Consequences of Metal–Oxide Interconversion for C–H Bond Activation during CH₄Reactions on Pd Catalysts

Abstract

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 H-abstraction 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 cation-lattice oxygen pairs (Pd(2+)-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 Pd(2+)-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 Pd(2+) and vicinal O(2-), Pd(ox)-O(ox), cleave C-H bonds heterolytically via σ-bond metathesis, with Pd(2+) adding to the C-H bond, while O(2-) abstracts the H-atom to form a four-center (H3C(δ-)···Pd(ox)···H(δ+)···O(ox))(‡) transition state without detectable Pd(ox) 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.

Understanding complete oxidation of methane on spinel oxides at a molecular level

“…The thermodynamically and kinetically favourable oxidation step forms an intermediate, CH 3 O. This is different from the continuous dehydrogenation of CH 3 on Pd surface 14 . This preference of oxidation to CH 3 O instead of further dehydrogenation to CH 2 or CH species could be understood as follows: the low coordinated species, such as CH 2 or CH, need more than one binding sites to stabilize them; however the singly dispersed nickel cations on surface lattice of Co 3 O 4 are not an adsorption site consisting of continuously packed Ni atoms for CH 2 or CH.…”

“…In situ studies using ambient pressure photoelectron spectroscopy, vibrational spectroscopy, and isotope-labelled experiments show that (1) Computational studies suggest that the pathways from transferring CHO to CO 2 are different from that on metal surface 14,33,34 . Sub-pathway of CO oxidation and sub-pathway of OCHO dehydrogenation were both proposed for the transformation of CHO intermediate to CO 2 .…”

“…Pt and Pd supported on Al 2 O 3 are the well-studied catalysts 9 . Significant efforts have been made in fundamental understanding of mechanism of the complete oxidation of methane 14 and optimization of catalytic performances of these Pt-or Pd-based catalysts 9 . From reaction mechanism point of view, Neurock and Iglesia et al first elucidated the reaction mechanism on Pd-based catalysts at a molecular level 14 ; these Pd-based catalysts exhibit different mechanisms of activation of C-H along increase of chemical potential of oxygen.…”

Oxidation of Methane to Methanol over Single Site Palladium Oxide Species on Silica: A Mechanistic view from DFT

“…[16][17][18][19] The active phase of Pd for CH 4 combustion has been investigated by DFT calculations and in situ SXRD experiments. It was found that the PdO(101) facet develops preferentially during the oxidation of Pd(100) 17,18 and that the formation of PdO(101) coincides with increased rates of methane oxidation during reaction at millibar pressures. 19 In addition to these studies of PdO nanoparticles or surfaces, the gas-phase C-H bond activation of CH 4 by various single transition-metal-oxide ions and bare metal cations has been experimentally and theoretically investigated by a large number of groups.…”

Carbon-Induced Surface Transformations of Cobalt

“…5,13 Another example of reactivity enhancement is the conversion of methane to synthesis gas by oxide overlayers on Pd. 14, 15 In this paper we will study the reconstruction of the Co(0001) surface as a function of adsorbed C atom overlayer concentration. This is relevant to the kinetics of synthesis gas conversion in the Fischer−Tropsch reaction, which produces long hydrocarbon chains useful as liquid fuels.…”