DOI: 10.1002/cssc.200800246

Synergism of Activated Carbon and Undoped andNitrogen-doped TiO2 in the Photocatalytic Degradation ofthe Chemical Warfare Agents Soman, VX, and YperiteBogdan Cojocaru,[a] Stefan Neatu,[a] Vasile I. P�rvulescu,*[a] Vasile Somoghi,[b]

Nicoleta Petrea,[b] Gabriel Epure,[b] Mercedes Alvaro,[c] and Hermenegildo Garcia*[c]

Introduction

During the last years photocatalysis has attracted a renewedinterest owing to its potential applications in fields such aswater and air purification,[1–3] and also for the preparation oforganic compounds that are difficult to obtain by alternativeprocedures.[4–7]

Upon illumination of a semiconductor with photons havingenergy higher than or equal to the semiconductor band gapenergy, electron–hole pairs are generated in the bulk particle.Subsequent migration leads these charges to the external sur-face of the particle. As any catalytic process, the overall photo-catalytic process consists of a series of independent steps:(i) adsorption of reactants from the fluid phase onto the solidsurface, (ii) reaction in the adsorbed phase, and (iii) desorptionof reaction products from the surface into the fluid phase. Ma-terial modification to increase substrate adsorption in the vicin-ity of the photocatalytic sites can lead to a photocatalyst withenhanced efficiency.

The most widely used semiconductor is titanium dioxide,which has proven to be efficient for the degradation of a widerange of pollutants in water or air. This semiconductor featureshigh photostability and oxidative power, low costs, and is non-toxic.[8, 9] However, a major drawback is the fact that TiO2 doesnot appreciably absorb wavelengths above 370 nm. Becausethis semiconductor has a band gap at about 3.2 eV,[9] excitationwith visible light is not possible and only about 5 % of solarlight can be absorbed. Therefore, an improvement of its effi-ciency in the visible range of the spectrum is desirable toexpand the applicability of this semiconductor as photocata-lyst. In this respect, doping the semiconductor with metal ornonmetal (such as nitrogen or carbon) ions represents a viablechoice to make TiO2 sensitive to visible light.[10–12] Among non-metal ions, nitrogen has already been widely reported as agood dopant for tuning TiO2 visible absorption.[13–17] N-TiO2

photocatalysts can be prepared by many procedures, such assol–gel processing,[18] hydrothermal treatment,[19] pulsed laser

deposition,[20] sputtering,[21–23] atmospheric pressure plasma-en-hanced nanoparticle synthesis,[24] and others.

Finely dispersed TiO2 suspended in water is often difficult tobe recovered. TiO2 particles have also low adsorption abilities,which conduct to a usually low photodecomposition rate. As asolution to these drawbacks, the enhancement of the photo-degradation rate was achieved by supporting TiO2 on adsorb-ents that, besides allowing an easy separation of the photoca-talyst, could also cooperate to the success of the photocatalyt-ic reaction by concentrating the target substances aroundactive TiO2 centers.[25–29]

Carbons could be a very convenient support with a high ad-sorption capacity. Activated carbon (AC) is a large-surface-areamaterial that shows adsorption preference for covalent mole-cules that are slightly polar and nonionic (which is the case forthe majority of organic compounds). Therefore, it is consideredan almost universal adsorbent of organic molecules. Until now,activated carbon is one of the most common adsorbents forremoving volatile organic compounds or water pollutants, butthe fact that it is a nondestructive process represents a majorlimitation. However, the association of AC with TiO2 can sur-

Efficient photocatalytic decomposition of chemical warfareagents is a process that may find application in emergency sit-uations or for the controlled destruction of chemical warfarestockpiles. A series of heterogeneous photocatalysts compris-ing TiO2–activated carbon or N-TiO2–activated carbon compo-sites exhibit excellent photocatalytic activity to effect the com-

plete decomposition of yperite, soman, and VX in high concen-trations. The remarkable photocatalytic activity arises from thesynergism between adsorption on active carbon and photoac-tivity by titania. Nitridation makes the composite also activeunder visible-light irradiation.

[a] B. Cojocaru, S. Neatu, Prof. V. I. P�rvulescuDepartment of Chemical Technology and CatalysisUniversity of Bucharest, Faculty of ChemistryBd. Regina Elisabeta 4–12, Bucharest, 030018 (Romania)E-mail : v_parvulescu@yahoo.com

[b] Dr. V. Somoghi, N. Petrea, G. EpureChemical Centre of Defence NBC and EcologySos. Oltenitei 225, Bucharest, 041309 (Romania)

[c] Dr. M. Alvaro, Prof. H. GarciaInstituto de Tecnologia Quimica CSIC-UPV andDepartamento de QuimicaUniversidad Politecnica de ValenciaAv. De los Naranjos s/n, Valencia, 46022 (Spain)Fax: (+ 34) 963 877809E-mail : hgarcia@qim.upv.es

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pass this problem and provide an effective heterogeneous cat-alyst. Precedents describing the combination of TiO2 as photo-catalyst and active carbon can be found in the literature.[25–29]

The preparation of TiO2–AC systems can be carried out by dif-ferent methods: deposition of TiO2 on activated carbon byusing an ionized cluster beam,[30] carbonization of a mixture ofcoal and TiO2,[31] impregnation of the activated carbon withTiO2,[26, 32] microwave-assisted impregnation of the activatedcarbon with TiO2,[33] impregnation of the activated carbon withTiO2 in supercritical solvents,[34–36] and others.

In this study we combined the effect of nitridation of titaniawith the positive cooperation of an AC support, even at lowpercentages, to photocatalytic degradation. N-doping was per-formed by thermal treatment of titania nanoparticles withurea. The photocatalysts (TiO2–AC and N-TiO2–AC) were pre-pared using an original approach based on the adsorption ofpreformed (undoped or nitrogen-doped) titania nanoparticleswith monomodal size distribution on large-surface-area activat-ed carbon.

The TiO2–AC and N-TiO2–AC composites were applied to thephotocatalytic decomposition of real chemical warfare agents.Decomposition of these agents is of great interest as responseto emergency situations and also as an alternative technologyfor the destruction of chemical warfare agent stockpiles. Nerveagents such as GD (soman) and VX are organophosphorousesters similar to insecticides. They are all liquids with differentvolatilities, depending on their boiling points at atmosphericpressure; the lowest boiling point being as low as 60 8C. De-struction of these compounds is complicated by the presenceof heteroatoms that lead to phosphorous compounds, whichare persistent and can easily generate additional wastes. Themain difficulty in their degradation is the breaking of the P�Cbond. Yperite (sulfur mustard; bis-2-chloroethyl sulfide) is apowerful vesicant agent and its photochemical decompositionwill also be useful under some circumstances. Destructionmethods for chemical warfare agents can be noncatalytic (e.g.biodegradation, combustion, pyrolysis, hydrolysis) or catalytic(e.g. oxidation, dehydrohalogenation, enzymatic hydrolysis,methanolysis).[37–40] Photocatalytic oxidation appears to be aviable alternative for chemical warfare agents decompositionbecause it generates less waste and can be performed withoutenergy consumption or addition of any corrosive chemical.However, reported data using real warfare compounds is veryscarce and there is an interest in establishing suitable photoca-talysts for the complete detoxification of chemical warfareagents. Most of the previous studies on the use of photocatal-ysis for decontamination have employed model molecules asmethylphosphonic acid for nerve agents or diethylsulfide forsulfur mustard.[41–49] In addition, most of the reports on modelsare focused on contaminated solutions rather than dry surfacesor powders. Thus, there is a paucity of data demonstrating theapplicability of photocatalysis for decontamination of realchemical weapons. Considering these precedents, the most im-portant contribution of our report is to provide data on thephotocatalytic degradation activity of real chemical warfareagents. The presented data clearly shows that we have ob-tained efficient photocatalysts for the complete detoxification

of a wide range of structurally diverse chemical warfare agents.Therefore, our report supports the applicability of photocataly-sis for detoxification of vesicant and nerve agents.

Results and Discussion

Photocatalyst Characterization

Anatase-phase titania nanoparticles, typically used for thepreparation of solar cells, with and without N-doping were ad-sorbed onto activated carbon. Three different weight percen-tages of activated carbon were used: 1, 5, and 25 wt %. Ele-mental analysis of N-TiO2 revealed a nitrogen content of0.1 wt % . This nitrogen content was common for all the nitro-gen-doped samples, which differ on the percentage of AC.

Textural data of the photocatalysts are summarized inTable 1. The photocatalysts doped with nitrogen exhibit ahigher surface area compared with the undoped correspond-

ing catalysts. This is an indication that the nitridation processby thermal treatment of titania with urea has produced signifi-cant changes in the surface area and the pore volume of thetitania. Both the increase of the surface area and the pore sizecan be attributed to the decomposition of the stabilizingligand present in the commercial Ti-Nanoxide T20 sample usedin this study and the chemical attack of ammonia evolvedfrom urea, eroding the titania surface. In the two series of un-doped and nitrogen-doped titania photocatalysts, the materi-als having the highest surface area were the ones having thelargest percentage of AC. The variation of surface area with ACcontent indicates that, as could be expected, the AC contentof the photocatalyst plays a major role in controlling the sur-face area and also the adsorption properties of the composite.

A confirmation of the nitridation effect in the textural prop-erties of the photocatalyst is also provided by the pore sizedistribution (Figure 1). TiO2–AC photocatalysts showed a mon-omodal pore size distribution, in good agreement with the tex-tural properties of the Ti-Nanoxide T20 precursor. After the ni-tridation, the pore size distribution became multimodal andthe main peak shifted to larger pore sizes (Figure 1). For exam-ple, comparison of the pore diameters of undoped andN-TiO2–5 wt % AC photocatalysts shows a larger average poresize after nitrogen doping and also the distribution is signifi-

Table 1. Textural data of prepared photocatalysts.

Photocatalyst ABET [m2 g�1][a] VBJH [cm3 g�1][b] DBJH [�][c]

TiO2–1 wt % AC 45 0.17 109TiO2–5 wt % AC 37 0.15 119TiO2–25 wt % AC 132 0.45 110N-TiO2–1 wt % AC 137 0.54 172N-TiO2–5 wt % AC 108 0.31 150N-TiO2–25 wt % AC 221 0.30 175

[a] Specific area determined by the Brunauer–Emmett–Teller method.[b] Total pore volume determined by Barrett–Joyner–Halenda (BJH)method. [c] Average pore size determined by BJH method

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cantly broader, with contribution of micropores for the nitro-gen-doped sample.

IR spectra of the AC-supported titania (Figure 2) present theexpected combination of the absorption bands characteristicof each of the two components. For example, for the TiO2–ACseries the weak band assigned to Ti�O�Ti bond is present atvalues around 1298 cm�1, while the strongest band assignedto Ti�OH bond is located around 3670 cm�1. On the otherhand, the complex structure of the AC presents bands as-signed to many different types of bonds such as C=C (around1650 cm�1), C=O (around 1725 cm�1). The nitridation of titaniais accompanied by the disappearance of bands at around2938 cm�1 assigned to CH3, CH2, and CH stretching vibrationsand, correspondingly, the disappearance of the bands in the1250–1500 cm�1 region assigned to CH2 and CH3 deformation.These bands, characteristic of organic aliphatic groups, origi-nate from the stabilizing ligands present in the commercial Ti-Nanoxide T20 sample to avoid particle growth. Calcination

during nitrogen doping causes the disappearance of these or-ganic ligands. At the same time, bands corresponding to N�H,observed around 1500–1550 cm�1, appear as consequence ofthe nitridation.

Diffuse reflectance (DR) UV-vis spectra of AC and of pureand nitrogen-doped titania materials are presented in Figure 3.

Figure 1. Barrett–Joyner–Halenda pore diameter distribution obtained from the desorption branch of the isothermal N2 adsorption plot for a) TiO2–1 wt % AC,b) N-TiO2–1 wt % AC, c) TiO2–5 wt % AC, d) N-TiO2–5 wt % AC, e) TiO2–25 wt % AC, and f) N-TiO2–25 wt % AC (P.V. = pore volume; P.D. = pore diameter).

Figure 2. DRIFT spectra of prepared photocatalysts in A) the 3750–2500 cm�1 region and B) the 2000–700 cm�1 region. a) TiO2–1 wt % AC,b) TiO2–5 wt % AC, c) TiO2–25 wt % AC, d) N-TiO2–1 wt % AC, e) N-TiO2–5 wt %AC, and f) N-TiO2–25 wt % AC.

Figure 3. DR UV-vis spectra of A) AC and undoped TiO2 and B) AC and N-TiO2.

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Photocatalytic Degradation of Chemical Warfare Agents

The pure titania sample shows the typical absorption of ana-tase with an intense transition in the UV region of the spec-trum which is due to electron promotion from the valenceband to the conduction band (TiO2 trace in Figure 3 A).

Figure 3 B shows the DR UV-vis spectra of N-TiO2 and of theAC material. The spectrum of N-TiO2 clearly shows the charac-teristic absorption of TiO2 in the UV region and a new absorp-tion shoulder at 410–520 nm.[50] The presence of the secondabsorption in the visible-light region correlates very well withthe yellow color of the nitridated material and is in agreementwith the expected effect of thermal urea treatment. The valueof 410 nm (3.0 eV) represents a narrower band gap with re-spect to that of the pure anatase TiO2 of 380 nm (3.2 eV). Thisnarrower band gap is the expected result of nitrogen dopingas reported in the chemical literature.[50, 51]

The presence of AC in the materials is expressed by the con-tinuous band from 450–800 nm with increasing absorptivity to-wards the wavelengths characteristic of black solids. It shouldbe noted that while TiO2 powder is white and N-TiO2 is lightyellow, addition of AC is responsible for the grey color devel-oped by the solids; the intensity of the grey color dependingon the percentage of AC. However, as commented earlier, theabsorption band onset of the photoactive TiO2 component re-mains uncorrelated with the amount of AC, as it should beconsidering that they are independent phases.

Photocatalytic Tests

The first chemical warfare agent that was investigated was GD(soman). Prior to determining the photocatalytic activity, ad-sorption experiments were carried out to differentiate betweenphysisorption and real photocatalytic degradation. Thus, solu-tions of this nerve agent were put in contact with the contami-nant and left for 15 min in the dark. Using a 0.077 wt % di-chloromethane solution, corresponding to 5 mg of GD for20 mg of photocatalyst, analysis of the liquid phase after15 min contact time with the photocatalyst without light expo-sure revealed the disappearance of GD with a maximum ofabout 36 and 80 % for the TiO2–5 wt % AC and N-TiO2–5 wt %AC, respectively. This decrease in GD actually corresponded tophysisorption of the nerve agent onto the photocatalyst poresand not to a real photodecomposition process. This fact wasconfirmed by the complete recovery of the amount of GD ad-sorbed on the catalysts by percolation with CH2Cl2. The per-centages of GD physisorption actually paralleled the surfacearea of these materials (see Table 1). It is important to notethat GD that is adsorbed from solution onto the solid is com-pletely desorbed by percolation with CH2Cl2.

Upon UV irradiation of the samples containing GD, the pho-tocatalytic properties of the prepared materials dominate theadsorption properties. When an initial solution of 0.077 % isused, after the initial fast adsorption of GD a subsequentslower photocatalytic process takes place, leading to a realchemical degradation of GD. Figures 4 and 5 show the resultsof photodegradation of GD in the presence of the investigatedphotocatalysts after UV irradiation exposure times of 30 minand 2 h.

The results obtained in the photocatalytic degradation ofGD deserve two special comments. Firstly, nitrogen-doped tita-nia samples are more efficient than the analogous undopedsamples. For the titania photocatalysts doped with nitrogen analmost complete degradation was achieved after only 30 minof irradiation (Figure 5). Undoped catalysts lead to lower de-contamination rates, but still over 90 % GD degradation couldbe obtained at 120 min irradiation for photocatalysts with thehighest AC content (Figure 4). Secondly, there is a synergismbetween titania and AC. This important result is clearly demon-strated by the fact that the undoped and nitrogen-doped tita-nia without AC are incapable of effecting photocatalytic degra-dation to a measurable extent. Furthermore, for the samples ofundoped titania that are less active and at short irradiationtimes (30 min) a clear relationship between the amount of ACand the overall photocatalytic activity of the composite is ob-served (Figure 4). For highly active photocatalysts (N-TiO2

series) or at long exposure times, no differences are observedand photodegradation was complete in some cases. For N-TiO2

extraction of the samples under these conditions did not showany residual nondecomposed GD. Importantly, a control experi-ment under the same conditions using AC revealed no degra-dation; the amount of recovered GD being unaltered after 2 hexposure. Thus, the above results show that both components,

Figure 4. Decontamination of GD (5 mg) deposited onto TiO2–AC photocata-lysts (20 mg) upon different UV irradiation exposure times.

Figure 5. Decontamination of GD (5 mg) deposited onto N-TiO2–AC photo-catalysts (20 mg) upon different UV irradiation exposure times.

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photocatalyst and AC, cooperate in the photocatalytic degra-dation of GD and that there is a dramatic and remarkable syn-ergism with respect to the activity of the individual compo-nents.

As expected, increasing the concentration of GD to0.77 wt % in the initial dichloromethane solution, correspond-ing to 50 mg of GD per 20 mg of photocatalyst led to smallerdecontamination yields for the same irradiation times (Fig-ures 6 and 7). As for the 0.077 wt % solution, there was an

almost linear relationship between the AC loadings and GD de-contamination efficiency and this influence of AC content wasobserved for the TiO2 and N-TiO2 series at short and long irradi-ation times. Again, doping the photocatalysts with nitrogenled to an increased conversion compared to the undoped cata-lysts (Figure 7). Overall, the results shown in Figures 6 and 7demonstrate that it could be possible to effect the completedegradation of large amounts of GD; irradiation time being afunction of the GD concentration on the photocatalysts.

The turnover frequency calculated for the N-TiO2–25 wt % ACsample in the experiments carried out with GD in visible lightwas ca. 0.55 min�1; almost three times smaller than that mea-sured under UV irradiation (ca. 1.46 min�1). These values indi-cate that this material could be applicable to field chemicalwarfare agent decontamination by sunlight.

The above results obtained with GD show that doping tita-nia with nitrogen leads to an enhancement of the photocata-lytic activity of titania under UV irradiation. Because whenusing UV light there should not be difference in light absorp-tion by the titania photocatalysts, the observed influence of ni-trogen doping on the photocatalytic activity could be due tothe presence of ligand in Ti-Nanoxide T20 and/or to an en-hanced surface area and porosity volume introduced in thematerial as consequence of the nitridation (Table 1). Surfacearea and porosity should have a large influence on the interfa-cial electron transfer processes and on the mobility and life-time of the charge carrier. There are precedents describing theinfluence of TiO2 surface modification on the degradation rateand on the effect of the porosity increase in nitrogen-dopingprocesses on the photocatalytic activity of TiO2 photocata-lysts.[52]

Photocatalytic decontamination of GD was also performedunder visible light irradiation. As expected in view of the litera-ture data, the undoped TiO2 series was incapable of effectingGD photodegradation under these illumination conditions. Incontrast, under visible light irradiation N-TiO2–AC photocata-lysts exhibited a remarkable activity, although the yield of deg-radation decreased by 10–15 % compared with the results ob-tained under UV irradiation at the same irradiation times(Figure 8). These results can be interpreted based on current

knowledge of titania doping. Bulk doping of TiO2 by nonmetalscan introduce interior states in the band gap of TiO2, changinglight absorption onset. As can be seen from Figure 3, the onsetof nitrogen-doped titania samples is shifted from 380 to520 nm. This red-shift caused by nitrogen-doping should be re-sponsible of the remarkable difference in photocatalytic activi-ty towards GD degradation between the undoped and nitro-gen-doped samples. The synergism of AC cooperating to thephotocatalytic degradation can also be seen in the results dis-played in Figure 8, where N-TiO2 is again unable to promoteGD degradation under the described conditions.

The distribution of the reaction products for the N-TiO2–ACphotocatalytic degradation of GD remained unchanged underirradiation by UV or visible light. The compounds that havebeen identified by GCMS from the material present on the

Figure 6. Decontamination of 50 mg of GD using 20 mg of TiO2–AC catalyst.

Figure 7. Decontamination of 50 mg of GD using 20 mg of N-TiO2–AC cata-lyst.

Figure 8. Decontamination of GD (50 mg) using the N-TiO2–AC photocata-lysts (20 mg) under visible light irradiation.

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photocatalyst and in the gas phase are those shown inScheme 1 with the labels 1–5. Obviously, the percentage ofthe intermediate byproducts depended on the irradiation time,indicating that they undergo secondary degradation processes.Based on the compounds that have been identified and ontheir temporal evolution, a reaction pathway for the GD photo-catalytic destruction is proposed in Scheme 1. In a first step, pi-nacolyl methylphosphonate 1 is formed with elimination of hy-drofluoric acid. This is not a “dark” process because adsorptionand subsequent desorption allows to recover unchanged GD,as commented earlier, although it can be assisted thermally.Methylphosphonate 1 can suffer an oxidative breakdown withformation of 3,3-dimethylbutan-2-ol 2 and methylphosphonicacid 4, or it can bind with a 3,3-dimethylbutan-2-ol moleculeto form O-pinacolyl, O’-pinacolyl methylphosphonate 3. This O-pinacolyl,O’-pinacolyl methylphosphonate decomposes to formtwo molecules of 3,3-dimethylbutan-2-ol and one molecule ofmethylphosphonic acid. The methylphosphonic acid will formphosphoric acid 5. 3,3-dimethylbutan-2-ol follows a photo-oxi-dative pathway to mineralize inCO2 and H2O. Although the pina-colyl phosphate 6 has not beenidentified among the photo-products, the formation of thephosphoric acid in a largerextent than that expected frompinacolyl methyl phosphonatesuggests the intermediacy ofthis compound as indicated inScheme 1.

After the results presented forGD, we selected two representa-tive photocatalysts, namelyTiO2–5 wt % AC andN-TiO2–25 wt % AC, and testedthem for the destruction ofnerve agent VX under UV irradia-tion. The results are given inTable 2. As can be seen there,the relative activity of undoped

and nitrogen-doped titania pho-tocatalysts follows the trendfound for GD.

GCMS analysis of the materialadsorbed onto the N-TiO2–25 wt % AC after irradiation al-lowed detecting the compoundsshown in Scheme 2. Based onthis product distribution and itstemporal evolution, the reactionpathway for VX degradationshown in Scheme 2 can be pro-posed.

TiO2–5wt % AC and N-TiO2–25 wt % AC photocatalysts werealso tested in the decontamina-tion of 50 mg of vesicant HD

(yperite) agent on 20 mg of photocatalyst under UV light irra-diation. In both cases, the occurrence of a dark reaction to a

Scheme 1. Proposed reaction pathway for the photodecomposition of GD.

Table 2. Results of the photocatalytic degradation of chemical warfareagents after 120 min irradiation with UV light in the presence of AC con-taining catalysts.

Photocatalyst Amount ofWarfare Agent [mg]

DegradationPercentage [%]

VX nerve agentsTiO2–5 wt % AC 5 98TiO2-5 wt % AC 50 46N-TiO2–25 wt % AC 5 >99N-TiO2–25 wt % AC 50 95HD vesicant agentTiO2–5 wt % AC[a] 50 9.5TiO2–5 wt % AC 50 >99N-TiO2–25 wt % AC[a] 50 8.4N-TiO2–25 wt % AC 50 >99

[a] No irradiation.

Scheme 2. Proposed reaction pathway for the photodecomposition of VX.

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V. I. P�rvulescu, H. Garcia et al.

minor level (<10 %) due to chlorine substitution and dehydro-chlorination was observed in the adsorption/desorption con-trols. The results are also given in Table 2. As can be seen, after2 h HD was completely decomposed on both materials.

It has been proposed that the decomposition of HD over ti-tania–silica systems[40] follows two main reaction pathways(Scheme 3), both having in common the initial attack ofoxygen on this chemical and differing in the occurrence of asubsequent C�S bond splitting (pathway A in Scheme 3) or Satom oxidation (pathway B in Scheme 3).

In the first route, a stable compound, bis-(2-chloroethyl)di-sulfide is formed by coupling of two [ClCH2CH2S]· radicals. bis-(2-Chloroethyl)disulfide is a reaction byproduct and remains onthe catalyst surface. Finally, after 2 h, photocatalytic degrada-tion results in inorganic oxides, H2O, CO2, and SO2.

The oxidation of sulfur atom is the second route of HD pho-tocatalytic decomposition. The products of this oxidation path-way, sulfoxide and sulfone, remain adsorbed on the surface ofthe catalyst, where they could be further oxidized leading tothe formation of inorganic compounds.

DRIFT spectroscopy of the tested N-TiO2–25 wt % AC photo-catalyst clearly showed that some photoproducts remain ab-sorbed onto the surface (Figure 9). For example, after testingN-TiO2–25 wt % AC with GD and VX, bands characteristic of aP=O bond can be observed in the 1100–1200 cm�1 region, andpossibly a P�CH3 bond in the 830–960 cm�1 region. In thesame manner the C�S bands (in the 1050–1200 cm�1 region)can be observed for tested photocatalysts with HD. The impor-tant decrease of the H-bonded O�H band can be consideredas a sign that the decontamination is a photocatalytic process.The oxidative photocatalytic activity of TiO2 particles is attribut-ed mostly to the reactive oxygen species ·OH as well as H2O2

generated by chemisorbed OH� or H2O,[53] therefore, the disap-pearance of the surface hydroxyl groups from the used photo-catalysts as observed by IR spectroscopy can be interpreted asevidence that these groups have undergone transformationinto reactive oxygen species (mainly ·OH radicals) and eventu-ally have been incorporated into the molecule undergoingphotodegradation.

At the same time, the relative intensity of the OH groups onthe surface of the fresh titania samples before their use can becorrelated with the photocatalytic activity of these samples,there being an optimum of surface OH for an optimum photo-catalytic activity. Thus, the highest photocatalytic activity ofthe series corresponds to the N-TiO2–AC samples that weretreated at 400 8C in the doping step of the preparation, andthis activity can be correlated with its medium intensity of thesurface OH groups in the IR spectrum. The quantity of surfaceOH groups and crystallinity of TiO2 induced by thermal treat-ment surely must have an influence on the photocatalytic ac-tivity, with 450 8C being an optimal calcination temperature foranatase TiO2.[54] Excessive quantities of water species on theTiO2 surface have a detrimental effect on photocatalytic reac-tions, but small quantities are essential for a sustainable reac-tion rate.[55, 56]

Scheme 3. Proposed reaction pathway for the photodecomposition of HD.

Figure 9. DRIFT spectra of N-TiO2–25 wt % AC photocatalyst a) fresh, andafter 20 mg of the photocatalyst being tested for 120 min under UV irradia-tion of material containing 50 mg of b) GD, c) VX, and d) HD.

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Photocatalytic Degradation of Chemical Warfare Agents

Finally, one point that deserves special comment is theorigin of the synergism between the titania photocatalysts andthe active carbon. This synergism has been confirmed by com-paring the results of photocatalysts without AC and those inwhich the percentage of AC has been gradually increased.Based on the known adsorption ability of AC, we propose thatthe chemical warfare agents are initially adsorbed onto AC. Apart of the reactants adsorbed on AC have interfacial contactwith titania nanoparticles and will be degraded by electron,holes, and reactive oxygen species present on the titania sur-face. However, because the titania nanoparticles constituted aphase that was independent from AC, we propose that anoth-er fraction of substrates adsorbed onto AC and not havingcontact with titania nanoparticles undergo degradation byattack of reactive oxygen species that are generated on thesurface of the titania photocatalyst but migrate on the carbonsurface. It has been reported that these reactive oxygen spe-cies can diffuse over sub-millimeter distances from the surfaceof titania to the vapor phase.[57–64] If this is the case then thesereactive oxygen species could reach the target compound lo-cated on the surface of the carbon, where most of the sub-strate remains adsorbed. The combination of adsorption oncarbon and generation of aggressive oxygen species on thephotocatalyst without deactivation will be the most likelyreason for this remarkable synergism. In this manner the for-mation of byproducts on AC will not poison the photogenera-tion of hydroxyl radicals and other species on the photocata-lyst surface. Figure 10 summarizes our proposal to account forthe cooperation of the two components, (N)-TiO2 and AC,boosting the efficiency of the photocatalytic degradation.

It has to be mentioned that this synergism between N-TiO2

and AC arising from the large surface area and adsorption ca-pacity of AC has not been observed using other large surfacearea supports. Thus, variouscombinations of TiO2 withporous supports such as sili-ceous MCM-41 and MCM-48,and different zeolites (Beta, ZSM-5, silicalite) have also been inves-tigated without any synergismin the catalytic behavior.[66] All ofthese materials acted as simple

diluents for titania, leading to poorer results when comparedwith those obtained with pure titania or with those reportedherein.

Conclusions

The present work provides data with real chemical warfareagents, showing the efficiency of a series of photocatalysts forthe complete decontamination of these extremely toxic com-pounds. It has been found that while titania and nitrogen-doped titania are inactive to effect degradation of these chem-icals to any measurable extent under the conditions tested,the presence of active carbon produces a remarkable syner-gism that boosts the photocatalyst efficiency. In the presenceof active carbon and using nitrogen-doped titania it is possibleto effect complete decontamination of high chemical warfareagent loadings even under visible light irradiation. It is pro-posed that this synergism arises from cooperation of activecarbon adsorbing the chemical compound and the titania pho-tocatalyst generating oxygen reactive species that diffuse andreact with the molecules located on the active carbon.

Experimental Section

Materials

Ti-nanoxide T20 was purchased from Solaronix as an 11 wt % aque-ous suspension of nanocrystalline titanium dioxide consisting ofanatase particles with a diameter of 20 nm. The surface area (asgiven by the manufacturer) was ca. 60 m2 g�1. Activated carbonwas purchased from Aldrich (Norit Darco KB-B) and exhibited a sur-face area of ca. 1600 m2 g�1. Urea was 98 % reagent grade, suppliedby Aldrich. The test molecules GD, VX, and HD (Figure 11) weresupplied and manipulated by the N.B.C. Defence and Ecology Sci-entific Research Centre of the Romanian Ministry of Defence. Itshould be reminded that the production and use of these chemicalwarfare agents is strictly regulated under an international conven-tion for the prohibition of this type of weapons.[64]

Photocatalyst Preparation

The doping of anatase titania with nitrogen was carried out byusing a saturated solution of urea. The solution was added to theTiO2 suspension in a 1:1 weight ratio. The mixture was sonicatedfor 10 min for better homogenization and then was gently dried invacuum (40 8C) using a rotary evaporator. After that, the solid wasthermally treated for 5 h at 400 8C. A yellowish solid was obtained.TiO2 deposition on activated carbon was carried out by a low-tem-perature procedure.[65] Activated charcoal was added to the sus-pension of TiO2. The mixed quantities were calculated to have thedesired amount of TiO2 on AC. In order to have a proper distribu-

Figure 10. Proposal to rationalize the synergism between activated carbonand TiO2 photocatalysts.

Figure 11. Chemical structures of a) GD (soman), b) VX, and c) HD (yperite).

434 www.chemsuschem.org � 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim ChemSusChem 2009, 2, 427 – 436

V. I. P�rvulescu, H. Garcia et al.

tion of TiO2, the mixture was dispersed in an ultrasonic bath for1 h. After that, the mixture was dried in a rotary evaporator undervacuum. Samples TiO2–1 wt % AC, TiO2–5 wt % AC, and TiO2–25 wt % AC containing 1, 5, and 25 wt % of AC, respectively, wereprepared. The deposition of N-TiO2 on AC followed the same pro-cedure, starting with the N-TiO2 powder. Samples N-TiO2–1 wt %AC, N-TiO2–5 wt % AC, and N-TiO2–25 wt % AC containing 1, 5, and25 wt % of AC, respectively, were obtained.

Photocatalyst Characterization

Nitrogen-doped TiO2 was characterized by chemical analysis usinga Perkin–Elmer CHNSO analyzer (combustion elemental analysis)and by dissolving the solid with a mixture of 40 % aqueous solu-tion of 0.1:1 of HF and HNO3 at 60 8C and determining the concen-tration of the elements by inductively coupled plasma–atomicemission spectrometry (ICP–AES). Textural analysis of the solidswas carried out using a Micromeritics ASAP 2020 Surface Area andPorosity Analyzer. DR UV-vis measurements were carried out with aCary 5 spectrophotometer from Varian using an integrating sphereaccessory and MgO as reference. For DR UV-vis measurements, thesample material was mixed with MgO in a 5:95 ratio mix. The slitwas set at 4 nm. DR UV-vis spectra of the catalysts were recordedin reflectance units and were transformed in Kubelka–Munk remis-sion function F(R). DRIFT spectra were recorded at room tempera-ture with a Thermo Electron Nicolet 4700 using a diffuse reflec-tance accessory (Smart collector for DRIFT spectra). The final spec-tra correspond to an average of 100 spectra with 4 cm�1 resolu-tion.

Photocatalytic Tests

Photocatalytic tests with target compounds were carried out in aclosed quartz tube flushed with a constant air flow (50 cm3 min�1).As irradiation source a 125 W high-pressure germicidal black-bulblamp (HQV 125 W, Osram, Germany) with a maximum emission at365 nm and 3.0 W UVA radiated power 315–400 nm was used.Light intensity at the distance where the sample was placed was810 Lx, measured with an 840006 Speer Scientific luxmeter. Refer-ence tests were carried out under UV light using inert silica (Cabo-sil M-5 supplied by Riedel-de Ha�n), and also adsorbing the com-pounds on the photocatalyst without light radiation. The testswere carried out at two chemical warfare agent concentrations inpolyethylene vats containing 20 mg of photocatalyst to which100 mL solution of toxic compound in dichloromethane (a 0.077 or0.77 % solution) were added. The vat was introduced in the quartztube. (Caution: the use of soman, VX, or yperite causes death and isregulated under the NATO agreement and their production, storage,and use requires the corresponding authorization). Decontaminationtests under visible light irradiation were carried out in the same re-actor using a 200 W Ne lamp (F74–765, Tungsram, Hungary) with amaximum emission at 600 nm and a light intensity of 4300 Lx(measured with an 840006 Speer Scientific luxmeter) was used. Forthe sake of comparison, in all the photocatalytic tests titania (ana-tase) supplied by Degussa (P-25) was used as reference. For nitro-gen-doped samples, nitrogen-doped P-25 was prepared with ureaas indicated above and used as reference. The evolution of the re-action was followed by taking one vat at the required reactiontime, extracting the photocatalyst with dichloromethane, and thenconcentrating and analyzing the solution by Thermo Electron TraceGC-DSQ GCMS equipment. The initial sample, unexposed to UVlight, and the irradiated samples were subjected to identical extrac-tion procedure with 500 mL solvent mixture [N,N-bis(trimethylsilyl)-

trifluoracetamide in dichloromethane to produce volatile trimethyl-silyl derivatives of the degradation byproducts). Besides analyzingthe products present on the photocatalysts, samples of the gasphase inside the reactor were taken at regular intervals with a gas-tight syringe and analyzed by GCMS. The decontamination ratewas calculated as the percentage of compound consumed fromthe initial quantity.

Acknowledgements

The authors kindly acknowledge NATO’s Scientific Affairs Divisionin the framework of the Science for Peace Programme Sfp981476for the financial support. Partial financial support by the SpanishMinistry of Science and Education (CTQ2006–06857) is also grate-fully acknowledged.

Keywords: decontamination · doping · photochemistry ·titania · carbon

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Received: December 6, 2008

Published online on April 6, 2009

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V. I. P�rvulescu, H. Garcia et al.