Understanding the interaction of WO3 with CO2 and H2O is vital for clarifying its role in the photocata-lytic reduction of CO2. In this study, we employed density functional theory to investigate the interaction of CO2 andH20 with both perfect and defective monoclinic WO3（001） surfaces. The interactions of co-adsorbed CO2 and H2Owere also studied. The major finding is that the presence of oxygen vacancies and co-adsorbed CO2 or H2O can sig-nificantly increase the stability of CO2 and H2O on the WO3（001） surface. A defective WO3（001） surface is more ca-pable of adsorbing a single CO2 or H20 molecule than a perfect WO3（001） surface, and H20 adsorbed onto a defec-tive WO3（001） surface spontaneously dissociates into a hydrogen atom and a hydroxy group. The presence ofco-adsorbed H20 can increase the stability of CO2 on the WO3（001） surface, while the presence of the co-adsorbed CO2 can increase the stability of H20 on WO3（001） surface. The analysis of the bonding mechanisms shows that thecharge redistribution between the adsorbate and the WO3（001） surface containing oxygen vacancies and co-adsorbed CO2 or H2O is stronger than that between the adsorbate and the perfect WO3（001） surface; thus, adsorption energy ishigher in the former case. The results will be useful for designing WO3 photocatalysts, as well as for an atomis-tic-level understanding of the photocatalytic reduction of CO2.
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