Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH₄ Reactions on Pd Catalysts

Journal Article · Tue Oct 01 00:00:00 EDT 2013 · Journal of the American Chemical Society

DOI:https://doi.org/10.1021/ja405004m· OSTI ID:1233794

Chin, Ya-Huei; Buda, Corneliu; Neurock, Matthew; Iglesia, Enrique

Mechanistic assessments based on kinetic and isotopic methods combined with density functional theory are used to probe the diverse pathways by which C-H bonds in CH₄ react on bare Pd clusters, Pd cluster surfaces saturated with chemisorbed oxygen (O*), and PdO clusters. C-H activation routes change from oxidative addition to Habstraction and then to σ-bond metathesis with increasing O-content, as active sites evolve from metal atom pairs (*-*) to oxygen atom (O*-O*) pairs and ultimately to Pd cationlattice oxygen pairs (Pd²⁺-O^2-) 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-H dissociation prevails on bare (*-*) and O*- covered surfaces (O*-O*), while C-H bonds cleave heterolytically on Pd²⁺-O^2- pairs at PdO surfaces. On bare surfaces, C-H bonds cleave via oxidative addition, involving Pd atom insertion into the C-H bond with electron backdonation from Pd to C-H antibonding states and the formation of tight three-center (H₃C···Pd···H)‡ transition states. On O*-saturated Pd surfaces, C-H bonds cleave homolytically on O*-O* pairs to form radical-like CH3 species and nearly formed O-H bonds at a transition state (O*···CH3 •···*OH)‡ that is looser and higher in enthalpy than on bare Pd surfaces. On PdO surfaces, site pairs consisting of exposed Pd²⁺ and vicinal O^{2-, Pd_ox-O_{ox, cleave C-H bonds heterolytically via σ-bond metathesis, with Pd2+ adding to the C-H bond, while O2- abstracts the H-atom to form a four-center (H3Cδ-···Pdox···Hδ+···Oox) transition state without detectable Pdox reduction. The latter is much more stable than transition states on *-* and O*-O* 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-H 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-H bonds, and metal-ligand pairs, with the pair as electrophile and the metal as nucleophile, mediate σ-bond metathesis pathways.}}

• Research Organization: Pacific Northwest National Lab. (PNNL), Richland, WA (United States). Environmental Molecular Sciences Lab. (EMSL)
• Sponsoring Organization: USDOE
• DOE Contract Number: AC05-76RL01830
• OSTI ID: 1233794
• Journal Information: Journal of the American Chemical Society, Vol. 135, Issue 41; ISSN 0002-7863
• Publisher: American Chemical Society (ACS)
• Country of Publication: United States
• Language: English

Related Subjects

Environmental Molecular Sciences Laboratory