Influence of the surface chemistry of modified activated carbon on its electrochemical behaviour in the presence of lead(II) ions

Cyclic voltammetric studies of the influence of surface chemistry on the electrochemical behaviour of granulated and powdered activated carbon samples in the presence of lead(II) ions both in bulk solution and pre-adsorbed on carbon were carried out. Variety in surface chemical character was achieved through modification of carbon samples by heat treatment in vacuum, ammonia and ammonia-oxygen atmospheres, as well as by oxidation in moist air and with concentrated nitric acid. For the samples obtained, the surface area (BET), acid–base neutralization capacities and sorption capacity towards Pb²⁺ ions were estimated. The states of the deposited Pb species were assessed by means of FTIR and XPS spectra as well as cyclic voltammetry. The importance of the surface chemistry of the carbon electrode materials are discussed in terms of their electrochemical properties and the mechanism of adsorption processes. The Cπ-metal and heteroatom-metal interaction are dominant in amphoteric and basic carbons, but in oxidized samples adsorption take place mainly by ion-exchange. Other forms of adsorption, such as the formation metal hydroxide species, are also covered buy this paper. Various forms of adsorbed lead species exhibit different electrochemical activities.     

Effect of air oxidation of Rayon-based activated carbon fibers on the adsorption behavior for formaldehyde

The tailoring of pore surface chemistry of activated carbon fibers is shown to be an effective method for improving the adsorption efficiency of various volatile chemical compounds (VOCs). An oxidation treatment with air resulted in a significant increase in the adsorption capacities and breakthrough time for Rayon-based activated carbon fibers (ACFs) in removal of formaldehyde. The porous structure parameters of Rayon-based ACFs were determined with standard nitrogen adsorption analysis. The pore surface chemistry of samples under study was analyzed by Fourier Transform Infrared spectra (FTIR). Thus to some extent, the relationship between the adsorption properties, porous structure and pore surface chemistry was revealed.     

活性炭表面化学改性技术的研究进展与展望

活性炭表面官能团和杂原子的数量与种类是影响活性炭吸附性能的重要因素。国内外研究表明,通过对活性炭进行表面改性可以显著改善活性炭对特定物质的吸附性能。文章简要介绍了活性炭的物理和化学性质,并从活性炭材料的表面化学性质方面论述了近年来国内外在活性炭材料改性方面的研究进展,最后提出了活性炭表面改性技术的发展方向和趋势。     

改性活性炭的表面特性及其对苯酚的吸附性能

研究了表面改性对活性炭吸附苯酚性能的影响。研究发现,硫酸氧化可增加活性炭表面酸性基团的含量,提高了活性炭的表面亲水性,降低pHPZC值,因而对吸附水中的苯酚的性能产生明显影响,降低了对苯酚的吸附。     

Influence of progressive surface oxidation of nitrogen-containing carbon on its electrochemical behaviour in phosphate buffer solutions

The effects of the nitric acid surface oxidation of nitrogen-containing carbons (SCN-type) on the chemical structure as well as the electrochemical properties of powdered electrodes prepared from them were studied. The oxidation efficiency was dependent on the duration of the oxidative modification. The surface chemistry was characterized using standard neutralization techniques and spectroscopic methods (FTIR and XPS). Electrochemically active surface groups obtained on carbon materials during oxidation were investigated by cyclic voltammetry. Some noteworthy relations between the electrochemical activity of carbon electrode material and the degree of surface oxidation are reported.     

Powdered activated carbon and carbon paste electrodes: comparison of electrochemical behaviour

Commercial activated carbon (Norit R3ex), de-mineralised with conc. HF and HCl, was oxidised (conc. HNO₃) and heat-treated at various temperatures (180, 300 and 420 °C). The physicochemical properties of the samples obtained were characterised by selective neutralisation and pH-metric titration of surface functional groups (acid–base properties), thermogravimetry (thermal stability—TG), FTIR spectroscopy (chemical structure) and low-temperature nitrogen adsorption (BET surface area). Thermal treatment of the carbon materials caused the surface functional groups to decompose; in consequence, the chemical properties of the carbon surfaces changed. Cyclic voltammetric studies were carried out on all samples using a powdered activated carbon electrode (PACE) and a carbon paste electrode (CPE), as were electrochemical measurements in aqueous electrolyte solutions (0.1 M HNO₃ or NaNO₃) in the presence of Cu²⁺ ions acting as a depolariser. The shapes of the cyclic voltammograms varied according to the form of the electrodes (powder or paste) and to the changes in the surface chemical structure of the carbons. The electrochemical behaviour of the carbons depended on the presence of oxygen-containing surface functional groups. The peak potentials and their charge for the redox reactions of copper ions depended on their interaction with the carbon surface.     

活性炭材料改性及其在环境治理中的应用

从活性炭材料的表面结构性质和表面化学性质两方面论述了活性炭材料改性的研究进展,就活性炭材料在环境污水处理以及大气污染防治方面的应用进展进行了评述,并展望了活性炭材料改性的前景.     

Oxidation of pinane over phthalocyanine complexes supported on activated carbon: Effect of the support surface treatment

The oxidation of cis-pinane with tert-butyl hydroperoxide, at room temperature and atmospheric pressure, was carried out in the presence of iron-phthalocyanines supported on activated carbons. The carbon supports were prepared from a NORIT activated carbon, which was modified by different chemical and thermal treatments (including oxidation in the gas and liquid phases). The carbon samples were characterized by nitrogen adsorption, mass titration and temperature programmed desorption (TPD). The TPD profiles were analysed by a simple deconvolution method, allowing for the determination of the amount of oxygen containing functional groups on the carbon surface. The main reaction product is 2-pinane hydroperoxide (77% selectivity at 91% conversion). Formation of 2-pinanol, pinocampheol and verbanol and the respective ketones was also observed. The influence of the surface chemistry of the carbon supports on catalytic activity and product selectivity is studied. The catalysts prepared from supports with very high or very low oxygen content exhibit low activity, whereas for supports with intermediate oxygen contents a good correlation between the amount of phenols and lactones and catalytic activity is obtained.     

活性炭吸附处理含酚废水的研究进展

含酚废水对环境和生物有较大危害,是一种常见的化工废水。活性炭作为良好的吸附剂被广泛用于污水处理,也常被用于吸附处理含酚废水。最新的研究集中于开发利用各种含碳原材料,并探究活性炭制备和改性方法,以改善活性炭对酚类的吸附性能。部分机理研究则关注活性炭的孔隙结构和表面官能团及其对吸附酚类性能的影响。本文从活性炭的制备和改性出发,归纳整理活性炭吸附酚类的特性和机理,分析吸附过程的主要影响因素,并对研究发展方向进行推论和展望。分析表明含碳量高的原材料适合制备活性炭,尤其是含碳废弃物。活性炭的苯酚吸附性能受比表面积和表面官能团的共同影响,这对于活性炭的制备和改性有指导意义。活性炭吸附苯酚的具体应用中,需要控制粒度、pH、温度、吸附时间和竞争吸附等影响因素。     

Effect of activation on boron nitride coating on carbon fiber

Boron nitride (BN) thin coating has been formed on the surface of chemically activated polyacrylonitrile (PAN) carbon fibers by dip coating method. The chemical activation of PAN fibers was carried out by two different chemicals, i.e. nitric acid (HNO₃) and silver nitrate (AgNO₃) solution. The chemical activation changes the surface properties, e.g. surface area and surface microstructure of the carbon fibers. These surface modifications ultimately influence properties of boron nitride coating on carbon fibers. The boron nitride coating on carbon fibers showed better crystallinity, strength and oxidation resistance when carbon fibers were activated by HNO₃. This improvement in strength and oxidation resistance is attributed to better crystallinity of boron nitride coating on HNO₃ activated PAN fibers.     

Characterization of oxygen containing functional groups on carbon materials with oxygen K-edge X-ray absorption near edge structure spectroscopy

Surface functional groups on carbon materials are critical to their surface properties and related applications. Many characterization techniques have been used to identify and quantify the surface functional groups, but none is completely satisfactory especially for quantification. In this work, we used oxygen K-edge X-ray absorption near edge structure (XANES) spectroscopy to identify and quantify the oxygen containing surface functional groups on carbon materials. XANES spectra were collected in fluorescence yield mode to minimize charging effect due to poor sample conductivity which can potentially distort XANES spectra. The surface functional groups are grouped into three types, namely carboxyl-type, carbonyl-type, and hydroxyl-type. XANES spectra of the same type are very similar while spectra of different types are significantly different. Two activated carbon samples were analyzed by XANES. The total oxygen contents of the samples were estimated from the edge step of their XANES spectra, and the identity and abundance of different functional groups were determined by fitting of the sample XANES spectrum to a linear combination of spectra of the reference compounds. It is concluded that oxygen K-edge XANES spectroscopy is a reliable characterization technique for the identification and quantification of surface functional groups on carbon materials.     

The effect of the gradual thermal decomposition of surface oxygen species on the chemical and catalytic properties of oxidized activated carbon

The physicochemical properties, surface chemical structure and some catalytic properties of a series of carbons prepared by nitric acid oxidation of an activated carbon and subsequent heat treatment under vacuum and mild temperature conditions (423-573 K) were studied. The porous structure characteristics of the partially evacuated samples were estimated from low-temperature nitrogen adsorption data. The thermal analysis and the quantitative determination of surface functional groups by selective neutralization of bases and pH-metric titration were carried out. The dehydration of 2-methylpropan-2-ol was used as a test reaction. While gradual annealing in vacuum alters the surface only slightly, it does differentiate strongly the number and the acidic strength of the surface groups. Progressive heating under mild conditions removes mainly those surface groups that are located in macropores or on the outer surface of the carbon. According to TPD results, the decomposed surface groups are single carboxylic groups, as expected. The decomposition of single, strong carboxylic groups is accompanied by rearrangements of other carboxylic groups with the simultaneous formation of additional cyclic structures like anhydrides, lactones or lactols. Catalytic tests support our previous findings that oxidized carbons have a high dehydration activity. This activity is controlled not only by the number and the strength of acidic groups, but also by their accessibility. There exists an optimum concentration of surface acidic groups above which the activity decreases due to steric restrictions.     

Surface modification of a granular activated carbon by citric acid for enhancement of copper adsorption

In this study, citric acid was used to modify a commercially available activated carbon to improve copper ion adsorption from aqueous solutions. The carbon was modified with 1.0 M citric acid, followed by an optional step of reaction with 1.0 M sodium hydroxide. It was found that the surface modification reduced the specific surface area by 34% and point of zero charge (pH_pzc) of the carbon by 0.5 units. Equilibrium results showed that citric acid modification increased the adsorption capacity to 14.92 mg Cu/g, which was 140% higher than the unmodified carbon. Higher initial solution pH resulted in higher copper adsorption. The chemical surface modification adversely affected the copper adsorption rate. Adsorption kinetic mechanisms were investigated with an intraparticle diffusion model. It was found that the modification did not change both external diffusion and intraparticle diffusion.     

载体预处理对Pd/C催化剂催化性能的影响

研究了活性炭硝酸表面改性对以其为载体制备的负载钯催化剂性能的影响。利用表面官能团滴定、N₂物理吸附和扫描电镜对催化剂进行表征，以邻硝基氯苯催化加氢制备2，2'-二氯氢化偶氮苯反应为模型反应对催化剂的性能进行评价。结果表明，经过不同浓度的HNO₃处理，活性炭孔结构性能变化不大，但是活性炭表面酸性含氧基团的浓度有了较大程度的增长，为Pd金属粒子的沉积提供了大量的吸附位，提高了Pd金属的分散度，从而制得高活性的Pd/C催化剂。通过30%HNO₃ 60 ℃水浴中回流4 h处理的活性炭可以达到最佳效果，所制得催化剂的活性是以未经硝酸处理过的活性炭载体制备的催化剂活性的2.3倍。     

四氯乙烷脱HCl反应中炭基催化剂的失活机理

活性炭表面物化性质是炭基催化剂催化TeCA脱HCl合成TCE反应性能的重要影响因素。采用酸、碱对活性炭进行调变处理,通过XRF、BET、Boehm滴定、GC-MS等对处理前后活性炭的物理结构、表面基团、无机氧化物种类和数量,以及反应后活性炭的物理结构和表面残留有机物的表征分析,研究了活性炭催化TeCA脱HCl反应性能与其表面物化性质的对应关系。结果表明:酸、碱等处理对活性炭物理结构和表面基团影响较小,但显著改变了无机氧化物种类和数量;反应后活性炭比表面积下降明显,表面残留有五氯丁二烯;活性炭中铝、铁氧化物与五氯丁二烯含量、孔道堵塞程度和失活速率呈密切对应关系。铝、铁与HCl生成的AlCl₃和FeCl₃ Lewis酸中心是促进TCE聚合进而导致孔道堵塞而失活的主要原因。     

Adsorption of dyes on activated carbons: influence of surface chemical groups

The surface chemistry of a commercial activated carbon has been selectively modified, without changing significantly its textural properties, by means of chemical treatments, using HNO₃, H₂O₂, NH₃, and thermal treatments under a flow of H₂ or N₂. The resultant samples were characterized in terms of their surface chemistry and textural properties, and subsequently tested in the removal of different classes of dyes. It was shown that the surface chemistry of the activated carbon plays a key role in dye adsorption performance. The basic sample obtained by thermal treatment under H₂ flow at 700 °C is the best material for the adsorption of most of the dyes tested. For anionic dyes (reactive, direct and acid) a close relationship between the surface basicity of the adsorbents and dye adsorption was shown, the interaction between the oxygen-free Lewis basic sites and the free electrons of the dye molecule being the main adsorption mechanism. For cationic dyes (basic) the acid oxygen-containing surface groups show a positive effect but thermally treated samples still present good performances, showing the existence of two parallel adsorption mechanisms involving electrostatic and dispersive interactions. The conclusions obtained for each dye individually were confirmed in the colour removal from a real textile process effluent.     

Influence of differences in porous structure within granules of activated carbon on adsorption of aromatics from aqueous solutions

The adsorption of organics from aqueous solutions on the activated carbon samples obtained by gradual abrasion of granules is studied. The nitrogen adsorption/desorption isotherms are applied to determine the parameters of porous structure of investigated carbon samples. The adsorption isotherms of four organic substances: nitrobenzene, phenol, 4-nitrophenol, and 4-chlorophenol from dilute aqueous solutions are also measured. The influence of porous structure changes inside of a carbon particle on organic solute adsorption from aqueous solutions is analyzed and discussed. It was stated that the carbon porosity was a function of the distance from granule surface, however, the properties of carbon micropore and mesopore walls were preserved. The similarity of adsorption mechanisms over the whole carbon granule were found for liquid systems.     

煤化工VOCs吸附处理技术研究进展及展望

煤化工产生的挥发性有机物VOCs气体成分复杂且有毒有害,为了避免煤化工VOCs及其光化学产物对环境和人体健康产生危害,通过分析VOCs气体的排放控制及处理技术,指出煤化工VOCs吸附技术是可以控制VOCs排放、回收吸附材料及回收有价值VOCs的经济、有效的VOCs去除技术。通过分析煤化工VOCs吸附的物理与化学过程及其影响因素、解吸附的过程与方法,对常用的吸附材料的改性研究及发展进行了综述,通过对比不同吸附装置的结构、吸附特点及优缺点,将煤化工VOCs吸附技术与其他技术的组合实际工程应用进行了比较分析,并展望了吸附技术的未来研究方向。影响吸附过程的因素有吸附材料的结构特性、表面化学性质及亲疏性热稳定性等物理化学特性,被吸附物质VOCs的分子特性、吸附剂与吸附质之间的相互作用、不同吸附质之间的相互竞争、吸附环境等;物理吸附过程包括外表面传值吸附阶段、内部表面扩散阶段、不同孔径孔隙之间的平衡阶段;吸附剂微孔提供了主要的吸附位点,而中孔及大孔则增强了VOCs的扩散通道。吸附材料经过适当改性具有优异的VOCs吸附能力;采用H2O2浸渍法改性可提高活性炭纤维表面含氧官能团含量,吸附能力增强;采用具有强氧化性的浓硫酸等改性使活性炭表面具有含氧基团,增强活性炭对氮的吸附能力;用碱性氢氧化物改性的活性炭增加了比表面积,用酸改性可增加表面官能团,用KOH活化可获得更好的孔隙率。需要针对VOCs种类、浓度、流量及排放量等特性选择适合的吸附装置。吸附技术是控制煤化工VOCs排放和回收有价值VOCs再利用的经济、有效且具有前景的技术,可与其他技术组合处理VOCs气体,进行有利用价值VOCs气体的回收利用,实现VOCs废气排放达标。吸附技术未来研究重点是吸附材料改性(或定向改性)、新型改性方法及新型吸附材料研究、高效低成本吸附装置研究、多组分吸附质同时脱除研究,并提出了多组分VOCs吸附及解吸附的复合吸附装置研究思路。     

Activation of coal tar pitch carbon fibres: Physical activation vs. chemical activation

Activated carbon fibres (ACF) are obtained mainly by physical activation with steam or carbon dioxide. Additionally, there are many papers dealing with chemical activation of carbon fibres, or a polymeric raw material, with several chemical agents like for example, phosphoric acid, zinc chloride, aluminium chloride,… Nevertheless, although it is well known that hydroxides are good activating agents, there are few papers about the activation of carbon fibres with KOH or NaOH. In the present work, ACF with high surface area are obtained by chemical activation with KOH and NaOH. Both chemical agents present different behaviour; thus, NaOH developed the highest value of porosity and KOH developed samples with narrower micropore size distribution. In order to compare the results with those obtained by physical activation, some ACF have been prepared using CO₂ activation. The main conclusion of this work is that by using chemical activation it is possible to obtain similar, or even higher, porosity (∼1 ml/g, ∼3000 m²/g) than by physical activation. However, chemical activation presents two important advantages: (1) a much higher yield (27-47% for chemical activation and 6% physical activation for ∼2500 m²/g activated carbon fibres) and (2) the surface of the fibres prepared by chemical activation is less damaged than by physical activation.     

Oxygen plasma modification of submicron vapor grown carbon fibers as studied by scanning tunneling microscopy

Scanning tunneling microscopy (STM) has been employed to monitor the changes in surface structure induced by oxygen plasma treatments of submicron vapor grown carbon fibers (VGCFs). It is shown that the fibers preserve their general smoothness upon plasma oxidation and that the structural changes brought about by this treatment essentially take place only at the atomic scale, where the relatively ordered domains typical of the untreated material are replaced by atomically rough and disordered structures. These atomic-scale changes imply the modification of some physico-chemical properties of the fiber surface, such as concentration of oxygen functionalities. The STM results, together with those obtained from nitrogen physical adsorption measurements, suggest that the potential improvement of plasma treatment in VGCF-matrix adhesion for application in composite materials should proceed mainly from chemical bonding due to the addition of functional groups rather than from increased mechanical interlocking.