SURFACE-ENHANCED RAMAN-SPECTROSCOPY AS AN IN-SITU REAL-TIME PROBE OF CATALYTIC MECHANISMS AT HIGH GAS-PRESSURES - THE CO-NO REACTION ON RHODIUM

The utility of surface-enhanced Raman spectroscopy (SERS) as an in-situ mechanistic probe of heterogeneous catalytic systems at high gas pressures in conjunction with mass spectrometry (MS) is demonstrated for the GO-NO reaction on rhodium. As in our earlier studies, the SERS-active transition-metal surfaces are prepared by electrodepositing ultrathin Rh films onto electrochemically roughened gold. These surfaces display remarkably robust SERS activity, enabling intense Raman spectra to be obtained over a range of reactant pressures (here up to 1 atm) and at temperatures up to at least 400 degrees C. The low-frequency (<1000 cm(-1)) spectral region, where metal-adsorbate vibrations are located, proved to be especially informative in the present case. The dominant presence of adsorbed atomic nitrogen from dissociative NO adsorption at temperatures below 300 degrees C is diagnosed by a band at 315 cm(-1). Carbon monoxide adsorption yields a sharp metal-carbon stretch at 465 cm(-1), whereas surface oxidation produces features at 530 and 800 cm(-1). A small volume flow reactor was utilized, the real-time formation of specific gas-phase reaction products being monitored simultaneously by means of amass spectrometer. This simultaneous SERS-MS procedure enables the relationships between the formation of specific adsorbed species (as sensed by SERS) and reaction products (as detected by MS) to be explored on a common(<10 s) time scale. The exclusive gas-phase products were found to be CO2 and N-2. Such real-time SERS/MS spectral sequences were obtained for the GO-NO reaction on rhodium both during temperature ramps and following abrupt changes in the gas-flow composition. The former condition enabled the relationship between the presence of adsorbed atomic nitrogen and CO2 production to be explored at ambient GO-NO pressures. Atomic nitrogen is inferred to be prevalent at temperatures well below the onset of detectable reaction at 250 degrees C and up to 320 degrees C, yet is absent at higher temperatures. The thermal removal of nitrogen is not accompanied by a marked rate acceleration, although the temperature-dependent kinetics are noticeably altered at this point. The dissociative adsorption of NO (but not CO) is seen to be activated at ambient pressure. The value of this combined SERS/MS approach for elucidating catalytic mechanisms for the GO-NO (and related GO-O-2) reaction and also, on a broader front, is discussed in the light of these findings.