Development of Metallic Nickel Nanoparticle Catalyst for the Decomposition of Methane into Hydrogen and Carbon Nanofibers

Metallic nickel nanoparticles were prepared primarily as catalysts for the thermal decomposition of methane to produce hydrogen. Nickel particle aggregates with controlled crystalline size and primary particle size were prepared first by the precipitation of nickel nitrate and oxalic acid in ethanol solution, followed by the thermal decomposition of nickel oxalate dihydrate under an oxygen-free atmosphere. The concentration of the reactants was found to play a critical role in the morphology and crystalline size of the nickel oxalate dihydrate and subsequently the resulting metallic nickel catalyst. The nickel oxalate dihydrate showed different crystalline sizes and morphologies including nanosheets, nanorods, and nanoneedles, depending on the concentration of the reactants. A series of decomposition atmospheres (CH4-N-2 in different ratios) were used to investigate their effects on the morphology and crystalline size of the metallic nickel particles. The addition of CH4 in the gas stream during the nickel oxalate reduction effectively prevented sintering and led to aggregates of smaller particles with larger surface areas. The metallic nickel particle aggregates were used as unsupported catalysts for methane decomposition, and these nickel particles showed promising catalytic activities due to their small particle sizes (30-40 nm). Transmission electron micrographs showed that under different reaction temperatures the nickel catalysts underwent a self-regulating process to stabilize themselves with appropriate particle sizes. Higher temperature led to the formation of smaller and more uniform nickel particles on the tips of the carbon nanofibers as smaller catalyst particles could be more easily activated.