Surface-enhanced Raman spectroscopy as an in situ real-time probe of heterogeneous catalytic reactions
Abstract
Surface-enhanced Raman spectroscopy (SERS) combined with parallel mass spectrometry has been employed as an in-situ real-time probe of surface speciation during several gas-phase heterogeneous catalytic reactions on transition-metal surfaces. The catalysts are ultrathin metallic films electrodeposited onto SERS-active gold, enabling surface vibrational spectroscopic information to be obtained with high temporal resolution ($\approx$1 s) at elevated temperatures (up to 500$\sp\circ$C) and under ambient-pressure flow-reactor conditions. Several reactions have been examined, including NO reduction processes on Rh, Pt and Pd, CO hydrogenation on Rh, methanol oxidation on Rh, and the reduction of surface Rh$\sb2$O$\sb3$ by H$\sb2$, CO, and CH$\sb3$OH. In the case of the NO reduction (by CO or H$\sb2$), a reaction of much importance for automobile emissions control, NO dissociation was directly monitored for the first time at high pressures. It was found that the extent of NO dissociation (Rh $>$ Pd $>$ Pt), as adjudged by the presence of surface atomic nitrogen, has a marked effect on N$\sb2$/N$\sb2$O selectivity. In the case of CO hydrogenation, evidence for CO dissociation as well as a proposed mechanism for the conversion of this species between adsorbed and gaseous states was obtained. For methanol oxidation on Rh, the reactivity of surface oxide (Rh$\sb2$O$\sb3$) was found to be much less than that observed for CO$\sb2$ and H$\sb2$O formation, suggesting that this species is not an intermediate. Furthermore, the amount of adsorbed CO formed under reaction conditions roughly correlates with observed CO/CO$\sb2$ selectivity. Finally, surface rhodium oxide reduction by H$\sb2$, CO, and CH$\sb3$OH is compared and contrasted, revealing several distinct mechanistic differences. These results indicate that CO is the likely methanol fragment which "scavenges" oxide from the surface under reaction conditions. New insight was afforded in each case by the ability of SERS to monitor surface-adsorbate vibrations and acquire spectra with high temporal resolution ($\approx$1 s) under "real world" conditions. Future directions for applying SERS to catalysis research are considered in light of these findings. In particular, preliminary results regarding the examination of supported silver catalysts and bimetallic transition-metal surfaces are discussed.
Degree
Ph.D.
Advisors
Weaver, Purdue University.
Subject Area
Chemical engineering|Chemistry|Analytical chemistry
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