Paramagnetic rhodium species in zeolites. Part 1.—RhNa-X and RhNa-Y

Abstract

At least five different paramagnetic Rh^II species have been detected by e.s.r. following activation by O₂ at 200–475 °C of ion-exchanged RhNa-Y prepared either from [Rh(NH₃)₅Cl]Cl₂(sample I) or from RhCl₃· 3H₂O (sample II). The conditions of formation of these species, their stability and their possible locations have been studied. Almost no paramagnetic species were detected in samples activated in O₂ at temperatures higher than 500 °C, indicating a total oxidation of rhodium. In samples activated in O₂ at temperatures > 200 °C and then evacuated at increasing temperatures, all Rh^II species disappeared progressively, while a new species E(Y) with g_∥= 2.068 and g _⊥= 2.674 developed. The same species E(Y) was also generated in sample II following evacuation at temperatures > 200 °C without O₂ activation. It is believed that species E(Y) is composed of isolated Rh^O atoms stabilized in the secondary channel of Y zeolite. Possible reduction mechanisms are discussed. A striking similarity between the overall chemistry of Rh in Na-X as revealed by e.s.r. and that of Rh in Na-Y was found.