Development and Mechanistic Study of Single Sites in 2D-Covalent Organic Frameworks for Co2 Electroreduction

El canvi climàtic i la dependència excessiva dels combustibles fòssils exigeixen solucions per reduir les emissions de CO2 i desenvolupar tecnologies d'energia sostenibles. La reducció electroquímica de CO2 té el potencial d'aconseguir un cicle d'energia neutre en carboni. No obstant això, la seva aplicació a nivell industrial encara presenta certs desafiaments que cal superar, com la baixa selectivitat, la curta durabilitat i les baixes densitats de corrent amb alts sobrepotencials. Per això és necessari el desenvolupament de noves plataformes catalítiques sostenibles, modulars, robustes i eficients. Aquesta tesi doctoral inclou una comprensió fonamental del mecanisme de reducció de CO2 utilitzant “Single Atom Catalysts” (SACs) introduïts en ”Covalent Organic Frameworks” (COFs). En aquesta dissertació s'investiga la reducció electroquímica de CO2 en aigua emprant nous COFs de Mn, amb un èmfasi especial en la comprensió de la relació entre l'estructura dels materials i l'activitat electrocatalítica. El centre d'interès inicial d'aquest treball és aumentar l'activitat i l'estabilitat dels catalitzadors de Mn utilitzant llocs actius de {Mn(CO)3} altament organitzats. En comparació amb els derivats de Mn equivalents, els COFs van exhibir més selectivitat i activitat.El cambio climático y la excesiva dependencia de los combustibles fósiles exigen soluciones para reducir las emisiones de CO2 y desarrollar tecnologías de energía sostenibles. La reducción electroquímica de CO2 tiene el potencial de lograr un “ciclo de energía neutro en carbono”. Sin embargo, para su aplicación a nivel industrial aún existen desafíos que se deben superar, como la baja selectividad, corta durabilidad y bajas densidades de corriente con altos sobrepotenciales. Por lo que es necesario el desarrollo de nuevas plataformas catalíticas sostenibles, modulares, robustas y eficientes. Esta tesis doctoral incluye una comprensión fundamental del mecanismo de reducción de CO2 utilizando “Single Aton Catalysts (SACs) introducidos en “Covalent Organic Frameworks (COFs). En esta disertación se investiga la reducción electroquímica de CO2 en agua utilizando nuevos COFs de Mn, con especial énfasis en la comprensión de la relación entre la estructura de los materiales y la actividad electrocatalítica. El centro de interés inicial de este trabajo es aumentar la actividad y estabilidad de los catalizadores de Mn utilizando sitios activos de {Mn(CO)3} altamente organizados.Global warming, climate change and our over-dependence on non-renewable fossil fuels demand long-term solutions to reduce CO2 emissions and develop sustainable energy technologies. The electrochemical CO2 reduction has the potential to accomplish a “carbon-neutral energy cycle”, which incorporates CO2 as the unlimited carbon source for the production of high-density fuels. However, for an industrial application, there are still challenges to overcome, such as low selectivity, short durability and low current densities along with high overpotentials. New sustainable, modular, robust and efficient catalytic platforms are needed. In this regard, this PhD dissertation entails a fundamental understanding of the CO2 mechanisms using Single Atom Catalyst within Covalent Organic Frameworks (COFs). This thesis focuses on the investigation of the electrochemical CO2 reduction using new Manganese based-COFs with emphasis on understanding the relationship between structure and electrocatalytic activity. The initial center of interest of this work is to accomplish active performance and durability for CO2 reduction using highly-organized {Mn(CO)3} active sites within COFs. Compared to equivalent Mn derivates, COFs exhibited higher selectivity and activity

Impact of the anodization time on the photocatalytic activity of TiO2 nanotubes

Titanium oxide nanotubes (TNTs) were anodically grown in ethylene glycol electrolyte. The influence of the anodization time on their physicochemical and photoelectrochemical properties was evaluated. Concomitant with the anodization time, the NT length, fluorine content, and capacitance of the space charge region increased, affecting the opto-electronic properties (bandgap, bathochromic shift, band-edge position) and surface hydrophilicity of TiO2 NTs. These properties are at the origin of the photocatalytic activity (PCA), as proved with the photooxidation of methylene blue

Toward the Understanding of the Structure–Activity Correlation in Single-Site Mn Covalent Organic Frameworks for Electrocatalytic CO₂ Reduction

The encapsulation of organometallic complexes into reticular covalent organic frameworks (COFs) represents an effective strategy for the immobilization of molecular electrocatalysts. In particular, well-defined polypyridyl Mn sites embedded into a crystalline COF backbone (COFbpyMn) were found to exhibit higher selectivity and activity toward electrochemical CO2 reduction compared to the parent molecular derivative noncovalently immobilized on carbon electrodes. In situ mechanistic studies revealed that the electronic and steric features of the reticular framework strongly affect the redox mechanism of the Mn sites, stabilizing the formation of a mononuclear Mn(I) radical anion intermediate over the most common off-cycle Mn0–Mn0 dimer. Herein, we report the study of a Mn-based COF (COFPTMn), introducing a larger phenanthroline building block, to explore how tuning the structural and electronic properties of the lattice may affect the catalytic CO2 reduction performance and the mechanism at the molecular level of the reticular system. The Mn sites encapsulated into the reticular COFPTMn exhibited a remarkable enhancement in the intrinsic catalytic CO2 reduction activity at near-neutral pH compared to that of the corresponding noncovalently immobilized molecular derivative. On the other hand, the poor crystallinity and porosity of COFPTMn, likely introduced by the lattice expansion and spatial dynamics of the phenanthroline linker, were found to limit its catalytic performances compared to those of the bipyridyl COFbpyMn analogue. ATR-IR spectroelectrochemistry revealed that the higher spatial mobility of the Mn sites does not completely suppress the Mn0–Mn0 dimerization upon the electrochemical reduction of the Mn sites at the COFbpyMn. This work highlights the positive role of the reticular structure of the material in enhancing its catalytic activity versus that of its molecular counterpart and provides useful hints for the future design and development of efficient reticular frameworks for electrocatalytic applications

Mechanically Constrained Catalytic Mn(CO)(3)Br Single Sites in a Two-Dimensional Covalent Organic Framework for CO2 Electroreduction in H2O

The development of CO2 electroreduction (CO2RR) catalysts based on covalent organic frameworks (COFs) is an emerging strategy to produce synthetic fuels. However, our understanding on catalytic mechanisms and structure-activity relationships for COFs is still limited but essential to the rational design of these catalysts. Herein, we report a newly devised CO2 reduction catalyst by loading single-atom centers, {fac-Mn(CO)(3)S}, (S = Br, CH3CN, H2O), within a bipyridylbased COF (COFbpyMn). COFbpyMn shows a low CO2RR onset potential (eta = 190 mV) and high current densities (>12 mA.cm(-2), at 550 mV overpotential) in water. TOFCO and TONCO values are as high as 1100 h-1 and 5800 (after 16 h), respectively, which are more than 10-fold higher than those obtained for the equivalent manganese-based molecular catalyst. Furthermore, we accessed key catalytic intermediates within a COF matrix by combining experimental and computational (DFT) techniques. The COF imposes mechanical constraints on the {fac-Mn(CO)(3)S} centers, offering a strategy to avoid forming the detrimental dimeric Mn-0-Mn-0, which is a resting state typically observed for the homologous molecular complex. The absence of dimeric species correlates to the catalytic enhancement. These findings can guide the rational development of isolated single-atom sites and the improvement of the catalytic performance of reticular materials