Intermetallic Compounds: Promising Inorganic Materials for Well

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Review pubs.acs.org/acscatalysis

Intermetallic Compounds: Promising Inorganic Materials for WellStructured and Electronically Modified Reaction Environments for Efficient Catalysis Shinya Furukawa*,† and Takayuki Komatsu* Department of Chemistry, School of Science, Tokyo Institute of Technology 2-12-1-E1-10, Ookayama, Meguro-ku, Tokyo, Japan, 152-8550 ABSTRACT: An overview of the catalytic properties of intermetallic compounds has been made to provide a comprehensive understanding regarding (1) what intermetallic catalysts can do, (2) their fundamental roles in enhanced catalysis, and (3) their advantages over other inorganic materials. A number of chemical transformations using intermetallic catalysts have been surveyed and classified into three major divisions hydrogenation/dehydrogenation, oxidation, and steam reforming and various subsections. The fundamental roles of intermetallic phases obtained from this survey were categorized into four types of effects: (a) electronic, (b) geometric, (c) steric, and (d) ordering effects. The unprecedented steric effects governed by the specific surface structures of intermetallic compounds highlight the unique capabilities of intermetallic materials. On the basis of this overview, we have concluded that intermetallic compounds have the following advantages for fine catalyst design: (i) control of the electronic structure, (ii) a specific and ordered atomic-level structure, and (iii) homogeneity of geometric and electronic structures. Thus, intermetallic compounds are promising inorganic catalyst materials capable of creating a well-designed reaction environment and suitable for developing efficient catalytic systems. KEYWORDS: intermetallic compound, catalyst material, catalyst design, geometric effect, electronic effect

1. INTRODUCTION Catalytic conversion by metallic materials is a key methodology used for various chemical transformations in industrial chemistry, organic synthesis, gas purification, and energy conversion. To improve the performance of metallic catalysts, secondary metallic elements have been frequently added to catalysts as modifiers. The development of such bimetallic catalytic materials remains an intriguing research area in catalytic chemistry.1−3 In bimetallic systems, two types of metals are present in various states, such as alloys,4,5 core−shell structures,6,7 and metal−oxide composites.6,7 Alloys are classified into two categories depending on their structure: solid-solution alloys and intermetallic compounds.8 The former, typically substitutional solid-solution alloys, consist of metals of similar atomic size and electronic character with a crystal structure identical to that of the parent metal with random atomic arrangements (Figure 1a). If the atoms of one element is sufficiently small to fit within the lattice void of the counterpart element, interstitial solid-solution alloys can be formed (Figure 1b). The latter involves the opposite situation to Figure 1a, where the component metals have significantly different characters and comprise distinct crystal structures with highly ordered atomic arrangements (Figure 1c).8 In general, the similarity of two metals can be roughly estimated by their relative positions in the periodic table. As is well-known, solidsolution alloys have been extensively tested as catalyst materials for a long time.9−13 In contrast, intermetallic compounds have been investigated for their applications to bulk materials with © XXXX American Chemical Society

Figure 1. Structure of bimetallic alloys: (a) substitutional and (b) interstitial solid-solution alloys and (c) intermetallic compound. The word “alloying” suggests the formation of alloy phase (a, b, or c) or simply, the mixing of two metal elements in an atomic level.

Received: September 11, 2016 Revised: December 5, 2016 Published: December 9, 2016 735

DOI: 10.1021/acscatal.6b02603 ACS Catal. 2017, 7, 735−765

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ACS Catalysis unique physical properties such as superconductivity,14,15 shape-memory effects,16 and hydrogen storage capability.17,18 However, until recently, they have rarely been studied for use in catalytic reactions, leaving a so-called “blue ocean” area for developments in the chemistry of intermetallic catalysts. This situation is clearly represented in Figure 2, which shows the

section 4, the fundamental aspects of intermetallic or alloy phases that govern catalysis are categorized into several parts, including electronic and geometric effects. Other observed features that are unique to intermetallic phases are also introduced. On the basis of the nature of these features, we demonstrate the advantages of intermetallic compounds in catalyst design. Finally, in section 5, we discuss future perspectives on this research field and describe several challenges to be overcome. For better categorizing in this review, we describe bimetallic combinations in order of the catalytically active metal (M) first and the counterpart metal (N) second. Specific intermetallic and solid-solution alloy phases are designated as MmNn and MxN(1−x) (where m, n are integer numbers and x is a decimal number), respectively. When we do not indicate a specific intermetallic or alloy phase, the bimetallic designation combined with a dash (M−N) is used. This designation indicates a composite of various phases, a generic term for a bimetallic system, and/or a simple mixture of two metals. Note that some descriptions in this manuscript differ from the original ones in the literature.

Figure 2. Annual increases in the number of literature works in which titles include “catalyst” and “metallic”, “alloy”, or “intermetallic”, as determined by Web of Science searches.

2. CATALYST PREPARATION In general, an intermetallic catalyst should contain a catalytically active element, such as one of the late transition metals (Au, Pt, Pd, Ni, Rh, Co, Ru, and Fe). Therefore, the counterpart metal should be an early transition metal or a typical metallic element, which is well-separated from the active metals in the periodic table (Figure 3). The combination of a

annual increase in the number of publications on alloys and intermetallic catalysts. Note that the actual numbers of the corresponding studies may be much longer than those shown in Figure 2, which only roughly reflects the trend in research. The upward trend for alloy catalysts follows that for metallic catalysts, although the total number is not as high. This upward trend is particularly prominent after 2000, making the chemistry of bimetallic catalysis as a “hot topic”. In contrast, the number of literature works on intermetallic catalysts is much smaller than that of alloy or metal catalysts. However, the recent increase in publications suggests that this field has also received growing attention. Moreover, the insights obtained from these studies suggest that the potential of intermetallic compounds for catalyst materials is high compared to conventional inorganic catalyst materials. To facilitate development and work toward establishing the chemistry of intermetallic catalysts in the near future, it is necessary to produce and share a comprehensive understanding about (1) what intermetallic catalysts can do, (2) their fundamental roles in enhanced catalysis, and (3) their advantages over other inorganic materials. To this day, a few reviews of intermetallic compounds focused on a specific topic (metal−support interaction)19 and reactions (acetylene semihydrogenation20,21 and methanol steam reforming)21,22 have been reported. We have also published a minireview summarizing our recent studies on intermetallic catalysts.23 However, no cyclopedic literature covering a broad range of catalytic chemistry of intermetallic compounds has been reported. With this context as our motivation, we summarize the current status of intermetallic compounds to provide an overview of their role as catalyst materials in this review. In section 2, we describe the typical methodologies for preparation of intermetallic compound catalysts and outline several points where care should be taken in their preparation. In section 3, we classify a number of catalytic systems that use intermetallic compounds as effective catalysts with respect to the type of reaction and clarify what intermetallic compounds can do. In

Figure 3. Active and second metal components of catalytically active intermetallic compounds represented in the periodic table. Shaded parts indicate elements that are rarely used.

later transition metal and a typical metal or a half metal (Al, Si, Zn, Ga, Ge, In, Sn, Sb, Te, Tl, Pb, or Bi) is commonly employed, whereas combinations containing an early transition metal (Sc, Ti, V, Y, Zr, Nb, La, Hf, or Ta) have been less frequently reported. For the former combinations, most intermetallic compounds can be easily prepared as highly dispersed nanoparticles on a certain catalyst support using conventional impregnation methods with hydrogen reduction.23 A stoichiometric supply and homogeneous dispersion of the two metal precursors are important in order to obtain a high phase-purity of the desired intermetallic phase. Ideally, the intermetallic catalyst should be single-phase so that the catalytic performance of the phase in question can be well understood. To avoid inhomogeneous distribution of metal precursors during drying, it is preferable to use the least amount of solvent possible, as is typically employed for pore-filling impregnation. 736

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Figure 4. Schematic illustration of various preparation method of intermetallic compounds. (a) Co-impregnation, (b) successive impregnation, (c) chemical vapor deposition, (d) hydride reduction, (e) polyol processsis, and (f) metallurgical alloying.

involving both the metallic phase and the support.27,28 Al2O3supported intermetallic compounds can successfully be prepared by liquid-phase reduction (Figure 4d) using borohydride as a strong reducing agent.24 It has been suggested that rapid coreduction of the two metal cations is effective for synthesizing an intermetallic phase on Al2O3.24 Liquid-phase reduction can also be applied for the colloidal synthesis of intermetallic nanoparticles without any catalyst support.29 Some capping agents are used to avoid aggregation. As a reducing agent, NaBH4,30 LiBH4,24,27 LiBHEt3,31−33 or sodium naphthalenide34 has often been used. As well as the aforementioned methods, chemical vapor deposition (CVD, Figure 4c)35−39 and polyol process (Figure 4e) have also been employed for the preparation of intermetallic nanoparticles. For the CVD method, the second metal precursor is fed as a vapor directly to the surface of the parent metal particles, followed by reductive deposition and alloying.35 At an appropriate temperature, the reductive deposition occurs preferentially on the active metal surface but not on the inert support, allowing selective deposition. The precursors having sufficiently high vapor pressures, such as silane39 and tetraalkyl compounds of group 14 elements,38 are typically employed for CVD. Note that because the loading amount depends strongly on the temperature and time of the CVD procedure, it is necessary to find an appropriate condition to control exactly the loading amount of the second metal.35 This method often enables high phase purities and minimal aggregation or sintering of the parent metal particles under an appropriate condition. Polyol process is a unique and useful method for synthesizing intermetallic nanoparticles by successive alloying of the parent metal particles in liquid phase. Multivalent alcohols such as ethylene glycol, propanediol, and glycerol (so-called polyols) act as a (a) high-boiling solvent (bp 200−350 °C), (b) capping agent, and (c) reductant (hydrogen donor).40 Both colloidal and supported nanoparticles can be used as the parent materials (Figure 4e). This method enables high phase-purity of the resulting

The two metal precursors are loaded on the catalyst support simultaneously or successively (Figure 4). For the former fashion, which is called coimpregnation (Figure 4a), the precursors simultaneously undergo the gas-phase reduction procedure typically at high temperatures. However, in most cases, the reduction of two metal precursors does not occur simultaneously because of the significant deviation in their reduction potentials.24 The reduction of noble metal precursor occurs (typically below 200 °C) before the furnace temperature is elevated sufficiently to reduce the base metal precursor. This trend can be observed clearly in the temperature programed reduction (TPR) profiles of some intermetallic catalysts.25 For the successive impregnation fashion (Figure 4b), the second metal precursor is loaded on the prereduced monometallic catalyst. The reduction of the second metal occurs at the surface or neighboring sites of the parent active metal nanoparticles by surface or spillover hydrogen, followed by diffusing into the bulk to form the intermetallic phase. The size of the resulting intermetallic nanoparticles depends roughly on that of the parent metal. Therefore, this procedure is suitable for the preparation of size-controlled intermetallic nanoparticles. Instead of the supported monometallic particles, Raney-type porous bulk metals can also be used as the parent materials for the successive impregnation procedure.26 Appropriate choice of the catalyst support is also a significant factor to prepare a supported intermetallic compound with high phase purity. Catalyst supports that tend to strongly capture the metals or metal precursors, such as γ-Al2O3, are less suitable for synthesizing single-phase intermetallic nanoparticles because they often result in less-homogeneous dispersions of the metal precursors and/or low surface mobility of the metals during the alloying process.24 Using an inert support such as SiO2 or carbon is suitable not only for obtaining high phase-purity but also for clarifying the role of the intermetallic phase in enhanced catalysis due to the absence of contributions from the support itself. Of course, the use of other supports such as TiO2 or MgO is also valid when one expects concerted catalysis 737

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N-alkylation of amines, and (5) hydrogenation of carbon oxides. 3.1.1. Chemoselective Hydrogenation. Hydrogenation of organic molecules having two reducible functional groups can yield several different products because hydrogenation may occur at any functional groups. If one aims to exclusively obtain a certain hydrogenated product, the targeted functional group should be selectively hydrogenated with the other functional group remaining. Occasionally, however, the other functional group may be kinetically and thermodynamically more easily hydrogenated than the target functional group. In such a case, a drastic modification of the catalytic property is required in order to suppress the undesired hydrogenation of the other group. 3.1.1.1. α,β-Unsaturated Aldehyde to Allylic Alcohol. Allylic alcohols such as crotyl alcohol, prenol, geraniol, cinnamyl alcohol, and furfuryl alcohol are valuable intermediates for the production of perfumes, flavoring, pharmaceuticals, and resins.49−51 These alcohols can be produced by preferred hydrogenation of the CO group of the corresponding α,βunsaturated aldehyde (i.e., crotonaldehyde, prenal, citral, cinnamaldehyde, and furfral, respectively). Selective catalytic hydrogenation of α,β-unsaturated aldehydes to allylic alcohols is a key step in the industrial production of fine chemicals and is one of the most widely studied chemoselective hydrogenations in fundamental research on catalysis.48 Conventional monometallic catalysts such as Pt52 Ni,39 or Ru53,54 typically catalyze undesired hydrogenation of the CC bond to form a saturated aldehyde or alcohol. In this research field, Sn-modified bimetallic systems represented by Pt−Sn have been reported to be effective (typically 70−90% selectivities for allylic alcohols) since the 1980s.55−57 At the early stage (prior to 2000), however, most studies proposed that cationic Sn species adjacent to Pt sites act as anchoring sites of the CO moiety for selective hydrogenation and made little mention of the involvement of alloy or intermetallic species.55−58 The adsorption properties of Pt−Sn alloys (including surface alloys and bulk intermetallic Pt3Sn) have been extensively studied by a combination of density functional theory (DFT) calculations and surface science techniques such as high-resolution electron energy loss spectroscopy (HREELS), low-energy electron diffraction (LEED), and temperature programed desorption (TPD) in the 2000s.59−62 These studies revealed that alloying of Pt with Sn significantly weakened the adsorption of α,β-unsaturated aldehydes in side-on conformations, resulting in CC hydrogenation, whereas it enhanced that in top coordination on Sn sites, enabling CO hydrogenation. Very recently, Hou et al. reported the catalytic properties of a supported Pt3Sn intermetallic phase.63 They prepared Pt3Sn intermetallic nanoparticles on SnO2 islands dispersed on reduced graphene oxide (rGO). This catalyst exhibits good to excellent (75− 98%) yields for 12 different unsaturated alcohols, including crotyl, cinnamyl, and furfuryl alcohols under relatively mild conditions (2 MPa H2, 70 °C, Scheme 1).63 Ni-based intermetallic catalysts such as Ni3Sn2/TiO264 and NiIn/MgO65 were also reported to be effective for chemoselective hydrogenation of various unsaturated aldehydes. These catalysts show good catalytic performances comparable to that of Pt3Sn/SnO2/rGO, although more severe reaction conditions are required (3 MPa H2, 110−140 °C). This might be due to the intrinsic hydrogenation ability of Pt, which is superior to that of Ni. X-ray adsorption fine structure (XAFS)

intermetallic nanoparticles, where the size, shape, and dispersity of the parent particles are retained. Schaak et al. reported that a variety of intermetallic nanopartilces were prepared successfully using this method: for example, Au−Cu,41 Pt−M (M = Sn, Sb, Fe, Bi, Pb, Cu),42,43 and M′−Sn (M′ = Fe, Ni, Co, Pd).42 For bimetallic combinations containing early transition metals, the preparation of supported intermetallic compounds is a challenging task. This is because the precursors of these metals are difficult to reduce to the metallic states by a typical hydrogen reduction treatment at high temperatures (400−800 °C) due to the very low reduction potentials (typically < −1.5 V vs normal hydrogen electrode (NHE)).44, For example, a reduction temperature of 900 or 1000 °C with flowing hydrogen is required to convert Pt−Ti4+ or Pt−Nb5+ composite to Pt3Ti45 or Pt3Nb46 phase, respectively. A strong reducing agent such as LiHBEt333,47 or sodium napthalenide34 in the liquid phase (Figure 4d) was reported to be effective for preparing intermetallic Pt3M (M = Ti, Zr, and Nb). Metal precursors with highly negative reduction potentials such as Sc3+ or lanthanide cations (typically < −2.0 V vs NHE)44 are almost impossible to be reduced using chemical agents. Intermetallic compounds containing such metals can be prepared as bulk materials by conventional metallurgical alloying (Figure 4f). However, considering the very low reduction potentials of these metals, the resulting intermetallic phase is likely to be highly sensitive to air and moisture. For instance, the surface of Pd-based intermetallic compounds containing Ti or Nb undergoes oxidative decomposition into monometallic Pd and the oxide phases of the counterpart metal even in purified Ar atmosphere containing only a trace amount of O2 (0.6 MPa) and high reaction temperatures (>110 °C) to work well. In contrast, Rh-based intermetallic compounds such as RhIn/SiO281 and Rh0.75Ni0.25 nanoparticles capped by octadecylamine84 were reported to be

Scheme 1. Selective Hydrogenation of Various Unsaturated Aldehydes over Pt3Sn/SnO2/rGO

analysis and DFT calculations for the NiIn system revealed that charge transfer from In to Ni occurs upon alloying, which produces electron-rich Ni and electron-deficient In sites.65 The authors propose that such polarized sites prefer CO adsorption rather than CC adsorption, allowing preferential hydrogenation of the CO moiety. They also point out that isolation of Ni atoms by In (the so-called ensemble effect) decreases CC adsorption. Intermetallic compounds with different Ni/In ratio (Ni2In, Ni3In, NiIn, and Ni2In3) were also tested in this system. As the Ni/In ratio decreased, the selectivity for CO hydrogenation increased, whereas the catalytic activity decreased. This trend is consistent with the rationale they proposed. These interpretations are essentially similar to those found for the aforementioned Pt−Sn system. Other than Sn or In-containing intermetallic compounds, RuTi/SiO237 and unsupported PtZn66 were also tested for crotonaldehyde hydrogenation, in which the selectivity to crotyl alcohol was ca. 40%. Interestingly, cofeeding ppm levels of

Scheme 2. Hydrogenation of Various Nitroarenes to Aminoarenes Using RhIn/SiO2 and Rh0.75Ni0.25 Catalysts

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ACS Catalysis Table 1. Catalytic Performances of the Reported Intermetallic and Alloy Catalysts for Acetylene Semihydrogenation

a

entry

catalyst phase/support

temp (°C)

feeding ratio (C2H2:H2:C2H4)

GHSV (mL gcat−1 h−1)

conv (%)

sel. (%)a

ref

1 2 3 4 5 6 7 8 9 10 11 12 13

Pd0.2Ag0.8 PdAu/TiO2 PdGa PdGa/Al2O3 Pd2Ga Pd2Ga/Al2O3 Pd2Ga/C PdZn/ZnO Ni3Sn Ni3Ge/MCM-41 Ni0.25Zn0.75/MgAl2O4 Ni3Ga/MgAl2O4 Fe4Al13

200 200 200 200 200 200 200 80 250 250

1:10:100 1:3:96 1:10:100 1:10:100 1:10:100 1:10:100 1:10:100 1:10:20 1:5:0 1:2:0 1:10:20 1:20:100 1:10:100

9000 63 000 4500 24 000 180 000 18 000 7 500 000 180 000 closed

85 60 66 84 95 88 90 100 100 94 97 90 81

49 (88) 77 82 75 66 58 (96) 68(>99) 89(98) (98) 80 84(90)

32 94 32 32 32 32 97 98 99 100 101 102 104

200 200

40 000 90 000

Ethylene selectivity in all products. Values in parentheses indicate ethylene selectivity in C2 molecules.

3.1.1.3. Alkyne to Alkene. Semihydrogenation of alkyne to alkene is also one of the most widely studied reaction categories in the area of fundamental and applied chemistry. Acetylene semihydrogenation is an important industrial process used to purify crude ethylene containing a small amount of acetylene, which terminally deactivates ethylene polymerization catalysts.89 Conversion of aliphatic and functionalized alkynes to the corresponding alkenes is also frequently employed in organic syntheses.90 Protecting the produced CC bond, which is easily hydrogenated to a saturated C−C bond in hydrogenation conditions, is the key concern for this reaction. In general, conversion of alkyne proceeds successively to alkene and alkane. However, direct conversion to ethane via surface ethylidene species has also been proposed for acetylene hydrogenation.91 Pd- and Ni-based catalysts are known to be effective for selective alkyne semihydrogenation. For example, a Pd−Ag alloy has been utilized as an acetylene semihydrogenation catalyst for ethylene purification in industrial process.92,93 A number of intermetallic compounds have also been reported to be effective catalysts: e.g., AuPd/TiO2,94 PdmGa (bulk and Al2O3- or C-supported, m = 1 or 2),31,32,95−97 PdZn/ZnO,98 Ni3Sn,99 Ni3Ge/MCM-41,100 Ni−Zn/MgAl2O4,101 and Ni3Ga/ MgAl2O4.102 In addition, other 3d transition-metal-based intermetallic compounds such as CoM (M = Al, Co, and Ge)103 and Fe4Al13104 catalyze acetylene hydrogenation and display high selectivity to ethylene. The enhanced selectivity to ethylene has often been explained by the weakened adsorption of ethylene.105,106 The adsorption energy of alkene on Pd typically becomes less negative in the presence of the second metal atoms adjacent to Pd atoms, which accelerates alkene desorption, thereby inhibiting overhydrogenation.106 However, alkyne adsorption is also weakened, which lowers the hydrogenation rate of alkyne. Therefore, alkene yields generally exhibit a volcano-type relationship to the adsorption properties of the surface. Table 1 summarizes the catalytic performance of reported intermetallic and alloy catalysts for acetylene semihydrogenation. Detailed information such as the list of feed gas space velocities can be found in the literature. In most cases, small amounts of C4 byproducts such as butenes are yielded by dimerization of C2 molecules. Note that ethylene selectivity is sometimes reported as the percentage of ethylene in C2 molecules (not including C4 molecules), which is shown in parentheses in Table 1. In general, Ni-based catalysts require higher reaction temperatures than Pd-based catalysts to obtain

active in very mild conditions. RhIn/SiO2 acts as a highly active and selective catalyst for hydrogenation of various nitroarenes including nitrostyrene under 1 atm H2 at 75 and 25 °C. As shown in Scheme 2 (left part), a variety of nitro arenes with vinyl, cyano, styryl, acetoxy, acetyl, formyl, chloro, bromo, and iodo groups were converted into the corresponding aminoarenes with high yields.81 Note that monometallic Rh/SiO2 preferentially catalyzes undesired CC hydrogenation and gave a negligible amount of the desired aminoarene. Chemoselective conversion can be achieved using a specific surface structure of RhIn, which is explained in detail in section 4.3. Although Rh0.75Ni0.25 displays similar catalytic performance for various functionalized nitroarenes, no substrate containing an alkenyl group was tested (Scheme 2, right part). For the Rh−Ni system, alloys with Ni contents higher than 0.33 show low selectivities (99%, above 40 °C), while the parent Ni2Ce was almost inactive (∼0% at 40 °C). The drastic enhancement in hydrogenation ability was explained by a direct involvement of the adsorbed hydrogen at the surface in ethylene hydrogenation. During hydrogenation of inner alkenes, isomerization to another stereoisomer also proceeds as a side reaction. This reaction takes places by C−C rotation of the alkyl intermediate, followed by β-hydrogen elimination as a reverse reaction of the initial hydrogen addition to alkene. Scheme 3 outlines the

high conversion rates. Among these catalysts, PdZn/ZnO exhibits a remarkably high catalytic activity and selectivity even at a reaction temperature of 80 °C (Table 1, entry 8), which is much lower than that for other Pd-based catalysts, typically 200 °C. The outstanding high catalytic activity is explained by a high H2 dissociation ability and the capacity for moderate di-σ coordination of acetylene to Pd, unlike in the case of other Pdbased intermetallic compounds such as PdGa107 or PdAl.108 The barrier for H2 dissociation on PdZn(100) of 39 kJ mol−1 is lower than that on PdGa(210) (49 kJ mol−1 at minimum)109 and close to that on Pd(111) (20 kJ mol−1).110 Armbrüster and the corroborators reported that PdGa and Pd2Ga could be prepared as supported or unsupported nanoparticles that display good ethylene selectivities (58−82%) at sufficiently high acetylene conversions (66−95%) (Table 1, entries 3−7). It is worth mentioning that a Ni0.25Zn0.75 catalyst (Table 1, entry 11) was discovered through theoretical predictions by Nørskov et al. They employed the heat of adsorption of methyl as a single descriptor representing catalytic activity and selectivity and surveyed the heat of adsorption for a number of bimetallic combinations. They discovered that intermetallic NiZn displayed a similar heat of adsorption of methyl to that of PdAg and demonstrated that high selectivity was indeed obtained when supported Ni−Zn catalysts were used. It should also be noted that Fe4Al13 shows a very high catalytic activity that is comparable to Pd-based catalysts despite its low specific surface area (∼20 m2g−1), and it has sufficient selectivity (Table 1, entry 13) compared to that of an industrial benchmark. The use of ubiquitous metal elements and their good catalytic performances makes them a fascinating alternative to noble metal catalysts. Liquid-phase hydrogenation of aliphatic alkynes was studied using Pd2Ga,111 and PdZn/ZnO,112 which showed high alkene selectivities. For functionalized alkynes, selective hydrogenation to the corresponding alkenes was achieved using Pd3Pb/SiO2 and Pd3Bi/SiO2.106 Intermetallic Pd3Pb shows higher alkene selectivity than other Pd-based bimetallic catalysts such as Pd0.5Ag0.5, Pd2Ga, PdZn, and the commercial Lindlar catalyst (Pd−Pb/CaCO3). Note that the states of Pb atoms in intermetallic Pd3Pb and Pd−Pb/CaCO3 differ significantly: Pb atoms comprise the lattice of Pd3Pb but are deposited as oxides on the surface of monometallic Pd, respectively.113 A DFT study on these Pb-containing materials suggested that the Pd3Pb(111) surface had much lower heat of adsorption of alkyne and alkene than the Pb-deposited Pd(111) surface.106 3.1.1.4. Diene to Monoene. Selective hydrogenation of diene to monoene requires inhibition of overhydrogenation of the product alkene to alkane. Similarly for alkyne semihydrogenation, diminishing the alkene adsorption by alloying has been often employed. Liquid-phase hydrogenation of 1,5hexadiene was performed using a Pd−Ag alloy supported on alumina.114,115 Alkene selectivity was significantly improved by alloying with a small amount of Ag (Pd0.8Ag0.2), resulting in a 77% yield of hexenes at 100% conversion. However, inner alkenes such as 2-hexenes were also formed by 1-hexene isomerization. Gas-phase hydrogenation of 1,3-butadiene was studied using PtxGe/SiO2 (x = 1, 2, and 3).116 Pt3Ge exhibited excellent selectivity (95%) to butenes at 100% conversion. Although Pt2Ge and PtGe showed good selectivities, the reaction rate significantly decreased as the Ge content increased. In addition, some Pt-based alloys (Pt0.8M0.2, M = Ni, Co, or Fe) were tested for 1,3-butadiene hydrogenation,

Scheme 3. Reaction Mechanism of Hydrogen-Mediated Alkene Isomerization and Hydrogenation Known as Horiuchi−Polanyi Mechanism

hydrogenation and isomerization of alkene by hydrogen, which is known as the Horiuchi−Planyi mechanism.121 The thermodynamically stable (E)-isomer is typically generated as an isomerization product. However, for monometallic and usual bimetallic catalysts, overhydrogenation to alkane inevitably occurs because hydrogen atoms can attack the alkenyl carbon from various directions. Therefore, it is difficult to selectively catalyze alkene isomerization in the presence of hydrogen. Recently, Rh- and Ru-based intermetallic compounds belonging to the orthorhombic Pnma space group, such as RhSb, were reported to selectively catalyze (Z) to (E) isomerization of various inner alkenes.122,123 A unique surface stereochemistry governed by a specific bimetallic structure allows one-atom 741

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experimental133,134 and theoretical135,136 studies revealed that the intermetallic Pt3Sn phase shows both high catalytic activity and selectivity among various Pt−Sn phases such as Pt−Sn alloys (Sn content