PLATINUM AND PALLADIUM HYDROSOLS - CHARACTERIZATION BY X-RAY PHOTOELECTRON-SPECTROSCOPY AND TRANSMISSION ELECTRON-MICROSCOPY

Particle size distributions and surface speciation in borohydride-, citrate-, CO gas-, and electric arc-generated (Bredig) platinum and palladium hydrosols have been investigated using transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS). Average particle sizes of all platinum and palladium hydrosols studied were in the range 2-12 nm. X-ray photoelectron spectra of the platinum hydrosols showed evidence for partially oxidised platinum species and platinum(II) oxides (PtO and Pt(OH)2). Platinum(IV) oxides (PtO2) were detected only on the Bredig platinum sol particles and macroscopic platinum particles generated by PtO2.xH2O reduction by CO gas. In contrast, X-ray photoelectron spectra of the palladium hydrosols revealed that palladium(II) and palladium(IV) oxides were present on the surfaces of the particles with the possible exception of the hydrosol generated by reduction of PdCl2 by CO gas which may have both PdO2 and/or [PdCl4]2- species on the surface. The surface platinum and palladium oxides in the hydrosol systems studied were confirmed by changes in the Pt 4f and Pd 3d XPS profiles after argon-ion sputtering and H-2 reduction which were indicative of oxide removal and/or reduction.