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 (q_ad) with the hydrogen uptake (N_H) was obtained from the isosteres using the Clausius–Clapeyron equation. A linear ln P_H2vs.T^–1 Clausius–Clapeyron relationship was found at low hydrogen uptakes. From the slope of these linear relationships, a decrease in –q_ad 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 –q_ad 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.