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.     

Elaboration by high-energy ball milling of copper/palladium composite materials – characterization and electrocatalytic activity for the reduction of nitrate in alkaline medium

In this study, an original approach was explored to decorate copper particles with palladium and well-defined bimetallic copper/palladium powders were elaborated through a two-step ball milling procedure. First, copper powder was milled with previously determined optimal conditions (ball-to-powder mass ratio of 2, milling duration of 6 h under argon) in order to obtain spherical nanocrystalline copper particles with an average diameter of 800 μm. Then, an additional milling in presence of 1 at.% of palladium powder was performed, leading to the formation of Cu–Pd composite materials. Palladium surface concentrations from 3 to 62 at.% were obtained by varying both the ball-to-powder mass ratio (2:1 or 10:1) and the milling duration (from 5 to 30 min). Scanning electron microscopy, optical microscopy, X-ray photoelectron spectroscopy and X-ray diffraction analyses confirmed that the more intense the milling is, the easier the palladium diffuses into the copper matrix and smaller the palladium concentration on copper particles is. Cyclic voltammetry and electrolysis experiments showed that palladium inclusions on copper improve greatly the electrocatalytic activity for nitrate reduction in alkaline media. The key role of Pd in the Pd–Cu composite electrodes is to accelerate the reduction of nitrite, formed by the electrochemical reduction of nitrate on Cu sites. Also different nitrate electroreduction behaviors were observed at copper and copper–palladium electrodes leading to the preferential formation of nitrite or ammonia depending on the applied potential and the Pd surface concentration.     

In situ Raman and in situ XRD analysis of PdO reduction and Pd° oxidation supported on γ-Al2O3 catalyst under different atmospheres

Reduction of Pd° and decomposition of palladium oxide supported on γ-alumina were studied at atmospheric pressure under different atmospheres (H(2), CH(4), He) over a 4 wt% Pd/Al(2)O(3) catalyst (mean palladium particle size: 5 nm with 50% of small particles of size below 5 nm). During temperature programmed tests (reduction, decomposition and oxidation) the crystal domain behaviour of the PdO/Pd° phase was evaluated by in situ Raman spectroscopy and in situ XRD analysis. Under H(2)/N(2), the reduction of small PdO particles (<5 nm) occurs at room temperature, whereas reduction of larger particles (>5 nm) starts at 100 °C and is achieved at 150 °C. Subsequent oxidation in O(2)/N(2) leads to reoxidation of small crystal domain at ambient temperature while oxidation of large particles starts at 300 °C. Under CH(4)/N(2), the small particle reduction occurs between 240 and 250 °C while large particle reduction is fast and occurs between 280 and 290 °C. Subsequent reoxidation of the catalyst reduced in CH(4)/N(2) shows that small and large particle oxidation of Pd° starts also at 300 °C. Under He, no small particle decomposition is observed probably due to strong interactions between particles and support whereas large particle reduction occurs between 700 and 750 °C. After thermal decomposition under He, the oxidation starts at 300 °C. Thus, the reduction phenomenon (small and large crystal domain) depends on the nature of the reducing agent (H(2), CH(4), He). However, whatever the reduction or decomposition treatment or the crystal domain, Pd° oxidation starts at 300 °C and is completed only at temperatures higher than 550 °C. Under lean conditions, with or without water, the palladium consists of reduced sites of palladium (Pd°, Pd(δ+) with δ < 2 or PdO(x) with x < 1) randomly distributed on palladium particles.     

Formation of Nanosized Catalysts Based on Palladium Phosphine Complexes and the Nature of Their Activity

Findings on the formation and features of nanosized particles based on palladium complexes, which are active in hydrogenation catalysis, are summarized. Depending on the nature of a reducing agent, nanosized particles formed by the reduction of palladium(II) phosphine complexes are either metallic nuclei stabilized by organophosphorus ligands or associates of polynuclear phosphido or phosphinideno palladium complexes whose surface contains immobilized Pd(0) clusters. The ensembles of the Pd(0) atoms are active in hydrogenation.     

PEDOT-supported Pd nanoparticles as a catalyst for hydrazine oxidation

Composite poly-3,4-ethylenedioxythiophene (PEDOT)/palladium (Pd) films were obtained by chemical deposition of dispersed palladium nanoparticles into PEDOT conducting polymer matrix. The amounts of palladium particles incorporated into PEDOT films were estimated by electrochemical quartz crystal microbalance measurements. It was shown that palladium loading depends on the time a PEDOT film is exposed in the solution, containing Pd(II)-ions, on the concentration of Pd(II) ions and the film thickness. X-ray photoelectron spectroscopy data have confirmed the presence of metallic palladium in the polymer. The morphology of pristine and composite films as well as the size of Pd nanoparticles and their distribution were characterized using scanning electron microscopy (SEM) and transmission electron microscopy (TEM). From SEM images, it was found that Pd particles decorated PEDOT globular structures as quasi-spherical particles, and their mean size was dependent on synthesis conditions. The nanoparticles were non-uniformly dispersed on the polymer surface. The comparison of TEM images of composite PEDOT/Pd films obtained for different times of metal loading was made. The remarkable effect of loading time on the size of particles has been established: the mean size of dominating palladium particles was close to 6–10 nm for 30 s of metal deposition, and it was getting larger with the increase of deposition time (close to 15–30 nm for 120 s). It is most likely that with prolongation of synthesis time, the deposition of palladium predominantly proceeds on the already deposited palladium clusters, resulting in the extension growth of their size. Catalytic properties of PEDOT/Pd composite films were studied in respect to hydrazine oxidation by cyclic voltammetry and voltammetry on rotating disk electrode. The obtained data allow to conclude that the process of hydrazine oxidation on PEDOT/Pd composites takes place predominantly on palladium particles, located on the surface or in the near-surface layers of the polymer. The diffusion nature of the limiting current of hydrazine oxidation on composite PEDOT/Pd film in phosphate buffer solution рН = 6.86 was confirmed, and hydrazine diffusion coefficient was calculated. The increase of the limiting currents of hydrazine oxidation with the increase of Pd deposition time was observed, resulting from the increase of the active surface area of palladium particles, acting as microelectrodes. The electroanalytical applications of these nanocomposite materials for the determination of hydrazine were demonstrated.     

Effect of the reduction conditions of the supported palladium precursor on the activity of Pd/C catalysts in hydrogenation of sodium 2,4,6-trinitrobenzoate

The study addresses the effect of the reduction conditions of palladium polynuclear hydroxo complexes (PHC) supported on the Sibunit carbon material on the dispersion of the metal particles and the activity of 0.5%Pd/Sibunit catalysts in the selective hydrogenation of sodium 2,4,6-trinitrobenzoate to 1,3,5-triaminobenzene in an aqueous solution (temperature of 323 or 343 K, pressure of 0.5 MPa). The palladium PHC were reduced using the most common methods pertaining to catalyst preparation: liquid-phase reduction with sodium formate and reduction in a hydrogen flow at elevated temperature. It was found that high-temperature reduction in the gas phase gives rise to Pd particles with a markedly lower dispersion compared with the sample obtained under mild liquid-phase reduction conditions. The catalytic activity of the sample containing large Pd particles proved to be higher than the activity of the catalyst obtained by reduction with sodium formate.     

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.     

Structure and catalytic properties of nanosized alumina supported platinum and palladium particles synthesized by reaction in microemulsion

Mixtures of nanosized platinum and palladium particles have been prepared by reduction of salt-containing microemulsion droplets using hydrazine as the reducing agent. To avoid possible negative effects of the presence of sulfur compounds during the preparation the microemulsion was made using the sulfur-free nonionic polyoxyethylene 4 lauryl ether surfactant. Transmission electron microscopy showed that the as-prepared mixtures contained crystalline platinum particles of fairly homogeneous size (20 to 40 nm) with adsorbed amorphous palladium particles 2 to 5 nm in size. Catalyst samples were prepared by depositing the nanoparticles on a gamma-Al(2)O(3) support followed by heating in air at 600 degrees C. Alloyed particles of platinum and palladium with sizes ranging from 5 to 80 nm were obtained during the heating. The majority of the particles had the fcc structure and their compositional range was dependent upon the Pt:Pd molar ratio of the microemulsion. A catalyst prepared from a microemulsion with a 20:80 Pt:Pd molar ratio showed the highest catalytic activity for CO oxidation, while pure platinum and palladium catalysts showed higher sulfur resistance. These results differ from the performance of conventional wet-impregnated catalysts, where a 50:50 Pt:Pd molar ratio resulted in the highest catalytic activity as well as the highest sulfur resistance.     

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.     

Role of polyaniline morphology in Pd particles dispersion. Hydrogenation of alkynes in the presence of Pd-polyaniline catalysts

Two polyaniline (PANI) samples of various molecular masses were used for the preparation of palladium catalysts (with 2 mass % of Pd). The physicochemical features of starting polyanilines were found to substantially affect the size and extent of palladium nanoparticles aggregation. Strongly aggregated large palladium particles appeared in the PANI sample of more compact morphology (PANI-H), higher crystallinity and lower specific surface area. Pd nanoparticles of a definitively smaller size were formed in the more amorphous PANI sample of looser morphology (PANI-L) and the extent of particles aggregation was markedly lower. The catalytic properties of Pd/PANI samples were studied in a liquid phase hydrogenation of unsaturated triple bond (C≡C) in alkynes reactants, phenylacetylene, and cyclohexylacetylene. The 2 mass % Pd/PANI-L catalyst prepared using polymer of less compact texture exhibited much higher activity in both reactions. In the presence of the 2 mass % Pd/PANI-L catalyst, alkene products were formed with a high selectivity (approximately 90 %) attained at the almost complete conversion of alkynes. This highly selective hydrogenation of the C≡C to the C=C bond was related to the presence of an electroactive polymer, PANI, in close proximity with Pd active sites. Polyaniline could have a role in a steric effect as well as in a modification of adsorptive properties of Pd centres.     

Application of Laser Induced Breakdown Spectroscopy as a Novel Approach for Monitoring of the Activity of Nano Palladium Catalyst as Compared to Two Well-known Methods

Catalyst deactivation is an unavoidable process that occurs in catalytic chemical reactions. Laser Induced Breakdown Spectroscopy (LIBS) is used here as a novel approach to investigate the activity of palladium supported with carbon catalyst (Pd/C) over the hydrogenation of cinnamic acid with tetralin. Their outputs for four catalyst samples are reported for different time intervals of 0, 5, 10, 15 min during the reaction. The results of LIBS analysis are compared to Inductively Coupled Plasma Mass Spectrometry (ICP-MS), which shows a good agreement. Experimental data specify that line intensities of palladium (Pd) are decreased significantly with an increment of the reaction time. Moreover, the Field Emission Scanning Electron Microscope with energy dispersive spectroscopy (FESEM-EDS) of catalysts samples show aggregation of palladium particles for some places in the catalyst surface. The changes of Pd content and sintering of Pd particles in the catalyst during the reaction play substantial roles in catalyst deactivation.     

纳米碳纤维负载钯催化剂在Heck反应中的应用——载体相互作用的影响

使用多元醇还原法制备了均匀分散的钯纳米颗粒.将钯纳米颗粒负载于板式、鱼骨式和管式纳米碳纤维,得到稳定、可重复使用的非均相催化剂.实验结果表明,钯纳米胶粒同载体之间的电位差对钯在载体上的负载量、粒子大小以及Heck反应中钯的溶失量有很大的影响.在制备过程中,增加钯纳米胶粒同纳米碳纤维表面的电位差能够大大降低钯在Heck反应中的流失.催化剂的反应活性随钯粒子的增大而降低.     

Hydrogen storage properties of isocyanide-stabilized palladium nanoparticles

Monodispersed palladium nanoparticles protected with n-octyl isocyanide were prepared, and their hydrogen absorption behavior was evaluated. The formation of the nanoparticles has been confirmed by means of 1H NMR and elemental analysis. Fourier transform infrared (FT-IR) showed that three distinct bands (2156, 1964, and 1611 cm(-1)) assigned to mono-, double-, and triple-bridged isocyanide ligands on the palladium surface. The average diameter of the particles was estimated to be 2.1 +/- 0.7 nm from observation by transmission electron microscopy (TEM). X-ray photoelectron spectroscopy (XPS) analysis revealed that the particles contained Pd(0) with little amounts of Pd(II) or Pd(IV), in sharp contrast to the thiol- or phosphine-stabilized palladium nanoparticles. The absorption and desorption of hydrogen were reversible, and the reactions were much faster for the nanoparticles than for the bulk palladium metal, whereas the storage capacity was almost the same, 0.6 wt %.     

In situ electron microscopy studies of the sintering of palladium nanoparticles on alumina during catalyst regeneration processes.

Sintering of a palladium catalyst supported on alumina (Al2O3) in an oxidizing environment was studied by in situ transmission electron microscopy (TEM). In the case of a fresh catalyst, sintering of Pd particles on an alumina surface in a 500 mTorr steam environment happened via traditional ripening or migration and coalescence mechanisms and was not significant unless heating above 500 degrees C. After the catalyst was used for the hydrogenation of alkynes, TEM coupled with convergent beam electron diffraction and electron energy loss spectroscopy analysis revealed that most of the Pd particles were lifted from the alumina surface by hydrocarbon buildup. This dramatically different morphology totally changed the sintering mechanism of Pd particles during the regeneration process. Catalytic gasification of hydrocarbon around these particles in an oxidizing environment allowed the Pd particles to move around and coalesce with each other at temperatures as low as 350 degrees C. For catalysts heating under 500 mTorr steam at 350 degrees C, steam stripped hydrocarbon catalytically at the beginning, but the reaction stopped after 4 h. Heating in air resulted in both catalytic and noncatalytic stripping of hydrocarbon.     

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.     

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.     

多重栾晶钯颗粒的可见光简易合成及对乙醇的电催化氧化

以氯化钯(PdCl₂)为金属前驱体, 乙醇为还原剂, 聚乙烯吡咯烷酮(PVP)为稳定剂和导向剂, 利用普通市售节能灯产生的光热作用, 辅助制备多重栾晶钯纳米颗粒。用HRTEM、FFT、PXRD、XPS、UV-Vis和FT-IR等技术对产品的形貌、晶体结构、光学性质和稳定性进行了表征, 并通过循环伏安法研究了多重栾晶Pd修饰玻碳电极对乙醇的电催化氧化活性。结果表明, 多重栾晶钯结构的形成依赖于光和热的协同作用。该材料的表面等离子共振吸收峰在可见光区域, 对乙醇有较好的电催化活性和抗中毒能力。     

多重栾晶钯颗粒的可见光简易合成及对乙醇的电催化氧化

以氯化钯(PdCl₂)为金属前驱体,乙醇为还原剂,聚乙烯吡咯烷酮(PVP)为稳定剂和导向剂,利用普通市售节能灯产生的光热作用,辅助制备多重栾晶钯纳米颗粒.用HRTEM、FFT、PXRD、XPS、UV-Vis和FT-IR等技术对产品的形貌、晶体结构、光学性质和稳定性进行了表征,并通过循环伏安法研究了多重栾晶Pd修饰玻碳电极对乙醇的电催化氧化活性.结果表明,多重栾晶钯结构的形成依赖于光和热的协同作用.该材料的表面等离子共振吸收峰在可见光区域,对乙醇有较好的电催化活性和抗中毒能力.     

The influence of thermal treatment on the electrochemical properties of carbon–Ni–Pd composites

Resorcinol–formaldehyde (RF) hydrogel and RF–nickel–palladium (RF–Ni–Pd) hydrogel were synthesized by sol–gel polycondensation followed by ambient drying. Carbon gel and carbon–nickel–palladium doped gels were prepared by carbonizing the RF and RF–Ni–Pd gels at 900 °C under a nitrogen atmosphere. The goal of this study was to determine the effect of oxidative thermal treatment on the electrochemical activity of nickel–palladium doped carbon gels (C–Ni–Pd). The scanning electron microscopy analysis, adsorption and X-ray diffraction measurements showed that the admixture of Ni and Pd to carbon matrix resulted in the modification of morphological, porous and crystalline features. It has been demonstrated that composite C–Ni–Pd composed of sphere-like granules incrusted with well-crystalline nickel and palladium particles exhibits electrochemical activity in 6 M KOH aqueous solution. Thermal treatment of the composite carried out in air at 450 °C brought about the improvement of electrochemical activity in the potential range of the hydrogen sorption/desorption reaction.