Al2O3-Coated Wire-Mesh Honeycombs for

Development of Al/Al2O3-Coated Wire-Mesh Honeycombs for Catalytic .... and corrugated (triangle shaped with side lengths of approximately 6 mm) ones...
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Ind. Eng. Chem. Res. 2004, 43, 907-912

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Development of Al/Al2O3-Coated Wire-Mesh Honeycombs for Catalytic Combustion of Volatile Organic Compounds in Air Kyung Shik Yang, Jin Seong Choi, Soo Hyun Lee, and Jong Shik Chung* Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31, Hyoja Dong, Nam Gu, Pohang 790-784, Korea

A wire-mesh honeycomb (WMH) catalyst was constructed from alternating layers of flat and corrugated wire-mesh sheets packed within a frame. The wire-mesh sheets were first coated with aluminum particles using an electrophoretic deposition method. The coated layer was suitably porous and showed a good adherence to the wire surface because of sintering of aluminum particles after thermal treatment at 800 °C. An additional calcination at 500 °C formed a thin layer (∼50 nm in thickness) of oxide phase on each aluminum particle in the deposited layer. Then, Al/Al2O3-coated WMH modules were coated with a Pt (1.0 wt %)/TiO2 catalyst. Their catalytic activities were compared with the conventional ceramic honeycomb (CH) modules with the same catalyst loading for catalytic combustion of various volatile organic compounds (VOCs) in air. The WMH modules showed a better performance than the CH modules for all of the VOCs tested, especially at high temperatures. Kinetic study has shown that the better performance with WMH was caused by an improved external mass-transfer rate as a result of the existence of additional flow across the channel walls of the wire mesh. Introduction Volatile organic compounds (VOCs) have been treated as one of the main categories of air pollutants because they produce direct (toxic and carcinogenic) and secondary (photochemical smog) environmental effects. Many technologies have been investigated for the removal of VOCs such as thermal combustion, catalytic combustion, adsorption, pervaporation, biofiltration, etc.1-5 Catalytic combustion of various air pollutants under relatively lower temperatures than thermal combustion is one of the promising options for air pollution control because of its high efficiency, low energy consumption, and low NOx formation.7,8 A common bed design for purposes of air pollution control is the catalyst monolith, which is now popular in the automobile afterburner and in selective catalytic reduction (SCR) of NOx in flue gases.9 The catalyst is deposited on the surface of, or incorporated into, the body of a block (the so-called honeycomb module) containing parallel, nonintersecting channels aligned in the direction of the gas flow. The honeycomb structure can provide readily an interphase area comparable to that of a pellet-packed bed, yet the pressure drop across the bed is several magnitudes lower at high flow rates.10,11 It is best fit for circumstances where a low pressure drop is required at high gas flow rates or where the dust exits, e.g., in off-gas from the power station, because the dust can flow through the reactor without inhibition. Meanwhile, the coated or incorporated catalyst layers are quite thin compared to the size of the pellet catalyst, which enables a much higher effectiveness factor. On the other hand, there are also some innate disadvantages in honeycomb reactors, mainly, the high price of the substrate based on geometric surface area, the low interphase masstransfer rate,12 the absence of radial mixing,13 and accumulations of powder in corners of the channels.14 * To whom correspondence should be addressed. Tel.: +8254-279-2267. Fax: +82-54-279-5528. E-mail: [email protected].

Use of wire mesh as the substrate of the monolithic catalyst in air pollution control was also reported recently in the literature. Ahlstro¨m-Silversand and Odenbrand applied a layer of porous alumina powder on the substrate of the stainless steel meshes by a thermal spray method;15 the coat was then impregnated with catalytic components. To attain a satisfactory conversion for CO and hydrocarbon combustion, up to five layers of wire mesh were stacked in series in their tests. A wire-mesh catalyst has excellent mass- and heat-transfer performance of a pellet-type catalyst with merits of low pressure drop and high effectiveness factor in the monolith, therefore exhibiting a higher efficiency than the present honeycomb modules.12 Applying thin catalytically active zeolite coatings to the fibrous substrate by several novel approaches has been suggested.16,17 A typical preparation procedure consists of immersing the support structure in an aqueous solution containing the reactant for the in situ hydrothermal synthesis of zeolite, after which the system is heated and the zeolite crystals grow on the surface of the support. Zeolite-coated wire meshes were used for SCR of NOx with NH3. The electrophoretic deposition (EPD) method has been considered as another technique because it requires a short deposition time and a simple apparatus. It is based on the deposition of particles or colloid suspensions on the surface of an electrode driven by an electric field.18 Vorob’eva et al. have tried to prepare a wire mesh coated with active alumina using EPD of alumina sol.19 In this study, the wire meshes were precoated with aluminum metal particles using the EPD method as suggested by Yang.20 The coated layer was subjected to thermal treatment to form a sintered but somewhat porous layer that could be strongly adhered on the wire surface. Subsequent calcination created a shallow layer of the oxidized Al2O3 phase on the surface of individual aluminum particles. To this final layer of the Al2O3/Al composite particles on the wire-mesh surface were

10.1021/ie034022g CCC: $27.50 © 2004 American Chemical Society Published on Web 01/17/2004

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deposited active catalyst particles such as Pt/TiO2 by washcoating with a solution of catalyst slurry. With these catalyst-deposited wire meshes, a honeycomb-type module was manufactured by stacking alternately the corrugated and flat types of the wire-mesh sheet within a frame as proposed by Chung.21 The catalyst-loaded wire-mesh honeycomb (abbreviated as WMH hereafter) modules were tested for the catalytic combustion of various VOCs. They exhibited superior catalytic performances compared to the ceramic honeycomb (CH) modules having similar geometric surface area and catalyst loading, especially in the hightemperature region. The better performance of the WMH can be explained in terms of improved masstransfer rate inside the channels because of flows in three-dimensional direction. Experimental Section Deposition of an Al/Al2O3 Layer onto Wire Mesh. Aluminum powder (99% purity, spherical, Angang Steel Co.) was used as a deposition material, with a mean particle size of 4.5 µm and a specific surface area of 1.83 m2/g. The aluminum powder sample was also subjected to chemical cleaning with a 1.0 wt % NH4OH solution to remove oxidized layers. After the cleaning, the color of the aluminum powder was changed into black. The cleaned aluminum powder (less than 5 wt %) was added to ethanol (99.9%, Aldrich) under stirring, together with a suitable amount of additives (for improving suspension and adhesion). The slurry solution was well mixed using an ultrasonic vibrator for 5 min. A commercial wire mesh made of stainless steel (SUS 316L, 20 mesh) was used as the substrate. It has an opening pore size of around 0.9 mm and a wire diameter (dw) of 0.27 mm. The surface of the wire-mesh substrate was treated with a 10 wt % H2SO4 solution for 2 min to remove dirt and grease as well as to bring about a roughness on the wire surface. For particle deposition at the cathode, the wire mesh was cut into pieces (5 cm × 10 cm). The stainless steel plate was used as an anode, and the size is the same as that of the cathode. The distance between the two electrodes was kept at 10 mm, and the dc voltage was changed from 50 to 300 V. The aluminum-coated samples were dried at room temperature for 12 h, sintered at 800 °C for 3 h with a He gas purge, and then calcined at 500 °C for 5 h. The final calcination step was intended for the formation of a shallow passivated layer of Al2O3 on the surface of individual aluminum particles. The specific surface area was determined by nitrogen adsorption in a constant-volume adsorption apparatus (Micrometrics, ASAP 2021C). X-ray photoelectron spectroscopy (XPS; VG Scientific, ESCA Lab. 220XL) was used for the determination of the surface structure and the phase of the coated sample. Preparation of Honeycomb Modules. Two types of Al/Al2O3-coated wire-mesh sheets (4.5 × 5.0 cm in size) were used for manufacturing WMH: flat sheets and corrugated (triangle shaped with side lengths of approximately 6 mm) ones. The two sheets were packed alternately to a final thickness of about 4.5 cm. The WMH manufactured in this way has a rectangular shape with a size of 4.5 × 4.5 cm and 5.0 cm in length. It, therefore, has parallel passage channels along the longitudinal direction, shaped as an equilateral triangle with a side length of approximately 6 mm (Figure 1). For the above WMH module weighing 44.3 g, the cells

Figure 1. Pictures of WMH: (a) top; (b) side view.

per square inch (CPSI) are 25 with a geometric surface area of 620 m2/m3. For comparison of catalytic activities, a conventional honeycomb catalyst module made of ceramic (cordierite) was also prepared. The CH having channels of square cross section (4 × 4 mm in channel size, 0.8 mm in wall thickness) was cut to a size of 4.5 × 4.5 cm and 5.0 cm in length. The CH contains 25 CPSI with a geometric surface area of 690 m2/m3, slightly higher than that of WMH. For the combustion of VOCs, we selected Pt/TiO2 as an active component. Particles of TiO2 (anatase; Samchun Co., Brunauer-Emmett-Teller surface area of 55 m2/g) were impregnated with an aqueous solution of H2PtCl6 (Aldrich). The impregnated sample was dried at 110 °C for 12 h and reduced at 450 °C for 8 h. For the washcoating of the Pt/TiO2 catalyst to the WMH, a slurry solution was made by mixing the catalyst powder with an appropriate amount of water, along with an inorganic binder such as colloidal silica (Aldrich, LUDOX AS30, less than 10 wt %). The honeycomb was dipped into the slurry solution and then dried at 120 °C for 12 h and calcined at 500 °C for 3 h. Similarly, the same amount of the Pt/TiO2 catalyst was loaded to the CH. To measure the platinum dispersion, hydrogen chemisorption was carried out for 0.5 g of the catalyst in a static volumetric adsorption apparatus, equipped with a high vacuum pump. The dispersion was expressed at the H/Pt ratio at monolayer coverage obtained by extrapolation of the linear portions of the adsorption isotherms to zero pressure.22 Catalytic Combustion of VOCs. The honeycomb catalyst module was packed inside of a square-shaped reactor (4.7 × 4.7 cm) made of stainless steel. A stream of reactant was generated by bubbling air through a saturator containing liquid reactant. It was then mixed with another air stream in a mixing chamber to achieve a desired concentration of reactant in the feed that enters the reactor. The inlet concentration of each reactant was adjusted to 1000 ppm at 298 K. The rates of the gas flows were controlled by a mass flow controller to attain the required concentration. The reaction temperatures were varied from 300 to 600 K under atmospheric pressure. Inside the reactor were isothermal conditions because the differences of the inlet and outlet temperatures were controlled below 1 K. All of the streamlines were maintained at around 100 °C to prevent the condensation of the reactant. The exhaust gas from the reactor was analyzed by a gas chromatograph (Hewlett-Packard HP5890) equipped with a HP-5 capillary column (30 m length × 0.32 mm i.d.) and a flame ionization detector.

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Figure 2. Al2p XPS spectrum of the Al-coated WMH: (a) the outer surface of the coatings; (b) after 15 nm etching with ion sputtering; (c) after 30 nm etching; (d) after 45 nm etching.

Results and Discussion Precoating of an Al/Al2O3 Layer onto Wire Mesh. Because the bare surface of a metal wire can hardly attach catalyst powder, the wire-mesh sheets were coated first with aluminum powder using the EPD method. The thickness of the deposited Al layer was usually fixed between 60 and 100 µm by controlling the voltage and deposition time. The Al-deposited wire meshes were then thermally treated at 800 °C for 8 h with a He gas purge to induce sintering among particles. This was followed by calcination at 500 °C and 5 h, which was intended to form a thin passivated layer of aluminum oxide on the surface of individual aluminum particles. XPS with ion-sputtering analysis was performed in order to confirm the existence and thickness of an Al2O3 layer in the deposited Al particles (Figure 2). Both metallic aluminum and aluminum oxide phases are detected at the outer surface of the coated layer. Both phases are observed even after several times of ion sputtering at 15 nm intervals. Further sputtering decreases the Al2O3 phase. The results show that the existence of the Al2O3 layer lasts up to a depth of around 50 nm, after which the oxide layer begins to disappear. Thus, we conclude that a thin Al2O3 layer was formed at the outer surface of the Al particle, completely encapsulating the Al particle. Catalytic Combustion of Various VOCs. The Al/ Al2O3-coated WMH was loaded with Pt (1.0 wt %)/TiO2 catalyst (with 20 wt % loading of WMH). The Pt dispersion was estimated from the ratio of the hydrogen atoms adsorbed per total metal atoms in the catalyst, H/Pt, which was determined from equilibrium static

Figure 3. Catalytic combustion of various VOCs with Al/Al2O3coated WMH catalysts. Loading of Pt (1.0 wt %)/TiO2 ) 20 wt %, GHSV ) 2000 h-1, and concentration of reactant ) 1000 ppm.

chemisorption of hydrogen. Details of the calculation procedure are given elsewhere. The physical properties of Al/Al2O3-WMH and Pt/TiO2 catalyst powder are summarized in Table 1. The combustion experiments for various VOC pollutants (1000 ppm) in air were carried out at a gas hourly space velocity (GHSV) of 2000 h-1 (refers to 1 atm and 298 K). Figure 3 shows the results for the catalytic destruction of various VOCs tested using the above-mentioned WMH module. Methyl alcohol is the most combustible, the conversion is over 90% at temperature below 373 K, but complete combustion is difficult because the conversion turns flat after ∼95%, which is different from the other reactants. Interestingly, a similar phenomenon was reported in previous literature,15 where the wire-mesh gauze was used over an Al2O3-based Pd/Pt catalyst. The catalytic combustion of hydrocarbon usually generates a large amount of CO at temperatures below 408 K during the reaction. The combustion of other components is severely retarded as long as the CO inhibition continues.15 The combustion of methyl alcohol occurred below the temperature; therefore, the reaction was inhibited by the generated CO. It was also observed that CO2 was the only product during the combustion of VOCs at temperatures over 423 K. According to experimental results in Figure 3, the sequence of the ease of combustion of VOCs is methyl alcohol > benzene > toluene > ethyl alcohol > n-hexane > ethyl acetate, which coincides with the results published in the literature, except n-hexane.24 Normal alkanes were found to be much more refractory than aromatic compounds with similar molecular weight, and the stability increases with carbon number if the number is over 3. The same tests were also carried out using the CH modules, which were

Table 1. Physical Properties of Al/Al2O3-Composite Particles That Were Coated on a Wire Mesh and Pt (1.0 wt %)/TiO2 Catalyst That Was Deposited on WMH and CH

item Al/Al2O3 layer

Pt/TiO2 layer

thickness of the Al2O3 phase on an individual Al particle thickness of the coated layer BET surface area (m2/g) average pore diameter (Å) Pt dispersion (H/Pt) thickness of the coated layer BET surface area (m2/g) average pore diameter (Å)

Al/ Al2O3-coated wire mesh

Pt/TiO2 catalyst deposited on Al/Al2O3-coated WMH

Pt/TiO2 catalyst deposited on CH

80 12.23 110 0.36 40 55 100

0.36 45 55 100

50 80 12.23 110

910

Ind. Eng. Chem. Res., Vol. 43, No. 4, 2004 Table 2. Rate Constants and Mass-Transfer Coefficients for Benzene Combustion over WMH and CH Coated with Pt (1.0 wt %)/TiO2 at a Linear Velocity of 8.16 cm/s module WMH

CH

Figure 4. Catalytic combustion of various VOCs with CH catalysts. Loading of Pt (1.0 wt %)/TiO2 ) 20 wt %, GHSV ) 2000 h-1, and concentration of reactant ) 1000 ppm.

WMH

CH

loaded with the same amount of catalyst, and the results are shown in Figure 4. The sequence of the ease of combustion among VOCs was found to be exactly the same as that with the WMH module. However, the threshold temperatures (the temperature at which more than 90% conversion is achieved) are higher than those observed with the WMH modules by 10-30 °C, confirming that the WMH modules are superior to the conventional CH modules. Comparison between WMH and CH. Benzene, one of the common VOCs, was selected for the comparison of the activities between WMH and CH in more detail. Figure 5 shows the benzene conversion plotted as a function of the linear velocity at two different temperatures of 413 and 433 K for both the WMH and CH modules. The conversion with WMH is always higher than that with CH, especially at high temperatures and low linear velocities. This can be attributed to the influence of the mass-transfer rate on the overall reaction rate at higher temperatures and lower linear velocities. At a low-temperature region, the reaction will be typically controlled by intrinsic kinetics, whereas at a high-temperature region, the rate of external mass transfer begins to exert an effect owing to faster reaction on the catalyst surface. To have a better understanding of the importance of interphase mass transfer, the external mass-transfer coefficients (km) of WMH and CH were calculated from the results of benzene conversion

exp conv (%)

ko

ln kr

kr

kmam

km (cm/s)

397.65 399.15 410.65 429.65 450.15 406.15 415.65 425.65 447.15 465.15

5.45 6.20 16.70 57.30 97.70 3.45 7.80 17.20 63.20 95.40

0.057 0.065 0.185 0.862 3.851 0.036 0.082 0.191 1.012 3.118

-2.868 -2.735 -1.684 -0.134 1.428 -3.335 -2.494 -1.645 0.068 1.335

0.057 0.065 0.186 0.875 4.170 0.036 0.083 0.193 1.070 3.800

51.303 50.299 53.304 56.297 50.291 17.597 18.120 19.426 18.727 17.361

8.275 8.113 8.597 9.080 8.111 2.550 2.626 2.815 2.714 2.516

Table 3. Temperatures (K) Required for 50% and 99% Conversion of Benzene and External Mass-Transfer Coefficients (km) of WMH and CH at Different Flow Rates

module

Figure 5. Activity comparison between WMH and CH. Loading of Pt (1.0 wt %)/TiO2 ) 20 wt %, and benzene concentration ) 1000 ppm.

temp (K)

linear velocity (cm/s)

T50% (K)

T99% (K)

external mass-transfer coefficient, km (cm/s)

1.36 2.72 4.08 8.16 1.36 2.72 4.08 8.16

415 420 424 427 429 435 438 443

430 441 447 455 445 452 460 471

0.80 ( 0.05 1.77 ( 0.30 3.13 ( 0.70 8.50 ( 0.80 0.35 ( 0.06 0.71 ( 0.20 1.39 ( 0.26 2.70 ( 0.70

that were measured at various temperatures and flow rates. Details of the calculation equation are given elsewhere.6 The effective diffusivity, De, can be estimated using the Chapman-Enskog formula.23 The effective diffusivity, De, was found to be 0.016 cm2/s, with a molecular diffusivity value of 0.32 cm2/s for benzene-air, a Knudsen diffusivity of 0.23 cm2/s (for a 100 Å pore in the catalyst layer), and a porosity of 0.35. The value of kr for the benzene oxidation was in the range of 0.35-19.6 at the present test conditions. With an estimated L value of 40 µm, we obtained the value of the Thiele modulus in the range of 0.01-0.22. Thus, ηs was found to be in the range of 0.98-0.99. We therefore conclude that the internal diffusion resistance is negligible at the present experimental conditions. This is mainly because the thickness of the catalyst layer is very thin. We then measured the intrinsic rate constant, kr, at the low-temperature region, where the external mass transfer can possibly be avoided. It was found that, at temperatures of less than 423 K, the measured value of kr remained more or less constant regardless of the flow-rate change. The kr values measured at this masstransfer-eliminated region were used for regression analysis to estimate the values of Ea and ln kr0. The regressed values remained more or less constant with Ea ) 121 ( 12 kJ/mol and kr0 ) (4.51 ( 1.08) × 1014 s-1 for both the WMH and CH modules. With these intrinsic parameters, we then calculated the external masstransfer coefficient, km, at the high-temperature region. With measured values of ko and external surface areas (am) of 620 and 690 m2/m3 for WMH and CH (the cell density of 20 CPSI), respectively, the km values that were calculated at a flow rate of 8.16 cm/s are shown in Table 2. Similarly, the km values obtained at other flow rates are listed in Table 3. Also listed in Table 3 are temperatures required for obtaining 50% and 99% conversion for the benzene combustion at different flow rates. From these results, it can be easily

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Acknowledgment The authors gratefully acknowledge the support from the Brain Korea 21 project. Notation

Figure 6. Activity comparison among powder catalyst, WMH, and CH. Pt (1.0 wt %)/TiO2, WHSV ) 12 000 cm3/g‚h, and benzene concentration ) 1000 ppm.

established that a better catalytic performance of WMH than of CH results from an improved mass-transfer resistance inside honeycomb channels. The conversion patterns of the WMH and CH modules were compared with that of the powder catalyst. The intrinsic activity of the powder catalyst was the same as those of WMH and CH from a similar calculation. The activity of the Pt (1.0 wt %)/TiO2 catalyst powder was tested at the same weight hourly space velocity (WHSV ) 12 000 cm3/g‚h) using a quartz tubing reactor. The results in Figure 6 show that there exists a relatively small difference in the curve pattern between the WMH and powder catalyst. In contrast to this, the distance between the curves of the CH and powder catalyst enlarges quickly with temperature, which implies the existence of a strong mass-transfer resistance in the CH. The WMH module has channels whose walls are interconnected with each other through the openings in the wire-mesh sheet. Thus, the gas feed that enters through the channels of WMH can have flows in three-dimensional direction, inducing a turbulent effect to facilitate the mass-transfer rate of molecules inside the channels. The result shows a good benefit of WMH over CH for fast reactions (probably at the hightemperature conditions) where the mass-transfer resistance becomes important. Conclusions WMH is a new generation of monolithic reactor that can be used for substitution of conventional CH catalysts. An Al/Al2O3 composite layer was successfully formed on the wire-mesh sheets using EPD of Al powder and additional thermal treatment. The aluminum oxide layer of about 50 nm was formed and encapsulated each aluminum particle. The Al/Al2O3 composite layer was suitably porous with a large surface area, so it is profitable for the deposition of catalyst on it. The benefit of WMH over the traditional CH catalyst was proved during the combustion of several typical VOCs. For example, the benzene conversion with WMH was always higher than that with CH. It was revealed that a better catalytic performance of WMH than of CH resulted from the difference in the mass-transfer resistance inside the channels because of the additional effect of free radial mixing across the channel walls of the wire mesh.

am ) external surface area of per unit volume (m3) of honeycomb, m2/m3 dw ) diameter of the wire mesh, mm Ea ) activation energy, J/mol kr ) intrinsic reaction rate constant, cm3/g/s km ) external mass-transfer coefficient, cm/s ko ) overall reaction rate constant, cm3/g/s kr0 ) preexponential constant, cm3/g/s De ) effective diffusivity in the porous catalyst, cm2/s Q ) gas flow rate at the reaction conditions, cm3/s Fc ) apparent density of the catalyst layer, g/cm3 L ) thickness of the catalyst layer, cm T ) absolute temperature, K Tr ) reaction temperature, °C V ) reactor volume, cm3 X ) fractional conversion of the reactant

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Received for review July 25, 2003 Revised manuscript received September 29, 2003 Accepted December 17, 2003 IE034022G