Effects of the Extent of Silica Doping and the Mesopore Size of an

Article pubs.acs.org/IECR

Effects of the Extent of Silica Doping and the Mesopore Size of an Alumina Support on Activity as a Diesel Oxidation Catalyst Junko Uchisawa,*,† Takeru Tango,‡ Alessio Caravella,§ Shigeki Hara,§ Masaaki Haneda,∥ Tatsuro Murakami,‡ Hideyuki Nakagawa,‡ Tetsuya Nanba,⊥ and Akira Obuchi† †

Energy Technology Research Institute, Engine Combustion and Emission Control Group, National Institute of Advanced Industrial Science and Technology (AIST), 16-1 Onogawa, Tsukuba 305-8569, Japan ‡ Research and Development Department, Mizusawa Industrial Chemicals Ltd., 1-1 Mizusawa, Tainai 959-2638, Japan § Research Institute for Innovation in Sustainable Chemistry, Membrane Separation Processes Group, National Institute of Advanced Industrial Science and Technology (AIST) , Central 5, 1-1-1 Higashi, Tsukuba 305-8565, Japan ∥ Advanced Ceramics Research Center, Nagoya Institute of Technology, 10-6-29 Asahigaoka, Tajimi, Gifu 507-0071, Japan ⊥ Fukushima Renewable Energy Institute, Hydrogen Energy Carrier Team, National Institute of Advanced Industrial Science and Technology (AIST), 2-2-9 Machiikedai, Kouriyama 963-0215, Japan ABSTRACT: Both the extent of SiO2 doping and the mesopore size of alumina were investigated with the aim of developing a support that maintains high activity during the catalytic oxidation of diesel exhaust hydrocarbons and NO over 0.75 wt % Pt/0.25 wt % Pd even after high temperature treatment. By controlling the SiO2 doping ratio and heat treatment conditions during the preparation process, 24 types of SiO2−Al2O3 were obtained, with SiO2 levels from 0 to 5.9 wt % and mesopore sizes from 6.7 to 10.2 nm. The highest catalytic activity was obtained at a SiO2 loading of approximately 4 wt %, and the beneficial effects of SiO2 doping were thought to result from optimization of the basicity of the support. Catalytic activity improved as the mesopore size increased up to approximately 10 nm. Computational simulations confirmed that the diffusion limitations of small mesopore structures may affect catalytic activity when the mesopores are below 10 nm.

1. INTRODUCTION Diesel engine vehicles are in widespread use because of their high fuel efficiency. The emissions resulting from these engines, however, represent significant health hazards, and environmental pollutant sources and regulations concerning exhaust emission are continually being strengthened. Accordingly, there is a significant demand for the development of after-treatment catalysts with improved efficiencies.1 Diesel oxidation catalysts (DOCs) are necessary for the removal of the soluble organic fraction (SOF) in the particulate matter (PM), as well as the removal of CO and hydrocarbons (HCs) present in the gas phase.2,3 Additionally, DOCs work to oxidize NO to NO2 to promote the regeneration of the diesel particulate filter (DPF) by oxidation of the PM and the selective catalytic reduction of NOx using NH3 as a reductant. The temperatures of exhaust gases resulting from diesel engines are decreasing with each new model because of the introduction of exhaust gas recirculation (EGR) systems to reduce engine-out NOx levels. As a result, DOCs must allow the complete oxidation of potential pollutant gases to CO2, H2O, and NO2 at increasingly lower temperatures. Precious metals (primarily Pt and Pd) are commonly used as the active components of DOCs, but because of their cost and rarity, much effort has been applied to reducing the required quantities of these metals. One approach is to increase the efficiency of the precious metal as a catalyst for the required reactions, and extensive research has been carried out to improve both the activity and durability of metal catalysts. Al2O3 is commonly used as a support material,4−7 and © 2014 American Chemical Society

enhancement of its performance by doping with secondary components has often been investigated. It has been reported that the thermal stability of Al2O3 is increased by SiO2 doping.8,9 Furthermore, an improvement of the catalytic activity is also expected by optimizing the mesopore size because this increases diffusion within the material.10,11 From these results, it is likely that significant improvements of the catalytic activity of Al2O3 will result from optimization of both SiO2 doping and the mesopore size. However, individual and systematic studies of these two effects on diesel oxidation activity have not been carried out to date because these two factors affect one another. To enable individual evaluation of these two factors, we prepared 24 different types of Al2O3 supports having different mesopore sizes and SiO2 contents using the sol−gel method and investigated both factors separately by systematic comparison of the samples. The aim of this study was to optimize both the extent of SiO2 doping and the mesopore size and thus develop a high efficiency alumina-based DOC.

2. EXPERIMENTAL SECTION 2.1. Catalyst Preparation. Mesoporous Al2O3 supports containing SiO2 were prepared by the sol−gel method. Sodium silicate (Japan Industrial Standard (JIS) No. 3 grade) was Received: Revised: Accepted: Published: 7992

February April 11, April 14, April 14,

10, 2014 2014 2014 2014

dx.doi.org/10.1021/ie5005724 | Ind. Eng. Chem. Res. 2014, 53, 7992−7998

Industrial & Engineering Chemistry Research

Article

the HC and NO conversions were measured at each temperature step using an FT-IR spectrometer (Nexas 470, Nicolet; resolution set at 0.5 cm−1) equipped with a multireflectance gas cell with an optical length of 2 m. The catalytic activity was calculated in terms of T50, defined as the temperature at which 50% of the HC was converted to CO2. Additionally, to examine the validity of the simulated gas test results, the relationship between the mesopore size of the support material and the catalytic activity was simulated by computational fluid dynamics (CFD) calculations, following the same strategy and settings as those adopted in a previous study, using ANSYS CFX simulation software.12,13 The simulated system consisted of a three-dimensional BCC structure composed of mesoporous catalyst particles with a uniformly distributed diameter of 2 μm. Moreover, it was assumed that the reactants move through the particle mesopores by Knudsen diffusion and by ordinary molecular diffusion between particles (i.e., within the macropores). The overall reaction rates, including diffusion during decane oxidation, were calculated over the mesopore range of 1−50 nm for a feed gas consisting of 2000 ppm (in terms of C) ndecane, 10% H2O, 5% O2, and N2 as the balance.

utilized as the source of SiO2 and was mixed with sulfuric acid to give an acidic sol solution. This sol solution was mixed into an aqueous aluminum sulfate solution (>8 wt %) and then neutralized with an aqueous sodium hydrate solution (5 wt %) while maintaining pH above 12, which resulted in the formation of a gel. The extent of SiO2 doping of each support was controlled by varying the concentration of sodium silicate in the mixture. The stabilized gels were rinsed with deionized water and then dried at 150 °C for 24 h and subsequently calcined at 620 °C for 2 h in air. The mesopore size of the support was tailored by regulating the hydrothermal conditions during the drying and calcination processes. To generate a hydrothermal atmosphere, the cap of the sample container was kept closed during the heat-treatment process, whereas atmospheric conditions were obtained by heat-treatment of the sample without the cap in place. The resulting materials were crushed and sieved to obtain granules ranging in size from 0.10 to 0.25 mm. Using these procedures, 24 kinds of SiO2− Al2O3 supports having different SiO2 contents and mesopore sizes were obtained. These supports were loaded with platinum and palladium using Pt(NO2)2(NH3)2 and Pd(NO2)2(NH3)2 nitric acid solutions (Tanaka Kikinzoku Industry Co., Ltd.) as the precursors and employing the incipient wetness method. Following impregnation, samples were dried and reduced under a flow of 10% H2 in N2 at 400 °C for 1 h and then calcined in air at 750 °C for 50 h to simulate thermal aging. 2.2. Catalyst Characterization. Pore size distributions of the prepared samples were determined from nitrogen physisorption data acquired using gas sorption instrumentation (Tristar II, Micromeritics Co., Ltd.) and applying BJH and BET analyses. A transmission electron microscope (TEM; Tecnai Osiris, FEI) equipped with energy dispersive X-ray spectrometry (EDX) was used to investigate the distributions of Si, Pt, and Pd on Al2O3. Analysis by X-ray diffraction (XRD; SmartLab, Rigaku Corp.) was used to investigate the crystallographic structures of the support materials. The extent of platinum dispersion was estimated from the specific CO adsorption, COadsorption/Pttotal (mol/mol), obtained from pulsed CO chemisorption (R6015H, Ohkura Riken Inc.). Prior to the chemisorption of CO at 50 °C, the samples were oxidized in air and then reduced in H2 at 400 °C. The acidity and basicity of the samples were characterized by temperature-programmed desorption of NH3 and CO2 (NH3TPD, CO2-TPD), respectively. After heat treatment in He at 500 °C for 1 h, the sample (0.30 g) was cooled to 100 °C under a He flow of 160 mL·min−1. Either 1000 ppm of NH3 or 1000 ppm of CO2 was then introduced for 20 min, and the sample was heated to 600 °C in He flow at a heating rate of 5 °C· min−1. Concentrations of NH3 or CO2 in the outlet stream were analyzed with a Fourier-transform infrared spectrometer (FT-IR; Nicolet, Magna 560; resolution set at 0.5 cm−1) equipped with a multireflectance gas cell with an optical path length of 2 m. 2.3. Catalytic Activity Test. The catalytic activity of each sample was determined using a fixed-bed flow reactor. Catalyst samples (0.040 g) were packed in a quartz glass tube (4 mm inner diameter and 310 mm long), and a simulated diesel exhaust gas composed of 1780 ppm (in terms of C) n-decane, 220 ppm (in terms of C) 1-methylnaphthalene, 200 ppm of NO, 10% H2O, and 5% O2 with the balance as N2 was passed through at a flow rate of 400 mL·min−1. The reactor temperature was lowered stepwise from 500 to 100 °C, and

3. RESULTS AND DISCUSSION 3.1. Characterization. Table 1 summarizes the characterizations of 0.75 wt % Pt/0.25 wt % Pd supported on a series of SiO2−Al2O3, and Figure 1 shows the pore size distribution profiles, from which it is evident that the pore distributions Table 1. Characterization Data for 0.75% Pt/0.25% Pd Catalysts on a Series of SiO2−Al2O3 Supports

composition of support material Al2O3

0.5 wt % SiO2−Al2O3





a

7993

preparation atmospherea

drying calcination A A H H A A H H A A H H A A H H A A H H A A H H

A H A H A H A H A H A H A H A H A H A H A H A H

BET

meso pore volume

meso pore diameter

PtPd disp

m2·g−1

mL·g−1

nm

%

180 170 137 136 199 179 157 150 212 200 178 171 221 213 216 207 250 237 212 220 245 235 255 243

0.36 0.38 0.34 0.35 0.39 0.39 0.36 0.37 0.41 0.42 0.40 0.42 0.40 0.42 0.44 0.46 0.44 0.46 0.44 0.50 0.44 0.46 0.50 0.52

7.8 8.8 9.7 10.2 7.6 8.5 9.1 9.8 7.5 8.3 8.9 9.6 7.1 7.8 8.0 8.8 6.7 7.7 8.2 8.9 6.9 7.7 7.6 8.4

13.9 21.7 22.0 19.5 19.5 19.6 19.2 17.6 13.3 16.3 15.9 11.9 12.6 14.3 15.1 11.9 16.0 14.7 16.2 11.8 11.8 14.0 14.5 10.9

A: atmospheric. H: hydrothermal. dx.doi.org/10.1021/ie5005724 | Ind. Eng. Chem. Res. 2014, 53, 7992−7998


Article

Figure 1. Effects of drying and calcination conditions on the pore size distributions of 0.75% Pt and 0.25% Pd loaded on SiO2−Al2O3 containing (a) 0, (b) 0.5, (c) 2.2, (d) 3.2, (e) 3.9, and (f) 5.4 wt % SiO2. ○, drying under atmospheric conditions and calcination under atmospheric conditions; ●, drying under atmospheric conditions and calcination under hydrothermal conditions; ◊, drying under hydrothermal conditions and calcination under atmospheric conditions; and ⧫, drying under hydrothermal conditions and calcination under hydrothermal conditions.

interactions with increasing degrees of SiO44− doping.14 It is also seen that the mesopore size increased as more vigorous hydrothermal conditions were applied, and thus, hydrothermal treatment with drying and calcination processes (⧫ in Figure 2) resulted in the largest mesopores. We believe that shrinkage of the alumina framework during drying and calcination processes was restrained for a longer period of time by the presence of water molecules in the gel. Figure 3 shows the distributions of Si and precious metal particles as determined by TEM-EDX analysis of the 3.9 wt % SiO2−Al2O3 sample. It was observed that Si was uniformly distributed over the Al2O3 along with platinum and palladium particles in 10 to 20 nm in diameter. Similar results were obtained for all other samples, which suggest that the starting materials (sodium silicate and aluminum sulfate) were mixed uniformly at the point at which the gelation state occurred. Additionally, the platinum and palladium showed almost the same distribution pattern in all samples. According to EDX spot measurements, the Pt/Pd ratio in each particle was within the range of 2.5 to 4.6, a span that is very close to the theoretical value of three. These two metals may therefore exist as an alloy,15−17 although precise details of the specific form of the two metals have not yet been obtained. 3.2. Catalytic Activity. 3.2.1. Effect of SiO2 Content. Figure 4 shows the temperature dependence of HC and NO oxidation over Al2O3 and 3.9 wt % SiO2−Al2O3 catalysts, while Table 2 summarizes the catalytic activity for all samples. In all cases, CO emissions were negligible because of the strong catalytic activity of the precious metals for CO oxidation. For all samples in Figure 4, HC oxidation began at approximately 200 °C and reached 100% conversion at around 300 °C. In contrast, the NO oxidation profile shows that the conversion increased up until approximately 350 °C and then decreased at higher temperatures, in agreement with the theoretical NO⇄NO2 equilibrium curve under 5% O2.

were significantly changed by varying the drying and calcination conditions. In all cases, the γ-Al2O3 structure was maintained even after the aging process, as confirmed by XRD analyses (not shown). Figure 2 shows the relationship between SiO2 content and mesopore size for these catalysts. The mesopore size generally decreased as the SiO2 content increased. We speculate that the structural framework of the mesoporous alumina became increasingly dense as the result of electrical

Figure 2. Effects of SiO2 content on the pore size distributions of SiO2−Al2O3. ○, drying under atmospheric conditions and calcination under atmospheric conditions; ●, drying under atmospheric conditions and calcination under hydrothermal conditions; ◊, drying under hydrothermal conditions and calcination under atmospheric conditions; and ⧫, drying under hydrothermal conditions and calcination under hydrothermal conditions. 7994



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Figure 3. TEM-EDX images of 0.75% Pt/0.25% Pd loaded on 3.9% SiO2−Al2O3 support after heat treatment at 750 °C for 50 h in air.

Figure 4. Temperature dependencies of diesel HC and NO oxidation over 0.75% Pt/0.25% Pd loaded on (a and b) Al2O3 and (c and d) 3.9% Si− Al2O3. ○, drying under atmospheric conditions and calcination under atmospheric conditions; ●, drying under atmospheric conditions and calcination under hydrothermal conditions; ◊, drying under hydrothermal conditions and calcination under atmospheric conditions; and ⧫, drying under hydrothermal conditions and calcination under hydrothermal conditions.

7995



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dispersions were almost equal in these seven samples, except in the case of the 0.5 wt % SiO2−Al2O3, it appears that certain properties of the support material influenced the activity. Figure 6 presents the NH3 and CO2-TPD profiles for a series of SiO2−Al2O3 with mesopore sizes of 7.6 ± 0.2 nm, after aging and without precious metal loading, while Figure 7 shows the relationship between SiO2 content and the total uptake of NH3 and CO2, normalized by the weight of the sample. It is evident that, as the SiO2 content in the catalyst was increased, CO2 uptake decreased dramatically, whereas there was only a minimal increase in the NH3 uptake. On the basis of these results, a decrease in the quantity of basic sites was suspected to be among the primary factors associated with improvements in the catalytic activity for HC and NO oxidation. Sasaki et al.18 have reported that the diesel NO oxidation activity of Pt/Al2O3 is decreased by increases in the quantity of basic sites on the support material and have suggested that acidic chemisorbed intermediates such as NO2 are stabilized over surface basic sites at which Pt oxides are formed. This results in the inhibition of the reaction over Pt surfaces because the activity of Pt oxide is lower than that of metallic Pt. Additionally, Haneda et al.19 have reported that diesel HC oxidation activity is also decreased by increases in the quantity of basic sites and have determined that a high surface density of acidic sites is important in terms of stabilizing the surface of metallic Pt and thus essential for high catalytic activity. Haneda suggested that the formation of intermediate acrylate species (−CHCH−COO−) was suppressed over Pt catalysts supported on Al2O3 with greater surface concentrations of basic sites. Taken together, these results suggest that a similar effect associated with lower basicity is exhibited by the SiO2− Al2O3 catalyst. Accordingly, we consider that one of the major effects of SiO2 doping is to decrease the basicity of the Al2O3 support. It is still not clear what causes the appearance of the maximum activity at 4 wt % SiO2 because basic sites continued to decrease as the SiO2 content increased. Zhong et al.8 found that the optimum SiO2 content is 2 wt % and have speculated that greater catalytic activity coincides with the addition of SiO2 because of the increasing number of active sites. We ourselves believe that the acidic/basic properties of the support also influence the catalytic activity but expect that more detailed investigations will be necessary in the future. 3.2.2. Effect of Mesopore Size. Figure 8 shows the relationship between mesopore size and catalytic activity for all 24 SiO2−Al2O3 samples investigated. It was found that both HC and NO oxidation activities were improved with increasing mesopore size over the range from 6 to 10 nm for all samples with the same SiO2 content. The highest activity (T50 = 235 °C, NO = 47%) was obtained with the 3.9 wt % SiO2−Al2O3 having the largest mesopores (8.9 nm). To estimate the effects of gas diffusion on this reaction system, the relationship between the mesopore size and diffusivity was investigated using CFD calculations. Figure 9 shows the calculation results for reaction rate at a temperature of 260 °C, which demonstrate that the rate increases significantly with increases in mesopore size up to approximately 10 nm, after which it reaches a plateau. Although the assumed gaseous conditions used for the simulation were not exactly the same as the experimental ones, the calculated trend in which increased activity correlates with greater mesopore sizes agrees with the experimental results. Consequently, it was concluded that the observed activity increase was due to the

Table 2. Catalytic Activity of 0.75% Pt/0.25% Pd Catalysts on a Series of SiO2−Al2O3 Supports preparation atmospherea composition of support material Al2O3






a

drying calcination A A H H A A H H A A H H A A H H A A H H A A H H

A H A H A H A H A H A H A H A H A H A H A H A H

catalytic activity HC conv. T50 %/°C

NO conv. % at 350 °C

256 248 237 242 263 252 239 241 252 240 235 236 263 247 246 244 251 239 240 235 250 245 247 239

35 38 38 40 31 32 36 37 36 40 41 41 35 40 38 42 40 44 40 47 38 44 38 44

A: atmospheric. H: hydrothermal.

Figure 5 shows the relationship between SiO2 content and activity for SiO2−Al2O3 catalysts having a mesopore size of 7.6 ± 0.2 nm, in which both HC and NO oxidation extents increased up to approximately 4 wt % SiO2 and leveled off afterward. From these results, we concluded that the optimum SiO2 level in the Al2O3 was 4 wt %. Because the Pt/Pd

Figure 5. Effects of SiO2 content on the catalytic activity during HC and NO oxidation and the dispersion of Pt on a series of SiO2−Al2O3 supports. The mesopore mean diameters of the supports were 7.5 ± 0.2 nm. 7996



Article

Figure 6. (A) NH3 and (B) CO2-TPD profiles of a series of SiO2−Al2O3 supports containing (a) 0, (b) 0.5, (c) 2.2, (d) 3.2, (e) 3.9, and (f) 5.4 wt % SiO2.

improved gas diffusion associated with mesopore size expansion. Because the calculations showed that the impact of increasing the mesopore size with regard to activity enhancement was minimal above 10 nm, it appears that the optimum mesopore size is around 10 nm. We therefore anticipate that the catalytic activity could be further improved by increasing the mesopore size to 10 nm in the case of the above-noted 3.9 wt % SiO2−Al2O3 sample.

4. CONCLUSIONS The effects of the extent of SiO2 doping and mesopore size of Al2O3 on the activity of Pt/Pd-based diesel oxidation catalysts were investigated. For this purpose, 24 different SiO2−Al2O3 catalysts with differing levels of SiO2 doping and various mesopore sizes were prepared and tested. The following conclusions were reached on the basis of our experimental results. (1) The optimum level of SiO2 addition to the Al2O3

Figure 7. Effects of SiO2 content on the NH3 and CO2 uptake of the series of SiO2−Al2O3 supports.

Figure 8. Effects of mesopore sizes and Si contents of 0.75% Pt/0.25% Pd/SiO2−Al2O3 on the catalytic activity during (a) HC and (b) NO oxidation. 7997



Article

(7) Matam, S. K.; Kondratenko, E. V.; Aguirre, M. H.; Hug, P.; Rentsch, D.; Winkler, A.; Weidenkaff, A.; Ferri, D. The Impact of Aging Environment on the Evolution of Al2O3 Supported Pt Nanoparticles and Their NO Oxidation Activity. Appl. Catal., B 2013, 129, 214. (8) Zhong, F.; Zhong, Y.; Xiao, Y.; Cai, G.; Wei, K. Effect of SiDoping on Thermal Stability and Diesel Oxidation Activity of Pt Supported Porous γ-Al2O3 Monolithic Catalyst. Catal. Lett. 2011, 141, 1828. (9) Beguin, B.; Garbowski, E.; Primet, M. Stabilization of Alumina toward Thermal Sintering by Silicon Addition. J. Catal. 1991, 127, 595. (10) 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. (11) 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. (12) Caravella, A.; Hara, S.; Hara, N.; Obuchi, A.; Uchisawa, J. Computational Fluid Dynamics Simulation of a Three-Dimensional Catalytic Layer for Decane Oxidation: Case Study of Reaction on Particle Surface. Ind. Eng. Chem. Res. 2011, 50, 11879. (13) Caravella, A.; Hara, S.; Hara, N.; Obuchi, A.; Uchisawa, J. ThreeDimensional Modeling and Simulation of a Micrometer-Sized Particle Hierarchical Structure with Macro- And Meso-Pores. Chem. Eng. J. 2012, 210, 363. (14) Uchisawa, J.; Tango, T.; Murakami, T.; Nakagawa, H.; Hara, S.; Nanba, T.; Obuchi, A. Diesel Hydrocarbon Oxidation over Platinum on Mesoporous Silica Doped with Secondary Component Metals via a Sol-Gel Methodology. React. Kinet. Mech. Catal. 2013, 108, 359. (15) Morlang, A.; Neuhausen, U.; K. Klementiev, V.; Shutze, F.-W.; Miehe, G.; Fuess, H.; Lox, E. S. Bimetallic Pt/Pd Diesel Oxidation Catalysts: Structural Characterisation and Catalytic behaviour. Appl. Catal., B 2005, 60, 191. (16) Ward, M. R.; Boyes, E. D.; Gai, P. L.; Hyde, T. Nanostructural Studies of Fresh and Road-Aged Practical Pt/SiO2 and Pt-Pd/Al2O3 Diesel Oxidation Catalysts by using Aberration-Corrected (Scanning) Transmission Electron Microscopy. Chem. Catal. Chem. 2012, 4, 1622. (17) Kaneeda, M.; Iizuka, H.; Hiratsuka, T.; Shinotsuka, N.; Arai, M. Improvement of Thermal Stability of NO Oxidation Pt/Al2O3 Catalyst by Addition of Pd. Appl. Catal., B 2009, 90, 564. (18) Sasaki, M.; Suzuki, K.; Sultana, A.; Haneda, M.; Hamada, H. Effect of Acid-Base Properties on the Catalytic Activity of Pt/Al2O3 Based Catalysts for Diesel NO Oxidation. Top. Catal. 2013, 56, 205. (19) Haneda, M.; Sasaki, M.; Hamada, H.; Ozawa, M. Effect of Pt Dispersion on the Catalytic Activity of Supported Pt Catalysts for Diesel Hydrocarbon Oxidation. Top. Catal. 2013, 56, 249.

Figure 9. Results of CFD calculations of the relationship between mesopore size and reaction rate during HC oxidation.

was 4 wt %. The positive effect induced by adding the silica is likely due to the optimization of the basicity of the support. (2) The activity was increased as the mesopore size increased up to a value of approximately 10 nm. It was confirmed by CFD calculations that diffusion limitations associated with small mesopore structures may affect the activity at mesopore sizes below 10 nm. (3) The Al2O3 mesopore structure can be controlled by the use of the sol−gel synthesis method followed by hydrothermal treatments. This approach appears to hold significant promise with regard to practical applications because it offers a convenient means of improving catalytic activity by the simple addition of optional metallic elements.

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AUTHOR INFORMATION

Corresponding Author

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

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was financially supported by the New Energy and Industrial Technology Development Organization (NEDO). We thank Dr. N. Yoshizawa and Mr. A. Takatsuki (AIST) for the TEM-EDS analyses.

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REFERENCES

(1) Russell, A.; Epling, W. S. Diesel Oxidation Catalysts. Catal. Rev. 2011, 53, 337. (2) Twigg, M. V. Catalytic Control of Emissions from Cars. Catal. Today 2011, 163, 33. (3) Hauff, K.; Tuttlies, U.; Eigenberger, G.; Nieken, U. A Global Description of DOC Kinetics for Catalysts with Different Platinum loadings and Aging Status. Appl. Catal., B 2010, 100, 10. (4) Haneda, M.; Sasaki, M.; Hamada, H.; Ozawa, M. Platinum-Based Catalyst for Diesel Hydrocarbon Oxidation. Chin. J. Catal. 2010, 32, 777. (5) Haneda, M.; Sasaki, M.; Hamada, H.; Ozawa, M. In Situ FT-IR Study of Diesel Hydrocarbon Oxidation over Pt/Al2O3 Catalyst. Catal. Lett. 2011, 141, 1262. (6) Diehl, F.; Barbier, J.; Duprez, D.; Guibard, I.; Mabilon, G. Catalytic Oxidation of Heavy Hydrocarbons over Pt/Al2O3. Influence of the Structure of the Molecule on Its Reactivity. Appl. Catal., B 2010, 95, 217. 7998


Effects of the Extent of Silica Doping and the Mesopore Size of an

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