Photocatalytic Hydrogen Production under Water Vapor FeedingA Minireview

Recent advances in photocatalytic water splitting in particulate semiconductor systems have been tremendous in terms of efficiency and scalable photoreactor design. In general, photocatalyst powders are immersed in liquid water, leading to higher efficiency than gas-fed water splitting systems. Conducting a gas-phase photocatalytic reaction under water vapor, however, has several advantages versus liquid water, such as easy scalability and suppression of leaching of the catalyst components. Here, we offer a historical overview of photocatalytic water splitting under vapor feeding. Moreover, the principles of water adsorption are reviewed to understand the basic phenomena occurring on the surface of semiconductor powders. The critical role of relative humidity is identified, including water on the surface for ion conductivity, which can close the electric circuit between cathodic and anodic reaction sites. Finally, on the basis of the cross-sectional discussion of reported works in both water vapor splitting and physical property of the adsorbed water layer, how to develop the photocatalytic water vapor splitting system and the perspective of the CO2 and N2 reduction system using water vapor as an electron donor are provided.