Sandrine Cottrino
email: sandrine.cottrino@insa-lyon.fr

Thomas Gaudisson

Stéphane Pailhes

Emanuela Archina Ferrara

Shashank Mishra

Stéphane Daniele

Mohamed Mézouar

Alain Largeteau

Yann Le Godec

Sylvie Le Floch
email: sylvie.le-floch@univ-lyon1.fr

Stéphane Pailhès

In situ X-ray diffraction study of a TiO2 nanopowder Spark Plasma Sintering

Keywords:

Introduction

Nanocrystalline TiO 2 has attracted increasing interest as a functional ceramic and therefore appropriate sintering mechanisms have been widely studied. The highly active and large surface area induced by nanoscale is a key parameter for catalytic, photocatalytic [START_REF] Levy | Photochemistry of nanostructured materials for energy applications[END_REF] and gas sensing applications [START_REF] Ferroni | Preparation and characterization of nanosized titania sensing film[END_REF]. It has been demonstrated that the high pressure preparation induces an improvement of the photocatalytic activity [START_REF] Akrami | High-pressure TiO2-II polymorph as an active photocatalyst for CO 2 to CO conversion[END_REF] and that the high pressure TiO 2 phase (columbite) opens new opportunities for photocurrent generation under visible light [START_REF] Wang | Visible-light photocurrent in nanostructured high-pressure TiO 2 -II (Columbite) phase[END_REF]. Slightly doped TiO 2 ceramics are promising candidates as high temperature thermoelectric devices working in oxidising environments [START_REF] Verchère | Optimum in the thermoelectric efficiency of nanostructured Nb-doped TiO2 ceramics: from polarons to Nb-Nb dimers[END_REF] and capacitor -varistor ceramics [START_REF] Li | Effect of sinter temperature on the electrical properties of TiO2-based capacitor-varistors[END_REF]. The electrical properties of TiO 2 are deeply affected by the microstructure, showing an increase of the conductivity by reducing the grain size [START_REF] Demetry | Grain size-dependent electrical properties of rutile (TiO2)[END_REF]. To develop nanoceramics, processes typically start with a nanocrystalline powder that can give a faster densification kinetic with a lower sintering temperature, but on the other hand, leads to a higher grain growth rate. Conventional sintering does not allow the elaboration of dense nanoceramics without the use of grain growth inhibitors. Spark Plasma Sintering (SPS) technique combines uniaxial pressure and high intensity pulsed current. It has been found that this process allows a significant reduction in the densification temperature [START_REF] Lee | Preparation of nanostructured TiO2 ceramics by spark plasma sintering[END_REF]. Indeed, SPS is one of the techniques among the fast sintering approaches allowing to obtain bulk nanoceramics by tuning the kinetics and the uniaxial pressure to prevent grain coarsening and to favour densification phenomena. Despite its renowned strengths, in many cases, SPS fails to retain nanoscale grain sizes while achieving full densification. In fact, the current SPS technique is limited to a pressure of 150 MPa (or 400 MPa with tungsten carbide (WC) mould). However, it has been shown that very high pressure (P > 1 GPa) can significantly lower the powder densification temperature [START_REF] Liao | Transformation-assisted consolidation of bulk nanocrystalline TiO2[END_REF]. Therefore, interest in high pressure SPS is growing. By replacing graphite punches and die with silicon carbide in standard SPS device, a pressure of 1 GPa can be achieved [START_REF] Ratzker | Optical and mechanical properties of transparent alumina fabricated by high-pressure spark plasma sintering[END_REF][START_REF] Anselmi-Tamburini | Fast low-temperature consolidation of bulk nanometric ceramic materials[END_REF]. To reach higher pressures special high pressure cells currently used in high pressure labs have been adapted by coupling pulsed current heating to a belt press (6 GPa-1800 °C) [START_REF] Balima | High pressure pulsed electric current activated equipment (HP-SPS) for material processing[END_REF] or a toroidal anvil device (8 GPa-1800 °C) [START_REF] Knaislová | High-pressure spark plasma sintering (HP SPS): a promising and reliable method for preparing Ti-Al-Si alloys[END_REF]. Coupling the very high pressure and SPS techniques, we developed a high pressure SPS (HP-SPS) device (up to 10 GPa) [START_REF] Godec | Device for sintering by pulsating current and associated method[END_REF] based on a Paris-Edinburgh press. In this report, in complement to our previous study carried out on a 2 mol% Nb-TiO 2 powder [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF], the objective is to understand the effect of pressure on pure titanium oxide powder during SPS sintering on densification, phase transition and grain growth phenomena. In order to observe this effect, three experiments were performed at different pressures (76 MPa, 400 MPa, 500 MPa) with the same heating cycle at 700 °C. Then, in order to further investigate the effect of very high pressure, two SPS sintering at 1 GPa were performed, and we followed by in situ synchrotron X-ray diffraction the grain growth and the phase transition of a TiO 2 nanopowder during sintering under 1.5 and 3.5 GPa. This type of real-time analysis is not possible on any other SPS device. It is a powerful means to study the mechanisms involved in SPS process and optimise the conditions of material sintering. At the end of the study, a calculation of the activation energies of the sintering and the grain growth is proposed to identify the main densification mechanism [START_REF] Fang | Coarsening, densification, and grain growth during sintering of nano-sized powders-a perspective[END_REF].

Materials and methods

TiO 2 nano-powder synthesis

The [Ti(OEt) 4 ] 3 (Strem Chemicals, 99.9 %) was distilled prior to use whereas the anhydrous n-pentane, obtained from a solvent purification system (MB SPS-800 from MBRAUN), was stored under argon over 4 Å molecular sieves. In a typical sol-gel synthesis of titania nanoparticles, the solution of [Ti(OEt) 4 ] 3 (26 g, 38 mmol) in 50 mL of n-pentane was added to 450 mL of boiling deionized water containing 3.70 g (11.5 mmol, 0.1 eq/Ti) of tBu 4 NBr. After stirring the obtained suspension for 2 h at 100 °C, it was centrifuged to obtain a white solid. The asprepared precipitate was first washed with deionized water (3 × 80 mL) and then with ethanol (1 × 70 mL). After drying the pow-der at 80 °C for 12 h, it was calcined at 500 °C for 4 h in air to remove all the organic residues. The obtained powder was crystallised in the anatase phase (JCPDS no. 00-021-1272) with a brookite fraction of 17 %. The average crystallite size, determined by Rietveld refinement, was around 15 nm.

SPS densification

The titanium dioxide powders were sintered by Spark Plasma Sintering (SPS) HPD 25 (FCT Systeme GmbH, Germany). The sintering temperature was measured using a K-type thermocouple (1 mm diameter) introduced into the die. The pulse pattern consisted of two 10 ms pulses followed by a 5 ms pause period. First of all, the powder was pressurised at the targeted pressure (76 MPa, 400 MPa, 500 MPa and 1 GPa). Depending on the pressure, different types of dies were used. For 76 MPa, we used a usual graphite die [START_REF] Verchère | Optimum in the thermoelectric efficiency of nanostructured Nb-doped TiO2 ceramics: from polarons to Nb-Nb dimers[END_REF]. For 400 MPa, a tungsten cemented carbide (WC-Co) die with an inner diameter of 10 mm was used [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF]. A silicon carbide die (10 mm diameter) placed in a graphite die [START_REF] Ratzker | Non-uniform microstructure evolution in transparent alumina during dwell stage of highpressure spark plasma sintering[END_REF] allowed to reach 500 MPa. For 1 GPa, a special designed Paris-Edinburgh-SPS high pressure cell (SPS-PE cell) was used [START_REF] Godec | Device for sintering by pulsating current and associated method[END_REF]. In this case, a pellet (7 mm in diameter and 3 mm thick) of pre-compacted powder (at 370 MPa) is inserted in a graphite tube surrounded by a gasket made of fired pyrophyllite. This assembly is placed between two opposed anvils with a pseudo-conoidal profile which reduces the uniaxial stress (Fig. 1a). The anvils are made of Cobalt-cemented tungsten carbide supported by a steel binding ring. The pressure is generated by applying load on the anvils. The gasket plastically flows and transfers a quasi-hydrostatic pressure to the sample. A thermoplastic PEEK (polyether ether ketone) supporting ring around the gasket enhances the pressurisation efficiency. The calibration of the pressure was previously done following the measurement of the equation of state of NaCl by in situ neutron diffraction at ISIS facility [START_REF] Le Godec | Neutron diffraction at simultaneous high temperatures and pressures, with measurement of temperature by neutron radiography[END_REF]. Using a Paris-Edinburgh portable press (Fig. 1c), the pressure on a 7 mm diameter sample can reach 5 GPa for a 700 kN load. As the maximum load applied with the FCT-SPS HPD 25 apparatus is 250 kN, the maximum pressure achieved with the Paris-Edinburgh module mounted in the FCT-SPS HPD 25 chamber is 2 GPa (Fig. 1b). The temperature can be measured at the top of the sample with a thermocouple passing through the drilled anvil.

The heating cycle was applied for each experiment with a rate of 100 °C/min and 10 min of dwell time. After the holding time, the heating power was shut off and the pressure released. Three samples were sintered at 700 °C for 76 MPa, 400 MPa and 500 MPa, followed by two others under 76 MPa at 750 °C and 960 °C and two more under 400 MPa at 450 °C and 600 °C. Finally, two samples were sintered at 1 GPa with a temperature of 560 °C and 650 °C.

In situ synchrotron X-ray diffraction during SPS process

The Paris-Edinburgh press coupled to a pulsed current generator was installed on the ID27 beamline at the European synchrotron radiation facility. Two experiments were performed at 1.5 and 3.5 GPa. During heating to 725 °C with a rate of 8 °C/min, X-ray diffraction patterns of the sample were collected with a rate of one pattern per minute. The X-ray wavelength was fixed to 0.2468 Å. The diffraction patterns were collected in transmission geometry using a MAR345 image plate scanner (X-Ray Research Company GmbH, Norderstedt, Germany) and circularly integrated using FIT2D software [START_REF] Hammersley | Two-dimensional detector software: from real detector to idealised image or two-theta scan[END_REF]. The sample was heated by a low voltage high intensity pulsed current (up to 1000 A/cm 2 ).

Characterisation techniques

The density of sintered samples was measured using Archimedes method with distilled water. The values for the relative densities were calculated assuming a theoretical density of 3.89 g cm -3 for anatase phase and 4.24 g cm -3 for rutile phase.

X-ray powder diffraction (XRD) was used to analyse the crystallographic phase and to evaluate the crystallite size and their microdistortion. A Bruker D8 Advance A25 diffractometer (Cu k α radiation at 0.154184 nm) equipped with a Ni filter and 1-D fast multistrip detector (LynxEye, 192 channels on 2.95) was used. The diffractograms were collected at 2θ from 20°to 80°for a total acquisition time of 30 min. The data analyses were carried out with DIFFRAC.Eva software to identify the crystallographic phases.

The diffractograms collected on the laboratory and synchrotron Xray sources were analysed by TOPAS software. In both cases, the instrumental resolution function was fixed from a measurement on a standard. Using the Rietveld deconvolution method, the average lattice distortion and crystallite size were determined from the width of the Gaussian and Lorentzian function, respectively.

Scanning electron microscopy was used to observe the microstructure of the sintered samples. The average grain size for each pellet was obtained by measuring about 150 grains by a line intercept technique using an image analysis software (ImageJ).

Results

To clarify the effect of temperature/pressure on densification, phase transition and grain growth, various experiments were conducted and several samples have been elaborated. Their characteristics are listed in Table 1. After sintering, the samples are in the form of 10 mm diameter blue pellets.

Effect of pressure on densification and transition phase

In order to analyse the effect of pressure on the densification and phase transition, a series of three experiments were performed at the same sintering temperature equals to 700 °C under 76 MPa, 400 MPa and 500 MPa according to the experimental cycle explained in the 2.2 part. It can be observed on the XRD profiles of the 3 sintered samples that no brookite fraction remains and that the anatase to rutile phase transition occurred at a temperature lower than 700 °C for the samples Table 1 Characteristics of all samples produced in this study. Each pellet is sintered with 1 g of TiO 2 powder, except samples at 450 °C and 600 °C under 400 MPa for which 0.7 g of powder was used and for two samples under 1 GPa for which 0.5 g of powder was used. sintered at 400 and 500 MPa. Indeed, at 700 °C-76 MPa the pellet is approximately half composed of the rutile phase, whereas at higher pressure values, the transition is completed (Fig. 2). The anatase to rutile phase transition usually occurs within 600-850 °C with a volume reduction of 9 % [START_REF] Guidi | Effect of dopants on grain coalescence and oxygen mobility in nanostructured titania anatase and rutile[END_REF][START_REF] Hanaor | Review of the anatase to rutile phase transformation[END_REF]. In fact, anatase is the most stable phase at nanoscale whereas rutile is the thermodynamically stable phase in coarse grains [START_REF] Zhang | New kinetic model for the nanocrystalline anatase-torutile transformation revealing rate dependence on number of particles[END_REF]. Therefore, pressure-induced nanoanatase transformation may follow different pathways depending on the surface chemistry and size of the nanoparticles [START_REF] Machon | Pressureinduced polyamorphism in TiO2 nanoparticles[END_REF][START_REF] Machon | Interface energy impact on phase transitions: the case of TiO2 nanoparticles[END_REF]. The application of pressure during SPS sintering is known to induce the phase transition anataserutile at lower temperatures [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF]. Sintering steps of the four samples (700 °C-76 MPa, 400 MPa, 500 MPa and 650 °C-1 GPa) can be evidenced thanks to the piston displacement (Fig. 3a) and its rate (Fig. 3b) as function of temperature. In Fig. 3a, we can observe that the densification starts at lower temperature when high pressure is applied. Fig. 3b allows to evidence the different densification steps. At low pressure (76 MPa), individual phase transition events were not evident. However, at the higher pressures of 400 MPa, 500 MPa and 1 GPa we can distinguish different peaks corresponding to densification of the anatase phase, then phase transition and finally the densification of the rutile phase [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF]. In fact, at 76 MPa-700 °C it is evident that densification of the anatase phase occurs progressively until a final phase transition occurs at 700 °C (to retain 58 % Rutile phase in the final product). At 400 MPa, the phase transition occurs around 430 °C (as evidenced from the XRD analysis on the pellet sintered at 450 °C-400 MPa giving 80% rutile phase and 20 % anatase phase). Hence, the first peak represents the anatase densification and at the end the phase transition. The second peak can be assigned to the rutile densification. At 500 MPa, three steps can be observed which could correspond to anatase densification (peak 1 in Fig. 3b), phase transition (peak 2 in Fig. 3b) and rutile densification (peak 3 in Fig. 3b). A sample sintered at 400 °C under 500 MPa contains 20 % rutile, 65 % anatase and 15 % brookite. This confirms the transition anatase-rutile at 400 °C under 500 MPa. But we notice that the brookite phase is still present at this stage of the densification, which was not the case at lower pressure. The same three stages of densification, shifted to lower temperatures, are clearly observed at 1 GPa as three well-defined peaks in the displacement rate (Fig. 3b). This means that densification and phase transition are faster at high pressure and occur in distinct narrower distinct temperature ranges.

As seen in previous work [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF], the rutile phase has a higher density (4.24 g/cm 3 versus 3.89 g/cm 3 to the anatase phase), involving a volume reduction of 9 % at the transition anatase to rutile. A more compact granular arrangement with more constrained grains, can favour the nucleation of rutile that can occur either at grain surface or in the bulk. This may explain the shift of the phase transition towards lower temperatures [START_REF] Liao | Transformation-assisted consolidation of bulk nanocrystalline TiO2[END_REF]. Fig. 3c shows the temperature evolution of the two densification stages and the transition observed in Fig. 3b as a function of pressure. Up to 500 MPa, the effect of pressure is significant. The anatase densification, phase transition and rutile densification temperatures are lowered by 240 °C, 300 °C and 400 °C respectively. No further significant shift is observed above 500 MPa.

We may point out that the density of the green pellet after the application of the pressure and before heating varies from 29 % under 76 MPa to 64.4 % under 1 GPa (Fig. 12c). Regardless of the sintering technique used, the density of the green compact has an impact on grain growth and densification. The finer the nanopowders, less compactable they are. This may be due to the ability of ultrafine powders to agglomerate due to Van der Waals attraction [START_REF] Roca | Compacting and densification of TiO2 nanoparticles[END_REF]. We evaluated the relative density of our compacted powder under pressure at room temperature using a WC matrix and a Zwick testing machine. The plot of the relative density of the green compact as a function of the logarithm of the pressure (Fig. 4) shows two linear regimes with a intercept point at P b equal to 330 MPa. This is consistent with the P b determined equal to 380 MPa by Alvarez-Roca et al. [START_REF] Roca | Compacting and densification of TiO2 nanoparticles[END_REF] for a 10 nm anatase powder. According to their study with a compaction pressure above P b , the agglomerates are fragmented, while below P b they are only rearranged. We can identify two mechanisms of particles rearrangements during the compaction: inter-agglomerates below P b and intra-agglomerate above P b . This leads to higher relative density of the green compact when a pressure above 330 MPa is applied. This partially explains the difference of the densification pathway at 76 MPa as compared to the sintering at pressures higher than 330 MPa.

In situ observation during sintering at very high pressure

We performed two in situ synchrotron X-ray experiments at 1.5 and 3.5 GPa. The series of diffractograms recorded during heating is quite similar for both experiments. These diffractograms are shown for the experiment at 3.5 GPa in Fig. 5. In situ XRD highlights the anatase-rutile transition at 315 and 295 °C for the experiments at 1.5 and 3.5 GPa respectively. In both experiments the brookite phase remains until this transition and disappeared with the anatase phase.

Using TOPAS software, we evaluated the evolution of the crystallite size for anatase and rutile during heating (Fig. 6a andc). Before the anatase-rutile transition, at around 300 °C, the anatase crystallite size increases from 15 to 25 nm. Then, the conversion anatase to rutile induces a jump in the crystallite size to 40 nm. The nucleation of the ru- tile occurs at the interface of the anatase grains and the rutile phase grows to the inside of the grains. Therefore, the formation of rutile grains mainly occurs by the merger of two anatase particles leading to a rapid grain growth at the phase transition [START_REF] Xue | Kinetic study on the effect of iron on the grain growth of rutile-type TiO 2 under in situ conditions[END_REF]. For crystallite size higher than 50 nm, TOPAS failed in the evaluation of the crystallite size with the synchrotron instrumental resolution function. The calculated values reach an upper limit that does not correspond to the reality. The recovered samples were analysed on the laboratory diffractometer. In that case, TOPAS evaluates the rutile crystallite size at 312 nm and 166 nm for sintering at 1.5 and 3.5 GPa, respectively. These values are reported in Figs. 6a and6c. The growth of rutile crystallites is lower at 3.5 GPa. The crystallites microdistortions given by TOPAS are shown in Figs. 6b and6d. The microdistortions in anatase crystallites are 3 times higher at 3.5 GPa than at 1.5 GPa. They start to relax at 200 °C. The relaxation is complete after the phase transition.

Effect of pressure on grain size

In order to measure the grain size, SEM observation was carried out on the samples sintered at 700 °C with different pressures (76, 400, 500 MPa) and 650 °C-1 GPa. SEM images are shown in Fig. 7a-d. At 76 MPa-700 °C, the shape of the grains and the overall microstructure show that the pellet is not yet well densified compared to the others. Indeed, this sample remains mostly crystallised in anatase and its relative density (d r ) is only 90 %. At the same temperature but at higher pressure, the pellets are well densified and the grains have a characteristic polygonal shape. Indeed, the relative density is approximately 98 % for these pellets. At this level of density (last stage of densification) a coarsening of the grains occurs resulting in a heterogeneous microstructure. Nevertheless, the average size of the grains decreases with the pressure for the same sample density (383 nm, 333 nm and 266 nm respectively for 400 MPa, 500 MPa and 1 GPa). The SEM image at 1 GPa clearly shows the presence of two populations of grains.

If we observe the microstructure on SEM images (Fig. 8) of samples sintered by SPS at 1.5 GPa and 3.5 GPa during the synchrotron experiment, a finer microstructure is also observed at 3.5 GPa as compared to 1.5 GPa. Grain size values of samples sintered at synchrotron cannot be compared with the other ones as the heating rate was twelve times slower.

To quantify the broadening of grain size distributions, SEM images are analysed. Figs. 9 and 10, show the grain size distribution of 6 samples sintered at different pressures. If we do not consider the sample treated at 700 °C under 76 MPa, as it is not well-sintered (relative density equal to 90 %), the mean size decreases with increasing pressure. Small grain population increases with the pressure and the coarse grain population decreases. For both samples sintered at 1.5 GPa and 3.5 GPa, during the synchrotron experiment, the average grain size is larger due to a slower heating rate (8 °C/min), but the evolution between 1.5 GPa and 3.5 GPa is similar to the samples set 400 MPa to 1 GPa.

To clearly underline the fact that the application of strong pressure during sintering activates densification mechanism at lower temperature and allows to obtain a same densification state of the pellets with a finer microstructure, the evolution of grain size, crystallite size and density as function of pressure sintering are represented in Fig. 11. At 700 °C-76 MPa, the pellet density is only around 90 %. For 700 °C-400 MPa, the density pellet reaches 98.5 %. For 700 °C-500 MPa or 650 °C-1 GPa, the density level is similar but the grain size is smaller. It can be observed that the size of crystallite is lower than grain size. As shown in our previous study [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF], under high pressure the high stress level on the grains leads to their fracture. After the phase transition, different crystal domains appear inside the grains.

The evolution of grain size, crystallite size and relative density is plotted as function of temperature for same pressures (76 MPa, 400 MPa and 1 GPa) in Fig. 12. If we observe the evolution of the den- sity (Fig. 12a), it appears that under high pressure the densification is faster and a relative density higher than 95 % is reached as low as 400 °C. As shown in Fig. 4, the green density of the pellet is improved when the applied pressure exceeds 330 MPa. Above this pressure the agglomerates are broken allowing the rearrangement of the particles at room temperature. If we observe grain size, it appears that at low pressure (76 MPa), higher temperatures are needed to reach a high level of densification (> 98 %) and thus the microstructure is coarser. Indeed, at high temperature, mechanism of grain boundary diffusion is accelerated (high thermal energy) and thus leads to grain growth. In Fig. 12b, it appears that the crystallite size is lower than grain size, as previously observed for the samples sintered at 700 °C at different pressures. A marked increase of the crystallite size occurs during the transition phase. As we observed with in situ X-ray diffraction, the rutile crystallites formed are two times larger than the remaining anatase crystallites. But just after the transition a remarkable decrease of the rutile crystallite size is observed. The same evolution of rutile crystallite size is observed by Maglia et al. [START_REF] Maglia | Synthesis of fully dense anatase TiO2 through high pressure field assisted rapid[END_REF]. In fact, the final grain size of rutile is controlled by the competition between two rutile formation phenomena: nucleation in anatase surface and growth in bulk anatase grain [START_REF] Liao | Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2[END_REF]. Nucleation within anatase bulk grain requires a higher activation energy and thus occurs at higher temperature. Thus, the first nucleation process that occurs, is around the grain boundaries. Boundary nucleation gives rise to larger rutile grains. In fact, one or more anatase grains are transformed in single rutile particle. This explain that the first rutile crystallites formed at the beginning of the transition are larger as compared to those formed in the second part of the transition. Regarding the grain size (Fig. 12a), it increases steadily with increasing temperature after the phase transition with a lower growth rate as the pressure increases. This is explained by Liao et al. with a decrease of the diffusivity when very high pressure (at least 1 GPa) is applied leading to a decrease of the growth rate [START_REF] Liao | Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2[END_REF][START_REF] Liao | Retention of nanoscale grain size in bulk sintered materials via a pressure-induced phase transformation[END_REF].

C O R R E C T E D P R O O F

Apparent activation energy of grain growth

We observed in this study like in previous ones [START_REF] Verchère | Effect of high pressure spark plasma sintering on the densification of a Nb-doped TiO2 nanopowder[END_REF][START_REF] Grasso | Highly transparent pure alumina fabricated by high-pressure spark plasma sintering[END_REF][START_REF] Sokol | Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)[END_REF] that applied high pressure during the sintering decreases the sintering temperature by activating the densification and the grain growth mechanisms earlier and faster. To quantify the effect of pressure in terms of kinetic, the Arrhenius equation is used to calculate the apparent activation energy of these phenomena. Activation energy is the minimum amount of energy required to initiate a chemical reaction. The lower this energy is, the stronger the driving force of the reaction is. Depending on the mechanism studied, densification or grain growth, different values of activation energy have been calculated. Indeed, Arrhenius law establishes the dependence of the rate of a chemical reaction on temperature. Depending on whether we are interested in the grain growth rate or the densification rate, we will obtain activation energies specific to each mechanism.

The latter stage grain growth, when the relative density exceeds 90 %, can be analysed using the conventional power law of the grain growth [START_REF] Maria | Cold sintering: current status and prospects[END_REF][START_REF] Jin | Densification and fine-grain formation mechanisms of BaTiO3 ceramics consolidated by self-assembly sintering[END_REF][START_REF] Rossetti | Plasma-assisted rapid sintering of nanotitania powders[END_REF]:

with G the grain size value after sintering (m), t the dwell time (h), K 0 a constant, R the ideal gas constant (J mol -1 K -1 ), T the sintering temperature (K) and Q the activation energy (kJ/mol). The exponent, n, varies depending on the mechanisms [START_REF] Fang | Coarsening, densification, and grain growth during sintering of nano-sized powders-a perspective[END_REF]. Considering that the predominant mechanism is grain boundary controlled, we used n = 2. Thus, knowing the grain size and the sintering temperature, the apparent activation energy can be calculated by linear fitting (Fig. 13). In this study, activation energies at 76 MPa, 400 MPa and 1 GPa have been estimated.

First, it is important to state that the main grain growth occurs at the final stage of sintering, when a density of at least 90 % is reached [START_REF] Coble | Sintering crystalline solids. 1. Intermediate and final state diffusion models[END_REF]. Second, the transition from the metastable anatase phase to the stable rutile phase is accompanied by considerable grain growth [START_REF] Guidi | Effect of dopants on grain coalescence and oxygen mobility in nanostructured titania anatase and rutile[END_REF]. We estimated the activation energy of the grain growth of rutile during the final sintering stage when most of the grains are converted in rutile. At low pressure sintering, an activation energy of 229 kJ/mol has been estimated for rutile grain growth in the temperature range 700-960 °C. In literature, in the case of a conventional sintering, a value of 360 kJ/ mol, consistent with our result, has been evaluated, in the range 1000-1250 °C, for grain growth of a barium titanate nanopowder, with a bimodal distribution (200 and 80 nm) [START_REF] Jin | Densification and fine-grain formation mechanisms of BaTiO3 ceramics consolidated by self-assembly sintering[END_REF]. Under low pressure (76 MPa), sintering phenomena are slow and extended over a large temperature range and phase transition is only fully achieved for temperature higher than 700 °C. Furthermore, the final step of densification occurs above 700 °C. In this temperature range, ionic diffusion coefficients are higher and grain coarsening is facilitated. In contrast, the activation energies calculated at 400 MPa and 1 GPa are significantly lower. From the definition of Arrhenius law, this means that the grain growth occurs at lower temperature. This is shown in Fig. 12 where it can be seen that grain growth of rutile phase starts around 450 °C compared to 700 °C at 76 MPa.

C O R R E C T E D P R O O F

Apparent activation energy of sintering

To compare the SPS sintering behaviour of nanopowder under different pressures, we used the displacement of punch to obtain a dilatometric curve. Dilatometric analysis allows to estimate the densification energy activation thanks to the so-called constant rate of heating method (CRH) [START_REF] Weibel | Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: grain size and doping effects[END_REF]. A prerequisite for calculating the apparent activation energy is to be sure that the sintering mechanism is solid phase sintering [START_REF] Balice | Densification mechanisms of UO2 consolidated by spark plasma sintering[END_REF]. The CHR method, as explained in Weibel's study, is useful

C O R R E C T E D P R O O F

With

Where γ is the surface energy, Ω the free volume of grain boundary, R the ideal gas constant, D 0,GB the diffusion coefficient of grain boundary, r the particle radius and c the constant heating rate.

Like Weibel et al. [START_REF] Weibel | Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: grain size and doping effects[END_REF], considering the negligible influence of temperature in the logarithmic term, we determined the effective activation energy of densification process from a simple plot of versus (Fig. 13). Effective activation energies determined by CRH method At 76 MPa and 400 MPa, at the phase transition temperatures respectively, 700 °C and 400 °C (Fig. 14), an inflection of the curves is observed. This densification stage corresponds to the volume contraction of 9 % induced by the phase transition. The corresponding activation energy is lower at 400 MPa (Ea 3 = 7.2 kJ/mol) than at 76 MPa (Ea 4 = 15.8 kJ/mol). This can be explained because pressure promotes the anatase-rutile transition. A 44 % reduction in the energy barrier has been assessed at 1.5 GPa [START_REF] Liao | Retention of nanoscale grain size in bulk sintered materials via a pressure-induced phase transformation[END_REF].

Discussion

The application of high pressures during the sintering of nanopowders has an important impact on the phenomena of densification (rate, activation energy), phase transition and grain growth. In order to improve the electrical, thermoelectric or mechanical properties of a material, it is necessary to have a dense material with a fine microstructure and therefore to minimise grain growth. The difficulty is that densification and grain growth are concomitant processes. Different steps occur during sintering. At low temperature, a slight coarsening of the grain takes place at the same time as the formation of the first sintering necks, then very quickly the densification mechanisms become preponderant with increasing temperature. At high density (> 90 %), the grain coarsening mechanisms become dominant.

When a pressure is applied during a sintering treatment, kinetic of the different processes is modified. Indeed, a maximum densification rate is observed at 250 °C under 1 GPa against 500 °C under 76 MPa (Fig. 3). It has been shown that at pressure above 1 GPa, it is possible to densify (d r 95 %) a nanoanatase powder at low temperatures (400 °C) [START_REF] Liao | Transformation-assisted consolidation of bulk nanocrystalline TiO2[END_REF]. To be efficient, the sintering pressure has to be greater than the internal stress of the nanoparticles. For a nanopowder with a crystallite size of 15 nm and thus a radius pore of 6 nm, the calculated internal stress is around 150 MPa [START_REF] Weibel | Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: grain size and doping effects[END_REF]. So if the applied pressure is significantly greater than the internal stress of nanoparticles it is possible to densify at lower temperature by superplastic deformation [START_REF] Mayo | High and low temperature superplasticity in nanocrystalline materials[END_REF][START_REF] Chaim | Sintering and densification of nanocrystalline ceramic oxide powders: a review[END_REF]. Furthermore, the application of high pressure at room temperature improves the packing density of the green compact by the fragmentation of agglomerates (nanoparticles have a strong tendency to agglomeration due to very high Van der Waals forces). With the pressure increase (above 330 MPa), these agglomerates are broken allowing the rearrangement of the nanoparticles. Green compact relative density reaches 64 % at 1 GPa compared to less than 30 % at 76 MPa. Thus, for pressures above 330 MPa, particles are reorganised in the compact and perhaps plastically deformed for pressure above 1 GPa [START_REF] Weibel | Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: grain size and doping effects[END_REF][START_REF] Issa | Room temperature plasticity and phase transformation of nanometer-sized transition alumina nanoparticles under pressure[END_REF][START_REF] Calvié | Mechanical behavior law of ceramic nanoparticles from transmission electron microscopy in situ nano-compression tests[END_REF] allowing to start the thermal sintering from a quite high relative density. This impacts drastically the early sintering stages.

Using in situ XRD at 1.5 GPa and 3.5 GPa, no significant crystallite growth is observed at low temperature before the phase transition. During the transition an important grain coarsening occurs explained by the coalescence of anatase crystallites to form rutile crystallites. The phase transition has been shifted to lower temperature as observed in Figs. 2 and3. Under high pressure grains are highly constrained. To release these stresses, the nanoparticles undergo structure change from the anatase form to the denser rutile form leading to a 9 % volume decrease. This relaxation during the transition phase is observed in Figs. 6b and6d. The applied high pressure results in an increased nucleation rate of the rutile phase [START_REF] Liao | Theory of high pressure/low temperature sintering of bulk nanocrystalline TiO2[END_REF].

If we compare two samples with approximately the same conversion state (80 % rutile at 450 °C-400 MPa and 98 % rutile at 750 °C-76 MPa), the grain size is similar (225 nm) whereas the density is higher at 450 °C-400 MPa. To obtain the same level of density at 76 MPa, it is necessary to heat above 700 °C. At these temperatures, grain coarsening mechanisms become progressively dominant (grain boundary diffusion) over densification mechanisms, and titania is known to possess a high diffusion coefficient for oxygen ions above 500 °C [START_REF] Guidi | Effect of dopants on grain coalescence and oxygen mobility in nanostructured titania anatase and rutile[END_REF]. At 76 MPa, we obtain density superior to 98 % at temperature above 900 °C but with a grain size over 500 nm. In comparison, high pressure sintering produces samples with densities greater than 98 % but at temperatures equal or lower than 700 °C and therefore with grain sizes of 300 nm or even smaller.

As mentioned previously, at low temperature and very high pressure, nanoparticles densification occurs first by a granular rearrangement and then by superplastic deformation that allows to decrease the pore size. This configuration favours densification diffusion phenomena. As described in Section 3.1, at high pressure (400 MPa) activation energy are lower than at 76 MPa. At very low temperature (Ea 1 ), the densification can be explained by nanoparticles rearrangement and sliding and plastic deformation. Above 300 °C (Ea 2 ) mass transfer mechanism are facilitated and accelerated. During the initial stage of sintering, surface diffusion and evaporation-condensation together con-tribute to the formation of necks. [START_REF] Rankin | Unstable neck formation during initial-stage sintering[END_REF]. The evaporation-condensation mass transfer mechanism is governed by vapour pressures due to the radii of curvature of particle surface and neck. The gap between the particles is formed by a positive radius of curvature (surface) and negative radius of curvature (neck) at the junction of the particles. The mass transfer takes place from the concave surface to convex surface (sintering neck) by evaporation. The sintering neck is gradually filled while grains grow more or less quickly depending on their initial size [START_REF] Jin | Densification and fine-grain formation mechanisms of BaTiO3 ceramics consolidated by self-assembly sintering[END_REF].

Concerning the microstructure, we have clearly observed that for samples with the same density, the more the applied stress increases, the more the average grain size decreases. Therefore, the grain growth rate decreases with the increase of applied pressure. How can we explain that densifying mass transfer are favoured at high pressure? More the pressure increases, more the granular arrangement is constrained. The particles slide, turn and under very high pressures are plastically deformed (especially for nanometric particles). Deformation may increase the radii of curvature facilitating mass transfer. This may contribute to the activation of densification phenomena at lower temperatures during the high pressure sintering process. During the first stage of densification, pore shrinkage is a predominant phenomenon. Based on Kellett and Lange's model, a porosity is filled only if the pore coordination number (N) is smaller than a critical coordination number (Nc) which depends of particles size and thus dihedral angle [START_REF] Fang | Coarsening, densification, and grain growth during sintering of nano-sized powders-a perspective[END_REF]. The pore coordination number (N) is the number of particles surrounding a pore (Fig. 15). While N > Nc, the pore is stable. When N < Nc, the pore is unstable and will shrink. In conventional sintering, grain coarsening reduces N, allowing sintering and densification to continue. But different mechanisms can reduce N. Under high pressure, the pores can collapse and divide in smaller pores with N < Nc (Fig. 15) [START_REF] Balima | An in situ small angle neutron scattering study of expanded graphite under a uniaxial stress[END_REF]. N decreases with pressure, which favours the densification mechanisms over those of grain growth.

Under high pressure, a specific grain growth mechanism appears, the grain boundaries sliding (GBS) [START_REF] Sokol | Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)[END_REF][START_REF] Ratzker | Stress-enhanced dynamic grain growth during high-pressure spark plasma sintering of alumina[END_REF]. Under high stress the grains slide, rotate until align their atomic planes resulting in grain growth by grain coalescence. This grain growth mechanism needs less energy and occurs at lower temperature. Therefore, high pressure sintering must be performed at the lowest possible temperature to achieve full densification while minimising grain growth. Our portable HP-SPS device for in situ analysis is a powerful tool for optimising pressure, temperature and heating rate to achieve dense nanoceramics. Then, larger products can be obtained in a larger volume high-pressure device associated with a pulsed electric generator [START_REF] Guignard | High pressure (HP) in spark plasma sintering (SPS) processes: application to the polycrystalline diamond[END_REF]. We have shown that HP-SPS is effective in obtaining a finer microstructure in dense ceramics. All samples are plotted in the same graph of grain size versus density (Fig. 16). It is clear that the samples sintered at 1 GPa have the finest grain size with a density of over 98 %.

Conclusion

High pressure, low temperature SPS sintering of TiO 2 nanopowder was successfully carried out in order to obtain ceramics with finer microstructure and high density. Application of very high pressure on the green pellet allows to highly increase its compactness, by agglomerates organisation under 330 MPa and then fragmentation with intraagglomerates grain rearrangement at higher pressure. High initial packing density leads to efficient densification phenomena at low temperature. Strongly constrained system promotes a phase transition at lower temperature. Microdistortion are released thanks to phase transition. Very high pressure, higher than 400 MPa, has also an important effect on grain growth. The kinetics are remarkably slowed down. On the final densification step, after the phase transition, very high pressure stabilises finer microstructures. Effect of high pressure on kinetics phenomena has been confirmed by activation energy calculations. Activation energy of sintering decreases with the pressure, confirming the lowering of the sintering temperatures.

Fig. 1 .

 1 Fig. 1. (a) Section view of the sample cell assembly between two WC anvils (b) The Paris-Edinburgh HP-SPS module mounted in the SPS-HPD 25 (FCT) chamber, working up to 2 GPa (c) The Paris-Edinburgh press coupled with a pulsed current generator, working up to 5 GPa [6].

Fig. 2 .

 2 Fig. 2. XRD profiles for the initial powder calcined at 500 °C/4 h, pellets sintered by SPS at 700 °C under 76 MPa, 400 MPa and 500 MPa. Peaks are identified as anatase (a), brookite (b), rutile (r) and graphite peaks (g). The latter refers to graphite residue on the sample surface originating from the graphite foil placed between the die and TiO 2 powder during sintering.

Fig. 3 .

 3 Fig. 3. a) Dilatometric curves registered at 76 MPa, 400 MPa, 500 MPa up to at 700 °C with 100 °C/min heating rate and under 1 GPa up to 650 °C with 16°/min heating rate; b) Corresponding displacement rates; c) Temperature evolution of the 3 mechanisms observed during densification as a function of pressure.

Fig. 4 .

 4 Fig. 4. Green density plotted versus logarithm of applied pressure.

Fig. 5 .

 5 Fig. 5. In situ X ray diffractograms recorded during sintering at 3.5 GPa (λ = 0.2468 Å).

Fig. 6 .

 6 Fig. 6. a) and c) Crystallite sizes extracted from the diffraction patterns recorded during in situ SPS experiments at 1.5 and 3.5 GPa, respectively. Values corresponding to 740 °C were obtained from ex situ measurements on the recovered samples; b) and d) Crystallographic microdistortion (filled diamonds) and fraction of phases (open squares) evolution during sintering at 1.5 and 3.5 GPa. Black and red marks referred to the anatase and rutile phases, respectively.

Fig. 7 .

 7 Fig. 7. SEM images of sintered powder at 700 °C under 76 MPa -d r 90.1 % (a), 400 MPa -d r 98.5 % (b) and 500 MPa -d r 98.5 % (c) at 650 °C under 1 GPa -d r 98.1 % (d).

Fig. 8 .

 8 Fig. 8. SEM images of sintered powder at 725 °C under 1.5 GPa (a) and 3.5 GPa (b).

Fig. 9 .

 9 Fig. 9. Grain size distribution of pellets sintered at 700 °C for 10 min under different pressures. Grain size is the grain diameter measured on the SEM images (50-175 values per sample). The data are discretized in twelve 50 nm width intervals.

Fig. 10 .

 10 Fig. 10. Grain size distribution of pellets sintered at 725 °C under 1.5 and 3.5 GPa. Grain size is the grain diameter measured on the SEM images (50-175 values per sample). The data are discretized in twelve 50 nm width intervals. only during the intermediate stage of sintering, when open porosity is still present (relative density < 90 %). The following expression, derived from Arrhenius law, considers that the predominant mechanism is grain boundary diffusion.

Fig. 11 .

 11 Fig. 11. Evolution of grain size (blue dots), crystallite size (blue triangles) and relative density (green dots) as function of pressure at 700 °C and 650 °C.for samples sintered under 76 MPa and 400 MPa are presented in Fig.14. Different slopes are observed for both experiments as function of temperature showing an impact of pressure on densification process. For each pressure, different densification mechanisms take place during heating. The values of activation energy calculated here are close to the literature. Weibel et al.[START_REF] Weibel | Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: grain size and doping effects[END_REF] find values of 0.39 eV (37.5 kJ/mol) for a similar nanopowder (12 nm) sintered under pressure (125-600 MPa). In contrast to Weibel et al., we observe different stages of densification in the ranges 200-400 °C and 200-700 °C for a sintering under 76 MPa and 400 MPa, respectively. At very low temperature (< 300 °C), densification is only due to an increase in compactness through the arrangement of the grains and perhaps plastic deformation, as reported for nanocrystalline particles under very high pressure above 1 GPa[START_REF] Weibel | Hot compaction of nanocrystalline TiO2 (anatase) ceramics. Mechanisms of densification: grain size and doping effects[END_REF][START_REF] Issa | Room temperature plasticity and phase transformation of nanometer-sized transition alumina nanoparticles under pressure[END_REF][START_REF] Calvié | Mechanical behavior law of ceramic nanoparticles from transmission electron microscopy in situ nano-compression tests[END_REF][START_REF] Mayo | High and low temperature superplasticity in nanocrystalline materials[END_REF]. Higher pressure implies greater compactness and thus a lower activation energy of densification (Ea 1 ). Above 300 °C, diffusion mechanisms became predominant and the effective activation energy increases (Ea 2 ). The activation energy, Ea 2 , at 400 MPa (23.4 kJ/mol) is significantly lower than at 76 MPa (38.5 kJ/mol). This reflects a facilitated mass transfer phenomenon during the sintering of highly constrained particles.At 76 MPa and 400 MPa, at the phase transition temperatures respectively, 700 °C and 400 °C (Fig.14), an inflection of the curves is observed. This densification stage corresponds to the volume contraction of 9 % induced by the phase transition. The corresponding activation energy is lower at 400 MPa (Ea 3 = 7.2 kJ/mol) than at 76 MPa (Ea 4 = 15.8 kJ/mol). This can be explained because pressure promotes the anatase-rutile transition. A 44 % reduction in the energy barrier has been assessed at 1.5 GPa[START_REF] Liao | Retention of nanoscale grain size in bulk sintered materials via a pressure-induced phase transformation[END_REF].

Fig. 12 .

 12 Fig. 12. Evolution of grain size, crystallite size and relative density as a function of the sintering temperature at three constant pressures: 76 MPa, 400 MPa and 1 GPa. For each pressure, the anatase-rutile transition temperature range is represented by a coloured area.

Fig. 13 .

 13 Fig. 13. Arrhenius equation linear fitting diagram for sintered pellets with pure TiO 2 powder at 76 MPa (a), 400 MPa (b) and 1 GPa (c).

Fig. 14 .

 14 Fig. 14. Activation energies determined by CRH method for sintering at 76 MPa (blue), 400 MPa (red). Relative densities of the samples are indicated in the corresponding temperature ranges.

Fig. 15 .

 15 Fig. 15. Illustration of the decrease in pore coordination numbers (indicated in red) due to pore collapse under pressure.

Fig. 16 .

 16 Fig. 16. Grain size versus relative density for all samples sintered at different pressures, ranging from 76 MPa to 1 GPa.

Acknowledgements

We would like to thank Matthieu Mercury, Clément Albin and Jean-Michel Combes (ILM) for their help in the HP-SPS device. We thank Laurent Gremillard (MATEIS) for his assistance in using TOPAS. HP-SPS experiments were performed at the MATEIS-SPS platform and the PLECE-ILMTech platform.

Funding

The authors thank CNRS Prématuration Program 2019 and the institute of chemistry of Lyon university (ICL) for funding this study.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.


Frederic C Meunier
email: fcm@ircelyon.univ-lyon1.fr

Hydrogenation of CO and CO 2 : contributions of IR operando studies

Keywords: in situ, methanation, methanol, syngas, CO 2

The hydrogenation of CO and especially that of CO 2 will raise increasing interest as a means to manufacture base chemicals and fuels. This contribution discusses some of the aspects of CO and CO 2 hydrogenations that can be unravelled using in situ and operando FT-IR studies. This non-exhaustive mini-review gives a limited set of examples taken over Co-, Rh-, Ni-and Ru-based heterogeneous catalysts of the type of information that can be obtained through IR studies. It stresses the need to carry out quantitative IR studies, if relevant conclusions are to be drawn. Some limitations and pitfalls of diffuse reflectance spectroscopy are also discussed. The possibility to carry out quantitative DRIFTS studies is demonstrated on several examples. The potential of IR spectroscopy to unravel the properties of CO 2 trapping-methanation materials is briefly discussed.

Content:

1. Importance of the catalytic conversion of CO and CO 2 and potential benefits from operando IR studies.

2. Pitfalls arising from IR operando studies involving carbon oxides 3. IR studies of CO and CO 2 hydrogenation to methanol 4. IR studies of CO and CO 2 hydrogenation to hydrocarbons 5. IR studies of CO 2 capture and use. 6. Conclusions.

Importance of the catalytic conversion of CO and CO 2 and potential benefits from operando IR studies.

The hydrogenations of CO and CO 2 to a wide range of commodities (Figure 1) will become key elements of the chemical industries in the coming decades. A greater availability of H 2 obtained from renewable sources or derived from nuclear energy will lower the cost of the associated processes. The need to recycle CO 2 , formed from carbonaceous fuel burning and stored during peaks of energy demand, will require to adapt many processes and catalysts that had been primarily optimised for syngas (CO + H 2 ) conversion.

Figure 1. Main reaction products formed through the hydrogenation of carbon oxides depending on the nature of the metal (in red) used. CO and CO 2 are readily interconverted through the water-gas shift (WGS) reaction and its reverse reaction (RWGS). The main end-products are given (MTBE = methyl tert-butyl ether; TAME = tert-amyl methyl ether; MTO = methanol-to-olefins).

The hydrogenation of CO 2 to formic acid 1 , methanol 2,3,4,5 , methane 6 , 7 , 8 and liquid fuels through Fischer-Tropsch synthesis 9 , 10 , 11 has been discussed in details in many reviews and research papers. This short contribution will outline some of the advances that have been obtained through in situ and operando IR studies for some of these reactions.

Foremost, IR studies enable the identification of surface species, whether those are reaction intermediates or spectator species. 12,13,14,15,16 . This is because many vibrational modes associated with adsorbates bonds are highly IR sensitive and lead to sharp bands enabling detection at the nanomole levels using only milligrams of catalysts 17 .

The vibrational modes of solid catalysts are difficult to investigate by IR because the corresponding lattice vibrations exhibit intense broad absorption bands (typically located in the far-IR region) that often lead to full IR signal absorption. IR can yet lead to indirect information about the structure of the solid under reaction conditions, if an adsorbed intermediate exhibits bands that have site-dependent wavenumbers, as for instance CO adsorbed on metal atoms or cations 18,19,20 .

IR spectra analysis is yet not straightforward, as exemplified by recent misinterpretations involving CO or CO 2 reactions. For instance, the gas-phase rotovibrational spectrum of CO is often mistaken for bands of adsorbates 21,22 . The same mistake can be found in the case of water 23 , the presence of which on the catalyst surface may alter metal-CO spectra and lead to an apparent increase of CO surface coverage with increasing temperature 24,25 .

Not removing gas-phase spectrum of CO complexifies the analysis of the carbonyl band region, making more difficult spotting small but important spectral features. An example was discussed elsewhere 19,26 , regarding an overlooked band likely related to CO adsorbed on metallic Pt nanoparticles, close to a much larger band related to CO adsorbed on Pt single atoms. The presence of such small concentrations of metallic Pt nanoparticles should not be neglected in the case of CO oxidation, since the turnover frequency of Pt atoms in nanoparticles was reported to be 10 4 -fold higher than that of Pt single atoms 19,30 .

Metal-carbonyl band positions will depend on several parameters such as surface coverage, CO coordination mode, coordination number and oxidation state of the metallic site. In addition, the IR signal can be dominated by that of spectator species, often adsorbed on the support, that may have little or no role in the main reaction pathway. For instance, formates seen by IR were shown to be irrelevant to the main reaction pathway of the low-temperature water-gas shift (WGS) reaction over ceria-supported metal catalysts 14 .

In addition, adsorbates of a same type can exhibit very different reactivities on a given sample. This is exemplified by the case of formates used for the WGS reaction 27 or the Fischer-Tropsch synthesis (vide infra) 28,29 , for which two fractions of formates displayed rate constants of decomposition varying by more than an order of magnitude.

The difference of reactivity was ascribed to the vicinity of the support-bound formates to the interface with the metal. Another example is related to the case of CO oxidation at low temperatures, for which turnover frequencies (TOF) of CO adsorbed on Pt metallic nanoparticles were shown to be more than four orders of magnitude higher than those on Pt single atoms/cations 19,30 .

A schematic representation of a hypothetical working surface of an oxide-supported metal is given in Figure 2. This scheme emphasises the many sites that could be simultaneously present under reaction conditions, including reduced M° and oxidized MO x metal sites, as observed in several studies 10,31,32 . The interface between the metal and the support is of particular interest, since it may expose high activity sites or ensembles, especially when the support is more efficient than the metal to adsorb and activate one of the reactants. This point is particularly true for H 2 O and CO 2 , which are often best activated on the acid and basic sites of oxide supports rather than on the metal, the latter being more efficient in activating molecules such as CO, CH 4 and H 2 33,34 . Interfacial sites between Cu and ZnO have been proposed as the most active sites for methanol synthesis through formate hydrogenation in CO 2 -containing feeds 4,35,36 .

It is also important to stress that supports typically exhibit high specific surface areas and may store large amounts of unreactive adsorbates located far away from the metal/support interface. These remote adsorbates may not be accessible, for instance, to hydrogen species activated on the metal. This is particularly true for non-redox supports such as alumina, that does not allow any significant H-spillover in contrast to TiO 2 37 .

Significant amounts of products (such as methanol) may re-adsorb strongly at the surface of the support, complicating kinetic analysis based on isotopic transient methods 38 . These last two paragraphs stress that fact that the IR signal may be dominated by adsorbates that are irrelevant to the rate-determining step of the main reaction pathway.

The reactivity of adsorbates should be measured by inducing some kind of transient in the system, to discriminate potential intermediates from spectator species. Ideally, steady-state isotopic transient analyses (SSITKA 38 ) should be carried out, by replacing one of the reactants with a labelled compound (e.g. 12 CO with 13 CO, C 18 O 2 with C 16 O 2 ).

This method offers the advantage of keeping the chemical potential of each element identical in the reactor and prevent catalyst composition changes (e.g. reduction, oxidation, carburization). SSITKA methods maintain constant adsorbate surface coverages, though changes could yet be expected for H 2 -D 2 exchanges because of the significant kinetic isotope effects that may occur. SSITKA-types experiments coupled with IR spectroscopy have been reported for transmission IR 39,40 and DRIFTS 41,42 .

Chemical transient methods may also be employed, for instance by removing one the reactants 43 , although those may lead to catalyst compositional changes and surely to changes in surface coverages. In such case, the initial rates of decomposition of adsorbates should be used, to better reflect the rates prevailing at steady-state. The use of modulation-excitation spectroscopy, which consist in periodic alterations of the concentration of one reactant, allows maintaining coverages close to those existing at

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steady-state, while probing adsorbates reactivity 10 .

Finally, the selectivity of decomposition of each adsorbate should be estimated, yet this is almost unfeasible in most cases. It is indeed possible that, for instance, formate decomposition leads to CO, CO 2 or methanol depending on the proximity of a H 2activating centre. In the simple case of unpromoted MgO pre-covered with formate species, Tamaru and co-workers were able to determine the selectivity of decomposition by using an on-line gas-chromatograph connected to their IR cell 13 . This method is no longer useful when various formates or other species are also present at the surface. In this case, maximal decomposition rates are calculated by assuming total selectivity to the compound of interest.

Pitfalls arising from IR operando studies involving carbon oxides

Diffuse reflectance FT-IR spectroscopy (DRIFTS) is widely used as a means to investigate catalytic reactions, because this method allows the direct use of powders and flowing the feed through the catalyst bed. One major drawback is yet the large dead-volume present above the catalyst bed (Figure 3A), leading to a significant path, up to several centimeters, in the gasphase 24,44 . Gas-phase CO 2 and even CO will then give observable IR signals, the absorption of CO 2 being typically too intense to be corrected, in contrast to that of CO. The correction of the signal of gas-phase CO is not yet straightforward. Figure 3B shows gas-phase roto-vibrational spectra of CO collected over SiC in a cell whose reaction bed was heated at different temperatures. It can be clearly seen that the spectra widen at higher temperatures, due to an increased occupancy of rotational states with higher energies.

The temperature profile over the optical path in the gas-phase is complex because of the large temperature gradient that may exist between the (cool) IR windows and the catalyst bed, which will also be affected by the thermal conductivity of the gas used. This underlines the need to record reference IR spectra of the compounds for subtraction purposes under identical conditions over an inert solid, thus allowing perfect correction of the gas-phase contribution (Figure 3C).

It should be stressed that removing gas-phase CO to record the spectra of only adsorb species is not a satisfactory method to cancel gas-phase contributions, even at low temperatures. Weakly adsorbed CO, for instance on metallic copper, could readily desorb, while strongly bound species could be dissociated or react with redox oxides present, as observed with ceria-based catalysts near 45 or at room temperature 46 , 47 .

An insidious pitfall of using feeds containing CO is the potential presence of the (highly toxic 48 , 49 ) volatile compound Ni(CO) 4 , which may result in catalyst and cell contaminations.

An example can be found in a study dealing with CO reactivity over mixed oxides of ceria and zirconia 50 , in which the authors did not realize that their feed was contaminated with Ni(CO) 4 , as evidenced by the presence of a band near 2055 cm -1 typical of this compound 51 (Figure 4). This contaminant eventually led to the deposition of Ni nanoparticles over ceria, explaining the band at 2007 cm -1 , assigned to CO adsorbed on metallic Ni nanoparticles. Hot traps (at ca. 250 °C) filled with wire wool or a Cu-based catalyst can be used to remove iron and nickel carbonyls from a contaminated gas stream, as these compounds will readily decompose into CO and metal particles under these conditions.

Ni(CO) 4 can also be formed in situ, for instance inside a spectroscopic cell whose steel had been corroded by NOx, as reported elsewhere 52 . The formation of Ni(CO) 4 was actually only observed if the cell had been brought above 300 °C under H 2 , so that the oxidized nickel present could be reduced to the metallic state, and then exposed to CO at low temperature, to favor multiple CO adsorption on Ni° and Ni(CO) 4 formation. The presence of O 2 also prevented the contamination, likely due to the passivation of metallic nickel through oxidation. The corrosion of Ni° by CO can occur at temperatures as low as 85 K 51 and is likely to be a problem more common than thought, as suggested in a recent paper 53 .

IR studies of CO and CO 2 hydrogenation to methanol

The established methanol manufacturing process was developed by ICI in the 1960s and is based on Cu/Zn/Al 2 O 3 catalysts exposed to CO/CO 2 /H 2 mixtures and was briefly discussed by

Golunski and Burch in a recent review 4 . While CO is the main carbon oxide component of the feed, it has been established that the primary carbon-containing reactant was CO 2 . One of the roles of CO is to prevent over-oxidation of the copper surface. The activity is roughly proportional to the surface area of metallic copper, but highly-active sites have been proposed Wavenumber (cm -1 )

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to form in the presence of the ZnO support (e.g. Cu 0 -Zn 0 alloyed phases 54 , Zn-modified lattice-strained Cu 0 phases 55 , Cu 0 -ZnO interface sites 56 , 57 ).

Numerous IR studies have been reported on the Cu-catalyzed hydrogenation of CO 58,59 and CO 2 39 ,60,61,62,63 to methanol, with formates often proposed as a main reaction intermediate.

Yet, very few of these studies involved the quantitative comparison of formate decomposition rate to that of methanol formation. The work of Mims and co-workers should be highlighted, as these authors investigated the reactivity of formate species over a Cu/SiO 2 catalyst using operando IR and isotopic exchange techniques 39 . It was concluded, in their case, that formate mostly decomposed back into the reagents CO 2 and H 2 and that the rate of conversion to methanol was sufficient to propose that formates were the main reaction intermediate. Yet, the formate concentrations used were estimated from earlier studies.

A major difficulty in the development of IR quantitative analyses is the quantification of adsorbate concentration. This is even more so in the case of DRIFTS, because the optical pathlength inside the sample is not directly known, in contrast to transmission IR mode, for which the optical pathlength is the wafer thickness. These hurdles faced by DRIFTS can yet be overcome through various means, for instance by adding an internal standard to the sample, for instance a few weight percents of CaCO 3 64 or BaSO 4 65 . Structural bands of the solid can also be used to quantify or normalize the optical pathlength, such as overtones found in the 2100-1800 cm -1 region in the case of silica and zeolites 66,67 .

The availability of formate salts and the relative stability of adsorbed formates on most oxide surfaces enable drawing calibrations curves (Figure 5). Experimental details identical to those reported in reference 41.

Formates exhibits several bands in the carboxylate stretching (Figure 5A) and C-H stretching (Figure 5C) regions, which may be used depending on the presence of overlapping contribution from other adsorbates. The calibration standards were measured twice, showing a satisfactory reproducibility of bed packing (Figure 5B). The calibration standards led to spectra very similar to those of the adsorbates formed under reaction conditions (Figure 5C), enabling the determination of the actual adsorbate concentrations under reaction conditions (Figure 5D).

The role of formates in the hydrogenation of CO 2 has been the subject of a controversy over ZrO 2 -supported Cu. Two distinct pathways have been proposed (Figure 6). Larmier et al. 68 proposed a formate-based route, in which a ZrO 2 -bound formate located at the interface with Cu nanoparticles is hydrogenated by H 2 activated on Cu to methoxy, still on the support.

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In contrast, Kattel et al. 62 proposed a formate-free pathway, starting with the reverse WGS (RWGS) and the formation of a CO-OH hydroxycarbonyl intermediate. It must be emphasized that in neither of these studies were the rates of formate decomposition compared to that of methanol formation. It was also recently shown that two different ZrO 2 -bound formates could be observed on Cu/ZrO 2 , one of these formates exhibiting a 65-fold higher rate constant of decomposition 69 . A quantitative analysis of the rate of formate decomposition was carried out by Lorito et al., while investigating CO/H 2 reaction over alumina-supported cobalt 29 , using the formate calibration curves described in figure 5. The reactivity of formates was determined by inducing a chemical transient. Interestingly, two types of reactivity (Figure 7A) were observed for formates, despite the fact that both species exhibited identical DRIFTS spectra.

These facts were rationalized by proposing that these alumina-bound formates differed by their proximity to alumina-metal interface. The decomposition rate of the faster species was found to be sufficient to account for the methanol observed, while that of the slower species was negligible (Figure 7B).

RWGS + HO-CO pathway

Formate pathway Formate Methoxy

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These data are yet not sufficient to prove that the fast formates were the main reaction intermediates in the formation of methanol, since the product specifically formed from the decomposition of these fast formates could not be concomitantly measured (i.e. other products were also formed simultaneously from decomposition on other metal and oxide sites). These fast formates can in theory decompose into CO, CO 2 or methanol, and this will particularly depend on the availability of hydrogen.

IR studies of CO and CO 2 hydrogenation to hydrocarbons.

Cobalt is highly active for CO dissociation and is thus commonly used for CO hydrogenation in Fischer-Tropsch reactions. Cobalt-based catalysts typically produce linear alkanes and short alkenes during CO hydrogenation with H 2 70,71,72 . The IR spectrum in the carbonyl region obtained under reaction conditions is complex, but can be roughly decomposed in a set of two species: linear (above ca. 1950 cm -1 ) and multi-bonded CO(ads) (below ca. 1950 cm -1 ) (Figure 8.A). The linear CO(ads) IR signal between 2100 and 1950 cm - 1 relates to various linear carbonyls, mostly present on (111) and (100) planes) 70,73,74,75,76 . The multi-bonded CO (also noted Hollow-CO) below 1950 cm -1 relates to CO adsorbed at defective sites such as steps.

Methanol Fast formates

Slow formates

Fast formates Slow formates (A) (B)

The reactivity of the carbonyls present at the surface of a 15 wt.% Co/alumina and the corresponded Sn-modified samples (Figure 8A) was investigated using a combination of operando DRIFTS and transient methods (both chemical and isotopic transients) 15 . The rates of formation of methane and propene, the main hydrocarbons formed under these conditions, was shown to be proportional to the fraction of Hollow-CO (Figure 8B). Hollow-CO was proposed as a main reaction intermediate, while linear CO was unreactive, either desorbing as CO or migrating onto the hollow sites to be hydrogenated. The rate of CO methanation (in H 2 ) was in fact found to be significantly higher than that of CO desorption (in He). The rate of Hollow-CO hydrogenation was found to be about twice that of methane formation (Figure 8C) and since methane selectivity is typically around 50 % at atmospheric pressure, it was concluded that Hollow-CO was part of the main reaction pathway to hydrocarbons. The hydrogenation of CO and CO 2 mixtures was investigated over a similar 15 wt.

Co/Siralox 77 . High selectivity to CH 4 are usually observed with CO 2 -rich feed (Figure 9A) and hypothesized to arise from a lower surface coverage of CO, which in turn would lead to a higher surface coverage of hydrogen favoring methane formation 78,79 . This hypothesis was confirmed through an operando DRIFTS study that showed that the IR spectra obtained in the presence of CO 2 corresponded to the same metallic state of the surface, but with lower carbonyl coverages. This was demonstrated by noting that the CO(ads) band positions versus 9B and9C). These results suggests that cobalt should be modified to bind CO more strongly, if the selectivity to CH 4 is to be kept low when feeding CO 2 . Christopher and co-workers investigated the hydrogenation of CO 2 by DRIFTS over Rh-based catalyst at atmospheric pressure 80 . The DRIFTS spectra were particularly informative about the Rh speciation because CO adsorbed on Rh nanoparticles (Rh NP ) gave rise to two bands of linear CO and bridge CO, while Rh single atoms (Rh iso ) led to a typical gem-dicarbonyl doublet (Figure 10A). The authors could derive quantitative DRIFTS information over a series of catalysts (Figure 10B) that enabled the determination of the fraction of Rh in nanoparticles and as single atoms (Figure 10C). These data indicated that Rh single atoms were responsible for the formation of CO through the reverse WGS reaction, while Rh nanoparticles carried out the more demanding full hydrogenation of CO 2 to CH 4 (Figure 10D). CO 2 capture and utilization (CCU) will play a prominent role in the restructuring of the energy sector and the fight against global warming. CO 2 would be trapped within large industrial units (e.g., power plants, boilers, cement manufacture) and could be reprocessed onsite or remotely. Practical utilizations of this stored CO 2 include its hydrogenation using renewable H 2 to produce methane, which represents a pillar of the power-to-gas technology 82 .

Sequential processes of CO 2 trapping and methanation based on solid traps and catalysts have been recently proposed 83,84,85 . Bifunctional materials exhibiting both storage and catalytic functions were used for isothermal sequential trapping and methanation of CO 2 , based on alumina-supported metals (i.e. Ru, Pt, Ni) promoted by earth-alkali 83,86 and alkali 84,85 . The main bottleneck observed was the slow kinetics of the methanation step, requiring twice the time of the trapping step 84,86 . The nature of the rate determining step (e.g. metal reduction, CO 2 release, CO 2 dissociation, CO hydrogenation, H 2 activation) is yet unknown.

The use of operando spectroscopy will be valuable to determine the trapping and methanation properties of solid trap-catalysts. A recent example is given in Figure 11Left, in which various surface species can be observed upon exposure to various feeds 87 . In particular, the presence of various (hydrogeno)carbonates, likely located on the support, and carbonyls, on metallic Ru, can be observed. Time-resolved analyses can be carried out to determine the reactivity of the corresponding species and unravel kinetically slow steps with a view at improving the sample. It should be stressed that the reactivity of carbonates may strongly depend on the nature of the feed used and that of the support, as shown in the case of Pt/CeO 2 , for which ceria-bound carbonates were highly stable under reducing conditions 88 .

One caution has yet to be raised concerning the use of DRIFTS for this type of storage-type work, as only the top layer of the catalyst bed (less than 200 microns 89 ) is typically measured. This means that it may not be possible to relate reactor effluent to the observed DRIFTS signal.

Figure 2 .

 2 Figure 2. Schematic representation of adsorbates covering the surface of the various domains that could be present on a typical oxide-supported metal used for CO 2 -H 2 reactions. Adsorbates X and Y could be, for instance, carbonyls and formates, respectively. The red solid arrows relate to the rates of adsorbate decomposition into by-products, while the green solid arrows relate to those of decomposition into the desired product. The main reaction intermediate would be that (here, arbitrarily, species Y at the metal/support interface) whose decomposition rate to the product closely matches that of conversion of the reagents into the product (green dashed arrow).

Figure 3 .

 3 Figure 3. (A) Schematic representation of a DRIFTS cell showing the large pathlength in the dead volume above the catalyst bed. (B) In situ DRIFTS spectra recorded over SiC at 50, 250 and 400 °C in the presence of 30% CO + 60% H 2 + 10% Ar at atmospheric pressure. The spectra obtained over SiC at the corresponding temperatures under pure Ar were used as background to calculate the pseudo-absorbance. The spectrum intensities were normalized to better compare the corresponding widths. (C) In situ DRIFTS spectra recorded over 14 wt.%Co/alumina at 220 °C (blue) in the presence of 30% CO + 60% H 2 + 10% at atmospheric pressure Ar at steadystate, (red) same, but corrected for the signal of gas-phase CO using a spectrum collected over SiC under

Figure 4 .

 4 Figure 4. In situ DRIFTS spectra recorded over ceria under CO (bands at 2172 and 2117 cm -1 ) showing the presence of Ni(CO) 4 (band at 2055 cm -1 ) in the presence of 1 kPa CO (Adapted from reference 22 with permission, © 2019 Elsevier B.V.)

10 / 21 Figure 5 .

 10215 Figure 5. (A) DRIFTS spectra of alumina dry-impregnated with various concentrations of sodium formates and dried at 100 °C. Each sample was measured twice to ascertain the reproducibility of DRIFTS sampling. T = 100 °C. (B) Corresponding calibration curve for which the formate band area was integrated over the range 1411-1307 cm -1 . (Adapted from reference 29 with permission, © 2015 Elsevier B.V.) (C) Typical DRIFTS spectra in the C-H stretching region of (black) ceria impregnated with sodium formate and (red) in situ spectrum recorded under WGS reaction conditions (D) (black dots) calibration plot relating the area of the formate CH stretching DRIFT band over 2880-2800 cm -1 to the nominal formate loading on CeO 2 impregnated with sodium formate and (red dots) derived formate concentrations observed under reaction conditions at various temperature.

Figure 6 .

 6 Figure 6. Reaction pathways proposed on Cu/ZrO 2 . (Adapted with permission from reference 62, Kattel, et al. J. Am. Chem. Soc. 138 (2016) 12440-12450, © 2016 American Chemical Society and from reference 68, Larmier et al., Angew Chem. Int. Ed. Angew. Chem. Int. Ed. 2017, 56, 2318-2323 © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim).

Figure 7 .

 7 Figure 7. (A) Variation of the logarithm of the normalized intensity of the formate DRIFTS band as a function of time following the removal of CO from the CO/H 2 /Ar feed at 220 °C and (B) Rates of formation (at steadystate at 220 °C under 30% CO + 60% H 2 at atmospheric pressure) of methanol and the initial rates of decomposition (following CO removal from the feed) of fast and slow formates. Experimental details identical to those given in reference 29.

Figure 21 Figure 8 .

 218 Figure 8D give a schematic representation of CO hydrogenation over a 15 wt. Co/alumina, as discussed in references 15 and 29. Multi-bonded CO are the main reaction intermediates in the formation of hydrocarbons, while linear CO are merely stored species.Formates on the alumina in the vicinity of the cobalt particles are potentially the main intermediate in the formation of the small concentration of methanol observed, while formates far from the interface are spectators.

Figure 9 .

 9 Figure 9. (A) Selectivity to CH 4 versus the relative CO surface coverage over supported non-promoted cobalt (+) and Pt-promoted cobalt (x) supported on Siralox under various CO and CO 2 -containing feed at atmospheric pressure. See original reference for details. (B) Evolution of the carbonyl DRIFTS band under H 2 at 210 °C following the removal of CO after steady-state under the CO-H 2 feed. The green spectrum was collected at steady-state under CO 2 +H 2 . (C) Positions of the main carbonyl DRIFTS band versus the band total area measured at 210 °C under various CO and CO 2 -containing feed. The blue thick lines are the data corresponding the band decomposition following CO removal as shown in Figure 9B. (D) Schematic representation of the state of the surface under reaction conditions. The CO surface coverage under CO-H 2 is very high, limiting methanation. In contrast, CO surface coverage is much lower under CO 2 -H 2 , leading to increased selectivity to methane. The structure of the surface remains intact for all CO-CO 2 -H 2 mixtures. (Adapted from reference 77 with permission, © 2022 Elsevier B.V.)

Figure 10 .

 10 Figure 10. (A) Typical DRIFTS spectrum obtained over a 4 wt.% Rh/TiO 2 after saturation with 10 kPa CO at 27 °C, showing the doublet associated with the gem-dicarbonyl adsorbed on Rh iso single atoms, a band of linearly adsorbed CO on Rh nanoparticles and a band of CO bridging two Rh atoms. (B) DRIFTS spectrum obtained under identical conditions as in (a) over a series of TiO 2 -supported catalysts with Rh content varying from 0.5 up to 6 wt.%. (C) Site fraction (%) of Rh iso and Rh in nanoparticles (see original reference for details). (D) Schematic representation of the active sites over TiO 2 -supported Rh: Rh NP carries out CO 2 methanation while Rh iso is active for the RWGS. (Adapted with permission from reference 80, Matsubu et al.. J. Am. Chem. Soc. 137 (2015) 3076-3084 © American Chemical Society)

Steady-state CO 2 /H 2 Steady-state CO/H 2 Purge in H 2

	(A)				(B)	0,30
	Selectivity to methane	0% 20% 40% 60% 80% 100%	H2/CO H2/CO2/CO H2/CO2 H2/CO with Pt H2/CO2/CO with Pt H2/CO2 with Pt		Pseudo-absorbance	0,05 0,10 0,15 0,20 0,25
	(C)	70%	80% CO related coverage rate 90%	100%	(D)	0,00 2100	2000	1900 Wavenumber (cm -1 )	1800	1700

Conclusions.

IR spectroscopy can bring both qualitative and quantitative insights into the structure and operating mode of materials used for adsorption and catalysis. This is even true for diffuse reflectance FT-IR spectroscopy (DRIFTS), although it is less straightforward than for transmission IR. The need for the quantitative analyses of adsorbate concentrations and reactivity was stressed and was exemplified in various reactions involving CO and CO 2 .