Pd Oxide Research Articles

Self-Assembled Pd@CeO2 /γ-Al2 O3 Catalysts with Enhanced Activity for Catalytic Methane Combustion.

Pd@CeO2 /Al2 O3 catalysts are of great importance for real applications, such as three-way catalysis, CO oxidation, and methane combustion. In this article, the Pd@CeO2 core@shell nanospheres are prepared via the autoredox reaction in aqueous phase. Three kinds of methods are then employed, that is, electrostatic interaction, supramolecular self-assembly, and physical mixing, to support the as-prepared Pd@CeO2 nanospheres on γ-Al2 O3 . A model reaction of catalytic methane-combustion is employed here to evaluate the three Pd@CeO2 /γ-Al2 O3 samples. As a result, the sample Pd@CeO2 -S-850 prepared via supramolecular self-assembly and calcined at 850 °C exhibits superior catalytic performance to the others, which has a far lower light-off temperature (T50 of about 364 °C). Moreover, almost no deterioration of Pd@CeO2 -S-850 is observed after five sequent catalytic cycles. The analysis of H2 -TPR curves concludes that there exists hydrogen spillover related to the strong metal-support interaction between Pd species and oxides. The strong metal-support interaction and the specific surface areas might be responsible for the catalytic performance of the Pd@CeO2 samples toward catalytic methane combustion.

Electrochemical and Chemical Treatment Methods for Enhancement of Oxygen Reduction Reaction Activity of Pt Shell-Pd Core Structured Catalyst

Based on the accelerated durability test protocol (ADT, rectangular wave, 1.0V (3s)-0.6V (3s), 10,000 cycles, at 80°C), a high activation protocol (HAP) was developed as an electrochemical treatment method using rectangular wave potential cycling for a carbon supported Pt shell-Pd core structured catalyst (Pt/Pd/C), which mitigated the decay of electrochemical surface area (ECSA) and enhanced the mass activity for oxygen reduction reaction (ORR). It was found that a longer holding time and a less cycle number (300s and 30 cycles) compared with those used for the ADT protocol (3s and 10,000 cycles) effectively suppressed the ECSA decay and enhanced the ORR mass activity. A potential in the range of 0.2 to 0.6V was suitable as the lower limit potential of the HAP protocol for the ORR activity enhancement when the upper limit potential was fixed at 1.0V. Furthermore, H2-O2 and Cu-O2 chemical treatment methods, which chemically realize the HAP performed on GC electrode, were developed, where the equilibrium potentials of Cu/Cu2+, hydrogen and oxygen were utilized for surface oxidation and reduction of Pd and Pt. Both methods mitigated the ECSA decay and enhanced the ORR mass activity, and the Cu-O2 chemical treatment method gave a higher mass activity. These chemical treatment methods are useful for mass-production of highly active Pt core-shell structured catalyst.

Multiple-Hydrogen-Bond Approach to Uncommon Pd(III) Oxidation State: A Pd-Br Chain with High Conductivity and Thermal Stability.

A Br-bridged Pd chain complex with the Pd ion in an uncommon +3 oxidation state, [Pd(dabdOH)2Br]Br2 (3), was prepared using a new method involving multiple hydrogen bonds. The PdBr chain complex exhibited superior electrical conductivity and thermal stability. An in-plane ligand with an additional hydrogen donor group (hydroxy group), (2S,3S)-2,3-diaminobutane-1,4-diol (dabdOH), was used to create a multiple-hydrogen-bond network, which effectively shrinks the Pd-Br-Pd distance, stabilizing the Pd(III) state up to its decomposition temperature (443 K). 3 shows semiconducting behavior with quite high electrical conductivity (3-38 Scm-1 at room temperature), which is 106 times larger than the previous record for analogous PdBr chains. Indeed, 3 is the most conductive MX-type chain complex reported so far. The precise positional control of ions via a multiple-hydrogen-bond network is a useful method for controlling the electronic states, thermal stability and conductivity of linear coordination polymers.

Graphene Oxide: Microscopic Evidence for Strong Interaction between Pd and Graphene Oxide that Results in Metal‐Decoration‐Induced Reduction of Graphene Oxide (Adv. Mater. 15/2017)

Graphene Oxide: Microscopic Evidence for Strong Interaction between Pd and Graphene Oxide that Results in Metal‐Decoration‐Induced Reduction of Graphene Oxide (Adv. Mater. 15/2017)

Metastable Pd ↔ PdO Structures During High Temperature Methane Oxidation

Methane in the form of natural gas is increasingly used as a transportation fuel, but the treatment of methane in the exhaust is a challenge since methane is a potent greenhouse gas. Pd is one of the most active catalysts for methane oxidation. Previous work has shown that transformation of Pd into the oxide, and decomposition of the oxide to metallic Pd can occur as temperature is raised in an oxidizing atmosphere, causing profound changes in catalytic reactivity. Equilibrium thermodynamics predict that the phases Pd and PdO must be in equilibrium at a well-defined temperature and oxygen pressure, since the two phases are immiscible and do not form solid solutions. But catalytic data suggests the existence of metallic Pd under conditions where only PdO should be thermodynamically stable. In this study we have explored the Pd ↔ PdO transition at high temperature using in situ XRD, TGA and from TEM examination of Pd catalysts that were quenched in liquid nitrogen or in a heating TEM holder to prevent any changes in microstructure during cooling. Corresponding data was obtained during methane oxidation, helping shed light on the nature of the working catalyst. The results show that the oxidation of metallic Pd to PdO is kinetically-controlled at high temperatures, allowing Pd to co-exist along with PdO. We refer to these as metastable Pd ↔ PdO structures. TEM shows that Pd and PdO domains can co-exist within a single particle, forming a phase boundary but allowing both Pd and PdO to be exposed to the gas phase. This kinetically controlled oxidation of Pd explains why we do not see core–shell PdO–Pd structures at elevated temperatures.

The PtPdAg/C electrocatalyst with Pt-rich surfaces via electrochemical dealloying of Ag and Pd for ethanol oxidation

In this work, the PtAg2/C-D and PtPd3Ag5/C-D catalysts with Pt-rich surfaces were fabricated via chemical reduction following electrochemical dealloying. The as-prepared catalysts were characterized by X-ray diffraction (XRD), transmission electron spectroscopy (TEM), atomic adsorption spectroscopy (AAS), X-ray electron spectroscopy (XPS) and electrochemical methods It is found that external Ag atoms are dealloyed by the cyclic voltammetry (CV) to expose more Pt active sites on the surface, which enhances the electrocatalytic performance greatly. The addition of Pd as another sacrificing element triggers higher activity due to the synergistic effect between Pd and Pt. The mass activity of as-prepared PtAg2/C-D catalyst reaches 3.35 times as high as that of Pt/C. The PtPd3Ag5/C-D catalyst has the highest electrocatalytic activity of 4500mAmg−1, which is 6.22 and 1.87 times as high as those of mono-component Pt/C and Pd/C, respectively. Their high electrocatalytic activity is attributed to electronic effects, oxygen-containing species from Pd oxides and higher utilization of Pt.

In situ XAFS Study of Palladium Electrodeposition at the Liquid/Liquid Interface

We report the use of XAFS (X-ray absorption fine structure) as an in situ method to follow the electrochemically driven deposition of palladium nanoparticles at a liquid/liquid interface. A novel glass/plastic hybrid electrochemical cell was used to enable control of the potential applied to the liquid/liquid interface. In situ measurements indicate that the nucleation of metallic nanoparticles can be triggered through chronoamperometry or cyclic voltammetry. In contrast to spontaneous nucleation at the liquid/liquid interface, whereby fluctuations in Pd oxidation state and concentration are observed, under a fixed interfacial potential the growth process occurs at a steady rate leading to a build-up of palladium at the interface. Raman spectroscopy of the deposit suggests that the organic electrolyte binds directly to the surface of the deposited nanoparticles. It was found that the introduction of citric acid results in the formation of spherical nanoparticles at the interface.

Size-Controlled Pd Nanoparticle Catalysts Prepared by Galvanic Displacement into a Porous Si-Iron Oxide Nanoparticle Host.

Porous silicon nanoparticles containing both Pd and iron oxide nanoparticles are prepared and studied as magnetically recoverable catalysts for organic reductions. The Pd nanoparticles are generated in situ by electroless deposition of Pd(NH3)42+, where the porous Si skeleton acts as both a template and as a reducing agent and the released ammonia ligands raise the local pH to exert control over the size of the Pd nanoparticles. The nanocomposites are characterized by transmission electron microscopy, energy-dispersive X-ray spectroscopy, nitrogen adsorption, X-ray diffraction, superconducting quantum interference device magnetization, and dynamic light scattering. The nanocomposite consists of a porous Si nanoparticle (150 nm mean diameter) containing ∼20 nm pores, uniformly decorated with a high loading of surfactant-free Pd nanoparticles (12 nm mean diameter) and superparamagnetic γ-Fe2O3 nanoparticles (∼7 nm mean diameter). The reduction of 4-nitrophenol to 4-aminophenol by sodium borohydride is catalyzed by the nanocomposite, which is stable through the course of the reaction. Catalytic reduction of the organic dyes methylene blue and rhodamine B is also demonstrated. The conversion efficiency and catalytic activity are found to be superior to a commercial Pd/C catalyst compared under comparable reaction conditions. The composite catalyst can be recovered from the reaction mixture by applying an external magnetic field due to the existence of the superparamagnetic iron oxide nanoparticles in the construct. The recovered particles retain their catalytic activity.

Effect of high-temperature on the swellable organically-modified silica (SOMS) and its application to gas-phase hydrodechlorination of trichloroethylene

Pd catalysts supported on swellable organically-modified silica (SOMS) and high-temperature-treated swellable organically-modified silica (H-SOMS) were characterized and tested for gas-phase hydrodechlorination (HDC) of trichloroethylene (TCE) conditions. The high-temperature treatment on SOMS resulted in an increase in surface area and pore diameter as well as significant improvement of Pd dispersion on H-SOMS with smaller Pd particle sizes compared to the Pd/SOMS catalyst. Although the high-temperature treatment led to some alteration of the SOMS polysiloxane network, the hydrophobicity and organic vapor adsorption characteristics of SOMS were preserved. The reduction and oxidation characteristics of Pd on SOMS and HSOMS were investigated in situ using XANES technique. It was found that the Pd sites in the pores of SOMS was accessible to small molecules such as H2, facilitating the reduction of PdOx, whereas oxidation of metallic Pd was limited even at higher temperatures when O2 was used. This effect was only observed over Pd/SOMS catalyst. For Pd/H-SOMS, because the pores were more widely open than Pd/SOMS, both reduction and oxidation of Pd were observed. Finally, the catalytic activity of Pd/H-SOMS for gas-phase HDC of TCE was significantly better than Pd/SOMS. When water was added to the reactant stream (TCE+H2O), both Pd/SOMS and Pd/H-SOMS maintained its catalytic performances due to hydrophobic property of the supports.

Al-doped TiO2 mesoporous material supported Pd with enhanced catalytic activity for complete oxidation of ethanol

Pd catalysts supported on Al-doped TiO2 mesoporous materials were evaluated in complete oxidation of ethanol. The catalysts synthesized by wet impregnation based on evaporation-induced self-assembly were characterized by X-ray diffraction, measurement of pore structure, XPS, FT-IR, temperature programmed reduction and TEM. Characteristic results showed that the aluminium was doped into the lattice of mesoporous anatase TiO2 to form Al-O-Ti defect structure. Catalytic results revealed that Al-doped catalysts were much more active than the pristine one, especially at low temperature (≤200°C). This should be ascribed to the introduction of aluminium ions that suppressed the strong metal-support interaction and increased the active sites of Pd oxides, enhanced the stabilized anatase TiO2, improved well dispersed high valence palladium species with high reducibility and enriched chemisorption oxygen.

Microscopic Evidence for Strong Interaction between Pd and Graphene Oxide that Results in Metal-Decoration-Induced Reduction of Graphene Oxide.

Graphene oxide (GO) is reduced spontaneously when palladium nanoparticles are decorated on the surface. The oxygen functional groups at the GO surface near the nanoparticles are absorbed to the palladium to produce a palladium oxide interlayer. Palladium therefore grows on the GO with preferred orientations, resulting in unique microstructural and electrical properties.

Chemical and structural properties of Pd nanoparticle-decorated graphene—Electron spectroscopic methods and QUASES

Graphite (Gr) and carbon nanomaterials such as graphene oxide (GO) and reduced graphene oxide (RGO) and those decorated with Pd nanoparticles were investigated by photoelectron spectroscopy (XPS) aided with Quantitative Analysis of Surfaces by Electron Spectroscopy (QUASES) and reflected electron energy loss spectroscopy (REELS).Oxidation of Gr decreased the C/O ratio from 10 (Gr) to 2.2 (GO), whereas reduction of GO by N2H4 increased this ratio to 6.6 (RGO) due to decreasing number of oxygen groups (hydroxyl, epoxy, carbonyl and hydroxyl). Graphene materials and those after Pd decoration had 6–11 average number of layers in stacked nanostructures. Pd decoration using NaBH4-reducing agents formed nanoparticles of size 6.9nm (Pd/Gr)>5.3nm (Pd/RGO)>4.25nm (Pd/GO), with PdOx overlayer thickness of 2.20nm (Pd/GO)>1.42nm (Pd/Gr)>1.20nm (Pd/RGO), decreased number of oxygen groups and average number of layers. Smaller Pd nanoparticles of larger PdOx overlayer thickness were observed on highly hydrophilic substrates (functional oxygen groups content). Decoration accompanied by reduction using NaBH4 led to the removal of water attached by hydrogen bonding to graphene interplanes and the formation of PdOx overlayer from oxygen functional groups. Nanoparticle size obtained from QUASES was confirmed by Pd 3d5/2 spectra binding energy and full-width at half maximum. Various chemistry and mechanisms of graphene reduction using N2H4 and NaBH4 were observed, where NaBH4 was more efficient to remove water bonded by hydrogen bonding to oxygen groups and thus further graphene exfoliation. A substantial influence of substrate, functional group content on nanoparticle coverage, size and Pd oxide overlayer thickness was observed.

Mechanism of a Suzuki-Type Homocoupling Reaction Catalyzed by Palladium Nanocubes

The trans-2-phenylvinylboronic acid homocoupling reaction catalyzed by palladium nanocubes (Pd-NCs) was investigated by kinetics, spectroscopy, and poisoning experiments. The reaction was evidenced to be sensitive to the presence of the base, which acts synergistically with the substrate molecules and assists the leaching of Pd oxide (PdOx) species to the reaction medium. This species catalyzes the homocoupling reaction through the formation of Pd–Ox–B(OH)2R pretransmetalation intermediates, via coordination with the vinylboronic acid molecules, involving an oxo-palladium-type interaction. The reaction rate was not enhanced by the saturation of the reaction medium with O2, which is due to the oxidized nature of the Pd-NC surface.

Porous bimetallic PdNi catalyst with high electrocatalytic activity for ethanol electrooxidation

Porous bimetallic PdNi catalysts were fabricated by a novel method, namely, reduction of Pd and Ni oxides prepared via calcining the complex chelate of PdNi-dimethylglyoxime (PdNi-dmg). The morphology and composition of the as-prepared PdNi were investigated by scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD). Furthermore, the electrochemical properties of PdNi catalysts towards ethanol electrooxidation were also studied by electrochemical impedance spectrometry (EIS), cyclic voltammetry (CV) and chronoamperometry (CA) measurement. In comparison with porous Pd and commercial Pd/C catalysts, porous structural PdNi catalysts showed higher electrocatalytic activity and durability for ethanol electrooxidation, which may be ascribed to Pd and Ni property, large electroactive surface area and high electron transfer property. The Ni exist in the catalyst in the form of the nickel hydroxides (Ni(OH)2 and NiOOH) which have a high electron and proton conductivity enhances the catalytic activity of the catalysts. All results highlight the great potential application of the calcination-reduction method for synthesizing high active porous PdNi catalysts in direct ethanol fuel cells.

Topological Dirac semimetal phase in Pd and Pt oxides

Topological Dirac semimetals (DSMs) exhibit nodal points through which energy bands disperse linearly in three-dimensional (3D) momentum space, a 3D analogue of graphene. The first experimentally confirmed DSMs with a pair of Dirac points (DPs), Na$_{3}$Bi and Cd$_{3}$As$_{2}$, show topological surface Fermi arc states and exotic magneto-transport properties, boosting the interest in the search for stable and nontoxic DSM materials. Here, based on the {\it ab-initio} band structure calculations we predict a family of palladium and platinum oxides as robust 3D DSMs. The novel topological properties of these compounds are distinct from all other previously reported DSMs, they are rendered by the multiple number of DPs in the compounds. We show that along three unit axes of the cubic lattice there exist three pairs of DPs, which are linked by Fermi arcs on the surface. The Fermi arcs display a Lifshitz transition from a heart- to a diamond-shape upon varying the chemical potential. Corresponding oxides are already available as high-quality single crystals, which is an excellent precondition for the verification of our prediction by photoemission and magneto-transport experiments. Our findings not only predict a number of much robust 3D DSM candidates but also extend DSMs to transition metal oxides, a versatile family of materials, which opens the door to investigate the interplay between DSMs and electronic correlations that is not possible in the previously discovered DSM materials.

The synergy between atomically dispersed Pd and cerium oxide for enhanced catalytic properties

We report a photochemical synthesis of Pd/CeO2 catalysts with atomically dispersed Pd. Compared to atomically dispersed Pd/CeO2 with a cubic CeO2 support (Pd/CeO2-CP), atomically dispersed Pd/CeO2 with a truncated octahedral CeO2 support (Pd/CeO2-TOP) exhibited higher activity and selectivity, owing to the synergy between Pd atoms and the (111) surface of CeO2. When compared to Pd/CeO2 with Pd clusters and nanoparticles via chemical reduction, Pd/CeO2-TOP showed excellent activity with an enhancement factor of 324 in CO oxidation, as well as an activity enhancement by a factor of 344 in selective oxidation of benzyl alcohol.

Unusual 1,2‐Aryl Migration and Depalladation of Alkylpalladium Intermediates Containing a syn‐β‐Hydrogen Atom

AbstractIn general, alkylpalladium intermediates containing a syn β‐hydrogen atom are expected to undergo β–hydride elimination preferentially. However, this study unveils a new mechanistic mode in a Pd(II)‐catalyzed transformation, which leads to a series of both C−N/C−C bond forming and C−C bond forming/cleavage events. It has been demonstrated that a Pd(II)‐catalyzed oxidative cyclization of stilbenes bearing a pendent nucleophile such as 2‐alkenylanilines and 2‐alkenylphenyl 1,3‐diketones operates through an uncommon Pd−C bond cleavage assisted by a neighboring aryl group and CuCl2–assisted transient Pd oxidation, rather than generally preferred β–hydride elimination in alkylpalladium intermediates containing a syn β‐hydrogen atom. As a result, 1,2‐aryl migration occurs to lead to the unexpected formation of a variety of 3‐substituted indoles and 4‐aryl‐1‐naphthols. In addition, selective migration of an aryl group could be achieved from β,β‐disubstituted alkene substrates through alkylpalladium intermediates deprived of syn β‐hydrogen atom.

Efficient electro-oxidation of formic acid at Pd-MnOx binary nanocatalyst: Optimization of deposition strategy

Formic acid (FA) electro-oxidation (FAO) was investigated at a binary catalyst composed of Pd (nano-Pd) and manganese oxide (nano-MnOx) nanostructures assembled onto a glassy carbon (GC) electrode. The deposition strategy of the catalyst components is properly adjusted in such a way that could eventually improve its electrocatalytic activity and stability toward FAO. The highest catalytic activity and stability are obtained at the Pd-MnOx/GC electrode (for which nano-Pd and nano-MnOx were simultaneously deposited onto the GC electrode). Such enhancement is thought to originate from facilitating the direct oxidation of FA and/or minimizing the adsorption of poisoning species at the electrode surface. Herein, several techniques including cyclic voltammetry, chronoamperometry, electrochemical impedance spectroscopy, field-emission scanning electron microscopy, energy dispersive X-ray spectroscopy, and X-ray diffraction are used to track the catalyst activity and to reveal its surface morphology and composition.

Current–voltage–temperature characteristics of PEDOT:PSS/ZnO thin film-based Schottky barrier diodes

In this work, we report the temperature dependence of the electrical parameters of PEDOT:PSS/ZnO Schottky barrier diodes (SBDs) grown on glass substrates. To understand the current conduction mechanism, the current–voltage–temperature characteristics of PEDOT:PSS/ZnO thin film SBDs were studied. The electrical parameters were extracted with both thermionic emission and Cheung models. The obtained Richardson constant and effective barrier height were 5 A cm−2 °K−2 and 0.74 eV, respectively. The diode ideality factor was 1.5 and the series resistance was 36 Ω. All these electrical parameters turned out to be temperature independent which was associated with the dominant transport mechanisms of thermionic emission. The Richardson constant slightly deviates from theoretical values due to the presence of interfacial defects created by the preparation and deposition of PEDOT:PSS and the ZnO film crystallinity. The conductive polymer PEDOT:PSS, as a Schottky contact to ZnO, arises as an alternative to the expensive noble metals: Pt, Pd, Ag and metal oxides: IrOx, PdOx, PtOx.

Improving Formate and Methanol Fuels: Catalytic Activity of Single Pd Coated Carbon Nanotubes.

The oxidations of formate and methanol on nitrogen-doped carbon nanotubes decorated with palladium nanoparticles were studied at both the single-nanotube and ensemble levels. Significant voltammetric differences were seen. Pd oxide formation as a competitive reaction with formate or methanol oxidation is significantly inhibited at high overpotentials under the high mass transport conditions associated with single-particle materials in comparison with that seen with ensembles, where slower diffusion prevails. Higher electro-oxidation efficiency for the organic fuels is achieved.