Nanoscale Core–Shell Catalysts for H ₂ Production by Methane Decomposition: Supported Nickel Nanoparticles Ensheathed in Metal Oxides

Catalytic methane decomposition provides a potentially viable route to low-cost, CO_<i>x</i>-free H₂ from this abundant source. A remaining challenge is to facilitate the needed low-temperature C-H bond activation with high catalytic activity and stability. We now report the design and synthesis of supported Ni catalysts that address these limitations; they consist of silica-supported Ni nanoparticles ensheathed in nanoscale TiO₂ shells, forming an embedded core-shell structure that provides catalyst stability, maintaining a H₂ site-time yield of 3.6 mol/g_Ni/h at a space velocity of 24000 mL g_cat^-1·h^-1 at 550 °C for 6 h on stream in a flow reactor. In contrast, a comparable catalyst without the sheath deactivated after only 2.5 h, being characterized by a Ni-extracting tip-growth mechanism of coproduct carbon nanotube formation that destroyed the catalyst. An advantage of the ensheathed catalyst is that it facilitates a Ni-sparing base-growth mechanism. Infrared and Raman spectra, combined with other catalyst characterization data and calculations at the level of density functional theory, show that the TiO₂ sheaths barely hinder the C-H bond cleavage and carbon nanotube formation on the underlying nickel, while providing the advantages stated above. We postulate that these results provide guidance for the synthesis of catalysts for low-temperature H₂ production and related reactions requiring C-H bond activation.