A heterogeneous photocatalyst system that consists of a ruthenium complex and carbon nitride (C3N4), which act as the catalytic and light-harvesting units, respectively, was developed for the reduction of CO2 into formic acid. Promoting the injection of electrons from C3N4 into the ruthenium unit as well as strengthening the electronic interactions between the two units enhanced its activity. The use of a suitable solvent further improved the performance, resulting in a turnover number of greater than 1000 and an apparent quantum yield of 5.7% at 400 nm. These are the best values that have been reported for heterogeneous photocatalysts for CO2 reduction under visible-light irradiation to date.

A hybrid for the visible-light-driven photocatalytic reduction of CO2 using methanol as a reducing agent was developed by combining two different types of photocatalysts: a Ru(II) dinuclear complex (RuBLRu′) used for CO2 reduction is adsorbed onto Ag-loaded TaON (Ag/TaON) for methanol oxidation. Isotope experiments clearly showed that this hybrid photocatalyst mainly produced HCOOH (TN = 41 for 9 h irradiation) from CO2 and HCHO from methanol. Therefore, it converted light energy into chemical energy (ΔG° = +83.0 kJ/mol). Photocatalytic reaction proceeds by the stepwise excitation of Ag/TaON and the Ru dinuclear complex on Ag/TaON, similar to the photosynthesis Z-scheme.

A polymeric carbon nitride semiconductor is demonstrated to photocatalyse CO2 reduction to formic acid under visible light (λ > 400 nm) with a high turnover number (>200 for 20 hours) and selectivity (>80%), when coupled with a molecular ruthenium complex as a catalyst.

A highly efficient tetradentate PNNP-type Ir photocatalyst, Mes-IrPCY2, was developed for the reduction of carbon dioxide. The photocatalyst furnished formic acid (HCO 2 H) with 87% selectivity together with carbon monoxide to achieve a turnover number of 2560, which is the highest among CO 2 reduction photocatalysts without an additional photosensitizer. Mes-IrPCY2 exhibited outstanding photocatalytic CO 2 reduction activity in the presence of the sacrificial electron source 1,3-dimethyl-2phenyl-2,3-dihydro-1H-benzo[d]imidazole (BIH) in CO 2 -saturated N,N-dimethylacetamide under irradiation with visible light. The quantum yield was determined to be 49% for the generation of HCO 2 H and CO. Electron paramagnetic resonance and UV−vis spectroscopy studies of Mes-IrPCY2 with a sacrificial electron donor revealed that the one-electron-reduced species is the key intermediate for the selective formation of HCO 2 H.

A hybrid material consisting of CaTaO2N (a perovskite oxynitride semiconductor having a band gap of 2.5 eV) and a binuclear Ru(II) complex photocatalytically produced HCOOH via CO2 reduction with high selectivity (>99%) under visible light (λ>400 nm). Results of photocatalytic reactions, spectroscopic measurements, and electron microscopy observations indicated that the reaction was driven according to a two-step photoexcitation of CaTaO2N and the Ru photosensitizer unit, where Ag nanoparticles loaded on CaTaO2N with optimal distribution mediated interfacial electron transfer due to reductive quenching.

A hybrid photocathode that consists of a ruthenium complex catalyst and a p-type semiconductor composed of earth-abundant elements, N,Zn-codoped Fe2O3, with a multiheterojunction structure (TiO2/N,Zn-Fe2O3/Cr2O3) was developed for the reduction of CO2 in aqueous solution with application of an electrical bias under simulated solar light irradiation. The TiO2 layer prevents contact between N,Zn-Fe2O3 and the electrolyte, so that dissolution of N,Zn-Fe2O3 by photoelectrochemical (PEC) self-reduction cannot occur. Both TiO2 and Cr2O3 significantly enhanced the cathodic photocurrent by tuning the band bending in N,Zn-Fe2O3. The use of a Ru complex with an electronic network provided by polypyrrole improved the performance and resulted in a stable photocurrent of 150 μA cm–2 for the production of HCOOH, CO, and a small amount of H2 under 1 sun irradiation with application of 0.1 V vs the reversible hydrogen electrode (RHE). The total amount of generated HCOOH, CO, and H2, two-electron-reduction products, was equal to half the amount of photogenerated electrons. The functional combination of the hybrid iron-based photocathode with a reduced SrTiO3 (SrTiO3–x ) photoanode realized stoichiometric solar CO2 reduction coupled with the water oxidation reaction without an external electrical bias. The solar to chemical energy conversion efficiency was 0.15%, which is comparable to that of a reported tandem system using a Ru complex/single-crystalline InP photocathode.

A hybrid photocatalytic system consisting of a Ru(ii) binuclear complex and Ag-loaded TaON can reduce CO2 to HCOOH by visible light irradiation even in aqueous solution (TONHCOOH = 750, ΦHCOOH = 0.48%).

Electrocatalytic CO2 reduction over a Mn-complex catalyst in an aqueous solution was achieved at very low energy with a combination of multiwalled carbon nanotubes (MWCNTs) and K+ cations. Although the bare Mn-complex did not function as a catalyst in an aqueous solution, the combined Mn-complex/MWCNT cathode promoted electrocatalytic CO2 reduction at an overpotential of 100 mV where neither the bare MWCNTs nor bare Mn-complex were catalytically active. The Mn-complex/MWCNT produced CO at a constant rate for 48 h with a current density of greater than 2.0 mA cm–2 at −0.39 V (vs RHE). The MWCNTs with electron accumulation properties, together with surface adsorbed K+ ions, provided an environment to stabilize CO2 adjacent to the Mn-complex and significantly lowered the overpotential for CO2 reduction in an aqueous solution, and these results were consistent with density functional theory (DFT) calculations. Experiments clarified that the synergetic effect of the MWCNTs and K+ ions was also applicable to Co and Re complexes that were almost inert with regard to CO2 reduction in an aqueous solution.