Highly selective oxidation of methane to methanol has long been challenging in catalysis. Here, we reveal key steps for the promotion of this reaction by water when tuning the selectivity of a well-defined CeO2/Cu2O/Cu(111) catalyst from carbon monoxide and carbon dioxide to methanol under a reaction environment with methane, oxygen, and water. Ambient-pressure x-ray photoelectron spectroscopy showed that water added to methane and oxygen led to surface methoxy groups and accelerated methanol production. These results were consistent with density functional theory calculations and kinetic Monte Carlo simulations, which showed that water preferentially dissociates over the active cerium ions at the CeO2–Cu2O/Cu(111) interface. The adsorbed hydroxyl species blocked O-O bond cleavage that would dehydrogenate methoxy groups to carbon monoxide and carbon dioxide, and it directly converted this species to methanol, while oxygen reoxidized the reduced surface. Water adsorption also displaced the produced methanol into the gas phase.
Strong bonding interactions
between a transition metal and a substrate
or support is one of the most effective strategies to immobilize subnanometer
scale clusters or atoms in heterogeneous catalysis. We show that such
a type of phenomenon can take place on a Mo2N surface.
Combined experimental and theoretical studies show that strong metal–support
interactions between face-centered cubic-structured γ-Mo2N and cobalt have been confirmed to effectively anchor subnanometer
Co clusters and prevent their aggregation. The results of X-ray absorption
near edge structure, ambient pressure X-ray photoelectron spectroscopy,
and density functional theory revealed electronic perturbations in
the nitride-bonded cobalt not seen on a strongly active oxide such
as CeO2. A charge transfer from Co to Mo2N was
observed with a significant stabilization of the Co 3d levels, which
prevents the full decomposition of CO2. The subnanometer
Co loaded on γ-Mo2N catalysts exhibited very high
selectivity to the product CO, whereas the undesirable methanation
activity, typically inevitable on traditional Co/oxide catalysts,
was successfully suppressed. As a consequence of the electronic perturbations
induced by the nitride, the cobalt was not able to fully dissociate
the CO2 molecule to generate C or CH
x
fragments necessary for methane production. Under reaction
conditions, the strong bonding between Co and γ-Mo2N maintained the subnanometer geometry of Co, leading to a remarkable
selectivity and stability.
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