Synthesis of Ag nanoparticles on oxide and carbon supports from Ag diammine precursor

To determine whether the method of “strong electrostatic adsorption” (SEA) can be extended to the preparation of uniform and highly dispersed supported Ag catalysts, the adsorption of silver diammine, Ag(NH₃)₂⁺, also known as Tollen's reagent, has been examined over five supports (i.e., SiO₂, γ-Al₂O₃, ZrO₂, Nb₂O₅, and carbon) with different surface areas and PZCs. The speciation of Ag in solution was followed by UV–vis spectroscopy, Ag uptake as a function of the solution pH was determined by atomic absorption, and Ag particle size was determined by powder XRD and STEM. At the Ag concentrations, pH, and surface loadings employed, silver diammine complexes convert into soluble Ag(H₂O)₂⁺ aquo complexes below pH 11 and hydrolyze to insoluble Ag₂O above pH 11. The deposition of either Ag species over carbon at any pH appears to be reactive and results in large particles. The silver aquo complexes appear to adsorb via ion exchange near the PZC of alumina, zirconia, and niobia but not silica, while the silver diammine complexes appear to adsorb electrostatically at high pH over all the oxides tested - niobia, silica, zirconia, and alumina in the order of ascending PZC. Niobia, which has the lowest PZC, adsorbs the highest surface density of Ag via both mechanisms. The particles obtained via electrostatic adsorption of Ag(NH₃)₂⁺ at high pH are somewhat smaller than those formed from Ag(H₂O)₂⁺ ions at the lower pH values. In the absence of ammonia in solution at high pH, deposition of Ag aquo complexes occurs via surface precipitation and gives large particles. In sum, SEA of silver ammine is demonstrated to be a simple, reproducible way to synthesize small particles on all supports but carbon.