Fluorination of an Alumina Surface: Modeling Aluminum−Fluorine Reaction Mechanisms Richa Padhye, † Adelia J. A. Aquino,* ,‡,§,∥ Daniel Tunega, § and Michelle L. Pantoya* ,† † Department of Mechanical Engineering and ‡ Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, Texas 79409, United States § Institute for Soil Research, University of Natural Resources and Life Sciences, Peter-Jordan-Strasse 82, A-1190 Vienna, Austria ∥ School of Pharmaceutical Sciences and Technology, Tianjin University, Tianjin 300072, P. R. China * S Supporting Information ABSTRACT: 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, CH 3 F, and CF 4 . 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 4 > HF > CH 3 F. 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. KEYWORDS: DFT calculations, modified alumina structures, implicit solvent model, alumina, catalysis, fluoropolymer reactions, aluminum combustion, fluorides ■ INTRODUCTION The science of aluminum fuel particle reactivity with fluoropolymers has important implications toward new processing techniques in the additive manufacturing of energetic materials. Many binders used in energetic composites, such as Kel-F and Viton, 1 as well as poly(vinylidene fluoride) (PVDF), 2 contain fluorine and are processed with aluminum fuel powder for the purpose of creating tailorable architectures. The reactivity of aluminum with fluorinated materials is therefore becoming an important area of research toward advancing energetic material science from a processing as well as a combustion perspective. The aluminum powder used in aluminum/fluorine compo- sites is composed of Al particles that have a core−shell structure. The core of the Al particle is crystalline Al, and the shell is amorphous Al 2 O 3 . 3 The Al 2 O 3 shell surrounding the crystalline Al core acts as an oxygen diffusion barrier, limiting the oxidation of crystalline Al. 4 In traditional Al combustion, the Al 2 O 3 shell is inert (i.e., does not chemically react to produce heat) and absorbs heat (i.e., acts as a heat sink). Recently, the inert oxide shell has been shown to have potential for contributing to the overall energy generated in Al combustion by utilizing an exothermic reaction between fluorine and Al 2 O 3 . Osborne et al. 5 showed that the alumina shell on an aluminum particle reacts with fluorinated species from a decomposing fluoropolymer, producing an exothermic surface reaction that also facilitates decomposition of the fluoropolymer. The surface reaction occurs prior to the main aluminum fluorination reaction and is referred to as a “preignition reaction” or PIR. 5 Pantoya and Dean 6 further showed that the surface area exposure of alumina and the fluoropolymer is directly related to the amount of energy Received: April 17, 2017 Accepted: June 28, 2017 Published: June 28, 2017 Research Article www.acsami.org © 2017 American Chemical Society 24290 DOI: 10.1021/acsami.7b05372 ACS Appl. Mater. Interfaces 2017, 9, 24290−24297