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The kinetics of the reaction has been studied generally with chronoamperometry,
since the analysis of the data is simpler. The first studies with Pt single crystals ana-
lyzed the kinetics in the absence of CO [Franaszczuk et al., 1992; Herrero et al.,
1994]. An important isotopic effect was detected when CD
3
OH was used instead of
CH
3
OH; currents in the former case in the absence of poison were about 3 - 4 times
lower than those recorded for CH
3
OH. Although the effect is smaller than that
expected for a typical C22H bond breaking during the rate-determining step (RDS)
(a 6-fold effect), it clearly indicates that the breaking of a C22H bond is taking
place in the RDS. The smaller isotopic effect has been explained by the partial contri-
bution of other steps to the total kinetics [Jusys and Behm, 2001]. Tafel slopes were
also measured; approximately 120 mV was obtained for Pt(111) and Pt(110) electro-
des, whereas 60 mV was measured for Pt(100) electrodes (Fig. 6.20) [Herrero et al.,
1994]. The value 120 mV suggests that the RDS is the first electron transfer, whereas
60 mV is associated with a chemical step after the first electron transfer. The combi-
nation of the isotopic effect and the Tafel slopes indicates that the first step in methanol
oxidation is the formation of adsorbed CH
2
OH. This intermediate is different from that
found in ultrahigh vacuum (UHV) environments, where adsorbed CH
3
O is detected
[Franaszczuk et al., 1992]. The difference in the behavior between the UHV and
electrochemical environments has been explained by the presence of water, which
affects the interactions of methanol with the surface.
Figure 6.20 Extrapolated current density at t ¼ 0 obtained from chronoamperometric exper-
iments for Pt(111), Pt(100), and Pt(110) electrodes in 0.2 M HCOOH
þ
0.5 M H
2
SO
4
on elec-
trode. The straight lines show the regions where the Tafel behavior is observed. (Data taken from
Herrero et al. [1994].)
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