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 RhII. The amount of RhI increased with the activation temperature, whereas the amount of RhIII 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 RhIII in RhCa-X and RhCa-Y were larger than in RhNa-X and RhNa-Y; accordingly, the relative amounts of RhI were larger in the latter zeolites.