Development of a Lower Energy Photosensitizer for Photocatalytic CO2 Reduction: Modification of Porphyrin Dye in Hybrid Catalyst System

A series of Zn–porphyrin dyes was prepared and anchored onto a TiO2 surface to complete a dye-sensitized photocatalyst system, Zn–porphyrin-|TiO2|-Cat, and tested as lower energy photosensitizers for photocatalytic CO2 reduction. Three major synthetic modifications were performed on the Zn–porphyrin dye to obtain a lower energy sensitization and improve the catalyst lifetime. We found that incorporating acetylene and linear hexyl groups into the Zn–porphyrin core allowed facile lower energy sensitization, and the addition of the cyanophosphonic acid as an anchoring group gave the long-term dye stability on the TiO2 surface. Under irradiation with red light of >550 nm and a light intensity of 207 mW/cm2, the hybrid ZnP CNPA catalyst showed a TONRe of ∼800 over an extended time period of 90 h. The photocatalytic activities of porphyrin hybrids differ greatly with the binding strength of the anchoring groups of dye and spectral range of the irradiated light and its intensity.