Catalysis Today 57 (2000) 231–239 Vanadia/titania catalysts for gas phase partial toluene oxidation Spectroscopic characterisation and transient kinetics study D.A. Bulushev, L. Kiwi-Minsker * , A. Renken Institute of Chemical Engineering, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland Abstract Formation of vanadia species during the calcination of ball milled mixture of V 2 O 5 with TiO 2 was studied by Raman spectroscopy in situ and at ambient conditions. It is found that calcination in air leads to fast (1–3 h) spreading of vanadia over TiO 2 followed by a slower process leading to the formation of a monolayer vanadia. The calcinated catalyst showed higher activity during toluene oxidation than the uncalcinated one, but the selectivity towards C 7 -oxygenated products (benzaldehyde and benzoic acid) remains unchanged. The activity of the catalysts is ascribed to the formation of vanadia species in the monolayer. The details of the parallel–consecutive reaction scheme of toluene oxidation are presented from steady-state and transient kinetics studies. Different oxygen species seem to participate in the deep and partial oxidation of toluene. Coke formation was observed during the reaction presenting an average composition C 2n H 1.1n . The amount of coke on the catalyst was not dependent on the calcination step and the vanadium content in the catalyst. Coke formation was seen to be responsible for the deactivation of the catalyst. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Raman spectroscopy; Transient response technique; V/Ti oxide catalysts; Toluene partial oxidation 1. Introduction Vanadia/titania (V/Ti) catalysts are promising materials for the selective side chain oxidation of alkyl-aromatics [1,2]. It is known that the perfor- mance of V/Ti catalysts is sensitive to the nature and coverage by vanadia species, which depend on the method of the catalyst preparation. The choice of the vanadium precursor, the temperature and time of calcination, conditions of catalyst activation and surface impurities have also been shown to be impor- tant for catalyst performance. Due to the industrial application of V/Ti catalyst in the o-xylene oxidation to phthalic anhydride many attempts have been made * Corresponding author. Tel.: +41-21-693-31-82; fax:+41-21- 693-31-90. E-mail address: lioubov.kiwi-minsker@epfl.ch (L. Kiwi-Minsker). to correlate the catalytic performance of these cata- lysts with their surface properties [2]. However, many questions concerning the nature of the active sites remain still open. The nature of the vanadium oxy- gen bond, the specific role of the terminal (V=O) and bridging (V–O–V) active sites for selective oxidation of organic compounds has been discussed intensively in literature [1–3]. No clear consensus has emerged. FT-Raman or FT-IR spectroscopy of the catalyst sur- face coupled with transient technique studies should provide valuable information about the catalytic cycle and the active species leading to product formation [4,5]. This knowledge seems essential for the under- standing of the reaction mechanism and development of a more effective catalyst. Catalytic toluene oxidation on vanadia/titania catalysts gives benzaldehyde and benzoic acid as the important products, however CO x products are formed 0920-5861/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII:S0920-5861(99)00331-4