Surface-enhanced Raman spectroscopy as an in-situ real-time probe of no reduction over rhodium at high gas pressures
✍ Scribed by Christopher T. Williams; Anish A. Tolia; Michael J. Weaver; Christos G. Takoudis
- Publisher
- Elsevier Science
- Year
- 1996
- Tongue
- English
- Weight
- 845 KB
- Volume
- 51
- Category
- Article
- ISSN
- 0009-2509
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✦ Synopsis
Surface-enhanced Raman spectroscopy (SERS) has been utilized as an in-situ probe of adsorbed species and surface reaction intermediates during the reduction of nitric oxide by either carbon monoxide or hydrogen over polycrystalline Rh films. SERS-active Rh surfaces were prepared by electrodepositiou of ultrathin films on electrochemically roughened gold and display remarkably robust SERS activity over a wide range of temperatures (up to 400°C) and pressures (here up to 1 arm). Mass spectrometry, employed in conjunction with SERS, enabled simultaneous real-time measurement of reaction kinetics for the CO-NO reaction. A charge-coupled device detector (CCD) allowed Raman spectra to be recorded on a time-scale (< 10 s) commensurate with reactions occurring on the surface. Several central differences exist between these two reduction processes, most notably the mechanism for NO dissociation. While NO decomposition proceeds through a direct pathway (NO(ad~)+S =~ N(~h)+O(~!) and is largely unaffected by the relative amount of gaseous CO, a hydrogen-assisted pathway appears to ve prevatent during NO reduction in hydrogen-rich environments. The evidence suggests that thisprocess proceeds via a Rh-NOH intermediate (450 cm-1). Adsorbed atomic nitrogen (315 era-l) reacted only to form N2 during reduction with CO, adjudged by its removal temperature (325°C) and the absence of N20 formation. In contrast, hydrogen facilitated the reactive removal of this moiety at lower temperatures, most likely, via NH3 formation. While extensive surface oxidation was detected during reaction with varying NO/CO ratros, Rh203 formation was inhibited under hydrogen-rich mixtures. These differences in surface speciation and their probable roles in the determination of product selectivity are discussed.