Influence of adsorption and mass transfer effects on temperature-programmed desorption from porous catalysts

Abstract A theoretical analysis is presented for temperature-programmed desorption (TPD) from a bed of catalyst perfused by a flow of carrier gas. The bed is modeled either as a single CSTR or as multiple CSTR's connected in series. The number of CSTR's required is governed by the product of Schmidt and Reynold's number, ScRe . The model developed in this paper is used to calculate TPD spectra for the desorption of CO and H 2 from the surface of a Group VIII metal. It is established that the position and shape of the spectra are sensitive functions of catalyst particle size, catalyst bed depth, carrier flow rate, and carrier gas composition. The effects of nonuniformities in the initial distribution of adsorbate are also examined. For small particle diameters, such nonuniformities are rapidly annealed by intraparticle diffusion, and as a consequence have little effect on the shape or position of the TPD spectrum. For larger particle diameters, distortions of the spectrum due to nonuniform adsorbate distribution are expected. The theoretically generated TPD spectra are used to evaluate the applicability of a relationship developed for describing equilibrium desorption in the absence of mass transfer effects. Reasonably accurate estimates of the enthalpy of adsorption can be obtained from this relationship, but its use to determine the preexponential factor for desorption produces large errors when significant mass transfer effects are present.