Special IR properties of palladium nanoparticles and their aggregations in CO molecular probe infrared spectroscopy

Dispersed Pd nanoparticles (Pd_n ) have been synthesized by reducing H₂PdC_l4 with ethanol, and stabilized using poly(vinylpyrrolidone) (PVP). The Pd_n is applied to the glassy carbon substrate to form a thin film, and then the potential cyclic scanning at 50 mV·s^-1 from -0.25 to 1.25 V was carried out for about 30 min to form the aggregations of Pd_n (Pd_n^ag). FTIR spectroscopy of both transmission and reflection modes was employed to study CO adsorption on Pd_n and Pd_n^ag in both solid|liquid and solid|gas interfaces. It has been revealed that CO adsorption on Pd_n film yields two IR bands near 1964 and 1906 cm^-1, which are assigned to IR absorption of CO bonded on asymmetric and symmetric bridge sites, respectively. In contrast to the IR properties of CO adsorbed on Pd_n, only species of CO bonded on asym- metric bridge sites was determined on Pd_n^ag, and the direc- tion of the IR band near 1963 cm^-1 is completely inverted. The full width at half-maximum (FWHM) of the CO_B^as band near 1964 cm^-1 is measured to be 14 cm^-1 on Pd_n film, while it is 24 cm^-1 on Pdnag film. The results of the present study demonstrated that the inverting of the IR band direction is a general phenomenon that is closely related to the interaction between nanoparticles in aggregation of Pd_n.