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

• The photocatalytic reduction of CO2 under single-phase DMF/scCO2 condition in the presence of fac-ReCl(bpy)(CO)3 was investigated.
• The TONs and TOFs were strongly influenced by the CO2 pressure of CO2, DMF concentrations and light intensity.
• The optimized TONs up to 72 which is >5× greater than under low-pressure conditions were observed.
• The TOFinit of 56 h−1 represents a 2.3× improvement in the rate of CO formation compared to the condition of ambient CO2 pressure.

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 CO2 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).

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