Differential heat of reoxidation of reduced V₂O₅/γ-Al₂O₃

Four V/γ-Al₂O₃ catalysts with coverages ranging from 1.4 to 8.2 V atoms/nm² were examined with heat flow microcalorimetry, Raman spectroscopy, and chemical titration. Raman spectra showed that crystalline V₂O₅ formed on the Al₂O₃ surface between coverages of 6.1 and 8.2 V/nm². The heats and rates of reoxidation of the reduced catalysts were measured in a static adsorption calorimetry system. Both the reoxidation rates and heats were strong functions of the degree of reduction. They decreased as the sample was reoxidized, and the variation depended on the reoxidation conditions. At a degree of reduction equivalent to 0.5 O removed/V ion, the differential heats at 300°C were about 170 kJ/mol of O (or 85 kJ/mol of V) for the two higher loading samples and about 200 kJ/mol of O (100 kJ/mol of V) for the two lower loading samples. However, at low extents of reduction, the situation was reversed and the multilayer samples had higher reoxidation heats such that the integral reoxidation heats were relatively constant independent of V loading. The rates of reduction by H₂ and reoxidation by O₂ increased as the vanadia coverage increased. The variations in the reoxidation heats and rates with vanadia coverage were related to the structural characteristics of the samples.