Nitrogen Oxide Removal

Yang Bo

2012.8.30

Background Zeolites are commonly synthesized in aqueous gels that contain a silica source, an alumina source, a structure directing agent (SDA), and a mineralizer (F−or OH−), all heated under autogenous pressure at high temperature (usually 90 −200℃). Zeolite synthesis gels are usually difficult to characterize with most analytical methods, but investigations of the formation of zeolites in dilute clear sols has attracted particular interest because it may yield experimental analysis and provide insight into the molecular processes that underlie zeolite formation. These clear sols are visually transparent and when prepared from tetraethylorthosilicate (TEOS), the SDA, and water, they contain both nanoparticles and oligomeric silicate species.

Introduction

Lowering the fuel consumption of transportation vehicles could decrease both emissions of greenhouse gases and our dependence on fossil fuels. One way to increase the fuel efficiency of internal combustion engines is to run them “lean,” in the presence of more air than needed to burn all of the fuel. Commercially available vehicles with lean-burn technology have diesel or gasoline direct injection engines, and typically have additional cost $1000 to $5000 compared with stoichiometric engine vehicles. A large part of extra cost is due to engine modification and exhaust catalysts that enable compliance with emission regulation.

Lean engines produce oxygen-rich exhuast, which prevents the reduction of NOx via the “three-way” catalyst commonly used for stoichiometric engines. The new combustion and catalyst technologies for lean engines must meet NOx emission standards as well as those for carbon monoxide, hydrocarbons, and particulate matter(soot). For diesel engines, special exhaust filters have eliminated the unsightly plumes of soot emissions from diesel exhaust. So the catalyst for deNOx is the key problem of lean-burn engines.

Catalyst Technologies

Selective catalytic reduction(SCR)

Lean NOx trap(LNT)

Selective catalytic reduction

This kind of catalyst are urea-based(NH3-SCR).The main reactions of NH3-SCR are following:

1.The decompose of urea:NH2-CO-NH2+H2O→HNCO+NH3

(2)

HNCO+H2O →NH3 +CO2

2.The reduction of NOx:4NO +4NH3 +O2 →4N2 +6H2O (“standard ” SCR)(3)

NO +NO2 +2NH3 →2N2+3H2O (“fast ”SCR)

(2) 田海影 ，刘盛强等，山东建筑大学学报， Vol.25 ,633-641 (3) Nathalie Marcotte at el. Applied Catalysis B: Environmental 105 (2011) 373-376

Disadvantages:The requirement of onboard storage of the urea reductant .The need to heat the urea in cold weather.The resupply of the urea reductant.

Lean NOx trap(LNT)

The LNT is known as the NOx storage and reduction catalyst. LNTs use engine fuel for NOx reduction. Alumina(Al2O3) as catalyst support , Pt as catalyst . The common adsorption material of NOx in LNT is alkaline earth metal oxide(BaCO3) or alkali metal(Na,K,Cs,etr) oxides.(4)

Unlike “three-way” catalyst , which reduce NOx continuously , LNTs switch between two modes of operation .

(4)楼狄明等，小型内燃机与摩托车， Vol3,No.2,(2010),73-74

Most of the cycle is spent trapping NOx during lean operation.

Modes of operation

Periodically, control systems cause the engine to run fuel rich, and unburned fuel is used in the exhaust is used to liberate the stored NOx and reduce it to N2.

During lean operation ,platinum (Pt) affects NO oxidation to NO2; most of NOx is NO, but it must be oxidized to NO2 for storage. Pt oxidizes hydrocarbon to fully eliminate oxygen from the exhaust, which must occur if NOx is to be released from the trap. Finally, during the rich phase, Pt enables regeneration of the LNT for more NOx storage by assisting in the NOx release and reduction steps. Rhodium (Rh) is also a common PGM in LNTs and is often added to promote NOx reduction and hydrocarbon-processing reactions.

The roles of PGM

One key factor in LNT performance for passenger-car application is NOx reduction at the low exhaust temperatures generated by lean engines(diesel exhaust temperatures below 250℃ are common at low engine power). At low temperatures, the effect of Pt on NOx reduction performance is greater than at higher temperatures, but they require PGMs that add to the cost of lean engine vehicles. Volatility in PGM price has been a problem in minimizing the cost of the technology. During the last decade, monthly average Pt market price have ranged from about $400 to $2000 per troy ounce (31.1034768g).

The new catalysts

The catalysts developed by Kim et al. are based on the perovskite oxides, La1-xSrxCoO3 and La1-xSrxMnO3,where lanthanum(La) is a rare-earth cation, strontium(Sr) is an alkaline-earth cation, and cobalt(Co) and manganese(Mn) are transition-metal cations. In test with simulated diesel exhaust , the La1-xSrxMnO3 catalyst achieved conversion of NO to NO2 comparable to that of commercial Pt-based catalysts. These catalysts are less effective at hydrocarbon and carbon monoxide oxidation , and are also prone to deactivation by sulfur, a contaminant present in fuel. However ,the oxidation of the catalyst could be improved in the presence of sulfur by adding palladium(Pd), a PGM. Palladium increases the performance of the perovskite-based LNT as well.

The catalyst developed by Kim et al. greatly reduces the amount of PGM in LNTs while still maintaining their effectiveness for NOx reduction from lean engines. This alternative technology will allow engineers greater flexibility as they work to develop better catalysts in a market where volatile PGM prices have made commercial introduction of fuel-efficient lean vehicles challenging. It is possible that these catalysts may allow lean-burn technology to be used with minimal added cost conventional engines.

Conclusion and help

Help:对 LNT的机理有了一定了解；了解有关 LNT催化剂的最新的发展方向及进展。

Thanks for your attention