Reaction induced surface segregation in amorphous CuZr, NiZr and PdZr alloys - An XPS and SIMS depth profiling study

Amorphous alloys based on zirconium are used as precursors for the preparation of highly active CO2 and CO hydrogenation catalysts. The activation process of glassy CuZr, NiZr, and PdZr alloys is characterized by XPS and SIMS depth profiling. Upon exposure to CO2/H-2 or CO/H-2 reactant gas mixtures at 523 K, the formation of zirconium oxide occurs in spite of a large excess of hydrogen, indicating that the zirconium component is highly reactive towards the carbon oxides. In the Ni64Zr36, Cu30Zr70, and Pd25Zr75 systems, a distinct overlay of almost pure zirconium oxide develops, which contains traces of Cu and Pd respectively for the latter two alloys. The thickness of this layer increases with treatment time in the range 10 to 90 min, and varies between 5 and 30 nm (Cu30Zr70:up to 80 nm). With Ni91Zr9 the zirconium component is oxidized, but no compact surface zirconia layer is formed, while with Cu70Zr30 the formation of alternating Zr-rich and Cu-rich bilayers is observed. Upon prolonged exposure to reaction conditions, the depth over which segregation phenomena occur in this alloy increases to values in excess of 1 mu m. From an extensive set of sputter depth profiles, changes in the oxidation state of zirconium have been traced as a function of depth for Cu30Zr70, Ni64Zr36, and Pd25Zr75. At the interface to the unreacted alloy, the oxidation state of the zirconium is smaller than 4+, and the presence of zirconium suboxides is detected. With the zirconium-rich alloys Cu30Zr70 and Pd25Zr75, the relative contribution of the formal oxidation state Zr-(3+) is maximum close to the outermost ZrO2 layer, whereas Zr-(1+) is predominantly found close to the unreacted core of the alloy. In contrast, for Ni64Zr36, with a low content of zirconium, both Zr-(3+) and Zr-(1+) reach their maximum concentrations at the interface between the oxide and the bulk. These observations are system-specific and, therefore, not due to sputter-induced processes. After activation, the second metal is present in the metallic state for catalysts based on copper (Cu70Zr30 and Cu30Zr70) and on nickel (Ni64Zr36 and Ni91Zr9). In contrast, for the catalyst based on palladium (Pd25Zr75), there is strong evidence for the formation of PdO even in the subsurface region.