Low-temperature reduction of LaNiO, under hydrogen leads to new compounds such as La,Ni20, and LaNiO, before total reduction. Hydrogen consumption, X-ray diffraction and XANES demonstrate the presence of these new phases. Ni3+ in LaNi03 becomes Ni2+ in La2Ni205, and a pure monovalent nickel phase is obtained in LaNi02. After this stage nickel leaves the structure as nickel metal. Before total reduction, reversible reoxidation of La2Ni20, and LaNi02 leads to the original structure of the LaNiO, perovskite.Perovskite catalysts are widely used for the oxidation' of CO. LaNi03 in particular is a good candidate for use as a fuel-cell electrodem2 In this respect their stability during heat treatment in a reducing atmosphere is one of their major advantages.Recently, Nakamura et al.3 studied the stability of LaB03-type perovskites under reducing conditions (B = V, Cr, Mn, Fe, Co, Ni). From LaNi03, at 1273 K, they obtained a mixture of La2Ni04 and NiO for oxygen partial pressures in the range lop2 >p(02)/Torr > > p(02)/Torr > lo-"' and a total decomposition into La203 and NiO for p(02)/Torr < 10-10 Working at much lower temperature, between 470 and 700 K, and recirculating hydrogen we have found evidence for the existence of new phases. These have been characterized by three methods: (i) the stoichiometry is obtained from the consumption of hydrogen during the reaction; (ii) X-ray diffraction is used to insure that the new products are pure phases; (iii) the determination of the K adsorption edge permits an absolute determination of the nickel oxidation states in these new phases. a mixture of La2Ni04 and Nio in the range EXPERIMENTAL SYNTHESIS OF LaNiO, LaNiO, was synthesized from an equimolar solution of La(N0,),.6H20 and Ni(N0,),-6H20 in water. The hydroxides were coprecipitated by adding an excess of a strong base (tetraethylammonium hydroxide). This precipitate was washed and freeze-dried. It was then calcined under flowing oxygen at 1220 K for 15 h. The X-ray diffraction patterns of the resulting product showed exclusively LaNi0, peaks. No peaks corresponding to lanthanum or nickel oxide were detected.
