Metal coverage

Palladium clusters deposited on amorphous carbon have been studied by XPS and UPS [28] and both techniques show broadening of the d-band peak as cluster size increases. The d-threshold shifts towards Ep as cluster size increases. In UPS studies the d-emission of the single atom has its peak at 3.0 eV below Ep, whereas the d-threshold is 2eV below Ep. Palladium clusters evaporated onto Si02 have been studied by UPS [38]. At large coverages of the Pd metal evaporated (> 10 atoms/cm ), a high emission intensity at Ep excited with photons of 21.2 eV (He(I)) or 40.8 eV (He(II)) as excitation source, is observed. This feature is characteristic in the spectra from bulk Pd samples. At the lowest metal coverage (3 x 10 atoms/cm ),... 

While the above XPS results give the impression, that the electrochemical interface and the metal vacuum interface behave similarly, fundamental differences become evident when work function changes during metal deposition are considered. During metal deposition at the metal vacuum interface the work function of the sample surface usually shifts from that of the bare substrate to that of the bulk deposit. In the case of Cu deposition onto Pt(l 11) a work function reduction from 5.5 eV to 4.3 eV is observed during deposition of one monolayer of copper [96], Although a reduction of work function with UPD metal coverage is also observed at the electrochemical interface, the absolute values are totally different. For Ag deposition on Pt (see Fig. 31)... 

Thus the total gas response is a combination of the response at the metal-insulator interface (i), metal in contact with the insulator (m), and exposed insulator surface (s) [Figure 2.1(b)]. The constant y is a function of several parameters, for example metal coverage and temperature. 

Bulk Pt alloys for the electrooxidation of formic acid have been less frequently studied compared to underpotential deposition (upd) modified Pt surfaces. The Pt50Ru5o surface was again found to be one of the most active Pt-Ru surfaces. Underpotentially deposited metals, such as Bi, Se, Sb, were studied as reaction modifiers for Pt surfaces and provided significant electrocatalytic activity increases. Electronic factors (ligand effects) rather than bifunctional effects were held responsible for these activity modifications, because the metal coverages that caused the activity gains were extremely small. 

Activity and selectivity variations according to second metal coverage. 

Figure S. Change in the O(ls) spectra of Ag clusters exposed to 500 L 02 at 80K.The diameters of the clusters have been estimated from metal coverage. The lower binding energy peak at 531 eV corresponds to O while that at 533 eV arises due to molecular oxygen (reproduced with permission from ref. [26]).

Figure 13. C(ls) core-level spectra of CO adsorbed on Au particles supported on a ZnO substrate. The feature at 285 eV corresponds to molecuiarly absorbed CO. The diameters have been obtained from the metal coverages.

Core-level XP spectra of superlattices of Pt nanocrystals ( 5nm diameter) up to five layers deposited on an Au substrate are shown in Fig. 8.21. We see that the intensity of the Pt (4f) feature increases with the number of depositions, accompanied by a decrease in the Au (4f) intensity as the substrate becomes increasingly shadowed due to the limited escape depth of the photoelectrons. The intensities of the C (Is) and S (2p) levels of the dithiol (at 285.0 and 163.6 eV, respectively) also increase with the increasing number of depositions. A plot of the metal coverage versus the number of depositions gave a... 

In contrast, ethane hydrogenolysis, which is a structure sensitive reaction over bulk Ni, displayed marked structural effects on the Ni/W system (41). We have observed, as shown in Figure 5, that the specific rate, or rate per surface metal atom, but not the activation energy, is a strong function of metal coverage on the Ni/W(110) surface, suggesting that the critical... 

For the lower metal coverage considered ( 0.25 ML) (only reported for c-Zr02) we designed a 2x2 supercell, resulting in an intra-metal distance of 7.32 A (see Fig. 2). At this metal-metal distance there are neither Pd nor Pt interactions and the metal adlayer may be considered as composed of isolated atoms. As for = I (ML) three adsorption sites were investigated O, Zr and... 

The Pt-atom (represented by =0.25 ML) shows no difference in adsorption energy between the O, and the 0 - sites (Fig. 11), suggesting both that there will be high mobility of the Pt-atom on the Zr02 lll surface, and that the adsorption process will occur on both terrace and step sites. However, as the metal coverage increases tbe surface mobility decreases, and for ML, Pt favours the O., site by more than... 

Table 4 Surface relaxations in the outermost atomic layers of the (111) surface (for M on top of O, site), reported for two different metal coverages 6= and 0.25 ML, Surface displacements are calculated as the difference of the ideal (111) surface and the relaxed geometry of the Pd and Pt/Zr02 interfaces. Negative and positive values indicates inwardly and outwardly displacements, respectively. For 0=0.25 ML, O., denotes the surface ion to which an metal atom is bound, while Oj represent the non-bound surface oxygens equivalent notation for the other surface layers. Displacements are given in A.

Also for the lower metal coverage ( 0.25 ML), the surface relaxations are of the order of 0.1 A for the outermost surface oxygen ions (see Table 4). However, in the case when Pd or Pt is supported on top of the 1-fold oxygen site, Q the snrface oxygen ion to which the metal atom is adsorbed, is inwardly relaxed by 0.3 and 0.5 A for the Pd and Pt atoms, respectively. The general observation for 0.25 ML is that surface relaxations are smaller than those observed for the higher metal coverage. 

Table 7 Bond lengths optimised for Pd and Pt/Zr02 111 interfaces. The notations of the distances in the table are presented in the top figures for the three different adsorption sites discussed in the text. Two metal coverages are presented ( 1 and 0.25 ML) Bond lengths are given in A.

Erom analysis of the adsorption energies for different metal coverages on c-Zr02, we propose that the cluster growth for the Pt metal shows a considerable size dependence, which is not observed for Pd. Instead, the differences in AE js for Pd are... 

Copper and nickel were deposited from metal foil wrapped around a hot tungsten filament. Chromium was evaporated from a chrome plated tungsten wire. XPS measurements were made to determine the metal coverage as well as the electronic structure at the interface. The metal coverage was determined by substituting the experimentally measured areas under the XPS curves, core hole cross sections ( ), and electron mean free path in both the metal and the polymer and an instrument response... 

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