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

Abstract

The experimentally validated computational models developed herein, for the first time, show that Mn-promotion does not enhance the activity of the surface Na2WO4catalytic active sites for CH4heterolytic dissociation during OCM. Contrary to previous understanding, it is demonstrated that Mn-promotion poisons the surface WO4catalytic active sites resulting in surface WO5sites with retarded kinetics for C-H scission. On the other hand, dimeric Mn2O5surface sites, identified and studiedvia ab initiomolecular dynamics and thermodynamics, were found to be more efficient in activating CH4than the poisoned surface WO5sites or the original WO4sites. However, the surface reaction intermediates formed from CH4activation over the Mn2O5surface sites are more stable than those formed over the Na2WO4surface sites. The higher stability of the surface intermediates makes their desorption unfavorable, increasing the likelihood of over-oxidation to COx, 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 Na2WO4surface sites towards CH4activation but can yield MnOxsurface sites that activate CH4faster than Na2WO4surface sites, but unselectively.