ENERGETICS OF H-2 AND O-2 ADSORPTION ON IR GAMMA-AL2O3 AND IR/SIO2 CATALYSTS, DEPENDENCE ON SUPPORT AND ON METAL-PARTICLE SIZE/

The adsorption of hydrogen and oxygen on Ir/SiO2 and Ir/Al2O3 catalyst s of various metallic percentages and different metal particle sizes h as been studied by adsorption microcalorimetry. The hydrogen adsorptio n stoichiometries lay between 1.1 and 2.4, depending on the support an d on degree of dispersion. The variation of hydrogen adsorption heat w ith amount adsorbed revealed significant differences in surface hetero geneity between the various samples. The dependence of hydrogen adsorp tion stoichiometry and differential heat of adsorption vs. coverage cu rves on support and on particle size may be related to crystallite siz e and shape, and to differences in metal-support interaction. On the c ontrary, oxygen adsorption stoichiometries and differential heat of ad sorption vs. coverage curves were very similar for all samples. The am ount of oxygen adsorbed at the completion of the monolayer, clearly id entified in the differential calorimetric isotherms, coincided with th e amount of superficial iridium as determined by a volumetric method. (C) 1998 Elsevier Science B.V.