Identification of Crucial Photosensitizing Factors to Promote CO2 -to-CO Conversion.

The sensitizing ability of a catalytic system is closely related to the visible-light absorption ability, excited-state lifetime, redox potential, and electron-transfer rate of photosensitizers (PSs), however it remains a great challenge to concurrently mediate these factors to boost CO2 photoreduction. Herein, a series of Ir(III)-based PSs (Ir-1-Ir-6) were prepared as molecular platforms to understand the interplay of these factors and identify the primary factors for efficient CO2 photoreduction. Among them, less efficient visible-light absorption capacity results in lower CO yields of Ir-1, Ir-2 or Ir-4. Ir-3 shows the most efficient photocatalytic activity among these mononuclear PSs due to some comprehensive parameters. Although the Kobs of Ir-3 is ≈10 times higher than that of Ir-5, the CO yield of Ir-3 is slightly higher than that of Ir-5 due to the compensation of Ir-5's strong visible-light-absorbing ability. Ir-6 exhibits excellent photocatalytic performance due to the strong visible-light absorption ability, comparable thermodynamic driving force, and electron transfer rate among these PSs. Remarkably, the CO2 photoreduction to CO with Ir-6 can achieve 91.5 µmol, over 54 times higher than Ir-1, and the optimized TONC-1 can reach up to 28160. Various photophysical properties of the PSs were concurrently adjusted by fine ligand modification to promote CO2 photoreduction.

Single-cluster Functionalized TiO2 Nanotube Array for Boosting Water Oxidation and CO2 Photoreduction to CH3OH.

Solar-driven CO2 reduction and water oxidation to liquid fuels represents a promising solution to alleviate energy crisis and climate issue, but it remains a great challenge for generating CH3OH and CH3CH2OH dominated by multi-electron transfer. Single-cluster catalysts with super electron acceptance, accurate molecular structure, customizable electronic structure and multiple adsorption sites, have led to greater potential in catalyzing various challenging reactions. However, accurately controlling the number and arrangement of clusters on functional supports still faces great challenge. Herein, we develop a facile electrosynthesis method to uniformly disperse Wells-Dawson- and Keggin-type polyoxometalates on TiO2 nanotube arrays, resulting in a series of single-cluster functionalized catalysts P2M18O62@TiO2 and PM12O40@TiO2 (M=Mo or W). The single polyoxometalate cluster can be distinctly identified and serves as electronic sponge to accept electrons from excited TiO2 for enhancing surface-hole concentration and promote water oxidation. Among these samples, P2Mo18O62@TiO2-1 exhibits the highest electron consumption rate of 1260 µmol g-1 for CO2-to-CH3OH conversion with H2O as the electron source, which is 11 times higher than that of isolated TiO2 nanotube arrays. This work supplied a simple synthesis method to realize the single-dispersion of molecular cluster to enrich surface-reaching holes on TiO2, thereby facilitating water oxidation and CO2 reduction.

Two-body dissociation of C3H4 isomers investigated by 50 keV/u Ne8+ impact.

The fragmentation of two isomers of C3H4, propyne (CH3CCH) and allene (CH2CCH2), is investigated by 50 keV/u Ne8+ impact. Obvious isomer effects are observed by comparing the time-of-flight spectra generated from the two isomers. Six two-body fragmentation channels of C3H4 2+ dications are identified for each isomer. CH2 + + C2H2 + is found to be the most favored CC bond breaking channel for both isomers, indicating that CH3CCH2+ intends to rearrange to the structure containing the CH2 group before fragmentation. For CH bond breaking channels, it is found that the CH3CCH which contains a CH3 group is more efficient for H2 + and H3 + ejection. In addition, two-body dissociation channels of C3H4 3+ trications are identified. While the H+ + C3H3 2+ channel is observed in the fragmentation of both isomers, the H2 + + C3H2 2+ channel only occurs in the fragmentation of CH3CCH3+. For CH2CCH2 3+, the peak and shoulder structures in the kinetic energy release spectrum of the H+ + C3H3 2+ channel are attributed to different geometries of the C3H3 2+ product.

Dissociation of [HCCH]2+ to H2+ and C2+: a benchmark reaction involving H migration, H-H combination, and C-H bond cleavage.

We report the formation of H2+ and C2+ from dissociation of acetylene induced by α-particle irradiation. The unusual dissociation channel [C2H2]2+ → H2+ + C2+ is unambiguously identified by measuring the time-of-flight of both fragmented ions in coincidence. Our quantum chemical calculation confirms the existence of this dissociation pathway. It shows that [HCCH]2+ is firstly populated to the 3Π excited electronic state, followed by acetylene-vinylidene isomerization, and finally the vinylidene-like intermediate dissociates to H2+ and C2+. This dissociation channel is the simplest prototypical reaction involving H migration, H-H combination, and C-H bond cleavage. The current study plays an important role for understanding the H2+/H3+ formation reactions from organic di-cations in an interstellar medium.

Damaging Intermolecular Energy and Proton Transfer Processes in Alpha-Particle-Irradiated Hydrogen-Bonded Systems.

Although the biological hazard of alpha-particle radiation is well-recognized, the molecular mechanisms of biodamage are still far from being understood. Irreparable lesions in biomolecules may not only have mechanical origin but also appear due to various electronic and nuclear relaxation processes of ionized states produced by an alpha-particle impact. Two such processes were identified in the present study by considering an acetylene dimer, a biologically relevant system possessing an intermolecular hydrogen bond. The first process is the already well-established intermolecular Coulombic decay of inner-valence-ionized states. The other is a novel relaxation mechanism of dicationic states involving intermolecular proton transfer. Both processes are very fast and trigger Coulomb explosion of the dimer due to creation of charge-separated states. These processes are general and predicted to occur also in alpha-particle-irradiated nucleobase pairs in DNA molecules.

Water-Mediated Selectivity Control of CH3 OH versus CO/CH4 in CO2 Photoreduction on Single-Atom Implanted Nanotube Arrays.

Controllable methanol production in artificial photosynthesis is highly desirable due to its high energy density and ease of storage. Herein, single atom Fe is implanted into TiO2 /SrTiO3 (TSr) nanotube arrays by two-step anodization and Sr-induced crystallization. The resulting Fe-TSr with both single Fe reduction centers and dominant oxidation facets (001) contributes to efficient CO2 photoreduction and water oxidation for controlled production of CH3 OH and CO/CH4 . The methanol yield can reach to 154.20 µmol gcat -1 h-1 with 98.90% selectivity by immersing all the catalyst in pure water, and the yield of CO/CH4 is 147.48 µmol gcat -1 h-1 with >99.99% selectivity when the catalyst completely outside water. This CH3 OH yield is 50 and 3 times higher than that of TiO2 and TSr and stands among all the state-of-the-art catalysts. The facile gas-solid and gas-liquid-solid phase switch can selectively control CH3 OH production from ≈0% (above H2 O) to 98.90% (in H2 O) via slowly immersing the catalyst into water, where abundant •OH and H2 O around Fe sites play important role in selective CH3 OH production. This work highlights a new insight for water-mediated CO2 photoreduction to controllably produce CH3 OH.

An (e, 2e + ion) investigation of dissociative ionization of methane.

We present in this paper an (e, 2e + ion) investigation of the dissociative ionization of methane by 54 eV electron impact employing the advanced reaction microscope. By measuring two electrons and the ion in the final state in triple coincidence, the species of the ions are identified and the energies deposited into the target are determined. The species and the kinetic energies of the fragmented ion show strong dependence on the intermediate states of the parent ion. Possible decay pathways for the production of different species of ions are analyzed.

Fragmentation Dynamics of a Carbon Dioxide Dication Produced by Ion Impact.

The response of carbon dioxide to radiolysis is crucial for understanding the atmospheric chemistry of planets. Here, we present a combined experimental and theoretical investigation of the three-body fragmentation dynamics of CO22+ to C+ + O+ + O initiated by 1 keV/u Ar2+ impact. Taking advantage of the kinematic complete measurement employing a reaction microscope, three dissociation mechanisms are distinguished, and their branching ratios are determined. The concerted fragmentation with two C-O bonds breaking simultaneously is dominant, while the sequential pathway with CO+ as the intermediate also makes a significant contribution. Also, a novel isomerization pathway with transitory formation of O2+ is identified. The identified mechanisms can contribute to O+ and O escaping from the Martian atmosphere, since the kinetic energies of most of the fragments are observed to be higher than the escape energy of oxygen.