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⁻¹·h⁻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.