Applied Catalysis B: Environmental 100 (2010) 386–392 Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb Photocatalytic reduction of CO 2 with H 2 O on mesoporous silica supported Cu/TiO 2 catalysts Ying Li a,b,∗ , Wei-Ning Wang b , Zili Zhan b , Myung-Heui Woo c , Chang-Yu Wu c , Pratim Biswas b,1 a Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53211, USA b Department of Energy, Environmental and Chemical Engineering, Washington University in St. Louis, St. Louis, MO 63130, USA c Department of Environmental Engineering Sciences, University of Florida, Gainesville, FL 32611, USA article info Article history: Received 9 February 2010 Received in revised form 12 August 2010 Accepted 16 August 2010 Available online 21 August 2010 Keywords: Photocatalysis TiO2 Nanocomposite CO2 photoreduction abstract Photoreduction of CO 2 to hydrocarbons is a sustainable energy technology which not only mitigates emissions but also provides alternative fuels. However, one of the largest challenges is to increase the overall CO 2 photo-conversion efficiency when water is used as the reducing reagent. In this work, meso- porous silica supported Cu/TiO 2 nanocomposites were synthesized through a one-pot sol–gel method, and the photoreduction experiments were carried out in a continuous-flow reactor using CO 2 and water vapor as the reactants under the irradiation of a Xe lamp. The high surface area mesoporous silica sub- strate (>300 m 2 /g) greatly enhanced CO 2 photoreduction, possibly due to improved TiO 2 dispersion and increased adsorption of CO 2 and H 2 O on the catalyst. CO was found to be the primary product of CO 2 reduction for TiO 2 –SiO 2 catalysts without Cu. The addition of Cu species, which was identified to be Cu 2 O by the XPS, markedly increased the overall CO 2 conversion efficiency as well as the selectivity to CH 4 , by suppressing the electron–hole recombination and enhancing multi-electron reactions. A synergistic effect was observed by combining the porous SiO 2 support and the deposition of Cu on TiO 2 . The peak pro- duction rates of CO and CH 4 reached 60 and 10 mol g-cat -1 h -1 , respectively, for the 0.5%Cu/TiO 2 –SiO 2 composite that has the optimum Cu concentration; the peak quantum yield was calculated to be 1.41%. Deactivation and regeneration of the catalyst was observed and the mechanism was discussed. Desorption of the reaction intermediates from the active sites may be the rate limiting step. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Increasing levels of greenhouse gases in the atmosphere is the primary cause of global warming. Release of carbon diox- ide (CO 2 ) from fossil fuel combustion is the major contributor to this phenomenon. Recently, many efforts are made to reduce CO 2 emissions through pre- or post-combustion CO 2 capture followed by compression and geological sequestration [1]. These processes are energy intensive and thus costly; in addition, there are many uncertainties with regard to long-term storage of CO 2 in geologi- cal formations. An alternative and more preferable way is to recycle CO 2 as a fuel feedstock with energy input from cheap and abundant sources (e.g. solar energy). Recent innovations in photocatalysis technology have made CO 2 conversion a potentially promising application. This process uti- ∗ Corresponding author at: Department of Mechanical Engineering, University of Wisconsin-Milwaukee, Milwaukee, WI 53211 USA. Tel.: +1 414 229 3716, fax: +1 414 229 6958. E-mail addresses: liying@uwm.edu (Y. Li), pratim.biswas@wustl.edu (P. Biswas). 1 Tel: +1 314 935 5548; fax: +1 314 935 5464. lizes ultraviolet (UV) and/or visible light as the excitation source for semiconductor catalysts, and the photoexcited electrons reduce CO 2 with H 2 O on the catalyst surface and form energy-bearing products such as carbon monoxide (CO), methane (CH 4 ), methanol (CH 3 OH), formaldehyde (HCHO), and formic acid (HCOOH) [2].A variety of photocatalysts such as TiO 2 , CdS, ZrO 2 , ZnO, and MgO have been studied, and among them, wide band-gap TiO 2 catalysts (∼3.2 eV) are considered the most convenient candidates in terms of cost and stability [2–4]. To enhance reaction rate, increase solar utilization, and control the selectivity of products are the major challenges so far in CO 2 photoreduction technology. An increased CO 2 conversion efficiency was observed when the TiO 2 surface was loaded with metals, which function as “charge-carrier traps” and suppress recombination of photoexcited electron–hole pairs. Metals can be deposited on TiO 2 surface via methods of incipient wetness impregnation [5], sol–gel [6], pho- toreduction [7], and sputter coating [8], etc. Tseng et al. [6] used sol–gel derived Cu/TiO 2 catalysts for CO 2 photoreduction in aque- ous phase and found the yield of methanol is much higher than those without Cu loading. Yamashita et al. [5] reported forma- tion of CH 4 by TiO 2 photocatalysts in a CO 2 and H 2 O system, and Cu impregnated on TiO 2 resulted in additional formation of 0926-3373/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2010.08.015