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Activity, stability and characterization of NO oxidation catalyst CoKxTi2O5


Applied Catalysis B: Environmental 85 (2008) 10–16

Contents lists available at ScienceDirect

Applied Catalysis B: Environmental
journal homepage: www.elsevier.com/locate/apcatb

Activity, stability and characterization of NO oxidation catalyst Co/KxTi2O5
Qiang Wang a, So Ye Park b, Linhai Duan b,c, Jong Shik Chung a,b,*
a

School of Environmental Science and Engineering, POSTECH, Pohang 790-784, Republic of Korea Department of Chemical Engineering, POSTECH, Pohang 790-784, Republic of Korea c College of Petrochemical Engineering, Liaoning Shihua University, Fushun 113001, Liaoning, China
b

A R T I C L E I N F O

A B S T R A C T

Article history: Received 25 January 2008 Received in revised form 15 May 2008 Accepted 20 June 2008 Available online 1 July 2008 Keywords: NO oxidation Ion exchange Microcosmic processes K promotion

Co/KxTi2O5 catalyst with well-dispersed nano-Co3O4 particles, synthesized by an ion exchange method from K2Ti2O5 precursor, showed extremely high NO oxidation activity and good stability. Morphological changes and microcosmic processes during the course of catalyst preparation were examined using atomic absorption spectrometry (AAS), X-ray diffraction (XRD), and scanning electron microscopyenergy dispersive X-ray (SEM-EDX). During the ion exchange of K2Ti2O5 with Co precursor solution, K+ ions were replaced by H+ (H3O+). After calcination at 500 8C, a partially collapsed structure was obtained when the exchange was controlled to a certain degree. Fortunately, the Co that precipitated on this collapsed structure formed highly stable nano-particles of Co3O4. Catalytic activity for the NO oxidation was found to be highly dependent on the loading and particle size of Co and K remaining in the KxTi2O5 support. ? 2008 Elsevier B.V. All rights reserved.

1. Introduction NOx abatement is one of the key problems for lean-burn engines that show better fuel economy than conventional engines operated at stoichiometric conditions [1–5]. Technologies recently developed to remove NOx from exhaust gases include continuously regenerating trap (CRT), selective catalytic reduction (SCR), and NOx storage and reduction (NSR) [6–14]. There is increasing acceptance of the view that the oxidation of NO to NO2 is an important pre-requisite step for treating emission gases, as the oxidized NO2 assists the oxidation of soot and promotes the SCR reaction of NOx [15–21]. For NSR technology, NO is also ?rst oxidized to NO2 on platinum and then stored on BaO as nitrate [22– 24]. At present, NO oxidation is mainly achieved by Pt-based catalysts which possess high catalytic activity and have already been commercially available. However, some of their drawbacks including pronounced deactivations in the presence of SO2 or H2O, gradual oxidation of Pt to PtO by the product NO2 and relatively high cost suggest people to search substitutes which are more active and economical than Pt-based catalysts [25–31].

* Corresponding author at: San 31, Hyoja-Dong, Nam-Ku, Pohang 790-784, Republic of Korea. Tel.: +82 54 279 2267; fax: +82 54 279 5528. E-mail address: jsc@postech.ac.kr (J.S. Chung). 0926-3373/$ – see front matter ? 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.apcatb.2008.06.022

SO2 poisoning is the biggest obstacle for NOx abatement. It is believed that SO2 is ?rst oxidized to SO3 on the precious metal Pt; SO3 then reacts with the alumina support to form aluminum sulfate, which covers the surface of g-Al2O3 or plugs the micropores [32–34]. One way to avoid this problem is to ?nd a catalyst that performs the selective oxidation of NO in the presence of SO2 or to modify Pt by adding other transition metals such as Cu, Rh, and Co, among others [35–41]. An alternative approach is to change the solid base g-Al2O3 to TiO2. Takeuchi and Matsumoto [32] reported that TiO2 is the most promising candidate in terms of overcoming the sulfur poisoning, as the stability of sulfates on the TiO2 surface is weaker than those on other oxides. We recently synthesized a new type of catalyst Co/KxTi2O5 via an ion exchange method as a promising substitute for Pt-based catalysts for NO oxidation. Compared with Pt-based catalysts, it showed higher activity for the NO oxidation to NO2 and better resistance to the inhibitory effects of NO2 and SO2 [42]; however, the microcosmic processes that occurred during catalyst synthesis and the reason for its high NO-oxidation activity were not investigated in detail. In the present work, the activity and stability of Co/KxTi2O5 were ?rst studied and compared with Ptbased catalysts; the catalysts were then characterized using various techniques including BET, atomic absorption analysis (AAS), X-ray diffraction (XRD), and scanning electron microscopyenergy dispersive X-ray (SEM-EDX) in an attempt to understand the mechanism of catalyst synthesis and to relate its properties to the superior performance obtained in NO oxidation.

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2. Experimental 2.1. Catalyst synthesis K2Ti2O5 was ?rst synthesized by melting a stoichiometric mixture of K2CO3 (Yakuri Pure Chemical Co., Ltd.) and TiO2 (Hombikat UV 100) at 850 8C, followed by cooling and crushing to a ?ne powder [42]. The solid has a high content of K ion, 30.73 wt.%. Co/KxTi2O5 catalysts with similar amounts of Co loading (20 wt.%) but different amounts of K content were then prepared by changing the concentration of Co precursor and the ion exchange period. Highly active Co/KxTi2O5 catalyst with a high K content of 15.28 wt.% was prepared by soaking 2 g K2Ti2O5 in 200 ml of 0.03 M Co(NO3)2 (Junsei Chemical Co., Ltd.) for 0.15 h. The same types of catalysts with low K contents of 5.67 wt.% (designated Co/ KxTi2O5 (L1)) and 2.15 wt.% (designated Co/KxTi2O5 (L2)) were prepared using a higher concentration of 0.25 M Co(NO3)2 (Junsei Chemical Co., Ltd.) and longer soaking times of 27 and 60 h, respectively. For comparison, an impregnated catalyst (designated Co/ KxTi2O5 (I)) was also prepared. First, 2 g of K2Ti2O5 was soaked in 200 ml pure water for 0.15 h, which was intended to reduce the K content by exchanging with H+ (H3O+) in water. After ?ltration and calcination at 500 8C for 5 h, 20 wt.% of cobalt was doped on the KxTi2O5 support by incipient wetness impregnation; the same method was also used to prepare 20 wt.% Co/TiO2 and 2 wt.% Pt supported on Al2O3 (Merck KGaA) and TiO2 (Hombikat UV 100) with Co(NO3)2 (Junsei Chemical Co., Ltd.) and H2PtCl6?5.7H2O (Kojima Chemical Co., Ltd.) as precursors, respectively. 2.2. Catalyst characterization BET surface areas were measured with a Micromeritics ASAP 2010 sorption analyzer using a static volumetric technique, based on the amount of adsorbed N2 at liquid N2 temperature. The samples were degassed at 200 8C in a vacuum for 5 h before the absorption measurements. The compositions of the catalysts were examined by AAS (Model Spectra AA 800 PerkinElmer, US). The Xray diffraction patterns were obtained using an X-ray analyzer (XRD, M18XHF, Mac Science Co., Ltd., Yokohama, Japan). Ni-?ltered ? Cu Ka radiation (l = 1.5415 A) was used with an X-ray gun operated at 40 kV and 200 mA. Diffraction patterns were obtained within the range of 2u = 5–808 with a step size of 0.028. The morphologies of catalysts were investigated by ?eld emission scanning electron microscopy (FE-SEM, Hitachi, S-4200) and the chemical compositions were determined by energy dispersive Xray (EDX) analysis. 2.3. Catalytic activity tests The catalytic activity tests were performed in a ?ow-reactor, consisting of a packed-bed made of quartz tube (10 mm internal diameter). The reactor was controlled by a proportional-integralderivative (PID) temperature controller/programmer (Han Kook electronic Co.), and the temperature was measured by a K-type thermocouple (0.5 mm outer diameter). The feed gas consisted of 700 ppm of NO and 10% O2 balanced with He. For a typical test procedure, the ?ow rate was adjusted at 300 ml/min and 0.2 g catalyst was chosen to yield a space of velocity of 60,000 h?1. Various gas hourly space velocities (GHSV) can be attained by changing the amount of catalyst. The reacted gases at the reactor outlet were continuously analyzed by means of an NOx analyzer (Chemiluminescence NO-NO2-NOx analyzer, Model 42C, high level, Thermo environmental instruments Inc.). Long-term stability was also investigated by isothermal tests at 300 8C for 24 h.

Fig. 1. NO conversions to NO2 over Co/KxTi2O5, 2 wt.% Pt/Al2O3 and 2 wt.% Pt/TiO2. Feed gas: 700 ppm NO, 10% O2 in He, 300 ml/min; temperature 200–420 8C.

3. Results and discussion 3.1. Catalytic activity and stability Fig. 1 shows NO conversions as functions of temperature for the active Co/KxTi2O5 and Pt-based catalysts. Co/KxTi2O5 showed much higher catalytic activity for the NO oxidation than Pt-based catalysts. When GHSV was kept at 20,000 h?1, a maximum NO conversion of 86% was achieved over Co/KxTi2O5, which is much higher than 41% over Pt/Al2O3 or 59% over Pt/TiO2. The peak temperature of 280 8C at the maximum conversion of 86% is also much lower than the temperatures of 420 8C obtained for Pt/Al2O3 and 340 8C for Pt/TiO2. The existence of peak temperature is due to the thermodynamic equilibrium given in Eq. (1). Even with a much higher GHSV of 60,000 h?1, more than 80% of NO conversion was still achieved at around 300 8C. For Pt supported catalysts, the NO oxidation activities could be affected by various factors. Schmitz et al. [43] reported that the relative order of importance of the factors are support > pre-treatment > loading > calcinations atmosphere > calcination temperature > precursor. A large numbers of studies have further suggested a general agreement that the NO oxidation reaction is highly structure sensitive; the turnover frequency (TOF) increases with increasing the Pt particle size [30,43–46]. Xue et al. [25] also found that the activity is highly dependent on the support and the Pt exposure on the surface. In contrast, Denton et al. [47] reported that the Pt dispersion is a major factor affecting the intrinsic activity of the reactions, while the nature of the support, the porosity and impurities of the support, and the nature of the platinum precursor are less important and not easily discernable. In the present work, we found that Pt supported on TiO2 is more activity than that on Al2O3. According to literatures, the support and the nature of supported Pt (particle size, dispersion and exposure, et al.) might be the reasons and more detailed studies are needed to reveal the intrinsic mechanism. NO2 $ NO ? 1=2O2 (1)

The results obtained for Co/KxTi2O5 show that it is highly stable in NO conversion relative to the stabilities reported previously with Pt/Al2O3 catalyst [48]. The isothermal experiment carried out at 300 8C (Fig. 2) showed that NO conversion decreased only slightly after 1440 min operation, con?rming that Co/KxTi2O5 is stable for a long-term operation.

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Fig. 4. XRD patterns of (A) K2Ti2O5, (B) Co/KxTi2O5 and (C) Co/KxTi2O5 (L2). ($) K2Ti2O5; (&) Co3O4; (~) anatase TiO2. Fig. 2. Isothermal experiment for NO oxidation to NO2 over Co/KxTi2O5 carried out at 300 8C. Feed gas: 700 ppm NO, 10% O2 in He, 300 ml/min; GHSV = 60,000 h?1.

KxTi2O5, Co/KxTi2O5 (L1), and Co/KxTi2O5 (L2), all with similar Co loading but different K. For the active catalyst of Co/KxTi2O5, an appreciable amount of K (15.28 wt.%) remained, whereas for inactive Co/KxTi2O5 (L1) and Co/KxTi2O5 (L2), almost all the K was lost during the ion exchange, with only 5.67 and 2.15 wt.% of K remaining, respectively. The BET surface areas of these three catalysts are much higher than that of K2Ti2O5, indicating that the well-formed K2Ti2O5 collapsed into small pieces during ion exchange and calcination. 3.2.2. XRD analysis Fig. 4 shows the XRD patterns of K2Ti2O5, Co/KxTi2O5, and Co/ KxTi2O5 (L2). The diffraction pattern for synthesized K2Ti2O5 was consistent with that calculated; however, after ion exchange, most characteristic peaks of K2Ti2O5 disappeared or were greatly weakened, suggesting a collapse in the structure. The active catalyst of Co/KxTi2O5 showed a lower degree of structural collapse, with some weak peaks of K2Ti2O5 remaining; however, for the inactive Co/KxTi2O5 (L2), which underwent a longer ion exchange period with a higher Co(NO3)2 concentration, the K2Ti2O5 structure ?nally converted to TiO2 as the majority of the K (93%) was replaced. Co3O4 peaks were found in the XRD patterns of both Co/KxTi2O5 and Co/KxTi2O5 (L2). We therefore conclude that Co/KxTi2O5 is composed of Co3O4 supported on a Krich amorphous-like phase of KxTi2O5, while Co/KxTi2O5 (L2) is composed of Co3O4 supported on anatase TiO2. The XRD results suggested us to postulate that at least two processes occurred during catalyst synthesis: K ion exchange with H+ (H3O+) and cobalt precipitation on the surface. To prove this, we investigated the hydrolysis of K2Ti2O5 in water. A 2 g sample of K2Ti2O5 was soaked in 200 ml of distilled water for 0.15 h, after which it was dried at 105 8C overnight. Half of this dried sample was calcined again at 500 8C for 5 h in air. Both the dried and calcined samples were analyzed by XRD (see Fig. 5). In both the dried and calcined samples, the characteristic peaks of K2Ti2O5 disappeared, showing the phase transformation from K2Ti2O5 to an amorphous or low crystallized phase. A slight shift to the left of the main peak in the dried sample implied that the space between the

Fig. 3. NO conversions to NO2 over Co/KxTi2O5 catalyst during shutdown procedures. Feed gas: 700 ppm NO, 10% O2 in He, 300 ml/min; temperature 300 8C; GHSV = 60,000 h?1.

Mulla et al. [48] reported that the deactivation of Pt/Al2O3 catalyst could be caused by changing conditions and it would deactivate further on the shutdown procedures. In the present work, this shutdown effect was also evaluated. After a reaction for 60 min, the reactor was cooled down to room temperature in an N2 ?ow and then brought to test again, as shown in Fig. 3. For Co/ KxTi2O5 catalyst, no appreciable deactivation was observed, even after three cycles, thereby providing further evidence that this catalyst is highly stable. 3.2. Characterization of catalysts 3.2.1. Composition analysis Table 1 shows the synthesis conditions, BET surface areas, and the contents of K, Co, and Ti for each of the different catalysts Co/

Table 1 Synthesis conditions, surface areas, and the compositions of K, Ti and Co for each of the different catalysts K2T2O5, Co/KxTi2O5, Co/KxTi2O5 (L1) and Co/KxTi2O5 (L2) Catalysts K2Ti2O5 Co/KxTi2O5 Co/KxTi2O5 (L1) Co/KxTi2O5 (L2) Co(NO3)2 (mol/L) – 0.03 0.25 0.25 Initial pH – 5.8 3.8 3.8 Soaking time (h) – 0.15 27 60 K (wt.%) 30.73 15.28 5.67 2.15 Ti (wt.%) 37.74 37.31 34.16 34.48 Co (wt.%) 0 19.17 20.75 20.89 K exchange level (%) 0 50.28 81.54 93.00 BET (m2/g) 0.13 19.11 20.26 22.79

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anatase TiO2 for the support. Co2? ? 4H2 O ! Co?OH?2 ? 2H3 O? ?2-x?H3 O? ? K2 Ti2 O5 ! Kx ?H3 O?2?x Ti2 O5 ? ?2 ? x?K? (2) (3)

Fig. 5. XRD patterns of (A) K2Ti2O5, (B) K2Ti2O5 hydrolytic product in water, dried sample, (C) K2Ti2O5 hydrolytic product in water, calcined at 500 8C, (D) Co/KxTi2O5.

layers was expanded via the ion exchange of K+ (ionic radius ? ? 1.38 A) into H3O+ (ionic radius 1.40 A) [49]. After calcination, however, a similar XRD pattern to those in Co/KxTi2O5 was formed. By heating the sample at 500 8C, H2O in the interlayer was evaporated out, with only H+ remaining as a substitute for K+. The neighboring Ti2O5 layers were too close to each other to stabilize the repulsive force between the layers, leading to collapse of the structure [49]. A more direct line of evidence for the occurrence of K+–H+ (H3O+) ion exchange process was that the ?nal soaking solution became highly basic (pH 13) at equilibrium. The hydrolysis of K2Ti2O5 in acid solution (HCl) again con?rmed this process. Fig. 6 shows the effect of pH in the ?nal hydrolysis solution after the ion exchange of K2Ti2O5. The hydrolytic products are highly dependent on the equilibrium pH. At alkaline to neutral pH, the products were similar to the support in Co/KxTi2O5 with a K-rich amorphous-like phase. When the equilibrium pH was lowered to 3, the product changed to anatase TiO2. We believe that reactions (2) and (3) occurred during the soaking of K2Ti2O5 in Co precursor solution. After calcination at 500 8C for 5 h, Kx(H3O)2?xTi2O5 transformed to an amorphous-like structure of KxTi2O5, and Co(OH)2 is deposited onto surface sites of the support as Co3O4. Thus, increasing the Co(NO3)2 concentration must decrease the equilibrium pH, which results in a higher level of K exchange and the structural collapse of Co/KxTi2O5 (L2) to

3.2.3. SEM-EDX Fig. 7(a) shows an SEM image of the water hydrolysis product of K2Ti2O5 that was calcined at 500 8C for 5 h. Plate-like particles with uneven surfaces are observed, with morphologies the same as those for K2Ti2O5 used as precursor material [50]. Many cracks were found on the surface, indicating that the large particles began to disintegrate into smaller pieces. The chemical composition analyzed by EDX is shown in Fig. 7(b). Only 17.98 wt.% of K remains in the phase, con?rming that 12.7 wt.% of K has been replaced. Fig. 8(a) shows an SEM image of Co/KxTi2O5. Well-dispersed nano-size particles of Co3O4 are clearly seen on the surface. The morphology of the support is the same as that in the hydrolytic product of K2Ti2O5. Chemical composition analyses by EDX were carried out at two different points. Localized point 1 in Fig. 8(b), which represents the KxTi2O5 support phase, shows 12.27 wt.% of remaining K, which is similar to the value analyzed by AAS (15.28 wt.%). Compared with the hydrolytic product of K2Ti2O5 support in pure water (17.98%), more K has been replaced because of the more acidic condition in 0.03 M Co(NO3)2 solution (pH 5.8). The EDX analysis at point 2 is shown in Fig. 8(c). The 26.8 wt.% of Co found at this point is attributed to Co3O4 having been deposited on the surface.

Fig. 6. The hydrolytic products of K2Ti2O5 in acid solution (HCl) with equilibrium pH of 13, 7 and 3; (&) anatase TiO2.

Fig. 7. (a) SEM image of K2Ti2O5 hydrolytic product in water, calcined at 500 8C. (b) EDX analysis of K2Ti2O5 hydrolytic product in water, calcined at 500 8C.

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Fig. 8. (a) SEM image of Co/KxTi2O5 catalyst. (b) EDX analysis of point 1 of Co/KxTi2O5 catalyst. (c) EDX analysis of point 2 of Co/KxTi2O5 catalyst.

Fig. 10. (a) SEM image of Co/KxTi2O5. (b) SEM image of Co/KxTi2O5 (I). (c) SEM image of 20 wt.%Co/TiO2.

Fig. 9. NO conversion as a function of Co loading of Co/KxTi2O5 catalyst.

3.2.4. Cobalt loading and particle size effects on catalytic activity In order to check the cobalt loading effect on the NO oxidation activity, various catalysts with different Co loadings (4.5– 21.5 wt.%) were tested at 300 8C and the results were summarized in Fig. 9. It shows that the conversion of NO to NO2 is largely

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Fig. 11. NO conversions to NO2 over Co/KxTi2O5, Co/TiO2 and Co/KxTi2O5 (I). Feed gas: 400 ppm NO, 5% O2 in He, 300 ml/min; GHSV = 60,000 h?1.

dependent on the Co loading of Co/KxTi2O5. Up to 18 wt.%, the NO conversion shows a steady increase, beyond which a retardation in the conversion increase is observed, probably due to saturation of Co coverage on the support surface. Fig. 10 shows the results of a high-resolution FE-SEM analysis of the particle size of Co3O4 in the three catalysts (Co/KxTi2O5, Co/ KxTi2O5 (I), and Co/TiO2) with the same Co loading of 20 wt.%. For Co/KxTi2O5 (Fig. 10(a)), well-dispersed nano-sized Co3O4 particles are clearly seen on the surface, with an average size of 30–40 nm. For Co/KxTi2O5 (I) (Fig. 10(b)) and Co/TiO2 (Fig. 10(c)), synthesized by impregnation, the particle size is much larger (around 150 nm). Fig. 11 shows the results of reaction tests over these three catalysts. As expected, Co/KxTi2O5 exhibits far superior activity to either Co/ KxTi2O5 (I) or Co/TiO2, indicating that smaller particles of Co3O4 are responsible for the superior performance. 3.2.5. Potassium promotion effect on NO oxidation To assess the K effect on NO oxidation, Co/KxTi2O5 with high K content (15.28 wt.%) was compared with both Co/KxTi2O5 (L1) with 5.67 wt.% K and Co/KxTi2O5 (L2) with 2.19 wt.% K. The results of reaction tests over these three catalysts are shown in Fig. 12. NO oxidation activity gradually decreases with decreasing K content. Co/KxTi2O5 achieved 82% of NO conversion at around 300 8C, while Co/KxTi2O5 (L1) and Co/KxTi2O5 (L2) yielded only 63% at 335 8C and 42% at 345 8C, respectively. Fig. 13 shows a high-resolution SEM image of Co/KxTi2O5 (L2). In comparison with Fig. 10(a), we found that Co/KxTi2O5 (L2) had the same morphology as Co/KxTi2O5,

Fig. 13. SEM image of Co/KxTi2O5 (L2).

which also had nano-Co3O4 particles well dispersed on the support. The particle size is also very small, around 40 nm. The XRD data presented in Fig. 4 and AAS data shown in Table 1 reveal that the only difference between these two catalysts is the support. For Co/KxTi2O5, the support is a K-rich amorphous-like phase with 15.28 wt.% K; for Co/KxTi2O5 (L2), the support transformed to layered anatase TiO2, with only 2.15 wt.% K remaining. In a previous work, XPS analysis also revealed that the Co 2p3/2 binding energy of Co/KxTi2O5 (779.4 eV) was lower than that of Co/TiO2 (779.8 eV) [42]. The above ?ndings provide further con?rmation that K enhances NO oxidation activity. 4. Conclusions Co/KxTi2O5 achieved 86% of NO conversion to NO2 at a relatively low temperature (280 8C), and is even more active than Pt-based catalysts. Long-term operation up to 24 h and shutdown procedure tests revealed that Co/KxTi2O5 is highly stable. Both XRD and SEMEDX analyses con?rmed that at least two processes took place during synthesis: K ion exchanged with H+ (H3O+) to form a K-rich amorphous phase support, and cobalt precipitation, ultimately forming Co3O4 on the surface. The K exchange level was highly dependent on the equilibrium pH, with anatase TiO2 being obtained in acidic solution of pH 3. High-resolution FE-SEM images show that Co/KxTi2O5 contains well-dispersed nano-Co3O4 particles (size 30–40 nm), which is attributed to the ion exchange method; however, for 20 wt.% Co/KxTi2O5 (I) and 20 wt.% Co/TiO2 catalysts synthesized by impregnation, the Co3O4 particle size is much larger, around 150 nm. Finally, it was revealed that the superior performance of Co/KxTi2O5 is strongly related to Co loading, Co3O4 particle size, and the potassium content remaining in the support. Acknowledgements The authors are grateful for ?nancial support from the Korea Institute of Science and Technology Evaluation and Planning (KISTEP, M1-0214-00-0133), the Korea Science and Engineering Foundation (KOSEF, M02-2004-000-10512-0), and the BK21 program of Korea.

Fig. 12. NO conversions to NO2 over Co/KxTi2O5, Co/KxTi2O5 (L1) and Co/KxTi2O5 (L2). Feed gas: 400 ppm NO, 5% O2 in He, 300 ml/min; GHSV = 60,000 h?1.

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