Study of the catalytic destruction of dimethyl methylphosphonate(DMMP): oxidation over molybdenum(110)

Theoretical study of gas-phase detoxication of DMMP and DMPT using ammonia-borane and its analogous compound

The detoxication of DMMP (Dimethyl methylphosphonate) and DMPT (O, S-dimethyl methylphosphonothiolate) via hydrogenation have been investigated computationally employing density functional theory (DFT). In this present study, we aim to explore the direct molecular H₂ assisted as well as ammonia-borane (NH₃BH₃) and 3-methyl-1,2-BN-cyclopentane (denoted as cy-AB) assisted hydrogenation pathways of DMMP and DMPT in order to detoxify them. The detoxication of DMMP has been carried out by successive elimination of two -OMe groups. However, in the case of DMPT, two possibilities have been identified because of two different substituents, -OMe and -SMe. In possibility-I, the elimination of the -OMe group occurs at the beginning, followed by the -SMe group, whereas in possibility-II, the reverse order of elimination occurs for -OMe and -SMe groups. During the detoxication of DMMP using both NH₃BH₃ and cy-AB as the assisting reagents, the first step has been identified as the rate-determining step (RDS) in which the hydrogens attached to the N- and B-centers of NH₃BH₃ are transferred to the O-center of PO and P-center, respectively. In harmony with DMMP detoxication, for DMPT also, analyzing the activation barriers, it can be articulated that for both NH₃BH₃ and cy-AB assisted pathways, both the possibilities are equally feasible as in both the possibilities the common first step is the RDS. Therefore, our computational study is designed to explore the assisting efficiency of NH₃BH₃ and its cyclic analogue for detoxifying the OPCs.

Scattering Dynamics, Survival, and Dispersal of Dimethyl Methylphosphonate Interacting with the Surface of Multilayer Graphene

We explored the interaction of a molecular beam of dimethyl methylphosphonate with a multilayer graphene surface to better understand the fate of chemical warfare agents in the environment. The experiments were done at surface temperatures between 120 and 900 K and translational energies between 200 and 1500 meV. At the lowest temperatures, the dimethyl methylphosphonate is adsorbed, with the molecules next to the carbon surface held slightly more strongly than the bulk molecular film that grows with continued dosing. We measured the desorption energy for submonolayer coverage using modulated beam techniques and found a value of 290 meV (28 kJ/mol). At higher surface temperatures, where the residence times are very short, we measured the scattering of the dimethyl methylphosphonate as a function of angle and translational kinetic energy. For a surface temperature of 250 K, with translational kinetic energies between 200 and 1500 meV, much of the incident flux has nearly been accommodated by the surface temperature and has no memory of the incident momentum. The internal energy also seems to be at least partially accommodated. As the surface temperature increases, the scattering transitions to direct-inelastic reflection, where much of the incident translational energy is retained, and the intensity of the scattering peaks superspecularly toward glancing final angles. These results demonstrate the efficacy of using kinetic energy controlled molecular beams to probe the interactions of complex organic molecules with well-defined surfaces, extending our fundamental understanding of how the dynamics for such systems crossover from trapping-desorption to direct inelastic scattering. Moreover, these results indicate that simulations that model the dispersal of chemical warfare agents using common interfaces in the environment need to account for multiple bounce trajectories and survival of the impinging molecules.

Decomposition of O,S-dimethyl methylphosphonothiolate by ammonia on magnesium oxide: a theoretical study of catalytic detoxification of a chemical warfare agent

The adsorption of a model nerve agent, O,S-dimethyl methylphosphonothiolate (DMPT), on the hydroxylated and unhydroxylated nano-crystalline magnesium oxide surface followed by the nucleophilic attack of ammonia (NH3) is investigated at the M06-2X/6-311++G(d,p) level of theory using the representative cluster models. The geometries of DMPT and NH3 are fully optimized, while the geometry of the oxide fragment is kept frozen. The main insight of this study is the incorporation of the Eley-Rideal mechanism for the first time in the detoxification process, where one of the reactant molecules (DMPT) is adsorbed and the other one (NH3) reacts with it directly impinging from the gas phase. There are two possible pathways of nucleophilic detoxification, either concerted or stepwise. The nature of the first transition state of nucleophilic attack in both pathways is the vital step for degradation. Our calculated results predict that the reaction of DMPT with NH3 gives rise to both P-S and P-O bond cleavage completely. Also, the P-S cleavage is found to be the favorable one over P-O bond breaking. The exploration of the overall reaction mechanism has established the catalytic activity of nano-crystalline MgO in nucleophilic DMPT degradation, as in all cases the activation barriers have reduced compared to the previously reported aminolysis of DMPT in the gas phase. Interestingly, the hydroxylated model has better catalytic performance than the unhydroxylated one.

VIBRATIONAL SPECTROSCOPY OF CHEMICAL AGENTS SIMULANTS, DEGRADATION PRODUCTS OF CHEMICAL AGENTS AND TOXIC INDUSTRIAL COMPOUNDS

This paper focuses on the measurement of spectroscopic signatures of Chemical Warfare Agent Simulants (CWAS), degradation products of chemical agents and Toxic Industrial Compounds (TIC) using vibrational spectroscopy. Raman Microscopy, Fourier Transform Infrared Spectroscopy in liquid and gas phase and Fiber Optics Coupled-Grazing Angle Probe-FTIR were used to characterize the spectroscopic information of target threat agents. Ab initio chemical calculations of energy minimization and FTIR spectra of Chemical Warfare Agents were accompanied by Cluster Analysis to correlate spectral information of real agents and simulants.

Oxidative Catalytic Decomposition of Toxic Gases Using Hydroxyapatite and Fluorhydroxyapatite

An oxidative catalytic route for the decomposition of nerve gases was investigated using hydroxyapatite (HA, chemical composition Ca10(PO4)6(OH)2) and its partially fluorinated analog fluorhydroxyapatite (FHA, Ca10(PO4)6Fx(OH)2−x). Samples were prepared with surface areas ranging from 34 to 238 m2/g to study surface area effects; 1.2 wt. % platinum was deposited on one substrate to investigate the effect of a transition metal on activity and selectivity. Reaction studies were performed using dimethyl methylphosphonate (DMMP), a nerve gas simulant, in a stream of 80 percent nitrogen and 20 percent oxygen at 573 K and atmospheric pressure. High surface area FHA samples showed an increase in the "protection period" (period of 100% conversion) with increasing fluorine substitution; such an increase was not seen for low surface area FHA samples. In the absence of platinum, the reaction products were methanol and dimethyl ether; with platinum, CO2 was also obtained.

Decomposition of dimethyl methylphosphonate on Pt, Au, and Au-Pt clusters supported on TiO2(110)

The decomposition of dimethyl methylphosphonate (DMMP) was studied by temperature programmed desorption (TPD), X-ray photoelectron spectroscopy (XPS), and Auger electron spectroscopy (AES) on TiO(2)-supported Pt, Au, and Au-Pt clusters as well as on TiO(2)(110) itself. In agreement with previous work, TPD experiments for DMMP on TiO(2)(110) showed that methyl and methane were the main gaseous products. Multiple DMMP adsorption-reaction cycles on TiO(2)(110) demonstrated that active sites for DMMP decomposition were blocked after a single cycle, but some activity for methyl production was sustained even after five cycles. Furthermore, the activity of the TiO(2) surface could be regenerated by heating in O(2) at 800 K or heating in vacuum to 965 K to remove surface carbon and phosphorus, which are byproducts of DMMP decomposition. On 0.5 ML Pt clusters deposited on TiO(2)(110), TPD studies of DMMP reaction showed that CO and H(2) were the main gas products, with methyl and methane as minor products. The Pt clusters were more active than TiO(2) both in terms of the total amount of DMMP reaction and the ability to break C-H, P-O, and P-OCH(3) bonds in DMMP. However, the Pt clusters had no sustained activity for DMMP decomposition, since the product yields dropped to zero after a single adsorption-reaction cycle. This loss of activity is attributed to a combination of poisoning of active sites by surface phosphorus species and encapsulation of the Pt clusters by reduced titania after heating above 600 K due to strong metal support interactions (SMSI). On 0.5 ML Au clusters, CO and H(2) were also the main products detected in TPD experiments, in addition to methane and methyl produced from reaction on the support. The Au clusters were less active for DMMP decomposition to CO and H(2) as well as P-O bond scission, but surface phosphorus was removed from the Au clusters by desorption at approximately 900 K. Au-Pt bimetallic clusters on TiO(2)(110) were prepared by depositing 0.25 ML of Pt followed by 0.25 ML of Au, and the bimetallic surfaces exhibited activity intermediate between that of pure Pt and pure Au in terms of CO and H(2) desorption yields. However, there is evidence that the production of methane from DMMP decomposition occurs at Au-Pt sites.

Corner capping of silsesquioxane cages by chemical warfare agent simulants

The room-temperature uptake and reactivity of gas-phase methyl dichlorophosphate (MDCP) and trichlorophosphate (TCP) within trisilanolphenyl-polyhedral oligomeric silsesquioxane (POSS) Langmuir-Blodgett films are investigated. The halogenated phosphate molecules are found to readily diffuse into and react with the hybrid inorganic-organic silicon-oxide films under ambient conditions. Reflection absorption infrared spectroscopy (RAIRS), X-ray photoelectron spectroscopy (XPS), and fast atom bombardment-mass spectrometry (FAB-MS) measurements suggest that the chlorophosphates undergo hydrolysis with the silanol groups of the POSS LB-film. Substitution and elimination reactions appear to cap the corner of the POSS molecules, leaving a surface-bound phosphoryl group and a resulting structure that is highly stable at elevated temperatures.

Adsorption and Photodegradation of Dimethyl Methylphosphonate Vapor at TiO2 Surfaces

The adsorption and degradation of the nerve agent simulant dimethyl methylphosphonate (DMMP) over UV-irradiated TiO(2) powders and thin films has been investigated. Adsorption of vapor-phase DMMP on TiO(2) powder is characterized by diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS). Photochemically assisted oxidation of adsorbed DMMP is carried out in situ by irradiation of samples in the DRIFTS accessory, giving kinetic data and information on specific site binding of DMMP and catalyst poisoning. Gas-phase intermediates from a static vapor phase reaction are identified by gas chromatography-mass spectrometry analysis, and surface-bound intermediates and products are analyzed by high-performance liquid chromatography-mass spectrometry, and ion chromatography of both aqueous and organic extractions from the TiO(2). Adsorbed DMMP is photodegraded in a stepwise fashion to give methylphosphonic acid, PO(4)(3-), H(2)O, and CO(2) as products. A proposed reaction pathway is consistent with a rapid degradation of DMMP but with extensive poisoning of the catalyst by surface-bound phosphonate products.

Dimethyl methylphosphonate decomposition on Cu surfaces: supported Cu nanoclusters and films on TiO2(110)

The thermal decomposition of dimethyl methylphosphonate (DMMP), which is a simulant molecule for organophosphorus nerve agents, has been investigated on Cu clusters as well as on Cu films deposited on a TiO(2)(110) surface. Scanning tunneling microscopy studies were conducted to characterize the cluster sizes and surface morphologies of the deposited Cu clusters and films. Temperature-programmed desorption experiments demonstrated that the surface chemistry of DMMP is not sensitive to the size of the Cu clusters over the range studied in this work. DMMP reaction on an annealed 40 monolayer Cu film resulted in the desorption of H(2), methane, methyl, formaldehyde, methanol, and molecular DMMP, and reaction on the small (4.4 +/- 0.9 nm diameter, 1.8 +/- 0.6 nm height) and large (10.7 +/- 1.9 nm diameter, 4.8 +/- 1.0 nm height) Cu clusters generated similar products. Formaldehyde and methane production is believed to occur via a methoxy intermediate on the Cu surface. These products are favored on the higher coverage Cu films that completely cover the TiO(2) surface since competing reaction pathways on TiO(2) are suppressed. X-ray photoelectron spectroscopy studies showed that DMMP begins to decompose on the Cu clusters upon adsorption at room temperature and that atomic carbon, atomic phosphorus, and PO(x) remain on the surface after DMMP decomposition.