Chapter 3. NSR Catalytic Materials

Abstract

Imagined in the 90's, the NOx-storage and reduction (NSR) process operates with alternative cycles of long lean phases (NOx storage) and short rich phases (NOx reduction). NSR catalysts should possess at least two components: a basic oxide for trapping the NOx and a metallic component for the NO oxidation and the reduction of stored NOx. 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 NOx 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 of Ba-support interactions) and to sulphur poisoning but supports like TiO2 or cerium-based oxides can protect the catalyst from stable sulphate adsorption.

Tungsten-Based Catalysts for Environmental Applications

“…One of the technologies for NO x abatement in diesel engine exhausts is the NO x trap system, also called NO x storage reduction (NSR) or lean NO x trap (LNT) [254]. The catalyst (typically Pt/BaO-Al 2 O 3 ) works according to a sequential operation: (i) during one or two minutes, NO x from the exhaust gases is stored on the catalyst as nitrate and nitrite species; (ii) during a few seconds, hydrocarbons are added to the exhaust gases for the reduction step, and adsorbed nitrates and nitrites are reduced to N 2 .…”