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

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 RhII 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: O2- - Al3+ (g∥ = 2.034, g⊥ = 2.00) over Co/γ-Al2O3 samples, O2- - RhIII (g1 = 2.09, g2 = 2.03, g3 = 2.00) over Rh/γ-Al2O3 samples, O2- - RhIII and O2--Zr4+ (g⊥ = 2.03, g⊥ = 2.00) over Rh/ZrO2 samples and CO - RhII (ḡ = 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.