Support Effects and Chemical Gradients along the Catalyst Bed in NO_x Storage-Reduction Studied by Space- and Time-Resolved In Situ DRIFTS

Abstract

Support effects in NO x storage-reduction (NSR) have been investigated by means of space-and time-resolved in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) using Pt-Ba catalysts supported on six different metal oxides (MO x ) Al 2 O 3 , TiO 2 , SiO 2 , CeO 2 , ZrO 2 , and Ce 0.5 Zr 0.5 O 2 ) and hydrogen as reductant. The NSR performance was significantly influenced by the type of oxide supports. The NO x storage capacity followed the order CeO 2 > Ce 0.5 Zr 0.5 O 2 > Al 2 O 3 > ZrO 2 > TiO 2 > SiO 2 , which is in agreement with the basicity of the oxides. In contrast, the rate of NO x reduction showed the reverse order. The study shows that storage and reduction of NO x and spillover processes are influenced by the type of support. As a result of this, the support material also affects the concentration gradients along the catalyst bed which in turn has an effect on the NSR performance. Strong basicity is beneficial for efficient NO x storage during lean periods but decreases the rate of NO x reduction during rich periods by stabilizing surface and bulk nitrates. The high oxygen-storage ability of CeO 2 and Ce 0.5 Zr 0.5 O 2 further decreases the rate of NO x reduction due to the consumption of reductant with the stored oxygen to form water. In practice, there seems to be a trade-off between efficient NO x storage and NO x reduction rate determined by the nature of the support material. In this respect the most widely used catalyst based on γ-alumina as support seems to be a good compromise for achieving efficient storage and fast reduction of NO x . However, CeO 2 and Ce 0.5 Zr 0.5 O 2 have a potential to serve as suitable support materials for efficient NSR catalysts if they are modified to enhance the reduction rate.

AN XPS study of the interaction of model Ba/TiO2 AND Ba/ZrO2 NSR catalysts with NO2

“…Accumulation curves depicted in Fig. 3b show that barium nitrate forms more efficiently when ZrO 2 is used as the substrate, which is consistent with the representation of that the higher the basicity of the substrate the higher the formation rate of barium nitrate is [17]. At the same time, a drop in the efficiency of the interaction of the Ba/TiO 2 sample with NO 2 after the annealing in the vacuum can also be due to the formation of barium titanates.…”

Effects of temperature and reductant type on the process of NOx storage reduction over Pt/Ba/CeO2 catalysts

“…3a), the bidentate nitrates on barium sites (1585, 1294, and 1020 cm −1 ) formed quickly. Specifically, the bands at 1020 and 1294 cm −1 were assigned to the symmetric and asymmetric stretching mode of -NO 2 , and the band at 1585 cm −1 was characteristic of N O vibration [6,28]. Another significant band at 1110 cm −1 could be assigned to nitrates species on ceria [29].…”

Effects of temperature and rich-phase composition on the performance of a commercial NOx-Storage-Reduction material

“…The 6 understanding of NSR technology has been hampered by the fact that various (usually non-7 realistic) reaction conditions were often used and a very large number of model materials with 8 varying formulations were investigated [4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22]. 9 Several studies have been devoted to commercial NSR materials [23,24,25,26] and in 10 situ FTIR studies [27,28,29,30], but rarely the combination of the two has been reported. One 11 of the difficulties associated with commercial samples is the large number of components 12 present, making difficult the interpretation of in situ/operando IR spectra.…”