New insights on solar photocatalytic degradation of phenol over Fe-TiO 2 catalysts: Photo-complex mechanism of iron lixiviates Cristina Ada ´n a , Arturo Martı ´nez-Arias a , Sixto Malato b , Ana Bahamonde a, * a Instituto de Cata ´lisis y Petroleoquı´mica, CSIC. C/Marie Curie No. 2, 28049 Madrid, Spain b Plataforma Solar de Almerı´a-CIEMAT, Crta. Sene ´s s/n, 04200 Tabernas, Almerı´a, Spain 1. Introduction Advanced oxidation methods like those based on heteroge- neous photocatalytic processes are among most promising ones for waste water treatments with low-organic contents [1]. The key advantage of photocatalysis is its inherently destructive nature: it can be carried out under ambient conditions and may lead to complete mineralization of organic carbon into CO 2 . The most widely used photocatalyst is TiO 2 , due to its optical and electronic properties, low cost, chemical stability and non-toxicity; further- more, it is relatively easy to produce and use, while it is able to efficiently catalyse environmental reactions too [2]. However, contrasting results are reported about the influence of crystal phase composition and particle size of titania on the photocatalytic oxidative degradation of organic pollutants in air or water. Thus, some authors reported that anatase performs better than rutile [3]. However, others showed best results for rutile [4]; even the presence of synergistic effects in the presence of combined anatase-rutile heterostructures has been reported as beneficial for increasing the photocatalytic activity [5]. In any case, titania- based catalysts are most interesting from a practical point of view since they show a favorable balance between effective light handling and chemical and mechanical stabilities. The employment of combined photocatalysis and solar technologies constitutes a powerful tool for the reduction of water pollution by organic compounds because of the mild conditions required and its generally good efficiency in miner- alization processes [6]. Nevertheless, efficient use of the solar spectrum for these processes is limited by various factors among which the relatively large band gap of the pure titania semi- conductor system can be most relevant. To increase the efficiency during application of visible light to the photocatalytic degradation of environmental pollutants, many efforts have been concentrated on the development of visible light-responsive TiO 2 photocatalysts . In this sense, most of the investigations have been focused on titania doping either with metal cations, using both chemical [7] and physical [8] methods, or with non-metal anions like nitrogen [9], sulphur [10] or carbon; enhanced photocatalytic activity under visible light can be attained in all these cases [11]. Thus, transition metal doping of titania, in which the dopant cations are incorporated into the TiO 2 lattice, has shown to provide systems able to absorb efficiently visible light up to a wavelength of 400– 600 nm without losing its UV light absorption capability [8]. Nevertheless, transition metal doping can also introduce impurity Applied Catalysis B: Environmental 93 (2009) 96–105 ARTICLE INFO Article history: Received 22 June 2009 Received in revised form 4 September 2009 Accepted 9 September 2009 Available online 15 September 2009 Keywords: Titania Iron Solar Photocatalysts Phenol Photo-complex Lixiviates ABSTRACT The influence of iron content and calcination temperature applied during catalyst preparation on the solar light assisted photo-degradation of phenol over iron-doped titania catalysts have been studied. Pure titania catalysts were also analysed for comparative purpose. An improvement in phenol and TOC removal was detected upon increasing calcination temperature. Although subtle differences in evolution and distribution of short-organic acids were observed upon increasing the calcination temperature, phenol photo-degradation apparently proceeded through the same oxidation pathway over all analysed catalysts. A photo-complex mechanism of iron lixiviates by which the reduced Fe(II) ions in solution become re-adsorbed on the catalyst surface and subsequently re-oxidized by photogenerated holes, leaving the catalyst ready for a new photo-degradation cycle, has been postulated to explain the obtained results. Iron is proposed to be extracted from the catalysts surface through formation of a [Fe- carboxylic acid] 2+ complex within a process in which refractory carboxylic acids simultaneously undergo further oxidation. XPS results and the absence of differences in the iron content determined by chemical analysis when comparing fresh and used catalysts confirm the re-adsorption phenomena of iron lixiviates on the catalyst surface. ß 2009 Elsevier B.V. All rights reserved. * Corresponding author. Tel.: +34915855475; fax: +34915854760. E-mail address: abahamonde@icp.csic.es (A. Bahamonde). Contents lists available at ScienceDirect Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb 0926-3373/$ – see front matter ß 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2009.09.017