Investigation of Shock Wave Effects on Phase Transformation and Structural Modification of TiO$_2$ and Al$_2$O$_3$

Abstract

Titanium dioxide and aluminum oxide are conventional materials used in heterogeneous catalysis as catalyst support. The widely used crystalline phase of both supports is the metastable phase (anatase and γ-Al$_2$O$_3$) in which they possess a higher specific surface area compared to the thermodynamically stable phase (rutile and α-Al$_2$O$_3$). However, these phases have better thermal and mechanical stability than anatase and γ-Al$_2$O$_3$. A novel method to induce phase transformation and structural modification of crystalline materials is by applying shock waves. This study aims to experimentally investigate the effects of shock wave treatment on titania and alumina. A pressure-driven shock tube was used in this work to generate the shock waves. Two sets of experiments were carried out for TiO$_2$ and one for Al$_2$O$_3$. Titania samples were prepared in the form of pellets for the first set. Titania and alumina samples were maintained as powder for the second set of experiments. For titania, twenty shocks were applied at nominal temperature and pressure of ~ 1772 K and 23.3 bar in the first set of experiments, while thirty shocks of ~ 1572 K and 66 bar were applied in the second set of experiments. For alumina, twenty shock loadings were applied at the same conditions used for the second set of titania. Characterization techniques, such as XRD, Raman spectroscopy, TEM, SEM, XPS, and N$_2$ physisorption were employed on treated samples in order to understand the effects of shock wave treatment. Partial phase transformation was observed in shock treated TiO2 from Raman spectra and TEM images. Crystallite size reduction was observed in the first set of experiments, while increase in defects was observed by the enhanced Ti$^{+3}$ in XPS spectra in both sets of experiments. Partial phase transformation was also observed in shock treated Al$_2$O$_3$, when mixed with CNF (carbon nanofibers), from XRD patterns and confirmed with XPS. For alumina, TEM and SEM images showed the smallest particles in contact with carbon fibers, while the biggest particles exhibited agglomeration. Physisorption experiments showed a decrease of 40% in surface area and pore collapse.