A combined computational and experimental study of methane activation during oxidative coupling of methane (OCM) by surface metal oxide catalysts

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na(2)WO(4) catalytic active sites for CH(4) heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO(4) catalytic active sites resulting in surface WO(5) sites with retarded kinetics for C–H scission. On the other hand, dimeric Mn(2)O(5) surface sites, identified and studied via ab initio molecular dynamics and thermodynamics, were found to be more efficient in activating CH(4) than the poisoned surface WO(5) sites or the original WO(4) sites. However, the surface reaction intermediates formed from CH(4) activation over the Mn(2)O(5) surface sites are more stable than those formed over the Na(2)WO(4) surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to CO(x), in agreement with the experimental findings in the literature on Mn-promoted catalysts. Consequently, the Mn-promoter does not appear to have an essential positive role in synergistically tuning the structure of the Na(2)WO(4) surface sites towards CH(4) activation but can yield MnO(x) surface sites that activate CH(4) faster than Na(2)WO(4) surface sites, but unselectively.