A vibrational spectroscopic investigation of the interaction of NO and CO on the {100} surface of platinum

In this thesis vibrational spectroscopy is applied to the study of the interaction of nitric oxide (NO) and carbon monoxide (CO) on the {100} surface of single crystal platinum. The technique used is Infrared Reflection-Absorption Spectroscopy (IRAS), in which an infrared beam is singly reflected from the surface of a Pt crystal. Absorption spectra are computed by the Fourier transformation of the interferogram generated by a Michelson interferometer. Variations in background gas phase absorption in the optical path are reduced by placing both the spectrometer and detector chambers in vacuum. Heterogeneous reactions on single crystal surfaces at low pressure (P < 10-4 mbar) are relatively simple systems exhibiting kinetic oscillations and spatio-temporal pattern formation under certain conditions. As such, they have recently become an important area of research within the fields of non-linear dynamics and synergetics. With the aim of deducing the oscillatory mechanism of the reaction of NO + CO on Pt{100}, starting with the study of the simplest processes: the adsorption of CO and NO, the complexity of the system under investigation is incremented by degrees to include the interaction of CO + NO under non-reactive, reactive and, finally, oscillatory conditions. A new experimental method is presented which allows for real-time acquisition of IRAS spectra during any reproducibly recurring surface reaction. It involves adding interferograms recorded at equivalent points in successive oscillatory cycles; the phase stability is ensured by synchronising the oscillation with a small periodic temperature modulation applied to the sample. The method has been used to determine the phase relationship of the C-O and N-O absorption bands in the oscillatory CO + NO reaction on Pt{100} and to relate them to the partial pressures of the reactants and the products. Adsorption of both CO and NO on the reconstructed hex surface phase of Pt{100} result in a surface phase transition to the (1x1) truncation of the bulk crystal. The temperature-dependent adsorption on the reconstructed Pt{100}hex surface exposed to a constant partial pressure of CO reveals a true hysteresis in the total and local CO coverage between the heating and cooling cycle, the underlying cause of which is the lower local CO coverage required to sustain existing (1x1) areas than that at which (1x1) islands grow. The temperature-dependent adsorption behaviour of a mixture of CO and NO on Pt{100}hex is shown to be determined by three processes. At temperatures below those, at which reaction occurs, (T < 400 K) adsorption is controlled by the non-reactive displacement of NO by CO. The CO displaces pre-adsorbed NO and adsorbs in atop sites, forming mixed NO + CO islands. The proportion of CO in the linear configuration subsequently equilibrates into bridge site occupancy. Above 400 K reaction leads to vacant site creation, which, due to a partial pressure ratio of PNO : PCO > 1, results in an increase in NO coverage. Under these conditions the high rate of equilibration between the bridged and atop CO result in their exhibiting identical behaviour. It is the competition between the non-reactive displacement of NO by CO and the reactive replacement of CO by NO which determines the parameter space of the low temperature oscillatory regime. The high temperature oscillatory regime is characterised by the ability of CO (but not NO) to lift the reconstruction, and a high reaction rate which reduces adsorbate coverage sufficiently that the hex surface phase is reformed. In the low temperature oscillatory regime the presence of pure atop CO islands and mixed CO + NO islands are observed, these are the equilibrium and far from equilibrium states of the system, respectively. The pure atop CO islands exhibit no change over an oscillatory cycle. Bridged CO is shown to play an equivalent role in the oscillatory mechanism to atop CO. The predictions of a mathematical model, which considers the non-linear dynamics deriving from the vacant site requirement for NO dissociation, are in broad agreement tile results. Deviations occur, primarily, because the model neglects to consider the displacement of NO by CO. In particular this leads to an erroneous prediction for the equilibrium state. Spectra for the high temperature oscillatory regime exhibit the presence of 0.03 monolayer (ML) atop CO on the (1x1) surface phase. The results are shown to be incompatible with the assumption that reaction is occurring on the unreconstructed surface. Detects are created by the lifting and reforming of the reconstruction due to the large mass difference between the two surface phases and are also known to exhibit a high activity towards NO dissociation. A new mechanism for the high temperature oscillatory cycle is proposed ill which the existing models are extended to include the creation and subsequent annealing of defects. Isotopic substitution is applied in the investigation of O2 + CO on Pt{100}hex. An absorption band at 1610 - 1637 cm-1 is assigned to a species which is comprised of O(2), but does not contain C or 0 from CO. (author)