Shape dependence of support for NOx storage and reduction catalysts.

Pt/BaO/Al2O3 catalysts with different BaO loadings prepared from Al2O3 nanorods (Pt/BaO/Al2O3-nr) and irregular Al2O3 nanoparticles (Pt/BaO/Al2O3-np) were investigated for NOx storage and reduction (NSR). The Pt/BaO/Al2O3 materials derived from Al2O3 nanorods always exhibited much higher NOx storage capacity (NSC) over the whole temperature range of 100-400°C than the corresponding Pt/BaO/Al2O3-np samples containing the same BaO loading, giving the maximum NSC value of 966.9 μmol/gcat at 400°C, 1.4 times higher than that of Pt/BaO/Al2O3-np. Higher catalytic performance of nanorod-supported NSR samples was also observed during lean-rich cyclic conditions (90 sec vs. 5 sec), giving more than 98% NOx conversion at 300-450°C over the Pt/BaO/Al2O3-nr sample with 15% BaO loading. To reveal this dependence on the shape of the support during the NSR process, a series of characterization techniques including the Brunauer-Emmett-Teller (BET) method, X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), H2 temperature programmed reduction (H2-TPR), and in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) were also conducted. It was found that intimate contact of Ba-Al and Ba-Pt sites was achieved over the Pt/BaO/Al2O3 surface when using Al2O3-nr as a support. This strong interaction among the multi-components of Pt/BaO/Al2O3-nr thus triggered the formation of surface nitrite and nitrate during the lean period, and also accelerated the reverse spillover of ad-NOx species onto the Pt surface, enhancing their reduction and leading to high NSR performance.