J. Phys. Chem. C 2007, 111, 3233-3240
3233
Adsorption and Decomposition of Dimethyl Methylphosphonate on Y2O3 Nanoparticles Wesley O. Gordon, Brian M. Tissue, and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061-0212 ReceiVed: August 4, 2006; In Final Form: NoVember 7, 2006
Experimental studies explore the adsorption and decomposition pathways of the nerve agent simulant, dimethyl methylphosphonate (DMMP), on Y2O3 nanoparticles with mean diameters of 6 and 2 nm. DMMP adsorption on Y2O3 was studied in an ultrahigh vacuum surface science instrument with X-ray photoelectron spectroscopy and reflection absorption infrared spectroscopy. Uptake probability measurements reveal an initial sticking probability near unity, while vibrational spectroscopy indicates that adsorbed DMMP dissociates via cleavage of a P-OCH3 bond to form a bridging O-P-O surface species. The extent of DMMP decomposition is found to increase as the particle size decreases, suggesting that edge, corner, and defect sites play a major role in the outcome of the gas-surface interaction.
1. Introduction Decontamination of environmental toxins has been the focus of considerable research for several decades. In particular, chemical warfare agents are of great concern due to proliferation, spurring research into the adsorption and reactivity of chemical warfare agents and their analogues on a variety of materials. High-surface area metal oxides are one class of materials that show reactivity toward many environmental pollutants, including chemical warfare agents. Dimethyl methylphosphonate (DMMP) is a common simulant for organophosphorus nerve agents such as soman [O-pinacolyl methylphosphonofluoridate] or sarin [O-isopropyl methylphosphonofluoridate] because of its similar composition and structure. The adsorption and decomposition of DMMP on bulk oxides has been reported for WO3,1,2 FeOx,3,4 MnOx,5 TiO2,2,6-9 MgO,4,10,11 Al2O3,4,12-15 La2O3,4 and SiO216,17 and studied theoretically on MgO.18 In addition to these materials, DMMP has been studied on nickel, iron, copper, vanadium, and cerium oxides when supported on γ-Al2O3,15,19-21 and on titania-supported nickel and copper metal clusters.22,23 Room-temperature reactivity of DMMP on some of these metal oxides highlights their possible use in decontamination, filtration, or sensing of chemical warfare agents. Increased surface reactivity is expected for chemicals exposed to nanoscale metal-oxide particles. Enhanced reactive properties of very small metal oxide particles are attributed to higher surface areas, and more numerous reactive sites such as crystal edge, corner, or defect sites that represent a larger percentage of surface atoms when compared to analogous bulk materials.24,25 Studies of chlorinated hydrocarbons exposed to CaO nanoparticles demonstrated that smaller particles show increased adsorption and reactivity.26,27 Increasing surface area of CaO and MgO yielded more favorable reaction and adsorption characteristics for acidic gases28 and organics,29 and the shape of nanocrystals has been found to influence reactivity.24 DMMP interaction with metal-oxide nanomaterials has been studied with MgO30,31 and CaO32 nanoparticles and with heterogeneous CaO,32 MgO,33 and CeOx21 nanocomposites, confirming enhanced reactive properties for the nanomaterials. Actual chemical agents such as VX [O-ethyl S-(2-diisopropylaminoethyl) * Corresponding author. E-mail:
[email protected].
methylphosphonothioate] and soman were found to react at room temperature on nanosized Al2O3, CaO, and MgO.34-36 In addition, particle size enhanced reactivity for sarin was observed in a theoretical study on MgO.37 Y2O3 is among several metal oxides that have not been studied as possible reactive sorbants for chemical warfare agents. Yttria is an insulating material that can host optically active dopants38 and can stabilize ZrO2 for use in catalytic and ceramic materials.39 Although the literature is extensive for these applications, the reactive and catalytic properties of yttria have received little attention.40,41 This is surprising, given the large number of basic surface sites that yttrium oxide has41,42 that could result in adsorption and reactive properties similar to those of other commonly studied basic metal oxides, such as MgO. The basic character of yttrium oxide is underscored by its known propensity to react with CO2,43 forming surface carbonate groups.41,44 Our approach to investigate the sorption and reactive properties of chemical warfare agent simulants on yttria nanoparticles combines gas-phase condensation for particle synthesis with ultrahigh vacuum (UHV) surface analytical techniques. Laserheated gas-phase condensation in a clean vacuum chamber can be used to synthesize a wide variety of metal-oxide nanoparticles while simultaneously eliminating the need for purification to remove solvents, stabilizers, or reagents. Cleanliness of the synthetic method eliminates the requirement for thermal pretreatment and associated concern about changes to the metal oxide film that high temperatures may cause, such as annealing of defect sites. Our results indicate that these clean nanoparticulate metal oxide materials are reactive with organophosphorus chemical warfare agent simulants. 2. Experimental Section Nanoparticle Synthesis. The nanoparticle synthesis method has been described previously.45,46 In this study, we use a similar vaporization chamber that allows direct transfer of deposited nanoparticles into an UHV surface science analysis chamber. Y2O3 targets are made by cold pressing powder (Sigma-Aldrich, 99.99%) in a pellet press with a 13-mm die under 5 tons of pressure, followed by sintering overnight at 1000 °C. The synthesis chamber is first evacuated to a base pressure of 10-7
10.1021/jp0650376 CCC: $37.00 © 2007 American Chemical Society Published on Web 02/06/2007
3234 J. Phys. Chem. C, Vol. 111, No. 8, 2007
Figure 1. Schematic of the UHV surface analysis chamber. The nanoparticles are synthesized in vacuo via the laser-assisted gas-phase condensation technique and transferred directly into the main UHV chamber.
Torr, isolated from the pumps, and backfilled to either 1 or 10 Torr of N2 as measured with a nitrogen-calibrated Pirani gauge (Granville-Phillips). A 30 W cw-CO2 laser beam is focused to a ∼1 mm spot on the Y2O3 target. Under these conditions, a plume of evaporated material is cooled and confined by collisions with the background gas, leading to nucleation of nanoparticles from the supersaturated gas phase. Particles can grow, coalesce, and aggregate in the gas phase as they travel toward a collection substrate.47-49 The collector is a reflective 1-cm2 Au-coated glass substrate (EMF Corp.) that had been cleaned in piranha solution (conc. H2SO4/30% H2O2 7:3 by volume), rinsed with copious amounts of distilled, deionized water and ethanol, and then dried under a stream of ultrahigh purity N2. The substrate was then attached to a sample mount that was positioned 3 cm directly above the target. UHV Analysis Chamber. After 4 min of laser evaporation, the synthesis chamber was evacuated to base pressure, and the sample was transferred directly (in vacuo) into the UHV analysis chamber shown in Figure 1. The UHV chamber is pumped by a 2000 L/s turbomolecular pump and maintained at a base pressure of 8 × 10-10 Torr during experiments. The duration between nanoparticle synthesis and UHV transfer was typically less than 7 min. Once in UHV, sample position was controlled with a precision XYZ translation and rotation sample manipulator such that the sample was positioned into the beam path of a Bruker IFS 66v/S infrared spectrometer. This instrument was configured in the reflection mode where p-polarized light strikes the nanoparticle film at a grazing angle of approximately 86°, reflects from the gold substrate, and is detected by a liquidN2-cooled MCT (mercury-cadmium-telluride) detector located inside an external vacuum chamber. All reflection absorption infrared (RAIR) spectra are the average of 256 scans from 4000 to 800 cm-1 with a 2.5-mm aperture and 2-cm-1 resolution. X-ray photoelectron spectra were obtained (in the same UHV chamber) with a SPECs Phoibos 100 hemispherical analyzer equipped with an Mg KR radiation source. Sample cooling was achieved via a copper braid connection between the sample mount and the cold head of a helium compressor. Sample temperature was monitored with K-type thermocouple wire spotwelded to the sample mount. Exposure. After preliminary characterization of the samples by RAIRS and XPS, Ar was bubbled through the liquid simulant in a sealed glass doser, which was held at 80 °C. The simulant was entrained in the argon and introduced into the UHV
Gordon et al.
Figure 2. RAIR spectra of Y2O3 nanoparticles (a) before and (b) after exposure to DMMP. The peaks observed in spectrum (a) are due to hydroxyl and carbonate groups that are the result of adsorption and subsequent decomposition of trace background CO2 and H2O in the nanoparticle preparation chamber.
chamber using a precision leak valve. Partial pressures of gases were monitored with an SRS RGA 300 quadrupole mass spectrometer. Unless otherwise noted, the Y2O3 nanoparticle sample was held at room temperature (295 K). Materials. The chemical warfare agent simulants, dimethyl methylphosphonate (DMMP, 97%) and trimethyl phosphate (TMP, 99%), were obtained from Aldrich and purified by freeze-pump-thaw cycles prior to use. Further exposure experiments were performed with d6-deuterated dimethyl methyl phosphonate (OdP(OCD3)2CH3, d6-DMMP), which was synthesized following published procedures.13 HPLC-grade methanol (EMD), employed for comparison to DMMP adsorption, was used as received. Particle Characterization. For particle-size analysis, nanoparticles were dispersed in ethanol and dropped onto a carboncoated grid for electron microscopy or onto a silicon wafer for atomic force microscopy (AFM). The electron microscopy was performed with a Phillips EM 420 scanning transmission electron microscope (STEM) operated at 100 kV, and the AFM was performed with a Digital Instruments Multimode atomic force microscope operated in the tapping mode. Surface area was measured using the Brunauer-Emmett-Teller (BET) method with a Quantachrome NOVA 1000 analyzer at 77 K. A hemispherical collector plate was mounted 3 cm above the target to collect enough material for the BET analysis. Particle shape and crystallographic information was acquired with highresolution transmission electron microscopy (HRTEM) obtained with a Philips CM300FEG TEM. 3. Results Nanoparticle Characterization. Nanoparticle films were inspected by RAIR spectroscopy and XPS prior to DMMP exposure to determine the purity of the samples under investigation. We find that the time the particles remain in the synthesis chamber (
