Abstract
Transmission IR spectroscopy has been used to study very low CO coverages on Pd/Al 2O 3 surfaces. It has been shown that on clean Pd surfaces, the first CO molecules adsorbed are adsorbed on (111) facets, as evidenced by the close agreement of CO vibrational frequency with those observed for 3-fold bridging and 2-fold bridging CO species on Pd(111). In distinction to this behavior, when the Pd surface is precovered with ethylidyne (CCH 3), a species thought to form only in 3-fold sites, the IR features for low coverages of CO no longer correspond to those for CO adsorption on Pd(111). A model is proposed in which CCH 3 effectively blocks all sites for CO adsorption on (111) facets on Pd crystallites. CO adsorption isotherms indicate that these (111) facets comprise 30 ± 10% of the total Pd crystallite surface.
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