Page 97 - MODERN ASPECTS OF ELECTROCHEMISTRY

Claude LamyAet al,
80
After the reactions (20) or (20′), the formyl-like species is spontane-
ously dissociated on platinum according to the reaction:
•
•
+
Pt–( CHO) → Pt–( CO) +H +e – (21)
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aq
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The strongly adsorbed CO species was identified as the main poison-
ing species blocking the electrode active sites from further adsorption of
35
intermediates formed during methanol oxidation. The reaction repre-
sented by Eq. (21) is a rather fast process and it is the main reason for the
fast poisoning phenomena observed on a platinum surface. The configu-
ration of the adsorbed CO species depends on the electrode coverage and
on the electrode structure. On a smooth platinum polycrystalline electrode,
linearly bonded CO is the main species at intermediate to high coverages
(which prevails under operating conditions of a DMFC), whereas bridge
34
and multibonded CO are clearly seen at low coverages. Linearly bonded
CO is the main species adsorbed on Pt (110), whereas multibonded CO is
mainly formed on Pt(11l). 36
The vital step in the reaction mechanism appears to be the formation
•
of the intermediate ( CHO) , which facilitates the overall reaction. The
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kinetics of its further desorption and/or oxidation into reaction products
are the key steps of the mechanism, leading to complete oxidation. An
alternative path to the spontaneous formation of the poisoning species, Eq.
(21), is its oxidation, with OH species arising from the dissociation of
water according to the following reactions:

+
Pt+ H O → Pt–(OH) +H aq +e -
2 ads (22)
•
_
+
Pt ( CHO) + Pt–(OH) → 2Pt+ CO +2H +2e – (23)
2
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assq
One other reaction has been observed:
•
•
+
Pt–( CHO) + Pt–(OH) → Pt + Pt–( COOH) +H aq +e – (24)
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then leading also to the formation of carbon dioxide:
•
+
Pt–( COOH) → Pt + CO +H +e – (25)
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2
aq
On the other hand, adsorbed CO can be oxidized through the reac-
tions:
•
+
Pt–( CO) +Pt–(OH) → 2Pt+CO +H +e – (26)
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2
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aq