Preparation And CO₂ Adsorption Of Nitrogen-Doped Porous Carbons

The concentration of CO2 in the atmosphere has increased to 400 ppmv at January 2015, a level 120 ppmv higher than the preindustrial period. This has caused serious concerns for climate change, since CO2 is generally recognized as a greenhouse gas and therefore a major contributor to global warming. To mitigate CO2 emissions, carbon capture and sequestration (CCS) is considered to be one of the most promising solutions. Among all CO2 emission sources, fossil fuel-burning power plants are the single largest anthropogenic sources globally, accounting for approximately one-third of CO2 emissions. Accordingly, capture of CO2 from flue gas streams in fossil fuel based power plants has been considered one of the major strategies for reducing anthropogenic CO2 emissions. CO2 is normally vented as 10-15%component of flue gas that contains a balance of nitrogen, water vapor and other minor components. Effective capture of CO2 under this low CO2 concentration or partial pressure remains a challenge. Currently, the commercial process for post-combustion capture is amine scrubbing. However, this method suffers from high energy consumption during sorbent regeneration, along with other issues such as equipment corrosion, solvent loss and toxicity. To overcome such challenges, adsorption via solid sorbent has been proposed as an attractive alternative. Generally speaking, adsorption has the advantages of low investment in equipment and easy operation, less energy intensive, easy in sorbent regeneration, and no equipment corrosion problems. However, the key of this new technology is to find solid sorbents with superior CO2 adsorption properties. In this thesis, nitrogen-doped porous carbons have been prepared and their CO2 adsorption properties have been investigated. The summaries of the results are as follows:1. N-doped carbons were synthesized by KOH activation of urea-modified petroleum coke. A series of nitrogen-doped highly porous carbons were synthesized by changing the preparation parameters. The resulting carbons were characterized by elemental analysis, FTIR, TEM, SEM, XPS, XRD and nitrogen sorption etc. The CO2 adsorption properties of these sorbents were also tested at 25℃ and 0℃ under atmospheric pressure. It has been found that this series of carbons exhibits high CO2 adsorption capacities ranging from 3.69-4.40 mmol/g and 5.69-6.75 mmol/g at 25℃ and 0℃ under atmospheric pressure, respectively. Specifically, the sample UC-650-2 prepared at 650℃ with KOH/precursor ratio of 2 shows the highest CO2 uptake of 4.40 mmol/g at 25℃, which is among the best of the known nitrogen-doped porous carbons. This high CO2 capture capacity is due to the synergy of the sorbent’s high microporosity and nitrogen content. In addition, the CO2/N2 selectivity of the sorbent is 17, higher than that of many reported CO2 sorbents. The multi advantages of the sorbent including its easy synthesis, low cost, high CO2 uptake capacity and selectivity as well as regenerability demonstrates that the nanoporous nitrogen-doped carbon is promising for CO2 capture.2. Nitrogen-doped porous carbons were prepared by the above mentioned method but using coconut shell, a biomass resource, as the carbon precursor. The obtained sorbents were carefully characterized by various analytical techniques and their CO2 adsorption capacities were tested at 25℃ and 0℃ under atmospheric pressure. The results show that the optimal coconut shell-based nitrogen-enriched porous carbon possesses CO2 uptake of 4.78 mmol/g and CO2/N2 selectivity of 19 at 25℃, which is higher than the petroleum coke-based sorbents. Based on the results of structural characterizations and CO2 adsorption properties of the sorbents, it is found that CO2 adsorption capacity of these nitrogen-doped sorbents is determined by their microporosity and nitrogen content.3. To further increase the nitrogen doped content and microporosity of the sorbents, petroleum coke were pretreated by nitric acid then modified with urea, followed by KOH activation. The nitrogen-enriched porous carbons prepared through this method have a higher nitrogen content and microporosity but lower CO2 uptake under the same test conditions compared with analogue nitrogen-doped sorbents obtained without pretreatment of precursor. These results show that besides nitrogen content and microporosity, there are other factors that play a very important role in determining the CO2 adsorption capacity of sorbents. After careful analyzing the characterization results, we found that although the total nitrogen content is increased due to the acid pretreatment, the relative amount of pyrrolic-N, which has been documented to have a much greater contribution to CO2 capture, is small. This may cause the lower CO2 uptake capacity. Despite this, the optimal sample of this series showed higher CO2 adsorption capacity than the analogue sorbent prepared by KOH activation of petroleum coke directly.