1 3
Topics in Current Chemistry (2019) 377:11
Pt proceeds mostly through re-adsorption of acetaldehyde, yielding adsorbed carbon monoxide CO ads as an intermediate [2]. Carbon-containing species as CO ads are
especially strongly adsorbed on Pt surface, thus blocking Pt sites for further adsorption of reactants until they are removed by further oxidation. CO ads is oxidized above
0.6 V, when the Pt surface becomes covered by Pt–OH or Pt–O species produced by
oxidation of water, which further react with CO ads yielding CO 2 as the final product.
Thus, on pure Pt, carbon dioxide appears only at very high positive potentials [8].
Products and adsorbed intermediates of EOR in acidic solution have been verified
using several techniques [3, 7, 8]. A recent study showed that the ethanol oxidation on
Pt/C yields primarily acetic acid and acetaldehyde as the products, with carbon dioxide occurring in the range of only 0.7–7.5% depending on the initial concentration of
the alcohol [9]. Such a low selectivity of Pt toward CO 2 is a practical problem to be
addressed in designing DEFCs. In addition, it contrasts with the consensus in chemistry stating that kinetics of any catalytic reaction is expected to be faster if it leads to a
more stable product. Since CO 2 is significantly more stable than acetic acid and acetaldehyde, the low selectivity of the EOR to CO 2 is also a fundamental question.
Consequently, pure Pt is not capable of catalyzing oxidation of ethanol through
the total oxidation pathway. However, Pt is a necessary catalyst constituent to provide
efficient adsorption of ethanol, the first reaction step in its oxidation. Modifications of
pure Pt have been developed to improve activity of Pt by the addition of co-catalysts,
in various structures and compositions, including adatoms, adlayers and intermetallic Pt–M assemblies, bimetallic alloys, metal oxides, and core–shell nanostructures
[10–18]. The co-catalyst promotional effect is explained in terms of bifunctional
effect (promoted mechanism) [19] and electronic effect (ligand effect or intrinsic
mechanism) [20]. According to the bifunctional effect, the oxidative removal of the
strongly adsorbed poisoning species (i.e., species that cannot be further oxidized at
that anode potential) is facilitated in the presence of an oxyphilic co-catalyst (e.g.,
Ru, Sn, Ir, Re, Os, Pb, etc.) by supplying oxygen atoms at an adjacent site at a lower
potential than that accomplished by pure Pt. The electronic effect postulates that the
presence of the co-catalyst modifies the electronic structure of Pt, and therefore the
bonding strength of poisoning species and oxygen-containing species. Moreover,
the addition of the co-catalysts decreases Pt content in the electrocatalysts, therefore
reducing the noble metal content. Pt–Sn and Pt–Ru are found to be the most active
binary catalysts [12]. Despite the increase in overall activity, the addition of Sn or Ru
decreases platinum selectivity towards CO 2 formation [10, 21]. Therefore, the addition of the co-catalyst facilitates the products of partial oxidation pathway.
Because existing Pt-based binary catalysts could not achieve total oxidation of
ethanol to CO 2 at moderate positive potentials, the prospects were focused on addition of a third component. Pt–Ru–W, Pt–Sn–Ni and Pt–Sn–Rh further enhanced
activity, but the yield of carbon dioxide was still low [12, 22]. The first multi-functional ternary electrocatalyst effective in splitting the C–C bond in ethanol at room
temperature and capable of carrying out the reaction to CO 2 was designed by Adzic
et al. [23]. Combining the attributes of Pt for ethanol dehydrogenerative adsorption,
SnO 2 for the supply of oxygen-containing species and Rh that is known to be capable of splitting C–C bond as observed in metal/gas interface studies [24, 25], an
active ternary Pt/Rh/SnO 2 catalyst was obtained. Reports from other groups have
Reprinted from the journal
3
Topics in Current Chemistry (2019) 377:11
Pt proceeds mostly through re-adsorption of acetaldehyde, yielding adsorbed carbon monoxide CO ads as an intermediate [2]. Carbon-containing species as CO ads are
especially strongly adsorbed on Pt surface, thus blocking Pt sites for further adsorption of reactants until they are removed by further oxidation. CO ads is oxidized above
0.6 V, when the Pt surface becomes covered by Pt–OH or Pt–O species produced by
oxidation of water, which further react with CO ads yielding CO 2 as the final product.
Thus, on pure Pt, carbon dioxide appears only at very high positive potentials [8].
Products and adsorbed intermediates of EOR in acidic solution have been verified
using several techniques [3, 7, 8]. A recent study showed that the ethanol oxidation on
Pt/C yields primarily acetic acid and acetaldehyde as the products, with carbon dioxide occurring in the range of only 0.7–7.5% depending on the initial concentration of
the alcohol [9]. Such a low selectivity of Pt toward CO 2 is a practical problem to be
addressed in designing DEFCs. In addition, it contrasts with the consensus in chemistry stating that kinetics of any catalytic reaction is expected to be faster if it leads to a
more stable product. Since CO 2 is significantly more stable than acetic acid and acetaldehyde, the low selectivity of the EOR to CO 2 is also a fundamental question.
Consequently, pure Pt is not capable of catalyzing oxidation of ethanol through
the total oxidation pathway. However, Pt is a necessary catalyst constituent to provide
efficient adsorption of ethanol, the first reaction step in its oxidation. Modifications of
pure Pt have been developed to improve activity of Pt by the addition of co-catalysts,
in various structures and compositions, including adatoms, adlayers and intermetallic Pt–M assemblies, bimetallic alloys, metal oxides, and core–shell nanostructures
[10–18]. The co-catalyst promotional effect is explained in terms of bifunctional
effect (promoted mechanism) [19] and electronic effect (ligand effect or intrinsic
mechanism) [20]. According to the bifunctional effect, the oxidative removal of the
strongly adsorbed poisoning species (i.e., species that cannot be further oxidized at
that anode potential) is facilitated in the presence of an oxyphilic co-catalyst (e.g.,
Ru, Sn, Ir, Re, Os, Pb, etc.) by supplying oxygen atoms at an adjacent site at a lower
potential than that accomplished by pure Pt. The electronic effect postulates that the
presence of the co-catalyst modifies the electronic structure of Pt, and therefore the
bonding strength of poisoning species and oxygen-containing species. Moreover,
the addition of the co-catalysts decreases Pt content in the electrocatalysts, therefore
reducing the noble metal content. Pt–Sn and Pt–Ru are found to be the most active
binary catalysts [12]. Despite the increase in overall activity, the addition of Sn or Ru
decreases platinum selectivity towards CO 2 formation [10, 21]. Therefore, the addition of the co-catalyst facilitates the products of partial oxidation pathway.
Because existing Pt-based binary catalysts could not achieve total oxidation of
ethanol to CO 2 at moderate positive potentials, the prospects were focused on addition of a third component. Pt–Ru–W, Pt–Sn–Ni and Pt–Sn–Rh further enhanced
activity, but the yield of carbon dioxide was still low [12, 22]. The first multi-functional ternary electrocatalyst effective in splitting the C–C bond in ethanol at room
temperature and capable of carrying out the reaction to CO 2 was designed by Adzic
et al. [23]. Combining the attributes of Pt for ethanol dehydrogenerative adsorption,
SnO 2 for the supply of oxygen-containing species and Rh that is known to be capable of splitting C–C bond as observed in metal/gas interface studies [24, 25], an
active ternary Pt/Rh/SnO 2 catalyst was obtained. Reports from other groups have
Reprinted from the journal
3