The Optical PI .operties of Silver Microcrystallites in Dependence etc.this structure is fast, as demonstrated by the high estimated value of DE above. The system is comparable to the vinylferrocene films investigated by Daum and Murray [36]; in this case the swelling/deswelling transition is caused by the fact that in the oxidized state the film is polar while in the reduced state it is apolar.The kinetics of transition between the deswollen and the swollen state affects the current-voltage curves across the equilibrium potential region. Note the sharp peak at the slow sweep rate of Fig. 3, which originates from the structure change [36]. The rate of deswelling is determined by the rate at which ox is accumulated in the film, cf. the RRDE experiment, Fig. 6. This proceeds at anodic potentials by three mechanisms: (I) ox is produced by oxidation of red which has remained in the film, N:d(diff). (11) ox is formed anodically from red which arrives by diffusion from solution (cf. Zr(t), Fig. 6, with King = +0.393 V); (111) ox diffuses from solution into the film (cf.I,(t), with F', :ing = -0.22 V). Only at the slow sweeps the film remains in the anodic potential range sufficiently long for extensive deswelling to occur. At the fast sweeps of Fig. 3 the matrix apparently remains in the solvated state; the cyclic voltammetry peaks are symmetric and determined by diffusion of the redox-system in the solvated matrix. The diffusion co--efficients of ox and red are likely to be of comparable value in this state, as assumed in the analysis of the fast sweeps of Fig. 3, to determine V , , cussions and support of this project.We thank Professor Heinz Gerischer for continued stimulating dis- References

The steady and oscillatory regions of the CO oxidation reaction on the Pd(110) surface have been determined as a function of externally controlled parameters (flow rate, CO and oxygen partial pressures, temperature) over a wide range. At constant sample temperature and flow rate, the experiments yield a characteristic cross-shaped phase diagram separating regions of monostability, bistability, and oscillatory behavior. The existence of a cross-shaped phase diagram indicates the operation of a slow feedback process, which could be traced back to the (experimentally verified) formation and retarded removal of subsurface oxygen during the reaction. The diagram reflects one of the first well-defined oscillatory systems in heterogeneous catalysis and may provide a general basis for mechanistic studies and models of oscillatory surface reactions.

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