Heusler Alloys - ACS Publications - American Chemical Society

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19 Jan 2017 - the degree of chemical ordering in Heusler alloys is typically evaluated by .... electronic structure derived by calculation.13 Thus, propyne.
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Heusler Alloys: A Group of Novel Catalysts Takayuki Kojima,*,†,‡ Satoshi Kameoka,‡ and An-Pang Tsai‡ †

Frontier Research Institute for Interdisciplinary Sciences, Tohoku University, Aramaki aza Aoba 6-3, Aoba-ku, Sendai 980-8578, Japan ‡ Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, Katahira 2-1-1, Aoba-ku, Sendai 980-8577, Japan S Supporting Information *

ABSTRACT: In this study, we investigated the catalytic properties of various Heusler alloys for the hydrogenation of propyne and the oxidation of carbon monoxide. For propyne hydrogenation, Co2FeGe alloy showed a higher activity than that of elemental Co, where neither Fe nor Ge showed any activity. This clearly indicates an alloying effect. For the oxidation of carbon monoxide, although most alloys showed a significant change in catalytic activity during measurement due to an irreversible oxidation of the alloy, Co2TiSn alloy showed a very small change. The results indicate that the catalytic activity and stability of a Heusler alloy can be tuned by employing an appropriate set of elements.

1. INTRODUCTION A Heusler alloy is an intermetallic compound (ordered alloy) described as X2YZ with an L21 structure (Figure 1).1 It is called

Transition metals exhibit various catalytic functions, and their catalytic properties are dominated by the surface and electronic structures and defects. Alloying modifies the atomic configuration at the surface and the electronic structure of any given element, resulting in variation in the catalytic properties. An intermetallic compound is an extreme alloy that exhibits a specific atomic arrangement at the surface and an electronic structure completely different from that of its constituent elements. Thus, intermetallic compounds have recently attracted attention as new catalysts.7−9 Heusler alloys are potentially valuable catalysts because there are so many possible sets of elements and electronic tuning can be done by elemental substitution. To our knowledge, there have been no experimental studies on the catalytic properties of Heusler alloys; however, only one theoretical study has appeared very recently.10 In this article, we describe our experimental investigation of the catalytic properties of 12 Heusler alloys for the hydrogenation of propyne and the oxidation of carbon monoxide as a first step toward exploring new catalysts.

Figure 1. Crystal structure of (full-) Heusler alloy (X2YZ): L21 structure. Drawing was done using VESTA.1

a full-Heusler alloy in a narrow sense, whereas XYZ with a C1b structure is called a half-Heusler alloy. Typically, X and Y are transition metals in groups 3−8 and 8−12, respectively, and Z is in group 13−15.2 Because many Heusler alloys are ferromagnetic, they have been mainly studied as magnetic materials since the discovery of Cu2MnAl in 1903.3 In particular, they have recently attracted much attention in the field of spintronics.4 There are also of paramount importance in other fields, such as Ni2MnGa for a ferromagnetic shape memory alloy5 and Fe2VAl for a thermoelectric material.6 One interesting feature of Heusler alloys is that their electronic structure can be tailored by elemental substitution in accordance with the rigid-band approximation. This enables tuning of the functional properties. © 2017 American Chemical Society

2. RESULTS AND DISCUSSION 2.1. Structural Characterization. Powder samples of Heusler alloys were prepared by arc melting and annealing, followed by crushing. X-ray diffraction (XRD) was performed to characterize the ordered structure. The diffraction pattern of L21-phase was sufficiently observed for all of the samples, as shown in Figure 2. Although several samples showed unknown Received: October 8, 2016 Accepted: January 6, 2017 Published: January 19, 2017 147

DOI: 10.1021/acsomega.6b00299 ACS Omega 2017, 2, 147−153

ACS Omega

Article

peak for Ifund in eq 1, and the α value used was the average of the ones estimated using Ifund for the 220, 400, 422, and 440 fundamental peaks in eq 2. Table 1 shows the obtained S-

extra peaks, their intensities were negligible (Figure S2 and Table S1). Thus, samples were almost of single phase.

Table 1. Prepared Heusler Alloys and Their Ordering Factors, S and α, with Standard Deviation (SD)a material

S

Fe2TiSn Co2TiAl Co2TiGe Co2TiSn Co2MnSi Co2MnGe Co2MnSn Co2FeGe Ni2TiAl Ni2TiSn Ni2MnSn Cu2TiAl

1.0 0.97 1 0.99 1.00 1.0 0.97 0.97 0.98 1.0 1.0 0.95

SD 0.04 0.01 0.04 0.01 0.02 0.01