摘要
碳纳米管(CNTs)由于具有优异的导电性能、极高的比表面积及良好的热稳定性等特点,因而它是一种理想的直接甲醇燃料电池(DMFC)催化剂载体材料。现今用于直接甲醇燃料电池的催化剂以铂金为主(platinum, Pt)为主,但铂金属会在甲醇的氧化过程中会伴随着中问产物一氧化碳(CO)的生成,造成铂金属中毒。为了解决这个问题,目前主要以添加其他金属来加以改善,在进一步降低贵金属Pt的用量的基础上提高其催化活性。例如,Sn由于成本较低,且实验表明PtSn二元复合催化活性明显提高,因此受到广泛关注。目前,二元复合催化效果的最佳原子比例范围却鲜有报道。
本研究将采用Sn作为第二种金属来添加,以多壁碳纳米管(MWCNTs)为载体,考察不同原子比例的Pt-Sn/MWCNTs催化剂的催化活性,确定最佳催化活性下的PtSn比例范围,并结合微观结构信息探索催化活性与结构的内在关联。采用浸渍还原法制备了八种不同铂锡比例的担载在多壁碳纳米管上的Pt-Sn/MWCNTs催化剂,利用场发射扫描电子显微镜(Field Emission Scanning Electron Microscope,FESEM)、X-射线衍射(X-ray Diffraction,XRD)、能谱仪(Energy Dispersive Spectrometer,EDS)等对不同比例催化剂的结构和形貌进行了表征；采用循环伏安法和交流阻抗法对八种样品进行电化学测试,通过比较不同比例的催化剂对甲醇的电催化性能,了解PtSn原子比例和催化活性之间的联系。
结果表明：
(1)采用浸渍还原法制成的Pt-Sn/MWCNTs复合材料,担载的金属颗粒平均粒径范围约在3.5-6.5nm,基本符合最佳催化粒径范围。
(2)XRD和EDS测试结果表明,Pt-Sn/MWCNTs存在Pt-Sn合金和SnO2。Pt-Sn合金能够降低了CO的吸附能力,而Sn02能够在低电位下提供OHads在二者同时作用下,能有效地抑制催化剂CO中毒,进而提高其对甲醇的氧化活性。
(3)循环伏安和交流阻抗测得,当PtSn原子比为3：1时,Pt-Sn/MWCNTs的催化剂的电化学活性表现为最好。即当Sn载量在3.5：1(Pt：Sn)～2.5：1(Pt：Sn)的范围内时,由于粒子粒径能达到最佳尺寸,且催化剂的电荷传质阻力最小而活性物质扩散速度最大,因而呈现相对较好的催化活性状态。
Carbon nanotubes become an expected candidate of the catalyst carrier in Direct Methanol Fuel Cell(DMFC) in recent years owe to their excellent conductivity, high specific surface area and good thermal stability and so on. The common catalyst for DMFC is platinum, Pt, which is easily poisoned with carbon monoxide generated in the oxidation process of Methanol. In order to solve this problem, other metals are added to improve the catalytic activity and further reduce the usage amount of Pt. Sn has been widely concerned because its cost is low and Pt-Sn binary catalyst can improve the catalytic activity. At present, the best atomic ratio of binary catalyst is the focus of inner depth research.
In this paper we chose Sn as the second metal and used Multi-walled carbon nanotubes(MWCNTs) as the vector, to study the catalytic activities of Pt-Sn/MWCNTs catalysts with different atomic ratios and confirm the optimum atom ratio of Pt-Sn/MWCNTs with the best catalytic activity. Also we explore the internal relationship of the catalytic activity and microstructure. Eight kinds of PtSn with different atomic ratios were on the MWNTs by the impregnation reduction method to form Pt-Sn/MWCNTs catalysts. Field Emission Scanning Electron Microscope(FESEM), X-ray Diffraction(XRD), Energy Dispersive Spectrometer(EDS) spectrum were used to reveal the structures and morphology of the catalysts of different atom ratios, Cyclic voltammetry and AC impedance electrochemical were used to test the electrocatalytic properties of the eight samples. The relation between the catalytic activity and the atomic ratio of PtSn was confirmed by studying the effect of atomic ratios of catalysts on the electrocatalytic performance of methanol.
The following results were obtained:
(1)The complex catalyst Pt-Sn/MWCNTs was prepared by impregnation reduction method, in which the metal particle size was about 3.5-6.5nm and it was in the range of the best particle size.
(2)XRD and EDS results showed that A highly dispersed fabric with PtSn alloy and SnO2 on MWCNTs could be seen. Pt-Sn alloy could reduce the adsorption capacity of CO, and SnO2 could provide OHads in low electric potential, both could effectively inhibit CO poisoning of the catalyst, and improve its activity for methanol oxidation.
(3)Cyclic voltammetry and AC impedance reflected that when the atomic ratio of Pt&Sn was 3:1, the Pt-Sn/MWCNTs catalyst performed the best electrochemical activity.That is to say, when the atomic ratio of Pt/Sn was in the 2.5:1 (Pt:Sn)～3.5:1 (Pt:Sn) range, the particle size was optimal size, charge resistance of the catalyst was minimum, and the spread speed of the active substance was maximum, which led to the best catalytic activity relative to the state.
引文
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