Brain G. Demczyk

Synthesis and characterization of molybdenum nitride hydrodenitrogenation catalysts

Abstract Details concerning the relationships between the structural, chemical and catalytic properties of Mo nitrides have been elucidated. A series of Mo nitride catalysts were prepared by the temperature programmed reaction of MoO3 with NH3. The structural properties of these nitrides were complex functions of the heating rates and space velocities employed. Two reaction sequences were proposed to account for the synthesis of high, medium and low surface area materials. An interesting conclusion was that the degree of reduction of the molybdate precursor or intermediate governed the structural properties of the product. Some evidence is also presented to suggest that the nucleation and growth rates involved in the transformation of the oxide to the nitride were significantly influenced by the synthesis conditions. The Mo nitrides proved to be exceptional pyridine hydrodenitrogenation catalysts. Their catalytic properties were superior to those of a commercial sulfided Co-Mo hydrotreatment catalyst, having higher activities and better C-N bond hydrogenolysis selectivities. Hydrodenitrogenation over the Mo nitrides appeared to be structure-sensitive. While detailed relationships between the catalytic activity and surface stoichiometry could not be ascertained, there did appear to be a correlation between the activity, and the particle size and grain boundary length. We proposed that at least two types of HDN sites existed on the Mo nitride surfaces; modest activity sites on the particles and high activity sites at grain boundaries. The N/Mo stoichiometry of the highest activity catalyst was near unity suggesting that MoN was present perhaps localized at the grain boundaries. Finally structures near or at the surface were markedly different from those of the bulk. While the predominant bulk phase was γ-Mo2N, the surface appeared to consist of either non-stoichiometric β-Mo16N7 or mixtures of Mo and β-Mo16N7.

Surface structure and composition of high-surface-area molybdenum nitrides

Abstract The determination of surface structure is critical in elucidating structure-function relationships for catalytic materials. However, such materials often consist of active regions of very limited spatial extent, rendering conventional bulk structural characterization techniques of limited utility. In this work, we have employed high-resolution transmission electron microscopy coupled with Fourier analysis, and X-ray photoelectron spectroscopy to determine the near-surface structures and compositions of a series of molybdenum nitride catalysts. The results show the near-surface to differ both in crystal structure and composition from the bulk. The bulk structure was γ-Mo2N (fcc) while the lattice structure near the surface was body-centered. Many of the materials contained nitrogen in excess of that expected for phases in the Mo-N phase diagram. As the amount of nitrogen decreased, the oxygen content increased, and the calculated lattice parameter increased. Taken together, the results suggested the presence of Mo2N3-xOx, a hypothetical primitive cubic structure, near the surface. This structure, which would produce a diffraction pattern that approximates that of a body-centered lattice, accounts for the near-surface structural and compositional properties of the high-surface-area molybdenum nitrides.

Structure of coprecipitated gold–iron oxide catalyst materials

Abstract Ultra-fine gold particles supported on iron oxide have been synthesized by coprecipitation and subsequently heat treated in air at temperatures of 200°C and above. Structure analyses have revealed variations in the degree of crystallinity of the iron oxide support as well as gold particle size and size distribution with heat treatment temperature. Significantly, X-ray diffraction reveals an ultra fine crystallite structure in as-prepared and lower temperature heat treated samples, while Mossbauer spectroscopy suggests the possibility of gold incorporation into the α -Fe 2 O 3 phase in the lower temperature annealed samples.

Structure and morphology of magnetron sputtered CoCr thin films

Abstract The growth characteristics of magnetron sputtered Co-22%Cr thin films on amorphous glass or carbon substrates have been investigated utilizing transmission electron microscopy, X-ray diffraction and electrical resistivity measurements. Results indicate that the initial deposit is “amorphous”, but that small crystallites form before the film reaches 5 nm film thickness. By 10 nm, well oriented grains with hcp c-axis perpendicular to the film plane develop, and by 50 nm, a columnar microstructure is evident. A distinct subgrain structure was also observed in the thinner films (10–50 nm), and this is reflected in the electrical resistivity. These observations cannot be accounted for by an “evolutionary selection” growth scenario, but must be related to the low surface mobility of adatoms at these low substrate deposition temperatures (

Observation of a secondary structural phase in the heavy-fermion superconductor UPt3

Abstract Atomic-resolution transmission electron microscopy is used to investigate the microstructure of polycrystalline UPt3. While the host material is the previously reported hexagonal Ni3Sn structure, a second phase having the double-hexagonal Ni3Ti structure is also identified.

Stacking faults in UPt3

Abstract Atomic resolution transmission electron microscopy and X-ray diffraction measurements have been combined to investigate the microstructure of superconducting UPt3. Regions of a second double hexagonal phase with a typical dimension of 25–30 A are found to occupy approximately 3% of the total sample volume.

Surface and catalytic properties of molybdenum nitrides.

Abstract A series of Mo nitride catalysts were prepared by the temperature programmed reaction of MoO 3 with NH 3 . The structural properties of these nitrides were complex functions of the space velocities and heating rates employed. Solid-state reaction sequences have been developed to account for the structural and compositional properties of the Mo nitrides. It appeared that the degree of reduction of the molybdate precursor or intermediate significantly influenced the properties of the product. The Mo nitrides proved to be exceptional hydrodenitrogenation catalysts with catalytic properties that were superior to those of a commercial sulfided Co-Mo/γ-Al 2 O 3 hydrotreatment catalyst. Pyridine HDN appeared to be structure-sensitive over the Mo nitrides. Based on characterization using a variety of spectroscopic and microscopic techniques we concluded that at least two types of active sites were present in the Mo nitrides; modest activity sites on the particles and high activity sites at defect sites. The predominant bulk phase in the nitrides was γ-Mo 2 N, however, the surface appeared to consist of either non-stoichiometric β-Mo 16 N 7 or mixtures of Mo and β-Mo 16 N 7 . Furthermore, the surface Mo:N stoichiometry for the highest activity catalyst was near unity suggesting the presence MoN or nitrogen rich regions.

Annealing studies of magnetron-sputtered CoCr thin films

Abstract The microstructure and magnetic properties of annealed CoCr thin films of various thicknesses, deposited on glass substrates, were examined. In general, little grain growth upon annealing was observed. Low-angle grain boundaries, present in as-sputtered films of thickness ⩽50 nm, were absent in annealed films. This occurs simultaneously with a decrease in the measured electrical resistivity. A reduction in the in-plane film stress occurs with annealing, correspondingly the experimentally measured anisotropy field values change markedly. A noticeable feature of the microstructure is the appearance of Cr-rich regions in annealed films 50 nm thick and above. These regions are thought to account for the observed increases in the saturation magnetization and coercivity with annealing time.

Thermal analysis of SmTiFe11−xCox (x = 0, 11) and DyTiCo11 intermetallic alloys

Abstract Alloys of composition SmTiFe 11− x Co x ( x =0, 11) and DyTiCo 11 have been examined via differential thermal analysis. Thermal peaks found in the Sm-containing TiFe 11 sample were consistent with a 1–12 phase, with additional Fe 2 Ti and a-Fe phases present, as found in a previous work. In SmTiCo 11 , evidence for a 1–12, a 2–17 and a primarily Co-containing solid solution was found. When Dy is substituted for Sm in the Co 11 sample, an additional peak, possibly due to the formation of the hexagonal 2–17 phase from the rhombohedral phase was noted. In addition, peaks corresponding to the ferromagnetic ordering of Fe in SmTiFe 11 and of Co in DyTiCo 11 were observed.