Structure-sensitivity of direct oxidation methane to methanol over Rhn/ZrO2-x (1 0 1) (n = 1, 4, 10) surfaces: A DFT study

Abstract

The size-dependence of methane direct oxidation to methanol over Rhn/ZrO2-x (101) (n = 1,4,10) was studied by density functional theoretical (DFT) calculations. We envisioned several paths for methanol formation using H2O2 or O2 as oxidant respectively. The CH3* directly oxidized to methanol by OOH* was considered the optimal route for methanol formation over Rhn/ZrO2-x (101) in our present calculation. Moreover, the C-O bond formation via the O* as the oxidant shown a higher barrier than that of OH* or OOH*, indicating more active of H2O2 than O2 due to more available Rh sites. More importantly, the high stability of CH3* favors the CH3OH formation. With the increasing of the size of clusters, the activation energy of H* abstracted from CH3* was decreased, resulting in a reduction of methanol formation as Rh size growths. Besides, the larger Rh clusters were more facility for methanol decomposition to CO than Rh1/ZrO2-x (101), leading the lower productivity for methanol further. Especially for Rh4/ZrO2-x (101) and Rh10/ZrO2-x (101), the surface was even poisoned by CO. From the view of kinetic, our present results show that the methanol formation rate increased with increasing the Rh dispersion, particularly for the atomically dispersed Rh catalyst. So, the atomically dispersed Rh catalyst has the higher productivity and selectivity for the methanol production, which means a strongly structure-sensitivity for methanol formation catalyzed by supported Rh catalysts.