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

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 6 h. 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.