IR study on the photochemistry and adsorption states of iron carbonyls adsorbed on alumina

The behavior of iron carbonyls adsorbed on alumina has been studied in the dark and under UV illumination by FTIR spectroscopy. Photolysis products of adsorbed Fe(CO)5 are affected by hydration of the surface as well as by their stability on alumina. On hydrated surfaces the photolysis leads to the formation of [HFe3(CO)11]-, [HFe(CO)4]-, and Fe(CO)4, which can be thermally produced at a lower rate by base catalysis. The photo- and thermochemical formation of [HFe(CO)4]- requires stronger basic sites than that required for the formation of [HFe3(CO)11]-. The photolysis of Fe(CO)5 on dehydrated alumina results in predominant formation of Fe2(CO)9 with concomitant formation of a small amount of Fe3(CO)12, though the latter is exclusively formed under similar conditions on silica. This is because Fe3(CO)12 undergoes a disproportionation reaction on dehydrated alumina. These photoproducts appear to be formed not at the outermost surface of alumina but inside the sample disk. The results are discussed with reference to those obtained for silica in terms of catalytic properties of oxide surfaces.