Positive effect of coexisting SO2 on the activity of supported iridium catalysts for NO reduction in the presence of oxygen

Applied Catalysis B: Environmental 41 (2003) 157–169

Positive effect of coexisting SO2 on the activity of supported iridium catalysts for NO reduction in the presence of oxygen Tomohiro Yoshinari a , Kazuhito Sato a , Masaaki Haneda b , Yoshiaki Kintaichi b , Hideaki Hamada b,∗ b

a Tsukuba Laboratory, Petroleum Energy Center, AIST Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan National Institute of Advanced Industrial Science and Technology, AIST Central 5, 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan

Received 10 December 2001; received in revised form 13 April 2002; accepted 14 April 2002

Abstract Silica-supported iridium catalyst showed excellent activity for NO reduction with H2 in the presence of O2 and SO2 . This reaction took place only on Ir and Rh/SiO2 , whereas on Pt, Pd and Ru/SiO2 SO2 showed inhibiting effect on NO reduction. SiO2 was the only effective support for Ir. The most outstanding feature was that the coexistence of O2 and SO2 is essential for NO reduction to occur. The NO reduction activity did not change much in the presence of 10–150 ppm of SO2 , and H2 O did not affect the activity, either. Concerning O2 concentration, only a small amount of O2 was sufficient for the activity increase and further increase of O2 concentration decreased NO conversion. The increase of Ir loading and H2 amount enhanced NO conversion. Accordingly, NO conversion of more than 40% was obtained in the presence of 10% O2 at a space velocity of 50,000 h−1 . The FT-IR study revealed that the presence of O2 and SO2 promoted the adsorption of NO and formation of NHx species on Ir/SiO2 . © 2002 Elsevier Science B.V. All rights reserved. Keywords: Nitrogen monoxide; Selective reduction; Hydrogen; Iridium catalyst; Silica; Oxygen; SO2 promoting effect; Infrared study

1. Introduction Air pollution by nitrogen oxides (NOx ) emitted from various engines and combustors is a serious environmental problem. Among a number of measures to reduce NOx , catalytic removal of NO in exhaust gases is one of the most practical and efficient methods. The three-way catalyst for gasoline-fueled vehicles and the selective catalytic reduction using ammonia as a reductant for large-scale boilers have already been commercialized. However, these two ∗ Corresponding author. Tel.: +81-298-61-9329; fax: +81-298-61-4457. E-mail address: [email protected] (H. Hamada).

catalytic methods cannot be applied to the exhaust from diesel and lean-burn engines, because three-way catalyst does not work for oxygen-rich exhaust gases and ammonia is a poison to the catalyst. In this regard, the selective catalytic reduction of NO with hydrocarbons (HC-SCR) in the presence of oxygen has attracted much attention, for which quite a few catalysts have been reported [1–9]. Nevertheless, practical application of HC-SCR is not so easy because the catalytic performance is not sufficient. Particularly, catalyst deactivation and reaction inhibition by coexisting SO2 and H2 O are major problems to solve. On the other hand, there have been some papers reporting selective reduction of NO in the presence of

0926-3373/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 6 - 3 3 7 3 ( 0 2 ) 0 0 2 0 8 - 4

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O2 with reductants other than hydrocarbons and ammonia, such as H2 or CO. Ueda et al. reported the activity of Pd/TiO2 for NO reduction with H2 [10], although the effect of SO2 was not examined. Kikuchi and co-workers investigated the activity of supported iridium catalysts and found that NO can successfully be reduced to N2 with CO over 0.02% Ir/silicalite catalyst and that the catalytic activity is hardly influenced by coexisting SO2 [11]. Machida et al. reported the activity of Pt/TiO2 –ZrO2 for NO reduction with H2 at low temperatures [12]. Recently, we reported that Ir/SiO2 shows excellent catalytic activity for NO reduction with H2 in the presence of O2 and SO2 [13]. Although supported Ir catalysts for NO reduction with hydrocarbons were studied [14–18], there have been no reports concerning selective NO reduction with H2 . The most interesting feature of this catalytic reaction is that SO2 , which is usually a catalyst poison, remarkably promoted NO reduction in the presence of O2 , although SO2 -promoting effect was reported for some catalysts concerning HC-SCR [19,20]. It was also discovered that both O2 and SO2 are necessary for NO reduction to take place. In the present paper, we wish to report the detailed behavior of this new catalytic reaction system. A presumed reaction mechanism of NO reduction with H2 will also be discussed.

2. Experimental 2.1. Catalyst preparation SiO2 (Fuji Silysia Chemical, Cariact G-10, 300 m2 g−1 ), Al2 O3 (Mizusawa Chemical, GB-45, 190 m2 g−1 ), TiO2 (Degussa, P-25, 50 m2 g−1 ) and H-ZSM-5 (Tosoh, HSZ-830NHA, SiO2 /Al2 O3 = 29 and HSZ-890HOA, SiO2 /Al2 O3 = 1900) were used as catalyst support. Supported noble metal catalysts were prepared by impregnation of these supports with aqueous solutions of [IrCl(NH3 )5 ]Cl2 , RuCl3 ·1.5H2 O, PtCl4 ·5H2 O, Rh(NO3 )3 , or Pd(NO3 )2 . The impregnated catalyst precursors were dried at 110 ◦ C for 3 h and finally calcined at 600 ◦ C for 8 h in air. The loading of noble metals was normally fixed at 0.5 wt.%, although the loading of Ir was changed from 0.1 to 5 wt.% for Ir/SiO2 .

2.2. Catalytic activity measurement Catalytic activity was measured by using a fixed bed flow reactor made of quartz with a 10 mm diameter. The standard reaction gas contained 1000 ppm NO, 10% O2 , a reductant (H2 , CO, hydrocarbons, or oxygenated hydrocarbons), 10% H2 O and 20 ppm SO2 diluted in He. For some experiments, the concentration of each component gas was changed. Liquid phase reductants were added into the reaction gas either by using a gas bubbling device or by using a micro pump to supply an aqueous solution. The detailed reaction procedure was as follows. Measured 0.01–0.05 g of a catalyst placed in the reactor was pre-treated at 600 ◦ C in He flow. After that, the flow gas was switched from He to the reaction gas with a flow rate of 60–120 cm3 min−1 . Stable catalytic activity was measured as the reaction temperature was lowered from 600 to 200 ◦ C with a step of 100 ◦ C. The analysis of the effluent gas was made with two gas chromatographs (Shimadzu GC8A) equipped with a Molecular Sieve 5A column (for analysis of N2 , H2 and CO) and a Porapak Q column (for analysis of N2 O, CO2 and others). The catalytic activity was evaluated in terms of NO conversion to N2 (and to N2 O) and the conversion of the reductant. A chemiluminescence NOx analyzer was used to check the stability of the catalytic activity. In some cases, the effluent gas was also analyzed by FT-IR (Nicolet, NEXUS 670 FT-IR) equipped with a gas cell (Gemini Scientific, Long-Path Gas Cell Mercury Model #0.14L/2M) to detect by-products, such as NH3 and unreacted NO. 2.3. Catalyst characterization 2.3.1. XRD measurement X-ray diffraction measurements (Mac Science MXP18) were performed to get information on the bulk structure of the catalyst samples by using Cu K␣ radiation at 40 kV and 150 mA. The scanning was done from 2θ = 10 to 90◦ at a speed of 5◦ min−1 . Metal crystallite size was calculated by Scherrer’s equation (d = Kλ/β cos θ ), where K = 0.9, λ = 1.54056. 2.3.2. Metal dispersion measurement In order to know metal dispersion, CO chemisorption measurements were performed using a pulse-type

T. Yoshinari et al. / Applied Catalysis B: Environmental 41 (2003) 157–169

adsorption apparatus. A 0.5 g catalyst sample placed in a U-shaped reactor was first reduced with a gas flow of 10% H2 /He at 350 ◦ C for 30 min and then the gas flow was changed to He. After cooling to room temperature, several gas pulses of 10% CO/He with a 0.5 cm3 volume were introduced into the He flow to be contacted with the catalyst sample. The effluent gas was analyzed with a TCD detector. The introduction of the CO pulse was repeated until CO adsorption was no more detected. The metal dispersion was calculated by assuming that one CO molecule is adsorbed on each metal atom on the surface.

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3. Results and discussion 3.1. Effect of coexisting O2 and SO2 on the catalytic activity of noble metal catalysts 3.1.1. Activity of noble metal catalysts Table 1 summarizes the effect of coexisting 20 ppm SO2 on the catalytic activity of 0.5 wt.% noble metal/SiO2 for NO reduction with 3000 ppm H2 in the presence of 0.65% O2 , the reaction gas being in an oxidizing atmosphere. Ru/SiO2 hardly catalyzed NO reduction irrespective of the presence of SO2 . Pd/SiO2 and Pt/SiO2 catalyzed NO reduction in the absence of SO2 at low temperatures with especially high NO conversion for Pt/SiO2 , but the presence of SO2 destroyed the deNOx activity almost completely. On the other hand, the behavior of Rh/SiO2 and Ir/SiO2 was found quite different. Although NO reduction took place at moderate temperatures around 400 ◦ C with not so high NO conversion in the absence of SO2 , the presence of coexisting SO2 significantly promoted NO reduction. For example, NO conversion on Ir/SiO2 at 300 ◦ C was only 4% in the absence of SO2 , whereas the conversion reached 45% (to N2 ) and 28% (to N2 O) in the presence of 20 ppm SO2 . This fact is quite surprising because SO2 is known normally as a catalyst poison for most of metal catalysts. It seems from the dispersion data shown in the table that the

2.4. FT-IR study Diffuse reflectance FT-IR spectra were recorded with a Nicolet NEXUS 670FT-IR spectrometer to analyze adsorbed chemical species on catalyst surface. Prior to each experiment, 20 mg of a catalyst placed in a diffuse reflectance high-temperature vacuum cell (Spectra-Tech, P/N 0030-103) was reduced in situ by heating in a flow of 10% H2 /He at a flow rate of 60 cm3 min−1 at 400 ◦ C for 30 min and then He for 15 min, followed by cooling to desired temperatures. Observation of surface species was carried out after introducing a reaction gas containing one or several gas components of 1000 ppm NO, 3000 ppm H2 , 0.65% O2 and 20 ppm SO2 . The background spectrum of clean surface was measured in a He flow. Table 1 NO reduction with H2 over various silica supported metal catalysts Catalyst (dispersion)

SO2 (ppm)

0.5% Ru/SiO2 (0.25)

NO conversion to N2 (N2 O) (%) 150 ◦ C

H2 conversion (%)

200 ◦ C

300 ◦ C

400 ◦ C

500 ◦ C

600 ◦ C

0 20

0(0) 0(2)

0(0) 1(0)

1(0) 2(0)

2(0) 4(0)

0.5% Pd/SiO2 (0.37)

0 20

13(5) 1(0)

3(1) 2(1)

0(1) 1(1)

0.5% Pt/SiO2 (0.46)

0 20

5(10) 1(0)

1(2) 4(4)

0.5% Rh/SiO2 (0.53)

0 20

3(0) 5(3)

0.5% Ir/SiO2 (0.72)

0 20

0(0) 5(4)

16(34) 1(0)

150 ◦ C

200 ◦ C

300 ◦ C

1(0) 2(0)

0 1

1 4

13 23

50 61

74 75

3(0) 1(1)

5(1) 1(1)

100 16

100 10

100 100

100 100

100 100

2(1) 5(3)

2(1) 1(1)

1(1) 1(1)

100 4

100 100

100 100

100 100

100 100

17(2) 48(5)

10(1) 52(1)

8(0) 28(1)

6(1) 10(1)

11 21

80 89

100 100

100 100

100 100

4(0) 45(28)

11(2) 35(8)

11(1) 17(1)

14(0) 12(1)

0 6

2 90

28 97

66 100

80 100

77 2

400 ◦ C

500 ◦ C

600 ◦ C

Reaction conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 10%, H2 = 3000 ppm, SO2 = 0 or 20 ppm, W/F = 0.0267 g s cm−3 (GHSV = 75,000 h−1 ).

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Table 2 NO reduction with various reductants over 0.5% Ir/SiO2 catalyst Reductants

SO2 (ppm)

NO conversion to N2 (N2 O) (%)

Reductant conversion (%)

300 ◦ C

400 ◦ C

500 ◦ C

600 ◦ C

300 ◦ C

400 ◦ C

500 ◦ C

600 ◦ C

H2

0 20

4(0) 45(28)

11(2) 35(8)

11(1) 17(1)

14(0) 12(1)

2 90

28 97

66 100

80 100

CO

0 20

1(0) 23(7)

23(4) 49(9)

9(2) 31(3)

3(0) 4(1)

6 23

66 98

98 99

99 100

C3 H 6

0 20

0(0) 1(1)

1(0) 24(7)

61(2) 74(5)

91(1) 81(2)

1 0

3 27

37 76

98 100

C3 H 8

0 20

1(0) 0(0)

1(0) 0(1)

1(0) 1(1)

1(0) 1(1)

0 0

0 0

0 1

4 7

n-C10 H24

0 20

0(0) 1(1)

0(0) 3(2)

1(0) 70(3)

19(1) 37(3)

0 1

0 9

3 80

49 97

(CH3 )2 CO

0 20

0(0) 0(1)

1(0) 6(2)

4(0) 16(1)

10(1) 28(1)

0 1

0 22

16 63

45 100

Reaction conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 10%, H2 (CO, C3 H6 , C3 H8 , n-C10 H24 , (CH3 )2 CO) = 3000(3000, 1000, 1000, 300, 1000)ppm, SO2 = 0 or 20 ppm, W/F = 0.0267 g s cm−3 .

SO2 promoting effect is not related to the difference in metal dispersion but to the nature of metal species. 3.1.2. Efficiency of reductants Table 2 presents the efficiency of various reductants other than H2 for the selective reduction of NO over Ir/SiO2 in the presence and absence of SO2 . It is noted that the SO2 promoting effect was also observed for NO reduction with CO, propene, n-decane and acetone. However, propane was not an efficient reductant regardless of the presence or absence of SO2 . Since the reductant conversions also increased along with NO conversion by the presence of SO2 , the enhancement of NO reduction is not because the reductant oxidation with O2 , which is a side reaction, is inhibited by SO2 . 3.1.3. Effect of supports In order to get information on the effect of support, the catalytic performance of Ir on various supports for the selective reduction of NO with H2 or CO was investigated in the presence and absence of SO2 . The results are shown in Table 3. For NO reduction with H2 , the SO2 promoting effect was remarkable only on Ir/SiO2 . Ir/Al2 O3 , Ir/TiO2 , and Ir/ZSM-5 were not good catalysts for both with and without SO2 , although a slight SO2 promoting effect was noticed. For NO reduction with CO, Ir/SiO2 was also definitely the

most active catalyst for NO reduction in the presence of SO2 and O2 , the SO2 promoting effect being most significant. Metal dispersion shown in the table does not seem to have a direct relationship with the catalytic performance. The remaining part of this paper was devoted to the discussion on the performance and reaction mechanism of NO reduction over Ir/SiO2 mainly with H2 . 3.2. Catalytic performance of Ir/SiO2 for NO reduction with H2 3.2.1. Effect of SO2 From a practical viewpoint, especially for diesel applications, the effect of SO2 is very important. Fig. 1 shows the variation of NO conversion with a change of SO2 concentration for the selective reduction of NO with H2 in the presence of O2 . It is noted that both NO conversion and H2 conversion did not change much with SO2 concentration in the range between 10 and 150 ppm. This indicates that Ir/SiO2 shows good deNOx performance even in the presence of high concentrations of SO2 . Since the promoting effect of SO2 may be related to the change of supported Ir species, the response of the conversion of NO and H2 to the intermittent feed of 20 ppm SO2 was examined at 400 ◦ C. The results

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Table 3 NO reduction with H2 and CO over 0.5% Ir on various supports Reductant

H2

CO

Catalyst

Dispersion

SO2 (ppm)

NO conversion to N2 (N2 O)(%)

H2 or CO conversion (%)

300 ◦ C

400 ◦ C

500 ◦ C

600 ◦ C

300 ◦ C

400 ◦ C

500 ◦ C

600 ◦ C

Ir/SiO2

0.72

0 20

4(0) 45(28)

11(2) 35(8)

11(1) 17(1)

14(0) 12(1)

2 90

28 97

66 100

80 100

Ir/Al2 O3

0.46

0 20

2(0) 5(4)

5(1) 10(3)

10(1) 3(1)

3(0) 3(0)

3 25

51 58

100 94

100 100

Ir/TiO2

0.12

0 20

2(0) 12(4)

4(0) 14(4)

5(1) 2(1)

6(0) 3(0)

29 27

88 71

100 95

100 100

Ir/ZSM-5 (Si/Al2 = 1900)

0.60

0 20

4(0) 9(7)

4(0) 9(3)

6(0) 4(1)

5(0) 1(1)

14 44

83 80

100 100

100 100

Ir/ZSM-5 (Si/Al2 = 29)

0.70

0 20

4(1) 13(7)

3(1) 8(4)

1(0) 1(2)

1(0) 0(0)

25 56

62 68

86 80

100 91

Ir/SiO2

0 20

1(0) 23(7)

23(4) 49(9)

9(2) 31(3)

3(0) 4(1)

6 23

66 98

98 99

99 100

Ir/Al2 O3

0 20

2(0) 0(1)

5(1) 2(1)

5(1) 6(2)

5(0) 1(1)

5 1

75 15

100 93

100 97

Ir/TiO2

0 20

4(0) 8(0)

6(1) 13(2)

20(1) 5(1)

13(0) 2(1)

13 7

60 49

95 97

97 98

Ir/ZSM-5 (Si/Al2 = 1900)

0 20

1(0) 1(1)

4(1) 2(1)

2(1) 10(2)

2(0) 2(1)

35 2

94 27

98 91

100 100

Ir/ZSM-5 (Si/Al2 = 29)

0 20

1(0) 0(0)

1(0) 2(1)

1(0) 4(1)

2(0) 1(0)

5 7

35 39

82 7

98 93

Reaction conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 10%, H2 (CO) = 3000(3000) ppm, W/F = 0.0267 g s cm3 .

are given in Fig. 2. Although both the conversions increased after introduction of SO2 , the subsequent removal of SO2 did not result in an entire decrease of the conversions corresponding to the initial activity without SO2 . This means that the effect of coexisting SO2 was not completely lost even after removal of SO2 from the reaction gas, suggesting that the structure of catalyst surface underwent somewhat irreversible change by SO2 . 3.2.2. Effect of O2 The effect of O2 concentration is another interesting issue from a viewpoint of reaction mechanism. Fig. 3 presents the effect of O2 concentration on the activity of Ir/SiO2 for NO reduction with H2 in the presence of SO2 at 400 ◦ C. In the absence of SO2 , NO was reduced considerably when O2 was not present in the reaction gas. This means that direct NO reduction with H2 took place very efficiently. However, an increase of O2 concentration resulted in a severe decrease of NO

conversion. This is a typical characteristic of so-called unselective reduction. In the presence of SO2 , however, the situation was completely different. In the absence of O2 , the activity for direct reduction of NO with H2 was completely lost by the presence of SO2 . On the other hand, the presence of O2 shows a remarkable effect to promote NO reduction. It should be noted that only a small amount of O2 (850 ppm) is sufficient for the enhancement of deNOx activity (from 0 to 80% NO conversion). However, higher O2 concentrations decreased NO conversion. Fig. 4 shows the response of NO and H2 conversion to the intermittent feed of 0.65% O2 at 400 ◦ C. It is evident that both the conversions were high in presence of O2 and low in absence of O2 , clearly showing O2 promoting effect. The reaction response to the feed of O2 was very quick in contrast to the case of SO2 shown before. These results confirm that the presence of O2 is essential for NO reduction to take place in the presence of SO2 .

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Fig. 1. Effect of SO2 concentration on NO reduction with H2 over 0.5% Ir/SiO2 catalyst. Conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 10%, H2 = 3000 ppm, SO2 = 0–150 ppm, W/F = 0.0267 g s cm−3 . NO conversion to N2 : (䊉) 300 ◦ C, (䉱) 400 ◦ C. NO conversion to N2 O: (䊏)300 ◦ C, (䉲) 400 ◦ C. H2 conversion: (䊊) 300 ◦ C, (䉭) 400 ◦ C.

Fig. 2. Response of NO and H2 conversion to the intermittent feed of SO2 over 0.5% Ir/SiO2 catalyst. Conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 10%, H2 = 3000 ppm, SO2 = 0 or 20 ppm, T = 400 ◦ C, W/F = 0.0267 g s cm−3 .(䊏) NO conversion to N2 , (䉬) NO conversion to N2 O, (䉱) H2 conversion.

Fig. 3. Effect of O2 concentration on NO reduction with H2 over 0.5% Ir/SiO2 catalyst in the presence and absence of 20 ppm SO2 . Conditions: NO = 1000 ppm, O2 = 0–10%, H2 O = 10%, H2 = 3000 ppm, SO2 = 0 or 20 ppm, T = 400 ◦ C, W/F = 0.0267 g s cm−3 . NO conversion to N2 : (䊉) with SO2 , (䊊) without SO2 . NO conversion to N2 O: (䊏) with SO2 , () without SO2 . H2 conversion: (䉱) with SO2 ,(䉭) without SO2 .

Fig. 4. Response of NO and H2 conversion to the intermittent feed of O2 over 0.5% Ir/SiO2 catalyst. Conditions: NO = 1000 ppm, O2 = 0 or 0.65%, H2 O = 10%, H2 = 3000 ppm, SO2 = 20 ppm, T = 400 ◦ C, W/F = 0.0267 g s cm−3 . (䊏) NO conversion to (N2 ), (䉬) NO conversion to N2 O, (䉱)H2 conversion.

T. Yoshinari et al. / Applied Catalysis B: Environmental 41 (2003) 157–169

Fig. 5. Effect of H2 O concentration on NO reduction with H2 over 0.5% Ir/SiO2 catalyst in the presence of 20 ppm SO2 . Conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 0–10%, H2 = 3000 ppm, SO2 = 20 ppm, W/F = 0.0267 g s cm−3 . NO conversion to N2 : (䊉) 300 ◦ C, (䊊) 400 ◦ C. NO conversion to N2 O: (䉱) 300 ◦ C, (䉭) 400 ◦ C.

3.2.3. Effect of H2 O Water vapor is another major component in exhaust gases. Therefore, the effect of H2 O concentration on the catalytic activity was checked. As shown in Fig. 5, the catalytic activity was not influenced by H2 O at all with no change of NO conversion in the range of 0–10% H2 O at 300 and 400 ◦ C. The high resistance to H2 O vapor is a beneficial characteristic of this catalytic system concerning practical applications. 3.2.4. Effect of Ir loading The influence of Ir loading on the catalytic activity of Ir/SiO2 was then investigated. Fig. 6 shows the effect of Ir loading on the catalytic activity for NO reduction with H2 in the presence of O2 and SO2 . The catalytic NO reduction activity increased with increasing Ir loading, especially at low temperatures. Thus, the temperature giving the maximum NO conversion shifted to lower temperatures with an increase of Ir loading. NO conversion at 300 and 400 ◦ C reached a maximum on 2% Ir/SiO2 and 1% Ir/SiO2 , respectively. For diesel or lean-burn engine applications, good NO conversion in the presence of high concentrations of O2 is needed. Therefore, we tried to increase the concentration of H2 reductant used for NO reduction

163

Fig. 6. Effect of Ir loading on the catalytic activity of Ir/SiO2 for NO reduction with H2 in the presence of 20 ppm SO2 . Conditions: NO = 1000 ppm, O2 = 0.65%, H2 O = 10%, H2 = 3000 ppm, SO2 = 20 ppm, T = 250–400 ◦ C, W/F = 0.0267 g s cm−3 . NO conversion to N2 + N2 O: (䉲)0.1% Ir, (䊉) 0.25% Ir, (䉱) 0.5% Ir, (䊏) 1.0% Ir, (䉬) 2.0% Ir, (夹) 5.0% Ir.

in order to obtain high NO conversion. Fig. 7 indicates the variation of NO conversion with an increase of H2 concentration at 250 ◦ C in the presence of various concentrations of O2 over 5% Ir/SiO2 that has higher activity than 0.5% Ir/SiO2 . Although an increase of O2 concentration inhibited NO reduction, NO conversion increased nearly proportionally with H2 concentration, suggesting first order kinetics with respect to H2 . Accordingly, NO conversion reached as high as 40% even in the presence of 10% O2 when 1.2% H2 was used at a space velocity of 50,000 h−1 . 3.2.5. Catalyst characterization Since the existence of O2 and SO2 is essential for the deNOx activity of Ir/SiO2 , the catalyst was analyzed by several characterization techniques. Fig. 8 shows the XRD patterns of 5% Ir/SiO2 after use for NO–H2 –O2 –SO2 reaction. The diffraction lines of IrO2 and Ir metal crystals with a crystallite size of 5.6 nm were detected for the catalyst. Although the true species under the reaction conditions are not known, there were no other diffraction lines.

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However, SOx species seems to be present on the catalyst surface as will be shown later. 3.3. NO reduction mechanism on Ir/SiO2

Fig. 7. Effect of O2 and H2 concentration on NO reduction with H2 over 0.5% Ir/SiO2 . Conditions: NO = 1000 ppm, O2 = 0.65–10%, H2 O= 10%, H2 = 0–12000 ppm, SO2 = 20 ppm, T = 250 ◦ C, W/F = 0.040 g s cm−3 . NO conversion to N2 + N2 O: (䊏) 0.65% O2 , (䉱) 1.3% O2 , (䉬) 3.9% O2 , (䊉) 10% O2 .

3.3.1. Reactivity of NO2 and several unit reactions For the selective reduction of NO with hydrocarbons, the first reaction step is often the oxidation of NO to NO2 . Therefore, the reactivity of NO2 was checked for the present reaction system. In Table 4 are shown the results of reduction of NO and NO2 with H2 in the presence of O2 and with/without SO2 over Ir/SiO2 . It was found that NO can be reduced more easily than NO2 although SO2 promoted both NO reduction and NO2 reduction. This suggests that NO2 may not be a reaction intermediate for the present NO reduction with H2 in the presence of O2 and SO2 . In Table 4 are also presented the results of several unit reactions of selected components of the reaction gas. As was already described, SO2 inhibited the direct NO reduction with H2 without O2 . On the other hand, SO2 promoted NO reduction with H2 in the presence

Fig. 8. XRD spectrum of 5% Ir/SiO2 catalyst used for NO reduction with H2 in the presence of O2 and SO2 . (䊉) Ir metal, (䊊) IrO2 .

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Table 4 Results of several unit reactions over 0.5% Ir/SiO2 catalyst Reaction

NO(NO2 ) conversion to N2 (N2 O) (%)

H2 Conversion(%)

300 ◦ C

400 ◦ C

500 ◦ C

300 ◦ C

400 ◦ C

NO2 –H2 –O2 NO2 –H2 –O2 –SO2

1(0) 3(0)

2(0) 9(0)

5(0) 11(1)

12 20

23 56

48 85

NO–H2 –O2 NO–H2 –O2 –SO2

4(0) 45(28)

11(2) 35(8)

11(1) 17(1)

2 90

28 97

66 100

NO–H2 NO–H2 –SO2

44(0) 0(0)

89(0) 1(1)

98(0) 2(3)

82 2

47 7

40 22

23 69

43 83

68 100

H2 –O2 H2 –O2 –SO2

500 ◦ C

Reaction conditions: NO(NO2 ) = 0 or 1000(1000)ppm, O2 =0 or 0.65%, H2 O = 10%, H2 = 3000 ppm, SO2 = 0 or 20 ppm, W/F = 0.0267 g s cm3 .

of O2 . The oxidation of H2 with O2 is a side reaction to inhibit NO reduction with H2 . Since the promoting effect of SO2 is often to be due to its inhibition effect on the side reaction, the catalytic activity for this reaction was examined in the presence and absence of SO2 . It is noted that SO2 did not inhibit but enhanced H2 oxidation with O2 , indicating that SO2 also has a promoting effect on the reaction of H2 and O2 . Consequently, it is concluded that the SO2 promotion effect for the reaction system of NO–H2 –O2 is not due to the inhibition of the side reaction of H2 –O2 reaction. 3.3.2. Formation of NH3 and reaction of NO with NH3 By FT-IR analysis of the reaction effluent gas using a gas cell, the formation of NH3 was noticed. Therefore the change of NO (1912 cm−1 ) and NH3 (966 cm−1 ) concentrations under various reaction conditions were investigated. The results are shown in Fig. 9. It is evident that the concentration of unreacted NO decreased when SO2 was added to the reaction system of NO–H2 –O2 . In the absence of O2 (i.e. NO–H2 reaction), the formation of high concentrations of NH3 was detected. The temperature giving maximum NH3 formation was 300 ◦ C in the absence of SO2 and 600 ◦ C in the presence of SO2 . Even in the presence of O2 , a small amount of NH3 formation was detected at 300 ◦ C. These results suggest that NH3 or other related species may be reaction intermediates for the present reaction.

Table 5 shows the effect of coexisting SO2 on NO reduction with NH3 under various conditions. It is interesting that the presence of SO2 also promoted NO reduction with NH3 in the presence of O2 and NH3 oxidation with O2 , although NO reduction with NH3 in the absence of O2 was inhibited by SO2 .

Fig. 9. Outlet gas concentrations analyzed by FT-IR for NO reduction with H2 over 0.5% Ir/SiO2 catalyst. Conditions: NO = 1000 ppm, O2 = 0 or 0.65%, H2 O= 0%, H2 = 3000 ppm, SO2 = 0 or 20 ppm, W/F = 0.040 g s cm−3 . NO concentration: (䉬) NO–H2 –O2 –SO2 , (䉱) NO–H2 –SO2 , (䊉) NO–H2 –O2 , (䊏) NO–H2 . NH3 concentration: (䉫) NO–H2 –O2 –SO2 , (䉭) NO–H2 –SO2 , (䊊) NO–H2 –O2 , (䊐) NO–H2 .

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Table 5 Reaction of NH3 over 0.5% Ir/SiO2 catalyst Reactions

N2 (N2 O) formation (ppm) 300 ◦ C

400 ◦ C

500 ◦ C

NO–NH3 NO–NH3 –SO2 NO–NH3 –O2 NO–NH3–O2 –SO2

805(0) 230(280) 140(500) 400(180)

900(0) 740(50) 415(45) 515(105)

945(0) 795(15) 320(35) 460(60)

NH3 –O2 NH3 –O2 –SO2

470(0) 375(85)

365(5) 390(20)

230(5) 308(7)

Reaction conditions: NO = 0 or 1000 ppm, O2 = 0 or 0.65%, H2 O = 10%, NH3 = 1500 ppm, SO2 = 0 or 20 ppm, W/F = 0.0267 g s cm3 .

3.3.3. FT-IT study FT-IR study was performed to investigate the mechanism of the selective reduction of NO with H2 in the presence of O2 and SO2 . Figs. 10–12 show the changes of FT-IR spectra of 5% Ir/SiO2 contacted with a series of gas flows con-

taining selected gas components at 200 ◦ C. Fig. 10 presents the spectra change in the absence of SO2 . When H2 was introduced, no IR absorption peaks were detected. But, an absorption peak due to NO (1834 cm−1 ) [21–24]and a peak at 3239 cm−1 and broad bands around 3500 cm−1 appeared when the gas flow was switched to H2 –NO. The peak at 3239 cm−1 can be assigned to NH2 species from a reference data [25,26]. The broad band around 3500 cm−1 is due to H2 O, although peaks due to NH3 might be included. After that, O2 was added into the reaction gas. This resulted in a complete loss of the NH2 peak, and the NO peak became stronger and shifted to higher wavenumber (1839 cm−1 ), although the original peaks were resumed after removing O2 again. These results indicate that the formation of NH2 species was inhibited by the presence of O2 . This phenomenon agrees well with the typical inhibition effect of O2 for NO reduction. The shift of the NO absorption peak by the presence of O2 suggests that the supported Ir became more cationic, leading to the weaker NO adsorption on Ir. Of course,

Fig. 10. IR spectra of absorbed species over 5% Ir/SiO2 catalyst in the absence of SO2 at 200 ◦ C. Conditions: (a) He, (b) H2 , (c) H2 –NO, (d) H2 –NO–O2 , (e) H2 –NO, (f) H2 .

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Fig. 11. IR spectra of absorbed species over 5% Ir/SiO2 catalyst in the absence of O2 at 200 ◦ C. Conditions: (a) He, (b) H2 , (c) H2 –SO2 , (d) H2 –SO2 –NO, (e) H2 –NO.

all the peaks disappeared when NO was removed from the reaction gas. When fresh Ir/SiO2 was contacted with a gas flow of H2 , H2 –SO2 and then H2 –SO2 –NO, there was no peak formation corresponding to NO or NH2 as shown in Fig. 11. This means that SO2 strongly adsorbed on Ir inhibits the adsorption of NO. Unfortunately, adsorbed SO2 peaks could not be confirmed because of the strong absorption bands of SiO2 support. In the presence of SO2 and O2 , completely different behavior was observed as can be seen from Fig. 12. First, no peaks were detected in the flow of H2 and H2 –SO2 . However, some peaks around 920 cm−1 appeared when O2 was added to the flow of H2 –SO2 . This peak might be assigned to SOx [25]. Then, after NO was further introduced, strong peaks of NO (1844 cm−1 ) and NH2 (around 3239 cm−1 ) clearly appeared. Removal of O2 again decreased the peak intensity considerably. These observations indicate that O2 helps the formation of adsorbed NO and NHx

species even in the presence of SO2 . The experimental results agree well with the reaction data described before. 3.3.4. Presumed mechanism From the FT-IR measurements, surface NHx species was detected when NO reduction with H2 proceeds, suggesting that N2 is formed by reaction of NHx and NO. Actually, the formation of NH3 was confirmed as a by-product. Concerning NO reduction by H2 in the absence of SO2 , O2 seems to inhibit the formation of NHx . Also, NO reduction with H2 in the presence of SO2 but absence of O2 , the adsorption of NO on Ir surface is completely inhibited because of the strong SO2 adsorption, resulting in entire loss of NO reduction activity. In the presence of SO2 and O2 , however, the situation is different. When O2 is added in addition to SO2 , the Ir surface can adsorb NO, leading to the formation of NHx . The surface adsorbed SO2 may be oxidized to SO3 species by O2

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Fig. 12. IR spectra of absorbed species over 5% Ir/SiO2 catalyst in the presence of O2 and SO2 at 200 ◦ C. Conditions: (a) He, (b) H2 , (c) H2 –SO2 , (d) H2 –SO2 –O2 , (e) H2 –SO2 –O2 –NO, (f) H2 –SO2 –NO, (g) H2 –SO2 –NO–O2 .

and this might create vacant Ir surface, which allows NO adsorption and reaction with H2 .

4. Conclusions Ir/SiO2 and Rh/SiO2 show special activity for NO reduction with H2 in the presence of O2 and SO2 , while Ru, Pd, Pt/SiO2 do not. The co-presence of O2 and SO2 is essential for NO reduction over Ir/SiO2 and Rh/SiO2 . Over Ir/SiO2 , CO, propene, n-decane and acetone are also effective reductants in addition to H2 , whereas propane is a poor reductant. SiO2 is the only effective support for Ir an Rh for this reaction. As for NO reduction with H2 over Ir/SiO2 , although SO2 is necessary for NO reduction, the change of SO2 concentration does not affect NO conversion. A small amount of O2 is enough for promotion of NO reduction, and higher O2 concentration decreases NO conversion. H2 O does not affect the catalytic activity at all.

Since the reactivity of NO2 is lower than that of NO, NO2 does not seem to be a reaction intermediate. From FT-IR measurements, SO2 is considered to be adsorbed strongly on Ir surface in the absence of O2 , to inhibit NO reduction with H2 . On the other hand, the addition of O2 enables NO adsorption on SO2 -blocked Ir surface, leading to NO reduction with H2 . Acknowledgements This work has been supported by the New Energy and Industrial Technology Development Organization (NEDO) under the sponsorship of Ministry of Economy, Trade and Industry (METI) of Japan. References [1] M. Iwamoto, H. Yahiro, Y. Yu-u, S. Shudo, N. Mizuno, Appl. Catal. 69 (1991) L15.

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