Deactivation of Ni/TiO2 catalyst in the hydrogenation of nitrobenzene in water and improvement in its stability by coating a layer of hydrophobic carbon

Water as a green solvent to replace the conventional organic solvent presents many advantages in the organic synthesis. The hydrogenation of nitrobenzene in water has been investigated by using Ni/TiO2 catalyst in this work, and our main purpose was focused on the Ni/TiO2 catalyst activity and its stability improvement. The experimental results and analysis from the data of XRD, XPS, ICP revealed that the formation of nickel hydroxide from metallic nickel reacting with water caused a rapid deactivation of Ni/TiO2 catalyst. Based on these, we designed a catalyst with hydrophobic property to prevent the nickel active species to contact with water; thus, a hydrophobic carbon layer was coated on the surface of Ni/TiO2. As expected, the hydrophobic carbon was successfully coated on Ni/TiO2 catalysts by a hydrothermal method and they presented higher reactivity and improved stability in the present aqueous reaction system; nickel hydroxide was not detected on the used and water treated carbon-coated Ni/TiO2 samples. The improved abilities were attributed to the increased hydrophobicity of catalysts modified by carbon, which not only prevents water to contact with nickel catalytic species, but also protects the metallic nickel to be oxidized as it exposed to air.

The activity and stability of Ni/TiO2 catalyst were significantly improved after the catalyst was modified by a carbon coating. The surface hydrophilic Ni/TiO2 catalyst changed to hydrophobic (Ni/TiO2)@C after coated with carbon, which inhibited the transformation of active nickel metallic species to Ni(OH)2 in H2O.Figure optionsDownload high-quality image (108 K)Download as PowerPoint slideHighlights
► Formation of nickel hydroxide caused Ni/TiO2 deactivation in water.
► A hydrophobic carbon layer was successfully coated on the surface of Ni/TiO2.
► The activity and stability of the catalyst were improved significantly after carbon coating.
► The surface hydrophobicity prevented water to contact with nickel catalytic species forming nickel hydroxide.