Effect of Engine Exhaust Parameters on the Hydrothermal Stability of

Nov 19, 2012 - Technical Center, Caterpillar, Post Office Box 1875, Mossville, Illinois 61552, United States. ABSTRACT: A dual-catalyst bed composed o...
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Effect of Engine Exhaust Parameters on the Hydrothermal Stability of Hydrocarbon-Selective Catalytic Reduction (SCR) Catalysts for LeanBurn Systems Preshit Gawade,† Anne-Marie C. Alexander,† Ronald Silver,‡ and Umit S. Ozkan*,† †

Department of Chemical and Biomolecular Engineering, The Ohio State University, 140 West 19th Avenue, Columbus, Ohio 43210, United States ‡ Technical Center, Caterpillar, Post Office Box 1875, Mossville, Illinois 61552, United States ABSTRACT: A dual-catalyst bed composed of a reduction catalyst, Pd-sulfated zirconia, and an oxidation catalyst, CoOx/CeO2, was investigated for selective catalytic reduction (SCR) of NOx (NO and NO2) by hydrocarbons for use in lean-burn natural gas engines. The primary focus of this submission is to examine the hydrothermal stability of the dual-catalyst bed and improve its hydrothermal durability by studying the effect of the engine exhaust parameters. The main parameters investigated were the concentration and nature of the hydrocarbons and the reaction temperature. Both cyclic as well as time-on-stream experiments were conducted under various engine exhaust compositions and different reaction temperatures to establish the conditions that could improve the water tolerance of the dual-catalyst bed. The higher concentration of the hydrocarbon mixture in the simulated engine exhaust was seen to assist the water tolerance of the dual-catalyst bed. Experiments were performed to isolate and identify the primary component in the hydrocarbon mixture that contributed more toward water tolerance. This study revealed that ethane had a more prominent effect than methane or propane for improving the hydrothermal stability of the catalyst bed. Moreover, the effect of the reaction temperature confirmed a shift in the operating temperature window in the presence of water vapor, as higher reaction temperatures were seen to significantly improve the hydrothermal stability of the dual-catalyst bed. Ion-exchanged zeolite catalysts, such as In-ZSM5,9 CoMFI,10 Pd-MFI,11,12 and Pd-MOR,13 have been reported to be effective for the selective reduction of NOx species by methane. More effective hydrocarbon (HC)-SCR catalysts have been described which utilize cobalt or bimetallic cobalt and palladium supported on zeolites and other supports.14−16 However, the activity of most of these catalysts was severely impaired in the presence of water vapor, inevitably present in the exhaust of natural gas engines. The loss of catalytic activity is largely a result of Al ions leaching out of the zeolite framework in the presence of water vapor,17 leading to irreversible suppression of catalytic activity of the metal ions. Therefore, the development of catalysts that have improved tolerance to water vapor is a key subject. Although Pd-based catalysts seem to offer promising catalytic activity, much effort has been directed into the development of metal oxide supports, which are more hydrothermally stable and also have the acidic strength comparable to that of zeolites. Early studies by Resasco and co-workers compared the activity of various zeolitic- and non-zeolitic-supported Pd catalysts.18 It was reported that Pd-loaded sulfated zirconia has similar activity for NOx reduction by methane to that of Pd-supported zeolites. The advantage of using zirconia over other wellestablished supports is its enhanced thermal and chemical stability. The increased acidity of sulfated zirconia is significant because the catalytic activity and selectivity of de-NOx are

1. INTRODUCTION The development of more efficient de-NOx technologies has attracted significant attention in the last two decades, a consequence of more stringent environmental regulations.1 Selective catalytic reduction (SCR) of nitrogen oxides (NOx) by ammonia is perhaps one of the most effective and large-scale commercial technologies for NOx control from stationery sources and has been studied extensively. This process, however, requires a supply of reductants, such as ammonia or urea, which pose hazards within themselves and which are mainly related to ammonia slip, direct ammonia oxidation, and corrosion of equipment due to the formation of ammonium salts.2,3 The selective reduction of NOx by hydrocarbons is considered a potential after-treatment process for small-scale stationary emission sources, mobile sources that operate with a high oxygen/hydrocarbon ratio, and natural-gas-fired power stations. The SCR of NOx by hydrocarbons, particularly methane, offers an attractive alternative to conventional emission after-treatment technologies4−6 and is specifically suited for lean-burn natural gas engines. The exhaust from natural gas engines is mainly composed of methane and other hydrocarbons, which are capable of acting as reducing agents for NOx reduction. Hence, this process eliminates the need of injecting additional reducing agents, thus avoiding significant fuel penalties. Although there is a great advantage of using methane as a reductant for NOx reduction, its use in lean-burn engines is somewhat problematic. This is mainly due to the competition from the combustion of the hydrocarbon in the presence of excess oxygen and the inherent difficulty of methane activation.7,8 © 2012 American Chemical Society

Received: August 29, 2012 Revised: October 4, 2012 Published: November 19, 2012 7084

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Figure 1. Schematic diagram of the HC-SCR reaction system.

forward direction. The oxidation catalyst also serves to reoxidize NO present in the system, a consequence of the partial reduction of NO2. The oxidation catalyst further catalyzes the combustion of excess hydrocarbons and carbon monoxide, which have not been spent in the selective reduction of NO. The development of the dual catalyst approach has involved the simultaneous optimization of both the reduction and oxidation components, which are physically mixed together. Initial palladium catalysts supported on sulfated zirconia were prepared by the impregnation of a commercial zirconia support and were reported to give good NO2 reduction activity with CH4 under “dry” lean-exhaust conditions between 375 and 400 °C; however, when exposed to water vapor, both NOx reduction and hydrocarbon oxidation were suppressed. This led to the development of a “one-pot” sol−gel synthesis technique for the Pd/SZ catalyst, which exhibited a significantly higher NO2 reduction activity at higher operating temperatures (400−450 °C). Upon the introduction of 7% water vapor to the dual-catalyst bed, NOx conversion decreased from 94 to 74%; however, this effect was fully reversible upon removal of water vapor from the feed stream. We have also shown that the dual-catalyst scheme is effective in the presence of 7% water, achieving N2 yields in excess of 60% even after 40 h on stream. As a continuation of this work, we examine the effect of the temperature and hydrocarbon concentration on the interaction of water vapor with the oxidation and reduction catalyst components of a dual-bed catalyst scheme, namely, CoOx/ CeO2 and Pd/SZ, and their implications on the activity and hydrothermal stability of a dual bed comprised of these catalysts under simulated lean-exhaust conditions.

thought to be a consequence of the metallic phase and acidity of the support. Studies by Ohtsuka and co-workers have shown that the addition of platinum into a Pd/SZ catalyst19 improves both long-term hydrothermal stability and NOx reduction activity. Later studies by the same group have shown that NOx reduction activity can be further improved through the use of a Fe dopant.20 In these studies, NOx conversion was reported to be 60% after 700 h on stream in the presence of 9% water vapor. Similar cooperative effects have also been observed on Co-promoted Pd-supported catalysts.16,21,22 Quincoces et al. have reported that a Co−Pd/SZ catalyst undergoes a reversible decrease of both NOx reduction and CH4 oxidation activity in the presence of water vapor; it was also noted that, in the presence of water vapor, the optimum operating temperature shifts to higher temperatures. In our previous contributions, we have described a novel dual-catalyst bed approach for the selective reduction of NOx species by hydrocarbons in lean systems.23−29 This method combines separate oxidation and reduction catalyst components to perform three distinct catalytic functions, specifically, NO oxidation, NO2 reduction, and CO and hydrocarbon oxidation for after-treatment of lean-burn natural gas engine exhaust, minimizing or eliminating the need for additional fuel injection. The dual-catalyst approach for lean-burn exhaust after-treatment takes advantage of the stronger oxidizing potential of NO2 compared to NO, which, in turn, helps to use the reducing capability of unburned hydrocarbons in the exhaust. NO oxidation to NO2 is an exothermic and reversible reaction, which is thermodynamically limited at high temperatures; however, when this reaction occurs in close proximity to a reduction catalyst, NO2 is sequentially removed from the system by NO2 SCR, thus pushing the equilibrium in the 7085

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outlined in the previous section. For instance, a dual-catalyst bed was tested under simulated engine exhaust in 10% water vapor with over 40 h of time-on-stream, as described above; however, the CH4 concentration was increased to 2050 ppm to match the total hydrocarbon concentration of the CHx gas feed while maintaining the space velocity at 32 000 h−1 In a similar set of studies to those outlined above, separate hydrocarbon feed streams were used. In these experiments, either CH4 (2050 ppm), C2H6 (1025 ppm), or C3H8 (683 ppm) reductants were introduced along with other feed gases as previously described. Here, the concentrations of the various hydrocarbon reductants were carefully chosen on an equal carbon basis to rule out any artifact due to extra carbon in the stream. From these studies, ethane was shown to have the most significant effect on water tolerance, and hence, further studies were conducted in excess ethane. In these experiments, an extra 100 ppm C2H6 was externally injected into the simulated engine exhaust containing 2050 ppm CHx, as described in section 2.2.1.1. 2.2.2. Effect of the Reaction Temperature. Both cyclic and timeon-stream experiments were conducted at different reaction temperatures under wet exhaust. Cyclic experiments were performed at 425, 450, and 475 °C and were carried out in the presence of 7% water vapor and 2500 ppm CHx concentration while keeping the space velocity and other parameters constant. Time-on-stream experiments were performed at 450, 475, and 500 °C in the presence of 10% water vapor while keeping the CHx (2050 ppm) concentration and space velocity constant.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Palladium supported over sulfated zirconia was prepared using a one-pot sol−gel method with 0.3% Pd loading. The details of the catalyst preparation can be found elsewhere.23,24 The oxidation catalyst 2% CoOx/CeO2 was prepared through a wet impregnation method using water as the solvent. The ceria support, for the oxidation catalyst, was prepared using the precipitation method. The details of preparation for both the ceria support and the oxidation catalyst can be found elsewhere.30,31 2.2. Catalytic Activity Testing. Steady-state reactions were conducted in a 1/4 in. outer diameter stainless-steel packed-bed reactor. The schematic of the reaction system is illustrated in Figure 1. A physical mixture of reduction (Pd/SZ) and oxidation (2% CoOx/ CeO2) catalysts were carefully packed inside the reactor tube and held centrally using quartz wool plugs. Inert quartz powder was mixed with the dual bed to maintain a constant catalyst bed volume. The ratio of the reduction catalyst/oxidation catalyst was maintained at 8:1 (by weight), unless otherwise stated. Our previous work focused on the optimization of dual-catalyst bed composition, in which the ratio of the reduction catalyst/oxidation catalyst was varied from 2:1 to 8:1 (by weight).29 In this study, it was found that an 8:1 reduction catalyst/ oxidation catalyst ratio achieved the highest NOx conversion under simulated engine exhaust conditions. The reactor was heated using a resistively heated homemade furnace, and the temperature of the catalyst bed was monitored and controlled using a K-type thermocouple and an Omega (model CS232) proportional− integral−derivative (PID) controller. Moreover, the position of the thermocouple was carefully monitored to ensure that it was held at the center of the reactor and directly above the catalyst bed. Brooks mass flow controllers and control box (5850E) were used to regulate the gas flow rates. Water vapor was introduced to the reactor system by saturating a helium stream through a heated water bubbler. The experiments reported in this submission were conducted at a gas hourly space velocity (GHSV) of 32 000 h−1, unless stated otherwise. Effluent gases were analyzed using a chemiluminescence NOx analyzer (Thermo-Scientific 42i-HL) coupled with a Micro gas chromatograph (GC) (Agilent 3000A Series), which was equipped with a 0.32 mm molecular sieve and PLOT Q column with a thermal conductivity detector (TCD). A PermaPure gas dryer was installed before the analytical instruments to preferentially remove water vapor from the gas stream to avoid damage to the instruments. 2.2.1. Effect of the Hydrocarbon Concentration. 2.2.1.1. Effect of Mixed Hydrocarbons. Reaction studies were carried in the presence of simulated engine exhaust conditions as outlined below: 180 ppm NO2, 2050−3050 ppm CHx, 650 ppm CO, 6.5% CO2, 10% O2, and 0−10% water vapor. It should be noted that the feed mixture used in the study contained only NO2 and no NO. The choice for the feed composition is based on the fact that the majority of NOx coming out of a lean-burn gas engine is NO2. Furthermore, for all of the experiments reported here, only CHx and water vapor concentrations were varied; other components in the feed, such as NO2, CO, CO2, and O2 were held constant. The CHx mixture was composed of CH4 (∼85% vol), C2H6 (∼10% vol), and C3H8 (∼5% vol), unless stated otherwise. The absolute concentrations of CH4, C2H6, and C3H8 varied from 1740 to 2592 ppm, from 208 to 305 ppm, and from 104 to 152 ppm, respectively, in the feed, but the relative ratios remained the same. To understand the effect of the hydrocarbon concentration under wet exhaust, cyclic experiments were conducted in the presence of 7% water vapor over a dual-catalyst bed at 450 °C. Each cycle (dry or wet) was kept online for approximately 45−60 min, unless stated otherwise. The CHx concentration was varied from 2050 to 3050 ppm while keeping the space velocity constant (32 000 h−1). Time-on-stream experiments were conducted over 40 h in the presence of 10% water vapor at 450 °C. 2.2.1.2. Comparison of Methane, Ethane, and Propane as Reducing Agents. To establish whether hydrocarbons other than methane aided the hydrothermal stability of the catalysts, various experiments were carried under the wet exhaust. This was achieved by initially comparing a methane-only hydrocarbon feed against a mixed hydrocarbon feed, comprised of the hydrocarbon composition

3. RESULTS AND DISCUSSION 3.1. Effect of the Hydrocarbon Concentration on Hydrothermal Stability. 3.1.1. Effect of the Mixed Hydrocarbon. From our previous work,23,24 it was shown that the reduction catalyst was significantly affected by the presence of water vapor, while the negative effect of water vapor on the oxidation catalyst component was negligible. Therefore, the impact of water on NOx and hydrocarbon activity was expected to be due to inhibition of reaction steps that take place on the reduction catalyst. Figure 2 shows the reversible effect of water vapor on a dual-catalyst bed composed of Pd/SZ and 2% CoOx/CeO2 at three different hydrocarbon concentrations. Both NOx and CH4 conversions were reversibly affected in the presence of 7% water vapor. This observation was consistent with our previous reports23,24,29 on the dual-catalyst bed as well as studies reported by Quincoces et al.21 on the Pd−Co/SZ catalyst. As seen in Figure 2a, a drop in NOx conversion was observed in the presence of 7% water vapor, regardless of the CHx concentration. However, the negative effect of water vapor decreased considerably by increasing the CHx concentration from 2050 to 3050 ppm in the feed, as illustrated in Figure 2a. During the third wet cycle, NOx conversion was observed to be 65% for CHx = 2050 ppm, 78% for CHx = 2500 ppm, and 90% for CHx = 3050 ppm. This preliminary work indicates that, when the hydrocarbon concentration is increased, NO x reduction activity improves significantly in the presence of water vapor. This may be associated with the fact that more hydrocarbons are available in the feed stream to overcome water inhibition of the reaction. It is also expected that the higher hydrocarbon concentration helps the kinetics of the CHx + NO2 reaction. It should be noted that, under the given conditions, both C2H6 and C3H8 conversions were greater than 90%; however, both were also affected reversibly, to some extent, by the presence of water vapor, regardless of the hydrocarbon concentration. Moreover, CO conversion was complete and remained unaffected under the wet conditions, 7086

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that both CH4 + NO2 and CH4 + O2 reactions are of positive reaction order. Several studies4 (and reference therein)32−35 have previously reported the negative effect of water vapor on methane combustion over a Pd-based catalyst, which was associated with either the formation of an inactive Pd phase32 or water inhibition during methane oxidation.33,34 For instance, Cullis et al.32 suggested that the competition between methane and water for surface-active sites led to the formation of the Pd(OH)2 phase from PdO, which is an inactive phase for methane oxidation. This claim was further supported by Burch et al.36 In their findings, the decomposition of Pd(OH)2 was a rate-limiting step rather than the activation of the C−H bond in methane. Kinetic studies on methane oxidation over a Pd catalyst, reported by Ribeiro et al.,33 also reveal the inhibitory effect of water during methane oxidation. In their findings, the reaction order with respect to water was (−1) during methane oxidation. In addition, some of our earlier work24 indicate that water competes with methane adsorption on the catalyst surface rather than preventing the methane oxidation reaction taking place. With this in mind, it may be possible that, under wet conditions, only a limited amount of methane is adsorbed onto the catalyst surface, which can subsequently react further through either the SCR reaction or methane oxidation. Direct methane oxidation contributes more significantly to methane conversion compared to its consumption during the SCR reaction. This is mainly due to the fact that the NO 2 concentration is much lower in the feed compared to the hydrocarbon concentrations. Moreover, as will be discussed in the subsequent sections, not only methane but also ethane and propane contribute considerably to NO2 reduction. Hence, the reported methane conversion is mostly a product of the methane oxidation reaction rather than its contribution in NO2 reduction. Figure 3 shows the time-on-stream performance of the dualcatalyst bed in 10% water at two different hydrocarbon mixture

Figure 2. Effect of the hydrocarbon concentration: (black ○) CHx = 2050 ppm, (red △) CHx = 2500 ppm, and (blue ◇) CHx = 3050 ppm on (a) NOx conversion, (b) CH4 conversion, and (c) rate of CH4 conversion (mmol gcat−1 min−1) during NO2 reduction in the presence of H2O. Reaction conditions: [NO2], 180 ppm; [CHx], varying; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 7%; temperature, 450 °C; pressure, 1 atm; and GHSV, ∼32 000 h−1.

indicating that CO oxidation was not hindered by the presence of water over the dual-catalyst bed. During cycling experiments, CH4 conversion was also seen to be affected considerably, although reversibly, under simulated wet exhaust, as shown in Figures 2b and 2c. Although the conversion of CH4 did not vary much with the change in the overall hydrocarbon concentration (Figure 2b), when the rate of CH4 conversion was considered (Figure 2c), pronounced differences were observed with the changing CHx concentration, in both dry and wet feed conditions. It should be noted that the three CHx concentrations used correspond to methane concentrations of 1737, 2152, and 2539 ppm in the feed. The higher CH4 concentrations clearly give higher conversion rates, suggesting a positive rate order for CH4 oxidation. Since NO2 conversion also increased with CHx concentration, it is likely

Figure 3. Effect of the hydrocarbon concentration on hydrothermal stability of a dual-catalyst bed Pd/SZ and CoOx/CeO2 under simulated lean exhaust: (black ○) [CHx] = 2050 ppm and (red △) [CHx] = 2500 ppm. Reaction conditions: [NO2], 180 ppm; [CHx], varying; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 10%; temperature, 450 °C; pressure, 1 atm; and GHSV, ∼32 000 h−1.

concentrations. An initial drop in NOx was observed during the first 10−15 h of the experiment, stabilizing afterward. When the CHx concentration was increased from 2050 to 2500 ppm, NOx conversion increased from 43 to 55%, as shown after 40 h on stream. Typically, dispersed Pd2+ sites are considered to be active for NOx reduction, while the formation of PdO aggregates results in direct methane combustion.37 Adelman and Sachtler38 suggested the possibility of PdO formation via 7087

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hydrolysis of Pd2+ ions. Ohtsuka et al.39 have reported the formation of PdO via agglomeration of palladium in the presence of water vapor over a Pd/MOR catalyst; as a result, methane conversion was observed to increase over time. A similar observation was reported by Quincoces et al.21 over Pd/ SZ during NO reduction in the presence of 6% water. However, no such phenomenon was observed in the present study, suggesting the lack of a phase transformation in the presence of water vapor, in the time frame that was tested. 3.1.2. Comparison of Methane, Ethane, and Propane as Reducing Agents. In the previous section, it was shown that having a higher concentration of mixed hydrocarbons in the feed assisted the NO2 reduction in wet exhaust. This led to further studies investigating the effect of each hydrocarbon, methane, ethane, and propane independent of each other in the wet exhaust. Methane is the major component of natural gas engine exhaust, accounting for approximately 80−90% (by vol). Hence, time-on-stream experiments were conducted with methane as the only hydrocarbon in the feed; the performance was then compared to those experiments with the equivalent CHx concentration. Figure 4a illustrates NOx conversion, in

contributed to the improved NO2 conversion attained with the hydrocarbon mixture. Also shown in Figure 4b is the rate of methane conversion for the two different feed mixtures used. The methane conversion rate was significantly higher when the feed consisted of methane only. There may be two independent effects that could explain the differences. On one hand, the higher CH4 concentration helps with the kinetic rate. On the other hand, there may also be a competitive adsorption between methane and other hydrocarbons (ethane/propane). These other hydrocarbons compete with methane for adsorption on the catalyst surface, resulting in lower methane oxidation in their presence. This was confirmed in a separate study (Figure 5) where methane oxidation was examined in the presence and absence

Figure 5. Hydrocarbon oxidation over a dual-catalyst bed Pd/SZ:2% CoOx/CeO2 = 8:1 in the presence of 10% O2: (a) (black bars) CHx = 2050 ppm (CH4 = 1737 ppm, C2H6 = 208 ppm, and C3H8 = 104 ppm) and (blue striped bars) only CH4 = 1737 ppm and (b) CHx = 2050 ppm [(black bars) CH4 = 1737 ppm, (red striped bars) C2H6 = 208 ppm, and (green bars) C3H8 = 104 ppm]. Reaction conditions: pressure, 1 atm; and GHSV, ∼58 000 h−1.

Figure 4. Effect of either a methane-only hydrocarbon feed or a mixed higher hydrocarbon feed on the hydrothermal stability of a dualcatalyst bed Pd/SZ and CoOx/CeO2 under simulated lean exhaust: (blue ◇) [CH4] = 2050 ppm and (black ○) [CHx] = 2050 ppm. Reaction conditions: [NO2], 180 ppm; [CHx], varying; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 10%; temperature, 450 °C; pressure, 1 atm; and GHSV, ∼32 000 h−1.

of other hydrocarbons at three different temperatures (Figure 5a). In this experiment, the CH4 concentration was kept constant at 1737 ppm. A decrease in methane conversion was observed when small amounts of ethane and propane (208 and 104 ppm, respectively) were added to the feed mixture. Comparisons of hydrocarbon conversions are presented in Figure 5b. It is evident that both ethane and propane are more readily oxidized, as compared to methane, when they are all present in the gas feed. Since the inlet concentrations are different, a conversion comparison does not necessarily represent a comparison of the conversion rates. The fact that ethane and propane conversions reached or approached 100% did not allow us to analyze the data in terms of observed rates. We compared, however, the conversion rates for methane and

10% water vapor. An initial drop was observed in the NOx conversion during the first 10 h of the experiment, stabilizing afterward, and appears to be a common feature of the time-onstream experiments. After 40 h on stream, NOx conversion was 35% under a CH4 only (2050 ppm) feed, as compared to 43% in a CHx (2050 ppm) feed. This indicates that higher hydrocarbon species, such as ethane and propane, can help to achieve higher NOx conversions than methane alone in wet exhaust conditions, thus aiding water tolerance of the catalyst. It should be noted that the total number of carbon atoms in the two feed mixtures are not the same; only the moles of total hydrocarbons are kept constant. This may have also 7088

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ethane at 350 °C, when both conversions were far from being complete. The conversion rate of ethane, even with the much lower inlet concentration, was significantly higher than that of methane (14 × 10−4 and 5 × 10−4 mmol gcat−1 min−1). The propane conversion rate would be expected to be even higher. Although the CH4 versus CHx study demonstrated that higher hydrocarbon species played a significant role during NO2 SCR, possibly more so than methane in wet exhaust, it was unclear whether this was a result of more “total carbon” in the feed or whether ethane and propane had more reducing capability than methane in the presence of water. To address these questions, experiments were conducted in 10% water vapor with single hydrocarbons, methane, ethane, or propane, in which the hydrocarbon concentration was kept on an equal carbon basis, 2050, 1025, and 683 ppm, respectively. As observed from Figure 6a, the experiment with ethane had the

or are directly oxidized with the help of the oxidation catalyst. The methane activation via hydrogen abstraction was an essential step for methane participation in either the SCR process or the combustion reaction. Methane is considered the hardest molecule to activate via hydrogen abstraction, followed by ethane and then propane. Therefore, it is plausible that enough ethane molecules were activated by the reduction catalyst to participate in the SCR process, resulting in better NOx conversion, while propane was more reactive toward O2, thereby performing poorly during NO2 reduction. Finally, we verified our assertion that ethane could significantly improve the NOx conversion under wet exhaust conditions by injecting small amounts (100 ppm) of extra ethane externally into the simulated engine exhaust composition, as shown in Figure 7. Here, all other parameters, such as

Figure 7. Effect of supplementary ethane addition on the hydrothermal stability of a dual-catalyst bed Pd/SZ and CoOx/CeO2 under simulated lean exhaust: (black ○) [CHx] = 2050 ppm and (blue ◇) [CHx] = 2050 ppm + [C2H6] = 100 ppm. Reaction conditions: [NO2], 180 ppm; [CHx], varying; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 10%; temperature, 450 °C; pressure, 1 atm; and GHSV, ∼32 000 h−1.

engine composition, reaction temperature, and space velocity, were maintained constant, and an extra 100 ppm ethane was mixed just before the reactor inlet. An improvement in NOx conversion (from 43 to 53%, after 40 h on stream) was observed with extra ethane addition, as shown Figure 7. 3.2. Effect of the Reaction Temperature on Hydrothermal Stability. Previously, we have reported24,29 that, under dry reaction conditions, an increase in the reaction temperature, beyond 450 °C, results in a drop in the NOx conversion. This observation was mostly associated with significant hydrocarbon combustion above 450 °C and, as a consequence, having an insufficient supply of hydrocarbons for NOx reduction. However, the presence of water vapor was seen to cause a shift in the operating temperature window. This shift toward the higher temperatures could be associated with a decrease in water adsorption on the catalyst surface at higher reaction temperatures. As discussed earlier, water was seen to compete with NO and CH4 for surface-active sites, resulting in decreases in both NOx and methane conversion.24 It is plausible that an increase in the reaction temperature results in a decrease in water adsorption, hence freeing more sites for NO and CH4 adsorption on the catalyst surface, consequently improving NOx and methane conversion. To verify this assertion, preliminary cyclic experiments were conducted in 7% water vapor over a dual-catalyst bed at various reaction temperatures, as shown in Figure 8. It was observed that the optimum temperature window for NO2 reduction was 425−

Figure 6. Effect of different hydrocarbons, as a function of equal carbon basis, on the hydrothermal stability of a dual-catalyst bed Pd/ SZ and CoOx /CeO2 under simulated lean exhaust: (a) NOx conversion and (b) CHx conversion, with (blue ◇) [CH4] = 2050 ppm, (red △) [C2H6] = 1025 ppm, and (green □) [C3H8] = 683 ppm. Reaction conditions: [NO2], 180 ppm; [CHx], varying; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 10%; temperature, 450 °C; pressure, 1 atm; and GHSV, ∼32 000 h−1.

highest NOx conversion of 54% after 40 h on stream, which was significantly higher than those achieved with methane (35%) and, in particular, propane (20%). The much lower NOx conversion observed by propane is mainly due to its high reactivity for direct oxidation, which is evident in Figure 6b, in which 100% C3H8 conversion was achieved. It appears that propane readily gets oxidized with O2, as a result, leaving an insufficient amount of propane for NO2 SCR to occur. Our previous report29 suggests that a reduction catalyst assists the methane activation via hydrogen abstraction, resulting in the formation of CH3/CH2 species. These species subsequently either participate in the NOx reduction reaction 7089

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Figure 8. Effect of the reaction temperature: (red △) 425 °C, (black ○) 450 °C, and (blue ◇) 475 °C on (a) NOx conversion and (b) CH4 conversion during NO2 reduction in the presence of 7% H2O. Reaction conditions: [NO2], 180 ppm; [CHx], 2500 ppm; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 7%; pressure, 1 atm; and GHSV, ∼32 000 h−1.

450 °C under dry conditions, because the maximum NOx conversion (96%) was achieved in this range. However, a further temperature increase decreased the NOx conversion to 90% at 475 °C under dry feed. This observation could be associated with a significant increase in hydrocarbon combustion at higher temperatures. Under the wet conditions, however, the trend was reversed. An increase in the temperature gave a higher NOx conversion (75−78% during the third wet cycle), indicating the shift of the optimum temperature window to higher temperatures. As expected, higher temperatures resulted in higher methane conversions, in both dry and wet conditions. Under wet conditions, methane conversion improved from 11 to 38% with an increase in the temperature from 425 to 475 °C. Time-on-stream experiments were conducted in 10% water vapor to further evaluate the dual-bed performance in the temperature window of 450−500 °C, as shown in Figure 9. After 40 h on stream, it was observed that NOx conversion improved from 45 to 57%, when the reaction temperature was increased from 450 to 475−500 °C. Furthermore, a significant improvement in methane conversion was observed, as methane conversion increased from 17 to 52%, while over 80% ethane and propane conversions were achieved irrespective of the temperature investigated. This shift in operating temperature window under wet exhaust conditions is in agreement with study reported by Quincoces et al.21 over Pd/SZ and Co−Pd/ SZ catalysts. In their findings, a significant drop in NO and methane conversion was observed below 500 °C in 6% water during NO reduction with methane. However, the increase in the reaction temperature in wet exhaust was seen to improve NO conversion as well as methane and ethane conversions. In addition, the current findings are consistent with our previous work,24 in which we have demonstrated that water adsorption

Figure 9. Effect of the temperature on the hydrothermal stability of a dual-catalyst bed Pd/SZ and CoOx/CeO2 under simulated lean exhaust: (a) NOx conversion, (b) CH4 conversion, and (c) C2H6 conversion at (black ○) 450 °C, (red △) 475 °C, and (blue ◇) 500 °C. Reaction conditions: [NO2], 180 ppm; [CHx], 2050 ppm; [CO], 650 ppm; [CO2], 6.5%; [O2], 10%; [H2O], 10%; pressure, 1 atm; and GHSV, ∼32 000 h−1.

decreased with an increase in the temperature, leaving more of the sites free for reactant adsorption. Time-on-stream experiments conducted at various reaction temperatures, in wet exhaust demonstrated that the temperature window shifted to higher temperatures from 450 to 475− 500 °C. It also showed an increased hydrothermal stability if reactions are conducted within a narrow temperature window of 475−500 °C. It should be noted that time-on-stream experiments were not conducted above 500 °C in wet exhaust for two reasons. First, temperatures below 500 °C are more relevant for natural gas engine exhausts. Second, we have found that, above 500 °C, hydrocarbon combustion dominated significantly (even in wet streams); hence, poor NOx reduction performance was observed because of the lack of hydrocarbon availability.

4. CONCLUSION This study shows that it is possible to improve the hydrothermal stability of the dual-catalyst bed by modifying engine exhaust parameters under the lean conditions. The increase in the hydrocarbon concentration in the feed was seen 7090

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to improve NOx conversion considerably. This improvement was associated with the fact that more hydrocarbon molecules were available to overcome water inhibition of the reaction. The reaction studies confirmed that ethane had the highest reactivity for NOx SCR and injecting a small amount (∼100 ppm) of ethane can improve the water tolerance of the dualcatalyst bed significantly. Finally, a shift in the reaction temperature window was observed in wet exhaust, as higher reaction temperatures were seen to improve both NOx and hydrocarbon conversions.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 614-292-6623. Fax: 614-292-3769. E-mail: ozkan. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support provided by the U.S. Department of Energy and Caterpillar, Inc. is gratefully acknowledged.

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REFERENCES

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dx.doi.org/10.1021/ef301415b | Energy Fuels 2012, 26, 7084−7091