Consequences of Metal鈥揙xide Interconversion for C鈥揌 Bond Activation during CH4 Reactions on Pd Catalysts

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• 作者：Ya-Huei (Cathy) Chin ; Corneliu Buda ; Matthew Neurock ; Enrique Iglesia
• 刊名：Journal of the American Chemical Society
• 出版年：2013
• 出版时间：October 16, 2013
• 年：2013
• 卷：135
• 期：41
• 页码：15425-15442
• 全文大小：1201K
• 年卷期：v.135,no.41(October 16, 2013)
• ISSN：1520-5126

文摘

Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C鈥揌 bonds in CH₄ react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C鈥揌 activation routes change from oxidative addition to H-abstraction and then to 蟽-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*鈥?) to oxygen atom (O*鈥揙*) pairs and ultimately to Pd cation-lattice oxygen pairs (Pd²⁺鈥揙^{2鈥?/sup>) in PdO. The charges in the CH₃ and H moieties along the reaction coordinate depend on the accessibility and chemical state of the Pd and O centers involved. Homolytic C鈥揌 dissociation prevails on bare (*鈥?) and O*-covered surfaces (O*鈥揙*), while C鈥揌 bonds cleave heterolytically on Pd2+鈥揙2鈥?/sup> pairs at PdO surfaces. On bare surfaces, C鈥揌 bonds cleave via oxidative addition, involving Pd atom insertion into the C鈥揌 bond with electron backdonation from Pd to C鈥揌 antibonding states and the formation of tight three-center (H₃C路路路Pd路路路H) transition states. On O*-saturated Pd surfaces, C鈥揌 bonds cleave homolytically on O*鈥揙* pairs to form radical-like CH₃ species and nearly formed O鈥揌 bonds at a transition state (O*路路路CH₃鈥?/sup>路路路*OH) that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd2+ and vicinal O2鈥?/sup>, Pd_ox鈥揙_ox , cleave C鈥揌 bonds heterolytically via 蟽-bond metathesis, with Pd2+ adding to the C鈥揌 bond, while O2鈥?/sup> abstracts the H-atom to form a four-center (H₃C未鈭?/sup>路路路Pd_ox路路路H未+路路路O_ox) transition state without detectable Pd_ox reduction. The latter is much more stable than transition states on *鈥? and O*鈥揙* pairs and give rise to a large increase in CH₄ oxidation turnover rates at oxygen chemical potentials leading to Pd to PdO transitions. These distinct mechanistic pathways for C鈥揌 bond activation, inferred from theory and experiment, resemble those prevalent on organometallic complexes. Metal centers present on surfaces as well as in homogeneous complexes act as both nucleophile and electrophile in oxidative additions, ligands (e.g., O* on surfaces) abstract H-atoms via reductive deprotonation of C鈥揌 bonds, and metal鈥搇igand pairs, with the pair as electrophile and the metal as nucleophile, mediate 蟽-bond metathesis pathways.}