Ni-Mn-Al catalysts obtained from layered double hydroxides for CO2 hydrogenation

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). Preparation of the studied 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 their physical and chemical properties were evaluated. After calcination and in-situ reduction, the oxides were evaluated as catalysts in the CO₂ hydrogenation reaction, which is a possible alternative to the abatement of global warming. It was concluded that Mn increased the specific surface 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, 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. It was also stable during 16 h on stream, at the temperature of maximum conversion. At low temperatures, this catalyst was one of the best reported in the literature, with CO₂ conversion and CH₄ selectivity of 40 % and 100 % at 225 °C, respectively. Additionally, 50 % conversion was achieved at 248 °C (T₅₀). 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%).