Can Hydrated Electrons be Produced from Water with Visible Light?

2021

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In the past decade, polymeric carbon nitrides consisting of heptazine (Hz) building blocks emerged as highly promising materials for photocatalytic hydrogen evolution from water or sacrificial electron donors with near-ultraviolet light. However, the complexity of these materials and their poor characterization at the atomic level are detrimental to the unraveling of the detailed mechanisms of the reactions leading to hydrogen evolution. Recently, it has been shown that a derivative of the Hz molecule, trianisole-heptazine (TAHz), is a potent photobase, which readily oxidizes various derivatives of phenol and even water by an excited-state proton-coupled electron-transfer (PCET) reaction. Energy profiles along minimum-energy reaction paths and relaxed PCET potential-energy surfaces, which previously were computed with ab initio electronic-structure methods for complexes of Hz and TAHz with protic substrates, led to qualitative insights. To obtain more quantitative insight, reaction dynamics simulations are required. In the present work, the time scales of the electron and proton transfer processes and the branching ratios of competing channels were explored with ab initio on-the-fly quasiclassical surface-hopping trajectory simulations for the hydrogen-bonded Hz-H2O complex. By the analysis of representative trajectories, detailed insight into the interplay of various nonadiabatic electronic transitions, electron transfer, proton transfer, and vibrational energy relaxation is obtained. The HzH radicals which are formed by the photoreduction of Hz can disproportionate to Hz and HzH2 in an exothermic reaction with a low reaction barrier. The time scales and branching ratios of competing channels in H-atom photodetachment from the HzH2 molecule also were explored with ab initio surface-hopping simulations. These results delineate for the first time a quantitatively supported scenario of water oxidation and hydrogen evolution with a molecular carbon nitride photocatalyst.

Section: Hzh Hzhmentioning

confidence: 99%

Ab Initio Nonadiabatic Surface-Hopping Trajectory Simulations of Photocatalytic Water Oxidation and Hydrogen Evolution with the Heptazine Chromophore

Huang

2021

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“…In particular, heptazine (heptaazaphenalene) is the monomer of the well-known polymeric photocatalyst graphitic carbon nitride (g-C 3 N 4 ) which is widely employed in current water-splitting research. − It has recently been shown that a chemically and photochemically stable derivative of heptazine, tri-anisole-heptazine, can photooxidize water under irradiation at 365 nm, yielding the heptazinyl radical . Computational studies indicate that hydrated electrons can be generated by photodissociation of the heptazinyl radical in an aqueous environment with near-visible light . Since the absorption bands of organic chromophores can be systematically tuned over a wide range by chemical modification, it may be possible to synthesize organic photocatalysts that can generate hydrated electrons from water with visible light.…”

Section: Discussionmentioning

confidence: 99%

“…45 Computational studies indicate that hydrated electrons can be generated by photodissociation of the heptazinyl radical in an aqueous environment with near-visible light. 46 Since the absorption bands of organic chromophores can be systematically tuned over a wide range by chemical modification, it may be possible to synthesize organic photocatalysts that can generate hydrated electrons from water with visible light.…”

Section: Discussionmentioning

confidence: 99%

Reduction of CO₂ with Hydrated Electrons: Ab Initio Computational Studies for Finite-Size Cluster Models

Pios

2023

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In the present work, the mechanisms of the reduction of the CO2 molecule with hydrated electrons to the hydroxyl-formyl (HOCO) radical were studied with ab initio computational methods. Hydrated hydronium radicals, H3O(H2O) n (n = 0,3,6), are considered as finite-size models of the hydrated electron in liquid water. The investigation of cluster models allows the application of high-accuracy electronic-structure methods, which are not computationally feasible in condensed-phase simulations. Reaction paths and potential-energy (PE) profiles of the proton-coupled electron-transfer reaction from hydrated H3O radicals to the CO2 molecule were explored on the ground-state PE surface. The computationally efficient unrestricted second-order Møller–Plesset method is employed, and its accuracy has been carefully benchmarked in comparison with complete-active-space self-consistent-field and multi-reference second-order perturbation calculations. The results provide insights into the interplay of electron transfer from the diffuse Rydberg-type unpaired electron of H3O to the CO2 molecule, the contraction of the electron cloud by the re-hybridization of the carbon atom of CO2, and proton transfer from the nearest water molecule to the CO2 – anion, followed by Grotthus-type proton rearrangements to form stable clusters. Starting from local energy minima of hydrogen-bonded CO2–H3O(H2O) n complexes, the reaction to form HOCO–(H2O) n+1 complexes is exothermic by about 1.3 eV (125 kJ/mol). The reaction is barrier controlled with a barrier of the order of a few tenths of an electron volt, depending on size and conformation of the water cluster. This barrier is at least an order of magnitude lower than the barrier of the reaction of CO2 with any closed-shell partner molecule. The HOCO radicals can recombine by H-atom transfer (disproportionation), resulting in formic acid or a dihydroxycarbene product, as well as by the formation of a C–C bond, resulting in oxalic acid. The strong exothermicity of these radical–radical recombination reactions likely results in the fragmentation of the closed-shell products formic acid and oxalic acid, which explains the strong specificity for CO formation observed in recent experiments of Hamers and co-workers.

“…On the other hand, excitation of the 2 πσ* state of HzH in aqueous environments can lead to the formation of hydrated electrons. Recent calculations predict a barrierless reaction path devoid of conical intersections for the photoinduced formation of the H 3 O···(H 2 O) n −1 radical in HzH···(H 2 O) n clusters . The hydrated H 3 O radical is a convenient finite-size model of the hydrated electron in bulk water. , The lifetime of the hydrated electron at ambient conditions is of the order of a microsecond, which is sufficient for effective diffusion-controlled reactions.…”

Section: Discussionmentioning

confidence: 99%

Ab Initio Electronic Structure Study of the Photoinduced Reduction of Carbon Dioxide with the Heptazinyl Radical

Pios

2022

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The photocatalytic conversion of carbon dioxide to liquid fuels with electrons taken from water with solar photons is one of the grand goals of renewable energy research. Polymeric carbon nitrides recently emerged as metal-free materials with promising functionalities for hydrogen evolution from water as well as the activation of carbon dioxide. Molecular heptazine (Hz), the building block of polymeric carbon nitrides, is one the strongest known organic photo-oxidants and has been shown to be able to photo-oxidize water with near-visible light, resulting in reduced (hydrogenated) heptazine (HzH) and OH radicals. In the present work, we explored with ab initio computational methods whether the HzH chromophore is able to reduce carbon dioxide to the hydroxy-formyl (HOCO) radical in hydrogen-bonded HzH-CO 2 complexes by the absorption of a photon. In remarkable contrast to the high barrier for carbon dioxide activation in the electronic ground state, the excited-state proton-coupled electron transfer (PCET) reaction is nearly barrierless, but requires the diabatic passage of three conical intersections. The possibility of barrierless carbon dioxide activation by excited-state PCET has so far not been taken into consideration in the interpretation of photocatalytic carbon dioxide reduction on carbon nitride materials.