NOx storage-reduction behavior of Pt–Ba/MO2 (MO2 = SiO2, CeO2, ZrO2) catalysts

A series of Pt–Ba/MO2 (MO2 = CeO2, SiO2, ZrO2) catalysts, containing 1 wt.% Pt and different Ba-loadings (4.5–28 wt.%), was investigated concerning NOx storage and reduction of the stored NOx species by propene using pulse thermal analysis combined with mass spectrometry and temperature-programmed reaction-desorption (TPRD). Well-characterized standard Pt/Al2O3 and Pt–Ba/Al2O3 catalysts were used as references. Exposure of the catalysts to NO/O2 pulses showed that the NOx storage capacity of the Ba-free as well as Ba-loaded catalysts was strongly affected by the nature of the support. Diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) of CO adsorption on the Ba-free catalysts revealed that the distribution of CO chemisorbing sites also depends on the nature of the support. Among the Ba-free catalysts Pt/CeO2 exhibited the highest NOx uptake. No NOx uptake was observed with Pt/SiO2. With all supports the NOx storage capacity increased with the Ba-loading, but differently depending on the support. TPRD of catalysts after exposure to NOx pulses revealed that the thermal stability of the stored NOx species is affected by the nature of the support and the Ba-loading. Up to ca. 10 wt.% of Ba, Pt–Ba/CeO2 showed the highest NOx uptake, whereas at the highest Ba-loading (28 wt.%) Pt–Ba/Al2O3 and Pt–Ba/ZrO2 afforded highest NOx storage. The NOx storage capacity depended strongly on the relative abundance of the differently stable BaCO3 phases. To test the storage-reduction behavior, the catalysts were subjected to alternating pulses of NO/O2 and propene at 300 °C. Depending on the support different reduction behavior was observed. While reduction occurred rapidly on Pt–Ba(16.7)/Al2O3 and Pt–Ba(16.7)/CeO2, it was relatively slow and incomplete on Pt–Ba(16.7)/ZrO2. This behavior was attributed to the low surface area of the ZrO2 support which at higher Ba-loading seems to result in partial blocking of active Pt sites.