Characterization of Ni/TiO2 catalysts prepared by successive adsorption-reduction of Ni2+ ions

Three Ni/TiO2 samples have been synthesized by impregnation (sample Ni-i-Ti), adsorption of Ni2+ ions from alkaline media on TiO2 (sample Ni-1-Ti) and on reduced Ni-1-Ti (sample Ni-2-Ti). The samples have been characterized by TPR, IR spectroscopy of adsorbed CO and X-ray diffraction. Impregnation leads to a less homogeneous distribution of the active phase and a negligibly lower temperature of reduction as compared with the adsorption technique. Reduction of Ni-i-Ti results in the formation of nickel particles with an average diameter of 22 nm. Surface Ni-0 was detected by CO testing. The average diameter of the nickel particles in the reduced Ni-1-Ti sample is about 20 nm. In this case, however, no metal nickel was detected on the surface, i.e. the metal particles are encapsulated in the support. The majority of the active sites for adsorption are regenerated during the reduction of nickel in Ni-1-Ti, which allows Ni2+ ions to be adsorbed again on the sample. As a result, the nickel concentration increases. The mean diameter of the nickel particles on reduced Ni-2-Ti remains of the order of 20 nm, similar to the case of the reduced Ni-1-Ti sample. With Ni-2-Ti, however, surface metal nickel was detected by CO adsorption. It is concluded that successive absorption-reduction of Ni2+ ions might be used to prepare titania supported nickel catalyst with a desired nickel concentration and metal particles that are not encapsulated in the support. Some new aspects of the strong metal-support interaction (SMSI) mechanism are also discussed.