Benedikt Lassalle-Kaiser

Structure–Activity Correlations in a Nickel–Borate Oxygen Evolution Catalyst

An oxygen evolution catalyst that forms as a thin film from Ni(aq)(2+) solutions containing borate electrolyte (Ni-B(i)) has been studied by in situ X-ray absorption spectroscopy. A dramatic increase in catalytic rate, induced by anodic activation of the electrodeposited films, is accompanied by structure and oxidation state changes. Coulometric measurements correlated with X-ray absorption near-edge structure spectra of the active catalyst show that the nickel centers in activated films possess an average oxidation state of +3.6, indicating that a substantial proportion of nickel centers exist in a formal oxidation state of Ni(IV). In contrast, nickel centers in nonactivated films exist predominantly as Ni(III). Extended X-ray absorption fine structure reveals that activated catalyst films comprise bis-oxo/hydroxo-bridged nickel centers organized into sheets of edge-sharing NiO(6) octahedra. Diminished long-range ordering in catalyst films is due to their ostensibly amorphous nature. Nonactivated films display a similar oxidic nature but exhibit a distortion in the local coordination geometry about nickel centers, characteristic of Jahn-Teller distorted Ni(III) centers. Our findings indicate that the increase in catalytic activity of films is accompanied by changes in oxidation state and structure that are reminiscent of those observed for conversion of β-NiOOH to γ-NiOOH and consequently challenge the long-held notion that the β-NiOOH phase is a more efficient oxygen-evolving catalyst.

Simultaneous femtosecond X-ray spectroscopy and diffraction of photosystem II at room temperature.

One Protein, Two Probes A central challenge in the use of x-ray diffraction to characterize macromolecular structure is the propensity of the high-energy radiation to damage the sample during data collection. Recently, a powerful accelerator-based, ultrafast x-ray laser source has been used to determine the geometric structures of small protein crystals too fragile for conventional diffraction techniques. Kern et al. (p. 491, published online 14 February) now pair this method with concurrent x-ray emission spectroscopy to probe electronic structure, as well as geometry, and were able to characterize the metal oxidation states in the oxygen-evolving complex within photosystem II crystals, while simultaneously verifying the surrounding protein structure. A powerful x-ray laser source can extract the geometry and electronic structure of metalloenzymes prior to damaging them. Intense femtosecond x-ray pulses produced at the Linac Coherent Light Source (LCLS) were used for simultaneous x-ray diffraction (XRD) and x-ray emission spectroscopy (XES) of microcrystals of photosystem II (PS II) at room temperature. This method probes the overall protein structure and the electronic structure of the Mn4CaO5 cluster in the oxygen-evolving complex of PS II. XRD data are presented from both the dark state (S1) and the first illuminated state (S2) of PS II. Our simultaneous XRD-XES study shows that the PS II crystals are intact during our measurements at the LCLS, not only with respect to the structure of PS II, but also with regard to the electronic structure of the highly radiation-sensitive Mn4CaO5 cluster, opening new directions for future dynamics studies.

In Situ X-ray Absorption Spectroscopy Investigation of a Bifunctional Manganese Oxide Catalyst with High Activity for Electrochemical Water Oxidation and Oxygen Reduction

In situ X-ray absorption spectroscopy (XAS) is a powerful technique that can be applied to electrochemical systems, with the ability to elucidate the chemical nature of electrocatalysts under reaction conditions. In this study, we perform in situ XAS measurements on a bifunctional manganese oxide (MnOx) catalyst with high electrochemical activity for the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER). Using X-ray absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS), we find that exposure to an ORR-relevant potential of 0.7 V vs RHE produces a disordered Mn3(II,III,III)O4 phase with negligible contributions from other phases. After the potential is increased to a highly anodic value of 1.8 V vs RHE, relevant to the OER, we observe an oxidation of approximately 80% of the catalytic thin film to form a mixed Mn(III,IV) oxide, while the remaining 20% of the film consists of a less oxidized phase, likely corresponding to unchanged Mn3(II,III,III)O4. XAS and electrochemical characterization of two thin film catalysts with different MnOx thicknesses reveals no significant influence of thickness on the measured oxidation states, at either ORR or OER potentials, but demonstrates that the OER activity scales with film thickness. This result suggests that the films have porous structure, which does not restrict electrocatalysis to the top geometric layer of the film. As the portion of the catalyst film that is most likely to be oxidized at the high potentials necessary for the OER is that which is closest to the electrolyte interface, we hypothesize that the Mn(III,IV) oxide, rather than Mn3(II,III,III)O4, is the phase pertinent to the observed OER activity.

Taking snapshots of photosynthetic water oxidation using femtosecond X-ray diffraction and spectroscopy

The dioxygen we breathe is formed from water by its light-induced oxidation in photosystem II. O2 formation takes place at a catalytic manganese cluster within milliseconds after the photosystem II reaction center is excited by three single-turnover flashes. Here we present combined X-ray emission spectra and diffraction data of 2 flash (2F) and 3 flash (3F) photosystem II samples, and of a transient 3F′ state (250 μs after the third flash), collected under functional conditions using an X-ray free electron laser. The spectra show that the initial O-O bond formation, coupled to Mn-reduction, does not yet occur within 250 μs after the third flash. Diffraction data of all states studied exhibit an anomalous scattering signal from Mn but show no significant structural changes at the present resolution of 4.5 Å. This study represents the initial frames in a molecular movie of the structural changes during the catalytic reaction in photosystem II.

Room temperature femtosecond X-ray diffraction of photosystem II microcrystals

Most of the dioxygen on earth is generated by the oxidation of water by photosystem II (PS II) using light from the sun. This light-driven, four-photon reaction is catalyzed by the Mn4CaO5 cluster located at the lumenal side of PS II. Various X-ray studies have been carried out at cryogenic temperatures to understand the intermediate steps involved in the water oxidation mechanism. However, the necessity for collecting data at room temperature, especially for studying the transient steps during the O–O bond formation, requires the development of new methodologies. In this paper we report room temperature X-ray diffraction data of PS II microcrystals obtained using ultrashort (< 50 fs) 9 keV X-ray pulses from a hard X-ray free electron laser, namely the Linac Coherent Light Source. The results presented here demonstrate that the ”probe before destroy” approach using an X-ray free electron laser works even for the highly-sensitive Mn4CaO5 cluster in PS II at room temperature. We show that these data are comparable to those obtained in synchrotron radiation studies as seen by the similarities in the overall structure of the helices, the protein subunits and the location of the various cofactors. This work is, therefore, an important step toward future studies for resolving the structure of the Mn4CaO5 cluster without any damage at room temperature, and of the reaction intermediates of PS II during O–O bond formation.

Structural and Electronic Study of an Amorphous MoS3 Hydrogen‐Generation Catalyst on a Quantum‐Controlled Photosensitizer

The design and synthesis of catalysts, especially for the production of solar fuels, is a major challenge in developing sources of renewable energy. Catalyst development requires an understanding of the mechanism(s) involved and the nature of the active site. While platinum group metals have unrivalled activity for both hydrogen and oxygen evolution, they are scarce and expensive. Photocatalytic systems relying on earth-abundant materials are therefore desirable for large scale energy production. Herein, we examine the structure and electronic properties of an amorphous molybdenum sulfide species and its possible use for photocatalytic hydrogen evolution. The catalyst was grown on a seeded quantum-rod sensitizer, a model system for investigating the photophysics of solar fuel generation. This catalyst s activity is shown experimentally to be associated with under-coordinated molybdenum centers, and we document that a reduced form of MoS3 is an active species for hydrogen generation. Molybdenum sulfides are prevalent in both biological enzymes and industrial catalysts. Mo metalloenzymes are involved in carbon, nitrogen, and sulfur metabolism, while synthetic molybdenum sulfides serve as industrial hydrotreating catalysts and are proven electrocatalysts for the hydrogen evolution reaction (HER). MoS2, [13,14] incomplete cubane [Mo3S4] 4+ clusters, molecular molybdenum catalysts, and amorphous MoS2 made by a reduction of MoS3 [19] have been shown to be active HER catalysts. Highly active HER catalysts, including Pt, have a Gibbs free energy of H adsorption (DGH) close to zero. [14] Density functional theory calculations show that the equatorial sulfur atoms in Fe–Mo cofactors in nitrogenase enzymes as well as the bridging S atom on the edge sites of MoS2 bind H atoms with DGH 0. These calculations, coupled with scanning tunneling microscopy (STM) studies, have indicated that molybdenum sulfide based hydrodesulfurization and HER catalysts derive their activities from under-coordinated atoms. Recent investigations of MoS2 nanoparticles using STM combined with electrochemical measurements have revealed that HER activity scales with the number of edge sites, rather than nanoparticle area, adding substantial evidence that undercoordination is critical to activity. There is also substantial current interest in molecularly thick and structurally disordered metal oxide and sulfide layers supported on electrodes, surfaces, and nanoparticles as potential catalysts for the HER and oxygen evolution reaction (OER). Such ultrathin films can support a variety of unusual and possibly favorable bonding geometries and may retain flexibility in healing and recovering. Despite their potential, such systems remain very difficult to characterize, impeding reproducibility and the communication of results between groups. Mechanisms are difficult to pin down when structural and electronic characterization is lacking. In this work, we use X-ray absorption techniques to obtain structural information on a catalytically active disordered molybdenum chalcogenide species that was grown on a wellcontrolled seeded quantum rod photosensitizer system with very high surface area. The high surface area of the colloidal system enables us to employ a variety of X-ray characterization techniques. Yet the system is also well-defined: Amorphous layers of MoS3 are deposited on quantumcontrolled photosensitizers. We take advantage of recent work showing that cadmium chalcogenide nanocrystals can be engineered to systematically control the separation of photogenerated holes and electrons, thus allowing us to modulate the photochemical yield of hydrogen. Nanorods of CdS grown on CdSe seeds with varying diameters and pure CdS nanorods of differing length were synthesized by a seeded-growth method previously reported. These particles have been of interest as a model system for investigating photochemical HER because their [*] Dr. M. L. Tang, D. C. Grauer, Dr. L. Amirav, Prof. J. R. Long, Prof. A. P. Alivisatos Department of Chemistry, University of California, Berkeley Material Sciences, Lawrence Berkeley National Laboratory (LBNL) Berkeley, CA 94720 (USA) E-mail: alivis@berkeley.edu jyano@lbl.gov

Nanoflow electrospinning serial femtosecond crystallography

An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14-3.1 µl min(-1) to perform serial femtosecond crystallography (SFX) studies with X-ray lasers. Thermolysin microcrystals flowed at 0.17 µl min(-1) and diffracted to beyond 4 Å resolution, producing 14,000 indexable diffraction patterns, or four per second, from 140 µg of protein. Nanoflow electrospinning extends SFX to biological samples that necessitate minimal sample consumption.

Accurate macromolecular structures using minimal measurements from X-ray free-electron lasers

X-ray free-electron laser (XFEL) sources enable the use of crystallography to solve three-dimensional macromolecular structures under native conditions and without radiation damage. Results to date, however, have been limited by the challenge of deriving accurate Bragg intensities from a heterogeneous population of microcrystals, while at the same time modeling the X-ray spectrum and detector geometry. Here we present a computational approach designed to extract meaningful high-resolution signals from fewer diffraction measurements.

A High-Spin Iron(IV)–Oxo Complex Supported by a Trigonal Nonheme Pyrrolide Platform

We report the generation and characterization of a new high-spin iron(IV)-oxo complex supported by a trigonal nonheme pyrrolide platform. Oxygen-atom transfer to [(tpa(Mes))Fe(II)](-) (tpa(Ar) = tris(5-arylpyrrol-2-ylmethyl)amine) in acetonitrile solution affords the Fe(III)-alkoxide product [(tpa(Mes2MesO))Fe(III)](-) resulting from intramolecular C-H oxidation with no observable ferryl intermediates. In contrast, treatment of the phenyl derivative [(tpa(Ph))Fe(II)](-) with trimethylamine N-oxide in acetonitrile solution produces the iron(IV)-oxo complex [(tpa(Ph))Fe(IV)(O)](-) that has been characterized by a suite of techniques, including mass spectrometry as well as UV-vis, FTIR, Mössbauer, XAS, and parallel-mode EPR spectroscopies. Mass spectral, FTIR, and optical absorption studies provide signatures for the iron-oxo chromophore, and Mössbauer and XAS measurements establish the presence of an Fe(IV) center. Moreover, the Fe(IV)-oxo species gives parallel-mode EPR features indicative of a high-spin, S = 2 system. Preliminary reactivity studies show that the high-spin ferryl tpa(Ph) complex is capable of mediating intermolecular C-H oxidation as well as oxygen-atom transfer chemistry.

Evidence from in situ X-ray absorption spectroscopy for the involvement of terminal disulfide in the reduction of protons by an amorphous molybdenum sulfide electrocatalyst.

The reduction of protons into dihydrogen is important because of its potential use in a wide range of energy applications. The preparation of efficient and cheap catalysts for this reaction is one of the issues that need to be tackled to allow the widespread use of hydrogen as an energy carrier. In this paper, we report the study of an amorphous molybdenum sulfide (MoSx) proton reducing electrocatalyst under functional conditions, using in situ X-ray absorption spectroscopy. We probed the local and electronic structures of both the molybdenum and sulfur elements for the as prepared material as well as the precatalytic and catalytic states. The as prepared material is very similar to MoS3 and remains unmodified under functional conditions (pH = 2 aqueous HNO3) in the precatalytic state (+0.3 V vs RHE). In its catalytic state (−0.3 V vs RHE), the film is reduced to an amorphous form of MoS2 and shows spectroscopic features that indicate the presence of terminal disulfide units. These units are formed concomitantly with the release of hydrogen, and we suggest that the rate-limiting step of the HER is the reduction and protonation of these disulfide units. These results show the implication of terminal disulfide chemical motifs into HER driven by transition-metal sulfides and provide insight into their reaction mechanism.