Alkali Carbonate Stabilized on Aluminosilicate via Solid Ion Exchange

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Alkali Carbonate Stabilized on Aluminosilicate via Solid Ion Exchange as a Catalyst for Diesel Soot Combustion Riichiro Kimura,† S. P. Elangovan,† Masaru Ogura,‡ Hiroshi Ushiyama,† and Tatsuya Okubo*,† † ‡

Department of Chemical System Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Institute of Industrial Science, The University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan ABSTRACT: We report the successful preparation of alkali-carbonatesupported aluminosilicate catalysts for diesel soot combustion. From the views of practical feasibility as well as economic and environmental concerns, utilizing alkali metals is preferable for diesel soot combustion. We overcome the drawback that the alkali compounds used as catalysts are easily soluble in water. Na2CO3 was stabilized on sodium- and potassium-containing nepheline ((Na, K)-nepheline) with water tolerance. This result suggests that carbonates are stabilized on the supports with higher energy than hydration energy and van der Waals force by thermal treatment of K2CO3/Na-nepheline via solid ion exchange. Na2CO3 on the surface of thermally treated K2CO3/Na-nepheline is likely to be a catalytically active species for carbon black combustion as (Na, K) mixed carbonate. Na2CO3 stabilized on (Na, K)-nepheline is recovered by CO2, which is likely to be derived from catalytic action on combustion of carbon black.

1. INTRODUCTION As a way of addressing global warming, which has become a great concern for humanity, diesel engines provide immense benefits with their low CO2 emissions due to their excellent fuel economy. However, diesel engines represent an important source of particulate matter (PM).1,2 PM consists mostly of carbonaceous soot and a volatile organic fraction (VOF) of hydrocarbons adsorbed on the surface3 and is known to be harmful for human health and the environment.4 Recently, as emissions of PM have been strictly regulated in many countries and regions, most notably by the Euro IV and V standards and the US EPA 2007 and 2010 regulations, there is an urgent demand for the development of systems for exhaust aftertreatment.5 The use of diesel particulate filters (DPFs) is currently the most widely used approach to trap PM.6,7 However, DPFs must be regenerated to remove accumulated soot or the risk of high pressure loss.8 Two strategies, passive and active regeneration, have been proposed to regenerate DPFs.9 The former strategy uses nitrogen dioxide, a known reactive oxidant for soot.10 However, NOx may not be sufficiently available in every operating condition.6 The latter strategy uses oxygen, the most plentiful species in exhaust. Unfortunately, a temperature in the range of 550
600 °C is required.9 As it is not always possible to regenerate DPFs passively, the trap system must be subjected to an active regeneration strategy that combusts collected PM by raising the temperature of the trap itself with a supply of external energy.8 This requires significant fuel consumption and hence decreases the thermal efficiency of the diesel engine. Therefore, a catalytic regeneration to lower the combustion temperature of PM is desirable. Various catalysts such as noble metals,12 transition metals,13
18 perovskites,19,20 spinel oxides,21 alkali metals,22
24 and alkali earth metals25 have already been investigated for PM combustion.11 r 2011 American Chemical Society

This earlier work has shown that the contact between a catalyst and soot is an important factor.26 Therefore, the use of mobile catalytic compounds such as potassium-containing materials, in which potassium promotes tight contact and enhances catalytic activity,27
34 is receiving a greater amount of attention. The catalytic mechanism of potassium itself has been examined in detail, and the reduction of alkali compounds to metal is thought to catalyze the soot oxidation of carbon.35,36 However, one problem is that reduced alkali metal can sublime, which enhances the mobility of catalyst and ultimately leads to loss of the catalyst.23,37 Another is that alkali metal salts are easily soluble in water; hence, the alkalinity around the DPF wall increases, causing damage to the DPF material. To overcome this problem, supports with high catalytic activity and stability for potassium catalysts, such as K/MgO,38 potash glass,39 Mg(Al)
O
K,40 and K4Zr5O12,41 are receiving an increasing amount of attention. In our previous studies, microsized sodalite was proposed as a support to maintain both the activity and stability of potassium carbonate.42 It was found that the activity was further enhanced after thermal treatment of K2CO3 impregnated on nano-sized sodalite (K2CO3/nanosized sodalite) at 800 °C, in association with the appearance of potassium-enriched nepheline group materials, viz., nepheline, kalsilite, and kaliophilite.43 In that work, we found that the catalytic activity of synthesized nepheline was decreased to some extent after washing with distilled water at 80 °C. We suggested in our previous work that sodium carbonate might appear after thermal treatment of K2CO3/sodalite42 and Received: April 13, 2011 Revised: May 28, 2011 Published: July 13, 2011 14892

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The Journal of Physical Chemistry C are currently verifying this phenomenon in another study. Here, we use potassium ion-exchanged Na-LTA zeolite as an initial support for K2CO3 because potassium has higher catalytic performance than sodium. To investigate the phenomenon systematically, we also use both Na2CO3 and Na-LTA. Furthermore, we use nephelines obtained from LTAs as initial supports to investigate the effect of transformation from zeolite to nepheline for the appearance of active species. We examine the catalytic active species and water tolerance of catalysts obtained from thermal treatment of Na2CO3/K2CO3 supported Na-LTA, K-LTA, pure-sodium nepheline (Na-nepheline), and purepotassium nepheline (K-nepheline) with XRD, Raman spectroscopy, TG-MS, and FT-IR. We show that thermally treated K2CO3/Na-nepheline is a promising candidate for diesel soot combustion with high catalytic activity and superior water tolerance. Potential reasons for this superior performance and a new perspective are discussed in detail with comprehensive characterization of thermally treated K2CO3/Na-nepheline. Furthermore, we suggest that the carbonate might be recovered as an active species after catalytic combustion, a notion that gives us a new perspective for understanding the catalytic mechanism.

2. EXPERIMENTAL SECTION 2.1. Preparation of Support Materials. Na-Linde-type A zeolite (Na-LTA) obtained from Wako Pure Chemical Industries was used as a starting material. Threefold ion-exchange of NaLTA was carried out with an aqueous solution of KNO3 at 353 K under stirring for 12 h to obtain potassium ion-exchanged LTA (K-LTA). The concentration of Kþ was adjusted to be 100 times higher than that necessary to compensate for the framework charge. The solid was filtered and washed with distilled water. Both Na-LTA and K-LTA were thermally treated at 1000 °C for 4 h and 1100 °C for 5 h to obtain Na-nepheline46 and K-nepheline,45 respectively. We confirmed that the ion exchange was completed from the results of XRF measurement. 2.2. Loading Alkali Carbonates on Supports by Impregnation. Na2CO3 and K2CO3 were used as the sources of sodium and potassium compounds, respectively, which were confirmed to be thermally stable up to 800 °C without decomposition into alkali oxide and CO2. Each was loaded on the support by an impregnation method. The required amount of Na2CO3 or K2CO3 was added to the support slurried in distilled water and maintained at 80 °C for 24 h under vigorous stirring and dried at 100 °C, resulting in the physical deposition of the carbonate on the support. The overall content of Na2CO3 or K2CO3 on the support was calculated by the stoichiometrically identical amount with the structure, considering the adsorbed water and crystal water obtained with TG analysis. 2.3. Thermal and Washing Treatments for Catalysts. The alkali-carbonate-supported samples (designated as carbonate/ support) were calcined at 800 °C for 5 h in air atmosphere (designated as 800-carbonate/support). The samples thus obtained were washed with water at room temperature (designated as wash-carbonate/support and wash-800-carbonate/support). Each gram of thermally treated carbonate/support was suspended in 30 mL of distilled water placed in a polypropylene bottle and maintained at the required temperature for about 24 h under vigorous stirring. Thereafter, the samples were filtered and dried at 60 °C, following thermal treatment at 300 °C for 30 min. 2.4. Evaluation of Catalytic Performance. The catalytic activity of the samples for soot combustion was evaluated by

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temperature-programmed oxidation (TPO) with a thermogravimetric differential analyzer (Thermoplus TG-8120, Rigaku) connected directly to a mass spectroscope (Anelva). Carbon black #2600 with a diameter of 13 nm, supplied by Mitsubishi Chemicals Co., was used as a reference for the carbonaceous soot found in diesel exhaust. Elemental analysis showed the carbon black’s carbonaceous nature with 93 wt % C, 0.32 wt % H, 0.43 wt % N, 1.0 wt % S, 4.2 wt % O, and 0.030 wt % others (ash). In addition, the carbon black has a Brunauer
Emmett
Teller (BET) surface area of 310 m2/g. The catalysts and carbon black were simply mixed together with a spatula to achieve a loose contact that is near practical conditions.26 The carbon black/ catalyst mixtures had weight ratios of approximately 1:10. The mixtures were heated to 800 °C at a rate of 10 K/min in a flow of mixture gas containing 10% oxygen in helium. The catalytic activity for carbon combustion is typically evaluated by the temperature, Tstart, i.e., the point at which the CO2 (m/z = 44) elution curve starts to appear. Weight losses with exothermic peaks attributed to the combustion of carbon black and the generation of CO2 were simultaneously detected by TG-DTA and MS, respectively. The apparent activation energy of soot oxidation is determined by the Ozawa method47 using the following expression: d log(β)/d(1/TR) = 0.4567E/R, where β is the heating rate, TR is the temperature corresponding to R% carbon conversion, and E is the apparent activation energy in kJ/mol. E can be estimated from the slope of the least-squares straight-line fit of log(β) plotted against 1/TR. 2.5. Characterization of Catalysts. The structures of the obtained samples were examined with the powder X-ray diffractometer (XRD) M03X-HF (Bruker AXS) using Cu KR radiation (wavelength of 0.154 06 nm, 40 kV, 30 mA) at a scanning rate of 4°/min over a range of 5°
45° (2θ). Raman spectroscopy was performed by an NR-1800 (Jasco Corp., NR-1800) equipped with a green laser (JUNO-100, λ = 532 nm) to excite the samples. Raman spectra were recorded between 155 and 1350 cm
1 and integrated for 30 s twice. Thermogravimetry
mass spectrometry (TG-MS) measurements were performed to analyze the gaseous species released from the samples during heat treatment using the thermogravimetric analyzer Thermoplus TG8120 (Rigaku) equipped with a mass spectrometer (Anelva). Surface species were analyzed using Fourier transform infrared spectroscopy (FT-IR, Jasco Corp., FT-IR 6100). Samples were diluted with KBr to about 1:10 mass ratio. All spectra were recorded from 4000 to 400 cm
1 by accumulating 16 scans at a resolution of 4 cm
1.

3. RESULTS AND DISCUSSION 3.1. Characterization of Supports and Catalysis. 3.1.1. XRD. Figure 1 shows the XRD patterns of obtained samples along with their supports. Na-LTA, K-LTA, Na-nepheline, and K-nepheline are confirmed to exhibit XRD patterns similar to those reported in the literature.48 The amount of carbonate loading was determined as mentioned in the Experimental Section. All 800carbonate/supports exhibit XRD patterns based on nepheline group materials with slight differences. Na2CO3/supports, Na2CO3/Na-LTA (Figure 1a), and Na-nepheline (Figure 1b) show patterns that closely resemble the pattern of Na-nepheline with an additional peak at 20.78°. Na2CO3/K-LTA (Figure 1c) and K-nepheline (Figure 1d) show patterns that closely resemble to the pattern of K-nepheline with an additional peak at 22.18°. 14893

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Figure 1. XRD patterns of the obtained samples from the various supports: (a) Na-LTA, (b) Na-nepheline, (c) K-LTA, and (d) K-nepheline.

Increasing the amount of Na2CO3 loading on K-LTA and K-nepheline results in another additional peak at 33.62°. The higher intensity of the additional peak of Na2CO3/K-LTA compared to that of Na2CO3/K-nepheline may be due to a higher amount of sodium diffusing to the bulk structure during thermal treatment. This is because during the thermal treatment process zeolite first turns into an amorphous phase before crystallizing into nepheline,46 and sodium might diffuse easier to the amorphous phase than the bulk structure of nepheline. K2CO3/supports, K2CO3/Na-LTA, and Na-nepheline show patterns that closely resemble the pattern of Na-nepheline with additional peaks at 22.24° and 28.54°, which is similar to the pattern of nepheline containing sodium and potassium ((Na, K)nepheline). Increasing the amount of K2CO3 loading on the supports leads to higher intensity of these peaks. The higher intensity of the additional peaks of K2CO3/Na-LTA compared to those of K2CO3/Na-nepheline is due to the same reason as that in the comparison between K2CO3/K-LTA and K-nepheline. As can be seen from these results, all obtained samples belong to nepheline group materials differing to a certain extent. This may be due to differences in the diffusivity of alkali cations for zeolite and nepheline. In the following sections, we use each sample by loading lower amounts of carbonate. 3.1.2. Raman Spectroscopy. In order to examine surface species, Raman spectroscopy was conducted. Figure 2 shows the Raman spectra of the supports, carbonate/supports, washcarbonate/supports, 800-carbonate/supports, and wash-800carbonate/supports, along with those of Na2CO3 and K2CO3. The typical peaks of Na2CO3 and K2CO3 appear at 1079 and 1063 cm
1, respectively, as shown in the literature.44 The peak corresponding to Na2CO3 appears with peaks of its natural structure in the case of carbonate/supports, viz., Na2CO3/NaLTA (Figure 2a), Na2CO3/Na-nepheline (Figure 2c), Na2CO3/ K-LTA (Figure 2e), and Na2CO3/K-nepheline (Figure 2f). The peak corresponding to K2CO3 appears for K2CO3/Na-nepheline

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(Figure 2d) in the same way. On the other hand, the peak corresponding to K2CO3, unlike that of Na2CO3, does not appear on K2CO3/Na-LTA (Figure 2b). It is conceivable that Na-LTA was ion-exchanged to K-LTA in the K2CO3 solution. After washing the carbonate/supports to obtain wash-carbonate/ supports, the peak corresponding to the carbonate disappears in all samples. After thermal treatment, 800-Na2CO3/Na-LTA, 800-Na2CO3/Na-nepheline, and 800-Na2CO3/K-LTA show the peak corresponding to Na2CO3, whereas 800-K2CO3/NaLTA and 800-K2CO3/Na-nepheline show the peak for Na2CO3 and 800-Na2CO3/K-nepheline shows the peak for K2CO3. A possible explanation for the results is that alkali cations on the surface were consumed for the formation of nepheline during thermal treatment and the alkali cation in the structure appears on the surface, which is in good agreement with the XRD results. Even after washing thermally treated carbonate/supports, the peak of the carbonate remains in Na2CO3 on Na-nepheline and in K2CO3 on Na-LTA or Na-nepheline, whereas it disappears in Na2CO3 on K-LTA or K-nepheline. Although stabilized Na2CO3 might appear from solid ion exchange between carbonate and nepheline, we were not able to clarify differences in the Raman shift of carbonates with/without interaction with the support. For 800-Na2CO3/Na-nepheline, the catalytic activity remains the same (shown in the next section); however, the peak of Na2CO3 almost disappears after washing. This suggests that Na2CO3 may interact with the support in an inactive state for Raman spectroscopy. 3.2. Evaluation of Catalytic Activity. The catalytic performance of the reference samples is shown in Figure 3. As explained previously, the catalytic activity for carbon combustion is evaluated by the temperature, Tstart, the point at which the CO2 elution curve starts to appear. First of all, the support itself has no catalytic activity, as its MS profile closely resembles that of a noncatalytic (only carbon black); in addition, the catalytic performance of K2CO3 is superior to that of Na2CO3. The catalytic performance of the obtained samples is shown in Figure 4. For carbonate/supports and carbonate/LTAs, Tstart is lower than that of carbonate/nephelines. Possible explanations for these results are that the supports have 4 Å pores for Na-LTA, 3 Å pores for K-LTA, and no pores for nepheline; hence, Kþ, Naþ, and CO32
(for which the hydrated ionic radiuses are 3, 4, and 4 Å, respectively) from the carbonates could infiltrate the pores in case of LTAs, leading to poorer contact with carbon and, in turn, less catalytic performance. A slight peak that appears in the range of 100
200 °C is attributed to the formation of bicarbonates.24 It is noted that a portion of the carbon is oxidized by the assistance of the catalyst, whereas the rest of the carbon is oxidized by gaseous oxygen, as demonstrated in some samples. The MS spectra split into two maxima because of the catalytic and noncatalytic combustion of carbon black caused by the heterogeneous contact between the catalyst and carbon black in the case of loose contact. After washing carbonate/supports, wash-carbonate/LTAs nearly lose their catalytic activities. On the other hand, wash-carbonate/nephelines somewhat retain their catalytic activities, and wash-Na2CO3/Na-nepheline and wash-K2CO3/K-nepheline have better catalytic activity than wash-K2CO3/Na-nepheline and wash-Na2CO3/K-nepheline. After thermal treatment of carbonate/supports, carbon oxidation starts in the range of 350
400 °C with a maximum peak at around 500 °C for all samples. After washing thermally treated carbonate/supports, carbonate/Na-LTAs and Na-nepheline retain their catalytic activities, whereas carbonate/K-LTA and 14894

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Figure 2. Raman spectra of the obtained samples.

Figure 3. TG-MS of the reference samples.

K-nepheline show lower catalytic activities than the corresponding samples before washing. These results demonstrate that thermally treated carbonate/Na-LTA and Na-nepheline exhibit great water tolerance among the prepared samples because the shift in the starting temperature of oxidation between before and after washing is least. This means that carbonates are stabilized on the supports with a higher energy than the hydration energy and the van der Waals force. In the following section, we will focus on the analysis for carbonate/Na-LTA and Na-nepheline.

3.2.1. FT-IR. The presence of carbonate species was also examined by FT-IR analysis. Figure 5 (left) shows the FT-IR results of the obtained samples starting with Na-nepheline as a support along with K2CO3 and Na2CO3. The spectra of potassium and sodium carbonate display a strong, broad band centered around 1460 cm
1, which exhibits ν(CO32
) absorptions.49 Comparing Na-nepheline and K2CO3/Na-nepheline, the spectrum for the latter clearly shows the ν(CO32
) absorptions derived from K2CO3 at 1460 and 1390 cm
1. After thermal treatment, the sharp ν(CO32
) absorption seen at 1457 cm
1 indicates that the carbonate in the surface turned from K2CO3 to Na2CO3, which is in good agreement with the results of Raman spectroscopy. The ν(CO32
) absorption slightly remains after washing, which is also in good agreement with the results of Raman spectroscopy. Figure 5 (right) shows the spectrum of wash-800-K2CO3/Na-nepheline mixed with carbon black and that following carbon black combustion. The ν(CO32
) absorption of wash-800-K2CO3/Na-nepheline mixed with carbon black almost disappeared because of the strong absorptions by carbon black. Interestingly, the ν(CO32
) absorption appears at the same 14895

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Figure 4. TG-MS of the obtained samples.

position as that of the spectra before washing, whereas thermally treated wash-800-K2CO3/Na-nepheline alone shows no changes after thermal treatment. Wash-800-K2CO3/K-nepheline and Wash-800-K2CO3/K-LTA mixed with carbon black after TG-MS analysis have no corresponding absorptions. For Na2CO3/Na-nepheline, the ν(CO32
) absorptions disappears in wash-800-Na2CO3/Na-nepheline, although the catalytic activity remains according to the TG-MS results. The ν(CO32
) absorptions did not appear after TG-MS analysis mixed with carbon black. This phenomenon was characteristic in the case of K2CO3/Na-nepheline. We derive the following speculation from these results. Na2CO3 stabilized on (Na, K)-nepheline is recovered by CO2, which is likely derived from catalytic action on combustion of carbon black. Another possibility is that new active sites appear along with chemisorption of CO2 on alkali cation sites. We believe that these

results will help us understand the mechanism of this catalysis in future work. 3.2.2. Ozawa Method. To speculate upon the active species on the surface of nepheline, the apparent activation energy of soot oxidation is determined by the Ozawa method47 using the following equation: d logðβÞ E   ¼ 0:4567 1 R d TR where β is the heating rate, TR is the temperature corresponding to R% carbon conversion calculated from the TG curve, and E is the apparent activation energy in kJ mol
1. E can be estimated from the slope of the least-squares straight-line fit of log(β) plotted against 1/TR. 14896

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Figure 5. FT-IR of the obtained samples.

Figure 6. Ozawa plots over wash-800-K2CO3/Na-nepheline at different carbon black conversion levels.

Figure 6a shows an example of the analytical procedure for estimating E for wash-800-K2CO3/Na-nepheline. The carbon black oxidation was conducted at different heating rates (β = 1, 3, 5, and 10 K/min). The plots of the logarithmic heating rates (log β) versus the inverse temperatures (1/TR) at various carbon black conversion levels (R = 10
90%) show good linear fits. The E estimated from the slope of the T50 plot is 113 kJ/mol, which is identical to the average E value (113 ( 3 kJ) estimated from all slopes in Figure 6. The E values of K2CO3 and Na2CO3 estimated from the slope of the T50 plot in the same way are 110 and 115 kJ/mol, respectively. As there is a good agreement of the E values of wash-800K2CO3/Na-nepheline and Na2CO3 across conversion rates, Na2CO3 stabilized on the surface of nepheline is likely to be

the active species for carbon black combustion. Slight differences in the E values between wash-800-K2CO3/Na-nepheline and Na2CO3 may be due to the status of the carbonate, which might be stabilized on the nepheline in (Na, K) CO3 due to the aforementioned solid ion exchange. For K2CO3/K-LTA, the E value shows good agreement with that of K2CO3; in addition, 800-K2CO3/K-LTA shows an E value between Na2CO3 and K2CO3. A possible explanation for this behavior is that K2CO3 interacts with the support in 800-K2CO3/K-LTA but not in K2CO3/K-LTA, which is in accord with the results of TG-MS analysis. It is conceivable that if there is an interaction between the carbonate on the surface and the structure, some of the alkali cations are included in the structure and thus increase the activation energy. For K2CO3/K-nepheline, one reason why both K2CO3/K-nepheline and 800-K2CO3/K-nepheline have similar activation energies may be that the K2CO3 molecules on both surfaces have some interaction with the structure. This is in accord with the results of TG-MS, which showed slight water tolerance, i.e., some interaction between the carbonate and the support. From these results, the Na2CO3 on the surface of 800-K2CO3/ Na-nepheline is likely to be the catalytically active species for carbon black combustion as a (Na, K) mixed carbonate.

4. CONCLUSION In this study, we report the successful preparation of alkalicarbonate-supported aluminosilicate catalysts for diesel soot combustion. Na2CO3 stabilized on sodium- and potassiumcontaining nepheline ((Na, K)-nepheline), obtained from thermal treatment of K2CO3/Na-nepheline via solid ion exchange, has great water tolerance. Na2CO3 on the surface of thermally treated K2CO3/Na-nepheline is likely to be the catalytically active species for carbon black combustion as a (Na, K) mixed carbonate. Na2CO3 stabilized on (Na, K)-nepheline is recovered 14897

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The Journal of Physical Chemistry C by CO2, which is likely derived from catalytic action on carbon black combustion. We believe that these results will be helpful in elucidating the mechanisms of this catalysis in future work.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT The authors gratefully thank Mr. Mizutani from DENSO Corporation for fruitful discussions. R. K. also acknowledges the support by a Grant-in-Aid for JSPS Fellows. ’ REFERENCES (1) Heck, R. M.; Farrauto, R. J. Appl. Catal., A 2001, 221, 443–457. (2) Twigg, M. V. Appl. Catal., B 2007, 70, 2–15. (3) Maricq, M. M. J. Aerosol Sci. 2007, 38, 1079–1118. (4) Sydbom, A.; Blomberg, A.; Parnia, S.; Stenfors, N.; Sandstr€ om, T.; Dahlen, S.-E. Eur. Respir. J. 2001, 17, 733–746. (5) Fino, D.; Specchia, V. Powder Technol. 2008, 180, 64–78. (6) van Setten, B. A. A. L.; Makkee, M.; Moulijn, J. A. Catal. Rev.— Sci. Eng. 2001, 43, 489–564. (7) Walker, A. P. Top. Catal. 2004, 28, 165–170. (8) Koltsakis, G. C.; Stamatelos, A. M. Prog. Energy Combust. Sci. 1997, 23, 1–39. (9) G€orsmann, C. Mon. Chem. 2005, 136, 91–105. (10) Cooper, B. J.; Jung, H. J.; Thoss, J. E. U.S. Patent 4902487, 1990. (11) Neeft, J. P. A.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 1996, 8, 57–78. (12) Uchisawa, J. O.; Obuchi, A.; Zhao, Z.; Kushiyama, S. Appl. Catal., B 1998, 18, 183–187. (13) Liang, Q.; Wu, X.; Weng, D.; Xu, H. Catal. Today 2008, 139, 113–118. (14) Liu, J.; Zhao, Z.; Liang, P.; Xu, C.; Duan, A.; Jiang, G.; Lin, W.; Wachs, I. E. Catal. Lett. 2008, 120, 148–153. (15) Villani, K.; Brosius, R.; Martens, J. A. J. Catal. 2005, 236, 172–175. (16) Machida, M.; Murata, Y.; Kishikawa, K.; Zhang, D.; Ikeue, K. Chem. Mater. 2008, 20, 4489–4494. (17) Aneggi, E.; Llorcab, J.; de Leitenburga, C.; Dolcettia, G.; Trovarellia, A. Appl. Catal., B 2009, 91, 489–498. (18) Shimizu, K.; Kawachi, H.; Satsuma, A. Appl. Catal., B 2010, 96, 169–175. (19) Teraoka, Y.; Nakano, K.; Shangguan, W.; Kagawa, S. Catal. Today 1996, 27, 107–113. (20) Fino, D.; Russo, N.; Saracco, G.; Specchia, V. J. J. Catal. 2003, 217, 367–375. (21) Fino, D.; Russo, N.; Saracco, G.; Specchia, V. J. J. Catal. 2006, 242, 38–47. (22) Wu, X.; Radovic, L. R. Carbon 2005, 43, 333–344. (23) An, H.; McGinn, P. J. Appl. Catal., B 2006, 62, 46–56. (24) Zhang, Y.; Zou, X. Catal. Commun. 2007, 8, 760–764. (25) Castoldi, L.; Matarrese, R.; Lietti, L.; Forzatti, P. Appl. Catal., B 2009, 90, 278–285. (26) Neeft, J. P. A.; van Pruissen, O. P.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 1997, 12, 21–31. (27) Jelles, S. J.; van Setten, B.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 1999, 21, 35–49. (28) Shangguan, W. F.; Teraoka, Y.; Kagawa, S. Appl. Catal., B 1998, 16, 149–154. (29) Miro, E. E.; Ravelli, F.; Ulla, M. A.; Cornaglia, L. M.; Querini, C. A. Catal. Today 1999, 53, 631–638. (30) Aneggi, E.; de Leitenburg, C.; Dolcetti, G.; Trovarelli, A. Catal. Today 2008, 136, 3–10.

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(31) Jimenez, R.; Garcia, X.; Lopez, T.; Gordon, A. L. Fuel Process. Technol. 2008, 89, 1160–1168. (32) Hleisa, D.; Labakib, M.; Laversina, H.; Courcot, D.; Aboukaïsa, A. Colloids Surf., A 2008, 330, 193–200. (33) Zhang, Y.; Zhang, Y.; Xub, J.; Jing, C.; Zhang, F. Thermochim. Acta 2008, 468, 15–20. (34) Gross, M. S.; Ulla, M. A.; Querini, C. A. Appl. Catal., A 2009, 360, 81–88. (35) McKee, D. W. Fuel 1983, 62, 170–175. (36) Janiak, C.; Hoffmann, R.; Sjovall, P.; Kasemo, B. Langmuir 1993, 9, 3427–3440. (37) Saber, J. M.; Falconer, J. L.; Brown, L. F. Fuel 1986, 65, 1356–1359. (38) Jimenez, R.; Garcia, X.; Cellier, C.; Ruiz, P.; Gordon, A. L. Appl. Catal., A 2006, 297, 125–134. (39) An, H.; Su, C.; McGinn, P. J. Catal. Commun. 2009, 10, 509–512. (40) Zhang, Z.; Zhang, Y.; Wang, Z.; Gao, X. J. Catal. 2010, 271, 12–21. (41) Wang, Q.; Sohn, J. H.; Park, S. Y.; Choi, J. S.; Lee, J. Y.; Chung, J. S. J. Ind. Eng. Chem. 2010, 16, 68–73. (42) Ogura, M.; Morozumi, K.; Elangovan, S. P.; Tanada, H.; Ando, H.; Okubo, T. Appl. Catal., B 2008, 77, 294–299. (43) Kimura, R.; Wakabayashi, J.; Elangovan, S. P.; Ogura, M.; Okubo, T. J. Am. Chem. Soc. 2008, 130, 12844–12845. (44) Bates, J. B.; Brooker, M. H.; Quist, A. S.; Boyd, G. E. J. Phys. Chem. 1972, 76, 1565–1571. (45) Kosanovic, C.; Buljan, I.; Bosnar, S.; Subotic, B.; Novaktusar, N.; Ristic, A.; Gabrovsek, R.; Kaucic, V. Acta Chim. Slov. 2008, 55, 960–965. (46) Dimitrijevic, R.; Dondur, V.; Vulic, P.; Markovic, S.; Macura, S. J. Phys. Chem. Solids 2004, 65, 1623–1633. (47) Krishna, K.; Bueno-Lopez, A.; Makkee, M.; Moulijn, J. A. Appl. Catal., B 2007, 75, 189–200. (48) Smith, J. V.; Tuttle, O. F. Am. J. Sci. 1957, 255, 282–305. (49) Iordan, A.; Zaki, M. I.; Kappenstein, C. J. Chem. Soc., Faraday Trans. 1993, 89, 2527–2536.

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