Preparation, reduction and CO chemisorption properties of cyanide-derived NixFe/Al2O3 catalysts Review Article

A series of nickel-iron catalysts with Ni-to-Fe ratios of 1.0, 1.5, and 2.0 has been prepared by precipitation of three different nickel-iron cyanide complexes, viz. NiFe(CN)5NO, Ni3[Fe(CN)6]2 and Ni2Fe(CN)6, onto a γ-Al2O3 support. Oxidative decomposition of these stoichiometric heteronuclear cyanide complexes followed by reduction in hydrogen resulted in the formation of single phase fcc alloys. The reduction of the oxidic precursors was studied by means of magnetization measurements and Mössbauer spectroscopy. With respect to a monometallic iron catalyst the reduction of the bimetallic catalysts proceeds at significantly lower temperatures. The surface of the catalysts in different stages of the reduction process has been studied by infrared spectroscopy of adsorbed carbon monoxide. Interaction of CO with the oxidic precursors at 300 K resulted in partial reduction of the particle surface and formation of hydrogen-carbonate species. The spectra recorded with the reduced catalysts demonstrate an interesting difference between the three alloys in the adsorption of hydrogen and carbon monoxide. With the NiFe and Ni3Fe2 catalyst the infrared spectrum is dominated by a band at 2020–45 cm−1 assigned to irreversibly linearly adsorbed CO. Bands in the 1960–1700 cm−1 range indicate the presence of some multicentered adsorbed CO species. With the Ni2Fe catalyst, multicentered CO is the dominant species, whereas the band due to linearly adsorbed CO is now recorded at 1980 cm−1. Furthermore, adsorbed formate species were detected. The formation of the formate species together with the low wave number of linearly adsorbed CO suggests that the reduced and evacuated alloy particle is still covered by hydrogen.