Dioxygen activation routes in Mars-van Krevelen redox cycles catalyzed by metal oxides

Catalytic redox cycles involve dioxygen activation via peroxo (OO*) or H2O2 species, denoted as inner sphere and outer-sphere routes respectively, for metal-oxo catalysts solvated by liquids. On solid oxides, O-2 activation is typically more facile than the reduction part of redox cycles, making kinetic inquiries difficult at steady-state. These steps are examined here for oxidative alkanol dehydrogenation (ODH) by scavenging OO* species with C3H6 to form epoxides and by energies and barriers from density functional theory. Alkanols react with O-atoms (O*) in oxides to form vicinal OH pairs that eliminate H2O to form OO* at O-vacancies formed or react with O-2 to give H2O2. OO* reacts with alkanols to re-form O* via steps favored over OO* migrations, otherwise required to oxidize non-vicinal vacancies. C3H6 epoxidizes by reaction with OO* with rates that increase with C3H6 pressure, but reach constant values as all OO* species react with C3H6 at high C3H6/alkanol ratios. Asymptotic epoxidation/ODH rate ratios are smaller than unity, because outer-sphere routes that shuttle O-atoms via H2O2(g) are favored over endoergic vacancy formation required for inner-sphere routes. The relative contributions of these two routes are influenced by H2O, because vacancies, required to form OO*, react with H2O to form OH pairs and H2O2. OCrmediated routes and epoxidation become favored at low coverages of reduced centers, prevalent for less reactive alkanols and lower alkanol/O-2 ratios, because H2O2 then reacts preferentially with O* (forming OO*), instead of vacancies (forming O*/H2O). Such kinetic shunts between two routes compensate for lower barriers required to form H2O2 than OO*. These re-oxidation routes prefer molecular donor (H2O2) or acceptor (alkanol) to perform stepwise two-electron oxidations by dioxygen, instead of kinetically demanding O-atom migrations. The quantitative descriptions, derived from theory and experiment on Mo-based polyoxometalate clusters with known structures, bring together the dioxygen chemistry in liquid-phase oxidations, including electro-catalysis and monooxygenase enzymes, and oxide surfaces into a common framework, while suggesting a practical process for epoxidation by kinetically coupling with ODH reaction. (C) 2018 Elsevier Inc. All rights reserved.