radiation. The radiation-chemical yields of CO and. CO2 were determined by ... Institute of Surface Chemistry, National Academy of Sciences of Ukraine,.
High Energy Chemistry, Vol. 36, No. 3, 2002, pp. 157–162. Translated from Khimiya Vysokikh Energii, Vol. 36, No. 3, 2002, pp. 185–190. Original Russian Text Copyright © 2002 by Doroshenko, Kaurkovskaya, Yakubenko, Pobokin, Entinzon, Ogenko.
RADIATION CHEMISTRY
Radiation-Induced Transformations of Iron(II) Formate V. N. Doroshenko, V. N. Kaurkovskaya, E. P. Yakubenko, D. I. Pobokin, I. R. Entinzon, and V. M. Ogenko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, ul. generala Naumova 17, Kiev, 03164 Ukraine Received December 10, 1999
Abstract—The solid-phase decomposition of the iron formate crystal hydrate Fe(HCOO)2 · 2H2O under exposure to 60Co γ-rays or 3.5-MeV electrons was studied. It was found that the irradiation of this salt to absorbed doses of 0.1–2 MGy resulted in the radiolysis of water of crystallization and the HCOO– anion and in the reduction or oxidation of the Fe2+ cation. The composition of the solid-phase (γ-Fe, α-Fe, FeO, γ-Fe ε-Fe2O3, Fe3O4, and FeCO3) and gaseous (H2O, CO, CO2, HCOOH, and CH4) radiolysis products of the substance was determined.
The metal salts of weak organic acids (oxalates, acetates, and formates) and metal hydroxides are highly sensitive to the action of ionizing radiations, and they decompose to form metal clusters, oxides, and gaseous products [1–6]. Using nickel formate as an example, we found [5] that the irradiation of salt crystal hydrates (absorbed doses of 0.1–2.5 MGy) caused the degradation of crystals and the formation of finely dispersed Ni and NiO particles. It was also found that the solid-state radiolysis of the compound at room temperature included the simultaneously occurring processes of dehydration and radiolyses of the cationic and anionic moieties of the nickel formate molecule. However, it is unknown whether such behavior is common to formates, in particular, iron formates. The aim of this work was to study the radiolysis of the iron formate crystal hydrate Fe(HCOO)2 · 2H2O under exposure to ionizing radiations in a dose range of 0.01–2 MGy. EXPERIMENTAL A fine crystalline powder of reagent-grade Fe(HCOO)2 · 2H2O was used in this study. The samples dried at 350 K for 3 h were irradiated with 60Co γ-rays (I = 1.37 × 1017 eV g–1 s–1) and accelerated electrons with E = 3.5 MeV (I = 1.85 × 1017 eV g–1 s–1). The dose rates were measured with a ferrous sulfate dosimeter (60Co γ-radiation) and a solution of chlorobenzene in ethanol (ELU-4 electron accelerator) [7]. The irradiated samples (the sample mass is given in parentheses) were examined by gravimetry (30 mg), mass spectrometry (2 mg), X-ray diffraction, and elemental analysis (0.1 g) to determine the degree of conversion, the phase composition, and the chemical composition of the resulting products. Thermogravimetry was performed in a dynamic mode at a heating rate of 3 K/min at 300–670 K using a vacuum adsorption unit with a McBain quartz
balance. The specific surface areas of irradiated samples were measured chromatographically using the nitrogen desorption technique. The powder diffraction patterns of Fe(HCOO)2 · 2H2O were measured on a Dron-3 X-ray diffractometer using CuKα and CoKα radiation. The radiation-chemical yields of CO and CO2 were determined by chromatographing on a column packed with zeolite NaX. The composition of gaseous radiolysis products and the kinetics of their release from the bulk of irradiated substances on heating at a rate of 5 K/min within the temperature range 293–700 K were found by mass spectrometry as described in [5]. The kinetics of dissolution of unirradiated and irradiated salts were studied by measuring the mass loss of samples kept in paper bags immersed in vessels with distilled water for 24 h at 370 K. The samples of the liquid phase were chemically analyzed (by titrimetry and 2– photocolorimetry) for the ions HCOO–, CO 3 (0.05– 0.005 M NaOH; phenolphthalein), Fe2+, and Fe3+ (sulfosalicylic acid and potassium permanganate) [8]. The error in the determination of salt solubility was no higher than 5% upon holding the samples after irradiation for three months. RESULTS AND DISCUSSION The efficiency of the action of ionizing radiations on the decomposition of salt crystals was evaluated by comparing the thermal stabilities of irradiated and unirradiated samples (Fig. 1) in the temperature range 293– 600 K. According to thermogravimetric data, a sample of the unirradiated salt (Fig. 1, curve 1) exhibited two stages of thermal decomposition: the removal of water of hydration at 373–423 K and the decomposition of the salt at 575 K. In irradiated samples (Fig. 1; curves 2, 3), these regions were shifted toward lower temperatures as the absorbed dose was increased. The above changes can be explained by the radiolysis of the anion and
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T, K Fig. 1. Thermogravimetric curves plotted as the temperature dependence of the degree of decomposition α for (1) unirradiated Fe(HCOO)2 · 2H2O and γ-irradiated Fe(HCOO))2 · 2H2O at doses of (2) 0.8 and (3) 1.3 MGy (dynamic mode).
hydration components of the iron formate molecule and by the accumulation of the gaseous products of radiation-chemical reactions in the bulk (internal cavities) of crystals followed by the liberation of these products upon heat treatment [5, 9]. To determine the chemical composition of the gaseous products of iron formate decomposition upon radiolysis and to compare them with the thermolysis products, the kinetics of release of molecular ions in the temperature range 293–600 K was studied by mass spectrometry. Figure 2 demonstrates the temperature dependence of the rates of release of main molecular ions in the thermal desorption mass spectra of unirradiated and irradiated Fe(HCOO)2 · 2H2O. In both cases, the ions were released in temperature ranges that correspond to the steps of hydration component removal (373 K) and salt decomposition per se (576 K). We found that both unirradiated and irradiated samples produced the same composition of molecular ions at + m/e: 16 ( CH 4 ), 17 (OH+), 18 (H2O+), 19 (H3O+), 28 (CO+), +
29 (COH+), 43 (CH3CO+), 44 ( CO 2 ), 45 (COOH+), and 46 (HCOOH+). The presence of high concentrations of hydronium ions H3O+ along with H2O+ and OH+ ions over a temperature range of 323–423 K (Fig. 2a) indicates that the salts Fe(HCOO)2 · 2H2O and Fe(HCOO)2 · (OH–)(H3O+) undergo dehydration via molecular and hydrolytic mechanisms. Considerable concentrations of hydrate fragments (OH+, H3O+, and H2O+) were detected in the thermal decomposition products of the unirradiated salt at 576 K. This is likely due to the degradation of Fe(HCOO)2(OH)– · (H3O)+ molecules. The peaks of ions at m/e = 17, 18, and 19 at 576 K were not detected in the salt held at 430 K.
The mass spectrometric data show that the water content of samples decreases after irradiation in the range of dehydration temperatures, whereas the OH+ ion is primarily released from the irradiated salt at 576 K (Fig. 2b). This ion can be formed in the decomposition of hydroxide-containing iron compounds as intermediates of radiation-chemical reactions. These results are consistent with the thermogravimetric data, and thus suggest the occurrence of the radiation-chemical dehydration of Fe(HCOO)2 · 2H2O in a radiation field. Such radiation-chemical dehydration was observed previously for nickel formate [5]. The thermal desorption mass spectra of irradiated (0.8 MGy) iron formate samples (Figs. 2d, 2f) in the range of dehydration temperatures exhibited the release of the molecular ions HCOO+ (45 amu), HCOOH+ + (46 amu), and CO 2 (44 amu). It is likely that these ions were retained in internal cavities, the degradation sites of salt crystals. The decomposition of the irradiated salt at 576 K was different from that in an unirradiated sample. The difference consists in changes in the quantitative composition of the released molecular ions CO+ + (28 amu), HCO+ (29 amu), and CO 2 (44 amu) and in a decrease in the fraction of HCOO+ (45 amu) and HCOOH+ (46 amu), which resulted from the radiolysis of the HCOO– anion. Figure 3 demonstrates the X-ray diffraction patterns of γ-irradiated Fe(HCOO2) · 2H2O samples (absorbed doses of 0.2, 0.4, and 1 MGy) compared with the X-ray diffraction pattern of an unirradiated sample (dotted lines). The characteristics of diffraction reflections with interplanar spacings d (0.496, 0.471, 0.462, 0.434, 0.346, 0.334, 0.266, 0.261, 0.247, 0.220, 0.207, and 0.195 nm) and relative intensities I in Fig. 3 unambiguously characterize Fe(HCOO)2 · 2H2O [10]. The reflections with d = 0.884 and 0.617 nm correspond to β-Fe(HCOO)2, which was detected in both initial and irradiated samples. Note that transition metal formates have a monoclinic structure, and the unit cell parameters of Fe(HCOO)2 · 2H2O are a = 0.861, b = 0.734, and c = 0.9274 nm and β = 96°10′ [11]. The action of ionizing radiations on iron formate crystals resulted in either the appearance of new reflections due to decomposition products in the X-ray diffraction patterns (Fig. 3) or a change in the intensity ratios between the diffraction lines of the main substance. We identified α-Fe (d = 0.202, 0.143, 0.117, and 0.0907 nm; cubic structure with a = 0.2866 nm), FeO (d = 0.241, 0.214, 0.151, 0.129, 0.107, 0.0984, and 0.0959 nm with a = 0.4357 nm), γ-Fe2O3 (d = 0.295, 0.211, 0.185, 0.160, and 0.147 nm), Fe3O4 (d = 0.297, 0.253, 0.272, 0.171, and 0.121 nm), and FeCO3 (d = 0.279, 0.213, 0.196, 0.173, 0.101, and 0.097 nm) [10] as new phases. The above products were detected upon the radiolysis of iron formate at absorbed doses higher than 0.1 MGy. However, reflections due to FeCO3 along HIGH ENERGY CHEMISTRY
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I, arb. units 1
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5
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7 20
(f)
(e)
7 8
15
8
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T, K Fig. 2. Temperature dependence for the rate of release of the molecular ions (1) H2O+, (2) H3O+, (3) OH+, (4) CO+, (5) COH+, +
(6) CO 2 , (7) COOH+, and (8) HCOOH+ for (a, c, e) unirradiated and (b, d, f) irradiated Fe(HCOO)2 · 2H2O according to thermal desorption mass-spectrometric data (dose of 0.8 MGy).
with the initial and dehydrated salts predominated in the diffraction patterns at doses of 0.1–0.5 MGy, whereas reflections due to Fe and Fe2O3 reliably appeared at doses higher than 0.5 MGy and the oxide Fe3O4 was detected at doses of 1 MGy or higher. Along with the diffraction characteristics of α-Fe, the X-ray diffraction patterns exhibited reflections that can be attributed to γ-Fe (d = 0.207, 0.180, 0.126, and 0.108 nm). Moreover, the presence of ε-Fe2O3 in the solid products was observed (a set of diffraction lines with d = 0.298, 0.245, 0.224, 0.174, and 0.152 nm). Note that a great number of diffraction lines due to several modifications HIGH ENERGY CHEMISTRY
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of metals, oxides, and initial salts made the interpretation of X-ray diffraction patterns difficult to perform at equal values of d. Therefore, the intensity ratios between the corresponding sample lines and tabulated data [10] were important in the identification of FeCO3, metals, and oxides, especially, at low doses. Upon the thermolysis of the iron formate crystal hydrate in air, the starting Fe(HCOO)2 · 2H2O and its decomposition products (α-Fe(HCOO)2, β-Fe(HCOO)2, and γ-Fe2O3) were detected by X-ray diffraction analysis. The above composition is qualitatively different from the composition of the solid products of radiolysis. This effect is
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(b)
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0.250
0.175
0.140 d, nm
0.116
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Fig. 3. X-ray diffraction patterns of γ-irradiated Fe(HCOO)2 · 2H2O at absorbed doses of (a) 0.2, (b) 0.6, and (c) 1 MGy as compared with the X-ray diffraction pattern of the initial compound (dotted lines).
likely due to the fact that the decomposition of iron formate under exposure to ionizing radiation includes a step of the formation of iron carbonate FeCO3 and its subsequent decomposition into the metal and oxides. Note that under exposure to accelerated 3.5-MeV elec-
trons the rates of these reactions increased because of sample heating in the course of irradiation. Therefore, the reflections of the metal polymorphs and oxides were more pronounced in the X-ray diffraction patterns of iron(II) formate dihydrate samples. HIGH ENERGY CHEMISTRY
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I, arb. units (c)
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Fig. 3. (Contd.)
Figure 4 shows a kinetic curve for the solubility of γ-irradiated Fe(HCOO)2 · 2H2O (curve 1) in distilled water in the salt concentration (wt %)–absorbed dose (MGy) coordinates. These data show that the amount of unreacted salt in the solution decreases with absorbed dose. The concentration of HCOO– ions changes in a similar manner. The chemical analysis of the solution for Fe2+ ions demonstrated nonmonotonic changes in the concentration with dose. These changes are likely due to the transformations of Fe2+ and Fe(HCOO)+ into Fe(OH)(HCOO) and Fe(OH)2 and then into Fe(OH)3 with the precipitation of corresponding colored solids [8]. The radiation-chemical yield G of Fe(HCOO)2 · 2H2O decomposition was determined from a linear portion of the salt concentration (wt %)–radiation dose (MGy) plot. It was equal to 2.4 ± 0.1 ion/100 eV. According to chromatographic analysis, the mass loss of Fe(HCOO)2 · H2O samples irradiated in a vacuum at room temperature is a linear function of absorbed dose (Fig. 4, line 2). These data refer to gaseous products, which freely leave salt crystals in the course of irradiation. They primarily consist of CO2, CO, CH4, H2O, and the derivatives of the anion. The radiation-chemical yields of CH4, CO, and CO2 were equal to 0.02, 0.01, and 1.04 molecule/100 eV, respectively. HIGH ENERGY CHEMISTRY
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wt % 100
80
60
wt % 4
1 2 40
3 2
20 1
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Fig. 4. Dose dependence for (1) the solubility and (2) mass loss due to the release of gaseous radiolytic products by a Fe(HCOO)2 · 2H2O sample.
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We found that the specific surface areas of unirradiated and irradiated (to a dose of 0.3 MGy) Fe(HCOO)2 · 2H2O were 8 and 80 m2/g, respectively. For the solid thermolysis product, Ssp = 46 m2/g. Obviously, irradiation at low doses allows a considerable dispersion of iron formate to be achieved. In general, the salient features of product formation in the solid-state thermolysis and radiolysis of Fe(HCOO)2 · 2H2O are consistent with those for nickel formate [5]. The similarity consists in that, regardless of the initiation mode of formate degradation, the salts undergo dehydration and decomposition to form solid (M and MexOy) and gaseous (H2O, CO, CO2, HCOOH, and CH4) radiolytic products. However, iron(II) formate is characterized by the formation of iron carbonate as an intermediate decomposition product after γ-irradiation at room temperature. Because of this, the γ-radiolysis of the above formate and its electron-beam radiolysis differ in the efficiency as a result of temperature differences in the corresponding experiments. In summary, the action of ionizing radiations causes the radiation-chemical transformations of the crystallization water and cationic and anionic moieties of Fe(HCOO)2 · 2H2O. The formation of solid products of radiolysis (metals and oxides) opens up possibilities for the directed radiation-chemical synthesis of specific metal-containing adsorbents and catalysts.
2. Gordeeva, V.A., Egorov, E.V., Zhabrova, G.M., Kadenatsi, B.M., Kushnerev, M.Ya., and Roginskii, S.Z., Dokl. Akad. Nauk SSSR, 1961, vol. 136, no. 6, p. 1364. 3. Erofeev, B.V. and Protashchik, V.A., Trudy II Vsesoyuznogo soveshchaniya po radiatsionnoi khimii (Proc. II All-Union Conf. on Radiation Chemistry), Moscow: Akad. Nauk SSSR, 1962, p. 703. 4. Zhabrova, G.M., Kadenatsi, B.M., Zvonov, N.V., Egorov, E.V., Azizov, T.S., Batalov, A.A., Gordeeva, V.D., and Glazunov, P.Ya., Kinet. Katal., 1963, vol. 3, no. 4, p. 610. 5. Doroshenko, V.N., Yakubenko, E.P., Denisenko, V.A., Kaurkovskaya, V.N., Entinzon, I.R., and Ogenko, V.M., Khim. Vys. Energ., 1996, vol. 30, no. 5, p. 356 [High Energy Chem. (Engl. transl.) 1996, vol. 30, no. 5, p. 320]. 6. Nelipko, S.A., Fizicheskie svoistva malykh metallicheskikh chastits (Physical Properties of Small Metal Particles), Kiev: Naukova Dumka, 1985, p. 248. 7. Praktikum po radiatsionnoi khimii (Laboratory Works on Radiation Chemistry), Saraeva, V.V., Ed., Moscow: Mosk. Gos. Univ., 1982, p. 216. 8. Zharovs’kii, F.G., Pilipenko, A.T., and P’yatnits’kii, I.V., Analitichna khimiya (Analytical Chemistry), Kiev: Vishcha Shkola, 1982, p. 543. 9. Ryabykh, S.M., Khim. Fiz., 1990, vol. 9, no. 2, p. 191.
REFERENCES
10. Mirkin, L.I., Spravochnik po rentgenostrukturnomu analizu polikristallov (Handbook on X-ray Diffraction Analysis of Polycrystals), Moscow: Fizmatgiz, 1961, p. 893.
1. Supe, A.A., Cand. Sci. (Chem.) Dissertation, Riga: Latvian State Univ., 1987, p. 16.
11. Hoy, J.R., Barrous, S.de.S., Barrous, F.de.S., and Fridberg, S.A., J. Appl. Phys., 1963, vol. 14, no. 3, p. 936.
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