(600cj) 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

*amiridis@engr.sc.edu

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.