Microcalorimetric measurements of differential heats of adsorption on reactive catalyst surfaces

Techniques are presented for measurement of differential heats of adsorption on reactive catalyst surfaces using heat-flux calorimetry. Samples are prepared ex-situ in ultra-pure flowing gases and then sealed in Pyrex capsules. Special calorimetric cells are employed to break the sample capsule after thermal equilibration of the sample with the calorimeter. In this manner the clean sample is exposed rapidly to the adsorbing gas, minimizing surface contamination. Initial heats of CO and H-2 adsorption at 403 K on Pt/SiO2 catalysts obtained using the present technique (135 and 100 kJ/mol. respectively) were in agreement with results reported in the literature using standard calorimetric procedures. Initial heats measured in this study for CO adsorption at 308 K on reduced Ni powders (120 kJ/mol) and on nickel samples containing metallic potassium (200 kJ/mol) corresponded to values in the literature from ultrahigh vacuum studies of CO adsorption on Ni single crystal surfaces. The initial heat of N-2 adsorption at 453 K on reduced iron determined in this study (200 kJ/mol) was in agreement with results obtained in ultrahigh vacuum measurements of metallic iron single crystal surfaces. These results, for catalyst systems that are sensitive to traces of oxygen-containing species, provide strong evidence that the experimental techniques employed in the present study allow clean metallic surfaces to be maintained during microcalorimetric adsorption studies.