Kinetic instabilities in the NO/H ₂ reaction on platinum

We have studied the catalytic reduction of nitric oxide (NO) with hydrogen (H ₂ ) on platinum using both field ion microscopy (FIM) and pulsed field desorption mass spectrometry (PFDMS). Under certain conditions of temperature and gas pressure this reaction exhibits non‐linear behaviour, i.e. hysteresis effects and rate oscillations occur. Ensembles of kink sites of the zones are found to act as pacemakers, which is in line with earlier studies demonstrating {012} planes to ignite in a quasi‐concerted manner.1,2 Experiments with tips of varying radius of curvature always lead to the same sequence of image patterns. Large variations in the H ₂ partial pressure has little influence on the frequency of the rate oscillations. Using PFDMS under the reaction conditions we show that NO ⁺ rather than H ₂ O ⁺ /H ₃ O ⁺ (the neutrals are formed during the reaction process) is the imaging species in FIM. Interestingly, $\hbox{PtO}_{{x}}^{n+} (x \leq 3)$ are seen in the mass spectra, indicating that the reaction is controlled by a vacant site mechanism with the inclusion of partial oxidation of the Pt surface. Copyright © 2004 John Wiley & Sons, Ltd.