Environmental Engineering Reference
In-Depth Information
effectively.
Numerous investigations have shown that the catalytic dechlorination by
palladium usually leads to the formation of a lesser amount of chlorinated intermediates
(Grittini et al., 1995; Liang et al., 1997; Lien and Zhang, 1999, 2001). The
surface-mediated dechlorination reaction suggests the importance of the interactions
between the metal surface and reactants. It has been reported that molecular structures
may be responsible for the different surface intermediates as illustrated in Table 7.6. The
surface intermediates shown in Table 7.6 have been identified in either gas or aqueous
phases. These surface species have been detected in the gas phase using X-ray
photoelectron spectroscopy, ultraviolet photoemission spectroscopy and many other
surface characterization techniques (e.g., Mason and Textor, 1977; Cassuto et al., 1994)
and in aqueous solution with computational methods (Teppen et al., 1998).
Taken together, a conceptual model explaining the catalytic behavior of
palladium and surface-mediated processes in the dechlorination of chlorinated
hydrocarbons is proposed (Figure 7.15). The dissolution of water by iron nanoparticles
leads to the hydrogen evolution (Eq. 7.1) and is followed by the formation of atomic
hydrogen at the palladium surface. Atomic hydrogen then degrades chlorinated
hydrocarbons such carbon tetrachloride through surface-mediated processes involving a
combination of parallel, sequential and multi-step pathways (Figure 7.15). This proposed
model explains the intrinsic function of palladium, how the proton source gets involved
into the overall reaction and how long-chain hydrocarbons are formed at the surface.
C 2 Cl 6
C 2 Cl 6
C 2 Cl 6
C 2 Cl 4
C 2 Cl 4
C 2 Cl 4
CCl 3 .
CCl 3 .
CCl 3 .
CCl 3 . + e - + H +
CCl 3 . + e - + H +
CCl 3 . + e - + H +
CH 2 Cl 2 , CH 4 , etc.
CH 2 Cl 2 , CH 4 , etc.
CH 2 Cl 2 , CH 4 , etc.
CHCl 3
CHCl 3
CHCl 3
CCl 4
CCl 4
CCl 4
CCl 2 :+ 2e - + 2H +
CCl 2 :+ 2e - + 2H +
CCl 2 :+ 2e - + 2H +
CH 2 Cl 2
CH 2 Cl 2
CH 2 Cl 2
e -
e -
e -
H 2 O
H 2 O
Cl
C
Cl
C
Cl
C
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
Cl
C
C
C
C
C
C
C
C
C
C
C
C
CH 4 , C 2 H 6 , C 3 H 8
CH 4 , C 2 H 6 , C 3 H 8
CH 4 , C 2 H 6 , C 3 H 8
H
H
H
H 2
H 2
Pd
Pd
Pd
OH -
OH -
e- ( Fe Fe 2+
e- ( Fe Fe 2+
e- ( Fe Fe 2+
+ e - )
+ e - )
+ e - )
Fe 0
Fe 0
Fe 2+ + 2e -
Fe 2+ + 2e -
Figure 7.15 Mechanisms for transformation of carbon tetrachloride by bimetallic Pd/Fe
nanoparticles. Atomic hydrogen is expressed as .
H
 
 
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