Materials Chemistry
Catalytic Characteristics of Component Elements in Heusler Alloys for Hydrogenation of Propyne
2024 Volume 65 Issue 5 Pages 530-533
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2024 Volume 65 Issue 5 Pages 530-533
Intermetallic compounds can be novel catalysts due to their unique structures consisting of multiple elements that occupy specific atomic sites. Heusler alloys (X2YZ) are a ternary intermetallic group consisting of various sets of X, Y, and Z. This group is useful in investigating the catalytic roles of individual elements under the same crystal structure. In this study, we investigated the catalytic characteristics of 3d transition metals and the group 13,14 elements in Heusler alloys for hydrogenation of propyne (C3H4). A larger number of valence electrons for 3d transition metals seemed to result in higher activity for the hydrogenation reaction, as well as pure metal catalysts, the activity hierarchy of which was Ni > Co > Fe. For the group 13,14 elements, the alloys with Al and Si were slightly active, whereas the ones with Ga and Ge were active, in which the Ge alloys were highly selective for producing propylene (C3H6). All the Sn-containing alloys significantly caused side reactions producing C4 and C6 species. This indicates that Sn possesses the ability to crack and couple carbon chains.
Intermetallic compounds often exhibit novel catalytic properties due to unique (surface) crystal and electronic structures that are different from those of component elements, also maintaining high durability due to a thermodynamically stable state compared to pure metal states.1–6) Heusler alloys are ternary intermetallic compounds of X2YZ with the L21-type structure, called “full-Heusler” in a narrow sense, while there are other families of Heusler alloys.7) This alloy group is well-known as magnetic (spintronic), thermoelectric, shape memory, and topological materials.8–12) Recently, their catalytic properties have been investigated, such as for oxidation of carbon monoxide,13) hydrogenation of alkynes,13–16) steam reforming of methanol,17) electrochemical oxygen evolution reaction,18) dehydrogenation of 2-propanol,19) hydrogenation and electrochemical reduction of carbon dioxide,20–23) and ammonia decomposition.16) For selective hydrogenation of alkynes, we found that Co2(Mn or Fe)Ge exhibited high alkene selectivity, and the substitution of Ga for Ge increased the reaction rate while decreasing the selectivity.14) However, Heusler alloys usually involve easily oxidized elements; thus, investigation of essential catalytic properties in a metallic state was possible for a limited number of alloys after the reduction of surface oxides by heating in hydrogen atmosphere.13) In this study, we evaluated the essential catalytic properties of full-Heusler alloys in a metallic state for hydrogenation of propyne (C3H4) by conducting sample preparation and catalytic reaction in a glove box with Ar atmosphere.
A Heusler alloy ingot was synthesized by arc-melting of pure metal sources with purity >99.9%, followed by annealing basically at 800–900°C for 48 h. The ingot was crushed and sieved into 20–63 µm powders as a catalyst sample in the glove box with Ar atmosphere (O2 < 1 ppm). The powder X-ray diffraction (XRD, Rigaku MiniFlex 300 with Cu-Kα radiation) was performed to check whether a target Heusler alloy was obtained correctly. A catalyst weighing 0.5 g (0.75 g for Sn-containing alloys) was loaded into a quartz tube with an inner diameter of 7.5 mm and set to a flow catalytic reactor built in the glove box. Metallurgical synthesis and handling without air exposure enable accurate catalytic evaluation and comparison among different alloy samples.24) Before the catalytic reaction, the catalyst was heated at 600°C for 1 h under H2 gas flow to remove a strain due to crushing and oxygen/water impurity species adsorbed on the catalyst surface. The C3H4 hydrogenation reaction was conducted using a feed gas of [0.1%C3H4/40%H2/59.9%He] at a flow rate of 30 mL (STP) min−1. From room temperature up to 250°C, exhaust gas passed through the catalyst was analyzed by gas chromatography (Agilent 490 Micro GC with PoraPLOT Q column) after keeping the temperature every 25°C for 30 min. Analysis of products by mass spectroscopy (Canon Anelva M-101GA-DM) was also conducted for Ni2TiSn, in which the time for keeping the temperature constant was 15 min.
The conversion of C3H4, the selectivities of C3H6 and C3H8, and the fraction of carbon lost due to side reactions (“carbon loss”) were evaluated by
| \begin{equation} \textit{Conversion}\ (\%) = 100 \times (C_{\text{feed}} - C_{\text{C3H4}})/C_{\text{feed}}, \end{equation} | (1) |
| \begin{equation} \textit{C$_{3}$H$_{6}$ selectivity}\ (\%) = 100 \times C_{\text{C3H6}}/(C_{\text{feed}} - C_{\text{C3H4}}), \end{equation} | (2) |
| \begin{equation} \textit{C$_{3}$H$_{8}$ selectivity}\ (\%) = 100 \times C_{\text{C3H8}}/(C_{\text{feed}} - C_{\text{C3H4}}), \end{equation} | (3) |
| \begin{align} &\textit{Carbon loss}\ (\%)\\ &\quad = 100 \times (C_{\text{feed}} - C_{\text{C3H4}} - C_{\text{C3H6}} - C_{\text{C3H8}})/C_{\text{feed}}, \end{align} | (4) |
where Cfeed is C3H4 concentration in the feed gas (0.1%), and CC3H4, CC3H6, and CC3H8 are C3H4, C3H6, and C3H8 concentrations in the outlet gas, respectively. For catalytic evaluation of pure metals, commercial powders with purity >99.9% (Ni: Rare Metallic; <200 mesh, Co: Rare Metallic; <100 mesh, Fe: Kojundo Kagaku; <53 µm) were used after sieving into 20–63 µm.
The synthesized catalysts were basically a single phase of the full-Heusler (L21-type) structure.13,14) However, the XRD pattern for Fe2TiAl (Fig. 1(a)) displayed a weak peak of 111 superlattice reflection, where the integrated intensity ratio of 111 and 220 peaks was 1.1:100 against the theoretically calculated value of 3.6:100. This indicates significant disorder between Ti and Al atoms. Although XRD peaks of secondary phases were observed for several catalysts,13,14) including Fe2TiAl (Fig. 1(a)), their intensities were small. Even the most significant level of intensity in secondary phase peaks was as small as the peaks in Fig. 1(b) for Co2TiSi. Figure 2 shows differences in the C3H4 conversion depending on X of X2YZ. In X2TiAl and X2TiSn (Figs. 2(a), (c)), X = Ni was most active, X = Co was in second place, and X = Fe was inactive. In X2TiGa, X2MnGa, and X2MnSn (Figs. 2(b), (e), (f)), X = Ni was more active than X = Co, in which there is no thermodynamically stable full-Heusler phase of Fe2TiGa, Fe2MnGa, and Fe2MnSn. This conversion hierarchy on X, Ni > Co > Fe, was the same as pure metal catalysts (Fig. 2(d)). This result indicates that X atoms mainly catalyze the C3H4 hydrogenation reaction, which is the same conclusion for the oxidation of carbon monoxide.13) Figure 3(a) shows the C3H4 conversion by Co2(Ti, Mn, or Fe)(Ga or Ge). Comparisons among Figs. 2(b), (e) and 3(a), and between Figs. 2(c) and 2(f) indicate that the conversion hierarchy on Y is Fe ≥ Mn > Ti. It seems that a larger number of valence electrons for 3d transition metals results in higher activity for the hydrogenation reaction. Although the reason that the Co2TiSi showed a larger conversion than Co2MnSi is unclear (Fig. 3(c)), the secondary phases are the possible origin of the activity.
XRD patterns for (a) Fe2TiAl and (b) Co2TiSi. Insets are enlarged views of small peaks at low diffraction angles. Asterisks in insets indicate peaks from secondary phases, which could not be identified. Low noise levels around 111 and 200 peaks are due to averaging data accumulated by repeated scans.
Conversion in C3H4 hydrogenation by (a) X2TiAl, (b) X2TiGa, (c) X2TiSn, (d) pure X metals, (e) X2MnGa, and (f) X2MnSn with X = Ni, Co, and Fe.
(a) Conversion and (b) C3H6 selectivity in C3H4 hydrogenation by Co2(Ti, Mn, or Fe)(Ga or Ge) and (c) conversion by Co2(Ti or Mn)Si. Data for Co2TiGe in (b) is not shown because selectivity is not reliable when conversion is as small as 10%.
Although Z = Sn was anomalous, as mentioned later, it seems that Z = Al and Si are slightly active (Figs. 2(a) and 3(c)) while Z = Ga is active (Figs. 2(b), (e) and 3(a)). The Co2(Mn or Fe)Ga was more active (higher conversion at lower temperature) than Co2(Mn or Fe)Ge (Fig. 3(a)), although its difference was small compared to the previous work,14) in which the samples were treated in air. The C3H6 selectivity for Co2(Mn or Fe)Ge was obviously higher than that for Co2(Mn or Fe)Ga (Fig. 3(b)), maintaining over 90% when the C3H4 conversion was 100%, as well as the previous work.14) The high C3H6 selectivity for Co2(Mn or Fe)Ga when the conversion was below 100% is due to residual C3H4 molecules adsorbed on the catalyst surface, which hinder further hydrogenation of C3H6 molecules.25,26)
Interestingly, only Z = Sn showed a significant carbon loss, as shown in Figs. 4(a), (b). For example, over 40% of the C3H4 molecules were consumed by side reactions at 150°C in Ni2MnSn. These alloys with Z = Sn only produced C3H6 but no C3H8, as shown in Figs. 4(c), (d). Figure 5 shows the analysis using mass spectroscopy for the reaction by the Ni2TiSn catalyst. Aside from m/z = 40 (C3H4) and 42 (C3H6), signals of m/z = 56, 82, and 84 were significant (Fig. 5(b)). These m/z can be assigned to butene (C4H8, m/z = 56), hexadiene or cyclohexene (C6H10, m/z = 82), and hexene or cyclohexane (C6H12, m/z = 84). The C4H8 molecules were probably formed through a coupling of C1 and C3 species or between C2 species, which needs a cracking of C3H4 molecules. The formation of C6H10 and C6H12 molecules also needs a coupling of C3-C3, C2-C4, or C1-C5 species. Thus, Sn atoms are considered to possess carbon cracking and coupling abilities. In pure Sn samples, the surface is covered with oxides, which cannot be removed by the heating in H2 atmosphere due to the low melting point (232°C). The crushing of Sn in a glove box to obtain a metallic surface is also impossible due to high ductility. Thus, it is difficult to evaluate the catalytic properties of metallic Sn. This study unveiled the catalytic characteristics of Sn by using Heusler alloys. By revealing the byproducts and mechanisms of side reactions in future research, the development of novel intermetallic catalysts containing Sn is anticipated for cracking, hydrogenolysis, and coupling of carbon species.
Carbon loss in C3H4 hydrogenation by (a) Ni2YZ and (b) Co2YZ and selectivities of C3H6 and C3H8 by (c) Ni2YSn and (d) Co2YSn. YZ elements in (a) and (b) and Y elements in (c) and (d) are displayed in legends.
(a) Temperature and (b) ion current in mass spectroscopy measurement during C3H4 hydrogenation by Ni2TiSn.
The catalytic properties of Heusler alloys with various component elements were investigated for the C3H4 hydrogenation. The hierarchy of catalytic activity on X of X2YZ was Ni > Co > Fe, the same as pure X catalysts, indicating that X atoms mainly catalyze the reaction. The activity hierarchy on Y was basically Fe ≥ Mn > Ti, which parallels the tendency on X that the larger the number of valence electrons, the higher the activity. For characteristics of Z elements, Al and Si were slightly active, while Ga and Ge were active. Ga was more active but less selective for C3H6 than Ge. The alloys with Z = Sn produced a large fraction of C4 and C6 species, indicating that Sn possesses an ability for cracking and coupling of carbon chains. This study unveiled the catalytic characteristics of Sn using Heusler alloys as a platform for investigating the catalytic roles of different elements under the same crystal structure.
A part of this work was supported by JSPS KAKENHI Grant Numbers JP19H02452 and JP23H01707 and MEXT Leading Initiative for Excellent Young Researchers Grant Number JPMXS0320200014. The XRD measurement was conducted using research equipment shared in MEXT Project for promoting public utilization of advanced research infrastructure (Program for supporting construction of core facilities) Grant Numbers JPMXS0441000021, JPMXS0441000022, and JPMXS0441000023.
