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Microporous and Mesoporous Materials 106 (2007) 212–218

Mesoporous nanocrystalline magnesium oxidefor environmental remediation

B. Nagappa, G.T. Chandrappa *

Department of Chemistry, Central College Campus, Bangalore University, Bangalore 560 001, India

Received 4 July 2006; received in revised form 28 January 2007; accepted 28 February 2007Available online 12 March 2007

Abstract

Mesoporous MgO nanocrystals are prepared through combustion route using magnesium nitrate as oxidizer and glycine as fuel. Thepowder has been characterized using powder X-ray diffraction (PXRD), scanning and transmission electron microscopy (SEM/TEM),surface area and porosity measurements. The PXRD pattern confirms the crystallinity and phase purity of the powder and the particlesize obtained from Scherrer’s formula lies in the range 12–23 nm. The SEM result reveals that the powder is porous and agglomeratedwith polycrystallite nanoparticles. The pore diameter observed in TEM image is in the range 4–11 nm. The surface area of the powder is�107 m2/g and average pore diameter obtained from desorption is 7.8 nm. It is found that 0.15 g of as made MgO powder could remove97% of fluoride from standard sodium fluoride solution (10 ppm) and 75% of fluoride from tube well water. In this technique 90% min-imization of sludge could be achieved. The comparative studies on the fluoride removal capacity of as made, regenerated and commercialgrade MgO powders are found to be 97%, 76% and 17%, respectively.� 2007 Elsevier Inc. All rights reserved.

Keywords: Combustion synthesis; Mesoporous; Nanocrystalline MgO; Tube well water; Defluoridation

1. Introduction

The development of novel techniques for dehalogen-ation of industrial wastes containing halides has becomeimmediate task due to their high toxicity and significantecological hazard. Nanocrystalline alkaline earth metaloxides attract significant attention as effective chemisor-bents for toxic gases, HCl, chlorinated and phosphoruscontaining compounds [1]. Destructive sorption takes placenot only on the surface of oxide materials but also in theirbulk. Although many metal oxides and metal oxide mix-tures prepared and activated in the proper way may becapable of acting as adsorbents in surface chemistry.MgO, serves as a good model since it possesses a simple(NaCl) crystal structure and can be prepared with widelyranging surface areas [2]. It is important to note that the

1387-1811/$ - see front matter � 2007 Elsevier Inc. All rights reserved.

doi:10.1016/j.micromeso.2007.02.052

* Corresponding author. Tel.: +91 80 22961350.E-mail address: [email protected] (G.T. Chandrappa).

efficiency of the destructive sorption increases with adecrease in the size of the MgO crystallites. MgO worksmost efficiently in the destructive sorption reaction whenits particle size is on a nanometer scale. The high efficiencyof the nanoparticle oxides is caused not only by their highsurface area but also by the high concentration of low-coordinated sites and structural defects on their surface[1]. As the particle size is scaled down to a few nanometers,the constituting atoms exhibit highly defective coordina-tion environments. Most of the atoms have unsatisfiedvalencies and reside at the surface. In short, microstruc-tural features such as small grain size, large number ofinterfaces and grain boundary junctions, pores, and vari-ous lattice defects that result from the chosen routes fortheir synthesis contribute significantly to the unique physi-cal and chemical properties of nanomaterials [3–5].Depending on preparation methods, MgO exhibits quitedifferent reactivity toward adsorbed chemicals [6]. There-fore, high surface area material having the most defect sites

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B. Nagappa, G.T. Chandrappa / Microporous and Mesoporous Materials 106 (2007) 212–218 213

per unit area, should be of interest as destructive adsor-bent. Fine powder of MgO exhibits strong surface basicityand moderate acidity [7]. The most conventional method ofsynthesis of MgO is the decomposition of various magne-sium salts or magnesium hydroxide [8]. The MgO productobtained by this method has relatively large and variedgrain size with low surface area. Nanocrystalline MgO isusually synthesized through hydrothermal [9] and sol–gelmethods [10,11].

In continuation of our research programme on porousmaterials [12,13] we have attempted to synthesize a por-ous nanocrystalline MgO powder with large surface areaby low temperature solution combustion technique. Thecombustion technique takes advantages of exothermic,fast and self-sustaining chemical reaction between metalsalt and a suitable organic fuel. In this technique, mostof the heat required for the synthesis is supplied by thereaction itself [14]. These features make the combustionmethod an attractive route for the manufacture of techno-logically useful nanomaterials at lower cost in few minutesas compared to other synthetic routes. Besides, combus-tion synthesis requires relatively simple equipments andproducing high purity homogeneous products with energysaving [15].

To certain extent (0.6 ppm, as per the World HealthOrganization) fluoride ingestion is necessary for bone andteeth developments but excessive ingestion causes a diseaseknown as fluorosis [16]. The World Health Organization(WHO) standards and Bureau of Indian Standards (BIS):10500 1991 [17], permit only 1.5 mg/l as a safe limit of fluo-ride in drinking water for human consumption. Fluorosiscontinues to be an endemic problem, and more and moreareas are being discovered regularly that are affected byfluorosis in different parts of the world. Excess fluoride inground water has been encounted in many countries[18,19]. In India, as many as 200 districts spread across17 states are suffering from high fluoride concentration.More than 6 million people are seriously affected by fluoro-sis and another 62 million are exposed to it [20]. Thus thereis an urgent need for the development of feasible tech-niques that are efficient for fluoride removal.

Defluoridation of water has been carried out using var-ious methods like coagulation and precipitation process(named as Nalgonda technique) [21], use of locally derivedsample of silty clay [22], natural materials [23], soil sorbent[24] and oxide minerals [20]. The removal of excessive fluo-ride (F�) from drinking water was also attempted using ionexchange/adsorption process, in which commercially avail-able metal oxides like activated alumina, magnesia andother materials were used as adsorbents [25]. In the presentstudy we explore the use of combustion derived MgO pow-der as adsorbent for the removal of fluoride content andother ions in tube well water mainly used for drinking pur-pose by rural community in India. Finally, regeneration ofthe used MgO was also conducted using 0.1 N NaOH toevaluate whether regenerated MgO is amenable to efficientregeneration and reuse.

2. Experimental

2.1. Chemicals and reagents

Chemicals and reagents of AR grade, and double dis-tilled water were used in the preparation of solutions inthe present investigation. A stock solution of standard fluo-ride (1000 ppm) was prepared by dissolving 2.2111 g ofanhydrous sodium fluoride, NaF in 1000 ml double dis-tilled water. Fluoride solutions of 5 and 10 ppm were pre-pared by successive dilution of the stock solution and usedfor the calibration of a Thermo Orion make Portable pHand pH/ISE Meter coupled with fluoride ion selective elec-trode, model 290 Aplus.

2.2. Synthesis

An aqueous solution containing magnesium nitrate(Mg (NO3)2 � 6H2O) as oxidizer (O) and glycine(NH2CH2COOH) as fuel (F), (corresponding F/O ratioU = 1.11 as shown in Eq. (1)) [14,26,27] was taken in a petridish. Excess water was allowed to evaporate by heating ona hot plate until wet powder was left out. Then the petridish was introduced into a muffle furnace maintained at400 ± 10 �C. Initially the wet powder undergoes dehydra-tion and starts smoldering combustion which appeared atone end and propagated throughout the mass within a min-ute. Voluminous and porous nanocrystalline black coloredproduct was obtained and it was turned into white onallowing it in the muffle furnace at the same temperaturefor �30 min. This non-carbonaceous powder is termed as‘‘as made MgO’’. The theoretical equation assuming com-plete combustion of redox mixture (U = 1.11) used for thesynthesis of MgO may be proposed as

9MgðNO3Þ2ðaqÞ þ 10NH2CH2–COOHðaqÞ! 9MgOðsÞ þ 14N2ðgÞ þ 20CO2ðgÞ þ 25H2OðgÞ ð1Þ

2.3. Removal of fluoride

The process of removal of fluoride present in 10 ppm ofstandard sodium fluoride solution utilizing batch stirringprocess on magnetic stirrer was performed [28]. In eachexperiment, 100 ml of 10 ppm of standard sodium fluoridesolution was taken in 250 ml beaker to which a knownquantity of as made MgO (0.025–0.15 g) as adsorbentwas added. In a batch of experiments, the optimum valuesof variables like pH (the process is independent of pH),stirring time (10 min) and sedimentation time (20 min) weremaintained constant. The fluoride adsorbed MgO settled atthe bottom of the beaker was separated by decantation andit is termed as ‘‘used MgO’’ (sludge). The fluoride contentleft out in treated water sample was measured using cali-brated Fluoride ion meter and total ionic strength adjust-ing buffer (TISAB III) [22,23]. This standard procedurewas extended for the removal of fluoride in tube well water

30 40 50 60 70 80

)u.a( ytisnetnI

2θ (degrees)

• ••

•∗ ∗∗ ∗

Fig. 1. Powder XRD patterns of (a) as made MgO (b) used MgO and (c)regenerated MgO powders (s – MgO; d – MgF2; � – Mg(OH)2).

214 B. Nagappa, G.T. Chandrappa / Microporous and Mesoporous Materials 106 (2007) 212–218

samples collected from different parts of the Karnataka,India, with the same experimental parameters.

The comparative studies on the adsorption efficiency ofas made MgO with the regenerated MgO and the commer-cial grade MgO (Thomas Baker make, Mumbai, India) forthe removal of fluoride in standard sodium fluoride solu-tion were also carried out by following the above procedureand the results are shown in Fig. 5.

2.4. Regeneration and reuse

In order for the sorption process to be viable, efficientregeneration and reuse is necessary. The separated sludgewas dried in air and 0.5 g of it was washed thrice with15 ml portions of 0.1 N NaOH followed by 45 ml doubledistilled water. The washings were separated by filtrationand the filtrate was made up to 100 ml with double distilledwater. The fluoride content was measured using fluorideion meter and it was found to be 4.5 ppm. The same proce-dure was repeated for the regeneration of sludge with 15 mlportion of 0.1 N HCl instead of NaOH and the fluoridecontent was estimated. With acid washing, it was foundthat only 2 ppm of fluoride could be removed from thesludge. From this, it is possible to conclude that the sludgecan be regenerated completely from fluoride with NaOHtreatment. The regenerated sludge was dried at 500 �C for2 h and subjected for reuse as ‘‘regenerated MgO’’ for theremoval of fluoride.

2.5. Characterization

The powder X-ray diffraction patterns of as made MgO,used MgO and regenerated MgO powders were carried outon a Phillips X’pert PRO diffractometer operating with theCuKa (k = 1.54056 A). The surface morphology of pow-ders were examined using JEOL (JSM – 840 A) scanningelectron microscopy (SEM). Transmission electron micros-copy images were observed with a JEOL 100 CX electronmicroscopy. The surface area and porosity of the as madeMgO were determined by Quanta Chrome CorporationNOVA 1000 gas sorption analyzer. The fluoride contentin ground water was estimated using Thermo Orion makePortable pH and pH/ISE Meter coupled with fluoride ionselective electrode, model 290 Aplus [22,23].

3. Results and discussion

3.1. XRD features

The powder X-ray diffraction patterns of as made, usedand regenerated MgO powders are shown in Fig. 1. The asmade MgO powder exhibits completely crystalline cubicphase (Fig. 1a) and all the diffraction peaks can be readilyassigned to a pure phase of periclase MgO [29], which basi-cally proves the formation of a homogeneous powder withrock salt structure [30]. The broadness of PXRD peaksindicates the nanocrystalline nature of MgO powders and

the particle size estimated from Scherrer’s formula [31] isin the range 12–23 nm. PXRD pattern (Fig. 1b) of usedMgO (powder is dried at 105 �C for 2 h before collectingXRD) shows the presence of magnesium hydroxide, mag-nesium fluoride and parent MgO. The formation of magne-sium hydroxide and magnesium fluoride may be due to thereaction between MgO with water and fluoride ions inaqueous solution [8,32]. The PXRD pattern of regeneratedMgO powder (Fig. 1c) shows completely crystalline cubicphase and all the peaks can be attributed to a pure phaseof periclase MgO [28].

3.2. SEM

Combustion derived MgO powders (as made) are foamyproducts with large agglomerates of very fine particles. TheSEM (Fig. 2a) result reveals that the powder is porous andagglomerated with polycrystallite nanoparticles. The poresand voids can be attributed to the large amount of gassesescaping out of the reaction mixture during combustion.The porous structure examination of the used and regener-ated MgO powders can be clearly seen in the SEM images(Fig. 2b and c) revealing the presence of pores/open voids.The pores can be developed and further enhanced by chem-ical activation during etching reaction by NaOH, whichresults in the formation of some pores.

3.3. TEM

TEM micrograph (Fig. 3) shows randomly orientednanocrystalline MgO particles and this TEM imageappears to be similar to MgO powder reported by Klab-unde et al. [33]. The selected area electron diffraction(SAED) pattern (inset in Fig. 3) of the sample shows reflec-tions corresponding to (111), (200), (220) and (222)planes, indicating the presence of cubic MgO crystallinephase [29].

Fig. 2. SEM images of (a) as made MgO (b) used MgO (dried at 105 �C for 2 h) and (c) regenerated MgO powders.

Fig. 3. TEM micrograph and electron diffraction pattern (inset) ofultrafine as made MgO powder containing multi-nanocrystallites.

0.2 0.4 0.6 0.8 1.0

20

40

60

80

100

120

140

)g/cc(debrosd

A emulo

V

Relative pressure (P/P0)

Fig. 4. Nitrogen adsorption–desorption isotherm of as made MgOpowder. Inset: Corresponding pore size distribution curve determinedfrom the N2 – desorption isotherm.


3.4. N2 adsorption isotherm

Fig. 4 shows representative of nitrogen adsorption/desorption isotherm [34] and the corresponding pores sizedistribution curve (inset) of the as made MgO product.The product has high BET surface area of �107.24 m2/g[35]. As gaseous products liberated during combustion,the agglomerates disintegrate and more heat is carriedaway from the system, thereby hindering the particlegrowth, lead to high surface area. Distribution of poresvolume with their diameter calculated by BJH method,

MgO sample exhibited broad distribution with the impor-tant part of the pores at above pore diameter 4 nm [36].The average pore diameter obtained from desorption isfound to be 7.8 nm.

3.5. Effect of adsorbent quantity

Different quantities (0.025–0.15 g) of MgO powder asadsorbent were taken separately in 100 ml of 10 ppm stan-dard fluoride solutions and kept for batch stirring. There-after, the analysis of residual fluoride in the treated waterwas performed and a relationship between fluoride removalcapacity (percentage) and adsorbent quantity (Fig. 5) wasestablished. From the trend it can be seen that a maximum

0.00 0.05 0.10 0.150

20

40

60

80

100

c

b

a

)%( yticapac lavo

mer ediroulF

Weight of adsorbents (g)

Fig. 5. Fluoride removal capacity versus weight of adsorbents: (a) asmade MgO, (b) regenerated MgO and (c) commercial grade MgO powders(using 10 ppm of standard sodium fluoride solution.

40

60

80

100

)%( ediroulf fo yticapa


level (93–97%) of fluoride removal occurs in the presence of0.1–0.15 g of as made MgO. After this, removal ratebecomes almost constant with increase of adsorbent quan-tity and can therefore 0.1 g be considered as optimum dose.This standard procedure was extended for the removal offluoride in water samples collected from different tube wellswith same experimental parameters. The experimentalresults show that (50–75%) of fluoride could be removedfrom different tube wells water (Table 1). The decrease influoride removal capacity (50–75%) in the tube well watercompare to (97%) standard fluoride solution is due to theremoval of other predominant ions such as nitrate andchloride present in tube well water samples. For instance6–44% of nitrate content is removed along with fluoridefrom water samples of different tube wells.

0 10 20 30 40 50 600

20

c lavome

R

Stirring time (min)

Fig. 6. Fluoride removal capacity versus stirring time (using 10 ppm ofstandard sodium fluoride solution).

3.6. Effect of stirring time

The effect of contact time on fluoride removal rate ofadsorbent was determined by using optimum dosage of0.1 g MgO powder. The batch experiments for 10 ppm offluoride were conducted at different intervals of stirringtime in 100 ml standard fluoride solution. After every10 min of stirring, the residual fluoride in treated waterwas analysed. The fluoride removal gradually increased

Table 1Fluoride and nitrate contents in tube well water samples before and after trea

Parameters # Analysis of water samples before treatment

Sample numbers ! 1 2 3 4

Nitrate (as NO�3 ) (ppm) 3.8 19.0 4.5 5.0Fluoride (as F�) (ppm) 1.8 2.32 1.65 2.1Chloride (as Cl�) (ppm) 1012.0 424.0 76.0 384.0

a ND – not detected.

with time and attained constant value above 10 min stirring(Fig. 6) and this duration could be considered as optimumtime for maximum level of fluoride removal. This stirringtime is six times lower than the methods [22,23] reportedso far.

3.7. Effect of pH

The effect of pH (5–9) on fluoride removal was carriedout using optimum dosage (0.1 g) of adsorbent and stirringtime (10 min) and the results are shown in Fig. 7. It is evi-denced from the graph that the fluoride removal was inde-pendent of pH.

3.8. Comparative studies

Comparative studies on the adsorption efficiency of asmade, regenerated and commercial grade MgO powdersfor the removal of fluoride in water sample have been car-ried out according to the procedure used earlier. Differentquantities (0.025–0.15 g) of the above MgO powders wereadded as adsorbents separately into 100 ml of 10 ppm stan-dard fluoride solutions and kept for batch stirring. There-after, the analysis of residual fluoride in the treated waterwas performed and relationship between fluoride removalcapacity (percentage) and adsorbent quantities (Fig. 5a–c)

tment with MgO adsorbent

Analysis of water samples after treatment

5 1 2 3 4 5

1.22 3.0 10.5 4.13 4.7 NDa

2.0 0.9 0.75 0.8 0.65 0.584.0 1002.0 412.0 70.0 380.0 80.0

5 987680

90

100

)%( ediroulf fo yticapac lavo

meR

pH

Fig. 7. Fluoride removal capacity verses pH (using 10 ppm of standardsodium fluoride solution).


was established. From the trend it can be seen that a max-imum level of 97%, 76% and 17% of fluoride could beremoved when as made, regenerated and commercial gradeMgO (0.15 g) powders were used as adsorbents respec-tively. From the graph it was observed that the fluorideremoval capacity of combustion derived nanocrystallineMgO (as made) showed about 9.5-fold higher removalcapacity as compared to the commercial grade MgO atthe dosage of 0.1 g.

4. Conclusions

In this study, an extensive laboratory investigation hasbeen carried out to evaluate the performance of combus-tion derived MgO powder as adsorbent on fluorideremoval by adsorption. This new adsorbent, termed MgOpowder is found to be very effective in removing fluoridefrom the aqueous environment. The adsorbent MgO pow-der has been prepared by low temperature solution com-bustion process. The combustion method enables theproduction of impurities free MgO powder in large scalein short time. Due to porous nature, large surface areaand fine particle size; MgO exhibits good adsorption char-acteristics to remove fluoride, nitrate and other ions pres-ent in water. As eco-friendly and non toxic, combustionderived MgO removes 97% fluoride present in water ascompared to regenerated MgO (�76%) and commercialgrade MgO (�17%). This simple and cost effective adsorp-tion technique to remove fluoride from ground water usingfine grained combustion derived MgO can be easily imple-mented in household level to community water supplyschemes.

Acknowledgments

B. Nagappa expresses his gratitude to the KarnatakaState Pollution Control Board for deputation to take up

this research work. The authors are thankful to Prof.K.C. Patil, for his valuable discussions. G.T. Chandrappagratefully acknowledges the financial support by DST,New Delhi, under the scheme, NSTI to carryout this work.

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