Role of excited-state hydrogen bonding in CO2 photoreduction catalyzed by sodium magnesium chlorophyll.

In this paper, we report a joint experimental and computational study to elaborate the mechanism for the photocatalytic CO2 reduction reaction (CO2RR). Experimental results indicate that the catalyst (sodium magnesium chlorophyll, MgChlNa2), which has a well-defined structure for calculation and understanding, can achieve the photoreduction of CO2 to CO only using water as a dispersant, without adding any photosensitizer or sacrificial agent. Subsequently, a series of structural models of the hydrogen-bonded complexes of the catalyst were constructed and outlined via utilizing density functional theory (DFT) calculations, including photophysical and photochemical processes. The results confirm that the rate-limiting step of the whole CO2RR was the intersystem crossing process. The electron and proton transfers involved in photophysical and photochemical processes are induced by hydrogen bonds in the excited states. The combination of experiments and calculations will provide an important reference for the design of high-efficiency photocatalysts in the photocatalytic CO2RR.

Computational Study of Photodegradation Process and Conversion Products of the Antidepressant Citalopram in Water.

Citalopram (CIT) is a commonly prescribed medication for depression. However, the photodegradation mechanism of CIT has not yet been fully analyzed. Therefore, the photodegradation process of CIT in water is studied by density functional theory and time-dependent density functional theory. The calculated results show that during the indirect photodegradation process, the indirect photodegradation of CIT with ·OH occurs via OH-addition and F-substitution. The minimum activation energy of C10 site was 0.4 kcal/mol. All OH-addition and F-substitution reactions are exothermic. The reaction of 1O2 with CIT includes the substitution of 1O2 for F and an addition reaction at the C14 site. The Ea value of this process is 1.7 kcal/mol, which is the lowest activation energy required for the reaction of 1O2 with CIT. C-C/C-N/C-F cleavage is involved in the direct photodegradation process. In the direct photodegradation of CIT, the activation energy of the C7-C16 cleavage reaction was the lowest, which was 12.5 kcal/mol. Analysis of the Ea values found that OH-addition and F-substitution, the substitution of 1O2 for F and addition at the C14 site, as well as the cleavage reactions of C6-F/C7-C16/C17-C18/C18-N/C19-N/C20-N are the main pathways of photodegradation of CIT.

An insight into the reaction mechanism of CO2 photoreduction catalyzed by atomically dispersed Fe atoms supported on graphitic carbon nitride.

We report a combination of experimental and computational mechanistic studies for the photoreduction of CO2 to CO with water, catalyzed by single-atom Fe supported on graphitic carbon nitride (g-C3N4). Density functional theory (DFT) and time-dependent DFT (TDDFT) methods were utilized to explore the behavior of single-atom Fe in g-C3N4, which is of vital importance to the understanding of the CO2 reduction reaction (CO2RR) mechanism. The calculation results reveal that the rate-limiting step of the hydrogen-bonded complex in the absence of Fe atoms is the cleavage of C-O bonds in COOH radicals during the whole CO2RR, which includes the photophysical and photochemical processes. The presence of Fe atoms not only activated CO2 in the ground state and increased the rate constant of the limiting step in the photophysical process, but also functioned as the catalytic active center, lowering the reaction barrier of the C-O bond cleavage in COOH˙ in the photochemical process and resulting in improved photocatalytic activity. In addition, DFT calculations further demonstrated that the electron and proton transfer involved in the photophysical and photochemical processes is closely related to and induced by the hydrogen bonds in the excited state.

FeNi Alloy Nanoparticles Encapsulated in Carbon Shells Supported on N-Doped Graphene-Like Carbon as Efficient and Stable Bifunctional Oxygen Electrocatalysts.

The development of cost-effective and durable oxygen electrocatalysts remains highly critical but challenging for energy conversion and storage devices. Herein, a novel FeNi alloy nanoparticle core encapsulated in carbon shells supported on a N-enriched graphene-like carbon matrix (denoted as FeNi@C/NG) was constructed by facile pyrolyzing the mixture of metal salts, glucose, and dicyandiamide. The in situ pyrolysis of dicyandiamide in the presence of glucose plays a significant effect on the fabrication of the porous FeNi@C/NG with a high content of doped N and large specific surface area. The optimized FeNi@C/NG catalyst displays not only a superior catalytic performance for the oxygen reduction reaction (ORR, with an onset potential of 1.0 V and half-wave potential of 0.84 V) and oxygen evolution reaction (OER, the potential at 10 mA cm-2 is 1.66 V) simultaneously in alkaline, but also outstanding long-term cycling durability. The excellent bifunctional ORR/OER electrocatalytic performance is ascribed to the synergism of the carbon shell and FeNi alloy core together with the high-content of nitrogen doped on the large specific surface area graphene-like carbon.

Infrared spectra of neutral dimethylamine clusters: An infrared-vacuum ultraviolet spectroscopic and anharmonic vibrational calculation study.

Infrared-vacuum ultraviolet (IR-VUV) spectra of neutral dimethylamine clusters, (DMA)n (n = 2-5), were measured in the spectral range of 2600-3700 cm-1. The experimental IR-VUV spectra show NH stretch modes gradually redshift to 3200-3250 cm-1 with the increase in the cluster size and complex Fermi Resonance (FR) pattern of the CH3 group in the 2800-3000 cm-1 region. Ab initio anharmonic vibrational calculations were performed on low-energy conformers of (DMA)2 and (DMA)3 to examine vibrational coupling among CH/NH and to understand the Fermi resonance pattern in the observed spectra features. We found that the redshift of NH stretching mode with the size of DMA cluster is moderate, and the overtone of NH bending modes is expected to overlap in frequency with the CH stretching fundamental modes. The FR in CH3 groups is originated from the strong coupling between CH stretching fundamental and bending overtone within a CH3 group. Well-resolved experimental spectra also enable us to compare the performance of ab initio anharmonic algorithms at different levels.

A 3D Covalent Organic Framework with Exceptionally High Iodine Capture Capability.

Using porous materials to cope with environmental issues is promising but remains a challenge especially for removing the radioactive vapor wastes in fission because of harsh adsorption conditions. Here we report a new, stable covalent organic framework (COF) as a porous platform for removing iodine vapor-a major radioactive fission waste. The three-dimensional COF consists of a diamond topology knotted by adamantane units, creates ordered one-dimensional pores and are highly porous. The COF enables the removal of iodine vapor via charge transfer complex formation with the pore walls to achieve exceptional capacity. Moreover, the 3D COF is "soft" to trigger structural fitting to iodine while retaining connectivity and enables cycle use for many times while retaining high uptake capacity. These results set a new benchmark for fission waste removal and suggest the great potential of COFs as a designable porous material for challenging world-threatening pollution issues.

Pseudohalogen-Based 2D Perovskite: A More Complex Thermal Degradation Mechanism Than 3D Perovskite.

(MA)2Pb(SCN)2I2, a new pseudohalogen-based 2D perovskite material, was reported as a very stable and promising photo-absorber in PSCs previously. However, the later researchers found that MA2Pb(SCN)2I2 was not as stable as claimed. Thus, it is very critical to clarify the controversy and reveal the degradation mechanism of MA2Pb(SCN)2I2. On the other hand, a large number of studies have indicated that adding a small amount of SCN- improves surface topography and crystallinity. However, whether SCN- ions can be incorporated into a 3D perovskite film remains debatable. In this work, the thermal degradation pathway of (MA)2Pb(SCN)2I2 is revealed by thermal gravimetric and differential thermal analysis coupled with quadrupole mass spectrometry and density functional theory calculations. The decomposition of (MA)2Pb(SCN)2I2 has been proved experimentally to be more complex than that of MAPbI3, involving four stages and multi-reactions from room temperature to above 500 °C. By combining the experimental results and theoretical calculations, it is found that 2D (MA)2Pb(SCN)2I2 actually is unstable when serving as photo-absorber in PSCs. Moreover, the role of SCN- in improving the crystallinity of 3D perovskite has also been discussed in detail.

Efficient Synthesis of Nitrogen- and Sulfur-co-Doped Ketjenblack with a Single-Source Precursor for Enhancing Oxygen Reduction Reaction Activity.

Promoting the oxygen reduction reaction (ORR) catalytic activities of cost-effective catalysts is of great significance for the development of various energy conversion and storage systems. Herein, we describe the preparation of a highly active N- and S-co-doped ketjenblack (Kb) by facile pyrolysis of a mixture of thiourea and Kb in the presence of FeCl3 ⋅6 H2 O followed by an acid-leaching process. This novel synthetic approach was rationally designed with the consideration that thiourea could easily introduce both N and S heteroatoms into the carbon matrix by a heat-treatment method by releasing plentiful reactive N- and S-containing gases, which could simultaneously optimize the porous structure of the resultant catalyst. Physical characterization revealed that N and S were homogeneously incorporated into the nanostructure of Kb and formed a hierarchical porous structure with a high specific surface area. The N/S-Kb catalyst showed impressive ORR activity, with an onset potential of 0.08 V at 0.1 mA cm-2 , which is 20 mV more positive than that of commercial 20 wt. % Pt/C catalyst. This was coupled with long-term durability and superior methanol tolerance in alkaline media. The improved ORR performance can be mainly ascribed to synergistic contributions of highly efficient active sites arising from high contents of thiophene S and pyridinic N along with the high specific surface area and the favorable mass-transport properties arising from the hierarchical porous structure. The remarkable ORR performance and facile preparation method make the N/S-Kb catalyst a promising substitute for Pt in electrochemical energy devices.

Infrared-Vacuum Ultraviolet Spectroscopic and Theoretical Study of Neutral Methylamine Dimer.

The methylamine dimer, (CH3NH2)2, is a model system to study the CH3 and NH2 spectral patterns in the neutral microsolvated systems relevant to chemical biology, atmospheric chemistry, and catalysis. We report infrared-vacuum ultraviolet spectroscopic measurements to probe the neutral (CH3NH2)2. Quantum chemical calculations and ab initio molecular dynamics simulations were performed to understand the observed spectral features. Experimental and theoretical results indicate the likely coexistence of both cis and trans structures. A salient feature of this work is that the peak widths are not significantly affected by the structural transformation and the fluctuation of hydrogen bond distance, allowing the stretching modes to be clearly resolved.

Quantum chemical investigation on photodegradation mechanisms of sulfamethoxypyridazine with dissolved inorganic matter and hydroxyl radical.

Sulfamethoxypyridazine (SMP) is one of the commonly used sulfonamide antibiotics (SAs). SAs are mainly studied to undergo triplet-sensitized photodegradation in water under natural sunlight with other coexisting aquatic environmental organic pollutants. In this work, SMP was selected as a representative of SAs. We studied the mechanisms of triplet-sensitized photodegradation of SMP and the influence of selected dissolved inorganic matter, i.e., anions (Br-, Cl-, and NO3-) and cations ions (Ca2+, Mg2+, and Zn2+) on SMP photodegradation mechanism by quantum chemical methods. In addition, the degradation mechanisms of SMP by hydroxyl radical (OH) were also investigated. The creation of SO2 extrusion product was accessed with two different energy pathways (pathway-1 and pathway-2) by following two steps (step-I and step-II) in the triplet-sensitized photodegradation of SMP. Due to low activation energy, the pathway-1 was considered as the main pathway to obtain SO2 extrusion product. Step-II of pathway-1 was measured to be the rate-limiting step (RLS) of SMP photodegradation mechanism and the effect of the selected anions and cations was estimated for this step. All selected anions and cations promoted photodegradation of SMP by dropping the activation energy of pathway-1. The estimated low activation energies of different degradation pathways of SMP with OH radical indicate that OH radical is a very powerful oxidizing agent for SMP degradation via attack through benzene derivative and pyridazine derivative ring.

Transition metal selenides as efficient counter-electrode materials for dye-sensitized solar cells.

Exploiting an alternative of the Pt-based counter-electrode materials for the triiodide reduction reaction has become a major interest in the fundamental research of dye-sensitized solar cells. Transition-metal selenides have recently been demonstrated as promising non-precious metal electrocatalysts for the triiodide reduction reaction. Herein, we prepared a series of transition-metal selenides via a free-reductant solvothermal method and used them as counter-electrodes in high efficiency dye-sensitized solar cells. The electrochemical results showed that these selenides had excellent catalytic activity for the reduction of the triiodine/iodine couple, and except for MoSe2, the conversion efficiencies of the corresponding dye-sensitized solar cells were comparable to the sputtered Pt counter-electrode. Theoretical investigation clearly revealed that the unsatisfactory performance of MoSe2 mainly originated from the processes of adsorption and charge-transfer. These findings can help to better understand the electrocatalytic processes and thus offer some useful guidelines to develop more efficient electrochemical catalysts.

Interaction between formaldehyde and luminescent MOF [Zn(NH2bdc)(bix)]n in the electronic excited state.

The hydrogen bond between formaldehyde and the luminescent metal-organic framework (MOF) [Zn(NH2bdc)(bix)]n was investigated using density functional theory and time-dependent density functional theory. The frontier molecular orbitals and electronic configuration demonstrate that the origin of the luminescence can be attributed to ligand-to-ligand charge transfer. Examination of the hydrogen bond behavior in the electronic excited state, with comparison of the electronic transition energies, bond distances, binding energy, (1)H-NMR chemical shifts, and infrared spectra with those of the ground state, demonstrate that the hydrogen bond is stronger when in the electronic excited state. Strengthening of the hydrogen bond weakens the radioactive transition of [Zn(NH2bdc)(bix)]n, which thus leads to a luminescence decrease or quenching phenomenon, meaning that the luminescent MOF [Zn(NH2bdc)(bix)]n may be applied to the detection of formaldehyde.

Role of the electronically excited-state hydrogen bonding and water clusters in the luminescent metal-organic framework.

The electronically excited state and luminescence property of metal-organic framework Zn(3-tzba)(2,2'-bipy)(H2O)·nH2O have been investigated using the density functional theory (DFT) and time-dependent DFT (TDDFT). The calculated geometry and infrared spectra in the ground state are consistent with the experimental results. The frontier molecular orbitals and electronic configuration indicated that the origin of luminescence is attributed to a ligand-to-ligand charge transfer (LLCT). We theoretically demonstrated that the hydrogen bond H47···O5═C is weakened in the excited state S1; the weakening of the excited-state hydrogen bonding should be beneficial to the luminescence. To explore the effect of the water clusters on the luminescence, we studied four complexes Zn(3-tzba)(2,2'-bipy)(H2O)·3H2O, Zn(3-tzba)(2,2'-bipy)(H2O)·2H2O, Zn(3-tzba)(2,2'-bipy)(H2O)·H2O, and Zn(3-tzba)(2,2'-bipy)(H2O). The results reveal that the presence of water should play an important role in the emission characteristics of the MOF. Also, the UV-vis absorption and emission spectra of Zn(3-tzba)(2,2'-bipy)(H2O)·3H2O are in good agreement with the experimental results.

Insight into the mechanism of selective catalytic reduction of NO(x) by propene over the Cu/Ti(0.7)Zr(0.3)O2 catalyst by Fourier transform infrared spectroscopy and density functional theory calculations.

The mechanism of selective catalytic reduction of NOx by propene (C3H6-SCR) over the Cu/Ti0.7Zr0.3O2 catalyst was studied by in situ Fourier transform infrared (FTIR) spectroscopy and density functional theory (DFT) calculations. Especially, the formation and transformation of cyanide (-CN species) during the reaction was discussed. According to FTIR results, the excellent performance of the Cu/Ti0.7Zr0.3O2 catalyst in C3H6-SCR was attributed to the coexistence of two parallel pathways to produce N2 by the isocyanate (-NCO species) and -CN species intermediates. Besides the hydrolysis of the -NCO species, the reaction between the -CN species and nitrates and/or NO2 was also a crucial pathway for the NO reduction. On the basis of the DFT calculations on the energy of possible intermediates and transition states at the B3LYP/6-311 G (d, p) level of theory, the reaction channel of -CN species in the SCR reaction was identified and the role of -CN species as a crucial intermediate to generate N2 was also confirmed from the thermodynamics view. In combination of the FTIR and DFT results, a modified mechanism with two parallel pathways to produce N2 by the reaction of -NCO and -CN species over the Cu/Ti0.7Zr0.3O2 catalyst was proposed.

Microscopic Modes and Free Energies for Topoisomerase I-DNA Covalent Complex Binding with Non-campothecin Inhibitors by Molecular Docking and Dynamics Simulations.

Topoisomerase I (Topo1) has been identified as an attractive target for anticancer drug development due to its central role in facilitating the nuclear process of the DNA. It is essential for rational design of novel Topo1 inhibitors to reliably predict the binding structures of the Topo1 inhibitors interacting with the Topo1-DNA complex. The detailed binding structures and binding free energies for the Topo1-DNA complex interacting with typical non-camptothecin (CPT) Topo1 inhibitors have been examined by performing molecular docking, molecular dynamic (MD) simulations, and binding free energy calculations. The computational results provide valuable insights into the binding modes of the inhibitors binding with the Topo1-DNA complex and the key factors affecting the binding affinity. It has been demonstrated that the - stacking interaction with the DNA base pairs and the hydrogen bonding with Topo1 have the pivotal contributions to the binding structures and binding free energies, although the van der Waals and electrostatic interactions also significantly contribute to the stabilization of the binding structures. The calculated binding free energies are in good agreement with the available experiment activity data. The detailed binding modes and the crucial factors affecting the binding free energies obtained from the present computational studies may provide valuable insights for future rational design of novel, more potent Topo1 inhibitors.

Photodegradation fate of different dissociation species of antidepressant paroxetine and the effects of metal ion Mg2+: Theoretical basis for direct and indirect photolysis.

Paroxetine (abbreviated as PXT) has been widely used as one of the standard antidepressants for the treatment of depression. PXT has been detected in the aqueous environment. However, the photodegradation mechanism of PXT remains unclear. The present study aimed to use density functional theory and time-dependent density functional theory to study the photodegradation process of two dissociated forms of PXT in water. The main mechanisms include direct and indirect photodegradation via reaction with ·OH and 1O2 and photodegradation mediated by the metal ion Mg2+. Based on the calculations, PXT and PXT-Mg2+ complexes in water are photodegraded mainly indirectly and directly. It was found that PXT and PXT-Mg2+ complexes were photodegraded by H-abstraction, OH-addition and F-substitution. The main reaction of PXT indirect photolysis is OH-addition reaction, while the main reaction of PXT0-Mg2+ complex is H-abstraction. All the reaction pathways of H-abstraction, OH-addition and F-substitution are exothermic. PXT0 reacts more readily with ·OH or 1O2 in water than PXT+. However, the higher activation energy of PXT with 1O2 indicates that the 1O2 reaction plays a minor role in the photodegradation pathway. The direct photolysis process of PXT includes ether bond cleavage, defluorination, and dioxolane ring-opening reaction. In the PXT-Mg2+ complex, the direct photolysis process occurs via a dioxolane ring opening. Additionally, Mg2+ in water has a dual effect on the direct and indirect photolysis of PXT. In other words, Mg2+ can inhibit or promote their photolytic reactions. Overall, PXT in natural water mainly undergo direct and indirect photolysis reactions with ·OH. The main products include direct photodegradation products, hydroxyl addition products and F-substitution products. These findings provide critical information for predicting the environmental behavior and transformation of antidepressants.

Luminescent properties of metal-organic framework MOF-5: relativistic time-dependent density functional theory investigations.

The electronically excited state and luminescence property of metal-organic framework MOF-5 were investigated using relativistic density functional theory (DFT) and time-dependent DFT (TDDFT). The geometry, IR spectra, and UV-vis spectra of MOF-5 in the ground state were calculated using relativistic DFT, leading to good agreement between the experimental and theoretical results. The frontier molecular orbitals and electronic configuration indicated that the luminescence mechanism in MOF-5 follows ligand-to-ligand charge transfer (LLCT), namely, π* → π, rather than emission with the ZnO quantum dot (QD) proposed by Bordiga et al. The geometry and IR spectra of MOF-5 in the electronically excited state have been calculated using the relativistic TDDFT and compared with those for the ground state. The comparison reveals that the Zn4O13 QD is rigid, whereas the ligands BDC(2-) are nonrigid. In addition, the calculated emission band of MOF-5 is in good agreement with the experimental result and is similar to that of the ligand H2BDC. The combined results confirmed that the luminescence mechanism for MOF-5 should be LLCT with little mixing of the ligand-to-metal charge transfer. The reason for the MOF-5 luminescence is explained by the excellent coplanarity between the six-membered ring consisting of zinc, oxygen, carbon, and the benzene ring.

Promoting the Oxygen Evolution Reaction via Morphological Manipulation of a Lamellar Nanorod-Assembled Ni(II)-Pyrazolate Superstructure.

Nanoscale pyrazolate-based coordination polymers (CPs) are becoming increasingly popular as electrocatalysts owing to their customizable compositions and structures. However, using them for oxygen evolution reaction (OER) is highly challenging due to their unsatisfactory catalytic efficiency and relatively low stability. Herein, a simple one-step solvothermal process was employed for the fabrication of polycrystalline nickel-pyrazolate [Ni(Pz)] with an unusual lamellar nanorod-assembled microsphere morphology for the first time using ethanol as a green organic solvent via controlling other physical parameters. Meanwhile, the Ni(Pz) structure and morphology are investigated to derive its formation process following the different monomeric feed ratios relying on the metal/ligand interactions of CP. Shaping the Ni(Pz) electrocatalyst in well-oriented lamellar nanorod-assembled microspheres brings the advantage of porosity and high specific surface area, which expedites mass/charge transport and contact with the electrolyte as well as creates less tortuous pathways for charge distribution, thus improving the charge homogeneity. These high-class structural features and polycrystalline nature of Ni(Pz)-E-PVP facilitate amazing catalytic OER activity with a low overpotential of 290 mV at 10 mA cm-2 and a Tafel slope of only 94 mV dec-1 beyond the yardstick material (i.e., RuO2) in alkaline solution. A suite of measurements, entailing X-ray photoelectron spectroscopy and density functional theory calculations, suggest that the rich Ni-N4 moieties in Ni(Pz)-E-PVP are central species providing adsorption sites for OER intermediates. This facile protocol is prophesied to commence the imminent development of noble metal-free, effective, and low-priced electrocatalysts for OER.

Single-atom-catalyst with abundant Co-S4 sites for use as a counter electrode in photovoltaics.

Herein, a 7.35 wt% Co loading C-SAC is synthesized by pyrolysis of Co-MOF-74 in a strongly polar molten salt system. In dye-sensitized solar cells, this SAC based counter electrode shows higher photoelectric conversion efficiency than the Pt counter electrode. This work provides new insights for the preparation and application of C-SACs.

Salt melt synthesis of Chlorella-derived nitrogen-doped porous carbon with atomically dispersed CoN4 sites for efficient oxygen reduction reaction.

Carbon-supported single-atom catalysts (C-SACs) demonstrate great potential in various key electrochemical reactions. Nevertheless, the development of facile and economical strategies is highly appealing yet challenging given that the commonly used pyrolysis method has strict requirements on the structure and composition of precursors. Here, we demonstrate for the first time a facile and low-cost pyrolysis strategy assisted by molten salts at high temperature for preparing porous C-SACs with well-dispersed Co-N4 sites directly from a Chlorella precursor. Based on the X-ray absorption fine structure results and aberration-corrected scanning transmission electron microscopy images, we show that single atom Co-N4 moieties are anchored on a carbon matrix. A porous structure with a large specific surface area (2907 m2 g-1) and atomically dispersed active sites of Co provide the as-prepared Co-N/C-SAC with excellent electrocatalytic activity and stability for the ORR. The electrochemical measurements show that the half-wave potential and limited current density of this material are 0.83 V vs. RHE and 5.5 mA cm-2, respectively, which are comparable to those of commercial Pt/C.