Paramagnetic metal and oxygen species observed with [CO4(CO)12] and [Rh4(CO)12] carbonyl clusters pyrolysed on γ-alumina and zirconia supports

Abstract

The pyrolysis of γ-Al₂O₃- and ZrO₂-supported [Co₄(CO)₁₂] and [Rh₄(CO)₁₂] gives highly dispersed metal systems whose electronic properties have been studied by e.s.r. spectroscopy. Paramagnetic formal Rh^II species (g_∥= 2.21, g_⊥= 2.11) were observed on Rh/γ-Al₂O₃ samples. The pattern of the hyperfine structure suggests that metal–metal interactions are present and that paramagnetic transition-metal carbonyl clusters can be assumed as model compounds. No paramagnetic species involving the metal were observed for either γ-Al₂O₃- or ZrO₂-supported cobalt samples. This behaviour is consistent with a higher acidic character of γ-Al₂O₃ with respect to ZrO₂. On contact with O₂ and CO the following paramagnetic species were observed: O^–₂—Al³⁺(g_∥= 2.034, g_⊥= 2.00) over Co/γ-Al₂O₃ samples, O^–₂—Rh^III(g₁= 2.09, g₂= 2.03, g₃= 2.00) over Rh/γ-Al₂O₃ samples, O^–₂—Rh^III and O^–₂—Zr⁴⁺(g_∥= 2.03, g_⊥= 2.00) over Rh/ZrO₂ samples and CO—Rh^II(g= 2.034) over Rh/γ-Al₂O₃ samples. The O₂ and CO bond strengths are higher for the γ-Al₂O₃-supported samples than for the ZrO₂-supported samples and are tentatively related to the catalytic activity of these metal systems.