CO₂ Adsorption Properties Of Nitrogen-doped Porous Carbon Synthesized By Low-temperature Sodium Amide Activation

The prospect of a deteriorating climate situation originating from global warming is a subject of widespread public concern.Carbon dioxide is one of the main greenhouse gases and its emissions are still increasing year by year.The uncontrolled emission of CO₂ will seriously affect the ecological environment and human life.There are many ways to separate and capture carbon dioxide,among which adsorption through solid materials is one of the most promising methods.The key to this method is the adsorbents with excellent adsorption performances.Among various adsorbents,nitrogen-doped porous carbon have shown great promise.In prevrious studies,KOH is widely used to obtain carbons with highly developed porous struture,which in turn possess very good CO₂ adsorption capacities.However,the high corrosivity of KOH under high temperature limits its further application.To solve this issue,in this thesis,sodium amide was explored as chemical activator to synthesis porous carbons under relatively low temperature.Sodium amide has strong nucleophilicity and alkalinity,which make it effectivley activate the precursor to achieve porous carbons under the temperature ranges of 400-500?.Also,sodium amide possesses lower corrosivity than KOH,which will not damage the equipment during the synthesis process.Moreover,sodium amide can act as both activator and nitridizing agent,which can produce N-doped porous carbon by a single reaction step.In this thesis,the nitrogen-doped porous carbons were synthesized by low-temperature sodium amide activation using lotus stalk shell,water chestnut shell,and petroleum coke as carbon precursors,respectively.The CO₂ adsorption properties of the as-synthesized carbons were carefully investigated.The corresponding CO₂adsorption mechanism for these sorbents was further explored based on various characterization results.The results are summarized as follows:1.Nitrogen-doped porous carbons were synthesized by carbonization of biomass lotus stalk followd by sodium amide activation.In this section,a series of carbonaceous adsorbents with different porous structures and surface chemical properties were obtained by changing the alkali/precursor ratio?1:1,2:1,3:1?and activation temperature?400?,450?,500??.The adsorbents were carefully characterized by different techniques such as nitrogen sorption,SEM,TEM,XRD,elemental analysis,and XPS etc.The CO₂ adsorption capacities of this series of samples was tested at 1 bar,25?and 0?,respectively.The corresponding CO₂uptake of these sorbents range from 3.12 to 3.88mmol/g and from 3.96 to 5.62mmol/g under 25?and 0?,respectively.The sample?LSC-500-1?obtained at alkali/precursor ratio of 1 and activation temperature of 500?exhibits the maximum CO₂ uptake among this series of samples under ambient conditions.Detailed studies showed that the combination of narrow micropore,nitrogen content and pore size distribution of the sorbents determine their CO₂ adsorption capacities under ambient conidtions.In addition to the excellent CO₂ adsorption capacity,this series of samples also have other multiple advantages like fast adsorption kinetics,good recycling,high CO₂/N₂ selectivity,moderate heat of adsorption,and excellent dynamic CO₂ capture capabilities.The excellent CO₂ adsorption performance,low-cost raw precursor and simple synthesis process make this series of nitrogen-doped porous carbon adsorbents promising CO₂ capture.2.To further improve the CO₂ adsorption capacities of the adsorbents,in this part,water chestnut shell,with harder texture than lotus stalk,were used as the precursor.Nitrogen-doped porous carbons were prepared under the same conidtions as the above section.It turns out that the maximum CO₂ uptake for these water chestnut shell-derived carbons is 4.50 mmol/g and 6.04 mmol/g under 1 bar,25?and 0?,respectively.To the best of our knowledge,this CO₂ adsorption capacity?4.50 mmol/g?is one of the highest values obtained by carbonacous adsorbents synthesized by non-KOH activation.Further investigation shows that the synergetic effect of narrow micropore,nitrogen content and pore size distribution decides the CO₂ uptake of these carbons.The improved CO₂ adsorption capacities greatly increase the potential application of this series of nitrogen-doped carbonaceous materials in the realistic CO₂ capture.3.The above two sections have shown that sodium amide has great advantages in both activation and nitridation.However,two step reactions i.e.carbonization and activaion were explored in both processes.In order to further simplify the preparation process,in this part,petroleum coke was used as the carbon precursor,and sodium amine was used as nitrding and activator agent.Thus,nitrogen-doped porous carbon were synthesized by a genuine single-step method.The maximum CO₂ adsorption capacities for this series of samples was 3.84 mmol/g and 5.93 mmol/g under 1 bar,25?and 0?,respectively.This single-step strategy can greatly simpligy the synthesis procedure and thus save the sorbent preparation cost.Furthermore,these nitrogen-doped porous carbon adsorbents have multiple merits such as high CO₂uptake,easy regeneration,good recyclability,fast adsorption kinetics,high CO₂/N₂selectivity,and excellent dynamic CO₂ capture capabilities.The simple preparation method,the inexpensive and widely available carbon precursor make these petroleum coke-based nitrogen-doped porous carbon have a wider application prospect.