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, RhP(T)/γ-Al2O3, RhII formal centres are stable only after decarbonylation at 250 °C; contact with O2 produces [RhIIIn–O2]˙–(n= 1,2) and Al3+—O2–, with an increasing amount of Al3+—O–2 as the decarbonylation temperature increases. RhP(T)/ZrO2 does not show paramagnetic species before contact with O2: the treatment with O2 stabilizes both [Rh2III—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, RhH(T)/γ-Al2O3, formal RhO centres formed in the decarbonylation temperature range 250–500 °C, while no paramagnetic species were observed on RhH(T)/ZrO2. Contact with O2 shows behaviour of RhH(T)/γ-Al2O3(ZrO2) very similar to that of RhP(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.