Abstract
Variable-temperature infrared (VTIR) spectroscopy is an instrumental technique that enables structural characterization of gas-solid adsorption complexes by analysis of meaningful vibrational modes, and simultaneous determination of the standard enthalpy change (ΔH0) involved in the gas adsorption process, which allows one to quantify the stability of the corresponding complex. This is achieved by a van’t Hoff analysis of a set of IR spectra recorded over a sufficiently large temperature range. Herein, the use of this versatile spectroscopic technique is demonstrated by reviewing its application to the study of carbon monoxide, carbon dioxide and dinitrogen adsorption on several (alkaline) zeolites, which can be regarded as the archetype of periodic porous solids.
Highlights
Gas adsorption in porous solids can be used in industrial processes such as oxygen separation from air [1,2,3,4], sweetening of natural gas, upgrading of biogas [5,6,7,8] and purification of hydrogen obtained from synthesis gas or from steam reformation of hydrocarbons [9,10,11], to name only a few examples
Precise knowledge about the structure and stability of the corresponding gas adsorption complex is of the utmost importance; and that is the case when seeking the optimum porous adsorbent for gas transport and delivery
In several of the cases reviewed periodic DFT calculations were used in order to elucidate the structure of gas adsorption complexes [65]
Summary
Gas adsorption (physisorption) in porous solids can be used in industrial processes such as oxygen (and argon) separation from air [1,2,3,4], sweetening of natural gas, upgrading of (landfill) biogas [5,6,7,8] and purification of hydrogen obtained from synthesis gas (syngas) or from steam reformation of hydrocarbons [9,10,11], to name only a few examples. Precise knowledge about the structure and stability of the corresponding gas adsorption complex is of the utmost importance; and that is the case when seeking the optimum porous adsorbent for gas transport and delivery. For such a purpose, classical infrared (IR) spectroscopy can give valuable structural information derived from analysis of the wavenumber shifts undergone by meaningful vibrational modes of the adsorbed molecule, and from the (relative) intensity of the corresponding IR absorption bands, but determination of the gas-solid interaction energy is out of reach unless a complementary technique (such as adsorption calorimetry) is used. Sci. 2020, 10, 8589; doi:10.3390/app10238589 www.mdpi.com/journal/applsci to both, structural characterization of the adsorption complex and (simultaneous) measurement of the standard enthalpy change (ΔH0) involved in the gas adsorption process, as will be shown below by reviewing several enlightening case studies
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