Infrared spectra of carbon monoxide adsorbed on supported palladium and palladium-silver alloys

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Infrared spectra of carbon monoxide adsorbed on supported PdAg alloys were investigated. CO is strongly chemisorbed only on the Pd atoms. The infrared spectra provide information of the relative abundance of diadsorbed (bridge bonded) and monoadsorbed (linear) CO (of importance for the “ensemble effect” of alloy catalysts); they also show to what extent the band frequency depends on the chemical nature of the metal atoms adjacent to the adsorbing Pd atom (“the ligand effect” of alloy catalysts). Three CO absorption bands were observed on Pd and on the alloys at ~2060, 1960 and 1920 cm −1. The most marked result is that the band frequency remains almost constant for Pd and PdAg alloys, but the relative intensities change in a very drastic manner. The 2060 cm −1 band which is ascribed to the linear CO complex and is rather weak for CO on Pd becomes the most important feature of the spectrum of CO on PdAg alloys where the bands characteristic for bridged CO are very faint. It thus appears that for this system the geometric “ensemble effect” is more pronounced than the “ligand effect.” We further conclude that Eischens' assignment of CO bands on transition metals is correct.