Guido Wasserschaff

Structure-Activity Relationships of Nickel-Hexaaluminates in Reforming Reactions Part II: Activity and Stability of Nanostructured Nickel-Hexaaluminate-Based Catalysts in the Dry Reforming of Methane

Ni–hexaaluminates exhibiting a high magnetoplumbite or β“‐alumina phase content (>80 wt %) and high specific surface areas (10–30 m2 g−1) were investigated under dry reforming conditions. Ni content and choice of mirror plane cation are the key factors controlling the structure–property relationship in the dry reforming reaction of CH4. The Ni content is favorably kept below a threshold of y=0.25 in ANiyAl12‐yO19−δ, (A=Ba, La, Sr) to ensure controlled nanoparticle formation and to avoid uncontrolled Ni0 nanoparticle growth apart from the support. Sr,Ni and Ba,Ni–hexaaluminates promote high activity of the catalyst in the dry reforming reaction of CH4, but show fast deactivation if the Ni content is maladjusted in the hexaaluminate framework (y≥0.5). La,Ni–magnetoplumbites display much lower activity accompanied by fast deactivation. The use of very high calcination temperatures (1600 °C) resulting in low specific surface area is detrimental to the activity in the dry reforming of CH4, simultaneously higher hexaaluminate phase content obtained undoes catalytic stability, reasoned by Ni0 nanoparticles produced after reduction cannot be stabilized over surface defects typically found on hexaaluminate platelets calcined at moderated temperatures (<1300 °C). As a result, larger metallic Ni ensembles are built up, selectivity to coke is increased and catalytic stability is compromised.

Structure–Activity Relationships of Nickel–Hexaaluminates in Reforming Reactions Part I: Controlling Nickel Nanoparticle Growth and Phase Formation

The controlled synthesis of hexaaluminates ANiyAl12−yO19−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 m2 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.