Paramagnetic metal and oxygen species observed with Rh/γ-Al2O3 and Rh/ZrO2. Dependence on the decarbonylation temperature of [Rh4(CO)12] on alumina and zirconia supports

Abstract

Paramagnetic metal and oxygen species observed when alumina-(or zirconia-supported rhodium, Rh/γ-Al₂O₃(ZrO₂), is obtained by the decarbonylation of [Rh₄(CO)₁₂]-Al₂O₃(ZrO₂) are discussed as a function of the decarbonylation temperature (250–600 °C) and the method of decarbonylation (pyrolysis in vacuo or reduction in an H₂ stream). On vacuum-pyrolysed samples, Rh_P(T)/γ-Al₂O₃, Rh^II formal centres are stable only after decarbonylation at 250 °C; contact with O₂ produces [Rh^III_n–O₂]^˙–(n= 1,2) and Al³⁺—O₂^–, with an increasing amount of Al³⁺—O^–₂ as the decarbonylation temperature increases. Rh_P(T)/ZrO₂ does not show paramagnetic species before contact with O₂: the treatment with O₂ stabilizes both [Rh₂^III—O₂]^˙– and Zr⁴⁺—O^–₂ centres at a decarbonylation temperature of 250 °C, while essentially Zr⁴⁺—O₂^– alone is formed at higher temperature. On H₂ reduced samples, Rh_H(T)/γ-Al₂O₃, formal Rh^O centres formed in the decarbonylation temperature range 250–500 °C, while no paramagnetic species were observed on Rh_H(T)/ZrO₂. Contact with O₂ shows behaviour of Rh_H(T)/γ-Al₂O₃(ZrO₂) very similar to that of Rh_P(T)/γ-Al₂O₃(ZrO₂). The stability of the Rh—O₂ bond is a function of the metal positive charge and it is related to the decarbonylation temperature. The strength of the interaction between O₂^– and the support acidic centres, Al³⁺ and Zr⁴⁺, is always high in γ-Al₂O₃-supported samples, and dramatically increases in ZrO₂-supported samples on increasing the decarbonylation temperature. Different pathways of electron transfer can explain the observed behaviour of Rh/ZrO₂.