1 3
Topics in Current Chemistry (2019) 377:11
4d orbitals, the up-shifted white line in the samples suggests a less filled d-band of
Rh. Concurrently, the lower white line for Pt suggests more populated d-band. Both
effects have been observed before in PtRh random alloys and are in line with our
DFT calculations that predicted the electron transfer from Rh to Pt [23, 86].
Relative changes in white-line peaks of Pt L 3 and Rh K-edges of the two ternary
catalysts as a function of applied potential are shown in Fig. 14. As seen, the intensity of the white line of Pt shows no effect on potential excursions to 0.41 V in either
catalyst (Fig. 14a). Further increase of the potential affects the white lines of both
catalysts, but the one with higher Sn content shows more abrupt change [31, 107].
On the other hand, while the white line of Rh in the catalyst with lower Sn content
is unaffected by the potential excursion up to 0.41 V, that of Pt 21 Rh 5 Sn 39 /C starts
to change as early as 0.21 V and continues to increase with a steeper slope over the
whole potential excursion (Fig. 14b).
It appears that a too high content of SnO 2 at the catalysts surface can be detrimental for the catalyst’s activity. The excess of oxygen-containing species uninvolved in
oxidation of alcohols are spilled over from SnO 2 to Pt and Rh, blocking Pt sites for
adsorption of ethanol and Rh sites for adsorption of the intermediate CH 2 CH 2 O*. In
addition, the high content of SnO 2 in the Pt 21 Rh 5 Sn 39 /C ternary catalyst affects the
Rh oxidation, which is evident at Rh XANES spectra obtained during the potential
excursion from 0.06 to 0.91 V.
Figure 15 shows the first-shell fitting results of the Pt–Rh–SnO 2 /C electrocatalyst
with Pt:Rh:Sn = 3:1:4 atomic ratio at the potential of 0.41 V where all parameters
except the passive electron reduction factors (S 0
2
) for the two metals were allowed to
vary with no constraints. The fitting results are summarized in Table 2. As expected,
the coordination numbers and bond lengths clearly show that the Pt and Rh form random alloy, as the ratio of their partial coordination numbers are consistent with mole
fraction ratio obtained by ICP (Pt/Rh = 3.0), the total coordination numbers are close
together, and the bond lengths are between those of the bulk values. Finally, the
nanoparticle size can be estimated from the total coordination numbers of the two
metals. Taking the average total coordination number N(Pt–M) = N(Rh–M) = 6.9,
the particle size is estimated to be 1.4 nm, which is in good agreement with the HRSTEM data.
4.3 Ir as an Alternative to Rh
Despite the superior activity of Pt–Rh–SnO 2 catalyst, further work was aimed to
substitute the noble metals with cheaper and possibly more abundant metals so that
the electrocatalyst can be used in mass production of portable fuel cells. While Pt
cannot be replaced due to the unprecedented oxidative adsorption of ethanol, lower
content of Rh in catalysts would significantly reduce the total cost as the current
price of Rh is higher than that of Pt. Iridium and rhodium are in the same group of
the periodic system and are expected to share adsorptive and catalytic properties.
Regardless of the low abundance of Ir, it is considerably cheaper than Pt and Rh.
Although Ir was studied as an EOR catalyst earlier, its capability in splitting the
C–C bond was not investigated [108–110].
Reprinted from the journal
25
Topics in Current Chemistry (2019) 377:11
4d orbitals, the up-shifted white line in the samples suggests a less filled d-band of
Rh. Concurrently, the lower white line for Pt suggests more populated d-band. Both
effects have been observed before in PtRh random alloys and are in line with our
DFT calculations that predicted the electron transfer from Rh to Pt [23, 86].
Relative changes in white-line peaks of Pt L 3 and Rh K-edges of the two ternary
catalysts as a function of applied potential are shown in Fig. 14. As seen, the intensity of the white line of Pt shows no effect on potential excursions to 0.41 V in either
catalyst (Fig. 14a). Further increase of the potential affects the white lines of both
catalysts, but the one with higher Sn content shows more abrupt change [31, 107].
On the other hand, while the white line of Rh in the catalyst with lower Sn content
is unaffected by the potential excursion up to 0.41 V, that of Pt 21 Rh 5 Sn 39 /C starts
to change as early as 0.21 V and continues to increase with a steeper slope over the
whole potential excursion (Fig. 14b).
It appears that a too high content of SnO 2 at the catalysts surface can be detrimental for the catalyst’s activity. The excess of oxygen-containing species uninvolved in
oxidation of alcohols are spilled over from SnO 2 to Pt and Rh, blocking Pt sites for
adsorption of ethanol and Rh sites for adsorption of the intermediate CH 2 CH 2 O*. In
addition, the high content of SnO 2 in the Pt 21 Rh 5 Sn 39 /C ternary catalyst affects the
Rh oxidation, which is evident at Rh XANES spectra obtained during the potential
excursion from 0.06 to 0.91 V.
Figure 15 shows the first-shell fitting results of the Pt–Rh–SnO 2 /C electrocatalyst
with Pt:Rh:Sn = 3:1:4 atomic ratio at the potential of 0.41 V where all parameters
except the passive electron reduction factors (S 0
2
) for the two metals were allowed to
vary with no constraints. The fitting results are summarized in Table 2. As expected,
the coordination numbers and bond lengths clearly show that the Pt and Rh form random alloy, as the ratio of their partial coordination numbers are consistent with mole
fraction ratio obtained by ICP (Pt/Rh = 3.0), the total coordination numbers are close
together, and the bond lengths are between those of the bulk values. Finally, the
nanoparticle size can be estimated from the total coordination numbers of the two
metals. Taking the average total coordination number N(Pt–M) = N(Rh–M) = 6.9,
the particle size is estimated to be 1.4 nm, which is in good agreement with the HRSTEM data.
4.3 Ir as an Alternative to Rh
Despite the superior activity of Pt–Rh–SnO 2 catalyst, further work was aimed to
substitute the noble metals with cheaper and possibly more abundant metals so that
the electrocatalyst can be used in mass production of portable fuel cells. While Pt
cannot be replaced due to the unprecedented oxidative adsorption of ethanol, lower
content of Rh in catalysts would significantly reduce the total cost as the current
price of Rh is higher than that of Pt. Iridium and rhodium are in the same group of
the periodic system and are expected to share adsorptive and catalytic properties.
Regardless of the low abundance of Ir, it is considerably cheaper than Pt and Rh.
Although Ir was studied as an EOR catalyst earlier, its capability in splitting the
C–C bond was not investigated [108–110].
Reprinted from the journal
25