The effect of the metal particle size on NO2 reduction

B. PASCUCCI; G. S. OTERO; P. G. BELELLI; M.M. BRANDA

Montevideo

Congreso; QUITEL 2016, 42nd International Congress of Theoretical Chemists of Latin Expression; 2016

Quitel 2016

The formation of NOx products is a consequence of incomplete combustion of fossil fuels being these oxides the most dangerous and harmful pollutants in our atmosphere. When NOx are combined with moisture they produce nitric acid, one of the major constituents of the acid rain and photochemical smog in the planet. One way of avoiding their formation is to achieve the reduction of NOx to innocuous products such as N2 and O2. Some recent works [1] have proved that the use of metal nanoparticles significantly enhances the catalytic activity compared to extended surfaces. Moreover, several research groups have concluded that size plays a key role in the catalytic activity of the nanoparticle. In particular, while gold extended surfaces show low reactivity, small Au nanoparticles (1-6 nm) are extremely reactive materials [2].In this work, the NO2 dissociative reaction to produce NO and O on Cu, Ag and Au nanoparticles of 19, 38, 55, 79 and 116 atoms has been studied from theoretical calculations. This reaction was comparatively analyzed on the respective perfect (111) metal surfaces. Molecule-substrate systems were studied from spin polarized DFT formalism by using the commercial Vienna Ab-Initio Simulation Package (VASP) with periodical conditions. Exchange and correlation effects were described from Generalized Gradient Approximation (GGA) using the Perdew-Wang (PW91) functional.Findings show that the most favorable geometrical configuration for NO2 adsorption on all the nanoparticles and (111) surfaces is with both oxygen atoms linked to the surface metal atoms. Adsorption energies obtained on nanoparticles have been ~50% greater than on the surfaces. Furthermore, nanoparticles of 38 atoms have been found to be the most reactive. Metal adsorption order is: Cu>Ag>Au. NO2 dissociation to NO + O has shown once again that the most reactive metal is Cu, with exothermic dissociation energies on nanoparticles of 55, 79 and 116 atoms. However, the order of reactivity is slightly different from the adsorption process, being in this case: Cu>Au>Ag. NO molecule places preferentially on bridge sites while O atom always places on hollow sites. Activation barrier calculations were performed using NEB and Dimer methods, for the most favorable reactions on the surfaces (111), Me19 and Me38 nanoparticles.