Hydrocarbons at Ag Films and Au Electrodes

Since hydrocarbons are important reactants in many heterogeneous catalytic processes on metallic catalysts at both the metal/gas and the metal/electrolyte interfaces, the SERS of alkenes, alkynes, and aromatic hydrocarbons have been investigated at these interfaces. For the metal/gas interface, strong SERS for hydrocarbons has been observed on low-temperature Ag films under UHV conditions, while for the metal/electrolyte interface, pretreated Au electrodes give strong SERS for hydrocarbons adsorbed from aqueous solutions. The SERS of ethylene, propylene, butenes, acetylene, benzene, and benzene derivatives adsorbed on Ag films in the UHV has been discussed by Moskovits and Dilella and Pockrand.  

Although the SERS of benzene has been reported at an Ag electrode, it has been difficult to duplicate, and absorbed hydrocarbons at Ag electrodes appear to give very weak SERS. Nonpolar unsaturated hydrocarbon molecules can chemisorb to metal surfaces via 7r-bonding interactions, and these interactions seem to be stronger on Au than on Ag. In addition, the SERS-active sites seem to be more stable at lower adsorbate coverage on Au than on Ag. Alkenes and alkynes adsorb on pretreated Au electrodes from saturated aqueous solutions primarily by donation of electron density from TT-orbitals to vacant Au orbitals. This process leads to lower-frequency bands for the C=C stretching modes. 

SERS under Nonstandard Conditions and in Nonaqueous Media 

The usual conditions for the observation of SERS spectra in electrochemical cells have been for molecules adsorbed from aqueous solutions at close to standard temperature and pressure. Of course, there are numerous observations of SERS at the solid/UHV interface, where the pressure is quite low ( 2 X 10 torr) and temperatures are maintained around 120 K. Some investigations have also been made of the applicability of using SERS to study interfaces at high pressures.  

SERS studies at 12 K for Li surfaces and at liquid He temperatures using matrix-isolated Ag, Na, and K metal clusters have been reported. Molecules such as CO and metal spheres formed by gas aggregation can be trapped in a solid matrix at 4 K. The SERS bands of CO chemisorbed on Ag particles were observed in this way. Ethylene and acetylene were co-condensed in argon and colloidal silver, and SERS spectra were obtained of the organic molecules adsorbed on the Ag particles.  

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