Determining the Adsorption of Methane by Activated Carbon using the IMI-HTP

Adsorption by nanoporous materials has a number of applications, including gas separation, purification and storage. The suitability of a particular material for any given application can be assessed by determining its adsorption properties, such as absolute uptakes with respect to pressure and temperature, and adsorption enthalpy with respect to surface coverage. High pressure measurements are required for gas storage.

The accuracy of absolute uptake depends on the experimental data quality and the conversions of excess adsorption to absolute adsorption. Also, it is necessary to choose a suitable isotherm model to analytically represent and interpolate data for determining the isosteric enthalpy of adsorption,ΔH_iso,由Van't Hoff情节。

本文展示了对Langmuir，Freundlich，SIPS（Langmuir-Freundlich）和Toth isotherm使用AN的方程式IMI-HTP高压吸附分析仪。这些方程可用于描述通过商业活性碳的吸附甲烷的吸附，以在307 K和329 K之间的压力大约6.5 MPa。_iso就表面覆盖范围或甲烷吸收而言，也使用从SIPS和TOTH表达式获得的数据进行。

等温方程

Langmuir方程式给出了以下方式，给出了：

(1)

wheren_a是吸附的金额，并且n_m是单层容量，θ=n_a/n_m是表面覆盖范围，P是压力，bis the adsorption coefficient, which is exponentially related to the positive value of the energy of adsorption.

Freundlich方程式如下：

(2)

whereK和n是经验参数。Freundlich方程本质上是经验的。但是，多站点的langmuir方程也可以通过假设指数吸附位点的能量分布来使用弗朗迪奇方程。

Langmuir-Freundlich等温线被给出：

(3)

wherek是相同的b在Langmuir方程式中，nis a measure of the heterogeneity of the adsorbent. Bradley initially proposed this equation as an empirical relation. However, Sips later derived this equation by assuming Langmuir isotherm behaviour and a near-Gaussian distribution of adsorption site energies.

tóth方程使用另一个指数，t，代表异质性，由以下方式给出：

(4)

whereb和t特定于特定的吸附物吸附剂系统。

实验程序

使用全自动进行测量IMI-HTP测压分析仪，如图1所示。可以使用仪器确定最多20 MPa压力的材料的气体吸附性能。欧洲杯足球竞彩1.755克Filtrasorb F-400was used to determine the methane adsorption isotherms with ±0.1 K stability at temperatures of 307 K, 312 K, 316 K, 320 K, 325 K and 329 K. The calibrated dosing manifold and the sample cell have their respective volumes of 6.32 cm³和16.47厘米³, determined by pycnometry, in which helium is used as the calibration fluid.

Figure 1.IMI-HTP高压吸附分析仪器

The manifold temperature was maintained constant at 308 K while stability was maintained at ±0.1 K. A 10.0 MPa range transducer, with an accuracy of ±0.04% of full scale, was used to measure the pressure. Methane compressibility was determined using a linear interpolation between tabulated compressibility values produced using the NIST REFPROP database. The NIST database uses the Setzmann and Wagner equation for methane.

结果和讨论

图2显示了根据使用IMI-HTP记录的过量摄取等温线计算出的绝对摄取等温线。使用方程式（1）至（4）拟合它们，并根据使用最佳拟合参数构建的van't Hoff图计算了吸附的同性焓。图3显示了在316 K下测得的绝对吸附的比较数据拟合。SIPS方程最适合所有过量吸附等温线。SIP和Tóth方程提供的拟合度明显优于Langmuir和Freundlich方程，以实现过量和绝对吸附。SIPS和Tóth方程适用于在测得的温度范围内提供绝对吸附数据，如图4所示。较低图中的残差表明Tóth方程是在SIPS方程上更好的拟合度。

The Sips and Tóth equation fits for the excess adsorption data over the measured temperature range is shown in the Figure 5.

图5的较低图中的残差表明，SIPS方程比Tóth方程更好。但是，低表面覆盖范围的残差大小看起来大致相等。

图6显示ΔH_isoas a function of coverage, calculated using the parameters from the Tóth and Sips model and isotherm data recorded at different temperatures. TheΔH_iso在这项研究中计算的差异超过大约23至25 kjmol^-1在其他地方报道了甲烷吸附在活化碳中的甲烷，尽管ΔH_iso已知会随温度和表面覆盖范围的变化而变化。从图6的不同趋势中可以明显看出，未达到计算出的焓与等温温度或特定表面覆盖率之间的明显相关性。这表明了不同的值的不同值ΔH_iso可以使用相同的实验数据根据选择插值数据的模型来计算。

Figure 2.Absolute methane adsorption by 1.755 g activated carbon (Filtrasorb F-400) at temperatures between 307 K and 329 K.

Figure 3.Langmuir（蓝色），Freundlich（红色），SIPS（灰色）和Tóth（绿色）拟合，顶部和残留物，底部，用于316 K.的绝对吸收数据。

图4。SIPS（蓝色）和Tóth（红色）拟合，顶部和残留物，底部，以在307 K和329 K之间的温度下绝对吸收等温线

图5。sips（蓝色）和tóth（红色）拟合，顶部和残留物，底部，在307 K和329 K之间的温度下过量吸收等温线。

图6。甲烷在F-400碳上的吸附等于焓，它是根据使用SIPS和Tóth方程拟合的各种等温线计算得出的。

结论

The Toth equation was found to be the best fit for all absolute adsorption isotherm data. However, the excess adsorption data is best modeled using the Sips equation. Also, calculation of a wide range ofΔH_isovalues using the same experimental data but different analysis methods indicates thatΔH_isovalues calculated from one method could result in a large potential error. The significant effect of the isotherm model and the selected data temperature range on the calculated values of ΔH_iso指示在报告吸附等等距焓时清楚地说明该方法的重要性。

该文章还证明了使用IMI-HTP分析仪中的活化碳成功确定甲烷的高压吸附。

This information has been sourced, reviewed and adapted from materials provided by Hiden Isochema.

有关此消息来源的更多信息，请访问Hiden Isochema。