Eh

HHa

b

c

ARRAA

KZHRZ

1

ftktrct1ta[trh

0h

Applied Surface Science 280 (2013) 360– 365

Contents lists available at SciVerse ScienceDirect

Applied Surface Science

jou rn al h omepa g e: www.elsev ier .com/ locate /apsusc

ffect of active zinc oxide dispersion on reduced graphite oxide forydrogen sulfide adsorption at mid-temperature

oon Sub Songa,c, Moon Gyu Parkb, Eric Croiseta,∗, Zhongwei Chena, Sung Chan Namc,o-Jung Ryuc, Kwang Bok Yib,∗∗

Department of Chemical Engineering, University of Waterloo, 200 University Avenue West, Waterloo, Ontario, N2L3G1, CanadaDepartment of Chemical Engineering Education, Chungnam National University, 99 Daehak-ro, Yuseong-gu, Daejeon, 305-764, Republic of KoreaGreenhouse Gas Department, Korea Institute of Energy Research, 152 Gajeong-ro, Yuseong-gu, Daejeon, 305-343, Republic of Korea

a r t i c l e i n f o

rticle history:eceived 12 March 2013eceived in revised form 23 April 2013ccepted 29 April 2013vailable online 9 May 2013

eywords:nO/rGO composite2S adsorptioneduced graphite oxide

a b s t r a c t

Composites of Zinc oxide (ZnO) with reduced graphite oxide (rGO) were synthesized and used as adsor-bents for hydrogen sulfide (H2S) at 300 ◦C. Various characterization methods (TGA, XRD, FT-IR, TEM andXPS) were performed in order to link their H2S adsorption performance to the properties of the adsor-bent’s surface. Microwave-assisted reduction process of graphite oxide (GO) provided mild reductionenvironment, allowing oxygen-containing functional groups to remain on the rGO surface. It was con-firmed that for the ZnO/rGO synthesize using the microwave-assisted reduction method, the ZnO particlesize and the degree of ZnO dispersion remained stable over time at 300 ◦C, which was not the case foronly the ZnO particles themselves. This stable highly dispersed feature allows for sustained high surfacearea over time. This was confirmed through breakthrough experiments for H2S adsorption where it was

inc oxide found that the ZnO/rGO composite showed almost four times higher ZnO utilization efficiency than ZnOitself. The effect of the H2 and CO2 on H2S adsorption was also investigated. The presence of hydrogenin the H2S stream had a positive effect on the removal of H2S since it allows a reducing environment forZn O and Zn S bonds, leading to more active sites (Zn2+) to sulfur molecules. On the other hand, thepresence of carbon dioxide (CO2) showed the opposite trend, likely due to the oxidation environmentand also due to possible competitive adsorption between H2S and CO2.

. Introduction

Hydrogen sulfide (H2S) is the most common sulfur componentound in natural gas. It is an undesirable component in most indus-rial applications and is deleterious to the environment. H2S isnown to be a major contributor to acid rain when oxidized inhe atmosphere to SO2. In industrial processes, sulfur impuritiesapidly deactivate or poison catalysts, which are widely used in thehemical or petrochemical industries [1]. Therefore, the removal ofoxic gases (SO2, H2S etc.) has become a critical issue. Since the9th century, various approaches to remove H2S, such as sorp-ion, catalysis or condensation, have been applied [2]. Among thosepproaches, different adsorbents, such as activated carbon, zeolites3,4], modified alumina [5] or metal oxides [6,7], have been inves-

igated for H2S removal process. Historically, the high temperatureange (500–800 ◦C) H2S sorbents were extensively developed forot-gas desulfurization (HGD) [2]. Zinc oxide (ZnO) is considered

∗ Corresponding author. Tel.: +1 519 888 4567; fax: +1 519 888 4567x36472.∗∗ Corresponding author. Tel.: +82 42 821 8583; fax: +82 42 821 8583.

E-mail addresses: ecroiset@uwaterloo.ca (E. Croiset), cosy32@cnu.ac.kr (K.B. Yi).

169-4332/$ – see front matter © 2013 Elsevier B.V. All rights reserved.ttp://dx.doi.org/10.1016/j.apsusc.2013.04.161

© 2013 Elsevier B.V. All rights reserved.

as a very effective sorbent for removal of H2S from hot gas steams,from a thermodynamic point of view, with the formation of zincsulfide (ZnS) [8] (see Eq. (1)). An important drawback when usingZnO for hot-gas H2S removal process is its thermal instability tovolatile metallic zinc [9]. However, for lower temperature appli-cations, thermal stability is no longer an issue and ZnO can beconverted to ZnS at ambient condition [10].

ZnO(s) + H2S(g) → ZnS(s) + H2O(g) (1)

Graphene (2 dimensional, mono-atomic thick sp2-carbon struc-ture) has recently received increasing attention as a material ofinterest due to its high electronic conductivity, large surface areaand high mechanical strength [11,12]. Because of those benefits,most of graphene-based material studies focused on the electro-chemistry field, such as battery [13,14] or super-capacitors [12,15].More recently, graphite oxide (GO) with metal oxide compositeshave been extensively studied as adsorbents [16–18]. Graphiteoxide-based or graphene-based materials are known to be useful

for water purification, toxic gas removal and ammonia adsorp-tion applications [19–21]. Graphite oxide which possesses oxygenfunctional groups attached on both sides of the surface receivedattention due to its ability to modify the physical properties and

face S

smGtosw

owacp(fa

2

2

oo(agw5whdtaCtwri3oa

2(

Afrcatsrr1ca7r

2

p

H.S. Song et al. / Applied Sur

urface chemistry in order to enhance the interactions with targetolecules [22]. The presence of oxygen groups on the surface ofO makes (or anchors) bonds with active metal oxides. Therefore,

hose oxygen functional groups are able to modify the availabilityf active sites on the surface of adsorbents depending on the disper-ion of those active metal oxides and their chemical heterogeneityith GO [17].

When considering metal oxides for gas-solid adsorption, onef the major issues is to avoid aggregation of nano-sized particlesith temperature. Indeed, particle aggregation leads to losses in

ctive surface area, with consequences of lower utilization effi-iency of the adsorbent. Based on the above, the objective of theresent study is to evaluate zinc oxide with reduced graphite oxiderGO) composites as adsorbents of H2S. The morphology and sur-ace chemistry changes from those composites are expected toffect the H2S removal efficiency.

. Experimental

.1. Synthesis of graphite oxide

A modified graphite oxide (GO) synthesis procedure basedn our previous work [23], was used in this study. Graphitexide was synthesized using a mixture of 360 mL of sulfuric acidSigma-Aldrich, ACS reagent, 95.0–98.0%) and 40 mL of phosphoriccid (Sigma-Aldrich, ACS reagent, ≥85 wt% in H2O), and 3.0 g ofraphite powder (Sigma-Aldrich, <45 �m, ≥99.99%). This mixtureas placed in an ice bath and when the temperature reached below◦C, 18.0 g of KMnO4 (Samchun Chemical, 99.3%) was added drop-ise. The mixture was stirred for 1 h and then transferred to aeating mantle to provide isothermal conditions at 50 ◦C. The oxi-ation process was conducted for 18 h. The system was then cooledo room temperature naturally, and then placed in an ice bathgain. 400 mL of de-ionized water and 15 mL of 30% H2O2 (OCIompany Ltd, 30 wt% in H2O) were added gradually. The mixtureurned bright yellow and generated copious bubbles. The mixtureas stirred for 1 h and then centrifuged at 3500 rpm for 3 min. The

emaining solid paste was washed with a mixture of 100 mL of de-onized water and 100 mL of 30% HCl (Sigma-Aldrich, ACS reagent,7%) twice. The product was then rinsed twice again with 200 mLf de-ionized water. After the washing steps, the paste was freeze-nd vacuum-dried overnight.

.2. Synthesis of nano-zinc oxide (ZnO)/reduced graphite oxiderGO) composite

400 mg of GO was dissolved in 200 mL of ethylene glycol (Sigma-ldrich, ReagentPlus®, ≥99%) and then underwent ultra-sonication

or 30 min. 100 mL of 0.1 M aqueous NaOH (Sigma-Aldrich, ACSeagent, ≥97.0%) solution was added, and the mixture was soni-ated for an additional 30 min. Then, 100 mL of 0.07 M aqueous zinccetate (Sigma-Aldrich, ACS reagent, ≥98%) solution was added intohe mixture drop-wise (2.0 mL/min) for 50 min. 300 �L of hydrazineolution (Sigma-Aldrich, 35 wt% in H2O) was added before theeduction process. The zinc acetate/GO (ZnAc/GO) mixture waseduced by microwave irradiation for 3 min (1 min irradiation with

min break, 3 times). This reduction process produces ZnO/rGOomposite. After cooling down, the ZnO/rGO mixture was filterednd washed with DI-water three times until the pH reached around.0. Finally, the paste was freeze- and vacuum-dried overnight. Foreference, ZnO itself powder was also prepared.

.3. H2S adsorption breakthrough tests

Dynamic breakthrough tests were conducted at moderate tem-erature (300 ◦C). 0.5 cm3 of the adsorbents diluted with 1.0 cm3 of

cience 280 (2013) 360– 365 361

Al2O3 (Sigma-Aldrich, ∼ 150 mesh) for a total of 1.5 cm3 of bedwere packed into a quartz tube (internal diameter 10 mm). Themass of the adsorbents were measured around 0.158 g for ZnO/rGOand 0.355 g for ZnO. In a typical test, a flow of H2S (10.7 mL/min,3.01 vol% of H2S balanced with N2) was mixed with 186.3 mL/minof N2 gas before passing through the adsorbent bed. The initial H2Sconcentration was 1638 ppm with a total flow rate of 197.0 mL/min.The product stream was further diluted with 1800.0 mL of N2 beforeinjection to the H2S analyzer due to the limitation of the H2Sanalyzer (Fluorescence H2S Analyzer, Model 101E, Teledyne). Theexperiments were carried out until the output H2S concentrationreached ∼ 100 ppm. The effects of CO2 and H2 on the H2S adsorptioncapacity of the sorbents were examined. 2.8 vol% of CO2 or H2 gases(5.0 mL/min) were mixed with the H2S/N2 stream (total flow rateof 197.0 mL/min) and passed through the adsorbent bed. In order toquantify the reactivity of ZnO with H2S, sorbent utilizations werecalculated as follows:

Sorbent utilization(%) = T/Tt ∗ 100% (2)

where T is the experimental breakthrough time (min/g of ZnO), andTt is the theoretical breakthrough time (min/g of ZnO), which canbe obtained from Eq. (3).

Tt

(min

gZnO

)= 1 mol H2S

1 mol ZnO× 1 mol ZnO

81.4 g ZnO× 22400 ml H2S

1 mol H2S

× 1VH2S(ml/ min)

(3)

where VH2S is 0.32 mL/min here. From Eq. (3), the theoretical break-through time (T) is calculated as 890 min/g ZnO. The experimentalbreakthrough time is determined when the outlet H2S concentra-tion reaches 1 ppm.

2.4. Characterizations

X-ray diffraction (XRD, Rigaku, 40 kV/100 mA of X-Ray, step size:0.02◦) was used for adsorbent characterization; 5–60◦ of 2� rangewas measured. Scanning electron microscopy (FE-SEM) imageswere obtained using a Hitachi S-4700 with an accelerating voltageof 5.0 kV. Thermo-gravity analysis (TGA, TA Instruments, TGA-2050) was used with an air flow of 50 mL/min. Fourier-transforminfrared (FT-IR) spectroscopy was carried out using a Nicolet 6700(Thermo Scientific) with KBr dilution at 1:300 weight ratio. X-rayphotoelectron spectroscopy (XPS, AXIS Ultra DLD, KRATOS Inc.)was used with Mono chromatic A1 Ka (1486.6 eV) for X-ray source,0.05 eV/step; and no surface treatment were used.

3. Results and discussions

3.1. Characterizations of adsorbents

The crystal structure of the adsorbents was examined by XRDanalysis (Fig. 1). The 2� characteristic peak of graphite oxide (GO)is shown at 9.7◦, corresponding to the (001) reflection. It is cor-related to an interlayer spacing of 9.2 A. Since it is well knownthat the typical interlayer spacing of graphite is 3.4 A, it is con-firmed that the GO possess a larger interlayer spacing than graphitedue to various hydrophilic oxygenated functional groups such ashydroxyl, carboxyl and epoxy. Among those functional groups,carboxyl groups are known to be ionized and create electrostaticrepulsion between neighboring GO sheets [24]. The XRD peaks forZinc oxide (ZnO) are shown at 31.6, 34.3, 36.1, 47.4 and 56.4◦ cor-

responding to the hexagonal phase of ZnO (JPCDS 36-1451); andthose peaks are matched with other references [25,26]. After theZnO/rGO composite was prepared by microwave-assisted method,it is confirmed that the crystal structure of ZnO was not affected.

362 H.S. Song et al. / Applied Surface Science 280 (2013) 360– 365

HtfZptgG

pfaotTlcraauirc

u

4000 3500 3000 2500 2000 1500 1000 500

44

0

10

41

12

12

13

92

15

54

17

16

ZnO/r GO

Tra

nsm

itta

nce (

a.u

.)

-1

ZnO

33

59

289.4 eV for epoxy (C O) and carboxyl (C( O)OH), respectively,have been detected; and those data were matched with other refer-ences [24,29]. The amount of oxygen functional groups attached on

Fig. 1. XRD analysis of (A) GO, (B) ZnO and (C) ZnO/rGO composite.

owever, the GO peak at 9.7◦ disappeared which implies thathe microwave reduction process removed the oxygen-containingunctional groups attached on the GO surface. There are also strongnO peaks. Therefore, the main diffraction peaks for ZnO/rGO com-osite is similar to that of pure hexagonal ZnO. It is confirmedhat the present microwave-assisted reduction process not onlyenerates ZnO nano-particle but also simultaneously reduces theO.

In order to determine the ZnO-H2S adsorption activity, pure ZnOarticles and ZnO/rGO composite have been used in this study. Forurther investigation of the H2S adsorption capacity per gram ofdsorbent, the amount of ZnO which was deposited on the surfacef rGO should be quantified. Therefore, from TGA analysis (Fig. 2),he actual ZnO loading on the ZnO/rGO composite was measured.hree major weight losses were found. Around 100 ◦C, the weightoss is from the removal of surface moisture. Around 200 ◦C, itan be assumed that the oxygen-containing functional groups onGO were removed, yielding CO, CO2 and steam [27]. However, atround 300 ◦C, carbon elements in rGO were combusted with air;nd no further decrease in weigh of the composite was observedp to 600 ◦C. It was assumed that the final weight of the compos-

te corresponds to the weight of ZnO that was deposited on theGO surface. The weight percentage of ZnO particles in ZnO/rGO

omposite were obtained (∼63.7 wt%).

The functionality of the microwave-assisted method was eval-ated with FT-IR spectra (Fig. 3). From FT-IR spectra of ZnO and

0 50 100 150 200 250 300 350 400 450 500 550 600

0

10

20

30

40

50

60

70

80

90

100

99.7 oC

198.4 oC

300.2 oC

Temperatu re (oC)

wt%

63.7 wt% ZnO

-0.4

-0.3

-0.2

-0.1

0.0

Dif

fere

nti

al (

wt%

/oC

)

Fig. 2. TGA analysis of ZnO/rGO.

Wavenumb er (cm )

Fig. 3. FTIR spectra of ZnO and ZnO/rGO composite.

ZnO/rGO composite, a strong and broad absorption peak is locatedat 3359 cm−1; and it was assigned to the stretching vibrations ofO H. It implies that O H bond from ZnO is related to the functionalgroups attached on the surface of ZnO. In more details, absorbancepeaks at 1041 cm−1 for alkoxy, 1212 cm−1 for epoxy, 1392 cm−1

for carboxy and 1716 cm−1 for C O were observed. The absorp-tion due to the C C aromatic bond (the skeletal vibration of thegraphene sheet [28]) is located at 1554 cm−1. This confirms thatthe microwave reduction process generates a mildly reduced rGOwhich still contains oxygen functional groups. However, after thereduction process, the amount of the oxygen functional groupswas decreased; therefore, the FT-IR peak intensity for the C O(1716 cm−1) was weak. The very weak C O peak in the FT-IR spec-tra could be originated from COOH groups situated at the edge ofrGO sheets [25]. From C1s XPS analysis (Fig. 4), it is confirmed thatthe ZnO/rGO composite possesses a considerable degree of oxygen-containing functional groups, such as epoxy, carbonyl and carboxylgroups. More specifically, the binding energy of 286.5, 287.8 and

rGO surface is able to be quantified by comparing carbon/oxygen

300 298 296 294 292 290 288 286 284 282 280

Bind ing energy (eV)

C-C

C-O

C-OOH

Fig. 4. C1s XPS analysis for ZnO/rGO composite.

H.S. Song et al. / Applied Surface Science 280 (2013) 360– 365 363

nalys

rcZbhet

d(btrahadlo

otoTagodrZ

3

gbt3atopamIp

Fig. 5. TEM (A) and EDS-Zn a

atios. Compared to our previous results [23], the area ratio ofarbon-carbon to carbon-oxygen containing functional groups fornO/rGO composite (0.39) is lower than for graphite oxide (2.69),ut higher than for graphene (0.15). The larger ratio impliesigher oxygen contents on the surface of rGO. Therefore, it can bexplained that ZnO/rGO composite by microwave-assisted reduc-ion is more mildly reduced than from other methods.

The direct evidence of the formation of nano-sized ZnOeposited on the surface of rGO is provided by TEM/EDS analysisFig. 5). It is observed that nano-size ZnO particles (represented inlue dots) were deposited on the surface of rGO. Those ZnO par-icles are well dispersed and separated from each other on theGO surface which displays a good combination between rGO sheetnd ZnO nanoparticles. Additionally, the shape of ZnO particles isexagonal wurtzite structure which is expected from XRD peaksnalysis[30]. It is observed that some of ZnO nanoparticles areeposited on the inside interlayer of rGO sheet to form sandwich-

ike ZnO/rGO structure since Zn2+ could diffuse into the interlayerf GO during the reduction process [31].

In summary, it was confirmed that a mildly reduced graphitexide (rGO) could be obtained from a microwave-assisted reduc-ion process. This mildly reduced rGO allows for some amount ofxygen-containing functional groups to remain on the rGO surface.he presence of those functional groups was confirmed from FTIRnd XPS analysis. It was already proposed that oxygen functionalroups are anchoring metal ions on the surface and help dispersionf metal ions [32]. From TEM, it can be visually seen that uniformlyistributed nano-size ZnO particles are deposited on the surface ofGO. From an application point of view, those uniformly distributednO particles should be able to promote H2S adsorption capacity.

.2. H2S adsorption breakthrough tests

From our preliminary experiments, it was found that reducedraphite oxide (rGO) itself did not adsorb any H2S. It implies thatreakthrough experiments should be able to determine the func-ionality of active ZnO for H2S adsorption. For pure ZnO particles, at00 ◦C, aggregations of the nano-particles were observed. However,s previously proposed in this study, from ZnO/rGO composite,he critical functionality of rGO anchoring metal oxide has beenbserved (Fig. 6). The average ZnO particle size for pure ZnO sam-le and ZnO/rGO composite for fresh samples was about 110 nm

nd 95 nm, respectively. After 300 ◦C calcination in N2 environ-ent for 2 h, the morphology of the samples changed dramatically.

t was clearly observed that the average ZnO particle size of theure ZnO sample was increased to 201 nm. However, for ZnO/rGO

is (B) of ZnO/rGO composite.

composite, the average particle size change was almost negligi-ble (96 nm). Therefore, it could be confirmed that the rGO playsa critical role in the dispersion of the active ZnO particles for H2Sadsorption by avoiding aggregation of ZnO at 300 ◦C, thus providingconstant higher specific surface area of ZnO to the reactant H2S gas.Those phenomena have been confirmed by the H2S breakthroughadsorption tests (Fig. 7). One may notice that the ZnO particle sizerange (∼100 nm) from SEM analysis is different from that from TEManalysis (∼35 nm). Since SEM provides lower resolution than TEM,this shows the limitation of using only SEM to determine the rangeof particle size. Nonetheless, this does not affect the qualitative dis-cussion mentioned above. At 300 ◦C, the pure ZnO sample showedonly 15.0% of ZnO utilization efficiency (Table 1). This low utiliza-tion efficiency could be caused by the aggregation effect on ZnOparticles. However, with the help of rGO, the ZnO/rGO compositeshowed 60.3% of ZnO utilization efficiency. About 4 times higherZnO utilization efficiency has been observed. Those critical func-tionality of rGO as a substrate for gas adsorption had been proposedin previous studies [16–18].

H2S → H+ + HS− (4)

HS− → H+ + S2− (5)

ZnO + 2H+ → Zn2+ + H2O (6)

Zn2+ + S2− → ZnS (7)

ZnS + H+ → Zn2+ + HS− (8)

The effect of the presence of carbon dioxide (CO2) and hydro-gen (H2) has been investigated as well (Fig. 7 and Table 1). In thisstudy, 2.8 vol% of H2 with 3 vol% of H2S/N2 was passed through theadsorbent bed. Overall, for H2S adsorption onto ZnO, H2S should bedissociated into H+ and HS− followed by the diffusion of the sulfurinto the oxide lattice [33,34]. The additional supply of hydrogenat 300 ◦C might promotes the reducibility of Zn-O; simultaneouslydecomposing ZnS [35,36]. It should be able to provide more activeZn2+ for sulfur molecules (Eq. (4)–(8)). However, it was seen thatthe reaction temperature of 300 ◦C was relatively low for pureZnO particles from the H2S breakthrough experiments. Therefore,at 300 ◦C, the increase of H2S adsorption capacity on the pureZnO particles was almost negligible since the 17.4% of the ZnO

utilization efficiency (in H2S/N2) was just increased to 17.7% (inH2S/H2/N2 environment). However, from the ZnO/rGO composite,the effects of H2 were clearly shown. The H2S breakthrough timein H2S/N2 was 400 min; however, in H2S/H2/N2, it increased to

364 H.S. Song et al. / Applied Surface Science 280 (2013) 360– 365

Fig. 6. SEM images for fresh and spent samples at 300 ◦C in N2 for 2 h: (A) ZnO fresh, (B) ZnO spent, (C) ZnO/rGO fresh and (D) ZnO/rGO spent.

Table 1Comparison of H2S adsorption capacity and utilization.

ZnO ZnO/rGO

N2 N2/CO2 N2/H2 N2 N2/CO2 N2/H2

47attfutob

Fi

Breakthrough time (min/g ZnO) 155 134

Total sulfur adsorbed (mg of S/g of ZnO) 70.9 61.3

Utilization (%) 17.4 15.0

94 min. Accordingly, ZnO utilization efficiency was increased from0.6% in H2S/N2 to 87.3% in H2S/H2/N2. From a qualitative evalu-tion based on the data obtained in this work, it was found thathe presence of CO2 leads to lower H2S adsorption capacity of ZnOhan in its absence. The H2S breakthrough time has been decreasedrom 628 min (in N2) to 537 min (in CO2/N2). In addition, the ZnO

tilization efficiencies are accordingly decreased from 70.6% (in N2)o 60.3% (in CO2/N2). It implies that CO2 showed an inhibiting effectn H2S adsorption by ZnO. This could be explained by two possi-le reasons. First, as mentioned above, in order to adsorb H2S onto

0 100 200 300 400 500 600 700 800 9000

20

40

60

80

100

Ou

tlet

H2S

Co

ncen

trati

on

(p

pm

)

min / g of ZnO

ZnO (N2)

ZnO (CO2/N 2

)

ZnO (H2/N 2

)

ZnO/rGO (N2)

ZnO/rGO (CO2/N 2

)

ZnO/rGO (H2/N 2

)

ig. 7. Dynamic H2S breakthrough tests for ZnO and ZnO/rGO composite at 300 ◦Cn presence of different gases.

158 628 537 77772.2 305.7 245.5 355.217.7 70.6 60.3 87.3

ZnO, the decomposition products of H2S (i.e. HS− and S2−) shouldbe substituted for oxygen atoms in the lattice of ZnO [35]. However,supplying CO2 reduces the reducing effect of Zn-O which hindersproviding active Zn2+ states to the sulfur elements. Second, sinceboth H2S and CO2 are classified as acidic gases, those gases maystrongly interact with the sorbents which have a basic character[37]. Therefore, competitive adsorption between H2S and CO2 couldhappen [35]. The competitive adsorption of CO2 on ZnO limits thediffusion of HS− and S2− into the bulk of unreacted ZnO.

4. Conclusion

The critical role of the reduced graphite oxide (rGO) for activeZnO nano-particle dispersion has been investigated. XPS and FT-IRanalysis confirmed that the microwave-assisted reduction processprovided a mild reduction environment to GO; therefore, amountof oxygen functional groups remained attached on the rGO sur-face. Those oxygen functional groups were anchoring metal oxide,thus helping the dispersion of the active ZnO particles on the sur-face. From calcination experiments, it was shown that ZnO/rGOprevented the aggregation effect on ZnO at 300 ◦C which couldallow for higher specific surface area of the active ZnO to H2S gas.From H2S breakthrough tests, it was confirmed that the ZnO/rGOcomposite showed almost 4 times higher ZnO utilization efficiencythan the pure ZnO particle at 300 ◦C. In addition, it also showed

that the presence of H2 in H2S/N2 environment, the H2S break-through time had been increased since the hydrogen moleculesprovided the reducing environment to the product Zn-S. It leadedto the decomposition of the Zn-S and provided active Zn2+ for sulfur

face S

mab

A

EETb2

R

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

[

H.S. Song et al. / Applied Sur

olecules. On the other hand, the presence of CO2 inhibited the H2Sdsorption. This could be explained by the competitive adsorptionetween H2S and CO2.

cknowledgement

This research was supported by the National Sciences andngineering Research Council of Canada (NSERC); and the Energyfficiency & Resources Program of the Korea Institute of Energyechnology Evaluation and Planning (KETEP) under a grant fundedy the Korea government Ministry of Knowledge Economy (No.011201020004B).

eferences

[1] J.A. Rodriguez, A. Maiti, Adsorption and decomposition of H2S on MgO (100),NiMgO (100), and ZnO (0001) surfaces: A first-principles density functionalstudy, Journal of Physical Chemistry B 104 (2000) 3630–3638.

[2] A. Samokhvalov, B.J. Tatarchuk, Characterization of active sites, determina-tion of mechanisms of H2S, COS and CS2 sorption and regeneration of ZnOlow-temperature sorbents: past, current and perspectives, Physical ChemistryChemical Physics 13 (2011) 3197–3209.

[3] C.L. Garcia, J.A. Lercher, Adsorption of hydrogen sulfide on ZSM 5 zeolites,Journal of Physical Chemistry 96 (1992) 2230–2235.

[4] H.G. Karge, R. János, Hydrogen sulfide adsorption on faujasite-type zeoliteswith systematically varied Si-Al ratios, Journal of Colloid and Interface Science64 (1978) 522–532.

[5] J.P. Wakker, A.W. Gerritsen, J.A. Moulijn, High temperature H2S and COSremoval with MnO and FeO on �-Al203 acceptors, Industrial and EngineeringChemistry Research 32 (1993) 139–149.

[6] T.-H. Ko, H. Chu, L.-K. Chaung, The sorption of hydrogen sulfide from hot syngasby metal oxides over supports, Chemosphere 58 (2005) 467–474.

[7] P. Dhage, A. Samokhvalov, D. Repala, E.C. Duin, M. Bowman, B.J. Tatarchuk,Copper-promoted ZnO/SiO2 regenerable sorbents for the room temperatureremoval of H2S from reformate gas streams, Industrial and Engineering Chem-istry Research 49 (2010) 8388–8396.

[8] V.M. Twigg, M.S. Spencer, Deactivation of copper metal catalysts for methanoldecomposition, methanol steam reforming and methanol synthesis, Topics inCatalysis 22 (2003) 191–203.

[9] S. Lew, K. Jothimurugesan, M. Flytzani-Stephanopoulos, High-temperatureH2S removal from fuel gases by regenerable Zinc Oxide-Titanium diox-ide sorbents, Industrial and Engineering Chemistry Research 28 (1989)535–541.

10] X.Y. Kong, Y. Ding, Z.L. Wang, Metal-semiconductor Zn-ZnO core-shellnanobelts and nanotubes, Journal of Physical Chemistry B 108 (2004)570–574.

11] S. Park, J. An, I. Jung, R.D. Piner, S.J. An, X. Li, A. Velamakanni, R.S. Ruoff, Colloidalsuspensions of highly reduced graphene oxide in a wide variety of organicsolvents, Nano Letters 9 (2009) 1593–1597.

12] M.D. Stoller, S. Park, Y. Zhu, J. An, R.S. Ruoff, Graphene-based ultracapacitors,Nano Letters 8 (2008) 3498–3502.

13] N. Li, G. Liu, C. Zhen, F. Li, L. Zhang, H.-M. Cheng, Battery performance andphotocatalytic activity of mesoporous anatase TiO2 nanospheres/graphenecomposites by template-free self-assembly, Advanced Functional Materials 21(2011) 1717–1722.

14] J.K. Lee, K.B. Smith, C.M. Hayner, H.H. Kung, Silicon nanoparticles-graphenepaper composites for Li ion battery anodes, Chemical Communications 46(2010) 2025–2027.

15] Y. Zhu, S. Murali, M.D. Stoller, K.J. Ganesh, W. Cai, P.J. Ferreira, A. Pirkle, R.M.Wallace, K.A. Cychosz, M. Thommes, D. Su, E.A. Stach, R.S. Ruoff, Carbon-

based supercapacitors produced by activation of graphene, Science 332 (2011)1537–1541.

16] M. Seredych, T.J. Bandosz, Reactive adsorption of hydrogen sulfide ongraphite oxide/Zr(OH)4 composites, Chemical Engineering Journal 166 (2011)1032–1038.

[

cience 280 (2013) 360– 365 365

17] M. Seredych, O. Mabayoje, T.J. Bandosz, Visible-light-enhanced interactions ofhydrogen sulfide with composites of Zinc (Oxy)hydroxide with graphite oxideand graphene, Langmuir 28 (2012) 1337–1346.

18] M. Seredych, O. Mabayoje, M.M. Kolesnik, V. Krstic, T.J. Bandosz, Zinc(hydr)oxide/graphite based-phase composites: Effect of the carbonaceousphase on surface properties and enhancement in electrical conductivity, Jour-nal of Materials Chemistry 22 (2012) 7970–7978.

19] S.-T. Yang, Y. Chang, H. Wang, G. Liu, S. Chen, Y. Wang, Y. Liu, A. Cao, Fold-ing/aggregation of graphene oxide and its application in Cu2+ removal, Journalof Colloid and Interface Science 351 (2010) 122–127.

20] V. Chandra, J. Park, Y. Chun, J.W. Lee, I. Hwang, K.S. Kim, Water-dispersiblemagnetite-reduced graphene oxide composites for arsenic removal, ACS Nano4 (2010) 3979–3986.

21] C. Petit, M. Seredych, T.J. Bandosz, Revisiting the chemistry of graphite oxidesand its effect on ammonia adsorption, Journal of Materials Chemistry 19 (2009)9176–9185.

22] O. Mabayoje, M. Seredych, T.J. Bandosz, Enhanced reactive adsorption ofhydrogen sulfide on the composites of graphene/graphite oxide with copper(Hydr)oxychlorides, ACS Applied Materials Interfaces 4 (2012) 3316–3324.

23] H.S. Song, C.H. Ko, W. Ahn, B.J. Kim, E. Croiset, Z. Chen, S.C. Nam, Selectivedibenzothiophene adsorption on graphene prepared using different methods,Industrial and Engineering Chemistry Research 51 (2012) 10259–10264.

24] B. Adhikari, A. Biswas, A. Banerjee, Graphene oxide-based hydrogels tomake metal nanoparticle-containing reduced graphene oxide-based functionalhybrid hydrogels, ACS Applied Materials Interfaces 4 (2012) 5472–5482.

25] Q.-P. Luo, X.-Y. Yu, B.-X. Lei, H.-Y. Chen, D.-B. Kuang, C.-Y. Su, Reducedgraphene oxide-hierarchical ZnO hollow sphere composites with enhancedphotocurrent and photocatalytic activity, Journal of Physical Chemistry C 116(2012) 8111–8117.

26] T. Lv, L. Pan, X. Liu, T. Lu, G. Zhu, Z. Sun, Enhanced photocatalytic degrada-tion of methylene blue by ZnO-reduced graphene oxide composite synthesizedvia microwave-assisted reaction, Journal of Alloys and Compounds 509 (2011)10086–10091.

27] S. Stankovich, D.A. Dikin, R.D. Piner, K.A. Kohlhaas, A. Kleinhammes, Y. Jia, Y. Wu,S.T. Nguyen, R.S. Ruoff, Synthesis of graphene-based nanosheets via chemicalreduction of exfoliated graphite oxide, Carbon 45 (2007), 1588-1565.

28] W. Zou, J. Zhu, Y. Sun, X. Wang, Depositing ZnO nanoparticles onto graphenein a polyol system, Materials Chemistry and Physics 125 (2011) 617–620.

29] S. Wang, S.P. Jiang, X. Wang, Microwave-assisted one-pot synthesis ofmetal/metal oxide nanoparticles on graphene and their electrochemical appli-cations, Electrochimica Acta 56 (2011) 3338–3344.

30] X.-P. Shen, A.-H. Yuan, Y.-M. Hu, Y. Jiang, Z. Xu, Z. Hu, Fabrication, characteriza-tion and field emission properties of large-scale uniform ZnO nanotube arrays,Nanotechnology 16 (2005) 2039–2043.

31] T. Lu, L. Pan, H. Li, G. Zhu, T. Lv, X. Liu, Z. Sun, T. Chen, D.H.C. Chua,Microwave-assisted synthesis of graphene–ZnO nanocomposite for electro-chemical supercapacitors, Journal of Alloys and Compounds 509 (2011)5488–5492.

32] P. Kundu, C. Nethravathi, P.a. Deshpande, M. Rajamathi, G. Madras, N. Rav-ishankar, Ultrafast microwave-assisted route to surfactant-free ultrafine Ptnanoparticles on graphene: Synergistic co-reduction mechanism and high cat-alytic activity, Chemistry of Materials 23 (2011) 2772–2780.

33] T. Baird, P.J. Denny, R. Hoyle, F. Mcmonagle, D. Stirling, Modified Zinc Oxideabsorbents for low-temperature gas desulfurisation, Journal of the ChemicalSociety, Faraday Transactions 88 (1992) 3375–3382.

34] W. Bai, K. Yu, Q. Zhang, Y. Huang, Q. Wang, Z. Zhu, N. Dai, Y. Sun, Chemicalsolution-process method to synthesize ZnO nanorods with ultra-thin pinheadsand ultra-thin nanobelts, Applied Physics A 87 (2007) 755–759.

35] I.I. Novochinskii, C. Song, X. Ma, X. Liu, L. Shore, J. Lampert, R.J. Farrauto, Low-temperature H2S removal from steam-containing gas mixtures with ZnO forfuel cell application. 1. ZnO particles and extrudates, Energy and Fuels 18 (2004)576–583.

36] S. Lew, A.F. Sarofim, M. Flytzani-Stephanopoulos, Sulfidation of Zinc Titanateand Zinc Oxide solids, Industrial and Engineering Chemistry Research 31 (1992)

1890–1899.

37] H.a.J. van Dijk, S. Walspurger, P.D. Cobden, R.W. van den Brink, F.G. de Vos,Testing of hydrotalcite-based sorbents for CO2 and H2S capture for use in sorp-tion enhanced water gas shift, International Journal Greenhouse Gas Control 5(2011) 505–511.