Chemical Engineering Journal 126 (2007) 119–130 Preparation and characterization of Ce-Zr and Ce-Mn based oxides for n-hexane combustion: Application to catalytic membrane reactors G. Picasso, M. Guti´ errez, M.P. Pina ∗ , J. Herguido Department of Chemical & Environmental Engineering, University of Zaragoza, 50009 Zaragoza, Spain Received 13 July 2006; received in revised form 11 September 2006; accepted 12 September 2006 Abstract Ce-Mn and Ce-Zr catalytic samples with different Ce/metal molar ratio have been prepared by coprecipitation followed by calcination at moderate temperatures (350–450 ◦ C) and characterized by XRD, XPS, BET, TPR and SEM–EDX techniques. The preparation procedure of analogous catalytic membranes over -Al 2 O 3 ceramic supports has been also studied in order to control loading, distribution and composition of the catalytic material inside the membrane thickness. To evaluate their catalytic performance, the combustion of n-hexane from air-diluted streams has been carried out in both a conventional fixed bed reactor and a flow-through catalytic membrane reactor operating in Knudsen diffusion regime. Surface area alone cannot account for the reaction performance achieved; however, the redox properties and oxygen mobility of the mixed oxides have been revealed as key parameters controlling the catalytic activity. Although Ce-Mn powdered catalysts appear more active in n-hexane combustion than their Ce-Zr counterparts; a strong deactivation phenomenon, more severe for Mn rich samples, is observed with time on stream in contrast with Ce-Mn based membranes showing catalytic stability under equivalent reaction conditions. © 2006 Elsevier B.V. All rights reserved. Keywords: Ce-Zr mixed oxides; Ce-Mn mixed oxides; Redox properties; Hexane combustion; Catalytic membrane reactors 1. Introduction Volatile organic compounds are an important class of pol- lutants responsible of photochemical ozone in the ground level and other oxidants. They are found in urban and industrial areas due to the emissions of vehicles, industrial processes and human activities. The total removal of highly toxic volatile organic com- pounds (VOC’s) without secondary pollutants requires the best achievable technology (BAT) in particular when their elimina- tion has to be ensured at moderate temperatures for savings in energy costs [1]. During last years, catalytic combustion has acquired an increasing importance for VOCs abatement due to NO x and other secondary atmospheric hazards [2] formation is substantially depressed. The most common catalysts used to control VOC emissions can be classified in two main groups: noble metals and metal oxides. Most of catalytic systems used in VOC control inciner- ators are still based in supported noble metals such as Pt [3–5], Pd [6], Au [7,8]. Recently, metal oxide based catalysts such as ∗ Corresponding author. Tel.: +34 976 76115; fax: +34 976 762142. E-mail address: mapina@unizar.es (M.P. Pina). Cr 2 O 3 , CuO [9,10] are increasingly applied in combustion sys- tems in order to reduce costs associated to catalyst inventario. Among several metal oxides, particular attention is being paid to the application of CeO 2 based catalysts for environmental purposes due to the unusual redox behavior of ceria and its high oxygen storage/transport capacity (OSC) [11]. Incorporation of other oxides such as ZrO 2 into ceria lattice enhances redox prop- erties providing high thermal stability [12]. The CeO 2 -ZrO 2 mixed oxides are well known in TWC technology and have found some interesting applications as industrial catalysis [13]. A great part of the research efforts has been focused on cat- alyst preparation in order to obtain Ce-Zr mixed oxides with elevated surface area. Among the synthesis methods, the copre- cipitation procedure from the corresponding salts has found recurrent application owing to the high oxygen storage capacity achieved by the final material [14–17]. Several papers dealing with Mn based catalysts have been published over the last 5 years. Mn confined in perovskite structures [18,19], supported on alumina-coated monoliths [20] or just employed as simple oxide MnO x [21–23] have been well studied as VOC removal catalysts due to the redox properties of the system. Moreover, Ce-Mn catalysts have been investigated for the combustion of CO [24], n-butane [25] and for pollutants removal from 1385-8947/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.cej.2006.09.005