Influence of metal addition to Ni-based catalysts for the co-production of carbon nanotubes and hydrogen from the thermal processing of waste polypropylene

This paper investigates the co-production of hydrogen and carbon nanotubes from the pyrolysis–catalytic gasification of waste plastics (polypropylene). We report on the influence of a range of metal additions to a nickel based catalyst based on ternary mixed oxide types Ni–Metal–Al, where the metal was Zn, Mg, Ca, Ce or Mn. The results showed that of the different metal–nickel catalysts investigated, the Ni–Mn–Al catalyst was the most promising catalyst in relation to the co-production of hydrogen and CNT. For example, the Ni–Mn–Al catalyst produced 71.4mmolhydrogeng−1 plastic, while the hydrogen production using Ni–Ca–Al, Ni–Ce–Al and Ni–Zn–Al catalysts were 68.5mmolg−1, 63.1mmolg−1 and 45.9mmolhydrogeng−1 plastic respectively. In addition, carbon deposition on the catalyst was highest in the order of: Ni–Mn–Al>Ni–Ca–Al>Ni–Zn–Al>Ni–Ce–Al>Ni–Mg–Al. The carbon deposition for the Ni–Mn–Al catalyst was found to consist of mostly carbon nanotubes. Further investigation of the Ni–Mn–Al catalyst demonstrated that the interaction between Ni and catalyst support plays a significant role in the gasification process; weak metal support interaction (for the Ni–Mn–Al catalyst calcined at 300°C) resulted in a lower hydrogen production and much higher yield of carbon products. In addition, the influence of steam injection rate on hydrogen and carbon nanotube production was investigated for the Ni–Mn–Al catalyst. Increasing the steam injection rate significantly increased hydrogen production and decreased carbon deposition. However, at lower steam injection rates, the quality of the product carbon nanotubes was improved. [Display omitted] •Multi-walled carbon nanotubes (CNTs) and hydrogen are produced from waste plastic.•Mn addition to a Ni–Al catalyst produced large yields of CNTs and good yields of H2.•Steam significantly increases H2 production and decreases CNT yield.•Optimum steam injection can improve CNT quality.