1 3
Topics in Current Chemistry (2018) 376:41
than on the corner or flat sites. Sarfraz et al. showed that the Cu surface, when decorated with Sn, resulted in a CO FE of > 90% and a current density of − 1.0 mA/cm
2
at − 0.2 V [28]. DFT calculations for the parent Cu(100) and Cu(111) facets, with
Cu atoms being replaced with Sn atoms, showed that the binding of hydrogen atoms
can be reduced by 0.18 eV while CO adsorption was not affected.
Sun et al. showed that the AuFe core/shell NPs could reduce CO 2 to CO with
ultrahigh selectivity (99% at − 0.5 V vs RHE) and mass activity (150 mA/mg at
− 0.9 V vs RHE) [39]. AuFe core/shell NPs were formed by surface Fe leaching of
the AuFe alloy during the first hour of electrolysis. This catalyst showed little to no
current drop after 90-h tests. The surface defects from Fe leaching, as well as the
subsurface alloy effect could be used to explain the increased selectivity and activity
at low potential, compared with Au NPs and Au foils. PBE (Perdew–Burke–Ernzerhof) DFT calculations based on a Schottky defect on Au surface suggested that the
formed Au defects could decrease the formation energy of *COOH and thus lower
the onset potential.
2.2 Hydrocarbon Selective Bimetallic Electrocatalysts
Catalysts with the ability to convert CO 2 to hydrocarbons are more appealing in the
field of CO 2 RR. Various Cu-based bimetallic materials have been reported to show
enhanced C 2+ products yields. Table 2 summarizes recent work related bimetallic
electrocatalysts for CO 2 reduction to hydrocarbons.
CO was identified as the key intermediate for further hydrocarbon formation [10,
54]. Metals with low CO binding energy, such as Au and Ag, might not form hydrocarbons because CO could be released easily from surface. In theory, strong CO
binding energy could benefit the further reduction. However, for monometallic particles, only Cu appeared to have the proper CO binding energy to produce hydrocarbon. The CO binding on Ni, Pd, Pt and Rh was too strong, which could easily result
in CO poisoning [54, 55]. The following mechanism was based on the reaction on
Cu surfaces.
For the C1 hydrocarbon (methane) formation, the most widely accepted pathway
was sequential hydrogenation of adsorbed CO* until CH 4 . The potential rate determine step was the formation of CHO*, and the subsequent reaction steps were exergonic. The formation of CO* required COOH* as an intermediate from initial CO 2
reduction [56] (Fig. 4a). If HCOO* formed, the final product would be HCOOH
[38]. Another C1 product, methanol, was also observed during CO 2 RR. However, in
this case, the intermediate was considered to be COH*, which required that hydrogen should bond to the oxygen instead of carbon (Fig. 4b). The formation of COH*
usually showed a higher kinetic barrier than that of CHO*. For the Cu(111) facet,
methanol formation required 0.18 eV higher energy than the methane formation at
− 1.15 V [57].
The formation of C2 products was usually seen when using Cu based materials. The most important steps were the ability for C–C coupling. There were several different hypotheses about the intermediates before C–C coupling. Nie et al.
calculated that CH and CH were possibly first coupled on the Cu(111) facet [57,
Reprinted from the journal
111
Topics in Current Chemistry (2018) 376:41
than on the corner or flat sites. Sarfraz et al. showed that the Cu surface, when decorated with Sn, resulted in a CO FE of > 90% and a current density of − 1.0 mA/cm
2
at − 0.2 V [28]. DFT calculations for the parent Cu(100) and Cu(111) facets, with
Cu atoms being replaced with Sn atoms, showed that the binding of hydrogen atoms
can be reduced by 0.18 eV while CO adsorption was not affected.
Sun et al. showed that the AuFe core/shell NPs could reduce CO 2 to CO with
ultrahigh selectivity (99% at − 0.5 V vs RHE) and mass activity (150 mA/mg at
− 0.9 V vs RHE) [39]. AuFe core/shell NPs were formed by surface Fe leaching of
the AuFe alloy during the first hour of electrolysis. This catalyst showed little to no
current drop after 90-h tests. The surface defects from Fe leaching, as well as the
subsurface alloy effect could be used to explain the increased selectivity and activity
at low potential, compared with Au NPs and Au foils. PBE (Perdew–Burke–Ernzerhof) DFT calculations based on a Schottky defect on Au surface suggested that the
formed Au defects could decrease the formation energy of *COOH and thus lower
the onset potential.
2.2 Hydrocarbon Selective Bimetallic Electrocatalysts
Catalysts with the ability to convert CO 2 to hydrocarbons are more appealing in the
field of CO 2 RR. Various Cu-based bimetallic materials have been reported to show
enhanced C 2+ products yields. Table 2 summarizes recent work related bimetallic
electrocatalysts for CO 2 reduction to hydrocarbons.
CO was identified as the key intermediate for further hydrocarbon formation [10,
54]. Metals with low CO binding energy, such as Au and Ag, might not form hydrocarbons because CO could be released easily from surface. In theory, strong CO
binding energy could benefit the further reduction. However, for monometallic particles, only Cu appeared to have the proper CO binding energy to produce hydrocarbon. The CO binding on Ni, Pd, Pt and Rh was too strong, which could easily result
in CO poisoning [54, 55]. The following mechanism was based on the reaction on
Cu surfaces.
For the C1 hydrocarbon (methane) formation, the most widely accepted pathway
was sequential hydrogenation of adsorbed CO* until CH 4 . The potential rate determine step was the formation of CHO*, and the subsequent reaction steps were exergonic. The formation of CO* required COOH* as an intermediate from initial CO 2
reduction [56] (Fig. 4a). If HCOO* formed, the final product would be HCOOH
[38]. Another C1 product, methanol, was also observed during CO 2 RR. However, in
this case, the intermediate was considered to be COH*, which required that hydrogen should bond to the oxygen instead of carbon (Fig. 4b). The formation of COH*
usually showed a higher kinetic barrier than that of CHO*. For the Cu(111) facet,
methanol formation required 0.18 eV higher energy than the methane formation at
− 1.15 V [57].
The formation of C2 products was usually seen when using Cu based materials. The most important steps were the ability for C–C coupling. There were several different hypotheses about the intermediates before C–C coupling. Nie et al.
calculated that CH and CH were possibly first coupled on the Cu(111) facet [57,
Reprinted from the journal
111