Raman spectroscopy of palladium oxide on γ-alumina applicable to automotive catalysts

Raman scattering is used to analyze a series of model catalysts consisting of γ-alumina powder with up to 20 wt.-% Pd, and two monolithic catalyst samples with palladium concentrations typically employed in automotive emission control. By using the 514.5 nm Ar laser line to take advantage of a resonant Raman effect, the 651-cm−1 Raman line in PdO can be detected in the oxidized catalysts with palladium loadings as small as 0.2 wt.-%. A strong fluorescence signal is observed in reduced pure γ-Al2O3. This signal is quenched by the addition of palladium, admixed either from aqueous solution or from Pd black, provided the palladium was allowed to interact with the γ-Al2O3 during at least one oxidation treatment at 600°C, followed by reduction in hydrogen at 300°C. Raman spectroscopy is found to be applicable for a quantitative, nondestructive analysis of PdO on γ-alumina, including automotive formulations, within certain limitations. At low concentrations, some palladium may interact with the support and thus may not contribute to the signal associated with crystalline PdO. The analysis is also suited for studying sintering and reduction/oxidation kinetics in situ.