This application note details the method for obtaining the isosteric heat of adsorption of CO2 on a range of adsorbent-grade activated carbons. CO2 adsorption isotherms below saturation vapor pressure were collected using a Micromeritics® 3Flex Surface Analyzer equipped with an iso-controller unit for temperature control. Isosteric heat of adsorption is calculated across the fullest range of surface coverage possible (zero to saturation) using the Micromeritics MicroActive software.
This application note details the method for obtaining the isosteric heat of adsorption of CO2 on a range of adsorbent grade activated carbons. CO2 adsorption isotherms below saturation vapor pressure were collected using a Micromeritics® 3Flex Surface Analyzer equipped with an iso-controller unit for temperature control. Isosteric heat of adsorption is calculated across the fullest range of surface coverage possible (zero to saturation) using the Micromeritics MicroActive software.
With current growing concerns over climate change and the importance of environmental protection, the capture, retention, and sequestration of CO2 is receiving huge interest in literature, research, and public policy. The activation of carbon to form porous materials of high surface area and pore volume, including micro-pore content (pores < 2 nm width) provides materials capable of being used for the containment of high volumes of CO2 per unit mass, thus reducing release into the atmosphere. Furthermore, source materials for carbonization and activation are abundant and represent a relatively inexpensive approach to material development and manufacture.
Carbons represent an incredibly diverse family of materials. Average pore size, size distribution, volume and area are heavily dependent on the source of the carbonaceous material and the precise method of activation. The number of potential sources of carbonaceous material is huge and includes including: wood and plant-based materials, shell, petroleum products and agricultural by-products, with new potential sources being frequently reported. This wide range of source materials, combined with numerous approaches and conditions of activation, give rise to the incredible diversity of porous characteristics of activated carbons. Not all carbons will be suitable.
This application note details the method for obtaining the isosteric heat of adsorption of CO2 on a range of adsorbent-grade activated carbons. CO2 adsorption isotherms below saturation vapor pressure were collected using a Micromeritics® 3Flex Surface Analyzer equipped with an iso-controller unit for temperature control. Isosteric heat of adsorption is calculated across the fullest range of surface coverage possible (zero to saturation) using the Micromeritics MicroActive software. With current growing concerns over climate change and the importance of environmental protection, the capture, retention, and sequestration of CO2 is receiving huge interest in literature, research, and public policy. The activation of carbon to form porous materials of high surface area and pore volume, including micro-pore content (pores < 2 nm width) provides materials capable of being used for the containment of high volumes of CO2 per unit mass, thus reducing release into the atmosphere. Furthermore, source materials for carbonization and activation are abundant and represent a relatively inexpensive approach to material development and manufacture. Carbons represent an incredibly diverse family of materials. Average pore size, size distribution, volume and area are heavily dependent on the source of the carbonaceous material and the precise method of activation. The number of potential sources of carbonaceous material is huge and includes: wood and plant-based materials, shell, petroleum products and agricultural by-products, with new potential sources being frequently reported. This wide range of source materials, combined with numerous approaches and conditions of activation, gives rise to the incredible diversity of porous characteristics of activated carbons. Not all carbons will be suitable or effective for a particular use and it is therefore essential that the desired characteristics are determined for a given application.
Gas adsorption isotherms are widely used when characterizing porous materials. The Micromeritics 3Flex Surface Analyzer and Micromeritics ASAP® Surface Area and Porosimetry Analyzer are ideal for determining pore size, volume and area in sizes extending from micro-pores, through meso-pores and into the macro-pore range together with the measurement of surface area. Indeed, BET surface area, total pore volume and average pore size are routinely reported in literature and product specifications.
Full characterization of the porous nature of candidate materials for CO2 capture and sequestration is essential to their development and selection. Additionally, it is also essential that the affinity of the materials specifically for CO2 adsorption is understood, as this ultimately dictates the ability to capture and, as importantly, retain CO2. Notably, this can be determined through measurement of the heat (enthalpy) of CO2 adsorption. The calculation of the isosteric heat of adsorption is typically made through application of the Clausius-Clapeyron equation to gas adsorption isotherms collected at different temperatures. A basic calculation can be made from just two isotherms collected at different temperatures, from which a single common adsorption quantity can be selected. This method will provide a single value for the heat of adsorption, usually at very low surface coverage.
Greater accuracy and considerably more information can be obtained through the collection of isotherms at more than two different temperatures and by applying a range of adsorption quantities which is common to all of the collected isotherms. Such isotherm analyses are possible using Micromeritics gas adsorption instruments and the isosteric heat of adsorption can be calculated and presented using the MicroActive software. Essentially, heat of adsorption is determined isosterically from plots of In(P) against 1/T at constant coverage (adsorption quantity). The slope of each isostere may then be used to calculate the heat of adsorption at that particular adsorption quantity. For illustration, Figure 1 shows a simplified isostere plot generated from isotherms collected at three different temperatures (shown by the three sets of vertical red circles) and generated at ten adsorption volumes (quantities). This arrangement would, therefore, provide ten heat of adsorption values, one for each adsorption quantity.
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