Determinação de propriedades e estruturas de catalisadores de sulfeto de molibdênio suportados em MgO por cálculos ab initio

Abstract

With respect to the reduction of pollutant emission of vehicles, the main process of hydrotreatment is the hydrodesulfurization, HDS, in which the sulfur atom of the organosulfur molecules is adsorved on the catalyst and reacts with hydrogen forming hydrogen sulfide (H2S) and heteroatomfree hydrocarbons. The HDS reactions are exothermic and irreversible and their mechanisms involve hydrogenolysis reactions – the break of CS bond – and the hydrogenation – saturation of double bonds. There are many divergences in the literature about the mechanism of these reactions. The interest about these types of reactions and the heterogenous catalysis in the material field is in constant expansion due to the possibility of producing several types of materials with great applicabilities and inferior costs. The studies about HDS seek the development of catalysts with a higher capacity to promote a more effective removal of sulfur besides the elucidation of their mechanisms. The catalysts more commonly used are compounds which possess MoS2 structures as the active phase. Even though it has catalytic activity in the bulk structure, the MoS2 is generally supported on a surface with an extended area, usually oxides such as alumina, which is the most utilized. There are many recent studies γ reporting the use of other types of oxides such as TiO2, silica, zeolites, ZrO2, MgO and mixed oxides. In this present work, a proposition of two types of catalyst structures of molybdenum sulfides supported on MgO is done by means of ab initio calculations. It is commonly accepted nowadays that the activity of HDS catalysts is greatly related to the existence of anionic vacancies of sulfur located majorly on the edges of the active phase, since the sulfurs of the basal planes are strongly bonded to permit the formation of these vacancies. Taking into account that the vacancy formation is a crucial step for HDS, we sought to obtain more precise structural information to assist the understanding of this reaction step. Thereby, we presented the study of the formation of the coordenative unsaturated sites on the edge of the molybdenum sulfide supported on MgO. Moreover, we discussed about the interaction between the sulfide and the support by varying the amount of the sulfide layers and the presence of the cobalt atom on the edge. Thereby, we sought to provide structural information on the theoretical models of MoS2 supported on MgO using DFT in order to contribute with studies in this context. In order to do that, we calculated the energy of the vacancy formation, the difference of the charge densities, pDOS and Bader charge analysis. We concluded that the support and the promoting atoms influence the formation of the vacancies on the edge of the molybdenum supported on MgO. The influence of these variables depends on the size of the layer, the proportions of the sulfur atoms on the edge and the type of the interaction of the sulfide on the support. In short, we can confirm that both act to decrease the energy of the vacancy formation, thus contributing to the improvement of this step.