In Situ Investigation of Methane Dry Reforming on Metal/Ceria(111) Surfaces: Metal–Support Interactions and C?H Bond Activation at Low Temperature

Article abstract of DOI:10.1002/anie.201707538

Studies with a series of metal/ceria(111) (metal=Co, Ni, Cu; ceria=CeO₂) surfaces indicate that metal–oxide interactions can play a very important role for the activation of methane and its reforming with CO₂ at relatively low temperatures (600–700 K). Among the systems examined, Co/CeO₂(111) exhibits the best performance and Cu/CeO₂(111) has negligible activity. Experiments using ambient pressure X-ray photoelectron spectroscopy indicate that methane dissociates on Co/CeO₂(111) at temperatures as low as 300 K—generating CH_x and CO_x species on the catalyst surface. The results of density functional calculations show a reduction in the methane activation barrier from 1.07 eV on Co(0001) to 0.87 eV on Co²⁺/CeO₂(111), and to only 0.05 eV on Co⁰/CeO_2?x(111). At 700 K, under methane dry reforming conditions, CO₂ dissociates on the oxide surface and a catalytic cycle is established without coke deposition. A significant part of the CH_x formed on the Co⁰/CeO_2?x(111) catalyst recombines to yield ethane or ethylene.

Full text of DOI:10.1002/anie.201707538

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Accepted Article
Title: In situ Investigation of Methane Dry Reforming on M-CeO2(111)
{M= Co, Ni, Cu} Surfaces: Metal-Support Interactions and the
activation of C-H bonds at Low Temperature
Authors: Jose A Rodriguez, Zongyuan Liu, Pablo Lustemberg, Ramon
Gutierrez, John Carey, Robert Palomino, Mykhailo Vorohta,
David Grinter, Pedro Ramirez, Vladimir Matolin, Michael
Nolan, M. Veronica Ganduglia-Pirovano, and Sanjaya
Senanayake
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201707538
Angew. Chem. 10.1002/ange.201707538
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http://dx.doi.org/10.1002/ange.201707538

10.1002/anie.201707538
Angewandte Chemie International Edition
In situ Investigation of Methane Dry Reforming on M-CeO2(111)
{M= Co, Ni, Cu} Surfaces: Metal-Support Interactions and the
activation of C-H bonds at Low Temperature
Zongyuan Liu,[a] Pablo Lustemberg,[b] Ramón A. Gutiérrez,[c] John J. Carey,[d] Robert M. Palomino,[a]
Mykhailo Vorokhta,[e] David C. Grinter,[a] Pedro J. Ramírez,[c] Vladimír Matolín,[e] Michael Nolan,[d] M.
Verónica Ganduglia-Pirovano,* [f] Sanjaya D. Senanayake,* [a] and José A. Rodriguez*[a]
Abstract: Studies with a series of M-CeO2(111) {M= Co, Ni, Cu}
surfaces indicate that metal-oxide interactions can play a very
important role for the activation of methane and its reforming with
CO2 at relatively low temperatures (600-700 K). Among the systems
examined, Co-CeO2(111) exhibits the best performance and Cu-
CeO2(111) has negligible activity. Experiments using ambient
pressure XPS indicate that methane dissociates on Co-CeO2(111),
at temperatures as low as 300 K, generating CHx and COx species
on the catalyst surface. The results of density-functional calculations
show a reduction in the methane activation barrier from 1.07 eV on
Co(0001) to 0.87 eV on Co2+/CeO2(111), and to only 0.05 eV on
Co0/CeO2-x(111). At 700 K, under methane dry reforming conditions,
CO2 dissociates on the oxide surface and a catalytic cycle is
established without coke deposition. A significant part of the CHx
formed on the Co0/CeO2-x (111) catalyst recombines to yield ethane
or ethylene.
In recent studies, we found that a Ni2+/CeO2(111) system
activates CH4 at room temperature as a consequence of metal-
support interactions.[5,6] The methane reforming with CO2 (DRM;
CH4 + CO2 → 2CO + 2H2) then takes place at a moderate
temperature of about 700 K. Over this surface, Ni and O sites of
ceria work in a cooperative way during the dissociation of the
first C-H bond in methane. Can this useful phenomenon be seen
with other admetal-ceria combinations? In this article we
compare the behavior of Co, Ni and Cu on CeO2(111) using
ambient-pressure X-ray photoelectron spectroscopy (AP-XPS),
kinetic testing, and theoretical calculations based on density-
functional theory.
The deposition of small amounts of Co (< 0.3 ML) on a
CeO2(111) film at 300 K produced a partial reduction of the
oxide surface and adsorbed Co/CoOx species (Figure S1 in
Supporting Information). Upon annealing from 300 to 700 K,
most of the Co0 transformed into Co2+ (Figure S2). This
particular type of metal-oxide surface was exposed to methane
at 300, 500 and 700 K. Figure 1 shows C 1s XPS spectra
collected before and after exposing a Co/CeO2(111) surface to 1
Torr of methane at 300 K for 5 minutes. The strong peak near
285 eV can be attributed to CHx groups formed by the partial
Natural gas can transform the energy landscape of the
world since it is a cheap and abundant fuel stock and a good
source of carbon for the chemical industry. CH4 is the primary
component of natural gas but is difficult to convert it to upgraded
fuels or chemicals due to the strength of the C-H bonds in the
molecule (104 kcal/mol) and its non-polar nature.[1] Enabling low-
CH4 1 Torr, 300 K
C 1s
[a]
Dr Z. Liu, Dr R. Palomino, Dr D. Grinter, Dr S.D. Senanayake, Dr
J.A. Rodriguez
Chemistry Department, Brookhaven National Laboratory
Upton, NY 11973 (USA)
CHx
COx
E-mail: ssenanay@bnl.gov
E-Mail: rodrigez@bnl.gov
[b]
[c]
Dr P. Lustemberg
Instituto de Fisica Rosario (IFIR), CONICET (Argentina)
MSc R.A. Gutierrez, Prof P.J. Ramírez
Facultad de Ciencias, Universidad Central de Venezuela, Caracas
1020-A (Venezuela)
Ce 4s
[d]
[e]
[f]
Dr. J.J. Carey, Dr. M. Nolan
Tyndall National Institute, University College Cork, Lee Maltings,
Cork (Ireland)
0.2 ML of Co
No cobalt
Dr M. Vorokhta, Prof V. Matolin
Department of Surface and Plasma Science, Faculty of Mathematics
and Physics, Charles University, Prague, (Czech Republic)
Dr M.V. Ganduglia-Pirovano
Instituto de Catálisis y Petroleoquímica, CSIC
C/Marie Curie 2, 28049 Madrid (Spain)
E-mail: vgp@icp.csic.es
290
285
Binding Energy (eV)
280
Figure 1. C 1s XPS spectra collected before and after exposing a CeO2(111)
surface containing 0.2 ML of Co to 1 Torr of methane at 300 K for 5 minutes.
Supporting information for this article is given via a link at the end of
the document
dissociation of methane on the metal/oxide interface.[5,6] This
peak was not seen when a pure CeO2(111) substrate was
exposed to CH4 at 300 K. In Figure 1 there is a second strong
temperature activation of methane is a major technological
objective. It is known that enzymes such as the methane
monooxygenase and some copper- and zinc-based inorganic
compounds can activate C−H bonds near room temperature.[2-4]
peak near 289.5 eV. This likely corresponds to
a COx
species.[5,6] Some of the CH4 molecules fully dissociated
producing C atoms that eventually reacted with oxygen atoms of
This article is protected by copyright. All rights reserved.

10.1002/anie.201707538
Angewandte Chemie International Edition
the ceria to yield COx species. The intensity of the C 1s peak for
the CHx species increased with Co coverage up to 0.15-0.2 ML,
and then decreased at higher admetal coverages. Thus, small
clusters of Co on ceria are the best for for C-H bond activation.
The dissociative adsorption of methane on the Co2+/CeO2(111)
surface at room temperature did not induce a change in the
oxidation state of Co2+ or Ce4+. Such changes were only seen
when the dosing of methane was done at temperatures of 500
and 700 K.
Figure 2 displays Ce 3d and Co 2p AP-XPS spectra
recorded while exposing a CeO2(111) surface with 0.2 ML of Co
to 50 mTorr of CH4 at different temperatures. Both ceria and
Co2+ species undergo reduction at 500-700 K as indicated by the
emergence of Ce3+ and Co0 features. Once the first hydrogen is
removed from the reactant molecule, a quick CH3 → CH2 → CH
→ C transformation occurs on the surface and oxygen atoms
from the sample react to form CO and H2O gas.
100
80
60
40
20
0
Cu/CeO2(111)
Ni/CeO2(111)
Co/CeO2(111)
300
400
500
600
700
Temperature (K)
Ce3+Ce3+
Ce3+
Ce 3d
Co 2p
Figure 3. Ce3+ concentration measured in XPS as a function of temperature
under reaction conditions (i.e. exposure to 50 mTorr of methane) on ceria pre-
covered with ~ 0.2 ML of Co, Ni or Cu.
0
Co
Co2+
reforming or by the generation of hydrocarbons (2CH4
→
Co2+
Co
C2H6/C2H4 + nH2).[7,8] CO and C2H4 also can be obtained through
the reaction: 2CH4 + 2CO2 → 2CO + C2H4 + 2H2O. At the
maximum of catalytic activity in Figure 4, one can estimate a
turnover frequency (TOF) of 6-7 molecules/Co atom · sec for
methane dry reforming. At Co coverages above 0.2 ML, there
0
700 K
500 K
300 K
Ce3+
3.0
UHV
1 Torr CH , 1 Torr CO
4
2
650 K
Co on ceria
2.5
2.0
1.5
1.0
0.5
0.0
920
900
880
800
780
Binding Energy (eV)
Figure 2. Ce 3d + Co 2p spectra for a Co/CeO2(111) (ΘCo ≈ 0.2 ML)
surface under 50 mTorr of CH4 at 300, 500 and 700 K.
H
2
CO
Similar experiments to those shown in Figures 1 and 2
were performed for Cu/CeO2(111). The results of XPS and
Auger spectroscopy indicate that the interaction of Cu with
CeO2(111), Figures S3 and S4, is not as strong as that seen for
Co. The dissociation of methane on Cu/CeO2(111) surfaces was
negligible at temperatures between 300 and 700 K (Figure S5).
In this aspect, the behavior of these surfaces is very similar to
that found for clean CeO2(111).[5,6] In Figure 3, we compare the
reduction of ceria (i.e. formation of Ce3+) after dosing methane to
C H /C H
2
6
2
4
Co-CeO2(111), Cu-CeO2(111) and
a Ni-CeO2(111) system
examined in a previous study.[5] In the temperature range of
500-700 K, Co/CeO2(111) reacts better with methane than
Ni/CeO2(111) or Cu/CeO2(111). As we will see below, the partial
reduction of ceria is important for the activation of CO2 and
closing the catalytic cycle for methane dry reforming.
0.0
0.1
0.2
0.3
0.4
0.5
Co coverage on CeO2(111) / ML
Figure 4. Catalytic activity for methane dry reforming and ethane production
on Co-ceria catalysts as a function of Co coverage. The figure reports the
amount of CO/H2 and ethane/ethylene formed after exposing the Co-ceria
surfaces to 1 Torr of CH4 and 1 Torr of CO2 at 650 K for 5 minutes.
In the case of Co/CeO2(111), catalytic activity for methane
dry reforming and C2 (ethane/ethylene) production was seen at
650 K (Figure 4). Clean CeO2(111) did not display significant
catalytic activity. However, the catalytic activity substantially
increased when Co was added, reaching a maximum for the
generation of CO/H2 at a coverage of ~ 0.15 ML. A maximum for
the production of ethane/ethylene was seen at a Co coverage of
0.1 ML. At these small Co coverages, the Co/CeO2(111) system
had no problem dissociating CH4 (Figures 1-3). The CH3 groups
generated on the surface underwent full decomposition to yield
syngas or formed carbon-carbon bonds to produce ethane or
ethylene. In Figure 4, the hydrogen is produced by methane dry
was a steady decline in the catalytic activity. At the same time,
postreaction characterization of the catalysts with XPS showed
an increase in the amount of atomic carbon present in the
surface (Figure S6). This carbon could eventually lead to the
formation of coke and catalyst deactivation. Thus, the optimum
Co coverage is below 0.2 ML, when the interactions with the
oxide support are important and the strength and number of the
Co-Co interactions is limited.
This article is protected by copyright. All rights reserved.

10.1002/anie.201707538
Angewandte Chemie International Edition
AP-XPS was used to study the chemical changes in the
best Co/CeO2(111) catalyst under reaction conditions. Figure 5
shows Ce 3d and Co 2p spectra collected while the catalyst is
exposed to CH4 or a mixture of CH4/CO2 at 700 K. Under pure
methane one sees a surface with Co0 and strong peaks for Ce3+.
The addition of CO2 to the reactant gas leads to a weak
reoxidation of Co and a substantial Ce3+ → Ce4+ conversion.
Two reaction paths are possible for the re-oxidation of the Ce3+
in the support: CO2(g) + Vac → CO(g) + O-oxide, or CO2(g) +
H(a) + Vac → HOCO(a) → HO-Vac + CO(g) and HO-Vac → O-
oxide + H(a). Both of them could close the catalytic cycle for
methane dry reforming after the process: CH4(g) → C(a) +
4H(a); C(a) + O-oxide → CO(g) + Vac.
3.0
2.5
2.0
1.5
1.0
0.5
0.0
Co 2p
Ce 3d
Cu/CeO2
Ni/CeO2
Co/CeO2
Co0
Co2+
Ce3+
Figure 6. Catalytic activity for methane dry reforming on Cu-, Ni-, Co-ceria
catalysts (θAdmetal ~ 0.15 ML). The figure reports the amount of CO/H2 formed
after exposing the catalysts to 1 Torr of CH4 and 1 Torr of CO2 at 650 K for 5
minutes.
Co0
Co2+
+
50 mTorr CH
4
clusters on CeO2(111), Figure S7. We found that Co2+ species
(3d7) adsorb most favorably at O-hollow sites in CeO2 (111), with
the transfer of two electrons to the reducible support. Cu atoms
transfer only one electron, yielding Cu1+ species (3d10). The Cu4
species also reduce the support, with the formation of two Ce3+.
50 mTorr CO , 700K
2
Ce3+
Ce3+
Ce3+
50 mTorr CH , 700K
4
The CeO2(111) supported Co1 and Cu4 species behave similarly
[5,6,9]
to the corresponding Ni1 and Ni4 ones.
Moreover, low-
930 920 910 900 890 880 870
810
800
790
780
loaded Co/CeO2-x(111) with metallic cobalt, is the active phase
for methane dry reforming, which will be modeled using single
metal Co atoms on Ce2O3(0001) (Figure S7). Hence, these M-
ceria {M=Co, Cu} model surfaces mimic the essential features of
the experimental catalysts as seen in the XPS data shown in
Figures 3 and 5.
Binding Energy (eV)
Figure 5. Ce 3d + Co 2p XPS spectra of the surface at 700 K under 50 mTorr
of methane w/o the addition of 50 mTorr CO2, The scale of the Co 2p region
has been multiplied by a factor of 3 (θCo ~ 0.2 ML).
The molecular binding of methane to Co or Cu surfaces is
very weak and dissociation, CH4(a) → CH3(a) + H(a), is difficult
due to large energy barriers.[10,11] Our calculated barriers are
1.07 and 1.64 eV (Figure S8), respectively, in agreement with
previous studies.[10,11] This is similar to Ni(111) with a barrier of
about 0.91.1 eV.[5,12] The molecular binding of CH4 to Co2+ and
Cu4 species on CeO2(111) lies within the 0.1-0.2 eV range
(Figures 7a and S9). On the Cu4/CeO2(111) surface, similar to
Cu(111), methane dissociation is hindered by a large energy
barrier of 1.45 eV. This is consistent with the negligible methane
dissociation observed for Cu-ceria systems at room temperature.
However, on Co1/CeO2(111), the barrier is reduced from 1.07 to
0.87 eV, as compared to Co(0001) (and from 1.02 eV if fcc
Co(111) is considered, Figure S8). Therefore, the energy barrier
for ceria supported small Co nanoparticles is accessible at lower
temperatures than on the extended metal surface and methane
dissociation is expected to occur, in agreement with the
experiments shown in Figure 1. Here, metal and support work in
a cooperative way in the dissociation of the CH bond. Note that
the final states shown in Figure 7a do not necessarily
correspond to the lowest energy structures of the dissociated
methane (Figure S9), but to local minima geometrically close to
the transition state structures.
Upon increasing oxygen removal from the ceria support by
reaction with methane, the Co2+ species gradually recover their
metallic state. Chemisorbed methane molecules on both
M0/Ce2O3(0001) {M=Co, Ni} model systems are more stable than
on the corresponding M2+/CeO2(111) model systems (Figure 7),
and thus the probability of reaction is expected to increase on
the actual active dry reforming metal-CeO2-x catalysts. We
observe that the distances between methane and the
M0/Ce2O3(0001) {M=Co, Ni} surfaces, as measured by the CM
distances, are reduced by approx. 0.6 (Co) to 1.0 (Ni) Å with
respect to the same distances in the M2+/CeO2(111) systems
(Figures S9 and S10). Moreover, CH4 adsorption on the M0/
Figure 6 compares the catalytic activity for methane dry
reforming of Co-, Cu- and Ni-CeO2(111)[6]. The surface with Co
is clearly the best catalyst, in agreement with the trends seen in
Figure 3 for the activation of pure methane. Among these
systems, Co-CeO2(111) is the only one able to catalyze the
2CH4 → C2Hx + {(8-x)/2}H2 reaction (x= 4,6). The negligible
catalytic activity of Cu/CeO2(111) results from a very poor
reaction with CH4 without the generation of the Ce3+ sites
necessary for the activation of CO2, as indicated in Figure 3,
which shows that reducibility increases in the order Cu < Ni < Co.
In a set of experiments, we deposited small Co coverages (5-10
wt%) on a ceria powder and tested the catalyst activity for DRM
in a flow reactor at temperatures between 700 and 975 K. The
powder system did not show signs for deactivation and the
conversion of methane through dry reforming was always close
to that determined by equilibrium thermodynamics.[8b] These
results are in agreement with the behaviour seen for
Co/CeO2(111) at 700 K.
Methane decomposition is frequently cited as the most
difficult step for the DRM process.[7,8] Here, we apply the spin-
polarized DFT+U approach to investigate the dissociative
adsorption of CH4 on Co and Cu nanoparticles deposited on
stoichiometric and reduced cerium oxide surfaces, plus the
extended Co(0001), Co (111) and Cu(111) surfaces. Results will
be compared to those recently obtained for Ni-ceria systems.[5,6]
The metal-ceria surfaces used for the experiments are quite
complex. Co/CeO2(111) displays high activity for methane
dissociation at low metal coverages with Co atoms in close
contact with the ceria support in a 2+ oxidation state, whereas
Cu/CeO2(111) is not active, with Cu1+ atoms aggregating to form
larger metallic nanoparticles. Thus, we model these systems
using single Co atoms and small tetrahedral Cu4
This article is protected by copyright. All rights reserved.

10.1002/anie.201707538
Angewandte Chemie International Edition
Ce2O3(0001) surfaces is aided by substantial hydrogen-metal
larger by about 0.7 eV on Co than on Ni (Figure S12). Thus,
both Co- and Ni-ceria systems are able to cleave CH bonds at
room temperature. However, it is only for Co-ceria that as the
temperature increases, and methane decomposes and reacts
interactions that are more pronounced compared to the M2+
CeO2(111) systems;
/
with the CeO2 support, accompanied by the Co2+/CeO2

Co0/CeO2-x transformation, that CH bonds are more easily
cleaved. Therefore, more vacant sites and more Ce3+ ions are
expected to form on Co-ceria catalysts as compared to Ni-ceria,
in agreement with the experimental observations (Figure 3).
Our results on M-ceria {M=Co, Ni, Cu} model catalysts
show that not only the nature of the metal is crucial for DRM
activity and system stability, as recently pointed out for Ni, Co
and Co-Ni nanoparticles,13,14 but also the oxide support can play
an essential role. An oxide support can modify the electronic
properties of an admetal in substantial ways making its chemical
properties very different from those of the corresponding bulk
metal.[5,6,15] Single Co and Ni atoms on CeO2 interact strongly
with the reducible support while adopting a +2 oxidation state,
and exhibit room temperature activity for CH bond dissociation.
Moreover, reducing the ceria support stabilizes metallic Co and
Ni atoms and the systems are active for methane activation and
dry reforming, with Co-CeO2-x being much more active than Ni-
CeO2-x. It is also seen that a low metal loading, below 0.2 ML, is
crucial for the catalyst activity and stability since deactivation
due to carbon deposition is observed at higher loading. This is
consistent with the calculated trend in the adsorption energy of
C atoms on the supported metal clusters of varying size (Figure
S13), for example, Co1/Ni1-CeO2 (-4.98/-4.12) < Co4/Ni4-CeO2 (-
6.86/-6.54 eV). Here, we show that by choosing the "right"
metal-oxide combination and manipulating metal-oxide
interactions, as well as controlling the effects of metal loading,
an improved catalytic activity can be obtained. Our findings
should be useful in the rational design of catalysts for reactions
involving CH bond dissociation. Cobalt-ceria can be added to
the short list of oxide-based systems that can activate methane
at room temperature,[6,16] opening the possibility for new and
exciting chemistry.
Acknowledgements
The work carried out at Brookhaven National Laboratory was
supported by the US Department of Energy (Chemical Sciences
Division, DE-SC0012704). The theoretical work was supported by
the MINECO-Spain (CTQ2015-78823-R) and the European
Commission Framework 7 project BIOGO (Grant No: 604296). The
COST action CM1104 is gratefully acknowledged. Computer time
provided by the SGAI-CSIC, CESGA, BIFI-ZCAM, RES, SNCAD
(Sistema Nacional de Computación de Alto Desempeño, Arg),
ICHEC, and the DECI resources BEM based in Poland at WCSS
and Archer at EPCC with support from the PRACE aislb, is
acknowledged. M. Vorokhta thanks the Ministry of Education, Youth
and Sports of the Czech Republic for financial support under project
LH15277.
Figure 7. Reaction energy profile for the CH4  CH3 + H reaction on: a) Cu4,
Co1 and Ni1 on CeO2(111) and b) Co1 and Ni1 on Ce2O3(0001). The activation
barriers are hardly affected by inclusion of vdW interactions (Figure S14). The
structures shown on the left, middle and right of the reaction pathways,
correspond to the side views of the molecularly adsorbed, transition and
dissociated states, respectively (Supporting information Figures S9 and S10).
All energies are referenced to the total energy of CH4(g) and the M/ceria
{M=Co, Ni, Cu} surfaces. Atoms color scheme: Ni in blue, Cu in brown, Co in
violet, Ce3+ in grey, Ce4+ in white, surface/subsurface oxygen atoms in
red/green. The CH4-Ni0/Ce2O3(0001) structure is by 0.15 eV more stable than
the corresponding one in Ref. 5.
The closer approach to the M0/Ce2O3(0001) surfaces facilitates
charge transfer to methane, e.g., the increase in the Bader
charge for the C atom upon CH4 adsorption is 0.03 and 0.16
electrons for Co2+/CeO2(111) and Co0/Ce2O3(0001), respectively,
with respect to the gas-phase CH4 molecule (Table S1).
Furthermore, the energy barrier for the dissociative adsorption of
methane on Co0/Ce2O3(0001) is substantially reduced compared
to Co2+/CeO2(111), becoming almost negligible – Ea = 0.05 eV.
This is not the case for the corresponding Ni-ceria systems for
which the barrier remains unchanged (~0.8 eV). We interpret
this unique Co behavior by inspecting the transition state
structures for the M0/Ce2O3(0001) {M=Co, Ni} surfaces (Figure
7b): the marked differences in activation barriers relate to the
ability of the metals to form strong MH bonds. Figure 7b shows
that on Co0/Ce2O3(0001), the Co sites work alone during the
dissociation of the first CH bond. By contrast, on
Ni0/Ce2O3(0001), Ni and O sites work cooperatively. This is also
consistent with the calculated adsorption energy for hydrogen
atoms on the M0/Ce2O3(0001) {M=Co, Ni} surfaces, which is
Keywords: cobalt• ceria • methane dissociation • X-ray
photoelectron spectroscopy • density functional theory
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10.1002/anie.201707538
Angewandte Chemie International Edition
TOC
Text for Table of Contents:
Low-loaded Co-CeO2 is a highly efficient, stable and non-
expensive catalyst for methane activation at RT and dry
reforming at relative low temperatures (700 K), as revealed by
experiments of ambient pressure XPS in combination with DFT
calculations. Ethane/ethylene formation is also observed. Upon
temperature increase the Co2+/CeO2  Co0/CeO2-x
transformation occurs, making the latter extremely active. The
DRM activity strongly depends on the metal-ceria combination,
with Co-ceria > Ni-ceria, and Cu-ceria being inactive.
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