COADSORBATE INDUCED COMPRESSION OF SULFUR OVERLAYERS ON RE(0001) AND PT(111) BY CO

Low coverages of sulfur chemisorbed on the rhenium(0001) and platinum( 111) surfaces were studied in UHV by scanning tunneling microscopy (ST M) and low-energy electron diffraction (LEED). On both of these surfac es of triangular symmetry the lowest coverage (almost-equal-to 0.25 mo nolayers) ordered structure is p(2 x 2). Exposure of this ordered sulf ur overlayer on either surface to low pressures (10(-9) Torr) of CO in duces compression of the sulfur layer to a structure associated with a higher local coverage and CO chemisorbs in the holes created in the s ulfur layer. The reordering was observed by both a change in the LEED pattern and by real space STM imaging of the surface. On the Re surfac e the new overlayer has (3 square-root 3 x 3 square-root)R30-degrees s ymmetry, while on the Pt surface it has (square-root 3 x square-root 3 )R30-degrees symmetry. There was no increase in the amount of sulfur o n the surface during this reordering. On both surfaces the overlayers could be returned to the original p(2 x 2) by annealing for several se conds at 600-degrees-C, during which CO desorbs and sulfur atoms reocc upy the vacant metal sites. This phenomenon of the compression of atom s in a strongly chemisorbed layer upon coadsorption of another molecul e provides a mechanism for carrying out catalytic reactions on metal s urfaces that are covered with strongly chemisorbed layers that do not participate in the reaction.