Structure-activity relationships of nickel-hexaaluminates in reforming reactions part i: Controlling nickel nanoparticle growth and phase formation

The controlled synthesis of hexaaluminates ANi_yAl _12-yO_19-δ (A=Ba, La, Sr, and y=0.25, 0.5, 1) is reported by a freeze drying route. This route allows the use of moderate temperatures of approximately 1200 °C to obtain hexaaluminates of high phase purity (>80 wt %) as well as high specific surface areas (10-30 m ² g^-1). Under reducing conditions at elevated temperatures, nickel expulsion from the hexaaluminate framework can be observed. High stability of the crystalline phase is observed even if all substitution cations leave the hexaaluminate framework. The moderate calcination temperature of 1200 °C facilitates the reducibility of the Ni-hexaaluminates compared to Ni-hexaaluminates calcined at 1600 °C. SEM and TEM imaging revealed that Ni-hexaaluminates with low Ni loading (y=0.25) and calcined at moderate temperature (1200 °C) lead under reducing atmosphere to the formation of strong textural growth and highly disperse and highly textured Ni⁰ nanoparticles. Nanoparticle growth is associated to surface defect sites occurring on the hexaaluminate platelets. Mission nickel part I: Nickel nanoparticle growth on Ni-hexaaluminate is assessed by STEM and X-ray powder diffraction methods before and after reduction. The study focuses on the influence of parameters such as Ni substitution degree, mirror plane cation (La, Sr, and Ba), and calcination temperature. A reduction mechanism is proposed (see picture).