Fenton-like degradation of azo-dye Orange II catalyzed by transitionmetals on carbon aerogels

Filipa Duarte a, F.J. Maldonado-Hodar b,*, A.F. Perez-Cadenas b, Luis M. Madeira a

a LEPAE, Departamento de Engenharia Quımica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugalb Departamento de Quımica Inorganica, Facultad de Ciencias, Universidad de Granada, 18071 Granada, Spain

Applied Catalysis B: Environmental 85 (2009) 139–147

A R T I C L E I N F O

Article history:

Received 15 April 2008

Received in revised form 1 July 2008

Accepted 3 July 2008

Available online 12 July 2008

Keywords:

Oxidation

Fenton

Dyes

Hydrogen peroxide

Carbons

Transition metals

A B S T R A C T

This works deals with the non-biodegradable azo-dye Orange II degradation using the heterogeneous

Fenton-like reaction. Two catalyst series based on transition metals (Fe, Co, Ni) were prepared trying to

improve not only the catalytic performance but also to diminish the metal leaching. Thus, metal-doped

carbon aerogels and supported catalysts, the latter prepared by classical wet impregnation, were tested.

The catalysts were characterized by different techniques: N2 adsorption, mercury porosimetry, X-ray

diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM). Chemical oxidation of

the azo-dye Orange II was carried out in a batch reactor, at atmospheric pressure and 30 8C. The influence

of the preparation method, the porous characteristics of supports and metal dispersions on the metal

leaching and catalytic performance were analyzed. Impregnated catalysts are in general more active than

metal-doped carbon aerogels, because metal particles are more accessible to reactants, while the doped-

carbon aerogels present a smaller leaching tendency. The behaviour of impregnated catalysts is

determined not only by the metal used but also by the porous texture of the supports. Thus,

microporosity strongly favours the Orange II adsorption capacity and tends to reduce the metal leaching,

while mesoporosity strongly increases the metal dispersion and consequently, the catalytic performance.

Iron is seemingly the most active metal tested, exhibiting simultaneously the lower leaching levels for all

of the supports. Although cobalt is an interesting alternative in terms of activity, it exhibits unacceptable

leaching degrees.

� 2008 Elsevier B.V. All rights reserved.

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1. Introduction

The textile industry has an important role in several countries,particularly in Portugal. This is positive at the economic level but itcontributes significantly to the water pollution because of theunwanted wastes that are present in its effluents. These wastes arecomplex organic molecules that are often difficult to degrade, likedyes. To fight this problem there are several possible approaches,requiring the use of processes that can be classified as physical,biologic or chemical. The physical ones that include, for example,liquid–liquid extraction, adsorption or ion exchange are notefficient enough because they do not destroy pollutants, butsimply transfer them from one phase to another. On the otherhand, the biologic treatments occur at low rates and, beyond that,many of the compounds usually present in these effluents (as is thecase of several dyes) are often not biodegradable and can be even

* Corresponding author. Tel.: +34 958240444; fax: +34 958248526.

E-mail address: fjmaldon@ugr.es (F.J. Maldonado-Hodar).

0926-3373/$ – see front matter � 2008 Elsevier B.V. All rights reserved.

doi:10.1016/j.apcatb.2008.07.006

toxic to the micro-organisms used in biological systems, thusbecoming inhibitors of these processes. To overcome theselimitations chemical processes have been proposed, in particularthe advanced oxidation processes (AOPs) that allow degrada-tion, and in some cases almost total mineralization, of organicpollutants in a fast, effective and inexpensive way [1]. Theseprocesses are based on the generation of highly reactive specieslike hydroxyl radicals that are capable of attacking the bonds inmost organic compounds. The Fenton’s reagent is one of such AOPs,where the reaction between a transition metal like iron ion (II) andhydrogen peroxide is used for HO� radicals generation. In a simpleway, this process can be described by the following reactions:

H2O2þ Fe2þ ! HO� þ OH� þ Fe3þ (1)

HO� þ organic matter ! oxidation products (2)

being the Fe2+ species then restored by several well know reactionsthat have already been disclosed in the literature [2,3].

The main advantages of the Fenton process are the simplicity ofthe pollutant destruction and the fact that it can be carried out

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147140

under soft operating conditions (temperatures closed to roomtemperature and atmospheric pressure), therefore not requiringspecific equipment [4]. The Fenton and the Fenton-like processes(the latter departing from Fe (III)) can be carried out in ahomogeneous system, where the iron catalyst is dissolved in theaqueous solution, or in a heterogeneous way. However, thehomogeneous system is less appropriate since it leads to highmetal (iron or other transition metal) concentrations in the finaleffluent. In the case of Fe, homogeneous process can originatemetal concentrations around 50–80 ppm, which overcomes thelegal limit imposed by the directives of the European Union of only2 ppm [5]. Even though the metal recovery is possible, it would risethe costs and the complexity of the process.

Thus, several studies have been made to find efficientheterogeneous systems, including oxides [6,7], zeolites [8,9], clays[10,11] or carbons materials [12]. Carbon materials and inparticular carbon aerogels are a good choice, for instance whencompared with pillared clay catalysts, because a much fasterreaction rate is achieved [12]. In addition, carbon materials havelow cost and high surface areas, the surface chemistry or porositycan be easily modified and the active metal phase can be recoveredupon deactivation, by burning the carbon support. Carbon aerogelsare one of the most interesting types of the new carbon materials.These materials are prepared by the carbonization of organicaerogels obtained from the sol–gel polycondensation of certainorganic monomers, commonly resorcinol (R) and formaldehyde(F), following the Pekalas method [13]. An important feature ofthese materials is that metal-doped organic aerogels can be readilyprepared, by adding a soluble metal salt to the initial mixture. Aftergelation, the metal salt is trapped within the gel structure and themetal ions can be chelated by the functional groups of the polymermatrix [14]. The added metal salts can also catalyze to a varyingdegree the polymerization or gelation process, which can in turnaffect the morphology and pore texture of the organic aerogel.After carbonization of this aerogel, the metal-containing phase isdistributed throughout the porosity of the carbon phase [15]. Thismethod yields a metal-doped monolithic carbon aerogel, whichcan be used as a catalyst. Alternatively, the metal catalyst can bedeposited on the carbon aerogel by other methods, such asimpregnation [16].

In a previous study we have reported the usefulness ofimpregnated carbon–Fe catalysts for the Fenton’s process, usingtwo different carbon supports: an activated carbon prepared fromagricultural by-products and a carbon aerogel [12]. The laterrevealed to be particular promising, but the level of leaching wasnot low enough. Therefore, in this paper, two series of aerogelcatalysts based on other transition metals (Fe, Co, Ni) wereprepared, characterized by different experimental techniques andused in the azo-dye Orange II degradation by a Fenton-likeprocedure. In the first series, metal-doped carbon aerogels wereprepared according to previous works [14]. The goal is to decreasethe leaching since metal particles are part of the global structure.The second catalysts series was prepared by wet impregnation,using two different carbon aerogel supports. In this case, metalparticles are formed and distributed during catalyst pre-treat-

Table 1Textural characteristics of doped carbon aerogels

Sample Metal content (wt.%) r (g cm�3) V2 (cm3

AFe–500 3.3 0.44 0.00

ACo–500 4.2 0.50 0.89

ANi–500 3.3 0.57 0.82

r: bulk density; V2: volume of pores with a diameter between 3.7 and 50 nm; V3: volume

diameter larger than 3.7 nm; SBET BET surface area obtained by N2 adsorption.

ments on the carbon surface. The correlation between preparationmethod, metal nature or textural characteristics and the leachingor catalytic performance is analyzed. Orange II, an azo dye, waschosen as a model compound because it is widely used in thetextile and paper industries, due to its low price.

2. Materials and methods

2.1. Catalysts preparation and characterization

Two different kinds of catalysts were prepared, varying themetal deposition method (doping and impregnation techniques).Both of them are based on the synthesis and posteriorcarbonization of an organic aerogel from resorcinol–formalde-hyde (R–F) mixtures. In all cases, the polymerization develops inaqueous solutions for 1 day at room temperature, 1 day at 50 8Cand 5 days at 80 8C using a R/F molar ratio of 1/2. After curing, thesamples were exchanged with acetone for 2 days and dried insupercritical CO2. This process yield organic aerogels which arefinally carbonized or even activated to produce carbon aerogels oractivated carbon aerogels, respectively.

In the preparation of metal-doped carbon aerogels [14] thecorresponding metal precursor (acetates) was dissolved in thestarting polymerization solutions (R + F + water + metal precur-sor) and the solution followed the cure and drying stepspreviously commented. After carbonization at 500 8C, thesesamples contain a metal loading of about 3.5% by weight(Table 1), which are homogeneously distributed along thecarbon matrix and can be used directly as catalysts becausemetal precursors are decomposed during the carbonizationprocess. In these samples, a rigorous control of the final metalcontent cannot be achieved because, even though all the startingsolutions have identical metal concentration, the final metalcontent in the catalyst depends on the weight loss that eachsample undergoes along the preparation step (curing, drying andcarbonization).

Two carbon aerogels without any transition metal were alsoprepared, and catalysts were prepared in this case by wetimpregnation. In the preparation of the so-called support M,Na2CO3 was used as polymerization catalyst (C) in a molar ratio ofR/C = 800 (Na is undetectable in the final carbon aerogel), while incase of the support AS3 no polymerization catalyst was added. Asdone previously, the sample M was carbonized at 500 8C but in thecase of the AS3 support, the carbonization temperature was1000 8C and then it was activated by vapour stream at 900 8C, todevelop the surface area.

Then, these two aerogels carbon supports were impregnatedwith Fe, Co and Ni acetates with a final metal content of 7 wt.%, aload similar to that employed in previous works [11,12]. Theseimpregnated samples were finally treated in inert atmosphere at300 8C for 2 h for acetates decomposition.

The doped aerogels will be labelled as AX-500, indicating thecarbonization temperature and where X refers to the metal that theaerogel contains. Similarly, the impregnated samples will bereferred as M–X or AS3–X. For all samples, the metal loading is

g�1) V3 (cm3 g�1) Sext (m2 g�1) SBET (m2 g�1)

1.52 7 482

0.12 132 660

0.03 176 669

of pores with a diameter larger than 50 nm; Sext: external surface area of pores with a

Fig. 1. Pore size distribution of doped catalysts obtained by mercury porosimetry.

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147 141

estimated by TG, by burning a fraction of the catalysts untilconstant weight is obtained.

Textural characterization of the catalysts was done byadsorption of N2 at �196 8C and mercury porosimetry. Fromnitrogen adsorption the apparent BET surface area values wereobtained. The Dubinin–Raduskevich Eq. (3) was also applied to N2

adsorption isotherms [17]:

W ¼W0 exp�A

bE0

� �2

(3)

where W is the amount adsorbed at relative pressure P/P0; W0 isthe limiting amount filling the micropores; A is the differentialmolar work given by A = RT ln P/P0; b is the affinity coefficient(b = 0.33 for N2) and E0 is the characteristic adsorption energy. Themicropore volume obtained from CO2 adsorption at 0 8C yields thevolume of narrower micropores (below 0.7 nm width) [18],whereas the total micropore volume is obtained from N2

adsorption at �196 8C if there are no pore constrictions. Themean micropore size, L0, was calculated from the characteristicenergy, E0, by applying the Stoeckli equation [19]:

L0ðnmÞ ¼ 10:8

E0ðkJ=molÞ � 11:4(4)

and the surface area of these pores from the following equation:

Smiðm2 g�1Þ ¼ 2000 W0ðcm3 g�1ÞL0ðnmÞ (5)

Mercury porosimetry was carried out using a QuantachromeAutoscan 60 equipment up to 4200 kg cm�2. Thus it was possibleto obtain the following parameters: pore size distribution, externalsurface area, Sext, for pores with diameters greater than 3.7 nm;pore volume corresponding to pores with a diameter between 3.7and 50 nm, V2, referred to as ‘‘mesopore’’ volume (one should notethat the mesopore volume range is defined as 2–50 nm); porevolume of pores with a diameter greater than 50 nm, or macroporevolume, V3; and particle density, r. Metal dispersion and naturewere studied by high-resolution transmission electron microscopy(HRTEM) using a Philips CM-20 electron microscope and by X-raydiffraction (XRD) using a Bruker D8 Advance diffractometer.

2.2. Catalytic activity

Chemical oxidation of the azo-dye Orange II (OII, Fluka p.a.) wascarried out in a batch reactor, at atmospheric pressure and 30 8C,using 1.0 L of a 0.1 mM dye solution, which is in the range of azodyes’ concentrations usually found in industrial waste streams[10]. Temperature was controlled by a thermostatic bath (Huber,Polystat CC1), and the reactor was provided with a magneticstirrer. After stabilization of temperature and pH (adjusted with a0.1 mM H2SO4 solution), the powder catalyst and the H2O2 solution(30 wt.%, Merck) were added, this being considered the initialinstant of the reaction (t = 0). Absorbance is continuously read by aPhilips PU8625 UV/vis spectrophotometer at 486 nm (character-istic wavelength of the Orange II molecule and corresponding to aregion wherein interference by oxidation products does not exist[11]), what required the use of a flow-through cell and recircula-tion of the reaction mixture by a peristaltic pump. Temperature, pHand absorbance are registered and recorded using the LabVIEW 5.0software. All experiments were carried out at T = 30 8C, pH 3 and[H2O2] = 6 mM, conditions that were selected based on previousresults [12]. Some runs were performed in duplicate and it waspossible to conclude that for the same instant, OII concentrationsdo not differ more than 6%.

Along the runs several samples were collected, and the totalmetal leached into the solution was determined using a UNICAM939/959 atomic absorption spectrophotometer. Total organiccarbon (TOC) was measured by catalytic oxidation followed by IRspectrometry for CO2 quantification using a Shimadzu 5000Ainstrument, model TOC-5000 CE. Reaction was stopped by filtratingthe catalyst (by means of 0.8 mm glass fibre-paper) and addingexcess Na2SO3, which instantaneously consumes the remaininghydrogen peroxide [10–12]. Samples were finally submitted tocentrifugation before TOC analysis.

3. Results and discussion

3.1. Catalysts characterization

3.1.1. Doped catalysts

As was previously pointed out [14–16], the nature of the metalspresent in the original polymerization solution has a greatinfluence in the R–F polymerization and consequently in themorphology and textural characteristics of the final catalysts. Fig. 1shows the pore size distribution (PSD) of doped-carbon aerogelsobtained by mercury porosimetry. It is observed that the samplesANi–500 and ACo–500 are mainly mesoporous while AFe–500 ismacroporous. The values for the meso and macropores volume,obtained by mercury porosimetry, and the corresponding surfaceareas are given in Table 1. This table shows that sample ACo has aslightly higher metal content and, on the other hand, that sampleswith higher mesoporous volume (V2) have higher BET surface areas(ACo and ANi), because macropores do not contribute significantlyto the surface area values.

Regarding the dispersion of the transition metals, XRD patterns(Fig. 2) suggest that they are well dispersed. Only in the case of theAFe-500 sample some X-ray diffraction peaks at 2u = 35.6, 57.1 and62.78, assigned to g-Fe2O3 (JCPDS 13-534) or Fe3O4 (JCPDS 88-0866), were noticed. In the other samples, metal particles are toosmall or are poorly crystalline to be detected by XRD.

3.1.2. Impregnated catalysts

The preparation procedure (reactants’ proportion, carboniza-tion temperature and activation processes) was designed to obtainsupports with very different pore structures. This is one of the mainadvantages of this kind of synthesis: the porosity of the materialscan be tailored, knowing in advance the influence of the differentsynthesis parameters [20]. The PSD obtained by mercuryporosimetry for both supports and their Fe-derivatives catalysts,as an example, is shown in Fig. 3 and the corresponding macro–mesopore volumes compiled in Table 2. Support M is basically amesoporous material that presents consequently a high externalsurface area; on the contrary, the AS3 support is a macroporousmaterial, which in spite its high macropore volume presents very

Fig. 2. X-ray diffraction patterns of metal-doped carbon aerogels.

Table 2Textural characteristics, obtained by mercury porosimetry, of carbon aerogels

supports and their Fe-derivatives catalysts

Sample r (g cm�3) V2 (cm3 g�1) V3 (cm3 g�1) Sext (m2 g�1)

M 0.48 0.96 0.22 335

M–Fe 0.55 0.74 0.19 269

AS3 0.63 0.00 0.77 3

AS3–Fe 0.66 0.00 0.75 4

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147142

low external surface area. After impregnation with the transitionmetal, the mesopore volume of support M significantly decreases,although the PSD is not shifted. However, the macropore volumeremains practically unaffected. Similar conclusions are obtainedfor the AS3 support and the derived catalysts.

The microporosity was studied by N2 adsorption, as previouslydescribed. Nitrogen adsorption isotherms at �196 8C weremeasured for both M and AS3 supports (Fig. 4a), and also forthe corresponding catalysts. Fig. 4b shows the results for the ironcatalysts, as an example. The results obtained from the analysis ofthese isotherms are compiled in Table 3.

Support AS3 exhibits type I isotherms, which are typical ofmicroporous solids; when the micropores are saturated there is aplateau and there is not hysteresis phenomena (Fig. 4a). On theother hand, M support exhibits isotherms of type IV, typical ofmesoporous materials and there is an important hysteresis cycle(Fig. 4a). Broadly speaking, the amount adsorbed in the P/P0 range

Fig. 3. Pore size distribution of carbon aerogels supports and their Fe-derivatives

catalysts.

Fig. 4. Nitrogen adsorption/desorption isotherms for AS3 and M supports (a) and

corresponding Fe-based catalysts (b).

between 0 and 0.01 corresponds to the filling of micropores up to0.8 nm, between 0.01 and 0.4 are filled the bigger micropores (0.8–2 nm) and at P/P0 between 0.4 and 0.95 adsorption in mesoporesoccurs. Thus, the strong adsorption capacity increase observed athigh P/P0 for M support and the hysteresis cycle are due to thelarge mesoporosity of this sample, as previously pointed out bymercury porosimetry. Analyzing the results compiled in Table 3, itis observed that SBET and Smi present very high and close values in

Table 3Micropore characteristics of the supports and their impregnated catalysts

derivatives

Sample SBET (m2 g�1) Smi (m2 g�1) W0 (cm3 g�1) E0 (kJ mol�1) L0 (nm)

AS3 1032 1000 0.41 24.5 0.8

AS3–Fe 944 916 0.38 24.4 0.8

AS3–Co 788 765 0.31 24.8 0.8

AS3–Ni 914 847 0.36 24.1 0.9

M 611 293 0.24 18.0 1.6

M–Fe 543 236 0.21 17.5 1.8

M–Co 525 234 0.20 17.7 1.7

M–Ni 535 239 0.21 17.6 1.8

Fig. 6. X-ray patterns of AS3–X catalysts (a) and M–X catalysts (b).

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147 143

the case of support AS3, while in the case of sample M thedifference between both parameters is more pronounced (in therange of the Sext value, Table 2), as consequence of the mesoporecontribution to the surface area values. Moreover, the microporesof the AS3-support are also narrower than those of M-support.

In the case of the impregnated catalysts (Fig. 4b), the shape ofthe isotherms is maintained regarding their correspondingsupports, and only a certain adsorption capacity decrease can benoticed, indicating in all cases the porosity blockage by metalparticles. Consequently, surface area decreases with the impreg-nation (Table 3). However, while in the case of the AS3–X catalystsL0 do not changes regarding AS-3 support, indicating that the metalparticles are located at the pore entrance completely blocking thepore, in the case of M–X catalysts, due to the larger size ofmicropores, some metal particles can be located inside these largemicropores.

In such a manner, the dispersion of the metallic phase onthe supports is clearly determined by their porous structure.The largest external surface (mesoporosity) of support M leads toa better dispersion of the metal. As can be seen in the HRTEMimages (Fig. 5), Fe particles are much larger in the AS3–Fe catalyst(15–20 nm vs. 1–2 nm for M–Fe where they cannot be welldistinguished), in spite of being also well distributed. The sameoccurs for nickel or cobalt catalysts (photos not shown).

It is important to remark that the acetates decompositionis complete after the pre-treatments. This was analyzed by TGand the XRD patterns (Fig. 6) do not show any diffraction peak

Fig. 5. HRTEM photos of the Fe-impregnated catalysts: AS3–Fe (a) and M–Fe (b).

corresponding to the precursors. The X-ray patterns furtherillustrate that the best dispersion apparently occurs with themesoporous support (M), where particles are too small to bedetected, as observed by HRTEM. In this catalyst series it is possibleto detect just two broad peaks at 37.28 and 43.28, in the M–Nicatalyst, which corresponds to NiO (JCPDS 4-835), indicating also apoor crystallinity. When the support is microporous (AS3), themetal that seems to be worse dispersed is iron, which present thenarrowest diffraction peaks. Diffraction peaks at 2u = 35.58, 62.68,30.18, 57.08 and 43.18 were assigned to Fe3O4 (JCPDS 88-0866). Incase of the nickel sample, peaks at 37.38, 43.28 and 62.88 are due toNiO, while the peak at 44.68 could indicate the presence of metallicNi (JCPDS-0850). For this catalyst series, i.e. AS3-impregnatedaerogels, metal particles are larger than the micropore size(L0 < 2 nm), and so they are formed on the very small externalsurface area provided by the macropores, thus favouring sintering.It is noteworthy that impregnated aerogels are pre-treated at alower temperature than in the case of carbon-doped catalysts.However, in the latter samples the metal loading is about half andthe organic matrix can avoid sintering in some extent [14,15].

3.2. Catalytic activity

3.2.1. OII degradation using doped aerogels

Firstly, a blank run was performed, using hydrogen peroxidealone, which showed negligible dye degradation (<1% after 15 h).

Doped carbons with about 3.5 wt.% of each metal (Fe, Co, Ni)were used to oxidize the OII dye solution in a slurry batch reactorand the different performances are shown in Fig. 7. It is interestingto realise that mesoporous ACo and ANi catalysts remove thepollutant more quickly than macroporous AFe at the beginning,indicating possibly the contribution of OII adsorption that isfavoured by the larger mesopore volume and surface areas of thesesamples (Table 1). Experiments carried out in the same conditions,but in the absence of H2O2, were carried out to confirm this aspect.Dye removal after reaching equilibrium was always around 15% forACo and ANi, and in the case of the AFe sample it was smaller than5%. In the last case, therefore, OII elimination proceeds mainly viacatalytic degradation, and the higher slope of the curve observedafter the adsorption process, indicates that Fe is the most activemetal tested, in spite that is worse dispersed (Fig. 2) than Co or Ni.

Fig. 7. Evolution of Orange II degradation by doped carbons (T = 30 8C, pH 3,

CH2O2¼ 6 mM, Ccat = 0.1 g L�1).

Fig. 9. Adsorptive (absence of H2O2) and catalytic behaviour of AS3 and M supports

in the Orange II elimination (T = 30 8C, pH 3, Ccat = 0.2 g L�1).

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147144

Nevertheless, ACo is an interesting alternative while Ni seems to beonly slightly active in the Fenton’s process. Although othertransition metals (Cu, Mn) have been tested by several authorsto catalyse the Fenton-like process [21–24], additional experi-ments carried out with Cu-, Mn- or W-doped carbon aerogels usedin this work, revealed already poor catalytic performances [25].

The leaching degree (i.e., the percentage of metal initiallypresent in the catalyst that was lost into the solution) wasmeasured and the results (% of leaching vs. t) are illustrated inFig. 8. Iron is the metal with the lowest leaching level, with a valueof only 0.6% (concentration of 0.02 ppm) after 22 h of reaction. Onthe other hand the cobalt sample has leaching values around 25–30% for the same period, which leads to a very high metalconcentration in solution (�1 ppm) and prevents the catalystreutilization.

3.2.2. OII degradation using impregnated carbon aerogels

Firstly, the supports’ adsorption capacity was studied inexperiments carried out in absence of H2O2, thus in this conditionsthe dye can be only removed by adsorption. The results are shownin Fig. 9. It should be pointed out the huge adsorption capacity of

Fig. 8. Leaching of metals during OII degradation using doped carbons. Operating

conditions are the same as those in Fig. 7.

AS3 support, which is able to completely decolourize the pollutedsolution in 12 h thanks to its microporosity and consequently highsurface area (Table 3). The support M removes, in the same period,around 40% of the initial pollutant concentration (which ispractically the removal capacity at equilibrium). The catalyticperformance of the supports was also tested in the same conditionsas their catalyst derivatives, the results being also shown in Fig. 9.In the case of support M, the amount of pollutant removedincreases regarding the process developed without H2O2, indicat-ing a certain catalytic activity of the carbon surface. On thecontrary, support AS3 becomes less effective when H2O2 is addedto the process, which is probably due to competition between dyeand hydrogen peroxide molecules by the carbon surface. Also, theformation of oxygenated surface groups by H2O2 at the entrance ofthe micropores can difficult the accessibility of the dye molecules.

The elimination rate of OII from the solution strongly increasesin the presence of some transition metals, mainly Fe and Co, whileNi seems to not improve the supports behaviour (Fig. 10). Theseresults are in agreement with Costa et al. [21] who found thatmetals such as Fe, Co or Mn favour the formation of HO� radicals,whereas Ni inhibited the H2O2 reaction. With these metals OIIshould therefore be removed mainly by oxidation. Both Fe and Coare specially actives when supported on the M carbon aerogel,because a higher dispersion degree is reached, in such a mannerthat the degradation of the pollutant is obtained after ca. 2 h ofreaction. This time value is similar to that reported in the literaturefor this and other heterogeneous AOPs [e.g. 12,26,27], although ingeneral much higher catalyst doses are employed in such studiesfor OII degradation (in the range of 1 g/L). In some studies the timeof reaction is slightly smaller [e.g. 28,29], but the reason for that isattributed to the use of radiation in the photo-activated processesemployed (e.g. photo-Fenton). Longer reaction times are neededfor AS3–X catalysts. The behaviour of these catalysts is stronglyinfluenced by the large adsorption capacity of the AS3 support. TheAS3–Fe catalyst is able to remove the pollutants in around 5 h, i.e. itis roughly two times slower than the homologue M–Fe in spite theadsorption contribution, which is clearly related to the worsedispersion degree obtained in this case. Unexpectedly, AS3–Copresents the worse behaviour of the series, which can be related tothe greater surface area decrease observed (Table 3) and the bigleaching degree observed even at short reaction times (see below).

Leaching was also studied for these experiments and theresults (Fig. 11) show that support M usually leads to highervalues of leaching (M–X vs. the corresponding AS3–X catalysts). Inaddition, when iron is used in the impregnation, much lower

Fig. 10. Orange II elimination catalysed by M (a) and AS3 (b) supports and by the carbons impregnated with Fe, Co and Ni (T = 30 8C, pH 3, CH2O2¼ 6 mM, Ccat = 0.2 g L�1).

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147 145

metal concentrations in solution are attained with both supports,whereas Co-based catalysts revealed to be weakly stable, inagreement with the results obtained using doped aerogels (Fig. 8).The use of this metal led to leaching degrees of around 35% in onesingle run.

Thus, Fe yields more reactive (Fig. 10) and simultaneously morestable (Fig. 11) catalysts. However, although Fe nanoparticlesformed into mesopores lead to the more promising catalyst (M–Fesample), it also implies higher values of leaching than whenincorporated in the microporous support (AS3–Fe), forming largerand less active particles (it is remarkable the low leaching values ofonly 0.2% for the AS3–Fe sample after 7.5 h). These trends put intoevidence the need for finding the most appropriate pore size(tunning) for the supports in order to get a better balance betweengood dispersion, high activity and good stability.

3.2.3. OII degradation using doped aerogels vs. impregnated aerogels

Finally it is important to compare the catalytic behaviour ofdoped and impregnated catalysts. For that, experiments with thesame metal amount were carried out, using in this case onlyiron catalysts, those that revealed better catalytic performancesand simultaneously better stabilities. Thus, and because sampleshave different metal loads, the amount of active phase (Fe)was homogenized decreasing the amount of AS3-Fe and M-Fe

Fig. 11. Leaching of Fe, Co and Ni from both carbon supports during OII degradation.

Operating conditions are the same as those in Fig. 10.

catalysts in a proportion of 7.0/3.3. The results obtained areshown in Fig. 12. The impregnation revealed to be more efficientthan the doping technique. In the latter, the transition metalbecomes part of the aerogels structure and some particles can besurrounded by the carbon matrix, becoming less accessible to theH2O2 molecules for radicals formation (Eq. (1)). This behaviourwas also previously observed when comparing the activity of Pt-doped and Pt-impregnated catalyst used in the VOC’s combus-tion reaction [16].

Regarding leaching, Fig. 13 shows that in spite of having thebest catalytic results, catalysts supported on M–carbon aerogelpresent the higher leaching values. Both macroporous AFeand AS3–Fe catalysts present a similar behaviour in terms ofstability. Using doped catalysts, low leaching values were inadvance expected since metal particles are part of the supportmatrix, being in principle more difficult to remove. Experimentaldata corroborate this. It is finally worth highlighting the excellent(very low) leaching degree of the AS3-Fe sample, which issimultaneously reasonably active (Fig. 12). Thus, to improve thecatalytic activity of Fe it should be impregnated on supports withlarge external surface area (mesopores or large micropores) toallow the development of small nanoparticles. However, thisseems to favour also the metal leaching. Doping could avoid tosome extent the metal sintering and leaching, but the carbon

Fig. 12. Orange II degradation catalysed by Fe when included in the supports by the

doping or by the impregnation techniques (T = 30 8C, pH 3, CH2O2¼ 6 mM,

Cmetal = 3.3 mg/L).

Fig. 13. Iron leaching during Orange II degradation when doped or impregnated in

different supports. Operating conditions are the same as those in Fig. 12.

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147146

matrix encapsulates some metal particles, avoiding the contactwith the reactants.

3.2.4. M–Fe reutilization for OII degradation

With the most promising catalyst, M–Fe, some experimentshave been conducted to evaluate its catalytic stability inconsecutive reaction cycles. For that, the solid was recovered byfiltration after each run. Fig. 14 shows that its performance issignificantly affected, with half-lives of 0.73, 1.21 and 2.14 h for the1st, 2nd and 3rd cycle, respectively. The iron leaching is not theonly reason for such activity decay, as can be deduced from thestrong decrease in the corresponding apparent kinetic constants(from 81 to 48 and then to 32 dm3/(gFe h) in the three cycles),obtained from a pseudo-first order fit [12] and taking into accountthe initial amount of iron in each cycle. Therefore, catalystdeactivation is obtained, which reactivation will be the aim offuture work. This affected also the mineralization degree attained(evaluated in terms of TOC reduction), which amounted to 74%after the 1st cycle and to 44% and 43% in the subsequent ones.

Fig. 14. Orange II degradation catalysed by M–Fe in 3 consecutive reaction cycles

(T = 30 8C, pH 3, CH2O2¼ 6 mM, Ccat = 0.2 g/L).

4. Conclusions

This work dealt with the azo-dye Orange II degradation using aFenton-like process, the catalysts consisting in doped or impreg-nated carbon aerogels with different transition metals (Fe, Co, Ni).The catalytic performance mainly depends on the metal present:catalysts based on Co and particularly on Fe are highly active, whileNi is almost inactive or only slightly active. This occurs for bothdoped and supported catalysts.

In the case of doped catalysts, ACo and ANi are mesoporousmaterials with significantly high surface areas, which somewhatinduces the adsorption process. ACo present a very goodperformance, based on both adsorption and catalytic processes.However, this sample undergoes significant leaching duringreaction (above 25%), which limits its application. The AFe aerogelis a macroporous material with lower surface area than ACo or ANiand, therefore the pollutant is mainly removed by catalyticoxidation. This sample has the lower leaching degree, only 0.6%after 22 h, which makes it very appropriate for the dye removal.Actually, these concentrations (0.02 ppm) are within EU legisla-tion, also allowing the catalysts reutilization. Such low leachinglevels are remarkable for heterogeneous catalysts employed inFenton-like processes.

Two carbon aerogels were used as catalysts supports, one ofthem (sample M) highly mesoporous, and the other one (sampleAS3), highly microporous. The high surface area of the laterstrongly induces the OII adsorption process. The characteristicsand performance of impregnated catalysts are determined by thetransition metal used and by the porous nature of the supports. Themesoporous carbon aerogel favours the formation of small metalnanoparticles, in comparison with the microporous one, in spite ofits smaller surface area. Metal particles cannot be formed insidemicropores of AS3, and thus sintering is favoured on the smallexternal surface area that provides the large macropore volume,blocking some micropores. The best dispersion reached on supportM enhances the catalytic behaviour, but also the metal leaching.Nevertheless, this provided the most promising catalyst for thisprocess (M–Fe), with an almost complete colour removal in just 2–3 h of reaction and leaching values around 1–2%, in soft operatingconditions (T = 30 8C, pH 3, CH2O2

¼ 6 mM, Ccat = 0.2 g L�1). It isnoteworthy the low loads of catalysts employed, much belowthose usually found in the literature (typically in the order of 1 g/L).However, the catalytic performance of this sample is not stable inconsecutive reaction cycles, possibly due to catalyst deactivationrather than to metal loss.

To improve the catalytic activity of this type of heterogeneouscatalysts, transition metals should be impregnated on supportswith large external surface area (mesopores or large micropores) topermit the development of small nanoparticles and simulta-neously decrease the mass transfer resistance for the reactants toaccess the carbon surface. However, this seems to favour also themetal leaching. Doping could avoid in some extent the metalsintering and leaching, but the carbon matrix encapsulates somemetal particles, avoiding the contact with reactant and thusdiminishing the catalytic performance. This can however beovercome employing more drastic conditions, or a photo-catalyticprocess (e.g., photo-Fenton).

Accordingly, better results require finding a better tuning of thecatalyst’ properties, particularly in terms of pore size distribution,in order to optimise the trade-off between reactivity and stability.

Acknowledgments

The authors deeply acknowledge Prof. Fernao Magalhaes(LEPAE-FEUP) for the careful revision of the text. The researchers

F. Duarte et al. / Applied Catalysis B: Environmental 85 (2009) 139–147 147

from University of Granada acknowledge the Spanish MEC and theJunta de Andalucıa for the projects CTQ-2007-61324 and RNM 547,respectively.

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