合金纳米颗粒原子级分散制备的双位点催化剂中单点Ag1对Pd1在炔烃双烷氧羰基化反应中的促进作用

炔烃烷氧羰基化反应是一种原子经济反应,可以生成α,β-不饱和羧酸、二羧羧酸酯及其衍生物.其中,乙炔的双烷氧羰基化产物(丁烯二酸二酯)可通过两段加氢制备1,4-丁二醇,该反应路径对于生产生物可降解材料聚丁二酸丁二醇酯(PBS)具有重要意义.然而,目前广泛应用的均相Pd基催化体系存在催化剂分离困难、金属流失以及需要添加有机膦配体或各种磺酸助剂等问题,这不仅增加了制备成本,还可能引入不必要的杂质.此外,尽管纳米催化剂在催化领域有着广泛的应用,但大多数纳米催化剂在炔烃双烷氧基羰基化反应中仍表现出反应活性低和稳定性差的问题,这限制了其在工业生产中的应用.相比之下,多相单金属点催化剂因其100%的原子利用率和较好的催化活性而备受关注.然而,由于多相单金属点催化剂具有相对简单的结构和配位环境,其应用仍受到一定的限制.为了克服这一难题,合成具有不同金属的双位点催化剂成为了一个研究热点.双位点催化剂有望进一步释放单金属位点催化剂的潜力,提高反应活性和稳定性.然而,尽管双位点催化剂具有巨大的应用前景,但目前仍面临着制备方法的挑战.如何设计并实用可行地制备出具有高效催化性能的双位点催化剂,是当前科研领域亟待解决的问题.本文提出一种由CO/CH3I混合物诱导分散Pd-Ag合金纳米颗粒的处理方法,用于制备Pd1-Ag1/AC双位点催化剂.利用X射线衍射、X射线光电子能谱和高角环形暗场扫描透射电子显微镜等方法对分散过程进行表征.结果表明,在分散过程中,CO和CH3I与金属发生协同作用,通过配位生成一种独特的双核络合物(PdI2(CO)-I2-AgI)结构.这种络合物结构逐一从合金纳米颗粒上剥落,直至完成原子级的分散.乙炔双烷氧羰基化反应结果表明,与单金属Pd1/AC相比,在相同反应条件下,双位点Pd1-Ag1/AC催化剂上的乙炔转化率提高了2倍.同时,在保持丁烯二酸二酯选择性为98%的情况下,该催化剂循环使用10次后催化剂性能未见明显变化.炔烃底物拓展结果表明,Pd1-Ag1/AC的协同作用不仅可以提高催化活性,而且在高位阻存在的情况下可以提高产物的立体选择性,这表明双位点协同促进作用对炔烃双羰基化反应具有良好的普适性.微分吸附量热表征结果证明了通过碘配体桥连的Pd1和Ag1单金属点之间的协同效应可以增强催化剂对乙炔的吸附能力.此外,结合拓展边X射线精细结构等实验结果,对催化剂的结构变化和反应机理进行了进一步的探究.第一性原理计算得到的反应能垒和过渡态结果表明,乙炔吸附是反应的速率决定步骤.与Pd1/AC相比,Pd1-Ag1/AC催化剂在反应决速步上表现出了更低的活化能,这从热力学和动力学两个层面解释了其具有更好催化性能的原因.Bader电荷分析和投影态密度计算的结果表明,由于Ag1位点的引入,使Pd1-Ag1/AC催化剂的电荷转移能力和吸附能力均得到增强.相较于Pd1/AC,Pd1-Ag1/AC具有更低的形成能,表明Ag1位点不仅可以提升催化活性,也可以增强双位点结构的稳定性.因此,在催化反应中,Ag1位点的存在起到了重要的促进作用,使得Pd1-Ag1/AC催化剂表现出更优异的性能.综上,本文制备的双位点Pd1-Ag1/AC催化剂在乙炔双烷氧羰基化反应中表现出较好的催化活性和稳定性,证明了双位点协同效应的重要性,为高效催化剂的设计和应用提供了新的思路.

Oxygen vacancy-rich amorphous FeNi hydroxide nanoclusters as an efficient electrocatalyst for water oxidation

In this work,a one-pot strategy is presented to directly synthesize amorphous Fe_(x)Ni_(y) hydroxide nanoclusters(denoted as ANC-Fe_(x)Ni_(y),<2 nm)with oxygen vacancies induced by ionic liquids.The ANC-Fe_(x)Ni_(y) catalyst presents abundant catalytic sites and high intrinsic conductivity.As such,the optimized ANC-Fe_(1)Ni_(2) exhibits high activity in oxygen evolution reaction(OER)with a Tafel slope of 39 m V dec^(–1) and an overpotential of 266 m V at 10 m A cm^(-2).Notably,the optimized ANC-Fe_(1)Ni_(2) shows an extraordinarily large mass activity of 3028 Ag_(FeNi)^(–1) at the overpotential of 300 m V,which is~24-fold of commercial RuO_(2) catalyst.The superior activity of these Fe_(x)Ni_(y) hydroxide nanoclusters is ascribed to(i)the amorphous and distorted structure with abundant oxygen vacancies,and(ii)enhanced active site density by downsizing the ANC-FexNiyclusters.This strategy provides a novel route for enhancing OER electrocatalytic performance and highly encouraging for the future application of amorphous metal hydroxides in catalysis.

Maximizing the mechanical performance of Ti_(3)AlC_(2)-based MAX phases with aid of machine learning

Mechanical properties consisting of the bulk modulus,shear modulus,Young’s modulus,Poisson’s ratio,etc.,are key factors in determining the practical applications of MAX phases.These mechanical properties are mainly dependent on the strength of M–X and M–A bonds.In this study,a novel strategy based on the crystal graph convolution neural network(CGCNN)model has been successfully employed to tune these mechanical properties of Ti_(3)AlC_(2)-based MAX phases via the A-site substitution(Ti_(3)(Al1-xAx)C_(2)).The structure–property correlation between the A-site substitution and mechanical properties of Ti_(3)(Al1-xAx)C_(2)is established.The results show that the thermodynamic stability of Ti_(3)(Al1-xAx)C_(2)is enhanced with substitutions A=Ga,Si,Sn,Ge,Te,As,or Sb.The stiffness of Ti_(3)AlC_(2)increases with the substitution concentration of Si or As increasing,and the higher thermal shock resistance is closely associated with the substitution of Sn or Te.In addition,the plasticity of Ti_(3)AlC_(2)can be greatly improved when As,Sn,or Ge is used as a substitution.The findings and understandings demonstrated herein can provide universal guidance for the individual synthesis of high-performance MAX phases for various applications.

Evolution of precipitate and precipitate/matrix interface in Al-Zn-Mg-Cu(-Ag)alloys

Evolution of precipitate and precipitate/matrix interface in artificially aged Al-Zn-Mg-Cu(-Ag)alloys has been systematically studied.In the early stage of ageing,Ag,as a fast diffuser,can promote the formation of solute pairs and small clusters.Solute clusters are further demonstrated to be able to act as precursors forη’precipitates by in-situ STEM heating.With prolonged ageing time,the precipitate/matrix interface evolves from the Zn-dominated interface between early-stageη’and Al matrix to the Zn and Mg co-segregatedη’/Al andη_(2)/Al interfaces.Theη’/Al interfacial layers are shown to precede the formation ofη’,while theη_(2)/Al interfaces are found to be closely related to the thickening process ofη_(2)and the involved particular atomic movements are specified.Experimental observations and DFT calculations re-veal that forη’andη_(2),Ag can dissolve into the precipitate as well as locate at the precipitate/matrix interface without showing preference.For Cu,its dissolution in the precipitate and segregation on the interface mainly occur forη_(2)rather thanη’.The incorporation of Ag and Cu does not change the defined precipitate structure.

Morphology effects in MnCeO_(x)solid solution-catalyzed NO reduction with CO:Active sites,water tolerance,reaction pathway

Morphological effects of nanoparticles are crucial in many solid-catalyzed chemical transformations.We herein prepared two manganese-ceria solid solutions,well-defined MnCeO_(x)nanorods and MnCeO_(x)-nanocubes,exposing preferentially(111)and(100)facets of ceria,respectively.The incorporation of Mn dopant into ceria lattice strongly enhanced the catalytic performance in the NO reduction with CO.MnCeO_(x)(111)catalyst outperformed MnCeO_(x)(100)counterpart due to its higher population density of oxygen vacancy defects.In-situ infrared spectroscopy investigations indicated that the reaction pathway over MnCeO_(x)and pristine CeO_(2)is similar and that besides the direct pathway,an indirect pathway via adsorbed hyponitrite as an intermediate cannot be ruled out.X-ray photoelectron and Raman spectroscopies as well as first-principles density functional theory(DFT)calculations indicate that the enhanced catalytic performance of MnCeO_(x)can be traced back to its“Mn–OL(VÖ)–Mn–OL(VÖ)–Ce”connectivities.The Mn dopant strongly facilitates the formation of surface oxygen vacancies(VÖ)by liberating surface lattice oxygen(OL)via CO*+OL→CO_(2)*+VÖand promotes the reduction of NO,according to NO*+VÖ→N*+OL and 2N*→N_(2).The Mn dopant impact on both the adsorption of CO and activation of OL reveals that a balance between these two effects is critical for facilitating all reaction steps.