Photocatalytic conversion and kinetic study of CO 2 and CH 4 over nitrogen-doped titania nanotube arrays Saeed Delavari 1 , Nor Aishah Saidina Amin * , Mehrorang Ghaedi 2 Chemical Reaction Engineering Group (CREG)/Low Carbon Energy Group, Faculty of Chemical Engineering, Universiti Teknologi Malaysia, 81310 UTM, Skudai, Johor, Malaysia article info Article history: Received 20 December 2014 Received in revised form 13 July 2015 Accepted 14 July 2015 Available online 22 July 2015 Keywords: Photocatalytic CO 2 conversion Response surface methodology Titania nanotube arrays Nitrogen-doped abstract The performance of highly ordered nitrogen-doped titania (TiO 2 ) nanotube arrays, fabricated by anod- ization method, was tested for photocatalytic CO 2 conversion with CH 4 . Nitrogen-doped titania nanotube arrays were characterized using field-emission scanning electron microscopy (FESEM), X-ray diffraction (XRD), transmission electron microscopy (TEM), BrunauereEmmetteTeller (BET), X-ray photoelectron spectroscopy (XPS) and Ultravioletevisible (UVevis) spectra. The photoreduction products were iden- tified using residual gas analyzer (RGA) and GC spectra. The effects of important parameters such as UV light power, initial ratios of CO 2 :CH 4 :N 2 in feed and distance between UV lamp and reactor on CO 2 and CH 4 conversions were analyzed using response surface methodology (RSM). FESEM images of titania nanotube arrays indicated highly ordered and vertically oriented morphology with inside diameter ranging from 3 to 50 nm. The optimal conditions for maximum CO 2 conversion of 41.5% were determined as 250 W UV light power, 10% CO 2 initial ratio and 2 cm distance between UV lamp and reactor. H 2 and CO were the main products with selectivities being 80.5% and 18.9%, respectively. CO 2 and CH 4 molecules were competitively activated by the charge transfer excited complexes and the values of feed ratios influenced the selectivity for the formation of the desired products. The kinetic model based on Langmuir eHinshelwood, incorporated photocatalytic adsorptive reduction and oxidation reactions over the catalyst surface fitted-well with the experimental data. © 2015 Elsevier Ltd. All rights reserved. 1. Introduction The drastic rise of greenhouse gas (i.e. CO 2 , CH 4 ,O 3 and N 2 O) concentrations, induced by human activities, raised urgent needs to address environmental problems related to climate change. The development along with application involving cleaner environ- mental technologies present multiple rewards such as reduced emissions, less waste and cost savings by reducing energy during production and also as a result of improved methods of recycling where possible. The reduction of conventional energy sources and increasing concerns on global warming are supporting the use of renewable and clean energy sources (Davis and Martín, 2014). Solar thermal energy is considered as one of the cleanest energy sources (Koroneos and Nanaki, 2012; Ozalp et al., 2010) and the production cost of solar energy is reducing with advances in technology (Fu et al., 2015; Trudewind et al., 2014). In this regard, many re- searchers consider that reduction of CO 2 and CH 4 can alleviate the effects of global warming. There are many methods available for CO 2 and CH 4 conversions but photocatalytic process offers many benefits over the others. Using titania (TiO 2 ) photocatalysts in the reduction process is ad- vantageous since the catalysts are resistant to chemical and pho- tocorrosion, can operate at low temperature, are not costly and consume significantly low energy (Zhao et al., 2009; Mahmodi et al., 2013). However, titania (TiO 2 ) has a bandgap of 3.2 eV and only high energy UV irradiation can excite it with a wavelength shorter than 387.5 nm. Tan et al. (2006) reported a maximum rate of methane from moist carbon dioxide, about 0.25 mmol/h by using titania pellets under monochromatic ultraviolet illumination at 253.7 nm wavelength. * Corresponding author. Tel.: þ60 7 553 5579; fax: þ60 7 558 816. E-mail addresses: delsaeed2009@gmail.com (S. Delavari), noraishah@cheme. utm.my (N.A.S. Amin). 1 Department of Chemical Engineering, Gachsaran Branch, Islamic Azad Univer- sity, Gachsaran, Iran. 2 Analytical Chemistry Department of Chemistry, Yasouj University, Yasouj, Iran. Contents lists available at ScienceDirect Journal of Cleaner Production journal homepage: www.elsevier.com/locate/jclepro http://dx.doi.org/10.1016/j.jclepro.2015.07.077 0959-6526/© 2015 Elsevier Ltd. All rights reserved. Journal of Cleaner Production 111 (2016) 143e154