不同温度下乙醇电氧化过程的原位红外光谱研究

直接乙醇燃料电池因其优异的性能备受关注。乙醇的电催化氧化并非简单的燃烧,涉及多种催化反应过程。乙醇的C-C键断裂选择性低,以及乙醇氧化中间产物C1分子由于没有及时氧化离开催化剂表面而造成的催化剂中毒,是制约其应用的瓶颈问题。电化学原位红外光谱是在电化学反应的同时,原位采集反应物种特定官能团的振动信息,可在分子水平揭示反应过程,推测反应机理。不同温度条件下乙醇电氧化过程的研究,有助于合理的设计高性能乙醇燃料电池催化剂。选用高性能的PtRh/RGO催化剂,结合同位素示踪法和电化学原位红外光谱技术,研究了不同温度下乙醇的电氧化过程。循环伏安研究表明,乙醇电氧化性能及其C-C键断裂的程度为PtRh/RGO (45℃)>PtRh/RGO (25℃)>商业Pt/C。电化学原位红外光谱从分子水平跟踪了乙醇的电氧化过程,观察到随着电位的增加, CO2, CO,-CH3,-C-O特征峰的强度逐渐增加。CO2和CH3COOH分别归属于乙醇完全氧化和不完全氧化的终产物,因此红外光谱中两种物质特征峰积分面积的比值CO2]/CH3COOH]可做为CO2选择性的量度。用来定量标定CH3COOH的特征峰是位于1 280 cm-1的-C-O振动峰,但对于PtRh/RGO催化剂的红外光谱而言,它的乙酸特征峰振动峰位1 280 cm-1附近出现1 214 cm-1甲醇衍生物的振动峰,通过一种反射红外光谱与标样透射红外光谱差减扣除叠加峰方法,定量计算了叠加峰中1 280 cm-1特征峰的积分强度,从而计算出PtRh/RGO的CO2选择性。结果表明对比25℃时, 45℃下PtRh/RGO具有更高的选择性, 0.3 V时提高48.1%, 0.5和0.6 V时略有提高, 0.4 V时降低,这可能是乙醇中β-C和水中OH竞争吸附所致。在两种反应温度条件下, CO2选择性都在电位高于0.4 V时呈现下降趋势。为了进一步研究CO2来源于α-C或β-C的完全氧化,使用同位素标记的13CH312CH2OH做为探针分子,通过电化学原位红外光谱研究了25和45℃下PtRh/RGO电极上乙醇电氧化过程。结果表明,β-C完全氧化为CO2的起始电位与温度无关,都为0.3 V。通过用13CO2/12CO2积分面积的比值定量分析,发现45℃下,该比值在电位0.3~0.5 V时相比于25℃下分别增加0.11, 0.18和0.22,表明随着温度或电位的增加,β-C完全氧化的选择性增加。

Isotopically-Labeled in-situ FTIR Study of PtRh Catalyst under Different Temperatures

Direct ethanol fuel cells are attracting much attention due to their excellent performance. Electro-oxidation of ethanol is not a combustion process, which involves multiple reaction processes. The low C-C bond cleavage ability and the poisoning caused by ethanol oxidation intermediate C1 molecules adsorb on the surface of catalysts are bottlenecks which restrict its application. Electrochemical in-situ fourier transform infrared spectroscopy(in-situ FTIRS) is to collect the vibration information of the specific functional groups of the reaction species in situ and reveal the reaction process at the molecular level, helping to understand the reaction mechanism. High performance PtRh/RGO catalyst was used, to investigate the electro-oxidation of ethanol at different temperatures through the combination of isotope tracer and electrochemical in-situ FTIRS. Cyclic voltammetry studies revealed that the electro-oxidation properties of ethanol and the selectivity of C-C bond cleavage ability decreased in the order of: PtRh/RGO(45 ℃)>PtRh/RGO(25 ℃)>commercial Pt/C. Electrochemical in-situ FTIRS revealed the electro-oxidation process at the molecular level. It was found that CO2, CO, -CH3 and -CO characteristic bands increased gradually with the increase of potential. CO2 and CH3COOH are the products of complete oxidation and incomplete oxidation of ethanol, respectively. Therefore, the ratio of the integrated area of the characteristic bands in the infrared spectrum CO2]/CH3COOH] is the measurement of CO2 selectivity. The band at 1 280 cm-1 was used to quantitatively calibrate CH3COOH, but for the infrared spectra of PtRh/RGO catalyst, the superposition of band CH3COOH at 1 280 cm-1 and methanol derivative appeared at 1 214 cm-1. The superposition band subtraction method was developed to calculate the CO2 selectivity of PtRh/RGO. The selectivity of CO2 on PtRh/RGO at 45 ℃ was improved compared with that at 25 ℃, increased 48.1% at 0.3 V, slightly increased at 0.5 and 0.6 V, but decreased at 0.4 V, which might ascribe to the competitive adsorption of β-C in ethanol and -OH in water. At both reaction temperatures, CO2 selectivity show a downward trend at potentials above 0.4 V. To further investigate the complete oxidation of CO2 derived from α-C or β-C, isotopically-labeled 13CH312CH2OH was used as the probe molecule, combined with electrochemical in-situ FTIRS to study the electro-oxidation of ethanol on PtRh/RGO electrodes at 25 and 45 ℃. The results show that the initial potential of complete oxidation of β-C is independent of temperature, both of which are 0.3 V. By quantitatively analyze the ratio of the integrated area of 13CO2/12CO2, it was found that the ratio under 0.3~0.5 V at 45 ℃ increased 0.11, 0.18 and 0.22 compared with that at 25 ℃, which indicated that the selectivity of β-C increased with the increase of temperature or potential.