Lean burning

Chen FI Y and Sachtler W M FI 1998 Activity and durability of Fe/ZSM-5 catalysts for lean burn NOx reduction in the presence of water vapor Catal. Today 42 73-83... 

The strength and interrelation of catalysis, classical promotion and electrochemical promotion is illustrated in Fig. 2.3. The reaction under consideration14 is the reduction of NO by CO in presence of 02. This is a complex reaction system but of great technological importance for the development of efficient catalytic converters able to treat the exhaust gases of lean burn and Diesel engines. 

Anderson, R. and J.R. Asik, Ignitability Experiments in a Fast Burn, Lean Burn Engine. SAE, 830477, 1983. 

The PAG system utilizes plasma to oxidize NO to NO2, which then reacts with a suitable reductant over a catalyst. LNC, NSR, and PAG systems have still several challenging tasks to be solved. Gonsequently, all these technologies are not yet appropriate for commercial applications to diesel and lean-burn engine exhausts [47]. 

The fact that Fischer-Tropsch fuels contain neither sulfur nor aromatics may become a strong selling point for the process. Less sulfur in the fuel has, of course, a direct effect on the sulfur oxides in the emissions, and the newly developed exhaust purification systems for lean burning engines that can be introduced means that all emissions, including GO2 and NOx, will diminish. Aromatics promote particulate formation in the combustion of diesel fuels and are therefore undesirable. We discuss this further in Ghapter 10. 

NOx Storage-Reduction Catalyst for Lean-burning Engines... 

One of the most straightforward methods to reduce carbon dioxide emissions is to enhance the fuel efficiency of engines. The three-way catalyst, although very successful at cleaning up automotive exhaust, dictates that engines operate at air-to-fuel ratios of around 14.7 1. Unfortunately, this is not the optimum ratio with respect to fuel efficiency, which is substantially higher under lean-burn conditions at A/F ratios of about 20 1, where the exhaust becomes rich in oxygen and NOx reduction is extremely difficult (Fig. 10.1). 

The NOx storage-reduction (NSR) catalyst, developed by Toyota and other companies, offers a solution based on a two step process, in which the engine switches periodically between a long lean-burn stage and a very short fuel-rich stage. The NSR catalyst combines the oxidation activity of platinum with a NOx storage compound based on barium oxide. Figure 10.10 illustrates the principle of operation. 

In the lean-burn stage all exhaust components are oxidized by the platinum particles in the catalyst. In particular, NO is oxidized to NO2. The latter reacts with BaO getter to form Ba(N03)2- In the rich mode, which only lasts for seconds, the exhaust stream is deficient in oxygen, and reducing components such as CO, H2 and hydro-... 

Describe how NOx can be removed from the exhaust when a car operates under lean-burn conditions (i.e. oxygen rich). Why is it attractive to drive cars under lean-burn conditions ... 

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. 

For similar motivations, there are limited incentives to develop an alternative SCR process for stationary sources based on methane (CH4-SCR) or other HCs, or based on NTP technologies, if not for specific, better applications. The situation is instead quite different for mobile sources, and in particular for diesel engine emissions. The catalytic removal of NO under lean conditions, e.g. when 02 during the combustion is in excess with respect to the stoichiometric one (diesel and lean-burn engines, natural gas or LPG-powered engines), is still a relevant target in catalysis research and an open problem to meet future exhaust emission regulations. 

It is believed that SCR by hydrocarbons is an important way for elimination of nitrogen oxide emissions from diesel and lean-burn engines. Gerlach etal. [115] studied by infrared in batch condition the mechanism of the reaction between nitrogen dioxide and propene over acidic mordenites. The aim of their work was to elucidate the relevance of adsorbed N-containing species for the F>cNOx reaction to propose a mechanism. Infrared experiments showed that nitrosonium ions (NO+) are formed upon reaction between NO, NOz and the Brpnsted acid sites of H—MOR and that this species is highly reactive towards propene, forming propenal oxime at 120°C. At temperatures above 170°C, the propenal oxime is dehydrated to acrylonitrile. A mechanism is proposed to explain the acrylonitrile formation. The nitrile can further be hydrolysed to yield... 

Burch, R., Breen, J.P. and Meunier, F.C. (2002) A review of the selective reduction of NO, with hydrocarbons under lean-burn conditions with non-zeolitic oxides and platinum group metal catalysts, Appl. Catal. B, 39, 283. 

Takahashi, N., Shinjoh, H., Iijima, T. et al. (1996) The new concept 3-way catalyst for automotive lean-burn engine NO, storage and reduction catalyst, Catal. Today, 27, 63. 

Miyoshi, N., Matsumoto, S., Katoh, K. et al. (1995) Development of New Concept Three-Way Catalyst for Automotive Lean-Burn Engines, SAE Technical Paper 950809. 

IMPACT OF OSC MATERIALS IN LEAN-BURN AND DIESEL CATALYSIS... 

One of the most promising processes is the active DeNO based on NO -trap materials. It has been developed for lean-burn gasoline engines. Cerium compounds are thought to intervene in different steps of the whole process (1) NO oxidation, (2) NO storage, (3) Nitrate desorption and NO reduction. Most probably, the main role of OSC materials is to accelerate HC partial oxidation during rich-spikes (giving CO and H2 as NO reducers). However, this beneficial effect of OSC compounds competes with a detrimental reaction,... 

Gold as alternative in I)eNO catalysis under lean burned conditions... 

Frank, B., Emig, G. and Renken, A. (1998) Kinetics and mechanism of the reduction of nitric oxides by H, under lean-burn conditions on a Pt-Mo-Co/a-ARO, catalyst, Appl. Catal. B 19, 45. 

Burch, R., Fornasiero, P. and Southward, B.W.F. (1999) An investigation into the reactivity, deactivation and in situ regeneration of Pt-based catalysts for the selective reduction of NOx under lean burn conditions, J. Catal. 182, 234. 

Burch, R. and Watling, T.C. (1997) Kinetics and mechanism of the reduction of NO by C3H8 over Pt/Al203 under lean-burn conditions, J. Catal. 169, 45. 

Mowery, D.L., Graboski, M.S., Ohno, T.R. et al. (1999) Deactivation of Pd0-Al203 oxidation catalyst in lean-burn natural gas engine exhaust aged catalyst characterization and studies of poisoning by H20 and S02, Appl. Catal. B 21, 157. 

---