Simultaneous impregnation of Ni and an additive via one-step melt-infiltration: Effect of alkaline-earth metal (Ca, Mg, Sr, and Ba) addition on Ni/γ-Al₂O₃ for CO₂ methanation

As part of carbon energy recycling, the CO₂ methanation process in the power to gas technology, which converts the electricity grid into a gas grid [CO₂ + 4H₂ (produced by surplus electricity) → CH₄] using a catalytic process, has been developed in the countries such as Germany and Denmark. To achieve an efficient catalytic process, the design of low-cost and high-efficiency catalysts is required. In this study, highly loaded Ni/Al₂O₃ catalysts (30 wt%) were prepared using the alkaline-earth metals (Mg, Ca, Sr, and Ba) via one-step melt-infiltration (OSMI) method. The Ni active sites were investigated, and the effectiveness of each alkaline-earth metal (Ca, Mg, Sr, and Ba) was determined. The catalytic activity was investigated in the temperature range of 275–400 °C and weight gas hourly velocity (WHSV) of 160,000 mL·g_cat⁻¹·h⁻¹ under atmospheric pressure. Ni and alkaline-earth metals were uniformly dispersed into the pore structure of the alumina support. The average metallic Ni particle size of each catalyst was similar (~11 nm), and no severe Ni sintering was observed even at a Ni loading of 30 wt%. Mg-promoted Ni/Al₂O₃ catalyst was unsuitable for the OSMI method because the reduction of the Ni active sites was not completely accomplished at 400 °C. Ca-promoted Ni/Al₂O₃ catalyst afforded the best catalytic activity among other metal-promoted Ni/Al₂O₃ catalysts. Ca increased the CO₂ adsorption amount and reducibility of the Ni active sites, and the optimum Ca loading was 5 wt%. However, the over-loading of Ca (10 wt%) was harmful to the catalytic activity of the Ni active sites.