Mohamad El-Roz
email: mohamad.el-roz@ensicaen.fr

Monika Kus

Pegie Cool

Pegie Cool b Frederic Thibault-Starzyk

New Operando IR Technique to Study the Photocatalytic Activity and Selectivity of TiO 2 Nanotubes in Air Purification: Influence of Temperature, UV Intensity, and VOC Concentration

Keywords: TiO 2 nanotubes, Photocatalysis, operando-IR, VOC and carbon monoxide photooxidation, temperature and VOC concentration effect

scientifiques de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The decontamination of air in closed spaces such as aircrafts, air-conditioned spaces, etc..., from volatile organic contaminants/compounds (VOC) attracts special attention due to its economic and health interests. The use of heterogeneous catalysts, largely employed for the control and treatment of exhaust gases, presents one of the key features for such treatment.

Indeed, the reaction on the catalyst can be induced either by heating, or by UV light when using a photocatalyst. This latter presents many advantages such as low cost and low environmental impact, and TiO 2 is considered as the most promising catalyst for this purpose. TiO 2 is cheap, resistant to photocorrosion, photostable and non toxic. It is easy to prepare, environmentally friendly and has a low band gap energy. 1,2,[START_REF] Fujishima | 2 Photocatalysis. Fundamentals and Applications[END_REF] With new environmental issues, it is expected that applications for TiO 2 will grow with a high speed. In particular, the improvement of TiO 2 activity in organic photodegradation processes is needed for large scale industrial treatment processes such as waste water treatment and de-VOCs [START_REF] Macak | [END_REF]5,6 . Titania nanotubes (TNTs) have been proven to be highly promising for potential applications in various fields 7,8 . Titania nanotubes layers were shown recently to be more efficient in the photocatalytic conversion of organic azo dyes than common photocatalysts [START_REF] Macak | [END_REF]9 . In the present work, TiO 2 -nanotubes (TNTs) have been prepared by a hydrothermal method according to S.

Ribbens et al. 10 , and studied in the photooxidation of methanol, n-hexane and carbon monoxide by a new operando IR technique. The common point of the air pollutants chosen (CH 3 OH, C 6 H 14 and CO) is their toxicity even at low concentration. The comparison of methanol and n-hexane photooxidation allows investigating the influence of hydrophilic/hydrophobic ratio and hydrocarbon chain length on the photooxidation activity.

In order to estimate the activity of TNTs in gas phase photooxidation reactions, a well known photocatalyst, TiO 2 -P25 from Degussa, has been used. The influence of various parameters, such as the temperature, the UV irradiation intensity and the concentrations of VOCs or CO was studied in real time. The present work shows the interest of operando IR to perform photocatalytic studies, on self supporting wafers of the catalysts and in gas flow in conditions very close to those of the real reaction.

Experimental part

Synthesis

Titanium based nanotubes were synthesised using a hydrothermal method. In a typical preparation 4,5g of commercial TiO 2 powder was dispersed in 80ml of 10M NaOH solution under vigorous stirring. The mixture was stirred for 1 hour and transferred to an autoclave with internal volume 150ml, followed by hydrothermal treatment at 150˚C for 48 hours.

After 48 hours the autoclave was quenched to room temperature and the product was recovered by centrifugation. The precipitation was washed 3 times with distilled water.

Afterwards the wet cake was dispersed in 240ml of 0,1M HCl solution and stirred for 30 min.

H-tubes were recovered by centrifugation and washed until the pH reached 1. The precipitate was washed 3 times with distilled water and 2 times with ethanol. The product was calcined at 350˚C for 6h to obtain mixed phase nanotubes.

Characterisation

X-ray diffraction

In order to investigate the crystallinity and phase purity of the samples, calcined and assynthesized TiO 2 -nanotubes crystals have been characterized, and compared to TiO 2 -P25 and TiO 2 -rutile (99.9% of purity), by X-ray diffraction (XRD) on a PANanalytical (X-Pert Pro) diffractometer with CuKα1 source (λ=0.15406 nm, 40 kV, 30 mA).

SEM and TEM

The morphological features and the chemical analyses of the samples were studied using Scanning Electron Microscope (SEM, Hitachi S3460) and Transmission Electron Microscope (TEM, Phillips CM 20) operating at 200kV.

Nitrogen sorption

Nitrogen sorption measurements of TiO 2 samples were carried out at -196°C on a Micromeritics ASAP 2020 micropore analyse. The samples were degassed at 150°C K for 24 h. Surface areas were calculated using BET equation.

UV-vis DR

UV-Vis DR measurements were performed on Thermo-electron evolution 500 UV-Vis spectrometer equipped with RSA-CU40 Diffuse Reflectance cell. From UV-Vis DR spectra information over the band gap energy can be obtained.

Raman

Raman spectra were recorded on a Nicolet Nexus FT-Raman spectrometer with an InGaAs detector and a 1064 nm Nd:YAG laser

Acidic properties

The acidic properties of the resulting materials were investigated by temperature desorption (TD) of pyridine followed by IR. Powders were pressed (~10 7 Pa) into self-supported discs (2 cm 2 area, 9-10 mg.cm -2 ) and placed in an IR cell equipped with KBr windows. IR spectra have been recorded using a Nicolet 6700 IR spectrometer equipped with a MCT detector and an extended-KBr beam splitter. Spectra were recorded in the 400-5500 cm -1 range at 4 cm -1 with 128 scans. A movable quartz sample holder allowed placing the self-supported discs in the infrared beam for recording spectra, and moving it into a furnace at the top of the cell for thermal treatment. 11 Pyridine (Py) adsorption was performed by introduction of doses inside the infrared cell containing the previously activated (under vacuum at 200°C for 5 h) selfsupported discs. After introduction of each dose of Py, the samples were heated at 200°C for 10 min to allow diffusion toward all accessible sites before recording the spectrum. Infrared spectra were recorded after Py saturation (1.33 mbar at equilibrium) followed by evacuation at 200°C to remove physisorbed species. In all experiments, analytical grade pyridine (Aldrich) was used after water trapping with molecular sieve 3A. The calculation of the IR band surface area has been performed using Omnic v8.2 software. The values were normalized for the same samples weight (20 mg).

Conditions for Photocatalytic operando test

The photocatalytic oxidation of methanol has been followed by IR spectroscopy. The powder of the desired catalyst was pressed into self-supported wafers (Ø = 16 mm, m~10 mg/cm 2 ). IR spectra were collected with a Nicolet 5700 FT-IR spectrometer (64 scans/spectrum) equipped with a MCT detector. The operando system is connected to a flow set-up. Gases are introduced into the lines (heated at 60°C) by mass flow controllers. The two gas mixtures, so called "activation" and "reaction" flows, can be prepared and sent independently to the reactor cell. The "Sandwich" reactor-cell used in this study is an evolution of the operando cell developed by Saussey et al. 12 , which has proven its reliability over many years of operando studies 12,13 . It is made of a stainless steel cylinder that carries a toroidal sample holder in its centre, where the catalyst self supporting wafer is placed.

Tightness is obtained by Kalrez© O-rings, and the dead volume (typically defined as the residual space between each sample face and the windows) is reduced to about 0.4 ml by filling the empty space with KBr windows placed on each side of the sample holder. The surface analysis is made possible without superposition of the gas phase signal and fluid dynamics is very similar to that inside a honeycomb monolith. Gases are introduced on the sample by 1/8" OD pipe and collected on the opposite side of the sample holder. More details can be found in the following references 14,15 . For this specific photocatalytic oxidation study, UV irradiation was carried out with a polychromatic light of a Xe-Hg lamp (LC8 spot light Hamamatsu, L10852, 200 W). It has been performed by using a UV-light guide (A10014-50-0110) mounted at the entrance of modified IR operando cell as presented in Scheme 1 in order to establish a "homogeneous" irradiation. UV irradiation intensity (I 0 ) has been measured using a light power meter (from Hamamatsu).

In such a configuration, at atmospheric pressure, and small amounts of VOC to be removed, a low partial pressure of VOC was established using a saturator at a carefully controlled temperature. The gas mixture composition was then fixed at 0.01 to 0.5 vol.% methanol, n-hexane or CO, 20 vol.% O 2 in Ar and the total flow was adjusted to 25 cm 3 /min. The analysis of the outlet gases was performed by means of a Pfeiffer Omnistar mass spectrometer. Likewise, FT-IR spectra of the gas phase were collected using a gas microcell. The photooxidation of methanol (MeOH) has been performed using TiO 2 -P25 and TNTs as photocatalyst under the same conditions (concentration, temperature, flow rate…). The concentration was fixed at 5040 ppm of MeOH in 20% O 2 in Ar at 25 cm 3 /min. The UV intensity (I 0 ) was estimated to ~2 mW/cm 2 (~20% of the lamp intensity).

Results and discussion

Characterization

The nanomorphology of calcined TiO 2 nanotubes (TNTs) is confirmed using scanning electron microscope (SEM) and transmission electron microscopy (TEM). The results are presented in Figure 1. The average length of the tubes was found around 90 nm.

< Figure 1 here> Figure 2 presents the X-ray diffraction patterns of TiO 2 -rutile, TiO 2 -P25, as synthesized and calcined TNTs. The results show that X-ray diffraction patterns of TNTs correspond to TiO 2 anatase phase and no significant modification has been observed after calcination. The broadening of X-ray peaks observed, in comparison with TiO 2 -P25 could be assigned to the nanocrystalline nature of the TNTs nanotubes. Moreover, after calcination at 350°C, no rutile phase has been observed. For reasons of comparison also the XRD pattern of rutile phase is reported in Figure 2.

< Figure 2 here>

Raman spectra are presented in Figure 3. Spectra of the calcined sample are compared to the spectra of the sample before calcination. The peaks at Raman shifts 188, 273, 369, 449 and 663 cm -1 reveal the presence of a trititanate structure for the non-calcined form. After the calcination process at 350˚C peaks at 399, 510 and 639 cm -1 are appearing which are characteristic for the anatase form of TiO 2 . This indicates that the sample undergoes incomplete transformation from H 2 Ti 3 O 7 to anatase TiO 2 during the calcination procedure.

< Figure 3 here>

The N 2 -sorption results show a surface area of TNTs (230 m 2 /g) 4 times higher with respect to TiO 2 -P25 (55 m 2 /g) (Figure 4). The hysteresis present at 0.8 to 0.9 in the case of TNTs assigned to the textural porosity of this nanotube.

< Figure 4 here>

The photocatalytic activity of a material is determined by the band gap energy, defect sites and surface area of the sample. Band gap energy can be calculated from UV-Vis DR spectra.

In Figure 5 16 The photocatalytic activity depends also on the surface area which is much higher for the TiO 2 -nanotube samples.

< Figure 5 here>

Acidic properties

The IR spectra (collected under dry air at room temperature) showed that the water content (calculated using the band at 1630 cm -1 ) is about three times higher in TNTs than in TiO 2 -P25. After activation under vacuum at 200°C, the IR spectra showed too separate bands at ~3720 cm -1 and ~3670 cm -1 (Figure 6) assigned to Ti(III)OH and Ti(IV)OH respectively 17,18 . The TiOH band area is twice higher in TNTs material which explains the higher hydrophilicity of this photocatalyst in comparison to TiO 2 -P25.

< Figure 6 here>

The IR spectra of the photocatalyst surfaces after pyridine adsorption, and evacuation at 200°C under vacuum (~10 -6 mbar), are presented in Figure 7. New bands appeared at 1610, 1575 and ~1445 cm -1 , assigned to pyridine molecules (Py) adsorbed on Lewis acid centers (Ti cation sites), and at 1490 cm -1 , assigned to Py on Brønsted (Py-B) and/or Lewis (Py-L) acidic sites 19 . The bands at 1610 cm -1 and 1445 cm -1 are assigned to pyridine molecules chemisorbed by the interaction between the nitrogen lone pair and the substrate surface 19,20,21 .

Table 1 summarizes the area of Py-B and Py-L bands. More Py was adsorbed on the TNTs than on TiO 2 -P25 surface (1.5 to 2.5 times), possibly because of the higher amount of TiOH on TNTs (Figure 6). Looking at the intensities for the OH bands, the Ti(III)OH (TNTs) / Ti(III)OH (TiO2-P25) ratio was about 1.3, close to the ratio of the area of the band at 1610 cm -1

(1.4) on the two solids. For Ti(IV)OH (TNTs) / Ti(IV)OH (TiO2-P25) (band at 3670 cm -1 ), a value of 2.6 has been found, close to that obtained for the area of the band at 1448 cm -1 (2.5) on the two samples (Table 1). This indicates that the bands at 1610 and 1448 cm -1 could be assigned to pyridine molecules adsorbed on Ti(III)OH sites and on Ti(IV)OH sites, respectively. A thermo desorption would be useful to confirm this hypothesis, but it is unfortunately not possible due to the degradation of pyridine on TiO 2 surface at T>225°C (coke formation on TiO 2 surface has been observed at high temperature). On the other hand the evolution of OH peak height, on TNTs sample, after pyridine adsorption at different temperatures has been followed (Figure 7B). The result shows an increase of the band at 3720 cm -1 assigned to pyridine desorption. A slow decrease of the band at 3670 cm -1 was observed, assigned to coke formation. This is in good agreement with a lower acidity for Ti(III)OH than for Ti(IV)OH sites.

< Figure 7 here>

Methanol photooxidation

Photooxidation reactivity and temperature effect

The IR spectra of TNTs during methanol adsorption are presented in Figure 8A. A decrease of the band at 3820-3600 cm -1 is observed (ν(TiO-H) vibration mode), simultaneously with an increase of a broad band at 3660-3000cm -1 , assigned to the vibration band of H-bonded TiOH (methanol adsorption on TiOH). Figure 8B shows the IR spectra of TiO 2 -P25 and TNTs samples monitored at RT after saturation with methanol (MeOH) before UV irradiation. Four main bands are observed between 3000 and 2700 cm -1 assigned to CH 3 (methanol) vibration modes. The bands at 2920 and 2820 cm -1 correspond respectively to the ν s (CH 3 ) and ν as (CH 3 ) vibration modes of dissociatively chemisorbed methanol (with rupture of the O-H bond) on TiO 2 surface 22,23,24 . The relatively high intensity of these two bands on TNTs compared to those on TiO 2 -P25 shows a high methanol chemisorption on TNTs surface (in agreement with the high amount of Ti(IV)OH sites on this material). The additional bands situated at 2945 and 2840 cm -1 are attributed to ν s (CH 3 ) and ν s (CH 3 ) vibration modes of methanol molecularly adsorbed on TiO 2 surfaces 22,23,24 .

< Figure 8 here> Figure 9 shows the IR spectra vs. time (and temperature) for the gas phase during the photooxidation of methanol on TNTs (A) and TiO 2 -P25 (B) as photocatalysts. It shows a spontaneous decrease of methanol bands after UV irradiation. New bands appearing during the photooxidation were assigned to CO 2 (2400-2200 cm -1 ), -C=O (1820-1670cm -1 ) and C-H vibration modes (1275-1100 cm -1 ) 25 . The IR-gas spectra during photooxidation (at RT) are presented in Figure 10. IR spectra of formaldehyde, methylformate and formic acid are also presented, and show that carbonyl species could be a mixture of the three: the CH band at 1275-1100 cm -1 assigned to r(CH) vibration mode is not present in the case of pure formaldehyde 26 . The quantification of carbonyl species is not possible due to the overlapping IR bands.

< Figure 9 Figure 11 illustrates the effect of temperature on methanol conversion and CO 2 selectivity in the case of TNTs (Figure 11-A) and TiO 2 -P25 (Figure 11-B). TNTs are 1.5 to 2 times higher compared to TiO 2 -P25, with higher CO 2 -selectivity (~1.5 times more, see Table 2). The increase of temperature led to an increase of methanol conversion with a maximum at ~180°C (Figure 11). Experiments have been performed at less than 200°C in order to prevent the transformation of the TiO 2 phase, especially in the case of TiO 2 -P25, as mentioned in the literature. A decrease in CO 2 selectivity (from 85% to 60%) on TNTs has been observed at T>35 °C, with a plateau at ~60 % for T>90°C. For TiO 2 -P25, no significant effect of the temperature on CO 2 selectivity was observed. The small time lag between CO 2 detection and methanol conversion (~1 min) is assigned to the ignition of methanol desorption from TiO 2 surface leading to a momentary increase of methanol in the gas phase. This is confirmed by the IR spectra of TiO 2 before and during methanol photooxidation when significant decrease of adsorbed methanol has been observed immediately upon switching UV-irradiation on.

< Figure 11 here>

In order to confirm these results, methanol conversion has been calculated using the MS signal for methanol before and during irradiation: In addition, the MS signal for H 2 O at m/z=18 and for CO 2 at m/z=44 (amplified 5 times) are presented in Figure 12. The results confirmed those obtained by gas-IR: methanol conversion and CO 2 production increased 1.5 to 2 times on TNTs in comparaison to TiO 2 -P25 (Figure 12). < Figure 12 here>

Concentration effect

In order to highlight the concentration effect on methanol photooxidation, 2000 ppm of methanol were sent under the same conditions on TNTs. The gas-IR and MS results are presented in Figure 13. Photooxidation activity was higher than with 5040 ppm of methanol.

The CO 2 selectivity is also higher and reached 100% for T>90°C, possibly because of an increase of the Oxidant-species/methanol ratio due to the decrease of methanol concentration. < Figure 13 here> Figure 14 presents the IR spectra of TNTs (and the gas phase of the reaction) before and during the photooxidation at RT and at 120°C. After UV irradiation new bands appeared at 2870, 1715, 1650, 1560 and 1370 cm -1 . The bands detected at 2870, 1560 and 1370 cm -1 are assigned respectively to ν s (CH), ν as (COO) and ν s (COO) vibrations of bidentate formate species adsorbed on the catalyst surface. Those observed at 1715 and 1650 cm -1 are assigned to ν(C=O) vibrations of formic acid (or methylformate) hydrogen bonded and co-ordinately bonded to Lewis acid sites, respectively 13,25,26,27 (the IR spectra vs. time and temperature during photooxidation of methanol at different temperature are presented in Figure I in the supplementary materials). The IR band at 1570 cm -1 reached a maximum at 120°C which means that the temperature favours the formation of bidentate formate, thus explaining the increase of methanol photooxidation. These results suggest that the bidentate formate ions are common intermediates and that the photooxidation mechanism could be presented as follows:

TiO 2 + hν h + + e - (h + + e -= electron-hole pair) (1)
In the presence of O 2

O 2 + e -O 2 •- (2)
Methanol and methoxy species adsorbed (CH 3 O) on the catalyst surface can react directly with the "h + " holes as already reported in the literature 28,29 :

CH 3 O -(CH 3 O (a) ) + h + CH 3 O • (3) CH 3 O • CH 2 O •-+ H + (4) CH 2 O •- H 2 CO +e - (5)
In the presence of O 2

•-formaldehyde could be oxidized into CO 2 and H 2 O (photocombustion):

H 2 CO + O 2 •- CO 2 + H 2 O + e - (6)
CH 3 O and CH 3 OH can be oxidized to form bidentate formate species HCOO:

H 3 O + O 2 •- HCOO + H 2 O + e - (7)
HCOO in presence of CH 3 O • leads to the formation of methyl formate:

Ti O O Ti H CH 3 O O O Ti O Ti + (8)
HCOO in presence of CH 3 OH leads to the formation of formic acid and methoxyl radical:

HCOO • + CH 3 OH CH 3 O • + HCOOH (9)
< Figure 14 here>

UV-intensity effect

Figure 15 presents methanol (5040 ppm) photooxidation at RT on TNTs, increasing with UVirradiation intensity (under the same condition as above). Methanol conversion was complete at intensity higher than 3.5 mW/cm 2 . The selectivity was also affected by intensity: complete photocombustion was obtained at higher than 5 mW/cm 2 . These results could be explained by the increase of the production of oxidant species when intensity increased. It is noteworthy that a high amount of carbonyl species was always detected in the first period of irradiation (whatever the intensity, Figure II, supplementary information). It could be assigned to the high concentration of methanol in the gas phase due to desorption phenomena mentioned above (decrease of [oxydants]/[methanol] ratio). Therefore, an increase of UV-irradiation intensity led to an increase of activity and CO 2 -selectivity of the photocatalyst.

< Figure 15 here> n-hexane photooxidation and temperature n-hexane photooxidation was performed under the same condition as for methanol and the concentration used was 5040 ppm. The UV intensity (I 0 ) used was ~2 mW/cm 2 . The influence of temperature on n-hexane conversion and on CO 2 production is presented in Figure 16.

Once more TNTs showed higher activity than TiO 2 -P25 for n-hexane photodegradation (~1.5 times, Table 2). A significant decrease of n-hexane conversion was observed with the increase of temperature (Figure 15). Only CO 2 and H 2 O were observed in the gas phase (Figure IIIsupplementary material). This selectivity confirms that carbonyl species observed in methanol photooxidation were produced from the bidentate formate species.

< Figure 16 here> increase in CO 2 production at temperatures between 45 and 75°C (Figure 16) is assigned to the incomplete photooxidation of species adsorbed on TiO 2 (n-hexane conversion decreased in this temperature range). The increase of the UV intensity from 2 to 3.5 mW/cm 2 led to an increased photooxidation. For I 0 >3.5 mW/cm 2 , no significant effect has been observed, showing that residual species on the photocatalyst were poisoning the surface rather than filtering UV light.

< Figure 17 here>

Carbon monoxide photooxidation

Intensity effect

The photooxidation of 1000 ppm of CO was performed at room temperature at various UV intensities under the same flow conditions as previously. Results are presented in Figure 18.

TNTs showed again higher activity than TiO 2 -P25 (1.5-2 times). These results were in good agreement with CO 2 production.

< Figure 18 here>

Concentration effect

The influence of CO concentration on photooxidation at ~11 mW/cm 2 is presented in Figure 19. CO conversion decreased with increasing CO concentrations. TNTs were again more active than TiO 2 -P25 (Table 2) and the difference was higher when the CO concentration was lower.

< Figure 19 here> < Table 2 here>

Conclusion

Operando-IR proved to be the ideal technique to study photocatalytic air purification.

Coupling this technique to on line analysis (gas-IR and MS) allowed real time monitoring of the species adsorbed on the surface of the photocatalyst and of those formed in the gas phase.

Using self supporting photocatalyst wafers and homogenous irradiation gives access to the activity, selectivity and mechanism of the process. Different parameters could be changed (temperature, VOC concentration…) and their influence on the photocatalysis reaction was evaluated. In order to demonstrate the reliability of this technique, two photocatalyst were studied in this work; TiO 2 -nanotubes (TNTs) as new photocatalyst and TiO 2 -P25 as reference.

The IR characterization of TNTs and TiO 2 -P25 allowed understanding the role of surface TiOH groups in the reaction. Our results indicate an influence of the amount of TiOH groups on the TiO 2 band gap. On the other hand, these sites (especially Ti(IV)OH sites) improve chemisorption of (hydrophilic) methanol and favors its photooxidation. The photooxidation of methanol, n-hexane and carbon dioxide using TNTs and TiO 2 -P25 was performed. TNTs was 1.5 to 2 times more active and selective than TiO 2 -P25 in methanol photooxidation, with improved results at higher temperature and UV-irradiation intensity or at lower methanol concentration. A negative influence of temperature was observed for n-hexane photooxidation, assigned to surface poisoning by unsaturated species (coke). TNTs material were also more active and selective than TiO 2 -P25 for photooxidation of CO into CO 2 , with a negative influence of CO concentration. As conclusion, TNTs material were generally more active and selective than the reference TiO 2 -P25 because of higher surface area.

Graphical abstract

Photocatalyst pellet

Reactivity, Selectivity Mechanism

≠ T ( C) ≠ I 0 ≠

Tables Table 1. Height of TiOH IR-bands and area bands of pyridine adsorbed on the TNTs and TiO 2 -P25 surfaces (per 10 mg/cm 2 of sample).

band heigth (a.u.) band area (cm -1 ) band (cm -1 ) 3720 cm -1 3690 cm -1 1610 cm -1 1575 cm -1 1491 cm -1 1448 cm -1 TNTs 0,13 0,164 3 0,22 0,44 2,8 TiO2 0,10 0,062 2,1 0,14 0,28 [CO] (ppm)

Scheme 1 .

 1 Scheme 1. The "sandwich" reactor-IR

  spectra of TiO 2 -P25 (a), TiO 2 -nanotube after calcination (b) and TiO 2 -nanotube before calcination (c) are compared. The band gap energies for the samples are respectively 3.07, 3.21 and 3.27eV. TiO 2 -P25 sample has lower band gap energy than the prepared nanotube samples. According to the literature, this slight difference could be assigned to the TiOH content and Ti(III)/Ti(IV) ratio present in the different samples which could affect the TiO 2 band gap values.

  here> < Figure 10 here>The quantification of methanol has been performed using the area of the 1090-950 cm -1 band assigned to the ν(C-O) vibration mode of methanol. Methanol conversion has been estimated with the following formula: the band area of MeOH (in the gas phase) before photooxidation A t(T) = the band area of MeOH during photooxidation at time "t" (or at Temperature "T")The selectivity of MeOH transformation to CO 2 has been calculated using the band area of CO 2 measured during a complete and total photocombustion of 5040 ppm of MeOH under special irradiation conditions (I 0 =11 mW/cm 2 ): photocombustion is then 100 % selective (CO 2 production) and the observed band area corresponds to ~5000 ppm of CO 2 . This result has been used for estimating CO 2 concentration during the photooxidation of 5040 ppm MeOH under different conditions. The CO 2 -selectivity has been calculated as follows: methanol converted in mole.

  the MS intensity of m/z=31 signal detected by MS before photooxidation I t(T) = the MS intensity of m/z=31 signal detected by MS during photooxidation at time t (or temperature T).

Figure 17 presents

 17 Figure 17 presents the evolution of the IR spectra of the photocatalyst surfaces before and

New operando-IR techniques to study the photocatalytic activity and selectivity of TiO 2 -

 2 nanotubes in air purification: influence of temperature, UV intensity and VOC concentration. Mohamad El Roz, Monika Kus, Pegie Cool, Frederic Thibault-Starzyk A new operando-IR technique has been used to investigate the photocatalytic activity and selectivity of TiO 2 -nanotubes. Real time IR-monitoring of the photocatalyst surface (identification of surface species) and of the gas phase of the reaction (products identification) gives access to the reaction mechanism and photocatalyst activity and selectivity. Coupling this technique with mass spectrometry gave complementary information on the gas phase during the photocatalytic reaction. The possibility to change and control several parameters (VOCs concentration, UV-intensity and temperature…) led to understand their influence on the reaction, showing the interest of operando-IR technique in photocatalysis studies. TiO 2nanotubes were always more active and selective than the reference TiO 2 -P25.

Figure 1 .

 1 Figure 1. SEM pictures of TiO 2 -P25 (a) and TNTs powders. "(c)" correspond to the TEM picture of TNTs powders.

Figure 2 .

 2 Figure 2. XRD results of TiO 2 -rutile (a) TiO 2 -P25 (b) TiO 2 -nanotube after calcination (c) and TiO 2

Figure 3 .

 3 Figure 3. Raman results of TiO 2 -nanotube after calcination (a) and TiO 2 -nanotube before calcination (b).

Figure 4 .

 4 Figure 4. N 2 -sorption isotherms of TNTs and TiO 2 -P25 samples.

Figure 5 .

 5 Figure 5. UV-Vis DR spectra of TiO 2 -P25 (a) TiO 2 -nanotube after calcination (b) and TiO 2 nanotube

Figure 6 .

 6 Figure 6. IR spectra of TiO 2 -P25 (a) and TNTs (b) under air at RT (1) and after activation under vacuum at 200°C (2) (spectra collected after cool down to RT).

Figure 7 .

 7 Figure 7. (A) Subtraction results of IR spectra of TiO 2 -P25 (a) and TNTs (b) after pyridine adsorption and evacuation under vacuum at 200°C (subtraction from the IR spectra of the photocatalysts after

Figure 8 .

 8 Figure 8. (A) Evolution of TNTs IR spectra during the methanol adsorption. (B) IR spectra of TNTs

Figure 9 .

 9 Figure 9. The IR spectra vs. time (and temperature) of the gas phase during the photooxidation of

Figure 10 .

 10 Figure 10. IR-gas spectra of the methanol photooxidation reaction before (a) and during (b)

Figure 11 .

 11 Figure 11. Evolution of the methanol conversion vs. temperature (a) and CO 2 selectivity (b), determined using IR spectra of the gas phase, during the methanol photooxidation using TNTs (A) and TiO2-P25 (B) as photocatalysts. ([MeOH]=5040ppm, I 0 ~2mW/cm2, Flow=25 cm 3 /min).

Figure 12 .

 12 Figure 12. Evolution of the methanol conversion (calculated using the m/z=31 MS signal) and MS

Figure 13 .

 13 Figure 13. IR-gas (A) and MS (B) results obtained during the methanol photooxidation at different

Figure 14 .

 14 Figure 14. IR spectra of the TNTs (A) and the gas phase (B), before (a) and during irradiation at RT

Figure 15 .

 15 Figure 15. (A) Effect of the UV lamp intensities on the methanol conversion (square) and on CO 2 (circle) and carbonyl species (diamond) production determined using IR spectra of the gas phase. (B)

Figure 16 .

 16 Figure 16. Evolution of the n-hexane conversion vs. temperature (determined using the IR-gas

Figure 17 .

 17 Figure 17. (A) Evolution of the IR spectra of TNTs (I) and TiO 2 -P25 (II) vs temperature during the photooxidation of n-hexane. (B) Subtraction results of the IR spectra of TNTs (I) and TiO 2 -P25 (II) during the photooxidation of n-hexane at RT (a) and 200°C (b), from the IR spectrum before UV irradiation. (I 0 ~ 2mW/cm 2 ; [n-hexane] = 5040 ppm; 20%O 2 /Ar, flux=25 cm 3 /min).

Figure 18 .

 18 Figure 18. (A) Evolution of the CO photooxidation (close symbol) and CO 2 production (open symbol) vs lamp intensities using TiO 2 -P25 (square) and TNTs (circle) as photocatalysts.Figure (B) corresponds to the CO 2 evolution in real time at different intensities.

Figure 19 .

 19 Figure 19. Evolution of the CO photooxidation vs. the CO concentration using TiO 2 -P25 (square) and TNTs (circle) as photocatalysts. I 0 = 2 mW/cm 2 .

Table 2 .

 2 Reactivity and selectivity of methanol, n-hexane and carbon dioxide photooxidation reactions using TNTs and TiO 2 -P25 as photocatalysts. (Conv.= conversion; Select. = selectivity.)

				Photooxidation of
	I 0 ~2 mW/cm 2
		Methanol		n-hexane	CO
	T (°C)
		Conv.	CO 2 -select.	Conv.	CO 2 -select.	Conv.
	photocatalyst
		(%)	(%)	(%)	(%)	(%)
	30	30%	85%	23%	100%	25%
	TNTs
	200	75%	60%	5%	100%	-
	30	14%	47%	12%	100%	10%
	TiO 2 -P25
	200	56%	44%	2%	100%	-

Acknowledgement

The authors wish to thank Prof. S. Bals and Ms. J. Pauwels from EMAT, physics department of the University of Antwerp, Belgium, for recording the TEM picture of TNT.


W Zeidan

N Mazellier
email: nicolas.mazellier@univ-orleans.fr

E Guilmineau

A Kourta

Turbulent wake characteristics of a simplified road vehicle: simulation vs experiment

Keywords: Aerodynamics, road vehicles, numerical simulation, experiments

A Simplified road vehicle (Ahmed body) with a square-backed shape was placed in a freestream flow velocity 40m/s corresponding to a high Reynolds number Re= 7.6 × 10 5 and experienced in a wind tunnel, in the same time in parallel a simulation was done in the same real conditions with different turbulence models. The main goal from this study was to examine the effectiveness of the DDES and k -ω SST turbulence models in presenting such complex flows. To certify the accuracy of the turbulent models many physical parameters were compared to experimental data, from boundary layer parameters to the wake characteristics, shear layer and mass entertainment. A great agreement were found between experiment and DDES model. In other hand a simulation was done to certify the effects of ground clearance and 5% of drag reduction was obtained by changing it from 50mm to 45mm.

Introduction

The aerodynamics of road vehicles becomes nowadays a common interest for environmental and economical issues. Industries are directed towards developing strategies in order to improve aerodynamic performance [START_REF]Aerodynamics of Road Vehicles[END_REF]. Better understanding of the flow around a road vehicle could be the main goal for better designs. This flow has a great effect on parameters such as drag, lift and fuel consumption [START_REF] Volpe | Experimental characterization of the unsteady natural wake of the full-scale square back ahmed body: flow bi-stability and spectral analysis[END_REF]. [START_REF] Ahmed | Some salient features of the time-averaged ground vehicle wake[END_REF] introduced a well representative model for road vehicles called Ahmed body which attracted several designers to initiate their researches. In fact, numerical simulation is essential in analyzing bluff body flow mechanisms, where experiments may face difficulties. An example is the sensitivity of the near wake region to any probe that could perturb the flow. [START_REF] Gaylard | Simulation of rear surface contamination for a simple bluff body[END_REF] and by using very large eddy simulation (VLES) have shown that numerical simulations is a well tool to estimate the aerodynamics of simple square-backed bluff body called Windsor body, see [START_REF] Volpe | Forces and flow structures evolution on a car body in a sudden crosswind[END_REF], [START_REF] Perry | Influence of short rear end tapers on the wake of a simplified square-back vehicle: wake topology and rear drag[END_REF].

Less computational calculation in the literature that shows the flow around squared-backed Ahmed body at high Reynolds number, [START_REF] Lucas | A numerical investigation of the asymmetric wake mode of a squareback ahmed body effect of a base cavity[END_REF] have done simulations for Ahmed body under similar condition (high Reynolds number) and analyzed the mean topology of the wake. This study will shed lights on the comparison between experimental and simulation analysis for the flow around Ahmed body, pointing that simulation done using real conditions similar to experimental ones. Deep discussion has been done to qualify the numerical simulation and turbulence models. In this abstract we focus on discussions on the mean flow properties. Additional results will be presented during the conference.

Experimental Setup

The experiment was held at PRISME laboratory in Orleans University (France) in wind tunnel of test section 2m by 2m wide, and 5m long. Full scale Ahmed body used in [START_REF] Ahmed | Some salient features of the time-averaged ground vehicle wake[END_REF], with length L=1044mm, width W=389mm, height H=288mm, ground clearance h=45mm (15% of the model height). The model was mounted via a plate located 0.47m from ground within a rotation ability in order to be aligned. The x-axis is aligned with 1 the streamwise direction, z-axis is vertical and positive upwards, and the y-axis followed the right handed coordinate system, the origin is at the bottom of the base of the model. An external strain gauge balance was used to measure the aerodynamic forces, located under the working section of the tunnel see figure 1a. The chosen sampling frequency is 1000Hz.

The pressure at the rear base was measured by covering the back-square face with an array of 81 pressure taps connected by tubing system to pressure scanner (length 600mm with internal diameter d=1.56mm) and with a manufacturer quoted accuracy of ±6P a with a sampling frequency 50Hz. In addition to that, 2D-2C PIV measurements were performed to analyse the topology of the flow downstream the body for x-z plane (Y=0), the size is 392×603mm 2 with spatial resolution around 2mm, the measurements obtained at 2.49HZ. The data were processed using DPIV software see [START_REF] Passaggia | Transverse instability and low-frequency flapping in incompressible separated boundary layer flows: an experimental study[END_REF] and [START_REF] Meunier | Analysis and treatment of errors due to high velocity gradients in particle image velocimetry[END_REF]. Numerical setup: Same simplified vehicle model was used with computational domain dimensions 13.2L × 1.7L × 1.9L see figure 1b, setting 40m/s at the inlet, which leads to Re H = 7.6 × 10 5 based on height and free-stream velocity, no slip condition was chosen for the body and the road in order to be enable the boundary layer's growth. Two full unstructured hexahedral meshes done using HEXPRESS, the first with 32 million cell and second with 56 million cell, 2% of drag reduction confirmed us the convergence of our mesh. In order to have better capturing near the wall a special refinement done by defining three boxes covering the whole body and zooming on the rear part in which cell size reached 1.7mm. Note that the computational time is around t =278, where t = tU ∞ /H, t time in seconds, 480 processor were used in the simulation. Two turbulence models were used, k -ω SST (SST, shear stress transport) RANS model, in addition to the Delayed Detached Eddy Simulation (DDES), which is a hybrid RANS-LES model that used the k -ω SST at the boundary layer for a less consumption and switch into LES in the rest of the region. Further information can be found in [START_REF] Gritskevich | Development of ddes and iddes formulations for the k-w shear stress transport model flow turbulence and combustion[END_REF] and [START_REF] Flynn | Detached-eddy simulation of the slipstream of an operational freight train[END_REF].

Results and discussions

Global values: Table 1 shows the mean values for the pressure coefficient (using ??) for the three studied cases. 4% and 11% is the difference that observed between the reference case (experiment) with DDES and k -ω SST models respectively. Same for the drag coefficient at the base estimated using equation ??, while the total drag coefficient difference is 1.2% for DDES, while 7% for k -ω SST . Its interesting to have comparable results between both models and experiment, but in the same time looking into the physics near the simplified vehicle will give more significance for the comparison, especially the wake which is important in the square-backed bodies, the recirculation region length l r was estimated and shown in figure 1, 2% and 22% difference for DDES and k -ω SST models respectively. DDES model is more comparable than k -ω SST in terms of characteristic lengths for the wake. Moreover, these results gave a sign to look at the boundary layer profile in order to check effects of boundary layer on these differences, the boundary layer was discussed at the rear top of the simplified vehicle and will be presented at the conference. Wake topology: To go further in the comparison, wake is analyzed in the following section. For that, the time averaged flow downstream the simplified vehicle is shown in figure 2. Separation of flow localized at the rear of the body causing a large recirculation region, experimental results are well compared to DDES in terms recirculation length as discussed before and in also the shape of the mean wake is similar, the mean separation line is well estimated in the DDES model as well, while SST kω is not well presenting the wake in comparison with experiment, the vertical plots of figure 3, confirmed this comparison. The vertical wake asymmetry appearing in figure 2 came from the presence of the ground perturbing the flow, which leads to flow features that is different from the upper and lower part of the wake. An agreement was obtained in comparing drag coefficients and characteristic lengths of the wake between the experimental data and DDES. Moreover, statistical and dynamical analysis will be shown in the presentation at the conference.

Figure 1 :

 1 Figure 1: Experimental setup (a) Simulation mesh and domain (b) DDES

Figure 2 :Figure 3 :

 23 Figure 2: Mean velocity field U normalized by free stream velocity along Z-plane for (a) PIV experiment, (b) DDES , (c) SSTkω

Table 1 :

 1 Pressure coefficient(C pb ) and pressure drag (C db ) at the base, and total drag (C d ), l r : recirculation region length.

		experiment ddes kω -sst
	C pb	-0.2735	4%	11%
	C db	0.273	4%	11%
	C d	0.333	1%	7%
	l r	1.5709	2%	22%


Matthias Ide

Mohamad El-Roz

Els De Canck

Aurélie Vicente

Tom Planckaert

Thomas Bogaerts

Isabel Van Driessche

Frederic Lynen

Veronique Van Speybroeck

Frédéric Thybault-Starzyk

Frédéric Lynen

Frédéric Thybault- Starzyk

Pascal Van Der Voort
email: pascal.vandervoort@ugent.be

Quantification of silanol sites for the most common mesoporous ordered silicas and organosilicas: total versus accessible silanols

Keywords: Silanol number, ordered mesoporous materials, grafting, spray dried

Introduction

The silanol number is a topic of interest and discussion since the 1950s until today. A good knowledge of the number of silanols on the silica surface is important in areas such as catalysis and adsorption. Also in emerging disciplines, such as drug delivery, tissue engineering by silica and organosilica scaffolds, thin film technology, etc. , the exact knowledge of the silanol number is needed or at least desirable. More specifically, in the field of chromatography, the silanol groups are of major importance for two reasons. In the first place, they are the anchoring point for the stationary phase functional groups. In this way, they have a direct influence on the retention mechanism of a chromatographic packing material. Secondly, once the functional groups are grafted on the silica surface, it is important that the residual silanol groups are removed from the system by endcapping with e.g. a trimethylsilylgroup. This, because residual silanol groups can, for example, lead to asymmetric or tailing signals in the chromatography of basic compounds. On the other hand, in a technique such as capillary electrochromatography (CEC) the remaining silanol groups help to create the electro osmotic flow, which drives the separation. [1] The determination of the silanol number is complicated by the easy adsorption of water on silica. Therefore the exact quantification with MAS NMR, FTIR, selective chemisorption, deuterium exchange and mass spectroscopy is always complicated by the need for a very controlled atmosphere during sample treatment and measurement. Many models appeared in the 1970s and 80s on the determination of the silanol number of silica gels or fumed silicas. [2][3][4] For silica gels, many different datasets on the silanol number are available, with a large number originating from the former USSR. The silanol number is often referred to as the Zhuravlev number, in honor of his pioneering work in the field. [START_REF] Zhuravlev | Colloids and Surfaces A-Physicochemical and Engeneering Aspects[END_REF][START_REF] Zhuravlev | [END_REF][START_REF] Zhuravlev | Colloids and Surfaces A-Physicochemical and Engeneering Aspects[END_REF][START_REF] Nawrocki | [END_REF] In 1995, Van Der Voort et al. published an overview of the silanol number of silica gels as a function of temperature with a specification of the number of isolated, geminal and vicinal silanols. [START_REF] Vansant | Characterization and Chemical Modification of the Silica Surface[END_REF] However for the mesoporous ordered (organo)silica Page 3 of 39

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Several standard methods have been used to characterize and quantify the density of silanol groups of both amorphous and mesostructured silica materials. Thermogravimetric analyses [10], diffuse reflectance fourier transform infrared spectroscopy (DRIFT) [11][12], 29 Si and 1 H NMR [13][14], microcalometry [15] or a combination of these techniques were combined with the use of probing molecules such as silanes [16][17][START_REF] Knözigner | The Hydrogen Bond[END_REF][START_REF] Kruk | [END_REF], water [20][21][22] or pyridine [23] adsorption. The silanol concentrations that have been reported on MCM-41 are hardly comparable, due the fact that each paper reports a different silanol number for MCM-41 without considering synthesis and analysis parameters. [24][25][26][27][28][29] For example the silanol number is strongly dependent on the temperature of any post synthesis treatment. [START_REF] Vansant | Characterization and Chemical Modification of the Silica Surface[END_REF] However in six different reports where MCM-41 was post treated at the same temperature, silanol values varying from 0.9 up to 3 groups per nm 2 were found. [10,23,[30][31][32][33] After a thorough search of literature no systematic determination of the silanol number of periodic mesoporous organosilicas could be found. Also a systematic comparison of the most common mesoporous ordered silicas is not available in literature.

Therefore in this contribution, we report the accessible surface silanols for various chromatographic relevant functional groups together with the total amount of silanol groups, whether they are on the surface or not. We believe this information will be useful for many colleagues in various fields of research and for the chromatographic colleagues in particular. The silanol numbers have been quantified using previously described IR techniques [34] combined with solid state NMR and elemental analysis of grafting procedures. Nucleosil 300-5 (mean pore diameter: 30 nm; 5μm particle size) was purchased from Macherey-Nagel. Hexadecyltrimethylammonium bromide (99%) (CTAB), tetraethyl orthosilicate (98% reagent grade) (TEOS), ammonium hydroxide, potassium chloride (>99%) acetonitrile (HPLC grade), 1-butanol (99.8%), pluronic PEO20PPO70PEO20 (P123), pluronic PEO106PPO70PEO106 (F127), Vinyltriethoxysilane (VTES) and Grubbs' 1 st generation were acquired at Sigma-Aldrich.

Fuming hydrochloric acid 37 w/w% was purchased from Roth, anhydrous toluene (> 99,9%) for the grafting was obtained from Acros. n-Octadecyldimethylchlorosilane(97%), n-Octyldimethylchlorosilane and Bis(trimethylsilyl) amine were supplied by ABCR. All chemicals were used without further purification.

Synthesis of the supports

SBA-15 [35], SBA-16 [36], MCM-41 [37], MCM-48 [38], spray dried MCM-41 [39] and ethene bridged periodic mesoporous organosilica (PMO) [40] were all prepared according to methods found in literature. All syntheses were carried out under hydrothermal conditions, except for the spray dried MCM-41 type material, denoted as MCM-41 (SD). Two types of pore systems were synthesized, a hexagonal and a cubical pore system. MCM-41, MCM-41(SD), SBA-15 and the ethene bridged PMO have a hexagonal pore system. The MCM-48 and the SBA-16 both exhibit a cubical pore system. All the synthesis procedures can be found in the supplementary information.

Nucleosil 300-5 is commonly used as a packing material in chromatographic HPLC applications. MCM-41, MCM-41(SD), MCM-48, SBA-15 and SBA-16 were calcined at 550 °C for 6 hours with a heating rate of 1.5°C/min to remove the surfactant. The surfactant of the ethene bridged PMO was removed by a soxhlet extraction with acetone at 120 °C. [41] After the synthesis all the samples were kept dry and under inert atmosphere. Prior to grafting water was removed from the samples by a heating procedure at 120°C under vacuum conditions for 24 hours. Three different silanes with increasing tail length were used as probing molecules to determine the amount of accessible silanol sites on the surface of the different silica materials. A trimethylsilyl (C3), an octyldimethylsilyl (C8) and an octadecyldimethylsilyl (C18) group were anchored to the surface. The reaction time was 24h for all the grafting reactions and the composition of the final mixture was 0.5 g silica/1.4 mmol silane/30 ml solvent. The trimethylsilyl and the octyldimethylsylil group were grafted by means of hexamethyldisilazane and octyldimethylchlorosilane, respectively, in dry acetonitrile and left to stir for 24 hours at room temperature.

Grafting procedures

The octadecyldimethylsilyl group was grafted by means of octadecyldimethylchlorosilane in dry toluene under the same conditions as cited above.

After reaction the materials were filtered off, washed thoroughly with acetonitrile or toluene and acetone, dried at 120°C under vacuum and analyzed with elemental analysis.

Characterization

Nitrogen adsorption experiments were measured at -196 °C using a Belsorp mini II gas analyzer.

X-ray powder diffraction (XRPD) patterns were collected on an ARL X'TRA X-ray diffractometer with Cu Kα radiation of 0.15418 nm wavelength and a solid-state detector. Elemental analyses (C, H, N) were performed on a Thermo Scientific Flash 2000 CHNS-O Analyzer.

For the determination of the silanol number, a special transmission FTIR setup was used.

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The silica powders were pressed (~10 7 Pa ) into self-supported discs (2 cm 2 area, 7-8 mg/cm 2 ). They were placed in a cell equipped with KBr windows. A movable quartz sample holder allowed placing the pellet in the infrared beam, for recording spectra, and moving it into a furnace at the top of the cell for thermal treatment (see figure 1).

A Nicolet 6700 IR spectrometer equipped with a DTGS detector and an extended-KBr beam splitter was used for the acquisition of spectra recorded in the 400-5500 cm -1 range. Spectra were recorded at 4 cm -1 and 250 scans were co-added for each spectrum.

Solid-state MAS NMR spectra were recorded on a Bruker Avance-400 (9.4T) spectrometer using 4 mm-OD zirconia rotors and a spinning frequency of 12 kHz. Single pulse excitation (30° flip angle) and 20 s recycling delay was used for 29 Si MAS NMR experiments. { 1 H}- 29 Si cross-polarization (CP) MAS experiments were performed using a contact time of 6 ms and a recycle time of 3 s. Tetramethylsilane (TMS) was used as chemical shift reference 29 Si nuclei.

Results and discussion

In order to simplify the discussion of the effect of the synthesis method on the material properties, the main differences between synthesis conditions and template removal are summarized in Table 1. understanding the effect of hydrothermal and spray dried synthesis on the material behavior. For comparison, Nucleosil was purchased from Macherey-Nagel in order to be used as reference.

The pore properties of the materials

Three types of pore structures were synthesized, a hexagonal P6/mm structure (MCM-41-HT, MCM-41-SD, SBA-15 and ethene bridged PMO) and two cubical structures (MCM-48 (Ia3d) and SBA-16 (Im3m)). Additionally there are substantial differences between MCM type materials and SBA type materials in general. MCM type materials exhibit pore sizes between 2 and 4 nm with a wall thickness of about 1 nm. No significant difference in the pore properties can be discerned between the hydrothermally synthesized and the spray dried MCM-41 material. This is small compared to the SBA and PMO materials where pore sizes vary between 6 and 10 nm with a wall thickness between 3 and 7 nm. This difference in pore size properties can be assigned to the use of a different surfactant as pore generating agent. An additional property of the materials

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The various pore properties are shown in table 2, the surface area (SA(BET)), pore volume (Vp)

and pore diameter (dp) were determined with N2-sorption. The unit cell (a0) was determined from the XRD spectra of the materials and the wall thickness (h) was calculated with the following formulas:

MCM-41, SBA-15 and ethene bridged PMO [42]: ;

MCM-48 [43]: ; SBA-16 [44]: ;

Note that for Nucleosil no pore diameter could be calculated due to its broad pore size distribution. This material is also unordered, and does not produce any XRD reflections. According to the literature [34], adsorbed water species are characterised by a δH2O band near 1630 cm -1 (Figure 2 -a). The spectrum of the dried sample (Figure 2 -b) presents a narrow band at 3740 cm -1 characterizing isolated silanol groups. The band of inner hydroxyl group is situated at about 3650 cm -1 . The band of H-bonded hydroxyl groups is situated at about 3540 cm -1 . From the area of the (ν+δ) OH band at 4560 cm -1 (inset of figure 2), we have estimated the total concentration of Si-OH groups (both on the surface and in the wall) (Table 3, column 1), using the methodology described in reference [34]. In this way, there is no bias from a possible contribution of physisorbed water.

3.3

The various OH species according to MAS 29 Si NMR

The OH species were quantified using MAS 29 Si NMR and the NMR band was located using CP MAS 3 A and B, respectively. Figure 4 depicts the NMR spectra for the silica materials with the Q 2 , Q 3 and Q 4 sites, as well as schematic explanation of these three species. It is interesting to see differences in intensity and width in the three Q n peaks in the direct spectra (figure 3A) as well as in the cross polarized spectra (figure 3B). The amount of SiO2(OH)2 (geminal silanol), SiO3OH (single silanol) and SiO4 species, respectively, are then estimated for all samples by using deconvolution, as presented in figure 3, and comparison with peak intensity in CPMAS. From the integration of the three different peaks obtained in each sample it has been possible to estimate the relative amount of each species. The quantification results are then presented in table 3.

Table 3:

The silanol number and the Q 2 , Q 3 and Q 4 species in the material.

Column 1 shows the silanol concentration acquired with infrared spectroscopy. This is the total silanol content, so both the surface silanols as the ones in the wall. A general trend visible in these data is that thick walled materials (SBA type materials) exhibit a higher number of total silanol groups. Columns 2 and 3 depict the Q 3 :Q 4 and Q 2 :Q 4 ratios of the various materials. The thinwalled MCM materials exhibit a much higher percentage of surface silicon atoms, due to thinner walls. A rough estimate of the percentage of surface silicon atoms is shown in the supplementary information (table S1). They therefore also have a much higher percentage of Q 3 -sites that the thick-walled SBA-type materials. Since silanol groups are preferentially at the surface it can be understood that MCM materials exhibit a slightly higher Q 3 :Q 4 ratio than SBA materials. Columns 4 and 5 depict the separate percentages of SiO2(OH)2 and SiO3OH species respectively indicated as geminal and single silanols. The total percentage of hydroxylated silicon atoms (SiO2(OH)2 + SiO3OH) is shown in column 6. The last column [START_REF] Zhuravlev | Colloids and Surfaces A-Physicochemical and Engeneering Aspects[END_REF] is the complement of the total percentage of hydroxylated silicon atoms and is designated as the condensation degree. The condensation degree is larger for hydrothermally synthesized SBA materials in comparison with hydrothermal MCM materials.Because the wall thickness of SBA materials is higher than MCM materials this indicates that the total amount of hydroxylated silicon atoms lowers with the wall thickness.

Q 3 /Q 4 Q 2 /Q 4 Q 3 / (Q 2 +Q 3 ) Q 2 / (Q 2 +Q 3 ) (Q 2 +Q 3 )/ (Q 2 +Q 3 +Q 4 ) Q 4 / (Q 2 +Q 3 +Q
Therefore it can be concluded that the main part of the silanol groups is situated at the pore wall/air interface.

Remarkably the spray dried MCM-41 has a higher condensation degree than the other MCMtype materials. This higher degree of condensation cannot be correlated to the wall thickness, because this is comparable with the other MCM type materials. However, this can be explained by the synthesis procedure. During the spray drying process, sol droplets are atomized and evaporated at 220 °C. This is a much higher condensation temperature than for the hydrothermal synthesis of MCM and SBA type materials, that typically does not exceed 110°C. Additionally the ageing step usually applied to promote further condensation and reordering of the structure after precipitation is not present in spray dried samples. This high temperature of spray drying combined with the rapid and constant evaporation of the solvent and the ethanol produced by the Page 13 of 39

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The OH-species for ethene PMOs

In comparison with the chemical shifts of the silicon atoms of pure silica materials in 29 Si MAS NMR, a significant and distinct shift occurs for organosilica materials. Typically, the signals for the silicon atoms with attached ethene bridges are located between -60 and -90 ppm. [45] The spectrum and a schematic overview of the different T sites are depicted in figure 5. No Q-sites were measured which indicates that no ethene bridges were broken during the synthesis of the silica material. When the total amount of OH species (columns 1 and 6) is compared to these on pure silica materials (table 3: columns 1 and 6), there are more hydroxyl groups present in the organosilica material. This is reflected in a much lower (below 50%) degree of condensation (column 7). Two different mechanisms lead to these differences. First of all the polycondensation reaction of the ethene bridged precursor molecule does not progress to completion because there are six siloxane bridges to be formed per precursor molecule instead of the four of TEOS. (See the T 3 sites in Figure 5). This is sterically more difficult than the polycondensation reaction with TEOS, used in the synthesis of MCM and SBA type materials. The second and probably the largest influence on the degree of condensation, is the temperature of the post treatment. While pure silica materials are calcined at 550°C, organosilicas are obviously not. The surfactant that acts as the porogen inside the Periodic Mesoporous Organosilicas is typically extracted by Soxhlet extraction with acetone or ethanol. [41] The materials are never exposed to temperatures above 120°C. When

PMOs are treated at temperatures above 200°C, the residual silanols will be further reduced by interaction with the ethene bridge, via an autohydrophobisation reaction, as described by Ozin [46] and Vercaemst [47]. These reactions can be shown as follows:

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T 2 /T 3 T 1 /T 3 T 2 / (T 1 +T 2 ) T 1 / (T 1 +T 2 ) (T 1 +T 2 )/ (T 1 +T 2 +T 3 ) T 3 / (T 1 +T 2 +T 3 )
EthenePMO 16.9 1.16 0.06 95 5 0.55 0.45

Determination of the maximum loading of the probing molecules

The maximum loading of the materials was determined via the grafting procedure of three silanes that differ in carbon chain length. Through the accessible silanol groups, the probing molecules were anchored to the surface. The amount of probing molecules was quantified on the carbon amount determined with elemental analysis. A first interesting observation can be made when SBA-15 is compared with SBA-16 and MCM-41

with MCM-48. The cubical pore systems (SBA-16 and MCM-48) systematically have a larger amount of C18 groups anchored to the surface. This is due to the fact that a cubical pore system is accessible from three dimensions while a hexagonal pore system just from one dimension.

Therefore it is clear that the inside of an SBA-16 and an MCM-48 particle can be reached more easily and thus faster than the inside of an SBA-15 and MCM-41 particle. This easy access to the whole of the pore system from multiple dimensions is the reason the carbon loading on cubic pore systems is higher than the loading on hexagonal pore systems. The difference between the hexagonal and the cubical system is higher for the SBA materials than for the MCM materials. A possible explanation for this might be that in the case of SBA-16 some trapping of the C18 chains into the pores occurs as SBA-16 has an Im3m cage like pore system. This would not happen in the MCM-48 system because it does not have a central cage with smaller pore windows, but can be described as a single sheet that winds though space following a gyroid surface. [49] This theory is confirmed by Nucleosil where the amount of grafted groups per nm² is much higher in comparison with the other materials. The Nucleosil material consists of perfect spheres with large disordered pores (~ 30 nm). This much higher loading for Nucleosil confirms that the grafting procedure for porous systems is strongly diffusion regulated. The influence of diffusion is Page 17 of 39

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However when the molar amount of groups per weight unit (mmol/g) is considered it is immediately clear that the ordered mesoporous materials exhibit a much higher loading than the commercial Nucleosil material. A second observation is the higher surface silanol loading of the SBA-type materials, compared to the MCM-type materials, regardless of the mesoscopic structure. This is particulary evident for the silanols that are accessible to the smallest silanes (C3, HMDS). This fact finds its origin in the synthesis methods. The SBA-type materials are synthesized by Pluronic surfactants, which are ethyleneoxide/propyleneoxide triblock copolymers. The ethenylene oxide side chains create microporous perforations in the SBA-walls. The SBA-type materials (and the PMO material that was also synthesized using the Pluronic 123 surfactanct) therefore have an important fraction of micropores. [50] Table 5 shows an overview of the micropore volume and and micropore surface area of these materials. It is clear that these micropores contribute very significantly to the total pore area and lie at the origin of the higher concentration of surface Page 18 of 39

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Computational considerations

To get a better view on the behavior of silane groups on the silica surface, theoretical calculations were performed to determine the geometry of a carbon chain substituent. All results

were obtained with the gaussian09 software package [START_REF] Frisch | Revision A[END_REF] using a B3LYP functional [START_REF] Becke | [END_REF]53] with a 3-21G Pople basis set. Since the primary focus was set on determining geometries this level of theory was chosen.

First, a random silica slab was made with approximately 5.3 hydroxyl groups per nm 2 . This model is designed to represent an amorphous silica surface, thus a random cluster was built with the required silanol-density. To calculate the density the total number of hydroxyl groups was divided by the exposed surface of the cluster, hydroxyl groups at the edge are only counted as half since they are also part of neighboring clusters. The proposed approach was chosen since there are no surface models available for silicas with this high silanol loading. The edges of the slab are saturated with hydrogen atoms, which were unrestricted during the optimization. One of the hydroxyl groups was exchanged with a C8 silane group. Geometry optimizations with the described methodology show the tail is oriented to minimize the interactions with the surface. This is as expected, since the apolar tail cannot interact with the polar surface (Figure 7). Similar calculations, where an implicit solvent model (IEFPCM [54]) was added, characterized by the dielectric constant of toluene, gave comparable results. The geometrical results show that only the -Si(CH3)2-moiety imposes a limit on the number of possible substituents per surface area.

From the result it can be seen that the -Si(CH3)2-moiety blocks at least two extra hydroxyl groups. Since those groups can also be overlapped by another silane group from a more distant position, they are only counted as half. So it can be concluded that, on this dense cluster, at least two groups are needed to accommodate the substituent leading to a maximum possible loading of 2.65 groups per nm 2 . On a less dense surface with about 2.5 hydroxyl groups per nm 2 there are no neighboring positions blocked (Figure 8). surface is more open this does not block the site. As an upper bound, on this type of surface it is possible to have 2.5 silane substituents per nm 2 . However, when the surface is almost full, the close interaction (2.35Å) will fully shield one of the hydroxyl groups and the theoretical maximum will never be reached.

This means the theoretical density in this case is 2.5 groups per nm 2 . On average, 2.5 groups per nm 2 is the theoretical maximal loading on a silica surface with only single silanols present, this is close to the value that can be estimated from the kinetic diameter of the silane and that was reported previously by Van Der Voort. [55] Page 21 of 39

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To assess the influence of geminal silanols a slight modification on the random surface was made, an extra hydroxyl group was added on a silicon atom and one Si-O-Si bond on that atom was broken. The other silicon atom was saturated by hydrogen. With this approach the surface contains a single geminal group. Two silane groups were placed on this site and after geometry optimization it appeared that at least two extra silanol groups were blocked by this. (Figure 9). For the same reason as before they only count as half. With the extra hydroxyl group from the geminal silanol, the surface silanol density is 6.7 groups per nm 2 (the surface area remains unchanged but an extra hydroxyl group is added). This means that when one extra hydroxyl group is blocked by the silane moiety, the density of substituents on the surface can be calculated as:

A fully hydroxylated silica surface (single silanol groups) can have maximum of 4.6 ± 0.5 groups per nm 2 . This means that roughly only half of the single silanol groups is sterically available for grafting which coincides with previous reports on that matter. [START_REF] Vansant | Characterization and Chemical Modification of the Silica Surface[END_REF] In Table 7 we summarize the total silanol number, differentiated as total, geminal and single silanols and the silanol that are accessible for the small HMDS molecule, expressed in mmol per gram and in groups per nm². The amount of single and geminal silanols (columns 2 and 3 in table 7) are calculated by multiplying the relative amount of silanols with the total number of OH-groups determined by IR spectroscopy (i.e. αOH x (Q 3 /(Q 2 +Q 3 ) = αOH,single). This gives the absolute amount single and geminal silanols, irrespective of their location. They could be on the pore wall or in the bulk silica.

Column 4 in table 7 then shows the silanols that were available to the small HMDS (C3 silane).

Because none of these values exceed the theoretical maximum values they can be considered as the "surface" or "reachable" silanols.

Conclusion

We have established that the amount of silane groups (expressed as groups per nm²) that can be grafted on the surface of ordered mesoporous silicas (MCM-41, MCM-48, SBA-15, SBA-16) is much smaller than on the surface of the commercial materials, which are typically used as a packing material for HPLC columns. The main reason for this is that the grafting reaction is strongly diffusion limited, as the surface area of these materials is basically internal surface area.

For the mesoporous silicas, it can be concluded that the amount of surface silanols is larger for the micropore containing SBA-type materials than for the non microporous MCM-type materials.

The amount of internal silanol groups is proportional, however not linearly, to the wall thickness.

Silica materials with thicker walls (SBA-15, SBA-16 and ethene PMO) also have more total silanol groups than the thin walled MCM-materials. In general, for the thick walled mesoporous silicas, one should keep in mind that up to 70% of the silanols is unreachable for a small silane.

Spray dried MCM-41 exhibits a more condensed silica structure due to the higher synthesis temperature. The PMO material (ethene PMO) on the other hand exhibits a lower condensation degree. This can be attributed to the different condensation behavior of (EtO)3-Si-CH=CH-Si(OEt)3 compared to Si(OEt)4 and to the absence of a high temperature calcinations procedure.

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Figure 1 :

 1 Figure 1: The In-Situ IR-cell used to

Figure 2 :

 2 Figure 2: Infrared spectra of SBA-15 sample in the 1500-5500 cm -1 range. a) Spectrum recorded

Figure 3 :Figure 4 :

 34 Figure 3: The 29 SiMAS NMR (A) and the CPMAS { 1 H}-29Si (B) spectrum for MCM-41HT (a), MCM-

Figure 5 :

 5 Figure 5: The allocation of the T 1 , T 2 and T 3 site on the NMR spectra of the PMO material.

Figure 6 :

 6 Figure 6: The polycondensation of silicates (A) and the autohydrophobisation illustrated for

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Figure 7 :

 7 Figure 7: Random silica surface with 5.3 hydroxyl group per nm 2 and one C8 silane substituent.

Figure 8 :

 8 Figure 8: Surface model with about 2.5 hydroxyl groups per nm 2 the surface is based on an

Figure 9 :

 9 Figure 9: The original cluster is modified to incorporate a geminal silanol, the Si-O bond between

Table 1 :

 1 Overview of the materials: synthesis conditions and template removal.

		Synthesis conditions	Template
				removal
	SBA-15	Hydrothermal	Acid	Calcination
	SBA-16	Hydrothermal	Acid	Calcination
	MCM-41	Hydrothermal	Base	Calcination
	MCM-48	Hydrothermal	Base	Calcination
	MCM-41(SD)	Spray dried	Acid	Calcination
	EthenePMO	Hydrothermal	Acid	Extraction

As mentioned above in the experimental part, the template removal has been performed by conventional calcination at 550°C, except the PMO which was activated by soxhlet extraction at 120°C using acetone as solvent. The comparaison of the MCM-41 and MCM-41(SD) allows

Table 2 :

 2 The

	Material	SABET	Vp	dp	a0	h
		m 2 /g	cm 3 /g	nm	nm	nm
	SBA-15	656	0.7	6.9	10.7	4.2
	SBA-16	738	0.6	6.2	14.6	6.4
	MCM-41	1144	0.6	2.9	3.8	1.1
	MCM-48	1293	0.8	2.8	7.3	1.0
	MCM-41(SD) 719	0.4	2.9	3.6	1.2
	EthenePMO 923	1.0	6.2	9.0	3.1
	Nucleosil	88	0.4	_	_	_
	3.2 The total concentration of silanol-groups from in-situ IR

pore properties of the various materials, with SABET the surface area, Vp the pore volume, dp the pore diameter, a0 the unit cell and h the wall thickness.

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  1 H 29 Si. It is well known that29 Si MAS NMR is a sensitive technique to show the presence of These species present a very different chemical shift. The CP MAS { 1 H}-29 Si is a technique that highlights the presence of OH groups, which elucidates the29 Si MAS spectrum. Just as it is the case with the in-situ IR measurements, solid state NMR is a bulk analysis tool. It measures both the silanols on the surface and in the walls of the materials.

	3.3.1 The OH species for MCM and SBA materials
	First the pure silica materials (MCM and SBA-type) are compared. Spectra obtained by direct
	polarization and by CP-MAS are presented in figure
				0,01
	0,5
				4700	4600	4500 Wavenumber (cm -1 )	4400	4300
	a
	b
	4500	4000	3500	3000	2500	2000	1500
				Wavenumber (cm -1 )

different types of silicon sites in solids. In siliceous zeolite, silicon species are depicted as Q

4

if the silicon is surrounded with four -O-Si groups, Q 3 if one -O-Si group is replaced with one OH group,

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							4 )
	SBA-15	3.5	0.30 0.04	88	12	0.25	0.75
	SBA-16	8.3	0.52 0.02	96	4	0.35	0.65
	MCM-41	3.6	0.62 0.04	94	6	0.40	0.60
	MCM-48	5.2	0.67 0.04	94	6	0.42	0.58
	MCM-41(SD) 3.2	0.35 0.01	97	3	0.26	0.74

Table 4 :

 4 The silanol number and the T 1 , T 2 and T 3 species in the material.

		1	2	3	4	5	6	7
		Silanol			SiO2OH	SiO1(OH)2	Total SiOH	Condensatio
		number (IR)			(%)	(%)	ratio	n degree
	Material	αOH
		(mmol/g)

Table 6

 6 depicts the loading capacity of the various (organo)silica materials for a trimethylsilyl (C3), an octyldimethylsilyl (C8) and an octadecyldimethylsilyl (C18) group. The pore diameter has been added for reference. From this table it can be observed that for the same reaction time the amount of grafted groups drops with the length of the aliphatic chain. When the hydrothermally synthesized pure silica materials are compared, the general tendency is that with smaller pores a lower amount of groups are grafted on the surface (MCM vs SBA vs Nucleosil). The same tendency has been shown as a function of the pore size of MCM materials by Waksburg et al.[48]

Table 5 :

 5 Overview of the micropore volume (Vpμ,t-plot) and surface area (SAμ,t-plot) by t-plot analysis; the mesopore volume (Vpm,BJH) and surface area (SAm,BJH) by BJH analysis; the total pore volume (Vp, 0.95) taken at P/P° = 0.95 and surface area (SABET) by BET analysis of SBA-15, SBA-16 and ethene PMO.

	Material	SABET	SAμ,t-plot	SAm,BJH	Vp, 0.95	Vpμ,t-plot	Vpm,BJH
		m 2 /g	m 2 /g	m 2 /g	cm 3 /g	cm 3 /g	cm 3 /g
	SBA-15	655	460	429	0.84	0.07	0.74
	SBA-16	738	469	398	0.63	0.11	0.47
	EthenePMO 923	649	598	0.99	0.07	0.87

Table 6 :

 6 The loading capacity for relevant chromatographic groups for various ordered

	mesoporous (organo)silica materials.
	Material	Pore size C3		C8		C18
		nm	mmol/g	groups/nm 2 mmol/g	groups/nm 2 mmol/g	groups/nm 2
	SBA-15	6.9	1.8	1.7	0.7	0.7	0.1	0.1
	SBA-16	6.2	2.3	1.9	0.8	0.7	0.5	0.4
	MCM-41	2.9	2.2	1.2	0.9	0.5	0.4	0.2
	MCM-48	2.8	2.5	1.2	1.2	0.6	0.5	0.3
	MCM-41(SD) 2.9	1.9	1.6	1.0	0.8	0.3	0.3
	EthenePMO	6.2	1.1	0.7	0.8	0.5	0.4	0.2
	Nucleosil	~ 30	0.4	2.8	0.5	3.3	0.3	1.9

Table 7 :

 7 The bulk silanol number, the amount of single and geminal silanols and the surface silanol groups, depicted with the pore diameter (dp) and pore wall (h).

		1	2	3	4	5	6	7
	Material	αOH,bulk	αOH,single,bulk	αOH,geminal,bulk	αOH,surface	αOH,surface	h	dp
		(mmol/g)	(mmol/g)	(mmol/g)	(mmol/g)	(gr/nm 2 )	(nm)	(nm)
	SBA-15	3.5	3.1	0.4	1.9	1.7	4.2	6.9
	SBA-16	8.3	8.0	0.4	2.3	1.9	6.4	6.2
	MCM-41HT 3.6	3.4	0.2	2.3	1.2	1.1	2.9
	MCM-48	5.2	4.9	0.2	2.6	1.2	1.0	2.8
	MCM-41SD 3.2	3.1	0.1	1.9	1.6	1.2	2.9
	EthenePMO 16.9	14.3	2.6	1.1	0.7	3.1	6.2

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Acknowledgements: M.I. acknowledges the IWT-Flanders (Agentschap voor Innovatie door Wetenschap en Technologie) grant nr. IWT/SB/71325 for financial support. T.B. is indebted to Ghent University, GOA-grant, project number 01G00710 and the European Research Council (FP7(2007-2013) ERC grant nr. 240483). Computational resources and services were provided by Ghent University.

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