Probing the core and surface composition of nanoalloy to rationalize its selectivity: Study of Ni-Fe/SiO2 catalysts for liquid-phase hydrogenation

Rationalization of the catalytic performance of bimetallic Ni-Fe catalysts in selective hydrogenation reactions is based on the Ni and Fe distribution within the nanoparticles and at their surface. By applying a combination of element-specific and surface-specific characterization techniques (57Fe Mo<spacing diaeresis> ssbauer spectroscopy, X-ray absorption spectroscopy, and low-energy ion scattering) to a series of Ni-Fe/SiO2 catalysts differing by their Ni and Fe molar proportions, we showed that reduced Ni- Fe nanoparticles exhibit a gradient of Ni concentrations from a Ni- enriched core to Nidepleted, Fe- enriched outer shells. A surface proportion of 35-45 Ni atom % showed the highest yield of furfuryl alcohol in liquidphase hydroconversion of furfural. These results point to the need for Ni surface domains of limited size among Fe atoms to restrict the hydroconversion process to its first stage rather than to nominal compositions of the catalyst or to surface sites that would appear to be particularly selective per se.