Pd Nanocatalysts Research Articles

Bimetallic Pt–Pd nano-catalyst: size, shape and composition matter

The goal of this work is to understand how the phase diagram (PHAD) of the platinum–palladium (Pt–Pd) alloy changes with size and shape and how it correlates with catalytic properties. By using nano-thermodynamics, the size and shape effects on the PHAD of Pt–Pd nanoparticles were determined theoretically. The PHAD of some nanoparticles (sphere, tetrahedron, octahedron, decahedron, cube, cuboctahedron and rhombic dodecahedron) exhibits a congruent melting point that becomes more and more pronounced when the size decreases. At the right of the congruent melting point i.e. close to the Pt-rich side, the coexistence region exhibits a contraction while an expansion is noticed at larger palladium concentrations. From the Gibbs free energy analysis, the stability of all the considered shapes has been determined versus temperature and composition. Furthermore, the surface segregation was also calculated and it is shown that the surface segregation is reversed at very small sizes. Indeed, below a critical size, Pd does not segregate anymore at the surface like it normally does for larger nanoparticles; but Pt does. The critical size range has been determined for each considered shape; and within this range Pt and Pd co-exist at the surface. Finally, the most catalytically active shapes are predicted to be the tetrahedron and the cube in agreement with the available experimental data and other theoretical results.

Fabrication and Electrocatalytic Performance of Graphene-fullerene Ammonium Iodide Composite Supported Pd Nanocatalyst for Ethanol Oxidation†

Fabrication and Electrocatalytic Performance of Graphene-fullerene Ammonium Iodide Composite Supported Pd Nanocatalyst for Ethanol Oxidation†

Palladium nanoparticles hosted in graphene-based 2-dimension polyelectrolyte brushes for enhanced hydrogenation selectivity of o-chloronitrobenzene

Pd nanocatalysts were successfully synthesized by the ethylene glycol reduction method using graphene as supports and applied to the selective hydrogenation of o-chloronitrobenzene (o-CNB) to o-chloroaniline (o-CAN). The results indicate that the low transfer resistance and strong metal-support interactions between RGO and Pd nanoparticles of Pd/RGO can efficiently improve the selectively. The low transfer resistance can shorten the stay time of the product molecule o-chloroaniline (o-CAN) on the surface of the catalyst sites and then reduce the dechlorination side-reaction of o-CAN. The strong metal-support interactions can lead to the electron-deficient state of the Pd nanoparticles which weakened the extent of electron feedback from the Pd nanoparticles to the aromatic ring and suppressed the hydrodechlorination of o-CAN. Moreover, the selectivity can be further improved for the hydrogenation of o-CNB after grafting sodium polystyrenesulfonate on the Pd/RGO to cover the low-coordinated surface Pd atoms.

Efficient 1,3-Oxazolidin-2-one Synthesis through Heterogeneous PdII -Catalyzed Intramolecular Hydroamination of Propargylic Carbamates.

Herein, we present an operationally simple protocol for the cycloisomerization of propargylic carbamates in which a heterogeneous catalyst consisting of Pd species immobilized on amino-functionalized siliceous mesocellular foam (PdII -AmP-MCF) is used. This Pd nanocatalyst displayed high efficiency at low catalyst loading and reaction temperatures, which allowed for the efficient and mild synthesis of a wide range of 1,3-oxazolidin-2-one derivatives and related compounds. Moreover, it proved possible to re-use the Pd nanocatalyst for several reactions, although a gradual decrease in activity was observed in the subsequent cycles.

Pd Nanocatalyst Adorning Coral Reef Nanocomposite for the Synthesis of Nitriles: Utility of Cucurbita pepo Leaf Extract as a Stabilizing and Reducing Agent.

A simple procedure for the palladium-catalyzed cyanation of aryl halides is described via a nucleophilic non-toxic cyanide source, K4[Fe(CN)6] in the presence of Pd/coral reef nanocomposite as a heterogeneous catalyst; the protocol provides a useful and easy method for the synthesis of aryl nitriles that are generated from the corresponding variant aryl halides, with sodium carbonate as a base. The nanocatalyst was prepared by a biological process using aqueous extract of leaves of Cucurbita pepo as a stabilizing and reducing agent and coral reef as a natural support, without deploying any hazardous chemicals. The catalyst, that is easily separable from the reaction mixture and reused multiple times, was characterized by FT-IR (Fourier-Transform Infrared Spectroscopy), ICP-AES (Inductively Coupled Plasma Atomic Emission Spectroscopy), XRD (X-ray Diffraction), TEM (Transmission Electron Microscopy), FE-SEM (Field Emission Scanning Electron Microscopy), EDS (Energy Dispersive X-ray Spectroscopy) and elemental mapping.

Glycerol oxidation on Pd nanocatalysts obtained on PEDOT-coated graphite supports

Glycerol oxidation is studied at Pd nanoparticles (NPs) obtained by electroless metal deposition on PEDOT-coated graphite electrodes. Pre-reduced PEDOT coatings with different polymerization charges and different doping ions (polystyrenesulfonate, PSS or dodecyl sulfate, SDS) are used to drive the electroless deposition of Pd NPs. Pd nanocatalysts with metal loadings in the order of 15–30 μgcm−2 and high electroactive surface area (EASA) are obtained. Oxidation of glycerol is studied in dependence of the type of doping (PSS or SDS) and polymerization charge. It is found that at constant Pd loading PEDOT/PSS-supported catalysts have a three times higher glycerol peak oxidation currents. The mass activity (MA) diminishes with increasing Pd loading on both types of PEDOT but for different reasons. Impeded access of glycerol to NPs deposited deeply in the polymer structure is suggested for the porous and hydrophilic structure of PEDOT/PSS. Aggregation of NPs and therefore smaller EASA due to the upper surface confined metal deposition process are operative at PEDOT/SDS coatings. These are also the reasons for the low MA values observed on this type of PEDOT support. MA values at PEDOT/PSS-coated graphite are found to range between 1100 and 1200 mAmg−1 and are between the best values obtained on Pd-supported catalysts.

Selective semi-hydrogenation of phenyl acetylene by Pd nanocatalysts encapsulated into dendrimer networks

A series of heterogeneous catalysts, based on Pd nanoparticles encapsulated into the networks, obtained from poly(propylene imine) (PPI) dendrimers, has been synthesized. Dendritic networks were designed using bi- and ter-functional coupling agents of various size, rigidity and polarity, influencing the catalyst morphology and hydrogenation activity. The catalysts obtained have demonstrated significant increase in selectivity on styrene in comparison with traditional Lindlard catalyst, maintaining at prolonged reaction time and quantitative conversions achieved. A positive dendritic effect was revealed when passing from dendrimers of the 1st generation in the carrier to those of the 3rd generation. The best results were obtained for the catalyst, synthesized using 3,3′-dimethoxy-4,4′-diphenyl diisocyanate in the presence of polymeric template (the selectivity of 90–95% at conversion of 98–100% and surface turnover frequency, TOFs, of 160–300 s−1). The same catalyst has demonstrated the best stability at recycling.

Atmospheric hydrogenation of α, β-unsaturated ketones catalyzed by highly efficient and recyclable Pd nanocatalyst

A thermoregulated phase-transfer Pd nanocatalyst was explored firstly and shown to be highly efficient and recyclable in the atmospheric hydrogenation of α, β-unsaturated ketones. Under optimized reaction conditions, the conversion of chalcone and the selectivity of dihydrochalcone were 99% and 98%, respectively. The catalyst can be easily separated from the product and used directly for four times without evident loss in activity and selectivity. The turnover frequency (TOF) for the atmospheric hydrogenation of chalcone was 870 h−1, which to the best of our knowledge was the highest value ever reported among transition metal nanocatalysts.

Hierarchical Pd@Ni catalyst with a snow-like nanostructure on Ni foam for nitrobenzene hydrogenation

Heterogeneous catalysts with a considerable active surface area are critically important to achieve the excellent catalytic activity. In this work, hence, a hierarchical Pd@Ni catalyst with a snow-like Ni nanostructure was prepared by combining the hydrothermal synthesis method with spontaneously galvanic replacement reaction. The snow-like Ni nanostructure on nickel foam not only effectively increased the specific surface area, but also ensured good dispersion of Pd nanocatalyst. Experimental results confirmed that the specific surface area of nickel foam was increased from 1 m2/g to 33 m2/g and the metallic active surface area was promoted from 0.34 m2/g to 1.31 m2/g. The catalytic activity and lifetime of the hierarchical Pd@Ni catalyst was also evaluated by nitrobenzene hydrogenation. It was demonstrated that, benefited from the higher catalytic surface area and the strong interaction between Pd and NiO, the hierarchical Pd@Ni catalyst could present both higher catalytic performance (TOF = 242 h−1) and lifetime than the conventional catalyst.

2D & 3D in situ study of the calcination of Pd nanocatalysts supported on delta-Alumina in an Environmental Transmission Electron Microscope

The quality of metallic nanoparticles (NPs) used in heterogeneous catalysis relies through many aspects on their small size, on the homogeneity of their spatial distribution on their supports and on their ability to resist to sintering or coalescence. It is thus very important to quantify these parameters and understand the mechanisms controlling the growth of NPs during the genesis process of the catalyst. Whereas conventional Transmission Electron Microscopy (TEM) is currently used for these purposes, it most frequently remains a ‘static’ method where results are obtained in high vacuum and post mortem, i.e. after the typical drying, calcination and reduction steps without the possibility to follow directly the evolution of both NPs and supports during those treatments. Environmental TEM (ETEM) unlocks this blocking and allows elementary mechanisms, such as Ostwald Ripening and coalescence to be unravelled through direct in situ observations. We report here an ETEM study of the preparation of Pd-based narrow NPs, less than 5 nm in size, deposited on a δ-alumina support. We focused on 3 main objectives: (i) quantifying the sizes of NPs at each preparation step performed in situ under environmental (i.e. respectively oxygen or air and hydrogen atmospheres at working temperatures) and comparing them to post mortem measurements; (ii) identify the oxidation state of the NPs through an in situ High Resolution imaging study of their crystallographic structure; (iii) explore the possibilities of environmental tri-dimensional (3D) studies by tilt series based Electron Tomography. This last item represents a challenging breakthrough in the characterization of nanocatalysts; it will be demonstrated that the use of modern instruments (microscope and accessories) allows tomographic acquisitions to be performed very fast, within a few minutes and even seconds, which opens the way to the 3D tracking of microstructures almost in real time during their evolution under gas and at high temperature.

Rapid Room-Temperature Synthesis of a Metastable Ordered Intermetallic Electrocatalyst.

Metal alloys with atomic scale ordering (ordered intermetallics) have emerged as a new class of high performance materials for mediating electrochemical reactions. However, ordered intermetallic nanostructures often require long synthesis times and/or high temperature annealing to form because a high-activation energy barrier for interdiffusion must be overcome for the constituent metals to equilibrate into ordered structures. Here we report the direct synthesis of metastable ordered intermetallic Pd31Bi12 at room-temperature in minutes via electrochemical deposition. Pd31Bi12 is highly active for the reduction of O2 to H2O, delivering specific activities over 35× higher than those of commercial Pt and Pd nanocatalysts, placing it as the most active Pd-based catalyst, to the best of our knowledge, reported under similar testing conditions. Stability tests demonstrate minimal loss of activity after 10,000 cycles, and a retention of intermetallic crystallinity. This study demonstrates a new method of preparing ordered intermetallics with extraordinary catalytic activity at room temperature, providing a new direction in catalyst discovery and synthesis.

Chemical Design of Palladium-Based Nanoarchitectures for Catalytic Applications.

Palladium (Pd) plays an important role in numerous catalytic reactions, such as methanol and ethanol oxidation, oxygen reduction, hydrogenation, coupling reactions, and carbon monoxide oxidation. Creating Pd-based nanoarchitectures with increased active surface sites, higher density of low-coordinated atoms, and maximized surface coverage for the reactants is important. To address the limitations of pure Pd, various Pd-based nanoarchitectures, including alloys, intermetallics, and supported Pd nanomaterials, have been fabricated by combining Pd with other elements with similar or higher catalytic activity for many catalytic reactions. Herein, recent advances in the preparation of Pd-based nanoarchitectures through solution-phase chemical reduction and electrochemical deposition methods are summarized. Finally, the trend and future outlook in the development of Pd nanocatalysts toward practical catalytic applications are discussed.

Highly active and durable Pd nanocatalyst promoted by an oxygen-deficient terbium oxide (Tb4O7−x) support for hydrogenation and cross-coupling reactions

Oxygen-deficient Tb4O7−x as an effective promoter and support for Pd nanocatalysts holds great potential in hydrogenation and cross-coupling reactions.

A carboxylic acid functionalized SBA-15 supported Pd nanocatalyst: an efficient catalyst for hydrogenation of nitrobenzene to aniline in water

The catalytic performance of a PdNPs/SBA-COOH has been investigated for the first time in the selective hydrogenation of nitrobenzene with hydrous hydrazine at RT in water medium.

Well-dispersed Pd nanoparticles on porous ZnO nanoplates via surface ion exchange for chlorobenzene-selective sensor.

The extensive use of chlorobenzene in chemical, pharmaceutical, and agrochemical industries poses a severe health hazard to human beings, because it is highly toxic. The detection of chlorobenzene by metal oxide gas sensors is difficult, owing to its chemically inert molecular structure. In this study, well-dispersed Pd nanoparticles were deposited on porous ZnO nanoplates via surface ion exchange, followed by H2 reduction. The preparation process effectively prevented the aggregation and uncontrollable growth of Pd particles. A gas-sensing test was conducted, and the modification of size-controlled Pd nanoparticles was found to effectively enhance the sensing properties of porous ZnO nanoplates to chlorobenzene over 300 °C (higher sensitivity at a low operating temperature). At 440 °C, 5% Pd@ZnO sensor showed a drastic increase in response by nearly 4.5-fold, as well as excellent sensing selectivity to chlorobenzene. Its repeatability and stability were acceptable. As known, Pd nanocatalysts contribute to the oxidation of chlorinated aromatic compounds. Pd@ZnO sensors generated more catalytic sites and oxygen species (confirmed by XPS), thus enhancing chlorobenzene oxidation and improving the sensitivity of ZnO-based gas sensors.

Good Pd Nanocatalysts Will Be Rewarded with Good Applications

Good Pd Nanocatalysts Will Be Rewarded with Good Applications

Pd nanocones supported on g-C3N4: An efficient photocatalyst for boosting catalytic reduction of hexavalent chromium under visible-light irradiation

Herein, a facile poly-l-lysine (PLL)-mediated one-pot hydrothermal approach was developed for large-scaled synthesis of Pd nanocones enclosed by the {1 1 1} facets, followed by uniformly dispersing them on graphitic carbon nitride (g-C3N4). The Pd nanocones/g-C3N4 demonstrated dramatic enhancement for photocatalytic transformation of Cr(VI) to Cr(III) via the dehydrogenation pathway under visible-light irradiation, which outperforms the {1 0 0} facets enclosed Pd nanocubes/g-C3N4, Pd black/g-C3N4, and bare g-C3N4 catalysts. Meanwhile, the as-prepared nanocomposite showed dramatically enhanced conversion up to 99.9% and improved reusable ability via the recycling test. It would advance the development of largely efficient g-C3N4 supported Pd nanocatalysts via morphology- and composition-engineering.

Influences of Reducing Agents on Performance of Carbon Supported Pd Nanocatalyst for Hydrogenation of Nitrobenzene

Influences of Reducing Agents on Performance of Carbon Supported Pd Nanocatalyst for Hydrogenation of Nitrobenzene

Synthesis of Highly Dispersed Pd Nanocatalysts Through Control of Organic Ligands and Their Electrochemical Properties for Oxygen Reduction Reaction in Anion Exchange Membrane Fuel Cells

In anion exchange membrane fuel cells, Pd nanoparticles are extensively studied as promising non-Pt catalysts due to their electronic structure similar to Pt. In this study, to fabricate Pd nanoparticles well dispersed on carbon support materials, we propose a synthetic strategy using mixed organic ligands with different chemical structures and functions. Simultaneously to control the Pd particle size and dispersion, a ligand mixture composed of oleylamine(OA) and trioctylphosphine(TOP) is utilized during thermal decomposition of Pd precursors. In the ligand mixture, OA serves mainly as a reducing agent rather than a stabilizer since TOP, which has a bulky structure, more strongly interacts with the Pd metal surface as a stabilizer compared to OA. The specific roles of OA and TOP in the Pd nanoparticle synthesis are studied according to the mixture composition, and the oxygen reduction reaction(ORR) activity and durability of highly-dispersed Pd nanocatalysts with different particles sizes are investigated. The results of this study confirm that the Pd nanocatalyst with large particles has high durability compared to the nanocatalyst with small Pd nanoparticles during the accelerated degradation tests although they initially indicated similar ORR performance.

Front Cover: Tailoring the 3D Structure of Pd Nanocatalysts Supported on Mesoporous Carbon for Furfural Hydrogenation (ChemNanoMat 11/2018)

Front Cover: Tailoring the 3D Structure of Pd Nanocatalysts Supported on Mesoporous Carbon for Furfural Hydrogenation (ChemNanoMat 11/2018)