Co-precipitation, impregnation and so-gel preparation of Ni catalysts for pyrolysis-catalytic steam reforming of waste plastics

Dingding Yao,Haiping Yang,Hanping Chen,Paul T Williams

doi:10.1016/j.apcatb.2018.07.075

Abstract

Three Ni/Al2O3 catalysts prepared by co-precipitation, impregnation and sol-gel methods were investigated for the pyrolysis-steam reforming of waste plastics. The influence of Ni loading method on the physicochemical properties and the catalytic activity towards hydrogen and carbon monoxide production were studied. Three different plastic feedstocks were used, high density polyethylene (HDPE), polypropylene (PP) and polystyrene (PS), and compared in relation to syngas production. Results showed that the overall performance of the Ni catalyst prepared by different synthesis method was found to be correlated with the porosity, metal dispersion and the type of coke deposits on the catalyst. The porosity of the catalyst and Ni dispersion were significantly improved using the sol-gel method, producing a catalyst surface area of 305.21 m2/g and average Ni particle size of 15.40 nm, leading to the highest activity among the three catalysts investigated. The least effective catalytic performance was found with the co-precipitation prepared catalyst which was due to the uniform Ni dispersion and the amorphous coke deposits on the catalyst. In regarding to the type of plastic, polypropylene experienced more decomposition reactions at the conditions investigated, resulting in higher hydrogen and coke yield. However, the catalytic steam reforming ability was more evident with polystyrene, producing more hydrogen from the feedstock and converting more carbon into carbon monoxide gases. Overall the maximum syngas production was achieved from polystyrene in the presence of the sol-gel prepared Ni/Al2O3 catalyst, with production of 62.26 mmol H2 g−1plastic and 36.10 mmol CO g−1plastic.

Highlights

World production of waste plastic grows year by year, as a consequence of the huge demand for plastic materials in every commercial field [1]
According to the Scherrer equation, the average crystallite size of Ni based on the main peak at around 2θ at 44.5° was determined to be 26.17, 52.28 and 19.69 nm for the Ni/Al-Co, Ni/Al-Im and Ni/AlSg catalysts, respectively. This indicates that a higher Ni dispersion and smaller Ni particles were found for the catalyst prepared by the sol-gel method compared with impregnation and co-precipitation
The Ni/Al catalyst prepared by the sol-gel method generated higher H2 and CO yields from waste plastics than the catalysts prepared by coprecipitation and impregnation, due to the higher surface area and fine nickel particle size with uniform dispersion

Summary

Introduction

World production of waste plastic grows year by year, as a consequence of the huge demand for plastic materials in every commercial field [1]. There is an urgent need to develop more effective methods to process waste plastics and improve its utilization efficiency. Chemical recycling processes such as pyrolysis are an effective option to recover energy from waste plastic. Erkiaga et al [8] compared the products from pyrolysis (500 °C)-steam reforming of HDPE with those from a gasification (900 °C)-steam reforming system by the same authors [9] Results show that the former one produced a high H2 yield of 81.5% of the maximum stoichiometric value, which was a little bit lower than the later one (83%) but enabling a more energy-efficient technology for plastics utilization. The pyrolysis and in-line steam reforming of waste plastics has been reviewed by Lopez et al [10], and they reported that more than 30 wt.% of H2 yield with up to 70 vol.% of concentration could be obtained

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Conclusion