Bimetallic Fe-Ni/SiO2 catalysts for furfural hydrogenation: Identification of the interplay between Fe and Ni during deposition-precipitation and thermal treatments

Supported Fe-Ni catalysts have been reported for their activity and selectivity in the hydrogenation of unsaturated organic molecules. However, the control of the size and composition of the bimetallic nanoparticles remains a bottleneck when oxide-supported catalysts are prepared by impregnation, and alternative procedures should be investigated. Starting with Ni(II) and Fe(II) sulfates as precursor salts, deposition-precipitation with urea (DPU) on SiO2 in an inert atmosphere initially leads to the formation of an ill-crystallized Fe-containing Ni(II) 1:1 phyllosilicate, which reduces under hydrogen at 700°C into bimetallic fcc Fe-Ni nanoparticles of 5.4 nm in average. Compared with the composition of the DPU solution (50 Fe at %, 50 Ni at %), an excess of Ni is detected on the catalyst (32 Fe at %, 68 Ni at %), due to the preferential reaction of Ni2+ ions with silica. In situ X-ray absorption spectroscopy and 57Fe Mössbauer spectroscopy show that the reduction of Fe ions to the metallic state is triggered by the formation of reduced Ni centers above 350°C, and, from then, proceeds progressively, resulting in an excess of Fe in the outer shells of the bimetallic particles. The composition of individual Fe-Ni particles evidences a standard deviation of 8%. The bimetallic Fe-Ni/SiO2 catalyst gives high yields in furfuryl alcohol in the hydrogenation of furfural, in contrast with an analog Ni/SiO2 catalyst that favours side-reactions of etherification, hydrogenolysis and hydrogenation of the furan ring.