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Effect of the preparation methods and alumina nanoparticles on the catalytic performance of Rh/Zrx Ce1−x O2 –Al2 O3 in methane partial oxidation
Sara Boullosa-Eiras, Tiejun Zhao, De Chen, Anders Holmen ∗
Department of Chemical Engineering,Norwegian University of Science and Technology Sem Sælands vei 4, N-7491 Trondheim, Norway
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Rh supported on Zrx Ce1−x O2 –Al2 O3 (x = 1, 0.5, 0.25, 0) catalysts are studied for catalytic partial oxidation of methane to synthesis gas. Alumina based nanocomposites are prepared through a citrate mediated route by modifying the alumina support with cerium andzirconyl nitrates via a simple evaporation-drying or a spray drying method. The effect of the commercial alumina types and the preparation method on the structure and thermostability of the nanocomposites has been studied based on the characterization by XRD, Raman spectroscopy, DTA, TEM and nitrogen adsorption–desorption measurements. Evonik Aeroxide AluC based nanocomposites prepared by spraydrying give the highest thermostability concerning the sintering and phase transformation of the composites. Rh with a 0.1 or 0.5 wt.% loading is deposited on these nanocomposites by incipient wetness impregnation method. After calcination at 1173 K for 5 h, these nanocomposites supported Rh catalysts are tested in the ﬁxed-bed reactor for methane partial oxidation. It is found that the smallerc-CeO2 crystal size in the nanocomposites, i.e. the higher oxygen vacancy concentration, would be responsible of the lower ignition temperature due to the enhanced reducibility, whereas the higher Rh dispersion would be responsible of the higher methane conversion and selectivity to synthesis gas. Additionally the Rh dispersion is found to be linked to the BET surface area. The stability of thenanocomposites is also studied under reaction conditions. © 2011 Elsevier B.V. All rights reserved.
Article history: Received 25 October 2010 Received in revised form 6 April 2011 Accepted 12 April 2011 Available online 25 May 2011 Keywords: Methane partial oxidation Rhodium Ceria Zirconia Alumina
1. Introduction Catalytic partial oxidation of methane (CPO) is an attractive route for theproduction of synthesis gas compared to the conventional steam methane reforming process (SMR). It gives a favorable H2 /CO (about 2) ratio of the produced synthesis gas, which is appropriate for processes such as methanol or Fischer–Tropsch synthesis . In addition, CPO can be operated in a millisecond contact-time reactor, being able to provide a compact hydrogen processor for its use in fuel cellelectric vehicles . Various transition metals such as Ni [3,4], Co , Ru , Pt , Ir  and Rh  are found active for this reaction. Among them, Rh based catalysts are presented as the most promising materials due to a better catalytic performance with higher activity, selectivity towards H2 and CO, and a good stability . The catalytic partial oxidation of methane to synthesis gas isthermodynamically favored at elevated temperatures to obtain a higher conversion and selectivity to H2 and CO . The mechanism has been frequently studied in the literature [1,4,11–14]. The discussion has mainly focused in the occur-
∗ Corresponding author. +4773594151. E-mail addresses: firstname.lastname@example.org, email@example.com (A. Holmen). 0920-5861/$ – see front matter © 2011Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2011.04.021
rence of the direct or the indirect routes. In the direct route H2 and CO are the primary reaction products [12,13] whereas in the indirect one CO2 and H2 O are initially produced by complete oxidation. Afterwards the reforming reaction and water gas shift (WGS) take place to produce syngas [11,14]. However, even though the...