Influence of Basic Solution on Carbonaceous Alumina Supported Nickel Catalysts for CO ₂ Methanation

In this study, chitosan was employed as a biopolymer template for the preparation of alumina supports via a sol-gel method for nickel-based catalysts in CO₂ methanation. The effects of ammonium hydroxide (NH₄OH) and sodium hydroxide (NaOH) coagulation agents on the structural formation of alumina were investigated using TGA, FTIR, CHN analysis, FESEM, XRD, N₂ adsorption-desorption isotherms, CO₂-TPD, NH₃-TPD, H₂-TPR, and XRF. The C-Al-NH₄OH support exhibited superior textural properties and enhanced reducibility of nickel species. However, it was observed that increasing the NH₄OH concentration accelerated chitosan adsorption onto aluminum species but hindered the formation of the alumina network, leading to reduced BET surface area and pore volume. Furthermore, TGA results indicated substantial aluminum loss during preparation in a NaOH coagulation bath due to the formation of water-soluble sodium aluminate species. The catalytic activity of Ni/C-Al catalysts for CO₂ methanation was evaluated in terms of CO₂ conversion and CH₄ selectivity. Process optimization was carried out using a face-centered central composite design response surface methodology (FCCCD-RSM) with reaction temperature, H₂/CO₂ ratio, and gas hourly space velocity (GHSV) as the primary variables. The optimized 30Ni/C-Al-3NH₄OH catalyst achieved a maximum CO₂ conversion of 93.2% and CH₄ selectivity of 97.1% under the optimal conditions at 386 °C, a H₂/CO₂ ratio of 6, and a GHSV of 7851 h^-1. In addition, the catalyst demonstrated excellent stability over a 48-h continuous operation.