NOx storage and reduction over flame-made M/MgAl2O4 (M = Pt, Pd, and Rh): A comparative study

Various NOx storage–reduction (NSR) catalysts with 1 wt% noble metal loading (Pt, Pd, or Rh) dispersed on MgAl2O4 have been synthesized by single step flame spray pyrolysis. The as-prepared powders consisting of nonporous nanoparticles were characterized by X-ray diffraction, scanning transmission electron microscopy, nitrogen adsorption–desorption, CO chemisorption and thermogravimetric methods. Different reduction agents were applied in the fuel rich cycles: H2, CO or C3H6. Processes occurring during fuel rich and lean cycles were analyzed in detail by means of time-resolved diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), and thermogravimetry combined with mass spectroscopy. With H2 as reductant, Pt/MgAl2O4 showed the best performance at short regeneration time (<50 s), followed by Rh/MgAl2O4 and Pd/MgAl2O4. However, if CO or C3H6 were used as reductant, Rh/MgAl2O4 outperformed both other catalysts, especially at longer regeneration times (>50 s). NOx storage on these catalysts was mainly restricted to surface species, although bulk nitrate species were identified by DRIFTS as well. The latter were not completely reduced under the experimental conditions used.

M/MgAl2O4 (1 wt% M: Pt, Pd, or Rh) catalysts were prepared by single step flame spray pyrolysis. The thermal stability of the stored NOx and the storage–reduction performance during lean-rich cycling with different reductants, H2, CO and C3H6, was investigated by means of TG and time-resolved in situ DRIFTS.Figure optionsDownload as PowerPoint slideHighlights
► M/MgAl2O4 catalysts (M = Pt, Pd or Rh) were prepared by flame spray pyrolysis.
► NSR performance of the catalysts decreased according to Rh > Pt > Pd.
► NO oxidation activity decreased according to Pt > Rh > Pd.
► Stability of NOx species adsorbed decreased in the order Pd > Pt > Rh.
► NOx reduction activity decreased according to Rh > Pd > Pt.