At room temperature CO is adsorbed strongly on Cu⁺ sites (band at 2131 cm⁻¹) whereas NO is preferably adsorbed on Cu²⁺ sites (band at 1882 cm⁻¹). Coadsorption of CO and NO allows simultaneous and selective detection of both kinds of cations. This observation is used to follow the changes on the sample surface occurring in the presence of oxygen. Addition of small amounts of O₂ to the CO–NO–Cu/SiO₂ system first leads to the oxidation of the Cu⁺ sites to Cu²⁺. This process is followed by formation of surface nitrates which block the Cu²⁺ sites for NO adsorption. Adsorption of CO at 85 K allows detection of Cu²⁺ cations (ca. 2200 cm⁻¹) in addition to the Cu⁺, CO is replaced by NO from these sites. Cu⁰ sites form carbonyls which, when copper is highly dispersed, can absorb at the same frequency at which Cu⁺–CO carbonyls are detected. In this case both kinds of species could be distinguished by their stability: the Cu⁰–CO species are easily destroyed during evacuation.

Adsorption of CO₂ on MIL-53(Al) and NH₂-MIL-53(Al) has been studied by Fourier transform infrared (FTIR) spectroscopy at different temperatures and equilibrium pressures. For better interpretation of the spectra ¹³CO₂ was also utilized. It is established that with both samples at low coverages CO₂ forms O-bonded complexes with the structural OH groups (OH···O¹²CO). These species are characterized by ν₃(¹²CO₂) at 2337–2338 cm^–1 and two ν₂(¹²CO₂) modes around 662 and 650 cm^–1. Simultaneously, the ν(OH) modes of the hydroxyl groups are red-shifted, while the δ(OH) modes are blue-shifted. At higher coverages (OH···O¹²CO)₂ dimeric species are formed and this leads to a decrease of the ν₃(CO₂) frequency by 2–4 cm^–1. This change is due to vibrational interaction as proven by the observation that the frequency remains practically unaffected for (OH···O¹²CO) (OH···O¹³CO) dimeric species. Interaction between dimers leads to additional slight decrease of the value of ν₃(CO₂). In parallel with the CO₂ adsorption a partial transformation of the material from large-pore to narrow-pore form occurs. Far before CO₂ interacts with all hydroxyl groups, polymeric CO₂ species are produced within the MIL-53(Al) sample. They are characterized by a split ν₃(CO₂) mode with a pronounced component at 2340 cm^–1. The formation of these species involves some of the dimers and is accompanied by a reopening of the MIL-53 structure. Analysis of the shift of the OH modes led to the conclusion that the polymeric moiety interacts strongly with one OH group and more weakly with several other hydroxyls. No polymeric species were observed with the NH₂-MIL-53(Al) sample which is associated with the more stable narrow-pore structure of this material. However, evidence of interaction between CO₂ and the hydroxyls H-bonded to amino groups was found.