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.     

Modification of a phase-inhomogeneous alumina support of a palladium catalyst. Part II: the effect of palladium dispersion on the formation of hydride forms,electronic state,and catalytic performance in the reaction of partial hydrogenation of unsaturated hydrocarbons

It was shown for the first time that amorphous phase in an alumina support promotes the formation of palladium particles in a wide size range. This catalyst has a low selectivity to butenes in the 1,3-butadiene hydrogenation. It was suggested that surface palladium aluminates contribute to an increase in butene selectivity up to 99.5% at a hydrogenation temperature of not more than 65 °C. At higher reaction temperatures, the catalyst based on phase-homogeneous γ-Al₂O₃ has the highest activity and butene selectivity. This catalyst was obtained by the traditional impregnation method and contains highly dispersed palladium particles with a sufficiently high electron density. It was shown that the formation of hydride forms on palladium particles with a size of less than 1 nm was detected by temperature-programmed reduction with hydrogen.     

Selectivity control for the catalytic 1,3-butadiene hydrogenation on Pt(111) and Pd(111) surfaces: Radical versus closed-shell intermediates

The hydrogenation of 1,3-butadiene to different C4H8 species on both Pd(111) and Pt(111) surfaces has been studied by means of periodic slabs and DFT. We report the adsorption structures for the various mono- and dihydrogenated butadiene intermediates adsorbed on both metal surfaces. Radical species are more clearly stabilized on Pt than on Pd. The different pathways leading to these radicals have been investigated and compared to those producing 1-butene and 2-butene species. On palladium, the formation of butenes seems to be clearly favored, in agreement with the high selectivity to butenes observed experimentally. In contrast, the formation of dihydrogenated radical species seems to be competitive with that of butenes on platinum, which could explain its poorer selectivity to butenes and the formation of butane as a primary product.     

Nature of the modifying action of white phosphorus on the properties of nanosized hydrogenation catalysts based on bis(dibenzylideneacetone)palladium(0)

The catalytic properties and nature of the nanoparticles forming in the system based on Pd(dba)₂ and white phosphorus are reported. A schematic mechanism is suggested for the formation of nanosized palladium-based hydrogenation catalysts. The mechanism includes the formation of palladium nanoclusters via the interaction of Pd(dba)₂ with the solvent (N,N-dimethylformamide) and substrate and the formation of palladium phosphide nanoparticles. The inhibiting effect exerted by elemental phosphorus on the catalytic process is due to the conversion of part of the Pd(0) into palladium phosphides, which are inactive in hydrogenation under mild conditions, and the formation of mainly segregated palladium nanoclusters and palladium phosphide nanoparticles. By investigating the interaction between Pd(dba)₂ and white phosphorus in benzene, it has been established that the formation of palladium phosphides under mild conditions consists of the following consecutive steps: Pd(0) → PdP₂ → Pd₅P₂ → Pd₃P. It is explained why white phosphorus can produce diametrically opposite effects of on the catalytic properties of nanosized palladium-based hydrogenation catalysts, depending on the nature of the palladium precursor.     

Gas Phase Catalysis by Metal Nanoparticles in Nanoporous Alumina Membranes

Nanoporous alumina membranes, loaded with palladium and ruthenium nanoparticles of various size, were used for gas phase hydrogenation of 1, 3‐butadiene and for oxidation of carbon monoxide, respectively. Those membranes contain 10⁹ ‐ 10¹¹ pores per cm², all running perpendicular to the surface. Membrane discs of 20 mm in diameter and only 60 μm thick, incorporated in a reactor in which the reactants can be pumped in a closed circuit through the pores, turned out to very actively catalyze hydrogenation of butadiene (Pd) and oxidation of CO (Ru). The activity of the Pd catalysts depends characteristically on the particles size, the gas flow, and of the educts ratio. As could be expected, larger particles are less active than smaller ones, whereas increasing gas flows in case of hydrogenation accelerates the reactions. Excessive hydrogen reduces selectivity with respect to the various butenes, but favours formation of butane.     

Influence of phosphorus concentration on the state of the surface layer of Pd–P hydrogenation catalysts

Phase composition and surface layer state of the Pd–P hydrogenation catalyst formed at various P/Pd ratios from Pd(acac)₂ and white phosphorus in a hydrogen atmosphere were determined. Palladium on the catalyst surface is mainly in two chemical states: as Pd(0) clusters and as palladium phosphides. As the P/Pd ratio increases, the fraction and size of palladium clusters decrease, and also the phase composition of formed palladium phosphides changes: Pd₃P_0.8 → Pd₅P₂ → PdP₂. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.     

Influence of CeO2 addition on the physicochemical and catalytic properties of Pd/TiO2 catalysts in CO oxidation

The influence of CeO₂ addition on the formation of the microstructure, electronic state, and catalytic properties of Pd/TiO₂ supported catalysts in CO oxidation were investigated. It was shown that, when Pd is supported on titanium dioxide modified with cerium dioxide, annealing at 500°C results in the formation of Pd/(CeO₂-TiO₂) catalysts with a nanocrystalline structure composed of incoherently intergrown fine anatase crystals and interblock boundaries in which palladium and cerium are stabilized. The higher catalytic activity of Pd/(CeO₂-TiO₂) catalysts compared to Pd/TiO₂ catalysts is explained by the smaller size of Pd particles and the higher proportion of palladium in the Pd^δ+ state.     

Synthesis and characterization of Sibunit-supported Pd–Ga,Pd–Zn,and Pd–Ag catalysts for liquid-phase acetylene hydrogenation

Pd/Sibunit and Pd–M/Sibunit (M = Ga, Zn, or Ag) catalysts have been synthesized, and their catalytic properties in liquid-phase acetylene hydrogenation have been investigated. Doping of the palladium catalyst with a metal M leads to the formation of the Pd₂Ga, PdZn, or Pd_0.46Ag_0.54 bimetallic compound. The bimetallic particles are much smaller (1.6–2.0 nm) than the monometallic palladium particles (4.0 nm). Doping with zinc raises the ethylene selectivity by 25% without affecting the activity of the catalyst. Specific features of the effect of each of the dopants on palladium are reported.     

Au/ZrO2催化剂中ZrO2的尺寸效应:1,3-丁二烯加氢反应

在不同温度(673～1073K)下,于流动N2气中焙烧ZrO(OH)2醇(乙醇)凝胶,制备了不同尺寸的ZrO2-AN纳米晶(6～30nm).采用沉积-沉淀方法制备了相应的质量分数为0.7%的Au/ZrO2-AN催化剂.用XRD,XRF,TEM/HRTEM,EDS,N2吸附和1,3-丁二烯加氢反应对ZrO2-AN和Au/ZrO2-AN催化剂进行了表征.结果表明,在所有的Au/ZrO2-AN样品中,Au粒子的平均尺寸为4～5nm,ZrO2-AN的颗粒大小没有因为负载Au粒子而发生改变.1,3-丁二烯在Au/ZrO2-AN催化剂催化下能以100%的选择性进行加氢反应生成单烯烃.随着Au/ZrO2-AN催化剂中ZrO2-AN纳米晶尺寸的增加或“载体”焙烧温度的升高,1,3-丁二烯的转化率明显降低;1-丁烯的选择性先增加后减小,2-丁烯中反/顺异构体的摩尔比在0.5～1.0的范围内逐渐增大,TEM/HRTEM表征结果清楚地表明,Au/ZrO2-AN催化剂中Au粒子与ZrO2-AN颗粒接触界面/周边随ZrO2-AN颗粒尺寸的减小而明显增加,这很可能是含有更小尺寸ZrO2-AN纳米粒子的Au/ZrO2-AN催化剂具有更高的催化活性的重要原因.     

A photoelectron spectroscopic study of the effect of particle size on the adsorbed state of carbon monoxide over supported palladium catalysts

UPS experiments on the adsorbed state of carbon monoxide over Pd/graphite model catalysts at 77–653 K have been performed. The CO-derived levels, (5σ + 1π) and 4σ, shift to higher with binding energy with decrease in the palladium particle size. Upon warming, the included levels on smaller Pd particles disappeared at much lower temperature than those on larger palladium particles.     

超高稳定性的等级孔碳负载高分散铜基选择性加氢催化剂

负载型Pd,Pt,Au等贵金属催化剂由于具有较高活性而被广泛应用于选择性加氢催化领域,但资源稀缺、价格昂贵等问题严重制约了其在催化领域的长远发展.目前大量研究结果表明,非贵金属催化剂也具有较高的选择性催化加氢能力,在已被报道的非贵金属加氢催化剂中,铜基催化剂由于在选择性加氢反应中表现出较高加氢选择性和活性引起了人们的广泛关注.然而,早期研究的负载型铜基催化剂普遍存在催化稳定性较低的问题,所以提高铜基催化剂的使用寿命成为了问题关键.本文以铜基有机金属框架HKUST-1作为合成目标催化剂的前驱体,首先探究了水热合成条件对HKUST-1合成结构完整性及结晶度的影响,再通过精确调控HKUST-1的原位碳化过程,利用金属有机框架高温分解自还原行为,成功制备出了等级孔碳负载的高分散铜基催化剂,并将所制备的催化剂应用于1,3-丁二烯选择性加氢反应中.扫描电子显微镜、高分辨透射电子显微镜、X射线衍射、氮气吸脱附、傅里叶红外吸收光谱、X射线光电子能谱等技术用来表征了碳化前后催化剂载体结构的变化,铜粒子尺寸、价态及其在载体中分布的变化.文中也深入探究了以上因素对催化剂选择性催化加氢性能的影响.研究表明:120℃水热合成18 h能获得尺寸在15μm左右,结晶度高且形貌规整的HKUST-1前驱体.随后通过合理地控制金属有机框架分解过程,可实现对碳载体的等级孔结构和活性铜纳米粒子的分散程度的精确调控,获得高效等级孔载体结构和高分散铜位点的催化剂.不仅如此,通过一步碳化自还原HKUST-1制备的等级孔碳负载Cu的催化性能表现出对碳化温度高度的关联性.其催化活性随碳化处理温度的升高呈现先增强后减弱的趋势,但所有获得的催化剂对单烯烃都具有很高的选择性(>98%).特别地,本文发现在600℃碳化合成的催化剂在低温75℃反应可实现对1,3-丁二烯的100%转化,对丁烯的选择性为100%.同时,该催化剂在恒温75℃下持续反应120 h以上,其对丁二烯转化率和对丁烯选择性依然保持100%,表现出了超高的催化稳定性和潜在的商用价值.本文展示了通过简单地调控金属有机骨架的碳化过程是获得具有优异选择性催化加氢性能的铜基催化剂的有效途径.     

Oxidation of 1,3-butadiene over Pd/C and Pd-Te/C catalysts in polar media

The oxidation of 1,3-butadiene over the Pd/C and Pd-Te/C heterogeneous catalysts occurs in organic solvents containing water at a temperature of 100°C and an oxygen partial pressure of $P_{\left[ {O_2 } \right]} = 4$ atm. Crotonaldehyde dominates among the three major products of oxidation over the Pd catalyst. The introduction of Te into the catalyst increases the methyl vinyl ketone yield, the furan yield being the lowest in all cases. X-ray photoelectron spectroscopy (XPS) showed that the active catalyst components can be in a partially oxidized state, particularly after storing the catalysts in air. Additional hydrogen treatment results in almost complete reduction of the active components to metals and enhances the catalytic activity. It is supposed that the oxidation of 1,3-butadiene over the Pd-Te catalysts proceeds via the activation of dioxygen over the Pd⁰ sites, with oxidized Pd and Te participating in subsequent chemical transformations.     

Synthesis,properties, and activity of nanosized palladium catalysts modified with elemental phosphorus for hydrogenation

The applicability of elemental phosphorus as a modifier of palladium catalysts for hydrogenation was demonstrated, and the conditions for the synthesis of nanoparticles that are highly efficient in hydrogenation catalysis were optimized. The modifying effect of elemental phosphorus depends on the P/Pd ratio; it is associated with changes in the catalyst dispersity and the nature of the formed nanoparticles containing various palladium phosphides (PdP₂, Pd5P₂, and Pd₆P) and Pd(0) clusters. The main stages of the formation of palladium catalysts for hydrogenation were determined, and a model of an active catalyst, in which the Pd₆P phosphide is the core of a nanoparticle and Pd(0) clusters form a shell, was proposed.     

The effect of Nafion ionomer on electroactivity of palladium–polypyrrole catalysts for oxygen reduction reaction

Present studies concentrated on the preparation, characterization, and electroactivity of palladium–polypyrrole (Pd/PPY) catalysts for oxygen reduction reaction. In particular, the effect of Nafion ionomer on their electroactivity was evaluated. In all catalysts prepared by “water-in-oil” microemulsion method, the Pd nanoparticles of ca. 7 nm in size appeared regardless of the Pd content (ranging from 2 to 20 wt.%). For comparison, carbon black-supported (Vulcan XC-72) catalyst (20 wt.% Pd) was also synthesized. Coating of the Pd/PPY samples with Nafion ionomer reduced their surface area and porosity. Chemical interaction due to Nafion acid functionalities affected the N-state of pyrrole as well as electron state of Pd in the Pd/PPY catalysts. As a result, the contribution of more oxidized palladium (Pd^δ+) increased. These interactions played an essential role in the electroactivity of Pd/PPY for oxygen reduction reaction. The increased amount of Nafion relative to that of PPY reduced limiting current density whereas the half-wave potential shifted to a more positive value and the fraction of hydrogen peroxide remarkably decreased.     

Reactivity of metal-containing monomers 66. Hydrogenation of nitrotoluene derivatives in the presence of polymer-immobilized Pd nanoparticles

A new approach to the synthesis of immobilized catalysts of the mixed type was developed: frontal polymerization of metal-containing monomers in the presence of a highly dispersed inorganic support. The synthesis of the acrylamide complex of Pd^II nitrate on the SiO₂ surface followed by polymerization and reduction results in the formation of a polymer-inorganic composite with inclusions of Pd nanoparticles stabilized by the polymer matrix on the support surface. The study of the catalytic properties in the hydrogenation of nitrotoluene derivatives showed that the polymer-immobilized Pd nanoparticles on the inorganic support are efficient catalysts for the reduction of the nitrocompounds.     

Electrocatalytic hydrogenation of 4-chlorophenol on the glassy carbon electrode modified by composite polypyrrole/palladium film

A polypyrrole/palladium composite film was prepared on a glassy carbon electrode by the electrochemical deposition method. The palladium particles were uniformly dispersed on a polypyrrole film that was previously electrodeposited on a glassy carbon electrode. By controlling the polymerization process of pyrrole, a highly porous polypyrrole film was obtained; this kind of structure provided more surface areas for depositing palladium particles. The sizes of Pd particles deposited on the porous polypyrrole film are about 15-30 nm. The X-ray photoelectron spectroscopy results showed there was strong interaction between polypyrrole film and palladium particles. This modified electrode showed excellent current efficiency (49.5%) for electrochemical hydrogenation of 4-chlorophenol and the phenol was the sole product. The potential effect on the dechlorination process was also investigated.     

金属有机骨架高温自还原制备高性能等级孔碳负载Co基催化剂

以钴基金属有机框架为前驱体, 利用一步高温碳化自还原法, 通过精确调控碳化过程, 实现等级孔道结构及钴纳米颗粒分散性的可控调节, 制备出高催化活性及产物选择性的等级孔碳负载Co基催化剂. 研究发现, 600 ℃碳化后的催化剂为具有高比表面积的等级孔道结构和高分散的钴纳米颗粒, 在选择性催化1,3-丁二烯加氢反应中, 丁二烯完全转化温度低至60 ℃, 对应丁烯的选择性高达61%, 实现了低温高选择性催化加氢.     

Formation and Properties of Hydrogenation Catalysts Based on Palladium Complexes with Primary Phosphines

The formation and catalytic properties of hydrogenation catalysts based on palladium(II) complexes with primary phosphines were studied. With the use of IR and UV spectroscopy, XRD analysis and GLC, it was found that the interaction of bis(acetylacetonato)palladium(II) or palladium(II) acetate with primary phosphines in an inert atmosphere resulted in the formation of polynuclear palladium complex associates mainly containing μ³-PR and a coordinated phosphine. Polynuclear palladium complexes and the palladium phosphide Pd₆P, which is formed from these complexes in an atmosphere of hydrogen, serve as supports for Pd(0) clusters. The effects of the ratio between initial components and the nature of the acido ligand at the palladium atom on the optimum conditions of catalyst formation were considered.__________Translated from Kinetika i Kataliz, Vol. 46, No. 4, 2005, pp. 609–614.Original Russian Text Copyright © 2005 by Belykh, Goremyka, Gusarova, Sukhov, Shmidt.     

Hydrogenation of olefins in supercritical CO(2) catalyzed by palladium nanoparticles in a water-in-CO(2) microemulsion

Nanometer-sized metallic palladium particles can be synthesized by hydrogen reduction of Pd2+ ions dissolved in the water core of a water-in-CO2 microemulsion. The Pd nanoparticles, stabilized by the micromeulsion and uniformly dispersed in the supercritical fluid phase, are effective catalysts for hydrogenation of olefins. Examples of rapid and efficient hydrogenation of water-soluble and CO2-soluble olefins catalyzed by the Pd nanoparticles in supercritical CO2 are given.     

New Catalysts of the Metal-Filamentary Carbon Type: From Fundamental Research to Technology

A principally new basic catalytic system is developed. The system is based on dispersed metal particles incorporated into carbon filaments by the catalytic decomposition of hydrocarbons on these particles. The basic system enables one to synthesize a series of catalysts for different processes. Mechanisms and methods for controlling the catalytic properties of this system are studied. The role of the crystal faces of the metal particles and of chemisorbed hydrogen species in selective hydrogenation is established. The chemical composition of the active component is optimized for the removal of acetylene from ethylene and of butadiene from butenes. A technology for catalyst precursor preparation is developed, which includes a mechanochemical activation stage and the shaping of the resulting powder. Optimum conditions are found for hydrocarbon decomposition in order to obtain metal-carbon catalysts for selective hydrogenation.