Pd Atoms Research Articles

Adsorption and gas-sensing properties of Pdn-GaNNTs to C2H2 and H2 gases

Based on DFT calculations, GaNNTs and Pd doped GaNNTs were proposed as gas-sensing materials for detecting the characteristic dissolved gases: C 2 H 2 and H 2 in insulating oil. The gas-sensing responses of GaNNTs, and Pd n -GaNNTs (n = 1–2) to C 2 H 2 and H 2 were studied. It is found that Pd n -GaNNTs shows higher reactivity and sensitivity to C 2 H 2 and H 2 than pristine GaNNTs. The electrical conductivity of Pd n -GaNNTs decreased obviously after the adsorption of C 2 H 2 and H 2 , and the gas-sensing responsiveness of Pd n -GaNNTs to C 2 H 2 and H 2 was improved with the number of the doped Pd atoms. Both of pristine GaNNTs and Pd n -GaNNTs show higher response to C 2 H 2 than that to H 2 . This study provides a theoretical basis for the experimental development of a high-performance gas sensor for on-line monitoring the characteristic dissolved gases in transformer oil. • Pd n cluster modified GaNNTs (Pd n -GaNNTs, n = 1–2) is used to detect the dissolved gases of insulating oil: C 2 H 2 and H 2 . • The sensing characteristics of GaNNTs, and Pd n -GaNNTs (n = 1–2) to C 2 H 2 and H 2 modified GaNNTs were calculated by DFT. • Pd n -GaNNTs (n = 1–2) show good adsorption properties to C 2 H 2 and H 2 by physisorption and chemisorption.

Strong Metal-Support Interaction Boosts Activity, Selectivity, and Stability in Electrosynthesis of H2O2.

Noble metals have an irreplaceable role in catalyzing electrochemical reactions. However, large overpotential and poor long-term stability still prohibit their usage in many reactions (e.g., oxygen evolution/reduction). With regard to the low natural abundance, the improvement of their overall electrocatalytic performance (activity, selectivity, and stability) was urgently necessary. Herein, strong metal-support interaction (SMSI) was modulated through an unprecedented time-dependent mechanical milling method on Pd-loaded oxygenated TiC electrocatalysts. The encapsulation of Pd surfaces with reduced TiO2-x overlayers is precisely controlled by the mechanical milling time. This encapsulation induced a valence band restructuring and lowered the d-band center of surface Pd atoms. For hydrogen peroxide electrosynthesis through the two-electron oxygen reduction reaction (ORR), these electronic and geometric modifications resulted in optimal adsorption energies of reaction intermediates. Thus, SMSI phenomena not only enhanced electrocatalytic activity and selectivity but also created an encapsulating oxide overlayer that protected the Pd species, increasing its long-term stability. This SMSI induced by mechanical milling was also extended to other noble metal systems, showing great promise for the large-scale production of highly stable and tunable electrocatalysts.

Discretization and Encapsulation of Palladium inside the Cavity of Crown Ether within the Interlayer of Layered Double Hydroxide for Enhanced Activity: A Case Study with Hydrogenation Reaction

AbstractThe activity of the noble metal‐based heterogeneous catalysts is limited by weak metal–support interactions, aggregation, and low surface to volume (S/V) ratio. The activity can be augmented in many ways. Among them, the discretization of the active sites and redistribution of electron density around the metal atom is an important one. In this work, these two phenomena are studied concerning a model reaction, hydrogenation of p‐nitrophenol (p‐NP). Herein, 1,4,7,10,13‐pentaoxacyclopentadecane ether is introduced in the basal space of layered double hydroxide (LDH) to encapsulate noble Pd0 atom inside the cavity of the crown molecule strategically. The modified LDH (Pda‐ECC‐L0.10@in situ CoAl LDH) augments the properties, like, high S/V ratio and nonaggregation of active sites by forming nonagglomerative discrete catalytic (DNSC) sites within the cavity of crown ether in the basal space. The developed catalyst exhibits higher turnover frequency demonstrating the improved activity due to the formed DNSC sites and redistributes electron density around the Pd atoms by LDH layers and crown molecules. Thus, the present material synthesis route can be considered as a stand‐alone method for preparation of the supported sub‐nanometer noble metal catalyst with higher activity and can be exploited for reactions where noble metal catalyst are used.

O3 titration for synthesis of Pt-Pd core-shell structural catalyst and its catalytic performance on C-H bond activation

A novelty catalyst synthesis method is introduced in this investigation, which employs O3 pulse to induce a segregation of Pd-Pt species to synthesize Pt-Pd core shell structural catalyst. In Pt-Pd alloy, O3 releases highly active oxygen (O*) species (O3→O2 + O·), which combine with Pd species or Pt species selectively. This combination induces a relative migration between Pd and Pt atoms, forming Pt-core@PdO-shell structural catalyst, which meets the thermodynamic stability of Pt-Pd alloy. Core shell structural catalyst displays a better catalytic performance for C-H bond activation. Experimental results indicate core-shell structural catalyst produces more highly active oxygen species than alloy catalyst does. These highly active oxygen species activate C-H bond in alkanes, contributing to a higher catalytic reactivity. In addition, the interaction between Pt-core and PdO-shell lowers the binding energy of ionic Pd species, which facilitates the release of reactive oxygen species.

C–H Bond Activation Mechanism by a Pd(II)–(μ-O)–Au(0) Structure Unique to Heterogeneous Catalysts

We focused on identifying a catalytic active site structure at the atomic level and elucidating the mechanism at the elementary reaction level of liquid-phase organic reactions with a heterogeneous catalyst. In this study, we experimentally and computationally investigated efficient C–H bond activation for the selective aerobic α,β-dehydrogenation of saturated ketones by using a Pd–Au bimetallic nanoparticle catalyst supported on CeO2 (Pd/Au/CeO2) as a case study. Detailed characterization of the catalyst with various observation methods revealed that bimetallic nanoparticles formed on the CeO2 support with an average size of about 2.5 nm and comprised a Au nanoparticle core and PdO nanospecies dispersed on the core. The formation mechanism of the nanoparticles was clarified through using several CeO2-supported controlled catalysts. Activity tests and detailed characterizations demonstrated that the dehydrogenation activity increased with the coordination numbers of Pd–O species in the presence of Au(0) species. Such experimental evidence suggests that a Pd(II)–(μ-O)–Au(0) structure is the true active site for this reaction. Based on density functional theory calculations using a suitable Pd1O2Au12 cluster model with the Pd(II)–(μ-O)–Au(0) structure, we propose a C–H bond activation mechanism via concerted catalysis in which the Pd atom acts as a Lewis acid and the adjacent μ-oxo species acts as a Brønsted base simultaneously. The calculated results reproduced the experimental results for the selective formation of 2-cyclohexen-1-one from cyclohexanone without forming phenol, the regioselectivity of the reaction, the turnover-limiting step, and the activation energy.

A density functional theory study on the effect of acetylene on vinyl acetate synthesis from acetoxylation of ethylene over Pd-Au catalysts

In this work, the reaction network of acetylene on Pd-Au catalyst was constructed firstly, and the single adsorption configurations and adsorption energies and electronic properties of key species in the reaction network were calculated with Density Functional Theory (DFT). It was found that most of the species tend to be adsorbed on the sites around Pd atoms and acetylene does not occupy the active site of ethylene. Then the energy barrier and reaction energy of the related reaction of acetylene on the Pd-Au surface and the geometric configuration of the transition state were calculated through DFT. Acetylene is highly reactive and oxidation, hydrogenation and polymerization were easy to occur. Among all the reactions, the oxidation reaction had the most competitive advantage. CO2 was the most in the final product, followed by the intermediate product C2H3. CO2 and C2H3 have no adverse effect on the reaction system, so the trace of acetylene in the raw material ethylene had a very weak effect on the vinyl acetate reaction system.

PdPtRu nanocages with tunable compositions for boosting the methanol oxidation reaction.

PtRu/C is a well-known commercial electrocatalyst with promising performance for the methanol oxidation reaction (MOR). Further improving the MOR properties of PtRu-based electrocatalysts is highly desirable, especially through structure design. Here we report a facile approach for the synthesis of PdPtRu nanocages with different components through a seed-mediated approach followed by chemical etching. The Pd@PtRu nanocubes were first generated using Pd nanocubes as the seeds and some Pd atoms were subsequently etched away, leading to the nanocages. When evaluated as electrocatalysts for the MOR in acidic media, the PdPtRu nanocages exhibited substantially enhanced catalytic activity and stability relative to commercial Pt/C and PtRu/C. Specifically, PdPt2.5Ru2.4 achieved the highest specific (8.2 mA cm−2) and mass (0.75 mA mgPt−1) activities for the MOR, which are 2.2 and 4.2 times higher than those of commercial Pt/C. Such an enhancement can be attributed to the highly open structure of the nanocages, and the possible synergistic effect between the three components.

Palladium clusters, free and supported on surfaces, and their applications in hydrogen storage.

Palladium is a late transition metal element in the 4d row of the periodic table. Palladium nanoparticles show efficient catalytic activity and selectivity in a number of chemical reactions. In this paper, we review the structural and electronic properties of palladium nanoclusters, both isolated and deposited on the surface of different substrates. Careful experiments and extensive calculations have been performed for small Pd clusters which provide ample information on their properties. Work on large Pd clusters is less abundant and more difficult to perform and interpret. Cluster deposition is a method to modify material surfaces for different applications, and we report the known results for the deposition of Pd clusters on the surfaces of a number of interesting substrates: carbonaceous substrates like graphene and some layered novel materials related to graphene, metal oxide substrates, silicon and silicon-related substrates and metallic alloy substrates. Emphasis is placed on revealing how the structural, electronic and magnetic properties change when the clusters are deposited on the substrate surfaces. Some examples of chemical reactions catalyzed by supported Pd clusters and nanoparticles are reported. An issue discussed in detail is the influence of Pd on the storage of hydrogen in porous materials. Experimental work shows that the amount of stored hydrogen increases when the absorbing material is doped with Pd atoms, clusters and nanoparticles, and a spillover mechanism from the metal particle to the substrate is usually accepted as the explanation. To shed light on this issue, a critical analysis based on density functional simulations of the mechanisms of hydrogen spillover in perfect and defective graphene doped with palladium clusters is presented.

Effect of single atomic layer graphene film on the thermal stability and hydrogen permeation of Pd-coated Nb composite membrane

Pd coated Nb-base composite membranes are preferable in the fields of hydrogen permeation. However, the rapid reduction of hydrogen permeability caused by high-temperature interfacial diffusion of Pd and Nb atoms hinders their large-scale application. In this paper, a single atomic layer graphene film was used for improving the thermal stability of a hydrogen-permeable composite membrane comprising a Pd coating on the Nb substrate. First, the graphene film was transferred onto the surface of the “clean” niobium substrate. Then a thin palladium coating was deposited on it by magnetron sputtering to form the niobium/graphene/palladium (Nb/Gr/Pd) composite membrane. The interfacial stability was evaluated in the temperature range of 673–973 K under vacuum, and the hydrogen permeation behavior was studied by gas-driven permeation method at 573–823 K. The results show that the single atomic layer graphene film can effectively compress the interdiffusion of Pd coating and Nb substrate and achieve a good hydrogen permeability below 823 K. However, it would be broken due to the micro-deformation of Nb substrate, the high mobility of Pd atoms, and the grain growth at a higher temperature. Therefore, it is concluded that the single atomic layer graphene film is unsuitable as an intermediate hindering layer for Nb-based hydrogen-permeable membranes.

Pd speciation on black phosphorene in a CO and C2H4 atmosphere: a first-principles investigation.

Deposited transition metal clusters and nanoparticles are widely used as catalysts and have long been thought stable in reaction conditions. We investigated the electronic structure and stability of freestanding and black phosphorene supported small Pd clusters containing 1 to 6 atoms in a CO or C2H4 atmosphere by extensive first-principles based calculations. We showed that, driven by the thermodynamics, subnanometric Pd clusters and single Pd atom on phosphorene may evolve for a better balance among metal-metal, metal-support and metal-adsorbate interactions, etc., resulting in atomic dispersion of Pd in reaction conditions. The strong interfacial Pd-P interactions would deform preformed Pd clusters into atomic strips of various lengths with enhanced stability comparable to bulk Pd. The diffusion barriers of terminal Pd atoms in the zigzag direction on phosphorene are small (<0.3 eV) and vary within ∼0.1 eV with the length of these atomic strips. Further adsorption of CO or C2H4 alters the Pd-P and Pd-Pd interactions and forms thermodynamically stable surface Pd species with decreased diffusion barriers, suggesting that atomic dispersion of Pd can be achieved on phosphorene, especially in a CO or C2H4 atmosphere. The current work may help to understand the superior catalytic performance of supported subnanometric transition metal catalysts in reaction conditions and pave the way for fabrication of single atom catalysts with the desired performance.

A casting combined quenching strategy to prepare PdAg single atom alloys designed using the cluster expansion combined Monte Carlo method.

In this work, the surface structure of a PdAg alloy is investigated by cluster expansion (CE) combined Monte Carlo (MC) simulations. All systems with different component proportions show an obvious component segregation corresponding to the depth from the surface. A significant amount of Ag is observed on the first layer, and Pd is concentrated significantly on the second layer. The Pd distribution on the PdAg surfaces is closely related to the temperature and composition ascribed to the concentration and configurational entropy effects, which are explicitly treated in MC simulations. The vacancies mainly distribute separately. The simulation results show good agreement with the experimental evidence. Moreover, we demonstrated a general and highly effective casting combined quenching strategy for controlling the ensemble size and chemical composition of alloy surfaces which could successfully be applied to the large-scale production of SAA.

Growth of size-matched nanoalloys – a comparison of AuAg and PtPd

The gas-phase growth of AuAg and PtPd clusters up to sizes ~3 nm is simulated by Molecular Dynamics. Both systems are characterized by a very small size mismatch and by a tendency of the less cohesive element to segregate at the nanoparticle surface. The aim of this work is to figure out the differences in the behavior between these two bimetallic systems at the atomic level. For each system, three simulation types are performed, in which either one species or both species are deposited on preformed bimetallic seeds. Our results show that core@shell and intermixed chemical ordering arrangements can be obtained, in agreement with the available experimental data. In the case of core@shell arrangement, the purity of the surface layer is perfect for Ag-rich and Pd-rich nanoparticles, whereas in Au-rich and Pt-rich ones, some tendency to surface migration of minority atoms (Ag or Pd) is observed. This tendency is somewhat stronger for Ag than for Pd. The analysis of the internal arrangement of the nanoparticles indicates that in the growth process the mobility of Pd and Ag minority atoms is stronger than that of Au and Pt minority atoms.

Bimetallic single atom promoted α-MnO2 for enhanced catalytic oxidation of 5-hydroxymethylfurfural

Bimetallic single atoms (Pd and Ni) work in synergy to promote the catalytic performance of α-MnO2, enabling a near unity conversion of HMF to DFF under mild conditions.

A computational study of direct CO2 hydrogenation to methanol on Pd surfaces.

The reaction mechanism of direct CO2 hydrogenation to methanol is investigated in detail on Pd (111), (100) and (110) surfaces using density functional theory (DFT), supporting investigations into emergent Pd-based catalysts. Hydrogen adsorption and surface mobility are firstly considered, with high-coordination surface sites having the largest adsorption energy and being connected by diffusion channels with low energy barriers. Surface chemisorption of CO2, forming a partially charged CO2δ-, is weakly endothermic on a Pd (111) whilst slightly exothermic on Pd (100) and (110), with adsorption enthalpies of 0.09, -0.09 and -0.19 eV, respectively; the low stability of CO2δ- on the Pd (111) surface is attributed to negative charge accumulating on the surface Pd atoms that interact directly with the CO2δ- adsorbate. Detailed consideration for sequential hydrogenation of the CO2 shows that HCOOH hydrogenation to H2COOH would be the rate determining step in the conversion to methanol, for all surfaces, with activation barriers of 1.41, 1.51, and 0.84 eV on Pd (111), (100) and (110) facets, respectively. The Pd (110) surface exhibits overall lower activation energies than the most studied Pd (111) and (100) surfaces, and therefore should be considered in more detail in future Pd catalytic studies.

A novel PdC monolayer with fully dispersed Pd atoms and a rigid carbon backbone: an intrinsic versatile electrocatalyst for overall water splitting and the corresponding reverse reaction.

The electrocatalytic overall water splitting and the corresponding reverse reaction play a vital role in future renewable energy systems and, hence, are frontiers of catalysis research. In this work, we identify a heretofore unknown two-dimensional palladium carbide using the structure swarm intelligence algorithm. The proposed monolayer, named α-PdC, consists of fully dispersed Pd atoms and a rigid carbon backbone, exhibiting high mechanical, dynamical, and thermal stability with desirable electrical conductivity. Further calculations show that the proposed monolayer is an intrinsic multifunctional electrocatalyst. It possesses an excellent catalytic performance toward the hydrogen evolution reaction (HER), the oxygen evolution reaction (OER), and the oxygen reduction reaction (ORR) with low overpotentials. Specifically, the overpotential for the HER is only -0.01 V, and the significantly low activation energy barrier (0.16 eV) on α-PdC elucidates the fast kinetics. Moreover, α-PdC could also be highly active towards the OER and ORR with comparable overpotentials (0.38 and 0.27 V, respectively). This study identifies an intrinsic versatile electrocatalyst with potential applications in the fields of energy conversion and storage.

Ambient hydrogenation of carbon dioxide into liquid fuel by a heterogeneous synergetic dual single-atom catalyst

Ambient hydrogenation of CO2 into liquid fuels is a potential economical solution for reducing CO2 emissions. Here, we report a highly active dual single-Pd-atom catalyst for ambient hydrogenation of CO2 to formate using a step-by-step catalyst design strategy. The theoretically predicted catalyst is synthesized experimentally and verified to capture a significant amount of CO2 (5.05 mmol/g, 273 K), and it can efficiently convert CO2 to formate under ambient conditions with a turnover frequency (TOF) as high as 13.46 h–1. Two major factors contributing to this extraordinary catalytic activity include the pore enrichment effect of the microporous structures and the ternary synergetic effect among two neighboring Pd atoms and the rich nitrogen environment. Our work may aid the development of heterogeneous catalysts to produce other commonly used fuels from CO2 under ambient conditions.

Effects of 4d transition metal Pd doping on the magnetic and superconducting properties of 112-type iron pnictide EuFeAs2

The recently discovered 112-type EuFeAs2 compound that shows complex Eu-spin magnetism provides a new platform to study the interplay between superconductivity (SC) and magnetism in iron pnictide superconductors. In this paper, by substituting Fe with the 4d transition metal (TM) Pd, we have synthesized a series of EuFe1−x Pd x As2 (0 ⩽ x ⩽ 0.08) samples and studied the doping effect on SC and magnetism by means of electrical transport and magnetization measurements. The systematic evolution of the lattice parameters indicates that the Pd atoms have been successfully substituted into the Fe sites. With Pd doping, the Fe-related spin density wave transition at in the parent phase is rapidly suppressed, and SC simultaneously emerges, exhibiting a domelike shape with a maximum onset transition temperature of around 18.5 K at x = 0.04. On the other hand, the Eu-related antiferromagnetic order at is suppressed very slowly and persists up to x = 0.08, covering the whole SC dome region. Also, the reentrant spin-glass and spin-reorientation transitions below remain unchanged with Pd doping. All these results indicate that the Eu spins have little effect on SC in EuFe1−x Pd x As2. Finally, based on the resistivity and magnetization data, the T − x phase diagram of EuFe1−x Pd x As2 is constructed and compared with those of 3d TMs Co/Ni and La doped ones.

Pd/Fe2O3 with Electronic Coupling Single-Site Pd-Fe Pair Sites for Low-Temperature Semihydrogenation of Alkynes.

Dispersing single palladium atoms on a support is promising to minimize the usage of palladium and improve the selectivity for alkyne semihydrogenation, but its activity is often very low as a result of unfavorable H2 activation. Here, we load palladium onto α-Fe2O3(012) to construct highly active and stable single-site Pd-Fe pairs with luxuriant d-electron domination near the Fermi level driven by strong electronic coupling and prove that Pd-Fe pairs cooperatively adsorb H2 and dissociate an H─H bond, whereas solo Pd sites enable preferential desorption of C═C intermediate, thus achieving both high activity and high selectivity for alkyne hydrogenation. This catalyst exhibits state-of-the-art performance in purifying acetylene of ethylene stream, with 99.6% and 100% conversion and 96.7% and 94.7% selectivity at 353 and 393 K, respectively, and excellent stability with negligible activity decay after a 200 h test. This single-site pair inherits the advantage but overcomes the weakness of both Pd ensemble and single Pd atoms, enabling ultralow-Pd-loading catalysts for selective hydrogenation.

The effect of CO treatment on the surface structure of bimetallic Pd-Au/HOPG and Pd-In/HOPG nanoparticles: A comparative study

Bimetallic nanoparticles have been attracting more and more attention as catalysts in recent years since they often provide improved catalytic performance when compared to their monometallic analogues. Trying to design the optimum configuration of active sites in such catalytic systems, researchers resort to different strategies, one of which consists of a specific pretreatment of pre-synthesized nanoparticles in a particular gaseous environment prior to the catalytic reaction, resulting in adsorption-induced segregation. However, such a strategy does not always lead to any significant changes in the surface structure, so the identification of all factors responsible for the segregation process is of high fundamental and applied interest for the targeted design of bimetallic catalytically active nanoparticles. Here we show that for two types of bimetallic nanosystems, a stoichiometric intermetallic compound of Pd and In, and a substitutional solid solution of Pd and Au, the exposure to a CO atmosphere leads to completely different results, i.e., a noticeable surface segregation of Pd atoms being observed exclusively for the latter system. Driven by adsorption of CO molecules, the process of segregation is enhanced in the studied Pd-Au/HOPG catalyst (initial Pd/Au ratio of 1.48) when the temperature of the CO treatment is increased to 150 °C. In contrast, the Pd-In/HOPG nanoparticles (initial Pd/In ratio of 1.35) show no considerable change in the apparent surface atomic composition under a CO atmosphere, irrespective of temperature. Our results, alongside previous findings, demonstrate that CO adsorption-induced segregation as a tool for target-oriented modification of active sites in bimetallic nanoparticles for catalytic applications is appropriate only for substitutional solid solutions having a reasonable number of contacts between atoms of one type prone to segregation.

Atomically Precise Palladium Nanoclusters with 21 and 38 Pd Atoms Protected by Phenylethanethiol

Atomically Precise Palladium Nanoclusters with 21 and 38 Pd Atoms Protected by Phenylethanethiol