Theoretical study on the gas-phase reaction mechanism between palladium monoxide and methane.

The gas-phase reaction mechanism between palladium monoxide and methane has been theoretically investigated on the singlet and triplet state potential energy surfaces (PESs) at the CCSD(T)/AVTZ//B3LYP/6-311+G(2d, 2p), SDD level. The major reaction channel leads to the products PdCH₂ + H₂O, whereas the minor channel results in the products Pd + CH₃OH, CH₂OPd + H₂, and PdOH + CH₃. The minimum energy reaction pathway for the formation of main products (PdCH₂ + H₂O), involving one spin inversion, prefers to start at the triplet state PES and afterward proceed along the singlet state PES, where both CH₃PdOH and CH₃Pd(O)H are the critical intermediates. Furthermore, the rate-determining step is RS-CH₃PdOH → RS-2-TS1cb → RS-CH₂Pd(H)OH with the rate constant of k = 1.48 × 10¹² exp(−93,930/ RT). For the first CH bond cleavage, both the activation strain Δ E^≠_strain and the stabilizing interaction Δ E^≠_int affect the activation energy Δ E^≠, with Δ E^≠_int in favor of the direct oxidative insertion. On the other hand, in the PdCH₂ + H₂O reaction, the main products are Pd + CH₃OH, and CH₃PdOH is the energetically preferred intermediate. In the CH₂OPd + H₂ reaction, the main products are Pd + CH₃OH with the energetically preferred intermediate H₂PdOCH₂. In the Pd + CH₃OH reaction, the main products are CH₂OPd + H₂, and H₂PdOCH₂ is the energetically predominant intermediate. The intermediates, PdCH₂, H₂PdCO, and t-HPdCHO are energetically preferred in the PdC + H₂, PdCO + H₂, and H₂Pd + CO reactions, respectively. Besides, PdO toward methane activation exhibits higher reaction efficiency than the atom Pd and its first-row congener NiO. © 2011 Wiley Periodicals, Inc. J Comput Chem, 2011 [ABSTRACT FROM AUTHOR]