This critical review describes the reactivity of heterogeneous catalysts from the point of view of four simple, but essential for Chemistry, molecules (namely dihydrogen, carbon monoxide, nitrogen monoxide and ethylene) that are considered as probes or as reactants in combination with “in situ” controlled temperature and pressure Infrared spectroscopy. The fundamental properties of H₂, CO, NO and C₂H₄ are shortly described in order to justify their different behaviour in respect of isolated sites in different environments, extended surfaces, clusters, crystalline or amorphous materials. The description is given by considering some “key studies” and trying to evidence similarities and differences among surfaces and probes (572 references).

Among microporous systems metal organic frameworks are considered promising materials for molecular adsorption. In this contribution infrared spectroscopy is successfully applied to highlight the positive role played by coordinatively unsaturated Cu²⁺ ions in HKUST-1, acting as specific interaction sites. A properly activated material, obtained after solvent removal, is characterized by a high fraction of coordinatively unsaturated Cu²⁺ ions acting as preferential adsorption sites that show specific activities towards some of the most common gaseous species (NO, CO₂, CO, N₂ and H₂). From a temperature dependent IR study, it has been estimated that the H₂ adsorption energy is as high as 10 kJ mol⁻¹. A very complex spectral evolution has been observed upon lowering the temperature. A further peculiarity of this material is the fact that it promotes ortho–para conversion of the adsorbed H₂ species.

The microporous metal–organic framework Ni₂(dhtp) (H₄dhtp = 2,5-dihydroxyterephthalic acid) shows distinct end-on CO₂ coordination to coordinatively unsaturated nickel sites giving rise to high CO₂ adsorption capacity at sub-atmospheric pressures and ambient temperatures.