Surface Raman spectroscopic study of NO transformation over Pd-based catalysts

Correlations between in situ Raman spectroscopic and catalytic measurements on palladium based three-way catalysts during the reduction of NO by CO have been tentatively established. Particular attention to the selectivity towards the transformation of NO into N2O has been paid in order to explain why N2O is the main N-containing product during the engine cold start. A comparative study of the selectivity from temperature-programmed and steady-state experiments on bulk and supported palladium catalysts shows that bulk Pd is significantly more selective towards the production of N2 than Pd/Al2O3 at low conversion and low temperature. In addition, the subsequent reduction of N2O by CO occurs more readily on bulk palladium. In parallel to this catalytic information, Raman spectra recorded in comparable experimental conditions reveal different spectral features relative to the nature of chemisorbed species, and to the development of surface PdO islands mainly on Pd/Al2O3. Both differences have been compared and discussed in the light of a previous mechanism proposed earlier in the literature.