The electrocatalytic water decomposition has been investigated in this thesis by means of its two half standard reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These reactions occur in different locations in a typical electrochemical cell: the anode and the cathode, respectively. Motivated by the lack of understanding about the reaction mechanisms occurring at the anodes and cathodes, we have proposed first: novel representations of typical TiO2 surfaces, based on small cluster systems, which can be used for a quick and more detailed assessment of the OER activities at modified TiO2 surfaces, and secondly we investigated the HER in two sets of model surfaces which represent recently synthesized materials, based on Mo2C and graphene with promising activities toward the HER. We have employed Density Functional Theory (DFT) based methods within both localized and extended basis sets, as implemented in GAMESS and VASP packages, respectively, to examine the structural, electronic and vibrational properties of the proposed models. We propose new reaction mechanisms for the OER on a number of molecular representations of TiO2 electrodes. For each reaction pathway, the free energy profile is computed, at different biases, from the DFT energies, the entropic and the zero-point energy contributions. The mechanisms explored in this thesis are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on molecular representations of the TiO2 surfaces. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. We subsequently investigate the influence of doping TiO2 surfaces with transition metals (TMs) on the performance of TiO2 -based electrodes for the water splitting electrochemical reaction. Two cluster models of the TM-doped active sites which resemble both the TiO2 anatase (001) and rutile (110) surfaces, respectively, are considered for the evaluation of the water decomposition reaction when a Ti is replaced by a TM atom. A set of TMs spanning from Vanadium to Nickel is considered. The late TMs explored here: Fe, Co and Ni are found to reproduce the observed experimental trends for the overpotentials in TiO2-doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. The vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO2-based cluster models. The last part of this work is devoted to the theoretical study of the HER on recently found materials based on the synergistic combination of molybdenum carbide and graphene layers. We propose two major structural models to describe the HER mechanism within the framework of DFT: Mo2C-based clusters adsorbed on carbon nanosheets and the Mo2C (001) surface covered by pure and nitrogen-doped graphene layers. The former system evaluates the influence of Mo2C nanoparticles adsorbed on carbon nanosheets towards the HER. The second one is employed to gain insight about the high HER activity observed in molybdenum carbide anchored on nitrogen-doped porous carbon nanosheets (Mo2C@2D-NPC), recently synthesized. The H-adsorption free energy has been used as a principal descriptor to asses the HER activity at the proposed model active sites. It resembles the value for the best state of the art catalyst for the HER (i.e., platinum at carbon substrate Pt@C) in some of the proposed structural models. Furthermore, a pH-correction is added within a simplified model, to the H-adsorption free energy barrier in every proposed structure. The pH dependence of the H-adsorption free energy barriers allows the assessment of the HER at acidic and alkaline conditions simultaneously. An overall agreement with experimental results is found and further predictions, promoting the development of better HER catalysts, have been done.

No description available.

[ES] La presente tesis doctoral se ha basado en el estudio del uso de MOFs como fotocatalizadores para la producción de combustibles solares. Específicamente, los fotocatalizadores basados en MOF se han utilizado para la reacción de descomposición del agua y la reducción de CO2 en ausencia de agentes de sacrificio o disolventes orgánicos. MIL-125(Ti)-NH2 se puede utilizar como fotocatalizador para la reacción de descomposición del agua en presencia de UV-Vis o irradiación natural de la luz solar. La actividad de este material se puede potenciar mediante el uso de Pt y NPs de RuOx como co-catalizadores. Además, la actividad fotocatalítica de MIL 125(Ti)-NH2 se puede mejorar mediante un tratamiento de plasma de oxígeno que introduce defectos estructurales dando lugar a un material optimizado para catalizar la reacción de descomposición del agua. La presente tesis ha mostrado por primera vez la posibilidad de utilizar MOFs como fotocatalizadores para la metanación de CO2. En particular, un Zn-MOF y un Ti-MOF, MOF(Zn)-1 y MIP 208 respectivamente, se pueden utilizar como fotocatalizadores para promover la metanación de CO2 en condiciones de reacción suaves. Además, la actividad fotocatalítica de estos MOFs se incrementa en presencia de pequeñas NPs de Cu2O y, especialmente, por NPs de RuOx en la estructura de estos materiales. Es de destacar que el material compuesto por NPs de RuOx soportadas en MIL-125(Ti)-NH2 puede considerarse un fotocatalizador de referencia para la metanación de CO2 mediada por energía solar y en flujo continuo. / [CA] La present tesi doctoral s'ha basat en l'estudi de l'ús de MOFs com a fotocatalitzadors per a la producció de combustibles solars. Específicament, els fotocatalitzadors basats en MOF s'han utilitzat per a la reacció de descomposició de l'aigua i la reducció de CO¿ en absència d'agents de sacrifici o dissolvents orgànics. MIL-125(Ti)-NH2 es pot utilitzar com fotocatalitzador per a la reacció de descomposició de l'aigua sota UV-Vis o irradiació natural de la llum solar i la seua activitat pot ser augmentada mitjançant l'ús de Pt i NPs de RuOx com co catalitzadors. A més, l'activitat fotocatalítica de MIL-125(Ti)-NH2 es pot millorar mitjançant un tractament de plasma d'oxigen que introdueix defectes estructurals resultant en un material optimitzat per a la reacció de descomposició de l'aigua. La present tesi ha mostrat per primera vegada la possibilitat d'utilitzar MOFs com fotocatalizador per a la metanació de CO¿. En particular, un Zn-MOF y un Ti MOF, MOF(Zn)-1 y MIP-208 respectivament, es poden utilitzar com fotocatalitzadors per a promoure la metanació de CO¿ en condicions de reacció suaus. A més, l'activitat fotocatalítica d'aquests MOFs pot ser realçada per la presència de xicotetes NPs de Cu2O i, especialment, per les NPs de RuOx en l'estructura d'aquestos materials. És de destacar que el material composat per NPs de RuOx suportades en MIL-125(Ti)-NH2 es pot considerar un fotocatalitzador de referència per a la metanació de CO2 amb energia solar i en flux continu. / [EN] The present doctoral thesis studied the use of MOFs as photocatalysts to produce solar fuels. MOF-based photocatalysts were used for overall water splitting and CO2 reduction in the absence of sacrificial agents or organic solvents. MIL 125(Ti)-NH2 can be used as photocatalyst for overall water splitting under both UV-Vis or natural sunlight irradiation. The activity of this material can be enhanced using Pt and RuOx NPs as co-catalysts. Also, the photocatalytic activity of pristine MIL 125(Ti)-NH2 can be enhanced by oxygen-plasma treatment, which introduces structural defects and produces an optimized material for overall water splitting. This thesis has shown for the first time the possibility of using MOFs as photocatalysts for CO2 methanation. More specifically, a Zn and Ti MOF materials, MOF(Zn)-1 and MIP-208 respectively, can be used as photocatalysts to promote CO2 methanation under mild reaction conditions. The photocatalytic activity of these MOFs can be enhanced in the presence of small Cu2O NPs, and, especially, RuOx NPs in their structure. RuOx NPs supported on MIL-125(Ti)-NH2 can be envisioned as a benchmark photocatalyst for solar-driven CO2 methanation in continuous-flow operations. / Cabrero Antonino, M. (2021). Metal-Organic Frameworks as Heterogenous Photocatalysts for the Production of Solar Fuels [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/176660 / TESIS

L’objectif principal de ce travail de thèse était de mettre en évidence et de mesurer l’effet co-catalytique de complexes moléculaires organo-métalliques à base de métaux de transition adsorbés sur des semi-conducteurs dopés photo-actifs vis-à-vis des réactions de photo-dissociation de l’eau et de photo-réduction du dioxyde de carbone, en en vue d’applications dans des cellules photochimiques et photo-électrochimiques. Nous avons tout d’abord identifié et sélectionné les matériaux (deux semi-conducteurs photo-actifs et deux co-catalyseurs moléculaires électroactifs) les plus adaptés (les résultats sont présentés dans le chapitre I). Nous avons ensuite conçu, développé et mis au point un bâti expérimental permettant la détection et le dosage en continu des produits de réaction lors des réactions d’intérêt (les résultats sont présentés dans le chapitre II). La détection des produits de réaction se fait à l’aide d’un chromatographe en phase gazeuse couplé au réacteur. Nous avons ensuite élaboré/synthétisé et mesuré les propriétés intrinsèques des matériaux sélectionnés (les résultats sont présentés dans le chapitre III). Finalement, nous avons mis en évidence l’activité co-catalytique des complexes utilisés et mesuré un ensemble d’indicateurs de performance tels que les cinétiques de réaction et les fréquences de « turn-over » (les résultats sont présentés dans le chapitre IV). / The main objective of this research work was to put into evidence the co-catalytic effect of organo-metallic molecular complexes containing transition metals as reactive centers, adsorbed at the surface of doped semiconductors with photo-activity with regard to water photo-dissociation and carbon dioxide photo-reduction, in view of practical applications in photochemistry and photo-electrochemistry. First, appropriate materials (two photoactive semiconductors and two molecular co-catalysts) have been identified and selected (results are presented in chapter I). Then, we have designed, constructed and optimized a specific test bench that can be used for the continuous detection and titration of reaction products (results are presented in chapter II). Product analysis was achieved by coupling a gas-phase chromatograph to the photo-electrochemical reactor. Then, photoactive semiconductors and molecular co-catalysts have been elaborated/synthesized and their intrinsic properties have been measured (results are presented in chapter III). Finally, the co-catalytic activity of molecular complexes has been put into evidence and several performance indicators such as reaction kinetics and turn-over frequency have been measured (results are presented in chapter IV).

The electrocatalytic water decomposition has been investigated in this thesis by means of its two half standard reactions: the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER). These reactions occur in different locations in a typical electrochemical cell: the anode and the cathode, respectively. Motivated by the lack of understanding about the reaction mechanisms occurring at the anodes and cathodes, we have proposed first: novel representations of typical TiO2 surfaces, based on small cluster systems, which can be used for a quick and more detailed assessment of the OER activities at modified TiO2 surfaces, and secondly we investigated the HER in two sets of model surfaces which represent recently synthesized materials, based on Mo2C and graphene with promising activities toward the HER. We have employed Density Functional Theory (DFT) based methods within both localized and extended basis sets, as implemented in GAMESS and VASP packages, respectively, to examine the structural, electronic and vibrational properties of the proposed models. We propose new reaction mechanisms for the OER on a number of molecular representations of TiO2 electrodes. For each reaction pathway, the free energy profile is computed, at different biases, from the DFT energies, the entropic and the zero-point energy contributions. The mechanisms explored in this thesis are found to be energetically more feasible than alternative reaction pathways considered in previous theoretical works based on molecular representations of the TiO2 surfaces. The representation of the surface of specific, commonly occurring, titanium dioxide crystals (e.g., rutile and anatase) within the small cluster approximation is able to reproduce qualitatively the rutile (110) outperforming of the anatase (001) surface. We subsequently investigate the influence of doping TiO2 surfaces with transition metals (TMs) on the performance of TiO2 -based electrodes for the water splitting electrochemical reaction. Two cluster models of the TM-doped active sites which resemble both the TiO2 anatase (001) and rutile (110) surfaces, respectively, are considered for the evaluation of the water decomposition reaction when a Ti is replaced by a TM atom. A set of TMs spanning from Vanadium to Nickel is considered. The late TMs explored here: Fe, Co and Ni are found to reproduce the observed experimental trends for the overpotentials in TiO2-doped electrodes. In the case of Cr and Mn, the present study predicts an enhancement of the OER activity for the anatase-like clusters while a reduction of this activity is found for the rutile-like ones. The vanadium-doped structures do not show relevant influence in the OER activity compared to pure TiO2-based cluster models. The last part of this work is devoted to the theoretical study of the HER on recently found materials based on the synergistic combination of molybdenum carbide and graphene layers. We propose two major structural models to describe the HER mechanism within the framework of DFT: Mo2C-based clusters adsorbed on carbon nanosheets and the Mo2C (001) surface covered by pure and nitrogen-doped graphene layers. The former system evaluates the influence of Mo2C nanoparticles adsorbed on carbon nanosheets towards the HER. The second one is employed to gain insight about the high HER activity observed in molybdenum carbide anchored on nitrogen-doped porous carbon nanosheets (Mo2C@2D-NPC), recently synthesized. The H-adsorption free energy has been used as a principal descriptor to asses the HER activity at the proposed model active sites. It resembles the value for the best state of the art catalyst for the HER (i.e., platinum at carbon substrate Pt@C) in some of the proposed structural models. Furthermore, a pH-correction is added within a simplified model, to the H-adsorption free energy barrier in every proposed structure. The pH dependence of the H-adsorption free energy barriers allows the assessment of the HER at acidic and alkaline conditions simultaneously. An overall agreement with experimental results is found and further predictions, promoting the development of better HER catalysts, have been done.

To achieve sustainable production of H2 fuel through water splitting, low-cost electrocatalysts for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER) are required to replace Pt and IrO2 catalysts. Here, for the first time, we present the interface engineering of novel MoS2/Ni3S2 heterostructures, in which abundant interfaces are formed. For OER, such MoS2/Ni3S2 heterostructures show an extremely low overpotential of ~218 mV at 10 mA cm-2, which is superior to that of the state-of-the-art OER electrocatalysts. Using MoS2/Ni3S2 heterostructures as bifunctional electrocatalysts, an alkali electrolyser delivers a current density of 10 mA cm-2 at a very low cell voltage of ~1.56 V. In combination with density function theory (DFT) calculations, this study demonstrates that the constructed interfaces synergistically favor the chemisorption of hydrogen and oxygencontaining intermediates, thus accelerating the overall electrochemical water splitting.

[ES] La presente tesis doctoral ha centrado la atención en la síntesis de nuevos materiales MOFs obtenidos por intercambio post-sintético de MOFs descritos anteriormente o por heterounión de MOFs estables que tienen estructura UiO66. Estos materiales se han aplicado como fotocatalizadores para la prometedora reacción de división completa del agua que pretende obtener H2 y O2 en forma estequiométrica a partir de agua en ausencia de agentes sacrificantes o nanopartículas metálicas. El nuevo material UiO66 (Zr/Ce/Ti) mostró resultados sorprendentes para la reacción general de división del agua en ambas condiciones de luz UV-Visible e irradiación solar. También se obtuvieron resultados interesantes en el caso del nucleo-corteza UiO66(Zr)-NH225@UiO66(Ce) que, en presencia de agua, cuando se irradió con luz ultravioleta y con luz solar, permitió obtener 550 𝝁 mol*g- 1 de H2 y 350 𝝁 mol*g-1 de H2 en 24 h respectivamente, en las mismas condiciones de trabajo definidas para UiO66(Zr/Ce/Ti). En este trabajo de tesis se presentó por primera vez la posibilidad de reducir 4-NP a través de una metodología tándem que involucra primero la producción de H2 a partir de agua en presencia de metanol, como agente de sacrificio, y UiO66(Zr)-NH2, como fotocatalizador, y la posterior reducción de 4-NP a 4-AP. Por otro lado, se ha demostrado que los materiales defectuosos con estructura de UiO66 pueden reducir selectivamente los dobles enlaces polarizados X=Y en presencia de gas H2. / [CA] La present tesi doctoral ha centrat l'atenció en la síntesi de nous materials MOFs obtinguts per intercanvi post-sintètic de MOFs descrits anteriorment o per heterounió de MOFs estables que tenen estructura UiO66. Estos materials s'han aplicat com fotocatalitzadors per a la prometedora reacció de divisió completa de l'aigua que pretén obtindre H2 i O2 en forma estequiomètrica a partir d'aigua en absència d'agents sacrificants o nanopartícules metàl·liques. El nou material UiO66 (Zr/Ce/Ti) va mostrar resultats sorprenents per a la reacció general de divisió de l'aigua en ambdós condicions de llum UV-Visible i irradiació solar. També es van obtindre resultats interessants en el cas del core-shell UiO66 (Zr) - NH225@UiO66 (Ce) que, en presència d'aigua, quan es va irradiar amb llum ultravioleta i amb llum solar, va permetre obtindre 550 𝝁 mol*g-1 de H2 i 350 𝝁 mol*g-1 de H2 en 24 h respectivament, en les mateixes condicions de treball definides per a UiO66 (Zr/Ce/Ti). En este treball de tesi es va presentar per primera vegada la possibilitat de reduir 4-nitrofenol a través d'una metodologia tàndem que involucra primer la producció de H2 a partir d'aigua en presència de metanol, com a agent de sacrifici, i UiO66 (Zr) -NH2, com fotocatalizador, i la posterior reducció de 4-NP a 4-AP. D'altra banda, s'ha demostrat que els materials defectuosos amb estructura d'UiO66 poden reduir selectivament els dobles enllaços polaritzats X=Y en presència de gas H2. / [EN] The present doctoral thesis has focused the attention on the synthesis of new MOFs materials obtained either by post-synthetic interchange of previously described MOFs or by heterojunction of stable MOFs having UiO66 structure. These materials have been applied as photocatalysts for the promising Overall Water Splitting reaction which claims to obtain H2 and O2 stoichiometrically starting from water in the absence of sacrificial agents or deposited metals nanoparticles. The new material UiO66 (Zr/Ce/Ti) showed surprising results in both UV- Visible light and sunlight irradiation conditions. Interesting results were also obtained in the case of the core-shell named UiO66(Zr)-NH225@UiO66(Ce) which, in the presence of water, when irradiated with both ultraviolet and solar light, allowed to obtain 550 𝝁 mol*g-1 of H2 and 350 𝝁 mol*g-1 of H2 in 24 h respectively, in the same working conditions defined for UiO66 (Zr/Ce/Ti). In this thesis was presented, for the first time, the possibility of reducing 4NP through a tandem methodology which, first, involves the production of H2 from water in the presence of methanol as a sacrificial agent and UiO66(Zr)-NH2 as a photo-catalyst, and subsequent the 4 NP reduction to 4 AP. On the other hand, it has been shown that defective materials with UiO66 structure can selectively reduce polarized X=Y double bonds in the presence of H2 gas. / Melillo, A. (2022). Metal-Organic Hybrid Materials with Catalytic and Photocatalytic Applications [Tesis doctoral]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/182744 / TESIS

No description available.

No description available.

A catalytic route to hydrogen production via thermochemical water splitting is highly desirable because it directly converts thermal energy into stored chemical energy in the form of hydrogen and oxygen. Recently, the Davis group at Caltech reported an innovative low-temperature (max 850°C) catalytic cycle for thermochemical water splitting based on sodium and manganese oxides (Xu, Bhawe and Davis, PNAS, 2012). The key steps are thought to be hydrogen evolution from a Na₂CO₃/MnO mixture, and oxygen evolution by thermal reduction of solids formed by Na⁺ extraction from NaMnO₂. Our work is aimed at understanding the fundamental chemical processes involved in the catalytic cycle, especially the hydrogen evolution from water. In this project, efforts are made to understand the interactions between the key components (Na, O₂, CO₂, and water) in the hydrogen evolution steps on a well-defined MnO(100) single crystal surface, utilizing x-ray photoelectron spectroscopy (XPS), low energy electron diffraction (LEED) and temperature programmed desorption (TPD). While some of the behavior of the catalytic system is observed with the model system developed in this work, hydrogen is only produced from water in the presence of metallic sodium, in contrast to the proposal of Xu et al. that water splitting occurs from the reaction of water with a mixture of Na₂CO₃ and MnO. These differences are discussed in light of the different operating conditions for the catalytic system and the surface science model developed in this work. / Ph. D.