Phase evolution, speciation and solubility limit of aluminium doping in zinc oxide catalyst supports synthesized via co-precipitated hydrozincite precursors

Abstract

The preparation of Al-doped ZnO via thermal decomposition of crystalline precursors, with a particular emphasis on kinetic effects on the solubility limits, was studied. The promoting effect of Al³⁺ on the catalyst system is discussed for methanol synthesis where ZnO:Al is employed as a support material for copper nanoparticles. The synthesis of the Al-doped zinc oxides in this study was inspired by the industrial synthesis of the methanol synthesis catalyst via a co-precipitated crystalline precursor, here: hydrozincite Zn₅(OH)₆(CO₃)₂. To determine the aluminium speciation and the solubility limit of the aluminium cation on zinc positions, a series of zinc oxides with varying aluminium contents was synthesized by calcination of the precursors. Short precipitate ageing time, low ageing temperature and aluminium contents below 3 mol% metal were advantageous to suppress crystalline side-phases in the precursor, which caused an aluminium segregation and non-uniform aluminium distribution in the solid. Even if zinc oxide was the only crystalline phase, TEM revealed such segregation in samples calcined at 320 °C. Only at very low aluminium contents, the dopant was found preferably on the zinc sites of the zinc oxide structure based on the signal dominating the ²⁷Al NMR spectra. The solubility limit regarding this species was determined to be approximately x_Al = 0.013 or 1.3% of all metal cations. Annealing experiments showed that aluminium was kinetically trapped on the site and segregated into ZnAl₂O₄ upon further heating. This shows that lower calcination temperatures such as applied in catalyst synthesis conserve a higher aluminium doping concentration on that specific site than is expected thermodynamically.