Sonochemically improved regeneration of mechanically amorphized Ca2Cr-layered double hydroxides – Synthesis, characterization and photocatalytic lidocaine decomposition

Abstract

Our aim was to extend the structure recovery method previously seen for mechanochemically amorphized Ca₂Fe-layered double hydroxides (LDH) to additional LDHs and to further improve it by utilizing ultrasonic irradiation. Work commenced with the synthesis of Ca₂Cr-LDH, under the developed optimal conditions of unusually high alkali concentration (9 M NaOH) and mild heating (50 °C). Comparing ultrasonic treatment with mechanical mixing, sonication was able to recover the original structure twice as fast, reduce the direct optical band gaps of LDHs by larger extent, while the surface Cr^VI content decreased by nearly 20 %. Without base addition, Ca₂M-hydrocalumites (M^III: Al, Sc, V, Cr, Fe, Ga, In) was used as sacrificial templates in lidocaine photomineralization by forming active M^III hydroxides and oxide particles. LDH-derived Cr(OH)₃ proved to be a robust and reusable catalyst, with efficiencies ahead of many semiconductors such as SnO₂, BiOBr, WO₃, SrTiO₃ and ZnO. At pH 12, dissolution of Ca₂Cr-LDHs was inhibited, so that the catalytic effects of pre-milling and structural restoration could be studied. Performance of Ca₂Cr-LDHs nearly doubled after grinding, while a small increase was observed for Mg₂Cr- and a decrease for Zn₂Cr-LDHs. However, with appropriate structural recovery, activity of all LDHs was enhanced compared to the initial or ground solids. Benefits of ultrasonic recovery in catalysis over mechanical stirring have also been demonstrated and highlighted key aspects of the efficiency of structure recovery in the significantly different hydrocalumite−hydrotalcite systems.