Abstract
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 electrodeposition 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 atm). Mass spectrometry, employed in conjuction 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 (ads)+S ⇒ 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 reduction in hydrogen-rich environments. The evidence suggests that this process proceeds via a Rh-NOH intermediate (450 cm −1). Adsorbed atomic nitrogen (315 cm −1) reacted only to form N 2 during reduction with CO, adjudged by its removal temperature (325°C) and the absence of N 2O formation. In contrast, hydrogen facilitated the reactive removal of this moiety at lower temperatures, most likely via NH 3 formation. While extensive surface oxidation was detected during reaction with varying NO/CO ratios, Rh 2O 3 formation was inhibited under hydrogen-rich mixtures. These differences in surface speciation and their probable roles in the determination of product selectivity are discussed.
Published Version
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