Chiang Mai J. Sci. 2010; 37(1) : 99-105 www.science.cmu.ac.th/journal-science/josci.html Contributed Paper 1. I NTRODUCTION Activated carbon is generally known as the exceptional and adaptable adsorbents [1-3]. It is employed to remove the dissolved organic and inorganic substance from gas and liquid phase. The activated carbon has also been the traditional supports used in heterogeneous catalysis as its porous structure leading to large internal surface area per unit weight. High surface areas of the activated carbon can accommodate adsorbates. It is also excellent electrical conductivity, impurity free and resistance to corrosion. Activated carbon cloths (ACCs) potential- ly present a number of significant advantages with respect to conventional activated carbon in powder or granule for instance high rate of mass transfer from liquid phase, no need of decantation or filtration and high flexibility to fit into any reactor size [4]. The porous Self-Reduction of Gold on Activated Carbon Cloth Surin Saipanya * and Thapanee Sarakonsri Department of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand. *Author for correspondence; e-mail: [email protected]Received: 27 November 2008 Accepted: 13 March 2009 ABSTRACT Activated carbon cloths (ACCs) were used as a support for preparation of Au nanoparticles. Pretreatment of ACCs was performed by using oxidizing agent HNO 3 in order to introduce surface oxygen complex. Au nanoparticles were prepared by impregnation of tetrachloroauric acid (HAuCl 4 ) without reducing agents in deionized water and ethanol. It was found that the preparation in ethanol solution achieved uniformly dispersed Au nanoparticles with diameter 10-100 nm on the support surfaces. X-ray diffraction (XRD) was employed to verify structural characteristics of Au metal adsorption on oxidized ACC surfaces. Morphology and composition were carried out by Scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS), respectively. Keywords: Au nanoparticles, activated carbon cloth. networks with plentiful functional groups on the surface of carbon fiber are formed by a uniform distribution of micropore providing faster adsorption-desorption rate, faster equilibrium rate and high fluid permeability [5]. As ACCs are so reductive, it can reduce metal ions into metallic elements. An acid-base character is associated with oxygen functionalities [6, 7]; the acidic character is associated with oxygen functionali- ties such as carboxyls, lactones and phenol whilst pyrones, chromenes, ethers and carbonyls imply the basic properties. The acidic and basic character of carbon surface depends on the conditions of preparation. HNO 3 is one of well known oxidizing agent for ACC pretreatment. Our work has used HNO 3 to treat ACCs before adsorption. Metallic catalyst (e.g. Pt, Pd, Au) on various
Transcript
Chiang Mai J. Sci. 2010; 37(1) 99
Chiang Mai J. Sci. 2010; 37(1) : 99-105www.science.cmu.ac.th/journal-science/josci.htmlContributed Paper
1. INTRODUCTIONActivated carbon is generally known as
the exceptional and adaptable adsorbents[1-3]. It is employed to remove the dissolvedorganic and inorganic substance from gasand liquid phase. The activated carbon hasalso been the traditional supports used inheterogeneous catalysis as its porous structureleading to large internal surface area per unitweight. High surface areas of the activatedcarbon can accommodate adsorbates. It isalso excellent electrical conductivity, impurityfree and resistance to corrosion.
Activated carbon cloths (ACCs) potential-ly present a number of significant advantageswith respect to conventional activated carbonin powder or granule for instance high rateof mass transfer from liquid phase, no needof decantation or filtration and high flexibilityto fit into any reactor size [4]. The porous
Self-Reduction of Gold on Activated Carbon ClothSurin Saipanya* and Thapanee SarakonsriDepartment of Chemistry, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand.*Author for correspondence; e-mail: [email protected]
Received: 27 November 2008Accepted: 13 March 2009
ABSTRACTActivated carbon cloths (ACCs) were used as a support for preparation of Au
nanoparticles. Pretreatment of ACCs was performed by using oxidizing agent HNO3 in orderto introduce surface oxygen complex. Au nanoparticles were prepared by impregnation oftetrachloroauric acid (HAuCl4) without reducing agents in deionized water and ethanol. It wasfound that the preparation in ethanol solution achieved uniformly dispersed Au nanoparticleswith diameter 10-100 nm on the support surfaces. X-ray diffraction (XRD) was employed toverify structural characteristics of Au metal adsorption on oxidized ACC surfaces. Morphologyand composition were carried out by Scanning electron microscopy (SEM) and Energydispersive spectroscopy (EDS), respectively.
Keywords: Au nanoparticles, activated carbon cloth.
networks with plentiful functional groups onthe surface of carbon fiber are formed by auniform distribution of micropore providingfaster adsorption-desorption rate, fasterequilibrium rate and high fluid permeability[5]. As ACCs are so reductive, it can reducemetal ions into metallic elements.
An acid-base character is associated withoxygen functionalities [6, 7]; the acidiccharacter is associated with oxygen functionali-ties such as carboxyls, lactones and phenolwhilst pyrones, chromenes, ethers andcarbonyls imply the basic properties. Theacidic and basic character of carbon surfacedepends on the conditions of preparation.HNO3 is one of well known oxidizing agentfor ACC pretreatment. Our work has usedHNO3 to treat ACCs before adsorption.
Metallic catalyst (e.g. Pt, Pd, Au) on various
100 Chiang Mai J. Sci. 2010; 37(1)
supports (e.g. metal oxide and carbon) hasbeen accomplished in many applications [3].Au catalyst is one of attractive catalystsrecently as it is active under mild conditionsand could enhance the selectivity of thereaction, moreover it has a lot of applicationsfor pollution such as air cleaning and light-off autocatalysts and purification of hydrogenstream in fuel cell [8]. As standard reductionof Au is lowest compared with Pt and Pd, inthe beginning of research, Au is the first targetto start with, before the further study of othermetal catalysts on ACC support.
In this work, ACCs were treated andimpregnated in solution of tetrachloroauricacid for the Au nanoparticle preparationwithout reducing agent.
2. MATERIALS AND METHODSACCs as a support used in the present
work were obtained from AmeriAsia (madein China Jiansu), coded as ACC MY-QW-025having a specific area of 1500 m2g-1 andHAuCl4 precursor was from Merck.
Elimination of contaminations can beacomplished by over night stirring ACCs in 6
Figure 1. SEM of ACCs (a) before (magnification 65x) and (b)after (magnification 100x) oxidative treatment by 1M HNO3 .
Chiang Mai J. Sci. 2010; 37(1) 101
M HCl (12-24 h), subsequently washed withdeionized water until pH is constant and thendried in oven at 100 oC.
Modification of ACC surface can bemade via oxidation. The pretreated ACC wasstirred in 1.6 M HNO3 solution for 6 h, thenwashed with deionized water for neutraliza-tion and the supports lastly were dried attemperature 100 oC overnight.
Au adsorbed cloths were prepared byincipient wetness impregnation methodusing 3 % wt Au/ACC solution. Deionizedwater and ethanol were chosen to be a solventfor the precursor solutions. The oxidizedACCs were vigorously stirred in brown-yellowsolution for 30 min as the colored solutionturned to be colourless indicating goldadsorbed on the surface of ACC support.After drying at 90oC in the oven overnight,SEM ( JEOL JSM-5910LV) and EDS(JEOL JSK-6335F) were used to charac-terized morphology and composition ofnanoparticles, respectively.
X-ray diffraction patterns of ACCs weretaken on a Siemens D-500 using Cu ka
radiation (l1.54, Ni filter, 2q=10-90o). TheACCs were cut into fine pieces formeasurements.
3. RESULTS AND DISCUSSIONPretreatment of ACC was performed by
using 6 M HCl to get rid of some mineralimpurities such as K, Mg, Ca, Al and Fe whichare usually existed in an activated carbon cloth[9]. After that the surface of fabric wasfuntionalized by oxidative treatment.
The oxidative treatment of ACCs wasperformed by 1.6 M HNO3 in order toproduce oxide functional groups. Figure 1shows ACCs before and after treatment byHNO3 solution by SEM. Not only to get ridof an impurities covered on ACC surface, butcarbon surface is also inert, functionalizationof carbon surface or chemical modificationis required to get favourably adsorbatesadsorbed on the surface. Acid treatmentintroduces oxygen components formed(such as carboxylic, anhydride, phenolicand carbonyl groups) which generate thesurface of ACC to be such reducing regions.
Figure 2. X-ray diffraction pattern of Au adsorptionon Activated carbon cloths (ACCs).
102 Chiang Mai J. Sci. 2010; 37(1)
Functionalities perform as holder foradsorbates and support their adsorption oncarbon surface by numerous mechanismsincluded ion-exchange, coordination andadsorption reactions [6].
SEM images of ACC before and aftertreatment are shown in Figure 1 show thatmacro physical structures of ACC aremaintained after acid treatment. Pretreatmentof ACC resulted in flat surface of fiber as nohollow or bump of solid on the surface in
the magnified image (image not shown).The smoothly ACCs can be attributed to bea favorable host for metal particles and finedispersed particles to be adsorbed.
After impregnation, several techniqueswere used to characterize Au nanoparticlesadsorbed activated carbon cloths. As it is wellknown that ACCs consist of randomlyoriented graphitic microcrystalline unit, X-raydiffraction is suited for the structuralcharacteristic of ACCs. X-ray diffraction
Figure 3. 3%wtAu/ACC by preparation in aqueous solution (a) SEM image(magnification 1500x) and (b) EDS of Au nanoparticles on ACC support.
Chiang Mai J. Sci. 2010; 37(1) 103
spectrum of Au-ACC composites is shownin Figure 2. The diffraction peak at 20-25o
observed in all the diffraction patterns ofcarbon-supported metals is attributed to the(002) plane of the hexagonal structure ofcarbon [10]. The broadening of the peaks isan indication of structural disorder of thesample. Prepared Au nanoparticle samplesshow crystalline structure.
It is obviously shown main characteristicpeaks of Au which match with JCPDS cardno 4-784 at 2q : 38.2o (111), 44.4o (200), 64.6o
(220) and 77.5o (311) indicating the successfulreduction of Au ion to Au metal. XRD peaksat 25.9o are also observed resulting in
hexagonal graphite structure (002). Thissignifies that oxidized ACCs have a highelectrical conductivity and could be the supportmaterial for electrode application.
SEM image in Figure 3 shows 3% wtAu/ACC prepared in water. The mean particlesize of metal was with wide range of sizedistribution ca. 80-200 nm. The results provethat nanoparticle size of Au can simply beprepared by our method. Hydrated halidesare soluble HAuCl4 (III) dissociate to AuCl4
-,and H+. Those anions can be adsorbed onporous activated carbon and then reduced tobe metal (Au0) resulting in the color changeof solution from color to colorless. The
Figure 4. SEM images of Au nanoparticles on ACC from 3% wt Au/ACC in ethanol solution(a) various sizes of Au nanoparticles (magnification 2,000x), (b) another selected area image(magnification 2,000x) and (c) magnification image of Au nanoparticles adsorbed inside thefabric surface (magnification 50,000x).
B C
A
104 Chiang Mai J. Sci. 2010; 37(1)
adsorption rate of anion on activated carboncloth is high as the color is clear in very shorttime.
It can also propose that metal firstlydeposits into the micropores and furtheraggregates to the outer surface of carbonfibers as macroparticles observes on thesurface. Figure 3(b) confirmed Au particlesby EDS spectra.
It can be obviously seen the smaller Aunanoparticles adsorbed with size 5-80 nmafter impregnation in ethanol (Figure 4 (a) and(b)). Higher magnification image shows othersmall structures on ACC surfaces and insidethe surface (Figure 4(c)). White areas in lowerlevel in Figure 4(c) could be Au clustersadsorbed inside the fiber surfaces. This makesprepared catalyst more interesting as theremight be other kinds of catalytic mechanismscan occur on this area which will be the nexttarget for further research.
From the result of particles’ size on SEM,the polarity can be taken an acount for particlesize distribution. HAuCl4 is high polar due toit is ionic substance which is more soluble inwater with high polarity (dielectric constant80.1 at 20oC [11]). It could also discuss in anatomic level that weakening force on chargesresults from water molecules forminghydration shells around those AuCl- ions. Ionicbond is weakened in the presence of watersolvent allowing ions to separate from thecrystal and separate through solution. Ethanolis less polar than water (dielectric constant is25.3 at 25oC[11]) indicating the reduction ofcharge screening. As the activated carboncloths were oxidized, the functional groupsintroduced on ACCs support must becompatible with Au ion in ethanol. Therefore,the nanoparticle size prepared in ethanol issmaller than that in water. So, the polarity ofsolvent affects the size of metal adsorptionon ACC. The ethanol performed such a goodsolvent for Au3+ adsorption on oxidized ACC
surface and metal ion can also be reduced tobe Au and adsorbed the surfaces.
4. CONCLUSIONSActivated carbon cloths were oxidized
by 1.6 M HNO3 and loaded with gold metalby impregnation in HAuCl4 solution resultingin Au nanoparticles adsorbed with variousdiameters. It was found that the type ofsolvent affects the particle size and particledistribution. Ethanol is a promising solventas given smaller nanoparticles and uniformdispersion of nanoparticles adsorbed on theoxidized ACC support. The further study ofmetal adsorption preparation on ACC supportis to be continued.
ACKNOWLEDGEMENTSThis work was financially supported by
Ministry of Science and Technology, Thailand.
REFERENCES
[1] Pradhan B.K. and Sandle N.K., Effectof different oxidizing agent treatmentson the surface properties of activatedcarbons. Carbon. 1999; 37(8): 1323-32.
[2] Polovina M. and Babic B., KaluderovicB, Dekanski A. Surface characterizationof oxidized activated carbon cloth.Carbon. 1997; 35(8): 1047-52.
[3] Auer E., Freund A., Pietsch J. and TackeT., Carbons as supports for industrialprecious metal catalysts. Applied CatalysisA: General. 1998; 173(2): 259-71.
[4] Carrott P.J.M., Nabais J.M.V., RibeiroCarrott M.M.L. and Pajares J.A.,Preparation of activated carbon fibresfrom acrylic textile fibres. Carbon. 2001;39(10): 1543-55.
[5] de Miguel S.R., Vilella J.I., Jablonski E.L.,Scelza O.A., Salinas-Martinez de LeceaC. and Linares-Solano A., Preparation ofPt catalysts supported on activated
[6] Guha A., Lu W., Zawodzinski Jr. T.A.and Schiraldi D.A., Surface-modifiedcarbons as platinum catalyst support forPEM fuel cells. Carbon. 2007; 45(7): 1506-17.
[7] El-Molla S.A., El-Shobaky G.A. andSayed Ahmed S.A., Catalytic Promotionof Activated carbon by Treatment withSome Transition Metal Cations. ChineseJournal of Catalysis. 2007; 28(7): 611-6.
