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

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