Chapter 3: NSR Catalytic Materials

Imagined in the 1990s, the NO_x-storage and reduction (NSR) process operates with alternative cycles of long lean phases (NO_x storage) and short rich phases (NO_x reduction). NSR catalysts should possess at least two components: a basic oxide for trapping the NO_x and a metallic component for the NO oxidation and the reduction of stored NO_x. The reference material is composed of barium oxide (10–20 wt%) deposited on a Pt/Al₂O₃ (1–2% Pt). Improvements of NSR catalysts were obtained by changing (i) the basic component (especially potassium but it can also damage the catalyst substrate; other alkaline or alkaline-earth oxides were also envisaged); (ii) the support (especially cerium-based supports, or titania, hydrotalcites, perovskites); (iii) the metal (Rh is often added to improve the NO_x reduction, and combinations of precious metals are considered). Noble metal-free catalysts were also developed, mainly based on cobalt, copper or manganese oxides. Perovskites including these transition metal oxides and potassium are also interesting for NSR applications. However, complete replacement of noble metal catalysts cannot be feasible yet. Usual NSR catalysts are sensitive to aging (sintering; strong Ba–support interactions) and to sulphur poisoning but supports like TiO₂ or cerium-based oxides can protect the catalyst from stable sulphate adsorption.