Theoretical Study on the Mechanism of Photoreduction of CO2 to CH4 on the Anatase TiO2(101) Surface

Artificial photosynthesis of CO, has recently attracted intense attention as a potential solution for the energy crisis and global warming. However, the molecular mechanism of the reaction is quite complicated and is far from understood. We performed a first-principles calculation on the thermodynamically feasible formaldehyde pathway: CO2 -> HCOOH -> H2CO -> CH3OH -> CH4. The interconversion of the Cl molecules has been systematically investigated. We find that a two-electron process has a lower barrier than a one-electron process for the photoreduction of all of the molecules under investigation except for methanol. On the basis of the full potential energy surface for photoreduction of CO, to methane, the rate-limiting step is found to be the photoreduction of formic acid to formaldehyde, which contains the elementary step that has the largest kinetic barrier. It will be more efficient if CO instead of formic acid is the precursor of formaldehyde. Then the rate-limiting step becomes the photoreduction of CO, to CO. However, the barriers for the photoreduction of the organic molecules are all higher than the barriers for their photodecomposition reaction, which suggests that all of the Cl organic molecules are more easily oxidized than reduced. Thus, charge separation is crucial for improving the efficiency and selectivity of the reaction. The intertwining of photoreduction and photooxidation reactions might be one of the major reasons for the complexity and low efficiency of the reaction. On the basis of the calculations, a new mechanism for the reaction is proposed.