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TiO2 on magnesium silicate monolith: effects of different preparation techniques on the photocatalytic oxidation of chlorinated hydrocarbons
Energy 29 (2004) 845–852 www.elsevier.com/locate/energy
TiO2 on magnesium silicate monolith: effects of different preparation techniques on the photocatalytic oxidation of chlorinated hydrocarbons ´ vila c, Ana I. Cardona a,, Roberto Candal b, Benigno Sa´nchez a, Pedro A Moise´s Rebollar c a
Departamento de Energı´as Renovables, CIEMAT, Avda. Complutense 22, 28040 Madrid, Spain INQUIMAE, Univ. de Buenos Aires Ciudad Universitaria, Pabello´n 2, 1428 Buenos Aires, Argentina c Consejo Sup. de Inv. Cientı´ficas ICP-CSIC Campus de la UAM. Cantoblanco, 28049 Madrid, Spain
b
Abstract In this article, the comparative results of the photocatalytic oxidation of trichloroethylene (TCE) alone and a mixture of chlorinated hydrocarbons (trichloroethylene, perchloroethylene and chloroform) in gas phase, obtained with three different monolithic catalysts in a flat reactor frontally illuminated with a Xenon lamp are presented. The three catalysts incorporate titanium dioxide (TiO2) as active phase on a magnesium silicate support, by means of different procedures: (i) incorporation of commercial TiO2 powder into the silicate matrix (‘‘massic monolith’’); (ii) sol–gel coating of the silicate support; (iii) impregnation with a commercial TiO2 aqueous suspension of the same silicate support. In the first case, the massic monolith was made from a 50:50 w/w mixture of magnesium silicate and ‘‘Titafrance G5’’ TiO2 powder. In the second case, a magnesium silicate monolith was coated with several layers of an aqueous TiO2 sol prepared from hydrolysis and condensation of titanium tetra-isopropoxide (Ti(OC3H7)4) in excess of acidified water (acid catalysis). The third catalyst was prepared by impregnating the same silicate support with several layers of ‘‘Titafrance G5’’ TiO2 powder water suspension. All the catalysts were thermal treated under comparable conditions in order to fix the TiO2 active phase to the silicate support. Although the performance of the massic monolith was better than the sol–gel monolith, the latter is of great interest because this technique allows the chemical composition of the active films to be easily modified. # 2003 Elsevier Ltd. All rights reserved.
Corresponding author. Tel.: +34-91-3466177; fax: +34-91-3466037. E-mail address: [email protected] (A.I. Cardona).
0360-5442/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0360-5442(03)00184-1
846
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1. Introduction Because of stricter requirements on hazardous waste disposal and increased concern for the environment, for the last 20 years, photocatalytic oxidation (PCO) has been investigated in response to the need for innovative technologies to handle these problems. Much of the rapidly growing literature on heterogeneous photocatalytic oxidative degradation of organic compounds in gas phase is summarized in a recently collective publication [1]. Among other applications, a major motivation for the development of photocatalytic processes is their potential use in air purification [2,3] and other applications that require the treatment of pollutants in low concentrations. It is clear, in view of work published to date, that the efficiency of this technology in the elimination of pollutants depends on the nature of the organic compounds to be destroyed. Furthermore, the use of different reactor configurations, ways in which catalysts are prepared and the experimental conditions have made the comparison of results found in the literature difficult. An important part of research in the field of photocatalysis applied to the destruction of organic contaminants has focused on the improvement of catalyst performance. Thus different methods of applying the active components (in the majority of cases, titanium dioxide, TiO2) on a variety of supports have been developed [4–6]. The two basic techniques for the incorporation of TiO2 in fixed-bed and packed-bed reactors, or in the surface of those supports tested to date are: (i) dispersion of the active phase throughout the entire volume of the support (massic catalyst) and (ii) deposition of films on different supports. Supports can be coated with TiO2 films by different processes, such as, chemical vapour deposition, physical vapour deposition, sputtering, dip-coating with TiO2 suspensions, dip-coating sol–gel, among others. Dip-coating is a very attractive methodology because it is simple and does not require expensive equipment. However, special care should be taken in the selection of the appropriate preparation conditions to avoid excessive cracking of the film and the consequent delamination of the TiO2 layer. The selection of the support material is critical because it directly affects the activity, homogeneity and adhesion of TiO2 to the surface. Materials like ceramics tiles, paper, glass, fibreglass and stainless steel have been employed and other materials are currently being developed for possible future photocatalytic applications. An extensive variety of organic compounds have been tested for PCO, showing different sensibilities to this process. Chlorinated hydrocarbons, like trichloroethylene (TCE) perchloroethylene (PCE) and chloroform have probably been studied the most [7–12]. These organics are widely used industrial solvents polluting water and air. In this paper, we compare the efficiency of the destruction of TCE alone and in a mixture of chlorinated hydrocarbons with a sepiolite-based monolithic photocatalyst on which the active phase, TiO2, was incorporated in different ways: (i) by the incorporation of commercial TiO2 powder into a magnesium silicate monolith (sepiolite ‘‘massic monolith’’); (ii) sol–gel dip-coating of magnesium silicate supports (‘‘sol–gel monolith’’) and (iii) impregnation of the same supports with a commercial TiO2 aqueous suspension (‘‘impregnated monolith’’).
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2. Experimental system 2.1. Photocatalytic system Continuous-flow-mode tests lasting several hours were conducted to evaluate conversion efficiencies. The photoreactor is stainless steel and has an effective aperture diameter of 4 cm enclosed by a Pyrex window. A Xenon lamp illuminates the front window and the catalyst monoliths. The gas carrier (CO2-free air) containing the contaminants flowed through the illuminated catalyst, and was then analysed by direct on-line sampling using a GC-HP 6890 with FID. At different time intervals, samples of outlet gases from manually opened valves were adsorbed through Tenax, 2,6-diphenyl-p-phenylene oxide, for further GC/MS identification of generated by-products. The experimental set-up, methodologies and diagrams of the monolithic photoreactor and experimental apparatus have been previously described [9,10]. 2.2. Reaction parameters Parameters of the photocatalytic reaction are shown in Table 1. 2.3. Catalyst preparation The preparation of monolith supports without TiO2 has been previously described [13]. The v v square-cell monoliths, after drying at 110 C and treating at 500 C for 4 h in air, had a 3.1-cm 2 pitch, wall thickness of 0.11 cm and geometric surface of 832 m /m3. 2.3.1. Massic monolith The synthesis of the TiO2-containing magnesium silicate monolith has been described before [13]. Briefly, appropriate amounts of TiO2 precursor (Titafrance G5) -titanium dioxide hydroxylated gel supplied by Rho¨ne Poulenc, BET 250 m2/g and natural magnesium silicates, such as an inorganic binder, were thoroughly mixed with the necessary amount of water to make an extrudable green paste. The green paste was extruded through the templates to produce the silicate monolith containing TiO2. The monoliths were dried and thermally treated as described above. This photocatalyst was labelled the ‘‘massic monolith’’. Table 1 Reaction parameters used in the comparative study of the different photocatalysts tested v
Temperature ( C) Concentration (ppmv) Incident photonic intensity (Ei/m2 s) Residence time (s) Area velocity (m/h) Linear velocity (cm/s) Flow rates (l/min) Type of catalyst Irradiated area (cm2)
30–45 177–545 4.23 E03 0.182–0.364 4.098–8.195 8.24–16.48 0.7–1.4 Massic, sol–gel, impregnation 102.50
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2.3.2. Sol–gel monolith This catalyst was prepared by dip-coating the monolith support with a TiO2 sol precursor. The TiO2 sol was prepared by hydrolysis of titanium isopropoxide (Aldrich) in excess of water v acidulated with nitric acid (Merck, PA), followed by peptization at 80 C for 16 h [14]. This synthesis yields ca. 2% TiO2 sols with 5 to 10-nm-diameter particles aggregated in 100-nm clusters. The first layer of TiO2 sol was applied on a monolith previously saturated in the dispersing phase (water þ HNO3 , pH 1), by dipping it in the TiO2 sol at 1 cm/s followed by immediate withdrawal at the same speed to avoid excessive sol absorption by the dry porous monolith walls. The coating of the walls not exposed to UV light was prevented by wrapping the monoliths with Teflon film. These ‘‘coating-free’’ areas help the solvent to evaporate during drying. v The coated monoliths were dried overnight in an oven at 40 C. During drying, a film of TiO2 gel is formed, which further evolves into a mesoporous xerogel film. The TiO2 film should be stabilized by an appropriate thermal treatment that allows the particles to be sintered with their closest neighbours and with the magnesium silicate substrate. Firing conditions are critical to avoid sudden evaporation of water that may peel off the TiO2 v v layer. In this study, the thermal treatment was 2 h at 110 C (ramp: 1.5 C/min), followed by v v 4 h at 500 C (ramp: 3 C/min). The following layers of TiO2 were applied by dip-coating the coated monoliths (without previous saturation in the dispersing phase), at a withdrawal speed of 1 cm/min. Drying and firing v were as before at 500 C. 2.3.3. Impregnated monoliths The ‘‘impregnated monolith’’ was coated by immersion in a basic aqueous suspension of TiO2-G5 Rho¨ne Poulenc for 20 s at a rate of 20 cm/s. The process was repeated three times and v in all the cases the impregnated monolith was dried at room temperature (ca. 22 C). 2.4. Catalyst characterization Samples of monolithic catalysts before and after use in the reaction were studied with the following characterization techniques: textural and morphological properties were studied by nitrogen adsorption and mercury intrusion porosimetry (MIP). BET surface areas were measured by nitrogen adsorption–desorption in a Micromeritics ASAP 2000. Pore-size distribution was determined by MIP using a Thermo Quest Pascal 140/240 porosimeter. The crystallite structure phases present in the catalysts were determined by X-ray diffraction (XRD) in a Philips PW1710 v powder diffractometer in the range 5–75 (2h) region using CuKa radiation (k ¼ 0:15418 nm). The TiO2 content and overall composition of the monoliths was determined by inductively coupled plasma (ICP) optical emission spectroscopy (Perkin-Elmer Optima 3300DV) of a dissolution of the ground catalysts in acid solution. 3. Results and discussion The characteristics of the catalyst samples are shown in Table 2. The massic monolith has the greatest BET surface area and porosity. This phenomenon is a consequence of the interpenetration of the TiO2 particles among the magnesium silicate fibres. The sol–gel and the impreg-
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Table 2 Characteristics of the catalyst samples used in this study Catalyst samples
Method of preparation
TiO2 (wt%)
BET Surface area (m2/g)
Pore volume (cm3/g)
Support ‘‘Massic’’ ‘‘Impregnated’’ ‘‘Sol–gel’’
Extrusion Extrusion Washcoated Sol–gel dip-coating
0 44.5 2.3 4.5
88 98 84 82
0.74 0.70 0.62 0.61
nated monoliths have a smaller surface area because pores are clogged by the TiO2 particles deposited on the monolith surface. X-ray diffraction analysis of the catalysts showed that the main phase of TiO2 in the monolith was anatase. In the case of the impregnated monolith, a fraction of rutile was determined. The surface of the sol–gel monolith contains only anatase with a crystallite size of 5 nm, as determined by the Scherrer equation. TCE was used as the model to study the photocatalytic performance of the three different photocatalysts. Fig. 1 shows that under the conditions studied, the best performance was observed for the massic monolith, followed by the sol–gel coated and the impregnated monolith. By comparing the TiO2 content of each catalyst with the above results, it is clear that the larger amount of TiO2 and higher BET area and porosity are responsible for the better photocatalytic activity of the massic monolith. However, it is also noticeable that, in spite of the small amount of TiO2 supported on the other catalysts, they showed activity comparable to the massic monolith. This observation confirms that in the case of the massic monolith, TiO2 that remains in the bulk of the magnesium silicate support does not show significant catalytic activity. The higher activity of the sol–gel monolith in comparison to the impregnated monolith may be
v
v
Fig. 1. Percentage of TCE conversion for monolithic catalysts tested: massic, sol–gel 500 C and impregnated 500 C. Initial concentration of TCE 545 ppmv, flow rate 1.4 l/min, residence time 0.182 s, linear velocity 16.48 cm/s, area velocity 8.195 m/h, irradiated area 102.5 cm2 and incident photonic intensity 4.23E03 Ei=m2 s.
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Fig. 2. Conversion of a mixture of TCE, PCE and chloroform, in air (initial concentrations: TCE 187 ppmv; PCE 213 ppmv chloroform 177 ppmv) for the three type of monolithic catalysts tested. Flow rate 1.4 l/min, residence time 0.182 s, linear velocity 16.48 cm/s, area velocity 8.195 m/h, irradiated area 102.5 cm2 and incident photonic intensity 4.23E03 Ei=m2 s.
a consequence of both the larger amount and the smaller particle size of the TiO2 present in these catalysts. Fig. 2 shows the performance of the different catalysts for the PCO of a mixture of TCE, PCE and chloroform in air. These results indicate that the three catalysts are more selective to the oxidation of TCE than PCE, and that chloroform is refractive to oxidation under the conditions in Fig. 2. It should be noted that the experiments presented in Fig. 1 (Experiment 1) and Fig. 2 (Experiment 2) were carried out with air containing a similar total number of micromols (lmols) of organic compounds per liter of air (ca. 22 lmols in Experiment 1 and ca. 23 lmols in Experiment 2). The apparently higher yield in the degradation of TCE, when mixed with other chlorinated compounds (Fig. 2) compared to TCE alone, is due to the smaller amount of TCE in Experiment 2. Fig. 3 shows that there is competition between TCE and PCE for the adsorption sites and that the total number of lmols degraded in the experiments carried out with the mixture of gases is lower than in the experiments with TCE alone. That result may be a consequence of poisoning of the surface by by-products produced during the oxidation of PCE or by adsorption of chloroform, which required a longer residence time to start the oxidation process. This is supported by the results shown in Fig. 4, which demonstrate that with longer residence times it is possible to degrade small amounts of chloroform.
4. Conclusions Taking into account the different amounts of TiO2 in the two catalysts with the best results, massic monolith and sol–gel monolith, it may be said that the sol–gel catalyst yields greater net activity due to the fact that a large percentage of the TiO2 in the massic monolith is inside the
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Fig. 3. Micromols of TCE, PCE and total organic compounds degraded in Experiment 2, and micromols of TCE degraded in Experiment 1 showing the different performance of the catalysts when a mixture of organic compounds was used. The impregnated monolith had the poorest performance compared to TCE alone. Those results may be related to the different adsorption/desorption behaviour of the organics on different surfaces.
walls and, therefore, ineffective for reaction. However, the massic catalyst has better total performance and its fabrication is simpler than the sol–gel catalyst. On the other hand, the results discussed in this paper demonstrate that by coating monolith supports with TiO2 it is possible to obtain photocatalysts with acceptable efficiency. Since with the sol–gel process it is simple to incorporate doping agents that change the properties of the
Fig. 4. Conversion of a mixture of TCE, PCE and chloroform in air (initial concentrations: TCE 187 ppmv; PCE 213 ppmv ,chloroform 177 ppmv) for the three types of monolithic catalysts tested. Flow rate 0.7 l/min, residence time 0.364 s, lineal velocity 8.24 cm/s, area velocity 4.098 m/h, irradiated area 102.5 cm2 and incident photonic intensity 4.23 E03 Ei/m2 s.
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original catalyst, the combination of sepiolite supports coated with metal oxide sols is a promising method for the preparation of novel photocatalysts. Degradation of mixtures of chlorinated compounds seems to be strongly affected by the competition of the organics for adsorption sites. In order to improve our understanding of the process that takes place during gas-phase photocatalysis of complex systems, experiments with catalysts with selectively modified surfaces should be performed. In this context, sol–gel monolith catalysts may be useful for providing photocatalysts with modified surfaces. For example, more acidic surfaces may be synthesized by incorporation of acidic metal cations with TiO2. Acknowledgements The authors wish to thank the Spanish Ministry of Science and Technology (Project PPQ2001-0692-CO2-O2 and Profit FIT-140100-2001-158), the University of Buenos Aires, Argentina (Projetc JX85) and the V.G. and VIII.G. CYTED Networks for supporting this study.
References ´ xidos Semiconductores y Materiales rela[1] Collective text elaborated by the CYTED Network VIII-G. Usos de O ´ cionados para Aplicaciones Ambientales y Opticas. Eliminacio´n de Contaminantes por Fotocata´lisis Heteroge´nea. Ed. Miguel A. Blesa, 2001. ISBN: 987-43-3809-1. [2] Peral J, Dome`nech X, Ollis DF. Heterogeneous photocatalytic oxidation for purification, decontamination and deodorization of air. J Chem Technol Biotechnol 1997;70:117–40. [3] Pichat P, Disdier J, Hoang-Van C, Mas D, Goutailler G, Gaysse C. Purification/deodorization of indoor air and gaseous effluents by TiO2 photocat. Catal Today 2000;63:363–9. [4] Pal B, Nogami G, Sharon M. Preparation and characterization of TiO2/Fe2O3 binary mixed oxides and its photocatalytic properties. Mater Chem Phys 1999;59(3):254–61. [5] Ding Z, Hu X, Yue PL, Lu GQ, Greenfield PF. Synthesis of anatase TiO2 supported on porous solids by chemical vapor deposition. Catal Today 2001;68:173–82. [6] Zhang Q, Gao L, Guo J. Effects of calcination on the photocatalytic properties of nanosized TiO2 powders prepared by TiCl4 hydrolysis. Appl Catal B Environ 2000;26(3):207–15. [7] Jacoby WA, Blake DM, Noble RD, Koval CA. Kinetics of the oxidation of trichloroethylene in air via heterogeneous photocatalysis. J Catal 1995;157:87–96. [8] Hung C-H, Marin˜as BJ. Role of chlorine and oxygen in the photocatalytic degradation of trichloroethylene vapor on TiO2 films. Environ Sci Technol 1997;31:562–8. [9] Sa´nchez B, Cardona AI, Romero M, Avila P, Bahamonde A. Influence of temperature on gas-phase photoassisted mineralization of TCE using tubular and monolithic catalysts. Catal Today 1999;54:369–77. [10] Cardona AI, Sa´nchez B, Romero M, Fabrellas B, Garcı´a E, Blanco J, et al. Solarized photoreactors for degradation of chlorinated organics in air. J Phys IV France 1999;9:Pr3–271. [11] Yamazaki S, Tsukamoto H, Araki K, Tanimura T, Tejedor-Tejedor MI, Anderson MA. Photocatalytic degradation of gaseous tetrachloroethylene on porous TiO2 pellets. Appl Catal B Environ 2001;33:109–17. [12] Ku Y, Ma C-M, Shen Y-S. Decomposition of gaseous trichloroethylene in a photoreactor with TiO2-coated nonwoven fiber textile. Appl Catal B Environ 2001;34:181–90. [13] Avila P, Bahamonde A, Blanco J, Sa´nchez B, Cardona AI, Romero M. Gas-phase photo-assisted mineralization of volatile organic compounds by monolithic titania catalysts. Appl Catal B Environ 1998;17:75–88. [14] Bischoff BL, Anderson MA. Peptization properties in the sol–gel preparation of porous anatase (TiO2). Chem Mater 1995;7:1772–8.
TiO2 on magnesium silicate monolith: effects of different preparation techniques on the photocatalytic oxidation of chlorinated hydrocarbons ´ vila c, Ana I. Cardona a,, Roberto Candal b, Benigno Sa´nchez a, Pedro A Moise´s Rebollar c a
Departamento de Energı´as Renovables, CIEMAT, Avda. Complutense 22, 28040 Madrid, Spain INQUIMAE, Univ. de Buenos Aires Ciudad Universitaria, Pabello´n 2, 1428 Buenos Aires, Argentina c Consejo Sup. de Inv. Cientı´ficas ICP-CSIC Campus de la UAM. Cantoblanco, 28049 Madrid, Spain
b
Abstract In this article, the comparative results of the photocatalytic oxidation of trichloroethylene (TCE) alone and a mixture of chlorinated hydrocarbons (trichloroethylene, perchloroethylene and chloroform) in gas phase, obtained with three different monolithic catalysts in a flat reactor frontally illuminated with a Xenon lamp are presented. The three catalysts incorporate titanium dioxide (TiO2) as active phase on a magnesium silicate support, by means of different procedures: (i) incorporation of commercial TiO2 powder into the silicate matrix (‘‘massic monolith’’); (ii) sol–gel coating of the silicate support; (iii) impregnation with a commercial TiO2 aqueous suspension of the same silicate support. In the first case, the massic monolith was made from a 50:50 w/w mixture of magnesium silicate and ‘‘Titafrance G5’’ TiO2 powder. In the second case, a magnesium silicate monolith was coated with several layers of an aqueous TiO2 sol prepared from hydrolysis and condensation of titanium tetra-isopropoxide (Ti(OC3H7)4) in excess of acidified water (acid catalysis). The third catalyst was prepared by impregnating the same silicate support with several layers of ‘‘Titafrance G5’’ TiO2 powder water suspension. All the catalysts were thermal treated under comparable conditions in order to fix the TiO2 active phase to the silicate support. Although the performance of the massic monolith was better than the sol–gel monolith, the latter is of great interest because this technique allows the chemical composition of the active films to be easily modified. # 2003 Elsevier Ltd. All rights reserved.
Corresponding author. Tel.: +34-91-3466177; fax: +34-91-3466037. E-mail address: [email protected] (A.I. Cardona).
0360-5442/$ - see front matter # 2003 Elsevier Ltd. All rights reserved. doi:10.1016/S0360-5442(03)00184-1
846
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1. Introduction Because of stricter requirements on hazardous waste disposal and increased concern for the environment, for the last 20 years, photocatalytic oxidation (PCO) has been investigated in response to the need for innovative technologies to handle these problems. Much of the rapidly growing literature on heterogeneous photocatalytic oxidative degradation of organic compounds in gas phase is summarized in a recently collective publication [1]. Among other applications, a major motivation for the development of photocatalytic processes is their potential use in air purification [2,3] and other applications that require the treatment of pollutants in low concentrations. It is clear, in view of work published to date, that the efficiency of this technology in the elimination of pollutants depends on the nature of the organic compounds to be destroyed. Furthermore, the use of different reactor configurations, ways in which catalysts are prepared and the experimental conditions have made the comparison of results found in the literature difficult. An important part of research in the field of photocatalysis applied to the destruction of organic contaminants has focused on the improvement of catalyst performance. Thus different methods of applying the active components (in the majority of cases, titanium dioxide, TiO2) on a variety of supports have been developed [4–6]. The two basic techniques for the incorporation of TiO2 in fixed-bed and packed-bed reactors, or in the surface of those supports tested to date are: (i) dispersion of the active phase throughout the entire volume of the support (massic catalyst) and (ii) deposition of films on different supports. Supports can be coated with TiO2 films by different processes, such as, chemical vapour deposition, physical vapour deposition, sputtering, dip-coating with TiO2 suspensions, dip-coating sol–gel, among others. Dip-coating is a very attractive methodology because it is simple and does not require expensive equipment. However, special care should be taken in the selection of the appropriate preparation conditions to avoid excessive cracking of the film and the consequent delamination of the TiO2 layer. The selection of the support material is critical because it directly affects the activity, homogeneity and adhesion of TiO2 to the surface. Materials like ceramics tiles, paper, glass, fibreglass and stainless steel have been employed and other materials are currently being developed for possible future photocatalytic applications. An extensive variety of organic compounds have been tested for PCO, showing different sensibilities to this process. Chlorinated hydrocarbons, like trichloroethylene (TCE) perchloroethylene (PCE) and chloroform have probably been studied the most [7–12]. These organics are widely used industrial solvents polluting water and air. In this paper, we compare the efficiency of the destruction of TCE alone and in a mixture of chlorinated hydrocarbons with a sepiolite-based monolithic photocatalyst on which the active phase, TiO2, was incorporated in different ways: (i) by the incorporation of commercial TiO2 powder into a magnesium silicate monolith (sepiolite ‘‘massic monolith’’); (ii) sol–gel dip-coating of magnesium silicate supports (‘‘sol–gel monolith’’) and (iii) impregnation of the same supports with a commercial TiO2 aqueous suspension (‘‘impregnated monolith’’).
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2. Experimental system 2.1. Photocatalytic system Continuous-flow-mode tests lasting several hours were conducted to evaluate conversion efficiencies. The photoreactor is stainless steel and has an effective aperture diameter of 4 cm enclosed by a Pyrex window. A Xenon lamp illuminates the front window and the catalyst monoliths. The gas carrier (CO2-free air) containing the contaminants flowed through the illuminated catalyst, and was then analysed by direct on-line sampling using a GC-HP 6890 with FID. At different time intervals, samples of outlet gases from manually opened valves were adsorbed through Tenax, 2,6-diphenyl-p-phenylene oxide, for further GC/MS identification of generated by-products. The experimental set-up, methodologies and diagrams of the monolithic photoreactor and experimental apparatus have been previously described [9,10]. 2.2. Reaction parameters Parameters of the photocatalytic reaction are shown in Table 1. 2.3. Catalyst preparation The preparation of monolith supports without TiO2 has been previously described [13]. The v v square-cell monoliths, after drying at 110 C and treating at 500 C for 4 h in air, had a 3.1-cm 2 pitch, wall thickness of 0.11 cm and geometric surface of 832 m /m3. 2.3.1. Massic monolith The synthesis of the TiO2-containing magnesium silicate monolith has been described before [13]. Briefly, appropriate amounts of TiO2 precursor (Titafrance G5) -titanium dioxide hydroxylated gel supplied by Rho¨ne Poulenc, BET 250 m2/g and natural magnesium silicates, such as an inorganic binder, were thoroughly mixed with the necessary amount of water to make an extrudable green paste. The green paste was extruded through the templates to produce the silicate monolith containing TiO2. The monoliths were dried and thermally treated as described above. This photocatalyst was labelled the ‘‘massic monolith’’. Table 1 Reaction parameters used in the comparative study of the different photocatalysts tested v
Temperature ( C) Concentration (ppmv) Incident photonic intensity (Ei/m2 s) Residence time (s) Area velocity (m/h) Linear velocity (cm/s) Flow rates (l/min) Type of catalyst Irradiated area (cm2)
30–45 177–545 4.23 E03 0.182–0.364 4.098–8.195 8.24–16.48 0.7–1.4 Massic, sol–gel, impregnation 102.50
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2.3.2. Sol–gel monolith This catalyst was prepared by dip-coating the monolith support with a TiO2 sol precursor. The TiO2 sol was prepared by hydrolysis of titanium isopropoxide (Aldrich) in excess of water v acidulated with nitric acid (Merck, PA), followed by peptization at 80 C for 16 h [14]. This synthesis yields ca. 2% TiO2 sols with 5 to 10-nm-diameter particles aggregated in 100-nm clusters. The first layer of TiO2 sol was applied on a monolith previously saturated in the dispersing phase (water þ HNO3 , pH 1), by dipping it in the TiO2 sol at 1 cm/s followed by immediate withdrawal at the same speed to avoid excessive sol absorption by the dry porous monolith walls. The coating of the walls not exposed to UV light was prevented by wrapping the monoliths with Teflon film. These ‘‘coating-free’’ areas help the solvent to evaporate during drying. v The coated monoliths were dried overnight in an oven at 40 C. During drying, a film of TiO2 gel is formed, which further evolves into a mesoporous xerogel film. The TiO2 film should be stabilized by an appropriate thermal treatment that allows the particles to be sintered with their closest neighbours and with the magnesium silicate substrate. Firing conditions are critical to avoid sudden evaporation of water that may peel off the TiO2 v v layer. In this study, the thermal treatment was 2 h at 110 C (ramp: 1.5 C/min), followed by v v 4 h at 500 C (ramp: 3 C/min). The following layers of TiO2 were applied by dip-coating the coated monoliths (without previous saturation in the dispersing phase), at a withdrawal speed of 1 cm/min. Drying and firing v were as before at 500 C. 2.3.3. Impregnated monoliths The ‘‘impregnated monolith’’ was coated by immersion in a basic aqueous suspension of TiO2-G5 Rho¨ne Poulenc for 20 s at a rate of 20 cm/s. The process was repeated three times and v in all the cases the impregnated monolith was dried at room temperature (ca. 22 C). 2.4. Catalyst characterization Samples of monolithic catalysts before and after use in the reaction were studied with the following characterization techniques: textural and morphological properties were studied by nitrogen adsorption and mercury intrusion porosimetry (MIP). BET surface areas were measured by nitrogen adsorption–desorption in a Micromeritics ASAP 2000. Pore-size distribution was determined by MIP using a Thermo Quest Pascal 140/240 porosimeter. The crystallite structure phases present in the catalysts were determined by X-ray diffraction (XRD) in a Philips PW1710 v powder diffractometer in the range 5–75 (2h) region using CuKa radiation (k ¼ 0:15418 nm). The TiO2 content and overall composition of the monoliths was determined by inductively coupled plasma (ICP) optical emission spectroscopy (Perkin-Elmer Optima 3300DV) of a dissolution of the ground catalysts in acid solution. 3. Results and discussion The characteristics of the catalyst samples are shown in Table 2. The massic monolith has the greatest BET surface area and porosity. This phenomenon is a consequence of the interpenetration of the TiO2 particles among the magnesium silicate fibres. The sol–gel and the impreg-
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Table 2 Characteristics of the catalyst samples used in this study Catalyst samples
Method of preparation
TiO2 (wt%)
BET Surface area (m2/g)
Pore volume (cm3/g)
Support ‘‘Massic’’ ‘‘Impregnated’’ ‘‘Sol–gel’’
Extrusion Extrusion Washcoated Sol–gel dip-coating
0 44.5 2.3 4.5
88 98 84 82
0.74 0.70 0.62 0.61
nated monoliths have a smaller surface area because pores are clogged by the TiO2 particles deposited on the monolith surface. X-ray diffraction analysis of the catalysts showed that the main phase of TiO2 in the monolith was anatase. In the case of the impregnated monolith, a fraction of rutile was determined. The surface of the sol–gel monolith contains only anatase with a crystallite size of 5 nm, as determined by the Scherrer equation. TCE was used as the model to study the photocatalytic performance of the three different photocatalysts. Fig. 1 shows that under the conditions studied, the best performance was observed for the massic monolith, followed by the sol–gel coated and the impregnated monolith. By comparing the TiO2 content of each catalyst with the above results, it is clear that the larger amount of TiO2 and higher BET area and porosity are responsible for the better photocatalytic activity of the massic monolith. However, it is also noticeable that, in spite of the small amount of TiO2 supported on the other catalysts, they showed activity comparable to the massic monolith. This observation confirms that in the case of the massic monolith, TiO2 that remains in the bulk of the magnesium silicate support does not show significant catalytic activity. The higher activity of the sol–gel monolith in comparison to the impregnated monolith may be
v
v
Fig. 1. Percentage of TCE conversion for monolithic catalysts tested: massic, sol–gel 500 C and impregnated 500 C. Initial concentration of TCE 545 ppmv, flow rate 1.4 l/min, residence time 0.182 s, linear velocity 16.48 cm/s, area velocity 8.195 m/h, irradiated area 102.5 cm2 and incident photonic intensity 4.23E03 Ei=m2 s.
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Fig. 2. Conversion of a mixture of TCE, PCE and chloroform, in air (initial concentrations: TCE 187 ppmv; PCE 213 ppmv chloroform 177 ppmv) for the three type of monolithic catalysts tested. Flow rate 1.4 l/min, residence time 0.182 s, linear velocity 16.48 cm/s, area velocity 8.195 m/h, irradiated area 102.5 cm2 and incident photonic intensity 4.23E03 Ei=m2 s.
a consequence of both the larger amount and the smaller particle size of the TiO2 present in these catalysts. Fig. 2 shows the performance of the different catalysts for the PCO of a mixture of TCE, PCE and chloroform in air. These results indicate that the three catalysts are more selective to the oxidation of TCE than PCE, and that chloroform is refractive to oxidation under the conditions in Fig. 2. It should be noted that the experiments presented in Fig. 1 (Experiment 1) and Fig. 2 (Experiment 2) were carried out with air containing a similar total number of micromols (lmols) of organic compounds per liter of air (ca. 22 lmols in Experiment 1 and ca. 23 lmols in Experiment 2). The apparently higher yield in the degradation of TCE, when mixed with other chlorinated compounds (Fig. 2) compared to TCE alone, is due to the smaller amount of TCE in Experiment 2. Fig. 3 shows that there is competition between TCE and PCE for the adsorption sites and that the total number of lmols degraded in the experiments carried out with the mixture of gases is lower than in the experiments with TCE alone. That result may be a consequence of poisoning of the surface by by-products produced during the oxidation of PCE or by adsorption of chloroform, which required a longer residence time to start the oxidation process. This is supported by the results shown in Fig. 4, which demonstrate that with longer residence times it is possible to degrade small amounts of chloroform.
4. Conclusions Taking into account the different amounts of TiO2 in the two catalysts with the best results, massic monolith and sol–gel monolith, it may be said that the sol–gel catalyst yields greater net activity due to the fact that a large percentage of the TiO2 in the massic monolith is inside the
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Fig. 3. Micromols of TCE, PCE and total organic compounds degraded in Experiment 2, and micromols of TCE degraded in Experiment 1 showing the different performance of the catalysts when a mixture of organic compounds was used. The impregnated monolith had the poorest performance compared to TCE alone. Those results may be related to the different adsorption/desorption behaviour of the organics on different surfaces.
walls and, therefore, ineffective for reaction. However, the massic catalyst has better total performance and its fabrication is simpler than the sol–gel catalyst. On the other hand, the results discussed in this paper demonstrate that by coating monolith supports with TiO2 it is possible to obtain photocatalysts with acceptable efficiency. Since with the sol–gel process it is simple to incorporate doping agents that change the properties of the
Fig. 4. Conversion of a mixture of TCE, PCE and chloroform in air (initial concentrations: TCE 187 ppmv; PCE 213 ppmv ,chloroform 177 ppmv) for the three types of monolithic catalysts tested. Flow rate 0.7 l/min, residence time 0.364 s, lineal velocity 8.24 cm/s, area velocity 4.098 m/h, irradiated area 102.5 cm2 and incident photonic intensity 4.23 E03 Ei/m2 s.
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original catalyst, the combination of sepiolite supports coated with metal oxide sols is a promising method for the preparation of novel photocatalysts. Degradation of mixtures of chlorinated compounds seems to be strongly affected by the competition of the organics for adsorption sites. In order to improve our understanding of the process that takes place during gas-phase photocatalysis of complex systems, experiments with catalysts with selectively modified surfaces should be performed. In this context, sol–gel monolith catalysts may be useful for providing photocatalysts with modified surfaces. For example, more acidic surfaces may be synthesized by incorporation of acidic metal cations with TiO2. Acknowledgements The authors wish to thank the Spanish Ministry of Science and Technology (Project PPQ2001-0692-CO2-O2 and Profit FIT-140100-2001-158), the University of Buenos Aires, Argentina (Projetc JX85) and the V.G. and VIII.G. CYTED Networks for supporting this study.
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