The influence of adsorbate–adsorbate interactions on surface structure: The coadsorption of CO and H2 on Rh(100)

Publisher Website
Google Scholar
Abstract
The coadsorption of CO and H2 on Rh(100) at 100 K has been studied using temperature programmed desorption (TPD), low energy electron diffraction (LEED), electron energy loss spectroscopy (EELS), and temperature programmed EELS (TP-EELS). The preferred binding sites, long ranged order, and degree of segregation are dependent on the order of adsorption. When H2 is exposed to a CO preexposed surface, segregation of the surface species (atomic H and CO) is observed. The postdosed H2 causes isolated CO molecules to change from the top site to the bridge site, and compresses the c(2×2) CO islands that develop during the CO preexposure. When CO is exposed to a H2 saturated surface (one hydrogen per surface Rh atom) an intimately mixed c(2×2) CO and H structure is formed with all the CO molecules occupying the top site. Strong repulsive CO–H interactions in this mixed adlayer result in two new low temperature H2 TPD states. During the desorption of the lowest temperature H2 TPD peak, the c(2×2) LEED pattern streaks and the CO molecules shift from the top site to the bridge site. It is proposed that the preferred binding site for hydrogen in the c(2×2) bridge bonding CO structure is different from the fourfold hollow site preferred in the c(2×2) top bound CO structure. After the second H2 TPD peak, the remaining adsorbed H and CO segregate, and the CO regions are compressed. The compression is relaxed as the hydrogen desorbs. The development of the surface structures and their influence on the H2 TPD can be qualitatively understood in terms of precursor adsorption of both CO on H covered surfaces and H2 on CO covered surfaces, strong CO–H repulsions, and local poisoning of H2 dissociation by CO.