Layered double hydroxides (LDHs) comprise a class of inexpensive structured materials that are finding an increasing number of applications across a range of industries. Specifically as precursors of mixed oxides catalysts for environmental applications such as hydrogen production and valorization of biomass, or LDH-phase catalysts for photocatalytic aerobic degradation of pollutants, photocatalytic water splitting and CO2 photoreduction, and other reactions. The uniqueness of LDHs is in its tunable electronic and morphological properties. Tuning the composition of LDHs leads to variable anion-exchange capacity and strength, as well as specificity of catalytic activity. However, the systematic study of composition-property relationships that need to be elucidated to allow tunability requires ability to synthesize LDHs with precisely controlled properties. This currently poses a significant challenge, as the most commonly employed process of LDH synthesis, co-precipitation under batch conditions, does not afford acceptable reproducibility. In this work we report the optimized flow synthesis protocols for HTs doped with various transition metals, which are of significant interest in catalytic applications. The synthesis was performed as a two-step process, using a mm-scale flow reactor with short residence time, followed by aging at ambient pressure and different temperatures. To help build the systematic understanding of the composition-synthesis-property relationships we have carried out extensive characterization of these materials to elucidate the structure, properties and molecular-level transitions that occur during their calcination. The obtained samples exhibited surface areas (80-150 m2 g-1), higher than typically attained in batch syntheses, a high degree of crystallinity and small crystallite sizes, in the range of 9.5-11.9 nm. A systematic characterization of HTs doped with different metals using PXRD, TEM and gas sorption data provide insights into the mechanism of the pore structure formation. The crystallites agglomerate into disk-like platelets, and agglomeration of the platelets gives rise to the HT pore structure.

Synthesis of doped layered double hydroxides in a meso-scale continuous flow reactor

256th National Meeting (2018)

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