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Kinetic Consequences of Chemisorbed Oxygen Atoms during Methane Oxidation on Group VIII Metal Clusters


Molecular insights and the kinetic relevance of reaction elementary steps for methane activation on Group VIII metal and oxide clusters are established based on kinetic, isotopic, and theoretical assessments. These fundamental understandings enable accurate prediction of complex rate dependencies and cluster size effects during methane conversion reactions in catalytic partial oxidation, reforming, and combustion processes.

Kinetics of methane reactions with oxygen are described by several regimes, each with unique rate dependencies and kinetic requirements, as the identities of the kinetically-relevant step and the most abundant surface intermediates vary with the surface and bulk oxygen contents of Pt and Pd clusters. C-H bond activation is the kinetically-relevant step in all regimes except for one that occurs immediately before the complete oxygen depletion. C-H bond activation steps may, however, proceed via mechanistically distinct paths of oxidative insertion of metal atom, oxidative insertion coupled with H abstraction, or H abstraction routes, over metal-metal, oxygen-metal, or oxygen-oxygen site pairs, respectively, thus exhibiting different activation enthalpies and entropies. The predominant route for C-H bond activation is dictated by the coverages and reactivities of oxygen on cluster surfaces and accessibility of metal atom to methane reactants. In a narrow regime before the complete oxygen consumption, C-H bond activation becomes kinetically inconsequential on oxygen-depleted surfaces and oxygen dissociative-adsorption steps limit methane conversion rates.

The relation among oxygen coverages, oxygen reactivities, and methane reaction paths leads to a single-valued functional dependence of reactive methane collision probabilities on oxygen chemical potentials at the cluster surfaces. The oxygen chemical potentials are given by kinetic coupling of the generation and removal of reactive oxygen atoms and thus are kinetic properties of methane reactions; they become a thermodynamic property only in the limiting case of equilibrated oxygen dissociative-recombination steps.

The fate of oxygen during catalysis was rigorously defined as the reactive collision probabilities for CO oxidation relative to those for methane and was measured at low oxygen coverages on Pt in which CO is most likely to desorb before encountering an oxygen atom and undergoing further oxidation to carbon dioxide. The reactive collision probabilities are much larger for CO oxidation than for methane oxidation; these results have unequivocally confirmed that CO and hydrogen, if formed on and desorbed from catalytic surfaces, rapidly undergo sequential oxidation to form carbon dioxide and water and that direct CO and hydrogen formation via molecular coupling of methane and oxygen is impractical at any residence time required for practical extents of methane conversion.

Thermodynamics of oxygen dissolution from cluster surfaces into the bulk, cluster size and metal coordination effects on thermodynamic tendencies of bulk oxidation, and their catalytic consequences are established on Pd clusters. Oxidation of Pd clusters occurs via gradual dissolution of chemisorbed oxygen atoms into the bulk phase over a wide range of oxygen chemical potentials. The oxygen dissolution steps initiate and complete at lower oxygen chemical potentials in small than large clusters, indicating that small clusters exhibit a higher thermodynamic tendency for bulk oxidation. Oxygen dissolution leads to more weakly bound surface oxygen atoms and to exposed Pd atoms. These Pd atoms, together with vicinal lattice oxygen atoms, form Pd-oxygen site pairs that are more effective for C-H bond activation than O*-O* sites prevalent on metallic Pd cluster surfaces via concerted steps of an oxidative insertion of Pd atoms into the C-H bonds and oxygen assisted H abstraction. As oxygen binding strength decreases and Pd atoms become accessible with increasing oxygen contents in the clusters, C-H bond activation rate constants increase over the entire range of O-to-Pd atomic ratios throughout the Pd-to-PdO phase transition.

This fundamental study describes how oxygen thermochemical properties influence active site structures and, in turn, dictate the kinetics of methane oxidation reactions. The direct relation between the oxygen thermochemical properties and methane oxidation kinetics has not been previously interpreted at the atomic scale; this relation appears to be general for alkane oxidation reactions over transition metal and oxide clusters, as has been shown also in our recent work on ethane oxidation.

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