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

Abstract

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 H{sub 2}SO{sub 4} solution to ultrahigh vacuum (UHV) produces a stable ({radical}3 x {radical}3)R 30{degree} (bi)sulfate surface structure. 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 {radical}3). 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. 58 refs., 10 figs., 2 tabs.