【作者】 王孝广；

【导师】 王伟民； 张忠华；

【作者基本信息】 山东大学 ， 材料加工工程， 2011， 博士

【摘要】 本文中,我们采用Al作为初始合金中的次贵金属组元,与一系列贵金属组元(Ag,Pd,Au/Pt等)按照一定成分比例熔炼并通过快速凝固方法获得相应的铝基初始合金,在相关腐蚀溶液中(本文中采用5 wt.%HCl溶液和20wt.% NaOH溶液)进行化学去合金化处理获得了相应的纳米多孔单金属、双(三)金属纳米多孔合金。通过X-射线衍射、扫描／透射电镜等分析手段发现,对于Al-Ag体系,快速凝固Al-15-50Ag合金主要由a-Al(Ag)和Ag2Al两个相组成,而Al-60Ag合金则完全由单一的Ag2Al组成。a-Al(Ag)和Ag2Al两个相共存时,由于产生的协同去合金效应以及银原子具有高的扩散速率使得去合金后获得的纳米多孔银表现出均匀分布的典型双连续韧带-通道多孔结构；在5 wt.%HCl溶液中,Al-15-40Ag合金可以发生完全去合金化形成单相面心立方银多孔结构,当银含量继续增大,则部分Ag2Al相不能被去合金化而保存到韧带内部；随着初始合金中银含量的增大,去合金后韧带尺寸呈现逐渐变大的趋势；Al-60Ag单相合金仅能在热的浓盐酸环境下发生表面去合金行为,随后即发生钝化现象,说明此时合金成分已接近Al-Ag系合金的去合金临界成分。对于Al-Pd体系,快速凝固Al80Pd20合金主要由α-Al和Al3Pd两相组成,去合金过程中α-Al相首先被腐蚀形成大尺寸通道,Al3Pd相则发生去合金化形成纳米多孔钯,当选用5 wt.%HCl溶液和20wt.% NaOH溶液进行去合金化处理时,分别获得了韧带尺寸为～20 nm和～5 nm的纳米多孔钯材料。这种通过去合金化获得的纳米多孔钯材料具有与钯纳米粒子类似的电化学性质,具有大的电化学表面积,对甲醇、乙醇和甲酸等有机小分子表现出优越的电催化氧化活性,有望在相应的电催化及燃料电池领域获得实际的应用。并且我们发现经过球磨处理的Al77Pd23合金粉末主要由Al3Pd相组成,α-Al相的含量进一步减少,通过在碱性溶液中去合金化处理获得粉末状雷尼钯,这种雷尼钯粉末显然可以较容易的大量制备,并且同样对乙醇等有机小分子表现出良好的电催化活性,有望投入到实际的工业生产中去。通过在单相Al2Au初始合金中添加适量的Pt、Pd等元素替代等量的Au,发现其相组成不发生变化,Pt, Pd原子占据Au的晶格位点,仅发生了微量的晶格收缩,形成相应的单相Al2(Au,Pt), Al2(Au,Pd), Al2(Au,Pd,Pt)金属间化合物。这种经过掺杂的初始合金去合金化过程中由于掺杂原子对于金原子的钉扎作用,使得形成多孔合金的韧带尺寸明显减小,且腐蚀溶液种类和掺杂量对韧带尺寸存在调制作用,当采用氢氧化钠作为腐蚀溶液时可以获得3-5 nm的超细孔结构,最佳的掺杂量在10 at.%到20 at.%之间,并且Pt掺杂具有比Pd掺杂更优越的韧带细化效果。由于Pt、Pd掺杂原子与周围金原子环境的协同作用,这类纳米多孔合金材料可以获得比单纯的多孔金更优越的甲醇和甲酸电催化活性。通过快速凝固方法获得的Al70Pd30合金由Al3Pd和Al3Pd2两种金属间化合物相组成,其中Al3Pd相可以发生去合金化形成纳米多孔钯,而Al3Pd2则表现出完全惰性,保留下来并被埋置于纳米多孔钯基体中,其致密结构导致这种纳米多孔钯复合材料的电化学活性面积较纯纳米多孔钯减小。Al75Pd17.5Au7.5三元合金由两种经过掺杂的金属间化合物相组成,即Al3(Pd,Au)和Al2(Au,Pd),去合金化后形成了np-Pd(Au)和np-Au(Pd)两种多孔固溶合金结构交替结合在一起的多孔复合结构,由于低扩散速率原子的掺入,np-Au(Pd)区域的韧带／通道尺寸明显低于相应腐蚀溶液中获得的纯多孔金所表现的尺寸。双相合金去合金化行为与相组成以及去合金化过程中相与相之间是否存在协同效应密切相关,造成了不同结构的纳米多孔金属(复合)材料。更多还原

【Abstract】 In the present thesis, the Al-based precursor alloys were prepared from pure Al and a serial of noble metals (Ag,Pd,Au/Pt) through rapid solidification method. And then, the dealloying treatment was performed in the 5 wt.% HCl and 20 wt.% NaOH aqueous solution, respectively. After dealloying, the Al element was selectively leached away and the corresponding nanoporous metals/alloys were obtained.By virtue of XRD, SEM, TEM and etc, it is found that there exist two distinct phases in the rapidly solidified Al-15～50Ag alloys, that isα-Al(Ag) and Ag2Al. However, the Al-60Ag alloy is entirely composed of a single Ag2Al phase. Due to the synergy effect betweenα-Al(Ag) and Ag2Al as well as the fast diffusion of silver atoms along alloy/solution interface, a kind of nanoporous Ag with a typical uniform bicontinuous ligament-channel structure can be obtained. When the Ag content was located between 15 at.% and 40 at.%, the Al-Ag alloys can be fully dealloyed and contribute to a single fcc Ag porous structure. With the increasing of Ag content in precursors, part of Ag2Al cannot be dealloyed and embedded in the Ag ligaments. And thus, the ligament/channel size of as-dealloyed porous Ag became larger progressively. Moreover, the Al-60Ag alloy with a single Ag2Al phase cannot be dealloyed in dilute HCl solution. Even performing the dealloying in the concentrated hydrochloric acid, only the Al atoms in surface layer can be selectively leached away at a high temperature, and then passivation occurs. It indicates that 60 at.% Ag is infinitely close to the parting limit for the dealloying of Al-Ag alloys.In the Al-Pd systems, the rapidly solidified Al80Pd20 alloy comprises two kinds of phase, and that is a-Al and Al3Pd. Theα-Al is firstly leached away, following that the Al3Pd was dealloyed and contributes to the formation of nanoporous Pd. The length scale of ligaments/channels can be modulated to～20 nm and～5 nm when performing the dealloying in the 5 wt.% HCl and 20 wt.% NaOH solution, respectively. Compared with Pd nanoparticles, the as-dealloyed nanoporous Pd reveals similar electrochemical features, such as showing a large electrochemical active surface area and exhibiting excellent electrocatalytic activities toward methanol, ethanol, formic acid and etc. Thus it will find promising applications in fuel cells and electrocatalysis-related areas. In addition, it is found that the as-milled Al77Pd23 powder is mainly composed of Al3Pd, only a little amount of a-Al exists. After dealloying in the alkaline solution, the Raney Pd catalyst can be obtained. This kind of Pd catalyst also reveals excellent activities toward several small organic molecules electro-oxidation, such as ethanol. Obviously, such Pd catalyst can be facile synthesized at a large scale and is expected to put into practical industrial production.When adding an appropriate amount of Pt, Pd into a single Al2Au precursor to replace the same amount of Au, the as-doped alloy also exhibits a single-phase intermetallic compound with an Al2Au-type structure. Due to the fact that Pt or Pd exists in a solid solution form partially substituting for Au in the AI2Au lattice, the as-formed phases with a little lattice contraction can be denoted as Al2(Au,Pt), Al2(Au,Pd), Al2(Au,Pd,Pt), respectively. In virtue of the pinning effect caused by these dopants with lower atomic diffusivity, the length scale of as-dealloyed nanoporous alloys can be significantly reduced. There exists distinct refine effect in the alkaline solution as compared with that in the acid solution. The porous structure with ligament as low as 3-5 nm can be obtained when dealloying in the NaOH solution. The optimal amount of dopants should be 10 at.% to 20 at.% and the Pt doping reveals a more superior ligament refining effect as compared with Pd doping. Except for refining effect, a kind of bi- or ternary nanoporous alloy structure evolves owing to the addition of Pt or/and Pd atoms. Due to the synergistic effect between Au and dopants,these nanoporous alloy materials exhibit more superior electrocatalytic activities toward methanol and formic acid electro-oxidation than the pure nanoporous Au.There exist two distinct intermetallic compounds in the rapidly solidified Al70Pd30 alloy, that is Al3Pd and Al3Pd2. The dealloying of Al3Pd contributes to the formation of continuous nanoporous Pd matrix, however, the Al3Pd2 is chemically inert and preserved in the as-dealloyed matrix. Although a decreased electrochemical active surface area can be depicted, this kind of Pd composite still reveals similar electrochemical properties as pure nanoporous Pd. The ternary Al75Pd17.5Au7.5 alloy comprises two kinds of doped intermetallic compounds, namely Al3(Pd,Au) and Al2(Au,Pd). After dealloying, two kinds of binary nanoporous alloys, np-Pd(Au) and np-Au(Pd), are alternated one after another to form a bimodal porous composite. The size of ligaments/channels in the np-Au(Pd) region is significant smaller than that of pure np-Au, that is consistent with the slower diffusivity of Pd atoms. The dealloying process of bi-phasic alloys is closely related to the phase composition, distribution and whether there exists a synergy between these two phases. And thus, varieties of nanoporous composites can be obtained in different cases.更多还原