The microcomposite structure of catalysts prepared by oxidation of amorphous Ni-Ta-Pd alloys

Amorphous Ni-40Ta-Pd alloys are known as the precursor of the catalysts for the decomposition of nitrogen monoxide. Since the catalytic activity appears after oxidation, the relationship between the activity and structure was investigated after oxidation in air at 750°C for different periods of time. The structure developed by oxidation was characterized with special interest in the dispersion state of palladium phase in the matrix oxides by means of SEM/EDX, EPMA and TEM/EDX. At the early stage of the oxidation, NiO, Ta₂O₅ and PdO are first formed. The oxides consist of two layers; comprising an outer NiO layer and an inner layer containing Ni, Ta and Pd ions. The outer NiO is developed by the outward diffusion of nickel ion during oxidation. Prolonged oxidation leads to the development of porosity of the specimen and to the formation of double NiTa₂O₆ oxide by the reaction of NiO with Ta₂O₅ in the inner layer. The inner layer formed after the complete oxidation for the amorphous Ni-40Ta-Pd alloy has sponge-like morphology, and consists of PdO, NiO and NiTa₂O₆. The addition of palladium prevents the diffusion of nickel to the surface, leading to the decrease in the thickness of the surface NiO layer and to the decrease in the grain size of matrix oxides. No change in the supporting oxides was observed but PdO is decomposed to Pd after the decomposition of NO gas. Palladium particles of 50-100 nm in diameter were dispersed homogeneously in the matrix oxide. The high catalytic activity for the decomposition of NO gas is attributable to the palladium dispersed on the matrix oxides with fine grain size and porous structure developed by the oxidation of amorphous Ni-Ta-Pd alloys.