【作者】 龙冉；

【导师】 熊宇杰；

【作者基本信息】 中国科学技术大学 ， 无机化学， 2014， 博士

【摘要】 理解催化剂的结构-性能的关系,对催化剂的设计具有指导性意义。金属纳米晶体(尤其是贵金属纳米晶体),由于其具有与块材截然不同的多种反应活性,在近几十年里得到了飞速的发展。在前期的异相催化体系研究中,涉及的贵金属催化剂通常是表面状态复杂、结构并不均一,给传统的单晶催化模型表面化学的研究与实际催化体系之间带来了巨大的鸿沟。然而,随着纳米材料合成技术的发展,可控合成结构均一的金属纳米晶体成为可能的现实。这种结构均一的纳米晶体,其表面状态与单晶催化模型具有一定可比性,将是具备高活性与高选择性催化剂的理想材料。本论文旨在探索纳米级尺度下晶面可控的钯纳米晶体的设计、可控合成及其相关催化性能研究。在本论文中,我们设计并合成了具有不同单一晶面的钯纳米晶体,并通过与密度泛函理论模拟和先进表征手段“三位一体化”相结合的方法,对不同晶面与小分子的相互作用机理、作用后的状态对后续反应的影响进行了充分的验证。本文中的研究结果对基于纳米晶体的金属催化剂的模型构建,以及具有光响应的催化剂的设计具有积极的指导意义。本论文主要包括以下几方面的内容：1.纳米晶体的晶面在反应分子的吸附及活化方面具有决定性的作用,因此纳米晶体的晶面控制成为了调控反应活性及选择性的关键因素。为了实现晶面控制,我们使用Pd作为一个模型体系,探究了动力学控制生长模式的原子附着添加行为。在Pd体系中,主要的产物是单晶纳米晶体,大大地减少了孪晶的影响。我们的研究结果表明,通过维持体系中Pd原子的低浓度来实现动力学控制,新还原的Pd原子更趋向于添加到截角八面体晶种的{100}面上,从而在表面上保留{111}面形成八面体结构。基于以上结果,我们得以将暴露{111}面的八面体与暴露{100}面的立方体做对比,充分探究催化反应的晶面选择性,体现了纳米晶体结构设计与可控合成对催化反应的重要意义。2.我们基于上一章的金属纳米晶体表面晶面调控技术,以无机化学与理论化学、物理化学、有机化学、同步辐射技术等学科的交叉合作模式,通过分子探针技术、X射线吸收精细结构谱表征和理论模拟相结合,首次揭示了氧分子在不同钯晶面的吸附与活化行为。通过合理的晶面选择,在吸附过程中金属表面可以给氧分子提供部分电子,引发氧分子磁矩的改变,从而自发地发生氧分子活化的过程。基于该晶面选择性的发现,阐明了晶面选择性在有机氧化催化剂以及癌症治疗剂的设计中的重要性,也揭示了有机化学界将金属催化剂广泛用于催化氧分子参与氧化反应的机理。3.鉴于钯纳米晶体和氧分子间的电荷转移对于氧分子活化的作用,金属纳米颗粒的表面电子状态将是调控吸附分子状态的一个有效途径。在此之前,有机化学家普遍认为半导体氧化物作为金属催化剂的载体,在有机氧化反应中主要是起到催化剂模板等作用。我们基于金属催化剂表面电子态和分子活化的构效关系,提出金属纳米晶体与半导体载体形成复合结构的思路和方法,通过复合结构中肖特基势垒作用使得半导体光生电子迁移到金属表面,从而有效地调控其表面电子态。在该工作中,我们首次以超快光谱和动力学测量为探针,揭示了金属表面等离激元导致的热电子会直接注入n-型半导体导带,与肖特基势垒驱动的电荷转移形成竞争关系。在阐明微观作用机制的基础上,我们得以通过光强调控这一简单而有效的手段,优化催化剂在氧分子活化和有机氧化反应中的活性。这项突破性研究进展,有助于加深人们对光催化剂复合材料设计的认识,也对阐明有机化学界在氧化反应中广泛使用氧化物载体的原理具有重要意义。4.利用光能-热能转换是一种极具应用前景的新型反应模式,是实现太阳能驱动催化反应的另一途径。我们通过Ru离子的辅助合成出了一种新型的Pd纳米晶体结构,该结构不仅具有优异的氢气响应,而且在紫外-可见较广谱范围具有光吸收。这两个特性的结合不仅有利于高效加氢反应,而且能够通过表面等离子体效应光热转换实现光能—热能—化学能的反应驱动。这是一种本质上不同于半导体的光能转化的新型光催化方式。更多还原

【Abstract】 The fundmental understanding of structure-catalytic activity relationship is of great importance in the design of efficient catalysts. Nanoscale metal crystals (particularly noble metal nanocrystals) are a class of materials that can be used in catalytic organic reactions. Previously, the catalysts involved in the heterogeneous catalytic reaction systems usually possess multiple crystal facets on surface. This feature formed an obstacle to performing reliable investigations at the intersection of surface science and realistic catalytic systems. Along with the rapid development of materials synthesis,it becomes feasible tocontrollably synthesize metal nanocrystals with desired structures. The metal nanocrystals with uniform and well-defined surface facetswould be ideal candidates for catalysts to achieve high activity and selectivity.In this dissertation, the metal of palladium (Pd)has been chosen as the model system. As a result, we have been able to investigate the molecule-nanocrystal interactionsas well as the structure-activity relationship in various reaction systems.These findings provide fresh insights into the design and synthesis of metal nanocrystals for various catalytic reactions. The specific findings include:1. In general, surface facets of a nanocrystal play an important role in determining the species adsorption and reaction activation, and in turn, hold the key to tailoring its activity and selectivity in catalysis. In the work, we use Pd as a model system to perform the investigation where kinetics of atomic addition can be precisely controlled by simply using a syringe pump. In the Pd system, the majority of products is single-crystal, largely simplifying the influence from twinned structures. By manipulating the kinetics of atomic addition, we figure out that newly formed Pd atoms are preferentially added to the{100} facets of cuboctahedral seeds when the Pd atomic concentrations are intentionally controlled very low, leaving{111} facets on the resulted octahedral nanocrystals. The{111} facets formed at the surface of octahedrons enable us to investigating facet-dependent catalytic effects, with well-developed{100}-bound Pd nanocubes as a reference.2. Based on the controlled synthesis of Pd nanocrystals in the last chapter, we first employ single-facet Pd nanocrystals as a model system to investigate the facet-dependent behavior for molecular oxygen activation. In our investigation, two types of nanocrystals with different surface facets are used:nanocubes enclosed by{100} facets and octahedrons by{111} facets. The yield of singlet O2, characterized by probe molecules in the presence of various scavengers, exclusively demonstrates that singlet-02-analogous species is preferentially formed on{100} facets. Both the simulations and characterizations further elucidate that O2is more activated on the{100} facets via chemisorption. As facet control enables to tuning the capability of activating O2, we have been able to demonstrate that the surface facet of metal nanocrystals is a critical parameter to designing catalysts for organic oxidation and therapy agents for cancer treatment.3. Since the Pd→O2electron transfer is responsible for O2activation, the charge state of metal surface may offer a knob for tuning its efficiency, which is fundamentally important to optimizing catalysts design for organic oxidations. Prior to our work, oxide semiconductors were generally considered as support materialsfor metal catalysts in organic reactions. In our work, we demonstrate using Pd-TiO2hybrid structures as a proof-of-concept model that plasmonic hot electrons can be injected into the conduction band of TiO2, in opposite to the function of Schottky junction, lowering the electron density of Pd surface. By varying the illumination intensity, it is feasible to modulate the charge state of Pd surface in such a metal-semiconductor hybrid configuration. This modulation enables enhancement of O2activation and in turn efficiency improvement of catalytic glucose oxidation by shedding appropriate light on the Pd-TiO2hybrid structures.4. Based on plasmon excitation, catalytic reactions can befacilitated through both plasmon-driven catalysis and photothermal conversion. In our work, we demonstrate the direct light harvesting in UV-to-visible range for hydrogenation reactions using the Pd nanostructures that are synthesized viaa one-pot Ru-assistedroute. Under the light illumination, hydrogenation reactions can be driven to different extentat room temperature depending on the light intensity. This unique characteristic of plasmonic nanostructures suggests that metal nanocrystals with high-index facets can be used in many catalytic heterogeneous reactions by harvesting solar energy instead of heat energy.更多还原