The influence of synthesis method and Mg-Al-Fe content on the thermal stability of layered double hydroxides

Layered double hydroxides (LDHs) and their thermally derived mixed oxides have reached growing attention in past decades due to their wide application as catalysts or catalyst supports in organic/pharmaceutical synthesis, clean energy and environmental pollution control (decomposition of volatile organic compounds, photodecomposition, DeNo(x) and DeSO(x)). Desired properties of LDHs can easily be tailored using different synthesis methods and introducing different bivalent and trivalent constituting metals. In this study, Mg-Al and Mg-Al-Fe LDHs were synthesized by low supersaturation (LS) and high supersaturation (HS) coprecipitation methods. The content of trivalent ions was varied in a wide range between 0.15 < x < 0.7 exceeding the optimal range for the single LDH phase synthesis (0.20 < x < 0.33). The intention was to induce the formation of different LDHs and consequently obtain, after thermal treatment, different multiphase mixed oxides. The properties of the precipitates were investigated by structural (XRD), chemical (AAS and EDS) and thermal analysis (TG-DTA). The study revealed that the LS method allows the formation of LDHs with an extended M(III) substitution (x = 0.5). Although, a more disordered structure in the stacking of layers was detected for HS samples, LS samples with the same initial composition showed lower thermal stability estimated by lower temperature of both LDH thermal decomposition transition stages. The thermal stability of LDHs was not influenced considerably with the introduction of a small amount of iron as ternary metal even though lower crystallinity of Mg-Al-Fe LDHs was observed.