Liuming Yan

Synthesis, Spectra, and Electron-Transfer Reaction of Aspartic Acid-Functionalized Water-Soluble Perylene Bisimide in Aqueous Solution

An aspartic acid-functionalized water-soluble perylene bisimide, N,N-di(2-succinic acid)-perylene-3,4,9,10-tetracarboxylic bisimide (PASP) was synthesized and characterized. It has absorbance maximum A(0-0) and A(0-1) at 527 and 498 nm (ε ≈ 1.7 × 10(4) L cm(-1) mol(-1)) respectively in pH 7.20 HEPES buffer. Two quasi-reversible redox processes with E1/2 at -0.17 and -0.71 V (vs Ag/AgCl) respectively in pH 7-12.5 aqueous solutions. PASP can react with Na2S in pure aqueous solution to form monoanion radical and dianion species consecutively. PASP(-•) has EPR signal with g = 1.998 in aqueous solution, whereas PASP(2-) is EPR silent. The monoanion radical formation is a first-order reaction with k = 8.9 × 10(-2) s(-1). Dianion species formation is a zero-order reaction and the rate constant is 4.3 × 10(-8) mol L(-1) s(-1). The presence of H2O2 greatly increases the radical formation rate constant. PASP as a two-electron transfer reagent is expected to be used in the water photolysis.

The modulation of metal–insulator transition temperature of vanadium dioxide: a density functional theory study

The modulation of metal–insulator transition (MIT) temperature and phase stability of thermochromic materials based on all the transition metal doped VO2 were systematically studied using density functional theory (DFT) calculations. The free energies, formation enthalpies, and Fermi energies of transition metal doped VO2 were evaluated from DFT calculations; the cell volumes and bulk moduli were obtained by fitting the free energies to the Birch–Murnaghan equation of states; and the decomposition enthalpies and entropies of the transition metal doped VO2 were calculated using both experimental data and DFT calculations. Based on these results, the MIT temperature was associated with lattice distortion of VO2 (M1) upon doping, the expansion of cell volume and the decrease in β-angle were associated with the decrease in MIT temperature, and the shrinkage of cell volume and the increase in β-angle were associated with the increase in MIT temperature. And it was also concluded that VO2 (M1) doped with high valence cations is more stable than those doped with low valence cations. These conclusions are consistent with experimental facts that W-, Mo-, and Re- are the most studied and the most effective dopants for the reduction of MIT temperature, and La-, Hg-, and Ag-doped VO2 undergoes phase separation. In addition, DFT calculations without spin-polarization were also carried out, and the influence of spin-polarization was evaluated. Finally, scandium was proposed as a potential dopant for VO2 in view of balanced comprehensive performance.

Proton Transport Pathways in an Acid–Base Complex Consisting of a Phosphonic Acid Group and a 1,2,3-Triazolyl Group

The proton transport pathways in an acid-base complex consisting of a phosphonic acid group and a 1,2,3-triazolyl group were studied using density functional theory (DFT) calculations in terms of stable configurations and transition states of the molecular or ionic dimers and trimers and verified by proof-of-concept experiments including experimental measurements of overall conductivity and (1)H NMR and FTIR spectroscopy of the methylphosphonic acid (MPA) and 1,2,3-triazole (Tri) complex as well as overall proton conductivity of polymeric blend of poly(vinylphosphonic acid) (PVPA) and poly(4-vinyl-1H-1,2,3-triazole) (PVTri). From the DFT calculations of dimers and trimers composed of ethylphosphonic acid (EPA), Tri, and their deprotonated counterparts, it was concluded that the intermolecular hydrogen bonds of the transition states corresponding to proton transport are much shorter than those of stable configurations, but the O-H and N-H bonds are much longer than those of stable configurations. The tautomerization activation energy decreases from 0.927-1.176 eV in Tri-Tri dimers to 0.336-0.444 eV in the EPA-Tri dimers. From the proof-of-concept experiments, about a 50 fold increase in overall conductivity was observed in the MPA-Tri complex consisting of 10% (molar ratio) MPA compared to pure Tri, and the calculated activation energy is consistent with the experimental activation energy evaluated from temperature dependence of proton conductivity of pure Tri and the MPA-Tri complex. In addition, the fast proton exchange between MPA and Tri, consistent with the DFT calculations, was verified by (1)H NMR and FTIR spectroscopy. Finally, a polymeric blend of PVPA and PVTri was prepared, and its proton conductivity at about 2.1 mS·cm(-1) in anhydrous state at 100 °C was observed to be significantly higher than that of PVPA or of poly(VPA-co-1-vinyl-1,2,4-triazole). The proton conductivity of the polymeric PVPA and PVTri blend in humidity state is in the same range as that of NAFION 117.

Methanol Distribution and Electroosmotic Drag in Hydrated Poly(perfluorosulfonic) Acid Membrane

The methanol distribution and electroosmotic drag in hydrated poly(perfluorosulfonic) acid electrolyte membrane are studied using molecular dynamics simulations under various electric fields applied. The results indicate that the methanol molecules are preferentially distributed near the hydrophobic PFSA backbones with their methyl groups in contact with the fluorine atoms and their hydroxyl groups pointing to the hydrophilic subphase. As the hydroxyl groups of methanol forming hydrogen bonds, hydroxyl groups are more likely to accept hydrogen atoms than to donate hydrogen atoms. The calculated methanol diffusion coefficient is in good correspondence with experimental values, and the electroosmotic drag coefficient for methanol is much smaller than that of water molecules.

Molecular dynamics simulations of electroosmosis in perfluorosulfonic acid polymer.

An atomistic MD simulation method has been developed to study the electroosmotic drag in the hydrated perfluorosulfonic acid polymer. The transport characteristics of the hydroniums and water molecules are evaluated from their velocity distribution functions with an electric field applied. It is shown that the microstructure of the hydrated perfluorosulfonic acid polymer is not perturbed significantly by the electric field up to 2 V/microm, and the velocity distribution functions obey the peak shifted Maxwell velocity distribution functions. The evaluated peak shifting velocities are only about 1% of the average thermal motion. The hydronium flow and water flow are evaluated from the average transport velocities or the peak shifting velocities. The electroosmotic drag coefficients from the MD simulations are in good correspondence with the experimental values. It is also shown that the electroosmotic drag coefficient has no or weak temperature dependence.

Highly Active Carbon Supported Pd–Ag Nanofacets Catalysts for Hydrogen Production from HCOOH

Hydrogen is regarded as a future sustainable and clean energy carrier. Formic acid is a safe and sustainable hydrogen storage medium with many advantages, including high hydrogen content, nontoxicity, and low cost. In this work, a series of highly active catalysts for hydrogen production from formic acid are successfully synthesized by controllably depositing Pd onto Ag nanoplates with different Ag nanofacets, such as Ag{111}, Ag{100}, and the nanofacet on hexagonal close packing Ag crystal (Ag{hcp}). Then, the Pd-Ag nanoplate catalysts are supported on Vulcan XC-72 carbon black to prevent the aggregation of the catalysts. The research reveals that the high activity is attributed to the formation of Pd-Ag alloy nanofacets, such as Pd-Ag{111}, Pd-Ag{100}, and Pd-Ag{hcp}. The activity order of these Pd-decorated Ag nanofacets is Pd-Ag{hcp} > Pd-Ag{111} > Pd-Ag{100}. Particularly, the activity of Pd-Ag{hcp} is up to an extremely high value, i.e., TOF{hcp} = 19u202f000 ± 1630 h(-1) at 90 °C (lower limit value), which is more than 800 times higher than our previous quasi-spherical Pd-Ag alloy nanocatalyst. The initial activity of Pd-Ag{hcp} even reaches (3.13 ± 0.19) × 10(6) h(-1) at 90 °C. This research not only presents highly active catalysts for hydrogen generation but also shows that the facet on the hcp Ag crystal can act as a potentially highly active catalyst.

Synergetic proton conducting effect in acid–base composite of phosphonic acid functionalized polystyrene and triazolyl functionalized polystyrene

The synergetic proton conducting effect with three orders of magnitude improvement in proton conductivity was observed in an acid–base composite composed of phosphonic acid functionalized polystyrene (PS-PA) and triazolyl functionalized polystyrene (PS-Tri). In addition, a new method for the development of proton conducting materials by the combination of different acidic and basic polymers is proposed. The PS-PA was synthesized by the bromination of polystyrene on the para-position of the phenyl ring followed by phosphonation and hydrolysis. The PS-Tri was synthesized by the chloromethylation of polystyrene on the para-position of the phenyl ring followed by azidation and 1,3-dipolar cycloaddition or ‘click’ reaction. A maximum proton conductivity of 11.2 mS cm−1, which is three orders of magnitude higher than that of pristine PS-PA or PS-Tri, a tensile strength of 16.3 MPa, and a minimum water uptake of 15.1% (90 °C, 90% RH) were observed in the PS-PA/PS-Tri composite composed of 66.7% PS-PA. Finally, a mosaic-like morphology model and space-charge effects were proposed to explain the synergetic proton conducting effect.

Structural and transport characteristics of UCl3 in molten LiCl-KCl mixture: a molecular dynamics simulation study

To obtain suitable data for the pyrometallurgical post-processing in the fusion-fission hybrid reactor, the structure and transport characteristics of molten LiCl-KCl mixture containing UCl3 were studied by molecular dynamics simulation. The radial distribution functions, densities, and self-diffusion coefficients were investigated at various molar fractions of UCl3. In the molten LiCl-KCl-UCl3 salt mixture, the first peak for gU-Cl(r) was located at 0.266 nm, which was slightly left-shifted than the X-ray diffraction data, i.e., 0.285 nm for pure molten UCl3. The preexponential factors for U3+ decreased from 46.2×10−5 cm2/s to 32.2×10−5 cm2/s as the molar fraction of U3+ increased from 0.005 to 0.05.

Hydrogen Bond and Proton Transport in Acid–Base Complexes and Amphoteric Molecules by Density Functional Theory Calculations and 1H and 31P Nuclear Magnetic Resonance Spectroscopy

Intermolecular and intramolecular hydrogen bond (H-bond) and proton transport in acid-base complexes and amphoteric molecules consisting of phosphonic acid groups and nitrogenous heterocyclic rings are investigated by density functional theory calculations and (1)H NMR and (31)P NMR spectroscopy. It is concluded that a phosphonic acid group can act both as H-bond donor and H-bond acceptor, while an imine nitrogen atom can only act as H-bond acceptor and an amine group as H-bond donor. And the intramolecular H-bond is weaker than the intermolecular H-bond attributing to configurational restriction. In addition, the strongest H-bond interaction is observed between a phosphonic acid and a 1H-indazole because of the formation of double H-bonds. The (1)H NMR and (31)P NMR chemical shifts for the acid-base complexes are consistent with the density functional theory calculations. From the (1)H NMR chemical shifts, fast proton exchange is observed between a phosphonic acid and 1H-benzimidazole or 1H-indazole. Finally, it is proposed that polymeric material tethered with 1H-benzimidazole or 1H-indazole rings is a favorable component for high-temperature proton exchange membranes based on acid-base complexes or acid-base amphoteric molecules.

Optical super-resolution microscopy and its applications in nano-catalysis

The resolution of conventional optical microscopy is only ∼200 nm, which is becoming less and less sufficient for a variety of applications. In order to surpass the diffraction limited resolution, super-resolution microscopy (SRM) has been developed to achieve a high resolution of one to tens of nanometers. The techniques involved in SRM can be assigned into two broad categories, namely “true” super-resolution techniques and “functional” super-resolution techniques. In “functional” super-resolution techniques, stochastic super-resolution microscopy (SSRM) is widely used due to its low expense, simple operation, and high resolution. The principle process in SSRM is to accumulate the coordinates of many diffraction-limited emitters (e.g., single fluorescent molecules) on the object by localizing the centroids of the point spread functions (PSF), and then reconstruct the image of the object using these coordinates. When the diffraction-limited emitters take part in a catalytic reaction, the activity distribution and kinetic information about the catalysis by nanoparticles can be obtained by SSRM. SSRM has been applied and exhibited outstanding advantages in several fields of catalysis, such as metal nanoparticle catalysis, molecular sieve catalysis, and photocatalysis. Since SSRM is able to resolve the catalytic activity within one nanoparticle, it promises to accelerate the development and discovery of new and better catalysts. This review will present a brief introduction to SRM, and a detailed description of SSRM and its applications in nano-catalysis.