Topics in Current Chemistry (2019) 377:5
1 3
⇄ CO 2 (g) + 6H
+
+ 6e
−
is E
0
≃ 0.016 V SHE and the theoretical fuel cell efficiency
is ≃ 96.7%. At platinum electrodes, it is argued that the electrochemical oxidation
reaction of C 1 molecules, as methanol, formaldehyde and formic acid, can proceed
through two (parallel) pathways, termed direct and indirect [62, 63]. The terminologies for these reaction pathways have the CO ads as a watershed in the reaction mechanism, and the preferred pathway depends on the nature and composition of the catalyst materials, as well as on the precise local geometric arrangement of the atoms
at the catalyst surfaces, as deduced from the studies on the single crystalline surfaces [64, 65]. The direct pathway is kinetically faster and it is believed that the C 1
molecules goes to CO 2 without going through CO ads . The indirect one is kinetically
slower, and the C 1 molecules are finally transformed in CO 2 , but through CO ads .
In the case of formic acid (HCOOH), the direct pathway involves its dehydrogenation and subsequent evolution to CO 2 . The formate species adsorbed (HCOO ads )
through the carbon atom has been identified and its possible role in kinetics—if it
acts like a catalytic poison or intermediate reaction—has been warmly debated in
literature [66–68]. In the indirect pathway, the C 1 molecules are dehydrated toward
CO ads , which displays the role of a catalytic poison, as it is oxidized to CO 2 at high
potentials.
The use of Pt single crystals in studying the electrocatalysis of methanol oxidation has revealed that the reaction is strongly sensitive to the catalyst surface structure [69]. Then, on platinum, the reaction yields formic acid and formaldehyde
as soluble products, while CO ads is detected at the electrode surface [69]. After a
kinetic study employing stepped Pt surfaces, it was concluded that methanol dissociation takes place exclusively at the step sites [65]. In these unpoised terraces, it
has been shown that the direct path of methanol electro-oxidation is a site-demanding process. The quantification of this statement was experimentally determined
[70] employing a cyanide-Pt(111)-modified electrode, as shown in Fig. 6 [70].
In the series of in situ FTIR spectra in Fig. 6c, no bands due to the intermolecular stretching frequencies of adsorbed CO ads were detected (which should appear
at ~ 2060–2075, and ~ 1850 cm
−1
, due to the linearly and bridge-bonded CO, respectively [71]). The absence of CO ads in the mechanism of methanol electro-oxidation
on cyanide-Pt(111)-modified electrode also explains the stability of the voltammetry
in the hydrogen region, even in the presence of methanol in solution (Fig. 6a). The
band due to CO 2 appears in the in situ FTIR spectra at 2343 cm
−1
at ~ 0.6 V RHE ,
that is the onset potential for the methanol oxidation to CO 2 on cyanide-Pt(111)modified electrode.
Concerning the surface structure of the cyanide-modified Pt(111) electrode,
Fig. 7a shows the pattern of the Pt sites occupied by cyanide. The configuration of
the cyanide-modified Pt(111) electrode is a (2√3 × 2√3)R30
o
structure and provides a limited arrangement of contiguous Pt atoms [72]. The formation of CO ads
requires a large atomic ensemble, at least three contiguous atoms of platinum [70].
This specific atomic configuration is not observed on the cyanide-modified Pt(111)
electrode, explaining the direct oxidation of methanol toward CO 2 , without going
through CO ads . Figure 7b, c displays the possible matches between the configurations involving three Pt atoms on the catalyst surface and the possible reaction
pathways.
Reprinted from the journal
90
1 3
⇄ CO 2 (g) + 6H
+
+ 6e
−
is E
0
≃ 0.016 V SHE and the theoretical fuel cell efficiency
is ≃ 96.7%. At platinum electrodes, it is argued that the electrochemical oxidation
reaction of C 1 molecules, as methanol, formaldehyde and formic acid, can proceed
through two (parallel) pathways, termed direct and indirect [62, 63]. The terminologies for these reaction pathways have the CO ads as a watershed in the reaction mechanism, and the preferred pathway depends on the nature and composition of the catalyst materials, as well as on the precise local geometric arrangement of the atoms
at the catalyst surfaces, as deduced from the studies on the single crystalline surfaces [64, 65]. The direct pathway is kinetically faster and it is believed that the C 1
molecules goes to CO 2 without going through CO ads . The indirect one is kinetically
slower, and the C 1 molecules are finally transformed in CO 2 , but through CO ads .
In the case of formic acid (HCOOH), the direct pathway involves its dehydrogenation and subsequent evolution to CO 2 . The formate species adsorbed (HCOO ads )
through the carbon atom has been identified and its possible role in kinetics—if it
acts like a catalytic poison or intermediate reaction—has been warmly debated in
literature [66–68]. In the indirect pathway, the C 1 molecules are dehydrated toward
CO ads , which displays the role of a catalytic poison, as it is oxidized to CO 2 at high
potentials.
The use of Pt single crystals in studying the electrocatalysis of methanol oxidation has revealed that the reaction is strongly sensitive to the catalyst surface structure [69]. Then, on platinum, the reaction yields formic acid and formaldehyde
as soluble products, while CO ads is detected at the electrode surface [69]. After a
kinetic study employing stepped Pt surfaces, it was concluded that methanol dissociation takes place exclusively at the step sites [65]. In these unpoised terraces, it
has been shown that the direct path of methanol electro-oxidation is a site-demanding process. The quantification of this statement was experimentally determined
[70] employing a cyanide-Pt(111)-modified electrode, as shown in Fig. 6 [70].
In the series of in situ FTIR spectra in Fig. 6c, no bands due to the intermolecular stretching frequencies of adsorbed CO ads were detected (which should appear
at ~ 2060–2075, and ~ 1850 cm
−1
, due to the linearly and bridge-bonded CO, respectively [71]). The absence of CO ads in the mechanism of methanol electro-oxidation
on cyanide-Pt(111)-modified electrode also explains the stability of the voltammetry
in the hydrogen region, even in the presence of methanol in solution (Fig. 6a). The
band due to CO 2 appears in the in situ FTIR spectra at 2343 cm
−1
at ~ 0.6 V RHE ,
that is the onset potential for the methanol oxidation to CO 2 on cyanide-Pt(111)modified electrode.
Concerning the surface structure of the cyanide-modified Pt(111) electrode,
Fig. 7a shows the pattern of the Pt sites occupied by cyanide. The configuration of
the cyanide-modified Pt(111) electrode is a (2√3 × 2√3)R30
o
structure and provides a limited arrangement of contiguous Pt atoms [72]. The formation of CO ads
requires a large atomic ensemble, at least three contiguous atoms of platinum [70].
This specific atomic configuration is not observed on the cyanide-modified Pt(111)
electrode, explaining the direct oxidation of methanol toward CO 2 , without going
through CO ads . Figure 7b, c displays the possible matches between the configurations involving three Pt atoms on the catalyst surface and the possible reaction
pathways.
Reprinted from the journal
90