Investigation of Support-Effects in VOx-Catalysts using Electrochemical Methods

• SFB 546 Struktur, Dynamik und Reaktivität von Übergangsmetalloxid-Aggregaten

• Teilprojekt B7: Untersuchung der Aktivkomponente-Träger-Wechselwirkung

• PhD Thesis: “Investigation of Support-Effects in VO_x-Catalysts using Electrochemical Methods”

Introduction

Crude oil is the basis for most of the products in the chemical industry. As crude oil is getting short prises are rising and therefore new resources like oil sands become profitable and offshore platforms for the exploration of oil fields deep under the sea are increasingly build. Thus, for economical and ecological reasons the resources have to be used more efficiently. For this purpose new catalysts are developed in order to reduce the amount of waste and to raise the yield of the desired products. But for the design of new catalysts it is necessary to understand the details of the reaction mechanism. Therefore numerous reactions have been investigated over the recent decades to elucidate the mechanisms of heterogeneous reactions.

Figure 1: Reduction and reoxidation step in the Mars-van-Krevelen mechanism

© TUB, ES3

Vanadium oxide (VO_x) has been investigated for various catalytic reactions like the oxidative dehydrogenation of propane (ODP) [1, 2]. This is a heterogeneous catalytic reaction in which the surface of the VO_x catalyst reacts with propane to form propene and water. This process follows a redox cycle known as Mars-van-Krevelen mechanism [3]. In this mechanism donation of oxygen from VO_x plays a pivotal role. In the reduction cycle lattice oxygen accepts hydrogen from the alkane and desorbs as water leaving a vacancy in the catalyst (figure 1, step I.). The role of oxygen as a hydrogen acceptor is the reason for the reaction to be exothermal and therefore energetically favourable in contrast to normal dehydrogenation reactions [4]. In the reoxidation cycle these vacancies are then refilled from the gas phase oxygen through diffusion in the lattice (figure 1, step II.). Therefore the capability of oxygen defect formation of a catalyst should have an influence on the reaction properties. But pure VO_x is mostly not applied as a catalyst. Usually it is combined with other metal oxides in order to improve the properties of those catalysts. These metal oxides are called “support” and interact with the active compound and hence alter their chemical performance. Therefore the influence of the support on the defect formation and the catalytic properties are investigated in this work.

Method

Figure 2: Impedance plot of a catalyst. 1: Volume conduction 2: Diffusion process 3: Formation of a double-layer

© TUB, ES3

A wet impregnation technique is used for the preparation of the supported catalysts. Different supports in varying concentrations are applied (e.g.: α-Al₂O₃, TiO₂ (Rutile), MgO, SiO₂ and ZrO₂). For the investigation of defect formation and oxygen diffusion electrochemical impedance spectroscopy (EIS) is used. This is an AC conductivity measurement, which gives more information than a standard DC experiment. With EIS it is possible to resolve different processes in the sample like diffusion of charged species (electrons, ions) and reactions occurring at the electrodes. Furthermore it allows the investigation of the diffusion mechanism.
Figure 2 depicts an example of an impedance plot giving valuable information. The arc at position 1 can be attributed to a relaxation process occurring in the volume of the sample giving the bulk conductivity. Position 2 shows a deformation of the perfect arc indicating a diffusion process what is supported by the formation of a charged double layer at the electrodes (position 3). The separation into these processes permits the determination of the bulk conductivity without influences due to electrode effects.

References [1] B. M. Weckhuysen, D. E. Keller: Catal. Today 78 (2003) 25 [2] G. C. Bond and F. Tahir: Appl. Catal. 71 (1991) 1 [3] P. Mars and D. W. van Krevelen: Chem. Eng. Sci. Special Suppl. 3 (1954) 41 [4] Jerzy Haber in: G. Ertl, H. Knözinger, F. Schüth, J. Weitkamp (Eds.), Handbook of Heterogeneous Catalysis, Wiley-VCH, Weinheim (2008) 3367