Surface bridging bonded

The representation of molecular properties on molecular surfaces is only possible with values based on scalar fields. If vector fields, such as the electric fields of molecules, or potential directions of hydrogen bridge bonding, need to be visualized, other methods of representation must be applied. Generally, directed properties are displayed by spatially oriented cones or by field lines. 

Figure 5 shows the utility of HREELS in establishing the presence of both bridge-bonded and atop CO chemisorbed on Pt(l 11) and two SnPt alloy surfaces, and also serves to emphasize that HREELS is very useful in studies of metal alloys. The v o pc ks for CO bonded in bridge sites appear at 1865, 1790, and 1845 cm on the Pt(lll), (2 x 2) and 3 surfaces, respectively. The VCO peaks for CO... 

Studies of the bonding of carbon monoxide to the metal surfaces produced structures in which the carbon atom is linked to one, two, or three metal atoms. The existence of bonds to two or three atoms (bridged bonds) has been questioned on the basis of theoretical calculations. None of these bondings, however, clarify the mechanism to any extent. 

Solla-Gullon et al. [Ill] carried out FT-IRs experiments of adsorbed CO for PdPt nanoparticles prepared by reduction of H2PtCl6 and K2PdCl4 with hydrazine in a w/o microemulsion of water/poly(ethyleneglycol) dodecyl ether (BRIJ(R)30)/ -heptane. The experiments gave information on the relative amount of linear- and bridge-bonded CO, which is known to depend on the surface distribution of the two elements. 

Figure 6.16 Attenuated total reflection surface enhanced infrared reflection absorption spectroscopy (ATR-SEIRAS) spectra for the oxidation of 0.1 M HCOOH in 0.5 M H2SO4 on a polycrystaUine electrode. The bands at 2055 -2075 and 1800-1850 cm are assigned to linear- and bridge-bonded CO, whereas the band at 1323 cm corresponds to adsorbed formate. (Reproduced from Samjeske et al. [2006].)...

In order to reconcile the inconsistency, we analyzed the spectral disappearance of all CO surface forms—atop, bridge, and 3-fold—as a function of electrode potential (Fig. 12.20). While the spectra are noisy, the bridge-bonded CO survives at the Pt... 

Quite complex kinetic behavior has been identified on some surfaces. For instance, on Ir(100), the TPD data from NO-saturated surfaces display two N2 desorption peaks, one at 346 K from the decomposition of bridge-bonded NO, and a second at 475 K from the decomposition of atop-bonded NO molecules [13], Interestingly, the first feature is quite narrow, indicating an autocatalytic process for which the parallel formation of N20 appears to be the crucial step. An additional complication arises from the fact that this Ir(100) surface undergoes a (1x5) reconstruction, and that NO adsorbed on the metastable unreconstructed (lxl) phase leads to N2 desorption at lower temperatures. In another example, on the reconstructed hexagonal Pt(100) surface, when a mixed NO + CO adsorbed layer is heated, a so-called surface explosion is observed where the reaction products (N2, C02 and N20) desorb simultaneously in the form of sharp peaks with half-widths of only 7 to 20 K. The shape of the TPD spectra suggests again an autocatalytic mechanism [14],... 

In principle, electron-withdrawing isocyanides on metal surfaces are more likely to favor the bridge-bonding mode. Since the migration of a terminal isocyanide to a bridging position formally results in the oxidation of the metal (M) atoms, easily-oxidized metals should favor the bridge-bonding adsorption mode. 

Oxygen binding to a metal can be represented by models sketched on Scheme 5, and it is understandable that the two atoms of the oxygen molecule linked to two metal atoms of the surface (bridge) is the most favorable position for weakening the 0—0 bond, leading to its dissociation. Recent ab initio calculations (DFT) have shown that the (111), (100), and (110) surfaces of Pt have different properties for the adsorption of O2, O, OH, OOH, and H2O2, which should influence differently the mechanism of ORR [39]. 

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