Carbon dioxide, which is a by-product of fuel processing and mainly released as one of the flue gases from fossil fuel combustion processes and industrial power plants, is the main reason for global warming and climate change. Adsorption is one of the most promising technologies in capture and storage of CO2 in terms of its low cost, low energy demand, ease of applicability and reusability after many processes. Carbonaceous materials, such as activated carbon (AC) based adsorbents are ideal candidates for CO2 adsorption due to their high surface area, big pore volume, high CO2 selectivity, stable adsorption capacity, adequate adsorption/desorption kinetics and high mechanical strength after repeated adsorption/desorption cycles at ambient pressure and temperature. Thus, modification of AC through different thermal and/or chemical pretreatments has been a great interest for researchers aiming to enhance effectiveness of AC’s specific properties in selective capture of CO2.
The objective of this preliminary study is to increase the pure and selective CO2 adsorption capacity of the AC-based adsorbents by introducing basic N-containing surface groups through liquid ammonia treatment methods. The effect of wet-NH3 treatment methods, ammonia concentration and thermal pretreatments were investigated. The effect of gaseous ammonia (amination) and ammonia-oxygen (ammoxidation) treatment at high temperatures to produce highly CO2-selective AC-based adsorbents are currently studied.
Commercial activated carbon supplied by NORIT ROX 0.8 was crushed and sieved into 355-250 μm (45-60 mesh) particle size and referred to as AC0. Then, approximately 20 gram of AC0 was placed in an extraction thimble and was washed with 200 ml of 2 N HCl acid solution for 12 hours in a Soxhlet apparatus under reflux to remove some ash content and sulfur accompanied with it. As the next step, the slurry was rinsed with 250 ml deionized (DI) water and washed again for 6 hours in the Soxhlet apparatus to remove HCl remaining on the support surface. Finally, the slurry was dried at 115 °C overnight. This support is referred to as AC1.
Current experimental procedure on wet NH3 treatment methods were based on the former studies of ammonia-modified AC-based adsorption [1,3]. Samples were exposed to different thermal and/or chemical pretreatments prior to being used as adsorbent. 2 g of AC1 samples were placed in 150 mL of 10 wt.% and 25 wt.% NH3 solutions (Merck-analytical reagent grade) and left for 48 h at room temperature. After this time, the solutions were washed with DI water and vacuum-filtered until the filtrate showed a pH value of 7. These supports were referred to as AC1-10NH3w and AC1-25NH3w. Finally, the samples were dried at 115 °C for 24 h. Another NH3 treatment technique is incipient-to-wetness-impregnation method. 2 g of AC1 samples were placed in Büchner flasks and impregnated with 10 wt.% and 25 wt.% NH3 solutions (2.1 ml DI water/g AC) and were dried at 115 °C for 24 h. Finally, the samples were calcined under 150 ml/min N2flow at 250 °C for two hours. These supports were referred to as AC1-10NH3i-250 and AC1-25NH3i-250. AC1-25NH3w sample was subjected to thermal treatment under 50 ml/min helium flow for 2 h at 600 °C and this support is denoted as AC1-25NH3w-600He.
The adsorption/desorption tests of AC-based adsorbents were conducted in a Gravimetric Analyzer (IGA-Hiden Isochema) in the upstream for collecting pressure versus change in the adsorbent weight data and a Mass Spectra (Hiden Analytical) in the downstream for analyzing the gases leaving the adsorption unit, i.e., unadsorbed gas streams. The adsorption/desorption isotherms were obtained for 70-75 mg AC-based samples within 0-1000 mbar pressure range for each 100 mbar step, at room temperature (RT) and at 120 °C. Four different feed streams are used in the adsorption studies, which are namely; 50 ml/min CO2 (pure CO2), 50 ml/min CH4 (pure CH4), 5ml/min CO2-45 ml/min CH4 (10% CO2-90% CH4) and 25 ml/min CO2-25 ml/min CH4 (50% CO2-50% CH4). Prior to tests, samples were outgassed overnight at RT to eliminate humidity and trapped gasses. The performance of the adsorbents were tested and compared on the basis of their adsorption capacity (mg adsorbed/g adsorbent), mass uptake values (%) and selective adsorption ability of CO2 over CH4.
The adsorption/desorption isotherms of pure and mixture gases were obtained in order to investigate the changes in CO2 adsorption behavior upon the modifications, heat/chemical treatments and/or impregnations applied to the AC-based adsorbents. When adsorption/desorption isotherms of pure gases at room temperature are compared, it was observed that mass uptake values of pure CO2 isotherms (~10%) were six folds of the pure CH4 isotherms (~1.7%) at 1000 mbar. When the adsorption temperature was increased from RT to 120 °C, almost 80% of the CO2 adsorption capacity was lost, and at that temperature mass uptake values for pure CO2 gas dropped down to 2%, while CH4 adsorption capacity decreased down to 0.4%. The adsorbed amounts on all samples decreased with temperature. When pure CO2 gas was sent to the gravimetric analyzer, it was seen that after adsorption process complete desorption occurred, i.e, both isotherms overlapped. However, as the composition of CH4 in the feed stream increased, desorption has been more difficult and deviations from the adsorption data were observed. A maximum of 100 mg CO2 and 17 mg CH4 gases were adsorbed per gram of adsorbent at 1000 mbar and RT. When the adsorption capacity of the adsorbents for gas mixtures were compared, in case of the feed mixture of 50% CH4-50% CO2 (65 mg adsorbed/g adsorbent) the mass uptake values and adsorption capacities were almost two folds with respect to the feed mixture of 90% CH4-10% CO2(30 mg adsorbed/g adsorbent).
Pure and selective CO2adsorption capacity of AC1-10NH3w sample was found to be slightly higher than AC1-25NH3w sample. High temperature treatment with helium (AC1-25NH3w-600He) increased the CO2 adsorption capacity both at RT and at 120 °C. It was also found that samples prepared through incipient-to-wetness-impregnation method (AC1-25NH3i-250) had higher adsorption capacities compared to samples prepared through solution and filtration method (AC1-25NH3w).
Pure CO2 isotherms at room temperature and at 120 °C were fitted to Langmuir, Freundlich and Dubinin-Radushkevich (D-R) models to describe the adsorption mechanism, surface properties and affinity of the adsorbent. Langmuir model, which quantitatively describes the formation of a monolayer adsorbate on the outer surface of the adsorbent, gave as an average of 4.02 mmol/g adsorbent for theoretical monolayer saturation capacity and 0.00168 for Langmuir isotherm constant with correlation coefficients greater than 0.99. The increase in temperature decreased the two parameters of the Langmuir model, since the adsorption rate decreased. Fitting Freundlich model to experimental CO2 adsorption data, which assumes a heterogeneous surface (multilayer adsorption) with a non-uniform distribution, resulted in an average of 0.07414 and 0.6354 for k and 1/n constants at RT (adsorption capacity and strength of adsorption) with correlation coefficients greater than 0.99, respectively. The increase in temperature resulted in higher 1/n values, which are very close to 1, indicating relatively uniform surfaces. The best fit was obtained in Dubinin-Radushkevich model with correlation coefficients greater than 0.999. This model was used to estimate the characteristic porosity of the adsorbent and the apparent energy of adsorption. The micropore capacity was found to be 0.221 and the obtained characteristic energy of the adsorbents with an average of 7.48 kJ/mol, which is less than 8 kJ/mol, confirmed the adsorption process to be a physical one in this study.
Based on the results of these preliminary tests, the effect of ammonia concentration on CO2 adsorption by treating AC1 samples with 1wt.% and 5 wt.% NH3solutions, enriching of AC surface with nitrogen by treatment with gaseous ammonia and a mixture of ammonia and oxygen at high temperatures are currently studied. The adsorbents will be characterized by Scanning Electron Microscopy/Electron dispersive X-Ray (SEM-EDX) for studying microstructural properties, High Resolution Transmitting Electron Microscopy (HRTEM) and HRTEM-EDX for detailed characterization of phases and dispersion of additives, and Fourier Transform Infrared Spectroscopy (FTIR) for studying type of AC surface groups.
Financial support provided by TÜBİTAK through project 113M263 and ROX 0.8 type activated carbon provided by NORIT are gratefully acknowledged.
 Zhang, Z., M. Xu, H. Wang and Z. Li, 2010, "Enhancement of CO2 Adsorption on High Surface Area Activated Carbon Modified by N2, H2 and Ammonia", Chemical Engineering Journal, Vol. 160, pp. 571-577.
 Shaarani, F. W. and B. H. Hameed, 2011, "Ammonia-modified Activated Carbon for the Adsorption of 2,4-dichlorophenol", Chemical Engineering Journal, Vol. 169, pp. 180-185.
 Przepiorski, J., M. Skrodzewicz and A. W. Morawski, 2004, "High Temperature Ammonia Treatment of Activated Carbon for Enhancement of CO2 Adsorption", Applied Surface Science, Vol. 225, No. 1-4, pp. 235-242.
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