Effect of Ion-Exchanger Monoporosity in the Kinetics of Oxygen Sorption by Silver-Containing Nanocomposites

Abstract

The results of a study of the kinetics of oxygen sorption from water by silver-containing nanocomposites synthesized on the base of macroporous ion exchangers with different pore sizes are presented. In the case of the Lewatit K 2620 ion exchanger, the pore size was fixed (41 nm), and for KU-23, it varied in the range from 10 to 100 nm. The nanocomposite materials Ag⁰⸱KU-23 and Ag⁰⸱Lewatit K 2620 were prepared by chemical precipitation. Using the different physicochemical methods, it was found that due to the monoporosity of the ion exchanger, the average size of the silver particles in the Ag⁰⸱Lewatit K 2620 nanocomposite is smaller than for KU-23. This effect contributes to the intensification of oxygen absorption and is proved by the results of studying the rate and degree of oxygen sorption by nanocomposites in the entire studied range of their capacity on metal. On the other hand, the polyporosity of the KU-23 ion exchanger, due to its better diffusion permeability, contributes to the more uniform distribution of silver over the volume of nanocomposite grains and ensures the steady state of the sorption process. Based on the presented experimental results, the synthesized silver-containing nanocomposites can be recommended as multifunctional materials with bactericidal action and catalytic effect for different industrial applications, including the deep removal of dissolved oxygen in the production of ultrapure water for energetics and microelectronics.

1. Introduction

The use of nanomaterials with specific physicochemical characteristics contributes to an increase in the efficiency of various chemical, electrochemical and sorption processes [1,2,3]. It is known that the catalytic activity and selectivity of such materials are significantly influenced by the nature of the metal, the size and structural features of the nanoparticles, as well as the nature of the matrix or substrate [4,5]. For instance, a decrease in the size of dispersed metal particles leads to a decrease in its melting point [6]; the inclusion of silver in the silica structure increases the hydrophilicity of its surface and, as a result, contributes to a multiple increase in specific adsorption [7]; and a macroporous sorbent modified with platinum nanoparticles acquires the ability of donor–acceptor interactions with water and alcohol molecules [8]. The introduction of metal nanoparticles into an ion-exchange polymer plays a decisive role in the formation of qualitatively new properties of the synthesized nanocomposite (NC). This method of obtaining nanomaterials makes it possible not only to purposefully change their properties by varying the amount and size of the metal component but also to stabilize its nanoscale state [9,10,11,12,13,14]. Nanocomposites are most often used in catalysis, electrocatalysis, electrochemical sensing and sorption processes.

Metal ion-exchanger nanomaterials play a special role in the implementation of technologies for the deep removal of dissolved oxygen in the production of ultrapure water [15,16]. Molecular oxygen O₂ is a strong oxidizer, so its presence is highly undesirable in many physical and chemical processes, even in minimal concentrations. The most stringent requirements for the quality of deoxygenated water are put forward by the semiconductor industry: in some cases, the required level of O₂ content should not exceed 10 μg/dm³ [17,18]. In microelectronics, ultrapure water is often used to wash silicon substrates in the manufacture of integrated circuits. In the presence of dissolved oxygen, it becomes possible to form an oxide layer on the substrate surface even in ultrapure water with a concentration of dissolved oxygen of 40–600 μg/dm³. The solution of the urgent problem of deep deoxygenation of water is possible with the use of metal-containing nanocomposites as sorbents of dissolved oxygen. The corresponding devices can be used in water treatment processes for the energy and microelectronic industries [18,19]. The advantage of water deoxygenation by metal-containing nanocomposites is that the treatment products remain inside the nanocomposite and do not pollute the treated water.

The copper/sulfocation-exchanger system is the most well known for the deep deoxygenation of water [18]. The polymer matrix ensures the transport function due to a developed system of channels, and the ionogenic groups of the matrix are a source of ions necessary for the reaction and simultaneously absorb products. The pore size and ion-exchange capacity of the matrix determine the nature of the size distribution of the metal, as well as its content in the ion-exchanger phase [20]. According to the electrochemical series, the introduction of silver instead of copper into the sulfocation exchanger should predictably lead to a decrease in the intensity of the oxygen sorption process due to a decrease in the chemical activity of the metal component. However, the specific bactericidal and catalytic properties inherent in silver can be useful in polyfunctional filters for water deoxygenation. At the same time, the main factors influencing the catalytic activity of the silver/sulfocation exchanger in the oxygen reduction are the size [21,22] and concentration of Ag nanoparticles, as well as the nature of their distribution in the ion-exchange matrix.

The purpose of this work is to synthesize the silver-containing nanocomposites based on the sulfocation exchangers KU-23 and Lewatit K2620 with different contents and distribution of the metal component by grain volume and to identify the role of the polymer base of these materials in the processes of sorption and reduction in oxygen dissolved in water.

2. Materials and Methods

The objects of this study were silver-containing nanocomposites based on the macroporous ion exchangers KU-23 and Lewatit K2620 with the following structure of a styrene and divinylbenzene copolymer functionalized with the -SO₃ − H⁺-group:

The main characteristics of KU-23 and Lewatit K 2620 are shown in

Table 1. The main physical and chemical characteristics of the ion-exchange resins used for Ag/sulfocation-exchanger nanocomposite synthesis.

Property	KU-23	Lewatit K2620
Grain size d, mm	0.5–1.0	0.4–0.6
Surface area, m2/g	30–40	33
Pore size, nm	10–100	41
Ion-exchange capacity ε, meq/cm3	1.25	1.9

.

It can be seen that, with other similar characteristics of the matrices, Lewatit K2620 (unlike KU-23) has a fixed pore size, that is, it is monoporous, and also has an increased ion-exchange capacity.

The preparation of the ion-exchange matrices for synthesis consisted of fractionation and the acid–base conditioning of grains. NC samples were obtained by the chemical deposition of silver into an ion-exchange matrix according to the following procedure described in [23]: ion exchangers were treated with 0.05 M AgNO₃ solution followed by the chemical reduction of silver ions inside the matrix to a metallic state. A solution of 0.09 M NaBH₄ + 0.63 M NaOH was used as a reducing agent. The nanocomposites based on the ion exchangers KU-23 and Lewatit K2620 are denoted in this paper as Ag⁰⸱KU-23 and Ag⁰⸱Lewatit K2620, respectively. To determine the redox capacity of the silver nanocomposites, 10% (by weight) was dissolved in hot nitric acid and then the metal ion content was determined by the titrimetric method. Iron (III) ions (iron ammonium alum) were used as an indicator [24]. The structure, size and radial distribution of the metal particles in the silver-containing nanocomposites were determined by X-ray diffraction analysis (XRD, ARL X’TRA (Thermo Scientific, Basel, Switzerland), scanning electron microscopy (SEM, JSM 6380LV (JEOL, Akishima, Japan)) and transmission electron microscopy (TEM, Libra 120 (Carl Zeiss, Oberkochen, Germany)). An elemental analysis was performed with the use of an INCA Energy 250 (Oxford Instruments, Oxford, UK) attachment for an energy dispersion analysis (EDAX).

The kinetics of oxygen sorption were studied by the gasometric method, which is based on a change in the volume of the gas phase in equilibrium with the volume of water being stirred [25]. Before the test, the samples were converted to the H⁺ form and washed with deoxygenated water. Oxygen was then injected, maintaining a constant temperature and waiting for thermal equilibrium between the gas and liquid phases.

3. Results and Discussion

3.1. Nanocomposites Characterization

During the synthesis, the samples Ag⁰⸱KU-23 and Ag⁰⸱Lewatit K2620 were obtained by the chemical deposition of silver. In order to increase the capacity of the NC, the metal deposition process was repeated several times (cycled). Having determined the capacity with silver, we obtained the linear dependencies of the NC capacity for metal on the number of deposition cycles N (Figure 1).

In the case of both studied matrices, the amount of silver in the ion-exchanger phase increases with an increase in the number of deposition cycles. However, the metal saturation in a KU-23-based nanocomposite proceeds more intensively than for Lewatit K2620, which contradicts the ratio of the values of their ion-exchange capacity. Apparently, the monoporous base of Lewatit K2620, due to diffusion restrictions, prevents the reduction in silver in the pores of the matrix. In order to confirm this hypothesis and to establish the structure of synthesized nanocomposites, the structure of silver-containing nanocomposites was studied (Figure 2 and Figure 3). Analyzing micrographs, at least 100 particles were selected, and their size was determined according to the scale of the images. Next, a curve was constructed for the distribution of the metal particles by size and a range was determined, which corresponded to the size of 66% of the particles. The obtained data on the particle sizes are presented in

Table 2. Physicochemical characteristics of synthesized nanocomposites.

N	Capacity, meq/cm3		Particles Size d, nm
			XRD		SEM		TEM
	KU-23	Lewatit K2620	KU-23	Lewatit K2620	KU-23	Lewatit K2620	KU-23	Lewatit K2620
1	0.65 ± 0.15	0.75 ± 0.12	12–16	9–13	90–150	70–90	5–15	2–10
3	2.25 ± 0.17	1.95 ± 0.18	17–23	15–19	90–300	80–110	3–10	4–16
5	3.18 ± 0.09	2.81 ± 0.11	19–25	16–22	120–360	80–120	3–15	4–18
7	4.45 ± 0.15	3.63 ± 0.09	27–33	23–29	150–390	120–150	10–35	6–28

.

As the metal content in the ion-exchanger phase increases, particle enlargement is observed. The particle size on the KU-23 matrix is larger than for Lewatit K 2620-based nanocomposites. In addition, nanocomposites based on the KU-23 matrix are characterized by the formation of large dendrites, while in the case of Lewatit K2620, there is a uniform distribution of small agglomerates. The absence of a dendritic structure and a clearer separation of agglomerates of the metal particles in the Lewatit K2620 ion exchanger is evidently due to its monoporosity, which contributes to the spatial separation of metal nanoparticles.

Using an elemental analysis of silver-containing nanocomposites, it was found that with increasing metal deposition cycles, the percentage of silver in the nanocomposite increases. For example, for three and five cycles of deposition silver in Ag⁰⸱KU-23, this parameter is approximately 50% of the grain weight. In addition to the metal component, the EDAX method identifies other elements such as carbon, oxygen, sodium and sulfur, which can be explained as follows. The appearance of oxygen is caused by the oxidative activity of metal nanoparticles. Sodium and sulfur are contained in the composition of the reducing agent. Carbon is the basis of the ion-exchange framework.

The metal radial distribution curves (Figure 4) for the samples with different matrices and different metal contents are plotted based on the silver content in the nanocomposite grain center (radial coordinate R = 0), in the middle (R = ½R₀, where R₀ is the grain radius) and on the grain surface (R = R₀). The analysis shows that the metal distribution is predominantly uniform. However, for Ag⁰⸱KU-23, the silver content near the grain surface is slightly lower than in its volume, which is especially noticeable for a low-capacity nanocomposite. For Ag⁰⸱Lewatit K2620, on the contrary, the metal content near the surface is higher, which is clearly manifested with an increase in the NC capacity: the mass content of silver increases under these conditions. The observed effects can also be explained by the monoporosity of the Lewatit K2620 ion exchanger, which helps to reduce the diffusion permeability of the matrix and prevents the uniform spatial distribution of metal nanoparticles over the grain volume.

To confirm the revealed effects and accurately determine the average size of the metal particles, the TEM method was used with multiple magnifications (Figure 5 and Figure 6). For this purpose, the aqueous gelatin suspension of the nanocomposite powder was studied by light-field and dark-field transmission electron microscopy.

After analyzing the obtained photographs, a histogram of the size distribution of the silver particles was constructed and the average range of the particle sizes was determined. The particle size data determined by the TEM method are shown in Table 2. They confirm that the size of the metallic component of NC increases with an increase in the number of cycles; however, the values of the particle size are an order of magnitude lower than those detected by the SEM method. The difference is probably due to the higher resolution of the TEM method, which makes it possible to isolate the particles that make up the agglomerates detected in the SEM micrographs.

In order to establish the most reliable method of particle size determination used in this work, an additional X-ray diffraction analysis of the NC samples was performed. During the analysis, there was no fragmentation or other physical changes in the nanocomposite grains. The processing of the X-ray images obtained using the Scherrer equation [25] allowed us to find the average size of the metal nanoparticles (Table 2). The analysis shows that according to the XRD data, the enlargement of the particles with an increase in the metal content in the nanocomposite, which was previously detected by electron microscopic methods, is confirmed. In addition, the X-ray phase analysis revealed the presence of metal particles similar in size to the particles recorded using the TEM method.

Based on the experimental data obtained by various physical methods, it can be concluded that with an increase in the number of metal deposition cycles, an increase in the size of silver particles is observed. Consequently, the chemical activity of the nanocomposites will decrease with increasing metal concentration. The resulting metal particle sizes are in the nanometer range. The size of the silver particles in the Lewatit K2620 matrix is smaller than for KU-23, which is explained by the smaller pore size, which prevents the formation of agglomerates of the dendritic structure. We believe that the ion-exchange capacity does not influence the size of the metal particles formed, and its value determines only the number of metal ions entering the pores of the polymer matrix (Figure 1).

3.2. Kinetics of Molecular Oxygen Sorption in the Sythesized Nanocomposites

The difference in grain sizes for the two ion exchangers studied should affect the rate of oxygen sorption; however, the results obtained earlier [26] did not reveal a significant effect of the size of the fractions of the studied grains on the kinetics of the absorption of molecular oxygen. To determine the sorption rate, we used the concept of the degree of sorption α, which characterizes the amount of absorbed oxygen Q (meq/cm³) per unit of NC metal capacity ɛ (meq/cm³):

α = Q/ɛ.

(1)

The obtained curves of the degree of oxygen reduction are shown in Figure 7.

A characteristic type of curve is observed: at the initial stage (up to 100 min), there is a more abrupt rise in the curve, followed by a smooth and prolonged increase in time. With an increase in the silver content in the nanocomposite, the amount of oxygen absorbed from the water increases. However, the observed dependence is not linear, although Figure 1 indicates a linear increase in the metal content in the ion-exchanger phase with an increase in the number of silver deposition cycles.

With an increase in the silver content, the volume of absorbed oxygen increases, and the degree of its sorption decreases, while for 3, 5 and 7 cycles the losses are minimal. The nanocomposites with the lowest capacity obtained by the single deposition of silver into ion-exchange matrices have the highest sorption activity. At the same time, the analysis of the kinetic curves for the different matrices demonstrates that the largest volume of absorbed oxygen is achieved on nanocomposites synthesized on the Lewatit K2620 matrix. According to the volume and degree of oxygen absorption, it can be concluded that the oxygen sorption rate for the Lewatit K2620 matrix will be higher due to the higher dispersion of silver particles in this matrix.

The specific morphology and capacitance characteristics of the Lewatit K 2620 matrix contribute to an increase in the rate of the sorption process. Due to the monoporosity of the ion exchanger and its lower oxygen permeability, diffusion to the center of the grain occurs more slowly, which should reduce the O₂ sorption rate. However, the amount of metal near the surface is slightly higher than for KU-23, which provides an increase in the intensity of oxygen sorption. Also, the high ion-exchange capacity for hydrogen ions maintains an acidic environment during the process, which leads to the formation of soluble oxidation products. Such conclusions are confirmed by the representation of kinetic curves in the coordinates dα/dt–t (Figure 8), that is, for the dependence of the oxygen sorption rate on time.

It can be seen that in the initial section, where the capacities of the ion exchangers on metal and hydrogen ions are commensurate, and the amount of oxygen absorbed is comparatively small, the rate of O₂ sorption and the rate of metal oxidation have the largest value (Figure 8, curve 1). As the content of the metal component increases, the amount of oxygen absorbed increases, but at the same time, the rate of the process and the degree of oxygen absorption decrease, and the differences between the nanocomposites synthesized on different matrices are leveled.

Based on the presented experimental results, recommendations can be made for the synthesis of silver-containing nanocomposites, depending on the intended purposes of their use. To obtain composite materials with a minimum silver particle size, but with maximum chemical activity, a monoporous ion exchanger Lewatit K2620 should be used. On the contrary, to obtain materials with the most uniform distribution of silver, it is more efficient to use a polyporous ion-exchange base KU-23. With the help of such materials, it is possible to achieve a stationary mode of oxygen sorption processes.

Both types of the studied nanocomposites can be useful as catalysts for different chemical and electrochemical reactions and also allow for intensifying the processes of the deep removal of dissolved oxygen in the production of ultrapure water, including water treatment for microelectronic industries. Despite the fact that the nanocomposites are oxidized in the process of deoxygenation, they can be regenerated by a chemical treatment with a reducing agent solution. Although the nanocomposites containing copper as a dispersed metal component are the most effective for the deoxygenation of water (traces of oxygen in deoxygenated water do not exceed 1–10 g/dm³), Ag-containing nanocomposites have such additional advantages as bactericidal action and catalytic effect for some processes, so these composite materials become multifunctional.

4. Conclusions

The silver/sulfocation-exchanger Ag⁰⸱KU-23 and Ag⁰⸱Lewatit K2620 nanocomposites were synthesized by the chemical deposition of Ag into KU-23 and Lewatit K2620 matrices, and their physicochemical characteristics were obtained. A certain enlargement of the silver particle size with an increase in the number of metal deposition cycles has been established by different physicochemical methods. In general, the size of silver particles in the Lewatit K 2620 matrix is smaller than in KU-23, which is due to the monoporosity of the Lewatit K 2620 matrix.

This study of the rate and degree of oxygen-reductive sorption by the synthesized nanocomposites showed that the rate of the process is higher on the Lewatit K2620 matrix, which is due to the greater dispersion of silver particles, which is provided by the monoporosity of the ion-exchange base. This leads to an increase in the chemical activity of the metal and makes the deoxygenation process more effective. In turn, the polyporosity of the KU-23 ion-exchange base contributes to a more uniform distribution of metal across the grain of the ion exchanger, which may be important for the steady-state management of the process of deoxygenation of liquid media.