温度对乙醇电催化氧化的影响

升高温度可以提高反应速率和增加物质的输运,因此通过不同温度下反应机理的研究可以深入理解电催化过程,对催化剂的设计具有指导意义。本工作初步建立了变温原位红外测定方法。采用温控电极,用电势测温法进行温度的校准,实验得出控温仪器加热温度Th与电极表面温度TS的关系为T_S=0.57T_h+7.71 (30°C T_h≤50°C);T_S=0.62T_h+5.12 (50°C T_h≤80°C),误差分析最大温差为1°C。利用该方法我们研究了商业Pt/C催化剂在不同温度下乙醇的电氧化过程。从循环伏安图可以明显看到随着温度的升高整体氧化电流增大,起始电位、峰电位均负移,说明热活化使得氧化反应更容易进行;第一个峰电流与第二个峰电流的比值用于定性评估CO_2的选择性,对比25°C,商业Pt/C催化剂在65°C下第一峰提高30%,说明高温有利于C―C键的断裂。对比25°C的原位红外谱图,我们发现35°C及50°C下商业Pt/C催化剂上CO_2产物的起始电位负移200 m V,说明高温下,Pt/C催化剂能在更低的电位提供含氧物种;而CH_3CHO、CH_3COOH起始电位不随温度变化。用CO_2与CH_3COOH的积分面积比来评估CO_2选择性,发现高温低电位其选择性最高,说明高温低电位有利于乙醇完全氧化生成CO_2,而高温高电位下表面吸附含氧物种占据了活性位,阻碍C―C键断裂。

Effect of Temperature on the Electrocatalytic Oxidation of Ethanol

The electrocatalytic activity of commercial Pt/C for ethanol oxidation is relatively low, and the C―C bond is difficult to break. Thus, the complete oxidation process is not easy, and the fuel utilization efficiency becomes considerably reduced. Increasing the temperature can increase the reaction rate and enhance the mass transport; therefore, a temperature-controlled electrode was used during our in situ FTIRS (Fourier Transform Infrared Spectroscopy) investigation. The temperature sensor was placed at a certain distance from the surface of the electrode; thus, the surface temperature needed to be corrected. The temperature was calibrated using the "potentiometric" measurement method, which was because the potential-temperature coefficient of the redox couple is constant under certain conditions, and the electrode surface temperature was obtained by potential conversion at different temperatures during the experiment. The experimental results showed that the relationship between the heating temperature, T_h, and the surface temperature, T_S, was T_S = 0.57T_h + 7.71 (30 ℃ < T_h ≤ 50 ℃) and T_S = 0.62T_h + 5.12 (50 ℃ < T_h ≤ 80 ℃), and according to error analysis, the maximum error was 1 ℃. The temperature-controlled electrode was applied to investigate the electrooxidation of ethanol using both in situ FTIRS and cyclic voltammetry using a commercial Pt/C catalyst at different temperatures. Clearly, based on the CV curve for the oxidation of ethanol, with increasing temperature, the overall oxidation current increased, and the onset potential and peak potential both negatively shifted, indicating that thermal activation allows the oxidation reaction to proceed easier. Electrooxidation of ethanol showed two positive oxidation peaks, and the ratio of the first peak current to the second peak current was used to qualitatively evaluate the selectivity of CO₂. Compared with at 25 ℃, the first peak current increased by 30% at 65 ℃, indicating that the high temperature was conducive to C―C bond cleavage. Comparing the in situ FTIRS recorded at 50 ℃, 35 ℃, and 25 ℃, we found that the onset potential of CO₂ on the commercial Pt/C catalyst was lower by 200 mV, indicating that Pt/C can provide oxygen-containing species at lower potentials at high temperature; however, the onset potentials of CH₃CHO and CH₃COOH did not change with temperature. The CO₂ selectivity was semi-quantitatively calculated by the area of CO₂ compared with the area of CH₃COOH from the FTIRS data. It was found that CO₂ had the highest selectivity at high temperature and low potential, indicating that high temperature is conducive to complete ethanol oxidation during CO₂ formation, possibly because both the ethanol bridge adsorption pattern and adsorbed OH (OH_ad) increased with temperature, enhancing subsequent CO_ad and OH_ad oxidation reactions. The low selectivity of CO₂ at the high potential was due to the adsorption of oxygen-containing species that occupied the surface-active site, blocking the adsorption of ethanol.