A DFT study on N₂O oxidation and methanol synthesis over Bi₄O₆: single-site catalytic model of α-Bi₂Mo₃O₁₂

Catalytic conversion of methane to methanol is one of the most promising processes for effective natural gas resource utilization. In this work, Bi₄O₆-catalyzed oxidation of methane to methanol with N₂O as an oxidizing reactant has been investigated by DFT calculation. For the overall reaction mechanism of three N₂O molecules on Bi₄O₆ catalyst, two catalytic cycles were proposed. Cycle 1 involved the consecutive decomposition of the first two N₂O molecules. Cycle 2 corresponded to the decomposition of the third N₂O molecule. The activation barriers of the first and second N₂O decomposition were computed to be 27.6 and 35.0 kcal/mol, respectively. The third N₂O decomposition in cycle 2 required 36.1 kcal/mol activation barriers. Thus, cycle 1 was the main catalytic cycle for N₂O as the cycle required lower in barriers than those of the other. Oxidation of methane to methanol on Bi₄O₇ and Bi₄O₈ catalysts was supposed to be a two-step mechanism consisting of H₃C-H bond breaking and CH₃-OH formation. The activation energies of the two steps were 32.7, 41.1, and 21.6, 17.2 kcal/mol for Bi₄O₇ and Bi₄O₈, respectively. Thus, methane oxidation over Bi₄O₈ was found to be more energetically favorable than those of Bi₄O₇, in which C-H bond breaking is the RDS. The present catalyst could be a promising material for the oxidation of methane to methanol. In summary, the single-site catalytic model study would be beneficial for guiding and searching for potential catalysts in heterogeneously catalyzed N₂O decomposition and methanol synthesis as green as possible. However, theoretical investigation of this kind of catalyst's extended model system must be taken into account.