Electrochemical and in situ FTIR spectroscopic studies of CO2 reduction on polycrystalline Rh surface
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The reduction of carbon dioxide on polycrystalline Rh electrode is studied by using programmed potential sweep method and in situ FTIR spectroscopy. Emphases are laid on the study of surface processes involved in the reduction. The adsorbed species derived from CO2 reduction (r-CO2) have been determined by in situ FTIR as bridge(COB) and linear(COL) bonded CO2 which yield IR absorption bands respectively around 1905 and 2020 cm(-1). The onset potential of CO2 reduction has been determined at -0.05 V. The programmed potential sweep experiments demonstrated that the oxidation of r-CO2 occurred in a current peak at about 0.36 V, from which the charge of r-CO2 oxidation(Q(ox)) has been measured quantitatively. It has been revealed that the Q(ox) varies with the potential (E-r) and the time (t(r)) applied for CO2 reduction. At a given t(r), Q(ox) increases along with the decrease of E-r from -0.15 V to -0.40 V. At each E-r, Q(ox) reaches its saturation value (Q(ox)(s)) when t(r) is longer than 250 s. In comparison with the oxidation charge(498 mu C . cm(-2)) for a saturation adsorption of CO on Rh electrode, the small value of Q(ox)(s) (e. g., 270 mu C . Cm-2 even for E-r at -0.40 V) indicates that the quantity of adsorbed CO species produced in CO2 reduction is far from that of a monolayer coverage. The ratio of the intensity of IR band of bridge bonded CO to that of linear bonded CO is served to figure out the surface site occupancy by r-CO2. In considering that the number of surface site occupied by bridge and linear bonded CO is 2 and 1 respectively t the surface site occupancy by r-CO2 has been evaluated at only 73% for CO2 reduction at -0.25 V for 600 s. It has been demonstrated that the subsequent adsorption of CO on the 27% vacancy surface sites yields mainly linear bonded CO species, implying that the reduction of a CO2 molecule may need the assistance of a few adjacent surface sites. The in situ FTIR results also confirmed that the submonolayer of r-CO2 is in a uniform distribution over Rh electrode surface. Finally, a reduction mechanism of CO2 on Rh electrode has been proposed based on results of both programmed potential sweep method and in situ FTIR spectroscopy, in which the hydrogen adsorption is considered as an important step assisting the reduction.