A method for calculating the activation energy distribution for desorption of ammonia using a TPD spectrum obtained under desorption control conditions

Abstract

A method, called the dc-TPD method, was presented for calculating the density distribution function of the activation energy for ammonia desorption by utilizing the temperature-programmed desorption (TPD) spectrum of ammonia from solid catalysts measured under desorption control conditions. TPD spectra were measured in a helium stream under low pressure (about 0.13 kPa) to realize desorption control conditions by removing ammonia desorbed from a catalyst quickly. The measured TPD spectra showed only one peak corresponding to stronger acid sites, and were found to be dependent not on the time factor (ratio of the catalyst weight to the gas flow rate), but only on the heating rate. The TPD spectrum thus obtained was assumed to be the sum of the spectra from a series of acid sites with different acid strengths distributed on the catalyst. The desorption of ammonia from the acid sites was approximated as an irreversible first-order reaction rate equation with the activation energy and frequency factor whose values correspond to the acid strength of the acid sites. Using these assumptions, the relation between the TPD spectrum and the activation energy distribution for desorption was derived. The distribution functions obtained can be used for characterizing the acidic properties of solid catalysts. The shape of the distribution function was found to have little dependence on the heating rate in the TPD experiment. The acidic properties of silica-alumina, H-mordenite, and HY-type and MFI-type zeolites were well characterized by the distribution function of the activation energy for ammonia desorption.