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/CeO2 Catalysts.

ABSTRACT

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/CeO2 surfaces, methane easily fully decomposes, CH4 → C + 4H, and water dissociates, H2O→ OH + H. However, in important reactions such as the direct oxidation of methane to methanol (MTM), where complex interplay exists between reactants (CH4, O2), it is desirable to avoid the complete dehydrogenation of methane to carbon. Remarkably, the barrier for the activation of C-H bonds in CHx (x = 1-3) species on Ni/CeO2 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. CH4 activation occurs only on Ni sites, and H2O activation occurs on both Ni and Cu sites. The MTM reaction pathway for the example of the Ni3Cu1/CeO2 model catalyst predicts a higher selectivity and a lower activation barrier for methanol production, compared with that for Ni4/CeO2. These findings point toward a possible strategy to design active and stable catalysts which can be employed for methane activation and conversions.