OSCILLATORY BEHAVIOR OF THE NO-H-2 REACTION OVER RH(533)

Waves moving over the surface of a Rh field emitter tip in an oscillatory way during the NO-H-2 reaction have been visualized earlier by field electron microscopy (FEM). An autocatalysis model has been proposed to describe the oscillatory behaviour of the reduction of NO by H-2. To study the oscillatory behaviour and the effect of the surface structure in more detail a large Rh(100) surface and a large stepped Rh(533) surface, Rh[4(111)*(100)], have been selected. The first results show that, in correspondence with the FEM experiments, rate oscillations could be observed over the Rh(533) surface in the 10(-6) mbar pressure regime around 470 K. No oscillatory behaviour was obtained on the Rh(100) surface under these conditions. The structure-sensitivity of the process is related to the large dependence of the Rh-N bond strength on the surface structure. Nitrogen desorbs at a much higher temperature from Rh(100) than from Rh(533).