Abstract
The paper describes a new resistance thermometer calorimeter, suitable for the measurement of heats of adsorption on powdered catalysts. The calorimeter, which is constructed as a single unit, is capable of being outgassed at 500 °C and is designed to measure heat liberations down to the order of magnitude of a few millicalories. In this respect it is particularly suited to the measurement of heats of adsorption over small coverage ranges (Δθ from 0.01 to 1%), but it is equally adaptable for conventional studies with large increments. The calorimeter has been used to measure the heats of adsorption of isopropanol and acetone on a nickel oxide catalyst over a range of ca. 1% coverage at room temperature. The heat of adsorption of isopropanol on an outgassed surface is 16 kcal/mole, independent of coverage. On a surface carrying presorbed oxygen, the heat is again constant, but at a slightly higher value (18 kcal/mole). Although the adsorption under these conditions is primarily physical, hydrogen bonding with oxygen atoms on the surface is invoked to explain the absolute magnitude of the heat and its slight enhancement in the presence of presorbed oxygen. With acetone, the heat of adsorption for the outgassed surface is 11 kcal/mole, but on the oxygenated surface an initial heat of 21 kcal/mole is found, falling to 11 kcal/mole at 0.5% coverage. This result is interpreted as normal physical adsorption for the outgassed surface, but a contribution from dissociative chemisorption is considered as one way to account for the high heats at low coverages on the oxygenated surface. It is suggested that presorbed oxygen may lower the activation energy for the dissociative adsorption and desorption of acetone, and that this may be the mechanism whereby at higher temperatures oxygen enhances the rate of dehydrogenation of isopropanol.
Published Version
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