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1 Microporous nd Mesoporous Mterils 156 (2012) Contents lists vilble t SciVerse ScienceDirect Microporous nd Mesoporous Mterils journl homepge: Systemtic evlution of therml nd mechnicl stbility of different commercil nd synthetic photoctlysts in reltion to their photoctlytic ctivity S. Ribbens, E. Beyers, K. Schellens b, M. Mertens c,x.ke d, S. Bls d, G. Vn Tendeloo d, V. Meynen,, P. Cool University of Antwerpen (UA), Deprtment of Chemistry, Lbortory of Adsorption nd Ctlysis, Universiteitsplein 1, 2160 Wilrijk, Belgium b University of Hsselt, Lbortory of Inorgnic nd Physicl Chemistry, Universiteitsln 1, 3590 Diepenbeek, Belgium c Flemish Institute for Technology Reserch, VITO N.V., Boeretng 200, B-2400 Mol, Belgium d University of Antwerpen (UA), Deprtment of Physics, EMAT, Groenenborgerln 171, 2020 Antwerpen, Belgium rticle info bstrct Article history: Received 14 November 2011 Received in revised form 29 December 2011 Accepted 27 Jnury 2012 Avilble online 8 Februry 2012 Keywords: Therml nd mechnicl stbility Mesoporous titnium dioxide P25 Evonik Ò nd Millenium PC500 Ò The effect of therml tretment nd mechnicl stress on the structurl nd photoctlytic properties of eight different (synthetic nd commercil) photoctlysts hs been thoroughly investigted. Different mesoporous Ti-bsed mterils were prepred vi surfctnt bsed synthesis routes (e.g. Pluronic 123, CTMABr = Cetyltrimethylmmonium bromide) or vi templte-free synthesis routes (e.g. trititnte nnotubes). Also, the stbilizing effect of the NOH/NH 4 OH post-tretment on the templted mesoporous mterils nd their photoctlytic ctivity ws investigted. Furthermore, the therml nd mechnicl properties of commercilly vilble titnium dioxides such s P25 Evonik Ò nd Millenium PC500 Ò were studied. The vrious photoctlysts were nlyzed with N 2 -sorption, X-ry diffrction (XRD), high resolution trnsmission electron microscopy (HR-TEM), differentil scnning clorimetry (DSC) nd therml grvimetric nlysis (TGA) to obtin informtion concerning the specific surfce re, pore volume, crystl structure, morphology, phse trnsitions, etc. In generl, results show tht the NOH post-tretment leds to n incresed control of the crystlliztion process during clcintion resulting in higher therml stbility, but t the sme time diminishes the photoctlytic ctivity. Mesoporous mterils in which presynthesized nnoprticles re used s titni source hve the best mechnicl stbility wheres the mechnicl stbility of the nnotubes is the most limited. At incresed tempertures nd pressures, the tested commercil titnium dioxides lose their superior photoctlytic ctivity cused by decresed ccessibility of the ctive sites. The observed chnges in dsorption cpcities nd photoctlytic ctivities cnnot be ssigned to one single phenomenon. In this respect, it shows the need to define generl/ stndrd method to compre different photoctlysts. Furthermore, it is shown tht the photoctlytic properties do not necessrily deteriorte under therml stress, but cn be improved due to crystlliztion, even though the initil mteril is (prtilly) destroyed. It is shown tht the usefulness of specific type of photoctlyst strongly depends on the ppliction nd the temperture/pressure to which it needs to resist. Ó 2012 Elsevier Inc. All rights reserved. 1. Introduction Environmentl stndrds (ISO 14001, October 1996) re becoming more stringent. Therefore, wide vriety of technology ws developed (dsorption [1,2], biodegrdtion [3], ozontion [4,5], Fenton rections [6]...) for the removl of orgnic pollutnts present in ir- nd wter strems. Among these techniques, heterogeneous photoctlysis hs ppered to be very promising. Indeed, photoinduced dvnced oxidtion processes (AOP) cn led to Corresponding uthor. Tel.: E-mil ddress: ver.meynen@u.c.be (V. Meynen). complete minerliztion of most orgnic pollutnts, like dyes nd romtic hydrocrbons, tht re emitted by industril nd domestic ctivities [7,8]. Therefore, semiconductor photoctlysis becme n estblished, significnt nd still expnding re of reserch [9 11]. Titnium dioxide hs been proven to be the most efficient photoctlyst for the degrdtion of hrmful pollutnts in wter nd ir [12 15]. Although, commercil vilble titnium dioxide powders (e.g. P25 Evonik Ò, TiO 2 Riedel-De-Hen Ò, Millenium PC500 Ò ) show high photoctlytic efficiency, these nnocrystlline titni powders hve often low surfce re nd re often hrd to recover out of solution due to their smll prticle sizes. Therefore, up till now, the number of industril pplictions /$ - see front mtter Ó 2012 Elsevier Inc. All rights reserved. doi: /j.micromeso

2 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) is rther limited. To overcome these problems, lot of effort hs been done in mking mesoporous titni mterils with sufficiently lrge prticle sizes in order to increse the surfce re nd to fcilitte the recupertion step respectively [16 18]. Nowdys, lot of synthesis routes re known to prepre these high surfce, highly photoctlytic ctive Ti-bsed mesoporous mterils [19 23]. In mny of these synthetic methods, n orgnic templte is used (f.i. Cetyltrimethylmmonium bromide, Pluronic 123,...), but lso templte free methods re vilble [24,25]. The structurl chrcteristics of these mesoporous mterils hve been described in gret detil in literture. Nevertheless, comprble/systemtic informtion concerning their therml nd mechnicl stbility is lcking. High therml stbility is of gret importnce to (1) void collpse of the structure during clcintion, necessry to remove the orgnic templte in mesoporous photoctlysts [16]. (2) The structure needs to withstnd high clcintion tempertures in order to induce (re)crystlliztion which is crucil in vrious cses (e.g. mesoporous photoctlysts) to enhnce the photoctlytic ctivity [25]. (3) Furthermore, during photoctlysis, products my irreversible dsorb on the ctlyst surfce leding to serious dectivtion of the photoctlyst s function of time [26]. One of the best known methods to regenerte the photoctlyst is to burn off the dsorbed compounds. This process demnds good therml stbility. (4) In order to fixte the ctlyst powder on support mteril (e.g. membrnes, thin films, foms,...), high therml stbilities re likewise importnt [27,28]. Besides therml stbility, lso the mechnicl stbility of the photoctlyst is essentil. Indeed, if the production process reches n industril level, it should be possible (1) to store the ctlyst in high silos or wrehouses without chnging the chrcteristics of the mteril under the mechnicl stress, (2) to llow formultion of the synthesized powders into pellets, extrudtes, etc. which re for exmple required to fcilitte seprtion from queous medium nd (3) to withstnd wter pressures or vpor strems in purifiction plnts. Although the therml [29 31] nd mechnicl stbility [32 34] of different titni bsed mterils hve lredy been studied, the vrious stbility tests re often incomplete nd performed under different conditions. This vrition mkes it difficult to mke systemtic comprison between the different photoctlysts with respect to performnce nd stbility of these mterils. Therefore, in this study, the therml nd mechnicl (unilterl pressure) limittions of the best known commercil nd synthetic photoctlysts re studied. It is the first systemtic nd comprtive study of the therml nd mechnicl stbility in reltion to the photoctlytic ctivity of totl of eight different Ti-bsed photoctlysts (commercilly vilble or prepred t lbscle bsed on templted nd templte free synthesis routes). This study shows the need nd forms bsis for further comprtive investigtions on the therml nd mechnicl stbility of other vilble nd newly developed photoctlysts in reltion to their performnces. 2. Experimentl section 2.1. Synthesis Chemicl regents Chep commodities were used for the preprtion of the synthetic mesoporous titni mterils: commercil crystlline TiO 2 (Riedel De Hën Ò, P25 (Evonik Ò ); PC500 Millenium (Crystl Globl Ò )); Titnium (IV) isopropoxide (97%, Sigm Aldrich); NOH pellets (98.5%, Acros); HCl solution (37% solution in wter, Acros); Cetyltrimethylmmonium bromide (CTMABr) (99+%, Acros); EO 20- PO 70 EO 20 (P123) (Sigm Aldrich); NH 4 OH wt% solution of NH 3 in wter p.. (Acros) nd ethnol (p.., Merck). All chemicls were used s received Templted synthesis nd post-tretment For the synthesis of mesoporous titnium dioxide, two different templted synthesis strtegies were pplied. The Evportion Induced Self Assembly (EISA) method is common pplied method for the synthesis of mesoporous trnsition metl oxide mterils [16,35]. A first solution ws prepred by dissolving 2.36 g (CTMAB) in 24 ml ethnol. In second solution ml EtOH, 4.7 ml HCl nd ml of Ti(O i Pr) 4 were mixed. These two mixtures were stirred nd dded to ech other, followed by the ddition of 8.2 ml of H 2 O. After 1 h of stirring, the solution ws trnsferred into Petri dish nd dried in n oven for 1 week t 60 C. Krtini-method: Insted of using moleculr Ti-precursor, presynthesized ntse nnoprticles re being used s titni source [16,36]. Therefore, 5 ml Ti(O i Pr) 4 ws dded to mixture of 6.6 ml H 2 O nd 10 ml EtOH. The mixture ws stirred t room temperture for 2 h nd hydrothermlly treted t 80 C for 4 h in Teflon lined utoclve. The formed nnoprticle solution ws quenched nd dded to n queous solution of Pluronic 123 (3.2 g EO 20 PO 70 EO 20 in 40 ml H 2 O). This suspension ws stirred for 1 h t C, followed by hydrotherml tretment in Teflon lined utoclve t 100 C for 20 h. After quenching nd filtrtion, the smples were wshed 3 times with 25 ml H 2 O. Stbiliztion: In order to stbilize the mesoporous structures during clcintion, smples (both EISA nd Krtini) were post-treted by refluxing 1 g of solid in 50 ml of bsic, queous solution of NOH or NH 4 OH for period of 48 h (the reflux temperture is tht of the queous solution (100 C)) [16]. NOH tretment took plce in M NOH solution. For the NH 4 OH post-tretment, the ph ws kept constnt between 9 nd 10 by ddition of NH 4 OH (Acros 28 30% p..) to the solution Templte free synthesis Trititnte nnotubes (TNT) re rolled-up mesoporous mterils, prepred without ddition of templtes [25,35,37,38]. Here, 4.5 g TiO 2 Riedel-De-Hen Ò (ntse powder, 12 m 2 /g) ws dispersed into 80 ml of 10 M NOH solution under vigorous stirring during 1 h. The mixture ws trnsferred into n utoclve nd hydrothermlly treted t 150 C for 48 h. After hydrotherml tretment, the solid ws recovered by centrifugtion. The precipitte ws wshed three times with deionized wter. In this wy, sodium trititnte nnotubes re obtined. The sodium nnotubes cn be ion-exchnged to hydrogen trititnte nnotubes by cid wshing. Therefore, N-TNT ws dispersed in 240 ml 0.1 M HCl nd stirred for 30 min. The solid ws recovered by centrifugtion nd dried for 2 dys. Afterwrds, the dried powder ws dispersed gin in 100 ml 0.1 M HCl nd stirred for 5 min. The precipitte ws seprted by filtrtion nd wshed three times with wter. Finlly, the wshed solid ws dried t 100 C for severl dys Chrcteriztion The surfce re nd porosity of the prepred mterils were determined vi N 2 -sorption on Quntchrome Qudrsorb SI utomted gs dsorption system. H-TNT nd N-TNT were outgssed t 150 C nd Millenium PC500 ws degssed t 100 C to prevent structurl chnges upon degssing. All the other described smples were outgssed t 200 C for 16 h. After degssing, N 2 - sorption ws crried out t 196 C. The Brunuer Emmet Teller (BET) method ws pplied to clculte the specific surfce re. The pore size distribution ws clculted from the dsorption brnch using the Brret Joyner Hlend (BJH) model. The volume dsorbed t reltive pressure P/P 0 = 0.98 ws pplied to determine the totl pore volume. The error on ech mesurement is round 5 10%, which hs to be tken into ccount, especilly when the determined surfce res re low.

3 64 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) X-ry diffrction ptterns (XRD) were collected on Pnlyticl X Pert PRO MPD diffrctometer using CuK rdition in 2h mode with brcket smple holder. Mesurements were done t scnning speed of 0.04 /4s continuous mode. The crystl sizes of ntse were clculted vi the Scherrer eqution. By performing electron energy loss spectroscopy (EELS), the crystllinity of the s-prepred smples ws determined. Reference spectr of trititnte nd morphous TiO 2 were recorded. All the reference signls hve been bckground substrcted, deconvoluted nd normlized. The spectr for the unknown smples were then interpreted s liner combintion of the two references spectr, tking into ccount bckground nd multiple scttering. The use of EELS for the determintion of crystl nd morphous phse compositions hs been described in detil in previous pper [39]. The mount of sodium in the NOH treted smples ws determined with electron probe micro nlysis (EPMA, JEOL JXA 733 superprobe). Using Scnning electron microscopy (SEM) nd high resolution trnsmission electron microscopy (HR-TEM), the morphology of the smples could be exmined. HR-TEM pictures were tken on Philips CM30 t 300 kv. Low bem intensities were pplied in order to preserve the structure. SEM ws performed using JSEM 5510 system. The smples were sputtered with thin film of gold to minimize the chrging effects. Differentil scnning clorimetry (DSC) gve informtion bout the therml stbility of the smples, their composition nd possible phse trnsformtions. The mesurements were performed on SDT2960 module (TA-instruments). Smples were heted t heting rte of 5 C/min under N 2 tmosphere (flow rte of 50 cm 3 /min) up to 800 C Photoctlysis The photoctlytic ctivity ws tested by photodegrdtion of n rtificil ctionic pollutnt (rhodmine-6g = R6G or pyridine chloride = PyCl) in queous solution. The ctlyst (8 mg of EISA/ Krtini, 16 mg of H-TNT/N-TNT or commercil ctlyst) ws dded to suspension of 50 ml dye solution nd stirred in drk in order to estblish n dsorption desorption equilibrium between the dye molecules nd the ctlyst surfce. The stirring speed ws djusted in such wy tht the ctlyst remined well dispersed in the solution. The stirring speed ws kept constnt in ll experiments. A lrge mgnetic stirrer ws pplied in order to ssure good dispersion of the ctlyst. After estblishing equilibrium, the solution ws irrdited for 60 min with UV light (wvelength 365 nm) emitted by 100 Wtt Hg-lmp (Sylvni Pr 38; 21.7 mw/cm 2 t 5 cm). The distnce between the pollutnt solution nd the UV-lmp is fixed. During this illumintion, smples with volume of 5 ml were tken out of the suspension t fixed time intervls (10 min) nd nlyzed by UV Vis spectroscopy (Thermo-electron evolution 500, double bem UV Vis spectrometer) nd brought bck to the pollutnt solution fter nlysis. The bsorbnce ws mesured t 526 nm (R6G) or t 256 nm (PyCl) with wter s reference. The dissolved oxygen demnd ws mesured by n oxygen electrode nd determined to be 7.45 ± 0.04 mg/l for the rhodmine 6G solution nd 7.52 ± 0.16 mg/l for the pyridine chloride solution. The obtined vlues were converted to concentrtions by pplying the Lmbert Beer s lw. In this wy, the photobleching could be studied. In order to study the effect of the post-tretment stbiliztion on the photoctlytic ctivity of the ctlysts prepred by the EISAor Krtini method in more detil, dditionl totl orgnic crbon (TOC) mesurements (Shimdzu TOC-V CPH equipped with gs injection kit using high precision syringe with Chney dpter) were performed [40]. In this wy, next to photobleching, determined by UV Vis nlysis, the photominerliztion (oxidtion towrds CO 2 ) could be evluted. Mesurements were performed by tking 0.6 ml of the 5 ml UV Vis smples. The remining (4.4 ml) of the smple ws brought bck to the illuminted pollutnt solution to prevent lrge chnges in volume/ctlyst rtio. Using this 0.6 ml volume, two seprte mesurements were performed in which (i) the totl mount of crbon (TC) ws mesured nd (ii) the totl mount of inorgnic crbon (IC) ws determined. For both TC nd IC mesurements, 3 sequentil injections were performed. For TC nlysis, the smples were directly injected on the combustion oven. For IC nlysis, the smples were directly injected on the cid contining vile. The totl orgnic crbon-vlue ws clculted using the following eqution: TOC ¼ TC IC: The photobleching process (UV) ws nlyzed for period of 60 min, totl orgnic crbon nlysis ws performed for 120 min in order to hve more detiled informtion on the photominerliztion cpcity of the different ctlysts [40]. All described vlues of dsorption, photodegrdtion nd totl conversion (= sum of dsorption nd photodegrdtion) hve been obtined by subtrcting the mesured concentrtion of the pollutnt/dye in solution t given time from the initil pollutnt/dye concentrtion (PyCl of R6G) in solution before dsorption (dsorption nd totl conversion; stock solution) or t dsorption desorption equilibrium (photodegrdtion; 0 min, the moment the UV-light is put on the smple). Blnc mesurements hve been performed (not shown) to ensure tht ll observed effects were true photoctlytic degrdtion rections. Photolysis of R6G nd PyCl only tkes plce fter period of 5 h or longer which is much longer thn the experiments described herein Therml nd mechnicl stbility Therml stbility To study the therml stbility of the templted mesoporous mterils, the smples were heted. A temperture of 450 C ws tken s strting temperture in order to ssure complete removl of the templte. In cse of templte free mesoporous photoctlysts (N-TNT nd H-TNT), the therml stbility tests were strted t 350 C. The temperture ws then incresed in steps of 100 Cin Lenton furnce under mbient conditions with heting rmp of 1 C/min. The desired temperture ws kept constnt for 6 h. Further increse in heting ws done up to the point where complete collpse of the structure ws observed (chosen by the uthors s less thn 50% of the originl S BET ) Mechnicl stbility Mechnicl tests were performed by pelletizing the smples into disc with dimeter of 13 mm. The pressure ws incresed in steps of 740 MP nd ws kept constnt for 2 min. After pelletizing the smples, the discs were softly grinded by hnd in mortr, resulting in fine powder. All mterils will be regrded s completely degrdted if more thn 50% of the initil surfce re is lost due to therml tretment or mechnicl pressure. 3. Results nd discussion 3.1. Therml stbility Templted mterils synthesized vi the Krtini nd EISA method The s-prepred Krtini nd EISA smples which re post-treted by refluxing in n queous NOH or NH 4 OH solution will be

4 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Tble 1 Influence of the clcintion temperture: () results of N 2 -sorption (surfce re (BET) nd totl pore volume (BJH)); (b) crystl sizes (XRD) of non-treted nd lkline posttreted EISA nd Krtini smples. (NM: not mesured). Temp. ( C) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) EISA nt C NM C C EISA TNH 4 OH C C C EISA TNOH C C C KrtininT C C C Krtini TNH 4 OH C C C Krtini TNOH C C C Crystl size (nm) b Specific surfce re (S BET ) nd totl pore volume (Tot. V p ) determined by N 2 - sorption t 196 C. b Crystl size derived from XRD t h pplying the Scherrer eqution. respectively referred to s EISA TNOH, EISA TNH 4 OH, Krtini TNOH nd Krtini TNH 4 OH. The non-treted smples will be indicted by respectively EISA nt nd Krtini nt. In Tble 1, the surfce res, totl pore volumes nd crystl sizes of the non-treted nd lkline post-treted EISA nd Krtini smples re plotted s function of the pplied clcintion temperture. It cn be observed tht the Krtini smples re chrcterized with smller surfce res s compred to the EISA smples (e.g. Krtini TNOH C450: 136 m 2 /g nd EISA TNOH C450: 383 m 2 /g). Beyers et l. [16] ssigned the lower mesoporosity of the Krtini smples to the limited interction between the pre-synthesized ntse nnoprticles nd the surfctnt. The lkline post-tretments re pplied in order to preserve the structure during clcintion by controlling the crystl growth [16,41,42]. Indeed, s observed in Tble 1, excessive crystl growth of the untreted EISA nt nd Krtini nt smples results in complete loss of the mesoporous structure t 550 C (loss of more thn 50% of the initil surfce re). Although the structure of both the EISA nd Krtini smple completely collpse t 550 C, it cn be seen tht the Krtini smple is chrcterized by higher therml stbility. Indeed, t 450 C, the surfce re of EISA nt diminished with 68% wheres decrese of only 24% is observed for Krtini nt. The use of pre-synthesized ntse nnoprticles in Krtini nt mterils during synthesis in contrst to the bsence of mesurble crystls for the EISA smples, results in internl stbiliztion [43]. Due to the low therml stbility of the untreted mterils, only the lkline post-treted smples will be further discussed. Considering the EISA smples, it cn be seen tht 53% of the initil surfce of EISA TNOH is lost fter clcintion t 650 C compred to surfce loss of 48% for EISA TNH 4 OH. It is cler tht both post-tretments hve the sme efficiency towrds the therml stbility of the EISA smple. However, cler differences between the posttretments cn be observed for the Krtini method: the loss of surfce re of the NOH treted Krtini smple fter clcintion t 650 C (28%) is much less compred to the NH 4 OH treted Krtini smple (53% t 650 C). Considering the crystl growth of the Krtini smples t elevted tempertures (see Tble 1), it is cler tht the NOH post-tretment llows for better inhibition of uncontrolled crystlliztion during therml tretment (6 nm t 650 C) s compred to NH 4 OH (11 nm t 650 C). Indeed, the presence of NOH ions on the surfce fter post-tretment (3 wt% N; mesured with EPMA) limits the crystlliztion nd growth of ntse during clcintion s described by Nm [43,44]. The fct tht this post-tretment is not s effective for the EISA smples s it is for the Krtini smples, lthough crystl growth is efficiently controlled, indictes tht other phenomen ply n dditionl role like for instnce porosity, wll thickness, degree of crystllinity, presence of ntse nnoprticles,...therefore, it is difficult to predict the stbility of the developed ctlysts. From the bove results it is cler tht generl method to stbilize different mterils is difficult to find nd every stbiliztion method needs to be dpted to the type of mteril nd synthesis method. Furthermore, it shows the importnce of this systemtic study nd the lck of nunces in literture. Fig. 1 shows the effect of crystlliztion, post-tretment nd loss of vilble surfce re on the dsorption cpcity nd photoctlytic ctivity in function of the clcintion temperture. It cn be seen tht the dsorption cpcity of the NOH post-treted EISA smple decresed by 47% fter clcintion t 650 C (Fig. 1A). This decrese coincides with the observed loss in surfce re (53%) (see Tble 1). Although, EISA TNH 4 OH C650 hs similr loss in surfce re fter incresing the temperture from 450 to 650 C (48%), the dsorption cpcity isn t ffected by this decrese. The sme trend cn be observed for the NH 4 OH treted Krtini smples (Fig. 1B). It is cler tht the NH 4 OH treted smples hve lower dsorption cpcity in generl, however it is much more thermlly stble. This points to the influence of other, multiple mteril chrcteristics other thn surfce re such s crystllinity, ccessibility nd diffusion/mss trnsport, prticle sizes nd distribution, surfce dsorption kinetics etc. tht need to be tken in ccount to unrvel the influence of therml stbility on the dsorption cpcity. For exmple, stronger increse in crystllinity hs tken plce for the NH 4 OH post-treted smples s compred to the NOH treted smples, which cn be observed by the increse in intensity of the ntse diffrction peks with incresing temperture in XRD. The diffrction peks remin brod confirming tht no strong crystl growth tkes plce (Fig. S1 nd Tble 1). Wheres morphous TiO 2 shows only short-rnge order nd lot of structurl defects (lrge mount of under- or overcoordinted structurl Tiunits nd dngling bonds) [44], the ntse phse hs well formed lttice with smll mount of structurl defects nd high mount of hydroxyl groups which results in higher dsorption cpcities [45,46]. However, this effect does not explin ll obtined differences in therml stbility. It needs to be noted tht by compring the NOH nd NH 4 OH post-treted smples, it becomes cler tht the NOH treted smples show higher ffinity towrds R6G s compred to the NH 4 OH treted smples. This cn be ssigned to the high number of bsic hydroxyl groups due to the reflux in NOH [47]. This effect is very cler for the Krtini smples wheres for the EISA smples the vilble surfce re lso contributes to the lrge dsorption differences. Therefore, chnges in dsorption cpcity cnnot be ssigned to one single prmeter since during clcintion, the surfce re, ccessibility, crystl size, crystllinity nd other structurl chrcteristic chnge in different wy depending on the originl synthesis pplied. Compring the influence of the employed stbiliztion method on the ctlytic ctivity, both EISA nd Krtini smples treted with NOH show low photoctlytic ctivities s compred to the NH 4 OH treted smples, irrespective of the therml tretment. The presence of sodium ions on the surfce seems to hve negtive effect s the NOH treted smples show limited photoctlytic ctivity (middle grey brs in Fig. 1) compred to the NH 4 OH treted smples. Indeed, previous studies showed tht sodium cts s electron/hole pir recombintion center [44]. At incresed clcintion tempertures, the photoctlytic ctivity increses,

5 66 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Conc (10-5 M) Conc (10-5 M) EISA TNOH C450 EISA TNOH C650 EISA TNH4OH C450 EISA TNH4OH C650 Fig. 1. (A) Results of photoctlytic test using UV-nlysis in which 8 mg of ctlyst ws dded to 50 ml M R6G-solution. Adsorption cpcity determined fter 60 min (EISA TNH 4 OH) nd 120 min (EISA TNOH) dsorption desorption equilibrium without UV illumintion. After equilibrium, photoctlytic experiments were performed t room temperture during 60 min of UV-illumintion. : Adsorption, : Photobleching : Totl Conversion. (B) Results of photoctlytic test using UV nlysis in which 8 mg of ctlyst ws dded to 50 ml M R6G-solution. Adsorption cpcity determined fter 60 min (Krtini TNH 4 OH) nd 120 min (Krtini TNOH) dsorption desorption equilibrium without UV illumintion. After equilibrium, photoctlytic experiments were performed t room temperture during 60 min of UV-illumintion. : Adsorption, : Photobleching : Totl conversion performed in order to study the effect of the lkline post-tretment on the photominerliztion to CO 2 in more detil [40]. As shown in Fig. 2, the obtined totl orgnic crbon-results re in correspondence with the UV Vis mesurements nd the sme trends re observed. For the Krtini smples, enhnced photobleching nd enhnced photominerliztion ctivity cn be observed s compred to the EISA smples. This proves tht the further degrdtion of the dye nd its intermedites into CO 2 cn tke plce, lthough the ctivity towrds photominerliztion is lower. In generl, it cn be concluded tht the Krtini smples show higher therml stbility nd photoctlytic ctivity compred to the EISA smples due to the use of smll pre-synthesized crystls. Post-tretment by NOH is the most efficient wy to control the crystl growth during clcintion resulting in more stbilized structures irrespective of the preprtion method (Krtini/EISA). Even though post-tretment with NOH results in more stbilized structures, these sodium ions ct s recombintion center resulting is low photoctlytic ctivities. From ctlytic point of view, the NH 4 OH post-tretment is superior. Although the Krtini smples re chrcterized by lower surfce re s compred to the EISA smples, there high crystllinity results in higher photominerliztion nd photobleching cpcities s compred to EISA. However, s stted bove, it is cler tht lso other structurl prmeters nd chnges influence the observed differences. It is difficult to distinguish ll Templte free mesoporous Ti-bsed mterils Nnotubes. Tble 2 presents the surfce res of the thermlly treted N-TNT nd H-TNT smples (obtined by ion-exchnge of N-TNT). Here, lrge differences in initil surfce re of N-TNT nd H-TNT re observed. The undelying reson for these differences is still uncler, but it could be prtilly ttributed to the higher molr mss of N compred to the interclted protons nd the fct tht surfce re is given s squre meters per grm. Furthermore, s described by Qmr et l. [48], it is possible tht higher mount of nitrogen gs (kinetic dimeter of nm) is dsorbed between the lyers of H-TNT (0.62 nm) compred to N-TNT (0.74 nm), due to the much smller dimensions of the hydrogen ions. In function of incresed temperture, it is cler tht the surfce re nd totl pore volume of both 0 Krtini TNOH C450 Krtini TNOH C650 Krtini TNH4OH C450 Krtini TNH4OH C650 6 Fig. 1 (continued) Photobleching lthough the specific surfce re decreses. Whtever the synthesis method, the photoctlytic ctivity of the NH 4 OH post-treted smples increses fter clcintion t 650 C (see Fig. 1). This increse is much lrger in NH 4 OH stbilized mterils s compred to NOH stbilized mterils nd seems to coincides with the previously reported differences in crystllinity (EISA TNH 4 OH C450: 68 ± 3%, determined by EELS nd EISA TNOH C450: 40 ± 6%, determined by EELS [39]) nd differences in crystl size (Tble 1 nd XRD in Fig. S1) upon heting. The rther limited increse in photoctlytic ctivity of the NOH treted Krtini smples could be correlted to the inhibited crystlliztion (see Fig. S1) during therml tretment in combintion with the presence of sodium ions. The photoctlytic ctivity of the NOH post-treted EISA smples is higher compred to the NOH-treted Krtini smples, but still very low compred to the other investigted photoctlysts. It is hrd to subscribe this observtion to one single prmeter becuse the impct of other structurl chnges cnnot be excluded. Besides UV Vis mesurements, in which the photoinduced bleching of R6G is studied, totl orgnic crbon-nlysis hs been PPM Rhodmine 6G EISA T NH4OH C EISA T NH4OHC EISA TNOHC EISA TNOH C Krtini TNH4OH C Krtini TNH4OH C Photominerliztion Fig. 2. Results of photoctlytic test (8 mg of ctlyst in 50 ml M R6Gsolution): photobleching (UV Vis nlysis, 60 min) nd photominerliztion (lv- TOC nlysis, 120 min) Krtini TNOHC Krtini TNOH C

6 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Het flow/mw Temperture/ C Fig. 3. DSC of H-TNT nc nd N-TNT nc. H-TNT nc N-TNT nc N-TNT nd H-TNT seriously decreses round 450 C (see Tble 2). This effect is lrger for H-TNT thn for N-TNT. In order to rtionlize this decrese, DSC nd HR-TEM mesurements were performed. The DSC spectr of both N-TNT nd H-TNT, presented in Fig. 3, show brod pek t C ttributed to the loss of structurl wter nd due to condenstion of hydroxyl groups. For H- TNT, the exothermic pek between 350 nd 420 C is ssigned to recrystlliztion of trititnte to ntse [25]. No chnges occur t this temperture for N-TNT. HR-TEM shows tht clcintion of H-TNT t tempertures higher thn 350 C results in the destruction of the nnotube morphology of H-TNT (see Fig. 4A C) due to recrystlliztion into ntse prticles. The lst pek in DSC round 500 C, origintes from the recrystlliztion of ntse prticles into rutile [25]. At the sme temperture, the sodium trititnte nnotubes (N 2 Ti 3 O 7 ) strt to recrystllize into sodium hextitnte (N 2 Ti 6 O 13 ) round 450 C [49,50] (Fig. 4). Furthermore, the morphology of the sodium trititnte nnotubes chnges during this recrystlliztion (Fig. 4D nd E) lthough it is much less drstic s for H-TNT. In generl, it is cler tht sodium nnotubes re more stble compred to hydrogen nnotubes. The influence of the chnges in morphology, crystllinity, crystl size, crystl phse nd surfce re on the dsorption cpcity nd photoctlytic ctivity is shown in Tble 2. The dsorption cpcity of both H- TNT nd N-TNT decreses t higher clcintion tempertures. Although this cn be directly correlted to the decresing surfce re, other prmeters like the presence of the sodium ions nd recrystlliztion could lso influence the dsorption chrcteristics. Concerning the photoctlytic ctivity of the different smples, it cn be concluded tht ll N-TNT smples hve limited ctivity despite their similr dsorption cpcity due to the presence of sodium nd its high bnd gp energy (3.4 ev). In contrst to N-TNT, the photoctlytic ctivity of the H-TNT is reltively high nd increses in function of elevted clcintion temperture (>350 C). This cn be ttributed to recrystlliztion of the structure into highly photoefficient ntse. Although, lso other structurl chnges could tke prt in the enhnced ctivity. So, lthough sodium nnotubes hve much better therml stbility thn hydrogen nnotubes, the presence of sodium ions severely diminishes its ctlytic performnce s compred to the less stble hydrogen nnotubes. Hydrogen nnotubes recrystllize into more photoctlytic ctive nnoprticles t tempertures higher thn 350 C. However, this is not very interesting since the ctlyst is destroyed nd the nnoprticles re hrd to recover out of solution Commercil titniumdioxide (P25 Evonik Ò ). P25 Evonik Ò is very fine, commercilly vilble powder (prticle size 20 nm) with low surfce re (50 60 m 2 /g). It hs high photoctlytic ctivity due to its crystllinity nd the blnced rtio ntse/rutile [51]. However, little is known bout the therml stbility of the structure nd the effect on the photoctlytic ctivity. In Tble 3, it cn be seen tht the clcintion temperture hs minor influence on the surfce re (difference of 12 m 2 /g between P25 nc nd P25 C550). In function of incresed clcintion temperture, cler decrese in totl pore volume cn be observed t elevted tempertures, ssigned to strong ggregtion of the prticles during clcintion (see SEM picture; Fig. S2). Note tht in this cse the totl pore volume origintes from interprticle porosity s P25 is not porous. This results in lower dsorption cpcity Fig. 4. HR-TEM: () H-TNT nc, (b) H-TNT C350, (c) H-TNT C450, (d) N-TNT nc nd (e) N-TNT C550.

7 68 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Tble 2 Influence of clcintion temperture: () results of N 2 -sorption (surfce re (BET) nd totl pore volume (BJH)); (b) results of photoctlytic test (16 mg of ctlyst in 50 ml M R6G-solution) of H-TNT nd N-TNT. Temp. ( C) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) Ads. b (10 5 M) Ct. b (10 5 M) Totl Conv. b (10 5 M) H-TNT nc C C C N-TNT nc C C C b Specific surfce re (S BET ) nd totl pore volume (Tot. V p ) determined by N 2 -sorption t 196 C. Adsorption (Ads.), ctlysis (Ct.) nd totl conversion (Totl. Conv.) determined by photoctlytic test. Tble 3 Influence of clcintion temperture: () results of N 2 -sorption (surfce re (BET) nd totl pore volume (BJH)); (b) results of photoctlytic test (16 mg of ctlyst in 50 ml M R6G/PyCl); (c) quntifiction of crystl phses (Rietveld method, XRD) of P25 Evonik Ò nd Millenium PC500 Ò. Temp. ( C) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) Ads. b (10 5 M) Ct. b (10 5 M) Totl Conv. (10 5 M) b R6G R6G PyCl R6G PyCl Antse c % Rutile c % P25 nc C C NM NM C Temp. ( C) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) Ads. b (10 5 M) Ct. b (10 5 M) Totl Conv. (10 5 M) b XRD t h c (nm) R6G PyCl R6G PyCl Millenium PC500 nc C C C NM: not mesured. Specific surfce re (S BET ) nd totl pore volume (Tot. V p ) determined by N 2 -sorption t 196 C. b Adsorption (Ads.), ctlysis (Ct.) nd totl conversion (Totl Conv.)determined by photoctlytic test. c Percentge crystl phse determined by pplying the Rietveld method XRD. (19% t 550 C) nd decresed photoctlytic ctivity (t 550 C, 24% of its originl photoctlytic ctivity is lost). To clrify the effect of ggregtion on the photoctlytic ctivity, the photoctlytic test ws repeted using pollutnt with smller dimensions (pyridine chloride) compred to R6G. Now, it cn be seen tht the decrese in photoctlytic ctivity of P25 fter clcintion t 650 C is very limited (6%) compred to R6G (24%). The differences in photoctlytic ctivity of the thermlly treted P25 towrds PyCl nd R6G clerly shows tht the use of certin ctlyst strongly depends on the tretment to which the photoctlysts ws subjected to. By using Rietveld quntifiction (XRD) the effect of the clcintion temperture on the rtio ntse/rutile ws studied (Tble 3). It is cler tht temperture hs negligible effect on the crystllinity of P25 s well s on the crystl size of the ntse Commercil titniumdioxide (Millenium PC500 Ò ). Millenium PC500 Ò is porous, commercilly produced titnium dioxide powder. Tble 3 shows tht the surfce re seriously diminishes (84%) in function of incresed clcintion temperture wheres the decrese in totl pore volume is less drstic (41%). After clcintion t 350 C, more thn 60% of the initil surfce re hs lredy been lost. The corresponding isotherms nd pore size distributions (see Fig. S3) show tht the smller pores re disppering s temperture increses, wheres lrger pores nd interprticle spces re being formed, explining the discrepncy in the decrese in specific surfce re nd totl pore volume. These chnges in porosity coincide with substntil growth in crystl size. Despite drstic structurl chnges during therml tretment, the dsorption cpcity towrds R6G isn t influenced. This could be cused by the enlrgement of the pores llowing much esier diffusion of the R6G molecules nd/or by the incresed crystllinity [45,52]. Since the dsorption cpcity is only 0.1 (10 5 M) despite the high specific surfce re of Millenium PC500, the dsorption seems to only pper t the externl surfce (cfr. P25). This might explin why differences in inner porosity upon incresed temperture, do not result in chnges in dsorption cpcity. By using pyridine chloride (much smller dimensions) s rtificil pollutnt insted of R6G, the importnce of the ccessibility of the ctive sites is clerly shown (see Tble 3). Due to the loss of vilble surfce re (84%), the dsorption of pyridine chloride drops by 80% wheres no effect ws observed for R6G confirming tht the R6G cn t enter into the mjority of the pores of Millenium PC500 nc. If the photoctlytic ctivity of the initil nd thermlly treted titnium dioxide towrds R6G is compred, minimum cn be observed t 350 C (see Tble 3). Chnges in photoctlytic ctivity throughout the different therml tretments cn t be ssigned to one single sset, but re combintion of chnging vilble surfce re, crystl size (7 nm for the unclcined PC500 Ò up to 17 nm t PC500 Ò C550), crystllinity nd ccessibility. Upon incresed clcintion temperture, the surfce re of millennium PC500 Ò decreses wheres the pore sizes nd crystllinity [52] increse Mechnicl stbility Besides high tempertures, photoctlysts need to be ble to withstnd pressure (f.i. during storge, formultion, etc...) without losing their specific surfce re, morphology nd photoct-

8 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Tble 4 Influence of pressure: () results of N 2 -sorption (surfce re (BET) nd totl pore volume (BJH)); (b) results of photoctlytic test (8 mg of ctlyst in 50 ml M R6G/PyCl) of Krtini TNH 4 OH, Krtini NOH, EISA TNH 4 OH nd EISA NOH. Pressure (MP) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) Ads. b (10 5 M) Ct. b (10 5 M) Totl Conv. b (10 5 M) R6G PyCl R6G PyCl EISA TNOH NM 0.02 NM EISA TNH 4 OH NM 0.40 NM NM 0.49 NM NM 0.41 NM 1.09 Krtini TNOH NM 0.34 NM NM 0.37 NM NM 0.39 NM 1.75 Krtini TNH 4 OH NM 0.54 NM NM 0.57 NM NM 0.51 NM 1.12 NM: not mesured. Specific surfce re (S BET ) nd totl pore volume (Tot. V p ) determined by N 2 -sorption t 196 C. b Adsorption (Ads.), ctlysis (Ct.) nd totl conversion (Totl Conv.) determined by photoctlytic test. Tble 5 Influence of pressure: () results of N 2 -sorption (surfce re (BET), totl pore volume (BJH)); (b) results of photoctlytic test (16 mg of ctlyst in 50 ml M R6Gsolution) of H-TNT nd N-TNT. Pressure (MP) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) Ads. b (10 5 M) Ct. b (10 5 M) Totl Conv. b (10 5 M) H-TNT N-TNT b Specific surfce re (S BET ) nd totl pore volume (Tot. V p ) determined by N 2 -sorption t 196 C. Adsorption (Ads.), ctlysis (Ct.) nd totl conversion (Totl Conv.) determined by photoctlytic test. lytic ctivity. Therefore, the mechnicl stbility of the s-synthesized mterils is tested by pelletizing the smples t 296 nd 740 MP Templted Ti-bsed mterils synthesized vi the Krtini nd EISA method As discussed in previous section, the surfctnt, present in the lkline post-treted EISA nd Krtini smples, cn be completely removed t 450 C. Therefore, ll the smples were clcined t 450 C, in order to void dditionl stbiliztion coming from smll mounts of surfctnt left in the pores. Considering the surfce re of both EISA nd Krtini smples fter pressurizing (Tble 4), it is cler tht EISA NOH hs lower mechnicl stbility s compred to EISA NH 4 OH nd both Krtini smples. This cn be clerly seen from the incresed loss in surfce re in cse of EISA NOH. Since the NOH nd NH 4 OH post-tretment control the degree of crystlliztion nd crystl growth, it is expected to hve mjor influence on the mechnicl stbility. For the EISA smples, cler differences cn be observed depending on the bse tretment pplied. Wheres EISA TNOH losses 28% of its initil surfce fter pplying pressure of 740 MP, EISA TNH 4 OH losses only 3% of its initil surfce. It is possible tht, due to the higher degree of crystlliztion of EISA TNH 4 OH C450 (68 ± 3%, determined by EELS) s compred to EISA TNOH C450 (40 ± 6%, determined by EELS), the NH 4 OH post-treted structure is more stbilized nd cn withstnd higher mechnicl pressures. In cse of the Krtini smples, the crystllinity is lredy present due to the use of pre-crystllized ntse nnoprticles (5 10 nm) resulting in more condensed titni network which might explin why in cse of Krtini smples there is no difference depending on the bse tretment. The sme trend cn be seen in the totl pore volume of the treted EISA nd Krtini smples nd in the chnges observed in the isotherms (Figs. S5 nd S6). Chnges in pore dimeter pper upon incresing pressure, resulting in mterils with prtilly nd loclly nrrowed pores t 740 MP (two-step desorption) [53]. This might influence the photoctlytic ctivity nd dsorption cpcity of the smples. Tble 4 shows tht the dsorption cpcity nd photoctlytic ctivity of EISA TNOH C450 decreses s function of incresed pressure. This is in greement with the drop of vilble surfce re of EISA TNOH in function of incresed pressure. Moreover, the ccessibility of the ctive sites within the pores diminishes due to pore nrrowing (Fig. S6A). Even if smller pollutnt molecules (pyridine chloride) re pplied (Tble 4), decresed dsorption cpcities cn be observed in function of pressure. The dsorption cpcity nd photoctlytic ctivity of ll other smples does not lter significntly since the loss of surfce re is very limited for these smples. The pores seem still well ccessible despite the presence of locl pore nrrowing Templte free mesoporous photoctlysts Nnotubes. Tble 5 nd Fig. S7 show the N 2 -sorption results of pelletized H-TNT nd N-TNT. The surfce re nd totl pore volume of the smples, pelletized t 296 MP, decreses tremendously. At 740 MP, 44% of the initil surfce of H-TNT nd 68% of the initil surfce of N-TNT is lost, which my imply tht H- TNT hs higher mechnicl stbility compred to N-TNT. Nevertheless, s the pore volume nd surfce re of the nnotubes is for lrge percentge due to interprticle porosity between the ggregted species, HRTEM imges were tken to study the impct of pressure on the morphology of the nnotubes itself. In Fig. 5A nd B, it cn be seen tht the morphology of H-TNT is completely destroyed t 296 MP wheres the nnotube morphology of N- TNT is still intct t 740 MP. The interstitil porosity between the ggregtes is however strongly reduced. Even t pressure of 740 MP, sodium nnotubes prtilly retined their rolled up,

9 70 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Fig. 5. HR-TEM picture: () H-tube pelletized t 296 MP, (b) H-tube pelletized t 740 MP nd (c) N-TNT pelletized t 740 MP. Tble 6 Influence of pressure: () results of N 2 -sorption (surfce re (BET) nd totl pore volume (BJH)); (b) results of photoctlytic test (16 mg of ctlyst in 50 ml M R6G/ PyCl) of P25 Evonik Ò nd Millenium PC500 Ò. Pressure (MP) S BET (m 2 /g) Loss of S BET (%) Tot. V p (ml/g) Ads. b (10 5 M) Ct. b (10 5 M) Totl Conv. b (10 5 M) R6G R6G PyCl R6G PyCl P Millenium PC b Specific surfce re (S BET ) nd totl pore volume (Tot. V p ) determined by N 2 -sorption t 196 C. Adsorption (Ads.), ctlysis (ct.) nd totl conversion (Totl Conv.) determined by photoctlytic test. multiwlled structure (Fig. 5C). The sodium nnotubes hve therefore higher mechnicl stbility of the morphology of the tubes compred to the hydrogen nnotubes. The reson for this might be the better interction of the sodium with the surrounding negtive loded Ti-O lyers compred to the interclted protons [54]. The loss of the nnotube morphology of H-TNT nd the limited ccessibility of the ctive sites due to the lrge decrese in interprticle porosity, results in tremendous decrese in photoctlytic ctivity nd dsorption cpcity (Tble 5). Even though the N-TNT hve lrger decrese in surfce re nd totl pore volume, the decrese of the dsorption cpcity is more limited compred to H-TNT, which is in greement with the preservtion of the nnotubes morphology. Despite the higher mechnicl stbility of the N-TNT compred to hydrogen nnotubes, their originl photoctlytic ctivity is very low, which mkes them not very interesting s photoctlysts Commercil titniumdioxide (P25 Evonik Ò ). In Tble 6, the influence of pressure on the surfce re nd totl pore volume of P25 Evonik Ò is presented. Although, non-porous P25 Evonik Ò becomes more dense in function of pressure s shown in Fig. 6, there seems to be no loss of externl surfce re under the pplied pressure. Nevertheless, considering the results of the photoctlytic test (Tble 6), cler influence of the incresed pressure on the dsorption cpcity nd photoctlytic ctivity cn be observed. Indeed, the dsorption cpcity decreses by 50% nd the photoctlytic ctivity decreses with 65% fter pplying pressure of 740 MP. Similr results re obtined if smller rtificil pollutnt like PyCl is pplied. Therefore, it cn be concluded tht P25 Evonik Ò losses its high photoctlytic ctivity fter pressurizing. Even though nitrogen hs still ccess to the totl specific surfce re fter pressurizing, the pollutnts seem to suffer from limited ccessibility of the ctive sites (for both R6G nd PyCl). Agin, it is cler tht the suitbility of the ctlyst under the pplied conditions depends on the size of the pollutnt Commercil titniumdioxide (Millenium PC500 Ò ). Although Millenium PC500 Ò is highly porous photoctlyst, the dsorption of R6G is reltively smll (see Tble 6). This cn be relted to the limited ccessibility of the pores by R6G s described in previous

10 S. Ribbens et l. / Microporous nd Mesoporous Mterils 156 (2012) Fig. 6. SEM: () P25 0 MP nd (b) P MP. section (Millenium PC500 Ò is chrcterized by very smll pores, see Fig. S8). If pressure of 296 MP is pplied, the totl pore volume decreses by 32% nd the vilble surfce re decreses by 35%. This results in severely diminished dsorption cpcity nd photoctlytic ctivity. At mechnicl pressure of 740 MP, 39% of the initil surfce re nd 41% of the totl pore volume is lost resulting in decresed dsorption cpcity (58%) nd the photoctlytic ctivity (75%). Nevertheless, if pyridine chloride is used s rtificil pollutnt, the dsorption cpcity decreses with only 11% nd the photoctlytic ctivity with 36% fter pressure of 740 MP suggesting the esier ccessibility of pyridine chloride compred to R6G. In generl, it is cler tht structure suffers severely from mechnicl stress. If the mechnicl stbility of the commercil titni ctlysts nd the s-prepred photoctlysts is compred, it is cler tht the porous post-treted Krtini nd EISA smples hve superior properties over the hydrogen nnotubes nd the commercil powders. For the hydrogen nnotubes nd Millenium PC500 Ò, cler structurl collpse cn be observed t incresed pressures. Although, this cn t be observed for P25 Evonik Ò (since it is nonporous), both P25 nd Millenium PC500 seem to suffer from diminished ccessibility of the ctive sites fter pressure hs been pplied, resulting in reduced photoctlytic ctivity. 4. Conclusion In this study, the therml nd mechnicl properties of eight different s-prepred nd commercilly vilble photoctlysts were studied. It is shown tht the chnges in clcintion temperture/mechnicl stress hve lrge impct on different chrcteristics of the mteril. Besides chnges in vilble surfce re nd pore size, the morphology of the smples s well s the crystl size, prticle size distribution, mss trnsport, crystllinity nd crystl phse cn be ltered. Therefore, the observed chnges in dsorption cpcities nd photoctlytic ctivities cnnot be ssigned to one single phenomenon. This mkes it hrd to predict the stbility of the dsorption cpcity nd photoctlytic ctivity of the photoctlyst with respect to therml nd mechnicl stress. In this respect, it shows the need to define generl/stndrd method to compre different photoctlysts s ws exemplified here. Furthermore, it is shown tht the photoctlytic properties do not necessrily deteriorte under therml stress, but cn be improved due to crystlliztion, even though the initil mteril is (prtilly) destroyed. Therefore, the usefulness of specific type of photoctlyst strongly depends of the ppliction nd the temperture/ pressure to which it needs to resist. In generl, it is cler tht the NOH post-tretment is beneficil for the mechnicl nd therml stbiliztion of mesoporous titni. However, it hs negtive effect on the photoctlytic ctivity. Other bses re therefore to be preferred. Hydrogen nnotubes re chrcterized by low mechnicl nd therml stbility s the nnotube morphology cn be completely destroyed t incresed temperture or mechnicl stress s low s 296 MP. Sodium nnotubes re more stble, but show low photoctlytic ctivities. Although, P25 Evonik Ò is recognized s highly efficient photoctlyst, its ctivity cn be seriously reduced by therml or mechnicl stress, which seems originte from the reduced ccessibility of the ctive sites. Acknowledgements Prof. B. Vn Der Veken is grtefully cknowledged for the DSC mesurements. Joke Vn Ler nd Mtthis Llemn (Shimdzu) re cknowledged for their technicl support with respect to the development of the lv TOC. This work hs been performed in the frme of the FWO project (G ) nd the GOA-project (41/FAO70200/799). Appendix A. Supplementry dt Supplementry dt ssocited with this rticle cn be found, in the online version, t doi: /j.micromeso References [1] W. Sun, Y. Qu, Q. Yu, J. Ni, J. Hzrd. Mter. 154 (2008) [2] Y.X. Zho, M.Y. Ding, D.P. Chen, Anl. Chim. Act. 542 (2005) [3] J. Michizoe, H. Ichinose, N. Kmiy, T. Mruym, M. Goto, J. Biosci. Bioeng. 99 (2005) [4] M.I. Mldondo, S. Mlto, L.A. Pérez-Estrd, W. Gernjk, I. Oller, X. Doménech, J. Perl, J. Hzrd. Mter. 138 (2006) [5] A.C. Silv, J.S. Pic, G.L. Snt Ann, M. Dezotti, J. Hzrd. Mter. 169 (2009) [6] D.H. Brenner, R. Molin, F. Mrtinez, J.A. Melero, Y. Segur, Appl. Ctl. B: Environ. 90 (3 4) (2009) [7] O. Crp, C.L. Huismn, A. Reller, Prog. Solid Stte Chem. 32 (2004) [8] M. Klvrioti, D. Mntzvino, D. Kssinos, Environ. Intern. 35 (2009) [9] S. Lio, H. Donggen, D. Yu, Y. Su, G. Yun, J. Photochem. Photobiol. A: Chem. 168 (2004) [10] H. Xi, H. Zhung, T. Zhng, D. Xio, Mter. Let. 62 (2008) [11] U.I. Gy, A.H. Abdullh, Z. Zinl, M.Z. Hussein, J. Hzrd. Mter. 168 (2009) [12] D. Robert, S. Mlto, Sci. Totl Environ. 291 (2002) [13] P.A. Pekkis, N.P. Xekoukoulotkis, D. Mntzvinos, Wter Res. 40 (2006) [14] F.B. Li, C.H. Ao, S.C. Lee, M.F. Hou, Chemosphere 59 (2005) [15] S. Josset, J. Trnto, N. Keller, V. Keller, M. Lett, M.J. Ledoux, V. Bonnet, S. Rougeu, Ctl. Tody 129 (2007) [16] E. Beyers, P. Cool, E.F. Vnsnt, Microp. Mesopor. Mter. 99 (2007) [17] S. Agrwl, G.W. Ho, Mter. Let. 63 (2009) [18] J. Yu, M. Zhou, B. Cheng, H. Yu, X. Zho, J. Mol. Ct. A-Chem. 227 (2005) [19] J. Du, Z. Liu, Z. Li, B. Hn, Y. Hung, Y. Go, Microporous Mesoporous Mter. 83 (2005) [20] K.S. Yoo, T.G. Lee, J. Kim, Microporous Mesoporous Mter. 84 (2005) [21] D.S. Kim, S.Y. Kwk, Appl. Ctl. A-Gen. 323 (2007) [22] S. Yun, Q. Sheng, J. Zhng, H. Ymshit, D. He, Microporous Mesoporous Mter. 110 (2008)

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