【作者】 李振宇；

【导师】 李晓天；

【作者基本信息】 吉林大学 ， 材料物理与化学， 2021， 博士

【摘要】 贵金属在有机合成,碳碳偶联,加氢,电催化等多相催化反应中处于重要的地位。为了提高贵金属催化剂的性能,轻合金元素（C、H、O、N和B）和过渡金属元素作为改性剂被加入到贵金属催化剂晶格间隙中,从而调控贵金属的晶相和电子结构。与过渡金属元素相比,轻合金元素硼（B）具有以下几点特异性:第一、B原子由于原子半径小使其很容易的溶解在贵金属的晶格间隙中,而过渡金属倾向于替换贵金属母体晶格内的原子。第二、贵金属-过渡金属形成的合金是由贵金属元素的d轨道和过渡金属的d轨道杂化主导其电子相互作用,与此不同的是,贵金属和B形成的合金采用贵金属的d轨道和B的（s,p）轨道杂化主导其电子相互作用。这些特异性可能带来新奇的效应。因此,研究间隙B原子对贵金属催化剂的结构调控和性能优化具有重要意义。本论文以电催化析氢反应为研究模型反应,通过固相反应实现了固定原子比的贵金属-B间隙化合物的精准合成,并结合理论和实验研究阐明了贵金属晶格间隙内的B原子连接模式、B浓度、B有序、贵金属晶相、B（s,p）-贵金属（d）轨道杂化度与催化性能之间的关系。本论文的主要研究内容如下:1、本文结合理论和实验研究了Pd-B合金催化剂,揭示了Pd主体晶格中有序排列的间隙B原子对优化Pd-B合金的近表面电子结构和表面析氢催化具有重要意义。我们理论计算了间隙B原子的分布和浓度对Pd母体的d带电子结构（如d带宽度、d带中心）和表面析氢催化性质的调控作用,理论结果表明次表面位置的Pd:B原子比为2:1时,Pd-B合金模型具有合适的（s,p）-d轨道杂化和最优的近表面电子结构,有利于电催化析氢反应（HER）。基于理论结果,我们利用了一种固相合成方法,制备了高结晶度和高纯度的Pd₂B间隙化合物,证实了Pd₂B间隙化合物具有有序的B原子排列结构和上述优化的电子结构。该Pd₂B催化剂实现了类似Pt的HER催化活性,并具有超过七天的催化稳定性。2、本文在固相合成策略的基础上,通过调整实验参数（温度、前驱体比例和加热时间）实现了含有不同硼浓度、硼分布和不同电子性质的多晶相Pd-B金属间化合物的合成,包括Pd₂B、Pd₅B₂和Pd₆B。它们的晶相调控为钯基催化材料的设计提供了新的思路。在这里,我们研究了Pd、Pd₆B、Pd₅B₂和Pd₂B的电催化产氢活性,结果表明随着Pd晶格间隙内B浓度的增加催化剂的HER催化性质随之提升。我们结合理论和实验结果证明了Pd₂B的高催化性能主要来自以下几个方面:首先、Pd（d）-B（s,p）之间的强轨道杂化导致Pd₂B的d带中心与Pd、Pd₅B₂和Pd₆B相比明显下降,这表明被吸附H^*中间产物与催化剂的结合作用更弱,使被吸附分子更容易活化和解离。第二、随着Pd宿主晶格间隙位置的B浓度增加,Pd-B键长按照Pd、Pd₆B、Pd₅B₂和Pd₂B的顺序缩短,Pd₂B中较短的Pd-B键长使得Pd-B键相互作用得到优化,确保了催化过程中Pd原子的稳定性,使Pd₂B获得了优异的催化性能。据此,Pd-B间隙化合物的催化性能是由Pd主晶格中B的浓度以及B（s,p）-Pd（d）轨道杂化程度共同决定。3、本文利用了固相合成方法,合成了间隙化合物硼化铑（RhB）,该结构含有不对称应变的hcp Rh亚晶格。我们通过理论计算研究了在Rh B中形成非常规的hcp-Rh亚晶格的可能因素（晶格膨胀、不对称应变和B相关的电子效应）,计算结果表明一维B链中B原子间的共价相互作用是在Rh B间隙化合物中产生不对称应变和实现hcp Rh亚晶格稳定存在的主要原因。电催化HER结果表明Rh B在过电位为15 m V时产生10 m A/cm²的电流密度,与Pt作为电催化剂时的电流密度相似,证实了Rh B具有类Pt的HER催化性能。Rh B的ΔG_H*绝对值与Pt接近,证实了Rh B理论上也具有优异的HER性能。进一步的计算结果阐明了Rh B间隙化合物中Rh（d）-B（s,p）轨道杂化优化了电子结构,使其成为具有优异催化性能的HER电催化剂。更多还原

【Abstract】 Noble metals play an important role in organic synthesis,carbon-carbon coupling,hydrogenation,electrocatalysis and other heterogeneous catalytic reactions.The addition of light metal or transition metal element to lattice of noble metals can effectively regulate their electronic structure and catalysis performance.The peculiarities of light metal element such as B rely on the followings:（i）the light metal atom with relatively smaller atomic size dissolved easily into the lattice interstitial of parent metal,while transition metal atom is more inclined to replace the parent metal atom.（ii）Different from metal-metal alloys with d-d orbital hybridization,alloy with light metal element is usually by orbital hybridization between their s,p orbitals and d orbitals of parent metal atoms.These peculiarities of light metal atom gives us the opportunity to optimize the catalysis perforamnce of noble metal via geometric and electronic effect.In this paper,we realize the precise synthesis of noble metal B interstitial compound with a fixed atomic ratio by solid phase replacement strategy,and the electrocatalytic hydrogen evolution reaction as the research model reaction.The relationship between the bonding mode of boron atoms,boron concentration and boron order in the noble metal crystal lattice and the corresponding catalytic performance of noble metal-B interstitial compound expounded theoretically and experimentally.Besides,the crystal phase and B（s,p）-noble metal（d）orbital hybridization on the influence on the catalysis performance of noble metal-B interstitial compound are also researched.The main contents of this paper are as follows:1.The parer present a joint theoretical and experimental study that reveals the importance of ordering of interstitial boron atoms in Pd-B alloy’s near-surface electronic structure and catalytic performance.We theoretically calculated the regulation effect of the distribution and concentration of interstitial B atoms on the d-band electronic structure（such as d-band width and d-band center）of Pd parent and the catalytic properties of hydrogen evolution on the surface.The theoretical results demonstrate that the sub-surface location of interstitial boron atoms with a Pd:B atomic ratio of 2:1 is necessary to ensure an appropriate degree of interatomic s,p-d orbital hybridization,and thereby an optimal surface electronic structure for the hydrogen evolution reaction（HER）.We adopt the solid phase synthesis strategy to prepare this highly crystalline intermetallic Pd₂B,and it can realize the desired Pd-B alloy structure.Due to its optimal electronic structure and ordered arrangement of interstitial boron atoms,the intermetallic Pd₂B exhibits Pt-like catalytic activity for HER and has excellent catalytic stability for over seven days.2.We selectively synthesized Pd₆B,Pd₅B₂,and Pd₂B via the solid-phase reaction between Na₂Pd Cl₄and Mg B₂based on the adjustment of experimental parameters,including:（i）the molar ratio of Na₂Pd Cl₄and Mg B₂;（ii）the reaction time;and（iii）the reaction temperature.The regulation of their crystal phases may construct new design principle for Pd-based catalysis materials.The electrocatalytic HER results identify that catalysis performance follows the increased order of Pd,Pd₆B,Pd₅B₂and Pd₂B as the increase of B concentration in Pd lattice interstitial site.We combined DFT theoretical calculation and experiment result demonstrated the reason of enhanced catalysis performance originated form the following several aspects.The strong orbital hybridization between Pd（d）–B（s,p）contributed to the Pd₂B exhibits significantly descend d-band center compared to Pd,Pd₆B,Pd₅B₂,suggesting much weaker binding interaction between the absorbed molecules and catalyst,and then enables absorbed H^*easily activated and dissociated.The bond length of Pd-B decreases in the sequence Pd<Pd₆B<Pd₅B₂<Pd₂B,which is followed by an increase in the B concentration in lattice interstitial site of Pd host lattice.The shorter Pd-B bond length in Pd₂B results in the optimized interaction of Pd-B bond,promoting the stability of Pd atom during catalysis process.Accordingly,a relation was proposed wherein the catalysis performance of Pd-B interstitial compound is co-determined by the B concentration in Pd host lattice,as well as of degree of B（s,p）-Pd（d）orbital hybridization.3.The paper developed the solid phase synthesis method,and synthesized intermetallic rhodium boride（Rh B）comprising an asymmetrically strained hcp Rh sublattice.We investigated the three possible factors（lattice expansion,asymmetric strain,and boron-relevant electronic effect）for the formation of unconventional hcp-Rh sublattice in Rh B through theoretical calculations.The DFT calculation and crystal structure analysis results demonstrated that the covalent interaction of interstitial boron atoms is found to be the main contributor to the generation of asymmetric strains and the stabilization of the hcp Rh sublattice.The electrocatalytic HER results exhibit that the Rh B generates 10 m A/cm²current density at an overpotential of 15 m V,which is similar to that in present of Pt as the catalyst.The absolute value ofΔG_H*of Rh B is close to that of Pt,which proves that Rh B has excellent HER performance in theory.The further theoretical calculation results demonstrate that the Rh（d）–B（s,p）orbital hybridization induces an optimized electronic structure,which contributes to Rh B as an excellent hydrogen-evolving eletrocatalyst with Pt-like activity.更多还原