Strong electrostatic adsorption (SEA)
Strong electrostatic adsorption (SEA) is a special case of wet impregnation by which the pH of the precursor solution is set to the pH that yields the strongest electrostatic interactions between the metal ions in solutions and the support [3].
Figure 1. Mechanism of electrostatic adsorption [3]
Brunelle described that the adsorption of precious metal complexes on oxide supports always has electrostatic interactions between electrically charged particles in nature. The hydroxyl groups that inhabit oxide surfaces in contact with aqueous solutions become protonated and therefore positively charged when solution pH is below the point of zero charge (PZC) of the oxide. This positively charged surface adsorbs anions such as hexachloroplatinate [PtCl6]2-. Conversely, the surfaces become deprotonated or negatively charged when solution pH is above their PZC. The oxide surfaces will then adsorb cations like platinum tetraammine [(NH3)4Pt]2+. The hydroxyl groups are neutral when solution pH equals to the PZC of the oxide, at which point the interactions between the metal precursor and support are weakest. This surface adsorption is illustrated in Figure 1 [3].