Platinum stretching modes

Narayanan and El-Sayed examined the binding of reactants to the platinum nanoparticle surface with Raman spectroscopy [42], There are two possible adsorption configurations of the thiosulfate ions to the platinum nanoparticle surface (through the S" ion or the O" ion). A shift in the symmetric S-S stretching mode (bound form through S") as well as an unshifted S-S stretching mode (free form in solution) indicates the presence of both bound and unbound forms of thiosulfate. 

A study of methanol adsorption on platinum under UHV conditions or at a gas/solid interface is also of interest. Not many papers dealing with methanol adsorption in a UHV chamber [135,136] are available. The adsorption takes place without a reaction on Pt( 111) at lo w temperatures (100 K), and based on thermal desorption experiments it was concluded that a monolayer of methanol adsorbate desorbs at 180 K. The heat of adsorption of molecular methanol was estimated to be 46 kJ mol-1 on unreconstructed Pt(l 11) [137]. Infrared spectroscopy has been applied for the study of methanol adsorption on Pt(l 11) [138], and it was shown that a 0.36 monolayer of methanol corresponds to the saturation of the desorption peak found at 180 K. The methanol multilayer coverages were also found, but had different infrared frequencies that were associated with the methyl and C-0 stretching modes (Scheme 11.1). 

Pig. 19. Normalized absorbance vs. thickness plot for a layer of water in contact with a platinum reflector measured using an infrared beam at the wavelength of the maximum absorptin of the O-H stretching mode and incident at an angle of 45°, calculated using (a) Beer s Law and (b) the full Fresnel reflectance equations. 

The low-frequency CO stretching vibration of this band was attributed to interaction of the oxygen atom with another surface nickel atom, the close proximity of the oxygen atom and the third surface nickel atom being caused by the open structure of the films. Only in the case of platinum could the authors find and evidence of a band in the region that could possibly be attributed to a metal carbon stretching mode. 

The assignment of the feature at 2240 cm is difficult to make because the frequency is too high to be readily assi ed to any of the surface CO species, since the frequency of CO stretch mode in gas phase is only 2143 cm. However, such a high frequency CO stretch has been reported when the molecule is bound to positively charg platinum. Consequently, because of the frequency regime, and the presence and the formation of carbon dioxide species on the surface, this feature could also be assigned to a surface COj related species. 

A significantly improved time resolution on the picosecond scale has been reported [307, 308]. The method has been applied to study the potential jump at the electrochemical interface platinum/aqueous solution of 0.1 M HCIO4 saturated with carbon monoxide induced by pulse irradiation with visible light. Position and intensity of the infi ared absorption caused by the CO stretching mode were measured as a function of time after the 532 nm pump pulse. 

Fig. 5.61. Time-resolved SEIRAS. Pump pulse 532 nm, 35 ps duration, 3 mJ cm , temporal profile indicated by dotted line position and infrared absorption intensity of CO stretch mode plotted as a function of time, platinum electrode, aqueous solution of 0.1 M HCIO4 (based on data in [310])...

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