Dynamics of redox behavior of nano-sized VO x species over Ti–Si-MCM-41 from time-resolved in situ UV/Vis analysis Olga Ovsitser, Maymol Cherian, Angelika Brückner, Evgenii V. Kondratenko * Leibniz-Institut für Katalyse e. V. an der Universität Rostock, Albert-Einstein-Str. 29a, D-18059 Rostock, Germany article info Article history: Received 5 February 2009 Revised 1 April 2009 Accepted 3 April 2009 Available online 9 May 2009 Dedicated to Professor Dr. Manfred Baerns on the occasion of his 75th birthday. Keywords: In situ UV/Vis Vanadia Silica Titanium Propane Propene ODP N2 O Reaction mechanism abstract Time-resolved in situ UV/Vis spectroscopy has been successfully applied for analyzing the reduction degree of highly dispersed VO x (0.4–1.1 V/nm 2 ) species over Ti–Si-MCM-41 (Ti/Si = 0–1.5) materials in the oxidative dehydrogenation of propane (ODP) as well as for deriving the kinetic parameters of reduc- tion (with C 3 H 8 ) and reoxidation (with O 2 and N 2 O) of these species. This quantitative kinetic study enabled to recognize at least two types of differently reducible VO x species in the samples with titanium. Three effects of titanium on redox properties of VO x species were identified: (i) the constant of reduction of oxidized VO x species by C 3 H 8 increases, (ii) the constant of oxidation of reduced VO x species by O 2 decreases; and (iii) the constant of oxidation of reduced VO x species by N 2 O increases. These effects are highly relevant for the ODP performance. The intrinsic ODP activity increases linearly with an increase in the constant of reduction of oxidized VO x species. Propene selectivity is determined by the ratio of the constants of reduction and reoxidation of VO x species; the higher the ratio, the higher the selectivity. This ratio can be tuned by the oxidizing agent (O 2 vs. N 2 O) and by the Ti/Si ratio. Ó 2009 Elsevier Inc. All rights reserved. 1. Introduction Monitoring the events taking place over catalytic materials un- der real reaction conditions is crucial for elucidating the reaction mechanisms, and for establishing the relationships between cata- lytic performance (reactivity/selectivity) and physico-chemical properties of the catalysts. This knowledge is of great importance for the rational design of new or improved catalytic materials. UV/Vis spectroscopy nowadays is one of a very few methods for characterizing heterogeneous catalysts under their working condi- tions [1–8]. When combining it with mass-spectroscopic and/or gas-chromatographic analysis, the state of catalytic materials, their activity, and selectivity can be simultaneously determined in one reactor system [9]. As a result, problems arising from differences in reaction conditions and cell designs are avoided. Therefore, a di- rect link between the physico-chemical and catalytic properties can be established. UV/Vis spectroscopy in different modifications has been applied to investigate the extent of reduction of VO x species under different reaction conditions [5,7,10–12]. To our best knowledge, only a cou- ple of studies has been aimed to analyze the kinetics of reduction and reoxidation of active surface species by in situ UV/Vis spectros- copy. The kinetics of reduction of CrO 3 by CO has been investigated in [13]. Kinetic parameters of reduction of VO x /MCM-41 by hydro- gen have been determined in [14]. Argyle et al. [15] have moni- tored the dynamics of reduction of VO x species over Al 2 O 3 with propane from transient analysis of UV/Vis spectra in the near-edge region. In situ UV/Vis spectroscopic study in the range of d–d tran- sitions of reduced VO x species enabled to determine redox kinetics of VO x species in VO x /TiO 2 catalysts used in oxidative scission of butane to acetic acid [16]. Two different processes, a fast one and a slow one, have been identified and attributed to reduction/reox- idation of VO x species on the outer surface and in the subsurface layers. Very recently, UV/Vis spectroscopy was combined with in situ X-ray absorption near-edge spectroscopy (XANES) to deter- mine the temporal evolution of coverage by hydroperoxo species over gold/titanosilicalite catalysts in the propane oxidation with H 2 and in the propylene epoxidation [17,18]. Moreover, these authors have demonstrated that the reaction rates determined by XANES were close to the turnover rates measured in a catalytic flow reactor, indicating that the hydroperoxo species were true intermediates in these reactions. In our previous study [11], we have qualitatively demonstrated that the reduction of VO x species 0021-9517/$ - see front matter Ó 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.jcat.2009.04.005 * Corresponding author. Fax: +49 381 1281 51290. E-mail address: evgenii.kondratenko@catalysis.de (E.V. Kondratenko). Journal of Catalysis 265 (2009) 8–18 Contents lists available at ScienceDirect Journal of Catalysis journal homepage: www.elsevier.com/locate/jcat