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Chelyabinsk Physical and Mathematical Journal. 2023. Vol. 8, iss. 4- P. 605-616.
DOI: 10.47475/2500-0101-2023-8-4-605-616
MECHANOСHEMICAL SYNTHESIS OF ION-EXCHANGE SILVER FORMS OF POLYANTIMONIC ACID
F.A. Yaroshenko, V.A. Burmistrov, A.E. Silova, Yu.A. Lupitskayaa, E.M. Filonenko, P.V. Timushkov, M.N. Ulyanov, S.I. Saunina
Chelyabinsk State University, Chelyabinsk, Russia a [email protected]
The possibility of synthesising ion-substituted forms of hydrated silver antimonates Ag27H2-2Y Sb2O6-2H2O has been studied. For the synthesis, the method of mechanochemical activation of the components of an inorganic mixture consisting of polyantimonic acid (PAA) with H2Sb2O6-2H2O composition and silver nitrate with the concentration range (y) from 0.0 to 1.0 has been applied. The results of studies of the phase composition of the synthesized compounds and their structural features are presented. Using the Rietveld method, the parameters of the crystal lattice of PAA hydrated ion-substituted silver forms with a pyrochlore-type structure have been refined. The model for the occupation of metal ions by crystallographic positions has been proposed: the framework of the structure of the compounds is formed by 16c- and 48f-positions, in which Sb5+ and O2- are statistically located; hydrated oxonium ions (H3O+) and silver ions occupy 16d- and 8b-positions respectively. It has been shown that the synthesis of silver forms of PAA is preferably carried out by the mechanochemical synthesis, which results in the complete substitution of proton groups by silver ions in the structure of the compounds.
Keywords: polyantimonic acid, mechanochemical synthesis, hydrated ion-substituted forms, pyrochlore-type structure, ion-exchange properties.
Introduction
At present, one of the priority directions in the development of modern material science is the working at design for new promising materials with special electrical properties and, to a greater extent, the synthesis of solid electrolytes with high proton conductivity values [1]. Solid proton-conducting electrolytes include a wide class of compounds from low-temperature crystalline hydrates of inorganic compounds — heteropolyacids and their salts — to high-temperature oxides [2]. Polyantimonic acid (PAA) and compounds based on it are of particular interest as promising ion-exchange materials [3], which can find practical application as low- and medium-temperature composite ion-exchange membranes [4-6].
It is known [3-6] that PAA and its derivatives are characterized by a pyrochlore-type structure (sp. gr. Fd-3m), the framework of which is a three-dimensional network formed by antimony-oxygen polyhedra with gross composition [Sb(V)O6/2]-of octahedral coordination. In this structure, Sb5+ and O2- are statistically located in the 16c- and 48f-positions respectively, and the counterions (hydrated oxonium ions) H3O+ occupy the 16d-positions. It was established in [4] that the ion-exchange properties
This work has been supported by the grant of the Russian Science Foundation, grant number 23-23-00140.
of PAA are determined by the diffusion mobility of proton groups, which in alkaline and salt solutions can be completely substituted by mono- and divalent metal ions [3]. In this case, a change in the structural parameters of the PAA crystal lattice and a change in thermal, proton-conducting, and ion-exchange properties should be expected.
The diffusion mobility of proton groups (ion-exchange properties) of hydrated compounds can be significantly affected not only by the nature of the cation that substitutes the counterions in the structure of compounds but also by the method of their preparation. Thus, in many works it is noted that the synthesis of ion-substituted forms of PAA was carried out by precipitation in solution [4-7], however, the implementation of the method can result in only partial substitution of the metal ion by proton-containing hydrogen groups. An alternative way to obtain materials for hydrogen storage is the mechanochemical synthesis of inorganic compounds, the use of which will reduce the number of stages for sample preparation.
In this regard, the purpose of this work is to study the conditions for the synthesis of PAA hydrated forms by the mechanochemical activation method when proton groups are replaced by silver ions in a wide concentration range as well as studying the structural features of the synthesized compounds and their ion-exchange properties.
1. Objects and Methods 1.1. Research objects
The initial PAA of H2Sb2O6-2H2O composition was obtained by co-precipitation [3] in several successive stages of chemical transformations in accordance with the reaction (1) when the solution was heated to 60 °C:
SbCls + 2HNO3 + 3HCl ^ H[SbCl6] + 2NO2 f +2H2O f . (!)
As a result of the implementation of the first stage, the complete dissolution of antimony trichloride in concentrated hydrochloric acid was observed; subsequently, for the complete oxidation of antimony ions to the pentavalent state, concentrated nitric acid was poured into the resulting solution in small portions and with constant stirring according to [1]. The reaction was accompanied by active evolution of brown gas, while the solution turned orange-brown. The resulting product was subjected to hydrolysis, after which the solution with the resulting white precipitate was thoroughly washed with distilled water until chloride ions were completely removed by centrifugation and dried in a muffle furnace at 110 °C for two hours. All reagents used were of analytical grade. Hydrated ion-substituted forms of PAA with Ag2YH2-2ySb2O6-2H2O (0.0 ^ 7 ^ 1.0 and Ay = 0.2) composition were obtained as a result of ion-exchange reaction (2) by mechanochemical activation of the initial components of inorganic mixture consisting of hydroantimonate H2Sb2O6-2H2O and silver nitrate:
H2Sb2O6 ■ 2H2O + 2yAgNO3 ^ Ag2YH2-27Sb2O6 ■ 2H2O + 2YHNO3. (2)
For this purpose, the weighed portions of the reagents H2Sb2O6-2H2O and AgNO3 were placed in a porcelain mortar and thoroughly ground for 20 minutes. The ground samples were a fine powder, the color of which varied from rich white to bright yellow, depending on the content of silver ions in the composition of compounds. Subsequently, the powders under study were dried in a muffle furnace at 110 °C for an hour, and evolution of brown gas was observed.
1.2. Research methods
The content of chloride ions in the solution was determined by titration of the analyzed sample using silver nitrate. The elemental analysis was performed by the energy dispersive X-ray spectroscopy (EDXRF-spectrometer ARL QUANT'X) in the combination with scanning electron microscopy data (microprobe analysis, JEOL JSM-6510).
X-ray data were obtained at room temperature on Bruker D8 ADVANCE diffractometer (CuKai-radiation) in the range of diffraction angles (20) from 10 ° to 70 °. The qualitative X-ray diffraction analysis was used to determine the phase composition of the studied samples after each mechanical grinding and to control the single-phase nature of the obtained compounds. The crystallographic parameters of the structure were refined by the Rietveld method in the PowderCell for Windows 2.4 software environment. The diffraction profile was approximated using the pseudo-Voigt analytical function, which is a linear combination of the Gauss and Lorentz (Cauchy) functions:
PV(x,xo,n,bL,bc,A) = A[(1 - n) x G(x,xo,bG) + n x L(x,xo,bL)], (3)
where x is a variable corresponding to the angle of reflection 20; xo is a parameter that determines the position of the function maximum; n is a specific share of the Lorentz function; A is a normalizing factor; bG and bL are the parameters of the Gaussian function G(x,x0,bG) and Lorentz function L(x,x0,bL) respectively [8]. Part of the geometric calculations, including the graphical representation of the unit cell of the pyrochlore-type structure in the polyhedral interpretation of PAA hydrated compounds, was performed using the VESTA data visualization program [9].
2. Results and Dicussion
It follows from the analysis of X-ray data that the process of ion exchange in hydrated silver forms of PAA obtained by mechanochemical activation of reagents includes several stages of phase transformations and is accompanied by structural changes (Fig. 1).
Thus, as a result of the first mechanical grinding of samples, the duration of which was 20 minutes, a mixed set of diffraction maxima is observed in the X-ray diffraction patterns of hydrated compounds Ag2YH2-2ySb2O6-2H2O in the range of change 0.0 ^ y ^ 1.0 (Fig. 1,c). In this case, in a given range of Bragg angles, a strictly defined sequence of diffraction maxima is recorded satisfactorily described by the quadratic form for crystals of the cubic system, sp. gr. Fd-3m (Fig. 1,a); yet a separate group of reflections belonging to AgNO3 composition phase (Fig. 1,b) is also registered, indicating the partial completion of the ion-exchange reaction between the components of the mixture. To complete the ion exchange in the studied compounds and, as a result, to remove the impurity phase of silver nitrate from the final products of synthesis, the samples were subjected to repeated grinding in a porcelain mortar for another 20 minutes, followed by one hour heat treatment at 110 °C. The diffraction patterns of the synthesized mixtures are characterized by a set of maxima of the same name, the shape and half-width of which do not change within the error of experimental measurements (Fig. 1, d). This allows to conclude that the symmetry of the crystal lattice of the cationite is retained in all the studied samples during ion exchange within the structure of the pyrochlore type (Fig. 2).
Thus, as a result of the synthesis of the hydrated ion-exchange silver forms of PAA by mechanochemical activation, the optimal conditions required to obtain single-phase samples with partial and/or complete substitution of proton groups for silver ions were
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Fig. 1. X-ray patterns of air-dry samples of various compositions: H2Sb2O6-2H2O (a); AgNO3 (b); Ag0.93H1.0rSb2O6-2H2O (* -Sb2O5 phase (sp. gr. Fd-3m) and # - AgNO3 phase) (c); Ag2Sb2O6 (sp. gr. Fd-3m) (d)
a „ A " c\T II CN CO Ci 1 (422) ¡> (511) ! > (440) |> (531) >(622) I (444) f (551) !> (731)
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e -A. A A . A x .
f .................. , -A...........,
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10 20 30 40 50 60 70 2®, degree
Fig. 2. X-ray patterns of air-dry samples of hydrated compounds of Ag2YH2_2y Sb2O6-2H2O composition in the range (0.0 < y < 1.0): H2Sb2O6^2H2O (a); Ago.4HL6Sb2O6^2O (b); Ago.sHi.2Sb2O6^2H2O (c); Agi.2Ho.sSb2O6^2H2O (d); Ag1.6H0.4Sb2O6-2H2O (e); Ag2Sb2O6^O (f)
determined. As the degree of ionic substitution (7) increases in the hydrated compounds that are isomorphic to pyrochlore, a regular decrease in the relative intensity value (I(hk1)) of the reflection group with odd indices (hkl) is observed on the X-ray patterns for Ag27H2-27Sb2O6•2H2O (Fig. 2) and a monotonic decrease in the unit cell parameter (a) from 10.350±0.005 A to 10.280±0.005 A are observed (Fig.3).
Fig. 3. Change in the unit cell parameter (a) of hydrated compounds of Ag27H2_27Sb2O6-2H2O composition in the concentration range 0.0 ^ y ^ 1.0 on the degree of ionic substitution (y)
At the same time the greatest changes in magnitude (I(hki)) is characterized by the ratio of diffraction maxima (I(3ii)/1(222)), the values of which vary from 0.85 to 0.07 depending on the content of silver ions (7) in the studied samples (Fig. 4).
Fig. 4. Change in the relative intensity of diffraction peaks (I(3ii)/I(222) ) on X-ray diffraction patterns of hydrated compounds of Ag27H2-27Sb2O6-2H2O composition in the concentration range 0.0 ^ y ^ 1.0 on the degree of ionic substitution (y)
As can be seen from Fig. 4, the curve of the dependence of the change in the diffraction maxima relative intensity (I(311)/I(222) ) for air-dry samples on the concentration of silver ions (7) is represented by lines, the slope of which depends on the substitution interval and the nature of the cation involved in the ion exchange reaction [4; 6]. On the given experimental dependence (Fig. 4), two linear segments
can be distinguished, forming an inflection point (7 = 0.5), equivalent to a sample of AgHSb2O6-2H2O composition obtained by co-precipitation [3]. This indicates the staging of ion-exchange processes in the compounds under study, apparently due to the partial substitution of counterions (hydrated oxonium ions) at first, and later of single isolated protons by silver ions [4]. An analysis of experimental data on the structural changes in hydrated ion-exchange silver forms of PAA with partial and/or complete substitution of proton groups by silver ions made it possible to propose a model for the occupation of metal ions within sp. gr. Fd-3m (Fig. 5).
Fig. 5. A fragment of the pyrochlore-type structure unit cell in the polyhedral interpretation of the silver forms of PAA
According to [3], the framework of the structure is formed by Sb5+ and O2- ions statistically located in the 16c- and 48 f -positions respectively (Fig. 5). Each Sb5+ cation is surrounded by six O2- anions, whereas each oxygen anion belongs to two Sb5+ cations at a time and acts as a link between two antimony-oxygen octahedrons of [Sb(V)O6/2]- — composition with the total charge equal to -1. To compensate the excess negative charge and maintain electroneutrality of the system it is necessary to have positively charged particles — hydrated oxonium ions (H3O+) — and/or neutral water molecules H2O, occupying 16d- and 8b-positions in the structure respectively.
It follows from X-ray diffraction data that substitution of proton groups by silver ions in hydrated PAA compounds leads to weight profile factor error reduction (wRp < 15%) in the case of 16d - and 8b - positions occupation by silver ions (Fig. 5). As it is known in [4] silver ions can occupy only 16d - positions in the structure, however such distribution of ions at crystallographic positions within Fd-3m pr. gr. increases the parameter value (wRp > 20%). The difference in the value of the correlation factor (wRp), apparently, is caused by differences in the technology of compound synthesis.
In the proposed model involving the Rietveld method for PAA and its silver forms of Ag2YH2-2ySb2O6-2H2O composition, the theoretically calculated X-ray diffraction patterns coincide with the experimental ones (Fig. 6-8). In this case, the correlation factor does not exceed 10%. The refined parameters of the crystal structure of the compounds under study, obtained by the mechanochemical activation of the initial
Fig. 6. Experimental, theoretical and difference X-ray diffraction patterns of hydroantimonate of H2Sb2O6-2H2O composition
Fig. 7. Experimental, theoretical and difference X-ray diffraction patterns of silver hydroantimonate of Ago,sHi.2Sb2O6-2H2O composition
components, do not contradict the data of [6], in which ion-substituted silver forms of PAA were synthesized by co-precipitation in solution. The structural analysis of the occupation of crystallographic positions by metal ions in accordance with the proposed model showed that most of silver ions (82.3%) substituting protons/proton groups are located in 16d-positions of the PAA structure. In this case, the remaining amount of silver ions (17.7%) is distributed over 8b-positions. This ion occupation is possible due to the close location of the conduction channels (16d- and 8b-positions) in a pyrochlore-type structure [10], which facilitates the transport of silver ions along a continuous network of hydrogen bonds in the crystal lattice of compounds obtained as a result of the mechanochemical synthesis. Thus, during ion exchange, the sorption of silver ions
2©, degree
Fig. 8. Experimental, theoretical and difference X-ray diffraction patterns of silver hydroantimonate of Ag2Sb2O6^2H2O composition
occurs not due to the chemical interaction between PAA and silver nitrate, but due to the mutual diffusion of metal ions into the bulk of the crystal structure and proton groups (hydrated oxonium ions) from the structure respectively. The interdiffusion of these ions, which have different ionic radii but the same charge, leads to a change in the basis of the unit cell of a pyrochlore-type structure, while the local electrical neutrality and symmetry of the crystal lattice are preserved. The occupation of metal ions in the structure is indicated not only by a change in the value of the unit cell (a) parameter but also by the extinction of the reflections group with odd indices (hkl), similar to [10], conditioned by both the arrangement of silver ions and an increase in the occupancy of crystallographic positions by these ions having a larger X-ray scattering factor [6] than the counterion (hydrated oxonium ion). The occupation degree of 16d-positions with silver ions is determined by the linear dependence of the unit cell (a) parameter on the content of silver ions (7) in the composition of compounds, and the monotonic decrease in the (a) value, apparently, is conditioned by the fact that silver ions occupy nonequivalent crystallographic positions of the structure (8b-positions), as in the KWSbO6 compound [10]. Based on the experimentally obtained and theoretically calculated structural analysis data in the hydrated PAA compounds, the values of interionic distances R1 (Ag — O) and R2(Sb — O) as functions of the substitution degree (7) were determined (Fig. 9).
As can be seen from the Fig. 9, the curves R1 and R2, characterizing the change in the distances between metal ions on the substitution degree (7) in the samples under study, have a nonlinear form. In this case, the calculated values of Ri = 0.252 (for Ag2Sb2O6-2H2O composition at 7 = 1.0) and R2 = 0.210 (for H2Sb2O6-2H2O composition at 7 = 0.0) are in good agreement with the data of other works [11]. The presented dependence demonstrates that, in the range of change (0.2 < 7 < 0.5), an increase in the value of R1 (a) is observed, correlating antibatically with the change in the values of the unit cell (a) parameter (Fig. 3) and, at the same time, there is a symbatic change in R2 (b) and (a) parameter (Fig. 3). An increase in R1 and a decrease in R2 in a given concentration range indicate the substitution of hydrated oxonium ions (H3O+) by silver ions in the 16d-positions, which, in turn, is conditioned by the
Fig. 9. Change in interionic distances Ri(Ag — O) (a) and R2(Sb — O) (b) on the substitution degree (7) in PAA hydrated compounds of Ag2YH2-2ySb2O6-2H2O composition in the concentration range 0.2 ^ 7 ^ 1.0
decrease in the diameter of the hexagonal channels of the structure. A further increase in the substitution degree (7) in the range of (0.6 < 7 < 1.0) leads to an observed decrease in the value of R1 and, at the same time, a growth of R2, which indicates a partial substitution of single isolated protons by silver ions in the structure of compounds. The antibatic change in the values of Ri and R2 determines the stability of the pyrochlore framework formed by antimony-oxygen octahedra.
Conclusion
The paper has presented the results of the mechanochemical synthesis of hydrated compounds based on polyantimonic acid in a wide range of ionic substitution of proton-containing groups for silver ions. The conducted studies of synthesized ion-substituted silver forms allow to conclude that the symmetry of the crystal lattice is not changed by the ion exchange and that the sorption of silver ions by polyantimonic acid is volumetric in nature and occurs due to the mutual diffusion of silver ions and protonic groups (hydrated oxonium ions). The monotonic dependence of the unit cell parameter and the regular decrease in the relative intensity of the reflections group with odd indices on the ionic substitution degree indicate that the silver forms of PAA should be considered as a solid solution of hydrated silver antimonates of Ag2YH2_27Sb2O6•2H2O composition 0.0 < y < 1.0. Within the space group Fd-3m, based on the data of X-ray diffraction analysis, a model of occupation by metal ions has been proposed, according to which silver ions in the structure of PAA compounds are statistically distributed over the 16d-and 8b-positions respectively. It has been demonstrated that the introduction of silver ions into the PAA structure leads to a decrease in the distance between the Sb5+ and O2- ions, and, as a result, to a decrease in the diameter of the hexagonal channels of the structure, which contributes to the transport of protons along the system of conduction channels (16d- and 8b-positions) in the crystal lattice ion-substituted forms of PAA.
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Article received 22.02.2023.
Corrections received 22.08.2023.
Челябинский физико-математический журнал. 2023. Т. 8, вып. 4- С. 605-616.
УДК 546.865-31:543.442.3:543.573:538.911:538.956 Б01: 10.47475/2500-0101-2023-8-4-605-616
МЕХАНОСИНТЕЗ ИОНООБМЕННЫХ СЕРЕБРЯНЫХ ФОРМ ПОЛИСУРЬМЯНОЙ КИСЛОТЫ
Ф. А. Ярошенко, В. А. Бурмистров, А. Е. Силова, Ю. А. Лупицкая", Е. М. Филоненко, П. В. Тимушков, М. Н. Ульянов, С. И. Саунина
Челябинский государственный университет, Челябинск, Россия а [email protected]
Исследована возможность синтеза ионзамещённых форм гидратированных антимо-натов серебра Ag27И2-27Sb2O6•2H2O методом механохимической активации компонентов неорганической смеси полисурьмяной кислоты (ПСК) состава H2Sb2O6-2И2О и нитрата серебра в интервале изменения концентраций (7) от 0.0 до 1.0. Приведены результаты исследований фазового (химического) состава синтезированных соединений и изучены их структурные особенности. Методом Ритвельда проведено уточнение параметров кристаллической решётки гидратированных ионзамещённых серебряных форм ПСК со структурой типа пирохлора и предложена модель заселения ионами металлов по кристаллографическим позициям: каркас структуры соединений формируют 16с- и 48/-позиции, в которых статистически располагаются Sb5+ и О2-, гидратированные ионы оксония (ИзО+) и ионы серебра занимают 16^- и 86-позиции соответственно. Показано, что синтез серебряных форм ПСК предпочтительно проводить методом механосинтеза, результатом которого является полное замещение протонных группировок на ионы серебра в структуре соединений.
Ключевые слова: полисурьмяная кислота, механохимический синтез, гидратированные ионзамещённые формы, структура типа пирохлора, ионообменные свойства.
Поступила в редакцию 22.02.2023. После переработки 22.08.2023.
Сведения об авторах
Ярошенко Федор Александрович, кандидат химических наук, доцент кафедры химии твёрдого тела и нанопроцессов, Челябинский государственный университет, Челябинск, Россия.
Бурмистров Владимир Александрович, доктор физико-математических наук, профессор, декан химического факультета, Челябинский государственный университет, Челябинск, Россия.
Силова Длена Евгеньевна, младший научный сотрудник, Челябинский государственный университет, Челябинск, Россия.
Лупицкая Юлия Александровна, кандидат физико-математических наук, доцент, доцент кафедры физики конденсированного состояния, Челябинский государственный университет, Челябинск, Россия; [email protected].
Филоненко Елена Михайловна, младший научный сотрудник кафедры физики конденсированного состояния, Челябинский государственный университет, Челябинск, Россия.
Тимушков Петр Викторович, ассистент кафедры химии твёрдого тела и нанопроцес-сов, Челябинский государственный университет, Челябинск, Россия.
Работа поддержана грантом Российского научного фонда, проект № 23-23-00140.
Ульянов Максим Николаевич, кандидат физико-математических наук, доцент кафедры общей и теоретической физики, Челябинский государственный университет, Челябинск, Россия.
Саунина Светлана Ивановна, кандидат физико-математических наук, доцент, доцент кафедры физики конденсированного состояния физического факультета, Челябинский государственный университет, Челябинск, Россия.
