SURFACE-ENHANCED RAMAN-SPECTROSCOPY AS AN IN-SITU REAL-TIME PROBE OF CATALYTIC MECHANISMS AT HIGH GAS-PRESSURES - THE CO-NO REACTION ON PLATINUM AND PALLADIUM

Surface-enhanced Raman spectroscopy (SERS), combined with simultaneous mass spectrometric measurements, has been utilized to probe the react ive nature of surface species present during the reduction of NO by CO on Pt and Pd. As in our earlier studies, the SERS-active transition-m etal surfaces are prepared by electro-depositing ultrathin films onto electrochemically roughened gold. These surfaces display remarkably ro bust SERS activity, enabling intense Raman spectra to be obtained over a range of reactant pressures (here up to 1 atm) and at temperatures up to at least 400 degrees C. During nitric oxide adsorption at 1 atm on Pt, both terminal (240 and 470 cm(-1)) and bridged (325 cm(-1)) sta tes of molecular NO were detected at lower temperatures (25-200 degree s C), with some dissociation occurring at higher (ca 250 degrees C) te mperatures as evidenced by the presence of atomic nitrogen (295 cm(-1) ). Similarly, a bridged NO species (310 cm(-1)) was observed on Pd und er similar conditions, with dissociation detected in the form of atomi c nitrogen (285 cm(-1)) and surface oxide (450 and 665 cm(-1)). Go-dos ing of reactants on platinum produced a surface dominated by NO and CO (470 and 2080 cm(-1)), whereas the former was adsorbed preferentially on Pd. Simultaneous SERS/MS measurements were performed during reacti on of an equimolar reactant mixture at 1 atm of total pressure over bo th metals. Both CO2 and N2O were formed during reaction on Pt, with on set of detectable product formation correlating with depletion of adso rbed CO and NO, respectively. In contrast, CO2 was the only product de tected over Pd, with the depletion of surface oxygen suggesting that N O dissociation may be rate limiting at higher temperatures (ca 300 deg rees C). The extent of dissociation on these surfaces is compared and contrasted, with particular emphasis placed on its role in determining reaction selectivity. Furthermore, the overall behavior of these cata lysts is compared with our former observations regarding this reductio n process on rhodium. (C) 1996 Academic Press, Inc.