MICROFACETTING OF PT(210) INDUCED BY OXYGEN-ADSORPTION AND BY CATALYTIC CO OXIDATION

The interaction of a Pt(210) surface with oxygen and the facetting process this surface undergoes in catalytic CO oxidation have been investigated with LEED, thermal desorption spectroscopy (TDS), work-function measurements and scanning tunneling microscopy (STM). At high oxygen coverages one observes a (5x2)-rect superstructure in LEED which in TDS is associated with the appearance of an additional low-lying adsorption state. The formation of the (5x2)-rect structure is thermally activated and proceeds at T greater-than-or-equal-to 550 K. STM revealed that the (5x2)-rect pattern is in fact due to a microfacetting of Pt(210) into 1x2-reconstructed (110) terraces and another orientation which is most likely (310). The two facet orientations build up an asymmetric sawtooth whose lateral spacing of approximately 25 angstrom in the [120BAR] direction represents the fivefold periodicity of the superstructure. The twofold periodicity in [001] direction results from the 1x2 reconstruction of the (110) terraces. If one exposes the (210) surface to a CO/O2 atmosphere in the 10(-4) Torr range the surface facets into the same orientations that have already been obtained with oxygen alone but now with much larger dimensions of the facets. This latter finding suggests that the oxygen-induced microfacetting of Pt(210) might represent the primary step in the facetting of Pt(210) during catalytic CO oxidation.