In situ TEM investigation of processes catalyzed by Pd-Ni alloy particles on carbon substrates in the presence and absence of oxygen
- Universitaet Hamburg (Germany)
Processes catalyzed by small Pd-Ni alloy particles (10-20 nm in diameter) vapour-deposited on thin carbon films were investigated by in situ experiments in a transmission electron microscope. During exposure to elevated partial pressures of oxygen, and in the regime of high concentration of Pd, the carbon support was oxidized at the edges of the particles at temperatures from 770 to 870 K, as was apparent by holes developing in the substrate. The activity was highest for pure Pd, and decreased strongly with increasing content of Ni. At concentration values of Ni from 5 to 10 at.%, segregation of Ni and oxidation to NiO was found, while the remaining Pd particle continued to beactivee in oxidizing the carbon. For further increasing concentrations of Ni, the tendency of NiO segregation decreased, and at about 25%, the particles appeared to be rather stable and no carbon oxidation could be detected any longer. Instead, a weak activity of graphitization of the carbon support was observed, which increased strongly with further increasing concentration of Ni. This was most pronounced for pure Ni, and was apparent in the transmission electron microscopy image by the formation of graphite shells around the particles, and of graphitized areas in the surrounding substrate area by dispersing Ni. At 870 K, this behaviour was found to be similar to the well known case without the presence of oxygen. The dependence on composition of the oxidation or graphitization of the carbon substrate is interpreted as being caused by competitive processes: dissociative adsorption of oxygen on alloys rich in Pd, segregation and oxidation of Ni at the surface of alloys with low concentrations of Ni, and dissolution and diffusion of carbon from the substrate in alloys with medium to high concentrations of Ni, leading to the formation of segregated graphite on the surface of the particles, as well as to graphitization of the substrate. 26 refs., 12 figs.
- OSTI ID:
- 98938
- Journal Information:
- Journal of Catalysis, Vol. 149, Issue 2; Other Information: PBD: Oct 1994
- Country of Publication:
- United States
- Language:
- English
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