et al. 2017). Generally, the second metal is scattered as small clusters on the surface
of the particles (Yan et al. 2013; Ling and Zhang 2014c).
The use of a second metal more noble than iron (E M > E Fe ) presents several
advantages. It can: (1) enhance the release of electrons at a faster rate by the
formation of a galvanic cells in which the second metal acts as the main cathode
(Xu and Zhang 2000); (2) prevent the formation of oxide film (Wang and Zhang
1997); and (3) catalyze reactions with hydrogen (hydrodechlorination and hydrogenation reactions) (Schrick et al. 2002; Chaplin et al. 2012). Metals like palladium
(Pd) and nickel (Ni) dissociate hydrogen produced by anaerobic iron corrosion to
form highly reactive atomic hydrogen H
à at the surface of the particles (Kim and
Carraway 2003; Li et al. 2017c). Cwiertny et al. (2006) suggest that bimetallic
particles enhanced the dechlorination rates primarily by reactions with atomic
hydrogen (hydrodechlorination), as the pseudo rate constants are correlated with
the solubility of atomic hydrogen with each additive.
Muftikian et al. (1995) have first shown the efficiency of palladized iron for the
dechlorination of C 1 and C 2 chlorinated compounds. Lien and Zhang (2007) have
shown the catalytic effect of Pd deposited on iron particles for TCE reduction, with a
rate constant 60–70 times higher than the rate obtained with traditional iron particles
when the particles contain 1–5% by mass of Pd. Beyond, reaction rates decreased
due to a diminution of iron active surface for TCE reduction, until showing an
absence of reactions when Pd represents 50% or more of the particle. A similar effect
has been observed for Ni/Fe, Ag/Fe, or Cu/Fe particles for TCE or HCB dechlorination (Xu and Zhang 2000; Tee et al. 2005; Nie et al. 2013; Fang et al. 2017).
Kim and Carraway (2003) have tested different bimetallic combinations for TCE
dechlorination, and the particles can be classified in the following order, according to
the specific surface area-normalized rate constant k SA : Pd/Fe > Ni/Fe > Cu/Fe >
Fe. Four noble metals—Pd, ruthenium (Ru), platinum (Pt), and gold (Au)—have
also been studied by Lin et al. (2004), and the catalytic activity on the dechlorination
of TCE is ranked in the order Pd ) Ru > Pt > Au. These results show the best
overall catalytic effect on iron-based particles compared to zinc-based ones, and the
highest constant rate is obtained with Pd for both metals. For the dechlorination of
1,1,1-trichloroethane, Cwiertny et al. (2006) have observed the following ranking:
Ni/Fe % Pd/Fe > Cu/Fe > Co/Fe > Au/Fe % Fe > Pt/Fe. The authors suggest that
these differences in ranking can be attributed to a different mechanism of reaction
between unsaturated (e.g., chlorinated alkenes) and saturated (e.g., chlorinated
alkanes) compounds.
Among all catalysts, Pd is more favorable due to its high efficiency in H*
generation, and its adsorption on Pd surface and its absorption into Pd crystal lattice,
resulting in the formation of Pd hydride (Chaplin et al. 2012). Among the different
hydrogen species, i.e. H
*
abs , H
*
ads and H 2 bubbles, H
*
ads is the only active hydrogen
species in the catalytic hydrodechlorination process (Jiang et al. 2017; He et al.
2018b), and the presence of defects on Pd surface facilitates the efficient
hydrodechlorination by H
*
ads (Liu et al. 2018). Another beneficial effect of this
metal is the production of a smaller number of reaction intermediates, especially
310
R. Rodrigues et al.
of the particles (Yan et al. 2013; Ling and Zhang 2014c).
The use of a second metal more noble than iron (E M > E Fe ) presents several
advantages. It can: (1) enhance the release of electrons at a faster rate by the
formation of a galvanic cells in which the second metal acts as the main cathode
(Xu and Zhang 2000); (2) prevent the formation of oxide film (Wang and Zhang
1997); and (3) catalyze reactions with hydrogen (hydrodechlorination and hydrogenation reactions) (Schrick et al. 2002; Chaplin et al. 2012). Metals like palladium
(Pd) and nickel (Ni) dissociate hydrogen produced by anaerobic iron corrosion to
form highly reactive atomic hydrogen H
à at the surface of the particles (Kim and
Carraway 2003; Li et al. 2017c). Cwiertny et al. (2006) suggest that bimetallic
particles enhanced the dechlorination rates primarily by reactions with atomic
hydrogen (hydrodechlorination), as the pseudo rate constants are correlated with
the solubility of atomic hydrogen with each additive.
Muftikian et al. (1995) have first shown the efficiency of palladized iron for the
dechlorination of C 1 and C 2 chlorinated compounds. Lien and Zhang (2007) have
shown the catalytic effect of Pd deposited on iron particles for TCE reduction, with a
rate constant 60–70 times higher than the rate obtained with traditional iron particles
when the particles contain 1–5% by mass of Pd. Beyond, reaction rates decreased
due to a diminution of iron active surface for TCE reduction, until showing an
absence of reactions when Pd represents 50% or more of the particle. A similar effect
has been observed for Ni/Fe, Ag/Fe, or Cu/Fe particles for TCE or HCB dechlorination (Xu and Zhang 2000; Tee et al. 2005; Nie et al. 2013; Fang et al. 2017).
Kim and Carraway (2003) have tested different bimetallic combinations for TCE
dechlorination, and the particles can be classified in the following order, according to
the specific surface area-normalized rate constant k SA : Pd/Fe > Ni/Fe > Cu/Fe >
Fe. Four noble metals—Pd, ruthenium (Ru), platinum (Pt), and gold (Au)—have
also been studied by Lin et al. (2004), and the catalytic activity on the dechlorination
of TCE is ranked in the order Pd ) Ru > Pt > Au. These results show the best
overall catalytic effect on iron-based particles compared to zinc-based ones, and the
highest constant rate is obtained with Pd for both metals. For the dechlorination of
1,1,1-trichloroethane, Cwiertny et al. (2006) have observed the following ranking:
Ni/Fe % Pd/Fe > Cu/Fe > Co/Fe > Au/Fe % Fe > Pt/Fe. The authors suggest that
these differences in ranking can be attributed to a different mechanism of reaction
between unsaturated (e.g., chlorinated alkenes) and saturated (e.g., chlorinated
alkanes) compounds.
Among all catalysts, Pd is more favorable due to its high efficiency in H*
generation, and its adsorption on Pd surface and its absorption into Pd crystal lattice,
resulting in the formation of Pd hydride (Chaplin et al. 2012). Among the different
hydrogen species, i.e. H
*
abs , H
*
ads and H 2 bubbles, H
*
ads is the only active hydrogen
species in the catalytic hydrodechlorination process (Jiang et al. 2017; He et al.
2018b), and the presence of defects on Pd surface facilitates the efficient
hydrodechlorination by H
*
ads (Liu et al. 2018). Another beneficial effect of this
metal is the production of a smaller number of reaction intermediates, especially
310
R. Rodrigues et al.