Avaliação de catalisadores de cobre derivados de hidróxidos duplos lamelares para reação de deslocamento água-gás

Abstract

Hydrogen is considered the fuel of the future, due to its high energy density and its application in fuel cells. Hydrogen is mainly obtained by light hydrocarbons reforming. In this process, the reaction product is a gas mixture composed by hydrogen, carbon monoxide and dioxide. However, carbon monoxide (CO) concentration in this mixture is extremely high for some applications, such as ammonia synthesis and fuel cells processes, where CO acts as a poison for some types of catalysts. To remove CO excess and improve hydrogen yield in reforming processes, it is used the Water-Gas Shift Reaction, which occurs in two steps: at high temperatures, around 400°C with Fe-Cr catalysts; and at low temperatures, around 250°C with Cu-Zn catalysts. The goal of this work was to develop and to evaluate copper, zinc and aluminum based catalysts derived from Lamellar Double Hydroxides (LDH) for the low temperature Water-Gas Shift Reaction. LDH, also called hydrotalcite-like materials, have useful properties for catalysis, such as homogeneous metal distribution throughout its structure, improving thermal stability and catalytic activity. An experimental design for the composition variables, Cu/Zn and (Cu+Zn)/Al molar ratios, was used, besides the aging treatment influence over LDH, which was verified by replicating the experiments in two groups: with and without aging. The catalysts were prepared by the co-precipitation method; and it was analyzed the influence of preparation conditions over their physical-chemical properties, such as composition, specific area, porous volume and diameter, as well as catalytic activity in low temperature Water-Gas Shift Reaction. The presence of Cu/Zn/Al LDH phase in all prepared catalysts was verified. In some samples, there were a few impurities, like malachite and aurichalcite in aged catalysts, and zinc and copper oxides in not aged materials. The aging step also affected the way the Cu/Zn and (Cu+Zn)/Al molar ratios influence the textural properties and the catalytic activity. In aged catalysts, the specific area does not show a strong dependency on the composition. However, these catalysts appear to have higher thermal stability in long period reactions. Experimental design allowed a better study of the influence of composition variables on catalysts final characteristics, aiding future studies in the synthesis of this type of material.