Surface Studies of Heat-treated γ-Aluminas in the Adsorption of Sulfur Dioxide below Monolayer Coverage

Calorimetric heats of adsorption of sulfur dioxide at surface coverages from 0.1 to 3.5 µmol m−2 have been measured on a series of γ-aluminas heat-treated at 500, 700, and 900 °C. Essential features of the structures of the adsorbents have been characterized by analysis of nitrogen adsorption/desorption isotherms and by both direct and indirect electron microscopy.Heats of adsorption of 423 °K for adsorbed amounts of 0.1 µmol m−2 were 48.0, 59.0, and 85.0 kcal mol−1 for the 500, 700 and 900 °C samples, respectively. The values for the 500 and 700 °C samples fell rapidly with increasing coverage to around 19.0 kcal mol−1 at 3.2 to 3.3 µmol m−2, while the values for the 900 °C sample decreased slowly to 15.0 kcal mol−1 at the same coverage.The pores had circular cross sections and mean diameters of 25, 35, and 40 Å were observed for the 500, 700, and 900 °C samples, respectively. Examination of carbon/platinum replicas of the surface showed the presence of agglomerates of spherical particles approximately 400 Å in diameter.It is proposed that the high initial heats on the 500 and 700 °C samples are due to the formation of a sulfate-like species on the surface and at higher coverage extensive hydrogen bonding is present. The very high heats obtained on the 900 °C sample are believed to be due to interactions with high-energy defect centers in the oxide which were created by heat treatment.