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Latin American applied research

versión impresa ISSN 0327-0793versión On-line ISSN 1851-8796

Lat. Am. appl. res. vol.43 no.4 Bahía Blanca oct./dic. 2013

 

Washcoating of MnOx on FeCralloy monoliths

M.A. Peluso, L. Costa-Almeida, O. Sanz, J.E. Sambeth, H. Thomas and M. Montes

Centro de Investigación y Desarrollo en Ciencias Aplicadas (CINDECA), La Plata, 1900, Argentina.apelu@quimica.unlp.edu.ar
Applied Chemistry Department, University of the Basque Country (UPV-EHU),San Sebastián, 20080, Spain

Abstract— Two MnOx catalyst as powder, prepared by decomposition of two different MnCO3, have been deposited on a FeCrAlloy® metallic monolith by means of washcoating using slurry concentration of 25 and 40 wt(%) (PM25, PM40 and AM25). When increase slurry concentration increase the amount of deposited material, but lower adhesion of solid retained is observed. The obtained monoliths showed excellent catalytic activity in the combustion of ethyl acetate and toluene, being the monoliths activity order: PM40 > AM25 > PM25. Nevertheless, the conversion g-1 and conversion m-2 show that PM25 is the more active monolith.

Keywords— MnOx; FeCralloy; Monoliths; VOCs.

I. INTRODUCTION

Volatile organic compounds (VOCs) are a class of contaminants produced in various industrial processes such as printing graphic, the paint and coatings industry, metallurgical industries, chemical and electronics. There are numerous methods employed to destroy these emissions, being the catalytic incineration one of the most often used for gaseous systems. To deal with large flows and avoid problems such as the pressure drop are used catalysts supported on monolithic structures, which can be metal or ceramic (Heck et al., 2001; Alvarez et al., 2002).

Metal monoliths began to be used due to the greater resistance and thermal conductivity than that the ceramic monoliths, and also presented thinner wall thicknesses, which implies lower pressure drop. The preparation of the metal monoliths is based generally on the generation of a very thin oxide film on the surface of the metal, which serves as a substrate of the catalytic coating (Avila et al., 2005). The manufacturing technology of steel monoliths is essentially based on the oxidation of Fe-Cr alloys at high temperature to encourage the migration of cation at the surface and its oxidation to a-Al2O3 (Avila et al., 2005).

γ-MnO2 (nsutite) is the best-know polymorphs of MnO2 used by the battery industry. The structure of γ-MnO2 is considered to be a random intergrowth of 1x1 tunnels of pyrolusite and 2x1 tunnels of ramsdellite, which are constructed of MnO6 octahedral units with edge or corner sharing. Besides, this solid is characterized by point defects such as Mn4+ vacancies, Mn3+ cations replacing Mn4+ and OH- species replacing O2- anions (Chabre and Pannetier, 1995). These properties make this oxide interesting from the catalytic point of view, due to their high electrical conductivity. Previous works have shown that the nsutite is very active for oxidation reactions, in particular for oxygenated VOCs as ethanol (Lamaita et al., 2005a; Lamaita et al., 2005b).

Cryptomelane and mixed Mn-Cu oxides have been successfully deposited on Fecralloy monoliths through the washcoating technique (Barbero et al., 2008; Frias et al., 2007). However, no studies of the nsutite phase supported on metallic monoliths for being used as a catalyst for VOCs removal are found in the literature.

In this study, the objective is to develop a structured catalysts prepared by deposition of manganese oxide onto Fecralloy monoliths by the washcoating method and thus obtain a catalytic systems suitable for use in VOCs abatment. As VOCs representing molecules were used compounds found primarily in the printing industry as ethyl acetate and toluene.

II. METHODS

A. Sample Preparation
Metallic monoliths made of Fecralloy (typical analysis Cr 22%, Al 4.8%, Si 0.3%, Y 0.3%, C 0.03% Fe balance) were used as support of manganese oxides. Cylindrical monoliths were prepared rolling up a corrugated and a flat 50 μm steel sheet (L=30 mm, d= 16 mm, V= 6 cm3, cell density 55 cells cm-2). Before catalysts coating, Fecralloy monoliths were heated in air at 900 °C for 22 hs to produce a surface composition and roughness convenient to assure adherence.

γ-MnO2 or nsutite, was synthesized through the oxidative decomposition of 100 g of MnCO3.xH2O (Panreac PRS, SBET 45 m2g-1) at 350 °C under flowing oxygen saturated with water at RT (500 cm3 min-1) for 24 hs, and with a heat rate of 20 °C min-1. The resulting powder was dried and calcined at 400 °C for 2 h. This oxide was named MnOx-P.

In order to study the effect of two different MnCO3 precursors, another MnOx powder was synthesized by the same procedure but using an anhydrous MnCO3 (Alfa Aesar, 99.9%, SBET 10 m2g-1) as precursor (MnOx-A).

The monoliths were washcoated with aqueous suspensions of 25 and 40 wt(%) of the prepared manganese oxide. The powders were previously grinding in a ball mill for 5 hs. The pH of the slurry was adjusted at 8 with diluted NaOH, in accord to the zeta potential study of the manganese oxide. The washcoating of the pre-treated monoliths were carried out by dipping and withdrawing the monoliths in the slurry at constant speed (3 cm min-1). Afterward, the monoliths were centrifuged at 400 rpm for 10 min to eliminate the excess slurry, and then dried at 120 °C for 1 h. This procedure was repeated four times successively and finally the monoliths were calcined in air at 400 °C for 2 h. The manganese oxide content was determined by measuring the weight of the samples both before and after the impregnation process.

B. Characterization
The MnOx powders were characterized by X-ray diffraction using a Phillips PW 1390 instrument with a Ni filter and Cu Kα radiation (λ =1.540589 A) in the 2θ range between 5 and 60°. Textural properties of the powders were determined by nitrogen adsorption using a Micromeritics ASAP 2000. TPR tests of the powders were analyzed in an Thermofinnigan TPDRO 1100, using a 5% H2/Ar reducing mixture carrier flowing at 40 cm3 min-1 at a heating rate of 10 °C min-1 from room temperature to 900 °C using 28 and 15 mg of sample for P-MnOx and A-MnOx, respectively. A calibration curve of the TCD response signal expressed by the integral area of peaks as a function of the amount of the hydrogen consumption was established by reducing known amounts of pure CuO to Cu.

Particle size distribution of the powder was determined with laser particle size analysis trhough a Malvern Instruments Mastersizer 2000 equipped with a sample suspension unit. 100 mg of solid were dispersed in 10 ml H2O in an ultrasonic bath for 1 h before the measurement.

The Zeta Potential of the manganese oxides was determined using a Zetasizer Nano serie of Malvern Instruments. Tipically, 10 mg of powder was dispersed in aqueous solution of 0.003M NaCl, and the pH was adjusted with diluted NaOH or HNO3 solutions.

Viscosities of the suspension were measured in a Haake viscosimeter with a NV sensor of 2-103 mPa s at 25 °C.

Textural properties of the monoliths were determined by nitrogen adsorption using a using a Micromeritics ASAP 2000 with a homemade cell that allows analyzing the complete 6 cm3 monoliths.

Adherence of the coating was measured by an ultrasound test. The adhesion is evaluated by calculating the amount of coating lost after dipping the samples in a beaker containing petroleum ether placed in a bath of supersonic cleaner for 30 min.

The sample morphology was examined with a scanning electron microscope (SEM) Hitachi S-2700. The samples were previously gold sputtered for 2 minutes in a SC 500 Sputter Coater.

C. Catalytic activity
The catalytic activity of the prepared monoliths was measured for the complete oxidation of ethyl acetate and toluene in air in a 1000 cm3 min-1 (GHSV = 12000 h-1) air stream containing 1000 mg C N-1 m-3 of ethyl acetate or toluene. The ignition curves were obtained under a heating ramp (100 to 400 °C at 2.5 °C min-1) after the catalysts were pretreated in air at 400 °C. Conversion was calculated by measuring VOCs disappearance by Gas Chromatography (HP 5890 chromatograph, with a TR Wax 30 m column), for ethyl acetate and by Mass Spectroscopy (Balzer Omnistar) for toluene, together with the CO2 measurement by an on-line IR detector (Sensotrans IR).

The catalytic activity of the powders was measured in the same way as the monoliths but using 250 mg of catalyst in an air stream of 500 cm3 min-1 and 500 mg C N-1 m-3 of ethyl acetate or toluene.

III. RERULTS AND DISCUSSION

A. Powder Characterization
The MnOx XRD pattern of the MnOx powders (Fig. 1) was characterized by some intense diffraction lines and some broad and poorly resolved peaks, which could correspond to a α-MnO2 phase (JCPDS 44-0141) or α-MnOx.nH2O hydrated oxide (JCPDS 81-1946). These manganese oxide phases were characterized by the presence of Mn4+ and Mn3+ cations (Post, 1999).


Fig. 1. DRX patterns of MnOx powders.

TPR experiments of the MnOx powders are represented in Fig. 2. For both samples, P-MnOx and A-MnOx, two reduction peaks were observed. The first peak is centered at 299 and 327 °C for the A-MnOx and P-MnOx, respectively, and the second reduction peak is centered at nearly 446 °C for both samples. The low reduction peak can be explained as the reduction of MnO2 and or Mn2O3 to Mn3O4, and the high reduction peak represented the reduction of Mn3O4 to MnO (Gil et al., 2004; Tang et al., 2008). Based on the O/Mn relationship (Table 1), it is deduced that Mn presented an average oxidation state of 3.6, indicating the presence of Mn4+ cations together with Mn3+ or Mn2+ cations. These results are in agreement with the manganese oxide phase detected by XRD, which contained Mn4+ and Mn3+ (Lamaita et al., 2005a).


Fig. 2. TPR of MnOx powders

Table 1. Textural properties and TPR quantification of MnOx powder samples.


Fig. 3. SEM Micrographs of FeCrAlloy surface after termic treatment.

Differences in the powders were observed in the textural properties (Table 1). These differences could be due to the fact that the specific area of the MnCO3 used to prepared P-MnOx were higher than that of the MnCO3 used to prepare A-MnOx, the specific areas of the MnCO3 used as precursors were different

Once milling, in both oxides, P-MnOx and A-MnOx, no significant changes were observed neither in the textural properties nor in the structure of the manganese oxide phase.

B. Preparation of structured support and manganese oxide slurry
The most critical points to deposit a catalyst on a base metal structured support using washcoating are adhesion and uniformity of the coating. The adhesion of the coating is very important because the catalytic systems used in environmental applications for the removal of VOCs must treat effluents with high space velocities. To promote the adhesion of the catalyst on the metal support, Fecralloy is treated in air at 900 °C for 22 h, producing a rough surface of aluminum oxide, α- Al2O3 (Fig. 3), suitable for the anchoring of fine particles of a catalyst powder from a suspension (Almeida et al., 2010).

The suspension of catalyst that is prepared must meet certain characteristics that depend mainly on: particle properties, solvent and additives, and solid content of the suspension. Various authors have shown that the adhesion of the coating depends mainly on the particle size of the catalyst deposited (Agrafiotis et al., 2000; Avila et al., 2005; Zamaro et al., 2005). They affirmed that the smaller the particle size the better the adhesion of the coating. Sanz et al. (2008) came to a similar conclusion. They pointed out that to improve the mechanical adhesion of the coating layer, the particle size should be less than the surface roughness of the medium.

The particle size of the P-MnOx at the end of the synthesis process was too big to get a good adhesion of the coating (d90=5μm). Therefore, the solid was grinding for 5 h in a ball milling. After the crushing, the 90% of the particles were smaller than 5 μm (d90=1μm). The particle size of the A-MnOx at the end of the synthesis process was smaller than the P-MnOx with a d90=2μm. Nevertheless, the solid was grinding for 5 h in a ball milling. After the crushing, the 90% of the particles were smaller than 1 μm (d90=0.2μm).

To deposit a catalyst powder on monoliths, it must be dispersed in a dispersing medium, usually water, and the suspension must be stable. The measured of the Z potential of the catalyst suspension conduce to an isoelectric point of 6.2. The suspension of catalyst was prepared in aqueous medium with a pH of 8.5, a value that ensures a high potential for both high Z potential and repulsions between particles that favors stability of the dispersion.

On the other hand, the viscosity of suspensions must have an appropriate value to achieve an uniform coating. In this work four different suspensions in which varied the percentage of solid, 25 and 40 wt(%) were prepared. Figure 4 shows the rheograms for the different suspensions. For both MnOx prepared, the more concentrated suspension has a viscosity greater than the diluted suspension. The suspension of 25 wt(%) of A-MnOx, presented a greater viscosity than the 25 wt(%) P-MnOx suspension, with a value closed to the 40 wt(%) P-MnOx suspension. The greater viscosity of the A-MnOx suspension is due to the smaller particle size of the A-MnOx powder. The 40 wt(%) A-MnOx suspension presented a viscosity too great that resulted inappropriate to used for the waschcoating technique.


Fig. 4. Variation of viscosity with shear rate.

Four dives coated monoliths were performed with the 25 and 40 wt(%) P-MnOx suspensions, named PM25 and PM40, respectively, and with 25 wt(%) A-MnOx suspension (AM25). The total amount of manganese oxide loaded on each monolith are presented in Table 2. As it is expected, monolith prepared with the more concentrate suspension, PM40, retained more catalyst than PM25. Nevertheless, the AM25 monolith retained almost the same manganese oxide than PM40, product of the higher or similar viscosity of 25 wt(%) A-MnOx suspension compared to the 40 wt(%) P-MnOx suspension.

Table 2. Textural properties of monoliths

The amount of solid loaded during the washcoating procedure strongly depends on the manganese oxide concentration in the slurry and the number of inmersions. Figure 5 shows the influence of the number of inmersions and the solid concentration in the slurry on the cumulative total solid load (Fig. 5A) and the load achieved at each immersion stage (Fig. 5B). This last load was calculated as the increase in weight after each inmersion. As is spected, the cumulative load is higher when increase the number of inmersions. For the P-MnOx powder, the manganese oxide loaded in four stage with the 25 wt(%) suspension is less than the solid loaded in one step with the 40 wt(%) suspension (Fig. 5A). Fig. 5B shows that the amount of catalyst deposited increase in every immersion stage.


Fig. 5. Effect of slurry concentration upon MnOx loading in the washcoat (A) MnOx load in each inmersion, and (B) cumulative loadings.

This effect could be due to the more chemical compatibility between layers of manganese oxide than to the interface manganese oxide/ structured support, and to the increased porosity of the coating layer to be deposited.

Textural characteristics of the manganese oxide coated monoliths were measured. The values are the mean of two samples, with an error 10%. The results are summarized in Table 2. For the monoliths prepared from P-MnOx suspensions, the specific area and pore volume per monolith increase with the increase in the amount of loaded manganese oxide. For the two classes of monoliths, the specific area per gram of retained solid decrease, the pore volume almost do not change and the pore diameter decrease with respect to the starting bulk MnOx oxide.

Scanning electron micrographs lateral views of the prepared monoliths are presented in Fig. 6, showing the morphology of the loaded manganese oxide phase. The coating covers the entire structured support, and in the PM40 is observed cumulus of manganese oxides. In monoliths prepared from P-MnOx, the effect that adherence decrease when increase the amount of manganese oxide suggests that the adherence to the metal fiber is better than that occurring on the coating itself.


Fig. 6.SEM micrographs of FeCrAlloy surface covered with MnOx: (A) PM25; (B) PM40 y (C) AM25.

The results of the adherence test are shown in Table 2. The adherence of the MnOx layer follows the order: PM25>AM25>PM40. The use of more diluted suspension to prepare a coated monolith conduces to a more adherent layer of catalytic phase on the monolith support. The difference between PM25 and AM25 could be due to different amount of manganese oxide deposited.

C. Catalytic activity
The catalytic activity of the powders in the oxidation of ethyl acetate and toluene are represented in Fig. 7. These catalysts did not produce significant amounts of intermediate products, yielding only CO2 and H2O. The oxidation of ethyl acetate is characterized by a T50 of 162 and 166 °C and a T90 of 196 and 212 °C for the A-MnOx and P-MnOx, respectively. The oxidation of toluene is characterized by a T50 of 218 and a T90 of 254 °C for both catalysts.


Fig. 7. Catalytic activity of MnOx powders.

The results of the supported manganese monoliths in the reactions of total oxidation of ethyl acetate and toluene are presented in Fig. 8. The ignition of ethyl acetate starts at 125 °C (Fig. 8A) for the most active monoliths, and total conversion is achieved at approximately 250 °C. In the case of toluene combustion (Fig. 8B), ignition begins at 200 °C and is completed at temperatures below 340 °C. In both experiments the only reaction products detected were CO2 and H2O. The results of catalytic activity indicate that the reactivity order in the combustion of the different VOC molecules is the same for the three monoliths: ethyl acetate > toluene


Fig. 8. Ignition curves over monoliths of (A) ethyl acetate and (B) toluene.

It is expected that a higher amount of MnOx loading causes an increase in catalytic activity. This is confirmed in general terms comparing both monoliths prepared from P-MnOx, where it has been observed for the two VOCs studied, that the greater the amount of manganese oxide loaded the higher catalytic activity. This is because the number of active sites is higher, making it easier removal of VOCs molecules (Barbero et al., 2008; Sanz et al., 2008).

Figure 9 and Fig. 10 show the conversion per MnOx gram and m2, respectively. They were constructed dividing the conversion values by the MnOx grams deposited over the monoliths and by the monoliths SBET. It is observed that PM25 monolith presents higher conversion at temperatures superior than 200 and 275 °C for ethyl acetate and toluene, respectively. This is coincident whit the results of Morales et al. (2011) in their studies of oxidation of VOCs on MnCu washcoated FeCralloy monoliths.


Fig. 9. Conversion per MnOx mass over monoliths of (A) ethyl acetate and (B) toluene.


Fig. 10. Conversion per m2 over monoliths of (A) ethyl acetate and (B) toluene.

IV. CONCLUSIONS

MnOx supported on Fecralloy monoliths are able to eliminate ethyl acetate and toluene at temperatures below 350 °C. The activity order of the monoliths were PM40 > AM25 > PM25. The activity of the PM40 is higher than that of the AM25, although they have the same manganese content. When the conversion divided the MnOx mass and divided the SBET is compared, the PM25 monolith presents the higher VOC conversion. Additionally, PM25 present the lower mass lossing during the adherence test.

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Received: July 19, 2012
Accepted: December 3, 2012
Recommended by Subject Editor: María Lujan Ferreira

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