Mechanistic consequences of composition in acid catalysis by polyoxometalate keggin clusters

The kinetics and mechanism of ether and alkanol cleavage reactions on Br?nsted acid catalysts based on polyoxometalate (POM) clusters are described in terms of the identity and dynamics of elementary steps and the stability of the transition states involved. Measured rates and theoretical calculations show that the energies of cationic transition states and intermediates depend on the properties of reactants (proton affinity), POM clusters (deprotonation enthalpy), and ion-pairs in transition states or intermediates (stabilization energy). Rate equations and elementary steps were similar for dehydration of alkanols (2-propanol, 1- and 2-butanol, tert-butanol) and cleavage of sec-butyl-methyl ether on POM clusters with different central atoms (P, Si, Co, Al). Dehydration rates depend on the rate constant for elimination from adsorbed alkanols or ethers and on the equilibrium constant for the formation of unreactive reactant dimers. Elimination involves E1 pathways and late carbenium-ion transition states. This is consistent with small kinetic isotope effects for all deuterated alkanols, with strong effects of substituents on elimination rates, and with the similar alkene stereoselectivities measured for alkanol dehydration, ether cleavage, and alkene double-bond isomerization. n-Donor reactants (alkanols, ethers) and products (water) inhibit dehydration rates by forming stable dimers that do not undergo elimination; their stability is consistent with theoretical estimates, with the dynamics of homogeneous analogues, and with the structure and proton affinity of the n-donors. Elimination rate constants increased with increasing valence of the central POM atom, because of a concurrent decrease in deprotonation enthalpies (DPE), which leads to more stable anionic clusters and ion-pairs at transition states. The DPE of POM clusters influences catalytic rates less than the proton affinity of the alkene-like organic moiety at the late carbenium-ion-type transition states involved. These different sensitivities reflect the fact that weaker acids typically form anionic clusters with a higher charge density at the transition state; these clusters stabilize cationic fragments more effectively than those of stronger acids, which form more stable conjugate bases with lower charge densities. These compensation effects are ubiquitous in acid chemistry and also evident for mineral acids. The stabilization energy and the concomitant charge density and distribution in the anion, but not the acid strength (DPE), determine the kinetic tolerance of n-donors and the selectivity of reactions catalyzed by Br?nsted acids.     

多元多金属含氧簇合物在催化化学中的应用*

多金属含氧簇合物在催化化学中的重要性与日俱增,在原子经济反应和环境友好催化方面有着诱人的实用前景。这类物种的催化活性、理化性质、分子结构和固态体相结构以及制备方法均能在分子水平上给出详细信息,因而被视为混合金属氧化物催化剂的分子设计材料。本文拟对能够直观反映出这种信息的多元多金属含氧簇合物催化的研究作一评述。     

Catalytic oxidation of methanol on Pd metal and oxide clusters at near-ambient temperatures

Supported Pd clusters catalyze methanol oxidation to methyl formate with high turnover rates and >90% selectivity at near ambient temperatures (313 K). Metal clusters are much more reactive than PdO clusters and rates are inhibited by the reactant O(2). These data suggest that ensembles of Pd metal atoms on surfaces nearly saturated with chemisorbed oxygen are required for kinetically-relevant C-H bond activation in chemisorbed methoxide intermediates. Pd metal surfaces become more reactive with increasing metal particle size. The higher coordination of surface atoms on larger clusters leads to more weakly-bound chemisorbed species and to a larger number of Pd metal ensembles available during steady-state catalysis. Chemisorbed oxygen removes H-atoms formed in C-H bond activation steps and inhibits methoxide decomposition and CO(2) formation, two functions essential for the high turnover rates and methyl formate selectivities reported here.     

DFT Studies of Palladium Model Catalysts: Structure and Size Effects

An important task for theory is the multi-scale modeling of catalytic properties of nanocrystallites with sizes ranging from clusters of few metal atoms to particles consisting of 10³–10⁴ atoms. To explore catalytic properties of nanosized metal catalysts, we developed an approach based on three-dimensional symmetric model clusters of 1–2 nm (~100 metal atoms) with fcc structure, terminated by low-index surfaces. With this modeling technique one is able to describe at an accurate DFT level various catalytic and adsorption properties of metal nanoparticles in quantitative agreement with experimental studies of model catalysts deposited on thin oxide films. Metal nanocrystallites exhibit properties that can significantly vary with their size and shape.     

Self‐Assembly and Structural Evolvement of Polyoxometalate‐Anchored Dendron Complexes

A cationic dendritic molecule that has alkyl chains has been synthesized and employed to encapsulate anionic polyoxometalates through electrostatic interactions. The prepared surfactant‐encapsulated polyoxometalate (SEP) complexes were used as building blocks to fabricate self‐assemblies in solution and the solid state. Monodispersion, lamellar, and columnar assemblies of SEP complexes have been characterized in detail. With increasing the number of peripheral cationic dendrons on inorganic clusters, the SEPs undergo changes from globular assemblies to monodispersions in solution and from lamellar assemblies to hexagonal columnar structures in the solid state, depending on the amounts of cationic dendrons in the complexes. The structural evolvement was simulated through consideration of the size and shape of the cationic dendron and polyanionic clusters, and the experimental results are in good agreement with the interpretation of the simulations. The present research demonstrates a new kind of dendritic complex and provides a route for controlling their assembling states by simply alternating the number of cationic dendrons in the complexes.     

Preparation parameter development for layer-by-layer assembly of Keggin-type polyoxometalates

Polyoxometalates possess many useful properties for electrochemical catalysis. These molecule-size clusters can be assembled into thin films through the layer-by-layer method. In this study, we determined a cluster concentration range within which layer-by-layer (LbL) films have been successfully fabricated. We also find the influence of salt added to the deposition solutions. In an attempt to understand the self-assembly process at the molecular level, thermodynamic arguments, derived from complexation between nanoscale particles and oppositely charged polyelectrolyte chains, have been employed to interpret the adsorption of polyoxometalate clusters onto a cationic polymer layer. The scaling results describe the contact mode between a polymer chain and a cluster. The assembly can be visualized with assistance by understanding the contact between the polymer chain and the cluster.     

低碳烷烃选择氧化金属氧化物催化剂的结构与其催化性能的关系

基于金属氧化物催化剂的低碳烷烃选择氧化是催化研究的一个热点和难点, 而认识催化剂的结构与其催化性能之间的关系对于获得高效催化剂, 解决低碳烷烃选择氧化反应中存在的问题至关重要. 本文围绕催化剂的结构对其催化性能的影响, 概述了V-P-O、V-Sb-O、Mo-V-Te-Nb-O和V-Mg-O等几类典型的氧化物催化剂体系在这方面的研究进展. 结合我们开发的Re-Sb-O催化剂体系, 分析了多功能中心、活性位分离、两相协同效应等方面对金属氧化物催化剂的催化性能的影响. 这些认识将有助于设计合成性能优异的催化剂及实现低碳烷烃的选择氧化活化和定向转化.     

金团簇二十面体结构融合过程中其催化活性的演变

近年来,由有机配体保护的原子精确金属团簇在合成方面已取得了重要进展,其独特的原子结构对一些化学反应产生独特的催化效果.原子精确的团簇催化剂明显不同于纳米颗粒催化剂和单原子催化剂,是一种关联均相和多相的、原子数目确定、尺寸均一、结构精确的新型催化剂.从原子尺度上精确构筑团簇催化剂,探究亚纳米尺度的微观结构对催化性能的影响...     

可溶液加工的聚(多金属氧簇降冰片烯)和聚(己酸降冰片烯)嵌段和无规共聚物的合成及催化功能的研究

将一种可有机功能化的Wells-Dawson POM与降冰片烯相连接,制备了多金属氧簇降冰片烯单体.再利用活性可控的开环易位聚合方法(ROMP),在Grubbs 3~(rd)催化剂的作用下,合成了聚(多金属氧簇降冰片烯)-聚(己酸降冰片烯)的杂化嵌段和无规共聚物(H-CPs),分别简写为Poly(POM)_m-b-Poly(COOH)_n和Poly(POM)_m-r-Poly(COOH)_n.采用~1H-NMR、~(31)P-NMR和FTIR等方法对共聚物结构进行表征,确认我们成功地合成了由共价键连接这2种单体形成的H-CPs.最后,利用带有光散射和红外探测器的凝胶渗透色谱(SEC)测定聚合物的绝对分子量和分子量分布,证明所得到的H-CPs不仅分子量可控,而且分子量分布系数较窄.最后,研究了H-CPs催化氧化四氢噻吩(THT)成环丁亚砜(THTO)反应,结果表明,相比于聚(多金属氧簇)的均聚物(Poly(POM)),H-CPs的催化活性有所下降,原因是POM催化剂含量较低以及H-CPs在催化介质中溶解性的差异.     

Stereoselective hydrolysis of amino acid esters by dipeptide catalysts and by N-decanoyl-histidine in surfactant aggregate domains

The rate constants of hydrolysis of the enantiomers of amino acid nitrophenyl esters by catalytic domains composed of cationic surfactant aggregates and dipeptide catalysts or Ndecanoyl-L -histidine have been determined at pH 7.30. The dipeptide catalysts shows large rate enhancement and stereoselectivity in aggregate domains. The surfactant structural effects are examined by investigation of the rate constants and stereoselectivities, and the nature of stereoselective catalysis is discussed.     

多金属氧酸盐催化的液相氧化反应

介绍了多金属氧酸盐的结构和性能, 综述了这类化合物具有的多功能性和可调性,并对它们与环境温和、经济廉价的条件（溶剂,氧化剂等）的良好兼容性等特点进行了概述。多金属氧酸盐可以在原子/分子层次上进行催化剂的设计,在绿色催化和清洁生产等方面具有良好的应用前景。文本总结了多金属氧酸盐在液相清洁催化氧化方面的进展,主要针对以过氧化氢和分子氧为氧化剂的烯烃类化合物的环氧化反应、烷烃和芳香族化合物的氧化反应,并详细探讨了此类氧化反应的反应机理。同时也关注了多金属氧酸盐在仿生催化领域的应用。     

Influence of stoichiometry and charge state on the structure and reactivity of cobalt oxide clusters with CO

Cationic and anionic cobalt oxide clusters, generated by laser vaporization, were studied using guided-ion-beam mass spectrometry to obtain insight into their structure and reactivity with carbon monoxide. Anionic clusters having the stoichiometries Co2O3(-), Co2O5(-), Co3O5(-) and Co3O6(-) were found to exhibit dominant products corresponding to the transfer of a single oxygen atom to CO, indicating the formation of CO 2. Cationic clusters, in contrast, displayed products resulting from the adsorption of CO onto the cluster accompanied by the loss of either molecular O 2 or cobalt oxide units. In addition, collision induced dissociation experiments were conducted with N 2 and inert xenon gas for the anionic clusters, and xenon gas for the cationic clusters. It was found that cationic clusters fragment preferentially through the loss of molecular O 2 whereas anionic clusters tend to lose both atomic oxygen and cobalt oxide units. To further analyze how stoichiometry and ionic charge state influence the structure of cobalt oxide clusters and their reactivity with CO, first principles theoretical electronic structure studies within the density functional theory framework were performed. The calculations show that the enhanced reactivity of specific anionic cobalt oxides with CO is due to their relatively low atomic oxygen dissociation energy which makes the oxidation of CO energetically favorable. For cationic cobalt oxide clusters, in contrast, the oxygen dissociation energies are calculated to be even lower than for the anionic species. However, in the cationic clusters, oxygen is calculated to bind preferentially in a less activated molecular O 2 form. Furthermore, the CO adsorption energy is calculated to be larger for cationic clusters than for anionic species. Therefore, the experimentally observed displacement of weakly bound O 2 units through the exothermic adsorption of CO onto positively charged cobalt oxides is energetically favorable. Our joint experimental and theoretical findings indicate that positively charged sites in bulk-phase cobalt oxides may serve to bind CO to the catalyst surface and specific negatively charged sites provide the activated oxygen which leads to the formation of CO 2. These results provide molecular level insight into how size, stoichiometry, and ionic charge state influence the oxidation of CO in the presence of cobalt oxides, an important reaction for environmental pollution abatement.     

Origin of Oxide sensitivity in gold-based catalysts: a first principle study of CO oxidation over Au supported on monoclinic and tetragonal ZrO2

The catalytic performance of Au/oxide catalysts can vary significantly upon the change of oxide species or under different catalyst preparation conditions. Due to its complex nature, the physical origin of this phenomenon remains largely unknown. By extensive density functional theory calculations on a model system, CO oxidation on Au/ZrO2, this work demonstrates that the oxidation reaction is very sensitive to the oxide structure. The surface structure variation due to the transformation of the oxide phase or the creation of structural defects (e.g., steps) can greatly enhance the activity. We show that CO oxidation on typical Au/ZrO2 catalysts could be dominated by minority sites, such as monoclinic steps and tetragonal surfaces, the concentration of which is closely related to the size of oxide particle. Importantly, this variation in activity is difficult to understand following the traditional rules based on the O2 adsorption ability and the oxide reducibility. Instead, electronic structure analyses allow us to rationalize the results and point toward a general measure for CO + O2 activity, namely the p-bandwidth of O2, with important implications for Au/oxide catalysis.     

Size and structure dependence of carbon monoxide chemisorption on cobalt clusters

We have carried out a series of ab initio calculations to investigate changes in the structural and magnetic properties of pristine cobalt clusters upon CO chemisorption. Our results show that binding energies of CO to 13-55 atom (0.5-1.5 nm) cobalt nanoparticles and preferred chemisorption sites depend on the cluster structure (whether fcc or icosahedral), size, and surface coverage. In addition, we find a strong influence of CO on the magnetism of the cluster, leading to magnetic moments smaller than in the bulk, at variance with pristine clusters which have magnetic moments larger than the bulk. Our findings suggest important changes in catalytic properties of cobalt at the nanoscale. Our theory suggests that at the nanoscale cluster size and surface coverage might control catalysis.     

多金属氧簇催化研究进展

多金属氧簇由于其组成和结构易于调控、具有酸性、氧化还原性、低毒性和低腐蚀性等优点,作为工业催化剂具有广阔的应用前景,是多酸化学领域的研究热点之一。本文综述了近5年来多金属氧簇在催化领域中研究的新进展,主要包括多金属氧簇的酸催化、氧化催化、双功能催化、加氢和活化二氧化碳合成碳酸酯等催化反应以及多金属氧簇的工业化应用等,并对未来发展趋势进行了展望。     

无机-有机多金属砷钒氧簇合物的合成及研究进展

多金属砷钒氧簇合物由于具有结构多样性、优良的物理特性以及广泛的应用前景而引起人们的关注。该类簇合物的合成战略是将有机配体和过渡金属配合物连接到砷钒氧骨架上以获得各种新奇结构。按此策略人们合成出一系列新奇的无机有机砷钒氧簇合物。本文综述了多金属砷钒氧簇合物的合成方法、结构性质等方面的研究进展,并对其今后研究前景进行了展望。     

Structural and electronic properties of neutral and ionic Ga(n)On clusters with n = 4-7

We report the results of a theoretical study of neutral, anionic, and cationic Ga(n)On clusters (n = 4-7), focusing on their ground-state configurations, stability, and electronic properties. The structural motif of these small gallium oxide clusters appears to be a rhombus or a hexagonal ring with alternate gallium and oxygen atoms. With the increase in the cluster size from Ga4O4 to Ga7O7, the ground-state configurations show a transition from planar to quasi-planar to three-dimensional structure that maximizes the number of ionic metal-oxygen bonds in the cluster. The ionization-induced distortions in the ground state of the respective neutral clusters are small. However, the nature of the LUMO orbital of the neutral isomers is found to be a key factor in determining the ordering of the low-lying isomers of the corresponding anionic clusters. A sequential addition of a GaO unit to the GaO monomer initially increases the binding energy, though values of the ionization potential and the electron affinity do not show any systematic variation in these clusters.     

脲醛树脂衍生的含氮碳层包覆的磷掺杂碳化钨(P-W2C@NC)全pH析氢催化剂

碳化钨由于其具有独特的类Pt电子结构和催化性能,使得其在电催化析氢领域吸引广泛关注。杂原子掺杂以及构筑高比表面积的碳化钨纳米材料是进一步提升其性能的重要策略。本研究以表面含有丰富极性官能团的脲醛树脂作为基底,在其支链骨架上均匀固载多酸阴离子簇,通过可控碳化获得了具有高比表面积的杂原子掺杂的碳化钨(P-W2C@NC)全pH析氢催化剂。该材料的BET比表面积高达136 m2 g-1。XRD、XPS、SEM和TEM表征证明催化剂是由含有磷掺杂的碳化钨纳米颗粒和包覆在碳化钨颗粒表面的薄层氮掺杂碳层共同构成。P-W2C@NC在全pH值电解液都具有高效的析氢性能,在0.5 M硫酸、1 M氢氧化钾和0.1 M磷酸缓冲液中,分别仅仅需要过电位83 mV、63 mV和179 mV就可以达到电流密度10 mA cm-2,并且具有超过20 h的长期催化稳定性。     

Structure and Stability of P1Om Cages and Their Highly Charged Protonated Clusters P1OmHn^（n＋）：Insight from Density Functional Calculations

Density functional calculations are used to determine structural and electronic properties of P4,P4O6,P4O10,P20O30 and P20O50 clusters and their protonated derivatives.These oxygen-rich phosphorus oxides are predicted to have relatively high stabilities with respect to their components P4 and O2,and their unsaturated P and end-on O atoms as the proton acceptor can accommodate multiple protons to generate highly positively charged cationic clusters,such as P20O30H1010+.Calculations indicate that P4O6 and P20O30 have higher reactivity toward the proton capture than the P4,P4O10 and P20O50 clusters,and the most stable protonated clusters among these different series of cationic clusters are P4H22+,P4O6H22+,P4O10H33+,P20O30H44+ and P20O50H44+,respectively.The cage skeleton of the phosphorus oxide clusters shows high stability for the consecutive protonation,and the unsymmetrical stretching of the skeletal P-O bond and the wagging mode of P-H coupled with the P-O bond stretching have strong adsorptions.These computational findings are useful for further experimental and theoretical studies of novel phosphorus oxide clusters and their highly positively charged derivatives.     

Effect of charge state and stoichiometry on the structure and reactivity of nickel oxide clusters with CO

The collision induced fragmentation and reactivity of cationic and anionic nickel oxide clusters with carbon monoxide were studied experimentally using guided-ion-beam mass spectrometry. Anionic clusters with a stoichiometry containing one more oxygen atom than nickel atom (NiO₂⁻, Ni₂O₃⁻, Ni₃O₄⁻ and Ni₄O₅⁻) were found to exhibit dominant products resulting from the transfer of a single oxygen atom to CO, suggesting the formation of CO₂. Of these four species, Ni₂O₃⁻ and Ni₄O₅⁻ were observed to be the most reactive having oxygen transfer products accounting for approximately 5% and 10% of the total ion intensity at a maximum pressure of 15 mTorr of CO. Our findings, therefore, indicate that anionic nickel oxide clusters containing an even number of nickel atoms and an odd number of oxygen atoms are more reactive than those with an odd number of nickel atoms and an even number of oxygen atoms. The majority of cationic nickel oxides, in contrast to anionic species, reacted preferentially through the adsorption of CO onto the cluster accompanied by the loss of either molecular O₂ or nickel oxide units. The adsorption of CO onto positively charged nickel oxides, therefore, is exothermic enough to break apart the gas-phase clusters. Collision induced dissociation experiments, employing inert xenon gas, were also conducted to gain insight into the structural properties of nickel oxide clusters. The fragmentation products were found to vary considerably with size and stoichiometry as well as ionic charge state. In general, cationic clusters favored the collisional loss of molecular O₂ while anionic clusters fragmented through the loss of both atomic oxygen and nickel oxide units. Our results provide insight into the effect of ionic charge state on the structure of nickel oxide clusters. Furthermore, we establish how the size and stoichiometry of nickel oxide clusters influences their ability to oxidize CO, an important reaction for environmental pollution abatement.