Periodic Density Functional Theory Study of Methane Activation over La2O3: Activity of O2-, O-, O22-, Oxygen Point Defect, and Sr2+-Doped Surface Sites

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Abstract
Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at , , , point defect, and Sr2+-doped surface sites on La2O3(001). The results show that the anionic species is the most active surface oxygen site. The overall reaction energy to activate methane at an site to form a surface hydroxyl group and gas-phase •CH3 radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen at an site is −102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium into the oxide by a direct Sr2+/La3+ exchange at the surface. The O--like nature of the surface site is reflected in a calculated hydrogen binding energy of −109.7 kcal/mol. Calculations indicate that surface peroxide ( ) sites can be generated by adsorption of O2 at surface oxygen vacancies, as well as by dissociative adsorption of O2 across the closed-shell oxide surface of La2O3(001). The overall reaction energy and apparent activation barrier for the latter pathway are calculated to be only 12.1 and 33.0 kcal/mol, respectively. Irrespective of the route to peroxide formation, the intermediate is characterized by a bent orientation with respect to the surface and an O−O bond length of 1.47 Å; both attributes are consistent with structural features characteristic of classical peroxides. We found surface peroxide sites to be slightly less favorable for H-abstraction from methane than the species, with ΔErxn(CH4) = 39.3 kcal/mol, Eact = 47.3 kcal/mol, and ΔEads(H) = −71.5 kcal/mol. A possible mechanism for oxidative coupling of methane over La2O3(001) involving surface peroxides as the active oxygen source is suggested.