Journal of Molecular Catalysis A: Chemical 327 (2010) 51–57 Contents lists available at ScienceDirect Journal of Molecular Catalysis A: Chemical journal homepage: www.elsevier.com/locate/molcata Enhancement of photocatalytic oxidation of oxalic acid by gold modified WO 3 /TiO 2 photocatalysts under UV and visible light irradiation V. Iliev a,∗ , D. Tomova a , S. Rakovsky a , A. Eliyas a , G. Li Puma b a Institute of Catalysis, Bulgarian Academy of Sciences, 1113 Sofia, Bulgaria b Photocatalysis & Photoreaction Engineering, Department of Chemical and Environmental Engineering, Faculty of Engineering, The University of Nottingham, University Park, Nottingham NG7 2RD, United Kingdom article info Article history: Received 30 March 2010 Received in revised form 14 May 2010 Accepted 17 May 2010 Available online 24 May 2010 Keywords: TiO2 WO3 Composite photocatalysts Gold Oxalic acid UV–vis light irradiation abstract The photooxidation of oxalic acid, catalyzed by nanosized TiO 2 or WO 3 and composite photocatalysts: Au/TiO 2 , Au/WO 3 , WO 3 /TiO 2 , Au/WO 3 /TiO 2 was studied under irradiation with UV, visible and combined UV–visible light. The catalysts were characterized by the XRD, XPS, SEM and TEM methods. The photocat- alytic mineralization of oxalic acid, catalyzed by WO 3 /TiO 2 or Au/WO 3 /TiO 2 , proceeded at a significantly higher rate under UV-A irradiation than that under visible light. This is due to the lower specific surface area of the WO 3 and its small amount in the composite catalyst. Doping of the semiconductor materials with gold nanoparticles more than doubles the rates of mineralization of oxalic acid, compared to the un-doped samples, and more significantly in the case of Au/WO 3 /TiO 2 . The higher rate constants of oxalic acid decomposition under UV, visible or UV–visible light irradiation with the WO 3 /TiO 2 and Au/WO 3 /TiO 2 catalysts, compared with those measured with the individual oxide photocatalysts, are due to the more efficient separation of the electron–hole charges generated upon irradiation. Especially efficient is the charge separation in the case of the Au/WO 3 /TiO 2 photocatalyst under irradiation with UV or combined UV–visible light, when the rate constants of oxalic acid destruction are approximately 1.7 times higher than that of the process catalyzed by Au/TiO 2 and 3 times higher than that catalyzed by pure TiO 2 . © 2010 Elsevier B.V. All rights reserved. 1. Introduction Photocatalytic reactions on semiconductor materials, irradiated with solar or artificial light, are of great interest because of their applicability for the removal of a large variety of pollutants, both in aqueous and in gaseous phase [1–7]. Band-gap excitation of a semiconductor produces photoinduced electrons and holes, which can respectively reduce and oxidize species, adsorbed on the sur- face of semiconductor particles [8]. However, a major disadvantage in most semiconductor materials is the high degree of recombina- tion between the photogenerated charge carriers, which ultimately decreases the photocatalyst effectiveness and the photonic effi- ciency of the redox process. Titanium dioxide (TiO 2 ) is one of the most widely recognised semiconductor photocatalyst, triggering off the oxidative destruction and mineralization of a wide range of organic substrates. The processes of catalytic oxidative destruction with TiO 2 are accomplished under UV light irradiation, due to the wide band gap of the anatase form of TiO 2 (3.2 eV). Sunlight may conveniently be used as the source of photon energy, however, the fraction of UV light in the solar energy radiation spectrum is only ∗ Corresponding author. Tel.: +359 2 9792514; fax: +359 2 9712967. E-mail address: iliev@ic.bas.bg (V. Iliev). about 4%. During the last years, studies in the field of photocatal- ysis have therefore focused on the preparation of semiconductor materials capable of utilizing, effectively, both the UV and the vis- ible component of solar light. The aim has been to increase the photonic efficiency of the processes of destruction of water and air contaminants [9–11]. In consequence, single semiconductor mate- rials [12–14] have been studied, as well as, doped semiconductors [13–15] or coupled semiconductors systems, in which one of them is excited by visible light irradiation (organic or inorganic semicon- ductors) [12,13,15–20]. Another approach to promote the photonic efficiency of a semiconductor photocatalyst has been the surface modification with nanosized particles of noble metals [18,21–23]. A number of authors have investigated coupled WO 3 /TiO 2 sys- tems under illumination with UV [24–27] or visible light [28–30] with the purpose of promoting the photonic efficiency of TiO 2 in the decomposition of water and air contaminants. WO 3 is a semicon- ductor photocatalyst with a band gap of 2.8 eV, which is activated by visible light illumination. The basic disadvantage of WO 3 as a photocatalyst is its low photonic efficiency. It has been reported [26,27,30], that the activity of WO 3 /TiO 2 photocatalysts, under UV light irradiation, is highest at an optimum WO 3 content in the com- posite material of about 3–4 wt%. The promoting effect of noble metals (in particular gold) on the WO 3 /TiO 2 system efficiency, has received little attention so far in the literature. 1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.molcata.2010.05.012