Heats of physisorption and the predictions of chi theory

Abstract

It is demonstrated that the heats of physical adsorption as a function of amount adsorbed may be calculated using the quantum mechanically based chi theory. This calculation is compared to that which utilizes the Dubinin “thermodynamic criterion” for which there exists a mapping relationship between the isotherm and the heats of physisorption, q st= RTln( P/ P s). This latter method, however, does not yield the functional form of either the isotherm or q st. The chi theory, on the other hand, utilizes the isotherm to determine the number of moles in a monolayer, n m, and an energy value, E χ , which is unique for any particular adsorbate–adsorbent pair. This determination is possible provided the background gas pressure is low enough during the measurement. Once determined from the isotherm, n m and E χ are used to precisely calculate the heat of adsorption as a function of coverage. Such parameterless calculations have never before been reported in the literature. In examining the literature where both the isotherm and the heats of adsorption are presented together, usually E χ cannot be obtained from the isotherm due to the presence of background gases. In spite of the background gases, the value for n m is extractable and excellent one-parameter fits to the heats of adsorption are possible. In several cases, both n m and E χ are extractable from the isotherm. In these cases the calculation for the heats of adsorption with no parameters what-so-ever are in excellent agreement with the experimental values and well within experimental error.