TiO2 Washcoated Cordierite Minimonoliths for Hydrodechlorination

Ind. Eng. Chem. Res. 2007, 46, 7961-7969

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Pd/TiO2 Washcoated Cordierite Minimonoliths for Hydrodechlorination of Light Organochlorinated Compounds Carlos A. Gonza´ lez,† Alba N. Ardila,† C. Montes de Correa,*,† Miguel A. Martıńez,‡ and Gustavo Fuentes-Zurita‡ EnVironmental Catalysis Research Group, Sede de InVestigacio´ n UniVersitaria (SIU), UniVersidad de Antioquia, Cra53 No. 61-30 Torre 2-332/333 Medellıń, Colombia, and Departamento de Ingenierıá de Procesos e Hidra´ ulica, UAMsIztapalapa, A.P. 55-534, 09340 Me´ xico, D.F., Me´ xico

Pd over TiO2 (sol-gel and Hombikat uv-100) slurries with and without binders have been deposited on square-channel cordierite honeycombs, and vibration, heat, and abrasion resistance of washcoated minimonoliths have been evaluated. Activity tests were carried out in a microreactor operated at differential conditions. Fresh and used catalyst samples were characterized by elemental analysis, XRD, BET, H2-chemisorption, H2-TPR, NH3-TPD, UV-vis, and SEM in order to assess not only the properties but also the homogeneity of catalyst layers. The results show that washcoat loading and catalytic performance are affected by the kind of binder used in the slurry. Washcoat differences are also found regarding vibration, heat, and abrasion resistance. 1. Introduction The development of monolithic catalysts and/or reactors has been one of the major achievements in the field of heterogeneous catalysis and catalytic reaction engineering.1-10 For more than 30 years monolithic catalyst manufacturers have been successful in stationary and automotive catalyst applications, where gasphase detoxification must be fast with contact times of seconds, since high volumes of gas have to be treated.11-21 According to expectations, monoliths will have increasing applications in chemical and biochemical processes; in mass production of chemicals, fine chemicals, and specialty chemical products; in the treatment of fuel and flue gases; and in other multiphase processes.1,6,9,19,22,23 This paper focuses on the assessment of Pd/TiO2 washcoated minimonoliths for hydrodechlorination (HDC) of organochlorinated compounds. HDC over noble metals, such as Pd24-27 in a large variety of supports, Al2O3, TiO2, SiO2, carbon, and MgO, is of great economic potential due to the low reaction temperature required (generally below 250 °C) and the production of useful and/or harmless products.24 It must be taken into account that few studies of HDC have focused on the influence of structured materials in these reactions and their interactions with catalysts, especially under operating conditions of interest for industrial application. Chlorinated hydrocarbons (CHCs) are among the most hazardous organic compounds emitted from different sources including chemical plants, textiles, waste incinerators, and many other sources.28-30 Heterogeneous HDC has been reported for liquid-phase31-35 and gas-phase36-39 operations, involving aliphatic40,41 and aromatic reactants.41-44 There have been several attempts at kinetic modeling,45-47 but most have reported HDC activity and selectivity over a particular, often narrow, range of operating conditions. From the technical point of view monolithic catalysts have several advantages compared with catalysts in pellet form, such as faster mass and heat transfer in the thin catalyst

layer,48-50 minimum axial dispersion,51 high catalyst specific surface area, ease of scale-up,13,48 mechanical strength, lower pressure drop, and ease of orientation in the reactor.1,9,12,14,15,18,52 Monolithic structures also provide a tool for downscaling catalyst testing units, contributing to microreactor technology and combinatorial catalysis.13 This reduction in reactor size saves both cost and weight, which has the secondary benefit of having a smaller reactor to heat.11 Conventionally structured catalytic reactors are constituted by low surface area (∼0.5 m2/g) ceramic or metallic monoliths whose interior walls are covered by a porous coating (washcoating) on which the active metal is deposited.13,53 Once a powder catalyst has been developed at the laboratory level, i.e., it has adequate activity and selectivity for the abatement of a contaminant, the following step is to washcoat it on a ceramic monolith. The objective is to obtain a monolithic system with at least the same performance as the powder. Usually this is not an easy task, since there are several problems to be solved. The coated catalyst should be thermally and mechanically stable, and this property could be met with the help of a binder.11,54-57 The washcoat is usually applied on the honeycomb by impregnation in a suspension (slurry), containing finely ground catalyst powder, and subsequent drying and calcination (dip-coating or washcoating).55,58,59 The adhesion and aggregation of fine particles can occur during slurry preparation, depending on adhesive forces between particles causing uniformity changes by cluster formation, which must be analyzed.20,60,61 In the present study, Pd-loaded TiO2’s both prepared by the sol-gel method and from a commercial source (Hombikat) have been evaluated as coating materials on cordierite honeycomb minimonoliths.40,62 In this study, an attempt is made to assess both the effect of the coating method on cordierite minimonoliths using proposed resistance tests (with and without binders)9,20,63 and catalytic activity tests for HDC of dichloromethane and tetrachloroethylene. 2. Experimental Section

* To whom correspondence should be addressed. Tel.: +57+4 210 6605. Fax: +57+4 210 66 09. E-mail: [email protected]. † SIU. ‡ UAMsIztapalapa.

2.1. Catalyst Preparation. Minimonoliths (10 × 10 × 12 mm, 36 channels) were obtained from commercial cordierite straight-channel monoliths with 400 square cross section cells

10.1021/ie070713r CCC: $37.00 © 2007 American Chemical Society Published on Web 10/26/2007

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per square inch (cpsi). Cordierite minimonoliths were first treated in nitric acid (20% w/w) for 5 h and washed with water until neutral. Afterward, minimonoliths were introduced into acetone for 2 h, dried in an oven at 100 °C, and finally calcined under static conditions at 600 °C for 2 h. Washcoats of titania sol-gel, titania Hombikat uv-100, Pd/TiO2 (sol-gel), and Pd/ TiO2 (Hombikat uv-100) were prepared. Alumina sol (obtained from pseudoboehmite),64 titania sol (solutions at pH 9 and 3 obtained during TiO2 sol-gel synthesis),40 and commercial titania (Aldrich) were used as binders to washcoat minimonoliths in two ways: (a) minimonolith was first immersed in a slurry prepared with only one of the binders before its immersion in another slurry containing the active phase; (b) one of the binders was mixed with the active phase before minimonolith washcoating. Titania powder materials were wetness impregnated with palladium(II) acetylacetonate dissolved in acetone to obtain a 0.8 wt % palladium loading. Pd elemental analysis indicates that the actual palladium content is 0.78 wt %, which is close to the nominal value (0.8 wt %). Different slurries were prepared using a 2.3 ratio of deionized water and catalyst powder (with and without binder) and stirring at 800 rpm using a mixer (Ultra-turrax T 25). Wet mixing was continued by ball-milling the slurry during a period of 36 h at room temperature maintaining pH 4 with diluted nitric acid solution. Minimonoliths were washcoated as follows: (1) Minimonoliths were dipped into a slurry (25 wt % solids) for 20 s; (2) excess slurry was softly blown off with compressed air (5 psi) keeping the monoliths in a vertical position for 10 s and avoiding shaking of the minimonolith to keep washcoat uniformity; (3) samples were dried in a microwave oven to obtain a uniform Pd distribution, and then weighed. Steps 1-3 were repeated until a 15 wt % washcoat loading was achieved; then (4) washcoated minimonoliths were calcined in air at 2 °C/ min from room temperature to 400 °C for 2 h. 2.2. Resistance Tests. Minimonolith washcoat adhesion was assessed through different resistance tests. First, heated samples at 400 °C were subjected to ultrasonic vibration in an aqueous medium for 1 h in a Branson 3510 ultrasonic vibration cleaner. Then, samples were dried and weighed. In the second test, samples were heated in a tubular stainless steel reactor which could be handled through a mobile support in the inside of a tubular furnace kept at 500 °C. Minimonolith samples were exposed to high and low temperatures for periods of 10 min. This operation was repeated during 10 cycles to qualitatively determine thermal shock coating resistance. Finally, washcoat abrasion resistance was examined by flowing air at high velocity (>1000 mL/min) through minimonolith channels. The decrease in minimonolith weight after each test was taken as a measure of washcoat abrasion resistance. 2.3. Catalytic Tests. Experiments were carried out under differential conditions in a fixed bed quartz tubular reactor operating at atmospheric pressure (ratio of reactor diameter to catalyst particle >10). The reactor was placed inside an electric oven equipped with an automatic temperature controller, temperature range 120-220 °C. The reaction mixture consisted of 550 ppmv dichloromethane or tetrachloroethylene, 5500 ppmv H2, 5500 ppmv toluene, and N2 balance. The gas flow rate was maintained at 145 mL/min. K-type thermocouples were used for temperature measurements. The target flow rate and feed composition were obtained by means of electronic mass flow controllers (Brooks 5850 TR series). Minimonoliths were wrapped with fiberglass layers while powder catalyst samples were supported on a glass frit and a layer of fiberglass above the catalyst bed served as a preheating zone. All powder catalysts

were ground to a small particle size (0.18-0.25 mm) and mixed with silicon carbide (SiC). Prior to reactions, catalyst samples were reduced in flowing 5% H2/N2 (75 cm3/min), ramping the temperature from 20 to 300 °C (at 2 °C/min) and kept at 300 °C for 1 h. An FTIR gas analyzer (Temet) equipped with a 2 L cell, 240 cm optical step, and operated at 120 °C was used to monitor reactants and products. The reaction mixture was allowed to stabilize for about 8 h until steady-state conditions. Preliminary experiments indicated that no diffusional limitations were present within the range of operating conditions.40,65,66 2.4. Catalyst Characterization. Fresh and used catalyst samples (powder or washcoated minimonoliths) were characterized by several techniques. Minimonolith samples were ground and calcined in air before characterization. Crystalline phases were determined by X-ray diffraction (XRD) in a diffractometer Siemens D-500, using Cu KR radiation at 2θ ) 4-70°. Palladium loading was determined by atomic absorption spectroscopy using a Varian Spectra AA-20FS. Textural and morphological properties were analyzed by nitrogen adsorptiondesorption by BET analysis in a Micromeritics ASAP 2010. UV-vis experiments in the diffuse reflectance mode were carried out at room temperature in a Varian Cary 5E spectrometer with 1 nm resolution between 200 and 1200 nm. The spectra are displayed using the Kubelka-Munk F(R) function. Temperature-programmed reduction (H2-TPR), ammonia temperature programmed desorption (NH3-TPD), and chemisorption studies were carried out in a Micromeritics AutoChem II 2920 equipped with a thermal conductivity detector (TCD). TPR and TPD were taken without any pretreatment or heating. In exceptional cases heating under dry 4% O2/Ar at 200 °C for 1 h was necessary to remove undesired impurities from air. For TPR experiments, reduction in flowing 5% H2/Ar was conducted from -15 to 800 °C at 10 °C/min. For TPD experiments, catalyst samples were kept in a flow of NH3/He for 3 h. Then, they were heated to 800 °C in flowing He to remove physically adsorbed ammonia. Palladium dispersion by hydrogen chemisorption was determined at 100 °C. Catalyst samples were initially reduced in flowing 5% H2/Ar heating from room temperature to 300 °C at 2 °C/min and kept at this temperature for 1 h. Afterward, samples were cooled at 100 °C in flowing Ar, and finally, hydrogen pulses were introduced until saturation. The morphology of catalyst samples was examined by scanning electron microscopy (SEM) using a high-vacuum Carl Zeiss Model DSM-940A, with 30 kV acceleration and 6 nm resolution. Minimonolith samples were carefully cut with a microtome (Accutom-50 from Struers Organization). Parallel and perpendicular cuts to the minimonolith axis were performed to analyze both homogeneity and thickness of washcoat layers. Samples were glued to the sample holder with silver paint and covered with a thin gold layer to improve image resolution. 3. Results and Discussion 3.1. Minimonolith Washcoats. As illustrated in Figure 1, similar percent washcoat was obtained using different slurries. It is worth noting that although the rate of blowing air and suspension viscosity can affect loading, film thickness, and shape,13,67 high reproducibility with different slurries can be observed. Washcoated minimonolith samples were submitted to several resistance tests for different periods and weighed before and after treatment in order to simulate vibrations, high temperature, and flow during operation.55,68,69 It is important to point out that test conditions were more extreme than those employed in HDC reactions. Results are summarized in Tables 1 and 2. Weight losses increased after ultrasonic treatment,

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7963

Figure 1. Washcoat loading as a function of number of immersions. H, Hombikat; sg, sol-gel; A, Aldrich; s, sol. Table 1. TiO2 Washcoat Stability without Binders after Ultrasonic Vibration (uv), Thermal Shock (ts), and Abrasion (a)a

washcoat

wt losses uv (%)

wt losses after uv, ts, a (%)

wt diff (%)

TiO2 (H) without milling TiO2 (sg) without milling TiO2 (H) milled TiO2 (sg) milled

4.6 5.0 0.7 1.4

8.2 11.5 1.2 2.2

3.6 6.5 0.5 0.8

a

H, Hombikat; sg, sol-gel.

Table 2. Effect of Binders on TiO2 and Pd/TiO2 Washcoat Stability after Ultrasonic Vibration (uv), Thermal Shock (ts), and Abrasion (a)a

(binder)-washcoat

wt losses uv (%)

wt losses after uv, ts, a (%)

wt diff (%)

(Al2O3 s)-TiO2 (H)b (TiO2 A)-TiO2 (H)b (TiO2 sg-pH 9)-TiO2 (H)b (Al2O3 s)-TiO2 (H)c (TiO2 sg-pH 9)-TiO2 (H)c (TiO2 sg-pH 3)-TiO2 (H)c Pd/TiO2 (sg)d (Al2O3 s)-Pd/TiO2(sg)d Pd/TiO2 (H)d (Al2O3 s)-Pd/TiO2 (H)d

3.6 2.3 10.0 2.3 12.4 4.1 3.5 1.2 2.3 0.3

8.2 7.2 15.8 2.4 13.6 4.2 4 1.2 3.0 0.3

4.6 4.9 5.8 0.1 1.2 0.1 0.5 0.0 0.7 0.0

a H, Hombikat; s, sol; A, Aldrich; sg, sol-gel. b Minimonolith submerged in slurries prepared with binders of (Al2O3 s), (TiO2 A) or (TiO2 sg-pH 9) before being submerged in TiO2 (H) slurries without milling. c Minimonolith submerged in mixtures of slurries with binders and TiO2 (H) without milling. d Minimonolith submerged onto both slurries of milled Pd/TiO (Hombikat 2 or sol-gel) and milled mixtures of binders and Pd/TiO2 slurries.

where adherence of the samples can be weakened due to vibration and the washcoat may become less resistant to other test conditions.63 Initially the adherence of titania sol-gel, synthesized in our laboratory and sieved to an average diameter of 125 µm, was compared with that of a fine commercial titania, Hombikat (particle size, 20 nm), without any binder. As can be observed in Table 1, Hombikat washcoat exhibited lower weight loss after all resistance tests. However, milling allowed considerably better results. This confirms that washcoat adherence increases by decreasing the size of aggregates deposited on the minimonolith, improving interaction between the particles and facilitating a convenient filling of the surface cordierite macropores, producing a more effective anchorage.67 Several authors have found that the use of a binder is not essential for washcoating a small particle size powder.70 On other hand, Beers et al.71,72 obtained BEA zeolite clusters of 4-20 µm washcoated on cordierite monoliths, which seem to be somehow bigger than

cordierite macropores. However, good adhesion results were reported, probably because a binder and optional extra components such as surfactants were used.72 Agrafiotis et al.73 found that the combination of high solid content and fine particles can lead to high viscosity slurries. In this way, the influence of using a binder in fine washcoats of titania Hombikat without any milling operation was tested. In the present work we just used 5 wt % binder on the washcoat, keeping in mind that higher quantities might affect slurry viscosity (loss of adherence capacity) or blocking active sites.20 In most experiments (see Table 2) titania (H) washcoat stabilities were better than those reported for similar slurries (see Table 1) without either milling or binders. Binders of synthesized titania sol-gel at pH 9 showed higher material losses. Noda et al.74 reported amorphous TiO2 as an appropriate rutile-type TiO2 binder. In this work, similar results were obtained with anatase TiO2 samples, which can be associated with the smaller size of the amorphous binder; different properties were provided by the binder or a pH effect. For example, γ-alumina slurries with a natural pH are unstable and settled fast.54 At the same time, for values lower than the isoelectric point, the powder surface is positively charged, whereas the opposite happens for pHs higher than 7.7. Therefore, this situation could occur with binders containing particles of different charges, leading to different adhesion capabilities. Comparison between methods of adding the binder in the slurry showed that the lowest weight losses were obtained when Al2O3 sol and TiO2 sol (pH 3) were simultaneously mixed. It has been reported that better adhesion with Al2O3 sol is due to its acidity. As can be observed in Table 2, the use of Al2O3 sol leads to less than 3 wt % adhesion losses under more severe conditions than those employed during catalytic reactions. 3.2. Catalytic Tests. In blank tests, passage of each reactant through the empty reactor (in the absence of catalyst) did not result in any detectable conversion. As can be observed in Figure 2, dichloromethane (DCM) HDC over cordierite and titania (sol-gel or Hombikat) supported on cordierite minimonoliths led to DCM conversions lower than 0.4%. The catalytic HDC of DCM over both Pd/TiO2 powder and Pd/TiO2 minimonolith samples (see Figure 3) showed similar reaction rates, suggesting that the reactor operated in the kinetic control regime. Previously, diffusional limitations were assessed for powder catalysts by means of different kinetic criterions.40,65,66 It is important to notice that similar activity was reached on both powder and minimonolith catalyst samples under the same reaction conditions. This suggests accessibility of the reacting molecules to washcoat active sites and, consequently, poor contact due to

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Figure 5. Effect of temperature on HDC of CH2Cl2 of both CH2Cl2 and C2Cl4 over Pd/TiO2 minimonolith samples. Figure 2. Blank tests. HDC of CH2Cl2 over supports. 65 mg of washcoat, 550 ppm CH2Cl2, H2/CH2Cl2 ) 10. H, Hombikat; sg, sol-gel.

Figure 6. X-ray diffraction pattern of cordierite.

Figure 3. Reaction rate mmol/(g of catalyst min). HDC of CH2Cl2. 65 mg of Pd/TiO2 both powdered and washcoated, CH2Cl2 550 ppm, H2/CH2Cl2 ) 10. C6H5CH3/CH2Cl2 ) 10. f, fresh catalyst; w, catalyst washed with water, filtered and calcined at 400 °C; H, Hombikat; sg, sol-gel.

Figure 4. HDC of CH2Cl2 over Pd/TiO2 washcoated minimonoliths: Pd impregnated by method A (MA) and Pd/TiO2 (sol-gel or Hombikat) impregnated by method B (MB). sg, sol-gel; H, Hombikat.

dense layers63 appears to be overcome. In other reports the use of either monolith or powder brings different results at more severe conditions75,76 (high temperature, high washcoating loading, etc). However, our results lead us to anticipate the potentiality of Pd/TiO2 washcoated minimonoliths for HDC reactions. Taking into account that water could remove or change the active phase during washcoating, several powder Pd/TiO2 samples were washed with water, filtered, dried, and calcined under the same conditions explained in the Experimental Section. As can be observed in Figure 3, the reaction rate was not significantly affected. Selected Pd/TiO2 minimonoliths with and without Al2O3 sol binder were examined for HDC reactions. The influence of the palladium impregnation method on cordierite minimonoliths for dichloromethane HDC is shown in Figure 4. Comparing minimonolith samples in which palladium was impregnated on titania washcoats (method A) with minimonolith samples washcoated with Pd/TiO2 (method B) indicates that HDC

reaction rate values at 200 °C over Pd/TiO2 washcoated minimonoliths are about 4 times higher than those obtained over minimonolith samples washcoated using method A. Additionally, repeated reactions with different minimonolith samples prepared by method A (see Figure 4, I1 and I2) delivered poorly reproducible results while the reproducibility was higher than 95% using minimonolith samples prepared by method B. Regarding the use of either titania sol-gel or titania Hombikat (with and without binder) as Pd carriers in Pd/TiO2 washcoated minimonoliths (method B), we observed almost the same activity under different reaction conditions. Also, the amount of binder (5 wt %) did not have a significant influence on catalytic activity. The most active catalysts for DCM hydrodechlorination were tested for tetrachloroethylene HDC in order to check both catalytic results under similar experimental conditions and possible changes in their properties after being used. Figure 5 compares DCM and tetrachloroethylene (TTCE) HDC on Pd/ TiO2 minimonoliths (Hombikat with Al2O3 sol binder). TTCE shows a greater reaction rate than DCM (nearly 4 times), reaching a maximum of 4.2 × 10-3 mmol/(g of catalyst min) at 200 °C. This catalytic behavior is similar to that previously observed over both Pd/γ-Al2O3 and Pd/TiO2 powder catalysts under different reaction conditions.46,66,77 Higher reaction rates for aliphatic and olefinic compounds have been explained based on the bond energy between carbon and chlorine atoms77 as well as on the interaction between chlorine atoms and the catalytic surface.46 Therefore, the number of chlorine atoms (adsorption strength) and the presence of double bonds (olefins) increase catalytic performance. 3.3. Characterization Results. 3.3.1. Structural and Textural Characteristics. Figure 6 shows the XRD pattern of cordierite, and Figure 7 shows the XRD of powder samples of TiO2 sol-gel (pH 9), TiO2 Hombikat, 0.8 wt % Pd/TiO2, Hombikat, TiO2 sol-gel (pH 3). Both the original cordierite minimonolith and powdered titania show characteristic peaks for these materials.12,78 Peaks attributed to cordierite appear at 2θ ) 10.5, 18.30, 21.70, 26.4, 28.4, 29.6, 34.1, and 54.5°. TiO2 exhibited the anatase phase only; 2θ ) 27.51, 36.159, 41.3, and 54.46°.12 TiO2 (sol-gel) synthesized at pH 9 was much more crystalline than that synthesized at pH 3. Palladium phases


Figure 7. X-ray diffraction patterns of fresh powdered catalyst samples: (a) TiO2 sol-gel (pH 9); (b) TiO2 Hombikat; (c) 0.8 wt % Pd/TiO2 Hombikat; (d) TiO2 sol-gel (pH 3). Table 3. BET Surface Area, Pore Volume, and Average Pore Diameter of Different Catalyst Samplesa catalyst TiO2 (sg) Pd/TiO2 (sg) TiO2 (H) Pd/TiO2 (H) a

BET surf. area

(m2/g)

44 39 195 127

pore vol

(cm3/g)

0.15 0.10 0.52 0.37

pore diam (nm) 7.8-9.7 5.0-7.3 5.3-6.3 5.5-6.3

sg, sol-gel; H, Hombikat.

are not distinguished by XRD, most probably due to the small Pd loading (0.8 wt %), which was quite low for the sensitivity limits of this technique. XRD of Pd/TiO2 washcoated minimonoliths are not shown since cordierite peaks mask those of TiO2. Textural characteristics of fresh catalysts are illustrated in Table 3 and Figure 8. TiO2 Hombikat exhibited both larger surface area and pore volume before and after Pd impregnation than TiO2 synthesized by sol-gel method. The decrease of the surface area and pore volume of Pd-loaded samples is attributed to the incorporation of the active phase.79 From the t-plot and high resolution isotherms (not shown), no microporosity was detected in the range of relative pressures between 10-5 and 10-1, respectively.80 The observed pore size is in the mesopore range. Adsorption isotherms (not shown) were type IV, according to the IUPAC classification. 3.3.2. H2 Chemisorption. H2 chemisorption was used to determine metal particle size and dispersion. Particle size was determined as d (nm) ) 112/D (%), assuming spherical particles.24,81,82 The results are listed in Table 4. H2 chemisorption indicates average particle sizes of 2.3 and 6.3 nm, corresponding to a dispersion of 46.6 and 18.3 wt %, respectively. Metal dispersion was higher for catalysts prepared by the TiO2 sol-gel method, in spite of the lower surface area of this support. The support influence originates from its interaction with palladium species affecting oxidation state,83 dispersion,84 thermal stability,85 and other characteristics of palladium species, and consequently catalytic activity. During thermal treatments TiOx phases with Ti3+ species having high mobility might be formed.86,87 These species interact with Pd and can block the active surfaces. On the other hand, powdered Pd/TiO2 shows higher dispersion than minimonolith washcoated Pd/TiO2. This can be associated with drying operations.21,60 Vergunst et al.88 and Hepburn et al.89 have observed a macroscopic redistribution of the active phase after drying operations of Ni- and Rhimpregnated catalysts on alumina monoliths. The impregnation of Pd over minimonolith washcoated titania (method A) leads to samples of lower dispersion than those prepared by method B. This behavior can be related to both

Figure 8. Pore volume distribution of catalyst samples as a function of pore diameter. Table 4. Hydrogen Chemisorption of Pd Samples Supported over Titania in Both Powder and Minimonoliths catalyst 0.8% Pd/TiO2-H (powder)b 0.8% Pd/TiO2-H (minimonolith)b 0.8% Pd/TiO2-sg (powder)b 0.8% Pd/TiO2-sg (minimonolith)b 0.8% Pd/TiO2-sg (minimonolith)c 0.8% Pd/TiO2-sg (minimonolith used), HDC of dichloromethaneb

dispersion (%)

particle size (nm)

38.1 33.6 46.6 43.5 18.3 13.4

2.9 3.3 2.3 2.6 6.3 8.9

a H, Hombikat, sg, sol-gel. b Palladium impregnated over titania powder before preparing the slurry. c Palladium impregnated over titania washcoat/ minimonolith.

uniformity of the active phase and amount of supported Pd. The precursor salt palladium(II) acetylacetonate dissolved in acetone needed more time to be deposited by method A. It has been discussed that the solubility of the precursor must be sufficiently high for the required metal loading. If the solubility is insufficient, the best way is to repeat this procedure a number of times.20 Also, it should be taken into account that the active phase does not interact as easily with minimonolith washcoated titania as with powder titania, where the support was stirred for a long time in the precursor solution. Therefore, in agreement with these observations, perhaps a way to obtain the best results with method A is to use multi-impregnations to obtain the desired Pd loading. Another recommendation20 is to use a vessel as large as the monolith itself to prevent liquid outside the monoliths causing an excess of metal loading at the external surface of the monolith. On the other hand, the unique distribution of material in the monolith demands that special precautions must be taken to avoid heterogeneity problems, which is especially important when dealing with monoliths of high cell density where the surface tension of the impregnating solution can cause difficulties in its distribution within the monolith channels.13,60 In the case of used catalysts (prepared by method B), decreased dispersion and increased particle size are attributed to active phase agglomeration by migration due to HCl produced during the HDC reaction, which can cause some mobility of Pd over the support.37,90 3.3.3. Temperature-Programmed Desorption of Ammonia, NH3-TPD. NH3-TPD profiles of fresh and used catalyst samples are presented in Figure 9. Fresh catalysts show similar acidity, i.e., desorption peaks in the same regions. In all cases, the introduction of Pd leads to an increase of the total amount of desorbed ammonia compared to the single, less intense NH3 desorption peak from TiO2 support. Additionally, TPD of a used catalyst shows a desorption peak at temperatures higher than 700 °C, which is attributed to HCl adsorbed on the support.91

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Figure 12. UV-visible spectra: (a) TiO2 only, (b) TiO2 washcoated minimonolith, and (c) Pd/TiO2 Hombikat washcoated minimonlith.

Figure 9. NH3-TPD profiles of Pd-loaded titania catalysts. H, Hombikat; sg, sol-gel.

Figure 10. TPR profiles of cordierite, titania sol-gel, and Pd/TiO2 catalysts. sg, sol-gel.

Figure 11. TPR profiles of titania Hombikat and Pd/TiO2 Hombikat (H).

3.3.4. H2-TPR. The reducibility of fresh and used catalysts was studied by temperature-programmed reduction. TPR profiles of selected catalyst samples are shown in Figures 10 and 11. Cordierite did not exhibit any reduction peak, while titania solgel and Hombikat underwent partial reduction, showing a peak at about 530 °C. H2 consumption was higher on Hombikat support.40,92-94 The low temperature H2 consumption peaks (at 30 °C) observed for Pd/TiO2 samples have been attributed to reduction of superficial PdO to metallic Pd,95,96 while peaks around 350 °C are associated with reduction of surface capped oxygen of TiO2.97,98 These results suggest that the presence of palladium facilitates the reduction of oxygen species on TiO2 lattice by the spillover effect.86 Therefore, the reduction temperature can greatly affect catalyst activity. H2-TPR of Pd/TiO2 shows that reduction temperatures higher than 350 °C may lead

to strong metal-support interaction, which might be an important factor for the high activity of Pd/TiO2. This possibility was observed in part during the relevant increase in H2 consumption over Pd/TiO2 (Hombikat) compared to Pd/TiO2 (sol-gel) and seems to indicate some support contribution to the reduction behavior. Li et al.86 have explained that metal particles can be blocked by the mobile TiOx phase formed due to the facile reduction of TiO2 in the presence of metal and hydrogen, which affects hydrogen chemisorption. Consequently, different movements of these phases affect H2 consumption on both TiO2 support types, as was observed in chemisorption results. No significant changes are observed in TPR profiles of fresh and used samples of Pd/TiO2 sol-gel. This suggests that catalytic species kept their reducibility properties after reaction, even though they suffered redistribution as observed by chemisorption. Besides, it is possible that the acidity change of the support observed by TPD did not modify the active phase. 3.3.5. UV-Vis. Figure 12 shows UV-vis diffuse reflectance spectra of TiO2 only, TiO2 washcoated minimonolith, and Pd/ TiO2 Hombikat washcoated minimonolith. These spectra are dominated by the strong adsorption edge (between 200 and 400 nm) characteristic of semiconductor materials like TiO2.81,99 However, Pd/TiO2 exhibits a less intense band than TiO2 in the same region. Therefore, palladium decreased the absorption capacity of TiO2, indicating that impregnated palladium covered a significant surface area of titania. Bands corresponding to palladium species were not distinguished in the UV-vis spectra. Bands in the 800-1200 and 1840-2000 nm regions are attributed to cordierite components. 3.3.6. SEM. Sample characterization by SEM allowed us to assess the homogeneity of catalyst coating on channel walls, while cross sections of channel walls were useful to determine the layer thickness. All images show that the surface of cordierite is covered with a TiO2 film (Figure 13). In Figure 13a, the interface between cordierite minimonolith and TiO2 can be observed. However, it can be seen that the texture of TiO2 is different from that of cordierite. The typical layered structure of cordierite is no longer visible, neither in the channels nor in the pores inside the channel walls, indicating a complete minimonolith coverage (Figure 13b). In parts b-d of Figure 13 multilayers of spherical crystals of palladium are observed and uncovered regions in the minimonolith are not observed. The thickness of TiO2 layers is between 80 and 90 nm. This is probably due to the irregular surface of the original cordierite support itself and/or TiO2 accumulation in some areas.20 SEM observations of perpendicular cuts conducted at several distances from the top of the minimonolith did not show significant differences between the TiO2 washcoat layer in the center of the minimonolith and in the channel open ends, indicating slurry access to the whole channel of the minimonolith. Figure 13e shows the TiO2 layer on the walls of the minimonolith channels.


Vibrations of high intensity and duration did not damage the Pd/TiO2 washcoat. Acknowledgment The authors are grateful to Colciencias and Universidad de Antioquia for financial support through the project Cod 111505-12426. Literature Cited

Figure 13. SEM micrographs: (a) transversal cut of Pd/TiO2 washcoated minimonolith, (b) internal walls of Pd/TiO2 washcoated minimonolith, (c) external walls of Pd/TiO2 washcoated minimonolith, (d) palladium clusters, (e) longitudinal cut of Pd/TiO2 washcoated minimonolith, and (f) corners of Pd/TiO2 washcoated minimonolith.

It is worth mentioning that SEM micrographs show that the structures of the fresh and used catalysts were not modified after resistance tests and hydrodechlorination reactions. Also, the presence of binders did not affect the washcoat structure.57 4. Conclusions Pd/TiO2 catalysts supported on cordierite minimonoliths are mechanically stable and catalytically active for dichloromethane and tetrachloroethylene HDC. Tetrachloroethylene is more easily dechlorinated than dichloromethane under the same reaction conditions. The preparation method (Pd impregnation and washcoating) significantly influences both catalytic activity and reproducibility. Therefore, under the conditions of this study, Pd/TiO2 washcoated minimonoliths give better HDC performance than minimonolith samples where palladium was impregnated over washcoated titania. Resistance tests showed that washcoats prepared with fine particles (submitted to milling operations) are more tightly supported on cordierite honeycomb and mechanical stability does not depend on the TiO2 type. The incorporation of Al2O3 sol binders into slurries of Pd/TiO2 increases adhesion properties (weight loss less than 2 wt %). Also, TiO2 sol is another good option as a binder. In general, major weight losses during ultrasonic vibration indicated that washcoat losses are mainly produced by erosion. H2-TPR and H2-chemisorption suggest Pd interaction with TiO2 prepared either by the sol-gel method or obtained from a commercial material (Hombikat). This influences both Pd dispersion and support reduction. However, similar activities were exhibited by these catalysts. SEM results indicate excellent adherence between deposited catalyst layers and ceramic support with and without binder.

(1) Tomasˇic´, V. Application of the Monoliths in DeNOx Catalysis. Catal. Today 2007, 119, 106. (2) Saracco, G.; Specchia, V.; Cybulski, A.; Moulijn, J. A. Structured Catalysts and Reactors; Marcel Dekker, Inc.: New York: 1998; p 417. (3) Sua´rez, S.; Yates, M.; A Ä vila, P.; Blanco, J. New TiO2 monolithic supports based on the improvement of the porosity. Catal. Today 2005, 105, 499. (4) Groppi, G.; Tronconi, E.; Honeycomb supports with high thermal conductivity for gas/solid chemical processes. Catal. Today 2005, 105, 297. (5) Liu, H.; Zhao, J.; Li, C.; Ji., S. Conceptual design and CFD simulation of a novel metal-based monolith reactor with enhanced mass transfer. Catal. Today 2005, 105, 401. (6) Boger, T.; Heibel, A. K.; Sorensen, C. M. Monolithic Catalysts for the Chemical Industry. Ind. Eng. Chem. Res. 2004, 43, 4602. (7) Frauhammer, F.; Eigenberger, G.; Hippel, L. V.; Arntz, D. A new reactor concept for endothermic high-temperature reactions. Chem. Eng. Sci. 1999, 54, 3661. (8) Heck, R. M. The application of monoliths for gas phase catalytic reactions. Chem. Eng. J. 2001, 82, 149. (9) William, J. L. Monolith structures, materials, properties and uses. Catal. Today 2001, 69, 3. (10) Pe´rez-Cadenas, A. F.; Zieverink, M. P.; Kapteijn, F.; Moulijn, J. A. High performance monolithic catalysts for hydrogenation reactions. Catal. Today 2005, 105, 623. (11) Giroux, T.; Hwang, S.; Liu, Y.; Ruettinger, W.; Shore, L.; Monolithic structures as alternatives to particulate catalysts for the reforming of hydrocarbons for hydrogen generation. Appl. Catal., B: EnViron. 2005, 56, 95. (12) A Ä vila, P.; Sańchez, B.; Cardona, A. I.; Rebollar, M.; Candal, R.; Influence of the methods of TiO2 incorporation in monolithic catalysts for the photocatalytic destruction of chlorinated hydrocarbons in gas phase. Catal. Today 2002, 76, 271. (13) Kapteijn, F.; Nijhuis, T. A.; Heiszwolf, J. J.; Moulijn, J. A. New non-traditional multiphase catalytic reactors based on monolithic structures. Catal. Today 2001, 66, 133. (14) Yashnik, S. A.; Ismagilov, Z. R.; Koptyug, I. V.; Andrievskaya, I. P.; Matveev, A. A.; Moulijn, J. A. Formation of textural and mechanical properties of extruded ceramic honeycomb monoliths: An 1H NMR imaging study. Catal. Today 2005, 105, 507. (15) Cybulski, A.; Moulijn, J. A. Monoliths in Heterogeneous Catalysis. Catal. ReV.sSci. Eng. 1994, 36, 179. (16) Kapteijn, F.; Heiszwolf, J.; Nijhuis, T.; Moulijn, J. Monoliths in multiphase catalytic processes: aspects and prospects. CaTTech 1999, 3, 24. (17) Koptyug, I. V.; Fenelonov, V. B.; Khitrina, Y. L.; Sagdeev, R. Z.; Parmon, V. N. In Situ NMR Imaging Studies of the Drying Kinetics of Porous Catalyst Support Pellets. J. Phys. Chem. B 1998, 102, 3090. (18) Tomasˇic´, V.; Zrncˇevic´, S.; Gomzi, Z. Direct decomposition of NO in a monolith reactor: comparison of mathematical models. Catal. Today 2004, 90, 77. (19) Bakker, J. J.; Kreutzer, M. T.; Lathouder, K.; Kapteijn, F.; Moulijn, J. A.; Wallin, S. A. Hydrodynamic properties of a novel ‘open wall’ monolith reactor. Catal. Today 2005, 105, 385. (20) Nijhuis, T. A.; Beers, A. E.; Vergunst, T.; Hoek, I.; Kapteijn, F.; Moulijn, J. A. Preparation of monolithic catalysts. Catal. ReV. 2001, 43, 345. (21) Vergunst, T.; Linders, M.; Kapteijn, F.; Moulijn, J. A. CarbonBased monolithic structures. Catal. ReV.sSci. Eng. 2001, 43, 291. (22) Roy, S.; Bauer, T.; Al-Dahhan, M.; Lehner, P.; Turek, T. Reactors, kinetics, and catalysis. Monoliths as multiphase reactors: a review. AIChE J. 2004, 50, 2918. (23) Tomasˇic´, V.; Jovic´, F. State-of-the-art in the monolithic catalysts/ reactors. Appl. Catal., A: Gen. 2006, 311, 112. (24) Figueira de Souza, A. G.; Bentes, A. M. P.; Rodrigues, A. C. C.; Borges, L. E. P.; Monteiro, J. L. F. Hydrodechlorination of carbon

7968


tetrachloride over PtNaX zeolite: Deactivation studies. Catal. Today 2005, 493, 107-108. (25) Yuan, G.; Keane, M. A. Liquid phase hydrodechlorination of chlorophenols over Pd/C and Pd/Al2O3: a consideration of HCl/catalyst interactions and solution pH effects. Appl. Catal., B: EnViron. 2004, 52, 301. (26) Golubina, E. V.; Lokteva, E. S.; Luni, V. V.; Turakulova, A. O.; Simagina, V. I.; Stoyanova, I. V. Modification of the supported palladium catalysts surface during hydrodechlorination of carbon tetrachloride. Appl. Catal., A: Gen. 2003, 241, 123. (27) Go´mez-Sainero, L.; Seoane, X. L.; Arcoya, A. Hydrodechlorination of carbon tetrachloride in the liquid phase on a Pd/carbon catalyst: kinetic and mechanistic studies. Appl. Catal., B: EnViron. 2004, 53, 101. (28) Shafiei, M.; Richardson, J. T. Dechlorination of chlorinated hydrocarbons by catalytic steam reforming. Appl. Catal., B: EnViron. 2004, 54, 251. (29) Khaleel, A.; Catalytic activity of mesoporous alumina for the hydrolysis and dechlorination of carbon tetrachloride. Microporous Mesoporous Mater. 2006, 91, 53. (30) Jujjuri, S.; Ding, E.; Hommel, E. L; Shore, S. G.; Keane, M. A. Synthesis and characterization of novel silica-supported Pd/Yb bimetallic catalysts: Application in gas-phase hydrodechlorination and hydrogenation. J. Catal. 2006, 239, 486. (31) Zinovyev, S.; Perosa, A.; Yufit, S.; Tundo, P. Hydrodechlorination and Hydrogenation over Raney-Ni under Multiphase Conditions: Role of Multiphase Environment in Reaction Kinetics and Selectivity. J. Catal. 2002, 211, 347. (32) Murena, F.; Gioia, F. Catalytic hydrodechlorination of decachlorobiphenyl. Appl. Catal., B: EnViron. 2002, 38, 39. (33) Aikawa, B.; Burk, R. C.; Sithole´, B. B. Catalytic dechlorination of 1-chlorooctadecane in supercritical carbon dioxide. Appl. Catal., B: EnViron. 2001, 32, 269. (34) Shindler, Y.; Matatov-Meytal, Y.; Sheintuch, M. Wet Hydrodechlorination of p-Chlorophenol Using Pd Supported on an Activated Carbon Cloth. Ind. Eng. Chem. Res. 2001, 40, 3301. (35) Tundo, P.; Zinovyev, S.; Perosa, A. Multiphase Catalytic Hydrogenation of p-Chloroacetophenone and Acetophenone. A Kinetic Study of the Reaction Selectivity toward the Reduction of Different Functional Groups. J. Catal. 2000, 196, 330. (36) Ordoń˜ez, S.; Dıéz, F. V.; Sastre, H. Catalytic Hydrodechlorination of Chlorinated Olefins over a Pd/Al2O3 Catalyst: Kinetics and Inhibition Phenomena. Ind. Eng. Chem. Res. 2002, 41, 505. (37) Heinrichs, B.; Schoebrechts, J. P.; Pirard, J. P. Multiphase Catalytic Hydrogenation of p-Chloroacetophenone and Acetophenone. A Kinetic Study of the Reaction Selectivity toward the Reduction of Different Functional Groups. J. Catal. 2001, 200, 309. (38) Rioux, R. M.; Thompson, C. D.; Chen, N.; Ribeiro, F. H. Hydrodechlorination of chlorofluorocarbons CF3-CFCl2 and CF3-CCl3 over Pd/carbon and Pd black catalysts. Catal. Today 2000, 62, 269. (39) Thompson, C. D.; Rioux, R. M.; Chen, N.; Ribeiro, F. H. Turnover Rate, Reaction Order, and Elementary Steps for the Hydrodechlorination of Chlorofluorocarbon Compounds on Palladium Catalysts. J. Phys. Chem. B 2000, 104, 3067. (40) Gonza´lez, C. A.; Bustamante, F.; Montes de Correa, C. Hidrodecloracioń catalı´tica de diclorometano, cloroformo y tetracloroetileno. ReV. Fac. Ing. 2006, 38, 73. (41) Keane, M. A.; Pina, G.; Tavoularis, G. The catalytic hydrodechlorination of mono, di- and trichlorobenzenes over supported Nickel. Appl. Catal., B: EnViron. 2004, 48, 275. (42) Keane, M. A, Murzin, D. Y. Kinetic treatment of the gas phase hydrodechlorination of chlorobenzene over nickel/silica: beyond conventional kinetics. Chem. Eng. Sci. 2001, 56, 3185. (43) Gopinath, R.; Lingaiah, N.; Sreedhar, B, Suryanarayana, I.; Prasad, P. P. S.; Obuchi, A. Highly stable Pd/CeO2 catalyst for hydrodechlorination of chlorobenzene. Appl. Catal., B: EnViron. 2003, 46, 587. (44) Lingaiah, N.; Prasad, P. S. S.; Rao, P. K.; Smart, L. E.; Berry, F. J. Studies on magnesia supported mono- and bimetallic Pd-Fe catalysts prepared by microwave irradiation method. Appl. Catal., A: Gen. 2001, 213, 189. (45) Murena, F. Catalytic hydroprocessing of chlorobenzene: the effect of thiophene. J. Hazard. Mater. 2000, 75, 49. (46) Lo´pez, E.; Ordoń˜ez, S.; Sastre, H.; Dıéz, F. V. Kinetic study of the gas-phase hydrogenation of aromatic and aliphatic organochlorinated compounds using a Pd/Al2O3 catalyst. J. Hazard. Mater. 2003, 97, 281. (47) Menini, C.; Park, C.; Shin, E. J.; Tavoularis, G.; Keane, M. A. Catalytic hydrodehalogenation as a detoxification methodology. Catal. Today 2000, 62, 355.

(48) Kreutzer, M. T.; Du, P.; Heiszwolf, J. J.; Kapteijn, F.; Moulijn, F. Mass transfer characteristics of three-phase monolith reactors. Chem. Eng. Sci. 2001, 56, 6015. (49) Vandu, C. O.; Ellenberger, J.; Krishna, R. Hydrodynamics and mass transfer in an upflow monolith loop reactor. Chem. Eng. Process. 2005, 44, 363. (50) Van Baten, J. M.; Krishna, R. CFD simulations of mass transfer from Taylor bubbles rising in circular capillaries. Chem. Eng. Sci. 2004, 59, 2535. (51) Winterbottom, J. M.; Marwan, H.; Stitt, E. H.; Natividad, R. The palladium catalysed hydrogenation of 2-butyne-1,4-diol in a monolith bubble column reactor. Catal. Today 2003, 79, 391. (52) Heiszwolf, J. J.; Engelvaart, L. B.; Van den Eijnden, M. G.; Kreutzer, M. T.; Kapteijn, F.; Moulijn, J. A. Hydrodynamic aspects of the monolith loop reactor. Chem. Eng. Sci. 2001, 56, 805. (53) Reymond, J. P. Structured supports for noble catalytic metals: stainless steel fabrics and foils, and carbon fabrics. Catal. Today 2001, 69, 343. (54) Jiang, P.; Lu, G.; Guo, Y.; Zhang, S.; Wang, X. Preparation and properties of a γ-Al2O3 washcoat deposited on a ceramic honeycomb. Surf. Coat. Technol. 2005, 190, 314. (55) Biamino, S.; Fino, P.; Fino, D.; Russo, N.; Badini, C. Catalyzed traps for diesel soot abatement: In situ processing and deposition of perovskite catalyst. Appl. Catal., B: EnViron. 2005, 61, 297. (56) Agrafiotis, C.; Tsetsekou, A. Deposition of meso-porous γ-alumina coatings on ceramic honeycombs by sol-gel methods. J. Eur. Ceram. Soc. 2002, 22, 423. (57) Agrafiotis, C.; Tsetsekou, A. The effect of powder characteristics on washcoat quality. Part I: Alumina washcoats. J. Eur. Ceram. Soc. 2000, 20, 815. (58) Valentini, M.; Groppi, G.; Cristiani, C.; Levi, M.; Tronconi, E.; Forzatti, P. The deposition of γ-Al2O3 layers on ceramic and metallic supports for the preparation of structured catalysts. Catal. Today 2001, 69, 307. (59) Musialik-Piotrowska, A.; Syczewska, K. Catalytic oxidation of trichloroethylene in two-component mixtures with selected volatile organic compounds. Catal. Today 2002, 73, 333. (60) Avila, P.; Montes, M.; Miro´, E. E. Monolithic reactors for environmental applications: A review on preparation technologies. Chem. Eng. J. 2005, 109, 11. (61) Bueno-Lo´pez, A.; Lozano-Castello´, D.; Such-Basań˜ez, I.; GarcıáCorte´s, J. M.; Illań-Go´mez, M. J.; Salinas-Martıńez de Lecea, C. Preparation of beta-coated cordierite honeycomb monoliths by in situ synthesis: Utilisation as Pt support for NOx abatement in diesel exhaust. Appl. Catal., B: EnViron. 2005, 58, 1. (62) Aristiza´bal, B.; Gonza´lez, C. A.; Barrio, I.; Montes, M.; Montes, C. Screening of Pd and Ni supported on sol-gel derived oxides for dichloromethane hydrodechlorination. J. Mol. Catal. A: Chem. 2004, 222, 189. (63) Zamaro, J. M.; Ulla, M.; Miro´, E. Zeolite washcoating onto cordierite honeycomb reactors for environmental applications. Chem. Eng. J. 2005, 106, 25. (64) Bustamante, F. Desempenõ de sistemas catalı´ticos bimeta´licos en la reduccioń de NOx en condiciones hu´medas. Master’s Thesis, UdeA, 2000; p 33. (65) Santamarıá, D. C. Efecto de las condiciones de pretratamiento de los catalizadores de paladio soportados en alu´mina sol-gel en la actividad de para la hidrodecloracioń catalı´tica de diclorometano gaseoso. Thesis, UdeA, 2006; p 45. (66) Gonza´lez, C. A. Hidrodecloracioń catalı´tica de diclorometano en presencia de trazas de cloroformo y/o tetracloroetilenosEstudio cine´tico. Master’s Thesis, UdeA, 2006; p 90. (67) Boix, A. V.; Miro´, E. E.; Lombardo, E. A.; Mariscal, R.; Fierro, J. L. G. Binder effect upon the catalytic behavior of PtCoZSM5 washcoated on cordierite monoliths. Appl. Catal., A: Gen. 2004, 276, 197. (68) Wu, X.; Weng, D.; Xu, L.; Li, H. Structure and performance of γ-alumina washcoat deposited by plasma spraying. Surf. Coat. Technol. 2001, 145, 226. (69) Zamaro, J. M.; Ulla, M. A.; Miro´, E. E. The effect of different slurry compositions and solvents upon the properties of ZSM5-washcoated cordierite honeycombs for the SCR of NOx with methane. Catal. Today 2005, 107-108, 86. (70) Agrafiotis, C.; Tsetsekou, A.; Ekonomakou, A. Effect of particle size on the adhesion properties of oxide washcoats on cordierite honeycombs. J. Mater. Sci. Lett. 1999, 18, 1421. (71) Beers, A. E. W.; Nijhuis, T. A.; Aalders, N.; Kapteijn, F.; Moulijn, J. A. BEA coating of structured supportssperformance in acylation. Appl. Catal., A: Gen. 2003, 243, 237.

Ind. Eng. Chem. Res., Vol. 46, No. 24, 2007 7969 (72) Beers, A. E. W.; Nijhuis, T. A.; Kapteijn, F.; Moulijn, J. A. Zeolite coated structures for the acylation of aromatics. Microporous Mesoporous Mater. 2001, 48, 279. (73) Agrafiotis, C.; Tsetsekou, A. The effect of processing parameters on ceramic honeycombs. J. Mater. Sci. 2000, 35, 951. (74) Noda, N.; Suzuki, J. Catalyst carrier and catalyst body. U.S. Patent 0261127A, 2005. (75) Zamaro, J. M.; Ulla, M.; Miro´, E. Improvement in the catalytic performance of In-mordenite through preferential growth on metallic monoliths. Appl. Catal., A: Gen. 2006, 308, 161. (76) Irandoust, S.; Andersson, B. Monolithic catalysts for nonautomobile applications. Catal. ReV.sSci. Eng. 1988, 30, 341. (77) Chen, N.; Rioux, R.; Ribeiro, F. H. Investigation of Reaction Steps for the Hydrodechlorination of Chlorine-Containing Organic Compounds on Pd Catalysts. J. Catal. 2002, 211, 192. (78) A Ä vila, P.; Bahamonde, A.; Blanco, J.; Sańchez, B.; Cardona, A.; Romero, M. Gas-phase photo-assisted mineralization of volatile organic compounds by monolithic titania catalysts. Appl. Catal., B: EnViron. 1998, 17, 75. (79) Romero, A. R.; Ramı´rez, J.; Cedenõ, L. Estudios de la activacioń y caracterizacioń de catalizadores de Mo soportados en alu´mina modificada con Nb. ReV. Mex. Ing. Quı´m. 2003, 2, 75. (80) Storck, S.; Bretinger, H.; Maier, W. F. Characterization of microand mesoporous solids by physisorption methods and pore-size analysis. Appl. Catal., A: Gen. 1998, 174, 137. (81) Belver, C.; Lo´pez-Munõz, M. J.; Coronado, J. M.; Soria, J. Palladium enhanced resistance to deactivation of titanium dioxide during the photocatalytic oxidation of toluene vapors. Appl. Catal., B. EnViron. 1996, 46, 497. (82) Anderson, J. R.; Pratt, K. C. Introduction to Characterization and Testing of Catalysts; Academic Press: Sydney, 1985. (83) Okumura, K.; Matsumoto, S.; Nishiaki, N.; Niwa, M. Support effect of zeolite on the methane combustion activity of palladium. Appl. Catal., B: EnViron. 2003, 40, 151. (84) Widjaja, H.; Sekizawa, K.; Eguchi, K.; Arai, H. Oxidation of methane over Pd/mixed oxides for catalytic combustion. Catal. Today 1999, 47, 95. (85) Ciuparu, D.; Pfefferle, L. Support and water effects on palladium based methane combustion catalysts. Appl. Catal., A: Gen. 2001, 209, 415. (86) Li, Y.; Fan, Y.; Yang, H.; Xu, B.; Feng, L.; Yang, M.; Chen, Y. Strong metal-support interaction and catalytic properties of anatase and rutile supported palladium catalyst Pd/TiO2. Chem. Phys. Lett. 2003, 372, 160. (87) Gopinath, R.; Lingaiah, N.; Babu, N. S.; Suryanarayana, I:, Prasad, P. S.; Obuchi, A. A highly active low Pd content catalyst synthesized by deposition-precipitation method for hydrodechlorination of chlorobenzene. J. Mol. Catal. A: Chem. 2004, 223, 289. (88) Vergunst, T.; Kapteijn, F.; Moulijn, J. A. Monolithic catalystss non-uniform active phase distribution by impregnation. Appl. Catal., A 2001, 213, 179.

(89) Hepburn, J. S.; Stenger, H. G., Jr. In situ XPS investigations of Cu1-xNixZnAl-mixed metal oxide catalysts used in the oxidative steam reforming of bio-ethanol. Appl. Catal., B: EnViron. 1989, 55, 287. (90) Heinrichs, B.; Noville, F.; Schoebrechts, J. P.; Pirard, J. P. Palladium-silver sol-gel catalysts for selective hydrodechlorination of 1,2dichloroethane into ethylene: IV. Deactivation mechanism and regeneration. J. Catal. 2003, 220, 215. (91) Lo´pez, E.; Ordoń˜ez, S.; Dıéz, F. V. Deactivation of a Pd/Al2O3 catalyst used in hydrodechlorination reactions: Influence of the nature of organochlorinated compound and hydrogen chloride. Appl. Catal., B: EnViron. 2006, 62, 57. (92) Peng, J.; Wang, S. Performance and characterization of supported metal catalysts for complete oxidation of formaldehyde at low temperatures. Appl. Catal., B: EnViron. 2007, 73, 282. (93) Caceres, C. V.; Fierro, J. L. G.; Lazaro, J.; Agudo, A. L.; Soria, J. Effect of support on the surface characteristics of supported molybdena catalysts. J. Catal. 1990, 122, 113. (94) Maity, S. K.; Rana, M. S.; Bej, S. K.; Ancheyta-Jua´rez, J.; Murali, Dhar, G.; Prasada Rao, T. S. R. Studies on physico-chemical characterization and catalysis on high surface area titania supported molybdenum hydrotreating catalysts. Appl. Catal., A: Gen. 2001, 205, 215. (95) Su, S. C.; Carstens, J. N.; Bell, A. T. A Study of the Dynamics of Pd Oxidation and PdO Reduction by H2 and CH4. J. Catal. 1998, 176, 125. (96) Lin, W.; Zhu, Y. X.; Wu, Z. N.; Xie, Y. C.; Murwani, I.; Kemnitz, E. Total oxidation of methane at low temperature over Pd/TiO2/Al2O3: effects of the support and residual chlorine ions. Appl. Catal., B: EnViron. 2004, 50, 59. (97) Epling, W. S.; Cheekatamarla, P. K.; Lane, A. M.; Reaction and surface characterization studies of titania-supported Co, Pt and Co/Pt catalysts for the selective oxidation of CO in H2-containing streams. Chem. Eng. J. 2003, 93, 61. (98) Pe´rez-Hernańdez, R.; Go´mez-Corte´s, A.; Arenas-Alatorre, J.; Rojas, S.; Mariscal, R.; Fierro, J. L. G.; Dıáz, G. SCR of NO by CH4 on Pt/ZrO2TiO2 sol-gel catalysts. Catal. Today 2005, 107-108, 149. (99) Pestryakov, A. N.; Lunin, V. V.; Fuentes, S.; Bogdanchikova, N.; Barrera, A. Influence of modifying additives on the electronic state of supported palladium. Chem. Phys. Lett. 2003, 367, 102.

ReceiVed for reView May 18, 2007 ReVised manuscript receiVed August 28, 2007 Accepted August 30, 2007 IE070713R

TiO2 Washcoated Cordierite Minimonoliths for Hydrodechlorination

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