Isolated Rh atoms in dehydrogenation catalysis.

Isolated active sites have great potential to be highly efficient and stable in heterogeneous catalysis, while enabling low costs due to the low transition metal content. Herein, we present results on the synthesis, first catalytic trials, and characterization of the Ga9Rh2 phase and the hitherto not-studied Ga3Rh phase. We used XRD and TEM for structural characterization, and with XPS, EDX we accessed the chemical composition and electronic structure of the intermetallic compounds. In combination with catalytic tests of these phases in the challenging propane dehydrogenation and by DFT calculations, we obtain a comprehensive picture of these novel catalyst materials. Their specific crystallographic structure leads to isolated Rhodium sites, which is proposed to be the decisive factor for the catalytic properties of the systems.

X-ray characterization of physical-vapor-transport-grown bulk AlN single crystals.

AlN slices from bulk crystals grown under low thermomechanical stress conditions via the physical vapor transport (PVT) method were analyzed by X-ray methods to study the influence of the growth mode on the crystal quality. Defect types and densities were analyzed along axial [0001] as well as lateral growth directions. X-ray diffraction (0110) rocking-curve mappings of representative wafer cuts reveal a low mean FWHM of 13.4 arcsec, indicating the generally high crystal quality. The total dislocation density of 2 × 103 cm-2 as determined by X-ray topography is low and dislocations are largely threading edge dislocations of b = 1/3〈1120〉 type. The absence of basal plane dislocations in homogeneous crystal regions void of macroscopic defects can be linked to the low-stress growth conditions. Under the investigated growth conditions this high crystal quality can be maintained both along the axial [0001] direction and within lateral growth directions. Exceptions to this are some locally confined, misoriented grains and defect clusters, most of which are directly inherited from the seed or are formed due to the employed seed fixation technique on the outer periphery of the crystals. Seed-shaping experiments indicate no apparent kinetic limitations for an enhanced lateral expansion rate and the resulting crystal quality, specifically with regard to the growth mode on a-face facets.

Complementary Molecular Dynamics and X-ray Reflectivity Study of an Imidazolium-Based Ionic Liquid at a Neutral Sapphire Interface.

Understanding the molecular-level behavior of ionic liquids (ILs) at IL-solid interfaces is of fundamental importance with respect to their application in, for example, electrochemical systems and electronic devices. Using a model system, consisting of an imidazolium-based IL ([C2Mim][NTf2]) in contact with a sapphire substrate, we have approached this problem using a complementary combination of high-resolution X-ray reflectivity measurements and atomistic molecular dynamics (MD) simulations. Our strategy enabled us to compare experimental and theoretically calculated reflectivities in a direct manner, thereby critically assessing the applicability of several force-field variants. On the other hand, using the best-matching MD description, we are able to describe the nature of the model IL-solid interface in appreciable detail. More specifically, we find that characteristic interactions between the surface hydroxyl groups and donor and acceptor sites on the IL constituents have a dominant role in inducing a multidimensional layering profile of the cations and anions.