TiO_2纳米管电极上电化学还原CO_2生成CH_3OH

采用原位阳极氧化-煅烧法制备TiO_2纳米管(TiO_2NTs)电极,运用X射线衍射(XRD)、电场发射扫描电子显微镜(FESEM)、X射线光电子能谱(XPS)、双电位阶跃测试等对制备电极进行表征,考察了其在0.1mol?L~(-1) KHCO_3水溶液中电化学还原CO_2的催化活性。结果表明TiO_2NTs电极上电化学还原CO_2的主产物为CH_3OH,CH_3OH由HCOOH和HCHO进一步还原而来。电极制备的最佳煅烧温度为450℃(TiO_2NTs-450),电解电位-0.56 V(vs RHE(可逆氢电极))时反应120 min后,生成CH_3OH的法拉第效率和分电流密度分别为85.8%和0.2 m A?cm~(-2)。与550和650℃煅烧的电极相比,TiO_2NTs-450电极具有更高的催化活性,归因于电极表面更多的三价钛活性位,有利于CO_2吸附,从而对·CO_2-起到稳定的作用,速率控制步骤转变为·CO_2-的质子化反应。

Electrochemical Reduction of CO2 to Methanol at TiO2 Nanotube Electrodes

A series of highly ordered TiO₂ nanotube (TiO₂NTs) electrodes are prepared via potentiostatic anodization of Ti foil followed by calcining in air. X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), X-ray photoelectron spectroscopy (XPS), and potential steps determination are used to characterize the electrodes. The electrochemical reduction of CO₂ on these three TiO₂NTs electrodes is investigated by cyclic voltammetry and potentiostatic electrolysis in 0.1 mol·L^-1 KHCO₃ aqueous solution. Methanol is found to be the major product in electrochemical CO₂ reduction, while formic acid, formaldehyde, methane, and CO are formed as minor products. Compared with the electrodes sintered at 550 and 650℃, the optimal TiO₂NTs electrode is found to be the one calcined at 450℃ (TiO₂NTs-450). After 120 min of reaction, the Faradaic efficiency and partial current density of methanol is 85.8% and 0.2 mA·cm^-2 at -0.56 V vs. reversible hydrogen electrode (RHE), respectively. The trivalent titanium in TiO₂ serves as an efficient site for adsorption of CO₂ and stabilization of the adsorbed ·CO₂^- radical. Consequently, the reduction of CO₂ on TiO₂NTs electrodes involves a fast first electron and proton transfer followed by a slow second proton transfer as the rate-limiting step.