CO₂ + O on Ag(110):  Stoichiometry of Carbonate Formation, Reactivity of Carbonate with CO, and Reconstruction-Stabilized Chemisorption of CO₂

Surface carbonate species CO 3 (a) forms when exposing oxygen-covered Ag(110) surface to CO 2 . The stoichiometry of this reaction has been a subject of study for 20 years. Here, using a combined approach of TPRS (temperature programmed reaction spectroscopy), LEED (low energy electron diffraction), and STM (scanning tunneling microscopy), we establish the 1:1 stoichiometry, in contrast to conclusions reached by others. Further, we find that carbonate reacts readily with CO, resulting in a chemisorbed CO 2 state. The reactivity of carbonate is as high as that of atomic oxygen under the same conditions. The chemisorbed CO 2 desorbs at 485 K, distinct from physisorbed CO 2 which desorbs at 130 K. The (1 × 2) reconstruction induced by carbonate formation appears essential to CO 2 chemisorption. Direct CO 2 exposure at 300 K on Ag(110)−(1 × 2) partially covered by chemisorbed CO 2 leads to additional CO 2 adsorption, whereas on clean Ag(110)−(1 × 1) no uptake is observed at this temperature. As a result of these reactions, background CO inherent in ultrahigh vacuum converts carbonate into chemisorbed CO 2 within a few minutes in UHV or with large CO 2 exposures (>30 L). STM images we have acquired of pure chemisorbed CO 2 on Ag(110)−(1 × 2) bear strong resemblance to those previously assigned to carbonate by others; we reinterpret these STM features as due to chemisorbed CO 2 . Mobility and disorder of chemisorbed CO 2 are also observed. At a sufficiently long time or large CO exposures chemisorbed CO 2 is depleted from the surface, which reverts to clean Ag(110)−(1 × 1).