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.
- DFT calculation
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