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 71.4 mmol hydrogen g^{- 1} plastic, while the hydrogen production using Ni-Ca-Al, Ni-Ce-Al and Ni-Zn-Al catalysts were 68.5 mmol g^{- 1}, 63.1 mmol g^{- 1} and 45.9 mmol hydrogen g^{- 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.