Comparison of the coverage of the linear CO species on Pt/Al2O3 measured under adsorption equilibrium conditions by using FTIR and mass spectroscopy

There is controversy in the literature about the relationship between the I R band area (denoted by A) of the adsorbed CO species (i.e., linear (denote d by L) and bridged (denoted by B) CO species) on noble-metal-containing so lids (supported metal catalysts and single crystals) and the coverage (deno ted by B) of the surface by CO. Usually, a straight line is observed at low coverages (theta < 0.5) but various profiles are observed at high coverage s. This constitutes a limitation on performing quantitative analysis using FTIR spectroscopy. Several explanations for the various profiles for the cu rves A = f(<theta>) (i,e,, dipole-dipole coupling, nonadsorption equilibriu m) have been suggested. In the present study we show that this also may be due to the fact that the coverage values involve the amounts of several ads orbed CO species. The coverages (denoted by BL)of the L species adsorbed on a 2.9% Pt/Al2O3 catalyst are determined at several adsorption temperatures (adsorption equilibrium, T-a range 300-740 K, P-a approximate to 10(3) Pa) by using two analytical methods: FTIR and mass spectroscopy. It is shown t hat at high adsorption temperatures (T-a > 550 K) the two analytical method s give the same coverage values in the range theta = 1-0.6. This leads to t he conclusion (a) that there is a proportionality between the IR band area of the L species and its amount on the Pt surface and (b) that quantitative studies involving the L species can be performed by using FTIR spectroscop y (i.e., determination of the heats of adsorption of the L species at sever al coverages). (C) 2000 Academic Press.