Modelling the oxidation of elemental sulfur in soils

Direct and recursive estimation models for the oxidation rate of elemental sulfur (S°) in soil have been proposed, both essentially based on a constant oxidation rate per unit area of exposed surface. Fertilizer S° is taken to consist largely of blocky shaped particles, i.e. having similar dimensions along three axes, which can be treated as equivalent spheres. The most important implication in applying the rate assumption to these shaped particles is that the mass at any time is related to the cube of the time. This has been verified experimentally for oxidation by thiobacilli. Although the assumption is less likely for heterotrophs, experiments involving four soils conformed to the cubic relation. Implications for the particle variables of size and size distribution have been given more limited testing. The data are generally consistent with theory, such as independence of the rate constant with particle size. Assuming an activation energy for the oxidation process implies, in addition to the above, an exponential relation of rate constant with temperature. This is supported by experiment. Values for the activation energy are approximately 85 kJ mol⁻¹, and therefore consistent with the rate limiting step for the oxidation being a chemical or biochemical reaction, rather than a diffusion process. Because absolute rate constants are generated by the models, they are useful for examining the effects of environmental variables not hitherto included. Empirical relationships, once established, can then be included in the model, such as the quadratic relation between rate constant and soil moisture, with the maximum at approximately field capacity. The delay time (the time to reach maximum oxidation rate) was useful, together with the rate constant, for distinguishing species of oxidizing microorganisms. Typically, under optimum conditions at 25°C, thiobacilli have a delay time of several days and a rate constant of 50μg cm⁻² day⁻¹ S, while heterotrophs have a negligible delay time but a rate constant of only 5μg cm⁻² day⁻¹ S. The cubic model with a single rate constant gave a surprisingly good fit to the oxidation rate over 12 months in New Zealand pastoral soils under field conditions of varying temperature and moisture. This was attributed to the balancing effect of moisture and temperature on the rate constant under the cool temperate climate. A knowledge of the annual average soil temperature is sufficient to provide advice on the optimum particle size for S° fertilizer.     

Catalytic oxidation of carbon monoxide under periodic operation

A growing literature demonstrates resonance of the rate of CO oxidation with frequency of composition switching and that a global increase in rate under composition modulation is possible. Differences are found for the various metal catalysts used: CO oxidation is stimulated at frequencies between 0.1 and 1.0 Hz over noble metals, whereas stimulation occurs at frequencies below 0.05 Hz with oxide catalysts. Multiple resonances with an apparent harmonic relationship, “wrong-way” response, or very slow relaxation to reproducible cycles have been observed. These differences suggest mechanisms through which rate improvements arise are not the same for all catalysts.     

HABR反应器硫酸盐型厌氧氨氧化启动特性研究

试验探讨了复合式厌氧折流板反应器(HABR)里硫酸盐型厌氧氨氧化启动特性及不同COD对系统的影响。通过提升进水基质浓度成功启动硫酸盐型厌氧氨氧化。结果表明,硫酸盐型厌氧氨氧化是分步进行的,一号、三号和四号隔室所进行的主要反应各不相同。NH4+-N、SO42-、COD去除效果稳定,最高值分别为52.2%、53.7%、60.9%。较低的COD(60~150 mg/L)能促进硫酸盐型厌氧氨氧化的进行,COD过高会抑制厌氧氨氧化菌活性。     

The kinetics and mechanism of the oxidation of carbon monoxide over an industrial vanadium oxide catalyst

The rate of catalytic oxidation of carbon monoxide was studied in a fixed bed reactor over the following range of conditions: 325 to 396°C, 0.11 to 1.75 mole % oxygen, 9.50 to 10.20 mole % carbon monoxide. Oxygen was the limiting component, most of the runs were taken to greater than 10% oxygen conversions, and therefore an integral approach was taken in the data analysis. The catalyst was vanadium oxide/potassium sulfate on silica, an industrial preparation which had been used in previous kinetic studies. The data were correlated successfully by the steady state adsorption model with Arrhenius activation energies of 22.1 k.cal/g. mole for the oxygen chemisorption step and 18.0 k.cal/g. mole for the reaction step. The rate constants obtained provide further support for the steady state adsorption model, previously used to evaluate the oxidation kinetics of naphthalene, toluene, benzene and orthoxylene on the same catalyst. The integrated rate equation appears to well describe the relationship between the various parameters with the conversion of the limiting component varying from less than 5% to over 50%. A brief comparison is reported between the steady state adsorption model and the more complex triangular mechanism postulated by Hirota and co-workers.     

Kinetic study of CO oxidation on step decorated Pt(1 1 1) vicinal single crystal electrodes

In this work, surface modification at atomic level was applied to study the reactivity of step sites on platinum single crystal surfaces. Stepped platinum single crystal electrodes with (1 1 1) terraces separated by monoatomic step sites with different symmetry were decorated with irreversibly adsorbed adatoms, without blocking the terrace sites, and characterized in 0.1 M HClO₄ solution. The kinetics of CO oxidation on the different platinum single crystal planes as well as on the step decorated surfaces has been studied using chronoamperometry. The apparent rate constants, which were determined by fitting the experimental data to a mean-field model, decrease after the steps of platinum single crystal electrodes have been blocked by the adatoms. This behavior indicates that steps are active sites for CO oxidation. Tafel slopes measured from the potential dependence of the apparent rate constants of CO oxidation were similar in all cases. This result demonstrates that the electrochemical oxidation of the CO adlayer on all the surfaces follows the same Langmuir–Hinshelwood model, irrespectively of step modification.     

Kinetics of aqueous oxidation of tantalum from chronopotentiometric measurements

Theoretical mechanisms and growth laws for the initial oxidation of metals are briefly reviewed. The oxidation of tantalum was studied chronopotentiometrically at temperatures between 0°C and 40°C in an oxygen-free electrolyte.

The probable rate-determining steps for the two different growth laws observed are deduced by comparison of the time dependence of the experimentally determined local oxidation current to theoretical local-current/time functions derived from assumed growth laws.

Tantalum oxidizes initially according to a direct logarithmic law, the slow step being tunnelling of electrons through the oxide film. At 40°C a transition from a direct logarithmic to a parabolic growth law is observed. The interpretation is that the diffusion of electrons which have been injected thermionically into the oxide film becomes rate-determining at film thicknesses for which current due to electron tunnelling becomes negligible.     

Infrared studies of the reactive adsorption of organic molecules over metal oxides and of the mechanisms of their heterogeneously-catalyzed oxidation

The use of IR spectroscopic techniques to provide information on the mechanisms of catalytic oxidation over metal oxide catalysts is briefly discussed. The data published on studies of the catalytic oxidation of methanol, of linear C₄ hydrocarbons and of methylaromatics over different metal oxide surfaces are reviewed and discussed. Lattice oxygen appears to act as the active oxygen species in both selective and total oxidation. Generalized mechanisms of these complex oxidation reactions are proposed and the catalyst features affecting selectivities in these reactions are discussed. The reaction network is apparently essentially governed by the organic chemistry of the reacting molecule (thus being substantially the same over the different oxide catalysts). However, the catalyst surface governs the rate of the different steps, favoring some paths over others. Thus, selectivity is determined by the catalyst chemical behavior and by the reaction variables (contact time, temperature, gas-phase composition, presence of steam, etc.). IR studies, if performed under conditions where some intermediates are actually detectable and jointly with other techniques, can give valuable information on the catalysis mechanisms. On the other hand, it has been concluded that in situ studies frequently do not give reliable information on reaction mechanisms, because under reaction conditions spectators rather than intermediates are detected.     

Deuterium isotope analysis of methanol oxidation on mixed metal anode catalysts

The widely accepted mechanism for methanol oxidation on Pt based catalyst surfaces has held that the rate determining step is activation of water, and/or oxidation of surface-bound CO to CO₂. In fact on pure Pt, water activation is always rate limiting at potentials negative of 0.6 V. Anode potentials greater than 0.4 V are outside the useful potential window of direct methanol fuel cells when using Nafion 117 at 60 °C. Enhancement of the water activation kinetics on Pt has been effected by the use of oxophilic transition metal promoters including Ru, W and Sn. For decades the search for improved methanol oxidation electrocatalysts has focused on water activation. A systematic deuterium isotope study on Pt black and two active mixed metal catalysts (PtRu and PtRuOsIr) shows that for each catalyst there is a characteristic transition potential above which the primary reaction in the rate-determining step changes from water activation to CH bond activation. On the mixed metal catalysts, this crossover potential is ca. 0.35 V, which is within the direct methanol fuel cell potential window (0-0.400 V). This study confirms that on these active catalysts there is a potential above which further improvements in water activation must be concomitant with acceleration of CH bond activation. Thus the catalyst search strategy involving Pt promoter metals must also consider the kinetic importance of CH bond activation.     

环己烷液相非催化/催化氧化反应动力学特性比较

分别以不同氧浓度气体及纯氧作为氧源,利用鼓泡塔反应器在1.1 MPa、不同温度、不同气体流量条件下比较了环己烷液相非催化氧化和催化氧化反应特性。通过比较分析,讨论了环己烷液相氧化的链式反应机理。理论分析和实验数据表明,环己烷液相催化氧化反应诱导期短,其表观速率与环己烷浓度和氧气浓度都为一级;而非催化氧化反应诱导期长,是由烃直接氧化引发过程控制,只有当过氧化氢积累到一定浓度以后,过氧化物分解产生自由基速率变得显著,反应才表现出自催化反应特征。而在催化剂存在的情况下,环己基过氧化氢分解非常迅速,环己基过氧化氢浓度在整个反应过程中稳定在一个很低的水平,反应由氧化生成酮的链传递反应过程控制。     

对二甲苯氧化用钴锰溴铪催化体系研制

以氧氯化铪为原料,实验制备了PX氧化反应用醋酸铪、溴化铪催化剂;并在PX氧化性能实验装置中评价了催化剂性能,比较了醋酸铪、溴化铪所形成钴锰溴铪催化剂的性能及铪浓度对催化性能的影响;实验表明醋酸铪与溴化铪的性能相当,铪浓度增加,反应速度加快,铪对PX氧化反应各步的催化幅度不同,在工业生产条件下,加入60μg/g铪,PX消失速度增加了36%,关键组分PT酸速度增加了7%。     

Determining the limiting reaction in anaerobic digestion processes. How has this been tackled?

The rate‐limiting step concept has been widely used in anaerobic digestion processes as well as in many different biological processes. Sometimes it is described as part of the study introduction, but many times is used to support the specific objectives of the work. However, there is no clear consensus of how to define this step or how to determine it. This review presents a critical revision of several methods and procedures proposed to determine the rate‐limiting step in the anaerobic digestion field. The review is structured as follows: first, the justification of this revision is presented, followed by the very basic original definition and the latest applications in determining the rate‐limiting step. It ends with a discussion about the existing methodologies and the possible directions for addressing this issue. Different experimental approaches have been used for determining the rate‐limiting step which, in general terms, can be grouped into two procedures: steady states and batch test evaluation. Modeling applications have rarely been employed despite them allowing a more detailed picture of the anaerobic digestion system Copyright © 2012 Society of Chemical Industry     

Wet oxidation of active carbon

As a preliminary step to studies on the regeneration of active carbon, its wet oxidation has been followed in order to obtain information about the different parameters involved: agitation, temperature and pressure. The oxidation reaction takes place in the liquid phase and it does not proceed at significant rates below 200°C, even with high initial oxygen pressures. The oxidation proceeds rapidly beyond 275°C at pressures as low as 30 atm. The only detectable reaction product is carbon dioxide. Although mass transfer effects are minimized at stirring speeds higher than 400 rpm, a visible abrasion of the carbon was apparent beyond 300 rpm. Consequently, the study was carried out at around 200 rpm. Mass transfer effects had to he considered then. Estimation of the different transfer coefficients was done from the experiments and through existing correlations. In this way the kinetics could be obtained. The rate of reaction can be represented by a first order kinetic expression. Comparison of the rates has been done at fixed conversions. The intrinsic rate constant has a surprisingly low activation energy of 8.4 Kcal/mole suggesting that the oxidation reaction proceeds via a free radical mechanism. The overall rates show a maximum at conversions varying between 0.10 and 0.40. The specific surface area decreases regularly with reaction time, indicating that the reaction takes place throughout the particle.     

Kinetic Monte Carlo simulation of the preferential oxidation of CO using normally distributed rate probabilities

This study presents results from a Kinetic Monte Carlo (KMC) simulation of the catalytic preferential oxidation of CO reaction (PROX) on 3D Pt crystallites. A new method of constructing normal distributions of event probabilities is presented based on the log transformation of calculated kinetic rates (log-KMC). This method allows for solving problems where the time scales of particular events are very different (stiff problems) and it was applied to the simulation of the PROX reaction on supported Pt catalysts to probe the experimental results on a catalyst with different crystallite sizes. It was found that the simulation replicated the trends from experimental results when it was assumed that the limiting step of the PROX reaction was oxygen adsorption. Lower Pt dispersions gave place to catalysts showing higher turnover frequencies, or TOF, which is due to large crystallites having lower numbers of less active base sites. This limiting effect is simulated in larger crystallite sizes with more available sites on (1 1 1) faces.     

Oxidation of coal in industrial conditions. 1. Kinetics of natural oxidation

In the light of research on the artificial oxidation of coal in the laboratory, earlier results on the natural oxidation of coal in open storage at different coke Ukrainian plants are analyzed. On the basis of the kinetics of coal oxidation in natural conditions, the need to correct existing standards for open storage is evaluated. Since the dependence of the degree of oxidation d _o on the storage time is sigmoid, the results are analyzed on the basis of the Avrami-Erofeev equation for the kinetics of heterogeneous reactions. The constants of that equation and the rate constants of oxidation are calculated for enriched coal of different ranks (G, Zh, K, and OS) in summer and winter. The influence of the degree of metamorphism on the rate constant of natural oxidation in summer and winter is mathematically described. On the basis of the Avrami-Erofeev equation, limiting open-storage times for the coal are established. The earlier limiting storage times must be corrected.     

Gold supported CeO2/Al2O3 catalysts for CO oxidation: influence of the ceria phase

A series of low loading gold supported ceria/alumina catalysts have been prepared by the deposition–precipitation method, varying the pH of the synthesis. The catalysts were characterised by means of XRD, TEM, S_BET, XRF and UV–Vis techniques, and their catalytic activity towards CO oxidation in the absence and in presence of water in the stream, were tested. It has been found that in this low loading gold catalysts, where the metallic particles are far away one from another and the oxygen transportation is not the limiting step of the reaction, the electronic properties of the ceria phase and the structure of the metal-support perimeter more than the diameter of the gold nanoparticles is the determinant factor in the catalytic performances of the solid.     

化学氧化强化湿法冶金清洁生产：进展与展望

湿法冶金具有能耗低、污染小等优点，广泛应用于低品位复杂矿石处理。金属浸出是湿法冶金的首要环节，但存在金属回收率低和反应时间长等问题。化学氧化可加速金属硫化物转化为金属离子或改变金属的价态，有利于后续目标金属的分离富集，在此过程中还可以通过介质强化、外场强化提高金属氧化浸出率。主要介绍了五种典型的低腐蚀性化学氧化剂(Fe3+, O2, H2O2, O3和过硫酸盐)，以及相关的协同氧化方法在金属浸出中的应用和机理分析，介绍了加压强化、介质强化、微波和超声等强化方法，对比分析了各方法的优缺点及适用范围。Fe3+广泛应用于硫化矿的酸性浸出，独特的离子对循环使Fe3+可与多种氧化剂形成协同氧化浸出机制。O2常通过加压强化提升氧化浸出效率，可促进难处理硫化矿氧化分解。H2O2氧化性强，氧化产物清洁无污染，受到广泛关注，近年来多用于电子废弃物资源处理领域。臭氧预氧化处理含硫含砷难处理金精矿，可有效解除难浸硫化矿对金的包裹，促进金的溶出。过硫酸盐性质稳定，氧化能力强，可活化生成更高氧化性的活性氧。协同氧化可结合各氧化剂的优点，提高氧化能力，降低综合成本。四种强化方法可为化学氧化过程提供能量、加强传质或提高金属分离选择性，有助于提高金属浸出率，缩短反应时间。展望了化学氧化强化金属浸出技术的发展前景和技术挑战，对湿法冶金清洁生产技术开发有指导意义。     

Voltammetric oxidation of Hantzsch 1,4-dihydropyridines in protic and aprotic media: relevance of the substitution on N position

A detailed investigation on the electrochemical oxidation of some Hantzsch 1,4-dihydropyridine derivatives with the aim of study the influence of the hydrogen substituent on the N1 position of the heterocyclic ring have been carried out in protic and aprotic media. For this objective we have synthesized two series of compounds wherein the difference was the substituent (H or ethyl) on the N1-position of the heterocyclic ring. Voltammetry, UV-Vis spectroscopy, Controlled potential electrolysis, EPR, ¹H NMR and gas chromatography-mass spectrometry techniques in order to obtain evidences for postulate oxidation mechanisms in both protic and aprotic media have been used. Compounds having the ethyl substituent in the N1 position follow an oxidation mechanism obeying the sequence ECE with the second step as the r.d.e. in both, protic and aprotic media, thus producing the corresponding ethyl substituted pyridinium cation. On the other hand compounds having H in the N1 position follow the same ECE sequence only at acidic media. At basic media, the mechanism consisted of a DISP1 scheme in which rate determining step (r.d.s.) is the uptake of the proton in the N1 position by the OH⁻ ion of the media. In aprotic media both type of compounds follow the same ECEC mechanism with the second step as the r.d.s. but only the H-substituted compounds generates an anionic species that is more easily oxidized than the parent compounds.     

Development of a quick screening staining test for detecting the oxidation potential of iron sulfide-bearing aggregates for use in concrete

Oxidation of iron sulfide-bearing aggregates is pointed out as responsible of concrete deterioration of numerous buildings in Trois-Rivières area, Canada. Several researchers have been focusing their efforts to fully understand the mechanisms involved in such reaction and to ultimately establish accurate guidelines for the quality control of aggregates. Though this testing programme is already rather well-established, a quick staining test will allow complementing the chemical approach that serves as its initial step. This paper presents a series of attempts to define the most accurate staining method by testing the efficiency of ASTM C 641 and the Midgley test, but also by optimizing them, namely by using different oxidizing reagents. The use of 6% bleach was able to correctly identify the oxidation potential of the investigated aggregates when used as reagent in the Midgley test. Based on the observations made in staining tests, a promising temperature test was also developed.     

甲醛催化氧化反应机理的研究进展

催化氧化技术具有环境友好、节省能源、操作简单等优点,在治理室内甲醛污染方面显示出非常有潜力的应用前景。本文综述了近年来甲醛催化氧化反应机理的研究进展,阐述了在甲醛催化氧化过程中,氧的活化、可能产生的反应中间体以及反应路径。重点介绍了贵金属催化剂（Au、Pt、Pd 和Ag）和过渡金属（Mn、Co和Ce等）氧化物催化剂在甲醛氧化反应过程中,不同金属种类、载体性质和添加剂等对反应机理的影响。介绍了已经商业化的除甲醛产品所采用的反应机理。最后,指出了甲醛催化氧化反应机理存在的问题并对其未来研究发展方向进行了展望。