Oscillating CO oxidation

Bouillon et al. (1982) examined the oscillating CO oxidation on Pt as a possible candidate for the model by Takoudis, et al. (1981), see Section E.10. The results of Monte Carlo simulation of this model reaction were compared with their numerical results. 

Figure 3.21. FEM images of the oscillating CO oxidation over the (1 0 0)-oriented Pt-tip. (E.I. Latkin, V. I. Elokhin, V.V. Gorodetskii, Chemical Engineering Journal 91 (2003) 123-131).

An example of FEM-images of the oscillating CO-oxidation reaction is given in Figure 8.27. 

Gorodetskii, V., Drachsel, W., and Block, J.H., Imaging the oscillation CO-oxidation on Pt-siurfaces with field ion microscopy, Catal Lett., 19, 223-231, 1993. 

Fig. 9.34 FEM images of the oscillating CO oxidation reaction on Pt (1 0 0). (A) schematic view, (B)-(F) images at different time (s). (From E.I. Latkin, VI Elokhin, V.V. Gorodetskii, Spiral concentration waves in the Monte Carlo model of CO oxidation over Pd(l 1 0) caused by synchronisation via COads diffusion between separate parts of catalytic surface, Chem. Eng. J. 91 (2003) 123. Copyright 2003 Elsevier).

P. Moller, K. Wetzl, M. Eiswirth, G. Ertl. Kinetic oscillations in the catalytic CO oxidation Computer simulations. J Chem Phys 55 5328-5334, 1986. 

A. P. J. Jansen, R. M. Nieminen. A Monte Carlo study of CO oxidation with oscillations induced by site blocking. J Chem Phys 706 2038-2044, 1997. 

V. N. Kusovkov, O. Kortluke, W. von Niessen. Kinetic oscillations in the catalytic CO oxidation on Pt single crystal surfaces Theory and simulation. J Chem Phys 705 5571-5580, 1998. 

As shown on Figs. 8.31 to 8.33 the rate and UWR (or 0) oscillations of CO oxidation can be started or stopped at will by imposition of appropriate currents.33 Thus on Fig. 8.31 the catalyst is initially at a stable steady state. Imposition of a negative current merely decreases the rate but imposition of a positive current of200 pA leads to an oscillatory state with a period of 80s. The effect is completely reversible and the catalyst returns to its initial steady state upon current interruption. 

Figure 8.31. Induction of self-sustained rate and catalyst potential, or work function, oscillations by NEMCA during CO oxidation on Pt. Inlet composition Pco 0.47 kPa, Po2=16 kPa, T=297°C.33 Reprinted with permission from Academic Press.

Figure 8.33. Effect of applied constant current on the frequency of the self-sustained rate and Uwr oscillations during CO oxidation on Pt conditions as on Fig. 8.32 Filled circles on the frequency vs current diagram are oscillatory states of this figure open circles include states shown on Fig. 8.31.33 Reprinted with permission from Academic Press.

A striking feature of the effect of current on the CO oxidation oscillations is shown in Fig. 8.33. It can be seen that the frequency of oscillations is a linear function of the applied current. This holds not only for intrinsically oscillatory states but also for those which do not exhibit oscillations under open-circuit conditions, such as the ones shown on Fig. 8.31. This behaviour is consistent with earlier models developed to describe the oscillatory behaviour of Pt-catalyzed oxidations under atmospheric pressure conditions which are due to surface Pt02 formation35 as analyzed in detail elsewhere.33... 

ImhM R, Cox MP, Ertl G, Muller H, Brenig W. 1984. Kinetic oscillations in the catal34ic CO oxidation on Pt(lOO) Theory. J Chem Phys 83 1578. 

Cutlip and Kenney (44) have observed isothermal limit cycles in the oxidation of CO over 0.5% Pt/Al203 in a gradientless reactor only in the presence of added 1-butene. Without butene there were no oscillations although regions of multiple steady states exist. Dwyer (22) has followed the surface CO infrared adsorption band and found that it was in phase with the gas-phase concentration. Kurtanjek et al. (45) have studied hydrogen oxidation over Ni and have also taken the logical step of following the surface concentration. Contact potential difference was used to follow the oxidation state of the nickel surface. Under some conditions, oscillations were observed on the surface when none were detected in the gas phase. Recently, Sheintuch (46) has made additional studies of CO oxidation over Pt foil. 

The N20 decomposition, CO oxidation, and H2 oxidation reactions are known to exhibit concentration oscillations over noble metal catalysts. Flytzani-Stephanopoulos et al. (47) have observed oscillations for the oxidation of NH3 over Pt. The effects are dramatic and lead to large temperature cycles for the catalyst wire. Heat and mass transfer effects are important. 

Ortho-para deuterium, 27 25, 50 Ortho-para hydrogen conversion, 27 23 Oscillatory catalytic reactions, 37 213-215, 271-272 see also Platinum catalytic CO oxidation on Pt(l 11) and Pt(llO) surfaces COj formation, 37 216-217 kinetic oscillation mechanism, 37 220-228... 

PbOj anode, 40 155-156 oxygen evolution, 40 109-110 PCE, catalytic synthesis of, l,l,l-trifluoro-2,2-dischloroethane, 39 341-343 7t complex multicenter processes of norboma-diene, 18 373-395 PdfllO), CO oxidation, 37 262-266 CO titration curves, 37 264—266 kinetic model, 37 266 kinetic oscillations, 37 262-263 subsurface oxygen phase, 37 264—265 work function and reaction rate, 37 263-264 Pd (CO) formation, 39 155 PdjCrjCp fCOljPMe, 38 350-351 (J-PdH phase, Pd transformation, 37 79-80 P-dimensional subspace, 32 280-281 Pdf 111) mica film, epitaxially oriented, 37 55-56... 

Figure 7 Rate and e.m.f oscillations for CO oxidation over platinum at 337°C and inlet P02 = 5.43 x 10 2 bar (from 58). (a) Oscillations obtained on a preoxidised surface (b) oscillations obtained on a pre-reduced surface. (Reproduced from J. Catal, 1988,11, 152, by permission from Academic Press Inc.)...

Fig. 7. Variations in the near-edge fine structure at the Pt Lm edge of a Pt/zeolite catalyst during CO oxidation exhibiting chemical oscillations [adapted from Hagelstein et at. (45)).

In the last decade we have performed some thousands of experiments in packed adiabatic tubular reactors (CO oxidation), however, we have never observed oscillations. If on certain catalysts (e.g., Pt/Al203) the oscillation are caused by the kinetic mechanism then, apparently, the interactions of heat and mass transfer with chemical reaction suppress the occurence of periodic activity in tubular reactors. 

Self-oscillations have also been revealed for heterogeneous catalytic reactions. Hugo and Jakubith [7] and Wicke and co-workers [8] found self-oscillations for CO oxidation on platinum. In the period 1973-1975, M.G. Slinko and co-workers studied self-oscillations in hydrogen oxidation on nickel [9,10]. 

Critical effects in CO oxidation over Pt catalysts were obtained [33, 34, 63-85] in various catalytic systems over wires, foils and gauzes, on single pellets and fixed beds, in isothermal and adiabatic reactors (differential and integral). The literature also reported the oscillating behaviour of the homogeneous oxidation of CO [86, 87]. 

Studies of CO oxidation over Pt/Al203 [72-74] showed that oscillations are observed only in the presence of admixed hydrocarbons or water. If the reaction mixture is thoroughly purified, self-oscillations vanish. These data are evidently an argument in favour of the fact that reversible formation steps of non-reactive species play a special role in the appearance of selfoscillations. In accordance with the assertions of Marshneva et al. [163,164],... 

Qualitative studies of this dynamic model with three variables, i.e. surface concentrations of CO and the two forms of oxygen (surface and subsurface), showed [170] the possibility of interpreting self-oscillations in this catalytic system. Recently a comprehensive analysis of this model [170] has been carried out [177], Sales et al. [178, 179] determined experimentally the parameters for the oxidation and reduction of the Pt subsurface layer. The application of these parameters and those for the CO oxidation over Pt that are close to the values measured in high-vacuum experiments, made it possible to perform the quantitative reproduction, by using the model [180], of almost the whole of the experimentally observed characteristics for the self-oscillations in the reaction rate of CO oxidation over Pt. 

The studies of Ertl and co-workers showed that the reason for self-oscillations [142, 145, 185-187] and hysteresis effects [143] in CO oxidation over Pt(100) in high vacuum ( 10 4 Torr) is the existence of spatio-temporal waves of the reversible surface phase transition hex - (1 x 1). The mathematical model [188] suggests that in each of the phases an adsorption mechanism with various parameters of CO and 02 adsorption/desorption and their interaction is realized, and the phase transition is modelled by a semi-empirical method via the introduction of discontinuous non-linearity. Later, an imitation model based on the stochastic automat was used [189] to study the qualitative characteristics for the dynamic behaviour of the surface. 

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