Understanding and Tuning the Effects of H2O on Catalytic CO and CO2 Hydrogenation

Catalytic CO_x (CO and CO₂) hydrogenation to valued chemicals is one of the promising approaches to address challenges in energy, environment, and climate change. H₂O is an inevitable side product in these reactions, where its existence and effect are often ignored. In fact, H₂O significantly influences the catalytic active centers, reaction mechanism, and catalytic performance, preventing us from a definitive and deep understanding on the structure-performance relationship of the authentic catalysts. It is necessary, although challenging, to clarify its effect and provide practical strategies to tune the concentration and distribution of H₂O to optimize its influence. In this review, we focus on how H₂O in CO_x hydrogenation induces the structural evolution of catalysts and assists in the catalytic processes, as well as efforts to understand the underlying mechanism. We summarize and discuss some representative tuning strategies for realizing the rapid removal or local enrichment of H₂O around the catalysts, along with brief techno-economic analysis and life cycle assessment. These fundamental understandings and strategies are further extended to the reactions of CO and CO₂ reduction under an external field (light, electricity, and plasma). We also present suggestions and prospects for deciphering and controlling the effect of H₂O in practical applications.