Calorimetric Measurement of the Heat of Hydrogen Adsorption on Palladium (Part 2) -The Analysis of the Anomaly in the Plot of Heat of Adsorption vs. Hydrogen to Palladium Ratio-

Abstract

We reported previously an anomaly that the heat of hydrogen adsorption on palladium increased showing a peak at a certain ratio of H to Pd. In this paper, the origin of this anomaly (peaks in Figs.1-3) was studied. Measurement of the X-ray diffraction revealed that there was no correspondence between these peaks and the α → β phase transition of Pd-H₂ system. The peak appeared only in the region of β phase (Fig.3), irrespective of the species of supporting material of Pd (Figs.1-3). The height of the peak decreased with the decrease in the temperature (Fig.4). When the specific surface area of palladium increased, the atomic ratio of hydrogen adsorbed to palladium at the peak increased and the equilibrium pressure of hydrogen decreased (Fig.6). The effects of the pretreatment of palladium by hydrogen (Figs. 7, 8) and the pretreatment by carbon monoxide, as well as the adsorption of hydrogen on the highly dispersed palladium (Fig.9)indicated that the heat of strong adsorption of hydrogen on the surface of palladium was not the origin of the peak. The peak probab ly resulted from the direct dissolution of hydrogen into β phase.