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We report the temperature dependence of the Yb valence in the geometrically frustrated compound${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$from 12 to 300 K using resonant x-ray emission spectroscopy at the Yb$L_{{\alpha }_1}$transition. We find that the Yb valence,v, is hybridized between thev = 2 andv = 3 valence states, increasing from$v=2.61\pm 0.01$at 12 K to$v=2.67\pm 0.01$at 300 K, confirming that${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$is a Kondo system in the intermediate valence regime. This result indicates that the Kondo interaction in${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$is substantial, and is likely to be the reason why${\mathrm{Y}\mathrm{b}\mathrm{B}}_4$does not order magnetically at low temperature, rather than this being an effect of geometric frustration. Furthermore, the zero-point valence of the system is extracted from our data and compared with other Kondo lattice systems. The zero-point valence seems to be weakly dependent on the Kondo temperature scale, but not on the valence change temperature scaleT_v.

 

Platinum is used extensively as a catalyst for a wide variety of chemical reactions, though its scarcity and price present limitations to expansions of its use. To understand the origin of platinum's versatility—with the goals of both improving the efficiency of existing catalysts and mimicking its reactivity with more abundant metals—the mechanisms of platinum-catalyzed chemical reactions must be understood via structural and spectroscopic characterization of these catalysts under operando conditions. Such data, typically consisting of complex mixtures of species, often prove challenging to interpret, inviting the aid of chemical theory. DFT calculations in particular have proven successful at predicting structural and spectroscopic parameters of transition metal species, though a thorough investigation of how these methods perform for platinum-based complexes has yet to be undertaken. Herein, we evaluated the performance of geometry optimization for five commonly used functionals (BP86, PBE, B3LYP, PBE0, and TPSSh) in combination with various ligand basis sets, relativistic approximations, and solvation and dispersion models. We applied these DFT methods to a training set of 14 platinum-containing complexes with varying sizes, oxidation states, and number and type of ligands and determined that the best-performing method was the PBE0 functional together with the def2-TZVP basis set for the ligand atoms, the ZORA relativistic approximation, and solvation and dispersion corrections. The ability of this DFT methodology to accurately predict metrical parameters was confirmed using two case studies, most notably by comparing the DFT optimized geometry of a previously uncharacterized complex to newly collected EXAFS data, which showed excellent agreement. 

Overcoming slow kinetics and high overpotential in electrocatalytic oxygen evolution reaction (OER) requires innovative catalysts and approaches that transcend the scaling relationship between binding energies for intermediates and catalyst surfaces. Inorganic complexes provide unique, customizable geometries, which can help enhance their efficiencies. However, they are unstable and susceptible to chemical reaction under extreme pH conditions. Immobilizing complexes on substrates creates single‐molecule catalysts (SMCs) with functional similarities to single‐atom catalysts (SACs). Here, an efficient SMC, composed of dichloro(1,3‐bis(diphenylphosphino)propane) nickel [NiCl₂dppp] anchored to a graphene acid (GA), is presented. This SMC surpasses ruthenium‐based OER benchmarks, exhibiting an ultra‐low onset and overpotential at 10 mAcm⁻²when exposed to a static magnetic field. Comprehensive experimental and theoretical analyses imply that an interfacial charge transfer from the Ni center in NiCl₂dppp to GA enhances the OER activity. Spectroscopic investigations reveal an in situ geometrical transformation of the complex and the formation of a paramagnetic Ni center, which under a magnetic field, enables spin‐selective electron transfer, resulting in enhanced OER performance. The results highlight the significance of in situ geometric transformations in SMCs and underline the potential of an external magnetic field to enhance OER performance at a single‐molecule level.

 

Metal oxide semiconductors have attracted much attention due to their versatility in different applications, ranging from biosensing to green energy-harvesting technologies. Among these metal oxides, oxide-based diluted magnetic semiconductors have also been proposed for fuel cell applications, especially for the oxygen reduction reaction (ORR) and the oxygen evolution reaction. However, the catalytic mechanism has been proposed to follow a two-electron pathway, forming hydrogen peroxide, instead of the four-electron pathway. Herein, we report cobalt-doped zinc oxide (CoxZn1–xO, 0 < x < 0.018) materials prepared using a co-precipitation method suitable for the electrocatalytic production of hydrogen peroxide. The electrocatalytic performance of CoxZn1–xO materials showed up to 60% hydrogen peroxide production with onset potentials near 649 mV, followed by the two-electron ORR mechanism. Ex situ X-ray absorption spectroscopy experiments at the Co K-edge demonstrated the presence of Co(II) ions at tetrahedral sites within the ZnO lattice. 

Proton transfer reactions are of central importance to a wide variety of biochemical processes, though determining proton location and monitoring proton transfers in biological systems is often extremely challenging. Herein, we use two‐color valence‐to‐core X‐ray emission spectroscopy (VtC XES) to identify protonation events across three oxidation states of the O₂‐activating, radical‐initiating manganese–iron heterodinuclear cofactor in a class I‐c ribonucleotide reductase. This is the first application of VtC XES to an enzyme intermediate and the first simultaneous measurement of two‐color VtC spectra. In contrast to more conventional methods of assessing protonation state, VtC XES is a more direct probe applicable to a wide range of metalloenzyme systems. These data, coupled to insight provided by DFT calculations, allow the inorganic cores of the Mn^IVFe^IVand Mn^IVFe^IIIstates of the enzyme to be assigned as Mn^IV(μ‐O)₂Fe^IVand Mn^IV(μ‐O)(μ‐OH)Fe^III, respectively.

 

Proton transfer reactions are of central importance to a wide variety of biochemical processes, though determining proton location and monitoring proton transfers in biological systems is often extremely challenging. Herein, we use two‐color valence‐to‐core X‐ray emission spectroscopy (VtC XES) to identify protonation events across three oxidation states of the O₂‐activating, radical‐initiating manganese–iron heterodinuclear cofactor in a class I‐c ribonucleotide reductase. This is the first application of VtC XES to an enzyme intermediate and the first simultaneous measurement of two‐color VtC spectra. In contrast to more conventional methods of assessing protonation state, VtC XES is a more direct probe applicable to a wide range of metalloenzyme systems. These data, coupled to insight provided by DFT calculations, allow the inorganic cores of the Mn^IVFe^IVand Mn^IVFe^IIIstates of the enzyme to be assigned as Mn^IV(μ‐O)₂Fe^IVand Mn^IV(μ‐O)(μ‐OH)Fe^III, respectively.