Catalytic Properties of Heusler Alloys for Steam Reforming of Methanol.

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.

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., X2YZ0.5Z0.5′). 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 (Co2MnFe1–GaGe1–).[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.

Results and Discussion

SRM with the Reactant CH3OH/H2O = 2:3

Figure 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 a), 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 a,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 . 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 b). The crystal structure and composition of these fine particles were not observed by XRD (Figure a) and energy-dispersive X-ray spectrometry (EDX) analyses. However, the fine particles in Figure b (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 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 b, 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 c); therefore, the smaller H2 production rate at 500 °C in the cooling phase than in the heating phase for Ni2TiAl (Figure b) 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 c). 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 c). SEM images revealed homogeneously precipitated fine particles (Figure ). However, XPS analysis revealed a very small surface composition of Cu (Table ), 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 ).[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/nMO) or 1 mol of H2O (m/nM + H2O(g) = 1/nMO + 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 d) and formed several phases after the reaction (Figure a). Co2TiSn and Ni2TiSn basically exhibited similar production rates although Ni2TiSn was slightly active compared to Co2TiSn (Figure e,f). In addition, Co2TiSn and Ni2TiSn exhibited similar structural changes, affording marginal X3Sn2 (Figure a), as well as similar SEM images revealing fine particles (Figure b) 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 ) 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 g,h). In addition, these alloys exhibited similar structural changes, affording marginal X3Sn2 and MnO (Figure a). SEM images after the reaction revealed fine particles in some parts of Co2MnSn and all parts of Ni2MnSn (Figure b). 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 i–k). All these Mn-containing alloys formed MnO during the reaction (Figure a). 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 b), and XPS analyses (Table ). 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 ), 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 l). Fine particle formation was not observed by SEM (Figure b). Only this alloy clearly exhibited TiO2 peaks in the XRD pattern (Figure a). The surface Ti composition was considerably greater than the stoichiometry even before the reaction (Table ). 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 m). SEM revealed fine particles with diameters of ∼400 nm (Figure a: region 1), and grains with diameters less than 100 nm are apparently precursors of the 400 nm particles (Figure a: 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 b and Table ). 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 a), 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 b). However, XPS analysis revealed that most of the surfaces are covered by Ga oxides (Table ). 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 b). 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.

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 ), 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 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 a,b), but it was less than that for 2:3 SRM (Figure a), 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 c), indicating that fine particle formation is considerably enhanced by O2 instead of H2O. Ni2TiAl exhibited a similar tendency with Co2TiAl (Figure d–f), although similar results were obtained for 2:1 and 2:3 SRM reactions (Figures e 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 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 d), although the structure of fine particles was not observed by XRD analysis (Figure a). 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 ), 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 e). After 2:1 and 2:3 SRM, similar surfaces (Figures f and 2b) as well as catalytic properties (Figures e and 1b) were observed. After oxygen-pretreated SRM, the surface clearly exhibited large numbers of fine particles (Figure g), which was verified by XRD analysis, with the strongest intensity peaks observed for Ni in the series (Figure b). 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 h, Table ). 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 m,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 ). For Co2FeGe, rather than fine particle formation, only surface roughening was observed in the SEM images (Figure a). XPS analysis indicated that the chemical composition among metals is not changed (Table ). 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 m 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 b). XPS analysis revealed an increase in the composition of Ga among metals (Table ). 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 c) 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.

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).

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.