Catalisadores de Ni-Mn-Al oriundos de HDL nas reações de metanação de CO2 e de deslocamento gás-água

Abstract

It is widely agreed among researchers that global warming is directly linked to increased concentrations of carbon dioxide (CO2) in the atmosphere. As such, technologies aimed at reducing CO2 emissions have been extensively studied. In this study, bimetallic (Ni-Mn, Ni-Al, Mn-Al) and trimetallic (Ni-Mn-Al) precursors were synthesized by a coprecipitation method to obtain layered double hydroxides (LDH). The preparation of these catalysts is easily scalable, due to the relative simplicity and reproducibility of LDH synthesis. The effects of pH and molar proportion of the metals on the catalyst physical and chemical properties were evaluated. After calcination and in-situ reduction, the oxides were evaluated as catalysts in the CO2 hydrogenation and water gas shift reactions, which are possible alternatives to the abatement of global warming. It was concluded that Mn increased the specific area of the oxides and the content of basic sites in the catalyst, as well as facilitated the onset of the materials reduction. The most active catalyst for CO2 methanation, obtained at lower pH, had only 6 mol% Mn and showed CO2 conversion and CH4 selectivity equal to 82% and 100% at 350 °C, respectively, remaining stable for 16 h in this condition. At low temperatures, this catalyst was one of the best performers compared to others in the literature, with CO2 conversion and CH4 selectivity of 40% and 100% at 225 °C, respectively. At temperatures higher than 400 °C, the reverse water gas shift reaction, which competes with methanation, was inhibited in solids with higher Mn contents (≥ 40 mol%). In WGSR, the bimetallic catalyst, composed of Ni and Al, was the one that presented the best catalytic performance (XCO = 86%, = 84%) at 300°C. This fact is interesting for catalysts applied in the WGSR in a single step, which should have good performance at medium temperatures. In addition, at 370°C, its performance was better than that of the commercial iron-based catalyst.