An investigation of the porosity of carbons prepared by constant rate activation in air

Carbon 41 (2003) 571–578

An investigation of the porosity of carbons prepared by constant rate activation in air E.A. Dawson a , *, G.M.B. Parkes a , P.A. Barnes a , M.J. Chinn b a

Centre for Applied Catalysis, University of Huddersfield, Queensgate, Huddersfield HD1 3 DH, UK b Dstl, Porton Down, Salisbury SP4 0 JQ , UK Received 21 May 2002; received in revised form 7 August 2002; accepted 16 October 2002

Abstract Nutshell carbon was activated in air / N 2 mixtures using controlled rate (CR) methods and the porosity characteristics compared with carbons activated conventionally in CO 2 at 800 8C to the same degree of burn off. The advantages of CR activation in air include the use of lower temperatures and the avoidance of thermal runaway. It was also possible to prepare activated carbons with significant microporosity, showing that excessive external burn off was prevented. In the CR experiments, the rate of evolution of CO 2 was controlled and constrained at a set level, either by altering the furnace temperature or the concentration of air in the activating gas. Although the highest micropore volumes (0.4 cm 3 g 21 ) were obtained at 40% burn off with the conventional method, at 20% burn off, the CR method using air concentration to control CO 2 evolution yielded carbons with similar micropore volumes (0.2 cm 3 g 21 ) to those activated conventionally.  2002 Elsevier Science Ltd. All rights reserved. Keywords: A. Activated carbon; B. Activation; C. Thermal analysis (DTA and TGA); D. Porosity, Surface areas

1. Introduction The preparation of activated carbons from nutshells or other precursors is usually performed on their chars in steam or CO 2 at temperatures of 800–1000 8C [1] to yield microporous materials in which high surface areas are developed as the carbon is burnt off. Air is not common as the oxidant although lower activation temperatures could be used, because the carbon–oxygen reactions are exothermic and can result in thermal runaway and excessive burn off. Furthermore, because of its high reactivity, reactions in oxygen can result in external carbon removal at the expense of creating internal porosity. However, it has been shown previously [2,3] that some of the above problems can be overcome by controlled rate thermal analysis (CRTA) [4]. This comprises a set of techniques where the reaction conditions (e.g., heating rate, atmosphere, etc.) are altered as some function of the rate of a chemical or physical change [4,5] in the sample. Normally, CRTA is used with low reaction rates and *Corresponding author. Tel.: 144-1484-472-174; fax: 1441484-472-182. E-mail address: [email protected] (E.A. Dawson).

small sample masses, so the effects of enthalpy changes can be assumed to be minimal and control of the rate is effectively achieved by manipulating the furnace temperature. However, once larger masses and / or faster reaction rates are used especially with highly exothermic reactions, thermal lag in the sample and furnace cause the system response to be sluggish and unable to cope with any temperature changes required. The alternative approach of controlling a process via the concentration of a reactant gas [6] does not suffer from this drawback as it is self-limiting. In the case of an oxidation process a higher reaction rate will consume more of the reactant gas, so reducing its concentration and giving a stabilising negative feedback. In CRTA the rate of reaction is pre-set at some target value and either the temperature or the concentration of active gas is altered to force the reaction rate to remain constant. The rate itself can be measured in terms of rate of mass loss, or rate of evolution / uptake of a gaseous species. In previous work, these methods have been shown to affect some of the properties of the samples produced [7,8]. When used preparatively these methods have been employed in the production of air activated carbons [2,3]. Their application in the formation of microporosity during carbon activation in air is an extension of these inves-

0008-6223 / 02 / $ – see front matter  2002 Elsevier Science Ltd. All rights reserved. PII: S0008-6223( 02 )00366-4

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tigations. A further aim is to investigate the differences if any, between the two types of rate control (thermal or gas concentration) and conventional activation methods in the surface area and porosity generated. To do this, in this paper we compare the effect on the porosity and surface area in a nutshell carbon activated conventionally in CO 2 with constant rate (CR) methods in air / nitrogen mixtures, and show that, at a low degree of activation only, gas concentration control of the rate of evolution of CO 2 during air activation can yield carbons of similar porosity to standard methods. 2. Experimental

spectrometer. The program allowed a target level of CO 2 to be set (expressed in terms of the magnitude in mV above its baseline value of the mass spectrometer signal for m /e544) at the start of an experiment, together with the total area under the curve of this signal, representing the extent of reaction (or burn off). The target CO 2 level was then maintained by PC control of either the temperature programmer or mass flow controllers. During the experiment, the cumulative area between the CO 2 curve and its initial baseline value was calculated and when the total area set was achieved, the experiment was automatically terminated, either by furnace cooling or cutting the supply of active gas (in this case, air) and data collection ceased.

2.1. Apparatus

2.2. Calibration

A schematic diagram of the apparatus for CR experiments is shown in Fig. 1. The sample (500 mg unactivated nutshell carbon) was placed in a silica U-tube within the water-cooled furnace (Fine Work Co.), the temperature of which was controlled by the temperature programmer (Eurotherm 818P). Water cooling ensures the temperature response of the furnace allows rapid sample cooling if required. Air and nitrogen (BOC) flows were controlled and maintained at a combined total value of 40 cm 3 min 21 using mass flow controllers (Brooks). A constant fraction of the effluent gas was admitted into the mass spectrometer (Hiden, HPR20) via a heated capillary and bypass inlet. The CO 2 signal from the mass spectrometer was relayed to the personal computer (PC) for purposes of rate control, using dynamic data exchange (DDE) between the Hiden software and our own CR program. Since N 2 was the major diluent gas, it was not possible to obtain a significant signal for CO as both species have m /e 28. However, the O 2 breakthrough signal (m /e 32) was monitored. In-house software was used both for data collection and control. Furnace and sample temperatures were saved, together with up to four m /e signals from the mass

Provided the temperature is too low for secondary reactions (between the evolved CO 2 and carbon or gas phase oxidation of evolved CO), there is a relationship between the CO 2 area output of the CR software and the amount of carbon lost during an experiment. In order to calculate real rates of reaction in terms of mg min 21 carbon reacted, the following method was adopted. A calibration curve of the amount of carbon lost against the CO 2 peak area was constructed for samples activated at the same temperature in flowing air for increasing periods of time. This is shown in Fig. 2. Using this graph, the mass represented by the area under the CO 2 profile between two times (t 1 and t 2 ) during the constant rate condition of any experiment was calculated, (see Fig. 3) and hence a real rate of carbon burn off (expressed in mg min 21 ) was related to the CO 2 target expressed in mV, as shown in Table 1. The small positive intercept is due to mass loss not associated with CO 2 evolution and is probably due to loss of water and / or CO. The differences in the actual burn off rates and the CO 2 levels in mV required to achieve them during the two types of control are due to the relative CO 2 baselines in 100%

Fig. 1. Constant rate schematic showing thermal and gas concentration feedback loops.

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control) or the mass flow controllers (gas concentration control).

2.3.1. Thermal control Carbons were prepared at constant gas (100% air) concentration with variable furnace temperature to control the reaction rate. Experiments were run at three reaction rates (see Table 1) and terminated at 20 or 40% burn off.

Fig. 2. Air activation of nutshell carbon: CO 2 peak area calibration.

2.3.2. Gas concentration control Carbons were prepared at constant temperature (400 8C) with variable air concentration to control the reaction rate. A delay of 25 min was allowed while samples were heated to the reaction temperature in flowing N 2 before blending was begun. Experiments were run at similar reaction rates (see Table 1) to those of the thermal control experiments and also terminated at 20 or 40% burn off. It is known that carbons activated conventionally in CO 2 tend to have narrow microporosity [9] and therefore for comparison, samples of activated nutshell carbon were prepared from the same unactivated nutshell carbon at 800 8C in CO 2 also to a burn off of 20 and 40% and designated CO2-20 and CO2-40. 2.4. Gas adsorption

Fig. 3. Thermal rate control: air activation of nutshell carbon: calculation of actual rate of burn off.

air for thermal control experiments and 100% nitrogen used during the set-up stage of the gas concentration control experiments.

2.3. Controlled rate reactions Experiments were conducted using one of the possible feedback loop options operating on the rate of CO 2 evolution either via the temperature programmer (thermal Table 1 Relationship between rate targets set for CO 2 evolution and actual rates of carbon burn off Rate control regime

Mass spectrometer CO 2 target (mV)

Rate of burn off (mg min 21 )

Thermal

15 30 50 70

0.26 0.51 0.87 1.20

50 70 90

0.80 1.25 1.58

Gas concentration

Measurements for N 2 and CO 2 adsorption were made using an automated volumetric instrument, the Omnisorp 100CX (Beckman Coulter). This instrument is capable of measuring adsorption at very low pressures as it has one pressure transducer calibrated in the range 0–10 Torr in addition to a second transducer for pressures up to one atmosphere (1 Torr5133.322 Pa). Samples were outgassed at 250 8C for 8 h at a pressure of 10 25 Torr before adsorption. The N 2 adsorption isotherm was measured at 2196 8C up to a relative pressure of 1 and the CO 2 isotherm was measured at 0 8C to a relative pressure of 0.04, using a value of P0 at this temperature of 26 142 Torr. BET surface areas were calculated from the N 2 adsorption data. Although the BET equation is widely used in the adsorption analysis of microporous solids, the concept of multilayer adsorption in very small pores makes the quoted surface areas nominal only. However, they are useful for comparison purposes. The cross sectional area of the N 2 molecule used for the calculation was 0.162 nm 2 . Microporosity was estimated from the Dubinin–Radushkevich (D–R) plots [10,11] for both N 2 and CO 2 adsorbates, using Eq. (1): log W 5 log W0 2 D log 2 (P0 /P)

(1)

where W is the volume of pores filled at relative pressure P/P0 , W0 is the total micropore volume and D is a constant dependent on the temperature and pore structure of the adsorbent. A plot of log W against log 2 (P0 /P) will therefore have an intercept on the y-axis of log W0 . The

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densities of liquid N 2 and CO 2 at the temperatures of measurement were 0.808 and 1.203 g cm 23 and their similarity constants ( b ) 0.33 and 0.35, respectively. One advantage of the D–R treatment is that extrapolation to P/P0 51 of the data gained for CO 2 adsorption (typically to a P/P0 value of only 0.03) in D–R co-ordinates is not as relatively large as it would be in the adsorption isotherm format [12]. A further empirical method of estimating microporosity used was the a s -method, where the adsorption is measured first on a non-porous reference sample of similar material [13]. The method is described fully elsewhere [14]. Suitable reference samples for carbon adsorbents have been also been described and evaluated by RodriguezReinoso et al. [15]. Mesopore volume was calculated from the N 2 desorption data using the BJH method [16].

3. Results and discussion Samples were designated Control Method (T or GC)Rate (mV setting)-Burn off (20 or 40%). Hence a carbon prepared under thermal rate control, at a target CO 2 evolution signal of 30 mV, to a final burn off of 20% is identified as T-30-20.

3.1. Reaction profiles The CO 2 evolution profiles for experiments in which samples were reacted to 20% burn off are shown in Fig. 4 (thermal control) and Fig. 5 (gas concentration control). In addition, all the thermal control reactions showed significant O 2 breakthrough, i.e., O 2 was in excess throughout the reaction. However, only baseline values were observed for the gas concentration control reactions indicating that the O 2 concentration was in fact rate limiting. Fig. 4 shows that at the highest rate, control has been lost and the system has developed oscillatory behaviour. This is due to the exothermic nature of the carbon–oxygen

Fig. 4. CO 2 levels during CR activation under thermal control at increasing target rates of burn off.

Fig. 5. CO 2 levels during CR activation under gas concentration control at increasing target rates of burn off.

reaction and the relatively high rate set which here cause thermal runaway. The system has been discussed more fully elsewhere [2]. Although oscillations could be avoided by changing the PID parameters of the furnace, the object here was to use control of the chemistry to prevent runaway, rather than use of the hardware and this is demonstrated in Fig. 5 showing gas concentration control at similar rates. The initial CO 2 evolution on start up in flowing nitrogen is due to thermal desorption of surface species and represents about 2.6% or 5.7% of the total CO 2 area depending on the degree of burn off. This phenomenon is not visible during thermal control (Fig. 4) as it occurs within the rate control regime. The mean reaction temperatures for thermal rate control and oxygen concentrations for gas concentration control are shown in Table 2 which shows that as the target rate of reaction is increased, it forces the mean temperature or oxygen concentration to rise to achieve that rate. The effect of increasing burn off on these parameters is less clear, but the increase in temperature or oxygen concentration at all Table 2 Variation in temperature and oxygen concentration required to maintain target reaction rates Sample

Mean temperature (8C)

Mean O 2 concentration (%)

T-15-20 T-30-20 T-50-20

267 270 306

20 20 20

T-15-40 T-30-40 T-50-40

272 293 296

20 20 20

GC-50-20 GC-70-20 GC-90-20

400 400 400

3.5 5.2 6.8

GC-50-40 GC-70-40 GC-90-40

400 400 400

3.9 5.7 6.6

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Fig. 6. (a) Nitrogen adsorption isotherms for carbons activated under thermal control to 20% burn off. (b) Nitrogen adsorption isotherms for carbons activated under thermal control to 40% burn off.

but the highest rate may reflect the relative activity of the carbon reaction sites, i.e., the less reactive sites are involved in the higher burn off experiments.

3.2. Surface areas Inspection of Figs. 6 and 7 indicates that as the N 2 adsorption isotherms are all type 1, the carbons prepared by both CR methods are predominantly microporous, with nearly horizontal plateau regions, indicating little mesoporosity. However the BET surface areas derived from them are affected by the activation conditions (see Table 3). The micropore volumes obtained from the N 2 D–R plots were converted to monolayer equivalent areas for comparison and were slightly higher than the BET surface areas. This is probably a reflection of the different treatment of adsorption in micropores by the BET and D–R theories. Samples activated under thermal control of the rate had lower mean surface areas at the two degrees of burn off than the analogous gas concentration control samples. The highest rate at both levels of burn off (T-50-20 and

575

T-50-40) yielded the highest surface areas, but as the values were within experimental error for the gas adsorption measurement technique, this was thought not significant and the rates studied here therefore had no effect on the surface area. However, across the two thermal control groups, the mean surface area fell with increased burn off, presumably due to burn out of the micropore walls or preferential reaction at the external surface. With the gas concentration control group however, the mean surface area increased with burn off, indicating continuing development of the microporosity. The expected increase in mean reaction temperature demanded by the increased rates under thermal control was relatively small (Table 2), which could underlie the nondependence of surface area development on rate. However, the significant difference in surface area between the thermal and gas concentration control samples is likely to be due to the effects of oxygen concentration and temperature of reaction and their differences across the two methods of rate control. The rates of burn off chosen were achieved by each control method using different reaction conditions (temperature and oxygen concentration). Thermal control operated at a lower mean temperature (267– 306 8C) and higher O 2 concentration (20%) than gas concentration control which operated at a constant temperature of 400 8C and mean O 2 concentrations of 3.5– 6.8% (Table 2). One explanation of these observations is that at the higher temperature in gas concentration control, increasing numbers of internal sites are reactive, either intrinsically energetically or via increased O 2 diffusion within the developing pore system. It is possible that once the external sites have reacted, the relatively low O 2 concentration controls the rate and prevents thermal runaway at the surface, allowing the internal centres to react. During thermal control however, at temperatures at least 100 8C lower, the reaction appears to be limited to the surface. The high O 2 concentration either has no access to the internal carbon or these sites are not reactive at these temperatures.

3.3. Microporosity

Fig. 7. (a) Nitrogen adsorption isotherms for carbons activated under gas concentration control to 20% burn off. (b) Nitrogen adsorption isotherms for carbons activated under gas concentration control to 40% burn off.

Microporosity is often characterised by both N 2 and CO 2 adsorption since the higher temperature used for CO 2 adsorption will overcome any activated diffusion effects. However, a limitation in the CO 2 method is due to the low maximum relative pressure possible in standard laboratory equipment, which enables filling of only the smaller micropores. Although it is generally accepted that the smallest micropores are filled by N 2 at 77 K at low pressures, albeit slowly in cases of activated adsorption, the mechanism of CO 2 adsorption at 273 K seems less clear, with authors divided between pore filling and monolayer adsorption [17]. Several authors have described the use of both N 2 and CO 2 adsorption to characterise more fully the microporosity in activated carbons [18–20].

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Table 3 Surface area and pore volume data for carbons activated under rate control in air and conventionally in CO 2 Sample

Surface area (m 2 g 21 )

Micropore volume (cm 3 g 21 )

BET

D–R (N 2 )

D–R (CO 2 )

a-plot

D–R (N 2 )

Mesopore volume (cm 3 g 21 )

T-15-20 T-30-20 T-50-20

337 325 356

382 367 406

0.14 0.13 0.14

0.16 0.17 0.18

0.14 0.14 0.15

0.003 0.001 0.002

T-15-40 T-30-40 T-50-40

178 205 209

207 238 239

0.07 0.08 0.12

0.13 0.15 0.17

0.08 0.09 0.13

0.006 0.005 0.006

GC-50-20 GC-70-20 GC-90-20

517 492 499

588 560 568

0.21 0.21 0.20

0.20 0.20 0.20

0.21 0.20 0.21

0.005 0.007 0.007

GC-50-40 GC-70-40 GC-90-40

619 645 638

699 728 720

0.24 0.25 0.25

0.22 0.23 0.23

0.25 0.27 0.26

0.021 0.016 0.015

CO2-20 CO2-40

574 990

626 1091

0.23 0.39

0.21 0.35

0.23 0.40

0.006 0.032

Fig. 8. (a) Representative D–R plots for carbons activated conventionally and by CR methods, calculated from N 2 adsorption data. (b) Representative D–R plots for carbons activated conventionally and by CR methods, calculated from CO 2 adsorption data.

Table 3 shows that there is good agreement between the micropore volumes calculated by the a s method (N 2 adsorption) and the D–R (N 2 ) method, for all samples. As in the case of the surface area data, the thermal control samples had significantly lower micropore volumes than the gas concentration samples. This supports the theory that little microporosity is being created because of external carbon burn off and low rates of diffusion under the conditions of the thermal control experiments. Inspection of the D–R plots (Fig. 8) shows that while the CO 2 adsorption data yields plots which are very nearly linear over the whole relative pressure range studied (0– 0.04) the N 2 data plots have a significant negative deviation at very low relative pressure [,4310 25 , log 2 (P0 /P) value 20] and a slight positive deviation at high pressure associated with multilayer formation or capillary condensation in the mesopores [14]. Changes of slope or deviations from linearity in D–R plots are common, making estimates of the micropore volume dependent on the portion of the plot used for extrapolation. Marsh and Rand [12] have classified these deviations as of three types, type A (negative deviation at low P/P0 ) often being exhibited by activated carbons with relatively low burn off (,30%). As evidence for activated adsorption, these carbons were demonstrated by Masters and McEnaney [21] to show less deviation if allowed longer to equilibrate during adsorption measurement. Negative deviation at low pressures is generally acknowledged to be due to activated diffusion of N 2 in the narrow micropores [22,23]. The linear part of the D–R (N 2 ) plots used for extrapolation in Fig. 8a was therefore restricted to the portion between log 2 (P0 /P) values of 5–15. In the case of the D–R (CO 2 ) plots (Fig. 8b), the sample

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activated conventionally in CO 2 showed the most deviation from linearity. As a result of this, the calculation of the micropore volume was made from extrapolation of points in the log 2 (P0 /P) range 0–10 only. This gave a value for the micropore volume of 0.35 cm 3 g 21 , which was in good agreement with the D–R (N 2 ) and a-plot values (see Table 3). This range was therefore used for the other samples. Evidence for activated diffusion is also provided by a comparison of the D–R (N 2 ) and D–R (CO 2 ) data for the micropore volume of the thermal control samples (Table 3). In these samples, the micropore volumes calculated from CO 2 adsorption at 0 8C are higher than the corresponding N 2 values. As has been previously described the higher temperature of CO 2 adsorption is important in overcoming these activated diffusion effects. It would appear that thermal control, while favouring external burn off permits the initiation of limited numbers of narrow micropores. In contrast to this, the micropore volumes of the gas concentration samples calculated by both N 2 and CO 2 D–R methods are in good agreement, and substantially higher than the thermal control carbons, suggesting that in these, the pores are being steadily widened in a controlled manner. However, ultramicropores are still apparent in the negative deviation at low pressure of the D–R (N 2 ) plots.

577

Fig. 9. Nitrogen adsorption isotherms for carbons activated conventionally in CO 2 to 20 and 40% burn off.

ing that the micropore size distribution was not significantly widened compared to the CR samples. Therefore the porosity created in CR and conventional activation appears to differ only in the number and not the size of the micropores. This is also supported by the negative deviation of the N 2 D–R plots (Fig. 8a) at very low pressure which is also evident in the CO 2 activated samples, showing the continued presence of ultramicropores.

3.6. Effect of sample size 3.4. Mesoporosity As expected from the shape of the adsorption isotherms, the samples had very low mesopore volume, with the gas concentration samples activated to 40% burn off having the highest values (up to 0.02 cm 3 g 21 ). This is a factor of 10 approx. smaller than the micropore volume. Therefore under neither rate control regime did significant pore enlargement take place at these levels of burn off.

3.5. Comparison with conventional activation in CO2 The BET surface area of the sample activated to 20% burn off in CO 2 (574 m 2 g 21 ) was only slightly higher (ca. 10%) than the highest of the analogous gas concentration control samples (517 m 2 g 21 , see Table 3), but much higher than the thermal control sample mean (339 m 2 g 21 ). Its porosity was also very similar to the gas concentration control samples. However, the sample activated to 40% burn off in CO 2 (CO2-40) had much higher surface area (990 m 2 g 21 ) and micropore volume (up to 0.4 cm 3 g 21 , depending on the method of calculation). It appears therefore, that at the lower activation level, the gas concentration CR method and conventional CO 2 activation yield carbons with similar adsorption capacity. The micropore volume of the CO 2 activated samples, calculated from N 2 adsorption data (D–R and a-plot) was slightly higher than the CO 2 value. Inspection of the isotherms (Fig. 9) however did not reveal a broadening of the knee, indicat-

The experiments described were conducted on relatively small samples and as has been seen, thermal control of the rate of CO 2 evolution yielded lower surface areas and porosity than both the gas concentration control and conventional activation methods. This has been attributed to external burn off at the expense of creation of internal porosity. In larger samples, where the thermal lag becomes significant both for the furnace and within the sample bed, the effect would be expected to be even greater. However, by using gas concentration control and a fluidised bed reactor, uniform porosity could be maintained due to the homogenising effect of the reactor design and the selflimiting effect of the oxygen concentration control. The latter has been shown here to be effective in creating microporosity and to provide good reaction control at the higher burn off rates.

4. Conclusions These studies have shown that the rate of air activation of carbon can be controlled and thermal runaway prevented by use of CR preparative techniques. It was demonstrated that for the rates chosen, good control was best achieved by gas concentration control, since under thermal control, oscillatory behaviour developed. The CR method was successfully used to prepare carbons to a specified burn off at a specified rate.

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The carbons prepared by the above routes were all microporous, but there were significant differences between the two types of control method, due to the combinations of temperature and oxygen concentration employed to achieve the target reaction rates. Much lower BET surface areas (180–350 m 2 g 21 ) were produced by the thermal control method (relatively lower temperature, but higher O 2 concentration), indicating preferential external burn off. However, the gas concentration control method (relatively higher temperature, but lower O 2 concentration) yielded carbons of 20% burn off comparable in surface area and micropore volume (500 m 2 g 21 , 0.2 cm 3 g 21 ) to carbons activated conventionally in CO 2 to the same level of burn off. The aim of this paper was to compare the rate control and conventional activation processes and to characterise any differences in the porosity of their activated carbon products. However, if the objective were to develop an alternative method of carbon activation for commercial purposes, ways of increasing the microporosity in the gas concentration control regime would need to be considered. It is possible that this could be done by further increasing the extent of burn off, since the micropore volume increased in these experiments when burn off was raised from 20 to 40%. Alternatively, it appears that following the above trend in temperature and oxygen concentration and their effect on porosity, an increase in temperature and the associated reduction in oxygen concentration during gas concentration control of the rate of activation, would also yield higher micropore volumes. The energy saving advantages of air activation would then be dependent on the final temperature chosen for optimum creation of microporosity.

Acknowledgements P.A.B., E.A.D. and G.M.B.P. would like to thank Dstl for financial support.

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