Investigating the Crystallization of a Hydroxycarbonate Precursor for a Ni–Mg–Catalyst through In Situ and Ex Situ Methods

In this study, the crystallization of a coprecipitated precursor for a Ni catalyst suitable for green methanation is investigated. The catalyst is derived from a basic hydroxycarbonate where Ni2+ is partially replaced by Mg2+-cations forming a mixed solid-solution crystal of the composition (Ni1-yMgy)12(CO3)8(OH)6O·mH2O (with y = 0-0.5). The hydrothermal aging was identified as a key step in its synthesis, which previously remained unexplored. Ex situ PXRD, IR, EDX, SEM, and ICP-OES as well as in situ IR and pressure monitoring were used to study this reaction. In this aging step, the amorphous precipitate begins to transform into a crystalline solid-solution compound within 30 min after reaching hydrothermal conditions. The initial amorphous precipitate was examined, and its sum formula was established as (Ni1-xMgx)2(CO3)(OH)2·2.5H2O. The multimethod approach clarified the metal ion ratio evolution and the crystallization and carbonate incorporation as a function of time as the two hydroxycarbonate phases interconverted, enhancing understanding of the aqueous precursor chemistry and catalyst synthesis and showing the importance of dissolved species in the mother liquor in this reaction.