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

Unravelling the Super-Reductive State of Iridium Photoredox Catalysts

Harnessing the excited state of reduced species has long posed a challenge in the field of photocatalysis. This study presents the isolation and characterization of 1-electron reduced iridium complexes commonly employed in photoredox catalysis. Stochiometric reactions unveiled an unprecedented super-reductant ability for the isolated complexes under light irradiation, reaching potentials below 3 V vs SCE. Notably, the reduced iridium complex can also be electrochemically generated in situ with analogous super-reductant ability, enabling electro(photo)catalysis. Experimental and computational studies reveal that photoreactivity rises from intrinsic excitation of the reduced (bpy●‒)* ligand within the iridium complex, while the metal center acts as a spectator. Corroborating this finding, the organic salt Li+bpy●‒ exhibited equivalent super-reducing reactivity under photochemical conditions. Our findings shed light on the access to the super-reductant states of iridium photoredox catalysts and other metalated bipyridines, opening new opportunities for electro(photo) synthetic methodologies