Deposition and characterisation of TiO2 coatings on various supports for structured (photo)catalytic reactors

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Abstract

Different types of aqueous TiO2–P25 suspensions were prepared and their properties, including rheological behaviour and zeta potential, were studied. Then, TiO2 suspensions were used to elaborate TiO2 coatings on various substrates (cordierite monolith, stainless steel plates and β-SiC foam) using a dip-coating method. The relationship between the suspension stability and the coating adhesion has been considered. The structural and morphological characterisation of TiO2 coatings has been performed using scanning electron microscopy, X-ray diffraction and other methods. Finally, the photocatalytic properties of the materials have been determined by studying the photooxidation of aqueous ammonia.

Graphical abstract

Different types of aqueous TiO2–P25 suspensions were prepared and their properties were studied. Then, TiO2 suspensions were used to elaborate TiO2 coatings on various substrates (cordierite monolith, stainless steel plates and β-SiC foam) using a dip-coating method. Finally, the photocatalytic properties of the materials have been determined by studying the photooxidation of aqueous ammonia.

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Introduction

In recent years, the amount of literature dealing with structured and coated reactors has greatly increased. For (photo)catalytic applications and water treatment, catalyst is usually dispersed in aqueous solution and used in slurry reactors. But this type of reactors has a major drawback: the catalyst particles must be separated from the liquid phase which involves a time and energy consuming process. To avoid the filtration/separation and catalyst recovery steps, the active phase is anchored on a suitable support and used in fixed or fluidized bed reactors [1], [2], [3]. For multiphasic system applications, mass transfer, heat transfer and/or diffusion problems may occur. In order to resolve or minimize these phenomena, the use of (micro)structured reactors has been proposed [4], [5], [6], [7]. In the case of heterogeneously catalysed reactions, the microstructured components may be entirely made of a catalytically active material like palladium, silver or rhodium. However, for some reactions, the inner geometric surface area is too small to provide a sufficient number of “active centres”. It is therefore necessary to cover the wall reactor or the internal structuration with porous material (e.g. oxides such as SiO2, Al2O3, TiO2, … or activated carbon) for subsequent impregnation with catalytic active species (e.g. noble metals).

In this work we report on the deposition of TiO2 on various structured supports. Titanium dioxide was chosen as a model porous material for two main reasons. First, since the late 1960s and the pioneering works of Fujishima and Honda who evidenced the electrochemical photolysis of water upon illuminating TiO2 electrode [8], titanium dioxide has found wide range of applications. Indeed, it has been used for photocatalytic applications [9], gas-sensing [10], water purification [11] and solar cells elaboration [12]. Second, titanium dioxide is widely used as support for catalytically active substance in heterogeneous catalysis [13], [14], [15].

Different types of substrates have been used to be coated by titanium dioxide in this study. Cordierite monolith was chosen because it is widely used in environmental applications [16] and it is also a widely used support for catalytic gas phase reactions [17]. Stainless steel, because of its mechanical properties and its good oxidation resistance, has been widely used as a TiO2 support for photocatalytic applications [18], [19], [20]. It is also a first-class material for the design of structured reactors [4], [21], [22]. Finally, we used a promising catalyst support material: β-SiC foam [23], [24], [25]. Silicon carbide, crystallized in its cubic structure, displays several interesting properties required for catalyst support materials: a high thermal conductivity, a high mechanical strength, a high resistance towards oxidation and chemical inertness. This material would be suitable to be used as internal structuration of classical reactors.

Current developments concentrate on coating methods that lead to coating layers with a large surface area, a sufficient mechanical/thermal stability and a good adhesion to the support of the coating layer. One of these methods is the dip-coating of structured supports into a suspension containing the desirable porous material [26]. Among its advantages, this method is easier to implement than chemical or physical vapour deposition methods and electrochemical depositions. Moreover, dip-coating allows the use of commercially available catalysts with high (photo)catalytic activity (such as Degussa P25) and none of the expensive precursors commonly used in the sol/gel process are required.

In this report, we present results concerning the preparation of TiO2 suspensions, their characterisation and their use for the deposition of titanium dioxide on structured substrates. The morphological and structural characterisation of the deposits is also reported. In the dip-coating process, the suspension containing the desirable porous material is usually used freshly prepared (i.e. the first day of ageing). For such dispersions, the electrostatic stability of particles within the liquid is a key parameter for avoiding a fast flocculation. Do the most stable suspensions lead to the most resistant coatings? In order to answer to this frequently asked question, we will discuss the relationship between the suspension stability and the TiO2 coating adhesion. Finally, the photocatalytic properties of the obtained materials have been determined by studying the photooxidation of aqueous ammonia.

Section snippets

Chemicals

TiO2–P25 (Degussa), regarded as a reference molecule in photocatalytic degradation of organic pollutants and water treatment, was used in the preparation of TiO2 suspensions. The X-ray diffraction (XRD) showed that this powder contains two crystalline phases: 90.7 vol.% anatase (crystallite size: 21.5 nm, determined by Scherrer equation) and 9.3 vol.% rutile (31.5 nm). The specific BET surface area of the TiO2 powder used in this study was determined to be 56.9 m2 g−1. Table 1 shows the particle size

Particle size

Table 1 presents the particle size distribution of type I suspensions with various concentrations determined by laser scattering. Suspensions with a relatively low concentration (125<C250 g/L) exhibit a particle size distribution very similar to the TiO2–P25 starting powder one. The mean diameter of the particles (D50) is about 4.2±0.1μm and the particle size distribution is unimodal. For suspensions having concentration less-than or equal to 125 g/L, D50 slightly decreases. This trend suggests

Conclusions

TiO2 coatings were prepared on various structured substrates by dip-coating using TiO2–P25 suspensions. The rheological behaviour of the suspensions was studied and we highlighted that in our concentration range and whatever their preparation mode, the suspensions had Bingham behaviour. We also studied the influence of additives (surfactant, aqueous ammonia) on the suspension stability. We discussed the relationship between the suspension stability and the coating adhesion and we highlighted

Acknowledgements

The authors would like to thank Laurence Burel (IRCELYON – CNRS) for her helpful assistance in the SEM observations, Andre C. van Veen (IRCELYON – CNRS) for the access to the particle size analyser and Jacques Dazord (LMI – Université de Lyon) for the AFM experiments. We acknowledge Virginie Giraudeau and Pascal Pitiot from Rhodia Company for fruitful discussions. Finally, we wish to thank Patrick Nguyen from SICAT Company for the β-SiC foam samples. This work was financially supported by

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