Towards an oscillation mechanism for the NOCO reaction on Rh〈110〉: NO dissociation kinetics and oxygen subsurface diffusion

The reaction between NO and CO on Rh{l10} has been found to show hysteresis effects as well as self-sustained rate oscillations at high temperatures (above 900 K). For certain CO/NO partial pressure ratios two modes of oscillation with different periods and amplitudes may be superimposed. In order to understand the oscillation mechanism, we studied the NO dissociation kinetics by means of temperature programmed desorption (TPD), monochromatized X-ray photoelectron spectroscopy (MXPS), ion scattering spectroscopy (ISS) and static secondary ion mass spectroscopy (SSIMS). At higher NO coverages the dissociation process has been 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 experiments with CO have provided evidence that oxygen atoms released during NO dissociation can diffuse into the subsurface region of the crystal. The reversibility of this process may play an important role in the oscillation mechanism of the NO-CO reaction on Rh{l10} although some of the results might be explained by assuming a subtle balance with an oxidation process.