Bimetallic Nanoparticles - Nanotechnologies for Water Environment Applications - page 170

range of 330 nm in diameter and a specific surface area of 59 m 2 /g (Schrick et al.,
2002). Studies conducted by Schrick et al. (2002) indicated that bimetallic Ni/Fe
nanoparticles increased the surface area normalized rate constant of the dehalogenation
reaction by a factor of 5080 as compared to iron nanoparticles or iron filings. Although
bimetallic Ni/Fe nanoparticles exhibit a relatively high reactivity toward chlorinated
organic compounds (Feng and Lim, 2005, Tee et al., 2005; Wu and Ritchie 2006), they
still might be of environmental concern because of the toxicity of nickel. Nevertheless, if
a contaminated site already has nickel already existed, the use of iron nanoparticles
alone becomes a win-win strategy for groundwater remediation. Iron itself serves as an
effective adsorbent for removal of metallic ions through reduction, surface adsorption,
precipitation and co-precipitation (Table 7.2). Thus, in the site contaminated with nickel,
it is likely that iron nanoparticles not only remove nickel but also form bimetallic Ni/Fe
nanoparticles.

In this section, the focus is on the bimetallic Pd/Fe nanoparticles. They are
among the most successful bimetallic iron nanoparticles to date and have been applied to
groundwater remediation in the field (Elliott and Zhang, 2001; Zhang, 2003). A
summary of bimetallic Pd/Fe nanoparticles for the degradation of chlorinated aliphatic
compounds is presented. Formation of long-chain hydrocarbons involved in the
degradation of chlorinated aliphatic compounds and the reaction mechanisms are
elucidated. In situ formation of bimetallic iron nanoparticles is discussed from both the
theoretical and practical aspect of groundwater remediation.

7.3.1 Bimetallic Pd/Fe Nanoparticles

7.3.1.1 Characterization

An SEM image in Figure 7.7 illustrates that iron nanoparticles are comprised of
spherical particles assembled in chains. In the absence of dispersants, the aggregation of
iron nanoparticles as a chain of beads has been widely reported (Nurmi et al., 2005; Tee
et al., 2005; Yuan and Lien, 2006). XRD analyses of bimetallic Pd/Fe nanoparticles
containing palladium at 1% and 5% are shown in Figure 7.8. The predominant features
are that the samples consist primarily of palladium metal, iron metal and iron corrosion
products. The main peaks of palladium and iron corresponding to the diffraction angles
(2θ) are assigned to 40.0 r , and 44.9 r , respectively. The observation of palladium metal on
the bimetallic Pd/Fe surface is consistent with X-ray photoelectron spectra (XPS) studies
where the Pd 3d spectrum confirms that the palladium is present on the iron surface in
the elemental state (Muftikian et al., 1996).

The widths of the XRD lines can be used to determine the grain size of Pd on the
surface of iron nanoparticles using the Scherrer's equation (Cullity and Stock, 2001):