Breaking through the stoichiometry barrier: As the diameter of silver particles is decreased below a critical size of 32 nm, the molar ratio of aqueous HgII to Ag0 drastically increases beyond the conventional Hg/Ag ratio of 0.5:1, leading to hyperstoichiometry with a maximum ratio of 1.125:1 (see figure). Therein, around 99% of the initial silver is retained to rapidly form a solid amalgam with reduced mercury. |
Anyone
who thinks amalgams are limited to tooth fillings is missing something:
Amalgams, which are alloys of mercury and other metals, have been used
for over 2,500 years in the production of jewelry and for the extraction
of metals like silver and gold in mining operations. These days, the
inverse process is of greater interest: the removal of mercury from
wastewater by amalgamation with precious metals in the form of
nanoparticles. Kseniia Katok and colleagues have now reported new
insights in the journal Angewandte Chemie:
if the diameter of silver nanoparticles is made even smaller,
significantly more mercury can be extracted relative to the amount of
silver used.
In
the conventional process, two silver atoms react with one mercury ion,
which carries a twofold positive charge, to produce two silver ions,
which go into solution, and a neutral mercury atom, which is taken up by
the metallic silver particles. The stoichiometric ratio of mercury to
silver is thus 1:2.
The
researchers at the University of Brighton, U.K., and colleagues in
Kazakhstan, France and Japan have now determined that the stoichiometry
of the reaction changes if the diameter of the silver nanoparticles
drops below a critical 32 nm. This effect, known as “hyperstoichiometry”
depends on the size of the nanoparticles. With particles that have a
diameter around 10 nm, the ratio can reach between 1.1:1 and 1.7:1,
depending on the mercury counterion. In these cases, the reaction is
clearly occurring differently than it does with silver particles of
“normal” size. The researchers postulate that the initially produced
silver ions are absorbed into the silver nanoparticles and, under the
catalytic influence of the tiny silver nanoparticles, are “recycled”
back to elemental silver by the negatively charged counterions of the
mercury salts, which in these experiments were nitrate or acetate. It
has often been observed that very small nanoparticles have a higher
catalytic activity than larger ones because their surface properties
dominate over their bulk properties. The hyperstoichiometric effect
suggests new approaches for the purification of runoff as well as
catalysis.
To
produce the necessary extremely small silver nanoparticles, the
scientists equipped a silicon dioxide surface with individual silicon
hydride (-SiH) groups. These are able to reduce silver ions to neutral
silver atoms, which are bound to the surface and probably act as
nucleation sites for the further aggregation of silver. The density of
SiH groups and reaction time can be used to control the size of the
particles. In contrast to conventional processes, this requires no
stabilizers, which stick to the silver nanoparticles and alter their
physical and chemical properties.
Hyperstoichiometric Interaction Between Silver and Mercury at the Nanoscale
