Effect of hydrothermal treatment on the physicochemical characteristics of Pd/C composites prepared <Emphasis Type="Italic">via</Emphasis> pyrolysis of sawdust impregnated with palladium nitrate

Palladium supported on carbon (Pd/C) catalysts (0.55–0.65 wt.% of Pd) were synthesized by pyrolysis of birch sawdust under inert atmosphere proceeded by prolonged impregnation of sawdust in aqueous solution of palladium nitrate. In some cases, hydrothermal treatment (HT) of the pristine sawdust was conducted to modify the specific surface area of the final carbon material applied as a catalyst support. Based on low-temperature nitrogen adsorption technique, it was postulated that HT of sawdust in the liquid phase increases. while HT in the gaseous phase decreases the specific surface area of Pd/C. The obtained catalysts contained Pd particles (size ranged from 2 to 10 nm) both coated and not coated with carbon shell as evidenced by XPS and TEM techniques. The synthesized Pd/C composites provide high conversion of chlorobenzene and high selectivity in respect to benzene in hydrodechlorination reaction performed in a flow fixed-bed reactor in the presence of H₂. XPS data for Pd/C composites tested in the catalytic reaction indicate their high resistance to HCl. A minor part of metal Pd was found to transform into PdCl₂ and PdO.     

Preparation of Pd/C catalysts via deposition of palladium hydroxide onto Sibunit carbon and their application to partial hydrogenation of rapeseed oil

Pd/Sibunit catalysts were prepared by deposition of palladium hydroxide onto the support surface in an alkaline medium. It was found that the palladium distribution throughout the catalyst grain, and the dispersion of Pd particles depend on (i) the order of the addition of H₂PdCl₄ and Na₂CO₃ to carbon suspension, (ii) Na₂CO₃ to H₂PdCl₄ ratio, and (iii) aging time of the mixture H₂PdCl₄ + Na₂CO₃ before its addition to the carbon. The catalysts were tested in the hydrogenation of cyclohexene and rapeseed oil under static conditions. The yield of trans-isomers as products of partial hydrogenation of rapeseed oil was found to decrease with decreasing the Pd particle size in the catalysts, as well as with increasing the Pd concentration on the periphery of the support grains.     

Thermoanalytical and spectroscopic studies of the synthesis of suppokted palladium catalyst in A H2/PdCl2/SiO2 parent system

A 5 wt% Pd/SiO₂ catalyst was synthesized by heating PdCl₂-impregnated SiO₂ in H₂ at 300°C for 2 h. It was found that the metal particle dispersion is improved when the reduction step is preceded by calcination at 300°C for 2h. Thermogravimetry of the impregnated support in air, N₂ and H₂ atmospheres was used to monitor the interactions occurring during the various preparative steps (i.e. drying, calcination and reduction) of the catalyst. The solid prduct of each preparative step was characterized by X-ray diffractometry and UV/Vis diffuse reflectance spectroscopy. The results indicate that following the drying step (at 110°C in air) the palladium occurs in two detectable forms: PdCl₂ particles and Si?O?Pdⁿ⁺ surface species. The calcination appears to transform the PdCl₂ particles into the latter surface species. The H₂-reduction eventually converts the surface species into finely-dispersed Pd° metal particles (average size=8–14 nm). No other reduction products, such as Pd_ySi_x, were detected.     

Pd immobilized on polyamide based on melamine and terephalic acid as an efficient and recyclable catalyst for Suzuki‐Miyaura coupling reaction

A simple synthetic strategy of polyamide was described from melamine and terephalic acid via one‐step polycondensation. PdCl₂ was then immobilized on the polyamide (denoted as Pd/MPA). Melamine and terephalic acid not only acted as monomers but also provided the ligand sites to help the polyamide to coordinate with Pd(II). The Pd/MPA catalyst was characterized by FT‐IR, TGA, SEM, TEM, XPS, N₂ adsorption‐desorption and atomic absorption spectroscopy. The catalyst was used in Suzuki‐Miyaura coupling reaction of various aryl halides, including less reactive chlorobenzene and benzyl chloride, to give the coupling products in moderate to excellent yields. High turnover frequencies (TOF) up to 29400 h^‐1 can be also obtained. In addition, it behaved truly as a heterogeneous catalyst with high reusability after being recycled 6 times and palladium leaching was negligible during the process. This work provides a practical polyamide support to develop heterogeneous palladium catalysts with simple synthetic procedure and low cost.     

Effect of the nature of the acido ligand in the precursor on the properties of nanosized palladium-based hydrogenation catalysts modified with elemental phosphorus

The effect of the nature of the acido ligand in the precursor and the modifying action of elemental phosphorus on palladium catalysts for hydrogenation are reported. The large turnover frequency (TOF) and turnover number (TON) values observed for styrene hydrogenation on the Pd blacks prepared in situ by PdCl₂ reduction with hydrogen in DMF are due to the formation of fine-particle catalyst with a base particle size of 6–10 nm. This is explained by the high PdCl₂ reduction rate and by the formation of a palladium cluster stabilizer—dimethylammonium chloride—in the reaction system via the catalytic hydrolysis of the solvent (DMF). The modifying action of elemental phosphorus on the properties of the palladium catalysts depends on the nature of the acido ligand in the precursor. In the case of oxygen-containing precursors at small P/Pd ratios, elemental phosphorus exerts a promoting effect, raising the TON and TOF values by a factor of about 9. In the case of palladium dichloride as the precursor, white phosphorus exerts an inhibiting effect. At the same time, it enhances the stability of the catalyst, raising the TON value at P/Pd = 0.3. The causes of these distinctions are considered.     

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.     

Formation and electrocatalytic properties of Pd deposits on Mo obtained by galvanic displacement

A Pd-Mo electrocatalytic system was obtained by forming palladium particles on the Mo surface that contacted a PdCl₂ solution under open-circuit conditions. The state of palladium on the electrode surface depended on the contact displacement time. Palladium particles 5–10 nm in size formed on the surface of the Pd(Mo) electrode after palladium deposition for 1 min. The specific rates of formic acid oxidation on the Pd(Mo) electrode were smaller than those on the Pd/Pt electrode. On the Pd(Mo) electrode, anode currents of methanol oxidation were recorded at a potential of 0.4 V. The difference in the effects of the Mo substrate on the activity of Pd particles in the electrooxidations of HCOOH and CH₃OH was explained by the difference in the mechanisms of these reactions.     

Effect of the Structure of Carbon Nanofibers on the State of an Active Component and on the Catalytic Properties of Pd/C Catalysts in the Selective Hydrogenation of 1,3-Butadiene

The state of highly dispersed palladium particles supported on filamentous carbon was studied using high-resolution electron microscopy, XPS, and X-ray diffraction analysis. Three types of filamentous carbon were used, in which the basal planes of graphite were arranged along, across, and at an angle to the nanofiber axis. The amount of supported palladium was 0.25–5.8 wt %. The structure of the carbon support was found to affect the properties of the active component. Highly dispersed palladium particles exhibited the strongest interaction with a carbon surface formed by the butt ends of graphite (002) layers. This interaction resulted in electron transfer from the metal to the support and in the stabilization of palladium in the most dispersed state. A change in the properties of palladium particles caused a change in the catalytic properties of Pd/C catalysts in the reaction of selective 1,3-butadiene hydrogenation to butenes. The strong interaction of Pd²⁺ with the butt ends of graphite resulted in the stabilization of palladium in an ionic state. An increase in the fraction of Pd²⁺ in the catalysts was responsible for a decrease in both the overall activity and selectivity of Pd/C catalysts in the reaction of 1,3-butadiene hydrogenation to butenes.     

Simple Electrodeposition of Dendritic Pd Without Supporting Electrolyte and Its Electrocatalytic Activity Toward Oxygen Reduction and H2O2 Sensing

Metallic palladium (Pd) electrocatalysts for oxygen reduction and hydrogen peroxide (H₂O₂) oxidation/reduction are prepared via electroplating on a gold metal substrate from dilute (5 to 50 mM) aqueous K₂PdCl₄ solution. The best Pd catalyst layer possessing dendritic nanostructures is formed on the Au substrate surface from 50 mM Pd precursor solution (denoted as Pd‐50) without any additional salt, acid or Pd templating chemical species. The Pd‐50 consisted of nanostructured dendrites of polycrystalline Pd metal and micropores within the dendrites which provide high catalyst surface area and further facilitate reactant mass transport to the catalyst surface. The electrocatalytic activity of Pd‐50 proved to be better than that of a commercial Pt (Pt/C) in terms of lower overpotential for the onset and half‐wave potentials and a greater number of electrons (n) transferred. Furthermore, amperometric i–t curves of Pd‐50 for H₂O₂ electrochemical reaction show high sensitivities (822.2 and ?851.9 µA mM^?1 cm^?2) and low detection limits (1.1 and 7.91 µM) based on H₂O₂ oxidation H₂O₂ reduction, respectively, along with a fast response (<1 s).     

Structure of the CO bond on supported Pd particles: influence of size and surface state

We have investigated the surface of supported palladium particles by static secondary ion mass spectrometry (SSIMS). Pd particles were grown in situ on alumi na (oxide layer and sapphire surfaces) and stabilized by heating treatment. The particle size, density and crystallographic structure were determined in previous studies by transmission electron microscopy and diffraction (TEM and TED). Various ionic species are detected by SSIMS analysis which makes it possible to characterize the CO absorbed layer: Pd _n CO⁺ (n=1, 2) for molecular adsorption and Pd _n C⁺ for CO dissociation. The size dependence of the bonding state of CO (linear, bridge, ...) was monitored by: PdCO⁺/σ _n Pd _n CO⁺ signal ratio over various size particles (mean diameter in the 2–9 nm range). Investigations were performed as a function of CO coverage (adsorption at room temperature) and also under CO dissociation conditions: heating under CO atmosphere or CO+O₂ (catalysis). The data analysis shows that on clean Pd particles smaller than 3 nm the CO molecules give rise mainly to PdCO⁺ species. We have interpreted this result by the adsorption of CO on two palladium atoms, the carbon end being tightly bonded to a low coordination Pd atom and the oxygen end weakly bonded to a neighbour Pd atom. These couples of Pd atoms form the specific sites for CO dissociation, the density of which depends on the roughness of the particle surface.     

Catalytic Active Site for NO Decomposition Elucidated by Surface Science and Real Catalyst

Comprehensive studies combining surface science and real catalyst were performed to get further insight into catalytic active site and reaction mechanism for NO decomposition over supported palladium and cobalt oxide-based catalysts. On palladium single-crystal model catalysts, adsorption, dissociation and desorption behavior of NO was found to be closely related to the surface structures, the stepped surface palladium being active for dissociation of NO. In accordance with this result, the activity of powder Pd/Al₂O₃ catalysts for NO decomposition was directly related to the number of step sites exposed on the surface, suggesting that the step sites act as the catalytic active site for NO decomposition on Pd/Al₂O₃. NO decomposition over cobalt oxide was found to be significantly promoted by addition of alkali metals. Surface science study and catalyst characterization led to the same conclusion that the interface between the alkali metal and Co₃O₄ serves as the catalytic active site. From the results of in situ Fourier transform infrared (FT-IR) spectroscopy and isotopic transient kinetic analysis, a reaction mechanism was proposed in which the reaction is initiated by NO adsorption onto alkali metals to form NO₂⁻ species and then NO₂⁻ species react with the adsorbed NO species to form N₂ over the interface between the alkali metal and Co₃O₄.     

State of active components on the surface of the PdCl<Subscript>2</Subscript>-CuCl<Subscript>2</Subscript>/γ-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> catalyst for the low-temperature oxidation of carbon monoxide

The state of the active constituents of the freshly prepared PdCl₂-CuCl₂/γ-Al₂O₃ catalyst for the low-temperature oxidation of the carbon monoxide by molecular oxygen was studied by X-ray absorption spectroscopy (XAS), powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and diffuse reflectance IR Fourier transform spectroscopy (DRIFTS). It was shown that copper in the form of a crystalline phase of Cu₂Cl(OH)₃ with the structure of the mineral paratacamite and palladium chloride in an amorphous state occurred on the surface of γ-Al₂O₃. According to XAS data, the local environment of palladium consisted of four chlorine atoms, which formed a flat square with an increased distance between palladium and one of the chlorine atoms. The evolution of the local environments of copper and palladium upon a transition from the initial salts to the impregnating solutions and chlorides on the surface of γ-Al₂O₃ was considered. The role of γ-Al₂O₃ in the formation of the Cu₂Cl(OH)₃ phase was discussed. It was found by the DRIFTS method that linear (2114 cm⁻¹) and bridging (1990 and 1928 cm⁻¹) forms of coordinated carbon monoxide were formed upon the adsorption of CO on the catalyst surface. The formation of CO₂ upon the interaction of coordinated CO with atmospheric oxygen was detected. Active sites including copper and palladium were absent from the surface of the freshly prepared catalyst.     

Selective Semihydrogenation of Alkynes on Shape‐Controlled Palladium Nanocrystals

A systematic study on the selective semihydrogenation of alkynes to alkenes on shape‐controlled palladium (Pd) nanocrystals was performed. Pd nanocrystals with a cubic shape and thus exposed {100} facets were synthesized in an aqueous solution through the reduction of Na₂PdCl₄ with L ‐ascorbic acid in the presence of bromide ions. The Pd nanocubes were tested as catalysts for the semihydrogenation of various alkynes such as 5‐decyne, 2‐butyne‐1,4‐diol, and phenylacetylene. For all substrates, the Pd nanocubes exhibited higher alkene selectivity (>90 %) than a commercial Pd/C catalyst (75–90 %), which was attributed to a large adsorption energy of the carbon–carbon triple bond on the {100} facets of the Pd nanocubes. Our approach based on the shape control of Pd nanocrystals offers a simple and effective route to the development of a highly selective catalyst for alkyne semihydrogenation.     

Polyacrylonitrile fiber mat‐supported palladium catalyst for Mizoroki–Heck reaction in aqueous solution

A novel palladium catalyst immobilized on polyacrylonitrile fiber mats (Pd/PAN) was prepared by electrospinning. The catalytic activity and recyclability of the microwave‐assisted Pd/PAN fiber mats were examined for the Mizoroki–Heck cross‐coupling of aryl iodides with three different acrylates in aqueous solution. The morphology of the prepared Pd/PAN fiber mats was characterized by scanning electron microscopy. The large size of the PAN fiber mat‐supported palladium catalyst enables much easier separation from the reaction mixture by simple filtration. Density functional theory calculation indicates that the chelation energy of palladium chloride (PdCl₂) with propionitrile (model of PAN) is considerable smaller than that of PdCl₂ with water, suggesting that the stability and reactivity of the Pd/PAN fiber mats catalyst could be improved through the surface derivatization with polar functional groups. Copyright © 2011 John Wiley & Sons, Ltd.     

Synthesis of highly dispersed Pd/C electro-catalyst with high activity for formic acid oxidation

In this study, we present a simple process to obtain highly dispersed palladium nanoparticles on Vulcan XC-72R carbon support without any protective agent. To obtain high metal loading Pd/C catalyst without any surfactant, we modified the polyol process by employing NH₃ species as a mediation to control the reaction pathway to avoid the precipitation of Pd(OH)₂, and hence the agglomeration of Pd nanoparticles. The obtained Pd/C sample was characterized by X-ray diffraction (XRD) and transmission electron microscope (TEM) techniques. The results show that highly dispersed Pd/C catalyst with an average diameter of 3.0 nm could be obtained in this novel process. The activity of formic acid oxidation on this Pd/C catalyst was examined via cyclic voltammetry technique and it is found that the catalytic activity is greatly enhanced due to the reduced particle size and the improved dispersion of palladium nanoparticles on the carbon surface.     

Palladium‐catalyzed carbonylation of quaternary ammonium halides to tertiary amides

Catalytic carbonylation of quaternary ammonium salts under anhydrous conditions was investigated using palladium catalyst. The carbonylation of tetramethylammonium iodide was chosen as a model reaction and studied systematically. Ligand‐free PdCl₂ showed efficient catalytic performance for this transformation. A palladium catalyst loading as low as 0.05 mol% was sufficient for high yield (96.9%) of N,N‐dimethylacetamide, corresponding to a turnover frequency of 242 h^?1. Under optimum conditions, several other quaternary ammonium halides were also carbonylated to corresponding tertiary amides in moderate to excellent yields. The catalytic activity of commercial palladium on activated carbon (Pd/C) catalyst was also evaluated. The Pd/C catalyst exhibited high activity for this carbonylation reaction and could be recycled six times with a slight decrease in activity. Furthermore, mechanistic considerations concerning Pd‐catalyzed carbonylation of quaternary ammonium halides were also discussed. Copyright © 2014 John Wiley & Sons, Ltd.     

乙二胺功能化树脂负载Pd(0)配合物：可重复使用的高活性Heck芳基化催化剂

The ethylenediamine-functionalized resin-supported Pd(0)complex was prepared from PdCl_2 and ethylenedia-mine-functionalized chloromethylated polystyrene,followed by reduction with KBH_4.The complex was character-ized by FT-IR,XRD,BET,SEM and EDS.The resin-supported catalyst exhibited high catalytic activity in theHeck reaction and could be reused up to 17 times in NMP or 16 times in DMF at 90 ℃ in the Heck reaction of io-dobenzene with acrylic acid.The leaching investigation disclosed that the palladium leaching was caused by the in-teraction of iodobenzene with the metal Pd(0)on supported catalyst.The leached palladium species in filtrate wasvery stable and could be reused five times after the solid catalyst was filtered off.A cross-transfer test in recyclingin the presence of additional carbon disclosed that the soluble leached palladium species had much higher catalyticactivity than supported and/or adsorbed palladium in solid-solution heterogeneous Heck reaction.     

Periodic mesoporous organosilica grafted palladium organometallic complex: efficient heterogeneous catalyst for water‐medium organic reactions

Water‐medium organic reactions were studied over periodic mesoporous silica (PMO) containing Pd(II) organometallic complex. This heterogeneous catalyst was achieved by Pd(II) compound coordinated with the PPh₂‐ligand onto the pore surface of phenylene‐bridged PMO support. This catalyst displayed ordered mesoporous channels, which ensured the high dispersion of Pd(II) active sites and the convenient diffusion of reactant molecules into the pore channels. Meanwhile, the phenyl group in the pore wall of PMO could enhance the surface hydrophobicity which promoted the adsorption of organic reactant molecules on the catalyst in aqueous environment. As a result, this elaborated catalyst exhibited comparable activity and selectivity with the corresponding PdCl₂(PPh₃)₂ homogeneous catalyst in the water‐medium organic reactions, and could be used repeatedly, showing a good potential in industrial applications. Copyright © 2010 John Wiley & Sons, Ltd.     

碳纳米管上原位沉积钯-钴-镍纳米颗粒及其对乙醇氧化的电活性

以水合肼为还原剂,在水和乙醇的混合溶液中制备多壁碳纳米管(MWCNT)负载的纳米镍(Ni/MWCNT)和纳米镍钴(Ni-Co/MWCNT)颗粒,然后将它们分别与氯化钯溶液反应,形成的钯纳米颗粒原位沉积在MWCNT表面,从而得到MWCNT负载的Pd-Ni/MWCNT和Pd-Ni-Co/MWCNT催化剂。SEM和TEM图像显示,MWCNT上的催化剂颗粒是由5~10 nm的小颗粒团聚而成的30~100 nm的大颗粒,三金属催化剂的粒径比双金属的粒径小,在MWCNT上的分散度更高。ICP和EDS分析显示,Pd直接还原并包覆在纳米镍和纳米镍钴表面;采用循环伏安和计时电流技术,研究了催化剂在碱性溶液中对乙醇氧化的电催化活性,结果表明,Pd-Ni-Co/MWCNT催化剂对乙醇氧化具有强的电催化活性,乙醇氧化对应的峰电流密度达101.8 mA·cm^-2,并且催化剂催化活性稳定。     

XAFS characterization of supported PdCl2?CuCl2 catalysts for CO oxidation

A heterogenized Wacker catalyst system in which pores of a high surface area alumina were filled with an aqueous solution of PdCl₂–CuCl₂ was active for the oxidation of CO near room temperature. The structure of thecatalyst was studied by XRD and XAFS. The active phase of Pd was a molecular Pd species whose structure was similar to PdCl₂, probably modified by a carbonyl ligand. The active phase of copper was found to be solid Cu₂Cl(OH)₃ particles. The presence of Cu was essential to keep the Pd in the Pd(II) state during the reaction.