Kinetics of Gold Dissolution in Thiosemicarbazide Solutions

The kinetics of gold dissolution in thiosemicarbazide solutions was studied relative to the ratio of ferric ion and thiosemicarbazide concentrations as well as pH. The composition of the complex formed was found to be [Au(NH₂)₂NHC=S₃]⁺. The rate constants for gold dissolution were determined at 278-298 K along with the stability, dissociation, and equilibrium constants at 288 K. The activation energy at 288 K was also found.     

Kinetics and Mechanism of the Dissolution of Gold in Solutions of Thiocarbamide

The kinetics and mechanism of the dissolution of metallic gold in solutions of thiocarbamide in the presence of iron(III) were investigated. The dependence of the dissolution rate of gold on the pH of the solution and the ratio of the concentrations of iron(III) and thiocarbamide was determined, and the optimum conditions for the dissolution of gold were determined. The first-order rate constant for the dissolution of gold and the equilibrium constants for the formation of the complex cations Au[SC(NH₂)₂]⁺ ₂ and the formation of formamide disulfide S₂C₂(NH)₂(NH₂)₂ were calculated.     

Kinetic Parameters of the Dissolution of Gold in Thiourea Solutions

The temperature dependence of the rate constants for the dissolution of gold in thiourea solutions was determined. The activation energies calculated suggest that the rate of gold dissolution is diffusion-controlled at 5-22 °C but is kinetically-controlled at 25-60 °C (E _a = 42.83 kJ/mol). Auger spectroscopy was used to study the composition of the surface adsorption layers and a mechanism for the passivation of the reactive surface during the gold dissolution process was proposed.     

Kinetics of the Dissolution of Silver in Thiocarbamide Solutions

The dependence of the rate of solution of silver on the pH of the solution, the ratio of the iron(III) and thiocarbamide concentrations, and the temperature has been determined. The rate constants for the solution of silver (k _i = 2.3·10^–4 to 9.6·10^–4s^–1) at temperatures from 283-298 K have been calculated and from the temperature dependence of the rate constant the activation energies have been calculated: 68.84 kJ/mol for kinetic control of the rate of solution and 26.06 kJ/mol in the adsorption inhibition region.     

Electrode Kinetics of Concentrated Sulfide Ion in Basic Solutions

The diffusion coefficients of sulfide ion in 5 M NaOH solution with concentrations of (0.02–0.3)M at temperatures 24–50°C were determined using a rotating-ring-disc Pt electrode to be (0.5–2.0)×10^?5 cm₂/sec. The reaction order of the sulfide oxidation to sulfur was calculated to be 0.42 which implies one-electron transfer as the rate-limiting step and the reaction rate constant was found to be k_f=2.57×10^?5 exp(-0.14ZF_π/RT).     

Mechanism of Gold Activation in Supported Gold Catalysts for CO Oxidation

Based on the structure of characterization of Au/Fe₂O₃ catalysts, the mechanism of gold activation in supported gold catalysts was proposed as follow:

PVP 稳定的纳米 Au 溶胶对葡萄糖液相选择氧化的催化性能

 采用化学还原法制备了聚乙烯吡咯烷酮 (PVP) 稳定的纳米 Au 溶胶, 这种 Au 溶胶在葡萄糖空气氧化制葡萄糖酸反应中具有良好的催化性能. 考察了 PVP 加入量和氯金酸前驱液的浓度对反应活性的影响. 紫外-可见吸收光谱和透射电镜分析结果表明, 含有较小 Au 粒子的 Au 溶胶体系具有较高的催化活性. 当 PVP/Au 质量比为 40, 氯金酸浓度为100 μg/ml 时, 得到稳定的 Au 溶胶体系具有金粒子尺寸小、分布均匀的特点, 对葡萄糖氧化反应活性高, 葡萄糖的转化率达到 54.4%.     

Theoretical Study on the Pyrolysis Mechanism and Kinetics of 3-Hydroxy-3-Methyl-2-Butanone

Thethermolysisofpyruvicacid2Ieadstotheeliminationofcarbondioxideandacetaldehydeisformedasanotherproduct.Hong'hasstudiedthemechanismofthereactionintheMINDO/3molecularorbitaltheory.Al-AwadiandEL-Dusouqui'havestudiedthepyrolysisofthe3-hydroxy-3-methyl-2-butanoneinordertoexaminetheinfluenceofthehydrogenatominvolvedintheTransitionStateoftheeliminationprocessandtoassessitscontributiontotherate-controllingstep.TheygaveoutthethermalrateconstantsandhypothesizedthatthereactionmightproceedthroughafO…     

过氧化氢溶液中单和双金属金-银纳米粒子氧化L-亮氨酸的动力学(英文)

The catalytic activity of surfactant stabilized mono-and bimetallic Au and Ag nanoparticles for the oxidation of an amino acid,L-leucine,was studied using hydrogen peroxide as the oxidant.The Au and Ag nanoparticle catalysts exhibited very good catalytic activity and the kinetics of the reaction were found to be pseudo-first order with respect to the amino acid.The effects of several factors,such as oxidant concentration,ionic strength,pH,and catalyst concentration on the reaction,were also investigated.In particular,optimal oxidant and catalyst concentrations were determined.Very high concentrations of the metal nano-catalysts or the oxidant led to a dramatic increase in reaction rate.Moreover,bimetallic Au-Ag catalysts provided higher selectivity than pure Au or Ag.     

肾上腺素氧化反应的研究——氧化反应动力学和机理以及自由及中间体

用紫外吸收光谱法结合计算机模拟研究了肾上腺素(Adjn)在Cu(Ⅱ)离子催化下, 用H_2O_2氧化产生肾上腺素红(Adom)的反应动力学, 考查了反应的速率随体系中肾上腺素的初始浓度, Cu(Ⅱ)离子浓度和H_2O_2浓度的变化情况。在流动法或添加自由基稳定剂的条件下, 利用顺磁共振法检测丁肾上腺素自氧化, 光氧化和Cu(Ⅱ)离子催化H_2O_2氧化产生的半醌自由基。考查了Cu(Ⅱ)离子催化下自由基的相对浓度随反应条件的变化情况。在反应动力学和顺磁共振实验的基础上所提出的可能的反应机理, 半定量地解释此反应动力学实验结果和顺磁共振实验中的自由基行为。     

滤压式电解槽中氨氮间接电氧化反应动力学

研究了模拟氨氮废水在滤压式电解槽中的电化学氧化过程. 结果表明, 在一定浓度Cl-的存在下, 氨氮化合物的间接电氧化过程符合二级反应动力学规律. 利用Hatta准数进一步分析电解槽内反应的空间分布得知, 不同条件下的Hatta准数均小于0.3, 这说明氨氮化合物的间接电氧化过程主要发生在慢反应动力学区域, 即发生在本体溶液中.     

碱性溶液中酚酞褪色的动力学研究

酚酞是最常见的酸碱指示剂之一,在pH12.0时,酚酞会慢慢褪色,直至呈无色.利用酚酞在碱性溶液中慢慢褪色的过程,通过测定溶液吸光度的变化,测定酚酞在碱性溶液中反应的速率常数和反应级数,使学生进一步理解化学反应动力学的基本概念.     

纳米金催化剂的存放失活

 采用共沉淀法制备了2.5%Au/ZnO催化剂,并用扫描隧道显微镜、 X射线衍射、 X射线光电子能谱和热重研究了纳米金催化剂在空气中存放失活的机理. 结果表明,纳米金催化剂在空气中存放失活是部分可逆的. 纳米金催化剂的存放失活有两方面原因: 一是纳米金粒子的长大,导致不可逆失活; 二是碳酸盐在催化剂表面的累积,这种失活可以通过高温焙烧来恢复,是可逆的.     

Kinetics and Mechanism of the Racemization of Aryl Allenes Catalyzed by Cationic Gold(I) Phosphine Complexes

The kinetics of the racemization of aromatic 1,3‐disubstituted allenes catalyzed by gold phosphine complexes has been investigated. The rate of gold‐catalyzed allene racemization displayed first‐order dependence on allene, and catalyst concentration and kinetic analysis of gold‐catalyzed allene racemization as a function of allene and phosphine electron‐donor ability established the accumulation of electron density on the phosphine atom and the depletion of electron density on the terminal allenyl carbon atoms in the rate‐limiting transition state for racemization. These and other observations were in accord with a mechanism for allene racemization involving rapid and reversible inter‐ and intramolecular allene exchange followed by turnover‐limiting, unimolecular conversion of a chiral gold η²‐allene complex to an achiral η¹‐allylic cation intermediate through a bent and twisted η¹‐allene transition state. With respect to proper ligand selection, these studies reveal that both electron‐poor phosphine ligands and polar solvents facilitate racemization.     

Kinetics and Mechanism of Oxidation of 1-Phenylsemicarbazide by Peroxydisulphate

Kinetics of oxidation of 1-phenylsemicarbazide (PSC) by peroxydisulphate ion (PDS) have been carried out where by the pseudo first order condition was verified at large excess of PDS concentration. The rate of the reaction was followed spectrophotometrically, The stoichiometry was found to be 1:1 where 1-phenylazoformamide is the oxidation product. The effect of acidity on the rate of oxidation was investigated for different temperatures. The parameters of activation ΔG*, ΔH* and ΔS* were computed for both hydrogen ion depedent and hydrogen ion independent reaction pathes. A free radical mechanism was proposed.     

Mechanism of Producing Metallic Nanoparticles,with an Emphasis on Silver and Gold Nanoparticles,Using Bottom-Up Methods

Bottom-up nanoparticle (NP) formation is assumed to begin with the reduction of the precursor metallic ions to form zero-valent atoms. Studies in which this assumption was made are reviewed. The standard reduction potential for the formation of aqueous metallic atoms—E⁰(Mⁿ⁺_aq/M⁰_aq)—is significantly lower than the usual standard reduction potential for reducing metallic ions Mⁿ⁺ in aqueous solution to a metal in solid state. E⁰(Mⁿ⁺_aq/M⁰_solid). E⁰(Mⁿ⁺_aq/M⁰_aq) values are negative for many typical metals, including Ag and Au, for which E⁰(Mⁿ⁺_aq/M⁰_solid) is positive. Therefore, many common moderate reduction agents that do not have significantly high negative reduction standard potentials (e.g., hydrogen, carbon monoxide, citrate, hydroxylamine, formaldehyde, ascorbate, squartic acid, and BH₄⁻), and cannot reduce the metallic cations to zero-valent atoms, indicating that the mechanism of NP production should be reconsidered. Both AgNP and AuNP formations were found to be multi-step processes that begin with the formation of clusters constructed from a skeleton of M⁺-M⁺ (M = Ag or Au) bonds that is followed by the reduction of a cation M⁺ in the cluster to M⁰, to form M_n⁰ via the formation of NPs. The plausibility of M⁺-M⁺ formation is reviewed. Studies that suggest a revised mechanism for the formation of AgNPs and AuNPs are also reviewed.     

Cooperative Ligands in Dissolution of Gold

Development of new, environmentally benign dissolution methods for metallic gold is driven by needs in the circular economy. Gold is widely used in consumer electronics, but sustainable and selective dissolution methods for Au are scarce. Herein, we describe a quantitative dissolution of gold in organic solution under mild conditions by using hydrogen peroxide as an oxidant. In the dissolution reaction, two thiol ligands, pyridine-4-thiol and 2-mercaptobenzimidazole, work in a cooperative manner. The mechanistic investigations suggest that two pyridine-4-thiol molecules form a complex with Au⁰ that can be oxidized, whereas the role of inexpensive 2-mercaptobenzimidazole is to stabilize the formed Au^I species through a ligand exchange process. Under optimized conditions, the reaction proceeds vigorously and gold dissolves quantitatively in two hours. The demonstrated ligand-exchange mechanism with two thiols allows to drastically reduce the thiol consumption and may lead to even more effective gold dissolution methods in the future.     

循环伏安和现场FTIR反射光谱研究甲醛在金电极上的氧化

The electro-oxidation of formaldehyde on a gold electrode in a gold electrode in a solution containing 0.1mol •L -1 Na2CO3＋0. 1mol•L - 1NaHCO3＋0. 1 mol•L - 1 HCHO was investigated by cyclic voltammetry and in -situ FTIR spetroscopy . The experimental results demonstrated that the oxidation of formaldehyde at different potential range connected with different surface species of gold. At lower potentials, the main product of formaldehyde oxidation was HCOO-, and at higher potentials, the products HCOO- and CO2 were detected simultaneously. From the results, a possible reaction mechanism was proposed.     

Kinetics and Mechanism of the Oxidation of Molybdenum Sulphide

This paper reports on the results attained in the determination of the mechanism of oxidation of molybdenum sulphide under non-isothermal conditions in an air atmosphere. The mechanism of the process was determined by simultaneous DTA-TG-DTG, and the kinetic parameters of the reactions involved were obtained according to the methods of Kissinger and Ozawa. This revised version was published online in July 2006 with corrections to the Cover Date.