Josefine Schnee
email: josefine.schnee@ensicaen.fr

Marco Daturi

Mohamad El-Roz
email: mohamad.elroz@ensicaen.fr.

Ultrafast time-resolved quantum cascade laser diagnostic for revealing the role of surface formate species in the photocatalytic oxidation of methanol

Keywords: Operando IR, Quantum cascade laser, Photocatalysis, Kinetic constants, Anatase

come

Ultrafast time-resolved quantum cascade laser diagnostic

for revealing the role of surface formate species in the photocatalytic oxidation of methanol

Introduction

Photocatalysis is an important and promising approach for green chemistry and sustainable energy solutions. [START_REF] El-Roz | On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study[END_REF][START_REF] Yoon | Visible Light Photocatalysis: The Development of Photocatalytic Radical Ion Cycloadditions[END_REF][START_REF] Chen | Titanium Dioxide Photocatalysis in Atmospheric Chemistry[END_REF][START_REF] Schneider | Understanding TiO2 Photocatalysis: Mechanisms and Materials[END_REF] In particular, TiO2 photochemistry is one of the most important and widely studied approaches for the catalytic conversion of organic compounds, [START_REF] Schneider | Understanding TiO2 Photocatalysis: Mechanisms and Materials[END_REF][START_REF] Panayotov | Photooxidation Mechanism of Methanol on Rutile TiO2 Nanoparticles[END_REF][START_REF] Tamaki | Femtosecond Visible-to-IR Spectroscopy of TiO2 Nanocrystalline Films: Elucidation of the Electron Mobility before Deep Trapping[END_REF][START_REF] Shen | Transfer of Photoinduced Electrons in Anatase-Rutile TiO2 Determined by Time-Resolved Mid-Infrared Spectroscopy[END_REF] and methanol photochemistry on TiO2 has often been used as a model reaction to study photocatalytic reaction mechanisms. [START_REF] El-Roz | On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study[END_REF][START_REF] Chuang | Photochemistry of Methanol and Methoxy Groups Adsorbed on Powdered TiO2[END_REF][START_REF] Wolski | Insight into methanol photooxidation over mono-(Au, Cu) and bimetallic (AuCu) catalysts supported on niobium pentoxide -An operando-IR study[END_REF][START_REF] El-Roz | Operando Infrared (IR) Coupled to Steady-State Isotopic Transient Kinetic Analysis (SSITKA) for Photocatalysis: Reactivity and Mechanistic Studies[END_REF] A powerful tool for such mechanistic investigations is operando Fourier-transform infrared (FT-IR) spectroscopy, to unravel surface adsorbed species and reaction intermediates. [START_REF] Vimont | Analysing and understanding the active site by IR spectroscopy[END_REF][START_REF] Mino | IR spectroscopy as a tool to investigate photocatalytic reactions at oxide surfaces[END_REF][START_REF] Ftouni | TiO2/Zeolite Bifunctional (Photo)Catalysts for a Selective Conversion of Methanol to Dimethoxymethane : On the Role of Brønsted Acidity[END_REF] The term operando means "working/operating" in Latin; and operando spectroscopy is a methodology wherein the spectroscopic characterization of materials undergoing reaction is coupled simultaneously with the measurement of catalytic activity and selectivity (typically by mass spectrometry and/or gas-IR). [START_REF] Bañares | Operando methodology: combination of in situ spectroscopy and simultaneous activity measurements under catalytic reaction conditions[END_REF][START_REF] Schnee | Elucidating and exploiting the chemistry of Keggin heteropolyacids in the methanol-to-DME conversion: enabling the bulk reaction thanks to operando Raman[END_REF] The primary concern of this methodology is to establish structure-activity/selectivity relationships of catalysts in order to gain insight into reaction mechanisms.

In the case of methanol photooxidation over TiO2, it was previously shown, indirectly through operando FT-IR coupled to steady state isotopic exchange kinetic analysis (SSITKA), that the main pathway goes through the chemisorption of methanol as surface methoxy species, then their oxidation into formates and, finally, into CO2 (with the conversion of chemisorbed formates being the rate-limiting step). In parallel, neighboring adsorbed formates and methoxy groups are supposed to give rise to methylformate, as a secondary species. [START_REF] El-Roz | On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study[END_REF][START_REF] El-Roz | Operando Infrared (IR) Coupled to Steady-State Isotopic Transient Kinetic Analysis (SSITKA) for Photocatalysis: Reactivity and Mechanistic Studies[END_REF] Still, the apparent rate constants of the different reaction steps remained to be determined, and, to our knowledge, there is to date no direct evidence for the key role of formate species in the conversion of methoxy species to CO2 and methylformate.

To provide this direct evidence is precisely the aim of the present work, in which the apparent rate constants for the formation and reaction of surface formate species on three different TiO2 photocatalysts (P25, commercial anatase, and homemade anatase) in the gas phase photooxidation of methanol at 25 °C were directly evaluated by using a new quantum cascade laser-assisted operando FT-IR setup, [START_REF] Schnee | Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information[END_REF] and were unprecedentedly correlated with the overall catalytic performance in terms of methanol conversion and selectivity to CO2 and methylformate. Quantum cascade lasers (QCLs) have been invented in 1994.

They are electrically pumped unipolar semiconductor lasers, emitting light through "intersub-band" optical transitions, in contrast to bipolar semiconductor lasers being "interband" lasers. As the photon energy resulting from intersub-band transitions is independent of the band gap of the material in the active region, it can be tuned by adapting the so-called quantum well thickness. [START_REF] Waried | Synchronization of quantum cascade lasers with mutual optoelectronic coupling[END_REF][START_REF] Tittel | Recent Advances in Trace Gas Detection Using Quantum and Interband Cascade Lasers[END_REF] As described elsewhere, [START_REF] Schnee | Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information[END_REF] the new setup used in the present work combines a standard FT-IR spectrometer operating in rapid-scan mode (which consists of moving the mobile mirror of the interferometer continuously, and not step by step as in the step-scan mode) with an optical bench containing four QCLs emitting in different (but slightly overlapping) wavenumber ranges within 1876 -905 cm -1 . Within the latter ranges, each of the QCLs is tunable to one single chosen wavenumber. Thanks to an appropriate arrangement of the mirrors, the spectrometer is able to easily switch from the full standard rapid-scan FT-IR to a QCL-irradiation mode tuned to one of the wavenumbers of interest. This is typically of interest when wishing to measure the evolution of one single species -namely formate species in the present workamong others within an interesting spectral region which contains several molecular/structural fingerprints. The rapid-scan mode can provide spectral evidences of the reaction species (on the catalyst or, alternatively, in the gas phase) at the ms time resolution. The QCL-irradiation mode can then be used to follow the IR absorbance of the most relevant species versus time with a time resolution of a few microseconds only, which is not achievable with rapid-scan FT-IR, namely depending only on the response time of the detector (here 50 MHz), the QCL pulse rate (settable up to 1 MHz), and the data acquisition system (able to operate with a best time resolution of 3 μs).

The results of the present work demonstrate, to our knowledge for the first time, that the catalytic performance of the photocatalysts directly relies on the conversion rate of the formate species at their surface. Furthermore, by comparing the apparent rate constants determined in the absence vs. presence of oxygen in the reaction feed, the crystalline nature of the TiO2 photocatalyst was enlightened to be a key parameter dictating the reactivity of TiO2 lattice oxygen towards methanol.

Experimental section

TiO2 photocatalysts

TiO2 P25 and commercial anatase (CA hereafter) were purchased from Evonik-Degussa and Sigma-Aldrich, respectively. Homemade anatase (HA hereafter) was prepared as follows. 25 mL of precursor (titanium isopropoxide, TTIP) were introduced into a polypropylene bottle, followed by 9 mL of hydrofluoric acid (40% in water) and 10 mL of acetic acid (stabilizing agents), which were added dropwise while stirring for 30 minutes with a magnetic agitator. The mixture was transferred afterwards to a Teflon reactor, sealed in a stainless-steel autoclave, and then placed into an oven at 200 °C. After 12h of crystallization, the solid recovered in the form of a cloudy mixture was washed with distilled water and ethanol until reaching a neutral pH. The suspended sample was then filtrated, dried at 80 °C for 6h, and finally calcined at 550 °C for 2h (6°C/min).

To evaluate their specific surface area, all three samples were analyzed through nitrogen physisorption at -196 °C with a Micrometrics Model ASAP 2020 volumetric adsorption analyzer. The samples were outgassed at 250 °C overnight before analysis. The specific surface area was determined from the Brunauer-Emmet-Teller (BET) equation.

IR monitoring and photocatalytic testing

The TiO2 powders were pressed into self-supported discs, all with a similar surface and thickness (2 cm 2 area; 10 mg cm -2 ; ≈ 65 µm). The resulting pellets were placed into a "Sandwich" transmission IR reactor-cell equipped with KBr windows at each side of the sample holder and with a heating system, and positioned in the sample compartment of the IR spectrometer (Thermo Scientific Nicolet iS50). The IR reactor-cell, described in more details elsewhere, [START_REF] Lesage | Studying the NO-trap mechanism over a Pt-Rh/Ba/Al2O3 catalyst by operando FT-IR spectroscopy[END_REF] was connected to a gas distribution system equipped with mass flow controllers to introduce gases into the lines (heated at 100 °C). On the other side, to analyze the composition of the outlet gas flow, the reactor-cell was connected to an auxiliary bench of the IR spectrometer equipped with a gas microcell. So, the setup allowed simultaneously IRobserving the surface of the working catalyst pellet and evaluating its catalytic performance, i.e., it allowed operando IR characterization.

The IR spectrometer was equipped with a 50 MHz mercury cadmium telluride (MCT) cryo-detector suitable for both standard FT-IR analysis and QCL-time-resolved absorbance diagnostic. As described in more details in reference [START_REF] Schnee | Coupling a Rapid-Scan FT-IR Spectrometer with Quantum Cascade Lasers within a Single Setup: An Easy Way to Reach Microsecond Time Resolution without Losing Spectral Information[END_REF] , the spectrometer was coupled with a MIRcat-QT system (Daylight Solutions Inc., San Diego, USA) incorporated into a Thermo Scientific iS50 Research Module, and containing four QCLs. The latter had respectively a spectral tuning range from 1876.2 to 1675.0 cm -1 , 1736.1 to 1310.6 cm -1 , 1459.9 to 1175.1 cm -1 and 1225.5 to 905.0 cm -1 . The associated spot size and wavelength accuracy were respectively < 2.5 mm and ≤ 1 cm -1 . The linewidth was ≤ 1 cm -1 (FWHM).

The laser beams were directed to the sample compartment of the spectrometer through the latter but without crossing the interferometer. They were pulsed instead of continuous, in order to avoid local sample heating. The data acquisition system for the QCL mode was provided by STYREL Technologies, partner of National Instruments (NI). It consisted of a NI PXIe-1073 chassis and a PXIe-5122 high resolution oscilloscope card (100 MHz bandwidth) controlled through a PCIe interface integrated within a Ci5 mini-PC configuration, as well as of a custom-made LabVIEW application. The LabVIEW application was designed in such a way that the data acquisition was triggered at each laser pulse (width of 300 ns here). So, in all experiments described in the present work, the time resolution corresponded to the time in-between two successive laser pulses (100 µs here, by setting the pulse rate at 10000 Hz).

For each pulse, the amplitude (= maximum -minimum) of the recorded signal curve was automatically exported to a text file vs. time, to finally get a curve of amplitude as a function of time consisting of one value every 100 µs. In the standard FT-IR mode (thus with the standard IR source on instead of the QCLs), spectra were collected through the Thermo Scientific OMNIC software, with a spectral resolution of 4 cm -1 and 64 scans/spectrum. Photocatalytic tests of the TiO2 pellets were performed at 25 °C and atmospheric pressure, under a continuous 30 mL/min reaction flow containing 0.1 vol.% of methanol (Sigma-Aldrich -anhydrous, 99.8%) and 20 vol.% of oxygen carried in argon (continuous flow and high excess of oxygen to minimize the influence of the gas concentrations). The partial pressure of methanol was established using a saturator. As explained in reference [START_REF] El-Roz | An operando-IR study of photocatalytic reaction of methanol on new *BEA supported TiO2 catalyst[END_REF] , UV irradiation of the pellets was carried out with a Xe-Hg lamp (LC8 spot light Hamamatsu, L10852, 200 W), by using a UV-light guide (A10014-50-0110) mounted at the entrance of the IR reactor-cell.

IR monitoring of formate species upon photocatalytic reaction was done in a first series of experiments, with the polychromatic UV light. In a typical experiment, the TiO2 pellet was first dehydrated by exposure to the reaction flow at 25 °C (to replace water by methanol) without UV irradiation. FT-IR spectra were measured continuously until complete disappearance of the band at 1630 cm -1 characteristic 21 of H2O bending vibrations. At this stage, the UV lamp was already on, but the shutter in-between the lamp and the UV-light guide directed to the IR cell was closed. Then, the photocatalytic test was launched by 1) turning the IR source of the spectrometer and the QCL tuned to the desired wavenumber respectively off and on, and 2) starting the QCL data acquisition system synchronized with the UV shutter in such a way that the shutter opened after 5 s of QCL data acquisition. So, the initial QCL amplitude was measured during 5 s, and then the photocatalytic reaction was launched by enabling UV irradiation of the pellet (already under reaction flow since the dehydration pre-treatment).

Depending on the experiments, the shutter remained open for 300 ms or 5 s, after which it was automatically closed again, still under QCL data acquisition to monitor the evolution of remaining adsorbed species once UV irradiation was off. The irradiation on-off cycles were repeated successively three times in each experiment to check the repeatability. At the end, the QCL was turned off again, and a standard FT-IR spectrum was measured in order to get a full picture of the photocatalyst surface after reaction. As blank tests, three additional experiments were also performed following the same procedure as described above but in the absence of one component per experiment during the whole period of QCL monitoring, namely the component 1) methanol, 2) oxygen, and 3) UV irradiation.

The catalytic activity of the photocatalysts -in terms of methanol conversion and selectivities/yields to/of formaldehyde, methylformate and carbon dioxide -was evaluated in a second step over fresh pellets. The latter were first dehydrated at 25 °C under irradiation with the polychromatic UV light, and then exposed to the reaction flow under irradiation at 365 nm (thanks to a monochromatic 365 nm band pass filter, I0 (365nm) ≈ 15 mW/cm 2 ). Using the latter filter aimed at keeping the methanol conversion low enough to enable a proper comparison of the different samples.

Results and Discussion

Figure 1 shows the operando IR spectra at steady state of TiO2 P25, CA and HA upon photooxidation of methanol at 25 °C and atmospheric pressure (under a reaction flow of 30 mL/min containing 0.1 vol.% of methanol and 20 vol.% of oxygen in argon). The spectra are shown after subtraction of the initial spectrum recorded after dehydration pre-treatment under the reaction flow (to replace water by methanol), before UV irradiation. The corresponding photocatalytic activities (discussed later) are given in Table 1. Besides the bands in the region from 1050 to 1300 cm -1 associated with adsorbed methanol [START_REF] El-Roz | On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study[END_REF] (not further discussed here), the spectra on Figure 1 Based on these results (Figure 1), the kinetic behavior (apparent rate constants) of the surface formate species was monitored by following the intensity of their characteristic IR bands through µs timeresolved QCL diagnostic under transient conditions. More precisely, each of the three TiO2 samples was submitted to several photocatalytic tests consisting each of three successive UV on-off cycles under reaction flow (30 mL/min, with 0.1 vol.% of methanol and 20 vol.% of oxygen in argon) at 25 °C, and differing from each other by the wavenumber to which the QCL was tuned for monitoring the formation (UV on) and conversion (UV off) of the formate species (at 1360, 1380, 1564 or 1580 cm -1 ). The formates were always monitored with a QCL pulse rate of 10000 Hz, leading to a time resolution of 100 µs, since it was unnecessary to reach a higher time resolution in this case. The results are shown here only for the mode at 1360 cm -1 , the three other wavenumbers showing a similar evolution. For each of the three TiO2 samples, Figure 2 shows the QCL beam absorbance at 1360 cm -1 as a function of time during the first UV on-off cycle (UV on in Figures 2a-b-c, UV off in Figures 2d-e-f). Figures S3 to S7 in the Supplementary Information show the QCL signal at 1360 cm -1 vs. time over each of the three TiO2 samples in the whole three repetitions of the UV on-off cycle. From each QCL signal curve, an apparent rate constant was extracted following the methodology described below for Figure 2. From the three repetitions for each photocatalyst (Figures S3 to S7), the relative error on these apparent rate constants was estimated to 10%, reflecting a good reproducibility. On Figures 2a-b-c, the observed QCL absorbance curve (shown again over longer time scales on Figures S3, S5 and S7, respectively) is the result of both the formation and conversion of surface formate species.

First, the absorbance increases as a function of time, reflecting that formate species accumulate at the surface of TiO2. So, in that initial period, the rate of formation is higher than the rate of reaction. Then, after 400 ms to 2.6 s depending on the photocatalyst, steady state is reached. For each photocatalyst, the best fit curve allowed evaluating the initial rate of QCL beam absorption "(dA/dt)0" (slope of the tangent at time = 0) directly related to the rate constant "kf" of formates formation through Equation 1 (with A the QCL beam absorbance due to formate species, ε the integrated molar extinction coefficient at 1360 cm -1 , L the optical distance, C0 the initial concentration of methoxy species getting converted to formates, n the reaction order, and kapp,f the apparent rate constant of formates formation). As ε and C0 are unknown here, direct access to kf is not provided. So, hereafter, (dA/dt)0 is treated as a whole as an apparent rate constant "kapp,f".

! !" !# " $ = -𝜀𝐿 ! !% !# " $ = 𝜀𝐿𝑘 & 𝐶 $ ( = 𝑘 )**,& (1)
On Figures 2d-e-f, the QCL absorbance decreases with time. Indeed, as UV irradiation is off, the formation of formate species is no longer possible. However, those formate species having accumulated at the surface are converted by an oxidation or crosslinking process with the residual holes or methoxy species, respectively, 1 thereby leading to a progressive decrease of the intensity of the IR band characteristic of formate species. In this case, (dA/dt)0 extracted from the best fit curve is directly related to the rate constant "kr" of formates reaction through Equation 2(with C'0 the initial concentration of formates remaining at the surface of TiO2 at the moment when UV irradiation is turned off, m the reaction order, and kapp,r the apparent rate constant of formates reaction). Again, as ε and C'0 are unknown here, -(dA/dt)0 is treated as a whole as an apparent rate constant of formates reaction "kapp,r".

-! !" !# " $ = -𝜀𝐿 ! !% !# " $ = 𝜀𝐿𝑘 , 𝐶 $ -. = 𝑘 )**,, (2)
The kapp,f and kapp,r rate constants contain three contributions that can change from one TiO2 photocatalyst to another, and therefore need to be kept in mind for comparing the photocatalytic activities: 1) the intrinsic rate constant of the formation (kf) and of the conversion (kr) of surface formate species, 2) the surface coverage of the photocatalyst by the reactants, depending on the concentration of active sites per m² of catalyst (directly impacting on C0 and C'0), and 3) the specific surface area of the catalyst (m²/g) (also directly impacting on C0 and C'0).

Table 1 compares the kapp,f and kapp,r average values over TiO2 P25, CA and HA, as well as the specific surface areas of the three photocatalysts and their photocatalytic activities. The latter activities (determined from the operando IR study) are expressed in terms of methanol conversion, selectivity to formaldehyde (FA), methylformate (MF) and carbon dioxide (CO2), and total yield of MF+CO2. The latter total yield is the main parameter to be considered for investigating the relationship between the catalytic activity measured at the outlet of the reactor and the kinetics of formates formation-conversion occurring at the surface of the photocatalysts. Indeed, in the literature, both MF and CO2 are supposed to result from the conversion of formate species, in turn resulting from the conversion of methoxy species through formaldehyde as an intermediate. 85 m²/g, respectively). Nevertheless, it reaches slightly higher kapp,r and kapp,f average values than TiO2 CA (0.15 vs. 0.13 s -1 and 0.30 vs. 0.23 s -1 , respectively). So, given that the kapp values depend on the three parameters described earlier (i.e. intrinsic rate constant, surface coverage, and specific surface area; the two latter impacting on C0 and C'0), as compared to TiO2 CA, TiO2 P25 must have higher intrinsic rate constants for the formation and reaction of formates, and/or a higher concentration of active sites per m² of its surface (thus a higher surface coverage by the reactants), compensating for its lower specific surface area. In any case, in parallel to its slightly faster formates kinetics, TiO2 P25 also reaches a slightly higher (MF+CO2) yield than TiO2 CA (0.44 vs. 0.42, respectively). Regarding TiO2 HA, it has a 4.5 times lower specific surface area than TiO2 CA (19 vs. 85 m²/g, respectively), a 14 times lower kapp,r, and a 3.8 times lower kapp,f (0.0091 vs. 0.13 s -1 and 0.06 vs. 0.23 s -1 , respectively). So, as compared to TiO2 CA, TiO2 HA must have a considerably lower intrinsic rate constant for the reaction of formates (with a similar surface coverage and a similar intrinsic rate constant for the formation of formates, as the factors 3.8 and 4.5 for the kapp,f and the specific surface area, respectively, are of the same order of magnitude). Again, in parallel to its slower formates kinetics, TiO2 HA also shows a lower (MF+CO2) yield than TiO2 CA, precisely 15 times lower, which agrees with its 14 times lower kapp,r. So, in both the comparisons "TiO2 P25 vs. TiO2 CA" and "TiO2 HA vs. TiO2 CA", the (MF+CO2) yield appears to be directly correlated with the kapp,r value associated with the reaction of formates, as illustrated on Figure 3a. Notice that the conversion of methanol -reflecting the overall activity of the photocatalysts -is linearly correlated with the kapp,r/kapp,f ratio, as shown on Figure 3b. Despite the difficulty to discuss these results quantitatively based on only three photocatalysts, they demonstrate, to our knowledge for the first time here, that the formation of MF and CO2 occurs through the conversion of formate species at the surface of the photocatalysts, indeed. More generally, they give a direct proof of the relationship between the intrinsic kinetic behavior of the active sites and the overall catalytic performance of the photocatalyst.

The results reveal another important information: the kapp,r is 0.15 to 0.56 times lower than the kapp,f. This proves that the formates conversion is the limiting step of the reaction, in agreement with our previous observation. [START_REF] El-Roz | On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study[END_REF] To our knowledge, the kr/kf (equivalent to kapp,r/kapp,f) is determined here for the first time for a heterogeneous photocatalytic reaction. Regarding the fact that the kapp,r increases with the kapp,f (Figure 3c), it can be mainly assigned to the charge diffusion. The higher the charge separation is, the higher is the activity of the active sites in the formation and conversion of the intermediates (in other words, the higher are the intrinsic rate constants kf and kr of the formation and conversion of the intermediates, here the formate species). In the same context, the fact that the formates kinetics, and thus the photocatalytic activities, are similar for TiO2 P25 and TiO2 CA although TiO2 P25 has a 2 times lower specific surface area, is likely due to a higher charge separation (in other words to higher intrinsic kr and kf) in the case of TiO2 P25, ensured by the interface of rutile/anatase phases.

Notice that the kapp,r values in Table 1 are independent from the UV irradiation time within the UV onoff cycles. Indeed, as demonstrated for TiO2 P25, the QCL absorbance curve versus time in the UV off period following an UV on period of 300 ms is the same as that in the UV off period following an UV on period of 5 s (Figure S8 in the Supplementary Information, with extracted kapp,r values of 0.12 and 0.15 s -1 , respectively). In the case of TiO2 P25 and CA, the formation-reaction of surface formate species was also monitored in the absence of oxygen in the photocatalytic reaction flow. Figure 4 compares the kapp,f and kapp,r values of TiO2 P25 and CA in the absence vs. presence of flowing oxygen. These values were extracted as previously from the curves of QCL absorbance at 1360 cm -1 vs. time, shown here only for TiO2 P25 on Figure 5. It should be noticed that the conversions were negligible in the absence of oxygen.

In the case of TiO2 P25, the kapp,f is the same in the absence as in the presence of oxygen in the flow (0.3 s -1 ). This indicates that formate species are able to form with lattice oxygen of TiO2 as rapidly as with flowing oxygen. However, the kapp,r is about three times lower in the absence than in the presence of oxygen in the flow (0.053 vs. 0.15 s -1 ). This reflects that, once formed, the formate species reach lattice oxygen of TiO2 less easily than flowing one. The fact that, in the absence of flowing oxygen, the QCL absorbance at 1360 cm -1 continues increasing after 400 ms of UV irradiation (Figure 5b) while, in the presence of flowing oxygen, steady state is reached (Figure 5a), is attributed to a raise of baseline due to the accumulation of electrons at the surface of TiO2 (as those electrons do not meet flowing oxygen to react with). [START_REF] Guzman | Tracing the Reaction Steps Involving Oxygen and IR Observable Species in Ethanol Photocatalytic Oxidation on TiO2[END_REF] Indeed, as shown on Figure S9 in the Supplementary Information, the QCL absorbance at 1310 cm -1 , where no specific absorption band appears here, also increases continuously until the end of the experiment in the absence of oxygen in the flow (whereas in the presence of flowing oxygen it slightly increases only in the first 200 ms before stagnating until the end). In the case of TiO2 CA, the absence of flowing oxygen has an even greater impact on kapp,r, with additionally a significant impact on kapp,f (Figure 4). More precisely, kapp,r and kapp,f are thirteen and two times lower in the absence than in the presence of flowing oxygen, respectively (0.01 vs. 0.13 s -1 , and 0.11 vs. 0.23 s -1 ). This suggests that lattice oxygen of TiO2 CA is less available for both the formation and reaction of formates species than lattice oxygen of TiO2 P25. So, changing the crystallinity of a TiO2

photocatalyst has an impact on the reactivity of its surface, here towards methanol oxidation.

,f [s-1] kapp,r [s-1] P25 With O2 Without O2 k app,f [s -1 ] k app,r [s -1 ] With O 2 Without O 2 0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 kapp,f [s-1] kapp,r [s-1] CA With O2 Without O2 With O 2 Without O 2 k app,f [s -1 ] k app,r [s -1 ]

Conclusions

Three TiO2 photocatalysts -TiO2 P25, commercial anatase and homemade anatase -were compared in terms of both photocatalytic activity and formation-conversion kinetics of surface formate species in the photooxidation of methanol at 25 °C and atmospheric pressure. The formates kinetics were monitored with a time resolution of 100 µs, thanks to a QCL tuned to 1360 cm -1 (one of the characteristic IR absorption wavenumbers of formates), upon turning UV irradiation on (formates formation) and off (formates conversion) under continuous reaction flow. The photocatalytic activity in terms of total yield of methylformate + carbon dioxide appeared to be directly correlated to the apparent kinetic constant for the reaction of formates "kapp,r" (= -the initial rate of QCL beam absorption due to formate species when UV irradiation was turned off). Indeed, both followed the same trend, namely TiO2 P25 ≈ commercial anatase > homemade anatase. So, the faster the conversion of the surface formates is, the higher is the catalytic methanol oxidation activity. This demonstrates, to our knowledge for the first time, that the formation of methylformate and carbon dioxide occurs through the conversion of formate species at the surface of the photocatalysts, indeed. Furthermore, the conversion of formates was revealed to be the rate-limiting step of the reaction, thanks to kr/kf (= intrinsic rate constant of formates conversion/intrinsic rate constant of formates formation) ratios determined here for the first time, to our knowledge. Finally, we also found that the crystallinity of TiO2 impacts on the availability of its lattice oxygen to contribute for both the formation and reaction of formate species. Indeed, removing oxygen from the reaction flow reduces the kapp,f over TiO2 CA by a factor 2 while it does not affect that over TiO2 P25, and it reduces the kapp,r over TiO2 CA about 4 times more than that over TiO2 P25.

Associated content

Supporting Information. FT-IR spectra of TiO2 P25 before and during UV irradiation with only oxygen in the flow (no methanol) at 25 °C, and plots of the amplitude of the QCL signal versus time in various conditions.

  show bands at 1360, 1380, 1564 and 1580 cm -1 associated with ns(O-C-O), δ(C-H), nas(O-C-O)' and nas(O-C-O) vibrations of surface formatespecies, respectively.[START_REF] El-Roz | An operando-IR study of photocatalytic reaction of methanol on new *BEA supported TiO2 catalyst[END_REF][START_REF] Groff | An infrared study of formate formation and reactivity on TiO2 surfaces[END_REF][START_REF] Andersson | Microemulsion-Mediated Room-Temperature Synthesis of High-Surface-Area Rutile and Its Photocatalytic Performance[END_REF] The nas(O-C-O)' vibrations are those of formate species bonded to O vacancies in the TiO2 lattice (so sharing one formate O atom with the TiO2 surface structure) instead of Ti atoms.[START_REF] Andersson | Microemulsion-Mediated Room-Temperature Synthesis of High-Surface-Area Rutile and Its Photocatalytic Performance[END_REF] As shown by FiguresS1 and S2in the Supplementary Information, the four bands associated with formate species appear neither under UV irradiation alone (without methanol in the flow), nor under reaction flow alone (without UV irradiation). They thus result from the photocatalytic reaction of methanol.

Figure 1 .

 1 Figure 1. Operando FT-IR spectra at steady state of TiO2 P25, TiO2 CA and TiO2 HA upon photooxidation of methanol at 25 °C and atmospheric pressure, shown after subtraction of the initial spectrum recorded after dehydration pre-treatment under flowing methanol before UV irradiation. Conditions: flow = 30 mL/min; 0.1 and 20 vol.% of methanol and oxygen in argon, respectively; Hg-Xe lamp (200 W) filtered at λ = 365 nm; I0 (365nm) ≈ 15 mW/cm 2 . "Abs." means absorbance, and is expressed in arbitrary units.

Figure 2 .

 2 Figure 2. QCL beam absorbance at 1360 cm -1 vs. time over TiO2 P25 (a and d), TiO2 CA (b and e) and TiO2 HA (c and f) during the first UV on-off cycle (UV on: a-b-c, UV off: d-e-f) under reaction flow at 25 °C and atmospheric pressure. Conditions: flow = 30 mL/min; 0.1 and 20 vol.% of methanol and oxygen in argon, respectively; Hg-Xe lamp (200 W); I0 ≈ 200 mW/cm 2 . In (a-b-c), time = 0 corresponds to the moment when the shutter in-between the lamp and the UV-light guide directed to the IR cell was opened to irradiate the sample. During the preceding 200 ms, the sample was already under reaction flow but still in the dark. In (d-e-f), time = 0 corresponds to the moment when the shutter was closed again, still under reaction flow. The fall times to open and to close the shutter were around 6.5 ms and 3.2 ms, respectively, with a minimum pulse width of 13 ms.

Figure 3 .

 3 Figure 3. (a) Evolution of the (MF+CO2) yield vs. the kapp,r of formate species over TiO2 HA (circle), CA (square) and P25 (losange) (the specific surface area of the three photocatalysts is also shown, as a reference). (b) Methanol conversion vs. kapp,r/kapp,f ratio of formate species. (c) kapp,r vs. kapp,f of formate species. Conditions: reaction flow = 30 mL/min containing 0.1 and 20 vol.% of methanol and oxygen in argon, respectively; Hg-Xe lamp (200 W) filtered at λ = 365 nm; I0 (365nm) ≈ 15 mW/cm 2 .

Figure 4 .

 4 Figure 4. For TiO2 P25 and CA: apparent rate constants for the formation and reaction of formate species (kapp,f and kapp,r, respectively) at 25 °C and atmospheric pressure in the absence vs. presence of oxygen in the reaction flow. Conditions: flow = 30 mL/min; 0.1 vol.% of methanol in argon, with 20 vol.% of oxygen if present; Hg-Xe lamp (200 W); I0 ≈ 200 mW/cm 2 .

Figure 5 .

 5 Figure 5. QCL beam absorbance at 1360 cm -1 vs. time over TiO2 P25 during a UV on-off cycle (UV on: a-b, UV off: c-d) under flowing methanol at 25 °C and atmospheric pressure in the presence (a and c) or absence (b and d) of oxygen. Conditions: flow = 30 mL/min; 0.1 vol.% of methanol in argon, with 20 vol.% of oxygen if present; Hg-Xe lamp (200 W); I0 ≈ 200 mW/cm 2 . In (a-b), time = 0 corresponds to the moment when the shutter in-between the lamp and the UV-light guide directed to the IR cell was opened to irradiate the sample. During the preceding 200 ms, the sample was already under flowing methanol (with oxygen in a, without oxygen in b) but still in the dark. In (c-d), time = 0 corresponds to the moment when the shutter was closed again, still under the same flow as in (a-b). The times to open and close the shutter were around 6.5 ms and 3.2 ms, respectively.

1 Table 1 .

 11 For the three TiO2 photocatalysts studied in this work: specific surface area (SSA), apparent rate constants for the formation and conversion of formate species -kapp,f and kapp,r, respectively (mean values over the three UV on-off cycles)under reaction flow at 25 °C and atmospheric pressure, and overall photocatalytic activity in terms of methanol conversion (CH3OH conv.) and selectivity to formaldehyde (FA), methylformate (MF) and carbon dioxide (CO2) under the same reaction conditions. Estimated error on the conversion and selectivity values: 5 and 10%, respectively. Estimated error on the kapp,f and kapp,r values: 10%. Conditions: reaction flow = 30 mL/min containing 0.1 and 20 vol.% of methanol and oxygen in argon, respectively; Hg-Xe lamp (200 W) filtered at λ = 365 nm; I0 (365nm) ≈ 15 mW/cm 2 . "Nd" means not detected.

	TiO2	SSA		Formates kinetics		CH3OH		Selectivity	(MF+CO2)
	type	[m²/g]	kapp,f [s -1 ] kapp,r [s -1 ] kapp,r/ kapp,f	conv.	FA	MF	CO2 Other	yield
	P25	48	0.30	0.15	0.5	0.51	0.17 0.31 0.55	0.01	0.44
	CA	85	0.23	0.13	0.56	0.59	0.10 0.29 0.42	0.19	0.42
	HA	19	0.06	0.0091	0.15	0.05	0.17 0.16 0.38	Nd	0.027
		As shown by

Table 1 ,

 1 TiO2 P25 has an almost 2 times lower specific surface area than TiO2 CA (48 vs.

Acknowledgements

J. Schnee and M. Daturi acknowledge the LabEx EMC3 and the European Union for the cofunding via the contract "DRUID" and the operational program FEDER/FSE 2014-2020, respectively. The authors acknowledge Yoann Levaque and Oumaima Thili (Laboratoire Catalyse et Spectrochimie, Université de Caen, France) for the technical support and for providing the homemade anatase sample, respectively.

Author information

All authors have given approval to the final version of the manuscript.

Competing interests

The authors declare no competing interests.


Houeida Issa Hamoud

Mama Lafjah

Fatima Douma

Oleg I Lebedev

Fatiha Djafri

Valentin Valchev

Marco Daturi

Mohamad El-Roz
email: mohamad.elroz@ensicaen.fr

Photo-assisted SCR over highly dispersed silver sub-nanoparticles in zeolite T under visible light: An Operando FTIR study

Keywords: Photo-assisted SCR, Silver clusters, Zeolite, Photocatalysis, Plasmon

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

Introduction

Nitrogen oxides (NOx) from diesel engines contribute substantially to air pollution [START_REF] Larrubia | An FT-IR study of the adsorption and oxidation of N-containing compounds over Fe2O3-TiO2 SCR catalysts[END_REF]. Selective Catalytic Reduction (SCR) with NH3 is one of the most effective technologies to control automotive NOx emissions. Most of the SCR catalysts possess a high activity at the intermediate and high temperature ranges (200-500 °C) [START_REF] Gao | Morphology-dependent properties of MnOx/ZrO2-CeO2 nanostructures for the selective catalytic reduction of NO with NH3[END_REF].

The development of active SCR catalyst at a lower temperature (< 200 °C), during the engine start-up, is very challenging due to the presence of excess H2O and adsorption competition between ammonia and combustion residuals on active sites [START_REF] Malpartida | An operando IR study of the unburnt HC effect on the activity of a commercial automotive catalyst for NH3-SCR[END_REF]. In this regard, a combined strategy between LNT (Lean NOx Trap) and SCR is necessary, which significantly increases the SCR operational costs and complexity [START_REF] Castoldi | Catalytic behaviour of hybrid LNT/SCR systems: Reactivity and in situ FTIR study[END_REF][START_REF] Xu | LNT+ in situ SCR catalyst system for diesel emissions control[END_REF].

Among the catalysts reported for the low-temperature SCR, single and mixed metal oxides catalysts containing Cr, Mn, Ce, Fe, Co, Ag, V, Ni and/or Cu oxides have been studied, combining the synergetic catalytic effect of the different components [START_REF] Thirupathi | Co-doping a metal (Cr, Fe, Co, Ni, Cu, Zn, Ce, and Zr) on Mn/TiO2 catalyst and its effect on the selective reduction of NO with NH3 at low-temperatures[END_REF][START_REF] Park | Effect of sulfur on Mn/Ti catalysts prepared using chemical vapor condensation (CVC) for low-temperature NO reduction[END_REF][START_REF] Huang | Alkali-and sulfur-resistant tungsten-based catalysts for NOx emissions control[END_REF]. For example, silver clusters (Agn δ+ ) have previously been identified as the prime species for the activity of Ag/Al2O3 in H2-assisted NH3-SCR (Strömet al., 2018), and it has been shown that the activity is linearly proportional to the relative amount of these clusters. However, these catalysts still present several drawbacks, such as the need of H2 to form a proton on Al2O3, which directly participates in the reaction mechanism, the poor N2 selectivity and low deNOx performance at low temperature (<200 °C) [START_REF] Ström | Surface species and metal oxidation state during H2-assisted NH3-SCR of NOx over alumina-supported silver and indium[END_REF].

Photo-assisted selective catalytic reduction of NOx under visible light is a promising approach for a significant decrease in the SCR reaction temperature. The active species can be generated at a relatively low temperature ensuring the oxidation-reduction reaction involved in the SCR process. Tanakaet al. have studied the photo-assisted selective catalytic reduction of NO with NH3 over a serial of TiO2 based photocatalysts such as: TiO2 modified by transition metal (i.e, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Zr, Nb, Mo, Ta or W) [START_REF] Yamazoe | Promotion effect of tungsten oxide on photo-assisted selective catalytic reduction of NO with NH3 over TiO2[END_REF]Yamamoto et al. 2015) and dye-modified TiO2 (Yamamoto et al., 2014a(Yamamoto et al., , 2015b)). In the TiO2 system, more than 80 % of NO conversion and approximately 100 % of selectivity can be achieved at a gas hourly space velocity (GHSV) of 8000 h -1 at 160 °C. However, the TiO2 has a low absorbance and activity in the visible range of light. The modification of the TiO2 with dyes can enhance significantly its activity under visible light. On the other hand, these molecules have weak stability under the reaction condition required for photo-SCR reaction (the presence of oxidant species promotes the degradation of the dyes at relatively high temperature). Recently, Lia et al. (2018) have studied the photo-SCR of NO over rare-earth natural clay coupled with reduced graphene oxide under simulated solar-light irradiation.

About 70 % of NO conversion and 100% N2 selectivity were obtained at room temperature.

Unfortunately, the complexity of such a system and their high-cost multi-step preparation method limit their practical use in diesel cars. Thus, developing new stable, and low-cost materials which can work at low-temperatures and high-GHSV SCR conditions remains a great challenge. More recently, we have successfully synthesized sub-nanometer silver clusters with enhanced narrow size distribution using the uniform size cages of FAU-type zeolite as a scaffold [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF]. The sub-nanometer Ag clusters show an exceptional photocatalytic activity and selectivity for the reforming of formic acid to H2 and CO2 under visible light. The photocatalytic activity was assigned to the plasmonic behavior of the Agn δ+ clusters. The high performance of these materials in photocatalysis under visible light makes them very promising candidates for the photo-assisted SCR reaction especially at low temperatures, in the so-called cold-start domain.

In this study, sub-nanometer silver cluster in FAU-type zeolite were studied in the photoassisted reaction at room temperature and 150 °C using operando FTIR spectroscopy. The first part of the paper is dedicated to the preparation and the physicochemical and textural characterization of the catalysts. Then, the performance of the prepared catalysts for the SCR-NH3 reaction under different conditions of temperature and irradiation is reported, and a possible reaction mechanism is discussed.

Experimental part

Materials

Sub-nanometer silver clusters in nanosized zeolite ZX-V (FAU-type, 10-30 nm, Si/Al = 1.63) were prepared in two-steps, as described in reference [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF]. In the first step, the nanosized vanadium-containing zeolite X (ZX-V) was synthesized from a clear precursor suspension with a molar composition: 10 SiO2: 1.1 Al2O3 : x V2O5 : 9 Na2O : 122 H2O. Sodium vanadate was selected as a precursor of the photoactive sites due to the efficiency of the vanadate sites for silver ion reduction versus water oxidation (formation of OH radicals and oxidant species are prevented). The properties of this precursor are fully compatible with the synthesis of the organic-template-free zeolite (same counter ions, low polymerization at high pH, etc.). The initial reactants were mixed to prepare two initial solutions denoted A and B. Solution A was prepared by dissolving 0.296 g aluminum powder (325 mesh, 99.5%, Alfa Aesar) and 2.5 g NaOH (Aldrich, 97%) in 3 g double distilled H2O. Vanadium containing samples were prepared by adding 0.122 g of VO3Na (2 wt.%, in respect to the Silica and Alumina content) to this solution and stirring for 3 h. Solution B was prepared by mixing 10 g colloidal silica (Ludox-HS 30, Aldrich) with 1.077 g of NaOH and 1 g double distilled H2O.

The resulting turbid suspensions were placed in an oven at 100 °C for 5 minutes to obtain water clear suspensions. Solution A was added drop wise under vigorously stirring to the solution B at 4 °C using an ice-bath. The resulting colorless suspensions were stirred 24 h at room temperature followed by dehydration of 6 to 6.5 g H2O (≈ 40 M H2O from the general gel composition). Hydrothermal crystallization was conducted at 50 °C for 36 h. The nanoparticles of ZX-V were recovered by centrifugation (25 min, 20000 rpm) followed by redispersion in double distilled water. The procedure was repeated till reaching pH ~ 8. The second step was devoted to the preparation of Ag@ZX-V by dispersing ZX-V (zeolite concentration = 3 mg/ml) in water solutions containing ethanol as an electron donor (water:ethanol=3:1; with total volume = 20 mL) and silver nitrate as a silver precursor (3 mM).

The reduction of silver ions is performed by UV irradiation of the solution for 5 min under stirring using a polychromatic Hg-Xe lamp (200 W). Less than 3 minutes of UV irradiation was enough to change the color of the milky suspension to green. This typical color modification is due to the formation of reduced silver particles in the solution. The essential role of vanadate and ethanol confirms the photocatalytic behavior of the process. The experiments were repeated many times until obtaining 200 mL of Ag@ZX-V solution. The solid was recovered by centrifugation (25 min, 20000 rpm) followed by re-dispersion in double distilled water and the procedure was repeated two times in order to remove the excess of free silver and silver precursor. Then, the suspension was dried at 50 °C under atmospheric condition.

Techniques

X-Ray Diffraction (XRD) analyses of the ZX-V and Ag@ZX-V materials were carried out with PANalytical X'Pert Pro diffractometer with CuKα irradiation (λ = 1.5418 Å). Rietveld refinements were then performed with the JANA2006 software [START_REF] Palatinus | SUPERFLIP-a computer program for the solution of crystal structures by charge flipping in arbitrary dimensions[END_REF]) on PXRD data recorded on a Bruker D8-vario1 diffractometer equipped with a primary germanium (111) Johansson monochromator (λKα1 = 1.5406 Å) and a LynxEye detector. The PXRD diagrams were recorded at room temperature between 5 and 50° (2θ) with a step size of ~0.014°. Variable divergent slits with a constant illuminated sample length of 6 mm were used.

Phase identifications were performed with the PANalytical High Score plus program. The crystal structure of Ag@ZX-V was solved by ab initio method using the SUPERFLIP software [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF].

Nitrogen adsorption/desorption measurements were performed with an ASAP 2020 MP instrument. The specific surface area was calculated with the BET equation while the pore volumes were determined by the BJH method. Prior to the measurements, samples were outgassed in situ at 150 ºC for 3 h.

Advanced transmission electron microscopy (TEM), high-angle annular dark field scanning TEM (HAADF-STEM) together with EDX mapping experiments were carried out on an aberration probe and image corrected JEM ARM200F cold FEG microscope operated at 200 kV equipped with a CENTURIO EDX detector and GIF Quantum spectrometer. DR-UV-Vis spectra of the dried powder samples were recorded by using a Cary 4000 UV-Vis spectrophotometer and a RSA-CU40 Diffuse Reflectance cell.

In situ FTIR spectroscopy of CO chemisorption was performed in order to determine the nature and the oxidation state of active sites and their accessibility. The samples (pellet of ~20 mg) were first heated under vacuum (<10 -6 mbar) at 150 °C during 2 h. Doses of CO (between 1 and 40 torr) were then put in contact with each sample until saturation. The IR spectra were recorded with a Nicolet 6700 FTIR spectrometer equipped with a DTGS detector at a spectral resolution of 4 cm -1 by accumulating 128 scans.

(Photo)Catalytic tests

For the operando experiments, a 'sandwich like' IR cell-reactor was used to analyze the catalysts surface and measure their SCR activity at the steady-state (El-Roz et al., 2017). First, the catalyst (shaped as a pellet of ~ 20 mg) was pretreated at room temperature under Ar and irradiation using a Xe-lamp (LC8 Hamamatsu, 200 W) with cut-UV filter (l > 390nm). The lamp irradiance was measured using ILT950 spectrilight spectroradiometer from International Light Technologies. Then, the reaction was studied at RT and 150 °C in the dark or under irradiation. The full flow gas composition is reported in Table 1.

Table1.

The total flow rate was fixed at 31 cc.min -1 . The relative concentrations of the effluent gas were continuously measured by Mass Spectrometry (Quadrupole Pfeiffer Omnistar GSD 301) and stabilized before being sent to the cell. Finally, the composition of the output gas from the IR reactor cell, including NO, NO2 and NOx, was analyzed simultaneously by Chemiluminescence (Thermo-Scientific, model 42 i-HL) and IR spectrometer (ThermoNicolet NEXUS 670 FTIR) equipped with an MCT detector with a spectral resolution of 4 cm -1 and accumulating 64 scans. The NOx conversions were calculated from the equations (1) at the steady-state (after at least 1 hour of reaction). It should be noted that these conversions are corrected by those obtained for the same experiment performed in dark and at RT (no possible reaction is expected in this condition for both catalysts).

NOx conversion

[%] = ("#$,&'()*+"#$,,-*()*) ("#$,&'()*)
x 100 (Eq. 1)

Results and discussion

Characterizations of ZX-V and Ag@ZX-Vcatalysts

The X-ray diffraction patterns of the ZX-V and Ag@ZX-V samples are displayed in Figure 1(A).

Figure 1.

The ZX-V sample shows the typical Bragg pattern of FAU-type zeolite. The Rietveld refinements (Table S1 and S2 in the supplementary information) show the following chemical formula Na82Al73Si119O384(H2O)139. A drastic change of the relative intensities of the diffraction peaks can be clearly observed after loading Ag into the ZX-V structure. This is mainly related to the change of the cationic composition between the two Faujasites. The deduced chemical formula of the Ag@ZX-Vis Ag27Na55Al73Si119O384(H2O)120 with the stoichiometric sum of the Agn d+ and Na + cations equal to 82, as previously observed for ZX-V. It should be noted that no diffraction peaks of metal oxide (M = V, Ag) phases were detected in the XRD patterns. This results show that the V content (0.25 wt.% V)) is below the detection limit of XRD. The absence of silver peaks is due to the small size of highly dispersed silver phase in the fresh sample.

Figure 1(B) shows that both ZX-V and Ag@ZX-V samples exhibit a combination of Type I and IV isotherms with a H3-type hysteresis. The surface areas and pore volumes of ZX-V and Ag@ZX-V samples are summarized in Table 2.

Table 1.

The Ag@ZX-V catalyst exhibits a significantly decreased specific surface area (369 m 2 /g) and micropore volume (0.091 cm 3 /g) as compared with the parent ZX-V catalyst (SBET = 689 m 2 /g; Vmic = 0.185 cm 3 /g). The mesoporosity was also decreased from 1.098 to 0.839 cm 3 /g after Ag loading into ZX-V structure. This decrease can be attributed to the partial pores blockage by Ag species and the higher density of silver particles.

The high resolution HAADF-STEM images of Ag@ZX-V sample are presented in Fig. 2.

The Ag sub-nanoparticles appear as bright white dots within the pores of FAU-type structure.

Ag-free ZX-V crystallites can also be observed in Figure 2(a) with a crystal size lower than 10 nm. The distribution of silver clusters in the zeolite is not perfectly uniform (some pores are empty or not completely filled; Fig. 2c) which in agreement with the loss of external surface area of ZX-V after Ag loading (Table 2). It should be noted that segregation of Ag nanoparticles from the pores into larger Ag nanoparticles can be obtained after the second scan by e-beam inside the microscope during imaging (Fig. S2). The EDS mapping of the Ag@ZX-V is reported in Fig. S3. The result confirms the good dispersion of the silver in the zeolite.

However, this result should be interpreted with precaution due to the low stability of the silver particles under the electron beam (Fig. S2). More details concerning this phenomenon are reported in our previous paper [START_REF] El-Roz | Uniform Generation of Sub-nanometer Silver Clusters in Zeolite Cages Exhibiting High Photocatalytic Activity under Visible Light[END_REF].

Figure 2.

The DR-UV-Vis spectra of ZX-V and Ag@ZX-V and activated Ag@ZX-V at 150°C under Ar are illustrated in Fig. 3. For ZX-V, only one broad band was observed in the region of 250-285 nm, with a maximum at 270 nm. This is attributed to the low-energy charge transfer band (LCT) associated with O to V 4+ electron transfer. This result points out that the amount of polymeric V 5+ oxides is negligible and that namely isolated interstitial V 4+ species are present.

The ICP and EDX analyses of the ZX-V sample showed that vanadium content is around 0.25 wt. %. The characterization and localization of vanadate species was difficult due to their relatively low amount in the zeolite. It is clear, however, that oxalate-vanadate species are highly dispersed within the zeolite structure as clusters and no large aggregates are formed.

Figure 3.

The DR-UV-Vis spectrum of Ag@ZX-V powder shows the appearance of new UV-visible bands at 270, 295, 325, 377, 414nm. The bands between 270 and 380 nm are characteristic of the electronic transitions of reduced Ag species (Agn d+ ) with sub-nanometer particles size [START_REF] Gurin | Metal clusters and nanoparticles assembled in zeolites: an example of stable materials with controllable particle size[END_REF][START_REF] Severance | Evolution of silver nanoparticles within an aqueous dispersion of nanosized Zeolite Y: mechanism and applications[END_REF]). In addition, new bands at 432 and 562 nm attributed to the formation of silver nano-particles with a particle size >12 nm and >20 nm are also observed [START_REF] Cuong | Theoretical modeling of optical properties of Ag 8 and Ag 14 silver clusters embedded in an LTA sodalite zeolite cavity[END_REF][START_REF] Zou | Controlled growth of silver nanoparticles in a hydrothermal process[END_REF]. The presence of narrow UV bands at specific wavelength indicates that the silver clusters have well-defined size and charge. The visible bands (400-530 nm) can be attributed to the external silver nanoparticles as well as to the connected silver clusters within the network of the zeolite pores.

It should be noted that the same bands are observed in the Ag@ZX-V suspension (Fig. S4).

This demonstrates the good stability of the prepared clusters in their solid forms under aerated condition. The shift observed in some absorption band can occur due to a solvatochrom effect.

After in-situ activation of the sample at 150°C (under Ar), an increase of the broad plasmonic band has been observed in the visible range. This is most probably due to the migration of the silver clusters and their aggregation promoting the formation of silver nanoparticles. This phenomenon was already observed under the TEM beam (Fig. S2).

The infrared spectroscopy of adsorbed CO was used to determine the oxidation state of silver nanoparticles (Fig. 4). It is well known that surface carbonyls with zero-valent silver atoms cannot be formed at room temperature (Hadjivanov et al., 2002). No characteristic bands of carbonyl groups are observed in the ZX-V spectra, while an intense band at 2185 cm -1 is obtained after CO adsorption on Ag@ZX-V (Fig. 4(A)). This band is attributed to Ag + ions, mainly located inside the pores (Hadjivanov et al., 2002). It is formed by multiple components

Catalytic activity in the photo-assisted SCR-NH3 reaction

The physicochemical properties, such as specific surface area, Si/Al ratio, acidity, stability and redox ability of metal-exchanged zeolite catalysts are strongly correlated to the catalytic performances for NOx removal during the SCR process. It is generally recognized that both adsorbed ammonia on Lewis acid sites and ammonium ions associated with Brønsted acid sites are precursors to an activated site that can selectively reduce NOx via different possible mechanisms of low-temperature NH3-SCR reaction, combining redox and acid-base steps [START_REF] Gao | A review on selective catalytic reduction of NOx by NH3 over Mn-based catalysts at low temperatures: catalysts, mechanisms, kinetics and DFT calculations[END_REF][START_REF] Iwasaki | Mechanistic Aspect of NO-NH3-O2 Reacting System[END_REF]I. Nova et al.;[START_REF] Iwasaki | Mechanistic Aspect of NO-NH3-O2 Reacting System[END_REF]. These concepts can be summarized and described through an Eley-Rideal (ER) mechanism, where the gaseous NO first reacts with activated NH3-absorbed species to form intermediates and then decomposes into N2 and H2O, or via a Langmuir-Hinshelwood (LH) mechanism, where the gaseous NO is adsorbed on a basic site and further combine with the adjacent activated NH3 species to form N2 and H2O.

According to previous reports regarding the SCR process over metal-exchanged zeolites [START_REF] Deka | Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5[END_REF][START_REF] Long | Reaction mechanism of selective catalytic reduction of NO with NH3 over Fe-ZSM-5 catalyst[END_REF], the presence of Brønsted acid sites is essential for achieving a high NOx conversion and high N2 selectivity. The Brønsted acid sites are involved in the reaction by chemically binding and activating the ammonia to form ammonium ions, which then participates in the SCR reaction.

On the other hand, [START_REF] Bin | Structural characterization and selective catalytic reduction of nitrogen oxides with ammonia: a comparison between Co/ZSM-5 and Co/SBA-15[END_REF] show that the existence of Brønsted acid sites contributes to an excellent dispersion of metal species within the zeolite structure. More recent studies have lowered the importance of these sites in the SCR mechanism [START_REF] Janssens | A consistent reaction scheme for the selective catalytic reduction of nitrogen oxides with ammonia[END_REF]. In our study, the FAU-X based samples do not possess any Brønsted acid site due to its sodium form with a low Si to Al molar ratio (Si/Al = 1.63). This is confirmed by the IR spectra of the ZX, ZX-V and Ag@ZX-V samples (Fig. S5), showing only one band at 3738 cm -1 in the O-H region characteristic of terminal Si-OH group [START_REF] Deka | Influence of NO2 on the selective catalytic reduction of NO with ammonia over Fe-ZSM5[END_REF]. However, the formation of acid sites during the reaction on Ag@ZX-V cannot be excluded due to the presence of NH3 and H2O in the gas phase. Thus, the monitoring of ZX-V and Ag@ZX-V samples in the photoassisted SCR-NH3 reaction at RT and at relatively low temperature (T = 150 °C) is of outmost importance. Then, some possible structure-activity relationships can be discussed by comparing the data obtained during the catalytic test with those relevant to their physicochemical properties.

NOx conversions during the photo-SCR

Figure 5 shows the conversion of NO, NO2, and total NOx during the photo-SCR at RT over the ZX-V and Ag@ZX-V catalysts. The values are determined by chemiluminescence at the steady state after 4 hours of visible-light irradiation. The NOx values were subtracted from those obtained at the steady state in dark at RT. Each experiment was repeated at least two to three times, with an estimated uncertainty of about ± 5%. It should be noted that it was difficult to estimate the ammonia conversions during the photo-SCR, due to the low intensity of the NH3 IR band in the gas phase. There is no significant activity (in the error range) observed over ZX-V, while negative conversions were obtained over Ag@ZX-V catalyst. The negative values obtained over Ag@ZX-V correspond to an increase of the NO, NO2 and NOx amount in the gas flow due to ammonia photooxidation. It should be noted that Ag + -ZX-V, prepared by a simple ionic exchange (no silver reduction), was also tested under similar previous conditions and no photocatalytic activity was observed (Fig. S6). This result indicates that the highly dispersed Agn d+ clusters in the zeolite matrix play a crucial role in the properties of a plasmonic photocatalyst. The significant amount of NO2 (14.4 % of conversion) detected at the steadystate of the photocatalytic reaction at RT (Fig. 5), indicates possible oxidation of NO into NO2 over Ag@ZX-V. The capacity of silver nanoparticles in the ammonia oxidation at relatively low temperature is already reported in the literature by using Ag/Al2O3 catalyst [START_REF] Wang | Nanosize effect of Al2O3 in Ag/Al2O3 catalyst for the selective catalytic oxidation of ammonia[END_REF]. The authors show that both the valence state and particle size of Ag species had a significant influence on the catalyst activity for the ammonia oxidation to NO.

Increasing the temperature from RT to 150 °C does not influence the performance of ZX-V in both dark and under visible light irradiation conditions, and no significant activity is observed (Figure 6(A)). This result is due to the absence of active sites for SCR on ZX-V and the lack in the active species with absorption edges in the visible-light region. Same behavior of Ag@ZX-V can be observed at 150°C in dark without any significant catalytic activity. On the contrary, the activity of the Ag@ZX-V sample was radically changed under visible irradiation at 150°C (Fig. 6(B)). Loading of uniform, sub-nanosized and stable silver particles in the ZX-V material improves the NOx conversion from 0.2 ±5 % (in dark) to more than 14.7 ±5 % (under visible light). It is to highlight that during the SCR experiments, no side products as N2O were detected in the outlet, witnessing for 100 % selectivity to N2. The CO and HC conversions were also investigated during the photo-assisted SCR reaction at RT and T = 150 °C (supplementary data, Fig. S7). All conversions are negligible over both ZX-V and Ag@ZX-V samples under the different conditions used in this work.

Figure 6.

Evolution of surface species during the photo-SCR

The FTIR spectra of the catalysts at the steady-state of the SCR-NH3 reaction are presented in Fig. 7.

Figure 7.

In the absence of irradiations at RT (spectra a, Fig. 7), a band centered at 1642 cm -1 , characteristic of NH3 coordinated to Lewis acid sites [START_REF] Rasmussen | Transient operando study on the NH3/NH4 + interplay in V-SCR monolithic catalysts[END_REF][START_REF] Hadjiivanov | Selective Catalytic Reduction of NOx over Cu-and Feexchanged zeolites and their mechanical mixture[END_REF], is observed for both samples. It can be strongly overlapped with the vibration band of adsorbed water. It is important to note that this band is more intense (saturated) and pronounced in case of ZX-V than for Ag@ZX-V. In addition, traces of band of ammonium cations can be observed, associated with the weak band at 1442 cm -1 (spectra a, Fig. 7), due to the presence of silica impurities that provide weak Brønsted acid sites [START_REF] Gallardo Amores | An FT-IR study of ammonia adsorption and oxidation over anatase-supported metal oxides[END_REF]. Such silica phase (quartz) was detected in the X-ray diffraction patterns of ZX-V and Ag@ZX-V (see part 2.1.).

No significant changes were observed in the IR spectra of ZX-V under visible irradiation at RT (spectrum b, Fig. 7(A)). This is in agreement with the absence of any photocatalytic activity of this sample at RT.

In contrast, the intensity of the band at 1642 cm -1 of Ag@ZX-V is significantly decreased when the visible light is switched on (spectrum b, Fig. 7(B)). This is probably related to water desorption and/or NH3 desorption/conversion. Under visible irradiation, a new broad band appeared at 1400 cm -1 belonging to the asymmetric vibration of adsorbed NH3on the Brønsted acid sites (formation of NH4 + ) (Strömet al. 2018). This is confirmed by the less pronounced absorption bands at 3260 and 3300 cm -1 , assigned to the symmetric and asymmetric N-H stretching vibrations of NH3 hydrogen bonded to surface OH (Fig. S8). Thus, the development of the band at 1400 cm -1 indicates the in-situ formation of new B.A.S. on the zeolite during the reaction, promoted by water dissociation and/or ammonia oxidation. The in situ formation of B.A.S. has been already reported in the literature [START_REF] Rasmussen | Transient operando study on the NH3/NH4 + interplay in V-SCR monolithic catalysts[END_REF][START_REF] Ström | Surface species and metal oxidation state during H2-assisted NH3-SCR of NOx over alumina-supported silver and indium[END_REF] and authors showed that water can hydrolyze metal sites (Al, Agn d+ clusters and/or vanadia species) of the catalyst creating new Brønsted acid sites. Also, the adsorbed ammonia is either directly consumed by the SCR reaction or converted into ammonium, which then participates in the SCR.

As the temperature increased from RT to 150 °C, the intensity of the band at 1642 cm -1 decreased rapidly, indicating the water/ammonia desorption. The other band located at 1400 cm -1 is appeared clearly in the spectra of ZX-V and is become more intense (saturated) in the case of Ag@ZX-V (spectrum (c), Fig. 7). In fact, water molecules might strongly adsorb and fully occupy the adsorption sites of ZX-V at RT, preventing the adsorption of NH3 on the B.A.S (Fig. S9). However, when the reaction temperature increases to 150 °C, more water molecules would desorb and release the active sites. As a result, the formation of NH4 + on the Brønsted acid sites of ZX-V is now clearly visible at T = 150 °C. However, the negligible catalytic activity of ZX-V and Ag@ZX-V at 150 °C (in dark) reveals that the formation of NH4 + is not enough for promoting the deNOx process. Under visible light irradiation at 150 °C, no modification in the IR spectrum of ZX-V surface is observed, as expected. However, a significant decrease of the bands at 1642 cm -1 and 1400 cm -1 are observed on the Ag@ZX-V (spectrum (d), Fig. 7).

The operando FTIR results on Ag@ZX-V reveal more information on the evolution of the surface species as a function of time during the photo-SCR reaction. The results are reported in Fig. 8. The data help to envisage the interplay between the pre-adsorbed species, ammonia and ammonium ions. While the band of NH3 bending vibrations at Lewis acid sites continuously decreases, a significant increase of the band at 1400 cm -1 , assigned to the vibrations of the ammonium ions at Brønsted acid sites, is observed during the reaction at RT under visible light (Fig. 8(A)). The irradiation of the Ag@ZX-V with visible light at 150°C under reaction flow (Fig. 8(B)) led to a quick decrease of water, ammonium and ammonia bands.

Figure 9.

The time-evolution of NH4 + (1400 cm -1 ) and NH3 overlapped by water adsorption (1642 cm -1 ) band areas over Ag@ZX-V during SCR-NH3 reaction at RT and 150 °C is shown on Fig. 10. The main observation from this experiment is the significant and sudden drop of all species (and especially ammonia) when the visible light is turned on, at all temperatures, except for NH4 + at RT (Fig. 10(A)). In contrast, no changes in ammonia evolution were observed over ZX-V under similar conditions (supplementary data, Fig. S10). Otherwise, the NH4 + ions are continuously formed on ZX-V during the reaction at 150 °C, confirming that the only conversion of ammonia into ammonium cannot promote the SCR at 150 °C. These results with the catalytic activity confirm the photo-assisted SCR reaction over Ag@ZX-V with high selectivity.

Figure 10.

Mechanism

In summary, the set of the previous experimental data revealed that the ZX-V sample was completely inactive during the photo-assisted SCR-reaction at RT and at T = 150 °C. In contrast, the reaction under visible irradiation seems to favor the oxidizing pathway to generate

NO and NO2 at RT over Ag@ZX-V, while the photo-assisted NOx reduction is promoted at T = 150 °C. In this part, the physico-chemical properties of Ag@ZX-V, as well as the reaction mechanism proposed in the literature, can be used to get insights in the photo-assisted SCR pathway over Ag@ZX-V at T = 150 °C.

Progressive hydroxylation for supported zeolite with pure Lewis acidity indicates that a significant amount of water molecules will adsorb dissociatively, hydroxylating the zeolite This can hinder the access of NH3 molecule to the zeolite and partially inhibits the formation of NH4 + on the Brønsted acid sites. Under visible irradiation and/or by increasing the temperature, the formation of ammonium species is promoted by partial desorption of water.

On the other hand, NH3 can also be linked to the free Ag Lewis sites. The presence of both adsorbed NH3 and NH4 + is indispensable for the photo-SCR reaction.

In a classical semiconductor photocatalyst [START_REF] Shishido | A unique photo-activation mechanism by "in situ doping" for photo-assisted selective NO reduction with ammonia over TiO2 and photooxidation of alcohols over Nb2O 5[END_REF][START_REF] Li | Photo-assisted selective catalytic reduction of NO by Z-scheme natural clay based photocatalyst: Insight into the effect of graphene coupling[END_REF], the UV or visible excitation can promote the formation of electron and holes by the excitation of an electron occupying the valence band, transferring it into the conduction band. The produced holes promote the oxidation, and the conduction band electrons promote the reduction of the adsorbed species. However, in the case of metallic clusters, the localized surface plasmon resonance (LSPR) is the unique optical phenomenon that occurs. When the frequency of incident light matches the oscillation frequency of the electrons of the conduction band of the metal, it triggers a collective excitation of the electrons. These excited surface electrons (hot electrons) oscillate coherently with the incident electromagnetic field. Then, hot electrons relax through a multi-step process initially involving electron-electron scattering, and then on a much slower timescale, transferring energy from the thermalized electron gas to the metal lattice via electron-phonon coupling, causing an interaction between the metal lattice and its surroundings (phonon-phonon interactions) (Liu & Li;2016). This non-radiative LSPR relaxation process converts the kinetic energy of hot electrons into thermal energy heating up quickly the surroundings of a metal surface. The generated heat can promote redox reactions that require a high temperature in dark conditions. One of the possible explanations of the photocatalytic activity of Ag@ZX-V at RT is the formation of oxidant species able to oxidize ammonium into NO, and NO into NO2. Recently, [START_REF] Planas | Distance-dependent plasmon-enhanced singlet oxygen production and emission for bacterial inactivation[END_REF] demonstrated that silver nanoparticles could enhance the formation of singlet oxygen ( 1 O2). More recently, Macia et al.

(2018) demonstrated an impressive plasmon-enhanced singlet oxygen production on hybrid silver nanocubes. However, in the previously cited works, the formation of singlet oxygen through direct sensitization by metal nanoparticles was not investigated, and the role of the plasmon was limited to the enhancement of the singlet oxygen production via a grafted photosensitizer (Rose Bengal). The unprecedented and well demonstrated observation of singlet oxygen formation through direct sensitization by metal nanoparticles (Ag, Au, Pt) is, however, demonstrated by [START_REF] Vankayala | Metal nanoparticles sensitize the formation of singlet oxygen[END_REF]. The singlet oxygen is well known for his capacity to generate highly oxidant OH and/or peroxide in the presence of labile hydrogen (in our case, the hydrogen of the ammonium) (Insug et al., 2015;[START_REF] Rinalducci | Formation of radicals from singlet oxygen produced during photoinhibition of isolated light-harvesting proteins of photosystem II[END_REF].

Therefore, the active site for the photo-oxidation at RT should be the Lewis acid site on Ag clusters, and the adsorption of NH3 onto the Lewis acid site would be the first step in the photooxidation. The adsorbed NH3 on Ag clusters (Lewis acid sites) could be activated by singlet oxygen to the NH2 radical under visible light (Eq. 2). The NH2 radical further reacts with the singlet oxygen to form NO (Eq. 3). The singlet oxygen can also oxidize NO into NO2 (Eq. 4).

NH3 + 1 O2 → NH2 • + H + + O2 -(Eq. 2) NH2 • + 1 O2 → NO + H2O (Eq. 3) NO + ½ 1 O2 → NO2 (Eq. 4)
When the temperature increases from 25 to 150 °C under visible irradiation, the route of NH3 reduction is more favorable than the ammonia oxidation pathway. The NH3 reduction is promoted by the local plasmonic heating under visible light, which induces the thermal decomposition of NH3-NOx intermediates. In fact, the irradiation by visible light at 150°C leads to significant heating of the catalyst and thus the apparent temperature at the contact point would be higher than 150 °C. Consequently, the SCR can operate at the low-temperature range in the presence of visible irradiation without increasing the temperature. This is confirmed by comparing the SCR activity of Ag@ZX-V at 150°C under visible light and at 200°C in dark (Fig. S11). The catalyst shows a good SCR activity under both conditions, with however higher and H2O (Eq. 6) seems to be doubtful in our case, since no indication of such species was observed in our FTIR spectra. Alternatively, we can assume that they were rapidly decomposed during the reaction. However, it is expected that surface NH2NO species are the major intermediates participating in the formation of N2 over the Ag-based catalyst [START_REF] Kondratenko | Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2[END_REF]Strom et al., 2018;Shimizu et al,. 2017;[START_REF] Kondratenko | Mechanistic aspects of N2O and N2 formation in NO reduction by NH3 over Ag/Al2O3: The effect of O2 and H2[END_REF].

NH2 • + NO (NO2) ⇌ NH2NO (NH2NO2) (Eq. 5)

NH2NO → N2 + H2O (T>150°C) (Eq. 6)

Based on this information and our previous results, the mechanism of the oxidation of NH3 and reduction of NO, at room temperature and 150 °C, respectively, can be proposed as follow: at room temperature, the excitation of the silver particle promotes the formation of singlet oxygen, this later oxidize NH3 first into NH2 and then to NO. The oxidation of NO by singlet oxygen forming NO2 can also take place. H + can also be produced from this oxidation process to form the NH4 + species detected on the surface of the catalyst. At 150 °C, the produced NH2 under visible light can interact with NO and/or NO2 to form NH2NO and NH2NO2, respectively. The dissociation of these intermediates to N2 and H2O is promoted by the local heating and traduced by a photo-asssisted SCR process. The mechanism of the reaction is illustrated in scheme 1.

Scheme 1.

Conclusions

In summary, sub-nanometer silver clusters in FAU-type zeolite containing vanadate clusters were synthesized and evaluated during the photo-assisted SCR-NH3 reaction under visible light at RT and 150 °C. The Ag-free sample was completely inactive during the photoassisted SCR-reaction under studied conditions.,In contrast, Ag@ZX-V revealed a photocatalytic activity under visible light promoted by the high dispersion and the good accessibility of the generated silver nanoparticles. The operando FTIR technique revealed the in-situ formation of Bronsted acid sites on Ag@ZX-V catalyst which play the role of storing and supplying the NH3 species to the active sites. Further the ammonia activation by singlet oxygen to possibly yield NH2 radicals is most probably a result of the direct sensitization by Agn d+ clusters/nanoparticles under visible irradiation. At room temperature, the singlet oxygen can further oxidize NH2 radical to NO as well as NO to NO2. However, the main reaction pathway leading to N2 is the thermal decomposition of NH2NO intermediates, formed from NO and adsorbed NH2 fragments. This SCR pathway is probably promoted by the local plasmonic heating of the catalyst at 150 °C and under visible light. However, further experiments are required to investigate the reaction intermediates that are not detected in the present study, likely due to their short lifetime.

(

  Fig. 4(B)) likely due to different silver clusters inside the zeolite. As the equilibrium pressure increases (Fig. 4(B)), another component appears at ~2135 cm -1 , which is characteristic of physisorbed CO on Agn d+ .

  Figure 4.

  Figure 5.

Figure 5

 5 Figure 5 illustrates the activity of both catalysts at RT temperature under visible light.

Figure 8 .

 8 Figure 8.

  support and hydrolyzing the M-O bonds (M = Al or Ag), thus leading to the co-existence of Lewis and Brønsted acid sites. The formation of new Brønsted acid sites by hydroxylation induces the transformation of adsorbed NH3 into ammonium species. Temperature and presence/absence of visible light conditions modulate the Lewis/Brønsted ratio up to the eventual total solvation of the zeolite by molecular adsorption of water. At RT and in the dark, the water molecules might strongly adsorb and fully occupy the adsorption sites of the catalyst.

  photo-assisted SCR reaction. The adsorbed NH3 on Lewis acid site is most likely activated to

Acknowledgment

Authors acknowledge the PHC-Tassili project (17MDU983) and the Normady Region (RHIN, RAPHYD project) for supporting this work.