279684 Investigation of Ag Diammine Impregnation On Low/High PZC Oxides and Carbon Using Strong Electrostatic Adsorption
Investigation of Ag Diammine Impregnation on Low/High PZC Oxides and Carbon Using Strong Electrostatic Adsorption
Eleni A. Kyriakidou, Oleg S. Alexeev, John R. Regalbuto, and Michael D. Amiridis*
Department of Chemical Engineering
University of South Carolina, Columbia, SC 29208
Unlike common impregnation techniques, where usually there is no metal precursor-support interaction, strong electrostatic adsorption (SEA) is focused on the determination of an optimal pH where metal complex-surface interactions are strongest. The SEA method is based on an electrostatic mechanism in which the surface functional groups of the support (typically hydroxyl) can be protonated and deprotonated and thus, positively or negatively charged as a function of pH. At the pH of strongest metal precursor-support interaction, oppositely charged metal coordination complexes adsorb in well dispersed monolayers. After the completion of the impregnation step, the wet slurries are dried and the ligands are removed usually with thermal treatments in various oxidizing/reducing environments, leading to the formation of reduced metal particles.
Our specific objective was to investigate the adsorption of silver diammine (Ag(NH3)2+), also known as Tollen's Reagent, onto six different supports (silica, oxidized carbon, alumina, ceria, zirconia and niobia) as a function of pH. pH shift control experiments were performed to determine the point of zero charge (PZC) of these supports. Moreover, adsorption-pH surveys of Ag diammine on these supports were also carried out to determine the pH of maximum uptake using Atomic Absorption Spectroscopy. The ability of these supports to protonate and deprotonate as a function of pH leads to an electrostatic attraction of the +1 silver cation in a pH range where the supports are negatively charged. Adjusting for the differences in surface area of the various supports, and taking into account that Al2O3, ZrO2 and CeO2 exhibit a basic PZC, whereas SiO2, Nb2O5 and Oxidized Carbon exhibit an acidic PZC, it is observed that the adsorption behavior differs between the high and low PZC supports. Since Ag exhibits a strong tendency to sinter at elevated temperatures, plasma treatments were used at low temperatures for the ligand removal and reduction step. The particle size and distribution were determined by High Resolution Transmission Electron Microscopy (HRTEM), and the metal distribution by Energy-Dispersive X-ray Spectroscopy (EDXS). In addition, the samples were characterized by XPS with in an effort to probe the oxidation state of the Ag species formed. SEA offers an opportunity to prepare a variety of 2 – 4 nm sized supported silver metal catalysts.
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