Continuous hydrothermal synthesis of Ca 2 Al-NO 3 layered double hydroxides: The impact of reactor temperature, pressure and NaOH concentration on crystal characteristics.

Continuous hydrothermal synthesis (CHS) of nanoparticles is most commonly associated with the production of metal oxides and ceramics. However, recent work has demonstrated that layered double hydroxides (LDH) can also be synthesised via this method. This research investigates how altering temperature, pressure and precursor base concentration affects growth and nucleation rates which impact on LDH characteristics. Experiments examined the separate effects of increased temperature, pressure and NaOH concentration on crystal domain length (CDL) and surface area. Adjustments to temperature and pressure in the reactor system resulted in variations in CDL. High temperature (200°C) with increasing pressure resulted in an increase in CDL between 50bar and 100bar, then a decrease up to 200bar. Crystal domain length of samples synthesised at 75°C and 150°C showed increases between 50bar and 150bar but a decrease at higher pressure. Variation in CDL showed little impact on specific surface area (4-7m2 g-1 ). Increasing NaOH decreased CDL. High precursor NaOH causes rapid nucleation to occur to the detriment of crystal growth. Samples with 1M and 0.5M NaOH exhibit Ca(OH)2 impurities as the increased NaOH causes precipitation of Ca2+ .