Catalytic Properties of Heusler Alloys for Steam Reforming of Methanol

Abstract

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

1. Introduction

Intermetallic compounds exhibit unique atomic ordered structures and electronic structures, even at the surface, rendering unique catalytic properties. Currently, they are gaining increasing popularity as new catalysts.¹⁻³ Almost all of the reported intermetallic catalysts are binary systems. However, ternary intermetallic compounds can render synergistic effects among three elements, which serve as novel catalysts different from ordinary binary catalysts. Heusler alloys (X₂YZ) constitute a group of ternary intermetallic compounds with an L2₁ structure (other types are also present).⁴ Typically, the main element X belongs to groups 8–12, while subelements Y and Z belong to groups 3–8 and 13–15, respectively, which is similar to most of the intermetallic catalysts reported thus far. Various possible elemental sets of X, Y, and Z can afford new catalysts, and catalysis can be tuned by utilizing in-between compositions (e.g., X₂YZ_0.5Z_0.5^′). However, Heusler alloys have not been identified as catalysts until only recently. Typically, catalysts are prepared by a liquid process in a form of supported nanoparticles, which requires considerable trials and errors to establish preparation conditions for each elemental system. This fact makes it difficult to screen ternary intermetallic compounds, including Heusler alloys, as catalysts.

Most of the stable intermetallic compounds, even in ternary (or higher) systems, can be easily prepared by metallurgical methods such as arc-melting. By this method, Heusler catalysts in a form of unsupported powders have been screened by our group.⁴⁻⁶ Previously, the oxidation of carbon monoxide over Heusler catalysts revealed that X mainly exhibits catalytic activity and Y and Z modulate the activity and durability.⁵ Our group has also investigated the selective hydrogenation of alkynes and reported that unlike practical Pd-based catalysts, non-noble metal catalysts such as Co₂(Mn or Fe)Ge exhibit high alkene selectivity, and catalysis can be systematically controlled by elemental substitution (Co₂Mn_xFe_1–xGa_yGe_1–y).⁶

It would be desirable and beneficial to screen these emerging ternary intermetallic catalysts for other reactions to develop new catalysts and gain fundamental knowledge. Hence, the steam reforming of methanol (SRM) is conducted herein, for which intermetallic catalysts are typically applied.^1−3,7−11 The reaction CH₃OH + H₂O → 3H₂ + CO₂ is one possibility for producing hydrogen from safe storage for use as a clean energy source in society.^12,13 The catalytic properties of 14 Heusler alloys with X = Fe, Co, Ni, or Cu; Y = Ti, Mn, or Fe; Z = Al, Si, Ga, Ge, or Sn, which were easily prepared equilibrium phases, and structural changes occurring during the reaction were examined in this report.

2. Results and Discussion

2.1. SRM with the Reactant CH₃OH/H₂O = 2:3

Figure 1 shows production rates of H₂, CO, CO₂, and CH₄ for the SRM with the reactant CH₃OH/H₂O = 2:3 during heating and cooling processes. For Co₂TiAl (Figure 1a), the H₂ production rate started to significantly increase at 500 °C with the heating, but it decreased at 600 °C. A higher H₂ rate was observed during cooling than during heating. Similarly, for Ni₂TiAl, the H₂ rate suddenly started to increase at 450 °C. Furthermore, the H₂ production rate at ≤400 °C was greater during cooling than during heating. Studies reported that Ni₃Al and Ni₃(Si,Ti) form Ni fine particles on the surface because of the selective oxidation of Al and Si, respectively, during methanol decomposition, thereby exhibiting high activity.¹⁴⁻¹⁶ Similarly, after SRM, Ni₂TiAl exhibited Ni fine particles, as can be observed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) analyses (Figure 2a,b, respectively); these fine particles were probably formed because of the selective oxidation of Ti as revealed by X-ray photoelectron spectroscopy (XPS) analysis summarized in Table 1. The increased H₂ production rate at ≥450 °C and the higher H₂ production rate at 400 °C during cooling than during heating were attributed to the Ni fine particle formation. SEM images of Co₂TiAl after SRM mainly revealed a number of carbon nanofibers (CNF) with fine particles, while occasionally observed fine particles on the surface (Figure 2b). The crystal structure and composition of these fine particles were not observed by XRD (Figure 2a) and energy-dispersive X-ray spectrometry (EDX) analyses. However, the fine particles in Figure 2b (region 2) were probably Co precipitated via the selective oxidation of Ti; this precipitated Co could serve as active centers, which produced an increased amount of H₂ during cooling than during heating. This mechanism was deduced from Table 1 and the subsequent experiments discussed in the next section.

Figure 1.

Production rates of H₂, CO, CO₂, and CH₄ in SRM with the reactant of CH₃OH/H₂O = 2:3 during heating and cooling processes for (a–c) X₂TiAl with X = Co, Ni, Cu; (d–f) X₂TiSn with X = Fe, Co, Ni; (g,h) X₂MnSn with X = Co, Ni; (i–k) Co₂MnZ with Z = Ge, Ga, Si; (l) Co₂TiGe; and (m,n) Co₂FeZ with Z = Ge, Ga. Circles, triangles, squares, and crosses represent H₂, CO, CO₂, and CH₄, respectively. The reaction was conducted with an LHSV of ∼20 h^–1 for powders sieved to 20–63 μm. Each data point was obtained after maintaining constant for 30 min at each reaction temperature.

Figure 2.

(a) XRD patterns, (b) SEM images of X₂TiAl after SRM with the reactant of CH₃OH/H₂O = 2:3, and (c) feeding rate of the reactant mixture and experimental and ideal maximum H₂ production rates. In (a), asterisks represent X₂TiAl peaks. In (b), two representative regions are shown for Co₂TiAl. In (c), red (heating) and pink (cooling) circles represent the feeding rate (left axis) and the ideal H₂ production rate assuming methanol decomposition (CH₃OH → 2H₂ + CO) (right axis), and blue (heating), and pale blue (cooling) represent the experimentally obtained H₂ production rate.

Table 1. XPS Analysis of the Chemical Compositions before and after SRM with Reactants of CH₃OH/H₂O = 2:3 and 2:1, Methanol Decomposition, and SRM with Oxygen Pretreatment.

		chemical compositions [%]
		all elements					among metals
catalyst	reaction	X	Y	Z	O	C	X	Y	Z
Co2TiAl	before reaction	2.5	3.4	12.6	40.8	40.8	13.3	18.6	68.1
	2:3 SRM	1.2	2.3	1.6	12.1	82.9	22.9	46.0	31.1
	MeOH deco	0.1	0.3	0.5	3.5	95.5	13.7	34.7	51.6
	2:1 SRM	0.2	0.9	1.0	5.1	92.9	11.1	41.3	47.6
	O2-pre SRM	1.6	3.1	2.2	14.6	78.4	23.2	44.6	32.1
Ni2TiAl	before reaction	1.5	3.6	14.3	42.7	37.9	7.6	18.6	73.9
	2:3 SRM	1.9	5.3	7.5	34.7	50.6	13.0	35.9	51.1
	MeOH deco	0.2	1.0	1.6	11.8	85.6	5.6	36.1	58.4
	2:1 SRM	1.5	3.9	5.9	24.0	64.7	13.5	34.7	51.8
	O2-pretreat	9.8	5.7	3.1	32.4	49.1	52.8	30.5	16.7
	O2-pretreat (black)	1.2	2.0	1.6	9.6	85.7	24.9	41.6	33.5
Cu2TiAl	before reaction	3.6	3.2	21.1	27.5	44.6	12.9	11.3	75.7
	2:3 SRM	0.4	4.0	5.5	17.4	72.8	3.7	40.5	55.8
Co2MnGe	before reaction	5.4	4.6	0.7	34.1	55.2	50.7	42.8	6.5
	2:3 SRM	0.0	21.2	0.0	42.9	36.0	0.0	100.0	0.0
Co2TiGe	before reaction	3.7	10.8	0.0	34.7	50.9	25.5	74.5	0.0
	2:3 SRM	0.2	1.2	0.0	6.9	91.7	16.4	83.6	0.0
Co2FeGe	before reaction	7.0	3.4	1.3	62.2	26.1	60.1	28.7	11.2
	2:3 SRM	0.0	6.7	0.2	17.9	75.3	0.0	97.5	2.5
	MeOH deco	0.5	0.2	0.1	3.6	95.6	64.9	27.0	8.1
Co2FeGa	before reaction	7.9	3.5	5.8	51.5	31.4	45.8	20.6	33.6
	2:3 SRM (gray)	0.7	2.7	16.6	44.8	35.2	3.5	13.6	82.9
	2:3 SRM (black)	0.9	1.7	3.4	13.7	80.2	15.5	28.4	56.1
	MeOH deco	0.2	0.2	0.3	2.2	97.1	23.8	23.8	52.4

The decrease in the H₂ production rate at 600 °C for Co₂TiAl was mainly related to the decrease in the methanol feeding because of the blocking of the extensive growth of CNF (Figure 2b, region 1). Compared to Ni₂TiAl, Co₂TiAl exhibited a larger number of CNF, as indicated by the SEM images and by the higher surface area of Co₂TiAl (3.26 m² g^–1) than that of Ni₂TiAl (0.32 m² g^–1) after the reaction. Compared to Co₂TiAl, Ni₂TiAl exhibited a smaller decrease in methanol feeding (Figure 2c); therefore, the smaller H₂ production rate at 500 °C in the cooling phase than in the heating phase for Ni₂TiAl (Figure 1b) is related to the coarsening of Ni particles at ≥550 °C and not to the decrease in methanol feeding. Besides H₂, CO was the main product over these alloys, which was typical for pure Co and Ni.¹³ Thus, H₂ is predominantly produced by methanol decomposition (CH₃OH → 2H₂ + CO), and most of the methanol is consumed at ≥550 °C for both alloys as the H₂ rates almost reach their ideal maximum values (assuming decomposition) (Figure 2c). The increase in the CO₂ production rate at high temperatures confirms the growth of CNF via the disproportionation of CO (2CO → CO₂ + C).

Conversely, Cu₂TiAl did not catalyze the reaction (Figure 1c). SEM images revealed homogeneously precipitated fine particles (Figure 3). However, XPS analysis revealed a very small surface composition of Cu (Table 1), indicating that active fine particles are not formed by selective oxidation. Co and Ni exhibited similar Gibbs energies of oxidation, but Cu exhibited a considerably more positive Gibbs energy (Table 2).¹⁷ Fine particle formation by selective oxidation possibly depended on a balance of component oxidizability; thus, Cu is probably difficult to precipitate on the surface of alloy with Ti and Al.

Figure 3.

SEM image of Cu₂TiAl after SRM with the reactant of CH₃OH/H₂O = 2:3.

Table 2. Standard Gibbs Energies of Oxide Formation at 400 and 600 °C for the Reaction with 1 mol of O₂ (2m/nM + O₂ = 2/nM_mO_n) or 1 mol of H₂O (m/nM + H₂O(g) = 1/nM_mO_n + H₂)¹⁷.

		ΔG° [kJ (O2 1 mol)−1]		ΔG° [kJ (H2O 1 mol)−1]
element	oxide	400 °C	600 °C	400 °C	600 °C
Fe	Fe3O4	–444	–413	–11	–7
Co	CoO	–373	–344	24	28
Ni	NiO	–355	–320	33	40
Cu	Cu2O	–239	–209	91	95
Ti	TiO2 (rutile)	–821	–786	–200	–193
Mn	MnO	–665	–636	–122	–118
Al	Al2O3 (α)	–976	–935	–278	–268
Si	SiO2 (quartz)	–788	–752	–184	–176
Ga	Ga2O3	–581	–537	–80	–68
Ge	GeO2 (hexagonal)	–437	–402	–8	–1
Sn	SnO2	–442	–399	–10	0

In case of X₂TiSn, Fe₂TiSn exhibited extremely low activity (Figure 1d) and formed several phases after the reaction (Figure 4a). Co₂TiSn and Ni₂TiSn basically exhibited similar production rates although Ni₂TiSn was slightly active compared to Co₂TiSn (Figure 1e,f). In addition, Co₂TiSn and Ni₂TiSn exhibited similar structural changes, affording marginal X₃Sn₂ (Figure 4a), as well as similar SEM images revealing fine particles (Figure 4b) after the reaction. EDX analysis indicated that their alloy compositions are stoichiometric in most areas after the reaction. Similar to X₂TiAl, oxidizability (Fe > Co ≅ Ni; Table 2) likely played a role in the structural change through the reaction, consequently dominating the catalytic properties.

Figure 4.

(a) XRD patterns and (b) SEM images of X₂TiSn after SRM with the reactant of CH₃OH/H₂O = 2:3. In (a), asterisks, “CS”, “NS”, “TO,” and “Un” represent X₂TiSn, Co₃Sn₂, Ni₃Sn₂, TiO₂ (rutile), and unknown peaks, respectively.

Similar to X₂TiSn, Co₂MnSn and Ni₂MnSn exhibited similar catalytic properties (Figure 1g,h). In addition, these alloys exhibited similar structural changes, affording marginal X₃Sn₂ and MnO (Figure 5a). SEM images after the reaction revealed fine particles in some parts of Co₂MnSn and all parts of Ni₂MnSn (Figure 5b). Even in the case of X₂MnSn, X = Co and Ni exhibited similar results, as well as for X₂TiAl and X₂TiSn. However, Co₂MnSn and Ni₂MnSn mainly produced CO₂ besides H₂, unlike (Co or Ni)₂TiAl and (Co or Ni)₂TiSn. In addition, the main byproduct of Co₂MnGe, Co₂MnGa, and Co₂MnSi was CO₂ (Figure 1i–k). All these Mn-containing alloys formed MnO during the reaction (Figure 6a). Especially, in Co₂MnGe, strong XRD peaks for MnO were observed, and the formation of MnO fine structures on the surface was observed by SEM, EDX (Figure 6b), and XPS analyses (Table 1). MnO catalyzes the water-gas shift reaction (CO + H₂O → CO₂ + H₂).¹⁸⁻²⁰ Thus, MnO most likely catalyzed the reaction between H₂O and CO produced by methanol decomposition in the Mn-containing alloys. The water-gas shift reaction is less favorable at higher temperature.²¹ Actually, the CO₂ selectivity (molar ratio of CO₂ in all byproducts) decreased with the increase in the temperature for Mn-containing alloys (Table 3), supporting the proposed mechanism.

Figure 5.

(a) XRD patterns and (b) SEM images of X₂MnSn after SRM with the reactant of CH₃OH/H₂O = 2:3. In (a), asterisks, “CS,” and “NS” represent X₂TiSn, Co₃Sn₂, and Ni₃Sn₂ peaks, respectively.

Figure 6.

(a) XRD patterns of Co₂MnZ after SRM with the reactant of CH₃OH/H₂O = 2:3 and (b) SEM image of Co₂MnGe after the reaction with atomic concentrations evaluated by EDX analysis of the represented image. In (a), asterisks represent X₂TiSn peaks, “CoGe?” peaks were likely CoGe with the C2/m space group, and “Un” peak was unknown, but possibly hcp-Co.

Table 3. CO₂ Selectivities at 550 and 600 °C for SRM with the Reactant of CH₃OH/H₂O = 2:3.

	CO2 selectivity [%]
catalyst	550 °C	600 °C
Co2MnSn	74.0	70.8
Ni2MnSn	81.8	73.6
Co2MnGe	81.5	74.9
Co2MnGa	68.5	61.1
Co2MnSi	83.9	78.5

Co₂TiGe did not catalyze the reaction (Figure 1l). Fine particle formation was not observed by SEM (Figure 7b). Only this alloy clearly exhibited TiO₂ peaks in the XRD pattern (Figure 7a). The surface Ti composition was considerably greater than the stoichiometry even before the reaction (Table 1). These facts indicated that active sites are not formed on the surface covered by inert TiO₂.

Figure 7.

(a) XRD pattern and (b) SEM image of Co₂TiGe after SRM with the reactant of CH₃OH/H₂O = 2:3. In (a), asterisks represent Co₂TiGe peaks, “CoGe?” peaks were likely CoGe with the C2/m space group, and the “Un” peak was unknown, but possibly hcp-Co.

Co₂FeGe exhibited an unusual change in the H₂ production rate: it increased with heating but decreased at 550 °C (Figure 1m). SEM revealed fine particles with diameters of ∼400 nm (Figure 8a: region 1), and grains with diameters less than 100 nm are apparently precursors of the 400 nm particles (Figure 8a: region 2). The precipitation of small grains possibly increased the H₂ production rate at 450 °C. XRD and XPS analyses revealed that most of the surfaces are covered by Fe₃O₄ (Figure 8b and Table 1). Thus, the coarsening of the grains and the formation of Fe₃O₄ possibly decrease the H₂ production rate at 550 °C.

Figure 8.

(a) SEM images captured from different regions and the (b) XRD pattern of Co₂FeGe after SRM with the reactant of CH₃OH/H₂O = 2:3. In (b), asterisks and “FO” represent peaks of Co₂FeGe and Fe₃O₄ with the Fd3̅m space group, respectively.

All of the alloy powders changed their colors after the catalytic reaction, but the metallic gray color of Co₂FeGa did not change, although the color of the bottommost region (ca. 10 vol %) of the catalyst bed changed to black. XRD (Figure 9a), SEM, and EDX results did not reveal changes in structures and compositions of the metallic gray powders during the reaction, except for the presence of minute CNFs in some areas of SEM images (Figure 9b). However, XPS analysis revealed that most of the surfaces are covered by Ga oxides (Table 1). Although studies on the SRM and methanol decomposition over Ga oxides at high temperatures of greater than 500 °C have not been reported, studies revealed that Ga oxides strongly interact with methanol molecules at a lower temperature.²² Thus, the H₂ production over Co₂FeGa is likely related to the presence of surface Ga oxides. The SEM image of the bottommost black powders revealed a significant growth of CNF (Figure 9b). As these black powders were obtained at the bottommost region and the H₂ production rate only exhibited a marginal difference between the heating and cooling processes, the fine particles on CNFs did not contribute to the H₂ production.

Figure 9.

(a) XRD patterns and (b) SEM images of Co₂FeGa powders with metallic gray and black colors after SRM with the reactant of CH₃OH/H₂O = 2:3. In (a), asterisks represent Co₂FeGa peaks.

2.2. Further Investigation on Co₂TiAl, Ni₂TiAl, Co₂FeGe, and Co₂FeGa

Co₂TiAl and Ni₂TiAl apparently exhibited high activity for methanol decomposition, which corresponded to the first step of SRM, because of the fine particle formation. Pure Co and Ni powders sieved to 20–63 μm exhibited low activity (Figure 10), possibly related to the large particle size for methanol decomposition that is structure-sensitive.²³ To validate the importance of fine particle formation, effects of steam and oxygen were investigated. Figure 11 shows the results of methanol decomposition using CH₃OH as the reactant without H₂O, the SRM using CH₃OH/H₂O = 2:1 as the reactant, and the SRM using CH₃OH/H₂O = 2:3 as the reactant after the oxygen pretreatment at 500 °C. For Co₂TiAl, a H₂ production rate for 2:1 SRM was greater than that for methanol decomposition (Figure 11a,b), but it was less than that for 2:3 SRM (Figure 1a), indicating that active fine particles are formed via oxidation by H₂O. The highest H₂ production rate was observed for 2:3 SRM after the oxygen pretreatment (Figure 11c), indicating that fine particle formation is considerably enhanced by O₂ instead of H₂O. Ni₂TiAl exhibited a similar tendency with Co₂TiAl (Figure 11d–f), although similar results were obtained for 2:1 and 2:3 SRM reactions (Figures 11e and 1b).

Figure 10.

Production rates of H₂, CO, CO₂, and CH₄ in SRM with the reactant of CH₃OH/H₂O = 2:3 during heating and cooling processes for (a) pure Co (RARE METALLIC, 99.9%, −100 mesh) and (b) pure Ni (RARE METALLIC, 99.9%, −200 mesh) powders sieved to 20–63 μm. Circles, triangles, squares, and crosses represent H₂, CO, CO₂, and CH₄, respectively. LHSV was 20 h^–1. Each data point was obtained after remaining constant for 30 min at each reaction temperature.

Figure 11.

Production rates of H₂, CO, CO₂, and CH₄ for (a–c) Co₂TiAl and (d–f) Ni₂TiAl in (a,d) methanol decomposition, (b,e) SRM with the reactant of CH₃OH/H₂O = 2:1, and (c,f) SRM with the reactant of CH₃OH/H₂O = 2:3 after oxygen pretreatment during heating and cooling processes. Circles, triangles, squares, and crosses represent H₂, CO, CO₂, and CH₄, respectively. LHSV of methanol was 13 h^–1 in all three reactions, which corresponded to the methanol part of LHSV (20 h^–1) for CH₃OH/H₂O = 2:3. Each data point was obtained after remaining constant for 30 min at each reaction temperature.

Figure 12 shows SEM images after these reactions. Co₂TiAl exhibited significant CNF growth in all reactions, where almost all areas were covered by CNF in case of the methanol decomposition and 2:1 SRM. In case of the surface after oxygen-pretreated SRM, fine particle formation was occasionally observed, which was likely enhanced by O₂ pretreatment (Figure 12d), although the structure of fine particles was not observed by XRD analysis (Figure 13a). As indicated by SEM and XPS analyses, significant CNF growth likely increased in the order of methanol decomposition >2:1 SRM >2:3 SRM without and with oxygen pretreatment (Table 1), but the H₂ production rate followed the reverse order. Thus, the active centers of Co₂TiAl are likely fine particles formed by the selective oxidation of Ti, not the fine particles on CNF. For Ni₂TiAl, only methanol decomposition afforded an excess growth of CNF (Figure 12e). After 2:1 and 2:3 SRM, similar surfaces (Figures 12f and 2b) as well as catalytic properties (Figures 11e and 1b) were observed. After oxygen-pretreated SRM, the surface clearly exhibited large numbers of fine particles (Figure 12g), which was verified by XRD analysis, with the strongest intensity peaks observed for Ni in the series (Figure 13b). The H₂ production rate clearly depended on the number of fine particles, indicating that the active centers are fine particles formed by the selective oxidation of Ti as well as in Co₂TiAl. Note that the topmost region (ca. 20 vol %) of the catalyst bed changed the color to black after SRM with oxygen pretreatment and revealed significant CNF growth (Figure 12h, Table 1). However, our conclusion was not altered as CNFs were apparently not grown at 400 °C during heating, while a higher H₂ production rate was observed during heating than during cooling at this temperature. Jang et al. reported that the activation of Ni₃Al catalysts by the selective oxidation of Al is enhanced by the addition of some steam (CH₃OH/H₂O = 10:1) in methanol decomposition, but it is significantly suppressed by the addition of more steam (CH₃OH/H₂O = 2:1 and 1:1).²⁴ The activation of Ni₂TiAl was more enhanced by the addition of more steam and using more severe oxidant (O₂). As Ni fine particle formation required the selective oxidation of Ti in the presence of Al, more severe oxidation conditions were likely preferred.

Figure 12.

SEM images of (a–d) Co₂TiAl and (e–h) Ni₂TiAl after (a,e) methanol decomposition, (b,f) SRM with the reactant of CH₃OH/H₂O = 2:1, and (c,d,g,h) oxygen-pretreated SRM with the reactant of CH₃OH/H₂O = 2:3. For the oxygen-pretreated reaction, two representative regions are shown for (c,d) Co₂TiAl, and observations of (g) less black Ni₂TiAl powders and (h) black Ni₂TiAl powders at the bottom and topmost regions of the catalyst bed, respectively, were separately conducted.

Figure 13.

XRD patterns of (a) Co₂TiAl and (b) Ni₂TiAl after methanol decomposition (MeOH deco), SRM with the reactant of CH₃OH/H₂O = 2:1 (2:1 SRM), and oxygen-pretreated SRM with the reactant of CH₃OH/H₂O = 2:3 (O₂ pret). For the oxygen-pretreated catalysts in (b), measurements of the less black powders (green line) and black powders (yellow line) at the bottom and topmost regions of catalyst bed, respectively, were separately conducted. Asterisks represent Co₂TiAl and Ni₂TiAl peaks.

In terms of application, the activity of oxygen-pretreated Ni₂TiAl is herein compared to those of reported catalysts. The turnover frequency (TOF) is often evaluated to compare the activity. TOF values have been reported for many transition-metal-supported catalysts, however, strongly depending on surface areas and dispersion of metals.^25,26 Thus, activities cannot be compared by TOF between supported small catalysts and our unsupported large catalysts. A H₂ production rate per catalyst weight is crucial for applications. Supported Cu catalysts exhibited H₂ production rates of ∼200 μmol g^–1 s^–1 at ∼250 °C and ∼300 μmol g^–1 s^–1 at ∼300 °C.^13,27 In our previous study, Raney Pd exhibit a H₂ production rate of 87 μmol g^–1 s^–1 at 300 °C.²⁸ However, herein, Ni₂TiAl exhibited a H₂ production rate of 8.6 μmol g^–1 s^–1 at 300 °C. Considering that the size of Ni fine particles was up to 400 nm and that their precursors were powders sieved to 20–63 μm, a considerably higher H₂ production rate is expected by using, for example, precursor powders with a diameter of ∼2 μm or foils with a thickness of ∼2 μm.

Co₂FeGe and Co₂FeGa exhibited relatively high H₂ production rates for 2:3 SRM (Figure 1m,n). As the main byproduct was CO, H₂ was supposedly produced by methanol decomposition. Actually, these catalysts were active for the methanol decomposition without steam (Figure 14). For Co₂FeGe, rather than fine particle formation, only surface roughening was observed in the SEM images (Figure 15a). XPS analysis indicated that the chemical composition among metals is not changed (Table 1). As H₂ was produced in the low-temperature region of ≤400 °C, metallic Co₂FeGe was considered to be active for methanol decomposition. Lower H₂ production rates observed at ≤400 and ≥550 °C for 2:3 SRM than those for methanol decomposition (Figures 1m and 14a) were likely related to the oxidation by H₂O at ≤400 °C and to the significant formation of Fe₃O₄ at ≥550 °C. Metallic Co₂FeGe with a diameter of 20–63 μm was active for the structure-sensitive reaction possibly due to the specific electronic structure or a specific reaction field constructed by atomic ordered structures. For Co₂FeGa, the H₂ production rate was slightly less for the methanol decomposition than that for 2:3 SRM. SEM only revealed surface roughening, not fine particle formation (Figure 15b). XPS analysis revealed an increase in the composition of Ga among metals (Table 1). However, it was less significant than that observed for 2:3 SRM. Thus, Ga oxides are considered to play a key role in catalysis of methanol decomposition. By the comparison of all results, secondary phases that might be FeO for Co₂FeGe and Co₂C for Co₂FeGa (Figure 15c) were not likely to contribute to the methanol decomposition without steam.

Figure 14.

Production rates of H₂, CO, CO₂, and CH₄ in methanol decomposition by (a) Co₂FeGe and (b) Co₂FeGa. Circles, triangles, squares, and crosses represent H₂, CO, CO₂, and CH₄, respectively. LHSV was 13 h^–1. Each data point was obtained after remaining constant for 30 min at each reaction temperature.

Figure 15.

SEM images of (a) Co₂FeGe and (b) Co₂FeGa after methanol decomposition and their (c) XRD patterns. In (c), asterisks represent Co₂FeGe and Co₂FeGa peaks.

3. Methods

Heusler alloy ingots were prepared by arc-melting and subsequent annealing, the conditions of which were described previously.^5,6 First, the ingots were crushed using a mortar and pestle. Second, the obtained powders were sieved to 20–63 μm for catalytic measurements. The alloys were (almost) a single phase with a highly ordered L2₁ Heusler structure,^5,6 which was examined by XRD for powders with a size of less than 20 μm post-annealed at 600 °C for removing strains.

A catalytic reaction was conducted in the gas phase using a fixed-bed flow reactor. A powder catalyst was held on quartz wool fixed in a quartz tube with an internal diameter of ∼7.5 mm. Considering formula weights, catalyst amounts were 0.60, 0.75, and 0.90 g for alloys with Z = Al, Si; Z = Ga, Ge; Z = Sn, respectively, so that the catalyst layer height was ∼5 mm. The catalyst was preheated at 600 °C for 1 h under H₂ flow for removing surface oxides and strains introduced because of crushing. A reactant mixture of CH₃OH and H₂O with a molar ratio of 2:3 was fed using a plunger pump at a constant rate of 0.1 mL min^–1, where the actual measured rate was typically 0.07 g min^–1 corresponding to 0.08 mL min^–1 and a liquid hourly space velocity (LHSV) of ∼20 h^–1. In addition to the reactant, 30 mL min^–1 of N₂ carrier gas was introduced during the reaction. The reaction temperature was controlled using an electric furnace surrounding the reactor. Gaseous products (H₂, CO, CO₂, and CH₄) were analyzed by gas chromatography (Shimadzu GC 14A).²⁹ Their production rates were estimated from the analyzed concentrations and downstream flow rate measured at room temperature using a flow meter. These analyses were performed 30 min after achieving the measurement temperatures from every 50 °C from 300 °C when heating up to 600 °C and every 100 °C when cooling down to 300 °C.

For selected samples, the effect of steam was investigated by the SRM using the reactant CH₃OH/H₂O = 2:1 and 1:0 (methanol decomposition), with a methanol feeding rate of 0.04 g min^–1, corresponding to that in the SRM with the reactant CH₃OH/H₂O = 2:3. In addition, the effect of oxygen pretreatment was investigated by the addition of a heating process at 500 °C for 1 h under a 100 mL min^–1 flow of [1% O₂/He balance] gas before the reaction.

The surface area was determined by the Brunauer–Emmett–Teller (BET) method with Kr adsorption (BELSORP-max volumetric adsorption instrument, MicrotracBEL); the BET surface area was 0.05–0.13 m² g^–1 before the reaction, mainly depending on formula weights.^5,6 Structural changes occurring because of the reaction were examined by XRD (Ultima IV diffractometer, Rigaku), SEM (field-emission type, SU6600, Hitachi) equipped with EDX (INCA x-act, Oxford Instruments), and XPS (PHI 5600, ULVAC-PHI).

4. Conclusions

Fourteen Heusler alloy catalysts were examined for SRM. Depending on elemental sets, various catalytic properties were observed because of structural changes occurring during the reaction; these structural changes apparently depended on a balance of component oxidizability. The H₂ production rates over Co₂TiAl and Ni₂TiAl significantly increased because of the formation of fine particles via the selective oxidation of Ti during the reaction, although most of H₂ was produced by methanol decomposition. H₂ production was enhanced by oxygen pretreatment, promoting fine particle formation. All of the Mn-containing alloys formed MnO, which catalyzed the water-gas shift reaction and produced CO₂ from CO generated by methanol decomposition. Co₂FeGa apparently exhibited activity for methanol decomposition because of the formation of Ga oxides. By choosing appropriate elemental sets, Heusler alloys can be good precursors of catalysts that exhibit high activity and desirable selectivity. On the other hand, Co₂FeGe apparently catalyzed methanol decomposition in the initial metallic states, possibly related to its unique electronic structure or a unique atomic ordered reaction field.

The authors declare no competing financial interest.

Notes

^§ An-Pang Tsai passed away on May 25, 2019.