Cr3+ exchanged zeolites can be either oxidized to Cr5+ by oxygen or reduced by H2 or CO to a lower valence state mainly Cr2+. E.s.r. measurements showed the formation of a 3d5NO+ complex upon adsorption of NO on reduced chromium samples. It was suggested that this complex has a Cr+NO+ structure and i.r. spectroscopy provided unambiguous evidence of the formation of the nitrosyl complex. Under certain conditions a diamagnetic complex [graphic omitted] was also formed. Adsorption of NO2 gave rise to the 3d5 complex Cr+NO+2. Similarly NO and NO2 adsorb on Ni2+ exchanged zeolites to form 3d9 complexes. Simultaneously at high pressures NO adsorbs irreversibly on Na+ ions of either the exchanged Y zeolites or even the unexchanged NaY. NO adsorption could also be used as a rough but easy means to locate the exchanged cations and to show cation migration from hidden sites towards accessible ones. The behaviour of the zeolite framework as a polar solvent and its probable role in the stabilization phenomenon of some cations in rather unusual valence states is emphasized.
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Naccache et al. (1973) studied this question.