A METHOD FOR CALCULATING THE ACTIVATION-ENERGY DISTRIBUTION FOR DESORPTION OF AMMONIA USING A TPD SPECTRUM OBTAINED UNDER DESORPTION CONTROL CONDITIONS

A method, called the dc-TPD method, was presented for calculating the density distribution function of the activation energy for ammonia des orption by utilizing the temperature-pro rammed desorption (TPD) spect rum of ammonia from solid catalysts measured under desorption control conditions. TPD spectra were measured in a helium stream under low pre ssure (about 0.13 kPa) to realize desorption control conditions by rem oving ammonia desorbed from a catalyst quickly. The measured TPD spect ra showed only one peak corresponding to stronger acid sites, and were found to be dependent not on the time factor (ratio of the catalyst w eight to the gas flow rate), but only on the heating rate. The TPD spe ctrum thus obtained was assumed to be the sum of the spectra from a se ries of acid sites with different acid strengths distributed on the ca talyst. The desorption of ammonia from the acid sites was approximated as an irreversible first-order reaction rate equation with the activa tion energy and frequency factor whose values correspond to the acid s trength of the acid sites. Using these assumptions, the relation betwe en the TPD spectrum and the activation energy distribution for desorpt ion was derived. The distribution functions obtained can be used for c haracterizing the acidic properties of solid catalysts. The shape of t he distribution function was found to have little dependence on the he ating rate in the TPD experiment. The acidic properties of silica-alum ina, H-mordenite, and HY-type and MFI-type zeolites were well characte rized by the distribution function of the activation energy for ammoni a desorption. (C) 1997 Elsevier Science B.V.