重塑位于火山曲线右支的弱吸附金属单原子位点的配位环境及电子结构

金属单原子位点独特的电子结构使其在燃料电池阴极氧还原催化反应(ORR)的应用引起广泛兴趣.金属单原子中心作为催化剂的活性位点遵循Sabatier原则,即位于火山型曲线右侧的金属中心与ORR中间产物的结合能力弱,使中间产物难以被位点活化,与之相反,左侧的位点则与中间产物的结合力太强,使中间产物难以脱附.因此,构建中间产物结合强度适中的位点可以提高催化剂活性.研究表明,通过改变金属中心的配位环境可以调控金属的电子结构,进而调节对ORR中间产物的吸附能力,提高催化剂活性.例如,对于位于火山曲线左侧的FeN4位点,通过在配位原子中引入高电负性的基团,如OH,S,Cl,减弱其对ORR中间产物的结合强度,使其活性向右侧活性顶点移动.然而,目前还没有提出有效的策略来改善火山曲线右侧位点的弱吸附能力.例如,Pd和Pt的纳米催化剂具有较高的ORR活性,而其单原子MN4位点却对ORR呈惰性,原因为高电负性的N使M具有较高的电荷密度,使其对中间产物的吸附能力太弱而位于火山曲线的最右边,具有较差的本征活性.为了缓解此类金属与配位N之间的不匹配的问题,我们提出通过利用电负性低的C(χp=2.55)代替N(χp=3.04)配位,以提升弱吸附位点的本征活性.本文制备了Ir-N-C,Pd-N-C,Pt-N-C,Ir-C,Pd-C和Pt-C六种催化剂;X射线衍射、透射电极和球差电镜结果表明,催化剂不含金属颗粒;同步辐射结果表明,催化剂呈M-N/C配位形式存在,且价态为氧化态.飞行时间二次离子质谱法(ToF-SIMS)测试可以检测到MNXCY和MCX的位点结构,结合同步辐射结果,认为活性位点分别是IrNXC((4-X)),IrC4,PdN_(2)CX和PdN3CX,PdC4,PtN3C1和PtN4,PtC4.电化学结果表明,相比N配位的催化剂,C配位的催化剂具有更高的半波电位、起始电位及本征活性,表明C配位更适合配位金属Ir,Pd和Pt,证明本文策略的可行性.为了进一步研究催化活性提升的原因,本文采用密度泛函理论计算探究催化机理,根据实验结果(X射线吸附光谱和ToF-SIMS)模拟了催化剂的活性位点结构,并计算反应中间产物的吸附能ΔGOH*.将ΔGOH*作为横坐标,反应的起始电位即催化活性作为纵坐标,绘制图谱得到火山型曲线.结果表明,相比N配位的金属位点,C配位的金属位点向强吸附端移动并靠近活性顶点,与实验结果一致,说明催化活性的提升是因为配位环境的改变改善了位点吸附能力.随后,通过计算活性位点的Bader电荷密度探究配位环境对电子结构的调控.结果显示,低电负性的C由于吸电子能力弱于高电负性的N,使MCX位点具有较低的Bader电荷密度,从而具有较强的中间产物吸附能力,进而提高催化活性.综上,本文提出了一个普遍性的原则,通过利用低电负性的元素配位调节具有弱结合能的金属(不仅是Ir,Pd,Pt)位点活性,为不同性质的金属匹配合适的配位环境,以实现高催化活性.

Improved Staggered Algorithm for Phase-Field Brittle Fracture with the Local Arc-Length Method

The local arc-length method is employed to control the incremental loading procedure for phase-field brittle fracture modeling.An improved staggered algorithm with energy and damage iterative tolerance convergence criteria is developed based on the residuals of displacement and phase-field.The improved staggered solution scheme is implemented in the commercial software ABAQUS with user-defined element subroutines.The layered system of finite elements is utilized to solve the coupled elastic displacement and phase-field fracture problem.A one-element benchmark test compared with the analytical solution was conducted to validate the feasibility and accuracy of the developed method.Our study shows that the result calculated with the developed method does not depend on the selected size of loading increments.The results of several numerical experiments show that the improved staggered algorithm is efficient for solving the more complex brittle fracture problems.

Boron separation by adsorption and flotation with Mg-Al-LDHs and SDBS from aqueous solution

Layered double hydroxides(LDHs)have been shown to be effective adsorbents for boron.However,solid-liquid separation is still a problem when separating boron from industrial radioactive waste liquid.In this research,three types of Mg-Al-LDHs including Mg-Al-LDH(NO_(3)^(-)),Mg-Al-LDH(Cl^(-))and Mg-Al-LDH(SO_(4)^(2-))were applied to adsorb boron,and moreover sodium dodecylbenzenesulfonate(SDBS)was used to float the LDH particles from aqueous solution after boron adsorption.The results showed that 60 min was sufficient for the equilibrium adsorption of the three LDHs.The boron adsorption capacity of three LDHs was determined as follows:Mg-Al-LDH(NO_(3)^(-))>Mg-Al-LDH(Cl^(-))>Mg-Al-LDH(SO_(4)^(2-)),and was 2.0,0.98 and 0.2 mmol·g^(-1),each ranging from 0 to 80 mmol·L^(-1)with the initial boron concentration.The efficiency of boron removal by Mg-Al-LDH(NO_(3)^(-))and SDBS can reach up to 89.7%.Furthermore,the boron flotation mechanism of SDBS and LDHs has been studied,since SDBS as a flotation agent can react with LDHs and penetrate into the interlayer of LDHs in addition to electrostatic attraction.Therefore,LDHs in solution can be floated onto the foam layer to be separated from the solution,and the clarified solution was obtained.The method is simple and promising for boron removal from aqueous solution.

视网膜疾病基因疗法:全球研究进展和挑战

过去30年来,基因疗法给多种遗传性和获得性视网膜疾病患者带来了很大希望。在大量临床前研究的基础上,全球已进行了60多项视网膜疾病基因疗法的各期临床试验,且Luxturna,一种用于由RPE65突变导致的2型Leber先天性黑朦(LCA2)的基因治疗产品已批准用于临床。目前的基因传递系统虽然在临床研究中取得了一些成功,但为保证安全性和有效性,仍然需要在载体和眼内给药方式上加以改进和变通。新的腺相关病毒载体技术的发展使大基因传递和小创伤载体注射方式的研究都有所进展。规律间隔成簇短回文重复序列(CRISPR)技术也丰富了基因疗法的手段,必将在视网膜疾病的治疗中得到广泛应用。虽然基因疗法仍面临许多挑战,但预计在未来若干年中,它将成为目前许多不可治性视网膜疾病的临床治疗选择。

Dynamic behavior of the Qinghai-Tibetan railway embankment in permafrost regions under trained-induced vertical loads

The unfrozen water content and ice content of frozen soil change continuously with varying temperatures,resulting in the temperature dependence of mechanical properties of frozen soil.Thus the dynamic behavior of embankment in permafrost regions under train loading also alters with seasons.Based on a series of strong-motion tests that were carried out on the traditional embankment of Qinghai-Tibet Railway(QTR)in permafrost regions,the acceleration waveforms recorded at the embankment shoulder and slope toes were obtained.Testing results show an obvious attenuation effect on the vertical train loading from road shoulder to slope toes.Furthermore,numerical simulations of a traditional embankment under vertical train loading in different seasons were conducted,and the dynamic behavior of the embankment was described.The results show that the vibration attenuation in the cold season is greater than that in the warm season.The maximum acceleration of vibration drops to about 5%when the train vibration load is transferred through the embankment into the permafrost,and the high-frequency components are absorbed when the vibration transmits downward.Moreover,the dynamic stress under the dynamic train loading decreases exponentially with an increasing depth in different seasons.The results can be a reference for design and maintenance of embankments in permafrost regions.

Numerical modelling of vibration response in loess hills due to a high-speed train on railway viaduct

In order to accurately analyze vibration characteristics and site effects of loess hills under moving load of a highspeed train,four types of loess hill models under railway viaduct was established by ABAQUS of finite element analysis software by field test.The dynamic response and stability of loess hills under two different vibration sources under high-speed train load were studied by using two-dimensional equivalent linear response timehistory analysis,and the influence of the mechanical parameters of loess on the vibration of different types of loess hill was analyzed.Results show that there are obvious differences between peak displacement cloud maps of loess hills under the railway viaduct under gravity and train load action.We analyzed the influence of the change of elastic modulus on vibration propagation of soil of foundation and loess knoll,and found that the change of elastic modulus of soil in different position of foundation has more effect on vibration propagation than that of loess knoll soil.At the same time,the vertical acceleration cloud maps of the four types of loess hills are obviously different.

Mechanical properties and strengthening mechanism of silty sands stabilized with steel slag-based geopolymer binder

Geopolymer binder has the advantages of early strength,fast solidification,high volume stability,and low permeability.It is beneficial to improve the mechanical performance of silty sands,saving cement consumption and being environmentally friendly.However,the strength improvement of silty sand stabilized with steel slag-based geopolymer was significantly controlled by their material composition and technical parameters.This study conducted a series of unconfined compression tests to investigate the material composition of steel slag-based geopolymer binders and their reasonable mixing ratio for silty sand stabilization.The optimum mixing ratio of precursor(steel slag)to alkaline activator(the combination of Na2SiO3 and CaO)and the optimum dosage of steel slag-based geopolymer for silty sand stabilization were explored.The strengthening mechanism of geopolymer-stabilized silty sands was discussed based on microstructural images and elemental concentrations of primary components observed by SEM and EDS.The results show that when the mass ratio of steel slag:Na2SiO3:CaO was 80:35:21,and the steel slag-based geopolymer material was 15%,the silty sand could achieve the best mechanical performance improvement.The microstructural characteristics of geopolymer-stabilized silty sands at different curing ages illustrated that the compactness and integrity of silty sand structures were enhanced over the curing age.The improving cementitious contact among particles and enlarging particle size was responsible for the strength improvement of silty sand.This research can provide a reference for applying steel slag-based geopolymer in silty sand stabilization in engineering practices.

Dehydrogenation behavior and mechanism of LiAlH_(4) adding nano-CeO_(2)with different morphologies

Complex hydride LiAlH_(4),as a hydrogen storage material,possesses high theoretical hydrogen storage capacity(10.5 wt.%).However,highly efficient additives are urgently required to modify its thermal stability and sluggish kinetics.Some additives exhibit unique morphology-dependent characteristics.Herein,the efficient rare earth oxide nano-CeO_(2)additives with different morphologies(nanoparticles,nanocubes,and nanorods)are prepared by the hydrothermal method,and the intrinsic properties are characterized.The three different morphologies of nano-CeO_(2),which are different in the Ce^(3+)content and specific surface area,are added to LiAlH_(4)to improve the dehydrogenation behavior.The LiAlH_(4)-CeO_(2)-nanorod composite exhibits the optimal dehydrogenation behavior,which begins to desorb hydrogen at 76.6℃ with a hydrogen capacity of 7.17 wt.%,and 3.83 wt.%hydrogen is desorbed within 30 min at 140℃.The dehydrogenation process of the composites demonstrates that hydrogen release is facilitated by the in-situ formed CeH_(2).73 and the facile transition between the oxidation states of Ce^(4+)and Ce^(3+).Combined with density functional theory calculations,the addition of nano-CeO_(2)can weaken the Al-H bond and accelerate the decomposition of[AlH_(4)]^(4-)tetrahedron,which is consistent with the reduction of the decomposition activation energy.

CO驱动的燃料电池可实现H2车载净化

由于CO优先吸附并毒化催化剂表面,质子交换膜燃料电池(PEMFC)即使在CO以ppm级别(<10 ppm)存在时性能也会因严重毒化而大幅降低.本文报道了一种原子级分散的Rh基催化剂,使得CO不仅不毒化电池,还成为了PEMFC的燃料.CO的起始氧化电位为0 V,且纯CO驱动的PEMFC功率密度达到了空前的236 mW cm^(−2),其最大转化频率(TOF,64.65 s^(−1),363 K)远远超过了目前所报道的任何化学或者电化学催化剂.利用这一功能,可以选择性地使用PEMFC技术纯化氢气中的少量CO杂质.仅通过运行一个单电池,CO的浓度可以降低1个数量级.这种Rh单位点催化剂对CO的催化转化行为归因于弱的CO吸附和两个相邻Rh位点在CO和H_(2)O分子之间的共活化的相互作用.

Methylammonium cation deficient surface for enhanced binding stability at TiO2/CH3NH3PbI3 interface

Heterojunction interfaces in perovskite solar cells play an important role in enhancing their photoelectric properties and stability. Till date, the precise lattice arrangement at TiO2/CH3NH3PbI3 heterojunction interfaces has not been investigated clearly. Here, we examined a TiO2/CH3NH3PbI3 interface and found that a heavy atomic layer exists in such interfaces, which is attributed to the vacancies of methylammonium （MA） cation groups. Further, first-principles calculation results suggested that an MA cation-deficient surface structure is beneficial for a strong heterogeneous binding between TiO2 and CH3NH3PbI3 to enhance the interface stability. Our research is helpful for further understanding the detailed interface atom arrangements and provides references for interfacial modification in perovskite solar cells.

Crystallographic facet-dependent stress responses by polyhedral lead sulfide nanocrystals and the potential ＂safe-by-design＂ approach

The particular physicochemical properties of nanomaterials are able to elicit unique biological responses. The property activity relationship is usually established for in-depth understanding of toxicity mechanisms and designing safer nanomaterials. In this study, the toxic role of specific crystallographic facets of a series of polyhedral lead sulfide （PbS） nanocrystals, including truncated octahedrons, cuboctahedrons, truncated cubes, and cubes, was investigated in human bronchial epithelial cells （BEAS-2B） and murine alveolar macrophages （RAW 264.7） cells./100} facets were found capable of triggering facet-dependent cellular oxidative stress and heavy metal stress responses, such as glutathione depletion, lipid peroxidation, reactive oxygen species （ROS） production, heme oxygenase-1 （HO-1） and metallothionein （MT） expression, and mitochondrial dysfunction, while {111} facets remained inert under biological conditions. The {100}-facet-dependent toxicity was ascribed to {100}-facet-dependent lead dissolution, while the low lead dissolution of {111} facets was due to the strong protection afforded by poly（vinyl pyrrolidone） during synthesis. Based on this facet-toxicity relationship, a ＂safe-by-design＂ strategy was designed to prevent lead dissolution from {100} facets through the formation of atomically thin lead-chloride adlayers, resulting in safer polyhedral PbS nanocrystals.

Sinter-resistant and high-efficient Pt/CeO2/NiAl2O4/Al2O3@SiO2 model catalysts with “composite energy traps”

The stability of nanosized catalysts at high temperature is still a challenging topic and is a crucial criterion to evaluate their suitability for industrial use. Currently, the strategy to improve the high-temperature stability of nano-sized catalysts is to restrict the migration of particles on the surface, which, however, lacks theoretical knowledge and directions. Herein, we reported a new approach that can effectively inhibit the migration and agglomeration of supported nanoparticles by fabrication of a model catalyst Pt/CeO2/NiAl2O4/Al2O3@SiO2. This catalyst is highly stable with the microstructure unchanged even after being aged at 1000 °C. Density functional theory calculations indicate that two types of confinement effects exist in the catalyst and their mechanisms were well explained from the viewpoint of "energy traps" which can also be applied to other supported catalysts.