You Han

Plasma application for more environmentally friendly catalyst preparation

The present status of catalyst preparation using nonthermal plasma treatment has been summarized in this paper. Improved dispersion, better low-temperature activity, enhanced stability, and better anti-carbon deposition performance can be achieved with nonthermal plasma-treated catalysts. The improvement in catalyst preparation with nonthermal plasma treatment can reduce or avoid the use of hazardous chemicals. Nonthermal plasma catalyst treatment has especially induced a new development of nonthermal plasma for catalyst reduction. The reduction using hydrogen at high temperatures or using hazardous liquid chemicals can be replaced by the developed plasma reduction process. The mechanism for nonthermal plasma treatment has been presented. An analog between the man-made gas discharge plasmas and the environment inside the zeolite pores and around catalyst surface defects is also proposed.

Deactivation mechanism of AuCl3 catalyst in acetylene hydrochlorination reaction: a DFT study

The deactivation mechanism of AuCl3 catalyst in the reaction of acetylene hydrochlorination was studied by using AuCl3 dimer model and the density functional theory (DFT) method. Four possible paths for the acetylene hydrochlorination reaction catalyzed by AuCl3 were illustrated with corresponding transition states. The activation free energies and reaction rate constants of the four paths were also analyzed. It is apparent that when HCl and C2H2 coadsorbed on the AuCl3 dimer, the C2H2 was co-catalyzed by HCl and the AuCl3 dimer to produce C2H3Cl and the reaction energy barrier was as low as 23.35 kcal mol−1. If the HCl in the gas phase could not adsorb on the Au site within the set time, the intermediate chlorovinyl was difficult to desorb from the AuCl3 catalyst as its desorption energy was as high as 41.336 kcal mol−1. As the reaction temperature increased, C2H2 became easier to be adsorbed on the AuCl3 catalyst prior to HCl, which resulted in the side reaction and the rapid deactivation of the AuCl3 dimer due to the loss of Cl atoms. Our calculations are necessary for us to clearly understand the experimental results, which indicate a great dependence of activity and stability of AuCl3 catalysts on the HCl : C2H2 ratio as well as the temperature.

Growth Mechanisms of Fluorescent Silver Clusters Regulated by Polymorphic DNA Templates: A DFT Study

The aggregation behaviors of silver atoms modulated by polymorphic DNA templates involving i-motif, G-quadruplex, and the Watson-Crick duplex, were investigated by using the density functional theory (DFT) calculations, combining with the experimental characterizations of CD, UV, fluorescence measurements and TEM, in order to understand the reason in the molecular level that polymorphic DNA templates affect the fluorescence emitting species of Ag nanomaterials. First, the affinity sites of silver ions on different DNA templates were analyzed by using DFT calculations, and the conformational variations of DNA templates caused by silver ions and atoms were disclosed. Second, the aggregation behaviors of silver atoms constrained by the polymorphic DNA templates were studied by DFT modeling, and distinct fluorescence property of nanosilvers templated by polymorphic DNA were evaluated using the time-dependent DFT calculations. It is illustrated that with the DNA template adopting i-motif or the duplex the silver atoms tend to aggregate inside the encapsulated spaces of nucleobases, and the formed silver nanoclusters are positively charged with high fluorescent spectral features; whereas with the template of the G-quadruplex the silver atoms are preferential to aggregate outside of the G-tetrad, which results in the formation of larger silver crystals without fluorescence property. The results obtained here are useful to explore the nucleation and growth mechanism of silver nanomaterials regulated by the structure-specific DNA templates, which is important to rational design of desirable fluorescent emitters for sensing in the field from biology to nanoscience.

Development, applications and challenges of ReaxFF reactive force field in molecular simulations

As an advanced and new technology in molecular simulation fields, ReaxFF reactive force field has been developed and widely applied during the last two decades. ReaxFF bridges the gap between quantum chemistry (QC) and non-reactive empirical force field based molecular simulation methods, and aims to provide a transferable potential which can describe many chemical reactions with bond formation and breaking. This review presents an overview of the development and applications of ReaxFF reactive force field in the fields of reaction processes, biology and materials, including (1) the mechanism studies of organic reactions under extreme conditions (like high temperatures and pressures) related with high-energy materials, hydrocarbons and coals, (2) the structural properties of nanomaterials such as graphene oxides, carbon nanotubes, silicon nanowires and metal nanoparticles, (3) interfacial interactions of solid-solid, solid-liquid and biological/inorganic surfaces, (4) the catalytic mechanisms of many types of metals and metal oxides, and (5) electrochemical mechanisms of fuel cells and lithium batteries. The limitations and challenges of ReaxFF reactive force field are also mentioned in this review, which will shed light on its future applications to a wider range of chemical environments.

Highly Active Subnano Palladium Clusters Embedded in i-Motif DNA

Highly active subnano Pd clusters were synthesized using i-motif DNA as the template through characterization via ESI MS, DLS, XPS, UV-vis, and FTIR. It is indicated that Pd1-Pd5 clusters are generated at a [Pd]/[base] ratio of 0.2, Pd8 to Pd9 clusters are generated at a [Pd]/[base] ratio of 0.5, and large nanoparticles with the size about 2.6 nm are formed at a [Pd]/[base] ratio of 2.0. The i-motif-stabilized Pd8-Pd9 clusters show high catalytic activity toward the reduction of 4-nitrophenol with a relative rate constant value of 2034 min(-1) (mM Pd)(-1). DFT calculations disclose that the unique structure of the i-motif with consecutive hemiprotonated CH(+)·C pairs can efficiently ligate Pd ions at the N3 sites of cytosines and that the synthesized Pd clusters consist of metallic Pd atoms as well as positively charged Pd that is ligated by nucleobases, which is capable of facilitating the activation of the nitryl group of 4-nitrophenol. This work suggests a promising pathway to preparing subnano metal catalysts with enhanced catalytic activity using programmable DNA scaffolds.

Effect of supercritical water on the stability and activity of alkaline carbonate catalysts in coal gasification

Abstract The stability and activity of alkaline carbonate catalysts in supercritical water coal gasification has been investigated using density functional theory method. Our calculations present that the adsorption of Na 2 CO 3 on coal are more stable than that of K 2 CO 3 , but the stability of Na 2 CO 3 is strongly reduced as the cluster gets larger. In supercritical water system, the dispersion and stability of Na 2 CO 3 catalyst on coal support is strongly improved. During coal gasification process, Na 2 CO 3 transforms with supercritical water into NaOH and NaHCO 3 , which is beneficial for hydrogen production. The transformation process has been studied via thermodynamics and kinetics ways. The selectively catalytic mechanism of NaOH and the intermediate form of sodium-based catalyst in water-gas shift reaction for higher hydrogen production has also been investigated. Furthermore, NaOH can transform back to Na 2 CO 3 after catalyzing the water-gas shift reaction. Thus, the cooperative effects between supercritical water and Na 2 CO 3 catalyst form a benignant circle which greatly enhances the reaction rate of coal gasification and promotes the production of hydrogen.

Analysis of degradation mechanism of disperse orange 25 in supercritical water oxidation using molecular dynamic simulations based on the reactive force field.

Abstract4-[N-(2-cyanoethyl)-N-ethylamino]-4′-nitroazo-benzene (disperse orange 25, DO25) is one of the main components in dyeing wastewater. In this work, supercritical water oxidation (SCWO) process of DO25 has been investigated using the molecular dynamic simulations based on the reactive force field (ReaxFF). For the SCWO system, the effects of temperature, the molecular ratio of DO25, O2 and H2O as well as the reaction time have been analyzed. The simulated results showed that the aromatic rings in DO25 could be attacked by hydroxyl radical, oxygen molecule, and hydroxyl radical together with oxygen molecule, respectively, which caused the aromatic ring-opening reaction to happen mainly through three different pathways. The hydroxyl radicals were mainly from water clusters and H2O2 (which was produced from oxygen molecules reacting with water clusters). However, for the SCW system as comparison, the aromatic rings in DO25 could be attacked by hydroxyl radical only, and the OH radicals just come from water clusters. During the DO25 SCWO degradation process, we also found that N elements in one DO25 molecule were difficult to be converted into environmentally friendly N2 molecules because of steric hindrance, but increasing the number of DO25 molecules could improve the possibility for the connection of N elements, thus promoting N element converting into N2. Extending reaction time could also improve N elements in DO25 to transform into N2 rather than carbonitride. Graphical AbstractThe processes of making DO25 wastewater by SCWO into clean water

Novel Heating-Induced Reversion during Crystallization of Al-based Glassy Alloys.

Thermal stability and crystallization of three multicomponent glassy alloys, Al86Y7Ni5Co1Fe0.5Pd0.5, Al85Y8Ni5Co1Fe0.5Pd0.5 and Al84Y9Ni4Co1.5Fe0.5Pd1, were examined to assess the ability to form the mixture of amorphous (am) and fcc-aluminum (α-Al) phases. On heating, the glass transition into the supercooled liquid is shown by the 85Al and 84Al glasses. The crystallization sequences are [am] → [am + α-Al] → [α-Al + compounds] for the 86Al and 85Al alloys, and [am] → [am + α-Al + cubic AlxMy (M = Y, Ni, Co, Fe, Pd)] → [am + α-Al] → [α-Al + Al3Y + Al9(Co, Ni)2 + unknown phase] for the 84Al alloy. The glass transition appears even for the 85Al alloy where the primary phase is α-Al. The heating-induced reversion from [am + α-Al + multicomponent AlxMy] to [am + α-Al] for the 84Al alloy is abnormal, not previously observed in crystallization of glassy alloys, and seems to originate from instability of the metastable AlxMy compound, in which significant inhomogeneous strain is caused by the mixture of solute elements. This novel reversion phenomenon is encouraging for obtaining the [am + α-Al] mixture over a wide range of high temperature effective for the formation of Al-based high-strength nanostructured bulk alloys by warm working.

Mechanistic insight into the selective crystallization of the metastable polymorph of tolbutamide in ethanol–water solution

The rapid-cooling crystallization of tolbutamide (TB) was carried out from ethanol and ethanol–water solutions at different initial supersaturations (3.5 and 2.7). PXRD and FTIR were used to characterize the polymorphs. It was found that the metastable Form IV with advanced solubility and bioavailability was obtained from ethanol–water solutions at the higher supersaturation, while stable Form III directly crystallized from ethanol at both supersaturations and from ethanol–water solution at the lower supersaturation. Hirshfeld surface analysis and the associated 2D fingerprint plots of the five TB polymorphs clearly quantify the interactions within the crystal structures. The mechanism of selective crystallization of the metastable polymorph of tolbutamide in ethanol–water solution was disclosed by molecular dynamics simulation. It was indicated that stronger interactions between TB and solvents weaken the TB–TB intermolecular NH⋯O hydrogen bonds and thus promote TB molecules to form dimers by π⋯π stacking. This work provides a feasible approach, combining experimental and molecular dynamics simulation methods, to better understanding the effect of solvent and supersaturation on polymorph outcomes by studying the competitive relationship between solute–solute and solute–solvent interactions, which is fundamental to the rational design of experimental work for controlling organic crystal polymorphs by simply varying solvents and supersaturations.

Treatment of penicillin with supercritical water oxidation: Experimental study of combined ReaxFF molecular dynamics

Supercritical water oxidation (SCWO) of penicillin (PCN) was investigated under different operating conditions. The chemical oxygen demand (COD) removal rate could reach 99.4% at 400 °C, 24 MPa, 1min and oxidation coefficient (OC) of 2. Experimental results showed that COD removal had no significant dependence on temperature and pressure variations. By contrast, COD removal could be significantly promoted with OC increasing from 0 to 2.0, but the effect was negligible as the OC further increased; similarly, longer residence time than a definite value seemed to contribute little to COD removal. Initial and deeper degradation pathways of penicillin were proposed based on the reactive force field (ReaxFF) molecular dynamics (MD) simulations. By tracing the evolution of intermediates, the migration routes of S and N during the SCWO process were obtained with H2S and NO2 produced as the corresponding products. Simulation results showed that SCW and oxidant not only accelerated the degradation by producing highly reactive radicals or molecules, but also participated in reactions by serving as H and O sources. Moreover, catalysis of water clusters in C-heteroatom bond cleavage was also observed.