Nonthermal-Plasma-Assisted Selective Catalytic Reaction of NO by

May 12, 2009 - conventional gasoline engine.1,2 The selective catalytic reduction of nitric oxide with ... (1) Liu, Z. M.; Seong, I. W. Catal. ReV. 20...
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Energy & Fuels 2009, 23, 2967–2973

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Nonthermal-Plasma-Assisted Selective Catalytic Reaction of NO by CH4 over CuO/TiO2/γ-Al2O3 Catalyst Hui-Juan Li, Xiao-Yuan Jiang,* Wei-Ming Huang, and Xiao-Ming Zheng* Institute of Catalysis, Department of Chemistry, Faculty of Science, Zhejiang UniVersity, Hangzhou 310028, People’s Republic of China ReceiVed February 3, 2009. ReVised Manuscript ReceiVed April 14, 2009

Removal of NO and CH4 gases was examined using nonthermal-plasma (NTP)-assisted 12% CuO/15% TiO2/γ-Al2O3 under four reaction conditions (NO + CH4, NO + CH4 + O2, NO + CH4 + NTP, and NO + CH4 + O2 + NTP). Methods of Brunauer-Emmett-Teller (BET), transmission electron microscopy (TEM), X-ray diffraction (XRD), H2-temperature-programmed reduction (H2-TPR), and X-ray photoelectron spectroscopy (XPS) were used to investigate the catalyst characteristics before and after the reactions. The results showed that plasma discharge improved low-temperature reaction activity and addition of O2 increased NO and CH4 conversion. Pore size distribution of 15% TiO2/γ-Al2O3 was a typical texture of the alumina support. The specific surface area of 12% CuO/15% TiO2/γ-Al2O3 changed little before and after the NO + CH4 reaction, but its surface crystalline CuO reduced to Cu2O and Cu, which was found by XPS analysis. After the reaction, highly dispersed CuO and bulk CuO reduction peak of the catalyst vanished after the NO + CH4 reaction. Cu2+ still existed to some extent on the surface of the catalyst after the reaction, compared to a significant increase in Cu+ and Cu0 species.

1. Introduction A fuel-efficient vehicle that emits less CO2 is desired for the relief of the energy crisis and global warming. Diesel and leanburn gasoline engines have been the alternatives to the conventional gasoline engine.1,2 The selective catalytic reduction of nitric oxide with hydrocarbons (HC-SCR) recently received increasing attention because of the need to replace the NH3-SCR process.3 Among hydrocarbons, methane offers the benefits of low cost and wide availability compared to other hydrocarbons. Its relative inertness provides, however, lower activity than other reduction gases of most catalytic systems.4,5 Therefore, efforts have been focused on the development of CH4-SCR catalysts that are active, selective, and stable. The main reaction can be described as 2NO + CH4 + O2 f N2 + CO2 + 2H2O.6 The Cu-based catalysts are widely used for NO removal, such as NH3-SCR,7 NO + CO,8 NO + CxHy,9 and NO decomposition.10 A nonthermal-plasma (NTP) technique using electrical discharges gives an innovative approach to economical solution * To whom correspondence should be addressed. Telephone/Fax: +86571-88273272. E-mail: [email protected]. (X.-Y.J.); [email protected] (X.-M.Z.). (1) Liu, Z. M.; Seong, I. W. Catal. ReV. 2006, 48 (1), 43–89. (2) Kaspar, J.; Fornasiero, P.; Hickey, N. Catal. Today 2003, 77 (4), 419–449. (3) Iwamoto, M.; Yahiro, H.; Torikai, T. Chem. Lett. 1990, 11, 1967– 1970. (4) Vesecky, S. M.; Paul, J.; Goodman, D. W. J. Phys. Chem. 1996, 100 (37), 15242–15246. (5) Matthew, M. Y.; Erik, M. H.; Umit, S. O. J. Catal. 2007, 247 (2), 356–367. (6) Mark, D. F.; Jackie, Y. Y. Catal. ReV. 2001, 43 (1-2), 1–29. (7) Sullivan, J. A.; Doherty, J. A. Appl. Catal., B 2005, 55 (3), 185– 194. (8) Jiang, X. Y.; Li, H. J.; Zheng, X. M. J. Mater. Sci. 2008, 43 (19), 6505–6512. (9) Meunier, F. C.; Zuzaniuk, V.; Breen, J. P.; Olsson, M.; Ross, J. R. H. Catal. Today 2000, 59 (3-4), 287–304.

of gas cleaning. In nonthermal discharge, the majority of electrical energy goes into the production of high-energy electrons rather than into gas heating.11,12 The discharge achieves nonthermal conditions through the production of short-lived microdischarges.13 Another possibility is to combine catalysts and plasmas to induce specific reactions under mild thermal conditions.14 Most designs of mixing plasma and catalyst include two independent processes: (1) reaction occurs to some extent at the plasma stage, and (2) primary products transform into final products at the catalyst stage.15,16 Hueso et al.17 found that the plasma reaction converted NO into N2 plus O2, CH4 oxidized into CO + H2O with a hybrid system integrating plasma and La1-xSrxCoO3-d catalyst, and incorporation of the catalyst favored oxidation of CH4 into CO2 at 190 °C. Chen et al.18 investigated the NO + CH4 reaction by dielectric barrier discharge (DBD) plasma combined with γ-Al2O3 catalysts and found that NO conversion increased with increasing CH4 content and reached 94% at 500 °C in a reaction system of 250 ppm NO + 3000 ppm CH4 + 3% O2. Niu et al.19 examined a combination of DBD plasma with Co-HZSM-5 catalyst for (10) Ishihara, T.; Anami, K.; Takiishi, K.; Yamada, H.; Nishiguchi, H.; Takita, Y. Chem. Lett. 2003, 32 (12), 1176–1177. (11) Kim, S. S.; Lee, H.; Na, B. K.; Song, H. K. Catal. Today 2004, 89 (1-2), 193–200. (12) Heejoon, K.; Han, J.; Kawaguchi, I.; Minami, W. Energy Fuels 2007, 21 (1), 141–144. (13) Miessner, H.; Francke, K.; Rudolph, R. Appl. Catal., B 2002, 36 (1), 53–62. (14) Holzer, F.; Roland, U.; Kopinke, F. D. Appl. Catal., B 2002, 38 (3), 163–181. (15) Heintze, M.; Pietruszka, B. Catal. Today 2004, 89 (1-2), 21–25. (16) Tonkyn, R. G.; Barlow, S. E.; Hoard, J. W. Appl. Catal., B 2003, 40 (3), 207–217. (17) Hueso, J. L. E.; Gonza´lez, A. R.; Cotrino, J.; Caballero, A. J. Phys. Chem. 2007, 111 (6), 1057–1065. (18) Chen, Z.; Mathur, V. K. Ind. Eng. Chem. Res. 2003, 42 (26), 6682– 6687. (19) Niu, J. H.; Yang, X. F.; Zhu, A. M.; Shi, L. L.; Sun, Q.; Xu, Y.; Shi, C. Catal. Commun. 2006, 7 (5), 297–301.

10.1021/ef9001007 CCC: $40.75  2009 American Chemical Society Published on Web 05/12/2009

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Li et al. NO and CH4 conversions were defined as follows:

NO (conversion) ) ([NO]in - [NO]out)/[NO]in × 100% CH4 (conversion) ) ([CH4]in - [CH4]out)/[CH4]in × 100% N2 (selectivity) ) 2N2/([NO]in - [NO]out) × 100% N2O (selectivity) ) 2[N2O]/([NO]in - [NO]out) × 100% Figure 1. Schematic diagram of the reaction apparatus.

CH4-SCR and found that N2 was the main product of NOx removal and NOx mostly decomposed below 300 °C. To date, there is limited information about the changes in catalyst phase after simultaneous removal of NO and CH4. This study aimed to examine the NO reduction by CH4 under a combination of DBD and 12% CuO/15% TiO2/γ-Al2O3 and to assess the effect of reaction conditions on NO and CH4 products. Phase changes of 12% CuO/15% TiO2/γ-Al2O3 before and after various cases were also discussed for reaction mechanisms. 2. Experimental Section 2.1. 15% TiO2/γ-Al2O3 Preparation. The 15% TiO2/γ-Al2O3 supports with different titanium dioxides were obtained by a precipitation method. TiO2 was prepared by hydrolysis of TiCl4 in the presence of γ-Al2O3 support (60 min). Probably, some Cu2O grains were coated by a fast-reduced (20) Kim, J. Y.; Rodriguez, J. A.; Hanson, J. C.; Frenkel, A. I.; Lee, P. L. J. Am. Chem. Soc. 2003, 125 (35), 10684–10692.

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Energy & Fuels, Vol. 23, 2009 2969 Table 1. BET Results of 12% CuO/15% TiO2/γ-Al2O3 before and after NO + CH4 Reactions

sample γ-Al2O3 fresh sample after NO + CH4 after NO + CH4 after NO + CH4 after NO + CH4

Figure 2. XRD spectra of 12% CuO/15% TiO2/γ-Al2O3 under different reaction conditions: (9) γ-Al2O3, (0) anatase TiO2, (2) Cu2O, (b) CuO, and (O) Cu, with treatments of (1) fresh, (2) after the NO + CH4 reaction, (3) after the NO + CH4 + NTP reaction, (4) after the NO + CH4 + 5% O2 reaction, and (5) after the NO + CH4 + 5% O2 + NTP reaction.

Cu film, leaving oxygen trapped. The CuO + H2 f H2O + Cu happened near 280 °C, and CuO was mainly reduced to Cu0. Alfonso et al.21 reported an X-ray absorption spectroscopy investigation of the chemical state of copper in Cu/ZrO2 catalysts under CH4/NO and CH4/NO/O2 mixture reaction conditions. The original catalyst consisted mainly of Cu2+ species, after treatment with CH4 from 573 to 773 K. The percentage of the different copper species in the Cu/ZrO2 catalyst was dominated by Cu+ (50%) and Cu2+ species (50%) to Cu0 (30%), Cu+ (60%), and Cu2+ (10%) and turned to Cu0 (20%), Cu+ (55%), and Cu2+ (25%) after treatment with CH4 + NO at 773 K. Drouet et al.22 examined the NO + CO reaction using NixCuyMn(3-x-y)O4 catalysts and found existing species related to catalyst pretreatment. The mainly copper species was Cu+ after treatment with 1% CO at 300 °C, and Cu+ and Cu0 coexisted at 450 °C, meaning that Cu0 preferred higher temperatures. In our study, the 12% CuO/15% TiO2/γ-Al2O3 catalyst was treated with NO + CH4 mixed gases at 773 K, with much Cu0 produced because of the CH4 reductive effect. The co-existence of Cu+ and Cu0 after NO + CH4 + NTP treatment was because of much CO production by plasma discharge at low temperature, and the resultant reductive atmosphere turned CuO into Cu2O and Cu0. In the NO + CH4 + O2 reaction, VCH4/ VO2 ) 4:5 with O2 scarcity and the reductive atmosphere at higher temperature caused Cu2+ reduction to Cu0. In the NO + CH4 + O2 + NTP reaction, O2 was ionized very quickly and produced at lot of CO, and at the same time, the Cu2O + CO f 2Cu + CO2 reaction occurred, which increased the Cu0 content with the appearance of the Cu2O phase. 3.2. BET Measurement. Table 1 shows N2 adsorption by 12% CuO/15% TiO2/γ-Al2O3 before or after NO + CH4 reactions. After NO + CH4 reactions, catalyst surface areas were all decreased, except for the case after the NO + CH4 + O2 reaction. The pore volumes were also decreased especially after the NO + CH4 reaction, but the average pore diameter changed little. (21) Caballero, A.; Morales, J. J.; Cordon, A. M.; Holgado, J. P.; Espinos, J. P.; Gonzalez-Elipe, A. R. J. Catal. 2005, 235 (2), 295–230. (22) Drouet, C.; Alphonse, P.; Rousset, A. Appl. Catal., B 2003, 33 (1), 35–43.

pore Vm pore BET average (N2) volume 2 (m /g) (nm) (mL/g) (mL/g)

177.10 141.07 reaction 123.59 + NTP reaction 135.72 + O2 reaction 144.07 + O2 + NTP reaction 132.24

13.20 12.58 12.70 12.86 12.75 12.27

40.71 31.13 27.58 30.39 31.79 29.43

0.475 0.358 0.285 0.301 0.335 0.305

3.3. TEM Results. TEM is also employed to observe the shape and size of 12% CuO/15% TiO2/Al2O3 catalyst fresh and after various NO + CH4 reactions. From the five micrographs, we can see that there are many pores dispersed on the 12% CuO/15% TiO2/Al2O3 catalysts fresh and after the four reaction and no obvious pore agglomeration appeared, expect for the catalyst after the NO + CH4 reaction, which maybe caused the pore volumes to decrease. The TEM data agrees with our BET results. 3.4. H2-TPR. Figure 4 showed the H2-TPR profiles of different catalysts. In our previous study, CuO was reduced at 356 °C and two reduction peaks occurred at 240 and 289 °C for 12% CuO/γ-Al2O3 and at 318.9 and 387.5 °C for 12% CuO/ TiO2.23 The reduction peak temperature decreased after loading on the γ-Al2O3 and TiO2 supports. It seemed that the synergistic effect of γ-Al2O3 and TiO2 promoted CuO reduction. In addition, no reduction peak appeared for the TiO2/γ-Al2O3 catalyst along test temperatures. With increasing CuO loading, the number of reduction peaks increased gradually. The R and γ peaks became larger gradually and became the largest for 12% CuO/15% TiO2/γ-Al2O3, The β and δ peaks appeared until 9 and 12% CuO loading. When CuO loading was 15%, the β and γ peaks turned small. There was a broad reduction peak named Φ at 425 °C, and its peak area changed little with CuO loading. Larsson et al.24 reported that the TiO2 reduction temperature was about 650 °C, and Xu et al.25 studied the H2-TPR of the Cu-TiO2 system, obtained similar conclusions, and suggested that the titania surface reduction was caused by the spillover of hydrogen from copper metal during the reduction process. When the finely dispersed CuO of the 12% CuO/15% TiO2/γ-Al2O3 sample was reduced, the atomic hydrogen generated by dissociate adsorption on the metallic Cu surface could spill over onto the surface of the lattice TiO2, thereby decreasing the TiO2 reduction temperature. Hu et al.26 reported that CuO-TiO2 had a stronger interaction than CuO-Al2O3 when CuO loaded on the TiO2/γ-Al2O3 support and CuO prior to being disperse on the TiO2 surface and the copper ions may be inserted into the cavity of TiO2. As shown in panel 1 of Figure 4, the γ peak first appeared with increasing CuO loading and, at the same time, the Φ peak changed little with CuO loading. Namely, the γ reduction peak devoted to copper ions may be inserted into the cavity of TiO2. In panel 2 of Figure 4, there was no δ reduction peak after reactions for all 12% CuO/15% TiO2/γ-Al2O3 catalysts and the total peak area decreased, indicating that the content of copper oxide could (23) Li, H. J.; Jiang, X. Y.; Zheng, X. M. Acta Phys.-Chim. Sin. 2006, 22 (5), 584–589. (24) Larsson, P. O.; Andersson, A.; Wallenberg, L. R.; Svensson, B. J. Catal. 1996, 163 (2), 279–293. (25) Xu, B.; Dong, L.; Fan, Y. N.; Chen, Y. J. Catal. 2000, 193 (1), 88–95. (26) Hu, Y. H.; Liu, T. D.; Shen, M. M.; Zhu, H. Y.; Wei, S. T.; Hong, X.; Ding, W. P.; Dong, L.; Chen, Y. J. Solid State Chem. 2003, 170 (1), 58–67.

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Figure 3. TEM micrograph of 12% CuO/15% TiO2/Al2O3 before and after NO + CH4 reactions: (1) fresh, (2) NO + CH4, (3) NO + CH4 + NTP, (4) NO + CH4 + O2, and (5) NO + CH4 + NTP + O2.

Figure 4. TPR spectra of 12% CuO/15% TiO2/γ-Al2O3 with different CuO loadings: (1) CuO loading of (a) 0%, (b) 3%, (c) 6%, (d) 9%, (e) 12%, and (f) 15%; (2) CuO loading of (a) 0%, (b) 3%, (c) 6%, (d) 9%, (e) 12%, and (f) 15% with NO + CH4 treatment; and (3) 12% CuO/15% TiO2/γ-Al2O3 with different NO + CH4 treatments of (a) fresh, (b) NO + CH4, (c) NO + CH4 + NTP, (d) NO + CH4 + O2, and (e) NO + CH4 + NTP + O2.

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Figure 5. XPS spectra of 12% CuO/15% TiO2/γ-Al2O3 under different NO + CH4 conditions: (1) fresh (2) after the NO + CH4 reaction, (3) after the NO + CH4 + NTP reaction, (4) after the NO + CH4 + 5% O2 reaction, and (5) after the NO + CH4 + 5% O2 + NTP reaction.

be reduced by H2 below 300 °C. Luo et al.27 studied the H2-TPR profile of Cu/CeO2 catalysts and found three-reduction peaks along test temperatures, belonging to highly dispersed CuO reduction, Cu2+-incorporated CeO2 lattice reduction, and bulk CuO reduction. After the catalyst was treated by HNO3, only the second reduction peak appeared. In our experiment, much water was produced during the four NO removal reactions. Water and NO might involve the 4NO + 2H2O + 3O2 f 4HNO3 reaction, and the produced HNO3 could dissolve the highly dispersed CuO and bulk CuO, causing a disappearance of R and δ reduction peaks. On the basis of the literature and our findings, we conferred the R reduction peak to the highly dispersed CuO reduction and the β and γ peaks to Cu2+incorporated γ-Al2O3 and TiO2 lattice reductions, respectively. The δ and Φ peaks were attached to bulk CuO and TiO2 reduction, respectively. Panel 3 of Figure 4 indicated that there were no R and δ peaks, except for fresh 12% CuO/15% TiO2/ γ-Al2O3 catalysts. When catalysts were treated under different conditions, the corresponding peak temperatures did not move obviously but the peak areas were different from each other. This might be caused by the content of copper oxide for the catalyst after different reactions. The Φ reduction peak of 12% CuO/15% TiO2/γ-Al2O3 after the NO + CH4 + NTP reaction was larger than others, probably because of the increase of the TiO2 reduction by much Cu2O. 3.4. XPS Measurement. The XPS spectra of the 12% CuO/ 15% TiO2/γ-Al2O3 catalyst after different NO + CH4 reactions showed Cu0, Cu+, or Cu2+ states of copper (Figure 5). The Cu 2p XPS spectrum of CuO could be easily identified by the 2p3/2 main peak at 934.2 eV and the satellite at about 940-945 eV. The binding energy of Cu+ was 1.3 eV lower than that of Cu2+, but no satellite peak appeared. The Cu0 binding energy was 0.3 eV smaller than Cu+.28 There were four kinds of copper species

for 12% CuO/15% TiO2/γ-Al2O3, and the binding energy was 932.4, 932.8, 933.1, and 934.8 eV for Cu (a), Cu (b), Cu (c), and Cu (d), respectively (Figure 5). The main copper species were Cu (c) and Cu (d) for fresh 12% CuO/15% TiO2/γ-Al2O3 catalyst. Cu (a) appeared and Cu (c) disappeared after all of the NO + CH4 reactions. Cu (b) had a binding energy of 932.8 eV and appeared after the NO + CH4 + NTP reaction. In comparison to the standard binding energy of Cu, Cu2O, and CuO and our XRD and H2-TPR results, we devoted Cu (a) to Cu0, Cu (b) to Cu+, Cu (c) to CuO, and Cu (d) to Cu2+ incorporated in Cu-O-Ti-O lattice. The surface copper species of 12% CuO/15% TiO2/γ-Al2O3 were different under different treatments (Table 2). CuO and Cu2+ were the main copper species. Cu0 took up 51.8 and 59.2% for the catalysts after NO + CH4 and NO + CH4 + O2 reaction, respectively. Maybe CH4 was involved in the two reactions, and the copper transform was reduced by CH4. Adding O2 inhabited the reductive atmosphere but promoted the CH4 active ability at higher temperatures, increasing Cu0

(27) Luo, M. F.; Song, Y. P.; Lu, J. Q.; Wang, X. Y.; Pu, Z. Y. J. Phys. Chem. 2003, 111 (34), 12686–12692.

(28) Morales, J.; Caballero, A.; Holgado, J. P.; Juan, P. E.; Gonza´lezElipe, A. R. J. Phys. Chem. 2002, 106 (39), 10185–10190.

Table 2. XPS Results of 12% CuO/15% TiO2/γ-Al2O3 Catalysts after Various NO + CH4 Treatments sample treatment fresh NO + NO + NO + NO +

position and Cu (a) Cu (b) Cu (c) Cu (d) fwhm of content content content content Cu 2p3/2 (%) (%) (%) (%)

934.8 (3.01) 933.1 (2.02) 934.8 (2.90) CH4 932.5 (3.02) 935.0 (3.30) 932.3 (3.35) CH4 + NTP 932.8 (2.94) 934.8 (2.94) CH4 + O2 932.5 (2.93) 934.8 (2.80) CH4 + O2 + NTP 932.4 (3.08)

0

0

60.6

39.3

51.8

0

0

48.2

37.6

38.5

0

53.9

59.2

0

0

40.8

65.2

0

0

34.4

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species on the surface of 12% CuO/15% TiO2/γ-Al2O3 after NO + CH4 + O2. For the catalysts after NO + CH4 + NTP and NO + CH4 + O2 + NTP reactions, the amount of Cu0 and Cu+ species enhanced greatly, indicating that the reductive gases increased under plasma discharge. The reduced gases of CO from the two reactions augmented obviously. Fernandez-Garcia et al.29 studied the nature of copper species in the NO + CO reaction with Cu/Al2O3 catalysts by XANES measurement and found that the content of surface Cu0 was determined by the reaction temperature and reactive gases component; i.e., the higher temperature and reactive gases, the more Cu0 produced. Otherwise, more Cu+ would be produced. When the NO + CH4 + NTP and NO + CH4 + O2 + NTP reactions were compared, the CO selectivity of the former reaction was higher than the latter (Table 2). The CO produced from NO + CH4 + NTP at low temperature reduced Cu2+ to Cu+, compared to less CO at higher temperature and more Cu+ on the surface of catalysts.30 Drouet et al.22 investigated the copper species of the NixCuyMn(3-x-y)O4 catalyst pretreated with 1% CO and found that the main copper species was Cu+ at 300 °C and Cu+ and Cu0 co-existed at 450 °C. The former catalytic reactivity was much higher than the latter and fresh catalysts; this meant that Cu+ played a key role in the NO + CO reaction. Hu et al.31 related the H2-TPR results to NO + CO reaction activity and found that better catalytic activity by Cu/CeO2-Al2O3 than CuO/ Al2O3 was because the former produced Cu+ at lower temperature than the latter and Cu+ was the activity component. In addition, XPS detected Cu+ species after the NO + CO reaction at 200 °C, and Cu2+ turned to Cu+ and Cu0 completely at 300 °C. They believed that the catalytic component was Cu2+ and Cu+ at 200 °C and Cu+ and Cu0 at 300 °C. According to our results, the activity component was Cu+ at lower temperatures and Cu0 at higher temperatures. The NO + CH4 + NTP and CH4 + NO + O2 + NTP reactions had better lower temperature activity than the others and more Cu0 and Cu+ species, and at higher temperatures, the catalysts had a lot of Cu0 species and higher catalytic activity. 3.5. Catalytic Activity. The conversion of NO and CH4 by the 12% CuO/15% TiO2/γ-Al2O3 catalyst is a function of the reaction temperature (Figure 6). NO conversion increased with increasing temperature. At T ) 500 °C, NO conversion was 17.1% in the NO + CH4 reaction and reached 26.8% in the NO + CH4 + O2 reaction. With plasma discharge, NO conversion over catalyst increased greatly. At T ) 300 °C, the efficiency of NO removal was 42.6% in the NO + CH4 + NTP reaction and 39.6% in the NO + CH4 + O2 + NTP reaction and there was almost no NO conversion for the NO + CH4 or NO + CH4 + O2 reaction, which indicated that plasma discharge could improve NO activity at low temperatures. At the same time, the N2 and N2O selectivity was 76.3 and 23.7% in the NO + CH4 + NTP reaction and 63.8 and 36.2% in the NO + CH4 + O2 + NTP reaction, respectively. The total selectivity was 100% in the two reactions, which meant that no other NOx species were involved. The total NO conversion was obtained in the NO + CH4 + NTP reaction at 450 °C, whereas NO (29) Fernandez-Garcia, M.; Marquez, A. C.; Rodriguez-Ramos, I.; Guerrero-Ruiz, A.; Haller, G. L. J. Phys. Chem. 1995, 99 (44), 16380– 16382. (30) Jiang, X. Y.; Ding, G. H.; Lou, L. P.; Chen, Y. X.; Zheng, X. M. J. Mol. Catal. 2004, 218 (2), 187–195. (31) Hu, Y. H.; Dong, L.; Shen, M. M.; Liu, D.; Wang, J.; Ding, W. P.; Chen, Y. Appl. Catal., B 2001, 31 (1), 61–69.

Li et al.

Figure 6. NO conversion (1) and CH4 conversion (2) by 12% CuO/ 15% TiO2/γ-Al2O3 under different NO + CH4 reaction conditions.

conversion was only 17.1% in NO + CH4, 26.8% in NO + CH4 + O2, and 61.4% in NO + CH4 + O2 + NTP reactions at 500 °C. This indicated that NO conversion was enhanced by plasma discharge in NO + CH4 + NTP and NO + CH4 + O2 + NTP reactions. As shown in panel 2 of Figure 6, CH4 conversion in the NO + CH4 reaction increased with temperature and plasma discharge or O2 existence enhanced CH4 removal greatly. The above results suggested that NO and CH4 reacted as selective catalytic reduction. As shown in Figure 7, N2 selectivity increased with the reaction temperature and plasma discharge or adding O2 promoted N2 production. The pattern of N2O selectivity with temperature appeared as a volcano type for all of the reactions. These phenomena illustrated that N2O was the midproduct only. The conversion of CH4 into CO2 and CO can be accelerated by plasma discharge and O2 (Figure 8). CO2 was produced at 350 °C in the NO + CH4 reaction and at 300 °C in the NO + CH4 + O2 reaction, which was concurrent with N2 production. Interestingly, CO was produced with CO2 in the plasma-catalyst reaction, and CO selectivity decreased at higher temperature, probably occurring as the 2CO + O2 f 2CO2 reaction. With only catalyst, NO converted into N2 and CH4 converted into CO2. When plasma discharge and catalyst co-existed, CH4 reacted with NO as CH4-SCR and with O2 as a partial oxide reaction to produce CO. At the same time, NO conversion increased remarkably, indicating that plasma discharge, which could produce CO, improved the NO conversion. In our prior

NTP-Assisted SelectiVe Catalytic Reaction

Figure 7. Effects of the temperature on the selectivity of N2 (1) and N2O (2) produced in the NO + CH4 reaction under different conditions.

findings,23 we concluded that the 12% CuO/15% TiO2/γ-Al2O3 catalyst had the better NO + CO reaction activity. In addition, adding O2 had a significant effect on CH4-SCR. However, the mechanism of plasma discharge and O2 effluence on NO conversion was complex. NO conversion increased in the NO + CH4 reaction but restrained in the NO + CH4 + NTP reaction with adding O2. In the C3H6-SCR reaction catalyzed by Sn/ γ-Al2O3, NO conversion increased when the concentration of O2 increased from 2 to 8% and the TPD-NOx results of Sn/ γ-Al2O3 showed that NOx gases were easily adsorbed by catalyst because of the presence of O2.32 Niu et al.19 investigated O2 adding to the NO + CH4 reaction by plasma-assisted CoHZSM-5 catalyst and found that NO conversion decreased from 50 to 30% when the O2 concentration changed from 0 to 12%. It seemed that NO decomposition (2NO f N2 + O2) was the (32) Li, J. H.; Hao, J. M.; Fu, L. X.; Liu, Z. M.; Cui, X. Y. Catal. Today 2004, 90 (3-4), 215–221.

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Figure 8. Effects of the temperature on the selectivity of CO2 (1) and CO (2) produced in the NO + CH4 reaction under different conditions.

main reaction under the plasma-discharge conditions and would be restricted by existing O2. 4. Conclusions (1) The catalytic activities of 12% CuO/15% TiO2/γ-Al2O3 were investigated in the NTP-assisted NO + CH4 reduction. The catalyst accelerated NO and CH4 removal, and adding O2 into NO + CH4 mixed gases also increased NO and CH4 conversion. (2) The specific surface area of the catalyst changed little before and after the reaction. The bulk CuO phase of fresh 12% CuO/15% TiO2/γ-Al2O3 and more Cu0 or Cu2O appeared after various NO + CH4 reactions, but highly dispersed CuO and bulk CuO reaction peaks vanished. There was much Cu2+ on the surface of the catalyst before and after different NO + CH4 reactions, whereas Cu+ and Cu0 were increased remarkably by plasma discharge. (3) The activity component was Cu+ at lower temperature and Cu0 at higher temperature. The plasmaassisted catalyst reaction could enhance Cu+ and Cu0 production and thus NO and CH4 conversions. EF9001007