A Highly Active “NiO-on-Au” Surface Architecture for CO Oxidation

Supported Au–Ni nanocatalysts consisting of Au nanoparticles decorated with Ni/NiO nanostructures were synthesized using a two-step method and characterized by X-ray diffraction, transmission electron microscopy, X-ray photoelectron spectroscopy, and X-ray absorption spectroscopy. The structural characterization indicates that a small part of surface Ni atoms can diffuse into Au cores upon reduction at 600 °C, while the alloyed Ni atoms segregate onto the Au nanoparticle surfaces when oxidizing at a similar temperature. The inward and outward diffusion of Ni atoms at Au surfaces are reversible with response to cycled reduction and oxidation treatments. CO oxidation reactions over the Au–Ni catalysts suggest that Au nanoparticles decorated with highly dispersed NiO nanopatches are the active surface architecture. Optimum activity has been observed by maximizing the density of boundaries between NiO patches and Au surfaces, which can be achieved through suitable redox pretreatments of the Au–Ni catalysts. NiO surface structures in the “NiO-on-Au” inverse catalysts not only enhance Au-catalyzed CO oxidation performance but also prevent sintering of Au nanoparticles at elevated temperatures. Similar high activity for the CO oxidation has been observed in the “CoO x -on-Au” and “FeO x -on-Au” inverse catalysts.