The theoretical maximum isosteric heat of adsorption in the Henry’s law region for slit-shaped carbon nanopores

The isosteric heat of adsorption in the Henry’s law region is calculated as a function of the width of slit-shaped pores. We determine the pore width where the isosteric heat is a maximum, which is shown to be a strong function of the solid-fluid collision diameter σ_sf and a weak function of the solid-fluid well depth potential ?_sf. Thus, general results are reported for the pore size where the isosteric heat of adsorption is a maximum that apply to a wide variety of gases. We compare our values of isosteric heat with those in the Henry’s law region determined from adsorption data for nitrogen, argon, carbon dioxide, and methane on various activated carbons. The isosteric heats of adsorption for helium and hydrogen in carbon slit pores are also calculated, but are not compared with experimental data. Reasons for differences between the theoretical maximum and the experimental values are discussed.     

吸附热预测吸附等温线

实验测定了N2 在沸石分子筛、C2 H6 在活性炭、CO2 在硅胶上的吸附等温线 ,研究用Clausius Clapeyron方程求得等量吸附热、再利用所得的吸附热预测其它温度的吸附等温线数据的方法。将吸附热预测的等温线与实验值及插值法内插得到的吸附等温线数据进行了比较 ,结果表明吸附热预测值与实验值吻合较好。此外还对文献数据利用等量吸附热预测较高压力 ( 65 0kPa)下的等温线 ,均与文献中的实验值一致。为吸附工业操作需要不同温度下的等温线数据和吸附过程的模拟与设计提供了简便、准确的计算方法     

Isosteric heats of adsorption in the Henry’s law region for carbon single wall cylindrical nanopores and spherical nanocavities

The isosteric heat of adsorption in the Henry’s law region is calculated as a function of the pore width for carbon single wall cylindrical nanopores and spherical nanocavities. The maximum isosteric heat of adsorption is obtained for six gas molecules: argon, methane, carbon dioxide, hydrogen, helium, and nitrogen. In addition, the results for cylindrical carbon nanopores are compared with adsorption data on single-wall carbon nanotubes from the literature. We find the pore width where the isosteric heat of adsorption is a maximum for both geometries. The effect of solid–fluid parameters on the pore diameter for the maximum isosteric heat of adsorption is determined for any system described by a Lennard–Jones potential. Constant relationships between the pore diameters for the maximum isosteric heat of adsorption and the specific solid–fluid parameters are found for cylindrical nanopores, spherical nanocavities, and parallel-wall slit-shaped pores. Surface mean curvature has a significant influence on the isosteric heat of adsorption.     

Experimental study and artificial neural network simulation of methane adsorption on activated carbon

The adsorption of methane on two activated carbons with different physical properties was measured. Adsorption isotherms were obtained by static volumetric method at different temperatures and pressures. The experimental results sow the best gas storage capacity was 113.5 V/V at temperature 280 K and pressure 8.5MPa on an activated carbon with surface area 1,060 m²/gr. An artificial neural network (ANN) based on genetic algorithm (GA) was used to predict amount of adsorption. The experimental data including input pressure, temperature and surface area of adsorbents as input parameters were used to create a GA-ANN simulation. The simulation results were compared with the experimental data and a good agreement was observed. The simulation was applied to calculate isosteric heat of adsorption by using the Clausius-Clapeyron equation. Comparison of the calculated adsorption heat showed different surface heterogeneity of the adsorbents.     

氢在多壁碳纳米管上的等量吸附热

为说明氢在多壁碳纳米管(MWCNTs)上所受吸附作用的强弱,文中基于氢和77 K氮的吸附数据,比较了由非局域密度泛函理论(NDFT)、等量吸附线和归一化等温线线性化确定的等量吸附热。结果表明,氢在MWCNTs上等量吸附热随温度变化且表现出在弱的能量不均匀表面吸附的特点,平均值约为3—4 kJ/mol;由77 K氮吸附等温线确定的孔大小分布(PSD)和比表面积影响NDFT计算初始吸附时的精度;在吸附量和吸附温度都较低时,等量吸附线标绘结果与归一化等温线线性化方程确定的等量吸附热之间的偏差较小,在200—290 K温度区间则与ND-FT计算结果更为接近。然而,当温度大于310 K时,3种方法计算结果间的偏差较大。     

Molecular modeling and adsorption properties of porous carbons

In this work, we calculate the adsorption isotherms and isosteric heat of argon in molecular models of saccharose coke obtained via the Hybrid Reverse Monte Carlo method. In the first route (method A), the molecular models were built by considering only carbon atoms, and all other heteroatoms present were neglected. In the second route (method B), the molecular models were built by considering carbon and hydrogen atoms. We find that the models obtained via method B have smaller pores as compared to the models obtained via method A. This is reflected in the adsorption properties. The amount adsorbed is less in models obtained via method B as compared to method A. We also find that the isosteric heat calculated in the models obtained via method B match the experimental data more closely as compared to models obtained from method A.     

Experimental and theoretical study of methane adsorption on granular activated carbons

The experimental and theoretical study of methane adsorption on granular activated carbons is presented. The adsorption data are modeled by various isotherm equations. Toth equation is found to have the best fit. The isosteric heat decreases with loading and increases weakly with temperature, which is an indication of heterogeneity of the methane and granular activated carbon system. Using optimized parameters from Toth equation, a novel procedure is developed to calculate the integral heat of adsorption, which is the total amount of isosteric heat of adsorption at a given temperature and pressure during the adsorption process. © 2011 American Institute of Chemical Engineers AIChE J, 2012     

Charged soc metal-organic framework for high-efficacy H2 adsorption and syngas purification: Atomistic simulation study

H₂ adsorption and syngas purification in charged soc metal-organic framework are investigated using atomistic simulations. As experimentally observed, the extraframework NO₃⁻ ions are entrapped in carcerand-like capsule with negligible mobility. At low pressure, H₂ adsorption occurs concurrently at multiple sites near the exposed indium atoms and organic components. The capsule is accessible at high pressure through the surrounding channels by restricted windows. Adsorption sites identified are remarkably consistent with inelastic neutron scattering measurements. The isotherm and isosteric heat of H₂ adsorption predicted match well with experimental data. As loading rises, the isosteric heat remains nearly constant, revealing the homogeneity of adsorption sites. CO₂/H₂ selectivity in syngas adsorption is up to 600 and substantially higher than other nanoporous materials. With a trace of H₂O, the selectivity increases slightly at low pressure due to promoted adsorption of CO₂ by H₂O bound proximally to the exposed indium atoms, but decreases at high pressure as a consequence of competitive adsorption of H₂O over CO₂. © 2009 American Institute of Chemical Engineers AIChE J, 2009     

Effect of heat transfer on the transient dynamics of temperature swing adsorption process

The effect of radial heat transfer on temperature swing adsorption (TSA) was studied by using an air-drying TSA experiment. The experimental dynamics of water adsorption and thermal regeneration in a fixed bed packed with zeolite 13X were used to evaluate the predicted results from the developed models. One-and two-dimensional models for energy balance with various equations describing internal velocity were compared in terms of the prediction of transient dynamics of TSA. Since the heat effect in adsorption step depended on the isosteric heat of adsorption, a dynamic simulation was performed under adiabatic, near-adiabatic, and constant wall temperature conditions. A comparison between one-and two-dimensional models was also made under near-adiabatic condition, which reflected on the experimental condition. There was little difference between adsorption breakthrough curves predicted by the one- and two-dimensional models because the radial distribution of temperature was negligible at the adsorption step. In the case of the regeneration step, a small difference between two models was expected just at the early period of time because the radial effect disappeared with time. One-dimensional model could provide an adequate prediction of the transient dynamics in this system when the wall energy balance was included.     

CO2 adsorption on carbonaceous surfaces: a combined experimental and theoretical study

We present an experimental and theoretical study to provide further insight into the mechanism of CO₂ chemisorption on carbonaceous surfaces. The differential heat of CO₂ adsorption at low and high coverages was determined in the temperature range 553-593 K. We found that the heat profile has two distinct energetic zones that suggest two different adsorption processes. In the low-coverage region, the heat of adsorption decreases rapidly from 75 to 24 kcal/mol, suggesting a broad spectrum of binding sites. In the high-coverage region, the heat becomes nearly independent of the loading, from 9 to 5 kcal/mol. A systematic molecular modeling study of CO₂ chemisorption on carbonaceous surfaces was performed. Several of the carbon-oxygen complexes that have been proposed in the literature were identified and characterized. The calculated adsorption energies are within the experimental uncertainty of the heat of adsorption at low coverage. Pre-adsorbed oxygen groups decrease the exothermicity of CO₂ adsorption. In the high-coverage region, our theoretical results suggest that CO₂ molecules are likely to adsorb on surface oxygen complexes and on graphene planes.     

Preparation of powdered activated carbon from rice husk and its methane adsorption properties

Success of adsorbed natural gas (ANG) storage process is mainly based on the characteristics of the adsorbent, so various synthesized adsorbents were analyzed for methane adsorption on a thermodynamic basis. Activated carbon from rice husk (AC-RH) was synthesized and its methane adsorption capacities were compared with phenol based activated carbons (AC-PH₂O and AC-PKOH). The adsorption experiments were conducted by volumetric method under various constant temperatures (293.15, 303.15, 313.15 and 323.15 K) and pressure up to 3.5MPa. Maximum methane adsorption was observed in AC-RH as its surface area is higher than the other two adsorbents. The experimental data were correlated well with Langmuir-Fruendlich isotherms. In addition, isosteric heat of adsorption was calculated by using Clausius-Clapeyron equation.     

Grand canonical Monte Carlo simulation of adsorption of nitrogen and oxygen in realistic nanoporous carbon models

Adsorption of nitrogen and oxygen in nanoporous carbons (NPC) is simulated using grand canonical Monte Carlo simulations, where the Steele potential (developed for gas interactions with graphite) is used to represent gas–carbon interactions. NPC models used for the adsorption simulations are developed using an isothermal‐isobaric (constant NpT) ensemble Monte Carlo algorithm whereby an initial polymer chain is evolved through a series of atomic displacement and bond rearrangement steps into the final carbon structure. These constant NpT carbon models are representative of real NPCs in terms of local structure and chemical composition. Predictions of nitrogen and oxygen sorption from our model NPCs show good agreement with experimental data. The isosteric heats of adsorption of both adsorbates lie within the range of experimental values for NPCs. Furthermore, the adsorption isotherms of the two gases showsemi‐quantitative match with experimental adsorption isotherms. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

温度-压力-吸附方程在计算煤岩等量吸附焓的应用

用鄂尔多斯盆地的长焰煤、肥煤、瘦煤和贫煤4个煤样的系列等温吸附实验数据来验证一个温度-压力-吸附方程,并探讨等量吸附时吸附平衡压力与温度之间的函数关系。用克劳修斯-克拉佩龙方程求等量吸附焓。提出单位等量吸附焓的概念及计算方法。结果表明：煤样等量吸附焓为负值,证实吸附过程是个放热过程;对于同一煤样,单位等量吸附焓随吸附量的增加而下降是由于吸附介质表面的能量不均匀性所造成。正因为吸附过程是个放热过程,所以吸附先发生在能量较高的位置上,以便放出更多能量;在相等吸附温度和压力条件下,高阶煤有较大的单位等量吸附焓,必有较大的吸附量。同时高阶煤吸附量随着吸附温度的升高而快速衰减,而中低阶煤吸附量衰减却没有那么明显。     

Effect of osmosis and ultrasound pretreatment on the moisture adsorption isotherms of quince

The effect of pretreatment osmotic–ultrasonic dehydration on the sorption isotherms of quince was determined by static gravimetric method at temperatures of 30 °C, 45 °C, and 60 °C. The curves obtained can be considered as type II according to the Brunauer–Emmett–Teller (BET) classification. Adsorption data were fitted into seven isotherm models. The best fit of the experimental data was obtained with Peleg for both fresh- and pretreated dried quince slices.Thermodynamic properties such as net isosteric heat, differential entropy, enthalpy–entropy compensation, and spreading pressure were determined from moisture adsorption isotherm data of quince. The net isosteric heat of sorption and differential entropy decreased with increasing moisture contents in an exponential function. A plot of differential heat versus entropy satisfied the enthalpy–entropy compensation theory. The spreading pressure increased with increasing water activity, and decreased with increasing temperature. The value of net isosteric heat, differential entropy and spreading pressure of untreated samples is higher than that of pretreated samples of quince.     

Effect of organic groups on hydrogen adsorption properties of periodic mesoporous organosilicas

We investigate the hydrogen adsorption properties of periodic mesoporous organosilicas (PMOs) and focus, in particular, on how these properties are affected by diverse organic groups embedded in the walls. PMOs with π electrons on the pore surface adsorb more hydrogen molecules per unit area and have a higher isosteric heat of hydrogen adsorption (Q_st). The number of adsorbed hydrogen molecules per unit area correlates well with the density of organic groups on the pore surface. We attribute the high Q_st to the high polarizability of organic groups with π electrons, which enhances the dispersion force. The molecular order of organic groups affects the adsorption-site affinity to hydrogen molecules as well as the location of adsorption sites. For phenylene-bridged PMOs with crystal-like pore walls, Q_st decreases rapidly with increasing hydrogen loading, which indicates two types of adsorption sites with different affinities to hydrogen molecules: one is an exposed CH bond and the other is a siloxane bond. However, Q_st for phenylene-bridged PMOs with amorphous pore walls exhibits a moderate slope, which might be caused by the random order of organic groups; this results in several types of adsorption sites with various affinities.     

Hydrogen adsorption studies on single wall carbon nanotubes

Hydrogen adsorption data on as-grown and heat-treated single walled carbon nanotubes (SWNTs) obtained by a volumetric procedure using a Quantachrome Autosorb-1 equipment are presented. The amounts of hydrogen adsorbed at atmospheric pressure reach approximately 0.01 wt.% at 298 K and 1 wt.% at 77 K. The isosteric heat of adsorption has been calculated for both samples from H₂ equilibrium adsorption data at three temperatures, having initial values of 7.42 and 7.75 kJ mol⁻¹. Studies in porous structure by N₂ adsorption and density measurements in helium pycnometer are reported.     

Characterization of single-walled carbon nanohorns using neon adsorption isotherms

Detailed volumetric neon adsorption isotherms were measured between 22.670 and 31.444 K on a sample of closed, mostly bud-like, aggregates of single-walled, carbon nanohorns, produced by NanoCraft, Inc. The results are compared to those recently reported on closed dahlia-like nanohorn aggregates. While for the closed dahlia-like aggregates the data shows that there are two groups of sites with different binding energies present at relatively low loadings, only one group of adsorption energies is present for the closed bud-like aggregates at comparable loadings. We measured the isosteric heat of adsorption and its dependence on substrate loading. From the low-loading limit of this quantity, we determined that the binding energy of the sites present on the bud-like aggregates have a value intermediate between that obtained for adsorption on planar graphite and that measured on the grooves of close-ended bundles on nanotubes for the same adsorbate.     

Adsorptive separation of ethylene/ethane mixtures with CuCl@HY adsorbent: equilibrium and reversibility

In this work, we investigate the potential of CuCl-functionalized HY zeolite (CuCl@HY) as an effective adsorbent for the ethylene/ethane separation. The CuCl@HY adsorbents were prepared with CuCl₂ as precursor by a solid-state dispersion method, followed by the activation with CO. The CuCl@HY adsorbents with different CuCl loadings were investigated for ethylene and ethane adsorptions, and evaluated the reversibility for multiple ethylene adsorption/desorption cycles. The experimental results reveal that the optimal adsorbent with copper loadings of 5 mmol/g HY zeolite displays high ethylene adsorption capacity, high C₂H₄/C₂H₆ adsorption selectivity and good reversibility. In addition, the adsorption equilibrium isotherms of ethylene and ethane on CuCl@HY at temperatures up to 333 K can be well correlated by the Sips models, and the corresponding isosteric heats of adsorption are calculated using the Clausius–Clapeyron equation. The value of isosteric heat of adsorption suggests that the interaction of CuCl@HY with ethylene molecules is between physisorption and chemisorption.     

In-situ temperature-depth profile evaluation of South African unmineable coal seams to determine CO2 sequestration potential

This study aimed to investigate the sorption behaviour of South African coal seams with relation to the effect of temperature during CO₂ sequestration. The excess adsorption isotherms of CO₂ adsorption were undertaken using a high-pressure volumetric system for four coals of different coal rank (denoted by Somkele [SK], anthracite KZN [AN], Tshikondeni [TD], and Syferfontein [SF]). The volumetric pressure step method was conducted at increments of system temperature of 35, 45, 55, and 65°C for pure CO₂ adsorption at incremental pressures up to 93 bar. The results showed that high temperatures have a very significant negative effect on the amount of CO₂ adsorbed on the coal samples. The high-rank coal samples (SK and AN) demonstrated elevated CO₂ adsorption capacity across all tested temperatures due to their high vitrinite content. The medium-rank coals (TD and SF) exhibited comparatively lower CO₂ adsorption capacity, attributed to the presence of adsorption hindrances such as higher ash content and volatile and mineral matter. The isosteric heat of adsorption revealed an increasing trend with coverage for all coal samples, with higher rank coals displaying greater slopes. The determined range of the isosteric heat of adsorption, spanning from 10 to 59 kJ/mol, indicated that the adsorption process is primarily of a physisorption nature. Three theoretical models (Langmuir, Freundlich, and Temkin) were evaluated and fitted to the sorption experimental data. The Temkin model exhibited superior fitting compared to the Langmuir and Freundlich isotherms. The Temkin isotherm parameters suggest that the adsorption of CO₂ onto coal is a physisorption process.     

Characterization of Cabot non-graphitized carbon blacks with a defective surface model: Adsorption of argon and nitrogen

A grand canonical Monte Carlo simulation (GCMC) is used to study the adsorption of argon and nitrogen on non-graphitized carbon black. The surface is assumed to be finite in length and composed of three graphene layers, the top layer of which contains defects. The isotherm obtained for the non-graphitized carbon shows a smooth S-shaped type while that obtained for the perfect graphene layer shows a wavy type. The isosteric heat is also affected by the defect; its behaviour versus loading exhibits a decrease at the beginning and then slightly increases once the first layer has been formed. The decreasing behaviour of isosteric heat at low loadings is not observed in the case of graphitized carbon black. The simulated results are compared against the experimental data of argon and nitrogen at 77 and 87.3 K on the Cabot carbon black BP 280, 460 and 2000. It is found that the defected finite surface describes well the data of these blacks. For the case of BP 2000 we have found that besides the defects of the surface, this sample contains a small population of small micropores having a width of 8.2 Å and its specific pore volume of 0.08 cm³/g.