PERGAMON Carbon 38 (2000) 1985–1993

Preparation of activated carbons from oil-palm-stone chars bymicrowave-induced carbon dioxide activation

*Jia Guo, Aik Chong LuaDivision of Thermal and Fluids Engineering, School of Mechanical and Production Engineering, Nanyang Technological University,

Singapore 639798, Singapore

Received 28 July 1998; accepted 15 February 2000

Abstract

A novel method of preparing activated carbons from oil-palm-stone chars by microwave-induced CO reaction was2

studied in this paper. The effects of processing parameters (gas flow rate, input microwave power and exposure time tomicrowave energy) and the presence of CuO receptors on the characteristics of the activated carbons were investigated inorder to determine and optimise the control parameters for the process. Experimental results showed that it was feasible toprepare activated carbons with high density and predominant microporosity from oil-palm-stone chars by microwave heating.These activated carbons are to be used as gas-phase adsorbents or carbon molecular sieves after a further modification ofcarbon deposition. CO gas flow rate, input microwave power and exposure time were found to be important processing2

parameters that would significantly affect the quality of the final products. Adding CuO receptors to the char samplesincreased the surface temperature and significantly reduced the processing time. However, these receptors caused an externalheating that resulted in heterogeneous pore structures as shown by the scanning electron microscope micrographs. 2000Elsevier Science Ltd. All rights reserved.

Keywords: A. Activated carbon; B. Activation, Pyrolysis; C. Adsorption; D. Microporosity

1. Introduction days, even up to a week to reach the desired level ofactivation and this of course will incur extra processing

Commercial activated carbons can be prepared from a costs. Another problem lies in the furnace such that thewide range of naturally occurring carbonaceous materials surface heating from the hearth wall does not ensure asuch as peat, coal, lignite, wood and various agricultural uniform temperature for different shapes and sizes ofby-products by a thermal process that normally involves chars. This generates a thermal gradient from the hottwo stages, namely pyrolysis and activation. In the first surface of the char particle to its interior and impedes thestage, the starting materials (sometimes pretreated with effective removal of gaseous products to the ambient,acidic or alkaline solution to inhibit tar formation) are thereby resulting in a detrimental effect on the quality ofpyrolysed under an inert atmosphere at a moderate tem- the activated carbons prepared [2].perature (about 600–8008C) to release the volatile matters Microwave-induced chemical reactions can be used toand produce chars with rudimentary pore structures. Sub- solve the above problems associated with conventionalsequently, in the second stage, the resulting chars are surface heating because microwave heating is both internalsubjected to a partial gasification at a higher temperature and volumetric heating. Therefore, the tremendous thermal(usually above 9008C) with oxidising gases, such as steam gradient from the interior of the char particle to its coolor carbon dioxide, to produce activated carbons with well- surface allows the microwave-induced reaction to proceeddeveloped and accessible internal porosities [1]. However, more quickly and effectively at a lower bulk temperature,in some cases, this activation process may take several resulting in energy savings and shortening the processing

time. In fact, microwave-induced chemical reactions havebeen used in a large number of applications, such as*Corresponding author. Tel.: 165-790-5535; fax: 165-791-pyrolysis of high-volatile bituminous coal with nitrogen [3]1859.

E-mail address: maclua@ntu.edu.sg (A.C. Lua) and coal gasification in water vapour [4]. However, no

0008-6223/00/$ – see front matter 2000 Elsevier Science Ltd. All rights reserved.PI I : S0008-6223( 00 )00046-4

1986 J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993

Nomenclaturec specific heat of material (J /kg 8C)C capacitance of capacitor (F)D distance between electrodes (m)E electric field strength (V/m)max

f voltage frequency (Hz)m mass of material (kg)

3p power dissipated per unit volume (W/m )P power supplied (W)t time (s)T temperature (8C)

2S area of plane capacitor (m )v voltage of alternating current (V)V voltage amplitude (V)max

Y admittance of capacitor (1 /V)Greek letters´* relative permittivity of material´9 real part of the complex permittivity´0 imaginary part of the complex permittivity

212´ dielectric constant of free space (8.85310 )0

u total porosity of solid (%)3

r apparent density of solid (g /cm )a3

r solid density of solid (g /cm )s

studies on the preparation of activated carbons from between electrodes D) without fringing effects [7]. Whenagricultural solid wastes by microwave heating have been the capacitor is in air, its admittance Y is j2pfC with:reported in the literature.

C 5 ´ S /D (1)0Oil-palm stone is one of the main solid wastes frompalm-oil processing mills. In Malaysia which is the largest

where ´ is the dielectric constant of free space, 8.85430palm oil producer in the world, more than one million tons 21210 F/m.of oil palm stones are estimated to be generated annuallyWhen the capacitor is filled with a material with a[5]. Preparation of activated carbons from oil-palm stones

relative complex permittivity,will be an economical way to utilise these agriculturalby-products, if viable. The objective of this work was to

´* 5 ´9 2 j´0 (2)study the technical feasibility of preparing high-qualityactivated carbons from oil-palm-stone chars utilising a the admittance of the capacitor becomes:novel method of microwave-induced char–CO reactions.2

Y 5 j2pfC´9 1 2pfC´0. (3)It has been found that char is a good receptor of micro-wave energy [6]. By applying a certain amount of micro-wave energy, the char may reach the minimum reaction Consequently, an electric power P is dissipated in theconditions. The effects of processing parameters (gas flow material such that:rate, input microwave power and exposure time to micro-

2P 5 ´0V pfC. (4)wave energy) on the characteristics (such as density, total max

porosity, proximate analysis, yield, specific surface areaand pore size distribution) of the activated carbons were In the ideal situation of a uniform electric field (theinvestigated in order to determine and optimise the control electric field strength E 5V /D) within the material,max maxparameters for the process. Also, the effect of adding the power dissipated per unit volume is:copper oxide powder (size 100–150 mm) to the char

2sample as receptors of microwave energy to speed up p 5 ´ ´0E pf. (5)0 max

microwave-induced CO reactions was studied.2

Assuming no heat transfer occurs, the increase intemperature DT as a function of the specific heat c and the

2. Principle of microwave heating mass m of the material can be expressed as:

2Analysis of microwave heating is given for a sine DT 5 ´ ´0E pft /cm (6)0 max

voltage v(t) with an amplitude V and a frequency f asmax

applied to an ideal plane capacitor (area S and distance where t is the time of exposure to microwave energy.

J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993 1987

3. Experimental The sample was heated in the furnace from room tempera-ture to 1108C in an inert atmosphere of nitrogen until

3.1. Raw material and char preparation complete dehydration and followed by decomposition at8508C for 7 min to determine the quantity of volatile

Oil-palm stones were obtained from a palm-oil process- matters. The furnace atmosphere was then changed toing mill in Selangor, Malaysia. The stones were crushed oxidising conditions by passing air through it. Thereafter,and sieved to a particle size range of 2.0–2.8 mm. Chars the sample was cooled to 8008C and maintained at thiswere prepared in a vertical tube furnace (818P, Lenton) temperature until its weight remained unchanged. The

3subjected to a flow (150 cm /min) of purified nitrogen gas. weight loss during this period was due to the reaction ofThe temperature was increased at a rate of 108C/min from the fixed carbon with oxygen and the final remainingroom temperature to 7008C and held at this temperature for residue was ash.3 h. The characteristics of the raw oil-palm stone andresulting char are shown in Table 1. 3.5. Gas adsorption

Adsorption characterisations of samples were deter-3.2. Microwave heating systemmined by nitrogen adsorption at 21968C with an acceler-ated surface area and porosimeter (ASAP-2000, Mi-Microwave-induced char–CO reactions of the resulting2cromeritics). The BET surface area was calculated from Nchars were carried out in a 2.45 GHz commercial micro- 2

adsorption isotherms using the Brunauer–Emmett–Tellerwave oven (EME2662, Electrolux) with suitable modi-(BET) equation [10]. The cross-sectional area for nitrogenfications. The oven has a power controller to select

2molecule was assumed to be 0.162 nm . The Dubinin–different power levels (80 W, 150 W, 300 W, 450 W andRadushkevich (DR) equation was used to calculate the750 W) and a timer for various exposure times (5 to 60micropore volume from the N adsorption data. Themin) at a set microwave power level. About 2 g of chars 2

micropore surface area was then determined from the valuewere spread on a rotary plate that was located in theobtained for the micropore volume [11]. The total porechamber of the oven. Purified carbon dioxide gas (99.999volume was found from the amount of nitrogen adsorbedvol.% purity) or nitrogen gas at a pre-set flow rate wasat a relative pressure of 0.99. The micropore and mesoporeused to purge any air in the chamber before the start of thesize distributions were calculated using a HK model [12]microwave heating process and it continued to flowand a BJH model [13], respectively.through the chamber during the whole process. As the

temperature of the sample in the microwave energy field is3.6. Scanning electron microscopynearly impossible to measure accurately, the input micro-

wave power was used as a process parameter instead of theA scanning electron microscope (S360, Cambridgesample temperature. Only the surface temperature of the

Instruments) was used to observe the surface pore structuresample after exposure to microwave energy was measuredof the activated carbons [14]. In this study, the magnifica-using an infrared pyrometer (505, Minolta). As a result oftion of the SEM was 3000 to 5000 times. Although onlythe internal and volumetric heating nature of microwavepart of the macroporosities could be seen by the SEM dueheating, the sample internal temperature may be tens orto its resolution limitation, the SEM micrographs werehundreds of degrees higher than that of the sample surfaceused to verify the uniformity of the surface pore structure[8].which was related to the homogeneity of the reaction.

3.3. Density measurement

4. Results and discussionSolid and apparent densities (r and r respectively) ofs a

the samples were measured by an ultra-pycnometer (UPY-Starting materials with high solid density are desirable1001, Quantachrome) and a mercury intrusion porosimeter

for the preparation of activated carbons. It can be seen(PoreSizer-9320, Micromeritics), respectively. For knownfrom Table 1 that raw oil palm stone has relatively highsolid and apparent densities, the total porosity u can besolid density, high volatile matter and high fixed carboncalculated as follows:contents but low ash content, making it suitable for further

u 5 (r 2 r ) /r 3 100%. (7) preparations into activated carbon. After pyrolysis ats a s

7008C for 3 h, the solid density of the char increased but3.4. Proximate analysis its apparent density decreased due to the release of low-

molecular-weight volatile matters (such as H , CO, CO2 2

A thermogravimetric analyser (TA-50, Shimadzu) was and CH ), resulting in the development of rudimentary4

used to determine the proximate analysis of sample in pore structures. Consequently, the total porosity hadterms of volatile matter, fixed carbon and ash contents [9]. increased up to 19.5%. The BET and micropore surface

1988 J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993

Table 1Characteristics of raw oil palm stone and char

Sample Solid Apparent Total Volatile Fixed Ash BET surface Micropore Total poredensity density porosity matter carbon area surface area volume

3 3 2 2 3(g /cm ) (g /cm ) (%) (%) (%) (%) (m /g) (m /g) (cm /g)

Stone 1.53 1.47 3.9 80.8 17.3 1.9 1.6 0.2 0.01Char 1.64 1.32 19.5 21.4 71.1 7.5 194.3 89.5 0.14

areas and the total pore volume also increased signifi- reaction had occurred during this microwave heatingcantly. process. Pajares et al. also observed that more

Table 2 shows the effects of N and CO gas flow rates mesoporosities were developed by CO gasification when2 2 23on the characteristics of the activated carbons prepared by a high CO flow rate (500 cm /min) was used [15]. The2

3exposure to 750 W microwave power for 30 min. When N CO flow rate of 200 cm /min with the highest BET and2 2

was introduced into the microwave oven to flush through micropore surface areas was chosen for subsequent experi-the char sample, the yields after exposure to microwave ments.heating were 100% (i.e. no weight loss), irrespective of the The density of the activated carbon depends not only ongas flow rates used. The BET and micropore surface areas the nature of the starting material but also the activationand the total pore volume of the activated carbons re- process [16]. The solid and apparent densities of activatedmained almost unchanged from those values for the carbons prepared by exposure to microwave power levelsstarting chars. This indicated no microwave-induced re- of 80 to 750 W for various times are shown in Table 3.actions had occurred in the absence of any oxidising gas; a Under low microwave power levels of 80 W, 150 W andresult of good thermal and chemical stabilities of nitrogen 300 W, and with exposure times as long as 60 min, thegas molecules. However, when CO was used instead of densities and total porosities of the activated carbons2

N , increasing CO gas flow rate lowered the yield of remained almost unchanged, indicating that no continual2 2

activated carbons. Similarly, the effects of CO flow rate carbonisation and no reaction between the char and carbon2

on the BET and micropore surface areas and the total pore dioxide had occurred. For a higher microwave power levelvolumes were significant. When the CO flow rate was as such as 450 W or 750 W, the total porosities of the2

3low as 50 cm /min, the increase in the BET surface area activated carbons increased substantially and progressivelyover that for the char (Table 1) was marginal because of with increasing exposure time. For instance, increasing theinsufficient CO gas to remove the gas products (e.g. CO exposure time from 5 min to 1 h at a heating power of 7502

and volatile matters) effectively and to react with the char W, the total porosity of the activated carbon increased byto produce internal pore structures. But, if the flow rate as much as 20.9%. This trend indicated that the densities

3was too high, such as 500 cm /min, the char–CO and porosities were related to a combined effect of2

reaction was so severe that much carbon contents were continual carbonisation and carbon–CO reaction, enlarg-2

burnt, resulting in detrimental effects on the quality of the ing the pores created during carbonisation and formingactivated carbons. The weight loss of the char during further new pores as reaction time increased.microwave heating was 52.9%, and this loss could not be Proximate analyses of activated carbons prepared underdue only to the continuation of the carbonisation phase different input microwave powers for various exposurebecause the volatile matter in the starting char was only times are shown in Table 4. The constituents of the21.4% (Table 1). Therefore, this indicated a carbon–CO activated carbons prepared under microwave powers of 802

Table 2Effects of N and CO gas flow rates on the characteristics of activated carbons2 2

Type of gas Gas flow rate Yield of BET surface Micropore surface Total pore3 2 2 3(cm /min) process (%) area (m /g) area (m /g) volume (cm /g)

N 50 100 195.2 90.3 0.142

200 100 193.0 88.4 0.13500 100 194.7 89.8 0.14

CO 50 76.0 218.3 142.6 0.162

100 61.8 255.9 163.7 0.19200 56.6 320.6 183.5 0.21300 55.3 314.5 182.1 0.21500 47.1 227.1 136.2 0.15

J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993 1989

Table 3 continual carbonisation and carbon–CO reaction occur2Densities and total porosities of activated carbons prepared from during microwave heating. The occurrence of microwave-oil palm stones induced carbon–CO activation is responsible for the2

further development of pore surface and structure.Exposure Solid density Apparent density Total porosity3 3conditions (g /cm ) (g /cm ) (%) Table 4 also showed that the losses were more signifi-

a cant for a higher microwave power (e.g. 750 W) than a80 W–60 m 1.64 1.32 19.5lower power (e.g. 450 W) and for a longer exposure time150 W–60 m 1.64 1.34 18.3(e.g. 60 min) than a shorter time (e.g. 40 min). For300 W–60 m 1.65 1.32 20.0instance, for a fixed exposure time of 60 min, the volatile

450 W–5 m 1.66 1.30 21.7 content decreased by 60.3% and 87.2% for 450 W and 750450 W–20 m 1.78 1.23 30.9 W, respectively, whilst the fixed carbon decreased by450 W–40 m 1.82 1.19 34.6 14.7% and 46.5% for 450 W and 750 W, respectively.450 W–60 m 1.83 1.17 36.1 Also, for a fixed microwave power of 750 W, the volatile

content decreased by 38.5% and 87.2% for exposure time750 W–5 m 1.73 1.28 26.0 of 5 min and 60 min, respectively, whilst the fixed carbon750 W–20 m 1.87 1.14 39.0

decreased by 4.4% and 46.5% for 5 min and 60 min,750 W–40 m 1.91 1.06 44.5respectively. Hence, this demonstrates that both microwave750 W–60 m 1.92 1.02 46.9power and exposure time are two important parameters for

a The notation 80 W–60 m means that the char was subjected to both continual carbonisation and carbon–CO reaction.2microwave energy under the input microwave power of 80 W for Fig. 1 shows the surface temperatures (detected by the60 min.

infrared pyrometer) of the chars after being subjected toinput microwave powers of 300 W, 450 W and 750 W for

W, 150 W or 300 W for exposure times of 60 min, were various times. For the microwave power of 300 W, as thesimilar to those of the chars (Table 1). Consequently, there exposure time was increased from 5 to 60 min, the samplewere no volatile matter and fixed carbon losses observed surface temperature increased slightly over this duration to(the last two columns). If the microwave power was just over 1008C. For the microwave power of 450 W orincreased to 450 W and beyond, the volatile contents 750 W, the sample surface temperature increased continu-decreased whilst the fixed carbon contents increased. For a ally with increasing exposure time. In relation to themicrowave power of 750 W and exposure time of 60 min, microwave powers used, it could be observed from Table 4the volatile matter loss (based on its starting weight in the that powers of 80 W, 150 W and 300 W did not producechar sample) was 87.2% whilst the corresponding fixed any weight loss in the volatile matter or fixed carboncarbon loss was 46.5%. This showed that a large propor- content. Hence, it could be deduced that a minimum powertion of the volatile matters had been removed after of 450 W was required for continuation of the carbonisa-exposure to microwave energy, leaving the final product tion process and carbon–CO reaction to take place.2

rich in carbon contents. The results also prove that both In manufacturing commercial activated carbons, rela-

Table 4Proximate analyses (%, dry basis) and weight losses (wt, %) of activated carbons

Exposure Volatile Fixed Ash Volatile matter Fixed carbonb bconditions matter carbon loss loss

a80 W–60 m 21.7 70.9 7.4 – –150 W–60 m 21.5 71.0 7.5 – –300 W–60 m 21.3 71.0 7.7 – –

450 W–5 m 16.2 75.8 8.0 26.9 –450 W–20 m 14.5 77.3 8.2 40.4 4.3450 W–40 m 12.9 78.8 8.3 51.0 9.9450 W–60 m 11.3 80.1 8.6 60.3 14.7

750 W–5 m 14.9 77.0 8.1 38.5 4.4750 W–20 m 9.8 81.6 8.6 69.7 24.1750 W–40 m 7.8 83.2 9.0 81.0 39.2750 W–60 m 6.2 84.6 9.2 87.2 46.5

a The notation 80 W–60 m means that the char was subjected to microwave energy under the input microwave power of 80 W for 60 min.b The last two columns refer to the volatile matter and fixed carbon weight losses (based on their respective individual starting weight in

the char sample) during microwave heating only. The symbol ‘–’ used in these columns denotes no significant weight loss was observed.

1990 J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993

Fig. 1. Surface temperatures of activated carbons after exposure Fig. 3. BET surface areas of activated carbons prepared byto microwave powers of 300 W, 450 W and 750 W for various exposure to microwave powers of 300 W, 450 W and 750 W fortimes. various times.

tively high yields of the final product are expected. The and micropore surface areas of activated carbons preparedeffects of microwave power and exposure time on the by exposure to input microwave powers of 300 W, 450 Wyields of the activated carbons are shown in Fig. 2. This and 750 W for various times are shown in Figs. 3 and 4,yield was based on the initial weight of the char sample. respectively. For 300 W, the BET and micropore surfaceThe figure shows that for the microwave power of 300 W, areas remained almost constant for different exposureas the exposure time increased from 5 to 60 min, the yield times up to 60 min, indicating no pore development hadof the activated carbons remained unchanged because of occurred. For a power of 450 W, both the BET andinsufficient amount of microwave energy to initiate re- micropore surface areas increased progressively with in-actions. When the microwave power of 450 W or 750 W creasing exposure time as a result of the combination ofwas used, increasing the exposure time decreased the yield microwave-induced continual carbonisation and carbon–of the activated carbons progressively because of continual CO reaction. This is in agreement with results by other2

carbonisation and carbon–CO reaction that resulted in the researchers [2,18,19]. However, for a power of 750 W,2

release of volatile matters and burn-off of some carbon increasing the exposure time increased the BET surfacecontents, respectively. area as expected, but long exposure times such as 50 min

The most important property of the activated carbon is and greater, the rate of micropore surface area increaseits adsorptive capacity which is related to the specific decreased due to conversion of some microporosities intosurface area, pore volume and pore size distributions. mesoporosities. For the exposure times shown, the increaseGenerally, the greater the surface area of the activated in the BET surface area was steeper for 750 W than 450 Wcarbon, the larger is its adsorptive capacity [17]. The BET

Fig. 4. Micropore surface areas of activated carbons prepared byFig. 2. Yield of activated carbons prepared by exposure to input exposure to microwave powers of 300 W, 450 W and 750 W formicrowave powers of 300 W, 450 W and 750 W for various times. various times.

J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993 1991

because of the higher carbon–CO reaction rate and higher value of 5468C as compared to about 1708C for the same2

rate of release of volatile matter. power and time, but without CuO added. The BET (469.12 2Some metal oxides like CuO or Fe O can be used as m /g) and micropore (225.6 m /g) surface areas also3 4

microwave energy receptors to mix together with the increased as compared to those results without CuO addedsamples and induce rapid chemical reactions [20]. In this (Figs. 3 and 4 for 10 min exposure time). However, thepaper, the effect of adding CuO particles to the char char–CO reaction induced by microwave energy in the2

sample to increase the sample surface temperature and presence of CuO receptors involves a totally differentspeed up the char–CO reaction was also studied. After heating mechanism, i.e. external heating instead of internal2

microwave heating, the activated carbons were separated heating when using microwave alone. The extremely highfrom the CuO powder by flotation in carbon tetrachloride temperature of the CuO particles that mixed with the char(CCl ). With the addition of CuO receptors to the char samples generated high local temperatures, resulting in a4

sample, more microwave energy could be absorbed and severe char–CO reaction of the char sample around the2

therefore the samples could reach a higher temperature in a CuO receptors. SEM micrographs were used to show theshorter time. It was found that the surface temperature of effects of CuO particles on the surface pore structures.the mixture (the ratio of CuO to char is 1:1) after exposure Fig. 5a shows the surface of the oil-palm-stone charto 750 W microwave power for only 10 min, reached a whilst Figs. 5b and c show those for activated carbons

Fig. 5. SEM micrographs: (a) oil-palm-stone char carbonised at 7008C for 3 h; (b) activated carbon prepared by exposure to 750 Wmicrowave power for 30 min; (c) activated carbon prepared by exposure to 750 W microwave power for 10 min with CuO receptors.

1992 J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993

prepared by microwave heating without and with theaddition of CuO receptors, respectively. For the charcarbonised at 7008C for 3 h (Fig. 5a), pores of varioussizes could be observed on the surface, indicating to someextent the heterogeneous nature of the char. The mi-crograph of the activated carbon prepared by exposure to750 W microwave power for 30 min (Fig. 5b) showed thatthere were many orderly pores with round shape anduniform sizes all over the surface as opposed to theheterogeneous surface of the char. This suggests that ahomogeneous char–CO reaction has occurred as a result2

of the nature of microwave heating, i.e. internal andvolumetric heating. For the case in which CuO receptorswere added, the surface of the final product (Fig. 5c) wasextremely heterogeneous. This was probably because of Fig. 7. Mesopore size distributions of activated carbons preparedthe external heating caused by the CuO receptors that with and without adding CuO receptors.absorbed more energy and reached higher temperatures.The high local temperatures had a detrimental effect on theuniformity of the surface pore structures and also changedthe internal pore structures as shown by the pore size between the samples activated with and without CuOdistributions (Figs. 6 and 7). receptors, with both having predominantly micropores.

Pores in porous materials are classified as micropores However, if CuO was added as microwave receptors, the(,2 nm), mesopores (2–50 nm) and macropores (.50 mesopore size distribution of the sample was differentnm), in accordance to the classification adopted by the from that without CuO. The higher numbers of mesoporesInternational Union of Pure and Applied Chemistry developed in the sample with CuO added was due to the(IUPAC) [21]. Activated carbons used for gas phase high local temperatures, resulting in the conversion ofadsorption usually have predominantly micropores since some microporosities into mesoporosities. The mesoporesmost molecules of gaseous pollutants range in diameters in the sample without CuO were less developed. Thefrom about 0.4 to 0.9 nm whilst liquid-phase activated narrow micropore distributions of these activated carbonscarbons have significantly more mesopores due to the suggest applications as gas-phase adsorbents or carbonlarger sizes of liquid molecules. molecular sieves after a further modification of carbon

Figs. 6 and 7 show the micropore and mesopore size deposition [22].distributions, respectively, for activated carbons preparedby exposure to 750 W microwave power (exposure time of30 min without CuO receptors and 10 min with CuO). 5. ConclusionsFrom the micropore size distribution curve (Fig. 6), itcould be seen that there were only slight differences It is feasible to prepare activated carbons with relatively

high densities and predominant micropores from oil-palm-stone chars by microwave-induced continual carbonisationand carbon–CO reaction. The densities, total porosities2

and proximate analyses of the activated carbons weredependent on the microwave power and exposure time. Amicrowave power of 450 W was considered as theminimum power required to cause any significant changesin these properties of the activated carbons. The effects ofCO flow rate, input microwave power and exposure time2

at a certain microwave power level (larger than theminimum power) on the BET and micropore surface areaswere significant. The maximum BET surface area of 412.5

2m /g was obtained by exposure to the microwave powerof 750 W for 60 min with a CO flow rate of 2002

3cm /min. Adding CuO receptors reduced the exposuretime to 10 min and increased the maximum BET surface

2area to 527.6 m /g, but created more mesoporosities asshown by the pore size distributions and the SEM mi-Fig. 6. Micropore size distributions of activated carbons prepared

with and without adding CuO receptors. crographs.

J. Guo, A.C. Lua / Carbon 38 (2000) 1985 –1993 1993

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