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.
- Amorphous precursor
- Electron probe micro analysis
- NO decomposition
- Nickel-tantalum-palladium alloy
- Oxidation treatment
- Scanning electron microscopy/energy dispersive X-ray spectrometer
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