Characterization and properties of metallic iron nanoparticles: Spectroscopy, electrochemistry, and kinetics

There are reports that nano-sized zero-valent iron (Fe⁰) exhibits greater reactivity than micro-sized particles of Fe⁰, and it has been suggested that the higher reactivity of nano-Fe⁰ may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. In an effort to better understand the reactivity of nano-Fe⁰, we have used a variety of complementary techniques to characterize two widely studied nano-Fe⁰ preparations: one synthesized by reduction of goethite with heat and H₂ (Fe ^H2) and the other by reductive precipitation with borohydride (Fe^BH). Fe^H2 is a two-phase material consisting of 40 nm α-Fe⁰ (made up of crystals approximately the size of the particles) and Fe₃O₄ particles of similar size or larger containing reduced sulfur; whereas Fe^BH is mostly 20-80 nm metallic Fe particles (aggregates of <1.5 nm grains) with an oxide shell/coating that is high in oxidized boron. The Fe^BH particles further aggregate into chains. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe₂O₃, Fe₃O₄, micro-sized Fe⁰, or a solid Fe⁰ disk, which is consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone - which presumably probes inner-sphere surface reactions - reacts more rapidly with Fe^BH than Fe^H2, whereas carbon tetrachloride reacts at similar rates with Fe^BH and Fe^H2, presumably by outer-sphere electron transfer. Both types of nano-Fe⁰ react more rapidly than microsized Fe⁰ based on mass-normalized rate constants, but surface area-normalized rate constants do not show a significant nano-size effect. The distribution of products from reduction of carbon tetrachloride is more favorable with Fe^H2, which produces less chloroform than reaction with Fe^BH.