Hydroxy and surface oxygen effects on 5-hydroxymethylfurfural oxidation to 2,5-furandicarboxylic acid on ß-MnO2: DFT, microkinetic and experiment studies.

Manganese dioxide, ß-MnO2, has shown potential in catalyzing the oxidation of 5-hydroxymethylfurfural (HMF) to 2,5-furandicarboxylic acid (FDCA), a monomer of bioplastic polyethylene furanoate (PEF). Herein, the insight into the hydroxy (OH) and surface oxygen effects on the HMF-to-FDCA reaction over ß-MnO2 is clarified through a comprehensive investigation using density functional theory (DFT) calculations, microkinetic modeling, and experiment. Theoretical analyses revealed that both active surface oxygen and OH species (from either base or solvent) facilitate C-H bond breaking and OH insertion, promoting the catalytic activity of ß-MnO2. Microkinetic modeling demonstrated that the FFCA-to-FDCA and DFF-to-FFCA steps are the rate-limiting steps of the hydroxylated and non-hydroxylated surfaces, respectively. These theoretical results agree well with the experiment when water and dimethyl sulfoxide (DMSO) were used as solvents. In addition, the synthesized ß-MnO2 catalyst showed high stability and activity, maintaining stable HMF conversion (≥99 mol%) and high FDCA yield (85-92 mol%) during continuous flow oxidation for 72 hours at pO2 of 1 MPa, 393 K and LHSV of 1 h-1. Thus, considering both hydroxy and surface oxygen species is a new strategy for enhancing the catalytic activity of Mn oxides and other metal oxide catalysts for the HMF-to-FDCA reaction.

Synergistic Induction of Solvent and Ligand-Substitution in Single-Crystal to Single-Crystal Transformations toward a MOF with Photocatalytic Dye Degradation.

External-stimuli responsiveness as found in natural organisms and smart materials is attractive for functional materials scientists who attempt to design and imitate fascinating behavior into their materials. Herein, we report a couple of new solvent-responsive isostructural two-dimensional cationic metal-organic frameworks (MOFs) of Mn(II) (1a) and Zn(II) (2a) that undergo unprecedented single-crystal to single-crystal (SCSC) transformation toward the corresponding isostructural three-dimensional MOFs of Mn(II) (1b) and Zn(II) (2b). The 2D MOFs 1a and 2a have been effortlessly and rapidly synthesized via the microwave-heating technique. The SCSC transformations are synergistically induced by solvent and ligand-substitution reactions and able to be triggered by water, methanol, ethanol, and n-propanol. Time-dependent SCSC transformations were studied by in situ X-ray diffraction. Investigations on photodegradation of methyl orange showed that Zn-MOF 2b has higher efficiency than Mn-MOF 1b under UV-C irradiation at 300 min, 94.27%, and 21.91%, respectively. The influence of charge on the dye molecules, heterogeneity of the catalysis, and •OH radical-scavenging test was studied. First-principles computations suggest that the high photocatalytic activity of 2b may be attributed to its suitable band-edge position for redox reactions.

Ecofriendly alkali metal cations diffusion improves fabrication of mixed-phase titania polymorphs on fixed substrate by chemical vapor deposition (CVD) for photocatalytic degradation of azo dye.

Controlling the nanoscale synthesis of semiconductor TiO2 on a fixed substrate has fascinated the curiosity of academics for decades. Synthesis development is required to give an easy-to-control technique and parameters for TiO2 manufacture, leading to advancements in prospective applications such as photocatalysts. This study, mixed-phase TiO2(B)/other titania thin films were synthesized on a fused quartz substrate utilizing a modified Chemical vapor depodition involving alkali-metal ions (Li+, Na+, and K+) solution pre-treatment. It was discovered that different cations promote dramatically varied phases and compositions of thin films. The films had a columnar structure with agglomerated irregular-shaped particles with a mean thickness of 800-2000 nm. Na+ ions can promote TiO2(B) more effectively than K+ ions, however Li+ ions cannot synthesize TiO2(B). The amounts of TiO2(B) in thin films increase with increasing alkali metal (K+ and Na+) concentration. According to experimental and DFT calculations, the hypothesized TiO2(B) production mechanism happened via the meta-stable intermediate alkaline titanate transformation caused by alkali-metal ion diffusion. The mixed phase of TiO2(B) and anatase TiO2 on the fixed substrate (1 × 1 cm2) obtained from Na+ pre-treated procedures showed significant photocatalytic activity for the degradation of methylene blue. K2Ti6O12, Li2TiO3, Rutile TiO2, and Brookite TiO2 phase formations produced by K+ and Li + pretreatment are low activity photocatalysts. Photocatalytic activities were more prevalent in NaOH pre-treated samples (59.1% dye degradation) than in LiOH and KOH pre-treated samples (49.6% and 34.2%, respectively). This revealed that our developed CVD might generate good photocatalytic thin films of mixed-phase TiO2(B)/anatase TiO2 on any substrate, accelerating progress in future applications.

Role of inorganic layers on polysulfide decomposition at sodium-metal anode surfaces for room temperature Na/S batteries.

Sodium metal is a promising anode material for room-temperature sodium sulfur batteries. Due to its high reactivity, typical liquid electrolytes (e.g. carbonate-based solvents and a Na salt) can undergo reduction to form a solid electrolyte interphase (SEI) layer, with inorganic components such as Na2CO3, Na2O, and NaOH, covering the anode surface along with other SEI organic products. One of the challenges is to understand the effect of the SEI film on the decomposition of soluble sodium polysulfide molecules (e.g., Na2S8) upon shuttling from the cathode to anode during battery cycling. Here, we use ab initio molecular dynamics (AIMD) simulations to study the role of an inorganic SEI used as a model passivation layer in polysulfide decomposition. Compared to other film chemistries, it is found that the Na2CO3 film can suppress decomposition with the slowest reduction rate and the smallest amount of charge transfer towards Na2S8. The Na2CO3 film can maintain its structural properties during the simulations. In contrast, Na2O and NaOH allow some decomposed polysulfide fragments to be inserted into the SEI layer. Moreover, the decomposition of Na2S8 on both Na2O and NaOH SEI layers is more reactive with more charge transfer to Na2S8 when compared to that of Na2CO3. Thus, the ability of the SEI to suppress polysulfide decomposition is in the order: Na2CO3 > NaOH ∼ Na2O. Analyses of the density of states reveal that the Na2S8 molecule receives electrons from the Na metal directly in the presence of n-type semiconductor films of Na2CO3 and NaOH, while the charge migration behavior is different in a p-type semiconductor Na2O with the SEI film donating its electrons to the polysulfide solely. Thus, this work adds new insights into charge transfer behavior of inorganic thin film SEIs that could be present at the initial stages of SEI formation.

Polysulfide cluster formation, surface reaction, and role of fluorinated additive on solid electrolyte interphase formation at sodium-metal anode for sodium-sulfur batteries.

Room-temperature sodium-sulfur batteries are promising next-generation energy storage alternatives for electric vehicles and large-scale applications. However, they still suffer from critical issues such as polysulfide shuttling, which inhibit them from commercialization. In this work, using first-principles methods, we investigated the cluster formation of soluble Na2S8 molecules, the reductive decomposition of ethylene carbonate (EC) and propylene carbonate (PC), and the role of fluoroethylene carbonate (FEC) additive in the solid electrolyte interphase formation on the Na anode. The clustering of Na2S8 in an EC solvent is found to be more favorable than in a PC solvent. In the presence of an electron-rich Na (001) surface, EC decomposition undergoes a two-electron transfer reaction with a barrier of 0.19 eV for a ring-opening process, whereas PC decomposition is difficult on the same surface. Although the reaction kinetics of an FEC ring opening in the EC and PC solvents are quite similar, the reaction mechanisms of the open FEC are found to be different in each solvent, although both lead to the production of NaF on the surface. The thick NaF layers reduce the extent of charge transfer to Na2S8 at the anode/electrolyte interface, thus decelerating the Na2S8 decomposition reaction. Our results provide an atomistic insight into the interfacial phenomena between the Na-metal anode surface and electrolyte media.

Controlling the Photocatalytic Activity and Benzylamine Photooxidation Selectivity of Bi2WO6 via Ion Substitution: Effects of Electronegativity.

Doping or ion substitution is often used as an effective strategy to improve photocatalytic activities of several semiconductors. Most frequently, the dopants provide extra states to increase light absorption, alter the electronic structure, or lower the carrier recombination. This work focuses on ion substitution in Bi2WO6, where the dopants modify band-edge potentials of the catalysts. Specifically, we investigate how the electronegativity (EN) of the dopant could be used to tune the band-edge potentials and how such changes influence the photocatalytic mechanism. Compared to Te that has a lower EN, I lowers the band-edge potentials. While substitutions with both ions enhance Rh B photodegradation and benzylamine photooxidation, the modified band potentials of I-doped Bi2WO6 influence the benzylamine photooxidation pathway, resulting in higher selectivity. Additionally, substitution of I7+ in the Bi2WO6 lattice improves the morphologies, decreases the band-gap energy, and reduces the carrier recombination. As a result, I-doped Bi2WO6 shows almost 3 times higher %conversion while maintaining 100% selectivity in the oxidative coupling of benzylamine. The findings here signify the importance of the choices of dopants on the photocatalytic reactions and would benefit the design of other related materials for such applications.

First-principles-driven catalyst design protocol of 2D/2D heterostructures for electro- and photocatalytic nitrogen reduction reaction.

Ammonia synthesis from nitrogen is a vital process and a necessity in a variety of applications including energy, pharmaceutical, agricultural, and chemical applications. The electro- and photocatalytic nitrogen reduction reactions (NRRs) are promising sustainable processes operated under milder conditions than the conventional Haber-Bosch process. However, the main pain points of these catalytic processes are their low selectivity and low efficiency. This perspective presents the recent status and the design protocols for developing promising 2D/2D heterojunction catalysts for the NRR, using the first-principles approach. The current theoretical studies are briefly discussed, and available methods are suggested for the development and design of new potential 2D/2D heterojunctions as efficient electro- and photo-NRR catalysts.

Conductive Co-triazole metal-organic framework exploited as an oxygen evolution electrocatalyst.

A Co-triazole metal-organic framework (Co-trz) endowed with electrical conductivity was synthesized effortlessly via a microwave-based method. Providing a high density of catalytic centers with electrically conductive features, as suggested by DFT calculations, the framework exhibited a low overpotential for the oxygen evolution reaction (OER) with good kinetics. A mechanistic reaction pathway was proposed based on monitoring alterations in the oxidation state and local coordination environment of Co centers upon the occurrence of the OER. Due to its performance and its chemical and electrochemical robustness, the framework was highlighted as a promising MOF electrocatalyst for the OER.

Copper(II) Compounds of 4-Nitrobenzohydrazide with Different Anions (ClO4 - , NO3 - and Br- ): Synthesis, Characterization, DFT Calculations, DNA Interactions and Cytotoxic Properties.

Copper complexes containing 4-nitrobenzohydrazide (4-NH), [Cu(4-NH)2 (ClO4 )2 ] (1), [Cu(4-NH)2 (NO3 )2 ] (2) and [Cu(4-NH)2 Br2 ] (3), were synthesized and characterized by various spectroscopy. DFT calculations have revealed a distorted octahedral for 1, 2 and 3. DNA interactions of 1-3 toward calf-thymus DNA were investigated by absorption titration, viscosity, fluorescence spectroscopy and gel electrophoresis. All complexes bind to DNA via intercalation mode. The Kb values have shown the binding ability in the order of 3>2>1. The in vitro cytotoxicity of 1-3 was tested against two human cancer cells (HeLa and MCF-7) and normal Vero cells by MTT assay. All complexes show anticancer ability upon increasing time, but the ligand is inactive. Complex 3 with the Br- anion gives the lowest IC50 values of 19.26±2.43 µg/mL toward MCF-7. Moreover, all complexes were not arrested at any cell cycle phases. However, the sub-G1 populations were enhanced upon increasing the complex concentration. Therefore, they may induce apoptotic cell death for both cancer cells. The antibacterial activity of 1-3 was tested against E. coli, Salmonella and Campylobacter. Complex 3 gives the best activity toward Campylobacter (0.156×103  µg/mL).

Promoting Electrochemical Performance of Ti3C2O2 MXene-Based Electrodes of Alkali-Ion Batteries via S Doping: Theoretical Insight.

Ti3C2O2 MXene has been proposed as a promising electrode material for alkali-ion batteries owing to its tunable physical and chemical properties without sacrificing the excellent metallic conductivity. However, it still suffers from low specific capacity due to its limited interlayer spacing, especially for a larger ion like sodium (Na). Sulfur doping was suggested as a viable strategy to improve the electrode's storage performance. Herein, first-principles calculations and kinetic Monte Carlo (kMC) simulations were carried out to study the role of S doping on Li/Na intercalation. Based on experimental findings, two different doping sites, C (SC) and O (SO), with various S concentrations were reported and therefore used as the models in this study. Computations reveal that S doping on both C and O sites improves the electronic conductivity of the MXenes as their densities of states at the Fermi level are increased. In addition, the doped MXenes reveal an expanded lattice parameter in the normal direction, which agrees with experimental observations. However, only the SO-doped MXenes display an enlarged interlayer spacing, whereas doping at the C site only increases the layer thickness. The enlarged interlayer spacing in the SO-doped MXenes improves stabilities and transport kinetics of ion intercalation as indicated by their significantly lower insertion energies and diffusion barriers when compared with those of the pristine system. The kMC simulations were carried out to account for anisotropic diffusion in the SO-doped system. The obtained macroscopic properties of diffusion coefficients and apparent activation energies of the SO-doped system clearly confirm its superior transport kinetics. The estimated diffusion coefficients of Li(Na) are improved by 4(8) orders of magnitude upon SO doping. A fundamental understanding of the role of S doping on the improved capacitive kinetics serves as a good guide for developing MXene-based electrode materials for Li- and Na-ion batteries.

Electronic and thermodynamic properties of native point defects in V2O5: a first-principles study.

The formation of native point defects in semiconductors and their behaviors play a crucial role in material properties. Although the native defects of V2O5 include vacancies, self-interstitials, and antisites, only oxygen vacancies have been extensively explored. In this work, we carried out first-principles calculations to systematically study the properties of possible native defects in V2O5. The electronic structure and the formation energy of each defect were calculated using the DFT+U method. Defect concentrations were estimated using a statistical model with a constraint of charge neutrality. We found that the vanadyl vacancy is a shallow acceptor that could supply holes to the system. However, the intrinsic p-type doping in V2O5 hardly occurred because the vanadyl vacancy could be readily compensated by the more stable donor, i.e., the oxygen vacancy and oxygen interstitial, instead of holes. The oxygen vacancy is the most dominant defect under oxygen-deficient conditions. However, under extreme O-rich conditions, a deep donor of oxygen interstitial becomes the major defect species. The dominant oxygen vacancy under synthesized conditions plays an important role in determining the electronic conductivity of V2O5. It induces the formation of compensating electron polarons. The polarons are trapped at V centers close to the vacancy site with the effective escaping barriers of around 0.6 eV. Such barriers are higher than that of the isolated polaron hopping (0.2 eV). The estimated polaron mobilities obtained from kinetic Monte Carlo simulations confirmed that oxygen vacancies act as polaron-trapping sites, which diminishes the polaron mobility by 4 orders of magnitude. Nevertheless, when the sample is synthesized at elevated temperatures, a number of thermally activated polarons in samples are quite high due to the high concentrations of oxygen vacancies. These polarons can contribute as charge carriers of intrinsic n-type semiconducting V2O5.

Mg-Ni-La based small hydrogen storage tank: kinetics, reversibility and reaction mechanisms.

The improvement of de/rehydrogenation kinetics and reversibility of a Mg-Ni-La based small hydrogen storage tank by doping with TiF4 and MWCNTs is reported for the first time. During sample preparation, MgH2 milled with 20 wt% LaNi5 and 5 wt% TiF4 and MWCNTs produces Mg2NiH4 and LaH3. Two-step dehydrogenation of Mg2NiH4 and MgH2 is detected at 295 and 350 °C, respectively. Hydrogen desorption and absorption of the tank complete within 150 and 16 min, respectively, together with reversible hydrogen storage capacity up to 4.00 wt% H2 (68% of theoretical value) upon 16 de/rehydrogenation cycles. Heat release from exothermic hydrogenation is removed effectively at the end of a double tube heat exchanger, where the reaction heat and heat transfer fluid are first in contact. Co-catalytic effects of Mg2NiH4 and LaH3 as well as good hydrogen diffusion benefit dehydrogenation kinetics and reversibility of the tank.

Electrochemical oxidation of resorcinol: mechanistic insights from experimental and computational studies.

This work investigates the mechanisms of resorcinol oxidation by density functional theory (DFT) calculation and cyclic voltammetry measurements. Complementary data from experimental and computational studies provide new insights into the reaction mechanisms. At both macro- and micro-electrodes, cyclic voltammetry of resorcinol is chemically and electrochemically irreversible over the whole pH range (1-14). Resorcinol molecules undergo a 1H+ 1e- oxidation at pH < pK a1 and a 1e- oxidation at pH > pK a2 to form radicals. The radicals then readily react to form dimers/polymers deposited on the electrode surface. All of the experimental findings are consistent with the proposed mechanisms, including the apparent transfer coefficient (ß) of 0.6 ± 0.1, the slope of the peak potential (E p) against pH of -54 mV pH-1, the peak-shaped responses at micro-electrodes, and the fouling of the electrodes upon the oxidation of resorcinol. DFT calculation of the reaction energy of elementary steps and the eigenvalues of the highest occupied molecular orbital (HOMO) of the radical intermediates confirms that the (1H+) 1e- oxidation is the energetically favorable pathway. In addition to mechanistic insights, an electrochemical sensor is developed for resorcinol detection at microelectrodes in low ionic strength samples with the sensitivity of 123 ± 4 nA µM-1 and the limit of detection (3 sB m-1) of 0.03 µM.

Transport properties of electron small polarons in a V2O5 cathode of Li-ion batteries: a computational study.

Employing the first-principles plane-wave approach, we explored the behavior of electron transport in the V2O5 cathode. Polaron migrations along different crystallographic directions in the presence and absence of Li+ ions were systematically examined using linear interpolation (LE) and nudged elastic band (NEB) methods. We find that the NEB calculations, based on structural optimizations of TS structures, generally exhibit lower hopping barriers than those obtained from the LE calculations. Both methods consistently predict that the [010] hopping, in the presence and absence of a nearby Li+ ion, is kinetically least favorable since the migration involves displacements of rigid 3-coordinated O atoms. Computations based on the LE method reveal anisotropic polaron mobilities where the estimated hopping frequencies within the layer are approximately one order of magnitude higher than the normal. The prediction based on the LE calculations is consistent with the experimental results. Lithiation dramatically affects the behavior of polaron movement. It significantly increases the reaction energies and hopping barriers due to the strong polaron-ion interaction. In addition, it is predicted that polaron hopping in the V2O5 cathode is non-adiabatic where lithiation has negligible effects on the adiabaticity.

Theoretical investigation of H2 oxidation on the Sr2Fe(1.5)Mo(0.5)O6 (001) perovskite surface under anodic solid oxide fuel cell conditions.

Periodic density functional theory (DFT) calculations and microkinetic modeling are used to investigate the electrochemical oxidation of H2 fuel on the (001) surface of Sr2Fe1.5Mo0.5O6 (SFMO) perovskite under anodic solid oxide fuel cell conditions. Three surface models with different Fe/Mo ratios in the topmost layer-identified by ab initio thermodynamic analysis-are used to investigate the H2 oxidation mechanism. A microkinetic analysis that considers the effects of anode bias potential suggests that a higher Mo concentration in the surface increases the activity of the surface toward H2 oxidation. At operating voltage and anodic SOFC conditions, the model predicts that water desorption is rate-controlling and that stabilizing the oxygen vacancy structure increases the overall rate for H2 oxidation. Although we find that Mo plays a crucial role in improving catalytic activity of SFMO, under fuel cell operating conditions, the Mo content in the surface layer tends to be very low. On the basis of these results and in agreement with previous experimental observations, a strategy for improving the overall electrochemical performance of SFMO is increasing the Mo content or adding small amounts of an active transition metal, such as Ni, to the surface to lower the oxygen vacancy formation energy of the SFMO surface.