How do layered double hydroxides evolve? First in situ insights into their synthesis processes.

Despite the importance of layered double hydroxides (LDHs) in catalysis, medicine and water treatment, the crystallisation process of these materials is seldom investigated. In this study, in situ characterisation techniques granted unprecedented experimental access to the formation dynamics of carbonate-intercalated Mg2+/Al3+ LDHs as model system when applying the most relevant co-precipitation approaches by exploring the effects of temperature and concentration of reactants. For this purpose, a combinatorial multi-modal characterisation approach was applied involving in situ measurements of pH, ion conductivity and light scattering, as well as synchrotron-based in situ X-ray diffraction (XRD). Shortly after beginning the addition of basic solutions (i.e., sodium carbonate and sodium hydroxide) to the solutions of magnesium nitrate hexahydrate and aluminium nitrate nonahydrate, a stable pH was reached due to the uptake of hydroxyl ions for nuclei formation. Shortly after, crystal growth phase was detected by an increase in the light scattering signal and confirmed via in situ XRD. Increasing the concentration of reactants accelerated the onset of crystal growth by 70% without significantly changing the crystallite size. On the other hand, increasing the temperature up to 65 °C showed a smaller influence on the reaction kinetics but resulted in a two-fold increase in crystallite size. Adding the solution of metal precursors to the basic solution, saturation was rapidly reached, without an induction period, favouring the formation of very small crystallites of approximately 10 nm.

Model for erosion-deposition patterns.

We investigate through computational simulations with a pore network model the formation of patterns caused by erosion-deposition mechanisms. In this model, the geometry of the pore space changes dynamically as a consequence of the coupling between the fluid flow and the movement of particles due to local drag forces. Our results for this irreversible process show that the model is able to reproduce typical natural patterns caused by well-known erosion processes. Moreover, we observe that, within a certain range of porosity values, the grains form clusters that are tilted with respect to the horizontal with a characteristic angle. We compare our results to recent experiments for granular material in flowing water and show that they present a satisfactory agreement.