Alkaline earth metal modified nickel nanoparticles supported on exfoliated g-C3N4 for atmospheric CO2 methanation

Abstract

Alkaline earth metals such as magnesium (Mg) and calcium (Ca) are promising catalyst promoters for CO2 methanation owing to their exceptional basic characteristics. This study details the development of nickel-based catalysts promoted with Mg and Ca supported by exfoliated graphitic carbon nitride (eg-C3N4). The catalysts were synthesised using a facile impregnation technique and characterised using N2 physisorption, XRD, TEM, XPS, H2-TPR and CO2-TPD. The Mg-promoted catalyst showed a greater number of mesopores, better dispersion of small Ni NPs, stronger metal-support interaction and enhanced moderate basic sites compared to Ni/eg-C3N4. Conversely, the presence of bulk CaCO3 species in the Ca-doped catalyst negatively affected the textural properties and basicity. Ni-Mg/eg-C3N4 catalyst demonstrated superior methanation activity with CO2 conversion (XCO2) of 77% and CH4 selectivity (SCH4) greater than 99% at 322 °C. This is mainly attributed to the creation of a NiO-MgO solid solution which enhances the interaction between metal and support, leading to the promotion of small and highly dispersed Ni NPs. Moreover, the incorporation of an Mg promoter generates Lewis basic sites and alters the basicity of the eg-C3N4 support, increasing the number of effective CO2 adsorption sites. The synergy between the Mg dopant and eg-C3N4 support significantly contributes to the high activity and stability for low-temperature CO2 methanation.