Adsorption effects during temperature-programmed desorption of carbon monoxide from supported platinum

Abstract

Experimental data are presented for the temperature-programmed desorption (TPD) of CO from porous Pt Al 2O 3 into a vacuum. Most of the preadsorbed CO desorbs in a peak between 380 and 550 K. Other workers have measured desorption at substantially higher temperatures during TPD of CO into a carrier gas rather than a vacuum. A comparison of the experimental conditions suggests that the competition of CO adsorption with CO desorption may contribute to the differences between the TPD results. To investigate the effects of CO adsorption, we develop a mathematical model and use it to compute desorption spectra for the TPD of CO from Pt dispersed over a porous support into (a) an inert carrier gas and (b) a vacuum. Over the realistic parameter range considered, our model predicts that adsorption effects, caused by high concentrations of gaseous CO in the system, are always an important feature, broadening the desorption peaks and shifting them to higher temperatures. Indeed, we find that adsorption competes with desorption to the extent that adsorption equilibrium is always approached closely within the porous supported Pt samples. For desorption into a carrier gas, the adsorption effects result from limitations to the flow of CO from the sample cell, whereas for desorption into a vacuum, the adsorption effects result from limitations to the diffusion of CO from the porous sample. Our results suggest that significant adsorption effects will also be present during the TPD of CO from other Group VIII precious metals dispersed over porous supports.