*2.1. Materials*

Nano-sized MgO•*n*Al2O3 powders with 0.95 < *n* < 2.45 were synthesized by the solution combustion method [28]. This entailed the mixing of appropriate amounts of magnesium and aluminum nitrate, Mg(NO3)2·6H2O 96% metal basis, Al(NO3)3·9H2O (96% metal basis, Fluka Analytical, Sigma Aldrich, St. Louis, MO, USA) in 200 mL of deionized water. To this solution, 30 g of citric acid (ACS reagen<sup>t</sup> ≥99.5%) and 6 mL of ethylene glycol (anhydrous, 99.8%, Sigma Aldrich, St. Louis, MO, USA) were added. The solution was dried on a hot plate at 120 ◦C under agitation by magnetic stirring until high-viscosity foam-like colloids were formed. The foams were crushed using a mortar and pestle to a fine light brown powder, which served as precursors for the appropriate spinel. The precursors were calcined in air at 850 ◦C for 72 h to obtain fine white powders.

To study the effects of disorder on the adsorption process, some of the samples were heat-treated in the presence of an electric field, in order to reduce "residual inversion" defects. The samples were heated in air to 800 ◦C, and maintained at this temperature for 30 min. The heating rate was 10 ◦C/min, and the furnace was naturally cooled. An electric field of 200 V/cm was applied at all treatment stages. Figure 1 shows the crucible and capacitor setup used for the treatment. As can be seen, there was no contact between the powder and the electrodes, and there was no flow of current.

**Figure 1.** Crucible and cell used for the electric field heat treatments.
