Secondary nucleation in the formation of methane crystal hydrate

The kinetic data on crystallization and a morphological analysis of a layer of CH₄ · 6H₂O hydrate crystals formed on the surface of water as a result of methane absorption showed that secondary nucleation occurred during hydrate crystallization. The mutual arrangement of crystals in the layer revealed photographically in situ was evidence that part of nuclei produced on the surface of previously formed crystals went away from the surface into solution and grew there independently of “mother” crystals, although the probability of such transfer into an immobile solution remained low. In view of this, a model of crystal growth generating secondary crystals was developed.     

Reaction of Hg2+ Insertion into Cysteine Pairs Within Bovine Insulin Crystals Followed via Raman Spectroscopy

Chemical modifications of protein crystals may be achieved via soaking of reactants from their precipitating solution, through the solvent channel, into the protein matrix. We describe a Raman microscopy approach to follow mercury insertion into cysteine pairs within protein single crystals, via soaking in an aqueous Hg²⁺ solution. The method has been developed using bovine insulin as the model system. Applying an efficient mercuration protocol, consisting of a first step of disulphide bridge TCEP-induced reduction within the crystal, followed by overnight reaction with a HgCl₂ solution, we obtained Hg-derivative crystals. Raman spectra collected on these derivative crystals, kept in the mother liquor, reveal a characteristic Raman band at 335 cm^?1, which has been assigned to a –S–Hg–S– bridge. The analysis provides Raman-based markers of mercury binding to cysteines, and thus of mercury intoxication.     

Initial Stages of Copper Electrocrystallization on a Glassy-Carbon Ring–Disk Electrode from Sulfate Electrolytes of Various Acidity: Potentiostatic Current Transients

Initial stages of copper electrocrystallization on glassy carbon from sulfuric acid electrolytes of pH 0.3 and 3.7 are studied by the method of potentiostatic current transients on rotating and stationary ring–disk electrode. The rate of copper deposition in a 0.5 M Na₂SO₄ + 0.01 CuSO₄ (pH 3.7) solution is marginally higher than in a 0.5 M H₂SO₄ + 0.01 CuSO₄ acid electrolyte (pH 0.3) at the expense of adsorption of sulfate and hydroxide ions on the substrate surface and the copper crystals. Regularities governing the multistage discharge of copper ions, the formation of the new phase nuclei, and the deposit dissolution are analyzed. The results of the study are compared with the data on the kinetics of copper electrocrystallization on a platinum electrode. The acceleration of the copper deposition on glassy carbon in the acid solution of pH 0.3, as compared with platinum, is due to accelerated discharge of copper ions and increased number of univalent copper ions in the near-electrode layer of solution. The oxygen-containing surface groups of glassy carbon (quinone–hydroquinone, carbonyl, etc.) are probably active centers for the discharge of copper ions and three-dimensional nucleation.     

Modification of surface crystal formation on mother crystal by additive molecules

Additive molecules and excess reactant species (NaOH and NH4F) in NaF crystallization were used to control surface crystal formation on a mother crystal. When the surface integration was inhibited or bulk diffusion facilitated, more surface crystals were found on the mother crystal due to an increase in the interfacial supersaturation on the crystal. The inclusion of KCl as an additive and increases in the reactant concentration and agitation speed resulted in a higher population of surface crystals on the mother crystal, because surface integration was inhibited. However, surface crystal formation was suppressed by the polymer gelatin due to retardation of the bulk diffusion by the gelatin layer formed around the crystal. In addition, surface crystal formation significantly changed according to the feeding mode, and the excess species of NaOH originating from the NH4F feeding mode were more effective in facilitating surface crystal formation than the excess species of NH4F coming from the NaOH feeding mode due to the smaller ionic radii of NaOH than of NH4F.     

Motion of agglomerates on the surface in the formation of methane hydrate crystals

A study of the formation of a crystal hydrate film CH₄ · 6H₂O on the surface of an immobile solution of methane in water have shown that, at early stages of film formation, hydrate crystals and their agglomerates are in motion at a velocity of 2–5 mm/s caused by crystal growth. Collisions of agglomerates are accompanied by their rotations at a Ω ～ 0.2–2 rad/s rate. The motion of crystals and agglomerates is explained using the hydrodynamic model that takes into account the surface tension and the difference in the pressure exerted by the medium on the growing and nongrowing faces of crystals, which sets them in chemoreactive motion.     

Simultaneous impedance measurements of two one-face sealed resonating piezoelectric quartz crystals for in situ monitoring of electrochemical processes and solution properties

Through admittance measurements of two piezoelectric quartz crystals in parallel on one impedance analyzer and then non-linear fitting according to an equivalent circuit of two parallel Butterworth-Van Dyke circuits, we have simultaneously obtained accurate and precise impedance responses of two one-face sealed crystals to changes in solution density and viscosity, temperature, conductance, and/or electrode mass. A series of sucrose aqueous solutions, ferri-/ferrocyanide redox switching, hot water cooling, a series of NaClO₄ aqueous solutions, bovine serum albumin adsorption and silver electrodeposition/stripping were selected as model systems for such purposes. Galvanostatic charging/discharging reactions at positive and negative poles in a Ni-Zn battery were synchronously monitored, with some quartz crystal microbalance (QCM) insights into the second reduction process of nickel hydroxide film. In all cases, the crystal immersion angle effect was found to be negligible. The present method as a versatile one is highly recommended for informative two-electrode monitoring of two concurrent chemical or biological events, or for check and/or compensation of effects due to solution density, viscosity, temperature and/or conductance during QCM researches.     

硫酸铵母液除氯及其对结晶影响

硫酸铵母液中Cl^-不断循环富集导致设备腐蚀严重,同时影响硫酸铵结晶及品质。本文运用硫酸钙铝法和脱硫灰铝法分别对硫酸铵母液进行除氯研究,采用筛分法对晶体粒径进行分析、扫描电子显微镜（Scanning Electron Microscope, SEM）对晶体尺寸、形貌进行表征,X射线衍射仪（X-ray Diffractometer,XRD）分析晶体物相。研究表明：除氯剂最佳投加量为3.0 g硫酸钙和0.8 g偏铝酸钠,3.0 g脱硫灰和0.8 g偏铝酸钠,对应除氯率分别为31.70%和36.38%。在转速200 r/min,反应温度为75 ℃,两种除氯剂加入会使ρ（Cl^-）快速下降,此为Cl^-与Ca²⁺和AlO₂^-反应形成了不溶钙铝氯化合物;除氯剂加入过量会使NaAlO₂发生双水解,解离出OH^-,抑制Cl^-与Ca²⁺、AlO₂^-反应,导致Cl^-去除率下降。硫酸钙铝法所产生的钙铝氯化合物会附着在晶粒活性表面进而增大硫酸铵结晶介稳区宽度,抑制晶体正常生长,导致结晶量减少;脱硫灰铝法中杂质金属可将OH^-消耗和减小硫酸铵结晶介稳区宽度,所含大量SO₄^2-会使硫酸铵结晶量增加,但晶体纯度降低。相关研究结果可为减少氨法脱硫设备腐蚀及优化硫酸铵结晶提供参考。     

The effects of trace elements on the crystal properties of TiO2

The effects of trace element doping of TiO₂ on the crystal growth and on the anatase-to-rutile phase transformation of TiO₂ were investigated. The co-precipitation process, from sulfate solution, of doped (Cr, Fe, V, Nb, Si, P) TiO₂ was also studied. The heating temperatures were 473, 673, 873, 993, 1133 K and a higher temperature needed to achieve a rutile content of 98–99%. Traces of reduced titanium were found in freshly calcined anatase by X-ray diffraction. Pure anatase structure was found in 85% of the samples heated below 1000 K. Anatase-to-rutile transformation was accelerated by the mmol% content of Nb, Cr, Si, and Fe in TiO₂. Interaction of co-precipitated or impregnated cations was found critical in the phase transformation process. Nb retarded the crystal growth during calcination. Sulfate ions minimized the specific surface area of TiO₂ heated at low temperatures. These results of doped TiO₂ serve to promote the development of new high-technology TiO₂ products for photocatalytic purposes.     

Treatment of ferric sulfate waste solutions for the production of ammonium ferric sulfate dodecahydrate

The process of precipitation of ammonium ferric sulfate dodecahydrate (NH₄Fe(SO₄)₂·12H₂O) from waste solutions, obtained during autoclave oxidation of pyrite concentrate has been studied. A special feature of these solutions is the high concentration of Fe(III) ions (>60 g L^–1) and sulfuric acid (> 61 g L^–1). Based on comprehensive laboratory tests, the study determined the optimal conditions for the precipitation process of ammonium ferric sulfate dodecahydrate (AFS) by salting out with ammonium sulfate: reagent excess (100% over stoichiometric, temperature 276 K, time 1 h). The process should be conducted under continuous slow stirring which would not allow forming of large crystals that are difficult to remove from the reactor. The test work confirmed that high quality crystals can be produced by prior oxidation of Fe(II) to Fe(III) ions using hydrogen peroxide and copper removal from the solution.     

X-射线单晶衍射仪易风化晶体低温显微上样系统的研制

易风化晶体离开母液就会风化并失去结晶溶剂,导致结构塌陷,由单晶变成粉末,从而无法完成测试.X-射线单晶衍射仪易风化晶体低温显微上样系统利用晶体在低温下比较稳定的原理,通过维持选样与上样过程中的低温环境,使易风化晶体能够得以测试,从根本上解决了风化晶体的测试问题.系统的研制为易风化晶体化合物结构的测试提供有力的技术保障,是单晶衍射仪功能开发的重要技术创新.     

The onset of chain folding in ultralong n-alkanes: An electron microscopic study of solution-grown crystals

Crystallization of extremely pure n-C₁₉₈H₃₉₈ from dilute solution is investigated by electron microscopy and differential scanning calorimetry. This long-chain alkane is capable of crystallizing in the extended form or of folding once or twice and thus provides useful insights into the fundamental process of chain-folded crystal growth. Crystallization conditions that give rise to the different crystal types are identified by measuring the variation in dissolution temperature with crystallization temperature and crystallization time. Examples of each type of crystal are examined in the electron microscope using various techniques. From shadow length measurements we find the crystals to have thicknesses corresponding to integer fractions of the extended chain length, in agreement with previous findings on this and similar materials. However, contrary to all past experience with polyethylenes, refolding was found to occur while in the solvent. This refolding, which here takes place isothermally, also occurs in integral steps, with wider implications discussed. The crystal morphologies are similar in many respects to those obtained from polyethylene; in particular the crystals containing once-folded chains are often lozenge shaped with the folds along {110} growth planes, thus producing clearly defined sectorization made visible by several techniques. Electron diffraction studies show that the chains are substantially perpendicular within the crystals in all cases. In addition, differences in surface regularity between the different crystal types revealed by decoration techniques are discussed.     

Number Density of Liquid Inclusions Formed in Frozen Aqueous Electrolyte

Frozen aqueous chlorides (≤50 mM ) are characterized by using confocal fluorescence microscopy and small angel X‐ray scattering (SAXS). The former method allows us to determine the size of a liquid inclusion formed in the ice matrix at temperatures above the eutectic point of the system (t_eu). Isolated liquid inclusions of a uniform size are formed when the temperature of a frozen electrolyte increases past t_eu. The size of the liquid inclusions depends on the observation temperature as well as on the concentration (c_salt) and type of salt dissolved in the original unfrozen solution. However, the number density of liquid inclusions is almost constant and independent of these experimental parameters, particularly when an electrolyte is frozen in liquid nitrogen. Salt accumulation can then occur at the imperfections of the ice crystals. The occurrence probability of the imperfections is independent of the nature of an incorporated salt. The amount of a salt confined in each inclusion ranges from 7 to 240 fmol, depending on c_salt. SAXS measurements provide information on the size of individual salt crystals formed at temperatures below t_eu. The radius of gyration of a salt crystal ranges from 2 to 2.8 nm, and does not depend significantly on c_salt. Thus, each inclusion is formed from 10⁶–10⁹ nanocrystals, which can act as seeds. When doped ice is prepared at higher temperatures, for example ?16 °C, the isolation of liquid inclusions is not sufficient and coalescence occurs more easily upon an increase in temperature or c_salt. However, when c_salt is lower than 10 mM , the number density of liquid inclusions is almost constant, irrespective of the freezing temperature.     

Zeolite ZSM-5 containing copper ions: The effect of the copper salt anion and NH<Subscript>4</Subscript>OH/Cu<Superscript>2+</Superscript> ratio on the state of the copper ions and on the reactivity of the zeolite in DeNO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>

Isotherms of copper cation sorption by H-ZSM-5 zeolite from aqueous and aqueous ammonia solutions of copper acetate, chloride, nitrate, and sulfate are considered in terms of Langmuir’s monomolecular adsorption model. Using UV-Vis diffuse reflectance spectroscopy, IR spectroscopy, and temperatureprogrammed reduction with hydrogen and carbon monoxide, it has been demonstrated that the electronic state of the copper ions is determined by the ion exchange and heat treatment conditions. The state of the copper ions has an effect on the redox properties and reactivity of the Cu-ZSM-5 catalysts in the selective catalytic reduction (SCR) of NO with propane and in N₂O decomposition. The amount of Cu²⁺ that is sorbed by zeolite H-ZSM-5 from aqueous solution and is stabilized as isolated Cu²⁺ cations in cationexchange sites of the zeolite depends largely on the copper salt anion. The quantity of Cu(II) cations sorbed from aqueous solutions of copper salts of strong acids is smaller than the quantity of the same cations sorbed from the copper acetate solution. When copper chloride or sulfate is used, the zeolite is modified by the chloride or sulfate anion. Because of the presence of these anions, the redox properties and nitrogen oxides removal (DeNO _x ) efficiency of the Cu-ZSM-5 catalysts prepared using the copper salts of strong acids are worse than the same characteristics of the sample prepared using the copper acetate solution. The addition of ammonia to the aqueous solutions of copper salts diminishes the copper salt anion effect on the amount of Cu(II) sorbed from these solutions and hampers the nonspecific sorption of anions on the zeolite surface. As a consequence, the redox and DeNO _x properties of Cu-ZSM-5 depend considerably on the NH₄OH/Cu²⁺ ratio in the solution used in ion exchange. The aqueous ammonia solutions of the copper salts with NH₄OH/Cu²⁺ = 6–10 stabilize, in the Cu-ZSM-5 structure, Cu²⁺ ions bonded with extraframework oxygen, which are more active in DeNO _x than isolated Cu²⁺ ions (which form at NH4OH/Cu2+ = 30) or nanosized CuO particles (which form at NH₄OH/Cu²⁺ = 3).     

Effect of the displacement reaction on the composition of the magnetic layer in electrodeposited layered nanostructures Co-Cu/Cu and Ni-Cu/Cu

Methods of stripping voltammetry, quartz microgravimetry, and x-ray-fluorescence analysis are employed to explore the copper displacement process on thin electrodeposited layers of pure cobalt and nickel and on deposits of these metals with inclusions of copper. The displacement reaction proceeds at a considerable rate on cobalt in the sulfate and sulfosalicylate solutions and virtually does not proceed on nickel in both the sulfate or sulfate-chloride solutions. An estimate of the rate of the copper displacement reaction following a change in the concentration of copper ions in the solution and in the pH of the sulfosalicylate solution is given. A decrease in the contact exchange rate is facilitated by a decrease in the concentration of copper ions in solution and their participation in the formation of complexes.     

Complex of Copper(II) with Ethylenediamine and Decyl Sulfate Ions. Formation in Water and Crystal Structure

Decyl sulfate ions form strong associates with ethylenediammonium cations and copper(II) ethylenediamine complexes. A solid compound of the composition copper(II):ethylenediamine:decyl sulfate ion = 1:2:2 was isolated from the aqueous solution; its crystal structure was studied to find that it comprises a regular combination of formula units of an inner-sphere complex bis(ethylenediamine)bis(decyl sulfato)copper(II) and a complex associate of two decyl sulfate ions with diaquabis(ethyenediamine)copper(II).     

Surface and Subsurface Oxygen on Platinum in a Copper Perchlorate Solution

The formation of an adatom layer on polycrystalline platinum and the three-dimensional nucleation of copper in a copper perchlorate solution are studied by cyclic voltammetry at 0.1 V s^–1 while varying potential ranges and by recording potentiostatic current transients. About 0.6 monolayers of copper adatoms are deposited when cycling with anodic limit E _a = 1.35 V, the process is slower than that in an acid sulfate solution. Decreasing E _a accelerates the process (nearly one monolayer forms for E _a = 0.80–0.95 V in a cathodic scan) due to an increased number of active centers (metastable copper oxides) and, probably, to a change in the platinum surface microstructure. Oxygen for copper oxides is presumably supplied by water molecules adsorbed on a monolayer of copper adsorbed atoms and by subsurface oxygen (O_ss), which appears on the platinum surface after the destruction of complexes O_ss–Pt _n –ClO₄. Both the copper nucleation and the deposit growth accelerate at higher concentrations of copper oxides, which form at low E _a. High cathodic overvoltages decrease the number of active crystallization centers due to reduction or removal of copper oxides.     

Stability of Ultradisperse Copper in a Sulfo Cation Exchanger Matrix

The recrystallization of ultradisperse copper chemically deposited onto a sulfo cation exchanger matrix was studied by the potentiometric method. The stationary value of the electrode potential of the copper-sulfo cation exchanger composite was established during a long period of time, which depended on the ionic form of the composite (H⁺, Cu²⁺, or Na⁺), solution composition (CuSO₄, H₂SO₄, and Na₂SO₄), and solution concentration. Recrystallization was favored by copper(II) counterions, which entered the composite as a result of ion exchange, nonexchange absorption of copper sulfate, or preliminary composite transformation into the Cu²⁺ form. In the quasi-equilibrium state, the concentration of copper(II) counterions was maintained at a high level by the Donnan interfacial potential. At all the copper(II) sulfate concentrations used, the potential of the Cu²⁺/Cu ion—metal pair in the ion-exchange matrix remained at virtually the same level, which was indicative of the stable state of copper particles. In the absence of an external source of copper ions, recrystallization was significantly hindered; therefore, the potential exhibited only a slight drift. Copper ions formed in the solution of small crystals were localized in the vicinity of ionogenic matrix centers, which decreased the mobility of these particles as counterions; therefore, the dispersity of particles remained unchanged.     

ZSM-5单晶生长的研究

本文研究了DEA(二乙醇胺)-Na2O-SiO2-Al2O3-H2O体系中由晶种水热生长的ZSM-5单晶形貌和线性生长速度. 晶体外部晶面为轴面(100)、(010)及坡面(101)、(011). 在互相贯穿连生的各单晶之间, 其对称轴互成倾角. 在160-220℃范围内, 晶体线性生长速度保持恒定. 观察到单晶生长的诱导期. 由线性生长速度与温度的关系计算了生长活化能. 研究了反应物组成对晶面法线生长速度的影响、母液成分变化, 并分析了晶体的化学成分. 按晶体生长的基本原理及ZSM-5结构, 构想了实际上可能存在的ZSM-5单晶的面结构, 并估算了不同晶面上生长位的密度及其分布. 从面结构及沸石生长基元角度讨论了单晶生长机理的DEA的作用, 支持了生长基元为高聚态硅(铝)阴离子团的假设.     

Precipitation of monodispersed basic iron(III) sulfate (sodium jarosite) particles

Uniform sodium jarosite particles were obtained by forced hydrolysis of ferric salt solutions to which copper sulfate, sodium sulfate, and sodium nitrate were added. It was found that at the same concentrations of ferric and sulfate ions the particle size and yield decreased with the lowering of the concentration of sodium ions, but the morphology remained the same. At a sufficiently small content of sodium in the reacting solution, no precipitation of sodium jarosite particles was observed. Finally, chemical and XRD analyses showed that small amounts of copper ions are incorporated in the crystal lattice which, with the change in the Na⁺/H₃O⁺ ratio, slightly affected the structure of this mineral, but not its other properties. Received: 12 July 2000 Accepted: 18 August 2000