Photochemical Reduction of CO2 Using TiO2: Effects of Organic Adsorbates on TiO2 and Deposition of Pd onto TiO2

Reduction of CO2 using semiconductors as photocatalysts has recently attracted a great deal of attention again. The effects of organic adsorbates on semiconductors on the photocatalytic products are noteworthy. On untreated TiO2 (P-25) particles a considerable number of organic molecules such as acetic acid were adsorbed. Although irradiation of an aqueous suspension of this TiO2 resulted in the formation of a significant amount of CH4 as a major product, it was strongly suggested that its formation mainly proceeded via the photo-Kolbe reaction of acetic acid. Using TiO2 treated by calcination and washing procedures for removal of the organic adsorbates, CO was photocatalytically generated as a major product, along with a very small amount of CH4, from an aqueous suspension under a CO2 atmosphere. In contrast, by using Pd (>0.5 wt %) deposited on TiO2 (Pd-TiO2) on which organic adsorbates were not detected, CH4 was the main product, but CO formation was drastically reduced compared with that on the pretreated TiO2. Experimental data, including isotope labeling, indicated that CO2 and CO32- are the main carbon sources of the CH4 formation, which proceeds on the Pd site of Pd-TiO2. Prolonged irradiation caused deactivation of the photocatalysis of Pd-TiO2 because of the partial oxidation of the deposited Pd to PdO.