Density functional theory (DFT) calculations were performed to examine exothermic surface chemistry between alumina and four fluorinated, fragmented molecules representing species from decomposing fluoropolymers: F, HF, CHF, and CF. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. It was hypothesized that the alumina surface structure could be transformed due to hydrogen bonding effects from the environment that promote surface reactions with fluorinated species. In this study, the alumina surface was analyzed using model clusters as isolated systems embedded in a polar environment (i.e., acetone). The conductor-like screening model (COSMO) was used to mimic environmental effects on the alumina surface. Four defect models for specific active -OH sites were investigated including two terminal hydroxyl groups and two hydroxyl bridge groups. Reactions involving terminal bonds produce more energy than bridge bonds. Also, surface exothermic reactions between terminal -OH bonds and fluorinated species produce energy in decreasing order with the following reactant species: CF > HF > CHF. Additionally, experiments were performed on aluminum powders using thermal equilibrium analysis techniques that complement the calculations. Consistently, the experimental results show a linear relationship between surface exothermic reactions and the main fluorination reaction for Al powders. These results connect molecular level reaction kinetics to macroscopic measurements of surface energy and show that optimizing energy available in surface reactions linearly correlates to maximizing energy in the main reaction.

The stabilizing, amorphous alumina (Al2O3) passivation layer surrounding aluminum (Al) particles participates in reactions that lower barriers to bulk Al oxidation. The behavior has been observed in thermites comprised of nanoscale Al particles (nano-Al) dispersed within a fluoropolymer matrix. Studies reported herein show the oxide passivation shell on nano-Al particles is affected by the polarity and hydrogen bonding properties of the solvent employed for thermite dispersal, resulting in enhanced thermal energy propagation during Al combustion in nano-Al + poly­(tetrafluoroethylene) (PTFE) mixtures. Relative to conventional treatments that employ hexane for thermite dispersal, the speed of flame front movement measured in a Bockmon Tube apparatus under steady-state conditions increased more than 2-fold following treatments in acetone or 2-proponal. Differential scanning calorimetry and infrared spectroscopy measurements indicate contact with the polar solvents increases the amount and accessibility of hydroxyl species on the nano-Al oxide shell, which in turn participates in a preignition reaction (PIR) that activates PTFE and likely weakens the Al passivation layer. A molecular-scale mechanism is proposed for the PIR that derives from catalytic reactions of alumina and halocarbon fluorinating reagents. Additionally, infrared spectra show evidence for a greater fraction of disordered, liquid-like hydrogen-bonded water molecules within the alumina layer of nano-Al particles after treatment in the polar solvents studied. The O–H vibrational features suggest solvent treatment may affect the structure of nano-Al surface oxides and PIR kinetics. This study reveals potential strategies for optimizing fuel particle reactivity that include modification of the Al particle passivation shell using polar solvents to promote early preignition exothermic reaction.

Density functional theory (DFT) calculations were performed to understand molecular variations on an alumina surface due to exposure to a polar environment. The analysis has strong implications for the reactivity of aluminum (Al) particles passivated by an alumina shell. Recent studies have shown a link between the carrier fluid used for Al powder intermixing and the reactivity of Al with fluorine containing reactive mixtures. Specifically, flame speeds show a threefold increase when polar liquids are used to intermix aluminum and fluoropolymer powder mixtures. It was hypothesized that the alumina lattice structure could be transformed due to hydrogen bonding forces exerted by the environment that induce modified bond distances and charges and influence reactivity. In this study, the alumina surface was analyzed using DFT calculations and model clusters as isolated systems embedded in polar environments (acetone and water). The conductor-like screening model (COSMO) was used to mimic environmental effects on the alumina surface. Five defect models for specific active -OH sites were investigated in terms of structures and vibrational -OH stretching frequencies. The observed changes of the surface OH sites invoked by the polar environment were compared to the bare surface. The calculations revealed a strong connection between the impact of carrier fluid polarity on the hydrogen bonding forces between the surface OH sites and surrounding species. Changes were observed in the OH characteristic properties such as OH distances (increase), atomic charges (increase), and OH stretching frequencies (decrease); these consequently improve OH surface reactivity. The difference between medium (acetone) and strong (water) polar environments was minimal in the COSMO approximation.

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Remarkable strides in the past few years have provided the chemical engineer with valuable information and tools for use in selecting and improving catalysts. Briefly these include three major theories and several measuring techniques useful in gaining insight into catalyst characteristics. THEORIES