Peculiarities of Thermodynamic Behaviors of Xenon Adsorption on the Activated Carbon Prepared from Silicon Carbide

Abstract

An activated carbon prepared from silicon carbide by thermochemical synthesis and designated as SiC-AC was studied as an adsorbent for xenon. The examination of textural properties of the SiC-AC adsorbent by nitrogen vapor adsorption measurements at 77 K, powder X-ray diffraction, and scanning electron microscopy revealed a relatively homogeneous microporous structure, a low content of heteroatoms, and an absence of evident transport macropores. The study of xenon adsorption and adsorption-induced deformation of the Si-AC adsorbent over the temperature range of 178 to 393 K and pressures up to 6 MPa disclosed the contraction of the material up to −0.01%, followed by its expansion up to 0.49%. The data on temperature-induced deformation of Si-AC measured within the 260 to 575 K range was approximated by a linear function with a thermal expansion factor of (3 ± 0.15) × 10−6 K−1. These findings of the SiC-AC non-inertness taken together with the non-ideality of an equilibrium xenon gaseous phase allowed us to make accurate calculations of the differential isosteric heats of adsorption, entropy, enthalpy, and heat capacity of the Xe/SiC-AC adsorption system from the experimental adsorption data over the temperature range from 178 to 393 K and pressures up to 6 MPa. The variations in the thermodynamic state functions of the Xe/SiC-AC adsorption system with temperature and amount of adsorbed Xe were attributed to the transitions in the state of the adsorbate in the micropores of SiC-AC from the bound state near the high-energy adsorption sites to the molecular associates.

In Situ Dilatometry Measurements of Deformation of Microporous Carbon Induced by Temperature and Carbon Dioxide Adsorption under High Pressures

“…Figure 6 depicts the results of the temperature-induced strain of Sorbonorit 4 measured under vacuum; the constant error determined from the measurements with a quartz mockup was taken into consideration according to a route described in recent work [20]. One can see that, within the entire temperature range from 213 to 573 K, an increase in temperature causes a non-monotonic change in its value (up to 267-280 K), while the relative deformation magnitude decreases to 0.08-0.10%.…”

“…Figure 7 shows the results of these calculations compared with those for a silicon carbide-derived activated carbon (denoted as ASC) [20] and the data for a graphite crystal in a plane parallel and perpendicular to a main crystallographic axis [70,71]: α II , and α ⊥ , respectively. The thermal linear expansion coefficient (TLEC) of Sorbonorit 4, αT [1/K], was calculated using the experimental plot Δl/l0 = f(T):…”

“…Figure 7 shows the results of these calculations compared with those for a silicon carbide-derived activated carbon (denoted as ASC) [20] and the data for a graphite crystal in a plane parallel and perpendicular to a main crystallographic axis [70,71]: αII, and α⊥, respectively. As follows from Figure 7, when T > 300 K, α ⊥ < α ASC < α II < α Tsorbonorit .…”

“…Therefore, in making efforts to develop new highpressure adsorption-based technologies and to improve the existing ones, researchers need to consider the adsorption-induced deformation (AID) of adsorbents. This phenomena should be taken into account both when predicting the performance of the adsorbent, including its integrity [17,18], and evaluating the thermodynamic state functions of adsorption processes [19,20].…”

“…Figure 7. Temperature dependence of TLEC of Sorbonorit 4 under vacuum conditions (violet line)compared with the data for a graphite crystal in a plane parallel and perpendicular to the main crystallographic axis: α II and α ⊥ (respective black lines)[70,71]; microporous carbon adsorbent ASC (dashed black line)[20].…”