Oscillating Surface Reactions

Oscillating reactions between small molecules such as CO and O2, CO and NO, and NO and H2 on surfaces proceed through relatively simple mechanisms in which the concentration of surface vacancies usually plays an important role. 

Oscillations have been observed in surface reactions also. This phenomenon in the oxidation of CO on the surfaces of palladium and platinum has been extensively studied (Conrad et ai, 1974 Engel and Ertl, 1978). Whether oscillations actually arise depends sensitively on the reaction temperature and the gas-phase concentrations of CO and O2. Although different mechanistic reasons for the occurrence of oscillations exist, autocatalytic steps are always involved. 

The reaction sequence of this mechanism is as follows ( and represent sites on the unreactive and reactive surfaces, respectively). At the start, where the surface is empty and in the less reactive (1x2) form, we have the following reaction steps O2 + 2 j 2 — 2 Ogjjj slow 6q 0 ( ) 

If Oqq exceeds a critical value, the surface switches to the unreconstructed form, where we have the following reactions  

If 9qq decreases below the critical value, the surface reconstructs back to the unreactive form, where the oxidation reaction proceeds until all Oads is removed  

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How relevant are these phenomena First, many oscillating reactions exist and play an important role in living matter. Biochemical oscillations and also the inorganic oscillatory Belousov-Zhabotinsky system are very complex reaction networks. Oscillating surface reactions though are much simpler and so offer convenient model systems to investigate the realm of non-equilibrium reactions on a fundamental level. Secondly, as mentioned above, the conditions under which nonlinear effects such as those caused by autocatalytic steps lead to uncontrollable situations, which should be avoided in practice. Hence, some knowledge about the subject is desired. Finally, the application of forced oscillations in some reactions may lead to better performance in favorable situations for example, when a catalytic system alternates between conditions where the catalyst deactivates due to carbon deposition and conditions where this deposit is reacted away. 

Imaging on a larger scale, in particular when large numbers of adsorbate molecules organize themselves in patterns of micrometer size, is possible by using recently developed methods based on photoemission (PEEM) and ellipsometry (EMSI). These techniques have provided us with spectacular movies of reaction wave fronts moving over surfaces. These phenomena are characteristic of oscillating surface reactions, a subject that has fascinated many in catalysis and surface science [81. 

Suchorski, Yu., Imbihl, R., and Medvedev, V.K., Compatibility of field emitter studies of oscillating surface reactions with single-crystal measurements catalytic CO oxidation on Pt, Surf. Set, 401, 392-399, 1998. 

Eiswirth, M. Krischer, K. Ertl, G. 1988. Transition to Chaos in an Oscillating Surface Reaction, Surf. Sci. 202, 565-591. 

Very recently, considerable effort has been devoted to the simulation of the oscillatory behavior which has been observed experimentally in various surface reactions. So far, the most studied reaction is the catalytic oxidation of carbon monoxide, where it is well known that oscillations are coupled to reversible reconstructions of the surface via structure-sensitive sticking coefficients of the reactants. A careful evaluation of the simulation results is necessary in order to ensure that oscillations remain in the thermodynamic limit. The roles of surface diffusion of the reactants versus direct adsorption from the gas phase, at the onset of selforganization and synchronized behavior, is a topic which merits further investigation. 

R. J. Gelten et al. Monte Carlo simulation of a surface reaction model showing spatio-temporal pattern formations and oscillations. J Chem Phys 705 5921-5934, 1998. 

The oxidation of CO on Pt is one of the best studied catalytic systems. It proceeds via the reaction of chemisorbed CO and O. Despite its complexities, which include island formation, surface reconstruction and self-sustained oscillations, the reaction is a textbook example of a Langmuir-Hinshelwood mechanism the kinetics of which can be described qualitatively by a LHHW rate expression. This is shown in Figure 2.39 for the unpromoted Pt( 111) surface.112 For low Pco/po2 ratios the rate is first order in CO and negative order in 02, for high pco/po2 ratios the rate becomes negative order in CO and positive order in 02. Thus for low Pcc/po2 ratios the Pt(l 11) surface is covered predominantly by O, at high pco/po2 ratios the Pt surface is predominantly covered by CO. 

McKarnin, M. A., Schmidt, L. D., and Aris, R. (1988). Forced oscillations of a self-oscillating bimolecular surface reaction model. Proc. R. Soc., A417, 363-88. 

A comparative study was done by Kevrekidis and published as I. G. Kevrekidis, L. D. Schmidt, and R. Aris. Some common features of periodically forced reacting systems. Chem. Eng. Sci. 41,1263-1276 (1986). See also two papers by the same authors Resonance in periodically forced processes Chem. Eng. Sci. 41, 905-911 (1986) The stirred tank forced. Chem. Eng. Sci. 41,1549-1560 (1986). A full study of the Schmidt-Takoudis vacant site mechanism is to be found in M. A. McKamin, L. D. Schmidt, and R. Aris. Autonomous bifurcations of a simple bimolecular surface-reaction model. Proc. R. Soc. Lond. A 415,363-387 (1988) Forced oscillations of a self-oscillating bimolecular surface reaction model. Proc. R. Soc. Lond. A 415,363-388 (1988). 

L. Forced Oscillations of a Self-Oscillating Bimolecular Surface Reaction Model... 

Takoudis, C. G., Schmidt, L. D. Aris, R. 1981 Isothermal sustained oscillations in a very simple surface reaction. Surf. Sci. 105, 325. 

FORCED OSCILLATIONS OF A SELF-OSCILLATING BIMOLECULAR SURFACE REACTION MODEL... 

The effects of forced oscillations in the partial pressure of a reactant is studied in a simple isothermal, bimolecular surface reaction model in which two vacant sites are required for reaction. The forced oscillations are conducted in a region of parameter space where an autonomous limit cycle is observed, and the response of the system is characterized with the aid of the stroboscopic map where a two-parameter bifurcation diagram for the map is constructed by using the amplitude and frequency of the forcing as bifurcation parameters. The various responses include subharmonic, quasi-peri-odic, and chaotic solutions. In addition, bistability between one or more of these responses has been observed. Bifurcation features of the stroboscopic map for this system include folds in the sides of some resonance horns, period doubling, Hopf bifurcations including hard resonances, homoclinic tangles, and several different codimension-two bifurcations. 

The model that will be used for forced oscillation studies is one which was first proposed by Takoudis et al. (1981) as a simple example of an isothermal surface reaction without coverage dependent parameters in which limit cycles can occur. The bimolecular reaction between species A and B is presumed to occur as a Langmuir-Hinshelwood bimolecular process except that two adjacent vacant sites on the surface are required for the reaction to take place. 

Isothermal sustained oscillations in a very simple surface reaction (with C.G. Takoudis and... 

Isothermal oscillations in surface reactions with coverage independent parameters (with C.G. Takoudis and L.D. Schmidt). Chem. Eng. Sci. 37, 69-76 (1982). 

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