Rapid and complete degradation of sulfur mustard adsorbed on M/zeolite-13X supported (M = 5 wt% Mn, Fe, Co) metal oxide catalysts with ozone

Abstract

Degradation of sulfur mustard (HD) adsorbed on zeolite-13X as well as M/zeolite-13X metal oxide materials (M = 5 wt% Mn, Fe, Co) was studied with and without gaseous ozone as the oxidizing agent under ambient reaction conditions. In the absence of ozone gas HD degradation was very slow over bare zeolite-13X as well as on the metal oxide impregnated zeolite-13X catalysts. Introduction of ozone gas remarkably enhanced the rate of degradation of HD and complete oxidation of HD was achieved on M/zeolite-13X materials in less than 10 minutes of reaction time. The influence of metal content, and catalyst quantity on the distribution of HD degradation products was also investigated. The results revealed that slightly longer reaction times (>5 minutes) and higher catalyst quantity favor the complete oxidation of HD. Carbon oxides were identified as the major degradation products of HD in the presence of ozone. GC-MS techniques were used for the analysis of the gas phase, condensed and surface extracted products. The HD degradation mechanism in the presence and absence of ozone was proposed based on GC-MS analysis results. The M/zeolite-13X catalysts were prepared using a wet impregnation method and characterized by different characterization techniques.

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1. Introduction

Protection against harmful chemicals is of major social and military concern. Particularly, protection against chemical warfare agents (CWA's) such as sarin (GB), sulphur mustard (HD), etc. is essential. The chemical protective equipment widely utilized for individual protection like gas-mask filters, protective over garments, canisters, etc. contain activated carbon or metal impregnated carbon systems as the adsorbent material. On the other hand, collective protection from contaminated field shelters, concealed areas, etc. can be achieved by using large size filters which also contain adsorbent material like activated alumina, impregnated carbon systems, transition metal oxide/nano metal oxides, etc. However, their protection efficiency is limited and the service life of the filters is also very short in both the case of individual and collective protection equipment.

These draw backs can be addressed by utilizing Pressure Swing Adsorption (PSA) based filtration systems which are well explored in the industry for separation of gases mixtures as well as environmental cleanup applications.^1–4 The main advantage of the PSA system is better protection against most of the harmful chemicals and enhanced service life of the filters. The adsorbent material utilized in PSA systems are zeolites and activated alumina.

Unfortunately, most of the CWA's have low vapor pressure and posses high affinity towards surfaces such as activated carbon, transition metal oxides, zeolite, etc. The adsorbed CWA's on activated carbon materials are often stable for hours to many days^5–7 and leads to the long-term environmental risk due to these contaminated chemical protective equipments such as gas mask filters, canisters, and protective over garments etc. On the other hand, increasing concern for the environment as well as safe disposal of contaminated adsorbent dictates that techniques for the degradation of adsorbed CWAs on the activated carbon/zeolite materials is of great interest to the scientific community to decompose the adsorbed CWA's to harmless products. It is believed that the decontaminant used to destroy the sulfur mustard efficiently, can also be used to decontaminate the other chemical warfare agents. The kinetics of the degradation of various CWA's on impregnated carbon⁸ transition metal oxides/nano metal oxides^9,10 and on bare zeolites^11–13 is usually very slow and takes a longer reaction time for degradation (half life 2–3 days) through a series of oxidation, elimination and hydrolysis reactions there by rendering these toxic chemicals to harmless products by utilizing the high surface area, Lewis acidity and surface hydroxyl groups of the catalyst materials.

Osovsky et al.¹⁴ studied the hydrothermal treatment of sulfur mustard adsorbed on activated carbon spheres and they reported that complete degradation of HD adsorbed was achieved within 0.5 to 2.5 h at a temperature of 120–160 °C by using water/carbon ratio of 0.3 : 1 to 5 : 1. But, the bottleneck is that the reactive adsorbents often take longer reaction time for the degradation process. Furthermore, some of the degradation products formed in the process (mustard sulfone and divinyl sulfone etc.) is potentially toxic to the human health as well as to the environment.

S. Popiel¹⁵ et al. reported the destruction of sulfur mustard in liquid phase using ozone, UV, hydrogen peroxide and their combination in buffered aqueous media. However, this method is applicable only for the laboratory study and did not find any practical importance. Furthermore, they did not report the degradation products identified in their study. Therefore, development of new methodology which has high practical importance to completely degrade/decompose the CWA's at ambient conditions in a shorter time is of immense important to the scientific community.

To the best of our knowledge, so far there are no reports on the complete degradation/mineralization of sulfur mustard on M/zeolite-13X (M = 5% Mn, Fe, Co) metal oxide materials using gaseous ozone. Therefore, we have chosen Mn, Fe, Co/zeolite-13X as the adsorbent materials for this study due to their excellent ozone decomposition ability at ambient conditions and are also proven as active catalysts for the removal of many organic pollutants as well as for waste water treatment under ozonation conditions.^16–18 Furthermore, zeolite-13X support has high thermal stability, resistance to chemicals, inexpensive, environmental friendly and readily available. The high kinetic diameter (1.1 nm) is also the key factor for the adsorption of most of CWA's. In this study we reported the complete degradation of HD adsorbed at ambient conditions on M/zeolite-13X materials by the utilizing the strong oxidizing potential of ozone gas. We also focused on the influence of metal content, and catalyst quantity on the complete mineralization of HD adsorbed on M/zeolite-13X catalytic systems.

2. Experimental procedures

2.1. Materials

The chemicals Mn (NO₃)₂·4H₂O (purity > 97%), Fe(NO₃)₃·9H₂O (purity > 97%), Co(NO₃)₂·6H₂O (purity > 97%), and acetonitrile AR grade (purity > 99.99) were purchased from M/s Sigma-Aldrich India Pvt. Ltd. Zeolite-13X materials (composition Na₂O·Al₂O₃ (2.8 ± 0.2) SiO₂·(6–7)H₂O) were from M/s Sorbead India Ltd. Mass flow controller (MFC, precession ± 1%) were procured from M/s Sierra, Switzerland. CO (range 1–2000 ppm, ± 1 ppm), CO₂ (range 10–20 000, ± 10 ppm) analyzers (from M/s Technovation Analytical Instruments Ltd India) were used to measure the concentrations of carbon oxides. O₂ (Purity 99.9% from M/s Everest Kanto cylinders Ltd India) gas was used as the source for the production of gaseous ozone. He (99.999%) gas which was used in this study for GC-MS analysis was supplied by M/s Everest Kanto cylinders Ltd India. Ozone generator (M/s. Eltech Engineers Ind. Ltd) and ozone analyzers were used to generate and monitor the ozone gas respectively. Ozone analyzer (M/s. Eltech Eng. India, range 0–200 ng m⁻³) was used to estimate the generated ozone concentration.

2.2. Preparation and physicochemical characterization of catalysts

Zeolite-13X (particle size, sieved to BSS 18/25 mesh) supported Mn, Fe, Co metal oxide catalysts of different metal loadings were prepared by wet impregnation method and activated by calcinations in air for 4 h at 400 °C.¹⁹ The specific surface area and pore size distributions were obtained by using Brunauer Emmett Teller and Barrett Joyner Halenda methods respectively for both bare zeolite-13X as well as impregnated M/zeolite-13X (M = 5 wt% Mn, Fe, Co) catalysts. N₂ adsorption–desorption experiments were performed using Micromeritics-ASAP-2010 unit at liquid nitrogen temperature (−196 °C). All the catalysts were degassed for 60 min at 120 °C prior to the analysis. Rigaku Miniflex (M/s. Rigaku corporation, Japan) was used for recording the XRD patterns of all the catalysts with Ni filtered Cu Kα radiation (λ = 1.5406 Å) source in the 2θ range from 10–80° with a scan speed of 2° min⁻¹ at 30 kV and 50 mA. TPR/TPD Nuchrom Unit with a thermal conductivity detector was utilized to carry out the TPR and TPD-NH₃ experiments. Approximately 100 mg of catalyst sample in a quartz reactor was used for the TPR studies. In a flow of 50 mL min containing 5% H₂–Ar gas was used to study the reduction profile of the catalysts in the temperature range of 40–750 °C with 10 °C min⁻¹ heating rate. NH₃-TPD experiments were performed on the catalyst samples which were preheated for 1 h at 300 °C and then the samples were allowed to cool at 40 °C. The catalyst was saturated by ammonia gas by pulsing onto the catalyst reactor at 40 °C. After base line restoration, the samples were heated from 40–750 °C with a heating rate 10 °C min in helium flow of 50 mL min⁻¹. Surface morphology of the catalysts were obtained by scanning electron microscope (SEM, JOEL, JAX-840) at 20 kV by coating the gold sputtering on the samples.

2.3. Reaction studies for HD degradation

Complete degradation of sulfur mustard adsorbed on zeolite-13X as well as on metal oxides impregnated zeolite-13X materials with gaseous ozone was carried out at ambient conditions using a quartz reactor of (5 mm id, 150 mm length) interfaced with an online GC-MS. The schematics of experimental setup is represented in Fig. 1. Ozone generator and oxygen gas was used in this study to generate the gaseous ozone of required concentrations (ozone conc. 4 mg min⁻¹ in 50 mL min⁻¹ O₂ flow) for HD degradation experiments. Hamilton 5μL syringe was used to introduce the HD onto the surface of the catalyst. 100 mg of catalyst (zeolite-13X and M/zeolite-13X) diluted with glass beads was packed in a quartz reactor with glass wool and mounted vertically in a laboratory stand with clamp for reaction studies. Prior to the experiments, the catalysts were activated at 400 °C in air for 1 h. 3μL of HD was adsorbed onto the catalysts bed with the help of Hamilton 5μL syringe. The quantity of HD adsorbed was far less the adsorption capacity of zeolite-13X materials and desorption of HD adsorbed in the 50 mL min⁻¹ flow of O₂ was not observed in the present study.

Fig. 1 Schematic of typical experimental set-up used for complete degradation of HD over M/zeolite-13X supported catalysts (M = 5 wt% Mn, Fe, Co).

The operating conditions (ozone gas concentrations 4 mg min⁻¹ and gas flow rates 50 mL min⁻¹ oxygen flow) were selected based on our previous studies on decontamination of sulfur mustard and its simulants using ozone gas in liquid phase.²⁰

The HD adsorbed was treated with gaseous ozone concentration of 4 mg min⁻¹ in a 50 mL min⁻¹ oxygen flow at temperature of 37 ± 3 °C for a fixed period of time. Later on the residual HD was extracted in 3 mL acetonitrile solvent and the solutions were subjected to gas chromatograph coupled with mass spectrometry. Bruker Scion MS-SQ gas chromatography coupled with Mass Spectrometry (GC-MS) was used for the monitoring of degradation of HD at selective intervals. Analysis was performed using HP-5 column of 30 m × 0.25 mm × 0.25 μm dimensions in Electron Impact (EI) mode. Ion source (70 eV electron energy) and transfer line temperatures were maintained at 200 °C, and 250 °C respectively. GC injection port was kept at 220 °C, split ratio of 1 : 15 and solvent delay of 2 minutes were maintained in all the experiments and a constant column flow of 1 mL min⁻¹ He (>99.999% purity) was used as carrier gas. GC-MS was used in temperature programming mode from 60–230 °C at 15 °C min⁻¹ heating rate. Ions were scanned in the range from 35 to 250 amu. GC-MS peak areas were used to estimate the % HD degradation with time using eqn (1). For reference experiments, the same quantity of HD was dissolved in acetonitrile solvent and standardized the HD concentration with chromatogram peak area. The gaseous CO and CO₂ concentrations were analyzed by using calibrated CO, CO₂ analyzers respectively. The product mixture of condensable phase was trapped in acetonitrile solvent at −5 °C and also analyzed by GC-MS. The exiting gaseous stream is passed through KI trap for the decomposition of residual ozone if present on the gaseous stream.

The conversion of HD = {([C^HD]_initial − [C^HD]_final)/([C^HD]_initial) × 100} (1)

Caution: HD is a carcinogen, vesicant, and cytotoxic agent; therefore this compound should be handled by the trained and authorized persons with proper personal protective measures in an efficient fuming hood only. To avoid any accident, sufficient amount of decontamination solution should be available at working place.

3. Results and discussions

3.1. Characterization of M/zeolite-13X catalysts (M = 5wt% Mn, Fe, Co)

3.1.1. BET surface area and pore size distribution studies. The N₂-adsorption and desorption isotherms of zeolite-13X as well as M/zeolite-13X catalysts were measured at −196 °C. The catalysts samples were pretreated at 120 °C for 60 min prior to the adsorption measurements. BET surface area, pore volume and average pore diameters were calculated from N₂-adsorption–desorption isotherms and the data were presented in Table 1. Total surface area was calculated according to the BET and Langmuir methods.²¹ The nature of the displayed N₂-adsorption–desorption isotherms (Fig. 2) are more or less similar type IV adsorption isotherms to typical mesoporous materials with H3 type hysteresis. 5 wt% Fe/zeolite-13X isotherms indicate the presence of slit type of pores formed due to aggregation of iron oxide species. The surface area of the M/zeolite-13X catalysts was in the range of 550 to 350 m² g⁻¹. From the table it is important to note that, impregnation of metal oxides on zeolite-13X supports makes the decrease of the BET surface area and average pore volume of all the metal oxide catalysts. The pore size distributions (Fig. 3) data clearly indicates that the pore size distributions are in narrow range and most of the area falls under the mesoporous range i.e. 20 to 50 A° which is ascribed to the zeolite-13X support. The surface area and pore volumes are decreased after impregnation with the metal oxide catalysts. However, the pore diameter is slightly increased in the case of Co and Mn metal oxide impregnated catalysts. But, 5 wt% Fe/zeolite-13X exhibitted higher pore diameter compared to other materials. This might be due to the slight aggregation of iron oxide species on zeolite-13X support. The reduction in surface area and pore volumes of all M/zeolite-13X materials is attributed as a result of impregnation of low surface area metal oxide species on zeolite-13X support. The N₂ adsorption–desorption studies help us to conclude that the decrease of surface area and pore volumes of the catalysts are due to impregnation of low surface area metal oxides on porous zeolite-13X support.²²

Table 1 Physicochemical properties of zeolite-13X and 5 wt% M/zeolite-13X (M = Mn, Fe, Co) metal oxides materials

Catalyst	Mn content (wt%)	BET surface area (m^2 g^−1)	Pore diameter (Å)	Pore volume (cm^3 g^−1)	TPR			H2 uptake (mL g^−1)	TPD-NH3 total acidity (mmol g^−1)
					Peak 1	Peak 2	Peak 3
					Tmax (°C)	Tmax (°C)	Tmax (°C)
Zeolite-13X	0	746	23	0.437	—	—		—	4.78
5% Fe	5	350	28.18	0.250	353	553	650	5.95	6.14
5% Co	5	467	22.57	0.264	323	464	586	9.56	8.72
5% Mn	5	547	24.44	0.334	344	515	—	8.42	9.12

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Fig. 2 N₂-adsorption–desorption isotherms of zeolite-13X and M/zeolite-13X catalysts (M = 5 wt% Mn, Fe, Co).

Fig. 3 Pore size distributions of zeolite-13X and M/zeolite-13X catalysts (M = 5 wt% Mn, Fe, Co).

3.1.2. X-ray diffraction studies. The major X-ray diffraction signals (Fig. 4) at 2θ = 15.4, 20.1, 23.3, 26.7, 31 are corresponding to the planes of {331}, {440}, {533}, {642}, {662} and their d values 5.742, 4.424, 3.817, 3.345, 2.871 respectively of zeolite-13X only.²³ Apart from the high intensity diffraction signals, several medium to low intensity diffraction signals are also observed in the XRD patterns of the 13X system. However, there are no separate diffraction patterns for the impregnated metal oxides observed in the present M/zeolite-13X systems. It might be attributed to the overlapping of the some of 13X diffraction signals with M_xO_y (M = Mn, Fe, Co) species or it could be the high dispersion of impregnated metal oxides on zeolite-13X support. However, the huge background of zeolite-13X diffraction patterns which hinders the diffraction peaks of the metal oxides catalysts cannot be ruled out (small quantity (5 wt%) of the metal loadings). Other possibility for not detecting the diffraction patterns is that the crystallites might be less than 5 nm size, which is beyond the detection limits of powder X-ray diffraction technique. The decrease in the intensity of some of the diffraction signals were noted in the XRD patterns of M/zeolite-13X catalysts. It is attributed to the adsorption of impregnated metal oxides (M = Mn, Fe, Co) onto the frame work of zeolite-13X support.

Fig. 4 XRD patterns of zeolite-13X and M/zeolite-13X (M = 5 wt% Mn, Fe, Co) supported catalysts calcined at 400 °C.

3.1.3. SEM analysis. The scanning electron micrograms (Fig. 5) of bare zeolite-13X along with 5 wt% M/zeolite-13X (M = Mn, Fe, Co) are taken at 10 000× magnifications. SEM data clearly indicates that the Mn and Co impregnated metal oxides distribution is quite uniform on the surface of the zeolite-13X support materials and particle aggregation is also not observed on Mn, Co/zeolite-13X materials. On the other hand little aggregation was observed in case of iron oxide impregnated zeolite-13X system. The SEM data is in agreement with textural and physicochemical properties of M/zeolite-13X catalysts as shown in the Table 1.

Fig. 5 Scanning electron micrographs of zeolite-13X and M/zeolite-13X (M = Mn, Fe, Co) supported catalysts.

3.1.4. Temperature programmed reduction (TPR) studies. TPR experiments are performed on the calcined catalysts to investigate the oxidation states of M/zeolite-13X (M = 5 wt% Mn, Fe, Co) supported catalysts deposited on the zeolite-13X support and to relate these oxidation states with activity studies of the catalysts (Fig. 6). The reducibility of metal oxide catalysts depends on the morphological properties of supported materials, because the support material determines the reactivity of the bridging M-support functionalities.²⁴ Apart from the morphological properties of the support, impurity levels, preparation procedures and the reduction conditions like H₂ partial pressure and heating rate also determines the reducibility of metal oxide catalysts. Therefore the reduction temperatures obtained may be different from the literature reports.²⁵ All metal oxide catalysts exhibited a multistep reduction patterns (Fig. 6) in the temperature region of 250 to 680 °C which indicates that the reduction profiles of impregnated metal oxides on zeolite-13X support are similar to that of bulk metal oxide particles with low interaction with the zeolite support.

Fig. 6 Temperature-programmed reduction (H₂-TPR) profiles of zeolite-13X and M/zeolite-13X (M = 5 wt% Mn, Fe, Co) supported catalysts.

In the case of Fe/zeolite-13X catalyst, one broad peak (T_2max) centered between 300 to 600 °C (corresponding to consumption of hydrogen) is attributed to the reduction of Fe₂O₃ to Fe₃O₄. The broadening of the peak indicating that iron oxide over zeolite-13X support is present as isolated iron oxide species which could interact strongly with the support. Besides broadening, a shoulder is observed on the low temperature side of the second peak at 353 °C (T_1max) first peak. It could be attributed to the reduction of hydroxilated iron oxide species.^26,27 The third high temperature peak (T_3max) centered between 570–710 °C corresponding to the subsequent reduction of Fe₃O₄ to metallic iron. Whereas in case of Mn/zeolite-13X catalysts, the first reduction peak (T_1max) (corresponding to consumption of hydrogen) could be assigned to the reduction of MnO₂ to Mn₂O₃ and the second peak corresponds to the reduction of Mn₂O₃ to Mn₃O₄. The third and less intensity peak at 565 °C represents the reduction of Mn₃O₄ to MnO.²⁸

In the similar fashion Co/zeolite-13X catalyst also exhibited a multi step reduction pattern (Fig. 6). The first reduction peak (T_1max) at 323 °C could be assigned to the reduction of CoO₂/Co₂O₃ to Co₃O₄ ^29,30 and the second reduction at 464 °C corresponded to the reduction of Co₃O₄ to CoO. The third reduction peak at 586 °C indicates the reduction of CoO to metallic cobalt or it might be due to strong interaction of Co-zeolite-13X support.³¹ H₂-TPR studies helped us to conclude the presence of different metal oxide species and also their interactions with zeolite-13X support.

3.1.5. Acidic properties of Mn/zeolite-13X catalyst. The quantity and strength of acidic sites present on the surface of the catalyst influences the performance of the catalyst. The acidic strengths of the zeolite-13X support as well as for all M/zeolite-13X catalysts are ascertained by performing the NH₃-TPD experiments in the temperature range from 40–700 °C (Fig. 7). The total acidities of the support and the catalysts are given in Table 1. Zeolite-13X support is exhibited only one broad peak in the region of 100–200 °C, whereas three broad peaks are observed in the NH₃-TPD patterns of all M/zeolite-13X catalysts: the first peak is in the temperature region of 100–250 °C and the second one is at 200–400 °C. The third bread peak is in the temperature region of 450–700 °C.

Fig. 7 Temperature programmed desorption profiles (TPD-NH₃) of bare zeolite-13X and M/zeolite-13X catalysts (M = 5 wt% Mn, Fe, Co).

The peak in the temperature region of 100–200 °C represents the ammonia desorbed from the weaker acidic sites (physisorbed ammonia). The high temperature peak in the temperature region of 200–400 °C represents the ammonia desorbed from Bronsted acidic sites present on the surface of the catalyst. Whereas the broad peak in the high temperature region of 400–700 °C represents the ammonia desorbed from Lewis acidic sites³² present on the surface of the catalysts. But, zeolite-13X support contains very little amount of stronger acidic sites in the high temperature region (Bronsted and Lewis acid sites). The increases of acidity values of all the M/zeolite-13X catalysts are attributed to the impregnation of metal oxides on the surface of the zeolite-13X support material.

The overall results (Table 1) indicate that the impregnated catalysts have more total acidic sites than that of bare zeolite-13X catalyst. The desorbed amount of ammonia per gram was in the order of acidity was Mn > Co > Fe > 13X.

3.2. Catalytic properties of M/zeolite-13X (M = 5 wt% Mn, Fe, Co) systems

3.2.1. Effect of reaction time on degradation of HD adsorbed with ozone. Fig. 8a depicts the results of HD degradation with and without metal oxides impregnated zeolite-13X catalysts in the presence and absence of ozone at ambient reaction conditions and at atmospheric pressure. The results reveal that HD degradation is very slow and no degradation products of HD is observed up to 30 minutes in absence of ozone or oxygen gas in the GC-MS analysis of the extracted HD from the bare zeolite-13X as well as M/zeolite-13X catalysts (M = 5 wt% Mn, Fe, Co). However, traces of mustard sulfoxide, sulfone, divinyl sulfoxide, hemi mustard, etc. are observed in GC-MS analyses of the extracted HD after 1 hour. This is ascribed to the oxidation, hydrolysis and elimination reactions of HD with hydroxyl groups present on the surface of the M/zeolite-13X catalysts.^9,33,34 Similar observations are made when treating the HD adsorbed in presence of oxygen gas. It might be due to the low oxidation potential of the oxygen gas (O₂ E^o = 0.68 eV) which might not be sufficient to oxidize/degrade the adsorbed sulfur mustard at ambient temperature and at atmospheric pressure.

Fig. 8 (a) Effect of reaction time on degradation of HD with bare zeolite-13X and M/zeolite-13X catalysts (M = 5 wt% Mn, Fe, Co) at GHSV: 30 000 h⁻¹, O₃ conc.: 4 mg min⁻¹, 3μL HD adsorbed on 100 mg catalyst, total flow: 50 mL min⁻¹, reaction temp: 37 ± 3 °C. (b) Total Ion Chromatogram (TIC) of HD degradation with time.

On the other hand, in presence of ozone as the oxidizing agent, HD degradation is very fast and complete degradation is achieved within one minute of time at same reaction condition as shown in Fig. 8b. This remarkable tendency is attributed to the strong oxidizing potential of the gaseous ozone (E^o = 2.07 eV) and its ability to react with most of the organic compounds at room temperature. The influence of various impregnated M/zeolite-13X catalysts on HD adsorbed degradation is presented in the Fig. 8a. From the figure it is concluded that all the zeolite-13X catalysts are capable to completely degrade the HD adsorbed within one minute of time in the presence of ozone at ambient reaction conditions. This is due to the high ozone decomposition capacity of these catalysts materials at mild to moderate reactions temperature^35,36 which produces highly reactive atomic oxygen species. It was also reported that, the O₃ was decomposed to O₂ on supported metal oxides catalysts according to the following reaction steps (eqn (2)–(4)), where * refers to the catalytic active sites.^37,38 The active oxygen species (O* and O₂*) formed on the catalyst surface by ozone decomposition were responsible for the HD degradation.

O₃ + * → O₂ + O* (2)

O* + O₃ → O₂ + O₂* (3)

O₂* → O₂ + * (4)

3.2.2. Distribution of degradation products of HD adsorbed over M/zeolite-13X (M = 5wt% Mn, Fe, Co) catalyst. The distribution of HD degradation products over 5 wt% Co/zeolite-13X catalysts with ozone at ambient conditions are described in Fig. 9a. The major degradation products identified by GC-MS during degradation of HD adsorbed on zeolite-13X catalysts are mustard sulfoxide and mustard sulfone, CO_X. Trace quantities of (<5% by GC area) of SO_2, acetaldehyde, HCl, divinyl sulfone, thiodiglycol, hemi mustard, etc. are also identified. Therefore, the calculations of selectivity's are made based on the major identified products and ignoring the concentration of minor products. It is noteworthy to mention that the selectivity towards mustard sulfoxide is decreased with time and there is no mustard sulfoxide in the reaction mixture after 3 minutes of time. In contrast, the selectivity to mustard sulfone is increased initially and attained a maximum of 40% selectivity in 2 minutes. Thereafter, the selectivity to mustard sulfone also goes down to zero percent after 5 minutes of time. The distribution of HD degradation products with other catalysts also studied under the same experimental conditions and found that, there are no significant differences between the applied catalysts towards the distribution of HD degradation products. It might be attributed to the 100% ozone decomposition ability of all the catalysts under employed conditions.

Fig. 9 Distribution of HD degradation products with time (a) conversion/selectivity to mustard sulfoxide and sulfone over 5 wt% Co/zeolite-13X catalysts at GHSV: 30 000 h⁻¹, O₃ conc.: 4 mg min⁻¹, 3μL HD adsorbed on 100 mg catalyst, total flow: 50 mL min⁻¹, reaction temp: 37±3 °C. (b) Conc. of CO, CO₂ with time over M/zeolite-13X and bare zeolite-13X.

Fig. 9b describes the concentrations of carbon oxides formed during the degradation of HD adsorbed over metal oxides impregnated and bare zeolite-13X catalysts. The results inferred that with time the concentration of CO, CO₂ are increased and reached a maximum concentration with all the impregnated and bare zeolite-13X catalysts in the presence of ozone gas. Thereafter, the CO_X concentrations are declined with time and approached to almost zero after 10–15 minutes of time. It indicated the complete degradation of HD adsorbed over the catalysts materials with ozone. From the results it is concluded that with increasing the reaction time total oxidation products TOPs (CO_X) are predominates in the degradation of HD adsorbed on M/zeolite-13X catalyst in the presence of ozone at ambient reaction conditions.

3.2.3. Influence of metal loading and catalyst quantity on degradation of HD adsorbed and its product selectivity. The influence of metal content on the degradation of HD adsorbed and its product distribution after 15 s reaction time is illustrated in Fig. 10a. With increase in the metal loadings from 1–7 wt% Co/zeolite-13X catalyst, the % degradation of HD adsorbed is increased from 90% HD degradation with 1% Co/zeolite-13X catalyst to 100% HD degradation with 7 wt% Co/zeolite-13X catalyst at the same reaction time. This tendency is attributed to the increase of active metal oxide species which enhances the ozone decomposition. In the same fashion, the selectivity to mustard sulfoxide is decreased/mustard sulfone is increased with increasing the active metal content from 1–7 wt% Co/zeolite-13X catalyst at the same reaction time. Fig. 10b depicts the effect of catalysts quantity on the degradation of HD adsorbed and its products distribution after 15 s reaction time. With increasing the catalyst quantity from 20 mg catalyst to 100 mg catalyst, the% degradation of HD adsorbed is increased from 90% HD degradation with 20 mg 5% Co/zeolite-13X catalyst to 100% HD degradation with 80 mg 5 wt% Co/zeolite-13X catalyst at the same reaction time. Thereafter, there is no influence of catalyst quantity on HD degradation but the selectivity to sulfoxide decreased or sulfone selectivity increased with 100 mg 5 wt% Co/zeolite-13X. Here also, the increase of active metal oxide quantity increases the degradation of HD adsorbed as well as the ozone decomposition capacity of the catalyst material due to increase of total acidity of the catalyst. Furthermore the selectivity to mustard sulfoxide/mustard sulfone also follows the same tendency as that of metal loading. The results conclude that although 80 mg of 5 wt% Co/zeolite-13X catalyst is sufficient for complete degradation of HD adsorbed under employed conditions, little excess of catalysts quantity (100 mg) was used for the HD degradation studies in order to completely decompose the ozone gas by considering its toxicity and harmful effects on human beings as well as on environment.

Fig. 10 Degradation of adsorbed HD and its products distribution after 15 s reaction time (a) HD conversion/selectivity to mustard sulfoxide and sulfone over 1–7 wt% Co/zeolite-13X catalysts (b) effect of catalyst quantity on HD conversion/selectivity to mustard sulfoxide and sulfone over 5 wt% Co/zeolite-13X at GHSV: 30 000 h⁻¹, O₃ conc.: 4 mg min⁻¹, 3μL HD adsorbed on 20–100 mg of catalyst, total flow: 50 mL min⁻¹, reaction temp: 37 ± 3 °C.

3.2.4. Probable mechanism. The total degradation products identified of HD adsorbed over M/zeolite-13X catalyst with ozone are listed in the Table 2. The analysis comprises of effluent gases of the reaction products mixture by CO, CO₂ analyzers as well as by GC-MS analyses of gaseous stream exiting form the reactor and the condensed products in the acetonitrile solvent at −5 °C. However, there are no products identified in the GC-MS analyses of condensable phase of the reaction mixture exiting through acetonitrile solvent system cooled at −5 °C temperatures. The major HD degradation products identified are carbon oxides, mustard sulfoxide and mustard sulfone. Traces of acetaldehyde, HCl, SO_2, 2-chloro ethyl vinyl sulfide, 1,4-oxathian, divinyl sulfide, divinyl sulfone, hemi mustard, thiodiglycol, 2-hydroxy ethyl vinyl sulfide, etc. as shown in the Table 2 are also identified in the degradation of HD adsorbed. But, these compounds could not be detected after treating the HD adsorbed with ozone after 2 minutes. The reason might be due to the high ozone concentration and oxidizing power of the catalyst which further oxidizes the degraded products to complete oxidation products like carbon oxides and sulfur dioxide.

Table 2 List of products detected due to degradation of adsorbed HD over M/zeolite-13X (M = 5 wt% Mn, Fe, Co) metal oxides catalysts in presence of ozone

Product name	Molecular formula	m/z values
Acetaldehyde	CH3CHO	44, 43, 31, 29, 26, 15
Carbon dioxide	CO2	44, 28, 22, 16
2-Chloro ethyl vinyl sulfide	C4H7SCl	122, 94, 73, 65, 45, 27
1,4-Oxathian	C4H8OS	104, 74, 61, 6, 34
Di vinyl sulfide	C4H6S	86, 85, 59, 58, 45
Di vinyl sulfone	C4H6SO2	118, 75, 66, 47, 27, 26
Ethanol-2-(2-chloroehyl thio)	C4H9OSCl	140, 141, 111, 109, 104
Half mustard		73, 63, 45, 27, 29
Thiodiglycol	C4H10OSCl	122, 104, 91, 61, 43
2-Hydroxy ethyl vinyl sulfide	C4H8OSl	104, 91, 61, 43
Bis (2-chloro ethyl) sulfone	C4H8SO2Cl2	127, 128, 92, 63, 27
Bis (2-chloro ethyl) sulfoxide	C4H8SOCl2	174, 158, 127, 104, 63, 27
Sulfur dioxide	SO2	64, 48, 32

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A plausible degradation mechanism is proposed based on the products identified in the GC-MS analyses of HD adsorbed over M/zeolite-13X catalysts with and without treatment from ozone gas.

In absence of ozone. In the absence of ozone, HD reacts with surface hydroxyl groups of M/zeolite-13X catalysts very slowly by means of hydrolysis, elimination and cyclisation reactions to yield various products (Table 2). The possible reactions of HD degradation in absence of ozone were reflected in Scheme 1. According to the Scheme 1, HD molecules react with surface of the M/zeolite-13X catalysts in the following two ways.

Scheme 1 Elimination and hydrolysis reactions of sulfur mustard on the surface of the M/zeolite-13X catalyst.

In first way they react with the physisorbed (the moisture adsorbed while transferring the samples into the reactor) or intercalated water molecules and surface hydroxyl groups that are present on the surface of the catalyst to form hemi mustard followed by thiodiglycol. This reaction is proposed to proceed through the formation of cyclic sulfonium ion intermediate.³⁹ Cyclisation of thiodiglycol by elimination of water molecule leads to the formation of 1,4-oxathiane.

In the second way, HD molecules react with Lewis acid sites of metal oxide catalyst (Mn⁺⁴, Co⁺³, Fe⁺³) to form surface bound alkoxyl species followed by hydrolysis, elimination and surface complexation of HD which poison the active sites of the catalysts and leads the reduction of reactivity towards HD degradation.⁴⁰ HCl elimination from HD molecule leads the formation of vinyl chloro ethyl sulfide and divinyl sulfide respectively. Hydrolysis of vinyl chloro ethyl sulfide results the formation of hydroxy vinyl ethyl sulfide (Scheme 1).

In the presence of ozone. As the decontamination reactions are found to be very slow on M/zeolite-13X catalyst (M = 5 wt% Mn, Co, Fe), ozone assisted catalytic degradation is opted for the faster and complete decontamination of HD adsorbed. The reaction mechanism proposed for the degradation of HD adsorbed in the presence of ozone over M/zeolite-13X catalysts based on the products identified by GC-MS analyses as displayed in (Scheme 2).

Scheme 2 Reaction mechanism indicating the degradation of adsorbed HD over Mn/zeolite-13X in presence of ozone.

It is proposed that presence of ozone significantly alters the catalytic mechanism from molecular oxygen by ozone decomposition in the presence of catalyst in gas phase reactions which proceeds mostly through surface mechanisms. Oyama et al.⁴¹ reported that adsorbed active oxygen species are formed on the surface of the M_yO_x catalysts during O₃ decomposition and these adsorbed oxygen species play the major role while oxidizing the VOCs to complete mineralized products due to its remarkable oxidizing potential.⁴² Peroxide ion species formed during the ozone decomposition on manganese oxide catalyst was identified by in situ Raman spectroscopy.⁴³ It is observed that addition of ozone significantly increases the oxidation efficiency of metal oxide catalysts and reduces the oxidation temperature. It might be due to the active oxygen species formed during the O₃ decomposition on the metal oxide catalysts and its further reaction with HD molecules.

Decomposition of ozone on the surface of the metal oxide catalyst results the formation of catalytic active sites O⁻M⁺⁵ as shown in the Scheme 2. Initially, the HD adsorbed interacts with the actives sites derived during the ozone decomposition and forms the HDO which on further oxidation yields the HDO₂ by ‘S’ atom oxidation. Several studies on degradation of reduced sulfur compounds using transition metal oxide catalysts evidenced the formation of sulfoxide and sulfone on the surface of the catalyst material.^44–46 The formed HDO and HDO₂ on extended oxidation reactions with adsorbed active oxygen species results the formation of carbon oxides, sulfur dioxide, HCl, H₂SO₄ and water molecules. In the present investigation, initially HDO and HDO₂ are observed as major products in GC-MS analyses. With time the concentrations of HDO and HDO₂ are declined and the concentrations of carbon oxides increased. This is attributed to the higher ozone concentration which leads the extended oxidation of formed HDO and HDO₂ to total oxidation products by breakage of C–S bond. As the time progresses the available surface adsorbed oxygen species are more and the HD adsorbed concentration is also decreases which favors the extended oxidation reactions to proceeds even at ambient reaction conditions. Literature reports support the cleavage of C–S bond on the metal oxide catalysts and the extended oxidation of partial oxidation products to completely mineralized products in the presence of ozone.^44,47–51

In another way, formation of acetaldehyde, carbon oxide and small amounts of acetic acid, etc. can be attributed to the oxidation and cleavage of C–S bond of HD by superoxide anion radical eqn (5)–(10). It should be noted that, the mechanism is highly complex and can also follow combination of mechanisms such as oxidation of S-atom, C-atom, cleavage of C–S bond, hydrolysis of C–Cl bond or elimination of HCl and free radical mechanisms as well. However, based on the quantities of products observed, it is under stood that C–S bond cleavage and S atom oxidation contributes to a major extend for the degradation of HD adsorbed over M/zeolite-13X catalysts at ambient temperature and atmospheric pressure.

(Cl-C₂H₄)₂S + O₂˙⁻ + H₂O → CH₃CHO + CH₃CH₂SH (5)

CH₃CH₂SH + O₂˙⁻ → CO₂ + H₂O + H₂SO₄ (6)

CH₃CHO + O₂˙⁻ → CH₃COOH (7)

CH₃COOH + O₂˙⁻ → CO₂ + H₂O (8)

ClCH₂CH₂OH + O₂˙⁻ → CO₂ + H₂O + HCl (9)

HSCH₂CH₂OH + O₂˙⁻ → CO₂ + H₂O + H₂SO₄ (10)

It is observed that the selectivity of catalyst is governed by the nature of oxygen species; electrophilic oxygen species such as O²⁻ and O⁻ are responsible for total oxidation of hydrocarbons, whereas the nucleophilic lattice stabilized O²⁻ is responsible for the formation of partial oxidation products.⁵² Here, the major products identified in the HD degradation with other M/zeolite-13X (M = 5 wt% Co, Fe) catalysts are also mainly carbon oxides, mustard sulfoxide and sulfone. Therefore, it is assumed that the HD degradation mechanism on Fe and Co catalyst might be proceeding through the formation of O–Fe⁺³ and O–Co⁺⁴ active sites respectively in the presence of ozone.^{38,41–43,53}

3.3. Reusability of the catalysts for degradation of HD adsorbed

The catalysts spent for the degradation of HD adsorbed in presence of ozone were activated by calcination in the atmospheric air flow of 1 lpm for 1 h at 400 °C. After activation, the spent catalysts were tested by adsorbing the same quantity of sulfur mustard followed by degradation of HD adsorbed in presence of ozone. The degradation efficiency of the Co/zeolite-13X catalysts are tested with ozone for several times (Fig. 11) and the results reveal that the catalysts exhibit 100% efficiency for the degradation of HD adsorbed with ozone after using it for 4 times. Similar results are obtained in case of other (5 wt% Mn, Fe/zeolite-13X) catalysts also except with bare zeolite-13X catalyst. Based on the results it is concluded that the impregnated metal oxide catalysts on zeolite-13X support can be reused several times without loss of its efficiency for the degradation of HD adsorbed using ozone as the oxidizing agent. The reason for not getting 100% efficiency with zeolite-13X materials might be due to decline of ozone decomposition efficiency of bare zeolite-13X at ambient temperature which is the key factor for activity of the catalyst with ozone. The reusability results concludes that impregnated zeolite-13X materials are the better choice for HD degradation compared to bare zeolite-13X materials.

Fig. 11 Reusability of 5 wt% Co/zeolite-13X catalyst for degradation of adsorbed HD in the presence of ozone as the oxidizing agent at GHSV: 30 000 h⁻¹, O₃ conc.: 4 mg min⁻¹, 3μL HD adsorbed on 100 mg of catalyst, total flow: 50 mL min⁻¹, reaction temp: 37 ± 3 °C.

4. Conclusions

The present study describes the complete degradation of HD adsorbed over bare zeolite-13X and M/zeolite-13X (M = 5 wt% Mn, Fe, Co) catalysts in presence of gaseous ozone. Depending on the reaction time the HD adsorbed can be degraded to its sulfoxide and sulfone as well as to total oxidation products using ozone gas. Little longer reaction times (>5 minutes) yields the complete oxidation products, whereas short reaction times (<1 minute) leads the formation of mustard sulfoxide and sulfone which are somewhat toxic compounds compared to HD. Nevertheless, 100% HD degradation is achieved in both the cases at ambient conditions. M/zeolite-13X materials exhibited 100% ozone decomposition efficiency even at ambient conditions for more than 15 minutes, but bare zeolite-13X materials exhibits the same efficiency only for 5 minutes. Furthermore, the metal oxides impregnated zeolite-13X materials are reusable for several times without loss of its efficiency. Therefore, M/zeolite-13X materials in presence of ozone can successfully be used for the complete degradation of adsorbed HD in Pressure Swing Adsorption (PSA) based NBC filtration systems for in situ degradation to non toxic compounds. This method can also applicable for the degradation of other CWA as well. In conclusion, the present method has more practical importance for the in situ degradation of CWA's and can be used for the decontamination of adsorbed CWA's in PSA based regenerable NBC filtration systems in combination with small quantity of gaseous ozone and metal oxides impregnated zeolite-13X materials.

Acknowledgements

The authors would like to thank Dr Lokendra Singh, Director DRDE for his keen interest and encouragement to carry out this study and also to permit the work for publishing.

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