CONVERSION OF ALMOND SHELL TO ACTIVATED CARBONS:
METHODICAL STUDY OF THE CHEMICAL ACTIVATION BASED ON A
EXPERIMENTAL DESIGN AND RELATIONSHIP WITH THEIR
CHARACTERISTICS
Mª Teresa Izquierdoa, *, Alicia Martínez de Yusob, Begoña Rubioa, Mª Rosa Pinob.
aInstituto de Carboquímica, CSIC. c/Miguel Luesma, 4. 50018 Zaragoza. Spain bUniversity of San Jorge. Autovía A23 Zaragoza-Huesca km 510. 50830 Villanueva de
Gállego, Zaragoza. Spain.
*corresponding author e-mail [email protected] voice 34 976 733977 fax 34 976 733318 ABSTRACT
The objective of this work is to carry out a methodical study of the preparation conditions of almond shell based chemically activated carbons and its influence on the characteristics of the activated carbons. An experimental design was used to optimize the preparation conditions of activated carbons from almond shell via chemical activation with phosphoric acid. Temperatures from 400º to 800ºC, impregnation ratios in the range 0.5-1.5 and carbonization times varying from 30 to 120 min were defined as continuous parameters to be introduced in a simplex mixture experimental design giving a total of 12 experiments to carry out.
The response surface methodology was applied in order to study the influence of all the production parameters on the selected responses. The optimization of all the characteristics of the activated carbons under the same experimental conditions is not possible because the influence of activation temperature, impregnation ratio and activation time is different. However using the response surface methodology it is possible to determine the ranges of each experimental preparation condition to obtain optimal characteristics of the activated carbon.
KEYWORDS: Agricultural waste, chemical activation, activated carbon, porosity, functional groups, response surface
2 1. INTRODUCTION
Activated carbons can be produced from virtually any carbonaceous precursor. Coal and lignite [1-4], wood [5-9], agricultural wastes [10-15] and industrial wastes [16-23] are raw materials widely used to produce activated carbons by different routes: physical or chemical activation [24, 25].
Activated carbons have recognized adsorption capacity and versatility for different applications [26, 27] not only as adsorbents but also as catalysts and catalysts supports.
The adsorption capacity of an activated carbon will depend on surface area, pore volume and microporosity [25]. Another important factor for the removal of polar compounds is surface chemistry [28]. These characteristics can be modified by controlling the production process giving best features of activated carbons depending on their use: gas or liquid phase emissions purification.
Last years a great amount of papers regarding the preparation of activated carbons using low cost precursors as waste materials can be found in the literature (a review can be found in [29]). Recent studies of activated carbons production cost [30] reveal that one important factor to minimize the cost of the activated carbon is the raw material cost and other relevant factor for the economy of the overall process is the quality (adsorption capacity) of the activated carbon because selling price by weight is very sensitive to the quality. Research is on the way to use waste materials because their low cost. Moreover, the demand of high adsorption capacity of the activated carbons for the different
3 applications involves the development of a methodical methodology for the preparation of activated carbons and its relationship with the physicochemical characteristics of them.
Chemical activation has recognized advantages for the production of activated carbons with high developed porosity [31]; in addition, this procedure leads to high yields and the use of low activation temperatures. Main parameters of the preparation method affecting the characteristics of activated carbons are impregnation ratio, activation temperature and activation time [32]. Other authors have also studied the effect of the activating atmosphere [33].
The impregnation ratio (defined as the ratio of the weights of chemical agent and the precursor) is a variable that highly affects not only the pore size distribution of the resulting activated carbon but also the surface area. A general trend is that at increasing impregnation ratio the surface area of the activated carbon increases; however it seems that high impregnation ratios lead to a reduction in pore volume and surface area [34], indicating a collapse of micropores due to weakness of pore walls after intensive dehydration.
The activation temperature in chemical activation ranges from 400ºC to 800ºC for different lignocellulosic precursors [29]. As a general trend, for a given impregnation ratio an increase in the activation temperature leads to an increase in surface area due to the volatilization process. This process highly affects the product yield, an important factor from an economical point of view [30].
The activation time, i.e. the time that the sample is maintained at the fixed activation temperature, should be enough to allow the evolution of volatiles from the precursor in order to enable the pore development. However, too long activation times enables pore enlargement causing decrease in surface area. Moreover, the control of the activation time
4 minimizes the energy required for activated carbon production improving the economy of the process [30].
According to the compilation of information carried out in this work, no study has been done on optimization of the production of activated carbons from almond shells by experimental design, considering simultaneously as variables impregnation ratio, activation temperature and activation time. In this context, the objective of this work is to carry out a methodical study of the preparation conditions of almond shell based chemically activated carbons and its influence on the characteristics of the activated carbons. An experimental design was used for the preparation conditions of activated carbons from almond shell via chemical activation with phosphoric acid and the response surface methodology was applied to optimize the experimental conditions.
2. EXPERIMENTAL
2.1. Preparation of activated carbons
Raw material for the preparation of activated carbons was almond shell (from Vera del
Moncayo, Zaragoza, Spain) crushed and sieved at 0.2-1 mm. The ash content of the almond shell is 2.5 %. More details of the characterization of raw material are given elsewhere [35].
Chemical activation with (orto)-phosphoric acid was carried out by impregnation method followed by one step carbonization-activation in N2 atmosphere. An experimental design was used to obtain the conditions to prepare the activated carbons. A commercial software was used to obtain the experimental conditions to prepare the activated carbons. A mixture design simplex based on a centroid design with interior points and two center runs was
5 used. The three mixture variables introduced were impregnation ratio (defined as: weight of phosphoric acid / weight of precursor), temperature and time of activation, defined all of them as a continuous function in the range of 0.5-1.5, 400ºC-800ºC and 30-120 min, respectively, giving a total of 12 experiments. As non-design variable either BET surface area (as representative of development of porosity) or CO or CO2 evolved groups from
TPD experiments (as representative of surface chemistry) were used as response of the system.
The procedure to prepare the activated carbons was as follows: the almond shell was mixed with a determined amount of phosphoric acid of 89 wt % concentration to reach the impregnation ratio desired. The suspension was shaken at room temperature during 1 h.
The impregnated samples were further thermally treated at different temperatures ranging from 400º to 800ºC during times of carbonization ranging from 30 to 120 min. The sample was abruptly brought to the desired temperature. An inert flow of N2 was passed through the sample during carbonization step, which was kept during both heating and cooling.
Solid pyrolysis residues were water washed in Soxhlet until 6 Table 1 reports the label of the samples with their preparation conditions. 2.2. Characterization of activated carbons Samples were characterized by ultimate analysis, temperature programmed desorption coupled to mass spectrometry (TPD-MS), N2 adsorption and thermogravimetry (TGA). Ultimate analysis of the activated carbons was carried out in a Thermo Flash 1112 microanalysis apparatus. Oxygen content was obtained by difference. Table 2 gives results 6 for elemental analysis composition obtained for the activated carbons as well as yield to activated carbon (calculated as 100 x mass of AC/mass of precursor). The TPD runs were carried out with a custom built set-up, consisting of a tubular quartz reactor placed inside an electrical furnace. TPD experiments were carried out by heating the samples up to 1100ºC in Ar flow at a heating rate of 10ºC /min, recording the amounts of CO and CO2 evolved at each temperature with a quadrupole mass spectrometer from Pfeiffer. The calibrations for CO and CO2 were carried out by standards diluted in Ar. In a typical run 0.5 g of carbon was placed in a vertical quartz tube reactor under a stream of 30 ml/min of Ar. Nitrogen adsorption-desorption isotherms at -196ºC were obtained in a Micromeritics ASAP 2020 automatic adsorption apparatus. Prior each analysis the samples were outgassed at 150ºC and up to a vacuum of 10-6 mm Hg. The volume of adsorbed nitrogen was measured from a relative pressure of 10-7 up to 0.995. Surface area was determined by BET method [36] applied to the adsorption branch of the isotherm. Pore volume corresponding to micropores was calculated using the t-plot method. Volume of mesopores was obtained from BJH method [37]. The total pore volume was taken from the measure of adsorbed nitrogen at a relative pressure of 0.995. Thermogravimetric (TG) curves were obtained in a TA Instruments thermobalance. Approximately 20 mg of sample was placed in a Pt crucible and was heated at 10ºC/min up to 1000ºC under a flow of Ar. 3. RESULTS AND DISCUSSION 7 The conditions used to prepare each activated carbons according to the experimental design change not only surface area but also other important characteristics of the activated carbons as porosity and surface chemistry. Results concerning the evaluation of these properties may help to discuss the influence of the preparation method on the final performance of an activated carbon when it is further applied to gas or water emissions control. BET surface area was chosen as a common non-design variable for the experimental design because is easy to compare with that reported in the literature. Moreover, BET surface area gives a rapid way to follow the development of the textural characteristics of the activated carbons with the different experimental conditions. The amount of CO and CO2 evolved during TPD experiments was chosen as a response for surface chemistry characteristics of the activated carbons. Before the application of the response surface methodology, a preliminary study of the characteristics of the activated carbons was carried out in order to help fixing axis in the response surface graphs and thus to choose most influencing variable on response surface among those studied. 3.1. Characterization of samples. Lignocellulosic materials consist mainly of hemicellulose, cellulose and lignin [38]. Lignin and hemicellulose are the components that decompose first. Lignin decomposes at low temperature and at low rate up to about 900ºC. Hemicellulose is a light fraction that decomposes at low temperature and cellulose decomposes at higher temperature. In the case of almond shells [39], DTG curves gives three zones of decomposition; the first peak 8 observed corresponds to hemicellulose (centred around 330ºC), the second peak corresponds to cellulose (centred around 380ºC) whereas the decomposition of lignin takes place over a broad range of temperatures starting around 220ºC, overlapping with the temperatures of decomposition of hemicellulose and cellulose. However, the addition of chemicals to some of the components of almond shell or to the almond shell itself lead to a variation in the temperatures of the decomposition and in the amount of residue obtained [39, 40]. Figure 1 a and b shows DTG curves for activated carbons obtained at low activation temperature and at high activation temperature, respectively. The inspection of these curves can help to discuss the results obtained from surface chemistry and porosity analysis. At the temperatures used to prepare the activated carbons, the process corresponding to the decomposition of the hemicellulose is not present in any of the activated carbons prepared. Despite of the cellulose decomposes around 300ºC and this temperature is lower than those used to prepare the activated carbons, DTG curve of sample AT400R15t30 exhibits a small peak at 300ºC and a wide broad band that is not present in any of the activated carbons prepared. The peak can be attributed to cellulose decomposition because activation time has been not enough to complete degradation at low temperature. The broad band centred at about 450ºC can indicate that the degradation process of the lignin during activation has been incomplete due to the low activation time used. Increasing the activation time for the same activation temperature it can be observed a shift of this broad band to higher temperatures, indicating that the degradation of the lignin is being completed. At increasing activation temperature, this broad band disappears and a band around 800ºC is becoming more intense (Figure 1 a). This fact can indicate that the 9 crosslinking process to produce polyaromatic units becomes important. This band around 800ºC is shifted to higher temperatures when the highest activation temperatures are used (Figure 1 b). At activation temperatures higher than 600ºC the crosslinking process has started during activation of the samples and during TGA experiments this process is completed at temperatures around 900ºC. These results can be compared with those obtained by TPD experiments (detailed below), where the most important evolution of CO is obtained with samples prepared at the higher activation temperatures. The conditions used to prepare the activated carbons change the amount and type of surface oxygen functional groups present on them. Figure 2 and 3 show the profiles of CO and CO2 evolution during a TPD experiment for low and high temperature activated carbons, respectively. The total amount of CO and CO2 evolved up to 1100ºC in a TPD run for each activated carbon is reported in Table 3. The amounts of CO and CO2 desorbed and the temperatures at which these gases released have been found to be characteristics of various oxygen complexes. In general it is agreed that each type of oxygen surface group decomposes to a defined product. It has been proposed that CO2 derives from complexes like carboxylic acids, anhydrides and lactones and CO derives from complexes like phenolic and quinonic groups. The complexes yielding CO2 have been shown to decompose typically over a range of temperatures 150º- 600ºC and complexes yielding CO at temperatures in the range of 600º-1000ºC [28]. The variation of the operation conditions used to prepare the activated carbons lead to a change in TPD spectra of the activated carbons not only in the amount evolved but also the shape of CO2 and CO peaks. The profiles of CO2 of samples obtained at lower temperatures of carbonization show quite symmetric peaks centered at about 500º and 10 800ºC. First peak can be attributed to lactones with contribution of carboxyls and second peak to anhydrides. Sample series AT400- exhibit only a small shoulder not a well defined peak at 800ºC, indicating that more stable functional groups are only formed at increasing severity of activation conditions. The profiles of CO of samples obtained at lower temperatures of carbonization show two peaks. First peak, near 600ºC, can be attributed to phenolic and ether groups. This peak is more asymmetric and is shifted to about 625ºC for samples obtained at increasing activation temperatures, indicating the destruction of less thermally stable phenolic and ether groups. Second CO peak, near 800ºC, can be attributed to anhydrides, carbonyls and quinones decomposition. Similar temperatures of evolution of second CO peak are obtained compared with second CO2 peak. However, the shape and the intensity changes because of the contribution of other surface complexes rather than anhydrides. The contribution of carbonyls and quinone-like groups seem to be higher for sample series AT467- and sample AT533-R083-t60, indicating the presence of more stable oxygen functional groups at increasing the severity of the activation conditions. The profiles obtained for CO2 evolution in TPD experiments for the samples obtained at higher temperatures of carbonization (600 to 800ºC) show a low intensity broad band up to approximately a temperature of desorption of 600ºC, indicating the disappearance of less stable functional groups, as carboxyls and lactones, at higher activation temperatures. The peak around 850ºC is attributed to anhydrides, increasing its intensity compared to that exhibit by carbons obtained at lower temperature of carbonization. The profiles obtained for the evolution of CO during TPD experiments show a intense peak at about 850ºC and a small shoulder at around 1000ºC. The peak at 850ºC exhibit asymmetry at low temperatures, indicating small contribution of phenols and ethers. Moreover, the intensity 11 of the peak is twice the CO2 peak, attributed to anhydrides, indicating the presence of thermally stable groups as quinones and carbonyls. The introduction of surface groups that yield CO increase as a general trend at increasing carbonization temperature as can be seen in Table 3. In the case of CO2 evolution, there is not a relationship between carbonization temperature and the total amount of CO2 evolved. The impregnation ratio and the time of activation are important parameters influencing the introduction of CO2-evolving groups. Table 4 summarizes the characterization of porosity of samples. The percentage of microporosity can be obtained as the ratio between total micropore volume, calculated by applying Dubinin-Radushkevich equation [43] to the adsorption branch of the N2 isotherm and volume adsorbed at relative pressure of 0.995 [44]. As can be seen, at intermediate impregnation ratios, lower carbonization temperatures are needed to obtain higher surface areas. On the other hand, the use of impregnation ratios in the extreme of the range studied need higher carbonization temperatures to obtain high surface areas. Yield to activated carbons reported in Table 2 have not a direct relationship with the development of porosity. It can be obtained activated carbons high surface areas and high yields an important factor from an economical point of view. Summarizing, BET surface area reach highest values in the range of impregnation ratios 1.0-1.17, independently of the activation temperature used; the amount of oxygen surface groups yielding CO highly depend on the temperature whereas the amount of oxygen surface groups yielding CO2 does not exhibit a clear relationship with a fixed preparation condition variable. 12 3.2. Response analysis and interpretation In order to study and describe the effect of the preparation experimental conditions on the differenr non-design variables studied, the response surface methodology was chosen using a second-order polynomial model. In a first approach, the complete model was used: 2 2 2 Y=A0+A1T+A2R+A3t+A12TR+A13Tt+A23Rt+A11T +A22R +A33t [1] being Y the response (either SBET or CO-CO2 TPD evolving groups), T, R and t, activation temperature, impregnation ratio and activation temperature, respectively and A0 is the constant, A1, A2, A3 are the linear coefficients, A12, A13, A23 are the interactive coefficients and A11, A22, A33 are the quadratic coefficients. Based on the analysis of variance (ANOVA) of the complete quadratic model, terms of equation [1] with no significant effect on the response can be established. The observation of p-value (significance level) is used to assess the significance of the coefficients on the response. The limits taken were 0.05 In present case based on the complete model, quadratic effect of the activation time as well as the interactive effect between temperature and impregnation ratio exhibit p-values greater than 0.15 for the responses SBET and amount of CO2 evolved in TPD. These parameters were removed from the model, resulting: 2 2 Y1=B0+B1T+B2R+B3t+ B13Tt+B23Rt+B11T +B22R [2] 13 being Y1 responses either SBET or the amount of CO2 evolved in TPD and Bij the coefficients for each variable or combination of variables. In the case of the amount of CO evolved in TPD used as response, p-values of coefficients of interactions between temperature and time as well as the quadratic term of impregnation ratio are higher than 0.15. These parameters were removed from the complete model, resulting: 2 2 Y2=C0+C1T+C2R+C3t+C12TR+ C23Rt+C11T + C33t [3] being Y2 the response amount of CO evolved in TPD and Cij the coefficients for each variable or combination of variables. In order to check the quality of the model predicted values are plotted versus measured values for SBET and CO and CO2 evolved groups in Figure 4. Typical error for each sample is also depicted. Using those limited quadratic models, isoresponse surface are represented graphically relating each response and two factors, keeping the third factor constant. The optimization of all responses (SBET and CO and CO2 evolved groups) under the same experimental conditions is impossible because the influence of activation temperature, impregnation ratio and activation time is different. However, it is possible to determine the ranges of each experimental preparation condition to obtain the desired activated carbon. Main characteristic desired for an activated carbon is large surface area, and it has been shown in the previous section that the main factor influencing surface area is impregnation ratio, existing a maximum in SBET between impregnation ratio 1.00 and 1.17. So, in Figure 5 it can be found the response surface of surface area as a function of activation temperature and time, keeping constant impregnation ratio at different values in the range studied (0.95-1.20). As can be deduced from this Figure 5 the optimal range for BET 14 surface area indicated by the model is activation temperature in the range 400º-550ºC (common for the range of impregnation ratio studied) and activation time in the range 85 min (lower value for impregnation ratio 0.95) and 120 min (higher value for the range of impregnation ratio studied). Despite there are not methodological studies in the literature concerning the production of activated carbons from chemical activation of almond shells, some data of activated carbons production with this precursor and using phosphoric acid can be found in order to compare the results obtained in this study [15, 41, 45]. Following the goal to obtain an ideal activated carbon, the volume of micro and mesopore can be an important parameter depending on the type of pollutant to be removed as well as the media from they have to be removed. Thus, the response surface methodology for Vmicro and Vmeso using the same model than that used for response SBET was applied. The idea is to test if the range of preparation conditions to obtain an optimal Vmicro and Vmeso are in the range of those obtained to obtain optimal SBET. Figure 6 depicts the response surface for volume of micro and mesopore when impregnation ratio fixed at 1.10. Optimal Vmeso is obtained in similar range of activation temperature and time than that obtained for SBET. However, optimal Vmicro are obtained at higher activation temperatures in the same range of activation time. Thus, in order to obtain an activated carbon with high surface area and well developed structure (high mesopore and micropore volumes) it is needed to work at the temperature intersection among intervals of the three responses studied, i.e. 550ºC and activation times of 100 min. However, depending on further application of the activated carbon (in particular, molecular size of the pollutant to be removed) activation temperature can be shifted to lower values giving higher surface areas as well as mesopore volumes. Surface chemistry in an important characteristic of activated carbons for the removal of polar compounds [28]. Thus, the amount of CO and CO2 evolving groups from TPD 15 experiments has been used as a response not only for the amount of surface groups present on the activated carbon but also the type of surface groups present depending on the activation conditions. Figure 7 depicts estimated response surface for the amount of CO2 evolving groups at impregnation ratios 1.00 and 1.20. The optimal range for this response indicated by the model is activation temperature in the range 400º-550ºC (common for the range of impregnation ratio studied) and activation time in the range 95-120 min. Both ranges can be combined with the previous responses studied. Figure 8 depicts estimated response surface for the amount of CO evolving groups at impregnation ratios 1.00 and 1.20. The optimal range for this response indicated by the model is activation temperature in the range 400º-665ºC (common for the range of impregnation ratio studied) and activation time in the range 108-120 min. Both ranges can be combined with the previous responses studied. Summarizing, the analysis all the responses allows to obtain a compromise among the preparation conditions of the activated carbons in the range studied in this paper. CONCLUSION Experimental design methodology has been used to study the influence of activation temperature, impregnation ratio and activation time on the surface area, porosity and surface chemistry of the activated carbons prepared. Response surface methodology has allowed the definition of a range of optimal experimental conditions for the preparation of activated carbon. This range is wide if only surface area and mesopore volume has to be optimal (being the intersection of the intervals for both responses temperature 400-550ºC, 16 time 90-120 min). When a well developed porous activated carbon is needed for a determined application, the range of temperature shifts to higher temperatures and the range of activation time becomes narrower (temperature 600-800ºC, time 112-120 min) to reach the optimal micropore volume. The amount of less stable acidic surface groups that evolves CO2 during TPD experiments is optimal in similar range of preparation condition than that used to optimize surface area response. However, the amount of more stable acidic surface groups that evolves CO during TPD experiments is optimal in a wide range of temperatures but a narrow interval of activation times (temperature 400-665ºC, time 108-112). 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Eds. Marcel Dekker, NY. Vol. 10, 455-522. 20 43. Dubinin, M.M., 1989. Fundamentals of the theory of adsorption in micropores of carbon adsorbents – Characteristics of their adsorption properties and microporous structures. Carbon 27, 457-467. 44. Lillo-Rodenas, M.A, Cazorla-Amoros, D., Linares-Solano A., 2005. Behaviour of activated carbons with different pore size distributions and surface oxygen groups for benzene and toluene adsorption at low concentrations. Carbon 43, 1758-1767. 45. Bansode, R.R., Losso, J.N., Marshall, W.E., Rao, R.M., Portier R. J., 2003. Adsorption of volatile organic compounds by pecan shell- and almond shell-based granular activated carbons. Biores. Technol. 90, 175-184 21 FIGURE CAPTIONS Figure 1. (a) DTG curves for decomposition of low temperature activated carbons. 1: AT400R05t120; 2: AT400R1t75; 3: AT400R15t30; 4: AT467R067t90; 5: AT467R117t45; 6: AT533R083t60. (b) DTG curves for decomposition of high temperature activated carbons. 7: AT600R05t75; 8: AT600R1t30; 9: AT667R067t45; 10: AT800R05t30; 11: AT800R15t120. Figure 2. : Profiles of CO2 and CO evolved in TPD experiments for low temperature activated carbons. 1: AT400R05t120; 2: AT400R1t75; 3: AT400R15t30; 4: AT467R067t90; 5: AT467R117t45; 6: AT533R083t60; Figure 3. Profiles of CO2 and CO evolved in TPD experiments for high temperature activated carbons. 7: AT600R05t75; 8: AT600R1t30; 9: AT667R067t45; 10: AT800R05t30; 11: AT800R15t120. Figure 4. Predicted values (using the corresponding model) versus measured values. a) SBET; b) amount of surface groups yielding CO during TPD experiments; c) amount of surface groups yielding CO2 during TPD experiments. Figure 5. Response surface for BET surface area at fixed impregnation ratios: a) 0.95 b) 1.00 c) 1.10 d) 1.20 Figure 6. Response surface for a) mesopore volume and b) micropore volume at fixed impregnation ratio 1.10. Figure 7. Response surface for CO2 evolving groups (in TPD experiments) at fixed impregnation ratios: a) 1.00 b) 1.20 Figure 8. Response surface for CO evolving groups (in TPD experiments) at fixed impregnation ratios: a) 1.00 b) 1.20 22 Table 1. Preparation conditions of the activated carbons and labeling of samples Sample T R * t (ºC) (min) AT400R05t120 400 0.50 120 AT400R1t75 400 1.00 75 AT400R15t30 400 1.50 30 AT467R067t90 467 0.67 90 AT467R117t45 467 1.17 45 AT533R083t60 533 0.83 60 AT533R083t60(2) 533 0.83 60 AT600R05t75 600 0.50 75 AT600R1t30 600 1.00 30 AT667R067t45 667 0.67 45 AT800R05t30 800 0.50 30 AT800R15t120 800 1.50 120 *impregnation ratio: amount phosphoric acid (g)/ amount almond shell (g) Table 2. Ultimate analysis of activated carbons (% in dry basis) 23 Sample C H N S O* Yield** AT400R05t120 69.10 3.57 0.40 n 26.93 72.5 AT400R1t75 75.20 2.42 0.31 n 22.07 68.4 AT400R15t30 72.40 3.13 0.49 n 23.98 69.2 AT467R067t90 69.90 2.81 0.34 n 26.95 71.7 AT467R117t45 70.00 2.72 0.30 n 26.98 70.6 AT533R083t60 68.70 2.12 0.38 n 28.80 72.9 AT533R083t60(2 68.11 2.57 0.50 n 28.82 73.6 ) 67.57 2.12 0.45 n 29.86 74.1 AT600R05t75 68.47 2.05 0.42 n 29.06 73.2 AT600R1t30 66.97 2.02 0.55 n 30.46 74.8 AT667R067t45 60.30 1.33 0.37 n 38.00 71.3 AT800R05t30 71.44 1.93 0.37 n 26.26 67.3 AT800R15t120 *by difference **mass of activated carbon/mass of precursor x 100 n: negligible 24 Table 3. Total amount of CO and CO2 evolved up to 1100ºC in TPD experiments. Sample CO CO2 mmol/g mmol/g AT400R05t120 1.98 0.57 AT400R1t75 2.17 1.32 AT400R15t30 1.80 0.75 AT467R067t90 2.59 1.04 AT467R117t45 2.18 1.18 AT533R083t60 2.23 0.39 AT533R083t60(2 2.18 0.36 ) 2.83 0.52 AT600R05t75 3.04 0.45 AT600R1t30 2.86 0.48 AT667R067t45 4.13 1.92 AT800R05t30 3.72 1.29 AT800R15t120 25 Table 4. Results from N2 physisorption 1 2 3 Sample SBET Vmeso Vmicro Vp (p/p0=0.995) Microporosity (m2/g) (cm3/g) (cm3/g) (cm3/g) % AT400R05t120 626±29 0.124 0.202 0.376 89.1 AT400R1t75 1128±50 0.385 0.259 0.670 77.8 AT400R15t30 789±34 0.268 0.176 0.482 71.0 AT467R067t90 785±33 0.259 0.209 0.497 78.9 AT467R117t45 1117±44 0.494 0.181 0.724 70.9 AT533R083t60 891±36 0.341 0.194 0.571 74.6 AT533R083t60(2) 903±36 0.378 0.182 0.593 72.0 AT600R05t75 502±23 0.106 0.176 0.301 86.7 AT600R1t30 926±37 0.375 0.182 0.624 69.7 AT667R067t45 624±28 0.176 0.183 0.440 71.4 AT800R05t30 723±34 0.147 0.253 0.425 88.9 AT800R15t120 990±38 0.511 0.145 0.694 64.6 1 BJH method 2 t-plot method 3 according to Lillo-Rodenas et al.. 2005 26 Figure 1 1,5 a 1,25 ) 1 - 1 n i 3 m ( t d / 0,75 ) 1 0 M / M 2 4 ( 0,5 d 5 0,25 6 0 200 400 600 800 1000 T (ºC) 3 b 2,5 9 10 ) 1 - 2 n i m 7 ( t d / 1,5 ) 0 M / 11 M 8 ( 1 d 0,5 0 200 400 600 800 1000 T (ºC) Figure 2 27 2 CO2 1,5 2 5 % 1 4 3 0,5 1 6 0 0 200 400 600 800 1000 1200 Tª (ºC) 4,5 CO 4 3,5 3 5 2,5 2 % 2 1,5 1 1 4 6 0,5 3 0 0 200 400 600 800 1000 1200 Tª (ºC) Figure 3 28 5 CO2 4 3 10 % 2 11 9 8 1 7 0 0 200 400 600 800 1000 1200 Tª (ºC) 10 9 CO 8 10 7 11 9 6 % 5 4 8 3 2 7 1 0 0 200 400 600 800 1000 1200 Tª (ºC) Figure 4 29 1200 4,4 a b 1100 4,0 1000 3,6 ) T ) E O B 900 C S ( ( 3,2 d d e e 800 t t c c i i 2,8 d d e e 700 r r p p 2,4 600 500 2,0 400 1,6 400 500 600 700 800 900 1000 1100 1200 1,6 2,0 2,4 2,8 3,2 3,6 4,0 4,4 m easured (SBE T) m easured (CO) 2,2 c 1,8 ) 2 O 1,4 C ( d e t c i d 1,0 e r p 0,6 0,2 0,2 0,6 1,0 1,4 1,8 2,2 m easured (CO2) 30 Figure 5 a b c d Figure 6 31 b a 32 Figure 7 a b 33 Figure 8 a b 34