Dynamic Evolution of Methane Oxidation on Pd-Based Catalysts: A Reactive Force Field Molecular Dynamics Study

Abstract

Studying the dynamic evolution of the reaction process that identifies the key intermediate is important for elaborating the reaction mechanism, but it is still challenging from the point of view of the experiment. Methane oxidation over Pd-based catalysts including Pd (111) with different oxygen coverages and PdO ( 101) is studied by reactive force field molecular dynamic simulations in this work to explore the dynamic character of oxygen species like chemisorbed oxygen and lattice oxygen on the C−H bond activation and the C−O bond formation. The results show that methane oxidation on the Pd (111) surfaces is initiated by the direct dissociation of methane molecules (CH 4 → CH 3 + H) rather than the oxygen-assisted dissociation (CH 4 + O → CH 3 + OH), which is in agreement with the first-principle electronic structure calculation results. A comparison of methane oxidation at different oxygen coverages shows that the catalytic activity increases with the increase of oxygen pressure in general: There is no CO at low oxygen coverage and CO is formed at higher oxygen coverage up to 3/9 ML, but no CO 2 is formed. This is because the C−O bond formation barrier is higher than the H− O bond formation barrier. Therefore, it is in favor of the formation of OH species instead of the CO 2 formation for methane oxidation on metallic Pd. On the PdO (101) surface, we notice that the lattice oxygen species are more active compared to the chemisorbed oxygen species in both the activation of methane and the oxidation of CH x species due to its strong hydrogen affinity and high electron density, leading to high methane oxidation activity and ultimately CO 2 formation. Importantly, a novel path for the CO 2 formation via a dioxymethylene (H 2 COO) intermediate (CH 2 O + O → H 2 COO) or a hydroxymethoxy (H 2 COOH) intermediate (CH 2 O + OH → H 2 COOH) has been identified from our present reactive force field molecular dynamic simulations, which is generally ignored in the static electronic structure scheme like density functional theory. It is hoped that the present results would be a guide for the rational design of the methane oxidation catalysts by controlling the active phase of Pd (or Pt) or provide more essential knowledge for a better understanding of the methane oxidation or methane reformation reaction mechanism.

Redox dynamics and surface structures of an active palladium catalyst during methane oxidation

“…While previous studies have focused typically only on the first C–H activation step of methane (owing to the high gas-phase barrier for this step requiring 2.5 eV 59 ), we simulated all four C–H activation steps involved in methane oxidation to gain a better understanding of the complete reaction path. We found the reaction proceeds by the sequential transfer of H from the adsorbed CH 4 to oxygen on both the metal (adsorbed O*) and oxide (lattice oxygen sites), although other pathways may be possible on the pristine metal as O* blocks metal sites 60 , 61 . The adsorbed oxygen on the metal facilitates the C–H bond breaking, which is structure insensitive on the oxide 62 .…”

Mechanisms of CH4 activation over oxygen‐preadsorbed transition metals by ReaxFF and AIMD simulations

“…Similar to the case of Pt(111), the oxygen insertion reaction and the oxygen-assisted dissociation of CH 3 on Pd(111) need to overcome higher activation barriers of 1.60 and 1.64 eV than that of the direct dissociation of CH 3 (1.03 eV). 56 Thus, CH 3 will proceed via the direct dissociation to produce CH 2 .…”