SURFACE TOPOGRAPHICAL CHANGES ON PT(100) AND PT(111) IN THE NO+CO REACTION - A REFLECTION ELECTRON-MICROSCOPY STUDY

The structural modifications which are induced by the NO + CO reaction on Pt surfaces were studied using reflection electron microscopy (REM) to image atomic steps. Small single crystal spheres of Pt (diameter almost-equal-to 0.3 mm) were prepared and then exposed in a UHV chamber to a flow of CO and NO at 10(-6) Torr. The sample temperature was cycled between 300 and 700 K over an exposure time of typically approximately 1 h. On the (100) orientations, the reaction led to a strong roughening of the substrate and to the development of a domain structure. As was demonstrated in adsorption experiments with CO (NO) alone, the major contribution to the roughening originated from the 1 X 1 reversible hex phase transition of Pt(100). On the (111) facets the flat terraces were only weakly affected by the reaction, but the step edges were quite strongly attacked leading to a meandering of the initially straight step edges. The observations made on Pt(111) are consistent with a mechanistic picture in which the NO + CO reaction takes-entirely place at step and kink sites.