Improvement of the photocatalytic activity of TiO2 induced by organic pollutant enrichment at the surface of the organografted catalyst

doi:10.1016/j.colsurfa.2015.09.022

Colloids and Surfaces A: Physicochemical and Engineering Aspects

Volume 485, 20 November 2015, Pages 73-83

https://doi.org/10.1016/j.colsurfa.2015.09.022 Get rights and content

Highlights

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Grafting of HTS reduces the amount of photocatalytic sites.
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TiOH₂⁺ groups make a major contribution in the creation of radicals.
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Photodegradation and adsorption affected by grafting density and pH.
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Acidic pH: HTS monolayer promotes the adsorption of organic pollutants.
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Pollutant adsorption plays major role in the photocatalytic efficiency.

Abstract

The photocatalytic activity of TiO₂ nanoparticles modified with hydrophobic hexadecyltrichlorosilane (HTS) is investigated. The strategy developed aims to concentrate the organic pollutants onto the surface. The impact of the grafted layer on the number and the nature of the surface photocatalytic sites involved in the production of radicals is analyzed. For this purpose, the surface of the TiO₂ particles has been thoroughly characterized by total organic carbon analysis, Fourier transform infrared spectroscopy, wettability (capillary rise and flotation), nitrogen adsorption, UV/vis spectroscopy and zeta potential. The adsorption capability and the photocatalytic activity of the materials have been tested in the degradation of salicylic acid and methyl orange at pH 3 and 10. The organic surface treatment reduces the overall production of radicals due to the diminution of the number of photocatalytic sites induced by the grafting of the HTS molecules onto the surface hydroxyl groups. Our analysis reveals that the TiOH₂⁺ surface groups are mainly involved in the creation of radicals. The photodegradation performance and the adsorption capacity are significantly affected by the grafting density and the pH. At basic pH, the pollutant adsorbed amount remains low and roughly similar for all the systems. The hydrophobized particles exhibit lower degradation efficiency than that of the untreated material. At acidic pH, the superficial HTS monolayer promotes the adsorption of the pollutants onto the modified titania due to the hydrophobic interactions between the HTS molecules and the organic contaminants. The photocatalytic activity considerably depends on the pollutant enrichment at the surface of the catalyst. Despite the rather low number of photocatalytic sites, the greater degradation at larger pollutant adsorbed amount implies that the contaminant surface coverage on the catalyst plays a major role in the efficiency of the photocatalytic process.

Graphical abstract

Introduction

Environmental problems such as hazardous wastes and toxic water pollutants still attract much attention. Wastewater treatment remains an issue because of the presence of a large variety of chemical species at various concentrations. Heterogeneous photocatalysis appears as a potential technology for the removal of organic species from the aquatic environment. In particular, it is well suited to degrade the aromatic compounds which present a potential hazard to the environment [1], [2], [3], [4]. Titanium dioxide (TiO₂) remains the most commonly used semiconductor photocatalyst due to its unique photochemical properties [4].

Organic pollutants are usually hydrophobic and highly diluted in water. They become harmful even at these extremely small concentrations. The very low surface coverage of the organic contaminants on the TiO₂ catalyst appears as the key factor to explain the limited photocatalytic efficiency [5], [6], [7]. This weak adsorption can be attributed to the surface structure of the titanium dioxide, with dominantly hydrophilic groups. In parallel, the hydrophilicity of the titania surface is enhanced under illumination [8]. This latter property is of great importance for the photocatalytic degradation of apolar organic compounds highly diluted in water. It is now well established that the surface hydrophobicity plays a major role in the photocatalytic decomposition of organic molecules in water [4], [5], [6], [7].

In this regard, many efforts have been made to enhance the adsorption capability of TiO₂ based photocatalysts. The first way consists of using mesoporous TiO₂ [9], [10], [11] or combining titania with inorganic materials such as zeolite [12], [13], mesoporous silica [14], graphene [15], activated carbon [16], as well as hydrophobic clay [17], [18]. Another approach to concentrate the organic contaminants at the surface of the catalyst deals with organic surface modification. As a matter of fact, the organic parts add the adsorption ability and selectivity to the titania material. Then, organic/TiO₂ hybrid nanoparticles are expected to enhance the removal of organic contaminants from water due to the synergistic effect of adsorption and photocatalytic degradation of apolar pollutants. Consequently, organic coating has been found to be more efficient than inorganic ones in enhancing the photodegradation rate of organic molecules. Different strategies, including ascorbic acid [19], surfactants [20], [21] and polymers [22], [23] have been tested for the surface modification with organic species.

In this study, a different strategy for the preparation of hydrophobic organic/TiO₂ photocatalysts is adopted. As hydrophobizing agent, an organosilane has been chosen. It enables the chemical link of the hydrophobic methyl groups to the surface titanium atoms through Si spacers in order to increase the stability of hydrophobization against the photoinduced destruction [24], [25]. We are aware, however, that the grafting of organosilane molecules onto TiO₂ can lead to weak photocatalytic activity [26], [27]. This behavior has been attributed to the coverage of the active photocatalytic sites (hydroxyl groups on TiO₂) by the silanol groups [26], [27]. In addition, a particular drawback related to aminosilane attachment on titanium dioxide comes from the tendency to form multilayers instead of the desired monolayer [28]. This hinders the adsorption of the pollutant molecules at the surface of the catalyst [26]. Nevertheless, we have identified hexadecyltrichlorosilane (HTS) as valuable hydrophobizing agent since some previous measurements indicate that it forms a monolayer at the surface of the catalyst [29].

The aim of the present study is to investigate the impact of hydrophobic hexadecyltrichlorosilane molecules on the photocatalytic activity of TiO₂. To characterize the photocatalytic performance of the materials, the degradation of two organic pollutants, namely methyl orange and salicylic acid, is investigated at pH 3 and 10. In addition, the adsorption capacity of the surface-organografted TiO₂ is also analysed. Another aim of the paper will be to discuss the relationship between the HTS surface coverage and the number, and the nature of the photocatalytic sites involved in the production of radicals.

Section snippets

Surface modification

Titanium dioxide powder P25 was purchased from Evonik–Degussa, N-hexadecyltrichlorosilane (HTS) was obtained from ABCR Karlsruhe–Germany (purity 95%) while cyclohexane was supplied by Sigma–Aldrich (purity 99%). The grafting of the organosilane onto the photocatalyst was conducted using the following procedure. First, the particles were heated at 120 °C during 8 h in order to decrease the physisorbed amount of water on the oxide surface [30], [31]. Then, 1 g of particle was dispersed into 40 mL of

Characterization of the organografted TiO₂ photocatalyst

The carbon mass measurement data are used to estimate the amount of HTS grafted onto TiO₂ (Fig. 1). The HTS uptake initially increases with the organosilane concentration and then reaches a plateau. The maximum surface coverage equals 2.24–2.34 μmol/m² which appears to correspond to the saturation of the surface. This indicates that the surface becomes totally covered with a molecular cross sectional area of 0.7 nm². The HTS layer can be considered as non-compact because a compact layer of

Conclusions

In this paper we evaluate the potential of the surface modification of TiO₂ with hexadecyltrichlorosilane (HTS) molecules for the photocatalytic removal of organic contaminants from water. The aim of the hydrophobic layer is to favor the adsorption of the pollutants onto the catalyst surface. The relationship between the HTS surface coverage and the nature and the number of the photocatalytic sites involved in the production of radicals is discussed. To this end, the chemical and surface

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This improved adsorption increases the photocatalytic efficiency [20,21]. At the same time, increasing the number of grafted organic groups at the surface can reduce the reaction rate constant by lowering the number of surface hydroxyl groups and the adsorption of water molecules and hydroxide ions, necessary for the formation of radicals [21]. In the case of surface modification by organophosphates, the presence of the phosphate anions could enhance the activity by facilitating the charge transfer between the formed h+ with H2O, resulting in free hydroxyl radical formation [22].
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Meanwhile, compared with the low concentration of POPs in the waters, the co-existent high-concentration hydrophilic contaminants were more likely to be adsorbed and oxidized preferentially, which further decreased the removal efficiency of POPs. To solve these problems, it was promising to improve the adsorption capacity of POPs on the TiO2 surface in order to enhance the photocatalytic oxidation ability (Thongkon, 2017; Kassir et al., 2015; Gaya and Abdullah, 2008; Li et al., 2006; Inumaru et al., 2004). In this regards, lots of strategies for modification of TiO2 based photocatalysts have been proposed to improve the adsorption capacity (Gaya and Abdullah, 2008; Li et al., 2006; Inumaru et al., 2004; Fujishima et al., 2000).
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The objective of this chapter is to describe our work in developing various strategies of surface modification in order to (1) enhance the adsorption capacity of the organic pollutants, (2) accelerate the kinetics of degradation, (3) shift the light activation toward visible and solar light, and (4) facilitate the catalyst recovery to be able to reuse easily the catalyst. The different options are compared for the degradation of methyl orange under the same experimental conditions. In order to enhance the adsorption capacity of the organic pollutants (1), two original approaches have been developed. The first one leads to the modification of the hydrophilic/hydrophobic properties of the catalyst via the grafting of organosilane onto TiO₂. The aim of the second approach is to increase the surface area of the photocatalyst thanks to the immobilization of TiO₂ onto bentonite clay. To accelerate the kinetics of degradation of the pollutant (2), the mixing of TiO₂ and ZnO to produce ZnO/TiO₂ systems has been evaluated. To improve the photochemical properties of the catalyst (3), the grafting of photosensitizers (PSs) such as monocarboxylic tetraphenyl porphyrin, chlorin e6, tetrakis(4-carboxyphenyl)porphyrin, and protoporphyrin IX has been conducted. These PSs serve as visible light antenna to modify the ultraviolet-limited photoresponse properties of the hybridized TiO₂ nanoparticles toward visible light activation. In order to facilitate the catalyst recovery (4), the use of thermoresponsive photocatalyst has been developed. Thermoresponsive copolymers based on 2-(2-methoxyethoxy) ethyl methacrylate and oligo (ethylene glycol) methacrylate have been grown from the surface of the ZnO photocatalyst. The recovery of the particles is based on the aggregation of the particles at high temperature and their redispersion at low temperature.
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Improvement of the photocatalytic activity of TiO2 induced by organic pollutant enrichment at the surface of the organografted catalyst

Highlights

Abstract

Graphical abstract

Introduction

Section snippets

Surface modification

Characterization of the organografted TiO2 photocatalyst

Conclusions

Appl. Catal. B: Environ.

J. Photochem. Photobiol. C: Photochem. Rev.

J. Photochem. Photobiol. C: Photochem. Rev.

J. Hazard. Mater. B

Appl. Catal. B

J. Catal.

Appl. Catal. B: Environ.

Microporous Mesoporous Mater.

Appl. Clay Sci.

J. Mol. Catal. A: Chem.

Appl. Catal. B.

J. Hazard. Mater.

Appl. Catal. A

J. Photochem. Photobiol. A: Chem.

Appl. Surf. Sci.

Mater. Chem. Phys.

Powder Technol.

Talanta

J. Colloid Interface Sci.

Colloids Surf. A

Colloids Surf. A

Colloids Surf. A

Carbon

Desalination

Improvement of the photocatalytic activity of TiO₂ induced by organic pollutant enrichment at the surface of the organografted catalyst

Characterization of the organografted TiO₂ photocatalyst