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 CH 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 CH 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 CH 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 CH 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 MH bonds. Figure 7b shows
that on Co0/Ce2O3(0001), the Co sites work alone during the
dissociation of the first CH 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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