Elucidation of Pathways in the Conversion of CO2-Rich Syngas to Methanol over Cu-Based Catalysts

The efficient conversion of CO2-rich syngas to fuels remains challenging because of distinct thermodynamics between CO2 and CO hydrogenation. In this work, we investigate the structure–performance relationship and reaction mechanism of CO2-rich syngas conversion to methanol over a series of Cu-based catalysts, with CO2 contents ranging from 4% to 25%. We find that the CuZnZr catalyst exhibits the best overall performance because of the enhanced adsorption/activation of CO and CO2, resulting from the strengthened Cu–oxide interaction and promoted Cu–ZnO–ZrOx interface. The catalytic performance of CuZnZr is highly sensitive to both the Zn/Zr ratio and the CO2 content in the feed gas, with a Cu/Zn/Zr ratio of 6:3:1, giving the highest activity. We find that CO is mainly adsorbed on Cu sites, whereas CO2 is preferentially activated at Cu–ZnO/ZrOx interfacial sites to form formate and methoxy species. In CO2-containing syngas, CO*, HCOO*, and CH3O* coexist on the surface, and a small amount of CO2 effectively promotes the hydrogenation of HCOO* to CH3O*. Isotope-labeled experiments further confirm that CO2 not only undergoes the reverse water–gas shift reaction to form CO but also directly participates in methanol formation through oxygen-containing intermediates. Both the carbon and oxygen atoms in methanol can originate from the CO2 in feed gas, demonstrating a coupled hydrogenation mechanism involving both CO and CO2. In addition, a trace amount of water vapor increases the surface coverage of both HCOO* and CH3O* and promotes methanol formation, rather than only acting as a spectator. Therefore, the improved catalytic performance of CuZnZr originates from the optimized Cu–ZnO–ZrOx interface and the cooperative transformation of CO*, HCOO*, and CH3O*. These findings provide mechanistic insights and guidance for the design of efficient Cu-based catalysts in the direct methanol synthesis from CO2-rich syngas.