Unraveling Thermal Depolarization Phenomena in Biphasic Polarized Calcium Phosphate Catalyst

Abstract

Permanently polarized biphasic calcium phosphate composed of hydroxyapatite and brushite (pp‐HAp/Bru), which is prepared by applying the thermally stimulated polarization treatment to calcined HAp, is used as a sustainable catalyst to transform CO2 into value‐added products. In this work, the stability of pp‐HAp/Bru is studied from structural, electrical, and catalytic perspectives, applying a thermal depolarization process with temperatures (Td) ranging from 200 to 1000 °C. Results show that the Bru phase is not stable when Td ≤ 400 °C, while the structure of the HAp refines and transforms into Bru when Td ≥ 600 °C. Besides, the electrical resistance and capacitance of the pp‐HAp/Bru increase with Td, evidencing the progressive electrical depolarization of the material. Thermal depolarization also influences the specific orientation of the OH– ions, which is partially lost (≈50%). All such changes affect the catalytic efficiency of pp‐HAp/Bru, which is proven using a reaction that transforms CO2 gas into acetic acid and formic acid. Results show that the total reaction yield linearly decreases with increasing Td. Based on such observations, a simple process is designed that allows the reconstitution of the structure and restores the activity of such green catalysts.