DeNOx oxide support

Here, we will first present briefly the general chemistry at work in deNOx catalysis. Then we wiU focus on the various specific nanometric issues in the domain. NOx decomposition mainly takes place on a metal center, as do most redox reactions in heterogeneous catalysis. We will therefore focus on how nanometric size influences the role of metal particles in this domain. It is very much linked to interaction with the oxide support, which wiU be the subject of the following part Zeolites are very specific oxide supports, controlling the distance between reactants and imposing chemical reactivity within the nanometer scale. Metal particles formed in zeoHtes have their size controlled between 0.5 and several dozens of nm, sometimes with Angstrom accuracy. We wiU next deal with three-way catalysts, mainly used for automotive deNOx. They involve very complex solids, where particle size is a key issue for activity. Finally, we wiU mention quickly new nanometric sohds like carbon nanotubes, which have appeared recently in deNOx catalysis. 

In conclusion, Cu on Ti02 or Zr02 show a unique and interesting behaviour since their deNOx activity is promoted and not inhibited by the presence of sulfur in the feed. This effect can hardly be attributed to a selective inhibition of the oxidation of decane, and is better explained by the promotion of a bifunctional mechanism involving the acid sites created on the support by the reaction of SO2. 

The difference in deNOx activity between the two differently prepared samples further highlights their differences in structure and surface chemistry. The catalytic activity of supported gallium oxide is likely to be governed by the surface concentration ratio of acidic/basic functional groups. 

It may be, however, that no single phase zeolite catalyst is able to fulfil the role, and there has also been interest in bifunctional lean burn-deNOx catalysts where the zeolite is mixed with another catalyst. A zeolitic catalyst may be used together with a supported Pt/Al203 catalyst that is more active for NOx reduction at low temperatures. Other examples include Mn203/Ce-ZSM-5, where the manganese oxide catalyses NO oxidation to NO2, which reacts rapidly over the metal-containing zeolite, and Pt/H-ZSM-5, where the acid sites activate the hydrocarbon and promote its reaction with the NOx-... 

Berland et al. also studied the combination of a model NSR catalyst (1 % Pt/ 10 % Ba0/Al203, denoted as Pt/Ba-Al) with oxides-based SCR samples [84]. WO3 supported over ceria-zirconia oxides (W03/Ce-Zr) were studied as the active NH3-SCR catalysts. The effect of the composition of the ceria-zirconia mixed oxides was studied with a constant WO3 loading (10 wt.% of W, added by impregnation). It is demonstrated that Pt/Ba-Al NSR catalyst can release important amount of ammonia, until over 50 % of selectivity at 300 °C (Fig. 19.8a). SCR materials W03/Ce-Zr, with different Ce-Zr ratio, were associated downstream to the Pt/Ba-Al NSR catalyst. In the NSR -I- SCR combined system, the DeNOx efficiency is strongly improved. An enhancement of 24 points in NOx conversion was obtained at 300 °C for the better SCR sample (W03/Ce-Zr(2o-go)) (Fig. 19.8b) [83]. 

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