Characterization and catalytic behavior of VO x –CeO 2 catalysts for the oxidative dehydrogenation of propane W. Daniell a , A. Ponchel a, , S. Kuba a , F. Anderle a , T. Weingand a , D.H. Gregory b and H. Kno¨zinger a a Department Chemie, Physikalische Chemie, Ludwig-Maximilians-Universita ¨t Mu ¨nchen, Butenandtstr. 5-13 (E), D-81377 Mu ¨nchen, Germany E-mail: Helmut.Knoezinger@cup.uni-muenchen.de b School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, UK A series of ceria-supported vanadium catalysts was prepared via impregnation of the support with an ammonium metavanadate solution. The 723K calcined samples were tested for propane oxydehydrogenation (ODH) activity and selectivity. The sample exhibiting the highest propane conversion was found to be the ceria support material itself, although this showed essentially no selectivity towards propene. An optimum propene yield of 4.2% was obtained at 673K for the 6wt% V 2 O 5 –CeO 2 sample. Con- version decreased with increasing V loading which was attributed to the formation of cerium vanadate (CeVO 4 ). This phase was found in all samples after calcination, its abundance rising in proportion to the V loading. In the 6wt% V 2 O 5 catalyst hydrated surface VO x species were present, although they underwent conversion to CeVO 4 at temperatures above 573K. The low reducibility of these surface vanadates was linked to the oxidation activity. It is inferred that surface polyvanadate species are responsible for the selective ODH of propane with V–O–V and/or V–O–Ce being the active oxygen species. KEY WORDS: vanadium; ceria; cerium vanadate; propane oxidative dehydrogenation 1. Introduction The efficient conversion of alkanes to alkenes, which can serve as key intermediates in various important industrial chemical processes, is one of today’s major catalytic goals [1]. Although there are several dehy- drogenation processes currently available, most involve endothermic reactions that must be carried out at high temperatures and lead to reduced selectivity (through hydrocarbon cracking) and deactivation of the catalyst (through coking) [2]. An alternative route is via oxida- tive dehydrogenation (ODH), an exothermic process requiring a selective catalyst to prevent complete oxi- dation of the alkane or further oxidation of the gener- ated alkene (see scheme 1). Supported vanadium oxides have proven themselves highly suitable catalysts for a variety of reactions involving the selective oxidation of hydrocarbons [3,4]. More specifically, numerous studies have been success- fully carried out using supported vanadium catalysts for the ODH of short-chain alkanes, in particular over alumina [5], zirconia [6,7] and magnesium oxide [8,9] supports. Furthermore, the activity and selectivity of these supported vanadium catalysts has been shown to be dependent upon both the nature of the support [10,11] and the dispersion of the vanadium component over the support [12]. Although the reaction mechanism for ODH has yet to be fully established, it is widely believed that the breaking of the C–H bond in the alkane is the rate determining step [13], and that the reaction proceeds via a Mars–Van Krevelen (redox)-type reaction mechanism [7]. Moreover, it is generally agreed that hydrocarbon oxidation over oxide catalysts involves the participation of lattice oxygen species or oxygen vacancies [14]. Cerium oxide (ceria) is well known as an oxygen-ion conductor due to its high concentration of oxygen vacancies and its oxygen mobility [15]. These properties have been linked to its catalytic activity in CO and methane combustion [16,17]. More recently ceria has been used for iso-butane ODH over Cr–Ce–O catalysts [18]; propane ODH over CeNi x O y mixed oxides [19]; and ethane ODH (with CO 2 ) over CaO-modified ceria systems [20]. In the present work a series of vanadium-loaded ceria catalysts has been prepared and their activity and selectivity towards propane ODH investigated. A vari- ety of characterization techniques have been employed to determine the nature of the vanadium species present in each catalyst. propane propene k 1 k 3 k 2 COx Scheme 1. Reaction routes for propane ODH. Topics in Catalysis Vol. 20, Nos. 1–4, July 2002 (# 2002) 65 1022-5528/02/0700-0065/0 # 2002 Plenum Publishing Corporation