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

Abstract

Reduction of CO(2) 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 TiO(2) (P-25) particles a considerable number of organic molecules such as acetic acid were adsorbed. Although irradiation of an aqueous suspension of this TiO(2) resulted in the formation of a significant amount of CH(4) as a major product, it was strongly suggested that its formation mainly proceeded via the photo-Kolbe reaction of acetic acid. Using TiO(2) 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 CH(4), from an aqueous suspension under a CO(2) atmosphere. In contrast, by using Pd (>0.5 wt %) deposited on TiO(2) (Pd-TiO(2)) on which organic adsorbates were not detected, CH(4) was the main product, but CO formation was drastically reduced compared with that on the pretreated TiO(2). Experimental data, including isotope labeling, indicated that CO(2) and CO(3)(2-) are the main carbon sources of the CH(4) formation, which proceeds on the Pd site of Pd-TiO(2). Prolonged irradiation caused deactivation of the photocatalysis of Pd-TiO(2) because of the partial oxidation of the deposited Pd to PdO.

Understanding the Reaction Mechanism of Photocatalytic Reduction of CO2 with H2O on TiO2-Based Photocatalysts: A Review

“…Second, electron transfer could be promoted in the direction from TiO 2 CB to external trappers or carriers. The electron trappers can be noble metals (e.g., Pt, Pd, Au, Ag) (Sasirekha et al, 2006;Iizuka et al, 2011;Yui et al, 2011;An et al, 2012;Uner and Oymak, 2012;Wang et al, 2012b) or metal oxides (e.g., CuO, FeO x , CeO 2 ) (Tseng et al, 2004;Qin et al, 2011;Srinivas et al, 2011;Wang et al, 2011b;Truong et al, 2012;Zhao et al, 2012b), and the electron carriers are often carbon materials (e.g., graphene) (Liang et al, 2011(Liang et al, , 2012Tu et al, 2012). Third, the incorporation of TiO 2 with another semiconductor, i.e., photo-sensitizer (e.g., AgBr, CdSe, PbS) Asi et al, 2011;Wang et al, 2011a;An et al, 2012) or n-type semiconductor (e.g., ZnO) (Xi et al, 2011), promotes electron transfer between the CB of the second semiconductor and the CB of TiO 2 .…”

“…It was proposed that the energy for band separation of the smaller Pt particles was higher due to the quantum confinement, thus preventing electron transfer from TiO 2 CB to Pt. Yui et al (2011) suggested that the addition of Pd to TO 2 induced the reduction of CO 2 to CH 4 , while the oxidation of Pd to PdO caused the catalyst deactivation. The above studies indicate the importance of choosing an appropriate noble metal, tailoring the particle size in an appropriate range, and avoiding the oxidation of noble metal to metal oxide.…”

A perspective on perovskite oxide semiconductor catalysts for gas phase photoreduction of carbon dioxide

“…The organic adsorbed on the surface of TiO 2 could participate in a so-called photo-Kolbe reaction (CH 3 COOH, for example) [36] :…”

Photodegradation of Dye Pollutants Catalyzed by H₃PW₁₂O₄₀/SiO₂ Treated with H₂O₂ under Simulated Solar Light Irradiation

“…Because of its band gap of 3.2 eV, TiO 2 is activated only by ultraviolet (UV) light (wavelength<387 nm). Pure TiO 2 can't be effectively activated under solar light irradiation, which limits its practical application, because UV light just accounts for less than 5 % of the solar energy that reaches the surface of the earth [12][13][14]. In recent years, visible light responsive photocatalysts have been extensively studied.…”