Joanna Wojciechowska

Dr Marcin Jędrzejczyk

Dr Jacek Grams

Dr Nicolas Keller
email: nkeller@unistra.fr

Dr Agnieszka M Ruppert
email: agnieszka.ruppert@p.lodz.pl

Enhanced production of 𝛾-valerolactone with internal source of hydrogen on Ca-modified TiO 2 supported Ru catalysts

Keywords: platform molecules, 𝛾-valerolactone, Ru/Ca-TiO2 catalysts, photo-assisted preparation method, formic acid

In this work, calcium-modified titania supported Ru catalysts were synthesized and evaluated in the levulinic acid hydrogenation with formic acid as internal hydrogen source and water as green solvent. A new elegant photo-assisted method allowing the synthesis of uniform size and evenly distributed Ru particles on the titania surface was developed. Compared to the counterpart catalysts prepared by classical wet impregnation, enhanced levulinic acid conversion and 𝛾-valerolactone yield were reached, and further exalted thanks to the support modification via the calcium introduction into the titania support. This synthesis approach resulted in a change of the surface and bulk properties of the support, namely a decrease in the anatase crystallite size and the formation of new calcium titanate phase. In consequence the properties of the catalysts were modified, and smaller ruthenium particles interacting stronger with the support were developed. This affected the strength of CO adsorption on the catalyst surface and facilitated the reaction performance. The optimum size of Ru particles allowing for most efficient levulinic acid conversion was established.

Introduction

The depletion of fossil fuel reserves associated to the necessity of limiting the worldwide CO2 release has been the driving force over the last decades in the search for alternative value chains for producing sustainable fuels and chemicals. Value added platform molecules issued from the conversion of lignocellulosic biomass feedstock are expected to play a pivotal role in the move towards future 'renewable-based chemicals' biorefinery schemes. [1] In this frame, 𝛾-valerolactone (GVL) has attracted a significant attention since it is used in many potential versatile applications, as biofuel additive thanks to a low volatility, a minimum toxicity and a good stability, as well as eg. as solvent or precursor in chemical syntheses. [2] GVL can be obtained by catalytic hydrogenation of levulinic acid (LA) issued from the hydrolytic conversion of lignocellulosic biomass, most of the studies being performed with a pressurized external source of hydrogen. [3] Ruthenium remains often a metal of choice for LA hydrogenation in water solvent due to its outstanding performance, which is related to the decrease of its energetic span in the presence of a protic solvent. [4] The nature of the catalyst support was also reported to be of high importance, and TiO2 was proved to be a promising candidate for supporting Ru nanoparticles for producing GVL. Indeed, Lange indicated that TiO2 was a support of high interest due to good mechanical and thermal stability under hydrothermal, acidic conditions. [5] Later on, we evidenced that the phase composition of the TiO2 support directly influenced the catalytic performance of the Ru catalysts, the highest activity being observed for an anatase/rutile mixed phase support when compared to the counterpart catalysts on the pure anatase or rutile phase supports. [6] This behavior was related to the selective deposition of Ru with optimum size distribution on the rutile fraction of the mixed phase support. By contrast, the formation of either too small Ru crystallites too strongly interacting with pure large surface area rutile, or of too large aggregates of Ru on the pure anatase phase, resulted in both cases in catalysts with low activity.

Kuwahara et al. studied the catalytic transfer hydrogenation of levulinate esters with alcohols as hydrogen donors using Ru(OH)x/TiO2 catalysts, and evidenced that high surface area anatase TiO2 supported highly-dispersed Ru(OH)x nano-clusters were the most active catalyst. [7] Furthermore, Tan et al. compared commercial titania and zirconia materials as supports for Ru catalyst for performing the reaction in water phase. They claimed that titania is more suitable than ZrO2, as the strong metal-support interaction between Ru and TiO2 facilitates the metal dispersion and therefore increases the catalyst stability. [8] The proper modification however allows also to prepare Ru/ZrO2 of high stability and activity. [9] Optimum dispersion and distribution of Ru on the TiO2 support was recently proposed for explaining the high activity and stability displayed by a 5%Ru/TiO2 catalyst in the continuous flow hydrogenation of methyl levulinate when compared to commercial Ru/C and Ru/Al2O3 reference catalysts. [10] In addition, Li et al. showed that the high activity and selectivity achieved on anatase TiO2 supported Ru catalysts were attributed to enhanced metal-support interaction thanks to intimate contact between Ru and anatase nanoparticles and to the Ru nano-size. A stronger electron donation from TiO2 to Ru and an easier reducibility of Ru species were proposed to modify both activity and selectivity patterns and to boost Ru for the selective transformation of substrates. [11] So far, most of the studies used externally supplied H2 for performing the hydrogenation of LA into GVL. However, a step forward in the design of sustainable biomass hydrogenation processes can be achieved by using formic acid (FA) as internal source of H2. Indeed, the acidic hydrolysis of lignocellulosic biomass forms an LA/FA equimolar mixture, while FA can be further selectively decomposed into H2. So far this internal H2 supply approach was not investigated to such extend as the LA hydrogenation using high pressure H2, and is more challenging, since the strategy requires that both the hydrogenation of LA to GVL and the FA dehydrogenation can be catalyzed by a single material in similar reaction conditions. [12] Taking into account that FA can decompose either via dehydrogenation into H2 and CO2 or via dehydration into CO2 and CO -with possible side reactions like the water-gas-shift reaction or Fischer-Tropsch with methane production, the first step H2 formation from FA is often considered as a limiting factor. Indeed, FA dehydration and side reactions negatively impact on the H2 selectivity while CO acts as a poison for the active sites, so that designing selective and robust catalysts still remains a crucial challenge.

Although being very active for LA hydrogenation, Ru catalysts are known to also catalyze side reactions that affect strongly the selectivity to hydrogen of the FA decomposition, so there the implementation of adequate modification of Ru catalysts remains actually an approach of high interest in this respect. One of the strategies for selectivity improvement relates with the introduction of bases to the reaction system, either by addition of free bases to the reaction mixture or by introducing directly basic sites to the catalyst surface through material modification. For instance, high TOF up to 1590 h -1 were achieved at 50°C on Au/ ZrO2 with the addition of NEt3 maintaining a low CO level below 5 ppm. [13] On another hand, the design of heterogeneous catalysts with basic sites is an elegant and more sustainable approach for avoiding separation issues resulting from the continuous supply of the reaction media with high amounts of amine additives. Further, the presence of suitable surface groups in a close vicinity of metal active centers was already proposed to positively impact on the catalytic activity. Recently, great improvement was achieved in terms of hydrogen production from formic acid in vapour phase with a very high selectivity thanks to the addition of K to a Au/Al2O3 and Pd/C catalysts through incipient wetness impregnation. [14,15] A proof of concept for the use of Ru catalysts with supports exposing basic surface sites was already demonstrated in the LA hydrogenation with external hydrogen supply. Indeed, Ru supported on Mg-LaO mixed oxide obtained from hydrotalcite was efficiently used as catalyst for producing GVL. [16] However no explanation for the role of basic sites in this system was proposed. Further, Cao et al. showed the beneficial effect of adding potassium to Ru/C and Ru/Al2O3 catalysts via KNO3 impregnation in the LA hydrogenation at 220°C under 13.8 bars of H2 using 1,4-dioxane as solvent. [17] The higher activity of Kmodified Ru catalysts was ascribed to an increase of electron donation from the K promotor to the Ru nanoparticles, providing enhanced metallic character to Ru, as well as to the decreased CO adsorption on K-doped materials. They proposed that LA hydrogenation is structure sensitive and depends on the Ru particle size, with an activity maximum at about 1.5 nm.

Here, we show the synthesis of Ca-modified TiO2 supported Ru catalysts and their application in the hydrogenation of LA towards GVL with FA as internal hydrogen source and water as green solvent. We evidenced that the controlled modification of the TiO2 support by calcium influenced both bulk and surface properties of the Ru catalysts and positively impacted on the FA dehydrogenation step as well as on the LA hydrogenation reaction. Further, an elegant photo-assisted synthesis method under solar light allowing the synthesis of Ru catalysts with enhanced GVL formation was developed for subsequently investigating the effect of the Ru particle size in the transfer hydrogenation as well as in both FA dehydrogenation and LA hydrogenation separated reactions, and for getting insight on the catalyst features playing a role in the achievement of enhanced performances in the process.

Results and Discussion

Materials characterization

Characterization of TiO2 and Ca-modified TiO2 supports

Figure 1 shows the powder XRD patterns of the TiO2-based supports. Whether TiO2 was modified with Ca or not, they exhibited diffraction reflexes corresponding to anatase TiO2 (JCPDS card 21-1272), with the characteristic most intense peak at 25.3° corresponding to the diffraction of (101) planes, no rutile phase being observed, except in the case of the TiO2 P25 commercial reference. For low Ca content, no phase related to the presence of Ca such as oxides or hydroxides was observed, in agreement with previous works on the modification of sol-gel TiO2 by Ca ions. [19,20] Increasing the Ca content resulted in the progressive appearance of a weak and broad diffraction signal at 2θ = 31.0° assigned to the Ca2Ti2O6 phase (JCPDS card 40-0103), and in the observation of clear diffraction peaks corresponding to both CaTiO3 (JCPDS cards 78-1013) and Ca2Ti2O6 phases) for the 20Ca-T500. It is however difficult to distinguish between those two phases of calcium titanate due to signal overlaps. This was accompanied with a strong decrease in the intensity of the anatase diffraction peaks, in agreement with the work of Al-Salim et al. that showed that increasing the content of alkaline earth ion such as Ca 2+ in the sol-gel synthesis of TiO2 strongly increased the amorphous content. [20] Figure 1. Powder XRD pattern of the support materials a) TiO2 P25 ;b) T500 ; c) 1Ca-T500 ; d) 5Ca-T500 ; e) 10Ca-T500 ; f) 20Ca-T500. (vertical lines) diffraction peaks corresponding to the anatase TiO2 phase ; (!) most intense peaks corresponding to the titanate phases.

Whether calcium titanate phases were observed or not, the modification of titania with Ca 2+ ions caused also some distortion in the TiO2 lattice, as a result of the intermediate ionic radius of Ca 2+ at 1.06 Å being larger than that of Ti 4+ (0.64 Å) and smaller than that of O 2-(1.31 Å), so that Ca 2+ ions could be either isomorphously substituted or interstitially introduced into the TiO2 lattice with the corresponding formation of oxygen vacancies or of Ti 3+ centers, respectively. In agreement with Al-Salim et al. [20] and Hameed et al., [19] this was experimentally visualized by an increase in the a cell parameter and a decrease in the c parameter, resulting in an overall change in the anatase cell volume (Table 1). Both commercial TiO2 P25 and sol-gel TiO2 T500 displayed the largest anatase mean crystallite size at 21 and 25 nm, respectively, whereas the Ca-modified TiO2 materials exhibited a lower mean crystallite size at 13-16 nm, with no significant differences between the different Ca contents. This indicated that the modification with Ca 2+ can inhibit the TiO2 crystallite growth, due to lower nucleation. This TiO2 crystal size decrease upon doping with alkaline earth ions was previously reported by Al-Salim et al. and Fu et al., [20,21] as well as in the case of the cationic modification of TiO2 and more generally of transition metal oxides nanomaterials. The smaller mean anatase crystallite size of Ca-modified samples compared to that of the Ca-free reference correlated well with their higher non-microporous BET specific surface area in the 40-60 m 2 /g range vs. 27 m 2 /g for the TiO2 T500 (Table 1). The decrease in surface area when increasing the Ca content could be explained by the slight increase in the mean crystallite size of anatase. It is worth noting that the surface area and the pore volume of Ca-TiO2 materials remained lower than those of the TiO2 P25, probably due to the existence of grain boundaries between crystallites in the samples that lower the overall specific surface area. One could note that modifying TiO2 with Ca 2+ led to a decrease in the isoelectric point of the supports, with values within the 2.4-3.0 range for Ca-modified TiO2 materials compared to 3.4 for the and 4.5 for the TiO2 T500 reference and the TiO2 P25.

This decrease in isoelectric point might be related to the formation of new basic sites at the support surface. Indeed, it has been reported that the doping of metal oxide by Ca promotor can cause an increase in the number and strength of basic sites, in the form of coordinately unsaturated oxygen ions O 2-. [22,23] For instance, the nature of alumina supports has been reported to be strongly shifted from acidic to basic by addition of growing amount of Ca. [22,23] Therefore the Ca-modified samples required the addition of a higher amount of H + during the zeta potential measurement for neutralizing the more negative surface by charge compensation, so resulting in the measurement of a lower isoelectric point. Further, the slight increase of the isoelectric point for 10 wt.% of Ca might result from the formation in large amount of CaTiO3 phase at the support surface, or from the increase in the amorphous content of the support.

The SEM images shown in Figure 2 revealed the morphological changes observed when modifying the TiO2 with calcium. While the Ca-free TiO2 reference displayed the classical morphology of sol-gel TiO2 materials, the incorporation of calcium into the support increased the heterogeneity of the material surface. This property may be assigned to the smaller TiO2 crystallite size and the progressive appearance of the calcium titanate phases when increasing the calcium content. Indeed, the sample with 20 wt.% of Ca revealed the most altered morphology with the appearance of melted-like small size particles. Whatever the Ca content, the Ti, Ca and O elemental mapping evidenced the homogeneous distribution of the calcium heteroatoms in the TiO2-based supports, whether the Ca ions distributed within the TiO2 lattice or the Ca ions from the titanate phase are concerned. In addition, the Ca contents derived from the EDS analysis were in good agreement with the theoretical values from the modified sol-gel syntheses, confirming that the Ca species were evenly distributed in the samples. [a] The mean TiO2 crystallite size, i.e. the average size of the coherent diffracting domains, was derived from XRD measurement through the Scherrer equation applied to the (101) peak of anatase at 25.7° with the usual assumption of spherical crystallites. [b] calculated on the anatase phase ; [c] Derived from XRD patterns (recorded for 12 h) by the Rietveld refinement method using the FULLPROF software. [18] Characterization of Ru catalysts

Figure 3 shows representative TEM images of Ru-based catalysts prepared via impregnation and thermal reduction under hydrogen, along with the corresponding Ru nanoparticle size distributions. The Ru/P25-I sample exhibited the smallest mean particle size of 2.9 nm with the narrowest size distribution (FWHM = 1.9). By contrast, the Ru/T500-I sample was characterized by a broader nanoparticle size distribution (FWHM = 6.2), centered on a much larger mean Ru nanoparticle size at 5 nm, together with the contribution of very large particles, up to 18 nm in size. Modifying the support with 10% of Ca improved the dispersion of the supported nanoparticles, with a reduced mean Ru particle size at 3.8 nm and a more homogeneous size distribution (FWHM = 2.4) The TEM images of the Ru/TiO2 catalysts prepared via the photodeposition method are presented in Figure 4, together with the corresponding Ru nanoparticle size distributions. Whatever the TiO2 support, the nanoparticle size distributions strongly differed from those achieved for Ru catalysts prepared by classical incipient wet impregnation. The supported nanoparticles were homogeneously dispersed and no ruthenium aggregates were observed. Their metallic character was evidenced by TEM and X-Ray Photoelectron Spectroscopy analyses shown in Supporting Information SI 1 and SI 2. Small Ru nanoparticles were synthesized on the supports, with sharp nanoparticle size distributions centered on remarkably small particle sizes. Recently, sub-nanometric and narrow particle size distributions were obtained for Ru/TiO2 catalysts. [24] Similarly here the smallest and sharpest Ru nanoparticle size distribution was obtained on the Ru/P25-P catalyst with a sub-nanometric mean size of 0.8 nm and a very small FWHM of 0.3 nm, while the Ru/T500-P catalyst displayed the largest particle size centered on 3 nm with the largest FWHM of 1.8 nm. Modifying the TiO2 support with 1-5 wt.% of Ca led to reduce the mean Ru particle size down to 1.4-1.5 nm with a much sharper size distribution (FWHM=0.6-0.8 nm) when compared to that obtained on the sol-gel TiO2 reference. Only in the case of the Ru/10Ca-T500-P catalyst, a bimodal distribution was identified, with the significant contribution of larger particles centered around 5 nm in size in parallel to the well-calibrated small size particles at 1.8 nm. This bimodal distribution might result from the increased amount of amorphous phase in the material when increasing the Ca content or from different bulk and surface charge recombination rates between both CaTiO3 and TiO2 semiconductor phases, that in both cases might impact the photogenerated charge dynamics at the support surface during the UV-A light-assisted preparation method. It could be evidenced that the use of the photodeposition preparation method allowed to prepare Ru/TiO2 catalysts with much smaller Ru nanoparticle size than when using the classical incipient wet impregnation. The photo-deposition method takes advantage of the redox photoactivity developed under UV-A light by the TiO2 semi-conductor support for promoting electrons to the conduction band of the semi-conductor, further able to directly reduce at room temperature highly dispersed preadsorbed metal ions at the surface of the support. This lowtemperature photoassisted redox strategy has been reported to offer a potential control over the Ru nanoparticle size and their size distribution, allowing the synthesis of well-calibrated metallic nanoparticles to be achieved. [24][25][26] The high resolution TEM image of the Ru/5Ca-T500-P catalyst shown in Figure 5 confirmed the coexistence of both the anatase TiO2 and the calcium titanate phase inside the support, and of the nanosized metallic Ru nanoparticles.

Temperature programed reduction (TPR) measurements

To study the reducibility of the Ru/TiO2 catalysts prepared by impregnation and photodeposition as well as to understand the influence of the Ca modification on the physicochemical properties of the catalysts, temperature programed reduction (TPR) measurements were performed. TPR profiles are only shown for Ru catalysts prepared by impregnation, since the photodeposition method led to directly synthesize metallic Ru nanoparticles on the support (SI 1 and SI 2). Figure 6 show that all the catalysts prepared by impregnation exhibit one main reduction maximum. On both commercial and sol-gel titania, the profiles look very similar, with a maximum at 110°C that could be most probably attributed to the reduction of well dispersed crystalline RuO2. [27,28] The very low H2 consumption with no clear maximum observed at lower temperature before the main signal could be assigned to the presence of well dispersed, small and amorphous RuO2. [29] With the addition of Ca to the support, the reduction maximum is shifted towards higher temperatures, from 140°C for 1%, to 200-210°C for the highest Ca content, indicating stronger interactions of RuOx with the support. This trend with increasing the amount of Ca could be related to the increase in the content of the CaTiO3 phase on which Ru is evenly distributed. This can result in stronger interaction with the support, leading to the shift in the reduction peak. Similar effect of strengthening of the metal-support interaction was observed by Carrero et al. in Mg and Ca-promoted Ni-Cu/SiO2 catalysts. [30] Moreover in the case of the Ca-modified samples, a third smaller and very broad shoulder is observed between 190°C-300°C, which increases with the Ca loading. This could be associated with the RuOx with strong interactions with the CaTiO3 phase.

CO-FTIR studies

FTIR spectra of adsorbed CO are presented in Figure 7 for catalysts prepared by impregnation. The spectra show bands at frequencies 2138 cm -1 , 2084 cm -1 and 2016 cm -1 with a shoulder at 1980 cm -1 that are attributed to CO adsorbed on Ru crystallites. In addition, in the spectra of samples containing 5%, 10% and 20% of Ca, a band at 1620 cm -1 is visible. It has to be mentioned that in the case of supported Ru catalysts, there is an ongoing discussion on the interpretation of the FTIR spectra and on the peak assignment. [31][32][33][34][35][36][37] Both bands at 2084 cm -1 and 2138 cm -1 are assigned to the multicoordinated CO adsorption on electron deficient ruthenium particles Ru δ+ . [31][32][33][34][35][36][37] The presence of positively-charged metal particles may result from its strong interaction with the carrier or the presence of chlorine. [34,36,37] Another mechanism involves the oxidation of metallic Ru induced by the reaction of adsorbed CO and hydroxyl groups of the support. [31] The band at 2016 cm -1 with a shoulder at 1980 cm -1 is attributed to CO linearly adsorbed on Ru sites. [35][36][37] In this region, there are bands which can be ascribed to the linear CO adsorption on faces and steps of Ru crystallites. It is however difficult to clearly differentiate between them as their exact position depends on the CO coverage.

On one hand, according to Blyholder et al., the shoulder at 1980 cm -1 represents the CO adsorption on edges, corners or dislocations of Ru particles. CO adsorption on lower coordination sites (like corners or edges) is stronger due to the reduced competition for electrons involved in the back-donation from the central Ru atom to carbon. [38] On another hand, this band was assigned by Elmasides at al. to CO adsorbed on metal sites strongly interacting with the support in Ca-doped Ru/ TiO2 materials. [39] This is in agreement, first with Figure 7, evidencing that the contribution of this shoulder becomes more prominent (in respect of the 2016 cm -1 band) for samples containing more Ca, and further with the TPR results, showing that increasing the Ca content in the support facilitates stronger Ru-support interactions.

Furthermore, the band located at 1620 cm -1 , which can be assigned to the carbonate type species formed on the surface of the available calcium cations, was evidenced in the case of the samples containing 5%, 10% and 20% of Ca.

When it comes to the intensity of different bands, it is necessary to point out that several aspects such as the metal particle size or the metal-support interactions can be responsible for the level of CO adsorbed. For Ru/P25-I, the highest CO adsorption was observed most probably due to the smallest Ru particle size. For Ru/T500-I, larger metal particles caused a drop in the amount of the adsorbed CO. Furthermore, a clear trend is observed, CO adsorption decreased with increasing the Ca content. Based on the TPR results, increasing contribution of Ca-modified support cause stronger Ru-support interactions. Therefore, it could be concluded that the CO adsorption on Ru strongly interacting with the support remains limited.

Figure 8 shows CO-FTIR spectra of the samples prepared by the photodeposition method. Analogous bands to those obtained for the materials prepared by impregnation were observed, however with a the red shift for each band of about 10-20 cm -1 . The red shift could be explained either by the smaller Ru particle size for samples prepared by photodeposition or by a decrease in the dipole-dipole coupling effect of adsorbed CO molecules with decreasing coverage. [40] The band in the region 2000-1990 cm -1 is attributed to the linear CO adsorption on Ru, while the two bands at 2063 cm -1 and 2126 cm -1 represent the multicoordinated CO adsorption on Ru δ+ . The relationship between the particle size and the CO adsorption was similar for samples prepared by photodeposition, namely the highest intensity of the bands was identified on smallest particles of Ru/P25-P. A similar CO adsorption was observed whatever the Ca content of the modified TiO2 materials, in agreement with the TPR analyses, for which it was concluded that all the tested samples exhibited the same type of metal-support interactions.

Signals at 1458 cm -1 and 1554 cm -1 which are especially identified for Ru/20Ca-T500 correspond to the carbonate species adsorbed on the support.

Analysis of the catalysts surface by ToF-SIMS analysis

ToF-SIMS analysis of the catalyst surface was performed in order to understand the effect of the calcium modification and the ruthenium interactions with the support (Table 2).

It was observed that the signals from ruthenium and the Ru + / Ti + ratio are the lowest in the case of Ru/T500-I and Ru/1Ca-T500-I among the samples prepared by the impregnation method. Moreover, by comparing the catalysts prepared by the two different methods, a slightly lower Ru intensity is observed for the ones prepared by impregnation. This can be explained by the partial coverage of Ru by titania migrating during the thermal treatment which occurs to higher extent for samples prepared by impregnation. [41] This suggests also that the modification with Ca could stabilize the support to some extent. The smaller Ru intensity could be also related with the presence of larger crystallites.

It is worth noting that the intensities of Ru signal observed for those samples prepared by photodeposition are similar to each other meaning that metal particles deposited on the surface are within the same size range. Based on the comparison of the Ru + /Ti + ratio, it is possible to say that either Ru is more exposed on the surface of TiO2 in the case of the catalysts with higher Ru/10Ca-T500-P 2.6 9.0 7.0 1.9

Ru/20Ca-T500-P 2.2 6.0 9.1 2.2 amount of Ca (5, 10 and 20%). This effect is more pronounced in the case of the catalysts prepared by impregnation. Further, the intensities of the signal originating from Ca + ion confirmed that there is more Ca on the surface when a higher amount of Ca was used for the support preparation whatever the synthesis method. It is however important to note that generally the intensity of the signals coming from Ca + are lower for the samples prepared by photodeposition, indicating that Ca is less exposed on the catalyst surface. The presence of CaRuO2 + ion confirms the existence of the interactions between the Ca-based phase and Ru, the intensity of the CaRuO2 + being globally higher for Ca-rich catalysts whatever the preparation method.

Catalytic activity

The catalytic activity of the different materials was evaluated in the simultaneous FA decomposition and LA hydrogenation towards GVL (FALA). Figure 9 shows both liquid and gaseous products formed in the reaction using Ru catalysts prepared by impregnation. First, full decomposition of FA was achieved with all catalysts. Further, the Ca-modified Ru catalysts performed much better than Ru supported on bare sol-gel TiO2 and commercial P25, with the highest LA conversion (38%) and the highest GVL yield (30%) obtained for Ru/5Ca-T500-I. When it comes to distribution of gaseous products, expressed as vol%, it can be concluded that FA is decomposed most probably via two pathways, i.e. dehydration and dehydrogenation, leading to the formation of both CO and CO2. Methane was also identified in the gas phase as a result of a secondary reaction (CO + 3H2 → CH4 + H2O). The higher amount of CO detected in the case of Ru/5Ca-T500-I could be most probably explained by the fact that less CO remained adsorbed on the catalyst surface. CO could be also consumed for the methane formation.

The catalysts prepared by the photodeposition method performed much better in the FALA reaction, while they exhibited the same trend in terms of LA conversion and GVL yield as their counterparts prepared by impregnation (Figure 10). The highest yield also occurred for Ru/5Ca-T500-P giving almost 50% of the final product, however this maximum was less pronounced than for the catalysts prepared by impregnation.

Whatever the Ru catalyst, negligible Ru leaching to the solution was observed, and Ru concentrations ranging from 0 to 0.34 ppm were released to the reaction mixture. Further, since the catalysts were used in the reaction without any thermal reduction, it was interesting to check how they behave when additionally reduced in the same conditions than the ones prepared by impregnation. The reduced sample Ru/ 5Ca-T500-P showed much lower performance and two times lower LA conversion and GVL yield was achieved despite a full decomposition of formic acid. The distributions of gaseous products are quite similar to those obtained on their counterparts prepared by impregnation, meaning that two decomposition pathways of FA also took place. In all cases, formation of CH4 was evidenced. On one hand, the much higher volume of H2 recorded for Ru/5Ca-T500-P and Ru/10Ca-T500-P samples as well for Ru/P25-P could mean that a small Ru size improves the selectivity of FA decomposition, while by contrast the hydrogenation reaction was blocked by strongly adsorbed CO in this case.

The catalysts were also tested separately in both FA decomposition and LA hydrogenation (Figure 11 and12) to evaluate in more details the influence of the Ca modification and of the new preparation method in each step of the GVL synthesis. Compared to FALA reaction, similar trends were observed for both types of materials. In both separated reactions, the catalysts prepared by photodeposition led to higher catalytic performances compared to their counterparts prepared by impregnation, in terms of levulinic acid conversion and valerolactone yield in the LA hydrogenation, and of FA conversion with higher H2 production in the FA decomposition. First, in FA decomposition (Figure 11) it was observed that the modification with Ca 2+ improved the catalytic performance forming a clear volcano-type profile for both series of Ru catalysts, with Ru/5Ca-T500 as the best materials.

Moreover, it was also shown that the selectivity to H2 of the FA decomposition was improved using Ca-based catalysts prepared by impregnation (with less CO produced) compared to Ru/T500-I. On the other hand, the selectivity was not significantly changed in the case of catalysts prepared by photodeposition and remained similar to the Ca-based impregnated ones except for Ru/20Ca-T500 where quite a lot of CO was recorded.

Figure 12 shows the results of LA hydrogenation for both series of catalysts. It is very important to notice that the modification with Ca has a positive effect in the case of the hydrogenation reaction as well. Again, the sample containing 5% of Ca allowed to reach the highest LA conversion and the highest GVL yield, whatever the preparation method.

Discussion

In this work, we investigated the influence of several factors (Ru particle size, support modification, novel synthesis method) on the hydrogenation of LA into GVL with FA as internal hydrogen source at low temperatures in water.

Focusing firstly on the Ru particle size effect it can be seen that for both P25 and T500 supported materials the GVL yield is higher when smaller Ru nanoparticles were tested meaning those prepared by the photodeposition method. (Figure 12a) Moreover it is worth noting that small Ru size is beneficial for both processes FA decomposition (Figure 12b) and LA hydrogenation (Figure 12c). The size effect in FA dehydrogenation was investigated using mainly Pd based catalysts, although the optimum size letting to obtain highest conversion of FA is still under the debate as depends also on the reaction conditions. Generally the smallest particles showed higher activity. Although in the work of Navlani-García et al. [42] the volcano type relationship with optimum moderate Pd nanoparticle size of 3.9 nm was associated with the suitable proportion of low-coordinated (small particles) and high-coordinated (large particles) palladium atoms. The mechanism of the FA dehydrogenation on metals and metal oxides was proposed by Mars. [43] In the initial step formic acid is adsorbed on the surface and donates a proton to the catalyst leading to the formation of formate intermediate. [44,45] Then the metal-formate species undergo β-hydride elimination producing CO2 and the H -that forms H2 with previously adsorbed H + on the catalyst. [46][47][48] According to the work of Navlani-Garcia et al. regarding the Pd catalysts the adsorption of HCOO -is favored on the smaller NPs owing its high reactivity to a large number of unsaturated atoms. Therefore in our case it could be said that higher yield of FA decomposition achieved on smaller Ru supported on TiO2 is due to the larger proportion of unsaturated low coordinated Ru atoms.

In case of LA hydrogenation the higher activity of smaller Ru nanoparticles is most probably attributed to the higher number of metal active sites available. [49,50] When we consider also Ru particles supported on Ca-based supports a clear maximum of catalytic performance was observed with the NP of 1.5 nm being the most active in all reactions. Interestingly for the most active catalyst Ru/5Ca-T500 the GVL yield dropped from ca. 50% to ca. 20% after reducing at 200 o C which resulted in an increase in nanoparticle size.

The optimal Ru particles size is however not the only factor responsible for reaching high activity. Comparing two samples Ru/1Ca-T500 and Ru/5Ca-T500 with almost the same Ru particle size (1.4 and 1.5 nm) the catalyst with 5% of Ca exhibited higher activity suggesting that addition of Ca 2+ has a crucial effect as well. Based on XRD results titania modification with Ca 2+ resulted in decreased anatase crystallite size which could favor the formation of smaller metal particles on the surface of the support. [51] However the role of calcium can not be limited to the effect of the nanoparticle size. Ca 2+ ions are incorporated into the TiO2 crystal lattice together with the formation of new calcium titanate phase. This in turn has an effect on the supported Ru nanoparticles which are interacting stronger with the support when the calcium content is increasing. This is well evidenced on TPR profiles for the catalysts prepared by the impregnation method as well as on CO-FTIR spectra with the presence of the band at 1980 cm -1 assigned to the adsorption of CO on Ru strongly interacting with the support. Another prove of Ru being in direct interactions with the Ca-modified support is given by ToF-SIMS. The presence of CaRuO2 + ions was identified for catalysts containing more Ca 2+ . Changes in the metal-support interaction strength resulted in a change of the nature of CO molecule adsorption. Apparently for Ca-based materials the CO adsorption was lower which could be one reason for their generally better performance since CO could act as poison for the active metal centers.

Generally the better activity of the catalysts prepared by the photodeposition method could be explained by several factors with the smaller Ru nanoparticle size and their higher distribution being one of them.

The role of modification with Ca 2+ is also strongly associated with the increase in the concentration and strength of surface basic sites, [22,23] The presence of basic sites explains well the improved catalytic performance of Ca-based catalysts in both reactions FA decomposition and LA hydrogenation despite the catalyst preparation method. Dehydrogenation of FA is facilitated by the presence of basic sites which act as a proton scavenger in O-H bond cleavage step. [13,52,53] On the other hand the mechanism of improved catalytic performance in LA hydrogenation could be related to the activation of the carbonyl compound where basic sites would interact with π* orbital of C=O group. [54,55]

Conclusions

We have reported on the synthesis and application of novel Camodified TiO2 supported Ru catalysts in the hydrogenation of LA towards GVL with FA as internal hydrogen source and water as green solvent. We developed a sustainable one-pot photoassisted synthesis method that allowed to obtain TiO2 and Camodified TiO2 supported Ru catalysts with enhanced catalytic activity towards GVL production with FA as hydrogen source in comparison to the reference counterparts prepared by impregnation. This was possible owing to the formation of uniform, small and well-dispersed metal crystallites.

Further, we evidenced that the controlled modification of the TiO2 support by calcium addition improved the catalytic performance in both the FA dehydrogenation step and the LA hydrogenation reaction. Such modification resulted in a decreased anatase crystallite size, together with the formation of a new calcium titanate phase, leading to the stabilization of smaller ruthenium particles on the support. Stronger interaction of ruthenium with the support was observed, which affected the strength of CO adsorption on the surface of the Ca-modified catalyst, thus facilitating the reaction performance. Thanks to the newly develop method it was possible to control precisely the Ru nanoparticle size which in turn allowed to tune the size of Ru crystallites for the transfer hydrogenation reaction.

Experimental Section

Materials preparation Support synthesis. Ca-modified TiO2 supports were synthesized according to a sol-gel method in basic conditions using titanium tetraisopropoxide (TTIP, 97%, Sigma-Aldrich) and calcium nitrate (Ca(NO3)2•4H2O, 99%, Sigma-Aldrich) as precursors. First, TTIP was added to a 40 mL ethanolic solution of calcium nitrates, with calcium contents corresponding to 1, 5, 10 and 20 wt. % relatively to TiO2, before a similar volume of water was added dropwise to solution. After rectification of the pH to 9 by adding ammonia (Carlo Erba, 30% aqueous solution), the solution was left under agitation at room temperature until dry paste was obtained. The resulting powder was further dried and finally calcined at 500°C in static air for 3 h with a heating rate of 5°C/min to form the Ca-TiO2 supports. A reference TiO2 support was synthesized according to a similar sol-gel method in the absence of any calcium nitrate, and the resulting material after calcination at 500°C was labelled as T500.

Incipient wet impregnation. Ruthenium catalysts were prepared by incipient wet impregnation from aqueous RuCl3 (Sigma-Aldrich, min 40% Ru content) solutions on commercial TiO2 (Aeroxide© P25, Evonik), TiO2 prepared by sol-gel method (T500) and Ca-modified TiO2 supports to obtain the 5wt. % of the Ru loading. The catalysts were then reduced for 1 h in hydrogen flow at 200°C and labelled as 5%Ru/P25-I, 5%Ru/T500-I and 5%Ru/Ca-T500-I.

Photodeposition. Ruthenium (III) chloride hydrate (RuCl3•xH2O, min 40% Ru content, Sigma-Aldrich) was used as ruthenium precursor salt for the catalyst preparation by the photodeposition method. RuCl3 was dissolved under stirring in 100 ml of methanol for 12 h, prior to the addition of 900 ml of distilled water to give a methanol:water ratio of 1:9 v/v. Subsequently the TiO2 support was dispersed under stirring in 1 L of ruthenium solution in a beaker-type glass reactor at a 1 g/L TiO2 concentration, and with a ruthenium precursor concentration corresponding to 5 wt. % of Ru relative to TiO2. After stirring for 1 h in the dark, the pH was adjusted with NaOH to pH=8 before the suspension was subsequently stirred in the dark for another 1 h to ensure the establishment of the adsorption-desorption equilibrium prior to irradiation. The TiO2 suspension was further exposed under stirring to a 500 W/m 2 solar light irradiation within an ATLAS Suntest XLS+ reaction chamber equipped with a Xenon arc lamp NXE 2201. At each time interval, 1 mL of solution was sampled and filtrated through a 0.20 µm porosity filter (Aireka Cells) to remove the TiO2 powder if any. The deposition was followed by UV-vis spectrophotometry using a Cary 100 Scan Varian spectrophotometer monitoring the disappearance of the main absorption peak at λ=324 nm assigned to the RuCl3 precursor. After completion of the process, the catalysts were recovered by filtration and dried at 100°C for 1 h. The catalysts were labelled as 5%Ru/P25-P, 5%Ru/T500-P and 5%Ru/Ca-T500-P.

Material characterisation

The surface area measurements have been carried out on a Micrometrics Tristar 3000 using N2 as adsorbent at -196°C with a prior outgassing at 200°C overnight in order to desorb the impurities or moisture. The Brunauer-Emmett-Teller (BET) specific surface area and the total pore volume have been calculated from the N2 adsorption isotherm.

Zeta Potential measurements were conducted on a Malvern Zetasizer NanoZS system with irradiation from a 632.8 nm He-Ne laser. The zetapotential was determined from the measured electrophoretic mobility using the Smoluchowski approximation. X-ray diffraction (XRD) patterns were recorded on a D8 Advance Bruker diffractometer in a θ/θ mode and using the Kα1 radiation of a Cu anticathode (λ = 1.5406 Å). The anatase mean crystallite size was calculated from the Scherrer equation applied with the usual assumption of spherical crystallites. Anatase cell parameters and cell volume were calculated using FullProf software.

Scanning electron microscopy (SEM) and elemental mapping was performed in secondary electron mode on a JEOL JSM-6700 F FEG microscope.

Temperature-programmed reduction (TPR) was performed on AMI1 system (Altamira Instruments) equipped with a thermal conductivity detector (TCD) and was used for examining the catalyst reducibility. The catalysts were submitted to H2/Ar (5:95 v/v) flow at a weight hourly space velocity (WHSV) of 0.161 h -1 and using a heating rate of 10°C min -1 . For studying the catalyst reducibility, the TPR profiles were recorded after performing a slight in-situ oxidative treatment in air (200°C for 30 min) on the impregnated supports after a drying step, allowing the decomposition of the chloride precursor.

CO adsorption and desorption measurements were carried out with Nicolet 6700 FTIR spectrometer equipped with MCT detector made by Thermo Scientific, using transmission cell. Number of scans was chosen as 64 and spectral resolution was set as 4 cm -1 . Before the measurements, the samples were first reduced in H2 flow at 200°C for 1 h, before to be cooled down to 40°C in argon. Then carbon monoxide sorption was performed for 20 min and during this process spectra were collected every 5 min. After that time CO desorption was performed for 20 min in Ar flow. During desorption, spectra was collected every 2 minutes. Gas flow was set every time as 20 ml/min. The Ru nanoparticle size distribution of Ru/TiO2 samples was determined by transmission electron microscopy (TEM) performed using a JEOL 2100F with a point resolution of 0.2 nm. The samples were sonically dispersed in an ethanol solution before a drop of the solution was deposited onto a copper grid covered by a holey carbon membrane for observation. The size distributions were calculated for each nanoparticle sample by averaging 300 particles from the TEM images using ImageJ software. They were also characterized by the Full Width at Half Maximum (FWHM). Time-of-flight secondary ion mass spectrometry (ToF-SIMS) measurements were performed using ION-TOF GmbH instrument (ToF-SIMS IV) equipped with a 25 kV pulsed Bi + primary ion gun in static mode. The samples were pressed into pellets before the measurements and attached to the sample holder using double-sided tape. The analyzed area corresponded to a 500 µm × 500 µm size square. For each sample, three spectra were collected. A pulsed electron flood gun was used for the charge compensation. Depth of the analysis did not exceed 1 nm, mass resolution at m/z=29 was in the range of 4000-7000. The samples were stored and transported in disposable containers for avoiding any contamination.

Atomic absorption spectroscopy (AAS) was used to determine the Ru concentration in the post-reaction mixtures using SOLAAR M6 Unicam Atomic Absorption Spectrometer.

Catalytic tests

Levulinic acid hydrogenation. In a typical levulinic acid (LA) hydrogenation experiment, 1 g of LA, 0.15 g of catalyst and 30 ml of water were mixed in a stainless steel autoclave (Berghof, Germany), equipped with a Teflon insert allowing a reaction volume of 45 ml. The reactor was pressurized with H2 to 5 bar and the temperature was maintained at 190°C for 30 min. At the end of the reaction, the reactor was cooled down, the remaining pressure was released and the reaction mixture was centrifuged to separate the solid catalyst and the product solution.

Formic acid decomposition. In a typical experiment, 0.4 ml of formic acid (FA), 0.15 g of reduced catalyst, and 30 ml of distilled water were mixed in a stainless steel autoclave (Berghof) equipped with a Teflon insert allowing a reaction volume of 45 ml. The reactor was flushed with H2 for 30 s and Ar for 15 s. The reactor was then heated to 190°C for 2 h.

LA hydrogenation with formic acid as a hydrogen source (FALA). In a typical LA hydrogenation experiment, 1 g of LA, 0.4 ml of FA, 0.6 g of a catalyst and 30 ml of water were mixed in a stainless steel autoclave (Berghof) equipped with a Teflon insert allowing a reaction volume of 45 ml. The temperature was maintained at 190°C for 5 h. At the end of the reaction the reactor was cooled down, the remaining pressure was released and the reaction mixture was centrifuged to separate the solid catalyst and the product solution.

Reaction product analysis. The gaseous products were analyzed by gas chromatography (VEB Chromatrom, Berlin) equipped with a TCD detector. Argon was used as the carrier gas with a flow rate of 15 ml min -1 and the injections were performed with a volume of 2 ml. The liquid products were analyzed by high-performance liquid chromatography (Perlan) equipped with a refractive index detector and a Rezex ROA column, and 0.0025 mol dm -3 H2SO4 was used as an eluent.

Figure 2 .

 2 Figure 2. SEM images and the corresponding O, Ti and Ca element mapping images of selected support materials : a) T500; b) 1Ca-T500; c) 5Ca-T500; d) 20Ca-T500.

Figure 3 .

 3 Figure 3. TEM images and the corresponding histograms of the Ru nanoparticle size distribution for the catalysts prepared via classical incipient wet impregnation with final thermal treatment under hydrogen : a) Ru/P25-I; b) Ru/T500-I; c) Ru/10Ca-T500-I.

Figure 4 .

 4 Figure 4. TEM images and the corresponding histograms of the Ru nanoparticle size distribution for the catalysts prepared via the photo-assisted synthesis approach : a) Ru/P25-P; b) Ru/T500-P; c) Ru/1Ca-T500-P; d) Ru/ 5Ca-T500-P; e) Ru/10Ca-T500-P.

Figure 5 .

 5 Figure 5. High resolution TEM images of the Ru/5Ca-T500-P catalysts, with the interplane distances of (101) planes of metallic Ru, of (101) planes for anatase TiO2 and of (101) planes for CaTiO3.

Figure 6 .

 6 Figure 6. TPR profiles for Ru catalysts prepared by the impregnation method a) Ru/P25-I; b) Ru/T500-I; c) Ru/1Ca-T500-I; d) Ru/5Ca-T500-I; e) Ru/10Ca-T500-I; f) Ru/20Ca-T500-I.

Figure 7 .

 7 Figure 7. TFTIR spectra of adsorbed CO obtained for the catalysts prepared by the impregnation method a) Ru/P25-I; b) Ru/T500-I; c) Ru/1Ca-T500-I; d) Ru/5Ca-T500-I; e) Ru/10Ca-T500-I; f) Ru/20Ca-T500-I.

Figure 8 .

 8 Figure 8. FTIR spectra of adsorbed CO obtained for the catalysts prepared by the photodeposition method a) Ru/P25-P; b) Ru/T500-P; c) Ru/1Ca-T500-P; d) Ru/5Ca-T500-P; e) Ru/10Ca-T500-P; f) Ru/20Ca-T500-P.

Figure 9 .

 9 Figure 9. Catalytic activity of the Ru catalysts prepared by the impregnation method in the FALA reaction, in terms of a) LA hydrogenation, FA decomposition and GVL yield ; and b) Gaseous product distribution.

Figure 10 .

 10 Figure 10. Catalytic activity of the Ru catalysts prepared by the photodeposition method in the FALA reaction, in terms of a) LA hydrogenation, FA decomposition and GVL yield ; and b) Gaseous product distribution.

Figure 11 .

 11 Figure 11. Catalytic activity of the Ru catalysts prepared by a) impregnation and b) photodeposition in the FA decomposition.

Figure 12 .

 12 Figure 12. Catalytic activity in terms of LA hydrogenation, FA decomposition and GVL yield of the Ru catalysts prepared by a) impregnation and b) photodeposition in the LA hydrogenation with external H2.

Figure 12 .

 12 Figure 12. a) Influence of the Ru nanoparticle size on a) the GVL yield in FALA, b) the FA decomposition and c) the GVL yield in LA hydrogenation with external hydrogen, for Ru catalysts prepared by impregnation and photodeposition.

Table 1 .

 1 Physicochemical characterization of the TiO2-based supports

	Sample	P25	T500	1Ca-T500	5Ca-T500	10Ca-T500	20Ca-T500
	Amount of Ca, %	0	0	1	5	10	20
	Anatase average crystallite size, nm a	21 b	24	13	15	15	16
	BET surface area, m 2 g -1	50	27	60	54	47	40
	Pore volume, cm 3 g -1	0.23	0.14	0.12	0.10	0.09	0.13
	Isoelectric point	4.5	3.4	2.6	2.4	3.0	-
	Cell parameter a, Å c	-	3.7816	3.7831	3.7828	3.7830	3.7839
	Cell parameter c, Å c	-	9.5020	9.4840	9.4650	9.4820	9.4770
	Cell volume, Å 3	-	135.88	135.73	135.44	135.70	135.69

Table 2 .

 2 Normalized intensities of ions calculated on the basis of ToF-SIMS spectra collected from the surface of catalysts prepared by the impregnation and the photodeposition method.

		Ion intensities
	Sample	Ru + /total +.	Ru + /Ti +.	Ca + /total +.	CaRuO2 + /total +.
		× 10 -2	× 10 -1	× 10 -2	× 10 -4
	Ru/T500-I	1.3	1.8	-	-
	Ru/1Ca-T500-I	1.2	1.8	3.8	1.3
	Ru/5Ca-T500-I	1.9	6.1	14.0	3.9
	Ru/10Ca-T500-I	2.0	7.1	13.0	4.4
	Ru/20Ca-T500-I	2.1	8.0	18.0	3.0
	Ru/T500-P	2.3	7.7	-	-
	Ru/1Ca-T500-P	2.2	5.6	2.7	1.2
	Ru/5Ca-T500-P	2.5	11	5.6	1.8

Acknowledgements

The French Embassy in Poland is thanked for supporting the PhD work of J.W. via a French Government Grant. T. Dintzer (ICPEES, Strasbourg) and D. Ihiawakrim (IPCMS, Strasbourg) a r e a c k n o w l e d g e d f o r p e r f o r m i n g S E M a n d T E M characterization, respectively.

A new elegant photo-assisted method allowing the synthesis of uniform size and evenly distributed Ru particles on the titania surface was developed. Enhanced catalytic performance compared to the counterparts catalysts prepared by classical wet impregnation were reached, and further exalted thanks to the support modification via the calcium introduction into the titania support.


Nizar Doss

Gaëlle Carré

Valérie Keller
email: nkeller@unistra.fr

Philippe André

Nicolas Keller

Photocatalytic decontamination of airborne T2 bacteriophage viruses in a small-size TiO 2 /β-SiC alveolar foam LED reactor

Keywords: LED, Photocatalysis, Titanium dioxide, T2 bacteriophage virus, Solid alveolar foams, Energy effectiveness

Light emitting diodes (LEDs) emitting at 392 nm were successfully used as an irradiation light source and associated to TiO2/b-SiC solid alveolar foams for designing a small-size, flow-through structured photocatalytic device for purifying air from airborne T2 bacteriophage viruses. Light emitting diodes are characterized by a high electricity-to-light yield, strength, a long lifetime, to ability to use a direct current power source, an almostcomplete recycling rate, and a lack of mercury.

Irrespective of the number of LEDs, we showed that the decontamination efficiency associated with removing airborne T2 bacteriophage viruses resulted from both the photocatalytic activity and the passive filtration effect of the TiO2/b-SiC solid alveolar foams.

A high photocatalytic filtration efficiency was observed with 56 LEDs and a logarithmic abatement of 3 was achieved for 60 min of run time, with an apparent time constant of 11.0 min after correcting for the natural decay of the bioaerosol. The pure filtration effect corresponded to a logarithmic abatement of 1, with an apparent time constant of 43.1 min.

The interest in using 56 LEDs vs. 40 LEDs was highlighted in terms of the logarithmic abatement as well as energy effectiveness.

Introduction

Indoor air quality control is garnering growing interest due to public concerns over human health.

The United States Environmental Protection Agency (EPA) considers indoor air pollution to be one of the primary environmental risks to public health given that humans spend roughly 80% of their time indoors (i.e., in confined places) where pollutant concentrations are higher [START_REF] Epa | Exposure factors handbook[END_REF]. Malodorous, toxic or global warming-related chemical pollutants have been the most studied targets, but the removal of airborne microorganisms is also a challenging field. Indeed, the continuously growing resistance of microorganisms to chemical treatments and their dissemination due to the intensification of transport mechanisms are responsible for threats associated with biological pollutants. Although many airborne microorganisms are not virulent or only minimally so, it is well known that many kinds of airborne microorganisms such as bacteria, viruses and fungi are considered to be hazardous to human health. These airborne microorganisms have a pronounced societal impact in terms of costs and mortality [START_REF] Who | Fact sheet N°104[END_REF].

The removal of chemical or airborne biological pollutants is therefore a challenging task.

Oxidative photocatalysis has attracted attention in recent decades as an efficient air treatment technology because of the strong oxidizing power of UV-A-irradiated semiconductors at room temperature [START_REF] Herrmann | Heterogeneous photocatalysis: fundamentals and applications to the removal of various types of aqueous pollutants[END_REF][START_REF] Ohtani | Photocatalysis A to Z-What we know and what we do not know in a scientific sense[END_REF][START_REF] Ollis | TiO2 photocatalysis-25 years[END_REF]). The organic nature of microorganism constituents has resulted in the establishment of a meaningful analogy between biological and chemical targets: indeed, e.g., cell walls are viewed as a complex assembly of high molecular-weight organic compounds that photocatalysis can oxidize via oxidative photoholes or hydroxyl radicals.

Therefore, irradiated photocatalysts cause oxidative damages to cell membranes, which are considered to be the first barrier to maintaining vital cell functions. As a result of this process, inactivation of bacteria, viruses, spores and yeasts can be achieved (Matsunaga et al. 1985;[START_REF] Blake | Application of the photocatalytic chemistry of titanium dioxide to disinfection and the killing of cancer cells[END_REF][START_REF] Gogniat | The bactericidal effect of TiO2 photocatalysis involves adsorption onto catalyst and the loss of membrane integrity[END_REF][START_REF] Josset | Numeration methods for targeting photoactive materials in the UV-A photocatalytic removal of microorganisms[END_REF][START_REF] Herrmann | Fundamentals and misconceptions in photocatalysis[END_REF][START_REF] Nakata | TiO2 photocatalysis: Design and applications[END_REF].

While photocatalysis has been already widely applied for removing chemical pollutants in environmental remediation efforts in water, air as well as on surfaces, the application of photocatalysis to biological objects mainly concerns the disinfection of water and the implementation of selfdecontaminating coatings [START_REF] Blake | Application of the photocatalytic chemistry of titanium dioxide to disinfection and the killing of cancer cells[END_REF]. The photocatalytic disinfection of contaminated air (i.e., bioaerosols) has been rarely investigated despite a large range of applications and pronounced public health concerns. This situation largely stems from the difficulty of working with bioaerosols.

However, studies with bioaerosols have focused on a wide range of microorganisms such as Escherichia coli, Legionella pneumophila, Microbacterium sp., Bacillus subtilis, Bacillus cereus, Staphylococcus aureus, Aspergillus brasiliensis, a Candida famata yeast, and MS2 and l phage viruses [START_REF] Goswami | Photocatalytic disinfection of indoor air[END_REF][START_REF] Keller | Biological agent inactivation in a flowing air stream by photocatalysis[END_REF][START_REF] Vohra | Enhanced photocatalytic disinfection of indoor air[END_REF][START_REF] Grinshpun | Control of aerosol contaminants in indoor air: Combining the particle concentration reduction with microbial inactivation[END_REF][START_REF] Josset | UV-A photocatalytic treatment of high flow rate air contaminated with Legionella pneumophila[END_REF][START_REF] Josset | UV-A photocatalytic treatment of Legionella pneumophila bacteria contaminated airflows through three-dimensional solid foam structured photocatalytic reactors[END_REF][START_REF] Pal | Photocatalytic Inactivation of Airborne Bacteria in a Continuous-Flow Reactor[END_REF][START_REF] Yu | Removal of bioaerosols by the combination of a photocatalytic filter and negative air ions[END_REF].

Therefore, the development of three-dimensional photocatalytic media such as alveolar solid foams that could take advantage of the simultaneous filtration and photocatalytic removal of airborne microorganisms is of interest. These media benefit from a static mixer role and a very low pressure drop, and they enable suitable light transmission to the reactor core. For these reasons, they are worth studying for designing flow-through structured reactors [START_REF] Kato | Photocatalytic degradation of gaseous sulfur compounds by silver-deposited titanium dioxide[END_REF][START_REF] Ochuma | Three-phase photocatalysis using suspended titania and titania supported on a reticulated foam monolith for water purification[END_REF][START_REF] Yuan | Promotion effect of Al2O3-SiO2 interlayer and Pt loading on TiO2/nickel-foam photocatalyst for degrading gaseous acetaldehyde[END_REF][START_REF] Yao | Antibacterial performance of a novel photocatalytic-coated cordierite foam for use in air cleaners[END_REF]Ochiai and Fujishima 2012;[START_REF] Ishiguro | Photocatalytic activity of Cu 2+ /TiO2-coated cordierite foam inactivates bacteriophages and Legionella pneumophila[END_REF][START_REF] Masson | b-SiC alveolar foams as a structured photocatalytic support for the gas phase photocatalytic degradation of methylethylketone[END_REF]. In the field of light sources, high-output, inexpensive light emitting diodes (LEDs) have garnered strong interest as a promising photochemical light source for replacing widespread traditional fluorescent lamps in environmental remediation applications [START_REF] Wang | Photocatalytic degradation of Reactive Red 22 in aqueous solution by UV-LED radiation[END_REF][START_REF] Chen | Photodecomposition of o-cresol by UV-LED/TiO2 process with controlled periodic illumination[END_REF][START_REF] Shie | Photodegradation kinetics of formaldehyde using light sources of UVA, UVC and UVLED in the presence of composed silver titanium oxide photocatalyst[END_REF][START_REF] Lapkin | Photo-oxidation by singlet oxygen generated on nanoporous silicon in a LED-powered reactor[END_REF]Gosh et al. 2008Gosh et al. , 2009;;[START_REF] Liu | Simple fabrication and photocatalytic activity of Sdoped TiO2 under low power LED visible light irradiation[END_REF][START_REF] Subagio | Photocatalytic degradation of bisphenol-A by nitrogen-doped TiO2 hollow sphere in a vis-LED photoreactor[END_REF][START_REF] Doss | Photocatalytic degradation of butanone (methylethylketone) in a small-size TiO2/β-SiC alveolar foam LED reactor[END_REF]. Indeed, the advantages exhibited by LEDs relate to their high electricity-to-light yield with minimal heating, their low energy consumption and a more sustainable Hg-free nature. Nearly complete recycling of LEDs is possible, and they benefit from a long lifetime and a small size that enables system miniaturization and configuration flexibility.

Ultraviolet and visible-light LEDs have been already successfully implemented in gas-and liquid-phase photocatalytic reactors for the degradation of model molecules such as Reactive Red 22 and o-cresol [START_REF] Wang | Photocatalytic degradation of Reactive Red 22 in aqueous solution by UV-LED radiation[END_REF][START_REF] Chen | Photodecomposition of o-cresol by UV-LED/TiO2 process with controlled periodic illumination[END_REF]), 4-chlorophenol (Gosh et al. 2008, 2009), orange methyl [START_REF] Liu | Simple fabrication and photocatalytic activity of Sdoped TiO2 under low power LED visible light irradiation[END_REF]), bisphenol-A in water [START_REF] Subagio | Photocatalytic degradation of bisphenol-A by nitrogen-doped TiO2 hollow sphere in a vis-LED photoreactor[END_REF], and formaldehyde [START_REF] Shie | Photodegradation kinetics of formaldehyde using light sources of UVA, UVC and UVLED in the presence of composed silver titanium oxide photocatalyst[END_REF]) and methyl ethyl ketone [START_REF] Doss | Photocatalytic degradation of butanone (methylethylketone) in a small-size TiO2/β-SiC alveolar foam LED reactor[END_REF] in air. Due to cost limitations in the case of highirradiance UV-A LEDs, studies have been mainly performed using more intense and cheaper visible LEDs.

Our previous investigations of airborne microorganisms were concerned with the UV-A photocatalytic treatment of flowing air contaminated by L. pneumophila, Escherichia coli, T2 bacteriophage viruses and B. atrophaeus using conventional fluorescent UV-A lamps and a classical reactor geometry [START_REF] Keller | Biological agent inactivation in a flowing air stream by photocatalysis[END_REF][START_REF] Josset | UV-A photocatalytic treatment of high flow rate air contaminated with Legionella pneumophila[END_REF][START_REF] Josset | UV-A photocatalytic treatment of Legionella pneumophila bacteria contaminated airflows through three-dimensional solid foam structured photocatalytic reactors[END_REF]. We previously notably highlighted the importance of the aerodynamics in air decontamination, directly resulting from the microorganism morphology, so that the results obtained from studies performed toward chemical pollutants, cannot be simply transposed to airborne microorganisms. Therefore, the aim of this paper was to report the first use of a flow-through LED photocatalytic reactor structured by open-cell β-SiC alveolar foams for decontaminating air from airborne T2 bacteriophage viruses. β-SiC alveolar foams are open cellular structures benefiting from a static mixer role, from very low pressure drops and from an increased surface-to-reactor volume ratio while maintaining a suitable light transmission to the reactor core, so that they are good candidates for elaborating flow-through photoreactors (Keller et al. 20013;[START_REF] Doss | Photocatalytic degradation of butanone (methylethylketone) in a small-size TiO2/β-SiC alveolar foam LED reactor[END_REF][START_REF] Masson | b-SiC alveolar foams as a structured photocatalytic support for the gas phase photocatalytic degradation of methylethylketone[END_REF]. In addition, their β-SiC nature provides a strong chemical and thermal stability to the material, allowing chemical/thermal regeneration treatments to be applied if necessary.

Here, we therefore demonstrated the ability of the foam-structured LED reactor to decontaminate flowing air, which resulted from a combined photocatalytic and filtration effect.

Experimental Section

Material preparation

The TiO2 MPT623 material was supplied by Ishihara Sangyo Kaisha. TiO2 MPT623 is a visible light-responsive Pt chloride-modified rutile TiO2 photocatalyst that has been already used for practical indoor air remediation applications [START_REF] Yu | An Efficient Visible-Light-Sensitive Fe(III)-Grafted TiO2 Photocatalyst[END_REF][START_REF] Nishikawa | Reinvestigation of the Photocatalytic Reaction Mechanism for Pt-Complex-Modified TiO2 under Visible Light Irradiation by Means of ESR Spectroscopy and Chemiluminescence Photometry[END_REF].

Medium surface area, open-cell β-SiC alveolar foam was synthesized by the SICAT Company (Willstätt-Germany) according to the Shape Memory Synthesis (SMS) replica method. In this method, a pre-shaped polyurethane foam is transformed into its corresponding carbide at 1360°C in an argon atmosphere [START_REF] Nguyen | Innovative porous SiC-based materials: From nanoscopic understandings to tunable carriers serving catalytic needs[END_REF]. This SMS replica synthesis reflects the fact that the original macrostructural features of the pre-shaped polyurethane foam were retained after synthesis. Therefore, the β-SiC solid foams can be manufactured with different shapes to conform to a particular photoreactor's geometry. These foams were modeled by Edouard and colleagues [START_REF] Huu | Towards a more realistic modeling of solid foam: Use of the pentagonal dodecahedron geometry[END_REF] as the packing of regular pentagonal dodecahedrons. The most characteristic parameters of the β-SiC foam are considered to be their cell size (f),pore or window diameter (a)-which correlates with the pore density (i.e., the number of pores per linear inch, ppi)-and the strut diameter (ds).

The deposition of the TiO2 MPT623 photocatalyst onto the alveolar β-SiC foam was performed by plunging the β-SiC foam support in a TiO2-containing water suspension and then drying the sample at ambient temperature. The impregnation suspension, with a TiO2 concentration of 100 g/L, was stirred for 30 min before the foam was plunged. The impregnation process was replicated, with a pressurized air flow treatment between each step, until the desired TiO2 mass was obtained. The TiO2 wt.% content was determined by weighting the β-SiC foam-coated TiO2 material after a final drying at 100°C and subsequent ultrasonic treatment.

Characterization techniques

X-ray diffraction (XRD) measurements were carried out on a D8 Advance Bruker diffractometer in q/q mode with Ka1 radiation of Cu at 1.5406 Å.

Surface area measurements were carried using a Micrometrics Tristar 3000 with N2 as an adsorbant at -196°C. The samples had been previously outgassed at 200°C for 8 h to desorb impurities or moisture. The BET-specific surface area was calculated from the N2 adsorption isotherms, and the micropore surface area was derived using the t-plot method.

Scanning electron microscopy (SEM) was carried out in secondary electron mode on a JEOL-JSM-6700 F microscope equipped with a field emission gun, and operated with an extraction potential ranging from 1-10 kV. Prior to the analysis, the samples were coated with gold.

UV-visible absorption spectra of the materials were recorded on a Varian Cary 100 Scan spectrophotometer equipped with a DRA-CA-301 Labsphere diffuse reflectance cell.

UV-A irradiance measurements were performed using a wideband RPS900-W Rapid Portable Spectroradiometer from International Light Technologies (Peabody, MA, USA). The measurements were performed as close as possible to a single LED. The UV-A irradiance values suffered from a non-negligible uncertainty because of difficulties recording the irradiance close to the LED surface;

the LED geometry required averaging the data recorded for different locations on the LED surface.

The light transmission through the β-SiC foam was defined as the fraction of the incident light, in terms of UV-A irradiance, transmitted through the material as the foam thickness was varied.

LED light sources and photocatalytic reactors

The LEDs (ref. were purchased from Roithner Lasertechnik (Austria). These

InGaN-based LEDs had a viewing angle of 15° (technical data provided by the supplier).

A photocatalytic reactor was engineered to incorporate 40 or 56 LEDs and a 65 x 40 mm (d x h) disk-shaped TiO2/β-SiC foam photocatalyst with a mean cell cell size of 5440 µm (Fig. 1). The photocatalytic reactor also included an inner fan functioning at 10 m 3 /h and the printed circuit boards for LEDs. The TiO2/β-SiC foam photocatalyst disk occupied a total volume of 133 mL, so that the tests were performed with a residence time of 0.048 s.

T2 bacteriophage preparation and photocatalytic test procedure

The recirculation tests were operated at an airflow rate of 10 m 3 /h in a 0.8-m 3 glovebox adapted for airborne microorganisms that acted as a reaction chamber. The test chamber was homogenized via the use of an additional fan operating at a higher airflow rate (30 m 3 /h).

We used nanometer-size T2 bacteriophage viruses as model viruses for airborne viruses.

Bacteriophages are viruses that infect specific bacteria, and T2 bacteriophages evolute on an E. coli host bacteria (strain: BAM). To obtain the bacteriophage suspension, an aliquot of a -80°C high titre stock of T2 bacteriophages (roughly 10 10 T2/mL) was first diluted to the tenth using a sterile MgSO4 solution (10 mM), and subsequently till a dilution factor of 10 8 (4 replicates for each dilution). 0.1 ml of each dilution of bacteriophages was poured onto a Luria Bertani (LB) agar plate and a soft LB agar (0.75% LB agar) containing the E. coli host strain was prepared simultaneously using the following methodology. One aliquot (100 µL) of the E. coli BAM suspension in an exponential growth phase was diluted in physiological water to an optical density of 0.5 at 600 nm and was mixed with 50 mL of soft LB agar in surfusion (50°C). 5 mL of this mix was sprayed onto previously prepared LB agar plates. The plates were incubated at 37°C for 24 h. Viral infection resulted in clear zones (or plaque) due to lysis by bacteriophages, and zones where bacterial growth occurred were turbid. After 24 h of incubation at 37°C, 5 mL of a sterile MgSO4 solution (10 mM) was poured on the surface of the plates with 1000-5000 lysis plaques. These plates were left an agitating plate at 4°C for 2 h, which allowed the viruses to diffuse into the liquid phase free of bacterial debris. The supernatants were collected, and chloroform was added to lyse the bacterial cells and liberate the phages within to a final concentration of 0.2% v/v. This stock suspension was stored at 4°C was then titrated following a standard lysis plaque procedure.

The contaminated bioaerosol was obtained by aerosolizing a 5 mL aliquot of a 6 × 10 6 T2 bacteriophages/mL suspension into the reaction chamber as droplets in a high-flow rate air stream (3.8 × 10 7 bacteriophages per m 3 of air). At the reactor outlet, T2 bacteriophages impacted the surface of the LB agar plates for 1 min at 10-min time intervals. Then, the surface was recovered with 5 mL of soft LB agar in surfusion (50°C) mixed with 100 µL of a E. coli BAM suspension in an exponential growth phase, which had been previously diluted in LB to an optical density of 0.5 at 600 nm. The plates were then incubated for 24 h at 37°C prior to the numeration of lysis plaques.

Results and Discussion

Material characterization

The XRD pattern of TiO2 MPT623 shown in Figure 2A exhibits diffraction peaks corresponding to rutile TiO2, with main peaks at 27.5°, 36.1° and 54.4° corresponding to the (110), ( 101) and ( 211) planes, respectively [START_REF] Dai | Preparation of Highly Crystalline TiO2 Nanostructures by Acid-assisted Hydrothermal Treatment of Hexagonal-structured Nanocrystalline Titania/Cetyltrimethyammonium Bromide Nanoskeleton[END_REF]. TiO2 MPT623 has a non-microporous specific surface area of 60 m 2 /g. Figure 2B shows the LED emission spectral range compared with the absorption range of TiO2 MPT623. The LEDs have been labeled here as LED-392 according to their measured wavelength of emission maximum at 392 nm. The total measured irradiance is 11.7 ± 2.0 mW/cm 2 with a spectral bandwidth of 15 nm. Those LEDs are considered to be UV-Vis LEDs, and they allow an activation of the rutile TiO2 photocatalyst in the UV-A range and in the beginning of the visible light region.

Indeed, rutile TiO2 is reported to have a band gap of 3.0 eV and a band edge at 413 nm [START_REF] Kavan | Electrochemical and Photoelectrochemical Investigation of Single-Crystal Anatase[END_REF][START_REF] Bosc | Room temperature visible light oxidation of CO by high surface area rutile TiO2-supported metal photocatalyst[END_REF]), whereas the LEDs had a spectral bandwidth of 15 nm and a maximum emission wavelength of 392 nm.

The optical and SEM images in Figure 3A show the macroscopic features of the open-cell TiO2/β-SiC foam catalyst and the uniformity of the TiO2 coating. The parameters of the solid β-SiC alveolar foam were measured using optical imaging to be 5440 ± 720 µm, 2290 ± 630 µm and 575 ± 80 µm in terms of cell size, window size and strut diameter, respectively. These parameters are as defined in the model developed by Edouard [START_REF] Huu | Towards a more realistic modeling of solid foam: Use of the pentagonal dodecahedron geometry[END_REF], with a 95-97% open porosity. Figure 3B shows the light transmission through the β-SiC foam as a function of foam thickness, with a light transmission following a 𝑇 " = 𝑒 %"/' first-order decreasing exponential model as a function of the foam thickness t, with λ = 0.47 ± 0.02 cm representing the characteristic length for the β-SiC foam.

We selected a β-SiC foam thickness of 0.4 cm to ensure auitable light penetration to the core of the foam.

The β-SiC foam used as a structured support had a specific surface area of 20 m 2 /g; after the impregnation process the resulting TiO2/β-SiC foam catalyst had a TiO2 content of 9.2 ± 0.2 wt.% corresponding to 1.6 g of TiO2 within the reactor, and exhibited a specific surface area of 24 m 2 /g.

Decontamination efficiency

Figure 4 shows the decontamination kinetics of the T2 phage virus bioaerosol obtained with the 40-and 56-LED TiO2/β-SiC foam-structured photocatalytic reactors, expressed as the evolution with time of both the contamination and the logarithmic abatement. The decontamination kinetics resulting from a pure filtration effect were reported, as well as the blank test results, which represent the stability of the bioaerosol within the test chamber as a function of time.

The degradation of chemicals is typically expressed following the Langmuir-Hinshelwood model for deriving the apparent kinetic rate constant in an apparent first-order reaction for highly diluted samples [START_REF] Cunningham | Adsorption of model pollutants onto TiO2, particles in relation to photoremediation of contaminated water[END_REF][START_REF] Minero | Photocatalytic Mechanisms and Reaction Pathways Drawn from Kinetic and Probe Molecules[END_REF]. Similarly, we used an apparent first-order decontamination kinetic for the T2 virus bioaerosol. We adopted an apparent time constant (t) to characterize the decontamination performance (Eq. 1):

%𝐶(𝑡) = 100% × 𝑒 % " 0 1 (1)
where %C(t) is the contamination of the test chamber as a percent.

The evolution of the contamination with time within the test chamber during blank tests was characterized by defining t b , the apparent time constant. The t filtration apparent time constant was derived from the tests using the non-irradiated TiO2/β-SiC foam-structured reactor irrespective of the number of LEDs. This constant has to be corrected from the natural decay of the bioaerosol for not determining the abatement resulting only from a filtration effect. We accordingly obtained the corrected t filtration corr apparent time constant (Eq. 4). Similarly, the t photo apparent time constant was derived from the photocatalytic tests and had to be corrected from the natural decay of the bioaerosol for determining the abatement resulting only from the photocatalytic filtration effect. We consequently obtained the corrected t photo corr apparent time constant (Eq. 5). Therefore, we can derive the following data:

%𝐶 2344 567"48"639 (𝑡) = 100% × :𝑒 % " 0 ;<== >?@A=BA?<C D E = %F >?@A=BA?<C (")

%F G (") = HII%×J K A L >?@A=BA?<C 1 HII%×J K A L G 1 = 100% × M𝑒 % " 0 >?@A=BA?<C 1 + 𝑒 " 0 G 1 O (2) %𝐶 2344 PQ3"3 (𝑡) = 100% × :𝑒 % " 0 ;<== RS<A< D E = %F RS<A< (") %F G (") = HII%×J K A L RS<A< 1 HII%×J K A L G 1 = 100% × M𝑒 % " 0 RS<A< 1 + 𝑒 " 0 G 1 O (3)
The corrected apparent time constants were derived as follows:

𝜏 2344 567"48"639 = 0 G ×0 >?@A=BA?<C 0 G %0 >?@A=BA?<C (4)

𝜏 2344 PQ3"3 = 0 G ×0 RS<A< 0 G %0 RS<A< (5)
The energy effectiveness (Ee) was calculated from Eq. 6 for the removal of T2 airborne viruses, as adapted from the works of [START_REF] Shie | Indoor and Built Environment. Photodegradation Kinetics of Toluene in Indoor Air at Different Humidities Using UVA, UVC and UVLED Light Sources in the Presence of Silver Titanium Dioxide[END_REF] on the degradation of Volatile Organic Compounds, as follows:

𝐸𝑛𝑒𝑟𝑔𝑦 𝑒𝑓𝑓𝑒𝑐𝑡𝑖𝑣𝑒𝑛𝑒𝑠𝑠 (𝐸 J , 𝑙𝑜𝑔. 𝑘𝑊 %H ℎ %H ) = gh i64jk 73l846"Qm62 8n8"JmJ9" 69Pj" P3oJ4 (pq.Q) = 73l P×"×r (6)
where V is the chamber volume (L), p is the consumed energy power (kW) and t is the reaction time (h).

Table 1 reports the decontamination results obtained with 40-and 56-LED TiO2/β-SiC foamstructured photocatalytic reactors in terms of the apparent time constant, and the energy effectiveness coefficients of the different systems (calculated for 60 min of run time), which corresponded to the duration of use necessary to attain a logarithmic abatement of 3 with the most efficient system (i.e., the photocatalytic filtration with 56 LEDs).

It is worth noting that a natural decay was observed during the blank tests, which resulted from (i) the possible instability of the bioaerosols during the test and (ii) the high recirculation flow rate used, which led to some microorganisms impacting the chamber walls. Therefore, natural decay during the blank tests was characterized with an apparent time constant of 52.1 min. It is worth noting that the non-irradiated TiO2/β-SiC foam-structured reactor-irrespective of the number of LEDs-displayed a pure filtration efficiency characterized by a logarithmic abatement of 1. The reactor was characterized by an apparent time constant of 23.6 min before correction and an apparent time constant of 43.1 min after correcting for bioaerosol stability.

A logarithmic abatement of 1.75 was achieved after 60 min with the system with 40 LEDs, with an apparent time constant of 20.5 min before correction and an apparent time constant of 33.8 min after correction. This finding revealed the contribution of the photocatalytic activity of foams in addition to their filtration efficiency obtained with 40 LEDs. On the other hand, a higher photocatalytic filtration efficiency with a logarithmic abatement of 3 was achieved after 60 min of run time using the TiO2/β-SiC foam-structured reactor irradiated by 56 LEDs with an apparent time constant of 9.1 min before correction and an apparent time constant of 11.0 min after correction.

These results highlighted the utility of using large cell-size foams for structuring reactors to take advantage of a large cell size for providing illumination to the core of the reactor while still enabling a decontamination efficiency based on the pure filtration effect within the alveolar foam. As a result, the global airborne microorganism abatement directly resulted from both photocatalytic activity and passive filtration.

After correcting for bioaerosol stability and despite an increased energy need, increasing the number of LEDs from 40 to 56 was beneficial in terms of the energy effectiveness coefficient. Indeed, the foam-structured reactor with 56 LEDs exhibited an energy effectiveness of 5.5 x 10 3 log.kW -1 h -1 vs.

4.5 x 10 3 log.kW -1 h -1 for a similar system with 40 LEDs. After correcting for the bioaerosol stability within the chamber, the energy effectiveness was boosted to 4.6 x 10 3 log.kW -1 h -1 using 56 LEDs vs. 3.2

x 10 3 log.kW -1 h -1 for the set-up with 40 LEDs. Therefore, the increase in the energy need associated with increasing the number of LEDs was largely overcome by the pronounced increase in the logarithmic abatement.

Conclusion

A small-size flow-through photocatalytic device incorporating a medium surface area, large-cell size β-SiC alveolar foam as a structured photocatalyst support and LEDs as an irradiation light source has been engineered to develop an air-purifying technology that is efficient at decontaminating airborne T2 bacteriophage viruses. Using TiO2 MPT623 as a photocatalyst, a system incorporating 56

LEDs emitting at 392 nm exhibited a high photocatalytic filtration activity, with a logarithmic abatement of 3 for 60 min of run time, for which a 10-fold abatement resulted from the passive filtration effect of the alveolar foam.

Irrespective of the number of LEDs, the decontamination effect resulted from both the photocatalytic activity and the passive filtration effect of the TiO2/b-SiC solid alveolar foams. The utility of using 56 LEDs vs. 40 LEDs was highlighted in terms of the logarithmic abatement as well as the energy effectiveness. Our results have illuminated interest in using a supporting solid alveolar foam and small-size LEDs as a valuable alternative for searching for efficient and energy-saving technologies.

Apparent time constantt (min)

Corrected apparent time constant

Figure 1

 1 Figure 1

B

  Photocatalytic filtration and filtration decontamination apparent time constants for the T2 bacteriophage virus bioaerosol obtained

Acknowledgments

The authors are grateful to DGA (Direction Générale de l'Armement) and Alsace regional council for financially supporting this work in the frame of the PhD grant of G. Carré. Prof. M.-C. Lett is deeply thanked for her participation to this study. Drs. C. Pham and P. Nguyen from SICAT Catalyst are thanked for providing β-SiC alveolar foams.