高(111)晶面暴露的Pd纳米颗粒的制备与表征及其加氢性能

通过液相氢气还原法,在不同温度下制备出了不同（111）晶面占比的Pd单晶纳米颗粒,用活性炭吸附制备成Pd/C纳米催化剂。通过透射电子显微镜（TEM）、傅里叶变换（FFT）、X射线衍射（XRD）表征证实了低温下制备的Pd纳米颗粒具有较高的（111）晶面占比。氢氧脉冲滴定（H₂-O₂）和H₂-程序升温脱附（H₂-TPD）结果显示,上述催化剂表面吸附氢气量与其Pd（111）晶面占比呈线性关系。此外,该系列Pd/C催化剂具有相似的粒径4.3 nm以及较窄的尺寸分布,相近的孔隙参数和Pd负载量,从而可对比（111）晶面比例差异对其加氢性能的影响。3个探针反应（苯乙烯、环己烯和对硝基甲苯的加氢反应）的实验结果表明,相比于低（111）晶面暴露比例的Pd/C催化剂,含有高（111）晶面暴露比例的Pd/C催化剂显示出更高的加氢活性,且Pd（111）晶面比例与氢气消耗速率呈一定的线性关系,这归因于H₂优先吸附于Pd（111）晶面促进了活性氢原子的形成。基于以上分析,高（111）晶面暴露的Pd基催化剂有利于加氢性能的提高。     

Competing Reaction Pathways in Heterogeneously Catalyzed Hydrogenation of Allyl Cyanide: The Chemical Nature of Surface Species

We present a mechanistic study on the formation of an active ligand layer over Pd(111), turning the catalytic surface highly active and selective in partial hydrogenation of an α,β-unsaturated aldehyde acrolein. Specifically, we investigate the chemical composition of a ligand layer consisting of allyl cyanide deposited on Pd(111) and its dynamic changes under the hydrogenation conditions. On pristine surface, allyl cyanide largely retains its chemical structure and forms a layer of molecular species with the CN bond oriented nearly parallel to the underlying metal. In the presence of hydrogen, the chemical composition of allyl cyanide strongly changes. At 100 K, allyl cyanide transforms to unsaturated imine species, containing the C=C and C=N double bonds. At increasing temperatures, these species undergo two competing reaction pathways. First, the C=C bond become hydrogenated and the stable N-butylimine species are produced. In the competing pathway, the unsaturated imine reacts with hydrogen to fully hydrogenate the imine group and produce butylamine. The latter species are unstable under the hydrogenation reaction conditions and desorb from the surface, while the N-butylimine adsorbates formed in the first reaction pathway remain adsorbed and act as an active ligand layer in selective hydrogenation of acrolein.     

密度泛函理论计算研究表面应力对钯催化乙炔选择性加氢活性与选择性的影响（英文）

在石油催化裂解过程中,除了生成乙烯、丙烯及丁烯等烯烃,还会产生部分炔烃.目前工业上通常采用炔烃选择性加氢转化为对应的单烯烃,以除去其中炔烃.由于产品烯烃中的炔烃等杂质含量需极低,这就对用于加氢催化剂的活性和选择性提出了很高的要求,即催化剂需要选择性吸附炔烃并加氢,而不损失其中的烯烃.经过前期大量的基础研究工作,目前工业中炔烃选择性加氢应用最广泛的催化剂是负载型钯基催化剂.然而,单独的钯金属选择性并不理想,因而对其选择性以及活性进行调控成为了当前关注的研究课题.本文采用密度泛函理论计算结合微观反应动力学模拟手段,研究了钯金属表面应力存在条件下的活性与选择性,以及形成次表层物种的可能性和形成后的活性与选择性.研究发现,改变钯金属的晶格参数与表面应力,反应物、表面反应中间体和产物的吸附能都会产生相应的变化,且吸附能与晶格参数的变化存在线性关系,晶格参数越大,吸附越强.利用表面反应过渡态能量与初始态能量之间的线性关系,相应的乙炔加氢生成乙烯的反应速率可以通过微观反应动力学模拟得到.结果显示,不同晶格参数的钯催化剂催化乙炔加氢生成乙烯的反应活性位于相应火山型曲线的强吸附侧,即减弱乙炔和氢的吸附强度可提高乙烯的生成速率.在此基础上,本文研究了不同表面应力的钯催化剂在次表面吸附不同覆盖度碳原子和氢原子的情况,发现晶格参数越大越有利于碳原子和氢原子在次表面的吸附.同时,研究发现在次表面碳掺杂的条件下,不同表面应力条件下的钯催化剂的活性均有所增强.此外,由于乙烯在所有研究的钯催化剂表面脱附比进一步加氢容易,因而乙烯都可以选择性生成.     

纳米Pd的形貌和尺寸可控制备及其1,4-丁炔二醇加氢性能

采用化学还原法合成Pd纳米立方体,并将其作为晶种,进一步合成大尺寸的纳米Pd立方体以及具有不同{100}和{111}晶面比例的纳米Pd多面体.将形貌和尺寸可控的纳米Pd溶胶应用于1,4-丁炔二醇催化加氢的反应中,反应结果表明,纳米Pd的催化性能取决于其尺寸和形貌.{111}晶面的催化活性高于{100}晶面,PVP稳定的Pd胶体对1,4-丁烯二醇均具有较高选择性,具有适当{100}和{111}晶面比例的纳米Pd多面体对1,4-丁烯二醇的选择性可达96%.     

Fabrication of Ultrafine Palladium Phosphide Nanoparticles as Highly Active Catalyst for Chemoselective Hydrogenation of Alkynes

Monodisperse palladium phosphide nanoparticles (Pd–P NPs) with a smallest size ever reported of 3.9 nm were fabricated using cheap and stable triphenylphosphine as phosphorous source. After the deposition and calcination at 300 °C and 400 °C, the resulting Pd–P NPs increased in size to 4.0 nm and 4.8 nm, respectively. Notably, the latter NPs probably crystallized with a single phase of Pd₃P_0.95, which acted as a highly active catalyst in semi‐ and stereoselective hydrogenation of alkynes. X‐ray photoelectron spectroscopy analysis determined a positive shift of binding energy for Pd(3d) in Pd–P NPs compared to that in Pd on carbon. It indicated the electron flow from metal to phosphorus and the larger electron deficiency of Pd in Pd–P NPs, which suppressed palladium hydride formation and subsequently increased the selectivity. Thus, this result may also indicate the applications of Pd–P and other metal–P NPs in various selective hydrogenation reactions.     

Palladium Nanoparticles Bonded to Two‐Dimensional Iron Oxide Graphene Nanosheets: A Synergistic and Highly Reusable Catalyst for the Tsuji–Trost Reaction in Water and Air

Low cost, high activity and selectivity, convenient separation, and increased reusability are the main requirements for noble‐metal‐nanocatalyst‐catalyzed reactions. Despite tremendous efforts, developing noble‐metal nanocatalysts to meet the above requirements remains a significant challenge. Here we present a general strategy for the preparation of strongly coupled Fe₃O₄ and palladium nanoparticles (PdNPs) to graphene sheets by employing polyethyleneimine as the coupling linker. Transmission electron microscopic images show that Pd and Fe₃O₄ nanoparticles are highly dispersed on the graphene surface, and the mean particle size of Pd is around 3 nm. This nanocatalyst exhibits synergistic catalysis by Pd nanoparticles supported on reduced graphene oxide (rGO) and a tertiary amine of polyethyleneimine (Pd/Fe₃O₄/PEI/rGO) for the Tsuji–Trost reaction in water and air. For example, the reaction of ethyl acetoacetate with allyl ethyl carbonate afforded the allylated product in more than 99 % isolated yield, and the turnover frequency reached 2200 h^?1. The yield of allylated products was 66 % for Pd/rGO without polyethyleneimine. The catalyst could be readily recycled by a magnet and reused more than 30 times without appreciable loss of activity. In addition, only about 7.5 % of Pd species leached off after 20 cycles, thus rendering this catalyst safer for the environment.     

Highly dispersed palladium nanoclusters incorporated in amino‐functionalized silica spheres for the selective hydrogenation of succinic acid to γ‐butyrolactone

Highly dispersed palladium nanoclusters incorporated on amino‐functionalized silica sphere surfaces (Pd/SiO₂‐NH₂) were fabricated by a simple one‐pot synthesis utilizing 3‐(2‐aminoethylamino)propyltrimethoxysilane (AAPTS) as coordinating agent. Uniform palladium nanoclusters with an average size of 1.1 nm can be obtained during the co‐condensation of tetraethyl orthosilicate and AAPTS owing to the strong interaction between palladium species and amino groups in AAPTS. The palladium particle size can be controlled by addition of AAPTS and plays a significant role in the catalytic performance. The Pd/SiO₂‐NH₂ catalyst exhibits high catalytic activity for succinic acid hydrogenation with 100% conversion and 94% selectivity towards γ‐butyrolactone using 1,4‐dioxane as solvent at 240°C and 60 bar for 4 h. Moreover, the Pd/SiO₂‐NH₂ catalyst is robust and readily reusable without loss of its catalytic activity. Copyright © 2015 John Wiley & Sons, Ltd.     

Acrolein hydrogenation on Pt(211) and Au(211) surfaces: a density functional theory study

Partial hydrogenation of acrolein, the simplest α,β-unsaturated aldehyde, is not only a model system to understand the selectivity in heterogeneous catalysis, but also technologically an important reaction. In this work, the reaction on Pt(211) and Au(211) surfaces is thoroughly investigated using density functional theory calculations. The formation routes of three partial hydrogenation products, namely propenol, propanal and enol, on both metals are studied. It is found that the pathway to produce enol is kinetically favoured on Pt while on Au the route of forming propenol is preferred. Our calculations also show that the propanal formation follows an indirect pathway on Pt(211). An energy decomposition method to analyze the barrier is utilized to understand the selectivities at Pt(211) and Au(211), which reveals that the interaction energies between the reactants involved in the transition states play a key role in determining the selectivity difference.     

In Situ Raman Monitoring and Manipulating of Interfacial Hydrogen Spillover by Precise Fabrication of Au/TiO2/Pt Sandwich Structures

The spillover of hydrogen species and its role in tuning the activity and selectivity in catalytic hydrogenation have been investigated in situ using surface‐enhanced Raman spectroscopy (SERS) with 10 nm spatial resolution through the precise fabrication of Au/TiO₂/Pt sandwich nanostructures. In situ SERS study reveals that hydrogen species can efficiently spillover at Pt‐TiO₂‐Au interfaces, and the ultimate spillover distance on TiO₂ is about 50 nm. Combining kinetic isotope experiments and density functional theory calculations, it is found that the hydrogen spillover proceeds via the water‐assisted cleavage and formation of surface hydrogen–oxygen bond. More importantly, the selectivity in the hydrogenation of the nitro or isocyanide group is manipulated by controlling the hydrogen spillover. This work provides molecular insights to deepen the understanding of hydrogen activation and boosts the design of active and selective catalysts for hydrogenation.     

Revealing the Dynamic Structure of Complex Solid Catalysts Using Modulated Excitation X‐ray Diffraction

X‐ray diffraction (XRD) is typically silent towards information on low loadings of precious metals on solid catalysts because of their finely dispersed nature. When combined with a concentration modulation approach, time‐resolved high‐energy XRD is able to provide the detailed redox dynamics of palladium nanoparticles with a diameter of 2 nm in 2 wt % Pd/CZ (CZ=ceria–zirconia), which is a difficult sample for extended X‐ray absorption fine structure (EXAFS) measurements because of the cerium component. The temporal evolution of the Pd(111) and Ce(111) reflections together with surface information from synchronous diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) measurements reveals that Ce maintains Pd oxidized in the CO pulse, whereas reduction is detected at the beginning of the O₂ pulse. Oxygen is likely transferred from Pd to Ce³⁺ before the onset of Pd re‐oxidation. In this context, adsorbed carbonates appear to be the rate‐limiting species for re‐oxidation.     

Properties of Pd–Ag/C catalysts in the reaction of selective hydrogenation of acetylene

Samples of Pd/C and Pd–Ag/C, where C represents carbon nanofibers (CNFs), are synthesized by methane decomposition on a Ni–Cu–Fe/Al₂O₃ catalyst. The properties of Pd/CNF are studied in the reaction of selective hydrogenation of acetylene into ethylene. It is found that the activity of the catalyst in hydrogenation reaction increases, while selectivity decreases considerably when the palladium content rises. The obtained dependences are caused by the features of palladium’s interaction with the carbon support. At a low Pd content (up to 0.04 wt %) in the catalyst, the metal is inserted into the interlayer space of graphite and the catalytic activity is zero. It is established by EXAFS that the main share of palladium in catalysts of 0.05–0.1 wt % Pd/CNF constitutes the metal in the atomically dispersed state. The coordination environment of palladium atoms consists of carbon atoms. An increase in the palladium content in a Pd/CNF catalyst up to 0.3 wt % leads to the formation of highly dispersed (0.8–1 nm) Pd particles. The Pd/CNF samples where palladium is mainly in the atomically dispersed state exhibit the highest selectivity in the acetylene hydrogenation reaction. The addition of silver to a 0.1 wt % Pd/CNF catalyst initially probably leads to the formation of Pd–Ag clusters and then to alloyed Pd–Ag particles. An increase in the silver content in the catalyst above 0.3% causes the enlargement of the alloyed particles and the palladium atoms are blocked by a silver layer, which considerably decreases the catalytic activity in the selective hydrogenation of acetylene.     

Crystal‐Plane‐Controlled Selectivity of Cu2O Catalysts in Propylene Oxidation with Molecular Oxygen

The selective oxidation of propylene with O₂ to propylene oxide and acrolein is of great interest and importance. We report the crystal‐plane‐controlled selectivity of uniform capping‐ligand‐free Cu₂O octahedra, cubes, and rhombic dodecahedra in catalyzing propylene oxidation with O₂: Cu₂O octahedra exposing {111} crystal planes are most selective for acrolein; Cu₂O cubes exposing {100} crystal planes are most selective for CO₂; Cu₂O rhombic dodecahedra exposing {110} crystal planes are most selective for propylene oxide. One‐coordinated Cu on Cu₂O(111), three‐coordinated O on Cu₂O(110), and two‐coordinated O on Cu₂O(100) were identified as the catalytically active sites for the production of acrolein, propylene oxide, and CO₂, respectively. These results reveal that crystal‐plane engineering of oxide catalysts could be a useful strategy for developing selective catalysts and for gaining fundamental understanding of complex heterogeneous catalytic reactions at the molecular level.     

Pd/TiN nanocomposite catalysts for selective hydrogenation of phenol and its derivatives

Pd/TiN nanocomposite catalysts were fabricated for one-step selective hydrogenation of phenol to cyclohexanone successfully. High conversion of phenol (99%) and selectivity of cyclohexanone (98%) were obtained at 30℃ and 0.2 MPa H₂ for 12 h in the mixed solvents of H₂O and CH₂Cl₂. The Pd nanoparticles were stable in the reaction, and no aggregation was detected after four successive runs. The catalytic activity and selectivity depended on slightly the Pd particle sizes. The generality of the catalysts for this reaction was demonstrated by the selective hydrogenation of phenol derivatives, which showed that the catalyst was selective for the formation of cyclohexanone.     

Size control and catalytic activity of bio-supported palladium nanoparticles

The development of nanoparticles has greatly improved the catalytic properties of metals due to the higher surface to volume ratio of smaller particles. The production of nanoparticles is most commonly based on abiotic processes, but in the search for alternative protocols, bacterial cells have been identified as excellent scaffolds of nanoparticle nucleation, and bacteria have been successfully employed to recover and regenerate platinum group metals from industrial waste. We report on the formation of bio-supported palladium (Pd) nanoparticles on the surface of two bacterial species with distinctly different surfaces: the gram positive Staphylococcus sciuri and the gram negative Cupriavidus necator. We investigated how the type of bacterium and the amount of biomass affected the size and catalytic properties of the nanoparticles formed. By increasing the biomass:Pd ratio, we could produce bio-supported Pd nanoparticles smaller than 10nm in diameter, whereas lower biomass:Pd ratios resulted in particles ranging from few to hundreds of nm. The bio-supported Pd nanoparticle catalytic properties were investigated towards the Suzuki-Miyaura cross coupling reaction and hydrogenation reactions. Surprisingly, the smallest nanoparticles obtained at the highest biomass:Pd ratio showed no reactivity towards the test reactions. The lack of reactivity appears to be caused by thiol groups, which poison the catalyst by binding strongly to Pd. Different treatments intended to liberate particles from the biomass, such as burning or rinsing in acetone, did not re-establish their catalytic activity. Sulphur-free biomaterials should therefore be explored as more suitable scaffolds for Pd(0) nanoparticle formation.     

Diene‐based polymer nanoparticles: Preparation and direct catalytic latex hydrogenation

The diene‐based polymer nanoparticles represented by poly(butadiene‐co‐acrylonitrile) were prepared in the semibatch emulsion polymerization system using Gemini surfactant (GS) trimethylene‐1,3‐bis(dodecyldimethylammonium bromide) as the emulsifier. The nanoparticles within the range of 17–54 nm were achieved with narrow molecular weight and particle size distributions. A spherical morphology was observed for the produced nanoparticles. The effects of GS concentration on the particle size, molecular weight, polymerization conversion and solid content, and composition of copolymer were investigated. The semibatch process using monomeric and conventional surfactant sodium dodecyl sulfate (SDS) was compared. At the second stage of this study, the prepared unsaturated nanoparticles were employed as the substrates for the latex hydrogenation in the presence of Wilkinson's catalyst, that is, RhCl(P(C₆H₅)₃)₃. The effects of the particle size and catalyst concentration on the latex hydrogenation rate were investigated. The particle size is found to have a significant effect on the reaction rate. When the 17‐nm nanoparticles were used as the substrates, a high conversion of 95 mol % was obtained within 18 h using only 0.1 wt % RhCl(P(C₆H₅)₃)₃. The latex hydrogenation process was completely free of organic solvents. The present synthesis and following “green” hydrogenation process can be extended to latices made from semibatch emulsion containing other diene‐based polymers. This study shows great promise for decreasing the demanded quantity of expensive catalyst and eliminating the organic solvent in the hydrogenation process. © 2012 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Selective hydrogenation of phenylacetylene into styrene on gold nanoparticles

Gold nanoparticles (2–10 nm) supported on γ-Al₂O₃ exhibit high activity and stability in the hydrogenation of phenylacetylene into styrene in the phenylacetylene-styrene mixture. The selectivity of the catalyst is particle size-dependent: the styrene-to-ethylbenzene molar ratio in the reaction products increases from 2 to 30 as the average gold particle size decreases from 8 to 2.5 nm. The selectivity of phenylacetylene hydrogenation correlates with the selectivity of phenylacetylene adsorption on Au/γ-Al₂O₃ from the phenylacetylene-styrene mixture.     

The Role of Nitrogen-doping in the Catalytic Transfer Hydrogenation of Phenol to Cyclohexanone with Formic Acid over Pd supported on Carbon Nanotubes

Highly selective one-step hydrogenation of phenol to cyclohexanone, an important intermediate in the production of nylon 6 and nylon 66, is desirable but remains a challenge. Pd nanoparticles supported on nitrogen- and oxygen-functionalized carbon nanotubes (NCNTs, OCNTs) were prepared, characterized, and applied in the hydrogenation of phenol to cyclohexanone to study the effect of N-doping. Almost full conversion of phenol with high selectivity to cyclohexanone was achieved over Pd/NCNT under mild reaction conditions using either H₂ or formic acid (FA) as a hydrogen source. The effects of reaction temperature and FA/phenol ratio and the reusability were investigated. Separate FA decomposition experiments without and with the addition of phenol were performed to investigate the reaction mechanism, especially the deactivation behavior. Deactivation was observed for both catalysts during the FA decomposition, while only Pd/OCNT rather than Pd/NCNT was deactivated in the transfer hydrogenation with FA and the FA decomposition in the presence of phenol, indicating the unique role of N-doping. Therefore, we assume that deactivation is caused by the strongly bound formates on the active Pd sites, suppressing further FA decomposition and/or transfer hydrogenation on Pd. The nonplanar adsorption of phenol on NCNTs via weak O−H⋅⋅⋅N interactions enables the occurrence of the subsequent hydrogenation by adsorbed formate on Pd.     

Small Cobalt Nanoparticles Favor Reverse Water-Gas Shift Reaction Over Methanation Under CO2 Hydrogenation Conditions**

Cobalt-based catalysts are well-known to convert syngas into a variety of Fischer–Tropsch (FTS) products depending on the various reaction parameters, in particular particle size. In contrast, the reactivity of these particles has been much less investigated in the context of CO₂ hydrogenation. In that context, Surface organometallic chemistry (SOMC) was employed to synthesize highly dispersed cobalt nanoparticles (Co-NPs) with particle sizes ranging from 1.6 to 3.0 nm. These SOMC-derived Co-NPs display significantly different catalytic performances under CO₂ hydrogenation conditions: while the smallest cobalt nanoparticles (1.6 nm) catalyze mainly the reverse water-gas shift (rWGS) reaction, the larger nanoparticles (2.1–3.0 nm) favor the expected methanation activity. Operando X-ray absorption spectroscopy shows that the smaller cobalt particles are fully oxidized under CO₂ hydrogenation conditions, while the larger ones remain mostly metallic, paralleling the observed difference of catalytic performances. This fundamental shift of selectivity, away from methanation to reverse water-gas shift for the smaller nanoparticles is noteworthy and correlates with the formation of CoO under CO₂ hydrogenation conditions.     

Activation and Spillover of Hydrogen on Sub‐1 nm Palladium Nanoclusters Confined within Sodalite Zeolite for the Semi‐Hydrogenation of Alkynes

The search for efficient nontoxic catalysts able to perform industrial hydrogenations is a topic of interest, with relevance to many catalytic processes. Herein, we describe a mechanistic phenomenon for the activation and spillover of hydrogen for remarkable selectivity in the semi‐hydrogenation of acetylene over sub‐1 nm Pd nanoclusters confined within sodalite (SOD) zeolite (Pd@SOD). Specifically, hydrogen is dissociated on the Pd nanoclusters to form hydrogen species (i.e., hydrogen atoms and hydroxyl groups) that spill over the SOD surfaces. The design and utilization of the small‐pore zeolite SOD (six‐membered rings with 0.28×0.28 nm channels) is crucial as it only allows H₂ diffusion into the channels to reach the encapsulated Pd nanoclusters and thus avoids over‐hydrogenation to form ethane. Pd@SOD exhibits an ethylene selectivity of over 94.5 %, while that of conventional Pd/SOD is approximately 21.5 %.     

Selective Hydrogenation of Citral over a Carbon‐titania Composite Supported Palladium Catalyst

A novel carbon‐titania composite material, C/TiO₂, has been prepared by growing carbon nanofibers (CNFs) on TiO₂ surface via methane decomposition using Ni‐Cu as a catalyst. The C/TiO₂ was used for preparing supported palladium catalyst, Pd/C/TiO₂. The support and Pd/C/TiO₂ catalyst were characterized by BET, SEM, XRD and TG‐DTG. Its catalytic performance was evaluated in selective hydrogenation of citral to citronellal, and compared with that of activated carbon supported Pd catalyst. It was found that the Pd/C/TiO₂ catalyst contains 97% of mesopores. And it exhibited 88% of selectivity to citronellal at citral conversion of 90% in citral hydrogenation, which was much higher than that of activated carbon supported Pd catalyst. This result may be attributed to elimination of internal diffusion limitations, which were significant in activated carbon supported Pd catalyst, due to its microporous structure.