NONLINEAR BEHAVIOR 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 1:3 0. On cooling the sample in a flow of the gases, an increase in the ra te of formation of N-2 occurred between 500 and 450 ii which indicated the presence of an autocatalytic step in the reaction sequence. A hea t-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 w as 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 synchronisatio n of the oscillations, as only irregular macroscopic rate oscillations could be observed over Rh(111) under these conditions. Furthermore, t he introduction of microscopic defects on the Rh(111) surface by means of Ar+ ion sputtering led to an increase in the synchronisation of th e oscillations, these defects are presumably playing a similar role to the steps on the Rh(533) surface.