Catalysis-tunable Heusler alloys in selective hydrogenation of alkynes: A new potential for old materials

Mechanistic Insights into the Direct Deoxygenation of Phenolic Compounds over Novel Heusler Alloy Catalysts

The catalytic deoxygenation of phenolic compounds is a crucial step in the valorization of biomass resources, which can effectively enhance the heating value and stability of primary biofuel. In this study, the catalytic mechanism of four Heusler alloy catalysts for the direct deoxidation pathway of phenol was studied through electronic structure regulation by element occupation. We found that Heusler alloys catalysts exhibit excellent catalytic activity in the dissociation activation of H2 and the cleavage of aryl hydroxyl bond (CAr-OH) bonds. The energy barriers for the direct cleavage of the CAr-OH bond in phenol on Ni2MoAl, Co2MoAl, Ni2NbAl and Ni2MoGa catalysts are 0.86, 0.95, 1.09, and 1.28 eV, respectively. And Y element of the X2YZ catalyst has a significant impact on this reaction, while the X element has a complex influence on the hydrogenation step of the unsaturated benzene ring. Microkinetic analysis further substantiates that the phenol (CAr-OH) bond cleavage step in the reaction exhibits a fast reaction rate and high extent of reaction. The reaction of hydroxyl hydrogenation to produce water exhibits the highest energy barrier, serving as the rate-determining step of the entire reaction. This issue could potentially be addressed by further fine-tuning the electronic structure.

Nanoporous copper titanium tin (np-Cu2TiSn) Heusler alloy prepared by dealloying-induced phase transformation for electrocatalytic nitrate reduction to ammonia

Heusler alloys are a series of well-established intermetallic compounds with abundant structure and elemental substitutions, which are considered as potentially valuable catalysts for integrating multiple reactions owing to the features of ordered atomic arrangement and optimized electronic structure. Herein, a nanoporous copper titanium tin (np-Cu2TiSn) Heusler alloy is successfully prepared by the (electro)chemical dealloying transformation method, which exhibits high nitrate (NO3-) reduction performance with an NH3 Faradaic efficiency of 77.14 %, an NH3 yield rate of 11.90 mg h-1 mg-1cat, and a stability for 100 h under neutral condition. Significantly, we also convert NO3- to high-purity ammonium phosphomolybdate with NH4+ collection efficiency of 83.8 %, which suggests a practical approach to convert wastewater nitrate into value-added ammonia products. Experiments and theoretical calculations reveal that the electronic structure of Cu sites is modulated by the ligand effect of surrounding Ti and Sn atoms, which can simultaneously enhance the activation of NO3-, facilitate the desorption of NH3, and reduce the energy barriers, thereby boosting the electrochemical nitrate reduction reaction.

From Charge to Spin: An In-Depth Exploration of Electron Transfer in Energy Electrocatalysis

Catalytic materials play crucial roles in various energy-related processes, ranging from large-scale chemical production to advancements in renewable energy technologies. Despite a century of dedicated research, major enduring challenges associated with enhancing catalyst efficiency and durability, particularly in green energy-related electrochemical reactions, remain. Focusing only on either the crystal structure or electronic structure of a catalyst is deemed insufficient to break the linear scaling relationship (LSR), which is the golden rule for the design of advanced catalysts. The discourse in this review intricately outlines the essence of heterogeneous catalysis reactions by highlighting the vital roles played by electron properties. The physical and electrochemical properties of electron charge and spin that govern catalysis efficiencies are analyzed. Emphasis is placed on the pronounced influence of external fields in perturbing the LSR, underscoring the vital role that electron spin plays in advancing high-performance catalyst design. The review culminates by proffering insights into the potential applications of spin catalysis, concluding with a discussion of extant challenges and inherent limitations.

Active site tuning based on pseudo-binary alloys for low-temperature acetylene semihydrogenation

The development of an efficient catalytic system for low-temperature acetylene semihydrogenation using nonnoble metals is important for the cost-effective production of polymer-grade pure ethylene. However, it remains challenging owing to the intrinsic low activity. Herein, we report a flexibly tunable catalyst design concept based on a pseudo-binary alloy, which enabled a remarkable enhancement in the catalytic activity, selectivity, and durability of a Ni-based material. A series of (Ni1-xCux)3Ga/TiO2 catalysts exhibiting L12-type pseudo-binary alloy structures with various Cu contents (x = 0.2, 0.25, 0.33, 0.5, 0.6, and 0.75) were prepared for active site tuning. The optimal catalyst, (Ni0.8Cu0.2)3Ga/TiO2, exhibited outstandingly high catalytic activity among reported 3d transition metal-based systems and excellent ethylene selectivity (96%) and long-term stability (100 h) with near full conversion even at 150 °C. A mechanistic study revealed that Ni2Cu hollow sites on the (111) surface weakened the strong adsorption of acetylene and vinyl adsorbate, which significantly accelerated the hydrogenation process and inhibited undesired ethane formation.

Co2 FeGe Heusler Alloy Nanoparticle Catalysts for Propyne Hydrogenation and Ammonia Decomposition

Heusler alloys (X2 YZ) can be a candidate for new catalysts as well as other intermetallic compounds. We previously found good catalytic properties of Co2 FeGe for selective hydrogenation of alkynes and developed nanoparticles of Co2 FeGe supported on SiO2 . However, the average diameter of the nanoparticles was 23 nm, which is not small enough compared to those of state-of-the-art nanoparticle catalysts. In this study, we developed SiO2 -supported Co2 FeGe nanoparticles of <10 nm in diameter. A catalytic test for selective hydrogenation of propyne indicated a partial formation of sites with low selectivity including excess Co atoms. For ammonia decomposition, enhancement of turnover frequency was achieved by reducing the particle size.

H2 -free Semi-hydrogenation of Butadiene by the Atomic Sieving Effect of Pd Membrane with Tree-like Pd Dendrites Array

H2 -free semi-hydrogenation at room temperature shows great advantage for replacing the thermocatalytic process in industry owing to the high energy and resource saving, however, remains great challenges. Herein, a tree-like Pd dendrites array decorated Pd membrane was constructed as the core device in an electrochemistry assisted gas-fed membrane reactor for butadiene semi-hydrogenation. It reveals that hydrogen atomic sieving effect of this Pd-based membrane under electrochemical condition was the key for semi-hydrogenation. The configuration study of Pd nanostructured membrane demonstrates that the penetration of hydrogen atoms through Pd membrane from electrochemical side to chemical side is affected by the consumption of hydrogen atom in semi-hydrogenation step. Such atomic sieving property of nanostructured Pd membrane with 5.1 times increase in catalytic active surface area brings above 14 times higher in butadiene conversion than that of bare Pd foil, with ≈90 % of butenes selectivity at butadiene conversion ≈98 % over 300 h of H2 -free reaction under 15 mA cm-2 .

Tuning hydrogenation chemistry of Pd-based heterogeneous catalysts by introducing homogeneous-like ligands

Noble metals have been extensively employed in a variety of hydrotreating catalyst systems for their featured functionality of hydrogen activation but may also bring side reactions such as undesired deep hydrogenation. It is crucial to develop a viable approach to selectively inhibit side reactions while preserving beneficial functionalities. Herein, we present modifying Pd with alkenyl-type ligands that forms homogeneous-like Pd-alkene metallacycle structure on the heterogeneous Pd catalyst to achieve the selective hydrogenolysis and hydrogenation. Particularly, a doped alkenyl-type carbon ligand on Pd-Fe catalyst is demonstrated to donate electrons to Pd, creating an electron-rich environment that elongates the distance and weakens the electronic interaction between Pd and unsaturated C of the reactants/products to control the hydrogenation chemistry. Moreover, high H₂ activation capability is maintained over Pd and the activated H is transferred to Fe to facilitate C-O bond cleavage or directly participate in the reaction on Pd. The modified Pd-Fe catalyst displays comparable C-O bond cleavage rate but much higher selectivity (>90%) than the bare Pd-Fe (<50%) in hydrotreating of diphenyl ether (DPE, modelling the strongest C-O linkage in lignin) and enhanced ethene selectivity (>90%) in acetylene hydrogenation. This work sheds light on the controlled synthesis of selective hydrotreating catalysts via mimicking homogeneous analogues.

Self-Optimized Ligand Effect of Single-Atom Modifier in Ternary Pt-Based Alloy for Efficient Hydrogen Oxidation

Modifying the atomic and electronic structure of platinum-based alloy to enhance its activity and anti-CO poisoning ability is a vital issue in hydrogen oxidation reaction (HOR). However, the role of foreign modifier metal and the underlying ligand effect is not fully understood. Here, we propose that the ligand effect of single-atom Cu can dynamically modulate the d-band center of Pt-based alloy for boosting HOR performance. By in situ X-ray absorption spectroscopy, our research has identified that the potential-driven structural rearrangement into high-coordination Cu-Pt/Pd intensifies the ligand effect in Pt-Cu-Pd, leading to enhanced HOR performance. Thereby, modulating the d-band structure leads to near-optimal hydrogen/hydroxyl binding energies and reduced CO adsorption energies for promoting the HOR kinetics and the CO-tolerant capability. Accordingly, PtPdCu₁/C exhibits excellent CO tolerance even at 1,000 ppm impurity.

Tripodal Pd metallenes mediated by Nb2C MXenes for boosting alkynes semihydrogenation

2D metallene nanomaterials have spurred considerable attention in heterogeneous catalysis by virtue of sufficient unsaturated metal atoms, high specific surface area and surface strain. Nevertheless, the strong metallic bonding in nanoparticles aggravates the difficulty in the controllable regulation of the geometry of metallenes. Here we propose an efficient galvanic replacement strategy to construct Pd metallenes loaded on Nb₂C MXenes at room temperature, which is triggered by strong metal-support interaction based on MD simulations. The Pd metallenes feature a chair structure of six-membered ring with the coordination number of Pd as low as 3. Coverage-dependent kinetic analysis based on first-principles calculations reveals that the tripodal Pd metallenes promote the diffusion of alkene and inhibit its overhydrogenation. As a consequence, Pd/Nb₂C delivers an outstanding turnover frequency of 10372 h^-1 and a high selectivity of 96% at 25 ^oC in the semihydrogenation of alkynes without compromising the stability. This strategy is general and scalable considering the plentiful members of the MXene family, which can set a foundation for the design of novel supported-metallene catalysts for demanding transformations.

Development of a ReaxFF potential for Au-Pd

The bimetallic alloys often outperform their single-component counterparts due to synergistic effects. Being widely known, the Au-Pd alloy is a promising candidate for the novel heterogeneous nanocatalysts. Rational design of such systems requires theoretical simulations under ambient conditions.Ab initioquantum-mechanical calculations employ the density functional theory (DFT) and are limited to the systems with few tens of atoms and short timescales. The alternative solution implies development of reliable atomistic potentials. Among different approaches ReaxFF combines chemical accuracy and low computational costs. However, the development of a new potential is a problem without unique solution and thus requires accurate validation criteria. In this work we construct ReaxFF potential for the Au-Pd system based onab initioDFT calculations for bulk structures, slabs and nanoparticles with different stoichiometry. The validation was performed with molecular dynamics and Monte-Carlo calculations. We present several optimal parametrizations that describe experimental bulk mechanical and thermal properties, atomic order-disorder phase transition temperatures and the resulting ordered crystal structures.

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.

Heusler alloy catalysts for electrochemical CO2 reduction

Developing efficient catalysts for electrochemical CO2 reduction reaction (ECO2RR) to hydrocarbons is becoming increasingly important but still challenging due to their high overpotential and poor selectivity. Here, the famous Heusler alloys are investigated as ECO2RR catalysts for the first time by means of density functional theory calculations. The linear scaling relationship between the adsorption energies of CHO (and COOH) and CO intermediates is broken and, thus, the overpotential can be tuned regularly by chemically permuting different 3 d, 4 d, or 5 d transition metals (TMs) in Heusler alloy Cu2TMAl. Cu2ZnAl shows the best activity among all the 30 Heusler alloys considered in the present study, with 41% improvement in energy efficiency compared to pure Cu electrode. Cu2PdAl, Cu2AgAl, Cu2PtAl, and Cu2AuAl are also good candidates. The calculations on the competition between hydrogen evolution reaction and CO2RR indicate that Cu2ZnAl is also the one having the best selectivity toward hydrocarbons. This work identifies the possibility of applying the Heusler alloy as an efficient ECO2RR catalyst. Since thousands of Heusler alloys have been found in experiments, the present study also encourages the search for more promising candidates in this broad research area.

Activity switching of Sn and In species in Heusler alloys for electrochemical CO2 reduction

The electrochemical CO₂ reduction reaction (CO₂RR) activity of Ni₂MnIn and Ni₂MnSn Heusler alloys was investigated. Although pure In, Sn and Ni₂MnIn generated formate as the major product, Ni₂MnSn generated H₂ as the major product. The CO₂RR selectivity could be controlled by selecting the constituent elements of the intermetallic catalysts.

Revealing the Potential of Ternary Medium-Entropy Alloys as Exceptional Electrocatalysts toward Nitrogen Reduction: An Example of Heusler Alloys

With less energy consumption and environmental pollution, electrochemical ammonia synthesis is regarded as the most promising way to replace the industrial Haber-Bosch process, which greatly contributes to global energy consumption and CO₂ emission. At present, the best metal electrocatalyst for N₂ fixation is ruthenium although its performance still suffers from a low Faradaic efficiency and a high overpotential. Alloy engineering is a promising way to discover more metal-based electrocatalysts for dinitrogen reduction reaction (N2RR), and almost all reported alloy catalysts so far are binary alloys. In this work, we proposed a large group of ternary alloy electrocatalysts (Heusler alloys) for N2RR and demonstrated their superior catalytic performance. As an example, alloying Ru with Mn and Si led to a reduced Ru-Ru distance on the surface, which facilitates an uncommon horizontal adsorption mode of N₂ and results in effective activation of N₂ molecules. The theoretical overpotential of N2RR on Ru₂MnSi(100-Ru) is only around 0.28 V, which ranks among the best reported results, and the usage of precious Ru is greatly reduced. Meanwhile, the adsorption of N₂ on Ru₂MnSi(100-Ru) was much stronger than that of protons, and it also took less energy to drive N2RR than the hydrogen evolution reaction (HER), making HER less competitive on this catalyst. Considering the successful synthesis of numerous Heusler alloys including the six members mentioned here, our work provided a wider range of practical and excellent N2RR electrocatalysts in terms of both catalytic performance and economical cost.

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.

Transport properties of Heusler compounds and alloys

Heusler compounds are a large group of intermetallic compositions with versatile material properties. In recent times, they are found to be important for their practical applications in the fields of spintronics and shape memory effect. Interestingly, their physical properties can be easily tuned by varying the valence electron concentration through proper doping and substitution. Empirical laws concerning the valence electron concentration, such as Slater-Pauling or Hume-Rothery rules are found to be useful in predicting their electronic, magnetic and structural properties quite accurately. Electrical transport measurements are simple laboratory-based techniques to gather a handful of information on the electronic properties of metals and semiconductors. The present review aimed to provide a comprehensive view of the transport in 3dand 4dtransition metal-based bulk Heusler compositions. The main emphasis is given on resistivity, magnetoresistance, Hall effect, thermopower and spin-dependent transport in spintronics devices. The review primarily focuses on magnetic Heusler compounds and alloys, albeit it also addresses several non-magnetic materials showing superconductivity or large thermopower.

Synthesis of Co2FeGe Heusler alloy nanoparticles and catalysis for selective hydrogenation of propyne

Although intermetallic compounds are attracting attention of catalysis researchers, ternary intermetallic catalysts have scarcely been investigated due the difficulty of synthesizing supported nanoparticles. In this study, we successfully synthesized SiO2 supported Co2FeGe Heuslar alloy nanoparticles. This catalyst exhibited high catalytic performance for selective hydrogenation of propyne by nano-sizing.

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.

In Situ Surface Structures of PdAg Catalyst and Their Influence on Acetylene Semihydrogenation Revealed by Machine Learning and Experiment

PdAg alloy is an industrial catalyst for acetylene-selective hydrogenation in excess ethene. While significant efforts have been devoted to increase the selectivity, there has been little progress in the catalyst performance at low temperatures. Here by combining a machine-learning atomic simulation and catalysis experiment, we clarify the surface status of PdAg alloy catalyst under the reaction conditions and screen out a rutile-TiO₂ supported Pd₁Ag₃ catalyst with high performance: i.e., 85% selectivity at >96% acetylene conversion over a 100 h period in an experiment. The machine-learning global potential energy surface exploration determines the Pd-Ag-H bulk and surface phase diagrams under the reaction conditions, which reveals two key bulk compositions, Pd₁Ag₁ (R3̅m) and Pd₁Ag₃ (Pm3̅m), and quantifies the surface structures with varied Pd:Ag ratios under the reaction conditions. We show that the catalyst activity is controlled by the PdAg patterns on the (111) surface that are variable under reaction conditions, but the selectivity is largely determined by the amount of Pd exposure on the (100) surface. These insights provide the fundamental basis for the rational design of a better catalyst via three measures: (i) controlling the Pd:Ag ratio at 1:3, (ii) reducing the nanoparticle size to limit PdAg local patterns, (iii) searching for active supports to terminate the (100) facets.

Porous intermetallic Ni2XAl (X = Ti or Zr) nanoparticles prepared from oxide precursors

Porous intermetallic Ni₂XAl (X = Ti or Zr) nanoparticles with small crystallite sizes (24-34 nm) and high Brunauer-Emmett-Teller (BET) surface areas (10-71 m² g^-1) were prepared from oxide precursors by a chemical route. CaH₂ acted as a template to form the porous morphologies and assisted the reduction.

Tunable eg Orbital Occupancy in Heusler Compounds for Oxygen Evolution Reaction*

Heusler compounds have potential in electrocatalysis because of their mechanical robustness, metallic conductivity, and wide tunability in the electronic structure and element compositions. This study reports the first application of Co2 YZ-type Heusler compounds as electrocatalysts for the oxygen evolution reaction (OER). A range of Co2 YZ crystals was synthesized through the arc-melting method and the eg orbital filling of Co was precisely regulated by varying Y and Z sites of the compound. A correlation between the eg orbital filling of reactive Co sites and OER activity was found for Co2 MnZ compounds (Z=Ti, Al, V, and Ga), whereby higher catalytic current was achieved for eg orbital filling approaching unity. A similar trend of eg orbital filling on the reactivity of cobalt sites was also observed for other Heusler compounds (Co2 VZ, Z=Sn and Ga). This work demonstrates proof of concept in the application of Heusler compounds as a new class of OER electrocatalysts, and the influence of the manipulation of the spin orbitals on their catalytic performance.

Durability and activity of Co2YZ (Y = Mn or Fe, Z = Ga or Ge) Heusler alloy catalysts for dehydrogenation of 2-propanol

Using dehydrogenation of 2-propanol as a test reaction for Heusler alloy catalysts, durability against oxidation and a relationship between activity and electronic structures were revealed.

Catalytic Properties of Heusler Alloys for Steam Reforming of Methanol

Intermetallic compounds have attracted research attention in catalysis because of their unique catalytic properties. Recently, a group of intermetallic compounds, referred to as Heusler alloys (X₂YZ), has been investigated as new catalysts. In this study, catalytic properties of 14 Heusler alloys with X = Fe, Co, Ni, or Cu; Y = Ti, Mn, or Fe; Z = Al, Si, Ga, Ge, or Sn for the steam reforming of methanol were examined. Co₂TiAl and Ni₂TiAl alloys exhibited relatively high H₂ production rates because of the formation of fine particles via the selective oxidation of Ti. X₂MnZ alloys exhibited high CO₂ selectivity because of a water-gas shift reaction catalyzed by using MnO that was formed during the reaction. Crystal phases, surface microstructures, and surface compositions of most alloys were changed because of the reaction, and the formation of fine particles possibly assisted in the observation of catalytic activity. Heusler alloys can be beneficial as catalyst precursors by the selection of appropriate elemental sets depending on target reactions.