CoPd Alloy Research Articles

Balancing the Binding of Intermediates Enhances Alkaline Hydrogen Oxidation on D-Band Center Modulated Pd Sites.

Exploring efficient alkaline hydrogen oxidation reaction (HOR) electrocatalysts is of great concern for constructing anion exchange membrane fuel cells (AEMFCs). Herein, d-band center modulated PdCo alloys with ultralow Pd content anchored onto the defective carbon support (abbreviated as PdCo/NC hereafter) are proposed as highly efficient HOR catalyst. The as-prepared catalyst exhibits exceptional HOR performance compared to the Pt/C catalyst, achieving thermodynamically spontaneous and kinetically preferential reactions. Specifically, the resultant PdCo/NC demonstrates a marked enhancement in alkaline HOR performance, with the highest mass and specific activities of 1919.6 mA mgPd-1 and 1.9 mA cm-2, 51.1 and 4.2 times higher than those of benchmark of Pt/C, along with an excellent stability in a chronoamperometry test. In the analysis of in situ Raman spectra, it was discovered that tetrahedrally coordinated H-bonded water molecules were formed during the HOR process. This indicates that the promotion of interfacial water molecule formation and enhancement of HOR activities in PdCo/NC are facilitated by defect engineering and the turning of d-band center in PdCo alloy. The essential knowledge obtained in this study could open up a new direction for modifying the electronic structure of cost-effective HOR catalysts through electronic structure engineering.

Origin of the Magnetic-Optical Properties of Palladium Compounds, an Ab Intio Study

Abstract: The structural, magnetic, and magneto-optical properties of the PdM (M=Fe, Co, Ni) alloy are investigated using first-principles calculations with the exciting code. The study employs the full potential linearized augmented plane wave method (FP-LAPW) based on density functional theory (DFT). The exchange-correlation energy is described in the generalized gradient approximation developed by Perdew-Burke-Ernzerhof (PBE-GGA). The calculated lattice parameters are in good agreement with theoretical and experimental results. The magnetic properties reveal a ferromagnetic stability phase for all compounds. Additionally, the total (TDOS) and partial (PDOS) density of states, along with the band structure, indicate that PdM (M = Fe, Co, Ni) exhibits metallic behavior. The total and local magnetic moments are investigated. We conducted a study on the polar magneto-optical Kerr effect and calculated Kerr spectra. The microscopic origins of the peaks found in Kerr's angle are interpreted as interband transitions, primarily arising from the states (d to d) in the PdNi and (S to d) in the PdFe and PdCo alloys. Keywords: P-MOKE, DFT, GGA-PBE, Intermetallic.

Highly efficient dehydrogenation of dodecahydro-N-ethylcarbazole over Al2O3 supported PdCo bimetallic nanocatalysts prepared by galvanic replacement

NEC is one of the most promising liquid organic hydrogen storage materials. The highly efficient and low cost dehydrogenation catalysts are crucial for large-scale hydrogen storage. In this paper, Co/Al2O3 precursors were prepared by chemical reduction, and then a series of Pd1Con/Al2O3 catalysts for 12H-NEC dehydrogenation was prepared by galvanic replacement of Pd2+ using Co as a template and reducing agent. The prepared catalysts were characterized by TEM, FT-IR, XPS, H2-TPR and H2-TPD. The results demonstrated that Pd1Con NPs were uniformly dispersed over the surface of Al2O3 in the form of Co@Pd core-shell structure, CoPd alloy and PdNPs, with an average particle size of 2.78 nm. The electron transfer occurred between Co and Pd. For the dehydrogenation of H12-NEC, the produced Pd1Co5/Al2O3 catalyst exhibited excellent catalytic activity. The dehydrogenation efficiency was 100% at 180 °C for 3 h and 98.5% at 150 °C for 6 h, and TOF reached 1.414 gH2/gPd min−1, which was significantly higher than that of monometallic Pd/Al2O3 catalysts and that reported in the literature. The kinetic studies of the catalytic 12H-NEC dehydrogenation showed that 4H-NEC→NEC is the rate-limiting step with the highest apparent activation energy of 68.72 kJ/mol. The hydrogen release efficiency remained essentially unchanged after four cycles.

Co-Pd alloy nanoparticles supported on activated carbon for High-efficiency production of vanillin from 4-methylguaiacol

Producing vanillin from lignin model compounds in an elegant and efficient manner is a focus of chemical utilization in lignocellulosic biomass. The facile synthesis of a cobalt-palladium bimetallic alloy anchored on activated carbon (Co-Pd/C), which excels in the selective oxidation of the lignin model compound 4-methylguaiacol to vanillin, is reported here. The characterization results of Co-Pd/C indicate that the catalysts enhance the catalytic efficiency by forming a cobalt-palladium alloy to modulate the number of d-band cavities and influence the O2 adsorption. Meanwhile, the synergistic effect between the alloys causes the Co-Pd/C catalyst exhibits good activity and stability in cycling experiments. A series of controlled experiments offer insight into the mechanism of catalytic generation of vanillin from 4-methylguaiacol. This work opens a new opportunity for achieving the valuable aromatic products from lignin.

Manifolding surface sites of compositional CoPd alloys via pulsed laser for hydrazine oxidation-assisted energy-saving electrolyzer: Activity origin and mechanism discovery

Energy-driven overall water splitting (OWS) for renewable hydrogen generation is critical for carbon neutrality. However, the OWS electrolyzer efficiency is significantly affected by the kinetically sluggish anodic oxygen evolution reaction (OER). An efficient approach is proposed to promote energy-saving hydrogen production by using a composition-dependent multifunctional CoPd alloy via pulsed laser ablation in liquids, where the OER is replaced with the hydrazine oxidation reaction (HzOR). The CoPd alloy improves N2H4 chemisorption onto the surface via dative bonding between the lone pair electrons of N and the empty 5 s orbital of Pd and 3d of low spin Co. The optimal Co1Pd9 alloy sample demonstrates excellent hydrogen evolution reaction and HzOR with the lowest overpotentials of 0.224 and 0.329 V at 10 mA/cm2 in 1.0-M KOH and 1.0-M KOH/0.5-M N2H4, correspondingly. An in situ/operando Raman study reveals that metal active site and structural transformation occurred during the oxidation (M*-O) and reduction (M*-H) reactions under an alkaline medium. Besides, the overall hydrazine splitting performed using the Co1Pd9 ‖ Co1Pd9 electrolyzer requires a cell voltage of ∼ 0.228 V only at 10 mA/cm2 with remarkable electrochemical and structural stability over 10 h, which is significantly lower than that of the traditional OWS electrolyzer (∼1.837 V). This study demonstrates the practical use of CoPd alloys in direct N2H4 fuel cells to produce both H2 fuel and electrical energy.

Effect of Fractal Topology on the Resistivity Response of Thin Film Sensors.

We discuss the effect of topological inhomogeneity of very thin metallic conductometric sensors on their response to external stimuli, such as pressure, intercalation, or gas absorption, that modify the material's bulk conductivity. The classical percolation model was extended to the case in which several independent scattering mechanisms contribute to resistivity. The magnitude of each scattering term was predicted to grow with the total resistivity and diverge at the percolation threshold. We tested the model experimentally using thin films of hydrogenated palladium and CoPd alloys where absorbed hydrogen atoms occupying the interstitial lattice sites enhance the electron scattering. The hydrogen scattering resistivity was found to grow linearly with the total resistivity in the fractal topology range in agreement with the model. Enhancement of the absolute magnitude of the resistivity response in the fractal range thin film sensors can be particularly useful when the respective bulk material response is too small for reliable detection.

Two-channel anomalous Hall effect originating from the intermixing in Mn2CoAl/Pd thin films

The anomalous Hall effect (AHE) is an electronic transport phenomenon with rich physics. In thin magnetic films and multilayers, an unusual peak observed in AHE is often identified as the topological Hall effect (THE), induced by topologically nontrivial spin textures. However, it is a challenge to determine whether this unusual peak truly originates from such spin textures or is an artifact due to inhomogeneities. In this work, we systematically study the Pd thickness and temperature dependence of the Hall effect in ${\mathrm{Mn}}_{2}\mathrm{CoAl}$/Pd thin films. We observe an unusual peak in the Pd thickness dependence and rule out that the origin of the peak is from the topologically nontrivial spin textures by directly imaging the magnetic domain structures. We also build a model for the AHE based on the sum of contributions from ${\mathrm{Mn}}_{2}\mathrm{CoAl}$ and an intermixed CoPd alloy. These materials have opposite signs of the AHE and different coercive fields, which successfully explains the thickness and temperature behaviors of the unusual peak. Our results show that the interface mixing layer can play an important role in the interpretation of the AHE in ferromagnet/heavy metal systems.

Chirality-Induced Noncollinear Magnetization and Asymmetric Domain-Wall Propagation in Hydrogenated CoPd Thin Films.

Array-patterned CoPd-based heterostructures are created through e-beam lithography and plasma pretreatment that induces oxidation with depth gradient in the CoPd alloy films, breaking the central symmetry of the structure. Effects on the magnetic properties of the follow-up hydrogenation of the thin film are observed via magneto-optic Kerr effect microscopy. The system exhibits a strong vertical and lateral antiferromagnetic coupling in the perpendicular component between the areas with and without plasma pretreatment, and asymmetric domain-wall propagation in the plasma-pretreated areas during magnetization reversal. These phenomena exhibit evident magnetic chirality and can be interpreted with the Ruderman-Kittel-Kasuya-Yosida coupling and the Dzyaloshinskii-Moriya interaction (DMI). The sample processing demonstrated in this study allows easy incorporation of lithography techniques that can define areas with or without DMI to create intricate magnetic patterns on the sample, which provides an avenue toward more sophisticated control of canted spin textures in future spintronic devices.

Highly efficient hydrogenation and dehydrogenation of N-Heteroarenes catalyzed by mesoporous graphitic carbon nitride supported CoPd alloy nanoparticles

The present study reports the preparation of a multifunctional nanocatalyst, namely mesoporous graphitic carbon nitride (m-gCN) supported cobalt-palladium (CoPd) alloy nanoparticles (m-gCN/CoPd), and its catalysis for two distinct organic transformations (hydrogenation and dehydrogenation of quinolines and tetrahydroquinolines) in order to obtain biologically important N-heterocyclic building blocks. The m-gCN/CoPd nanocatalysts were prepared by following a two-step protocol comprising the synthesis of colloidal CoPd alloy nanoparticles (NPs) via surfactant-assisted solvothermal co-reduction of metal precursors and the deposition of as-prepared CoPd NPs on exfoliated m-gCN nanosheets in solution. After the characterization of m-gCN/CoPd nanocatalysts, they were first utilized as catalyst in the synthesis of a variety of 1,2,3,4-tetrahydroquinoline derivatives via an efficient and environmentally friendly transfer hydrogenation methodology using ammonia borane (H3NBH3) as a hydrogen source in water. Next, the oxidation of 1,2,3,4-tetrahydroquinoline analogues was realized by using tert-butyl hydroperoxide (TBHP) as a green oxidant and m-gCN/CoPd as catalysts. Additionally, m-gCN/CoPd nanocatalysts are reusable in both reactions up to the five consecutive runs. Besides being novel, the presented catalytic hydrogenation and dehydrogenation protocols provides distinct advantageous such as using green reagents (H3NBH3 and TBHP) and water as the solvent for the synthesis of quinoline derivatives in the context of sustainable chemistry.

In Situ Study of Nanoporosity Evolution during Dealloying AgAu and CoPd by Grazing-Incidence Small-Angle X-ray Scattering.

Electrochemical dealloying has become a standard technique to produce nanoporous network structures of various noble metals, exploiting the selective dissolution of one component from an alloy. While achieving nanoporosity during dealloying has been intensively studied for the prime example of nanoporous Au from a AgAu alloy, dealloying from other noble-metal alloys has been rarely investigated in the scientific literature. Here, we study the evolution of nanoporosity in the electrochemical dealloying process for both CoPd and AgAu alloys using a combination of in situ grazing-incidence small-angle X-ray scattering (GISAXS), kinetic Monte Carlo (KMC) simulations, and scanning transmission electron microscopy (STEM). When comparing dealloying kinetics, we find a more rapid progression of the dealloying front for CoPd and also a considerably slower coarsening of the nanoporous structure for Pd in relation to Au. We argue that our findings are natural consequences of the effectively higher dealloying potential and the higher interatomic binding energy for the CoPd alloy. Our results corroborate the understanding of electrochemical dealloying on the basis of two rate equations for dissolution and surface diffusion and suggest the general applicability of this dealloying mechanism to binary alloys. The present study contributes to the future tailoring of structural size in nanoporous metals for improved chemical surface activity.

Resistivity Testing of Palladium Dilution Limits in CoPd Alloys for Hydrogen Storage.

Palladium satisfies most of the requirements for an effective hydrogen storage material with two major drawbacks: it has a relatively low gravimetric hydrogen density and is prohibitively expensive for large scale applications. Pd-based alloys should be considered as possible alternatives to a pure Pd. The question is how much one can dilute the Pd concentration in a variety of candidate materials while preserving the hydrogen absorption capability. We demonstrate that the resistivity measurements of thin film alloy samples can be used for a qualitative high-throughput screening and study of the hydrogen absorbing properties over the entire range of palladium concentrations. Contrary to palladium-rich alloys where additional hydrogen scattering indicates a degree of hydrogen content, the diluted alloy films respond by a decrease in resistance due to their thickness expansion. Evidence of significant hydrogen absorption was found in thin CoPd films diluted to just 20% of Pd.

Catalytically active nano-porous cobalt-palladium alloys

The potential advantages of nano-alloys and particularly, nano-porous alloys, place them at the "spotlight" of heterogeneous catalysis. Nevertheless, controlling the precise compositions of these materials is still a synthetic challenge. Previous research introduced the fabrication of metals and alloys with a high nano-scale porosity and controllable compositions, via a hydrogen-mediated chemical reduction process of metal complex salts. We have used this procedure to obtain two magnetic nano-porous Co-Pd alloys, pure porous palladium and pure porous cobalt. Single crystal X-ray diffraction studies enabled structural determination of the two Co-Pd bi-complex salts that were used as precursors for these alloys. Powder X-ray diffraction studies determined the crystalline phases of the alloys and indicated the nanometric size of their crystallites. High-resolution scanning electron microscopy indicated that these alloys assemble as highly porous clusters of interconnected nano-crystallites. It also indicated that each alloy cluster preserves the micrometric morphologies of its salt precursor. Energy dispersive X-ray spectroscopy showed that the alloys exhibit uniform composition down to the micro-level, which preserved the Co/Pd ratio within the salts. Focused ion beam tomography enabled 3D structural representation of the alloys and metals. Geometrical analysis of the 3D reconstructed data determined 90% porosity and a specific surface area of ~100 m2/g for the alloys. In addition, the alloys showed improved catalytic activity in the semi-hydrogenation of phenylacetylene, compared to the pure metals and commercial Pd/C. Moreover, their magnetic properties enabled facile recovery at the end of the reaction. The yield for styrene in this reaction was increased using "design of experiments" (DOE), a method for optimization of reaction conditions. Furthermore, our experiments implied that a highly porous structure significantly improves the selectivity of styrene in the reaction. These results demonstrated the advantage of fabricating nano-porous alloys with uniform compositions that may exhibit special properties and serve as new and efficient catalysts.

Pd-Co alloy supported on TiO2 with oxygen vacancies for efficient N2 and O2 electrocatalytic reduction

Small molecules are converted into high-value products through electrochemical methods, which are considered as potential substitutes for traditional industrial methods. To reduce the use of precious metal and modulate the local environment by oxygen vacancies, Pd-Co nanoalloys supported on TiO2 with oxygen vacancies (Vo) are prepared through a facile pyrolysis approach. The superior ORR performance is obtained comparing to the commercial Pt/C (20 wt%) catalysts in alkaline solution. Moreover, the catalyst has amazing catalytic activity for NRR. The synergistic effect of Pd-Co alloys and the Vo greatly improves the yield of NH3 (4.11 µg/(h·cm2)) and the Faradaic efficiency (2.83%). The performance of small molecular reduction is enhanced by the strong metal-support interactions (SMSI), which agree with the density functional theory (DFT) calculations results. It is anticipated that the established nanoalloy electrocatalysts with modulation of metal–support interactions provide guidance for designing heterogeneous electrocatalysts with high efficiency.

Detailed investigation of structural and magnetic properties of multiphase binary Pd-Co alloys prepared by modified polyol process

Pd-Co based magnetic alloy nanoparticles were prepared by the modified polyol process and stabilized by polyvinylpyrrolidone (PVP) and 3-aminopropyl-trimethoxysilane (APES) capping agents and further reduction of metal salts with Sodium borohydride (NaBH4) at high temperature to form desired NPs. We have a detailed investigation of the effect of Pd concentration in the Pd-Co alloy NPs on structural and magnetic properties. XRD and Rietveld refinement analyses were confirmed that the multiphase structures of fcc-PdCo, fcc-Co, and hcp-Co phases coexist at low Pd loading samples. Over %50 of Pd loading resulted in a single fcc-PdCo phase with reduced lattice parameter to 4.0079 Å and d(111)-space to 2.31 Å. TEM and SEM images reveal well dispersed and uniformly distributed NPs with an average particle sizes of below 7 nm. The elemental compositions and the characteristic OH, CH, CO stretching peaks of capping agents were confirmed by EDS and FT-IR spectrums, respectively. M(T) and M(H) curves revealed that there are multi magnetic phase transitions in the Pd-Co structure as a function of Pd loading from superparamagnetic to ferromagnetic phase or back to superparamagnetic phase by reducing the temperature from 300 K to 5 K. We observed that the blocking temperature (TB) could not be detected due to 5% Pd loading below 300 K, while it reduced up to 55 K at high Pd loading of 62%. The coercive field (Hc) was increased to ~1900 Oe for Pd0.62Co0.38 sample due to the smallest particle size as 5.26 nm. The highest amount of Co resulted in maximum saturation magnetization (Ms) up to 65.5 emu/g for 5% Pd concentration. We measured the Mr/Ms ratios were less than 0.5, which is due to the internal stress that results in the uniaxial magnetic anisotropy in the structure. The maximum Keff and μf.u. values were found to be over 12.9 × 106 erg/cm3 and 0.72 μB at 5 K for 5% Pd concentration, respectively.

HRTEM study of microstructure-coercivity relationship in perpendicular Co25Pd75 thin films

The microstructure of 20 nm thick polycrystalline Co25Pd75 alloy films exhibiting perpendicular magnetic anisotropy (PMA), studied on the atomic scale by analytical transmission electron microscopy (TEM) techniques, is presented. Thin CoPd alloy films are considered promising candidates for pinning MgO-based perpendicular magnetic tunnel junctions (pMTJs) for spin- transfer torque magnetic tunnel junctions (STT-MRAM). The CoPd alloy films studied were grown by DC magnetron sputtering on four different seed layers: MgO, Ta, and Ta/Ru/Ta trilayer and Ta/Pd bilayer. The effect of the seed layers on the Co25Pd75 structural quality, the size and texture of crystallites as well as their compositional homogeneity and structural coherency of the grain boundaries formed between adjacent crystallites, are investigated. The detailed structural analysis reveals that Co25Pd75 layers grown on the Ta/Pd bilayer and Ta/Ru/Ta trilayer seeds consist of larger crystalline columns, with sizes ranging from 25 nm to 15 nm respectively, compared to the other two samples. Correlation of the Co25Pd75 crystalline quality with coercivity is suggested.

Designing ultrafine PdCo alloys in mesoporous silica nanospheres with peroxidase-like activity and catalase-like activity.

Nanomaterial-based artificial enzyme mimetics have attracted increasing attention because of their robust stability, adjustable activity, and cost-effectiveness. In this study, we developed a simple and effective method for the synthesis of highly dispersed ultrafine PdCo alloys with peroxidase- and catalase-like activities. The aberration-corrected transmission electron microscopy analysis verified that the cyanogel precursor in the mesoporous silica nanospheres (MSNs) was converted to PdCo alloy in NH3 at a high temperature. The PdCo alloy was homogenously distributed in MSNs as ultrafine and monodispersed particles. By selectively removing the Co species from the binary alloy through an acid-leaching approach, the role of each component in the enzyme-like mimetics was systematically studied. Using glutathione (GSH) as the model analyte, the potential application of PdCo@MSNs in GSH detection from complex cell media was confirmed via colorimetric assay. The ultrafine alloy size, double mimetic activities, and abundant loading space of PdCo@MSNs make them promising not only in clinical diagnosis but also in overcoming hypoxia-induced photodynamic therapy resistance in tumor treatment.

Enhanced syngas production from CO2 photoreduction over CoPd alloy modified NiAl-LDH under visible light.

We reported the modification of a NiAl-layered double hydroxide (LDH) by loading well-dispersed CoPd alloys through a NaBH4 reduction method. The modified NiAl-LDH achieved 14.5-fold and 2.1-fold improvements of the H2 and CO evolution rates and an applicable ratio of H2/CO (nearly 1 : 1) under visible light (λ > 420 nm). This study revealed the potential of alloys to adjust the H2/CO ratio and enhance syngas production for LDHs for the first time.

Evolution of superparamagnetism in the electrochemical dealloying process

In situ superconducting quantum interference device magnetometry provides insights into the electrochemical dealloying mechanism of a CoPd alloy. Charge-dependent measurements of magnetic moment allow the separation of primary and secondary dealloying contributions. Coercivity evolution revealed the transition from collective ferromagnetism to superparamagnetism of small alloy clusters evolving in the dealloying process, which is interpreted as an “inverse” magnetic percolation problem. Temperature-dependent magnetization curves enable a qualitative comparison of magnetic cluster size distributions in the nanoporous Pd framework, which are found to be strongly influenced by dealloying potential. The study underlines the potential of electrochemical dealloying as a promising method for the preparation of tailor-made magnetic nanostructures.

Positive vs negative resistance response to hydrogenation in palladium and its alloys

Resistive solid state sensors are widely used in multiple applications, including molecular and gas detection. The absorption or intercalation of the target species varies the lattice parameters and an effective thickness of thin films, which is usually neglected in the analyses of their transport properties in general and the sensor response in particular. Here, we explore the case of palladium-based thin films absorbing hydrogen and demonstrate that the expansion of thickness is an important mechanism determining the magnitude and the very polarity of the resistance response to hydrogenation in high resistivity films. The model of the resistance response that takes into account the modifications of thickness was tested and confirmed in three Pd-based systems with variable resistivity: thin Pd films above and below the percolation threshold, thick Pd–SiO2 granular composite films with different contents of silica, and Pd-rich CoPd alloys where resistivity depends on the Co concentration. The superposition of the bulk resistivity increase due to hydride formation and the decrease in the film resistance due to the thickness expansion provides a consistent explanation of the hydrogenation response in both continuous and discontinuous films with different structures and compositions.

Electronic and Catalytic Effects of Single-Atom Pd Additives on the Hydrogen Sensing Properties of Co3O4 Nanoparticle Films.

Atomically dispersed Pd additives significantly enhanced the hydrogen sensing performance of a Co3O4 nanoparticle film, and their electronic along with catalytic roles were comprehensively investigated based on a series of systematic experiments. Aggregates of Co3O4 nanoparticles (approximately 3 nm in size) with homogeneously dispersed Pd additives at concentrations in the range of 1-20% (on a molar basis with respect to Co) were generated in the gas phase via reactive pulsed laser ablation of Co-Pd alloy targets in He/O2 mixtures. The form of the Pd could be modified from single atoms to oxide clusters (1-2 nm), and the effects of these additives on the hydrogen sensing properties of thick films prepared by direct deposition were examined. The highest hydrogen sensing performance was obtained at 5% Pd loading, where single Pd atoms were present at the maximum density. Further Pd loading resulted in the formation of Pd oxide clusters and degraded the sensitivity. X-ray photoelectron spectroscopy and Pd K-edge X-ray absorption spectroscopy showed that single Pd atoms in the Pd4+ state at Co3+ sites on the Co3O4 nanoparticle surfaces donated electrons to the Co3O4 valence band. The greater concentration of free electrons led to an increase in the concentration of ionosorbed oxygen under dry air. Consequently, more ionosorbed oxygen was available for reaction with hydrogen, enhancing sensitivity. In situ X-ray absorption spectroscopy data confirmed that approximately 10% of the single Pd atoms in the Pd4+ state were reduced to Pd2+ during exposure to 1000 ppm H2, implying that a Pd4+ ↔ Pd2+ catalytic redox cycle accelerates the water formation reaction during hydrogen sensing. The present results provide deeper insights and understanding of the effects of noble metal additives on gas sensing, while highlighting the unique role of single-atom additives.