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Y. Feng et al. / Electrochimica Acta 55 (2010) 6991–6999
the cleavage of C–C bond at low potential for ordered intermetallic
compound PtPb/C-a.
An explanation for the occurrence of this phenomenon can be elu-
cidated: for electrooxidation of 2-propanol, the evidence obtained
during investigation of Markiewicz et al. [40] and Lin et al. [8] indi-
cates that the OH− group is connected to the second carbon atom
of the chain in the 2-propanol molecule, it will cause electronic
effect, making it very difficult for C–C bond of intermediate alco-
hols to break, so there is no COads absorption during the oxidization
of 2-propanol. The electron interaction between Pt and Pb or Ru can
bring change transfer from Pb to Pt or Pt to Ru. This electronic mod-
ification in unfilled d band states of Pt atoms may cause a stronger
bond between carbon atoms and surface Pt atoms, thus chang-
ing ability of cleavage of C–C bonds. Therefore, how to accelerate
cleavage of C–C bond in low potential lead to entirely oxidation of
2-propanol is a key of enhance activity of catalyst for 2-propanol
electrooxidation.
Kim et al. [28] indicates that the effect of second element on
Pt catalysts have been believed to be associated with modification
of geometric and electronic structures by Pt–M interactions, thus
aspect of geometric structure modification on the catalyst, the XRD
analyses indicated that the solid-solution of PtRu/C alloy catalyst
[50]. The lattice parameter and geometric environment of both cat-
alysts with different crystal structures is different. But ethylene
glycol and 2-propanol electrooxidation reaction is the structure
sensitive reaction [48,51]. Orts et al. [51] show that the activity
of Pt (1 0 0) and Pt (1 1 1) planes for the oxidation of ethylene glycol
is different. This also means that cleavage of C–C and C–O bonds
may be sensitive to geometric parameters, i.e. bond lengths and
On the other hands, the amount of “labile-bonded” oxygen-
containing species on the catalyst may facilitate the electrooxi-
dation of adsorbed carbonaceous residues to CO2. The previous
research [44,47,52] showed that the surface oxides of Pt metal or
Ru metal can be classified as active and poison species according to
the effect on oxidation of primary alcohols: for example, the RuO2
and Pt surface oxides of higher oxidation states (i.e. Pt(OH)3 and
PtO2) are poison species due to their preventing alcohol oxidation
or low activity, while Ru0, RuOxHy, Pt0 and the Pt surface oxides
of lower oxidation states, such as PtO, Pt(OH)2, are considered as
active species due to their promoting oxidation of primary alcohols.
It suggested that Pb0, Pb2+ may be oxidized when the potential pos-
itively moves, or reacts with electrolyte solution to forms Pb(OH)2
in the alkaline medium as SOMs electrooxidation proceed [35]. The
mediate, which was produced in ethylene glycol electrooxidation
process. Therefore, the amount of Pb0 and Pb2+ should improve the
activity of catalysts. Based on foregoing analysis, the surface species
on four catalysts can be divided into active and no-active species,
which results show in Table 1.
For electrooxidation reaction of ethylene glycol, the activity of
catalyst follows this order: PtPb/C-b > PtPb/C-a > PtRu/C > Pt/C, but
the result of XRD indicates that the crystalline size of PtPb/C-b cat-
alyst is larger than of other three catalysts. It is generally assumed
that the large crystalline size of catalyst can decrease the specific
surface area and activity of catalyst. However, this conclusion fails
to fit the facts obtained in our experiment. This result also confirms
that in addition to crystalline size and specific area of catalyst, other
factors can influence the activity of catalyst. From Table 1, it can
be seen that the amount of “active species” for four catalysts fol-
lows this order: PtPb/C-b > PtPb/C-a > PtRu/C > Pt/C. It means that
although the bond lengths of Pt–Pt in catalysts and the electronic
modification in unfilled d band states of Pt atoms can influence the
cleavage of C–C bond and desorption of COads, but amount of oxy-
genated functional groups plays a key role to improve activity of
catalysts.
4. Conclusions
In this study we have shown that ordered intermetallic com-
pound PtPb/C has better electrocatalytic activities for ethylene
glycol and 2-propanol electrooxidation in alkaline medium. The
results of EIS showed that in ethylene glycol electrooxidation,
the major role of intermetallic compound PtPb/C-b catalyst is to
enhance intermediate oxidation. In 2-propanol electrooxidation,
the good activity of ordered intermetallic compound PtPb/C-a could
be attributed to the cleavage of C–C bond at low potential for
ordered intermetallic compound PtPb/C-a.
Although the activity of electrocatalysts depends on many fac-
tors, such as modification of geometric and electronic structure by
Pt–Pb interaction, crystalline size and so on. But the key factor for
each electrooxidation reaction is different. For ethylene glycol elec-
trooxidation, the intermediate-COads poisonous species are likely
to be strongly adsorbed on the Pt surface, blocking the effective
and active catalyst sites for nest turnover, and making the anodic
reactions more sluggish. The effect of formation and desorption of
poisonous species on activity of catalyst is very significant. There-
fore the increased amount of surface oxygen-containing species
is a key factor to improve the performance of electrocatalyst. For
2-propanol electrooxidation, the main products were acetone and
small amount of CO2. The adsorbed COads does not appear in the
reaction process. The main challenge is the cleavage of the C–C
bond, what makes difficult its oxidation into CO2. Therefore the
modification of geometric and electronic structures may be play a
decisive role in the enhance activity of electrocatalyst.
Acknowledgements
Financial support for this work was provided by the National
Key Basic Research Program of China (No. 2008CB617502).
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