Identificador: TDX:891
Autores: Gómez Díaz, Jaime
Resumen:
In this dissertation, the mechanisms that govern four essential industrial processes have been studied by means of Density Functional Theory (DFT). The processes are the following: Ostwald (HNO3 production), Degussa and Andrussow (HCN production) and MacArthurForrest (gold recovery). In these processes, ammonia is the main raw material or a precursor of it.<br/>The KohnSham equations that describe the model systems have been solved by means of VASP and GPAW using the RPBE functional. These packages make use of periodic boundary conditions to simulate an infinite solid as model of the real metal surface.<br/>Ostwald Process: The first step of this process, NO formation, has been studied on the (111) and (211) surfaces of Au, Cu, Ir, Pd, Pt and Rh metals. The reactions involved have been analyzed as well in the PtRh alloy (the one used in industry). The thermodynamic stability of Pt and Rh oxides as a function of the temperature and the pressure of oxygen has been described. Finally, the volatility of these oxides has been calculated to determine catalyst loses in form of PtO2 and RhO2 gases.<br/>Good selectivity towards NO formation has been found for Pt metal. The desorption of NO has been found determinant for several metals that produce NO on the surface but hardly leaves it. The addition of Rh impurities to Pt does not increase its reactivity, but Rh oxides are more stable than Pt ones and Rh is less prone to form volatile oxides.<br/>Degussa and Andrussow Processes: The formation of CN bonds from individual atoms and hydrogenated moieties on a series of transition and noble metals has been studied there.<br/>A comprehensive study on all the possible couplings between partially hydrogenated species, CHx and NHy, on Pt(111) surface is shown. The differences between the mechanism under the distinct conditions (oxygen presence or absence) have been stressed. Under nonoxidative conditions, the coupling of partially hydrogenated species is found as the most effective way to form the CN bond on Pt. Under oxygen presence conditions, dehydrogenations are eased and C, CH and N are the most stable species.<br/>Thus, C+N and HC+N appear as the favoured paths for HCN formation.<br/>For the remainder metals, the basic steps under anaerobic conditions have been analysed. Correlations between the adsorption energy of the different species with the adsorption energy of nitrogen atom has been found. The dissociation energy for the most important reactions correlates as well with the adsorption energy of carbon atom.<br/>MacArthurForrest Process: In this Chapter, the interactions of cyanides with gold (111) and (211) surfaces have been analysed. A comparison with carbon monoxide interaction has been stated. Cyanides are the responsible of the solution of gold from its minerals by the formation of the watersoluble [Au(CN)2]molecule.<br/>On the other hand, CO affinity for metallic gold is very weak.<br/>A Bader analysis on the atomic charges and Localized Density Of States (LDOS) and Electronic Localization Function (ELF) studies are shown.<br/>Almost no interaction between CO and gold have been observed. CN bonding with gold have been described as covalent according to atomic charges, bond length and electronic analysis. The mechanism proposed by experimental works: CNadsorption, etransfer, second CNadsorption and elimination of the soluble compound, is supported by our theoretical approach.
Repositori URV