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.

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.

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.

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.