(29a) Selectivity of Photocatalytic Conversion of Carbon Dioxide Modulated By Surface Ligands on Cu2O/TiO2 Particles

Conversion of CO₂ into CO and hydrocarbon using solar irradiation has been intensively studied, yet efficiency and selectivity of CO₂ photoconversion remains to be improved. In photocatalysis, CO₂ and CO, which is intermediate state of CO2 reduction, adsorbed on the catalyst surface plays an important role in determining activity and selectivity because stabilized CO₂ and CO can react with adsorbed proton or hydroxyl groups without detachment. In this study, we carried out gas-phase conversion of CO₂ using Cu/TiO₂ nanoparticles as photocatalyst, showing significant enhancement when ligand is passivated on the surface. Interestingly, CH₄ production rate increased 2 times and CO production rate decreased when taurine passivated on Cu/TiO₂ photocatalyst. However, CO production rate increased when ethylenediamine treated. These results proved by in-situ Fourier-transform infrared spectroscopy (FT-IR) under photocatalytic CO₂ conversion condition. In addition, density functional theory (DFT) calculation shows surface ligand can control the selectivity of CO₂ conversion by changing of CO binding strength. These findings provide a platform for the design of surface chemistry on photocatalytic conversion of CO₂.

From this work, I demonstrate control for the selectivity of CO₂ conversion in photocatalyst using taurine and ethylenediamine ligands treatment. The taurine treatment enhance the CO binding strength of Cu₂O surface, so the CH₄ selectivity is 2 times enhanced. However, the CO selectivity is dramatically enhanced when ethylenediamine treated. These changes attributed to the control of CO and COOH binding strength of Cu₂O surfaces. Therefore, we investigate about the correlation between the surface interaction of the ligands and the selectivity of CO₂ photocatalysts. This work is proved by situ DRIFTS, photoactivity measurements and DFT calculation method. These DFT calculation results also proved by photocatalytic reaction and DRIFTS measurements. Also, the important finding was that CO₂^-, which is primary CO₂ reduction state, changed to different intermediate state. If the strong CO binding condition, the CO₂^- would be changed to bidentate or polydentate carbonate form in taurine passivation, but if the weak CO binding condition, the CO₂^- could converted to CO gas immediately in ethylenediamine passivation. This study has provided new information to correlate the surface chemistry with CO₂ reduction activity by Cu₂O/TiO₂. The ligand passivation appears to be a promising method for CO₂ reduction, and therefore we can control the activity and selectivity of CO₂ reduction.