Doctor of Philosophy

dissertationJP-10 is a synthetic fuel with high volumetric energy content. One problem with JP-10, is that its combustion kinetics can be too slow for efficient combustion in hypersonic flight applications. Chapter 2 presents a study on the thermal breakdown and catalytic combustion of JP-10 fuel using CeO 2 (ceria) nanoparticles, in a flow tube reactor. In-situ mass spectrometry was used to analyze decomposition products. In the absence of O2, CeO2 efficiently oxidizes JP-10, reducing decomposition onset temperatures by 300 K over that in a clean flow tube. Under conditions with O2 and CeO2 present, oxidation of JP-10 was found to be catalytic; i.e., oxidation is initiated by reaction of JP-10 with CeO2, which is then reoxidized by O2. Boron is of interest as a high energy density fuel as it has one of the highest volumetric heats of combustion known. A major difficulty in getting boron to burn efficiently is that boron surfaces are protected by a native oxide layer. Chapter 3 presents a simple, scalable, one-step, one-pot synthesis method for producing ∼50 nm boron nanoparticles that are largely unoxidized, made soluble in hydrocarbons through oleic acid functionalization, and optionally coated with ceria. Scanning electron microscopy (SEM) and dynamic light scattering (DLS) were used to investigate size distributions, with X-ray photoelectron spectroscopy (XPS) to probe the surface chemistry. Cryogenic methane has been proposed as a fuel for use in hypersonic engines, due to its relatively high energy content; however its poor ignition performance needs to be addressed through use of catalysts. Chapters 4 and 5 investigate the composition, structure, and surface chemistry of several types of Pd/PdO based nano-catalysts designed to be fuel soluble. A combination of high resolution transmission electron microscopy (HRTEM), electron diffraction, scanning transmission electron microscopy/energy dispersive x-ray spectroscopy (STEM/EDX), and XPS were used. In-situ generated particles were found to be primarily crystalline, metallic Pd, in a narrow size distribution around 8 nm. The ignition temperature was lowered ∼150 K by the catalyst, and evidence is presented showing that ignition is correlated with formation of a subnanometer oxidized Pd surface layer at higher temperatures

Surfactant- and Ligand-Free Synthesis of Platinum Nanoparticles in Aqueous Solution for Catalytic Applications

The synthesis of surfactant-free and organic ligand-free metallic nanoparticles in solution remains challenging due to the nanoparticles’ tendency to aggregate. Surfactant- and ligand-free nanoparticles are particularly desirable in catalytic applications as surfactants, and ligands can block access to the nanoparticles’ surfaces. In this contribution, platinum nanoparticles are synthesized in aqueous solution without surfactants or bound organic ligands. Pt is reduced by sodium borohydride, and the borohydride has a dual role of reducing agent and weakly interacting stabilizer. The 5.3 nm Pt nanoparticles are characterized using UV-visible spectroscopy and transmission electron microscopy. The Pt nanoparticles are then applied as catalysts in two different reactions: the redox reaction of hexacyanoferrate(III) and thiosulfate ions, and H2O2 decomposition. Catalytic activity is observed for both reactions, and the Pt nanoparticles show up to an order of magnitude greater activity over the most active catalysts reported in the literature for hexacyanoferrate(III)/thiosulfate redox reactions. It is hypothesized that this enhanced catalytic activity is due to the increased electron density that the surrounding borohydride ions give to the Pt nanoparticle surface, as well as the absence of surfactants or organic ligands blocking surface sites

Surfactant- and Ligand-Free Synthesis of Platinum Nanoparticles in Aqueous Solution for Catalytic Applications

The synthesis of surfactant-free and organic ligand-free metallic nanoparticles in solution remains challenging due to the nanoparticles’ tendency to aggregate. Surfactant- and ligand-free nanoparticles are particularly desirable in catalytic applications as surfactants, and ligands can block access to the nanoparticles’ surfaces. In this contribution, platinum nanoparticles are synthesized in aqueous solution without surfactants or bound organic ligands. Pt is reduced by sodium borohydride, and the borohydride has a dual role of reducing agent and weakly interacting stabilizer. The 5.3 nm Pt nanoparticles are characterized using UV-visible spectroscopy and transmission electron microscopy. The Pt nanoparticles are then applied as catalysts in two different reactions: the redox reaction of hexacyanoferrate(III) and thiosulfate ions, and H2O2 decomposition. Catalytic activity is observed for both reactions, and the Pt nanoparticles show up to an order of magnitude greater activity over the most active catalysts reported in the literature for hexacyanoferrate(III)/thiosulfate redox reactions. It is hypothesized that this enhanced catalytic activity is due to the increased electron density that the surrounding borohydride ions give to the Pt nanoparticle surface, as well as the absence of surfactants or organic ligands blocking surface sites

Hydrogen assisted magnesiothermic reduction of TiO2

The development of low cost titanium metal production processes has challenged the Ti research and industrial communities around the world for decades. The strong affinity of titanium to oxygen dictates that it is very difficult to produce low-oxygen Ti metal from TiO2 directly. In this paper, a hydrogen assisted magnesiothermic reduction (HAMR) process for producing Ti metal powder from TiO2 powder at relatively low temperatures (&lt;= 750 degrees C) is established. The overall approach is based on the thermodynamic tuning of the relative stability of MgO versus that of Ti-O solid solutions by temporarily alloying the system with hydrogen. It is shown that Ti-H-O solid solutions are less stable than their corresponding Ti-O solid solutions, which changes the reaction of Mg with Ti-O from being thermodynamically unfavorable to being favorable. The key steps for producing pure Ti metal powder from TiO2 involve Mg reduction of TiO2 in a hydrogen atmosphere which produces porous TiH2, a heat treatment procedure to consolidate the powder and reduce specific surface area of the powder, and the final step to deoxygenate the powder using Mg in a hydrogen atmosphere to further reduce the oxygen content. This paper systematically examines the changes of oxygen content, phase transformations, and the evolution of the morphology of the particles during the entire process. The results show that this approach has great potential to be a viable method for the production of low-oxygen Ti metal powder from TiO2. In addition, the effect of hydrogen on the oxidation of Ti powder is analyzed using XPS, which reaffirms that titanium hydride is more impervious to surface oxidation than Ti metal, another crucial advantage of using hydrogen atmosphere. (C) 2016 Elsevier B.V. All rights reserved.</p

Oxide-Free, Catalyst-Coated, Fuel-Soluble, Air-Stable Boron Nanopowder as Combined Combustion Catalyst and High Energy Density Fuel

Elemental boron has one of the highest volumetric heats of combustion known and is therefore of interest as a high energy density fuel. The fact that boron combustion is inherently a heterogeneous process makes rapid efficient combustion difficult. An obvious strategy is to increase the surface area/volume ratio by decreasing the particle size. This approach is limited by the fact that boron forms a ∼0.5 nm thick native oxide layer, which not only inhibits combustion, but also consumes an increasing fraction of the particle mass as the size is decreased. Another strategy might be to coat the boron particles with a material (e.g., catalyst) to enhance combustion of either the boron itself or of a hydrocarbon carrier fuel. We present a simple, scalable, one-step process for generating air-stable boron nanoparticles that are unoxidized, soluble in hydrocarbons, and coated with a combustion catalyst. Ball milling is used to produce ∼50 nm particles, protected against room temperature oxidation by oleic acid functionalization, and optionally coated with catalyst. Scanning and transmission electron microscopy and dynamic light scattering were used to investigate size distributions, with X-ray photoelectron spectroscopy to probe the boron surface chemistry

Mitigation of the Surface Oxidation of Titanium by Hydrogen

As a reactive metal, Ti is prone to surface oxidation spontaneously when exposed to environment containing oxygen-a phenomenon also known as surface passivation. It has also been known that titanium hydride (TiH2) is impervious to oxygen. However, it is not clear to date and there is little published report on how and why hydrogen affects the oxidation of Ti. &quot;Impervious&quot; may be an overstatement because TiH2 does oxidize. Because surface oxygen is also a part of the total oxygen in titanium in addition to bulk oxygen and the passivation film affects the properties of titanium, understanding the surface oxidation behavior of titanium and the effects of hydrogen is thus of considerable interest from both fundamental and practical perspectives. This article studies the effect of hydrogen on the surface passivation of titanium from different aspects including (I) the comparison of oxygen contents in alpha-Ti and TiH2 powders when exposed to air, (II) the oxidation states, their relative fractions, and the thickness of the oxidized layers as a function of hydrogen contents, and (III) the characterization of the passivation layer on the surface by high-resolution transmission electron microscope. The experimental data showed that the presence of hydrogen can indeed make titanium metal less prone to oxidation. The alloying of titanium with hydrogen can result in reduced thickness and the relative fraction of titanium in the form of Ti-IV in the passivated surface, effectively minimizing surface oxidation. The fundamental reason for the effect of hydrogen on the surface oxidation of titanium is discussed and attributed to the difference in oxidation behavior of alpha and delta phases.</p