Studies On Deactivation Of Catalyst And Evolution Of Carbon Deposition During Catalytic Decomposition Of Toluene Over Nickel-based Catalyst

The technology of biomass gasification can convert low-quality biomass materials into high value gasification gas,which has broad application prospects.However,tar is inevitably produced during the gasification process due to high content of volatiles in biomass,and tar seriously affects the safe operation of gasification equipment and the utilization of gasification gas.Therefore,it is very important to develop a high efficiency,economical and durable nickel-based catalyst for catalytic removal of tar.Nickel-based catalysts can be easily deactivated due to some factors such as carbon deposition and sintering during catalytic decomposition of tar.In this paper,catalytic decomposition of toluene as the model compound of tar over nickel-based catalyst was studied.The effects of deactivation factors on catalytic activity and the formation and evolution of carbon deposition during the reaction process were studied.Based on the excellent modification of iron additives to nickel-based catalysts,the similarities and differences between nickel-iron catalyst and nickel-based catalyst were compared in terms of catalytic activity and evolution of carbon deposition.The effect of iron additives on catalytic activity and carbon deposition was analyzed.First,we analyze the effects of carbon deposition,sintering of nickel particle and oxidation of nickel on the catalytic activity.In the six-hour of catalytic reforming reaction over Ni/?-Al₂O₃?NA?,the proportion of the above three deactivation factors in catalytic deactivation is 72.2%,11.1%and 16.7%,respectively.However,during the six-hour of reforming reaction over Ni-Fe/?-Al₂O₃?NFA?,the proportion of the three deactivation factors in catalytic deactivation is 80%,10%and 10%,respectively.And carbon deposition is still the factor that causes the most reduction in catalytic activity.The NA catalytic cracking,NA catalytic reforming and NFA catalytic reforming were carried out to study the evolution characteristics of different types of carbon deposition.The amorphous carbon is formed in the initial stage of the NA cracking reaction,and it converts into carbon networks in the middle and late stages of the reaction.At the initial stage of the NA reforming reaction and the NFA reforming reaction,amorphous carbon is mainly produced,and it converts into carbon nanotubes.Carbon networks and carbon nanotubes are two types of graphite carbon with different microstructure.The addition of water and iron addition promoted the formation of graphite carbon.After 60 minutes of the above three reactions,the amount of graphite carbon can reach 37.4mg/g-cata,200mg/g-cata and642mg/g-cata respectively.The infrared characterization indicates that the amorphous carbon in the cracking reaction is derived from the decomposition of toluene and a chain hydrocarbon intermediate,and the amorphous carbon formed in the middle and late stages of the reforming reaction is mainly derived from the oxygen-containing intermediate.The Raman analysis further confirms that the graphite carbon is aging in the later stage of NA cracking reaction and NA reforming reaction,and its amount is reduced by 42%and 17%in the two reactions respectively.The graphite carbon produced by the NFA reforming reaction is better quality,and aging phenomenon is not observed in the 120 minutes reaction.Finally,the kinetics of two types of carbon deposition in NA cracking,NA reforming and NFA reforming are calculated.It is concluded that the formation of amorphous carbon is more favorable in all three reactions.During 60¹20 minutes of the three reactions,amorphous carbon increased by 17%,5%,and 13%respectively.Combined with the gradually decreasing trend of catalytic activity in the middle and late stages of three reaction,it is known that the stable increase of amorphous carbon during the reaction is the main factor leading to the decrease of catalytic activity.