||This study investigated the photocatalytic reduction of CO2 in a self-designed closed circulated batch reactor system and a bench-scale batch photocatalytic reactor. The photocatalysts tested included titanium dioxide (TiO2, Degussa P-25) and zirconium oxide (ZrO2). The reductants investigated included hydrogen (H2), water vapor (H2O), and hydrogen plus water vapor (H2+H2O). The wavelengths of incident near ultra-violet (UV) and UV lights for the photocatalysis of TiO2 and ZrO2 were 365 nm and 254 nm, respectively. The initial concentrations of CO2 ranged from 0.2-5.0% and the reaction temperature ranged from 35-95 ○C. The incident near-UV (or UV) light with wavelength of 365 nm (or 254 nm) was irradiated by a 15-watt low-pressure mercury lamp. The photocatalytic reaction was conducted continuously for approximately two hours. Reactants and products were analyzed by a gas chromatography with a flame ionization detector followed by a methanizer (GC/FID-methanizer).|
Experimental results indicated that glass pellets coated with TiO2 had better photoreduction efficiency than ZrO2. The highest yield rates of the photoreduction of CO2 were obtained using TiO2 with H2+H2O and ZrO2 with H2. Photoreduction of CO2 over TiO2 with H2+H2O formed CH4, C2H6, and CO in the yield of 32.95~94.60, 0.80~18.55, 1.12~21.78 μmol/g, respectively, while the photoreduction of CO2 over ZrO2 with H2 formed CO in the yield of 0.34~4.99 μmol/g.
Results obtained from the operating parameter tests showed that the photoreduction rate increased with the initial concentration of carbon dioxide and resulted in more product accumulation. The photoreduction rate of carbon dioxide increased with reaction temperature, which promoted the formation of products. Concurred with previous researches, the reaction rate of major products over TiO2 and ZrO2 were higher than previous investigations of CO2 photoreduction.
Furthermore, the spectra of FTIR showed that formic acid (HCOOHads), methanol (CH3OHads), carbonate (CO32−ads), bicarbonate (HCO32−ads), formate (HCOO−ads), formic acid (HCOOH ads), formaldehyde (HCOHads) and methyl formate (HCOOCH3 ads) formed on the surface of TiO2 and ZrO2 photocatalysts. The detected reaction products supported the proposal of two reaction pathways for the photoreduction of CO2 over TiO2 and ZrO2 with H2 and H2O, respectively.
A modified bimolecular Langmuir-Hinshelwood kinetic model was developed to simulate the reaction temperature, CO2 initial concentration and relative humidity promotion and inhibition of the photoreduction of CO2. Additionally, the modified L-H kinetic model was successfully applied to simulate the photoreduction rate of CO2.
The result showed that CO2 could be reduced by used solar light over TiO2 and ZrO2 photocatalysts. The reaction products of CO2 photoreduction over TiO2 were CH4, C2H6, and CO in the yield of 2.16~2.995, 0.057~0.128, 0.078~0.134 μmol/g, respectively, while the photoreduction of CO2 over ZrO2 formed only CO in the yield of 0.023~0.051 μmol/g.
Furthermore, experimental results indicated that TiO2 gave the highest average photo energy efficiency (AEf) of ~4.13%, and apparent quantum efficiency (φA) of ~1.05%. However, the ZrO2 gave the highest average photo energy efficiency (AEf) of 5.07×10-3%, and apparent quantum efficiency (φA) of ~1.54×10-2%.