Enhanced NH₃ Selective Catalytic Reduction for NO_x Abatement

Abstract

Nitrogen oxides NO x , which include NO (nitrogen monoxide), NO 2 (nitrogen dioxide), and N 2 O (nitrous oxide), are currently considered to be among the most dangerous air pollutants, as they cause acid rain, contribute to photochemical smog, and have direct harmful effects on human health. For all these reasons, in the last decades NO x emissions have been regulated in industrialized countries by more and more restrictive legislation.[1] Selective catalytic reduction (SCR) technology is well established and used worldwide to control NO x emissions from power plants and other stationary sources. A hot exhaust gas is passed over an extruded honeycomb monolith catalyst consisting of V 2 O 5 -WO 3 /TiO 2 [2] in the presence of a nitrogenous reductant such as ammonia or urea (an ammonia carrier), which converts NO x to nitrogen according to the so-called standard SCR reaction (1).Currently NH 3 or urea SCR is also being increasingly employed to reduce NO x in the exhaust gases of internal combustion engines operated with an excess of air, such as Diesel engines installed in heavy-duty vehicles and passenger cars. Zeolite-based catalysts promoted by transition metals such as Fe and Cu are considered for this application. One problem of SCR systems for vehicles, however, is poor activity at low temperatures where most of the NO x is produced, for example, during cold startup and on traveling short distances. The chosen method to boost the DeNOx activity of SCR catalysts for mobile applications at low temperature is to increase the NO 2 /NO molar feed ratio (NO 2 accounts only for few percent of total NO x in the engine exhaust) and thus promote the fast SCR reaction (2).This is realized in practice by installing a Diesel oxidation catalyst (DOC), typically consisting of precious metals carried on a flow-through honeycomb support, to convert a portion of NO to NO 2 upstream of the SCR converter. Thus, considerable improvements in NO x conversion are achieved, and the highest DeNOx efficiencies correspond to the NO 2 /NO molar ratio of 1/1 associated with reaction (2).[4] However, the oxidation activity of the DOC is strongly dependent on temperature and flow rate of the exhaust gases, so that optimal NO 2 /NO feed ratio cannot be guaranteed for all possible engine operating conditions.Here we reveal that a superior NO conversion efficiency can also be achieved by means of an enhanced SCR reaction [5] involving selective reduction of NO to N 2 by NH 3 and nitrate species such as ammonium nitrate [reaction (3)].The new reaction has very high DeNOx efficiency at low temperatures, similar to that of fast SCR, even though no NO 2 is fed to the SCR catalyst; accordingly, it can replace reaction (2) in boosting NH 3 SCR activity without the necessity of pre-oxidizing NO. Figure 1 shows the temporal evolution of the NO, NH 3 , and NO 2 outlet concentrations during one run at 205 8C over an Fe-ZSM-5 washcoated monolith catalyst. Initially only equimolar amounts of NH 3 and NO (1000 ppm each) were fed to the catalyst in a nitro...

Mesostructured amorphous manganese oxides: facile synthesis and highly durable elimination of low-concentration NO at room temperature in air

“…Among them, 24 h is the optimized time period of material synthesis. The resultant Mn-24 h shows the best performance and the time durations for 100% and 60% removal of 10 ppm NO are as long as 10 h and 21 h, respectively, which is much better than that of previously reported mesoporous X-Mn materials (4 h and 5 h for 100% and 60% removal, respectively 7 ) and Co 3 Table S1, ESI †), the surface state of the synthesized Mn-t h materials determines their performances, which will be discussed below.…”

Ambient and laboratory observations of organic ammonium salts in PM1

“…3,4 This includes emissions from ships, diesel locomotives, or gas turbines, and of increasing importance are those from heavy-duty vehicles and passenger cars using diesel engines. 73 The ndings of this work emphasize the need for control measures for traffic emission of NH 3 , to reduce effects on aerosol mass through organic ammonium salt formation.…”

Bauxite-supported Transition Metal Oxides: Promising Low-temperature and SO2-tolerant Catalysts for Selective Catalytic Reduction of NOx

“…As shown in Figure S2, NO x conversion of Cu/bauxite is gradually enhanced when NO 2 /NO x ratio is increased from 0.25 to 0.5, and reaches the maximum at 0.5, subsequent drop of NO x conversion is observed at 0.75 and 1.0. It was reported that when NH 3 and NO 2 were fed into the reactor, NH 4 NO 3 can be formed at low temperature 113. Since NH 4 NO 3 cannot be detected directly by our apparatus, transient response method (TRM) of Cu/bauxite at 150 °C was conducted to indirectly estimate the amount of NH 4 NO 3 according to N-balance.…”

“…Nitrogen oxides (NO x ) are considered as one of serious air pollutants, they are mainly emitted from automobile exhaust gas and industrial combustion of fossil fuels 123. To meet for more and more stringent regulations of NO x emission, several promising techniques including NO x storage and reduction (NSR) and selective catalytic reduction (SCR) have been proposed for NO x post-treatment 23.…”