(629b) Synthesis and Catalytic Properties of Nickel Silicide Catalysts for Selective Hydrogenation

Hydrotreating research has played an important role in environmental catalysis studies worldwide. This has led to a strong interest in catalytic materials different from the classically utilized metal sulfide and noble metal catalysts. Metal carbides and nitrides are initially highly active, but are easily poisoned by a small amount of sulfur. Thermochemical calculations have indicated that metal silicides can tolerate much higher H₂S concentrations than the corresponding carbides and nitrides. Therefore, we explored the possibility of using metal silicides as novel catalysts for heterogeneous catalytic reactions. Nickel-silicon intermetallic catalysts (Ni₂Si, NiSi, and NiSi₂) were prepared by direct silification method using SiH₄ as the silicon source. The as prepared nickel silicides were characterized by X-ray diffraction, transmission electron microscopy, magnetic measurements, X-ray photoelectron spectroscopy, temperature-programmed reduction, temperature-programmed desorption measurements. The results show that nickel silicide formation involves the following sequence as a function of increasing temperature: Ni (cubic) → Ni₂Si (orthorhombic) → NiSi (orthorhombic) → NiSi₂ (cubic). The insertion of Si atoms into the interstitial sites between Ni atoms resulted in a significant change in the unit cell lattice of nickel. All of nickel silicide materials were ferromagnetic at room temperature, with saturation magnetization values drastically decreased when Si is present. Nickel silicide develops a thin silicon oxide layer during exposure to air, which can be removed by H₂-temperature programmed reduction. The catalytic hydrogenation of cinnamaldehyde and phenylacetylene over the nickel silicide nanoparticles was investigated. The repulsive force between the electronegative silicon atoms in the nickel silicide intermetallic and oxygen atoms in the C=O bond of cinnamaldehyde inhibit the hydrogenation of C=O bond to some extent. Nickel silicide catalyst presents 77% selectivity to hydrocinnamaldehyde, which is much higher than monometallic nickel catalyst. The nickel silicide catalyst also shows excellent (ca 93%) styrene selectivity in the hydrogenation of phenylacetylene due to the strong modification of electronic structure around the Fermi level caused by the interaction of nickel and silicon. These results indicate that these novel nickel silicides are promising catalysts for the selective hydrogenation of unsaturated hydrocarbons.

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