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-sit u mechanistic probe of heterogeneous catalytic systems at high gas pre ssures in conjunction with mass spectrometry (MS) is demonstrated for the GO-NO reaction on rhodium. As in our earlier studies, the SERS-act ive transition-metal surfaces are prepared by electrodepositing ultrat hin Rh films onto electrochemically roughened gold. These surfaces dis play remarkably robust SERS activity, enabling intense Raman spectra t o be obtained over a range of reactant pressures (here up to 1 atm) an d at temperatures up to at least 400 degrees C. The low-frequency (<10 00 cm(-1)) spectral region, where metal-adsorbate vibrations are locat ed, proved to be especially informative in the present case. The domin ant presence of adsorbed atomic nitrogen from dissociative NO adsorpti on at temperatures below 300 degrees C is diagnosed by a band at 315 c m(-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 fo rmation of specific gas-phase reaction products being monitored simult aneously by means of amass spectrometer. This simultaneous SERS-MS pro cedure enables the relationships between the formation of specific ads orbed species (as sensed by SERS) and reaction products (as detected b y 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 sp ectral 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 explore d at ambient GO-NO pressures. Atomic nitrogen is inferred to be preval ent at temperatures well below the onset of detectable reaction at 250 degrees C and up to 320 degrees C, yet is absent at higher temperatur es. The thermal removal of nitrogen is not accompanied by a marked rat e acceleration, although the temperature-dependent kinetics are notice ably altered at this point. The dissociative adsorption of NO (but not CO) is seen to be activated at ambient pressure. The value of this co mbined SERS/MS approach for elucidating catalytic mechanisms for the G O-NO (and related GO-O-2) reaction and also, on a broader front, is di scussed in the light of these findings.