REGULAR AND IRREGULAR SPATIAL PATTERNS IN THE CATALYTIC REDUCTION OF NO WITH NH3 ON PT(100)

The catalytic reduction of NO with NH3 on a Pt(100) surface, which exhibits kinetic oscillations under isothermal conditions in the 10(-6) mbar pressure range, has been studied by photoemission electron microscopy (PEEM) as a spatially resolved technique. Oscillations in the rate of product formation for N2 and H2O are observed between 425 and 450 K. During the rate oscillations, the surface reacts predominantly spatially uniformly. Towards the lower T-boundary for oscillations, however, fluctuating adsorbate islands (diameter almost-equal-to 10-50-mu-m) appear and one observes target patterns and rotating spirals. Below the lower T-boundary for oscillations, the reaction rate is stationary, but with PEEM one observes a spatially chaotic pattern in which the surface is still oscillating locally. The transition from macroscopic rate oscillations to unsynchronized oscillatory behavior can bc associated with the breakdown of long range synchronization via gas phase coupling. In the spatial patterns imaged by PEEM, one can clearly identify three distinct grey levels which undergo a cyclic transformation into each other via propagating reaction fronts. One can assign different mechanistic steps to these transformations, namely the lifting of the hex reconstruction through NO adsorption and the dissociation of NO on the 1 X 1 phase, decomposition of NH3 on the 1 X 1 O(ad)/NO(add) phase, and the restoration of the hex surface.