MOLECULAR MECHANISMS OF AMMONIA MOLECULE ADSORPTION ON AEROSIL

АННОТАЦИЯ

В данной статье исследованы механизм адсорбции молекул аммиака и изотермические характеристики на поверхности аэросила. Адсорбционный процесс измерялся объемным методом при температуре 303 K. Полученные данные изотермы позволили понять характер взаимодействия молекул аммиака с адсорбционными центрами на поверхности аэросила. Адсорбционная изотерма при различных давлениях отражает различные механизмы адсорбции и предоставляет информацию о природе адсорбционных центров.

ABSTRACT

In this study, the adsorption mechanism and isothermal characteristics of ammonia molecules on the surface of Aerosil were investigated. The adsorption process was measured using the volumetric method at a temperature of 303 K. The obtained isotherm data provided insight into the interaction of ammonia with adsorption centers on the Aerosil surface. The adsorption isotherm reflected different adsorption mechanisms at varying pressures and offered information about the nature of the adsorption centers.

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

Keywords: Aerosil, adsorption, thermodynamics, isotherm, differential heat.

Introduction. Hydrated Aerosil is a pure, amorphous, non-porous silicon dioxide consisting of colloidal-sized spherical particles and is characterized by a high specific surface area [1]. During the drying stage, when SiO₂ is obtained, the particles initially retain their individuality, and then, under the influence of weak binding forces, form aggregates of primary monodisperse spheres [2]. The aggregation of particles is facilitated by the presence of water and hydrogen chloride complexes in the surface layer, which determine the magnitude of the surface charge and the hydrogen index (pH) of the Aerosil hydrogel. During the drying process, acid is removed from the particle surface and restores the hydration shell [3]. This reduces the attraction between particles and prevents aggregation [2]. The formation of silanol groups during the production of Aerosil also prevents particle adhesion during their growth.

Existing studies have mainly made it possible to identify the chemical nature of the active centers present on the surface of hydrated Aerosil. These include silicon atoms carrying OH groups, strained siloxane bridges, silanediol groups, coordinatively unsaturated silicon atoms, isolated hydroxyl groups, as well as various forms of bound water [3–6]. Two types of OH groups exist on the surface of Aerosil: free and hydrogen-bonded. The latter enhances the activity of the former through its presence [4,7].

Free isolated silanol groups are more stable than bonded ones and begin to be removed from the surface at temperatures above 873 K [8]. They are less reactive; however, the electron systems of aromatic compounds predominantly interact with them [9]. These groups also exhibit higher sensitivity in reactions with chlorosilanes [4,7].

Silanol groups that are hydrogen-bonded (paired) or interact via an H₂O molecule are so strongly retained that their removal occurs only through condensation accompanied by the loss of water from the silanol groups, making them the strongest adsorption centers for water molecules [10]. These groups can participate in chemical modification reactions and are responsible for the properties of Aerosil as a filler and adsorbent. Their complete removal from the surface occurs up to a temperature of 773 K, inversely correlated with the formation of siloxane bonds [11].

Materials and Methods.

The adsorption of ammonia on Aerosil was measured using a volumetric static method. The experiment was conducted at a temperature of 303 K over a wide pressure range. Prior to the measurements, the Aerosil sample was dehydrated under vacuum at 200 °C. The adsorption volume was described as a function of the logarithm of pressure (ln p/p°). The amount of adsorbed ammonia (a) was expressed in terms of the number of ammonia molecules corresponding to the surface adsorption sites.

The composition of the studied zeolite is pyrogenic SiO₂. For water purification purposes, the water was passed through a column containing zeolite. Differential molar adsorption calorimetric studies of water adsorption on the Aerosil adsorbent were carried out using the apparatus described in references [9,10]. Dissolved gases were removed from the adsorbate by freezing and subsequent evacuation. The use of the Peltier effect in the heat compensation method significantly improved the accuracy of adsorption heat measurements. The calorimeter was capable of measuring the heat released over an indefinite period. Adsorption measurements were performed using a universal high-vacuum volumetric setup, which enabled precise quantification of adsorption and accurate dosing of the adsorbate. [13,15]

Results and discussion. The adsorption properties of ammonia molecules on surfaces, especially on highly dispersed materials such as silicon dioxide (Aerosil), are of great importance for a deep understanding of physical and chemical adsorption processes. Due to its large specific surface area and the presence of active hydroxyl groups, Aerosil exhibits significant adsorption interactions with various gases, including ammonia. In this study, the adsorption isotherm of ammonia on Aerosil was investigated, and based on the isothermal data, the molecular interaction characteristics were analyzed. The shape of the isotherm exhibited elements of both BET and Langmuir models. At low pressures, typical monomolecular adsorption was observed, while at higher pressures, polymolecular adsorption took place. This behavior is attributed to the microporous structure and surface properties of Aerosil.

The adsorption isotherm of ammonia molecules on the Aerosil adsorbent is shown in Figure 1 in semi-logarithmic coordinates. The isotherm starts at a relative pressure of ln(P/P⁰) = –8.398, corresponding to an initial adsorption capacity of 73.841 µmol/g. As adsorption progresses, the curve gradually rises, reflecting the sequential uptake of ammonia molecules. At an adsorption level of 300 µmol/g, the relative pressure reaches ln(P/P⁰) = –5.11. In this region, ammonia molecules are mainly immobilized at specific adsorption sites on the Aerosil adsorbent, occupying internal surface layers associated with clay mineral domains. Further adsorption leads to a capacity of 400 µmol/g, corresponding to ln(P/P⁰) = –3.97. Beyond this point, the isotherm exhibits a steep upward trend along the adsorption axis, ultimately approaching saturation near 700 µmol/g. The saturated vapor pressure of ammonia at 303 K is 8456.35 mmHg. [13,14]

Figure 1. Adsorption isotherms of ammonia molecules on the Aerosil adsorbent

When expressed in BET coordinates, the adsorption isotherm of ammonia molecules on the Aerosil adsorbent exhibits a linear relationship in the relative pressure range of 0.01 < P/P⁰ < 0.45. The calculated adsorption capacity at the active sites (aₘ) is 400 µmol/g, with a corresponding BET constant of 1.0.

For the adsorption of ammonia molecules on the Aerosil adsorbent, the specific surface area available is estimated to be approximately **89.5 m²/g**. The molecular area occupied by a single ammonia molecule in a densely packed monolayer (**ōₘ**) is evaluated to be **21 Ų**, which corresponds well with the parameters used in the **Langmuir adsorption model**. [13]

Figure 2. Differential Heat of Adsorption of Ammonia Molecules on Aerosil. Dashed lines indicate the heat of condensation of ammonia at 303 K.

The differential heat of adsorption (Qd) of ammonia molecules on the surface of Aerosil is shown in Figure 2, exhibiting a wave-like decreasing trend. In the initial stages of adsorption, the Qd value decreases from 60.10 kJ/mol to 43.88 kJ/mol. This behavior indicates the gradual saturation of high-energy adsorption sites and the corresponding decline in adsorption enthalpy.

At the terminal stage—under higher relative pressure—ammonia molecules primarily accumulate on the external (basal) surfaces of the Aerosil adsorbent. In this regime, the adsorption energy decreases to 21.16 kJ/mol and then gradually increases, approaching the enthalpy of ammonia condensation at 303 K, corresponding to a capacity of 717.9 µmol/g. The low heat values observed in this region suggest the predominance of weak, nonspecific physical adsorption mechanisms, with minimal involvement of cationic sites.

Figure 3 illustrates the differential entropy (ΔSd) of ammonia adsorption on Aerosil. These entropy values were calculated using the experimentally obtained isotherm and enthalpy data, applying the Gibbs–Helmholtz equation. The entropic characteristics provide deeper insight into the degree of molecular ordering and thermal interactions governing the adsorption process. [15]

Figure 3. Differential molar entropy of ammonia molecule adsorption on Aerosil. Dashed lines represent the average integral entropy.

The entropy of ammonia molecules in the liquid state is assumed to be zero

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 4. Equilibrium time of ammonia molecule 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 Aerosil exhibits a stepwise nature, indicating the presence of various adsorption centers on the surface. The isotherms made it possible to determine that the interaction of ammonia molecules with active centers initially involves hydrogen bonding, followed by the predominance of physical adsorption mechanisms. These results have practical significance for the development of sorbents based on Aerosil and for the advancement of gas separation technologies.

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