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• W2034330431 endingPage "15978" @default.
• W2034330431 startingPage "15958" @default.
• W2034330431 abstract "Kinetic and isotopic data and density functional theory treatments provide evidence for the elementary steps and the active site requirements involved in the four distinct kinetic regimes observed during CH4 oxidation reactions using O2, H2O, or CO2 as oxidants on Pt clusters. These four regimes exhibit distinct rate equations because of the involvement of different kinetically relevant steps, predominant adsorbed species, and rate and equilibrium constants for different elementary steps. Transitions among regimes occur as chemisorbed oxygen (O*) coverages change on Pt clusters. O* coverages are given, in turn, by a virtual O2 pressure, which represents the pressure that would give the prevalent steady-state O* coverages if their adsorption–desorption equilibrium was maintained. The virtual O2 pressure acts as a surrogate for oxygen chemical potentials at catalytic surfaces and reflects the kinetic coupling between C–H and O═O activation steps. O* coverages and virtual pressures depend on O2 pressure when O2 activation is equilibrated and on O2/CH4 ratios when this step becomes irreversible as a result of fast scavenging of O* by CH4-derived intermediates. In three of these kinetic regimes, C–H bond activation is the sole kinetically relevant step, but occurs on different active sites, which evolve from oxygen–oxygen (O*–O*), to oxygen–oxygen vacancy (O*–*), and to vacancy–vacancy (*–*) site pairs as O* coverages decrease. On O*-saturated cluster surfaces, O*–O* site pairs activate C–H bonds in CH4 via homolytic hydrogen abstraction steps that form CH3 groups with significant radical character and weak interactions with the surface at the transition state. In this regime, rates depend linearly on CH4 pressure but are independent of O2 pressure. The observed normal CH4/CD4 kinetic isotope effects are consistent with the kinetic-relevance of C–H bond activation; identical 16O2–18O2 isotopic exchange rates in the presence or absence of CH4 show that O2 activation steps are quasi-equilibrated during catalysis. Measured and DFT-derived C–H bond activation barriers are large, because of the weak stabilization of the CH3 fragments at transition states, but are compensated by the high entropy of these radical-like species. Turnover rates in this regime decrease with increasing Pt dispersion, because low-coordination exposed Pt atoms on small clusters bind O* more strongly than those that reside at low-index facets on large clusters, thus making O* less effective in H-abstraction. As vacancies (*, also exposed Pt atoms) become available on O*-covered surfaces, O*–* site pairs activate C–H bonds via concerted oxidative addition and H-abstraction in transition states effectively stabilized by CH3 interactions with the vacancies, which lead to much higher turnover rates than on O*–O* pairs. In this regime, O2 activation becomes irreversible, because fast C–H bond activation steps scavenge O* as it forms. Thus, O* coverages are set by the prevalent O2/CH4 ratios instead of the O2 pressures. CH4/CD4 kinetic isotope effects are much larger for turnovers mediated by O*–* than by O*–O* site pairs, because C–H (and C–D) activation steps are required to form the * sites involved in C–H bond activation. Turnover rates for CH4–O2 reactions mediated by O*–* pairs decrease with increasing Pt dispersion, as in the case of O*–O* active structures, because stronger O* binding on small clusters leads not only to less reactive O* atoms, but also to lower vacancy concentrations at cluster surfaces. As O2/CH4 ratios and O* coverages become smaller, O2 activation on bare Pt clusters becomes the sole kinetically relevant step; turnover rates are proportional to O2 pressures and independent of CH4 pressure and no CH4/CD4 kinetic isotope effects are observed. In this regime, turnover rates become nearly independent of Pt dispersion, because the O2 activation step is essentially barrierless. In the absence of O2, alternate weaker oxidants, such as H2O or CO2, lead to a final kinetic regime in which C–H bond dissociation on *–* pairs at bare cluster surfaces limit CH4 conversion rates. Rates become first-order in CH4 and independent of coreactant and normal CH4/CD4 kinetic isotope effects are observed. In this case, turnover rates increase with increasing dispersion, because low-coordination Pt atoms stabilize the C–H bond activation transition states more effectively via stronger binding to CH3 and H fragments. These findings and their mechanistic interpretations are consistent with all rate and isotopic data and with theoretical estimates of activation barriers and of cluster size effects on transition states. They serve to demonstrate the essential role of the coverage and reactivity of chemisorbed oxygen in determining the type and effectiveness of surface structures in CH4 oxidation reactions using O2, H2O, or CO2 as oxidants, as well as the diversity of rate dependencies, activation energies and entropies, and cluster size effects that prevail in these reactions. These results also show how theory and experiments can unravel complex surface chemistries on realistic catalysts under practical conditions and provide through the resulting mechanistic insights specific predictions for the effects of cluster size and surface coordination on turnover rates, the trends and magnitude of which depend sensitively on the nature of the predominant adsorbed intermediates and the kinetically relevant steps." @default.
• W2034330431 created "2016-06-24" @default.
• W2034330431 creator A5010884148 @default.
• W2034330431 creator A5039229575 @default.
• W2034330431 creator A5047051406 @default.
• W2034330431 creator A5086150545 @default.
• W2034330431 date "2011-09-15" @default.
• W2034330431 modified "2023-10-12" @default.
• W2034330431 title "Reactivity of Chemisorbed Oxygen Atoms and Their Catalytic Consequences during CH₄–O₂ Catalysis on Supported Pt Clusters" @default.
• W2034330431 cites W164226548 @default.
• W2034330431 cites W1965217145 @default.
• W2034330431 cites W1967038247 @default.
• W2034330431 cites W1967967489 @default.
• W2034330431 cites W1969883898 @default.
• W2034330431 cites W1971027276 @default.
• W2034330431 cites W1974026045 @default.
• W2034330431 cites W1974958441 @default.
• W2034330431 cites W1978883486 @default.
• W2034330431 cites W1982791686 @default.
• W2034330431 cites W1986188386 @default.
• W2034330431 cites W1989068066 @default.
• W2034330431 cites W1989614262 @default.
• W2034330431 cites W1990944504 @default.
• W2034330431 cites W1994017328 @default.
• W2034330431 cites W1999720804 @default.
• W2034330431 cites W2000168358 @default.
• W2034330431 cites W2002199513 @default.
• W2034330431 cites W2002885602 @default.
• W2034330431 cites W2007354670 @default.
• W2034330431 cites W2007395042 @default.
• W2034330431 cites W2009065662 @default.
• W2034330431 cites W2009962518 @default.
• W2034330431 cites W2010375408 @default.
• W2034330431 cites W2014907980 @default.
• W2034330431 cites W2016439995 @default.
• W2034330431 cites W2019833377 @default.
• W2034330431 cites W2023973838 @default.
• W2034330431 cites W2026742228 @default.
• W2034330431 cites W2028114648 @default.
• W2034330431 cites W2030276460 @default.
• W2034330431 cites W2031430816 @default.
• W2034330431 cites W2031928152 @default.
• W2034330431 cites W2036113194 @default.
• W2034330431 cites W2038803925 @default.
• W2034330431 cites W2039425128 @default.
• W2034330431 cites W2042627852 @default.
• W2034330431 cites W2044972566 @default.
• W2034330431 cites W2048626879 @default.
• W2034330431 cites W2049927357 @default.
• W2034330431 cites W2050413419 @default.
• W2034330431 cites W2053858998 @default.
• W2034330431 cites W2055912405 @default.
• W2034330431 cites W2056357832 @default.
• W2034330431 cites W2060954670 @default.
• W2034330431 cites W2065251521 @default.
• W2034330431 cites W2066832986 @default.
• W2034330431 cites W2072012009 @default.
• W2034330431 cites W2072837550 @default.
• W2034330431 cites W2075719948 @default.
• W2034330431 cites W2077994078 @default.
• W2034330431 cites W2079867776 @default.
• W2034330431 cites W2083222334 @default.
• W2034330431 cites W2083733070 @default.
• W2034330431 cites W2086054121 @default.
• W2034330431 cites W2087585288 @default.
• W2034330431 cites W2087698390 @default.
• W2034330431 cites W2088403169 @default.
• W2034330431 cites W2089400568 @default.
• W2034330431 cites W2093414965 @default.
• W2034330431 cites W2093786005 @default.
• W2034330431 cites W2094977330 @default.
• W2034330431 cites W2101665600 @default.
• W2034330431 cites W2108781471 @default.
• W2034330431 cites W2117242147 @default.
• W2034330431 cites W2119341942 @default.
• W2034330431 cites W2122427541 @default.
• W2034330431 cites W2123485699 @default.
• W2034330431 cites W2124318833 @default.
• W2034330431 cites W2124433213 @default.
• W2034330431 cites W2136135454 @default.
• W2034330431 cites W2136733582 @default.
• W2034330431 cites W2153502737 @default.
• W2034330431 cites W2157282307 @default.
• W2034330431 cites W2162251512 @default.
• W2034330431 cites W2171438195 @default.
• W2034330431 cites W2324248585 @default.
• W2034330431 cites W3144131531 @default.
• W2034330431 cites W3206606417 @default.
• W2034330431 cites W4245526875 @default.
• W2034330431 doi "https://doi.org/10.1021/ja202411v" @default.
• W2034330431 hasPubMedId "https://pubmed.ncbi.nlm.nih.gov/21919447" @default.
• W2034330431 hasPublicationYear "2011" @default.
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