Further electron spin resonance studies of RhNa-X. Effects of carbon monoxide, hydrogen and oxygen

Abstract

The effects of H₂, O₂ and CO on RhNa-X after various pretreatments have been studied by electron spin resonance (e.s.r.). Activation of RhNa-X under flowing O₂ at 200–600 °C followed by evacuation at >450 °C generated a paramagnetic species, E(X), in the inner sites of the zeolite, as well as small metallic particles in the α-cage. Species E(X) with g_∥= 2.029 and g⊥= 2.610 was assigned to isolated Rh^o atoms stabilized inside the β-cage or hexagonal prism. The reduction process taking place during the evacuation step is believed to involve O^2– from the zeolite framework. Adsorption of CO on such activated samples gave rise to two paramagnetic adducts assigned to two Rh^o–CO species in different sites. The disruption of the small metallic particles in the α-cage by CO is proposed to account for the formation of these adducts. The same metallic Rh crystallites in the α-cage are also regarded as the precursors of two different Rh^I–O^–₂ species obtained by adsorption of O₂. An alternative route for the formation of E(X) is by chemical reduction at fairly low temperatures of samples pretreated in O₂ at > 450 °C and then evacuated at 25 °C. The precursor of E(X) is believed to be an isolated Rh³⁺ cation located in an inner site of the zeolite. It is proposed that the reduction of such a precursor by H₂, CO and other reducing agents occurs via an electron-transfer mechanism.