In Situ Spectroscopic Characterization of Ni_1–<i>x</i>Zn_<i>x</i>/ZnO Catalysts and Their Selectivity for Acetylene Semihydrogenation in Excess Ethylene

Abstract

The structures of ZnO-supported Ni catalysts were explored with in situ X-ray absorption spectroscopy, temperature-programmed reduction, X-ray diffraction, high-resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy, and electron energy loss spectroscopy. Calcination of nickel nitrate on a nanoparticulate ZnO support at 450 °C results in the formation of Zn-doped NiO (ca. Ni_0.85Zn_0.15O) nanoparticles with the rock salt crystal structure. Subsequent in situ reduction monitored by X-ray absorption near-edge structure (XANES) at the Ni K edge reveals a direct transformation of the Zn-doped NiO nanoparticles to a face-centered cubic alloy, Ni_1–<i>x</i>Zn_<i>x</i>, at ∼400 °C with <i>x</i> increasing with increasing temperature. Both in situ XANES and ex situ HRTEM provide evidence for intermetallic β₁-NiZn formation at ∼550 °C. In comparison to a Ni/SiO₂ catalyst, Ni/ZnO necessitates a higher temperature for the reduction of Ni^II to Ni⁰, which highlights the strong interaction between Ni and the ZnO support. The catalytic activity for acetylene removal from an ethylene feed stream is decreased by a factor of 20 on Ni/ZnO in comparison to Ni/SiO₂. The decrease in catalytic activity of Ni/ZnO is accompanied by a reduced absolute selectivity to ethylene. H–D exchange measurements demonstrate a reduced ability of Ni/ZnO to dissociate hydrogen in comparison to Ni/SiO₂. These results of the catalytic experiments suggest that the catalytic properties are controlled, in part, by the zinc oxide support and stress the importance of reporting absolute ethylene selectivity for the catalytic semihydrogenation of acetylene in excess ethylene