Phase Transformation Processes in Coprecipitated Cu/Zn/Zr Methanol Catalyst Precursors—Insights into Suspension Aging Form Ultrafast Nucleation

Abstract

Catalyst precursors for methanol synthesis prepared by coprecipitation, such as Cu/Zn-based hydroxycarbonates, are generally formed within two steps. The initial precipitation phase (nucleation) leads to a suspension from which, in a subsequent aging phase, the final solid precursor is formed, thus providing the structural and morphological features required for later use in catalysis. Combing ultrafast continuous nucleation with sampling from batch-wise aging opens up the possibility to follow the evolution and transitions of solid phases during suspension aging. The temporal progression of the existence of the different phases in a Cu/ZnO/ZrO₂-based system is investigated by scanning electron microscopy, X-ray diffraction, and inductive coupled plasma optical emission spectroscopy . According to the findings of this study, the intermediate recrystallization reveals to be a yet unknown two-step process. The presence of an amorphous transient zinc depot of Na₂Zn₃(CO₃)₄ × 3 H₂O greatly influences the formation of the relevant zincian malachite catalyst precursor [(Cu,Zn)₂(OH)₂CO₃], which is partly in contrast to reports on Cu/ZnO/Al₂O₃ systems. Finally, a general mechanism including the relevant transformations during suspension aging in Cu/ZnO/ZrO₂ systems is proposed, relying on a general thermodynamic approach, explaining the transient and final species.