Photocatalytic conversion of carbon dioxide to methane on TiO_2/CdS in aqueous isopropanol solution

Abstract

The photocatalytic conversion of CO_2 to CH_4 on hybrid TiO_2/CdS catalysts in water in the presence of isopropanol (IPA) was explored. As compared to TiO_2, TiO_2/CdS increased the production of CH_4 by a factor of ∼10, whereas the production of H_2 and CO remained comparable. The amount of CdS loaded on the TiO_2 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 ^(13)CH_4 to be a dominant product in the early stages of the photocatalysis under ^(13)CO_2 atmosphere, whereas only ∼25% of the total observed methane accounted for ^(13)CH_4 resulting from ^(13)CO_2 in the prolonged photocatalytic reaction over 6 h. Although the remainder of the methane originated from unlabeled carbons (e.g., from ^(12)C-IPA and ^(12)C-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 CO_2 species (e.g., (bi)carbonate via mono- and bi-dentate modes at pH ∼4.5) to be enhanced by the coupling of CdS to TiO_2, 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.