DeNOx

The ever increasing demand to develop highly fuel efficient engines coincides with the need to minimize air pollution originating from the exhaust gases of internal combustion engines. Dramatically improved fuel efficiency can be achieved at air-to-fuel ratios much higher than stoichiometric. In the presence of oxygen in large excess, however, traditional three-way catalysts are unable to reduce NOx. Among the number of lean-NOx reduction technologies, selective catalytic reduction (SCR) of NOx by NH3 over Cu- and Fe-ion exchanged zeolite catalysts has been extensively studied over the past 30+ years. Despite the significant advances in developing a viable practical zeolite-based catalyst for lean NOx reduction, the insufficient hydrothermal stabilities of the zeolite structures considered cast doubts about their real-world applicability. During the past decade renewed interest in zeolite-based lean NOx reduction was spurred by the discovery of the very high activity of Cu–SSZ-13 (and the isostructural Cu–SAPO-34) in the NH3-SCR of NOx. These new, small-pore zeolite-based catalysts not only exhibited very high NOx conversion and N2 selectivity, but also exhibited exceptionally high hydrothermal stability at high temperatures. In this review we summarize the key discoveries of the past [similar]5 years that led to the introduction of these catalysts into practical applications. This review first briefly discusses the structure and preparation of the CHA structure-based zeolite catalysts, and then summarizes the key learnings of the rather extensive (but not complete) characterisation work. Then we summarize the key findings of reaction kinetic studies, and provide some mechanistic details emerging from these investigations. At the end of the review we highlight some of the issues that still need to be addressed in automotive exhaust control catalysis.

The NO x selective catalytic reduction of ethanol (EtOH-SCR) was studied using a complex gas mixture representative of a diesel exhaust, over zinc alumina mixed oxide supported silver catalysts (2 wt.% Ag/Zn x-Al 2 O 3 , with x = 10–20–33 at.%). The supports were obtained using a template assisted sol–gel route in order to achieve high surface areas. For the higher Zn loading (33%), the calcination temperature has been raised from 600 • C to 800 • C then 1000 • C in order to evaluate thermal stability of these materials. Addition of zinc in alumina network leads to the formation of the spinel-type zinc alumi-nate structure. Chemical and physical characterizations of the catalysts have been confronted with the EtOH-SCR results, in order to understand the respective influence of the metal and support in nitrogen formation. This study shows that addition of zinc and modification of the support calcination temperature both play upon the Lewis acidic sites (LAS) concentration and density, determined by pyridine adsorption monitored by FTIR spectroscopy. This parameter is shown to be related with ammonia emission at T ≥ 350 • C. Besides, formation of nitrogen at T ≤ 350 • C is shown to be dependent on (i) the rate of acetaldehyde formation and (ii) the reactivity of acetaldehyde in SCR of NO reaction. Modification of the alumina support directly impacts these parameters. Finally, it is demonstrated that Zn is a hopeful candidate to increase the EtOH-SCR activity at low temperature (T ≤ 300 • C).

Vanadium based catalysts supported on a mixture of tungsten and titanium oxide (V2O5/WO3–TiO2) are known to be highly active for ammonia selective catalytic reduction (NH3–SCR) of NOx species for heavy-duty mobile applications. However they are also known to be sensitive to high temperatures which leads to both sintering of the anatase TiO2 support and a first order phase transition to rutile at temperatures >600°C. Here we report our attempts to use SiO2 to stabilize the TiO2 anatase phase and to compare its catalytic activity with that of a non-stabilized V2O5/WO3–TiO2 catalyst after thermal aging up to 800°C. Detailed characterization using spectroscopic (Raman, UV–vis, X-ray absorption spectroscopy), scattering and techniques providing information on the catalytic surface (Brunauer–Emmet–Teller, NH3 adsorption) have also been performed in order to understand the impact of high temperatures on component speciation and the catalytic interface. Results show that non-stabilized V2O5/WO3–TiO2 catalysts are initially stable after thermal aging at 600°C but on heating above this temperature a marked drop in catalytic activity is observed as a result of sintering and phase transformation of Anatase into Rutile TiO2 and phase segregation of initially highly dispersed WO3 and polymeric V2O5 into monoclinic WO3 and V2O3 crystallites. Similar behavior was observed for the 4–5 wt-% of SiO2-stabilised sample after aging above 700°C, importantly therefore, offset by some ∼100°C in comparison to the unstabilised sample.