Studies on oscillations in the platinum catalyzed CO oxidation by means of an omegatron mass probe

Surface Science 107 (1981) L329-L333 North-Holland Publishing Company

SURFACE SCIENCE LETTERS STUDIES ON OSCILLATIONS IN THE PLATINUM CATALYZED OXIDATION BY MEANS OF AN OMEGATRON MASS PROBE

CO

D. BARKOWSKI, R. HAUL and U. KRETSCHMER Institut fiir Phsyikalische Chemie und Elektrochemie, D-3000 Hannover I, W.-Germany Received

29 December

1980; accepted

for publication

Universittit Hannover, Callinstr. 3A,

5 February

1981

A versatile all glass UHV apparatus for the study of catalytic reactions in a wide range of pressure is described. An omegatron in connection with different capillary leaks located close to the catalyst surface serves as a mass probe. As an example oscillating instabilities in the nonstationary CO/O* reaction in the mbar range are reported.

Various attempts have currently been made to overcome the pressure gap in catalytic studies between the conditions of UHV surface analysis techniques (p < 10m6 mbar) and of the industrial processes (p > 1 bar). Thus in one type of experiment the reaction is carried out at higher pressure in a separate reaction chamber and the catalyst is subsequently transferred by means of a vacuum lock to an UHV analysis system, e.g. refs. [l-3]. Alternatively the catalysis as well as the analysis is accomplished in the same UHV compatible apparatus which is evacuated before and after the reaction, e.g. ref. [4]. The different versions of these principles have their merits, however, they all suffer from the fact that the catalyst surface is no longer under working conditions when analyzed. A continuous mass spectrometric analysis of the reactants and reaction products can be achieved by means of a gas sampling leak while the catalytic reaction is proceeding [S]. The UHV flow apparatus, schematically shown in fig. 1, enables the immediate measurement of gas composition in direct vicinity of the adjustable catalyst surface in a wide range of pressure (10-‘-l@’ mbar) by means of a mass probe, consisting of a capillary leak in connection with an omegatron mass spectrometer. In order to avoid interactions of gases with the metal walls of conventional, large volume UHV systems, a compact all glass apparatus has been constructed. Gas analysis is achieved by an inert omegatron (OM) with gold plated components as developed by Gentsch [6]. The capillary leaks (L, diam. IO-SO, length --lo0 pm) are prepared by applying a directed electrical discharge to the thin walled, closed end of Duran glass tubes, alternatively quartz leaks could be used [7,8]. Leaks of larger diameter for measurements at lower pressures are obtained

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D. Barkowski et al. /Oscillations in platinum catalyzed CO oxidation

‘H,TC Fig. 1. Omegatron

mass probe

system

for catalytic

studies in a wide pressure

range; schematic.

by conventional glass blowing techniques. The tubes with the leak are interchangeably mounted on a small stainless steel flange (F) which is protected from the reaction chamber by a ground glass joint (GJ). The catalyst (C) may be used as a suspended wire or as a polycrystalline foil, single crystal plate and thin layer of fine particle material supported on a glass or quartz sample holder (S) with a plane thin walled end. The temperature of the catalyst can be controlled by programmed heating (H) either directly or by means of a heating coil embedded in the top of the sample holder. Thermocouples (TC) which may be spotwelded to metal catalyst surfaces serve for temperature measurement. In order to avoid temperature gradients between the catalyst surface and the ambient gas, the inflowing reaction mixture is preheated and the reactor is kept at the reaction temperature. The distance between the catalyst surface and leak is adjustable by a driving mechanism (D) sealed on to the sample holder and can be measured microscopically through a plane window (W). The reaction chamber as well as the omegatron can be baked out and are evacuated by turbomolecular pumps (TMP). Argon ion bombardment is not provided, rather it is relied on the formation of a reproducible state of the catalyst surface under stationary reaction conditions. The flow of reactants is controlled by metal leak valves (LV) which can be separated from the reaction chamber by an UHV glass valve (GV). The total pressure is measured with an ionization gauge (IM 2, low7

1 mbar). For quantitative kinetic studies - not dealt with in this short communication calibration of the omegatron is required, taking into account, e.g.: transition from

D. Barkowski et al. /Oscillations 235 I

VI +: s

Fig. 2. Onset

of oscillations

in platinum catalyzed CO oxidation 240 I

with increasing

245,,260 I

temperature;

L331

T / Y

2.5’C/min, p(tota1) = 16 mbar, 1.3%

co. viscous to molecular flow along the leak, influence of the temperature on gas density, calibration of ionization gauges. The capability of the versatile catalytic flow reactor and mass probe device is exemplified by results obtained in a study on oscillating instabilities in the platinum catalyzed CO oxidation. Typical results of non-stationary flow experiments with a polycrystalline platinum foil (0.05mm thickness, 0.9cm2 area, distance from leak
Fig. 3. Oscillations

at increasing

oxygen

pressure;

0.5 mbar 02/s,

p(C0)

= 0.2 mbar,

26O’C.

Fig, 4. Intluence of oxygen (a) 01 carbon monoxide (b) preadsorption oscillations: 26O’C.

on the evolution of

escalations suddenly appear at a critical gas composition of 1.3% CO with decreasing amplitude and increasing frequency. Particularly instructive are experiments in which the platinum surface is previously exposed to either 02 or CO with sudden addition of the other reactant, When oxygen is preadsorbed the oscillations set in practically irlstantaneously at full ampfitude and constant frequency (fig. 4a). The small time lag is mainly caused by gas mixing. Contrar~y, in the case of CO preadsorption a conside~ble delay is observed and the osculations develop only gradually (fig. 4b). While so far oscillations in the catalytic CO/O2 reaction have only been observed near atmospheric pressure, alike instabihties were found here in the range of a few mbar. Recently, Adlhoch, Lintz and Weisker [lo] reported the occurrence of osculations even at pressures below 7 X lo4 mbar for the oxidation of CO with NO on platinum. The present findings in transient flow experiments are in agreement with results obtained in comprehensive studies, particularly by Wicke et al. [ 1 I]. These authors give an interpretation based on kinetic argurnents for the cyclic surface reaction which proceeds almost iso~ermal~y. Two different sites are assumed, both of which can adsorb CO with different energies, while only one is able to competitively adsorb oxygen atoms. Reaction OGGWS between species adsorbed on adjacent sites (~ngmuir-~~inshelwood ~~echanism~. The oscillations in the rate of CO2 pruduction are caused by alternations between the forn~at~on of CO clusters subsequent to a high reactivity state, and stimulated CO desorption subsequent to a low reactivity state. While the CO adsorption on a preadsorbed oxygen layer is readily possible (WITIpression of the surface layer, surface mobility of CO [12]), oxygen adsorption in the reverse case requires prior CO desorption {fig. 4).

D. Barkowski et al. / ~s~ill~~ions in platinum catalyzed CO oxidation

I.333

The nature and distribution of the different adsorption sites is as yet not clear. In principle, two concepts may be considered: (i) the bonding configuration of the adsorbate depends on the crystallographic structure and topography of the substrate surface; (ii) different bonding configurations on a uniform surface may be due to mutual interactions between adjacent adsorbed species of the same or different kind; correspondingly the surface coverage is a determining factor. For instance, in CO adsorption on platinum it is known [12] that the adsorption energy as well as the structure of the overlayer changes abruptly at certain coverages. Further information on the oscillation mechanism could be obtained from studies on single crystal faces in a wide pressure range, in connection with results obtained particularly from TDS, work function change, and photoelectron spectroscopy. We wish to thank Prof. Dr. H. Gentsch, Universitst Essen, for his readiness to leave the omegatron set up at our disposal and for helpful suggestions. Financial support by “Fonds der Chemischen Industrie” and “Max Buchner Forschungsstiftung” are gratefully acknowledged.

References [I] H.D. Polaschegg, A. Jungel and E. Schirk, Vakuum-Tech. 28 (1979) 227. [ 21 M. Seidl, E. SteinheiI and W. Scherber, Deut. Pat. Off. 2620941 (1977). [3] D. KitzeImann, W. Vieistich and T. Dittrich, Chem. Ing.-Tech. 49 (1977) 463. [4] D.W. Blakely, E.I. Kozak, B.A. Sexton and G.A. Smorjai, J. Vacuum Sci. Technol. 13 (1976) 1091. [5 ] H. Hammarquist, B. Kasemo, K.E. Keck and T. Hogberg, Ned. Tijdschr. Vacuumtech. 16 (1978) 67 (presented at ECOSS I, Amsterdam, 1978). [6] H. Gentsch, W. Grossmann, N. GuiIlen, M. Kijpp and H. Rinne, Vakuum-Tech. 23 (1974) 230. [7] F.P. Lossing and A.W. Tickner, J. Chem. Phys. 20 (1952) 907. [8] B. Kasemo, Rev. Sci. Instr. 50 (1979) 1602. [9] E.g., LeyboldHeraeus, K&t, IM-Type 110. [lo] W. AdIhoch, H.G. Lintz and T. Weisker, presented at: Kinetics of Physicochemical OsciIIations, Discussion Meeting by Deutsche Bunsenges. Physik. Chem., Aachen 1979. Abstract in: Ber. Bunsenges. Physik. Chem. 84 (1980) 410. [ 1I] E. Wicke, P. Kummann, W. Keil and J. Schiefler. Ber, Bunsenges. Physik. Chem. 84 (1980) 315; W. KeiI and E. Wicke, Ber. Bunsenges. Physik. Chem. 84 (1980) 315. 1121 T. Engel and G. Erti, Advan. Catalysis 28 (1979) 1.