The admittance of a simple electrocatalytic process coupled with an additional electrochemical adsorption reaction

The admittance of the process which is under activation control, is analysed in stationary conditions for the case of coupling in parallel with an additional electrochemical adsorption reaction which influences the electrode activity for the main process.The whole system has two time constants. As its equivalent circuit, a two-time constant electrical RC circuit of a particular structure is proposed; this structure consists of two subcircuits coupled in parallel.The use of a multi-time constant RC equivalent circuit for a faradaic process results in most cases in an ambiguous interpretation of the process, since the majority of structures of such circuits have multiple solutions. The choice of the correct interpretation needs reference to arguments outside the scope of ac measurements.It is indicated that the above ambiguity does not appear if one relinquishes the use of equivalent circuits in the modelling of complex faradaic systems.     

Estimation of adsorption isotherm parameters with inverse method--possible problems

In recent years the inverse method (IM) has been frequently applied to estimate of isotherm parameters. The IM has been used for adsorption process modeling for one, two and even three component chromatography. This method requires only a few injections with various sample concentrations, so the solute consumption and time requirements are very modest. The successful estimation of isotherm parameters with IM depends on applied chromatography column model and a numerical method used to solve the model. For HPLC column the classical equilibrium-dispersive (ED) model can be used. This model is solved frequently with very fast Rouchon finite difference method. However, the accuracy of computations with Rouchon method is decreasing with increase of the number of analyzed components. The aim of this work is the comparison of the results obtained with inverse method when ED model was solved with Rouchon or orthogonal collocation on finite element (OCFE) scheme. Assuming that solution of ED model with OCFE method can be regarded as real a solution, it was found that the Rouchon scheme may not give satisfactory results even for column with 10,000 theoretical plates for three component chromatography. Moreover, the optimal conditions for separation, calculated with Rouchon method, can be remarkably different from that obtained with the OCFE method. The next aim of this work is the presentation of Craig method application to estimation of model parameters with IM.     

Chemical production of activated carbon from green coffee with adsorption isotherm support by Taguchi model

In this study, the aim was to produce the activated carbon from green coffee for use in liquid phase applications with adding zinc borate which was a boron chemical. Phosporic acid was chosen as the chemical activation material and different reaction parameters (percent of phosporic acid, amount of zinc borate) were tested during the process of chemical activation. The experimental sets were determined by using Taguchi optimization method and optimal conditions were obtained. Taguchi optimization method was preferred to reach optimum process parameters by using time and material in the most beneficial way. The effects of the process parameters (microwave drying time, temperature of carbonization and duration of carbonization) were investigated to determine the optimal sample. The characteristic properties of the obtained activated carbons were determined with Fourier transform infrared spectroscopy (FT-IR), Brunauer-Emmett-Teller surface area analysis (BET), Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis. The samples of activated carbon produced were used for determination of the iodine number and the adsorption of heavy metal Cr (VI) ions from solution. Analysis and studies of adsorption showed that activated carbon was produced successfully. The activated carbon was evaluated in liquids phase applications and Langmuir isotherm was found more applicable and experimental data was proper with the second-order kinetic model.     

A thermodynamically consistent explicit competitive adsorption isotherm model based on second-order single component behaviour

A competitive adsorption isotherm model is derived for binary mixtures of components characterized by single component isotherms which are second-order truncations of higher order equilibrium models suggested by multi-layer theory and statistical thermodynamics. The competitive isotherms are determined using the ideal adsorbed solution (IAS) theory which, in case of complex single component isotherms, does not generate explicit expressions to calculated equilibrium loadings and causes time consuming iterations in simulations of adsorption processes. The explicit model derived in this work is based on an analysis of the roots of a cubic polynomial resulting from the set of IAS equations. The suggested thermodynamically consistent and widely applicable competitive isotherm model can be recommended as a flexible tool for efficient simulations of fixed-bed adsorber dynamics.     

Theoretical model for moisture adsorption on ionic liquids: A modified Brunauer-Emmet-Teller isotherm approach

The adsorption of atmospheric water on the air-liquid interface of ionic liquids is analyzed by means of a modified version of the Brunauer-Emmet-Teller (BET) multilayer adsorption isotherm including lateral interactions between adsorbed molecules, treated in a mean-field fashion. Recently reported experimental results of water adsorption on hydrophobic ionic liquids of the 1-alkyl-3-methyl imidazolium tetrafluoroborate (C_nMIM-BF₄) family are analyzed in the present theoretical framework. The calculated values of the lateral interaction are seen to be compatible with the Keesom dipole-dipole interaction in water, confirming the validity of the multilayer assumption hypothesis. A somehow surprisingly ordered hydrophilic-like adsorption of atmospheric water is suggested to take place in the free surface of hydrophobic ILs of the imidazolium family.     

电聚合制备聚苯胺@石墨毡复合电极及其在电芬顿过程中的高效电催化性能

作为一种高级氧化技术(AOPs),芬顿氧化法(Fenton)因其操作简单、绿色高效而备受关注.其基本原理是Fe2+催化H2O2产生的羟基自由基(?OH)进攻有机物使之降解为无机小分子或盐.电芬顿法(Electro-Fenton,E-Fenton)是利用电化学方法原位生成H2O2的Fenton衍生法,其优点在于不需要从外界加入H2O2、高效节能、无选择性、并且易于和其他处理技术耦合,是一种非常有价值和应用前景的新型水处理技术.电芬顿技术的理论探究和工艺优化,是当今高级氧化技术的理论和实践研究的重要内容.E-Fenton过程的关键步骤是阴极材料上氧还原反应(Oxygen reduction reaction,ORR)持续生成H2O2.由于析氢过电位高、稳定性好、性能优异,碳材料成为ORR反应最常用的电催化阴极材料.石墨毡作为一种三维多孔立体材料,具有电化学活性面积大、传质好、导电性强、价格低等优点,是ORR的理想阴极材料.聚苯胺材料作为一种导电高分子材料,价格便宜、加工性好、且含有丰富的N原子,在基础研究和实际应用领域都十分活跃.我们创新性地采用电聚合的方法合成了聚苯胺@石墨毡(PANI@GF)复合电极,并通过降解邻苯二甲酸二甲酯(dimethyl phthalate,DMP)研究了其在电芬顿过程中的电催化性能.通过扫描电镜、X射线光电子能谱分析对电极表面结构和杂原子掺杂性进行了物化表征.结果显示PANI@GF复合电极同时具有宏观和微观的三维多孔结构,这种结构蓬松的多孔结构为氧气提供了合适的传递通道和足够的反应面积.所制备复合电极中N原子含量约为1.9%,且吡啶N和吡咯N的含量相对较高.这些N原子来自聚苯胺分子中含有的大量N原子,并能够促进ORR反应.石墨毡和聚苯胺两种材料的在结构和组分上的特点,使得PANI@GF复合电极具有优异的电芬顿降解DMP的性能.在DMP浓度为50 mg/L、电位0.5 V(vs.SCE)、氧气流速为0.4 L/min的条件下,其DMP降解反应表观动力学常数达0.0753 min-1,是石墨毡电极表观动力学常数(0.0151 min-1)的5倍.PANI@GF复合电极制备的最优聚合时间和碳化温度分别为1 h和900℃.这是因为聚合时间太长,可能导致聚苯胺层厚度大,微孔结构被堵塞,进而降低了反应活性面积和影响氧气传质效果,使得电极性能下降;而聚合时间太短,可能导致电极复合不充分.高温碳化可以使石墨毡表面聚苯胺层形成更多的孔结构,从而有利于ORR过程.DMP降解过程中氧气流速、Fe2+用量以及pH值等工艺条件对电极性能有一定的影响,结果表明其相应的优化值分别为0.4 L/min、1.0 mmol/L和3.0.当氧气流速过低时,溶液中低浓度的溶解氧使ORR过程受传质过程限制,导致电极不能充分反应;当氧气流速过大时,并不会增加已经达到饱和的溶液中的氧气浓度,而过大的氧气速率会冲击电极表面,降低电极稳定性而影响其催化性能.对Fe2+.用量而言,E-Fenton过程有多种Fe循环途径,不同的铁含量对于电极性能影响不明显.因此,1.0 mmol/L的Fe含量足够满足实验需要.pH值对E-Fenton过程至关重要,pH较高时,铁离子会形成配合物,阻碍铁循环,并且会导致H2O2的分解,从而降低电极DMP降解性能;而当pH太低时,较多的酸增加成本,且需要后续处理过程以消除酸的影响.实验结果表明3.0是最优pH值,与传统Fenton方法的最适pH相符.PANI@GF复合电极具有高效催化降解DMP的能力,在电芬顿技术处理有机废水中有潜在应用.     

Role of adsorption phenomena in the electrocatalytic reduction of nitric acid at a platinized platinum electrode

The electrocatalytic reduction of nitric acid was studied at a platinized platinum electrode in the presence of different supporting electrolytes. It has been found that at low HNO₃ concentrations the polarization behaviour, the shape of the polarization curves and the reduction rate depend significantly upon the supporting electrolyte. (For instance, one maximum in HClO₄, two maxima in H₂SO₄.)With increasing nitric acid concentrations the differences in the character of the polarization curves gradually disappear. The phenomena observed can be explained by the role of competitive adsorption between nitric acid and ions of the supporting electrolyte. Strongly adsorbing chloride ions exert a very pronounced influence on the reduction rate and induce periodical phenomena under certain experimental conditions. The adsorption of chloride ions was studied by a radiotracer method. Results of this study confirm the view on the importance of adsorption competition in the polarization behaviour. On the basis of simple adsorption models an attempt was made to explain the phenomena observed.     

On the temperature behaviour of an adsorption system described by a Frumkin-type isotherm

A theory is developed describing stepwise coverae changes with temperature in a system of adsorbed species interacting on the surface. The temperature patterns of coverage is shown to be significantly more interesting and variegated than the concentration pattern. The existence of double transitions in the system is predicted and the conditions for their realization are analyzed.     

Electrodeposited silver nanodendrites electrode with strongly enhanced electrocatalytic activity

Highly uniform dendritic silver nanostructures as a new electrode material, have been synthesized by electrodeposition on the glassy carbon (GC) electrode with assistance of polyethylene glycol 400 (PEG-400) as a soft template, to achieve a superior electrocatalyst with enhanced detection sensitivity in electroanalysis compared to conventional bulk Ag electrodes. The effects of the growth conditions such as concentrations of the reagents and applied potentials on the morphology and structure of as-prepared tree-like nanostructures have also been investigated by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray diffraction (XRD). In the silver nanodendrites (AgNDs), the diameter of the trunk is around 100-200 nm with length up to 10-40 μm, and the length of its branches can reach 10 μm. In addition, the electrocatalytic behavior of this modified electrode was exploited as a sensitive detection system for the reduction of RDX high explosive, hydrogen peroxide and hexacyanoferrate (HCF) by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) techniques. Also, the obtained results were compared to multiwalled carbon nanotubes (MWCNTs) and bulk silver electrodes. These studies show that the nanodendritic silvers significantly increase the electron-transfer rate of the electrochemical reactions by as much as 1-2 orders of magnitude.     

Extension of theHüttig isotherm on mixed gas adsorption

Hüttig's theory was extended for mixed gas adsorption. The derived equation is the same as the one that has been obtained byGonzalez andHolland for bilayer adsorption.

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Erweiterung der Hüttig-Isotherme für Gasmischungen (Kurze Mitteilung)

Zusammenfassung Die Theorie vonHüttig wurde auf gemischte Gase ausgedehnt. Die dabei abgeleitete Gleichung ist dieselbe wie die vonGonzalez undHolland für Zweischichtadsorption.

     

Thermodynamic quantities of adsorption described by Freundlich isotherm

Thermodynamic analysis of the Freundlich adsorption isotherm was performed. Equations describing the thermodynamic characteristics of adsorption as a function of the adsorbed quantity are presented, and relationships between the excess differential and average molar thermodynamic adsorption quantities were obtained. It was shown that the thermodynamic approach does not contradict the molecular statistical theory and provides more general equations. A model of adsorption described by the Freundlich isotherm is discussed. Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 175–177, January, 2000.     

Lightweight,flexible, nanorod electrode with high electrocatalytic activity

We report on a dropcasting method to fabricate lightweight and mechanically flexible electrodes from gold nanorods. The nanorod electrodes possess extremely high electroactive areas, which lead to greatly enhanced electrocatalysis towards oxygen reduction reaction and methanol oxidation reaction when compared to conventional gold disk electrodes.     

Analysis of low vapor pressure region of adsorption isotherm

Summary In order to explain the low relative vapor pressure region in experimental adsorption isotherms of polar small molecules by polar adsorbents — as is the case with the protein-water system — the statistical thermodynamics of adsorption by a multi-component surface was studied on the basis of the BET model. The derived adsorption isotherm gives the same expression as the simple summation of the BET equation over all kinds of polar sites. The mathematical examination reveals a condition necessary for the approximation of the derived adsorption isotherm to the usual straight BET line. Besides, according to the present theory, it is possible to derive the number and weight averages of adsorption energy factors (C of BET theory) by the analysis of low vapor pressure region.

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Zusammenfassung Die Adsorption kleiner polarer Moleküle an polaren Adsorbentien, z. B. Wasser an Proteinen, wurde auf der Basis des BET-Modells im Bereich kleiner relativer Dampfdrücke mit den Methoden der statistischen Thermodynamik behandelt.Nach der entwickelten Theorie ist es möglich, die Anzahl und das durchschnittliche Gewicht von Adsorptionsenergiefaktoren zu ermitteln.

     

An evaluation on S-type adsorption isotherm in the model of crosslinked polyhydroxamates/oxazine dyes/water interactions

The effects of crosslinker and dye type on swelling and S-type adsorption properties of crosslinked polyhydroxamates (CHP) were investigated. CHPs containing N,N?-methylenebisacrylamide (N), or ethylene glycol dimethacrylate (E) were used in the swelling, diffusion, and adsorption experiments in solutions of oxazine dyes such as Brilliant Cresyl Blue, Nile Blue, and Cresyl Violet. Swelling and diffusion parameters of CHPs in dye solutions (such as equilibrium swelling, half time of swelling, swelling value at half time, network parameter, diffusion exponent, and diffusion constant) were calculated. It is understood from the time of swelling to reach equilibrium that CHPs swell very fast. CHP-E in all dyes solutions swelled considerably more than CHP-N. Dye solution diffusion into CHPs was determined to be of non-Fickian character. It has been observed that the swelling properties of hydrogels are highly influenced by the crosslinker type. The adsorption of oxazine dyes onto CHPs is similar to the S-type adsorption in the Giles classification system. When it was seen that the experimental data fit the Sigmoidal 4 parameter equation with a high correlation (r²?>?0.995), the use of this equation determined the adsorption parameters such as the highest bonding rate or monolayer coverage, the transition point of the isotherm, the magnitude of the absorbent's absorbability and the slope parameter. Site-size, maximum fractional occupancy, the binding ratio at the transition point, binding constant, the initial binding constant, partition coefficient, and adsorption free energy values were also calculated by using the found adsorption values. Dye adsorption from all dyes solutions to CHP-E is considerably higher than CHP-N. An increasing linear relationship was found between swelling and adsorption. In conclusion, the sigmoidal equation approach can be a useful tool for chemists, chemical, agricultural and environmental engineers, polymer scientists to find the adsorption parameters of polymer adsorbents, and at the same time, it can be said that CHP can be used as a good sorbent in the removal of some chemical agents (such as dye molecules, organic molecules, biologically active molecules).