Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on reactant adsorption at vanadium oxide substrate

Abstract

Extended cluster models together with density-functional theory are used to evaluate geometric, energetic, and electronic properties of different adsorbate species that can occur at a vanadium oxide surface where the selective catalytic reduction (SCR) of NO in the presence of ammonia proceeds. Here, we focus on atomic hydrogen, nitrogen, and oxygen, as well as molecular NO and NHx, x = 1, 4, adsorption at a model V2O5(010) surface. Binding sites, oxygen and vanadium, at both the perfect and reduced surface are considered where reduction is modeled by (sub-) surface oxygen vacancies. The reactants are found to bind overall more strongly at oxygen vacancy sites of the reduced surface where they stabilize in positions formerly occupied by the oxygen (substitutional adsorption) compared with weaker binding at the perfect surface. In particular, ammonia, which interacts only weakly with vanadium at the perfect surface, binds quite strongly near surface oxygen vacancies. In contrast, surface binding of the NH4 adsorbate species differs only little between the perfect and the reduced surface which is explained by the dominantly electrostatic nature of the adsorbate interaction. The theoretical results are consistent with experimental findings and confirm the importance of surface reduction for the reactant adsorption forming elementary steps of the SCR process.

Elementary steps of the catalytic NOx reduction with NH3: Cluster studies on reaction paths and energetics at vanadium oxide substrate

“…The electronic structure of the present clusters is evaluated employing density functional theory (DFT) as implemented in the cluster code StoBe 33 using Gaussian basis sets of double zeta valence plus polarization (DZVP) quality, for further basis set details see 20 . Electron exchange and correlation is accounted for by the gradient corrected revised Perdew, Burke, and Ernzerhof (RPBE) functional [34][35][36] as introduced by Hammer et al 37 .…”

“…In our preceding theoretical studies 20,21 we focused on parts (i) and (ii) of the reaction. We could…”

“…Our theoretical study showed that surface reduction strongly affects the adsorption properties and reduced sites may play an important role for the SCR reaction. In particular, the presence of NH 3 surface species stabilizing near Lewis acid sites, so called "coordinated ammonia", can be explained only by considering reactive vanadium sites, such as those near oxygen vacancies 20 . Further we could show that surface reduction facilitates the dehydrogenation of NH 3 to form (NH 2 + H) 21 as suggested in the 'amidenitrosamide' SCR reaction mechanism proposed earlier 22 .…”

“…Thus, it is very likely that oxygen vacancies occur under reaction conditions and provide reduced surface sites. Further, we investigated the adsorbate binding of different species at the vanadium oxide surface 20 . This includes hydrogen, nitrogen, NO, ammonia, as well as NH x species, x= 1, 2, 4 generated by (de)hydrogenation processes at the surface.…”

“…In previous studies considering elementary reaction steps that have been discussed so far 7 we divided the SCR reaction into three parts 20,21 (i) The initial interaction and adsorption of gas phase NH 3 and NO resulting in different reaction species at the catalyst surface. Here relative surface binding energies of the adsorbate species have important implications for possible reaction mechanisms.…”