S. Pascarelli

X-ray optics of a dynamical sagittal-focusing monochromator on the GILDA beamline at the ESRF

The performance of a dynamical sagittal-focusing monochromator for hard X-rays is described. It consists of a flat first crystal and a diamond-shaped ribbed second crystal which is clamped by its central rib and dynamically bent by applying a force on its two apices. The system has proved to perform very well on the GILDA beamline at the ESRF. The horizontal acceptance varies with energy and with focusing geometry as predicted theoretically; the total available horizontal fan of radiation (3.6 mrad) is in fact collected in the 1:3 geometry. The system is routinely run in a dynamical focusing mode for XAFS spectroscopy in the energy range 5-30 keV with Si(311) crystals, with a constant spot size FWHM ~1 mm on the sample and without degradation of energy resolution or reproducibility. Using simple geometrical considerations we calculate the variations of the horizontal profile of the reflected beam during rocking-curve scans in different focusing geometries and find them in agreement with observed ones. Not only is this is a practical aid in alignment but it illustrates the X-ray optics of sagittal focusing in an elegant way.

In Situ Redispersion of Platinum Autoexhaust Catalysts: An On-Line Approach to Increasing Catalyst Lifetimes?†

Supported precious metals, such as platinum (Pt), rhodium (Rh), and palladium (Pd), are used to facilitate many industrial catalytic processes. Pt in particular is found at the core of catalysts used throughout the petrochemical industry: from bifunctional catalysts (isomerization/dehydrogenation) used for refining of hydrocarbon fuel stocks, to three-way (CO and hydrocarbon oxidation/NOx reduction) conversions within car exhausts. In this latter, ubiquitous application— commercialized in the USA and Japan in 1977—Pt has always been a pivotal component in the abatement of harmful gas emissions from gasolineor diesel-driven engines. The ever-increasing appreciation of the damage that noxious gas emissions are doing to our environment and the finite availability of noble metals provide strong drivers for the continued study and optimization of the behavior of Pt-based three-way catalysts (TWCs). Central to technological progress in this area is a fundamental understanding of how these materials behave, which may allow us to stop them degrading or deactivating during operation. A longstanding problem, affecting many applications that use highly dispersed metal nanoparticles, is loss of active surface area in the metal components as a result of “sintering”. This is a particularly pernicious problem in applications in which catalysts have to experience high temperatures—in excess of 800 8C in the case of modern car catalysts. This deleterious process causes the particle size of the metal to increase massively—through either particle diffusion or agglomeration or through “ripening” processes. The result is that a large fraction of the active metal is effectively “hidden away” within the bulk of these larger particles where it cannot be used to affect the desired chemical conversions that occur on the particle surface. This central issue of exhaust catalyst deactivation has long been recognized in the hydrocarbon reforming and emission abatement industries. In the former industry, “oxidative redispersion” has been utilized to reverse the effects of sintering and regenerate spent Pt-based reforming catalysts. However, whereas other noble metal particles such as Pd or Rh can be effectively redispersed by gaseous oxygen at certain temperatures, this method is efficient for Pt catalysts only when Cl is present either in the catalyst formulation or as an adjunct added during the redispersion process: in the absence of Cl, redispersion in Pt/Al2O3 by oxygen is limited both to a narrow temperature window (of around 500 8C) and a low level of redispersion. 6] Further, a continuous oxidative treatment over time is required for this redispersion process. Exhaust gases exiting from gasoline engines change quickly and dramatically during operation. Temperatures can rise transiently to around 1000 8C, and the exhaust gas composition itself fluctuates quickly between oxidative and reductive compositions. Clearly, the conventional approach to redispersion and reactivation is highly unsuitable on many counts for “on-board” redispersion and regeneration of TWCs. Other regeneration phenomena have recently been shown in some related cases. The “intelligent” catalyst system of Daihatsu shows in-built structural reversibility of the noble metal component. In this case, it is the structure of the perovskite support that provides the foundation for this extremely elegant piece of applied catalyst design. The possibility of forming very large particles is intrinsically reduced and, under some circumstances, this technology has been successfully commercialized. However, this approach is very much dependent upon the structure of a particular and low surface area support material and is limited in this sense. [*] Dr. Y. Nagai, K. Dohmae, T. Tanabe, Dr. H. Shinjoh TOYOTA Central R&D Labs., Inc. Nagakute, Aichi 480-1192 (Japan) Fax: (+ 81)561-63-6150 E-mail: e1062@mosk.tytlabs.co.jp

Energy-dispersive absorption spectroscopy for hard-X-ray micro-XAS applications.

Originally developed for time-resolved X-ray absorption spectroscopy (XAS), energy-dispersive absorption spectroscopy offers new opportunities for applications such as fluorescence detection and microbeams for scanning probe spectroscopy, thanks to recent developments in both instrumentation and optics. In this context, this paper presents a first example of chemical mapping recorded at ID24, the energy-dispersive XAS beamline at the ESRF. Attributes of this geometry for microanalysis are addressed. Finally, present and future plans are discussed and developed in the light of the evolution of the focal spot on this instrument in the past ten years.

Dynamics of the Magnetic and Structural alpha-epsilon Phase Transition in Iron

We have studied the high-pressure iron bcc to hcp phase transition by simultaneous x-ray magnetic circular dichroism and x-ray absorption spectroscopy with an x-ray energy dispersive spectrometer. The combination of the two techniques allows us to obtain simultaneously information on both the structure and the magnetic state of iron under pressure. The magnetic and structural transitions simultaneously observed are sharp. Both are of first order in agreement with the theoretical prediction. The pressure domain of the transition observed (2.4+/-0.2 GPa) is narrower than that usually cited in the literature (8 GPa). Our data indicate that the magnetic transition slightly precedes the structural one, suggesting that the origin of the instability of the bcc phase in iron with increasing pressure is to be attributed to the effect of pressure on magnetism as predicted by spin-polarized full-potential total energy calculations.

Measurement of femtometre-scale atomic displacements by X-ray absorption spectroscopy

The frequencies of extended X-ray absorption fine-structure (EXAFS) measurements, which are oscillations occurring on the high-energy side of an X-ray absorption edge, can be used to identify interatomic distances in materials. We have used a dispersive X-ray spectrometer, which has no moving components, to make rapid measurements with minimal energy drift of the difference in EXAFS from the Fe K edge in an iron-cobalt thin film undergoing periodic strain through magnetostriction. We show that magnetostriction can be detected by differential X-ray absorption. The magnitude of the recorded signal relative to the noise shows a sensitivity to mean differential atomic motion of one femtometre: a factor of 100 times more sensitive than that normally available.

Portable laser‐heating system for diamond anvil cells

The diamond anvil cell (DAC) technique coupled with laser heating has become the most successful method for studying materials in the multimegabar pressure range at high temperatures. However, so far all DAC laser-heating systems have been stationary: they are linked either to certain equipment or to a beamline. Here, a portable laser-heating system for DACs has been developed which can be moved between various analytical facilities, including transfer from in-house to a synchrotron or between synchrotron beamlines. Application of the system is demonstrated in an example of nuclear inelastic scattering measurements of ferropericlase (Mg(0.88)Fe(0.12))O and h.c.p.-Fe(0.9)Ni(0.1) alloy, and X-ray absorption near-edge spectroscopy of (Mg(0.85)Fe(0.15))SiO(3) majorite at high pressures and temperatures. Our results indicate that sound velocities of h.c.p.-Fe(0.9)Ni(0.1) at pressures up to 50 GPa and high temperatures do not follow a linear relation with density.

Turbo-XAS: dispersive XAS using sequential acquisition

A new experimental technique for time-resolved X-ray absorption studies in the sub-second range has been successfully tested on the dispersive XAS beamline (ID24) at the ESRF. It consists of a sequential acquisition of energy points using the dispersive optics scheme installed on the beamline. Turbo-XAS takes full advantage of the properties of third-generation radiation sources, overcoming many of the problems encountered in the classical dispersive XAS mode, based on position-sensitive detectors. The new technique benefits from the basic assets of the dispersive set-up, i.e. the absence of movement of the optics and the extremely small and stable horizontal focal spot. In addition, it features simultaneous recording of I0 and I1 and the possibility of performing fluorescence and electron detection.

The time-resolved and extreme conditions XAS (TEXAS) facility at the European Synchrotron Radiation Facility: the general-purpose EXAFS bending-magnet beamline BM23

BM23 is the general-purpose EXAFS bending-magnet beamline at the ESRF, replacing the former BM29 beamline in the framework of the ESRF upgrade. Its mission is to serve the whole XAS user community by providing access to a basic service in addition to the many specialized instruments available at the ESRF. BM23 offers high-signal-to-noise ratio EXAFS in a large energy range (5–75 keV), continuous energy scanning for quick-EXAFS on the second timescale and a micro-XAS station delivering a spot size of 4 µm × 4 µm FWHM.

STRUCTURAL AND OPTICAL INVESTIGATION OF INASXP1-X/INP STRAINED SUPERLATTICES

We report a complete characterization of InAsxP1−x/InP (0.05<x<0.59) superlattices epitaxially grown by low pressure metalorganic chemical vapor deposition and by chemical beam epitaxy. Samples were obtained by both conventional growth procedures and by periodically exposing the just-grown InP surface to an AsH3 flux. Using the latter procedure, very thin InAsxP1−x/InP layers (10–20 A) are obtained by P↔As substitutions effects. Arsenic composition of the so obtained layers depends both on AsH3 flux intensity and exposure times. Samples have been characterized by means of high resolution x-ray diffraction, high resolution transmission electron microscopy, 4 K photoluminescence, and extended x ray absorption fine structure spectroscopy. The combined use of high resolution x-ray diffraction and of 4 K photoluminescence, with related simulations, allows us to predict both InAsP composition and width, which are qualitatively confirmed by electron microscopy. Our study indicates that the effect of the formatio...

Optimization of Paris–Edinburgh press cell assemblies for in situ monochromatic X-ray diffraction and X-ray absorption

We describe some important improvements allowed by the development of new cell assemblies coupled to opposed conical sintered diamond anvils in the Paris–Edinburgh press. We provide X-ray absorption and diffraction experiments carried out at pressures up to 16.5 GPa. The maximum temperature reached was 1800 K for P<10 GPa and 1300 K for higher pressures. The sintered diamond anvils are X-ray transparent and give access to a much larger X-ray window than the tungsten carbide anvils, even at the highest pressure. Therefore, X-ray measurements are performed using in situ cross-calibration simultaneously. We also describe a new heating setup used to reach high temperatures, despite the low conductivity of the sintered diamond core by deviating the electrical current using copper strips. These improvements are illustrated by recent data collected using angle dispersive in situ X-ray diffraction on liquid Fe-18%wt S and using EXAFS at the barium K-edge on Ba8Si46 silicon clathrates and at the iodine K-edge on iodine-intercalated nanotubes.