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The technologically important heterogeneous catalytic reactions are also interesting as systems open to the environment, exhibiting selforganisation. Concentration oscillations, spatial patterns and chaos have been observed in many heterogeneous catalytic systems within suitable parametric regimes. In the present thesis oscillatory mechanisms which take place in surface reactions are studied through Kinetic Monte Carlo (KMC) simulations. Themechanismof oscillations that emerge in the catalytic oxidation of CO on Pt(100) under low pressure conditions is studied first. The oscillations that are observed in this system have been attributed to changes in the surface structure depending on the CO coverage. On the two structures of Pt(100): 1?1 and hex, oxygen adsorbs with a different sticking probability (it is much lower on hex), affecting the surface reactivity. Thus, restructuring between these two configurations produces oscillations of the reactivity and hence of concentrations also. F ...
The technologically important heterogeneous catalytic reactions are also interesting as systems open to the environment, exhibiting selforganisation. Concentration oscillations, spatial patterns and chaos have been observed in many heterogeneous catalytic systems within suitable parametric regimes. In the present thesis oscillatory mechanisms which take place in surface reactions are studied through Kinetic Monte Carlo (KMC) simulations. Themechanismof oscillations that emerge in the catalytic oxidation of CO on Pt(100) under low pressure conditions is studied first. The oscillations that are observed in this system have been attributed to changes in the surface structure depending on the CO coverage. On the two structures of Pt(100): 1?1 and hex, oxygen adsorbs with a different sticking probability (it is much lower on hex), affecting the surface reactivity. Thus, restructuring between these two configurations produces oscillations of the reactivity and hence of concentrations also. Following the example of the ZiffGulariBarshad (ZGB) model, a pioneer in KMC simulations of surface reactions, in our simulations we first implement the LangmuirHinshelwood reaction steps for CO oxidation on a square lattice. Poisoning by CO or O and "reactive" regions are found, as in the ZGB model. The model is then modified by using a hexagonal lattice as substrate, and further by implementing lattice "reconstruction" (square-hexagonal switching). Introducing lattice reconstruction in the model produces oscillations in a narrow parametric range. The oscillatory parametric regime is extended to a broader range when the sticking coefficient of oxygen is assumed to be low on the hexagonal lattice. Second, the catalytic CO oxidation at high pressure conditions is investigated. Oscillations that appear under these conditions are related to surface oxidation. When oxygen is present in excess, metal atoms cooperate with adsorbed oxygen atoms in a nucleation mechanism, to form the surface oxide. In the simulations, the requirement of high oxygen concentration is introduced through the overall coverage, that is adsorbed oxygen atoms are transformed to oxide with some probability when the overall coverage on the lattice is high. The new oxide species is allowed to react with CO at a different rate than the adsorbed oxygen. The oxide was in the past believed to have very low reactivity, while later studies have indicated that it may be more reactive towards CO oxidation than the Ophase on metal. The reaction between oxide and CO in the latter case is believed to follow a Marsvan Krevelenmechanism(instead of LangmuirHinshelwood). In the present simulations the LangmuirHinshelwood mechanism is implemented at all times. However we investigate not only the parametric regions that correspond to oxide reactivity being low, but also those that correspond to oxide reactivity being high. Higher amplitude oscillations are observed the farther apart the reactivities of oxide and Oatoms are, while in general oscillations are more prominent when the adsorbed O on metal is assumed to be more reactive. The oscillatory NO + CO system on metal surfaces, such as on Pt(100), is the third system that is explored. The oscillatory mechanism in this case is not related to a surface transition but is believed to follow an "elementary steps model", consisting of the particle interactions mechanism alone. This mechanism has been named the ‘‘vacancy model’’ due to the central role that the autocatalysis of surface vacancies plays in the emergence of oscillations. The rate determining step is the dissociation of NO, which requires the presence of vacant sites in order to proceed. Thus in the absence of vacancies this step is inhibited, while when a few vacant sites are created (through reaction or desorption), suddenly a large number of them are created through the steps that are coupled to NO dissociation and in which vacancies are produced. The vacancy concentration explosions are followed by periods in which the coverage builds up, thus completing the oscillation cycle. An important aspect of the system’s behaviour is also the increase of the rate of CO and NO desorption with increasing coverage. .................................................................................................................................................
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