Pd Ga Research Articles

Ternary gallide Zr7Pd7–xGa3+x (0 ≤ x ≤ 1.8): Synthesis, crystal and electronic structures

The new ternary compound Zr7Pd7-xGa3+x (0 ≤ x ≤ 1.8) was synthesized by arc melting the elements under argon and subsequent annealing the ingots at 870 K for 720 h. The polycrystalline samples were characterized by powder X-ray diffraction (XRD) and the crystal structure of the compound was determined from single crystal X-ray diffraction data. Zr7Pd7Ga3 crystallizes with a ternary version of the Zr7Ni10 type of structure exhibiting statistical mixtures of palladium and gallium atoms on three crystallographically independent nickel sites (oS68, Cmce, a = 12.997(3), b = 9.623(2), c = 9.630(2) Ǻ, Z = 4, R1 = 0.033, wR2 = 0.067 for 719 unique reflections with Io> 2σ(Io) and 48 refined parameters). The crystal structure of Zr7Pd7Ga3 is represented by a 3D Pd–Ga framework built of sinusoidal layers of Pd and Ga stacking along the b axis sandwiching the Zr atoms. The homogeneity range of the compound Zr7Pd7–xGa3+x has been refined from powder XRD and EDX data: 0 ≤ x ≤ 1.8; variation of the lattice parameters is the following: a = 13.0174(8)–12.904(3), b = 9.6306(8)–9.559(8), c = 9.6348(8)–9.700(5) Å. Electronic structure calculations have been performed for an idealized model Zr7Pd10 as well as a model compound Zr7Pd6.5Ga3.5 revealing that the Ga-free Zr7Pd10 is significantly destabilized by strong Pd–Pd anti-bonding interactions. Substitution of Pd by Ga reduces those, stabilizing Zr7(Pd,Ga)10 which features strong heteroatomic bonding between Zr and the Pd/Ga substructure, putting it into the class of polar intermetallics.

Crystal and electronic structures of the new ternary gallide Zr12Pd40−xGa31+y (x = 0–1.5, y = 0–0.5)

Zr12Pd40Ga31 was prepared from the elements by arc melting under argon and subsequent annealing at 870 K for 720 h. Single-crystal X-ray diffraction reveals that Zr12Pd40Ga31 crystallizes in a new hexagonal structure type: Pearson symbol (PS) hP166, space group P6/mmm, a = 18.7670(6) Ǻ, c = 8.6634(6) Ǻ). The crystal structure consists of three types of atomic layers – two flat sheets at z = 0 (layer A) and z = 0.5 (C) and one corrugated at z = 0.25 and z = 0.75 (B), which stack in the sequence … ABCB … along the [001] direction. The structure shows close vicinity to a series of hexagonal structures with PS hP164–hP171. These compounds show peculiar structural variability expressed in the different atomic occupations of the Wyckoff positions along and around the 3-fold and 6-fold axes. Homogeneity range and lattice parameters of new ternary compound Zr12Pd40−xGa31+y (x = 0–1.5, y = 0–0.5) have been refined from EDX and powder XRD data. Electronic structure calculations and bonding analysis have been performed for an idealized model revealing domination of the Pd–Ga and Ga–Ga interactions.

Isotermal section at 870 K of Zr–Pd–Ga phase diagram in the range 45–75 ат. % Ga

Isotermal section at 870 K of Zr–Pd–Ga phase diagram in the range 45–75 ат. % Ga

Surface oxidation-induced restructuring of liquid Pd-Ga SCALMS model catalysts.

We have examined model systems for the recently reported Pd-Ga Supported Catalytically Active Liquid Metal Solutions (SCALMS) catalysts using near-ambient pressure X-ray photoelectron spectroscopy (NAP-XPS) under oxidizing conditions. Gallium is known to be highly prone to oxidation and in practical applications, handling of the catalyst material in air or the presence of traces of oxygen in the reactor are unavoidable. Therefore, we expect our results to be of high relevance for the application of Ga-based SCALMS catalysts. Pd-Ga alloy samples of 1.3 and 1.8 at% Pd content were exposed to molecular oxygen at different pressures between 3 × 10-7 and 1 mbar and a temperature of 550 K. We observe the formation of wetting Ga2O3 films upon exposure to molecular oxygen. The absolute thicknesses of the oxide films depend on oxygen pressure, with values ranging from ∼12 Å at 10-7 to 10-5 mbar to ∼50 Å at 1 mbar. The formed metal-oxide interface leads to a redistribution of Pd, which accumulates at the boundary between the wetting oxide film and the metal substrate as a response to the oxide film growth. A maximum Pd 3d intensity is observed at an oxide thickness of 5 Å. For thicker films, the Pd 3d signal and the Ga 3d signal ascribed to the metallic substrate decrease in parallel, which is attributed to the oxide layer growing on top of the liquid metal alloy. From this observation, we conclude that no significant amount of Pd is bound in the newly formed oxide film. Density-functional theory (DFT) calculations support the experimental observations.

Nanometallurgy in solution: organometallic synthesis of intermetallic Pd-Ga colloids and their activity in semi-hydrogenation catalysis.

Nanoparticles (NPs) of Pd1--xGax (x = 0.67, 0.5, 0.33), stabilized in non-aqueous colloidal solution, were obtained via an organometallic approach under mild conditions using [Pd2(dvds)3] and GaCp* as all-hydrocarbon ligated metal-precursor compounds (dvds = 1,1,3,3-tetramethyl-1,3-divinyl-disiloxane; Cp* = η5-C5Me5; Me = CH3). The reaction of the two precursors involves the formation of a library of molecular clusters [PdnGamCp*y(dvds)z], as shown by liquid injection field desorption ionization mass spectrometry (LIFDI-MS). Full characterization of the catalytic system (HR-TEM, EDX, DLS, PXRD, XPS, NMR, IR, Raman) confirmed the formation of ultra-small, spherical NPs with narrow size distributions ranging from 1.2 ± 0.2 nm to 2.1 ± 0.4 nm (depending on the Pd : Ga ratio). The catalytic performance of the Pd1--xGax NPs in the semi-hydrogenation of terminal and internal alkynes and the influence of the gallium content on product selectivity were investigated. The highest activities (65%) and selectivities (81%) are achieved using colloids with a "stoichiometric" Pd/Ga ratio of 1 : 1 at 0 °C and 2.0 bar H2 pressure. While lower Ga ratios lead to an increase in activity, higher Ga contents increase the olefin selectivity but are detrimental to the activity.

Liquid Metals in Catalysis for Energy Applications

Summary To keep up with the fast-paced transitioning of the global energy sector, which is constantly thriving to enable reliable, economic, and sustainable energy production, catalysis research has been required to continuously evolve in response. The challenges in the existing systems are predominantly due to dependencies on heterogeneous solid catalysts that are susceptible to coking. In this respect, liquid-metal (LM) catalysts have been demonstrated to have a critical advantage over conventional catalysts. Recently, LMs acquired a place in catalysis, with a reputation often synonymous with interesting properties and a remarkable ability to break trade-offs between homogeneous and heterogeneous catalysis. This review bridges the fundamental principles of LM research and the recent advances in LM-based thermal and electrochemical catalysis for energy applications. Moreover, emerging approaches for the improved utilization of LMs are outlined, and the concepts requiring greater research attention that could enable the development of exciting energy solutions are highlighted.

Novel sensor to investigate microstructural contributions to corrosion of high‐palladium dental alloys

AbstractThere is incomplete understanding of corrosion mechanisms for high‐palladium dental alloys used for metal‐ceramic restorations. This investigation employed two complementary techniques as a novel sensor for an initial investigation of corrosion tendencies for phases in representative high‐Pd alloys with Pd–Cu–Ga and Pd–Ga compositions. After the initial oxidation step and simulated porcelain‐firing cycles, specimens were polished for examination by Scanning Kelvin Probe Force Microscopy and Atomic Force Microscopy to evaluate the Volta potential difference across the surfaces and correlate potential differences with microstructural phases. The Ru‐rich phase, present in high‐Pd alloys from use of Ru as a grain‐refining element, was clearly more noble than the Pd solid solution matrix in the Pd–Cu–Ga alloys but had much less Volta potential difference than the matrix in the Pd–Ga alloy. Other precipitates in the alloy microstructures had minimal observable differences in Volta potential compared to the matrix. While high‐Pd alloys have been found previously to show excellent overall in vitro corrosion resistance, the Ru‐rich phase may cause localized galvanic corrosion in Pd–Cu–Ga alloys leading to potentially problematic Pd ion release, but have less effect on corrosion in Pd–Ga alloys.

The direct synthesis of hydrogen peroxide from H2 and O2 using Pd–Ga and Pd–In catalysts

The direct synthesis of hydrogen peroxide is investigated using PdGa/TiO2 and PdIn/TiO2 catalysts prepared by an acid-washed sol-immobilisation procedure, which allows for enhanced catalytic selectivity.

The nature of the active sites of Pd–Ga catalysts in the hydrogenation of CO2 to methanol

The Pd/Ga ratio influences the phases formed during catalysis. The best catalyst necessitates the formation of Pd–Ga intermetallic compounds and also a low content of Ga2O3, whose excess tend to block surface sites.

The electrooxidation of formic acid catalyzed by Pd–Ga nanoalloys

Pd–Ga nanoalloys exhibit enhanced catalytic activity and stability towards the HCOOH oxidation reaction.

ZrNiAl-type gallides with pronounced metal-metal bonding, and the dimorphism of ScPdGa

Abstract The ZrNiAl-type (space group P6̅2m) gallides TIrGa (T=Zr, Nb, Hf, Ta), ZrNiGa and ScPdGa were obtained by arc-melting of the elements, followed by annealing in sealed silica tubes. The samples have been characterized through their Guinier powder patterns. The structures of ZrNiGa, NbIr1.08Ga0.92 and TaIr1.10Ga0.90 were refined from single-crystal X-ray diffractometer data. Refinements of the occupancy parameters indicate the formation of solid solutions for the niobium and the tantalum compound. Within the huge family of ZrNiAl-type phases, NbIr1.08Ga0.92 and TaIr1.10Ga0.90 have the smallest c/a ratios; however, no structural distortions or superstructure formation have been observed. Electronic structure calculations were exemplarily carried out for NbIrGa, substantiating the dominance of the Ir–Ga and Nb–Ir bonding interactions. Temperature dependent magnetic susceptibility measurements of LT-ScPdGa, ZrIrGa, NbIrGa and HfIrGa have shown Pauli paramagnetism. ScPdGa is dimorphic with an orthorhombic TiNiSi-type high-temperature modification (arc-melting and quenching; space group Pnma) which transforms to the hexagonal ZrNiAl-type low-temperature modification upon annealing at 1023 K. Both structures were refined from single-crystal X-ray diffractometer data. HT-ScPdGa exclusively shows Pd–Ga bonding within the three-dimensional [PdGa] polyanionic network, while additional weak Pd–Pd and Ga–Ga interactions occur in LT-ScPdGa.

Frontispiece: Spectroscopic Observation and Molecular Dynamics Simulation of Ga Surface Segregation in Liquid Pd–Ga Alloys

Frontispiece: Spectroscopic Observation and Molecular Dynamics Simulation of Ga Surface Segregation in Liquid Pd–Ga Alloys

Gallium-rich Pd-Ga phases as supported liquid metal catalysts.

A strategy to develop improved catalysts is to create systems that merge the advantages of heterogeneous and molecular catalysis. One such system involves supported liquid-phase catalysts, which feature a molecularly defined, catalytically active liquid film/droplet layer adsorbed on a porous solid support. In the past decade, this concept has also been extended to supported ionic liquid-phase catalysts. Here we develop this idea further and describe supported catalytically active liquid metal solutions (SCALMS). We report a liquid mixture of gallium and palladium deposited on porous glass that forms an active catalyst for alkane dehydrogenation that is resistant to coke formation and is thus highly stable. X-ray diffraction and X-ray photoelectron spectroscopy, supported by theoretical calculations, confirm the liquid state of the catalytic phase under the reaction conditions. Unlike traditional heterogeneous catalysts, the supported liquid metal reported here is highly dynamic and catalysis does not proceed at the surface of the metal nanoparticles, but presumably at homogeneously distributed metal atoms at the surface of a liquid metallic phase.

Observation of Systematic Variation in Yb Ion Valence as a Function of Interatomic Spacing in Icosahedral Approximant Crystals

We discovered two alloy systems, Pd–Ga–Yb and Pd–Ge–Yb, that belong to the Tsai-type 1/1 approximant to the Au–Al–Yb quasicrystal exhibiting the unusual quantum criticality and show a similar behav...

Synthesis and characterization of Sibunit-supported Pd–Ga, Pd–Zn, and Pd–Ag catalysts for liquid-phase acetylene hydrogenation

Pd/Sibunit and Pd–M/Sibunit (M = Ga, Zn, or Ag) catalysts have been synthesized, and their catalytic properties in liquid-phase acetylene hydrogenation have been investigated. Doping of the palladium catalyst with a metal M leads to the formation of the Pd2Ga, PdZn, or Pd0.46Ag0.54 bimetallic compound. The bimetallic particles are much smaller (1.6–2.0 nm) than the monometallic palladium particles (4.0 nm). Doping with zinc raises the ethylene selectivity by 25% without affecting the activity of the catalyst. Specific features of the effect of each of the dopants on palladium are reported.

The Electronic Structure and Bonding of Acetylene on PdGa(110)

We have studied the adsorption of acetylene on the PdGa(110) surface by Density Functional Theory calculation. Our results predict the hollow site is the most stable location for the adsorbate. In this site, both Pd and Ga atoms interact with acetylene. This molecule is bonded with the C–C bond almost parallel to the surface. A small tilt angle of 4.1° is computed. The C atoms present a rehybridization from sp → sp2. This rehybridization is also present in the bridge site but it is not present in the top configuration, where the C–C–H bond angle is about 160°. We computed the total density of state for the system and also the projection on Pd, Ga, C, and H atoms. These plots show a shift to lower energies on C and H projected states, which is an indication of stabilization after adsorption. At the same time, there is a reduction in the density for the Pd atom directly bonded to the C atom. The crystal orbital overlap population curves show an increase in the C–C overlap population (OP) after adsorption; while, the C2H2 rehybridizes to a near sp2 geometry. The acetylene withdraws charge from the surface indicating a donation–adsorption mechanism. The formation of Pd–C and Ga–C bonds and a decrease in OP for Pd–Pd and Pd–Ga bonds are detected. We also found, in the hollow site, a reduction in the C–C bond stretching vibrational frequency. This is an indication of the significant distortion in the adsorbed molecule. We have also found that the C–C bond breaking is unfavorable and that semihydrogenation is 2.27 eV more stable than C2H2 + H2 in the gas phase.

EXAFS study of Pd/Sibunit and Pd–Ga/Sibunit catalysts for liquid-phase hydrogenation of acetylene to ethylene

A series of catalysts Pd/Sibunit and Pd–Ga/Sibunit were prepared by impregnation with joint solutions of palladium and gallium nitrates in ethanol. The state of the active component and catalytic properties of the samples were studied in the liquid-phase hydrogenation of acetylene. The EXAFS method showed that the active component in the gallium-modified catalysts exists in the form of a non-stoichiometric Pd–Ga solid solution. The activity of the catalysts was found to be determined by the ratio Pd:Ga. It was shown that at the molar ratios Ga:Pd>0.5:1, the catalytic activity is reduced by blocking the surface of the bimetallic particles by gallium oxide.

Interfacial characterization of nickel–yttria-stabilized zirconia cermet anode/interconnect joints with Ag–Pd–Ga active filler for use in solid-oxide fuel cells

Nickel–yttria-stabilized zirconia cermet anode (Ni–YSZ)/interconnect joints with silver–palladium–gallium (Ag–9Pd–9Ga) active fillers are prepared by vacuum brazing. The joint structure and microstructure are analyzed by X-ray diffraction (XRD) and scanning electron microscopy coupled with energy dispersive spectroscopy (SEM/EDS), and by using an electron probe microanalyzer (EPMA). SEM observations of the joint show no cracks near the interface, confirming the compatibility of the Ag–9Pd–9Ga filler with a different anode and interconnect. The XRD pattern of the joint specimens oxidized at 800 °C for 250 h shows Cr2O3 and (Mn,Cr)3O4 surface layers. EPMA analysis of the cell/Ag–9Pd–9Ga/alloys joint at the cross section shows Cr, O, Fe, Zr, Ni, Ag, Pd, Ga, Y, and Mn. Overall, the results indicate that the bonding between metal and cermet is well established and the interface is smooth. By correlating the XRD and EPMA analysis results, we also analyzed the possible stages during joint oxidation. The joint strength was evaluated at 25 and 800 °C under shear and tensile loading conditions, respectively, and the brazed Ag–9Pd–9Ga sealant compared favorably with the commercially available glass-ceramic GC-9 counterpart.

In situ spectroscopy of complex surface reactions on supported Pd-Zn, Pd-Ga, and Pd(Pt)-Cu nanoparticles.

It is well accepted that catalytically active surfaces frequently adapt to the reaction environment (gas composition, temperature) and that relevant "active phases" may only be created and observed during the ongoing reaction. Clearly, this requires the application of in situ spectroscopy to monitor catalysts at work. While changes in structure and composition may already occur for monometallic single crystal surfaces, such changes are typically more severe for oxide supported nanoparticles, in particular when they are composed of two metals. The metals may form ordered intermetallic compounds (e.g. PdZn on ZnO, Pd2Ga on Ga2O3) or disordered substitutional alloys (e.g. PdCu, PtCu on hydrotalcite). We discuss the formation and stability of bimetallic nanoparticles, focusing on the effect of atomic and electronic structure on catalytic selectivity for methanol steam reforming (MSR) and hydrodechlorination of trichloroethylene. Emphasis is placed on the in situ characterization of functioning catalysts, mainly by (polarization modulated) infrared spectroscopy, ambient pressure X-ray photoelectron spectroscopy, X-ray absorption near edge structure, and X-ray diffraction. In the present contribution, we pursue a two-fold, fundamental and applied, approach investigating technologically applied catalysts as well as model catalysts, which provides comprehensive and complementary information of the relevant surface processes at the atomic or molecular level. Comparison to results of theoretical simulations yields further insight. Several key aspects were identified that control the nanoparticle functionality: (i) alloying (IMC formation) leads to site isolation of specific (e.g. Pd) atoms but also yields very specific electronic structure due to the (e.g. Zn or Ga or Cu) neighboring atoms; (i) for intermetallic PdZn, the thickness of the surface alloy, and its resulting valence band structure and corrugation, turned out to be critical for MSR selectivity; (ii) the limited stability of phases, such as Pd2Ga under MSR conditions, also limits selectivity; (iii) favorably bimetallic catalysts act bifunctional, such as activating methanol AND water or decomposing trichlorothylene AND activating hydrogen; (iv) bifunctionality is achieved either by the two metals or by one metal and the metal-oxide interface; (v) intimate contact between the two interacting sites is required (that cannot be realized by two monometallic nanoparticles being just located close by). The current studies illustrate how rather simple bimetallic nanoparticles may exhibit intriguing diversity and flexibility, exceeding by far the properties of the individual metals. It is also demonstrated how complex reactions can be elucidated with the help of in situ spectroscopy, in particular when complementary methods with varying surface sensitivity are applied.

Ensemble Effect Evidenced by CO Adsorption on the 3-Fold PdGa Surfaces

The atomic structure and composition of a catalyst’s surface have a major influence on its performance regarding activity and selectivity. In this respect, intermetallic compounds are promising future catalyst materials, as their surfaces exhibit small and well-defined ensembles of active metal atoms. In this study, the active adsorption sites of the 3-fold-symmetric surfaces of the PdGa intermetallic compound were investigated in a combined experimental and computational approach using CO as a test molecule. The PdGa(111) and (−1–1–1) surfaces exhibit very similar electronic structures, but have Pd sites with very different, well-defined atomic coordination and separation. They thereby serve as prototypical model systems for studying ensemble effects on bimetallic catalytic surfaces. Scanning tunneling microscopy and Fourier transform infrared spectroscopy show that the CO adsorption on both surfaces is solely associated with the topmost Pd atoms and Ga acts only as an inactive spacer. The different local configurations of these Pd atoms dictate the CO adsorption sites as a function of coverage. The experimental results are corroborated by density functional theory and illustrate the site separation and ensemble effects for molecular adsorption on intermetallic single crystalline surfaces.