Oxygen island

What happens to the methoxy formed by this process is strongly temperature dependent. At low temperature (up to - 340K) it is stable on the surface and forms the beautiful structures shown in fig.2. Since the active oxygen is used in such reactions then the methoxy must (i) not block the active site at its formation or (ii) diffuses away from the active site. Our evidence indicates the latter to be the case since methoxy is present at sites away from the oxygen islands. Above approximately 340 K the methoxy is unstable and decomposes to yield formaldehyde and hydrogen in the gas phase. Above approximately 400 K, the stoichiometry of the reaction changes to... 

Figure 4.14 STM images of 1 L of oxygen exposed to Pt(lll) at the temperatures indicated, emphasising the anisotropic growth of oxygen islands. The scale bar of the image at 160K is 30 A that at all other temperatures is 50 A. (Reproduced from Ref. 25).

In summary, in situ STM studies of CO titration on the oxygen precovered metal surfaces have demonstrated atomic details of CO oxidation on metal surfaces and have shown excellent agreement with macroscopic kinetic measurements. Moreover, in situ studies have revealed an interesting but not well-understood, nonlinear behavior of reaction kinetics. The accelerated reaction rate observed takes place only when surface oxygen islands, either compressed oxygen islands or surface oxide islands, are reduced to the nanometer size. The nonlinear reactivity of these nanoislands is in stark contrast with the large adsorbate layer and requires further investigations. 

While for Ru(OOOl) [22] and Rh(lll) [24] the attractive interactions lead to dense oxygen islands with a local (1 x 1) ordering below the surface, cf. Fig. 5.7, at Ag(lll) it already becomes more favorable to incorporate oxygen rather... 

C0(ads)+0(ads)— C02(ads)+, 4.C02(ads) — C02(gas)+. The probabilities of steps 1 and 2 are between 0 and 1, while probabilities of other steps are P(3) = 1, P 4) = 1, P(-l)= 0 P(-2)= 0, P(-4)=0. The ZGB-model shows the effect of heterogeneity in the adlayer because of the infinitely fast formation of C02, there is a segregation of the reactants in CO and oxygen islands. The original model has later been extended and modified by numerous people to include desorption of the reactants, diffusion, an Eley Rideal mechanism for the oxidation step, physisorption of the reactants, lateral interactions, an oxidation step with a finite rate constant, surface reconstruction and additional poisoning adsorbates. 

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