Environmental Engineering Reference
In-Depth Information
[Watanabe and Motoo, 1975a, b; Maillard et al., 2005]. However, a complete
picture of Pt/Ru catalytic function, including the CO removal process, is only now
emerging. In particular, the correlation between CO stripping voltammetry (reactivity)
and spectra—IR (see above) and/or nuclear magnetic resonance (NMR)—is key to
understanding the methanol (CO) oxidation process. It should be mentioned that the
spectroscopic and voltammetric measurements are basically different: IR spectro-
scopy and/or NMR [Tong et al., 2002] probe two different CO populations on the
heterogeneous Ru-island decorated Pt electrodes, but voltammetry is a probe of reac-
tivity of CO oxidation: the cyclic voltammogram split can only be shown when the
oxidizing CO is confined within the island borders [Tong et al., 2002].
The data in Figs. 12.11a, 12.19, and 12.20 indicate that while the chemisorbed CO
disappears from both Ru and Pt sites of Pt(111) and Pt(111)/Ru surfaces at suffi-
ciently positive potentials, the stability of bridge-bonded compared with atop CO is
greater on P(111)/Ru (Fig. 12.20) than on pure Pt(111). The degree of stability of
bridge-bonded CO (Fig. 12.20) is unexpected when compared with previous findings
with clean Pt(111) [Friedrich et al., 1966, 1996, 2002], and, as noticed above, brings
the electrochemical and infrared data to coherence. However, the voltammetric split
can only be observed in a narrow Ru coverage range of Ru on Pt(111), not higher
than 0.25 ML [Lu et al., 2002], while IR and NMR [Tong et al., 2002] mapping of
independent Pt(111)-CO and Pt(111)/Ru-CO populations continues (Fig. 12.21;
notice the background occurring from nonresonant SFG [Shen 1989]). At the same
time, at an Ru coverage of
0.25 ML, the voltammetric split of the CO stripping
from Pt(111)/Ru disappears [Lu et al., 2002].
The intriguing observation that voltammetric CO stripping disappears at a certain
Ru coverage, but the spectra from Ru-CO and Pt-CO patches are still measured at high
Ru coverage, will now be considered. It can be seen that three conditions must be met
simultaneously to obtain avoltammetric split of the type in Fig. 12.18:
†
Ru atoms need to be physically present as distinctive Ru (nano)islands on the Pt
surface rather than as an intermixture with Pt surface atoms of the Pt/Ru alloy
[Ianniello et al., 1994; Crown et al., 2000].
†
The Ru (and surrounding Pt) nanoislands need to develop independent CO
populations—a condition realized by solution CO dosing to high CO coverage
on both Pt and Ru of the Pt/Ru surface.
†
The island Pt/Ru edge needs to have some unique chemical properties compared
with the surrounding two-dimensional Ru and Pt phases [Lu et al., 2002].
Apparently, the two first conditions are always or easily met on heterogeneous
[Koper et al., 1999] Ru-island covered Pt surfaces. In contrast, the third condition is
fulfilled rarely, and, on Pt(111)/Ru, is lost at Ru coverages higher than about
0.25 ML [Lu et al., 2002]. The physical property of the Ru deposit above 0.25 ML
on Pt(111) that changes with Ru coverage is the transformation of the Ru islands
from essentially monoatomic to multilayer adlattices [Crown et al., 2002]. At the
same time, with increasing Ru coverage, both Ru and Pt surface atoms become less
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