Versatile control over size and spacing of small
mesopores in metal oxide films and catalytic
coatings via templating with hyperbranched
core–multishell polymers†‡
Denis Bernsmeier,
a
Erik Ortel,
a
J¨
org Polte,
b
Bj¨
orn Eckhardt,
a
Sabrina Nowag,
c
Rainer Haag
c
and Ralph Kraehnert*
a
Controlling the pore structure of metal oxide films and supported catalysts is an essential requirement for
tuning their functionality and long-term stability. Typical synthesis concepts such as “Evaporation Induced
Self Assembly”rely on micelle formation and self assembly. These processes are dynamic in nature and
therefore strongly influenced by even slight variations in the synthesis conditions. Moreover, the
synthesis of very small mesopores (2–5 nm) and independent control over the thickness of pore walls
are very difficult to realize with micelle-based approaches. In this contribution, we present a novel
approach for the synthesis of mesoporous metal oxide films and catalytic coatings with ordered porosity
that decouples template formation and film deposition by use of hyperbranched core–multishell
polymers as templates. The approach enables independent control of pore size, wall thickness and the
content of catalytically active metal particles. Moreover, dual templating with a combination of
hyperbranched core–multishell polymers and micelles provides facile access to hierarchical bimodal
porosity. The developed approach is illustrated by synthesizing one of the most common metal oxides
(TiO
2
) and a typical supported catalyst (PdNP/TiO
2
). Superior catalyst performance is shown for the
gas-phase hydrogenation of butadiene. The concept provides a versatile and general platform for the
rational optimization of catalysts based e.g. on computational prediction of optimal pore structures and
catalyst compositions.
Introduction
Many applications in e.g. photovoltaics,
1,2
catalysis
3,4
and pho-
tocatalysis
5,6
rely on mesoporous oxide coatings with tailored
properties. In particular fast catalytic reactions require opti-
mized pore systems that facilitate fast diffusion and high
surface areas. Controlling the size and the shape of mesopores
as well as the crystallinity and wall-thickness of the framework
in such lms are key factors for tuning their functionality and
stability.
7,8
Several strategies for improved control over the nano-
structure of metal oxide coatings and supported catalysts have
been reported.
8,9
So-called nanocasting provides access to
tunable pore morphologies by replicating the nanostructure of a
template material into an ordered pore system.
10
Oxide
coatings with ordered and well-connected mesopores can be
formed by a strategy called evaporation-induced self assembly
(EISA).
11–13
Typical EISA-based syntheses employ micelles of amphi-
philic block-copolymers as pore templates and a reactive metal
precursor dissolved in volatile solvent(s). The volatile solvent
evaporates during lm deposition leading to increasing poly-
mer concentration, assembly of the template molecules into
micelles and nally formation of an ordered mesophase
comprised of micelles and condensed precursor. Subsequent
thermal treatments induce stiffening of the inorganic network,
crystallization and removal of the template.
9
EISA is a dynamic and delicate process. Due to the transient
nature of solvent evaporation and polymer self-assembly it
reacts very sensitive to the synthesis conditions and the ther-
modynamics of the employed block-copolymers. Mesoporous
TiO
2
lms based on e.g. the template Pluronic P123 were
reported to form either lamellar, hexagonal or cubic phases
14
depending on the polymer concentration, pH, temperature as
well as on relative humidity applied during and aer lm
deposition.
7
a
Department of Chemistry, Technische Universit¨
at Berlin, Straße des 17. Juni 124,
b
Department of Chemistry, Humboldt-Universit¨
at zu Berlin, Brook-Taylor-Str. 2, 12489
Berlin, Germany
c
Department of Chemistry, Freie Universit¨
at Berlin, Takustr. 3, 14195 Berlin, Germany
†Dedicated to the occasion of the 80th birthday of Prof. Dr. Manfred Baerns
‡Electronic supplementary information (ESI) available. See DOI:
10.1039/c4ta01842g
Cite this: J. Mater. Chem. A,2014,2,
13075
Received 15th April 2014
Accepted 17th June 2014
DOI: 10.1039/c4ta01842g
www.rsc.org/MaterialsA
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The strong sensitivity of EISA results in severe limitations of
this synthesis approach. In the preparation of larger samples or
thicker coatings local gradients in the evaporation conditions
lead to inhomogeneities across the formed materials. Attempts
to control pore size and wall thickness independently fail due to
close interactions between template and precursor during self
assembly.
15
Moreover, the dynamic nature of micelles formed
from amphiphilic block-copolymers typically makes the
formation of well-ordered pore systems with small mesopores
difficult to realize (see e.g. ref. 7 and 16 for TiO
2
).
Hyperbranched core–shell and core–multishell (“CMS”)
polymers offer a potential solution. They typically consist of a
core (e.g. hyperbranched glycerol) and one or more shells of a
different polymer (e.g. an alkyl layer and outer poly(ethylene
glycol) layer). Core–multishell polymers that feature a hydro-
phobic core or inner shell and a hydrophilic outer shell
resemble somehow the structure of polymer micelles typically
employed for EISA. However, the nature of the polymer bonds is
covalent, hence no polymer assembly into micelles is needed
anymore.
Mesoporous powders of different metal oxides have been
synthesized using CMS polymers as unimolecular pore
templates. Yin et al. obtained a 2D mesoporous titania network
employing amphiphilic core-double-shell polymers.
17
Nowag
et al. reported the synthesis of bimodal mesoporous Pt/SiO
2
powders using a mixture of a CMS polymer and micelles of a
PEO–PPO–PEO (Pluronic P123).
18
However, the obtained silica
showed separated domains of unordered CMS-templated pores
(d
pore
ca. 2 nm) and larger SBA-15-type mesopores (d
pore
ca.
6 nm) cast by P123 micelles. The studied CMS polymer also
stabilized the colloidal Pt nanoparticles employed in the
synthesis,
18,19
demonstrating bifunctionality as pore template as
well as particle stabilizer. However, the potential of CMS poly-
mers to generate oxide coatings and catalysts with small mes-
opores, ordered porosity, materials with tunable pore-wall
thickness and hierarchical porosity via dual templating remains
so far unexplored.
We present a new strategy for polymer-templated metal oxide
lms with small mesopores and tunable wall thickness. The
strategy gives direct access to hierarchical pore systems as well
as catalytic functionality. The synthesis combines the advan-
tages of EISA and CMS polymer templates. CMS polymers,
consisting of hyperbranched polyglycerol cores, hydrophobic
inner alkyl layers and a hydrophilic outer layer of mono-
methylated poly(ethylene glycol), are shown to control the
obtained pore size and to act as particle stabilizers. This novel
approach enables for the rst time (i) the synthesis of nano-
crystalline metal oxide lms with ordered pores and size control
between 3 and 5 nm, (ii) independent control over the thickness
of pore walls and the size of the mesopore by adjusting the
content of CMS polymer in the dip-coating solution, (iii) metal
oxide lms with hierarchical bimodal mesoporosity obtained by
combining CMS polymers with micelle-based templates, and
(iv) catalytic coatings PdNP/TiO
2
with small mesopores. The
obtained catalysts show the highest activity reported so far in
literature for selective gas-phase hydrogenation of 1,3-
butadiene.
Experimental
Chemicals and materials
Na
2
PdCl
4
$3H
2
O (99.95%) and NaBH
4
(98%) were obtained from
Alfa Aesar and TiCl
4
(99.9%) from Acros. Ethanol (99.9%) was
purchased from Roth. These chemicals were used without
further purication. Water was puried (18.2 MUcm MilliQ,
Millipore). Si wafers and steel plates (grade 1.4301) were
employed as substrates for lm deposition. Before dip-coating,
Si wafers were cleaned with ethanol. The surface of steel plates
was grinded with 180-grit sandpaper and thoroughly cleaned as
described in earlier publications.
4,20
Prior to coating, Si wafers
and steel plates were calcined in air for 2 h at 600 C.
Two different polymers with core–multishell structure,
called CMS5 and CMS10 hereaer, were employed as templates.
CMS5 consisted of a hyperbranched polyglycerol core with a
molar mass of approximately 5000 g mol
1
, an intermediate
shell of C
18
alkyl chains and an outer shell of monomethylated
poly(ethylene glycol) (mPEG
750
) with a total mass of the polymer
of M
w
¼93 300 g mol
1
.
18
CMS10 consisted of a hyperbranched
polyglycerol core with a molar mass of approximately
10 000 g mol
1
, an intermediate shell of C
12
alkyl chains and an
outer shell of monomethylated poly(ethylene glycol) (mPEG
750
,
M
w
¼750 g mol
1
) with a total mass of M
w
¼56 500 g mol
1
.
The synthesis of the CMS polymers was realized by amide
coupling of the shell molecules to the hyperbranched core as
described by Keilitz et al.
21
Amphiphilic block copolymers
poly(ethylene oxide)-b-poly(butadiene)-b-poly(ethylene oxide)
(PEO–PB–PEO containing 18 700 g mol
1
PEO and 10 000 g
mol
1
PB) were obtained from Polymer Service Merseburg
GmbH (see ref. 15 for details).
Mesoporous TiO
2
coatings
For the synthesis of mesoporous TiO
2
lms, CMS5, CMS10 or
micelles formed from PEO–PB–PEO were used as pore template
and TiCl
4
as precursor in an ethanolic solution. The amount of
CMS polymers was varied between 12 mg and 174 mg. The
amount of employed PEO–PB–PEO was 75 mg. The polymer was
dissolved in 3.68 ml of ethanol and 333 ml of water and stirred
for 12 h. Thereaer 2.32 ml of a homogeneous mixture of TiCl
4
(908 mM) and ethanol were added at room temperature. The
obtained mixture was stirred for 1 h.
Dip-coating of all samples was performed with a withdrawal
rate of 300 mm min
1
in a controlled atmosphere of RH ¼40% at
25 C. The lms were subsequently dried at 80 C for 4 h in a tube
furnace under owing air. To remove the templates the temper-
ature was then raised in owing air with 1 K min
1
to 300 C,
held constant for 1 h and followed by naturally cooling down to
room temperature. To further crystallize the TiO
2
framework, the
obtained lms were treated with a second calcination procedure
ramping with 3 K min
1
to 450 C and holding this temperature
for 5 min followed by cooling to room temperature.
PdNP/TiO
2
catalyst
Colloidal Pd nanoparticles were prepared by reduction of
Na
2
PdCl
4
in an ethanolic solution with CMS10 acting as
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stabilizer and NaBH
4
as reducing agent. The required amount
of Na
2
PdCl
4
was rst dissolved in ethanol (11 mM). Then
3.68 ml of this solution were mixed with 70 mg of CMS10 and
stirred for 12 h at room temperature. Thereaer 333 mlofa
freshly prepared aqueous solution of NaBH
4
(366 mM) were
quickly added to form a colloid with a dark brownish color. The
PdNP/TiO
2
catalysts were synthesized by adding to the CMS10-
stabilized PdNP colloid 2.32 ml of a homogeneous mixture of
TiCl
4
(908 mM) and ethanol. Dip-coating and thermal treat-
ments were performed as already described for the TiO
2
lm
synthesis. Aer calcination at 450 C the catalysts were reduced
for 6 h at 350 CinH
2
/Ar atmosphere (4 vol% H
2
).
Characterization
SEM images were collected with a JEOL 7401F scanning electron
microscope. To determine the lm thickness, coated samples
were split into two pieces and imaged at the cross-section.
Image J Version 1.45s (http://rsbweb.nih.gov/ij) was employed
to determine pore diameter, lm thickness and PdNP particle
diameter and to derive FFT plots from the SEM images. Size and
crystallinity of colloidal nanoparticles as well as lm
morphology and crystallinity of lm fragments were studied by
TEM (FEI Tecnai G2 20 S-TWIN instrument operated at 200 kV).
Lab scale SAXS analysis of dissolved CMS polymers in
ethanol was performed using a SAXSess instrument (Anton Paar
GmbH). Obtained scattering curves were analyzed with the
assumptions of spherical shape, homogeneous electron density
and a Schulz–Zimm size distribution.
2D-SAXS pattern with a beam incident angle of b¼13or 90
in respect to the lm surface were recorded at the HASYLAB B1
beamline at DESY Hamburg with a sample to detector distance
of 1338 mm and a calibrated radiation energy of 16 026 eV using
a 2D PILATUS 1M detector. The SAXS data were processed
employing the soware FIT2D Version V12.077. The modulus of
the scattering vector qis dened in terms of the scattering angle
qand the wavelength lof the radiation used: thus q¼4p/
lsin(q/2). XRD was measured on a Bruker D8 Advance (Cu Ka
radiation) with gracing incident beam (1). Reexes were
assigned using PDFMaintEx library Version 9.0.133. The
average crystallite size was calculated applying the Scherrer
equation. Obtained data were analyzed with the Rietveld
method using TOPAS V4.2 (Bruker AXS). Experimental settings
were considered by the Fundamental Parameter approach
incorporated in the program. Phases of anatase (I41/amd) and
palladium (Fm3m) were rened using scale factor, lattice
parameter and crystallite size. Inuence of four background
parameters and zero point error were taken into account. The
crystallite sizes were calculated from peak broadening based on
the volume-weighted column height.
The Pd concentration was determined using inductively
coupled plasma-optical emission spectroscopy (ICP-OES) using
a 715-ES-inductively coupled plasma (ICP) analysis system
(Varian).
Kr adsorption was measured at 77 K using the Autosorb-1-C
automated gas adsorption station from Quantachrome. The
surface area was calculated via Brunauer–Emmett–Teller (BET)
method. Before adsorption measurement the samples were
degassed at 150 C for 2 h in vacuum. To determine the coating
mass, the mass depth of each lm was calculated by
STRATAGem lm analysis soware (v 4.3) based on WDX
spectrums analyzed with the Cameca “Camebax-microbeam”
electron microprobe at ZELMI (TU-Berlin). In this study, BET
surface area values are related either to the coating mass
(m
2
g
1
) or to the geometric lm volume (m
2
cm
3
).
Catalytic testing
The catalytic performance of PdNP/TiO
2
catalyst lms in the
gas-phase hydrogenation of 1,3-butadiene was studied at
temperatures between 50 and 150 C. Films were coated on both
sides of steel plates (plate size 27 mm 30 mm). Five identical
steel plates were stacked parallel into the reactor housing with
1.5 mm distance between the plates. The total coating mass
amounted to 2.3 mg. A test setup and procedure similar to the
one described in ref. 4 and 22 was used. A reaction mixture
consisting of 10% butadiene (2.5 purity), 20% hydrogen
(5.0 purity), and 70% nitrogen (5.0 purity) was passed through
the reactor at a ow rate of 60 ml min
1
(STP) at 1.05 bar. The
catalyst was then heated to 150 C under reactive gas ow and
equilibrated to reaction conditions for 4 h. Thereaer, the
temperature was decreased stepwise in 10 K increments to 50 C
with a dwell time of 3 h for each temperature set point. Analysis
of the gas products was performed continuously every 7 min by
online gas chromatography (Agilent GC 7890 equipped with
FID, TCD and columns HP Plot Al
2
O
3
, Molsieve 5A, HP Plot Q
and DB FFAP.) The space-time yield (STY) was calculated as
produced moles of butenes per second per kilogram of the
catalyst [mol s
1
kg
1
].
Results and discussion
The following sections describe rst the physico-chemical
properties of dissolved CMS polymers and of mesoporous TiO
2
lms templated with CMS polymers. Thereaer, control over
the thickness of pore walls is demonstrated. Moreover, the
generation of hierarchical porosity by dual templating with
CMS polymers and micelles of PEO–PB–PEO is reported.
Finally, a mesoporous PdNP/TiO
2
catalysts templated by CMS
polymers is presented as well as its performance in butadiene
hydrogenation.
CMS polymers in solution
To proof that CMS polymers can act as unimolecular mesopore
templates, the size of CMS polymers dissolved in ethanol was
analyzed by SAXS. SAXS scattering curves of CMS5 and CMS10
and the corresponding ts are shown in ESI Fig. S1.‡Fitting the
curves with a theoretical model indicates diameters of the
polymer structures of 6.6 nm (CMS5) and 8.8 nm (CMS10) and
about 30% polydispersity. Hence, both studied CMS polymers
form individual dissolved entities with narrow size distribution
and diameters smaller than the micelles of typical template
polymers (see e.g. ref. 15).
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Mesoporous TiO
2
lm templated with CMS10
To proof the synthesis concept and understand the inuence of
calcination temperature titania lms were deposited from
solutions containing CMS10, TiCl
4
, ethanol and water and
calcined at 300 and 450 C, respectively. The resulting lms
have a thickness of circa 80 nm. Fig. 1A–C and G presents
analysis of the sample calcined at 300 C by SEM, SAXS and
XRD. The lm surface features an abundance of pores with
diameters of about 4 to 5 nm (Fig. 1A, SEM). The FFT image
corresponding to the SEM micrograph (inset in Fig. 1A) shows a
distinct ring corresponding to periodic distances of about 9 nm.
Hence, the lm surface consists of locally ordered mesopores
imprinted by the hyperbranched CMS polymer.
The 2D SAXS pattern recorded perpendicular to the
substrates surface (b¼90, Fig. 1B) features an isotropic ring,
which conrms the ordered pore structure. The corresponding
d-spacing in x-direction d
x
z8.4 nm (q
x
z0.75 nm
1
) corre-
sponds well with the periodicity of 9 nm observed by SEM on the
lm surface. The pattern recorded at a smaller angle (b¼13,
Fig. 1C) shows an ellipsoidal shape. Such patterns are typically
attributed to an ordered mesostructure aer isotropic
shrinkage in the direction normal to the substrate.
23,24
A
comparison of d
x
with the d-spacing in z-direction (q
z
z
1.93 nm
1
;d
z
z3.3 nm) suggests an anisotropic shrinkage of
approximately 61%. XRD analysis of the sample (Fig. 1G) does
not show distinct reections that could be assigned to a crys-
talline titania phase.
The observed lm properties are consistent with data
reported for mesoporous EISA-based TiO
2
lms synthesized
from block-copolymer templates, except for the smaller pore
size and d-spacing. Anisotropic shrinkage of about 60% was
reported for TiO
2
lms templated by micelles of F127,
24
ca. 70%
for PEO–PB–PEO polymers.
15
The circular (b¼90) and ellip-
tical scattering features (b¼13) are similar to scattering
patterns previously assigned to a distorted cubic arrangement
of micelle-templated mesopores.
23
However, the pore ordering
in lms templated by CMS polymers appears to be less
pronounced than reported for many titania lms templated by
micelles of e.g. F127,
24
P123,
14
Brij 58.
16,25
The lower degree of
pore ordering could be related to the polydispersity of the
employed CMS polymer which have a polydispersity index of
approximately 1.5. Also the low crystallinity is typical for
micelle-templated TiO
2
lms calcined at 300 C.
26
Impact of calcination temperature
CMS10-templated lms calcined at 300 C were heated in air to
450 C (3 K min
1
, 5 min dwell time) to induce further crys-
tallisation. Fig. 1 presents corresponding SEM images (D), 2D-
SAXS patterns (E and F) and XRD data (G). SEM analysis of the
lm evidences pores with a diameter of about 4 to 5 nm
(Fig. 1D), i.e. the same size as for the sample calcined at 300 C.
However, pore walls appear to be more compact as for the
300 C calcined lm (Fig. 1A). The FFT image corresponding to
the SEM data (inset in Fig. 1D) contains a distinct ring corre-
sponding to a periodic distance of 9 to 10 nm. 2D-SAXS recorded
in transmission at b¼90features an isotropic ring (Fig. 1E).
The SAXS pattern at b¼13(Fig. 1F) shows reections in x- and
z-direction, but no full ellipsoidal shape. The pattern suggests a
loss of pore periodicity in z-direction during calcination at
450 C, possibly resulting from pore degradation due to crys-
tallite growth. XRD analysis of the 450 C calcined sample
(Fig. 1G) conrms the presence TiO
2
anatase as indicated by the
reections at 2 theta angles of 25.28(101), 36.95(103), 37.80
(004), 38.58(112), 48.05(200) [PDF 21-1272]. The average
crystallite size determined by Rietveld renement amounts to
about 14 nm. This observation agrees well with evidence derived
from the microscopy and SAXS (Fig. 1D and F) that the pore
systems order slightly degrades at 450 C.
15,27
The surface area (Kr sorption) of the CMS10-templated lms
amounts to 182 m
2
g
1
or 703 m
2
cm
3
(T
calc
¼300 C) and
59 m
2
g
1
or 236 m
2
cm
3
(T
calc
¼450 C), respectively. In
comparison, micelle-templated anatase lms (PEO
213
–PB
184
–
PEO
213
, TiCl
4
) calcined at 475 Coffer a surface area of
85 m
2
g
1
.
15
113 m
2
g
1
were reported by Yu et al. for TiO
2
anatase lms synthesized from titanium tetraisopropoxide/
Pluronic P123 aer calcination at 400 C.
28
Hence, the surface
area of CMS10-templated TiO
2
is in the same order of magni-
tude as observed for micelle-templated TiO
2
. The high surface
area of the CMS-templated lms implies that mesopores are
interconnected and accessible to krypton gas.
The combined data conrms that hyperbranched core–
multishell polymers can template ordered mesopores of about
4–5 nm size in TiO
2
lms. Pore sizes are smaller than typically
obtained with micelles of amphiphilic block-copolymers.
Additional tests on the inuence of relative humidity during
dip-coating (see ESI Fig. S2‡) prove that the synthesis is also
more robust than typical EISA syntheses. In contrast to Plur-
onic-templated lms,
29
the pore morphology of CMS-templated
TiO
2
did not change for relative humidities between 12
and 80%.
Fig. 1 SEM, SAXS and XRD analysis of CMS10-templated TiO
2
films.
The top row shows results for films calcined at 300 C, the bottom row
films calcined at 450 C. (A and D) Top-view SEM images with FFT
insets. 2D-SAXS pattern recorded in transmission mode with an inci-
dent angle of b¼90(B and E) and 13(C and F). (G) XRD pattern
recorded in grazing incidence mode (1in respect to film surface).
Assigned (hkl) indices correspond to crystalline TiO
2
anatase (PDF 21-
1272).
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Controlling the thickness of pore walls
The wall thickness of a porous material is a critical parameter
for its thermal stability. Mesoporous titania lms with different
wall thicknesses were synthesized by changing the concentra-
tions of the CMS10 polymer template in the dip-coating solu-
tion while keeping the amount of precursor constant. Fig. 2A–F
shows top-view SEM and FFT images of the respective lms aer
calcination at 300 C. Fig. 2G summarizes the inuence of the
mass ratio between CMS template and precursor on pore
diameter and on the periodic distances between pores.
Increasing concentrations of the CMS10-template result in
lms with similar morphologies (Fig. 2A–F). The resulting lm
thicknesses range from 70 nm to 95 nm. Templated mesopores
with about 4 nm diameter can be observed for all concentra-
tions. FFTs for all images indicate the preserved pore ordering.
However, the pore spacing systematically changes as indicated
by the changing diameter of the ring seen in the FFTs. The
periodic spacing decreases monotonously from 10 nm to 6 nm
with increasing polymer content, which indicates a decrease in
the wall thickness of the templated pores. Hence, the developed
synthesis enables for the rst time for pore diameter smaller
5 nm the control of the pore-wall thickness independent of the
templated mesopore size.
Controlling pore size
The control of pore size is particularly important for catalyst
design. For micelle-templated lms the obtained pore size can
be controlled by the employed structure-directing agent. We
therefore tested a second template polymer CMS5 in order to
establish pore-size control in a similar fashion also for small
mesopores. CMS5 features a signicantly smaller hyper-
branched core (5000 g mol
1
) than CMS10 (10 000 g mol
1
).
Fig. 3 (lecolumn, 1 : 0) displays SEM (a and b) and FFT (c)
images of a TiO
2
lm templated with CMS5 polymer. The lms
morphology strongly resembles that of CMS10 templated TiO
2
(Fig. 1A) showing an abundance of locally ordered mesopores.
However, pore diameter (3 nm) and periodic distance (7.5 nm)
are apparently smaller than for the lms templated with CMS10
(4–5 nm and 9 nm, respectively). The smaller structural features
produced by CMS5 agree well with the observation that CMS5
forms also smaller polymer entities already in solution (SAXS:
6.6 nm) than CMS10 (SAXS: ca. 8.3 nm, see ESI S1‡). Hence, the
size of templated mesopores can be controlled by the size of the
dissolved polymer template, which can be related to the molar
weight of its hyperbranched core.
Hierarchical porosity by dual templating with CMS5 and
micelles of PEO–PB–PEO
Hierarchical pore systems can show superior performance in
applications that rely on fast mass transport such as catalysis.
Fig. 2 Influence of the content of CMS10 polymer on pore size and pore spacing in TiO
2
films calcined at 300 C. (A–F) SEM and FFT, (G) image
evaluation. CMS10 content increases from left to right. Values are given as mass of template per 100 mg TiCl
4
in the dip-coating solution. (G)
Plots the measured pore diameter (SEM) and regular distances (from FFT) vs. the amount of template. Increasing polymer content decreases the
spacing between pores while the pore size remains almost constant.
Fig. 3 Influence of the ratio between CMS5 and PEO–PB–PEO
polymer in dual templating with two different porogens for TiO
2
films
calcined at 300 C. (A and B) SEM images, (C) FFT. The amount of
PEO–PB–PEO increases from left (1 : 0, no micelles) to right (0 : 1, no
CMS5, pores from PEO–PB–PEO only). Mass ratios of 1 : 1 and 1 : 3
induce bimodal porosity with separate pore domains, at a ratio 1 : 6 all
smaller mesopores are located inside the wall formed by larger
mesopores.
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The feasibility of synthesizing TiO
2
lms with hierarchically
organized bimodal porosity was tested combining different
amounts of the smaller core–multishell polymer CMS5 with
micelles of PEO
213
–PB
184
–PEO
213
block-copolymers in the same
dip-coating solution.
Fig. 3 presents SEM images in low (A) and high (B) magni-
cation and corresponding FFTs (C) of calcined titania lms
templated with either CMS5 (“1:0”), with different mixtures of
CMS5 and PEO–PB–PEO (mass ratio 1 : 1, 1 : 3, 1 : 6), and with
PEO–PB–PEO only (“0:1”). The mass ratio between CMS5 and
PEO–PB–PEO increases from leto right in Fig. 3. This was
realized by varying the amount of CMS5 in the employed dip-
coating solutions. The lm thicknesses range from 70 nm to
100 nm.
SEMs image of the lm templated with CMS5 polymer
(Fig. 3, 1 : 0) show mesopores on the outer lm surface with a
pore diameter of ca. 3 nm. The periodic distance from FFT
amounts to 7.5 nm. Templating with micelles of PEO–PB–PEO
in the absence of CMS templates produces TiO
2
structures with
large mesopores of about 20 nm diameter and a periodic
distance of 29.3 nm (Fig. 3, ratio 0 : 1) (see ref. 15 for more
details). For all lms synthesized with the template mixtures
(Fig. 3, 1 : 1, 1 : 3, 1 : 6) a bimodal porosity can be clearly
recognized. All systems show mesopores with the desired pore
diameters of about 3 nm originating from CMS5 and 20 nm
from PEO–PB–EPO micelles, respectively. However, the lms
differ in the local distribution of the differently templated
pores. At a mass ratio CMS5 to PEO–PB–PEO of 1 : 1 separated
pore domains for each template are formed (Fig. 3, 1 : 1). FFT
images show therefore two distinct rings with periodic
distances of 7.0 nm and 28.0 nm corresponding to individual
domains of the respective polymer template.
The size of the individual domains decreases with increasing
PEO–PB–PEO content as indicated by SEM and FFT (Fig. 3,
1 : 3). The individual domains of PEO–PB–PEO nally disappear
at a ratio CMS5 to PEO–PB–PEO of 1 : 6. Large 20 nm pores are
homogeneously distributed across the whole lm, with smaller
CMS5-templated pores located in all the walls of the larger
mesopores (Fig. 3, 1 : 6). The respective FFT shows one clear
ring which corresponds to the periodic distances of the PEO–
PB–PEO-templated pore structure and a broad halo originating
from the smaller pore spacings. Hence, the combined data
suggest that a hierarchically organized bimodal porosity is
obtained.
It should be noted that also the available surface area changes
signicantly with the introduction of hierarchical porosity.
The BET surface area of a CMS5-templated lm amounts
to 1050 m
2
cm
3
,whereasalm prepared with a CMS5 to
PEO–PB–PEO ratio of 1 : 3 shows about 160 m
2
cm
3
.The
decrease in surface area originates from the effect that additional
larger mesopores are introduced at the expense of smaller ones.
In conclusion, hyperbranched CMS polymers act as unim-
olecular template species and enable in combination with PEO–
PB–PEO polymers a simple one-pot synthesis of metal oxides
with hierarchically organized bimodal mesoporosity. Both
desired pore sizes can be controlled individually by the size of
each respective template.
Mesoporous PdNP/TiO
2
catalysts
CMS-type polymers have been reported to stabilize Pd nano-
particles in colloidal solutions.
30
We exploited this ability for the
synthesis of a PdNP/TiO
2
catalysts from a solution containing
preformed colloidal PdNP, CMS10 and the TiCl
4
precursor.
Deposited lms were calcined (450 C, air) and subsequently
reduced (350 C, H
2
/Ar). Fig. 4 shows SEM and TEM images for
the resulting PdNP/TiO
2
catalyst. The top-view SEM image
(Fig. 4A) strongly resembles that of the CMS10/TiO
2
lm
without PdNP (Fig. 1D), indicating that the presence of the Pd
colloid did not change the CMS10-templated pore geometry.
The cross-section SEM image (Fig. 4B) reveals a lm thickness
of 80 nm. A bright-eld TEM image (Fig. 4C) and the corre-
sponding Z-contrast TEM image in high-angle annular dark-
eld mode (HAADF) (Fig. 4D) of the identical part of the catalyst
reveal that the preformed colloidal PdNP (BF: black spots,
HAADF: bright spots) are well distributed in a mesoporous
structure (BF and HAADF: grey area). High-resolution TEM
images (Fig. 4E and F) conrm the presence of crystalline TiO
2
anatase and Pd. (101)-planes of anatase (d¼0.352 nm) (PDF 21-
1272) can be clearly distinguished in the pore walls (Fig. 4E).
Fig. 4F presents a typical Pd particle showing lattice fringes
corresponding to metallic Pd, i.e. (111)-planes with d-spacing of
0.225 nm, and (200)-planes with d-spacing of 0.195 nm (PDF 46-
1043). The detected PdNPs possess nearly spherical shape. The
average PdNP size amounts to 5.6 1.5 nm (Fig. 4H). SAED
(Fig. 4G) shows ring positions that are consistent with anatase
(PDF 21-1272).
XRD analysis (Fig. 4I) reveals broad reections at 2 Theta
angles of 25,36–39and 47that can be assigned to crystalline
TiO
2
anatase (PDF 21-1272). At 2 theta of 40another weak
reection is observed. The reection can be assigned to a
metallic Pd phase (PDF 46-1043). Other phases, e.g. PdO (PDF
46-1211), were not detected. The average crystallite size (derived
from Rietveld renement) amounted to 11.8 nm for anatase and
5.3 nm for Pd. The values are consistent with crystallite and
particle sizes extracted from TEM. Compositional analysis of
the PdNP/TiO
2
catalyst with ICP-OES indicates a content ratio of
Pd to TiO
2
of approximately 2.4 wt%. This value matches closely
with the 2.5 wt% Pd expected from the composition of the dip-
coating solution.
Activity and selectivity of the PdNP/TiO
2
catalyst in the gas
phase hydrogenation of 1,3-butadiene are illustrated in Fig. 5
showing (A) the inuence of temperature on 1,3-butadiene
conversion and (B) selectivity to 1-butene and to selectivity to all
butenes (sum of S
1-butene
,S
trans-2-butene
and S
cis-2-butene
)vs. buta-
diene conversion. Both graphs contain corresponding bench-
mark data for a previously synthesized 0.5 wt% PdNP/TiO
2
catalyst with a similar PdNP size as reported in ref. 4. The
benchmark catalyst was synthesized with F127 as mesopore
template, titanium(IV) bis(ammonium lactato)dihydroxide
(TALH) as TiO
2
precursor and colloidal PdNP.
4
Note that a
loading of size-controlled PdNP higher than 0.5 wt% could not
be obtained in this previous study due to the fact that the
particle stabilizer (PVP) degraded the micelle-templated pore
structure.
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Both catalysts are active and selective. Butadiene conversion
increases with increasing temperature for both catalysts
(Fig. 5A). The butadiene conversion at 70 C amounted to 26%
for PdNP/CMS10/TiCl
4
and to 5.6% for PdNP/F127/TALH,
respectively. Thus, the CMS10-templated catalyst shows a ca.
ve times higher activity than the TALH-based reference cata-
lyst. This ve times higher activity correlates well with the ve
times higher Pd loading (2.5 wt%) that could be achieved only
with the new CMS-based catalyst.
Almost identical product selectivities to 1-butene, trans-2-
butene, cis-2-butene and small amounts of n-butane were
observed for both catalysts (Fig. 5B). At butadiene conversions
up to about 50% the selectivity for 1-butene is 55% which is in
line with values reported in literature for other Pd-based cata-
lysts.
22
Moreover, the Arrhenius plots constructed for both
catalysts in the interval 50 to 80 C were tted by straight lines
and yielded the same activation energy for both catalyst of about
62 kJ mol
1
. The equivalent trends in selectivity and activation
energy indicate that both catalysts possess the same intrinsic
behavior.
Generally, variations of the pore system (pore size, bimodal
porosity) can be used to inuence the catalytic performance in
cases when pore diffusion is limiting. For very small pores the
reactant transport in the pore system of the catalyst can be
limited by Knudsen diffusion. However, all data presented here
were recorded in the kinetic regime. Hence, the data charac-
terize the materials intrinsic catalytic properties, not the effects
of pore diffusion.
The space time yield (STY) calculated at 50 Cforthenew
mesoporous PdNP/CMS10/TiO
2
catalyst corresponds to
0.122 mol s
1
kg
1
. Hence, the developed catalytic coating shows
in the kinetic regime and under comparable conditions space-
time-yields for butenes that are at least two to six times higher
than values reported in literature (e.g. 0.016 mol s
1
kg
1
,
31
0.019 mol s
1
kg
1
,
4
0.061 mol s
1
kg
1
(ref. 32)).
Fig. 4 SEM, TEM and XRD analyses of a PdNP/TiO
2
catalyst film calcined at 450 C and reduced in H
2
/Ar at 350 C. Top-view SEM image with FFT
inset (A). Cross-section SEM image (B). TEM image in bright-field (C) and high-angle annular dark-field mode (D). HR-TEM images of TiO
2
(E) and
Pd (F) crystallites with corresponding FFT insets. SAED (G) and XRD pattern (I). Assigned (hkl) indices correspond to crystalline TiO
2
anatase (PDF
21-1272) and cubic Pd metal (PDF 46-1043), respectively. A histogram of PdNP diameters from TEM analysis is given (H).
Fig. 5 Catalytic performance of mesoporous PdNP/TiO
2
catalytic coatings. (A) Butadiene conversion vs. temperature. (B) Selectivity to 1-butene
and total butane selectivity vs. butadiene conversion. The CMS10-templated catalyst with 2.5 wt% Pd loading (circles) is compared to a previously
reported mesoporous F127-templated TiO
2
catalyst film with 0.5 wt% Pd based on TALH (squares). [4] The five times higher PdNP loading
enabled by CMS-templating results in five times higher catalytic activity while the catalyst selectivity remains unaffected.
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Conclusion
The developed synthesis strategy based on pore templating with
core–multishell polymers allows unprecedented control and
exibility in the synthesis of oxide coatings and supported
catalysts with small mesopores and with hierarchical porosity.
The pore size can be controlled by the size of the template
between 3 nm (CMS5) and 4 to 5 nm (CMS10), i.e. in a pore size
range that is difficult to access with polymer-micelle templates.
Due to the templates preformed covalent structure the pore wall
thickness was easily adjusted by the ratio between CMS polymer
and precursor in the dip-coating solution while keeping the
pore size constant. Metal oxide lms with hierarchical bimodal
porosity prepared by dual so-templating with CMS polymers
and micelle-based templates are accessible for the rst time.
Extending the synthesis approach to use CMS polymers as
bifunctional NP stabilizer and porogen produces PdNP/TiO
2
catalytic coatings with controlled mesoporosity, a high acces-
sible surface area and high Pd loading. No detrimental effects of
the synthesis on the properties of the catalyst support (lm
integrity, pore templating, pore ordering) or the active PdNP
(particle size, activity and selectivity in butadiene hydrogena-
tion) were observed.
Catalytic activity and pore diffusion within the support can
be easily tuned with the presented approach. The concept thus
provides a versatile and general platform for the rational opti-
mization of catalysts based e.g. on computational prediction of
optimal pore structures.
33
The synthesis also paves the way to
model-type catalysts with well-dened pore structure, particle
size and high metal loading for the investigation of structure–
activity relationships as well as practical applications.
Acknowledgements
The authors acknowledge Arno Bergmann, Maria Wuithschick,
Ulla Vanio and ZELMI at Technical University Berlin for support
in material analytics. RK thanks in particular Einstein Foun-
dation Berlin for generous support provided by an Einstein-
Junior-Fellowship (EJF-2011-95). DB, EO and RK acknowledge
also funding from BMBF (FKZ 03EK3009). JP acknowledges
generous funding by the Deutsche Forschungsgemeinscha
within Project PO 1744/1-1. Portions of this research were con-
ducted on beamline B1 at light sources DORIS III and PETRA III
at DESY, a member of the Helmholtz Association (HGF).
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