Carbon dioxide enabled hydrogen storage by methanol: Highly selective and efficient catalysis with well-defined heterogeneous catalysts

The use of hydrogen can reduce CO2 emissions and alleviate energy shortages, but large-scale storage and transfer of hydrogen remain obstacles to utilization. Hydrogenation of CO2 to CH3OH and dehydrogenation of CH3OH to H2 and CO2 constitutes a "carbon neutral" cycle for hydrogen storage and release with CO2 and CH3OH. Highly efficient heterogeneous catalysts are required for optimal hydrogen storage systems and improved cycling efficiency. Herein, the goal of this review is to provide a comprehensive discussion for well-defined heterogeneous catalysts, focusing on their distinct active site structures, electronic states, and the consequential impact of these characteristics on catalytic activities when employed with different carrier materials. The structure–activity relationships and reaction mechanisms of the catalysts are investigated with advanced characterization techniques. Moreover, some promising but incipient pathways, such as photothermal catalysis, are briefly discussed. It is expected that this review will inspire the design of highly efficient heterogeneous catalysts for promising CO2-CH3OH hydrogen storage systems with closed carbon cycles.