Modelling and experimental verification of TPD from porous catalysts

Abstract

Model predictions of the coupled effects due to pore diffusion and readsorption of the desorbed species on temperature-programmed desorption (TPD) data have been compared with experimental observations recorded during TPD of physisorbed methanol from γ-Al 2O 3. The intrinsic rate constants for adsorption and desorption were estimated from data unaffected by diffusional intrusions by varying the carrier gas flow rate and the catalyst load. In the diffusion-controlled regime the model successfully describes both the observed shift of the TPD peak temperature and the decrease of the effective desorption rate constant as the size of the catalyst particle is increased. Model analysis along with numerical simulations of TPD experiments for a wide range of operating conditions have demonstrated how the observed desorption rate depends on diffusional limitations and readsorption. It has been shown that, when diffusion is slow and readsorption is fast, a drop in the net desorption rate occurs due to the formation of intraparticle gas-phase concentration gradients which enhance the local rate of readsorption.