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Article
Carbide-Modified Pd on ZrO2as Active Phase for
CO2-Reforming of Methane—A Model Phase
Boundary Approach
Norbert Köpfle 1, Kevin Ploner 1, Peter Lackner 2, Thomas Götsch 1,3, Christoph Thurner 1,
Emilia Carbonio 3,4, Michael Hävecker 3,5, Axel Knop-Gericke 3,5, Lukas Schlicker 6,
Andrew Doran 7, Delf Kober 6, Aleksander Gurlo 6, Marc Willinger 8, Simon Penner 1,
Michael Schmid 2and Bernhard Klötzer 1,*
1Institute of Physical Chemistry, University of Innsbruck, Innrain 52 c, A-6020 Innsbruck, Austria;
[email protected] (N.K.); [email protected] (K.P.); [email protected] (T.G.);
[email protected] (C.T.); [email protected] (S.P.)
2Institute of Applied Physics, TU Wien, Wiedner Hauptstr. 8-10/134, 1040 Wien, Austria;
[email protected] (P.L.); [email protected] (M.S.)
3Fritz-Haber-Institut der Max-Planck-Gesellschaft, Anorganische Chemie, Faradayweg 4–6,
D-14195 Berlin, Germany; [email protected] (E.C.); [email protected] (M.H.);
[email protected] (A.K.-G.)
4Helmholtz-Zentrum Berlin für Materialien und Energie GmbH, BESSY II, Albert-Einstein-Straße 15,
D-10623 Berlin, Germany
5Max Planck Institute for Chemical Energy Conversion, Department of Heterogeneous Reactions,
Stiftstraße 34-36, D-45470 Mülheim an der Ruhr, Germany
6Institut für Werkstoffwissenschaften und -technologien, Fachgebiet Keramische Werkstoffe,
Technische Universität Berlin, D-10623 Berlin, Germany; [email protected] (L.S.);
[email protected] (D.K.); [email protected] (A.G.)
7
Advanced Light Source, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA; [email protected]
8Scientific Center for Optical and Electron Microscopy, ScopeM, ETH Zürich, 8093 Zürich, Switzerland;
[email protected]
*Correspondence: [email protected]; Tel.: +43-512-507-58004
Received: 13 July 2020; Accepted: 24 August 2020; Published: 2 September 2020


Abstract:
Starting from subsurface Zr
0
-doped “inverse” Pd and bulk-intermetallic Pd
0
Zr
0
model
catalyst precursors, we investigated the dry reforming reaction of methane (DRM) using
synchrotron-based near ambient pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS),
in-situ X-ray diffraction and catalytic testing in an ultrahigh-vacuum-compatible recirculating batch
reactor cell. Both intermetallic precursors develop a Pd
0
–ZrO
2
phase boundary under realistic DRM
conditions, whereby the oxidative segregation of ZrO
2
from bulk intermetallic Pd
x
Zr
y
leads to a highly
active composite layer of carbide-modified Pd
0
metal nanoparticles in contact with tetragonal ZrO
2
.
This active state exhibits reaction rates exceeding those of a conventional supported Pd–ZrO
2
reference
catalyst and its high activity is unambiguously linked to the fast conversion of the highly reactive
carbidic/dissolved C-species inside Pd
0
toward CO at the Pd/ZrO
2
phase boundary, which serves the
role of providing efficient CO
2
activation sites. In contrast, the near-surface intermetallic precursor
decomposes toward ZrO
2
islands at the surface of a quasi-infinite Pd
0
metal bulk. Strongly delayed
Pd carbide accumulation and thus carbon resegregation under reaction conditions leads to a much
less active interfacial ZrO2–Pd0state.
Keywords:
palladium carbide; graphite; metal-support interaction; coking; palladium-zirconium
intermetallic phase; in-situ X-ray photoelectron spectroscopy; in-situ X-ray diffraction; high resolution
electron microscopy; dry reforming of methane; carbon dioxide activation
Catalysts 2020,10, 1000; doi:10.3390/catal10091000 www.mdpi.com/journal/catalysts
Catalysts 2020,10, 1000 2 of 29
1. Introduction
Among multiple worldwide and local approaches and strategies toward the mitigation of
global warming, the methane dry reforming reaction (DRM) is regarded as a potential method to
simultaneously deal with the two major climate-harming greenhouse gases, methane and carbon
dioxide, and to further convert them to useful syngas following the reaction
CH4+CO2→2H2+2CO
.
At 100% selectivity, a H
2
:CO =1:1 ratio is obtained, which is directly suitable for follow-up reactions like
carbonylation or hydro-formylation processes [
1
–
3
]. Higher H
2
contents can in principle be achieved
via membrane reactor operation or a combination with the water-gas shift reaction and/or methane
steam reforming to attain the optimum H
2
/CO ratio of 2 required for the synthesis of renewable
fuels [
4
,
5
]. In this respect, loss of H
2
selectivity at elevated pressures due to the water-gas shift
equilibrium [
6
], as well as coking issues, especially on Ni-based catalysts [
7
–
10
], represent the major
application-oriented obstacles.
From a fundamental scientific viewpoint, a group of highly active catalysts on noble metal
basis exhibiting enhanced coking resilience deserve particular attention [
8
,
9
,
11
–
17
]. In a recent
near-ambient-pressure in-situ X-ray photoelectron spectroscopy (NAP-XPS) study of our group,
the particular anti-coking and carbon-converting role of the Pd/ZrO
2
phase boundary was
highlighted [
18
]. Empirically, attempts to enhance the Ni coking resistance, while simultaneously
keeping a high activity, yielded promising bimetallic DRM catalysts. In this respect, the NiPd/ZrO
2
system stood out, particularly in terms of the desired combination of excellent catalytic performance
and coking resistance [
19
]. However, despite the promising empirical data that have been compiled
on the latter, the mechanistic benefit of alloying of Ni especially with Pd has not been resolved yet.
Regardingthemethane-activatingroleofthe intermetalliccomponent, ensembleandelectronic structure
effects at the bimetallic surface are conceivable [
20
–
24
], whereas for the potential mechanistic role of
intermetallic–oxidic phase boundary sites different levels of metal–oxide bi-functional synergisms can
be anticipated, which essentially depend on the intrinsic activities of the individual alloy components
toward CO
2
and CH
4
[
15
]. Focusing on a potential mechanistic difference between Ni and Pd, pure Ni
surfaces are in principle capable of simultaneous CH
4
and CO
2
activation [
24
–
27
], which might be
interpreted in terms of a less prominent co-catalytic role of the Ni–support interface, especially on
rather inert supports. Conversely, the degree of bi-functional operation can be steered by different
degrees of alloying of Ni with an outstanding methane activator such as Pd, which exhibits at the
same time inferior carbon dioxide activation properties in its pure state [
15
,
28
,
29
]. Consequently,
in the limiting “pure Pd case,” the promotion of CO
2
activation and subsequent CO product formation
requires particularly active and abundant oxide support–Pd interface sites, which are therefore essential
for a high dry reforming activity.
As a general strategy derived from these considerations, the knowledge-based empirical
development of DRM catalyst preparation should aim at an extended (bi)metal–oxide interface
with superior methane activation properties at the intermetallic and superior carbon dioxide activation
kinetics at the oxidic component. For the latter, surface reducibility toward CO
2
-activating vacancy
sites and/or reactivity of a basic oxidic component toward CO
2
to form reactive carbonate species
under reaction conditions are specifically important catalyst optimization parameters [30–32].
To unravel mechanistic details of a catalytically operating (bi)metal–oxide interface, the use of
model systems is highly advantageous, especially if further steps to close the material’s gap, that is,
the transfer of mechanistic ideas to technologically more relevant catalyst materials, are attempted.
With respect to studies employing intermetallic precatalyst samples, bulk intermetallic compounds
can be partially or quantitatively decomposed toward metal–oxide systems, e.g., using leaching
techniques-prior to reaction [
33
], yielding supported powder catalysts, or subjected to in-situ activation
in the respective reaction atmosphere. The latter approach is a particularly efficient way to generate a
large amount of phase-boundary sites, as has been shown both for the Cu–Zr and Pd–Zr systems [
34
].
Especially the near-surface regions of a bulk Pd–Zr precatalyst are oxidatively decomposed under
realistic DRM conditions, whereby the resulting Pd
0
nanoparticles provide an appropriate near-surface
Catalysts 2020,10, 1000 3 of 29
carbon loading at the resulting Pd
0
nanoparticle/ZrO
2
interface. This favors fast supply of reactive
carbon atoms toward the phase boundary, whereas redox-active Zr sites assist in CO
2
activation
and the transfer of CO
2
-derived oxygen to the latter [
18
]. In contrast, it has also been shown that a
CVD-prepared Zr
0
subsurface alloy state on a Pd foil was only slightly more active than its individual
components ZrO2and Pd0[35].
If this “in-situ corrosive” activation is limited to near-surface regions and does not suppress the
electric conductivity of the 3D corrosion layers below XPS-compatible levels, the resulting system will
still encompass the electronic advantages of fully bulk-metal-based “inverse” ultrahigh-vacuum (UHV)
model systems, thus allowing for in-situ spectroscopic characterization with vacuum-based electron
spectroscopic techniques without charging. Nevertheless, the selectively corroded top layers consist
typically of a quasi-3-dimensional oxide–metal composite, which resembles a supported catalyst
quite closely.
This “intermetallic precursor” concept is somewhat complementary to the ex-solution behavior of
complex oxide materials such as perovskites, which—especially under reductive treatments—show
ex-solution of metallic nanoparticles and the formation of a catalytically operating three-dimensional
interface on top of an oxidic material. Both approaches provide additional pylons supporting the
materials-gap-spanning bridge the between UHV/bulk-metal-based “inverse” model catalysts and
technologically used powder catalysts.
In the present work, we exemplify this strategy for the Pd–Zr system in the methane dry reforming
reaction—a Pd-metal-foil-based subsurface Zr–Pd alloy model system will be directly compared to
a bulk-intermetallic Pd–Zr intermetallic phase and a conventionally prepared, supported Pd/ZrO
2
reference catalyst.
Clearly, the detailed understanding and directional steering of the material-specific carbon
chemistry is one of the pending issues in the development of prospective DRM catalyst materials,
as coking is the most limiting factor in the catalyst lifetime. Various ways of controlling coke formation
and optimizing its conversion are discussed. Among others, these include efficient gasification of coke,
for example, by addition of water to the feed, adjustment of the surface chemical properties of support
materials with respect to basicity, redox activity and oxygen storage capacity, control of nanoparticle
size and interfacial stabilization (including exploiting metal-support interaction effects), directional
alloying to suppress nucleation and growth of graphitic and/or carbon nanotube (CNT)-type species,
redox-cycling for intermediate coke abatement, catalyst passivation by sulphur and the addition of
H
2
and/or H
2
O to the feed to suppress thermal gas phase composition of methane and other small
hydrocarbons. A comprehensive review of these strategies is provided in Reference [
36
]. However,
much more detailed knowledge about the growth, reactivity and lifetime of the distinct carbon species
or carbonaceous deposits can expected from in-situ model catalyst studies under realistic reaction
conditions. Hence, we provide a detailed in-situ XPS analysis focusing on the synopsis of all core-level
trends of the involved elements Zr, Pd, C and O and attempt to bridge the materials gap completely by
including the comparative assessment of a technologically relevant Pd/ZrO2catalyst.
In-situ spectroscopic and structure-determining methods such as synchrotron-based near ambient
pressure XPS (NAP-XPS at HZB/BESSY II) and in-situ X-ray diffraction (XRD) (at ALS/Berkeley)
were used to achieve complementary characterization of bulk-related phase changes vs. the active
surface/interface state of the working catalyst in a realistic DRM atmosphere.
2. Results and Discussion
2.1. Precatalyst Characterization
2.1.1. CVD Process toward the Pd(111) and Pd Foil-Based Subsurface Zr0–Pd Alloy Precatalysts
Figure 1shows the thermal evolution of the ZrO
x
layer prepared by chemical vapor deposition
(CVD; decomposition of Zirconium (IV) tert-butoxide at 400
◦
C, see Section 3.1) on Pd(111) upon
Catalysts 2020,10, 1000 4 of 29
annealing in ultrahigh vacuum (UHV base pressure <1
×
10
−10
mbar). The initial as-prepared CVD
state shows a mix of bulk- and ultrathin zirconia and already a minor contribution of intermetallic Zr
0
but no remaining C from the precursor, as evidenced by the absence of any C 1s intensity (not shown).
The subsequent intermediate stages of annealing show a decrease of the ZrO
x
components and finally
the full conversion of the last remaining ultrathin oxide patches to Zr
0
, leaving a purely bimetallic
(alloyed) state behind after a total time of 40 min at 450 ◦C.
Catalysts 2020, 10, x FOR PEER REVIEW 4 of 31
2. Results and Discussion
2.1. Precatalyst Characterization
2.1.1. CVD Process toward the Pd(111) and Pd Foil-Based Subsurface Zr0–Pd Alloy Precatalysts
Figure 1 shows the thermal evolution of the ZrOx layer prepared by chemical vapor deposition
(CVD; decomposition of Zirconium (IV) tert-butoxide at 400 °C, see Section 3.1) on Pd(111) upon
annealing in ultrahigh vacuum (UHV base pressure < 1 × 10−10 mbar). The initial as-prepared CVD
state shows a mix of bulk- and ultrathin zirconia and already a minor contribution of intermetallic
Zr0 but no remaining C from the precursor, as evidenced by the absence of any C 1s intensity (not
shown). The subsequent intermediate stages of annealing show a decrease of the ZrOx components
and finally the full conversion of the last remaining ultrathin oxide patches to Zr0, leaving a purely
bimetallic (alloyed) state behind after a total time of 40 min at 450 °C.
Figure 1. Pd 3d, Zr 3d and (Pd 3p3/2 + O 1s) X-ray photoelectron spectroscopy (XPS) regions during
annealing of a CVD-prepared Zr/ZrOx layer on a Pd(111) single crystal at 400–450 °C under UHV
conditions for the given periods of time: see panels (a–e). The as-prepared state was obtained by
preceding exposition to 10−3 mbar s ZTB within 200 s at 400 °C.
XPS quantification of panel B (annealed 10 min at 400 °C) revealed that ≈31% of the Zr 3d signal
originate from deposited Zr was fully reduced and alloyed with the Pd(111) substrate (binding
energy BE = 179.6 eV, very similar to Zr in Pt3Zr [37,38]. Another 25.5% of the Zr signal originate from
bulk-like ZrO2 (BE ≈ 182.8 eV when slightly substoichiometric [39]) and the remaining 43.5% are
present in the form of the ultrathin zirconia film (BE = 180.9 eV [37]). In contrast to previous work
[37,40,41], newer results indicate that oxygen-deficient ultra-thin ZrOx is obtained by reduction of
ZrO2 on Pt(111), Rh(111) and Ru(0001) [42], different from the previously assumed stoichiometric
ZrO2 “trilayer” film [37,40,41]. The intermediate occurrence of related substoichiometric ZrOx films
(Figure 1a–d, green component) appears also likely for our Pd substrates.
The scanning tunneling microscopy (STM)-observed structure of the final annealing state after
50 min at 450 °C corresponding to panel (e) of Figure 2 is shown in Reference [35]. It is strongly
Figure 1.
Pd 3d, Zr 3d and (Pd 3p
3/2
+O 1s) X-ray photoelectron spectroscopy (XPS) regions during
annealing of a CVD-prepared Zr/ZrO
x
layer on a Pd(111) single crystal at 400–450
◦
C under UHV
conditions for the given periods of time: see panels (
a
–
e
). The as-prepared state was obtained by
preceding exposition to 10−3mbar s ZTB within 200 s at 400 ◦C.
XPS quantification of panel B (annealed 10 min at 400
◦
C) revealed that
≈
31% of the Zr 3d signal
originate from deposited Zr was fully reduced and alloyed with the Pd(111) substrate (binding energy
BE =179.6 eV, very similar to Zr in Pt
3
Zr [
37
,
38
]. Another 25.5% of the Zr signal originate from bulk-like
ZrO
2
(BE
≈
182.8 eV when slightly substoichiometric [
39
]) and the remaining 43.5% are present in
the form of the ultrathin zirconia film (BE =180.9 eV [
37
]). In contrast to previous work [
37
,
40
,
41
],
newer results indicate that oxygen-deficient ultra-thin ZrO
x
is obtained by reduction of ZrO
2
on
Pt(111), Rh(111) and Ru(0001) [
42
], different from the previously assumed stoichiometric ZrO
2
“trilayer”
film [
37
,
40
,
41
]. The intermediate occurrence of related substoichiometric ZrO
x
films (Figure 1a–d,
green component) appears also likely for our Pd substrates.
The scanning tunneling microscopy (STM)-observed structure of the final annealing state after
50 min at 450
◦
C corresponding to panel (e) of Figure 1is shown in Reference [
35
]. It is strongly
reminiscent of a previously described subsurface V/Pd(111) alloy [
43
], as it also exhibits a modified
Pd (1
×
1) base lattice with similar chemical contrast phenomena and apparent height modulations.
The latter are likely due to the variable local presence of subsurface Zr
0
. The proof of its fully
subsurface Zr
0
-converted nature via low-energy ion scattering (LEIS) is also provided in Reference [
35
].
In combination with the XPS data of Figure 1e, showing exclusively metallic Zr
0
, the LEIS data provide
Catalysts 2020,10, 1000 5 of 29
unambiguous experimental evidence for the absence of any Zr species in the topmost metal layer or in
oxidic surface layers.
Generally, the reduction to and dissolution of Zr
0
appears to be kinetically enhanced by
the continuous removal of oxygen atoms resulting from the zirconia decay at the extended
zirconia-metal interface under the effectively strongly reducing, non-equilibrium UHV conditions.
Thermodynamically, the high enthalpy of Zr
0
dissolution is helpful for dissolving it in the Pd bulk,
as well as the availability of a large Pd bulk volume leading to a positive dissolution entropy change,
especially in case of “deep” dissolution. In 3D bulk alloys, the coordination of Zr
0
with Pd and Pt is
energetically strongly favorable [44].
We further emphasize that all Zr
0
species, irrespective of their structural environment prior to
DRM, become quantitatively oxidized under DRM reaction conditions toward an active state involving
only Zr
+4
species at the surface, as will be shown in the context of the NAP-XP spectra discussed in
Section 3.2.1. This implies that the clarification of the exact location of the Zr
0
atoms is catalytically not
crucial. A detailed microstructural characterization of the intermediate and final annealing states will
thus become part of an independent study based on DFT (density functional theory) and STM.
In view of catalytic relevance, the fully subsurface-Zr
0
reduced state (panel (e) in Figure 1) was
chosen as the most suitable precatalyst due to its higher structural uniformity and the rather isotropic
electronic modification of potential Pd active sites by bulk- and subsurface-dispersed Zr
0
. This high
atomic dispersion was also regarded as a useful prerequisite for in-situ growth of equally dispersed
oxidized Zr species under reaction conditions, with the expectation of a maximized Pd–ZrO
x
phase
boundaryinducedbythereaction. ForthecatalyticDRMtestsbothinthecombinedXPS—high-pressure
batch reactor setup (cf. Section 3.2.2) and in the NAP-XPS system at BESSY II (cf. Section 3.2.5),
the subsurface Zr
0
–Pd alloy was prepared on a Pd foil using the identical preparation conditions as
used for Pd(111). The reproducible formation of a fully bimetallic subsurface Zr
0
state was again
only achieved under excellent UHV conditions and could also be verified via the above-discussed
combination of XPS and LEIS (data not shown). In essence, the preparation routines on the single- and
polycrystalline substrates were found to be fully compatible.
2.1.2. Characterization of the PdxZryBulk-Intermetallic Precatalyst
In contrast to the Zr
0
–Pd subsurface alloy, which was prepared under excellent UHV conditions
as described in the section above, the co-melted bulk-intermetallic precatalyst was prepared under
high-vacuum conditions (base pressure ~10
−7
mbar), transferred in a Schlenk flask in inert gas
atmosphere and finally transferred through ambient air into the in-situ NAP-XPS chamber at BESSY
II (cf. Section 3.2.5). Figure 2a displays the Pd 3d
5/2
, Zr 3d, C 1s and O 1s core level spectra of this
precatalyst. Despite the dominance of Zr
+4
species in the Zr 3d region, the intermetallic Zr
0
component
is clearly visible at a BE of 179.6 eV [
37
,
38
]. Obviously, the short contact to the ambient during transfer
to the NAP-XPS chamber already caused environmental corrosion of bimetallic Zr
0
toward bulk-like
Zr
+4
O
x
at the surface. In contrast, the Zr
0
–Pd foil subsurface alloy sample (cf. Section 2.1.1) exhibited
a lower degree of oxidative Zr segregation after the similar transfer procedure (cf. NAP-XPS data
discussed in Section 2.3.1, “as-prepared” spectra), probably due to the subsurface location of Zr
0
protected by the pure Pd top layer. In the Pd 3d region, only metallic Pd is observed on the Pd
x
Zr
y
bulk-intermetallic precatalyst at a BE of 335.0 eV (we note that the Pd 3d
5/2
region is superimposed by
the second harmonic component of the O 1s signal at a nominal BE of 333.5 eV). In the C 1s region,
a certain amount of graphite-type C is already present before DRM. The XRD data of Figure 2b reveal
the coexistence of the intermetallic phases Pd
3
Zr and Pd
2
Zr in the sample bulk and the absence of
XRD-detectable amounts of Pd metal and/or ZrO2phases.
Catalysts 2020,10, 1000 6 of 29
Catalysts 2020, 10, x FOR PEER REVIEW 6 of 31
Figure 2. Room-temperature XPS and X-ray diffraction (XRD) characterization of the PdxZry bulk-
intermetallic precatalyst prior to heating in the respective CH4:CO2 reaction mixture. Panel (a)
displays the Pd 3d5/2, Zr 3d, C 1s and O 1s core level regions. The photoelectron kinetic energy was
adjusted to 400 eV for all regions by choosing the respective photon energy. Panel (b) displays the
XRD pattern of the PdxZry bulk-intermetallic precatalyst, along with the peak positions for clean Pd
and the intermetallic compounds Pd2Zr and Pd3Zr.
2.1.3. Characterization of the Supported Pd–ZrO2 Powder Catalyst
Figure 3 shows the ex-situ XRD data obtained on the supported reference catalyst after
calcination and before H2 reduction, as well as the post-DRM state.
Figure 2.
Room-temperature XPS and X-ray diffraction (XRD) characterization of the Pd
x
Zr
y
bulk-intermetallic precatalyst prior to heating in the respective CH
4
:CO
2
reaction mixture. Panel
(
a
) displays the Pd 3d
5/2
, Zr 3d, C 1s and O 1s core level regions. The photoelectron kinetic energy was
adjusted to 400 eV for all regions by choosing the respective photon energy. Panel (
b
) displays the XRD
pattern of the Pd
x
Zr
y
bulk-intermetallic precatalyst, along with the peak positions for clean Pd and the
intermetallic compounds Pd2Zr and Pd3Zr.
2.1.3. Characterization of the Supported Pd–ZrO2Powder Catalyst
Figure 3shows the ex-situ XRD data obtained on the supported reference catalyst after calcination
and before H2reduction, as well as the post-DRM state.
Catalysts 2020,10, 1000 7 of 29
Catalysts 2020, 10, x FOR PEER REVIEW 7 of 31
Figure 3. X-ray diffraction patterns (Mo Kα1 radiation, 0.7093 Å ) of the impregnated Pd–ZrO2 powder
catalyst. The black (lower) pattern shows the fully oxidized PdO–ZrO2 state after 5 h of calcination in
air at 800 °C. The gray (upper) curve displays the catalyst after H2 prereduction and five cycles of
methane dry reforming.
After calcination, a mixture of monoclinic and tetragonal ZrO2 species is present, together with
PdO but no Pd metal is detectable. Already after H2 prereduction at 600 °C in 1 bar pure H2 for 1h,
full conversion of PdO to Pd0 was obtained (not shown, practically identical to the post-DRM
diffractogram). The fully metallic state of Pd is not changed by the DRM reaction (light gray
diffractogram). After 5 DRM cycles to 800 °C, ZrO2 appears to contain a larger fraction of the
thermodynamically stable monoclinic polymorph, as can be deduced from the changed intensity
ratio of the main reflections of the two coexisting phases at 12.87 and 13.77°.
Figure 4 shows a summary of the electron microscopic characterization of the calcined Pd–ZrO2
powder catalyst before and after in-situ reduction (scanning transmission electron microscopy STEM
and energy-dispersive X-ray analysis EDX). The annular dark field image in Figure 4a reveals the
particulate structure of the calcined catalyst but does not show features with clear contrast due to the
presence of PdO and ZrO2, with similar scattering properties. According to the STEM EDX map, Pd
is present in relatively large patches that are irregularly distributed on the surface of the ZrO2
particles. Secondary electron imaging reveals the surface morphology and indicates that the PdO
islands are smoothly covering the ZrO2 particles. At high magnification, lattice fringe imaging
confirms that Pd is present in the form of PdO structures that show a good wetting of the ZrO2
particles. During in-situ reduction in H2 at 600 °C, shrinkage of PdO patches and simultaneous
increase in contrast due to the reduction of PdO is observed (see panel (b), Figure 4). Lattice fringes
Figure 3.
X-ray diffraction patterns (Mo K
α1
radiation, 0.7093 Å) of the impregnated Pd–ZrO
2
powder
catalyst. The black (lower) pattern shows the fully oxidized PdO–ZrO
2
state after 5 h of calcination
in air at 800
◦
C. The gray (upper) curve displays the catalyst after H
2
prereduction and five cycles of
methane dry reforming.
After calcination, a mixture of monoclinic and tetragonal ZrO
2
species is present, together
with PdO but no Pd metal is detectable. Already after H
2
prereduction at 600
◦
C in 1 bar pure
H
2
for 1h, full conversion of PdO to Pd
0
was obtained (not shown, practically identical to the
post-DRM diffractogram). The fully metallic state of Pd is not changed by the DRM reaction (light gray
diffractogram). After 5 DRM cycles to 800
◦
C, ZrO
2
appears to contain a larger fraction of the
thermodynamically stable monoclinic polymorph, as can be deduced from the changed intensity ratio
of the main reflections of the two coexisting phases at 12.87 and 13.77◦.
Figure 4shows a summary of the electron microscopic characterization of the calcined Pd–ZrO2
powder catalyst before and after in-situ reduction (scanning transmission electron microscopy STEM
and energy-dispersive X-ray analysis EDX). The annular dark field image in Figure 4a reveals the
particulate structure of the calcined catalyst but does not show features with clear contrast due to the
presence of PdO and ZrO
2
, with similar scattering properties. According to the STEM EDX map, Pd is
present in relatively large patches that are irregularly distributed on the surface of the ZrO2particles.
Secondary electron imaging reveals the surface morphology and indicates that the PdO islands are
smoothly covering the ZrO
2
particles. At high magnification, lattice fringe imaging confirms that Pd is
present in the form of PdO structures that show a good wetting of the ZrO
2
particles. During in-situ
reduction in H
2
at 600
◦
C, shrinkage of PdO patches and simultaneous increase in contrast due to the
reduction of PdO is observed (see panel (b), Figure 4). Lattice fringes recorded in HAADF STEM after
reduction confirm the formation of metallic Pd
0
(see panel (c), Figure 4). The resulting particles are
mostly rather large and show a partial de-wetting and thus more roundish shape but some are smaller
and still well interfaced to the surface of the ZrO2particles, as shown in Figure 4c.
Catalysts 2020,10, 1000 8 of 29
Catalysts 2020, 10, x FOR PEER REVIEW 8 of 31
recorded in HAADF STEM after reduction confirm the formation of metallic Pd0 (see panel (c), Figure
4). The resulting particles are mostly rather large and show a partial de-wetting and thus more
roundish shape but some are smaller and still well interfaced to the surface of the ZrO2 particles, as
shown in Figure 4c.
Figure 4. In panel (a), an annular dark field image of the calcined powder catalyst, an EDX map and
corresponding secondary electron image, as well as a high-resolution image with lattice fringes of
PdO are shown. Panel (b) shows an overview image recorded before and after in-situ reduction in a
DensSolutions gas-flow reactor (conditions: 400 mbar H2, 600 °C, 1 h). Panel (c) shows a high-
resolution example of a selected smaller Pd particle on ZrO2.
In conclusion, the chosen impregnation routine using an aqueous ZrO2 suspension (see Section
3.1) yielded, after calcination and reduction, a supported Pd–ZrO2 catalyst with rather low dispersion
(~0.6%) and—in average—rather large Pd metal domains. The average Pd particle size derived from
the dispersion calculation based on CO adsorption and temperature-programmed desorption (TPD)
measurements (cf. Section 3.1.) amounts to ~130 nm, assuming half-embedded spherical particles.
Thus, the catalyst exhibits a plethora of large particles. The size of the particular, rather small Pd
particle shown in Figure 4c is ~15 nm but it was chosen for high-resolution imaging as also a
potentially much more active local phase boundary region to ZrO2 is well resolved, allowing for
verification of the presence of a stabilized Pd metal-support interface. The catalytic importance of the
latter will be elaborated in detail in the following sections.
2.2. Catalytic Testing via Temperature-Programmed DRM Experiments
As shown in Figure 5, panel (a), only a slight promotion of DRM activity relative to phase-pure
ZrO2 was observed on the subsurface Zr0–Pd foil precatalyst prepared by CVD. The first peak of the
Figure 4.
In panel (
a
), an annular dark field image of the calcined powder catalyst, an EDX map and
corresponding secondary electron image, as well as a high-resolution image with lattice fringes of
PdO are shown. Panel (
b
) shows an overview image recorded before and after in-situ reduction in a
DensSolutions gas-flow reactor (conditions: 400 mbar H
2
, 600
◦
C, 1 h). Panel (
c
) shows a high-resolution
example of a selected smaller Pd particle on ZrO2.
In conclusion, the chosen impregnation routine using an aqueous ZrO
2
suspension (see Section 3.1)
yielded, after calcination and reduction, a supported Pd–ZrO
2
catalyst with rather low dispersion
(~0.6%) and—in average—rather large Pd metal domains. The average Pd particle size derived from
the dispersion calculation based on CO adsorption and temperature-programmed desorption (TPD)
measurements (cf. Section 3.1) amounts to ~130 nm, assuming half-embedded spherical particles.
Thus, the catalyst exhibits a plethora of large particles. The size of the particular, rather small Pd
particle shown in Figure 4c is ~15 nm but it was chosen for high-resolution imaging as also a potentially
much more active local phase boundary region to ZrO
2
is well resolved, allowing for verification of
the presence of a stabilized Pd metal-support interface. The catalytic importance of the latter will be
elaborated in detail in the following sections.
2.2. Catalytic Testing via Temperature-Programmed DRM Experiments
As shown in Figure 5, panel (a), only a slight promotion of DRM activity relative to phase-pure
ZrO
2
was observed on the subsurface Zr
0
–Pd foil precatalyst prepared by CVD. The first peak of
the pure ZrO
2
-CO signal is assigned to the partial oxidation of methane (POM) to CO due to an
increased level of O
2
in the admitted DRM reaction mixture. The subsequent DRM onset at ~700
◦
C
is nevertheless clearly visible. The activity of the undoped ultra-pure Pd foil is not shown, as it is
below the experimental detection level. Panel (b) shows the DRM experiment for the Pd
x
Zr
y
bulk
intermetallic precatalyst. As a result of using a batch reactor with fixed initial molar amounts of
Catalysts 2020,10, 1000 9 of 29
reactants, the decrease of the CO
2
formation rate in the isothermal region in panel (b) is caused by
progressive reactant consumption—the CH
4
/CO
2
conversion on the initially Pd
x
Zr
y
bulk intermetallic
catalystafter 28 min isothermalreactionat 800
◦
Camounts to ~90%. This effectis even morepronounced
for the Pd–ZrO
2
powder catalyst (panel C), on which quantitative conversion is attained already after
~20 min at 800 ◦C.
Catalysts 2020, 10, x FOR PEER REVIEW 9 of 31
pure ZrO2-CO signal is assigned to the partial oxidation of methane (POM) to CO due to an increased
level of O2 in the admitted DRM reaction mixture. The subsequent DRM onset at ~700 °C is
nevertheless clearly visible. The activity of the undoped ultra-pure Pd foil is not shown, as it is below
the experimental detection level. Panel (b) shows the DRM experiment for the PdxZry bulk
intermetallic precatalyst. As a result of using a batch reactor with fixed initial molar amounts of
reactants, the decrease of the CO2 formation rate in the isothermal region in panel (b) is caused by
progressive reactant consumption—the CH4/CO2 conversion on the initially PdxZry bulk intermetallic
catalyst after 28 min isothermal reaction at 800 °C amounts to ~90%. This effect is even more
pronounced for the Pd–ZrO2 powder catalyst (panel C), on which quantitative conversion is attained
already after ~20 min at 800 °C.
Figure 5. Temperature-programmed DRM reaction rate profiles obtained on (a) the CVD-prepared
subsurface Zr0–Pd foil precatalyst versus a single-phase ZrO2 film; (b) on the PdxZry bulk-intermetallic
precatalyst; (c) on 73 mg of the supported Pd–ZrO2 powder reference catalyst, corresponding to ~140
cm2 Pd surface area or ~ 2.3 × 1017 surface Pd atoms. Reaction conditions for (a) and (b): 50 mbar CH4,
50 mbar CO2, 977 mbar He; linear temperature ramp (25 °C/min) up to 800 °C, followed by isothermal
reaction for 30 min. Reaction conditions for (c): 15 mbar CH4, 15 mbar CO2, 950 mbar He; linear
temperature ramp (10 °C/min) up to 800 °C, followed by isothermal reaction for 30 min. Prior to DRM,
Figure 5.
Temperature-programmed DRM reaction rate profiles obtained on (
a
) the CVD-prepared
subsurface Zr
0
–Pd foil precatalyst versus a single-phase ZrO
2
film; (
b
) on the Pd
x
Zr
y
bulk-intermetallic
precatalyst; (
c
) on 73 mg of the supported Pd–ZrO
2
powder reference catalyst, corresponding to
~140 cm
2
Pd surface area or ~2.3
×
10
17
surface Pd atoms. Reaction conditions for (
a
) and (
b
): 50 mbar
CH
4
, 50 mbar CO
2
, 977 mbar He; linear temperature ramp (25
◦
C/min) up to 800
◦
C, followed by
isothermal reaction for 30 min. Reaction conditions for (
c
): 15 mbar CH
4
, 15 mbar CO
2
, 950 mbar
He; linear temperature ramp (10
◦
C/min) up to 800
◦
C, followed by isothermal reaction for 30 min.
Prior to DRM, the supported catalyst was cleaned by a cycle of oxidation with O
2
at 400
◦
C and
reduced to the Pd
0
state in 1 bar H
2
at 600
◦
C for one hour. From the molar product formation vs. time
data, the molar rates were obtained by differentiation and normalized to the geometrically estimated
total amount of surface Pd atoms of the respective catalyst to obtain the turnover frequency (TOF)
values. Numerical details of the TOF calculations are provided in the experimental Section 3.1. For the
phase-pure ZrO
2
sample (oxidized ZrO
2
metal foil of same geometrical size, 7.2 cm
2
) this normalization
is purely artificial.
Catalysts 2020,10, 1000 10 of 29
It is important to emphasize that the TOF differences at maximum rate between the initially
bulk-intermetallic Pd
x
Zr
y
precursor and the supported Pd–ZrO
2
reference catalyst are not only due to
different “intrinsic”(i.e., sample-specific)activities, but—to alargepart—also dueto externaldifferences
of the initial CH
4
:CO
2
partial pressures, the slopes of the temperature ramps and the recirculating batch
reactor volumes (Figure 5b: setup described in Section 3.2.2, 50:50 mbar, 25 K/min,
Vreactor =296.0 mL
;
Figure 5c: setup in Section 3.2.3, 15:15 mbar, 10 K/min, V
reactor
=13.8 mL). The initial molar amount of
reactants in Figure 5c was thus
≈
70 times lower than in Figure 5b. Obviously, the reactant conversion
on the supported reference powder catalyst is already beyond 50% at the beginning of the isothermal
period at 800
◦
C. This means that the remaining reactant pressures of only
≈
7 mbar strongly contribute
to the lowered maximum rate relative to the initially bulk-intermetallic Pd
x
Zr
y
catalyst. To partly
compensate for this difference, the corresponding TOF of the CO formation rate on the latter was read
out after 23 min isothermal operation, corresponding to a similar remaining CO
2
partial pressure of
~7 mbar. At this point it amounts to ~8 s
−1
, in contrast to only ~0.035 s
−1
on the Pd–ZrO
2
powder
catalyst. We emphasize that the direct comparison of these TOF values, although now determined
at comparable reactant pressures, still remains misleading for two reasons. Firstly, the inactivity
of ultra-clean Pd proves the pronounced structure sensitivity of the DRM reaction on Pd/ZrO
2
.
Consequently, normalization of the rates to the—unfortunately experimentally unavailable—number
of Pd–ZrO
2
interface sites on each catalyst system would be mandatory. Secondly, the estimation of
the number of Pd surface atom sites both on the bulk- and subsurface-intermetallic Pd
x
Zr
y
precursors
is based on the simplified consideration of the macroscopic geometric surface area (rough estimation
of ~1.2
×
10
16
surface Pd atoms within the reactor on each). This number is directly applicable to
the CVD-prepared subsurface-Zr
0
precatalyst but in case of the in-situ corroded bulk-intermetallic
precursor—although exhibiting the same geometrical size—both the effective Pd metal surface area
of the corroded near-surface region and the number of Pd–ZrO
2
interface sites within this region
are experimentally very difficult to access. During DRM, it nevertheless develops a similar total
number of XPS-detectable Pd atoms at the surface as being present on the foil-based CVD-prepared
catalyst, which was the basis of using the same number of surface Pd atoms for both systems. A reliable
quantitative comparison and therefore a direct numerical estimation of the total amount of Pd atoms
present on top of the corrosion layer is not possible based on XPS, due to different base intensities,
mainly caused by different carbon surface coverages on the post-DRM state, and due to the unknown
depth profile. Nevertheless, quantitative comparison of the atomic ratios of the activated states lead
to a surface composition of 68% Pd and 32% ZrO
2
for the post-DRM CVD-prepared catalyst and
a surface composition of 46% Pd and 54% ZrO
2
for the DRM-corroded bulk intermetallic system.
Thus, the (clearly oversimplified) assumption of an equal number of geometrically accessible surface
Pd sites appears not entirely unjustified. We admit that measuring this number by CO adsorption
would basically be preferable, as there might be some porosity and/or accessible “internal” Pd surface
within the corrosion layer but unfortunately, CO TPD on the post-DRM sample is experimentally
not accessible. Thus, we consider the TOF numbers on the bulk-intermetallic precursor the least
reliable ones.
Finally, the supported catalyst reproduces the salient kinetic features (w.r.t. light-offtemperature,
general activity trace in the T-programmed region, activation energy of around 140 kJ/mol) of the
initial Pd
x
Zr
y
bulk-intermetallic precatalyst quite well, although the TOF level is generally much lower.
Most likely, this can be explained by its quite low dispersion (~0.6%) and Pd dewetting tendency
upon H
2
prereduction (see Section 2.1.2) and thus the inherently low ratio of interfacial vs. surface Pd
atom sites.
Nevertheless, the direct rate comparison between Figure 5a,b is straightforward (identical reaction
conditions, same reactor) and clearly shows the strong promotion of DRM activity on the initially
bulk-intermetallic precatalyst when compared with the CVD-prepared initially subsurface Zr
0
system.
Anticipating the subsequently shown in-situ XPS results, the strong DRM promotion on the bulk Pd
x
Zr
y
precursor is caused by accumulation of reactive, dissolved/carbidic carbon species in oxidatively
Catalysts 2020,10, 1000 11 of 29
segregated Pd nanoparticles under DRM conditions. The absence of this type of reactive C accumulation
on the CVD-prepared model catalyst surface will also be shown.
The CVD preparation of sub-monolayer ZrO
x
islands on Pd(111) and in due course, of our
“inverse” subsurface Zr
0
-intermetallic precatalysts on bulk Pd foil, was originally motivated by a
potentially scalable promotional role of variable Pd–ZrO
2
phase boundary dimensions for the DRM
activity. But as a matter of fact, all “inverse” systems turned out to be (if at all) only slightly more
active than the sum of their individual oxidic and metallic surface components. Thus, also changes of
the initial bimetallic Zr
0
amount between 40% and 90% of a monolayer led only to minor differences in
the observed activities (not shown). In practice, these tiny differences are anyway not scalable with
the dimensions of the “inverse” ZrO
2
-on-Pd phase boundary, which is formed in situ under DRM
conditions on the initial subsurface Zr
0
–Pd foil precatalyst. This holds firstly because of the poor
signal-to-noise ratio of the rate measurements due to low overall activity (Figure 5a) and is secondly
due to the experimentally hardly accessible island density of reactively segregated ZrO
x
from the
subsurface Zr
0
–Pd precursor. From the surface microscopy viewpoint, the exact amount of this special
type of phase boundary sites can hardly be quantified, and—as these do not represent particularly
active interface sites for DRM, anyway—the highly demanding attempt to normalize such tiny rate
differences to an in-situ variable number of these interface sites appears to be of little use.
2.3. In-Situ Characterization under DRM Reaction Conditions
2.3.1. In-Situ Characterization of Precatalyst Activation
Figure 6displays the XPS analysis of the oxidative segregation behavior of Zr
0
species on both
the CVD-prepared initial subsurface Zr
0
–Pd foil catalyst and the initial Pd
x
Zr
y
bulk-intermetallic
precatalyst under close-to-real DRM conditions as a function of the sample temperature. As mentioned
in Section 2.1.2, the contact to the ambient during transfer to the NAP-XPS chamber already caused
corrosive segregation of Zr
0
toward a top layer of Zr
+4
O
x
on the Pd
x
Zr
y
bulk-intermetallic precatalyst,
along with some “adventitious” carbon deposition. This effect was less pronounced on the initial
Zr0–Pd foil subsurface alloy sample, most likely due to its protective Pd0surface termination.
The DRM-induced sequence of oxidatively segregated species on the CVD-prepared initial
subsurface Zr
0
–Pd foil catalyst is somewhat complex—the decrease of the intermetallic Zr
0
component
leads to an intermediate increase of the ultrathin ZrO
x
components around 200
◦
C. The surface-wetting
ultrathin film is subsequently converted to a bulk ZrO
2
species at and above 250
◦
C. Upon reaching
500 ◦C, almost all Zr0is converted to bulk-like ZrO2at the surface.
On the Pd
x
Zr
y
bulk-intermetallic precatalyst, the heating process leads to the loss of the residual
weak Zr
0
intensity in the spectra due to progressively “deeper” oxidation of the sample. As can be
seen in Figure 7, displaying the complementary in-situ XRD information obtained on the initial Pd
x
Zr
y
bulk-intermetallic precatalyst, a treatment at 500
◦
C in DRM atmosphere is not sufficient to induce
XRD-detectable deep bulk corrosion of the intermetallic precursor, since the intermetallic reflections
still remain dominant and the Pd
0
and ZrO
2
reflections below the detection limit. Nevertheless, we note
that the catalysis-relevant near-surface regions probed by NAP-XPS did already approach the fully
segregated Pd
0
–ZrO
2
state around 500
◦
C. The respective Pd 3d spectra are not shown in Figure 6,
as both precatalysts preserve their unaltered metallic Pd state and do not exhibit noticeable changes in
the Pd 3d spectra up to 500
◦
C. The as-prepared intermetallic initial state consists of a phase mix of
Pd
3
Zr and Pd
2
Zr and with coarse crystallites (spotty, that is, not completely continuous rings) and
exhibits no detectable contributions from segregated Pd
0
or ZrO
2
species, which means that they can
be present only within a few layers at the top. First signs of these species appear in XRD above 700
◦
C
and between 700
◦
C and 800
◦
C their contribution to the diffractogram becomes dominant, at the cost
of the parent intermetallic phases, meaning that “deep” corrosion happens around 750
◦
C. As the time
span between the diffractograms is 5 min, corrosion kinetics must be very slow up to ~700
◦
C. At this
point kinetics become fast enough for the Pd
0
and tetragonal (t-)ZrO
2
phases to clearly exceed the
Catalysts 2020,10, 1000 12 of 29
~1% XRD detection limit. The overall sequence confirms the buildup of a corrosion layer consisting of
crystalline Pd
0
(reflections shifted by ~0.2
◦
due to combined C-dissolution and thermal expansion)
and t-ZrO
2
, which grows in thickness with increasing temperature and time and finally extends over
most of the spatial regions accessible to X-ray diffraction. The catalytic data in Figure 5b suggest that
this in-situ corrosion process gives rise to a particularly DRM-active state.
Catalysts 2020, 10, x FOR PEER REVIEW 12 of 31
Figure 6. Zr 3d NAP-XP spectra recorded in situ under close-to-real dry reforming reaction of
methane (DRM) conditions (p(CH4): p(CO2) = 0.15 mbar: 0.15 mbar) between room temperature and
500 °C. The excitation energy was chosen as hν = 580 eV to obtain a photoelectron kinetic energy of
400 eV common to all experiments. Left panel: CVD-prepared initial subsurface Zr0–Pd foil catalyst;
right panel: initial PdxZry bulk-intermetallic precatalyst.
The DRM-induced sequence of oxidatively segregated species on the CVD-prepared initial
subsurface Zr0–Pd foil catalyst is somewhat complex—the decrease of the intermetallic Zr0
Figure 6.
Zr 3d NAP-XP spectra recorded in situ under close-to-real dry reforming reaction of methane
(DRM) conditions (p(CH
4
): p(CO
2
)=0.15 mbar: 0.15 mbar) between room temperature and 500
◦
C.
The excitation energy was chosen as h
ν
=580 eV to obtain a photoelectron kinetic energy of 400 eV
commontoallexperiments. Leftpanel: CVD-preparedinitialsubsurfaceZr
0
–Pdfoil catalyst; rightpanel:
initial PdxZrybulk-intermetallic precatalyst.
Both newly formed phases—Pd
0
and tetragonal (t-)ZrO
2
—appear as completely continuous rings
in the detector image and, therefore, exhibit a nanocrystalline morphology. Both the mean crystallite
Catalysts 2020,10, 1000 13 of 29
size and the lattice constant of Pd
0
were estimated via Rietveld analysis, yielding values of
≈
7.5 nm
and 3.914 Å, respectively, during DRM at 800 ◦C [18].
Figure 7.
Synchrotron-based in-situ X-ray diffraction analysis of the bulk-intermetallic Pd
x
Zr
y
catalyst
during DRM between
≈
20
◦
C and 800
◦
C. A gas flow of 2 mL/min of CH
4
/CO
2
(ratio 1/1) at ambient
pressure with a heating rate of 20 K/min was applied. The colored bars mark the positions of the
respective reference reflections of the most prominent involved (inter)metallic and oxidic phases
and are derived from International Centre for Diffraction Data (ICDD) database and labeled with
their corresponding #ICDD numbers. The diffraction angles of the reference phases are based on
room-temperature data, for example, for metallic Pd 2
θ
=12.6
◦
. The diffractogram at 800
◦
C shows this
reflection at ~12.5
◦
, which is in line with the expected thermal lattice expansion. The same holds for the
t-ZrO
2
reflections. Thus, these reflections can be clearly assigned to the room-temperature reflections
of the referenced phases.
2.3.2. In-Situ NAP-XPS under Transient Reaction Conditions
Subsurface Zr0–Pd Foil Precatalyst
Figure 8displays the isothermal in-situ NAP-XPS spectra on the CVD-prepared Zr
0
–Pd foil
catalyst (where Zr was initially subsurface) at 700
◦
C under realistic DRM conditions. The intensity
Catalysts 2020,10, 1000 14 of 29
trends (based on peak integration) associated with changing gas-phase conditions are depicted in the
bar graphs at the right side. Complete carbon clean-off, including the oxygenate and CH
x
components
above 285 eV [
45
–
47
], is observed upon switching to pure CO
2
. According to the relative intensity
trends, the mostly graphitic carbon present after growth in clean CH
4
appears to be rather located
on top of the ZrO
2
islands (Zr 3d increasing, Pd 3d hardly affected), which were already pre-formed
in situ under DRM conditions around 500
◦
C (cf. Figure 6, left panel). The principal possibility of
graphite deposition from thermally activated CH
4
on ZrO
2
surfaces above ~750
◦
C has been shown
previously [
48
] and is most likely enhanced by the strongly ionized gas environment of the NAP-XPS
experiment. A lower steady-state carbon concentration than that growing in pure CH
4
builds up in
the 1:1 DRM reactant mixture and eventually becomes time-independent. The Pd 3d spectra remain
qualitatively unchanged in the metallic state upon gas-phase variation and neither carbidic nor oxidic
Pd components are present.
Catalysts 2020, 10, x FOR PEER REVIEW 15 of 31
Figure 8 displays the isothermal in-situ NAP-XPS spectra on the CVD-prepared Zr0–Pd foil
catalyst (where Zr was initially subsurface) at 700 °C under realistic DRM conditions. The intensity
trends (based on peak integration) associated with changing gas-phase conditions are depicted in the
bar graphs at the right side. Complete carbon clean-off, including the oxygenate and CHx components
above 285 eV [45–47], is observed upon switching to pure CO2. According to the relative intensity
trends, the mostly graphitic carbon present after growth in clean CH4 appears to be rather located on
top of the ZrO2 islands (Zr 3d increasing, Pd 3d hardly affected), which were already pre-formed in
situ under DRM conditions around 500 °C (cf. Figure 6, left panel). The principal possibility of
graphite deposition from thermally activated CH4 on ZrO2 surfaces above ~750 °C has been shown
previously [48] and is most likely enhanced by the strongly ionized gas environment of the NAP-XPS
experiment. A lower steady-state carbon concentration than that growing in pure CH4 builds up in
the 1:1 DRM reactant mixture and eventually becomes time-independent. The Pd 3d spectra remain
qualitatively unchanged in the metallic state upon gas-phase variation and neither carbidic nor oxidic
Pd components are present.
Figure 8. C 1s, Zr 3d and Pd 3d5/2 AP-XPS data recorded in situ at 700 °C on the CVD-prepared initially
subsurface Zr0–Pd foil catalyst (excitation energies were chosen for 400 eV photoelectron kinetic
energy). Left spectra: 0.3 mbar pure CH4; middle spectra: 0.3 mbar pure CO2; right spectra: 0.15 mbar
CH4 + 0.15 mbar CO2. Right side: bar graphs of integrated peak intensities.
Figure 8.
C 1s, Zr 3d and Pd 3d
5/2
AP-XPS data recorded in situ at 700
◦
C on the CVD-prepared
initially subsurface Zr
0
–Pd foil catalyst (excitation energies were chosen for 400 eV photoelectron
kinetic energy). Left spectra: 0.3 mbar pure CH
4
; middle spectra: 0.3 mbar pure CO
2
; right spectra:
0.15 mbar CH4+0.15 mbar CO2. Right side: bar graphs of integrated peak intensities.
Catalysts 2020,10, 1000 15 of 29
Bulk-Intermetallic PdxZryPrecatalyst
From the combination of the reactivity data (Section 2.2), the XPS and the XRD information
(Section 2.3.1), we conclude that a composite Pd
0
-nanoparticle/t-ZrO
2
active phase is formed in situ
via oxidative decomposition of an increasing fraction of the parent intermetallic Pd
x
Zr
y
under DRM
conditions. This in-situ corrosion process starts in the near-surface regions and finally affects the entire
XRD-detectable bulk region of the parent intermetallic phase.
As can be indirectly deduced from the preceding Figures 7and 8, the main difference
between the initial subsurface and bulk-intermetallic precatalysts is the formation of two distinctly
different Pd
0
–ZrO
2
active states via DRM-induced “corrosive” activation. As one starts from minute
submonolayer amounts of Zr
0
atoms on the CVD-prepared catalyst, its segregated state must be
constituted of 2- to 3-dimensional ZrO
2
islands on top of an otherwise hardly changed, almost infinite
Pd
0
bulk, whereas the intermetallic Pd
x
Zr
y
exhibits extended 3-dimensional growth of ZrO
2
and
Pd and thus contains Pd
0
nanoparticles in intimate contact with 3D t-ZrO
2
domains. The combined
structural information of the XRD-derived Pd nanoparticle size, together with the chemical surface
composition from XPS analysis, strongly suggest the scenario of a “quasi-3D” decay toward a
conglomerate “selective-corrosion” layer on top of the PdZr substrate. In Figure 9, this fundamental
morphological difference manifests itself in the simultaneous occurrence of a carbidic/dissolved carbon
Pd
x
C component in the Pd 3d and C 1s spectra (C
bulk
at a binding energy of 283.0 eV and Pd
carbidic
at
335.6 eV [
49
,
50
], which becomes populated if methane is present in the gas phase. These components
are clearly absent on the CVD-prepared initial subsurface Zr
0
–Pd foil catalyst under otherwise identical
conditions (cf. Figure 8, left side, clean CH
4
). Regarding reactivity, the initially bulk-intermetallic
Pd
x
Zr
y
catalyst is ~30 times more active than its CVD-prepared counterpart (Section 2.2, panels (a) and
(b) in Figure 5), suggesting a major catalytic relevance of the observed carbidic/dissolved C species.
Strong support for their role as the most reactive C intermediates comes from the transient response to
the gas-phase composition. As soon as the gas supply is switched from CH
4
to CO
2
, the C
bulk
signal
disappears immediately, whereas the C
graphite
-component decreases at a much slower rate. Obviously,
C
bulk
is much more reactive than C
graphite
with respect to the carbon clean-offreaction in pure CO
2
.
This fast response (at least on the timescale of
≈
5 min, required for collecting two subsequent sets of
XPS spectra), also holds for the C
bulk
build-up upon re-activating the CH
4
supply. In the 1:1 DRM
reaction mixture, both the C
bulk
and C
graphite
species are reversibly populated and eventually become
time-independent by reaching a steady-state value. The intensity bar graphs displayed on the top
right side of Figure 9summarize the quantification of the relative C
bulk
vs. C
graphite
intensity trends
and highlight the faster and quantitative response of the carbidic carbon species upon switching from
CH
4
to CO
2
and to the 1:1 DRM mixture. The Pd 3d intensity trends are interpreted in terms of
preferential growth of the carbonaceous species on the Pd domains/particles, since the Pd 3d intensity
is clearly suppressed in the presence of CH
4
, both in clean methane and the DRM mixture. As the Zr
3d intensity exhibits little changes (or slightly tends toward the reverse trend), preferential shielding
of Pd by methane-induced carbon deposits is obvious. We emphasize that essentially the opposite
trend, namely preferential shielding of ZrO
2
domains, was observed on the initial subsurface Pd–Zr
0
precatalyst (Figure 8, Pd 3d bar graph).
In due course, the subsurface/dissolved character of the C
bulk
species vs. the preferential
accumulation of the C
graphite
species at the Pd surface was verified by depth profiling via photoelectron
kinetic energy variation.
As shown in Figure 10, the resulting intensity variations of the C
bulk
and C
graphite
intensities
clearly exhibit a relative enhancement of the C
bulk
component with increasing XPS probing depth.
We note that also the C1s region of oxygenated species (CO
x
, CO
x
H) exhibits an apparent intensity
increase at higher photoelectron kinetic energies. It is safe to say that a volume-dissolved species
such as C
bulk
, as compared to graphene/graphite layers at the surface, should exhibit reduced signal
attenuation with increasing probing depth. Interestingly, the oxygenated C 1s intensity appears to
be also less attenuated relative to C
graphite
. As we suggest that the corrosion of the Pd
x
Zr
y
bulk
Catalysts 2020,10, 1000 16 of 29
intermetallic leads to a “conglomerate“ 3D layer of nanosized Pd
0
and t-ZrO
2
domains, the latter may
inherently contain a large amount of “buried“ Pd-ZrO
2
interface sites. An enhanced concentration of
oxygenated species at these sites would result in a “quasi-3D“ depth distribution perpendicular to the
outer (geometrical) surface and thus to a similar response to photon energy variations as for C
bulk
.
As the poor signal-to-noise ratio and the ambiguity of the background subtraction in the oxygenate
C 1s region does not allow for reliable quantitative assignments of intensity trends, this explanation is
rather speculative.
Catalysts 2020, 10, x FOR PEER REVIEW 17 of 31
Figure 9. C 1s, Zr 3d and Pd 3d5/2 AP-XP spectra recorded in situ at 700 °C on the initially bulk-
intermetallic PdxZry precatalyst (excitation energies were chosen for 400 eV photoelectron kinetic
energy). Left spectra: 0.3 mbar pure CH4; middle spectra: 0.3 mbar pure CO2; right spectra: 0.15 mbar
CH4 + 0.15 mbar CO2. Right side: bar graphs of integrated intensities of the peak components.
In due course, the subsurface/dissolved character of the Cbulk species vs. the preferential
accumulation of the Cgraphite species at the Pd surface was verified by depth profiling via photoelectron
kinetic energy variation.
As shown in Figure 10, the resulting intensity variations of the Cbulk and Cgraphite intensities clearly
exhibit a relative enhancement of the Cbulk component with increasing XPS probing depth. We note
that also the C1s region of oxygenated species (COx, COxH) exhibits an apparent intensity increase at
higher photoelectron kinetic energies. It is safe to say that a volume-dissolved species such as Cbulk,
as compared to graphene/graphite layers at the surface, should exhibit reduced signal attenuation
with increasing probing depth. Interestingly, the oxygenated C 1s intensity appears to be also less
attenuated relative to Cgraphite. As we suggest that the corrosion of the PdxZry bulk intermetallic leads
to a “conglomerate“ 3D layer of nanosized Pd0 and t-ZrO2 domains, the latter may inherently contain
a large amount of “buried“ Pd-ZrO2 interface sites. An enhanced concentration of oxygenated species
at these sites would result in a “quasi-3D“ depth distribution perpendicular to the outer (geometrical)
surface and thus to a similar response to photon energy variations as for Cbulk. As the poor signal-to-
noise ratio and the ambiguity of the background subtraction in the oxygenate C 1s region does not
allow for reliable quantitative assignments of intensity trends, this explanation is rather speculative.
Figure 9.
C 1s, Zr 3d and Pd 3d
5/2
AP-XP spectra recorded in situ at 700
◦
C on the initially
bulk-intermetallic Pd
x
Zr
y
precatalyst (excitation energies were chosen for 400 eV photoelectron
kinetic energy). Left spectra: 0.3 mbar pure CH
4
; middle spectra: 0.3 mbar pure CO
2
; right spectra:
0.15 mbar CH
4
+0.15 mbar CO
2
. Right side: bar graphs of integrated intensities of the peak components.
By means of in-situ XRD analysis during O2re-oxidation of the initial bulk-intermetallic PdxZry
precatalyst directly after DRM, the verification of the “solid solution” character of C
bulk
within the Pd
particles was attempted. Figure 11 shows the changes of the respective diffraction data over time on
the post-DRM sample in a flow of 2 mL/min clean oxygen at ambient pressure and 600 ◦C.
Both a change of the Pd lattice parameter and of the particle size (determined via Rietveld
analysis) with time can be deduced. Between 25 min and 35 min oxygen exposure, that is, at the onset
of partial Pd
0
oxidation to PdO, a lowering of the lattice parameter by
≈
0.016 Å (from 3.914 Å to
3.898 Å, that is, ~0.4%) takes place and the mean crystallite size increases from
≈
7.5 nm to
≈
10.3 nm.
We conclude that the partial oxidation of Pd
0
toward PdO is accompanied by simultaneous carbon
Catalysts 2020,10, 1000 17 of 29
depletion of the Pd bulk, thus leading to a lowering of the lattice parameter of the metallic entities
due to C
bulk
-depletion. This result suggests that the DRM-induced carbidic C
bulk
resides not only
in the XPS-detectable near-surface regions but throughout the bulk of the Pd
0
particles. The 0.4%
change of the lattice parameter corresponds to a lowering of the post-DRM C
bulk
concentration from
≈
2% to zero, as deduced from the relation between the solid-solution C-concentration and the lattice
parameter [49,51].
Catalysts 2020, 10, x FOR PEER REVIEW 18 of 31
Figure 10. Left side: in-situ depth profiling of the C 1s signal under isothermal DRM conditions at
constant reactant pressures on the initially bulk-intermetallic PdxZry precatalyst by variation of the
photon energy. T = 700 °C, p(CH4): p(CO2) = 0.15 mbar: 0.15 mbar. Right side: bar graphs of the
integrated intensities of the peak components.
By means of in-situ XRD analysis during O2 re-oxidation of the initial bulk-intermetallic PdxZry
precatalyst directly after DRM, the verification of the “solid solution” character of Cbulk within the Pd
particles was attempted. Figure 11 shows the changes of the respective diffraction data over time on
the post-DRM sample in a flow of 2 mL/min clean oxygen at ambient pressure and 600 °C.
Figure 10.
Left side: in-situ depth profiling of the C 1s signal under isothermal DRM conditions at
constant reactant pressures on the initially bulk-intermetallic Pd
x
Zr
y
precatalyst by variation of the
photon energy. T =700
◦
C, p(CH
4
): p(CO
2
)=0.15 mbar: 0.15 mbar. Right side: bar graphs of the
integrated intensities of the peak components.
Finally, after isothermal variation of the reactant pressures, the reversibility of the steady-state
carbon chemistry as a function of the reaction temperature was tested under fixed 1:1 DRM gas-phase
conditions. Figure 12 displays the relative NAP-XPS intensity changes of the steady-state C
bulk
and
C
graphite
components as a function of temperature under in-situ DRM conditions. Between 600 and
≈
700
◦
C, both components show increasing intensities, beyond
≈
700
◦
C they both decrease, whereby the
relative response of C
bulk
appears to be generally stronger. The simultaneous presence of C
graphite
and
Catalysts 2020,10, 1000 18 of 29
C
bulk
in the so called “coking window” of DRM [
52
] represents a novel aspect for the mechanistic
interpretation of this temperature region and the enhancement of reversible de-coking processes at
higher temperatures.
Catalysts 2020, 10, x FOR PEER REVIEW 19 of 31
Figure 11. Time-resolved in-situ X-ray diffractograms (hν = 24 keV) of the Pd/PdO reflex region of the
initially bulk-intermetallic PdxZry catalyst during reoxidation in 2 mL/min oxygen at 600 °C after
DRM at 800 °C, taken in order to check the reversibility of the DRM-induced structural changes and
of the carbon uptake.
Both a change of the Pd lattice parameter and of the particle size (determined via Rietveld
analysis) with time can be deduced. Between 25 min and 35 min oxygen exposure, that is, at the onset
of partial Pd0 oxidation to PdO, a lowering of the lattice parameter by ≈0.016 Å (from 3.914 Å to 3.898
Å, that is, ~0.4%) takes place and the mean crystallite size increases from ≈7.5 nm to ≈10.3 nm. We
conclude that the partial oxidation of Pd0 toward PdO is accompanied by simultaneous carbon
depletion of the Pd bulk, thus leading to a lowering of the lattice parameter of the metallic entities
due to Cbulk-depletion. This result suggests that the DRM-induced carbidic Cbulk resides not only in
the XPS-detectable near-surface regions but throughout the bulk of the Pd0 particles. The 0.4% change
of the lattice parameter corresponds to a lowering of the post-DRM Cbulk concentration from ≈2% to
zero, as deduced from the relation between the solid-solution C-concentration and the lattice
parameter [49,51].
Finally, after isothermal variation of the reactant pressures, the reversibility of the steady-state
carbon chemistry as a function of the reaction temperature was tested under fixed 1:1 DRM gas-phase
conditions. Figure 12 displays the relative NAP-XPS intensity changes of the steady-state Cbulk and
Cgraphite components as a function of temperature under in-situ DRM conditions. Between 600 and
≈700 °C, both components show increasing intensities, beyond ≈700 °C they both decrease, whereby
the relative response of Cbulk appears to be generally stronger. The simultaneous presence of Cgraphite
and Cbulk in the so called “coking window” of DRM [52] represents a novel aspect for the mechanistic
interpretation of this temperature region and the enhancement of reversible de-coking processes at
higher temperatures.
Figure 11.
Time-resolved in-situ X-ray diffractograms (h
ν
=24 keV) of the Pd/PdO reflex region of the
initially bulk-intermetallic Pd
x
Zr
y
catalyst during reoxidation in 2 mL/min oxygen at 600
◦
C after DRM
at 800
◦
C, taken in order to check the reversibility of the DRM-induced structural changes and of the
carbon uptake.
Up to
≈
700
◦
C, the increasing carbon amount may be caused by the increase of C solubility and the
simultaneously accelerated CH
4
decomposition rate with increasing temperature, meaning that the Pd
particles exhibit enhanced (net) carbon dissolution. Beyond
≈
700
◦
C, thermodynamics generally favor
carbon depletion. The equilibrium of the methane decomposition reaction CH
4
2H
2
+C
graphite
is
shifted toward H
2
formation and carbon deposition with increasing temperature due to the respective
positive entropy change but to a lower extent than that of the equally exoentropic but much more
endothermic carbon-converting inverse Boudouard reaction CO
2
+C
graphite
2CO. The approximate
temperature below which C
graphite
deposition exceeds carbon oxidation by CO
2
is around 725
◦
C,
as reported in Reference [26].
A qualitative kinetic scenario accounting for increasingly fast C
bulk
depletion toward CO via
the metal-oxide phase boundary at higher temperatures is proposed in the following to explain
this trend, as well as to explain the differences between the “inverse” (foil-supported) and “real”
(nanoparticle-based) model catalysts with respect to carbon deposition and DRM activity. Figure 13
provides a simplified schematic representation of the three types of employed model catalysts and
their distinct chemical and structural development under DRM conditions.
Catalysts 2020,10, 1000 19 of 29
Catalysts 2020, 10, x FOR PEER REVIEW 20 of 31
Figure 12. Left side: response of the steady-state Cgraphite and Cbulk (PdxC) intensities to temperature
changes between 600 and 740 °C on the initially bulk-intermetallic PdxZry catalyst. Constant DRM
pressure conditions: 0.15 mbar CH4 + 0.15 mbar CO2. Excitation energy hν = 685 eV (photoelectron
kinetic energy 400 eV). Right side: bar graph of Cgraphite and Cbulk intensity trends.
Up to ≈700 °C, the increasing carbon amount may be caused by the increase of C solubility and
the simultaneously accelerated CH4 decomposition rate with increasing temperature, meaning that
the Pd particles exhibit enhanced (net) carbon dissolution. Beyond ≈700 °C, thermodynamics
generally favor carbon depletion. The equilibrium of the methane decomposition reaction CH4 ⇄ 2H2
+ Cgraphite is shifted toward H2 formation and carbon deposition with increasing temperature due to
the respective positive entropy change but to a lower extent than that of the equally exoentropic but
much more endothermic carbon-converting inverse Boudouard reaction CO2 + Cgraphite ⇄ 2CO. The
approximate temperature below which Cgraphite deposition exceeds carbon oxidation by CO2 is around
725 °C, as reported in Reference [26].
A qualitative kinetic scenario accounting for increasingly fast Cbulk depletion toward CO via the
metal-oxide phase boundary at higher temperatures is proposed in the following to explain this
trend, as well as to explain the differences between the “inverse” (foil-supported) and “real”
(nanoparticle-based) model catalysts with respect to carbon deposition and DRM activity. Figure 13
provides a simplified schematic representation of the three types of employed model catalysts and
their distinct chemical and structural development under DRM conditions.
On the CVD-prepared inverse model catalyst (top panel), finely dispersed ZrO2 islands are
formed on top of a quasi-infinite Pd bulk-metal substrate. Although many metal-oxide interface sites
are generated, the C supersaturation of Pd next to the latter remains low, since the C atoms can simply
diffuse away. As shown in Figure 5, Section 2.2., the DRM activity is effectively not boosted by the
Pd0–ZrO2 interface sites forming in situ under DRM conditions. In contrast, on the initially bulk-
intermetallic PdxZry precatalyst (middle panel) Cbulk-loaded and partially graphite-covered crystalline
Pd0 in close contact to t-ZrO2 is formed. Both newly formed phases—Pd and t-ZrO2—form a
nanocrystalline conglomerate on top of the polycrystalline PdxZry substrate. As both a high number
of active interface sites and a sufficient amount of dissolved carbon are available in close vicinity, a
Figure 12.
Left side: response of the steady-state C
graphite
and C
bulk
(Pd
x
C) intensities to temperature
changes between 600 and 740
◦
C on the initially bulk-intermetallic Pd
x
Zr
y
catalyst. Constant DRM
pressure conditions: 0.15 mbar CH
4
+0.15 mbar CO
2
. Excitation energy h
ν
=685 eV (photoelectron
kinetic energy 400 eV). Right side: bar graph of Cgraphite and Cbulk intensity trends.
On the CVD-prepared inverse model catalyst (top panel), finely dispersed ZrO
2
islands are
formed on top of a quasi-infinite Pd bulk-metal substrate. Although many metal-oxide interface
sites are generated, the C supersaturation of Pd next to the latter remains low, since the C atoms can
simply diffuse away. As shown in Figure 5, Section 2.2, the DRM activity is effectively not boosted
by the Pd
0
–ZrO
2
interface sites forming in situ under DRM conditions. In contrast, on the initially
bulk-intermetallic Pd
x
Zr
y
precatalyst (middle panel) C
bulk
-loaded and partially graphite-covered
crystalline Pd
0
in close contact to t-ZrO
2
is formed. Both newly formed phases—Pd and t-ZrO
2
—form a
nanocrystalline conglomerate on top of the polycrystalline Pd
x
Zr
y
substrate. As both a high number of
active interface sites and a sufficient amount of dissolved carbon are available in close vicinity, a highly
active catalyst evolves. The schematic representation of the reductive activation of the supported
PdO/t-ZrO
2
powder catalyst under DRM conditions toward Pd
0
/t-ZrO
2
is depicted in the lowest panel.
Based on the fact that the supported catalyst exhibits qualitatively the same catalytic profile as the
initially bulk-intermetallic Pd
x
Zr
y
catalyst (Figure 5, Section 2.2), its DRM-active state is suggested
to feature the same carbon–CO
2
conversion mechanism but at a strongly reduced number of active
interfacial sites close to relatively large Pd particles.
Catalysts 2020,10, 1000 20 of 29
Catalysts 2020, 10, x FOR PEER REVIEW 21 of 31
highly active catalyst evolves. The schematic representation of the reductive activation of the
supported PdO/t-ZrO2 powder catalyst under DRM conditions toward Pd0/t-ZrO2 is depicted in the
lowest panel. Based on the fact that the supported catalyst exhibits qualitatively the same catalytic
profile as the initially bulk-intermetallic PdxZry catalyst (Figure 5, Section 2.2), its DRM-active state is
suggested to feature the same carbon–CO2 conversion mechanism but at a strongly reduced number
of active interfacial sites close to relatively large Pd particles.
Figure 13. Schematic representations of the Zr0 subsurface alloy vs. PdxZry bulk intermetallic and vs.
supported PdO/ZrO2 precatalysts and their structural and chemical transformations under realistic
Figure 13.
Schematic representations of the Zr
0
subsurface alloy vs. Pd
x
Zr
y
bulk intermetallic and vs.
supported PdO/ZrO
2
precatalysts and their structural and chemical transformations under realistic
reductive/DRM conditions. The representation of the CVD-prepared “inverse” precatalyst (top panel)
is based on a Pd(100) fcc lattice plane containing Zr
0
atoms in the subsurface region. The simplified
schematic alloy and ZrO
2
lattices do not reflect the actual crystal lattice structures and stoichiometries.
As a general conclusion, carbon conversion during DRM is synergistically enhanced by the
combination of an extended metal–oxide phase boundary with C
bulk
-loaded nanoparticulate Pd
0
.
The XRD results of Figure 7in Section 2.3.1 show that the boundary forms in situ by oxidative t-ZrO
2
segregation from Pd
x
Zr
y
in the DRM gas phase and that this process leads to nanodispersed Pd
0
.
The related NAP-C1s core level spectra on the initial bulk Pd
x
Zr
y
precatalyst (shown in Figure 9)
provide clear evidence for considerable C
bulk
concentrations within the Pd
0
nanoparticles but only in
the presence of methane. This means that carbon needs to be permanently supplied to the Pd
0
surface
Catalysts 2020,10, 1000 21 of 29
via dissociative CH
4
activation, in order to populate the carbidic C
bulk
species via C antisegregation.
As soon as the methane supply is switched off, the C
bulk
signal disappears immediately, whereas the
C
graphite
component decreases at a much lower rate. Obviously, the atomically distributed C
bulk
species is much more reactive than Cgraphite with respect to the carbon clean-offreaction in pure CO2.
The CVD-prepared initial subsurface Zr
0
–Pd alloy does not allow for the near-surface accumulation
of carbidic/interstitial C
bulk
under DRM conditions (as shown in Figure 8), or this process would at
least take a much longer time than that of our DRM experiments. Clearly, the dimensions of the
respective Pd
0
bulk play a fundamental role for the local accumulation of the reactive C
bulk
intermediate.
The reduced Pd
0
bulk dimensions of the nanoparticles from local Pd
x
Zr
y
corrosion obviously facilitate
the near-surface and bulk accumulation of interstitially dissolved carbon. As a logical consequence,
also resegregation of C
bulk
atoms to the surface from sufficiently supersaturated near-surface regions
will be enhanced after a much shorter period of time. The effects of this re-segregation are twofold:
The (unwanted) nucleation and growth of C
graphite
domains will be facilitated [
53
], but—at the same
time—an enhanced rate of C
bulk
and/or C
ads
diffusion toward active interfacial sites at the Pd
0
–ZrO
2
phase boundary will result. Thus, the much higher reaction rates on the bulk intermetallic Pd
x
Zr
y
precursor effectively result from the combination of an extended phase boundary with the reduced Pd
0
metal dimensions. As can be deduced from Figure 13, the carbon clean-offrate at the phase boundary is
enhanced above 700
◦
C, resulting in a lower overall steady-state C1s signal but in particular lower C
bulk
intensity. This is likely due to a changed balance of graphitic C formation, redissolution and clean-off
rates, as sketched for the Pd
0
–ZrO
2
nanocomposite model systems in Figure 11. A possible mechanistic
explanation for the “coking window” [
26
,
52
,
54
,
55
] of DRM at lower/intermediate temperatures is
typically based on the expected shift of the Boudouard equilibrium toward carbon. Assuming a steeper
increase with temperature (i.e., a higher activation barrier) of the C clean-offrate at the phase boundary
in comparison to the net rate of partially reversible graphene/graphite accumulation, the Boudouard
process C +CO
2
2
·
CO at the phase boundary can be assumed to overtake the net C
graphite
deposition
rate at sufficiently high temperatures.
3. Materials and Methods
3.1. Model Catalyst Preparation
The preparation of the bulk-intermetallic Pd
x
Zr
y
sample was realized by resistive heating of
a stack of alternating small sheets of pure, clean Pd and Zr metal under high vacuum conditions
(
1×10−7mbar
) at a nominal ratio =2:1 in a Ta crucible [
35
]. When the sample was heated slightly
above the melting point of Pd (1555
◦
C), a spontaneous exothermic reaction between Pd and Zr took
place, leading to an intermetallic Pd–Zr melt. The heating was turned offimmediately and the melt
recrystallized to form the Pd–Zr bulk phase mixture characterized in Reference [35].
As reference catalyst and substrate for PdZr subsurface alloy formation, ultra-clean Pd foil
(Goodfellow, purity 99,95%, 0.1 mm thick, size 2 cm
×
1.8 cm, was used. Surface
preparation/cleaning
involved alternating cycles of Ar sputtering on both sides followed by thermal annealing
(
6.0 ×10−5mbar
Ar, 2 keV, 1
µ
A sample current; T
anneal
=700
◦
C), until XPS and AES spectra
without evidence for impurity traces were obtained. The resulting cleaned Pd surface area thus
amounted to 2
×
2 cm
×
1.8 cm =7.2 cm
2
. Based on the Pd surface atom density of 1.66
×
10
15
cm
−2
,
this corresponds to a total number of 1.2
×
10
16
surface Pd atoms, on which all TOF estimations for the
inverse catalysts are based. The catalytically measured molar turnover rates within an UHV-compatible
recirculating batch reactor (cf. Section 3.2.2) were normalized to this number. For the Pd/Zr
0
subsurface
alloy preparation, Zirconium (IV) tert-butoxide (Zr(O-t-C
4
H
9
)
4
(ZTB, Sigma Aldrich, purity: 99.999 %)
was used as the CVD precursor molecule, which was dosed via a leak valve onto the Pd foil substrate
in the ultrahigh vacuum chamber of Setup 1 (described below). After an exposure at 400
◦
C and a ZTB
pressure of 5.0
×
10
−6
mbar for 200 s, corresponding to 1.0
×
10
−3
mbar*s, an adsorbate coverage of
≈
0.5 ML was calculated using the post-deposition XPS data. The resulting ZrO
x
state on top of Pd
0
Catalysts 2020,10, 1000 22 of 29
was then thermally annealed at
≈
430
◦
C under excellent UHV conditions. Only if the base pressure
of the system was kept below 5
×
10
−10
mbar during thermal annealing, 100% conversion to the
Zr
0
subsurface alloy state was achieved, which represents a strong improvement of the preparation
routine relative to the partially oxidized Pd foil precatalyst state described in Reference [
35
], which was
obtained by thermal annealing at a higher base pressure (
≈
5
×
10
−9
mbar). As has been mentioned
in Section 2.1.1, the 100% subsurface alloy state was proven via the combination of low-energy ion
scattering, which showed no Zr signal after annealing and XPS, which proved the exclusively bimetallic
state of Zr0at a binding energy of 179.6 eV.
The preparation of the supported Pd
0
/ZrO
2
reference catalyst involved aqueous impregnation
of a ZrO
2
suspension (Alfa Aesar, 20% in H
2
O) with Pd(NO
3
)
2·
2H
2
O (Sigma Aldrich) to obtain a
final 10% (mass) loading of the catalyst after reduction to Pd
0
. Thereafter, calcination for 5 h at 800
◦
C
in air (fast heating and cooling) was performed. After this step, the BET surface area amounted to
15 m
2
/g. After subsequent prereduction in H
2
and quantitative thermal desorption of the latter, a Pd
metal surface area of ~0.2 m
2
per g catalyst was calculated on the basis of a 0.5 ML CO saturation
coverage, using a combination of volumetric CO adsorption and quantitative thermal desorption
spectrometry [
56
]. This corresponds to a rather low Pd dispersion of ~0.6%. Finally, catalytic testing
was started from the H
2
-prereduced state in a recirculating batch reactor setup using the reaction
parameters described in Section 3.2.3. The surface atom density of Pd(111) of 1.66
×
10
15
atoms per
cm
2
provides the basis of all turnover frequency (TOF) estimations. The CO-TPD-based calculations
mentioned above allow to extract the number of Pd surface atoms of the supported catalyst—73 mg
powder catalyst contain ~140 cm
2
Pd surface area, corresponding to ~2.3
×
10
17
Pd surface atoms.
The catalytically measured molar turnover rates within the reactor were normalized to this number.
3.2. Experimental Setups for in- and ex-Situ Characterization and Catalytic Testing of Model Catalysts
For catalytic testing, ex-situ structural and electronic characterization and complementary in-situ
structural vs. electronic characterization under close-to-real DRM conditions, six different experimental
setups, two of them being synchrotron end stations, were used:
3.2.1. STM/XPS/LEIS/LEED Setup for Subsurface Alloy Characterization (TU Vienna)
This UHV system consists of a preparation chamber with a base pressure below 10−10 mbar and
an analysis chamber for scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy
(XPS), low-energy electron diffraction (LEED) and low-energy ion scattering (LEIS) measurements,
with a base pressure of ~7
×
10
−11
mbar. For all measurements with this setup, a Pd(111) single
crystal (MaTecK) was used and prepared by cycles of 10 min sputtering (2 keV Ar
+
ions and a
sputter-current density of approx. 2
µ
A/cm
2
) and 10 min annealing (T
≈
873 K). To generate the XP
spectra, a non-monochromatized Mg K
α
source (h
ν
=1253.6 eV, 225 W) and a SPECS PHOIBOS
100 analyzer with a pass energy of 16 eV were utilized. The photoelectrons were collected at an angle
of 15
◦
with respect to the surface normal. Temperatures were measured with a K-type thermocouple
spot-welded near the sample support plate. STM measurements were conducted using an Omicron
µ
-STM with electrochemically etched W tips in constant current mode. Sample heating was performed
in the preparation chamber.
3.2.2. Combined XPS/High Pressure Batch Reactor Setup (University of Innsbruck)
The UHV system with an attached all-quartz recirculating batch reactor, described in detail
in Reference [
57
], was used for the CVD preparation of the subsurface Zr
0
/Pd-foil samples
explained in Section 3.1. Beyond its use as CVD coating chamber, it is designed for quantitative
catalytic/kinetic studies up to 1 bar on poly- or single-crystalline bulk samples, detecting products
by online mass-spectroscopy (MS) analysis (HP GC-MS System G1800A) via a capillary leak
and/or by conventional GC-MS analysis via column injection. MS signals of CH
4
, CO, and CO
2
(
m/z=15 +16, 28, and 44
) were externally calibrated and corrected for fragmentation. Hydrogen
Catalysts 2020,10, 1000 23 of 29
can be quantified using an additional detection line to a differentially pumped Balzers QMA 125
quadrupole mass spectrometer. Ex-situ surface analysis was performed using a XPS/AES/LEIS
spectrometer (Thermo Electron Alpha 110) and a twin Mg/Al-anode X-ray gun (XR 50, SPECS). All
DRM reactions were conducted with initial partial pressures of CH
4
:CO
2
=50:50 mbar. The batch
reaction cell was backfilled with pure He to a total pressure of 970 mbar in order to achieve efficient gas
intermixing via recirculation and fast heat transfer to the sample via sufficient thermal conductivity. For
DRM catalysis, the reactor was heated at a constant linear rate of 25 K/min to the final temperature of
800
◦
C and then kept isothermally at this temperature for
≈
30 min. From the product partial-pressure
vs. time plots, the reaction rates were obtained by differentiation. The partial-pressure changes per
minute [mbar/min] were then converted to molar rates and finally to turnover frequency values (TOF)
on the basis of the total Pd surface atom number of the respective (model) catalyst. A mean value of
the Pd metal surface atom density of 1.66 ×1019 atoms/m2was assumed.
3.2.3. Recirculation Batch Reactor for Supported Powder Catalysts (University of Innsbruck)
Catalytic testing of the powder catalysts was performed in a recirculating batch reactor connected
to a quadrupole mass spectrometer (QMS) arranged in cross-beam geometry, equipped with a
secondary electron multiplier (Balzers QMG 311). This setup is specialized for the measurement
of small sample amounts (approximately 100 mg) and conversions (reactor volume =13.8 mL).
The reactor and the sample holder are made of quartz glass and can be heated up to 1100
◦
C using
a Linn FRV-25/150/1100 high-temperature furnace. The temperature is monitored directly at the
sample position by means of a quartz-capillary-shielded K-type thermocouple (NiCr-Ni). For the DRM
measurements, a CH4:CO2=1:1 mbar mixture was used. One DRM cycle consists of three steps—(1)
pre-oxidation at 400
◦
C in 1 bar pure O
2
for 1 h, (2) pre-reduction at 600
◦
C in 1 bar pure H
2
for 1 h
and (3) DRM reaction. The latter is quantified on the basis of the addition of Ar to the initial DRM
mixture, whereby the Ar signal is used for correction of the intrinsic (reaction-independent) mass
spec signal changes due to the thermal expansion and the gas withdrawal through a capillary leak
to the QMS. In analogy to Section 3.2.2, the total pressure in the reactor is increased to 1 bar with He
for improvement of the recirculation efficiency and the thermal conductivity. Starting from 100
◦
C,
a heating ramp with 10
◦
C min
−1
is applied and the gas-phase composition is continuously monitored
by QMS. The signals of H
2
, CH
4
, CO
,
and CO
2
(m/z=2, 15, 28, and 44) were externally calibrated and
corrected for fragmentation.
3.2.4. In-Situ XRD at the Advanced Light Source (ALS) at LNBL
The bulk structural changes of the melt-prepared intermetallic PdZr sample were investigated in
situ during heating under DRM conditions by synchrotron-based XRD [
58
,
59
] at the Beamline 12.2.2
(Advanced Light Source ALS, Lawrence Berkeley National Laboratories-LBNL, California). X-ray
diffraction was performed with a monochromatic beam (30
µ
m spot size, 0.5172 Å, MAR345 detector,
300 s total acquisition time) in transmission mode with a beam energy of 24 keV, just below the Pd
K edge at 24.3503 keV, in order to achieve sufficient pattern intensity. The Fit2d software [
60
] was
used for calibration and integration. Detector-sample distance and wavelength were determined
using the NIST standard reference material 660b, LaB
6
. The sample was powder-ground before being
transferred to a 700
µ
m quartz capillary. The desired gas atmosphere was achieved by injection of a
1:1 CH
4
:CO
2
mixture (flow rate: 1 mL/min each, ambient pressure) and a heating rate of 20 K/min to
800
◦
C was applied for DRM testing. All references plotted in the XRD Figures are derived from the
ICDD database and are labeled with their corresponding #ICDD numbers.
3.2.5. In-Situ NAP-XPS at ISISS End-Station at HZB/BESSYII
The vacuum chamber operated at beamline ISISS-PGM at the BESSY II synchrotron/Helmholtz
Center for Materials and Energy Berlin (HZB) allowed us to perform NAP-XPS characterization
of the bulk- and surface-intermetallic PdZr model catalysts up to 1 mbar total DRM reactant
Catalysts 2020,10, 1000 24 of 29
pressures [
61
]. The subsurface Zr
0
/Pd-foil and bulk Pd
x
Zr
y
precatalyst samples were positioned
inside the high-pressure analysis chamber
≈
1 mm away from a 1 mm aperture, which is the entrance
to the differentially-pumped electrostatic lens system separating gas molecules from photoelectrons
focused toward the SPECS hemispherical analyzer. Binding energies were referenced to the Fermi edge
recorded after each scan. The temperature was measured by a K-type Ni/NiCr thermocouple directly
spot-welded to the sample and temperature-programmed heating was done by an IR laser from the
rear. The subsurface alloy sample was prepared in setup 3.2.2. as described above and then transferred
to BESSY II under dry inert Ar gas conditions provided inside a transportable Schlenk sample flask.
For final transfer to the NAP-XPS chamber, it was briefly exposed to ambient pressure conditions at
≈
293 K. Nevertheless, only minor signs of oxidative Zr segregation were visible thereafter, even at
low photoelectron kinetic energies (i.e., at high surface sensitivity). The bulk intermetallic samples
were also transferred to BESSY II in inert gas after the above-described melt preparation but due to
the coexistence of Zr
0
and Pd
0
metal in the surface, already the initial (pre-DRM) sample showed a
considerable degree of oxidative segregation toward ZrO
2
and Pd
0
after the contact to the ambient
(cf. Figure 6, Section 2.3.1).
Photon energies were chosen to result in kinetic energies of 400 eV (unless otherwise stated in
the Figure or text) of the ejected photoelectrons for the main peaks of all spectral regions recorded,
in order to extract information from a constant information depth and to yield the same attenuation of
the photoelectrons through the gas phase. Photoelectrons were collected in the direction normal to
the surface at a constant pass energy of 10 eV. Photoemission peak intensities are corrected for the
respective photon flux at a given photon energy. Since the BESSY II synchrotron operates in top-up
mode (constant ring current), no additional correction for the ring current was required. Since all
photoemission peaks were collected at the same kinetic energy of the photoelectrons, the overall
attenuation effects were the same for all-core levels and, thus, cancel out in quantification routines.
3.2.6. High-Resolution Transmission Electron Microscopy
Electron microscopic characterization and the in-situ reduction of the supported PdO/ZrO
2
powder catalyst from the calcined to the metallic state were performed using a double aberration
corrected JEOL Grand ARM transmission electron microscope. The instrument was operated at 300 kV.
In-situ reduction was performed using the Climate System of DensSolutions.
3.3. Details of XPS Analysis
All XPS data (ex- and in-situ) were analyzed using the CasaXPS software program, version 2.3.16
Pre-rel 1.4 (Casa Software Ltd., Teignmouth, United Kingdom) [
62
]. For quantification and peak fitting,
aShirleybackgroundwasappliedtoallspectraandtheassociatedScofieldrelativesensitivityfactors[
63
]
were used for quantification. All binding energies were referenced to the Fermi edge. For each spectral
region recorded at the ISISS station, the Fermi edge was recorded after the monochromator moved
to its new position, that is, whenever the incident photon energy was changed. Peak fitting of the
Pd 3d
5/2
, O 1s and C 1s peaks was conducted using a weighted sum of Gaussian and Lorentzian peak
shapes with a GL(30) contribution (corresponding to 30% Gaussian/Lorentzian character). Asymmetric
parameters were used for Pd 3d
5/2
fitting of Pd
0
(GL(30) with exponential tailing T(1.3)) and Pd
x
C
(GL(30), exponential tailing T(1.5)) as well as for C 1s fitting of graphite (GL(30)T(1.1)) and Pd
x
C
(GL(30)T(1.3)). Regarding the BE component assignment, all Pd 3d5/2spectra show a component at a
BE of 333.5 eV (labelled as O 1s 2nd harm.), which is caused by the O 1s signal excited by the second
diffraction order of the synchrotron radiation at the plane grating monochromator. Deconvolution
of all Pd 3d
5/2
spectra involved the metallic Pd component at a BE of 335.0 eV and the carbidic Pd
x
C
component at a BE of 335.6 eV [49,50].
Different C1s components are considered in all C 1s spectra, namely carbonates and
carbon-oxygenates above 287 eV and C
x
H
y
species at BEs of
≈
285.4 eV [
45
–
47
,
64
]. The component
at a BE of 283.0 eV is assigned to interstitial/carbidic C in Pd
x
C [
65
]. The O 1s components of the
Catalysts 2020,10, 1000 25 of 29
CVD-prepared Zr
x
O
y
layers were fitted with GL(30) parameters and assigned to binding energies taken
from References [
38
–
40
] to distinguish between ultrathin zirconia (
≈
530 eV) vs. bulk t-ZrO
2
(
≈
531 eV)
and are in good agreement with the fitted values of the corresponding Zr 3d region (cf. Figure 1). The fit
of the superimposed Pd 3p component was performed using the clean Pd surface as a reference and
GL(70)T(1.8) parameters and the plasmonic component according to reference [
66
]. All components
in the Zr 3d region were fitted with GL(30) peak shapes, only the metallic Zr
0
component was fitted
with LF(1, 1.8, 30, 90) parameters (asymmetric peak shape). Electron attenuation lengths were taken
from the NIST database SR 82 [
67
]. The orbital asymmetric parameters were taken from the ELETTRA
online database [68].
4. Conclusions
The near-surface regions of a bulk Pd
x
Zr
y
precatalyst are oxidatively decomposed under realistic
DRM conditions and the resulting Pd
0
nanoparticles provide an appropriate carbon loading at the
resulting Pd
0
/t-ZrO
2
interface at
≈
700
◦
C. In due course, this creates locally optimized conditions
for bifunctional catalyst operation—Pd
0
regions favor CH
4
activation and fast supply of reactive
C-atoms toward the phase boundary, whereas redox-active ZrO
x
sites [
15
,
28
] assist in CO
2
activation
and the transfer of CO
2
-derived oxygen to the latter, thus providing optimum conditions for high
CO activity. The CVD-prepared sub-surface Zr
0
–Pd alloy precatalyst does not become a catalyst
with these outstanding catalytic properties, mainly due to the absence of reactive carbidic/dissolved
carbon at the in-situ formed ZrO
2
–Pd
0
phase-boundary, as sufficient carbon supersaturation of an
infinite Pd bulk material is much harder to achieve under otherwise identical reaction conditions.
Consequently, reduced Pd nanoparticle dimensions facilitate and accelerate the accumulation of
reactive carbidic/dissolved carbon in the near-surface regions and, most importantly, at the active
ZrO2–Pd0phase boundary, due to kinetically enhanced carbon resegregation.
As a more general remark, empirical approaches to suppress coking of the less costly Ni catalysts
by specifically active supports such as CeZrO
x
[
7
,
69
,
70
] and La
2
O
2
CO
3
/La
2
O
3
[
32
] seem to make
use of the “reactive phase boundary” principle. As both Ni and Pd are capable of (re)dissolving
carbonaceous species, we suggest an analogous mechanism for the most favorable NiPd/ZrO
2
-based
catalysts [
19
]—faster C-depletion of the metallic component via an accelerated phase-boundary reaction
can directly lower the C concentration of the metal particles and thus initially disfavor nucleation
and growth of graphite-type C-species but also enhance the relative amount of redissolution of the
latter in the metal as C
bulk
under stationary reaction conditions. This mechanistic scenario suggests a
knowledge-based approach toward directional promotion of microkinetic steps leading to the required
combination of enhanced activity and improved coking control.
More specifically, our results provide strong additional support and/or rationalization of
the cooperative action of these already individually known general principles on our specific
bulk-intermetallic model system, which adds—at least in our view—an important aspect for
knowledge-based DRM catalyst design. We highlight that a fruitful combination of these principles
can be obtained with intermetallic precursors. Thus, for improving the DRM performance one needs
to simultaneously optimize (1) the phase boundary dimensions, (2) the CO
2
activation properties of the
support, (3) the (bi)metal particle size, (4) suppress nucleation and growth kinetics of graphitic C species,
(5) optimize the surface- and bulk-diffusion-controlled abundance and reactivity of interfacial C
ads
species and (6) optimize the CH
4
adsorption kinetics (reactive sticking probabilities) at the (bi)metallic
surface. As shown in this work, the use of intermetallic precursors such as Pd
x
Zr
y
represents an
efficient approach to match these criteria. While functions (1) and (3) can be controlled via the chosen
synthesis route, critical parameters for function (2) encompass surface reducibility, basicity, reactivity
of oxygen vacancies toward CO
2
and the overall CO
2
bond strength [
71
]. Functions (4) to (6) are linked
to the (bi)metallic component of the active catalyst and require a delicate balance between optimized
CH
4
activation, C
ads
reactivity via electronic modulation of for example, C bond strength [
27
] and low
barriers for Cbulk/Cads diffusion and in particular for redissolution of Cgraphite.
Catalysts 2020,10, 1000 26 of 29
The detailed reaction mechanism of CO
2
splitting toward reactive oxygen-, carbonate- and/or
formate-type species [
48
,
71
] at the phase boundary remains to be clarified. The XPS-fitted
C1s-oxygenated species (see Figures 9and 10), which appear to reside at the phase boundary
and/or the “inner” oxidic surface, exhibit a partially reversible dynamic response to gas-phase and
temperature changes. For example, support acidity vs. basicity has been proposed to influence their
specific nature and distribution at the phase boundary [
72
]. CO
2
splitting may proceed both via
reactive vacancies at the phase boundary or via intermediate vacancy-bonded oxygenates [73,74].
As a general note, we emphasize that intermetallic catalyst precursors such as PdZr feature a
double advantage—they can both serve as highly useful UHV- and electron spectroscopy-compatible
model catalysts and, at the same time, they exhibit superior catalytic performance.
Author Contributions:
Conceptualization, B.K., S.P.; methodology, E.C., A.K.-G., A.G., M.H., A.D.; validation,
K.P., N.K.; formal analysis, N.K., K.P., L.S., D.K., M.W., P.L., M.S.; investigation, N.K., K.P., A.D., T.G., C.T., P.L.,
M.W.; resources, K.P., N.K., P.L.; data curation, E.C., M.H., M.S.; writing—original draft preparation, B.K., N.K.,
S.P.; writing—review and editing, A.G.; visualization, N.K., K.P., L.S., M.W., M.S.; supervision, M.H., A.K.-G., E.C.;
project administration, B.K.; funding acquisition, B.K. All authors have read and agreed to the published version
of the manuscript.
Funding:
This work was financially supported by the Austrian Science Fund FWF through SFB F45 “FOXSI”
grants F4501-N16, F4503-N16 and F4505-N16. N. Köpfle acknowledges a PhD position via doctoral program
“Reactivity and Catalysis” of the University of Innsbruck. Financial support by the project CALIPSOplus under
the Grant Agreement 730872 from the EU Framework Programme for Research and Innovation HORIZON 2020 is
acknowledged. This research used resources of the Advanced Light Source, which is a DOE Office of Science User
Facility under Contract No. DE-AC02-05CH11231. L. Schlicker acknowledges the funding of his work by an ALS
doctoral fellowship. T. Götsch additionally acknowledges funding by the FWF (Austrian Science Fund) via project
number J4278.
Acknowledgments:
The authors thank the HZB/BESSY II stafffor their support of the in-situ XPS measurements
at beamline ISISS-PGM.
Conflicts of Interest: The authors declare no conflict of interest.
References
1.
Alyea, E.C.; He, D.; Wang, J. Alcohol synthesis from syngas. Appl. Catal. A Gen.
1993
,104, 77–85. [CrossRef]
2.
Wang, S.; Lu, G.; Millar, G.J. Carbon dioxide reforming of methane to produce synthesis gas over
metal-supported catalysts: State of the art. Energy Fuels 1996,10, 896–904. [CrossRef]
3.
Ghoneim, S.A.; El-Salamony, R.A.; El-Temtamy, S.A. Review on innovative catalytic reforming of natural gas
to syngas. World J. Eng. Technol. 2016,4, 116–139. [CrossRef]
4.
Minardi, E.; Chakraborty, S.; Curcio, S. Membrane reactors for dry reforming of methane. In Membrane
Reactors for Energy Applications and Basic Chemical Production; Elsevier B.V., Radarweg 29, 1043 NX; Elsevier:
Amsterdam, The Netherlands, 2015; pp. 99–144. [CrossRef]
5.
Bosko, M.; M
ú
nera, J.; Lombardo, E.; Cornaglia, L. Dry reforming of methane in membrane reactors using Pd
and Pd–Ag composite membranes on a NaA zeolite modified porous stainless steel support.
J. Membr. Sci.
2010,364, 17–26. [CrossRef]
6.
Oyama, S.T.; Hacarlioglu, P.; Gu, Y.; Lee, D. Dry reforming of methane has no future for hydrogen
production: Comparison with steam reforming at high pressure in standard and membrane reactors.
Int. J. Hydrogen Energy 2012,37, 10444–10450. [CrossRef]
7.
Wolfbeisser, A.; Sophiphun, O.; Bernardi, J.; Wittayakun, J.; Föttinger, K.; Rupprechter, G. Methane dry
reforming over ceria-zirconia supported Ni catalysts. Catal. Today 2016,277, 234–245. [CrossRef]
8.
Ashcroft, A.T.; Cheetham, A.K.; Green, M.L.H.; Vernon, P.D.F. Partial oxidation of methane to synthesis gas
using carbon dioxide. Nature 1991,352, 225–226. [CrossRef]
9.
Rostrupnielsen, J.; Hansen, J. CO
2
-Reforming of Methane over Transition Metals. J. Catal.
1993
,144, 38–49.
[CrossRef]
10.
Bradford, M.C.; Vannice, M.A. Catalytic reforming of methane with carbon dioxide over nickel catalysts I.
Catalyst characterization and activity. Appl. Catal. A Gen. 1996,142, 73–96. [CrossRef]
11.
Sakai, Y.; Saito, H.; Sodesawa, T.; Nozaki, F. Catalytic reactions of hydrocarbon with carbon dioxide over
metallic catalysts. React. Kinet. Catal. Lett. 1984,24, 253–257. [CrossRef]
Catalysts 2020,10, 1000 27 of 29
12.
Ferreira-Aparicio, P.; M
á
rquez-
Á
lvarez, C.; Rodr
í
guez-Ramos, I.; Schuurman, Y.; Ruiz, A.G.; Mirodatos, C.
A transient kinetic study of the carbon dioxide reforming of methane over supported Ru catalysts. J. Catal.
1999,184, 202–212. [CrossRef]
13.
Kehres, J.; Jakobsen, J.G.; Andreasen, J.W.; Wagner, J.B.; Liu, H.; Molenbroek, A.; Sehested, J.; Chorkendorff, I.;
Vegge, T. Dynamical properties of a Ru/MgAl
2
O
4
catalyst during reduction and dry methane reforming.
J. Phys. Chem. C 2012,116, 21407–21415. [CrossRef]
14.
Solymosi, F.; Kuts
á
n, G.; Erd˝ohelyi, A. Catalytic reaction of CH
4
with CO
2
over alumina-supported Pt metals.
Catal. Lett. 1991,11, 149–156. [CrossRef]
15.
Bitter, J.; Seshan, K.; Lercher, J. Mono and Bifunctional Pathways of CO
2
/CH
4
Reforming over Pt and Rh
Based Catalysts. J. Catal. 1998,176, 93–101. [CrossRef]
16.
Zhao, Y.; Pan, Y.-X.; Xie, Y.; Liu, C.-J. Carbon dioxide reforming of methane over glow discharge
plasma-reduced Ir/Al2O3catalyst. Catal. Commun. 2008,9, 1558–1562. [CrossRef]
17.
Rameshan,C.; Li,H.; Anic,K.; Roiaz, M.; Pramhaas,V.; Blume,R.; Hävecker,M.; Knudsen,J.; Knop-Gericke, A.;
Rupprechter, G.; et al. In situ NAP-XPS spectroscopy during methane dry reforming on ZrO
2
/Pt(111) inverse
model catalyst. J. Phys. Condens. Matter 2018,30, 264007. [CrossRef]
18.
Köpfle, N.; Götsch, T.; Grünbacher, M.; Carbonio, E.; Hävecker, M.; Knop-Gericke, A.; Schlicker, L.; Doran, A.;
Kober, D.; Gurlo, A.; et al. Zirconium-assisted activation of palladium to boost syngas production by methane
dry reforming. Angew. Chem. Int. Ed. 2018,57, 14613–14618. [CrossRef]
19.
Steinhauer, B.; Reddy, K.M.; Radnik, J.; Martin, A. Development of Ni-Pd bimetallic catalysts for the utilization
of carbon dioxide and methane by dry reforming. Appl. Catal. A Gen. 2009,366, 333–341. [CrossRef]
20.
Pakhare,D.; Spivey, J.J.Areviewofdry(CO
2
)reformingofmethaneovernoblemetalcatalysts.
Chem. Soc. Rev.
2014,43, 7813–7837. [CrossRef]
21.
Strasser, P.; Koh, S.; Anniyev, T.; Greeley, J.; More, K.L.; Yu, C.; Liu, Z.; Kaya, S.; Nordlund, D.; Ogasawara, H.;
etal. Lattice-straincontroloftheactivityindealloyedcore–shellfuelcellcatalysts. Nat. Chem.
2010,2, 454–460
.
[CrossRef]
22.
Liu, P.; Nørskov, J.K. Ligand and ensemble effects in adsorption on alloy surfaces. Phys. Chem. Chem. Phys.
2001,3, 3814–3818. [CrossRef]
23.
Gao, F.; Goodman, D.W. Pd–Au bimetallic catalysts: Understanding alloy effects from planar models and
(supported) nanoparticles. Chem. Soc. Rev. 2012,41, 8009–8020. [CrossRef]
24.
Zhang, J.; Wang, H.; Dalai, A. Development of stable bimetallic catalysts for carbon dioxide reforming of
methane. J. Catal. 2007,249, 300–310. [CrossRef]
25.
Wei, J.; Iglesia, E. Isotopic and kinetic assessment of the mechanism of reactions of CH
4
with CO
2
or H
2
O to
form synthesis gas and carbon on nickel catalysts. J. Catal. 2004,224, 370–383. [CrossRef]
26. Bradford, M.C.J.; Vannice, M.A. CO2Reforming of CH4.Catal. Rev. 1999,41, 1–42. [CrossRef]
27.
Foppa, L.; Silaghi, M.-C.; Larmier, K.; Comas-Vives, A. Intrinsic reactivity of Ni, Pd and Pt surfaces in dry
reforming and competitive reactions: Insights from first principles calculations and microkinetic modeling
simulations. J. Catal. 2016,343, 196–207. [CrossRef]
28.
Bitter, J.; Seshan, K.; Lercher, J. The State of Zirconia Supported Platinum Catalysts for CO
2
/CH
4
Reforming.
J. Catal. 1997,171, 279–286. [CrossRef]
29.
Foppa, L.; Margossian, T.; Kim, S.M.; Müller, C.R.; Coperet, C.; Larmier, K.; Comas-Vives, A. Contrasting
the Role of Ni/Al
2
O
3
Interfaces in Water–Gas Shift and Dry Reforming of Methane. J. Am. Chem. Soc.
2017,139, 17128–17139. [CrossRef]
30.
Kühl, S.; Düdder, H.; Girgsdies, F.; Kähler, K.; Muhler, M.; Behrens, M. Perovskites as Precursors for
Ni/La
2
O
3
Catalysts in the dry reforming of methane: Synthesis by constant pH co-precipitation, reduction
mechanism and effect of Ru-doping. Z. Anorg. Allg. Chem. 2017,643, 1088–1095. [CrossRef]
31.
Gallego, G.S.; Mondrag
ó
n, F.; Tatibouët, J.-M.; Barrault, J.; Batiot-Dupeyrat, C. Carbon dioxide reforming
of methane over La
2
NiO
4
as catalyst precursor—Characterization of carbon deposition.
Catal. Today
2008,133, 200–209. [CrossRef]
32.
Bonmassar, N.; Bekheet, M.F.; Schlicker, L.; Gili, A.; Gurlo, A.; Doran, A.; Gao, Y.; Heggen, M.; Bernardi, J.;
Klötzer, B.; et al. In situ-determined catalytically active state of LaNiO
3
in methane dry reforming.
ACS Catal.
2019,10, 1102–1112. [CrossRef]
33. Armbruster, M. Intermetallic compounds in catalysis. Encycl. Catal. 2011. [CrossRef]
Catalysts 2020,10, 1000 28 of 29
34.
Köpfle, N.; Mayr, L.; Schmidmair, D.; Bernardi, J.; Knop-Gericke, A.; Hävecker, M.; Klötzer, B.; Penner, S.
A comparative discussion of the catalytic activity and CO
2
-Selectivity of Cu-Zr and Pd-Zr (Intermetallic)
compounds in methanol steam reforming. Catalysts 2017,7, 53. [CrossRef]
35.
Köpfle, N.; Mayr, L.; Lackner, P.; Schmid, M.; Schmidmair, D.; Götsch, T.; Penner, S.; Klötzer, B.
Zirconium-palladium interactions during dry reforming of methane. ECS Trans.
2017
,78, 2419–2430.
[CrossRef]
36.
Wittich, K.; Krämer, M.; Bottke, N.; Schunk, S.A. Catalytic dry reforming of methane: Insights from model
systems. ChemCatChem 2020,12, 2130–2147. [CrossRef]
37.
Li, H.; Choi, J.-I.J.; Mayr-Schmölzer, W.; Weilach, C.; Rameshan, C.; Mittendorfer, F.; Redinger, J.;
Schmid, M.; Rupprechter, G. Growth of an ultrathin zirconia film on Pt
3
Zr examined by high-resolution
X-ray photoelectron spectroscopy, temperature-programmed desorption, scanning tunneling microscopy,
and density functional theory. J. Phys. Chem. C 2015,119, 2462–2470. [CrossRef]
38.
Lackner, P.; Hulva, J.; Köck, E.-M.; Mayr-Schmölzer, W.; Choi, J.I.J.; Penner, S.; Diebold, U.; Mittendorfer, F.;
Redinger, J.; Klötzer, B.; et al. Water adsorption at zirconia: From the ZrO
2
(111)/Pt
3
Zr(0001) model system to
powder samples. J. Mater. Chem. A 2018,6, 17587–17601. [CrossRef]
39.
Lackner, P.; Zou, Z.; Mayr, S.; Diebold, U.; Schmid, M. Using photoelectron spectroscopy to observe oxygen
spillover to zirconia. Phys. Chem. Chem. Phys. 2019,21, 17613–17620. [CrossRef]
40.
Choi, J.I.J.; Mayr-Schmölzer, W.; Mittendorfer, F.; Redinger, J.; Diebold, U.; Schmid, M. The growth of
ultra-thin zirconia films on Pd3Zr(0001). J. Phys. Condens. Matter 2014,26, 225003. [CrossRef]
41.
Antlanger, M.; Mayr-Schmölzer, W.; Pavelec, J.; Mittendorfer, F.; Redinger, J.; Varga, P.; Diebold, U.; Schmid, M.
Pt
3
Zr(0001): A substrate for growing well-ordered ultrathin zirconia films by oxidation. Phys. Rev. B
2012,86, 035451. [CrossRef]
42.
Lackner, P.; Choi, J.I.J.; Diebold, U.; Schmid, M. Substoichiometric ultrathin zirconia films cause strong
metal–support interaction. J. Mater. Chem. A 2019,7, 24837–24846. [CrossRef]
43.
Konvicka, C.; Jeanvoine, Y.; Lundgren, E.; Kresse, G.; Schmid, M.; Hafner, J.; Varga, P. Surface and subsurface
alloy formation of vanadium on Pd(111). Surf. Sci. 2000,463, 199–210. [CrossRef]
44.
Niessen, A.K.; Miedema, A.R. The enthalpy effect on forming diluted solid solutions of two 4d and 5d
transition metals. Ber. Bunsenges. Phys. Chem. 1983,87, 717–725. [CrossRef]
45.
Wolfbeisser, A.; Klötzer, B.; Mayr, L.; Rameshan, R.; Zemlyanov, D.; Bernardi, J.; Föttinger, K.; Rupprechter, G.
Surface modification processes during methane decomposition on Cu-promoted Ni–ZrO
2
catalysts.
Catal. Sci. Technol. 2014,5, 967–978. [CrossRef]
46.
Belton, D.N. Electron spectroscopic identification of carbon species formed during diamond growth.
J. Vac. Sci. Technol. A 1990,8, 2353–2362. [CrossRef]
47.
Bastl, Z. X-Ray photoelectron spectroscopic studies of palladium dispersed on carbon surfaces modified by
ion beams and plasmatic oxidation. Collect. Czechoslov. Chem. Commun. 1995,60, 383–392. [CrossRef]
48.
Kogler, M.; Köck, E.-M.; Klötzer, B.; Perfler, L.; Penner, S. Surface reactivity of YSZ, Y
2
O
3
, and ZrO
2
toward
CO, CO2, and CH4: A comparative discussion. J. Phys. Chem. C 2016,120, 3882–3898. [CrossRef]
49.
Balmes, O.; Resta, A.; Wermeille, D.; Felici, R.; Messing, M.E.; Deppert, K.; Liu, Z.; Grass, M.E.; Bluhm, H.;
Van Rijn, R.; et al. Reversible formation of a PdC
x
phase in Pd nanoparticles upon CO and O
2
exposure.
Phys. Chem. Chem. Phys. 2012,14, 4796–4801. [CrossRef]
50.
Teschner, D.; Borsodi, J.; Wootsch, A.; R
é
vay, Z.; Hävecker, M.; Knop-Gericke, A.; Jackson, S.D.; Schlögl, R.
The roles of subsurface carbon and hydrogen in palladium-catalyzed alkyne hydrogenation. Science
2008,320, 86–89. [CrossRef]
51.
Maciejewski, M.; Baiker, A. Incorporation of carbon into palladium during low-temperature
disproportionation of carbon monoxide over palladium/zirconia prepared from glassy palladium-zirconium.
J. Phys. Chem. 1994,98, 285–290. [CrossRef]
52.
Muraza, O.; Galadima, A. A review on coke management during dry reforming of methane.
Int. J. Energy Res.
2015,39, 1196–1216. [CrossRef]
53.
Rameshan, R.; Vonk, V.; Franz, D.; Drnec, J.; Penner, S.; Garhofer, A.; Mittendorfer, F.; Stierle, A.; Klötzer, B.
Role of precursor carbides for graphene growth on Ni(111). Sci. Rep. 2018,8, 2662. [CrossRef] [PubMed]
54.
Budiman, A.W.; Song, S.-H.; Chang, T.S.; Shin, C.-H.; Choi, M. Dry reforming of methane over cobalt catalysts:
A literature review of catalyst development. Catal. Surv. Asia 2012,16, 183–197. [CrossRef]
Catalysts 2020,10, 1000 29 of 29
55.
Gadalla, A.M.; Bower, B. The role of catalyst support on the activity of nickel for reforming methane with
CO2.Chem. Eng. Sci. 1988,43, 3049–3062. [CrossRef]
56. Dropsch, H.; Baerns, M. CO adsorption on supported Pd catalysts studied by adsorption microcalorimetry
and temperature programmed desorption. Appl. Catal. A Gen. 1997,158, 163–183. [CrossRef]
57.
Mayr, L.; Rameshan, R.; Klötzer, B.; Penner, S.; Rameshan, C. Combined UHV/high-pressure catalysis
setup for depth-resolved near-surface spectroscopic characterization and catalytic testing of model catalysts.
Rev. Sci. Instrum. 2014,85, 055104. [CrossRef]
58.
Doran, A.; Schlicker, L.; Beavers, C.M.; Bhat, S.; Bekheet, M.F.; Gurlo, A. Compact low power infrared tube
furnace for in situ X-ray powder diffraction. Rev. Sci. Instrum. 2017,88, 013903. [CrossRef]
59.
Schlicker, L.; Doran, A.; Schneppmüller, P.; Gili, A.; Czasny, M.; Penner, S.; Gurlo, A. Transmission in situ and
operando high temperature X-ray powder diffraction in variable gaseous environments. Rev. Sci. Instrum.
2018,89, 033904. [CrossRef]
60.
Hammersley, A.P.; Svensson, S.O.; Hanfland, M.; Fitch, A.N.; Häusermann, D. Two-dimensional detector
software: From real detector to idealised image or two-theta scan. High Press. Res.
1996
,14, 235–248.
[CrossRef]
61.
Bluhm, H.; Hävecker, M.; Knop-Gericke, A.; Kiskinova, M.; Schlogl, R.; Salmeron, M. In situ
X-ray photoelectron spectroscopy studies of gas-solid interfaces at near-ambient conditions. MRS Bull.
2007,32, 1022–1030. [CrossRef]
62. CasaXPS Version 2.3.16 Pre-rel 1.4, Casa Software Ltd.: Teignmouth, UK, 2011.
63.
Scofield, J. Theoretical Photoionization Cross Sections from 1 to 1500 keV; Office of Scientific and Technical
Information (OSTI), U.S. Department of Energy: Oak Ridge, TN, USA, 1973.
64.
Rodriguez, N.; Anderson, P.; Wootsch, A.; Wild, U.; Schlögl, R.; Pa
á
l, Z. XPS, EM, and catalytic studies of the
accumulation of carbon on Pt black. J. Catal. 2001,197, 365–377. [CrossRef]
65.
Ahn, B.S.; Jeon, S.G.; Lee, H.; Park, K.Y.; Shul, Y.-G. Hydrogenolysis of CFC-12 (CF
2
Cl
2
) over Pd/
γ
-Al
2
O
3
pretreated with HCFC-22 (CHF2Cl). Appl. Catal. A Gen. 2000,193, 87–93. [CrossRef]
66.
Gabasch, H.; Unterberger, W.; Hayek, K.; Klötzer, B.; Kleimenov, E.; Teschner, D.; Zafeiratos, S.; Hävecker, M.;
Knop-Gericke, A.; Schlögl, R.; et al. In situ XPS study of Pd(111) oxidation at elevated pressure, Part 2:
Palladium oxidation in the 10−1mbar range. Surf. Sci. 2006,600, 2980–2989. [CrossRef]
67.
Powell, C.J.; Jablonski, A. NIST Electron Effective-Attenuation-Length Database SRD 82; Institute of Standards
and Technology: Gaithersburg, MD, USA, 2011.
68.
Yeh, J.J. Atomic Calculation of Photoionization Cross-Sections and Asymmetry Parameters; Gordon and Breach
Science Publishers: Langhorne, PE, USA, 1993.
69.
Damyanova, S.; Pawelec, B.; Arishtirova, K.; Martinez-Huerta, M.; Fierro, J. The effect of CeO
2
on the surface
and catalytic properties of Pt/CeO
2
–ZrO
2
catalysts for methane dry reforming. Appl. Catal. B Environ.
2009,89, 149–159. [CrossRef]
70.
Makri, M.; Vasiliades, M.A.; Petallidou, K.; Efstathiou, A.M. Effect of support composition on the origin and
reactivity of carbon formed during dry reforming of methane over 5wt% Ni/Ce
1−x
M
x
O
2−δ
(M=Zr
4+
, Pr
3+
)
catalysts. Catal. Today 2016,259, 150–164. [CrossRef]
71.
Fan, M.-S.; Abdullah, A.Z.; Bhatia, S. Catalytic technology for carbon dioxide reforming of methane to
synthesis gas. ChemCatChem 2009,1, 192–208. [CrossRef]
72.
Simonov, M.N.; Rogov, V.A.; Smirnova, M.; Sadykov, V.A. Pulse microcalorimetry study of methane dry
reforming reaction on Ni/Ceria-Zirconia catalyst. Catalysts 2017,7, 268. [CrossRef]
73.
Grünbacher, M.; Götsch, T.; Opitz, A.K.; Klötzer, B.; Penner, S. CO
2
reduction on the pre-reduced mixed
ionic-electronicconducting Perovskites La
0.6
Sr
0.4
FeO
3-δ
and SrTi
0.7
Fe
0.3
O
3-δ
.ChemPhysChem
2017
,19, 93–107.
[CrossRef]
74.
Opitz, A.K.; Nenning, A.; Rameshan, C.; Kubicek, M.; Götsch, T.; Blume, R.; Hävecker, M.; Knop-Gericke, A.;
Rupprechter, G.; Klötzer, B.; et al. Surface chemistry of Perovskite-type electrodes during high temperature
CO
2
electrolysis investigated by operando photoelectron spectroscopy.
ACS Appl. Mater. Interfaces
2017,9, 35847–35860. [CrossRef]
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