Chromium ions within zeolites. Part 1.—Infrared, electron spin resonance and temperature-programmed reduction studies of the valence states of chromium ions

Abstract

It has been found that Cr^III ions introduced into NH₄Y zeolites and their stabilized form SY are not reduced in vacuo but readily reduced by hydrogen to Cr^II in the temperature range 570–870 K. The absorption of NO on Cr^III and Cr^II ions takes place with the formation of dinitrosyl complexes which exhibit, in both cases, practically identical i.r. and e.s.r. spectra. Thus the adsorption of NO cannot be used for the determination of Cr valence state in zeolites. Oxidation of Cr^III ions to Cr^V and Cr^VI occurs with retainment of zeolite crystallinity. Non-skeletal oxygen ligands bound in complexes of Cr^V and Cr^VI are easily released from oxidized zeolite. These complexes are not stable, Cr^VI being more readily reduced than Cr^V. The results were obtained by means of i.r. and e.s.r. spectra of adsorbed NO, e.s.r. spectra of Cr^V ions, t.p.r. of zeolites by hydrogen and mass-spectrometric detection of oxygen released from zeolites.