Theoretical study on the gas-phase reaction mechanism between nickel monoxide and methane for syngas production

The comprehensive mechanism survey on the gas-phase reaction between nickel monoxide and methane for the formation of syngas, formaldehyde, methanol, water, and methyl radical has been investigated on the triplet and singlet state potential energy surfaces at the B3LYP/6-311++G(3df, 3pd)//B3LYP/6-311+G(2d, 2p) levels. The computation reveals that the singlet intermediate HNiOCH(3) is crucial for the syngas formation, whereas two kinds of important reaction intermediates, CH(3)NiOH and HNiOCH(3), locate on the deep well, while CH(3)NiOH is more energetically favorable than HNiOCH(3) on both the triplet and singlet states. The main products shall be syngas once HNiOCH(3) is created on the singlet state, whereas the main products shall be methyl radical if CH(3)NiOH is formed on both singlet and triplet states. For the formation of syngas, the minimal energy reaction pathway (MERP) is more energetically preferable to start on the lowest excited singlet state other than on the ground triplet state. Among the MERP for the formation of syngas, the rate-determining step (RDS) is the reaction step for the singlet intermediate HNiOCH(3) formation involving an oxidative addition of NiO molecule into the C-H bond of methane, with an energy barrier of 120.3 kJ mol(-1). The syngas formation would be more effective under higher temperature and photolysis reaction condition.