Christian Bolzmacher
email: christian.bolzmacher@cea.fr@www-list.cea.fr

Nassim Benbara

Marc Rebillat

Nazih Mechbal

Piezoelectric transducer for low frequency sound generation on surface loudspeakers

Keywords: Piezoelectric actuators, Surface loudspeaker, Bending wave loudspeaker

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

In the foreseeable future, the adoption of multifunctional car structure (MCS) will offer the potential to radically upgrade the abilities of vehicles in terms of ecological requirements emerging from the social and legal environment. Integrating car structures with functional systems that monitor structural integrity and aging, change shape at local level, act as sound sources and tackle noise vibration harshness issues will eliminate many of the weight, volume, and signature penalties associated with the current approach of designing, manufacturing and maintaining vehicles and functional systems separately. One type of MCS that appears as very promising and which we propose to investigate in this paper is a Load-bearing Loudspeaker Structure (LLS). A LLS refers to structures that are equipped with active elements such as piezoelectric elements that allow for music reproduction and 3D sound spatialization. We consider the fact that the passenger cabin of cars consists of many large surrounding surfaces such as door panels, roof trim panel, trims and dashboards, which can act as membranes. The objective is thus to design surface loudspeaker able to efficiently reproduce sound in the low, medium and high frequency range depending on the sound panel size. Piezoelectric transducers chosen because of their small size, low weight and high force output are investigated in this paper. They are coupled to mechanical amplification units to increase displacement amplitudes to overcome the lack of low frequency sound production [START_REF] Bai | Optimal Implementation of Miniature Piezoelectric Panel Speakers Using the Taguchi Method and Genetic Algorithm[END_REF][START_REF] Sugimoto | PVDFdriven flexible and transparent loudspeaker[END_REF] and are compared to a piezoceramic ring. The goal is to efficiently excite the full audio spectrum.

Piezoelectric materials used in this work are ceramic perovskite materials made from lead zirconate titanate (PZT) that expands or contracts when an electrical charge is applied, generating motion and force called the inverse piezoelectric effect. Piezoelectric actuators use this motion to provide very short high force strokes with high frequency and fast response times at small size giving them a significant power-to-size ratio. Bulk piezo ceramics with external electrodes provide deformations in the range of 300 ppm with fields of 600 kV/m requiring high driving voltages. Multi-layer piezoelectric ceramics have been developed with the aim to reduce the required voltage by reducing the PZT layers to 50 to 100 µm thickness placed between internal electrodes. Those materials can be driven with a maximum electric field up to 2 MV/m with a voltage of less than 200 V achieving a deformation of about 1000 ppm.

Depending on the application, piezoelectric ceramics can be glued directly to a surface or a structure introducing a bending-type deformation. Alternatively, the high output force can be transformed into an amplified deformation at the expense of the size of the actuator due to the required amplification unit. If the amplification unit is rigidly coupled to the piezoceramic, these actuators are called flextensional actuators. Three different actuator types are described in the next sections:

• Piezoelectric rings • Flat flextensional piezoelectric actuators • Cymbal-type flextensional piezoelectric actuators

Piezoelectric ring

The first prototype of a sound-emitting panel uses a piezoceramic ring of dimension OD50 mm, ID20 mm, and TH 0.5 mm. It is made of a hard Navy I-type material called Pz26 from Meggitt Ferroperm with good coupling factors, high Curie temperature, high mechanical quality factor, low dielectric loss and very good stability over time. Table 1 shows the material characteristics. This piezoceramic ring is glued centered to a 300 x 200 x 3 mm3 plate made of polypropylene, a material often used for car passenger garnishments. The plate is screwed via a spacer (thickness 6.7 mm) to another stiffer plate made of Delrin creating a closed housing. The final assembly is shown in Figure 1.

Flat flextensional piezoelectric actuator

Flat flextensional piezoelectric actuators transform the high output force of piezoelectric ceramics into an amplified displacement. The actuator used in this work is called "Amplified Piezoelectric Actuator" (APA) developed by Cedrat Technologies based on a low voltage piezoelectric ceramics stack. The mechanical amplification is obtained due to an external monolithic shell of elliptic shape made of stainless steel, which magnifies the stack deformation in normal direction. This elliptical frame preloads the piezoelectric ceramic stack and protects it from tensile force. This kind of mechanical amplifiers is especially well-suited for dynamic drive conditions and provide relatively high force and large displacement while being compact in size. The APA400MML model has been chosen because of its good compromise between force and displacement output. Characteristics are given in Table 2. For the experimental tests, the actuator has been screwed in a reactive configuration centered between two plates. The sound emitting membrane of size 300 x 200 x 3 mm3 is made of polypropylene and the back plate is made of Delrin (thickness 6.7 mm) as shown in Figure 2.

Cymbal-type flextensional piezoelectric actuator

The cymbal transducer consists of a piezoelectric or ceramic disc (either in single layer or multilayer form) sandwiched between two metal endcaps, each having a truncated conical shape. These metal endcaps serve as a mechanical transformer for converting and amplifying the lateral displacement of the ceramic into an axial motion of the endcaps. Both the d33 and d31 piezoelectric coefficients contribute to the axial displacement of the composite. Displacement increases exponentially with increasing cavity diameter and increases linearly with increasing cavity depth. Displacement is inversely proportional to the endcap thickness. Displacement and generative force show a position dependent behavior.

TDK PowerHap™ piezoelectric actuators provide high performance in terms of acceleration, force, and response time. They are made of multilayer piezo plates with copper inner electrodes. This allows the actuators to be driven with relatively low operating voltages up to 120V. Once activated, the piezo plates only expand minimally in the z axis, but due to the constant volume of the piezo effect contract simultaneously in both the x and y axes. The actuators employ cymbals on both sides of the plate as levers to amplify the contraction by a factor of 15 in the z axis. The biggest actuator of the PowerHap™ series, the PowerHap™ 15G, is used in this paper with its characteristics given in Table 3. The actuator is also fixed in a reactive configuration between two plates of dimensions 300 x 200 mm3.

EXPERIMENTAL TEST SET-UPS

Dynamic displacement measurements have been carried out using a laser vibrometer to investigate the dynamic behavior of the actuators. Erreur ! Source du renvoi introuvable.

shows the experimental test set-up using a Polytec OFV-5000 laser vibrometer with an OFV-534 laser head mounted on a 3-axes cartesian robot. This experimental set-up allows to measure the dynamic displacement response of a single point on an actuator or a surface as well as the scanning of a complete surface to determine the mode shapes of the vibrating plate. Acoustic characterizations are carried out in a semi-anechoic chamber (i.e. whose floor is not covered with foam) in order to not alter the acoustic signal recorded by the microphone (Figure 4). The prototype plate is on a stand, at the same height and a distance of 1 m from the microphone, itself on a tripod (1 m from the ground). An APEX220 microphone from Apex Electronics is used for the measurements connected via an XLR cable to a Focusrite Scarlet2 6i6 soundcard connected via USB to the computer. Each set of measurements begins with a "blank" measurement of the room, with a sound level meter, to determine the sound level (about 29 dB within the room in general). The amplified piezoelectric actuators in this paper are driven with a Piezomechanik LE 150/100 EBW amplifier. It generates voltages between 0 V and 150 V to be obtained with a preset offset voltage from a 0 V to 5 V input. The gain is x30 and its no load bandwidth extends up to 70 kHz. The amplifier can supply up to 350 mA continuously and 1200 mA peak. This leads to a rapid decrease in its bandwidth with increasing load. The piezoceramic ring is driven with an ELBATEC T-500 symmetrical high-voltage amplifier. It generates voltages of ±200 V from a ±10 V input. The maximum current is 400 mA and the no-load bandwidth extends up to 100 kHz.

MEASUREMENT RESULTS

Piezoelectric ring

First, the displacement behavior at the center of the plate is measured with the laser vibrometer. Therefore, a 400 Vpp sine sweep is applied to the piezoelectric ring element. The loudspeaker is screwed to the base of the scanning table as shown in Figure 4 to prevent any unintended movement. Figure 5 shows the result for an excitation frequency from 50 Hz to 20 kHz. It can be seen that the absolute displacements are in the low micrometer range from 0.3 µm to 25 µm decreasing towards higher frequencies.

Next, the sound pressure level of the piezoelectric ring speaker has been characterized from 100 Hz to 20 kHz for a driving voltage of 400 Vpp. The sound pressure level (SPL, the reference level corresponds to the threshold of hearing, 20 μPa) is shown in Figure 6 measured in the semi-anechoic chamber. A 1/12 octave filter has been applied to the SPL curve. Due to the low capacitance of the ring actuator, high frequencies with relatively low power consumption can be attained. However, the low frequency range is quite limited. Despite of a few acoustic holes an average SPL of 80 dB is emitted up to 20 kHz. The lower cut-off frequency with this polypropylene is around 400 Hz reaching a SPL of 75 dB.

Flat flextensional piezoelectric actuator

As for the piezoelectric ring, laser vibrometer displacement measurements have been carried out for the APA400MML actuator. First, the frequency response has been measured for frequencies from 20 Hz to 20 kHz for the actuator only. A 7.5 Vpp sine sweep (offset 3.75 V) was applied to the piezoelectric actuator with a duration of 2 s. The result is shown in Figure 7. It can be seen that the APA400MML actuator has linear displacement behavior up to its first resonance frequency indicated at 634 Hz by the supplier. Figure 8 shows the displacement frequency response for a 150 Vpp (offset 75 V) sine sweep from 100 Hz to 500 Hz. This is the maximum driving voltage that can be applied to the actuator. At those driving voltages, the actuator cannot be actuated on its first resonance, which would cause failure to the ceramic due to excessive tensile stress. The supplier indicated a mean maximum peak to peak displacement amplitude of 368 µm up to the first resonance frequency. It has to be noted that one of the three piezoelectric ceramics composing the APA400MML actuator was electrically damaged and has been electrically removed from the actuator. Driven with only two ceramics the measured peak displacement amplitude is around 150 µm before the first resonance frequency but the voltage amplifier can now fully charge the actuator up to 500 Hz.

Next, the Cedrat Technologies APA400MML speaker has been characterized in the semianechoic chamber. A 7.5 Vpp sine sweep of 5 s has been applied to the actuator and measured with the microphone. In Figure 9, the 1/12 octave filtered sound pressure level from 100 Hz to 20 kHz is shown. It can be seen that even at very low driving voltages relatively high levels can be attained in the range from 400 Hz to 2 kHz. For higher frequencies, the membrane material as well as the characteristics of the actuator induce damping lowering the maximum sound pressure level. Figure 10 shows the transfer function for a 150 Vpp (75 V offset) sine sweep excitation from 100 Hz to 500 Hz. As described earlier, this kind of actuator cannot be driven in resonance, which would lead to breakdown of the ceramics. It can be seen that SPLs exceeding 75 dB can be achieved from 1500 Hz to 600 Hz using this kind of actuators and the current surface size. It may be convenient to increase the size of the membrane to achieve higher levels at lower frequencies.

Cymbal-type flextensional piezoelectric actuator

Finally, the displacement amplitude of the TDK PowerHap 15G actuator has been measured. Figure 11 shows the peak displacement amplitude from 100 Hz to 20 kHz with an almost linear zone below the first resonance frequency around 8 kHz. The mean peak amplitude at 7.5 V is around 10 µm. Figure 12 shows the frequency response for a 2 s sine sweep from 100 Hz to 1 kHz at 120 Vpp with an offset of 60 V (maximum driving voltage). The almost horizontal plateau has a peak displacement amplitude of 80 µm. This actuator can be fully charged up to 1 kHz by the amplifier used in this paper.

The sound pressure level of the TDK PowerHap 15G speaker has been characterized in the semi-anechoic chamber. A 120 Vpp (offset 60 V) sine sweep of 5 s has been applied to the actuator and measured with the microphone. Figure 13 shows the sound pressure level from 100 Hz to 20 kHz. Relatively high SPLs are achieved between 500 Hz and 2.5 Hz. There is a small hole present in the transfer function from 1.2 kHz to 1.6 kHz due to the insufficient charging of the actuator by the amplifier (limited to 1 kHz for this actuator). Next to the APA actuators, this actuator is an interesting candidate to excite the car structures because of its compact geometry, relatively low power consumption and high displacement amplitude. The study revealed that the piezoceramic ring can be used for exciting the medium to high frequency audio range. In practice, lower maximum voltages will be applied reducing the sound pressure level and thus affecting primarily the lower end of the excited frequencies. This will shift the addressable audio range to higher frequencies. This can be partially compensated by using multiple piezoceramic actuators. The advantage if these actuators is their minimal size requirement and surface conform integration.

The amplified piezoelectric actuators can address the medium low frequencies from 150 Hz to kHz. The APA series can easily be adapted to a targeted audio range but may introduce actuators of bigger size. The PowerHap actuators stand out by their small size and relatively high output displacement. Output force may be an issue depending on the LLS stiffness. In both cases, the voltage amplifiers are the main limitation factor for speaker applications. Using different membrane materials with higher Young's modulus and lower damping than the polypropylene material used in this paper can increase the sound pressure level but may introduce coloration to the created sound. In general, to achieve a proper radiation efficiency and radiated sound power, the Young's modulus cannot be too low. A tradeoff between Young's modulus and the density needs to be found to design efficient LLS.

Compared to classical electrodynamic speaker, the piezoelectric actuators are not visible and can be applied to structures that have initially other functions than creating sound. After equalization, LLS performance could be similar to electrodynamic speaker, at least subjectively. The process of equalization basically cancels out the lack of certain frequency band, and thus, is no longer affecting the performance. The use of sophisticated control schemes applied by a digital signal processor when using multiple actuators could significantly improve sound quality even on complex structures. The implementation of piezoelectric actuators is then a good choice because they can be more easily controlled then electrodynamic exciter technology.

CONCLUSIONS

Three different actuators have been compared in this paper in terms of maximum displacement amplitude and maximum sound pressure level on 300 x 200 x 3 mm3 speaker surfaces. A flat piezoelectric ring glued to a surface is compared to amplified piezoelectric actuators. The ring actuator produces good sound pressure levels at frequencies higher than 400 Hz but excites confuse mode patterns due to the stiff ceramic glued to the surface making control schemes more challenging. The other two actuators have been tested in a reactive configuration (fixed between two plates) to overcome the diffuse mode patterns created by a piezoceramic disc or ring glued to a surface. Highest output displacements can be obtained with the APA400MML flextensional actuator from Cedrat Technologies, which is limited to a maximum frequency of around 500 Hz due to its first mechanical resonance. Due to the high capacitance, this actuator is relatively power consuming. It's maximum output force of 200 N is not necessary for the speaker surface used in this paper but gives much design space for applications with complex and stiff structures. A smaller piezoceramic stack could be used to reduce power consumption. The flat cymbal actuator from TDK named PowerHap 15G produces a force of 20 N, which is enough to displace the membrane. Good acoustic power is produced between 500 Hz and 2.5 kHz. Power consumption is reduced by a factor of three compared to the APA400MML actuator because of the lower capacitance (3.6µF compared to 10µF). The very compact design (26 x 26 x 2.4 mm3) could make this actuator a good choice for medium frequencies in car applications. The limiting factor for high frequency sound generation using those piezoelectric actuators is the availability of high-power electric amplifiers. In order to cover a large audio spectrum, the combination of simple piezoelectric ceramics with amplified piezoelectric actuators on the same loudspeaker panel can be advantageous to maintain low power consumption.

Figure 1 :

 1 Figure 1: Polypropylene plate with centered piezoelectric ring (left) and prototype speaker with housing (right).

Figure 2 :

 2 Figure 2: APA400MML from Cedrat Technologies (left) and surface speaker prototype with centered actuator (right).

Figure 3 :

 3 Figure 3: TDK PowerHap piezoelectric actuators.

Figure 4 :

 4 Figure 4: Set-up for displacement measurements with a laser vibrometer (left) and acoustic measurements in a semi-anechoic chamber (right).

Figure 5 :

 5 Figure 5: Displacement response of the PZT ring speaker measured at the center of the plate.

Figure 6 :

 6 Figure 6: SPL of the PZT ring speaker measured at 1 m at 400 Vpp.

Figure 7 :

 7 Figure 7: Displacement amplitude frequency response of the APA400MML actuator for a 7.5 Vpp driving voltage (2 s sine sweep) from 10 Hz to 20 kHz.

Figure 8 :

 8 Figure 8: Displacement amplitude frequency response of the APA400MML actuator for a 150 Vpp driving voltage (2 s sine sweep) from 100 Hz to 500 Hz.

Figure 9 :

 9 Figure 9: SPL from 100 Hz to 20 kHz of the APA400MML speaker at 7.5 Vpp.

Figure 10 :

 10 Figure 10: SPL from 100 Hz to 500 Hz of the APA400MML speaker at 150 Vpp.

Figure 11 :

 11 Figure 11: Displacement amplitude frequency response of the TDK PowerHap 15G actuator for a 7.5 Vpp driving voltage (2 s sine sweep) from 100 Hz to 20 kHz.

Figure 12 :

 12 Figure 12: Displacement amplitude frequency response of the TDK PowerHap 15G actuator for a 120 Vpp driving voltage (2 s sine sweep) from 100 Hz to 1 kHz.

Figure 13 :

 13 Figure 13: SPL of the TDK PowerHap 15G speaker at 120Vpp.

Table 1 :

 1 Material parameters Meggitt Pz26.

	Relative dielectric permittivity k33	-	1300
	Curie temperature	°C	230
	Piezoelectric charge coefficient d33	pC/N	300
	Density	kg/m3

Table 2 :

 2 Characteristics of an APA400MML from Cedrat Technologies.

	Max. no load displacement	µm	295-434
	Blocked force	N	171-231
	Stiffness	N/µm	0.547
	Resonance frequency (blocked-free)	Hz	634
	Capacitance	µF	10
	Drive voltage	V	0-120
	Dimensions	HxLxW in mm	24x78x11.5
	Mass	g	47.5

Table 3 :

 3 Characteristics of the TDK PowerHap 15G actuator.

	Max. no load displacement	µm	200
	Blocked force	N	20
	Stiffness	N/mm	100
	Resonance frequency (blocked-free)	kHz	8
	Drive voltage	V	0-120
	Capacitance	µF	3.6
	Dimensions	LxWxH in mm	26x26x2.4

ACKNOWLEDGEMENTS

This work was financially supported by the French National Research Agency (ANR, contract ANR-17-CE33-0004).


Josefine Schnee
email: josefine.schnee@sorbonne-universite.fr

Marco Daturi

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

Ultrafast time-resolved quantum cascade laser diagnostic for enlightening 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 enlightening 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 ratelimiting 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 rates of 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 work -among 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

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 rates 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

1. 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 IR-observing 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) cryodetector 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 and flashing the full surface of the wafer. It was verified that the catalyst is homogeneously illuminated and it behaves as a true photo-reactor.

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 same spectra over a wider range of wavenumbers (4000 to 1000 cm -1 ) are provided in Figure S1 in the Supplementary Information. 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, among which the band at around 1120 cm -1 is assigned to the ν(CO) mode of the methoxy species linearly adsorbed on the TiO2 surface (not further discussed here), 1 the spectra on Figure 1 Based on these results (Figure 1), the kinetic behavior (apparent rates) 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 S4 to S8 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 was extracted following the methodology described below for Figure 2.

From the three repetitions for each photocatalyst (Figures S4 to S8), the relative error on these apparent rates 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 S4, S6 and S8, 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, i.e. when the rate of formates consumption tends to zero) 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 Rapp,f the apparent rate of formates formation). In fact, working in IR transmission conditions allows taking advantage of the fact that the beam absorbance is proportional to species concentration, so that Equation 1 can be written as a derivation of the Beer-Lambert law. 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 "Rapp,f", which is directly related with the rate of formates formation (ε and L in Equation 1 can reasonably be considered constant here from one TiO2 sample to the next).

Therefore, conclusions drawn later in the present work about the parameter "(dA/dt)0 = Rapp,f" and its relationship with the overall photocatalytic performance (Table 1) are directly applicable to the rate of formates formation.

' (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 derived from the Beer-Lambert law (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 Rapp,r the apparent rate of formates reaction). Again, as ε and C'0 are unknown here, -(dA/dt)0 is treated as a whole as an apparent rate of formates reaction "Rapp,r", which is directly related with the rate of formates reaction (ε and L in Equation 2 can reasonably be considered constant here from one TiO2 sample to the next).

Therefore, conclusions drawn later in the present work about the parameter "-(dA/dt)0 = Rapp,r" and its relationship with the overall photocatalytic performance (Table 1) are directly applicable to the rate of formates reaction.

'

(

The Rapp,f and Rapp,r rates 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 Rapp,f and Rapp,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 formationconversion 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. 1 As shown by Table 1, TiO2 P25 has an almost 2 times lower specific surface area than TiO2 CA (48 vs.

( d A d t ) 0 = -εL ( d c d t ) 0 = εL k f C n 0 = R app, f -( d A d t ) 0 = -εL ( d c d t ) 0 = εL k r C ′ m 0 = R app,r
85 m²/g, respectively). Nevertheless, it reaches slightly higher Rapp,r and Rapp,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 Rapp 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 Rapp,r, and a 3.8 times lower Rapp,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 Rapp,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 Rapp,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 Rapp,r value associated with the reaction of formates, as illustrated on Figure 3a. Notice that the conversion of methanolreflecting the overall activity of the photocatalysts -is linearly correlated with the Rapp,r/Rapp,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 Rapp,r is 0.15 to 0.56 times lower than the Rapp,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 Rapp,r/Rapp,f) is determined here for the first time for a heterogeneous photocatalytic reaction. Regarding the fact that the Rapp,r increases with the Rapp,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, if we assume that the concentration of active sites per m² of catalyst surface is similar for the two samples. [START_REF] Li | Synergistic effect between anatase and rutile TiO2 nanoparticles in dyesensitized solar cells[END_REF] Water is suspected to enable a so-called indirect hole transfer and it has been demonstrated through isotopic substitutions that the water content is influencing the reactions outcome. [START_REF] Chiarello | Effect of the CH3OH/H2O ratio on the mechanism of the gas-phase photocatalytic reforming of methanol on noble metal-modified TiO2[END_REF] However, in our case, the reaction was performed in "dry" conditions (after purging the surface via a methanol flow, so that it can be assumed that the only water present on the surface is produced by the reaction).

Therefore, the concentration of water is negligible with respect to the largely more concentrated adsorbed methanol, whose affinity with the TiO2 surface is higher than that of water. [START_REF] El-Roz | On the mechanism of methanol photooxidation to methylformate and carbon dioxide on TiO2: an operando-FTIR study[END_REF] Notice that the Rapp,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 S9 in the Supplementary Information, with extracted Rapp,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. In the case of TiO2 P25, the Rapp,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 Rapp,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 need to react with lattice oxygen coming from the bulk lattice, via complex diffusion pathways, as oxygen from the surface lattice is no longer regenerated by gaseous oxygen, as for example proposed by Ranran Feng et al. 26 Alternatively, an explanation can be found in the work by Panayotov et al., [START_REF] Panayotov | Photooxidation Mechanism of Methanol on Rutile TiO2 Nanoparticles[END_REF] where it was suggested that under both anaerobic and aerobic conditions, the major oxidation agent is the photogenerated hole, whereas the main role of adsorbed oxygen is to remove the photogenerated electrons by forming superoxide species. As a consequence of electron extraction, the separation of charges would be further improved, leading to a 5-fold increase in formate production, not far from the factor 3 here found. However, it should be noticed that in our case we do not observe superoxides.

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). 27 Indeed, as shown on Figure S10 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 Rapp,r, with additionally a significant impact on Rapp,f (Figure 4). More precisely, Rapp,r and Rapp,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. S 12

TOC graphic

  show bands at 1360, 1380, 1564 and 1580 cm -1 associated with νs(O-C-O), δ(C-H), νas(O-C-O)' and νas(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 νas(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 FiguresS2 and S3in 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 Rapp,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. Rapp,r/Rapp,f ratio of formate species. (c) Rapp,r vs. Rapp,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 compares the Rapp,f and Rapp,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 Figure5. It should be noticed that the conversions were negligible in the absence of oxygen.

Figure 4 .Figure 5 .

 45 Figure 4. For TiO2 P25 and CA: apparent rates of formation and reaction of formate species (Rapp,f and Rapp,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 S5 . 7 Figure S6 . 8 • 9 Figure S7 .

 S57S689S7 Figure S5. QCL signal at 1360 cm -1 vs time over TiO2 CA during repeated UV on-off cycles (UV on: a-b-c-d, UV off: e-f-gh; two panels on the same horizontal line forming one UV on-off cycle) under reaction flow at 25 °C and atmospheric pressure. (a) and (e) show the same UV on-off cycle as (b) and (f), but with a y axis expressed in absorbance instead of amplitude. 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-d), 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 (e-f-g-h), time = 0 corresponds to the moment when the shutter was closed again, still under reaction flow. The times to open and close the shutter were around 6.5 ms and 3.2 ms, respectively.

Figure S8 .

 S8 Figure S8. Same UV on(a)-off(b) cycle as shown on Figure S7 (b and f, respectively), but over a longer time scale.

Figure S9 .

 S9 Figure S9. QCL beam absorbance at 1360 cm -1 vs time over TiO2 P25 under reaction flow at 25 °C and atmospheric pressure during (a-b) a UV on-off cycle with on and off periods of 300 ms and (c) the same UV off cycle as shown on Figure S3e, i.e. after a UV on period of 5 s. 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), 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 (b-c), time = 0 corresponds to the moment when the shutter was closed again, still under reaction flow. The times to open and close the shutter were around 6.5 ms and 3.2 ms, respectively. Extracted Rapp,r values are 0.12 in (b) vs 0.15 in (c).

Figure S10 .

 S10 Figure S10. QCL beam absorbance at 1310 cm -1 (a and c) and 1360 cm -1 (b and d) vs time over TiO2 P25 getting UVirradiated under flowing methanol at 25 °C and atmospheric pressure in the absence (a-b) or presence (c-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 . 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 (without oxygen in a-b, with oxygen in c-d) but still in the dark. The time to open the shutter was around 6.5 ms.

Table 1 .

 1 For the three TiO2 photocatalysts studied in this work: specific surface area (SSA), apparent rates of formation and conversion of formate species -Rapp,f and Rapp,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 Rapp,f and Rapp,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.

  26. Feng, R.-r.; Liu, A.-a.; Liu, S.; Shi, J.; Zhang, R.; Ren, Z., In Situ Studies on the Dissociation and Photocatalytic Reactions of CH3OH on TiO2 Thin Film by Sum Frequency Generation Vibrational Spectroscopy. The Journal of Physical Chemistry C 2015, 119 (18), 9798-9804. 27. Guzman, F.; Chuang, S. S. C., Tracing the Reaction Steps Involving Oxygen and IR Observable Species in Ethanol Photocatalytic Oxidation on TiO2. Journal of the American

	Chemical Society 2010, 132 (5), 1502-1503.

Acknowledgements

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.

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 rate of reaction of formates "Rapp,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 Rapp,f over TiO2 CA by a factor 2 while it does not affect that over TiO2 P25, and it reduces the Rapp,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.

Author information

Competing interests

The authors declare no competing interests. • QCL signal at 1360 cm -1 vs time over TiO2 CA in 3 repetitions of the UV on-off cycle under reaction flow at 25 °C.

Supplementary Information