FIELD-ION MICROSCOPIC STUDIES OF THE CO OXIDATION ON PLATINUM - FIELD-ION IMAGING AND TITRATION REACTIONS

Elementary steps of the CO oxidation-which are important for understanding the oscillatory behavior of this catalytic reaction-are investigated simultaneously on different Pt-single crystal surfaces by field ion microscopy. Due to preferential ionization probabilities of oxygen as imaging gas on those surface sites, which are adsorbed with oxygen, these sites can be imaged in a lateral resolution on the atomic scale. In the titration reaction a COad-precovered field emitter surface reacts with gaseous oxygen adsorbed from the gas phase or, vice versa, the O-ad-precovered surface with carbon monoxide adsorbed from the gas phase. The competition of the manifold of single crystal planes exposed to the titration reaction at the field emitter tip is studied. The surface specificity can be documented in the specific reaction delay times of the different planes and in the propagation rates of the reaction-diffusion wave fronts measured on these individual planes during the titration reaction with a time resolution of 40 ms. At 300 K the COad-precovered surfaces display the {011} regions, precisely the {331} planes as the most active, followed by {012}, {122}, {001}, and finally by {111}. Reaction wave fronts move with a velocity of 8 Angstrom/s at {012}, with approximate to 0.8 Angstrom/s at {111}, and have a very fast ''switch-on'' reaction at the (001) plane with 500 Angstrom/s. At higher temperature, T=350 K, an acceleration of reaction rates is combined with shorter delay times. The titration reaction of a precovered O-ad surface with COgas at T=373 K shows the formation of CO islands starting in the {011} regions with a quickly moving reaction front into the other surface areas without showing particular delay times for different surface symmetries. The two reverse titration reactions have a largely different character. The titration of COad with oxygen adsorbed from the gas phase consists of three different steps, (i) the induction times, (ii) the highly surface specific reaction, and (iii) different rates of wave front propagation. The reaction of CPgas with a precovered O-ad layer on the other hand starts with nucleating islands around the {011} planes from where the whole emitter surface is populated with COad without pronounced surface specifity.