Studies in Adsorptiondesorption of Carbon Dioxide

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Studies in adsorption/desorption of carbon dioxide

Document by:Bharadwaj

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Abstract

The use of fuels in various forms generates huge quantities of carbon dioxide, a green house gasand reducing its emission has been accorded top priority in todays research to protect our

climate. The biggest challenge is how to effectively and efficiently capture carbon dioxide and

develop industrial scale process with lowest possible cost. In the present work we report studieson adsorptive removal of carbon dioxide using various adsorbents which include mainly activated

carbons- with and without modification. The effect of activation temperature has been studied in

detail. Temperature programmed desorption studies have been carried out which clearly indicate

shift in the mechanism of sorption on activated carbon surfaces with changes in the activation of

carbon and it also depends on the nature of surface/ modification. An attempt has been made to

explain the experimental results of adsorption and temperature programmed desorption using

various surface characterization techniques such as surface area, pore size and size distribution,

IR and XRD. The results of this work would help in understanding surface interactions during

adsorption/desorption and also in enhancing capacity of carbon dioxide sorption.

Introduction

The use of fuels for generation of energy has a major contribution in the release of green house

gas, carbon dioxide. A recent report by Nobel Prize winning Intergovernmental Panel on Climate

Change concluded that global carbon dioxide emission must be reduced to the order of 50-80%

by year 2050, if we have to avoid damage to the climate.

As far as current state of knowledge is concerned, there is no effective carbon dioxide capture

technology, as yet, which is not cost and energy intensive (Yue et al., 2008). Thus, the biggest

challenge today is how to effectively and efficiently capture carbon dioxide and develop

industrial scale process with lowest possible cost. This, in effect requires research to be carried

out on various fronts involving substantial investment in R&D for identifying and developingmost appropriate processes for carbon dioxide removal. Since burning of fuels can not be avoided

in view of our energy requirements and thus generation of green house gases is also inevitable, itbecomes imperative to develop technologies which have potential for reducing green house gas

emissions.

The processes for removal of gases through adsorption, such as pressure swing adsorption can

very well be used for removal of carbon dioxide (Yong et al., 2002; Liu et al., 2007). The process

has advantage of being cyclic in nature and carbon dioxide is adsorbed at high pressures and

desorbed by lowering the pressure. The process is less capital intensive and also requires lower

operating cost. Thermal swing adsorptive process is another alternative. However, primary

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requirement of any adsorption separation is that the adsorbent material should have high capacity

for removal and ease of regeneration. As far as carbon dioxide removal is concerned, this has

been a major impediment for application at a commercial level. Song et al. (1998) and Yong et al.

(2001) have reported some studies on modified and unmodified carbons and have found that

although carbon adsorbents have good capacity for CO2 at low pressures and low temperatures,the capacity decreases with increase in temperature. They have also suggested that chemical

modification of carbons can enhance the capacity especially at high temperatures. In view of this,the objective of this work is to evaluate the adsorption/desorption behaviour of various

adsorbents, both modified and unmodified and to study effect of various parameters of

adsorption/ desorption characteristics. The Temperature Programmed Desorption (TPD) studies

of this work can provide more insight into the sorption behaviour on the various surfaces and

nature of surface interactions. This is expected to further enhance our knowledgebase for suitably

modifying adsorbent surfaces and thereby in developing tailor made adsorbents.

Experimental

Two different adsorbents were used in this work from activated charcoal family. The unmodified

activated charcoal was procured from Fluka U.S.A., while the other activated charcoal, acid

washed with phosphoric acid and sulfuric acid was procured from Sigma, U. S. A.. Both thesamples were in powder form and were used without any further treatment/modification. For

studying the effect of activation temperature, activation of the samples was done in situ before

adsorption experiment under constant Helium flow. The activation time was 10 h. After the

activation, the sample was allowed to cool down to room temperature under He flow and then

adsorption of CO2 was studied.

Carbon dioxide adsorption was characterized by temperature programmed desorption

experiments (TPD). 20 to 30 mg of sample was placed in a quartz tube. After degassing the

sample, activation was carried out using the procedure described above. The adsorption of carbon

dioxide was carried out at room temperature of 30 0C for 4 h. The adsorbent bed was then flushed

with helium for about 30 min. The TPD tests were then carried out by heating the sample at 10 0C

/min up to 500 0C with constant helium flowing through the tube and desorbed carbon dioxidewas analyzed using CO2 detector. The activation temperature effect was studied in the range 1000C to 400 0C and adsorption without activation was also compared. All the gases used in the study

(Helium and carbon dioxide) were extra pure above 99.99 % purity.

Characterization of adsorbents

The specific surface area, pore size distribution and pore volume were determined on the basis of

nitrogen adsorption using Quantachrome Autosorb-1 Instrument (Quantachrome Inc., USA). X-

ray diffraction patterns of the samples were recorded in XPERT-PRO X-ray Diffractometer from

PANalytical Instruments using CuK radiation. The IR spectra of the samples were taken indiffused reflectance mode in Thermo Nicolet FTIR spectrophotometer.

Results and Discussion

Carbon based adsorbents generally have good capacity for adsorption of carbon dioxide,

especially at ambient temperatures and low pressures (Yong et al. 2001, 2002). The adsorption

capacity decreases with increase in the temperature. However, the adsorption capacity at highertemperatures can be greatly increased by suitable chemical modification. This is mainly because

of the fact that CO2 is slightly acidic gas and therefore is likely to have more, stronger adsorption

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on the basic sites with increased amounts. Thus, chemical modification to increase the basicity of

the surface can have better adsorption characteristics than unmodified carbon adsorbents. Also,

adsorption/ desorption mechanism at high temperatures involve more chemisorption for carbon

dioxide than mere physical adsorption. This is evident from the fact that surface area of the

carbon adsorbents has less effect on sorption capacity at high temperatures clearly indicatingmore surface interactions than pure physical adsorption. Similarly, it is expected that activation of

the material using different temperature can also affect the adsorption/ desorption behavior ofcarbon dioxide, through modification of surfaces/ adsorption sites.

The results of three different activation temperatures for the unmodified activated charcoal are

shown in Figure 1 for activation temperatures of 100, 200 and 300 0C respectively. The TPD plots

clearly show substantial difference in the sorption characteristics in these three cases. For

activation temperature of 100 0C, two peaks were observed in the TPD. The first desorption

observed at 80 0C and the peak was observed in the range 80 140 0C, while the 2nd desorption

peak was observed in the temperature range of 180-330 0C. The second peak was much larger

compared to the first. The occurrence of the two peaks here indicates difference in the nature of

chemisorption and also possibility of macropore/micropore desorption taking place. For the

activation temperature of 200 0C, the similar two peaks were observed. However, here there is a

shift in the desorption temperature and also in the temperature range of desorption. The first peakin this case is much smaller compared to that observed with 100 C activation temperature and can

be seen to initiate at ~100 0C and the 2nd peak at ~260 0C. The 2nd peak is much broader and is

observed in the range 260 430 0C. The change in the TPD behaviour clearly points to the

modification in the sorption sites with activation temperature. The TPD plot of activationtemperature 300 0C is also shown in Figure-1 again clearly demonstrates the shift in the

desorption temperature and sorption behaviour as described earlier. In order to further confirm

0 100 200 300 400 500

Temp (Deg. C)

Signal(mV)

AC-1

AC-1-100

AC-1-200

AC-1-300

Figure 1. TPD of CO2 for the unmodified activated charcoal and

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effect of activation temperature, the TPD of the unmodified activated charcoal (denoted by AC-1

in Fig.1) was carried out. It was found that the CO2 desorption started at 800C as observed in 100

0C activation, but the 2nd peak was almost merged with the first and practically only one large

peak of CO2 desorption was seen. This is clearly different from the three cases discussed earlier

depicting changes in the adsorption/desorption behaviour with activation temperature.

The results of three different activation temperatures for the acid treated activated charcoal(CHAC) are shown in Figure 2 for activation temperatures of 100, 200 and 300 0C respectively.

As seen earlier for unmodified carbon, here too the TPD plots clearly show substantial difference

in the sorption characteristics for different activation temperatures. The quantity of sorption of

carbon dioxide in this material is less as compared to the unmodified activated charcoal which is

logical as the surface in this case is more acidic compared to the unmodified charcoal. Further,

here it has been observed that adsorption of carbon dioxide occurs prominently at lower

temperatures of activation, such as 100 0C and very less adsorption takes place, as can be seen

form the TPD plots of Fig. 2, at higher temperatures of activation. This is quite different from the

earlier case of unmodified adsorbent and the observed difference can be mainly attributed to the

nature of acidic surface which probably gets deactivated at higher temperatures.

0 110 220 330 440 550

Temperature,oC

Signal,mV

(a)-CHAC-100

(b)-CHAC-200

(c)-CHAC-300

Figure 2. TPD of CO2 for the acid treated charcoal activated

at different temperatures.

Carbon adsorbents are highly porous containing both micropores and macropores. The pore size

distribution can have its effect on adsorption/ desorption characteristics of CO2. Macropores can

facilitate faster transport of CO2 as compared to micropores, whereas the surface and molecular

level interactions are more prominent in micropores. Since the overall sorption kinetics is pore

diffusion controlled, both macropore and surface diffusion play important role. The individual

contribution of these parameters can be very different for different carbon materials owing to the

changes in the type of surface interactions, internal capacity for sorption and pore size and pore

size distribution which can be reflected in the TPD plots of different materials. Since CO 2 is a

linear molecule with diameter of approx. 3.4 0A, porosity is likely to have little effect on diffusionand capture for average pore size greater than 5 0A. In such cases, surface area and surface

interactions play important role. For the materials wherein the sorption capacity lies mostly in

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micropores with sizes close to molecular size of the sorbate, surface interactions and diffusion

play crucial role in sorption kinetics. Further, there is a possibility that the adsorption of CO2 on

some carbon surfaces can be irreversible, at least partly indicating that certain amount of carbon

dioxide can remain trapped in the pores of carbon, especially in the micropores. Such behaviour

can be expected in highly microporous materials and TPD plots of such materials would depictslow and incomplete desorption of CO2 even at high temperatures. From the results of this work,

it is evident that there is shift in desorption to higher temperatures for higher activationtemperatures and also that delayed and slower desorption occurs in such cases. The reason for

such adsorption/ desorption behaviour can be explained using the plausible mechanism, wherein

higher sorption capacity can be generated, especially in the micropores when higher activation

temperatures are used. In the case of simple chemisorption, a single peak for CO 2 would occur,

whereas, the present behaviour clearly indicate a more complex sorption behaviour and effect of

activation temperature on adsorption/ desorption. The surface area obtained for the activated

carbons used in this work is of the order of 775 m 2/g and for acid treated charcoal, the average

pore size was quite large, of the order of 39 0A. Further, there is substantial pore size distribution.

It is necessary to correlate the surface area and pore size distribution to substantiate the above

postulated mechanism. A more quantitative study, especially in sorption isotherm is required to

evaluate and confirm the above sorption behaviour.

X-ray diffraction patterns of the acid treated activated charcoal were recorded in XPERT-PRO X-

ray Diffractometer from PANalytical Instruments using CuK radiation. Figure 3 shows the XRD

profile of the acid treated activated charcoal heated at different temperatures. The major phase

present in the sample CHAC is graphite, which corresponds to the diffraction peaks observed at 2

theta positions 26.2 and 54.8 in the XRD pattern [JCPDS 41-1487]. The diffraction peaks at 2

theta positions 20.8 and 22.02 corresponds to the phase fullerene. The broad humps near 25 and

45 degree 2 theta positions indicate partial amorphous nature of the sample.

10 20 30 40 50 60 70

2 Theta (Degree)

Intensity(a.u.)

Figure 3. X-ray diffraction patterns of the acid treated carbon

Activation Temperature: (a) 100 (b) 200 (c) 300 (d) 400 C

The IR spectra of the samples shows not much change in the IR-spectra when heated at different

temperatures indicating not much change in the surface character with temperature. These

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findings also indicate that the nature of adsorbent surface dictates the sorption capacity and the

adsorption/ desorption mechanism has contribution of surface area and pore size distribution

more important in the chemisorption of carbon dioxide on activated charcoal.

400140024003400

Wavenumber (Cm -1)

Intensity(a.u

(a)-100 (b)-200

(c)-300 (d)-400

Figure 4. DRIFT Spectra of the acid treated carbon activated

at different temperatures.

Conclusions

The temperature of activation is found to play a crucial role in the adsorption and desorption

characteristics of carbon dioxide on adsorbent materials belonging to class of carbons. There isclear shift in the mechanism of sorption/ desorption even when the overall sorption is

chemisorption. Further, there is marked difference in the modified and unmodified adsorbent

materials. The results of this work clearly highlight the need for study of adsorption and

desorption of carbon dioxide at high temperatures in order to understand the best methodology to

modify adsorbents for enhanced adsorption capacity and better kinetics and also to understand the

mechanism of sorption along with its dependence on various process parameters.

References

1. Liu X., Zhou L., Fu X., Sun Y., Su W., Zhou Y. Adsorption and regeneration study of themesoporous adsorbent SBA-15 adapted to the capture/ separation of CO2 and CH4. Chem.

Eng. Sci. 62, 1101-10 (2007).2. Song H. K., Lee K. H. Adsorption of carbon dioxide on chemically modified carbon

adsorbents. Separ. Sci. Technol. 33, 2039-57 (1998).3. Yong Z., Mata V. and Rodrigues A. E. Adsorption of carbon dioxide at high temperature- a

review. Separ. Purif. Technol. 25, 195-205 (2002).

4. Yong Z., Mata V. and Rodrigues A. E. Adsorption of carbon dioxide on chemically modified

high surface area carbon based adsorbents at high temperatures. Adsorption 7, 41-50 (2001).

5. Yue M. B., Sun L. B., Cao Y., Wang Z. J., Wang Y., Yu Q., Zhu J. H. Microporous and

Mesoporous Mater. 114, 74-81 (2008).

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Studies in Adsorptiondesorption of Carbon Dioxide