Merging Surface Organometallic Chemistry with Graphitic Carbon Nitride Photocatalysis for CO2 Photofixation

The creation and development of efficient solar-energy conversion systems for artificial photosynthesis is one of the challenges of modern chemistry and materials. Currently, the search for sustainable and stable photocatalytic systems for CO2 reduction by visible light is being actively pursued. Many useful and energy-rich chemicals (e.g. CH3OH and HCOOH) can be obtained by photocatalytic CO2 reduction, which can reduce the emissions of greenhouse gases. Recently, significant improvements have been achieved in the area of CO2 reduction by using a heterogeneous photocatalyst under visible-light irradiation. However, most of the catalysts screened did not exceed the state-of-the-art system with turnover numbers (TONs) of approximately 200 and apparent quantum yields (AQYs) of approximately 2 %. The introduction of melon-based graphitic carbon nitride (gC3N4) polymers as solar-energy transducers has significantly extended the scope of conventional inorganic photocatalysts to polymeric photocatalysts. The latter is even more promising than the conventional photocatalysts because it is metal-free, sustainable, and has demonstrated the ability to induce water splitting, CO2 reduction, and selective organic synthesis by means of visible light. The use of g-C3N4 for CO2 reduction is an emerging research topic that couples organic basic functionality to photocatalytic functionality and allows for activation/adsorption and reduction of CO2. In the previous system for CO2 to CO photocatalytic conversion the quantum yield is less than 1. Recently, Maeda and co-workers have developed a promising heterogeneous system for the reduction of CO2 into formic acid under visible-light irradiation by merging organometallic chemistry with polymer photocatalysis by using g-C3N4 and a ruthenium complex as light-harvesting units and catalytic active sites, respectively. By carefully optimizing the heterogeneous catalyst and the reaction conditions the AQY was remarkably enhanced to 5.7 with a high TON of >1000, which are the highest values for g-C3N4 and are better than other heterogeneous photocatalysts working with visible light. The authors promote surface kinetics for both the reduction reaction and charge transfer by chemically modifying g-C3N4 with a Ru complex by means of a surface-chemistry strategy. The surface Ru-complex catalysis is demonstrated to change reaction pathway to produce the more valuable product, formic acid, rather than carbon monoxide. Four different Ru-based complexes (Scheme 1), trans-(Cl)[Ru(bpyX2)(CO)2Cl2] (bpyX2 = 2,2’-bipyridine with substituent X in the 4-position, X = H, CH3, PO3H2, or CH2PO3H2), were used as kinetic promoters for CO2 reduction, along with a suitable reaction environment to increase the efficiency of the overall process. Mesoporous graphitic carbon nitride (mpg-C3N4) with a specific surface area of 180 m g¢1 and pore volume of 0.7 cm g¢1 was selected as a light-harvesting semiconductor. The surface of the porous organic photocatalyst is covered with amino groups, which can be easily functionalized by surface organic chemistry. This feature is unique and is not available for traditional inorganic photocatalysts. Results indicated that RuP and RuCP could adsorb on the surface of mpg-C3N4 owing to the intense interaction between acid and base functional groups on the two units. However, no adsorption was found to occur for RuH and RuMe, which do not have an anchoring group. The rich amino groups on the surface of the gScheme 1. Photocatalytic CO2 reduction by Ru-complex anchored g-C3N4 photocatalysts. C.B. = Conduction band, V.B. = Valance band, D = electron donor. Reprinted with permission from Ref. [1] .