Raman spectroscopic characterization of transition metal surfaces in heterogeneous catalytic reactors

Anish Arun Tolia, Purdue University


Surface Enhanced Raman Spectroscopy (SERS) combined with simultaneous mass spectrometry has been developed as a real-time in-situ probe for heterogeneous catalytic reactions at high gas pressures. Chemical reactions important for pollution control, particularly those occurring in automobile catalytic converters, have been examined using this technique. One part of this work deals with the surface oxidation of rhodium as probed by SERS and X-ray photoelectron spectroscopy (XPS). Two types of surface oxides were identified through Raman bands at 290 and 530 $\rm cm\sp{-1}$ and corresponding XPS peaks at 532 and 530 eV respectively. These were assigned to $\rm RhO\sb2$ and/or RhOOH and $\rm Rh\sb2O\sb3$ respectively. The reactivity of these oxygen species towards CO oxidation was determined. Based on the data, a quantitative model for spatial distribution of the oxides was proposed. The second part of the thesis relates to the application of the combined SERS-MS technique to probe the CO-NO reaction on Rh and Pt thin films at atmospheric pressure and temperatures from 25 to $350\sp\circ\rm C.$ Adsorption of NO on Rh was studied and estimates for rate and extent of NO dissociation were obtained from the temperature and time-dependent behavior of the 315 $\rm cm\sp{-1}$ peak from adsorbed atomic nitrogen. Through investigations of competitive and co adsorption of the reactants, NO was found to be preferentially adsorbed over CO under all conditions examined. Based on correlations between surface species and gas phase products, the role of atomic nitrogen in the reaction pathway was elucidated. $\rm N\sb2$ and $\rm CO\sb2$ were the only observed reaction products. Platinum was found to be much less efficient for NO dissociation compared to rhodium, with most adsorption occurring molecularly. Unlike rhodium, CO was found to adsorb competitively with NO under reaction conditions. For an equimolar ratio of CO and NO, both $\rm CO\sb2$ and $\rm N\sb2O$ were observed as reaction products at higher temperatures. Once again, correlations between surface and gas-phase species were used to elucidate the role of adsorbed molecules in the reaction. In the final part of this work, a related system, the NO-$\rm H\sb2$ reaction on rhodium, has been studied using SERS. This study provides an excellent example of how the real-time capabilities of this technique can be used to probe the dynamics of adsorbed species.




Weaver, Purdue University.

Subject Area

Chemical engineering

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