The infrared spectrum of carbon monoxide on reduced and oxidized palladium

Abstract

The spectra of carbon monoxide adsorbed on a silica-supported, 9% palladium catalyst has been studied in conjunction with adsorption studies of carbon monoxide and oxygen. Both bridged and linear species are observed. These species both react readily at room temperature with gas phase oxygen, and a pre-adsorbed oxygen layer reacts rapidly with gaseous carbon monoxide to yield the same spectrum as that on the reduced catalyst. If the catalyst is subjected to bulk oxidation, however, the spectrum of adsorbed carbon monoxide changes dramatically; the linear form is greatly enhanced and the bridged species becomes greatly reduced. No bands ascribable to adsorbed carbon dioxide are observed. Evidence is presented that some of the several separate bands seen for bridged species stem from the same species interacting to differing degrees with neighboring chemisorbed carbon monoxide. Estimates based on adsorbed amounts suggest that the bridged species has an extinction coefficient an order of magnitude greater than that for the firmly bound linear species but that a loosely bound linear species has an extinction coefficient more comparable to that of the bridged species. A model is suggested for the mode of adsorption that assumes both linear and bridged carbon monoxide bound to the same metal atom.