Pt d-band center

The activity for the ORR of Pt monolayers, deposited on different single-crystal surfaces, using the Cu UPD technique [14], were investigated in acid and in alkaline electrolytes [7, 8]. Figure 5.1a show the typical ORR curves obtained for pure Pt/C and Pt monolayer on Pd/C nanoparticles, and Fig. 5.1b shows the plot of ORR activity versus Pt d-band center on different surfaces [7]. As can be seen, the ft monolayer electrocatalysts exhibited support-induced tunable activity by arising either by structural and/or electronic effects. It can be observed that the most active of all surfaces is PtML/Pd(lll), and the least active is PtML/Ru(0001). The plots of the kinetic current on the platinum monolayers on various substrates at 0.8 V as a function of the calculated d-hznA center, e, generated a volcano-like curve, with PtML/Pd(lll) showing the maximum activity (Fig. 5.1a). 

For further fine-tuning of the monolayer Pt/Pd ORR activity, they further introduced mixed metal + Pt monolayer catalysts [115], which contained 0.2 monolayer of a foreign metal from selection of (Au, Pd, Rh, Ir, Ru, Os, and Re) combined with 0.8 monolayer of Pt co-deposited on Pd(lll) or on Pd/C nanoparticles. The foreign metals have either a weaker M-OH btmd (for the case of Au-OH), or a stronger M-OH bond (for the rest of the cases) than the Pt-OH bond. DFT calculations [115] showed that, in addition to altering the Pt d-band center energies, the OH( m) OH( pq (or 0( m>—OH( pt)) repulsion plays an important role in augmenting the ORR activity, as shown below in Fig. 10.9. 

Figure 9.12 Binding energies of O versus d-band center (relative to the Fermi level, - sp) of the Pt and Pt3Co alloy surfaces. Labels identify the adsorption sites. The line is the best hnear fit. (Reproduced with permission from Xu et al. [2004].)...

Figure 9.14 Kinetic current density (squares) at 0.8 V for O2 reduction on the Pt monolayer deposited on various metal single-crystal surfaces in a 0.1 M HCIO4 solution, and calculated binding energies (circles) of atomic oxygen (BEq), as a function of calculated d-band center (relative to the Fermi level, ej — sp) of the respective surfaces. The data for Pt(lll) were obtained from [Markovic et al., 1999] and are included for comparison. Key 1, PIml/ Ru(OOOl) 2, PtML/Ir(lll) 3, PtML/Rh(lH) 4, PtML/Au(lll) 5, Pt(lll) 6, PIml/ Pd(lll). (Reproduced with permission from Zhang et al. [2005a].)...

For one kind of transition metal atoms, the d band center can be varied by changing the structure. As mentioned above, the band width depends on the coordination number of the metal and this leads to substantial variations in the d band centers [19]. Atoms in the most close-packed (111) surface of Pt have a coordination number of 9. For the more open (100) surface it is 8 and for a step or for the (110)... 

Pt surfaces tend to restructure into overlayers with an even higher density of Pt atoms than the close-packed (111) surface [21]. The Pt atoms are closer to each other on the reconstructed surfaces than in the (111) surface. The overlap matrix elements and hence the bandwidth are therefore larger, the d bands are lower and consequently these reconstructed surfaces bind CO even weaker than the (111) surface. The reconstructed Pt surfaces are examples of strained overlayers. The effect of strain can be studied theoretically by simply straining a slab. Examples of continuous changes in the d band center and in the stability of adsorbed CO due to strain are included in Figure 4.10. The effect due to variations in the number of layers of a thin film of one metal on another can also be described in the d band model [22,23]. 

The importance of the Pt 5d states for the bonding of CHX fragments is in agreement with results reported by Norskov et at4. They show that the adsorption energy correlates with the energy of the d-band center across a transition metal series. The diffuse metal sp states effectively are the same across a transition metal series, and only changes in the d-band are reflected in the adsorption energies. 

Structural Characteristics of the Adsorption of Oxygenated Species on Pt, PtsCo, PtsNi, PtsV, and Pt4.3Rh. Atomic Oxygen is Adsorbed on fee Hollow Sites while Hydroxyl and OOH Adsorb on Top Sites. Interactions with either Co, Ni, V or Rh are Indicated in Bold Font. Distances are in A. The d-Band Center is an Average for all the Atoms in the Slab and is Referred to the Fermi Level. 

Stamenkovic et al. used the adsorption energy of O on alloy surfaces as a catalytic activity indicator. They use this approach to develop a volcano model of activity for the oxygen reduction reactivity and compare their computed predictions of activity to experimental results. They showed that Pt-3d alloy surfaces are near the top of the volcano, but more significantly identified the chemical descriptor (d-band center or O adsorption energy) that could be used to identify new alloy surfaces that could have higher activity. 

Fig. 3.1 UHV characterization of Pt Fe alloy (a) LEIS spectra during UHV treatment annealed and sputtered surfaces, (b) LEIS spectra red) after electrochemical treatment reveals dissolution of Fe atoms from the surface and formation of Pt-skeleton surface (c) UPS spectra (d) The d-band center position relative to the Fermi level from the valence band spectra measurements on PtjM alloy surfaces (red) and calculated values of d-band center position (blue), Schematic ball models for the sputtered surface and Pt-skin are included to help visuahze the structure and composition of Pt and M atoms in the near-surface regions. Reprinted with permission from [29], copyright 2007 Nature Pubhshing Group...

Fig. 3.2 Characterization of Ptj Ni(hkr) surfaces with (a) Auger Electron Spectroscopy (AES) (b) LEIS (revealed complete segregation of Pt for aU three orientations and the formation of the Pt-skin structure (c) UPS (confirmed that the position of d-band center is structure sensitive (d-f) LEED the Pt3Ni(l 11) surface exhibits a (1 X 1) pattern, Pt3Ni(100) has a (1 x 5) reconstruct pattern, and Pt3Ni(110) exhibits (1 x 2) periodicity. Reprinted with permission from [23], copyright 2007 by American Association for the Advancement of Science...

In the case of Pt3Ni(M0 surfaces UPS results (Fig. 3.2) show that the d-band density of states (DOS) shifts from -2.70 eV on Pt3Ni(110) to -3.10 eV on Pt3Ni(lll) to -3.14 eV on Pt3Ni(100). Furthermore, the DOS of the alloy surfaces are quite different from corresponding pure Pt single crystals the d-band center is downshifted by approximately 0.16, 0.24, and 0.33 eV, respectively. We have had six different single-crystal systems that could be studied specifically for the elusive electronic effect in electrocatalysis. 

In contrast to sputtered surfaces, UHV-annealed PtjM surfaces are rather stable in electrochemical experiments, implying that Pt-skin surfaces are structurally and compositionally identical in both UHV and electrochemical environments. Although the composition of the top most layer is the same (pure Pt) for both structures [21], there are three key differences between the Pt-skin and Pt-skeleton surfaces (1) electronic properties are different for the two surfaces i.e., for each PtjM alloy the corresponding Pt-skin surface has a larger d-band center shift from the Fermi level than the sputtered surface (2) Pt-skeleton is morphologically different than the Pt-skin i.e., the Pt surface atoms have a lower average coordination number and (3) the composition of the 3d element in the second layer of the skeleton surfaces... 

Figure 3.6 demonstrates on both the Pt-skin as well as Pt-skeleton surfaces the relationship between the specific activity and the d-band center position exhibits a volcano shape, with the maximum catalytic activity obtained for PtjCo. This behavior is apparently a consequence of the Sabatier principle discussed earlier, and published in many recent studies [77, 78]. For metal surfaces that bind oxygen too strongly, as in the case of Pt, the d-band center is too close to the Fermi level and the rate of the ORR is limited by the availability of spectator-free Pt sites. 

Fig. 3.6 Relationships between the catalytic properties and electronic structure of PtjM alloys [29, 31] (a-b) Specific activity (at 0.9 V) for the ORR on Pt M surfaces in 0.1 M HCIO at 333 K vs. experimentally measured d-band center positions for the Pt-skin and Pt-skeleton surfaces (c) The activity calculated from DFT vs. calculated d-band center for the Pt-skin slabs, (a-b) Reprinted from [29], Fig. 4...

If the d-band center is too far from the Fermi level, as in the case of PtjV and PtjTi, the intermediates and bind too weakly to the surface. For the PtjM systems, the experimental results (summarized in Fig. 3.6a) and componential screening of the same binary alloys (Fig. 3.6c) converge to the same optimal Pt to 3d ratio (e.g., PtjM) and the identity of the 3d element (e.g., Co or Ni). While it is tempting to conclude that the rationale for the variation in activity depends exclusively on the position of the metal d-states relative to the Fermi level. Fig. 3.6 clearly shows that for a similar position of the d-band center, a different activity is obtained on Pt-skin and Pt-skeleton surfaces. The relationships between the elec-trocatalytic activity of the ORR and the d-band center position were also recently demonstrated by Adzic and Mavrikakis [77, 78]. 

Now we come to the chemical consequences of the nanostructure peculiarities discussed above in relation to the catalytic properties of nanoparticles as presented in Section 17.1. It was recently suggested that a key parameter of the chemical activity of a transition metal surface was the mean valence d states relative position with respect to the molecular orbital involved in the molecule-surface interaction [98]. The higher the d-band center, the stronger the interaction with the adsorbed molecule. Moreover, it was shown that this parameter is directly related to the activation energy of the CH3-H bond breaking on various Ni-based surfaces [99]. As could be expected, the stepped Ni(211) surfaces exhibited the lowest activation energy. We note that step sites of the fee (211) surface are of the B5 type, as discussed in Section 17.1 in the case of Pt nanoparticles. The same trend was recently evidenced on localized defects on the Ni(lll) surface [lOOj. 

Fig. 17.6 Illustration of the isomerization mechanism dependence on the Pt particle s d-band center. The origin of the energy axis is the Fermi level.

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