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La7.33BaYSi6O25.5 catalysts for hydrocarbon species
Catalysis Today 164 (2011) 575–579
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Oxidation property of Pt/La7.33 BaYSi6 O25.5 catalysts for hydrocarbon species Takashi Wakabayashi a,∗ , Sumio Kato b , Yuunosuke Nakahara a , Masataka Ogasawara b , Shinichi Nakata b a b
Catalysts Division, Engineered Materials Sector, Mitsui Mining & Smelting Co., Ltd., 1013-1 Ageoshimo, Ageo, Saitama 362-0025, Japan Department of Engineering in Applied Chemistry for Environments, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegatagakuen-machi, Akita 010-8502, Japan
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 15 November 2010 Accepted 17 November 2010 Available online 15 December 2010 Keywords: Apatite-type silicate Hydrocarbon Three-way catalyst
a b s t r a c t Pt loaded La7.33 BaYSi6 O25.5 (Pt/La7.33 BaYSi6 O25.5 ) catalyst was investigated for oxidation of hydrocarbon species and for reduction of NO by hydrocarbon species. Pt/La7.33 BaYSi6 O25.5 catalyst exhibited higher activity for NO reduction by alkane (C3 H8 ), alkene (C3 H6 ) and aromatic (C7 H8 ), than Pt/Al2 O3 catalyst and Pt/CeO2 –ZrO2 (CZ, CeO2 /ZrO2 weight ratio was 1/1) catalyst. The oxidizing power for hydrocarbon species of carrier affected the catalytic activity for NO reduction. In addition, oxidation of adsorbed hydrocarbon on La7.33 BaYSi6 O25.5 was observed by FT-IR measurement. It was presumed that the catalytic activity for NO reduction is affected by adsorbed form of hydrocarbon. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The three-way catalysts (TWCs) oxidize hydrocarbons (HC), carbon monoxide (CO) and as well as reduces nitrogen oxides (NOx) leading to near-zero emissions for these pollutants, in the condition close to stoichiometric air-to-fuel ratio (A/F). It is known that the constituent parts of hydrocarbons emitted from engine are mainly alkanes, alkenes and aromatics [1]. TWCs are composed of Al2 O3 , CeO2 –ZrO2 and precious metals, such as Pt, Pd and Rh. Currently, TWCs are demanded from improvement of performance and efficient use of the precious metals to deal with the global new legislation and the fluctuation of the precious metal’s price. To solve these issues, improvement of light-off performance for removal of hydrocarbons and NOx, and transient NOx control are needed. For that purpose, it has been carried out that the development of supported precious metal catalysts which have strong metal-support interaction (SMSI) [2]. We have reported that apatite-type rare earth silicatessupported Pd and Pt catalysts exhibited high activity for selective catalytic reduction of NO by C3 H6 [3]. Apatite-type rare earth silicates have attracted much attention as oxide ion conductors whose conductivities are higher than that of Y2 O3 stabilized ZrO2 at low temperature. In this study, we have investigated catalytic performance of Pt/La7.33 BaYSi6 O25.5 for oxidation of hydrocarbon species and for reduction of NO by hydrocarbon species.
La7.33 BaYSi6 O25.5 was synthesized by sol–gel method [3]. The catalyst had 1 mass % Pt supported on La7.33 BaYSi6 O25.5 powder and was prepared by impregnation of Pt (NH3 )2 (NO2 )2 . The powder was heated at 600 ◦ C for 3 h in air. In this study, ceramic honeycomb catalyst coated with Pt/La7.33 BaYSi6 O25.5 slurry was used. Pt/Al2 O3 and Pt/CZ catalysts also were prepared by using commercial Al2 O3 and CZ (CZ, CeO2 /ZrO2 weight ratio was 1/1) powder. Aging of these catalysts were carried out by heating at 900 ◦ C for 25 h in air containing 10 vol% H2 O. The catalytic activities were evaluated at raising temperature from 100 to 500 ◦ C in a simulated stoichiometric (A/F = 14.6, = 1) gas using 400 ppm C3 H6 – 1550 ppm O2 – 500 ppm NO, 400 ppm C3 H8 – 1750 ppm O2 – 500 ppm NO or 171 ppm C7 H8 – 1300 ppm O2 – 500 ppm NO (N2 as balance). The flow rate of the gas during the experiments was 25 L/min (GHSV 100,000 h−1 ), and the heating rate of the furnace was 20 ◦ C/min. The concentration of HC and NO in the effluent gas was continuously monitored by an on-line gas analyzer (Horiba, MEXA7100 and Best sokki, SESAM3-N). A specific surface area was measured by N2 adsorption using Brunauer–Emmett–Teller (BET) method (Quantachrome Instrument, Quadrasorb SI). The dispersion of Pt on each catalyst was measured by using CO pulse method [4]. FT-IR experiments were carried out using a JASCO FT/IR-6200 spectrometer equipped with MCT detector and KBr beam splitter. Spectra were obtained with 64-scan data acquisition at resolution of 4 cm−1 in controlled gas atmosphere and temperature, employing a diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy cell. The carriers were pretreated in air flow at 700 ◦ C for 30 min and measurement was carried out after cooling down to 300 or 400 ◦ C under N2
∗ Corresponding author. Tel.: +81 048 775 3269; fax: +81 048 775 8126. E-mail address: t [email protected] (T. Wakabayashi). 0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2010.11.035
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T. Wakabayashi et al. / Catalysis Today 164 (2011) 575–579
atmosphere. The total gas flow rate through the DRIFT cell was 0.1 L/min.
3. Results and discussion The catalytic activities for NO reduction using C3 H6 , C3 H8 or C7 H8 as reductants over Pt/La7.33 BaYSi6 O25.5 and Pt/Al2 O3 , Pt/CZ catalysts were investigated. Fig. 1 shows conversions of C3 H6 and NO over each Pt catalysts for C3 H6 –NO–O2 reaction. The catalytic activities was evaluated by light-off temperature (LOT50 ) shown in Table 1, which is the temperature of the reaction exhibiting 50% conversion of HC and NO, and the conversion at 500 ◦ C (500). Pt/La7.33 BaYSi6 O25.5 catalyst exhibited the highest activity for both HC oxidation and NO reduction. The LOT50 of C3 H6 for Pt/La7.33 BaYSi6 O25.5 and Pt/CZ catalysts were 403 and 455 ◦ C, respectively. The conversion of C3 H6 for Pt/Al2 O3 catalyst was lower than 50% even at 500 ◦ C. On the other hand, NO conversion for Pt/La7.33 BaYSi6 O25.5 catalyst achieved 50% at 454 ◦ C. The 500 of C3 H6 and NO decreased in the sequence of Pt/La7.33 BaYSi6 O25.5 > Pt/CZ > Pt/Al2 O3 . Fig. 2 shows conversions of C3 H8 and NO over each Pt catalysts for C3 H8 –NO–O2 reaction. The LOT50 of C3 H8 for Pt/La7.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ catalysts were 377, 400 and 464 ◦ C, respectively. And the LOT50 of NO for Pt/La7.33 BaYSi6 O25.5 and Pt/Al2 O3 catalysts were 402 and 440 ◦ C, respectively. Both HC and NO conversions for C3 H8 –NO–O2 reaction over Pt/La7.33 BaYSi6 O25.5 and Pt/Al2 O3 catalysts were higher than those for C3 H6 –NO–O2 reaction. While, the catalytic activity for C3 H8 –NO–O2 reaction was slightly lower than that for C3 H6 –NO–O2 reaction. These results suggest that the carrier of each Pt catalysts affected the catalytic activities for NO reduction using C3 H8 and C3 H6 . Fig. 3 shows conversions of NO and C7 H8 over each Pt catalysts for C7 H8 –NO–O2 reaction. The NO conversions were lower than 50% for all Pt catalysts and LOT50 of C7 H8 for Pt/La7.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ catalysts were 397, 492 and 448 ◦ C, respectively. The LOT50 and 500 for C7 H8 conversions decreased in the sequence of Pt/La7.33 BaYSi6 O25.5 > Pt/CZ > Pt/Al2 O3 , which is same order in the case of C3 H6 –NO–O2 reaction. From Table 1, it can be seen that Pt/La7.33 BaYSi6 O25.5 catalyst exhibited higher activity for both oxidation of each hydrocarbon species and NO reduction by
Table 1 Catalytic activities over Pt catalysts in simulated gases.
◦
T50 ( C)
500 (%)
Carrier
C3 H6
NO
C3 H8
NO
C7 H8
NO
La7.33 BaYSi6 O25.5 Al2 O3 CZ La7.33 BaYSi6 O25.5 Al2 O3 CZ
403 U.D. 455 83.4 47.8 64.3
454 – – 71.1 29.7 36.2
377 400 464 94.8 87.4 61.2
402 440 U.D. 92.4 80.3 48.4
397 492 448 81.5 55.1 71.9
– – – 43.4 15.6 39.6
–: Conversion were lower than 50%. Table 2 Specific surface area and dispersion of Pt catalysts. Carrier
La7.33 BaYSi6 O25.5 Al2 O3 CZ
Surface area (m2 /g)
Dispersion (CO/Pt)
Fresh
Aged
Fresh
Aged
12.5 155.8 74.7
12.4 104.0 46.2
0.035 0.468 0.322
– 0.033 0.012
–: Not evaluated by CO adsorption method.
C3 H8 (alkane), C3 H6 (alkene) and C7 H8 (aromatic), than Pt/Al2 O3 catalyst and Pt/CZ catalyst. Table 2 shows the specific surface area and dispersion of Pt on each carrier for fresh and aged condition. The specific surface area and dispersion of Pt on each carrier for fresh and aged condition was lowered in the sequence of Pt/Al2 O3 > Pt/CZ > Pt/La7.33 BaYSi6 O25.5 . Especially, dispersion of Pt on La7.33 BaYSi6 O25.5 catalyst after the aging was too small to evaluate by CO adsorption method. These results suggest that properties of Pt on the catalyst such as chemical state, is more effective factor on the catalytic activity than dispersion of Pt. The catalytic activities for NO reduction over Pt catalysts were affected by a type of hydrocarbon and each carrier. Therefore, a form of adsorbed hydrocarbon on each carrier was investigated by FT-IR measurement. First, a form of adsorbed hydrocarbon on each carrier was investigated at 300 ◦ C for HC oxidation. All the Pt loaded catalysts exhibited small conversions at this temperature. The DRIFT spectra of CO were used to compare with the DRIFT spectra of C3 H6 or C3 H8 . Fig. 4(A) shows the DRIFT spectra of La7.33 BaYSi6 O25.5 after the treatment in the flow of 0.1%CO–N2 , 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C for 10 min. In the presence of CO (spec-
Fig. 1. Conversion curves of C3 H6 and NO over Pt/La9.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ for C3 H6 –NO–O2 reaction.
T. Wakabayashi et al. / Catalysis Today 164 (2011) 575–579
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Fig. 2. Conversion curves of C3 H8 and NO over Pt/La9.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ for C3 H6 –NO–O2 reaction.
trum a), band at 2181 cm−1 , which was assigned to vibration of CO adsorbed on the Ba or La sites was observed [5]. The broad bands observed at 1484 and 1400 cm−1 were assigned to the vas (CO) and vs (CO) stretching vibrations of carbonate ion (CO3 2− ) [6,7]. These bands were also observed in the presence of C3 H6 (spectrum b) and C3 H8 (Spectrum c). In the spectrum b, the weak band observed at 1667 cm−1 was assigned to the C C stretching vibration of the C3 H6 . In spectra b and c, the bands observed in the region of 2700–3100 cm−1 would be assigned to the C–H stretching vibration of methoxide group and adsorbed methanol molecule, and the bands observed at 1475, 1462 and 1420 cm−1 were assigned to ıas (CH3 ) and ıs (CH3 ) of bridging methoxide group and ıs (OH) of adsorbed methanol, respectively [8,9]. These results suggest that adsorbed hydrocarbon on La7.33 BaYSi6 O25.5 was partially oxidized to methanol, formaldehyde and dimethylether, and in addition,
a part of C3 H6 and C3 H8 was completely oxidized to carbonate. Fig. 4(B) shows the DRIFT spectra of Al2 O3 after the treatment in the flow of 0.1%CO–N2 , 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C for 10 min. In the presence of CO (spectrum a), the weak bands observed at 1594 and 1376 cm−1 were assigned to the vas (COO) and vs (COO) stretching vibration [10]. The shoulder observed at 1649 cm−1 in the presence of C3 H6 (spectrum b) was assigned to C C stretching vibration of C3 H6 [9], but the broad shoulders at 1575–1350 cm−1 were unidentified. In the presence of C3 H8 (spectrum c), the bands was observed at 1579, 1460 and 1397 cm−1 , which are assigned to vas (COO− ), vs (COO− ) and C–H bending of the acetate species [11]. These results suggest that adsorbed C3 H8 on Al2 O3 was slightly oxidized. Fig. 4(C) shows the DRIFT spectra of CZ after the treatment in the flow of 0.1%CO–N2 , 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C for
Fig. 3. Conversion curves of C7 H8 and NO over Pt/La9.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ for C3 H6 –NO–O2 reaction.
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T. Wakabayashi et al. / Catalysis Today 164 (2011) 575–579
Fig. 4. DRIFT spectra of La9.33 BaYSi6 O25.5 (A), Al2 O3 (B) and CZ (C) in a flow of 0.1%CO/N2 (a), 0.25% C3 H6 –N2 (b), and 0.25% C3 H8 –N2 (c) at 300 ◦ C for 10 min.
10 min. In the presence of CO (spectrum a), the shoulders observed at 1488 and 1408 cm−1 were assigned to the vas (CO) and vs (CO) stretching vibration of the carbonate species [7,12]. In the presence of C3 H6 (spectrum b), the bands at 2967, 2946, 2857 and 1430 cm−1 were assigned to the C–H stretching and bending vibration of the CH3 and CH2 group. The peaks at 1564, 1370 and 1360 cm−1 were assigned to the vs (COO− ) and vas (COO− ) of the formate species [13]. In the presence of C3 H8 (spectrum c), the weak band observed at 2967 cm−1 was assigned to the C–H stretching vibration of the CH3 group. From the comparison of the spectra a and b, the broad band observed at 1575–1350 cm−1 would be assigned to formate and
carbonate. These results probably suggest that adsorbed hydrocarbons on CZ were partially oxidized to propionic acid or acetic acid in C3 H6 –N2 , and to acetic acid in C3 H8 –N2 . In addition, a part of the adsorbed C3 H6 and C3 H8 were completely oxidized to carbonate. Next, a form of adsorbed hydrocarbon on each carrier was investigated at 400 ◦ C for HC oxidation and NO reduction. All Pt loaded catalysts exhibited apparent activities, more 5% HC conversions at this temperature. Fig. 5(A) shows the DRIFT spectra of La7.33 BaYSi6 O25.5 after the treatment in the flow of 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C or 400 ◦ C for 10 min. In the DRIFT spectra in the presence of C3 H6 (spectrum a) and C3 H8 (spectrum b),
Fig. 5. DRIFT spectra of La9.33 BaYSi6 O25.5 (A), Al2 O3 (B) and CZ (C) in a flow of 0.25% C3 H6 –N2 (a) and 0.25% C3 H8 –N2 (b) at 300 ◦ C or 400 ◦ C for 10 min.
T. Wakabayashi et al. / Catalysis Today 164 (2011) 575–579
observed bands were assigned to carbonate, methanol and methoxide, which is same spieces observed at 300 ◦ C (spectrum b and c in Fig. 4(A)). In spectrum a, intensity of the peaks assigned to methoxide were lower than those observed at 300 ◦ C. While, in spectrum b, intensity of the peaks observed in the region of 1200–1700 cm−1 were larger than those observed at 300 ◦ C. This indicates that an amount of carbonate on the carrier increased with the reacting temperature, indicating proceeding of HC oxidation. Fig. 5(B) shows the DRIFT spectra of Al2 O3 after the treatment in the flow of 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C or 400 ◦ C for 10 min. In the presence of C3 H6 (spectrum a), no remarkable difference between the observed bands in DRIFT spectra at 300 ◦ C and 400 ◦ C was observed. In the presence oh C3 H8 (spectrum b), intensity of the peaks observed in the region of 1200–1700 cm−1 were slightly larger than those observed at 300 ◦ C. Fig. 5(C) shows the DRIFT spectra of CZ after the treatment in the flow of 0.25%C3 H6 –N2 and 0.25%C3 H8 –N2 at 300 ◦ C or 400 ◦ C for 10 min. In the presence of C3 H6 (spectrum a), intensity of the peaks assigned to formate species were lower than those observed at 300 ◦ C. In the presence of C3 H8 (spectrum b), no remarkable difference between the DRIFT spectra at 300 ◦ C and 400 ◦ C was observed. From the above-mentioned results, it was found that C3 H6 and C3 H8 adsorbed on La7.33 BaYSi6 O25.5 or CZ as the oxidized form, such as carbonate, methanol and formate. And most of adsorbed species on Al2 O3 were C3 H6 and C3 H8 molecule. These results suggest that oxidation of hydrocarbon was affected by the active oxygen species in the oxides. Macleod et al. have noted the formate species is un-reactive in NO + O2 at low temperature [14]. Therefore, it is considered that formation of formate on Pt/CZ catalyst led to lower NO conversion than Pt/La7.33 BaYSi6 O25.5 catalyst
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4. Conclusions Pt/La7.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ catalysts were investigated for oxidation of hydrocarbon species and NO reduction using C3 H6 , C3 H8 and C7 H8 . Pt/La7.33 BaYSi6 O25.5 catalyst exhibited higher performance for NO reduction by C3 H8 , C3 H6 and C7 H8 , than Pt/Al2 O3 and Pt/CZ catalysts. From the results of DRIFT measurement, intermediate of adsorbed hydrocarbon on La7.33 BaYSi6 O25.5 were observed. On the basis of results, it was presumed that the catalytic activity for NO reduction is affected by adsorbed form of hydrocarbon. Further investigation is needed to clarify the reduction mechanism of NO and adsorbed form of hydrocarbon on La7.33 BaYSi6 O25.5 . References [1] Y. Takei, H. Hoshi, M. Okada, K. Abe, SAE paper, No. 932670, 1993. [2] H. Tanaka, M. Uenishi, I. Tan, M. Kimura, Y. Nishihata, J. Mizuki, SAE Paper, 2001-1-1301, 2001. [3] S. Kato, T. Yoshizawa, N. Kakuta, S. Akiyama, M. Ogasawara, T. Wakabayashi, Y. Nakahara, S. Nakata, Res. Chem. Intermed. 34 (2008) 703. [4] T. Takeguchi, S. Manabe, R. Kikuchi, K. Eguchi, T. Kanazawa, S. Matsumoto, Appl. Catal. A 91 (2005) 293. [5] C.L. Angell, P.C. Schaffer, J. Phys. Chem. 70 (1966) 1413. [6] S.M. Teleb, M.S. Refat, Bull. Chem. Technol. Macedonia 25 (2006) 57. [7] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1978. [8] E. Ozensoy, D. Herling, J. Szanyi, Catal. Today 136 (2008) 46. [9] P. Cheung, H. Liu, E. Iglesia, J. Phys. Chem. B 108 (2004) 18650. [10] A.M. Arias, M.F. Garcia, A.B. Hungria, A.I. Juez, K. Duncan, R. Smith, J.A. Anderson, J.C. Conesa, J. Soria, J. Catal. 204 (2001) 238. [11] H. He, C. Zhang, Y. Yu, Catal. Today 90 (2004) 191. [12] A. Kotsifa, D.I. Kondarides, X.E. Verykios, Appl. Catal. B 72 (2007) 136. [13] J.L. Flores-Moreno, G. Delahay, F. Figueras, B. Coq, J. Catal. 236 (2005) 292. [14] N. Macleod, R.M. Lambert, Appl. Catal. B 46 (2003) 483.
Contents lists available at ScienceDirect
Catalysis Today journal homepage: www.elsevier.com/locate/cattod
Oxidation property of Pt/La7.33 BaYSi6 O25.5 catalysts for hydrocarbon species Takashi Wakabayashi a,∗ , Sumio Kato b , Yuunosuke Nakahara a , Masataka Ogasawara b , Shinichi Nakata b a b
Catalysts Division, Engineered Materials Sector, Mitsui Mining & Smelting Co., Ltd., 1013-1 Ageoshimo, Ageo, Saitama 362-0025, Japan Department of Engineering in Applied Chemistry for Environments, Faculty of Engineering and Resource Science, Akita University, 1-1 Tegatagakuen-machi, Akita 010-8502, Japan
a r t i c l e
i n f o
Article history: Received 30 June 2010 Received in revised form 15 November 2010 Accepted 17 November 2010 Available online 15 December 2010 Keywords: Apatite-type silicate Hydrocarbon Three-way catalyst
a b s t r a c t Pt loaded La7.33 BaYSi6 O25.5 (Pt/La7.33 BaYSi6 O25.5 ) catalyst was investigated for oxidation of hydrocarbon species and for reduction of NO by hydrocarbon species. Pt/La7.33 BaYSi6 O25.5 catalyst exhibited higher activity for NO reduction by alkane (C3 H8 ), alkene (C3 H6 ) and aromatic (C7 H8 ), than Pt/Al2 O3 catalyst and Pt/CeO2 –ZrO2 (CZ, CeO2 /ZrO2 weight ratio was 1/1) catalyst. The oxidizing power for hydrocarbon species of carrier affected the catalytic activity for NO reduction. In addition, oxidation of adsorbed hydrocarbon on La7.33 BaYSi6 O25.5 was observed by FT-IR measurement. It was presumed that the catalytic activity for NO reduction is affected by adsorbed form of hydrocarbon. © 2010 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
The three-way catalysts (TWCs) oxidize hydrocarbons (HC), carbon monoxide (CO) and as well as reduces nitrogen oxides (NOx) leading to near-zero emissions for these pollutants, in the condition close to stoichiometric air-to-fuel ratio (A/F). It is known that the constituent parts of hydrocarbons emitted from engine are mainly alkanes, alkenes and aromatics [1]. TWCs are composed of Al2 O3 , CeO2 –ZrO2 and precious metals, such as Pt, Pd and Rh. Currently, TWCs are demanded from improvement of performance and efficient use of the precious metals to deal with the global new legislation and the fluctuation of the precious metal’s price. To solve these issues, improvement of light-off performance for removal of hydrocarbons and NOx, and transient NOx control are needed. For that purpose, it has been carried out that the development of supported precious metal catalysts which have strong metal-support interaction (SMSI) [2]. We have reported that apatite-type rare earth silicatessupported Pd and Pt catalysts exhibited high activity for selective catalytic reduction of NO by C3 H6 [3]. Apatite-type rare earth silicates have attracted much attention as oxide ion conductors whose conductivities are higher than that of Y2 O3 stabilized ZrO2 at low temperature. In this study, we have investigated catalytic performance of Pt/La7.33 BaYSi6 O25.5 for oxidation of hydrocarbon species and for reduction of NO by hydrocarbon species.
La7.33 BaYSi6 O25.5 was synthesized by sol–gel method [3]. The catalyst had 1 mass % Pt supported on La7.33 BaYSi6 O25.5 powder and was prepared by impregnation of Pt (NH3 )2 (NO2 )2 . The powder was heated at 600 ◦ C for 3 h in air. In this study, ceramic honeycomb catalyst coated with Pt/La7.33 BaYSi6 O25.5 slurry was used. Pt/Al2 O3 and Pt/CZ catalysts also were prepared by using commercial Al2 O3 and CZ (CZ, CeO2 /ZrO2 weight ratio was 1/1) powder. Aging of these catalysts were carried out by heating at 900 ◦ C for 25 h in air containing 10 vol% H2 O. The catalytic activities were evaluated at raising temperature from 100 to 500 ◦ C in a simulated stoichiometric (A/F = 14.6, = 1) gas using 400 ppm C3 H6 – 1550 ppm O2 – 500 ppm NO, 400 ppm C3 H8 – 1750 ppm O2 – 500 ppm NO or 171 ppm C7 H8 – 1300 ppm O2 – 500 ppm NO (N2 as balance). The flow rate of the gas during the experiments was 25 L/min (GHSV 100,000 h−1 ), and the heating rate of the furnace was 20 ◦ C/min. The concentration of HC and NO in the effluent gas was continuously monitored by an on-line gas analyzer (Horiba, MEXA7100 and Best sokki, SESAM3-N). A specific surface area was measured by N2 adsorption using Brunauer–Emmett–Teller (BET) method (Quantachrome Instrument, Quadrasorb SI). The dispersion of Pt on each catalyst was measured by using CO pulse method [4]. FT-IR experiments were carried out using a JASCO FT/IR-6200 spectrometer equipped with MCT detector and KBr beam splitter. Spectra were obtained with 64-scan data acquisition at resolution of 4 cm−1 in controlled gas atmosphere and temperature, employing a diffuse reflectance infrared Fourier transform (DRIFT) spectroscopy cell. The carriers were pretreated in air flow at 700 ◦ C for 30 min and measurement was carried out after cooling down to 300 or 400 ◦ C under N2
∗ Corresponding author. Tel.: +81 048 775 3269; fax: +81 048 775 8126. E-mail address: t [email protected] (T. Wakabayashi). 0920-5861/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.cattod.2010.11.035
576
T. Wakabayashi et al. / Catalysis Today 164 (2011) 575–579
atmosphere. The total gas flow rate through the DRIFT cell was 0.1 L/min.
3. Results and discussion The catalytic activities for NO reduction using C3 H6 , C3 H8 or C7 H8 as reductants over Pt/La7.33 BaYSi6 O25.5 and Pt/Al2 O3 , Pt/CZ catalysts were investigated. Fig. 1 shows conversions of C3 H6 and NO over each Pt catalysts for C3 H6 –NO–O2 reaction. The catalytic activities was evaluated by light-off temperature (LOT50 ) shown in Table 1, which is the temperature of the reaction exhibiting 50% conversion of HC and NO, and the conversion at 500 ◦ C (500). Pt/La7.33 BaYSi6 O25.5 catalyst exhibited the highest activity for both HC oxidation and NO reduction. The LOT50 of C3 H6 for Pt/La7.33 BaYSi6 O25.5 and Pt/CZ catalysts were 403 and 455 ◦ C, respectively. The conversion of C3 H6 for Pt/Al2 O3 catalyst was lower than 50% even at 500 ◦ C. On the other hand, NO conversion for Pt/La7.33 BaYSi6 O25.5 catalyst achieved 50% at 454 ◦ C. The 500 of C3 H6 and NO decreased in the sequence of Pt/La7.33 BaYSi6 O25.5 > Pt/CZ > Pt/Al2 O3 . Fig. 2 shows conversions of C3 H8 and NO over each Pt catalysts for C3 H8 –NO–O2 reaction. The LOT50 of C3 H8 for Pt/La7.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ catalysts were 377, 400 and 464 ◦ C, respectively. And the LOT50 of NO for Pt/La7.33 BaYSi6 O25.5 and Pt/Al2 O3 catalysts were 402 and 440 ◦ C, respectively. Both HC and NO conversions for C3 H8 –NO–O2 reaction over Pt/La7.33 BaYSi6 O25.5 and Pt/Al2 O3 catalysts were higher than those for C3 H6 –NO–O2 reaction. While, the catalytic activity for C3 H8 –NO–O2 reaction was slightly lower than that for C3 H6 –NO–O2 reaction. These results suggest that the carrier of each Pt catalysts affected the catalytic activities for NO reduction using C3 H8 and C3 H6 . Fig. 3 shows conversions of NO and C7 H8 over each Pt catalysts for C7 H8 –NO–O2 reaction. The NO conversions were lower than 50% for all Pt catalysts and LOT50 of C7 H8 for Pt/La7.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ catalysts were 397, 492 and 448 ◦ C, respectively. The LOT50 and 500 for C7 H8 conversions decreased in the sequence of Pt/La7.33 BaYSi6 O25.5 > Pt/CZ > Pt/Al2 O3 , which is same order in the case of C3 H6 –NO–O2 reaction. From Table 1, it can be seen that Pt/La7.33 BaYSi6 O25.5 catalyst exhibited higher activity for both oxidation of each hydrocarbon species and NO reduction by
Table 1 Catalytic activities over Pt catalysts in simulated gases.
◦
T50 ( C)
500 (%)
Carrier
C3 H6
NO
C3 H8
NO
C7 H8
NO
La7.33 BaYSi6 O25.5 Al2 O3 CZ La7.33 BaYSi6 O25.5 Al2 O3 CZ
403 U.D. 455 83.4 47.8 64.3
454 – – 71.1 29.7 36.2
377 400 464 94.8 87.4 61.2
402 440 U.D. 92.4 80.3 48.4
397 492 448 81.5 55.1 71.9
– – – 43.4 15.6 39.6
–: Conversion were lower than 50%. Table 2 Specific surface area and dispersion of Pt catalysts. Carrier
La7.33 BaYSi6 O25.5 Al2 O3 CZ
Surface area (m2 /g)
Dispersion (CO/Pt)
Fresh
Aged
Fresh
Aged
12.5 155.8 74.7
12.4 104.0 46.2
0.035 0.468 0.322
– 0.033 0.012
–: Not evaluated by CO adsorption method.
C3 H8 (alkane), C3 H6 (alkene) and C7 H8 (aromatic), than Pt/Al2 O3 catalyst and Pt/CZ catalyst. Table 2 shows the specific surface area and dispersion of Pt on each carrier for fresh and aged condition. The specific surface area and dispersion of Pt on each carrier for fresh and aged condition was lowered in the sequence of Pt/Al2 O3 > Pt/CZ > Pt/La7.33 BaYSi6 O25.5 . Especially, dispersion of Pt on La7.33 BaYSi6 O25.5 catalyst after the aging was too small to evaluate by CO adsorption method. These results suggest that properties of Pt on the catalyst such as chemical state, is more effective factor on the catalytic activity than dispersion of Pt. The catalytic activities for NO reduction over Pt catalysts were affected by a type of hydrocarbon and each carrier. Therefore, a form of adsorbed hydrocarbon on each carrier was investigated by FT-IR measurement. First, a form of adsorbed hydrocarbon on each carrier was investigated at 300 ◦ C for HC oxidation. All the Pt loaded catalysts exhibited small conversions at this temperature. The DRIFT spectra of CO were used to compare with the DRIFT spectra of C3 H6 or C3 H8 . Fig. 4(A) shows the DRIFT spectra of La7.33 BaYSi6 O25.5 after the treatment in the flow of 0.1%CO–N2 , 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C for 10 min. In the presence of CO (spec-
Fig. 1. Conversion curves of C3 H6 and NO over Pt/La9.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ for C3 H6 –NO–O2 reaction.
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Fig. 2. Conversion curves of C3 H8 and NO over Pt/La9.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ for C3 H6 –NO–O2 reaction.
trum a), band at 2181 cm−1 , which was assigned to vibration of CO adsorbed on the Ba or La sites was observed [5]. The broad bands observed at 1484 and 1400 cm−1 were assigned to the vas (CO) and vs (CO) stretching vibrations of carbonate ion (CO3 2− ) [6,7]. These bands were also observed in the presence of C3 H6 (spectrum b) and C3 H8 (Spectrum c). In the spectrum b, the weak band observed at 1667 cm−1 was assigned to the C C stretching vibration of the C3 H6 . In spectra b and c, the bands observed in the region of 2700–3100 cm−1 would be assigned to the C–H stretching vibration of methoxide group and adsorbed methanol molecule, and the bands observed at 1475, 1462 and 1420 cm−1 were assigned to ıas (CH3 ) and ıs (CH3 ) of bridging methoxide group and ıs (OH) of adsorbed methanol, respectively [8,9]. These results suggest that adsorbed hydrocarbon on La7.33 BaYSi6 O25.5 was partially oxidized to methanol, formaldehyde and dimethylether, and in addition,
a part of C3 H6 and C3 H8 was completely oxidized to carbonate. Fig. 4(B) shows the DRIFT spectra of Al2 O3 after the treatment in the flow of 0.1%CO–N2 , 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C for 10 min. In the presence of CO (spectrum a), the weak bands observed at 1594 and 1376 cm−1 were assigned to the vas (COO) and vs (COO) stretching vibration [10]. The shoulder observed at 1649 cm−1 in the presence of C3 H6 (spectrum b) was assigned to C C stretching vibration of C3 H6 [9], but the broad shoulders at 1575–1350 cm−1 were unidentified. In the presence of C3 H8 (spectrum c), the bands was observed at 1579, 1460 and 1397 cm−1 , which are assigned to vas (COO− ), vs (COO− ) and C–H bending of the acetate species [11]. These results suggest that adsorbed C3 H8 on Al2 O3 was slightly oxidized. Fig. 4(C) shows the DRIFT spectra of CZ after the treatment in the flow of 0.1%CO–N2 , 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C for
Fig. 3. Conversion curves of C7 H8 and NO over Pt/La9.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ for C3 H6 –NO–O2 reaction.
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Fig. 4. DRIFT spectra of La9.33 BaYSi6 O25.5 (A), Al2 O3 (B) and CZ (C) in a flow of 0.1%CO/N2 (a), 0.25% C3 H6 –N2 (b), and 0.25% C3 H8 –N2 (c) at 300 ◦ C for 10 min.
10 min. In the presence of CO (spectrum a), the shoulders observed at 1488 and 1408 cm−1 were assigned to the vas (CO) and vs (CO) stretching vibration of the carbonate species [7,12]. In the presence of C3 H6 (spectrum b), the bands at 2967, 2946, 2857 and 1430 cm−1 were assigned to the C–H stretching and bending vibration of the CH3 and CH2 group. The peaks at 1564, 1370 and 1360 cm−1 were assigned to the vs (COO− ) and vas (COO− ) of the formate species [13]. In the presence of C3 H8 (spectrum c), the weak band observed at 2967 cm−1 was assigned to the C–H stretching vibration of the CH3 group. From the comparison of the spectra a and b, the broad band observed at 1575–1350 cm−1 would be assigned to formate and
carbonate. These results probably suggest that adsorbed hydrocarbons on CZ were partially oxidized to propionic acid or acetic acid in C3 H6 –N2 , and to acetic acid in C3 H8 –N2 . In addition, a part of the adsorbed C3 H6 and C3 H8 were completely oxidized to carbonate. Next, a form of adsorbed hydrocarbon on each carrier was investigated at 400 ◦ C for HC oxidation and NO reduction. All Pt loaded catalysts exhibited apparent activities, more 5% HC conversions at this temperature. Fig. 5(A) shows the DRIFT spectra of La7.33 BaYSi6 O25.5 after the treatment in the flow of 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C or 400 ◦ C for 10 min. In the DRIFT spectra in the presence of C3 H6 (spectrum a) and C3 H8 (spectrum b),
Fig. 5. DRIFT spectra of La9.33 BaYSi6 O25.5 (A), Al2 O3 (B) and CZ (C) in a flow of 0.25% C3 H6 –N2 (a) and 0.25% C3 H8 –N2 (b) at 300 ◦ C or 400 ◦ C for 10 min.
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observed bands were assigned to carbonate, methanol and methoxide, which is same spieces observed at 300 ◦ C (spectrum b and c in Fig. 4(A)). In spectrum a, intensity of the peaks assigned to methoxide were lower than those observed at 300 ◦ C. While, in spectrum b, intensity of the peaks observed in the region of 1200–1700 cm−1 were larger than those observed at 300 ◦ C. This indicates that an amount of carbonate on the carrier increased with the reacting temperature, indicating proceeding of HC oxidation. Fig. 5(B) shows the DRIFT spectra of Al2 O3 after the treatment in the flow of 0.25%C3 H6 –N2 or 0.25%C3 H8 –N2 at 300 ◦ C or 400 ◦ C for 10 min. In the presence of C3 H6 (spectrum a), no remarkable difference between the observed bands in DRIFT spectra at 300 ◦ C and 400 ◦ C was observed. In the presence oh C3 H8 (spectrum b), intensity of the peaks observed in the region of 1200–1700 cm−1 were slightly larger than those observed at 300 ◦ C. Fig. 5(C) shows the DRIFT spectra of CZ after the treatment in the flow of 0.25%C3 H6 –N2 and 0.25%C3 H8 –N2 at 300 ◦ C or 400 ◦ C for 10 min. In the presence of C3 H6 (spectrum a), intensity of the peaks assigned to formate species were lower than those observed at 300 ◦ C. In the presence of C3 H8 (spectrum b), no remarkable difference between the DRIFT spectra at 300 ◦ C and 400 ◦ C was observed. From the above-mentioned results, it was found that C3 H6 and C3 H8 adsorbed on La7.33 BaYSi6 O25.5 or CZ as the oxidized form, such as carbonate, methanol and formate. And most of adsorbed species on Al2 O3 were C3 H6 and C3 H8 molecule. These results suggest that oxidation of hydrocarbon was affected by the active oxygen species in the oxides. Macleod et al. have noted the formate species is un-reactive in NO + O2 at low temperature [14]. Therefore, it is considered that formation of formate on Pt/CZ catalyst led to lower NO conversion than Pt/La7.33 BaYSi6 O25.5 catalyst
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4. Conclusions Pt/La7.33 BaYSi6 O25.5 , Pt/Al2 O3 and Pt/CZ catalysts were investigated for oxidation of hydrocarbon species and NO reduction using C3 H6 , C3 H8 and C7 H8 . Pt/La7.33 BaYSi6 O25.5 catalyst exhibited higher performance for NO reduction by C3 H8 , C3 H6 and C7 H8 , than Pt/Al2 O3 and Pt/CZ catalysts. From the results of DRIFT measurement, intermediate of adsorbed hydrocarbon on La7.33 BaYSi6 O25.5 were observed. On the basis of results, it was presumed that the catalytic activity for NO reduction is affected by adsorbed form of hydrocarbon. Further investigation is needed to clarify the reduction mechanism of NO and adsorbed form of hydrocarbon on La7.33 BaYSi6 O25.5 . References [1] Y. Takei, H. Hoshi, M. Okada, K. Abe, SAE paper, No. 932670, 1993. [2] H. Tanaka, M. Uenishi, I. Tan, M. Kimura, Y. Nishihata, J. Mizuki, SAE Paper, 2001-1-1301, 2001. [3] S. Kato, T. Yoshizawa, N. Kakuta, S. Akiyama, M. Ogasawara, T. Wakabayashi, Y. Nakahara, S. Nakata, Res. Chem. Intermed. 34 (2008) 703. [4] T. Takeguchi, S. Manabe, R. Kikuchi, K. Eguchi, T. Kanazawa, S. Matsumoto, Appl. Catal. A 91 (2005) 293. [5] C.L. Angell, P.C. Schaffer, J. Phys. Chem. 70 (1966) 1413. [6] S.M. Teleb, M.S. Refat, Bull. Chem. Technol. Macedonia 25 (2006) 57. [7] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordination Compounds, Wiley, New York, 1978. [8] E. Ozensoy, D. Herling, J. Szanyi, Catal. Today 136 (2008) 46. [9] P. Cheung, H. Liu, E. Iglesia, J. Phys. Chem. B 108 (2004) 18650. [10] A.M. Arias, M.F. Garcia, A.B. Hungria, A.I. Juez, K. Duncan, R. Smith, J.A. Anderson, J.C. Conesa, J. Soria, J. Catal. 204 (2001) 238. [11] H. He, C. Zhang, Y. Yu, Catal. Today 90 (2004) 191. [12] A. Kotsifa, D.I. Kondarides, X.E. Verykios, Appl. Catal. B 72 (2007) 136. [13] J.L. Flores-Moreno, G. Delahay, F. Figueras, B. Coq, J. Catal. 236 (2005) 292. [14] N. Macleod, R.M. Lambert, Appl. Catal. B 46 (2003) 483.