SURFACE-ENHANCED RAMAN-SPECTROSCOPY AS AN IN-SITU REAL-TIME PROBE OF CATALYTIC MECHANISMS AT HIGH GAS-PRESSURES - THE NO-H-2 REACTION ON RHODIUM

The reduction of nitric oxide by hydrogen on rhodium was studied by ut ilizing surface-enhanced Raman spectroscopy (SERS) to probe the nature of adsorbed species formed underreaction conditions. As in our earlie r studies, SERS-active transition-metal surfaces are prepared by elect rodepositing ultrathin Rh films onto electrochemically roughened gold. These interfaces display remarkably robust SERS activity, enabling te mporal sequences of surface Raman spectra to be obtained over a wide r ange of reactant pressures (here up to 1 atm) and at temperatures up t o at least 450 degrees C. As is also reported in our recent study of t he CO-NO reaction, heating Rh in pure NO yielded a surface dominated i ncreasingly by adsorbed atomic nitrogen up to 300 degrees C, as diagno sed by a 315-cm(-1) band due to surface-nitrogen stretching. The NO-H- 2 reaction was investigated at atmospheric pressure using various reac tant compositions. The desorption temperature of adsorbed atomic nitro gen was observed to sharply decline as the relative amount of H-2 was increased, indicating reactive removal by adsorbed hydrogen atoms. Und er conditions of either equimolar reactants or excess hydrogen, a feat ure at 450 cm(-1) was observed at temperatures above 100 degrees C. Su bstitution of H-2 by D-2 yielded no discernible downshift in the frequ ency of this vibration. This result, coupled with findings from a numb er of experiments involving transient spectral responses to changes in the gas-phase reactant composition, leads us to suggest that the 450- cm(-1) vibration arises from a Rh-NOH species formed via reaction betw een adsorbed NO and atomic hydrogen. The possible mechanistic importan ce of this species and how it may relate to reported gas-phase product s and catalyst selectivity are also discussed.