New reactor dedicated to in operando studies of model catalysts by means of surface x-ray diffraction and grazing incidence small angle x-ray scattering

International audienceA new experimental setup has been developed to enable in situ studies of catalyst surfaces during chemical reactions by means of surface x-ray diffraction (SXRD) and grazing incidence small angle x-ray scattering. The x-ray reactor chamber was designed for both ultrahigh-vacuum (UHV) and reactive gas environments. A laser beam heating of the sample was implemented; the sample temperature reaches 1100 K in UHV and 600 K in the presence of reactive gases. The reactor equipment allows dynamical observations of the surface with various, perfectly mixed gases at controlled partial pressures. It can run in two modes: as a bath reactor in the pressure range of 1-1000 mbars and as a continuous flow cell for pressure lower than 10−3 mbar. The reactor is connected to an UHV preparation chamber also equipped with low energy electron diffraction and Auger spectroscopy. This setup is thus perfectly well suited to extend in situ studies to more complex surfaces, such as epitaxial films or supported nanoparticles. It offers the possibility to follow the chemically induced changes of the morphology, the structure, the composition, and growth processes of the model catalyst surface during exposure to reactive gases. As an example the Pd8Ni92(110) surface structure was followed by SXRD under a few millibars of hydrogen and during butadiene hydrogenation while the reaction was monitored by quadrupole mass spectrometry. This experiment evidenced the great sensitivity of the diffracted intensity to the subtle interaction between the surface atoms and the gas molecules

Catalytic properties of supported gold nanoparticles: new insights into the size-activity relationship gained from in operando measurements

International audienceThe relationship between the catalytic activity and the size was studied in operando in the case of gold nanoparticles on TiO2(110) model catalyst during carbon monoxide oxidation. The geometrical parameters, the shape and the dispersion of the particles on the oxide support were examined in detail. The catalytic activity was found optimum for a nanoparticle diameter of about 2 nm and a height of six atomic monolayers. Above the maximum, it fits a power law of the diameter D^-2.4 +/-0.3. This indicates that the low-coordinated sites play a major role in the catalytic activity, however such a model still fails to explain the activity maximum. The nanoparticle sintering was also investigated since it is suspected of being responsible for the decrease of the catalyst activity in the course of time. It was clearly observed for particles with a size around the maximum of activity and smaller. At the very beginning of the CO conversion into CO2, the sintering is strongly activated. The nanoparticles mobility is dependent upon the TiO2(110) surface direction under consideration: it is higher along the [001]TiO2 than along the [1-10]TiO2. Then, the sintering greatly slows down. This could be explained by a nanoparticles' pinning at the step edges. The thermal energy released by the exothermic CO oxidation reaction was evaluated and it suggests that the sintering results from a more complex process than from a reaction-induced local heating

Carbon Monoxide Oxidation Promoted by a Highly Active Strained PdO Layer at the Surface of Au 30 Pd 70 (110)

International audienceThe evolution of the Au30Pd70(110) surface was studied by coupling grazing incidence X-ray diffraction and mass spectrometry under oxygen-rich conditions at moderate temperatures (300 to 470 K). This allows us to correlate the depth profile of its structure to its catalytic properties for carbon monoxide (CO) oxidation. Under increasing pressure from ultrahigh vacuum up to 100 mbar, both oxygen and CO induce Pd segregation, even at room temperature. However, in pure oxygen the surface is reorganized with a (1 × 2) missing row reconstruction, whereas in pure CO it is strongly roughened. When oxygen pressure is increased a phase corresponding to the initial step of the oxidation with oxygen dissolution in the subsurface region appears at first. Then, from about 400 K onward, an oxidized thin Pd layer (≤1 nm) is formed growing in the [100]PdO direction. This PdO phase is strained and does not coincide with the P42/mmc structure usually observed for this oxide under ambient conditions. It is more probably consistent with the high pressure I4/mmm PdO structure strained by epitaxy on the underneath alloy. For higher oxidizing conditions and layer thickness, the oxide will then relax to the usual PdO structure. This strained oxide is easily reduced by CO and exhibits a very high activity for CO oxidation. Its catalytic performance at 470 K is comparable to the one found on surfaces of pure palladium at higher temperatures. Furthermore, on the clean Au30Pd70(110) surface, surface oxidation is hindered up to 470 K if CO is introduced prior to oxygen. This indicates that when Pd is alloyed with gold, its binding with CO is stronger than with oxygen. The weakening of the Pd–O binding by surrounding gold atoms is the key of the formation of a well-ordered and very active thin PdO film on Au30Pd70(110)

Oxygen-Induced Changes of the Au 30 Pd 70 (110) Surface Structure and Composition under Increasing O 2 Pressure

SSCI-VIDE+ATARI+MAA:EEHInternational audienc

Looking by grazing incidence small angle x-ray scattering at gold nanoparticles supported on rutile TiO2 (110) during CO oxidation

International audienceThe catalytic activity of oxide-supported gold nanoparticles depends crucially on their size. The present work describes a dedicated set-up in which particle size is determined by grazing incidence small angle x-ray scattering (GISAXS) and reactivity is analysed via a mass spectrometer. Catalytically active gold nanoparticles supported on TiO2(110) of size ranging between 2.4 and 5 nm were characterized during the CO oxidation at pressures in the range 0.1-100 mbar. The growth was found 3D and the particles were best modelled by a truncated sphere. The reaction rate per Au atom measured at 470 K was seen to increase in a monotone manner as the cluster size decreases, without reaching any maximum. Particles of size lower than 3 nm were stable under oxygen but sintering occurs when CO is added at 470K. That dimension coincides with the switch which was previously observed from nucleation-growth, with particles pinned on defects, to coalescence where particles become independent of defects

Size and catalytic activity of supported gold nanoparticles: an in operando study during CO oxidation

International audienceThe origin of the catalytic activity of gold nanoparticles remains debated despite extensive studies. This in operando work investigates the relationship between catalytic activity and size/shape of gold nanoparticles supported on TiO2(110) during CO oxidation. The nanoparticles were synthesized by vapor deposition in ultrahigh vacuum. Their geometry was monitored in the presence of O2, Ar, or a mixture of O2 + CO and of Ar + CO by grazing incidence small-angle X-ray scattering simultaneously with the catalytic activity. The occurrence of CO oxidation induces a sintering directly correlated to the reaction rate. The catalytic activity is optimum for a nanoparticle's diameter of 2.1 +/- 0.3 nm and a height of about six atomic layers. Below this size, the activity drop corresponds to a height decrease. Rescaling of activities obtained in different experimental conditions shows consistency of these results with published data using both "model" and "real" catalysts

Soft x-ray resonant magnetic reflectivity studies for in-and out-of-plane magnetization profile in ultra thin films

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Epitaxial growth and structure of cobalt ferrite thin films with large inversion parameter on Ag(001)

International audienceCobalt ferrite ultrathin films with the inverse spinel structure are among the best candidates for spin filtering at room temperature. High-quality epitaxial CoFe 2 O 4 films about 4 nm thick have been fabricated on Ag(001) following a three-step method: an ultrathin metallic CoFe 2 alloy was first grown in coherent epitaxy on the substrate and then treated twice with O 2 , first at room temperature and then during annealing. The epitaxial orientation and the surface, interface and film structure were resolved using a combination of low-energy electron diffraction, scanning tunnelling microscopy, Auger electron spectroscopy and in situ grazing-incidence X-ray diffraction. A slight tetragonal distortion was observed, which should drive the easy magnetization axis in-plane due to the large magneto-elastic coupling of such a material. The so-called inversion parameter, i.e. the Co fraction occupying octahedral sites in the ferrite spinel structure, is a key element for its spin-dependent electronic gap. It was obtained through in situ resonant X-ray diffraction measurements collected at both the Co and Fe K edges. The data analysis was performed using FDMNES, an ab initio program already extensively used to simulate X-ray absorption spectroscopy, and shows that the Co ions are predominantly located on octahedral sites with an inversion parameter of 0.88 (5). Ex situ X-ray photoelectron spectroscopy gives an estimation in accordance with the values obtained through diffraction analysis

Textured YBCO films grown on wires: applications to superconducting cables

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