Comparison of Hombikat UV100 and P25 TiO2 performance in gas-phase photocatalytic oxidation reactions

Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 58–65

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Journal of Photochemistry and Photobiology A: Chemistry journal homepage: www.elsevier.com/locate/jphotochem

Comparison of Hombikat UV100 and P25 TiO2 performance in gas-phase photocatalytic oxidation reactions Angela Alonso-Tellez a , Romain Masson a , Didier Robert b , Nicolas Keller a , Valérie Keller a,∗ a b

Laboratoire des Matériaux, Surfaces et Procédés pour la Catalyse (LMSPC), CNRS, University of Strasbourg, 25 rue Becquerel, 67087 Strasbourg, France Saint-Avold Antenna, LMSPC, CNRS, Lorraine University and University of Strasbourg, rue Victor Demange, Saint-Avold, France

a r t i c l e

i n f o

Article history: Received 14 May 2012 Received in revised form 1 October 2012 Accepted 6 October 2012 Available online 13 October 2012 Keywords: TiO2 UV100 Hombikat Hydrogen sulfide Methylethylketone Photocatalysis Sulfates

a b s t r a c t The superior performance of TiO2 Hombikat UV100 coatings compared to those made of Aeroxide TiO2 P25 was evidenced in the gas-phase UV-A photocatalytic oxidation of two very different molecules with two different on-stream behaviors: MEK and H2 S. Despite its lower crystallinity, UV100 could take advantage of a higher light transmission through the TiO2 coating, of its smaller particle size that could lead to a better balance between surface and bulk recombinations, and also of its larger surface area. This last-mentioned characteristic is of considerable interest in regard to a possible increase of the coating’s pollutant adsorption capacity and its ability to generate more OH• radicals. In respect of the H2 S oxidation reaction, the UV100 coating could benefit from its larger surface area as a means to improve its ability to store larger amounts of sulfates and thus to enhance the photocatalyst’s resistance to deactivation by poisoning sulfates. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Among generally employed semiconductors, TiO2 is still considered to be the most attractive and efficient one. TiO2 P25 prepared via the chloride technology method is currently the de-facto commercial reference photocatalyst, even if the reason of its high performance level continues to be a matter of debate [1,2]. To a lesser extent, TiO2 Hombikat UV100, obtained via the sulfate technology, has been studied for photocatalysis purposes. Aside from studies by Vorontsov et al. on the gas phase diethylsulfide removal for which UV100 was a more efficient agent than P25 [3], the value of using UV100 has been demonstrated for liquid phase reactions [4–11], while other studies showed contrary results, with the predominance of the P25 standard [12,13]. The larger surface area offered by UV100 compared to P25 was reportedly the explanation for the higher resistance to deactivation shown by UV100 in the photoreduction of Cr(VI) [4–6]; whereas Wang et al. measured a photonic yield in the liquid phase methanol photooxidation, in favor of UV100 for low TiO2 concentrations, and inversely for high concentrations [7]. By contrast, the rare studies in relation to gasphase removal showed the superiority of TiO2 P25 in the oxidation of n-butanol [14] and no differences of performance in that of

∗ Corresponding author. Tel.: +33 36885 736; fax: +33 36885 761. E-mail addresses: [email protected], [email protected] (V. Keller). 1010-6030/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jphotochem.2012.10.008

acetone [15]. In most of the published cases, no hypotheses were put forward that would explain the results. The aim of this paper is to report on the comparison between TiO2 P25 and Hombikat UV100 photocatalysts in the gas phase, taking the degradation of two very different molecules as gas phase model reactions: the well-documented photocatalytic degradation of methylethylketone (MEK), for which stable on-stream performances are reached [16–18], and the more rarely investigated photocatalytic oxidation of H2 S, for which TiO2 displayed a different on-stream behavior. Indeed, the photocatalytic degradation of H2 S over TiO2 leads – according to reaction mechanisms that have continued until now not to be fully understood – to the formation of SO2 as the oxidation by-product and to the creation of sulfates as ultimate reaction products, accumulating at the photocatalyst surface and causing on-stream deactivation [19–25]. 2. Experimental 2.1. Characterization techniques X-ray diffraction (XRD) measurements were carried out on a D8 Advance Bruker diffractometer (K␣1 (Cu) with radiation set at 1.5406 A˚ in a /2 mode). Surface area and porosimetry measurements were carried out on a ASAP2010 Micromeritics using N2 as adsorbant at −196 ◦ C, and with a prior outgassing at 200 ◦ C for 1 h for desorbing impurities or moisture. The BET surface area was calculated from the N2

A. Alonso-Tellez et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 58–65

adsorption isotherms, the micropore surface area was derived using the t-plot method and the pore size distribution was obtained from the desorption branch by the BJH method. Thermal Gravimetry Analysis (TGA) and Temperature Programmed Oxidation/Mass Spectrometry (TPO–MS) were performed using a TGA5000 analyser and an OmniStar (Pfeiffer) spectrometer-coupled AutochemII chemisoption analyser (Micromeritics), respectively, with 20 ◦ C/min heating rate, 80/20 O2 /N2 mixture and 35 mL/min flow rate. X-Ray Photoelectron Spectroscopy (XPS) surface characterisation was performed on a ThermoVG Scientific apparatus with a Al K␣ (1486.6 eV) source (20 eV pass energy). The spectra were decomposed assuming Doniach–Sunjic shaped contributions and Shirley background subtraction. Sulfur-to-titanium (S/Ti) surface atomic ratios were calculated using the sensitivity factors determined by Scofield. The energy shift due to electrostatic charging was subtracted using contamination carbon C 1s band at 284.6 eV. The light transmission through the TiO2 coating at a given surface density was defined as the fraction of the incident light being transmitted through the TiO2 coating. It was directly measured by comparing the light irradiance transmitted through the TiO2 coated reactor to the incident light irradiance received by the coating, using a wideband RPS900-W Rapid Portable Spectroradiometer (IL Technology).

2.2. Experimental devices and procedures The photocatalytic tests were carried out in a 270 mm length single pass annular Pyrex reactor made of two coaxial tubes (i.d. 28 mm and e.d. 30 mm), through which the reactant mixture was passed. Details are reported elsewhere [26]. Depending on the test conditions, 25–500 mg of photocatalyst, corresponding to a surface density of 0.11–2.23 mg/cm2 , was evenly coated on the internal side of the 30 mm diameter external tube by evaporating a catalystcontaining aqueous suspension to dryness. The catalyst coated reactor was dried at 110 ◦ C for 1 h in air. The photocatalyst was first exposed to the polluted air stream with no illumination until dark-adsorption equilibrium was reached. Then, illumination provided by an 8 W blacklight tube (Philips TL8 W/08-BLB-F8T5) with a spectral peak centered at 380 nm and located inside the inner tube of the reactor, was switched on. The irradiance received by the TiO2 coating was 3.3 mW/cm2 . The performances of both TiO2 photocatalysts towards MEK and H2 S photocatalytic degradation reactions have been compared in similar reaction conditions, operating in the treatment of industrial gaseous effluents, and allowing the accurate measurement of efficiency and behavior differences. The inlet flow in the case of MEK photocatalytic oxidation was obtained by bubbling air through two MEK and water saturators, then mixed with additional air to obtain the required MEK–water–air ratios with a constant total air flow of 430 mL/min. The MEK content was set at 1500 ppm in flowing air. The relative humidity was set at 50% [26]. The reaction products were analyzed on-line every 3 min by a TCD detector on a Agilent-3000A micro-gas chromatograph, allowing quantification of MEK, water, CO2 , and organic byproducts on Stabilwax, PLOT U, OV1 and MS-5A columns. The efficiency of the remediation process was expressed in terms of MEK conversion (CMEK ), of both CO2 and acetaldehyde selectivities (SCO2 , SAc ) as well as of mineralization yield, calculated according to Eqs. (1)–(4):

CMEK (%) =

([MEK]in − [MEK]out ) × 100 [MEK]in

(1)

SCO2 (%) = SAC (%) =

([CO2 ]out /4) × 100 ([MEK]in − [MEK]out )

([AC]out /2) × 100 ([MEK]in − [MEK]out )

Yield (%) =

([CO2 ]out /4) × 100 [MEK]in

59

(2) (3) (4)

The photocatalytic oxidation of H2 S was carried out with an inlet feed composition as follows: H2 S (15 ppm), air (92 vol.%), and balanced He in dry conditions with a total flow of 500 mL/min (at 0.21 mg/cm2 surface density), and flow rates ranging from 200 mL/min to 500 mL/min (at 1.77 mg/cm2 surface density), corresponding to gas velocities in the 1.4–3.5 cm/s range, and to residence times ranging from 19 s to 7.6 s, respectively. H2 S and SO2 were analyzed on-line every 3 min by a PFP Detector coupled to a CP-Sil 5 CB column on a Varian 3800 gas-chromatograph. The efficiency of the remediation process was expressed in terms of H2 S conversion (CH2 S ), of SO2 selectivity (SSO2 ) – expected as low as possible since SO2 remained an unwanted hazardous gaseous by-product – and of duration at total sulfur removal, according to Eqs. (5) and (6). CH2 S (%) =

[H2 S]in − [H2 S]out × 100 [H2 S]in

(5)

SSO2 (%) =

[SO2 ]out × 100 [H2 S]in − [H2 S]out

(6)

3. Results and discussion 3.1. TiO2 photocatalysts From XRD pattern (not shown, see [5,6]), TiO2 P25 displayed an anatase/rutile ratio of 80/20 and a nonmicroporous specific surface area of 50 ± 3 m2 /g. By contrast, anatase was the sole crystallized TiO2 phase evidenced in the TiO2 UV100, with a high and mainly microporous specific surface area of 330 ± 15 m2 /g, micropores accounting for about 243 m2 /g (i.e. a microporous surface ratio of 0.74). The TiO2 UV100 also featured a smaller primary particle size of 8 nm vs. 19 nm for TiO2 P25 (considering the anatase phase). The large surface area of UV100 resulted from the small primary particle size and from the high agglomeration of such small subparticles into round-shaped particles, generating a high density of small pores, as reported by Colon et al. [5]. Fig. 1A shows the N2 adsorption–desorption isotherms and the derived pore size distributions of TiO2 P25 and of UV100. P25 displayed adsorption–desorption isotherms characteristic of non-microporous materials with the presence of both mesoand macroporosity, whereas the performance of UV100 was typical of a partly microporous material. The pore distribution confirmed the largely macroporous nature of the P25 porosity, with a very weak pore volume, and also showed a maximum at about 15 nm probably corresponding to the inter-particle mesoporosity within particle agglomerates. This was consistent with the 10 nm pore size reported by Nguyen et al., assigned to the porosity of 200–215 nm size agglomerates [27]. By contrast, UV100 had a small size porecentered distribution, with pores that were mainly smaller than 5 nm. Fig. 1B shows the light transmission through the TiO2 coating as a function of the surface density for both TiO2 photocatalysts, evidencing a higher light transmission through the UV100 coating at a similar surface density. This probably resulted from a different particle agglomeration and a different light scattering, due to strongly different crystallite and particle sizes. The microporosity of UV100 was already assigned by Hidalgo et al. to the presence of very small pores existing within the UV100 round-shaped particles, made from highly agglomerated

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Adsorbed N 2 (mmol/g)

8

6

4

2

A 0 0,0

0,2

0,4

0,6

0,8

1,0

Relative pressure (P/P0)

Light transmission (%)

100

B

P25 UV100

80

60

40

20

0 0,0

0,5

1,0

1,5

2,0

2,5 -2

Surface density (mg/cm ) Fig. 1. (A) N2 isotherms and pore size distributions as inset for P25 and UV100 and (B) light transmission within both coatings as a function of the surface density.

subparticles [6]. It should be added that the different TiO2 did not show any significative difference in terms of UV/vis absorption (not reported). Fig. 2 shows the TGA weight loss recorded on fresh TiO2 , with first at low temperature the desorption of molecularly adsorbed water, and then at higher temperature the surface dehydroxylation, 100

Weight (%)

95

Fig. 3. XPS spectra of (top) Ti 2p (middle) O 1s and (bottom) S 2p, of TiO2 P25 and UV100, fresh and after H2 S test. Test conditions: d(TiO2 ) = 1.77 mg/cm2 , total flow of 200 mL/min.

90 85 80

Fresh P25 Used P25 Fresh UV100 Used UV100

75 70 200

400

600

800

1000

Temperature (°C) Fig. 2. TGA of both fresh and H2 S-tested TiO2 P25 and UV100. Test conditions: d(TiO2 ) = 1.77 mg/cm2 , total flow of 200 mL/min.

till 300 ◦ C for P25 and even 500 ◦ C for UV100. The higher dehydroxylation temperature on UV100 could have resulted from the smaller size of the crystallites. The high specific surface of UV100 was put forward for explaining its large weight loss of 12% vs. only 2% for the medium specific surface P25, since the UV100/P25 weight loss ratio was in agreement with the corresponding specific surface ratio, i.e. 12/2 vs. 330/53. Fig. 3 shows the Ti 2p and the O 1s region of the XPS spectra recorded on fresh TiO2 . The Ti 2p spectra of P25 only evidenced the doublet related to the Ti 2p3/2 –Ti 2p1/2 spin–orbit components of surface Ti4+ , at 458.2 eV and 463.9 eV respectively, with no contribution attributed to Ti3+ species, indicating the presence of few surface defects [28]. By contrast, a high energy asymmetry was

A. Alonso-Tellez et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 58–65 Table 1 Averaged relative contents of O Ti, O H, H2 O and O S surface species as well as averaged S/Ti surface atomic ratio. Conditions of the H2 S degradation test: d(TiO2 ) = 1.77 mg/cm2 and total flow of 200 mL/min. O H (%)

67 61

26 31

48 42

24 32

O S (%)

H2 O (%)

S/Ti

– –

7 8

– –

14 18

14 7

0.25 0.23

observed in the Ti 2p3/2 –Ti 2p1/2 doublet of UV100. This could result from the presence of a non-negligible content of amorphous TiO2 in the sample, that shows a stronger charge effect than that of crystallized anatase during the XPS recording. However, the mean particle size calculated at 8 nm for UV100 does not allow totally ruling out the possible presence of some crystallites with largely smaller characteristic size, such as 2–3 nm, and for which quantum size effect with higher energy Ti4+ centers could thus be observed and would lead to asymmetric peaks. Whatever the TiO2 , the O 1s spectra was characterized by three contributions attributed to O Ti binding in TiO2 network, Ti O H binding corresponding to OH surface groups and oxygen from residual water molecules adsorbed at the surface, respectively. The different photocatalysts had similar relative surface contents, in the 61–67% and 26–31% ranges for O Ti4+ and OH surface species, respectively, resulting in the presence at the surface of similar amounts of residual adsorbed water (ca. 7–8%) (Table 1). This was consistent with TGA characterization, evidencing that weight losses were directly proportional to the specific surface. This showed that the different TiO2 photocatalysts displayed a similar density of OH surface group, acting as molecular adsorption sites for H2 O. 3.2. Influence of the TiO2 characteristics Fig. 4 shows the influence of the TiO2 surface density on the steady-state performances in the photocatalytic degradation of MEK, in terms of MEK conversion, selectivities to acetaldehyde and to CO2 and mineralization yield into CO2 . Both UV100 and P25 TiO2 coatings exhibited a similar behavior with increasing the TiO2 surface density, with a first increase of both MEK conversion and CO2 mineralization yield due to the increase in the amount of illuminated TiO2 particles, before they stabilized as a result of the screening effect of excess particles, which masks part of the TiO2 particles, due to the limited penetration thickness of UV-A illumination [29,30]. The main difference between both coatings was related to the level of performances observed on this plateau and to the optimal TiO2 surface density. Whereas the P25 TiO2 coating was able to develop a MEK conversion of 40% and a CO2 selectivity of 50% leading to a mineralization yield of 20%, the coating made from UV100 TiO2 produced nearly double the level of performance, with a MEK conversion of 70% and a CO2 selectivity of 58%, resulting thus in a doubled mineralization yield of 40%. With levels of 9% and 20% on the plateau for TiO2 UV100 and P25 coatings, respectively – a result also in favor of the UV100 coating, the evolution of the acetaldehyde selectivity with the increase of the TiO2 surface density followed the usual pattern corresponding to the evolution of an intermediate reaction by-product, with a decrease of the acetaldehyde selectivity for increased MEK conversions. It was worth noting that the plateau of maximum MEK conversion and mineralization yield was reached at a higher TiO2 surface density in the case of the UV100 coating, i.e. about 1.25 mg/cm2 compared to about 0.75 mg/cm2 for the P25 coating. This means that, at a similar surface density, the UV100 coating displayed higher performances, and also that, in a similar reactor configuration, a higher amount of

MEK conversion CO2 selectivity

A: UV100

Acetaldehyde selectivity Mineralization yield

80

60

(%)

Fresh P25 UV100 After test P25 UV100

O Ti (%)

40

20

0 0,0

0,5

1,0

1,5

2,0

Surface density (mg/cm 100

B: P25

2

)

MEK conversion CO2 selectivity Acetaldehyde selctivity Mineralization yield

80

60

(%)

Photocatalytic systems

100

61

40

20

0 0,0

0,5

1,0

1,5

2,0 2

Surface density (mg/cm ) Fig. 4. Influence of the surface density in the 0.11–2.2 mg/cm2 range on the MEK mineralization performances obtained on (A) UV100 and (B) P25 TiO2 photocatalysts.

TiO2 could be efficiently used per unit reactor volume in the case of the TiO2 UV100 photocatalyst. Fig. 5 shows the evolution with time on stream of the photocatalytic performances in the H2 S degradation on TiO2 P25 and UV100 coatings in two different operating conditions. Fig. 5A shows the H2 S conversion and the SO2 selectivity obtained on both coatings at a TiO2 surface density of 0.21 mg/cm2 and a total air flow of 500 mL/min. In that reaction condition, the UV100 coating maintained a H2 S conversion of 100% with no SO2 formation in the gas phase, corresponding to a complete conversion of H2 S into surface sulfates. By contrast, on-stream deactivation of the TiO2 P25 coating was observed down to 90% after 5 h of test, together with the release of unwanted SO2 to the gas phase (instead of total oxidation into the ultimely oxidized sulfates), corresponding to a detrimental increase in the SO2 selectivity to about 35–40%. In addition, it was noteworthy to observe the superiority of the UV100 coating in operating conditions corresponding, in terms of TiO2 surface density, to the plateau observed for the MEK degradation, i.e. a TiO2 surface density of 1.77 mg/cm2 , at a total flow in the 200–500 mL/min range, corresponding to flow rates ranging from 1.4 cm/s to 3.5 cm/s (Fig. 5B and C). In such less restrictive operating conditions for the photocatalyst coating, the H2 S conversion was complete and efficiency comparison was therefore performed in terms of on-stream evolution of the SO2 selectivity and of duration at total sulfur removal. Whatever the flow rate tested, TiO2 UV100 performed better than P25 in terms of total sulfur removal

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200

100

2x10

SO 2 (a.u.)

Conversion on P25 SO2 selectivity on P25

(%)

60

1x10

40

600

800

1000

SO2/P25

H2O/P25

SO2/UV100

H2O/UV100

1x10

-10

H2O (a.u.)

Conversion on UV100 SO2 selectivity on UV100

80

400

-12

-12

5x10

-11

20

A

0 0

1

2

3

4

5

0 200

Time on stream (h)

400

600

800

0 1000

Temperature (°C) 100

B

P25 UV100

Fig. 6. TPO–MS performed on both fresh and H2 S-tested TiO2 P25 and UV100. Test conditions: d(TiO2 ) = 1.77 mg/cm2 , total flow of 200 mL/min.

SO 2 selectivity (%)

80

60

40

37 h

6h

20

0 0

10

20

30

40

Time on stream (h)

Duration at total sulfur removal (h)

40

2

C

at a 1,77 mg/cm surface density

30

UV100 P25

20

10

0 1

2

3

Flow rate (cm/s) Fig. 5. Photocatalytic H2 S degradation over P25 and UV100 coatings. (A) Evolution with time on stream of H2 S conversion and SO2 selectivity at d(TiO2 ) = 0.21 mg/cm2 and total flow of 500 mL/min; (B) evolution with time on stream of SO2 selectivity and duration at total sulfur removal at d(TiO2 ) = 1.77 mg/cm2 and total flow of 200 mL/min; and (C) duration at total sulfur removal at d(TiO2 ) = 1.77 mg/cm2 for total air flows ranging from 200 to 500 mL/min, corresponding to flow rates in the 1.4–3.5 cm/s range.

efficiency. Indeed, the duration at total sulfur removal was extended from 6 h up to 37 h before SO2 was released into the outlet flow at a flow rate of 1.4 cm/s, whereas it was extended from 4 h to 30 h, and from 2 h to 20 h, at a flow rate of 2.1 cm/s and 3.5 cm/s, respectively. TGA, XPS and TPO–MS characterization of the both UV100 and P25 photocatalysts after testing in the operating conditions of

Fig. 5A (TiO2 surface density of 0.21 mg/cm2 and total flow of 200 mL/min) are reported in Figs. 2, 3 and 6, respectively. TGA curves showed that both TiO2 photocatalysts displayed, after test, a larger weight loss when compared to that of fresh materials, i.e. 25% and 7% for UV100 and TiO2 P25, respectively, with globally similar temperature ranges with H2 O and/or SO2 detection. At temperatures lower than 100–150 ◦ C, the weight loss corresponded to the desorption of water adsorbed on TiO2 similarly to fresh TiO2 materials, before a more marked weight loss occurred within the 200–400 ◦ C range attributed to the dehydration of the surface sulfates species. Analogy could be made with the temperature dependence of Ti(SO4 )/ZrO2 materials, for which Sohn et al. observed the dehydration of sulfate species linked to Ti4+ following several steps till 300 ◦ C [31,32]. Here, the TiO2 photocatalyst after test could thus be considered as a strongly hydrated Ti(SO4 )/TiO2 material, consistent with the marked release of water till 300–400 ◦ C and the water contribution in the XPS O 1s spectra. However, one should remain cautious, since TiO2 can catalyze the Claus reaction in the 200–250 ◦ C range, i.e. 2H2 S + SO2 ↔ 2H2 O + 3/nSn , between residual H2 S and SO2 adsorbed molecules, and thus impact on the MS signals.At higher temperatures, the weight loss observed was attributed to the decomposition of surface sulfates, with SO2 release and no formation of water. The sulfate decomposition was already observed on mixed phase anatase/rutile TiO2 within the 550–800 ◦ C range, depending on the photocatalyst nature [33]. Barraud et al. showed that the sulfate decomposition of bulk sulfated titania samples, obtained by calcination of a H2 SO4 -treated Ti(OH)4 amorphous gel, was responsible for the weight loss observed under air at 600–750 ◦ C [34]. Sohn et al. have evidenced that the nature of the interaction between the sulfates and the support causes strong changes in the sulfate decomposition temperature [32]. One could also propose that the different mono- and bi-chelated coordination modes to Ti4+ , observed notably by FTIR [25,33,35], could impact on the stability of the sulfates to a certain extent, or that the dehydration of sulfates at a low temperature could destabilize them and promote a decomposition at a lower temperature. In addition, the SO2 release from the sulfate decomposition in the 500–800 ◦ C range was more strongly pronounced on UV100 than on TiO2 P25, consistent with the higher weight loss observed. XPS characterization shown in Fig. 3 evidenced a slight shift to higher binding energies of Ti4+ for the different samples after photocatalytic test (E = +0.4 eV and +0.5 eV for TiO2 P25 and UV100, respectively). This was attributed to the increase in effective positive charge around Ti4+ surface species, suggesting the direct

A. Alonso-Tellez et al. / Journal of Photochemistry and Photobiology A: Chemistry 250 (2012) 58–65

coordination of titanium atoms to strongly electron-withdrawing SO4 centers and the existence of an electron transfer from TiO2 to sulfate anions. A similar 0.2–0.5 eV upward shift of Ti4+ 2p binding energies, depending on the sulfation conditions, was already observed by Barraud et al. during the study of sulfated titania photocatalysts [34] and in a parametric study of the H2 S photocatalytic oxidation over the TiO2 P25 reference [25]. It was suggested that the resulting possible titanium to sulfate electron transfer, causing delocalization and a new electron distribution at the TiO2 surface as reported by Jung and Grange [36], could be beneficial to the charge separation, and thus to the photocatalytic yield. In addition, the Ti 2p spectra of used TiO2 P25 showed a higher energy doublet at 459.7–465.5 eV, assigned to a Ti S binding, already observed by Grandcolas at the surface of multi-walled TiO2 nanotubes – so with a probably very different electronic environment – after the UV-A photocatalytic oxidation of diethylsulfide (DES) [33], and no reduction of Ti4+ to Ti3+ was also observed during the reaction. By contrast, the Ti 2p spectra of the UV100 showed a weak intensity doublet contribution at lower energy, attributed to the presence of small amount of Ti3+ , that could result from the reduction under the H2 S flow of a small part of Ti4+ surface species, as already observed by Grandcolas during the DES degradation over anatase/rutile mixed systems prepared in the presence of porogens [33]. The keypoint in the present study and that of Grandcolas – both reporting on the partial surface reduction into Ti3+ , could be the markedly lower crystallinity of investigated samples (UV100 and obtained by porogen-based sol–gel TiO2 ) when compared to that of TiO2 P25. Admittedly, the artificial asymmetry introduced previously increased the fit uncertainty in the case of UV100, so that one could not exclude the existence of a Ti S bonding doublet contribution, even it was not necessary to take it into account. In addition to the contributions observed on fresh materials, the O 1s spectra recorded on the TiO2 photocatalysts after test displayed an additional contribution arising at 532.0 eV energy, attributed to oxygen bonded to the central atom of sulfur within sulfate species, and thus assigned to the O S binding [25,33,36,37]. When compared to fresh TiO2 spectra, the main peak assigned to O Ti binding in TiO2 network was also shifted by +0.5 eV, from 529.4 eV to 529.9 eV and from 529.6 eV to 530.1 eV, for TiO2 P25 and UV100, respectively. This was attributed to the presence of the electron-withdrawing SO4 on the Ti4+ center and thus to possible electron transfer from Ti4+ to sulfate anions, that could indirectly lead to electron donation from the oxygen of O Ti to Ti4+ . Whatever the TiO2 nature, the S 2p region of the spectra of used TiO2 showed a broad signal, composed of the doublet related to the S 2p3/2 –S 2p1/2 spin–orbit components at 168.2 eV and 169.6 eV, assigned to surface S6+ sulfates [38]. One could not exclude that the S 2p spectra could be slightly enlarged by the presence of surface polysulfates (resulting from a partial and local polymerization of sulfates), or of sulfates with different coordination modes to Ti4+ , observed notably by FTIR as mono- and bi-chelated coordination modes [25,35,36], that could change the electronic environment of sulfur. Previous works on TiO2 P25 showed that light penetration and reactant diffusion across the coating resulted in the existence of a reactivity gradient depending on the TiO2 particle location within the coating (deep internal or external layers). This reactivity gradient should lead to a gradient in terms of S/Ti surface atomic ratio and of relative contents of surface oxygenated phases along the coating thickness. However, we have no access to such data, since the recovering of the whole used catalyst from the reactor wall leads to average the characterization data of used TiO2 , with a stronger influence in the case of high TiO2 surface density tests. Thus, calculated values derived from XPS spectra were taken as averaged S/Ti surface atomic ratios and averaged relative contents of surface oxygenated phases. For both used TiO2 photocatalysts, the

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averaged relative contents of surface oxygenated phases shown in Table 1 remained unchanged compared to fresh TiO2 (24–32% after test vs. 26–31% on fresh ones). The O Ti relative content decreased from 61–67% for fresh TiO2 down to 42–48% after test, simultaneously to the increase in the O S relative content up to 14–18%. In the case of P25, the results previously pointed out that O Ti4+ surface sites would be active sites for H2 S photooxidation, with a first molecular adsorption step on Ti4+ through the sulfur atom of H2 S [25]. There was here no evidence that it could occur differently on TiO2 UV100, and a similar hypothesis could be drawn. Despite the larger sulfate content of used UV100 compared to used P25, as observed by TGA, both tested TiO2 exhibited a similar averaged S/Ti surface atomic ratio (0.25 for P25 vs. 0.23 for UV100), so that they did not differentiate in terms of sulfate density at the TiO2 surface, i.e. a similar sulfate dispersion. This was explained by the higher surface area of UV100, and means that UV100 could accommodate larger amounts of sulfates or maintain a higher sulfur removal efficiency for a similar sulfate content. Two main hypotheses could be proposed for explaining the superiority of the UV100 particle coating in both photocatalytic oxidation reactions: (i) A higher degree of light transmission through the photocatalyst coating by comparison to that across the P25 coating. First, at a similar TiO2 surface density, the irradiance received by the UV100 TiO2 particles within the coating is greater than in the case of the P25 TiO2 coating. This increases the photocatalytic reaction rates which are proportional to the radiant flux. Secondly, the deeper light penetration within the UV100 TiO2 coating allows more TiO2 particles to be activated by light and thus to take part to reactions. That results in the very interesting possibility of increasing the amount of illuminated TiO2 per unit reactor volume, with a MEK conversion and mineralization yield plateau being obtained for a higher TiO2 surface density. (ii) A high surface area, resulting in an enhanced pollutant adsorption capacity and in the ability to produce more OH• radicals. In parallel to that, it could also be pointed out that the smaller size porosity of the UV100 coating – including a large fraction of microposity that is not present in the P25 coating – could also result in an artificial increase in the residence time of the pollutant within the photocatalytic coating of UV100 particles. This would increase the coating tortuosity for the flowing gas and thus be beneficial to the degradation performances. As a result, molecules to degrade (as well as reaction intermediates) would more easily escape from the P25 coating than from the UV100 one, due to a larger-size porous network, and therefore would have less probability to adsorb/re-adsorb on the TiO2 surface for reacting, resulting in a lower conversion and a lower mineralization yield to CO2 . This differs from the residence time within the reactor, set by the reaction conditions, and determined as the reactor volume-to-flow rate ratio. It should be pointed out that this pore-size aspect did not seem to be detrimental to the light penetration, probably as a result of the large macroporosity of the coating. Even if both TiO2 particle coatings differ in many aspects, one could not fully avoid that the smaller UV100 particle size (at 8 nm vs. 19 nm for anatase in P25) could play a positive role by allowing a possibly better balance between bulk and surface recombinations. Indeed, Zhang et al. have already observed an optimal particle size at about 10 nm in the chloroform decomposition [39], resulting from a compromise between bulk and surface recombinations, preponderant for too large and too small particle sizes, respectively [40–43]. Since the possible presence of some very small 2–3 nm diameter UV100 crystallites could not be totally ruled out, a beneficial quantum size effect could however occur in some very small

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size nanoparticles and thus impact on the activity, although probably in a very secondary way. In addition, and particularly in the case of the H2 S removal, UV100 might take advantage of its high surface area for also improving the photocatalyst resistance to sulfate deactivation. Indeed, averaged S/Ti surface atomic ratios were similar on P25 and UV100, i.e. a similar poisoning sulfate dispersion, whereas larger amounts of sulfates were however stored at the UV100 surface. This deactivation resistance was consistent with the less marked deactivation of UV100 during the Cr(VI) reduction compared to P25, UV100 being less affected by chromium deposition due to its high surface area [4,5]. By contrast to the above-reported aspects, UV100 could however suffer from a lower crystallinity of the material compared to P25, by containing – beside to the photocatalytic anatase crystallites – a non-negligible fraction of amorphous TiO2 phase [44], reported by Ohtani et al. to show a negligible activity [45]. It could also suffer from the absence of any rutile phase close to the main anatase phase as in P25, the presence of both phases being often reported as positively impacting on the photocatalytic efficiency, though this point is still strongly debated [2]. 4. Conclusion The value of using TiO2 Hombikat UV100 in place of the TiO2 P25 standard has been pointed out in two different gas-phase oxidative photocatalytic reactions under UV-A, the well-investigated MEK oxidation for which stable on-stream performances are reached, and the more rarely studied photocatalytic oxidation of H2 S, for which TiO2 is subjected to on-stream deactivation by sulfate surface poisoning. In the H2 S oxidation, like on TiO2 P25, O Ti4+ surface sites have been proposed to act as active sites for the reaction, on which initial molecular adsorption of H2 S takes place through the sulfur atom of H2 S, and on which the sulfates were proposed to be stored. Despite its lower crystallinity, the superiority of UV100 in both reactions could be mainly attributed to a higher light transmission across the photocatalytic coating allowing more illuminated TiO2 to operate and to its high specific surface area. This latter results in an enhanced pollutant adsorption capacity and in the capacity to generate more OH• radicals, as well as in the possibility of storing larger amounts of poisoning sulfates and to lower and delay the SO2 release to the gas phase, with the maintenance of a weak selectivity into the unwanted SO2 by-product. This explained the strong improvement in the photocatalyst deactivation resistance. The smaller crystallite size of the UV100 could also lead to a better balance between surface and bulk photogenerated charge recombinations. References [1] T. Ohno, K. Sarukawa, K. Tokieda, M. Matsumura, Morphology of a TiO2 photocatalyst (Degussa, P-25) consisting of anatase and rutile crystalline phases, Journal of Catalysis 203 (2001) 82–86. [2] B. Ohtani, O.O. Prieto-Mahaney, D. Li, R. Abe, What is Degussa (Evonik) P25? Crystalline composition analysis, reconstruction from isolated pure particles and photocatalytic activity test, Journal of Photochemistry and Photobiology A: Chemistry 216 (2010) 179–182. [3] A.V. Vorontsov, E.N. Savinov, L. Davidov, P.G. Smirniotis, Photocatalytic destruction of gaseous diethyl sulfide over TiO2 , Applied Catalysis B: Environmental 32 (2001) 11–24. [4] G. Colon, M.C. Hidalgo, J.A. Navio, Influence of carboxylic acid on the photocatalytic reduction of Cr(VI) using commercial TiO2 , Langmuir 17 (2001) 7174–7177. [5] G. Colon, M.C. Hidalgo, J.A. Navio, Photocatalytic deactivation of commercial TiO2 samples during simultaneous photoreduction of Cr(VI) and photooxidation of salicylic acid, Journal of Photochemistry and Photobiology A: Chemistry 138 (2001) 79–85. [6] M.C. Hidalgo, G. Colon, J.A. Navio, Modification of the physicochemical properties of commercial TiO2 samples by soft mechanical activation, Journal of Photochemistry and Photobiology A: Chemistry 148 (2002) 341–348.

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