The microstructure and physical properties of hot pressed (Hf, Ti) C have been investigated with the aim of producing a cutting tool material with similar hardness to that of WC-Co and TiC-based cermets. Sintered samples were hot pressed from HfC0.7 and TiC0.9 powders using powder metallurgical techniques and the processing cycle was optimized for this system. Ni was used as a binder in selected samples and C black was added to compensate for sub-stoichiometry and to aid in the reduction of oxides formed during milling. Microstructural analyses were performed by scanning and transmission electron microscopy (SEM and TEM) and the composition was determined from X-ray diffraction (XRD) and energy dispersive X-ray spectrometry (EDS). The physical properties measured are density and Vickers hardness, and the indentation fracture toughness was determined using the Shetty formula. The fundamental interactions between HfC, TiC and Ni during hot pressing were investigated and the results obtained used to explain the microstructure that develops in samples made from powder mixtures. The interactions studied are the inter-diffusion of HfC and TiC through the solid state, and the dissolution and re-precipitation rate of the carbides in a liquid Ni binder. EDS analysis revealed that the rate at which Ti diffuses into HfC is higher than the rate at which Hf diffuses into TiC. Upper limits to the diffusion coefficients for these processes are determined and show that solid solution carbides will form from HfC + TiC powder mixtures at 2000 ºC in 1 hour if the average powder particle size is less than 5 μm. The diffusion rates decrease with a decrease in hot pressing temperature but mass transport between the phases can be enhanced by addition of a metallic binder. TEM and EDS analysis shows that Ni wets TiC more efficiently than HfC and that the solubility of TiC in Ni is also higher than that of HfC. The grain size of the carbide phases increases with an increase in the rate at which they dissolve into and re-precipitate from the liquid binder. The crystal structure of the binder phase depends on the concentration of Ti and Hf that remain in the binder after cooling and the carbide phase in which the binder is embedded. Analysis of TEM electron diffraction patterns show that the binder phase consists of cubic solid solutions as well as intermetallic and cubic phases in which atomic ordering is observed.

Nitrides of late transition metals possess interesting properties leading to different technological applications, yet, due to many factors, synthesis and reliable characterization of the physical properties of these materials constitute a big challenge. In this work, we present a detailed firstprinciples investigation of the structural, the electronic and the optical properties of the bulk crystalline MNx (where M = Pd, Pt, Cu, Ag or Au; and x = 1/3, 1 or 2) and ZnN. The studied structural properties include energy-volume equation of state (EOS), equilibrium lattice structural parameters, cohesive and formation energies, relative phase stabilities, bulk modulus and its pressure derivative. By means of the enthalpy-pressure EOS, some possiblepressure-induced structural phase transitions are carefully examined. Electronic properties of the energetically most stable phases are investigated via the analysis of their band structure and their total and partial densities of states (DOSs). The frequency-dependent optical constants (absorption coefficient, reflectivity, refractive index, and energy-loss spectrum) of some phases are derived from the calculated frequency-dependent microscopic dielectric tensor. Our calculations of the structural and the electronic properties are based on density functional theory (DFT) within the projector-augmented wave (PAW) formulation and the generalised-gradient approximation (GGA) to the exchange-correlation functional. In order to improve the calculated electronic structure, and to investigate the optical spectra, we carry out expensive GW0 calculations within the the random-phase approximation (RPA) to the dielectric tensor. Obtained results are discussed within the employed theoretical methods of calculations. Whenever possible, our obtained results are compared with experiment and with previous theoretical studies. We report the discovery of some possible low-energy competitive phases which are more stable at zero pressure than the synthesized and other hypothetical structural phases. To the best of our knowledge, our calculated optical spectra may be considered as the first, and thus, the most accurate, calculations within the many-body perturbation GWA calculations, so far.

Proton exchange membrane or anion exchange membrane water electrolyzers and fuel cells are still expensive for large-scale commercialization. It requires more investigation and research on finding more economical and efficient electrocatalysts for reactions in these devices. This thesis investigates the performance of metal-modified transition metal carbides on hydrogen evolution reaction (HER) and ethanol oxidation reaction (EOR). The catalysts screening principles for HER and EOR in acid and alkaline are examined and developed by correlating density functional theory (DFT) calculations with experimental results. Metal-modified transition metal carbides can reduce the amount of platinum group metals required for HER, but it is unclear what descriptors are relevant for these materials for the HER under alkaline conditions. Several transition metal carbides (Mo2C, NbC, TaC, WC, VC) thin films were synthesized and modified with monolayers of platinum or gold. The experimentally measured HER exchange current densities were compared with DFT calculations of adsorbed hydrogen and hydroxyl binding energies. The plot of HER activity versus hydrogen binding energy showed a volcano shape for catalysts in both acid and alkaline electrolytes, but the hydroxyl binding energy did not form a strong correlation with alkaline HER activity. Relatively high surface area molybdenum carbide (Mo2C) particles was modified with 5 wt % silver, copper, nickel, platinum, and palladium and subsequently assessed for their HER activity in alkaline and acid electrolytes. DFT‐calculated hydrogen binding energies predicted that Pt–Mo2C and Pd–Mo2C should be most active, which was confirmed with experimental results. Similar activity trends were observed at both high and low pH values, with Cu/Mo2C being the least active. X‐ray photoelectron spectroscopy (XPS) confirmed that metal particles remained on the sample before and after HER testing. Pt‐modified nanocrystalline Mo2C showed superior HER activity compared with Pt‐modified commercial Mo2C, making it a potential replacement for bulk Pt in alkaline membrane electrolyzers. The positive effect on the HER activity of the metal contact with non‐passivated Mo2C surfaces was also demonstrated. Ethanol is an ideal fuel in low-temperature fuel cells. The EOR on platinum-modified tantalum carbide (TaC) was investigated using both model thin films and powder catalysts. The results demonstrated that the 1.5 wt% Pt-modified TaC catalyst obtained enhanced EOR activity compared to Pt. In-situ infrared reflection absorption spectroscopy (IRRAS) study revealed that the Pt surface was less poisoned by EOR intermediates and a higher CO2 selectivity (7~9%) was achieved on the 1.5 wt% Pt/TaC catalyst, compared to the 40 wt% Pt/C. DFT calculations revealed that the binding energies of EOR intermediates on the Pt/TaC(111) surface a weaker than on Pt(111), suggesting an enhanced poison-tolerance from the adsorption of these intermediates. The combined experimental and theoretical investigations strongly suggested that Pt/TaC should be a promising electrocatalyst for EOR. Palladium-modified tungsten carbide (Pd/WC) as an efficient catalyst was investigated for EOR through combined DFT, surface science and electrochemical measurements. Compared to the Pd(111) surface, DFT calculations suggested that the Pd/WC(0001) surface should be less poisoned by the ethanol decomposition intermediates, consistent with surface science results that desorption temperatures of the detected intermediates were lower on the Pd/WC surface. Electrochemical evaluation coupled with in-situ IRRAS measurements of 5 wt% Pd/WC/C powder catalysts were then conducted. The EOR activity of the 5 wt% Pd/WC/C-op catalyst synthesized by the one-pot (op) method was noticeably enhanced, compared to the benchmark 40 wt% Pd/C and 5 wt% Pd/WC/C-iwi that was synthesized using a conventional incipient wetness impregnation (iwi) method. The IRRAS results showed that the EOR products were detected at a lower onset potential on 5 wt% Pd-WC/C-op than on 40 wt% Pd/C. Overall, results from the current thesis demonstrated the feasibility of using metal-modified transition metal carbides as lower-cost and more efficient electrocatalysts for HER and EOR. These results identified descriptors that can be potentially used to design more cost-effective catalysts. Furthermore, results from this thesis also revealed the general similarities and differences of the activity and stability of carbide-based catalysts in acid and alkaline electrolytes.

Thesis (Ph.D.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references (ℓ. 134-138)

To sustain future population and economic growth, the global energy supply is expected to increase by 60% by 2040, but the associated CO₂ emissions are a major concern. Converting CO2 into a commodity through a CO₂-neutral process has the potential to create a sustainable carbon energy economy; however, the high stability of CO₂ requires the discovery of active, selective and stable catalysts. To initially probe the performance of catalysts for CO₂ reduction, CO₂ is activated with H₂, which produces CO and CH₄ as the primary products. For this study, CO is desired for its ability to be used in the Fischer-Tropsch process, while CH₄ is undesired because of its low volumetric energy density and abundance. Precious bimetallic catalysts synthesized on a reducible support (CeO₂) show higher activity than on an irreducible support (γ-Al₂O₃) and the selectivity, represented as CO:CH₄ ratio, is correlated to electronic properties of the supported catalysts with the surface d-band center value of the metal component. Because the high cost of precious metals is unsuitable for a large-scale CO₂ conversion process, further catalyst development for CO₂ reduction focuses on active, selective and low-cost materials. Molybdenum carbide (Mo₂C) outperforms precious bimetallic catalysts and is highly active and selective for CO₂ conversion to CO. These results are further extended to other transition metal carbides (TMCs), which are found to be a class of promising catalysts and their activity is correlated with oxygen binding energy (OBE) and reducibility as shown by density functional theory (DFT) calculations and in-situ measurements. Because TMCs are made from much more abundant elements than precious metals, the catalysts can be manufactured at a much lower cost, which is critical for achieving a substantial reduction of CO₂ levels. In the aforementioned examples, sustainable CO₂ reduction requires renewable H₂, 95% of which is currently produced from hydrocarbon based-feedstocks, resulting in CO₂ emissions as a byproduct. Alternatively, CO₂ can be reduced with ethane from shale gas, which produces either synthesis gas (CO + H₂) or ethylene with high selectivity. Pt/CeO₂ is a promising catalyst to produce synthesis gas, while Mo₂C based materials preserve the C-C bond of ethane to produce ethylene. Ethylene and higher olefins are desirable for their high demand as commodity chemicals; therefore, future studies into CO₂ reduction must identify new low-cost materials that are active and stable with higher selectivity toward the production of light olefins.

Lo, Kwok Cheung. / Thesis (M.Phil.)--Chinese University of Hong Kong, 2009. / Includes bibliographical references (leaves 114-118). / Abstracts in English and Chinese. / Thesis / Assessment Committee --- p.ii / ABSTRACT --- p.iii / ACKNOWLEDGEMENTS --- p.v / TABLE OF CONTENT --- p.vi / Chapter Chapter 1 --- Introduction --- p.1 / Chapter Chapter 2 --- Theoretical Background --- p.5 / Chapter Chapter 3 --- Results --- p.38 / Chapter Chapter 4 --- Discussion and Concluding Remarks --- p.85 / LIST OF TABLES / Table / Table la Electronic energies and geometrical parameters of scandium dicarbide by B3LYP/LANL2DZ and B3LYP/LANL2DZ-d --- p.41 / Table lb Comparison of literature results with current computational results of cyclic scandium dicarbide at equilibrium state by B3LYP/LANL2DZ and B3LYP/LANL2DZ-d --- p.42 / Table lc Comparison of literature results with current computational results of linear scandium dicarbide at equilibrium state by B3LYP/LANL2DZ and B3LYP/LANL2DZ-d --- p.43 / Table 2a Electronic energies and geometrical parameters of titanium --- p.46

Thesis (Ph.D.)--Texas Christian University, 2009. / Title from dissertation title page (viewed Oct. 30, 2009). Includes abstract. Includes bibliographical references.

The catalytic conversion of biomass-derived oxygenates into valuable fuels and chemicals is a promising route to address the current energy and environmental issues. The low-temperature catalytic conversion is particularly promising in that it does not require intense energy input and can yield a variety of value-added products. In this approach, the hydrodeoxygenation reaction removes excess oxygen in biomass-derived oxygenates to convert them into value-added fuels and chemicals. There are two promising conversion pathways under this category: the conversion of lignocellulosic biomass and the biodiesel production from waste cooking oils. In the first approach, furfural is an important platform chemical that can be further upgraded into value-added products. Transition metal carbide catalysts have been demonstrated to be highly active and selective in cleaving the aliphatic C-O bond to form 2-methylfuran from furfural. However, the stability of these catalysts needs further improvement. In the second approach of biodiesel production, glycerol is a major by-product. The upgrading of glycerol via the selective hydrodeoxygenation reaction is especially economically promising. Different numbers of C-O bonds of glycerol can be cleaved to form value-added compounds, such as allyl alcohol, propanal, and acetol. Mo₂C has previously been shown as a selective catalyst for C-O bond scission, but its interaction with oxygen is so strong that all the C-O bonds in glycerol are cleaved. In order to selectively break certain numbers of C-O bonds while preserving others, the Mo2C catalyst needs to be modified. In this dissertation, the strategies for modifying the Mo₂C surface to achieve enhanced stability for furfural conversion and tunable selectivity for glycerol upgrading are demonstrated. With the addition of cobalt, the interaction between the surface and the oxygen atom in furfural is lowered to a proper extent and therefore the stability of Mo₂C is enhanced. By using different coverages of copper to modify Mo₂C, the number of C-O bonds cleaved in glycerol can be controlled. The subsequent chapters then compare the reactions of glycerol over the corresponding transition metal nitride, Mo₂N, as well as the C-O bond scission over a modified transition metal oxide, WOx/Pt(111), surface. It is shown that while Mo₂C and Mo₂N both break all C-O bonds of glycerol to produce propylene, Mo₂N also selectively cleaves two C-O bonds of glycerol to form allyl alcohol and propanal, a phenomenon only observed on the Cu/Mo₂C interface. DFT calculations reveal that the surface nitrogen atoms in Mo₂N block some Mo sites, and therefore promote the selective C-O bond scission. In the case of C-O bond scission of isopropanol over WOₓ/Pt(111), it is shown that surface hydroxyl groups on WOx sites catalyze the reaction. DFT results also demonstrate the synergistic effect between WOx and Pt and predict the energetics of in situ acid sites formation, which are very useful for the optimization of relevant metal oxide/metal catalysts. Overall, this dissertation compares the similarities and differences regarding the active sites and reaction mechanisms of C-O/C=O bond scission of biomass-derived oxygenates over transition metal carbide, nitride, and oxide surfaces, which should in turn provide useful guidelines for the rational catalyst design for biomass upgrading reactions.

The rapid influx of solar energy and the desire to utilize carbon dioxide (CO2) will require large-scale energy storage and CO2 conversion technologies. Electrocatalytic devices can substantially impact both challenges, but improvements to electrocatalyst cost, activity, and selectivity are needed. Transition metal carbides provide a unique framework to reduce the loading of expensive catalyst metals while tuning the electrocatalytic activity and selectivity. Transition metal nitrides have many similar properties as carbides, and their synthesis inherently avoids the unwanted carbonaceous overlayer associated with carbide synthesis. Here it is shown that carbides and nitrides enable lower platinum-group metal (PGM) loadings and improve the activity and selectivity of electrocatalysts for reactions of water electrolysis and electrochemical CO2 reduction. Atom-thick layers of Pt were deposited onto niobium carbide (NbC) thin films to assess hydrogen evolution reaction (HER) activity. The Pt/NbC thin film, with one monolayer of Pt on NbC, performed similarly to bulk Pt. This correlated well with density functional theory (DFT) calculations of the hydrogen binding energy on the Pt/NbC surface. Potential applications of transition metal nitrides as electrocatalyst support materials were explored by synthesizing thin film nitrides of niobium and tungsten. The stability of each nitride was evaluated across broad potential-pH regimes to create a pseudo-Pourbaix diagram for each one. The films were each modified with atom-thick layers of Pt and were evaluated for HER performance in acid and alkaline electrolyte. Thin layers of Pt on WN and NbN showed Pt-like HER performance in acid and are promising candidates for high-surface area catalysts. To address the issue of high iridium (Ir) loading for the oxygen evolution reaction (OER) at the water electrolyzer anode, core-shell Ir-metal nitride particles were synthesized that contained 50% of the Ir mass loading of benchmark IrO¬2 particles. Iridium-iron nitride (Ir/Fe4N) showed increased activity on a mass-Ir basis and on a per-site basis, compared to IrO2. The core-shell morphology and stability under reaction conditions were confirmed with electron microscopy and in-situ X-ray absorption spectroscopy. Electrochemical reduction of CO2 to a mixture of CO and H¬2 (synthesis gas) was achieved on the palladium hydride (PdH) electrocatalyst. The product mixture can then be used as feedstock for the Fischer–Tropsch process and methanol synthesis. The syngas production performance was optimized by evaluating shape controlled PdH particles, bimetallic PdH, and PdH supported on transition metal carbides. At each step, the phase transition from Pd to PdH was monitored under reaction conditions with synchrotron-based X-ray absorption spectroscopy and X-ray diffraction. We also performed an overall carbon balance for catalytic transformation of CO2 to methanol via four reaction schemes, including one relying on electrocatalytic syngas production. The analysis revealed that hybrid electrocatalytic/thermocatalytic processes are most promising for resulting in overall CO2 reduction, but current densities of recently reported electrocatalysts need to increase to make the process economically feasible.

As our world continues to develop and contend with the impacts of climate change, the scale up renewable energy technologies has never been more urgent. Alternative fuels derived from biomass-derived oxygenates and water splitting offer promising solutions for the transition towards sustainable chemical feedstocks and integration of clean renewable energy sources. However, this technology continues to be hampered by the need for scarce and costly precious metal catalysts. The work done in this thesis explored the facet-dependence of glycerol electrooxidation and studied the application of earth-abundant transition metal carbides (TMCs) and nitrides (TMNs) for reducing precious metal catalyst loadings in water electrolysis and electrooxidation of methanol and glycerol. Glycerol valorization has drawn significant interest in recent years due to the growth in biodiesel production leading to the market saturation of glycerol. While this molecule can be converted into a variety of value-added products, the possibilities have been limited by poor selectivity for C-C bond scission. The breaking of the C-C bonds in glycerol allows for complete extraction of energy from the molecule via complete glycerol oxidation, thereby opening the door for utilizing glycerol as an electrochemical fuel. While platinum (Pt) has been among the most popular catalysts, its tendency for poisoning due to adsorbed CO has hindered its activity. Previously demonstrated to enhance the catalytic activity of platinum (Pt) by reducing CO binding energy and increasing C-C bond scission selectivity in ethanol electrooxidation, TMCs were employed as catalyst supports for the glycerol electrooxidation reaction. This work used electrochemical techniques and in-situ IRRAS to study various loadings of Pt/TaC and Pt/WC to find enhanced C-C bond scission activity at reduced Pt loading because of the synergistic effects between Pt and TMCs. While Pt has remained the benchmark catalyst for glycerol electrooxidation due to its high C-C scission activity, gold (Au) has also found popularity with its high catalytic activity attributed to greater resistance to CO poisoning, despite its favorability for partial glycerol oxidation. Previous studies have hinted at the significance of Au surface facets on glycerol oxidation activity and product selectivity, but none had used nanoparticles with controlled surface facets. This thesis sought to bridge the knowledge gap using precisely-synthesized Au nanocrystals with well-characterized {100}, {110}, and {111} surface facets to provide insight into glycerol electrooxidation on Au. Electrochemical techniques were used in parallel with in-situ IRRAS analysis to uncover the differences in product selectivity and oxidation activity between the three Au surfaces, with Au {111} exhibiting the greatest activity for C-C bond scission, while Au {110} showed the lowest onset potential due to facile AuOH- formation. Hydrogen (H₂) fulfills a critical role in modern society, not only as a renewable fuel, but also as a key chemical feedstock. Production of H₂ from water electrolysis creates opportunities for storing excess energy from renewable sources as an energy-dense fuel and reducing the environmental footprint of chemical processes requiring H₂. However, efforts have been hampered by the dependence on scarce Pt-group catalyst materials. This thesis explores the application of TMNs as an earth-abundant material for enhancing the activity of Pt in the hydrogen evolution reaction (HER). Combined with DFT calculations, the HER activity of monolayer Pt- and Au-modified TMN thin films was correlated with the ΔGH* values in a volcano-type relationship. Electrocatalytic experiments in acidic electrolyte showed that TMN-supported monolayer Pt exhibited similar HER activity to the Pt foil, correlating with intermediate hydrogen adsorption strength. TiN-supported Pt and Au powders were studied to extend the correlations from thin films. Furthermore, the electrochemical stability of TMNs was studied across a wide range of potentials and pH values to generate pseudo-Pourbaix diagrams and identify TMN candidates for HER, alcohol oxidation, ORR and OER applications. Using the pseudo-Pourbaix findings, Pt/TMN catalysts were selected for studying methanol electrooxidation activity. Methanol electrooxidation has drawn significant attention particularly due to interest in direct alcohol fuel cells. Much like the case for glycerol oxidation, while Pt has been the benchmark catalyst, it has been hindered by strong adsorption of CO. As the modification of Pt with other materials, such as ruthenium, has shown promising enhancements to methanol electrooxidation activity, the synergistic effects of Pt modification with TMNs were studied in this work. In the resulting electrochemical experiments, Pt/Mo₂N was found to exhibit negligible activity likely because of its oxidative instability. In contrast, Pt/TiN showed enhanced activity, and in-situ IRRAS experiments suggest that Pt/TiN enhanced the COads-free pathway leading to increased formic acid selectivity. This thesis demonstrated avenues for developing more optimized catalysts with reduced loadings of Pt and other precious metals for applications in alternative fuel production and utilization. The influence of Au surface facets on glycerol oxidation was examined and the synergistic effects between Pt and earth-abundant TMC and TMN materials were used to enhance the electrooxidation of biomass-derived oxygenates and H₂ production from water electrolysis. These electrochemical stability and activity trends can guide future catalyst design for other critical reactions such as oxygen evolution and challenging applications like glycerol electroreduction.