Regular and irregular spatial patterns in the catalytic reduction of NO with NH₃ on Pt(100)

Abstract

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 lowerT-boundary for oscillations, however, fluctuating adsorbate islands (diameter ≈ 10–50 μm) appear and one observes target patterns and rotating spirals. Below the lowerT-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 be 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 1x1 phase, decomposition of NH₃ on the 1x1 O_ad/NO_add phase, and the restoration of the hex surface.