Alexander V Yakimov

Christoph J Kaul

Yuya Kakiuchi

Sebastian Sabisch

Felipe Morais Bolner

Jean Raynaud

Vincent Monteil

Pierrick Berruyer

Christophe Copéret

may come from teaching and research institutions in France or abroad, or from public or private research

Keywords: Ziegler-Natta catalysts, solid-state NMR, DFT calculations, High-field 47/49 Ti NMR

Ziegler-Natta (ZN) catalysts -typically formulated as TiCl4/MgCl2/AlR3 and possibly containing additional organic ligands -are essential to the production of polyethylene and polypropylene. Despite their industrial relevance and years of research on these materials, the role of each constituent (support, organic ligands, post-treatment with organic or inorganic modifiers etc.) on the structure of Ti surface sites responsible for polymerization remains poorly understood, partly because of the high complexity of such materials. Herein, we show how high-field 47/49 Ti NMR can bring about new lights on the structures of the Ti surface sites in ZN pre-catalysts (prior to activation with alkyl aluminium) resulting from adsorption of TiCl4 on MgCl2 followed by a post-treatment with BCl3, an additive used to improve catalytic activity by increasing the amounts of active sites. The implementation of high-field NMR (900 MHz), low temperature (~100 K), magic angle spinning (10 kHz), CPMG echo train acquisition and DFT modelling, to study this material (TiCl4/MgCl2/BCl3) and molecular analogs, allows the detection of a 47/49 Ti NMR signature and the development of a molecular level understanding of the NMR signature of Ti surface sites. The extracted 49 Ti NMR parameters (δiso, exp = -170 ppm and CQ, exp = 9.3 MHz) from this signature analyzed by DFT modeling indicate the presence of one specific coordination sphere for Ti, namely a fully chlorinated hexacoordinated Ti site with a symmetric charge distribution, due to the post-treatment with BCl3 (that removes the alkoxide ligands) and the coordination environment provided by surface of an amorphous MgCl2.

To cite this version:

Alexander V Yakimov, Christoph J Kaul, Yuya Kakiuchi, Sebastian Sabisch, Felipe Morais Bolner, et al.. Well-Defined Ti Surface Sites in Ziegler-Natta Pre-Catalysts from 47/49Ti solid-state Nuclear Magnetic Resonance Spectroscopy. 2023. hal-04296050 Introduction Heterogeneous Ziegler-Natta (ZN) catalysts [START_REF] Ziegler | Das Mülheimer Normaldruck-Polyäthylen-Verfahren[END_REF][START_REF] Natta | Une Nouvelle Classe de Polymeres d′α-Olefines ayant une Régularité de Structure Exceptionnelle[END_REF][3] are central for the industrial production of polyethylene (PE grades such as HDPE & LLDPE) and polypropylene (PP such as isoPP). [START_REF] Claverie | Natta catalysis: 50 years after the Nobel Prize[END_REF] These catalysts are multicomponent hybrid materials that include organic (ether, alcohol and esters, referred to as internal donors) and inorganic parts (TiCl4/MgCl2), [START_REF] Kashiwa | The discovery and progress of MgCl2-supported TiCl4 catalysts[END_REF] which are activated with alkyl aluminum, e.g. Al(C2H5)3, in the presence or not of additional organic promoters (external donors), to provide polymers with specific properties resulting from the molar masses, the dispersity and the stereoselectivity when applicable (isoPP). [START_REF] Moore | Polypropylene handbook polymerization, characterization, properties, processing, applications[END_REF] Despite intensive studies over the past decades, [START_REF] Piovano | Electronic Properties of Ti Sites in Ziegler-Natta Catalysts[END_REF][START_REF] Magni | Preparation of a model Ziegler-Natta catalyst: electron irradiation induced titanium chloride deposition on magnesium chloride thin films grown on gold[END_REF][START_REF] Mori | Direct observation of MgCl2-supported Ziegler catalysts by high resolution transmission electron microscopy[END_REF][START_REF] Fregonese | MgCl2/TiCl4/AlEt3 catalytic system for olefin polymerisation: a XPS study[END_REF][START_REF] Schmidt | Characterization of a model Ziegler-Natta catalyst for ethylene polymerization[END_REF][START_REF] Kim | Surface science of single-site heterogeneous olefin polymerization catalysts[END_REF][START_REF] Andoni | A Preparation Method for Well-Defined Crystallites of MgCl2-Supported Ziegler-Natta Catalysts and their Observation by AFM and SEM[END_REF][START_REF] Groppo | The potential of spectroscopic methods applied to heterogeneous catalysts for olefin polymerization[END_REF][START_REF] Yakimov | A Formulation Protocol with Pyridine to Enable Dynamic Nuclear Polarization Surface-Enhanced NMR Spectroscopy on Reactive Surface Sites: Case Study with Olefin Polymerization and Metathesis Catalysts[END_REF][START_REF] Yakimov | DNP-SENS Formulation Protocols To Study Surface Sites in Ziegler-Natta Catalyst MgCl2 Supports Modified with Internal Donors[END_REF] establishing structure-activity relationships in ZN catalysts has remained elusive. The situation is particularly challenging because of the low Ti weight loading (2-4 wt. %), and the multistep catalyst preparation, where the state of Ti sites and the structure of the MgCl2 support undergoes changes at every stage. This process ultimately results in a small fraction of catalytically active Ti sites upon activation with alkyl aluminum reagents. [START_REF] Ashuiev | Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance[END_REF] EPR spectroscopy combined with DFT modeling has recently enabled to identify the spectroscopic signature of Ti active sites in activated ZN PE catalysts and to assign them to a specific structural motif, namely a bimetallic Ti(III)/Al alkyl species. [START_REF] Ashuiev | Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance[END_REF] Despite these advances, establishing a link between the Ti site structures before and after activation remains an important question in order to establish the role of MgCl2 on the structure of the surface sites and a detailed structure-activity relationship. While the preparational procedure of MgCl2 is known to be critical to generate highly active catalysts, [START_REF] Albizzati | Components and catalysts for the polymerization of olefins[END_REF] understanding the interaction of Ti with the support surface is complicated because MgCl2 loses crystallinity upon interaction with TiCl4. [START_REF] Wada | Revisiting the identity of δ-MgCl2: Part I. Structural disorder studied by synchrotron X-ray total scattering[END_REF] Thus, structural investigations based on XRD or other techniques that rely on long-range order, are not that informative. While optical spectroscopy (IR, UV-Vis, Raman, etc.) [START_REF] Groppo | The potential of spectroscopic methods applied to heterogeneous catalysts for olefin polymerization[END_REF] , XAS techniques [START_REF] Zarupski | Silica-Magnesium-Titanium Ziegler-Natta Catalysts. Part I: Structure of the Pre-Catalyst at a Molecular Level. Part I: Structure of the Pre-Catalyst at a Molecular Level[END_REF] and various multinuclear NMR ( 1 H, [START_REF] Andoni | A Preparation Method for Well-Defined Crystallites of MgCl2-Supported Ziegler-Natta Catalysts and their Observation by AFM and SEM[END_REF] C, [START_REF] Blaakmeer | The coordinative state of aluminium alkyls in Ziegler-Natta catalysts[END_REF] Mg, and [START_REF] Larsen | QCPMG-MAS NMR of Half-Integer Quadrupolar Nuclei[END_REF] Cl) [START_REF] Blaakmeer | Structural Characterization of Electron Donors in Ziegler-Natta Catalysts[END_REF][START_REF] Blaakmeer | Probing Interactions between Electron Donors and the Support in MgCl2-Supported Ziegler-Natta Catalysts[END_REF][START_REF] Tijssen | Solid-state NMR studies of Ziegler-Natta and metallocene catalysts[END_REF][START_REF] Blaakmeer | Solid-State NMR Investigations of MgCl2 Catalyst Support[END_REF][START_REF] Blaakmeer | The coordinative state of aluminium alkyls in Ziegler-Natta catalysts[END_REF] methods have been successfully used for addressing the structure of Ti sites on MgCl2, the results are very difficult to interpret and lead to rather poor molecular-level understanding. Furthermore, previous studies have also shown that the internal donors such as THF or ethanol, that are known to activate MgCl2, are non-innocent and react with TiCl4 resulting in the formation of surface Ti alkoxide species, [START_REF] Grau | Tetrahydrofuran in TiCl4/THF/MgCl2: a Non-Innocent Ligand for Supported Ziegler-Natta Polymerization Catalysts[END_REF][START_REF] D'anna | Structural Characterization of the EtOH-TiCl4-MgCl2 Ziegler-Natta Precatalyst[END_REF] that may explain the broad range of reactivity of surface sites. In fact, the addition of BCl3 (and other MClx additives) has been proposed to remove surface alkoxy species via transmetallation by forming B-alkoxide species. Note that this treatment also results in a significant removal of Ti sites (see Table S2 for elemental analysis), while increasing the number of active sites when the pre-catalyst is treated with triethylaluminum (TEA). [START_REF] Ashuiev | Spectroscopic Signature and Structure of the Active Sites in Ziegler-Natta Polymerization Catalysts Revealed by Electron Paramagnetic Resonance[END_REF][START_REF] Grau | Tetrahydrofuran in TiCl4/THF/MgCl2: a Non-Innocent Ligand for Supported Ziegler-Natta Polymerization Catalysts[END_REF][START_REF] D'anna | Structural Characterization of the EtOH-TiCl4-MgCl2 Ziegler-Natta Precatalyst[END_REF][START_REF] Humbert | Activity Enhancement of MgCl2-supported Ziegler-Natta Catalysts by Lewis-acid Pretreatment for Ethylene Polymerization[END_REF] Thus, resolving the structure of Ti surface sites formed upon adsorption of TiCl4 on activated MgCl2 is still of prime interest and calls for the development of characterization techniques, that can probe the local environment and electronic structure of the Ti sites. Considering that these sites are Ti(IV) and diamagnetic, 47/49 Ti NMR appears to be a perfect candidate. However, 47/49 Ti NMR spectroscopy suffers from a number of critical limitations, namely i) the low gyromagnetic ratio of both [START_REF] Correa | Key Elements in the Structure and Function Relationship of the MgCl2/TiCl4/Lewis Base Ziegler-Natta Catalytic System[END_REF] Ti and [START_REF] D'amore | Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler-Natta Catalysts[END_REF] Ti nuclei, ii) the low natural abundance of these nuclei (5.41 and 7.44 % correspondingly), iii) the quadrupolar nature of the nuclei (I = 7/2 and 5/2, respectively), iv) the rather large quadrupole moment of both nuclei (24.7 and 30.2 fm 2 , correspondingly), [START_REF] Stone | Table of nuclear electric quadrupole moments[END_REF] and v) close proximity of these isotopes in the NMR spectra (within 267 ppm), often yielding overlapping broad lines. [START_REF] Lucier | Chapter One -Reviewing 47/49 Ti Solid-State NMR Spectroscopy: From Alloys and Simple Compounds to Catalysts and Porous Materials[END_REF] All these factors contribute to the difficulty in obtaining high quality 47/49 Ti NMR spectra, usually associated with a poor signal-to-noise ratio and complex line shapes. The situation is further worsened for ZN catalysts due to the low amount of Ti (often about 2-4 wt. % in range), the expected heterogeneity of surface sites formed upon the adsorption of TiCl4 on specific sites of MgCl2 as well as the presence of Ti with both Cl and OR ligands. [START_REF] Grau | Tetrahydrofuran in TiCl4/THF/MgCl2: a Non-Innocent Ligand for Supported Ziegler-Natta Polymerization Catalysts[END_REF][START_REF] D'anna | Structural Characterization of the EtOH-TiCl4-MgCl2 Ziegler-Natta Precatalyst[END_REF] Pioneering work on 47/49 Ti NMR of Ziegler-Natta pre-catalysts led to observation of a very broad low-intense signal, which is difficult to interpret. [START_REF] Blaakmeer | Preactive Site in Ziegler-Natta Catalysts[END_REF] However, recent hardware developments (higher fields, low temperature) augmented with computational approaches have shown that it is possible to detect the NMR signature of Ti sites in Ti silicalite (TS-1), another industrially relevant materials containing only 1-2 wt% Ti. [START_REF] Lätsch | NMR Signatures and Electronic Structure of Ti Sites in Titanosilicalite-1 from Solid-State 47/49[END_REF] A specific challenge associated with the characterization of the ZN catalyst is MgCl2 itself. Its disordered character (upon adsorption of TiCl4) [START_REF] Wada | Revisiting the identity of δ-MgCl2: Part I. Structural disorder studied by synchrotron X-ray total scattering[END_REF] makes the development of meaningful DFT models particularly challenging, due to the lack of the corresponding crystal structure. Consequently, previously Ti surface models used for DFT computations were based on TiCl4 adsorbed on various facets of crystalline MgCl2. [START_REF] Blaakmeer | Preactive Site in Ziegler-Natta Catalysts[END_REF] However, recent findings related to the characterization of the corresponding V-based ZN pre-catalysts (prepared by adsorption of VOCl3 on similar MgCl2) have shed light on how the morphology of MgCl2 can affect the structure of the V sites and the NMR signatures, thanks to the high sensitivity of [START_REF] Widdifield | Understanding chemical shielding tensors using group theory, MO analysis, and modern densityfunctional theory[END_REF] V NMR combined with computations. [START_REF] Sabisch | Geometry and local environment of surface sites in Vanadium-based Ziegler-Natta catalysts from 51 V solid-state NMR Spectroscopy[END_REF] Namely, adsorption of VOCl3 on MgCl2-(THF)1.5 generates a well-defined V-alkoxide surface species, with a distorted local geometry due to loss of crystallinity of MgCl2. [START_REF] Sabisch | Geometry and local environment of surface sites in Vanadium-based Ziegler-Natta catalysts from 51 V solid-state NMR Spectroscopy[END_REF] Encouraged by the advances in solid-state NMR and computational modeling, along with our improved understanding of MgCl2 and its impact on the local geometry of surface sites, we investigate here the corresponding Ti-based ZN PE pre-catalysts by combining high magnetic field (900 MHz), magic angle spinning (MAS), low temperature (~100 K), higher filling factor (3.2 mm thin-wall rotor, ~40 µL sample volume), and probe optimized for low-gamma nuclei, with DFT computations benchmarked on molecular compounds. This multipronged approach unveils the structure of Ti sites in ZN PE catalysts, prepared from the adsorption of TiCl4 on MgCl2 followed by a BCl3 treatment. [START_REF] Humbert | Activity Enhancement of MgCl2-supported Ziegler-Natta Catalysts by Lewis-acid Pretreatment for Ethylene Polymerization[END_REF]

Results and discussions NMR measurements

We first conducted 47/49 Ti NMR measurements on two molecular complexes in order to assess the effect on NMR signatures of specific Ti local environments, representative of possible surface sites, namely [H2NMe2]2[TiCl6] 34 and [H2NMe2]2[TiCl5(O i Pr)]. For the NMR measurements, the Double Frequency Sweep -Quadrupolar Carr-Parcell-Meiboom-Gill (DFS-QCPMG) methodology was selected in order to gain from DFS (signal enhancement), MAS (narrowing down NMR line) and CPMG (signal enhancement by echo train acquisition). [START_REF] Larsen | QCPMG-MAS NMR of Half-Integer Quadrupolar Nuclei[END_REF][START_REF] Iuga | Population and Coherence Transfer Induced by Double Frequency Sweeps in Half-Integer Quadrupolar Spin Systems[END_REF] While [H2NMe2]2[TiCl6] shows two relatively narrow lines (<1 kHz) corresponding to the [START_REF] Correa | Key Elements in the Structure and Function Relationship of the MgCl2/TiCl4/Lewis Base Ziegler-Natta Catalytic System[END_REF] Ti and the [START_REF] D'amore | Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler-Natta Catalysts[END_REF] Ti isotopes (Fig. 1a), the Ti complex having one alkoxide ligand does not show any NMR signal under similar conditions, likely due to a too large quadrupolar coupling constant associated with a much larger asymmetric environment (vide infra). Fitting the spectrum for [H2NMe2]2[TiCl6] yields a CQ( 49 Ti) = 3.0 MHz, δiso( 49 Ti) = -250 ppm. On the basis of the narrow lines, this sample was used for the calibration of NMR measurement parameters, which was of highest importance for the spectra of ZN pre-catalysts. We next focused on ZN catalysts (TiCl4@MgCl2(THF)1.5) treated with BCl3, in order to remove possible alkoxide ligands and increase our chances to obtain an NMR spectrum with narrower lines (vide infra), a highly active in ethylene polymerization (see ESI Fig. S2). The 47/49 Ti DFS-QCPMG NMR spectrum of BCl3-treated sample shows only a broad feature centred at ca. -276 ppm with a number of sidebands (Fig. 1b), whose peak position and width is consistent with a surface Ti site in a similar Ti coordination environment as in [H2NMe2]2[TiCl6]. The signal can be fitted to a single Ti site (with both [START_REF] Correa | Key Elements in the Structure and Function Relationship of the MgCl2/TiCl4/Lewis Base Ziegler-Natta Catalytic System[END_REF] Ti and [START_REF] D'amore | Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler-Natta Catalysts[END_REF] Ti isotopes; we note that due to larger electric quadrupolar moment [START_REF] D'amore | Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler-Natta Catalysts[END_REF] Ti isotope leads to a very broad line on a level of background) with a δiso( 49 Ti) of -170 ppm and a CQ( 49 Ti) of 9.3 MHz, which indicate that Ti is in a distorted octahedral environment with likely only Cl ligands. Notably, no 47/49 Ti NMR signal could be obtained on the pristine sample (without prior treatment with BCl3) consistent with what we observe for the molecular compound and as expected for Ti sites having large CQ due to the presence of both Cl and OR ligands as revealed by [START_REF] Andoni | A Preparation Method for Well-Defined Crystallites of MgCl2-Supported Ziegler-Natta Catalysts and their Observation by AFM and SEM[END_REF] C MAS NMR (see ESI Fig. S3 for details).

DFT calculations

In this study, we investigated the contributions that affect the NMR signatures -namely the chemical shift anisotropy and CQ -of molecular model systems as well as model surface species. For this purpose, we conducted DFT calculations of the NMR parameters [START_REF] Te Velde | Chemistry with ADF[END_REF] -using B3LYP, [START_REF] Becke | Density-functional thermochemistry. III. The role of exact exchange[END_REF][START_REF] Stephens | Ab Initio Calculation of Vibrational Absorption and Circular Dichroism Spectra Using Density Functional Force Fields[END_REF] QZ4P, [START_REF] Van Lenthe | Optimized Slater-type basis sets for the elements 1-118[END_REF] and the all electron scalar relativistic zeroth-order regular approximation (ZORA) [START_REF] Wolff | Density functional calculations of nuclear magnetic shieldings using the zerothorder regular approximation (ZORA) for relativistic effects: ZORA nuclear magnetic resonance[END_REF] -to explore these contributions. For the purpose of benchmarking the computational protocol, we used a library of molecular compounds with previously reported solution 47/49 Ti NMR isotopic chemical shifts and observed a good agreement between computation and experiment (referenced against benchmark correlation, see ESI for details). [START_REF] Hao | Titanium-47 and -49 nuclear magnetic resonance spectroscopy: Chemical applications[END_REF][START_REF] Berger | 47/49 Ti NMR of some titanium compounds[END_REF][START_REF] Berger | NMR Data of Methyltitanium Trichloride and Related Organometallic Compounds. A Combined Experimental and Theoretical Study of MenXCl4-n (n = 0-4; X = C, Si, Sn, Pb, Ti)[END_REF] Building on this DFT protocol, the NMR calculations based on the crystal structures of the molecular compounds [H2NMe2]2[TiCl6] and [H2NMe2]2[TiCl5(O i Pr)], confirmed a very high sensitivity of the chemical shift and CQ towards substitution between chlorido and alkoxide ligands. Having one alkoxide ligand is enough to decrease the chemical shift from -203 ppm to -573 ppm, but also to induce a significant increase in the CQ from 2.1 to 37.9 MHz when compared to the hexachlorido compound, [H2NMe2]2[TiCl6]. Consequently, the lineshape of the Ti-alkoxy species is expected to be significantly broadened compared to the highly symmetric [H2NMe2]2[TiCl6] species, thus explaining the difficulty to observe and the absence of 47/49 Ti NMR signal for [H2NMe2]2[TiCl5(O i Pr)] and the related supported ZN catalysts prior to treatment with BCl3 under our experimental conditions. [START_REF] Grau | Tetrahydrofuran in TiCl4/THF/MgCl2: a Non-Innocent Ligand for Supported Ziegler-Natta Polymerization Catalysts[END_REF][START_REF] D'anna | Structural Characterization of the EtOH-TiCl4-MgCl2 Ziegler-Natta Precatalyst[END_REF] Next, the [START_REF] Correa | Key Elements in the Structure and Function Relationship of the MgCl2/TiCl4/Lewis Base Ziegler-Natta Catalytic System[END_REF] Ti NMR signatures of model ZN surface sites were investigated as shown in Table 1, inspecting the effect of directly bound ligands (Cl vs. OR) as well as the MgCl2 morphology, focusing on the difference between the representative crystalline (110)-facet (TiCl4-x(OR)x@MgCl2-110) vs. an disordered environment around Ti/Mg modeled by a nanoribbon structure (TiCl4-x(OR)x @MgCl2-NR). [START_REF] Sabisch | Geometry and local environment of surface sites in Vanadium-based Ziegler-Natta catalysts from 51 V solid-state NMR Spectroscopy[END_REF] The calculated [START_REF] Correa | Key Elements in the Structure and Function Relationship of the MgCl2/TiCl4/Lewis Base Ziegler-Natta Catalytic System[END_REF] Ti NMR signatures for the models built from the commonly employed crystalline MgCl2 (110)-facet surface [START_REF] Blaakmeer | Structural Characterization of Electron Donors in Ziegler-Natta Catalysts[END_REF][START_REF] Blaakmeer | Probing Interactions between Electron Donors and the Support in MgCl2-Supported Ziegler-Natta Catalysts[END_REF][START_REF] Correa | Theoretical Investigation of Active Sites at the Corners of MgCl2 Crystallites in Supported Ziegler-Natta Catalysts[END_REF][START_REF] Breuza | MgCl2-Supported Ziegler-Natta Catalysts: a DFT-D "Flexible-Cluster" Approach to Internal Donor Adducts[END_REF][START_REF] D'amore | Surface Investigation and Morphological Analysis of Structurally Disordered MgCl2 and MgCl2/TiCl4 Ziegler-Natta Catalysts[END_REF][START_REF] Seth | A Density Functional Embedded Cluster Study of Proposed Active Sites in Heterogeneous Ziegler-Natta Catalysts[END_REF][START_REF] Correa | Key Elements in the Structure and Function Relationship of the MgCl2/TiCl4/Lewis Base Ziegler-Natta Catalytic System[END_REF] show a similar trend as the molecular model systems discussed above: the chemical shift decreases upon introduction of terminal alkoxide ligands from -64 ppm for the fully chlorinated titanium (TiCl4@MgCl2-110)

to -489 and -763 ppm for the mono-and bis-alkoxide systems, respectively. This is accompanied by a significant increase in CQ from 15.3 MHz for the perchlorinated species to 27.5 MHz and 30.4 MHz for the mono-or bis-alkoxide model surface sites. Although, the trend of the isotropic chemical shift upon introduction of alkoxide ligands (300-500 ppm per alkoxide ligand) is consistent with what was observed for the Ti molecular systems and the corresponding V-based ZN pre-catalysts, [START_REF] Sabisch | Geometry and local environment of surface sites in Vanadium-based Ziegler-Natta catalysts from 51 V solid-state NMR Spectroscopy[END_REF] the isotropic chemical shift of all model systems based on the (110)-facet significantly diverged from the experimentally observed value (δiso, exp = -170 ppm), pointing out that this (crystalline) model does not capture the electronic structure of Ti sufficiently. Furthermore, these model systems do not reproduce the rather low CQ value of the measured Ti species observed for the ZN pre-catalyst (CQ, exp = 9.3 MHz) well. The specific difference between experimental and computational values, in particular in terms of CQ, indicates that Ti has to be in a more symmetrical environment than predicted from crystalline MgCl2 models, paralleling what has been recently proposed for the corresponding V-based Ziegler-Natta pre-catalysts. In the latter case, the NMR response is in better agreement when MgCl2 is modeled by a disordered model (nanoribbon) rather than crystalline (110)-facet models. It is noteworthy that the metal sites adopt a more symmetric environment when adsorbed on nanoribbon, whereby the fully chlorinated Ti site (TiCl4@MgCl2-NR) is associated with a calculated chemical shift and CQ value of -122 ppm and 11.7 MHz, respectively. Such calculated values show similar trends upon substituting Cl by OR ligands, namely increasing shielding and CQ values, and match within the level of uncertainty the experimental values (-170 ppm and 9.3 MHz). Overall, these results indicate that the ZN pre-catalysts treated with BCl3 contain a well-defined Ti surface site, best described as being fully chlorinated and having a more symmetric environment inherited from being absorbed on disordered MgCl2 (see Fig. 1 and2). This data also indicates that it is currently only possible to observe 47/49 Ti NMR signatures when Ti is solely decorated by Cl ligands as in the BCl3-treated ZN pre-catalyst, because of the associated lower CQ and thereby narrow lines. On the contrary, the untreated ZN pre-catalysts, that has been shown to contain Ti-alkoxy species by [START_REF] Andoni | A Preparation Method for Well-Defined Crystallites of MgCl2-Supported Ziegler-Natta Catalysts and their Observation by AFM and SEM[END_REF] C NMR, is expected to contain Ti sites with (extremely) large CQ leading to significant second-order quadrupolar line broadening, which would be currently impossible to observe with the state-of-the-art equipment even at prolonged measurement times. [START_REF] Blaakmeer | Structural Characterization of Electron Donors in Ziegler-Natta Catalysts[END_REF] We next examined the orientation of the chemical shielding tensor (CST) and carried out a Natural Chemical Shielding (NCS) 50- 53 analysis of the surface Ti sites in TiCl4-x(OR)x@MgCl2-NR with the goal to rationalize the chemical shift trends observed upon substitution of Cl for OR ligands (detailed analysis of TiCl4-x(OR)x@MgCl2-110-facet in ESI). Notably, the CST orientation strongly depends on the symmetry of the system and the number of terminal Cl/OR ligands. For the fully chlorinated system, TiCl4@MgCl2-NR, the most deshielded component (σ11/δ11) is oriented perpendicular to the plane containing Ti and the two terminal cis-Cl ligands (µ1-Cl), associated with short Ti-Cl bonds (Fig. 3a), with σ22 bisecting the µ1-Cl-Ti-µ1-Cl angle, and σ33 being perpendicular to the other two components. With one terminal alkoxide ligand, as in TiCl3OR@MgCl2-NR, σ11 is oriented along the Ti-OR bond, while σ22 is oriented perpendicular to the plane containing Ti and the terminal Cl and OR ligands. When both terminal ligands are alkoxides (TiCl2(OR)2@MgCl2-NR), the CST adopts a similar orientation as in the fully chlorinated system. Noteworthy, the chemical shift across the series of model systems is significantly influenced by the CST component perpendicular to the plane containing Ti and the two terminal µ1-X (Cl/OR) ligands. The subsequent NCS analysis shows that the chemical shift is mostly driven by the paramagnetic part of the chemical shifts and thereby the nature and the relative energies of high-lying occupied and low-lying unoccupied molecular orbitals, as evidenced by the Ramsey equation (Fig. 3d).

The NCS analysis reveals that the chemical shift originates predominantly from the coupling between the σ(Ti-µ1-X) and π*(Ti-µ1-X) orbitals, as well as to a smaller extent the corresponding π(Ti-µ1-X) and σ*(Ti-µ1-X) orbitals (see ESI). The introduction of more electronegative terminal alkoxide ligands results in a larger energy gap between the coupled σ and π* orbitals and consequently to a smaller deshielding as expected from the Ramsey equation 54 (see Fig. 3d).

Conclusion

Overall, we have established a protocol to measure and calculate the 47/49 Ti NMR spectroscopic signatures for Ti surface sites, focusing here on an industrially relevant ZN pre-catalysts based on TiCl4 adsorbed on MgCl2. We have found that the 47/49 Ti NMR signatures are highly sensitive to the presence of alkoxides bound to Ti and to the local coordination environment imposed by the morphology of MgCl2 support. Notably, employing DFS-QCPMG MAS NMR at 21.1 T and 100 K a single 47/49 Ti NMR signature with a chemical shift of -170 ppm and a low CQ of 9.3 MHz is observed in the ZN pre-catalyst treated with BCl3, that indicates the presence of a well-defined species with a rather symmetric environment, best described as a hexacoordinated Ti species adsorbed on a disordered MgCl2 with only chlorido ligands. We propose that the formation of these well-defined Ti sites results from the minimization of the charge distribution around Ti, only possible upon distortion of the Ti sites on the highly flexible ionic disordered MgCl2 support (model here using the nanoribbon-like structure).

Observing the structure of this surface species indicates the role of BCl3 and the support, and its overall well-defined structure is particularly noteworthy considering that ZN catalysts are multisite polymerization catalysts, possibly pointing that the multi-sites activity in these systems could be related to the activation steps with alkyl aluminium and transfer reactions rather than the complexity of the adsorbed Ti species on MgCl2. We are currently exploring the matter in more details.

FIGURE 1 .

 1 FIGURE 1. 47/49 Ti DFS-QCPMG MAS NMR spectra (10 kHz MAS, 21.1 T, 100 K) for (a) [H2NMe2]2[TiCl6] (512 scans) and (b) ZN precatalyst treated with BCl3 (370'496 scans) as well as modeled line shape (solid red line) for δiso = -170 ppm and CQ = 9.3 MHz. Magnitude correction was used for the representation.

FIGURE 2 .

 2 FIGURE 2. Two-dimensional map of calculated 49 Ti NMR signatures for the nanoribbon (green points) and (110)-facet model systems (violet points). Experimental value of BCl3-treated ZN pre-catalysts (red point). The deviations from the computed values were estimated from the benchmark sets for the chemical shift (ca. ±50 ppm) and CQ (ca. ±2 MHz).

FIGURE 3 .

 3 FIGURE 3. (a) Coordination environment of the Ti species with three distinct Cl ligands for TiCl4@MgCl2-NR (terminal: violet, cis to both terminal ligands: turquois, trans to one terminal ligand: red). Chemical shielding tensor orientation in the TiCl4-x(OR)x@MgCl2-NR model systems (σ11 in red, σ22 in green and σ33 in blue).[START_REF] Zurek | Density Functional Study of the 13C NMR Chemical Shifts in Single-Walled Carbon Nanotubes with Stone-Wales Defects[END_REF][START_REF] Autschbach | Analysis of electric field gradient tensors at quadrupolar nuclei in common structural motifs[END_REF] (b) Rationalisation of the chemical shielding through the visualisation of the corresponding σ -π* Kohn-Sham orbitals obtained from the DFT calculations of the fully chlorinated (TiCl4@MgCl2-NR) surface model, which were selected by reconstruction of the Natural Localized Molecular Orbitals (NLMO) orbitals that contribute the most to the deshielding (see ESI). (c) The influence on the MO scheme upon exchanging the terminal ligands rationalizes the large deshielding for the fully chlorinated species compared to alkoxide species. (d) The Ramsey equation describes the chemical shielding arising from the magnetic coupling of occupied (ψocc) and vacant (ψvac) orbitals with an associated energy difference ΔEvac-occ between these orbitals.[START_REF] Ramsey | Magnetic Shielding of Nuclei in Molecules[END_REF]

https://doi.org/10.26434/chemrxiv-2023-2hmzj-v2 ORCID: https://orcid.org/0000-0002-8624-1002 Content not peer-reviewed by ChemRxiv. License: CC BY-NC-ND 4.0

ACKNOWLEDGMENT

The authors thank Lyndon Emsley and Laura Piveteau for granting access to the 900 MHz LT NMR spectrometer at EPFL. The Euler supercomputer cluster is gratefully acknowledged for providing computational time. We are grateful to Dr. Matthieu Humbert and Dr. David Gajan (CRMN lab, UMR 5082) for fruitful discussions regarding 13 C CP-MAS spectroscopy.

Funding Sources

C.J. Kaul thanks the Swiss National Science Foundation (SNF IZLCZ0_206049) for financial support. Y. K. is grateful to the Swiss National Foundation (SNF) for financial support of this work (grant no. 200020B_192050). This project was partially funded through the ETH+ Project SynthMatLab. F. M. B. thanks the University of Lyon for funding his PhD.

ASSOCIATED CONTENT

Supporting Information. Synthesis protocols and corresponding solution NMR, single-crystal XRD structures, solid-state NMR measurement parameters and additional spectra, DFT calculation protocol for NMR parameters.

AUTHOR INFORMATION Corresponding Author

*Alexander Yakimov yakimov@inorg.chem.ethz.ch *Christophe Copéret ccoperet@ethz.ch ‡These authors contributed equally.


Sébastien Leroy

Jean-François Blach

Agnieszka Kopia

Sebastian Lech

Łukasz Cieniek

Nicolas Kania

Sébastien Saitzek
email: sebastien.saitzek@univ-artois.fr

Pr Sébastien Saitzek

Enhancement of the photocatalytic properties of nanosized La2Ti2O7 synthesized by Glycine-assisted sol-gel route

Nanosized lanthanum titanate (La2Ti2O7) have been synthetized by a new glycineassisted sol gel route in order to improve the photocatalytic properties. The effect of calcination temperature on crystallites size and their photocatalytic efficiency has been investigated, showing a best efficiency for a calcination temperature of 1000°C. Thus, the apparent rate constant of degradation of rhodamine B is greatly improved by a factor of 1.86 compared to commercial P25 TiO2 Degussa® and by a factor of 6.42 compared to a sol-gel synthesis without glycine.

In order to study the efficiency of the photocatalyst on industrial water discharges, the photodegradation of phenol has also been investigated, showing the effectiveness of the photocatalyst with an apparent rate constant of 5.49x10 -3 min -1 . The photocatalyst mass loading, the luminous flux received and the pH have been also investigated. The effect of different type of scavengers shows the importance of superoxide and hole in the photodegradation mechanisms. Finally, the coupling of ultrasound and light excitation was carried out in order to achieve a sonophotocatalysis effect. The apparent rate constant corresponding to the photodegradation of phenol increases slightly. Nevertheless, we have been able to observe that this is more important as the volume of the photoreactor is low.

Introduction

For a few decades, the development of new photocatalysts has been significant interest in the scientific community for their many potential applications, including: i) the photodegradation of organic pollutants such as dyes [START_REF] Arabatzis | Silver-modified titanium dioxide thin films for efficient photodegradation of methyl orange[END_REF][START_REF] Height | Ag-ZnO catalysts for UVphotodegradation of methylene blue[END_REF][START_REF] Wilhelm | Photodegradation of rhodamine B in aqueous solution via SiO2@TiO2 nanospheres[END_REF][START_REF] Hui Sun | Photodegradation of azo dye Congo Red from aqueous solution by the WO3-TiO2/activated carbon (AC) photocatalyst under the UV irradiation[END_REF], pharmaceutical wastes [START_REF] Awfa | Photodegradation of pharmaceuticals and personal care products in water treatment using carbonaceous-TiO2 composites: A critical review of recent literature[END_REF][START_REF] Thiruppathi | CuWO4 Nanoparticles: Investigation of Dielectric, Electrochemical Behaviour and Photodegradation of Pharmaceutical Waste[END_REF], industrial wastewater treatment [START_REF] Stepnowski | Enhanced photo-degradation of contaminants in petroleum refinery wastewater[END_REF], mineralization of hazardous organic product [START_REF] Schmelling | Photocatalytic transformation and mineralization of 2,4,6-trinitrotoluene (TNT) in TiO2 slurries[END_REF], agricultural wastewater treatment (pesticides) [START_REF] Burrows | Reaction pathways and mechanisms of photodegradation of pesticides[END_REF][START_REF] Bustos | Sunlight and UVC-254 irradiation induced photodegradation of organophosphorus pesticide dichlorvos in aqueous matrices[END_REF], polymer fibers degradation [START_REF] Yousif | Photodegradation and photostabilization of polymers, especially polystyrene[END_REF], ii) the hydrogen production by water-splitting [START_REF] Yeh | Graphite oxide as a photocatalyst for hydrogen production from water[END_REF][START_REF] Wang | A metalfree polymeric photocatalyst for hydrogen production from water under visible light[END_REF], iii) the photocatalytic redox reactions, for example the reduction of Cr(VI) to Cr(III) [START_REF] Yang | La2Ti2O7: An efficient and stable photocatalyst for the photoreduction of Cr(VI) ions in water[END_REF] or for the direct formation of metallic nanoparticles by photoreduction [START_REF] Alammar | Facile preparation of Ag/ZnO nanoparticles via photoreduction[END_REF] and iv) the development of anti-bacterial coating based on the production of Reactive Oxygen Species (ROS) at the photocatalyst surface [START_REF] Yemmireddy | Using Photocatalyst Metal Oxides as Antimicrobial Surface Coatings to Ensure Food Safety-Opportunities and Challenges[END_REF]. Among the photocatalyst oxides, TiO2 is widely used and studied [START_REF] Emilio | Phenol photodegradation on platinized-TiO2 photocatalysts related to charge-carrier dynamics[END_REF][START_REF] Sawunyama | Photocatalysis on TiO2 surfaces investigated by atomic force microscopy: photodegradation of partial and full monolayers of stearic acid on TiO2(110)[END_REF][START_REF] Hsien | Photodegradation of aromatic pollutants in water over TiO2 supported on molecular sieves[END_REF] due to its good chemical stability, nontoxicity, abundance of raw materials and strong oxidizing ability [START_REF] Schneider | -Understanding TiO2 Photocatalysis Mechanisms and Materials(2)[END_REF]. However, the growing interest has led researchers to explore new photocatalytic materials such as semiconductors ZnO [START_REF] Height | Ag-ZnO catalysts for UVphotodegradation of methylene blue[END_REF], SnO2 [START_REF] Bhattacharjee | A novel approach for the synthesis of SnO2 nanoparticles and its application as a catalyst in the reduction and photodegradation of organic compounds[END_REF], WO3 [START_REF] Liang | Preparation of orthorhombic WO3 thin films and their crystal quality-dependent dye photodegradation ability[END_REF], ZnS [START_REF] El-Kemary | Fluorescence modulation and photodegradation characteristics of safranin O dye in the presence of ZnS nanoparticles[END_REF], BiVO4 [START_REF] Malathi | A review on BiVO4 photocatalyst: Activity enhancement methods for solar photocatalytic applications[END_REF], BiPO4 [START_REF] Pan | Influence of OH-related defects on the performances of BiPO4 photocatalyst for the degradation of rhodamine B[END_REF], and La2Ti2O7 [START_REF] Wang | Sol-gel synthesis of La2Ti2O7 modified with PEG4000 for the enhanced photocatalytic activity[END_REF][START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF] (LTO) in order to probe their effectiveness under UV light. Currently, these studies are being continued to allow use under visible irradiation, for example by i) band gap engineering via crystallites shape control [START_REF] Smith | Semiconductor nanocrystals: Structure, properties, and band gap engineering[END_REF], stress/strain effect [START_REF] Yin | Effective band gap narrowing of anatase TiO2 by strain along a soft crystal direction[END_REF] or even by the development of a compound with a mixed anion such as oxynitrides [START_REF] Ahmed | A review of metal oxynitrides for photocatalysis[END_REF], oxyfluorides [START_REF] Kuriki | A Stable, Narrow-Gap Oxyfluoride Photocatalyst for Visible-Light Hydrogen Evolution and Carbon Dioxide Reduction[END_REF], oxyhydride,… ; ii) the effect of Localized Surface Plasmon Resonance (LSPR) via metallic nanoparticles as Au or Ag [START_REF] Zhu | Au/La2Ti2O7Nanostructures Sensitized with Black Phosphorus for Plasmon-Enhanced Photocatalytic Hydrogen Production in Visible and Near-Infrared Light[END_REF]; iii) The combination of light-absorbing materials such as CdS/La2Ti2O7 nanocomposites [START_REF] Soltani | Enhancement of visible light photocatalytic activity of ZnS and CdS nanoparticles based on organic and inorganic coating[END_REF] or g-C3N4/La2Ti2O7 [START_REF] Zhou | Synthesis of visible-light-driven g-C3N4/La2Ti2O7 heterojunction photocatalysts for improved photocatalytic performance[END_REF] and iv) the creation of all oxide n/p heterojunction such as BiOI/La2Ti2O7 [START_REF] Ao | Fabrication of novel p-n heterojunction BiOI/La2Ti2O7 composite photocatalysts for enhanced photocatalytic performance under visible light irradiation[END_REF] or SnS2/La2Ti2O7 [START_REF] Chen | New SnS2/La2Ti2O7 heterojunction photocatalyst with enhanced visiblelight activity[END_REF]. The ability of La2Ti2O7 to act as a photocatalyst has been highlighted since the early 2000s but this oxide has been known since the 1970s for these piezo-electric properties [START_REF] Kimura | Electrooptic and Piezoelectric Properties of La2Ti2O7 Single Crystal[END_REF]. Lanthanum dititanate oxide belongs to the layered perovskite structure with a general formula AnBnO3n+2 with (A=La et B=Ti). The La2Ti2O7, so-called LTO is characterised by a stack of 4 layers of [TiO6] octahedron along the c axis and infinite layers of octahedra belong a and b axis. In 1975, Gasperin et al. [START_REF] Gasperin | Dititanate de lanthane[END_REF] resolve the structure of La2Ti2O7, highlighting a monoclinic structure with a P21 space group whose lattice parameters are: a= 7.800(3) Å ; b= 5.546(2) Å ; c= 13.011(4) Å et β= 98. [START_REF] Zhang | Photocatalytic degradation of azophloxine on porous La2Ti2O7 prepared by sol-gel method[END_REF](2)°. At 780°C, this monoclinic phase evolves to an orthorhombic structure (CmC21 space group with following lattice parameters : a = 3.954(2) Å ; b = 5.607(2) Å et c = 25.952(8) Å [START_REF] Ishizawa | Compounds with perovskite-type slabs. V. A high-temperature modification of La2Ti2O7[END_REF]) while retaining ferroelectric properties.

These are then lost when the temperature becomes higher than Tc = 1482°C [START_REF] Yan | Piezoelectric ceramics with super-high curie points[END_REF] and the structure evolves again towards an orthorhombic structure with Cmcm space group (paraelectric phase). LTO is a ferro-/piezo-electric material with a high Curie temperature [START_REF] Nanamatsu | A new ferroelectric: La2Ti2O7[END_REF], allowing use in extreme environments as piezoelectric actuators for aeronautic or spatial applications [42].

Beyond their electronic applications, LTO can also be used as a host matrix for the development of luminescent materials. Indeed, a substitution of the site La, by another element whose near ionic radius is possible without structure modification. So, LTO acts as an excellent host matrix, especially for the substitution of lanthanides ions such as Pr 3+ [START_REF] Diallo | nti-Stokes luminescence and site selectivity in La2Ti2O7:Pr 3+[END_REF][START_REF] Zhang | Color tuning and white light emission in a single-phased La2Ti2O7:Pr 3+ , Tb 3+ phosphor[END_REF], Eu 3+ [START_REF] Park | Rapid visualization of latent fingerprints with Eu-doped La2Ti2O7[END_REF][START_REF] Szczepanski | Luminescence and upconversion properties in La2Ti2O7:Eu 3+ ,Er 3+ oxides under UV and NIR radiations towards a two-color sensor[END_REF], Sm 3+ [START_REF] Shao | Structural Characterization and Photoluminescent Properties of (La1-xSmx)2Ti2O7 Solid Solutions Synthesized by a Sol-Gel Route[END_REF][START_REF] Lakshminarayana | Photoluminescence of Sm 3+ , Dy 3+ , and Tm 3+doped transparent glass ceramics containing CaF2 nanocrystals[END_REF],… . In addition, a substitution of La2Ti2O7 with Er 3+ has made it possible to obtain an up conversion luminescence properties which may be of interest for improving the efficiency of solar cells [START_REF] Van Sark | Upconversion in solar cells[END_REF][START_REF] Schulze | Photochemical Upconversion Enhanced Solar Cells: Effect of a Back Reflector[END_REF] for phototherapy applications [START_REF] Sun | Hierarchical Plasmonic Nanorods and Upconversion Core-Satellite Nanoassemblies for Multimodal Imaging-Guided Combination Phototherapy[END_REF]. More recently, co-substitution also makes it possible to couple the two properties of classical luminescence and up-conversion to produce sensors sensitive to a double excitation [START_REF] Meenambal | Cosubstitution of Lanthanides (Gd 3+ /Dy 3+ /Yb 3+ ) in β-Ca3(PO4)2 for Upconversion Luminescence, CT/MRI Multimodal Imaging[END_REF].

In addition, La2Ti2O7 oxide has been studied for its activity in water-splitting either alone [START_REF] Kim | Effect of Precursors on the Morphology and the Photocatalytic Water-Splitting Activity of Layered Perovskite[END_REF] or co-catalyzed with metals [START_REF] Wang | Physicochemical properties and photocatalytic H2 evolution activity of Rh-doped La2Ti2O7 prepared by molten salt synthesis[END_REF], [START_REF] Hu | Visible light driven (Fe, Cr)-codoped La2Ti2O7 photocatalyst for efficient photocatalytic hydrogen production[END_REF], for photocatalytic degradation of organic pollutants CH3Cl [START_REF] Hwang | Photocatalytic Degradation of CH3Cl over a Nickel-Loaded Layered Perovskite[END_REF], CH3OH [START_REF] Hwang | Photocatalytic decomposition of watermethanol solution over metal-doped layered perovskites under visible light irradiation[END_REF] and dyestuff [START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF], [START_REF] Hou | Synthesis and characterization of La2Ti2O7 employed for photocatalytic degradation of reactive red 22 dyestuff in aqueous solution[END_REF], [START_REF] Zhang | Effects of calcination temperature on characterization and photocatalytic activity of La2Ti2O7 supported on HZSM-5 zeolite[END_REF], [START_REF] Zhang | Photocatalytic degradation of azophloxine on porous La2Ti2O7 prepared by sol-gel method[END_REF], for photodegradation of antibiotics molecules such as ofloxacin [START_REF] Wang | Role of PEG2000 on sol-gel preparation of porous La2Ti2O7 for enhanced photocatalytic activity on ofloxacin degradation[END_REF] and the oxidation of isopropanol [START_REF] Ku | Photocatalytic Oxidation of Isopropanol in Aqueous Solution Using Perovskite-Structured La2Ti2O7[END_REF].

In a previous work [START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF], we used La2Ti2O7 synthesized by Sol-Gel method and investigated its photocatalytic properties on the Rhodamine B photodegradation followed by the study of the influence the influence of various parameters such as temperature, catalyst loading, pH. We have demonstrated an efficient use of this oxide for photocatalysis processes under UV irradiation.

However, the photocatalytic efficiency is strongly affected by the degree of crystallinity and the exchange surface of the material. In recent years, the optimization of synthetic parameters in order to control the size of crystallites and their crystallinity have been the subject of much researches by the scientific community, in particular via: i) the nature of precursors employed [START_REF] Salavati-Niasari | Synthesis of nickel and nickel oxide nanoparticles via heattreatment of simple octanoate precursor[END_REF], ii) the use of polymer precursors for porosity control [START_REF] Wang | Role of PEG2000 on sol-gel preparation of porous La2Ti2O7 for enhanced photocatalytic activity on ofloxacin degradation[END_REF] ; ii) the diversification of synthesis processes such as the hydrothermal route [START_REF] Ahmadian-Fard-Fini | Hydrothermal green synthesis of magnetic Fe3O4-carbon dots by lemon and grape fruit extracts and as a photoluminescence sensor for detecting of E. coli bacteria[END_REF], the polymeric solgel route [START_REF] Li | Hydrothermal synthesis and photocatalytic properties of layered La2Ti2O7 nanosheets[END_REF], [START_REF] Kim | Photocatalytic Water Splitting over La2Ti2O7 Synthesized by the Polymerizable Complex Method[END_REF], microwave assisted synthesis [START_REF] Zhou | Preparation and Characterization of Nanocrystalline La2Ti2O7 by Microwave Assisted Process[END_REF], Molten Salt Synthesis (MSS) [START_REF] Huang | Onestep synthesis of dandelion-like lanthanum titanate nanostructures for enhanced photocatalytic performance[END_REF], spray pyrolysis and others ; iii) the use of template [START_REF] Xie | Review of Research on Template Methods in Preparation of Nanomaterials[END_REF] or iv) the design of microreactors at the nanometric scale [START_REF] Salavati-Niasari | Nanoscale microreactor-encapsulation 14-membered nickel(II) hexamethyl tetraaza: synthesis, characterization and catalytic activity[END_REF].

In this present work, we have sought to improve the photocatalytic activity by developing a new synthetic route limiting the coalescence of grains in the calcination step. For that, La2Ti2O7 were synthesized by a new route named Sol-Gel method assisted by glycine combustion. Firstly, we will demonstrate the improvement of the photocatalytic properties with respect to the sol-gel synthesis already described. Secondly, for further understanding of photodegradation mechanism performed on Rhodamine B and phenol pollutants, we used scavenger species and studied photodegradation kinetics by total organic carbon measurements.

The photocatalytic properties were studied as a function of catalyst loading, pH and luminous flux. Finally, a study in sonophotocatalysis was made by highlighting the influence of volume and the increase of catalytic efficiency.

Experimental section

Nanosized La2Ti2O7 oxide was synthesized by Sol-Gel method assisted by glycine combustion. Lanthanum (II) nitrate hexahydrate (Strem Chemicals, 99,99%), glycine (Acros Organics 99%) and Titanium (IV) n-butoxide (Alfa Aesar 98 + %) were used as starting precursors. First, a solution (A) was prepared according to the following protocol: 0.15 mol of lanthanum nitrate initially ground manually in an agate mortar is introduced into a mixture containing 10 mL of water and 10 mL of acetic acid. After 15 min of stirring, 0.30 mol of glycine is introduced into the reaction vessel of A solution and then homogenized for 15 min by stirring. At the same time, a second solution (B) was prepared by dissolving 0.15 mol of titanium precursor in 10 mL of absolute ethanol. The whole is homogenized by stirring for 15 min. Finally, A and B solutions were mixed and stirred for 30 min until a translucent gel was obtained. The solution was then placed in oven at 80°C until a viscous white gel was formed. This gel is then calcined in air at different temperatures for 10h to obtain La2Ti2O7 nanopowders.

X-ray diffraction (XRD) patterns were collected using a Rigaku ULTIMA IV diffractometer equipped with Cu anticathode, Soller slits to limit the divergence of X-ray beam and a nickel foil filter attenuates the Cu Kβ line. The registered angular range is 20°-80° (scanning rate of 0.1°. min -1 ) using the Bragg-Brentano configuration. In order to estimate the average crystallite size, we used Scherrer's relation [START_REF] Cenovar | Preparation of Nano-Crystalline TiO2 by Sol-Gel Method using Titanium Tetraisopropoxide (TTIP) as a Precursor[END_REF][START_REF] Ying | A rigorous comparison of X-ray diffraction thickness measurement techniques using silicon-on-insulator thin films[END_REF]:

D = (1)
Where λ is the X-ray wavelength (λ𝐾 (Cu) = 1.5418 Å), FWHMs corresponds to the broadening only induced by the size effect, which is deconvolved from the instrumental part, K is a dimensionless shape factor (with a typical value of 0.89) and θ is Bragg's angle corresponding to (hkl) reflection.

Transmission electron microscopy (TEM) examinations were performed using Tecnai G2-20 TWIN FEI microscope, equipped with a LaB6 cathode and operating at the accelerating voltage of 200 kV.

The surface morphology and the chemical composition of the powder was analysed using Scanning Electron Microscope (FEI Nova NanoSEM 450) equipped with Energy Dispersive Spectroscopy detector Octane Elect Plus (EDAX) working with GENESIS software.

Optical band gaps were calculated from the absorbance spectra recorded on UV-2600 spectrophotometer with integrating sphere (Shimadzu). For that, the Tauc plot based on following equation has been used [START_REF] Tauc | Optical properties and electronic structure of amorphous germanium[END_REF][START_REF] Murphy | Band-gap determination from diffuse reflectance measurements of semiconductor films, and application to photoelectrochemical water-splitting[END_REF]:

αhν = A (hν -Eg) (2)
Where α is the absorption coefficient, hν corresponds to photon energy, A is a proportionality constant, Eg is the optical band gap, and the exponent n is a factor linked to the nature of the transition (n = ½ for direct transition).

Porosity measurements were determined by nitrogen adsorption at -196 °C on a Tristar II apparatus from Micromeritics, after having degassed the sample under vacuum at 120°C. The specific surface areas (SBET) were calculated from the Brunauer-Emmett-Teller (BET) equation using P/P0 values in the 1.0 × 10 -2 and 2.5 × 10 -1 range and the pore size distributions were obtained from the desorption branch using the Barrett-Joyner-Halenda (BJH) method.

Each specific surface area measurement was performed on three consecutive batches of the same sample. The SBET values uncertainty is estimated to be less than 5%. Total Organic Carbon (TOC) analyses were carried out on a TOC-L total organic carbon analyzer (Shimadzu). The values presented are the averages obtained on 3 consecutive measurements.

The photocatalytic activity was determined by monitoring the degradation of Rhodamine B and phenol on UV-visible spectra using a usual Beer-Lambert law. The used photo-reactor consist of a 200 mL flask irradiated from above by a 40 W UV lamp (254 nm)

and an aluminium foil covers the system to prevent UV radiations. Before any measurements, the solution was stirred for 30 min in the dark in presence of catalyst to ensure an appropriate adsorption/desorption equilibrium. To describe the photocatalytic kinetic at solid-liquid interface the Langmuir-Hinshelwood (LH) model was used [START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF], [START_REF] Rahimi-Nasrabadi | Photocatalytically active La2Ti2O7 nanostructures, synthesis and characterization[END_REF]. In such model, it is commonly admitted that the reaction products and intermediates are negligible and that the rate of reaction depends directly to the coverage surface of the catalytic site (θ). This model also allows to consider the relative adsorption on the surface oxide and the photodegradation rate v can be expressed as follow:

𝑣 = 𝑘 𝜃 𝑤𝑖𝑡ℎ 𝜃 = ( 3
)
Where kph is the kinetic constant of photodegradation, KLH the adsorption constant which depends on the affinity between dye and catalyst and C the initial concentration of dye. For low adsorption and/or low concentration (i.e. KLHC<<1) the photodegradation rate become directly proportional to the dye concentration and can be expressed as follow:

𝑣 = 𝑘 𝐾 𝐶 = 𝑘 𝐶 ( 4
)
Where kapp is the apparent rate constant of the first order reaction and can be determine by the slope of the straight line when plotting Ln(C0/C) vs t.

Results and discussions

Structural characterizations

XRD patterns of obtained La2Ti2O7 are shown in Figure 1.a. where we can observe that a 900 °C temperature is required to obtained a pure LTO phase. Under this temperature, we observe reflections peaks of the LaTiO3 phase. The presence of this phase can be explained by the combustion of glycine, which induces a significant production of reducing gas, promoting the formation of Ti 3+ . All observed X-ray reflections were indexed with La2Ti2O7 layered-perovskite structure with P21 space group [START_REF] Gasperin | Dititanate de lanthane[END_REF]. There is an evolution of the Full Width at Half Maximum (FWHM) that become narrow as the temperature increase, which is due to the coalescence of grains and the average crystallite size is calculated using Scherrer's relation [START_REF] Cenovar | Preparation of Nano-Crystalline TiO2 by Sol-Gel Method using Titanium Tetraisopropoxide (TTIP) as a Precursor[END_REF][START_REF] Ying | A rigorous comparison of X-ray diffraction thickness measurement techniques using silicon-on-insulator thin films[END_REF] (Table 1).

We also know that La2Ti2O7 is a n type semiconductor with a direct allowed transition, so we characterise the band gap of synthesized materials with UV-vis diffuse reflectance and using Tauc equation described in experimental part (Table 1). The optical band gap for LTO-AG synthesized between 800 and 1 100°C are very close around 4 eV (Figure 1.b). The shallower slope observed for LTO-AG-800 probably indicates the level of defect states (Urbach tail induced by a disorder) [START_REF] Akshay | Visible range optical absorption, Urbach energy estimation and paramagnetic response in Cr-doped TiO2 nanocrystals derived by a sol-gel method[END_REF] greater in the case of a lower annealing temperature.

The specific surface area (SBET) was calculated from adsorption isotherm by BET method and significantly decrease when the calcination temperature is lowered which confirm the coalescence of LTO grains and will have an antagonist effect impact on the photocatalytic efficiency. The average pore size was determined by BJH method and show an increase when temperature decrease due to coalescence of grain. Finally, we observe that the total pore volume (determined from the amount of nitrogen adsorbed at a relative pressure (P/P0) of 0.95 in order not to consider the inter-particles porosity) decrease with temperature highlighting a decrease in the number of pores. All those results are summarised in Table 1. In this one, an increase of crystallites size can be observed with temperature, when this size increases the specific surface area (SBET), the average pore size and the total porous volume decrease significantly. Note that the uncertainties compiled in table 1 were obtained from a standard deviation performed on a series of 3 independent measurements. In order to compare the photocatalytic efficiency between the synthetic routes (usual Sol-Gel (SG) method and Sol-Gel Assisted by Glycine (SG-AG)) with respect to the TiO2 P25 Degussa®, the apparent rate constants were determined. For the SG method, an apparent rate constant of 1.79x10 -2 min -1 was obtained (figure 4.c), this rate increases strongly in the case of the SG-AG method at 11.49x10 -2 min -1 (more than six times greater). For comparison, this kapp is almost twice as high as the TiO2 reference, whose value is 6.16x10 -2 min -1 . This can be explained by the fact that the glycine has a surfactant role which allow a better dispersion of precursors in the gel and can react with the oxygen in air to create hot spots that leads to a massive gas release that limits the rise of temperature, avoid partial sintering of particles and generally allows to have a better specific surface area [START_REF] Khetre | Synthesis and characterization of nanocrystalline LaCrO3 by combustion route[END_REF][START_REF] Purohit | Ultrafine ceria powders via glycine-nitrate combustion[END_REF]. For the same calcination temperature of 1 000°C we observed for the Sol-Gel synthesis a specific surface area of 1 m².g - 1 [START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF] and for the assisted by glycine method a specific surface area which is nearly as double as the previous one (i.e. 1.9 m².g -1 ). In addition, we can note the total pore volume increases slightly from 3.7x10 -3 to 4.8x10 -3 cm 3 .g -1 for the SG [START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF] and AG pathways (present work), respectively. The important difference is the drastic decrease in the average pore size from 36 nm to 7 nm for the SG and AG pathways, respectively. This increase in the number of pores and the presence of straight channels may explain the increase in photocatalytic properties. B.

Guo et al. [START_REF] Guo | The study of the relationship between pore structure and photocatalysis of mesoporous TiO2[END_REF] describes this behavior in mesoporous TiO2 synthetized by sol-gel route using various surfactants.

Reaction intermediates have a negative role on the adsorption of the dye on the catalyst surface [START_REF] Matos | Influence of activated carbon upon titania on aqueous photocatalytic consecutive runs of phenol photodegradation[END_REF]. To study the influence of those intermediates two types of recyclability tests have been performed. In the first one, the solution was kept intact after a defined irradiation time (90 min) and Rhodamine B was then added to reach the same initial concentration before running another degradation test. In this one (Figure 4.d), we can observe a decrease of the percentage of discoloration at 90 min over 4 successive runs. In the second method particles were collected by centrifugation after 90 min and washed with water and ultrasounds without any thermic treatment. Then those particles were poured in a new batch with the same initial concentration of Rhodamine B and irradiate for another 90 min. For this second method, we can observe (Figure 4.d) that there is no decrease on the efficiency over 4 consecutives runs. We can conclude that the intermediate which are present in the solution before the mineralisation step can be adsorbed at the catalyst surface during photodegradation and reduce the efficiency of the catalyst on the dye degradation. It is also important to note that LTO-AG-X sample can be easily collected and remove from cleaned water by only centrifugation step, which means that this material can be recycling without complex treatment or procedure.

Initially, Rhodamine B was chosen as a model molecule in order to demonstrate the catalytic efficiency of the catalysts. Subsequently, phenol was chosen to continue our investigations given the fact that it is a toxic molecule present in many industrial wastewaters, the discharges of which are regulated. Indeed, this molecule is a very widespread pollutant in many industrial effluents from the petroleum [START_REF] Diya'uddeen | Treatment technologies for petroleum refinery effluents: A review[END_REF], resin and paint fabrication [START_REF] Wang | Synthesis of chelating resins with iminodiacetic acid and its wastewater treatment application[END_REF][START_REF] Latinwo | Removal of Phenol from Paint Wastewater by Adsorption onto Phosphoric Acid Activated Carbon Produced from Coconut Shell : Isothermal and Kinetic Modelling Studies[END_REF],

tannery [START_REF] Bharagava | Characterization and Identification of Recalcitrant Organic Pollutants (ROPs) in Tannery Wastewater and Its Phytotoxicity Evaluation for Environmental Safety[END_REF] or even pharmaceutical industries [START_REF] Shourian | Efficient phenol degradation by a newly characterized Pseudomonas sp. SA01 isolated from pharmaceutical wastewaters[END_REF]. For the drinking water, the World Health Organization advises a guideline concentration of phenol to 1 µg.L -1 [START_REF] Yan | Phenol biodegradation by the yeast Candida tropicalis in the presence of m-cresol[END_REF]. As the discharges are regulated according to the standards of the countries, it is necessary to ensure the effluent before discharge into natural environments (general effluent discharge standard of phenol is 0.05 ppm [START_REF] Aoudj | Simultaneous removal of chromium(VI) and fluoride by electrocoagulation-electroflotation: Application of a hybrid Fe-Al anode[END_REF]). The following paragraph aims to study the potential of the La2Ti2O7 photocatalyst synthesized by glycine on the UV photodegradation of Phenol.

Study on Phenol

Influence of photocatalyst loading

The photodegradation of phenol have been monitored by UV-visible spectroscopic method based on the maximum adsorption of the compound (λ = 270 nm) [START_REF] Roig | Monitoring of phenol photodegradation by ultraviolet spectroscopy[END_REF]. In Figure 5.a, we can observe the evolution of the apparent rate constant for several photocatalyst loading (the initial concentration of phenol is fixed to 50 mg.L -1 ). First, we can note a linear increase in the efficiency with the catalyst loading which has already been observed on other catalyst such as ZnO and TiO2 [START_REF] Sakthivel | Solar photocatalytic degradation of azo dye: Comparison of photocatalytic efficiency of ZnO and TiO2[END_REF] and is due to the increase of available photocatalytic sites. An optimal loading of 0,5 g.L -1 is obtained for the LTO-AG in the case of phenol degradation and the apparent rate constant corresponds to 5.5 x10 -3 min -1 i.e. a photodegradation rate of 47.0 µmol.gcat -1 .h -1 . For higher loading we can observe an exponential decrease in the photocatalytic efficiency which can be due to several phenomena such as an increase in the agglomeration of particles or a screening effect that does not allow the light flux to deeply penetrate the solution and reach all photocatalyst particles.

Influence of irradiated surface

It is commonly admitted that photocatalytic degradation is proportional to the luminous flux receive [START_REF] Herrmann | Heterogeneous photocatalysis: Fundamentals and applications to the removal of various types of aqueous pollutants[END_REF][START_REF] Ollis | Destruction of water contaminants[END_REF]. Thus, the kinetic of the reaction is modified according to the following equation [START_REF] Kim | Kinetic study for photocatalytic degradation of volatile organic compounds in air using thin film TiO2 photocatalyst[END_REF] :

𝑣 = 𝑣′𝜙 (5)
Where v' is the reaction rate independent of photon flux, Φ is the light density and the exponent n has a value between 0 and 1. Indeed, this n value can change from 1 for low flux to reach a value of ½ when the flux increases and 0 when the saturation is reached. For low intensities, the reaction rate is directly proportional to the light intensity. The electron-hole pairs are consumed and participate mainly in the chemical surface reaction. Beyond a certain limit value of luminous intensity, the dependence is carried out in square root. This evolution of n exponent probably results in a limitation of the charge transfer and a higher rate of recombination of the electron-hole pairs. Additionally, at even higher light intensity, a stationary state is reached, and the reaction rate saturates.

Several tests have then been made by using photoreactor with different diameter and the same loading of catalyst and light flux penetration. A linear evolution of the apparent rate constant as a function of irradiated surface can be observed in Figure 5.b. This evolution gives us some information on the fact that the saturation phenomenon has not been reached yet.

Furthermore, the linear evolution also indicates that the photoreaction takes place close to the surface of the photoreactor because it is directly proportional to this one, which can be explained by an absorption of UV radiations by the aqueous solvent.

Influence of pH

In photocatalytic processes, the pH could have an influence on the properties of semiconductor catalyst such as it is charge surface or dissociation in the solution [START_REF] Chiou | Influence of operating parameters on photocatalytic degradation of phenol in UV/TiO2 process[END_REF], but also on the species to be degraded and its adsorption [START_REF] Azeez | The effect of surface charge on photocatalytic degradation of methylene blue dye using chargeable titania nanoparticles[END_REF]. The study on the zeta potential in LTO oxides have already been described in our previous work [START_REF] Leroy | Photocatalytic and sonophotocatalytic degradation of rhodamine B by nano-sized La2Ti2O7 oxides synthesized with sol-gel method[END_REF]. The Point of Zero Charge (PZC) is reached for a pH value of 8, under this pH lanthanum dititanate oxide particles are positively charged and above this they are negatively charged which would have an importance because phenol can be observed under positively and negatively charged species depending on the pH of solution. Figure 5.c shows the influence of pH with and without catalyst on phenol.

This behavior is explained by the fact that phenol has two pKa values which are: pKa1(PhOH2 +/ PhOH)=-6.4 and pKa2(PhOH/PhO -)=9.95 at 25°C [START_REF] Combs | pH effects on the direct supercritical fluid extraction of phenols from aqueous matrices[END_REF]. Thus, for a pH greater than 9.95, the phenolate form is negatively charged. At pH = 12, as the surface of the photocatalyst is also negatively charged, the photocatalysis is disadvantaged. While at a pH below the PZC (i.e. pH = 8), photocatalysis is more favored (hence the best efficiency is observed at pH = 5.2 with kapp = 5.49x10 -3 min -1 )). For more acidic pH, the efficiency decreases because the phenol is naturally degraded; thus at pH = 2 the degradation rates with and without photocatalyst are practically identical.This requirement of neutral condition has been already observed in literature for TiO2 and La2Ti2O7 compounds [START_REF] Chiou | Influence of operating parameters on photocatalytic degradation of phenol in UV/TiO2 process[END_REF][START_REF] Yuan | Influence of co-doping of Zn(II) + Fe(III) on the photocatalytic activity of TiO2 for phenol degradation[END_REF][START_REF] Trillas | Phenol photodegradation over titanium dioxide[END_REF]. on phenol can be observe with an apparent rate constant of 1.78x10 -3 min -1 , for TiO2 this constant is about 2.84x10 -3 min -1 and for the AG method we obtained an apparent rate constant of 5.49x10 -3 min -1 . This increase is similar as the one obtained on Rhodamine B and can be explained the same way. This similarity in the improvement of the photocatalytic properties seems to be independent of the nature of the species to be photodegraded showing that the material performance depends mainly upon microstructure, size, morphology of this oxide.

Comparison between La2Ti2O7 and TiO2

Some intermediates are still present in solution and can have a negative role on photodegradation In Figure 6, the variation of the total organic carbon can be observed. The half-time of the reaction is determined at 10 hours.

Influence of scavengers and photodegradation mechanism

During photocatalytic process various species are created such as holes and radicals [START_REF] Scott | Photocatalytic degradation of phenol in water under simulated sunlight by an ultrathin MgO coated Ag/TiO2 nanocomposite[END_REF][START_REF] Wang | Fabrication of AgBr/La2Ti2O7 hierarchical heterojunctions: Boosted interfacial charge transfer and high efficiency visible-light photocatalytic activity[END_REF][START_REF] Wan | Construction of ultrafine Ag3PO4 nanoparticle and La2Ti2O7 nanosheet 0D/2D heterojunctions with improved photocatalytic performance[END_REF] .In order to have a better understanding of which one have an influence on the photodegradation mechanism some tests have been made with scavenger species. Three species where used, the ethylenediaminetetraacetic acid (EDTA), the ascorbic acid and isopropanol to collect holes (h + ), superoxide radicals (O2 •-) and hydroxyl radicals (HO • ), respectively. In Figure 6 no change has been observed in the apparent rate constant with the use of isopropanol which means the hydroxyl radicals don't have a role in photocatalytic process, the use of EDTA and ascorbic acid imply a variation of the apparent constant that decrease drastically which indicate that holes and superoxide radicals have an important role on the mechanism of degradation for La2Ti2O7. The important role of superoxide radicals has already been observed for LTO [START_REF] Wang | Fabrication of AgBr/La2Ti2O7 hierarchical heterojunctions: Boosted interfacial charge transfer and high efficiency visible-light photocatalytic activity[END_REF][START_REF] Wan | Construction of ultrafine Ag3PO4 nanoparticle and La2Ti2O7 nanosheet 0D/2D heterojunctions with improved photocatalytic performance[END_REF].

Sonophotocatalysis

In sonophotocatalytic processes both UV irradiation and ultrasounds are used to improve the efficiency of photodegradation. UV light coupled with the catalyst will create hole/electron pair that will be used to decompose pollutants. Ultrasonic waves will create cavitation bubbles, when those previous one collapse they produce important energy and heat that leads in the formation of free radicals issued from the water [START_REF] Joseph | Sonophotocatalysis in advanced oxidation process: A short review[END_REF][START_REF] Adewuyi | Sonochemistry: Environmental science and engineering applications[END_REF]. The used of ultrasonic wave also allow a better dispersion of catalyst that increase the active surface reachable by the species to be degraded [START_REF] Panda | Recent advancements in the sonophotocatalysis (SPC) and dopedsonophotocatalysis (DSPC) for the treatment of recalcitrant hazardous organic water pollutants[END_REF]. In order to study the influence of sonophotocatalysis the photoreactor containing 200 mL of aqueous solution of phenol with 0.5 g.L -1 of catalyst has been placed in an ultrasonic bath (37 kHz) and result are shown in Figure 8.a. The apparent rate constant of photocatalysis (PC), sonocatalysis (SC) and sonophotocatalysis (SPC) have been compared and are 3.21x10 -3 min -1 , 5.74x10 -4 min -1 and 3.58x10 -3 min -1 respectively. A small increase can be observed with the use of both ultrasound and UV irradiation, it's also important to note that the volume of solution can have a role on ultrasonic waves effect. Indeed, if the volume in reactor decrease, the intensity of ultrasound increase that could lead to an even better dispersion and desorption of product on catalyst. In order to study the influence of volume three test have been made with the exact same condition (catalyst loading, concentration of phenol, light flux, …) and three different volume of 200, 100 and 50 mL. Figure 8.b shows this influence, and a better efficiency was obtained in sonophotocatalysis for a small volume with an apparent rate constant that goes from 3.58x10 -3 min -1 for 200 mL to 6.43x10 -3 min -1 for 50 mL.

Conclusion

La2Ti2O nanoparticles were successfully synthesized by a new Sol-Gel coupled to glycine combustion route. These nanoparticles were characterized by XRD, BET, SEM, EDS and TEM and we found that a minimal calcination temperature of 900°C is required to obtain the pure La2Ti2O7 phase. The size of these particles was determined by using Scherrer's formula. The increase of crystallite size and the decrease of surface area with temperature was pointed out. Photodegradation tests have been made on Rhodamine B and phenol and shows a better photocatalytic efficiency for LTO-AG nanopowders synthesized at 1 000 °C. The experimental conditions such as catalyst loading and pH to obtain the better efficiency were determined. Total organic carbon measurements were carried out showing that there are intermediates present in solution before mineralisation step. We pointed out with recyclability tests that those intermediates have a counterproductive role on photodegradation. We investigate the role of radicals and holes produced on the mechanism of photodegradation with various scavenger's species and conclude that superoxide radicals and holes have the most important role on the photodegradation. We also made sonophotocatalysis study and pointed out the influence of volume in the efficiency of ultrasound and the increase in sonophotocatalytic activity with the use of both ultrasound and UV irradiation.

1 . 1 (

 11 photodegradation of rhodamine B (C0 = 4 mg.L -1 ) by following the kinetics by UV-Visible

Figure 5 .

 5 Figure 5.d shows the comparison of the degradation of phenol without catalyst and

Figure 1 .

 1 Figure 1.a) XRD patterns of LTO-AG obtained using different calcination temperature during synthesis and b) Tauc Plots of LTO-AG-800 and LTO-AG-1000.

Figure 2 .

 2 Figure 2. SEM image of La2Ti2O7 powder with EDS elements distribution for La, Ti and O.

Figure 3 .

 3 Figure 3. Microphotograph transmission electron microscopy a) particles of LTO-AG-1000 b) part of grain c) diffraction pattern with solution for [211] zone axis.

Figure 4 .

 4 Figure 4.a) Example of Ln(C0/C) vs Time plot to determine apparent rate constant for a sample of LTO-AG-1000 ; b) Total Organic Carbon analysis on the degradation of Rhodamine B with LTO-AG-1000 ; c) Photocatalytic efficiency of TiO2 P25, Sol-Gel and Assisted by glycine synthesis and d) Recyclability test of Rhodamine B with LTO-AG-1000 with the influence of intermediates of photodegradation.

Figure 5 .

 5 Figure 5.a) Evolution of apparent rate constant as function of catalyst loading with LTO-AG-1000 ; b) Evolution of apparent rate constant as function of irradiated surface with LTO-AG-1000 ; c) Evolution of apparent rate constant as function of pH with LTO-AG-1000 and d) Comparison of apparent rate constant without and with catalyst TiO2 P25 and LTO-AG-1000.

Figure 6 .

 6 Figure 6. Total organic carbon analysis on the degradation of phenol with LTO-AG-1000.

Figure 7 .

 7 Figure 7. Influence of scavenger on the photocatalytic process with LTO-AG-1000.

Figure 8 .

 8 Figure 8.a) Histograms obtained for three effects, PhotoCatalysis (PC), SonoCatalysis (SC) and SonoPhotoCatalysis (SPC) for LTO-AG-1000 ; b) Influence of the volume to be sonicated on the photocatalytiv efficiency for LTO-AG-1000.

Table 1 .

 1 Average crystallite size determined by XRD profile refinement, BET surface area, average pore size, total pore volume and optical band gap of the LTO-AG-X samples

	Sample	Average	SBET (m².g -1 )	Average	Total porous	Eg (eV)
		crystallites		pore size	Volume
		size (nm)		(nm)	(cm 3 .g -1 )
	LTO-AG-800	47  4	10.6  0.6	17  0.85	4.8x10 -2 	3.95  0.02
					2.4x10 -3
	LTO-AG-900	53  4	6.7  0.4	11  0.55	1.9x10 -2 	4.09  0.02
					1.0x10 -3
	LTO-AG-1000	70  5	1.9  0.2	7  0.35	4.8x10 -3 	4.04  0.02
					2.4x10 -4
	LTO-AG-1100	85  6	0.6  0.1	4  0.20	1.3x10 -3 	4.09  0.02
					6.5x10 -4
	The powder La2Ti2O7 was studied by both TEM and SEM coupled with EDS analyses.

Acknowledgments

The work was financially supported by « les Ministères de l'Europe et des Affaires Etrangères (MEAE) et de l'Enseignement Supérieur, de la Recherche et de l'Innovation (MESRI) » and the national agency for academic exchange of Poland (Narodowa Agencja Wymiany Akademickiej -NAWA) -PHC Polonium (N° 44701ZK). Chevreul Institute (FR 2638)", "Ministère de l'Enseignement Supérieur et de la Recherche", "Région Hauts-de-France", "FEDER" and "C'Nano Nord-Ouest compétence centre" are also acknowledged for supporting and funding this work.