Photocatalytic reduction of CO₂ under supercritical CO ₂ conditions: Effect of pressure, temperature, and solvent on catalytic efficiency

The photocatalytic reduction of CO₂ to CO in single-phase, high-pressure mixtures of supercritical CO₂ and N,N-dimethylformamide (DMF), using fac-ReCl(bpy)(CO)₃ as a catalyst in the presence of triethanolamine as a sacrificial electron donor has been investigated. The catalytic efficiency was found to be strongly influenced by both the CO ₂ and DMF concentrations. For example, the turnover number (TON) for CO formation increases linearly with CO₂ pressure up to 60 at 17.8 MPa/60 C. It also increases dramatically as the [DMF] is increased from 3.8 to 6.3 M, and then remains almost constant with further increases in [DMF]. This resulted in an optimized TON of 62 at 17.8 MPa CO₂/60 C and [DMF] = 6.3 M, and an initial turnover frequency (TOF) of ∼56 h^-1 at 17.8 MPa CO₂/60 C and [DMF] = 11.4 M. These values are 5× and 2.3× higher, respectively than those obtained under biphasic low-pressure conditions (0.1 MPa CO₂), demonstrating that significant improvements in catalytic performance are possible through the use of high concentrations of CO₂. Manipulation of the incident light intensity led to further improvements in both TON (up to 72 under low light levels) and TOF (up to 117 h^-1 under the highest light level investigated).