Characterization of the Rh(111) electrode by CEELS, AES, LEED, and voltammetry. Adsorption of (Bi)sulfate, perchlorate, and carbon monoxide

We studied the Rh(111) electrode in sulfuric and perchloric acid solutions, and in sulfuric acid solutions containing dissolved carbon monoxide, using voltammetry, core electron energy loss spectroscopy (CEELS), Auger electron spectroscopy (AES), and low-energy electron diffraction (LEED). Emersion of the electrode from the clean H2SO4 solution to ultrahigh vacuum (UHV) produces a stable (root 3 x root 3)R 30 degrees (bi)sulfate surface structure. The (bi)sulfate coverage is close to 0.33 ML, the valency of (bi)sulfate sulfur is +6, and the oxygen-to-sulfur ratio is 4, the proper value for the (bi)sulfate adsorbate. When (bi)sulfate is adsorbed from mixed sulfuric/perchloric acid solutions, the (bi)sulfate coverage is lower, namely 0.24 ML. This and some other measurements show that adsorption of perchlorate on rhodium is strong and successfully competes with (bi)sulfate adsorption. CO replaces (bi)sulfate adsorbate irreversibly. In agreement with recent STM investigations, we observe that the main CO structure is (2 x 2). However, depending on experimental conditions, a split (2 x 2) also appears, a LEED analogue of STM's Rh(111)(3 x root 3). The (2 x 2)-CO surface structure corresponds to Theta = 0.75 +/- 0.03 ML, while the split (2 x 2) structure corresponds to Theta = 0.65 +/- 0.08 ML. AES shows that at a low potential the linear-bonded CO molecules from the (2 x 2)-CO surface structure is substituted in part by the bridge-bonded CO, which increases back-donation of metal d-electrons to the CO. The stronger back-bonding increases the electron screening of the orbital (core) hole, leading to a more fully relaxed final state of the Rh(111)-CO system.