Oscillatory behaviour in the NO-H-2 reaction over Ir(510)

The non-linear behaviour in the NO-H-2 reaction over the stepped Ir(5 1 0) was studied using mass spectrometry, LEED and AES with NO pressures between 7.7 x 10(-8) and 7.7 x 10(-7) mbar and H-2/NO ratios ranging from 1 to 54. In addition. TDS experiments were carried out to study the interaction bet ween the species present on the surface during the reaction. During tempera ture programmed heating of a NO saturated surface in the presence of a hydr ogen flow an explosive formation of N-2. and to a lesser extent H2O, was ob served, indicative of an autocatalytic reaction. Furthermore. TDS experimen ts on a NO saturated surface that was precovered with O, showed that the pr esence of O-ads lowers the NO activation energy of desorption by 12 kJ mol( -1) During a heating-cooling cycle an adsorbate induced lifting of the surface reconstruction was observed under similar reaction conditions at which osci llations in rate were observed. The rates of N and H2O formation are in-pha se during these oscillations. No other products were observed under oscilla tory conditions. Possible mechanisms of the oscillations are discussed. Fin ally. a comparison between the non-linear behaviour over Ir(5 1 0), Pt(1 0 0) and other Ir and Ph surfaces was made. (C) 2001 Elsevier Science B.V. Al l rights reserved.