Direct Measurement of the Interaction Energy between Solids and Gases. IX. Heats of Adsorption of Ammonia and Pyridine on Several Solids at High Temperature

Abstract

Abstract Heats of adsorption of ammonia and pyridine on H–Y zeolites and other solids were measured using a microcalorimeter in the temperature range of 313 to 673 K. Infrared spectra of adsorbed ammonia and pyridine were obtained in order to interpret the calorimetric data. There appeared two kinds of the temperature dependence of heats of adsorption. One is that heats of adsorption on H–Y5.0 and silica-alumina do change with coverage to a greater extent at high temperature range (above 473 K) than low temperature range. The difference in the shape of the heat curves was found to be attributable to the difference in the selectivity of adsorption at different temperatures, that is, adsorption occurred on stronger acidic sites is preference to weaker or non-acidic sites in the high temperature range, while random adsorption occurred simultaneously on acidic and non-acidic sites in the low temperature range. The other kind of temperature dependence was recognized to appear as a slight decrease in heats of adsorption with a temperature rise, which were observed in cases of ammonia adsorption on H–Y zeolites above 473 K and on Na–Y and silica above 313 K while the shapes of heat curves were similar to one another irrespective of the adsorption temperature. Such decrease may be attributed to the fact that the temperature dependence of the heat of adsorption was thermodynamically defined by the difference in molar heat capacity between adsorbed state and gaseous state.