Dispersed Palladium Research Articles

Hydrodechlorination of 4-Chlorophenol on Pd-Fe Catalysts on Mesoporous ZrO2SiO2 Support.

A mesoporous support based on silica and zirconia (ZS) was used to prepare monometallic 1 wt% Pd/ZS, 10 wt% Fe/ZS, and bimetallic FePd/ZS catalysts. The catalysts were characterized by TPR-H2, XRD, SEM-EDS, TEM, AAS, and DRIFT spectroscopy of adsorbed CO after H2 reduction in situ and tested in hydrodechlorination of environmental pollutant 4-chlorophelol in aqueous solution at 30 °C. The bimetallic catalyst demonstrated an excellent activity, selectivity to phenol and stability in 10 consecutive runs. FePd/ZS has exceptional reducibility due to the high dispersion of palladium and strong interaction between FeOx and palladium, confirmed by TPR-H2, DRIFT spectroscopy, XRD, and TEM. Its reduction occurs during short-time treatment with hydrogen in an aqueous solution at RT. The Pd/ZS was more resistant to reduction but can be activated by aqueous phenol solution and H2. The study by DRIFT spectroscopy of CO adsorbed on Pd/ZS reduced in harsh (H2, 330 °C), medium (H2, 200 °C) and mild conditions (H2 + aqueous solution of phenol) helped to identify the reasons of the reducing action of phenol solution. It was found that phenol provided fast transformation of Pd+ to Pd0. Pd/ZS also can serve as an active and stable catalyst for 4-PhCl transformation to phenol after proper reduction.

Sintering Activated Atomic Palladium Catalysts with High-Temperature Tolerance of ∼1,000°C

Summary Sintering-induced aggregation of active metals is a major cause of catalyst deactivation. Catalysts that can operate above 800°C are rare. Here, we report an unusual noble metal catalyst with sintering-induced activation at temperatures up to 1,000°C. The catalyst consists of atomically dispersed palladium embedded in a reducible SnO2 support designated for lean methane combustion. High temperature reaction simultaneously causes favorable changes of palladium ensemble state combining synergistically with lattice oxygen activation. Such changes lead to at least one order of magnitude improvement of the intrinsic reactivity, which compensates the surface area loss. Extensive characterizations such as atom probe tomography, X-ray absorption spectroscopy, and isotope tracking together with theoretical calculations illustrate the structure and surface chemistry changes and their impacts on the reaction mechanism. The catalyst also shows notable long-term stability and facile regeneration after poisoning. Our work may provide new insights into designing active and thermally stable catalysts.

Highly efficient and enhanced sulfur resistance supported bimetallic single-atom palladium–cobalt catalysts for benzene oxidation

Catalytic oxidation is one of the effective pathways for completely eliminating volatile organic compounds (VOCs) emitted from industrial and transportation activities. Meanwhile, single-atom catalysts have excellent application prospects in numerous reactions due to their high metal atomic utilization efficiency. In this work, we adopted a novel strategy to prepare an active Pd/Co single-atom catalyst (i.e., Pd1Co1/Al2O3) for benzene oxidation. The successful formation of the atomically dispersed palladium and cobalt species on Al2O3 was verified by the aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption fine structure. By the in situ temperature-programmed techniques and in situ diffuse reflectance Fourier transform infrared spectroscopy, we observed a double effect of the palladium and cobalt oxide active sites, resulting in an enhanced performance for benzene oxidation. A benzene conversion of 90 % was achieved over the Pd1Co1/Al2O3 catalyst at 250 °C and a space velocity of 40,000 mL/(g h). Interestingly, the catalyst also possessed enhanced sulfur resistance performance. The good regeneration ability of the active sites in the catalyst was due to the single-atom dispersion of Pd and Co. In addition, we deduce that benzene oxidation might occur over Pd1Co1/Al2O3 via a pathway of benzene → cyclohexadiene → phenol → quinone → maleate → acetate → CO2 and H2O. We believe that the obtained results can provide a useful idea for rationally designing the double active site single-atom catalysts and understanding the mechanism of VOCs oxidation.

Selective Hydrogenation on a Highly Active Single-Atom Catalyst of Palladium Dispersed on Ceria Nanorods by Defect Engineering.

Single-atom catalysis represents a new frontier that integrates the merits of homogeneous and heterogeneous catalysis to afford exceptional atom efficiency, activity, and selectivity for a range of catalytic systems. Herein we describe a simple defect engineering strategy to construct an atomically dispersed palladium catalyst (Pdδ+, 0 < δ < 2) by anchoring the palladium atoms on oxygen vacancies created in CeO2 nanorods. This was confirmed by spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurement. The as-prepared catalyst showed exceptional catalytic performance in the hydrogenation of styrene (99% conversion, TOF of 2410 h-1), cinnamaldehyde (99% conversion, 99% selectivity, TOF of 968 h-1), as well as oxidation of triethoxysilane (99% conversion, 79 selectivity, TOF of 10 000 h-1). This single-atom palladium catalyst can be reused at least five times with negligible activity decay. The palladium atoms retained their dispersion on the support at the atomic level after thermal stability testing in Ar at 773 K. Most importantly, this synthetic method can be scaled up while maintaining catalytic performance. We anticipate that this method will expedite access to single-atom catalysts with high activity and excellent resistance to sintering, significantly impacting the performance of this class of catalysts.

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.

Atom-Scale Dispersed Palladium in a conductivePd0.1TaS2 Lattice with a Unique Electronic Structurefor Efficient Hydrogen Evolution

Noble metal catalysts have extraordinary catalytic activity but suffer from high cost; therefore single atom catalysis attracts tremendous attention. Here, we propose atom-scale dispersed palladium (Pd) in a layered TaS2 lattice to form new compounds of PdxTaS2, whose crystal structure was resolved by powder X-ray diffraction. The electrical conductivity of metallic Pd0.1TaS2 (1.36 * 104 S m-1) is twice as large as that of TaS2 (6.18 * 103 S m-1). This compound applied as a new electrocatalyst for the hydrogen evolution reaction (HER) exhibits excellent catalytic performance with an onset overpotential of 77 mV, superior to those of other catalysts such as TaS2 (295 mV) and Pd-loaded TaS2 (114 mV). Density functional theory calculations reveal that the interaction of Pd-4d and Ta-5d orbitals results in the upshift of the Fermi level (TaS2 versus Pd0.1TaS2) to decrease the barrier height for the HER. Furthermore, the hybridization of Pd-4d and S-3p orbitals builds up pathways along the c axis to improve the conductivity and promote HER performance. It is noted that the amount of Pd used in Pd0.1TaS2 is only 4 wt%, compared with 20 wt% in commercial Pd/C catalysts, which is beneficial for its practical application.

Direct Synthesis of Atomically Dispersed Palladium Atoms Supported on Graphitic Carbon Nitride for Efficient Selective Hydrogenation Reactions.

Heterogeneous catalysts with atomically precise metal sites have enabled unique insight into structure-property relationships in materials science. Herein, we report the construction and selective hydrogenation performance of a single-atom palladium catalyst by confining the palladium atoms into the six-fold N-coordinating cavities of graphitic carbon nitride (g-C3N4) through a facile spatial confinement-reduction approach under mild reducing conditions. Spherical aberration correction electron microscopy and extended X-ray absorption fine structure measurements confirm the presence of atomically dispersed palladium atoms stabilized by the g-C3N4 support. Its exceptional catalytic activity was demonstrated by the hydrogenation of styrene (98% conversion, 1.5 h) and furfural (conversion of 64% and selectivity of 99%, 4 h) and hydrodechlorination of 4-chlorophenol (99% conversion and 99% selectivity, 10 min). This palladium catalyst can be reused at least five times with negligible deterioration of its activity. Importantly, the palladium atoms retained their atomic dispersion following the thermal treatment. Moreover, this synthetic method can be scaled up while retaining similar catalytic activity. Fundamental insights are provided to elucidate how the material's structure significantly impacts the catalytic performance at the atomic scale.

Catalysis by platinum and palladium species confined in the bulk of glass fibre materials

The results of studies on the application of silicate glass fibre materials in catalysis are summarized and analyzed. Despite the very low noble metal content, catalysts based on these materials showed exceptionally high activities and selectivities in some catalytic reactions. This is due to specificity of the glassy state, which makes it possible, first, to confine highly dispersed palladium and platinum species in the bulk of glass fibres and, second, selectively absorb polar molecules, thus excluding the undesirable reactions involving non-polar molecules. The size dependences of the complete oxidation of propane and selective hydrogenation of acetylene, the nature of the structure sensitivity of these reactions and the reaction mechanisms are discussed. Ways for improving glass fibre catalyst performance are proposed and examples of the successful application of Pt/glass fibre catalysts for purification of industrial gases from volatile organic compounds are given.The bibliography includes 175 references.

Hierarchical porous TS-1/Pd/CdS catalysts for enhanced photocatalytic hydrogen evolution

The conversion of water to hydrogen through solar energy is considered as a promising solution to the energy shortage and environmental problems. In this work, the alkali-treated titanium silicalite-1 (TS-1) with a hierarchical porous structure was used as a carrier to prepare the TS-1/Pd/CdS catalysts with large specific surface area and wide photoresponse range. The hydrogen evolution rate of the 0.3NaOH-TS-1/Pd/CdS can reach 2556.0 μmol/h under visible light, which is about 3.5 times that of the Untreated TS-1/Pd/CdS. Herein, the catalysts with a composite structure are beneficial to the uniform dispersion of palladium and CdS quantum dots, the increase of the relative concentration of the active phase tetracoordinated titanium in the framework, and the enhancement of the reflecting and scattering of light in the multi-level pores, thus promoting the photocatalytic hydrogen production reaction.

One-step fabrication of a highly dispersed palladium nanoparticle-decorated reduced graphene oxide electrocatalyst for methanol electro-oxidation in acidic media

Reduced graphene oxide (RGO) was successfully decorated with homogeneous highly dispersed palladium nanoparticles (PdNPs) by a novel single-step hydrothermal-assisted formic acid reduction reaction without the use of any surfactants. The structure, surface morphology, and elemental composition of the as-prepared PdNP-RGO electrocatalyst were extensively characterized. The PdNP-RGO electrocatalyst demonstrated outstanding electrocatalytic activity (8.65 mA/cm2) toward the methanol oxidation reaction (MOR) in an acidic medium that is 1.4 times higher than PdNP-decorated Vulcan XC72 (6.2 mA/cm2) and also comparable with that of Pd-based electrocatalysts in alkaline media from the previous reports. Furthermore, the as-prepared PdNP-RGO electrocatalyst also shows remarkable stability performance for the MOR in an acidic medium and thus offer new insights into the processing method of an outstanding electrocatalyst at the anode electrode in the direct methanol fuel cell with reasonable cost, clean and facile synthesis approach.

In Situ Dispersion of Palladium on TiO2 During Reverse Water–Gas Shift Reaction: Formation of Atomically Dispersed Palladium

AbstractThe application of single‐atom catalysts (SACs) to high‐temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation. We demonstrate that Pd can be predominantly dispersed as isolated atoms onto TiO2 during the reverse water–gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO2 surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO2) with neat TiO2 prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared and X‐ray absorption spectroscopies and scanning transmission electron microscopy. The thermodynamic stability of Pd under high‐temperature rWGS conditions is associated with Pd‐Ti coordination, which manifests upon O‐vacancy formation, and the artificial increase in TiO2 surface area.

In Situ Dispersion of Palladium on TiO2 During Reverse Water-Gas Shift Reaction: Formation of Atomically Dispersed Palladium.

The application of single-atom catalysts (SACs) to high-temperature hydrogenation requires materials that thermodynamically favor metal atom isolation over cluster formation. We demonstrate that Pd can be predominantly dispersed as isolated atoms onto TiO2 during the reverse water-gas shift (rWGS) reaction at 400 °C. Achieving atomic dispersion requires an artificial increase of the absolute TiO2 surface area by an order of magnitude and can be accomplished by physically mixing a precatalyst (Pd/TiO2 ) with neat TiO2 prior to the rWGS reaction. The in situ dispersion of Pd was reflected through a continuous increase of rWGS activity over 92 h and supported by kinetic analysis, infrared and X-ray absorption spectroscopies and scanning transmission electron microscopy. The thermodynamic stability of Pd under high-temperature rWGS conditions is associated with Pd-Ti coordination, which manifests upon O-vacancy formation, and the artificial increase in TiO2 surface area.

Palladium nanoparticles dispersed on functionalized macadamia nutshell biomass for formic acid-mediated removal of chromium(VI) from aqueous solution

Driven by the need for sustainably sourced catalysts and the use of reaction systems that generate environmentally benign by-products, the present study aimed to deposit stable, dispersed palladium (Pd) nanoparticles on the modified surfaces of granular macadamia nutshell (MNS) biomass for catalytic reduction of hexavalent chromium (Cr(VI)) to trivalent chromium (Cr(III)). Through wet impregnation with Pd(II) ions and subsequent hydrazine-mediated reduction to Pd(0), Pd nanoparticles were embedded in a scaffold of polyethyleneimine grafted on bleached MNS previously coated with a chemically bound layer of polyglycidyl methacrylate. Imagery from scanning electron microscopy showed the formation of different layers of the polymeric coating and dispersed palladium resulting from surface modification and palladium nanoparticle synthesis, respectively. X-ray diffraction analysis confirmed the formation of Pd on the modified MNS surface and suggested an estimated crystallite size of 5.0 nm. The supported nanoparticles exhibited catalytic activity in formic acid-mediated Cr(VI) reduction and showed promising stability with consecutive reuse. These findings set the stage for advanced studies into performance optimization.

Fabrication and characterization of ternary sepiolite/g-C3N4/Pd composites for improvement of photocatalytic degradation of ciprofloxacin under visible light irradiation

The development of high-quality photocatalytic materials for the degradation of organic pollutants under visible light irradiation is a vital field of research. In the present study, a composite of natural sepiolite clay and synthetic graphitic carbon nitride (CN) mixed with dispersed palladium nanoparticles was developed for the efficient photocatalytic degradation of ciprofloxacin (CIP) under visible light irradiation. The sepiolite, CN, and composite materials were characterized by several techniques. The sepiolite/CN composite (SC30%) displayed superior activity than pristine sepiolite and CN, resulted from the generation of new electron trap states in the interfacial contract between sepiolite and CN to suppress the charge recombination of CN. Furthermore, the well-dispersed of 1 wt% Pd-nanoparticles in the SC30% composite collectively enhanced CIP degradation by avoiding the recombination of photogenerated electrons and holes. Additionally, the electron trap states on the surface of all samples were studied using novel reversed double-beam photoacoustic spectroscopy to understand electron transfer in the composites related to the photocatalytic degradation mechanism of CIP. The developed sepiolite/CN/Pd(0) composite can act as a potential catalyst for the degradation of organic pollutants in wastewater under visible light irradiation.

Atomic Dispersion and Surface Enrichment of Palladium in Nitrogen-Doped Porous Carbon Cages Lead to High-Performance Electrocatalytic Reduction of Oxygen.

Metal-nitrogen-carbon (MNC) nanocomposites have been hailed as promising and efficient electrocatalysts toward oxygen reduction reaction (ORR), due to the formation of MNx coordination moieties. However, MNC hybrids are mostly prepared by pyrolysis of organic precursors along with select metal salts, where part of the MNx sites are inevitably buried in the carbon matrix. This limited accessibility compromises the electrocatalytic performance. Herein, we describe a wet-impregnation procedure by facile thermal refluxing, whereby palladium is atomically dispersed and enriched onto the surface of hollow, nitrogen-doped carbon cages (HNC) forming Pd-N coordination bonds. The obtained Pd-HNC nanocomposites exhibit an ORR activity in alkaline media markedly higher than that of metallic Pd nanoparticles, and the best sample even outperforms commercial Pt/C and relevant Pd-based catalysts reported in the literature. The results suggest that atomic dispersion and surface enrichment of palladium in a carbon matrix may serve as an effective strategy in the fabrication of high-performance ORR electrocatalysts.

Modification of Ni/Al2O3 catalyst with Pd nanoparticles for selective phenylacetylene semihydrodenation

NiPd/Al2O3 (4.9 wt% of Ni–0.1 wt% of Pd), Ni/Al2O3 (5 wt% of Ni) and Pd/Al2O3 (0.1 wt% of Pd) catalysts were tested in the liquid-phase semihydrogenation of phenylacetylene in batch- and flow-type reactors. Catalysts were characterized by temperature-programmed reduction, high resolution transmission electron microscopy, and X-ray photoelectron spectroscopy. Differences in the formation of palladium particles during deposition of metals on the surface of the alumina were revealed. Relatively large Pd particles observed in Pd/Al2O3 transformed to β-PdHx phase in a hydrogen atmosphere. When Pd and Ni are co-supported, no monometallic palladium particles were detected. Pd was apparently atomically distributed in the close vicinity of some NiO particles. As a result, there were two types of NiO particles in NiPd/Al2O3: NiO particles isolated from Pd species and those in close contact with atomically dispersed palladium. This distribution of metals provided the formation of activated hydrogen, which, in turn, led to an increase in the phenylacetylene conversion compared to monometallic Ni/Al2O3, while maintaining high selectivity of styrene formation. The close interaction of metals and their co-action in the hydrogenation of phenylacetylene was confirmed by the kinetic data obtained in a batch-type reactor. These data showed the difference in the reaction mechanisms on Ni/Al2O3 and NiPd/Al2O3.

Palladium dispersion effects on wet methane oxidation kinetics

The catalytic activity for dry and wet methane oxidation over a series of palladium–alumina catalysts with systematically varied palladium loadings and PdO dispersions was measured and compared with conceptual multiscale simulations.

Mechanochemical Kilogram-Scale Synthesis of Noble Metal Single-Atom Catalysts

Summary Single-atom catalysts (SACs) have attracted broad interest recently due to their superior catalytic properties. However, a facile fabrication method for the large-scale synthesis of SACs is still in demand. This study reports a mechanochemical approach for the mass production of noble metal SACs. The successful formation of atomically dispersed palladium species on zinc oxide (Pd1/ZnO) was verified by aberration-corrected high-angle annular dark-field scanning transmission electron microscopy and X-ray absorption spectroscopy. Furthermore, our method exhibited little scaling-up effect on the mass production of Pd1/ZnO at ranges of 10–1,000 g, in which the catalyst structure and catalytic performance were retained. Meanwhile, the versatility of this approach was demonstrated by the large-scale fabrication of Rh and Ru SACs, and Pd1/Cu single-atom alloys. Thus, this promising strategy provides the potential for cost-effective mass production of SACs and subsequently may open a window for their industrial application.

Ni-based layered metal-organic frameworks with palladium for electrochemical dechlorination

Electrocatalytic hydrogenolysis (ECH) appears to be an effective method for remediation of toxic contaminants in natural water. In this work, metallic palladium was loaded on Ni-based layered metal-organic frameworks (NiMOF) using adsorption followed by an electroreduction method. Benefiting from high dispersion of palladium and enhanced adsorption of NiMOF towards 2,4-dichlorophenoxyacetic acid (2,4-D), the as-obtained electrode (Pd-NiMOF/Ni) offers large amounts of active hydrogen and high surface concentration of 2,4-D in the ECH process. Consequently, Pd-NiMOF/Ni displays enhanced activity and selectivity compared to traditional electrodes. In addition, the Pd-NiMOF/Ni cathode was extremely stable, as verified by six successive cycling tests. Co-existing ions, including NO3−, HCO3−, CO32− and Ca2+ exert a negative impact on the electrocatalytic dechlorination, whereas no effect of Cl− on the process. Nonetheless, the electrochemical dechlorination of 2,4-D in Evian water remained effective, demonstrating the potential application of this electrode in efficient dechlorination of chlorinated aromatic compounds.

Palladium Nanocatalysts Encapsulated on Porous Silica @ Magnetic Carbon‐Coated Cobalt Nanoparticles for Sustainable Hydrogenation of Nitroarenes, Alkenes and Alkynes.

AbstractPalladium nanoparticles were impregnated on porous silica shell carbon‐coated cobalt nanoparticles, resulting in a magnetically retrievable material that was evaluated in the catalytic hydrogenation of nitroarenes, alkenes and alkynes. The prepared material was characterized by HR‐XRD, HR‐TEM, elemental mapping EDX, ICP‐OES and XPS analyses, revealing highly dispersed palladium nanoparticles within the porous platform that could account for the high activity observed. Mild reaction conditions, easy retrievability of the catalyst with the aid of an external magnet, recycling in four runs with a total leaching of 19 ppm (1.2 % of the initially employed Pd amount), and high stability makes this material attractive for sustainable and environmentally benign applications.