Pd−Ga Intermetallic Compounds as Highly Selective Semihydrogenation Catalysts

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.

The PdGa intermetallic compound is a highly selective and stable heterogeneous hydrogenation catalyst for the semi-hydrogenation of acetylene. We have studied single crystals of PdGa grown by the Czochralski technique. The 69Ga electric-field-gradient (EFG) tensor was determined by means of NMR spectroscopy, giving experimental confirmation of both the recently refined structural model of PdGa and the theoretically predicted Pd–Ga covalent bonding scheme. The hydrogenation experiment has detected no hydrogen uptake in the PdGa, thus preventing in situ hydride formation that leads to a reduction of the catalytic selectivity. We have also determined bulk physical properties (the magnetic susceptibility, the electrical resistivity, the thermoelectric power, the Hall coefficient, the thermal conductivity and the specific heat) of single-crystalline PdGa. The results show that PdGa is a diamagnet with metallic electrical resistivity and moderately high thermal conductivity. The thermoelectric power is negative with complicated temperature dependence, whereas the Hall coefficient is positive and temperature-dependent, indicating complexity of the Fermi surface. Partial fulfillment of the NMR Korringa relation reveals that the charge carriers are weakly correlated. Specific heat measurements show that the density of electronic states (DOS) at the Fermi energy of PdGa is reduced to 15% of the DOS of the elemental Pd metal.

In situ study of the formation and stability of supported Pd2Ga methanol steam reforming catalysts

GaPd2, orthorhombic, Pnma (no. 62), a = 5.4829(8) A, b = 4.0560(4) A, c = 7.7863(8) A, V = 173.2 A, Z = 4, Rgt(F) = 0.022, wRref(F) = 0.029, T = 295 K. Source of material Starting from powder of Pd (99.9 %, Chempur) and granules of Ga (99.9999%, Chempur) in a ratio of 2:1 and 1.5 mg/ml GaI3 as transport agent (99.999%, Chempur), needle-shaped single crystals of Pd2Ga were synthesized by an exothermal chemical transport reaction in a temperature gradient from 673 K (source) to 873 K (sink) [1]. Experimental details Lattice parameters of the title compound were determined by least-squares fitting of 24 reflections from powder X-ray diffraction data obtained from ground single crystals (Huber Image Plate Guinier camera G670, CuK41 radiation, , = 1.54056 A, LaB6 as internal standard, a = 4.15692 A). Due to the significant homogeneity range reported for Pd2+xGa1–x [2], the occupancy values together with the anisotropic displacement parameters were allowed to vary in separate series of the structure refinement while the overall scale factor was fixed. The resulting occupancies were equal to unity within one e.s.d. 0.998(4), 1.003(4) and 1.006(5) for Pd1, Pd2 and Ga sites, respectively, confirming the 2:1 composition of the investigated specimen. In the final refinement series full occupancies were assumed for all positions. Discussion The crystal structure of Pd2Ga adopts an atomic arrangement of theCo2Si type of structure [3]. The present investigation provides a more precise refinement of the atomic as well as the displacement parameters of the previously reportedmodel obtained on the basis of photographic single crystal data [4] and X-ray powder diffraction [2]. The title compound represents the stoichiometric (x = 0) composition of Pd2+xGa1–x and thus falls into the reported homogeneity range of –0.04 ( x ( 0.02 at 873 K [2,4]. All atoms in the crystal structure of Pd2Ga are situated on one of the two mirror planes perpendicular to [010]. A clear gap at !3.00 A separates the first coordination sphere of each atom in the structure of Pd2Ga. Each Pd1 atom has distorted tetrahedral coordination by four Ga atoms with distances varying between 2.54 A and 2.56 A. One additional Ga atom is quite far away at a distance of 2.96 A. The environment of Pd2 is different: nonplanar trigonal coordination by three Ga atomswith d(Pd—Ga) = 2.56A – 2.62A and two other Ga atoms at a considerably longer distance of 2.84 A. The closest Pd—Ga contacts are comparable with the sum of the single bond radii of Pd (1.28 A) and Ga (1.25 A) [5]. The coordination sphere of each palladium site is completed by eight Pd atomswith d(Pd—Pd) between 2.82A and 2.99A, thus increasing the coordination number of both positions to 13. These contacts are slightly longer than the interatomic distance of 2.75A in ccpPdmetal [6]. Gallium atoms are surrounded exclusively by ten palladium species. The shortest Ga···Ga distance of 3.43 A is significantly longer than the average interatomic distance of 2.70 A in the 4-modification of Ga [6]. The environment of gallium atoms is formed by seven Pd atoms in the range of 2.54 A – 2.62 A and three Pd atoms at distances 2.84 A (2×) and 2.96 A (1×). Z. Kristallogr. NCS 223 (2008) 7-8 / DOI 10.1524/ncrs.2008.0004 7 © by Oldenbourg Wissenschaftsverlag, Munchen

The crystal structures of two ternary M x(Ga, As) y phases (M ≡ Rh, Pd) with Rh 5Ge 3- and Cr 12P 7-type derivative structures

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.

Propane Dehydrogenation on Single-Site [PtZn4] Intermetallic Catalysts

A study has been made of the catalytic and acidic properties together with the surface composition of the zeolite HZSM-5 modified by gallium by hydrothermal treatment with a sodium gallate solution and also by impregnation with gallium nitrate solution. It has been established that the first method of introducing gallium produces a more selective catalyst for aromatization of n-butane. It has been shown that under the conditions of the catalytic reaction several types of active centers incorporating Ga are formed: Ga2O3 on the zeolite surface, gallium ions inside the zeolite canals, and gallium atoms at the surface of and in the lattice that are bonded to OH groups.

Intermetallic compounds can be novel catalysts due to unique atomic-ordered arrangements and electronic structures. However, their elemental set, composition, and crystal structure cannot be freely selected, which limits the tunability of catalytic properties. In this study, we demonstrated an improvement in catalytic properties by adding a third element to intermetallic catalysts using selective hydrogenation of propyne by CoGa with Cu addition as a model reaction and catalyst. The selectivity was drastically improved by 5 %Cu addition. The surface segregation of Cu was revealed and considered to result in an ensemble effect for the selectivity improvement. The addition of NiGa to Cu does not result in segregation. Since Cu-Ni is miscible, but Cu-Co is immiscible, the third element should likely be immiscible with one component of intermetallic catalysts to utilize a segregation effect for improving catalytic properties. • Adding a small amount of Cu significantly improved the selectivity of propene. • Cu atoms segregated to the surface of CoGa, leading to the site-isolation effect. • The Cu segregation was not observed in NiGa with Cu. • Besides the low surface energy, the immiscibility of Cu with Co was critical.

Rare-earth cobalt pnictides, RCo2Pn2 (Pn = P, As), belong to the ThCr2Si2 structure type, which is ubiquitous among intermetallic compounds. The structural and magnetic properties of simple ternary RCo2P2 phosphides, which combine partially delocalized (itinerant) 3d magnetic moments of cobalt and localized 4f magnetic moments of lanthanides, were investigated extensively in 1980-1990s, predominantly by the Jeitschko group. Those studies established that LaCo2P2 shows ferromagnetic (FM) ordering of Co moments, while the other members of the series, with R = Ce, Pr, Nd, or Sm, exhibit antiferromagnetic (AFM) ordering in both R and Co magnetic sublattices. This observation also correlated with the larger separation between the [Co2P2] layers in the crystal structure of LaCo2P2 as compared to the decreased interlayer distances in the other structures of the RCo2P2 series. Our work over the past decade has focused on unraveling the rich magnetic behavior that can be observed in these systems when internal chemical and external physical factors are used to perturb their crystal and electronic structures. We began our foray into these materials by demonstrating that the preservation of FM ordering of Co 3d moments in the mixed La1-xR'xCo2P2 phases also forces the R 4f moments to adopt FM arrangement, although antiparallel to the Co moments. As an example, in La0.75Pr0.25Co2P2 such mutual influence of the 3d and 4f moments leads to a cascade of magnetic phase transitions. All these changes were traced back to the modification of the crystal structure and, consequently, the electronic band structure of these materials. The substitution of smaller R3+ ions for the La3+ ions leads to structural compression along the tetragonal c axis, perpendicular to the [Co2P2] layers, and an increase in the Co-Co distances within the layer. This structural effect is translated into more localized Co magnetic moments, stronger magnetic exchange between Co sites, and higher ordering temperatures. A more dramatic change in properties is observed in EuCo2Pn2, which exhibit AFM ordering of the localized 4f moments of Eu2+ ions and only paramagnetic behavior in the Co sublattice. Under applied pressure, these compounds undergo structural collapse, which causes a dramatic decrease in the separation between the [Co2Pn2] layers, an increase in the oxidation state of Eu, and magnetic ordering of Co moments. We further demonstrated that similar effects can be stimulated by chemical compression, which is achieved by doping Eu into the more constrained lattice sites, for example, in PrCo2P2 or CaCo2As2. In both cases, the induced mixed valence of Eu results in the change from AFM to FM ordering in the Co sublattice. A series of solid solutions Ca1-xEuxCo2As2 shows a fascinating evolution of magnetic behavior from AFM ordering of Co 3d moments to simultaneous FM ordering of Co 3d and Eu 4f moments to AFM ordering of Eu 4f moments as one proceeds from CaCo2As2 to EuCo2As2. Importantly, all these changes in magnetic properties are well justified by the analysis of electronic density of states and crystal orbital Hamilton population, providing the understanding of how chemical factors can be leveraged, in general, to modify properties of itinerant magnets.

Learning What Makes Catalysts Good

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.

OLCAO based Electronic Structure Calculations and Optical Properties of Ni_3Ga Intermetallic Alloy System

ConspectusIntermetallic nanoparticles (iNPs) have been the subject of many recent reports for their demonstrated applications as highly active and selective heterogeneous catalysts. As a subclass of alloys, intermetallic compounds possess ordered crystal structures and, therefore, well-defined atomic environments, unlike the solid solution of alloys whose atomic arrangements are random and locally unpredictable. Catalytically active iNPs typically contain a group 8–10 transition metals as the "active" metals. They usually also include an "inactive" metal that does not directly participate in the catalytic reaction but can significantly modify the active metal's behavior. The choice of the inactive metal component can range across the periodic table.A few general challenges remain to design iNPs as heterogeneous catalysts with outstanding performance. Synthetically, the high surface energy of small nanoparticles is prone to their aggregation, while maximizing the surface-to-volume ratios is highly desired for efficient noble metal utilization. Additionally, even though the formation of bulk intermetallic compounds has been extensively studied, the formation of intermetallic phases at the nanoscale can behave differently. For example, the formation temperatures of iNPs are often drastically different from those predicted from the bulk phase diagrams. This behavior often leads to further challenges in the synthesis of iNPs.In addition to synthetic challenges, it is also critical to demonstrate the performance of iNPs in catalysis and establish the structure–property relationships. Instrumental and computational techniques often assist the understanding of catalytic properties. Due to the long-range order of intermetallic structure, various electron and X-ray techniques are often used to precisely determine the structure of iNPs. Structural modeling in density functional theory (DFT) calculation can also benefit from such ordered structures. These techniques have siginificantly improved the understanding of enhanced catalytic properties of iNPs in thermo-, electro-, and photocatalysis. Hydrogenation of furfural to furfuryl alcohol, for example, is a model reaction where PtSn iNPs show enhanced activity and chemoselectivity in hydrogenating C═O rather than C═C bonds. This superior catalytic performance can be correlated to the change in the geometric and electronic surface structure of the PtSn iNPs based on careful instrumental and computational characterizations. Additionally, intermetallic surfaces can be further modified by ligands or defects. While adding complexity to iNP systems, these modifiers provide additional control over their catalytic properties.In this Account, taking encapsulated iNPs in mesoporous silica as an example, we review the current strategies to develop iNPs as high-performance heterogeneous catalysts, with insights on the distinct formation behavior of iNPs compared to bulk intermetallic materials. We then highlight thermo- and electro-catalysis reactions to which these iNP catalysts are applied. We also discuss the unique pairwise hydrogenation reaction with parahydrogen catalyzed by iNPs. In this reaction, iNPs show unparalleled potential. We anticipate that this Account could foster additional interests in studying intermetallic catalysts and lay the foundation for their applications.

Ru3Sn7, an intermetallic compound with advanced catalytic properties, exhibits a complex crystal structure and intriguing electronic properties, making it an attractive candidate for investigations under high-pressure (HP). The structural, vibrational and electronic band structure of this compound were investigated at HP up to ∼20 GPa using synchrotron x-ray powder diffraction, micro-Raman, and density functional theory, respectively. Despite the local structural changes implied by a discernible reduction in the compressibility and distinct slope changes in the pressure evolution of the symmetric stretching vibrations of the Ru and Sn atoms around 8 GPa, the cubic structure is found to be stable throughout the pressure range. In support, our calculated phonon dispersion relation confirmed the stability of the cubic phase till the highest pressures. A comprehensive analysis of the Raman spectrum reveals the signatures of the pressure induced sudden strengthening of electron-phonon coupling (EPC) as early as 3 GPa which is backed by a bounce in the phonon and electron density of states (DOS). Electronic structure calculations demonstrate that the metallic nature of Ru3Sn7is preserved in the studied pressure range with a minor redistribution of electronic DOS across the Fermi level (EF). The band structure calculations predict intriguing changes in the electronic structure, revealing the pressure inducedd-phybridization through the high symmetry point of the Brillouin zone which is largely responsible for the observed reduction in the compressibility and enhancement of the EPC in Ru3Sn7.