71
sites at lower potentials compared with the Pt sites; this accelerates CO oxidation
and lowers CO coverage, that is the poisoning effect.
Ru H O Ru OH
H e
ad
+
→
−
+
+ −
2
(6.19)
CO
OH
CO H e
ad +
→
+
+
−
+
−
2
(6.20)
For oxidation of CO, the optimal composition of PtRu alloy should have atomic
composition 50/50 at. %, assuming that one Pt atom adsorbs CO and one Ru atom
adsorbs OH. For oxidation of methanol, however, previously reported results provided evidence that the best activity of the PtRu electrodes contains about 10–20 at.
% of Ru surface atoms [56]. This can be roughly explained by the bifunctional
mechanism taking into account the number of surface sites necessary to accommodate
all the adsorbed intermediates, as observed by IR reflectance spectroscopy. Because
Ru is not involved in adsorption of methanol nor its dissociation, four surface Pt
atoms are needed for the activation of methanol only, that is one for dissociative
adsorption of CH 3 OH, two for the formyl species, and one for dissociation of
H 2 O. Taking into account the presence of other adsorbed species on Pt (e.g., –
COOH ads and bridge bonded > CO B ), the number of Pt atoms could vary from 5 to
7. Only one Ru surface atom is needed for H 2 O activation leading to the formation
of Ru-OH ads , so that the optimum surface atomic composition of the PtRu electrodes
should contain between 12.5 and 20 at. % Ru.
In addition to the bifunctional mechanism, also operative could be the electronic
effect (also called ligand effect) on Pt atoms caused by the interaction with Ru
atoms [53]. X-ray absorption spectroscopy (XAS) measurements demonstrated a
considerable increase in d-band vacancies on Pt in the PtRu/C electrocatalyst compared with that for pure Pt(111). This is likely to be caused by an effective transfer
of d-electrons from the Pt to the neighboring Ru. The increase in d-band vacancies
broadens the width of the d-band and lowers its center [58]. As a consequence, the
back donation of Pt d-electrons to the CO orbitals is hampered and CO bonding to
Pt is weakened. Identification of several intermediates and products in this and similar reactions is done best using in situ infrared (IR) spectroscopy.
It was found that even a smaller amount of noble metals is needed for successful
oxidation of methanol by decorating electrodes with monolayer or submonolayer of
Ru on Pt, or vice versa [59–62].
6.5 Ethanol Oxidation Reaction
Ethanol is a high-energy density fuel (8.0 kWh/kg), nontoxic, renewable energy
source requiring simple logistics for its applications. These features make it attractive for fuel cells and a very promising alternative power source for transportation,
6.5 Ethanol Oxidation Reaction
sites at lower potentials compared with the Pt sites; this accelerates CO oxidation
and lowers CO coverage, that is the poisoning effect.
Ru H O Ru OH
H e
ad
+
→
−
+
+ −
2
(6.19)
CO
OH
CO H e
ad +
→
+
+
−
+
−
2
(6.20)
For oxidation of CO, the optimal composition of PtRu alloy should have atomic
composition 50/50 at. %, assuming that one Pt atom adsorbs CO and one Ru atom
adsorbs OH. For oxidation of methanol, however, previously reported results provided evidence that the best activity of the PtRu electrodes contains about 10–20 at.
% of Ru surface atoms [56]. This can be roughly explained by the bifunctional
mechanism taking into account the number of surface sites necessary to accommodate
all the adsorbed intermediates, as observed by IR reflectance spectroscopy. Because
Ru is not involved in adsorption of methanol nor its dissociation, four surface Pt
atoms are needed for the activation of methanol only, that is one for dissociative
adsorption of CH 3 OH, two for the formyl species, and one for dissociation of
H 2 O. Taking into account the presence of other adsorbed species on Pt (e.g., –
COOH ads and bridge bonded > CO B ), the number of Pt atoms could vary from 5 to
7. Only one Ru surface atom is needed for H 2 O activation leading to the formation
of Ru-OH ads , so that the optimum surface atomic composition of the PtRu electrodes
should contain between 12.5 and 20 at. % Ru.
In addition to the bifunctional mechanism, also operative could be the electronic
effect (also called ligand effect) on Pt atoms caused by the interaction with Ru
atoms [53]. X-ray absorption spectroscopy (XAS) measurements demonstrated a
considerable increase in d-band vacancies on Pt in the PtRu/C electrocatalyst compared with that for pure Pt(111). This is likely to be caused by an effective transfer
of d-electrons from the Pt to the neighboring Ru. The increase in d-band vacancies
broadens the width of the d-band and lowers its center [58]. As a consequence, the
back donation of Pt d-electrons to the CO orbitals is hampered and CO bonding to
Pt is weakened. Identification of several intermediates and products in this and similar reactions is done best using in situ infrared (IR) spectroscopy.
It was found that even a smaller amount of noble metals is needed for successful
oxidation of methanol by decorating electrodes with monolayer or submonolayer of
Ru on Pt, or vice versa [59–62].
6.5 Ethanol Oxidation Reaction
Ethanol is a high-energy density fuel (8.0 kWh/kg), nontoxic, renewable energy
source requiring simple logistics for its applications. These features make it attractive for fuel cells and a very promising alternative power source for transportation,
6.5 Ethanol Oxidation Reaction