Elucidating the Photoredox Nature of Isolated Iron Active Sites on

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Elucidating the Photoredox Nature of Isolated Iron Active Sites on MCM-41 Laura Collado, Ingrid Jansson, Ana E. Platero-Prats, Virginia Pérez-Dieste, Carlos Escudero, Elies Molins, Lluís Casas i Duocastella, Benigno Sanchez, Juan M. Coronado, David P. Serrano, Silvia Suárez, and Víctor Antonio Antonio de la Peña O'Shea ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03208 • Publication Date (Web): 17 Jan 2017 Downloaded from http://pubs.acs.org on January 17, 2017

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Elucidating the Photoredox Nature of Isolated Iron Active Sites on MCM-41 Laura Collado†, §, Ingrid Janssonψ, Ana E. Platero-Prats‡, Virginia Perez-Dieste#, Carlos Escudero#, Elies Molins⊥, Lluis Casas i Doucastela□, Benigno Sánchez ψ, Juan M. Coronado§, David P. Serrano§, ║, Silvia Suarez ψ, Victor A. de la Peña-O’Shea†* †

Photoactivated Processes Unit, Institute IMDEA Energy, Avda. Ramón de la Sagra 3, 28935 Móstoles, Spain. Thermochemical Processes Unit, Institute IMDEA Energy, Avda. Ramón de la Sagra 3, 28935 Móstoles, Spain. ψ Photocatalytic Treatment of Pollutants in Air FOTOAIR-CIEMAT, Avenida Complutense, 22, 28040 Madrid, Spain. ‡ # X-ray Science Division, Advanced Photon Source, Argonne National Laboratory, 60439 Argonne, USA. ALBA Syn⊥ chrotron Light Source, carretera BP 1413 Km. 3.3, 08290 Cerdanyola del Vallès, Spain. Institute of Materials Science of Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain. □Departament of Geology, UAB, 08193 Bellaterra, Spain. ║ Department of Chemical and Environmental Engineering Group, URJC, C/ Tulipán, 28933 Móstoles, Spain. §

ABSTRACT: Photocatalytic performance is highly dependent on the nature and dispersion of the active sites, playing a crucial role in the optoelectronic and charge-transfer processes. Here, we report stabilized isolated iron on MCM-41 as highly active catalysts for a photoredox reaction. The unique nature of the single-atom centers exhibit a trichloroethylene (TCE) conversion per iron site that is almost 5 times higher than that of TiO2. Advanced characterization and theoretical calculations indicate the generation of hydroxyl radicals (OH•), through a photo-induced ligand-to-metal charge-transfer (LMCT) mechanism, which act as hole scavengers that lead to the formation of intermediate oxo-iron species (Fe=O). This intermediate specie is the key step to promote the photocatalytic reactions. Understanding the mechanistic photoredox pathway in isolated active site materials is imperative for developing highly efficient non-precious photocatalysts for environmental or energy applications. KEYWORDS: Isolated active sites materials, photoredox mechanism, charge transfer processes, structure-optoelectronic relationship, Density Functional Theory. Photocatalytic solar-energy conversion for energetic and environmental applications is one of the most important challenges of this century.1,2 The design and development of efficient, robust and low cost light harvesting materials is crucial for the achievement of industrially competitive photocatalysts. Semiconductors (such as oxides, quantum dots, perovskites) have been traditionally employed as photocatalysts due to their unique optoelectronic and catalytic properties.1-3 The major drawbacks for the ultimate application of these materials are their low quantum yields due to the high recombination rates, charge transport/transfer limitations, inadequate bandgaps, photocorrosion, as well as the use of expensive or harmful elements. A promising alternative is the development of low-cost isolated active sites (IAS) photocatalysts based on transition metal chromophores doping ordered silicon-based open-frameworks, such as silicates, zeolites and mesoporous ordered structures.4-9 The ability of IAS to induce photoredox reactions is attributed to the photogeneration, via ligand-to-metal charge-transfer (LMCT) excitations, of unusually longlived localized charges.10-13 The adequate selection of the IAS metal cation results in the development of lightharvesting materials in a wide range of the solar spectrum, which in turn allows to tune the redox potential of

the catalyst towards different target molecules and processes. Among the wide range of transition doping metals, iron is an interesting element due to its reversible redox behavior under illumination.10,14-17 Besides, it represents a cheaper and safer alternative for a variety of processes, such as H2 production as an energy vector16 and photooxidation of Volatile Organic Compounds (VOCs)16,17 that are harmful to the environment and the human health. Nonetheless, the structural and optoelectronic nature of the IAS, in both fundamental and excited states, still remains unclear as it does its role in the photocatalytic mechanism together with the reaction intermediates. Recent advances in the mechanistic study of water decomposition, using metal oxides, enabled the structural identification of oxo-species as reaction intermediates.18,19 These findings open up further discussions on the specific charge transfer processes that result from the activation of those IAS. However, there are still unresolved questions concerning the identification of some other intermediates and their corresponding reaction pathways, which still need to be better understood. In this work, we report for the first time a whole draw of the mechanism that converts Fe tetrahedral single sites, incorporated into an insulating nanostructured MCM-41, into a very efficient photoredox catalyst. Ad-

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vanced structural and optoelectronic characterization techniques were combined with theoretical calculations to truly understand the reaction pathways and intermediates involved in the photoxidation of trichloroethylene (TCE), one of the most abundant VOCs, when using isolated Fe active sites. Methods Synthesis of samples and iron doping degree. Isolated and Fe-doped MCM-41 materials were prepared by a hydrothermal method. Doped samples were labeled as FeX (where X is Si/Fe = 100 and 20 molar ratio). Iron oxide nanoparticles were deposited over MCM-41 by an incipient wetness impregnation procedure, according to a Si/Fe=20 molar ratio (see Supporting Information, Section 1). Iron content was determined by Inductive coupled Plasma-Optical Emission spectroscopy (ICP-OES). Mössbauer spectroscopy. The chemical state and local environment of the Fe species were evaluated using a transmission Mössbauer spectrometer, operating in constant acceleration mode, with a 57Co source in an Rh matrix. Further details are provided in Supporting Information, Section 2. Theoretical calculations and modeling. Density Functional Theory (DFT) calculations were performed to determine the energetic and structural properties. In addition TD-DFT was also employed to calculate the optoelectronic transitions (Methodology details are provided in the Supporting Information). The local structure of the single-sites was investigated by Pair Distribution Function (PDF) analyses. Samples were collected using a BRUKER D8 Advance diffractometer with a X-rays source of Ag (λ = 0.56083 Å). Differential PDF of the Fecontaining systems were obtained by subtracting the contribution of the bare MCM-41 to the Fe20 sample. Near Edge X-ray Absorption Fine Structure (NEXAFS). NEXAFS spectra were recorded at the CIRCE beamline of ALBA Synchrotron Light Source under UHV in total electron yield mode. The beamline resolving power is E/ΔE=8000 (ie. ΔE=83 meV for the Fe L-edge). NEXAFS experiments under illumination were conducted using a UV-LED (Hamamatsu Co.) source with a maximum emission wavelength centered at 365 nm. Photocatalytic activity. The photocatalytic oxidation of gas-phase trichloroethylene was studied in a continuous plug flow flat photoreactor. A gas mixture of TCE (25 ppm) and air was prepared using a gas cylinder of TCE/N2 (TCE concentration of 25 ppm) and compressed air free of water and CO2. The total gas flow was varied between 300 and 700 ml/min (residence time = 1.29-0.75 s). Samples were irradiated using two UVA Philips TL-8W/05 fluorescent lamps (365 nm, 11.5 mW/cm2). The gas-phase composition was continuously monitored using a FTIR ThermoNicolet 5700 spectrometer maintained at 110 ºC. The temporal evolution of TCE and reaction products was obtained by integrating the area under the characteristic IR band of each component. Commercial TiO2 (G-5 Millenium) was selected as a reference (for details see Supporting Information, Section 5).

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Results and discussion Iron-doped samples were prepared by optimizing the synthesis conditions in order to maximize the amount of iron incorporated into the MCM-41 framework (see Supporting Information, Section 1). All samples show mesoporous structure with a decrease in the surface area and pore size upon iron incorporation (Supporting Information, Figures S1A & S1B). High resolution Electron Microscopy (HR-TEM) confirms the presence of uniform and oriented hexagonal arrangement of mesopores in all materials. The increase of Fe content leads to disordered regions that result in additional porosity (Figure 1A). Lowangle XRD studies reveal the characteristic lattice reflections of the 2D-hexagonal mesophase with a partial loss of mesoporous structure and a concomitant expansion of the unit cell in Fe samples14,20,21 (Supporting Information, Figure S2 & Table S1). Due to the nano-nature and complexity of these materials, a combination of Mössbauer spectroscopy, theoretical calculations and PDF analysis using X-ray scattering data was used to provide detailed insights into the local structure of the single sites. Mössbauer spectroscopy data of Fe-doped MCM-41 samples show the hyperfine parameters characteristic of high spin tetrahedrally coordinated Fe3+ ions (Figure 1B). The isomer shifts (IS) (lower than 0.3 mm/s) and very low quadrupolar values (SI, Table S2) suggest a homogenous distribution of these tetrahedral Fe species. These species are coordinated to oxygen and/or hydroxyl groups.10,14,21 In contrast, the analysis of the supported Fe/MCM-41 sample reveals the formation of α-Fe2O3 nanoparticles.17,21 Active site environment. Theoretical calculations were performed considering a unit cell previously described by Ugliengo et al.,23 which is composed of 578 atoms (Fe1Si141O355H102) and covers three possible sets of surface site environments (see methodology in Supporting Information, Section S3). The iron-doped structures are less stable than the bare MCM-41 (Figure 1C). Both Band C-sites are the most stable configurations, showing similar energies. This suggests that the substitution of silanol positions by iron is energetically favored and, therefore, are the most accessible catalytically active sites. The Pair Distribution Functions of both bare and FeMCM-41 samples are dominated by atom-atom distances involving Si atoms. The main PDF peak observed for MCM-41 at ~1.63 Å corresponds to the Si-O bonding (Supporting Information, Figure S4 & S5). This value agrees with our and other theoretical calculations22,25 and PDF26,27 analyses (Supporting Information, Figure S4). Differential PDF of Fe-20 sample (Figure 1D), calculated by subtracting the PDF of Fe-20 to that of bare MCM-41, clearly shows new peaks associated with Fe-O distances of ~1.79 Å and Fe…Si distances of ~3.19 Å (Supporting Information, Figure S5). No evidence of the presence of iron aggregates was observed, proving the single nature of the iron sites. Additionally, PDF data did not show the appearance of new distances corresponding to Fe—Fe dimers; consistent with the single-site nature of the iron sites (see SI, Figure S5 and Figure 1D inset).

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Figure 1. (A) High Resolution TEM images; (B) Mössbauer spectra for iron doped and supported samples; (C) Calculated energies (∆E) for MCM-41 and Fe-MCM41 models; (D) Experimental PDF analyses for MCM-14 (black line) and Fe-20 (red line) and differential PDF (blue dashed line). Fe A-site model including bond distances is also depicted on the righthand side (Fe (orange) Si (blue) O (red) H (white)). Electronic versus local structure. Our studies confirm that the incorporation of Fe into an insulator matrix leads to significant changes in the electronic structure, which are intriguingly linked to the geometrical conformation and environment of the active site. Stereochemical analyses evidence that the local structures of the iron sites suffer a significant distortion from tetrahedral S(T) to square planar S(SP), following the minimal distortion interconversion pathway (Figure 2A). In bare MCM-41, Si sites (circles) exhibit a tetrahedral geometry with a small distortion.28,29 In contrast, Fe centers (square) exhibit intermediate distorted geometries with a deviation of 1.5% of the S(T)-S(SP) planarization pathway. This effect is particularly noticeable in the case of the C-site, which presents a higher planarization degree.

degree of distortion and surrounding moieties of the iron sites. As expected, MCM-41 shows an insulator behavior (Supporting Information, Figure S6), with a Valence Band (VB) composed of O 2p states and a Conduction Band (CB) formed by Si 2p. On the other hand, Fe-doped materials possess mid-gap states mainly formed by Fe 3d and O 2p hybridized orbitals, whose energy levels are distributed in a wide range of energies above and below the EF (known as band tail).32 A detailed analysis of the mid-gap states revealed the crucial role of the Fe local distortion and the surroundings in the electronic behavior. While the A−sites exhibit a metallic character without energy gap, both B- and C−sites have energy gaps that increase with the Fe distortion, due to an additional loss of degeneracy of the t2 orbitals (Figure 2B-D).

Density of states (DOS) reveals that the geometry distortion of Fe sites, tending towards square planar, stabilizes the orbitals with higher occupation while destabilizes those with lower occupation, thus breaking the degeneracy of the tetrahedral geometry24,30,31 (Figure 2B-C). The changes in the electronic structure also depend on the

The formation of hybridized orbitals indicates the presence of iono-covalent Fe-O bonding.24,25 This is also prevalent in the charge density plots (Figure 2E and Supporting Information, Figure S7A-C), where Si-O bonds are purely ionic whereas iron centers share charge with surrounding oxygen atoms.

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Figure 2. (A) Calculated distortion of shape tetrahedron-square plane pathways and polyhedral interconversion coordinate for MCM-14 (circle) and Fe-doped sites (square); Total DOS (black) and atom-projected (PDOS) for Fe (orange), total O (red line), O neighbouring to Fe (blue dash) and EF (black dash) of: (B) Fe-A, (C) Fe-B and (D) Fe-C sites; Schematic representation of the (E) isosurfaces of a section of charge density and (F) ELF for selected B-Fe active site. Atoms color: Fe (orange) Si (blue) O (red) H (white). Vertical dashed line correspond to the Fermi level. Electron localization function (ELF) (Figure 2F and SI Figure S7A-C) show an evident polarization in O atoms near the Fe sites, being higher for OH groups. This effect is not observed in Si-O bonds, even considering their localized character as result of the Hubbard correction.33 Optoelectronic. Our studies reveal that the Fe IAS play a crucial role in light mediated charge-transfer processes.10,14,34 Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis/DRS) studies performed on the doped samples show a broad absorption band between 250-450

nm (Figure 3A and SI, Figure S7). Time Dependent Density Functional Theory (TD-DFT) calculations indicate that the electronic transitions in the range 350-450 nm, characteristic of photocatalytic experiments, are associated with ligand-to-metal charge-transfer (LMCT) excitations from oxygen to iron atoms7,8,35 (Figure 3B). On the other hand, the lower energy transition corresponds to the infrared absorption and is composed of d-d transitions between mid-gap states (Figure 3B, TableS3).

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Figure 3. (A) UV–Vis spectra of Fe-20 and TD-DFT (6-31G**) calculated excitation energies (vertical bars) for A-Fe, B-Fe and C-Fe sites.; (B) Fe L-edge NEXAFS spectra of Fe-20 in the dark (left) and under UV illumination (right) and (C) selected molecular orbitals contributing to Fe d-d (visible) and LMCT (UV) transitions for C-Fe site. Atoms color: Fe (orange) Si (blue) O (red) H (white). Due to the insulator nature of SiO2, MCM-41 has a weak absorption below 250 nm (not shown), consistent with TD-DFT calculations (Supporting Information, Table S3). Fe/MCM-41 shows a broad band at 350-650 nm assigned to octahedral Fe d-d transitions within Fe2O3 clusters (Supporting Information, Figure S8).36,37 Near Edge X-Ray Absorption Fine Structure (NEXAFS) confirms the photoreduction of iron by charge-transfer (CT) processes. Under dark conditions, the Fe LIII-edge of Fe-20 sample is dominated by dipole allowed 2p-3d transitions, as evidenced by the broad band centered at ~711 eV with a shoulder at ~710 eV. These contributions are associated with t2 and e orbitals of high-spin Fe3+ species in a tetrahedral coordination with a crystal field splitting of ~0.9 eV.38-41 Under UV illumination (Figure 3B), the broadening of the bands suggests the existence of a new contribution at lower energy (708.3 eV), assigned to the presence of iron species with higher electron density (Feδ+). This suggests the photo-reduction of Fe3+ species through LMCT processes.41-42 Photocatalysis and proposed Mechanism. Photooxidation experiments show that Fe-doped samples show a remarkable trichloroethylene (TCE) conversion of ~90%

(Figure 4A). It is highly that Fe-100 sample shows a conversion rate per iron sites43 4.7 times higher than TiO2 (used as reference), demonstrating the efficiency of isolated iron as photo catalytically active sites. MCM-41 and Fe/MCM-41 samples show a negligible TCE conversion (< 3%), proving that both bulk SiO2 and Fe3+ octahedral species are inactive for this reaction. Reaction yields are higher towards intermediate oxidation products such as dichloroacetylchloride (DCAC), phosgene (COCl2) and traces of CO (Supporting Information, equations 1-5).44-46 This behavior changes for higher residence times and iron loadings (Figure 4B). The improved photocatalytic activity of the single active Fe-doped systems needs to be understood by considering a combination of textural, structural and optoelectronic aspects that complete the mechanism puzzle. The first step to consider is the reagent adsorption. Taking into account the high surface area of these materials, a significant high uptake of both TCE and reaction products is expected. However, IR data show that the TCE adsorption of Fe−doped samples is 4.8 times higher than bare MCM-41.

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Figure 4. Photocatalytic oxidation results for MCM-41 based materials and a reference TiO2 catalyst at different flow rates: (A) TCE conversion and (B) product yield of: CO2 (dash), COCl2 (square) and DCAC (solid); (C) Simulated adsorption isotherm of TCE in a Fe-C site and space-filling of TCE adsorption at selected pressures (inset); (D) Space-filling lateral views of TCE interaction with Fe active sites (right) and (E) ball-and-stick local view of the interaction between TCE and hydroxyl groups of a Fe active site (left). Dashed blue lines indicate H-bonds. Atoms color: Fe (blue) Si (yellow) O (red) H (white), Cl (green). This result indicates a higher interaction of TCE with iron active sites that favors the oxidation processes. This observation was clarified using simulated TCE isotherms for MCM-41 in the absence and presence of Fe (Figure 4C and Supporting Information, Figure S10A-B). These studies show that, although both systems can uptake similar amounts of TCE, the adsorption energy is 2-fold higher for the doped catalyst, corroborating the strong interaction with Fe sites. The nature of this interaction is explained by the formation of H-bonding between Cl and OH groups, which are favored in the case OHFe neighbors (Figure 4D-E and Supporting Information, Figure S10C-E). DFT calculations show that the presence of hydroxyl groups bonded to iron stabilizes the doped structures, thus affording materials with more accessible active sites that facilitate the TCE approach and the subsequent photo-oxidation. In addition, the presence of Fe increases the Lewis acidity of the MCM-41, which is higher when coordinated to OH groups acting as Brönsted basic centers and enabling the formation of HCl during the reaction.

states that ultimate lead to the critical step of intermediates formation, which are not clearly determined. Our TD-DFT calculations suggest that light absorption induces a LMCT that enables the photo−generation of localized charges within the Fe site, as well as the formation of hydroxyl radicals (OH•)(Figure 3 & Figure 5A). The hole injection converts the Fe-OH• into a Fe=O (1.612 Å) transition state (TS). On the other hand, the proton transfer leads to the production of HCl and the subsequent formation of a photogenerated oxo-radical Fe-O• (Figure 5A, C-site TS*). Although these species have not been reported for isolated sites, the critical role of these oxointermediates in the first steps of the photoredox reaction has been reported in recent water oxidation studies.18,19 The geometry of the C-site TS is near to tetrahedral, leading to changes in PDOS with a stabilization of t2 orbitals and the appearance of additional mid-band gap states (Figure 5B1). In addition, charge density is higher around oxyl-species and ELF plots exhibit a polarized electronic distribution around this oxygen.

TCE adsorption is followed by competitive multielectronic redox reactions, both in fundamental and excited

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Figure 5. (A) Energy calculations of proposed reaction pathways for Fe-MCM41 C-site and schematic representation of charge density isosurfaces (inset) and excited states (*) for selected intermediates calculated by TD-DFT (6-31G**); (B) Total DOS (black) and atom-projected DOS (PDOS) for (B1) C-site (TS), (B2) C-site(OU) and (B3) superoxide formation, where Fe (orange line), total O (red line), O neighboring to Fe (blue dash), and O form superoxide specie (black filled) vertical dashed line correspond to Fermi level. Electron localization function for same reaction centers are also depicted (C1-C3). Atoms color: Fe (orange) Si (blue) O (red) H (white). As a consequence of the TCE photoxidation (Supporting Information, equations 2-3) by a Fe=O intermediate, an open unsaturated (OU) Fe−site with trigonal planar structure is formed (Figure 5A). In addition, the loss of OH groups leads to the appearance of electronic holes (Figure 5B2), as well as the generation of additional midgap states close to the Fermi level. This also leads to the formation of an iono-covalent bond with a frustrated Lewis pair localized in the iron atom47 (Figure 5C2). Other authors18 described an intermediate step involving the spontaneous nucleophilic addition of water to oxo-sites (in dark conditions) to form hydroperoxide intermediates, which create radical Fe-OOH• species that act as photoxidation sites (Figure 5A). However, our studies show that this intermediate species is energetically unfavorable (Figure 5A and SI, Figure S10A). In subsequent steps, the generated C-sites (OU) can either continue reacting with TCE or intermediates to yield further oxidized products, or react with O2 and/or H2O getting re-oxidized and closing the redox cycle. DFT calculations show that the reaction with O2 is the most energetically favored event (Figure 5A), forming superoxide

intermediates (O-O bond length 1.31 Å) in which iron is 5coordinated. The local structural rearrangements on the C-site (OU) involve an orbital degeneracy loss that gives overlapped O 2p and Fe 3d bands and the disappearance of the hole associated with the O vacancy in the midband gap states (Figure 5B3-C3). On the other hand, the reaction with water leads to a nearly tetrahedral arrangement, with H2O located over the unsaturated iron (Figure 5A & Supporting Information, Figure S10B). PDOS analyses show that the hybridization between O 2p (H2O) and Fe 3d is lower than that with O2 (Figure 5B2 & B3). Finally, TCE interaction with the C-Site (OU) is very low and PDOS indicates that the hole is still present and the 2p C or Cl orbitals and Fe 3d overlapping is negligible (Supporting Information, Figure S10C). These findings point out that the high DCAC selectivity in Fe-MCM-41 samples may be associated with the limited number of surface active sites and the localized nature of the redox active sites, which hinder the multielectronic processes. This behavior was also evidenced by Zhang et al.,18 who showed that the absence of adjacent M-OH active sites decrease the photoxidation rate.

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Conclusion

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ACKNOWLEDGMENTS

As a result of both experiments and theoretical calculations, it is clear that isolated tetrahedral iron sites have unique textural, structural and optoelectronic properties and excellent stability, which result in TCE photoxidation rates that are five times higher than that of the common semiconductor (i.e. TiO2). This is the first study that establishes a photo-oxidation mechanism for isolated active sites, including intermediates and excited states. This reaction mechanism is based on the formation of highly reactive hydroxyl radicals that are coordinated to Fe. The first step of the photo-oxidation reaction is through LMCT processes, and the subsequent formation of an intermediate Fe=O state through hole injection. Afterwards, unsaturated Fe−sites are re-oxidized by O2 forming superoxide species. The existence of this intermediate step and the low metal density explain the high yield of intermediate oxidation products. These results provide a better understanding of the reactivity and the charge transfer processes in single sitebased photocatalysts and intermediate species. This information may be critical to develop a more rational design of these materials. We hope that the findings presented here would open new possibilities in the use of these low-cost and environmentally friendly photocatalysts in other light-mediated reactions such as CO2 reduction, water splitting or remediation of air and/or water contaminants.

ASSOCIATED CONTENT Supporting Information. Supporting Information accompanying this paper includes more information about the synthesis of the samples, textural properties, X-ray diffraction including PDF, Mössbauer spectroscopy, modeling (density of states, TD-DFT adsorption calculation) and photocatalytic activity. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected]

Author Contributions L.C., I.J., S.S and V.A.P.O. conceived and designed the experiments. L.C. performed the synthesis and characterization of the samples, D.S. participated in the data analysis, I.J., J.M.C and B.S. carried out the photocatalytic experiments, designed and supervised by S.S. A.E.P.P performed the PDF analysis and participated in the active site environment and mechanism sections. V.P.D and C.E. made the NEXAFS measurements. E.M. and L.C.D. conducted the Mössbabuer spectroscopy. V.A.P.O. made the theoretical calculations and mechanistic studies. L.C. and V.A.P.O. wrote the paper. All authors discussed the results and commented on the manuscript.

Notes The authors declare no competing financial interest.

The authors would like to thank Dr. J. Conesa from Catalysis and Petrochemistry Institute and Dr. J. Cirera from Barcelona University for the valuable discussions about the electronic and structural distortion studies, respectively. L.C. thanks funding from the FPI grant (BES-2010-032400). I.J, B.S and S.S acknowledge financial support from the Spanish Ministry of Economy and Competitiveness through the project (CTM2011-25093) and V.A.P.O to SolarFuel (ENE2014-55071JIN). A.E.P.P. acknowledges a Beatriu de Pinos fellowship (BP-DGR-2014) from the Agency for Management of University and Research Grants (Government of Catalonia). V.A.P.O. acknowledges support from the Centre of Supercomputacio de Catalunya (CESCA) ant to ALBA Cells Synchrotron facilities. This work, developed under the HyMAP project, has received funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation programme (grant agreement No. 648319). The results reflect only the authors’ view and the Agency is not responsible for any use that may be made of the information they contain.

ABBREVIATIONS TCE, Trichloroethylene; LMCT, Ligand-to-Metal Charge-Transfer; VOCs, Volatile Organic Compounds; IAS, Isolated Active Sites; ICP-OES, Inductive Coupled Plasma-Optical Emission Spectroscopy; PDF, Pair Distribution Function; DFT, Density Functional Theory; HRTEM, High Resolution Electron Microscopy; IS, Isomer Shifts; DOS, Density of States; VB, Valence Band; CB, Conduction Band; EF, Fermi Level; ELF, Electron Localization Function; UV-Vis/DRS, Ultraviolet-Visible Diffuse Reflectance Spectroscopy; TD-DFT, Time Dependent Density Functional Theory; DCAC, Dichloroacetylchloride; COCl2, Phosgene; TS, Transition State; OU, Open Unsaturated.

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