs. 2, S4

AND

e based on XRD of the catalysts (Fig. S3)

3.2.2 Ethanol oxidation

The side products ethylene and diethyl ether in the oxidation of ethanol to acetaldehyde reveal the

presence of extra acid/base functionalities at the catalyst surface of an oxidation catalyst. Ethanol

oxidation has been performed over all catalysts within the reaction temperature range from 280 to

310°C achieving conversion of ethanol from 5 to 15% (Fig. S3.7). Under these conditions

acetaldehyde as well as ethylene are formed over all catalysts, but the selectivity is different for the

catalysts with different aspect ratios of their primary particles. In Fig. 5 the product distribution is

Table

3.1 Specific surface area, results of shape analysis based on TEM, and crystallite size calculated from

anisotropic fitting in Rietveld

refinement of the XRD patterns

Catalyst

IDa

pHb

Surface area of

hydrothermal

productc

/g)

Surface

area of

catalysta

/g)

Number

particlesd

Mean particle

lengthd

(nm)

Mean

particle

diameterd

(nm)

Mean

aspect

ratiod

D<100>

(nm)

D<010>

(nm)

D<001>

(nm)

AR1.5

19112

6.3

27.7

25.9

132

53.8

35.7

1.5

27.1

27.3

41.4

AR1.7

19113

6.7

26.1

22.9

124

59.0

35.1

1.7

27.2

26.3

46.8

AR3.2

19114

8.0

23.3

22.1

104

32.6

3.2

28.5

25.6

70.3

AR3.9

19251

9.1

25.7

24.0

125

119

30.8

3.9

27.2

22.1

65.3

AR5.1

19116

9.9

31.0

28.7

131

122

24.4

5.1

24.0

17.3

55.4

Figure

3.4 Schematic representation of the proposed anisotropic mechanism of particle

growth.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

compared at 10% ethanol conversion. The ethylene selectivity is quite high over the catalyst AR1.5,

but decreases as the aspect ratio increases.

The selectivity to acetaldehyde decreases with increasing temperature and conversion, whereas

ethylene selectivity rises (Fig. S3.7). At constant temperature the selectivity to acetaldehyde decreases

in general slightly with time on stream, whereas ethylene selectivity is quite constant or even

increases (Fig. S3.7) suggesting no structural relations between redox and acid-base sites.

The specific rate of the acid-catalysed dehydration reaction decreases with increasing aspect ratio (Fig.

Figure

3.5 Selectivity in ethanol oxidation over the nanostructured MnWO4 catalysts at 10% ethanol conversion.

Figure

3.6 a) Specific reaction rates measured at T=310°C and normalized to surface area, and apparent activation

energy as a f

unction of the aspect ratio; Red solid circle: acetaldehyde formation rate; red open circle: apparent

activation energy of acetaldehyde formation; black solid triangle: ethylene formation rate; black open triangle: apparent

activation energy of ethylene fo

rmation; b) Rate of formation of ethylene as a function of Brønsted acid site density at

the catalyst surface determined by ammonia adsorption and specific surface area measurements.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

3.6). In contrast, the specific redox reaction rate exhibits a flat maximum at the aspect ratio 3.2. It

should be noted at this point that the rates are integral rates. Conclusions about the intrinsic activity

are not possible from these values. However, the apparent activation energy of ethanol oxidation is

quite constant (48±6 kJ/mol) suggesting that not the nature, but the number of active sites changes in

the current catalyst series.

The observed trends may be related to a change in the relative fraction of acid-base and redox species

at the catalyst surface that may be, again, related to differences in the termination of the catalyst

particles within the current series. The presence of a defect rich MnOx chain-like 2D over-layer,

particularly, at the (010) termination of particles in the catalyst AR5.1 has been verified by electron

microscopy and photoelectron spectroscopy.[104] These MnOx chains might bear oxygen defects,

which are believed to provide the active sites in the activation of propane. The same manganese oxide

sites might be relevant for oxidation of ethanol in the present experiments as well. Due to the

preferential growth of the particles along the <001> direction, the relative abundance of (010) planes

that host these manganese oxide sites for oxidation catalysis may increase with increasing aspect ratio

(Fig. 3.3). Therefore, an aspect ratio above three is reflected in increased integral rates of the

oxidation reaction compared to the integral rates of acid-catalysed dehydration reaction over the

corresponding catalysts. The sites responsible for dehydration may be, in contrast, preferentially

located at the two ends of the rods, which are terminated by (001) crystal planes. According to

structural considerations, Brønsted acid sites might be preferentially located at these planes (Fig.

S3.8). Therefore, the two catalysts AR1.5 and AR1.7 exhibit enhanced acid-base functionalities,

which is in agreement with the shape (Figs. 3.2, 3.3a)) and the proposed dissolution-recrystallization

mechanism (Figs. 3.4, S3.8).

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Experimentally, the acidity of the catalysts has been studied by infrared spectroscopy. After heating to

reaction temperature of ethanol oxidation (300°C) in vacuum, three bands located at 3470, 3400, and

3366 cm-1 are observed in the O-H stretching region on top of a broad feature (Fig. 3.7a)). The latter is

attributed to hydroxy groups that undergo hydrogen bonding or strongly adsorbed water molecules,

respectively. A residual band is observed at 1642 cm-1 that might be due to the bending mode of

molecular water, but discrimination of this band from overtones and combination vibrations of

MnWO4 is difficult. Based on the very low frequency and the unusual narrow bandwidth, the three

sharp bands at 3470, 3400, and 3366 cm-1 are tentatively assigned to hydroxy groups that form well-

ordered hydrogen bonding networks at the surface. Further investigations are necessary to clarify the

origin of these bands that are characterized by unusual low O-H stretching frequencies. To investigate

the acidity of the hydroxy groups, NH3 was adsorbed at the surface of the catalysts after evacuation at

300°C for 1 hour at 40°C. Unfortunately, the bands of adsorbed ammonia in the N-H stretching

vibration region between 3400 and 3000 cm-1 overlap with the hydroxy bands due to the low

frequency of the latter (Fig. 3.7a)). Therefore, the consumption of particular OH species in the

reaction with ammonia cannot be monitored. But weak acidity is confirmed by formation of a weak

but distinct band at 1439 cm-1 due to the asymmetric deformation vibration of ammonium ions formed

by reaction of ammonia with Brønsted acid sites at the surface of the catalysts AR1.5, AR1.7, and

AR3.9 (Fig. S3.9). No clear indications regarding redox processes of adsorbed ammonia molecules

have been observed in the spectra. The concentration of acid sites at the surface of catalyst AR5.1 was

below the detection limit. The small number of acid sites (in the range from 0 for AR5.1 to 1.2 μmol

Figure 3.7 a) Infrared spectra in the

region of OH stretching vibrations after thermal treatment of the catalyst in the

infrared cell in vacuum at 300°C; The measurement was performed in vacuum at 40°C; W 4f spectra (b)), and O 1s

spectra (c)) of the catalysts AR1.5 and AR5.1 measured by synchrotron-based near ambient pressure X-

ray

photoemission spectroscopy (NAP-XPS) at an inelastic mean free path (IMFP) of ca. 0.6 nm in 0.25 mbar in O2

/He

at a total gas flow of 4.2 sccm at 300°C.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

g-1 for AR1.5) suggests that most of the OH groups observed in the infrared spectra of the catalysts

after dehydroxylation at 300°C in vacuum (Fig. 3.7a)) are nonacidic in nature. Essentially all OH

groups observed at the surface of catalyst 5.1 are not able to protonate ammonia. Therefore, the band

at 3366 cm-1 that represents the dominant hydroxy species at the surface of catalyst AR5.1 is

tentatively assigned to Mn-OH groups. A sharp band below 3500 cm-1 has been attributed to Mn-OH

in the layered Mn2+ hydroxide Mn2(OH)2SO4.[105] The surface concentration of acid sites correlates

with the formation rate of ethylene (Fig. 3.6b), demonstrating the involvement of these sites in

dehydration of ethanol. The non-zero intercept of the fitting line in Fig. 3.6b) suggests that Lewis acid

sites may be involved in dehydration of ethanol as well. Pathways leading to ethylene from ethanol

adsorbed on Ce4+ have been discussed.[106] Similar reaction routes might also be possible at

cordinatively unsaturated manganese or tungsten sites that are present at the surface of the catalysts as

it becomes evident from ammonia adsorption (Fig. S3.9). The peak at 1591 cm-1 may arise from the

asymmetric bending vibration of ammonia molecules adsorbed at Lewis acid sites.

A high concentration of hydroxy groups at the surface of the catalyst AR1.5 is also confirmed by

NAP-XPS measured in presence of oxygen at 300°C. The O 1s core level spectra of the catalysts

AR1.5 and AR5.1 are shown in Fig. 3.7c). The main peak at 530.2 eV is assigned to lattice oxygen,

while the component at 531.6 eV is attributed to hydroxy groups. Fig. 3.7c) shows that catalyst AR

1.5 comprises a higher concentration of hydroxy groups than catalyst AR5.1. A detailed analysis of

the W 4f core level reveals that also the W 4f7/2 and W 4f5/2 doublet (Fig. 3.7 b)) can be deconvoluted

into two contributions. The low binding energy doublet at 35.4 and 37.5 eV is in good agreement with

the binding energy values reported in the literature for metal tungstates.[107, 108] The high binding

energy doublet at 36.0 and 38.1 eV is more pronounced in the AR 1.5 catalyst than in the AR5.1

catalyst, in line with the higher hydroxyl concentration observed in the O 1s spectrum of catalyst

AR1.5. Furthermore, the contributions of the high binding energy W 4f doublet as well as the

hydroxide component in the O 1s spectrum are decreasing with increasing probing depth (compare

Figs. 3.7 b), 3.7 c), S3.10 a), S3.10 b)). Therefore, we tentatively assign the high binding energy

doublet in the W 4f spectrum to modified tungsten atoms bearing hydroxy groups at the surface of the

catalyst. Thus, the XPS measurements indicate that more W-OH groups are present at the surface of

the catalyst containing primary particles with a low aspect ratio (AR1.5). These results are

complementary to the characterization of the catalysts by ammonia adsorption. The acid sites probed

by FTIRS of adsorbed ammonium ions are attributed to W-OH groups at the surface of the catalyst,

which is also plausible in terms of the acid-base chemistry of tungsten compared to manganese. Mn-

OH groups are apparently not resolved by XPS, neither in the O 1s spectra (Fig. 3.7c)), nor in the Mn

2p spectra (Fig. S3.11). The discrepancy between FTIRS and XPS concerning the presence of

hydroxy groups at the surface of the two catalysts might be due to the fact that the Mn-OH

contributions are enveloped by M-O contributions in the O1s (Fig. 3.7c)) and Mn2p (Fig. S3.11)

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

spectra, or due to differences in the pretreatment procedures before IR (vacuum, 300°C) and XPS

(0.25 mbar O2 at 300°C) measurements, respectively.

In summary, the variation of the initial pH in the hydrothermal synthesis of nanostructured MnWO4

affects the dissolution-recrystallization kinetics of the freshly formed particles during ageing at 180°C.

Although these dissolution and recrystallization processes are responsible for anisotropic particle

growth, more important is that ageing provides the basis for the formation of the catalytically active

sites. With increasing pH under hydrothermal conditions the fraction of acid W-OH groups at the

surface of the final catalyst decreases and, consequently, acid-base catalysed side reactions during

ethanol oxidation are increasingly suppressed, which is reflected in enhanced selectivity to

acetaldehyde with increasing starting pH (Fig. 3.5).

Selectivity is the major issue in oxidation catalysis.[58, 109] Activation of C-H bonds in saturated

hydrocarbons is challenging because the formed reaction products and intermediates easily undergo

consecutive and parallel reactions. Minimization of rates of undesired pathways requires co-ordinated

design of bulk electronic properties and surface dynamics of oxidation catalysts.[68] C-H bond

activation may involve multiple mechanisms including carbenium or carbonium intermediates and

homolytic splitting of C-H bonds at metal oxide surface functional groups under formation of radical

species.[110, 111] Model calculations, generally based on small cluster models, favour the homolytic

pathway over transition metal oxide catalysts.[112] However, Lewis acid sites in terms of

coordinatively unsaturated metal cations and Brønsted acid sites may be present at the surface of a

transition metal oxide under reaction conditions as well. Brønsted acid sites are particularly expected

since water is an unavoidable coproduct in oxidation reactions and, as demonstrated in the current

example, the dehydroxylation temperature is often above the reaction temperature.

Whereas acidity at the surface of nanostructured MnWO4 catalysts is attributed to the presence of W-

OH groups and coordinatively unsaturated metal cations, the origin of redox activity is not that

straightforward. During particle growth under hydrothermal conditions the formation of defects may

occur. In the course of dissolution and recrystallization chemical defect in terms of cationic vacancies

or OH- groups at anionic positions are generated that may be related to structural defects.[113]

Establishing relations between defect chemistry and redox activity requires further kinetic studies and

more comprehensive characterization of the catalysts, which is currently under way.

3.3 Conclusions

Nanostructured MnWO4 catalysts have been prepared by hydrothermal synthesis. The aspect ratio of

the primary MnWO4 particles increases with increasing pH during ageing at 180°C under

hydrothermal conditions. Electron microscopy revealed that the particles grow along the <001> axis.

A mechanism of particle growth by dissolution re-crystallization is proposed that leads to an increase

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

in the fraction of terminating (010) planes with increasing aspect ratio. At the same time, the fraction

of (001) planes, which are terminated by W-OH groups, is decreasing. These changes are reflected in

the selectivity patterns of the ethanol oxidation reaction that probes both, redox and acid-base sites.

Metastable structures are required to achieve catalytic activity over MnWO4. Solid-state synthesis

provides theses structures only on a limited scale. The implementation of chemical defects as basis for

catalytic activity of MnWO4 succeeded via the variation of the chemical potential under mild

hydrothermal conditions.

3.4 Experimental details

3.4.1 Hydrothermal synthesis

The hydrothermal synthesis of MnWO4 was performed in an analytical autoclave HPM-PT-040

(Premex Reactor GmbH) described before[99] adopting a synthesis method that has been reported

previously.[100] In the first step, a 0.2 M aqueous solution of Mn(NO3)2 (Mn(NO3)2·4H2O, 98%,

Roth) was added to a 0.2 M aqueous solution of Na2WO4 (Na2WO4·2H2O, 99%, Sigma Aldrich) while

stirring leading to a mixed solution of pH=6.7. Subsequently, the pH of the mixed solution was

adjusted to 6.3, 8.0, 9.1, and 9.9 by adding appropriate amounts of 0.1 M HNO3 (64-66%, Sigma

Aldrich) or 0.1 M NaOH (98%, Alfa Aesar), respectively. The mixtures were transferred to the

autoclave and the temperature was raised from 20°C to 180°C at a rate of 5 °C/min. The synthesis

temperature was kept at 180°C for 12 h. During hydrothermal synthesis the pH was recorded using a

pH probe (ZrO2 probe Model A2 and Ag/AgCl reference electrode, both with a 1/2“ outer tubing

made from Hastelloy C-276; Corr Instruments). The pH probes were calibrated by use of four buffer

solutions at the given reaction temperatures prior to the experiments. At the same time the Raman

spectra of the synthesis gels were recorded using a Raman probe (RAMAN RXN1, immersion optic

1/4”OD (HC-276); Kaiser Optical Systems). After cooling down the gel at a rate of 5 °C/min, the

products of hydrothermal synthesis were filtered by centrifugation and washed twice with de-ionized

water (MilliPore®). In the final step, the solids were dried in a muffle furnace in air at 80°C for 12 h.

Depending on the pH value before hydrothermal treatment, yellowish to brownish solids were

collected. The solids were annealed in Argon (flow rate: 50 mL/min) at 400°C (heating rate 5 °C/min)

for 2 h using a rotary tube furnace (XERION) resulting in five phase-pure MnWO4 catalysts with

different mean aspect ratios (AR) characterized by the identification numbers 19112 (AR=1.5), 19113

(AR=1.7), 19114 (AR=3.2), 19251 (AR=3.9), and 19116 (AR=5.1). The catalysts are called ARx.x,

whereas x.x corresponds to the mean aspect ratio of the particles in the materials after thermal

activation as determined by analysis of transmission electron microscopy images.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

3.4.2 Characterization of catalysts

Transmission electron microscopy (TEM) studies were conducted on a Philips CM200 FEG

transmission electron microscope operating at 200 kV. High resolution TEM (HRTEM) and high

resolution high angle annular dark field scanning transmission electron microscopy (HAADF-STEM)

were performed on a Cs corrected FEI TITAN 80-300 operated at 300 kV. TEM samples were

prepared by drop deposition from ethanolic suspensions onto lacey-carbon coated Cu grids, and by

cross section preparation.

Field emission scanning electron microscopy (FESEM) was carried out with a Hitachi S4800

instrument operating at 5 kV.

XRD measurements were conducted as described in section 2.4.2.2

The specific surface area was measured in a volumetric N2 adsorption device (Autosorb-6-B,

Quantachrome) at the temperature of liquid nitrogen. The sample was degassed in dynamic vacuum at

a temperature of 300°C for 2 h prior to adsorption. The relative N2 pressure was varied and 11 data

points were measured. The linear range of the adsorption isotherm (p/p0=0.05–0.3) was considered to

calculate the specific surface area according to the BET method.

Ethanol oxidation was performed at atmospheric pressure in a feed composed of 30.2 mL N2, 2.8 mL

O2 and 1.2 mL CH4. The feed was passed through a saturator at 15°C to achieve a concentration of 4

vol% ethanol. A laboratory quartz U-tube fixed bed reactor (4 mm inner diameter, 6 mm outer

diameter, 26 cm length) was used, which contained 200 mg of the catalyst previously pressed and

sieved to a particle size of 250-355 µm. Gas analysis was performed by online gas chromatography

(GC 6890A, Agilent) equipped with two channels. A combination of two capillary columns (GS-

Carbonplot and Plot Mole Sieve 5A) in connection with a thermal conductivity detector (TCD) was

used to analyse the permanent gases CO2, O2, N2 and CO. A combination of two capillary columns

(HP-FFAP and HP Plot Q) connected to a flame ionization detector (FID) was applied to analyse

alkanes, olefins and oxygenates.

Transmission Fourier transform infrared spectroscopy (FTIRS) measurements were carried out using

a Varian 670 spectrometer equipped with a MCT detector. The spectra were recorded at a resolution

of 4 cm-1 accumulating 512 scans. Self-supported wafers (area weight of 23-29 mg cm-2) were

transferred into an IR cell that was connected to a vacuum system, in which residual pressures of ca.

1·10-6 mbar can be employed. Prior to the adsorption of gases, the catalysts were heated in dynamic

vacuum at 300°C for 1h. A reference spectrum of the solid was taken after cooling down to 40°C.

Then, the sample chamber was charged with 7 mbar partial pressure of NH3 at 40°C. After 30 min,

spectra were recorded applying the spectrum of the pretreated MnWO4 as background. The Brønsted

acid site density was calculated applying the extinction coefficient 16 cm μmol-1.[114]

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Near-ambient-pressure X-ray photoelectron spectroscopy (NAP-XPS) was conducted at the end

station of the ISISS beam line at BESSY II/HZB (Helmholtz-Zentrum Berlin, Germany). Details of

the setup have been published earlier.[115] For the XPS measurements, Mn 2p, O 1s, and W 4f core

level spectra were collected at constant kinetic energies (KE) of the photoelectrons of 150 eV and 750

eV, resulting in inelastic mean free paths (IMFP) of the excited photoelectrons of ~0.6 nm (150 eV,

denominated as surface) and ~1.6 nm (750 eV, denominated as deep), respectively. The experiments

were performed at 300°C and a total pressure of 0.25 mbar in a 1/1.1 O2/He mixture with a total gas

flow of 4.2 sccm.

3.5 Supporting information

Figure S 3.1

SEM image of the intermediate formed by reaction of manganese nitrate with sodium tungstate at

room temperature.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Figure S 3.2

XRD patterns of the hydrothermal products; The pH of the starting solution is provided in the legend

of the figure. For allocation of the corresponding final catalyst, please refer to Table 3.1 in the main text.

Figure S 3.3

XRD patterns of the catalysts after activation by thermal treatment of the hydrothermal products in

flowing Ar at 400°C.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Figure S 3.4

TEM micrographs of as-synthesized (top row) and at 400 °C thermally activated (bottom row)

nanostructured MnWO

catalysts with different aspect ratio: 1.5 (a) and (f), 1.7 (b) and (g), 3.2 (c) and (h), 3.9 (d)

and (i), 5.1

(e) and (j).

Figure S 3.5

Distribution of A) diameter, B) length and C) aspect ratio of the nanostructured MnWO

catalysts

after thermal treatment; In each plot, a, b, c, d, and e represent the catalysts AR1.5, AR1.7, AR3.2, AR3.9, and

AR5.1, respectively.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Figure S

3.6

HAADF_STEM images of MnWO

nanoparticles viewed along <001>

with different aspect ratios: a)

AR1.5, b) AR5.1 and c) perspective model for a typical faceted nanoparticle.

Figure S

3.7

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Figure S

3.8

Schematic representation of the formation of W-OH groups at (001) planes during dissolution-

recrystallization under hydrothermal conditions at 180°C.

Figure S 3.9 FTIR spectra of NH3 adsorbed at the surf

ace of the catalysts AR1.5, AR1.7, and AR3.9 after pretreatment at

300°C for 1h in vacuum; Adsorption of ammonia was performed at 40°C; The spectra have been recorded in presence of gas

phase ammonia (p=6.508-7.042 mbar).

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

Figure S 3.10 W 4f spectra (a)), and O 1s spectra (b)) of the catalysts AR1.5 and AR5.1 measured by synchrotron-based

near ambient pressure X-ray photoemission spectroscopy (NAP-XPS) at an inelastic mean free path (IMFP) of ca. 1.6 nm

0.25 mbar in O2/He at a total gas flow of 4.2 sccm at 300°C.

Figure S 3.11 Mn 2p spectra of the catalysts AR1.5 and AR5.1 measured by synchrotron-based near ambient pressure X-

ray photoemission spectroscopy (NAP-XPS) at an inelastic mean free path (IMFP) of ca. 0.6 nm (a) and 1.6 nm (b) in 0.25

mbar in O2/He at a total gas flow of 4.2 sccm at 300°C.

Hydrothermal Synthesis of Bi-functional Nanostructured Manganese Tungstate Catalysts for Selective

Oxidation

3.6 Acknowledgement

This work was conducted in the framework of the BasCat collaboration between BASF SE, TU Berlin,

FHI, and the cluster of excellence “Unified Concepts in Catalysis” (UniCat). X.L. acknowledges the

Berlin International Graduate School of Natural Sciences and Engineering (BIG NSE) as part of

UniCat for financial support. The authors thank Maike Hashagen and Jasmin Allan for technical

assistance. We thank the HZB staff for their continual support of the electron spectroscopy activities

of the FHI at BESSY II.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

4 Hydrothermal Synthesis and Characterization of

Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Abstract: Phase-pure CoWO4 catalysts were prepared by hydrothermal synthesis followed by thermal

treatment in Ar at 400°C for 2 hours. The catalysts were characterized by different techniques, such as

N2 adsorption, X-ray diffraction, X-ray photoelectron spectroscopy, scanning electron microscopy,

transmission electron microscopy, and Raman spectroscopy. The tungstate and binary reference

oxides were tested in the oxidative dehydrogenation of propane. Higher pH value before hydrothermal

treatment gives rise to higher Co/W ratio in the near surface region. Moreover, a correlation between

the specific propane consumption rate and the Co/W ratio determined by XPS was observed.

Enrichment of the surface in W leads to low selectivity to propene. The selectivity to conversion

trajectory of such a catalyst resembles that of mesoporous tungsten oxide. The catalyst with the

highest Co/W ratio exhibits termination by a Co chain structure on the (010) crystal planes as

confirmed by HRTEM. FT-IR spectroscopy reveals surface Co bridging hydroxyl groups, which are

responsible for a band centered at 3387 cm-1. The presence of such an Co oxo-hydroxy terminating

layer might be responsible for the high activity and selectivity in oxidative dehydrogenation of

propane via a redox-type hydrogen abstraction.

KEYWORDS: cobalt tungstate, oxidative dehydrogenation of propane, hydrothermal synthesis,

nanostructure, site-isolation

4.1 Introduction:

Oxidative dehydrogenation of propane (ODP) is advantageous compared to catalytic dehydrogenation

of propane in view of several aspects including, for example, thermodynamic limitation, coke

formation, heat of reaction, and stability.[7, 16, 59]. However, consecutive oxidation of propene to

carbon oxides in ODP represents the major obstacle in terms of economic feasibility.[11, 59] Novel

catalytic materials are required, which are able to activate C-H bonds in propane selectively. A class

of molybdates has been investigated in the oxidative dehydrogenation of propane.[8] The study

revealed that mixed Ni-Co molybdates show most promising performance in terms of selectivity, but

at comparatively high reaction temperatures (550°C). Tungstates are generally less selective than

molybdates, but an acceptable selectivity was reported for CoWO4.[10] In a recent work dealing with

mixed Ni-W-O oxides, the author concluded that Ni oxide is more active and shows constant

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

selectivity in a wide range of propane conversion, whereas W oxide is less active and selective.[24]

However, in all previous studies, the preparation procedure is based on precipitation followed by a

calcination step at relatively high temperature (above 500°C) resulting in low surface area materials

and/or a mixture of phases.

When hydrothermal techniques are applied, the variation of process conditions may result in particles

with varying shape, bulk electronic structure, and surface termination. The shape of MnWO4 particles

influences acid base properties and the nature of the catalytically active sites on the surface.[104, 116]

The regular shape of the nanoparticles facilitates the application of a combination of integral methods

and local methods, such as TEM, to investigate the nature of active sites. Mn oxide chains on the (010)

surface of MnWO4 crystals have been proposed to be responsible for selective propane activation. The

unique nanostructure makes MnWO4 active at comparatively low temperature (400°C) and almost as

selective as VOx/SBA-15.[104] In our previous study, general structural requirements of an ODP

catalyst are proposed. It is postulated that bridge-type hydroxyl groups connecting redox active

elements, which are embedded in an inert bulk oxide, are ideal active site precursors. To verify our

hypothesis, nanostructured CoWO4 was prepared by hydrothermal synthesis and investigated in the

oxidative dehydrogenation of propane.

4.2 Results and discussion

The preparation of monoclinic cobalt tungstate composed of the wolframite-type structure was

performed by hydrothermal synthesis at 180°C for 12 hours in aqueous medium. The formation of the

solid inside the autoclave has been monitored by in-situ Raman spectroscopy (Figure 4.1). The

synthesis products are denominated as CoWx.x, whereas x.x indicates the pH value of the initial

suspension obtained after mixing the aqueous cobalt nitrate and sodium tungsten precursor solutions

in the autoclave at room temperature. During the synthesis of CoWO4, an intermediate is formed as

evidenced by the appearance of a band at 937-941 cm-1, which can be attributed to polytungstate

species (Figure S 1 and 2). With increasing temperature, mono- or poly- tungstate species are

transformed into the wolframite type metal tungstate as evidenced by the occurrence of a peak at 884-

885 cm-1. Phase transition is accelerated with increasing the pH.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Rietveld refinement of the XRD patterns (Figure 4.2) confirms phase-purity of all CoWO4 catalysts.

The aspect ratio (AR) of the catalysts (Summarized in Table 4.1) is estimated based on anisotropic

fitting of the data.

The SEM images (Figure 4.3) show that the hydrothermally synthesized CoWO4 materials are

nanostructured. It can be seen that CoW8.5 is more rod-shaped than the other CoWO4 catalysts in

agreement with the aspect ratio determined by XRD (Table 4.1). The ratio of the dimension in

different crystallographic directions (<001>/<010>) increases monotonically with increasing pH value.

Figure

4.1 In situ Raman spectra recorded during the synthesis of the catalysts a) CoW6.2, b) CoW7.6 and c)

CoW8.5.

Figure

4.2 XRD patterns of as-synthesized CoWO4 catalyst precursors

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative Dehydrogenation of Propane

Catalyst

Cataly

st IDa

pH before

hydrothermal

synthesis

Acidb or

Basec used

(g)

pH after

hydrothermal

synthesis

BET

(m2g-1)

<100>

(nm)

<010>

(nm)

<001>

(nm)

<100>

D<010>

<001>

D<010>

CoW6.2

22787

6.2

27.1b

1.6

42.0

20.1

37.1

12.8

0.5

0.3

CoW7.6

22785

7.6

n.d.

33.5

22.0

30.4

18.4

0.7

0.6

CoW8.1

23615

8.1

6.2c

n.d.

22.4

22.5

23.2

44.5

1.0

1.9

CoW8.5

22786

8.5

13.1c

9.5

35.6

13.6

13.1

35.8

1.0

2.7

a Solid sample after thermal treatment in Ar at 400°C for 2 h

b 1 mole L-1 aqueous HNO3

c 1 mole L-1 aqueous NaOH

d based on XRD of the catalysts (Figure 4.2)

Table

4.1 Specific surface area, synthetic parameters and crystallite size calculated from anisotropic fitting in Rietveld refinement o

f the XRD patterns.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

In addition to the rod shaped structure, particles with flat surfaces can be detected by STEM (Figure

4.4). Particles with a diameter of 10-20 nm and length of 20-60 nm can be observed. At higher

magnification, one image revealed a hexagonal shape of the cross section viewed along the <001>

direction (Figure 4.5 a) The side planes can be determined as (110), (100) and (010) facets. At an

atomic level, viewing from <100> direction, smaller bright dots with lower contrast at the outmost

surface are observed in zig-zag chains on top of the larger brighter dots (Figure 4.5 b). Because in

HAADF-STEM the contrast is due to Rutherford scattering proportional to approximately Z², the

smaller bright dots with less contrast should be attributed to Co atoms, suggesting surface termination

on the (010) crystal planes by Co chains. Another bright field image along the <100> direction also

indicates Co surface termination (Figure 4.5 c).

Figure

4.3 SEM images of the a) CoW6.2, b) CoW7.6, c) CoW8.1 and d) CoW8.5 catalysts

Figure

4.4 HAADF-STEM

images of the CoW8.5 catalyst.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Co/W

O/(Co+W)

Na/(Co+W)

C/(Co+W)

CoW6.2

0.83

2.53

0.034

0.16

CoW7.6

0.92

2.45

0.21

0.073

CoW8.1

1.24

2.00

0.10

0.065

CoW8.5

1.52

2.05

0.092

0.064

The Co/W ratio (Table 4.2) increases monotonically with higher pH before hydrothermal synthesis,

which might be caused by dissolution of tungsten oxide in alkaline medium under hydrothermal

condition. The high Co/W ratio of the CoW8.5 catalyst suggests an enrichment of Co surface. The Co

2p spectra indicate that cobalt exists in the oxidation state 2+ after thermal activation in He gas

atmosphere at 400°C (Figure S4.3).

Figure

4.5 a) HAADF-STEM image of one particle in the CoW8.5 catalyst viewed along the <001> direction,

-STEM-HAADF and c) HR-STEM-BF images of the surface of the same catalyst viewed along the <100>

direction.

Table

4.2 Surface-near molar ratios of CoWO4

according to XPS.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

The catalysts and reference oxides have been studied in propane oxidation at 400°C (Figure 4.6).

CoW6.2 and CoW7.6 are only slightly more active than m-WO3 (Figure 4.6). CoW8.5 shows 50%

selectivity to propene at 9% propane conversion. The reaction temperature was 160°C compared to

the reaction temperature applied in previous studies over molybdates and tungstates prepared by

precipitation.[10] The CoW8.5 catalyst also shows the highest specific propane consumption rate

(Figure 4.7). The steady state propane consumption rate correlates with the Co/W ratio in the near

surface region on the CoWO4 catalysts (Figure 4.7), indicating that cobalt oxide species might be

responsible for activity. Interestingly, the most active CoW8.5 catalyst, which is enriched in Co on the

surface, is much more selective than pure cobalt oxide. The latter is a combustion catalyst, giving ca.

6% selectivity to propene at 10% propane conversion. In analogy to MnWO4 catalysts, site isolation

of the redox active species might be responsible for the much higher selectivity of CoWO4 compared

to bulk Co3O4.

The superior performance of the CoW8.5 catalyst might be attributed to its surface termination by

CoOx chains on (010) planes as suggested by STEM and XPS. The very high CO2/CO ratio, (325 to

410), on this catalyst is another indication that CoOx species are active sites, because pure cobalt

oxide shows a CO2/CO ratio over 1000 (Figure S4.4). The CO2/CO ratio on the CoWO4 catalysts

series increases with increasing Co/W ratio in the near surface region. At the ODP reaction

temperature, it is highly likely that the formed CO can be consecutively oxidized to CO2. With higher

Figure

4.6 Catalytic performance of mesoporous WO3, commercial Co3O4 and nanostructured CoWO4

catalysts

(T=400°C, W/F=0.9

-2.4 g s mL-1) in the oxidative dehydrogenation of propane in a C3H8/O2/N2

feed (10:5:85).

The selectivity to propene is shown as a function of propane conversion. Other ca

rbon-

containing products are

mainly CO and CO

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

contact time, the propene formation rate decreases, whereas the CO2 formation rate increases (Figure

S4.5), suggesting a possible consecutive combustion of propene into carbon dioxide.

At room temperature, the infrared spectrum of CoW8.5 in the O-H stretching region is characterized

by a strong signal at 3377 cm-1 due to well-defined OH groups on the surface (Figure 4.8 a).

Interestingly, the band is not very much perturbed by adsorption of molecular water that causes the

broad base line in the room temperature spectrum in the range 3800-3000 cm-1. Dehydration and

perhaps also partial dehydroxylation at 400°C in He flow results in a sharp peak at 3387 cm-1

attributed to Co-(OH)-Co type hydroxyl groups. However, a direct correlation between the surface

concentration and the intensity of the peak is not observed. Dehydroxylation of the surface may result

in surface defect formation, which is essential for oxygen activation in the oxidative dehydrogenation

of propane. A partially de-hydroxylated, oxidized surface would then tend to abstract hydrogen from

propane. The catalysts CoW6.2, CoW7.6, and CoW8.1 show sharply decreasing selectivity with

increasing conversion (Figure 4.6). The infrared spectra of these catalysts exhibit a band at 3674 cm-1

due to acidic W-OH groups, which is missing on the surface of the catalysts CoW8.5 that displays the

best performance. Dehydroxylation of W-OH groups requires higher temperatures, since the intensity

of the band at 3674 cm-1 barely decreases at reaction temperature (Figure 4.8 b, Figures S4.6-S4.9).

Figure

4.7 Steady state propane consumption rate as a function of Co/W ratio in the near

surface region

determined by

XPS.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

The effects of varying concentrations of propane, oxygen and water on the formation rate of propene,

carbon monoxide and carbon dioxide were determined on m-WO3, CoW6.2 and CoW8.5 catalysts. On

the m-WO3 catalyst (Figure S4.10), the propene formation rate increases almost linearly with

increasing propane partial pressure, but only slightly with increasing oxygen partial pressure, and

decreases with increasing water partial pressure. On the CoW6.2 catalyst (Figure 4.9), the propene

formation rate increases almost linearly with increasing propane partial pressure, whereas the carbon

oxides formation rates increase to a lesser extent with increasing propane partial pressure. The

increasing oxygen partial pressure leads to unchanged propene formation rate but higher carbon

dioxide and carbon monoxide formation rates. On the CoW8.5 catalyst (Figure S 4.10), the propene

and carbon oxide formation rates increase almost to the same extent with increasing propane partial

pressure. With increasing oxygen partial pressure, the propene formation rate decreases whereas the

carbon oxides formation rates increase. Propene and carbon dioxide formation rates decrease with

Figure

4.8 FT-IR spectra of CoWO4 catalysts in He flow at a) room temperature and b) 400°C

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

increasing water partial pressure. The influence of water on carbon monoxide formation rates is very

weak.

Some general trends can be noted from this preliminary study. Increasing partial pressure of oxygen is

detrimental to the propene selectivity on all three studied catalysts indicating that oxygen activation is

crucial in terms of selectivity. Increasing water partial pressure is not beneficial for the selectivity to

propene on CoW8.5.

Figure

4.9

Influence of partial pressure of a) propane, b) oxygen and c) water on the specific product formation

rate on

CoW6.2 catalyst.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

m-WO

CoW6.2

CoW8.5

Apparent reaction order

with respect to p C3H8 p O2 p H2O p C3H8 p O2 p H2O p C3H8 p O2 p H2O

Consumption

0.78

0.11

-0.31a

0.82

0.32

-0.18

0.54

0.22

-0.18

a: in this study, only C3H6 is detected in reaction products

4.3 Conclusions

The size, shape and the surface termination of the cobalt tungstates can be controlled by the pH value

in the solution before the hydrothermal synthesis. The Co/W ratio in the near surface region of the

nanostructured CoWO4 catalysts increases monotonically as determined by XPS analysis with the

increasing initial pH. Nanostructuring of the cobalt tungstate enables the propane activation at a

Figure

4.10 Influence of partial pressure of a) propane, b) oxygen and c) water on the specific product

formation rate on CoW8.5 catalyst.

Table

4.3 Apparent reaction orders with respect to propane, oxygen and water on tungsten conta

ining catalysts.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

temperature as low as 400°C, whereas the cobalt tungstate prepared by precipitation requires a much

higher temperature as 560°C for C-H activation in propane. [10] In analogy to the nanostructured

MnWO4 synthesized at pH 9.9,[104] high resolution TEM and FT-IR spectroscopic analysis suggests

surface termination by Co-O(H)-Co chain structures on the (010) facets. Site isolation of the very

active Co atoms on a nanostructured crystalline CoWO4 might be responsible for the improvement of

the selectivity to propene in the ODP reaction, because the bulk Co3O4 is very active but non-selective

to propene. As has been illustrated in this study, such site isolated one dimensional chains consisted of

redox active transition metals on the surface termination layer might serve as a paradigm for the

construction of the active sites for selective propane activation.

4.4 Experimental details:

4.4.1 Synthesis of the catalysts

The hydrothermal synthesis of CoWO4 was performed in an analytical autoclave HPM-PT-040

(Premex Reactor GmbH). For the synthesis of CoWO4, in the first step, a 0.2 M aqueous solution of

Co(NO3)2 (Co(NO3)2·6H2O, 99+%, Acros Organics) was added to a 0.2 M aqueous solution of

Na2WO4 (Na2WO4·2H2O, 99%, Sigma Aldrich) while stirring leading to a mixed solution of pH=7.6.

Subsequently, the pH of the mixed solution was adjusted to 6.2, 7.9, 8.1 and 8.5 by adding 27.1 g of 1

M HNO3 (HNO3, 64-66%, Sigma Aldrich) or 6.2 and 13.1 g 1 M NaOH (NaOH, 98%, Alfa Aesar),

respectively. After the first step for both metal tungstates, the mixtures were transferred to the

autoclave and the temperature was raised from 20°C to 180°C at a rate of 5 °C/min. The synthesis

temperature was kept at 180°C for 12 h. After cooling down the gel at a rate of 5 °C/min, the products

of hydrothermal synthesis were filtered by centrifugation and washed twice with de-ionized water

(MilliPore®). In the final step, the solids were dried in a muffle furnace in air at 80°C for 12 h. The

solids were annealed in Argon (flow rate: 50 mL/min) at 400°C (heating rate 5 °C/min) for 2 h using a

rotary tube furnace (XERION). The catalysts are called CoWx.x, whereas x.x corresponds to the pH

value before the hydrothermal synthesis.

The synthesis of mesoporous WO3 is described elsewhere. Co3O4 (Aldrich, 99.995%) was used as

obtained.

4.4.2 Catalyst characterization

High resolution TEM (HRTEM) and high resolution high angle annular dark field scanning

transmission electron microscopy (HAADF-STEM) were performed on a double corrected JEOL

JEM-ARM200F equipped with CEOS CESCOR, and CEOS CETCOR hexapole aberration correctors

for probe and image forming lenses, respectively, and a cold field emission gun (CFEG). The

acceleration voltage was set to 200 kV. TEM catalysts were prepared by drop deposition from

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

ethanolic suspensions onto lacey-carbon coated Cu grids. Field emission scanning electron

microscopy (FESEM) was carried out with a Hitachi S4800 instrument operating at 5 kV.

The X-ray diffraction (XRD) measurement was performed in Bragg-Brentano geometry on a Bruker

AXS D8 Advance theta/theta diffractometer, using Ni filtered Cu Kα radiation and a position

sensitive LynxEye silicon strip detector. The sample powder was filled into the recess of a cup-shaped

sample holder, the surface of the powder bed being flush with the sample holder edge (front loading).

The XRD data were evaluated by whole powder pattern fitting according to the Rietveld method as

X-ray photoelectron spectra were recorded at room temperature, using non-monochromatized Al Kα

(1486.6 eV) excitation and a hemispherical analyzer (Phoibos 150, SPECS). The binding energy scale

was calibrated by the standard Au4f(7/2) and Cu2p(3/2) procedure. Theoretical cross sections from

references were used to calculate the elemental composition.

The catalytic tests were carried out using a setup for partial oxidation (Integrated Lab Solutions) with

8 fixed bed quartz reactors (6 mm inner diameter) in parallel. Each reactor was equipped with a

thermocouple for measuring the temperature inside the catalyst bed containing 20-300 mg of catalyst

previously sieved to a particle size of 250-355 µm and the catalytic performances were determined at

atmospheric pressure. The reactant feed, which comprised C3H8, O2, and N2 as diluent, was passed

through the reactors at a flow rate of 7.5-20 mL/min. The gas composition was 10 % C3H8, 5 % O2

and 85% N2. The product (and bypass) gas mixtures were analyzed by an online gas chromatograph

(Agilent 7890). A system of Plot-Q and Plot-molsieve columns connected to a thermal conductivity

detector (TCD) separated the permanent gases CO, CO2, N2, O2, and CH4. A system of a FFAP and a

Plot-Q columns connected to a flame ionization detector (FID) allowed the separation of C2-C3

hydrocarbons and oxygenates.

4.5 Supporting information

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S

4.1 Raman spectra of tungsten containing compounds measured as solids using 633 cm-1

excitation

wavelength.

Figure S

4.2 Raman spectra of ammonium paratungstate (APT), ammonium metatungstate

(AMT) and sodium

tungstate in aqueous solution

at room temperature

. Bands denoted with asterisks belong to the sapphire window

of the Raman probe.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S

4.3 Co2p(3/2), O1s and W4f (and 5p3/2) core level XPS spectra of the CoWO4

samples after Shirley

background subtraction and charging correction.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S

4.4 CO2/CO ratio in the ODP reaction over m-WO3, Co3O4 and CoWO4 catalysts. W/F=1.8-

2.4 g s

-1 in the oxidative dehydrogenation of propane in a C3H8/O2/N2 feed (10:5:85).

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S 4.5 a) Propene formation, b) CO2 formation and c) CO formation rates over m-WO3 and

CoWO4 catalysts. W/F=0.75-2.4 g s mL-1 in the oxidative dehydrogenation of propane in a

C3H8/O2/N2 feed (10:5:85).

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S

4.6 FT-IR

spectra of CoW6.2 catalyst in He flow.

Figure S

4.7 FT-IR

spectra of CoW7.6 catalyst in He flow.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S

4.8 FT-IR

spectra of CoW8.1 catalyst in He flow.

Figure S

4.9 FT-IR

spectra of CoW8.5 catalyst in He flow.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Figure S

4.10 Influence of partial pressure of a) propane, b) oxygen and c) water on the specific product

formation ra

te on m-WO3 catalyst.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

5 Conclusions

In the present work, hydrothermally synthesized, nanostructured metal tungstates were investigated in

the oxidative activation of lower alkanes. The issue of structure-reactivity relationship in oxidative

dehydrogenation of propane (ODP) over metal tungstate catalysts was addressed.

Rod-shaped nanostructured MnWO4 was found to be a new class of material, which can relatively

selectively activate propane at moderate temperature. By comprehensive and complementary

structural characterization, a two-dimensional manganese chain structure was observed to be the

surface-terminating layer on (010) crystal planes. In situ analysis during the hydrothermal synthesis

suggests the formation of this unique Mn surface structure by dissolvation of WOx in the alkaline

solution during aging. The outmost chain is composed of -Mn-(OH)2-Mn-. A redox type reaction is

indicated. Mn in the aforementioned chain structure might be the electron acceptor. “Site isolation” of

the active Mn chains might explain why a Mn-based oxide turns from a combustion catalyst into a

relatively selective oxidation catalyst.

Hydrothermal synthesis enables to achieve a family of nanostructured CoWO4 catalysts with varying

shape and surface termination. And they were also found to be highly active in ODP reaction. CoWO4

synthesized applying an intial pH of 8.5 shows the highest activity and selectivity, and this specific

catalyst shows superior performance than the MnWO4 catalyst. Dehydroxylation occurs at elevated

temperature and formation of the Co-O-Co site is expected to happen in the oxygen containing gas

feed. Kinetic investigation and structural characterizations suggest surface oxygen species in the “-

Co-O-Co-“ chains to be the active sites for C-H activation in propane activation.

Generally, on the CoWO4 catalysts, higher oxygen concentration in the reaction feed leads to higher

selectivity to COx, which suggests a crucial role of oxygen activation. The control of oxygen

activation might be the major challenge in the activation of propane since it is related to the

consecutive combustion of propene. An alternate feeding of propane and oxygen, which minimizes

the contact of propane with the unselective oxygen species, is likely to improve the overall selectivity

to propene.

In the supplementary experiments (results were not shown in the thesis) for the MnWO4 catalysts, the

influence of shape and surface structure in ODP over nanostructured MnWO4 was investigated.

Kinetic studies reveal a strong dependence of propane consumption rate on the initial pH of the

hydrothermal syntheses of the phase-pure catalysts. Catalyst synthesized with highest initial pH shows

very good long term stability, whereas the ones synthesized with lower initial pH endure severe

deactivation. Thorough structural characterization was conducted over the series of catalysts. (010)

planes were found to be partially terminated by manganese atoms in the form of chain structure by

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

transmission electron microscopy. Raman and IR spectroscopy resolve the manganese oxyhydroxide

surface structure. In situ IR spectroscopy demonstrates reversible dehydroxylation and hydroxylation

at high and low temperature, respectively. Gas phase ethanol oxidation probe reaction suggests a

redox type activation of propane. The abundance of the near-surface Mn determined by X-ray

photoelectron spectroscopy correlates to the initial propane consumption rate.

In another set of supplementary experiments (results not shown in the thesis), in situ UV-Vis and FT-

IR spectroscopy has been applied for estimating the oxidation degree of the surface layer on a series

of MnWO4 catalysts in the ODP reaction. And time resolved analysis enables to derive the kinetic

parameters of reoxidation of the surface layer. The quantitative kinetic study reveals a correlation

between the initial reoxidation rate constant and the apparent propene formation rate, suggesting the

important role of the reoxidation step of the catalyst. In oxygen partial pressure variation experiment

on one MnWO4 catalyst, increasing oxidation degree of the surface layer was found to accelerate the

propane consumption rate. This indicates the catalytic relevancy of the surface structural oxygen

species in propene activation. In situ FT-IR spectroscopy observes an increasing amount of hydroxyl

groups on the catalyst when switching from inert to reaction gas atmosphere. This hints the

transformation of the nucleophilic surface structural oxygen species into hydroxyl groups by

hydrogen abstraction from propane.

The work elucidates the importance of segregated termination layers on the surface of bulk mixed

metal oxides applied as catalysts in alkane oxidation. It is demonstrated that surface termination can

be established not only under reaction conditions [1, 42-45, 67], but also during synthesis under

specific hydrothermal conditions. The developed synthesis method might stimulate further synthetic

work resulting in the discovery of new catalytic materials.

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

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Appendix

Curriculum vitae

Name: Xuan Li

M/F: Male

Date of birth: 06.06.1988

Nationality: Chinese

Education

● PhD (2013-Present) on Heterogeneous Catalysis

- Institutes: Fritz-Haber-Institute of the Max-Planck-Society and BasCat Unicat BASF JointLab

● Master of Science (2010-2013) in Physical Chemistry in Nanjing University

● Bachelor of Science (2006-2010) in Chemistry in Nanjing University

Publication List

[1] X. Li, T. Lunkenbein, V. Pfeifer, M. Jastak, P. K. Nielsen, F. Girgsdies, A. Knop-Gericke, F.

Rosowski, R. Schlögl and A. Trunschke, Selective Alkane Oxidation by Manganese Oxide: Site

Isolation of MnOx Chains at the Surface of MnWO4 Nanorods, Angew. Chem. Int. Ed., 2016, 55,

4092-4096

[2] X. Li, T. Lunkenbein, J. Krohnert, V. Pfeifer, F. Girgsdies, F. Rosowski, R. Schlogl and A.

Trunschke, Hydrothermal synthesis of bi-functional nanostructured manganese tungstate catalysts for

selective oxidation, Faraday Discuss., 2015, DOI: 10.1039/C5FD00191A

[3] A. Y. Klyushin, M. T. Greiner, X. Huang, T. Lunkenbein, X. Li, O. Timpe, M. Friedrich, M.

Hävecker, A. Knop-Gericke and R. Schlögl, Is nanostructuring sufficient to get catalytically active

Au?, ACS Catalysis, 2016, DOI: 10.1021/acscatal.5b02631, 3372-3380

[4] X. Li, S. Ye, J. Zhao, L. Li, L. Peng and W. Ding, Selective oxidation of toluene using surface‐

modified vanadium oxide nanobelts, Chinese Journal of Catalysis, 2013, 34, 1297-1302

[5] H. Liu, C. Guan, X. Li, L. Cheng, J. Zhao, N. Xue and W. Ding, The Key Points of Highly Stable

Catalysts for Methane Reforming with Carbon Dioxide, ChemCatChem, 2013, 5, 3904-3909

Hydrothermal Synthesis and Characterization of Nanostructured CoWO4 as Catalysts for Oxidative

Dehydrogenation of Propane

Oral presentation List

[1] 49th Annual German Catalysis Meeting, Weimar, Germany, 2016, Identification of active site of

nanostructured MnWO4 in selective activation of propane, Xuan Li, Thomas Lunkenbein, Verena

Pfeifer, Jutta Kröhnert,Johannes Noack, Frank Girgsdies, Frank Rosowski, Robert Schlögl, Annette

Trunschke

[1] Designing New Heterogeneous Catalysts: Faraday Discussion, London, United Kingdom, 2016,

Hydrothermal synthesis of bi-functional nanostructured Paper manganese tungstate catalysts for

selective oxidation, Annette Trunschke, Xuan Li, Thomas Lunkenbein, Jutta Kröhnert, Verena

Pfeifer, Frank Girgsdies, Frank Rosowski, Robert Schlögl,

Poster presentation List

[1] 16th International Congress on Catalysis, Beijing, China, 2016, Selective Alkane Oxidation by

Manganese Oxide: Site Isolation of MnOx Chains at the Surface of MnWO4 Nanorods, Xuan Li,

Thomas Lunkenbein, Detre Teschner, Jutta Kröhnert, Christian Schluz, Frank Rosowski, Robert

Schlögl, Annette Trunschke

[2] 48th Annual German Catalysis Meeting, Weimar, Germany, 2015, Shape-dependent catalytic

properties of nanostructured MnWO4 in selective oxidation, Xuan Li, Thomas Lunkenbein, Verena

Pfeifer, Jutta Kröhnert, Frank Girgsdies, Raoul Naumann d’Alnoncourt, Frank Rosowski, Robert

Schlögl, Annette Trunschke