Lean-burning conditions

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). 

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. 

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. 

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. 

Diesel engines and some gasoline-fuelled engines operate under lean-burn conditions, where there is 10-15% more oxygen than is needed to burn all the fuel.8,9 The most important strategies for NO,c removal under these conditions are to use either (i) NO, storage-reduction catalysts or (ii) selective catalytic reduction (SCR) by an added reductant the term selective implies that the reductant attacks the NO, in preference to the oxygen. 

Mei, D.H., Ge, Q.F., Neurock, M., Kieken, L., Lerou, J. First-principles-based kinetic Monte Carlo simulation of nitric oxide decomposition over Pt and Rh surfaces under lean-burn conditions. Mol. Phys. 2004,102, 361-9. 

However, diesel engines and some gasoline engines are operated under lean-burn conditions, where the oxygen is fed in excess, i.e., 10-20% more than is required to meet the stoichiometry for combustion of the fuel [132,489]. Gold catalysts have therefore been examined for their potential in low-temperature activity to combat cold-start emission problems and removal of NOj, from lean-burn engines [202]. 

Roberts and co-workers [507] prepared a new class of spongy Au catalysts, that are unsupported powdered Au that consist of micrometre particles ( 5p) that have nanometre scale ( 10nm) internal skeletal structures. These catalysts were shown to be very active in the SCR of propene under lean-burn conditions. Alloying these catalysts with Pd leads to a significant widening of the temperature window of activity. 

However, as expected, reduction of NO to N2 under the more commercially interesting lean-burn conditions is more difficult with gold catalysts, since gold... 

Saruhan, B., Stranzenbach, M. and Mondragon Rodriguez, G.C. (2007) An integrated solution for N Ox-reduction and -control under lean-burn conditions. Materialwiss. Werhstojfftech., 38 (9), 725-33. 

For the first and the second type of lean bum engines, therefore, catalytic after-treatment systems are considered that include such a Cu/Zeolite catalyst to convert NO c under the lean burn conditions, and a conventional three-way catalyst to convert CO and HC as well as, with the second engine type, NO t under stoichiometric operation conditions. Some applications actually use a supported Iridium catalyst instead of the Cu/Zeolite catalyst. 

Kinetics of the Reduction of NO by CaHe and CaHe Over Pt Based Catalysts Under Lean-Burn Conditions... 

Platinum-titania-sepiolite monolithic catalysts for the reduction of nitric oxide with propene in lean-burn conditions... 

Finally, Ya-Huei Chin and Daniel Resasco (University of Oklahoma) review the catalytic oxidation of methane under lean-burn conditions. They focus on palladium-based catalysts, which are the most active for methane oxidation. They examine both the low temperature region (<800 °C), which is most relevant to exhaust control, and the high temperature region (>800 °C), which is applicable to gas turbines. 

Under lean-burn conditions the fuel efficiency is substantially higher due to the excess of oxygen. But the NOj. reduction is extremely difficult. 

Storage and reduction catalysts (NSR) offer the possibility of controlling NOx emissions from automobile sources while permitting operation under predominantly lean-burn conditions. °" The concept is based upon the storage of NO under lean conditions on an alkaline-earth oxide component, such as baria, which is then released during intermittent rich/stoichiometric periods where the stored NO is released and reduced by Hg, CO or HC over the noble metal component (Fig. 10.9). 

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