Unusual Physical and Chemical Properties of Ni in Ce1-xNixO2-y Oxides: Structural Characterization and Catalytic Activity for the Water Gas Shift Reaction

The structural and electronic properties of Ce1-xNixO2-y, nanosystems prepared by a reverse microemulsion method were characterized with synchrotron-based X-ray diffraction, X-ray absorption spectroscopy, Raman spectroscopy, and density functional calculations. The Ce1-xNixO2-y systems adopt a lattice with a fluorite-type structure with an acute local order where Ni displays a strongly distorted (oxygen) nearest-neighbor coordination and the presence of Ni atoms as first cation distances, pointing to the existence of Ni-O-Ni entities embedded into the ceria lattice. A Ni <-> Ce exchange within the CeO2 leads to a charge redistribution and the appearance of O vacancies. The Ni-O bonds in Ce1-xNixO2-y are more difficult to reduce than the bonds in pure NiO. The specific structural configuration of Ni inside the mixed-metal oxide leads to a unique catalyst with a high activity for the water gas shift (CO + H2O -> H-2 + CO2) reaction and a simultaneous reduction of the methanation activity of nickel. Characterization results indicate that small particles of metallic Ni at the interface position of a ceria network may be the key for high WGS activity and that the formate-carbonate route is operative for the production of hydrogen.