Applied Surface Science 338 (2015) 1–14 Contents lists available at ScienceDirect Applied Surface Science jou rn al h om ep age: www.elsevier.com/locate/apsusc Gold–indium modified TiO 2 nanocatalysts for photocatalytic CO 2 reduction with H 2 as reductant in a monolith photoreactor Beenish Tahir, Muhammad Tahir 1 , NorAishah Saidina Amin ∗ Chemical Reaction Engineering Group/Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor Baharu, Johor, Malaysia a r t i c l e i n f o Article history: Received 8 December 2014 Received in revised form 17 February 2015 Accepted 19 February 2015 Available online 26 February 2015 Keywords: Gold–indium–TiO2 Monolith photoreactor CO2 photoreduction H2 reductant RWGS reaction a b s t r a c t In this study, CO 2 photoreduction via reverse water gas shift (RWGS) reaction over gold (Au) and indium (In) modified TiO 2 nanocatalysts in a monolith photoreactor has been investigated. Crystalline nanopar- ticles of anatase TiO 2 were obtained in doped TiO 2 samples with metals deposited over TiO 2 as Au and In 3+ ions. The catalytic performance of metal-doped/TiO 2 catalysts was found to be considerably higher when compared to pure TiO 2 . The maximum production of CO as the main product was 8982 mol g- catal -1 h -1 at selectivity 99% and CO 2 conversion of 9.5% over 0.2 wt.% Au–3.5 wt.% In/TiO 2 , and CO 2 /H 2 feed ratio 1.5. The CO production over co-metals (Au–In) doped TiO 2 monolithic catalyst was 1.3 times higher than Au/TiO 2 , 4.39 times higher than In/TiO 2 and 76 times higher than un-doped TiO 2 catalysts. Significantly higher photoactivity of metal-doped TiO 2 was obviously due to fast electron transfer with hindered recombination rates and larger illuminated surface area in monolith channels. The quantum efficiency of CO production through RWGS reaction using Au–In/TiO 2 catalyst was considerably improved (0.79%) than Au/TiO 2 (0.53%) and In/TiO 2 (0.14%) monolithic catalysts. The stability of the reused cata- lysts for CO production sustained at cyclic runs. This development confirmed higher performance of metals-doped TiO 2 nanocatalysts supported over monolith channels for CO 2 photoreduction via RWGS reaction. © 2015 Elsevier B.V. All rights reserved. 1. Introduction Increasing carbon dioxide (CO 2 ) concentration in the atmo- sphere is a matter of great concern and any attempt to reduce its emission is of primary importance. The photocatalytic conver- sion of CO 2 into fuels and/or chemicals has grown into an intense area of research owing to global warming and fossil fuel shortages [1–3]. CO 2 can be reduced with water to CO, CH 4 , HCOOH, HCHO, and CH 3 OH with photo-technology [4–7]. However, H 2 O is a weak reductant and is hardly reducible, thus photoreduction of H 2 O to H 2 proceeds preferably through water splitting instead of CO 2 reduc- tion. Large amount of H 2 can be produced by water splitting under sunlight irradiations, thus, it can be available in excess [8,9]. There- fore, CO 2 photoreduction with H 2 through reverse water gas shift (RWGS) reaction is more effective to reduce CO 2 to fuels [10–12]. ∗ Corresponding author. Tel.: +60 7 553 5579; fax: +60 7 5588166. E-mail addresses: bttahir@yahoo.com (M. Tahir), noraishah@cheme.utm.my (N.S. Amin). 1 Permanent address: Department of Chemical Engineering, COMSATS Institute of Information Technology, Lahore, Pakistan. Among semiconductor materials, securing photocatalysts with high activity, selectivity and quantum yield efficiency is a critical challenge. TiO 2 is more promising owing to numerous advantages such as strong oxidative potential, low cost, available in excess, chemically/thermally stable and non-toxic [13–16]. However, TiO 2 photoactivity is lower because of the fast recombination of photo- excited electron (e - )–hole (h - ) pairs. The photocatalytic activity of titanium based photocatalysts can be enhanced by the incor- poration of metals. Among different single metals loaded systems Ag [4,17], Pt [18], I [19], Cu [20], Rh [21], and Fe [22] were widely reported for CO 2 photoreduction. Recently, indium (In) is consid- ered as the most effective metal to enhance TiO 2 photoactivity because of multiple oxidation sates and prevent charges recom- bination [23]. Tahir and Amin [24] reported photocatalytic CO 2 reduction with H 2 O vapors over In-doped TiO 2 photocatalyst and found much higher TiO 2 photoactivity in the presence of In-metal. Simultaneous doping or depositing two different types of metal atoms on TiO 2 has attracted considerable interest; since they can improve TiO 2 photoactivity expressively compared to single metal doped TiO 2 . Sasikala et al. [25] investigated In and N co-doped TiO 2 –Pd nanocomposites for H 2 generation and found significant improvement in TiO 2 activity in the presence of http://dx.doi.org/10.1016/j.apsusc.2015.02.126 0169-4332/© 2015 Elsevier B.V. All rights reserved.