EPR STUDIES OF PHOTOREDUCTION OF NI TIO2 CATALYSTS/

Irradiations of Ni/TiO2 catalyst by UV in hydrogen at 77 K produced no t only Ni+ ions on the catalyst surface, but also Ni3+ and Ti3+ specie s in bulk or near the interface between nickel and titania. These phot ogenerated species were detected and characterized by low temperature electron paramagnetic resonance (EPR) spectroscopy Relative spin conce ntrations of the photogenerated paramagnetic species (Nin+ and Ti3+) v aried with the nickel content in titania. A high nickel content in the sample resulted in a high peak intensity ratio of Nin+ to Ti3+. It wa s found that the photoinduced self-redox reaction of Ni2+ ions to form Ni+ and Ni3+ ions has a priority over the photoreduction of Ti4+ to T i3+ ions. The characteristic EPR spectrum of the Ni3+ (3d(7)) ions wit h g(1) = 2.268, g(2) = 2.237, and g(3) = 2.045 indicates that the Ni3 ions ate most likely located in the substitutional sites of TiO2, pos sibly near the surface rutile phase. The Ni+ species (3d(9)) with g(pa rallel to) = 2.130 and g(perpendicular to) = 2.063 are on the surface of TiO2. Both Ni+ and Ni3+ ions are quite stable in hydrogen. The Ni3 ions seem to be responsible for anchoring the nickel ions onto titani a and stablizing the Ni+ species on the surface. The Ni+ ions are thus free from oxygen poisoning and still show a high activity toward olef in oligomerization.