Metal Hexacyanometallate Nanoparticles: Spectroscopic Investigation on the Influence of Oxidation State of Metals on Catalytic Activity

Abstract

A series of metal hexacyanometallate nanoparticles KxMy[M′(CN)6]z·qH2O, with M=M′=Co (1) or Fe (2), M=Co and M′=Fe (3) and M=Fe and M′=Co (4) were prepared by either co-precipitation or reverse micro-emulsion. These nanoparticles were characterised by ATR FTIR, TEM and XPS, which revealed the presence of CoII/III and/or FeII/III within the compounds. The average binding energies found for the Co 2p3/2 photoelectron lines for CoII and CoIII were ca. 781.7 and 782.9 eV respectively, while the binding energies for FeII and FeIII of the Fe 2p3/2 photoelectron lines were located at 709.5 and 711.4 eV. TEM reveal the particle size ranged between 180 and 520 nm. The eight different compounds’ catalytic activity were tested for the solvent-free oxidation of benzyl alcohol using hydrogen peroxide as the oxidant. The progression of the reaction was monitored using ATR FTIR, by following the appearance of the carbonyl stretching frequency of the oxidation products, which showed the formation of not only benzyl aldehyde but also benzyl benzoate, which is the condensation product of benzyl alcohol and benzoic acid (another oxidation product). The TOF of the catalytic oxidation reaction was correlated with the mean particle diameter, % occurrence of CoII/III and/or FeII/III, total metal electronegativity and the degree of covalence of the Co- and Fe–ligand bond.