Adsorbed water molecules on a K-promoted catalyst surface studied by stimulated micro-Raman spectroscopy

Results are presented from a micro-Raman (confocal Raman) study of H2O adsorption on a K-promoted FeO (styrene production) catalyst. On such a material, Rydberg states of the K promoter atoms are formed as shown in previous studies. Due to the huge polarizability of the Rydberg species, stimulated Raman processes are observed, as expected. The stimulated Raman process is shown to give Raman gain at the positions of the plasma lines from the He-Ne laser. The main plasma line supported Stokes band from H2O(nu(2)) or a K-water complex at 1639 cm(-1) and a nearby non-Raman band at 1884 cm(-1) are shown to vary almost identically under changes of the vapor pressure of water and the sample temperature, confirming their origin from adsorbed H2O. The band intensities also depend strongly on the amount of K in the catalyst sample, which shows that the K atoms enhance the Raman scattering via Rydberg state formation. Several of the enhanced Stokes bands from the surface are interpreted as bending and stretching transitions in Rydberg complexes (K+-OH2)-e(-). The Gaussian 98 program package was used to help assign the observed Raman lines. The Raman gain method should be very useful for spectroscopy at alkali-promoted catalysts in general.