Simultaneous NOx and Hydrocarbon Emissions Control for Lean-Burn

Jan 30, 2012 - Simultaneous NOx and Hydrocarbon Emissions Control for Lean-Burn Engines Using Low-Temperature Solid Oxide Fuel Cell at Open Circuit. T...
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Simultaneous NOx and Hydrocarbon Emissions Control for Lean-Burn Engines Using Low-Temperature Solid Oxide Fuel Cell at Open Circuit Ta-Jen Huang,* Sheng-Hsiang Hsu, and Chung-Ying Wu Department of Chemical Engineering, National Tsing Hua University, Hsinchu 30013, Taiwan, ROC ABSTRACT: The high fuel efficiency of lean-burn engines is associated with high temperature and excess oxygen during combustion and thus is associated with highconcentration NOx emission. This work reveals that very high concentration of NOx in the exhaust can be reduced and hydrocarbons (HCs) can be simultaneously oxidized using a low-temperature solid oxide fuel cell (SOFC). An SOFC unit is constructed with Ni−YSZ as the anode, YSZ as the electrolyte, and La0.6Sr0.4CoO3 (LSC)−Ce0.9Gd0.1O1.95 as the cathode, with or without adding vanadium to LSC. SOFC operation at 450 °C and open circuit can effectively treat NOx over the cathode at a very high concentration in the simulated exhaust. Higher NOx concentration up to 5000 ppm can result in a larger NOx to N2 rate. Moreover, a higher oxygen concentration promotes NO conversion. Complete oxidation of HCs can be achieved by adding silver to the LSC current collecting layer. The SOFCbased emissions control system can treat NOx and HCs simultaneously, and can be operated without consuming the anode fuel (a reductant) at near the engine exhaust temperature to eliminate the need for reductant refilling and extra heating.

1. INTRODUCTION The reduction of greenhouse gas emissions depends on increasing energy efficiency, at least in the near future. Most of the required increase in energy efficiency can be achieved easily by converting automotive gasoline engines to lean burn at maximum fuel efficiency,1 as automobiles with gasoline engines consume a very large fraction of the fuel used worldwide. However, the fuel efficiency of gasoline engines has been constrained by stoichiometric combustion owing to the use of the three-way catalytic converter to meet environmental regulations.1 This tension between global and local environmental protections has not yet been resolved because of the difficulty in controlling the emission of NOx (NO+NO2) at high concentration. Note that the highly efficient lean-burn engine is associated with internal combustion at high temperature with excess oxygen, and so its exhaust contains a high NOx concentration; this problem is especially severe for spark-ignited gasoline engines.1 Therefore, a technology for controlling highconcentration NOx emission is required to realize highly efficient lean-burn gasoline engines. The NOx concentration in the exhaust of an automotive gasoline engine with spark ignition can be as high as 4000 ppm.1 However, the exhaust of the lean-burn engines contains excess oxygen and so the three-way catalytic converter for the stoichiometric-burn engine cannot function to reduce NOx. Urea-based ammonia selective catalytic reduction (SCR) is one of the most promising methods for NOx removal from diesel engine exhaust.2,3 Nevertheless, the urea-SCR after-treatment system is quite complex and raises concerns regarding urea distribution infrastructure, potential freezing of the urea © 2012 American Chemical Society

solution, ammonia slip, and the inconvenience and cost of refilling urea. Therefore, every possible means of achieving a low NOx concentration has been used, including the extensive use of exhaust gas recirculation (EGR),4 which reduces the fuel efficiency. Therefore, a technology for controlling highconcentration NOx emission is also required to realize highly efficient diesel engines, which requires the deletion of EGR. Notably, EGR deletion can lower the cost of the diesel-fueled automobile by simplifying its engine and the associated aftertreatment system. The method of electrochemical NO reduction without a reducing agent has been studied extensively to reduce NOx.5 However, the process of electrochemical NO reduction is based on oxygen pumping and is therefore performed with an applied current; the current efficiency is usually only a few percent because simultaneous O2 reduction consumes a substantial proportion of the electrical current.5 Thus, electrochemical NO reduction using an applied current is not an efficient means of treating an exhaust that contains excess oxygen. Notably, when a current (voltage, potential) is applied to a system of porous metal electrodes interfaced to a solid electrolyte, the effect of electrochemical promotion of catalysis (EPOC) can be observed.6 Surface science investigations have shown that the physical basis for the EPOC effect lies in the electrochemically induced spillover of oxygen onto the surface of the metal Received: Revised: Accepted: Published: 2324

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process.18 Suitable amounts of reagent-grade metal nitrates La(NO3)3·6H2O (Stream chemicals, USA), Sr(NO3)2 (Sigma, ST. Louis, MO) and Co(NO3)2·6H2O (Strem Chemicals, Newburyport, MA) were dissolved in deionized water. Glycine (Sigma) was also dissolved in deionized water. Then, these two solutions were mixed together to yield a glycine to NO3− ratio of 0.8:1. The mixture was heated at 110 °C with stirring until combustion occurred. The product was ground into a powder. The powder was calcined by heating to 500 °C and held for 2 h, then to 900 °C and held for 4 h, before being slowly cooled to room temperature. In this work, the powder was always heated in air at 5 °C min−1. GDC with composition Ce0.9Gd0.1O2 was prepared by coprecipitation. The details of the method have been described elsewhere.19 The GDC powders were calcined by heating to 900 °C, and held for 4 h before cooling. LSC−GDC composite was prepared by mixing the aboveprepared LSC and GDC powders in a LSC:GDC weight ratio of 2:1. The mixture was ground for 24 h, and then calcined by heating to 500 °C and held for 2 h, and then to 800 °C, held for 4 h before cooling. LSC−V was prepared by adding vanadium to LSC using impregnation. The V cation solution was prepared by dissolving NH4VO3 (Merck, Germany) in deionized water. After drying, the powders were calcined by heating to 800 °C, and held for 4 h before cooling. Notably, after calcination at 800 °C when O2 can be dissociated to oxygen atoms over V-added LSC, V can be fully oxidized to V2O5.20 The weight of V2O5 was 1% of that of LSC. The composite of LSC−V and GDC (denoted LSC−V−GDC) was prepared by mixing the above-prepared LSC−V and GDC powders in a (LSC−V):GDC weight ratio of 2:1 and was calcined by a procedure the same as that of LSC− GDC composite. 2.2. Construction of SOFC Unit Cell. A disk of anodesupported bilayers of yttria-stabilized zirconia (YSZ) and Ni− YSZ (NexTech, Lewis Center, OH) was utilized to make an anode-supported cell. The cathode side of the bilayers was spincoated with a thin interlayer of LSC−GDC to enhance adhesion; the coated cell was dried at 50 °C for 6 h, then heated in air at 10 °C min−1 to 500 °C and held for 2 h, and then heated at 5 °C min−1 to 1250 °C and held for 2 h before cooling. Then, the cell was spin-coated with LSC−GDC as the cathode functional layer; the thus-coated cell was dried at 50 °C for 6 h, then heated at 10 °C min−1 to 500 °C and held for 2 h, and then heated at 5 °C min−1 to 900 °C and held for 2 h. Then, the cell was spin-coated with LSC as the current collecting layer; the heat treatment was the same as that for the cathode functional layer. The SOFC unit cell thus prepared had a configuration of Ni−YSZ/YSZ/LSC−GDC/LSC−GDC/ LSC (denoted LSC cell). The thus-prepared cell was sealed in an electrochemical reactor, with a schematic diagram shown in Figure 1A. The cathode area was always 1 cm2. The other SOFC unit cell was prepared by the same procedure using LSC−V−GDC as the cathode material. For the current collecting layer of this cell, silver was added to the LSC (denoted LSC−Ag). LSC−Ag was prepared by adding silver using the same procedure as that of LSC−V except that AgNO3 was used as the precursor; the weight of Ag was 1% of that of LSC. This cell had a configuration of Ni−YSZ/YSZ/ LSC−GDC/LSC−V−GDC/LSC−Ag (denoted LSC−V/Ag cell). 2.3. Activity Test. The activity test was performed at 450 °C with the SOFC operating at open circuit. The anode gas was pure

electrodes. Electrostatic interactions in the double layer formed at the catalyst−gas interface are a key element of promotion in catalysis.7 The catalytic activity and selectivity of metals interfaced with solid electrolytes can be altered dramatically and reversibly via potential application. The increase in catalytic rate can be several orders of magnitude higher than that anticipated from Faraday’s Law.8 Therefore, electrochemical NO reduction may be performed in a way differently from the above-mentioned oxygen pumping. Simultaneous NOx reduction and power generation using solid oxide fuel cells (SOFCs) has been shown to be feasible;9−17 this method is denoted SOFC-DeNOx. Nevertheless, although SOFCs can generate an electrical current during the reduction of NOx, operation at the temperature of the engine exhaust, currently below 450 °C, would generate very little electricity and is therefore inefficient for power generation; otherwise, the operating temperature must be increased by extra heating. In addition, the consumption of the anode fuel increases cost and imposes the inconvenience of refilling the anode fuel if it differs from the automotive fuel. Therefore, an altenative SOFC operation at the exhaust temperature and open circuit is sought. Such an SOFC operation should require no extra heating and consume no anode fuel; it should support applications onboard automobiles without refilling the anode fuel; in this setup, the anode fuel acts merely as a reductant to generate the open-circuit voltage and can be enclosed in the anode side with or without circulation. This work primarily studies such operation to control highconcentration NOx emission. Since hydrocarbons (HCs) are usually formed in automotive gasoline engines owing to cylinder wall quenching and other effects,1 HCs must be treated in an exhaust after-treatment system. Therefore, simultaneous HCs emission control is also studied. In this work, an SOFC unit cell is constructed with Ni−YSZ as the anode, YSZ as the electrolyte, and the composite of La0.6Sr0.4CoO3 (LSC) and Ce0.9Gd0.1O1.95 (GDC), with or without vanadium added to LSC, as the cathode. Note that LSC is a well-known cathode material and GDC is a wellknown electrode material for intermediate-temperature SOFCs. Experimental results indicate that very high concentration of NOx can be treated using the SOFCs operated at open circuit and 450 °C, which is the operating temperature of lowtemperature SOFCs. The SOFC-based NOx emission control system can function without consuming any anode fuel (a reductant) and hence can be care free. It can also function at near the exhaust temperature and hence requires no extra heating. Complete oxidation of HCs, in terms of propylene conversion, can be achieved by adding silver to the LSC current collecting layer. Therefore, high-concentration NOx and HCs can be removed from the exhaust simultaneously by the lowtemperature SOFCs at open circuit. The existing SOFC technologies, already well-established and commercialized, can be fully used for emissions control of automobiles with leanburn engines. A care-free exhaust after-treatment converter that is based on SOFCs can be immediately implemented onboard automobiles.

2. EXPERIMENTAL SECTION 2.1. Preparation of Cathode Materials. There were two types of cathode materials used in this work: one was La0.6Sr0.4CoO3‑δ (LSC) and the other was vanadium-impregnated LSC (denoted LSC−V); both were composites with gadoliniadoped ceria (GDC). LSC was prepared by the glycine-nitrate 2325

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Figure 2. Variation of NO conversion and NOx to N2 rate with inlet NOx concentration in the LSC−V/Ag cell.

related to the NOx conversion to N2 in percent by: NOx to N2 rate =0.5 × (NOx conversion to N2 in percent) × (inlet NOx in ppm) × (flow rate in mL min−1)/(cathode area in cm2 × 0.082 × 298 × 1011); the NOx conversion to N2 is defined as [(inlet NOx − outlet NOx)/(inlet NOx)], which is based on the fact that no formation of N2O has been detected in the tests of this work. It is noted that a control experiment with helium (an inert gas) as the anode gas has been performed in a previous work;16 the result has shown that the DeNOx rate with pure hydrogen as the anode gas at open-circuit voltage is larger than that with helium as the anode gas. The NO conversion increases with the NOx concentration; however, the NOx to N2 rate shows a maximum at a NOx concentration of around 5000 ppm. Note that, at a NOx concentration of over 5000 ppm, the NO conversion increases as the NOx to N2 rate decreases, indicating an increase in the extent of NO oxidation to NO2. Although NO2 can be more efficient than O2 in oxidizing HCs and particulate matter, since NO2 is a stronger oxidant than O2,23 its presence may not be needed. Figure 3 shows that, in the LSC cell,that is, with LSC

Figure 1. Schematic diagrams of (A) the experimental electrochemical reactor and (B) simultaneous NOx and HCs treatments by the solid oxide fuel cell at open circuit.

hydrogen. The inlet cathode gas consisted of 10% H2O and 10% CO2 always, 14% O2 (except where noted otherwise), various concentrations of NO x, and the balance helium. This composition of the cathode gas was similar to that of leanburn engine exhausts, as reported elsewhere.21,22 For designated tests, 300 ppm C3H6 and/or 35 ppm SO2 were added to the cathode gas. The overall flow rate of either the anode or the cathode gas was always 150 mL min−1. Possible reactions of NOx and HCs are indicated in Figure 1B, as will be clarified later; these reactions are the targets for the application of this SOFC operation at open circuit. The tests were conducted by introducing a designated gas mixture to the cathode side of the SOFC unit cell. The voltage and current were measured by Digital Multi-Meter (PROVA 803, Prova Instruments, Taiwan). The open-circuit voltage was always measured during the test. The current was monitored and always showed a zero value indicating that the cell is indeed at open circuit. The NO and NO2 contents in the outlet cathode gas were measured by NO and NO2 analyzers (NGA 2000, Emerson, Germany), respectively. The N2 and C3H6 contents were measured by gas chromatographs (China Chromatography 8900, Taiwan) that were equipped with a thermal conductivity detector and a flame ionization detector, respectively. The CO content was measured by a CO NDIR analyzer (nondispersive infrared analyzers, Beckman 880).

Figure 3. Variation of NO conversion and NOx to N2 rate with inlet NOx concentration in the LSC cell.

3. RESULTS AND DISCUSSION 3.1. Effect of NOx Concentration. Figure 2 plots the variation of the NO conversion and the NOx to N2 rate with the inlet NOx concentration over the cathode of the LSC−V/ Ag cell. Notably, the NOx to N2 rate, in μmol N2 min−1 cm−2, is

as the cathode material without impregnating vanadium, both the NO conversion and the NOx to N2 rate become maximal at NOx concentration of around 5000 ppm. Thus, the behavior of the NOx to N2 rate is the same in either the LSC−V/Ag cell or the 2326

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LSC cell. Nevertheless, the NOx to N2 rate of the LSC cell becomes much larger than that of the LSC−V/Ag cell when the NOx concentration is larger than 2000 ppm. However, the NO conversion via the LSC−V/Ag cell can be larger than that via the LSC cell when the NOx concentration goes up to 9000 ppm. Notably, the lean-burn hydrogen-fueled internal combustion engines usually have NOx emissions of the order of 10 000 ppm when the air to hydrogen equivalence ratio is 1.3.24 The NOx concentration in the exhaust of the lean-burn gasoline engine with spark ignition can reach 4000 ppm.1 In this work, the condition for “lean-burn gasoline engine” means that with just enough excess air to achieve maximum fuel efficiency, but does not means that with too much excess air to result in a lowering of the exhaust temperature. This “lean-burn” with maximum fuel efficiency results in high combustion temperature and high NOx emission.1 Below 4000 ppm NOx, the NOx removal behavior in the LSC−V/Ag cell is the same as that in the LSC cell: both the NO conversion and the NOx to N2 rate increase with NOx concentration. However, at around 4000 ppm NOx, both the NO conversion and the NOx to N2 rate in the LSC cell are much larger than those in the LSC− V/Ag cell. Therefore, the LSC cell is better than the LSC−V/Ag cell at treating the exhaust of lean-burn gasoline engines. This difference is attributable to the fact that the added V species are not as active as LSC in NOx conversion. Therefore, LSC should be used as the cathode material without impregnated vanadium. Since the exhaust from the lean-burn gasoline engine may contain NOx with a concentration as high as 4000 ppm,1 only the SOFC-DeNOx technology can provide a solution for its NOx emssion control. The effective NOx emission control with the SOFC at open circuit shown in this work can increase the applicability of this technology for automobiles  that is, the SOFC-DeNOx device becomes care free when the anode fuel (the reductant) is enclosed in the anode side. Nevertheless, although the NO conversion can reach around 90% when 5000 ppm NOx is presented in the simulated exhasut, Figure 3 only present the results of a feasibility study. Further studies of cathode materials are expected to result in better NOx activity. Nevertheless, Figure 3 shows that the NOx to N2 rate will become constant when the NOx concentration becomes relatively small. This constant DeNOx rate has been reported previously,17 at a fixed voltage of 0.6 V; both NO and NOx conversions increase with decreasing NO concentration in this relatively low NOx concentration range; thus, 100% conversion can be achieved. Notably, the above-mentioned fixed voltage is a self-generated voltage, which is generated in the SOFCs during close-circuit operation. The voltage as observed in the present study was the open-circuit voltage, which was constant during the DeNOx operation. Figure 4 shows that, when 900 ppm NO passes the LSC cell, both the NO conversion and the NOx to N2 rate increase with O2 concentration from 6 to 14%. This finding indicates a promotion effect of oxygen, which has also been observed in the catalytic NO decomposition at 400 °C but without the presence of H2O and CO2.25 The promotion effect on SOFCDeNOx at open circuit may be attributed to the fact that the generated open-circuit voltage increases with O2 concentration;19 notably, the NOx conversion increases with voltage.13,16 Nevertheless, the correlation of the DeNOx rate with the experimental voltage is quite irregular.16 Therefore, Figure 4 shows the correlation of experimental DeNOx rate with the O2 concentration instead of the open-circuit voltage. The correlation of the DeNOx rate with the open-circuit voltage

Figure 4. Variation of NO conversion and NOx to N2 rate with inlet O2 concentration in the LSC cell. 900 ppm NOx in the cathode gas.

needs further studies for clarification. Since the simulated exhaust in Figure 4 contains no HCs or other reductant and no oxygen is transported from the cathode to the anode when the SOFC is operated at open circuit, NO reaction should be direct NO decomposition to N2 and O2; this confirms the proposed NO reaction shown in Figure 1B. It is noted that the NOx to N2 rate can be around 3 orders of magnitude larger than those achieved over conventional oxide catalysts of NO decomposition. In preliminary tests of this work, a comparison of the NOx to N2 rates between the LSC cell at open circuit and the conventional catalytic reactor using exactly the same LSC− GDC composite as the catalyst has shown such promotion effect. This promotion effect at open circuit is considered to be similar to that at a fixed voltage.16 It is the electrochemical enhancement of NO decomposition, but without the transport of oxygen from the cathode to the anode. It is noted that the SOFC at open circuit does not generate an electrical current and does not consume any anode fuel. The promotion of the rate of NO decomposition at open circuit is caused only by the generated open-circuit voltage. 3.2. Simultaneous Oxidation of Propylene. A highly efficient lean-burn gasoline engine should generate a relatively small amount of CO and particulate matter.1 The remaining pollutants are a high-concentration NOx and some HCs; the latter are produced at least by cylinder wall quenching since engine cooling is needed. Therefore, the need to remove NOx and HCs from the engine exhaust remains for emissions control. Figure 5A shows that, in the LSC cell, the NO conversion increases with the addition of propylene to the cathode gas. It is noted that the activity of C3H6 is usually used to represent that of HCs in the engine exhaust. Therefore, the presence of C3H6 promotes the conversion of NOx in the exhaust. This is attributed to hydrocarbon reduction of NO in a process that is similar to that in C3H6−SCR.26 Notably, Figures 1A and B show that hydrocarbon is oxidized over the SOFC cathode simultaneously with NOx decomposition. When the SOFC is operated at open circuit, there is no generation of the electrical current and thus no formation of the oxygen ion to be transported from the cathode to the anode. Therefore, the mechanism over the cathode of SOFC at open circuit is considered to be similar to that of catalytic reactions at the gas/ solid interface. The reaction between NOx and C3H6 is further verified by Figure 5B, which reveals that, in the LSC cell, the C3H6 2327

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This indicates that C3H6 is oxidized mostly by the O species that are formed by the dissociation of O2,13,26 according to:

O2 → 2O

(5)

In fact, reactions 3 and 4 should occur regardless of the source of the O species. Notably, the concentration of oxygen is very much larger than that of NOx; thus, the extent of reaction 5 should be much greater than those of reactions 1 and 2. Therefore, excess oxygen in the engine exhaust promotes not only NO conversion but also C3H6 conversion. Figure 6 shows that, in the LSC−V/Ag cell, when the NOx concentration decreases from 968 to 400 ppm, the C3H6

Figure 6. Profiles of NO and C3H6 conversions with time in the LSC− V/Ag cell.

conversion increases to almost 100%. Thus, HCs can be removed almost completely; this indicates that the presence of NOx inhibits C3H6 conversion, possibly because the adsorptivity of NOx is stronger than that of C3H6. This finding also confirms that C3H6 is oxidized mostly by the O species that are formed by O2 dissociation, as mentioned above. The presence of 35 ppm SO2 does not affect the complete oxidation of propylene. The above results demonstrate that the SOFC-based NOx emission control system can be operated close to the exhaust temperature without consuming any anode fuel. Complete oxidation of HCs, in terms of propylene conversion, can be achieved simultaneously. Therefore, high-concentration NOx and some HCs can be simultaneously removed from the engine exhaust using low-temperature SOFCs. Notably, a high concentration of NOx and some HCs are pollutants in the exhaust of highly efficient lean-burn gasoline engines. The simultaneous control of NOx and HCs emissions has been demonstrated to be effective using SOFCs at open circuit. Since existing SOFC technologies on both materials development and devices manufacturing are well-established and commercialized, they can be fully utilized for emissions control of automobiles. A care-free exhaust after-treatment converter that is based on SOFCs can be immediately implemented onboard automobiles. The novel technology of SOFC-DeNOx at open circuit has been shown to be effective for simultaneous NOx and HCs emissions control using a low-temperature SOFC operated at near the engine exhaust temperature to eliminate the need for extra heating. Higher NOx concentration up to 5000 ppm can result in a larger NOx to N2 rate. The cathode material can

Figure 5. Effect of inlet NOx concentration in the LSC cell on (A) NO conversion and (B) C3H6 conversion in association with outlet CO concentration. Base case in (A): 900, 1800, and 2900 ppm NOx, respectively, plus 10% H 2 O, 10% CO 2 and 14% O 2 . C 3 H 6 concentration was 300 ppm if added.

conversion increases with NOx concentration. Note that the reaction of NO starts with its dissociation,11,26 according to NO → N + O

(1)

as does that of NO2,14 according to NO2 → NO + O

(2)

This latter dissociation is followed by NO dissociation to produce additional O species. Then, the O species reacts with C3H6 by full oxidation to form CO2,26 according to C3H6 + 9O → 3CO2 + 3H2O

(3)

or by partial oxidation to form CO, according to C3H6 + 6O → 3CO + 3H2O

(4)

The occurrence of reaction 4 is supported by the results concerning the outlet CO concentration that are shown in Figure 5B. It can be seen that the extent of the partial oxidation of propylene exceeds that of the full oxidation. Notably, the formed CO can be removed by simultaneous CO and NOx removal using the SOFCs.15 The results of Figure 5 can reveal that the NO conversion is not equivalent to the C3H6 conversion but is much smaller. 2328

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(11) Huang, T. J.; Chou, C. L. Effect of voltage and temperature on NO removal with power generation in SOFC with V2O5-added LSCF−GDC cathode. Chem. Eng. J. 2010, 160, 79−84. (12) Huang, T. J.; Chou, C. L. Effect of temperature and concentration on reduction and oxidation of NO over SOFC cathode of Cu-added (LaSr)(CoFe)O3−(Ce,Gd)O2‑x. Chem. Eng. J. 2010, 162, 515−520. (13) Huang, T. J.; Hsiao, I. C. Nitric oxide removal from simulated lean-burn engine exhaust using a solid oxide fuel cell with V-added (LaSr)MnO3 cathode. Chem. Eng. J. 2010, 165, 234−239. (14) Huang, T. J.; Wu, C. Y.; Wu, C. C. Effect of temperature and concentration on treating NO in simulated diesel exhaust via SOFCs with Cu-added (LaSr)MnO3 cathode. Chem. Eng. J. 2011, 168, 672− 677. (15) Huang, T. J.; Wu, C. Y.; Wu, C. C. Simultaneous CO and NOx removal from simulated lean-burn engine exhaust via solid oxide fuel cell with La0.8Sr0.2Mn0.95Cu0.05O3 cathode. Electrochem. Commun. 2011, 13, 755−758. (16) Huang, T. J.; Wu, C. Y.; Lin, Y. H. Electrochemical enhancement of nitric oxide removal from simulated lean-burn engine exhaust via solid oxide fuel cells. Environ. Sci. Technol. 2011, 45, 5683− 5688. (17) Huang, T. J.; Wu, C. Y.; Wu, C. C. Lean-burn NOx emission control via simulated stack of solid oxide fuel cells with Cu-added (LaSr)MnO3 cathodes. Chem. Eng. J. 2011, 172, 665−670. (18) Chick, L. A.; Pederson, L. R.; Maupin, G. D.; Bates, J. L.; Thomas, L. E.; Exarhos, G. J. Glycine-nitrate combustion synthesis of oxide ceramic powders. Mater. Lett. 1990, 10, 6−12. (19) Huang, T. J.; Chou, C. L. Effect of O2 concentration on performance of solid oxide fuel cells with V2O5 or Cu added (LaSr)(CoFe)O3−(Ce,Gd)O2‑x cathode with and without NO. J. Power Sources 2009, 193, 580−584. (20) Kim, C. Y.; Bedzyk, M. J. Study of growth and oxidation of vanadium films on α-Fe2O3(0001). Thin Solid Films 2006, 515, 2015− 2020. (21) Li, L.; Chen, J.; Zhang, S.; Guan, N.; Wang, T.; Liu, S. Selective catalytic reduction of nitrogen oxides from exhaust of lean burn engine over in situ synthesized monolithic Cu−TS-1/cordierite. Catal. Today 2004, 90, 207−213. (22) Kašpar, J.; Fornasiero, P.; Hickey, N. Automotive catalytic converters: current status and some perspectives. Catal. Today 2003, 77, 419−449. (23) Shrivastava, M.; Nguyen, A.; Zheng, Z.; Wu, H. W.; Jung, H. S. Kinetics of soot oxidation by NO2. Environ. Sci. Technol. 2010, 44, 4796−4801. (24) Sopena, C.; Diéguez, P. M.; Sáinz, D.; Urroz, J. C.; Guelbenzu, E.; Gandía, L. M. Conversion of a commercial spark ignition engine to run on hydrogen: Performance comparison using hydrogen and gasoline. Int. J. Hydrogen Energy 2010, 35, 1420−1429. (25) Liu, Z.; Hao, J.; Fu, L.; Zhu, T. Study of Ag/La0.6Ce0.4CoO3 catalysts for direct decomposition and reduction of nitrogen oxides with propene in the presence of oxygen. Appl. Catal., B 2003, 44, 355− 370. (26) Kotsifa, A.; Kondarides, D. I.; Verykios, X. E. A comparative study of the selective catalytic reduction of NO by propylene over supported Pt and Rh catalysts. Appl. Catal., B 2008, 80, 260−270.

affect the NOx to N2 rate considerably. Moreover, a higher oxygen concentration promotes NO conversion. HCs can promote NOx conversion; however, the presence of NOx may inhibit HCs' oxidation; HCs are mostly oxidized by the O species from O2 dissociation. Complete oxidation of HCs, in terms of propylene conversion, can be achieved by adding silver to the LSC current collecting layer. It is potentially economically competitive with current emissions control technologies because it is based on current SOFC technology but can be operated close to the exhaust temperature at open circuit. The open-circuit operation makes it care free, that is, no consumption of the anode fuel (a reductant) and thus no refilling of the reductant. Notably, reductant refilling is required by current technology of urea-based SCR. Also note that SOFC-based auxiliary power unit (APU) has been commercialized. The low-temperature SOFC at open circuit can use the APU or can be manufactured with lower cost by deleting the current collecting parts. It is noted that the current device (SOFC) for this novel technology is more suitable for stationary power generation applications. However, the application of this technology can be extended from stationary emissions sources to “lean-burn engine” or even “lean-burn gasoline engine” by designing a more suitable device for the latter. A low-temperature SOFC operating at open circuit, as reported in this article, is a way to use the current device for the latter and also to show the general idea. Notably, one example of a technology extended successfully from stationary emissions sources to “lean-burn engine” (diesel engine) is selective catalytic reduction using ammonia.

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*Phone: +886 3 5716260; fax: +886 3 5715408; e-mail: tjhuang@ che.nthu.edu.tw.

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REFERENCES

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