488 / 2018-09-09 10:31:31

The use of Ni, Fe and Ni-Fe catalyst for co-production of hydrogen and carbon nanotubes from waste plastics

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Environmental crisis that comes from the plastic debris in the marine environment has become a serious issue and attracts the worldwide attention. Thermal chemical recycling via pyrolysis into chemical products is a promising technology for tackling waste issues related to plastics. The use of Ni-Fe catalysts for the catalytic pyrolysis of real-world waste plastics to produce hydrogen and high value carbon nanotubes (CNTs) has been investigated (Fig.1). 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. Monometallic Ni or Fe based catalyst was compared with the bimetallic Ni-Fe catalyst in terms of hydrogen yield and carbon nanotubes production. Results show that Fe/γ-Al2O3 produced more hydrogen and carbon nanotubes than Ni/γ-Al2O3 because of the higher cracking ability. The maximum hydrogen yield of 31.8 mmol H2/gplastic with content of 62.88 vol.%, and 287 mg g-1plastic of uniform carbon nanotubes with diameter around 20~40 nm were obtained in the presence of bimetallic Ni-Fe/γ-Al2O3. In addition, the effect of catalyst temperature on the catalytic-pyrolysis of waste plastic was studied to explore catalytic mechanism of Ni-Fe catalyst. The decomposed carbon from the catalytic decomposition of waste plastics was catalysed by Fe3C and Ni-Fe compounds in the catalyst to produce the graphite carbon. The process for the carbon nanotubes formation was suggested to follow the vapor-liquid-solid model. The tip-growth of carbon nanotubes was observed to be dominant at higher catalyst temperature, while it was the base-growth mechanism at relatively lower temperature.