Intermetallic Compounds: Promising Inorganic Materials for Well-Structured and Electronically Modified Reaction Environments for Efficient Catalysis

Hydrosilylation of carbonyls over electron-enriched Ni sites of intermetallic compound Ni3Ga heterogeneous catalyst

Nanoparticulate intermetallic compound Ni3Ga supported on SiO2 has emerged as a highly efficient catalyst for the hydrosilylation of carbonyls, such as aldehydes and ketones, at room temperature. Formation of electron-enriched Ni via alloying with Ga is the key to the catalytic performance.

Eutectic salt mixture-assisted sodium-vapor-induced synthesis of Pt-Ca nanoparticles, and their microstructural and electrocatalytic properties

We have fabricated Pt-Ca nanoparticles with oxygen reduction reaction catalytic activity via a sodium vapor-induced synthesis method. Prior addition of NaCl to form a eutectic mixture of CaCl2 and NaCl facilitated the formation of intermetallic Pt2Ca nanoparticles. Pt3Mg and Pt5Sr nanoparticles also were suggested to be producible.

Single-Crystal Cobalt Phosphide Nanorods as a High-Performance Catalyst for Reductive Amination of Carbonyl Compounds

The development of metal phosphide catalysts for organic synthesis is still in its early stages. Herein, we report the successful synthesis of single-crystal cobalt phosphide nanorods (Co₂P NRs) containing coordinatively unsaturated Co-Co active sites, which serve as a new class of air-stable, highly active, and reusable heterogeneous catalysts for the reductive amination of carbonyl compounds. The Co₂P NR catalyst showed high activity for the transformation of a broad range of carbonyl compounds to their corresponding primary amines using an aqueous ammonia solution or ammonium acetate as a green amination reagent at 1 bar of H₂ pressure; these conditions are far milder than previously reported. The air stability and high activity of the Co₂P NRs is noteworthy, as conventional Co catalysts are air-sensitive (pyrophorous) and show no activity for this transformation under mild conditions. P-alloying is therefore of considerable importance for nanoengineering air-stable and highly active non-noble-metal catalysts for organic synthesis.

Few-nm-sized, phase-pure Au5Sn intermetallic nanoparticles: synthesis and characterization

Nanoparticles of intermetallic compounds have attracted much interest because they can exhibit novel electronic and catalytic properties due to their specific crystal structure, ordered atomic arrangement, and quantum effect. Here, gold-tin (AuSn) bimetallic nanoparticles with various mixing ratios were prepared by a co-reduction method using various protective agents (e.g., polymer, amine, phosphine, carboxylic acid, and thiol). Powder X-ray diffractometry and transmission electron microscopy revealed that few-nm-sized, phase-pure Au5Sn intermetallic nanoparticles (IMNPs) were successfully synthesized when Au3+ and Sn2+ precursors with a ratio of 6 : 4 were co-reduced in the presence of oleylamine. The Au5Sn IMNPs thus prepared did not exhibit localized surface plasmon resonance, in contrast to pure Au nanoparticles of comparable sizes. This suggests that interband transition dominates the optical response due to an increase in the density of states near the Fermi level by introducing Sn. The Au5Sn IMNPs supported on mesoporous silica (SBA-15) catalyzed the aerobic oxidation reaction of indanol.

C-H bond activation in light alkanes: a theoretical perspective

Alkanes are the major constituents of natural gas and crude oil, the feedstocks for the chemical industry. The efficient and selective activation of C-H bonds can convert abundant and low-cost hydrocarbon feedstocks into value-added products. Due to the increasing global demand for light alkenes and their corresponding polymers as well as synthesis gas and hydrogen production, C-H bond activation of light alkanes has attracted widespread attention. A theoretical understanding of C-H bond activation in light hydrocarbons via density functional theory (DFT) and microkinetic modeling provides a feasible approach to gain insight into the process and guidelines for designing more efficient catalysts to promote light alkane transformation. This review describes the recent progress in computational catalysis that has addressed the C-H bond activation of light alkanes. We start with direct and oxidative C-H bond activation of methane, with emphasis placed on kinetic and mechanistic insights obtained from DFT assisted microkinetic analysis into steam and dry reforming, and the partial oxidation dependence on metal/oxide surfaces and nanoparticle size. Direct and oxidative activation of the C-H bond of ethane and propane on various metal and oxide surfaces are subsequently reviewed, including the elucidation of active sites, intriguing mechanisms, microkinetic modeling, and electronic features of the ethane and propane conversion processes with a focus on suppressing the side reaction and coke formation. The main target of this review is to give fundamental insight into C-H bond activation of light alkanes, which can provide useful guidance for the optimization of catalysts in future research.

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.

Exploring Cu/Al cluster growth and reactivity: from embryonic building blocks to intermetalloid, open-shell superatoms

Cluster growth reactions in the system [Cu5](Mes)5 + [Al4](Cp*)4 (Mes = mesitylene, Cp* = pentamethylcyclopentadiene) were explored and monitored by in situ LIFDI-MS and 1H-NMR. Feedback into experimental design allowed for an informed choice and precise adjustment of reaction conditions and led to isolation of the intermetallic cluster [Cu4Al4](Cp*)5(Mes) (1). Cluster 1 reacts with excess 3-hexyne to yield the triangular cluster [Cu2Al](Cp*)3 (2). The two embryonic [Cu4Al4](Cp*)5(Mes) and [Cu2Al](Cp*)3 clusters 1 and 2, respectively, were shown to be intermediates in the formation of an inseparable composite of the closely related clusters [Cu7Al6](Cp*)6 (3), [HCu7Al6](Cp*)6 (3H) and [Cu8Al6](Cp*)6 (4), which just differ by one Cu core atom. The radical nature of the open-shell superatomic [Cu7Al6](Cp*)6 cluster 3 is reflected in its reactivity towards addition of one Cu core atom leading to the closed shell superatom [Cu8Al6](Cp*)6 (4), and as well by its ability to undergo σ(C-H) and σ(Si-H) activation reactions of C6H5CH3 (toluene) and (TMS)3SiH (TMS = tris(trimethylsilyl)).

Rare-Earth Incorporated Alloy Catalysts: Synthesis, Properties, and Applications

To improve the performance of metallic catalysts, alloying provides an efficient methodology to design state-of-the-art materials. As emerging functional materials, rare-earth metal compounds can integrate the unique orbital structure and catalytic behavior of rare earth elements into metallic materials. Such rare-earth containing alloy catalysts proffer an opportunity to tailor electronic properties, tune charged carrier transport, and synergize surface reactivity, which are expected to significantly improve the performance and stability of catalysis. Despite its significance, there are only few reviews on rare earth containing alloys or related topics. This review summarizes the composition, synthesis, and applications of rare earth containing alloys in the field of catalysis. Subsequent to comprehensively summarizing and constructively discussing the existing work, the challenges and possibilities of future research on rare-earth metal compound materials are evaluated.

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.

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.

Interstitial and substitutional light elements in transition metals for heterogeneous catalysis

The addition of foreign element dopants to monometallic nanoparticle catalysts is of great importance in industrial applications. Both substitutional and interstitial doping of pure metallic phases can give profound effects such as altering electronic and transport properties, lattice parameters, phase transitions, and consequently various physicochemical properties. For transition metal catalysts, this often leads to changes in catalytic activity and selectivity. This article provides an overview of the recent developments regarding the catalytic properties and characterisation of such systems. In particular, the structure-activity relationship for a number of important chemical reactions is summarised and the future prospects of this area are also explored.

Hydrogenation of Carbon Dioxide to Methanol over Non-Cu-based Heterogeneous Catalysts

The increasing atmospheric CO₂ level makes CO₂ reduction an urgent challenge facing the world. Catalytic transformation of CO₂ into chemicals and fuels utilizing renewable energy is one of the promising approaches toward alleviating CO₂ emissions. In particular, the selective hydrogenation of CO₂ to methanol utilizing renewable hydrogen potentially enables large scale transformation of CO₂ . The Cu-based catalysts have been extensively investigated in CO₂ hydrogenation. However, it is not only limited by long-term instability but also displays unsatisfactory catalytic performance. The supported metal-based catalysts (Pd, Pt, Au, and Ag) can achieve high methanol selectivity at low temperatures. The mixed oxide catalysts represented by M_a ZrO_x (M_a =Zn, Ga, and Cd) solid solution catalysts present high methanol selectivity and catalytic activity as well as excellent stability. This Review focuses on the recent advances in developing Non-Cu-based heterogeneous catalysts and current understandings of catalyst design and catalytic performance. First, the thermodynamics of CO₂ hydrogenation to methanol is discussed. Then, the progress in supported metal-based catalysts, bimetallic alloys or intermetallic compounds catalysts, and mixed oxide catalysts is discussed. Finally, a summary and a perspective are presented.

Random alloy and intermetallic nanocatalysts in fuel cell reactions

Fuel cells that use small organic molecules or hydrogen as the anode fuel can power clean electric vehicles. From an experimental perspective, the possible fuel cells' electrocatalytic reaction mechanisms are obtained through in situ electrochemical spectroscopy techniques and density functional theory calculations, providing theoretical guidance for further development of novel nanocatalysts. As advanced nanocatalysts for fuel cells' electrochemical reactions, alloy nanomaterials have greatly improved electrocatalytic activity and stability and have attracted widespread attention. Enhanced electrocatalytic performance of alloy nanocatalysts could be closely related to the synergistic effects, such as electronic and strain effects. Depending on the arrangement of atoms, alloys can be classified into random alloy and intermetallic compounds (ordered structure). Intermetallic compounds generally have lower heats of formation and stronger heteroatomic bonding strength relative to the random alloy, resulting in high chemical and structural stability in either full pH solutions or electrochemical tests. Here, we summarize the latest advances and the structure-function relationship of noble metal alloy nanocatalysts, among which Pt-based catalysts are the main ones, as well as comprehensively understand why they significantly affect the electrocatalytic performance of fuel cells. Novel alloy nanocatalysts with a robust three-phase interface to achieve efficient charge and mass transfer can obtain desirable activity and stability in the electrochemical workstation tests, and is expected to acquire a higher power density on fuel cell test systems with harsh test conditions.

The impact of synthetic method on the catalytic application of intermetallic nanoparticles

Intermetallic alloy nanocrystals have emerged as a promising next generation of nanocatalyst, largely due to their promise of surface tunability. Atomic control of the geometric and electronic structure of the nanoparticle surface offers a precise command of the catalytic surface, with the potential for creating homogeneous active sites that extend over the entire nanoparticle. Realizing this promise, however, has been limited by synthetic difficulties, imparted by differences in parent metal crystal structure, reduction potential, and atomic size. Further, little attention has been paid to the impact of synthetic method on catalytic application. In this review, we seek to connect the two, organizing the current synthesis methods and catalytic scope of intermetallic nanoparticles and suggesting areas where more work is needed. Such analysis should help to guide future intermetallic nanoparticle development, with the ultimate goal of generating precisely controlled nanocatalysts tailored to catalysis.

Sodium Vapor-Induced Synthesis of Intermetallic Pt5Ce Compound Nanoparticles

We have developed a synthetic route that uses sodium for the production of intermetallic Pt₅Ce nanoparticles (ca. 6 nm average diameter) supported on carbon powder. Sodium melt was demonstrated to reduce a powder mixture of PtCl₂ and CeCl₃ to form submicrometer Pt₅Ce particles with the simultaneous formation of NaCl. The NaCl-CeCl₃ melt mixture and Na melt were formed during heating, which led to a uniform reaction between Pt and Ce, and the melt induced grain growth. The synthetic procedures were then modified to supply sodium vapor to the vicinity of the metal sources supported on carbon powder with an aim to suppress grain growth. Pt₅Ce nanoparticles were successfully formed on the carbon support with high loading and dispersity.

Al2 Pt for Oxygen Evolution in Water Splitting: A Strategy for Creating Multifunctionality in Electrocatalysis

The production of hydrogen via water electrolysis is feasible only if effective and stable catalysts for the oxygen evolution reaction (OER) are available. Intermetallic compounds with well-defined crystal and electronic structures as well as particular chemical bonding features are suggested here to act as precursors for new composite materials with attractive catalytic properties. Al2 Pt combines a characteristic inorganic crystal structure (anti-fluorite type) and a strongly polar chemical bonding with the advantage of elemental platinum in terms of stability against dissolution under OER conditions. We describe here the unforeseen performance of a surface nanocomposite architecture resulting from the self-organized transformation of the bulk intermetallic precursor Al2 Pt in OER.

Catalysis-in-a-Box: Robotic Screening of Catalytic Materials in the Time of COVID-19 and Beyond

This work describes the design and implementation of an automated device for catalytic materials testing by direct modifications to a gas chromatograph (GC). The setup can be operated as a plug-flow isothermal reactor and enables the control of relevant parameters such as reaction temperature and reactant partial pressures directly from the GC. High-quality kinetic data (including reaction rates, product distributions, and activation barriers) can be obtained at almost one-tenth of the fabrication cost of analogous commercial setups. With these key benefits including automation, low cost, and limited experimental equipment instrumentation, this implementation is intended as a high-throughput catalyst screening reactor that can be readily utilized by materials synthesis researchers to assess the catalytic properties of their synthesized structures in vapor-phase chemistries.

Palladium-Based Bimetallic Nanocrystal Catalysts for the Direct Synthesis of Hydrogen Peroxide

The direct synthesis of H2 O2 from H2 and O2 is a strongly desired reaction for green processes and a promising alternative to the commercialized anthraquinone process. The design of efficient catalysts with high activity and H2 O2 selectivity is highly desirable and yet challenging. Metal dopants enhance the performance of the active phase by increasing reaction rates, stability, and/or selectivity. The identification of efficient dopants relies mostly on catalysts prepared with a random and non-uniform deposition of active and promoter phases. To study the promotional effects of metal doping on Pd catalysts, we employ colloidal, bimetallic nanocrystals (NCs) to produce catalysts in which the active and doping metals are colocalized to a fine extent. In the absence of any acid and halide promotors, PdSn and PdGa NCs supported on acid-pretreated TiO2 (PdSn/s-TiO2 , PdGa/s-TiO2 ) were highly efficient and outperformed the monometallic Pd catalyst (Pd/s-TiO2 ), whereas in the presence of an acid promotor, the overall H2 O2 productivity was also further enhanced for the Ni-, Ga-, In-, and Sn-doped catalysts with respect to Pd/s-TiO2 .

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.

Surface Hexagonal Pt1Sn1 Intermetallic on Pt Nanoparticles for Selective Propane Dehydrogenation

A series of 2-3 nm Pt-Sn bimetallic nanoparticles with different Pt-Sn coordination numbers were synthesized by a stepwise approach including electrostatic adsorption and temperature-programmed reduction of metal precursors on the SiO₂ support. In situ synchrotron X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS) demonstrated a highly ordered hexagonal Pt₁Sn₁ intermetallic shell on Pt nanoparticles. The turnover rates (TORs), propylene selectivity, and stability of these bimetallic catalysts significantly surpass those of the monometallic Pt catalyst for propane dehydrogenation. At the same time, TORs increase with increasing the Pt-Sn coordination number, whereas propylene selectivity is not significantly influenced by the Pt-Sn coordination number. Combined with experiments and theoretical calculations, the high propylene selectivity of Pt-Sn bimetallic nanoparticles is attributed to the geometric effects of Sn that reduce the Pt ensembles, and the high TORs are due to the electronic effects that weaken Pt-hydrocarbon chemisorption energies.

Single-atom Pt in intermetallics as an ultrastable and selective catalyst for propane dehydrogenation

Propylene production via propane dehydrogenation (PDH) requires high reaction temperatures to obtain sufficient propylene yields, which results to prominent catalyst deactivation due to coke formation. Developing highly stable catalysts for PDH without deactivation even at high temperatures is of great interest and benefit for industry. Here, we report that single-atom Pt included in thermally stable intermetallic PtGa works as an ultrastable and selective catalyst for PDH at high temperatures. Intermetallic PtGa displays three-hold-Pt ensembles and single Pt atoms isolated by catalytically inert Ga at the surface, the former of which can be selectively blocked and disabled by Pb deposition. The PtGa-Pb/SiO₂ catalyst exhibits 30% conversion with 99.6% propylene selectivity at 600 °C for 96 h without lowering the performance. The single-atom Pt well catalyzes the first and second C–H activation, while effectively inhibits the third one, which minimizes the side reactions to coke and drastically improves the selectivity and stability.

Propylene production via propane dehydrogenation demands a highly stable catalyst that works without deactivation even at high temperatures. Here, the authors show that single-atom Pt included in thermally stable intermetallic PtGa works as an active and selective catalyst for propane dehydrogenation even at 600 °C for 96 h without deactivation.

Catalytic conversion of ethane to valuable products through non-oxidative dehydrogenation and dehydroaromatization

Chemical utilization of ethane to produce valuable chemicals has become especially attractive since the expanded utilization of shale gas in the United States and associated petroleum gas in the Middle East. Catalytic conversion to ethylene and aromatic hydrocarbons through non-oxidative dehydrogenation and dehydroaromatization of ethane (EDH and EDA) are potentially beneficial technologies because of their high selectivity to products. The former represents an attractive alternative to conventional thermal cracking of ethane. The latter can produce valuable aromatic hydrocarbons from a cheap feedstock. Nevertheless, further progress in catalytic science and technology is indispensable to implement these processes beneficially. This review summarizes progress that has been achieved with non-oxidative EDH and EDA in terms of the nature of active sites and reaction mechanisms. Briefly, platinum-, chromium- and gallium-based catalysts have been introduced mainly for EDH, including effects of carbon dioxide co-feeding. Efforts to use EDA have emphasized zinc-modified MFI zeolite catalysts. Finally, some avenues for development of catalytic science and technology for ethane conversion are summarized.

Structural trends in the dehydrogenation selectivity of palladium alloys

Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior. To provide molecular-level insights into these effects, a series of Pd intermetallic alloy catalysts with Zn, Ga, In, Fe and Mn promoter elements was synthesized, and the structures were determined using in situ X-ray absorption spectroscopy (XAS) and synchrotron X-ray diffraction (XRD). The alloys all showed propane dehydrogenation turnover rates 5–8 times higher than monometallic Pd and selectivity to propylene of over 90%. Moreover, among the synthesized alloys, Pd₃M alloy structures were less olefin selective than PdM alloys which were, in turn, almost 100% selective to propylene. This selectivity improvement was interpreted by changes in the DFT-calculated binding energies and activation energies for C–C and C–H bond activation, which are ultimately influenced by perturbation of the most stable adsorption site and changes to the d-band density of states. Furthermore, transition state analysis showed that the C–C bond breaking reactions require 4-fold ensemble sites, which are suggested to be required for non-selective, alkane hydrogenolysis reactions. These sites, which are not present on alloys with PdM structures, could be formed in the Pd₃M alloy through substitution of one M atom with Pd, and this effect is suggested to be partially responsible for their slightly lower selectivity.

Alloying is well-known to improve the dehydrogenation selectivity of pure metals, but there remains considerable debate about the structural and electronic features of alloy surfaces that give rise to this behavior.

Synthesis of Intermetallic Pt-Based Catalysts by Lithium Naphthalenide-Driven Reduction for Selective Hydrogenation of Cinnamaldehyde

Intermetallic nanoparticles (NPs) with a well-defined atom binding environment and a long-range ordering structure can be used as ideal models to understand their physical and catalytic properties. In this work, several kinds of nanostructured and carbon nanotube (CNT)-supported Pt-based intermetallic compounds (IMCs) have been synthesized by one-step lithium naphthalenide-driven reduction at room temperature without the use of surfactants in light of the reduction potential of metals. In the chemoselective hydrogenation of cinnamaldehyde, the second metal in Pt-M IMCs significantly creates a suitable reaction environment through construction of a good geometric and electronic structure. The Pt₃Sn/CNT catalyst presents highly efficient and good chemoselective hydrogenation of cinnamaldehyde to cinnamyl alcohol. This can be attributed to the fact that the incorporated Sn atoms effectively dilute large Pt ensembles and increase the electron density of Pt. The in situ-formed SnO_x interfaces as Lewis acid sites facilitate the coordination of C═O bonds, enhancing the selectivity to cinnamyl alcohol. In addition, the SnO_x interface as the joint between Pt₃Sn IMCs NPs and CNTs significantly improves the stability of the catalyst in the reaction environment.

Ultra-small intermetallic NiZn nanoparticles: a non-precious metal catalyst for efficient electrocatalysis

Intermetallics are long-range-ordered alloys traditionally synthesized by annealing nanoparticles of a random alloy, which results in the sintering of the nanoparticles and leads to the formation of polydispersed samples. It thus remains a challenge to achieve a monodispersion of tiny intermetallics. In the current work, ultra-small monodisperse intermetallic NiZn nanoparticles were synthesized based on a low-temperature solution chemistry route involving the chemical conversion of metal nanoparticles into an ordered alloy using an organometallic zinc precursor. During the transformation of single metal nanoparticles into the corresponding alloy, the particles retained their morphology. The resulting ordered alloy made up of earth-abundant materials demonstrated high electrocatalytic performance for the oxygen evolution reaction (OER) with a low overpotential of 283 mV at a current density of 10 mA cm^-2 and a small Tafel slope of 73 mV dec^-1, along with excellent stability and durability. The prepared intermetallic NiZn exhibited outstanding OER efficacy, better than those of a Ni_0.7Zn_0.3 alloy, pure Ni nanoparticles and even state-of-the art RuO₂. The atomic ordering as well as the modification of the electronic structure of Ni upon becoming alloyed with Zn, together with an atomic-scale synergistic effect produced from Ni and Zn, led to the enhanced intrinsic catalytic activity. The present findings point to a general route to produce nanoscale tiny alloys and also provide excellent electrocatalysts having exceptional energy conversion efficiency.