Catalytic Ammonia Oxidation on Noble Metal Surfacesa Theoretical Study

Resumen

This thesis is based on the study of ammonia oxidation on platinum group metals. The objectives of this thesis are accept or discard the diverse mechanisms proposed. Even suggest the most appropriate according to the data obtained. To carry out this work is necessary to know the geometry of each species that may exist on the surface of the catalyst and the transition states of the reactions that lead from one species (or combination of species) to another. This is know the key points of a reaction (activation energy and reaction enthalpy). With all data obtained was proposed a microkinetic model of the process and analysis this to obtain a reduced model, equivalent to a mechanism. With this model it is possible to obtain a simulation of the temporal evolution of each species, both in gas phase on the surface, depending on initial conditions. All this information is useful to know how the mechanism works and the evolution of products depending on the temperature or the oxygen-ammonia ratio. To carry out this thesis has used the density functional theory (DFT) implemented in VASP code on a model of a periodic cell of 2 ¡Á 2 with four layers of metal where the two more superficial are entirely free, being able to deform and adapt the molecule adsorbed. The Encut and k-points used are 400 eV and 5 ¡Á 5 ¡Á 1, respectively.This thesis is divided in three chapters. The first examines and compares the dehydrogenation of ammonia on platinum in the faces 100 and 111. The second chapter examines and compares the dehydrogenation on platinum, palladium and rhodium on both sides, 100 and 111. And the third chapter examines the process of ammonia oxidation on Pt(100).The first part has been carried out a systematic study of adsorption and the relative stability of the ammonia and the species of dehydrogenation on the surfaces of Pt (111) and Pt (100). Different adsorption geometries and positions have been studied. The vibrational spectra of various fragments of ammonia have been calculated and were compared with the experimental data available. The adsorption of NH3 is on top position and for the NH2 is on bridge and it is the most stable on Pt (100) than on Pt (111). For the NH and N are adsorbed on the hollow site. There is a considerable difference in the energy of adsorption of NH2 on both sides. This difference is mainly explained by the geometry that takes the kind on both sides. Being much more stable on the 100 side than on the face 111. Accordingly, the platinum surface determines the most stable species NHx: On Pt(100) has more affinity NH2 species, whereas species prefer NH Pt(111).The second part extends the study of the dehydrogenation to other metals such as Palladium and Rhodium. The different adsorption geometries and positions have been studied for the intermediate of ammonia dehydrogenation (NHx, x=0-2). The six surfaces studied, the NH3 adsorbs preferably on the top position, the NH2 on bridge, NH and N on hollow. However, the adsorption energies of the fragments NHx fluctuate considerably from one surface to another. All species absorbs more strongly on the face 100 than on face 111. The Rh(100) is the surface that provides maximum stability for the different NHx species. The reaction energy, the activation energy and the geometry of the transition state for the successive of ammonia dehydrogenation (NHx ¡ú NHx-1) have been determined, which allows calculating the rate coefficients. Our results prove that the reaction is structure sensitive. As a general trend, the first step of dehydrogenation is the limiting step, especially for palladium. According to the experimental data Rhodium is a good catalyst for the decomposition of NH3 compared to Pt and Pd. It has also been observed a linear relationship between the potential energy of the transition state and the adsorption energy of the products. The third part studies the ammonia oxidation on Pt(100). The conversion of NH3 leading to NHx intermediate species that reacts with adsorbed oxygen species and ultimately the formation of the products (NO, N2O, N2 and H2O) that it has been systematically calculated. The reaction comes through an imine mechanism, while the classical mechanisms postulated by Bodenstein and Andrussow (nytroxyl and hydroxilamine, respectively) as reaction intermediates can be discarded. The activation energy for the oxidative ammonia dehydrogenation on Pt(100) has been drastically reduced compared to the non-oxidative ammonia dehydrogenation. The barriers of ammonia dehydrogenation are greatly favored by the O-assisted way than the OH-assisted way. The final products are formed by recombination of adsorbed Nitrogen with N (N2), O (NO) and NO (N2O). The water is formed through the recombination of two adsorbed OH, regenerating adsorbed oxygen. The limiting step in the oxidative ammonia dehydrogenation is the first step, abstraction of the first proton of ammonia (NH3¡úNH2+H). While the nitric oxide desorption is the rate determining step (rds) of the process. We calculated the reaction rate coefficients of elementary steps involved in the reaction mechanism allows doing a microkinetic analysis. The simulations carried out with the microkinetic model describe well the experimental distribution of products obtained at different temperatures, depending on the time and the ratio of initial NH3/O2. Getting a temporal distribution of each species in gas phase and on the surface.