Daniel Herein

In situ X-ray powder diffraction analysis of the microstructure of activated iron catalysts for ammonia synthesis

Abstract The formation of the iron species catalysing the synthesis of ammonia was followed by in situ X-ray powder diffraction. It was shown that a complex sequence of reactions during the activation process (reduction of the magnetite precursor) leads to a metastable form of α-iron. A detailed examination of the diffraction lines of iron helped to elucidate the question, why “ammonia iron” is so much superior to “normal iron”. Unusual line profiles were observed which could be controlled by changing the reaction conditions. Platelet crystallites of defective alpha-iron form the outer shell of isotropic bulk iron particles which represent the main body of the catalyst material. A shoulder at higher d values of the Fe(110) reflection points to the formation of an iron/nitrogen phase during catalytic action. The crystallographic observations are in line with previous data on paracrystallinity, but their interpretation is of a different nature. The structural effects of oxygen poisoning were investigated. It is shown, why the poisoning is detrimental for the catalytic performance and how the speculation could arise that this effect may be beneficial for the reaction.

Synthesis, structure, and thermal behavior of [Cu70Se35(PEt2Ph)24]

We investigated the molecular and crystal structure as well as the thermal behavior of [Cu70Se35(PEt2Ph)24] using single-crystal X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), X-ray powder diffractometry, and thermal gravimetric analysis (TGA). The use of a He-cooled sample head in HRTEM makes possible for the first time the investigation of semiconductor cluster molecules under the extreme conditions of the experiment and, thus, comparison with the results of single-crystal X-ray diffraction. The lattice spacings and packing symmetry of the three-dimensional cluster molecule superlattice found in HRTEM are in good agreement with the ones determined by X-ray structure diffraction. The atomic ordering inside the cluster molecules, which resembles a hexagonal packing of the selenium atoms, could not be resolved in the HRTEM measurements, since upon irradiation with the electron beam the clusters start to move after a few seconds and the images go out of focus. This movement is possibly affected by cleavage processes of the phosphine ligands, as could be observed in thermal gravimetric analysis under comparably mild conditions. Powder diffraction measurements show that this cleavage of ligand molecules is accompanied by the formation of larger copper selenide cluster cores and ends up at high temperatures in the formation of bulk α-Cu2Se.

A new three-dimensional structural model for the CuCl2 graphite intercalation compound

Abstract The in-plane structure of stage-1 to stage-3 CuCl2 graphite intercalation compounds (GIC) was investigated using X-ray and electron diffraction. A new 3D structural model of these CuCl2–GIC was defined based on a monoclinic, face-centred pseudo-orthorhombic unit cell and the X-ray and electron diffraction patterns were computed using this model. The modulations of the (00l) intensities were well reproduced. The asymmetric shape of the CuCl2–GIC (hk0) reflections may be explained by including additional (hkl) lines into the calculations. This is attributed to the preferred orientation of the domain islands in the GIC. The exact atom positions remain unknown, because defects present in the noncommensurate GIC structure result in broad and asymmetric diffraction peaks. The internal structure of the CuCl2 layers in the GIC proved to be nearly the same as in the free compound.

Approaching the preparation of Pd-Al2O3 catalyst systematically: one step closer to reproducible catalyst synthesis

A systematic approach was developed to describe the impregnation of alumina with Pd-nitrate solution in a reproducible and defined way. Preparation of catalysts was represented by ten parameters and the influence of these parameters on catalytic activity in hydrogenation of 1,3-butadiene to 1-butene was measured. Based on these results, generic rules about preparation of the catalyst were identified by a regression model. The most important preparation parameter was the ratio between the amount of impregnating solution and the pore volume of the support.