Periodic density functional theory study of methane activation over La2O3: Activity of O2-, O-, O22-, oxygen point defect, and Sr2+-doped surface sites

Michael S. Palmer, Matthew Neurock, Michael M. Olken

Research output: Contribution to journalArticle

171 Scopus citations

Abstract

Results of gradient-corrected periodic density functional theory calculations are reported for hydrogen abstraction from methane at O(s)2-, O(s)-, O2(s)2-, point defect, and Sr2+-doped surface sites on La2O3(001). The results show that the anionic O(s)- species is the most active surface oxygen site. The overall reaction energy to activate methane at an O(s)- 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 O(s)- 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 (O2(s)2-) 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 O2(s)2- 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 O(s)- 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.

Original languageEnglish (US)
Pages (from-to)8452-8461
Number of pages10
JournalJournal of the American Chemical Society
Volume124
Issue number28
DOIs
StatePublished - Jul 17 2002

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