TiO 2 support

Over the past few decades, the scientific and engineering interest in the application of heterogeneous photocatalysis by using TiO2 powder for the decomposition of organic hazardous materials in water has grown exponentially. However, there are some drawbacks of the practical use of the powder TiO2 during the photocatalytic process (Sopyan et al., 1996):

(1) separation of TiO2 powder from water is difficult; (2) the suspended TiO2 powder tends to aggregate especially at high concentrations (Araña et al., 2003b). Therefore, to solve these problems, much attention has been paid to the development of supported TiO2 (Chun et al., 2001; Hosseini et al., 2007). Several studies have been published on the effect of the characteristics of the supported TiO2 photocatalysts, such as crystal structure (Song et al.,

45 2006), crystal size (Zhang et al., 2005a; Chen and Dionysiou, 2006), TiO2 loading (Zhang et al., 2006b), specific surface area (Chen and Dionysiou, 2006), and thickness of film (Jung et al., 2005) on their reactivity to obtain the best photocatalyst or optimize the operation parameters of the photocatalyst preparation process. Obviously, only TiO2 on the external surface of the support can be excited by light and induce the photocatalytic reaction, so the concentration of TiO2 on the external surface of the support is a very important parameter of the supported TiO2 photocatalyst (Jung et al., 2005; Zhang et al., 2005a,b,c).

Many supporting materials and coating methods were proposed for degradation of several organic compounds (Pozzo et al., 1997). One possible way was the use of materials such as silica, alumina, zeolites or clays (Tanguay et al., 1989), but no improvement of photo-efficiency was observed.

Among these particle supports, activated carbon (AC) is very promising for three reasons: (1) activated carbon is able to adsorb the pollutants and then release them onto the surface of TiO2. Consequently, a higher concentration of pollutants around the TiO2 than that in the bulk solution is created leading to an increase in the degradation rate of the pollutants (Tsumura et al., 2002; Tryba et al., 2003; Matos et al., 2009); (2) the charge transference between TiO2 and activated carbon can cause an acidification of TiO2 surface hydroxylic groups. This will enhance the interaction between some pollutants and TiO2 to further promote the degradation. Moreover, the ability to absorb visible light of the supported TiO2 is also enhanced (Arana et al., 2003a,b); and (3) the intermediates produced during degradation can be also adsorbed by activated carbon and then further oxidized. Other authors (Herrmann et al., 1999; Matos et al., 2001) have reported a synergistic effect for AC-supported TiO2

systems, refering to remarkable effects in the kinetics of pollutant degradation, each pollutant being more rapidly photodegraded in the mixed system which contained activated carbon.

This so called synergetic effect has been explained by the formation of a common contact interface between the different solid phases, in which AC acts as an efficient adsorption trap to the organic pollutant, which is then more efficiently transferred to the TiO2 surface, where it is immediately photocatalytically degraded by a mass transfer to the photoactivated TiO2

(Ao and Lee, 2003; Chiang and Huang 2001; Arana et al.,. 2003; Tryba et al., 2003; Colon et al., 2004; Ingaki et al., 2004; Tao et al., 2005 & 2006; Wang et al., 2007a). A mechanism for the enhanced activity of TiO2/AC was illustrated in Fig. I-15 (Liu et al., 2007; Lu et al., 2010).

46 Fig.I-15. Role of AC in the enhanced activity of TiO2 (Liu et al., 2007).

Zainal et al. (2008) reported that the removal efficiency of Methylene Blue by using immobilised TiO2/AC was found to be two times better than the removal by immobilised AC or immobilised TiO2 alone. Similarly, Liu et al. (2006) cited that TiO2/AC composite was much more active than P-25 for phenol degradation and exhibiting good decantability, less deactivation after several runs and less sensitivity to pH change. Moreover, Arana et al., (2003b) reported that AC could perform an acidification of surface hydroxylic groups in the supported TiO2 with the hydroxylic groups in TiO2 supported on AC being more acidic than those in the bare-TiO2 (P25). Since the intermediates are alkaline, they could interact strongly with TiO2/AC to give further degradation of the intermediates. In addition, AC has a large capability of adsorbing the intermediates preventing them from dissolving into the bulk solution in the case of TiO2/AC. Thus chain photodecomposition reactions can be preceded with the intermediates mineralized. On the contrary, the use of powder TiO2 would allow the dissolution of some intermediates into solution and the dissolved intermediates can be decomposed further only when they collided with TiO2 again.

Several authors argued that the development of TiO2 photocatalysts anchored on supporting AC materials with large surface areas, by which dilute polluted substances could be adsorbed would be of great significance, not only to avoid the disadvantages of filtration

47 and suspension of fine photocatalyst particles, but also to lead to high photodecomposition efficiency for large numbers of pollutant (Yoneyama and Torimoto, 2000; Matos et al., 2001;

Colon et al., 2003; Li et al., 2006; Zhang and Lei, 2008; Zhu and Zou, 2009a; Ravichandran et al., 2010). Consequently, TiO2/AC is considered to be a promising photocatalyst for industrial applications.

Many techniques have been developed for immobilizing TiO2 catalysts onto solid surface, for example in the past decade, various methods including sol–gel (Lee et al., 2004), hydrothermal (Toyoda et al., 2003), precipitation (Khan and Mazyck, 2003), dip coating (Sun et al., 2006), and hydrolysis (El-Sheikh et al., 2004). Recently, metal organic chemical vapour deposition (MOCVD), an extensively used surface coating technology, has been applied to the preparation of the TiO2 photocatalyst (Mills et al., 2002; El-Sheikh et al., 2004; Zhang et al. 2005a). MOCVD production of supported catalysts offers the following advantages (Ding et al., 2000; Aksoylu et al., 2003): (1) the produced materials are mainly on the external surface of the support; (2) it has little effect on the porous structure of the support due to the use of gases as precursors; (3) many of the traditional steps in catalyst preparation, such as saturation, drying, and reduction can be eliminated; (4) The TiO2 coating by MOCVD had a good adhesion on the surface of activated carbon; (5) the properties of the deposited material are easily controlled.

Consequently, MOCVD is a very promising method to prepare activated carbon supported TiO2 photocatalyst. A high loading of TiO2 is obviously needed in order to obtain a high catalytic efficiency. However, the deposition rate of supporting TiO2 onto activated carbon by MOCVD is commonly very low, so a long deposition time is required to obtain a high loading of TiO2 (Ding et al., 2000 & 2001; Zhang et al., 2004). Nevertheless, it is well known that a long deposition time will lead to several problems, such as destruction of the textural structure of AC and energy consumption.

These drawbacks limit the widespread use of MOCVD in the catalyst preparation on a large scale. There are two possible reasons for the low deposition rate (Moene et al., 1996;

Ding et al., 2001; Serp and Kalck, 2002; Choy, 2003): (1) the rate of the mass transport of the precursor with a large molecular diameter from the gas phase to the surface of activated carbon, which is the first and crucial step during deposition and plays an important role in the deposition rate, is very small due to the low mesoporous surface area of activated carbon; (2)

48 the precursor cannot be efficiently adsorbed onto the surface of activated carbon because oxygen bearing groups on the support, which are the anchoring sites for the precursor during deposition, are not sufficient. On the other hand, a high loading of TiO2 commonly results in the agglomeration of particles leading to the decrease in active surfaces for catalytic reaction.

Therefore, it is necessary to enhance the deposition rate of TiO2 by MOCVD as well as obtain smaller-sized particles well dispersed. A convenient way to enhance the mesopores surface area and the amount of oxygen bearing groups of activated carbon is performed by acid treatment such as HNO3 oxidization (Figueiredo et al., 1999; Aksoylu et al., 2001). Therefore, the modification of activated carbon support by HNO3 may lead to a significant increase in the deposition rate of MOCVD production of TiO2.

4. Photocatalytic treatment of organic compounds

The presence of wide variety of organic compounds, which are toxic and stable to natural decomposition in water supplies and industrial effluents, is an ever increasing problem for the global concern. Nowadays, high concentrations of these compounds are introduced into the water system from various agricultural activities and industrial wastewater discharges such as coal gasification, resin manufacturing, oil refining, coking plants, chemical synthesis, dyes, plastics, textiles, pharmaceuticals, paper mill, herbicides and fungicides production (Wang et al., 2009; Yang et al., 2009).

Conventional water treatment technologies such as solvent extraction, activated carbon adsorption, and chemical treatment process such as oxidation by ozone (O3) often produce hazardous by-products and generate large amounts of solid wastes, which require costly disposal or regeneration method (Venkatachalam et al., 2007). Biological treatment is often not convenient for treatment of phenolic wastewater as its toxicity may cause the phytotoxic effect on the active microorganisms (Robert and Malato, 2002). Due to these reasons, considerable attention has been focused on complete oxidation of organic compounds to harmless products such as CO2 and H2O by the advanced oxidation process (AOP) that appears to be the most emerging technology recently (Liotta et al., 2009). Photocatalytic degradation of such organic pollutants with TiO2 semiconductor has been proved to be the most efficient and popular method because it is a stable and low-cost photosensitized material (Lathasree et al., 2004; Barakat et al., 2005; Colon et al., 2006; Fabbri et al., 2006). A list for different organic compounds being treated by the heterogeneous photocatalysis is reported in Table I-6.

49 Table I-6

Photocatalytic degradation of different organic compounds by heterogeneous photocatalysis (Gaya et al., 2008)

Compound Results and observation reference

Aldehyde

(Acetaldhyde) mineralisation using film of F-TiO2 Kim and Choi (2007) Carboxylic acid

(phenoxyacetic acid and

2,4,5-phenoxyacetic acid) Degussa P-25 was more efficacy than

Millennium PC500 Singh et al.(2007) Chloroanilines

(2-Chloroaniline) Using UV/TiO2/H2O2 system, slow degradation rate, at low pH, excess of H2O2 decreased the

degradation Chu et al. (2007)

Chlorocarboxylic acid

(Monochloroacetic acid)

UV/ O3 system, O3 increases the degradation by

its decomposition by UV Mas et al.(2005)

Phenols

(Phenol) TiO2 is not favourable when the concentration

is more than100 ppm Pelizzetti and Minero (1993) Fluorophenols

(4-Fluorophenol) The efficiency of oxides was in the following order: IO4− > BrO3− > S2O82− > H2O2 > ClO3−

and Mg²⁺> Fe³⁺ > Fe²⁺ > Cu²⁺

Selvam et al.(2007)

Chlorophenols

(2,4Dichlorophenol) UV/TiO2 (Degussa P-25) Bayarri et al.

(2005) Herbicides

(Isoproturon) UV/TiO2 (Degussa P-25); degradation rat

increased by addition of electron accepteur Haque and Muneer (2003) Pharmaceutical

(Tetracycline) Solar photocatalysis; degradation rat follows

the pseudo-first order. Reyes et al .(2006) Ketones

(Acetone) UV/TiO2; UltraSonic has insignificant effect on

acetone photooxidation Vorontsov et al.

acetone, tert-butyl formate and tert-butyl alcool

were recorded as intermediates Bertelli and Selli (2004)

50 4.1. Phenols

Phenols are considered as priority pollutants since they are harmful to organisms at low concentrations and many of them have been classified as hazardous pollutants because of their potential to harm human health. It should be noted that the contamination of drinking water by phenols, at even a concentration of 0.005 mg L^-1 could bring about significant taste and odour problems making it unfit for use. A representative of this class of compounds is phenol. Sources of phenol include the discharges of chemical process industries such as coal gasification, polymeric resin production, oil refining, coking plants, paper mill, herbicides and fungicides production (Kahru et al., 1998). Human consumption of phenol-contaminated water can cause severe pains leading to damage of capillaries ultimately causing death.

Phenol containing water, when chlorinated during disinfection of water also results in the formation of chlorophenols. Advanced oxidation processes (AOPs), being able to solve the problem of phenol destruction in aqueous systems were more and more checked during the last decade. Among AOPs, heterogeneous photocatalysis using TiO2 as photocatalyst appears as the most emerging technology for phenol degradation (Grzechulska-Damszel, 2009).