Recently, there has been an increasing interest in the research of photocatalytic reduction of CO 2 with H 2 O, an innovative way to simultaneously reduce the level of CO 2 emissions and produce renewable and sustainable fuels. Titanium dioxide (TiO 2 ) and modified TiO 2 composites are the most widely used photocatalysts in this application; however, the reaction mechanism of CO 2 photoreduction on TiO 2 photocatalysts is still not very clear, and the reaction intermediates and product selectivity are not well understood. This review aims to summarize the recent advances in the exploration of reaction mechanism of CO 2 photoreduction with H 2 O in correlation with the TiO 2 photocatalyst characteristics. Discussions are provided in the following sections: (1) CO 2 adsorption, activation and dissociation on TiO 2 photocatalyst; (2) mechanism and approaches to enhance charge transfer from photocatalyst to reactants (i.e., CO 2 and H 2 O); and (3) surface intermediates, reaction pathways, and product selectivity. In each section, the effects of material properties are discussed, including TiO 2 crystal phases (e.g., anatase, rutile, brookite, or their mixtures), surface defects (e.g., oxygen vacancy and Ti 3+) and material modifications (e.g., incorporation of noble metal, metal oxide, and/or nonmetal species to TiO 2 ). Finally, perspectives on future research directions and open issues to be addressed in CO 2 photoreduction are outlined in this review paper.

NO reduction by CO was investigated over CuO/γ-Al2O3, Mn2O3/γ-Al2O3, and CuOMn2O3/γ-Al2O3 model catalysts before and after CO pretreatment at 300 °C. The CO-pretreated CuO-Mn2O3/γ-Al2O3 catalyst exhibited higher catalytic activity than did the other catalysts. Based on X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), UV/Vis diffuse reflectance spectroscopy (DRS), Raman, and H2-temperature-programmed reduction (TPR) results, as well as our previous studies, the possible interaction model between dispersed copper and manganese oxide species as well as γ-Al2O3 surface has been proposed. In this model, Cu and Mn ions occupied the octahedral vacant sites of γ-Al2O3, with the capping oxygen on top of the metal ions to keep the charge conservation. For the fresh CuO/γ-Al2O3 and Mn2O3/γ-Al2O3 catalysts, the -Cu-O-Cu- and -Mn-O-Mn- species were formed on the surface of γ-Al2O3, respectively; but for the fresh CuO-Mn2O3/γ-Al2O3 catalyst, -Cu-O-Mn- species existed on the surface of -Al2O3. After CO pretreatment, -Cu-□-Cu- and -Mn-□-Mn- (□ represents surface oxygen vacancy (SOV)) species would be formed in CO-pretreated CuO/γ-Al2O3 and CO-pretreated Mn2O3/γ-Al2O3 catalysts, respectively; whereas -Cu-□-Mn- species existed in CO-pretreated CuO-Mn2O3/γ-Al2O3. Herein, a new concept, surface synergetic oxygen vacancy (SSOV), which describes the oxygen vacancy formed between the individual Mn and Cu ions, is proposed for CO-pretreated CuO-Mn2O3/γ-Al2O3 catalyst. In addition, the role of SSOV has also been approached by NO temperature-programmed desorption (TPD) and in situ FTIR experiments. The FTIR results of competitive adsorption between NO and CO on all the CO-pretreated CuO/γ-Al2O3, Mn2O3/γ-Al2O3, and CuO-Mn2O3/γ-Al2O3 samples demonstrated that NO molecules mainly were adsorbed on Mn2+ and CO mainly on Cu+ sites. The current study suggests that the properties of the SSOVs in CO-pretreated CuO-Mn2O3/γ-Al2O3 catalyst were significantly different to SOVs formed in CO-pretreated CuO/γ-Al2O3 and Mn2O3/γ-Al2O3 catalysts, and the SSOVs played an important role in NO reduction by CO.