Periodic Density Functional Theory Study of Methane Activation over La2O3: Activity of O2-, O-, O22-, Oxygen Poi
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- 作者：Michael S. Palmer ; Matthew Neurock ; and Michael M. Olken
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：July 17, 2002
- 年：2002
- 卷：124
- 期：28
- 页码：8452 - 8461
- 全文大小：246K
- 年卷期：v.124,no.28(July 17, 2002)
- ISSN：1520-5126
文摘
Results of gradient-corrected periodic density functional theory calculations are reported forhydrogen abstraction from methane at 
e10001.gif">, e10002.gif">, e10003.gif">, point defect, and Sr2+-doped surface sites onLa2O3(001). The results show that the anionic e10004.gif"> species is the most active surface oxygen site. Theoverall reaction energy to activate methane at an e10005.gif"> 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 atan e10006.gif"> site is -102 kcal/mol. An oxygen site with similar activity can be generated by doping strontium intothe oxide by a direct Sr2+/La3+ exchange at the surface. The O--like nature of the surface site is reflectedin a calculated hydrogen binding energy of -109.7 kcal/mol. Calculations indicate that surface peroxide(e10007.gif">) sites can be generated by adsorption of O2 at surface oxygen vacancies, as well as by dissociativeadsorption of O2 across the closed-shell oxide surface of La2O3(001). The overall reaction energy andapparent 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 e10008.gif"> intermediate is characterized by abent orientation with respect to the surface and an O-O bond length of 1.47 Å; both attributes are consistentwith structural features characteristic of classical peroxides. We found surface peroxide sites to be slightlyless favorable for H-abstraction from methane than the e10009.gif"> species, with 
Erxn(CH4) = 39.3 kcal/mol, Eact= 47.3 kcal/mol, and Eads(H) = -71.5 kcal/mol. A possible mechanism for oxidative coupling of methaneover La2O3(001) involving surface peroxides as the active oxygen source is suggested.
CH3 radical is 8.2 kcal/mol, with an activation barrier of 10.1 kcal/mol. The binding energy of hydrogen atan Erxn(CH4) = 39.3 kcal/mol, Eact= 47.3 kcal/mol, and Eads(H) = -71.5 kcal/mol. A possible mechanism for oxidative coupling of methaneover La2O3(001) involving surface peroxides as the active oxygen source is suggested.