J. A. van Bokhoven

Evolution of Fe species during the synthesis of over-exchanged Fe/ZSM5 obtained by chemical vapor deposition of FeCl3

The evolution of iron in over-exchanged Fe/ZSM5 prepared via chemical vapor deposition of FeCl3 was studied at each stage of the synthesis. Different characterization techniques (EXAFS, HR-XANES, 57 Fe Mossbauer spectroscopy, 27 Al NMR, EELS, HR-TEM, XRD, N2 physisorption, and FTIR spectroscopy) were applied in order to correlate the changes occurring in the local environment of the Fe atoms with migration and aggregation phenomena of iron at micro- and macroscopic scale. Mononuclear isolated Fe-species are formed upon FeCl3 sublimation, which are transformed into binuclear Fe-complexes during washing. During calcination, iron detached from the Bronsted sites migrates to the external surface of the zeolite, finally leading to significant agglomeration. Nevertheless, agglomeration of Fe can be strongly suppressed by adequately tuning the conditions of the calcination.  2002 Elsevier Science (USA). All rights reserved.

Metal organic frameworks for photo-catalytic water splitting

Metal organic frameworks (MOFs) have recently debuted as participants and solid supports in catalytic water splitting. Their porosity and structural versatility offer a tantalising consolidation of the components needed for solar light harvesting and water splitting. Herein, we describe a selection of relevant contemporary investigations that employ electrocatalysis, chemically introduced redox partners, and photo-catalysts to generate dioxygen and dihydrogen from water. The role of semiconducting MOFs in these systems is addressed, in tandem with band gap control by linker functionalisation and doping. Considered holistically, MOFs offer an impressive physical, spatial and chemical versatility with which to support and sustain water splitting reactions. Major challenges toward practical implementation do remain, but opportunities for development are evidently numerous.

A von Hamos x-ray spectrometer based on a segmented-type diffraction crystal for single-shot x-ray emission spectroscopy and time-resolved resonant inelastic x-ray scattering studies

We report on the design and performance of a wavelength-dispersive type spectrometer based on the von Hamos geometry. The spectrometer is equipped with a segmented-type crystal for x-ray diffraction and provides an energy resolution in the order of 0.25 eV and 1 eV over an energy range of 8000 eV-9600 eV. The use of a segmented crystal results in a simple and straightforward crystal preparation that allows to preserve the spectrometer resolution and spectrometer efficiency. Application of the spectrometer for time-resolved resonant inelastic x-ray scattering and single-shot x-ray emission spectroscopy is demonstrated.

Optimization of zeolite Beta by steaming and acid leaching for the acylation of anisole with octanoic acid: a structure–activity relation

Abstract This paper presents a catalytic and spectroscopic study of the dealumination of a commercial zeolite, Beta (Zeolyst). After dealumination by hydrochloric or oxalic acid and steaming (up to 973 K), increased activity and selectivity were found in the acylation of anisole with octanoic acid. The treatments led to the extraction of aluminum from the crystallographic T-sites in the framework structure, giving amorphous extraframework aluminum(–silicon)–oxide species in the case of steaming, while treatment with mineral acids or complexing agents removed framework and extraframework aluminum from the zeolite. 27 Al MQ MAS NMR revealed a changing of the aluminum coordinations and a shift from framework to extraframework species after steaming. Steaming removes aluminum from other crystallographic T-sites than acid leaching. Using nitrogen physisorption, no differences in the texture properties could be observed and XRD showed that the crystallinity was preserved, indicating high stability of the commercial zeolite Beta. The nature of the enhanced activity is suggested to result from higher accessibility of the active sites that are proposed to be associated with framework-connected aluminum atoms.

In situ X-ray absorption spectroscopy as a unique tool for obtaining information on hydrogen binding sites and electronic structure of supported Pt catalysts: towards an understanding of the compensation relation in alkane hydrogenolysis

L2 and L3 X-ray absorption near edge spectra (XANES) on supported Pt particles, with and without chemisorbed hydrogen, are shown to reflect the type of hydrogen-binding site on the Pt surface. FEFF8 ab initio multiple scattering calculations are used to determine XANES spectral fingerprints for the atop vs threefold H binding sites on Pt. Comparison of the experimental XANES data with the theoretical fingerprints, and further theoretical results, show that the acid/base properties of the support have a profound influence on the hydrogen coverage, and therefore on the mode of hydrogen adsorption on the Pt surface. As the electron richness of the support oxygen atoms increases (i.e., with increasing alkalinity of the support), the H coverage increases and the hydrogen-binding site of the strongly adsorbed hydrogen changes from atop to threefold. This site change is primarily responsible for the observed changes in previously reported kinetic data, which show an increase in negative order (roughly from −1. 5t o−2.5) in hydrogen partial pressure for neopentane hydrogenolysis with increasing support alkalinity. This change in negative order directly reflects the greater number of vacant Pt sites that must be available to allow adsorption of the neopentane. A compensation relation is found in the kinetic data of Pt on different supports resulting directly from this change in hydrogen coverage. This implies that the experimentally determined kinetic parameters are apparent values. These apparent values are correlated to the intrinsic kinetic parameters via the thermodynamic properties of the sorption of the reactants, described by the Temkin equation. The TOF of neopentane hydrogenolysis over several catalysts, measured in previous work, decreases with the increasing alkalinity of the support. This can now be directly explained as the result of the change in hydrogen coverage using a Frumkin isotherm, implying that the neopentane adsorption becomes weaker with increased hydrogen coverage. These conclusions, that hydrogen drives the catalysis, are further supported by density functional calculations on small Pt 4 clusters, which show that the acid/base properties of the support have a much larger direct influence on Pt–H bonding than on Pt–CH n bonding.  2003 Elsevier Science (USA). All rights reserved.

Particle size and support effects in hydrogenation over supported gold catalysts

Particle size and support effects were determined for the hydrogenation of nitrobenzene over gold supported on alumina and titania by kinetic experiments, TEM and in situ high energy resolution fluorescence detected X-ray absorption spectroscopy (HERFD XAS). Especially when supported on alumina, the catalytic activity correlated well with the fraction of particles smaller than 2 nm. These particles are able to split hydrogen on undercoordinated gold atoms, hence the strong particle size effect. In addition, on titania the increased length of the metal–support interface for smaller particles leads to enhanced activity because of the beneficial effect of the metal–support interface on the splitting of hydrogen. Reporting the average particle size is not always relevant in describing catalytic performance.

Apparatus for In Situ X-Ray Absorption Fine Structure Studies on Catalytic Systems in the Energy Range 1000 eV < E < 3500 eV

A new apparatus for in situ x-ray absoprtion fine structure measurements in the medium energy range of 1000–3500 eV has been developed. Measurements can be performed in a gaseous environment (max. pressure 1 bar) at temperatures ranging from 80 to 750 K. Pre-treatments can be performed at 5 bar and 750 K in the same cell, after which XAFS measurements can be done without exposing the sample to ambient air. In a modular set-up several detector systems can be used: fluorescence detection using a gas proportional counter, a photodiode or a microstrip detector. All detectors are highly integrated into the cell, gaining solid angle for detection. Electron yield detection can be used simultaneously using conversion electron yield or total electron yield. The performance of the new apparatus is demonstrated by a study of the K edge of Al in Zeolite Beta. The Al content is as low as 2 wt%. It will be shown that octahedral framework Al is formed while adding gaseous water at room temperature after ammonia removal ...

Communication: The electronic structure of matter probed with a single femtosecond hard x-ray pulse

Physical, biological, and chemical transformations are initiated by changes in the electronic configuration of the species involved. These electronic changes occur on the timescales of attoseconds (10−18 s) to femtoseconds (10−15 s) and drive all subsequent electronic reorganization as the system moves to a new equilibrium or quasi-equilibrium state. The ability to detect the dynamics of these electronic changes is crucial for understanding the potential energy surfaces upon which chemical and biological reactions take place. Here, we report on the determination of the electronic structure of matter using a single self-seeded femtosecond x-ray pulse from the Linac Coherent Light Source hard x-ray free electron laser. By measuring the high energy resolution off-resonant spectrum (HEROS), we were able to obtain information about the electronic density of states with a single femtosecond x-ray pulse. We show that the unoccupied electronic states of the scattering atom may be determined on a shot-to-shot basis and that the measured spectral shape is independent of the large intensity fluctuations of the incoming x-ray beam. Moreover, we demonstrate the chemical sensitivity and single-shot capability and limitations of HEROS, which enables the technique to track the electronic structural dynamics in matter on femtosecond time scales, making it an ideal probe technique for time-resolved X-ray experiments.

Flexible aluminium coordination of zeolites as function of temperature and water content, an in-situ method to determine aluminium coordinations

The aluminium coordinations in zeolites H-Beta and H-Y have been quantitatively investigated as a function of temperature in the presence and absence of water. In-situ Al K edge X-ray Absorption Spectroscopy shows that a framework tetrahedrally coordinated aluminium is stable in inert to at least 725 K. However, in the presence of water, already at room temperature, part of the framework tetrahedral aluminium is converted to an octahedral coordination. This octahedral aluminium is not stable in inert at 375 K, where it quantitatively reverts to the tetrahedral framework coordination.

The dedicated QEXAFS facility at the SLS: Performance and Scientific Opportunities

The SuperXAS beamline at the Swiss Light Source (SLS) features a permanently installed monochromator for quick scanning EXAFS (QEXAFS) spectroscopy in series with a conventional double crystal monochromator (DCM). All installed optical components like collimating and focussing mirrors can be used by both devices. The remote exchange of the monochromators is possible in less than five minutes while maintaining the beam geometry on the sample. The QEXAFS system allows fast absorption scans down to the millisecond range for the investigation of time dependent processes. Using a Si(111) channel cut crystal the energy range from 5–16 keV can be covered, with a Si(311) cut the range 9.5–30 keV. Usually a quick scanning interval of 0.1°–2° in Bragg angle is selected, thus covering XANES, full EXAFS or multiple edge scans of e.g. all L‐edges of a heavy element. Up to about 80 spectra per second can be collected, corresponding to a time resolution of 12.5 ms. The high intensity of the beamline even facilitates flu...