Abstract
Ag films of few monolayers thickness have been prepared on polycrystalline Ti enriched AgTi 2 surfaces and characterized with Auger and photoemission spectroscopies. Irreversible CO adsorption at these Ag films is observed at T = 110 K, whereas at bulk Ag CO does not adsorb irreversibly above 50 K. UV photoemission spectra of CO covered Ag/AgTi 2 exhibit well separated 4σ, 1π, and 5σ photoemission peaks indicative of weak molecule-substrate bonding and effective screening as also observed with the CO/Ag(110) system. However, the energy shifts of these peaks with respect to free CO are different for different orbitals in constrast to physisorbed CO on bulk Ag, from which the action of weak chemisorption has to be concluded. Accordingly, the CO adsorption energy at the Ag films on the Ti rich support is increased to 10 kcal/mol, twice the value at (bulk) Ag surfaces. These observations are interpreted in terms of an electronic effect, known as the ligand effect in heterogeneous catalysis.
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