Catalytic Requirements for Alkanol, Alkane, Alkene, and CO Oxidation on Dispersed Metal Clusters
Weifeng Tu, Petar Lachkov, Yifei Yang, and Ya-Huei (Cathy) Chin*
Department of Chemical Engineering and Applied Chemistry, University of Toronto, Toronto, Canada
Reactions of alcohol, alkane, alkene, or CO with oxygen on dispersed metal clusters are important pathways for synthesizing value-added chemicals and fuels, producing energy, or eliminating pollutants. These reactions occur via kinetically coupled reductant and oxygen activation steps on metal cluster surfaces. Here, we utilize kinetic and isotopic studies in combination with surface titration techniques to probe the elementary steps and their kinetic relevance, report the catalytic involvement of reactive oxidant species, and then discuss the mechanistic synergies across these oxidation reactions. We report a direct relation connecting the oxidant and reductant pressures to the bulk chemical state of the metal clusters, coverages of reactive oxygen species, and their reactivities in these oxidation reactions, demonstrated here for the case of methanol oxidative dehydrogenation and methane oxidation reactions. The oxidant and reductant pressure ratio determines the relative rates of oxygen activation and oxygen removal on cluster surfaces and thus the bulk chemical state, instantaneous oxygen coverages, and the relative abundance of oxygen atom pair, oxygen atom-oxygen vacancy pair, and metal atom pair at cluster surfaces. These site-pairs activate methanol via kinetically distinct routes, thus leading to different rate dependencies. For the case of CO oxidation, turnover rates vary with not only O* but also O2* coverages and for propene epoxidation, the rates vary with reactive oxygen (O* and O2*) and also with OOH* intermediates formed from hydrogen abstraction by O2*. The presence of the diverse oxygen species and their distinct reactivities in activating the reductants lead to the marked difference in rates, selectivities, and the apparent complex kinetic dependence on reactant pressures.