SURFACE-ENHANCED RAMAN-SPECTROSCOPY AS AN IN-SITU REAL-TIME PROBE OF NO REDUCTION OVER RHODIUM AT HIGH GAS-PRESSURES

Surface-enhanced Raman spectroscopy (SERS) has been utilized as an in- situ probe of adsorbed species and surface reaction intermediates duri ng the reduction of nitric oxide by either carbon monoxide or hydrogen over polycrystalline Rh films. SERS-active Rh surfaces were prepared by electrodeposition of ultrathin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures in conjunction with SERS, enabled (up to 400 degrees C) and pressures (here up to 1 atm). Mass spectrometry, employed in conju nction with SERS, enabled simultaneous real-time measurement of reacti on kinetics for the CO-NO reaction. A charge-coupled device simultaneo us real-time measurement of reaction kinetics for the detector (CCD) a llowed Raman spectra to be recorded on a time-scale (less than or equa l to 10 s) commensurate with reactions occurring on the surface. Sever al central differences exist between these two reduction processes, mo st notably the mechanism for NO dissociation. While NO decomposition p roceeds through a direct pathway (NO(ads)+S double right arrow N-(ads) +O-(ads)) and is largely unaffected by the relative amount of gaseous CO, a hydrogen-assisted pathway appears to be prevalent during NO redu ction hydrogen-rich environments. evidence suggests that this process proceeds via a Rh-NOH intermediate (450 cm(-1)). Adsorbed atomic nitro gen (315 cm(-1)) reacted only to form N-2 during reduction with CO, ad judged by its removal temperature (325 degrees C) and the absence of N 2O formation. In contrast, hydrogen facilitated the reactive removal o f this moiety at lower temperatures, most likely via NH3 formation. Wh ile extensive surface oxidation was detected during reaction with vary ing NO/CO ratios, Rh2O3 formation was inhibited under hydrogen-rich mi xtures. These differences in surface speciation and their probable rol es in the determination cb product selectivity are discussed.