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Most recent developments in the production, properties and applications of borides and carbides are reviewed. The reactivity and sintering of finely-divided boron carbide with metal additives has been investigated. The additives (Fe, T i , Zr, V, Nb, Ta, Mo, W and Al) generally promote sintering of the boron carbide. Their effectiveness is reduced occasionally when there is some surface activation caused by the metals reacting vjith the boron carbide to form metal borides and carbides of different, crystal lattice type and molecular volume. The more metallic character of the bonding in the metal borides and carbides enhances surface and crystal lattice diffusion at the grain boundaries of the more covalent boron carbide. Iron is much more effective than the other metal addives tested in promoting sintering of the boron carbide at 1800 degree C since it forms the lowest-melting borides. It also enhances sintering during hot pressing of boron carbide at this temperature.

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.

As três famílias de ligas de maior importância comercial para a indústria de fundição apresentam a formação de eutéticos do tipo facetado/não-facetado durante a solidificação: os ferros fundidos com grafita (eutético austenita/grafita), os ferros fundidos brancos (eutético austenita/carboneto) e as ligas Al-Si (eutético alfa/silício). No caso dos ferros fundidos brancos, a morfologia dos carbonetos eutéticos exerce influência decisiva sobre o comportamento mecânico e a resistência à degradação (em função de solicitações como abrasão ou ciclos térmicos e/ou mecânicos) das ligas resistentes a desgaste pertencentes ao sistema Fe-C-X (em que X representa um ou mais dos elementos Cr, V, Mo, W e, mais raramente, Nb). Investigou-se o efeito do potencial nucleante do banho metálico sobre o eutético de ligas do sistema Fe-C-V, e conseqüentemente sobre a morfologia do carboneto. As alterações de morfologia do eutético austentita/carboneto VC em função da adição de alumínio ou titânio foram avaliadas por meio de microscopias óptica e eletrônica de varredura. Observou-se que a adição de alumínio ou titânio, em ligas com teores residuais de oxigênio ou nitrogênio, promoveu alteração do eutético VC de irregular para fibroso (no caso da adição de alumínio) e para divorciado (no caso da adição de titânio). Conclui-se que a presença de partículas nucleantes induziu a nucleação seletiva da fase facetada (carboneto VC) a temperaturas mais elevadas, diminuindo o grau de cooperação durante o crescimento do eutético. / The three most commercially important families of alloys for the foundry industry show the formation of faceted/non-faceted eutectics type during solidification: cast irons with graphite (austenite/graphite eutectic), white cast irons (austenite/carbide eutectic) and Al-Si alloys (alfa/silicon eutectic). In the case of white cast irons, the eutectic carbide morphology has decisive influence on mechanical behavior and on degradation resistance (due to solicitation such as abrasion or thermal and/or mechanical cycling) of wearing resistant alloys of the Fe-C-X system (where X stands for one or more of the following elements: Cr, V, Mo, W and, seldom, Nb). The metallic bath nucleant potential on the eutectic of the Fe-C-V system and, thus, on the carbide morphology, were investigated. It was evaluated the nucleant potential for aluminum and titanium addition to the bath. The morphologies changes for the austenite/carbide eutectic were evaluated by optical and scanning electronic microscopy. It was observed that aluminum or titanium addition in alloys with residual content of oxygen or nitrogen promoted VC eutectic morphology change from irregular to fibrous (when aluminum is added) and to divorced (when titanium is added). It is concluded that the presence of nucleant agents in the metallic bath induced the preferred nucleation of the faceted phase at higher temperatures, decreasing the cooperation during the eutectic growth.

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.

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Fischer-Tropsch synthesis reaction to produce sulphur free hydrocarbons has enjoyed a resurgent in interests due to increases in world oil prices. In this work, the suitability of Mo and Mo-W carbides has been investigated as a possible cost-effective alternative to noble metals in Fischer-Tropsch synthesis. The molybdenum and tungsten monometallic and bimetallic carbides were prepared through precipitation from homogeneous solution to the sulphide followed by carburization with a mixture of propane and hydrogen to produce the resulting metal carbide. A 23 factorial design strategy was employed to investigate the effect of three carburizing variables, namely, time, temperature and gas ratio on the resulting catalyst. In particular, the effect of supports was also examined through four common semiconductor oxide supports, namely: Al2O3, SiO2, TiO2 and ZrO2. Thermogravimetric analysis of the carburization reactions showed that the conversion from metal sulphide to the metal carbides is a multistep process producing different phases of carbides, namely ??-MoC1-x, ??-Mo2C, ?? -WC1-x and ??-W2C, depending on heating rate and temperature. The rate determining step of the carburising reaction is the diffusion of carbon atoms into the metal matrix, hence giving relatively low activation energy values. Statistical analysis of the factorial design revealed that all three carburizing variables affect the final physiochemical makeup of the catalyst. SEM analysis showed that the carbides are well dispersed on the surface of the support and catalyst particles produced are nanoparticles in the range of 25 to 220 nm depending on the support. Fischer-Tropsch activity test showed that monometallic molybdenum carbide is active under Fischer-Tropsch conditions while tungsten carbide is inactive for the conditions studied in this project. However, bimetallic carbide catalyst, consisting of the two mentioned metals gave overall higher reaction rates and decreased methane selectivity. Steady state analysis revealed that there are two active sites on the surface of molybdenum carbide catalyst resulting in two chain growth propagation values when analysed via the Anderson-Schulz-Flory kinetics. Overall, ZrO2 support appeared to be the most suitable support followed by SiO2, TiO2 and Al2O3. Finally, kinetic modelling of data showed that methanation and higher hydrocarbons formation path occurs via combination of the oxygenated intermediate and Eley-Rideal mechanism.