Coadsorption of atomic and molecular hydrogen on Rh/Al2O3
Abstract
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 (qad) with the hydrogen uptake (NH) was obtained from the isosteres using the Clausius–Clapeyron equation. A linear ln PH2vs.T–1 Clausius–Clapeyron relationship was found at low hydrogen uptakes. From the slope of these linear relationships, a decrease in –qad from 100 kJ (mol H2)–1 with an increasing uptake was noticed. This heat was attributed to the dissociative chemisorption of hydrogen. At high hydrogen uptakes, a curved Clausius–Clapeyron relationship was obtained. This curved relationship is attributed to the simultaneous presence of the two weakly adsorbed types, i.e. atomic and molecular adsorptions. The –qad of the atomic adsorption and the molecular adsorption in this weakly adsorbed region have a value of 42 kJ (mol H2)–1 and 21 kJ (mol H2)–1, respectively.