Photocatalytic conversion of carbon dioxide to methane on TiO2/CdS in aqueous isopropanol solution

Abstract

The photocatalytic conversion of CO2 to CH4 on hybrid TiO2/CdS catalysts in water in the presence of isopropanol (IPA) was explored. As compared to TiO2, TiO2/CdS increased the production of CH4 by a factor of ∼10, whereas the production of H2 and CO remained comparable. The amount of CdS loaded on the TiO2 was not observed to significantly affect the yields and distributions of the products. An electron impact time-of-flight mass spectrometry (TOF-MS) study revealed 13CH4 to be a dominant product in the early stages of the photocatalysis under 13CO2 atmosphere, whereas only ∼25% of the total observed methane accounted for 13CH4 resulting from 13CO2 in the prolonged photocatalytic reaction over 6h. Although the remainder of the methane originated from unlabeled carbons (e.g., from 12C-IPA and 12C-organic contaminants), the use of deuterated IPA in the TOF-MS study did not provide evidence for the contribution of the methyl groups of IPA. Furthermore, the diffuse reflectance infrared Fourier transform analysis showed the adsorption of aquated CO2 species (e.g., (bi)carbonate via mono- and bi-dentate modes at pH ∼4.5) to be enhanced by the coupling of CdS to TiO2, which was found to significantly weaken after the reactions. On the other hand, the IPA-associated IR bands were influenced to a lesser extent by the photoreaction.