Comparison of the Coverage of the Linear CO Species on Pt/Al2O3Measured under Adsorption Equilibrium Conditions by Using FTIR and Mass Spectroscopy

There is controversy in the literature about the relationship between the IR band area (denoted by A) of the adsorbed CO species (i.e., linear (denoted by L) and bridged (denoted by B) CO species) on noble-metal-containing solids (supported metal catalysts and single crystals) and the coverage (denoted by θ) of the surface by CO. Usually, a straight line is observed at low coverages (θ<0.5) but various profiles are observed at high coverages. This constitutes a limitation on performing quantitative analysis using FTIR spectroscopy. Several explanations for the various profiles for the curves A=f(θ) (i.e., dipole–dipole coupling, nonadsorption equilibrium) 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 adsorbed CO species. The coverages (denoted by θL) of the L species adsorbed on a 2.9% Pt/Al2O3catalyst are determined at several adsorption temperatures (adsorption equilibrium, Tarange 300–740 K, Pa≈103Pa) by using two analytical methods: FTIR and mass spectroscopy. It is shown that at high adsorption temperatures (Ta>550 K) the two analytical methods give the same coverage values in the range θ=1–0.6. This leads to the 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 spectroscopy (i.e., determination of the heats of adsorption of the L species at several coverages).