Selective Catalytic Reduction of Nitric Oxide with Ethylene on Copper

Elizabeth Vega , Hugo Ruiz , Gerardo Marti´nez-Villa , Gustavo Gonza´lez-A´ , Elizabeth Reyes , Jose´ Garci´a , Hugo Ruiz , Gerardo Marti´ne...
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Ind. Eng. Chem. Res. 1999, 38, 873-878

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KINETICS, CATALYSIS, AND REACTION ENGINEERING Selective Catalytic Reduction of Nitric Oxide with Ethylene on Copper Ion-Exchanged Al-MCM-41 Catalyst R. Q. Long and R. T. Yang* Department of Chemical Engineering, University of Michigan, Ann Arbor, Michigan 48109-2136

Selective catalytic reduction (SCR) of NOx by ethylene was investigated on copper and/or cerium ion-exchanged Al-MCM-41 in the presence of excess oxygen. Ce-Al-MCM-41 showed little activity, but Cu-Al-MCM-41 and cerium-promoted Cu-Al-MCM-41 (i.e., Ce-Cu-Al-MCM41) were found to be active in this reaction. Higher NOx conversions to N2 were obtained on Ce-Cu-Al-MCM-41 as compared with Cu-Al-MCM-41. The maximum activity of Ce-CuAl-MCM-41 was close to that of Cu-ZSM-5, but the former had a wider temperature window. TPR results indicated that only isolated Cu2+ and Cu+ ions were detected in the Cu2+-exchanged Al-MCM-41 samples, which may play an important role in the selective catalytic reduction of NOx to N2. After some cerium ions were introduced into Cu-Al-MCM-41, Cu2+ in the molecular sieve became more easily reducible by H2. This may be related to the increase of catalytic activity of NOx reduction by ethylene. Introduction The selective catalytic reduction (SCR) of NOx (NO + NO2) with hydrocarbons is an attractive route for NOx conversion to N2 from exhaust gas. The three-way catalyst (Pt, Pd, and Rh) has been used commercially in gasoline engines for reduction of NO to N2 by carbon monoxide and hydrocarbons under stoichiometric conditions, but it becomes ineffective in the presence of excess oxygen.1 Since the reports by Iwamoto et al.2 and by Held et al.3 that copper ion-exchanged ZSM-5 could selectively reduce NOx by hydrocarbons (e.g., C2H4, C3H6, C3H8, etc.) in the presence of excess oxygen, this topic has been extensively studied all over the world in recent years. Three comprehensive reviews on the reaction were recently presented.4-6 Many catalysts, such as Cu-ZSM-5,2,3 Co-ZSM-5 and Co-ferrierite,7-9 Cu2+-exchanged pillared clays,10,11 Pt/Al2O3,12,13 FeZSM-5,14,15 and so on, have been found to be active in this reaction. Copper ion-exchanged zeolites (including ZSM-5, ferrierite, mordenite, Y zeolite, L zeolite, etc.) were also studied and compared directly under the same conditions.2,16 It was found that Cu-ZSM-5 showed the highest NOx conversion but Cu-Y zeolite showed the lowest activity among the catalysts under the same conditions. It seems that the zeolite framework composition plays an important role for the copper ionexchanged zeolites in the SCR reaction. Considering that MCM-41, which is a new molecular sieve with pore dimensions between 1.5 and 10 nm,17,18 has a more open porosity and higher specific surface area than ZSM-5, we have tried using ion-exchanged or doped MCM-41 as the SCR (by NH3 and hydrocarbons) catalyst recently.19,20 Because MCM-41 has high thermal stability, large BET surface area, and large pore volume, it has * Corresponding author. Telephone: (734) 936-0771. Fax: (734) 763-0459. E-mail: [email protected].

been studied for potential use as catalysts, supports, or sorbents.21 We found that platinum-doped MCM-41 catalysts showed a higher specific activity than Pt/Al2O3 catalyst.20 In this work, we will present the first results on the activity of copper ion-exchanged Al-MCM-41 catalysts for selective catalytic reduction of NOx by ethylene, using cerium as the promoter. Experimental Section Preparation of the Catalyst. Fumed silica (99.8%, Aldrich), tetramethylammonium hydroxide pentahydrate (TMAOH, 97%, Aldrich), 25 wt % cetyltrimethylammonium chloride ((CTMA)Cl) in water (Aldrich), Al[C2H5CH(CH3)O]3 (97%, Aldrich), and NaOH (98.1%, Fisher) were used as source materials for preparing AlMCM-41. The Al-MCM-41 (Si/Al ) 10) sample was synthesized according to the procedure given by Borade and Clearfield.22 Solution A was prepared by dissolving 1.325 g of TMAOH in 100 mL of deionized water and then adding 5 g of fumed silica. Solution B was obtained by dissolving 0.72 g of NaOH in deionized water and adding 25 mL of (CTMA)Cl followed by adding 2.19 mL of Al[C2H5CH(CH3)O]3 at room temperature. The two solutions were stirred for 10-15 min; then, solution A was added to solution B. The reaction mixture had the following chemical composition: 1SiO2-0.05Al2O3-0.23(CTMA)Cl-0.11Na2O-0.089TMAOH-125H2O. After being stirred for 15 min, the mixture was transferred into a 250-mL three-neck flask and was then heated at 100 °C for 48 h. After the solution was filtered, the solid was washed, dried, and calcined at 560 °C for 10 h in a flow of air (150 mL/min). The copper and cerium ion-exchanged Al-MCM-41 were prepared by using a conventional ion-exchange procedure at room temperature. Solutions of 0.02 M

10.1021/ie980623+ CCC: $18.00 © 1999 American Chemical Society Published on Web 01/22/1999

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Cu(NO3)2 (adjusting pH value to 4.5 by 2 N HNO3) and 0.02 M Ce(NO3)3 were used for the exchange. The CeCu-Al-MCM-41 catalyst was obtained from Cu2+ exchange of the Ce-Al-MCM-41 sample; Ce exchange was done first because it was harder than copper exchange. All the exchange processes were repeated three times, and each time was carried out for 24 h. After the ion-exchange procedure, all the samples were calcined at 550 °C in air for 4 h. The Cu/Al in Cu-AlMCM-41 obtained by neutron activation analysis was 0.372 (i.e., 74.4% ion exchange), and Ce/Al in Ce-AlMCM-41 was 0.164 (i.e., 49.2% ion exchange). In the Ce-Cu-Al-MCM-41 sample, Ce/Al and Cu/Al were 0.053 and 0.37, respectively, i.e., 89.9% total ion exchange. Characterization of the Catalyst. The powder X-ray diffraction (XRD) measurement was carried out with a Rigaku Rotaflex D/Max-C system with Cu KR (λ ) 0.1543 nm) radiation. The samples were loaded on a sample holder with a depth of 1 mm. A Micromeritics ASAP 2010 micropore size analyzer was used to measure the N2 adsorption isotherm of the samples at liquid N2 temperature (-196 °C). The specific surface areas of the samples were determined from the linear part of the BET plots (P/P0 ) 0.05-0.20). The pore size distribution was also calculated from the desorption branch of the N2 adsorption isotherm using the Barrett-Joyner-Halenda (BJH) formula, as suggested by Tanev and Vlaev,23 because the desorption branch can provide more information about the degree of blocking than the adsorption branch, and the best results were obtained from the BJH formula. Prior to the surface area and pore volume measurements, samples were dehydrated at 350 °C for 4 h. The reducibility of catalyst was characterized by temperature-programmed reduction (TPR) analysis. In each experiment, 0.1 g of sample was loaded into a quartz reactor and then pretreated in a flow of He (40 mL/min) at 400 °C for 0.5 h. After the sample was cooled to room temperature in He, the reduction of the sample was carried out from 30 to 700 °C in a flow of 5.34% H2/N2 (27 mL/min) at 10 °C/min. The consumption of H2 was monitored continuously by a thermal conductivity detector. The water produced during the reduction was trapped in a 5-Å molecular sieve column. Catalytic Performance Evaluation of the Catalyst. The SCR activity measurement was carried out in a fixed-bed quartz reactor. The reaction temperature was controlled by an Omega (CN-2010) programmable temperature controller. Sample (0.5 g), as particles of 60-100 mesh, was used in this work. The typical reactant gas composition was as follows: 1000 ppm NO, 1000 ppm C2H4, 0-7.5% O2, 500 ppm SO2 (when used), 2.3% water vapor (when used), and balance He. The total flow rate was 250 mL/min (ambient conditions). The premixed gases (1.01% NO in He, 1.04% C2H4 in He and 0.99% SO2 in He) were supplied by Matheson Company. The NOx concentration was continuously monitored by a chemiluminescent NO/NOx analyzer (Thermo Electro Corporation, Model 10). The other effluent gases were analyzed by a gas chromatograph (Shimadzu, 14A) at 50 °C with a 5-Å molecular sieve column for O2, N2, and CO and a Porapak Q column for CO2, N2O, and C2H4. Results Characterization of the Catalyst. The powder XRD patterns of Al-MCM-41 and the ion-exchanged

Figure 1. XRD patterns of Al-MCM-41, Cu-Al-MCM-41, CeAl-MCM-41, and Ce-Cu-Al-MCM-41. Table 1. Main Characteristics of the Catalysts

sample Al-MCM-41 Ce-Al-MCM-41 Cu-Al-MCM-41 Ce-Cu-Al-MCM-41

BET pore pore spec surf vol, diam, area, m2/g cm3/g nm 917 904 885 871

1.05 1.09 1.03 1.03

4.3 4.2 4.1 4.8

H2/Cu (TPR), mol/mol 1st 2nd peak peak

0.41 0.36

0.51 0.49

Al-MCM-41 samples are shown in Figure 1. The pattern of Al-MCM-41 is consistent with that reported previously for the Al-MCM-41 molecular sieve,22 and all XRD peaks can be indexed on a hexagonal lattice with d100 ) 4.1 nm. According to the value of d100, the unit cell dimension (a ) 4.7 nm) was calculated by the formula a ) 2d100/x3. After the sample was exchanged with copper ions and/or cerium ions, the shapes of the XRD patterns were essentially unchanged, indicating that the ion-exchange process did not affect the framework structure of this molecular sieve. No oxide phase (CuO or CeO2) was detected in these samples. The BET specific surface area, pore volume, and average pore diameters of the Al-MCM-41 and ionexchanged Al-MCM-41 samples are summarized in Table 1. These samples were found to have narrow pore size distributions with a pore size of ca. 4.2 nm, high BET specific surface areas (ca. 900 m2/g), and high pore volumes (>1.00 cm3/g). TPR can be used to identify and quantify the copper species in copper ion-exchanged zeolites.24 As shown in Figure 2, no H2 consumption was found on the Ceexchanged Al-MCM-41 sample below 700 °C, indicating that the cerium ions in Al-MCM-41 are hard to be reduced to lower valence. For the Cu-Al-MCM-41 and Ce-Cu-Al-MCM-41 samples, the TPR profiles showed two reduction peaks, which suggests a two-step reduction process of isolated Cu2+ species.24 One peak appeared at a lower temperature, indicating that the process of Cu2+ f Cu+ occurred. The other peak at a higher temperature suggests that the produced Cu+ was further reduced to Cu0. According to these results, we can conclude that no CuO aggregates existed in the two

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Figure 2. TPR profiles of (a) Ce-Al-MCM-41, (b) Cu-Al-MCM41, and (c) Ce-Cu-Al-MCM-41.

copper ion-underexchanged Al-MCM-41 samples (i.e., Cu/Al < 0.5) because the CuO aggregates would be reduced to Cu0 by H2 in one step at about 230 °C if they existed in the two samples.24 This was in-line with the above XRD result that no CuO phase was detected in the two samples. Copper in Cu-Al-MCM-41 and CeCu-Al-MCM-41 mainly existed in the form of isolated copper ions. Delahay et al. also reported that copper was mainly present as isolated Cu2+ species in underexchanged Cu-Beta and Cu-MFI zeolites.24 The ratios of H2 consumption to Cu for the second peak (Cu+ f Cu0) in the two samples were close to 0.5 (Table 1); by comparison, the H2/Cu ratios for the first peak (Cu2+ f Cu+) were lower than 0.5 (Table 1). This phenomenon could be accounted for as the result of partial reduction of Cu2+ to Cu+ during the course of catalyst preparation (calcination at 550 °C) or pretreatment at 400 °C in a flow of He. In addition, the reduction temperatures of Ce-Cu-Al-MCM-41 were about 20 °C lower than those of Cu-Al-MCM-41 (323 vs 342 °C and 540 vs 561 °C, as shown in Figure 2), suggesting that the Cu2+ and Cu+ ions in the former are more easily reduced than those in the latter. NOx Reduction Activity of the Catalyst. The catalytic performance of the catalysts for the SCR reaction of NOx with C2H4 as functions of the reaction temperature is summarized in Figure 3 and Figure 4. Ce-exchanged Al-MCM-41 was found to be inactive in this reaction at 250-600 °C; almost no NOx was reduced to N2, but some C2H4 was oxidized to CO2 by O2 at higher temperatures. Over the Cu-Al-MCM-41 and Ce-Cu-Al-MCM-41 catalysts, only a small amount of NOx was converted to N2 at 250 °C. With the increase of reaction temperature, NOx conversion increased, passing through a maximum, and then decreased at higher temperatures. No nitrous monoxide was detected in the temperature range. The nitrogen balance was above 95% in this work. The decrease in NOx conversion at higher temperatures was due to the combustion of ethylene. Ce-Cu-Al-MCM-41 showed higher NOx conversions than Cu-Al-MCM-41 in the whole temperature range, indicating that cerium ions played an important promoting effect in the catalyst (because no activity was obtained on the Ce-Al-MCM-41 sample). The maximum NOx conversion reached 38% at 550 °C

Figure 3. Conversions of NOx for the SCR reaction on Ce-AlMCM-41, Cu-Al-MCM-41, Ce-Cu-Al-MCM-41, and CuZSM-5 (data obtained from ref 11). Reaction conditions: catalyst ) 0.5 g, [NO] ) [C2H4] ) 1000 ppm, [O2] ) 2%, He ) balance, total flow rate ) 250 mL/min, and space velocity ≈ 7500 h-1.

Figure 4. Conversions of C2H4 for the SCR reaction on Ce-AlMCM-41, Cu-Al-MCM-41, and Ce-Cu-Al-MCM-41. Reaction conditions are the same as in Figure 3.

over the Ce-Cu-Al-MCM-41 catalyst, which was about 15% higher than that over Cu-Al-MCM-41. The ethylene conversion reached 100% at 500 °C on CuAl-MCM-41 catalyst. The conversions of ethylene on Ce-Cu-Al-MCM-41 were found to be lower than those on Cu-Al-MCM-41 before they reached 100% (Figure 4). It is well-known that oxygen is important in the SCR reaction of nitric oxides by hydrocarbons.2 The effect of O2 concentration on NOx conversion to N2 was also investigated at 500 °C on Ce-Al-Cu-MCM-41 catalyst. As shown in Figure 5, almost no NOx reacted with ethylene in the absence of O2. After O2 was introduced into the feed mixture, NOx conversion was significantly increased. This indicates that oxygen plays an important role for the reduction of NOx by C2H4, as we expected. The maximum conversion reached ca. 38% in the presence of 1-1.6% O2. After that, NOx conversion was found to decrease slightly at higher O2 concentrations. C2H4 conversion always increased with the increase of O2 concentration. The effect of H2O and SO2 on the catalytic activity of the Ce-Cu-Al-MCM-41 catalyst was also studied in this work. As shown in Figure 6, Ce-Cu-Al-MCM-41 was stable in the absence of H2O and SO2. However, when 2.3% water vapor and 500 ppm of SO2 were added

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Figure 5. Effect of O2 concentration on the SCR activity on CeCu-Al-MCM-41 catalyst. Reaction conditions: temperature ) 500 °C, catalyst ) 0.5 g, [NO] ) [C2H4] ) 1000 ppm, He ) balance, total flow rate ) 250 mL/min, and space velocity ≈ 7500 h-1.

Figure 6. Effect of H2O and SO2 on the SCR activity over the Ce-Cu-Al-MCM-41 catalyst. Reaction conditions: temperature ) 500 °C, catalyst ) 0.5 g, [NO] ) [C2H4] ) 1000 ppm, [H2O] ) 2.3% (when used), [SO2] ) 500 ppm (when used), He ) balance, total flow rate ) 250 mL/min, and space velocity ≈ 7500 h-1.

to reaction gases, the NOx conversion was found to decrease from 36% to 21-22% at 500 °C on the catalyst. No deterioration of catalyst was observed after about 30 min of running under these conditions. The effect of H2O and SO2 was reversible. When H2O and SO2 were removed from the reactants, the catalytic activity was restored. Discussion When sodium ions in Al-MCM-41 were partially exchanged by copper ions or copper and cerium ions, the Cu-Al-MCM-41 and Ce-Cu-Al-MCM-41 catalysts were active in the SCR reaction of NOx by ethylene in the presence of excess oxygen. Since the samples without copper (i.e., Al-MCM-41 and Ce-Al-MCM-41) showed little or no activity in this reaction, copper ions clearly play an important role for the reduction of NOx to N2. Iwamoto and co-workers studied the effect of zeolite structure on the catalytic performance of SCR reaction by comparing NOx conversion on different zeolites that were subjected to copper ion exchange.2,16

In addition to Cu-ZSM-5 (Si/Al ) 11.7), Cu2+-exchanged ferrierite (Si/Al ) 6.2), mordenite (Si/Al ) 5.3), zeolite L (Si/Al ) 3.0), and zeolite Y (Si/Al ) 2.8) were also investigated. They found that the maximum activity was obtained on Cu-ZSM-5 and the lowest activity was on Cu-Y. The maximum NOx conversion (ca. 40%) was obtained on Cu-ZSM-5 at 250 °C under the conditions of catalyst ) 0.5 g, [NO] ) 1000 ppm, [C2H4] ) 250 ppm, [O2] ) 2%, and total flow rate ) 150 mL/ min.16 Delahay et al.24 and Corma et al.25 also reported that a similar catalytic activity for NOx reduction was obtained on Cu-Beta zeolite as compared with CuZSM-5. In this work, we found that Cu-Al-MCM-41 was also active for NOx reduction to N2 by ethylene in the presence of excess oxygen, but the activity was lower than that of Cu-ZSM-5. This may be related to their different framework compositions. However, on the Cepromoted Cu-Al-MCM-41 catalyst, we obtained 38% NOx conversion at 550 °C, which was close to the maximum value on the Cu-ZSM-5 catalyst under the same conditions, as shown in Figure 3. Cu-ZSM-5 was prepared by exchanging ZSM-5 (Si/Al ) 30) with Cu(NO3)2 solution and had similar copper content with Ce-Cu-Al-MCM-41 (4.07 wt % in Cu-ZSM-511 vs 3.47 wt % in Ce-Cu-Al-MCM-41). The maximum NOx conversion on Cu-ZSM-5 was obtained at a lower temperature (300 °C), but Ce-Cu-Al-MCM-41 catalyst had a wider temperature window (i.e., a window of 200 °C vs 95 °C at NOx conversion > 25%, as shown in Figure 3). Like Cu-ZSM-5, the catalytic activity on the Ce-Cu-Al-MCM-41 catalyst decreased in the presence of H2O and SO2. The inhibition of catalytic activity by H2O and SO2 has been extensively studied on CuZSM-5 catalyst.4-6,26-28 Two reasons were proposed for the deactivation. One was competitive adsorption for the active sites by H2O and SO2; the other was framework dealumination of ZSM-5 after it was subjected to prolonged exposure to wet exhaust gas at high temperatures.26-28 The former was reversible; the latter was irreversible. It has been reported that Al-MCM41 has a high hydrothermal stability.21,29 For example, it is stable after heating at 800-900 °C for 2 h in the presence of 2.3% H2O/O2.29 In this work, the Ce-CuAl-MCM-41 catalyst was only exposed to 2.3% H2O at 500 °C for a short time; hence, it was impossible for framework dealumination of Al-MCM-41. The reason for deactivation by H2O and SO2 was likely due to competitive adsorption on the copper sites by H2O and SO2. Our previous study also showed that SO2 could adsorb on the same sites as those by NOx over Cu2+exchanged pillared clays.11 H2 TPR and XRD results showed that the copper in Cu-Al-MCM-41 and Ce-Cu-Al-MCM-41 was mainly present in the form of isolated Cu2+ ions. Cu2+ ions could be partially reduced to Cu+ ions when the samples were treated at high temperatures. Almost no CuO aggregates were detected in the two catalysts. This is consistent with the results obtained in the zeolites (e.g., ZSM-5, Beta zeolite, etc.) that were underexchanged by Cu2+.5,24 The isolated Cu2+ ions may play an important role in the SCR reaction of NOx by ethylene. Several authors have claimed that the active species for the SCR reaction of NOx may involve a Cu2+/Cu+ redox cycle in Cu2+-exchanged zeolites.24,25,30 In the presence of hydrocarbon (e.g., C2H4, C3H6, C3H8, etc.), the Cu2+ would be reduced to Cu+ and then Cu+ was oxidized back to Cu2+ by NOx, thus completing the catalytic cycle. The

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X-ray absorption near-edge structure (XANES) study of Liu and Robota30 reported that a significant proportion of copper was in the form of Cu+ in the presence of propylene (one of the selective reducing agents), even in a large excess of oxygen. Furthermore, the concentration of cuprous ions followed the same trend with temperature as that of NOx conversion in the reaction. But the use of a nonselective reducing agent (e.g., methane) did not lead to the formation of Cu+. Hence, they concluded that cuprous ions are essential for this reaction. However, more recently, Haller and co-workers31 used in situ XANES analysis together with a comparison of the catalytic behaviors of Cu-ZSM-5 with CuO/Al2O3 and CuO/SiO2 systems and indicated that the rate-limiting step of the reaction took place on cupric oxides. They suggested that zeolite supports did not play an essential role in the reaction and that the mechanism might not involve a Cu2+/Cu+ redox cycle. Their conclusions were based on the fact that the CuO/Al2O3 sample showed a similar catalytic behavior in the reduction of nitric oxides as compared with Cu-ZSM-5 under their experimental conditions. On the other hand, reports also exist showing that Cu-ZSM-5 (with high exchange, e.g., 80-100%) was more active than CuO-doped Al2O3.4-6 In this work, only isolated Cu2+ and Cu+ ions were detected in the Cu-Al-MCM-41 and Ce-Cu-MCM41 catalysts, suggesting that CuO aggregates may not play an important role in this reaction on these two catalysts. After some cerium ions were introduced into Cu-Al-MCM-41, the copper ions in the molecular sieve become more easily reducible by H2. This may be related to the increase of catalytic activity of NOx reduction by ethylene on the Ce-promoted Cu-Al-MCM-41 catalyst.

Conclusions Both Cu-Al-MCM-41 and Ce-Cu-Al-MCM-41 were found to be active in the reduction of NOx to N2 by ethylene in the presence of excess oxygen. Due to the interaction between cerium and copper, the copper ions in Ce-Cu-Al-MCM-41 could be reduced by H2 more easily than those in Cu-Al-MCM-41. The former also showed higher NOx conversions than the latter in the temperature range of 250-600 °C. In the copper ionexchanged Al-MCM-41 catalysts, the copper species were mainly present in the form of isolated Cu2+ ions, which may play an important role in the selective catalytic reduction of NOx to N2. H2O and SO2 caused some deactivation of the catalyst, but the deactivation was reversible.

Acknowledgment This work was funded by U.S. Department of Energy (DE-FG22-96PC96206).

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Received for review September 28, 1998 Revised manuscript received December 7, 1998 Accepted December 10, 1998 IE980623+