Effect of Pd doping on CH4 oxidation mechanism over Pt clusters: A systematic DFT study

Pt-Pd bimetallic catalyst exhibits excellent activity and promising stability in CH4 oxidation. In this study, the monometallic Pt13, bimetallic Pt12-Pd and Pd@Pt12 clusters were built and optimized via quantum chemistry method to uncover the effect of Pd doping on CH4 oxidation mechanism over Pt clusters. The feasible reaction paths for CH4 oxidation were systematically considered and the parameters of every step of the elementary reaction were obtained with density functional theory (DFT) calculations. On the Pt13 and Pd@Pt12 clusters, the optimal reaction path for CH4 oxidation is CH4* → CH3* → CH2* (+O*) → CH2O* → CHO* → CO* (+O*) → CO2* and the rate-determining step (RDS) is CH3* → CH2* with an energy barrier of 1.28 eV (for Pt13) and 1.20 eV (for Pd@Pt12) respectively. On the Pt12-Pd cluster, the optimal reaction path changes into CH4* → CH3*(+O*) → CH3O* → CH2O* → CHO* → CO* (+OH*) → COOH* → CO2* and the RDS is CH4* → CH3* with an energy barrier of 1.28 eV. Doping Pd in the center of the Pt13 cluster (Pd@Pt12) will improve the catalytic performance while doping Pd in the shell (Pt12-Pd) will change the reaction mechanism.