Effect of Ba and Mg as promoter on Ru/g-C3N4 catalyst for CO2 methanation

Abstract

Catalytic hydrogenation of CO₂ to produce value-added hydrocarbons is a promising strategy for reducing greenhouse gas emissions and producing renewable fuels and chemicals. In this study, Ru- g-C₃N₄ catalysts were modified with barium (Ba) and magnesium (Mg) promoters to improve the conversion efficiency of CO₂ and tune the products selectivity of the Ru/g-C₃N₄ catalysts. Two different synthetic approaches were used; namely, co-impregnation with Ru and pre-incorporation of Ba or Mg into the g-C₃N₄ lattice prior to Ru loading. The study revealed that Ba modification significantly improved the catalyst's performance, resulting in higher CO₂ conversion rates and the selective formation of CH₄. This enhancement is attributed to the electronic and structural modifications induced by doping Ba into the g-C₃N₄ lattice that facilitate CO₂ activation and hydrogenation. In contrast, adding Ba via co-impregnation partially covered Ru sites, promoting CO formation by stabilizing isolated Ru sites and small clusters, as confirmed by in-situ CO adsorption studies. Conversely, Mg had a negative effect, likely due to more pronounced coverage of Ru active sites, which suppressed catalytic performance. These findings emphasise the importance of selecting suitable promoters to optimise CO₂ hydrogenation catalysts, with Ba-doped Ru/g-C₃N₄ emerging as an effective system for methane synthesis.