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

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.