Chemical Science Research Frontiers 2024 76 Metal oxide clusters consisting of several MO 6 metal oxide units (M: metal ions), called polyoxometalates, exhibit unique reactivities and physical properties owing to their electronic and geometric structures, unlike their bulk materials counterparts. We found that group 5 transition metal Nb/Ta oxide clusters, such as [M 6 O 19 ] 8– (MV = Nb, Ta) and [Nb 10 O 28 ] 6– , possess large negative charges and have demonstrated effectiveness as base catalysts [1,2]. Notably, the [Ta 6 O 19 ] 8– ( Ta6 ) cluster shows high activity for CO 2 fixation reactions with styrene oxide and amine compounds. DFT calculations predict that the terminal oxygen (Ta=O) on the surface of Ta6 acts as a Lewis base, facilitating monodentate CO 2 coordination [2]. Understanding the structural dynamics of CO 2 adsorbed on Ta6 is crucial to elucidating its high catalytic activity. The recent development of a high-energy-resolved fluorescence detection (HERFD) method has garnered attention for its ability to measure XANES spectra with high-energy resolution. This method detects fluorescent X-rays with specific wavelengths, achieving an energy resolution beyond the core electron lifetime width, thereby enhancing the clarity of peaks in XANES spectra [3]. This technique has revealed small peaks that conventional methods often miss and has been applied to evaluate the symmetry of Ce n oxide clusters ( n = 2, 6, 24, 38, 40) [4]. However, HERFD- XANES studies remain relatively limited. In this study, we measured in situ Ta L 3 -shell HERFD-XAFS of Ta6 in DMF during CO 2 adsorption at SPring-8 BL36XU beamline (Fig. 1) [5]. The HERFD- XANES measurements revealed previously undetected peaks within the white line region, that could not be observed by the conventional transmission method. In addition, we traced the CO 2 adsorption process on Ta6 and experimentally clarified that the ligand field splitting is changed by the local structural change of Ta due to CO 2 adsorption. The Ta L 3 -edge HERFD-XANES spectrum of Ta 6 prior to CO 2 adsorption in DMF solution is shown in Fig. 2(a). Peaks (A 1 and B 1 ) along with a shoulder peak (C 1 ), which were not visible using the conventional transmission method at SPring-8 BL01B1 with a Si(111) double crystal, were detected using the HERFD method. These peaks correspond to the electronic transition from 2 p to 5 d orbitals, which are expected to split due to the ligand field in the distorted octahedral Ta6 units. These distortions approximate C 4V symmetry, enabling a detailed discussion of local distortions using Ta L 3 -edge HERFD-XANES. From the in situ HERFD-XANES spectra, we observed gradual changes in the electronic state of Ta6 in DMF under CO 2 gas flow (Fig. 2(b)). The secondary differential spectra of Ta L 3 -edge HERFD XANES showed slight shifts in peaks A 1 and B 1 to A 2 (–0.2 eV) and B 2 (+0.2 eV), respectively, while peak C 2 emerged at 9885.6 eV, replacing the vanished C 1 peak (Fig. 2(c)). Experimental data revealed that five CO 2 molecules were adsorbed onto Ta6 , with spectral changes attributed to structural modifications around the Ta centers. To further elucidate the structural changes, DFT calculations were employed to CO 2 adsorbed Ta6 ( Ta6-CO 2 ) model. The surface Ta=O bonds within the {TaO 6 } unit elongated, while the bridged Ta– O bonds In situ HERFD-XANES study on CO 2 activation on niobium oxide clusters Fig. 1. Experimental setup for in situ HERFD-XANES measurement. Reaction vessel XAFS cell Peristaltic pump 100% 1% Glass tube N 2 CO 2 Peristaltic pump Incident X-ray Detector XAFS cell MFCs Analyzer crystal Fluorescent X-ray Ta L α 1 Research Frontiers 2024 77 shortened upon CO 2 adsorption (Fig. 3). This structural adjustment indicated a shift of the Ta atom toward the octahedral center, pushing the {TaO 6 } unit closer to O h symmetry. Energy calculations and electronic structure analyses were performed for the {TaO 6 } and CO 2 adsorbed one ({TaO 6 -CO 2 }), as shown in Fig. 3. In the {TaO 6 } unit, regions A 1 ( d xy , d yx , d zx ), B 1 ( d z 2 , d x 2– y 2 , d z 2 + s O ), and C 1 ( d x 2– y 2 + s O ) unit correlated with peaks A 1 , B 1 , and C 1 of Ta6 . Upon CO 2 adsorption, these evolved into regions A 2 ( d xy , d yx , d zx ), B 2 ( d z2 , d z 2 + s O ), and C 2 ( d x 2– y 2 ), corresponding to peaks A 2 , B 2 , and C 2 of Ta6-CO 2 . The disappearance of C 1 and the appearance of C 2 were linked to the destabilization of the d x 2– y 2 orbitals, including the hybrid orbital of Ta d x 2– y 2 + 2 p orbitals and stabilization of d z 2 orbitals caused by the structural centering effect. This is mainly due to the compression of the bridged Ta– O bonds and the elongation of the Ta=O bonds. In summary, we observed a change in the ligand-field splitting of the Ta 5 d orbitals in a Ta6 catalyst upon CO 2 interaction, using in situ Ta L 3 - edge HERFD-XANES. Theoretical calculations indicated that the three peaks observed before CO 2 adsorption correspond to transitions to molecular orbitals with significant contributions from the Ta 5 d orbitals, which are split by the ligand field of the distorted O h symmetrical {TaO 6 } units. The observed peak replacement reflects the destabilization of the d x 2– y 2 orbitals and the stabilization of the d z 2 orbital, induced by the centering effect from the off-center arrangement of the {TaO 6 } units upon CO 2 attachment to the terminal oxygen. We demonstrate that HERFD- XANES is an effective tool for tracking subtle changes in the electronic and geometric structures of materials in their working state. Seiji Yamazoe Department of Chemistry, Tokyo Metropolitan University Email: yamazoe@tmu.ac.jp References [1] S. Hayashi et al. : Chem. Asian J. 12 (2017) 1635. [2] S. Hayashi et al. : J. Phys. Chem. C 122 (2018) 29398. [3] K. Hämäläinen et al. : Phys. Rev. Lett. 67 (1991) 2850. [4] P. Estevenon et al. : Chem. Mater. 35 (2023) 1723. [5] T. Matsuyama, S. Kikkawa, N. Kawamura, K. Higashi, N. Nakatani, K. Kato, and S. Yamazoe: J. Phys. Chem. C 128 (2024) 2953. Fig. 3. Schematic illustration showing the contribution of Ta-based d -orbitals in TaO 6 and TaO 6-CO 2. Fig. 2. (a) Comparison of XANES spectra of Ta6 measured by HERFD and transmittance mode. (b) In situ HERFD-XANES spectra of Ta6 during CO 2 adsorption. (c) HERFD-XANES spectra of Ta6 and Ta6-CO 2 along with their corresponding second derivative spectra. (a) (b) (c) Photon Energy (eV) Photon Energy (eV) Photon Energy (eV) 9875 0 1 2 3 4 5 6 0 – 2 0 0 4 8 0 4 8 2 4 6 – 2 0 2 4 A A 1 1 B B 1 1 C C 1 1 A A 2 2 B B2 2 C C 2 2 6 1 2 3 4 5 6 9880 9885 9890 HERFD 9885.6 eV – 1.8 eV Ta6 Ta6 1eq. 2eq. 3eq. 4eq. 5eq. Ta6 – CO 2 (excess) Transmittance 9875 9880 9885 9890 9875 9880 9885 9890 Normalized Absorbance Normalized Absorbance Norm. abs. 2nd Deriv. 2nd Deriv. (smoothed) Norm. abs. Ta6 – CO 2 Ta6 Ta6-CO 2 {TaO 6 } Region C 1 Region B 1 Region A 1 Region C2 2 eV Region B 2 Region A 2 d x 2 – y 2 + σ O d x 2 – y 2 + σ O d x 2 – y 2 dx 2 – y 2 d yz dyz d xy d xy d zx d z 2 d z 2 d z 2 + σ O dz 2 + σ O O t O b 102.7° 103.1° 1.801 Å 2.022 Å 2.360 Å 1.987 Å Compression 1.868 Å Elongation 2.417 Å O c CO 2 Ta {TaO 6 -CO 2 } d zx Orbital Energy (eV)
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