Jeong-Gil Choi

Growth and characterization of chloronitroaniline crystals for optical parametric oscillators: I. XPS study of Mo-based compounds

Abstract The surface electronic structures and compositions of Mo-based compounds (oxides and nitrides) have been elucidated using X-ray photoelectron spectroscopy (XPS). The Mo nitride samples were synthesized using the temperature-programmed nitridation of a low surface area MoO3 with NH3. The Mo oxides were also used to compare XPS results. No significant change was observed in the surface electronic structure of core levels of Mo 3d, Mo 3p, N 1s, and O 1s for Mo nitrides after the hydrogen reduction. A species with a Mo 3d5/2 binding energy of 228.4±0.2 eV from Mo nitrides was newly observed and was calculated to be Mo1.3+. This value indicates that the absolute magnitude of charge transfer from Mo 4d states to N 2p states is 1.3 electrons per molybdenum in Mo nitrides. The binding energies of Mo 3d5/2 were shifted to lower ones (232.5→230.0→228.4 eV) as the oxides were transformed to nitride during nitridation (Mo6+→Mo4+→Moδ+). There are two different regions (nitride and oxide regions) present on the Mo nitride surface as shown from the deconvoluted Mo 3d peaks. For the Mo nitrides prepared in this study the nitride fraction was observed to be a majority on the surface.

Synthesis and catalytic properties of vanadium interstitial compounds

Vanadium nitrides and carbides were synthesized by the temperature-programmed reaction of vanadium oxide (V2O5) with pure NH3, and with pure CH4 or a mixture of 49.9% CH4 in H2, respectively. Based on the XRD results, the materials contained VN or VN with a negligible amount of V2O3 in the bulk after the nitridation of vanadium oxides and only V8C7 after the carburization. These results indicated that the structural properties of these materials were strong functions of the heating rate and space velocity employed. The vanadium nitrides and carbides proved to be active NH3 synthesis and decomposition catalysts. Since the activity varied with changes in the surface area and particle size, ammonia decomposition over the vanadium nitrides and carbides appeared to be structure-sensitive. This response was considered to be due to variations in the surface stoichiometry with particle size. In general, the activities of the vanadium carbides were about 1 to 2 orders of magnitude lesser than that of Mo nitride catalyst while vanadium carbides had higher activities by a factor of 1 or 2 than Pt/C catalyst. However, the activity of the lowest surface area vanadium carbide is similar to that of Mo nitride catalyst.

The surface properties of vanadium compounds by X-ray photoelectron spectroscopy

Abstract The electronic structures of vanadium carbides have been examined using X-ray photoelectron spectroscopy (XPS) before and after Ar sputtering. After Ar ion sputtering for 1 h, there was no significant variation observed in both the surface electronic structure and the amounts of core levels of V 2p, C 1s, and O 1s for vanadium carbides used in the present study. A species with a binding energy of 513.2±0.2 eV from vanadium carbides was newly observed, and was estimated to be V 1.3+ using the plot of the V 2p 3/2 binding energies against the oxidation number. This value indicates that the absolute magnitude of charge transfer from V 3d states to C 2p states is 1.3 electrons per vanadium in vanadium carbides. Among the deconvoluted three carbon peaks, the carbidic carbon peak appeared at 282.4±0.2 eV and was considered to be due to the photoelectrons ejected from the carbon in vanadium carbide lattice. For all of 1-h sputtered vanadium carbides except for V 8 C 7 -4, the average ratio of C c /V δ + was nearly unity. Using NH 3 decomposition reaction, the catalytic activity measurements exhibited that the vanadium carbide with the smallest C c /V δ + ratio had the highest activity. These results indicate that the most active catalyst was carbon-deficient at surface of the vanadium carbide used in this study.

Surface properties of high-surface-area powder and thin film molybdenum nitrides treated in H2 and H2S

Abstract The surface properties of the high-surface-area powder and thin film Mo nitrides were investigated using X-ray photoelectron spectroscopy (XPS). After H 2 and H 2 S treatments at 673 K for 3 h, these materials showed a variation in the distribution of Mo oxidation states and the atomic ratios (O/Mo, N/Mo, S/Mo), however, no change in the surface electronic structure was observed. The primary species for reduced and sulfided materials was Mo δ+ while that for as-prepared materials was Mo 6+ . For H 2 -treated thin film Mo nitrides, the degree of reduction in the O/Mo ratios was 94% for TF-1 with γ-Mo 2 N and that for TF-2 with δ-MoN was 64%. However, the N/Mo ratio decreased 65% for TF-1 while TF-2 exhibited the increase of N/Mo ratio to 63%. These results suggested that Mo nitrides exhibited a strong bulk structure dependence for the degree of variation of O/Mo and N/Mo ratios in H 2 treatment. Sulfur incorporation with Mo came at the expense of oxygen, showing that the S/Mo ratio increased with decreasing the O/Mo ratio. For high-surface-area Mo nitrides the sulfur tolerance was associated with differences in the predominant crystal faces exposed by the crystallites of γ-Mo 2 N. The estimation of S/Mo ratios suggested that the γ-Mo 2 N(200) was more sulfur-tolerable than γ-Mo 2 N(111). For thin film Mo nitrides, it was observed that the δ-MoN which had a hexagonal structure was more sulfur-tolerable than γ-Mo 2 N with a face-centered cubic structure.