Patricia Garcia-Muñoz
email: garciamunoz@unistra.fr

Fernando Fresno

Christophe Lefevre

Didier Robert

Nicolas Keller
email: nkeller@unistra.fr

+ Photo-Cwpo Photocatalysis

Tio

Highly robust La1-xTixFeO3 dual catalyst with combined photocatalytic and photo-CWPO activity under visible light for 4chlorophenol in water treatment

Keywords: dual Ti-substituted LaFeO3 catalyst, visible light, photoassisted CWPO, photocatalysis, water treatment

0.6 0.8 1.0 TOC/TOC 0 t (min) La x Ti 1-x FeO 3 LaFeO 3 La Fe La x Ti 1-x FeO

1-INTRODUCTION

Over the last decade, the increase in the volumes of polluted water and the presence of biorecalcitrant refractory pollutants in the water streams has been the driving force for the development of new cost-effective technologies for facing those new challenges in terms of pollutant abatement and for meeting the increasingly stringent environmental requirements of the govern regulations.

In polluted water treatment, Advanced Oxidation Processes (AOPs) have demonstrated their ability to oxidize the recalcitrant compounds that cannot been removed by conventional mechanical, biological or physico-chemical treatments. They gather a vast range of processes such as eg.

photolysis, photocatalysis, ozonation, homogeneous Fenton process or its analogous heterogenous Catalytic Wet Peroxide Oxidation (CWPO), based on the generation of oxidative HOx° radicals that enable the oxidation of the organic pollutants up to CO2 and H2O [START_REF] Malato | Decontamination and disinfection of water by solar photocatalysis: Recent overview and trends[END_REF][START_REF] Pliego | Trends in the Intensification of the Fenton Process for Wastewater Treatment: An Overview[END_REF]. Among the heterogeneous AOPs, a special attention has been given to both photocatalysis and CWPO processes, as a result of relatively low investment/running costs and of their high efficiency. Both AOPs operate under different optimum conditions and via specific reaction mechanism, so that both processes have to deal with their own advantages and drawbacks for being implemented. CWPO takes advantage of its heterogeneous nature that reduces the generation/precipitation of sludges and the production of wastes compared to homogeneous Fenton-like processes, and exhibits high reaction rates, however it still suffers from uncomplete mineralization yield due to the surface complexation of iron ions with short-chain acids. By contrast, complete mineralization of the substrate can be achieved with photocatalysis, but with far lower reaction rates and unfortunately with a UV-light dependence in the case of the most active TiO2 reference photocatalyst. Combining those processes for taking advantages of the higher mineralization degree of photocatalysis and of the higher reaction rates of the photoassisted CWPO is a promising strategy [START_REF] Pliego | Trends in the Intensification of the Fenton Process for Wastewater Treatment: An Overview[END_REF][START_REF] García-Muñoz | Sulfonamides photoassisted oxidation treatments catalyzed by ilmenite[END_REF][START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF]. Physical mixtures of catalyst have been reported, while a promising strategy could be the design of a single-phase catalyst enabling both photoassisted processes to take place simultaneously [START_REF] García-Muñoz | Sulfonamides photoassisted oxidation treatments catalyzed by ilmenite[END_REF][START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF]. This single-phase catalyst strategy has been scarcely reported for implementing combined processes [START_REF] Cai | Visible light-assisted heterogeneous Fenton with ZnFe2O4 for the degradation of Orange II in water[END_REF][START_REF] Mechakra | Use of natural iron oxide as heterogeneous catalyst in photo-Fenton-like oxidation of chlorophenylurea herbicide in aqueous solution: Reaction monitoring and degradation pathways[END_REF][START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF][START_REF] García-Muñoz | Ilmenite (FeTiO3) as low cost catalyst for advanced oxidation processes[END_REF][START_REF] García-Muñoz | Modified ilmenite as catalyst for CWPO-Photoassisted process under LED light[END_REF]. Natural iron oxide (α-Fe2O3) was employed by Mechakra et al. to remove the Linuron herbicide in water under both UV and solar light. While the total removal of the pesticide was reached, only 80% COD conversion was obtained [START_REF] Mechakra | Use of natural iron oxide as heterogeneous catalyst in photo-Fenton-like oxidation of chlorophenylurea herbicide in aqueous solution: Reaction monitoring and degradation pathways[END_REF]. Another natural oxide, ilmenite (FeTiO3), has been used for the photo-assisted catalytic removal of different pollutants in water [START_REF] García-Muñoz | Sulfonamides photoassisted oxidation treatments catalyzed by ilmenite[END_REF][START_REF] García-Muñoz | Modified ilmenite as catalyst for CWPO-Photoassisted process under LED light[END_REF]. Complete phenol depletion with about 95-97% mineralization degree and a low 0.5% iron leaching was obtained under visible and solar light in the presence of a stoichiometric amount of H2O2 [START_REF] García-Muñoz | Modified ilmenite as catalyst for CWPO-Photoassisted process under LED light[END_REF], while natural ilmenite was also efficiently used for the treatment of sulfonamide antibiotics in aqueous solution [START_REF] García-Muñoz | Sulfonamides photoassisted oxidation treatments catalyzed by ilmenite[END_REF]. Cai et al. used ZnFe2O4 single-phase ferrite as heterogeneous Fenton catalyst under visible light for removing Orange II from water in the presence of H2O2 [START_REF] Cai | Visible light-assisted heterogeneous Fenton with ZnFe2O4 for the degradation of Orange II in water[END_REF]. Although the catalyst enabled the decomposition of H2O2 and allowed to obtain a complete conversion of the Orange II substrate, only a 60% mineralization yield was achieved. Also, Ba-substituted BiFeO3 with an optimal 0.12 Ba/Bi molar ratio exhibited enhanced activity as heterogeneous photo-Fenton catalyst in weakly acidic conditions for the toluene removal using visible light irradiation, when compared to the pristine BiFeO3 counterpart, although no complete mineralization was obtained [START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF]. The enhancement was associated to an efficient redox iron cycle and to the presence of oxygen vacancies resulting from the substitution-induced network distortion.

Lanthanum ferrite (LaFeO3) is one of the most common ABO3-type perovskite used eg. as gas sensing, magnetic and optoelectronic materials, solid oxide fuel cell electrode, or heterogeneous catalysts [START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF]. It is a p-type semiconductor with a small 2.1-2.7 eV band-gap, that shows interest for being used as catalyst in different AOPs, in the presence or absence of light [START_REF] Rao | Efficient degradation of diclofenac by LaFeO3-Catalyzed peroxymonosulfate oxidation---kinetics and toxicity assessment[END_REF][START_REF] Parrino | Cu-substituted lanthanum ferrite perovskites: Preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation[END_REF][START_REF] Song | Enhanced photocatalytic decomposition of an organic dye under visible light with a stable LaFeO3/AgBr heterostructured photocatalyst[END_REF][START_REF] Afzal | High surface area mesoporous nanocast LaMO3 (M=Mn, Fe) perovskites for efficient catalytic ozonation and an insight into probable catalytic mechanism[END_REF]. The versatility of the LaFeO3 materials relates with the possibility of A-and/or B-cation partial substitution within the A1-xA'xB1-yB'yO3±d structure that induces geometrical and charge constraints, and consequently impacts strongly on the textural, structural, oxygen mobility and electronic properties of the material, and allows the surface redox properties of the catalysts to be tuned [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF][START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF][START_REF] Rao | Efficient degradation of diclofenac by LaFeO3-Catalyzed peroxymonosulfate oxidation---kinetics and toxicity assessment[END_REF][START_REF] Parrino | Cu-substituted lanthanum ferrite perovskites: Preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation[END_REF][START_REF] Song | Enhanced photocatalytic decomposition of an organic dye under visible light with a stable LaFeO3/AgBr heterostructured photocatalyst[END_REF][START_REF] Wang | Ethanol sensing characteristics of BaTiO3/LaFeO3 nanocomposite[END_REF] and so the catalytic properties [START_REF] Soltani | Improving heterogeneous photo-Fenton catalytic degradation of toluene under visible light irradiation through Ba-doping in BiFeO3 nanoparticles[END_REF][START_REF] Sannino | Structured catalysts for photo-Fenton oxidation of acetic acid[END_REF][START_REF] Phan | Heterogeneous photo-Fenton degradation of organics using highly efficient Cu-doped LaFeO3 under visible light[END_REF].

The interest of using the LaFeO3 perovskite as photocatalyst under visible light has been reported for the oxidative degradation of dyes in aqueous phase [START_REF] Peng | Perovskite LaFeO3/montmorillonite nanocomposites: synthesis, interface characteristics and enhanced photocatalytic activity[END_REF][START_REF] Li | Photoinduced charge property of nanosized perovskitetype LaFeO3 and its relationships with photocatalytic activity under visible irradiation[END_REF][START_REF] Su | Synthesis of large surface area LaFeO3 nanoparticles by SBA-16 template method as high active visible photocatalysts[END_REF][START_REF] Ding | Microwave-assisted synthesis of perovskite ReFeO3 (Re: La, Sm, Eu, Gd) photocatalyst[END_REF][START_REF] Thirumalairajan | Controlled synthesis of perovskite LaFeO3 microsphere composed of nanoparticles via self-assembly process and their associated photocatalytic activity[END_REF][START_REF] Tang | Microwave-assisted synthesis of nanoparticulate perovskite LaFeO3 as a high active visible-light photocatalyst[END_REF][START_REF] Ismael | Perovskite-type LaFeO 3 : Photoelectrochemical Properties and Photocatalytic Degradation of Organic Pollutants Under Visible Light Irradiation[END_REF][START_REF] Kumar | Facile hydrothermal synthesis and characterization of LaFeO3 nanospheres for visible light photocatalytic applications[END_REF] or for water decomposition [START_REF] Parida | Fabrication of nanocrystalline LaFeO3: An efficient sol-gel auto-combustion assisted visible light responsive photocatalyst for water decomposition[END_REF]. They were also scarcely used as photo-Fenton-like catalysts, notably for the degradation of RhB dye under visible light, or for that of methylparaben [START_REF] Orak | Photocatalytic and photo-Fenton-like degradation of methylparaben on monolith-supported perovskite-type catalysts[END_REF] or acetic acid [START_REF] Sannino | Structured catalysts for photo-Fenton oxidation of acetic acid[END_REF] in water under UV-A light. The influence of A or B sub-lattice substitution within the LaFeO3 photocatalysts or photo-Fenton catalysts remains even more scarcely documented. The Cu ® Fe cationic substitution in LaFeO3 was studied for the CWPO of organic compounds in water under visible light [START_REF] Phan | Heterogeneous photo-Fenton degradation of organics using highly efficient Cu-doped LaFeO3 under visible light[END_REF]. The partial substitution of La by Ca [START_REF] Li | Preparation of Ca-doped LaFeO3 nanopowders in a reverse microemulsion and their visible light photocatalytic activity[END_REF] or Li [START_REF] Hou | Preparation, characterization and investigation of catalytic activity of Li-doped LaFeO3 nanoparticles[END_REF] has been reported, in both cases for improving the elimination of organic dye compounds in aqueous solution, methylene blue and methyl blue, respectively. Similarly, the LaFeO3 perovskite with partial Zn or Mn incorporation within the Fe sub-lattice exhibited activity in the removing of methylene blue and methyl orange under UV and sunlight irradiation, respectively [START_REF] Wei | Synthesis, magnetization and photocatalytic activity of LaFeO3 and LaFe0.5Mn0.5-xO3-δ[END_REF][START_REF] Dong | Photocatalytic activities of LaFe1âˆ'xZnxO3 nanocrystals prepared by solâ€"gel auto-combustion method[END_REF]. This positive effect was proposed to result from the electronic unbalance caused by the partial cationic substitution, that consequently would be compensated by an increase in the oxidation state of a part of the Fe (III) to Fe (IV) , with consequently the creation of oxygen vacancies for maintaining the electroneutrality of the structure. In addition, the catalytic photo-Fenton properties of cobalt ferrite spinels were enhanced by partially substituting iron by rare earth metals in the removal of cationic and anionic dyes [START_REF] Dhiman | Enhanced catalytic properties of rare-earth substituted cobalt ferrites fabricated by sol-gel auto-combustion route[END_REF]. Besides ferrites materials, controlled cationic substitution in titanium-containing perovskite-type structures was also reported to be beneficial in terms of photocatalytic activity under visible light [START_REF] Fei | Visible-light activated titania perovskite photocatalysts: Characterisation and initial activity studies[END_REF][START_REF] Otsuka-Yao-Matsuo | Visible light-induced photobleaching of methylene blue aqueous solution using (Sr1-xLax)TiO3+δ-TiO2 composite powder[END_REF].

We recently first demonstrated the ability of a Ti-substituted LaFeO3 perovskite material to act simultaneously in similar reaction conditions as heterogeneous photo-Fenton catalyst for efficiently decomposing hydrogen peroxide into active HOx° radicals and as heterogeneous photocatalyst under UV-A light [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF]. Ti has been selected for substituting the trivalent cations from LaFeO3 due to its tetravalent nature in its highest oxidation state. The partial and exclusive substitution of La by Ti within the LaFeO3 perovskite network was achieved by using commercial Aeroxide© TiO2 P25 nanocrystals as titanium source during a Pechini sol-gel synthesis. The obtained La1-xTixFeO3 catalyst displayed an optimum in terms of activity and stability for a Ti content of 3.2 at%, taking the removal of 4-chlorophenol in water as test reaction under UV-A light. By combining both photo-assisted CWPO and photocatalysis processes, the incorporation of Ti atoms in the LaFeO3 structure allowed to improve the reaction rates of both chlorophenol and TOC removal, and to achieve complete TOC conversion at circumneutral pH conditions in comparison to the single AOP counterparts. However, although the incorporation of Ti atoms strongly increased the robustness of the catalysts compared to the pristine LaFeO3, the La1-xTixFeO3 catalyst still suffered from a certain iron leaching, corresponding to 0.05 mg/L in aqueous solution. Further, the performances of the dual catalysts were only assessed under UV-A light.

Therefore, we report here on the synthesis of a novel reusable La1-xTixFeO3 dual catalyst, for which the Ti incorporation was achieved during the Pechini sol-gel synthesis using a laboratory-made amorphous titania as titanium source in replacement of the Aeroxide© TiO2 P25 nanocrystals, for taking advantage of a favorable temperature-driven atomic diffusion mechanism during the final calcination treatment. Using the removal of 4-chlorophenol in water as test reaction, this novel dual catalyst allowed to increase further the reaction rates obtained through the combined photo-CWPO/photocatalysis AOP approach in comparison to those observed on the catalyst developed in Garcia-Munoz et al. [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF], and subsequently to totally inhibit Fe leaching. Further, the photoreactions were performed under pure visible light (>420 nm) for taking advantage of the reduced band gap of the material as well as under artificial solar light.

2-MATERIALS AND METHODS

Materials

Analytical grade chemicals were used without further purification. 4-Cl-Phenol (>99%) and H2O2 (>30%) were purchased from Sigma Aldrich. La(NO3)3 hexahydrate, Fe(NO3)3 nonahydrate, citric acid hydrate (C6H8O7xH2O, 99.5%) and Titanium tetraisopropoxide (TTIP, 97%) were purchased from Sigma Aldrich. Commercially available Aeroxide© TiO2 P25 was obtained from Evonik.

Synthesis of Ti-modified LaFeO3

Ti-modified LaFeO3 materials were obtained by modifying a classical sol-gel Pechini synthesis. In this method, Lanthanum nitrate hexahydrate and iron nitrate nonahydrate were dissolved in 30 mL distilled water at 108.3 g/L and 133 g/L, respectively, and the metallic ions undergo complexation by subsequent addition of citric acid at 70 g/L, corresponding to a stoichiometric molar ratio in respect of the metallic ions. 10% wt. of crystallized Aeroxide© TiO2 P25 or dried amorphous titania obtained via a sol-gel synthesis -in respect of the theoretical amount of LaFeO3 perovskite obtained -, was further directly introduced under stirring into the aqueous sol. The resulting suspensions were maintained at 80°C for 2 days (2°C/min heating rate) in order to induce the complete gel formation.

The calcination of the dried gel was performed using a heating cycle as reported by Gosavi et al. [START_REF] Gosavi | Pure phase LaFeO3 perovskite with improved surface area synthesized using different routes and its characterization[END_REF] with heating rates of 5°C/min, consisting in holding the temperature at 600°C for 2 h, before to further maintaining a temperature plateau at 800°C for 2 h.

Without considering the titania phase, the stoichiometric reaction forming the LaFeO3 perovskite structure is as follows (Eq.1) [START_REF] Qi | A simple way to prepare nanosized LaFeO3 powders at room temperature[END_REF]:

[Eq.1]
Non-modified lanthanum ferrite powder was synthesized as reference material by following the same protocol without any addition of titanium source, with a similar calcination procedure at 800°C. The reference titanium-free material was labelled as LFO, while the Ti-modified materials were labelled as P25-LFO and SG-LFO when using Aeroxide© TiO2 P25 and dried amorphous titania as titanium source, respectively.

Synthesis of the amorphous titania precursor

The amorphous titania material was synthesized according to a sol-gel method under basic conditions using TTIP as precursor. In a typical synthesis, 17.8 g of TTIP was first added to a 40 mL ethanolic solution at room temperature, before a similar volume of water was added dropwise to the solution. After rectification of the pH to 9 by adding ammonia (Carlo Erba, 30% aqueous solution), the solution was left under agitation at room temperature until a dry paste was obtained. The resulting powder was further dried at 100°C for 12 h.

Characterisation techniques

X-ray diffraction (XRD) pattern have been recorded at room temperature in a θ/θ mode on a Brucker D8 Advance diffractometer equipped with a monochromatic copper radiation source (Kα = 1.54056 Å) with a scan step of 0.02°. Some patterns have been investigated by Rietveld refinement with the Fullprof software for which the modified Thompson-Cox-Hasting function was chosen to generate the line shape of the diffraction peaks. Instrumental broadening has been previously determined by measuring the scattering from corundum (NIST standard SRM 1976b) [START_REF] Mccusker | Rietveld refinement guidelines[END_REF][START_REF] Rodríguez-Carvajal | Recent advances in magnetic structure determination by neutron powder diffraction[END_REF]. The mean crystallite size defined as the average size of the coherent diffracting domains, was derived from the whole XRD patterns after refinement and determined from the Scherrer equation with the usual assumption of spherical crystallites taking into account the intrinsic broadening of the peaks due to the instrumentation.

The surface area measurements were carried out on a Micrometrics Tristar 3000 using N2 as adsorbent at -196 °C with a prior outgassing at 250°C overnight to desorb the impurities or moisture.

The iron and titanium contents in the catalysts were determined by Inductively coupled plasma optical emission spectroscopy (ICP-OES) carried out on an Optima 7000 DV spectrometer (Perkin Elmer), after a microwave-assisted acidic dissolution in aqua regia at 185°C under autogenic pressure.

UV-vis diffuse reflectance spectra were acquired in a UV/Vis/NIR Perkin Elmer Lambda 1050 spectrometer, recording the reflectance between 250 and 800 nm. Band gaps were estimated from Tauc plots by representing (αhν) 2 vs. photon energy, where α= -Ln(Reflectance), in accordance with the direct semiconductor character of LaFeO3 [START_REF] Parrino | Cu-substituted lanthanum ferrite perovskites: Preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation[END_REF].

Fluorescence spectra of the powdered samples were recorded in a Fluorescence Spectrometer Perkin Elmer LS55, using an excitation wavelength of 450 nm and a cut-off filter at 515 nm.

Fluorescence lifetime measurements by time-correlated single photon counting (TCSPC) were carried out with a Mini-t equipment from Edinburgh Instruments using as excitation source a laser diode (model EPL375) with a wavelength of 445.2 nm and a pulse width of 90.7 ps, at repetition rate of 1 MHz, and including a band pass filter at 600 ± 25 nm.

X-ray photoelectron spectroscopy (XPS) characterization was performed on a ThermoVGMultilabESCA3000 spectrometer (Al Kα anode at hλ = 1486.6 eV). The energy shift due to electrostatic charging was subtracted using the adventious sp2 carbon C 1s band at 284.6 eV.

Contributions with Doniach-Sunjic shape and a' S-shaped' Shirley type background were used.

Typical reaction procedure

The 4-CP degradation experiments were carried out in a batch beaker-type glass reactor at atmospheric pressure, at a constant temperature of 20°C ± 2°C (using a using cooling bath) and using H2O2 as main oxidant. In a typical reaction procedure, 50 mg of catalyst was dispersed under stirring in a 100 mL aqueous solution of 4-chlorophenol at 25 mg/L concentration, corresponding to an initial organic carbon concentration of 14.8 ppm and to a catalyst load of 0.5 g/L. Prior to irradiation, the catalyst suspension has been stirred for 1 h in the dark to ensure the establishment of the adsorption/desorption equilibrium. The experiments were carried out within an Suntest XLS+ reaction chamber (Atlas Material Testing Technology BV, Gelnhausen, Germany) equipped with a Xenon arc lamp of 1700 W adjustable power and a Solar ID65 filter to limit the UV radiation at 300 nm for simulating solar exposition according to ICH Q1B guidelines. After adding hydrogen peroxide at the stoichiometric concentration corresponding to a complete mineralization of the initial pollutant (125 mg/L), the suspension was exposed to simulated solar light at 250 Wm -2 which corresponds to an average solar radiation in a summer day in southern Europe, or to pure visible light at 215 Wm -2 through the use of 420-nm cut-off filter (GG420 Yellow Schott Optical Filters). Solar light and visible light spectral distributions obtained with or without the use of the cut-off filter, respectively, are reported in Fig. S1.

At each time interval, 8 mL of solution has been sampled and then filtered through 0.20 μm porosity filter to remove the photocatalyst powder if any, before the concentration of 4chlorophenol has been determined by UV-Visible spectrophotometry by monitoring the disappearance of the main absorption peak at λ=224 nm. Total Organic Carbon measurements were performed using a Shimadzu TOC-L analyzer to determine the organic carbon load. Hydrogen peroxide concentration was measured by colorimetric titration using the TiOSO4 method [START_REF] Eisenberg | Colorimetric Determination of Hydrogen Peroxide[END_REF] and the Fe leached during time-course of reaction was quantified by the ortho-phenanthroline method [START_REF] Sandell | Colorimetric Determination of Traces of Metals[END_REF].

The efficiency of the UV cut-off filter was evidenced by performing a photocatalytic test of 4-Cl-Phenol degradation (at 25 mg/L) under pure visible light (ie. with an irradiation > 420 nm) using the commercially available Aeroxide© TiO2 P25 reference photocatalyst. As it can be seen in Figure S2, no activity was observed under pure visible light for the reference photocatalyst, neither in the 4-Cl-Phenol degradation nor in the TOC conversion.

In the reusability tests with sequential cycles, the catalyst was recovered after each run by powder filtration and dried overnight at 100°C.

RESULTS AND DISCUSSION

Ti-substituted LaFeO3 catalyst characterization

The elemental chemical analyses shown in Table 1 indicated that the P25-LFO and SG-LFO catalysts possess a titanium content of 3.2 at.% and 4.0 at.%, respectively, with a similar Ti/Fe atomic ratio.

The XRD patterns of the pristine and the Ti-modified LaFeO3 catalysts are shown in Figure 1 with their corresponding profile Rietvelt refinements. The refined cationic distribution in the LaFeO3 structure and the crystallographic data of the refined samples are provided in Table 1 and Table S1.

For the pristine titanium-free catalyst, the most intense diffraction peaks at 2θ = 22.6°, 32.2°, 39.7°, 46.1°, 57.4° and 67.3° corresponded to the diffraction of the (100), ( 110), ( 111), ( 200), ( 210) and (220) planes of the LaFeO3 phase (JCPDS card 070-7777), respectively. α-Fe2O3 in small amount (11%) was observed as extra phase (JCPDS card 033-06664).

The XRD patterns of both P25-LFO and SG-LFO catalysts exhibited diffraction reflexes indexed in the Pbnm orthorhombic unit cell of LaFeO3, while a slight increase in the amount of α-Fe2O3 extraphase was observed (15-18%). By contrast, the addition of the TiO2 powder during the Pechini sol-gel synthesis did not result in the presence of TiO2 or La2O3 extra-phases.

It has been proposed that a solid-solid diffusion occurs between the particles of TiO2, -whether they are crystallized in TiO2 P25 or amorphous in the dried precursor -, and the amorphous LaFeO3 crystallites before they crystallize during the calcination treatment, allowing the titanium atoms to be distributed within the LaFeO3 crystallites. The determination of the cationic distribution within the structures was performed considering that titanium can move towards both La and Fe cationic sites.

The separate powder XRD refinement of both Ti/La and Ti/Fe site occupations showed that the titanium atoms occupies exclusively the La A-site and the Ti-substituted LaFeO3 catalysts can be consequently labelled as La1-xTixFeO3. This result is rather surprising since in view of the involved However, there is no empirical rule on the location of the doping element as a function of steric or electronic criterion. Indeed, substitution on B site has been reported eg. for Ti, [START_REF] Iguchi | Electrical Transports of LaFe1-xTixO3 (x≤0.10)[END_REF][START_REF] Phokha | Structure, magnetic, and dielectric properties of Ti-doped LaFeO3 ceramics synthesized by polymer pyrolysis method[END_REF][START_REF] Sasikala | Transition metal titanium (Ti) doped LaFeO3 nanoparticles for enhanced optical structural and magnetic properties[END_REF][START_REF] Sasikala | Chemical, Morphological, Structural, Optical, and Magnetic Properties of Transition Metal Titanium (Ti)-Doped LaFeO3 Nanoparticles[END_REF] Ni [START_REF] Nguyen | Crystal structure and magnetic properties of LaFe1-xNixO3 nanomaterials prepared via a simple co-precipitation method[END_REF] or Ca [START_REF] Li | Preparation of Ca-doped LaFeO3 nanopowders in a reverse microemulsion and their visible light photocatalytic activity[END_REF],whereas substitution in A site has been reported eg. for Bi [START_REF] Li | Effect of Bi-doping on the electrocatalytic properties of LaFeO3 powders prepared by solâ€"gel method[END_REF], Pr [START_REF] Maity | Visible Light Driven Photocatalytic Activity of Granular Pr Doped LaFeO3[END_REF],Li [START_REF] Hou | Preparation, characterization and investigation of catalytic activity of Li-doped LaFeO3 nanoparticles[END_REF],Na [START_REF] Devi | Quantification of charge carriers participating antiferromagnetic to weak ferromagnetic phase transition in Na doped LaFeO3 nano multiferroics[END_REF] or 4f elements [START_REF] Sabeeh | Rare earth substituted nanocrystalline LaFeO 3 perovskites and their composites with reduced graphene oxide for enhanced photocatalytic and other potential applications[END_REF]. This predictive character seems thus to be more complex and should be dependent on other extrinsic parameters such as the synthesis temperature or the cooling rate which are well known to affect the cationic distribution in the oxides [START_REF] De Guire | The cooling rate dependence of cation distributions in CoFe2O4[END_REF][START_REF] Zhang | Temperature Dependence of Cation Distribution and Oxidation State in Magnetic Mnâˆ'Fe Ferrite Nanocrystals[END_REF].

Table 2 evidences that performing the ferrite synthesis in the presence of the crystallized TiO2 nanopowder resulted in a partial Ti ® La substitution rate of 5 at.% in the LaFeO3 crystallites, while by contrast using the dried amorphous precursor increased the substitution rate up to 11 at.%. Both A-and B-site substitutions are reported for the LaFeO3 structure to be possible, but in this case, the Ti ions is occupying the A-site. La 3+ (A) site substitution was also obtained by Hou et al. with the solgel synthesis in the presence of acetic acid of Li + -substituted LaFeO3 [START_REF] Hou | Preparation, characterization and investigation of catalytic activity of Li-doped LaFeO3 nanoparticles[END_REF], while Li et al. substituted La 3+ by Ca 2+ in the perovskite structure during a reverse emulsion synthesis, with the simultaneous formation of oxygen vacancies [START_REF] Li | Preparation of Ca-doped LaFeO3 nanopowders in a reverse microemulsion and their visible light photocatalytic activity[END_REF].

We assume that the temperature-driven solid-solid atomic diffusion of titanium within the amorphous LaFeO3 crystallites is favoured when using an amorphous source of titania when compared to well-crystallized 22 nm mean size TiO2 nanocrystals. Therefore, this difference can explain the increase in the substitution rate in the case of the SG-LFO catalyst.

The partial Ti ® La substitution within the LaFeO3 crystallites suppressed the growth of large crystallites during the final calcination at 800°C, so that both P25-LFO and SG-LFO catalysts exhibited an average crystallite size of 20-21 nm compared to 46 nm for the pristine LFO catalyst. Parrino et al.

observed a similar behavior in the case of Cu-substituted lanthanum ferrite perovskites, and proposed that it may result from a distortion of the perovskite lattice by oxygen defects [START_REF] Parrino | Cu-substituted lanthanum ferrite perovskites: Preparation, characterization and photocatalytic activity in gas-solid regime under simulated solar light irradiation[END_REF]. Sengele et al. have also reported a reduction in crystallite size through the doping of sol-gel TiO2 by Ta cations, that the authors have attributed to a lower nucleation rate, that favored the formation of smaller size spherical nanoparticles under slower growth conditions [START_REF] Sengele | Ta-doped TiO2 as photocatalyst for UV-A activated elimination of chemical warfare agent simulant[END_REF]. Here, the decrease in the mean crystallite size was accompanied by an increase in the specific surface area from 8 to 28 m 2 /g for the P25-LFO catalyst. However, despite a smaller mean crystallite size of 20 nm, the SG-LFO catalyst displayed a low specific surface area of 5 m 2 /g. This may result from the existence of more grain boundaries in the SG-LFO catalyst than in the P25-LFO counterpart.

Diffuse reflectance UV-vis spectroscopy shown in Figure 2 confirmed that La1-xTixFeO3 materials can potentially be used as photocatalysts under pure visible light. The band gap was not modified by the doping of the perovskite, and the band-gap values were graphically estimated from Tauc plots at 2.1 eV, whether the catalysts contained substitutional titanium or not (Figure 2).

The influence of the partial Ti ® La cation substitution in the LFO structure on the surface state of the SG-LFO catalyst is shown in Figure 3. The La 3d and Fe 2p core orbital XPS spectra recorded on the pristine LFO and the SG-LFO catalysts revealed the presence of La 3+ and Fe 3+ ions at the surface of both catalysts. In the case of the LFO catalyst, the La 3d5/2 -La 3d3/2 orbital doublet contribution observed at 833.4 eV and 850.1 eV, with a spin orbit splitting of 16.7 eV [START_REF] Thirumalairajan | Novel Synthesis of LaFeO3 Nanostructure Dendrites: A Systematic Investigation of Growth Mechanism, Properties, and Biosensing for Highly Selective Determination of Neurotransmitter Compounds[END_REF][START_REF] Humayun | Synthesis of ZnO/Bi-doped porous LaFeO3 nanocomposites as highly efficient nano-photocatalysts dependent on the enhanced utilization of visible-light-excited electrons[END_REF] and a multiplet splitting of 3.8 eV, was characteristic of La 3+ species [START_REF] Petrović | LaMO3 (M=Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation[END_REF]. The Fe 2p3/2 -Fe 2p 1/2 orbital doublet contribution observed at 710.3 eV and 723.7 eV with a 13.4 eV spin orbit coupling [START_REF] Petrović | LaMO3 (M=Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation[END_REF][START_REF] Faye | Influence of lanthanum stoichiometry in La1-xFeO3-δ perovskites on their structure and catalytic performance in CH4 total oxidation[END_REF] and with the low intensity and broad shake-up satellite peak at about 718 eV was characteristic of Fe 3+ [START_REF] Yamashita | Analysis of XPS spectra of Fe2+ and Fe3+ ions in oxide materials[END_REF].

The partial Ti ® La substitution in the LFO structure induced an upward (+0.8 eV) shift for the La 3d5/2 -La 3d3/2 orbital doublet contribution at 834.2 eV and 850.9 eV, as well as a downward (-0.7 eV) shift for the binding energies of Fe 2p3/2 and Fe 2p 1/2 orbital doublet contribution at 709.7 eV and 723.2 eV. Further, the Ti 2p3/2 -Ti 2p1/2 orbital doublet contribution observed at 457.3 eV and 463.0 eV on the Ti 2p core level spectra revealed the stabilization of Ti cations at the surface of the SG-LFO catalyst as Ti 3+ substituting La 3+ in the LFO network [START_REF] Petrović | LaMO3 (M=Mg, Ti, Fe) perovskite type oxides: Preparation, characterization and catalytic properties in methane deep oxidation[END_REF]. However, the surface reconstruction of the Ti 3+ -substituted LFO nanocrystals cannot be fully ruled out, so that this contribution might be attributed to Ti 4+ -O-M with a strongly lower binding energy than Ti 4+ in an usual TiO2 network environment [START_REF] Yu | Effects of F-Doping on the Photocatalytic Activity and Microstructures of Nanocrystalline TiO2 Powders[END_REF].

According to the electrostatic model, the upward shift of La 3+ binding energy observed in the La-O-Ti configuration corresponds to a O®Ti electronic transfer in comparison to the La-O-La reference configuration, since lanthanum is less electronegative than the substituting titanium. This led to a O¬La electronic transfer that results in an electronic depletion of the lanthanum ion and consequently in a shift towards higher energies of the binding energy of La core electrons. In a mirrored reversed manner, the downward shift of Fe 3+ binding energy in the Fe-O-Ti configuration corresponded to a shift towards lower energies of the binding energy of Fe core electrons, that was attributed to an electronic enrichment of Fe cations.

The fluorescence spectra using a 450 nm excitation wavelength displayed in the inset of Figure 4 shows an emission profile of LaFeO3 with two maxima at ca. 600 and 620 nm. Emission therefore appears at energies lower than the band gap, reflecting that excitonic photoluminescence is mainly occurring, which is usually associated with sub-band-gap transitions from electronic states related to surface defects or oxygen vacancies [START_REF] Liqiang | Review of photoluminescence performance of nano-sized semiconductor materials and its relationships with photocatalytic activity[END_REF]. Titanium doping induces small changes both in the position and the intensity of these features, which may be attributed to surface modifications with respect to the pristine LFO material. Thus, the modified positions may reflect different energy levels of vacancies/defects, while the slightly higher intensity can be related to a higher content of these features in the doped systems [START_REF] Liqiang | The preparation and characterization of La doped TiO2 nanoparticles and their photocatalytic activity[END_REF]. The decrease in the crystallite size, and therefore in the surfaceto-bulk ratio, upon doping, as shown in Table 1, is in good agreement with this assumption. The main body of Figure 4 shows the fluorescence decays of the three samples. Slower decay in doped samples is apparent from the curves. In view of the two maxima observed in the fluorescence spectra as mentioned above, the decay profiles were fitted to a bi-exponential decay curve, with the lifetimes shown in the figure obtained as the average of both components, calculated as indicated in the figure legend. Assuming, as described above, that the fluorescence emission arises from transitions from surface electronic states, the longer-lived emission observed for the titanium-substituted LFO can be a reflection of a longer lifetime of electrons trapped in these states, thus increasing their availability to react with adsorbed species [START_REF] Charanpahari | Enhanced photocatalytic activity of multi-doped TiO2 for the degradation of methyl orange[END_REF][START_REF] Varma | Dependence of photocatalysis on charge carrier separation in Ag-doped and decorated TiO2 nanocomposites[END_REF].

Catalytic activity under visible light

First, the activity of the pristine LaFeO3 material under visible light as photoassisted CWPO catalyst in the oxidation of 4-chlorophenol was assessed and Figure 5 depicts the main parameters followed for the reaction as a function of the time under irradiation. As it can be inferred from the data, the non-substituted LFO reference catalyst could not allow neither a complete mineralization of the 4-CP nor a complete degradation to be achieved. Only a low TOC conversion of 27% and a 4-CP conversion of 71% were obtained after 300 min of reaction, even with a total H2O2 decomposition.

Further, the catalyst suffered from a relatively high amount of iron leached (ca. 0.5 mg/L after 300 min of test, corresponding to 0.4 at.% in respect of the iron content of the catalyst), that can significantly contribute to the activity via a typical photo-Fenton process in homogeneous phase.

The activity of the pristine LFO and the La1-xTixFeO3 catalysts in pure photocatalysis under visible light as well as in CWPO in the dark is shown in Figures S3 andS4, respectively, for evaluating the contribution of both single processes to the pollutant and TOC conversion profiles. Pure photocatalysis on the pristine LFO catalyst led to a low 4-CP conversion of 4% after 240 min of, that increased to 10% and 20% by using the P25-LFO and the SG-LFO photocatalysts, respectively, with the corresponding increase in the apparent kinetic rate from 0.5 x 10 -3 min -1 , to 1.1 and 2.5 x 10 -3 min -1 , respectively (Figure S3). In all cases, negligible TOC conversions were achieved within the test duration time. Using the LFO reference as CWPO catalyst allowed a 4-CP conversion of 40% and a TOC mineralization of 11% to be obtained after 360 min of test, with an apparent kinetic constant for the phenolic compound abatement of 1.6 x 10 -3 min -1 (Figure S4). Similarly than in pure photocatalysis, the substitution improved the 4-CP removal -although in a moderate extent -, and a slightly higher 4-CP conversion of 48% and 55% was obtained after 360 min of test on P25-LFO and SG-LFO, respectively. The enhancement was more pronounced for the TOC conversion, that increased after 360 min of test to 23% and 33% for the P25-LFO and SG-LFO catalysts, respectively.

Figure 6 shows the influence of the partial substitution of La by Ti in the La1-xTixFeO3 catalysts on the evolution with time under pure visible light irradiation of the 4-Cl-phenol concentration, the TOC, the H2O2 conversion and the concentration of the leached Fe. The La1-xTixFeO3 catalysts allowed strongly faster chlorophenol and TOC removal to be achieved when compared to the unmodified LFO. The higher the Ti substitution degree, the shorter the reaction duration and the faster the decomposition rates (Table 3), with a complete chlorophenol and TOC conversion being obtained within 200 min and 300 min of reaction for SG-LFO and P25-LFO catalysts, respectively. By contrast, only 75% and 27% of chlorophenol removal and mineralization, respectively, were achieved for 300 min of test with the LFO reference catalyst. A complete H2O2 depletion was achieved within 240 min of test with the LFO reference, with an apparent rate constant for a pseudo first order reaction of 0.0071 min -1 . The cationic substitution led to an increase in the apparent kinetic constant for the H2O2 decomposition to ca. 0.0233-0.0245 min -1 , independently of the substitution degree, and complete H2O2 decomposition was achieved on both La1-xTixFeO3 catalysts for shorter test durations when compared to the LFO reference (Table 3).

By contrast, considering a pseudo first order reaction rate for chlorophenol conversion and a zero order reaction rate for the TOC conversion, the higher the substitution degree, the higher the kinetic rate constants. Indeed, the apparent kinetic constant for the 4-CP removal increased from 0.0042 min -1 for the LFO catalyst to 0.0096 min -1 and further to 0.0145 min -1 for the P25-LFO and SG-LFO catalysts, respectively, while the mineralization rate also increased with the substitution degree, with a rate constant of 0.0600 mg/L min and 0.0693 mg/L min for the P25-LFO and SG-LFO catalysts, respectively, vs. 0.0008 mg/L min for the pristine LFO catalyst.

Further, it must also be remarked that the Ti ® La partial substitution considerably increased the robustness of the LFO catalyst, with a strong decrease in the iron release from ca. 0.5 mg/L after 300 min of test on the pristine LFO catalyst, down to 0.03 mg/L (corresponding to 0.03% wt.) for the P25-LFO catalyst with a 5% substitution rate, and with a further reduction down to a non-detectable level for the SG-LFO catalyst with a 11% substitution rate (ie. below the 0.001 mg/L detection threshold, that corresponds to < 0.0009% wt.).

In the case of the titanium-free LFO catalyst, the apparent kinetic rate constants could not be ascribed to a pure heterogeneous process due to the low stability of the catalyst. The iron released to the solution reacts with the latest oxidation intermediates, like oxalic and formic acids, provoking the formation of iron-oxalate complexes [START_REF] Zazo | UV-LED assisted catalytic wet peroxide oxidation with a Fe(II)-Fe(III)/activated carbon catalyst[END_REF]. These complexes are slower photodegraded which led to an incomplete TOC conversion (Eq. ( 2)).

[Eq.2]

By contrast, reducing strongly the Fe leaching to the reaction media lowered the homogeneous photo-Fenton contribution to the H2O2 decomposition rate, and consequently to both chlorophenol and TOC removal rates. Therefore, the complete inhibition of Fe release for the SG-LFO catalyst evidenced that the strong increase in the catalytic activity under pure visible light observed for the SG-LFO catalyst could be ascribed to pure heterogeneous surface reactions, resulting from the physico-chemical properties developed by the catalyst.

The catalytic results showed that strongly higher kinetic rate constants were obtained under light in the presence of H2O2, compared to those obtained with pure photocatalysis, whereas both 4-CP removal and TOC conversion profiles were similar to that of the H2O2 decomposition. This suggested that the mineralization of the 4-CP substrate mainly occurred through the photoassisted catalytic H2O2 decomposition, that is proposed to produce the HOx° (HO° and HOO°) oxidative species via two different mechanisms.

First, at the surface of the La1-xTixFeO3 catalysts, the production of HOx° during the Fe III /Fe II redox cycle might be schematized as follows (Eqs. 3-4):

[Eq.3] [Eq.4]
In this mechanism, we propose that the electron-rich surface iron allowed a faster H2O2 decomposition by the Fe surface sites to be obtained, so that a higher amount of active radicals can be consequently produced at the surface of the La1-xTix FeO3 catalysts. Also, we assume that the electron-rich nature of the surface iron might favour a faster degradation of the short-chain acids, consequently providing a much faster carboxylic acid mineralization, together with the surface iron reduction to +2 state, known as the rate limiting step in the Fe III /Fe II redox cycle (Eq.5).

[Eq.5]

Further, the production of HO°x radicals was also proposed to occur through the photocatalytic decomposition of H2O2 that acts as charge acceptor and is consequently reduced or oxidized by the (e -/h + ) photogenerated charges (Eqs.6-8).

[Eq.6]

[Eq.7]

[Eq.8]

In addition to both mechanisms involving H2O2, the oxidizing HO° radicals can also be produced through the direct photocatalytic oxidation of adsorbed water by the photogenerated holes (Eq.9).

[Eq.9]

In the case of a photocatalytic mechanism, the enhanced availability of the photogenerated charge carriers in the Ti-substituted LFO catalysts compared to the LFO reference, evidenced by the longer lifetime of electrons for the doped samples measured by fluorescence, would lead to a more efficient production of HO°x radicals via both H2O2-and H2O-mediated photocatalytic pathways. The enhanced lifetime of charges was proposed to directly result from the structural and electronic changes induced by the partial substitution.

Finally, we propose that the formation of Fe II species might also result from the photoassisted reduction of Fe III (Eq.10) -more probably via the use a photogenerated electron from the conduction band of the ferrite -, since this was directly evidenced by the Casas's group at the surface of the FeTiO3 ilmenite under light in the absence of H2O2 [START_REF] García-Muñoz | Modified ilmenite as catalyst for CWPO-Photoassisted process under LED light[END_REF][START_REF] Silveira | UV-LED/ilmenite/persulfate for azo dye mineralization: The role of sulfate in the catalyst deactivation[END_REF]. The reduction might occur together with the oxidation of Ti 3+ to Ti 4+ as a plausible way of compensating the charge, so that Eq.10 might be also schematized as Eq.11. This is an alternative mechanism enabling the formation of Fe II species, consequently able to generate the HO° radicals via Eq.3. The Ti 4+ species might further be reduced back to Ti 3+ by a photogenerated electron from the conduction band.

[Eq.10]

[Eq.11]

The interest of using both La1-xTixFeO3 catalysts was further directly evidenced on the TOC conversion vs. H2O2 conversion curve shown in Fig. S5. The curve profiles in the high H2O2 conversion range confirmed that photocatalysis was clearly involved in the reaction process for allowing full TOC conversion to be obtained. As photocatalyst, La1-xTixFeO3 enabled the mineralization of the short chain acids, and consequently hindered the detrimental formation of stable Fe complexes. This effect was more pronounced on the SG-LFO catalyst, since the profile evidenced that higher mineralization degrees were achieved for lower H2O2 conversions.

Although the increase in surface area impacted on the amount of surface iron available for decomposing H2O2, with 28 m 2 /g vs. 8 m 2 /g for the P25-LFO vs. LFO, the specific surface area of the La1-xTixFeO3 catalyst cannot be considered as being the key factor driving the activity of the catalyst, since the most active SG-LFO catalyst displays a low surface area of 5 m 2 /g.

We propose therefore that a combination of surface and electronic modifications modulates the activity of the La1-xTixFeO3 catalysts under visible lightby influencing the activity in photo-CWPO and in photocatalysis -, and leads the SG-LFO catalyst to exhibit the best catalytic results. Both La1-xTixFeO3 catalysts exhibited a similar apparent rate constant for the H2O2 decomposition, so we can assume that the superiority of the SG-LFO catalyst over its P25-LFO counterpart in terms of 4-CP and TOC removal rates, may result from (i) its higher Ti ® La substitution rate -the resulting electronrich Fe surface sites favoring a much faster degradation of the Fe carboxylate complex intermediates (Eq.4) leading to a much faster carboxylic acid mineralization -and (ii) its superior photocatalytic activity, as evidenced in Fig. S3 in the absence of H2O2. Further, a higher amount of electron-rich Fe might also explain the enhanced 4-CP degradation obtained with SG-LFO in pure photocatalysis, the enhanced substitution degree allowing also the catalyst surface to be cleaned faster from the poisoning Fe carboxylate complex intermediates.

So, La1-xTixFeO3 takes great advantage of its ability to act as a dual catalyst, ie. to exhibit combined activity under pure visible light as heterogenized photo-Fenton catalyst and photocatalyst throughout the 4-CP mineralization test. First, it allowed faster 4-CP removal and TOC conversion rates to be obtained, while at the end of the test, it allowed full TOC conversion to be reached.

Stability and reusability of the La1-xTixFeO3 catalysts under visible light

To get insight into the catalyst stability, reusability tests with sequential cycles were performed for the most active catalyst. The catalytic results shown in Figure 7 for 5 reaction cycles evidenced the ability of the SG-LFO catalyst to be reused. Complete 4-CP removal was obtained, with no significant change in the apparent kinetic rate constant over 5 reuses, at 0.0145 ± 0.0004 min -1 , and only a slight decrease in the mineralization degree achieved after 210 min of test was observed after 5 consecutive runs (from 100% to 97%). Further, no iron release to water was detected during the reuse cycling which corroborates the high stability and robustness of the SG-LFO catalyst.

The results of the Rietveld refinements of the XRD patterns of the catalyst as a function of the test cycling evidenced the very good structural stability of the SG-LFO catalyst after the sequential photoreactions (Figure S6, Table S2 andTable 4). No change in the Ti ® La substitution rate within the LaFeO3 network, in the cationic distribution or in the mean crystallite size was observed. Further, no formation of new extra-phase such as TiO2 or La2O3 single oxides occurred during the test cycles, what is in good agreement with the unnoticeable leached iron from the materials during the course of the photoreactions. So, the SG-LFO catalyst exhibited further a better structural stability during the cycling procedure than its P25-LFO counterpart, for which the formation of rutile TiO2 as extra-phase in small amount (few percents) was accompanying the Fe leaching to the reaction media during the combined photo-assisted process [START_REF] Garcia-Muñoz | Ti-substituted LaFeO3 perovskite as photoassisted CWPO catalyst for water treatment[END_REF].

The high stability of the La1-xTixFeO3 catalysts -and more precisely of the SG-LFO catalystwas proposed to result from their higher photocatalytic activity, that allows the catalyst surface to be cleaned faster from the Fe carboxylate complex intermediates. Indeed, the formation at the surface of such refractory complexes between surface iron and carboxylic acids, that photodegrade with the release of the iron active phase, is known to be one of the main mechanisms involved in the iron leaching in AOP processes. The higher stability of the SG-LFO catalyst may also be attributed to the more homogeneous incorporation of the titanium atoms within the LFO network, as a result of a more favoured atomic diffusion within the amorphous LaFeO3 crystallites when using an amorphous source of titanium when compared to well-crystallized titania nanocrystals.

Catalytic activity of the La1-xTix FeO3 catalysts under solar light

To highlight further the viability of the La1-xTixFeO3 catalysts as a preliminary step to fields experiments, the photoassisted reactions were consequently performed also under simulated solar light (300-800 nm) with the pristine LFO and the most efficient Ti-substituted LFO catalyst, ie. SG-LFO, and the results are reported in Figure 8 and Table 5. Under solar light, the positive influence of the Ti ® La substitution on the catalytic performances was similar to that observed under pure visible light, whether the reaction was performed in the presence or absence of H2O2 (i.e. pure photocatalysis, Figure S7). Indeed, as far as pure photocatalysis is concerned, the SG-LFO outperformed as well the non-modified LFO reference.

Results showed in figure 8 indicated that neither a complete TOC conversion nor a complete 4-CP elimination were obtained with the pristine LFO catalyst, whereas the SG-LFO catalyst allowed full abatement of 4-CP to be achieved for 120 min of test, and full TOC removal for 150 min. Further, full decomposition of H2O2 was reached within 120 min with the SG-LFO catalyst vs. 180 min with the LFO reference catalyst. This corresponded to an increase in the apparent rate constants for H2O2, 4-CP and TOC conversion, the more pronounced effect being observed in the case of the 4-CP and TOC conversion, with more than two-and three-folds increase, respectively (Table 5).

In this case as well, the Fe release to the water was also completely inhibited thanks to the Ti ® La substitution within the LFO network, whereas a high Fe release of 1.4 mg/L was observed with the SGO reference material. Thus, the higher catalytic activity and the higher reaction rates observed under solar light for the SG-LFO catalyst did not involve any homogeneous photo-Fenton contribution, and resulted from pure heterogeneous surface reactions.

Table 6 enables the comparison of the catalyst activity under solar light (300-800 nm) and under pure visible light (l>420 nm), taking into account that the apparent rate constants under solar light result from the activation of the catalysts by both UV-A and visible light fractions. So, in a first approximation, the apparent rate constants under solar light can be considered as the sum of both corresponding rate constants. A higher apparent rate constant for 4-CP removal was obtained under solar light with the pristine LFO catalyst, while similar rate constants were noticed for the TOC conversion. By contrast, the SG-LFO catalyst led under solar light to higher apparent rate constants for both the 4-CP and the TOC conversion. As a result, the activity of the catalyst under visible light contributed approximatively for a half and a third to the overall apparent rate constant of 4-CP and TOC conversion obtained with the full solar spectra, respectively, so that the SG-LFO catalyst takes advantage of its ability to be active over an extended range of wavelength.

CONCLUSIONS

A new and highly robust, reusable, Ti-substituted La1-xTixFeO3 dual catalyst with combined photocatalytic and photoassisted CWPO activity under pure visible light has been synthesized. The influence of the nature of the titanium source upon both the cationic substitution degree and the photoactivity was evaluated, and, the higher the substitution degree, the higher the activity of the catalyst in the combined oxidation process. The La1-xTixFeO3 dual catalyst with a 11 at.% substitution degree obtained using a dried amorphous sol-gel precursor TiO2 as precursor outperformed its 5 at.% substituted counterpart obtained using crystallized nanocrystals as well as the non-modified LaFeO3 [a] the determination of the cationic distribution within the structures was performed considering that titanium can move towards both cationic sites. [a] the SG-LFO #1 #3 and #5 correspond to the SG-LFO catalyst after tests #1, #3 and #5, respectively.

  would have expected a substitution of Fe for Ti which has a closer radii value. Works on M-doped LaFeO3 (M = 4f, Mg, Na, Pb, Zn, Ti, Mn, Al, Pd) have been reported due to the great flexibility offered by the perovskite structure and the numerous possible substitution at both the A-and B-sites.

  reference catalyst. While the non-modified LaFeO3 catalyst could not reach neither a complete 4-CP removal nor a complete TOC conversion, complete 4-CP mineralization with enhanced H2O2 decomposition, 4-CP and TOC conversion rates was achieved using the 11 at.% Ti-substituted La1-xTixFeO3 catalyst. Further, the partial Ti àLa substitution in the LaFeO3 perovskite network led to pure heterogeneous surface reactions under pure visible light, with an increase in the catalyst robustness by more than two orders of magnitude compared to the unmodified LaFeO3 reference, visualized by the complete inhibition of the Fe release to water, the stability of the substituted structure and the stability of the catalytic performance over five test cycles. Finally, under simulated solar light, the dual La1-xTixFeO3 catalyst also displayed superior activity with a significative contribution issued from the visible part of the solar spectra. and the Sciences Offensive. Fernando Fresno thanks financial support from the Spanish Ministry of Science, Innovation and Universities (Project SOLPAC: ENE2017-89170-R, MCIU/AEI/FEDER, EU).

Figure 1 Figure 1

 11 Figure 1

Figure 2 Figure 2 .

 22 Figure 2

Figure 3 .

 3 Figure 3. XPS profiles recorded on both the Ti-free LFO and the SG-LFO catalysts, (A) wide scan survey

Figure 4 Figure 4 .

 44 Figure 4

Figure 5 Figure 5 .Figure 6 Figure 6 .Figure 7 Figure 7 .Figure 8 Figure 8 .

 55667788 Figure 5

Table 3 .

 3 Catalytic performances under pure visible light of pristine LFO and of La1-xTixFeO3 catalysts, expressed in terms of kinetic rate constants of H2O2 decomposition, 4-CP oxidation and TOC mineralization, Fe leached and TOC conversion.. Operating conditions: [4-CP]0 = 25 mg/L; [H2O2]0 = 125 mg/L; T=25°C; [cat] = 0.5 g/L; l > 420 nm. the H2O2 and 4-CP concentration evolution curves were fitted by a pseudo first-order kinetic as usually admitted, [b] the TOC concentration evolution curve was fitted with a zero-order kinetic model as usually admitted, [c] the detection limit of Fe leached to the solution has been measured at 0.001 ppm.

Table 5 .Table 6 .*

 56 Catalytic performances under solar light of LFO and SG-LFO, expressed in terms of kinetic rate constants of H2O2 decomposition, 4-CP oxidation and TOC mineralization, Fe leached and TOC conversion. Operating conditions: [4-CP]0 = 25 mg/L; [H2O2]0 = 125 mg/L; T=25°C; [cat] = 0.5 g/L; l =300-800 nm. the H2O2 and Cl-phenol concentration evolution curves were fitted by a pseudo first-order kinetic as usually admitted, [b] the TOC concentration evolution curve was fitted with a zero-order kinetic model as usually admitted, [c] the detection limit of Fe leached to the solution has been measured at 0.001 ppm at the end of each reaction. Approximate comparison of the apparent rate constants of 4-CP oxidation and TOC conversion obtained on the pristine LFO and the SG-LFO catalysts submitted to different irradiations. (min -1 ) r0 TOC (mg/L min) kCl-Phenol (min -1 ) r0 TOC (mg/L min) kCl-Phenol (min -1 )* r0 TOC (mg/L min)Data calculated from the experiments performed under solar light and pure visible light (obtained by filtrating the UV-A light fraction of the full solar spectra), considering in a first approximation that the apparent rate constants under solar light can be calculated as the sum of individual rate constants obtained under UV-A light (l<420 nm) and visible light (l>420 nm).

Table 1 .

 1 Main physico-chemical properties of the LaFeO3 and Ti-substituted LaFeO3 catalysts.

	Catalyst	Ti precursor	Fe (at. %)	Ti (at. %)	Ti/Fe at. ratio	dp (nm)	SBET (m 2 /g)
	LFO	-	23.0	-	-	46(1)	8
	P25-LFO	Crystallized Aeroxide© TiO2 P25	16.0	3.2	0.23	21(1)	28
	SG-LFO	Laboratory-made amorphous titania	22.7	4.0	0.21	20(1)	5

Table 2 .

 2 Refined cationic distribution in the La1-xTixFeO3 catalysts with the crystallographic cell parameters and the weight percentage of extra phases obtained after Rietveld refinement.

	Catalyst	a(±0

.001) (Å) b(±0.001) (Å) c(±0.001) (Å) La site distribution a Fe site distribution a a-Fe 2O3 (wt.%) TiO2 (wt.%)

					La	Ti	Fe
	LFO	5.547	5.560	7.861	1.00	0	1.00	11	0
	P25-LFO	5.525	5.553	7.860	0.95	0.05	1.00	18	0
	SG-LFO	5.528	5.561	7.856	0.89	0.11	1.00	15	0

Table 4

 4 Refined cationic distribution in the SG-LFO#1 SG-LFO#3 and SG-LFO#5 catalysts corresponding to the SG-LFO catalyst after 1 st , 3 rd and 5 th test, with the crystallographic cell parameters and the weight percentage of extra phases obtained after Rietveld refinement

	Catalyst	a(±0.001) (Å)	b(±0.001) (Å) c(±0.001) (Å)	La site distribution a	Fe site distribution a	a-Fe 2O3 (wt.%) TiO2 (wt.%)
					La	Ti	Fe
	SG-LFO	5.528	5.561	7.856	0.89	0.11	1	18	0
	SG-LFO#1 a	5.526	5.554	7.854	0.91	0.09	1	12	0
	SG-LFO#3 a	5.524	5.554	7.853	0.91	0.09	1	11	0
	SG-LFO#5 a	5.527	5.551	7.853	0.92	0.08	1	12	0

Acknowledgements.

The authors want to thank the European Fund for regional development (EFRE/FEDER) for the financial support of the PHOTOPUR project which is performed within the framework of Interreg V