【作者】 张亮亮；

【导师】 时康；

【作者基本信息】 厦门大学 ， 物理化学， 2019， 硕士

【摘要】 单原子锚定在固体表面兼具均相和异相催化特性,因而单原子催化剂是催化领域近年来的研究前沿,其中,发展金属单原子碳载催化剂是最重要的研究方向之一。然而,制备金属单原子碳载催化剂目前仅限于以石墨烯为载体。是否采用其它sp2杂化碳材料也能制得金属单原子碳载催化剂是值得探索的重要议题,这其中包括两个未解的关键科学问题:(1)sp2杂化碳材料具有何种关键界面特征方可用于制备金属单原子催化剂;(2)此种关键界面的成因及调控方法。本论文为解决上述两个关键科学问题,以最常用的两种sp2杂化碳材料玻碳和石墨为研究对象,采用电化学循环伏安方法,在0.5 M硫酸中氧化调控两种碳电极表/界面;发展了吸附-电化学还原方法在处理后的两种电极表面制备铂(Pt)催化剂,系统地表征了两种不同的碳载铂催化剂的分散性和电催化性能,研究了电化学调控玻碳和石墨的表/界面构-效关系,以及两种碳的本质差别。在论文第三章中,首先,测定了吸附在电化学处理后玻碳(CGCE)表面的铂离子的电化学还原起始电位与电解液pH的关系,并采用XPS技术表征了吸附的铂离子在电化学还原前后的价态变化;随后,采用电化学和原位红外等方法,系统调查了Pt/CGCE催化剂的电催化氧化甲酸(HCOOH)、CO和还原析氢性能;最后,采用分子探针方法,研究了铂离子在CGCE表面的吸附位点。实验结果发现:(1)CGCE在5 mM K2PtCl6水溶液中浸泡后,其表面吸附物种为[Pt(OH)2]2+;(2)在NaAc/HAc(pH=4.50)溶液中于析氢电位下(-0.2 V vs SCE)电化学还原后,铂仍以+2价的形式高度分散在CGCE表面,与文献报到的铂单原子电催化剂相似,具有极高的电催化活性,如:氧化CO的TOF约为0.13 s-1;(3)在CGCE表面,铂的吸附与沉积位点为电子性质紊乱的碳微晶basal面;(4)在处理后的石墨电极表面,采用同样的吸附-电化学还原方法制得的多为铂原子簇。进一步来讲,氧化改性后的玻碳和石墨表面关键差别在于:前者生成了致密的表面氧化层,层中仅含有高分散的、被表面含氧基团所包围的、面积极小的碳微晶basal面,因此还原的铂可保持高分散性;相比之下,后者表面氧化层则含有较多的大面积的碳微晶basal面,铂在这些面上还原后易发生聚集,形成原子簇。在酸性电解液中,电化学氧化玻碳电极将产生一对可逆的氧化还原峰。对此,在论文第四章中,以CGCE和恒电位氧化的玻碳(PGCE)为研究对象,系统调查了此表面氧化还原峰的特征。实验采用高真空热处理进一步调控表面,并结合XPS技术,检测了电极处于还原和氧化态时表面物种的差异;采用维他命C和Fe2+/3+为电化学分子探针,研究了电极表面电化学反应动力学与高真空热处理温度间的关系。实验结果表明,可逆的氧化还原峰与电极表面的含氧基团有关,同时也涉及sp2C和sp3C的电化学转换;低于400℃的高真空热处理将恶化电极表面动力学性质;但经高于600℃的真空热处理除去大部分含氧基团后,电极表面与最初的CGCE和PGCE有着相似的电子传导动力学性质,揭示玻碳电极表面的电子传导动力学性质主要决定于其表面难热解的羰基功能团、电化学活性的sp2C/sp3C,后者与含有缺陷的电子性质紊乱的碳微晶basal面密切相关。更多还原

【Abstract】 Because single atom anchored at on a solid surface has the homogeneous and heterogeneous catalytic properties,single atom catalyst(SAC)has been the research frontier in the field of catalysis in recent years.And one of the most important research directions is to develop single metal atom catalyst at the carbon surface.Nevertheless,the preparation of the SAC/carbon is limited to the use of graphene as the support so far.Whether other sp2 hybrid carbon materials can be used to fabricate the SAC/carbon is an important issue worth exploring,in which there are two key unsaved scientific questions,including:(I)the key interface characteristics of sp2 hybrid carbon materials for the preparation of SAC/carbon;(2)the cause and control method of those key interface characteristics.in this thesis,to solve the above two key scientific problems,two most commonly used sp2 hybrid carbon materials,glass carbon(GCE)and graphite,were taken as the research objects.A voltammetric method was developed to regulate the GCE and graphite electrode interfaces in 0.5 M H2SO4 electrolyte.And the SAC/carbon was prepared by means of the adsorption-electrochemical reduction approach.The dispersion and electrocatalytic properties of two SAC/carbon electrodes were systematically characterize,while the surface/interface structure-activity relationship and the essential differences between GCE and graphite were studied.In chapter 3,firstly,the relationship between the onset reduction potential of platinum ions adsorbed on the surface of glassy carbon(CGCE)and the pH of the electrolyte was determined,while the changes in the valence states of the adsorbed platinum ions before and after electrochemical reduction were examined by XPS technique;subsequently,the electrocatalytic behaviors of SAC/CGCE for the formic acid(HCOOH),CO and reduced hydrogen evolution were investigated using electrochemistry and IR methods;finally,the adsorption sites of Pt ions on CGCE surface were studied by molecular probe method.The results showed that:(1)after soaking CGCE in 5 mM K2PtCl6 aqueous solution,the adsorption species on its surface was[Pt(OH)2]2+;(2)after the electrochemical reduction in NaAc/HAc(pH=4.50)solution at the hydrogen evolution potential(-0.2v vs SCE),Pt in the form of+2 valence was still highly dispersed on the CGCE surface,and had the extremely high electrocatalytic activity,like as the SAC reported in the literature(for example,the TOF of CO oxidation was about 0.13s-1);(3)on the CGCE surface,the adsorption and deposition sites of Pt are the basal planes of carbon microcrystals with electrical disturbance;(4)on the treated graphite electrode surface,Pt clusters would be obtained by the same adsorption-electrochemical reduction method.Furthermore,the key difference between the surfaces of the oxidized glass carbon and graphite is that the former generates a dense surface oxidation layer,which only contains highly dispersed carbon microcrystalline basal planes surrounded by surface oxygen-containing groups,thus the reduced Pt can maintain high dispersion.In contrast,the latter surface oxidation layer contains larger basal planes,along which Pt aggregates are likely to occur to form atomic clusters after reduction.In acidic electrolyte,a couple of reversible redox peaks are generated by electrochemical oxidation of glassy carbon electrodes.In chapter 4,the characteristics of this surface redox peak on CGCE and PGCE were investigated systematically.High vacuum heat treatment(HVT)was used to further regulate the surface,and XPS technology was combined to detect the difference of species on the surface when the electrode was in the states of reduction and oxidation.The relationship between electrochemical reaction kinetics of electrode surface and HVT temperature was studied by using vitamin C and Fe2+/3+ as electrochemical molecular probes.The experimental results revealed that the reversible redox peak was related to the oxygen-containing groups on the electrode surface,as well as the electrochemical conversion of sp2C and sp3C.HVT below 400℃ would deteriorate the surface kinetics of the electrode.Nevertheless,after higher than 600℃ HVT to remove most of the oxygen containing groups,resulting electrodes had similar dynamics properties as the initial CGCE and PGCE,revealing that the electrical dynamics properties of GCE surface mainly depends on its surface carbonyl group and electroactive sp2C/sp3C.The latter is closely related to the defected basal plane of carbon microcrystals.更多还原