Effect of void structure of photocatalyst paper on VOC decomposition

Chemosphere 66 (2007) 2136–2141 www.elsevier.com/locate/chemosphere

Effect of void structure of photocatalyst paper on VOC decomposition Shuji Fukahori, Yumi Iguchi, Hideaki Ichiura, Takuya Kitaoka *, Hiroo Tanaka, Hiroyuki Wariishi Department of Forest and Forest Products Sciences, Graduate School of Bioresource and Bioenvironmental Sciences, Kyushu University, Fukuoka 812-8581, Japan Received 4 March 2006; received in revised form 12 September 2006; accepted 12 September 2006 Available online 12 December 2006

Abstract TiO2 powder-containing paper composites, called TiO2 paper, were prepared by a papermaking technique, and their photocatalytic efficiency was investigated. The TiO2 paper has a porous structure originating from the layered pulp fiber network, with TiO2 powders scattered on the fiber matrix. Under UV irradiation, the TiO2 paper decomposed gaseous acetaldehyde more effectively than powdery TiO2 and a pulp/TiO2 mixture not in paper form. Scanning electron microscopy and mercury intrusion analysis revealed that the TiO2 paper had characteristic unique voids ca. 10 lm in diameter, which might have contributed to the improved photocatalytic performance. TiO2 paper composites having different void structures were prepared by using beaten pulp fibers with different degrees of freeness and/or ceramic fibers. The photodecomposition efficiency was affected by the void structure of the photocatalyst paper, and the initial degradation rate of acetaldehyde increased with an increase in the total pore volume of TiO2 paper. The paper voids presumably provided suitable conditions for TiO2 catalysis, resulting in higher photocatalytic performance by TiO2 paper than by TiO2 powder and a pulp/TiO2 mixture not in paper form.  2006 Elsevier Ltd. All rights reserved. Keywords: TiO2; Photocatalysis; VOC; Papermaking technique; Paper structure

1. Introduction TiO2 has photo-induced catalytic ability, and especially anatase-type TiO2 demonstrates a high oxidizing power when illuminated by near-UV light (Hashimoto et al., 2001). During the past two decades, photocatalytic oxidation of organic substances by TiO2 photocatalysis has become attractive as an energy-saving chemical process (Benoit-Marquie´ et al., 2000; Ao et al., 2003). Hydroxyl radicals produced on hydrated TiO2 surfaces can oxidatively convert various organic compounds in contact with TiO2 to CO2 and H2O, and thus much research on TiO2 photo-oxidation has been carried out with regard to the ´ vila et al., 2002; decomposition of organic pollutants (A *

Corresponding author. Tel./fax: +81 92 642 2993. E-mail address: [email protected] (T. Kitaoka).

0045-6535/$ - see front matter  2006 Elsevier Ltd. All rights reserved. doi:10.1016/j.chemosphere.2006.09.022

Ao and Lee, 2003; Mohseni and David, 2003; Jiang et al., 2004; Parra et al., 2004; Lim and Kim, 2005). For environmental cleanup, practical TiO2-based products have been developed, such as TiO2-coated tiles, TiO2-glass (Sopyan et al., 1994), TiO2-film or sheet materials (Noguchi and Fujishima, 1998; Dhananjeyan et al., 2000; Vankelecom, 2002) and TiO2-containing paper (Matsubara et al., 1995). Of the various TiO2-based materials, paper-like composite containing photoactive TiO2 powders, called TiO2 paper or photocatalyst paper here, is promising because paper-shaped materials have a wide variety of applications in rooms. We have previously reported the preparation of zeolite- and/or TiO2-filled paper-like composites for treatment of environmental pollution (Ichiura et al., 2002, 2003a,b,c; Fukahori et al., 2003a,b). We also successfully prepared paper-based TiO2 composite having high photocatalytic performance and

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high durability, by a papermaking technique (Iguchi et al., 2003). In that photocatalyst paper, which contained ceramic fibers as a supporting matrix for TiO2 powders, most of the photoactive TiO2 particles were preferentially immobilized on the inorganic fibers and incorporated into the layered pulp fiber network out of contact with the organic pulp matrix, resulting in high durability. Such photocatalyst paper is unique in regard to its structural properties and photocatalytic phenomena. In this study, the void structure of TiO2 paper and its photocatalytic efficiency with respect to the decomposition of acetaldehyde gas were investigated. TiO2 paper sheets having a variety of void structures were prepared by using beaten pulp and/or ceramic fibers, and their photocatalytic performances were compared. The relationship between the total pore volume of TiO2 paper and the photodegradation efficiency is also discussed. 2. Materials and methods 2.1. Materials Anatase-type TiO2 powder (ST-01) was purchased from Ishihara Sangyo Kaisha, Ltd. Commercial softwood bleached kraft pulp was beaten to 460 or 700 ml of Canadian Standard Freeness (CSF) according to Technical Association of the Pulp and Paper Industry (TAPPI) Test Methods T200 and T227 with a standard beater for paper sheet making. Ceramic fibers (ISOWOOL 1260 BULK, ISOLITE Industry, Ltd.) were cut into ca. 0.5 mm lengths on average with a two-flute end mill. Two types of flocculant, polydiallyldimethylammonium chloride (PDADMAC, molecular weight (Mw): ca. 3 · 105; charge density (CD): 5.5 meq g 1; Aldrich, Ltd.) and anionic polyacrylamide (A-PAM, HH351; Mw: ca. 4 · 106; CD: 0.64 meq g 1; Kurita, Ltd.) were used as retention aids in a dual polyelectrolyte system (Iguchi et al., 2003). 2.2. Preparation of TiO2 paper Various photocatalyst paper sheets with a grammage of 60 g m 2 were prepared according to TAPPI Test Methods Pulp suspension (0.15%)

Pulp and ceramic fiber suspension (0.15%) PDADMAC

TiO2 powder

TiO2 powder A-PAM

Freeze-dried

Handsheet making Oven-dried (105 °C)

Pulp/TiO2 mixture TiO2 paper

Ceramic fiber/TiO2 paper

Fig. 1. Preparation of TiO2 paper.

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T205, as illustrated in Fig. 1. TiO2 powders were poured into pulp suspension (0.15% wt/vol) prepared from beaten pulp fibers with CSF = 460 or 700 ml, and TiO2 paper sheets without flocculants were made from the suspensions by using a standard sheet-forming machine equipped with a 200-mesh (0.074 mm) wire. The TiO2 content in the handmade paper was adjusted to 12.5% wt/wt of pulp by controlling the dosage of TiO2 powder. The wet sheets were pressed at 350 kPa for 5 min, and then dried in an oven at 105 C for 10 min. Anatase phase of original TiO2 remained unchanged by thermal drying. The pulp/TiO2 mixture was obtained by freeze-drying the paper stock containing pulp fiber (CSF = 460 ml) and the designated amounts of TiO2 powder. Ceramic fiber/TiO2 paper was made as follows: 0.15% fiber suspension (weight ratio 9:1 of ceramic fiber to pulp) was mixed with cationic flocculant PDADMAC (0.1% of fiber component), TiO2 powder (12.5% of fiber component) and then A-PAM (0.1% of fiber component), in that order, at intervals of 3 min. The mixed suspension was dewatered to form a paper web (Japanese Patent 3686875, 2005). The photocatalyst paper sheets obtained were conditioned for more than 24 h at 23 C and a relative humidity of 50%. 2.3. TiO2 and ceramic fiber contents The ash content of the photocatalyst paper was gravimetrically determined by calcination in an electric furnace at 700 C for 20 min (Iguchi et al., 2003). The retention of ceramic fibers was confirmed to be more than 98% under the papermaking conditions used in this study. The TiO2 content was calculated by subtracting the ceramic fiber content from the ash content, considering that the weight losses of hydrated TiO2 and ceramic fibers at 700 C are ca. 13% and trace, respectively. 2.4. Photocatalytic decomposition of acetaldehyde gas The photodegradation of gaseous acetaldehyde by TiO2 powder or TiO2 paper samples was investigated under weak UV irradiation at 365 nm; the actual UV intensity on the surfaces of photocatalyst samples was set at ca. 3.0 W m 2. The TiO2 powder, TiO2 paper (25 mm · 25 mm · 0.15 mm high) or cottony pulp/TiO2 mixture was placed in a cylindrical reaction vessel (80 mm in diameter and 40 mm in height, ca. 2.0 · 105 mm3) equipped with a quartz cover and a agitator with a four-bladed propeller. Acetaldehyde standard gas was injected into the vessel using a microsyringe. TiO2 powder and TiO2/pulp mixture were sufficiently scattered at the bottom of the reaction vessel. The initial concentration of acetaldehyde gas in the reaction vessel was adjusted to 250 ppm (vol/vol). The TiO2 photocatalyst was irradiated by UV light through the quartz cover and the injected gas (1 ml) was continuously stirred during the photocatalytic reaction. A gas chromatograph (GC-17 A, Shimadzu, Ltd.) equipped with a thermal conductivity detector and a 30 m · 0.53 mm

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Supel-Q PLOT column (SUPELCO, Ltd.) quantitatively measured the concentration of residual acetaldehyde in the reaction vessel. All experiments were carried out more than three times at room temperature (20 ± 1 C) and the standard deviations were within 5% in each photodegradation test. 2.5. Other analyses The surfaces of TiO2 paper samples were observed by scanning electron microscopy (SEM) in a JSM-5600 apparatus by JEOL, Ltd. after carbon coating. The electron accelerating voltage was set at 10 kV, and the SEM images were obtained under a high vacuum. Mercury intrusion analysis was carried out using a PoreMaster 33P (YUASA IONICS, Ltd.) to evaluate the pore volumes and size distributions of TiO2 paper samples, by using the pressurecumulative mercury intrusion volume curves (Landry, 2005). 3. Results and discussion 3.1. SEM observation of TiO2 paper surfaces TiO2 samples with identical amounts of TiO2 photocatalyst but different void structures were successfully prepared in a similar manner to that reported in our previous study (Iguchi et al., 2003). The TiO2 content was ca. 0.15 g per paper sheet (12.5% wt/wt). The SEM images of TiO2 paper sheets prepared with pulp fibers (CSF = 460 or 700 ml) and ceramic fiber/TiO2 paper are shown in Fig. 2. Pulp and ceramic fibers formed characteristic network structures, and fine TiO2 particles were scattered on the fiber matrix. 3.2. Photodecomposition of acetaldehyde gas by TiO2 paper The photocatalytic performance of TiO2 paper prepared from pulp fibers (CSF = 460 ml) was first compared with that of TiO2 powder and a pulp/TiO2 mixture. Fig. 3 shows the photodecomposition profiles of gaseous acetaldehyde treated by three types of TiO2 samples under UV irradiation (365 nm, 3 W m 2). The acetaldehyde concentration decreased over time under UV irradiation, but the photodegradation tendencies differed between the different types of TiO2 samples. In this case, it was confirmed that neither photolysis nor simple adsorption of acetaldehyde occurred (Iguchi et al., 2003). When TiO2 samples were placed in the reaction vessel, it was commonly observed that the acetaldehyde concentration decreased quickly in 5 min without UV irradiation. Almost no adsorption of acetaldehyde on fiber components and no photolysis of acetaldehyde were confirmed. Thus, the decrease in acetaldehyde concentration was attributed to simple adsorption onto TiO2 surfaces. Such adsorption leveled off after the initial 5 min, thus the UV lamp was switched on 5 min after acetaldehyde gas was injected into the reaction vessel. Of the three

Fig. 2. SEM images of surfaces of TiO2 paper sheets with CSF = 460 ml (a) or 700 ml (b) and ceramic fiber/TiO2 paper (c).

samples, TiO2 powders were expected to have the highest photocatalytic performance since they have the highest specific surface area, which could be assumed from the amounts of acetaldehyde physisorbed before UV irradiation. On the other hand, both TiO2 paper and the pulp/ TiO2 mixture had less accessible specific surface area. Thus,

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the mixture showed a lower photocatalytic performance than the powder, as expected. However, it was interesting to note that the TiO2 paper displayed remarkably high photocatalytic performance and quickly decomposed the acetaldehyde. After 80 min of UV irradiation, the concentration of acetaldehyde was below the detection limit. In this experimental procedure, the TiO2 powder might become poisoned by the initial VOC adsorption. Identical amounts of TiO2 photocatalyst were used in all experiments, and the TiO2 paper had the same composition as the pulp/TiO2 mixture. However, there was a significant difference in the photocatalytic performance before and after the paper formation. TiO2 paper has a characteristic pore structure originating from the layered pulp fiber network, and it was thought that the void structure might be a factor involved in the high photocatalytic efficiency. 3.3. Pore structures of TiO2 samples with different shapes Fig. 4 shows the pore size distributions of TiO2 paper and the pulp/TiO2 mixture obtained through mercury intrusion analysis. A clear peak around 10 lm was observed only for the TiO2 paper. The pulp/TiO2 mixture had a featureless pore size distribution, like that of a massive ball of string. The average pore size obtained for the TiO2 paper was almost in agreement with the SEM observation result shown in Fig. 2. Another peak around 8 nm was observed for both the TiO2 paper and the pulp/TiO2 mixture; thus these fine pores are presumably derived from the mesopores on the surfaces of the TiO2 particles. These two samples have the same amounts of TiO2 photocatalyst and showed similar acetaldehyde adsorptivities before UV irradiation. However, under UV irradiation the TiO2 paper demonstrated a remarkably higher VOC decomposition efficiency than the pulp/TiO2 mixture even though the components were exactly the same. Pore analysis revealed that the nanostructure of the TiO2 surface remained almost unchanged,

0.0 -3 10

-2

10

-1

10 1 Pore diameter (μm)

10

2

10

Fig. 4. Pore structures of TiO2 paper (open diamonds) and pulp/TiO2 mixture (closed squares).

but that the structure of macrovoid around 10 lm in diameter varied considerably. Thus, the contribution of the macrovoid in TiO2 paper is of great significance in VOC decomposition by TiO2 paper. 3.4. Effect of paper void structure on acetaldehyde photodecomposition TiO2 paper samples having different macrovoid structures were prepared by using pulp fibers with different CSF values or by using ceramic fibers. The CSF value is a fibrillation index of pulp fibers; a low CSF value means that pulp fibers are well fibrillated and flexible, resulting in a dense fiber network. On the other hand, pulp fibers with high CSF values are rigid and make the paper bulky. Inorganic ceramic fibers are slender and rigid, thus the ceramic fiber/TiO2 paper was more porous than the TiO2 paper made with pulp fibers with CSF = 700 ml, as shown in Fig. 2. The photodecomposition profiles of acetaldehyde gas treated by TiO2 paper sheets with different voids are given in Fig. 5. The apparent photocatalytic activity of TiO2 paper was affected by the CSF values of pulp fibers, and the TiO2 paper with CSF = 700 ml could decompose acetaldehyde more effectively than that with CSF = 460 ml. The ceramic fiber/TiO2 paper having more porous than TiO2 paper with CSF = 460 ml also showed the higher catalytic efficiency equivalent to the TiO2 paper with CSF = 700 ml. Neither adsorption of acetaldehyde onto ceramic fibers nor significant differences in the amounts of adsorbed acetaldehyde prior to UV irradiation were confirmed. Therefore, the high performance of TiO2 paper was attributed not to the fiber type but to the macrovoid structure of the TiO2 paper samples. The correlation between the total pore volume of TiO2 paper samples and their initial photodegradation rate is shown in Fig. 6. A higher CSF value and the combination use of ceramic fibers provided a larger total pore volume,

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interesting from a practical point of view. It is easy to control the paper void structure by controlling the degree of beating of pulp fibers and the combination of fiber components, and thus paper-structured photocatalyst materials are expected to be promising in practical applications.

Concentration of acetaldehyde (ppm)

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B

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Fig. 5. Photodecomposition of acetaldehyde gas by TiO2 paper with CSF = 460 ml (open diamonds) or 700 ml (open circles) and ceramic fiber/ TiO2 paper (open triangles).

Fig. 6. Total pore volumes (vertical bars) of TiO2 paper samples and initial degradation rates of acetaldehyde under UV irradiation (closed squares); TiO2 paper prepared using pulp fiber with CSF = 460 ml (A) or TiO2 paper prepared using pulp fiber with CSF = 700 ml (B) and ceramic fiber/TiO2 paper (C).

as seen in the SEM images (Fig. 2). The initial degradation rate from 5 min to 25 min improved with an increase in the total pore volume of TiO2 paper. The same amounts of TiO2 catalyst were used in all cases; thus these results indicate that the void structure of TiO2 paper greatly influences the apparent photocatalytic performance with respect to VOC decomposition. Such differences in the catalytic performances of TiO2 paper sheets suggest that the catalytic efficiency could depend on the surroundings. Recently, the preparation of novel catalyst materials having micrometer-scale pore structures has been actively investigated (Mukai et al., 2003; Nishihara et al., 2005; Takahashi et al., 2005), and a close relationship between catalytic performance and micrometer-scale pores of solid-type catalysts has been proposed (Takahashi et al., 2005). The micrometer-scale and semi-closed void structures in TiO2 paper are unique characteristics of paper materials, and might provide suitable conditions for TiO2 photocatalysis for acetaldehyde decomposition. Such improvement in the photocatalytic performance of a paper structure is

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