Topics in Current Chemistry (2019) 377:11
1 3
the total oxidation pathway of ethanol. This was ascribed to the electron donation
from Rh to Pt through which the d-band states of both metals were changed so that
the intermediates produced by splitting of C–C bond in ethanol become more mobile
and able to reach oxygen-containing species covering the SnO 2 sites at the surface of
the ternary catalyst and further oxidized to CO 2 [11].
To find the optimal SnO 2 sites, the mole fraction of Sn was varied while the Pt:Rh
ratio was kept fixed to 3:1 [107]. Two carbon-supported catalysts with stoichiometric
atomic ratio of Pt 21 Rh 5 Sn 39 and Pt 23 Rh 5 Sn 26 were obtained by simultaneous codeposition of all three metals from their soluble chlorides, followed by reduction in ethylene glycol and heating in air to oxidize Sn to SnO 2 . The synthesized PtRh particles
had an average size of 1.5 nm with a narrow distribution [84]. Their electrochemical activity was compared to the commercially available Pt/C and PtRu/C. When
normalized to the total noble metal loading, the ethanol oxidation current densities
of the two ternary catalysts are significantly higher and their oxidation onsets are
shifted to lower potentials than those of the commercial ones (Fig. 12). The comparison of the two ternary catalysts reveals that the one with the lower Sn content shows
higher activity and lower onset of ethanol oxidation. Subsequent chronoamperometric tests also showed that the catalyst with reduced Sn mole fraction has better stability. Because the two catalysts have roughly the same Pt:Rh ratio, the difference in
their activity is ascribed to the variance in the content of SnO 2 [107].
As described before, besides the supply of oxygen-containing species, the role
of SnO 2 is to keep Pt and Rh in metallic form. Figure 13 shows the in situ XANES
spectra for Pt L 3 -edge and Rh K-edge of the two catalysts at potentials ranging from
0.06 to 0.91 V. As seen in Fig. 13a, the spectra obtained from Pt 23 Rh 5 Sn 26 /C at Pt
edge at the three most negative potentials (0.06, 0.21, and 0.41 V) overlap and their
shape is close to that of Pt foil. The changes in Pt XANES spectra are observed
at potentials equal to or more positive than 0.71 V, but only in the intensity of the
white line; however, no obvious shift in the edge energy (E 0 ) is seen. On the other
hand, the changes in Rh XANES spectra are readily observed above 0.41 V, as white
line increases and edge position shifts to the higher energies (Fig. 13c). A close
observation of Fig. 13c further reveals isosbestic points at 23,256 and 23,300 eV,
suggesting that Rh exists in two different chemical forms having the same total concentration. Thus, all Rh K-edge spectra can be reproduced with a linear combination
of two XANES spectra, one of metallic Rh at 0.06 V, and the other of oxidized Rh at
0.91 V. During the potential excursion, Rh changes its oxidation state directly from
Rh
0
to Rh
n+
(n is probably 3), without passing through lower oxidation states.
Figure 13b, d show the Pt L 3 -edge and Rh K-edge XANES spectral region for the
Pt 21 Rh 5 Sn 39 /C catalyst obtained in situ at potentials ranging from 0.06 to 0.91 V.
Like the behavior of the Pt 23 Rh 5 Sn 26 /C catalyst, the changes in the intensity of the Pt
white line (Fig. 13b) are noticed above 0.41 V. In contrast, the Rh K-edge (Fig. 13d)
shows an abrupt oxidation that commences on or before 0.21 V, as observed from
the increase of white line as well as the shift of the edge position to higher photon
energies. It is interesting to note that the Pt white line for both catalysts is lower than
that of the Pt foil at potentials below 0.41 V (cf. Figure 13a, b), whereas that of Rh
foil is always lower than that of the catalysts (Fig. 13c, d). Since the white line of Rh
K-edge represents the transition of an electron from 1s orbital to hybridized 5p and
Reprinted from the journal
22
1 3
the total oxidation pathway of ethanol. This was ascribed to the electron donation
from Rh to Pt through which the d-band states of both metals were changed so that
the intermediates produced by splitting of C–C bond in ethanol become more mobile
and able to reach oxygen-containing species covering the SnO 2 sites at the surface of
the ternary catalyst and further oxidized to CO 2 [11].
To find the optimal SnO 2 sites, the mole fraction of Sn was varied while the Pt:Rh
ratio was kept fixed to 3:1 [107]. Two carbon-supported catalysts with stoichiometric
atomic ratio of Pt 21 Rh 5 Sn 39 and Pt 23 Rh 5 Sn 26 were obtained by simultaneous codeposition of all three metals from their soluble chlorides, followed by reduction in ethylene glycol and heating in air to oxidize Sn to SnO 2 . The synthesized PtRh particles
had an average size of 1.5 nm with a narrow distribution [84]. Their electrochemical activity was compared to the commercially available Pt/C and PtRu/C. When
normalized to the total noble metal loading, the ethanol oxidation current densities
of the two ternary catalysts are significantly higher and their oxidation onsets are
shifted to lower potentials than those of the commercial ones (Fig. 12). The comparison of the two ternary catalysts reveals that the one with the lower Sn content shows
higher activity and lower onset of ethanol oxidation. Subsequent chronoamperometric tests also showed that the catalyst with reduced Sn mole fraction has better stability. Because the two catalysts have roughly the same Pt:Rh ratio, the difference in
their activity is ascribed to the variance in the content of SnO 2 [107].
As described before, besides the supply of oxygen-containing species, the role
of SnO 2 is to keep Pt and Rh in metallic form. Figure 13 shows the in situ XANES
spectra for Pt L 3 -edge and Rh K-edge of the two catalysts at potentials ranging from
0.06 to 0.91 V. As seen in Fig. 13a, the spectra obtained from Pt 23 Rh 5 Sn 26 /C at Pt
edge at the three most negative potentials (0.06, 0.21, and 0.41 V) overlap and their
shape is close to that of Pt foil. The changes in Pt XANES spectra are observed
at potentials equal to or more positive than 0.71 V, but only in the intensity of the
white line; however, no obvious shift in the edge energy (E 0 ) is seen. On the other
hand, the changes in Rh XANES spectra are readily observed above 0.41 V, as white
line increases and edge position shifts to the higher energies (Fig. 13c). A close
observation of Fig. 13c further reveals isosbestic points at 23,256 and 23,300 eV,
suggesting that Rh exists in two different chemical forms having the same total concentration. Thus, all Rh K-edge spectra can be reproduced with a linear combination
of two XANES spectra, one of metallic Rh at 0.06 V, and the other of oxidized Rh at
0.91 V. During the potential excursion, Rh changes its oxidation state directly from
Rh
0
to Rh
n+
(n is probably 3), without passing through lower oxidation states.
Figure 13b, d show the Pt L 3 -edge and Rh K-edge XANES spectral region for the
Pt 21 Rh 5 Sn 39 /C catalyst obtained in situ at potentials ranging from 0.06 to 0.91 V.
Like the behavior of the Pt 23 Rh 5 Sn 26 /C catalyst, the changes in the intensity of the Pt
white line (Fig. 13b) are noticed above 0.41 V. In contrast, the Rh K-edge (Fig. 13d)
shows an abrupt oxidation that commences on or before 0.21 V, as observed from
the increase of white line as well as the shift of the edge position to higher photon
energies. It is interesting to note that the Pt white line for both catalysts is lower than
that of the Pt foil at potentials below 0.41 V (cf. Figure 13a, b), whereas that of Rh
foil is always lower than that of the catalysts (Fig. 13c, d). Since the white line of Rh
K-edge represents the transition of an electron from 1s orbital to hybridized 5p and
Reprinted from the journal
22