Abstract
The acidic properties of 0.5 and 1.0 wt% iron supported on silica were probed by adsorption of pyridine, as studied by gravimetric measurements and infrared spectroscopy. The state of iron on silica was monitored by Mössbauer spectroscopy. This latter technique showed evidence for three types of Fe cations on the surface of silica, i.e., trivalent iron and two states of divalent iron having low and high coordination, respectively. Addition of iron to the silica generated Lewis acid sites on the surface, while little or no acidity existed on the silica surface alone. These new acid sites were shown to be iron cations in sites of low coordination. Infrared spectroscopy and gravimetric measurements of adsorbed pyridine indicated that the trivalent iron cations were stronger Lewis acids than divalent iron cations. The strength differences of the different oxidation states is explained on the basis of electronegativity differences.
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