Adiabatic Calorimetric Studies of the Adsorption of Sulfur Dioxide at 423 °K on Heat-treated Magnesias Characterized by Electron Microscopy

Abstract

Calorimetric heats of adsorption of sulfur dioxide at surface coverages from 0.2 to 7.7 μmol m−2 have been measured on a series of magnesias heat-treated at 600, 700, and 800 °C. Structural features of the adsorbents have been characterized by analysis of nitrogen adsorption–desorption isotherms and by transmission electron microscopy.Heats of adsorption at 423 °K for adsorbed amounts of 0.3 μmol m−2 were respectively, 86.0, 87.0, and 77.0 kcal mol−1 for the 600, 700, and 800 °C samples. The values for the 600 and 700 °C samples fell rapidly above a coverage of 2.0 μmol m−2 to around 44.0 kcal mol−1 at 5.0 to 7.0 μmol m−2, while the values for the 800 °C sample decreased continuously to 39.0 kcal mol−1 at the same coverage.Crystallite shapes tended increasingly towards the cubic form as the temperature of heat-treatment was increased and average particle sizes were 80, 240, and 320 Å for the 600, 700, and 800 °C samples, respectively.It is proposed that the very high heats obtained on all the samples are due to interactions with high-energy defect centers in the oxide and with the formation of magnesium sulfite initially, and magnesium sulfate towards monolayer completion.

Heats and entropies of adsorption of sulfur dioxide at low surface coverages on chrysotile asbestos at 323 K

“…Tests to confirm the presence of microquantities of sulfite, in both chrysotile series, proved to be inconclusive although sulfite formation under similar experimental conditions in the magnesium oxide-sulfur dioxide system has been reported. 15 Figure 5 shows that the experimental differential entropy increased with surface coverage from a value much below the level expected for immobile adsorption (i.e., adsorption in which only one degree of freedom is left to the adsorbed molecule9) to a value which coincided with the model for immobile adsorption for the low activation temperature surfaces and which was slightly greater than the immobile adsorption model for the 500 and 700 °C surfaces. These observations are consistent with (a) chemisorption interactions at low coverages (0 < 0.1) for all activation ranges and (b) multipoint adsorption which would have the effect of reducing the mobility of the adsorbate.…”