Abstract
Infrared spectroscopy and thermogravimetric studies of adsorbed pyridine, combined with volumetric reduction/oxidation measurements, were used to investigate the acidic properties of molybdena-alumina samples reduced to different extents in hydrogen at 670 K. Pyridine was adsorbed at 420 K and studied after evacuation at progressively higher temperatures ranging from 370 to 670 K. The average molybdenum oxidation state was varied from +6 to +4.7, as monitored by measuring the consumption of hydrogen and the formation of water during reduction and the consumption of oxygen during reoxidation. Infrared spectra of adsorbed pyridine showed the presence of Brønsted acid sites and two types of Lewis acid sites on oxidized molybdena-alumina. The number of Brønsted acid sites increased at low extents of reduction (e.g., an average Mo oxidation state of +5.6) and decreased to zero with further reduction. The Lewis acidity also passed through a maximum with extent of reduction but was not eliminated with reduction. These results are explained by the accepted model for molybdena-alumina in which the surface of alumina is populated with small molybdena clusters. It is proposed that the electronegativity of alumina is altered in the vicinity of these molybdena clusters, thereby altering the acidic properties.
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