Nanoscale Core–Shell Catalysts for H2 Production by Methane Decomposition: Supported Nickel Nanoparticles Ensheathed in Metal Oxides

Abstract

Catalytic methane decomposition provides a potentially viable route to low-cost, CO_x-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.