CHEMICAL ENGINEERING TRANSACTIONS VOL. 45, 2015 A publication of The Italian Association of Chemical Engineering www.aidic.it/cet Guest Editors: Petar Sabev Varbanov, Jiří Jaromír Klemeš, Sharifah Rafidah Wan Alwi, Jun Yow Yong, Xia Liu Copyright © 2015, AIDIC Servizi S.r.l., ISBN 978-88-95608-36-5; ISSN 2283-9216 DOI: 10.3303/CET1545044 Please cite this article as: Tahir B., Tahir M., Saidina Amin N.A., 2015, Carbon dioxide reduction with hydrogen in a continuous catalytic monolith photoreactor, Chemical Engineering Transactions, 45, 259-264 DOI:10.3303/CET1545044 259 Carbon Dioxide Reduction with Hydrogen in a Continuous Catalytic Monolith Photoreactor Beenish Tahir , Muhammad Tahir , Nor A. S. Amin* Chemical Reaction Engineering Group/ Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor Baharu, Johor, Malaysia. noraishah@cheme.utm.my Photocatalytic CO2 reduction with H2 as a reductant over gold (Au)-doped TiO2 nanocatalysts in a continuous monolith photoreactor has been investigated. The nanocatalysts were characterized by XRD, SEM, N2 adsorption-desorption and UV-Visible spectroscopy. Crystalline nanoparticles of anatase phase TiO2 were obtained in the Au-doped TiO2 samples. CO was the major product over 0.5 wt. % Au-doped TiO2 with the yield rate of 12,305 ppm g-catal. -1 h -1 , 318 times higher than un-doped TiO2 catalyst. Significantly higher photoactivity of Au-doped TiO2 was obviously due to fast electron transfer with hindered recombination rates and larger illuminated surface area inside the monolith channels. The CO production rate was gradually reduced with increasing the space velocity. The stability of the reused catalysts for CO production sustained at cyclic runs. It is evident Au-doped TiO2 nanocatalyst supported over monolith channels is highly potential for continuous CO2 photoreduction to CO and hydrocarbons. 1. Introduction Photocatalytic reduction of carbon dioxide (CO2) to useful chemicals has grown into an intense area of research owing to global warming and energy crises (Wang et al., 2015). The reduction of CO2 to CO, CH4, HCOOH, HCHO, and CH3OH, with water as the reducing agent, has been reported for the first time more than three decades ago (Inoue et al., 1979). The photocatalytic CO2 reduction with water is a challenging task as H2O is a weak reductant and is hardly reducible. However, photoreduction of CO2 with H2 through reverse water gas shift (RWGS) reaction is more effective to produce fuels (Tahir et al., 2015). Among the semiconductors materials, TiO2 is a promising photocatalyst due to numerous advantages such as strong oxidative-reductive potential, cheaper, abundantly available, and chemically/thermally stable (Ruzmanova et al., 2013). However, TiO2 photoactivity is relatively lower due to the fast recombination rate of electron-holes pairs, which can be improved by modifying its structure with noble metals (Sacco et al., 2015). Among the noble metals, gold (Au) nanoparticles doping into TiO2 can efficiently enhance photoactivity. The purposes of Au-doping or depositing into TiO2 are: (i) to modify TiO2 surface morphologies, (ii) to improve e-/h+ pair’s separation by acting as electron trap and, (iii) to increase the surface electron activity by localized surface plasma resonance (Sellappan et al., 2013). Au-doped TiO2 catalysts, investigated for CO2 photoreduction with H2O to hydrocarbons, registered higher TiO2 photoactivity with Au-metal (Mei et al., 2013). Therefore, it is envisaged that gold-doped TiO2 system would efficiently reduce CO2 through RWGS reaction. Among the structured supports, monolith containing parallel straight channels takes advantages of distinctive structure, higher surface area to volume ratio, and efficient light harvesting. Monolith substrate provides up to 100 times higher specific surface area than other types of catalyst supports having the same outer dimensions (Liou et al., 2011). We reported monolith photoreactor for CO2 reduction and found good CO2 conversion efficiency and CO selectivity (Tahir and Amin, 2015b). However, monolith photoreactor was investigated in a batch mode of operation. Recently, enhanced photocatalytic CO2 reduction to CH4 via steam reforming over metal-doped TiO2 in a photocatalytic fluidized bed reactor has