Chemical Science Research Frontiers 2023 80 Synchrotron-radiation-based Mössbauer spectroscopy for investigating the structure of atomically dispersed NiN x sites in (Ni, N)-doped carbon electrocatalysts Metal- and nitrogen-doped carbons have garnered significant attention as potential catalysts for diverse chemical reactions due to the hypothesized activity of their atomically dispersed nitrogen-coordinated metal MN x sites. For instance, the catalytic potential of iron- and nitrogen-doped carbons (Fe–N–Cs) in various oxygen reduction reactions has been explored in both acidic and alkaline environments. Similarly, nickel- and nitrogen-doped carbons (Ni–N–Cs) are promising catalysts, exhibiting high selectivity for CO under electrochemical CO 2 reduction conditions. Historically, Mössbauer spectroscopy has proven effective in studying various iron-based catalysts since the 1960s [1], including investigations into Fe–N–Cs [2]. This method, utilizing the nuclear resonance of a probe nuclide allows element- selective study of electronic states of each chemical component even in materials with multiple compounds. Furthermore, Mössbauer spectroscopy is applicable even in gas atmospheres due to highly penetrating incident γ -rays from radioactive isotopes (RI). However, the commercial availability of RI sources for many elements poses limitations, restricting this method to the study of iron-containing materials. The advent of synchrotron radiation (SR) presented an alternative source for Mössbauer spectroscopy, owing to its capability to produce X-rays of any energy appropriate for Mössbauer spectroscopy. SR-based Mössbauer spectroscopy, developed in 2009 [3], enables the selection of numerous elements as probes and has also been applied to 61 Ni Mössbauer spectroscopy [4]. Recently, we employed this method to study the Ni–N–C catalysis in order to further understand the active site NiN x structure [5]. The herein report focuses on 61 Ni Mössbauer experiments in Ref. 5. The 61 Ni SR-based Mössbauer spectroscopy was conducted at SPring-8 BL09XU , utilizing the electron-storage ring’s “203 bunch” operating mode. The SR with an energy corresponding to the nuclear resonance of 61 Ni (67.4 keV) was selected using a Si (333) beamline monochromator and an additional Si (111) monochromator. The SR was then transmitted by a sample in a He-flow cryostat to regulate its temperature at 5 K. The samples consisted of NiN x synthesized on polyacrylonitrile (PACN), a material derived from 61 Ni-enriched metal powder (enrichment: 99.42%). Samples with different Ni contents, 0.1 wt%, 0.5 wt%, 1 wt% were synthesized, that is, primarily comprising PACN. However, the attenuation of 67.4 keV SR by "light" elements was minimal. To alleviate the disadvantage of low 61 Ni content in the sample with a large volume, a new sample holder (approximately 20 cm in length) was developed for the cryostat, illustrated in Fig. 1 . The highly penetrating SR facilitated the easy insertion of such an extraordinary environmental chamber. Proceeding downstream of the sample, the SR was scattered by Fig. 1. Photo of the customized sample holder. Fig. 2. 61 Ni Mössbauer spectra of 61 Ni PACN samples [5]. Raw data are presented as open black circles, fitted curves are depicted as shown as solid black lines, and individual transition locations and intensities are denoted by vertical black lines. SR IN SR OUT Velocity (mm/s) Counts – 6 – 4 1000 cts – 2 0 2 4 6 0.1 wt% ( × × 3) B fit = 5.44 ± 0.34 T 0.5 wt% B fit = 5.39 ± 0.24 T 1 wt% B fit = 8.25 ± 0.45 T Research Frontiers 2023 81 61 Ni 0.86 V 0.14 foil (enrichment: 86.2%) at typically 30 K in another vacuum cryostat. The nuclear resonance energy of the foil was systematically scanned by controlling its velocity using the Doppler effect. Scattering from the foil was detected using an eight- element Si avalanche photodiode detector on the Ni–V foil, and its velocity dependence yielded the Mössbauer spectrum. The 61 Ni Mössbauer spectra of 61 Ni PACN are shown in Fig. 2. A single spectrum was successfully acquired in a typical one-day measurement. The spectra revealed magnetic hyperfine fields of ~5.4 ± 0.4 T for the 0.1 wt% and 0.5 wt% samples, and a larger field of 8.25 ± 0.45 T for the 1 wt% sample. Therefore, the local state appeared similar in the former two samples, but distinct in the latter. Notably, the magnetic hyperfine field of 8.25 T closely resembles that of Ni metal around 8 T, indicating Ni aggregate-like behavior. To elucidate the magnetic hyperfine field of 5.4 T, we conducted density functional theory calculations to study the magnetic hyperfine field of the NiN x sites. Our calculations successfully interpreted the observations as the high-spin Ni 2+ character of the NiN 4 active sites with tetrahedrally distorted geometries. Recently, nanoparticles have been synthesized using various elements. We hold the belief that there is an opportunity to further the study of these cutting-edge samples using SR-based Mössbauer spectroscopy, which enables the use of various (heavy) elements as probe nuclides (Fig. 3). Moreover, post-2019, the Mössbauer activity in the shared- use beamline at SPring-8 transitioned from BL09XU to BL35XU, which means at least double the beam intensity is available for Mössbauer studies. This significant improvement serves to bolster frontier studies, providing a more robust platform for advanced investigations. Ryo Masuda Graduate School of Science and Technology, Hirosaki University Email: masudar@hirosaki-u.ac.jp References [1] For example, please see the textbook: F. J. Berry: in Mössbauer Spectroscopy Applied to Inorganic Chemistry vol. 1 ed. by G. J. Long (Plemum press, New York and London, 1984), Chap. 13, p. 391. [2] U. I. Kramm et al. : J. Am. Chem. Soc. 136 (2014) 978. [3] M. Seto et al. : Phys. Rev. Lett. 102 (2009) 217602. [4] R. Masuda et al. : Sci. Rep. 6 (2016) 20861. [5] D. M. Koshy, Md D. Hossain, R. Masuda, Y. Yoda, L. B. Gee, K. Abiose, H. Gong, R. Davis, M. Seto, A. Gallo, C. Hahn, M. Bajdich, Z. Bao and T. F. Jaramillo: J. Am. Chem. Soc. 144 (2022) 21741. Fig. 3. Table of elements for the Mössbauer effect. SR-based Mössbauer spectroscopy was conducted using elements highlighted green backgrounds. Lanthanide Actinide 1 H Li Be Na Mg K Ca Sc Ti V Cr Mn Fe Co Ni Cu Zn Ga Ge As Se Br Kr Rb Sr Y Zr Nb Mo Tc Ru Rh Pd Ag Cd In Sn Sb Te I La Ce Pr Nd Pm Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Ac A A A SR-based Mössbauer absorption spectroscopy has been performed Mössbauer effect has been observed Mössbauer effect has not been observed Th Pa U Np Pu Am Cm Bk Cf Es Fm Md No Lr Xe Cs Ba Fr Ra * ** 104~ Hf Ta W Re Os Ir Pt Au Hg Tl Pb Bi Po At Rn 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 He Ne F O N C B Ar Cl S P Si Al * **
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