CARBON NANOTUBES AND HYDROGEN FROM PYROLYSIS-GASIFICATION OF
WASTE PLASTICS
J. C. Acomb, C. Wu, and P. T. Williams*
Energy Research Institute,
The University of Leeds, Leeds LS2 9JT, UK.
(Tel: 44 1133432504; Email: p.t.williams@leeds.ac.uk)

Abstract
The production of hydrogen and carbon nanotubes (CNTs) from different plastics in a two stage pyrolysis-gasification reactor
has been investigated in relation to process conditions. CNTs grew on the nickel catalyst and were analysed by electron
microscopy, thermogravimetric analysis and Raman spectroscopy. One of the key process parameters was the introductionof steam which proved significant in the production of hydrogen with a rise observed at higher steam injection rates as steam
reforming reactions increase. CNTs were produced from low density polyethylene, polypropylene and polystyrene; however it
was found that the addition of water influenced the growth of CNTs differently depending on the plastic source. The largest
CNT yield was obtained for polystyrene at a steam injection rate of 0.25 g/hr, and gave a conversion rate to CNTs of 29% wt.
The CNTs produced were multiwalled CNTs and had a diameter of around 15nm. By simply changing the rate of steam
injected the major product produced from the plastics can be shifted between CNTs and hydrogen, giving the potential for anindustrial process which has great flexibility over its production.

Keywords: Carbon nanotubes, hydrogen, pyrolysis, gasification, plastics

1. Introduction
Carbon nanotubes (CNTs) are a material that havegenerated a great deal of scientific interest over recentyears as a result of their remarkable properties, including
a high strength, a large surface area and good electricalconductivity. The potential to produce them from a waste
source such as waste plastics holds the opportunity tosimultaneously tackle waste management issues andprovide a cheap source of CNTs. Plastics pose a serious
waste management problem since they make up asignificant proportion of municipal solid waste, up to 10%
by weight in England [1] but have a low recycling rate,
only 25% in Europe [2].
Whilst a number of studies have produced CNTs fromplastics before [3-7], to our knowledge this is the firststudy that has investigated the addition of steam toincrease the yield of CNTs from a plastic feedstock. The
use of steam for CNT production with an acetylene
feedstock was investigated by Hata et al [8]. The
injection of steam was found to be beneficial to theproduction of CNTs since reacts with amorphous carbons
which deactivate the catalyst and prevent further CNT
growth. This allowed larger yield and a higher purity ofCNTs to be produced. This study aims to investigate theeffect of different steam injection rates on the productionof CNTs and hydrogen from various plastic sources.

2. Materials and Methods
Three different plastics were investigated for their ability
to produce hydrogen and CNTs; low density
polyethylene, polypropylene and polystyrene. Thecatalyst used was Ni-Al2O3 prepared by impregnation,with a nickel loading of 5%.
The experimental procedure was conducted in a twostage pyrolysis-gasification reactor. Plastics werepyrolysed in nitrogen at 600°C, before the evolved gases
were passed to a second stage where steam wasinjected and the gases were heated at 800°C in thepresence of a nickel-alumina catalyst. Carbon depositionin the form of CNTs would then occur on the surface ofthe catalyst. Experiments were conducted without steam
injection and at injection rates between 0.25 g/hr and4.74 g/hr. The volatile products after the gasification
process were passed through two condensers, where
any condensed products were collected. The noncondensed gases were collected in a 25 L Tedlar™ gas
sample bag.
Gases were analysed by gas chromatography whilst theCNTs grown on the surface of the catalyst were analysedby scanning electron microscopy (SEM), transmissionelectron microscopy (TEM), temperature programmedoxidation and Raman spectroscopy.

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013

3. Results and Discussion
The introduction and variation of steam had a profoundeffect on the production of hydrogen. As can be seen in
figure 1 the yield was fairly low when no steam wasadded, however the addition of steam increased this,with further increases seen as the steam injection rate
was raised. This was most likely due to an increase insteam reforming reactions and the reaction of steam withamorphous carbons.

Figure 3: TEM image of CNT from plastic source

Figure 1: Hydrogen production from LDPE, PPand PS

CNTs were produced from all the plastic samples, withtypical SEM and TEM images of the nanotubes produced
shown in figures 2 and 3. From the SEM image in figure
2 it can be seen that a small amount of amorphouscarbons and a large amount of long thin filament type
carbons are present. The long thin deposits were thought
to be CNTs, and this was confirmed with the use of TEM.
As shown in figure 3, multi-walled carbon nanotubes witha diameter of around 15nm were observed.

Temperature programmed oxidation was also used todetermine the amount of CNTs that were produced. From
the derivative plot different peaks were observed foramorphous and nanotubular carbons, and so the weightassociated with the CNTs produced could be calculated.
It was observed that increasing the steam injection rateaffected each plastic differently in terms of producing
CNTs. The largest CNT yield was obtained forpolystyrene with a steam injection rate of 0.25 g/hr, with29% of the plastic sample converted into nanotubes.

4. Conclusions
CNTs and hydrogen were successfully synthesized fromthree different plastic feedstocks, showing a range ofplastic sources can be used to produce CNTs. Overall it
was seen that the steam injection rate had a strong
influence on production of both CNTs and hydrogen. As
a result the process can be tailored to maximize either
the production of hydrogen or CNTs from plastic sources.
To optimize either the production of hydrogen or CNTs,
the rate of steam injection simply needs altering, giving
the potential for good flexibility as an industrial process.

References

Figure 2: SEM image of carbon deposits on nickel
catalyst[1] 'Municipal Waste Composition: A Review of Municipal
Waste Component Analyses', (2009), Department forEnvironment, Food and Rural Affairs[2] 'Plastics - the Facts 2011: An analysis of European
plastics production, demand and recovery for 2010',(2011), Plastics Europe, EuPC, EuPR, EPRO
[3] Arena, U., et al., Polymer Degradation and Stability,
91, (2006), 763-768[4] Chung, Y.-H. and S. Jou, Materials Chemistry andPhysics, 92, (2005), 256-259[5] Kukovitskii, E.F., et al., Chemical Physics Letters,266, (1997), 323-328[6] Liu, J., et al., Polymer Degradation and Stability, 96,(2011), 1711-1719[7] Pol, V.G. and P. Thiyagarajan, Journal ofEnvironmental Monitoring, 12, (2010), 455-459[8] Hata, K., et al., Science, 306, (2004), 1362-4

7th International Symposium on Feedstock Recycling of Polymeric Materials (7th ISFR 2013)
New Delhi, India, 23-26 October 2013