COADSORPTION OF ATOMIC AND MOLECULAR-HYDROGEN ON RH AL2O3/

Adsorption isosteres of hydrogen on a 5 wt.% Rh/Al2O3 sample have been measured volumetrically in the temperature region between 273 and 523 K. The variation in the heat of adsorption (q(ad)) with the hydrogen uptake (N-H) was obtained from the isosteres using the Clausius-Clapey ron equation. A linear ln P-H2 vs. T-1 Clausius-Clapeyron relationship was found at low hydrogen uptakes. From the slope of these linear rel ationships, a decrease in -q(ad) from 100 kJ (mol H-2)(-1) with an inc reasing uptake was noticed. This heat was attributed to the dissociati ve chemisorption of hydrogen. At high hydrogen uptakes, a curved Claus ius-Clapeyron relationship was obtained. This curved relationship is a ttributed to the simultaneous presence of the two weakly adsorbed type s, i.e. atomic and molecular adsorptions. The -q(ad) of the atomic ads orption and the molecular adsorption in this weakly adsorbed region ha ve a value of 42 kJ (mol H-2)(-1) and 21 kJ (mol H-2)(-1), respectivel y.