Rh多晶表面 CO₂ 还原过程的电化学和原位FTIR反射光谱研究

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，而是形成均匀的亚单层分布。

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 CO2reduction (r-CO2) have been determined by in situ FTIR as bridge(COB) and linear(COL) bonded CO, which yield IRabsorption bands respectively around 1905 and 2020cm-1. The onset potential of CO2reduction has been determined at -0.05 V. The programmed potential sweep experiments demonstrated that the oxidation of r-CO2occurred in a current peak at about 0.36 V, from which the charge of r-CO2oxidation(Qox) has been measured quantitatively. It has been revealed that the Qoxvaries with the potential(Er) and the time(tr) applied for CO2reduction. At a given tr, Qoxincreases along with the decrease of Erfrom -0.15 V to -0.40 V. At each Er, Qoxreaches its saturation value (Qoxs) when tris longer than 250s. In comparison with the oxidation charge(498μCcm-2) for a saturation adsorption of CO on Rh electrode, the small value of Qsox(e.g., 270 Ccm-2even for Erat -0.40 V) indicates that the quantity of adsorbed CO species produced in CO2reduction is far from that of a monolayer coverage. The ratio of the intensity of IRband of bridge bonded COto 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, the surface site occupancy by r-CO2has been evaluated at only 73% for CO2reduction at -0.25V for 600 s. It has been demonstrated that the subsequent adsorption of COon the 27% vacancy surface sites yields mainly linear bonded COspecies, implying that the reduction of a CO2molecule may need the assistance of a few adjacent surface sites. The in situ FTIRresults also confirmed that the submonolayer of r-CO2is in a uniform distribution over Rh electrode surface. Finally, a reduction mechanism of CO2on 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.