Promoting the protonation step on the interface of titanium dioxide for selective photocatalytic reduction of CO2 to CH4 by using red phosphorus quantum dots

Enhancing the selectivity of hydrocarbon in CO2 is a great challenge. Herein, taking widely-used and highly-stable TiO2 as an example, we found that the protonation step, the key step for CH4 production, can change from endoergic to exoergic by using red phosphorus quantum dots. Consequently, the main product in CO2 reduction can be shifted from CO into CH4. The preparation method is very simple, which just ultrasonically treating the red P in the presence of TiO2. With an initial rate of CH4 production of 4.69 µmol·g−1h−1, under simulated solar light, it manifests a significant 49.4-fold enhancement of CH4 yield over TiO2. Density functional calculation indicates that the red P optimizes the surface electronic structure. The Gibbs free energy for CHO* formation (−1.12 eV) becomes lower than the desorption energy of the CO (−0.01 eV) when red P is introduced. This indicates that the CO intermediates on the surface are rapidly protonated to produce CHO*. Subsequently, the CHO* will be converted into CH4 instead of being desorbed from the surface to produce CO. This study demonstrates that red P quantum dot is a promising candidate for the development of efficient photocatalyst for CO2 photoreduction to CH4 under solar light irradiation.