2.2. Synthesis of LDH

In total, 12 LDH materials were prepared as listed in Table 1. The materials differed in the layer chemistry (Mg/Al or Mg/Fe), metal ratio (2:1, 3:1, 4:1, 6:1 and 8:1), and type of interlayer anion (Cl⁻ or NO₃). The materials were synthesized by the standard co-precipitation method using aqueous metal salts. The solution containing both M(II) and M(III) in stoichiometric amounts was prepared with 100 mL of DI water. Regardless of the M(II) to M(III) ratio, the sum of the chemicals’ mass was set at a constant. Thus, the solution density was also constant. This solution was slowly mixed with aqueous 5 M NaOH in a 2 L beaker which initially contained 400 mL of water at pH 10. The precipitate formed in the 2 L beaker was constantly mixed with magnetic stirrer at room temperature. The whole synthesis lasted about 30–40 min. The portions of metal solution and base solution were gradually injected to maintain a constant pH 10 throughout the synthesis. In the end, the formed precipitates were aged for 2 h at room temperature (22 °C) and were then washed with water and dried at 70 °C for 24 h. The materials with 6:1 and 8:1 metal ratios were not aged to prevent the formation of metal hydroxides (e.g., brucite) as indicated in our previous trials.

The layered double hydroxide (LDH) materials used in the study with the chemistry of the brucite-like layer.