The global emissions of greenhouse gases continue to rise by about 3% each year [1]. Rising levels of CO2 in the atmosphere can be subsided though CO2 utilization technologies such as conversion into fuels and different value-added products. Unlike other CO2 conversion techniques, photocatalysis utilizes a renewable and sustainable form of energy and doesn’t increase net CO2 emissions. A variety of different catalysts have already been tested in numerous studies for the photocatalytic reduction of gaseous CO2 [2]. However, researchers have yet to discover an efficient photocatalyst that can reduce CO2 to yield economical amounts of value-added products under the irradiation of solar light.

Hence, in this study, an attempt in enhancing the performance of TiO2-based photocatalysts for application in CO2 conversion was made. Surface modifications to help enhance the visible light absorption and the charge separation of TiO2 photocatalysts were pursued. More specifically, a novel TiO2-based photocatalyst, incorporating copper (Cu), platinum (Pt) and reduced graphene oxide (rGO), was synthesized. To investigate the intrinsic and enhanced properties of the photocatalytic composite, several advanced material characterization tools were employed and they include XRD, Raman, DRS, PL, TEM, STEM-EDX, BET, FT-IR, etc.

A custom-built reactor was used to test the photocatalytic activity of the catalyst in reducing CO2 under different parameters and conditions, most importantly is the photocatalytic reduction of liquid CO2 which has been rarely investigated [3]. The reactor was designed and constructed to allow for operational flexibility in terms of temperature, pressure, light source, and presence/absence of a reducing agent. Gas-phase and liquid-phase products from the different photoreactivity tests were analysed using GC-FID and HPLC, respectively. A mechanism for the CO2 photoreduction reaction was also proposed.

Results from this study show that CO2 was successfully reduced to CO, CH4, and other products under the irradiation of UV light, with the activity being much higher for liquid CO2 than for gaseous CO2. Furthermore, the proposed catalyst rGO-(Pt/Cu-TiO2) showed higher or comparable activity results during the photocatalytic reduction of liquid CO2 when compared to other tested catalysts, specifically in the formation of aldehydes.

[1] Al Jitan, S., Palmisano, G., & Garlisi, C., Catalysts, 2020, 10, 2, 227.
[2] Habisreutinger, S. N., Schmidt‐Mende, L., & Stolarczyk, J. K., Angewandte Chemie International Edition, 2013, 52, 29, 7372-7408.
[3] Kaneco, S., Kurimoto, H., Ohta, K., Mizuno, T., & Saji, A., Journal of Photochemistry and Photobiology A: Chemistry, 1997, 109, 1, 59-63.