Efficient photocatalytic CO2 methanation via promoted conversion of bridge *CO over surface K+ on ultrathin TiO2.

• Revealed the role of surface K+ for promoting the conversion of bridge *CO to -CHO on TiO 2. • Highly dispersed K+ was introduced on the surface of ultrathin TiO 2 nanosheets via an in situ topochemical reactions etching route. • The photoreduction rate for CH 4 of optimized photocatalyst is 3.39 times higher than that of P25 and the selectivity for methane was 96.02%. The photoreduction rate for CH 4 of optimized photocatalyst is superior to that of most of the reported TiO 2 -based photocatalysts. The efficient conversion of CO 2 to carbon-neural fuels by photocatalysts can effectively address the increasingly prominent challenges of environmental pollution and energy shortage. TiO 2 is considered a promising photocatalyst. However, its disadvantages, such as poor adsorption of CO 2 and low carrier separation efficiency, inhibit its performance and selectivity in photocatalytic CO 2 reduction. Herein, K+-modified anatase TiO 2 ultrathin nanosheet photocatalysts were obtained via a reverse etching method using K 2 Ti 8 O 17 as the precursor. The selectivity of the optimized photocatalyst for methane was 96.02% in a gas–solid system without additives, with an excellent photocatalytic rate of 42.3 μmol·g−1·h−1, which was 3.39 times higher than that of P25. The effects of K+ on photocatalytic behaviors and the selectivity of CH 4 was systematically investigated. The test results showed that during treatment with formic acid, a large amount of K+ from the layered K 2 Ti 8 O 17 were removed and the original structure was destroyed. Therefore, highly dispersed TiO 2 nanosheets were obtained, with a small amount of K+ remaining on the TiO 2 surface. The well-defined rectangular shapes of TiO 2 reduce the transmission distance of photogenerated carriers, whereas the presence of K+ increase the chemisorption of CO 2 molecules, thereby activating CO 2 , promoting the conversion of bridge *CO to -CHO, and increasing CH 4 production. This study provides insightful references for the preparation and modification of traditional photocatalysts for photocatalytic CO 2 reduction reactions. [Display omitted] [ABSTRACT FROM AUTHOR]