Catalytic Pyrolysis Of Polypropylene Plastic With Stainless Steel Mesh For Co-Production Of Hydrogen And Carbon Nanotubes And Study Of Lithium Storage Properties

At present,the rapid increase of plastic wastes has caused a series of problems such as environmental pollution and land occupation .Waste Polypropylene（PP）plastics can be converted into high-value carbon nanotubes（CNTs）and hydrogen by the technology of pyrolysis and catalytic deposition ,which is an effective way to realize their resource recovery and high-value utilization .However,powder catalysts commonly used for catalytic conversion of waste plastics are confronted with the problems of high preparation cost and difficult product separation .Large-scale preparation of CNTs arrays directly on stainless steel substrates has proved to be a feasible approach,but the low yield of CNTs limits its practical application .In this paper,inexpensive 316 stainless steel mesh was used as the catalytic substrate to transform PP into CNTs and H₂ with high purity and high yield.The pretreatment methods of catalyst for efficient conversion of PP were investigated,and the regulation mechanism of pyrolysis and catalytic reaction parameters on catalytic pyrolysis products was revealed.Rapid separation of solid products from substrate and rapid regeneration of catalyst were achieved,and the electrochemical properties of CNTs as anode materials of lithium ion battery were studied.The pretreatment of stainless steel 316（SS 316）catalyst was carried out by different thermochemical and wet-chemical methods.The effects of pretreatment on the physical properties of the catalyst and the catalytic pyrolysis of PP for co-production of CNTs and hydrogen were studied.The results showed that the combination of acid etching and thermal oxidation -reduction had the best pretreatment effect,with which a large number of metal particles were formed on the surface of SS316 catalyst,showing the highest root mean square roughness of 201 nm.The mole content of Fe on the surface increased from 7.21%to 25.52%while the content of Cr decreased from 8.80%to1.41%.Moreover,catalyst pretreatment significantly improved the yield of CNTs and hydrogen,which were 5.95 and 6.78 times of that without pretreatment,respectively.This effect was caused by the destruction of the original Cr₂O₃ protective layer by acid etching and thermal oxidation -reduction pretreatment,so that Fe₂O₃ and FeO were fully exposed on the surface of stainless steel,and the surface roughness of the material was greatly increased.In a pyrolysis-catalytic deposition system,the reaction parameters of CNTs and hydrogen co-produced by PP plastics were regulated.Taking CNTs and hydrogen as target products,the optimal condition s for simultaneous production of high yield and high purity CNTs and hydrogen were determined by regulating the pyrolysis method,rate and catalytic temperature.The results showed that the highest yield of carbon(653.70 mg/g _plastic)and yield of hydrogen(50.66 mmol/g _plastic)could be obtained by slow pyrolysis at a heating rate of 10℃/min and catalytic temperature of 800℃.CNTs had a higher degree of graphitization and fewer surface defects under these condition s.After CNTs were separated by ultrasound,the regeneration of SS 316 catalyst could be achieved without any treatment,and the catalytic activity remained at a high level after regeneration .The highest yields of CNTs and hydrogen reached 662.5 mg/g _plastic and 51.26 mmol/g _plastic in the course of 10cycles.Several possible reaction paths from PP plastics to final CNTs products were proposed by means of mold experiments.Among them,C1-C4 alkenes followed the"gas-liquid-solid"mechanism and showed the pattern of top growth.Monocyclic aromatic hydrocarbons followed the"gas-solid-solid"mechanism,and the bottom growth pattern was more extensive.The electrochemical properties of CNTs prepared from PP plastics as anode material for lithium ion batteries were studied.CNTs were prepared for thermal oxidation post-treatment,and the phase composition of the materials before and after treatment was characterized,and the lithium storage characteristics were studied.The results showed that the material before thermal oxidation treatment was identified as Fe-Ni alloy/MWCNTs composites.The results of the continuous charge-discharge cycles at the current density of 400 m Ag^-1 showed that the specific capacity of the material was low at the initial stage of cycles,and the reversible specific capacity of the first 50 cycles was less than250 mAh g^-1.However,as the cycle continued,the specific capacity of the material showed an increasing trend.After 500 cycles,the reversible specific capacity of the material remained 435.9mAh g^-1.The material treated by thermal oxidation was identified as Fe₂O₃/Ni O/MWCNTs composites.The results of continuous charge-discharge cycles at 400 m Ag^-1 showed that the specific capacity remained higher than 300 mAh g^-1 during 500 cycles.After 150 cycles,the reversible specific capacity reached the highest 452.6 mAh g^-1,which was 1.2 times of the theoretical value of commercial graphite.After that,it maintained a high capacity retention rate of more than 85%,showing a long cycle life.