Oscillatory Phenomena in Heterogeneous Catalysed Oxidation Reactions Oxidation Reactions

BY D. MUKESH, M. GOODMAN, C. N. KENNEY, AND W. MORTON

1 Introduction

The observation of oscillations in heterogeneous catalytic reactions is an indication of the complexity of catalyst kinetics and makes considerable demands on the theories of the rates of surface processes. In experimental studies the observed fluctuations may be in catalyst temperature, surface species concentrations, or most commonly because of its accessibility, in the time variation of the concentrations of reactants and products in contact with the catalyst. It is now clear that spontaneous oscillations are primarily due to non-linearities associated with the rates of surface reactions as influenced by adsorbed reactants and products, and the large number of experimental studies of the last decade have stimulated a considerable amount of theoretical kinetic modelling to attempt to account for the wide range of oscillatory behaviour observed.

Several homogeneous gas- and liquid-phase reactions are now also known to exhibit self oscillations and it is clear that many living organisms depend on coupled oscillatory reactions catalysed by enzymes to control biological functions. However, only heterogeneous oxidation reactions catalysed by noble metals are reviewed here. Experimental studies are first described, followed by a discussion of kinetic analyses which have been put forward to account for them. Particular attention is given to the most extensively studied system to date, the oxidation of CO over Pt catalysts.

2 CO Oxidation

Sheintuch and Schmitz have thoroughly reviewed oscillatory oxidation reactions up to 1977. A year later another review was published by Slinko and Slinko. Varghese et al. reported oscillations in the rate of CO production during the oxidation of CO over Pt supported on γ-[Al.sub.2][O.sub.3] catalyst between 100–150 °C. They also observed multiple peak limit cycles in gas-phase concentrations in the presence of hydrocarbon impurities. Oscillations have been observed by Plichta and Schmitz and by Sheintuch over Pt foil over a temperature range of 150–250 °C. The former authors also detected simultaneous oscillations in the catalyst temperature. The time period of oscillation observed by Sheintuch was of the order of 25 min. The oscillations were of single and multiple peak type. The feed gas concentration was varied between 0–10% CO and 10–20 0. Turner et al. observed rate oscillations on Pt wire over a wide range of temperature from 150–300°C. The oscillations were observed when the ratio of partial pressures of CO to 0 was in the range of 0.001 to 0.045. Dauchot and Van Cakenberghe have also observed oscillations on Pt wire. Gray et al. and Barkowski et al. have found oscillations in the production of CO2 when a feed of 5% CO in O2 was passed over polycrystalline Pt at 250°C. Hugo and Jakubith reported oscillations when a mixture of CO and air was passed over Pt gauze at 120°C. The time period of these oscillations was around 2min. Beusch et al., McCarthy et al., and Rathousky et al. have observed oscillations at 180 °C over a Pt catalyst supported on A10. Beusch et al. also found oscillations in catalyst temperature of the order of 2–3 °C. The oscillations observed by Rathousky et al. could be spikes or quasi-sinusoid al with a period of oscillations as high as 8h. Turner et al. found oscillations in C0 production under similar conditions even when the reaction was carried out over polycrystalline Pd or Ir wire. Sales et al. observed formation of an 'oxide' layer on the Pt wire during the reaction and they suggested this fact as the cause of the observed oscillations.

3 H2 Oxidation

The oxidation reaction of H2 has also been shown to exhibit relaxation and sinusoidal oscillations over noble metal catalysts for a wide range of temperatures. Thus Wicke et al. observed oscillations during the oxidation of H2 over 0.4% Pt on SiO2–Al2O3 support over a temperature range of 95–200 °C. The variation in the catalyst temperature was of the order of [+ or -] 20 °C. Rajagopalan and Luus observed oscillations over Pt wire in the presence of impurities. Beusch et al. observed similar oscillations when a mixture of 3.14% H2 in air was passed over Pt catalyst at 80 °C. Horak and Jiracekand Zuniga and Luus have also observed oscillations over Pt catalyst. The former authors observed that there was a very large difference between the catalyst and the gas temperature when oscillations occur. Kurtanjek et al. observed oscillations on Ni plate over a temperature range of 160–400 °C. Belyaev et al. found oscillations over Ni foil at 180°C in the presence of excess H. The period of oscillation varied between 6 and 120 s. Schmitz et al. observed multi-peak oscillations and chaotic behaviour when H2 oxidation was carried out on Ni catalyst in the presence of excess H2.

4 Hydrocarbon Oxidation

Vayenas et al. observed multiple peak oscillations during the oxidation of ethene over polycrystalline Pt film between 200–400 °C. Formation and disintegration of an oxide film on the catalyst was given as the reason for the observed periodic behaviour. Sheintuch and Luus recently observed oscillations during the oxidation of propene when 1% propene in 0 was passed over Pt wire over a temperature range of 175–228 °C. Krylov and his co-workers have also reported oscillations in cyclohexane oxidation on Zeolite NaX and in propene oxidation on the surface of CaO–MgO solid solutions.

5 NH3 Oxidation

Stephanopoulos et al, have reported oscillations in studies on the oxidation of NH3 over Pt wire or foils. In this case there were temperature fluctuations on the catalyst of the order of 20 °C. The feed consisted of 20&dnash;40% NHin air. The period of oscillation varied from 1–200s. The oxidation products consisted of NO, N2, and H2O.

6 Oxidation of CO/H2 or CO/Hydrocarbon Mixtures

Oscillations have been observed in this department when mixtures of gases were oxidized over supported Pt catalyst, although such behaviour was not found during the oxidation...