Variable temperature infrared spectroscopy: a convenient tool for studying the thermodynamics of weak solid-gas interactions

This tutorial review describes the use of variable temperature infrared spectroscopy of adsorbed species (VTIR), a recent method for studying the thermodynamics of weak solid–gas interactions. Examples show how a fundamental relationship of thermodynamics (the van't Hoff equation, used long since in several fields of physical chemistry) can describe equilibrium processes at the solid–gas interface. The VTIR method is fully exploited by measuring absorbance of an IR band, temperature and pressure over a wide temperature range: an estimation of the interaction energy is, however, possible even ignoring the equilibrium pressure. Precise thermodynamic characterization of solid–gas interactions is required in several fields: on the applied side, gas sensing, separation and storage, which involve such areas as work-place security, air pollution control and the energy sector; regarding fundamental knowledge, weak solid–gas interactions are relevant to a number of fields, including hydrogen bonding, coordination chemistry and surface phenomena in a broad sense. Infrared (IR) spectroscopy of (gas) molecules adsorbed on a solid is frequently used to characterize both, the adsorbed species and the adsorbing centres at the solid surface. The potential of the technique can be greatly enhanced by obtaining IR spectra over a temperature range, and simultaneously measuring IR absorbance, temperature and equilibrium pressure. When this is done, variable temperature infrared (VTIR) spectroscopy can be used not only for a more detailed surface characterization, but also for precise studies on the thermodynamics of solid–gas interactions. Furthermore, when weak interactions are concerned, the technique shows favourable features compared to adsorption calorimetry, or to other classical methods. The potential of the VTIR method is highlighted by reviewing recently reported studies on dihydrogen, dinitrogen and carbon monoxide adsorption on zeolites. To facilitate understanding, an outline of the basis of the method is also given, together with an appraisal of the critical points involved in its practical use.