Density functional theory calculations of surface thermochemistry in Al/CuO thermite reaction

This paper investigates the thermochemistry of the heterogeneous Al/CuO thermite reaction through density functional theory calculations. We examine the interactions of atomic Al, Cu, O, as well as ${\mathrm{O}}_{2}$, AlO, ${\mathrm{Al}}_{2}\mathrm{O}, \mathrm{Al}{\mathrm{O}}_{2}, {\mathrm{Al}}_{2}{\mathrm{O}}_{2}$ molecular species, with Al(111), Cu(111), and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ ($\ensuremath{\gamma}$ and amorphous) surfaces, all of which being condensed phase products during the thermite reaction. Al(111) exhibits a very high reactivity, characterized by adsorption energies ranging from 3 to 5.3 eV for atomic Al, Cu, O, and from 4 to 9.5 eV for all molecular species. This reactivity is associated to barrierless molecular decomposition, followed by the spatial spreading of adsorbate species across the surface facilitated by hot adatom migration processes. The ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surface also exhibits extremely high reactivity, with adsorption energies of 4.5 and 9.4 eV for atomic Cu and Al, respectively. Additionally, adsorption energies range from 7 to 15 eV for condensation of ${\mathrm{Al}}_{x}{\mathrm{O}}_{y}$ suboxides. Al-rich suboxides, namely ${\mathrm{Al}}_{2}\mathrm{O}$ and ${\mathrm{Al}}_{2}{\mathrm{O}}_{2}$, show the greatest adsorption energy with 15.05 eV for ${\mathrm{Al}}_{2}\mathrm{O}$, against 6.52 eV for $\mathrm{Al}{\mathrm{O}}_{2}$. In contrast, O and ${\mathrm{O}}_{2}$ exhibit no reactivity on ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$ surfaces exhibiting oxidation states being superior or equal to ${\mathrm{Al}}^{III}$. Finally, Cu(111) surface exhibits much lower reactivity compared to Al(111) and ${\mathrm{Al}}_{2}{\mathrm{O}}_{3}$, with adsorption energies ranging from 2 to 3.5 eV for Al, O, and Cu atoms, which effect is even more pronounced at high temperature due to entropic effects. Although energetic, molecular ${\mathrm{Al}}_{x}{\mathrm{O}}_{y}$ suboxides show nondissociative adsorption on Cu(111). These findings point to different modes of oxide nucleation on these surfaces, pleading for planar nucleation and growth onto Al(111), while being more localised onto Cu(111). They renew our understanding of the thermite reaction chemistry, quantitatively differentiating the various type of heterogeneous reactions, the effect of rising the temperature, and their implication on the overall reaction. They also provide valuable data for higher-level diphasic simulations of the computational fluid dynamics, aiming to achieve predictive capability.