Heats of adsorption of linear CO species adsorbed on reduced Fe/Al 2O 3 catalysts using the AEIR method in diffuse reflectance mode

Abstract

IR spectra in diffuse reflectance mode are quantitatively exploited to determine the heats of adsorption of two linearly adsorbed CO species formed on Fe 2+ and Fe° sites (denoted L Fe 2 + and L Fe° CO species) of reduced x% Fe/Al 2O 3 (wt%, x = 1 and 5) catalysts according to the AEIR procedure developed previously using the IR transmission mode. The IR transmission properties of iron containing catalysts are limited particularly for high iron loadings favoring the use of the diffuse reflectance mode. The heats of adsorption of the L Fe 2 + CO species linearly vary with the coverage of the sites from E L Fe 2 + ( 1 ) = 45 kJ / mol to E L Fe 2 + ( 0 ) = 66 kJ / mol at coverage 1 and 0, respectively. These values are modified by the presence of neither Fe° sites nor carbonaceous adsorbed species. The heats of adsorption of the L Fe° CO species on a C-free iron surface (for adsorption temperature T a < 456 K), linearly vary with its coverage from E L Fe ° ( 1 ) = 79 kJ / mol to E L Fe ° ( 0 ) = 105 kJ / mol at high and low coverages. For T a > 456 K, the CO dissociation overlaps the CO adsorption equilibrium leading to a C-containing iron surface. The presence of carbonaceous species has no significant impact on the heat of adsorption of the L Fe° species at high coverages whereas it increases significantly that at low coverages: E L Fe ° ( 0 ) = 120 kJ / mol. Considering reduced iron supported catalysts, the AEIR method is particularly useful because it allows the determination of the individual heats of adsorption of two adsorbed CO species that can be simultaneously present on the surface for high iron loadings. The study confirms that quantitative exploitations of DRIFT spectra are available for well designed experimental conditions offering an alternative for solid catalysts with low IR transmission properties.