Shape dependence of support for NO_x 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 NO_x storage and reduction (NSR). The Pt/BaO/Al₂O₃ materials derived from Al₂O₃ nanorods always exhibited much higher NO_x 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/g_cat 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% NO_x 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-NO_x species onto the Pt surface, enhancing their reduction and leading to high NSR performance.