Application of In Situ Surface-Enhanced Raman Spectroscopy (SERS) to the Study of Citrate Oxidation on Silica-Supported Silver Nanoparticles

Surface-enhanced Raman spectroscopy (SERS) was used to characterize citrate anions adsorbed on nanometer-sized particles of Ag supported on SiO2. The magnitude of the surface-enhancement effect was determined to be ∼3 × 102 on the as-prepared samples of Ag/SiO2. Upon heating in air above 373 K, the citrate anions undergo oxidation to uni- and bidentate carbonate species and then decomposition to CO2 and adsorbed O atoms. In the SERS of Ag/SiO2, a very strong enhancement of the ν(C=O) signal for the bidentate CO3 species was observed for temperatures between 398 and 448 K, which is accompanied by an increase in the UV–vis absorbance of the sample at the frequency of the laser line used for Raman spectroscopy. This phenomenon is attributed to an increase in the surface-enhancement effect caused by clustering of the Ag nanoparticles as they sinter at elevated temperatures. The present investigation shows that the proper interpretation of in situ SERS spectra requires an understanding of the changes occurring in the UV–vis spectrum of the sample.