The temperature dependence of the interaction of NO + CO on Pt{1 0 0}

Infrared reflection absorption spectroscopy together with mass spectrometry has been used to investigate the interaction of NO and CO on Pt{1 0 0}, initially prepared in the reconstructed ‘hex’ phase, under ambient pressures of these gases, in the temperature range 300–500 K. The results allow the local and total coverages of adsorbed CO and NO to be related to the rate of reaction to produce gas phase CO2, and provide insight into the species present on the surface during the so-called low temperature oscillatory reaction regime of this process. At temperatures below that at which NO dissociation occurs (approximately 390–400 K) adsorption is controlled by the non-reactive displacement of NO by CO and results in a CO-poisoned surface. Above 400 K when significant CO2 production occurs, the NO coverage increases to produce a surface with NO and CO fully intermixed; the increase in NO coverage is attributed to the higher rate of NO arrival from the gas phase (with a partial pressure ratio of PNO:PCO>1) at free surface sites created by NO dissociation and subsequent reaction with CO. The competition between these two processes of non-reactive NO displacement by CO and reactive displacement of CO by NO is proposed to determine the parameter space of the low temperature oscillatory regime. Rapid equilibration between bridged and atop CO species leads to them appearing to exhibit identical reaction behaviour. Particularly at the lowest reaction temperatures (around 400 K), islands of pure CO may coexist on the surface but not participate in the reaction. Under conditions corresponding to the high temperature oscillatory regime, small quantities of absorbed CO, but no NO, are seen on the surface.