17:15 〜 19:15
[SIT18-P01] Relationship between the oxidation state of FeS and its reactivity with hydrogen under high pressure and high temperature conditions
キーワード：高圧実験、X線回折、X線吸収微細構造解析、硫化鉄、水素
Troilite (FeS) is commonly found in primitive meteorites and therefore one of the promising components in the planetary cores. FeS has drawn attention as one of the candidates of hydrogen reservoirsbecause Phase V of FeS (hereafter FeS V), which is a high-pressure and high-temperature phase of FeS, has been reported to be hydrogenated to form FeSHx[1,2]. However, the reactions between hydrogen and FeS V are still under debate because some experiments[3,4] also reported the non-reactive behavior of FeS V with hydrogen. In this study, we focused on the effect of the state of the starting materials on its reactivity with hydrogen at high pressure and high temperature.
Three different iron sulfide samples (Sample-A, Sample-B, and Sample-C) were investigated extensively. Sample-A was synthesized from powder mixtures of iron and sulfur. Sample-B and Sample-C were commercially available reagents used in the previous studies, reporting the hydrogenation of FeS. Angle-dispersive X-ray diffraction profiles indicated that the main phase of each sample was troilite (Fe1-xS, 0≦x≦0.05).
The unit-cell volume of FeS in Sample-A was consistent with that of stoichiometric FeS, with the Fe/S ratios close to 1.00[5]. In contrast, the unit-cell volumes of Sample-B and Sample-C were about 5% lower than that of Sample-A. Those discrepancies suggest the ratio of Fe over S are different: Fe/S values of Sample-B and Sample-C are lower than that of stoichiometric FeS and Sample-A. Those observations suggested that Fe2+ in troilite of Sample-B and Sample-C was partially oxidized to Fe3+. Synchrotron X-ray Fe K-edge absorption spectra were obtained at BL-12C, PF, KEK. The absorption edge energies of each sample showed that the average Fe valence of samples B and C is higher than that of Sample-A, which were consistent with the results of the X-ray diffraction measurements. We also conducted time-resolved X-ray diffraction experiments of FeS with hydrogen at 5 GPa and 1000 K at BL04B1, SPring-8. The unit-cell volume of Sample-B and Sample-C increased about 2 %, while that of stoichiometric Sample-A expanded only 0.5 %. The recovered Sample-B and Sample-C were notably larger than those of starting materials, whereas the unit-cell volume change of troilite in Sample-A before and after the experiments was within the experimental error. The hydrogen-induced volume expansion of iron sulfide is caused by the increase in the Fe/S ratio, and not caused by the incorporation of hydrogen atoms into the crystal structure of iron sulfide. The discrepancy between previous studies on the reactivity of iron sulfide under high pressure and high temperature conditions can be attributed to the different oxidation states of the starting materials.
references
1. Shibazaki Y et al. Earth Planet, Sci. Lett. 2011 Jan 3;301(1–2):153–8.
2. Abeykoon S et al. J. Geophys. Res. Solid Earth., 2023 Sep 1;128(9).
3. Piet H et al. J. Geophys. Res. Planets., 2021 Nov 1;126(11).
4. Iizuka-Oku R et al. Sci. Rep., 2021 Dec 1;11(1).
5. Kusaba K et al. J. Phys. Chem. Solids., 1997 May 19; 58(2):241-246.
Three different iron sulfide samples (Sample-A, Sample-B, and Sample-C) were investigated extensively. Sample-A was synthesized from powder mixtures of iron and sulfur. Sample-B and Sample-C were commercially available reagents used in the previous studies, reporting the hydrogenation of FeS. Angle-dispersive X-ray diffraction profiles indicated that the main phase of each sample was troilite (Fe1-xS, 0≦x≦0.05).
The unit-cell volume of FeS in Sample-A was consistent with that of stoichiometric FeS, with the Fe/S ratios close to 1.00[5]. In contrast, the unit-cell volumes of Sample-B and Sample-C were about 5% lower than that of Sample-A. Those discrepancies suggest the ratio of Fe over S are different: Fe/S values of Sample-B and Sample-C are lower than that of stoichiometric FeS and Sample-A. Those observations suggested that Fe2+ in troilite of Sample-B and Sample-C was partially oxidized to Fe3+. Synchrotron X-ray Fe K-edge absorption spectra were obtained at BL-12C, PF, KEK. The absorption edge energies of each sample showed that the average Fe valence of samples B and C is higher than that of Sample-A, which were consistent with the results of the X-ray diffraction measurements. We also conducted time-resolved X-ray diffraction experiments of FeS with hydrogen at 5 GPa and 1000 K at BL04B1, SPring-8. The unit-cell volume of Sample-B and Sample-C increased about 2 %, while that of stoichiometric Sample-A expanded only 0.5 %. The recovered Sample-B and Sample-C were notably larger than those of starting materials, whereas the unit-cell volume change of troilite in Sample-A before and after the experiments was within the experimental error. The hydrogen-induced volume expansion of iron sulfide is caused by the increase in the Fe/S ratio, and not caused by the incorporation of hydrogen atoms into the crystal structure of iron sulfide. The discrepancy between previous studies on the reactivity of iron sulfide under high pressure and high temperature conditions can be attributed to the different oxidation states of the starting materials.
references
1. Shibazaki Y et al. Earth Planet, Sci. Lett. 2011 Jan 3;301(1–2):153–8.
2. Abeykoon S et al. J. Geophys. Res. Solid Earth., 2023 Sep 1;128(9).
3. Piet H et al. J. Geophys. Res. Planets., 2021 Nov 1;126(11).
4. Iizuka-Oku R et al. Sci. Rep., 2021 Dec 1;11(1).
5. Kusaba K et al. J. Phys. Chem. Solids., 1997 May 19; 58(2):241-246.