Adsorption of water vapour on α-Fe2O3

Abstract

The chemisorption of water vapour on de-hydroxylated α-Fe2O3 has been investigated by heats of immersion into water at 25°C. The properties of physically adsorbed multilayers on the hydroxylated surface have been studied using dielectric relaxation techniques in conjunction with adsorption isotherms. The heat of immersion of the completely hydroxylated substrate is –367 ergs/cm2 and increases with increased outgassing temperature to –1073 ergs/cm2 for outgassing at 375°C. Differentiation of the integral heat curve gives the initial heat of chemisorption to be at least –50 kcal/mol of water vapour. The electrical capacitance at 70 c/s to 300 kc/s of physically adsorbed water vapour on the hydroxylated substrate is constant within the B.E.T. monolayer, but rises sharply with the onset of the second layer. This behaviour suggests that the first layer of physically adsorbed water is immobile, but that succeeding layers are mobile. Confirming evidence as to the immobile nature of the physisorbed monolayer is given on an entropy basis. The integral entropy of adsorption obtained from multi-temperature isotherms at constant spreading pressure is –33.2 cal/mol K for half a monolayer. This value agrees favourably with the theoretical value of –36.8 cal/mol K calculated from statistical thermodynamics for a localized monolayer, but not with the value –15.9 cal/mol K derived for mobile adsorption. Characteristic relaxation frequencies of adsorbed water vapour were obtained from Cole-Cole are plots of dielectric constant against dielectric loss. The characteristic frequencies increase smoothly with coverage from 10 c/s to a value of 10 kc/s at B.E.T. coverages of three or more, suggesting the development of a hydrogen-bonded ice-like structure for those coverages.