Heusler alloys (X₂YZ) are well-established intermetallic compound materials in various fields because their function can be precisely adjusted by elemental substitution (e.g., X₂YZ_1−xZ′_x). Although intermetallic compound catalysts started attracting attention recently, catalysis researchers are not familiar with Heusler alloys. We report their potential as novel catalysts focusing on the selective hydrogenation of alkynes. We found that Co₂MnGe and Co₂FeGe alloys have great alkene selectivity. Mutual substitution of Mn and Fe (Co₂Mn_xFe_1−xGe) enhanced the reaction rate without changing selectivity. The substitution of Ga for Ge decreased the selectivity but increased the reaction rate monotonically with Ga composition. Elucidation of these mechanisms revealed that the fine tuning of catalytic properties is possible in Heusler alloys by separately using ligand and ensemble effects of elemental substitution.

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