REGULAR ARTICLE Heterogeneous reaction mechanisms of the reduction of nitric oxide on carbon surfaces: a theoretical analysis Ana Arenillas • Borja Arias • Fernando Rubiera • Jose ´ J. Pis • Ramo ´n Lo ´pez • Pablo Campomanes • Covadonga Pevida • M. Isabel Mene ´ndez Received: 9 July 2009 / Accepted: 23 November 2009 / Published online: 11 December 2009 Ó Springer-Verlag 2009 Abstract The mechanism of reaction between NO and two models of carbonaceous materials with active sites was investigated at the UB3LYP/6-31 ? G(d) and UM06-2X theory levels. The small model is the anthracene radical and the large one is also a monoradical built with ten benzene rings. The mechanistic routes found with both models lead to a satisfactory justification of the experi- mental data and showed the important role of the temper- ature and the oxygen and nitrogen surface complexes, generated in the carbonaceous material at intermediate steps of the mechanism, in the global process. The com- putational results presented in this work revealed that, at low temperatures, the high Gibbs energy barrier that appears after N 2 release from the (NO) 2 dimer, initially chemisorbed on the char surface, prevents the subsequent evolution of the system with the result that CO 2 emission does not take place. On the other hand, at high tempera- tures, the mean energy available to the reactants may be sufficient to overcome this energy barrier giving rise to the formation of N 2 and CO 2 as reduction products. The N 2 may come from two sources depending on the approach of the NO molecule at different points of the reaction coor- dinate. The best description of the carbonaceous surface through a larger model confirms the absence of N 2 O release in the reduction of nitric oxide on carbon surfaces. Keywords NO heterogeneous reduction  Carbonaceous surface  Reaction mechanisms  Density functional calculations 1 Introduction The reduction of nitrogen oxide (NO) by carbonaceous materials has been widely investigated by many research- ers in relation to the NOx and/or N 2 O emissions during coal combustion [1–5]. The global NO heterogeneous reduction can be schematized as follows: C ac þ xNO ! x=2 ð Þ N 2 þ CO x ð1Þ or C ac þ xNO ! x=2 ð Þ N 2 O þ 1=2 ð Þ CO x ð2Þ where C ac represents an active site on the char surface. However, the mechanisms involved in the process are not yet fully understood. Ever since the work of Smith et al. in 1959 [6] to the present day, two generally accepted con- clusions about the mechanism of the NO-carbon reaction have been drawn from experimental observations: (1) first step is chemisorption of NO on the carbon surface, and (2) oxygen complexes are important intermediates in the reaction. Published works on this subject agree that reduc- tion goes through two stages: NO adsorption, reversibly Electronic supplementary material The online version of this article (doi:10.1007/s00214-009-0708-8) contains supplementary material, which is available to authorized users. A. Arenillas  B. Arias  F. Rubiera  J. J. Pis  C. Pevida (&) Instituto Nacional del Carbo ´n, CSIC, Apartado 73, 33080 Oviedo, Spain e-mail: cpevida@incar.cisc.es R. Lo ´pez  M. I. Mene ´ndez (&) Departmento de Quı ´mica Fı ´sica y Analı ´tica, Universidad de Oviedo, C/Julia ´n Claverı ´a 8, 33006 Oviedo, Spain e-mail: Isabel@uniovi.es P. Campomanes Laboratory of Computational Chemistry and Biochemistry, E ´ cole Polytechnique Fe ´de ´rale de Lausanne (EPFL), 1015 Lausanne, Switzerland 123 Theor Chem Acc (2010) 127:95–108 DOI 10.1007/s00214-009-0708-8