Synthesis of aluminum-nickel based HTlcs using urea coprecipitation and an investigation into the coprecipitation mechanism

Nickel-aluminum based hydrotalcite-like compounds (HTlcs) doped with 2 wt % copper were synthesized by coprecipitation using a urea hydrolysis method. The average size of the coprecipitated particles ranged from 4 nm to 15 nm. The particles displayed a certain degree of coagulation and edge-surface platelet interactions. The effects of several parameters on the composition, physical morphology, average particle size and particle size distribution of the precipitates were systematically studied. Particle growth was found to be the reason for the relatively large size particle formed in low urea concentration environments, while agglomeration was believed to be responsible for large particle size in high urea concentration environments.The bulk pH curve of the coprecipitation process shows two distinct buffer regions. The first pH buffer region is considered to be controlled by the reaction of OH- with trivalent metal ions while the second pH buffer region, controlled by the reaction with divalent metal ions. When multiple divalent metals were present in the bulk solution, more buffer region may occur according to the coprecipitation properties of the divalent metals. From X-ray Diffraction (XRD) characterization and degree of coprecipitation determined by Atomic Absorption Spectroscopy (AAS), the formation of an aluminum hydro-complex, Al(OH)3n-1(H2O)m +, is proved by to be a possible intermediate before HTlc precipitates. A new tentative coprecipitation scheme in an Al-Ni-Cu system was derived from this study. The effect of pretreatment conditions of the precipitated HTlcs, inclusion of sodium ions and doping of copper on catalyst properties is also discussed.