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Topics in Current Chemistry (2019) 377:11
3.1.1 The Role of Rh in the Ternary Pt–Rh–SnO 2 Catalyst System
The catalytic property of the Pt–Rh–SnO 2 system is explained in terms of the synergistic effect among the three constituents [23]. Ethanol oxidation commences on
Pt sites by dehydrogenate adsorption; Rh cleaves the C–C bond in ethanol and SnO 2
supplies oxygen-containing species for further oxidation of the intermediates, and
also keeps Pt and Rh metallic. DFT calculations of ethanol decomposition on the
PtRh/SnO 2 (110) have explained that the path to the total ethanol oxidation pathway
leads through an oxametallacyclic conformation (*CH 2 CH 2 O) intermediate, which
further breaks into *CH 2 and *CH 2 O radicals and two protons with a reasonable
energy barrier of 1.29 eV. An oxidation pathway through acetaldehyde is unlikely,
as it requires an extremely high energy of 3.82 eV. Furthermore, separate DFT studies on Rh(111) have shown that Pt in Rh–Pt alloy weakens the C–Rh interaction
through electronic effect and helps the removal of carbonaceous species in the form
of CO 2 [87].
Experimental confirmation that the addition of Rh enhances EOR activity via
facilitating ethanol total oxidation to CO 2 was performed with single-crystal-based
model catalysts [i.e., SnO 2 /Pt(111) and RhSnO 2 /Pt(111)] and carbon-supported
nanoparticle catalysts (i.e., Pt–SnO 2 /C and Pt–Rh–SnO 2 /C) using IRRAS [21, 70].
For the single-crystal catalysts, Pt(111) single-crystal surface was first prepared by
annealing in H 2 /Ar mixture and cooling in the same stream, and then nanoclusters of
SnO 2 or RhSnO 2 were deposited on it. Carbon-supported catalysts were synthesized
via wet-chemistry methods, followed by the deposition of the nanocatalyst ink (catalysts dispersed in the mixture of ethanol, water, and Nafion) on an inactive support
(polycrystalline gold electrode) in a similar manner described by Markovic et al.
[88].
Figure 5a–d shows the in situ IRRAS spectra collected from the above catalysts
during EOR in perchloric acid supporting solution. The two most prominent bands
represent the electrolyte rearrangement near the electrode surface during the potential excursion, where perchlorate anions enter the electrolyte layer, producing the
positive-going band near 1100 cm
−1
, and replacing water molecules (negative-going
band around 1600 cm
−1
). The negative-going band at 1040 cm
−1
represents the ethanol consumption, whereas positive-going bands at 2343, 1280, and 933 cm
−1
represent the oxidation products consisting of CO 2 , CH 3 CHO, and CH 3 COOH. Band
assignments are listed in Table 1.
The selectivity of the catalyst towards the total oxidation pathway was evaluated
by the intensity of the positive-going peak near 2343 cm
−1
, the asymmetric stretch
vibration of CO 2 . It is evident that the intensity of the CO 2 band produced by the
two Rh-containing catalysts [RhSnO 2 /Pt(111) and Pt–Rh–SnO 2 /C] is much higher
when compared to the two Rh-free catalysts (SnO 2 /Pt(111) and Pt–SnO 2 /C). This
fact confirms that the addition of Rh enhances catalysts’ selectivity in C–C bond
splitting and CO 2 production. In addition, a bipolar band around 2040 cm
−1
, which
is particularly strong at SnO 2 /Pt(111) catalyst, appears due to the linearly bonded
CO on the catalyst surface (CO L ). The bipolar shape of the band comes from the
fact that CO L is present at both sample and the reference potential but with different vibration frequencies, according to the Blyholder mechanism [89]. The band can
Reprinted from the journal
11
Topics in Current Chemistry (2019) 377:11
3.1.1 The Role of Rh in the Ternary Pt–Rh–SnO 2 Catalyst System
The catalytic property of the Pt–Rh–SnO 2 system is explained in terms of the synergistic effect among the three constituents [23]. Ethanol oxidation commences on
Pt sites by dehydrogenate adsorption; Rh cleaves the C–C bond in ethanol and SnO 2
supplies oxygen-containing species for further oxidation of the intermediates, and
also keeps Pt and Rh metallic. DFT calculations of ethanol decomposition on the
PtRh/SnO 2 (110) have explained that the path to the total ethanol oxidation pathway
leads through an oxametallacyclic conformation (*CH 2 CH 2 O) intermediate, which
further breaks into *CH 2 and *CH 2 O radicals and two protons with a reasonable
energy barrier of 1.29 eV. An oxidation pathway through acetaldehyde is unlikely,
as it requires an extremely high energy of 3.82 eV. Furthermore, separate DFT studies on Rh(111) have shown that Pt in Rh–Pt alloy weakens the C–Rh interaction
through electronic effect and helps the removal of carbonaceous species in the form
of CO 2 [87].
Experimental confirmation that the addition of Rh enhances EOR activity via
facilitating ethanol total oxidation to CO 2 was performed with single-crystal-based
model catalysts [i.e., SnO 2 /Pt(111) and RhSnO 2 /Pt(111)] and carbon-supported
nanoparticle catalysts (i.e., Pt–SnO 2 /C and Pt–Rh–SnO 2 /C) using IRRAS [21, 70].
For the single-crystal catalysts, Pt(111) single-crystal surface was first prepared by
annealing in H 2 /Ar mixture and cooling in the same stream, and then nanoclusters of
SnO 2 or RhSnO 2 were deposited on it. Carbon-supported catalysts were synthesized
via wet-chemistry methods, followed by the deposition of the nanocatalyst ink (catalysts dispersed in the mixture of ethanol, water, and Nafion) on an inactive support
(polycrystalline gold electrode) in a similar manner described by Markovic et al.
[88].
Figure 5a–d shows the in situ IRRAS spectra collected from the above catalysts
during EOR in perchloric acid supporting solution. The two most prominent bands
represent the electrolyte rearrangement near the electrode surface during the potential excursion, where perchlorate anions enter the electrolyte layer, producing the
positive-going band near 1100 cm
−1
, and replacing water molecules (negative-going
band around 1600 cm
−1
). The negative-going band at 1040 cm
−1
represents the ethanol consumption, whereas positive-going bands at 2343, 1280, and 933 cm
−1
represent the oxidation products consisting of CO 2 , CH 3 CHO, and CH 3 COOH. Band
assignments are listed in Table 1.
The selectivity of the catalyst towards the total oxidation pathway was evaluated
by the intensity of the positive-going peak near 2343 cm
−1
, the asymmetric stretch
vibration of CO 2 . It is evident that the intensity of the CO 2 band produced by the
two Rh-containing catalysts [RhSnO 2 /Pt(111) and Pt–Rh–SnO 2 /C] is much higher
when compared to the two Rh-free catalysts (SnO 2 /Pt(111) and Pt–SnO 2 /C). This
fact confirms that the addition of Rh enhances catalysts’ selectivity in C–C bond
splitting and CO 2 production. In addition, a bipolar band around 2040 cm
−1
, which
is particularly strong at SnO 2 /Pt(111) catalyst, appears due to the linearly bonded
CO on the catalyst surface (CO L ). The bipolar shape of the band comes from the
fact that CO L is present at both sample and the reference potential but with different vibration frequencies, according to the Blyholder mechanism [89]. The band can
Reprinted from the journal
11