The reversible reduction–oxidation processes of Ag+ ions in Ag+-exchanged zeolites A (AgA) with H2 and O2 adsorption were studied by Fourier transform infrared (FTIR) spectroscopy. CO adsorption was carried out at low temperature to probe the functional groups present on AgA (before and after H2 adsorption) and their locations in the channels of zeolite. The hydroxy groups were formed upon H2 (D2) adsorption on AgA with different IR frequencies depending on the degree of Ag+-exchange. It was found that these generated hydroxy groups mainly locate inside the β-cages in AgA at 30%-exchange degree, as evidenced by the inaccessibility of CO due to the small six-oxygen ring windows. On the contrary, the hydroxy groups, which had already been formed before H2 adsorption due to the dissociation of H2O on Ag+ ions, could interact with CO, showing an IR band at 2156 cm−1. This indicates that of hydroxy groups of this type are situated inside the larger α-cages containing 8-membered rings or on the external surface. The CO stretching band of CO–Na+ adducts appeared at 2165 cm−1 and could be easily removed by evacuation, while the CO–Ag+ species was detected at 2182 cm−1 and was highly resistant toward evacuation even at ambient temperature.
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Wang et al. (2004) studied this question.