45 Heats of Chemisorption of Oxygen on Palladium and Palladium-Silver Alloys

Abstract

Heats of chemisorption of oxygen have been measured on palladium black and on palladium-silver alloys, in the temperature range 500-700° K. and fraction of surface coverages, θ, 1 × 10−4 to 0.7. Heat of chemisorption data were obtained by means of measurements of reaction equilibrium of water decomposition on palladium and palladium-silver alloys. For palladium black, a value of the adsorption heat of −24 kcal./mole oxide was obtained. This value is not much different from the value corresponding to the formation of bulk palladium oxide (PdO). A large increase in the value of the heat is obtained by alloying palladium with silver. This effect sets in at relatively low silver concentration (˜1%). This result is taken as an indication that bonding orbitals, used in the alloying process, do not contribute greatly to the adsorptive bond. Furthermore, from the behavior of the adsorption equilibrium function in the range of θ values studied, it can be concluded that oxygen chemisorption on palladium black is localized and noninteracting and occurs on an apparently thermodynamically uniform surface. On the other hand, surface nonuniformity is produced by alloying palladium with even 1% of silver.