Seminars Archive

Bistability and formation of chemical waves: The NO+H2 reaction on Rh as studies by Fast XPS and XPS Microscopy

Abstract
X-ray Photoemission Spectroscopy (XPS) with a Synchrotron Source as ELETTRA allows fast spectra acquisition with high energy resolution which can be used to study surface reaction mechanisms by monitoring the adsorbate coverages (Fast XPS) simultaneously with the partial pressures of reactants and products in the gasphase. Furthermore, focussing of the photon beam gives access to XPS microscopy with chemical contrast (Scanning Photoemission Microscopy, SPEM). These two methods have been used to study the NO+H2 reaction on Rh. In a first part, results of a fast XPS study of the global kinetics of this reaction on Rh(533), a stepped Rh(111) surface, will be presented. The three reaction products N2, NH3 and H2, show a large hysteresis in their formation rates. The XPS spectra taken at the SuperESCA beamline reveal at O 1s and N 1s level a total of four different adsorbates during the hysteresis. The reactive surface is covered by an atomic nitrogen species N(I), whereas the unreactive surface is covered by atomic oxygen O and another atomic nitrogen species N(II). Oxygen destabilises, also if present in only very small amounts, the nitrogen species N(I) drastically; at higher coverages it promotes the formation of the N(II) species. The nitrogen species N(II) might be attributed to atomic nitrogen adsorbed at step sites or to an NHx species. In the light of these results, a mechanistic scheme is suggested to explain hysteresis and isothermal rate oscillations. In a second part, first microscopy results of the chemical waves formed in the NO+H2 reaction will be shown, as studied on a microstructured Rh/Pt(100) crystal at the ESCAMicroscopy . The chemical contrast obtainable at this beamline allows for the first time to map selectively different adsorbates in the chemical waves. A synergistic effect of Pt and Rh which enhances the catalysts activity for light-off under conditions of carbon contamination will be presented.