CHARACTERIZATION OF MODEL 3-WAY CATALYSTS .1. DETERMINATION OF THE ACCESSIBLE METALLIC AREA BY CYCLOHEXANE AROMATIZATION ACTIVITY MEASUREMENTS

The accessible metallic area measured by hydrogen chemisorption on mod el ceria-alumina supported platinum or(and) rhodium catalysts has been compared to the results obtained in cyclohexane aromatization activit y tests. The benzene formation was studied at 573 K after reduction by hydrogen at the same temperature. The turnover frequency, calculated on the basis of hydrogen chemisorption data, was found to depend on th e type of support, ceria or alumina, in interaction with the precious metal particles. However, for the catalysts supported on ceria-alumina , the activity per metallic platinum or rhodium site was almost the sa me for both fresh catalysts or after hydrothermal aging at 1273 K. The possible use of the cyclohexane aromatization activity measurement fo r the determination of the metallic area of three-way catalysts is exa mined. Compared to hydrogen chemisorption, this method presents the ad vantage of a better sensitivity for measuring very low dispersions of aged catalysts, typically dispersions inferior to 3%. In the discussio n, the possible influence of hydrogen spill-over effects and of the co nnected role of chlorine are considered. Although not directly studied , the possible effect of sulphur present in commercial catalysts aged in vehicles is also discussed. (C) 1997 Academic Press.