Petrography and mineral chemistry of Ca, Al-rich inclusions of Ningqiang carbonaceous chondrite are studied and the highly Re-Os fractionated Pt-metal nuggets in it is discovered. The refractory inclusions are classif...Petrography and mineral chemistry of Ca, Al-rich inclusions of Ningqiang carbonaceous chondrite are studied and the highly Re-Os fractionated Pt-metal nuggets in it is discovered. The refractory inclusions are classified into melilite-spinel-rich, and spinel-pyroxene-rich types. The former probably crystallized from melts, while the latter could be aggregates of the nebular condensates. The strong f ractionation between Re and Os suggests a pre-solar origin of the Pt-metal nuggets, which probably experienced various processes of formation and evolution before being captured by the refractory inclusions as indicated by the depletions of W and Mo.展开更多
文摘Petrography and mineral chemistry of Ca, Al-rich inclusions of Ningqiang carbonaceous chondrite are studied and the highly Re-Os fractionated Pt-metal nuggets in it is discovered. The refractory inclusions are classified into melilite-spinel-rich, and spinel-pyroxene-rich types. The former probably crystallized from melts, while the latter could be aggregates of the nebular condensates. The strong f ractionation between Re and Os suggests a pre-solar origin of the Pt-metal nuggets, which probably experienced various processes of formation and evolution before being captured by the refractory inclusions as indicated by the depletions of W and Mo.
文摘质子交换膜燃料电池(PEMFCs)因其高能量密度、低操作温度和环保等特性,被视为极具潜力的能量转换系统.目前,碳载铂颗粒(Pt/C)是PEMFCs阴极氧还原反应(ORR)中使用最广泛的催化剂.然而,Pt与碳载体间的电子结构差异导致Pt纳米颗粒(Pt NPs)易从碳载体上脱落,严重降低了ORR的催化活性.此外,Pt的高成本和稀缺性也限制了其广泛应用.相比之下,Pt纳米枝晶(NDs)因具有高利用率的表面活性位点而备受关注.然而,Pt NDs的合成通常需要严格控制反应条件,且其与碳基底间的弱相互作用易导致活性位点损失和性能下降.因此,开发具有强金属载体相互作用的Pt复合碳催化剂对PEMFCs的实际应用至关重要.本文通过原位Cl-介导的生长策略,结合碳本征空位工程,成功制备了分散在富含碳本征空位的中空氮掺杂碳基底上的Pt NDs催化剂(Pt@HNC-V-800).拉曼光谱和电子顺磁共振光谱结果表明,碳本征空位的形成机制源于碳基底结构中氮原子的耗散,该过程引起碳原子的重新排列,进而产生了丰富的本征缺陷位点.X射线吸收光谱和X射线光电子能谱结果表明,与无碳空位的Pt@HNC催化剂相比,富含本征碳空位的样品(Pt@HNC-V-800)表现出较低的Pt-Pt键配位数(8.64)和更强的给电子效应.得益于Pt NDs丰富的活性位点及其与本征碳空位基底之间的强电子效应,Pt@HNC-V-800的ORR半波电位高达0.947 V,质量活性和比表面活性分别为1.55 A mg^(-1) Pt和1.85 mA cm^(-2),是商用Pt/C的8.2和6.8倍(0.191 A mg^(-1)Pt和0.27 mA cm^(-2)).加速耐久性测试结果表明,经20000次电势循环后,Pt@HNC-V-800的活性无明显变化,其活性损失远低于无碳本征空位的Pt@HNC材料和商业Pt/C催化剂.因此,与无碳本征空位的Pt@HNC材料相比,Pt@HNC-V-800的ORR活性和稳定性都有较大提升,进一步证实了碳本征空位工程协同Pt NDs策略的优越性.此外,密度泛函理论计算结果表明,Pt@HNC-V的丰富空位降低了氧中间体过电势,优化了ORR中间体在Pt NDs上的吸附能,进而提高了催化剂的ORR本征活性.同时,富碳本征空位的存在增强了Pt NDs在碳载体上的结合能,使Pt NDs不易在电势循环过程中脱离碳载体,从而增强了稳定性.综上所述,本文通过Pt NDs与碳本征空位工程协同效应策略,精准调控碳负载Pt基催化剂的结构,大幅提升其在酸性条件下的ORR性能,为进一步设计高性能的ORR电催化剂提供了新思路.
基金This work was supported by the National Natural Science Foundation of China(No.12075243)the National Key Research and Development Program of China(No.2017YFA0402800)。
