Aluminum-Phosphate Binder Formation in Zeolites as Probed with X‑ray Absorption Microscopy

In this work, three industrially relevant zeolites with framework topologies of MOR, FAU and FER have been explored on their ability to form an AlPO₄ phase by reaction of a phosphate precursor with expelled framework aluminum. A detailed study was performed on zeolite H-mordenite, using in situ STXM and soft X-ray absorption tomography, complemented with ²⁷Al and ³¹P magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy, XRD, FT-IR spectroscopy, and N₂ physisorption. Extraframework aluminum was extracted from steam-dealuminated H-mordenite and shown to dominantly consist of amorphous AlO­(OH). It was found that phosphoric acid readily reacts with the AlO­(OH) phase in dealuminated H-mordenite and forms an extraframework amorphous AlPO₄ phase. It was found that while AlPO₄ crystallizes outside of the zeolitic channel system forming AlPO₄ islands, AlPO₄ that remains inside tends to stay more amorphous. In the case of ultrastable zeolite Y the FAU framework collapsed during phosphatation, due to extraction of framework aluminum from the lattice. However, using milder phosphatation conditions an extraframework AlPO₄ α-cristobalite/tridymite phase could also be produced within the FAU framework. Finally, in steamed zeolite ferrierite with FER topology the extraframework aluminum species were trapped and therefore not accessible for phosphoric acid; hence, no AlPO₄ phase could be formed within the structure. Therefore, the parameters to be taken into account in AlPO₄ synthesis are the framework Si/Al ratio, stability of framework aluminum, pore dimensionality and accessibility of extraframework aluminum species

Direct Observation of Cr³⁺ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr³⁺ impurities in Al₂O₃ with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr³⁺ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L₃ and L₂ edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms

Direct Observation of Cr³⁺ 3d States in Ruby: Toward Experimental Mechanistic Evidence of Metal Chemistry

The role of transition metals in chemical reactions is often derived from probing the metal 3d states. However, the relation between metal site geometry and 3d electronic states, arising from multielectronic effects, makes the spectral data interpretation and modeling of these optical excited states a challenge. Here we show, using the well-known case of red ruby, that unique insights into the density of transition metal 3d excited states can be gained with 2p3d resonant inelastic X-ray scattering (RIXS). We compare the experimental determination of the 3d excited states of Cr³⁺ impurities in Al₂O₃ with 190 meV resolution 2p3d RIXS to optical absorption spectroscopy and to simulations. Using the crystal field multiplet theory, we calculate jointly for the first time the Cr³⁺ multielectronic states, RIXS, and optical spectra based on a unique set of parameters. We demonstrate that (i) anisotropic 3d multielectronic interactions causes different scaling of Slater integrals, and (ii) a previously not observed doublet excited state exists around 3.35 eV. These results allow to discuss the influence of interferences in the RIXS intermediate state, of core–hole lifetime broadenings, and of selection rules on the RIXS intensities. Finally, our results demonstrate that using an intermediate excitation energy between L₃ and L₂ edges allows measurement of the density of 3d excited states as a fingerprint of the metal local structure. This opens up a new direction to pump-before-destroy investigations of transition metal complex structures and reaction mechanisms

Mn and Co Charge and Spin Evolutions in LaMn_1–<i>x</i>Co_<i>x</i>O₃ Nanoparticles

The charge and spin-state evolution of manganese and cobalt in the LaMn_1–<i>x</i>Co_<i>x</i>O₃ (<i>x</i> = 0.00, 0.25, 0.50, 0.75, and 1.00) perovskite nanoparticles have been studied with soft X-ray absorption spectroscopy. The results show a gradual increase in the average oxidation state of both Mn and Co ions with cobalt doping. The average valence of the LaMn_1–<i>x</i>Co_<i>x</i>O₃ samples remains close to 3.0, with the Mn valence increasing from 3.1 to 4.0 and the Co valence increasing from 2.0 to 3.0. The symmetry of Mn and Co was determined using multiplet calculations. Calculating the intensity-area of the oxygen K pre-edge feature confirmed an increase in covalency with increasing Mn and Co oxidation state. The ground-state composition of Mn³⁺ in LaMnO₃, and Co³⁺ in LaCoO₃, was investigated, and it was found that Mn³⁺ (<i>D</i>_4<i>h</i>) and Co³⁺ (<i>O</i>_<i>h</i>) are mainly in their low-spin state, with 10–20% admixture of high-spin state contributions into a mixed spin ground state

Femtosecond Soft X-ray Spectroscopy of Solvated Transition-Metal Complexes: Deciphering the Interplay of Electronic and Structural Dynamics

We present the first implementation of femtosecond soft X-ray spectroscopy as an ultrafast direct probe of the excited-state valence orbitals in solution-phase molecules. This method is applied to photoinduced spin crossover of [Fe(tren(py)₃)]²⁺, where the ultrafast spin-state conversion of the metal ion, initiated by metal-to-ligand charge-transfer excitation, is directly measured using the intrinsic spin-state selectivity of the soft X-ray L-edge transitions. Our results provide important experimental data concerning the mechanism of ultrafast spin-state conversion and subsequent electronic and structural dynamics, highlighting the potential of this technique to study ultrafast phenomena in the solution phase

Charge-Transfer Analysis of 2p3d Resonant Inelastic X‑ray Scattering of Cobalt Sulfide and Halides

We show that with 2p3d resonant inelastic X-ray scattering (RIXS) we can accurately determine the charge-transfer parameters of CoF₂, CoCl₂, CoBr₂, and CoS. The 160 meV resolution RIXS results are compared with charge-transfer multiplet calculations. The improved resolution and the direct observation of the crystal field and charge-transfer excitations allow the determination of more accurate parameters than could be derived from X-ray absorption and X-ray photoemission, both limited in resolution by their lifetime broadening. We derive the crystal field and charge-transfer parameters of the Co²⁺ ions, which provides the nature of the ground state of the Co²⁺ ions with respect to symmetry and hybridization. In addition, the increased spectral resolution allows the more accurate determination of the atomic Slater integrals. The results show that the crystal field energy decreases with increasing ligand covalency. The L₂ edge RIXS spectra show that the intensity of the (Coster–Kronig induced) nonresonant X-ray emission is a measure of ligand covalency

Three-Dimensional Structure and Defects in Colloidal Photonic Crystals Revealed by Tomographic Scanning Transmission X-ray Microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy

Distorted Tetrahedral Co^II in K₅H[CoW₁₂O₄₀]·<i>x</i>H₂O Probed by 2p3d Resonant Inelastic X‑ray Scattering

The Co 2p_3/2 X-ray absorption spectroscopy and high-energy-resolution (∼0.09 eV fwhm) 2p3d resonant inelastic X-ray scattering (RIXS) spectra of the single-cobalt-centered polyoxometalate K₅H­[CoW₁₂O₄₀]·<i>x</i>H₂O were measured. The low-energy dd transition features at 0.55 eV, unmeasurable with ultraviolet–visible (UV/vis) spectroscopy, were experimentally revealed in 2p3d RIXS spectra. RIXS simulations based on ligand-field multiplet theory were performed to assess the potential cobalt tetragonal symmetry distortion, which is described with the ligand-field parameters 10<i>D</i>_q (−0.54 eV), <i>D</i>_s (−0.08 eV), and <i>D</i>_t (0.005 eV). Because 2p3d RIXS probes not only the optical spin-allowed transitions but also the spin-forbidden transitions, we show that the current 2p3d RIXS simulation enables a series of dd feature assignments with higher accuracy than those from previous optical data. Furthermore, by wave-function decomposition analyses, we demonstrate the more realistic and detailed origins of a few lowest dd transitions using both one-electron-orbital and term-symbol descriptions

Three-Dimensional Structure and Defects in Colloidal Photonic Crystals Revealed by Tomographic Scanning Transmission X-ray Microscopy

Self-assembled colloidal crystals have attracted major attention because of their potential as low-cost three-dimensional (3D) photonic crystals. Although a high degree of perfection is crucial for the properties of these materials, little is known about their exact structure and internal defects. In this study, we use tomographic scanning transmission X-ray microscopy (STXM) to access the internal structure of self-assembled colloidal photonic crystals with high spatial resolution in three dimensions for the first time. The positions of individual particles of 236 nm in diameter are identified in three dimensions, and the local crystal structure is revealed. Through image analysis, structural defects, such as vacancies and stacking faults, are identified. Tomographic STXM is shown to be an attractive and complementary imaging tool for photonic materials and other strongly absorbing or scattering materials that cannot be characterized by either transmission or scanning electron microscopy or optical nanoscopy

From Ligand Fields to Molecular Orbitals: Probing the Local Valence Electronic Structure of Ni²⁺ in Aqueous Solution with Resonant Inelastic X‑ray Scattering

Bonding of the Ni²⁺(aq) complex is investigated with an unprecedented combination of resonant inelastic X-ray scattering (RIXS) measurements and ab initio calculations at the Ni L absorption edge. The spectra directly reflect the relative energies of the ligand-field and charge-transfer valence-excited states. They give element-specific access with atomic resolution to the ground-state electronic structure of the complex and allow quantification of ligand-field strength and 3d–3d electron correlation interactions in the Ni²⁺(aq) complex. The experimentally determined ligand-field strength is 10<i>Dq</i> = 1.1 eV. This and the Racah parameters characterizing 3d–3d Coulomb interactions <i>B</i> = 0.13 eV and <i>C</i> = 0.42 eV as readily derived from the measured energies match very well with the results from UV–vis spectroscopy. Our results demonstrate how L-edge RIXS can be used to complement existing spectroscopic tools for the investigation of bonding in 3d transition-metal coordination compounds in solution. The ab initio RASPT2 calculation is successfully used to simulate the L-edge RIXS spectra