Palladium Phosphide Research Articles

Differential Selectivity of Palladium–Phosphorus Catalysts in the Competitive Hydrogenation of Isomeric Chloronitrobenzenes

The competitive hydrogenation of сhloronitrobenzene isomers in the presence of different palladium- containing catalysts was studied. The nature of catalytic activity carriers for the Pd–P nanoparticles containing both Pd(0) clusters and palladium phosphides was determined by the method of phase trajectories. It was found that the hydrogenation of сhloronitrobenzene isomers under mild conditions occurred on the clusters of Pd(0), and the dependence of the differential selectivity of Pd–P clusters in the hydrogenation of o- and m-сhloronitrobenzene on the P/Pd ratio was related to the dispersity of the Pd(0) clusters.

Inflating hollow nanocrystals through a repeated Kirkendall cavitation process

The Kirkendall effect has been recently used to produce hollow nanostructures by taking advantage of the different diffusion rates of species involved in the chemical transformations of nanoscale objects. Here we demonstrate a nanoscale Kirkendall cavitation process that can transform solid palladium nanocrystals into hollow palladium nanocrystals through insertion and extraction of phosphorus. The key to success in producing monometallic hollow nanocrystals is the effective extraction of phosphorus through an oxidation reaction, which promotes the outward diffusion of phosphorus from the compound nanocrystals of palladium phosphide and consequently the inward diffusion of vacancies and their coalescence into larger voids. We further demonstrate that this Kirkendall cavitation process can be repeated a number of times to gradually inflate the hollow metal nanocrystals, producing nanoshells of increased diameters and decreased thicknesses. The resulting thin palladium nanoshells exhibit enhanced catalytic activity and high durability toward formic acid oxidation.

Phosphorus-Modified Palladium Hydrogenation Catalysts: An Electron-Microscopy Study.

Size and phase composition of the Pd-P catalyst, which is highly active in the hydrogenation of unsaturated compounds have been established using X-ray powder diffraction (XRD) analysis, high resolution transmission electron microscopy (HRTEM) technique in combination with energy-dispersive X-ray (EDX) analysis and selected area electron diffraction (SAED) analysis. The reduction of Pd(acac)₂ with hydrogen in the presence of white phosphorus (P/Pd = 0.3) in N,N-dimethylformamide (DMF) at 80 °C results in formation of Pd-enriched polycrystalline nanoparticles with a predominant diameter of 5.7 nm (C(Pd) = 1 mmol·L⁻¹) and 5.1 nm (C( Pd) = 5 mmol · L⁻¹). The polycrystallites consist of palladium phosphides (Pd5P2, Pd3P0.8 and palladium crystallites. It is found that phase composition of the Pd-P catalyst is changed at the stage of Pd-P sample preparation for XRD analysis. It is established that palladium phosphide Pd6P detected by XRD technique is a product of the solid-phase reaction between Pd crystallites and Pd₅P₂, Pd₃P(0.8) palladium phosphides at the stage of high-temperature sample preparation (400 °C). The mechanism of low-temperature synthesis of palladium phosphides was proposed.

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: Pd3P0.8 → Pd5P2 → PdP2. The causes of the modifying action of phosphorus on the properties of palladium catalysts for hydrogenation of unsaturated compounds were considered.

Pd–P Hydrogenation Catalyst: Nanoparticle Nature and Surface Layer State

The Pd–P selective catalyst for liquid-phase hydrogenation of o-chloronitrobenzene (o-CNB) was obtained by the reduction of Pd(acac)2 with hydrogen at 80∘C in the presence of white phosphorus (P/Pd = 1) in N,N-dimethylformamide (DMF). It has been shown [(high-resolution transmission electron microscopy (HRTEM), energy dispersive X-ray spectroscopy (EDX), X-ray powder diffraction (XRD)] that such low-temperature synthesis of the Pd–P catalyst affords nanoparticles of palladium phosphides (Pd5P2, PdP2), the Pd5P2 phosphide being prevailing. On the nanoparticle surface, palladium is present as a phosphide (BE (Pd3d5∕2) = 336.2 eV; BE (P2p3∕2) = 130 eV) and as palladium clusters of ≈ 1 nm in diameter. The formation of the Pd–P catalyst proceeds through a number of stages: a redox process between Pd(acac)2 and white phosphorus affording mainly PdP2 nanoparticles, H3PO3 and acacH; next follows the reduction of unreacted Pd(acac)2 with hydrogen at 80∘C and the reaction of Pd(0) atoms with each other and with PdP2. It is assumed that formation of small palladium clusters on the surface of the Pd5P2 nanoparticles ensures the high selectivity of the Pd–P catalyst in the o-CNB hydrogenation.

ChemInform Abstract: Fabrication of Ultrafine Palladium Phosphide Nanoparticles as Highly Active Catalyst for Chemoselective Hydrogenation of Alkynes.

AbstractThe title catalyst is applied for the hydrogenation of terminal and internal alkynes to produce the corresponding alkenes with high yields and chemoselectivity.

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 Pd3 P0.95, 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.

Facile synthesis of palladium phosphide electrocatalysts and their activity for the hydrogen oxidation, hydrogen evolutions, oxygen reduction and formic acid oxidation reactions

We demonstrate a new approach for producing highly dispersed supported metal phosphide powders with small particle size, improved stability and increased electrocatalytic activity towards some useful reactions. The approach involves a one-step conversion of metal supported on high surface area carbon to the metal phosphide utilising a very simple and scalable synthetic process. We use this approach to produce PdP2 and Pd5P2 particles dispersed on carbon with a particle size of 4.5–5.5nm by converting a commercially available Pd/C powder. The metal phosphide catalysts were tested for the oxygen reduction, hydrogen oxidation and evolution, and formic acid oxidation reactions. Compared to the unconverted Pd/C material, we find that alloying the P at different levels shifts oxide formation on the Pd to higher potentials, leading to greater stability during cycling studies (20% more ECSA retained, 5k cycles) and in thermal treatment under air. Hydrogen absorption within the PdP2 and Pd5P2 particles is enhanced. The phosphides compare favourably to the most active catalysts reported to date for formic acid oxidation, especially PdP2, and there is a significant decrease in poisoning of the surface compared to Pd alone. The mechanistic changes in the reactions studied are rationalised in terms of increased water activation on the surface phosphorus atoms of the catalyst. One of the catalysts, PdP2/C is tested in a fuel cell as anode and cathode catalyst and shows good performance.

Liquid-phase hydrogenation of nitrobenzene and o-nitrochlorobenzene in the presence of phosphorus-containing palladium nanoparticles

The properties of phosphorus-modified palladium catalysts in the liquid-phase hydrogenation of nitrobenzene and o-nitrochlorobenzene have been investigated. A general regularity in the reduction of the nitro compounds in the presence of the phosphorus-containing palladium catalysts is an autoacceleration observed at large P: Pd ratios of 0.7–1.5. The autoacceleration is due to a redox process between the intermediate (azoxy derivative) and PdP2, a component of the catalyst nanoparticles. A promoting effect of elemental phosphorus on the chemoselectivity of the Pd-P catalysts in o-nitrochlorobenzene hydrogenation has been discovered. The main factors responsible for the changes in the quantitative characteristics of the phosphorus-modified palladium nanoparticles are considered. Among these factors there are the nature of the nanoparticles, their size, and the state of the surface layer. The properties of the Pd-P catalysts are determined by the proportions of palladium clusters and palladium phosphides (Pd(0) and Pd x P) in the nanoparticles. The formation of the clusters and phosphides has been confirmed by X-ray diffraction and X-ray photoelectron spectroscopy.

The Catalytic Properties of Pd Nanoparticles Modified by Phosphorus in Liquid-phase Hydrogenation of o-chloronitrobenzene

The new Pd-P catalysts for o-CNB hydrogenation obtained by interaction of Pd(acac)2 with white phosphorus in hydrogen, in N,N-dimethylformamide (DMF) under mild conditions were developed and studied. They exhibit higher selectivity to o-chloroaniline (74-94%) compared to Pd-black (56%). The introduction of phosphorus increases selectivity of palladium catalysts, however the activity values and P/Pd ratio are inversely related. The nature and the properties of Pd-P catalysts depend on P/Pd ratio. With the increase of the P/Pd ratio the average size of the particles decreases from 26 nm (for P/Pd = 0) to 5.6 nm (P/Pd = 0.3) and 4.8 nm (P/Pd = 1.0). Nanoparticles formed at P/Pd = 0.3 consist of palladium phosphide Pd6P and clusters Pd(0). Core-shell structure PdxP@Pdn was proposed for them. The Pd(0) clusters in the Pd-P catalyst (P/Pd = 0.3) possess higher electron density (Pd3d5/2 334.52 eV) compared to metallic palladium. With the increase of the P/Pd ratio the fraction of Pd(0) reduces and P-enriched palladium phosphides are formed. The Pd-P catalyst with P/Pd = 1.0 mainly consists of Pd5P2 and PdP2. The Pd(0) content for this catalyst is below 8%. Dimethylammonium dihydro- and hydrophosphates (phosphites) formed via the partial hydrolysis of the solvent – DMF contribute to the stabilization of Pd-P nanoparticles. We suggest that the nature of the catalyst nanoparticles consisting of Pd and PdxP as well as electronic state of Pd(0) clusters are the main factors determining catalytic properties of the Pd-P catalyst in o-CNB hydrogenation. Keywords: о-Chloronitrobenzene, hydrogenation, nanoparticles, palladium, phosphide, phosphorus.

Physical properties of new cerium palladium phosphide with C6Cr23-type structure

We have found that a cerium palladium phosphide crystallizes into a C6Cr23-type structure with atomic disorder. Prepared polycrystalline samples show a homogeneity range in the ternary Ce–Pd–P phase diagram. The physical properties of the highest-quality sample of Ce2.4Pd20.7P5.9 were investigated by measuring the magnetization, electrical resistivity and specific heat. No pronounced phase transition was observed down to 0.5K. The Kondo screening of localized 4f electrons in metallic Ce2.4Pd20.7P5.9 appears to be weaker than that in the isostructural compounds of Ce3Pd20Si6 and Ce3Pd20Ge6. By a comparative study of Ce2.4Pd20.7P5.9 and Ce3Pd20X6 (X=Si, Ge), the competition between the Kondo temperature and ordering temperatures including the quadrupolar ordering temperature is briefly discussed.

The nature of nanoparticles formed in the system PdCl2-elemental phosphorus

The state of palladium in nanoparticles formed in the system PdCl2 — elemental phosphorus in an inert atmosphere was identified using X-ray diffraction (XRD), high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS). Palladium phosphides of different composition and Pd(0) clusters are shown to form as a result of a redox process. According to the XPS data, the structures of nanoparticles are of the core-shell type. The interaction of PdCl2 with P4 is followed not only by the hydrolysis of the solvent (DMF) accelerated by hydrochloric acid, but also by the decomposition catalyzed by palladium cluster of organic compounds. The formed graphite and phosphoric acids stabilize the nanoparticles.

Microstructure and thermoelectric properties of Ca0.9Yb0.1MnO3/Pd nanocomposite prepared by electroless plating

We have fabricated a Ca0.9Yb0.1MnO3/Pd nanocomposite in which a palladium layer made up of particles with a size of about 50nm exists at grain boundaries in the Ca0.9Yb0.1MnO3 matrix. The nanocomposite was produced by plating Ca0.9Yb0.1MnO3 powder with palladium and then carrying out spark plasma sintering. The palladium particles contained 10wt.% phosphorus, independent of the palladium content, and had an amorphous structure. The as-plated phosphorous-containing palladium particles transformed into crystalline palladium and amorphous palladium phosphide after sintering at 1023K. The thermoelectric properties of the Ca0.9Yb0.1MnO3/Pd nanocomposite bulk degraded with increasing palladium content, mainly as a result of the high electrical resistivity. This is probably because the palladium-phosphorus phase located at the grain boundaries inhibits electrical conductance, which leads to lower electrical mobility.

Structural transitions at the nanoscale: the example of palladium phosphides synthesized from white phosphorus

Stoichiometric reactions of Pd(0) nanoparticles with various amounts of white phosphorus (P4) are an efficient route to convert them into the corresponding Pd phosphides Pd(x)P(y). Formation of crystallized palladium phosphide nanoparticles is a two-step process, which allows exploring in detail the phase transitions of the Pd(x)P(y) system, from amorphous Pd-P nanoparticles (formed in a first step at moderate temperature) to crystallization (at higher temperature). The second temperature was found to be strongly dependent on the Pd/P ratio: PdP2, Pd5P2 and Pd3P stoichiometries form the amorphous phases, but only PdP2 and Pd5P2 could be further crystallized from them. Although it exists as a bulk crystalline material, Pd3P could only be crystallized by starting from the more Pd-rich Pd6P composition. Phase-to-phase transformations from P-poor phosphides (Pd3P and Pd5P2) to the P-rich PdP2 were also demonstrated, and a first Pd-P phase diagram at the nanoscale was tentatively produced.

Synthesis and Hydrodesulfurization Properties of Noble Metal Phosphides: Ruthenium and Palladium

Silica-supported ruthenium and palladium phosphide catalysts (Ru2P, RuP, Pd3P, Pd5P2) were investigated for the hydrodesulfurization (HDS) of dibenzothiophene (DBT). The Ru and Pd phosphide catalysts were prepared by temperature-programmed reduction of hypophosphite-based precursors consisting of uncalcined or calcined Ru/SiO2 or Pd/SiO2 impregnated with ammonium hyposphosphite (NH4H2PO2). The Ru2P/SiO2 and RuP/SiO2 catalysts prepared from uncalcined precursors had smaller average crystallite sizes, higher CO chemisorption capacities, and higher HDS activities than the catalysts prepared from the calcined precursors, while the effect of preparation method on catalytic properties was less clear for the Pd3P/SiO2 and Pd5P2/SiO2 catalysts. Following HDS testing at 673 K, X-ray diffraction analysis revealed that the Pd5P2/SiO2 catalysts decomposed to give Pd3P on the silica support, while the other phosphides exhibited good stability during the testing period. At temperatures at which high DBT conversion was observed (>598 K), the Ru and Pd phosphide catalysts were less active than sulfided Ru/SiO2 and Pd/SiO2 catalysts prepared from the uncalcined metal precursors.

Formation of palladium phosphides in the reaction of bis(dibenzylideneacetone)palladium(0) with white phosphorus

The reaction of bis(dibenzylideneacetone)palladium(0) with white phosphorus was studied using the methods of NMR, UV spectroscopy, and X-ray powder diffraction. The products of the reaction are shown to be palladium phosphides, their composition depending on the ratio of the reagents. The mechanism of the formation of the palladium-enriched phosphides is suggested, which includes the formation of palladium diphosphide PdP2 that subsequently reacts with the excess of bis(dibenzylideneacetone)palladium(0) leading to palladium phosphides Pd5P2, Pd3P0.8, Pd4.8P, and free dibenzylideneacetone.

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.

Selective synthesis of nanosized palladium phosphides from white phosphorus

The reaction of white phosphorus with the palladium(0) complex [Pd2(dba)3] (dba is dibenzylideneacetone) leads to stable associates [Pd(P4)]n. The coordination polymer obtained is stabilized in a solid state via the formation of nanosized palladium phosphides Pd3P2 and Pd6P with average particle sizes of 140 and 65 nm, respectively.

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.

Reaction of palladium bisacetylacetonate with elemental phosphorus. Effect of water on the composition of palladium phosphides

Reaction of palladium bisacetylacetonate with elemental phosphorus in an inert atmosphere is shown to proceed as a redox process forming palladium phosphides of different compositions: PdP2, Pd5P2, Pd4,8P, and Pd12P3,2. The conversion of Pd(acac)2 and the composition of palladium phosphides formed in benzene is established to be affected by water. A tentative scheme of the formation of palladium phosphides is suggested.