A correlated X-ray photoelectron and electron spin resonance spectroscopic study of rhodium-exchanged X and Y zeolites

Abstract

X-Ray photoelectron (X.p.) and electron spin resonance (e.s.r.) spectroscopies have been employed to study the effect of the location and the Si/Al ratio on the distribution of the various Rh oxidation states in faujasite-type zeolites after various activation processes. The zeolites studied were RhNa-X, RhCa-X, RhNa-Y and RhCa-Y. The samples were activated in flowing oxygen to 200, 400 and 500 °C, activated in vacuum to 500 °C and reduced with H₂ at 500 °C after vacuum activation. Strong e.s.r. signals were observed only in the samples activated to 400–500 °C in flowing oxygen, indicating the presence of Rh^II. The amount of Rh^I increased with the activation temperature, whereas the amount of Rh^III was substantial in the samples activated in flowing oxygen. While no significant effects were attributed to the Si/Al ratio the co-cation had a considerable effect on the distribution of oxidation states, mainly in samples activated in flowing oxygen. The relative amounts of Rh_III in RhCa-X and RhCa-Y were larger than in RhNa-X and RhNa-Y; accordingly, the relative amounts of Rh^I were larger in the latter zeolites.