CO2+O on Ag(110):: Stoichiometry of carbonate formation, reactivity of carbonate with CO, and reconstruction-stabilized chemisorption of CO2

Surface carbonate species CO3(a) forms when exposing oxygen-covered Ag(110) surface to CO2. 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 CO2 state. The reactivity of carbonate is as high as that of atomic oxygen under the same conditions. The chemisorbed CO2 desorbs at 485 K, distinct from physisorbed CO2 which desorbs at 130 K. The (1 x 2) reconstruction induced by carbonate formation appears essential to CO2 chemisorption. Direct CO2 exposure at 300 K on Ag(110)(1 x 2) partially covered by chemisorbed CO2 leads to additional CO2 adsorption, whereas on clean Ag(110)-(1 x 1) no uptake is observed at this temperature. As a result of these reactions, background CO inherent in ultrahigh vacuum converts carbonate into chemisorbed CO2 within a few minutes in UHV or with large CO2 exposures (> 30 L). STM images we have acquired of pure chemisorbed CO2 on Ag(110)(1 x 2) bear strong resemblance to those previously assigned to carbonate by others; we reinterpret these STM features as due to chemisorbed CO2. Mobility and disorder of chemisorbed CO2 are also observed. At a sufficiently long time or large CO exposures chemisorbed CO2 is depleted from the surface, which reverts to clean Ag(110)-(l x 1).