Comprehensive study on Ni- or Ir-based alloy catalysts in the hydrogenation of olefins and mechanistic insight

Metal-alloyed atomic catalysts such as single-atom alloys have attracted much attention due to their high atom economy and their unique catalytic properties. Recently, we found that a SiO₂-supported Ni-Ir alloy (Ni-Ir/SiO₂) was an effective heterogeneous catalyst for the hydrogenation of styrene and that the isolated Ni atom surrounded by Ir metal atoms, an Ir-alloyed Ni single atom, in the Ni-Ir alloy was the main active site, showing much higher activity in comparison with the monometallic Ir/SiO₂ and Ni/SiO₂. Reports on such effective metal-alloyed non-noble-metal single-atom catalysts are limited, and their catalytic potential should be clarified. In this study, bimetallic catalysts composed of non-noble metals and noble metals were comprehensively scrutinized in the hydrogenation of styrene as a model reaction. Among various combinations, Ni-Ir/SiO₂ (introduced Ni/Ir molar ratio 1) showed the largest rate enhancement (7-fold) in comparison with the monometallic counterpart catalysts, while a SiO₂-supported Co-Ir alloy (Co-Ir/SiO₂, introduced Co/Ir molar ratio 4) and Ni-Pt alloy (Ni-Pt/SiO₂, introduced Ni/Pt molar ratio 0.25) were also effective. The activity orders were Co-Ir/SiO₂ > Ir/SiO₂ > Co/SiO₂ and Ni-Pt/SiO₂ > Pt/SiO₂ > Ni/SiO₂, and the (Co-Ir/SiO₂)/(Ir/SiO₂) and (Ni-Pt/SiO₂)/(Pt/SiO₂) activity ratios were 3.5 and 2.5, respectively. Moreover, Ni-Ir/SiO₂ showed higher activity for the hydrogenation of various olefins in comparison to the monometallic Ir/SiO₂ and Ni/SiO₂. The reaction mechanism of styrene hydrogenation over the Ir-alloyed Ni single atom of Ni-Ir/SiO₂ was proposed on the basis of spectroscopic studies such as FTIR, XPS, and XRD, kinetic studies such as the effect of substrate concentration and H₂ pressure, the isotopic effect of hydrogen, and the effect of the reaction temperature, and DFT calculations. The rate-determining step is the second hydrogenation of the half-hydrogenated styrene adspecies on an Ni atom by H species on Ir atoms in the Horiuti-Polanyi mechanism. The single Ni atom in Ir metal atoms has two roles: (1) formation of the reactive half-hydrogenated styrene adspecies on Ni atom and (2) formation of the reactive H species on Ir atoms by the electron transfer from an Ni atom to Ir atoms. These roles resulted in a low activation energy, leading to a large rate enhancement in comparison to the monometallic counterpart catalysts.