Unraveling the mechanism of the NO reduction by CO on gold based catalysts

José L. C. Fajín, ^≠ M. Natália D. S. Cordeiro, ^≠ and José R. B. Gomes^§*
Journal of Catalysis, Volume 289, May 2012

^≠ REQUIMTE, Faculdade de Ciências, Universidade do Porto, P-4169-007 Porto, Portugal

^§CICECO, Departamento de Química, Universidade de Aveiro, 3810-193 Aveiro, Portugal

*Corresponding Author. E-mail: [email protected]. Phone: +351 234 401 423. Fax: +351 234 401 470

Abstract

Periodic density functional theory (DFT) calculations have been used to unravel the mechanism of the NO reduction by CO (NO + CO → N₂ + CO₂) on clean and hydrogen covered gold based catalysts. The effects caused by the presence of low-coordinated atoms on the catalyst were taken into consideration by using the stepped Au(3 2 1) surface. A careful analysis of several reaction mechanisms was made and it is concluded that if hydrogen species are not available on the catalyst surface, the N–O bond cleavage will proceed through the ON₂O and N₂O intermediates while CO reacts directly with formed oxygen adatoms. If hydrogen species are available on the catalyst, the reaction will occur via the NOH and N₂O intermediates. However, the reaction has to compete with a more favorable route, where NH₃ instead of N₂ is obtained after formation of NOH and NH_xOH intermediates. The calculations agree also with the experimental observation of the NCO intermediate species which is formed without energy cost from combination of CO and N fragments. The NCO species is very probably a spectator at moderate temperatures since its evolution toward N₂ and CO₂ is less favorable than other possible routes studied in this work. Finally, calculated reaction rate constants at three different temperatures show that most of the reactions studied are only possible at moderately high temperature.

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