Study of experimental and theoretical procedures when using thermogravimetric analysis to determine kinetic parameters of carbon black oxidation

Combustion of carbon black (CB) in the crucible of a thermobalance is controlled by both carbon reactivity and oxygen transport from the oxidizing flux to the surface of the bed and within the porous bed. The kinetic constant of combustion has been determined using a fixed-bed reactor in which CB combustion is mainly under kinetic control. Then, modelling of oxygen transport in the thermobalance allowed determining the oxygen diffusivity within the CB pile. Fickian diffusion is a good approximate value of the diffusion coefficient for modelling of internal oxygen transport. The effects of the initial sample mass and of the sample containment on the initial combustion rate have been investigated. The effectiveness factor of the bed was calculated for different experimental conditions. Advices to correctly extract a kinetic constant from thermogravimetric experiments are given. According to the required precision, an experimental procedure is proposed. Limitations to oxygen transport within the bed may be ignored. They can be minimized by the use of an inert material to remove the stagnant atmosphere between the surface of the bed and the mouth of the crucible. It appears mandatory to account for oxygen transport limitations within the CB pile. It can be assumed that the sample temperature (not known) during reaction is the regulation temperature. Thermal effects are also minimized by use of the inert material. A 30–50 mg sample mass seems to be optimal for determination of the kinetic parameters.     

Thermal Oxidation of Carbonaceous Nanomaterials Revisited: Evidence of Mechanism Changes

Kinetic data, for example, activation energy and reaction order, are crucial for the understanding of chemical reactions and processes. Here, we describe a novel method for obtaining kinetic data based on thermogravimetric measurements (TGA) that exploits in each measurement multiple successive isothermal steps (SIS). We applied this method to the notoriously challenging carbon combustion process for vastly different carbons for oxygen molar fractions between 1.4 % and 90 %. Our obtained apparent E_A values are within the wide range of results in the literature and vary in a systematic way with the oxygen partial pressure. The improved accuracy and large amount of obtainable data allowed us to show that the majority of experimentally obtained apparent data for apparent E_A are neither in a kinetic regime nor in a diffusion‐controlled one but rather in a transition regime.     

燃烧反应机理构建的极小反应网络方法 氢氧燃烧

基于化学同时平衡原理, 提出复杂反应体系的极小反应网络方法(MRN), 在指定中间物种数目条件下, 构建反应步数最小的详细燃烧反应机理. 确定了8个物种的氢氧燃烧的6个独立反应, 对缺乏动力学参数的独立反应进行组合替代, 反应速率常数采用Arrhenius双参数形式. 采用构建的9步反应氢氧燃烧机理(MRN-C0)进行了点火延迟时间和层流火焰速度的模拟.     

Kinetics and mechanism of palladium(II) acetate reduction by hydrogen on the surface of a carbon support

The kinetics of the reduction of Pd(II) compounds by dihydrogen on the surface of a carbon support has been investigated for palladium acetate as an example. A kinetic model has been constructed for this reaction. An autocatalytic mechanism is suggested, in which the key role is played by Pd(0) compounds and their hydrides. The reaction occurring on the support surface is compared with the same reaction in solutions of palladium phosphine acetate complexes, where a similar mechanism is observed. One of the most important features of the surface reaction is the relatively slow reduction of the Pd(I) compounds to Pd(0). This makes it possible to obtain materials with a high Pd(I) content of 5% and above.     

Formation of carbon oxides during tobacco combustion: Pyrolysis studies in the presence of isotopic gases to elucidate reaction sequence

During the combustion of tobacco, carbon monoxide is formed by the thermal decomposition of tobacco with primary products such as carbon dioxide and water. These three processes occur in parallel and are interdependent. The temperature ranges over which each process occurs, and their relative importance have been assessed by pyrolysing tobacco in the presence of various isotopically labelled gases. Non-isothermal pyrolyses were conducted at a heating rate of 1.6 K s^?1 up to 1000°C, with the products analysed by mass spectrometer.Pyrolysis in the presence of oxygen-18 indicates that combustion of tobacco starts at 180°C. Carbon dioxide and water are formed by combustion at 180°C, while carbon monoxide is not formed as a combustion product until 460°C. The quantities of carbon monoxide and dioxide formed by thermal decomposition of tobacco above 400°C are significantly reduced by the occurrence of combustion.Pyrolysis in the presence of carbon-13 dioxide or carbon dioxide-18 shows that its major reaction, endothermic reduction to form carbon monoxide begins at 450°C. Pyrolysis in an oxygen-18/carbon-13 dioxide atmosphere has shown that this endothermic reduction of carbon dioxide occurs in parallel with the strongly exothermic oxidising reactions. 30% of the total carbon monoxide formed was produced by thermal decomposition of the tobacco. 36% was produced by combustion of the tobacco, and at least 23% was produced via carbon dioxide. The remainder was produced by an interaction of the carbon dioxide reduction and the oxidation. Similar proportion would be expected inside the reaction zone of a burning cigarette.Pyrolysis in the presence of heavy water has shown that the major reaction of the water is to quantitatively produce carbon monoxide and hydrogen above 600°C. Considerable isotopic exchange reactions also occur. Pyrolysis in the presence of carbon monoxide-18 has shown that carbon monoxide reacts with tobacco to a small extent at temperatures above 220°C mainly to abstract oxygen combined in the tobacco and produce carbon dioxide.A sequence of general chemical steps for the production of the carbon oxides and water during tobacco combustion has been deduced. This is based on the present work together with considerations of previously published studies on graphite and coal reactions.     

Burning Graphite Faster than Carbon Black: A Case of Diffusion Control

External diffusion may be exploited as a tool to purify materials in a way thought to be inaccessible from a chemical reactivity point of view. A mixture of two carbonaceous materials, graphite and carbon black, are thermally oxidized in either i) outside total diffusion-limited regime or ii) total diffusion-limited regime. Depending on the treatment applied it is possible to purify either graphite, a trivial task, or carbon black, a task thought impossible. Introducing geometrical selectivity, controlled total diffusion-limited chemistry exceeds by far the field of carbon materials and can be used as an engineering tool for many materials purification, original synthesis, or to introduce asymmetry in a system. Several examples for direct applications of the findings are mentioned.     

Temperature dependencies of high-temperature reduction on conversion products and their isotopic signatures

High-temperature reduction (HTR) is widely used for oxygen and hydrogen isotope determination. Decomposition of cellulose, sucrose and polyethylene foil by HTR is quantitative for temperatures around 1450 degrees C. For lower reaction temperature production of CO(2), water and the deposition of carbon inside the reactor are significant and thus the element of interest for isotopic analysis is split into different pools, leading to isotope fractionation. After reduction of cellulose or sucrose at 1125 degrees C less than 60% of the oxygen is found as CO, which is monitored with the isotope ratio mass spectrometer to determine the delta(18)O value. The remaining oxygen is unevenly distributed between CO(2) and H(2)O, preferentially as CO(2). Raising the reaction temperature to 1425 degrees C yields almost quantitative conversion of oxygen into CO and results in a 3 per thousand more positive delta(18)O value. Similarly, only 40-50% of the carbon of cellulose and sucrose is transformed into CO in the HTR reactor at 1125 degrees C. This is far from the stoichiometric expected value of 83% for quantitative carbon transfer for cellulose and 92% for sucrose. Of the carbon 40-50% is deposited in the reactor and the remainder can be found as CO(2). Based on the comparison of carbon isotope results from HTR and those obtained from combustion, we hypothesize that CO produced during the HTR originates partly from sample carbon and glassy carbon. A combined combustion and HTR carbon isotope determination may provide an insight into the intramolecular carbon distribution of organic substances. These results suggest that HTR should be carried out at temperatures above 1450 degrees C to make sure that fractionations associated with the reduction process are minimal. If this is not possible frequent calibration is required using reference materials of the same structure as the sample.     

Synthesis of Polybenzoquinolines as Precursors for Nitrogen‐Doped Graphene Nanoribbons

Graphene nanoribbons (GNRs) represent promising materials for the next generation of nanoscale electronics. However, despite substantial progress towards the bottom‐up synthesis of chemically and structurally well‐defined all‐carbon GNRs, strategies for the preparation of their nitrogen‐doped analogs remain at a nascent stage. This scarce literature precedent is surprising given the established use of substitutional doping for tuning the properties of electronic materials. Herein, we report the synthesis of a previously unknown class of polybenzoquinoline‐based materials, which have potential as GNR precursors. Our scalable and facile approach employs few synthetic steps, inexpensive commercial starting materials, and straightforward reaction conditions. Moreover, due to the importance of quinoline derivatives for a variety of applications, the reported findings may hold implications across a diverse range of chemical and physical disciplines.     

城市污水污泥燃烧特性和动力学特性分析

采用热重分析仪对城市污水污泥进行热重实验,通过TG(热重)、DTG(微分热重)和DSC(差示扫描量热)曲线的分析对比,获得污水污泥热解和燃烧不同阶段的特性。低于250℃,燃烧与热解的热重曲线基本吻合,说明在此之前,失重速率主要受控于有机物的分解以及析出,燃烧过程对失重影响不明显。燃烧DTG曲线呈现“W”峰型,第一个失重速率峰与热解DTG曲线基本对应,高于410℃,燃烧DTG曲线出现显著的加速。采用Coats-Redfern 积分法,假设不同的反应模型进行拟合,推断出污泥燃烧过程中两个DTG峰体现出不同的反应机理。借鉴煤燃烧机理分析认为,污泥燃烧反应初期挥发分的燃烧受控于化学反应速率,随着温度的不断提高,剩余挥发分的燃尽和固定碳的燃烧总反应速率逐渐受扩散因素控制。基于分析所得污泥燃烧机理,采用四个独立的平行反应模型模拟污泥的燃烧过程,拟合曲线与实验数据吻合良好。     

Photophysics and electrochemistry relevant to photocatalytic water splitting involved at solid–electrolyte interfaces

Direct photon to chemical energy conversion using semiconductor–electrocatalyst–electrolyte interfaces has been extensively investigated for more than a half century. Many studies have focused on screening materials for efficient photocatalysis. Photocatalytic efficiency has been improved during this period but is not sufficient for industrial commercialization. Detailed elucidation on the photocatalytic water splitting process leads to consecutive six reaction steps with the fundamental parameters involved: The photocatalysis is initiated involving photophysics derived from various semiconductor properties(1: photon absorption, 2: exciton separation). The generated charge carriers need to be transferred to surfaces effectively utilizing the interfaces(3: carrier diffusion, 4: carrier transport). Consequently, electrocatalysis finishes the process by producing products on the surface(5: catalytic efficiency, 6: mass transfer of reactants and products). Successful photocatalytic water splitting requires the enhancement of efficiency at each stage. Most critically, a fundamental understanding of the interfacial phenomena is highly desired for establishing "photocatalysis by design" concepts, where the kinetic bottleneck within a process is identified by further improving the specific properties of photocatalytic materials as opposed to blind material screening. Theoretical modeling using the identified quantitative parameters can effectively predict the theoretically attainable photon-conversion yields. This article provides an overview of the state-of-the-art theoretical understanding of interfacial problems mainly developed in our laboratory.Photocatalytic water splitting(especially hydrogen evolution on metal surfaces) was selected as a topic,and the photophysical and electrochemical processes that occur at semiconductor–metal, semiconductor–electrolyte and metal–electrolyte interfaces are discussed.     

Massenspektrometrische untersuchungen zur elementaranalyse organischer verbindungen—III: Verbrennungsvorgänge im leeren rohr

A measuring system is described which permits study of all stages of combustion processes as functions of carrier gas, temperature, residence time and tube filling. The organic sample is fed at constant speed into a stream of carrier gas. The mixture reaches the combustion chamber within a few milliseconds via a transfer capillary. With the help of a viscous inlet system, a sample of the resulting reaction products is taken and fed into a mass spectrometer. Reaction time and temperature can be adjusted within wide ranges or varied continuously. A plot of the extent of reaction of the various combustion products against temperature at a chosen reaction time yields an oxidation-thermogram which gives a clear picture of the combustion process. It is evident from thermograms of selected compounds that the samples decompose in the presence of oxygen at appreciably lower temperatures than in inert gas. The primary step of the decomposition is “oxidative pyrolysis” which often leads to other products than “inert pyrolysis”. The intermediate products found are partly structurally specific and, especially with nitrogen-containing samples, are numerous and long-lived (for example, carbon monoxide, nitric oxide, cyanogen, hydrocyanic acid, cyanic acid and methyl cyanate). The notorious “difficult combustibility” is largely due to the fact that carbon monoxide, cyanic and hydrocyanic acids undergo complete combustion only at very high temperature. The combustion properties of the “empty tube” can be improved noticeably by a filling of quartz wool and markedly by partly filling with platinum wool.     

Characterisation of the interface of sputter-deposited copper coatings on nitrogen plasma-treated carbon substrates

The adhesion of copper coatings to carbon substrates is very poor, because of lack of diffusion or reaction between the constituents. Because there is technological interest in enhancing the adhesion and improving the interface between copper and carbon, plasma treatment of the carbon substrate was employed in this study. For modification of the carbon surface a nitrogen plasma was used. It was confirmed by pull-off tests that the strength of adhesion of the copper coating can be improved by a factor of more than 10 by plasma pretreatment, even after treatment for a very short time (1 min). To obtain more information about the mechanisms of the processes occurring at the interface SIMS investigations were performed on samples which had been treated for different times (between 1 and 60 min). These measurements confirmed that nitrogen is located on the interface. With increasing pretreatment time the amount of nitrogen detected on the interface increased. Besides characterisation of as-deposited samples, another focus was to study the mechanisms of diffusion of nitrogen if the samples are heat treated at 500°C.     

Synthesis and characterization of nanocast silica NCS-1 with CMK-3 as a template

Nanocast silica (NCS-1) was synthesized by a casting process by employing the mesoporous carbon CMK-3 (the replica of SBA-15) as a template, tetraethoxysilane (TEOS) as the silica source, and hydrochloric acid (HCl) as the catalyst. The ordered carbon template was removed by employing different methods, such as calcination, thermal treatment followed by calcination, and controlled combustion. According to XRD and TEM characterization, NCS-1 exhibits an ordered structure with hexagonal symmetry and retains the morphology of the original SBA-15 used for the synthesis of CMK-3 over two replication steps on the nanometer scale. This demonstrates the well-connected porosity in CMK-3 type carbon, which can be used as a mold to synthesize mesostructured materials. The nitrogen adsorption isotherms generally show type IV shape, indicating mesoporous characteristics. The structure of NCS-1 is strongly influenced by variables of the nanocasting process, such as the loading amount of silica, hydrolysis temperature, and carbon removal methods. The surface area, pore size, and pore volume of NCS-1 can be tuned to a certain range by varying these parameters.     

Some aspects of diffusion: fluctuations in reaction diffusion,and geometric influences in nonuniform media

Summary We deal with two diffusion problems: Space-integrated conserved entities characterizing very fast - diffusion - controlled reactions, such as time lags, etc. are universal. They are given by relationships which do not reflect the failure of the mean field hydrodynamic equations. We present another application which does not reflect this failure, for determining the surface flux via a diffusion controlled reaction producing a colored product. Another anomalous diffusion process we considered is transport through cellular materials whose cell sizes are highly nonuniform. We have analyzed the effects of extreme nonuniformity by considering fractal-like models of cellular solids. The diffusion current through these models can exhibit anomalous time-dependencies which are not predicted by the diffusion equation. In particular, it is shown that the initial diffusion current can be characterized by a power-law dependence on the time. Furthermore, the exponent of the power law is given in terms of the distribution of cell sizes in the fractal-like cellular solid.     

Shape‐Controlled Growth of Carbon Nanostructures: Yield and Mechanism

Carbon nanostructures with precisely controlled shapes are difficult materials to synthesize. A facet‐selective‐catalytic process was thus proposed to synthesize polymer‐linked carbon nanostructures with different shapes, covering straight carbon nanofiber, carbon nano Y‐junction, carbon nano‐hexapus, and carbon nano‐octopus. A thermal chemical vapor deposition process was applied to grow these multi‐branched carbon nanostructures at temperatures lower than 350 °C. Cu nanoparticles were utilized as the catalyst and acetylene as the reaction gas. The growth of those multi‐branched nanostructures was realized through the selective growth of polymer‐like sheets on certain indexed facets of Cu catalyst. The vapor–facet–solid (VFS) mechanism, a new growth mode, has been proposed to interpret such a growth in the steps of formation, diffusion, and coupling of carbon‐containing oligomers, as well as their final precipitation to form nanostructures on the selective Cu facets.     

燃烧反应机理构建的极小反应网络方法： C1燃料燃烧

使用极小反应网络方法, 在指定中间物种条件下, 构建反应步数最小的详细燃烧反应机理. 确定了关 于C1燃烧机理的17个物种和14个独立反应, 其中包含氢气燃烧的8个物种6个反应, 对缺乏动力学参数的独立反应进行组合替代, 反应速率常数采用Arrhenius双参数形式. 采用构建的25步反应C1多燃料燃烧机理(MRN-C1)进行了点火延迟时间和层流火焰速度的模拟. 考虑到工程应用对机理组分数的限制, 以CH₄和CH₃OH单组分燃料为例, 考察了去除“滞留”物种后单组分机理与总机理的模拟结果差别.     

Algorithm to test the structural plausibility of a proposed elementary reaction

An algorithm is introduced that tests whether a proposed elementary reaction can be realized within a specified number of cleavages and formations of covalent bonds. This is related to the problem of computing the minimum chemical distance of a given stoichiometry, but differs from it in important ways that are exploited in the algorithm design. One application of the algorithm is as a filter in MECHEM—a computer aid for the elucidation of reaction pathways. In that application, reaction steps implying more changes to covalent bonds than a given threshold are ruled implausible, and in practice such tests need to be carried out many thousands of times. Future applications of the algorithm can be expected because the question addressed is a fundamental one: What elementary reactions can occur? © 1993 by John Wiley & Sons, Inc.     

元素化学课堂教学的设计：碳的结构与性能及其应用

以"碳的结构与性能及其应用"为例说明元素化学课堂教学中的方法,引导学生在复习碳元素的结构与性质的基础上,组织学生开展课堂讨论,引导学生对不断涌现的新型碳材料的关注和兴趣。同时,分组讨论碳氧化物的结构与化学性质之间关系及其有效利用,即模拟光合作用实现CO2的光催化还原和基于费托合成实现CO为原料制备燃料,鼓励和推进学生开展碳材料及其应用的本科生科研训练,实现课堂知识与科学研究衔接。     

2020 Roadmap on Carbon Materials for Energy Storage and Conversion

Carbon is a simple, stable and popular element with many allotropes. The carbon family members include carbon dots, carbon nanotubes, carbon fibers, graphene, graphite, graphdiyne and hard carbon, etc. They can be divided into different dimensions, and their structures can be open and porous. Moreover, it is very interesting to dope them with other elements (metal or non‐metal) or hybridize them with other materials to form composites. The elemental and structural characteristics offer us to explore their applications in energy, environment, bioscience, medicine, electronics and others. Among them, energy storage and conversion are extremely attractive, as advances in this area may improve our life quality and environment. Some energy devices will be included herein, such as lithium‐ion batteries, lithium sulfur batteries, sodium‐ion batteries, potassium‐ion batteries, dual ion batteries, electrochemical capacitors, and others. Additionally, carbon‐based electrocatalysts are also studied in hydrogen evolution reaction and carbon dioxide reduction reaction. However, there are still many challenges in the design and preparation of electrode and electrocatalytic materials. The research related to carbon materials for energy storage and conversion is extremely active, and this has motivated us to contribute with a roadmap on ‘Carbon Materials in Energy Storage and Conversion’.