Adsorption of bisulfate anion on the Au(111), Pt(111), and Rh(111) surfaces - A comparative study

We report on adsorption of bisulfate anion on the Bu(111), Pt(111), and Rh(111) electrodes in sulfuric acid media using electrochemistry, Auger electron spectroscopy, low energy electron diffraction and core-level electron energy loss spectroscopy, The key observations for the bisulfate adsorbate on all surfaces studied are: (i) the oxygen-to-sulfur ratio is 4, (ii) the S(LMM) Auger electron transitions and S2p energy loss spectra show signatures characteristic of the sulfate anion, (iii) the spectral data are typical of the S6+ valency in the adsorbate. These results indicate that no decomposition or dehydration of the adsorbed anion occur in UHV. On Pt(lll) and Rh(lll) the maximum anion coverage is very close to 1/3 of the monolayer (ML). This is in an excellent agreement with the Me(111)(root 3 x root 3)R30 degrees from electron diffraction. For Au(lll), the maximum coverage is 1/5 ML while the diffraction data indicate a diffuse AU(root 3 x root 3)R30 degrees structure. Whereas in the specific case of the Au(lll) electrode the low energy electron diffraction data provide only an indirect basis for structural assignment of the sulfate adsorbate, we may conclude from these data that the structure of bisulfate on gold is different from that on Pt(111) and Rh(lll). The core level energy loss spectra show that the electron density on surface sulfur is higher than in the salt, evidently due to backdonation of electrons from the substrate to the adsorbate. To complete the description of the anion-surface chemical bond requires the assessment of the electron donation contribution to the bond. This will be interrogated in a separate study.