Rh多晶表面CO_2还原过程的电化学和原位FTIR反射光谱研究

Electrochemical and in situ FTIR Spectroscopic Studies of CO2 Reduction on Polycrystalline Rh Surface

应用程序电位扫描法和电化学原位ＦＴＩＲ反射光谱从定量角度在分子水平上研究了ＣＯ２在Ｒｈ电极上的电催化还原性能．红外光谱结果指出ＣＯ２还原的吸附产物为线型和桥式吸附态ＣＯ物种．在所研究的还原电位范围（－０．１５～－０．４０Ｖ）和相同还原时间，ＣＯ２还原吸附物种的氧化电量随还原电位的负移而增大，在每个还原电位下，时间超过２５０ｓ时都可达到一个相应的饱和值．原位红外光谱和电化学研究结果均表明，ＣＯ２的还原与Ｒｈ电极表面氢吸附反应密切相关，同时需要一定数量相邻表面位的参与．因此生成的ＣＯ不能在Ｒｈ电极表面达到满单层吸附。

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 (rCO2) have been determined by in situ FTIR as bridge(COB) and linear(COL) bonded CO, which yield IR absorption bands respectively around 1 905 and 2 020 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 rCO2 occurred in a current peak at about 036 V, from which the charge of rCO2 oxidation(Qox) has been measured quantitatively. It has been revealed that the Qox varies with the potential(Er) and the time(tr) applied for CO2 reduction. At a given tr, Qox increases along with the decrease of Er from -015 V to -040 V. At each Er, Qox reaches its saturation value (Qsox) when tr is longer than 250 s. In comparison with the oxidation charge(498 Ccm-2) for a saturation adsorption of CO on Rh electrode, the small value of Qsox(e. g., 270 Ccm-2 even for Er at -040 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 rCO2. In considering that the number of surface site occupied by bridge and linear bonded CO is 2 and 1 respectively, the surface site occupancy by rCO2 has been evaluated at only 73% for CO2 reduction at -025 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 rCO2 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.