Estudo de catalisadores do tipo Ni-Ga para a síntese de metanol a partir da hidrogenação do CO>sub<2>/sub< em baixas pressões

Abstract

Given the industrial development and large population growth in the last years, the uncontrolled emission of greenhouse gases, especially CO2, has generated great concern for the environment and life on Earth. Thereby, policies for containment and CO2 utilization began to be widely studied, as in the case of some reactions where CO2 is converted into value-added products. Among several options for its use, the hydrogenation of CO2 to produce methanol has drawn a lot of attention because methanol is an important raw material for the industry and has a growing demand in the world market. Cu-based catalysts are the most used catalyst in this reaction, but more recently the discovery of Ni-Ga type catalysts with good activity at low pressures has grabbed the attention of the scientific community. Despite being promising, little has been studied about these catalysts and how they act in the reaction. In this context, this work aims to study supported and unsupported Ni-Ga alloy catalysts in methanol synthesis through CO2 hydrogenation at low pressures. The first part of this work seeks to understand the behavior of the unsupported Ni5Ga3 alloy, for example, which physicochemical characteristics of the alloy exert the biggest influence on the reaction, and which intermediates and reaction mechanisms take place on it. In this step, it was noticed that the particle size exerts a strong influence on the catalytic activity and that the reaction takes place through two reaction routes on the alloy, rWGS, and formate. The second part of the work explored the Ni5Ga3 alloy supported on basic oxides, in order to improve the catalytic activity, since the conversions and selectivity were low in the unsupported alloy. The use of supports improved the catalytic activity and the alloy supported on ZrO2 showed the best result, and it was concluded that it is due to the interaction strengths of the reaction intermediates and reagents with the Ni5Ga3-ZrO2 interface. In the third step, the behavior of the alloy on mesoporous inert supports with high surface area (MCM-41 and SBA-15) was studied. It was observed through in situ analysis techniques that the decrease in the density of metallic sites disfavored the adsorption of CO on the surface and, therefore, lower selectivity and conversion to methanol was obtained compared to the reference sample. The fourth and last part of the work united the best basic support used in the second part with the best among the high-area mesoporous supports used in the third part, preparing new catalysts through the ALD technique of ZrO2 on pure SBA-15 or an already impregnated SBA-15 with Ni5Ga3. Among the two groups of prepared materials, it was noted that more ALD cycles were favorable to produce methanol intermediates and that the impregnation of Ni5Ga3 after ALD of ZrO2 on SBA-15 led the reaction to follow only the formate route, different from what happened with the other group, being very desirable for a high selectivity for methanol.