Applied Catalysis A: General 360 (2009) 154–162

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

Philippe Rodriguez *, Valerie Meille, Stephanie Pallier, Mohamad Ali Al Sawah

Universite de Lyon, CNRS, Laboratoire de Genie des Procedes Catalytiques, CPE Lyon, 43 bd du 11 novembre 1918, BP 82077, 69616 Villeurbanne Cedex, France

A R T I C L E I N F O

Article history:

Received 10 October 2008

Received in revised form 19 February 2009

Accepted 9 March 2009

Available online 20 March 2009

Keywords:

Structured reactors

Titanium dioxide

Dip-coating

b-SiC foam

Stainless steel

Cordierite monolith

A B S T R A C T

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 b-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.

� 2009 Elsevier B.V. All rights reserved.

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Applied Catalysis A: General

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1. Introduction

In recent years, the amount of literature dealing with structuredand coated reactors has greatly increased. For (photo)catalyticapplications and water treatment, catalyst is usually dispersed inaqueous solution and used in slurry reactors. But this type ofreactors has a major drawback: the catalyst particles must beseparated from the liquid phase which involves a time and energyconsuming process. To avoid the filtration/separation and catalystrecovery steps, the active phase is anchored on a suitable supportand used in fixed or fluidized bed reactors [1–3]. For multiphasicsystem applications, mass transfer, heat transfer and/or diffusionproblems may occur. In order to resolve or minimize thesephenomena, the use of (micro)structured reactors has beenproposed [4–7]. In the case of heterogeneously catalysed reactions,the microstructured components may be entirely made of acatalytically active material like palladium, silver or rhodium.However, for some reactions, the inner geometric surface area istoo small to provide a sufficient number of ‘‘active centres’’. It istherefore necessary to cover the wall reactor or the internalstructuration with porous material (e.g. oxides such as SiO2, Al2O3,TiO2, . . . or activated carbon) for subsequent impregnation withcatalytic active species (e.g. noble metals).

* Corresponding author. Tel.: +33 472431753; fax: +33 472431673.

E-mail address: pro@lgpc.cpe.fr (P. Rodriguez).

0926-860X/$ – see front matter � 2009 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcata.2009.03.013

In this work we report on the deposition of TiO2 on variousstructured supports. Titanium dioxide was chosen as a modelporous material for two main reasons. First, since the late 1960sand the pioneering works of Fujishima and Honda who evidencedthe electrochemical photolysis of water upon illuminating TiO2

electrode [8], titanium dioxide has found wide range of applica-tions. 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 forcatalytically active substance in heterogeneous catalysis [13–15].

Different types of substrates have been used to be coated bytitanium dioxide in this study. Cordierite monolith was chosenbecause it is widely used in environmental applications [16] and itis also a widely used support for catalytic gas phase reactions [17].Stainless steel, because of its mechanical properties and its goodoxidation resistance, has been widely used as a TiO2 support forphotocatalytic applications [18–20]. It is also a first-class materialfor the design of structured reactors [4,21,22]. Finally, we used apromising catalyst support material: b-SiC foam [23–25]. Siliconcarbide, crystallized in its cubic structure, displays severalinteresting properties required for catalyst support materials: ahigh thermal conductivity, a high mechanical strength, a highresistance towards oxidation and chemical inertness. This materialwould be suitable to be used as internal structuration of classicalreactors.

Current developments concentrate on coating methods thatlead to coating layers with a large surface area, a sufficientmechanical/thermal stability and a good adhesion to the support of

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162 155

the coating layer. One of these methods is the dip-coating ofstructured supports into a suspension containing the desirableporous material [26]. Among its advantages, this method is easierto implement than chemical or physical vapour depositionmethods and electrochemical depositions. Moreover, dip-coatingallows the use of commercially available catalysts with high(photo)catalytic activity (such as Degussa P25) and none of theexpensive precursors commonly used in the sol/gel process arerequired.

In this report, we present results concerning the preparation ofTiO2 suspensions, their characterisation and their use for thedeposition of titanium dioxide on structured substrates. Themorphological and structural characterisation of the deposits isalso reported. In the dip-coating process, the suspension contain-ing the desirable porous material is usually used freshly prepared(i.e. the first day of ageing). For such dispersions, the electrostaticstability of particles within the liquid is a key parameter foravoiding a fast flocculation. Do the most stable suspensions lead tothe most resistant coatings? In order to answer to this frequentlyasked question, we will discuss the relationship between thesuspension stability and the TiO2 coating adhesion. Finally, thephotocatalytic properties of the obtained materials have beendetermined by studying the photooxidation of aqueous ammonia.

2. Experimental

2.1. Chemicals

TiO2–P25 (Degussa), regarded as a reference molecule inphotocatalytic degradation of organic pollutants and watertreatment, was used in the preparation of TiO2 suspensions. TheX-ray diffraction (XRD) showed that this powder contains twocrystalline 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 studywas determined to be 56.9 m2 g�1. Table 1 shows the particle sizedistribution of the initial TiO2–P25 powder determined by laserscattering. The span of the distribution measures the spreadbetween the 10% and 90% points of the cumulative undersizedistribution divided by the cumulative undersize distribution at50%: span ¼ ðD90 � D10=D50Þ. In the case of P25 powder, the spanvalue indicates that the particle size distribution is unimodal. Thedistribution of the particle diameters D10=D90 is 1.3 mm/14.5 mmwith a mean diameter of the particles (D50) of about 4.2 mm. Thislatter value is much higher than the values of 21.5 and 31.5 nmdetermined by Scherrer equation suggesting that primary particlesagglomerate in micro-scale clusters.

For the preparation of suspensions, distilled water was used assolvent and, in some cases, sodium hexametaphosphate (SHMP)(from Aldrich) was added as surfactant.

2.2. Supports and cleaning procedures

The cordierite monolith pieces supplied by Corning were cut toform a rectangular parallelepiped (approx. 5 cm� 1 cm� 1 cm);

Table 1Size distribution of TiO2–P25 starting powder and TiO2 particles of type I

suspensions with various concentrations determined by laser scattering.

Concentration

(g/L)

D10 (mm) D90 (mm) D50 (mm) Span

TiO2–P25 � 1.3 14.5 4.2 3.1

Type I suspensions 125 1.2 15.2 3.5 4

250 1.4 19.3 4.3 4.2

375 2.8 205.7 9 22.5

the wall thickness was �0:65 mm, and the cell dimensioncorresponded to 9 cells cm�2 (i.e. 64 cpsi). Each piece weighedabout 4.5 g. Before the catalyst deposition, the cordierite monolithpieces were sonicated in ethanol for 15 min and dried for 1 h at100 �C.

Stainless steel AISI 316L (ThyssenKrupp) is an austenitic alloywhich contains 17–19% of chromium, 11–13% of nickel, 2% ofmolybdenum and 0.02% of carbon. Nickel is added to stabilize theaustenitic structure whereas chromium and molybdenum preventcorrosion. A passive film which is a mixture of chromium and ironoxides (Cr2O3 and Fe2O3) is formed at the alloy surface. Plates,50 mm long, 25 mm wide and 1 mm thick; weighed about 10 g,were used as catalyst support. The cleaning and pretreatmentprocedures of the stainless steel plates will be described hereafter(Section 3.2.1).

Finally, silicon carbide (b-SiC) in a foam form with a specificsurface area of 22.1 m2 g�1 and a pore opening centred at around1000 mm was used as substrate. b-SiC support was synthesizedaccording to the shape-memory synthesis developed by Ledouxet al. [27,28] which is a gas–solid reaction between SiO and carbon.Samples used in this study were graciously provided by SICATCompany (Otterswiller – France). 40 mm long, 20 mm wide and10 mm thick pieces were cut and used as catalyst support. Eachpiece weighed about 2 g. Before the catalyst deposition, the b-SiCpieces were sonicated in ethanol for 15 min and dried for 1 h at100 �C.

2.3. Preparation of TiO2 suspensions

Three types of suspensions were prepared as follows:

� Type I suspensions were prepared by introducing 1–15 g of TiO2–P25 powder in 40 cm3 of distilled water leading to concentra-tions ranging between 25 and 375 g/L. The dispersions wereagitated by magnetic stirring for 1 h at room temperature. pHwas 5.8.� Type II suspensions were prepared using the same method

described for type I suspensions except that 140 mg of sodiumhexametaphosphate (0.35%, w/v) and 600 mL of NH4OH 0.2 Mwere initially added. pH was comprised between 9 and 10.� Finally, type III suspensions were prepared like type I suspen-

sions except that 140 mg of sodium hexametaphosphate (0.35%,w/v) were initially added. pH was 5.6.

2.4. Physicochemical analyses

The rheological behaviour of TiO2–P25 suspensions wasmeasured with a Gemini HRnano rheometer (Bohlin Instruments).All the measurements were performed at the same temperature(25 �C) using a spiral bob mobile which avoids particle settling.Particle size distributions have been determined by laser scattering(performed with a Microtrac S3500 Series Particle Size Analyser). Aspherical shape of particles was assumed. The samples weredispersed in water and sonicated for 20 s (40 W) in the analyser.Electrostatic stability of suspensions was studied using a zetapotential analyser (Zetasizer Nano ZS, Malvern Instruments).

BET measurements have been performed with a MicrometricsASAP 2010 apparatus by nitrogen adsorption at 77 K. All thesamples were first degassed at 120 � C for 1 h and then at 350 � C for3 h under vacuum. X-ray diffraction (Brucker D8 Advancediffractometer) using Cu K a radiation was used to assess theanatase to rutile ratio, the crystallite size and the crystallinity ofthe initial P25 powder and of the TiO2 coatings. The crystallite sizewas evaluated from the X-ray diffraction based on the Scherrerformula. Before XRD and BET analyses, the TiO2 films were scrapedfrom stainless steel plates. The morphology of the coatings was

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162156

examined using a JEOL JSM-5800 LV scanning electron microscope(SEM) coupled to a PGT Instruments energy dispersive spectro-meter (EDS).

Before TiO2 deposition, different pretreatments were per-formed on stainless steel plates. In order to study the effects ofthese pretreatments on their surface morphology, the plates wereobserved by atomic force microscopy (AFM) using a ScientecMolecular Imaging microscope in constant-force mode.

The scotch tape test (ASTM D3359) is a classical method forevaluating the adhesion of a coating to a substrate. This method isefficient for flat supports like plates but inappropriate forsubstrates in a foam or monolith form. Thus, the adherence (ormechanical resistance) of the TiO2 coatings was evaluated byimmersion of the coated structure, coupled with an ultrasonictreatment (Transsonic 275/H), in a beaker containing heptane ordistilled water [26]. An adherence percentage was determined bysuccessive weighing of the coated support after 1 min in heptaneand 1 min in distilled water. The adherence percentages given inthis manuscript represent the absolute amount of catalystremaining on supports and are the average values obtained fordifferent samples. For monolith ceramic and b-SiC foam, thestandard deviation (s) is about 1.5% while for stainless steel plates,s is about 2.5%.

2.5. Coating procedure

A dip-coating method was used to immobilize TiO2 particles onstructured supports. Substrates were placed into the TiO2 slurry for5 s, they were then withdrawn and shaken to remove the excess ofsuspension. After the deposition, coatings were dried at roomtemperature and then heat treatments under air flowing wereapplied. Heat treatments were done using a programmable furnace(Nabertherm, Model L40/11/B170). The furnace temperature wasincreased at a ramp rate of 2 �C/min until it reached 150 � C andwas held at this temperature for 2 h. Subsequently, the tempera-ture was increased at a ramp rate of 2 �C/min to 500 � C and held atthis temperature for 2 h. The high-temperature calcination stepallows fixing the coating to the support [29]. Finally, the furnacewas allowed to cool down naturally at room temperature. Thecooling down period took approximately 15 h.

2.6. Photocatalytic characterisation

The removal of nitrates from water represents a seriousecological and public health problem [30] thus, the catalytichydrogenation of nitrates (and nitrites) to nitrogen has been asubject of extensive investigation [31]. Currently, the mostefficient systems are based on Pd-Cu bimetallic catalysts [32–34]. But the main drawback is the low selectivity to N2 which leadsto the formation of ammonium ions as by-products. To overcomethis issue, our laboratory develops a new approach, taking intoaccount this limitation through a process combining two steps. Thefirst step consists in the catalytic hydrogenation of nitrates innitrogen and ammonium ions. The second step consists in thephotooxidation of ammonium into nitrates. The combination ofboth techniques in a recycle process should lead to a lower amountof nitrate at the outlet through a partial formation of gaseousnitrogen during each cycle. The second step of this process, i.e. thephotooxidation of aqueous ammonium/ammonia (NH4

+/NH3), hasbeen chosen to evaluate the photocatalytic activity of our samples.

The experiments were carried out at 25 � C in a cylindrical Pyrexglass reactor (75 cm3) and the UV illumination was provided by aPhilips HPK 125 W Hg lamp (240<l<440 nm). The volume ofaqueous slurry in all experiments was 25 mL. Ammoniumsolutions with an initial concentration of NH4

+ of 50 ppm wereprepared from NH4OH solution provided by Acros. The reaction

slurry was magnetically stirred to maintain a well-mixed TiO2

suspension during the experiments. Photocatalytic tests wereperformed using TiO2–P25 starting powder (reference), TiO2 filmsscraped from stainless steel plates and TiO2/ b-SiC material. For allexperiments, the effective amount of TiO2 was about 20 mg. Beforethe UV lamp was turned on, the suspension (TiO2 and aqueousammonia) was placed in the dark and stirred during 30 min untilthe pH was stable, indicating adsorption equilibrium. At regulartime intervals, samples (0.5–1 mL) were taken from the reactorwith a syringe and filtered through a 0.20 mm nylon membrane(Millex-GN, Millipore). The filtrate was used for measurement ofpH and NH4

+. The ion concentrations were determined using aDionex ion chromatography system (ICS-1000) with an IonPacCS12A cation-exchange column (4 mm � 250 mm). Methanesul-fonic acid (20 mM) was used as eluent and the elution was carriedout at 1 mL min�1.

3. Results and discussion

3.1. Study of TiO2–P25 suspensions

3.1.1. Particle size

Table 1 presents the particle size distribution of type Isuspensions with various concentrations determined by laserscattering. Suspensions with a relatively low concentration(125<C � 250 g/L) exhibit a particle size distribution very similarto the TiO2–P25 starting powder one. The mean diameter of theparticles (D50) is about 4:2� 0:1 mm and the particle sizedistribution is unimodal. For suspensions having concentrationless-than or equal to 125 g/L, D50 slightly decreases. This trendsuggests that magnetic stirring performed during the suspensionpreparation might lead to a partial deagglomeration of primaryparticles. On the other hand, for the most concentrated suspen-sions (C ¼ 375 g/L), the mean diameter of the particles increases(D50 ¼ 9 mm) and the size distribution span is wider whichcorresponds to a multimodal distribution of the particle size. Theseresults indicate that high concentrations enhance particle agglom-eration.

Comparison has been realized between type I and type IIIsuspensions having the same concentration (not shown here). Nosignificant differences in the particle size distribution wereevidenced suggesting that the addition of sodium hexametapho-sphate has no influence on the TiO2 particle size in theconcentration range investigated. The addition of SHMP probablytends to repulse TiO2 micro-clusters from each other but does notlead to a deagglomeration of the primary particles.

3.1.2. Rheological behaviour

Fig. 1 shows shear-stress and viscosity versus shear-rate curvesobtained for type I and type III suspensions having the sameconcentration (C ¼ 250 g/L). It should be noted that only theviscosity measured from the ramp-up mode is shown here forclarity purposes. Whatever the suspension type, the rheologicalbehaviour is almost identical. Beyond a certain critical level, i.e. theyield stress, suspensions exhibit a Newtonian behaviour. This typeof behaviour is called Bingham behaviour. For type III suspensions,at high shear-rate regime, viscosity seems to increase. Thisphenomenon should not be related to a shear-thickeningbehaviour but to sliding effects due to the low intrinsic viscosityof the suspension. Indeed, a similar phenomenon was evidencedfor distilled water reference curves (not shown here). The yieldstress values determined from Bingham model are very low,respectively, 0.02 Pa for type I suspension and 0.003 Pa for type IIIsuspension.

The shear-stress versus shear-rate curves show that suspen-sions required a higher stress level during the ramp-up strain stage

Fig. 1. Shear stress and viscosity vs. shear rate curves for type I (a) and type III (b)

suspensions (C ¼ 250 g/L).

Fig. 2. Variation of zeta potential as a function of pH for type I suspensions.

Table 2Zeta potential value of the different TiO2 suspensions.

Type of suspension Zeta potential (mV)

Type I susp. �3.5

Type II susp. �51.9

Type III susp. �50.4

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162 157

for the flow to occur than during the decreasing strain rate mode.Even if this behaviour could not be described as a thixotropic one,because it is limited, it indicates the existence of particleaggregation in the suspensions. When suspensions are relaxed(when no stress is applied), flocculation occurs and particles settle.This phenomenon is due to the existence of attractive van derWaals force between TiO2 particles. Beyond the yield stress, vander Waals interactions are eliminated and suspension flow occurs.

Concerning the evolution of viscosity with time, it should benoticed that even if particle settling occurs, suspension intrinsicviscosity appeared to be homogeneous in time: intrinsic viscosityhad not changed even after a month. Suspensions only need to beshaken in order to be rehomogenized.

The results obtained indicate that the addition of SHMP leads toa marked reduction in yield stress value of the TiO2 suspensions(one order of magnitude) and to a slight decrease of the intrinsicviscosity. This is a result of the adsorption of ionic surfactant on theTiO2 particles. Similar trends have already been reported fortitanium dioxide dispersions [35] and titania pigments [36].

3.1.3. Stability

According to the DLVO theory (the DLVO theory is named afterDerjaguin, Landau, Verwey and Overbeek who developed it in the1940s), an important factor influencing the stability of aqueousdispersions is the surface potential of the particles. It is well knownthat the pH of aqueous medium has a substantial effect on thestability of aqueous dispersion of TiO2 powders. Fig. 2 shows thepH dependence of a type I suspension on its zeta potential. The zeta

potential of suspensions is related to the electrostatic stability ofTiO2 particles, it will determine whether the particles within theliquid will tend to flocculate or not. The pH of the suspension wasadjusted with additions of 0.02 M H2SO4 or 0.2 M NaOH solution.

The zeta potential measurements have shown that the TiO2

particles exhibit a transition from positive zeta potential at low pH tonegative zeta potential at high pH. Positive sign of the zeta potentialis due to the adsorption of H+ ions onto the surface, while at high pHregion H+ ions are released out of the surface and OH� ions areadsorbed inducing negative sign of the zeta potential. Zeta potentialversus pH curve indicates that the isoelectric point (iep) of TiO2–P25particles, i.e. the pH at which the zeta potential vanishes to zero, isabout 5.9. This result is in good agreement with the values reportedin the literature [37]. The higher the absolute value of the zetapotential is, the higher the stability of suspensions is, i.e. the TiO2

particles within the liquid will not tend to flocculate. So our resultsindicate that TiO2–P25 suspensions are the most stable for basic pH.At low pH (<4), the zeta potential seems to decrease and leads to acorresponding deterioration in the stability of the suspension. Thisphenomenon might be a consequence of the co-ion adsorption ontothe TiO2 particle surface. It is well known that the potentialdetermining ions for oxides are the H+ and the OH�, but whenintroducing H2SO4 solution to decrease the pH, an adsorption ofSO4

2� counterions occurs at the particle surface yielding significantdifferences on the electrokinetic charge since the valence of sulphatecounterions doubles the one of H+ ions. Such a phenomenon has beenreported by Fernandez-Nieves et al. who studied the stability of TiO2

dispersions under the presence of several electrolytes [38]. Anotherexplanation is possible. At low pH range, the ionization of the particlesurface may have reached a saturation, but the double electricallayers around the particles are depressed due to the concomitantincrease of the electrolyte concentration with the decreasing pH,eventually resulting in the decrease of the zeta potential and thesuspension stability. A similar trend has been observed by Chen et al.for titanium dioxide dispersions at high pH [35].

Table 2 compares the zeta potential values obtained for thevarious types of suspensions. For type I suspensions the absolute

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162158

value of the zeta potential is very low which corresponds to anunstable suspension, i.e. the TiO2 particles will tend to sticktogether rapidly. Indeed, the pH of type I suspensions (5.8) matchesthe iep. For type II and type III suspensions, the absolute value ofthe zeta potential is higher which indicates that suspensions aremore stable, i.e. the flocculation of particles will be longer.Moreover, the values of the zeta potential are very similar. Withsuch polyphosphate concentration (3.5 mg/g), the pH of thesuspension has nearly no influence on the zeta potential value[36,39]. This trend suggests that the maximum adsorption capacityhas been obtained by adding the anionic surfactant and that addingOH� ions in the stabilized suspension has no supplementary effecton the zeta potential value.

The concentration of SHMP in type II and type III suspensions (e.g.0.35%, w/v) has been optimized by measuring the zeta potential ofTiO2 suspensions having various SHMP concentrations. It appearedthat the absolute value of the zeta potential reached a maximum forthe concentration of 0.35% (w/v). Below this value the maximumadsorption capacity of TiO2 particles has not been reached thus theaddition of SHMP increases suspension electrostatic stability. On theother hand, increasing the SHMP concentration above 0.35% (w/v)has nearly no influence on the zeta potential value.

It is well known that some industrial processes need stabledisperse systems [40]. Concerning the elaboration of structuredreactors, the adhesion of the coating is another major parameter. InSection 3.2.2 , we will discuss the relationship between thestability of the suspension and the adherence of TiO2 coatings.

Fig. 3. AFM images (90 mm � 90 m m areas) of stainless steel plates after acetone deg

treatment (d).

3.2. Study of TiO2 coatings

3.2.1. Preparation of stainless steel plates before TiO2 deposition

The surface morphology of stainless steel plates is too smooth,i.e. their developed surface area is too small for anchoring acatalyst. Moreover, chemical bonding is limited on untreatedsurface. Thus, before the TiO2 deposition, stainless steel plateswere prepared as follows. First, plates were degreased in acetonefor 15 min under sonication and then a heat treatment wasperformed. The furnace temperature was increased at a ramp rateof 2 �C/min until it reached 500 � C and was held at thistemperature for 4 h, finally the furnace was allowed to cool downnaturally at room temperature. After the heat treatment, stainlesssteel plates were chemically treated by immersion in a sulphuricacid solution (30 wt.%) during 3 h. In order to eliminate acidictraces before the TiO2 deposition, plates were immersed two timesin distilled water under sonication during 30 min. Finally, plateswere dried at 100 � C for 1 h.

Fig. 3 shows AFM images (90 mm � 90 mm areas) of thestainless steel plates after the different steps of the pretreatment.The z-range has been normalized at 2000 nm for all the AFMimages in order to accentuate the differences of roughnessbetween the samples.

Fig. 3(a) shows the surface morphology after acetone degreas-ing. This reference sample presents a surface morphology verysmooth and regular. Grains and grain boundaries, characteristic ofaustenitic alloys, are observable. After heat treatment (Fig. 3(b)),

reasing (a), heat treatment (b), chemical treatment (c) and after simple chemical

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162 159

the surface morphology is still smooth but grains and grainboundaries appear more distinctly. On smaller scans(20 mm � 20 mm areas, not shown here), small roughness appearsuggesting the formation of a superficial oxide layer. This trend issupported by the fact that at the macroscopic scale, stainless steelplates take a characteristic blue-black tone. Moreover, Fig. 3(b)shows the presence of holes which appear in dark on AFM images.Such holes are defects also observable on reference sample(depending on the observed area) and should not be ascribed tothe heat treatment. After chemical treatment (Fig. 3(c)), the surfacemorphology has completely changed. The important differencebetween grains and grain boundaries z-range indicates thatroughness has greatly increased. Moreover, AFM images highlightthat grain boundaries have been preferentially attacked by thesulphuric treatment. Thus, the surface morphology of stainlesssteel plates can be described as pillars of 100 nm height formed bythe grains. Grain surface appears rough suggesting that grains havealso been attacked. The surface morphology obtained after achemical treatment without previous heat treatment (Fig. 3(d)) issmoother than the one described previously. Grain boundaries areless attacked thus leading to a smaller altitude difference betweengrains and grain boundaries than the one observed in Fig. 3(c).

The AFM study of stainless steel plates after the different stepsof the pretreatment allows concluding that a heat treatmentfollowed by a chemical treatment seems to be more efficient toincrease the surface roughness of the plates than a simple chemicaltreatment.

3.2.2. Adherence of TiO2 coatings

Table 3 shows the adherence percentage of the TiO2 coatingsdeposited on cordierite monolith and b-SiC pieces as a function ofsuspension’s nature and concentration. Whatever the nature of thesuspension, the adherence percentage decreases with the con-centration increase. This is probably related to an increase of theintrinsic viscosity thus leading to thicker coatings which are lessresistant. It should be noticed that the adherence percentagevalues obtained for concentrations greater-than or equal to 250 g/L might be underestimated. Indeed, for such concentrations theexcess of suspension is not completely removed from thecordierite cells or b-SiC pores (capillary forces) during the coatingprocess thus leading to an accumulation of suspension which isfavourably eliminated during the adhesion tests. At a givenconcentration, TiO2 coatings from type I suspensions appear to bemore resistant than the ones obtained from type II suspensions.Similar results have been obtained with b-SiC foams comparingthe adherence of TiO2 coatings from type I and type III suspensions.

Even if type II and type III suspensions are more stable than typeI suspensions, the adsorption of polyphosphate surfactant onparticle surface seems to be unfavourable for the adherence of TiO2

coatings on cordierite and b-SiC foam supports. Increasingelectrostatic stability increases the dispersion stability, which isa positive point in order to avoid flocculation. But it also tends torepulse particles from each other, which appears not being helpfulfor the coating adhesion. Nevertheless, the results obtained fortype I suspensions indicate that TiO2 coatings exhibit a good

Table 3Adherence percentage of TiO2 coatings supported on cordierite monolith and b-SiC

pieces as a function of suspension type and concentration.

Support Suspension 25 g/L 125 g/L 187.5 g/L 250 g/L

Cordierite Type I 100% 100% 98.3% 89.5%

Type II 100% 98% 93.9% 83.9%

b-SiC Type I 98.3% 95.4% 92.1% 90.3%

Type III 92.3% 90.6% 89% 86.9%

resistance and that their adherence to these supports is compatiblewith (photo)catalytic applications.

Concerning stainless steel plates, the effect of suspensionconcentration on the coating resistance was similar to the onedescribed for cordierite and b-SiC supports, i.e. increasingsuspension concentration leads to a decrease of the TiO2 coatingadherence. This trend is related to the increase of coatingthickness. Indeed, thick titanium dioxide films deposited onstainless steel substrates have been reported to be fragile, whichraises a problem with respect to their long-term mechanicalstability [41]. For this support, the best compromise betweencoatings’ thickness and adherence was obtained for suspensionshaving a concentration of 250 g/L. It should be noticed that for suchconcentration an accumulation of suspension is observed at thestainless steel plate bottoms. As described in the previousparagraph this accumulation can lead to an underestimation ofthe adherence percentage.

For these substrates, the coating resistance was also success-fully investigated by the scotch tape test, i.e. tape is clear after thetest. Samples were also immersed in beakers containing distilledwater (without ultrasonics treatment) at room temperature and at80 �C. Whatever the temperature no weight loss has beenmeasured after 8 h. Even if this test was realized in a staticconfiguration, i.e. no liquid stirring or flows were applied, itdemonstrates that coatings exhibit a good thermal resistance.

The effect of suspension nature on the coating adherence isshown in Table 4. The use of type III suspensions allows obtainingcoatings with the best adherence (�90%) on the other hand type IIsuspensions lead to deposits with the worst resistance (�78%).Even if the stability of these two types of suspension is similar, theresults obtained in term of coating resistance are different. In thiscase, the presence of NH4OH in the suspension seems to beunfavourable for the adherence of TiO2 coatings. This trend hasbeen confirmed by tests consisting in the pretreatment of stainlesssteel plates with NH4OH or NaOH, the resulting TiO2 filmsexhibited a poor adherence.

Compared to type I suspensions, the presence of a surfactant intype III suspensions leads to coatings which are more homo-geneous on the plate surface, i.e. the average thickness of thedeposit is more homogeneous (macro- and microscopic observa-tions). Even if titania particles in type III suspensions are morerepulsed from each other than in type I suspensions, the resultingadhesion is better. For porous substrates like cordierite monolithand b-SiC foam, suspension is retained by the porous volumewhereas for stainless steel substrates the wettability plays a majorrole for coating adhesion. We believe that for this kind of substrate,van der Waals forces involved in particles repulsion are aparameter much less important than wettability effects. Thus,the homogeneity of deposit thickness seems to be a moreimportant parameter than the relative stability of suspensions,in order to improve the coating adherence on stainless steel plates.

TiO2 coatings prepared from type III suspensions at 250 g/L onstainless steel plates which have only been chemically treated(H2SO4 30% for 3 h) appears to be less resistant than coatingsdeposited on plates prepared with a heat treatment followed by achemical treatment. The adherence percentage is about 75%. Thistrend is in good agreement with the AFM study of stainless steelplates after the different steps of the pretreatment. A heattreatment followed by a chemical treatment allows obtaining a

Table 4Adherence percentage of TiO2 coatings deposited on stainless

steel plates as a function of suspension type (C ¼ 250 g/L).

Type I susp. Type II susp. Type III susp.

Adherence% 84 77.9 89.4

Fig. 4. X-ray diffraction spectra of TiO2–P25 starting powder and TiO2 films

obtained from type I and type II suspensions.

Fig. 5. Scanning electron micrographs of TiO2 films anchored on b-SiC foam

prepared with type I suspensions at 25 g/L (a), 75 g/L (b) and 125 g/L (c).

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162160

higher surface roughness thus leading to an improved spreading ofthe suspension and an increase of the coating resistance.

3.2.3. Structural and morphological characterisation of TiO2 coatings

Fig. 4 shows XRD spectra of TiO2–P25 starting powder and TiO2

films obtained from type I and type II suspensions. Results showedthat the whole coating procedure (including heat treatments) didnot affect the anatase to rutile ratio. Whatever the suspension typeused, the TiO2 films present two crystalline phases: 90% anataseand 10% rutile. In our case, the calcination step was performed at arelatively low temperature (500 �C) in order to avoid theconversion of anatase into the more stable rutile phase. The aimwas to preserve the anatase phase because it is generally acceptedthat anatase is the more active of the two phases, both inphotocatalysis and in photoelectrochemical studies and that forTiO2–P25 the anatase to rutile phase conversion begins at 400 � C[43,42]. Moreover, these results showed that the addition ofsurfactant did not affect the film composition, i.e. no crystallizedimpurities were found. We did not check, with other techniques,the presence of amorphous impurities. Nevertheless, Zhang et al.used SHMP in order to avoid agglomeration during the preparationof nanosize ceria powder and they did not mention the presence ofimpurities in their final product [44].

The intensity of the TiO2 film peaks was higher than the P25 onewhile the full width at half maximum was thinner: due to heattreatments, the crystallinity of TiO2 films was higher that the TiO2–P25 one. This trend was confirmed by BET measurements: thespecific BET surface area of TiO2 films (scraped from stainless steelplates) was determined to be 40.5 m2 g�1 which is lower than thevalue of 56.9 m2 g�1 determined for the TiO2–P25. We also noticeda slight increase of the crystallite size: 22 nm for the anatase phase(versus 21.5 nm for the starting powder) and 35.5 nm for the rutilephase (versus 31.5 nm). This trend coupled with the decrease ofthe specific surface area can be related to a slight sintering oftitanium dioxide particles during the coating process (probablyduring the heat treatments).

Fig. 5 shows the SEM pictures of TiO2 films anchored on b-SiCfoam prepared with type I suspensions at 25, 75 and 125 g/L. TheseSEM images show the influence of the suspension concentration onthe TiO2 film morphology. For concentrations less than or equal to50 g/L, TiO2 films appear homogeneous without micro-cracks(Fig. 5(a)). Increasing the suspension concentration increases thefilm thickness and leads to the formation of micro-cracks (Fig. 5(b)and (c)). For TiO2 films prepared from type I suspensions at 125 g/L,

the coating thickness reached 30 mm (SEM observations). It shouldbe noticed that even for TiO2/ b-SiC materials prepared withsuspensions having concentrations up to 250 g/L, the adherencepercentage of the TiO2 coating is still above 90% (Table 3). Thus,micro-cracks, which the origin will be discussed hereafter, do notaffect significantly the TiO2 film resistance.

The specific BET surface area of TiO2/ b-SiC compositeselaborated with a type I suspension at 125 g/L was determinedto be 23.1 m2 g�1. This value is very similar to the one obtained for

Fig. 6. Scanning electron micrographs of TiO2 films deposited on stainless steel plate

prepared with type III suspension at 250 g/L: (a) surface morphology and (b) coating

thickness.

Table 5Photooxidation of ammonium ions using different samples—results

obtained after 5 h of UV irradiation.

Catalyst Degradation (%) Initial pH Final pH

TiO2–P25 45.6 10 7.6

TiO2 from type I susp. 32.6 10.2 8.1

TiO2 from type II susp. 36.1 10 7.8

TiO2/ b-SiC 26.9 10 8.8

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162 161

the b-SiC substrate alone, the weight of the TiO2 coating beingmuch less important than the substrate one: 110 mg versus 1.5 g.As this value matches the expected increase of the specific surfacearea, it might indicate that the substrate porosity and the TiO2

coating one are both still accessible.Fig. 6 shows the SEM images of TiO2 film grafted on stainless

steel plate prepared with type III suspension at 250 g/L. Thesurface morphology of the TiO2 appears homogeneous without thepresence of agglomerates (Fig. 6(a)). At the microscopic scale,micro-cracks are observable. Their origin can be related tomicroscopic signs of strain relaxation due to the coating thicknessbut they can also be attributed to shrinkage effects due to heattreatments. Even if micro-cracks appeared on coating surface, nocracks and flaking off from the substrate were observed.

In order to determine the coating thickness we performed SEMobservations of the TiO2/stainless steel composite slices where theidentification of each layer was determined by EDS (Fig. 6(b)). Thecoating thickness (measured with a micrometer and observed bySEM) ranged between 2 and 15 mm depending on the suspensionnature (type and concentration) used to prepare the coatings.Moreover, these SEM observations highlighted that the coatingthickness appeared homogeneous all along the plates.

3.3. Photocatalytic activity

The photocatalytic activities of the TiO2–P25 starting powder(reference) and TiO2 films obtained from type I and type II

suspensions were compared using the photooxidation of ammo-nium ions as model reaction. The activity of TiO2/ b-SiC materialwas also studied. The results obtained after 5 h of UV irradiationare reported in Table 5. The aim of these photocatalytic tests was tocheck whether (or not) the suspension preparation has altered thephotocatalytic activity of TiO2–P25. A more detailed study of theinfluence of several parameters on the decomposition rate of NH4

+

will be the matter of a forthcoming paper.Using TiO2–P25, a degradation of NH4

+ by about 46% after 5 hwas obtained, which corresponds to a decrease of the pHdispersion from 10 to 7.6. It was checked that these results werenot obtained as a result of thermal effects or of a volatilization ofNH3. Indeed, when irradiating a solution of NH3/NH4

+ withoutTiO2, or when mixing a solution of NH3/NH4

+ with TiO2, withoutirradiation, no degradation of NH3/NH4

+ and no pH variation couldbe observed. Using TiO2 films obtained from type I and type IIsuspensions, a degradation of NH4

+ by 33–36% was obtained.Compared to P25 powder, the slight decrease of the photocatalyticactivity could be related to the sintering effects described inSection 3.2.3. Indeed, thermal treatments lead to a slight increaseof the primary particle size and to a decrease of the specific surfacearea. All these factors are known to reduce the photocatalyticactivity of titanium dioxide [42,45].

The photocatalytic activities of TiO2 films obtained from type Iand type II suspensions are very similar. Thus, the use of surfactantdoes not reduce the photocatalytic activity of titanium dioxidetowards oxidation of ammonia. Moreover, it should be noticedthat for TiO2 films obtained from type II suspensions no trace ofammonium ions could be detected by ion chromatography beforethe addition of NH4OH solution. Thus, the addition of ammoniaduring the preparation of type II suspensions has no effect on thephotocatalytic results since ammonia has been totally ‘‘out-gassed’’ (volatilization of gaseous NH3) during the coatingprocedure.

The result obtained with the use of TiO2/ b-SiC material ispromising. Indeed, we demonstrated that the use of a structuredcatalyst did not lead to a complete loss of photocatalytic activity.Nevertheless, the degradation of ammonium ions was lessimportant than the one obtained with P25 powder and scrapedTiO2 films. Different hypotheses are conceivable to explain theseresults. First, we assume that the decrease observed for thedegradation of NH4

+ should not be ascribed to diffusion problemsof the UV radiations into the foam structure (no masking effects)because TiO2/ b-SiC material was cut in small pieces (few mm3).Thus, the structuration is probably not responsible of this decrease.A more detailed study of the photocatalytic properties is necessaryin order to isolate the modifications induced by the preparationmethod from those of the structuration (mass and/or heattransfers, contact times. . .). On the other hand, due to the relativebrittleness of the b-SiC support, after 30 min of stirring thedispersion became opaque and took a grey tone (attrition effect)which could alter the diffusion of UV radiations and reduced thephotocatalytic activity of the sample. Moreover, the heat treat-ments realized during the coating procedure are also responsible ofthe reduction of the photocatalytic activity as discussed in theprevious paragraph.

P. Rodriguez et al. / Applied Catalysis A: General 360 (2009) 154–162162

The problem of attrition observed with the use of TiO2/ b-SiCcomposites might be solved if these materials were no more usedin a ‘‘slurry’’ configuration but as an internal structuration of aclassical reactor.

4. Conclusions

TiO2 coatings were prepared on various structured substratesby dip-coating using TiO2–P25 suspensions. The rheologicalbehaviour of the suspensions was studied and we highlightedthat in our concentration range and whatever their preparationmode, the suspensions had Bingham behaviour. We also studiedthe influence of additives (surfactant, aqueous ammonia) on thesuspension stability. We discussed the relationship between thesuspension stability and the coating adhesion and we highlightedthat the most stable dispersions do not lead systematically to themost resistant coatings. The nature of the substrate plays a majorrole. For porous supports (e.g. cordierite monolith, b-SiC foam)titania suspension is retained by the porous volume. In this case,increasing the electrostatic stability of dispersion leads to arepulsion of TiO2 particles which is unfavourable for the coatingresistance. On the other hand, for non porous supports (e.g.stainless steel), the wettability of the substrate appears moreimportant than van der Waals forces involved in particle repulsion.In this case, the homogeneity of the deposit thickness induced bythe use of a surfactant is an important factor for the coatingadhesion.

By studying the structural properties of titanium dioxidecoatings, we highlighted that the coating procedure described inthis report does not affect the anatase to rutile ratio of the TiO2–P25 starting powder. Moreover, the use of surfactant does notintroduce crystallized impurities in the titania films. Themorphological characterisations revealed the presence of micro-cracks for the coatings prepared using highly concentrateddispersions. These micro-cracks might be the sign of strainrelaxation due to the coating thickness or be related to shrinkageeffects due to heat treatments. Whatever their origin we observedthat micro-cracks do not affect the TiO2 film resistance.

The photocatalytic activity of the obtained materials wasevaluated by studying the photooxidation of aqueous ammonia.We highlighted that heat treatments performed during the coatingprocedure could be responsible of the reduction of photocatalyticactivity (sintering effects). We demonstrated that using astructured catalyst did not lead to a complete loss of photocatalyticactivity which is a promising result.

TiO2 anchored on b-SiC foam is a promising material for(photo)catalytic applications and could be used as an internalstructuration in classical reactors. Finally, we have developed areproducible method to coat stainless steel with a titanium dioxidefilm which might be used for the elaboration of (photo)catalyticstructured reactors.

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 sizeanalyser and Jacques Dazord (LMI – Universite de Lyon) for theAFM experiments. We acknowledge Virginie Giraudeau and PascalPitiot from Rhodia Company for fruitful discussions. Finally, wewish to thank Patrick Nguyen from SICAT Company for the b-SiCfoam samples. This work was financially supported by Axelera‘‘Process Intensification WP6’’.

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