Effect of Macropore Formation in Pt Catalyst Supports on the Oxidation

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Effect of Macropore Formation in Pt Catalyst Supports on the Oxidation Activity for Diesel Fuel Mist Junko Uchisawa,*,† Akira Obuchi,† Tetsuya Nanba,† Shigeki Hara,‡ Alessio Caravella,‡ Takeru Tango,§ Tatsuro Murakami,§ Hideyuki Nakagawa,§ Takahiro Kogawa,|| and Akira Abe|| †

)

Energy-saving System Team, Research Center for New Fuels and Vehicle Technology, National Institute of Advanced Industrial Science and Technology, 16-1 Onogawa, Tsukuba 305-8569, Japan ‡ Membrane Separation Processes Group, Research Institute for Innovation in Sustainable Chemistry, National Institute of Advanced Industrial Science and Technology, Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan § Adsorption Functional Materials Development Group, Research and Development Department, Mizusawa Industrial Chemicals Ltd., 1-1 Mizusawa, Tainai 959-2638, Japan Mitsui Mining & Smelting Co., Ltd., 1013-1 Ageoshimo, Ageo 362-0025, Japan ABSTRACT: The effect of macropore formation in catalyst supports on the oxidation activity for diesel fuel mist was examined with the goal of improving catalyst efficiency. To investigate the effect on a laboratory scale using granular samples, we designed a reaction system that supplied fuel mist to the catalyst bed directly by an air atomizing nozzle. The catalytic activity was found to be improved by macropore formation in the support. This effect was also demonstrated for a washcoated catalyst on a honeycomb with diesel engine exhaust. These results suggest that the macropore diffusion channels are secure even if fuel mist attaches to the support surface, and the reactant gas can diffuse to the interior of the support and reach the active sites. Macropore formation prevented the blockage of pores of the support surface and thus the active sites of the catalyst.

is obtained with relatively larger nanosized mesopores.7 Additionally, the high activity of VOC oxidation is maintained by the formation of micrometer-sized macropores, even after the Pt particles are agglomerated by aging and diffusivity in the mesopores decreases.8 On the other hand, when liquid fuel mist is included in the reactant gas, there is a possibility that the mist will attach to the support surface, blocking the pore channel entrances, especially at low temperatures. When this occurs, the active sites present in the interior of the support are isolated, and the catalytic activity decreases. It is expected that catalytic performance at low temperatures can be improved by using a support material having relatively large macropores in order to prevent pore blockage. There are other structural parameters, such as channel geometry9,10 and thickness of the washcoat,11 that could be manipulated toward improvement of the catalytic performance. Our focus in this work, however, is limited to studying the effect of macropore formation in the catalyst support for fuel mist oxidation. The main difficulty in conducting mist oxidation reactions on a laboratory scale is supplying the desired amount of fuel, due to its low volatility and high boiling point. Mati et al.12 investigated the influence of engine oil on the catalyst by vaporizing the oil and supplying it to the catalyst bed by a flow of N2 gas. In contrast, in this study, we tried to investigate the influence of the fuel mist on the catalytic activity. For this, we constructed a reaction system for supplying fuel mist to the catalyst bed directly

1. INTRODUCTION Diesel engine automobiles are in widespread use due to their high fuel efficiency. However, emissions from the engines are health hazards and environmental pollutants; thus, exhaust emission regulations are continuously being strengthened. As a result, development of aftertreatment catalysts with higher efficiency is strongly desired.1 Diesel oxidation catalysts (DOCs) are effective for the removal of the soluble organic fraction (SOF) in particulate matter (PM), and of CO and hydrocarbons in the gas phase, oxidizing them completely to CO2 and H2O. In addition, DOCs oxidize NO to NO2 to promote the regeneration of the diesel particulate filter (DPF), i.e., oxidation of PM, and selective catalytic reduction of NOx. Furthermore, for the regeneration of DPF by burnout of the filtered PM, DOCs with high catalytic activity for fuel oxidation are required for active heating, which is performed by the catalytic combustion of fuel in the DOC placed upstream of the DPF, supplied by retarded combustion in the engine, by postinjection into the engine cylinders, or by direct injection into the exhaust pipe. The precious metals Pt and Pd are commonly used as active catalyst components of DOCs.2 4 Since they are rare metals, a decrease in their consumption is strongly desired.5,6 The catalytic activity of these precious metals is very high compared to those of base metal catalysts and their amount can be reduced by using them more effectively. However, operation at high space velocity (SV) may result in pore diffusion limitations when these metals are loaded in a porous support for the purpose of high metal dispersion. The authors have studied the gas diffusivity interior of the pores of the support material and have found that the highest catalytic performance for complete oxidation of relatively small volatile organic compounds (VOC), such as propylene and toluene, r 2011 American Chemical Society

Received: August 24, 2011 Accepted: November 22, 2011 Revised: November 18, 2011 Published: November 22, 2011 719

dx.doi.org/10.1021/ie2018965 | Ind. Eng. Chem. Res. 2012, 51, 719–724

Industrial & Engineering Chemistry Research

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Table 1. Physical Properties of Granular Catalysts Prepared for Investigating the Effect of Macropore Formation on Fuel Mist Oxidation pore volume (mL 3 g 1) Pt dispersion

sample

BET

code

(m2 3 g 1)

0.002 0.1 μm

0.1 5 μm

(CO/Pt)

Al2O3-1

154

0.48

0.01

0.25

Al2O3-2

227

0.41

0.07

0.18

Al2O3-3

227

0.42

0.10

0.28

Al2O3-4

224

0.43

0.14

0.26

Table 2. Honeycomb Monolith (HM) Catalysts for Practical Scale Testsa sample

Al2O3

Pt/Pd

code

(g 3 L 1)

(g 3 L 1)

Figure 1. Experimental setup for mist oxidation activity tests under simulated gas conditions.

pore volume (>1 μm) (mL 3 g 1)

blankb

0

0/0

0.06

HM-1

70

1.26/0.32

0.21

HM-2

70

1.26/0.32

0.12

a

Substrate (blank); cylindrical cordierite honeycomb (NGK Insulators, Ltd.; ϕ 5.66 in. 3 in., 300 cells in. 2 b Cordierite honeycomb without loading catalyst.

by an air atomizing nozzle normally used in inductively coupled plasma (ICP) spectrometry. We report herein the effect of macropore formation in the support on fuel mist oxidation, both under laboratory scale conditions using granular catalyst samples and under practical conditions using a honeycomb washcoated with a catalyst having macropore structures.

Figure 2. Experimental setup of mist oxidation activity tests under practical exhaust gas conditions.

honeycomb as a substrate (blank; NGK Insulators, Ltd.; ϕ 5.66 in. 3 in., 300 cells in. 2). The obtained honeycomb sample was referred to as “HM-1”. For comparison, a mixture of the same composition of HM-1 without adding PMMA was crushed with a ball mill and washcoated on the same honeycomb, which was referred to as “HM-2”. These honeycomb samples were calcined at 750 °C for 50 h in air to simulate aging. 2.2. Catalyst Characterization. The pore size distributions of the prepared samples were measured with nitrogen physisorption using BJH and BET analyses with a gas sorption instrument (Tristar II, Micromeritics Co., Ltd.) and a mercury intrusion analyzer (Autopore IV, Micromeritics Co., Ltd.). The Pt dispersion was estimated from the specific amount of CO adsorption, COadsorption/Pttotal (mol/mol), by pulsed CO chemisorption (Ohkura Riken Inc., Model R6015H). Prior to the chemisorption of CO at 50 °C, the samples were oxidized in air and then reduced in H2 at 400 °C. To investigate the morphology of the sample, a transmission electron microscope (TEM; JEM2000FX2, JEOL Ltd.), a field emission scanning electron microscope (FE-SEM; S-4700, Hitachi Ltd.), and an optical microscope were used. 2.3. Catalytic Activity Test. 2.3.1. Simulated Reaction Using Granular Catalyst. The test for catalytic mist oxidation was conducted using a fixed-bed flow reactor as shown in Figure 1. Catalyst samples (0.1 g) were packed into a 10 mm i.d. quartz glass tube. Quartz wool was placed only in the lower part of the catalyst bed so that the fuel mist could reach the surface of the sample directly. The upper part of the reactor outside the electric furnace was covered with aluminum foil and kept warm to

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Three kinds of γ-alumina with different macropore structures, listed as Al2O3-2, Al2O3-3, and Al2O3-4 in Table 1, were made by controlling the pressure of a molding machine (AUTOTAB-500, Ichihashi-Seiki Co., Ltd.) to pelletize Al(OH)3 powder (Mizusawa Industrial Chemicals, Ltd.). The resulting Al(OH)3 pellets were calcined at 600 °C for 2 h in air and then were crushed and sieved to obtain granules with a mean diameter of 100 μm. Commercial γ-alumina without macropores, Al2O3-1 (Sumitomo Chemicals Co., Ltd., KHS-46), was used as a reference support. Using Pt(NO2)2(NH3)2 solution (Tanaka Kikinzoku Industry Co., Ltd.) as a Pt precursor, 1.0 wt % Pt was impregnated by the incipient wetness method. After impregnation, the samples were dried and reduced in 10% H2/N2 at 400 °C for 1 h, and then calcined in air at 500 °C for 1 h. The obtained catalyst samples were referred to by the abbreviation of the support material, e.g., Al2O3-1. Table 2 shows honeycomb samples for the engine test. Because the above pressure control technique could not be applied to washcoating, poly(methyl methacrylate) (PMMA; mean particle diameter = 3 μm, Sekisui Plastics Co., Ltd.) was used as a template for the macropores13,14 and control of the alumina grain diameter was performed to form macropores in the washcoat. A mixture of γ-alumina (GP-20; particle diameter = 20 μm, Mizusawa Industrial Chemicals, Ltd.), Pt(NO2)2(NH3)2 and Pd(NO2)2(NH3)2 solutions, 10 wt % PMMA and, alumina sol as a binder was washcoated on a cylindrical cordierite 720

dx.doi.org/10.1021/ie2018965 |Ind. Eng. Chem. Res. 2012, 51, 719–724


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Figure 3. Pore distribution of various granular alumina supports measured with nitrogen physisorption and a mercury intrusion.

Figure 5. SEM observation of Al2O3 supports without and with macropores. (a) Al2O3-1. (b) Al2O3-4.

Figure 4. TEM observation of Pt particles loaded on Al2O3 supports without and with macropores. The point of the arrow shows a Pt particle. (a) Al2O3-1. (b) Al2O3-4.

catalyst bed with a vaporizer set at 200 °C substituting for the nozzle. A U-tube packed with glass wool and cooled to 0 °C was placed downstream of the reactor tube in order to trap nonreacted fuel mist. Only gaseous components passed through and were introduced to the analyzer. The concentrations of gaseous components were continuously measured using a nondispersive IR gas analyzer (CGT-7000; Shimadzu Co., Ltd.) for CO and CO2. The catalytic activity was evaluated by the T50, which was defined as the temperature at which 50% of the fuel was converted to CO2. 2.3.2. Activity Test Using Honeycomb Catalyst. Activity tests with honeycomb catalysts were performed using a diesel engine (V2403, Kubota, displacement volume = 2.4 L) as shown in Figure 2, and the catalytic combustibility of the gas oil (diesel fuel) in the honeycomb was evaluated. The average gas composition was 260 ppm NOx, 280 ppm CO, 6.7% O2, 10.3% CO2, and N2 as a balance (SV = 58 000 h 1). Two thermocouples were installed at the entrance and exit of the honeycomb catalyst, and gas oil was atomized just before the honeycomb catalyst using a single-fluid nozzle. The engine speed was set at 1100 rpm. The temperature of the exhaust gas at the exit was adjusted to 600 °C by controlling the amount of the gas oil addition, and then the temperature of the entrance exhaust was lowered from 300 to 200 °C stepwise by

prevent fuel dew condensation on the inner reactor wall. A thermocouple (TC; K type) was inserted in the center of the quartz wool below the catalyst bed for continuous monitoring of the reaction temperature. Decane (C10H22) and gas oil [Japan Industrial Standard (JIS) No. 2 grade] as the fuel samples were pressurized and introduced to the liquid mass flow controller (STEC, LV-F20; full scale = 20 μL 3 min 1) and supplied into the air atomizing nozzle (Fujiwara Seisakujo, Ltd.). With 0.1 L 3 min 1 of air for atomization, the fuel mist was added to the top of the catalyst bed directly. Furthermore, 0.9 L 3 min 1 of supplemental air was added from the side of the nozzle, which was mixed with the fuel mist and atomizing air above the catalyst bed. In this manner, a simulated exhaust gas, composed of 5000 ppmC hydrocarbon, 10% O2, and N2 as a balance, was passed through the catalyst at a flow rate of 1.0 L 3 min 1. The reactor temperature was decreased at the rate of 3 °C 3 min 1 from 500 to 100 °C. To ensure consistency of the test conditions, especially the reaction temperature and the mist jetting pattern, the distances from the nozzle head to the heater and from the nozzle head to the upper part of the catalyst bed were always set to the same values (40 and 90 mm, respectively). For comparison, decane vapor was supplied to the 721



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Figure 6. Temperature dependence of fuel oxidation with granular catalysts under simulated gas conditions. Catalyst = 0.1 g, Pt = 1.0 wt %, flow rate = 1.0 L 3 min 1, gas composition = 5000 ppmC hydrocarbon + 10% O2 balanced with N2. (a) Al2O3-1. (b) Al2O3-4.

Figure 8. Proposed schematic of the improvement of fuel mist oxidation by macropore formation in the catalytic support.

TEM images of Al2O3-1 and Al2O3-4 are shown in Figure 4. For both samples, the Pt particles were present in the interior of bulk Al2O3, i.e., in the mesopores, and were finely distributed with a mean size of 5 nm or less. Their FE-SEM images are shown in Figure 5. Both samples were aggregates of secondary particles comprising primary particles of submicrometer size. It was confirmed that macropores on the micrometer scale were formed as void regions of the secondary particles in Al2O3-4, while macropores were barely seen in Al2O3-1. 3.1.2. Activity Test. Figure 6 shows the temperature dependence of the catalytic fuel oxidation for Al2O3-1 and Al2O3-4. For decane oxidation, similar activities were observed between the gas and mist oxidations for both samples. The conversion was maintained at 100% from 500 to 250 °C and decreased gradually to 0% at around 120 °C. For gas oil mist oxidation, the conversion was 100% down to 275 °C for Al2O3-4, but began to decrease at a higher temperature of 350 °C for Al2O3-1. Figure 7 summarizes the results of the activity tests. For decane, the four samples showed almost the same activities, with T50 = ca. 160 °C, for both gas and mist oxidations. This suggests that the catalytic properties among the four samples were almost the same even though their specific surface areas and Pt dispersions were different in some degree. The volatility of decane is relatively high, the boiling point is as low as 174 °C under atmospheric pressure, and the saturated vapor pressure at 10 °C estimated by Antoine’s equation is 538.2 ppm (=5382 ppmC). Accordingly, it is suggested that decane is vaporized immediately,

Figure 7. Comparison of fuel oxidation activity (T50) of the granular catalysts.

changing the engine loading. The lowest combustion temperature (critical temperature, Tc) of the entrance side of the exhaust gas that could raise the exit temperature to 600 °C was determined.

3. RESULTS AND DISCUSSION 3.1. Granular Catalyst. 3.1.1. Characterization. Table 1 shows the physical properties of the granular samples, and Figure 3 shows their pore size distributions. All the samples had mesopores approximately 10 nm in diameter, and the pore volume below 0.1 μm was nearly the same (0.41 0.48 mL 3 g 1). The prepared samples, Al2O3-2, -3, and -4, had relatively high specific surface areas and also had macropores above 0.1 μm; therefore, the formation of multidimensional pore structures was confirmed. The peak size of the macropores in the prepared samples was systematically changed from submicrometer to 2 μm as the pelletizing pressure decreased. This suggested that the pressure control method was effective for controlling the macropore size for the granular catalysts. In contrast, such macropores were not seen in the commercial alumina (Al2O3-1). 722



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Figure 9. Pore distributions of honeycomb catalysts measured with a mercury intrusion.

Figure 10. Optical microscope images of the surface of the honeycomb catalysts. (a) HM-1. (b) HM-2.

even as it is atomized from the nozzle in mist form. It is noted that the mean free path at 160 °C was estimated to be about 70 nm and the diameter of the mesopores in the alumina support (10 nm) was considerably smaller than that. From this, Knudsen diffusion should occur in the mesopores by the collision of reactant molecules with the pore walls, and this diffusion is considerably slower than molecular diffusion. Therefore, the diffusion in the mesopores becomes the limiting step. We consider this to be the reason why similar activities for decane oxidation were observed, even though the macropore size was different. On the other hand, for gas oil mist oxidation, the T50 values of the samples having larger macropores were approximately 20 °C lower than the samples with smaller or no macropores. The 90% distillation temperature of gas oil is approximately 350 °C, so most of the components are present in mist form around T50 (220 240 °C). This suggests that larger macropores are advantageous for gas oil mist oxidation, whereas the activity for gaseous decane is not significantly affected. Figure 8 shows a proposed schematic of the improvement of catalytic oxidation of fuel mist having low volatility by macropore formation in the support. When there are relatively large macropores, diffusion channels are secure even if fuel mist attaches to the support surface, and the reactant gas can diffuse to the interior of the support and reach the active catalyst sites present in the mesopores. However, when the macropores are smaller, the channel entrances are blocked by the attached mist which remains on the support surface, isolating the active sites present on the interior of the support, and finally decreasing the catalytic

Figure 11. Comparison of fuel oxidation activities (critical temperature, Tc) among honeycomb catalysts.

activity because the reactant gas can no longer reach the active sites. Evidence of this sort of pore blockage can be seen from incomplete conversion at high temperature, 350 °C, as shown in Figure 6a for Al2O3-1, which has no macropores. 3.2. Honeycomb Catalyst. 3.2.1. Characterization. Compositional information and pore volumes of the honeycomb catalysts are listed in Table 2, and the pore distributions are shown in Figure 9. A rather sharp peak was present at around 3 μm in both prepared honeycomb catalysts (HM-1 and HM-2), as shown in Figure 10. This peak was attributed to cracks in the alumina washcoat, which was confirmed by observation with an optical microscope. HM-2 had more macropores around submicrometer order (ca. 0.4 μm) than did HM-1. On the other hand, HM-1 had larger pores of around 1.2 μm, in addition to the appearance of a broad peak ranging from 3 to 10 μm, and the pore volume was almost twice as large as that of HM-2. Figure 11 shows optical microscope images of the washcoat surfaces of HM-1 and HM-2. It was found that the washcoat of HM-2 comprised agglomerations of finely crushed alumina particles (Figure 10b), whereas that of HM-1 comprised alumina maintaining the original secondary particle size (20 μm). 723



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These observations suggest that the submicrometer size pore distribution peak of HM-2 results from the void of the finely crushed alumina particles, while the broader peak in HM-1 results from the void of the original alumina secondary particle structure. 3.2.2. Activity Evaluation. Figure 11 shows the results of the Tc for gas oil combustion over the honeycomb catalysts, HM-1 and HM-2, using the diesel engine exhaust. It was found that HM-1 having larger macropore volume showed higher activity (Tc = 240 °C) than HM-2 (Tc = 260 °C). Because the distributions above 3 μm were very different between these two samples, it was suggested that the difference of activity depended on the macropore structures. Similar to the result of the granular catalyst test, this also illustrates the effect of macropore formation. That is, diffusion channels are secure even if fuel mist attaches to the washcoat surface, and the reactant gas can diffuse to the interior of the washcoat and reach the active sites. From this, it is expected that the amount of precious metals can be reduced by forming macropores in the washcoat layer. While it would be necessary to develop technology to produce durable large macropores in large quantities, this study suggests that macropore formation is a very promising strategy for practical use in catalyst support materials.

(5) Hinokuma, S.; Murakami, K.; Uemura, K.; Matsuda, M.; Ikeue, K.; Tsukahara, N.; Machida, M. Arc plasma processing of Pt and Pd catalysts supported on r-Al2O3 powders. Top. Catal. 2009, 52, 2108. (6) Taniguchi, M.; Tanaka, H.; Uenishi, M.; Tan, I.; Nishihata, Y.; Mizuki, J.; Suzuki, H.; Narita, K.; Hirai, A.; Kimura, M. The selfregenerative Pd-, Rh-, and Pt-perovskite catalysts. Top. Catal. 2007, 42, 367. (7) Uchisawa, J.; Nanba, T.; Masukawa, S.; Obuchi, A. Optimization of Pt catalyst silica support pore size for complete oxidation of volatile organic compounds. React. Kinet. Catal. Lett. 2008, 95, 391. (8) Uchisawa, J.; Kosuge, K.; Nanba, T.; Masukawa, S.; Obuchi, A. Effect of meso- and macropore structures of Pt-supported fibrous silica on the catalytic oxidation of toluene. Catal. Lett. 2009, 133, 314. (9) More, H.; Hayes, R. E.; Liu, B.; Votsmeier, M.; Checkel, M. D. The effect of catalytic washcoat geometry on light-off in monolith reactors. Top. Catal. 2006, 37, 155. (10) Ramanathan, K.; Balakotaiah, V.; West, D. H. Geometry effects on ignition in catalytic monoliths. AIChE J. 2004, 50, 1493. (11) Ramanathan, K.; West, D. H.; Balakotaiah, V. Optimal design of catalytic converters for minimizing cold-start emissions. Catal. Today 2004, 98, 357. (12) Mati, K.; Rahmouni, C.; Bianchi, D.; Gaillard, F. Performances and deactivation of supported precious metal catalysts in the oxidation of an engine oil vapour. Catal. Lett. 2009, 131, 170. (13) Cho, J.; Hirota, T.; Tanioka, K. Exhaust Gas Catalyst and Exhaust Gas Processing Apparatus Using the Same. World Patent WO2007116881A1, 2007. (14) Sadakane, M.; Horiuchi, T.; Kato, N.; Sasaki, K.; Ueda, W. Preparation of three-dimensionally ordered macroporous perovskitetype lanthanum-iron-oxide LaFeO3 with tunable pore diameters: High porosity and photonic property. J. Solid State Chem. 2010, 183, 1365.

4. CONCLUSION The effect of macropore formation in the catalytic support on the oxidation of diesel fuel mist was examined both by laboratory scale tests using simulated diesel engine exhaust gas and granular catalysts and by practical tests using diesel engine exhaust gas and honeycomb catalysts. The activity was improved by forming macropores on the order of 1 10 μm in the catalyst support. It was suggested that the macropores prevented the blockage of pores in the surface of the washcoat and the active sites of the catalysts by the fuel mist. Thus, it is expected that the amount of precious metals can be reduced by forming such macropores in the washcoat layer. ’ AUTHOR INFORMATION Corresponding Author

*Tel.: +81 29 861 8716. Fax: +81 29 861 8259. E-mail: [email protected].

’ ACKNOWLEDGMENT This study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). We thank Dr. Y. Soneda and Ms. M. Jona for FE-SEM image acquisition and Dr. N. Yoshizawa and Mr. A. Takatsuki for TEM image acquisition. ’ REFERENCES (1) Johnson, T. V. Diesel emission control in review SAE Tech. Pap. 2008, No. 2008-01-0069. (2) Haneda, M.; Sasaki, M.; Hamada, H.; Ozawa, M. Platinum-based catalyst for diesel hydrocarbon oxidation. Chin. J. Catal. 2010, 32, 777. (3) Xue, E.; Seshan, K.; Ross, J. R. H. Roles of supports, Pt loading and Pt dispersion in the oxidation of NO to NO2, and of SO2, to SO3. Appl. Catal., B 1996, 11, 65. (4) Kr€ocher, O.; Widmer, M.; Elsener, M.; Rothe, D. Adsorption and desorption of SOx on diesel oxidation catalysts. Ind. Eng. Chem. Res. 2009, 48, 9847. 724


Effect of Macropore Formation in Pt Catalyst Supports on the Oxidation

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