A fast X-ray photoelectron spectroscopy study of the NO-H2 reaction over Rh(533):: understanding hysteresis and oscillations in the reaction rate

The dynamic behaviour of the NO-H-2 reaction over Rh(533) was studied in situ using fast X-ray photoelectron spectroscopy (XPS) and mass spectrometry. H-2 rich NO:H-2 mixtures were investigated in the 10(-6) mbar pressure range, where the three reaction products N-2, NH3 and H2O show a large divergence in formation rates on the two branches of a heating-cooling cycle. This hysteresis is most prominent between 450 and 500 K;. The high energy resolution achieved by using synchrotron light allows us to distinguish five surface species at different stages during the hysteresis by analysing the lineshape and energy positions of the N Is and O Is photoelectron spectra. It can be established that the reactive surface is covered by an atomic nitrogen species N(I) and the unreactive surface by atomic oxygen O and another atomic nitrogen species N(II), which might be attributed to atomic nitrogen adsorbed at step sites or to an NH, species. During the hysteresis no NO is observed. The results show that the presence of very small amounts of oxygen drastically destabilizes the nitrogen species N(I); higher oxygen coverages result in the formation of the N(II) species. In the light of these results, a mechanistic scheme is suggested to explain hysteresis and isothermal rate oscillations. (C) 1998 Elsevier Science B.V. All rights reserved.