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Catalytic NO–H2–CO–O2 reactions over Pt-supported mesoporous yttrium oxide
Catalysis Communications 10 (2009) 1995–1999
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Catalytic NO–H2–CO–O2 reactions over Pt-supported mesoporous yttrium oxide Ahmed Jalal Samed, Takayuki Tanaka, Shin Hamada, Keita Ikeue, Masato Machida * Department of Nano Science and Technology, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
a r t i c l e
i n f o
Article history: Received 7 June 2009 Received in revised form 12 July 2009 Accepted 17 July 2009 Available online 23 July 2009 Keywords: Yttrium oxide Mesoporous Template synthesis Platinum NOx
a b s t r a c t Catalytic light-off of a stream of NO, H2, CO in an excess O2 has been studied over various metal oxides loading 1 wt% Pt. Because a low-surface area Y2O3 (<5 m2 g1) was found to exhibit the highest de-NOx activity, a mesoporous Y2O3 was then synthesized from an yttrium-based surfactant mesophase tem plated by dodecyl sulfate ðDS ¼ C12 H25 OSO 3 Þ, which was anion-exchanged by acetate (AcO = CH3COO ). The product showed a 3-D mesoporosity with a large surface area (396 m2 g1) and the Pt-supported catalyst achieved much improved light-off characteristics suitable for the low-temperature de-NOx in the presence of CO and excess O2. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The control of NOx emissions from automobiles and stationary sources is important due to the detrimental environmental consequences. Increasingly stringent environmental regulations for NOx abatement provide the impetus for new approaches to control the emission of NOx. On the other hand, lean-burn technology has been introduced for gasoline engines to minimize CO2 emission and fuel consumption. The situation thus requires the catalysis applicable to de-NOx under lean-burn conditions. In the last decade the catalytic NO–H2 reaction in excess oxygen has been extensively investigated [1–9]. Hydrogen is a promising reductant for NOx at low temperatures under lean-burn conditions when used in conjunction with Pt-based catalysts. However, even small concentration of CO causes a significant poisoning effect on NO–H2 reactions over the Pt/Al2O3 catalyst [1] in contrast to a promoting effect on the Pd/ Al2O3 catalyst [10,11]. We have previously studied the NO–H2 reactions in an excess O2 over Pt catalysts supported on various metal oxides with basic and/or acidic characters [12–17]. The oxide supports having both moderate acidic and/or basic characters are found to be suitable for the high activity and selectivity to N2. But, the validity of such catalysts is not confirmed in the presence of CO. In the present work we have studied the catalytic NO–H2–CO–O2 reaction over Pt catalysts supported on various metal oxides. The highest activity was observed for Y2O3, which has subsequently been synthesized as a mesoporous support having extremely high * Corresponding author. Tel./fax: +81 96 342 3651. E-mail address: [email protected] (M. Machida). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.07.016
surface areas. The correlation between the microstructure and catalytic properties of the Pt/Y2O3 catalyst for the target reaction is discussed for the development of novel selective de-NOx catalysts working at low temperatures in the CO-containing lean-burn exhaust.
2. Experimental 2.1. Catalyst preparation and characterization A series of metal oxide supports were synthesized by the precipitation method using aqueous solutions of metal nitrates and aqueous ammonia and subsequent air calcination (400 °C, 5 h). Supported Pt catalysts (1 wt% loading as Pt) were prepared by impregnation of a nitric acid solution of Pt(NO2)2(NH3)2, followed by drying and air calcination (400 °C, 5 h). Yttrium-based surfactant mesophases were synthesized by the homogeneous precipitation method using urea according to the literature [18,19]. Yttrium nitrate hexahydrate (Y(NO3)36H2O) was used as an yttrium source, and sodium dodecyl sulfate (SDS, CH3(CH2)11OSO3Na) as a templating agent. Urea was used as an agent to raise the pH of the solution, because on heating at above 60 °C it undergoes hydrolysis as follows,
ðNH2 Þ2 CO þ 3H2 O ! 2NHþ4 þ 2OH þ CO2
ð1Þ
Yttrium nitrate, SDS, urea and water were mixed at molar ratio of 1:2:30:60 and stirred at 40 °C for 1 h to yield a transparent mixed solution. The mixed solution was heated at 80 °C and then kept at that temperature for 72 h, where the precipitation
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A.J. Samed et al. / Catalysis Communications 10 (2009) 1995–1999
occurred. After the reaction, the precipitate (YDS) was collected by centrifugation, washed with water and then treated with an aqueous solution of CH3COONa to remove the surfactant (DS) via ion-exchange (YAcO). The YAcO supported Pt catalyst (1 wt% loading) 2 was prepared by the liquid-phase reduction of PtCl6 with N2H4 solution at pH10. Using X-ray fluorescence (Horiba MESA500W), XPS (VG Sigmaprobe spectrometer) and FT–IR (Jasco 610) measurements, it was demonstrated that the as prepared catalyst was free from residual sulfur and chlorine, which originated from 2 DS and PtCl6 , respectively. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku Multiplex instrument using monochromated Cu Ka radiation (30 kV, 30 mA). BET surface area and pore size distribution were determined from N2 adsorption isotherms measured at 77 K using a Belsorp Mini instrument (Bel Japan, Inc.). Average pore size analysis was calculated using the Dollimore and Heal method [20] on the desorption branch. Metal dispersion of Pt was measured by pulsed CO chemisorption at 50 °C after reduction treatment at 200 °C in H2. The microstructure of YAcO was observed by TEM (FEI TECNAI F20, 200 kV).
3. Results and discussion 3.1. Catalyst screening results Fig. 1 summarizes the catalytic activity of 1 wt% Pt supported on various metal oxides for NO–H2–CO–O2 reactions. The activity is expressed by the cumulative NOx (NOx = NO + NO2) removal during the heating ramp (40–200 °C) and the temperature where NOx conversion reached to 20% (T20%). Pt catalysts supported on MgO, Al2O3 and SiO2 having large surface areas (P100 m2 g1) initiate the NO reduction at the low temperatures of 6100 °C and achieve high NOx removals. On the other hand, Pt catalysts supported on La2O3 and other lanthanide oxides (Pr, Gd and Yb) exhibit higher T20% values (>100 °C) and thus low NOx removals (<20%). Among these oxides supports, Y2O3 achieves the highest NOx removal and low T20% (100 °C). According to our previous study [12–17], oxidatively adsorbed NOx species (NO2 and/or NO3) play a key role in the NO–H2 reactions over Pt in an excess O2 depending on the property of support materials. Although Pt on acidic or amphoteric oxides such as SiO2, ZrO2, Al2O3 and ZSM-5 forms reactive NO2/NO3 species suitable for
2.2. Catalytic reaction Catalytic NO–H2–CO–O2 reaction was carried out in a conventional flow reactor equipped with a water cooled infrared image furnace. A granulated catalyst (10–20 mesh, 0.05 g) was fixed by quartz wool in a Pyrex glass tube (4 mm i.d.). Prior to catalytic reactions, Pt-impregnated catalyst was preheated at 400 °C for 1 h in a flow of 5% H2/He except for Pt/YAcO, which was preheated at 200 °C for 1 h in a flow of 5% H2/He to avoid structural collapse. The treatment removed the acetate species (AcO) from YAcO as was evident from the FT–IR measurement. Temperature programmed reactions were carried out at a ramp of 10 °C/min in a stream of gas mixture consisting of 0.02% NO, 0.5% H2, 0.1% CO and 10% O2 balanced with He supplied to the catalyst bed at W/F = 0.03 g s cm3. The catalytic activity was then measured at the constant catalyst temperature where conversions of NO, H2 and CO reached to steady-states. The reaction was also carried out in the absence of CO (0.02% NO, 0.5% H2 and 10% O2 balanced with He, W/F = 0.03 g s cm3). Effluent gas was analyzed by using a Shimadzu NOA-7000 chemiluminescence NOx analyzer, a Horiba VA3000 NDIR analyzer and a Pfeiffer GSD30101 mass spectrometer.
a
b
× 4 0
1
2
3
4
5
6
7
8
9
10
2θ / deg Fig. 2. Powder X-ray diffraction patterns of (a) yttrium-based mesophase after anion-exchange (YAcO) and (b) 1 wt% Pt loaded catalyst (Pt/YAcO).
Fig. 1. Catalytic activity of supported Pt catalysts (1 wt% loading). NOx removal: cumulative removal in the heating ramp (40–200 °C). T20%: Temperature at which NO conversion reached to 20%. 0.02% NO, 0.5% H2, 0.1% CO and 10% O2 balanced with He, W/F = 0.03 g s cm3, Heating rate = 10 °C min1.
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the reaction with H2, considerably more N2O than N2 is produced. This is not the case of present Pt/Y2O3 catalyst, because its product selectivity to N2 was more than 60%. On the other hand, Pt becomes less reactive when supported on oxides having a high basicity, such as CaO and La2O3, which adsorbed NO as stable NO3 species in excess O2. A similar effect of acid/base character on the catalysis can be seen in the present NO–H2–CO–O2 system, because the NOx reduction route appears to be the same as discussed later. According to our CO2-TPD measurement (Supplementary data), Y2O3 exhibits a desorption peak located at about 300 °C, which is higher than ZrO2 (120–200 °C) and MgO (140 °C) and is lower than La2O3 (480 °C). One possible reason for the high activity and selectivity to N2 of Pt/Y2O3 is therefore due to the moderate
a
20nm
b
20nm
Adsorption / cm3 g-1
350 300 250
basicity of Y2O3, which may yield NO2/NO3 species having a higher reactivity toward H2. 3.2. Pt-supported mesoporous yttrium oxide It has been found that Pt/Y2O3 exhibits the high catalytic de-NOx activity in the NO–H2–CO–O2 system at low temperatures. Because Y2O3 in Fig. 1 is a dense phase with a very low BET surface area (<5 m2 g1), we have next tried to synthesize a mesoporous support to improve the catalytic activity. The mesoporous Y2O3 was prepared according to a template synthesis route which has been reported previously [18,19]. The homogeneous precipitation using urea in the presence of DS as a template produced a hexagonal mesophase (YDS), which was subsequently subjected to ion-exchange of DS with acetate ions (AcO). The replacement of the DS moiety in the mesostructure with much smaller AcO converts the mesophase into a mesoporous material (YAcO) with a large surface area. Fig. 2 shows the XRD patterns of as prepared YAcO and 1 wt% Pt/ YAcO. Both sample presented a single strong reflection at around 2h = 1.8° and a very weak shoulder at 2h = 2–4°. The d spacing, 4.8 nm, determined from the low-angle diffraction peak is consistent with the reported (1 0 0) reflection of a hexagonal structure [18], but the whole pattern implies that the mesostructure should be extremely disordered. No diffraction peaks were observed at 2h higher than 10°. The TEM photographs of these samples are shown in Fig. 3. Clearly, 3-D mesoporosity are developed thoroughly in the YAcO sample (a) after removing a DS moiety, but the structural ordering is pretty poor. Such a wormhole-like mesopores were preserved in the 1 wt% Pt loaded YAcO (b), which showed the deposition of Pt with the size of about 1–2 nm. This is consistent with the metal dispersion of Pt, 45% (diameter: 2.5 nm), which is higher than that of the Pt supported on a dense Y2O3 (5%), as measured by pulsed CO chemisorption. The porous structure was also characterized by N2 adsorption– desorption isotherm measured at 77 K (Fig. 3 and Table 1). The isotherm for YAcO was similar to type IV defined in the IUPAC classification with a small hysteresis loop (a). The adsorption in the range p/p0 = 0.2–0.4 is a clear indication of capillary condensation in mosopores. The shape of the isotherm was preserved after loading 1 wt% Pt (b), but the adsorption took place at higher pressure p/p0 = 0.5–0.7, compared to 0.2–0.4 observed for YAcO. This is consistent with the broadening of pore size distribution and the enlargement of average pore diameter from 2.5 nm (YAcO) to 3.2 nm (Pt/YAcO). The BET surface area decreased from 396 to 279 m2 g1 after loading Pt. Taking these results into consideration, Pt particles should mainly be deposited on the external surface of mesoporous YAcO as observed in the TEM photograph (Fig. 3b). The increase of pore size during the Pt impregnation process is associated with partial collapse of mesopores during the removal of water. We also proved that the base strength of the surface of Y2O3 is unlikely affected by mesoporous structure, because the CO2-TPD profiles from YAcO and dense Y2O3 are very similar. How-
200 150
Table 1 Specific surface area, pore diameter and pore volume calculated from N2 adsorption isotherms at 77 K.
a b
100 50 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
p/p0
YAcO 1 wt% Pt/YAcO a
Pore diameterb/nm
Pore volumec/mm3 g1
396 279
2.5 3.2
430 550
The specific surface area was determined using the BET method. The pore size distribution was calculated using the Dollimore and Heal method on the desorption branch. c Total pore volume was calculated form N2 adsorption at p/p0 = 0.99. b
Fig. 3. TEM images and N2 adsorption/desorption isotherms of (a) yttrium-based mesophase after exchange (YAcO) and (b) 1 wt% Pt loaded catalyst (Pt/YAcO).
Surface areaa/m2 g1
A.J. Samed et al. / Catalysis Communications 10 (2009) 1995–1999
ever, the number of CO2 adsorption sites per unit weight of YAcO is two orders of magnitude more than those of dense Y2O3. Finally, the catalytic activity of 1 wt% Pt/YAcO for the NO–H2– CO–O2 reaction was evaluated after pretreatment in 5% H2/He at 200 °C. Fig. 4 compares the steady-state conversion and selectivity measured at each reaction temperatures. Clearly, the mesoporous YAcO material showed much higher de-NOx activity. The Pt/YAcO exhibited the light-off about 20 °C lower than Pt/dense Y2O3 and thus the activity in a wide temperature range of 100–300 °C. The product selectivity at 140 °C was about 60% N2 and 40% N2O for both Pt catalysts. Above 150 °C, however, the de-NOx efficiency begins to decrease, because H2–O2 reaction becomes dominant with an increase of reaction temperatures. The drawback of the YAcO is its low thermal stability, because the wormhole-like mesopores shown in Fig. 3 are totally collapsed when heated above 400 °C, where a dense Y2O3 with a low-surface area (<5 m2 g1) is formed. Fortunately, however, the present NOx reduction using H2 as a reducing agent takes place at temperatures as low as 200 °C, where the mesoporosity can stably be preserved. For the reason, the present catalyst would be suitable for low-temperature stationary NOx removal rather than automobile emission control. To gain insight into the reaction route of the present system, the NO–H2 reaction over 1 wt% Pt/Y2O3 (a dense phase) was carried out in the absence CO or H2 with keeping the other gas concentrations unchanged. The negligible reduction of NO without H2 feed demonstrates that H2 plays an essential role as the reducing agent for NO. Fig. 5 compares the result of steady-state reactions without
100
a
NOx conv.
80
Conversion / %
NOx conv. to N2 NOx conv. to N2O
60
H2 conv./%
NOx conv.
80
NOx conv. to N2 NOx conv. to N2O
60
H2 conv./% 40
20
0 0
100
200
300
400
500
Temperature / °C Fig. 5. Steady-state conversion efficiency and product selectivity of NO–H2–O2 over 1 wt% Pt loaded on dense Y2O3. 0.02% NO, 0.5% H2 and 10% O2 balanced with He, W/ F = 0.03 g s cm3.
CO feed. The reaction in the absence of CO initiated the NOx reduction at around 50 °C, compared to 120 °C required in the presence of 1000 ppm CO as shown in Fig. 4a. On the other hand, CO in the feed did not influence the NOx reduction efficiency at temperatures higher than 180 °C, where CO was totally oxidized to CO2. The poisoning behavior of CO can be rationalized by the fact that strong CO adsorption onto Pt surface may inhibit the activation of H2 especially at lower temperatures as was reported previously [10,11]. The resulting shift of H2 and NOx conversions towards higher temperature is consequently the modification of the NO– H2 reaction in the presence of CO, but their reaction route should basically be the same. In case of Pt/YAcO, however, the light-off of CO at lower temperature of 100 °C renders the NO–H2 reaction more tolerant to poisoning by CO.
CO conv./% 40
4. Conclusion
20
0 0
100
200
300
400
500
Temperature /°C 100
b 80
Conversion / %
100
Conversion / %
1998
Among the variety of metal oxide supports, Pt supported on Y2O3 has been found to be the most active for de-NOx in the stream of NO, H2, CO and excess O2 at low temperatures (6250 °C). The catalytic activity was much improved by the use of mesoporous Y2O3, which was synthesized from an yttrium-based surfactant mesophase templated by dodecyl sulfate anion. Thanks to a large specific surface area (279 m2 g1) and a high Pt dispersion (45%), our catalyst achieved the efficient NOx removal in the wide range of 100–300 °C. Acknowledgement This study was supported by Elements Science and Technology Project from the Ministry of Education, Culture, Sports, Science and Technology.
60
40
Appendix A. Supplementary data 20
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2009.07.016. 0 0
100
200
300
400
500
Temperature /°C Fig. 4. Steady-state conversion efficiency and product selectivity of NO–H2–CO–O2 reaction over 1 wt% Pt loaded on (a) dense Y2O3 and (b) YAcO. 0.02% NO, 0.5% H2, 0.1% CO and 10% O2 balanced with He, W/F = 0.03 g s cm3.
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[12] M. Machida, S. Ikeda, D. Kurogi, T. Kijima, Appl. Catal. B: Environ. 35 (2001) 107. [13] M. Machida, T. Watanabe, S. Ikeda, T. Kijima, Catal. Commun. 3 (2002) 233. [14] M. Machida, S. Ikeda, J. Catal. 227 (2004) 53. [15] M. Machida, T. Watanabe, Appl. Catal. B: Environ. 52 (2004) 281. [16] S. Hamada, S. Hibarino, K. Ikeue, M. Machida, Appl. Catal. B: Environ. 74 (2007) 197. [17] S. Hamada, K. Ikeue, M. Machida, Appl. Catal. B: Environ. 71 (2007) 1. [18] M. Yada, H. Kitamura, M. Machida, T. Kijima, Inorg. Chem. 37 (1998) 6470. [19] M. Yada, S. Mouri, M. Machida, T. Kijima, Angew. Chem. Int. Ed. 38 (1999) 3506. [20] D. Dollimore, G.R. Heal, J. Appl. Chem. 14 (1964) 109.
Contents lists available at ScienceDirect
Catalysis Communications journal homepage: www.elsevier.com/locate/catcom
Catalytic NO–H2–CO–O2 reactions over Pt-supported mesoporous yttrium oxide Ahmed Jalal Samed, Takayuki Tanaka, Shin Hamada, Keita Ikeue, Masato Machida * Department of Nano Science and Technology, Graduate School of Science and Technology, Kumamoto University, 2-39-1 Kurokami, Kumamoto 860-8555, Japan
a r t i c l e
i n f o
Article history: Received 7 June 2009 Received in revised form 12 July 2009 Accepted 17 July 2009 Available online 23 July 2009 Keywords: Yttrium oxide Mesoporous Template synthesis Platinum NOx
a b s t r a c t Catalytic light-off of a stream of NO, H2, CO in an excess O2 has been studied over various metal oxides loading 1 wt% Pt. Because a low-surface area Y2O3 (<5 m2 g1) was found to exhibit the highest de-NOx activity, a mesoporous Y2O3 was then synthesized from an yttrium-based surfactant mesophase tem plated by dodecyl sulfate ðDS ¼ C12 H25 OSO 3 Þ, which was anion-exchanged by acetate (AcO = CH3COO ). The product showed a 3-D mesoporosity with a large surface area (396 m2 g1) and the Pt-supported catalyst achieved much improved light-off characteristics suitable for the low-temperature de-NOx in the presence of CO and excess O2. Ó 2009 Elsevier B.V. All rights reserved.
1. Introduction The control of NOx emissions from automobiles and stationary sources is important due to the detrimental environmental consequences. Increasingly stringent environmental regulations for NOx abatement provide the impetus for new approaches to control the emission of NOx. On the other hand, lean-burn technology has been introduced for gasoline engines to minimize CO2 emission and fuel consumption. The situation thus requires the catalysis applicable to de-NOx under lean-burn conditions. In the last decade the catalytic NO–H2 reaction in excess oxygen has been extensively investigated [1–9]. Hydrogen is a promising reductant for NOx at low temperatures under lean-burn conditions when used in conjunction with Pt-based catalysts. However, even small concentration of CO causes a significant poisoning effect on NO–H2 reactions over the Pt/Al2O3 catalyst [1] in contrast to a promoting effect on the Pd/ Al2O3 catalyst [10,11]. We have previously studied the NO–H2 reactions in an excess O2 over Pt catalysts supported on various metal oxides with basic and/or acidic characters [12–17]. The oxide supports having both moderate acidic and/or basic characters are found to be suitable for the high activity and selectivity to N2. But, the validity of such catalysts is not confirmed in the presence of CO. In the present work we have studied the catalytic NO–H2–CO–O2 reaction over Pt catalysts supported on various metal oxides. The highest activity was observed for Y2O3, which has subsequently been synthesized as a mesoporous support having extremely high * Corresponding author. Tel./fax: +81 96 342 3651. E-mail address: [email protected] (M. Machida). 1566-7367/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2009.07.016
surface areas. The correlation between the microstructure and catalytic properties of the Pt/Y2O3 catalyst for the target reaction is discussed for the development of novel selective de-NOx catalysts working at low temperatures in the CO-containing lean-burn exhaust.
2. Experimental 2.1. Catalyst preparation and characterization A series of metal oxide supports were synthesized by the precipitation method using aqueous solutions of metal nitrates and aqueous ammonia and subsequent air calcination (400 °C, 5 h). Supported Pt catalysts (1 wt% loading as Pt) were prepared by impregnation of a nitric acid solution of Pt(NO2)2(NH3)2, followed by drying and air calcination (400 °C, 5 h). Yttrium-based surfactant mesophases were synthesized by the homogeneous precipitation method using urea according to the literature [18,19]. Yttrium nitrate hexahydrate (Y(NO3)36H2O) was used as an yttrium source, and sodium dodecyl sulfate (SDS, CH3(CH2)11OSO3Na) as a templating agent. Urea was used as an agent to raise the pH of the solution, because on heating at above 60 °C it undergoes hydrolysis as follows,
ðNH2 Þ2 CO þ 3H2 O ! 2NHþ4 þ 2OH þ CO2
ð1Þ
Yttrium nitrate, SDS, urea and water were mixed at molar ratio of 1:2:30:60 and stirred at 40 °C for 1 h to yield a transparent mixed solution. The mixed solution was heated at 80 °C and then kept at that temperature for 72 h, where the precipitation
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occurred. After the reaction, the precipitate (YDS) was collected by centrifugation, washed with water and then treated with an aqueous solution of CH3COONa to remove the surfactant (DS) via ion-exchange (YAcO). The YAcO supported Pt catalyst (1 wt% loading) 2 was prepared by the liquid-phase reduction of PtCl6 with N2H4 solution at pH10. Using X-ray fluorescence (Horiba MESA500W), XPS (VG Sigmaprobe spectrometer) and FT–IR (Jasco 610) measurements, it was demonstrated that the as prepared catalyst was free from residual sulfur and chlorine, which originated from 2 DS and PtCl6 , respectively. Powder X-ray diffraction (XRD) measurements were performed on a Rigaku Multiplex instrument using monochromated Cu Ka radiation (30 kV, 30 mA). BET surface area and pore size distribution were determined from N2 adsorption isotherms measured at 77 K using a Belsorp Mini instrument (Bel Japan, Inc.). Average pore size analysis was calculated using the Dollimore and Heal method [20] on the desorption branch. Metal dispersion of Pt was measured by pulsed CO chemisorption at 50 °C after reduction treatment at 200 °C in H2. The microstructure of YAcO was observed by TEM (FEI TECNAI F20, 200 kV).
3. Results and discussion 3.1. Catalyst screening results Fig. 1 summarizes the catalytic activity of 1 wt% Pt supported on various metal oxides for NO–H2–CO–O2 reactions. The activity is expressed by the cumulative NOx (NOx = NO + NO2) removal during the heating ramp (40–200 °C) and the temperature where NOx conversion reached to 20% (T20%). Pt catalysts supported on MgO, Al2O3 and SiO2 having large surface areas (P100 m2 g1) initiate the NO reduction at the low temperatures of 6100 °C and achieve high NOx removals. On the other hand, Pt catalysts supported on La2O3 and other lanthanide oxides (Pr, Gd and Yb) exhibit higher T20% values (>100 °C) and thus low NOx removals (<20%). Among these oxides supports, Y2O3 achieves the highest NOx removal and low T20% (100 °C). According to our previous study [12–17], oxidatively adsorbed NOx species (NO2 and/or NO3) play a key role in the NO–H2 reactions over Pt in an excess O2 depending on the property of support materials. Although Pt on acidic or amphoteric oxides such as SiO2, ZrO2, Al2O3 and ZSM-5 forms reactive NO2/NO3 species suitable for
2.2. Catalytic reaction Catalytic NO–H2–CO–O2 reaction was carried out in a conventional flow reactor equipped with a water cooled infrared image furnace. A granulated catalyst (10–20 mesh, 0.05 g) was fixed by quartz wool in a Pyrex glass tube (4 mm i.d.). Prior to catalytic reactions, Pt-impregnated catalyst was preheated at 400 °C for 1 h in a flow of 5% H2/He except for Pt/YAcO, which was preheated at 200 °C for 1 h in a flow of 5% H2/He to avoid structural collapse. The treatment removed the acetate species (AcO) from YAcO as was evident from the FT–IR measurement. Temperature programmed reactions were carried out at a ramp of 10 °C/min in a stream of gas mixture consisting of 0.02% NO, 0.5% H2, 0.1% CO and 10% O2 balanced with He supplied to the catalyst bed at W/F = 0.03 g s cm3. The catalytic activity was then measured at the constant catalyst temperature where conversions of NO, H2 and CO reached to steady-states. The reaction was also carried out in the absence of CO (0.02% NO, 0.5% H2 and 10% O2 balanced with He, W/F = 0.03 g s cm3). Effluent gas was analyzed by using a Shimadzu NOA-7000 chemiluminescence NOx analyzer, a Horiba VA3000 NDIR analyzer and a Pfeiffer GSD30101 mass spectrometer.
a
b
× 4 0
1
2
3
4
5
6
7
8
9
10
2θ / deg Fig. 2. Powder X-ray diffraction patterns of (a) yttrium-based mesophase after anion-exchange (YAcO) and (b) 1 wt% Pt loaded catalyst (Pt/YAcO).
Fig. 1. Catalytic activity of supported Pt catalysts (1 wt% loading). NOx removal: cumulative removal in the heating ramp (40–200 °C). T20%: Temperature at which NO conversion reached to 20%. 0.02% NO, 0.5% H2, 0.1% CO and 10% O2 balanced with He, W/F = 0.03 g s cm3, Heating rate = 10 °C min1.
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A.J. Samed et al. / Catalysis Communications 10 (2009) 1995–1999
the reaction with H2, considerably more N2O than N2 is produced. This is not the case of present Pt/Y2O3 catalyst, because its product selectivity to N2 was more than 60%. On the other hand, Pt becomes less reactive when supported on oxides having a high basicity, such as CaO and La2O3, which adsorbed NO as stable NO3 species in excess O2. A similar effect of acid/base character on the catalysis can be seen in the present NO–H2–CO–O2 system, because the NOx reduction route appears to be the same as discussed later. According to our CO2-TPD measurement (Supplementary data), Y2O3 exhibits a desorption peak located at about 300 °C, which is higher than ZrO2 (120–200 °C) and MgO (140 °C) and is lower than La2O3 (480 °C). One possible reason for the high activity and selectivity to N2 of Pt/Y2O3 is therefore due to the moderate
a
20nm
b
20nm
Adsorption / cm3 g-1
350 300 250
basicity of Y2O3, which may yield NO2/NO3 species having a higher reactivity toward H2. 3.2. Pt-supported mesoporous yttrium oxide It has been found that Pt/Y2O3 exhibits the high catalytic de-NOx activity in the NO–H2–CO–O2 system at low temperatures. Because Y2O3 in Fig. 1 is a dense phase with a very low BET surface area (<5 m2 g1), we have next tried to synthesize a mesoporous support to improve the catalytic activity. The mesoporous Y2O3 was prepared according to a template synthesis route which has been reported previously [18,19]. The homogeneous precipitation using urea in the presence of DS as a template produced a hexagonal mesophase (YDS), which was subsequently subjected to ion-exchange of DS with acetate ions (AcO). The replacement of the DS moiety in the mesostructure with much smaller AcO converts the mesophase into a mesoporous material (YAcO) with a large surface area. Fig. 2 shows the XRD patterns of as prepared YAcO and 1 wt% Pt/ YAcO. Both sample presented a single strong reflection at around 2h = 1.8° and a very weak shoulder at 2h = 2–4°. The d spacing, 4.8 nm, determined from the low-angle diffraction peak is consistent with the reported (1 0 0) reflection of a hexagonal structure [18], but the whole pattern implies that the mesostructure should be extremely disordered. No diffraction peaks were observed at 2h higher than 10°. The TEM photographs of these samples are shown in Fig. 3. Clearly, 3-D mesoporosity are developed thoroughly in the YAcO sample (a) after removing a DS moiety, but the structural ordering is pretty poor. Such a wormhole-like mesopores were preserved in the 1 wt% Pt loaded YAcO (b), which showed the deposition of Pt with the size of about 1–2 nm. This is consistent with the metal dispersion of Pt, 45% (diameter: 2.5 nm), which is higher than that of the Pt supported on a dense Y2O3 (5%), as measured by pulsed CO chemisorption. The porous structure was also characterized by N2 adsorption– desorption isotherm measured at 77 K (Fig. 3 and Table 1). The isotherm for YAcO was similar to type IV defined in the IUPAC classification with a small hysteresis loop (a). The adsorption in the range p/p0 = 0.2–0.4 is a clear indication of capillary condensation in mosopores. The shape of the isotherm was preserved after loading 1 wt% Pt (b), but the adsorption took place at higher pressure p/p0 = 0.5–0.7, compared to 0.2–0.4 observed for YAcO. This is consistent with the broadening of pore size distribution and the enlargement of average pore diameter from 2.5 nm (YAcO) to 3.2 nm (Pt/YAcO). The BET surface area decreased from 396 to 279 m2 g1 after loading Pt. Taking these results into consideration, Pt particles should mainly be deposited on the external surface of mesoporous YAcO as observed in the TEM photograph (Fig. 3b). The increase of pore size during the Pt impregnation process is associated with partial collapse of mesopores during the removal of water. We also proved that the base strength of the surface of Y2O3 is unlikely affected by mesoporous structure, because the CO2-TPD profiles from YAcO and dense Y2O3 are very similar. How-
200 150
Table 1 Specific surface area, pore diameter and pore volume calculated from N2 adsorption isotherms at 77 K.
a b
100 50 0 0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0
p/p0
YAcO 1 wt% Pt/YAcO a
Pore diameterb/nm
Pore volumec/mm3 g1
396 279
2.5 3.2
430 550
The specific surface area was determined using the BET method. The pore size distribution was calculated using the Dollimore and Heal method on the desorption branch. c Total pore volume was calculated form N2 adsorption at p/p0 = 0.99. b
Fig. 3. TEM images and N2 adsorption/desorption isotherms of (a) yttrium-based mesophase after exchange (YAcO) and (b) 1 wt% Pt loaded catalyst (Pt/YAcO).
Surface areaa/m2 g1
A.J. Samed et al. / Catalysis Communications 10 (2009) 1995–1999
ever, the number of CO2 adsorption sites per unit weight of YAcO is two orders of magnitude more than those of dense Y2O3. Finally, the catalytic activity of 1 wt% Pt/YAcO for the NO–H2– CO–O2 reaction was evaluated after pretreatment in 5% H2/He at 200 °C. Fig. 4 compares the steady-state conversion and selectivity measured at each reaction temperatures. Clearly, the mesoporous YAcO material showed much higher de-NOx activity. The Pt/YAcO exhibited the light-off about 20 °C lower than Pt/dense Y2O3 and thus the activity in a wide temperature range of 100–300 °C. The product selectivity at 140 °C was about 60% N2 and 40% N2O for both Pt catalysts. Above 150 °C, however, the de-NOx efficiency begins to decrease, because H2–O2 reaction becomes dominant with an increase of reaction temperatures. The drawback of the YAcO is its low thermal stability, because the wormhole-like mesopores shown in Fig. 3 are totally collapsed when heated above 400 °C, where a dense Y2O3 with a low-surface area (<5 m2 g1) is formed. Fortunately, however, the present NOx reduction using H2 as a reducing agent takes place at temperatures as low as 200 °C, where the mesoporosity can stably be preserved. For the reason, the present catalyst would be suitable for low-temperature stationary NOx removal rather than automobile emission control. To gain insight into the reaction route of the present system, the NO–H2 reaction over 1 wt% Pt/Y2O3 (a dense phase) was carried out in the absence CO or H2 with keeping the other gas concentrations unchanged. The negligible reduction of NO without H2 feed demonstrates that H2 plays an essential role as the reducing agent for NO. Fig. 5 compares the result of steady-state reactions without
100
a
NOx conv.
80
Conversion / %
NOx conv. to N2 NOx conv. to N2O
60
H2 conv./%
NOx conv.
80
NOx conv. to N2 NOx conv. to N2O
60
H2 conv./% 40
20
0 0
100
200
300
400
500
Temperature / °C Fig. 5. Steady-state conversion efficiency and product selectivity of NO–H2–O2 over 1 wt% Pt loaded on dense Y2O3. 0.02% NO, 0.5% H2 and 10% O2 balanced with He, W/ F = 0.03 g s cm3.
CO feed. The reaction in the absence of CO initiated the NOx reduction at around 50 °C, compared to 120 °C required in the presence of 1000 ppm CO as shown in Fig. 4a. On the other hand, CO in the feed did not influence the NOx reduction efficiency at temperatures higher than 180 °C, where CO was totally oxidized to CO2. The poisoning behavior of CO can be rationalized by the fact that strong CO adsorption onto Pt surface may inhibit the activation of H2 especially at lower temperatures as was reported previously [10,11]. The resulting shift of H2 and NOx conversions towards higher temperature is consequently the modification of the NO– H2 reaction in the presence of CO, but their reaction route should basically be the same. In case of Pt/YAcO, however, the light-off of CO at lower temperature of 100 °C renders the NO–H2 reaction more tolerant to poisoning by CO.
CO conv./% 40
4. Conclusion
20
0 0
100
200
300
400
500
Temperature /°C 100
b 80
Conversion / %
100
Conversion / %
1998
Among the variety of metal oxide supports, Pt supported on Y2O3 has been found to be the most active for de-NOx in the stream of NO, H2, CO and excess O2 at low temperatures (6250 °C). The catalytic activity was much improved by the use of mesoporous Y2O3, which was synthesized from an yttrium-based surfactant mesophase templated by dodecyl sulfate anion. Thanks to a large specific surface area (279 m2 g1) and a high Pt dispersion (45%), our catalyst achieved the efficient NOx removal in the wide range of 100–300 °C. Acknowledgement This study was supported by Elements Science and Technology Project from the Ministry of Education, Culture, Sports, Science and Technology.
60
40
Appendix A. Supplementary data 20
Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.catcom.2009.07.016. 0 0
100
200
300
400
500
Temperature /°C Fig. 4. Steady-state conversion efficiency and product selectivity of NO–H2–CO–O2 reaction over 1 wt% Pt loaded on (a) dense Y2O3 and (b) YAcO. 0.02% NO, 0.5% H2, 0.1% CO and 10% O2 balanced with He, W/F = 0.03 g s cm3.
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