The future of artificial photosynthesis depends on economic and robust water oxidation catalysts (WOCs). Cobalt-based WOCs are especially promising for knowledge transfer between homogeneous and ...heterogeneous catalyst design. We introduce the active and stable {CoII 4O4} cubane CoII 4(dpy{OH}O)4(OAc)2(H2O)2(ClO4)2 (Co 4 O 4 -dpk) as the first molecular WOC with the characteristic {H2O-Co2(OR)2-OH2} edge-site motif representing the sine qua non moiety of the most efficient heterogeneous Co-oxide WOCs. DFT-MD modelings as well as in situ EXAFS measurements indicate the stability of the cubane cage in solution. The stability of Co 4 O 4 -dpk under photocatalytic conditions (Ru(bpy)32+/S2O8 2–) was underscored with a wide range of further analytical methods and recycling tests. FT-IR monitoring and HR-ESI-MS spectra point to a stable coordination of the acetate ligands, and DFT-MD simulations along with 1H/2H exchange experiments highlight a favorable intramolecular base functionality of the dpy{OH}O ligands. All three ligand types enhance proton mobility at the edge site through a unique bioinspired environment with multiple hydrogen-bonding interactions. In situ XANES experiments under photocatalytic conditions show that the {CoII 4O4} core undergoes oxidation to Co(III) or higher valent states, which recover rather slowly to Co(II). Complementary ex situ chemical oxidation experiments with Ru(bpy)33+ furthermore indicate that the oxidation of all Co(II) centers of Co 4 O 4 -dpk to Co(III) is not a mandatory prerequisite for oxygen evolution. Moreover, we present the CoII x Ni4–x (dpy{OH}O)4(OAc)2(H2O)2(ClO4)2 (Co x Ni 4–x O 4 -dpk) series as the first mixed Co/Ni-cubane WOCs. They newly bridge homogeneous and heterogeneous catalyst design through fine-tuned edge-site environments of the Co centers.
The transfer of charge at the molecular level plays a fundamental role in many areas of chemistry, physics, biology and materials science. Today, more than 60 years after the seminal work of R. A. ...Marcus, charge transfer is still a very active field of research. An important recent impetus comes from the ability to resolve ever faster temporal events, down to the attosecond time scale. Such a high temporal resolution now offers the possibility to unravel the most elementary quantum dynamics of both electrons and nuclei that participate in the complex process of charge transfer. This review covers recent research that addresses the following questions. Can we reconstruct the migration of charge across a molecule on the atomic length and electronic time scales? Can we use strong laser fields to control charge migration? Can we temporally resolve and understand intramolecular charge transfer in dissociative ionization of small molecules, in transition-metal complexes and in conjugated polymers? Can we tailor molecular systems towards specific charge-transfer processes? What are the time scales of the elementary steps of charge transfer in liquids and nanoparticles? Important new insights into each of these topics, obtained from state-of-the-art ultrafast spectroscopy and/or theoretical methods, are summarized in this review.
Accurate modeling of the X-ray absorption near-edge spectra (XANES) is required to unravel the local structure of metal sites in complex systems and their structural changes upon chemical or light ...stimuli. Two relevant examples are reported here concerning the following: (i) the effect of molecular adsorption on 3d metals hosted inside metal–organic frameworks and (ii) light induced dynamics of spin crossover in metal–organic complexes. In both cases, the amount of structural models for simulation can reach a hundred, depending on the number of structural parameters. Thus, the choice of an accurate but computationally demanding finite difference method for the ab initio X-ray absorption simulations severely restricts the range of molecular systems that can be analyzed by personal computers. Employing the FDMNES code Phys. Rev. B, 2001, 63, 125120 we show that this problem can be handled if a proper diagonalization scheme is applied. Due to the use of dedicated solvers for sparse matrices, the calculation time was reduced by more than 1 order of magnitude compared to the standard Gaussian method, while the amount of required RAM was halved. Ni K-edge XANES simulations performed by the accelerated version of the code allowed analyzing the coordination geometry of CO and NO on the Ni active sites in CPO-27-Ni MOF. The Ni–CO configuration was found to be linear, while Ni–NO was bent by almost 90°. Modeling of the Fe K-edge XANES of photoexcited aqueous Fe(bpy)32+ with a 100 ps delay we identified the Fe–N distance elongation and bipyridine rotation upon transition from the initial low-spin to the final high-spin state. Subsequently, the X-ray absorption spectrum for the intermediate triplet state with expected 100 fs lifetime was theoretically predicted.
The kinetics involved in a recently revealed ambient-temperature mechanism for the catalytic oxidation of carbon monoxide by oxygen over a 5 wt % Pt/Al2O3 catalyst are evaluated within a periodic, ...plug flow, redox operation paradigm using combined mass spectrometry (MS), diffuse reflectance infrared spectroscopy (DRIFTS), and time-resolved Pt L3-edge XAFS. The species that are the most active at room temperature are shown to be a high-wavenumber (ca. 1690 cm–1) carbonate that we associate directly with a room-temperature redox process occurring in a fraction of the Pt atoms present in the catalyst. Our results, however, do not exclude the participation of carbonate species native to the Al2O3 support, though these species tend to store CO at ambient temperature and become significant participants in CO oxidation catalysis only at slightly higher temperatures (323–333 K). Pt carbonate formation (1690 cm–1) under CO and the reaction to yield CO2 is shown to be extremely rapid and subject to an average apparent activation energy (E app), across the techniques applied, of 8.7 kJ mol–1, within the temperature range investigated (276–343 K). Reoxidation of Pt (XANES) and subsequent CO2 production mediated by Pt carbonates under O2 (MS/IR) displays a first-order dependence upon O2 partial pressure and a negative dependence upon the coverage of CO adsorbed on the Pt nanoparticles also present in this catalyst. This oxidative regeneration/CO2 production step is subject to an apparent activation energy (E app) of 56.5 (±5) kJ mol–1, is kinetically limited by the desorption of molecular CO from Pt nanoparticles, and also is shown to be dependent upon the partial pressure of O2 present in the oxidizing half of the cycle that we associate with the direct interaction of O2 with molecular CO adsorbed on the nanoparticles that promotes their desorption. Finally, a minority reactive state producing CO2 in the oxidizing cycle that displays no dependence upon the CO coverage of the nanoparticles can be induced through simple thermal treatment of the catalyst. These results are discussed in terms of the number and types of species present within the reactive system and in terms of the wider possibilities for the development of effective low-temperature CO oxidation using Pt/Al2O3 catalysts.
OLED technology beyond small or expensive devices requires light-emitters, luminophores, based on earth-abundant elements. Understanding and experimental verification of charge transfer in ...luminophores are needed for this development. An organometallic multicore Cu complex comprising Cu-C and Cu-P bonds represents an underexplored type of luminophore. To investigate the charge transfer and structural rearrangements in this material, we apply complementary pump-probe X-ray techniques: absorption, emission, and scattering including pump-probe measurements at the X-ray free-electron laser SwissFEL. We find that the excitation leads to charge movement from C- and P- coordinated Cu sites and from the phosphorus atoms to phenyl rings; the Cu core slightly rearranges with 0.05 Å increase of the shortest Cu-Cu distance. The use of a Cu cluster bonded to the ligands through C and P atoms is an efficient way to keep structural rigidity of luminophores. Obtained data can be used to verify computational methods for the development of luminophores.
Many important catalytic reactions take place at metal–oxide interfaces. However, their mechanisms are typically difficult to probe due to the low concentration of active sites and the lack of highly ...sensitive spectroscopic methods. In this work, we analyze the impact of oxide reducibility on the mechanism of low-temperature CO oxidation over platinum nanoparticles supported on ceria-based solid solutions. We demonstrate that the easier reducibility of Ce4+ at the Pt/Ce0.5Sn0.5O2 interface (in comparison to that for Pt/CeO2) increases the catalytic CO oxidation rate, lowers the apparent activation energy, and increases the reaction order in oxygen. Operando time-resolved X-ray absorption spectroscopy suggests that the Ce4+ reduction rate at the Pt/Ce0.5Sn0.5O2 interface is accelerated, while the Ce3+ oxidation rate becomes rate-limiting. Importantly, no reduction of Sn4+ in the ceria–tin solid solution and no formation of Pt/Sn alloys were detected under relevant reaction conditions using in situ X-ray absorption spectroscopy, ambient-pressure X-ray photoelectron spectroscopy, and infrared spectroscopy. This work provides a better understanding on the reactivity of interfaces. It demonstrates that the reducibility of the oxide close to a metal strongly influences catalytic rates, which provides ideas for the design of better catalysts.
Spectroscopic techniques to study the electron configuration and local coordination of a central atom by detecting inner-shell radiative decays following photoexcitation using hard X-rays are ...presented. The experimental setup requires an X-ray spectrometer based on perfect crystal Bragg optics. The possibilities arising from non-resonant and resonant X-ray emission spectroscopy are discussed when the instrumental energy broadenings of the primary (beamline) monochromator and the crystal spectrometer for X-ray emission detection are on the order of the core hole lifetimes of the intermediate and final electronic states. The small energy bandwidth in the emission detection yields line-sharpened absorption features. In transition metal compounds, electron–electron interactions as well as orbital splittings and fractional population can be revealed. Combination with EXAFS spectroscopy enables to extend the
k-range beyond unwanted absorption edges in dilute samples that limit the EXAFS range in conventional absorption spectroscopy.
We report on an element-selective study of the fate of charge carriers in photoexcited inorganic CsPbBr3 and CsPb(ClBr)3 perovskite nanocrystals in toluene solutions using time-resolved X-ray ...absorption spectroscopy with 80 ps time resolution. Probing the Br K-edge, the Pb L3-edge, and the Cs L2-edge, we find that holes in the valence band are localized at Br atoms, forming small polarons, while electrons appear as delocalized in the conduction band. No signature of either electronic or structural changes is observed at the Cs L2-edge. The results at the Br and Pb edges suggest the existence of a weakly localized exciton, while the absence of signatures at the Cs edge indicates that the Cs+ cation plays no role in the charge transport, at least beyond 80 ps. This first, time-resolved element-specific study of perovskites helps understand the rather modest charge carrier mobilities in these materials.
Understanding structure-performance relationships are essential for the rational design of new functional materials or in the further optimization of (catalytic) processes. Due to the high ...penetration depth of the radiation used, synchrotron-based hard X-ray techniques (with energy > 4.5 keV) allow the study of materials under realistic conditions (in situ and operando) and thus play an important role in uncovering structure-performance relationships. X-ray absorption and emission spectroscopies (XAS and XES) give insight into the electronic structure (oxidation state, spin state) and local geometric structure (type and number of nearest neighbor atoms, bond distances, disorder) up to ~5 Å around the element of interest. In this mini review, we will give an overview of the in situ and operando capabilities of the SuperXAS beamline, a facility for hard X-ray spectroscopy, through recent examples from studies of heterogeneous catalysts, electrochemical systems, and photoinduced processes. The possibilities for time-resolved experiments in the time range from ns to seconds and longer are illustrated. The extension of X-ray spectroscopy at the new Debye beamline combined with operando X-ray scattering and diffraction and further developments of time-resolved XES at SuperXAS will open new possibilities after the Swiss Light Source upgrade mid 2025.
We present a new setup for resonant inelastic hard x-ray scattering at the Bernina beamline of SwissFEL with energy, momentum, and temporal resolution. The compact R = 0.5 m Johann-type spectrometer ...can be equipped with up to three crystal analyzers and allows efficient collection of RIXS spectra. Optical pumping for time-resolved studies can be realized with a broad span of optical wavelengths. We demonstrate the performance of the setup at an overall ∼180 meV resolution in a study of ground-state and photoexcited (at 400 nm) honeycomb 5d iridate α-Li2IrO3. Steady-state RIXS spectra at the iridium L3-edge (11.214 keV) have been collected and are in very good agreement with data collected at synchrotrons. The time-resolved RIXS transients exhibit changes in the energy loss region <2 eV, whose features mostly result from the hopping nature of 5d electrons in the honeycomb lattice. These changes are ascribed to modulations of the Ir-to-Ir inter-site transition scattering efficiency, which we associate to a transient screening of the on-site Coulomb interaction.
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