Inhomogeneous microporous structures of carbonaceous adsorbents

Ways for theoretical calcultion of the geometric surface area of the micropore walls of carbonaceous adsorbents with inhomogeneous microporous structures are considered for the slitlike micropore model. Two calculations methods have been worked out based on the theory of volume filling of micropores (TVFM) from the parameters of the adsorption equation proposed by Stoeckli and from the parameters of the binomial TVFM equation. An independent method for determining the geometric surface area of the micropore walls from water vapor adsorption isotherms served as a reference. When we used an active carbon with a wide micropore distribution all the three methods led to practically identical results, which indicated that the concept of the geometric surface area of carbonaceous adsorbents is physically feasible. The physical unfeasibility of a specific surface determined by the BET method for a microporous adsorbent is shown.     

The complete pore structure analysis of fine porous solids

A new method is presented for the determination of the distribution of pore volume and surface in micropores. The analysis facilitates obtaining typical parameters of micropore structure from the adsorption isotherm. The calculated characteristic micropore radii and the micropore surface area for a model of cylindrical pores agree with the data from small-angle X-ray scattering. Micropore cumulative distribution curves for two typical active carbons were joined together with a high pressure mercury porosimetry data. The overall porosity cumulative distribution curve agrees with the Dubinin model of the porous structure of active carbons.     

Microporous structures of carbonaceous adsorbents

The simplest model of the porous structure of carbonaceous adsorbents is proposed, in which the microporous zones are formed by the totality of contacting and merged carbon crystallites. The micropores limited in extent resulted from the burnout on activation of several atomic layers of carbon in the crystallites by gaseous oxidizers (intercrystallite micropores) and from some development of the spaces between contacting crystallites (intercrystallite micropores). The micropore zones are shaped into regular cubes. The slit-like spaces between part of the micropore zones represent the micropore volume, while the total surface area of the walls of such pores is the mesopore surface. The parameters of the model for typical samples of microporous carbonaceous adsorbents are calculated and discussed. The formation of intercrystallite slit-like micropores as a result of the burnout of carbon atomic layers in crystallites on avtivation is considered. It is shown that only a wide micropore distribution leading to the formation of the supermicropores practically results in deviation from the applicability of the one-term adsorption equation of the theory of volume filling of micropores.     

利用标准等温线分析活性炭的完全孔分布

利用我们从标准等温线性质发展的孔分布计算两个新方法——完全无模型孔分布计算法和改进的微孔分析法对有代表性的九个活性炭样品进行了完全孔分布分析.用吸附仪测量样品的低温氮吸附等温线,用Pickett方程关联了这些吸附等温线,发现n值在1.1—1.4之间.因常数C值在100—300之间,选用标准等温线n_2作为完全孔分布分析的依据.获得了总表面积、微孔的表面积和体积、非微孔的表面积和体积以及平均微孔水力半径等主要孔结构参数,并获得了所有样品的完全孔分布.两个比值SR和VR在0.9—1.1之间,结果是令人满意的.分析结果指出,DX-09-1-1,7S及J三个样品基本上全是微孔,而其余的炭毡及BPL炭样品都有相当数量的非微孔.这些结果说明,利用标准等温线进行活性炭样品的完全孔分布分析是实用的.     

Theoretical analysis and experimental study on SO2 adsorption onto pistachio‐nut‐shell activated carbon

The adsorption study of SO₂ onto the activated carbon prepared from pistachio‐nut shell was studied theoretically and experimentally. A single‐particle sorption model known as concentration‐dependent surface diffusivity micropore, surface and macropore diffusion control model incorporating micropore, macropore and surface diffusions, together with a nonlinear isotherm at the micropore mouth, has been derived and solved by a finite difference method. The effects of different types of nonlinear isotherms and concentration dependent surface diffusivities have been thoroughly studied. The effects of adsorbate concentration and temperature on adsorption were studied experimentally. Good agreement was found between the model predictions and the experimental results. The value of the tortuosity factor and the extracted diffusion coefficients obtained are consistent with their corresponding values reported. © 2008 American Institute of Chemical Engineers AIChE J 2009     

Determination of porosity in pillared clays by N2 adsorption isotherms

N₂ adsorption isotherms of various pillared montmorillonites (PILCs) were analyzed for evaluation of their porosities. The quantitative data of the total and micropore volumes were calculated using the B.J.H. method and the t-plot, respectively. The volume of mesopores is the difference between the total volume and the micropore volume. The linear branch above the P/P ₀ of 0.5 should be used for the calculations of the micropore volume and external surface area.The evaluation of the specific surface area (SA) of the PILCs and influences of the porosity on the calculations were discussed in detail. An upper limit of the monolayer capacity of a porous solid is proposed, based on the adsorption on a nonporous solid. Due to the space restriction in the fine slit-like pores of the PILCs, the specific surface areas calculated with the B.E.T. equation using the adsorption data in a relative pressure region ranging from 0.01 to 0.1 are more accurate.The mean micropore widths of the PILCs, derived from the data of the multi-point B.E.T. SA and micropore volume consist reasonably with the pore widths obtained by XRD diffraction.The step-like curves of the t-plot in low-pressure region and of the logarithmic plot reveal presence of pores of various sizes in the pillared clays. The micropore size distribution can be derived by a new method using the N₂ adsorption data.     

Characterization of the textural properties of metal loaded ZSM-5 zeolites

The textural properties of metal ion over-exchanged zeolites, Co–, Cu–, Ni–ZSM-5, were compared with those of the parent ZSM-5 by nitrogen adsorption measurements and photoelectron spectroscopy. The treatment of the adsorption isotherms permitted a thorough characterization of the microstructure of the samples. A first estimation of the microporous volume was made by the Dubinin–Radushkevich method. The “t-plot” method was used to determine the external surface and to obtain another estimation of the microporous volume. Effective micropore size distribution was obtained with the Horvath–Kawazoe approach. The Dubinin–Radushkevich method overestimated the microporous volume of the zeolites, in particular when the solid had an important external surface. It is shown that Cu–ZSM-5 had micropore size and micropore size distribution very close to those of ZSM-5, whereas Co–ZSM-5 and Ni–ZSM-5 had lower microporous volumes and larger external surfaces. The external to internal (microporous) surface ratios, r_s, and the mesoporous to microporous volume ratios, r_v, are proposed as practical parameters to classify the solids with respect to their microstructure. ZSM-5, a pure microporous solid, has low values of both r_s and r_v parameters. Ni– and Co–ZSM-5 have high r_s ad r_v parameters, indicative of some degree of mesoporous character. Cu–ZSM-5 has intermediate characteristics, typical for micro-mesoporous solid. XPS measurements confirmed that copper ions penetrated to a greater extent in the channels of ZSM-5 zeolite than cobalt and nickel ions, which merely deposited on the external surface of ZSM-5 zeolite as oxidic aggregates.     

褐煤及其干馏半焦的微孔结构分析

用CO_2吸附法于298K下研究了大雁褐煤、黄县褐煤及其干馏半焦的微孔结构特性。用由Dubinin-Astakhov方程导出的关系式计算了所研究样品的微孔孔径分布和微孔有效表面积S_(micro),讨论了干馏温度对半焦的微孔孔容及平均当量半径的影响。     

Preparation and modification of activated carbon fibres by microwave heating

Thermal treatment of activated carbon fibres (ACF) has been carried out using a microwave device, instead of a conventional furnace. The results show that microwave treatment affects the porosity of the ACFs, causing a reduction in micropore volume and micropore size. More importantly, the results also show that microwave treatment is a very effective method for modifying the surface chemistry of the ACFs with the production of pyrone groups, detected by FTIR. As a result very basic carbons, with points of zero charge approximately equal to 11, are readily obtained.     

二苯并呋喃在活性炭上的吸附相平衡和动力学

The adsorption of dibenzofuran on three commercial granular activated carbons （ACs） was investigated by dynamic experiment to correlate the adsorption equilibrium and kinetics with the structure of activated carbons. Physical properties including surface area, average pore diameter, micropore area and micropore volume of the activated carbons were characterized by N2 adsorption experiment on ASAP2010. To calculate the adsorption parameters, adsorption isotherm data were fitted to the Langmuir equation, and adsorption kinetic data were fitted to the linear driving force （LDF） diffusion model. From the correlation results, it is concluded that the adsorption equilibrium and diffusion coefficient of dibenzofuran on activated carbon are controlled respectively by the total adsorbent surface area and the adsorbent pore diameter.     

A method for the estimation of micropore volume and micropore surface area

Micropore volumes have been estimated by comparing the results of mercury intrusion with helium displacement in porous samples. Differences in the surface area measured by gas adsorption and mercury porosimetry afford an estimation of micropore surface area and the micropore surface to volume ratio gives a mean micropore radius.     

Influence of pore structure and surface chemistry on electric double layer capacitance in non-aqueous electrolyte

The performance as electric double layer capacitors (EDLC) in non-aqueous electrolyte of a series of alkaline agent-activated carbons with high surface area is presented in this work. The results obtained show that, in general, capacitance increases with surface area. However, the results obtained in this study confirm that capacitance not only depends on surface area, but also on two other parameters: pore size distribution and surface chemistry. It has been shown that capacitance is higher for a sample with wider micropore size distribution than for a sample with higher surface area but too narrow micropore size distribution. In addition, it has been observed that the sample with a very high amount of surface groups presents very high capacitance values. In the present study, a KOH-activated carbon with a capacitance as high as 220 F/g was prepared. Finally, the results obtained with a mesoporous sample have shown that the presence of mesopores in activated carbons with very high surface area (e.g. >2000 m²/g), do not seem to be effective for double layer capacitors.     

A new approach to quantify the microporosity of activated carbons by analysing the N2/77 K and CO2/273 K adsorption data by the simplex flexible method

A mathematical model has been applied to N₂/77 K and CO₂/273 K adsorption isotherms for a series of activated carbons prepared by carbonising olive stones in N₂ and then activating them in CO₂ to six different levels of burn-off in the range 8–80%. Narrow and wide micropore volumes of activated carbons were calculated from the Dubinin-Radushkevich and Dubinin-Astakhov equations considering one, two and three micropore size distributions in each sample, and allowing a variation of the micropore volume and characteristic energy of each distribution with the burn-off. The flexible simplex method was applied to obtain the parameters of each distribution in the mathematical model. Generally, it was found that increasing the number of micropore size distributions above two did not significantly improve fits. Each isotherm was fitted using six parameters at most. However, various constraints were imposed, and the parameters were estimated from each isotherm using non-linear, least-squares regression analysis. The results obtained confirm the valuable use of CO₂/273 K adsorption to quantify the narrow microporosity of activated carbons. Differences between N₂/77 K and CO₂/273 K adsorption in microporous activated carbons were due to the wide microporosity. An agreement between micropore volumes obtained from CO₂/273 K adsorption and that corresponding to one of the two distributions of micropores obtained from N₂/77 K adsorption was obtained. The Dubinin-Radushkevich equation was more successful than the Dubinin-Astakhov equation in the quantification of the microporosity with N₂/77 K and CO₂/273 K. On the other hand, the exponent n of the Dubinin-Astakhov equation was better correlated with the burn-off of the carbons than with the parameter B.     

氧化时间对纯钛表面微弧氧化膜结构特性的影响

采用微弧氧化技术在纯钛表面制备了含钙磷的多孔复合氧化膜,用扫描电子显微镜、X射线衍射仪、电子探针分析等手段分析了处理时间对氧化膜形貌、厚度、成分、结构的影响。结果表明：随处理时间的延长,膜厚逐渐增大,微孔数量增多、孔径增大、分布趋匀;表面沉积的钙磷盐逐渐减少并消失,膜中钙磷摩尔比趋于稳定;膜中金红石相TiO2增多、锐钛...     

The determination of diffusion coefficients in activated carbons made from bituminous coal

The gas chromatographic method has been used to measure the effective diffusion coefficients D_e or the diffusion parameters $\text{D}{}_{\mathrm{e}}\text{d}{}^2$. Nitrogen was used as a carrier gas, krypton-85 as test gas. The tests were carried out on different activated carbons made from air-preoxidized bituminous coals whose pore structure as a function of the burn-off was known. The diffusion parameters for krypton were found to increase exponentially with the mean micropore diameter of the activated carbons which was varied in our tests between 5.8 and 7.0 Å. For activated carbon with the narrowest pores the activation energy of krypton diffusion was found to be 2.8 kcal/mol. In case of such a product the diffusion time is not rigidly dependent on the particle diameter as would have to be expected theoretically. With increasing micropore diameter this dependency is attenuated very rapidly, and in carbons with a mean micropore diameter above ca. 6.8 Å it exists no longer at all. To check the results, gravimetric studies on the adsorption kinetics of krypton in vacua and on the desorption kinetics using the gas flushing method were made. It was found that the results obtained by the gas chromatographic method and the gas flushing method were in good agreement, whereas the diffusion parameters obtained by measurements in vacuo were by one order of magnitude lower. The findings are discussed in relation to the pore structure of the adsorbent and the properties of the sorbate. The limits of the different measuring methods are shown.     

Some physical properties of Canadian coals and their effects on coal reactivity

The pore volume, surface area and compressibility of eleven Canadian coals, varying in rank from lignite to semianthracite, have been determined by mercury porosimetry, gas adsorption method and relations derived from helium and mercury densities. The total pore volume was measured in the diameter range of 0.2 nm–2.98 μm, which was subdivided into two groups, namely the micropore region (< 0.0036 μm) and the combined meso- and macropore region (0.0036–2.98 μm). It has been determined that the porosity of the eleven coals studied varies from 2 to 39%. It has been found that the total pore volume, micropore volume, surface area and the apparent compressibility of these coals decrease with increase in the carbon content, or the rank of the coals. The effect of the total pore volume, micropore volume and surface area on chemical reactivity of the coal is discussed separately. A good correlation was obtained between the carbon content and helium density of the coal after correction is made for the mineral content.     

常见层状硅酸盐矿物的表面特征

用N2吸附法系统测试了高岭石、蒙脱石、叶蜡石、滑石、凹凸棒石、海泡石、铁云母、绢云母、蛙石、钾长石等10种常见工业矿物材料的比表面积、孔结构以及表面能特征。初步讨论矿物表面特征、微孔结构与矿物晶体结构、化学成分以及加工工艺、性能之间的联系。研究结果表明：矿物的表面特征是由其矿物结构决定的，矿物成分的微小差异可能导致矿物表面特征的较大差别。矿物表面特征测试的结果可以为粘土类工业矿物的环境应用和材料开发提供基础数据。     

盐酸酸溶对蒙脱石结构的影响

通过X射线衍射分析、Brunauer-Eammett-Teller法、透射电镜分析研究了盐酸酸溶对蒙脱石结构的影响.结果表明:随着蒙脱石酸溶程度增加,层间可交换阳离子与八面体中Al3 逐渐溶出,蒙脱石晶体逐渐被破坏,四面体片残留逐渐转变为无定型SiO2.酸溶产物比表面积先增加到144m2/g,然后减小,酸溶过程中没有大孔产生.H 置换晶层间阳离子和八面体中部分Al3 ,使微孔表面积增大,酸溶程度过高则使晶体骨架崩塌,微孔大幅度减少.中孔是蒙脱石颗粒堆垛形成,蒙脱石酸溶在某种程度上强化了蒙脱石晶片的剥片作用,增加了中孔表面积.     

水蒸气活化法制备椰壳活性炭的孔结构特征

以农林废弃物椰壳在600℃炭化2h后的炭化料为原料,以水蒸气为活化剂,研究了活化温度、活化时间、水蒸气用量对活性炭的比表面积、微孔容积和收率等的影响。结果表明：椰壳炭化料的比表面积仅为185m^2／g,且以中孔为主。在活化过程中,通过提高活化温度和水蒸气用量缩短了活化时间,扩宽了孔径;当水蒸气用量和活化温度较为适宜时,延长活化时间,有利于微孔的形成。活性炭的比表面积、总孔容积、微孔容积可达：1465m^2／g,0．9703cm^3/g,0．7519cm^2／g。并通过非定域密度函数理论（NLDFT）对活性炭的孔径分布进行了表征。     

非均匀多孔介质表面变压过程的分形特征

多孔物质的表面结构可以用分形维数D来表征,D与微孔结构存在对应的关系,不同的表面分形维数对应不同的微孔结构.详细讨论了分形维数和微孔结构随压力的变化规律.随着压力的增大,多孔介质表面形成更小的吸附微孔,孔径分布广,表面变得更粗糙,分形维数增大,至到趋于稳定值.分形维数表达表面结构简洁明了.