Transition Metal-Free Alkyne Hydrogenation Catalysis with BaGa2, a Hydrogen Absorbing Layered Zintl Phase

Membrane-Free Selective Semi-Hydrogenation of Alkynes Over an In Situ Formed Copper Nanoparticle Electrode

Selective semi-hydrogenation of alkynes is a significant reaction for preparing functionalized alkenes. Electrochemical semi-hydrogenation presents a sustainable alternative to the traditional thermal process. In this research, affordable copper acetylacetonate is employed as a catalyst precursor for the electrocatalytic hydrogenation of alkynes, using MeOH as the hydrogen source in an undivided cell. Good to excellent yields for both aromatic and aliphatic internal/terminal alkynes are obtained under constant current conditions. Notably, up to 99% Z selectivity is achieved for various internal alkynes. Mechanistic investigations revealed the formation of copper nanoparticles (NPs) at the cathode during electrolysis, acting as the catalyst for the selective semireduction of alkynes. The copper NPs deposited cathode demonstrated reusable for further hydrogenation.

Zintl Ions and Phases Promote the Catalytic Hydrophosphination of Alkynes, Alkenes, and Imines

Although Zintl ions and phases have been known for more than a century, their application as tools to build organic molecules is underdeveloped. Here, a range of Zintl ions and phases were surveyed in the hydrophosphination of alkynes, alkenes, and imines with diphenylphosphine to afford useful organophosphine products. Further investigations with diphenylphosphine in the absence of the unsaturated organic substrates revealed the formation of the diphenylphosphide anion, allowing for the conclusion that the role of the Zintl species is as an initiator in these transformations.

Zintl Phases: From Curiosities to Impactful Materials

The synthesis of new compounds and crystal structures remains an important research endeavor in pursuing technologically relevant materials. The Zintl concept is a guidepost for the design of new functional solid-state compounds. Zintl phases are named in recognition of Eduard Zintl, a German chemist who first studied a subgroup of intermetallics prepared with electropositive metals combined with main-group metalloids from groups 13-15 in the 1930s. Unlike intermetallic compounds, where metallic bonding is the norm, Zintl phases exhibit a combination of ionic and covalent bonding and are typically semiconductors. Zintl phases provide a palette for iso- and aliovalent substitutions that can each contribute uniquely to the properties. Zintl electron-counting rules can be employed to interrogate a structure type and develop a foundation of structure-property relationships. Employing substitutional chemistry allows for the rational design of new Zintl compounds with technological properties, such as magnetoelectronics, thermoelectricity, and other energy storage and conversion capabilities. Discovering new structure types and compositions through this approach is also possible. The background on the strength and innovation of the Zintl concept and a few highlights of Zintl phases with promising thermoelectric properties in the context of structural and electronic design will be provided.

Experiment and Theory in Concert To Unravel the Remarkable Electronic Properties of Na-Doped Eu11Zn4Sn2As12: A Layered Zintl Phase

Low-dimensional materials have unique optical, electronic, mechanical, and chemical properties that make them desirable for a wide range of applications. Nano-scaling materials to confine transport in at least one direction is a common method of designing materials with low-dimensional electronic structures. However, bulk materials give rise to low-dimensional electronic structures when bonding is highly anisotropic. Layered Zintl phases are excellent candidates for investigation due to their directional bonding, structural variety, and tunability. However, the complexity of the structure and composition of many layered Zintl phases poses a challenge for producing phase-pure bulk samples to characterize. Eu11Zn4Sn2As12 is a layered Zintl phase of significant complexity that is of interest for its magnetic, electronic, and thermoelectric properties. To prepare phase-pure Eu11-xNaxZn4Sn2As12, a binary EuAs phase was employed as a precursor, along with NaH. Experimental measurements reveal low thermal conductivity and a high Seebeck coefficient, while theoretical electronic structure calculations reveal a transition from a 3D to 2D electronic structure with increasing carrier concentration. Simulated thermoelectric properties also indicate anisotropic transport, and thermoelectric property measurements confirm the nonparabolicity of the relevant bands near the Fermi energy. Thermoelectric efficiency is known to improve as the dimensionality of the electronic structure is decreased, making this a promising material for further optimization and opening the door to further exploitation of layered Zintl phases with low-dimensional electronic structures for thermoelectric applications.

Catalysis of Alloys: Classification, Principles, and Design for a Variety of Materials and Reactions

Alloying has long been used as a promising methodology to improve the catalytic performance of metallic materials. In recent years, the field of alloy catalysis has made remarkable progress with the emergence of a variety of novel alloy materials and their functions. Therefore, a comprehensive disciplinary framework for catalytic chemistry of alloys that provides a cross-sectional understanding of the broad research field is in high demand. In this review, we provide a comprehensive classification of various alloy materials based on metallurgy, thermodynamics, and inorganic chemistry and summarize the roles of alloying in catalysis and its principles with a brief introduction of the historical background of this research field. Furthermore, we explain how each type of alloy can be used as a catalyst material and how to design a functional catalyst for the target reaction by introducing representative case studies. This review includes two approaches, namely, from materials and reactions, to provide a better understanding of the catalytic chemistry of alloys. Our review offers a perspective on this research field and can be used encyclopedically according to the readers' individual interests.

Alkyne Hydrogenation Catalysis across a Family of Ga/In Layered Zintl Phases

Transition-metal-free Zintl-Klemm phases have received little attention as heterogeneous catalysis. Here, we show that a large family of structurally and electronically similar layered Zintl-Klemm phases built from honeycomb layers of group 13 triel (Tr) or group 14 tetrel (Tt) networks separated by electropositive cations (A) and having a stoichiometry of ATr2 or ATrTt (A = Ca, Ba, Y, La, Eu; Tr = Ga, In; Tt = Si, Ge) exhibit varying degrees of activity for the hydrogenation of phenylacetylene to styrene and ethylbenzene at 51 bar H2 and 40-100 °C across a variety of solvents. The most active catalysts contain Ga with, formally, a half-filled pz orbital, and minimal bonding between neighboring Tr2 or TrTt layers. A 13-layer trigonal polytype of CaGaGe (13T-CaGaGe) was the most active, cyclable, and robust catalyst and under modest conditions (1 atm H2, 40 °C) had a surface specific activity (590 h-1) comparable to a commercial Lindlar's catalyst. Additionally, 13T-CaGaGe maintained 100% conversion of phenylacetylene to styrene at 51 bar H2, even after 5 months of air exposure. This work reveals the structural design elements that lead to particularly high catalytic activity in Zintl-Klemm phases, further establishing them as a promising materials platform for hydrogen-based heterogeneous catalysis.

A novel ternary bismuthide, NaMgBi: crystal and electronic structure and electrical properties

A new ternary sodium magnesium bismuthide, NaMgBi, has been synthesized from the constituent metals, and its crystal structure was determined by single-crystal X-ray diffraction. NaMgBi crystallizes in a tetragonal PbFCl-type structure corresponding to the space group P4/nmm, where Z = 2, a = 4.7123(4) and c = 7.8158(7) Å. The structure is composed of layers formed by edge-sharing Bi tetrahedra centered with Mg stacked in the c-axis direction, and these layers sandwich the Na atoms. First-principles computations based on density functional theory calculations have verified that the most stable atomic configuration is the one in which the Na and Mg atoms occupy the 2a and 2c sites, respectively. The electrical resistivity measured for a sintered polycrystalline sample of NaMgBi with a relative density of 70% was found to gradually decrease from 868 to 26.4 mΩ cm upon increasing the temperature from 297 to 506 K, and the Seebeck coefficient decreased from 273 to 180 μV K−1 upon increasing the temperature from 298 to 496 K. Electronic structure calculations have revealed that NaMgBi must be a semiconductor with a small band gap of ∼0.1 eV.

Lucky Number 13: A 13-Layer Polytype of the Alkyne Hydrogenation Catalyst CaGaGe

Polytypism, the ability of materials to form crystal structures with different stacking sequences, occasionally causes materials with the same stoichiometry and similar local structures to have profoundly different properties. Herein, we discover a metastable 13-layer trigonal (13T) polytype of CaGaGe, a layered intermetallic phase comprised of [GaGe]^2- honeycombs separated by Ca²⁺. 13T-CaGaGe is synthesized from arc-melting the elements, and its structure is elucidated via neutron powder diffraction. Air-stable 13T-CaGaGe has one misaligned [GaGe]^2- layer for every 13 and transforms into the more stable 4-layer hexagonal (4H) CaGaGe polytype after annealing at 500 °C. Transition-metal-free 13T-CaGaGe shows remarkable activity in the catalytic hydrogenation of phenylacetylene to styrene and ethylbenzene, much higher than the 4H polytype. This work identifies the first 13-layer polytype for any crystal structure and further establishes the influence of polytypism on catalysis.

Converting copper sulfide to copper with surface sulfur for electrocatalytic alkyne semi-hydrogenation with water

Electrocatalytic alkyne semi-hydrogenation to alkenes with water as the hydrogen source using a low-cost noble-metal-free catalyst is highly desirable but challenging because of their over-hydrogenation to undesired alkanes. Here, we propose that an ideal catalyst should have the appropriate binding energy with active atomic hydrogen (H*) from water electrolysis and a weaker adsorption with an alkene, thus promoting alkyne semi-hydrogenation and avoiding over-hydrogenation. So, surface sulfur-doped and -adsorbed low-coordinated copper nanowire sponges are designedly synthesized via in situ electroreduction of copper sulfide and enable electrocatalytic alkyne semi-hydrogenation with over 99% selectivity using water as the hydrogen source, outperforming a copper counterpart without surface sulfur. Sulfur anion-hydrated cation (S²⁻-K⁺(H₂O)_n) networks between the surface adsorbed S²⁻ and K⁺ in the KOH electrolyte boost the production of active H* from water electrolysis. And the trace doping of sulfur weakens the alkene adsorption, avoiding over-hydrogenation. Our catalyst also shows wide substrate scopes, up to 99% alkenes selectivity, good reducible groups compatibility, and easily synthesized deuterated alkenes, highlighting the promising potential of this method.

Highly selective electrocatalytic semi-hydrogenation of alkynes over a noble-metal-free catalyst is highly desirable. Here, authors synthesize sulfur-containing copper nanowire sponges for selective electrocatalytic alkyne semi-hydrogenation using water as the hydrogen source.

High catalytic performance of Al-Pd-(Ru, Fe) icosahedral approximants for acetylene semi-hydrogenation

Three cubic crystalline icosahedral approximants (C phase: Al_72.0Pd_16.4Fe_11.6, P₄₀ phase: Al_72.0Pd_16.4Ru_11.6, P₂₀ phase: Al_70.0Pd_22.3Ru_7.7) exhibit high ethylene selectivity of over 90% for hydrogenating acetylene at 150 °C. Moreover, the powdered P₂₀ also demonstrates a high catalytic performance under an industry-like ethylene feed containing 0.5% acetylene as an impurity. Overall, icosahedral approximants in the Al–Pd–(Ru, Fe) systems are promising as a novel class of alloy catalysts.

The Al–Pd–(Ru, Fe) icosahedral approximants exhibited high catalytic ethylene selectivity and stability for semi-hydrogenation of acetylene.

Synthesis and Characterization of NaCd0.92Sn1.08, Na(Cd0.28Sn0.72)2 and Na2CdSn5 with Three-Dimensional Cd-Sn Frameworks

The crystal structures of three new ternary compounds, NaCd0.92Sn1.08 (I), Na(Cd0.28Sn0.72)2 (II), and Na2CdSn5 (III) synthesized in a sodium-cadmium-tin system were determined by single-crystal X-ray analysis to be the following: (I) LiGeZn-type structure (hexagonal, a = 4.9326(1) Å, c = 10.8508(3) Å, space group P-6m2); (II) CaIn2-type structure (hexagonal, a = 4.8458(2) Å, c = 7.7569(3) Å, P63/mmc); and (III) isotype with tI-Na2ZnSn5 (tetragonal, a = 6.4248(1) Å, c = 22.7993(5) Å, I-42d). Each compound has a three-dimensional framework structure mainly composed of four-fold coordinated Cd and Sn atoms with Na atoms located in the framework space. Elucidation of the electrical properties of the polycrystalline samples indicated that compounds (I) and (II) are polar intermetallics with metallic conductivity, and compound (III) is a semiconducting Zintl compound. These properties were consistent with the electronic structures calculated using the ordered structure models of the compounds.

Bimetallic Pd/Sn-based Nanoparticles and their Catalytic Properties in the Semihydrogenation of Diphenylacetylene

Multimetallic nanoparticles often enhance the catalytic performance of their monometallic counterparts by increasing reaction rates, catalyst selectivity, and/or stability. A prerequisite for understanding structure- and composition-associated properties, however, is the careful design of multimetallic nanoparticles with various structures and compositions. Here, bimetallic Pd/Sn-based nanoparticles are prepared with a tunable composition and structure exploiting ionic liquids (ILs) as reaction medium (i. e., methyltrioctylammonium bis(trifluoromethylsulfonyl)imide). The nanoparticles are obtained in a one-pot synthetic procedure by reducing the metal salt precursors with triethylborohydride in the IL. The results show that the reaction parameters, in particular the nature and ratio of the Pd2+ /Sn2+ precursors as well as the reaction temperature, influence NP formation and composition. X-ray diffraction with Rietveld analysis and transmission electron microscopy are employed to determine NP size and phase composition. Under optimized reaction conditions Pd2 Sn or PdSn nanocrystals are formed as single-phase products after introducing an additional annealing step at 200 °C. Nanocrystals with intermetallic composition reveal enhanced catalytic properties in the semihydrogenation of diphenylacetylene which was used as a model reaction.

Regulating Pd/Al2O3 catalyst by g-C3N4 toward the enhanced selectivity of isoprene hydrogenation

The catalytic performance of Pd NPs in the selective hydrogenation of isoprene is modulated by g-C₃N₄ deposits on commercial alumina supports.

Intermetallic compounds in catalysis - a versatile class of materials meets interesting challenges

The large and vivid field of intermetallic compounds in catalysis is reviewed to identify necessities, strategies and new developments making use of the advantageous catalytic properties of intermetallic compounds. Since recent reviews summarizing contributions in heterogeneous catalysis as well as electrocatalysis are available, this contribution is not aiming at a comprehensive literature review. To introduce the field, first the interesting nature of intermetallic compounds is elaborated - including possibilities as well as requirements to address catalytic questions. Subsequently, this review focuses on exciting developments and example success stories of intermetallic compounds in catalysis. Since many of these are based on recent advances in synthesis, a short overview of synthesis and characterisation is included. Thus, this contribution aims to be an introduction to the newcomer as well as being helpful to the experienced researcher by summarising the different approaches. Selected examples from literature are chosen to illustrate the versatility of intermetallic compounds in heterogeneous catalysis where the emphasis is on developments since the last comprehensive review in the field.

Synthesis, structural, and electronic properties of Sr1−xCaxPdAs

.This work maps out the structural and electronic phase diagram of Sr_1-xCa_xPdAs, a unique family of layered intermetallic honeycomb phases in which the PdAs layers distort away from ideal hexagonal symmetry.