We present the first structure determination by surface x-ray diffraction during the restructuring of a model catalyst under reaction conditions, i.e., at high pressure and high temperature, and correlate the restructuring with a change in catalytic activity. We have analyzed the Pt(110) surface during CO oxidation at pressures up to 0.5 bar and temperatures up to 625 K. Depending on the O 2 =CO pressure ratio, we find three well-defined structures: namely, (i) the bulk-terminated Pt(110) surface, (ii) a thin, commensurate oxide, and (iii) a thin, incommensurate oxide. The commensurate oxide only appears under reaction conditions, i.e., when both O 2 and CO are present and at sufficiently high temperatures. Density functional theory calculations indicate that the commensurate oxide is stabilized by carbonate ions (CO 2ÿ 3 ). Both oxides have a substantially higher catalytic activity than the bulk-terminated Pt surface.

Atomic steps at the surface of a catalyst play an important role in heterogeneous catalysis, for example as special sites with increased catalytic activity. Exposure to reactants can cause entirely new structures to form at the catalyst surface, and these may dramatically influence the reaction by 'poisoning' it or by acting as the catalytically active phase. For example, thin metal oxide films have been identified as highly active structures that form spontaneously on metal surfaces during the catalytic oxidation of carbon monoxide. Here, we present operando X-ray diffraction experiments on a palladium surface during this reaction. They reveal that a high density of steps strongly alters the stability of the thin, catalytically active palladium oxide film. We show that stabilization of the metal, caused by the steps and consequent destabilization of the oxide, is at the heart of the well-known reaction rate oscillations exhibited during CO oxidation at atmospheric pressure.

Using a combined experimental and theoretical approach, we show that a thin RhO 2 oxide film forms on a Pt 25 Rh 75 (100) surface at elevated oxygen pressures and temperatures prior to the bulk oxidation. By the use of in situ surface x-ray diffraction under realistic CO oxidation reaction conditions, we show that the onset of the growth of thin RhO 2 oxide film coincides with an increase in CO 2 production. During the reaction, the consumed oxide film is continuously re-grown by oxygen in the gas phase. Our theoretical results strongly suggest that the CO adsorbs on the metallic substrate but reacts with the O in the RhO 2 oxide film at the border between the RhO 2 oxide film and the metallic substrate. This scenario could explain the experimental observations of oxidation reactions on other late transition metal surfaces as well as on their corresponding nanoparticles under realistic conditions.

The oxidation of a vicinal Pd͑553͒ surface has been studied from ultrahigh vacuum ͑UHV͒ to atmospheric oxygen pressures at elevated sample temperatures. The investigation combines traditional electron based UHV techniques such as high resolution core level spectroscopy, low-energy electron diffraction, scanning tunneling microscopy with in situ surface x-ray diffraction, and ab initio simulations. In this way, we show that the O atoms preferentially adsorb at the step edges at oxygen pressures below 10 −6 mbar and that the ͑553͒ surface is preserved. In the pressure range between 10 −6 and 1 mbar and at a sample temperature of 300-400°C, a surface oxide forms and rearranges the ͑553͒ surface facets and forming ͑332͒ facets. Most of the surface oxide can be described as a PdO͑101͒ plane, similar to what has been found previously on other Pd surfaces. However, in the present case, the surface oxide is reconstructed along the step edges, and the stability of this structure is discussed. In addition, the ͑ ͱ 6 ϫ ͱ 6͒ Pd 5 O 4 surface oxide can be observed on ͑111͒ terraces larger than those of the ͑332͒ terraces. Increasing the O pressure above 1 mbar results in the disappearance of the ͑332͒ facets and the formation of PdO bulk oxide.

A versatile instrument for the in situ study of catalyst surfaces by surface x-ray diffraction and grazing incidence small angle x-ray scattering in a 13 ml flow reactor combined with reaction product analysis by mass spectrometry has been developed. The instrument bridges the so-called "pressure gap" and "materials gap" at the same time, within one experimental setup. It allows for the preparation and study of catalytically active single crystal surfaces and is also equipped with an evaporator for the deposition of thin, pure metal films, necessary for the formation of small metal particles on oxide supports. Reactions can be studied in flow mode and batch mode in a pressure range of 100-1200 mbar and temperatures up to 950 K. The setup provides a unique combination of sample preparation, characterization, and in situ experiments where the structure and reactivity of both single crystals and supported nanoparticles can be simultaneously determined.

Future large X-ray observatories in space will require mirrors with large effective areas and long focal lengths to accomplish the proposed science. ESA programs for developing lightweight optics based on modules of silicon pore optics (SPO) and slumped glass optics (SGO) were put in place for the IXO mission (f=20m, r≈1m). To test such optics the MPE PANTER X-ray test facility has been upgraded / extended with support from ESA to accommodate in-focus measurements of such optics modules. We describe the extension to PANTER and the first results obtained from measuring such SPO and SGO modules during commissioning.

We present the design and scientific motivation for Arcus, an X-ray grating spectrometer mission to be deployed on the International Space Station. This mission will observe structure formation at and beyond the edges of clusters and galaxies, feedback from supermassive black holes, the structure of the interstellar medium and the formation and evolution of stars. The mission requirements will be R>2500 and >600 cm 2 of effective area at the crucial O VII and O VIII lines, values similar to the goals of the IXO X-ray Grating Spectrometer. The full bandpass will range from 8-52Å (0.25-1.5 keV), with an overall minimum resolution of 1300 and effective area >150 cm 2 . We will use the silicon pore optics developed at cosine Research and proposed for ESA's Athena mission, paired with off-plane gratings being developed at the University of Iowa and combined with MIT/Lincoln Labs CCDs. This mission achieves key science goals of the New Worlds, New Horizons Decadal survey while making effective use of the International Space Station (ISS).

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