Dynamic instability at solid-gas interface

Earlier, it was difficult to produce a clean surface and to characterize its surface structure. However, with the development of electronic industry, techniques have been developed to produce clean surface with well-defined properties. It has been possible to investigate catalytic oxidation on metal surface in depth. Example of dynamic instability at gas-liquid interface is provided by such studies. Studies on chemical oscillations during oxidation of CO over surface of platinum group metals have attracted considerable interest [62-68]. 

Experimental studies of interaction have been made on well-defined Pt (100) and (1010) surfaces [64, 65]. The experiments were performed in a standard UHV system equipped with facilities for LEED, AES work function measurement () with a Quadruple Mass Spectrometer and self-compensation vibration capacitor method. 

Under the influence of the adsorbate, the surface structure may switch periodically between more (=non-reconstructed) and less reactive (=reconstructed) state, whereby the driving forces are the difference in surface free energy of the clean planes on the one hand and the difference in CO adsorption energy on the other. In other words, the reconstructed phase adsorbs CO more rapidly than it is reacted. Thus, the CO coverage increases beyond its critical value for nucleation of the structural transformation into the non-reconstructed state. The latter exhibits an increased oxygen coefficient so that CO is removed more rapidly from the surface. As a result, the CO coverage drops and the surface transforms back to reconstructed states. 

The clean Pt (100) and (110) surfaces are reconstructed and then construction is lifted if a critical CO coverage is reacted. Both modifications of the respective planes exhibit different sticking coefficients so that as a net result, the surface switches between the states of high and low reactivities. Thus, the rate of catalytic CO oxidation on defined Pt (100) and Pt (110) surfaces at low pressure under isothermal condition exhibits temporal oscillations which are coupled with periodic transformation of the surface structures between reconstructed and non-reconstructed phases [66], 

Reconstructed (1x2) structure is formed when crystal is cut, during the cleaning process at 800°C. During CO oxidation, the surface structure changes from (1 x 2) to non-structured (1x1) form. 

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