Supported nanoparticle synthesis with Au bis-Ethylenediamine: The mechanism of adsorption onto oxides and carbons

Gold bis-ethylenediamine ([Au(en)(2)](3+) or AuED) is an effective precursor for the preparation of small Au nanoparticles on common catalyst supports. By studying the adsorption of AuED over a wide variety of supports with surveys of uptake versus time and pH, including XAS studies of solution and adsorbed spe-cies, we have demonstrated that an electrostatic mechanism accounts for the data much better than the previously postulated cation exchange mechanism. In the electrostatic mechanism 1) uptake-pH curves are volcano-shaped and are higher in amplitude and broader in pH the lower the PZC of the support, 2) AuED complexes retain at least one hydration sheath with diameter 13.3 angstrom (0.72 complex/nm(2)) and adsorb as outer sphere complexes, 3) the monolayer limit in aqueous solutions basified with NaOH is not determined by cation exchange stoichiometry but is a steric limit of close-packed, hydrated com-plexes. They adsorb at a much lower surface density than that of the surface hydroxyl groups and never electrically saturate the surface. 4) The change in AuED speciation at high pH is consistent with a change in the coulombic attraction of a 3+ versus a 2+ complex. Over carbon supports, adsorption is predominantly electrostatic, but an additional parallel mechanism is apparent in proportion to the graphitic nature of the support. Synthesized Au nanoparticles over the oxide and carbon materials was characterized by high sensitivity powder XRD, STEM imaging, and TPR. The resultant nanoparticles were generally smaller than other synthesis methods reported in the literature for silica, titania, alumina, and carbon. The loading limit of electrostatic adsorption isa surface density of about 1 mu mol/m(2). At loadings less than or equal to this, electrostatic adsorp-tion provides a simple method with which to synthesize well dispersed Au nanoparticles. (C) 2020 Elsevier Inc. All rights reserved.

Automated summary

Gold bis-ethylenediamine (AuED) is an effective precursor for synthesizing small Au nanoparticles on common catalyst supports. The electrostatic mechanism is shown to account for the data better than the previously proposed cation exchange mechanism, with adsorption predominantly driven by electrostatic interactions on carbon supports. The synthesized Au nanoparticles were generally smaller than those obtained through other methods, with electrostatic adsorption providing a simple method for synthesizing well-dispersed Au nanoparticles.