Differential Heats Of Adsorption Research Articles

Accurate Measurements of NH3 Differential Adsorption Heat Unveil Structural Sensitivity of Brønsted Acid and Brønsted/Lewis Acid Synergy in Zeolites.

Differential adsorption heats of NH3 on a series of zeolites, including MOR, MFI, FER, and BEA, are accurately measured to probe their acidity using flow-pulse adsorption microcalorimetry. Initial adsorption heats of NH3 at Brønsted acid sites (BAS) vary between 105 to 136 kJ/mol, depending on framework aluminum amounts and topography structures of zeolites. A Brønsted/Lewis acid synergy between BAS and proximate tricoordinated framework-associated aluminum species is identified to generate super acid sites with initial adsorption heats of NH3 around 150 kJ/mol, but occurs only in the MFI zeolites and sensitively depends on the Si/Al ratio. These accurate data of NH3 differential adsorption heats unveil structural sensitivity of BAS and Brønsted/Lewis acid synergy in zeolites and provide experimental benchmark data for fundamental understanding of acidity and acid-catalysis of zeolites.

Experimental Study and Thermodynamic Analysis of Carbon Dioxide Adsorption onto Activated Carbons Prepared from Biowaste Raw Materials

Nutshells are regarded as cost-effective and abundant raw materials for producing activated carbons (ACs) for CO2 capture, storage, and utilization. The effects of carbonization temperature and thermochemical KOH activation conditions on the porous structure as a BET surface, micropore volume, micropore width, and pore size distribution of ACs prepared from walnut (WNS) and hazelnut (HNS) shells were investigated. As a result, one-step carbonization at 900/800 °C and thermochemical KOH activation with a char/KOH mass ratio of 1:2/1:3 were found to be optimal for preparing ACs from WNS/HNS: WNS-AC-3 and HNS-AC-2, respectively. The textural properties of the WNS/HNS chars and ACs were characterized by low-temperature nitrogen vapor adsorption, XRD, and SEM methods. Dubinin’s theory of volume filling of micropores was used to evaluate the microporosity parameters and to calculate the CO2 adsorption equilibrium over the sub- and supercritical temperatures from 216.4 to 393 K at a pressure up to 10 MPa. The CO2 capture capacities of WNS- and HNS-derived adsorbents reached 5.9/4.1 and 5.4/3.9 mmol/g at 273/293 K under 0.1 MPa pressure, respectively. A discrepancy between the total and delivery volumetric adsorption capacities of the adsorbents was attributed to the strong binding of CO2 molecules with the adsorption sites, which were mainly narrow micropores with a high adsorption potential. The high initial differential heats of CO2 adsorption onto ACs of ~32 kJ/mol confirmed this proposal. The behaviors of thermodynamic functions (enthalpy and entropy) of the adsorption systems were attributed to changes in the state of adsorbed CO2 molecules determined by a balance between attractive and repulsive CO2–CO2 and CO2–AC interactions during the adsorption process. Thus, the chosen route for preparing ACs from the nutshells made it possible to prepare efficient carbon adsorbents with a relatively high CO2 adsorption performance due to a substantial volume of micropores with a size in the range of 0.6–0.7 nm.

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

Adsorption-based carbon dioxide capture, utilization, and storage technologies aim to mitigate the accumulation of anthropogenic greenhouse gases that cause climate change. It is assumed that porous carbons as adsorbents are able to demonstrate the effectiveness of these technologies over a wide range of temperatures and pressures. The present study aimed to investigate the temperature-induced changes in the dimensions of the microporous carbon adsorbent Sorbonorit 4, as well as the carbon dioxide adsorption, by using in situ dilatometry. The nonmonotonic changes in the dimensions of Sorbonorit 4 under vacuum were found with increasing temperature from 213 to 573 K. At T > 300 K, the thermal linear expansion coefficient of Sorbonorit 4 exceeded that of a graphite crystal, reaching 5 × 10−5 K at 573 K. The CO2 adsorption onto Sorbonorit 4 gave rise to its contraction at low temperatures and pressures or to its expansion at high temperatures over the entire pressure range. An inversion of the temperature dependence of the adsorption-induced deformation (AID) of Sorbonorit-4 was observed. The AID of Sorbonorit-4 and differential isosteric heat of CO2 adsorption plotted as a function of carbon dioxide uptake varied within the same intervals of adsorption values, reflecting the changes in the state of adsorbed molecules caused by contributions from adsorbate–adsorbent and adsorbate–adsorbate interactions. A simple model of nanoporous carbon adsorbents as randomly oriented nanocrystallites interconnected by a disordered carbon phase is proposed to represent the adsorption- and temperature-induced deformation of nanocrystallites with the macroscopic deformation of the adsorbent granules.

Influence of Nanoporous Carbon Structures on High-Pressure Adsorption Separation of Light Hydrocarbons

Four activated carbons (ACs) prepared from polymers (PMB, P-300), wood waste (S-1), and silicon carbide (AUK) by chemical and physical activation were investigated as potential adsorbents for ethane/methane mixture separation. Their textural properties were compared by performing scanning electron microscopy, X-ray, and CHNS/O elemental analysis. N2 and CO2 adsorption interpreted in terms of the theory of volume filling of micropores, non-local density functional theory (NLDFT), and Brunauer–Emmett–Teller (BET) models revealed very different porosity features of these ACs. The adsorption of pure ethane and methane onto all of the ACs was measured at pressures up to 50 bar and temperatures of 303–333 K in order to evaluate their adsorption selectivity for a methane/ethane mixture using the IAST method. The calculations indicated that ACs with the largest mean pore sizes, P-300 and PBM, are least suitable for adsorption-based separation of the ethane/methane mixture since they demonstrated a relatively strong dependence of selectivity coefficient (SF) on pressure and temperature. AUK showed the highest value of SF for ethane at low pressures (up to 5 bar), reaching 18 at 303 K for the mixture 95/5% CH4/C2H6. Finally, S-1 was found to be the most efficient adsorbent for the ethane/methane separation since it demonstrated a relatively high separation performance in a pressure range of 2–50 bar with almost no reduction in separation performance. In general, the SF value of ACs for ethane correlated with the difference between the initial differential isosteric heats of ethane and methane adsorption as well as with the presence of high-energy adsorption centers as narrow micropores with size less than 1 nm and heteroatoms (N,O). Molecular dynamics simulations of the adsorption of the methane/ethane mixture onto the model carbon adsorbents also indicated the influence of pore width on selective capacity.

Разработка методов получения углеродных адсорбентов на основе Ангренского угля и нефтяных остатков

Currently, activated carbon adsorbents are widely used in the world for industrial wastewater treatment from various organic compounds, sorption of heavy metal ions and life-threatening radioactive substances. Carbon adsorbents with hydrophobic properties are mainly modified based on the brown coal, coke, scape, and bark of plants. The methods of manufacturing coal adsorbents based on Angren coal, petroleum coke and asphaltenes are listed. The physicochemical composition and complete thermodynamic properties of the activated carbon adsorbent were studied. It was found that during the heat treatment of mixtures of carbonaceous raw materials, its mass is lost with increasing temperature. At the same time, the adsorption activity of iodine increases with increasing temperature. Depending on the change in temperature of the obtained carbon adsorbent samples, the calculated surface area with respect to iodine is 280.7 m2/g with a weight loss of 0.2 g. The differential heat of adsorption, isotherm, molar entropy and thermokinetics of the benzene and toluene molecules studied in the article are calculated. The adsorption of 4.5 mmol/g of benzene and 3.5 mmol/g of toluene on the activated carbon adsorbent was determined. It is noted that during the adsorption of benzene and toluene by the obtained adsorbent from activated carbon, the differential heat of reaction gradually decreases. When recalculating the isothermal adsorption of benzene and toluene on activated carbon according to the equation of the volumetric theory of micropore saturation, it was obtained that 84.5 % of benzene and 94.2 % of toluene were adsorbed on the pores of the adsorbent.

Size-Dependent Energy and Adhesion of Pd Nanoparticles on Graphene on Ni(111) by Pd Vapor Adsorption Calorimetry

Carbon-supported late transition-metal nanoparticles are promising catalysts and electrocatalysts for wide-ranging applications. However, experimental investigations of the bonding energetics of metal nanoparticles on carbon supports are very limited. Here, we report heats of adsorption of Pd vapor deposited onto single-layer graphene(0001) supported on Ni(111) at 100 and 300 K as Pd grows particles of well-defined size in the range from three atom clusters to 6 nm diameter. Sizes were determined from He+ low-energy ion scattering (LEIS). In this size range, the differential heat of Pd adsorption increases from 228 kJ/mol to within 10 kJ/mol of the heat of sublimation of bulk Pd (377 kJ/mol). The chemical potential of metal atoms in these nanoparticles as a function of average particle size was determined from these results. The adhesion energy at the Pd/graphene(0001)/Ni(111) interface was extracted from these data and found to be 3.5 J/m2 for the largest Pd particles. For the three metal elements that have now been studied (Pd, Ni, and Ag), we found an increase in metal/graphene(0001)/Ni(111) adhesion energy with metal carbophilicity, which we defined here as the heat of C atom adsorption on that metal’s (111) surface estimated from published density functional theory calculations.

Basic thermodynamic characteristics of toluene adsorption in Cu2+ZSM-5 Zeolite

This paper presents the results of isotherms and basic (ΔH, ΔF and ΔS) thermodynamic characteristics of toluene adsorption in CuZSM-5 zeolite. For measurements of isotherms and differential heat of adsorption, a system consisting of a universal high-vacuum adsorption unit and an attached differential modified Tian-Calve microcalorimeter DAC-1-1A was used to provide direct quantitative and qualitative characteristics of the nature and forces of adsorption interaction. The correlation between the adsorption-energy characteristics wasfound and the molecular mechanismof toluene adsorption in CuZSM-5 zeolite in the whole filling region was revealed. Toluene adsorbed in CuZSM-5 zeolite is located in the first coordination sphere with Cu2+ cation, forming two-dimensional complexes. It was found that the charge density significantly affects the mechanism, the energy of adsorption and the number of adsorbed molecules. It was determined that the average molar entropy of toluene adsorption in CuZSM5 zeolite indicates that the mobility of toluene molecules in zeolite is below the liquid phase and close to that of the solid phase, indicating that the mobility of toluene molecules on cations is strongly inhibited. This work shows how calorimetric data can be used to complement crystal structure results and detect subtle adsorbent/adsorbate interactions at the molecular level.

Calorimetric study of methanol adsorption in LiZSM-5 and CsZSM-5 zeolites

This paper presents isotherms and basic (ΔH, ΔF, and ΔS) thermodynamic characteristics of methanol adsorption in Li3,37ZSM-5 and Cs3,17ZSM-5 zeolites obtained by the calorimetric method. For measurements of isotherms and differential heat of adsorption, a system consisting of a universal high-vacuum adsorption unit and an attached differential modified Tian-Calvet type microcalorimeter, DAC-1-1A, which directly quantifies and qualifies the nature and forces of adsorption interaction, was used. The adsorption isotherm is described by the equations of the volumetric micropore filling theory (VMOT). The correlation between the adsorption-energy characteristics is found, and the molecular mechanism of methanol adsorption in Li3,37ZSM-5 and Cs3,17ZSM-5 zeolites throughout the filling region is revealed. Methanol adsorbed in Li3,37ZSM-5 and Cs3,17ZSM-5 zeolites is located in the first coordination sphere with cations Li+ and Cs+, forming tetra- and trimeric complexes. It is found that the charge density significantly affects the adsorption mechanism, the adsorption energy, and the number of adsorbed molecules. It was determined that the average molar entropy of ammonia adsorption on LiZSM-5 and CsZSM-5 zeolites indicates that the mobility of methanol molecules in the zeolite is below the liquid phase and close to the mobility of the solid phase, indicating a strong inhibition of mobility of methanol adsorbed on cations. This work shows how calorimetric data can complement crystal structure results and detect subtle adsorbent/adsorbate interactions at the molecular level.

Thermodynamics of hydrogen sulfide adsorption in NaX zeolite

Sulfur-containing organic and inorganic compounds are the main harmful component of natural gas and oil, as well as products of their processing. Since sulfur compounds have acidic properties, their content leads to corrosion of technological parts in production processes. The content of hydrogen sulfide in the transported gas should not exceed 20 mg/m3. In Uzbekistan, motor vehicles run mainly on liquefied natural gas. Sulfur compounds in natural gas corrode gas cylinders, resulting in an explosion. If synthetic zeolites are used in manufacturing processes, as well as in small gas production, process degradation will be prevented. Synthetic zeolites are widely used in various industrial production processes due to their high sorption properties. This study makes it possible to study the localization and sorption structure of sulfur compounds in supercavities of zeolites of the faujasite type. The adsorption of hydrogen sulfide on NaX zeolite was carried out in a high vacuum adsorption unit. Based on the results obtained, the differential heat of adsorption, isotherm, entropy, and thermal equilibrium time (thermokinetics) were calculated.

Mechanism of H2O Vapor Adsorption in A Type Zeolites: A Model Based on Adsorption Calorimetry

In this study, the energy characteristics of water vapor adsorption in NaA and NaA(NaBO2) zeolites were measured by adsorption calorimetry. Information of differential heats, isotherms, entropy and kinetic of adsorption in the matrix of NaA and NaA(NaBO2) zeolites were obtained. Adsorption isotherms (a) and differential heats of adsorption (Qd) of water vapor by NaA and NaA(NaBO2) zeolites were measured at 303 K. The heats of adsorption are stepwise and each step corresponds to the stoichiometric formation of adsorption complexes of H2O molecules with Na+ or H+ ions, (H2O)n/Na+ or H+, (n = 1-4), which are located on the SIII, SII and SI crystallographic positions of NaA and NaA(NaBO2). Hydration mechanism of NaA and NaA(NaBO2) are complex and at saturation, the zeolites are occupied by the next water zeolitic host/guest systems: NaA – 2.2[(H2O)4/Na+III+H+], 3[(H2O)2/Na+II] and 6.6[(H2O)2/Na+I]; NaA(NaBO2) - 2[(H2O)4/Na+III], 3[(H2O)4/Na+II] and 6.86[(H2O)/Na+I]. The complete hydration of the α- and β cages of NaA amounts thus to 21,8 and 6,2 respectively H2O/u.c. and α-cage of NaA(NaBO2) – 26,3 H2O/u.c. The mean molar integral adsorption entropy of water is ~-30,5 J/mol K less than the entropy of the bulk liquid. This value nearly the same as entropy of crystalline water so the mobility of water in the zeolitic matrix is solidlike.

Differential heats, isotherm and entropies of n-pentane adsorption on LiZSM-5 and CsZSM-5 zeolites

Artificially synthesised zeolites are widely used in water purification devices as adsorbents, ion exchangers, molecular sieves; they are used as electron donors and acceptors. Also, zeolites are currently the most important catalysts for the processing of various hydrocarbon raw materials. Synthetic zeolites ZSM-5 are highly efficient catalysts of these processes. For o characterisation of zeolite channels and estimation of sorption volume npentane are the most suitable. n-Pentane fills all sorption space and adsorbed with higher energy. This paper presents the results of the basic thermodynamic characteristics and isotherms of n-pentane adsorption in Cs3,17ZSM- 5 and Li3,37ZSM-5 zeolites at 303 K. A system consisting of a universal high-vacuum adsorption unit and a Tian-Calvet type differential modified microcalorimeter, DAC-1-1A, coupled to it was used to measure isotherms and differential heats of adsorption. The correlation between adsorption-energy characteristics was found and the molecular mechanism of n-pentane adsorption in CsZSM-5 and LiZSM-5 zeolites in the whole filling region was revealed.

Water adsorption on acetylene black carbon

Differential heat, isotherm, thermokinetics, and entropy of water adsorption on the black carbon were studied by the adsorption-calorimetric method at 303 K. The adsorption mechanism, type, and number of formed ion-molecular complexes were determined. Thermokinetics of adsorption on the black carbon and the law of migration of exchange cations were determined, and the adsorption energy of gases and vapors was calculated. The adsorption isotherm is described by the two-and three-dimensional mathematical equations of the volume-saturation theory of micro-pores (TVSM).

Адсорбционные свойства по отношению к метану функционального пористого материала на основе отходов кофе

A new microporous carbon material (MUM-51) was obtained from waste coffee grounds by carbonization and activation of KOH. MUM-51 has been characterized by various methods, including N2 adsorption-desorption at 77 K, X-ray diffraction analysis, Fourier transform IR spectroscopy, and Raman spectroscopy. Methane adsorption was studied on the resulting adsorbent at pressures up to 10 MPa and temperatures of 298.15, 303.15, 308.15, 313.15, 318.15, and 323.15 K. The specific pore volume of the adsorbent according to DFT is VDFT = 1.604 cm3/g. The BET specific surface is SBET = 3456 m2/g. The maximum value of methane adsorption at a temperature of 298,15 K and a pressure of 10 MPa reaches ~ 19 mmol/g. Experimental data on methane adsorption on MUM-51 were analyzed using the Dubinin–Radushkevich adsorption model in the temperature range 298.15 – 323.15 K and pressures up to 10 MPa. It has been established that the average relative deviations between the experimental results and the results obtained using the Dubinin-Radushkevich model are less than 3%. The initial differential molar heat of methane adsorption on the MUM-51 adsorbent is 28.7 kJ/mol. The results of the work showed that the carbon material obtained from waste coffee grounds, which has a high specific surface area and porosity, can be effectively used as an adsorbent for greenhouse gases, in particular methane.

Thermodynamics of hydrogen sulfide adsorption in Zeolite LiX

Sulfur compounds adversely affect the technological process of oil and gas refining, as well as the release of sulfur-containing compounds into the atmosphere, polluting the environment. Oil and gas processing is carried out using microporous adsorbents with high sorption capacity in the purification of gases containing sulfur. This is one of the most efficient methods in oil refining and gas processing. In research work, the adsorption of gases containing sulfur in their composition is carried out in various ways. When determining the formation complex, the main centers of Li-ionic molecules, it was possible to obtain a more accurate idea of the adsorption mechanism at the stages of absorption of hydrogen sulfide molecules into Li-zeolites using a highly sensitive adsorption calorimeter. In our study, in the process of adsorption of hydrogen sulfide on the LiX zeolite, the results of differential adsorption heat, isotherm, entropy, and thermal equilibrium time (thermokinetics) based on obtained in a high vacuum adsorption device are accurately revealed. Based on exact formulas, it was proved how many hydrogen sulfide molecules are adsorbed in the zeolite that died under vacuum conditions.

Thermodynamics of hydrogen sulfide adsorption in Zeolite Lix

Sulfur compounds adversely affect the technological process of oil and gas refining, as well as the release of sulfur-containing compounds into the atmosphere, polluting the environment. Oil and gas processing is carried out using microporous adsorbents with high sorption capacity in the purification of gases containing sulfur. This is one of the most efficient methods in oil refining and gas processing. In research work, the adsorption of gases containing sulfur in their composition is carried out in various ways. When determining the formation complex, the main centers of Li-ionic molecules, it was possible to obtain a more accurate idea of the adsorption mechanism at the stages of absorption of hydrogen sulfide molecules into Li-zeolites using a highly sensitive adsorption calorimeter. In our study, in the process of adsorption of hydrogen sulfide on the LiX zeolite, the results of differential adsorption heat, isotherm, entropy, and thermal equilibrium time (thermokinetics) based on obtained in a high vacuum adsorption device are accurately revealed. Based on exact formulas, it was proved how many hydrogen sulfide molecules are adsorbed in the zeolite that died under vacuum conditions.

NalTA energy in molecular sieve guest-guest interactions energetics of host-guest interactions in NalTA molecular electrics

The paper analyzes the isotherm and differential heats of adsorption of carbon dioxide in NaLTA and establishes the molecular host-guest mechanism of interactions in the molecular sieve matrix.

The dehydrogenation of butane on metal-free graphene

The dehydrogenation of alkane feedstock to produce alkenes is a significant and energy intensive industrial process, generally occurring on metals and metal oxides. Here, we investigate a catalytic mechanism for the dehydrogenation of butane on single-layer, metal-free graphene using a combination of ab initio quantum chemical calculations and adsorption microcalorimetry. Dispersion-corrected Density Functional Theory (DFT) is employed to calculate transition states and energy minima that describe the reaction pathways connecting butane to the two possible products, but-1-ene and but-2-ene. The deprotonations occur with moderate energy barriers in the 0.54 eV-0.69 eV range. A strong agreement is observed between the results of the adsorption energies calculated by DFT (0.40 eV) and the measured differential heat of adsorption of n-butane on a graphitic overlayer. We conclude that the active-site for this catalytic reaction is a metal-free graphene vacancy, created by removing a carbon atom from a single-layer graphene sheet.

ZrBDC-Based Functional Adsorbents for Small-Scale Methane Storage Systems

Metal-organic frameworks (MOF), potentially porous coordination structures, are envisioned for adsorption-based natural gas (ANG) storage, including mobile applications. The factors affecting the performance of the ANG system with a zirconium-based MOF with benzene dicarboxylic acid as a linker (ZrBDC) as an adsorbent were considered: textural properties of the adsorbent and thermal effect arising upon adsorption. The high-density ZrBDC-based pellets were prepared by mechanical compaction of the as-synthesized MOF powder at different pressures from 30 to 240 MPa at 298 K without a binder and mixed with polymer binders: polyvinyl alcohol (PVA) and carboxyl methylcellulose (CMC). The structural investigations revealed that the compaction of ZrBDC with PVA under 30 MPa was optimal to produce the ZrBDC-PVA adsorbent with more than a twofold increase in the packing density and the lowest degradation of the porous structure. The specific total and deliverable volumetric methane storage capacities of the ZrBDC-based adsorbents were evaluated from the experimental data on methane adsorption measured up to 10 MPa and within a temperature range from 253 to 333 K. It was measured experimentally that at 253 K, an 100 mL adsorption tank loaded with the ZrBDC-PVA pellets exhibited the deliverable methane storage capacity of 172 m3(NTP)/m3 when the pressure dropped from 10 to 0.1 MPa. The methane adsorption data for the ZrBDC powder and ZrBDC-PVA pellets were used to calculate the important thermodynamic characteristic of the ZrBDC/CH4 adsorption system—the differential molar isosteric heat of adsorption, which was used to evaluate the state thermodynamic functions: entropy, enthalpy, and heat capacity. The initial heats of methane adsorption in powdered ZrBDC evaluated from the experimental adsorption isosteres were found to be ~19.3 kJ/mol, and then these values in the ZrBDC/CH4 system decreased at different rates during adsorption. In contrast, the heat of methane adsorption onto the ZrBDC-PVA pellets increased from 19.4 kJ/mol to a maximum with a magnitude, width, and position depended on temperature, and then it fell. The behaviors of the thermodynamic state functions of the ZrBDC/CH4 adsorption system were interpreted as a variation in the state of adsorbed molecules determined by a ratio of CH4-CH4 and CH4-ZrBDC interactions. The heat of adsorption was used to calculate the temperature changes of the ANG systems loaded with ZrBDC powder and ZrBDC pellets during methane adsorption under adiabatic conditions; the maximum integrated heat of adsorption was found at 273 K. The maximum temperature changes of the ANG system with the ZrBDC materials during the adsorption (charging) process did not exceed 14 K that are much lower than those reported for the systems loaded with activated carbons. The results obtained are of direct relevance for designing the adsorption-based methane storage systems for the automotive industry, developing new gas-power robotics systems and uncrewed aerial vehicles.

Adsorption-Based Hydrogen Storage in Activated Carbons and Model Carbon Structures

The experimental data on hydrogen adsorption on five nanoporous activated carbons (ACs) of various origins measured over the temperature range of 303–363 K and pressures up to 20 MPa were compared with the predictions of hydrogen density in the slit-like pores of model carbon structures calculated by the Dubinin theory of volume filling of micropores. The highest amount of adsorbed hydrogen was found for the AC sample (ACS) prepared from a polymer mixture by KOH thermochemical activation, characterized by a biporous structure: 11.0 mmol/g at 16 MPa and 303 K. The greatest volumetric capacity over the entire range of temperature and pressure was demonstrated by the densest carbon adsorbent prepared from silicon carbide. The calculations of hydrogen density in the slit-like model pores revealed that the optimal hydrogen storage depended on the pore size, temperature, and pressure. The hydrogen adsorption capacity of the model structures exceeded the US Department of Energy (DOE) target value of 6.5 wt.% starting from 200 K and 20 MPa, whereas the most efficient carbon adsorbent ACS could achieve 7.5 wt.% only at extremely low temperatures. The initial differential molar isosteric heats of hydrogen adsorption in the studied activated carbons were in the range of 2.8–14 kJ/mol and varied during adsorption in a manner specific for each adsorbent.

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

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.