Study On Biomass Activation And Ammonia Modification For Porous Nitrogen-doped Carbon

Introducing activation and ammonification into biomass pyrolysis is a vital way to prepare high value-added products,but actie oxygen-containing functional groups typical of biomass would initiate complex interaction effect during simultaneous biomass activation and NH₃ modification.Based on biomass pyrolysis and activation of carbon-based materials,this study discussed activation mechanism during biomass pyrolysis.Combined with biomass nitrogen-enriched pyrolysis,the interactions of three processes（biomass pyrolysis,chemical activation and NH₃ modification）were further explored to efficiently and greenly prepare porous nitrogn-doped carbon materials.The main research work is as follows:The effect of pyrolysis temperature（400-800°C at the ratio of 1:2）and the ratio of activator/biomass（1:8 to 1:1 at 600°C）into biomass pyrolysis with KOH,which is a typical and extensive activator,was performed in the fixed bed system.Bamboo wastes was choosed as the biomass material,due to its lower ash and nitrogen content.Chemical activation mechanism of KOH was explored through revealing the evolution mechanism of gaseous product,bio-oil,biochar and KOH,based on experiments and quantum calculations.Results showed that,during biomass pyrolysis process,KOH would react with active O-containing species and carbon fragments in biomass.At lower ratio（1:8-1:2）or lower temperature（400-600°C）,the reactions between KOH and O-containing species became the main reactions,where KOH was completely transformed to K₂CO₃,leading to forming large amounts of gaseous products and phenols（reaching 75%）in bio-oil.While the reactions between KOH and more stable carbon fragments were enhanced at higher ratio（>1:2）or higher temperature（700-800°C）,and became the main reactions.Hydrocarbons became the dominants compositions（reaching 57.43%）,while the content of phenols and O-species decreased largely.Besides,for biochar,the addition of KOH caused the great increase of oxygen content in biochar（reaching 23.68 wt.%）,especially at higher ratio.At higher temperature,the reactions between KOH and biomass were enhanced greatly,and the specific surface area（reaching1351.13 m²/g）increased sharply.And O-containing groups further transformed to more stable–OH,C-O,and–COOH groups.The synergistic effect of chemical activation and NH₃ modification on activated carbons（ACs）was explored via two contrasting bamboo pyrolysis strategies involving either two steps（activation followed by nitrogen doping in NH₃ atmosphere）or one step（activation in NH₃atmosphere）with several chemical activating reagents（KOH,K₂CO₃,KOH+K₂CO₃,and H₃PO₄）.Results showed that,for the one-step method,synergism between KOH/K₂CO₃ and NH₃ for pore structure forming is observed.The ACs attained larger surface are and pore diameters with about 90%small mesopores（2 to 4 nm）.As for the nitrogen-doping,there is great competition among activotor and ammonia to react with O-containing species,and activation also could etch carbon skeleton to expose many active sites for ammonia reacting to dope nitrogen-containing functional groups,leaving higher nitrogen content.Due to a promotion effect with the KOH+K₂CO₃ combination,the AC attained the great surface area(2417 m²g^-1)and highest nitrogen content（3.89 wt.%）,endowing the highest capacitance(175F g^-1).Bamboo H₃PO₄ activation followed by nitrogen doping in NH₃ atmosphere were performed to dope nitrogen-containing functional groups by reactions between NH₃ and phosphorus-containing functional groups.The effect of ratio（H₃PO₄/biomass）into porous nitrogen-doped carbon was investigated by comparing the pore structure and surface characteristics of biochar products in the first step and the second step.Results show that,the first step of H₃PO₄ activation made biochar products with higher specific surface area(～1000m² g^-1)and richer phosphorus-containing functional groups（mainly unstable C-P-O and C-O-P groups,～1.8 at.%）,especially at higher ratio（1.5-2.0）.During the second step of NH₃modification,NH₃ and its free radicals could react with phosphorus-containing functional groups to form functional pyridinic-N（～60%）and N-P groups,with higher nitrogen content（reaching 12.21 wt.%）.And due to the slight etching effect of NH₃ atmosphere,micopores in the biochar prepared by the first step were expanded to form small mesopores（2-6 nm）which is benefical for mass transport.The biochar prepared by the second step still remained higher specific area(～800 m² g^-1).