Weak interaction between TiO2 and CO2 molecules is detrimental to CO2 photoreduction. To alleviate this drawback, ceria is usually exploited as a basic promoter, but fundamental insights into the correlation of ceria-tuned CO2 adsorption and the resulting activity of photoreduction are lacking. In this work, highly dispersed CeO2/TiO2 and bare TiO2 catalysts were fabricated and their structural, surface, and optical properties and activity for CO2 photoreduction were explored. Microcalorimetric measurement and in situ infrared spectroscopy were used to reveal the strengths and states of CO2 adsorption and the course of photoreduction of CO2 with H2O vapor. Monodentate carbonate (m-CO32-), bidentate carbonate (b-CO32-), and bidentate bicarbonate (b-HCO3-) are found to be the main surface species for the coadsorption of CO2 and H2O on catalyst surfaces. The presence of CeO2 containing Ce3+ strengthens the bonding of CO2 with catalyst surfaces and increases the production of b-CO32- and b-HCO3- species. Unlike m-CO32-, b-CO32- and b-HCO3- surface species could readily be transformed to surface CO2- in the presence of H2O under simulated sunlight irradiation. This might be attributed to the fact that the CO2 segment in the two species is bound to Ti/Ce atoms that have reductive capabilities under photoirradiation. In addition, the presence of CeO2 containing Ce3+ facilitates photogenerated charge separation. As a result, ceria-tuned CO2 adsorption and enhanced charge separation are jointly responsible for the increased activity of CeO2/TiO2 catalysts. (C) 2016 Elsevier Inc. All rights reserved.