Al2O3 for NO reduction with CH4

Catalysis Communications 5 (2004) 527–531 www.elsevier.com/locate/catcom

Promoting effect of La2O3 and Nd2O3 on PdO/Al2O3 for NO reduction with CH4 Kazuhito Sato a, Yasushi Ozawa b, Ayako Watanabe a, Masatoshi Nagai

a,*

a

b

Graduate School of Bio-Applications and Systems Engineering, Tokyo University of Agriculture and Technology, 2-24-16 Nakamachi, Koganei, Tokyo 184-8588, Japan Yokosuka Research Laboratory, Central Research Institute of Electric Power Industry, 2-6-1 Nagasaka, Yokosuka, Kanagawa 240-0196, Japan Received 19 December 2003; accepted 29 June 2004 Available online 3 August 2004

Abstract The promoting effect of La2O3 and Nd2O3 addition on the catalytic activity of PdO/Al2O3 for the NO reduction with CH4 was studied in the absence of O2. The addition of La2O3 and Nd2O3 to PdO/Al2O3 is due to promoting the NO reduction to N2 and preventing the formation of HCN, NH3, N2O and carbons. It has been significantly observed by XRD that hydrogen-loaded Pd was formed. The La2O3 and Nd2O3-added PdO/Al2O3 catalysts were characterized by CH4-TPSR and NO reduction in the mixed stream of NO and CH4. Ó 2004 Elsevier B.V. All rights reserved. Keywords: Promoting effect; Palladium oxide; Lanthanum; Neodymium; NO reduction; Methane; Characterization

1. Introduction The reduction of NO with NH3 has been developed and established for exhausts from power plants fueled with natural gas. There are several problems regarding the toxicity, transportation, and storage of NH3 in cogeneration plants using gas engines established in urban area. The reduction of NO with CH4 is an attractive strategy for gas engines fueled with methane. The Pdexchanged H/MFI catalyst is well known to show high activity for CH4-SCR [1,2] due to the highly active Pd2+ ion sites which convert NO with CH4 to N2 [3,4]. However, the Pd-H/MFI was severely deactivated in the presence of water because it had a low Pd stability due to the transfer of Pd cations in the micropores of the zeolite to the outer shell of the surface [3] with the agglomeration of PdO [5,6]. On the other hand, alumina

*

Corresponding author. Tel./fax: +81-42-388-7060. E-mail address: [email protected] (M. Nagai).

1566-7367/$ - see front matter Ó 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.06.012

is more resistant to water than the H/MFI zeolite because of its structural stability. Pd/Al2O3 was active for the direct decomposition of NO to N2 [7,8]. The addition of small amounts of lanthanum, praseodymium, and neodymium to a palladium catalyst was reported to effectively reduce the sintering of alumina [9]. Also, the addition of La2O3 to PdO/Al2O3 shifted the temperature of the transformation of PdO to Pd from 110 to 190 K to react with alumina. Recently, Toops et al. [10] reported that La2O3/c-Al2O3 has an activity for NO reduction with CH4. Furthermore, the addition of Nd2O3 to PdO forms Nd4PdO7, which is weakly bonded Pd with Nd2O3 and more stable than PdO [11], and it shifted the transformation of PdO to Pd to a higher temperature. In a previous paper [12], the addition of both Nd2O3 and La2O3 to PdO/Al2O3 during CH4 combustion significantly prevents the agglomeration of PdO and the deactivation by forming water. In this study, we report the NO reduction with CH4 over both Nd2O3- and La2O3-added PdO/Al2O3 catalysts at temperatures of 473–773 K. The reaction

K. Sato et al. / Catalysis Communications 5 (2004) 527–531

products were analyzed by GC and mass spectrometry. The XRD analysis verified the hydrogen-loaded Pd after the reaction. The temperature-programmed surface reaction with methane (CH4-TPSR) was studied to determine the formation of graphitic carbons and the reactivity of the catalysts with methane.

2. Experimental Catalysts composed of PdO and Al2O3, and/or Nd2O3 and La2O3 (weight ratio of 23:100:3:3) were prepared. c-Alumina (BET surface area: 94 m2 g
1, mean particle diameter: 4.09 lm) was impregnated with an aqueous solution of Nd(NO3)3 Æ 6H2O and La(NO3)3 Æ 6H2O with stirring, dried at 383 K for 1 h, and calcined at 1273 K for 2 h. The resulting solid was impregnated with an aqueous solution of Pd(NO3)2, dried at 383 K for 1 h, and then calcined at 1073 K for 2 h. The solid was molded, crushed and sieved to 9–16 mesh. NO reduction was carried out using a conventional fixed-bed flow reactor made of quartz glass at atmospheric pressure with 0.03 g of catalyst. NO reduction was carried out at temperatures of 473–773 K and at the total rate of 1.0 ml s
1 of NO/ CH4 (1:3) in He for 1 h after the catalyst was heated to 873 K at the rate of 0.167 K s
1 in flowing 10% O2/He and kept at 873 K for 1 h. The NO and reaction products were measured by a TCD connected online with a Molecular Sieve 5A column to separate the N2 and CO and a Porapak Q column to separate the N2O and CO2. The concentration of NOx (NO and NO2) was continuously monitored by a chemiluminescence NOx analyzer (Shimadzu, NOA-305A) to check the catalytic activity. The reduction rate of NO to N2 was calculated using the following equation: v ¼ FCX =W ;

ð1Þ
1

where v represents the reaction rate (lmol s ), F the total gas flow rate (ml s
1), C the inlet mole concentration of NO (lmol ml
1), X the conversion of NO to N2 (
) and W the catalyst weight (g). The CO chemisorption of the catalysts was measured by the pulse method [13]. Before the CO adsorption was measured, the catalysts were heated in a stream of O2 from room temperature to 573 K at 25 K min
1, held at 573 K for 15 min, and then purged with He for 5 min. Helium gas was changed to H2. The catalysts were heated again from 573 to 673 K at 25 K min
1 and held at this temperature for 11 min. The catalysts were cooled to 323 K in He flow and then the amount of CO adsorption was obtained. The CO chemisorption was calculated based on the Pd metal loadings of the catalysts. The BET surface area of the catalysts was measured (Gemini 2360, Micromeritics). The formation of NH3 and HCN was determined using a Balzers quadrupole mass spectrometer (Quadstar 422) in the mixed stream

of NO and CH4 (1:1) at 723 K. The desorption of H2, NH3, H2O, HCN, N2 and CO2 was measured from m/ z = 2, 17, 18, 27, 28 and 44, respectively. The formation of H2 and H2O during the reaction above 373 K was characterized using the temperature-programmed surface reaction with CH4 (CH4-TPSR) under in situ conditions after oxidation of the catalysts. For the CH4-TPSR measurement, the catalysts were heated in the CH4 stream at 0.167 K s
1 to a final temperature of 1173 K. Before CH4-TPSR, the catalysts were heated from room temperature to 673 K for 3 h in dry air, purged in flowing He (0.25 ml s
1) for 0.5 h at 673 K, and cooled to room temperature. Gases desorbed during the reaction of both NO and CH4 were analyzed using the quadrupole mass spectrometer. The XRD spectra of the catalysts were taken using a RAD-II (Rigaku ˚ ). Co.) equipped with Cu Ka radiation (k = 1.542 A

3. Results and discussion The reduction of NO with CH4 over the various PdO/ Al2O3 catalysts at the reaction temperatures of 473–773 K is shown in Figs. 1 and 2. The N2 yields for the four catalysts were low below 523 K, but rapidly increased up to ca. 600 K and the NO conversion obtained by the NOx analyzer was 100%. N2 yields for all the catalysts gradually decreased at the higher temperatures up to 773 K, while the decrease in the N2 yields was more pronounced for PdO/Al2O3 and Nd–PdO/Al2O3 than for La Æ Nd–PdO/Al2O3 and La–PdO/Al2O3. The N2 yield for the undoped PdO/Al2O3 was the highest (12%) at 523 K, whereas that for the other catalysts, especially La Æ Nd–PdO/Al2O3 (44%), was higher than that for the PdO/Al2O3 at 773 K as shown in Fig. 1. The main product was N2 together with N2O, CO and CO2 during the reaction. N2O was detected from 523 to 723 K for

100

N2 and N2O Yield and NO conversion /%

528

80 60 40 20 0 450

550

650 Temperature /K

750

Fig. 1. Reduction of NO with methane (1:3) over various La Æ Ndadded PdO/Al2O3s. (d, s, ) PdO/Al2O3, (j, h, ) La Æ PdO/Al2O3, (N, M, ) Nd Æ PdO/Al2O3 and (r, , ) La Æ Nd–PdO/Al2O3. (Close) N2, (Open) N2O and (Gray) NO.

K. Sato et al. / Catalysis Communications 5 (2004) 527–531

100

CO and CO2 Yield /%

80 60 40 20 0 450

550

650

750

Temperature /K Fig. 2. Formation of CO and CO2 during reduction of NO with methane (1:3) over various La Æ Nd added PdO/Al2O3s. (d, s) PdO/ Al2O3, (j, h) La Æ PdO/Al2O3, (m, n) Nd Æ PdO/Al2O3 and (r, }) La Æ Nd–PdO/Al2O3. (close) CO and (open) CO2.

PdO/Al2O3, while for La Æ Nd–PdO/Al2O3 it was observed only at 473 and 523 K. As shown in Fig. 2, the CO formation for the La and/or Nd doped catalysts was higher than the undoped catalyst, while the CO2 formations for the four catalysts were similar. This result suggests that the addition of La and/or Nd leads to CO formation from the reaction of the carbon atom of CH4 with an oxygen, and not to carbon deposition (mentioned later). From the CO chemisorption (Table 1), the addition of Nd or both La and Nd decreased the number of active sites, although the BET surface areas of the four catalysts were similar. The La Æ Nd– PdO/Al2O3 catalyst was less active than the PdO/Al2O3 catalyst. Suhonen et al. [11] found that the addition of Nd formed a thermally stable species such as Nd4PdO7 which was less dispersed than PdO at low temperature. The addition of La and Nd probably formed a new thermally stable species such as Nd4PdO7 in the catalyst preparation, although small amounts of the species were not able to be identified using XRD. Therefore, the species lowered the activity of the PdO/Al2O3 catalyst at low temperature, whereas they promoted the reaction of NO to N2 at high temperature due to the thermal stability of the species. The mass balance of NO, N2 and N2O decreased with the increasing reaction temperature as shown in Fig. 1.

529

As HCN and NH3 were not analyzed by the GC, it was determined whether or not these compounds were formed during the reaction by mass spectrometry. Fig. 3 shows the profile of the mass spectroscopic experiment of both NO and CH4 over La Æ Nd–PdO/Al2O3 and PdO/Al2O3 at 723 K. La Æ Nd–PdO/Al2O3 (B) had lower peaks of NH3 and HCN than those for PdO/Al2O3 (A). This result showed that the mass balance deficit above 673 K is due to the formation of NH3 and HCN. The addition of both La2O3 and Nd2O3 prevented the formation of HCN and NH3, but promoted the NO reduction to N2. Furthermore, the small PdO peaks and large Pd peaks were observed for La Æ Nd–PdO/Al2O3 and PdO/ Al2O3 from the XRD data for both catalysts after the reaction (Fig. 4). It is noted that the La Æ Nd–PdO/ Al2O3 only exhibited the shoulder peak with each Pd peak which was ascribed as H-loaded Pd (2h = 39.06°, 45.41°, 60.17° and 79.60°). No peaks of graphitic carbons were observed for the two catalysts. Small particles of Pd were formed on which CH4 can dissociate into fragments (H and CHx (x = 1–3)). Hydrogen was subtracted in the Pd loading process. Therefore, the addition of La and Nd forms the H-loaded Pd which promotes the reduction of NO to N2 during the CH4 decomposition. In order to determine the deposition of graphitic carbons and the reactivity of the catalysts with CH4, the CH4-TPSR was carried out in a stream of CH4 over PdO/Al2O3 and La Æ Nd–PdO/Al2O3 (Fig. 5). Methane was decomposed to form hydrogen, water, and CO2 during the CH4-TPSR. For PdO/Al2O3 (A), the formation of H2 gradually increased up to 800 K and then rapidly, while the H2O and CO2 formations decreased above approximately 700 K. The formation of H2 for La Æ Nd–PdO/Al2O3 was higher than that for PdO/ Al2O3 and those of H2O and CO2 were lower up to ca. 950 K. On the contrary, above these temperatures, the relationship was reversed. The La Æ Nd–PdO/Al2O3 and PdO/Al2O3 catalysts after CH4-TPSR were determined by XRD in Fig. 6. For the Pd/Al2O3 catalyst, Pd metal and graphitic carbons were observed with a small amount of PdO after the reaction. For the La Æ Nd– PdO/Al2O3, no graphitic carbons were observed but Pd metal and a small amount of PdO formed. Consequently, methane was decomposed on La Æ Nd–PdO/

Table 1 Physical properties and catalytic activities of the four catalysts Physical properties and activity BET surface area (m2 g
1) CO chemisorption (lmol g
1) NO to N2 reduction rateb (lmol s
1 g
1) a b

Fresh catalyst. At 523 K.

Catalysta PdO/Al2O3

La–PdO/Al2O3

Nd–PdO/Al2O3

La Æ Nd–PdO/Al2O3

56 34 0.177

57 34 0.085

56 29 0.095

52 31 0.106

530

K. Sato et al. / Catalysis Communications 5 (2004) 527–531 -12

2.0x10

Intensity / a.u.

Intensity / a.u.

-12

1.5x10

-12

-8

4.0x10

-8

3.0x10

-8

2.0x10

-8

1.0x10

-8

H2

NH3

A

1.0x10

5.0x10

B

A

-13

1.2x10

-13

1.0x10

-13

8.0x10

-14

6.0x10

-14

4.0x10

-14

2.0x10

-14

HCN A

0.0

Intensity / a.u.

Intensity /a.u.

5.0x10

B

0

10

20

30

40

50

1.0x10

-8

8.0x10

-9

6.0x10

-9

4.0x10

-9

2.0x10

-9

60

Fig. 3. Formation of NH3 and HCN in MS profile during both NO and CH4 (1:1) reaction for: (A) PdO/Al2O3; (B) La Æ Nd-added PdO/ Al2O3 at 723 K.

Intensity / a.u.

Time on stream / min

B

B A

H2O

A

B B

A

1.2x10

-10

1.0x10

-10

8.0x10

-11

6.0x10

-11

4.0x10

-11

2.0x10

-11

A

CO2

B B

0.0 400

A 60 600 800 1000 Temperature / K

Fig. 5. CH4-TPSR profile over: (A) PdO/Al2O3; (B) La Æ Nd–PdO/ Al2O3. (h) H2, (d) H2O and (r) CO2.

Fig. 4. XRD patterns after reaction of NO and CH4 (1:1) over: (A) PdO/Al2O3; (B) La Æ Nd–PdO/Al2O3. (s) Pd, (d) PdO, (m) H-loaded Pd and (n) Al2O3.

Al2O3 to form water and carbon dioxide during the CH4-TPSR, but it was preferentially decomposed to hydrogen and graphitic carbons on PdO/Al2O3. Amorphous carbon for PdO/Al2O3 would be formed more than La Æ Nd–PdO/Al2O3 during the NO reduction with CH4, although no graphitic carbons were detected by XRD as shown in Fig. 4. Albers et al. [14] reported that carbon formation inhibited the formation of the b-phase Pd hydride, suggesting the poisoning of the H-loaded Pd formation with carbons in the same way. Thus, the addition of La and Nd prevented carbon formation and promoted the formation of H-loaded Pd which then catalyzed the NO reaction with hydrogen in the Hloaded Pd. The NO reduction with CH4 to N2 over the promoted PdO/Al2O3 catalysts proceeded as follows: NO reacted with CH4 to form N2, N2O, CO2 and H2O at low temperature. The NO reduction to N2 decreased with the increasing reaction temperature. Above 623 K, HCN

K. Sato et al. / Catalysis Communications 5 (2004) 527–531

531

temperature. The addition promoted the formation of CO and H2O and prevented that of HCN, NH3, N2O, H2 and carbons at high temperature. It was concluded that the La and Nd addition prevented carbon formation and promoted the formation of the thermally stable Pd compound and the H-loaded Pd which catalyzed the reaction of the oxygen atom of NO with hydrogen.

Acknowledgement This study was carried out as a research project with a Grant-in-Aid for Scientific Research from the Ministry of Education, Sport, Culture and Science, Japan.

References

Fig. 6. XRD patterns after CH4-TPSR for: (A) PdO/Al2O3; (B) La Æ Nd–PdO/Al2O3. (s) Pd, (d) PdO, (n) Al2O3 and (r) carbon.

and NH3 were additionally formed together with carbons for PdO/Al2O3. NO readily reacted with hydrogen in the H-loaded Pd which was promoted by the La and Nd addition. The addition attained a high level conversion of NO to N2 with preventing the formation of HCN, NH3 and N2O at high temperature.

4. Conclusion The addition of both La2O3 and Nd2O3 to PdO/ Al2O3 increased the NO conversion to N2 more than that with La2O3 or Nd2O3 and without them at high

[1] Y. Nishizaka, M. Misono, Chem. Lett. (1993) 1295. [2] B. Wen, Q. Sun, W.M.H. Sachtler, J. Catal. 204 (2001) 314. [3] C. Descorme, P. Gelin, C. Lecuyer, M. Primet, Appl. Catal. B 13 (1997) 185. [4] L.J. Lobree, A.W. Aylor, J.A. Reimer, A.T. Bell, J. Catal. 181 (1999) 189. [5] K. Okumura, J. Amano, N. Yasunobu, M. Niwa, J. Phys. Chem. B 104 (2000) 1050. [6] M. Ogura, S. Kage, M. Hayashi, M. Matsukata, E. Kikuchi, Appl. Catal. B 27 (2000) L213. [7] C.T. Yeh, R.J. Wu, T.Y. Chou, Appl. Catal. B 6 (1995) 105. [8] M. Haneda, Y. Kintaichi, I. Nakamura, T. Fujitani, H. Hamada, J. Catal. 218 (2003) 405. [9] F. Oudet, P. Courtine, A. Vejux, J. Catal. 114 (1988) 112. [10] T.J. Toops, A.B. Walters, M.A. Vannice, J. Catal. 214 (2003) 292. [11] S. Suhonen, M. Valden, M. Pessa, A. Savima¨ki, M. Ha¨rko¨nen, M. Hietikko, J. Pursiainen, R. Laitinen, Appl. Catal. A: Gen. 207 (2001) 113. [12] Y. Ozawa, Y. Tochihara, M. Nagai, S. Omi, Catal. Commun. 4 (2003) 87. [13] Shokubai (Catalyst) 31 (1989) 317, in Japanese. [14] P. Albers, J. Pietsch, S.F. Parker, J. Mol. Catal. A: Chem. 173 (2001) 275.