Tuning Selectivity in the Direct Conversion of Methane to Methanol: Bimetallic Synergistic Effects on the Cleavage of C-H and O-H Bonds over NiCu/CeO 2 Catalysts.

The efficient activation of methane and the simultaneous water dissociation are crucial in many catalytic reactions on oxide-supported transition metal catalysts. On very low-loaded Ni/CeO 2 surfaces, methane easily fully decomposes, CH 4 → C + 4H, and water dissociates, H 2 O→ OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH 4 , O 2 ), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of C-H bonds in CH x ( x = 1-3) species on Ni/CeO 2 surfaces can be manipulated by adding Cu, forming bimetallic NiCu clusters, whereas the ease for cleavage of O-H bonds in water is not affected by ensemble effects, as obtained from density functional theory-based calculations. CH 4 activation occurs only on Ni sites, and H 2 O activation occurs on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni 3 Cu 1 /CeO 2 model catalyst predicts a higher selectivity and a lower activation barrier for methanol production, compared with that for Ni 4 /CeO 2 . These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions.