A SIMPLE METHOD OF DETERMINING PORE AND SURFACE DIFFUSIVITIES IN ADSORPTION STUDIES

In this paper, a new graphical method of determining pore and surface diffusivities in adsorption systems is presented. The method involves only a simple linear plot of the inverse of half time of adsorption versus a concentration factor. The pore diffusivity and the surface diffusivity are determined directly from the intercept and the slope of the straight line, respectively. Experimental data of n-butane on Ajax activated carbon are used to illustrate the potential application of this new technique in the diffusivity parameter determination.     

活性炭孔结构对卷烟烟气过滤效率的影响

综述了活性炭孔径分布和比表面积对卷烟烟气过滤效率的影响,介绍了利用气体吸附法表征活性炭孔径分布的方法,并对活性炭滤嘴在低焦油卷烟设计中的应用进行了展望。     

Removal of volatile organic compounds by cryogenic condensation followed by adsorption

Removal of volatile organic compounds (VOCs) from gaseous effluents by cryogenic condensation and adsorption has been studied. Mathematical models have been developed to predict the extent of removal of a binary mixture of VOCs in air by these two methods under a wide range of operating conditions. The model results are verified with the published work. A model parametric study carried out in this work suggests that if the concentrations of VOCs in the effluent stream vary over a wide range, condensation followed by adsorption is an effective technique to control the emissions. Condensation is found to be suitable if the VOCs emission levels are high (>1%). On the other hand, if the emission levels are low i.e. parts per millions (ppm) or sub ppm, adsorption is a preferred technique for removing the VOCs from the effluent stream. The model results in this work have significance from the perspective of understanding the mechanism of removal of VOCs by these two methods, determining the key operating parameters that control the removal process and also, defining an effective VOC control strategy.     

The interfacial capacitance of Rh(111) in HCl solutions

The impedance spectra of Rh(111) in aqueous HCl solutions have been measured as function of potential. In the double layer region, just as for other platinum group metals, the equivalent circuit of the interface is a circuit of a R_ad-W-C_ad element combination parallely connected to a C_dl capacitance; whereas at the H⁺/Cl⁻ adsorbate exchange peak an additional parallel R_ad,2-C_ad,2 branch appears in the equivalent circuit. The (R_ad-W-C_ad) and the R_ad,2-C_ad,2 branches were demonstrated to be due to chloride and hydrogen adsorption, respectively. Due to the same anion appearing in the charging of C_ad and C_dl the two branches are coupled to each other.     

Zeta-potentials and enthalpy changes in the process of electrostatic self-assembly of cations on silica surface

Zeta potentials of silica were measured at different pH. With the increment of pH values, zeta potentials increased accordingly, resulting from the increase of the concentration of the negative charges on the silica surface. With further increment of pH values, however, zeta potentials decreased when pH value reached beyond 10.55. When HCl was added into the system to increase pH, negative charges on the silica surface reduced and so the zeta potential decreased. The isoelectric point of silica colloid appeared at pH 2.99. The density of negative charges was also measured in the process of electrostatic self-assembly of nine kinds of cations (LiCl, NaCl, KCl, MgCl₂, CaCl₂, SrCl₂, BaCl₂, AlCl₃, NH₄Cl). The increased ionity and number of charges of cations were favorable for coagulation of silica colloids. Only Al³⁺ can make the negative charges positive without causing colloid to coagulate. The changes of enthalpy and Gibbs free energy in the process of electrostatic self-assembly were determined by calorimetry and calculation, respectively. With the decreased disorder and exothermal change, the electrostatic self-assembly of cations on silica surface is regarded as an “enthalpy-driven” reaction.     

In-situ ESR spectroelectrochemical studies of overoxidation behaviour of poly(3,4-butylenedioxythiophene)

The behaviour of poly(3,4-butylenedioxythiophene) (PBuDOT), a relative of poly(3,4-ethylenedioxythipohnene) PEDOT within the poly(3,4-alkylenedioxythiophene) family, has been investigated at potentials above its electrochemical stability threshold using in situ ESR spectroelectrochemistry. The aim was to investigate the effect of electrochemical overoxidation on the charge carrying species, namely polarons, normally generated and annihilated during reversible redox doping and dedoping reactions, by determining the potential dependencies of spectroscopic parameters of the ESR spectra of the polymer over a selected potential range. Specific features of the trends of these dependencies allowed also for an evaluation of presence of the second type of charge carrying species—diamagnetic bipolarons and the effects of their interactions with polarons at different potentials. Around 1.5 V, where the boundary of electrochemical stability of the polymer lies, sharp drop of the concentration of paramagnetic centres has been observed together with a transitory narrowing of the ESR line. These changes were found to be irreversible as evidenced by the course of subsequent reduction half-cycle, which differed from the one for a not overoxidised polymer, observed in previous studies. Aided by the results of electrochemical studies it was concluded that the overoxidation process leads to a degradation of the polymer most probably due decrease of the conjugation length of the main chain π-bond through cross-linking or addition reactions. While the electrochemical results pointed to a non-complete degradation of the polymer, the specific parameters of the ESR line in the reduction half-cycle indicate that the remaining spins are confined to isolated segments of a partially degraded polymer where their behaviour resembles oligomer-like radicals.     

Dipoles and their possible effects on conductivity in polymer-ceramic composite electrolytes

This paper conceptualizes and discusses the physical existence of electrically active dipoles in polymer-ceramic composite electrolytes. The concept of physical existence of dipoles is supported by experimental evidences that include time dependence, dc field assisted variation, and mechanically induced effects on conductivity.     

Pore structures of multi-walled carbon nanotubes activated by air, CO2 and KOH

Multi-walled carbon nanotubes (MWNTs) synthesized by the catalytic decomposition of benzene were activated by KOH, CO₂ or air. The adsorption isotherms of the activated MWNTs were analyzed and their pore size distributions were obtained. The results showed that the specific surface areas of the MWNTs activated by KOH, CO₂ and air were increased to 785 m²/g, 429 m²/g and 270 m²/g, respectively. The MWNTs activated by KOH were rich in micropores and mesopores, especially high mesopores having volumes up to 1.04 cm³/g. The CO₂-activated MWNTs also had many micropores while the air-activated MWNTs had a much smaller micropore volume. The morphologies of the activated MWNTs were examined by transmission electron microscopy and high resolution transmission electron microscopy, and the activation mechanisms were discussed.     

Frequency response of adsorption of a gas onto bidisperse pore-structured solid with modulation of inlet molar flow-rate

We present a theoretical analysis of the frequency response of a continuous-flow adsorber for adsorption of a gas onto bidisperse pore-structured solid, forced by the periodic modulation of the inlet flow-rate. Spherical macroparticle and spherical microparticle geometries are considered. A local adsorption equilibrium of the gas on the exterior surface of microparticle is assumed. It is confirmed that the in-phase characteristic function of the frequency response of a flow adsorber is independent of the overflow parameter, but that the out-of-phase characteristic function is a strong function of the overflow in the lower frequency region. The overall characteristic functions in the microparticle diffusion regime have information of both microparticle diffusion and macropore diffusion. Hence, in this regime we can extract the macropore diffusion parameter as well as the microparticle diffusion parameter from the experimental data of the overall characteristic functions. In the microparticle diffusion regime the capacity parameter of microparticle affects also the characteristic functions.     

微波加热椰壳制备活性炭的表征及其对苯系物吸附性能的研究

采用微波加热椰壳水蒸气活化法制备了活性炭,采用H-K方程、DFT理论表征了活性炭的孔结构,该活性炭的BET比表面积889m^2/g,平均孔径0.55nm;采用自制的实验装置进行了微波椰壳基活性炭吸附苯系物的工艺探索,研究了气体流速和吸附时间对活性炭吸附苯系物量的影响,当气体流速15m^3/h,吸附时间8d,活性炭5g时,可将实验箱内300mg的苯系物全部脱除,达到了国标GB/T18883-2002对空气中苯系物浓度的要求。     

Synthesis of spherical mesoporous silica nanoparticles with nanometer-size controllable pores and outer diameters

Spherical mesoporous silica particles with tunable pore size and tunable outer particle diameter in the nanometer range were successfully prepared in a water/oil phase using organic templates method. This method involves the simultaneous hydrolytic condensation of tetraorthosilicate to form silica and polymerization of styrene into polystyrene. An amino acid catalyst, octane hydrophobic-supporting reaction component, and cetyltrimethylammonium bromide surfactant were used in the preparation process. The final step in the method involved removal of the organic components by calcinations, yielding the mesoporous silica particles. Interestingly, unlike common mesoporous materials, the particle with controllable pore size (4–15 nm) and particle diameter (20–80 nm) were produced using the method described herein. The ability to control pore size was drastically altered by the styrene concentration. The outer diameter was mostly controlled by varying the concentration of the hydrophobic molecules. Relatively large organic molecules (i.e. Rhodamine B) were well-absorbed in the prepared sample. Furthermore, the prepared mesoporous silica particles may be used efficiently in various applications, including electronic devices, sensors, pharmaceuticals, and environmentally sensitive pursuits, due to its excellent adsorption properties.     

Rehydration and microstructure of cement paste after heating at temperatures up to 300 °C

This paper is concerned with the evolution of the microstructure of cementitious materials subjected to high temperatures and subsequent resaturation in the particular context of long-term storage of radioactive wastes, where diffusive and convective properties are of primary importance. Experimental results obtained by mercury intrusion porosimetry (MIP) are presented concerning the evolution of the pore network of ordinary portland cement (OPC) paste heated at temperatures varying between 80 and 300 °C. The consequences of heating on the macroscopic properties of cement paste are evaluated by measures of the residual gas permeabilities, elastic moduli and Poisson's ratio, obtained by nondestructive methods. Resaturation by direct water absorption and water vapour sorption are used to estimate the reversibility of dehydration. The results provide some evidence of the self-healing capacity of resaturated cement paste after heating at temperatures up to 300 °C.