Search

• Two-electron reduced centers formed on metal oxides in redox cycles activate O 2 via inner or outer sphere routes. • Inner sphere routes form bound peroxo species at O-vacancies. • Outer sphere routes form H 2 O 2 (g) at vicinal OH pairs, formed via H 2 O dissociation on O-vacancies. • The O-atoms in more reducible oxides exhibit a greater preference for the inner sphere routes. • The large charge-balancing cations influence the O 2 activation selectivity of the vicinal O-atoms. The activation of dioxygen at typically isolated two-electron reduced centers can lead to the formation of electrophilic superoxo or peroxo species, providing an essential route to form reactive O 2 -derived species in biological, organometallic, and heterogeneous catalysts. Alternatively, O 2 activation can proceed via outer sphere routes, circumventing the formation of bound peroxo (OO*) species during oxidation catalysis by forming H 2 O 2 (g), which can react with another reduced center to form H 2 O. The electronic and binding properties of metal oxides that determine the relative rates of these activation routes are assessed here by systematic theoretical treatments using density functional theory (DFT). These methods are combined with conceptual frameworks based on thermochemical cycles and crossing potential models to assess the most appropriate descriptors for the activation barriers for each route using Keggin polyoxometalates as illustrative examples. In doing so, we show that inner sphere routes, which form OO* species via O 2 activation on the O-vacancies (*) formed in the reduction part of redox cycles, are mediated by early transition states that only weakly sense the oxide binding properties. Outer sphere routes form H 2 O 2 (g) via O 2 activation on OH pairs (H/OH*) formed by dissociation of H 2 O on O-vacancies; their rates and activation barriers reflect the rates of the first H-atom transfer from H/OH* to O 2. The activation barriers for this H-transfer step depend on the binding energy of more weakly-bound H-atom in H/OH* pairs (HAE 2) and on the OOH-surface interaction energy at its product state ( E int 0). The E int 0 values are similar among oxides unless a large charge-balancing cation is present and interacts with OOH; consequently, HAE 2 acts as an appropriate descriptor of the outer sphere dynamics. HAE 2 also determines the thermodynamics of H 2 O dissociation on O-vacancies, which influence the inner and outer sphere rates by setting the relative coverage of * and H/OH*. These results, in turn, show that HAE 2 is a complete descriptor of the reactivity and selectivity of oxides for O 2 activation; the O-atoms in more reducible oxides (more negative HAE 2) exhibit a greater preference for the inner sphere routes and for the formation of electrophilic OO* intermediates that mediate epoxidation and O-insertion reactions during catalytic redox cycles. Large charge-balancing cations locally modify E int 0 values that determine the outer sphere rates and thus can be used to alter the preference of O-atoms to either inner or outer sphere routes. [ABSTRACT FROM AUTHOR]

Catalytic redox cycles involve dioxygen activation via peroxo (OO ∗ ) or H 2 O 2 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 C 3 H 6 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 H 2 O to form OO ∗ at O-vacancies formed or react with O 2 to give H 2 O 2 . OO ∗ reacts with alkanols to re-form O ∗ via steps favored over OO ∗ migrations, otherwise required to oxidize non-vicinal vacancies. C 3 H 6 epoxidizes by reaction with OO ∗ with rates that increase with C 3 H 6 pressure, but reach constant values as all OO ∗ species react with C 3 H 6 at high C 3 H 6 /alkanol ratios. Asymptotic epoxidation/ODH rate ratios are smaller than unity, because outer-sphere routes that shuttle O-atoms via H 2 O 2 (g) are favored over endoergic vacancy formation required for inner-sphere routes. The relative contributions of these two routes are influenced by H 2 O, because vacancies, required to form OO ∗ , react with H 2 O to form OH pairs and H 2 O 2 . OO ∗ -mediated routes and epoxidation become favored at low coverages of reduced centers, prevalent for less reactive alkanols and lower alkanol/O 2 ratios, because H 2 O 2 then reacts preferentially with O ∗ (forming OO ∗ ), instead of vacancies (forming O ∗ /H 2 O). Such kinetic shunts between two routes compensate for lower barriers required to form H 2 O 2 than OO ∗ . These re-oxidation routes prefer molecular donor (H 2 O 2 ) 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. [ABSTRACT FROM AUTHOR]