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

Support effects in NOx 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 (MOx = Al2O3, TiO2, SiO2, CeO2, ZrO2, and Ce0.5Zr0.5O2) and hydrogen as reductant. The NSR performance was significantly influenced by the type of oxide supports. The NOx storage capacity followed the order CeO2 > Ce0.5Zr0.5O2 > Al2O3 > ZrO2 > TiO2 > SiO2, which is in agreement with the basicity of the oxides. In contrast, the rate of NOx reduction showed the reverse order. The study shows that storage and reduction of NOx 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 ail effect on the NSR performance. Strong basicity is beneficial for efficient NOx storage during lean periods but decreases the rate of NOx reduction during rich periods by stabilizing surface and bulk nitrates. The high oxygen-storage ability of CeO2 and Ce0.5Zr0.5O2 further decreases the rate of NOx 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 NOx storage and NOx reduction rate determined by the nature of the Support material. In this respect the most widely used catalyst based on gamma-alumina as support seems to be a good compromise for achieving efficient storage and fast reduction of NOx. However, CeO2 and Ce0.5Zr0.5O2 have a potential to serve as suitable support materials for efficient NSR catalysts if they are modified to enhance the reduction rate.