Symmetry, sulfate adsorption

Bisulfate ions in solution belong to the C3V symmetry group. Adsorption through three oxygen atoms does not change the symmetry. But if the ion is one-or two-fold coordinated upon adsorption, a change to a symmetry should be observed. Table 5 indicates four IR-active bands for this symmetry of sulfate ions in solution. 

The possible symmetries for adsorbed sulfate are (for adsorption through one or three oxygen atoms) and 2 (for adsorption through two oxygen atoms). 

The distance between two adjacent Pt atoms, 2.77 A, fits well with the distance between two oxygen atoms in the sulfate ion, c. 2.5 A (value given for bisulfate in [36]). So, from geometric considerations, a symmetry is very probable. A 2 symmetry has been proposed for the adsorption of sulfate on well-ordered Pt(lOO) electrodes (see below) where the threefold coordination is not possible due to mismatch between surface structure and threefold coordination. Two well-separated strong bands (1200 and 1100 cm ) are observed, which account for the Qv symmetry. There is definitely a difference in the behavior of the adsorbate on both single... 

A comparison of the polarization curves (positive sweeps) of the ORR on Cu(lll) and Cu(OOl) at the same rotation rate (1600 rpm). Fig. 31, shows relatively small, for example, less than a factor of 2, differences in activity, but very different reaction pathways below —0.4 V. Since very little is known about the nature of adsorbed anions on Cu(hkl) in sulfuric acid solution, or how the adsorption sites might differ between the different surfaces, it is difEcult to rationalize the structural sensitivity of the ORR on Cu hkl). For the Cu(lll)-S04 ad interface, both Wilms and coworkers [102] and li and Nichols [103] found by STM that a complex incommensurate ordered structure of sulfate anions formed at the Cu(lll) surface throughout the —0.1 to —0.4 V potential region in H2SO4. This ordered structure does not appear to lie along any high symmetry directions of the Cu(lll) substrate but it... 

Stimming and his coworkers found by in situ STM that adsorbed sulfate ions on Pt(lll) form the same adlayer structure as that found on Au(lll) [34]. Ordered domains with X V7) symmetry appeared in the potential range between 0.5 and 0.7 V versus a reversible hydrogen electrode (RHE) in 0.05 M H2SO4. As shown in Fig. 3, only the (111) surface shows the characteristic butterfly peaks at potentials slightly negative than 0.5 V. Their STM observations confirmed that the butterfly peaks are formed because of the adsorption and desorption of sulfate ions as indicated by the CO replacement technique used by Clavilier as described above [14]. STM images obtained on Pt(lll) were interpreted in terms of the coadsorption of sulfate anions and water. 

The surface morphology of zirconia and sulfated zirconia were recently studied using NO as a surface probe. When NO was co-adsorbed with molecular oxygen onto activated Zr02, the characteristic EPR spectrum of the O2 radical was observed. More interestingly, Volodin et aV observed a triplet S = 1) spin state due to an NO NO dimer on sulfated zirconia. The species was characterised by the spin-Hamiltonian parameters of = 1-993,3 = 1.942, D = 195 G, and 0 G. At lower NO pressures, the EPR spectra of NO monomers was clearly seen. It was suggested that the symmetry and distance (5.2 A) between two adjacent Zr + ions, was suitable as an adsorption site for the two NO molecules. 

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