NOX adsorber catalyst

B100 [100% biodiesel) with NOx adsorbing catalyst on uehicte... 

Another technology under consideration for NOx abatement in diesel vehicles is the LNT, which is also commonly called the NOx adsorber catalyst (NAC) or NOx storage/reduction catalyst (NSRC). 

Figure 4 shows the amounts of adsorbed NO from the feed of NO, NO-O2, NO2, or NO2-O2 on ln/H-ZSM-5, lr/H-ZSM-5, and lr/ln/H-ZSM-5. It is apparent from these results that lr/H-ZSM-5 adsorbed little NOx. NO could hardly be adsorbed on every catalyst in the absence of 02. It is interesting to note that lr/ln/H-ZSM-5 adsorbed larger amounts of NOx from the mixture of NO and O2 than from NO2, and it was also larger than the amount of NOx adsorbed on ln/H-ZSM-5 from NO2. 

Another important catalytic technology for removal of NOx from lean-burn engine exhausts involves NOx storage reduction catalysis, or the lean-NOx trap . In the lean-NOx trap, the formation of N02 by NO oxidation is followed by the formation of a nitrate when the N02 is adsorbed onto the catalyst surface. Thus, the N02 is stored on the catalyst surface in the nitrate form and subsequently decomposed to N2. Lean NOx trap catalysts have shown serious deactivation in the presence of SOx because, under oxygen-rich conditions, SO, adsorbs more strongly on N02 adsorption sites than N02, and the adsorbed SOx does not desorb altogether even under fuel-rich conditions. The presence of S03 leads to the formation of sulfuric acid and sulfates that increase the particulates in the exhaust and poison the active sites on the catalyst. Furthermore, catalytic oxidation of NO to N02 can be operated in a limited temperature range. Oxidation of NO to N02 by a conventional Pt-based catalyst has a maximum at about 250°C and loses its efficiency below about 100°C and above about 400°C. 

A lean NOx trap (LNT) (or NOx adsorber) is similar to a three-way catalyst. However, part of the catalyst contains some sorbent components which can store NOx. Unlike catalysts, which involve continuous conversion, a trap stores NO and (primarily) N02 under lean exhaust conditions and releases and catalytically reduces them to nitrogen under rich conditions. The shift from lean to rich combustion, and vice versa, is achieved by a dedicated fuel control strategy. Typical sorbents include barium and rare earth metals (e.g. yttrium). An LNT does not require a separate reagent (urea) for NOx reduction and hence has an advantage over SCR. However, the urea infrastructure has now developed in Europe and USA, and SCR has become the system of choice for diesel vehicles because of its easier control and better long-term performance compared with LNT. NOx adsorbers have, however, found application in GDI engines where lower NOx-reduction efficiencies are required, and the switch between the lean and rich modes for regeneration is easier to achieve. 

Sulfur Toxic, NOx Inhibits exhaust catalysts, 02 sensors, NOx adsorbers, particulate traps... 

The concept of trapping a contaminant in low concentration by adsorption with periodic regeneration of the adsorbent-catalyst has been applied commercially by Toyota for NOx-trap catalysts used in converting NOx in diesel or lean burn engine emissions, for example, for reduction of NOx in the presence of O2 [63]. The catalyst acts as absorbent of NOx (in the form of surface nitrate-like species) in the presence of O2 (lean conditions), but a periodic switch of the air to fuel ratio to rich conditions (deficit of O2 with respect to stoichiometry for the complete oxidation of CO and hydrocarbons present in the car emissions to CO2) leads to regeneration by reducing trapped NOx to N2. 

A new system, designed specifically for diesel engines, composed of a NOx adsorbent trap coupled to a lean NOx reduction catalyst is described. The trap adsorbs NOx between 150 and 500 C which is periodically desorbed thermally by a localized exotherm generated within the catalyst from the controlled injection and oxidation of diesel fuel (always maintaining the exhaust lean). The hydrocarbon injection temperature is controlled to allow for a downstream lean-NOx Pt catalyst to reduce the desorbed NOx. Significant increases in NOx reduction are possible. 

Fig. 1.13 An upstream passive NOx adsorber (PNA) captures NOx generated at T < 150 °C. A LT urea-SCR catalyst can then convert this NOx upon release at T > 150 °C [47]...

Examples of multi-disciplinary innovation can also be found in the field of environmental catalysis such as a newly developed catalyst system for exhaust emission control in lean burn automobiles. Japanese workers [17] have successfully merged the disciplines of catalysis, adsorption and process control to develop a so-called NOx-Storage-Reduction (NSR) lean burn emission control system. This NSR catalyst employs barium oxide as an adsorbent which stores NOx as a nitrate under lean burn conditions. The adsorbent is regenerated in a very short fuel rich cycle during which the released NOx is reduced to nitrogen over a conventional three-way catalyst. A process control system ensures for the correct cycle times and minimizes the effect on motor performance. 

Chemisorption data shown in Fig. 4 show that either NO or NO2 was hardly adsorbed on Ir site. Furthermore, chemisorption of pure NO was negligibly small on ln/H-ZSM-5 and lr/ln/H-ZSM-5 at 673 K, while NO in the presence of O2, as well as NO2, could significantly be adsorbed. These results also support our supposition that chemisorption of NO2 is important and that of NO is iess important on these catalysts. In the presence of O2, chemisorption of NOx, both NO and NO2, was enhcinced by the addition of Ir on to ln/H-ZSM-5. 

Here, 6 and V represent the surface coverage and the amount of adsorbed NOx, respectively. Calculated K and Vo are summarized in Tabie 2. If the important adsorption site on these catalysts consists of In species, and if NO2... 

Figure 6.7. Moles of adsorbed NOx (point) and fraction of Ba involved in the storage (square) as function of Ba loading at catalyst saturation on Pt—Ba(x)/y-Al203 catalysts.

Ammonia is adsorbed on the surface of an SCR catalyst in a diffusion limited laminar flow regime. The ammonia combines with vanadium pentoxide V2O5, a catalytic metal impregnated on the surface of the catalyst, to form a Bronsted acid site. NOx reduction takes place on this acid site to form nitrogen and water. The spent -OH site is restored to -OH via oxidation to repeat the catalytic cycle. Once the vanadium site can no longer revert back into the -1-5 oxidative state, then that site is no longer active for NO reduction. Figure 17.7 shows the catalytic cycle for the SCR reactions. 

Utilities using post-combustion SCR-supported ammonia injection for NOx control as well as those using ammonia conditioning to improve electrostatic precipitator performance will produce fly ash that contains ammonia compounds. The ammonia is primarily physically adsorbed onto the fly ash particles as sulphate and bisulphate species. In many cases, the residual ammonia levels are quite low (<50ppm) however, elevated concentrations can occur as the catalyst ages or due to mechanical problems with the ammonia injection system. While elevated ammonia concentrations in fly ash do not negatively impact pozzolanic properties, it can reduce ash marketability due to odour concerns. For this reason, several processes have been developed to remove or reduce the amount of ammonia in fly ash. 

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