Environmental Engineering Reference
In-Depth Information
The complete oxidation product CO
2
is only found to a minor extent [Chang et al.,
1990; Xia et al., 1997]. It is generally assumed that these products are formed by
the following general reaction scheme:
C
2
H
5
OH
C
2
H
5
OH
ads
(6
:
20)
C
2
H
5
OH
ads
!
CO
ads
þ
CH
x,ads
þ
(6
x)H
þ
þ
(6
x)e
(6
:
21a)
C
2
H
5
OH
ads
!
CH
3
CHO
ads
þ
2H
þ
þ
2e
(6
:
21b)
H
2
O
!
OH
ads
þ
H
þ
þ
e
(6
:
22)
CH
3
CHO
ads
CH
3
CHO
(6
:
23a)
CH
3
CHO
ads
!
CO
ads
þ
CH
x,ads
þ
(4
x)H
þ
þ
(4
x)e
(6
:
23b)
CH
3
CHO
ads
þ
OH
ads
!
CH
3
COOH
þ
H
þ
þ
e
(6
:
23c)
CO
ads
þ
OH
ads
!
CO
2
þ
H
þ
þ
e
(6
:
24)
CH
x
þ
2OH
ads
!
CO
2
þ
(2
þ
x)H
þ
þ
(2
þ
x)e
(6
:
25)
It should be noted that this scheme does not represent the elementary steps of the
ethanol oxidation mechanism. The reaction is initiated by the adsorption of an ethanol
molecule on a vacant surface site (6.20). After adsorption, ethanol can decompose
(6.21a) in strongly adsorbed CO and CH
x
, or be oxidized to weakly adsorbed acet-
aldehyde (6.21b), which can desorb and diffuse into the bulk of the solution
(6.23a). Alternatively, acetaldehyde can decompose into strongly adsorbed fragments
(6.23b). In the presence of adsorbed OH (6.22), the weakly adsorbed acetaldehyde and
the strongly absorbed CO fragments can be oxidized to acetic acid (6.23c) and CO
2
(6.24), respectively. The CH
x
fragment resulting from the dissociative adsorption of
ethanol or acetaldehyde can also be oxidized to CO
2
(6.25), most likely through a
CO-like intermediate [Shin et al., 1996].
The first molecular-level study of products and intermediates of ethanol electro-
oxidation was provided by Willsau and Heitbaum with the use of DEMS [Willsau
and Heitbaum, 1985]. They identified CO
2
and acetaldehyde as the primary reaction
products, although it should be noted that acetic acid is not volatile enough to be
detected by DEMS. By employing deuterium and
18
O labeling, this study showed
that acetaldehyde is formed by the cleavage of hydrogen atoms from the hydroxyl
group and the a-carbon of ethanol. Furthermore, it was found that CO
2
is formed
through a strongly bound intermediate, which is oxidized to give two CO
2
molecules,
one originating from the methyl group and the other from the alcohol group and still
containing the alcoholic O. Subsequent DEMS measurements [Iwasita and Pastor,
1994; Schmiemann et al., 1995] studied the C22C bond breaking in more detail. By
isotopic labeling of the carbons in ethanol, methane and ethane originating from the
methyl group of ethanol were detected during slow cathodic potential sweeps into
the hydrogen adsorption region after ethanol adsorption at potentials before the
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