TEMPERATURE-PROGRAMMED DESORPTION OF AMMONIA WITH READSORPTION BASED ON THE DERIVED THEORETICAL EQUATION

Temperature-programmed desorption (TPD) spectra were analyzed on the basis of the theoretical equation of TPD of ammonia with freely occurring readsorption, and two kinds of methods to determine the strength of zeolite acidity were proposed. The change of enthalpy upon desorption (Delta) was determined from the derived equation for the relationship between the temperature at the peak maximum and a parameter including the acid amount and the contact time of the carrier gas. The theoretical equation was confirmed experimentally not only from the precise determination of Delta H but also from an agreement between the values thus measured and those from microcalorimetry. Because the change of entropy upon desorption of ammonia (Delta S) was almost independent of the zeolite species, the constancy of Delta S was assumed; thus, Delta H was measured by a one-point experiment. Examples of determination of Delta H using this method were described. The proposed constant value of 150 J K-1 mol(-1) of Delta S was supported by thermodynamic considerations, Trouton's rule and entropy of ammonia. Because the simulated spectra based on the theoretical equation were similar to those obtained experimentally, a narrow distribution of zeolite acidity was expected.