31 October 2001 Recent advances in Raman scattering studies of exhaust-gas catalysts
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Raman scattering is a powerful technique for studying catalysts used in the treatment of automotive exhaust gas. It has the sensitivity and chemical specificity needed to identify the oxide phases of many of the precious metals (Pt, Pd, Rh) used in these catalysts, even when they are highly dispersed on high-surface-area supports such as (gamma) -Al2O3. Moreover, this technique can be employed in situ under temperature (300 - 600 degree(s)C) and pressure (1 atm) conditions typically encountered in normal operation. Bulk Pd oxide (PdO) is readily detected with visible excitation, because of a strong resonance Raman enhancement, and its formation and decomposition on Pd/(gamma) -Al2O3 and Pd/ZrO2 catalysts can be followed in real time. Pt does not oxidize as easily as Pd, but a layer of atomic O will form on Pt, and it produces a Raman signature that can be detected with UV excitation at 244 nm. Similarly, UV excitation enhances spectra from adsorbed NOx and SOx and hydroxyls on model Pt/(gamma) -Al2O3 catalysts. In situ UV Raman spectra of Ba-containing catalysts, being considered for use as NOx traps, show adsorbed NOx and SOx and, thus, can be used to characterize the NOx trapping, S poisoning, and regeneration of the trap. UV excitation has several advantages in addition to the eletronic resonance enhancment: the signal is increased because of the dependence of the Raman cross section on the fourth power of the frequency, the fluorescence is often Stokes shifted well beyond the range of the Raman lines, and the thermal background from a heated sample is negligible.
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Willes H. Weber, Willes H. Weber, Dairene Uy, Dairene Uy, } "Recent advances in Raman scattering studies of exhaust-gas catalysts", Proc. SPIE 4469, Raman Spectroscopy and Light Scattering Technologies in Materials Science, (31 October 2001); doi: 10.1117/12.447381; https://doi.org/10.1117/12.447381

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