Collisional Electron Transfer Route between Homogeneous Porphyrin Dye and Catalytic TiO₂/Re(I) Particles for CO₂ Reduction

Dye detachment issue in the TiO2-mediated dye-sensitized photocatalytic system engenders an electron injection route based on collisional quenching between the detached solution-phase dye and dispersed n-type TiO2 particles, in addition to the general fast electron injection pathway from dye chemisorbed on TiO2. For porphyrin-sensitized hetero-binary hybrids prepared by mixing a solution-phase Zn porphyrin sensitizer (ZnP) and heterogeneous Re(I) molecular catalyst-immobilized TiO2 particles (TiO2/Re(I)), we found that without any dye immobilization on the TiO2 surface, the dissolved porphyrin dye can effectively transport its excited-state electrons to the heterogeneous catalytic TiO2/Re(I) particles via a collisional electron transfer pathway, which is relatively unstudied and thus an interesting topic in the dye-sensitized semiconductor photocatalytic system. The better light-harvesting ability of porphyrin in solution and the TiO2-induced stabilization of the molecular Re(I) catalyst raised the photochemical CO2-to-CO conversion activity of the semiheterogeneous system (porphyrin + TiO2/Re(I) catalyst) above that of the homogeneous system (porphyrin + Re(I) catalyst) and the heterogeneous ternary hybrid (porphyrin/TiO2/Re(I) catalyst).