The emergence of Heusler alloy catalysts

ABSTRACT

For over a century, Heusler alloys (X₂YZ; X, Y: transition metals, Z: typical metals) have attracted attention as magnetic materials. Nowadays, they are also popular as thermoelectric and shape memory materials. However, until very recently, Heusler alloys had been almost unknown as catalysts. We conceived that they could be ideal candidates for catalysts because (1) they have so many elemental sets and (2) they can be partially substituted with other elements (e.g. X₂YZ_1–xZ’_x) in many cases. We first investigated a variety of Heusler alloys as catalysts and then discovered effective catalysts without precious metals for the selective hydrogenation of alkynes based on the aforementioned feature (1) and demonstrated a systematic tuning of catalytic properties by applying the feature (2). Heusler alloy catalysts have also recently been studied by other groups; hence, this traditional alloy group is becoming a new stream in terms of catalysis. In this article, we review the current progress on Heusler alloy catalysts and describe their future prospects.

Graphical Abstract

KEYWORDS:

• Heusler catalysts
• Heusler alloy catalysts
• Heusler compound catalysts
• intermetallic compound catalysts
• intermetallic catalysts
• selective hydrogenation
• semi hydrogenation
• partial hydrogenation
• tuning of catalytic
• noble metal free catalysts

CLASSIFICATION:

• 60 New topics / Others
• 106 Metallic materials
• 205 Catalyst / Photocatalyst / Photosynthesis

1. Introduction

The words ‘intermetallic compounds’ and ‘ordered alloys’ are often used synonymously. In a narrow sense, intermetallic compounds have no order–disorder transition temperature below a melting point and have a limited allowable composition range, while ordered alloys have the transition temperature and a relatively wide composition range, as shown in Figure 1(a,b) [1,2]. In an A-B alloy, an intermetallic compound with highly covalent bonds will form if the A-B bond is stronger than the A-A and B-B bonds, but an ordered alloy with a weakly covalent bonding will form if the A-B bond is moderately stronger. Thus, there are many intermetallic compounds in alloys consisting of transition metals and typical metals. However, many ordered alloys consist of only transition metals. Heusler alloys (Heusler compounds) (X₂YZ) typically consist of transition metals in groups 8–12 for X, 3–8 for Y, and typical metals in group 13–15 for Z, as shown in Figure 2 [3–5]. Depending on elemental sets of X, Y, and Z, they are typical intermetallics, typical ordered alloys showing transitions of L2₁ → B2 → A2 (body-centered cubic, bcc; Figure 3(a–c)), or in-between alloys only showing the L2₁ → B2 transition. Note that there are also D0₃ phases with disordering between X and Y atoms, half-Heusler, inverse Heusler, and quaternary Heusler alloys, as shown in Figure 3(d–g).

Figure 1. Phase diagrams of (a) Al-Pt system [1] forming intermetallic compounds and (b) Fe-Pt system [2] forming ordered alloys. The diagrams were traced from National Institute for Materials Science (NIMS) AtomWork <http://crystdb.nims.go.jp/> [1,2].

Figure 2. Schematic illustration of the elemental substitution of Heusler alloys (X₂YZ) [3] and typical components of X, Y, and Z (highlighted in periodic table). Other elements such as lanthanoids and those in groups 1 and 2 are also possible components (see Ref [4]). The figure was excerpted from Ref [5], ©The Authors, some rights reserved; exclusive license American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC http://creativecommons.org/licenses/by-nc/4.0).

Figure 3. Crystal structures for (a) L2₁ (full-Heusler, Fm–3m), (b) B2 (Pm–3m), (c) A2 (bcc, Im–3m), (d) D0₃ (Fm–3m), (e) C1_b (half-Heusler, F–43m), (f) Li₂AgSb or Hg₂CuTi (inverse Heusler, F–43m), and (g) LiMgPdSn (quaternary Heusler, F–43m) phases, where symbols in parentheses represent space groups [3]. In (b), the Y and Z atoms are mixed. In (c), all atoms are mixed. In (d), X and Y elements are mixed. In (e), half of X atoms are absent, so that the structure is called a half-Heusler (XYZ) in contrast to a full-Heusler (X₂YZ), in a narrow sense. In (f), additional Y atoms occupy the vacant sites of the half-Heusler. In (g), the fourth elements (A) occupy the vacant sites of the half-Heusler. (f) and (g) show the prototype names because their Strukturbericht symbols (like L2₁) are not commonly used. For (f), there are two prototype names for the same structure.

Heusler alloys have great features. There are so many possible sets of X, Y, and Z (Figure 2). This feature predicts a discovery of new catalysts, and enables them to substitute each other (e.g. X₂YZ_1–xZ’_x). The substitution of Heusler alloys is common practice in other fields, especially because their electronic structures can be controlled in a rigid band-like fashion [6,7]. Thus, the substitution would be useful for tuning catalytic properties through controlling electronic structures (ligand effect) and also through changing surface elements (ensemble effect), as shown in Figure 2. However, Heusler alloys were almost unknown as catalysts even though they have been popular as magnetic (spintronic), thermoelectric, and shape memory materials [6–8]. Especially, they have attracted attention of magnetic researchers since the discovery of Cu₂MnAl over a century ago [9,10] because they display ferromagnetism without ferromagnetic elements. Focusing on these features, we started a study on Heusler alloys as new catalysts, and experimentally investigated their catalytic properties [11,12]. In this study, we roughly revealed the catalytic properties of Heusler alloys by investigating 12 alloys for the hydrogenation of propyne and the oxidation of carbon monoxide. Recently, we discovered effective catalysts without noble metals for the selective hydrogenation of alkynes and achieved a systematic control of catalytic properties by elemental substitution, by using the features of Heusler alloys [5]. In this article, we review our studies in Sections 2 and 3, and briefly refer to other studies on Heusler catalysts in Section 4 [13–15]. Finally, we summarize our studies and describe future prospects in Section 5.

2. Experimental details

Details are described in the previous papers [5,11,12]. Alloy ingots were prepared by arc-melting from pure metallic pieces (purity ≥ 99.9%). They were annealed in the Ar atmosphere for homogenizing compositions (typically 1000 °C) and ordering atomic positions (typically 500–600 °C) and then crushed using a mortar and pestle. The powders obtained were sieved to a particle size of 20–63 μm, and then used as catalysts. The sieving was necessary to exclude as many uncertain factors as possible. X-ray diffraction (XRD) was performed for a powder sieved to <20 μm after annealing at 600 °C for 1h to remove the defects introduced by crushing. The XRD confirmed that all samples were (almost) a single phase of the L2₁ structure with high order. The Brunauer–Emmett–Teller method for Kr adsorption isotherms at 77 K yielded the surface area of catalysts as 0.05–0.13 m² g^–1 mainly depending on atomic weights.

Catalytic tests were conducted using a standard fixed-bed flow reactor built using Swagelok® parts made of 316 stainless steel. An appropriate amount of catalyst (typically 0.4 g) was placed into a quartz tube (4-mm internal diameter) and then inserted into an electric furnace. Before the reaction, gas lines, including gas regulators, were purged enough to remove air contamination accumulated due to a slight leakage at the joints, and the catalyst was heated at 600 °C for 1h under hydrogen flow for removing surface oxides and defects. After the introduction of a reaction gas at room temperature at a flow rate of 30 cm³ min^–1, heating was started to measure temperature dependence. Unreacted and produced gases were analyzed using a gas chromatograph.

3. Results and discussion

3.1. Oxidation of carbon monoxide [11,12]

The 12 Heusler alloys listed in Table 1 were investigated for the oxidation of carbon monoxide using a reactant of [1.2%CO/0.4%O₂/He balance], the concentration of which was CO rich to suppress an irreversible oxidation of catalysts. Figure 4 shows conversions of CO during continuous heating by 2.5 °C min^–1. In pure X, the hierarchy, Cu < Co < Ni < Fe, was observed of temperature, in which the CO conversion began to increase, while Co achieved a high conversion close to its limit (66.7%) at lower temperature than others, as shown in Figure 4(a). This temperature hierarchy of X and the high conversion of Co seemed to be basically followed by X in X₂TiSn, X₂TiAl, and X₂MnSn, as shown in Figure 4(b–d), although the hierarchy of Fe and Ni in X₂TiSn was opposite. This indicates that a main active element is X in X₂YZ for CO oxidation.

Table 1. Heusler alloy catalysts used in the subsections 3.1 and 3.2. Top row: X, left column: Y, and cell: Z, in X₂YZ.

X Y	Fe	Co	Ni	Cu
Ti	Sn	Al, Ge, Sn	Al, Sn	Al
Mn	–	Si, Ge, Sn	Sn	–
Fe	–	Ge	–	–

Figure 4. CO conversion in CO oxidation using [1.2%CO/0.4%O₂/He balance] reactant during a continuous heating, for (a) pure X powders, (b) X₂TiSn, (c) X₂TiAl, and (d) X₂MnSn. Colors and symbols in (b–d) correspond to those in (a). Horizontal line at 66.7% is a guide for describing the maximum conversion calculated from the CO:O₂ ratio. After Ref [11], reproduced with permission by ACS, https://pubs.acs.org/doi/10.1021/acsomega.6b00299. Further permissions related to the material excerpted should be directed to the ACS.

Figure 5 shows CO conversions during cycles of heating and cooling between 80 and 600 °C for catalysts with a total surface area of 0.027 m². Most Heusler alloys exhibit a hysteresis as Fe₂TiSn and Ni₂TiSn, as shown in Figure 5(a,b). This was due to the irreversible oxidation of catalysts, which was revealed by estimating an excess consumption of O₂ from a consumption ratio of CO and O₂. In contrast, Co₂TiSn showed a small hysteresis, as shown in Figure 5(c). This was due to high oxidation resistance, which was indicated by a small excess O₂ consumption and by a tiny XRD peak of oxide even after the reaction under O₂-rich conditions. These results indicate that we can control an activity by choosing X and a durability by Y and Z.

Figure 5. CO conversion in CO oxidation using [1.2%CO/0.4%O₂/He balance] reactant during heating and cooling cycles, for (a) Fe₂TiSn, (b) Ni₂TiSn, and (c) Co₂TiSn. After Ref [11], reproduced with permission by ACS, https://pubs.acs.org/doi/10.1021/acsomega.6b00299. Further permissions related to the material excerpted should be directed to the ACS.

3.2. Hydrogenation of propyne [11,12]

The alloys listed in Table 1 were investigated for the hydrogenation of propyne (C₃H₄) using a reactant of [1%C₃H₄/55%H₂/He balance]. Figure 6 shows conversions of C₃H₄ for catalysts with a total surface area of 0.027 m². Most Heusler alloys showed a too small conversion, as well as Co₂TiSn. This small conversion was indicated due to residual surface oxides and/or segregated Sn at the surface, because most alloys contain elements having very stable oxides such as Al, Si, and Ti, and because Sn atoms tend to segregate at the surface [11,12]. Co₂FeGe, Co₂MnGe, and pure Co showed a certain conversion, as shown in Figure 6, whereas pure Fe, Mn, and Ge revealed almost no activity. This indicates that a main active element was X for C₃H₄ hydrogenation as well as for CO oxidation. The conversion for Co₂FeGe (50%Co) was larger than that for pure Co (100%Co). This indicates that a new electronic structure formed by the alloying of Co, Fe, and Ge was more suited for the C₃H₄ hydrogenation than pure Co. These results indicate that we can control an activity by choosing not only the main active element, X, but also sub-elements, Y and Z, through the creation of new electronic structures.

Figure 6. C₃H₄ conversion in C₃H₄ hydrogenation using [1%C₃H₄/55%H₂/He balance] reactant. The data was obtained during the second cooling cycle of continuous heating and cooling, in which a temporal change in conversion was well settled. After Ref [11], reproduced with permission by ACS, https://pubs.acs.org/doi/10.1021/acsomega.6b00299. Further permissions related to the material excerpted should be directed to the ACS.

3.3. Selective hydrogenation of alkynes [5]

3.3.1. High alkene selectivity

The investigation in Section 3.2 indicated that only Co₂MnGe and Co₂FeGe had an activity for the hydrogenation of hydrocarbons in the alloys listed in Table 1. Thus, we further investigated them for selective hydrogenation of alkynes, focusing on selectivity. This reaction is important for removing alkyne impurities from alkene feedstocks because the impurities inhibit the polymerization process of alkenes [16,17]. In industry, Pd-based catalysts have been used for selective hydrogenations of ethyne (C₂H₂) in ethene (C₂H₄) and propyne (C₃H₄) in propene (C₃H₆) [16–18]. We found that non-noble metal catalysts, Co₂MnGe and Co₂FeGe, had a high alkene selectivity with these reactions.

Figures 7(a–c) present results on the C₃H₄ conversion and the C₃H₆ selectivity for C₃H₄ hydrogenation using a reactant of [0.1%C₃H₄/40%H₂/He balance]. For Co₂FeGa_0.75Ge_0.25 in Figure 7(c), the selectivity was high when the conversion was low, decreasing as the conversion increased, which is typical behavior for ordinary catalysts because of the strong adsorption of alkynes inhibiting re-adsorption of alkenes [19–21]. In contrast, Co₂MnGe and Co₂FeGe showed a high selectivity even when conversion was at 100%, as shown in Figure 7(a,b). Figure 7(d–f) show a C₃H₆ conversion for C₃H₆ hydrogenation without C₃H₄ using a reactant of [0.1%C₃H₆/40%H₂/He balance]. Co₂FeGa_0.75Ge_0.25 converted C₃H₆ to C₃H₈ in 100% even at room temperature, while Co₂MnGe and Co₂FeGe showed only a small conversion in the whole temperature. Thus, Co₂MnGe and Co₂FeGe do not have the ability to hydrogenate alkenes to alkanes, which can be called ‘intrinsic selectivity,’ unlike Pd-based catalysts that need precise controls of reaction conditions to ensure the selectivity [16,17].

Figure 7. (a–c) C₃H₄ conversion and C₃H₆ selectivity in C₃H₄ hydrogenation using [0.1%C₃H₄/40%H₂/He balance] reactant, and (d–f) C₃H₆ conversion in C₃H₆ hydrogenation using [0.1%C₃H₆/40%H₂/He balance] reactant, for (a,d) Co₂MnGe, (b,e) Co₂FeGe, and (c,f) Co₂FeGa_0.75Ge_0.25 catalysts. The figure was remade using the data in Ref [5], ©The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC, http://creativecommons.org/licenses/by-nc/4.0).

Owing to this property, they also showed high selectivity even for the hydrogenation of alkyne impurities in alkene feedstocks using a reactant of [0.1%C₃H₄ or C₂H₂/10%C₃H₆ or C₂H₄/40%H₂/He balance], as shown in Figure 8(a,b), in contrast to the example for Co₂FeGa shown in Figure 8(c). An excessive H₂ concentration in the reactants, alkyne:H₂ = 1:400, also indicated the great selectivity, because the alkyne:H₂ ratio is usually 1:10–20 in other reports, including the reports on binary intermetallic compounds [16,20,22,23].

Figure 8. Alkyne conversions and alkene selectivities in alkyne hydrogenations in the presence of alkenes for (a) Co₂MnGe, (b) Co₂FeGe, and (c) Co₂FeGa. The reactant was [0.1%C₃H₄ or C₂H₂/10%C₃H₄ or C₂H₄/40%H₂/He balance]. After Ref [5], © The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC, http://creativecommons.org/licenses/by-nc/4.0).

3.3.2. Control of catalytic properties by elemental substitution

We investigated a change in catalytic properties by elemental substitution (Co₂Mn_xFe_1–xGa_yGe_1–y). Figure 9 summarizes the effects due to substitutions of Mn for Fe and Ga for Ge in Co₂FeGe, using [0.1%C₃H₄/40%H₂/He balance] in reaction. For Mn substitution, the C₃H₆ selectivity showed no change, while the C₃H₄ reaction rate increased with Mn substitution, as shown in Figure 9(a). On the other hand, Ga substitution significantly decreased the selectivity, while increasing the reaction rate. In Figure 9(b), we can see a strong correspondence between the apparent activation energy (E_a) estimated from the rate, and a d-band center (ε_d) estimated from the density-of-state (DOS) curves calculated for bulk states. This means that the change in electronic structures affected the elementary steps of the reaction. Hard X-ray photoelectron spectroscopy confirmed that actual specimens had valence band structures corresponding to the calculated DOSs, which means that both the sample preparations and the calculations were well conducted. Note that the bulk ε_d can be used for relative comparison among the alloys, with the same structure consisting of similar elements because the change in DOSs, from bulk to surface, originates from the symmetry breaking at the surface [24]. In addition, Mn substitution for Fe in Co₂FeGa and Ga substitution for Ge in Co₂MnGe showed similar changes in catalytic properties to those in Figure 9(a), which indicates that the effects of Mn-Fe and Ga-Ge substitutions are independent of each other.

Figure 9. Elemental substitution effects on (a) C₃H₄ reaction rate at 50 °C and C₃H₆ selectivity at 200 °C, and (b) experimental E_a and calculated ε_d in Mn substitution (Co₂Mn_xFe_1–xGe) and Ga substitution (Co₂FeGa_yGe_1–y) for Co₂FeGe. The rate is per surface area and relative to that of Co₂FeGe. The figure was remade using the data in Ref [5], ©The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC, http://creativecommons.org/licenses/by-nc/4.0).

3.3.3. Possible mechanisms of substitution effects

The Mn substitution certainly changed the electronic structures, increasing ε_d and causing the monotonic decrease in E_a with the Mn composition, but not causing the monotonic increase in the reaction rate, as shown in Figure 9. We built a hypothesis that this lack of correspondence between E_a and rate was due to the enthalpy-entropy compensation [25]. For H₂ adsorption, the increase in ε_d strengthens the bond between H and catalyst atoms, which decreases (negatively increases) not only the adsorption enthalpy (ΔH_H2,ad), but also the adsorption entropy (ΔS_H2,ad) due to the reduction of freedom. This phenomenon is the compensation effect [25]. In the Arrhenius equation of the reaction rate, r = Aexp(–E_a/RT), the apparent frequency factor (A) and the E_a have the relations: A ∝ exp(ΔS/R) and exp(–E_a/RT) ∝ exp(–ΔH/RT), where ΔS and ΔH are entropy and enthalpy changes for related elementary steps. Thus, if a reaction mechanism is fixed with a change in ε_d, a reduction in E_a with ε_d should originate from a reduction in ΔH, which diminishes ΔS, and eventually A. We believe that such compensation was the reason why the rate did not increase monotonically with the reduction in E_a. See [5] for details.

However, the increase in the rate due to Ga substitution seemed reasonable. In addition, the selectivity significantly decreased, unlike in Mn substitution. These results can be explained by assuming that Ga atoms contribute to molecular adsorption, but Ge atoms do not. The high selectivity of Pd-based catalysts is believed to originate from small active sites surrounded by poisonous species, which sterically inhibit the adsorption of larger molecules, alkenes, but allow the adsorption of smaller molecules, alkynes [16,17]. Figure 10(a) shows schematic illustrations of (110) surfaces for Co₂FeGe, Co₂FeGa_0.5Ge_0.5, and Co₂FeGa. These indicate that the size of adsorption sites is small for Co₂FeGe, while it is enlarged by Ga substitution, if Ge atoms have no ability for molecular adsorption but Ga atoms do. Thus, Co₂FeGe should have a high alkene selectivity, while Ga substitution should decrease the selectivity. In Figure 10(a), we can also see an increase in the number of sites available for adsorption due to Ga substitution, which indicates an increase in the reaction rate. We believe that this hypothesis was the reason for the Ga substitution effects.

Figure 10. (a) Schematic illustrations of (110) surfaces, and (b) local partial DOS (LPDOS) of Ga 4p and Ge 4p bands at Co₂FeGa(110) and Co₂FeGe(110) surfaces, respectively. The figure was excerpted from Ref [5], ©The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. Distributed under a Creative Commons Attribution NonCommercial License 4.0 (CC BY-NC, http://creativecommons.org/licenses/by-nc/4.0).

We verified the difference of adsorption ability between Ga and Ge. Figure 10(b) shows local partial 4p DOSs of Ga and Ge at (110) surfaces for Co₂FeGa and Co₂FeGe, respectively. We can see that the Ga band has higher energy than the Ge band, which indicates that Ga atoms have a higher ability for molecular adsorption than Ge atoms. In addition, theoretical calculations have indicated that typical metals gain the adsorption ability through the hybridization of their p-bands with the d-bands of transition metals in intermetallic compounds [26,27].

4. Other research

There is a catalytic study on Cu₂MnAl published in 1935 [13]. We did not know the existence of this paper, written in German, until the recent appearance of a patent on Heusler alloy catalysts [14], which mentioned that 1935 work. Hedvall had intensively investigated the relationship between ferromagnetism and catalysis, and reported in this paper that an increase in CO₂ yield over Curie temperature was observed in the catalytic reaction of 2CO →  CO₂ + C by Cu₂MnAl, as well as in the hydrogenations of CO and C₂H₄ by Ni [13].

The patent was filed by BASF [14], which claimed the supported fine-particle catalysts of Heusler alloys with X = Mn, Fe, Co, Ni, Cu, and Pd; Y = V, Mn, Cu, Ti, and Fe; Z = Al, Si, Ga, Ge, In, Sn, and Sb. Hereafter, we will only mention the experimental data described in the patent. Table 2 lists the catalysts, which were prepared by impregnation using fumed silica or gamma alumina as a support, and methanol or water as a solvent, followed by annealing under a gas flow of pure H₂, 5%H₂/N₂, or 10%H₂/N₂. The XRD indicated that most samples consisted of A2 (bcc) main phase and A1 (face-centered cubic, fcc) secondary phase. The formation of L2₁ and B2 phases could not be monitored because their fingerprint peaks were hidden by the halo peak of amorphous silica. The XRD patterns for Samples 12 and 13 are likely from the A1 single phase. We cannot evaluate the phases of the catalysts supported on the alumina because the XRD patterns were mostly from the alumina. The distribution of particle size was evaluated for Sample 8. Figure 11(a) shows the size distribution for smaller particles (mainly < 400 nm) observed by high-angle annular dark field scanning transmission electron microscopy. The median particle size was evaluated as 86.6 nm in these particles. Figure 11(b) shows the size distribution for larger particles (mainly > 400 nm) observed by backscattered electron images from scanning electron microscopy, and indicates that the size was distributed around 800 nm for the particles of > 400 nm.

Table 2. List of Heusler catalysts with preparation conditions, including phases indicated by XRD and catalytic properties in Ref [14]. ‘Up’ and ‘Down’ for Knoevenagel condensation means that activity was higher or lower than that for SiO₂ support without Heusler catalysts, respectively. Phases in samples 14–17 were not determined (n.d.).

No.	Alloy	Support	Solvent	Annealing atmosphere	XRD	Activity for Knoevenagel condensation	Selective reduction of NO
1	Co2FeGa	SiO2	Methanol	H2	A2 + A1	Down	–
2	Co2FeAl	SiO2	Methanol	H2	A2 + A1	Up	–
3	Co2FeSi	SiO2	Methanol	H2	A2 + A1	Down	–
4	Co2FeIn	SiO2	Methanol	H2	A2 + little A1	Down	–
5	Co2FeGa	SiO2	Water	H2	A2 + A1	Down	–
6	Co2FeAl	SiO2	Water	H2	A2 + little A1	Up	–
7	Co2FeSi	SiO2	Water	H2	A2 + little A1	Up	–
8	Co2FeGa	SiO2	Water	5%H2/N2	A2 + A1	Up	–
9	Co2FeAl	SiO2	Water	5%H2/N2	A2 + A1	Up	–
10	Co2FeSi	SiO2	Water	5%H2/N2	A2 + little A1	Down	–
11	Co2FeIn	SiO2	Methanol	5%H2/N2	A2 + little A1	Probably down	–
12	Cu2FeAl	SiO2	Water	5%H2/N2	A1	–	Moderately active, N2O produced
13	Cu2FeSi	SiO2	Water	5%H2/N2	A1	–	Moderately active, N2O produced
14	Fe2MnGa	Al2O3	Water	10%H2/N2	n.d.	–	Highly active, no N2O
15	Fe2MnSi	Al2O3	Water	10%H2/N2	n.d.	–	Highly active, no N2O
16	Co2CuAl	Al2O3	Water	10%H2/N2	n.d.	–	Highly active, no N2O
17	Fe2TiGa	Al2O3	Water	10%H2/N2	n.d.	–	Highly active, no N2O

Figure 11. Size distribution for (a) smaller particles (mainly < 400 nm) and (b) larger particles (mainly > 400 nm) in Sample 8 listed in Table 2. The figure was excerpted from Ref [14]. with slight rearrangement. See Ref [14]. for details of how to estimate the minimum diameter.

Samples 1–10 (probably also 11) were tested for the Knoevenagel condensation using benzaldehyde and malononitrile. The fumed silica support showed a certain activity. Samples denoted ‘Up’ in Table 2 showed a higher activity than the pure fumed silica. Especially, Co₂FeAl showed the highest activity, while the inventors considered that it was due to the high activity of Al for this reaction. Samples 12–17 were tested for the selective reduction of NO using a reactant of [0.05%NO/0.05%NH₃/5%O₂/10%H₂O/N₂ balance]. Samples 12 and 13 showed a moderate activity for reducing NO and produced a certain amount of N₂O. Samples 14–17 showed a high activity with almost no production of N₂O.

In addition, a thesis exists on the first-principles calculation for NH₃ dissociation on Ni₂MnGa and Co₂CrGe surfaces [15]. The author, Senanayake, considered that a higher DOS at the Fermi energy would result in a lower activation energy for the dissociation, and explored such materials in Heusler alloys. The Co₂CrGe surface had a high DOS, lowering the activation energy to 0.862 eV. The Ni₂MnGa surface lowered it to only 1.309 eV, while calculations on 105 kinds of Ni-Mn-Ga compositions revealed that the Ni₄Mn₁₁Ga surface had a high DOS, lowering it to 0.575 eV. Although the Ni₄Mn₁₁Ga can no longer be called ‘Heusler alloy’ and the preparation is probably impossible, the value was lower than 0.73 eV for pure Fe.

5. Summary and future prospects

5.1. Our studies

Catalytic properties of many Heusler alloys were investigated. The results indicated that X is the main active element, and Y and Z have roles that can change the activity, selectivity, and durability. Especially for the selective hydrogenation of alkynes, non-noble metal catalysts with a great selectivity were discovered, based on the feature that there are so many sets of X, Y, and Z. In addition, the systematic control of catalytic properties by elemental substitution was demonstrated based on the feature that substitution is possible with various elements in a wide composition range. In Co₂(Mn,Fe)(Ga,Ge) for the selective hydrogenation, the Mn-Fe and Ga-Ge substitutions brought the ligand and ensemble effects, respectively, and they were independent of each other. Thus, we expect that the catalytic properties can be fine-tuned for specific target reactions. Aside from the practical benefit, this tunability would be useful for investigating general mechanisms of catalysis by intermetallic compounds, because the effects originating from electronic structures and surface elements can be investigated independently and systematically under the same crystal structure.

Since many Heusler alloys contain easily oxidizable elements, as shown in Figure 2, it is difficult to predict which elemental sets are suited for the reactions involving oxygen. Actually, we have also confirmed the metallurgical and microstructural changes due to oxidation in ongoing studies on the steam reforming of methanol, and the dehydrogenation of 2-propanol. However, some alloys show unique properties depending on the elemental set, like the Co₂TiSn for CO oxidation, which showed high oxidation resistance. Therefore, we believe that there is great promise in using Heusler alloys for various reactions.

5.2. Beyond our studies

Materials informatics could be relevant when seeking effective Heusler catalysts [28]. This method usually requires a large number of data sets, especially in the application of catalysts where the properties vary strongly depending on experimental processing conditions [29]. Heusler alloys are a well-defined platform where the components and compositions can be changed without structural change. This would decrease the number of descriptors, reducing data sets required. Recently, predictions using materials informatics from only several tens to hundreds of data sets were achieved even for catalysts [30]. Thus, effective Heusler catalysts could be found using the informatic approach with catalytic experiments under the same reaction conditions.

There have only been a few studies on Heusler catalysts, since their potential was only recently discovered. However, certain researchers have considered the application of catalysts as mentioned in Section 4, and we succeeded in spotlighting the potential of Heusler alloy catalysts. Thus, more researchers will start to study Heusler catalysts. For practical applications, the patent seems to have not succeeded in developing ‘good Heusler catalysts’ because the L2₁ single phase and a small average particle size (e.g. < 50 nm) with the sharp size distribution were not obtained, and because the properties were not compared to those of practical catalysts.

On the other hand, a certain number of groups have tried to prepare Heusler nanoparticles, however, not for catalytic application. The studies published before 2014 were summarized in the review paper by Felser’s group [31]. L2₁-Co₂FeGa nanoparticles with an average particle size of around 20 nm were successfully synthesized on fumed silica supports by impregnation, although a relatively small amount of extra phases existed in addition to the L2₁ phase [32–35]. The synthesis was also achieved by impregnation into facile carbon nanotubes [36]. The synthesis of L2₁-Co₂FeAl nanoparticles was also reported [37,38]. In addition, Fe₂CoGa nanoparticles were synthesized, while the structure was Li₂AgSb or Hg₂CuTi type, the so-called ‘inverse Heusler’ (Figure 3(f)) [39]. To our knowledge, for other L2₁-Heusler nanoparticles, the size was not small enough, or there was not sufficient information on the size distribution or no proof of the L2₁ phase, like Co₂FeSn [40] and Ni₂MnIn [41].

There are still only a few reports of successful, high-quality preparation of Heusler nanoparticles. However, this seems to reflect a scarcity of attempts at synthesis, because the applications have been considered limited. Now, we have a big application for catalysts. Therefore, we believe that many researchers will start developing a variety of Heusler nanoparticles as catalysts.

Acknowledgments

We are grateful to Prof. S. Fujii of Kagoshima University for the electronic structure calculation and to Dr S. Ueda of National Institute for Materials Science for collaborating with us for the electronic structure evaluation by hard X-ray photoelectron spectroscopy, which was performed at BL15XU at SPring-8 under the approval of NIMS Synchrotron X-ray Station (proposal no. 2017B4905).

Disclosure statement

The authors are inventors on a patent application related to this work filed by Tohoku University (JP patent application no. 2017–220616, filed on 16 November 2017).

Additional information

Funding

A part of this work was supported by the Hattori Hokokai Foundation, the Iwatani Naoji Foundation, the Noguchi Institute, and NIMS microstructural characterization platform as a program of ‘Nanotechnology Platform’ (project no. 12024046) of the Ministry of Education, Culture, Sports, Science and Technology (MEXT), Japan. We also thank the supports from the Program for Creation of Interdisciplinary Research from FRIS, Tohoku University, and from Dynamic Alliance for Open Innovation Bridging Human, Environment and Materials.