PHYSICOCHEMICAL MECHANISMS OF AMMONIA ADSORPTION ON AEROSIL

ABSTRACT

In this work, the adsorption behavior and isotherm properties of ammonia molecules on the Aerosil surface were systematically investigated. The adsorption measurements were performed using the volumetric method at a constant temperature of 303 K under equilibrium conditions. The obtained adsorption isotherms provided important information about the interaction mechanisms between ammonia molecules and the active adsorption centers located on the Aerosil surface. The analysis of the experimental data showed that the adsorption process depends significantly on pressure changes and surface heterogeneity. Furthermore, the adsorption isotherm indicated the existence of different adsorption regions corresponding to monolayer and possible multilayer adsorption at different pressure intervals. The results also made it possible to evaluate the energetic characteristics of the adsorption sites and the nature of adsorbate–adsorbent interactions. These findings contribute to a better understanding of the adsorption mechanism of polar molecules on highly dispersed silica surfaces and may be useful for the development of adsorption-based purification and separation technologies.

АННОТАЦИЯ

В данной работе исследованы адсорбционные свойства и изотермы адсорбции молекул аммиака на поверхности аэросила. Экспериментальные исследования проводились объемным методом при температуре 303 К в условиях адсорбционного равновесия. Полученные изотермы адсорбции позволили определить особенности взаимодействия молекул аммиака с активными центрами поверхности аэросила. Анализ экспериментальных данных показал, что процесс адсорбции существенно зависит от давления и энергетической неоднородности поверхности адсорбента. Кроме того, изотермы адсорбции свидетельствуют о наличии различных областей адсорбции, соответствующих формированию мономолекулярного слоя и возможной полимолекулярной адсорбции при различных диапазонах давления. Полученные результаты позволяют оценить энергетические характеристики адсорбционных центров и природу взаимодействия адсорбат–адсорбент. Результаты исследования могут быть использованы при разработке адсорбционных технологий очистки и разделения газовых смесей.

Keywords: Fumed silica, sorption, thermodynamic properties, adsorption isotherm, differential heat of adsorption.

Ключевые слова: аэросил, адсорбция, термодинамика, изотерма, дифференциальная теплота.

Introduction. Hydrated Aerosil represents a highly pure, amorphous, nonporous form of silicon dioxide composed of spherical colloidal particles and distinguished by its large specific surface area [1]. At the initial stage of drying during SiO₂ formation, the particles preserve their discrete structure; however, due to weak intermolecular forces, they subsequently combine into aggregates formed by primary monodisperse spheres [2]. The particle aggregation process is promoted by the presence of water molecules and hydrogen chloride complexes within the surface layer, which influence the surface charge as well as the pH value of the Aerosil hydrogel. During further drying, acid residues are eliminated from the particle surface, leading to the restoration of the hydration layer [3]. As a result, interparticle attraction decreases and further aggregation is inhibited [2]. Moreover, the generation of silanol (Si–OH) groups during Aerosil synthesis also hinders particle coalescence during their formation and growth.

Previous investigations have primarily enabled the determination of the chemical nature of active sites located on the surface of hydrated Aerosil. These sites include silicon atoms bonded with hydroxyl (OH) groups, distorted siloxane linkages, silanediol species, coordinatively unsaturated silicon centers, isolated hydroxyl functionalities, and different forms of physically and chemically bound water [3–6]. It is known that two categories of OH groups are present on the Aerosil surface: isolated (free) hydroxyl groups and hydrogen-bonded hydroxyl groups. The presence of hydrogen-bonded OH groups contributes to an increase in the reactivity of the free hydroxyl groups [4,7].

Isolated free silanol groups demonstrate greater thermal stability compared to hydrogen-bonded silanol groups and start to desorb from the surface at temperatures exceeding 873 K [8]. These groups generally show lower chemical reactivity; nevertheless, they preferentially interact with the π-electron systems of aromatic molecules [9]. In addition, such silanol groups display increased reactivity in their interactions with chlorosilane compounds [4,7].

Hydrogen-bonded (paired) silanol groups or those associated through water molecules (H₂O) are strongly bound to the surface and can only be eliminated through condensation processes that involve the release of water molecules from the silanol structures. Due to this strong binding, they serve as the most active adsorption sites for water molecules [10]. These functional groups are also capable of participating in surface chemical modification processes and significantly influence the performance of Aerosil as both a reinforcing filler and an adsorbent material. Their complete disappearance from the surface is observed at temperatures up to 773 K and is inversely related to the formation of siloxane (Si–O–Si) bridges [11].

Materials and Methods.

The adsorption behavior of ammonia on Aerosil was investigated using a static volumetric technique. The measurements were performed at 303 K across an extended pressure interval. Before conducting the adsorption experiments, the Aerosil specimen was degassed and dehydrated under vacuum at 200 °C. The adsorption capacity was analyzed as a function of the logarithmic pressure ratio (ln p/p°). The quantity of adsorbed ammonia (a) was quantified based on the number of ammonia molecules associated with the surface adsorption centers.

The investigated material consisted of pyrogenic silicon dioxide (SiO₂). For the purification process, water was filtered through a column packed with zeolite. Differential molar heats of water adsorption on the Aerosil adsorbent were determined using a calorimetric setup described in earlier studies [9,10]. Prior to the experiments, dissolved gases were eliminated from the adsorbate through a freeze–pump–thaw procedure. The application of the Peltier effect within the heat compensation technique considerably enhanced the precision of the adsorption heat determination. The calorimetric system allowed continuous monitoring of the heat evolved during the adsorption process. Adsorption experiments were also carried out using a universal high-vacuum volumetric apparatus, which provided accurate control of adsorbate dosing and reliable determination of adsorption values [13,15].

Results and discussion. The adsorption behavior of ammonia molecules on solid surfaces, particularly on highly dispersed materials such as silicon dioxide (Aerosil), plays a crucial role in understanding the fundamental aspects of physical and chemical adsorption phenomena. Owing to its high specific surface area and the presence of reactive hydroxyl functionalities, Aerosil demonstrates strong adsorption affinity toward various gaseous substances, including ammonia. In the present work, the adsorption isotherm of ammonia on Aerosil was examined, and the molecular interaction features were evaluated based on the obtained equilibrium data. The isotherm profile revealed characteristics consistent with both the BET and Langmuir adsorption models. In the low-pressure region, adsorption predominantly followed a monolayer mechanism, whereas at elevated pressures, multilayer adsorption was observed. Such behavior can be explained by the microporous nature and surface characteristics of the Aerosil material.

The adsorption isotherm describing the uptake of ammonia molecules by the Aerosil adsorbent is presented in Figure 1 using semi-logarithmic coordinates. The isotherm begins at a relative pressure of ln(P/P⁰) = –8.398, which corresponds to an initial adsorption capacity of 73.841 µmol/g. As the adsorption process continues, the isotherm curve shows a gradual increase, indicating the progressive accumulation of ammonia molecules on the surface.

When the adsorption amount reaches approximately 300 µmol/g, the relative pressure increases to ln(P/P⁰) = –5.11. Within this pressure range, ammonia molecules are predominantly localized at energetically favorable adsorption centers of the Aerosil surface, occupying internal surface regions related to clay-like structural domains. With further adsorption, the uptake rises to about 400 µmol/g, corresponding to ln(P/P⁰) = –3.97. After this point, the isotherm demonstrates a sharp increase along the adsorption coordinate, eventually approaching a saturation capacity close to 700 µmol/g. The saturated vapor pressure of ammonia at 303 K was determined to be 8456.35 mmHg [13,14].

Figure 1. Isotherm profile of ammonia adsorption on Aerosil

When represented in BET form, the adsorption isotherm of ammonia on the Aerosil adsorbent demonstrates a linear dependence within the relative pressure interval of 0.01 < P/P⁰ < 0.45. The monolayer adsorption capacity at the energetically active centers (aₘ) was determined to be 400 µmol/g, while the corresponding BET constant was found to be equal to 1.0.

Based on the adsorption data, the accessible specific surface area of the Aerosil adsorbent was calculated to be about 89.5 m²/g. The cross-sectional area occupied by a single ammonia molecule in a closely packed monolayer (ōₘ) was estimated as 21 Ų, which is in good agreement with the assumptions of the Langmuir adsorption theory [13].

Figure 2. Differential heat profile of ammonia adsorption on Aerosil; dashed lines correspond to the heat of liquefaction at 303 K.

The differential heat of adsorption (Qd) for ammonia molecules interacting with the Aerosil surface is illustrated in Figure 2 and shows an oscillating decreasing pattern. At the early stages of adsorption, the Qd values decline from 60.10 kJ/mol to 43.88 kJ/mol. This trend suggests the progressive occupation of the most energetically favorable adsorption sites, accompanied by a gradual reduction in the adsorption enthalpy.

At the final stage of adsorption, corresponding to higher relative pressures, ammonia molecules are mainly adsorbed on the outer (basal) surfaces of the Aerosil adsorbent. In this region, the adsorption energy drops to about 21.16 kJ/mol and then shows a slight increase, gradually approaching the condensation enthalpy of ammonia at 303 K, which corresponds to an adsorption capacity of 717.9 µmol/g. The relatively low heat values in this range indicate that weak, non-specific physical adsorption dominates, with only a minor contribution from cationic adsorption centers.

To characterize the thermodynamic aspects of ammonia molecule adsorption on the Aerosil adsorbent, key parameters — including changes in enthalpy (ΔH) and Gibbs free energy (ΔG) during condensation, absolute temperature (T), and the average differential heat of adsorption (Qd) — were systematically evaluated.

Figure 3. Kinetic equilibrium time of ammonia adsorption on Aerosil

The adsorption process of ammonia on Aerosil is important not only from a thermodynamic perspective but also from a kinetic point of view. The rate at which ammonia molecules adhere to the surface and the effect of temperature on this process are critical factors determining the technical efficiency of the sorbent. Through thermokinetic analysis, factors such as the mechanism of adsorption, diffusion, and reaction rates can be identified.

Conclusion. The adsorption of ammonia on the Aerosil surface demonstrates a step-like behavior, suggesting the existence of different types of adsorption sites. Analysis of the adsorption isotherms revealed that, at the initial stage, ammonia molecules interact with the active centers mainly through hydrogen bonding, whereas at later stages physical adsorption becomes the dominant mechanism. These findings are of practical importance for designing Aerosil-based sorbent materials and for improving gas separation and purification technologies.

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