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CHEMICAL PROBLEMS 2022 no. 3 (20) ISSN 2221-8688
277
UDC 621.315.592
PARAMAGNETIC CENTERS IN GAMMA IRRADIATED (RaO)x(SiO2)y SAMPLES WITH
ADSORBED WATER
Z.A. Mansimov
Institute of Radiation Problems, 9, B.Vahabzadeh str., AZ1143 Baku, Azerbaijan e-mail: zaur.mansimov@mail. ru
Received 06.06.2022 Accepted 29.08.2022
Abstract: The y- irradiated 77K samples (RaO)x(SiO2)y with adsorbed water were studied using EPR spectroscopy, and EPR spectrum with g=2.0028 and hyperfine splitting constant A(H)=505.3 G belonging to hydrogen atoms was registered. EPR, presumably attributed to paramagnetic hole (=Si-O)+, electronic (=Si-)- centers and products of their interaction with water molecules spectra was registered. It is assumed that the main part of atomic hydrogen was oxidized to the HO2' radical, and some of the H atoms participate in surface radiative hydrogenation reactions with the formation of hydrogen-containing radicals when irradiated samples were heated from 77 K to room temperature. It is also assumed that 'OH radicals are formed on the surface of y--irradiated at 77K (RaO)x(SiO2)y samples.
Keywords: (RaO)x(SiO2)y system, y-radiation, electron paramagnetic resonance, hydrogen atom, electron and hole centers
DOI: 10.32737/2221-8688-2022-3-2 77-281
Introduction
One of the most important and highly productive processes in nuclear-hydrogen energy is the fragment radiolysis. Fragments formed as a result of nuclear transformations perform 80-85% of the conversion process. Therefore, under the conditions of nuclear transformations, the use of fragmentation energy and radiant radiation for the direct production of hydrogen is one of the topical problems [1,2]. One of the decay products of uranium isotopes is radium. Radium is an alpha active isotope, like uranium. Therefore, the heterogeneous radiolysis of water in the presence of radium is of great model value.
Besides, natural isotopes of radium are widely distributed in the environment in the form of uranium fission products, for example, in various rocks, bottom sediments of oil and gas fields. Chemical transformations under the effect of radiation in terms of uranium-containing rocks contacting with water are relevant from the point of view of studying the mechanism of geochemical processes [3,4].
In this work, a silicate system, which is closer to the natural environment and has a high radiation resistance, was taken as a model; at that, various amounts of natural radium isotopes were introduced.
Experimental
The radium-silicate samples with size >20 p,m were prepared and filled into thin-walled "LUCH" glass ampoules, sealed and vacuumed up to 10-2 Pa. The samples were irradiated for 5, 10, 15 hours at a temperature of
77K using a 60Co isotope source with a dose rate of D=0.38 Gy/s). Measurements were made before and after influence of radiation at liquid nitrogen and room temperature, using an EMX Plus, Bruker. For more accurate determination
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CHEMICAL PROBLEMS 2022 no. 3 (20)
of the spectra, the spectrum accumulation method was used to increase the signal/noise ratio. The Cr3+ in ruby single crystals as reference sample was used to determine the number of paramagnetic centers. The EPR
spectrum of the reference sample at room and liquid nitrogen temperatures was taken and a two-step integration process was performed, and the amount of paramagnetic centers (Nx) was calculated by the following formula
N(XI
N( Etalon) SC*t)-Q[ft-)' L J AFH '
where, N(x) is the number of paramagnetic center in the samples with mass Am, S(x), S(et) numerical values of the areas under the absorption curve of the test and reference
samples, respectively, and Q(x), Q(et) -correction coefficients for the test and reference samples, respectively.
Results and discussion
The EPR spectrum of the sample (RaO)x(SiO2)y registered at 77 K is presented in Fig.1.
a)
b)
Fig.1. EPR spectrum at T=77K of irradiated for 15 hours (RaO)x(SiO2)y sample contacted with water (the activity A = 6100 Bk/g; mass of sample was 4*10-2 g) (a); central part of this spectra (b).
The EPR spectrum shown in Fig.1 consists of the superposition of a clearly distinguished doublet belonging to the EPR spectrum of hydrogen atoms and a number of signals in the central part of the spectrum. The recorded EPR spectrum of hydrogen atoms is characterized by g- factor =2.0028 and hyperfine splitting constant A(H^) = 505.3 G. The width of each hyperfine component of the EPR spectrum of hydrogen atoms is AH = 2.08 G. The central part of the EPR spectrum
(Fig.1,b) consists of the superposition of, at least, two signals. The main paramagnetic particles in y- irradiated silicon dioxide at 77K are exactly atomic hydrogen and can be hole (=Si-O)+ and electronic (=Si-)- centers. The total amount of these paramagnetic centers is approximately more than 60% of the total amount of paramagnetic centers. When the irradiated samples are heated, the main part of atomic hydrogen is oxidized to the HO2 radical, and a part of the H" atoms participate in the
reactions of radiation hydrogénation of the surface with the formation of hydrogen-containing radicals. In the process of radiolysis, during the homolytic dissociation of water
molecules located in the pores of silicon dioxide and its surface, two paramagnetic particles - a hydrogen atom and a hydroxyl radical are formed:
H2O -y- irradiation^ H + •QH
However, the EPR spectrum of the hydroxyl radical was not detected in any radiolyzed SiO2 matrix. According to [6], HO^ was highly reactive and its EPR spectrum cannot always be
recorded due to the degenerate orbital state. Fig. 2 shows the dependence of the concentration of paramagnetic centers as a function of irradiation time.
o.s - »
1 1 • I 11 a u H
Fig. 2. Dependence of the concentration of paramagnetic centers in the (RaO)x(SiO2)y with adsorbed water due to the central part of the EPR spectrum shown in Fig. 1 on the irradiation time
(D = 0.38 Gy/sec, T=77 K)
As can be seen from Fig. 2, the concentration of paramagnetic centers belong to the central part of the EPR spectrum which, as shown in Fig.2, increases as the irradiation time of the sample increases. It is assumed that under
the effect of gamma irradiation the electron and hole centers are formed and these formed centers interact with adsorbed water according to the following scheme [7,8]:
(RaO)x(SiO2)y^(RaO)x(SiO2)y+n+p
n+LA^L„ P+Ld^LP H2Os+p^ H2Os* H2Os+e^ H2Os*^H+OH H+S^Hs OH+S^OHs
where LA, LD, Ln and LP are free electrons and hydrogen and hydroxyl groups and their holes and their localized states (lines b and c in localized states. the EPR spectrum), H, OH, Hs and OHs are
Conclusion
The paramagnetic centers in y- irradiated these centers as a function of the irradiation at 77K samples (RaO)x(SiO2)y were identified time was studied. It was suggested that the main and the dependences of the concentration of part of atomic hydrogen is oxidized to the HO2^
radical, and a part of the H" atoms participate in the reactions of radiation hydrogenation of the surface with the formation of hydrogen-containing radicals during the heating from 77K to room temperature of the irradiated samples.
The formation of OH radicals is also suggested on the surface of y- irradiated at 77K (RaO)x(SiO2)y samples.
References
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3. Choppin G., Rydberg J. Nuclear Chemistry: Fundamentals of Theory and Applications. Moscow: Energoatomizdat Publ., 1984. 304 p. (In Russian).
4. Hala I., Navratil J. Radioactivity, Ionizing Radiation and Nuclear Power Engineering. Edited by Myasoedov B.F., Kalmykov S.N. Moscow, 2013. 432 p. (In Russian).
5. Weil J.A., Bolton J.R. and Wertz J.E. Electron Paramagnetic Resonance: Elementary Theory and Practical Applications, Wiley, New York,1994, 592 p.
6. Kajihara K., Hirano M., Skuja L. and Hosono H. Interstitial OH Radicals in F2-Laser-Irradiated Bulk Amorphous SiO2. J. Phys. Chem. B. 2006, vol. 110, no. 21, pp. 10224-10227.
7. LaVerne J.A. OH-radicals and oxidizing products in the gamma-radiolysis of water. Radiation Research. 2000, vol. 153, pp. 196200.
8. LaVerne J.A., Pimblott S.M. New mechanism for H2 formation in water. Journal of Physical Chemistry A. 2000, vol. 104, pp. 9820-9822.
SU iL9 ADSORBSiYA OLUNMU§, QAMMA §ÜALANMANIN T9SiREN9 M9RUZ QALAN (RaO)x(SiO2)y SiSTEMiND9 PARAMAQNiT M9RK9ZL9RiN T9DQiQi
Z.9. Mansimov
AMEA Radiasiya Problemhri institutu AZ1143, Baki, B.Vahavzadd küg.,9; e-mail:zaur.mansimov@mail.ru
Xülasa: 77K temperaturda y-§üalmaya maruz qalmi§ (RaO)x(SiO2)y+H2O sisteminda EPR spektroskopiya metodundan istifada edarak tadqiqatlar aparilmi§ va g = 2.0028 va ifrat inca qurulu§lu A(H) = 505.3 G olan hidrogen atomlarina maxsus olan EPR spektri müayyanla§dirilmi§dir. Eynu zamanda spektrin markazi hissasi tadqiq edilarak paramaqnit de§ik (ESi-O)+ va elektron (=Si-)- markazlarina aid edilan va onlarin su molekullari spektrlari ila qar§iliqli tasir mahsullari qeyda alinmi§dir. Müayyan edilmi§dir ki, atomar hidrogeninin asas hissasi HO2* radikalina oksidla§ir va H* atomlarinin bir hissasi isa §üalanmi§ nümunalarin 77 K-dan otaq temperaturuna qadar qizdirildiqda sathi radiasiyali hidrogenla§ma reaksiyalarinda i§tirak edarak hidrogen tarkibli radikallar amala gatirir.
Ham^nin müayyan edilmi§dir ki, 77 K temperaturda (RaO)x(SiO2)y nümunalarinda y-§üalanmanin tasirindan nümunalarin sathinda *OH radikallari amala galir.
A?ar sözlar: (RaO)x(SiO2)y sistemi, y-§üalanma, Elektron Paramaqnit Rezonans (EPR) metodu, elektron va de§ik markazlari.
ПАРАМАГНИТНЫЕ ЦЕНТРЫ В ГАММА-ОБЛУЧЕННЫХ ОБРАЗЦАХ (RaO)x(SiO2)y С АДСОРБИРОВАННОЙ ВОДОЙ
З.А. Мансимов
Институт Радиационных Проблем Национальной АН Азербайджана AZ1143 Баку, ул. Б.Вахабзаде, 9, e-mail:zaur.mansimov@mail.ru
Аннотация: С использованием метода ЭПР исследованы у-облученные при 77 К образцы (РаО)х(БЮ2)у с адсорбированной водой и идентифицирован спектр ЭПР с g=2.0028 и константой сверхтонкого расщепления А(Н)=505.3 Гс, принадлежащий атомам водорода. Зарегистрированы спектры ЭПР, приписанные, предположительно, парамагнитным дырочным (^ьО)+, электронным (^ь)-центрам и продуктам их взаимодействия с молекулами воды. Высказано предположение, что основная часть атомарного водорода окисляется до радикала НО2, а часть Н атомов участвует в реакциях радиационного гидрирования поверхности с образованием водородсодержащих радикалов при нагреве облученных образцов от 77 К до комнатной. температуры. Предполагается также образование радикалов •ОН на поверхности у-облученных при 77 К образцов ^аО)х(БЮ2)у. Ключевые слова: оксидная система (RaO)x(SiO2)y, у-излучение, электронный парамагнитный резонанс, атом аодорода, дырочные и электронные центры
