Study On Non-thermal Plasma-Catalytic Reforming Of Toluene Over Nickel-Based Catalysts

Biomass gasification is an efficient way of biomass energy utilization,but tar produced in the gasification process is a bottleneck restricting the development and application of gasification.The conventional approaches of tar removal mainly include physical method,thermal cracking and catalytic conversion,which are generally limited by low selectivity of product,high energy consumption and poor catalyst stability,etc.In recent years,non-thermal plasma(NTP)for tar reforming has attracted much attention due to its high reactivity at low temperatures and easy operation.However,the selectivity of targeted products in tar reforming still needs to be improved by the plasma alone system.Nickel-based catalysts are widely used in tar reforming due to their excellent C-H bond and CC bond breaking ability,low cost and easy access,but facing the problem of rapid deactivation caused by coke deposition.Based on this,a plasma-catalytic reforming system was constructed by combining NTP with nickel-based catalysts to realize the efficient conversion of tar to high value-added syngas,and effectively prevent the coke deposition on the catalyst surface,which provided a feasible technique for the removal and reuse of biomass tar.In this paper,toluene was selected as a tar model compound and reformed by a dielectric barrier discharge(DBD)reactor coupled with a series of supported nickel-based catalysts.The effects of different experimental parameters(such as nickel loadings,reaction temperature and discharge power)for toluene reforming were investigated,and the results were as follows:(1)Ni/HZSM-5 catalysts were prepared by hydrothermal synthesis assisted by the template method,and the reaction performance of plasma-catalytic reforming of toluene was investigated.The results showed that the combination of plasma and catalysis could significantly improve the toluene reforming in comparison with the catalyst alone and the NTP alone system.With the increase of reaction temperature and discharge power,the toluene conversion,H2 selectivity and CO selectivity all increased.When the temperature,discharge power and S/C were fixed at 525℃,75 W and 1,respectively,the activities of plasma-catalytic reforming with different catalysts were in the following sequence:15%Ni/HZSM-5>20%Ni/HZSM-5>10%Ni/HZSM-5>HZSM-5>commercial HZSM-5>NTP.15%Ni/HZSM-5 catalyst obtained the highest toluene conversion(97.58%),H2 selectivity(31.72%)and CO selectivity(42%).According to the results of XPS and H2-TPR analysis,15%Ni/HZSM-5 catalyst had abundant surface adsorbed oxygen(Osur),lower reduction temperature and higher mobility of Osur,which were conducive to the surface oxidation of toluene molecule and intermediates adsorbed on the catalyst surface,thus accelerating the reaction of toluene reforming.(2)The hierarchical Ni/HZSM-5 catalysts were obtained by modifying the pore structures of traditional HZSM-5 through steam assisted crystallization combined with the in-situ synthesis method,and then coupled with NTP for toluene reforming.The results showed that when the temperature and discharge power was 525℃ and 75 W,respectively,the toluene conversion activities of all catalysts were higher than 95%.Among them,the hierarchical 15%Ni/HZSM-5 showed the best performance of toluene reforming,with the highest toluene conversion of 99.18%,H2 selectivity of 48.28%and CO selectivity of 35.97%.Combined with the analysis of the physicochemical properties of the catalyst,it was confirmed that the hierarchical 15%Ni/HZSM-5 effectively improved the pore structure of the traditional HZSM-5.Its large specific surface area(361.8 m2/g)and abundant pore structure could provide more active sites for toluene reforming,which were conducive to mass transfer and diffusion of the reactants and reduce the intermediate products staying in the pore,so as to effectively reduce the coke deposition and improve the activity and stability of the catalyst.(3)The Ni/nitrogen-doped carbon(Ni/ZIF-8-C)catalysts were prepared by the deposition-precipitation method combined with glucose assisted hydrothermal method,and the performance of plasma-catalytic reforming of toluene was investigated.The results showed that when the discharge power was 75 W and reaction temperature was 525℃,the toluene conversion,H2 selectivity and CO selectivity of 15%Ni/ZIF-8-C catalyst were improved by 4.79%,35.62%and 26.83%,respectively,in comparison with those of the NTP system,respectively.After continuous reaction for 48 h,the 15%Ni/ZIF-8-C catalyst still maintained high catalytic activity and stability.Finally,according to the analysis of the optical emission spectrum and gas chromatography-mass spectrometry,the mechanism of plasma-catalytic reforming of toluene was proposed.Firstly,H2O and N2 undergo dissociation and collision to form·OH,·N and N2*free radicals and excited particles.Groups such as CH3 and ·HC=CH· were formed through dehydrogenation and destruction of the benzene ring.Through a series of combination and recombination,intermediate products such as xylene,ethylbenzene,benzaldehyde and benzonitrile were formed.Then,these intermediates were further oxidized by·O,·O2*and ·OH to form CO and CO2,and H radicals combined to generate H2.