Characterization and photocatalytic performance in air of cementitious materials containing TiO2. Case study of formaldehyde removal

Applied Catalysis B: Environmental 107 (2011) 1–8

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Applied Catalysis B: Environmental journal homepage: www.elsevier.com/locate/apcatb

Characterization and photocatalytic performance in air of cementitious materials containing TiO2 . Case study of formaldehyde removal Aurélie Hadj Aïssa a , Eric Puzenat a , Arnaud Plassais b , Jean-Marie Herrmann a , Claude Haehnel b , Chantal Guillard a,∗ a b

Université Lyon 1, CNRS, UMR 5256, IRCELYON, Institut de recherches sur la catalyse et l’environnement de Lyon, 2 avenue Albert Einstein, F-69626 Villeurbanne, France CTG Italcementi, Rue des Technodes, 78931.Guerville-Cedex, France

a r t i c l e

i n f o

Article history: Received 31 January 2011 Received in revised form 1 June 2011 Accepted 11 June 2011 Available online 17 June 2011 Keywords: Titania Cementitious materials Photocatalysis Raman spectroscopy mapping Diffuse reflectance spectroscopy Formaldehyde

a b s t r a c t This article demonstrates that photocatalytically self-cleaning mortars and concretes can additionally contribute to the elimination of volatile organic compounds (VOC’s) present in indoor and outdoor atmospheres. Formaldehyde was chosen as a representative VOC air pollutant. Titania (PC-105 from Millennium Chemicals) was added in a white Portland Cement (Société Ciment Calcia) to prepare mortar samples according to a normalized methodology. A normalized steady-state was chosen after aging the samples for three months. The distribution of titania at the exposed surface of the mortar sample was quantitatively examined, using Raman spectroscopy mapping and diffuse reflectance spectroscopy. Interestingly, when varying the weight percentage of titania (with respect to the cement weight) between 0 and 10 wt%, it was demonstrated that the “occurrence rate” (or presence probability) of 100% in TiO2 at the surface of the mortar was already reached at 5 wt% TiO2 . Direct linear correlation between UVabsorptance and Raman spectroscopy mapping spectroscopy was obtained. In parallel, the photocatalytic removal of formaldehyde, performed in a dynamic flow reactor with an in-line photoacoustic analytic spectrometer was established by the curve rate r = f(TiO2 wt%). Since concretes and mortars are solids which can be considered as “breathing materials”, which absorb and/or adsorb many compounds, it was carefully and quantitatively determined what part of formaldehyde was adsorbed and/or absorbed by the mortar, either in the dark or under UV. The real and true photocatalytic nature of the disappearance reaction relative to UV-irradiated TiO2 let us conclude that, in addition to their photocatalytic self-cleaning properties, such cementitious materials can possibly contribute to the elimination of atmospheric VOC’s which may come in contact with them. © 2011 Published by Elsevier B.V.

1. Introduction At the beginning of the 1990s, photocatalysis was described as an emerging technology for environmental applications [1] and was mainly focused on water and air purification [2–4]. TiO2 is commonly used as a photocatalyst for industrial applications because of its highest photocatalytic activity and its reasonably low cost. Moreover, TiO2 has a strong chemical stability in a large variety of environmental conditions: use in water, in air, in solvents, at high and/or low pH’s, etc. TiO2 exhibits two types of surface photo-assisted reactions under UV-light irradiation ( < 380 nm): (i) photocatalytic redox reactions in the adsorbed phase and (ii) photo-induced hydrophilicity [5,6]. The synergy of these two potentially simultaneous features

∗ Corresponding author. E-mail address: [email protected] (C. Guillard). 0926-3373/$ – see front matter © 2011 Published by Elsevier B.V. doi:10.1016/j.apcatb.2011.06.012

makes TiO2 particularly interesting for building materials. It was the basis for the development of new building materials with selfcleaning properties, such as ceramics, tiles, glasses, aluminium and stainless steel walls, paving blocks, mortars, concretes, etc. [7–11]. The present article focuses on concrete materials, basically on mortars, the cement being the only solid phase able to provide photoactive titania in the final product. The cement is already commercially available [12] and has been successfully used in several European constructions and monuments, such as the Romancatholic church “Dives in Misericordia” in Rome (Italy) and the “Cité des Arts et de la Musique” palace in Chambéry (France). The utilization of TiO2 -modified cement was studied in Europe under the European Project Photocatalytic Innovative Covering Applications for Depollution Assessment (PICADA) [13]. Both self-cleaning and depollution properties were evaluated at laboratory [10,14–16] and at large pilot scale [17–19]. The air decontamination was mainly focused on NOx removal [10,20,21], whereas some studies were devoted to the removal of volatile organic compounds (VOCs), such

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as BTEX [15,16,22]. However, only a few studies paid attention to the direct impact of the cementitious materials on the final photocatalytic performances [23,24] and to the characterization of TiO2 available on the surface to take part in photocatalytic reaction [14]. The present article is aim at correlating the physicochemical properties of TiO2 -containing mortar surface with their photocatalytic properties observed during the removal of formaldehyde. Formaldehyde was chosen as a model pollutant since it is known as one of the most abundant VOCs, found in urban areas [25]. Downtown, formaldehyde originates from anthropogenic sources and, more particularly, from vehicle exhaust gas. Formaldehyde has a negative impact on human health and is responsible for irritations, allergies and also cancer [26]. In this study, Raman spectroscopy and UV-diffuse reflectance spectroscopy were used as precious tools to characterize the surface of the samples (absorptance of UV-photons, distribution of titania and cementitious hydrated phases at the surface of the mortar sample). The spectroscopic results were then used to understand the impact of the quantity of TiO2 present at the surface of the final material to remove and destroy formaldehyde.

2. Experimental

2.3. Characterizations 2.3.1. Raman spectroscopy Raman spectra were recorded at 20 ◦ C with a confocal Raman microscope (LabRam HR UV–vis-Nir spectrometer of Jobin Yvon equipped with an Argon-Krypton laser fixed at 514 nm and a CCD detector cooled at −75 ◦ C). Measurements were carried out under a 50× microscope objective focussing the laser beam on a spot of about 2 ␮m diameter and collecting the scattered light. The laser was powered at 1 mW for each experiment with an optical density of 1, which means that the power received by the sample was divided by 10. It was previously checked that such a low laser power induced only a negligible heating of the samples. A 300 grooves/mm diffraction grating was chosen to get a maximum signal. Consequently, the positions of the bands were accurate within 4 cm−1 . The depth of analyze is around 1 ␮m. The Raman mappings of the different samples were established on 3 randomly chosen zones of 105 × 84 ␮m2 , divided into 490 rectangles of 18 ␮m2 each. In each rectangle, the analyzed spot area is 3 ␮m2 . In each zone, a spectrum was recorded from 250 to 1250 cm−1 with an exposure time of 40 s for 2 accumulations. An automatic data processing was used for the peak fitting of each map spectrum (Gauss-Lorentz function, 100 iterations, no omission point). The processing was made by the LabSpec5 software from Horiba Jobin Yvon.

2.1. Materials The photocatalyst used was titanium dioxide PC-105 from Millennium Chemicals, presently name Crystal. Physical data were provided by the manufacturer: specific surface area 85 ± 10 m2 /g, particle size 20 ± 5 nm, TiO2 content >95% with 99% anatase. White Portland cement was provided by Ciment Calcia (oxide composition: 67.05 wt% CaO, 22.02 wt% SiO2 , 4.02 wt% Al2 O3 , 2.52 wt% SO3 , 0.5 wt% MgO, 0.31 wt% Fe2 O3 , 0.17 wt% TiO2 , 0.09 wt% K2 O, 0.01 wt% Na2 O, 0.01 wt% MnO, 3.30 wt% others). Normalized sand from Société Nouvelle du Littoral and deionized water were used to prepare the mortar sample. Formaldehyde was delivered by a commercial permation tube (LNI, Switzerland).

2.3.2. Diffuse reflectance spectroscopy UV–vis diffuse reflectance spectra were recorded using an optical fiber spectrometer Avaspec-2048, equipped with a symmetrical Czerny-Turner design with 2048 pixel CCD detector array. The setup consists of a UV–vis light source, an optical fiber probe composed of 6 concentric fibers to deliver the light to the sample and a collecting central fiber. The spectra were recorded from 250 to 800 nm. A barium sulfate pellet was used as a blank white reference. For each sample, a series of 5 spectra was recorded to take into account the roughness of the surface sample. The data were recorded and processed with software Avasoft 7.0. 2.4. Photocatalytic degradation of formaldehyde

2.2. Preparation of standardized mortar sample The preparation of mortar was based on the methodology of the standard NF EN 196.1. The following weight ratios were used for all samples: 2/9 of cement + TiO2 , 6/9 of normalized sand and 1/9 of water. Powdered TiO2 was added to the dry cement before mixing and subsequent addition of water. The cementitious paste obtained was first mechanically stirred at 140 rpm with 62 planetary motion per minute for 30 s. Sand was subsequently added and the paste was mixed at 285 rpm with 125 planetary motion per minute for 30 s. A duration of 90 s for release time was observed before mixing again the paste at 285 rpm with 125 planetary motion per minute for 60 s. The paste was poured in a 8.6 cm-diameter Petri dish at full filling. The samples were stored 24 h in a room at 20 ◦ C under 70% relative humidity (RH) and, for the next 27 days, at 100% RH. The samples were aged during at least three months, before the characterization and the photocatalytic tests were performed. A series of specimens containing 0, 1, 2.5, 5 and 10 wt% titanium dioxide were prepared. The TiO2 content in % is defined as: %TiO2 =

mTiO2 mTiO2 + mcement

× 100

(1)

A Teflon disk with the same size as that of the mortar samples was used as a blank sample owing to its photo-inertness under UV.

2.4.1. Reactor and experimental set-up The photocatalytic tests were performed in a dynamic flow reactor (V ≈ 150 cm3 ), specially designed for evaluating the photoactivity of cement mortar samples. The reactor is a cylinder-shape chamber with 4 inlets located above the sample for a better homogenization of the gas flow inside the reaction volume. Since the outlet is located below the sample, a hole of 6mm-diameter was made in the center of each tested sample to enable the gas to pass through the reactor. The reactor is equipped with an optical window made of quartz (fused silica). The sample surface (area = 38 cm2 ) is simultaneously exposed to irradiation and gaseous pollutant. The cross section view of the reactor is illustrated in Fig. 1(a). The experimental set-up is described in Fig. 1(b). A permeation oven is used to generate formaldehyde (permeation rate = 3720 ng min−1 at 100 ◦ C). The carrier gas for formaldehyde is dry air provided by Air Liquide (O2 /N2 = 2/8, hydrocarbons <500 ppb and H2 O <3 ppm). A high-medium pressure mercury lamp (Philips HPK 125 W) was used as the source of the UV-irradiation light. A Pyrex disc was added as an optical filter (transmittance: wavelength >280 nm) to use UV-A and UV-B photons. A circulating-water cell eliminates any possible heating of the sample by the IR-beams emitted by the lamp. The irradiation source is placed 12 cm above the sample surface. A buffer volume of 285 cm3 is placed at the exit of the reactor to prevent any perturbation in the gas flow, possibly due to the sampling of the gas phase for analysis with a photoacoustic spectrometer.

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Fig. 1. (a) Schematic reactor cross section, (b) experimental set-up.

3.1. Characterizations 3.1.1. Raman spectroscopy The mortar samples used are mainly composed of the commonly expected hydrated cement phases due to their ages. The Raman spectrum of mortar (Fig. 2(a)) shows 7 bands attributed to the different hydrated and carbonated cement phases composing the cementitious materials. The peak at 853 cm−1 is relative to a vibration of SiO4 group in calcium silicate hydrates, C–S–H ((CaO)3.4 (SiO2 )2 (H2 O)8 mean composition) [27]. C–S–H is the main hydrate formed during the hydration of white Portland cement. The peak at 985 cm−1 is due to the vibration of SO4 2− in sulfoaluminate hydrates, AFm (Ca4 Al2 (OH)12 (SO4 )·6H2 O) [28,29]. The

640

985

1072

701

853

1090

519

399 356

3. Results and discussion

278

2.4.2. Analysis Formaldehyde was analyzed, every 5 min, by a photoacoustic spectrometer Innova 1213. The filter used for formaldehyde was the UA 0987 model, which has an excited wavelength at 2950 cm−1 . The measurements were carried out after an automatic flushing programmed for a 2-m tube length, and with a compensation for water interference. A sample integration time is equal to 20 s. The calibration of the apparatus was done with formaldehyde permeation cartridge.

356 cm−1 peak is characteristic of the Ca–O vibration in Ca(OH)2 . The bands at 278, 701, 1072 and 1090 cm−1 are attributed to the polymorphic forms of CaCO3 : vaterite, calcite and aragonite. The peak at 278 cm−1 is due to a Ca–O vibration in CaCO3 . The antisymmetric bending band (4 ) of O–C–O of calcite and aragonite appears at 701 cm−1 . The bands at 1072 and 1090 cm−1 originate from the degeneration of the symmetric C–O vibration 1 of vaterite. The symmetric mode of the C–O bond in aragonite and in calcite also contribute to the 1090 cm−1 peak [29–31]. CaCO3 is progressively formed from the carbonation of C–S–H and of Ca(OH)2 when exposed to CO2 naturally contained in the air. In Fig. 2(b), the TiO2 Raman spectrum clearly shows three bands at 399, 519 and 640 cm−1 . These bands are characteristic of the

Intensity [a.u.]

Before starting the photocatalytic test, the sample was pre-treated overnight under UV-irradiation (irradiance EUV (290 nm <  < 400 nm) = 3.7 mW cm−2 measured by CCD spectrometer Avaspec-2048) and under a dry air flow rate equal to 200 ml min−1 (residence time equal to 4 min). The formaldehyde concentration in air was first stabilized and homogenized outside the reactor (C = 20 ppm) before it was introduced in the reactor maintained in the dark. After a steady state of formaldehyde concentration in the dark, the light was then switched on and maintained for 5 h.

(c)

(b)

(a) 250

450

650

850

1050

1250

-1

Raman shift [cm ] Fig. 2. Raman spectra of (a) mortar, (b) TiO2 (PC-105), (c) mortar + TiO2 (2.5 wt%).

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Fig. 3. TiO2 Raman mappings of TiO2 -mixed mortar: (a) 0 wt%, (b) 1 wt%, (c) 2.5 wt%, (d) 5 wt%, (e) 10 wt%. The bright color corresponds to the maximum of intensity of the TiO2 Raman shift whereas the dark zones indicate the absence of TiO2 .

anatase form and correspond to B1g , A1g and Eg Raman-active mode, respectively [32–35]. In Fig. 2(c), which illustrates a spectrum of a 3 ␮m2 area of a TiO2 -mixed mortar zone, peaks from both TiO2 and mortar phases can be observed. For this particular spot, TiO2 is identified thanks to the three characteristic peaks of anatase. The bands relative to TiO2 appear clearly and can be easily distinguished from those belonging to the mortar phases. The hydrated cement phase bands are also present in the spectrum. The 356 and 701 cm−1 peaks appear as TiO2 -band shoulders because of the much better response of TiO2 in Raman spectroscopy. The band at 278 cm−1 , corresponding to CaCO3 phase, is not visible on the spectrum. The main reason for its absence is the heterogeneity of the material. Raman mapping is then carried out to overcome the heterogeneity of the material. The TiO2 Raman mapping for different TiO2 contents from 0 to 10 wt% are illustrated in Fig. 3. Each spot of the maps corresponds to the sum of the three characteristic TiO2 fit peak areas; this value is associated with a color scale. The absence of TiO2 is represented by a dark color and the bright color is associated with a higher presence at TiO2 on the surface of the sample. The color of the mapping becomes brighter where TiO2 content increases in the sample. The mappings show a non-uniform coloration, indicating a heterogeneous distribution of TiO2 at the surface of the mortar. An accurate quantification of TiO2 on the surface could not be done because of the heterogeneity of the material and the impossibility to use an internal standard. The TiO2 surface occurrence rate or presence probability  was calculated for each sample. It is defined as the ratio of the spot area containing TiO2 to the whole surface analyzed. It is given by: (2)

response of the zero catalyst sample. The occurrence rate  (in %) of TiO2 as a function of its content in the cement, expressed in wt% is presented in Fig. 4(a). TiO2 occurrence rate increases linearly up to 2.5 wt%. At TiO2 content ≥5 wt%, the curve  = f(TiO2 wt%) levels off with  tending to 100%. Thus, this indicates that the samples at 5 and 10 wt% TiO2 show a significant but variable response of TiO2 on the whole surface analyzed. The mean intensity of TiO2 bands for each 8800 ␮m2 scanned spot is represented in Fig. 4(b) as a function of the amount of TiO2 added to the cement. The intensity increases linearly with the quantity of TiO2 added to the mortar indicating that the surface TiO2 content in a volume of 8800 ␮m3 is proportional to the TiO2 content in the sample bulk. It can be noted that the signal intensity even increases between 5 and 10 wt%, although a full occurrence rate in TiO2 is already obtained at 5 wt%. This is due to Raman spectroscopic analysis, which actually takes into account the thickness of included titania. The different hydrated phases and TiO2 maps are represented in Fig. 5 for a mortar loaded with 5 wt% of TiO2 . The maps show the distribution of TiO2 , C–S–H, CaCO3 and Ca(OH)2 phases from Fig. 5(a)–(d), respectively. Fig. 5(e) exhibits the zones where the phases are mainly present. TiO2 , C–S–H, CaCO3 and Ca(OH)2 are represented by orange, blue, green and pink colors, respectively. From the image analyses of the different hydrated phases and of TiO2 , it seems that, where CaCO3 is present in a higher concentration (white color of the map), TiO2 is correlatively also present in a higher level. Two explanations could explain this observation: either TiO2 could promote the carbonatation of C–S–H and of Ca(OH)2 or CaCO3 renders TiO2 more sensitive to Raman spectroscopy.

Since white Portland cement contains 0.17 wt% TiO2 , the 0 wt% titania mortar was chosen as the blank reference sample. To determine if TiO2 is detected or not at the surface, the local Raman response of TiO2 must be higher than the TiO2 mean Raman

3.1.2. Diffuse reflectance spectroscopy To characterize the behavior of TiO2 -loaded samples under UV-light the diffuse reflectance spectra were recorded for the different cementitious samples (Fig. 6(a)). All the samples show the

=

area where TiO2 is detected total analyzed area

A.H. Aïssa et al. / Applied Catalysis B: Environmental 107 (2011) 1–8

a

b

100

5

40000 35000 30000

Intensity [a.u.]

[%]

80

60

40

25000 20000 15000 10000

20

5000 0

0

2

4

6

8

0

10

0

2

4

TiO2 [wt%]

6

8

10

TiO2 [wt%]

Fig. 4. (a) TiO2 occurrence rate (or presence probability) at the surface of mortar samples and (b) mean intensity of TiO2 peaks observed on the TiO2 -mapping as a function of the TiO2 wt content in the Portland cement. Error bars are defined considering standard deviations.

same behavior in the visible, with a reflectance of ca. 60%, with a very slightly lower value for 0 and 1 wt%. In the UV-region, the reflection decreases abruptly at  ≤ 390 nm when decreasing the wavelengths. This is typical of the UV-absorptance of TiO2 related to its energy band gap at EG = 3.2 eV. In addition, the reflectance increases with the TiO2 content in wt%. Concerning the 0 wt% TiO2 mortar, there is a continuous slightly decreasing absorptance inherent to the material itself with no gap present at  ≤ 390 nm. To determine the behavior under UV-light of TiO2 present on the mortar surface, the mean UV-absorptance of TiO2 , AUV , was calculated between 250 and 400 nm, according to Eq. (3) with the 0 wt% TiO2 mortar used as zero. Considering that the light scattering

is the same for all our samples, the UV-absorptance due to TiO2 in the material is estimated as:



400

AUV

1 = 400 − 250

(R0%

TiO2

− RX% TiO2 )d

(3)

250

UV-absorptance is plotted in Fig. 6(b) as a function of the TiO2 content. The absorptance increases substantially with the amount of deposited TiO2 till tending to a plateau for TiO2 % ≥5 wt%. This behavior shows that the increase of TiO2 wt% in the materials does not improve the absorptance of UV-light above 5 wt% in parallel to the variation of occurrence rate  expressed in Fig. 4(a). In Fig. 6(c),

Fig. 5. Raman mapping of: (a) TiO2 (orange); (b) C–S–H (blue); (c) CaCO3 (green); (d) Ca(OH)2 (pink) of 5 wt% TiO2 mortar and (e) localisation of the different solid phases. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)

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100

25

80

20

Cformaldehyde [ppm]

R [%]

a

60 (a1)

40

(a2) (a3)

20

Dark

(a)

15 (b) 10 (c)

(a4)

Introduction of formaldehyde in the reactor in the dark

5

(a5)

0 250

450

650

850

λ [nm]

UV

0

0

2

4

6

t [h]

b

0,35

Fig. 7. Temporal changes in formaldehyde concentration during different photocatalytic tests: (a) Teflon used as a blank sample; (b) 0 wt% TiO2 mortar; (c) 5 wt% TiO2 mortar (molar flow D = 9.4 ± 0.5 ␮mol h−1 ; UV irradiance EUV = 3.7 ± 0.4 mW cm−2 ; flow rate F = 200 ml min−1 , relative humidity 3%).

0,30

AUV [a.u.]

0,25

3.2. Photocatalytic degradation of formaldehyde

0,20

The photocatalytic activities of 5 mortar samples for formaldehyde removal in air and at room temperature (25 ± 3 ◦ C) were determined by following the change in the temporal concentration of HCHO as presented in Fig. 7.

0,15 0,10 0,05 0,00 0

2

4

6

8

10

TiO2 [wt%]

c

0,35 0,30

AUV [a.u.]

0,25 0,20 0,15 0,10 0,05 0,00 0

20

40

60

80

100

θ [%] Fig. 6. (a) UV–visible diffuse reflectance spectra of mortar containing (a1) 0 wt% TiO2 , (a2) 1 wt% TiO2 , (a3) 2.5 wt% TiO2 , (a4) 5 wt% TiO2 , (a5) 10 wt% TiO2 (the reference is based BaSO4 ), (b) UV-absorptance of mortars as a function of their weight percent of TiO2 , (c) Relationship between the UV-absorptance of TiO2 and its surface occurrence rate. Error bars are defined considering standard deviations.

a direct proportional relationship between the UV-absorptance and the occurrence rate of TiO2 at the surface of the mortar is observed, suggesting that all TiO2 detectable by Raman spectroscopy can absorb UV-photons.

3.2.1. Preliminary behavior in the dark Before starting the photocatalytic degradation reaction, formaldehyde was first introduced in the reactor in the dark. The blank test was carried out on the Teflon sample as illustrated in Fig. 7(a). The concentration of formaldehyde during the blank test initially decreases when the polluted gas is sent through the reactor but it rapidly returns to its initial value. This decrease corresponds to the filling of the reactor by the gas phase inducing a possible limited adsorption on the reactor walls. By contrast, when formaldehyde is introduced in the reactor in presence of the mortar sample, with and without TiO2 maintained in the dark, the concentration of formaldehyde decreases and reaches a stable value from which it does not return to its initial value (Fig. 7). Experiment performed during 72 h in dark shows that no return to the initial value of formaldehyde was occurred. The reason for this behavior is not due to the adsorption of formaldehyde on TiO2 since it similarly occurs on the pristine material without TiO2 . Several experiments have been performed in order to elucidate what kind of process is happening between the formaldehyde and the mortar, Raman spectroscopy and DRIFT of mortar with formaldehyde solution deposited. However, the results obtained did not allow to establish the presence of intermediates due to the complexity of the cementitious matrix. Moreover, no CO2 was evolved in the gas-phase with a sensibility around 1 ppm. Additionally, the basic character of mortars helps in trapping any acidic intermediates formed. The percentage of formaldehyde removal due to the cementitious matrix fluctuates between 40% and 65%. 3.2.2. Disappearance of formaldehyde under UV-illumination For the Teflon sample, when the light is switched on, formaldehyde concentration remains at the same level, meaning that formaldehyde is not degraded by pure photochemistry ( > 290 nm).

A.H. Aïssa et al. / Applied Catalysis B: Environmental 107 (2011) 1–8

rdeg [µmol.h-1.m-2]

a

600

after at least 2 h of UV-irradiation. The formaldehyde degradation rate was determined after 5 h of UV-irradiation. At steady-state it was defined as the quantity of formaldehyde degraded per time unit of and per unit of solid surface area, according to Eq. (4).

400

rdeg =

200

0

0

2

4

6

8

10

TiO2 [wt%]

rdeg [µmol.h-1.m-2]

b

7

600

400

200

0

0

20

40

60

80

100

c

600

rdeg [µmol.h-1.m-2]

θ [%]

400

(Cdark − CUV )F Sirr

(4)

where rdeg is the degradation rate in ␮mol h−1 m−2 , Cdark is the formaldehyde concentration in the reactor before irradiation, CUV is the concentration during irradiation (in ␮mol l−1 ), F is the gas flow rate (in l h−1 ) and Sirr is the irradiated surface area of the sample (in m2 ). Fig. 8(a) gives the formaldehyde degradation rate as a function of the TiO2 content added in the cement from 0 to 10 wt%. At low contents of TiO2 , the reproducibility of experiments is difficult to be achieved. One of the reasons is the important removal of formaldehyde induced by the cementitious matrix itself. Under the experimental conditions, the degradation rate increases proportionally with the TiO2 content. However, above 5 wt% TiO2 , the degradation rate levels off and reaches plateau at a rate of ca. 450 ␮mol h−1 m−2 . Strini et al. [15]. and Poon and Cheung [10] observed the same behavior of the photocatalytic activity of cement vs. TiO2 content during the degradation of BTEX and NO respectively. However, Husken et al. [20] found a linear increase of NO degradation in the same range of TiO2 percentage. The curve r = f(TiO2 wt%) in Fig. 8(a) is very important since it clearly demonstrates that the process is really catalytic and photocatalytic. Indeed, the curves parallels those obtained in all types of photocatalytic reactors, used in gaseous, liquid and aqueous phase reactions as previously established by one of us [1,4]. The photocatalytic degradation rate of formaldehyde is plotted as a function of the surface occurrence rate in TiO2 determined by Raman spectroscopy (Fig. 8(b)). The plot shows a linear dependency between the degradation rate and the occurrence rate in TiO2 . The photocatalytic degradation rate of formaldehyde is optimal at full occurrence rate in TiO2 . In Fig. 8(c), the formaldehyde photocatalytic degradation rate is represented as a function of the UV-absorptance measured by diffuse reflectance spectroscopy. The degradation rate is proportional to the UV-absorptance above a minimum absorptance value of 0.10 related to the own absorptance of the mortar. 4. Conclusions

200

0

0

0,1

0,2

0,3

AUV [a.u.] Fig. 8. (a) Photocatalytic rate of formaldehyde degradation as a function of TiO2 (D = 9.4 ± 0.5 ␮mol h−1 ; EUV = 3.7 ± 0.4 mW cm−2 ; F = 200 ml min−1 , 3% RH). Correlations between the photocatalytic degradation rate of formaldehyde and (b) the surface occurrence rate of TiO2 and (c) UV-absorptance of TiO2 for 0–10 wt% TiO2 mortar. Error bars are defined considering standard deviations.

Under UV-irradiation, the concentration of formaldehyde only decreases when in presence of TiO2 , putting in evidence the photocatalytic ability of TiO2 -containing mortar to photodegrade formaldehyde. The photocatalytic degradation of formaldehyde reaches a steady-state equilibrium, since CUV remains constant

It has been clearly demonstrated that cementitious materials containing titania could not only be self-cleaning as previously described but could also participate to the cleaning of ambient air containing VOC’s, such as formaldehyde which is the most commonly found air pollutant in outdoor urban atmospheres, provided it might come in contact with their surface by convection and turbulence. First, a thorough description of titania distribution at the surface of the mortar samples prepared in standardized conditions was successfully obtained by Raman spectroscopy mapping. The photocatalytic activities followed the same basic rules as those observed in laboratory experiments with respect to the mass and the exposed area of added titania. A full description of the reaction pathway of the total degradation reaction of formaldehyde was described. An optimum weight percentage of titania corresponding to 5 wt% with respect to the cement (before addition to the sand) was carefully determined. It was shown that formaldehyde can be absorbed by the mortar. Some intermediates such as formic acid can follow the same way as well as the final CO2 produced but this corresponds to the surface properties of mortars and concretes which are in permanent interactions with ambient atmospheres.

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Acknowledgments This project is funded by CTG Italcementi France. The authors thank the service of Raman spectroscopy of IRCELYON. References

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