Study on the Fuel-Reforming-Assisted Hydrocarbon Selective

Mar 22, 2010 - Technical Center, GM Daewoo Auto & Technology Co., Cheongcheon-dong, Bupyeong-gu,. Inchoen 403-714, Korea, and School of ...
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Ind. Eng. Chem. Res. 2010, 49, 3553–3560

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Study on the Fuel-Reforming-Assisted Hydrocarbon Selective Catalytic Reduction over Silver/Alumina Catalyst Jaewook Lee,† Soonho Song,*,‡ and Kwang Min Chun‡ Technical Center, GM Daewoo Auto & Technology Co., Cheongcheon-dong, Bupyeong-gu, Inchoen 403-714, Korea, and School of Mechanical Engineering, Yonsei UniVersity, Shinchon-dong, Seodaemoon-gu, Seoul 120-749, Korea

This study aims to assess the feasibility of fuel-reforming-assisted hydrocarbon selective catalytic reduction (HC-SCR) over 2 wt % Ag/Al2O3 catalyst. For this purpose, the effects of reduction agents and the presence of hydrogen on NOx reduction over silver catalyst were tested, and, after catalytic partial oxidation (CPOx) reforming performances were studied, the DeNOx performance of reformate-assisted HC-SCR was investigated. Although normal heptane/octane/dodecane showed better NOx reduction rates at 300 and 350 °C than the other C2-C4 range hydrocarbons, the performances were still unpractical at the temperature range similar to actual diesel exhaust. To enhance the low temperature activity of the Ag/Al2O3 catalyst, CPOx reforming was tested and applied in the SCR system. By the addition of CPOx products to the HC-SCR reactor, NOx reduction performance was increased to a great extent especially in the low temperature range between 200 and 400 °C. For the reductant condition of C/N ) 6 + CPOx/N ) 6 without water, about 52.5% of NOx reduction was recorded over C3H6-SCR at 300 °C, whereas nearly zero DeNOx efficiency was measured when only C3H6 was present in the reactants. Also, it was found that C/N ) 6 + CPOx/N ) 6 showed the better NOx reduction efficiency than C/N ) 3 + CPOx/N ) 9. Together with the hydrogen model gas tests results, it implies that there is an optimum hydrogen to hydrocarbon ratio in terms of low-temperature activity enhancement assuming that the total amount of fuel used in CPOx-assisted HC-SCR is the same. In addition, inhibition effect by water was reduced to an extent by CPOx products. C3H6-SCR was deactivated so seriously that no NOx conversions were measured at 300 and 400 °C in the presence of water vapor. However, through the addition of CPOx products to the C3H6-SCR reactants, NOx reduction was increased to 20% at 300 °C and 25% at 400 °C. 1. Introduction Modern turbocharged diesel engines operate with precisely controlled direct injection in excessive air at a high compression ratio so that it is possible to achieve higher thermal efficiency and lower CO2 emission than gasoline engines do. Also, the studies over the past decades have improved the engine performance and NVH (noise, vibration, and harshness) characteristics even further, which makes diesel-powered passenger cars quite common these days over the world. However, upcoming emission legislation such as Euro-6, that goes into effect in 2014, requires that NOx emission should be reduced by 55% from the values in Euro-5 in cars and light truck classes. Furthermore, the U.S. Federal Tier2 Bin5 NOx standard requires 65% more reduction of NOx from Euro-6 regulation. To comply with this stringent emission legislation, applying the catalystbased DeNOx systems in diesel exhaust seems unavoidable. Furthermore, the systems obviously should be able to perform NOx reduction reactions successfully at low temperatures and even in a lean environment having a substantial amount of oxygen.1,2 To meet the diesel DeNOx requirements, many different kinds of catalytic systems have been studied and proposed to date. Among them are urea-SCR (or simply SCR), HC-SCR, or lean NOx trap (LNT). Also, hybrids of these systems are being tried such as LNT + SCR, where the majority of engine-out NOx is reduced on LNT, and SCR helps the NOx reduction by using * To whom correspondence should be addressed. Tel.: 82-2-21232811. Fax: 82-2-312-2159. E-mail: [email protected]. † GM Daewoo Auto & Technology Co. ‡ Yonsei University.

the NH3 produced from LNT while the fuel-rich operating conditions aid the LNT regeneration.3 But from the perspective of compatibility or simplicity in vehicle integration, HC-SCR leads the others because it can use on-board diesel fuel as the reducing agent, and the fuel can be easily added into the diesel exhaust by in-cylinder postinjection or in-exhaust secondary injection. The early research about HC-SCR was conducted using a zeolite-based catalyst system as Ko¨nig et al. summarized.4 But concerns about the hydrothermal durability of zeolite catalysts5 have turned HC-SCR studies toward metal oxide catalysts as an alternative. Attention was given to PGM (platinum group metal) since these materials had been already used in conventional stoichiometric engine exhaust cleanup catalysts, and the stabilities and tolerance to typical poisons in the engine exhaust had been proven.6,7 But the important issue with the PGM catalyst is its higher yield of N2O from the SCR reaction, because N2O is another form of air pollutant emission impacting global warming.6,7 Meanwhile, Ag/Al2O3 has been considered as a strong candidate for HC-SCR as an alternative to PGM catalyst because it has high N2 selectivity in the SCR reaction with moderate NOx reduction efficiency. But in spite of extensive studies on Ag/Al2O3 catalysts, its NOx reduction performances still remain at an unpractical level in vehicle emission mode until now.8,9 One reason to which the low NOx conversion over silver catalyst is attributed is the low temperature that the catalyst experiences through the emission mode test. Klingstedt et al.9 have reported that the catalyst temperature on the cold-started NEDC cycle was mostly less than 200 °C, where the silver catalyst is normally inactive. Also, the dependency of Ag/Al2O3 catalytic activity on the hydrocarbon’s molecular structure might affect

10.1021/ie9015528  2010 American Chemical Society Published on Web 03/22/2010

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Figure 1. A schematic of CPOx-assisted HC-SCR test bench.

the insufficient NOx conversion to an extent. In regards to the low temperature activity enhancement of Ag/Al2O3, it has been reported that the coexistence of hydrogen and hydrocarbons in the reactants over Ag/Al2O3 catalyst highly promoted the SCR reaction.10-14 Satokawa et al.10 reported that when hydrogen and hydrocarbons are present in the SCR reactants, low temperature activity was significantly improved and the maximum NOx reduction rate was highly increased as well. However, for DeNOx promotion over Ag/Al2O3 by H2 to be viable in the vehicle, on-board H2 production from diesel fuel should be available to supply the aftertreatment with sufficient amount of hydrogen. For the production of H2, the fuel reforming process is necessary to convert hydrocarbon fuels to hydrogen and carbon monoxide, and the most representative reforming processes are partial oxidation (POx), steam reforming (SR), autothermal reforming (ATR), and plasma reforming.15 Also, there has been a study about exhaust gas fuel reforming (EGFR) that replaces some of hydrocarbon fuel with hydrogen for the purpose of engine performance and emission improvements.16 In view of design impact on conventional diesel exhaust, the EGFR system seems to be more feasible than the others because EGFR can work with on-board fuel and exhaust gas. Also, the presence of water in exhaust may facilitate the reforming reaction of EGFR so that the hydrogen concentration after the reaction could be even higher than with the POx because of the ATR-like reaction of fuel, oxygen, and water. In this study, the catalytic POx (CPOx) reforming reaction involving only oxygen and hydrocarbons was tested as a hydrogen production measure. It was expected that there would be an additional effect of CPOx producing olefinic or partially oxygenated hydrocarbons as a byproduct,17,18 which would enhance DeNOx over Ag/Al2O3 even further. This study aims to investigate the overall characteristics of HC-SCR over the Ag/Al2O3 catalyst and to promote the reaction by reforming hydrocarbons. For this purpose, a fixed-bed flow reactor system was designed and tested for HC-SCR and CPOx to investigate the overall operating characteristics of CPOx and HC-SCR over Ag/Al2O3 and the promotion effects of CPOx reformate on the SCR reactions by coupling two different catalyst systems. Also in the CPOx-assisted HC-SCR tests, total carbon to NO ratio was controlled to be the same to investigate what constitution of hydrocarbon and CPOx products shows the better NOx reduction at low temperature range.

2. Experiment 2.1. Catalyst. As a HC-SCR catalyst, a 2 wt % Ag/Al2O3 catalyst was tested. The silver catalyst was originally in a 400 cpsi honeycomb monolith form and was cut to the size of 19 mm wide × 15 mm long, and the approximated volume of the catalyst was 2.8 cm3. Sliver loading of the catalyst was selected because it has been widely accepted that 2 wt % of silver-loaded catalyst gives optimum performance.19-21 For the fuel reforming, the diesel fuel cracking (DFC) catalyst was supplied from Heesung Catalyst Co., Korea, and it was made by washcoating PGM (0.5 wt % Rh with the metal oxides of CeO2 and ZrO2)based metal oxides on a ceramic honeycomb monolith. A 400 cpsi catalyst brick was cut out to have a volume of ca. 1.4 cm3, assembled between the upstream and downstream blank monolith, and wrapped with 1/36-in. thick alumina paper to be placed within the quartz tube. 2.2. Experimental Setup. To assess the effects of fuel reforming on the enhancement of DeNOx performance over Ag/ Al2O3 catalyst, a CPOx reforming reactor and a HC-SCR reactor were installed in parallel as shown in Figure 1. In the CPOx reactor (left part of the Figure), 1.15 cm3 of DFC catalyst was placed in the middle of a 2 cm O.D. × 50 cm long quartz tube. Liquid hydrocarbons that were supplied to the reactor by using a syringe pump were partially oxidized in fuel-rich conditions to produce incomplete combustion products such as carbon monoxide, hydrogen, fuel fragments, and so forth. To prevent liquid droplet formation and its wetting along the inner wall of the quartz tube, supplied fuel was delivered into the reactor through the separately inserted 1/8-in. tube with 0.5 L/min pure N2 gas as carrier. The tip of the tube was located at about 7 cm upstream of the CPOx catalyst, and there were six 0.5 mmdiameter nozzle holes in the radial direction at the tip of the tube. By using this fuel supply system, possible prereaction which may take place before the reactants make contact with the catalyst could be prevented, and the homogeneous mixing of fuel and environmental reactants could be achieved successfully. When CPOx/HC-SCR coupled tests were being conducted, a little amount of CPOx products were bypassed to the HCSCR reactor. To make a slight pressure difference between the CPOx product line and the HC-SCR reactant line, a backpressure valve and a manometer were installed in the ventilation tube of CPOx. By closing the backpressure valve, the gas pressure at the inlet of MFC-3 was increased to 3.5-4.0 kPa. Since the water or the heavy fuel fragments that were not or incompletely reacted might cause unexpected control problems with the MFC

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when condensation took place, a NaCl-ice bath was installed upstream of MFC-3 to cool down the CPOx products in a bath temperature of around -4 °C. CPOx products which predominantly consisted of hydrogen, CO, and fuel fragments were then supplied to the inlet of the HC-SCR reactor. In the HC-SCR reactor side, O2/N2/NO gases were controlled by using MFC. When the hydrocarbons that consisted of the number of carbons per molecule equal to or less than 4 were used as the reducing agent, another MFC was installed to control the total gas flow rates. Otherwise, for the other alcoholic and nonalcoholic hydrocarbons that are liquid phase at room temperatures, a syringe pump was used to compose the required C/N ratio conditions. The syringe pump injected the required amount of liquid hydrocarbons into the N2/NO mixture, which was heated up to 250 °C to avoid possible condensation through the line. Meanwhile, pure oxygen was not mixed with other reactants but introduced into the reactor at 120 mm upstream of the catalyst through a separately installed 1/8-in. tube to prevent any premature reaction. 2.3. Reaction Condition and Test Procedure. For the tests of HC-SCR over 2 wt % Ag/Al2O3, the effects of the reducing agent, H2, and the presence of water vapor were investigated at a gas hourly space velocity (GHSV) of 50 000 h-1. GHSV was defined as the ratio of reactant gas flow rates at 25 °C and 1 atm to the volume of catalyst. Hence, for a given catalyst volume of 2.8 cm3, the total flow rate of reactant was controlled to 2.35 L/min. Given the NO concentration of 200 ppm, hydrocarbons as the reducing agents were controlled as 1200 ppm C1 to have the same C/N ratio of 6. The reaction temperature was controlled from 550 to 200 °C with a fixed temperature interval based on the thermocouple located at the catalyst inlet. At each reaction temperature, the SCR reaction lasted for at least 20 min in order for the emissions to be stabilized. After the measurements at 200 °C, the silver catalyst was exposed to 2 L/min of 10% O2 in N2 balance at 550 °C for 1 h to remove any deposited carbon or adsorbed reactants. H2 promotion effects were investigated with different H2/N ratios of 0, 1, 3, 5, and 10 which corresponded to 0, 200, 600, 1000, 2000 ppm H2 since all the tests were conducted with 200 ppm of NO. Meanwhile, the CPOx reaction was induced with n-C12H26 with a C/O ratio of 0.95. A C/O ratio of 0.95 was selected from the CPOx tests because the tests showed the maximum hydrogen at that C/O ratio. The GHSV of the CPOx reaction was controlled to be 100 000 h-1, and the concentration of O2 was 10% in the N2 balance. In the tests where CPOx products were not introduced, the amount of hydrocarbon addition was controlled on the basis of the C/N ratio. Alternately, for the condition of the simultaneous addition of hydrocarbons and CPOx products, the total amount of reducing agent added was controlled on the basis of the CO+CO2 concentration after the catalyst at 550 °C, since the hydrocarbon and CPOx products were presumed to be completely oxidized at 550 °C. The concentration of CO+CO2 was 2400 ppm which means the CPOx product flow rate was controlled to have 1800 ppmC when 600 ppmC C3H6 was added, and this condition was referred to as C/N ) 3 + CPOx/N ) 9 in this study. Such a limitation in the amount of total CO+CO2 emission was assumed because both the hydrocarbons and CPOx products must come from on-board fuel and, therefore, their total concentration should be confined within the fuel penalty tolerance. By this assumption, the reductant conditions tested in this study were C/N ) 12 + CPOx/N ) 0, C/N ) 9 + CPOx/N ) 3, C/N ) 6 + CPOx/N ) 6, C/N ) 3 + CPOx/N ) 9, and C/N ) 0 + CPOx/N ) 12.

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Figure 2. A polar diagram of NOx conversions at 300 °C (white symbols), 350 °C (gray symbols), and 400 °C (black symbols) from HC-SCR over Ag/Al2O3 with various reducing agents. Conditions: NO ) 200 ppm, reductant ) 1200 ppmC, O2 ) 5%, N2 as balance, SV ) 50 000 h-1.

2.4. Reactants and Gas Analysis. In the CPOx reaction, n-C8H18, iso-C8H18, and n-C12H26 were tested as the representative hydrocarbons of automotive gasoline and diesel fuel. Also, pure O2 and N2 mixture, and/or deionized pure water were used to simulate diesel exhaust gas conditions. As the reducing agent for the HC-SCR reaction, nine nonalcoholic hydrocarbons and three alcohols were tested. Nonalcoholic hydrocarbons were C2H2, C2H4, C3H6, C3H8, n-C4H10, n-C7H16, n-C8H18, i-C8H18 (2,2,4-TMP), and n-C12H26. Alcohols were CH3OH, C2H5OH, and C3H7OH. For all the SCR reactions, the NO concentration was 200 ppm, and the oxygen concentration was 5% in the pure N2 balance. To examine the effect of the presence of water on the SCR reaction, deionized water was injected into the reactor to make up a reactant condition comprising 5 vol % water. H2 produced over CPOx reaction was measured by using 2000A-EU TCA (Thermal Conductivity Analyzer) of Teledyne Analytical Instrument. Hydrocarbons, NOx, COx, and the other gaseous components were analyzed by using Nicolet Antaris IGS of Thermo System Co., which is an FT-IR analyzer specially designed for an internal combustion engine exhaust application. Before the products were sampled for the measurements, sampled flow was fed to a NaCl-ice cold bath and then to a particle filter to remove water or incomplete hydrocarbon fragments. 3. Results 3.1. Overall Characteristics of HC-SCR over Ag/Al2O3. 3.1.1. Effects of Hydrocarbon’s Structure. Figure 2 shows the NOx reduction efficiencies for the tested 12 hydrocarbons at 300, 350, and 400 °C reaction temperatures. As can be seen from the figure, ethanol-SCR showed the best NO reduction performance among the others. It showed a quite constant NOx conversion rate higher than 70% at all temperatures. In nonalcoholic hydrocarbons, acetylene (C2H2) showed a better performance than the others. C2H2-SCR over Ag/Al2O3 was activated from a temperature lower than 250 °C, and around 60% of the maximum NOx conversion was achieved at 450 °C.

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Figure 3. Effects of H2/N ratio on NOx conversion performance of C3H6SCR over Ag/Al2O3. Conditions: 200 ppm NO, 1200 ppmC1 C3H6, 5% O2, 0 ppm (b), 200 ppm (3), 600 ppm (9), 1000 ppm (]), 2000 ppm (2) H2, N2 as balance.

Figure 4. Effects of H2/N ratio on C3H6 oxidation to COx (CO and CO2) over Ag/Al2O3. Condition: 200 ppm NO, 1200 ppmC1 C3H6, 5% O2, 0 ppm (b), 200 ppm (3), 600 ppm (9), 1000 ppm (]), 2000 ppm (2) H2, N2 as balance.

Compared to light hydrocarbons having four or less carbons per molecule, midrange hydrocarbons such as n-heptane (C7H16), n-octane (C8H18), and n-dodecane (C12H26) showed better performance at the low temperature of 350 °C. With regard to the effect of the hydrocarbons’ nature, it has been reported that the activity of linear heavy alkane is better than that of isoalkane and unsaturated hydrocarbons than saturated ones as reducing agents for HC-SCR.20,21 Such characteristics were also confirmed in this study when various kinds of saturated, unsaturated, and alcoholic hydrocarbons were used as a reducing agents for the NO-SCR reaction over the Ag/Al2O3 catalyst. 3.1.2. Effects of Hydrogen. Figure 3 and Figure 4 show the NOx conversion rates for C3H6- and n-C8H18-SCR with varying H2/N ratios and a fixed amount of hydrocarbons of C/N ) 6. As the figures indicate, maximum NOx conversion rates and the low temperature activity were improved simultaneously to a great degree in the presence of hydrogen. The extent of improvement increased with increasing H2/N ratio. For the C3H6SCR over Ag/Al2O3, the activity at low temperature range between 200 and 400 °C was dramatically improved, but the high temperature activities over 400 °C remained almost similar to that in the absence of hydrogen. These results are consistent

Figure 5. Effects of H2/N ratio on NOx conversion performance of C8H18SCR over Ag/Al2O3. Conditions: 200 ppm NO, 1200 ppmC1 C8H18, 5% O2, 0 ppm (b), 200 ppm (3), 600 ppm (9), 1000 ppm (]), 2000 ppm (2) H2, N2 as balance.

with the results reported by Burch et al.11 and Arve et al.12 In their tests with C8H18 as reducing agent, the cofeed of hydrogen promoted NOx reduction efficiency to a great degree at a temperature below 450 °C. Such promotion effects by hydrogen are thought to be related to the oxidation of hydrocarbons. Shibata et al.14 suggested that the hydrocarbon activation process is accelerated in the presence of hydrogen, which results in the enhancement of low temperature activity of Ag/Al2O3 catalyst. C3H6 conversion shown in Figure 4 can be understood as the result of hydrocarbon activation promotion by hydrogen. Burch et al.11 also showed a similar C8H18 conversion in the presence of hydrogen. But the promotion effect of hydrogen seems to be confined within the particular temperature range between 200 and 400 °C because the DeNOx enhancement in C8H18-SCR only occurred also at the same temperature range as C3H6-SCR as shown in Figure 5. These results are thought to be related to the hydrogen availability in the reactants. Referring to Burch et al.,11,15 hydrogen tends to be oxidized at much lower temperatures than hydrocarbons, so that it can be presumed that hydrogen would not be available at higher temperatures resulting in negligible DeNOx promotion. Considering such a promotion effect of H2 on HC-SCR, it should be taken into account that hydrogen also comes from fuel only. Assuming that 100% of the hydrogen atoms in n-C12H16 fuel were converted to H2 by the reforming reaction in the absence of water, the amount of additional hydrocarbons required to make a H2/N ratio of 10 is equal to the amount for a C/N ratio of 9.23. Therefore, the concentration of H2 should be optimized, together with hydrocarbons, as to not cause a significant fuel penalty. 3.2. CPOx Performance. To investigate the effects of different fuel on reforming performances, i-C8H18 (2,2,4-TMP), n-C8H18, and n-C12H26 were tested at a GHSV of 100 000 h-1 with a different C/O ratio in the absence of water. As shown in Figure 6, i-C8H18 CPOx resulted in higher H2 yields and fuel conversions than the other fuels. Hydrogen selectivity of i-C8H18 CPOx was as high as ca. 78% at a C/O ratio of 1.0-1.2, and the fuel conversion was higher than 80% up to a C/O ratio of 1.2. Such a superiority of i-C8H18 fuel in hydrogen yields also has been reported by Williams et al.22 and Subramanian et al.23 On the other hand, normal hydrocarbons such as n-C8H18 and n-C12H26 showed quite lower H2 selectivity and fuel conversion

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Figure 6. Hydrogen selectivities of iso-C8H18 (2,2,4-TMP) (b), n-C8H18 (3), and n-C12H26 (9) CPOx over DFC as a function of C/O ratio at SV ) 100 000 h-1.

rates. The maximum H2 selectivity was around 70% at a C/O ratio of 1.0 for n-C8H18 and 64% at a C/O ratio of 0.85 for n-C12H18. Even though i-C8H18 showed the best reforming performance of the others, n-C12H26 was chosen as the reforming feedstock to assist SCR performances because commercial diesel fuel has high molecular weight. CPOx of n-C12H26 showed the highest fuel conversion, and H2 selectivity at 100 000 h-1 when tested with different GHSV numbers of 50 000 h-1, 75 000 h-1 and 100 000 h-1 and 10% of O2 concentration led to a more reliable and efficient reforming performance than 5% and 15%. Also, it was found that H2 selectivity was improved in proportion to water vapor concentration, increasing from 64% in the absence of water to 75% in the presence of 5 vol % of water vapor. Although the fuel reforming was conducted without water to assist HC-SCR in this study, H2 productivity improvement by water implies the possibility of less fuel penalty at the same H2 concentration or more enhancement of DeNOx over Ag/Al2O3 at the same amount of fuel for reforming. 3.3. DeNOx Enhancement over Ag/Al2O3 by CPOx. 3.3.1. nC12H26 CPOx-Assisted C3H6-SCR and C12H26-SCR. To evaluate the CPOx-assisted HC-SCR, some of n-C12H26 CPOx products were bypassed to C3H6-SCR reactants in the absence of water. NOx conversion rates and NO2/NOx ratios at the exit of the SCR catalyst for the various reductant conditions are presented in Figure 7 and Figure 8, where black symbols indicate the reductant conditions in the absence of CPOx products and white symbols indicate the reductant conditions in the presence of CPOx products. In the absence of CPOx products, the increase in the amount of propylene resulted in improved NOx reduction at temperatures above 350 °C, but the activity in the temperatures below 350 °C was not affected by the increase of C3H6. On the other hand, by the cofeed of CPOx products to the SCR reactants, the activity of C3H6-SCR over Ag/Al2O3 was highly improved. The active temperature window was dramatically decreased below 350 °C. Such improvement of SCR activity can be attributed to the addition of hydrogen which was produced from the reforming of n-C12H26 through the CPOx reaction. The ratio of NO2 to total NOx at the catalyst outlet showed also the relevancy to NO reduction. Since only NO was introduced to the SCR reactor, NO2 at the catalyst outlet is the SCR reaction products.23,25 Therefore, the higher the ratio of NO2 to NOx at the catalyst outlet, the more reactive is the SCR reaction. As shown in Figure 8, the NO2/NOx ratio increased

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Figure 7. NOx conversion rates of C3H6-SCR (black) and CPOx-assisted C3H6-SCR (white). Conditions: 200 ppm NO + 10% O2 + (reductant) + N2 as balance. Reductant conditions: 600 ppmC C3H6 (b)/1200 ppmC C3H6 (1)/2400 ppmC C3H6 (9), 1200 ppmC C3H6 + 1200 ppmC CPOx (3)/600 ppmC C3H6 + 1800 ppmC CPOx (O).

Figure 8. NO2/NOx ratios of C3H6-SCR (black) and CPOx-assisted C3H6SCR (white). Conditions: 200 ppm NO + 10% O2 + (reductant) + N2 as balance. Reductant conditions: 600 ppmC C3H6 (b)/1200 ppmC C3H6 (1), 1200 ppmC C3H6 + 1200 ppmC CPOx (3)/600 ppmC C3H6 + 1800 ppmC CPOx (O).

as the catalyst inlet temperature became higher than 350 °C in the absence of the CPOx product, and this temperature window of NO2 detection coincides with that of NOx conversion. When CPOx products were supplied to the SCR reactants, a higher NO2/NOx ratio was measured at low temperature ranges below 350 °C. Similar to the NO2 formation increase due to CPOx products, hydrocarbon conversion was also increased at a lower temperature range. Accordingly, it is expected that CPOx product gas highly promoted the SCR reaction over the Ag/ Al2O3 catalyst particularly in the low temperature window by enhancing the NO and hydrocarbon reaction over the catalyst. Reducing agent conditions of C/N ) 12, C/N ) 3 + CPOx/N ) 9 and C/N ) 6 + CPOx/N ) 6 in Figure 7 and Figure 8 indicate that the conditions are the same in the amount of hydrocarbon used for DeNOx because the total carbon to NOx ratio is constant but different in the amount of fuel reformed. C/N ) 12 implies the condition when all the hydrocarbons were fed to the SCR reactor only, but C/N ) 3 + CPOx/N ) 9 and C/N ) 6 + CPOx/N ) 9 mean that 25% and 50% of the fuel were reformed separately before the SCR. Interestingly, the

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Figure 10. NOx conversion rates of C12H26-SCR (black symbol) and C12H26 CPOx-assisted C12H26-SCR (white symbols) over Ag/Al2O3 catalyst. Conditions: 200 ppm NO + 10% O2 + (reductant) + N2 as balance. Reductant conditions: 600 ppmC C12H26 (b)/1200 ppmC C12H26 (1)/2400 ppmC C12H26 (9), 600 ppmC C12H26 + 1800 ppmC CPOx (0)/1200 ppmC C12H26 + 1200 ppmC CPOx (3).

Figure 9. NOx (left panels) and C3H6 (right panels) conversion in the absence (9) and presence (0) of H2. Conditions: (a,d) 200 ppm NO + 600 ppmC C3H6 + (1300 ppm H2) + 5% O2 + N2 as balance; (b,e) 200 ppm NO + 1200 ppmC C3H6 + (700 ppm H2) + N2 as balance; (c,f) 200 ppm NO + 1800 ppmC C3H6 + (100 ppm H2) + N2 as balance.

reductant condition of C/N ) 6 + CPOx/N ) 6 showed better performances in NOx conversion and NO2/NOx ratio. At the C/N ) 3 + CPOx/N ) 9 condition, SCR showed 43.8% of NOx conversion at 300 °C, but 52.5% of NOx conversion was achieved with C/N ) 6 + CPOx ) 6. Additionally, the superiority in NOx reduction of C/N ) 6 + CPOx/N ) 6 to C/N ) 3 + CPOx/N ) 9 was maintained for the temperature range of 300-500 °C. These characteristics were also confirmed from the tests using model gas mixtures. Instead of the CPOx products, hydrogen gas was directly added into the SCR reactants to form reductant conditions of C/N ) 3 + H2/N ) 6.5 (Figure 9 a,d), C/N ) 6 + H2/N ) 3.5 (Figure 9b,e) and C/N ) 9 + H2/N ) 0.5 (Figure 9c,f). By increasing H2 and lowering C3H6, NOx conversion at a high temperature of around 450 °C was decreased, but low temperature activity between 200 and 350 °C was significantly improved along with hydrocarbon conversion improvement. In terms of low temperature activity, the results shown in Figure 9 indicate that both H2 and hydrocarbon should be sufficient enough to achieve higher NOx reduction. When one of H2 or hydrocarbon is much higher than the other, sufficient NOx reduction at low temperatures could not be achieved. On the other hand, in terms of high temperature activity, it can be noted that fuel reforming is not beneficial because high temperature NOx reduction was clearly restricted by the amount of hydrocarbons. Accordingly, for the reforming-assisted HC-SCR to be effective in a vehicle exhaust, the amount of fuel to be reformed for the aftertreatment needs to be optimized. In low temperature operating conditions, about half of the fuel should be reformed to supply a sufficient amount of H2, but in high

exhaust temperatures the fuel needs to be fed to SCR directly to avoid an NOx reduction decrease due to insufficiency of hydrocarbons by reforming. Figure 10 shows the NOx conversion rates when n-C12H26 was used as the feedstock of CPOx and the reducing agent for the SCR reaction. Comparing with the results presented in Figure 7, the enhancement of the low temperature activity for the n-C12H26 CPOx-assisted C12H26-SCR was less dramatic than the cases of n-C12H26 CPOx-assisted C3H6-SCR. But as seen in CPOx-assisted C3H6-SCR, the active temperature range was broadened when CPOx products were present. When 2400 ppmC C12H26 was added as a reducing agent, peak NOx conversion of 66.1% was achieved at 450 °C. But, the conversion decreased with decreasing temperature, and only 23% of inlet NOx was reduced at 350 °C. Meanwhile, by increasing CPOx/HC ratio to C/N ) 6 + CPOx/N ) 6 and C/N ) 3 + CPOx/N ) 9, the NOx conversion rate at 350 °C was increased from 23% without CPOx to 33.9% and 38.4% as the reformate proportion increased. 3.3.2. DeNOx Inhibition by Water and Reversibility of CPOx Promotion Effect. To investigate the effects of water, 5 vol % of water vapor was added to the reactants of n-C12H26 CPOx-assisted C3H6-SCR. For comparison, different reductant conditions were tested. Condition 1 through condition 4 in Figure 11 and Figure 12 denote the reductants conditions of 1200 ppmC C3H6, 1200 ppmC C3H6 + 1200 ppm CPOx, 600 ppmC C3H6 + 1800 ppm CPOx, and 2400 ppm CPOx, respectively. Also, the figures show the results of C12H26 CPOxassisted C3H6-SCR at 300 and 400 °C. At condition 1, where only 1200 ppmC C3H6 was added, about 32% and about 4% NO conversion rates were measured at 400 and 300 °C in the absence of water. However, the activity of C3H6-SCR was seriously inhibited by water addition, so that the NOx conversion rate was decreased to nearly 0% in the presence of water at both 300 and 400 °C. When the CPOx products were introduced into the SCR reactor, C3H6-SCR activity in the presence of water was significantly improved. The NOx conversion rate was increased from about 0% without CPOx addition to about 22% with CPOx at 300 °C and 25% at 400 °C. Interestingly, it can be seen that the ratio of CPOx/HC has no significant effects on NOx reduction in the presence of water. Of course, there was a slight increase of NOx conversion from 20.4% to 21.8% by an

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Figure 11. NOx conversion rates at 300 °C in the absence (black bar)/ presence (gray bar) of 5% water vapor. Gas conditions: 200 ppm NO + 10% O2 + N2 as balance. Reductants conditions: (1) 1200 ppmC C3H6, (2) 1200 ppmC C3H6 + 1200ppmC CPOx, (2) 600 ppmC C3H6 + 1800 ppmC CPOx, (4) 2400 ppmC CPOx. C12H26 was reformed.

Figure 12. NOx conversion rates at 400 °C in the absence (black bar)/ presence (gray bar) of 5% water vapor. Gas conditions: 200 ppm NO + 10% O2 + N2 as balance. Reductants conditions: (1) 1200 ppmC C3H6, (2) 1200 ppmC C3H6 + 1200 ppmC CPOx, (2) 600 ppmC C3H6 + 1800 ppmC CPOx, (4) 2400 ppmC CPOx. C12H26 was reformed.

increase of the CPOx/HC ratio from condition 2 to condition 3 at a low temperature of 300 °C. However, the increase is too small to be noticed as activity enhancement. Although NOx reduction was improved by the addition of CPOx products, its level seems not to be practical, especially considering the required NOx reduction declared in emission legislation. Therefore, to make this study feasible in diesel NOx reduction, a more water-resistant catalyst system is required. The response or reversibility of CPOx promotion effect is another matter of concern. To answer this question, the flow of CPOx products to the SCR was ON and OFF periodically. Figure 13 and Figure 14 show the results of the NOx conversion and the NO2/NOx ratio, and CO, CO2, and CO+CO2 emissions at 400 °C over Ag/Al2O3 catalyst with the reductants conditions of 1200 ppmC n-C8H18 and in/out of 1200 ppmC CPOx product. By adding CPOx products, the NOx conversion rate was increased from 44% to 72%. It was then dropped to the initial NOx conversion rate of 44% by excluding the CPOx products. The NO2/NOx ratio (gray symbol in Figure 13) also went up to 32% and down to 13% according to in/out of CPOx products. By this experiment, it was found that the effect of CPOx is

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Figure 13. Time evolution of NOx conversion (black symbol) and NO2/ NOx (gray symbol) ratio in the presence/absence of CPOx products at 400 °C. Conditions: 200 ppm NO + 10% O2 + 1200 ppmC C8H18 + (1200 ppm CPOx) + N2 as balance.

Figure 14. Time evolution of CO (black symbol), CO2 (dark gray symbol), and CO+CO2 (gray symbol) concentration in the presence/absence of CPOx products at 400 °C. Conditions: 200 ppm NO + 10% O2 + 1200 ppmC C8H18 + (1200 ppm CPOx) + N2 as balance.

very reversible, and there was no aging effect by cofeed of CPOx products. 5. Conclusion By assisting C3H6-SCR and C12H26-SCR with CPOx, the reaction temperature was significantly lowered from above 350 °C to below 250 °C. In addition, it was also found that there is an optimum ratio of H2/HC/NOx when the total amount of fuel added for CPOx and the total amound added for SCR were assumed to be same. For a fixed amount of hydrocarbons of HC/NOx ratio of 12, C/N ) 6 + CPOx/N ) 6 condition showed the best performance, which was also confirmed by model gas experiments. Even though the low-temperature activity of silver catalyst was significantly promoted when there existed abundant hydrogen in the reactants, NOx conversion was essentially limited by the amount of hydrocarbons. Therefore, to achieve satisfactory NOx conversion efficiency over a wide range of temperatures, there should be a well-balanced amount of hydrocarbons and hydrogen available in the reactants. NOx conversion efficiency over silver catalyst was seriously inhibited by the presence of water. When CPOx products were not

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introduced into the SCR reactants, NOx conversion dropped down to nearly zero efficiency up to a high temperature of 400 °C. On the other hand, a cofeed of CPOx products enhanced NOx conversion to an extent even in the presence of water, so that about 20% and 25% of NOx reductions were recorded at 300 and 400 °C. Acknowledgment This work is part of the project “Development of Partial Zero Emission Technology for Future Vehicle” funded by the Ministry of Commerce, Industry and Energy, and we are grateful for its financial support. Literature Cited (1) Johnson, T. V. Diesel Emission Control in Review. Soc. Automot. Eng. 2009, Article No. 200901-0121. (2) DieselNet. http://www.dieselnet.com/standards/eu/ld.php. (3) Dykes, E. C. NOx performance of an LNT+SCR system designed to meet EPA 2010 emissions: results of engine dynamometer emission tests. Society of AutomotiVe Engineers. 2008, 200801-2642. (4) Ko¨nig, A.; Held, W.; Richter, T. Lean-burn catalysts from the perspective of a car manufacturer. Early work at Volkswagen research. Top. Catal. 2004, 28, 99. (5) Walker, A. P. Mechanistic studies of the selective reduction of NOx over Cu/ZSM-5 and related systems. Catal. Today. 1995, 26, 170. (6) Burch, R.; Millington, P. J. Selective reduction of nitrogen oxides by hydrocarbons under lean-burn conditions using supported platinum group metal catalysts. Catal. Today. 1995, 26, 185. (7) Meffert, M. W.; Lenane, D. L.; Openshaw, M.; Roos, J. W. Analysis of Nitrous Oxide Emissions from Light Duty Passenger Cars. Soc. Automot. Eng. 2000, 200001-1952. (8) Burch, R.; Breen, J. P.; Meunier, F. C. A review of the selective reduction of NOx with hydrocarbons under lean-burn conditions with nonzeolitic oxide and platinum group metal catalysts. Appl. Catal., B 2002, 39, 283. (9) Era¨nen, K.; Lars-Eric Lindfors, L.-E.; Niemi, A.; Elfving, P.; Cider, L. Influence of Hydrocarbons on the Selective Catalytic Reduction of NOx over Ag/Al2O3-Laboratory and Engine Tests. Soc. Automot. Eng. 2000, 200001-2813. (10) Klingstedta, F.; Era¨nena, K.; Lindforsa, L.-E.; Andersson, S.; Cider, L.; Landberg, C.; Jobson, E.; Eriksson, L.; Ilkenhans, T.; Websterd, D. A highly active Ag/alumina catalytic converter for continuous HC-SCR during lean-burn conditions: From laboratory to full-scale vehicle tests. Top. Catal. 2004, 30/31, 27. (11) Satokawa, S.; Shibata, J.; Shimizu, K.; Satsuma, A.; Hattori, T. Promotion effect of H2 on the low temperature activity of the selective reduction of NO by light hydrocarbons over Ag/Al2O3. Appl. Catal., B 2003, 42, 179.

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ReceiVed for reView May 23, 2009 ReVised manuscript receiVed February 18, 2010 Accepted February 24, 2010 IE9015528