XPS ANALYSIS OF AMORPHOUS NI-NB-SN-PT ALLOY CATALYSTS FOR ELECTROOXIDATION OF FORMALDEHYDE

When tin was substituted for a part of platinum, the catalytic activity of electro-oxidation of formaldehyde for HF-treated amorphous Ni-Nb-xSn-(3-x)Pt alloys increased and the maximum activity was observed at about 0.4 at.%Sn. An attempt was made to correlate such a composition dependence of the catalytic activity to the nature of the surface of the alloy catalysts by using X-ray photoelectron spectroscopy. XPS analysis revealed that the surface layer was rich in platinum and deficient in nickel, but the surface concentration of these elements decreased continuously with increasing tin content. The binding energies of platinum 4f and nickel 2p3/2 electrons of the metallic state exhibited almost no change with alloy tin content. Carbon 1s spectrum showed the presence of a small amount of chemisorbed intermediates of HCHO oxidation such as COH and COOH, but their relative intensities were almost independent of tin content.