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

• DOI: 10.1016/j.jcat.2018.05.016
• Publication Date: 2018-06-14T18:16:59Z
• Authors: Stephanie Kwon; Prashant Deshlahra; Enrique Iglesia
• Keywords: Vicinal; Coordination sphere

Abstract 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, O2 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 O2 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. OO∗-mediated routes and epoxidation become favored at low coverages of reduced centers, prevalent for less reactive alkanols and lower alkanol/O2 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.