In situ PM-IRRAS study of powder catalyst: Dynamic evolutions of species on catalyst and in gas phase during NOx storage-reduction

The potential of polarization-modulation infrared reflection-absorption spectroscopy (PM-IRRAS) for in situ and time-resolved studies using powder samples is demonstrated using as an example the NOx storage-reduction behaviour of a Pt-Ba/Al2O3 powder catalyst. The unique feature of PM-IRRAS, the simultaneous but separate detection of gas phase species and species on the catalyst surface, is illustrated. Bands due to the powder prominently appeared in the gas phase spectra due to transmittance and absorption of IR-light by the solid. The difference in the appearance of powder bands in the gas phase and surface spectra was explained by the distinct local sensitivity in the gas phase and surface spectra for powder samples and also by optoelectronic effects caused by the photoelastic modulator. The study shows that PM-IRRAS is not only suitable for in situ mechanistic studies on idealized model film catalysts, but also for the investigation of technically more relevant powder catalysts.