TOWARDS AN OSCILLATION MECHANISM FOR THE NO-CO REACTION ON RH(110) - NO DISSOCIATION KINETICS AND OXYGEN SUBSURFACE DIFFUSION

The reaction between NO and CO on Rh{110} has been found to show hyste resis effects as well as self-sustained rate oscillations at high temp eratures (above 900 K). For certain CO/NO partial pressure ratios two modes of oscillation with different periods and amplitudes may be supe rimposed. In order to understand the oscillation mechanism, we studied the NO dissociation kinetics by means of temperature programmed desor ption (TPD), monochromatized X-ray photoelectron spectroscopy (MXPS), ion scattering spectroscopy (ISS) and static secondary ion mass spectr oscopy (SSIMS). At higher NO coverages the dissociation process has be en found to be hindered and shifted to the range of temperatures where thermal desorption of nitric oxide and nitrogen molecules takes place (above 300 K) as well. Work function measurements and titration exper iments with CO have provided evidence that oxygen atoms released durin g NO dissociation can diffuse into the subsurface region of the crysta l. The reversibility of this process may play an important role in the oscillation mechanism of the NO-CO reaction on Rh{110} although some of the results might be explained by assuming a subtle balance with an oxidation process.