Oxidation of amorphous Ni–Zr alloys studied by XPS, UPS, ISS and XRD

Amorphous Ni36Zr64 and Ni24Zr76 alloys were treated under conditions close to those used for activating Ni–Zr alloys in catalytic reactions. The composition and the structure of the surface region were investigated by X-ray photoelectron spectroscopy XPS., ultraviolet photoelectron spectroscopy UPS., ion scattering spectroscopy ISS.and X-ray diffraction XRD.. The results show that exposing the amorphous alloys to 500 mbar O2 at 523 K led to oxidized surfaces consisting of zirconia and different oxidic nickel species. On the oxidized Ni24Zr76 surface, only small amounts of NiO and Ni2O3 were found, while Ni2O3 was detected as the major Ni-containing species on the oxidized Ni36Zr64 surface. Oxidation of Ni in the alloys appeared to be inhibited by the presence of metallic Zr and started only after complete oxidation of the surrounding Zr. Annealing the oxidized alloys to 773 K led to a reduction of the surface oxides. The liberated oxygen atoms presumably diffused into the bulk, releasing metallic Ni. At the topmost layer, zirconia was reduced by carbon atoms originating from the bulk to form a ZrC-like surface compound. The exchange of Ni and Zr atoms between the surface and the bulk was enhanced by the oxidation and the annealing. The preferential oxidation of zirconium, which attracts Zr atoms from the bulk to the surface, provides the driving force for such processes. q1998 Elsevier Science B.V. All rights reserved.