Š synthetic activated carbons for the removal of hydrogen cyanide from air

8/8/2019 Synthetic activated carbons for the removal of hydrogen cyanide from air

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Chemical Engineering and Processing 44 (2005) 11811187

Synthetic activated carbons for the removal of hydrogen cyanide from air

Terzic M. Oliver a,, Krstic Jugoslav b, Popovic Aleksandar c, Dogovic Nikola a

a Institute for Medical and Technical Protection, Kataniceva 15, 11000 Beograd, Serbia and Montenegrob Institute of Chemistry Technology and Metallurgy, Department of Catalysis and Chemical Engineering,

Njegoseva 12, 11000 Beograd, Serbia and Montenegroc Faculty of Chemistry, Studentski trg 16, 11000 Beograd, Serbia and Montenegro

Received 15 November 2004; received in revised form 4 March 2005; accepted 11 March 2005

Available online 23 June 2005

Abstract

Copper containing and copper free synthetic activated carbons produced from porous sulfonated styrene/divinylbenzene resin were studied

for assessing the removal efficiency of HCN vapors from air. The pore structures and surface chemistry of these activated carbons were

analyzed through N2 physisorption at 77 K and X-ray photoelectron spectroscopy (XPS), respectively. Incorporation of copper into starting

material significantly increased HCN breakthrough times, but decreased benzene breakthrough times. The surface area and pore volume of

the adsorbents also decreased with incorporation of copper. Results of XPS analysis revealed partial or complete reduction of the starting

divalent copper on the surface of the adsorbents confirmed by the lack of formation of (CN)2 during the adsorption of HCN. The performance

of copper containing water vapor activated adsorbent was compared to the performance of ASC Whetlerite carbon.

2005 Published by Elsevier B.V.

Keywords: Activated carbon; Heat treatment; Porosity; Chromatography; Hydrogen cyanide

1. Introduction

Hydrogen cyanide (HCN) is an acutely poisonous com-

pound, which might enter the human body by breathing con-

taminated air. The HCN vapors are commonly released to the

air from various sources including vehicle exhaust emissions,

chemical processing, extraction of gold and silver from low-

grade ores, metal plating, steel, iron, and finishing industries,

petroleum refineries, and waste disposed of in landfills. The

best-known adsorbents for the removal of HCN from air are

the metal salts impregnated activated carbons. These materi-

als are usually obtained by wetting of the non-impregnatedcarbon with a solvent containing the transition metal salt,

followed by drying. This method is used to produce ASC

Whetlerite carbons which contain salts of copper, chromium

and silver, and have been shown to be effective adsorbents

for removal of HCN, cyanogen chloride and hydrogen sul-

phide. However, these materials possess some disadvantages,

Corresponding author. Tel.: +381 64 261 0995.

E-mail address: [email protected] (T.M. Oliver).

such as lack of control of carbonaceous starting materials,

friability of active carbon support, complexity of impregna-

tion techniques [13], poor dispersion of impregnates in the

deeper pore system [4], and presence of hazardous chromium

species [5]. In order to overcome some of these shortfalls,

Barnes et al. [6] used approach based on the incorporation

of the desired metal(s) into the sodium carboxymethylcellu-

lose by ion exchange prior to carbonization and activation.

Although obtained materials showed some benefits, fibrous

nature of the precursor was retained and material exhibited

poor mechanical properties.

In this work, copper containing and copper free syn-thetic activated carbons produced from porous sulfonated

styrene/divinylbenzene resin were studied for assessing the

removal efficiency of HCN vapors from air. It has been

shown that adsorbents of this kind have improved mechanical

characteristics of beads and can be prepared with tailorable

structural properties, including pore size distribution and sur-

face area [7,8]. Furthermore, the ion exchange properties of

the resin enable easy incorporation of significant quantities

of copper into the material as an active ingredient. Two pairs

0255-2701/$ see front matter 2005 Published by Elsevier B.V.

doi:10.1016/j.cep.2005.03.003


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1182 T.M. Oliver et al. / Chemical Engineering and Processing 44 (2005) 11811187

of adsorbents were synthesized by thermal decomposition of

starting resins being in H+ and Cu2+ form, respectively. Dur-

ing heat treatment, one sample from each pair of resins was

activated with water vapor. Nitrogen adsorption at 77 K has

been used to characterize the pore structure of the adsorbents.

The external surfaces of the adsorbents have been studied

using X-ray photoelectron spectroscopy (XPS). The perfor-mances of synthetic activated carbons were evaluated by the

breakthrough data of HCNand benzene vapors andcompared

with the performances of commercial non-impregnated BPL

carbon and ASC Whetlerite carbon produced from material

similar to BPL carbon by the common method. Along with

the breakthrough of HCN, eventual concurrent formation of

cyanogen (CN)2 was also monitored. Dynamic rather than

static measurements were chosen in view of the greater rele-

vance of the data to filtration systems.

2. Experimental

2.1. Synthesis

Macroreticular styrene/divinylbenzene sulfonic acid ion

exchange resin Amberlite 200 (registered trademark of

Rohm and Haas Company) in the Na+ form was washed

with distilled water in a column, converted to the H+ form

by passing through 2 mol/dm3 hydrochloric acid solution,

and rinsed with distilled water in order to achieve neutral

pH. This resin was used as the starting material for preparing

the pair of adsorbents denoted as carbonized amberlite

resin in H form (CAH). For obtaining the pair of copper

containing carbons, designated as carbonized amberliteresin in Cu form (CAC), the resin was treated as described

above, saturated with a 0.5 mol/dm3 copper sulfate solution,

and rinsed with distilled water until no sulfate ions were

detected in the washing solution (via a barium chloride

test).

Heat treatments were performed in a horizontal metal tube

(3.40cm diameter) with gas inlet, in a furnace modified for

temperature programming. For all samples, approximately

20 g of resin beads, dried at 105 C for 16 h, were placed

between quartz wool plugs in a tube. The adsorbents denoted

as CAH1 andCAC1 were prepared from resin samples apply-

ing the heat treatment procedure which included a 1 h heat

up to 200 C and 1 h hold at 200 C. Then, the temperature

was raised by 100 C increments during 0.5 h and held at the

new temperature for 0.5 h. The temperature rise cycle was

repeated until the final heat treatment temperature of 800 C

was reached. After holding 1 h at this temperature, the sam-

ples were allowed to cool down to room temperature over

next 18 h. During the entire heating and cooling procedure

the argon at 0.4 dm3/min was passed through the tube.

The adsorbents denoted as CAH2 and CAC2 were pre-

pared in the same manner as previously described except that

after reaching 700 C the argon was replaced with the acti-

vation medium (mixture of argon and water vapor). These

samples were cooled in the argon atmosphere in the same

way as the first two samples. The total mass of water used

during activation process was 8.10 g, thus giving the flow of

5.4cm3/h at a superficial linear velocity of 0.595 cm/h. After

cooling, all samples were immediately placed in weighing

bottles and stored in a vacuum desiccator containing anhy-

drous CaSO4.

2.2. Methods

The copper content in CAC samples was estimated by

atomic absorption spectroscopy after total oxidation of the

pulverized samples and nitric acid dissolution.

Cu 2p, O 1s, C 1s and Cu Auger XPS experiments were

carried out on a Kratos 300 ESCA instrument supplied with

data acquisition facilities. The instrument was calibrated

using C 1s XP spectra, which were assumed to be 284.6 eV

[9]. Chemical shift data has been used to study the oxidation

states of incorporated copper.

The nitrogen adsorption/desorption isotherms were mea-sured at 77 K using a gassorptionapparatus Thermo Finnigan

Sorptomatic 1990. Carbon samples were outgassed 16 h at

300 C prior to the adsorption analysis. The various models

and appropriate software-WinADP version 4.0 CE Instru-

ments were used to analyze the obtained isotherms.

Breakthrough tests were carried out using an instrumen-

tal gas chromatography method for automatic determination

of the C6H6 or HCN/(CN)2 concentration in a flow sys-

tem, as described earlier [10]. Breakthrough tests were per-

formed with inlet vapor concentrations of 10.00 mg/dm3 for

C6H6 and 2.00 mg/dm3 for HCN. All measurements were

performed at (292 1) K. The carrier gas was air with rel-ative humidity RH = 45 5% and volumetric flow rate of

0.172 dm3/min. The ratio of concentrations at the outlet and

inlet of the adsorbent bed was monitored as function of time.

The activated carbon adsorbents used for comparison pur-

poses were a coal based, steam activated Chemviron BPL,

1230 mesh, and Chemviron Whetlerite, 1230 mesh. The

pore structures and surface chemistry of these activated car-

bons are described elsewhere [11,12].

3. Results and discussion

3.1. Synthesis

The yields (based on the weight of starting material) and

copper content (in mg/g of the adsorbent) for CAH and CAC

samples are presented in Table 1. It is obvious that in the case

of CAH2 and CAC2, a weight loss occurred indicating some

heterogeneous chemical reactions of oxidation of carbon with

water vapor at temperatures above 700 C [13]. The weight

yields for copper containing samples are higher than of their

corresponding copper free samples, as expected. In addition,

the copper content in CAC2 is higher than in CAC1 because

of higher loss of the organic part of the material.


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T.M. Oliver et al. / Chemical Engineering and Processing 44 (2005) 11811187 1183

Table 1

Weight yields and copper contents of the investigated carbons

Sample Weight yield (%) Copper content (mg Cu/g)

CAH1 36

CAH2 29

CAC1 46 256

CAC2 38 284

The particles of all synthesized activated carbons are

spheres of great physical integrity with slightly smaller

dimensions (mean diameter 0.18 mm) compared with the ion

exchange resin.

3.2. Surface chemistry

During heat treatment, the copper may be transformed to

CuO and/or be reduced to Cu1+ or Cu0. The decomposition

of the pure copper sulfate occurs at about 700 C [14],butitis

found that interaction with the carbon surface leads to prema-

ture low temperature breakdown of the compound according

to the following equation [15]:

CuSO4+C Cu + CO2+ SO2 (1)

The results of the stoichiometric calculations based on the

assumptions that the effect of the copper on the decomposi-

tion is insignificant, and that all copper in the samples is in

zero state, are consistent with the small differences (45%)

in the weight loss observed between analogous samples of

the CAC and CAH carbons.

The XPS data confirmed previous assumption by showing

that thecopper in both CAC samples is present predominantly

intheCu0 state. TheXPS scans of these samples, showing the

Cu 2p peak, and the Cu Auger spectrum, are almost identical

thus, only CAC1 scans are givenin Figs. 1 and 2, respectively.

Accordingly, we can assume that in the samples inves-

tigated divalent copper is reduced with carbon during the

heattreatmentof copper-containing resin. The introduction of

activation medium didnot produce notable changein thestate

Fig. 1. Cu 2p X-ray photoelectron spectrum of CAC1.

Fig. 2. Cu Auger X-ray photoelectron spectrum of CAC1.

of copper. Although reduced copper could undergo oxidation

with water vapor, it retrieves zero state due to prolonged con-

tact with carbon surface under argon gas.Nevertheless, activation did produce some changes in the

surface chemistry of the materials. The typical asymmetric

peaks of the C 1s core level XP spectrum obtained on the

CAH1 and CAH2 samples are given in the Figs. 3 and 4.,

respectively.

While XPS scan of CAH1 gives only the peak at 284.6 eV

assigned to graphitic carbon, the XPS scan shown in Fig. 4

evidences two additional peaks on the surface of the CAH2 at

286.0 and 287.3 eV for C O (phenolic, alcoholic or etheric)

and C O (carbonyl or quinone) functional groups, respec-

tively. Similar results are obtained with copper containing

samples. Obviously, the activation treatment performed with

a water vapor resulted in the chemisorption of oxygen on thesurface of the carbons [16].

3.3. Nitrogen adsorption

Fig. 5 shows the nitrogen adsorption/desorption isotherms

obtained with CAH and CAC samples. The predominant

Fig. 3. C 1s X-ray photoelectron spectrum of CAH1.


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Fig. 4. C 1s X-ray photoelectron spectrum of CAH2.

Fig. 5. Adsorption/desorption isotherms of nitrogen at 77 K for the CAH

and CAC carbons.

character of all these isotherms is type I [17] but with pos-

itive slope across much of the pressure range indicating the

presence of mesopores beside micropores in the materials.

Moreover, all isotherms exhibit a hysteresis loop usually

associated with the filling and emptying the mesopores by

capillary condensation. The shape of the loops is similar for

all samples.

The specific surface area (SBET), total pore volume (Vtot),

micropore volume (Vmic), and mesopore volume (Vmes) are

presented in Table 2. The specific surface area of sam-ples SBET, was calculated according to Brunauer, Emmett,

Teller method from the linear part of the nitrogen adsorp-

tion isotherms [18]. Vtot was given at p/p0 = 0.997. The pore

Table 2

Surface areas and pore volumes of investigated carbons

Sample SBET (m2/g) Vtot (cm

3/g) Vmic (cm3/g) Vmes (cm

3/g)

CAH1 446 0.472 0.220 0.251

CAH2 623 0.582 0.305 0.277

CAC1 320 0.322 0.156 0.192

CAC2 427 0.463 0.208 0.277

size distribution for mesopores was calculated according to

BJH method from desorption branch of isotherm. The val-

ues ofVmic are calculated using HorvathKawazoe equation

[17,19]. These results are in good correlation with values

obtained using the DubininRadushkevich (DR) equation

[20] that has been shown to successfully describe the adsorp-

tion of vapors by microporous carbons.It can be observed that in the synthesized samples specific

surface area, total pore volume, micropore volume and meso-

pore volume all increase significantly with activation. During

activation process, unsaturated carbon at the edge of the

graphite-like basal layers as well as the unorganized carbon

is burned out. The difference of burning speeds of different

parts of the layers cause an asymmetrical burning which

results in the development of microporous structure. The

widening of existing pores or/and burnout of walls between

adjacent micropores leads to an increase in the volume of

mesopores.

In the CAHsamples,water vapor activation increasesSBETfor 40%, while this increase in the CAC samples is 33%. Ontheother hand, theactivation increase of thetotal pore volume

is lower for CAH samples (23%) compared to the CAC sam-

ples (44%). In the CAH samples, increase in the volume is

obtained mainlyin themicropore region (39% compared with

10%for themesopore region),while in theCAC samples, sig-

nificant increase in volume (44%) is obtained in mesopore

region.

As can be seen from comparison of data for CAH1 and

CAC1, the incorporation of copper into the material caused

about 29% reduction ofSBET, Vmic, and Vmes, while Vtot was

reduced for 32%.

The HorvathKawazoe distribution of micropores in thecarbons is shown in Fig. 6. It is shown that in the process of

activation, micropore distribution of carbons has not changed

significantly, especially in the case of CAH samples. In the

both samples, about 70% of the cumulative micropore vol-

umeis attributed to theporeshavingpore half-widths between

0.2 and 0.3 nm. The median micropore half-widths for CAH1

and CAH2 are 0.252 and 0.253 nm, respectively. The incor-

poration of copper did not lead to significant change in the

median micropore half-width, only slightly decreased the

contribution of pores in the 0.20.3 nm region, and increased

in the 0.30.4 nm region. Therefore, it seems that copper only

blocks the entry of micropores and do not enter into micro-

pores, otherwise the micropore distribution should change

more significantly.

TheBJH poresizesplotsfor thesamples aregiven in Fig.7.

Only notably changein themesopore distribution obtained by

introduction of copper intothe material (CAC1 versus CAH1)

can be observed in the pores having half-widths between 1

and 5 nm. The volume contribution in this region decreased

markedly, probably because copper jams those mesopores.

Since the mesopore distribution in the whole did not undergo

drastic change it can be assumed that that the finely divided

elemental copper sits here uniformly dispersed in the matrix

of carbon material.


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Fig. 6. The HorvathKawazoe micropore distribution of the CAH and CAC carbons.

3.4. Removal of HCN

The experimentally determined breakthrough curves of

HCN vapors, correlated with asymmetrical S-shaped curves

according to the modified YoonNelson model [21], are

shown in Fig. 8.

The breakthrough time as well as the shape of the break-

through curve for HCN did not differ much among CAH

samples. The breakthrough curve was typically sigmoidal,

with a rapid initial increase in concentration followed by a

much slower asymptotic approach to the influent HCN level.

The breakthrough times were relatively short, as expected

Fig. 7. The BJH mesopore distribution of the CAH and CAC carbons.


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Fig. 8. The breakthrough curves of HCN vapors (292K, inlet concentration

2.00mg/dm3) for CAH1, CAH2, CAC1, CAC2, BPL and ASC Whetlerite.

from the assumption that mostly weak physical adsorption of

HCN is taking place on the surface. The similar assumption

can be made with non-impregnated BPL carbon which fol-

lowed the trend of CAH carbons and showed poor ability to

remove HCN.

Incorporation of copper into the resin beads generally led

to an increase in the HCN breakthrough times. This was par-

ticularly obvious in the case of CAC2 compared to CAH2.

Furthermore, the concentration of effluent increased grad-

ually with time and the HCN breakthrough profile became

more inclined, suggesting chemical activity in the CAC sam-

ples. It is known that the copper, added to the activatedcarbons, acts chemically with HCNtowardsformationof pre-

cipitate [22]. In the case of divalent copper, the precipitate is

copper (I) cyanide and the reaction is followed by the release

of (CN)2:

2CuO + 4HCN 2CuCN + (CN)2+H2O (2)

Carbons impregnated with mono- or zero-valent copper

species do not have a tendency to produce (CN)2 upon expo-

sure to HCN:

CuO+ HCN CuCN+ 12 H2 (3)

2Cu2O + 2HCN 2CuCN + H2O (4)

In the CAC pair of adsorbents, during HCN removal the

appearance of (CN)2 in the effluent stream was not observed.

This result is consistent with XPS data showing that the cop-

per is present on the surface of these adsorbents primarily in

the zero state.

The breakthrough time for HCN was much longer for

CAC2 than CAC1. According to the results listed in Table 2,

a heat treatment with water vapor caused significant increase

in Vmes as well as in the other parameters. Since the active

componentcopper appears to have an even distribution within

the mesopores of the material, this treatment also led to the

Fig. 9. The breakthrough curves of benzene vapors (292K, inlet concen-

tration 10.00 mg/dm3) for CAH1, CAH2, CAC1, CAC2, BPL and ASC

Whetlerite.

increase of the active surface for HCN adsorption. That is

why the CAC2 carbon is much better adsorbent than CAC1

carbon even though the difference in the total copper content

is not significant (Table 1).

The performance of ASC Whetlerite was only slightly

better than the performance of CAC2 carbon. Although ASC

Whetlerite carbon possesses better textural characteristics

[11] than CAC2 carbon, it seems that relatively large

copper loading (28%) of CAC2 carbon compared to ASC

Whetlerite carbon (7%) is the major factor contributing

to the obtained results. In the case of ASC Whetleritealong with breakthrough of HCN the evolution of (CN)2 is

detected.

3.5. Removal of benzene

The experimentally determined breakthrough curves of

benzene vapors, correlated with asymmetrical S-shaped

curves according to the modified YoonNelson model [21],

are shown in Fig. 9.

Benzene is physically adsorbed in the micropores of the

activated carbon and its breakthrough time depends very

much on the adsorbents surface area and micropore vol-

ume. The order of the breakthrough times of benzene vapors

on the carbons generally followed the order of increasing

micropore volumes, as would be expected. The decrease in

surface area and micropore volume caused by the introduc-

tion of copper in the starting material (Table 2) resulted in

lower breakthrough times of CAC samples compared to the

corresponding CAH samples (Fig. 9). Thus, the role of cop-

per seems to be completely different for HCN and benzene

adsorption.InHCNadsorptioncopperactsasanactivesitefor

the chemisorptions, while in benzene adsorption the presence

of copper diminishes the active adsorption area that provides

physisorption.


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4. Conclusion

The results of this work suggest that copper containing

synthetic activated carbons exhibit adsorptive and chemisorp-

tive properties. Identified method for the production of copper

containing carbons yielded hard, non-friable spheres with

mean diameter of 0.18 mm. Under the conditions used, thewater vapor activated sample produced a very encouraging

HCN performance which is comparable to the performance

of ASC Whetlerite carbons. The formation of poisonous

(CN)2 gas, common byproduct of removal by Cu(II) salts

impregnated activated carbons, is avoided without usingenvi-

ronmentally hazardous chromium in the composition of the

active phase of carbon. Copper is detected on the surface of

the adsorbent as predominantly Cu0 and appears to have an

even distribution within the mesopores of the material. The

ion exchange property of the starting resin enabled easy and

homogenous implantation of copper into the structure prior

to carbonization, while the activation was shown to affect the

accessibility of copper within the carbon particles.

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