FIELD-ION MICROSCOPY DURING AN OSCILLATING SURFACE-REACTION - NO H-2 ON PT/

The reduction of nitric oxide with hydrogen on Pt field emitter tips w as studied by means of field ion microscopy (FIM) in the temperature r ange of 440 to 530 K and at total pressures in the range of 10(-3) to 10(-2) Pa. Kinetic instabilities associated with the surface reaction could be observed in vivo by applying low viewing fields (usually belo w 9 V/nm). One type of experiment, performed at 444 K, was characteris ed by the appearance of striae and superimposed bright spots either st ationary or moving with variable size. Mass spectrometric evidence was obtained according to which the dynamic changes in the local image br ightness were due to variations in the NO ionisation rate brought abou t by the occurrence of water formation from adsorbed hydrogen and oxyg en. A field corrosion process was suggested to be responsible for the occurrence of the striae. In another type of experiment, performed at 525 K, regular self-sustained oscillations with respective variations in the local brightness of the images were detected. The change to hig h image brightness was found to occur in an explosive manner (millisec ond time scale) on planes formerly identified to be of {012} symmetry. After ignition, the spatio-temporal pattern evolution was seen to inv olve areas of the [100] zone until finally the central (001) pole was comprised. The catalytic cycle was found to stop after 0.1-0.2 seconds and to return in periods of some ten seconds with the identical patte rn sequence. The oscillation mechanism is not yet known but likely inv olves a reaction-diffusion step, i.e. surface diffusion of reactants ( NO or H-2) into regions of enhanced water formation. In addition, a fi eld-assisted morphological change of the emitter tip was found to take place in the initial stages of the experiment. The resulting morpholo gy was pyramidal and this shape must be considered a prerequisite for the occurrence of regular oscillations.