Field ion microscopy during an oscillating surface reaction: NOH2 on Pt

The reduction of nitric oxide with hydrogen on Pt field emitter tips was studied by means of field ion microscopy (FIM) in the temperature range 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 below 9 V/nm). One type of experiment, performed at 444 K, was characterised by the appearance of striae and superimposed bright spots either stationary or moving with variable size. Mass spectrometric evidence was obtained according to which the dynamic changes in the local image brightness were due to variations in the NO ionisation rate brought about by the occurrence of water formation from adsorbed hydrogen and oxygen. 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 high image brightness was found to occur in an explosive manner (millisecond time scale) on planes formerly identified to be of thesz012thesz symmetry. After ignition, the spatio-temporal pattern evolution was seen to involve 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 pattern sequence. The oscillation mechanism is not yet known but likely involves a reaction-diffusion step, i.e. surface diffusion of reactants (NO or H2) into regions of enhanced water formation. In addition, a field-assisted morphological change of the emitter tip was found to take place in the initial stages of the experiment. The resulting morphology was pyramidal and this shape must be considered a prerequisite for the occurrence of regular oscillations.