Co-production of hydrogen and carbon nanotubes from real-world waste plastics: Influence of catalyst composition and operational parameters

The use of Ni-Fe catalysts for the catalytic pyrolysis of real-world waste plastics to produce hydrogen and high value carbon nanotubes (CNT), and the influence of catalyst composition and support materials has been investigated. Experiments were conducted in a two stage fixed bed reactor, where plastics were pyrolysed in the first stage followed by reaction of the evolved volatiles over the catalyst in the second stage. Different catalyst temperatures (700, 800, 900 degrees C) and steam to plastic ratios (0, 0.3, 1, 2.6) were explored to optimize the product hydrogen and the yield of carbon nanotubes deposited on the catalyst. The results showed that the growth of carbon nanotubes and hydrogen were highly dependent on the catalyst type and the operational parameters. Fe/gamma-Al2O3 produced the highest hydrogen yield (22.9 mmol H-2/g(plastic)) and carbon nanotubes yield (195 mg g(plastic)(-1)) among the monometallic catalysts, followed by Fe/alpha-Al2O3, Ni/gamma-Al2O3 and Ni/alpha-Al2O3. The bimetallic Ni-Fe catalyst showed higher catalytic activity in relation to H-2 yield than the monometallic Ni or Fe catalysts because of the optimum interaction between metal and support. Further investigation of the influence of steam input and catalyst temperature on product yields found that the optimum simultaneous production of CNTs (287 mg g(plastic)(-1)) and hydrogen production (31.8 mmol H-2/g(plastic)) were obtained at 800 degrees C in the absence of steam and in the presence of the bimetallic Ni-Fe/gamma-Al2O3 catalyst.