The three-dimensional (3D) distribution of individual atoms on the surface of catalyst nanoparticles plays a vital role in their activity and stability. Optimising the performance of electrocatalysts ...requires atomic-scale information, but it is difficult to obtain. Here, we use atom probe tomography to elucidate the 3D structure of 10 nm sized Co
FeO
and CoFe
O
nanoparticles during oxygen evolution reaction (OER). We reveal nanoscale spinodal decomposition in pristine Co
FeO
. The interfaces of Co-rich and Fe-rich nanodomains of Co
FeO
become trapping sites for hydroxyl groups, contributing to a higher OER activity compared to that of CoFe
O
. However, the activity of Co
FeO
drops considerably due to concurrent irreversible transformation towards Co
O
and pronounced Fe dissolution. In contrast, there is negligible elemental redistribution for CoFe
O
after OER, except for surface structural transformation towards (Fe
, Co
)
O
. Overall, our study provides a unique 3D compositional distribution of mixed Co-Fe spinel oxides, which gives atomic-scale insights into active sites and the deactivation of electrocatalysts during OER.
Iron K-edge X-ray absorption pre-edge features have been calculated using a time-dependent density functional approach. The influence of functional, solvation, and relativistic effects on the ...calculated energies and intensities has been examined by correlation of the calculated parameters to experimental data on a series of 10 iron model complexes, which span a range of high-spin and low-spin ferrous and ferric complexes in O(h) to T(d) geometries. Both quadrupole and dipole contributions to the spectra have been calculated. We find that good agreement between theory and experiment is obtained by using the BP86 functional with the CP(PPP) basis set on the Fe and TZVP one of the remaining atoms. Inclusion of solvation yields a small improvement in the calculated energies. However, the inclusion of scalar relativistic effects did not yield any improved correlation with experiment. The use of these methods to uniquely assign individual spectral transitions and to examine experimental contributions to backbonding is discussed.
A novel restricted-open-shell configuration interaction with singles (ROCIS) approach for the calculation of transition metal L-edge X-ray absorption spectra is introduced. In this method, one first ...calculates the ground state and a number of excited states of the non-relativistic Hamiltonian. By construction, the total spin is a good quantum number in each of these states. For a ground state with total spin S excited states with spin S' = S, S - 1, and S + 1 are constructed. Using Wigner-Eckart algebra, all magnetic sublevels with MS = S,..., -S for each multiplet of spin S are obtained. The spin-orbit operator is represented by a mean-field approximation to the full Breit-Pauli spin-orbit operator and is diagonalized over this N-particle basis. This is equivalent to a quasi-degenerate treatment of the spin-orbit interaction to all orders. Importantly, the excitation space spans all of the molecular multiplets that arise from the atomic Russell-Saunders terms. Hence, the method represents a rigorous first-principles approach to the complicated low-symmetry molecular multiplet problem met in L-edge X-ray absorption spectroscopy. In order to gain computational efficiency, as well as additional accuracy, the excitation space is restricted to single excitations and the configuration interaction matrix is slightly parameterized in order to account for dynamic correlation effects in an average way. To this end, it is advantageous to employ Kohn-Sham rather than Hartree-Fock orbitals thus defining the density functional theory∕ROCIS method. However, the method can also be used in an entirely non-empirical fashion. Only three global empirical parameters are introduced and have been determined here for future application of the method to any system containing any transition metal. The three parameters were carefully calibrated using the L-edge X-ray absorption spectroscopy spectra of a test set of coordination complexes containing first row transition metals. These parameters are universal and transferable. Hence, there are no adjustable parameters that are used to fit experimental X-ray absorption spectra. Thus, the new approach classifies as a predictive first-principles method rather than an analysis tool. A series of calculations on transition metal compounds containing Cu, Ti, Fe, and Ni in various oxidation and spin states is investigated and a detailed comparison to experimental data is reported. In most cases, the approach yields good to excellent agreement with experiment. In addition, the origin of the observed spectral features is discussed in terms of the electronic structure of the investigated compounds.
A central question in biological water splitting concerns the oxidation states of the manganese ions that comprise the oxygen-evolving complex of photosystem II. Understanding the nature and order of ...oxidation events that occur during the catalytic cycle of five S
states (
= 0-4) is of fundamental importance both for the natural system and for artificial water oxidation catalysts. Despite the widespread adoption of the so-called "high-valent scheme"-where, for example, the Mn oxidation states in the S
state are assigned as III, IV, IV, IV-the competing "low-valent scheme" that differs by a total of two metal unpaired electrons (
III, III, III, IV in the S
state) is favored by several recent studies for the biological catalyst. The question of the correct oxidation state assignment is addressed here by a detailed computational comparison of the two schemes using a common structural platform and theoretical approach. Models based on crystallographic constraints were constructed for all conceivable oxidation state assignments in the four (semi)stable S states of the oxygen evolving complex, sampling various protonation levels and patterns to ensure comprehensive coverage. The models are evaluated with respect to their geometric, energetic, electronic, and spectroscopic properties against available experimental EXAFS, XFEL-XRD, EPR, ENDOR and Mn K pre-edge XANES data. New 2.5 K
Mn ENDOR data of the S
state are also reported. Our results conclusively show that the entire S state phenomenology can only be accommodated within the high-valent scheme by adopting a single motif and protonation pattern that progresses smoothly from S
(III, III, III, IV) to S
(IV, IV, IV, IV), satisfying all experimental constraints and reproducing all observables. By contrast, it was impossible to construct a consistent cycle based on the low-valent scheme for all S states. Instead, the low-valent models developed here may provide new insight into the over-reduced S states and the states involved in the assembly of the catalytically active water oxidizing cluster.
The sensitivity of iron Kβ X-ray emission spectroscopy (XES) to the nature of the bound ligands (σ-donating, π-donating, and π-accepting) has been explored. A combination of experiment and theory has ...been employed, with a DFT approach being utilized to elucidate ligand effects on the spectra and to assign the spectral intensity mechanisms. Knowledge of the various contributions to the spectra allows for a deeper understanding of spectral features and demonstrates the sensitivity of this method to the identity of the interacting ligands. The potential of XES for identifying intermediate species in nonheme iron enzymes is highlighted.
Despite decades of intense research, the core structure of the methane C-H bond breaking diiron(IV) intermediate, Q, of soluble methane monooxygenase remains controversial, with conflicting reports ...supporting either a "diamond" diiron core structure or an open core structure. Early extended X-ray absorption fine structure (EXAFS) data assigned a short 2.46 Å Fe-Fe distance to Q (Shu et al. Science 1997, 275, 515 ) that is inconsistent with several theoretical studies and in conflict with our recent high-resolution Fe K-edge X-ray absorption spectroscopy (XAS) studies (Castillo et al. J. Am. Chem. Soc. 2017, 139, 18024 ). Herein, we revisit the EXAFS of Q using high-energy resolution fluorescence-detected extended X-ray absorption fine structure (HERFD-EXAFS) studies. The present data show no evidence for a short Fe-Fe distance, but rather a long 3.4 Å diiron distance, as observed in open core synthetic model complexes. The previously reported 2.46 Å feature plausibly arises from a background metallic iron contribution from the experimental setup, which is eliminated in HERFD-EXAFS due to the increased selectivity. Herein, we explore the origin of the short diiron feature in partial-fluorescent yield EXAFS measurements and discuss the diagnostic features of background metallic scattering contribution to the EXAFS of dilute biological samples. Lastly, differences in sample preparation and resultant sample inhomogeneity in rapid-freeze quenched samples for EXAFS analysis are discussed. The presented approaches have broad implications for EXAFS studies of all dilute iron-containing samples.
Nitrogenase is a complex enzyme that catalyzes the reduction of dinitrogen to ammonia. Despite insight from structural and biochemical studies, its structure and mechanism await full ...characterization. An iron-molybdenum cofactor (FeMoco) is thought to be the site of dinitrogen reduction, but the identity of a central atom in this cofactor remains unknown. Fe Kβ x-ray emission spectroscopy (XES) of intact nitrogenase MoFe protein, isolated FeMoco, and the FeMoco-deficient ΔnifB protein indicates that among the candidate atoms oxygen, nitrogen, and carbon, it is carbon that best fits the XES data. The experimental XES is supported by computational efforts, which show that oxidation and spin states do not affect the assignment of the central atom to C⁴⁺. Identification of the central atom will drive further studies on its role in catalysis.
Valence‐to‐core X‐ray emission spectroscopy (V2C XES) has been applied to a series of compounds relevant to both homogeneous catalysts and intermediates in heterogeneous reactions, namely Fe(CO)5, ...Fe2(CO)9, Fe3(CO)12, Fe(CO)3(cod) (cod=cyclo‐octadienyl), Fe2Cp2(CO)4 (Cp=cyclo‐pentadienyl), Fe2Cp*2(CO)4 (Cp*=tetramethylcyclopentadienyl), and FeCp(CO)2(thf)B(ArF)4 (ArF=pentafluorophenyl). DFT calculations of the V2C XES spectra show very good agreement with experiment, which allows for an in depth analysis of the origins of the observed spectral signatures. It is demonstrated that the observed spectral features can be broken down into specific ligand and metal fragment contributions. The relative intensities of the observed features are further explained through a quantitative investigation of the metal 3p and 4p contributions to the spectra. The ability to use V2C XES to separate carbonyl, hydrocarbon, and solvent contributions is highlighted.
Valence‐to‐core X‐ray emission spectroscopy is applied to iron–carbonyl compounds. Analysis of the origins of the spectral features in terms of ligand and metal contributions is achieved by DFT calculations. The intensities of the observed features are explained through a quantization of the metal 3p and 4p contributions. This allows for the separation of carbonyl and hydrocarbon signals in catalytically relevant compounds and intermediates (see figure).
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