Visible-light C–heteroatom bond cleavage and detoxification of chemical warfare agents using titania-supported gold nanoparticles as photocatalyst

Abstract

Gold nanoparticles supported on TiO₂ effect the detoxification of soman and VX nerve gases and yperite vesicant agent at room temperature upon visible light illumination.

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Introduction

Organophosphorous esters such as pesticides and chemical warfare agents (CWAs) are liquids with different volatilities, depending on their boiling points at atmospheric pressure. Destruction of these compounds is complicated by the fact that phosphorous byproducts derived from the original compound are also persistent and can still exhibit high toxicity. The main difficulty in detoxification of these poisonous compounds is to effect the breaking of the P–C bond. Some of the toxic compounds used as chemical components of mass destruction weapons are derived from this category of substances. Other types also contain C–S bonds that must also be disconnected to ensure complete decontamination.

The production, storage and use of any CWAs are strictly controlled by the Chemical Weapons Convention.¹ This convention entered into force in 1997 and obliges all the signatory states to destroy any chemical weapon stockpile before 2012. The current World's declared stockpile of CWAs is about 40,000 metric tones, which gives an idea of the importance of the issue. In addition to the problem of destruction of massive inventories of extremely deadly chemicals, there is a large concern in western countries about the need for counter measures to neutralize emergency situations and any possible terrorist attack.²

The photoactivity of titania has been widely studied since the discovery of its photocatalytic activity.³ Over the years it has been firmly demonstrated that the photocatalysis technique can degrade to the stage of total or partial mineralization (CO₂ and H₂O) a wide range of chemical substances, and it was regarded as a most promising technology for air or aqueous pollution abatement.² Prior reports on the use the photocatalysis to detoxify model compounds of CWAs with UV light have demonstrated the potential of this method.^4–10 However, for obvious reasons the number of reports concerning the photodegradation of real CWAs is considerably more scarce and limited to the use of doped titania systems,¹¹ encapsulated photosensitizers¹² or titanate-based nanotubes.¹³ The main limitation of the photocatalysis that has to be overcome is the low efficiency of the process with visible light irradiation, this being a consequence of the wide band gap of titania in the most photocatalytically active anatase phase (3.2 eV).

The use of supported gold nanoparticles as heterogeneous catalysts is a topic of much current interest in chemistry, among other reasons because it constitutes a clear example of chemical activity derived from the nanometre particle size. Not surprisingly considering that TiO₂ is the most widely used photocatalyst and the importance of Au/TiO₂ as a heterogeneous catalyst, the photocatalytic activity of Au/TiO₂ to effect the degradation of organic compounds, water purification and even the photocatalytic synthesis of products of industrial relevance has attracted increasing interest.^14–17

However, most of these studies are limited to the degradation of simple organic compounds such as formaldehyde or methanol and cannot be used to predict the catalytic activity for the degradation of CWA.^18,19 Some reports on the use of Au/TiO₂ as photocatalyst for the degradation of more complex molecules have also described the photoactivity of this material for the decoloration of azo-dyes like methyl orange²⁰ or methylene blue.²¹ Some of these precedents have shown visible light photocatalytic activity of Au/TiO₂ arises from photon absorption by the gold surface plasmon band and subsequent electron injection into the valence band of titania.¹⁹ It was thus proposed that photoexcited semiconductor nanoparticles undergo charge equilibration when they are in contact with metal nanoparticles. Such a charge distribution has direct influence in dictating the energetics of the composite by shifting the Fermi level to more negative potentials.²²

In the present article we describe an extremely efficient system, environmentally benign that at the open atmosphere only needs visible light to effect the detoxification of contaminated surfaces. Other alternatives such incineration or chemical degradation are not applicable for spillovers or they require spreading corrosive and toxic chemicals. Thus, the use of visible light operating at ambient conditions employing a non-toxic solid can be an extremely advantageous alternative. We will show here that deposition of small gold nanoparticles (5 nm average sizes) as light harvester and catalytic sites on anatase (Au/TiO₂) renders a material that exhibits an extremely high visible-light photocatalytic activity that is able to effect detoxification of a wide range of CWA in a few minutes.

Experimental

Gold supported titania photocatalysts were prepared by the deposition-precipitation method, by adding the titania support (1 g) to an 100 cm³ aqueous solution of HAuCl₄ (0.2 M) previously adjusted at pH ≈ 8.5 with a NaOH solution (0.2 M). The slurry was maintained at 75 °C, under vigorous stirring for 5 h. After stirring, the sample was filtered, washed with deionised water until the elimination of chloride had occurred, and then dried under vacuum at 80 °C for 48 h. The samples have been investigated by using UV–Vis spectroscopy in the DRS mode, XPS and TEM. Diffuse reflectance spectra (DRS) in the range of 200–800 nm were taken on a Cary 5 spectrophotometer from Varian. The spectra were recorded against BaSO₄ as the baseline. The computer processing of the spectra with Bio-Rad Win-IR software consisted of conversion of wavelength (nm) to wavenumber (cm⁻¹) and calculation of the Kubelka–Munk function F(R) from the absorbance. Samples were granulated for DRS measurements, and the size fraction of 60–100 mesh was loaded in a quartz cell with a Suprasil window. The spectra of the samples were obtained under ambient conditions. XPS spectra were recorded at room temperature using a SSX-100 spectrometer, Model 206 from Surface Science Instrument. The pressure in the analysis chamber during the analysis was 1.33 mPa. Monochromatized Al–Kα radiation (hν = 1486.6 eV) was used. It was generated by bombarding the Al anode with an electron gun operated with a beam current of 12 mA and acceleration voltage of 10 kV. The spectrometer energy scale was calibrated using the Au 4f_7/2 peak centred at 83.98 eV. Charge correction was made with the C 1s signal of adventitious carbon (C–C or C–H bonds) located at 284.8 eV. An estimated error of ± 0.1 eV can be assumed for all measurements. TEM micrographs were taken using a TECNAI F20 instrument operated at 200 kV. Specimens were prepared by placing a drop of the sample material onto a copper grid with a perforated carbon film and then allowing the solvent to evaporate. The loading amounts of gold over TiO₂ Degussa P25 determined by ICP-AES analysis were 0.4, 0.7, and 1.5 wt%, respectively. The photocatalytic activity was measured by using an open naturally aerated quartz reactor. As irradiation source a 125 W high-pressure germicidal black-bulb lamp (HQV 125 W, Osram, Germany) with a maximum emission at 365 nm and 3.0 W UVA radiated power 315–400 nm was used. Light intensity at the distance where the sample was placed was 810 Lx. Light intensity measurement was performed with an 840006 Speer Scientific luxmeter. A general photocatalytic procedure was carried out as follows: 20 mg of photocatalyst to which a 100 μL 0.77 wt% solution of chemical warfare agent in dichloromethane were added are placed into an open polyethylene vat. The ensemble has been introduced in the quartz reactor, perpendicularly to the light propagation direction. (Caution:The use of soman, VX or sulfur mustard causes death and is regulated under the NATO agreement and their production, storage and use require the corresponding authorization.) The evolution of the reaction was followed by taking one vat at the required reaction time, extracting the photocatalyst with dichloromethane, concentrating and analyzing the solution with a GC-MS equipment from Thermo Electron. The initial sample, unexposed to UV light and the irradiated samples were subjected to identical extraction procedure with 500 μL solvent mixture (N,N-bis(trimethylsilyl)trifluoracetamide in methylene dichloride to produce volatile trimethylsilyl derivatives of the degradation by-products). The decontamination rate was calculated as the percentage of compound consumed from the initial quantity.

Results and discussion

To support that visible light absorption takes place at gold nanoparticles, diffuse reflectance UV–Vis spectra of the parent TiO₂ support and Au/TiO₂ samples were recorded (Fig. 1).

Fig. 1 Diffuse reflectance UV–Vis spectra of (a) 0.4% Au/TiO₂, (b) 0.7% Au/TiO₂, and (c) 1.5% Au/TiO₂. The inset presents an expansion of the region 500–650 nm of the spectra.

As expected, parent TiO₂ possesses an absorption threshold in the UV–Vis region at around 370 nm (3.2 eV), which is characteristic for the transition between the valence and conduction band of TiO₂ particles. After gold deposition over the titania support, the presence in the optical spectra of an absorption band in the visible region of the spectra at around 550 nm due to surface plasmon vibrations is observed. This absorption band in the visible region causes the pink-violet colour attained by three Au/TiO₂ samples and is responsible for visible light absorption of Au/TiO₂ that was assumed to be the origin of their photocatalytic activity.

For gaining understanding of the photocatalytic behaviour in the photodegradation of the three CWA, further characterization of the Au/TiO₂ materials was carried out. Fig. 2 shows the TEM micrographs of the investigated samples. Statistical analysis of the particle size distribution indicates no difference in the average diameter of the gold nanoparticles for the three samples that was about 5 nm. No change in the Au nanoparticles particle size distribution after the photocatalytic experiments were observed by TEM.

Fig. 2 TEM images and particle size distribution of (A) 0.4% Au/TiO₂, (B) 0.7% Au/TiO₂, and (C) 1.5% Au/TiO₂.

XP spectra of the investigated samples showed no significant difference in either of the Au(4f) or Ti(2p) profiles of the three Au/TiO₂ samples. Au(4f) XP spectrum of all samples exhibits peaks at the binding energies values of 84.3 and 87.9 eV for 4f_7/2 and 4f_5/2 electrons, respectively (Fig. 3A).

Fig. 3 XPS spectrum of the 0.7 wt% Au/TiO₂ sample in the Au(4f) region before (A) and after (B) performing the photocatalytic reaction.

These values match well, both in binding energies and peak width, with the corresponding data recorded for Au⁺.²³ Similarly, the Ti 2p_3/2 signals were observed at binding energy value of 458.9 eV, typical for TiO₂ Degussa P25 material (not shown for brevity). The analysis of the tested catalysts indicated a shift of the peaks assigned to the 4f_7/2 levels of gold to a higher energy (85.8 eV) suggesting an oxidation during the photocatalytic process (Fig. 3B). Based on these data we suggest the reaction mechanism presented in Fig. 6.

Preliminary experiments carried out under dark conditions showed that adsorption of these CWAs on Au/TiO₂ does not cause decomposition and that the amounts adsorbed on the solid can be recovered quantitatively by percolation with CH₂Cl₂. It is worth commenting that these experiments demonstrated a major difference between the behaviour of real CWAs and the DMMP stimulant, for which some thermal degradation by chemisorption has been observed.²⁴

In contrast to the dark controls, visible light irradiation led to a very spectacular and general efficiency for CWA degradation. Three reference Au/TiO₂ materials differing in the gold loading were used in the present study. The results obtained in the visible light photocatalytic degradation are listed in Table 1. They show that complete degradation of three structurally different CWA (Fig. 4) can be achieved in short exposure times. The photocatalytic activity of Au/TiO₂ increases with the gold loading and in some cases 30 min of exposure to visible light suffices to destroy the CWA (Table 1). Controls show that the photocatalyst temperature is about 42 °C and that the degradation effect cannot be attributed to a thermal degradation.

Table 1 Photocatalytic performance of the investigated catalysts

Substrate	Irradiation source	Time of exposure/min	Catalysts
			0.4% Au/TiO2	0.7% Au/TiO2	1.5% Au/TiO2
Soman	UV	30	15	14	21
		120	35	38	42
	Visible	30	40	42	47
		120	85	100	100
VX	UV	30	13	22	17
		120	39	40	45
	Visible	30	45	71	73
		120	78	100	100
Sulfur mustard	UV	30	11	28	40
		120	54	59	67
	Visible	30	21	32	37
		120	82	100	100

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Fig. 4 From left to right: chemical structures of soman (O-pinacolyl methylphosphonofluoridate), VX (O-ethyl-S2-diisopropylamino ethyl methyl phosphonothionate), and sulfur mustard (bis(2-chloroethyl)sulfide).

The photocatalytic data indicates that in general the gold loading in the range 0.4–1.5 wt% increases the photocatalytic activity of the material, although complete disappearance of the deadly CWA at 120 min under visible light irradiation had already been obtained at 0.7 wt% Au load. A control experiment using parent TiO₂ used as support showed that in the absence of Au nanoparticles, this material is devoid of any visible light photocatalytic activity.

The product distribution changes with the conversion indicating that after the initial degradation of the CWA, the primary by-products undergo consecutive degradation. What is important is that at final reaction times innocuous products such as phosphonates, sulfides, disulfides, sulfoxides and sulfones are present. Moreover, FT-IR spectrum of the used Au/TiO₂ photocatalyst shows that in addition to the expected vibration bands corresponding to the inorganic metal oxide, some peaks corresponding to minor amounts of strongly adsorbed and unrecoverable organic material is still present at 120 min visible light irradiation (Fig. 5). Therefore, complete detoxification of vesicant and nerve agents can be achieved under visible light irradiation under ambient conditions using Au/TiO₂ photocatalysts that are stable in the process.

Fig. 5 DRIFT spectra of the 0.7% Au/TiO₂ photocatalysts before (a) and after (b) performing soman detoxification.

We also carried out analogous photocatalytic experiments using UV light. The results are also included in Table 1. As it can be seen there, although the degradation products are identical to those observed in the visible light illumination, the Au/TiO₂ samples are less active under UV irradiation compared to visible light illumination. This is a remarkable result indicating that the light absorption by the gold surface plasmon band is responsible for the high photocatalytic efficiency. Otherwise, TiO₂ under visible light irradiation or Au/TiO₂ under UV irradiation should have exhibited identical behaviour. In addition, no influence of the Au loading should be observed if gold was not playing any role.

These data demonstrate that our photocatalytic system is based on the double aptitude of semiconductors to simultaneously adsorb chemicals and to absorb photons. When a photocatalyst is irradiated with photons of energy higher than (or equal to) its band gap energy, charge separation occurs and electrons and positive holes are created (Fig. 6). In a fluid reaction medium, reactants can adsorb on the semiconductor surface and react either with electrons (acceptor molecules such as O₂) or with holes (electron donor molecules).²

Fig. 6 Photodegradation of the neurotoxic compounds on the Au/TiO₂ catalysts under visible irradiation.

The preparation of finely dispersed gold nanoparticles is believed to be a crucial step in obtaining highly active gold catalysts for the reactions. Highly dispersed gold is regarded as a potentially useful material for various industrial and environmental applications.²⁵ In the research area of heterogeneous catalysis, Au/TiO₂ is one of the preferred catalysts exhibiting excellent activity for aerobic oxidations,^26–29 including the catalytic oxidative degradation of CWA simulants.²⁴ Thus, the oxidative degradation of dimethyl methylphosphonate (DMMP, a simulant of nerve gas) may occur on Au/TiO₂ catalysts under aerobic conditions without irradiation.²⁴ One related precedent study performed an in situ IR study of the photocatalytic reaction of tabun simulant by Au/TiO₂.³⁰ However, data with a series of real CWAs proving the photocatalytic efficiency as presented here are missing. This point is particularly important considering that in the literature there are contradictory data showing that the presence of Au nanoparticles increases or decreases the photocatalytic activity inherent to titania. For instance, it has been found that the decrease in the surface OH titanol groups caused by deposition of gold nanoparticles is highly detrimental in the photocatalytic oxidation of cyclohexane promoted by Au/TiO₂ as compared to TiO₂.³¹ Thus, the current available data do not allow the anticipation of the visible light photocatalytic activity of Au/TiO₂ with a wide range of real CWAs and the complete detoxification by C–heteroatom bond breaking. The present study is aimed at filling this information gap using vesicant (sulfur mustard) and nerve gas (soman and VX).

In conclusion the mechanism described in Fig. 6 accounts for an electron transfer from gold to titania, a cooperative effect of this electron with light from either UV irradiation or from visible light irradiation to generate O₂⁻˙ and OH˙ radicals that generate a total degradation of the investigated CWA. Remnant water and water formed in the photocatalytic degradation participate in the hydrolysis steps.

Conclusions

The data presented in this study show that we have obtained efficient photocatalysts for the complete detoxification of structurally different CWAs. Therefore, this work provides valuable data with real CWAs supporting the general applicability of Au/TiO₂ as an efficient visible light photocatalyst for detoxification of vesicant and nerve agents. Compared to incineration and chemical degradation, the photocatalytic detoxification of chemical weapons presented here can be taken as a clear example of the realization of the “green” chemistry principles since it only uses air, it is solventless makes an efficient use of energy and uses a stable and recyclable gold supported titania photocatalyst.

Acknowledgements

The authors kindly acknowledge financial support provided by the NATO's Scientific Affairs Division in the framework of the Science for Peace Program Sfp 981476, and PN-II- contract number 21-048/2007. This research has also been partially supported by the CNCSIS through the PN-II-RU-TD-2007-1 project (contract number 20/2007). Prof. Ryan Richards and Dr Zhi Li from Colorado School of Mines, Department of Chemistry and Geochemistry, Golden, Illinois, USA, and Prof. Simion Simon from Babes-Bolyai University, Cluj-Napoca, Romania, are gratefully acknowledged for performing the TEM and XPS investigations.

Notes and references

1. Convention on the Prohibition of the Development, Production, Stockpiling and Use of Chemical Weapons and on their Destruction, available online at http://www.opcw.org/.
2. http://cordis.europa.eu/fp7/dc/index.cfm .
3. A. Fujishima and K. Honda, Nature, 1972, 238, 37–38.
4. M. A. Fox, Y.-S. Kim, A. A. Abdel-Wahab and M. Dulay, Catal. Lett., 1990, 5, 369–376.
5. A. V. Vorontsov, E. V. Savinov, L. Davydov and P. G. Smirniotis, Appl. Catal., B, 2001, 32, 11–24.
6. A. V. Vorontsov, L. Davydov, E. P. Reddy, C. Lion, E. N. Savinov and P. G. Smirniotis, New J. Chem., 2002, 26, 732–744.
7. D. V. Kozlov, A. V. Vorontsov, P. G. Smirniotis and E. N. Savinov, Appl. Catal., B, 2003, 42, 77–87.
8. I. Martyanov and K. J. Klabunde, Environ. Sci. Technol., 2003, 37, 3448–3453.
9. D. A. Panayotov, D. K. Paul and J. T. Yates Jr, J. Phys. Chem. B, 2003, 107, 10571–10575.
10. D. A. Panayotov, P. Kondratyuk and J. T. Yates Jr, Langmuir, 2004, 20, 3674–3678.
11. S. Neatu, V. I. Parvulescu, G. Epure, E. Preda, V. Somoghi, A. Damin, S. Bordiga and A. Zecchina, Phys. Chem. Chem. Phys., 2008, 10, 6562–6570; S. Neatu, V. I. Parvulescu, G. Epure, N. Petrea, V. Somoghi, G. Ricchiardi, S. Bordiga and A. Zecchina, Appl. Catal., B, 2009, 91, 546–553.
12. B. Cojocaru, V. I. Parvulescu, E. Preda, G. Epure, V. Somoghi, E. Carbonell, M. Alvaro and H. Garcia, Environ. Sci. Technol., 2008, 42, 4908–4913.
13. M. Grandcolas, A. Louvet, N. Keller and V. Keller, Angew. Chem., Int. Ed., 2009, 48, 161–164.
14. A. A. Ismail, D. W. Bahnemann, I. Bannat and M. Wark, J. Phys. Chem. C, 2009, 113, 7429–7435.
15. E. Kowalska, R. Abe and B. Ohtani, Chem. Commun., 2009, 241–243.
16. H. Li, Z. Bian, J. Zhu, Y. Huo, H. Li and Y. Lu, J. Am. Chem. Soc., 2007, 129, 4538–4539.
17. W. Kubo and T. Tatsuma, J. Mater. Chem., 2005, 15, 3104–3108.
18. X. Chen, H. Y. Zhu, J. C. Zhao, Z. T. Zheng and X. P. Gao, Angew. Chem., Int. Ed., 2008, 47, 5353–5356.
19. Y. Tian and T. Tatsuma, J. Am. Chem. Soc., 2005, 127, 7632–7637.
20. I. M. Arabatzis, T. Stergiopoulos, D. Andreeva, S. Kitova, S. G. Neophytides and P. Falaras, J. Catal., 2003, 220, 127–135.
21. M. J. Uddin, F. Cesano, D. Scarano, F. Bonino, G. Agostini, G. Spoto, S. Bordiga and A. Zecchina, J. Photochem. Photobiol., A, 2008, 199, 64–72.
22. V. Subramanian, E. E. Wolf and P. V. Kamat, J. Am. Chem. Soc., 2004, 126, 4943.
23. F. Neatu, Z. Li, R. Richards, P. Y. Toulec, J.-P. Genet, K. Dumbuya, J. M. Gottfried, H.-P. Steinruck, V. I. Parvulescu and V. Michelet, Chem.–Eur. J., 2008, 14, 9412–9418 and references therein.
24. D. A. Panayotov and J. R. Morris, J. Phys. Chem. C, 2008, 112, 7496–7502.
25. G. C. Bond and D. T. Thompson, Catal. Rev. Sci. Eng., 1999, 41, 319–388.
26. S. Carrettin, P. Concepcion, A. Corma, J. M. Lopez Nieto and V. F. Puntes, Angew. Chem., Int. Ed., 2004, 43, 2538–2540.
27. A. Corma, P. Concepcion and P. Serna, Angew. Chem., Int. Ed., 2007, 46, 7266–7269.
28. C. Gonzalez-Arellano, A. Abad, A. Corma, H. Garcia, M. Iglesias and F. Sanchez, Angew. Chem., Int. Ed., 2007, 46, 1536–1538.
29. J. Guzman, S. Carrettin, J. C. Fierro-Gonzalez, Y. L. Hao, B. C. Gates and A. Corma, Angew. Chem., Int. Ed., 2005, 44, 4778–4781.
30. P. A. Kolinko and D. V. Kozlov, Environ. Sci. Technol., 2008, 42, 4350–4355.
31. J. T. Carneiro, C.-C. Yang, J. A. Moma, J. A. Moulijn and G. Mul, Catal. Lett., 2009, 129, 12–19.

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Footnote

† Electronic Supplementary Information (ESI) available: The mechanism of degradation under UV irradiation, degradation percentage of soman, VX and sulfur mustard, and photodegradation products of soman, VX and sulfur mustard. See DOI: 10.1039/c0jm00345j

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