The pyrophoric character of zerovalent iron nanoparticles
and cumbersome handling of this material has been a
drawback in practical applications, despite the expectation
of an enhanced reactivity. We have been interested in
how the iron nanoparticles can gain stability in air without
significantly sacrificing reactivity. The freshly synthesized
iron nanoparticles ignited spontaneously upon exposure
to air. However, when exposed slowly to air, an ∼5 nm
coating of iron oxide was formed on the surface of particles.
The oxide shell did not thicken for at least two months,
indicating no sign of further corrosion of iron particles. The
reactivity studies on nitrate reduction showed that the
freshly synthesized iron reacted at the fastest rate. After
formation of the oxide shell the rate constants decreased by
ca. 50% of that of fresh iron, but were still higher than
that of commercial grade micro- or milli-sized iron powder.
Nitrate (50 ppm/350 mL) can be recharged 6 times into a
bottle containing 0.5 g of iron nanoparticles. The reduction
rate of the second cycle was the fastest among the six
cycles, which can be attributed to the increase of surface
area and the fresh iron surfaces that were bared by the
dissolution of oxidized iron species on the surface. The oxidized