Non-linear behaviour in the NO-H-2 reaction over Rh(111)

The dynamic behaviour of the NO-H-2 reaction over Rh(111) was studied using mass spectrometry and Auger electron spectroscopy in the 10(-7)-10(-5) mbar total pressure range for NO:H-2 ratios between 1:5 and 1:30. On cooling the sample in a flow of the gases, an increase in the rate of formation of N-2 occurred between 500 and 450 ii which indicated the presence of an autocatalytic step in the reaction sequence. A heat-cool cycle of the sample actually produced a large hysteresis in the formation of all products: N-2, NH3 and H2O. This type of behaviour was also observed under similar conditions over the Rh(533) surface, as were regular macroscopic rate oscillations. During these oscillations, N-2 production was out of phase with that of NH3 and H2O. The Rh(533) surface consists of four-atom wide (111) terraces separated by (100) steps. The steps seem to Flay an important role in the synchronisation of the oscillations, as only irregular macroscopic rate oscillations could be observed over Rh(111) under these conditions. Furthermore, the introduction of microscopic defects on the Rh(111) surface by means of Ar+ ion sputtering led to an increase in the synchronisation of the oscillations, these defects are presumably playing a similar role to the steps on the Rh(533) surface.