Influences of Calcination Atmosphere on Nickel Catalyst Supported on Mesoporous Graphitic Carbon Nitride Thin Sheets for CO Methanation

Nickel (Ni) catalysts supported on mesoporous graphitic carbon nitride (mpg-C₃N₄) were synthesized through simple impregnation method with air and nitrogen calcination atmosphere for CO methanation. The effects of pretreatment gas on catalyst structure, surface characteristics, and Ni species reducibility were investigated. Under air-calcination condition, the increase in specific surface area of the catalyst can be ascribed to the creation of mesopores and exfoliation of bulk mpg-C₃N₄ to form thin sheets. However, excessive Ni content on the catalyst accelerated the decomposition of the mpg-C₃N₄ support during calcination. The catalysts calcined in nitrogen showed lower surface area and fewer number of pores compared to air-treatment. The Ni/mpg-C₃N₄ catalyst calcined in air with Ni loading 10% exhibited enhanced medium-temperature activity for CO methanation with 79.7% CO conversion and 73.9% CH₄ selectivity. This finding can be explained by the formation of mpg-C₃N₄ thin sheets, which increased the number of catalyst active sites. The CO methanation performance of Ni/mpg-C₃N₄ catalysts calcined in air was superior to those calcined in nitrogen. Interestingly, CO₂ formed by water-gas shift reaction at 320 °C also contributed to the overall methane formation through CO₂ methanation. Therefore, mpg-C₃N₄ thin sheets can be an interesting support for nickel catalyst for CO_x methanation.