Synthesis And Growth Mechanism Of Carbon Nanotubes Catalyzed From Waste Plastic Gasification Gas

Plastic products exhibited stable chemical properties,lightweight characteristics,exceptional impact resistance,and affordability,making them extensively utilized in both industrial manufacturing and daily life applications.Nevertheless,the consequential issue of"white pollution"has experienced a noteworthy escalation in severity.The global outbreak of the COVID-19 pandemic has further amplified the demand for plastic products in diverse industries,thereby intensifying this problem.The clean and high-value energy harnessing of waste plastics has emerged as a prominent research focus on a global scale.In this study,we employed the fixed-bed reactor chemical vapor deposition（CVD）method,utilizing a Ni-based catalyst,to accomplished the gasification and cracking of polyethylene.This process yields high-value carbon nanotubes（CNTs）and hydrogen-rich syngas.The catalytic efficiency was regulated through the addition of promoter,while a plasma pretreatment system was employed in conjunction with the chemical vapor deposition system to comprehensively investigated the growth mechanism of CNTs,controlled and optimized their structural development,and enhanced the quality of hydrogen-rich syngas.The specific research content was presented as follows:（1）In order to address the issues of low carbon nanotubes（CNTs）content and low carbon conversion rate during the process of preparing CNTs by waste plastic gasification used the chemical vapor deposition（CVD）method.This study investigated the influence of temperature on CNTs growth and hydrogen production.Experimental results demonstrated that an increase in catalytic temperature positively affects the CNTs yield,carbon conversion rate,and hydrogen production rate of the Ni/ZSM5 catalyst.Within the temperature range of 400～600°C,the CNTs yield increased by approximately 1.8～2.6 times,the carbon conversion rate improved by around1.8 times,and the hydrogen production rate increased by 11.5 times.As the catalytic temperature rose,the CNTs yield,carbon conversion rate,and hydrogen production rate gradually increased.However,when the catalytic temperature reaches 700°C,the catalyst’s activity becomes inhibited,resulting in a decline in CNTs yield,carbon conversion rate,and hydrogen production rate.At 800°C,the catalytic performance improved.The CNTs obtained under the experimental conditions were all multi-walled carbon nanotubes,and the CNTs content in the carbon deposits continuously increased with the temperature rose（400～800°C）,reaching a maximum of 98.08%at the highest temperature.The diameter of the CNTs ranges from 39.5 to 55.3 nm and gradually increases with the temperature rose.（2）In this study,Mn was incorporated into a monometallic Ni-based catalyst to investigated its effect on the growth mechanism of carbon nanotubes（CNTs）and the production of hydrogen-rich gas.The findings demonstrated that the introduction of Mn promote favorable conditions for CNTs growth,particularly at elevated growth temperatures ranging from 600 to750°C,resulting in a CNTs content exceeding 75%in the carbon deposits.Specifically,at650°C,the Ni Mn/ZSM5 catalyst achieved the highest carbon yield(2.9 g _CNTs/g _catalyst)and the highest H₂ content in the syngas（39 vol.%）.When comparing the carbon yield and the quality of hydrogen-rich gas produced by the Ni Mn/ZSM5 catalyst with that of the Ni/ZSM5 catalyst at the same catalytic temperature,an improvement in catalytic performance by a factor of 1.3and 1.8 was observed,respectively.The introduction of the promoter Mn enhanced the interaction between the Ni particles and the catalyst support,leading to a transition in the growth mechanism of CNTs from top growth to bottom growth.As a result,the CNTs grown on the Ni Mn/ZSM5 catalyst exhibited a more orderly and smoother structure.Additionally,the inclusion of Mn enhanced the dispersion of the metallic Ni on the support surface and increased the deposition-diffusion rate of free carbon on the active metal particles.（3）Plasma pretreatment was utilized to modify the catalyst and investigate its influence on the growth of carbon nanotubes（CNTs）and hydrogen production,as well as its impact on the deactivation phenomenon of the La Ni O₃ catalyst.Three Ni-based catalysts underwent low-temperature dielectric barrier discharge（DBD）plasma pretreatment used Ar gas as the carrier.Subsequently,the pretreated catalysts were employed in the CVD method to synthesized carbon nanotubes and conduct hydrogen production experiments.The magnitude of the metal-support interaction（MSI）among the three catalysts was determined as follows:Ni/ZSM5<Ni Mn/ZSM5<La Ni O₃.When these three catalysts were applied for the co-production of carbon nanotubes and hydrogen gas from waste plastic gasification,it was observed that the diameter of the CNTs grown on the catalysts decreased with increasing MSI.Ar plasma pretreatment of the catalyst led to enhanced catalytic activity,with the La Ni O₃ catalyst consistently exhibiting the highest carbon yield,gas component conversion rate,and H₂ content in the syngas under identical experimental conditions.The optimal catalytic temperature for the La Ni O₃ catalyst was determined to be 650°C,where the CNTs yield and carbon conversion rate reached their maximum values of 7.43 g _CNTs/g _catalyst and 35.94%,respectively.At the same temperature,the carbon nanotube yield and carbon conversion rate of the Ar plasma pretreated La Ni O₃ catalyst increased by 1.15 times and 1.28 times,respectively.At 700°C,the carbon nanotube yield and carbon conversion rate further increased to 10.79 g _CNTs/g _catalystand 58.62%,respectively,on the Ar-La Ni O₃ catalyst,while the H₂ content in the syngas reached its highest value of 66.15 vol.%.In contrast,the non-pretreated La Ni O₃ catalyst exhibited significant deactivation at 700°C.Plasma pretreatment improved the adsorption characteristics and catalyst structure,thereby mitigating the deactivation phenomenon of the La Ni O₃ catalyst at 700°C.The CNTs grown on the La Ni O₃ catalyst followed a bottom growth mechanism,while after Ar plasma pretreatment,both top and bottom growth mechanisms coexisted,with a higher proportion of top-grown CNTs.