Theoretical views on activation of methane catalyzed by Hf~(2+) and oxidation of CO (x~1σ~+) by N_2O (x ~1σ~+) catalyzed by HfO~(2+) and TaO ~(2+)

摘要

The mechanisms of activation of CH_4 catalyzed by ~(1/3)Hf~(2+) and oxidation of CO by N_2O catalyzed by ~(1/3)HfO~(2+) or ~(2/4)TaO~(2+) have been investigated using the B3LYP level of theory. For the activation of methane, the TSR (two-state reactivity) mechanism has been certified through the spin-orbit coupling (SOC) calculation and the Landau-Zener-type model. In the vicinity of the minimum energy crossing point (MECP), SOC equals 900.23 cm~(-1) and the probability of intersystem crossing is approximately 0.62. Spin inversion makes the activation barrier decline from 1.63 to 0.57 eV. NBO analysis demonstrates that empty 6s and 5d orbitals of the Hf atom play the major role for the activation of C-H bonds. Finally, CH_4 dehydrogenates to produce Hf-CH_2 ~(2+). For oxidation of CO by N_2O catalyzed by HfO~(2+) or TaO~(2+), the covalent bonds between transition metal atoms and the oxygen atom restrict the freedom of valence electrons. Therefore, they are all SSR (single-state reactivity). The oxygen atom is directly extracted during the course of oxygen transfer, and its microscopic essence has been discussed. The detailed kinetic information of two catalytic cycles has been calculated by referencing the "energetic span (δE)" model. Finally, TOF(HfO~(2+))/TOF(TaO~(2+)) = 2.7 at 298.15 K, which has a good consistency with the experimental result.