Pt monolayers

Zhang J, Mo Y, Vukmirovic MB, Klie R, Sasaki K, Adzic RR. 2004. Platinum monolayer electrocatalysts for O2 reduction Pt monolayer on Pd(lll) and on carbon-supported Pd nanoparticles. J Phys Chem B 108 10955-10964. 

Zhang J, Vukmirovic MB, Sasaki K, Nilekar AU, Mavrikakis M, Adzic RR. 2005a. Mixed-metal Pt monolayer electrocatalysts for enhanced oxygen reduction kinetics. J Am Chem Soc 127 12480-12481. 

The experiments showed that the properties of the Pt monolayer were modified differently by the different supporting metal (Fig. 9.13). PtML/Pd(lll) and PIml/ Ru(OOOl) are the most and least active of these surfaces, respectively. When the ORR kinetic currents obtained from Koutecky-Levich plots are plotted against the... 

Figure 9.13 Polarization curves for ORR on Pt monolayers supported on Ru(OOOl), Ir(lll), Rh(lll), Au(lll), and Pd(lll) in a 0.1 M HCIO4 solution on a disk electrode. The curve for Pt(lll) was obtained from [Markovic etal., 1999] and is included for comparison. The rotation rate was 1600rev/min and the sweep rate was 20mV/s (50mV/s for Pt(lll)). Key 1, PtMi,/ Ru(OOOl) 2, PtML/fr(lll) 3, PtML/Rh(lH) 4, PtML/Au(lll) 5, Pt(lll) 6, PtML/Pd(lH). (Reproduced with permission from Zhang et al. [2005a].)...

Similarly to the Pt monolayer catalysts, a series of Pd monolayers deposited on different metal single crystals were tested for ORR activity. The results are shown in Fig. 9.22. The ORR activity increases in the order PdML/Ru(0001) < PdML/fr(lll) < PdMi,/ Rh(lll)[Pg.299]

Pd ternary alloys, including Pd-Co-Au [Fernandez et al., 2005a, b] and Pd-Co-Mo [Raghuveer et al., 2005] have been developed to further improve the stability of the catalyst. The addition of 10% Au to the Pd-Mo mixture improved catalyst stability. Another promising way to improve the activity and durability of Pd-M alloys is to deposit a Pt monolayer on them. Recently, a Pt monolayer deposited on PdsFe/C was found to possess higher activity than that of Pt/C [Shao et al., 2007b]. 

Inoue H, Brankovic SR, Wang JX, Adzic RR. 2002. Oxygen reduction on bare and Pt monolayer-modified Ru(OOOl), Ru(lOl-O) and Ru nanostructured surfaces. Electrochim Acta 47 3777-3785. 

Shao M, Sasaki K, Liu P, Adzic RR. 2007b. Pd3pe and Pt monolayer-modified Pd3pe. Z Phys Chem221 1175-1190. 

In this reaction scheme, hydrogen adsorption (on an empty site ) can occur only after OH removal. Since the adsorption energy of hydrogen is gained only in the second step, this may shift the onset of the reaction to more cathodic potentials. We wUI see later that on a surface covered by Pt monolayer islands that allow an easy formation of Had (see below), a homolytic reaction according to... 

The onset of H pd OHad exchange at 0.1 V on the Pt monolayer island-modified Ru(0001) surface in either scan direction is the basis for our estimate that the thermodynamic equilibrium potential for H pd OHad exchange on Ru(OOOl) is at about this value (see the preceding section). Therefore, at a potential of 0.1 V, Hupd and OHad are about equally stable. With increasing potential, they become more weakly... 

At higher potentials, positive of the Hupd OHad exchange, the C Vs of the Pt island-modified Ru(OOOl) surface closely resemble those of the ft-fiee Ru(OOOl) electrode, except for the lower currents/charges in the characteristic features. This simply reflects the fact that at these potentials, the surface reactivity is dominated by the electrochemical properties of the remaining exposed Ru surface. As already mentioned, the Pt monolayer islands themselves contribute only little to the voltammetric behavior, which is due to the weak bonding and hence low adsorbate coverages on these islands. 

It is important to note that reaction of COad occurs only at sufficiently high coverages, equivalent to a reduced reaction barrier (see the discussion of CO oxidation on Ru(0001) above). The high coverage is maintained by continuous OHad formation, in competition with re-adsorption of CO. The Pt islands help in maintaining the high coverage via (14.8). Finally, additional CO adsorption on the Pt monolayer islands and reaction with OHad on the Ru(0001) areas may be possible as well, and this would further increase the overall reaction rate. At these potentials, however,... 

Because of the lower adsorption energies on the Pt monolayer islands, the steady-state coverage on the islands is relatively low, and surface blocking plays no role. 

The much steeper increase of the current with potential in Fig. 14.12 after the initial slow increase, as compared with bare or Pt monolayer island-modified Ru(0001), is attributed to an increased abundance of OHad/Oad species with increasing potential. In agreement with our previous arguments, this can be rationalized by the weaker adsorption of these species on the mixed Ru3 Pt sites, while on Ru(OOOl) the... 

Pseudomorphic Pt monolayers on Ru(0001) interact very weakly with H pd, OHad, or Oad, because of electronic ligand (vertical ligand effects) and strain effects (tensile strain), in agreement with results obtained under UHV conditions and in DPT calculations. Therefore, base CVs on these surfaces do not show pronounced voltammettic features. 

Ru(OOOl) surfaces partly covered by Pt monolayer islands exhibit very interesting catalytic effects. In reactions such as Hupd Oad/OHad exchange or CO... 

For the same reason, Ru(OOOl) modihcation by Pt monolayer islands results in a pronounced promotion of the CO oxidation reaction at potentials above 0.55 V, which on unmodified Ru(OOOl) electrodes proceeds only with very low reaction rates. The onset potential for the CO oxidation reaction, however, is not measurably affected by the presence of the Pt islands, indicating that they do not modify the inherent reactivity of the O/OH adlayer on the Ru sites adjacent to the Pt islands. At potentials between the onset potential and a bending point in the j-E curves, COad oxidation proceeds mainly by dissociative H2O formation/ OHad formation at the interface between the Ru(OOOl) substrate and Pt islands, and subsequent reaction between OHad and COad- The Pt islands promote homo-lytic H2O dissociation, and thus accelerate the reaction. At potentials anodic of the bending point, where the current increases steeply, H2O adsorption/OHad formation and COad oxidation are proposed to proceed on the Pt monolayer islands. The lower onset potential for CO oxidation in the presence of second-layer Pt islands compared with monolayer island-modified Ru(OOOl) is assigned to the stronger bonding of a double-layer Pt film (more facile OHad formation). 

Hosier H, Richter B, Behm RJ. 2004. Catalytic influence of Pt monolayer islands on the hydrogen electrochemistry of Ru(OOOl) studied by ultrahigh vacuum scanning tunneling microscopy and cychc voltammetry. J Phys Chem B 108 14780. 

FIGURE 2.1. Orbital-projected DOS plots for the surface Pt atoms of the Pt/Ox-alumina system. The shaded regions represent die DOS of the Pt (111) surface, the soUd (—) Unes are for the Pt monolayer, and the dashed (- -) lines are for the bilayer. 

Such anion adsorption can be prevented by chemisorbing a mono-layer of a strongly adherent thiol molecule to the Au surfaces [97,98]. 1-Propanethiol (PT) was used here because the gold nanotubules can still be wetted with water after chemisorption of the PT monolayer [97].t The Em versus applied potential curves for an untreated and PT-treated gold nanotubule membrane, with KBr solutions present on either side of the membrane, are shown in Fig. 13. The untreated membrane shows only cation permselectivity, but the permselectivity of the PT-treated membrane can be switched, exactly as was the case with the nonadsorbing electrolyte (Fig. 12). 

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