Levi T. Thompson

XPS study of as-prepared and reduced molybdenum oxides

Abstract The surface properties of Mo oxides (MoO 2 and MoO 3 ) were investigated using X-ray photoelectron spectroscopy (XPS). As-prepared MoO 3 showed two well resolved spectral lines at 232.5 and 235.6 eV, which were assigned to the Mo 3d 5/2 , 3d 3/2 spin-orbit components, respectively. After the sample was reduced with H 2 at 673 K for 3 h, 76% of the MoO 3 present on the surface was transformed to MoO 2 . Hydrogen reduction of MoO 2 produced a new state (Mo δ+ , 0 δ 3+ based on our binding energy assignments for Mo 0 , Mo 4+ , Mo 5+ , and Mo 6+ . The concentration of Mo δ+ state increased with increasing reduction time and this state was suggested to contain hydrogen, which may be a necessary requisite for desulfurization and denitrogenation reactions.

Molybdenum carbide catalysts for water-gas shift

Molybdenum carbide (Mo2C) was demonstrated to be highly active for the water–gas shift of a synthetic steam reformer exhaust stream. This catalyst was more active than a commercial Cu–Zn–Al shift catalyst under the conditions employed (220–295°C and atmospheric pressure). In addition, Mo2C did not catalyze the methanation reaction. There was no apparent deactivation or modification of the structure during 48 h on‐stream. The results suggest that high surface area carbides are promising candidates for development as commercial water–gas shift catalysts.

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.

High Activity Carbide Supported Catalysts for Water Gas Shift

Nanostructured carbides are refractory materials with high surface areas that could be used as alternatives to the oxide materials that are widely used as support materials for heterogeneous catalysts. Carbides are also catalytically active for a variety of reactions, offering additional opportunities to tune the overall performance of the catalyst. In this paper we describe the synthesis of molybdenum carbide supported platinum (Pt/Mo(2)C) catalysts and their rates for the water gas shift reaction. The synthesis method allowed interaction of the metal precursor with the native, unpassivated support. The resulting materials possessed very high WGS rates and atypical Pt particle morphologies. Under differential conditions, rates for these catalysts were higher than those for the most active oxide-supported Pt catalysts and a commercial Cu-Zn-Al catalyst. Experimental and computational results suggested that active sites on the Pt/Mo(2)C catalysts were located on the perimeter of the Pt particles and that strong interactions between Pt and the Mo(2)C surface gave rise to raft-like particles.

Molybdenum nitride catalysts: I. Influence of the synthesis factors on structural properties

Effects of the synthesis parameters on the structural properties of molybdenum nitride catalysts, prepared by the temperature-programmed reaction of MoO3 with NH3, have been examined. Molybdenum trioxide was heated in flowing NH3 through two linear heating segments (623 to 723 K then 723 to 973 K) with different space velocities in a 23 factorial design. The temperature limits for these heating segments were defined based on the results of in situ X-ray diffraction analysis of the gas-solid reaction. The resulting catalysts were characterized using BET surface area analysis, environmental scanning electron microscopy, ex situ X-ray diffraction, and oxygen chemisorption. The primary bulk phase present was γ-Mo2N. Some of the lower surface area catalysts also contained MoO2 and Mo, but there was no evidence of nitrides other than γ-Mo2N. The catalysts consisted of micrometersized, plate-like aggregates of nanometer-sized crystallites, and possessed surface areas ranging up to ≈140 m2/g depending on the synthesis and reduction conditions employed. Statistical analysis of the results revealed that the space velocity individually and the heating rates combined had the most significant effects on the structural properties. The production of catalysts with surface areas in excess of 50 m2/g required the use of slow heating rates during the first segment and high space velocities. We concluded that the key to producing the highest surface area Mo nitrides was channeling the reaction through HxMoO3 (x ≤ 0.34) and γ-Mo2OyN1-y intermediates. Passivation of the materials immediately following synthesis appeared to produce an oxynitride at the surface. Reduction of the passivated materials in H2 at temperatures up to 673 K caused a significant increase in the surface area and O2 uptake. The O2 uptake for the low and medium surface area catalysts varied linearly with the BET surface area and corresponded to an O:Mo stoichiometry of approximately 1:5. The oxygen site density for the highest surface area nitride was lower than those for the lower surface area catalysts, presumably due to differing surface structures.

Metal acetylacetonate complexes for high energy density non-aqueous redox flow batteries

This paper describes the design, synthesis, and fundamental characterization of a series of Cr and V acetylacetonate (acac) complexes for use in redox flow batteries (RFBs). These materials offer a significant improvement in theoretical energy density relative to state-of-the-art aqueous chemistries. A detailed assessment of the solubility, cyclic voltammetry, and charge–discharge behavior of the complexes is presented. Their solubilities in acetonitrile vary by more than four orders of magnitude based on the structure/substituents on the acac ligand. Complexes bearing acac ligands with ester substituents have solubilities of up to 1.8 M, a significant improvement over most other metal complexes that have been considered for non-aqueous RFB applications. While the acac ligand substituents have a dramatic impact on solubility, they do not, in most cases, impact the electrochemical properties of the complexes. For instance, voltammetry for all of the V(acac)3 derivatives examined exhibit two quasi-reversible redox events separated by approximately 2.1 V. Charge–discharge testing in static H-cell and laboratory-scale flow batteries yielded energy densities that were consistent with the voltammetry and coulombic and energy efficiencies of up to 92% and 87%, respectively. Overall, these studies provide the basis for the development of structure–function relationships that could lead to new and even better performing energy storage chemistries in the future.

Self-propagating high temperature synthesis and dynamic compaction of titanium diboride/titanium carbide composites

Self-propagating High-temperature Synthesis (SHS) of titanium and boron carbide (B4C) combined with explosively driven Dynamic Compaction (DC) was employed for the fabrication of composite TiB2/TiC compacts. A 23 factorially designed experiment set was used to examine the effects of the TiB2/TiC ratio, delay time and C/M ratio on the consolidation and properties of the compacts. The delay time is the time between completion of the SHS reaction and compaction. The C/M ratio, the ratio of the explosive mass to that of the flyer plate, influences the pressure applied to the samples during compaction. Composites with molar TiB2/TiC ratios of 2:1 or 1:2 were prepared using Ti and B4C or Ti, C and B4C, respectively, as reactants. The SHS/DC of Ti and B4C resulted in high quality, near fully dense TiB2/TiC composite compacts. Under best conditions, the densities were greater than 98% of the theoretical maximum. While the microhardness and densities of the compacts with TiB2/TiC ratio of 2:1 were comparable to those of monolithic TiB2 and TiC, compacts with TiB2/TiC ratios of 1:2 were poorly consolidated and contained extensive cracks. Given the high energy and time efficiency, high product quality and inexpensive reactants, the SHS/DC of Ti and B4C represents an attractive technique for the economical fabrication of TiB2/TiC composites.

Syngas and HDS catalysts derived from sulphido bimetallic clusters

The clusters, CP2′Mo2Fe2S2(CO)8 (MoFeS) and Cp2′Mo2CO2S3(CO)4 (MoCoS) (Cp′ = η-C5H4Me) have been supported on the refractory oxides, Al2O3, SiO2, TiO2, and MgO, and subjected to temperature programmed decomposition (TPDE) under flowing H2. Typically, CO evolution commences near 100°C, followed by evolution of 1–2 Cp-ligands from 180 to 400°C along with small amounts of CO2, CH4, and H2S or Me2S. The resulting compositions are shown to be active catalysts for CO hydrogenation and hydrodesulphurization (HDS) of thiophene. Methane is the principal hydrocarbon product from CO hydrogenation except for MoFeS/MgO where high selectivity for C2 products was observed. The activity and selectivity of MoCoS/Al2O3 for thiophene HDS closely resembles those of conventionally prepared “cobalt molybdate” catalysts. The cluster derived catalysts have been characterized by Mossbauer and X-ray absorption (XANES) and EXAFS) spectroscopies. It is concluded that the clusters undergo oxidation by the surface upon loss of organic ligands. The results obtained to date show that sulphido bimetallic clusters are excellent precursors for the formation of uniform catalytic surfaces. The uniformity of the surface species facilitates physical characterization of the active site(s). Our results show that the supported clusters are transformed to surface oxo-ensembles which are active for CO hydrogenation and HDS of organic sulphur compounds.

Structural and mechanical properties of TiB2 and TiC prepared by self-propagating high-temperature synthesis/dynamic compaction

Titanium-diboride and titanium-carbide compacts with diameters of 100 mm and thicknesses of 25 mm were fabricated by self-propagating high-temperature synthesis/dynamic compaction (SHS/DC) of the elemental powders. Under the best conditions, the densities were greater than 99% and 96.8% of the theoretical densities for TiB2 and TiC, respectively. The microhardness, compressive strength, and elastic modulus of the TiB2 prepared by the SHS/DC method were comparable to reported values for hot-pressed TiB2. While the microhardness and elastic modulus of the TiC compacts were comparable to those for hotpressed TiC, the compressive strength was lower due to extensive cracks in the compacts. The TiB2 prepared using a low-purity boron powder (1–5% carbon impurity) compacted to higher densities and had less cracking than that prepared using a high-purity boron powder (0.2% carbon). This result could have an impact on the cost of producing TiB2/TiC structural components by the SHS/DC process.

Temperature-programmed desorption and decomposition of NH3 over molybdenum nitride films

The surfaces of β-Mo16N7, γ-Mo2N, and δ-MoN films were characterized using NH3 temperature-programmed desorption (TPD). Ammonia adsorption at ∼ 280 K and TPD using a heating rate of 6 K/s produced NH3 peaks at ∼ 360 K. The desorption kinetics depended on the structure and composition of the film. Ammonia desorption from the β-Mo16N7 and γ-Mo2N films was first-order; however, desorption from the δ-MoN film appeared to be second-order. Assuming a pre-exponential factor of 1013 s−1, the desorption energy for the β-Mo16N7 and γ-Mo2N films was 22 kcal/mol. The NH3 saturation capacity increased in the following order: δ-MoN < β-Mo16N7 < γ-Mo2N. This order is similar to that expected for the Mo surface atom density. Some of the NH3 decomposed into H2 and N2. Two H2 desorption peaks were produced: a low-temperature peak due to recombination of surface hydrogen and a high-temperature peak due to hydrogen that emerged from the nitride subsurface. The N2 desorption spectrum consisted of a peak at ∼ 340 K and several peaks in the range 500–900 K. 15NH3 TPD experiments indicated that the low-temperature N2 desorption peak was due to NH3 decomposition while the origin of the high-temperature peaks was the nitride itself. The amount of N2 that desorbed in this high-temperature envelope increased with increasing NH3 dose. We believe that nitrogen desorption from the nitride was induced by the presence of hydrogen which altered the MoN bonding. Ammonia desorption and decomposition spectra for the films were similar to those for a series of bulk γ-Mo2N powders. Characteristics of the γ-Mo2N film resembled those of the low-surface-area powder (< 20 m2/g), while the behavior of the β-Mo16N7 and δ-MoN films was similar to that for the higher-surface-area powders.