OSCILLATORY BEHAVIOR IN THE CATALYTIC REDUCTION OF NO AND NO2 WITH HYDROGEN ON PT FIELD EMITTER TIPS

We studied the catalytic reduction of NO and NO2 with hydrogen on Pt u sing both Field Ion Microscopy (FIM) and Field Electron Microscopy (FE M) under various temperatures (395-530 K) and total pressures (10(-4)- 10(-2) Pa). Kinetic instabilities, including surface explosions and se lf-sustained oscillations of the catalytic production rate, were found for properly chosen external parameters and well developed tip morpho logies. Oscillatory behavior was only found for pyramidally shaped tip s. The {210} planes, most probably reconstructed under reaction condit ions, acted as pacemakers. Reaction-diffusion fronts were found to pro pagate along the [100] zone lines. Very regular oscillations were regi stered for NO2 reduction in FEM studies. The oscillation periods incre ased with increasing NO2/H-2 ratios without significant changes in the pattern formation. The mechanism of the rate oscillations is driven b y the autocatalytic decomposition and reaction of adsorbed NO.