Accepted Manuscript

Comparison of NO2 removal using date pits activated carbon and modifiedcommercialized activated carbon via different preparation methods: effect ofporosity and surface chemistry

Meriem Belhachemi, Mejdi Jeguirim, Lionel Limousy, Fatima Addoun

PII: S1385-8947(14)00568-3DOI: http://dx.doi.org/10.1016/j.cej.2014.05.004Reference: CEJ 12098

To appear in: Chemical Engineering Journal

Received Date: 24 January 2014Revised Date: 1 May 2014Accepted Date: 2 May 2014

Please cite this article as: M. Belhachemi, M. Jeguirim, L. Limousy, F. Addoun, Comparison of NO2 removal usingdate pits activated carbon and modified commercialized activated carbon via different preparation methods: effectof porosity and surface chemistry, Chemical Engineering Journal (2014), doi: http://dx.doi.org/10.1016/j.cej.2014.05.004

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1

Comparison of NO2 removal using date pits activated carbon and

modified commercialized activated carbon via different preparation

methods: effect of porosity and surface chemistry

Meriem Belhachemia,c, Mejdi Jeguirimb*, Lionel Limousyb, Fatima Addounc

aLaboratoire de Fiabilité des matériaux et des structures en région sahariennes. Université de Béchar,

B.P 417, 08000 Béchar, Algeria

bInstitut de Science des Matériaux de Mulhouse, UMR 7361 CNRS, 15, rue Jean Starcky 68057

Mulhouse, France

cLaboratoire d’étude physico-chimique des matériaux et application à l’environnement, Faculté de

Chimie, USTHB, B.P.32, El Alia 16111 Bab Ezzouar Alger, Algeria

*Corresponding author : mejdi.jeguirim@uha.fr;

Tel: +33-3-89608661.

Abstract

This study aims to evaluate the NO2 adsorption capacities of activated carbons

prepared by physical and chemical activation of date pits and to compare them with

modified commercialized activated carbons. The texture, morphology and surface

chemistry of the different carbons were evaluated by N2 and CO2 adsorption, scanning

electron microscopy (SEM), energy dispersive X-ray spectrometry (EDX),

temperature programmed desorption coupled with mass spectroscopy (TPD-MS),

Fourier transform infrared spectroscopy (FTIR) and X-ray fluorescence (XRF)

techniques. The obtained results show that activated carbon prepared from date pits

are an efficient adsorbents for the removal of NO2 from the exhaust gases at ambient

temperature. The obtained adsorption capacities are close to the ones obtained for the

2

commercialized activated carbons. The analysis of the correlation between NO2

adsorption capacities and the textural and the surface chemistry properties shows that

a higher microporosity volume, an homegenous microporosity and the presence of

stable oxygen groups contribute strongly to the adsorption of NO2 on the activated

carbon surface. In contrast, the presence of strong acidic groups such as carboxylic,

anhydrides and lactones inhibits the reduction of NO2 into NO, which represents a

crucial step for the NO2 adsorption.

Keywords: activated carbon, date pits, microporosity, surface chemistry, NO2

adsorption

1. Introduction

The economic development of contemporary societies is followed by the consumption

of greater amounts of fossil fuels generating important air polluting chemical

compounds such as nitrogen oxides (NO, NO2), CO and sulfur oxides (SO2) which are

considered as greenhouse gases.

Several control techniques were proposed to prevent NOx emissions [1-6]. Among

these techniques combustion control and selective catalytic reduction (SCR) are

widely used. Combustion control of NOx is achieved by optimizing the combustion

process during NOx generation. The installation and operating costs of these

technologies are relatively low. However, the removal efficiency is too low (generally

30–40%) [7]. Combustion control technology could not achieve the NOx higher

removal required by the stricter environmental standards. Therefore, SCR method was

preferred by using ammonia and a catalyst to reduce NOx by producing nitrogen and

water vapor. This technology gives high NOx elimination that can reach up to a 94%.

3

However, it remains the most expensive technology. In addition, catalysts have a

finite life in flue gas and some ammonia slides without being reacted [8, 9]. Thus,

NOx control technologies aim to identify cheaper and efficient techniques.

Although various purification methods are offered, adsorption technology remains the

most common and efficient technique for removing organic and inorganic micro-

pollutants. Among various adsorbents, activated carbon is the main material used for

numerous applications. Several works are currently available for their applications in

water treatment; air pollution control; organic and inorganic pollutants elimination;

solvents and hydrocarbons recuperation; pharmaceutical compounds separation;

catalysis; hydrogen and natural gas storage [10, 11].

Activated carbon can be prepared by chemical or physical methods. The latter

technique is carried out by carbonization in an inert gas followed by activation in an

oxidant gas (CO2 or H2O). In contrast, chemical activation is carried out by heating

the precursor in the presence of an added chemical agent. Among the different

chemical reagents, ZnCl2 is one of the commonly used agent for the activation of

various precursors [12, 13]. Impregnation of lignocellulosic precursors with ZnCl2

causes degradation of the cellulosic material and, on carbonization, dehydration.

These processes result in charring and aromatization of the carbon skeleton and

creation of a porous structure. Generally, both thermal activation with CO2 and

chemical activation with ZnCl2 led to microporous activated carbons.

The removal of NO2 using activated carbons has been a potential technique [14, 15].

As a consequence, many research groups examined the factors favoring the adsorption

of NOx in order to develop specific activated carbons for this application. For

example, Bashkova and Bandoz have modified activated carbons by urea and

4

subsequent heat treatment at high temperature in order to introduce basic groups.

They have found that the modification of activated carbons has a positive effect on

adsorption properties, with NO2 uptake capacity increasing from 35 to 66 mg/g in dry

condition at ambient temperature [16]. Gao et al have used a commercial activated

carbon treated at 900°C under CO2 flow for different temperatures. They have shown

that the micropores in the carbon structure play a role of reactors for the creation of

–C(ONO2) species. These oxygen surface groups are formed by the interaction of

adsorbed NO2 on –C(O) surface complexes of activated carbon [1]. Nowicki et al.

have studied the adsorption capacity of NO2 on activated carbons issued from plum

stones. The samples were activated according to two steps: carbonization at different

temperatures (400, 500 and 600°C) and activation using KOH at constant and

increasing temperatures. The results show high surface area of 3228 m2/g and pore

volume of 1.61 cm3/g. However, the adsorption capacity of NO2 on activated carbons

does not exceed 67 mg/g. The authors have reported that the removal of NO2 depends

on the parameters and conditions of preparation [17]. Moreover, activated carbon

gives prominence results as a catalyst support for NO catalytic adsorption [2, 18, 19].

Previous studies have used activated carbon as a catalyst support of metals (Cu, Fe, )

[2,4]. Lopez et al. examined the removal activity of NO using activated carbon doped

with copper. The catalyst improves NO adsorption in the presence of oxygen, by

increasing the breakthrough time and the adsorption capacity [2]. Recently, Sousa et

al. have studied the NO oxidation using modified activated carbon as a catalyst. The

materials were impregnated with HNO3, O2, urea and melamine to obtain different

surface chemistries. Their results show high NO conversion (88%) in relation to the

NO concentration and the amount of catalyst. They have also proved that the

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introduction of nitrogen species on activated carbons has a positive effect on the

catalytic activity [18].

The use of agriculture and agro-industrial residues as a precursor to produce activated

carbons was widely applied to convert them into value added products. Date stones

and palm tree wastes generated by the food industry are present in large amounts in

Algeria. Belhachemi et al. showed that date pits could be used as precursors to

produce activated carbons with a well-developed porosity and tailored oxygen surface

groups [20]. Hence, the purpose of this present investigation is to evaluate the

performance of these activated carbons for the removal of NO2 from air and to

compare them with modified commercialized activated carbons. The textural and

chemical surface properties of the different samples are determined and correlated to

the NO2 adsorption capacities. The main objective is to identify the activated carbon

characteristics that may improve the performance of activated carbons using date pits

as a precursor.

2. Experimental

2.1. Samples preparation

Two granular activated carbons were produced from Algerian date pits as a precursor

by physical and chemical activation using CO2 (CCO2) and ZnCl2 (CZn) agents

respectively. During CCO2 sample preparation, date stones were creased and separated

to obtain particle size between 0.5 and 1mm. After, carbonization and activation with

CO2 were performed. Raw date pits were carbonized at 825°C with a heating rate of

5°C/min under a constant flow rate of N2 (100mL/min). Then, the carbonized sample

was activated under a pure CO2 flow rate of 100mL/min at 825°C for 4 hours. Further

6

details on experimental conditions can be found in previous investigations [20]. The

second sample CZn was prepared by impregnation of 1g zinc per gram of precursor,

followed by a heat treatment at 550°C for 1.5 h (4°C/min) under nitrogen (flow rate of

100mL/min). After the sample was cooled down to room temperature, it was washed

firstly with a boiling dilute solution of HCl (to remove Zn) and finally with distilled

water until reaching a neutral pH.

In order to compare the performances of the prepared activated carbons from date

pits, a commercial activated carbon GAC (NORIT GAC 1240) was used in this study.

Moreover, in order to study the effect of surface chemistry properties, two modified

commercial activated carbons were elaborated from GAC. Hence, the commercial

activated carbon GAC was oxidized by a (NH4)2S2O8 liquid phase at 25°C for 24h to

obtain sample GAC-O. Then, the latter was washed and dried at 130°C and heated at

700°C under N2 (flow rate of 100mL/min) and kept for 1h in order to eliminate

selectively oxygen surface groups, which have a lower stability, to obtain sample

GAC-O-T. In fact, the main stable oxygen functions can be removed from carbon

surface in the range of temperature from 800 to 1000°C [21].

2.2. Characterization of activated carbons

2.2.1. Pore structure and morphology characterization

Technique of nitrogen and dioxide carbon adsorption at -196°C and 0°C respectively

was used to characterize the texture of activated carbons, using a static manometric

apparatus. The latter uses the BET method to determine the specific surface area. The

micropore volume (VN2), size less than 2 nm, was calculated from the N2 adsorption

data according to the Dubinin–Raduskevitch (DR) equation. The volume of narrow

micropore (VCO2), size less than 0.7 nm, was also determined by applying the DR

method to the CO2 adsorption isotherm. The mesoporous volume (Vmeso) was

7

obtained from N2 adsorption, by subtracting the microporous volume (VN2) from the

total porous volume (VT) adsorbed at p/po = 0.95, while the porous volume

corresponding to the wide microporosity (Vw) was obtained from the gap between the

volume of micropores (VN2) and the narrow microporous volume (VCO2) [22]. The

pore size distribution was obtained using BJH (Barrett–Joyner–Halenda) method.

Scanning electron microscopy (Philips model FEI model Quanta 400 SEM) and

energy dispersive X-ray spectrometry (EDX) were used to observe the morphology

and the surface elemental analysis of the activated carbons, which allows determining

the elemental mapping of the samples.

2.2.2. Characterization of carbon surface chemistry and composition

Qualitative and quantitative information concerning the carbon surface functionalities

were evaluated using a home-made temperature programmed desorption set-up

coupled with a mass spectrometer (TPD-MS). 10 mg of the sample was placed in a

quartz tube in a furnace and heat-treated with a linear heating rate of 5°C/min under

vacuum. The amount and nature of the oxygen functionalities present in each sample

was evaluated in the temperature range 25–900 °C.

FTIR spectra of carbon samples were measured by transmission using a Bruker

(IFS66/S) spectrometer equipped with an MCT detector cooled with liquid nitrogen.

Each sample was mixed with KBr to 0.5%wt concentration and a pellet of 1 mg was

made. All spectra were measured over the range of 4000 and 600 cm-1. A spectra

resolution of 8 cm-1 was considered and 100 scans per sample were taken.

While inorganic compounds present in the different activated carbon samples may be

at the origin of textural modification during the activation step, the main ones were

8

characterized by X-ray fluorescence (XRF) using a Magix from PHILIPS

spectrophotometer apparatus.

2.3. NO2 adsorption experiments

NO2 adsorption tests were performed in a fixed bed reactor presented in Figure 1.

These experiments were realized at room temperature (25°C) and dry conditions.

In each test 100 mg of activated carbon sample was put on a quartz frit (16 mm in

diameter) set in a vertical fused silica tube. In order to control the temperature of the

sample and the flow rate of the inlet gases, a thermocouple and mass flow meters

were used. The reactor total pressure was equal to 1 atm. Before adsorption tests,

activated carbons were outgassed at room temperature for 1h under a flow of N2.

Then, a gas stream mixture consisting of nitrogen and NO2 (500 ppmv in N2) was

admitted at a flow rate of 20 NL/h and injected through the reactor. Infrared analyser

(Rosemount NGA 2000) was used for the measurements of NO2, NO, CO2 and CO

quantities at the reactor outflow every 5 seconds.

The NO and NO2 concentrations, the flow rate, and the adsorbent mass were used to

calculate the NO2 uptake capacity in mg/g of adsorbent according to:

Where NO2ads is the adsorbed rate of NO2 in µmol/s. NO2inlet is the inlet NO2

concentration in ppmv. NO2outlet and NOoutel are, respectively, the outlet NO2 and NO

concentrations in ppmv. D is the flow rate in NL/s. VM is the molar volume at 0°C

9

(22.4 L/mol). MNO2 is the molar mass of NO2 (46000 mg/mol) and mAC is the mass of

activated carbon in g.

After adsorption experiment, temperature-programmed desorption were carried out to

evaluate adsorption capacity and to deduce the evolved gases present in each sample.

TPD results were achieved by heating the sample under nitrogen flow to 900°C at a

heating rate of 5°C/min.

3. Results and discussion

3.1 Activated carbon characterization

3.1.1. Morphology and pore structure characterizations

Scanning electron micrograph observations of the different activated carbon particles

are presented in figure 2 (a, b, c, d and e). The SEM pictures obtained with the CZn

and GAC samples present regular and smooth surfaces (Fig.2a and 2c), while the

other samples show an irregular cracked surface (Fig.2b, 2d and 2e). It means that,

according to the activation mode, the morphology of carbon particles differ. Other

SEM pictures with a lower enlargement (not show here) confirm that all the activated

carbon particles present a homogeneous diameter between 0.5 and 1 mm. In order to

go further, minerals contained in the different activated carbons were analyzed by X-

ray fluorescence. Results indicate that the main elements present in the commercial

activated carbon (GAC, GAC-O and GAC-O-T) correspond to Si, Al, S and Fe (see

Table 1). The different treatment performed on the GAC sample did not modify the

chemical composition of the activated carbons. At the opposite, an important

difference between the activated carbons obtained with the date pits after a chemical

or a physical activation is observed. The use of ZnCl2 leads to the specific extraction

of alkali and alkaline earth metals (see Table 1), while the activation with CO2

10

induces the elimination of Si, Al and Fe from the carbon matrix, and an important

decrease of the sulphur content. This result could be at the origin of the apparition of a

specific surface texture (corresponding to micropores or mesopores), while reactions

involved during the different activation mode seem to act on different sites of the

char.

N2 adsorption-desorption isotherms obtained with the different activated carbons at -

196°C are presented in figure.3. Textural characteristics obtained from both N2 and

CO2 adsorption isotherms are shown in Table 2. It is seen that the shape of the

adsorption isotherm and the adsorbed volume depend on the activation conditions.

The large knee of the N2 isotherm at low pressures (below P/P0=0.2) shows the

presence of a large micropore size distribution. All these materials are essentially

microporous, with a structure of porosity relatively wider for the samples CCO2 and

CZn. These results are confirmed in Table 2 by analyzing the microporous volume

values found with N2 (size <2 nm) and CO2 (size <0.7 nm) adsorption isotherms

(VN2>VCO2). CCO2 sample corresponding to a burn-off of 49% shows a larger knee and

the linear branch of isotherm is not parallel to the relative pressure axis with the

presence of a hysteresis loop. Such results indicate the existence of wide micropores

and mesopores, which is confirmed by the Vmes and Vw (see Table 1). However, N2

and CO2 adsorption isotherms show that the sample GAC exhibits relatively similar

values of microporous volumes indicating the presence of narrow and homogenous

micropores. It is well known that the values of VN2 and VCO2 give important

information about the micropore size distribution in the absence of kinetic restrictions.

Consequently, for similar values of VN2 and VCO2 carbons reveal a narrow and

homogenous microporosity, while for VN2>VCO2 the micropores become larger [23].

11

The activated carbons after oxidation and heat treatments (GAC-O and GAC-O-T)

keep the same shape than the parent sample GAC. This behavior means that the

modification of the chemical structure of activated carbons with the conditions used in

this study, has no significant effect on the microporosity of the parent sample (GAC).

In fact, the values of Vmes and VCO2 do not change, while the surface area SBET and the

microporous volume VN2 decrease (Table 2).

The oxidation of sample GAC to obtain sample GAC-O causes a decrease in both the

SBET and VN2. This behavior is attributed to the effect of fixation of oxygen surface

groups which increase the constrictions of pores at the entrance, producing diffusional

restrictions for the N2 molecule at -196°C. On the other hand, the SBET and VN2 values

increase after heat treatment (sample GAC-O-T), due to the elimination of less stable

oxygen surface complexes, which produces the gap of pores.

Pore size distribution curves (Fig.4) suggest that all activated carbons are microporous

materials due to the sharp increase of pore size distribution curves for pore diameters

less than 20 Å. According to the pore size distribution, sample CCO2 contains

important amount of narrow mesopores from 20 to 60Å than GAC and CZn. Whereas,

a minor change is noted in pore size distribution for the sample GAC after chemical

surface modifications. This result is confirmed in calculated microporous and

mesoporous volumes in Table 2.

3.1.2. Characterization of carbon surface chemistry

FTIR method is used to identify the functional groups on the activated carbon surface.

Fig.5 depicts FTIR spectrum of the different activated carbons. The FTIR assignments

of functional groups on carbon surface are listed on Table 3. All activated carbons

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show a strong absorption peak at 1068 cm-1 attributed to C-OH (stretching) of

phenolic groups. They have also a medium absorption peak at 3230 cm-1 attributed to

the O-H (stretching) groups. According to available data in literature, this absorption

band is attributed to the presence of ether groups [24]. An absorption band at 1710

cm-1 attributed to C=O stretching is observed for all the samples present in lactone,

anhydride and carboxylic groups. However, this peak is stronger for GAC and GAC-

O samples and decreases for the other samples, which is due to the heat treatment.

TPD-MS experiments give the evolution of CO2 and CO emissions, as a result of the

decomposition of the oxygen functionalities existing at the surface of the activated

carbons. The determination of the amount of CO and CO2 evolved gives an estimation

of the amount of surface oxygen groups on the activated carbons. Moreover, the

analysis of the evolution temperature indicates the presence of different oxygen

surface groups. Fig.6 shows CO and CO2 desorption profiles obtained during the

TPD-MS study. It is well known that the CO2 evolves from the decomposition of

carboxylic acids at low temperatures or lactones at high temperatures, anhydrides

decomposes to both CO and CO2, and carbonyl, phenols, ethers, quinones, pyrone and

chromene groups decompose to CO at temperatures up to 1000°C [21,25].

The comparison of the different samples show that the CO profile of GAC-O is

different from the others activated carbons. This oxidized sample (GAC-O) presents a

large peak at high temperature at about 600°C. The others samples show one peak at

high temperature (>800°C), suggesting the presence of very stable groups such as

ethers, carbonyls and quinones. The presence of ethers group was already observed

during FTIR analysis.

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The CO2 desorption profiles of the activated carbons show a peak at low

temperatures. However, this peak is more intense for GAC-O and CCO2 comparing to

the other samples suggesting that they contain more lactones. A second peak is

detected for the oxidized sample (GAC-O) at higher temperature at about 400°C,

which is attributed to anhydrides.

Table 4 reports the amounts of CO and CO2 released, obtained by integration of the

areas under the TPD-MS peaks. The activated carbon obtained after (NH4)2S2O8

treatment reveals higher amounts of CO and CO2. However, activated carbons CCO2

and GAC-O-T contain high amount of very stable oxygen complexes evolved as CO,

which may result essentially from basic groups (pyrone and chromene).

3.2. NO2 adsorption on the different activated carbons

3.2.1 NO2 adsorption capacities

Fig.6 presents a typical result of the profiles NO2 and NO concentrations during the

adsorption step (Fig. 7(a)) and gas composition during the TPD step (Fig 7 (b)) for

test performed with activated carbon prepared from date pits by physical activation

(CCO2). A release of NO was observed in the first minutes of adsorption which shows

the reduction of NO2 to NO on the carbon surface. This behavior was also observed

for the different examined activated carbons. Therefore, NO increases in the first 10

min, reaches a maximum and after decreases. The release of NO suggests that the

NO2 reduction occurred on the activated carbon surface.

From the profiles of the emitted NO2 and NO, the amount of the adsorbed NO2 is

calculated as function of time. The obtained curve is shown in Fig 7 (b) and indicates

14

that the departed NO2 is not completely transformed into NO. Thus, a significant part

of NO2 is adsorbed on activated carbons and the total amount of adsorbed NO2,

calculated according to equations (1) and (2), is 129 mg/g.

During the TPD step (Fig. 7 (b)), two sharp desorption peaks of NO2 are noted at 63

and 103°C, respectively. Such behavior may indicate that NO2 adsorbs on two

different sites and its release occurred from two different reaction pathways.

Moreover, a slight peak of CO2 occurred simultaneously with the first peak of NO2

desorption. The NO2 emission disappears completely at 190°C. Above 70°C, the NO

and CO2 emission curves have a similar trend with two desorption peaks occurring

simultaneously at 140°C and 320°C. Such behavior may indicate that these gases are

emitted from the same surface groups. Above 350°C the emission of NO decreases

with similar tendency than CO2 until 460°C but vanishes at 530°C. Desorption of CO2

decreases slowly until the end of the experiment at 900°C. The desorption of CO

shows a continuous emission between 60 and 200°C followed by a steadily increasing

emission between 200 and 900°C.

Similar tendency of the emitted NO2 and NO curves was observed upon the exposure

of 500 ppmv NO2 on the different activated carbons. However, the adsorption

capacities vary strongly for the different prepared samples. The evolution of the

adsorption capacities versus time for the different activated carbons is shown on

figure 8. Moreover, due to the reduction of NO2 into NO, the amount of the adsorbed

oxygen was calculated for the different activated carbons. The evolution of the

adsorbed oxygen resulting from NO2 reduction is shown on figure 9. The adsorption

NO2 capacities of the different activated carbons as well as the amount of the

adsorbed oxygen are available in Table 5.

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All adsorbent materials show important NO2 adsorption capacity. The best effective

samples are activated carbons CCO2 and GAC-O-T, prepared by thermal treatment.

However, the introduction of oxygen surface groups into GAC carbon by oxidation

with a (NH4)2S2O8 solution decreases the adsorption capacity of NO2 for this carbon.

The high adsorption capacities are reached by the samples CCO2 and GAC-O-T

correspond to 129 and 136 mg/g respectively. It is worth noting that the adsorption

capacities in this study are higher than activated carbons capacities found in the

literature. Thus, Kante et al. [26] studied adsorption of NO2 using wood based

activated carbon treated with chemicals such as sodium, cerium and lanthanum

chlorides. The adsorption capacity of the initial activated carbon was the highest with

39 mg/g. Deliyanni et al gave a breakthrough capacity of 49 mg/g after oxidation and

amination of wood based activated carbon [27]. In another study carried out by

Nowicki et al. [28], NO2 removal was examined using activated carbon prepared from

walnut shell. The produced activated carbon had a high surface area of 2305 m2/g and

pore volume of 1.15 cm3/g. Using walnut shell activated carbon in dry condition, a

NO2 adsorption capacity of 66 mgNO2/g was achieved. However, it is important to

note that these calculated capacities available in literature correspond only to the

breakthrough capacities and not to the total NO2 adsorption capacities as presented in

the present investigation.

The analysis of the adsorbed oxygen resultant from NO2 reduction (Fig. 9) shows

similar trend than the NO2 adsorption capacities. In fact, the GAC-O-T sample having

the highest NO2 adsorption capacity exhibits also the highest amount of the adsorbed

oxygen during the first 8000 seconds (where more than 80% of the total NO2 is

absorbed). In contrast, GAC-O sample having the lowest NO2 adsorption capacity

exhibits the lowest amount of the adsorbed oxygen. Such behavior confirms that the

16

amount of the adsorbed NO2 is correlated to the amount of adsorbed oxygen resulting

from the NO2 reduction on activated carbon surface.

3.2.2 Effect of porosity and chemical surface groups

In order to understand further the adsorption of NO2 and its reduction to NO on the

different activated carbons, their textural and chemical properties were analyzed as a

function of the adsorption and reduction capacities.

Figure 10 shows the adsorption capacities of the activated carbons as a function of

their specific surface area. It is noted that no significant relationship exists between

the NO2 adsorption and the specific surface area. In fact, the sample GAC-O-T (SBET

= 912 m2/g) has the highest NO2 adsorption capacities although its lower specific

surface area comparing to the other samples.

Furthermore, Figure 11 represents NO2 adsorption capacities (mg/g) as a function of

the microporous and the narrow microporous volumes obtained from nitrogen (-

196°C) and CO2 (0°C) adsorption isotherms, respectively. It is seen that no direct

correlation can be observed between the adsorption capacity of the samples and their

microporous volumes (VN2 and VCO2). However, the microporous volume VN2 (size

less than 2 nm) seems to affect the adsorption capacity rather than VCO2, volume of

narrow micropores (size less than 0.7 nm). Hence, the higher NO2 adsorption capacity

obtained for GAC-O-T may be attributed to the homogenous microporosity.

From this analysis, one may conclude that the microporosity and micropore size in

activated carbon play an important role in NO2 adsorption capacity. However, these

textural characteristics are not the only parameters affecting the interaction of NO2

with activated carbons. In fact, GAC sample has a lower adsorption capacity

comparing to GAC-O-T although its higher microporous volume. Therefore, chemical

17

surface groups may affect the NO2 adsorption capacity. The comparison of the

adsorption capacities with the chemical surface groups (Table 4) shows that the

highest adsorption capacities are obtained for the activated carbons emitting the

highest amount of CO during TPD-MS, namely, GAC-O-T and CCO2. Such behavior

may be attributed to the presence of stable groups such as ethers, carbonyls and

quinones as observed in TPD-MS study. It may be also attributed to the basic

character of GAC-O-T and CCO2 through the presence of pyrone and chromene

groups. In fact, extensive studies have confirmed that the heat treatment of activated

carbons at high temperatures rises the basicity of activated carbon [29, 30]. This

behavior is attributed to the elimination of the strong acidic complexes (carboxylic,

anhydrides and lactones) at lower temperatures and the others acidic functionalities

(carbonyl and phenol) at higher temperatures. The basic character is also due to

delocalized π electrons of graphene layers which proceed as Lewis bases [31].

The previous hypothesis is not available for GAC-O sample. In fact, although a higher

amount of stable groups (CO = 3.02 mmol/g) was observed during TPD-MS study,

this sample has the lowest NO2 adsorption capacity. Such behavior may be attributed

to the presence of higher amount of strong acidic complexes such as carboxylic,

anhydrides and lactones. In fact, the introduction of complexes on the surface of

carbon material by (NH4)2S2O8 oxidation increases the acidic groups. Therefore, these

acidic groups may inhibit the adsorption of NO2 on the activated carbon. Such

hypothesis is confirmed by the discrepancy between the NO2 adsorption capacity of

GAC-O-T and CCO2. In fact, these activated carbons have a similar amount of stable

groups (Table 4) but CCO2 has a higher amount of acidic groups. The latter may

explain the lower NO2 adsorption capacity of CCO2 comparing to GAC-O-T.

18

Moreover, the inhibitory effect of the acidic groups may be confirmed by the

comparison of the amount of CO2 emitted during TPD-MS analysis and the amount of

the adsorbed oxygen resulting from NO2 reduction. Figure 12 shows that the presence

of acidic groups leads to a decrease of the amount of adsorbed oxygen. Hence, these

acidic groups inhibit the reduction of NO2 to NO on the activated carbon surface. This

step is necessary for the adsorption of NO2 as shown previously. Therefore, the

presence of acidic groups does not restrain directly the NO2 adsorption but restrain

essentially the reduction of NO2 to NO and therefore the adsorption of oxygen. Such a

result was not evidenced previously in the literature.

Conclusions

Adsorption of NO2 at ambient temperature was studied using activated carbons

prepared by physical and chemical activation of date pits and modified commercial

activated carbons. The textural and chemical surface properties of the different

samples were analyzed and correlated to the NO2 interaction with the activated carbon

surface. The obtained results show that activated carbon prepared from date pits are

efficient adsorbents with adsorption capacities similar to the commercialized

adsorbents. Moreover, it is seen that the microporosity, the micropore size and the

presence of stable oxygen groups are the most important properties defining the

adsorption capacity of NO2. In contrast, the presence of strong acidic groups such as

carboxylic, anhydrides and lactones inhibits the reduction of NO2 into NO, which

represents a crucial step for the adsorption of NO2. In fact, it is observed that the

oxygen left on the carbon surface during NO2 decomposition plays an important role

on the NO2 adsorption.

19

Hence, the increase of basic group amounts and the elimination of strong acidic

groups may be a promising issue for improving further the performance of the

prepared activated carbons from date pits for NO2 removal.

Acknowledgments

The authors are grateful to Mrs Damaris Kehrli (Labroratoire Gestion des Risques et

Environnement), M. Joseph Dentzer and M. Yassine Elmay (Institut de Science des

Matériaux de Mulhouse) for carrying out some experiments.

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23

FIGURE CAPTIONS

Fig.1. Scheme of the set-up for the NO2 adsorption experiments

Fig.2. SEM photographs of the different activated carbons with the same enlargement:

a) CCO2 b) CZn c) GAC d) GAC-O e) GAC O-T

Fig.3. N2 adsorption-desorption at -196°C of activated carbons

Fig.4. Pore size distribution of activated carbons

Fig.5. CO and CO2 profiles during TPD-MS of the different activated carbons

Fig.6. FTIR spectra of the different activated carbons

Fig.7. (a) Outlet concentrations of NO2 and NO during the adsorption step of 500

ppmv NO2 on CCO2 at 25°C (b) Outlet concentrations of NO2, NO, CO2 and CO

during TPD step

Fig.8. NO2 adsorption capacities at 25°C for the different activated carbons

Fig.9. The amount of adsorbed oxygen resulting from NO2 reduction on the different

activated carbons

Fig.10. NO2 adsorption capacities as a function of surface area of activated carbons

Fig.11. Dependence of adsorbed amount of NO2 on micropore volume of activated

carbons

Fig.12. Dependence of the adsorbed amount of oxygen on the acidic surface groups

24

NO2

Thermocouple

Condenser

N2

BYPASS

Mass flow meters 1 2 3

4 5 6

7 8 9

ES

C

ENTE R

1000 ppm NO

1000 ppm NO2

NO/NO2 CO/CO2

analyser

furnace

Data acquisition system

Activated Carbon

Porous quartz layer

Fig.1. Scheme of the set-up for the NO2 adsorption experiments

25

a) b)

c) d)

26

e)

Fig.2. SEM photographs of the different activated carbons with the same enlargement: a) CZn b) CCO2 c) GAC d) GAC-O e) GAC O-T

27

0,0 0,2 0,4 0,6 0,8 1,0

0

100

200

300

400

500

CCO2

CZn

GAC GAC-O

GAC-O-T

V(c

m3/g

)

P/P0

Fig.3. N2 adsorption-desorption at -196°C of activated carbons. Filled and hollow symbols

indicate adsorption and desorption data, respectively.

28

0 20 40 60 80 100 120 140 160 180

0,00

0,05

0,10

0,15

0,20

0,25

0,30

0,35

CCO2

CZn

GAC

GAC-O

GAC-O-T

Po

re v

olu

me

(cm

3/g

)

Pore diameter (A)

Fig.4. Pore size distribution of activated carbons

29

0,0E+00

2,0E-07

4,0E-07

6,0E-07

8,0E-07

0 200 400 600 800

CO

deso

rpti

on

ra

te (

mo

l/s/

g)

Temperature (°C)

C-CO2

C-Zn

GAC

GAC-O

GAC-O-T

0,0E+00

1,0E-07

2,0E-07

3,0E-07

4,0E-07

5,0E-07

0 200 400 600 800

CO

2d

eso

rp

tion

ra

te (

mo

l/s/

g)

Temperature (°C)

C-CO2

C-Zn

GAC

GAC-O

GAC-O-T

Fig.5. CO and CO2 profiles during TPD-MS of the different activated carbons

30

100015002000250030003500

W avenumber cm-1

0.0

50

.10

0.1

50

.20

Ab

so

rba

nce

Un

its

Fig. 6. FTIR spectra of the different activated carbons.

GAC

GAC-O-T

GAC-O-T

CZn CCO2

31

(a)

0

200

400

600

800

1000

1200

0 200 400 600 800

Temperature (°C)

NO

2,

NO

, C

O,

CO

2 (

pp

mv

) NO NO2

CO CO2

(b)

Fig. 7. (a) Outlet concentrations of NO2 and NO during the adsorption step of 500

ppmv NO2 on CCO2 at 25°C (b) Outlet concentrations of NO2,NO, CO2 and CO during

TPD step

0

100

200

300

400

500

0 4000 8000 12000 16000

Time (s)

NO

2,

NO

, N

O2

ad

s (p

pm

v)

NO(ppm)

NO2(ppm)

NO2 ads (ppm)

32

0

30

60

90

120

150

0 4000 8000 12000 16000 20000 24000

Time (s)

NO

2 a

ds

(mg

/g)

C_CO2 C_Zn GAC

GAC-O GAC-O-T

Fig. 8. NO2 adsorption capacities at 25°C for the different activated carbons

33

0

10

20

30

40

50

60

0 4000 8000 12000 16000 20000 24000

Time (s)

Oa

ds

(mg

/g)

C_CO2 C_Zn GAC

GAC-O GAC-O-T

Fig. 9. The amount of adsorbed oxygen resulting from NO2 reduction on the different

activated carbons

34

800 900 1000 1100 1200 1300 1400

60

80

100

120

140

160

NO

2 a

dso

rbe

d (

mg

/g)

SBET

(m2/g)

Fig.10. NO2 adsorption capacities as a function of surface area of activated carbons

35

CCO2 CZnCl2 GAC GAC-O GAC-O-T

0

20

40

60

80

100

120

140

160

Mic

ropo

re v

olu

me

(cm

3/g

)

NO

2 a

dso

rbe

d (

mg/g

)

VN2

VCO2

1,0

0,5

0,75

0,25

0,0

Fig. 11. Dependence of the adsorbed amount of NO2 on micropore volume of activated carbons

36

0

0,4

0,8

1,2

0

20

40

60

GAC-O CCO2 GAC-O-T CZn GAC

CO

2(m

mol/

g)

Oa

ds

(mg

/g)

Fig. 12. Dependence of the adsorbed amount of oxygen on the acidic surface groups

37

TABLES CAPTIONS Table 1. Main inorganic compounds present in the different activated carbons determined by

X-ray fluorescence (% wt)

Table 2. Specific surface area, micropore volume and mesopore volume of the different

activated carbons.

Table 3. FTIR assignments of functional groups on activated carbon surface

Table 4. Amounts of CO and CO2 obtained by integration of the TPD-MS peaks for the

different activated carbons

Table 5. Amounts of the adsorbed NO2 and oxygen

38

Table 1. Main inorganic compounds present in the different activated carbons determined by

X-ray fluorescence (% wt)

Elements CZn CCO2 GAC GAC-O GAC-O-T Si 0.699 0.031 0.740 0.711 0.801 Al 0.597 0.018 0.619 0.611 0.636 S 0.395 0.011 0.398 0.467 0.310

Fe 0.134 0.036 0.166 0.150 0.148 K 0.049 1.290 Ca 0.045 0.111 Mg 0.045 0.491 Zn 0.024 0.060

39

Table 2. Specific surface area, micropore volume and mesopore volume of activated carbons

Activated

Carbon

SBET

(m2·g-1)

VN2

(cm3· g-1)

Vmeso

(cm3·g-1)

VT

(cm3·g-1)

VCO2

(cm3·g-1)

Vw

(cm3·g-1)

CCO2 1359 0.51 0.26 0.77 0.27 0.24

CZn 1172 0.48 0.16 0.64 0.35 0.13

GAC 1038 0.43 0.10 0.53 0.39 0.04

GAC-O 849 0.34 0.10 0.44 0.37 0.00

GAC-O-T 912 0.38 0.10 0.48 0.39 0.00

40

Table 3: FTIR assignments of functional groups on activated carbon surface

Wavenumber (cm-1)

Band intensities Assignement CCO2 GAC-O GAC-O-

T GAC Czn

3230 m m m m m O-H strech 2891 w w w w - C-H strech 1710 w s w s w C=O 1567 w s w m w C=C of benzene 1400 - m - m - C–C stretch 1068 s s s s s -C-OH 852 m f m f m C-H 779 m m - m m C=C-H

deformation 686 w w - f w C-H deformation W : weak, m: medium, s : Strong

41

Table 4: Amount of CO and CO2 obtained by integration of the TPD-MS peaks for activated

carbon

Material CO (mmol/g) CO2 (mmol/g)

CCO2 1.05 0.51

CZn 0.53 0.21

GAC 0.36 0.12

GAC-O 3.02 1.26

GAC-O-T 1.01 0.23

42

Table 5: Amounts of the adsorbed NO2 and oxygen

Activated

Carbon

NO2 adsorbed

(mg/g)

O adsorbed

(mg/g)

CCO2 129 40

CZn 124 56

GAC 127 60

GAC-O 78 23

GAC-O-T 136 46

43

Research Highlights

- Activated carbons (AC) were prepared from date pits and commercial

adsorbents

- Textural, morphological and surface chemistry properties were analysed

- NO2 adsorption capacities were evaluated and correlated to AC

characteristics.

- microporosity, micropore size and surface groups affect NO2 adsorption

capacity.

- Strong acidic groups inhibit the NO2 interaction with the AC surface