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

Abstract

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.

Computational and Variable‐Temperature Infrared Spectroscopic Studies on Carbon Monoxide Adsorption on Zeolite Ca‐A

“…The required values of A M , for which only an approximation is experimentally known, were chosen as those giving the best linear fits of Equation (4) for the whole set of experimental data for each IR absorption band, by following an iteration procedure explained in detail elsewhere. [39] From the determined values of A M , it was inferred that the maximum absorbance of the band at 2179 cm À1 is about ten times larger than that of the band at 2188 cm À1 . Also, for the species giving rise to the band at 2179 cm À1 (Figure 4 b), the corresponding surface coverage did not reach a value higher than q = 0.3; hence, dicarbonyls should not be formed in any significant proportion.…”

“…[39,40] Briefly, under adsorption equilibrium at any given temperature, the integrated intensity (absorbance) of a characteristic IR absorption band should be proportional to the fractional coverage q of the adsorbed species giving rise to that band, and therefore this integrated intensity gives information on the activity of both the adsorbed species and the empty adsorption sites (1Àq). At the same time, the equilibrium pressure gives the activity of the gas phase.…”

Interaction of Small Molecules within Metal Organic Frameworks Studied by In Situ Vibrational Spectroscopy

“…The earliest spectroscopic evidences for the formation of an electron-donor-acceptor complex between CO 2 and functional groups of MOFs was reported in a MOF of type MIL-53 (MIL stands for Materials Institute Lavoisier) and amino-based MOF Zn 4 O (NH 2 -BDC) 3 (IR-MOF-3) and (NH 2 -MIL-53(Al) [6,8]. Two bands at 669 and 653 cm , respectively, suggesting that oxygen atoms of hydroxyl group act as the electron donor.…”

“…Since large variation of dynamic dipole moment take place as a function of loading, due to the interaction among the adsorbed molecules and therefore the integrated areas of IR bands do not always correlate with the amount of molecules adsorbed. Cautions must be taken when using variable temperature IR to measure the absorbance of molecular hydrogen bands and estimate the adsorption energy [3].…”

A rapid microwave-assisted synthesis of a sodium–cadmium metal–organic framework having improved performance as a CO2 adsorbent for CCS

“…18,19 In essence, at any given temperature, the integrated intensity, A, of the IR absorption band of adsorbed CO 2 should be proportional to the surface coverage, , thus giving information on the activity (in the thermodynamic sense) of both the adsorbed species and the empty adsorption sites, 1-. Simultaneously, the equilibrium pressure provides information on the activity of the gas phase.…”