*2.1. Samples Synthesis*

The compounds were synthesized via the co-precipitation route (direct method), followed by hydrothermal treatment obtaining nanoscopic crystallites with a partially disordered (turbostratic) structure. The addition of urea to the reactants helps to keep the pH to the desired value and the development of CO2 allows to obtain a high exchange surface structure [15]. Both the LDHs were with the M<sup>2</sup>+/M3<sup>+</sup> ratio = 2.

The NiAl-NO3 LDH was synthesized following the pathway suggested by [18], starting from Ni(NO3)2·6H2O (99.0% purity, supplied by Merck KGaA, Darmstadt, Germany), Al(NO3)3·9H2O (98.8% purity, supplied by VWR CHEMICALS, Leuven, Belgium), and urea (99.8mass% purity, supplied by CARLO ERBA, Milan, Italy). The two salts and urea, in a proper stoichiometric amount, were dissolved in 200 mL of deionized water under magnetic stirring, then the solution was transferred in a Teflon vessel autoclave and kept at 100 ◦C for 24 h. After heating, the system was cooled at room temperature naturally, and the green solid compound obtained was vacuum filtered and washed with water and ethanol. Subsequently, it was dried in a stove at 60 ◦C for 24 h. To synthesize the MgAl-CO3 LDH, as suggested by [19], the reagents Al(NO3)3·9H2O (98.8% purity, supplied by VWR CHEMICALS, Leuven, Belgium), Mg(NO3)2·9H2O (98.9% purity, supplied by VWR CHEMICALS, Leuven, Belgium), and urea (99.8 mass% purity, supplied by CARLO ERBA, Milan, Italy) were used. After the dissolution of the due amount of the reagents, as a function of the Mg/Al desired ratio, the reaction was continued in a Teflon vessel autoclave at 180 ◦C temperature for one hour. The white compound obtained was separated from the solution by centrifugation at 7000 rpm 10 min−1, repeatedly washed with water, and dried in a stove at 60 ◦C for 24 h.
