On the origin of methane in the RWGS: Insights into selectivity on Mo2C catalysts

The reverse water-gas shift (RWGS) reaction is thermodynamically limited due to its endothermic nature in terms of CO 2 conversion. While higher reaction temperatures and increased H 2 /CO 2 feed ratios can enhance CO 2 conversion, a more efficient approach involves reaction intensification via the removal of the by-product (H 2 O), shifting the equilibrium per Le Chatelier’s principle. Ex situ H 2 O removal, combined with recycling of unreacted gases leads to CO-enriched feeds, which in turn promotes methanation, especially at lower temperatures. In this study, the origin of CH 4 formation was investigated under both conventional and CO-enriched RWGS feed conditions using (isotopically labelled) transient, breakthrough, experiments over a Mo 2 C catalysts, known for its high RWGS activity. In CO-free feeds, methanation is highly suppressed, even under elevated pressure. In contrast, a CO enriched feed significantly enhanced CH 4 formation, particularly during reaction initiation. Using isotopically labelled C 13 O 2 , two key insights were revealed: (1) CH 4 is primarily formed via adsorbed *CO species (either adsorbed from the gas phase or via CO 2 dissociation), and (2) in the presence of an extra reducing agent in the feed (CO), the activity of the catalysts is improved by facilitating the removal of surface *O species. These findings underscore the importance of improving the hydrogenation (or *O removal) capacity of Mo 2 C catalysts to boost performance under both conventional and intensified RWGS conditions. • Labeled CO 2 is used to elucidate the origin of methane in the RWGS reaction on Mo2C catalysts. • Methane is predominantly formed from *CO surface species. • Increasing the hydrogenation function is a viable strategy to improve catalyst performance.