THE EFFECT OF ADDED SALTS ON CARBON-MONOXIDE ADSORPTION ON PLATINUM AND PALLADIUM HYDROSOLS - AN FTIR STUDY

Fourier transform infrared spectroscopy has been used to investigate the effect of added cations and anions on CO adsorption on platinum and palladium hydrosols. In general, anion effects on CO adsorption may be classified into three categories. In the first category, poisoning anions (e.g. CN-, SH-) block the surface against CO adsorption so that infrared spectra exhibit either no nu(CO)ads bands or nu(CO)ads bands at lower frequencies (< 2070 cm-1). In the second category, inert anions (e.g. the halides, citrate, EDTA2-) have no apparent effect on nu(CO)ads. The third category is characterised by anions such as PO4(3-), CO3(2-), or stearate ion which affect nu(CO)ads via the pH change that the dissolved anions cause in the metal hydrosol dispersion medium. Infrared spectra of platinum and palladium hydrosols in cyanide media were found to exhibit bands due to cyanate, tetracyanoplatinate(II), and tetracyanopalladate(II) arising from cyanide corrosion of the metal hydrosols. Ultraviolet/visible spectra of iodide ion in platinum hydrosols indicated that iodoplatinate(II) species had been produced. The band shape of nu(CO)ads at 2070 cm-1 in platinum hydrosols is found to be sensitive to the state of dispersion of the hydrosol particles. Bands due to CO adsorbed on hydrosol particles in either a completely or partially aggregated state are weaker and broader with a decreased value of nu(CO)ads relative to that of the unaggregated hydrosol.