DETERMINATION OF THERMODYNAMIC PARAMETERS FOR THE DISSOCIATIVE ADSORPTION OF DIHYDROGEN ON RH/AL2O3

High-pressure infrared spectroscopy has been used to study the interaction of H-2 with Rh/Al2O3 catalyst films as a function of temperature and pressure. The interaction process was found to be dissociative and fit Langmuir adsorption isotherms. Two dissociative Langmuir models were employed in fitting the data. A model in which reversible dissociation of H-2 on Rh, together with reversible H spillover to react with hydroxyl groups on the Al2O3 support was included, provided a DELTAH(ads) of -7.7 kcal mol-1 for the process. This low enthalpy of adsorption is indicative of weak Rh-H bonding and may explain why Rh functions well as a hydrogenation catalyst. This work demonstrates the utility of Fourier transform infrared spectroscopy in obtaining thermodynamic data for supported catalysts.