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Aug 5, 2009 - In the present study, we focus on the thermal decomposition of another important ... DMMP is a simulant for the nerve agent, Sarin (GB)...
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15684

J. Phys. Chem. C 2009, 113, 15684–15691

Thermal Decomposition of a Chemical Warfare Agent Simulant (DMMP) on TiO2: Adsorbate Reactions with Lattice Oxygen as Studied by Infrared Spectroscopy Dimitar A. Panayotov and John R. Morris* Department of Chemistry, Virginia Tech, Blacksburg, Virginia 24061 ReceiVed: April 20, 2009; ReVised Manuscript ReceiVed: June 2, 2009

The thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on high surface area TiO2 nanoparticles (Degussa P25) has been studied by transmission infrared spectroscopy. The dominant reaction channel in the low-temperature regime from 295 to 400 K is the nucleophilic attack of adsorbed DMMP by neighboring oxygen to produce Ti-OCH3 and a variety of P-Ox surface bound groups. Arrhenius studies reveal an activation energy of ∼48 kJ mol-1 for the conversion of surface bound phosphoryl (PdO) groups into P-Ox species. Above 400 K, thermally activated lattice oxygen begins to play a dominant role in driving the oxidation of surface Ti-OCH3 groups. The electrons left behind following the extraction of lattice oxygen are observed by a broad rise in the infrared absorbance as the electrons are excited from shallow traps into the conduction band. Tracking lattice oxygen removal as a function of temperature reveals an activation energy of ∼33 kJ mol-1, over the temperature range ∼400-600 K, for the high-temperature oxidation of strongly bound surface adsorbates. These measurements can be employed to provide a more complete understanding of the key role that lattice oxygen plays in the degradation of adsorbates on the surface of active nanoparticulate semiconductor oxides. Introduction Research into thermally driven heterogeneous reactions on nanoparticulate metal-oxide surfaces is of fundamental importance in the development of new materials for chemical and photochemical catalysis, including the decontamination of chemical warfare agents (CWAs).1 A number of recent studies have provided insight into the mechanisms of thermally2 and photochemically3 driven reactions between TiO2 and 2-chloroethyl ethyl sulfide (2-CEES), a simulant for the sulfur-based CWA, mustard gas (HD). In the present study, we focus on the thermal decomposition of another important CWA simulant, dimethyl methylphosphonate (DMMP), when it reacts on the surface of Degussa P25 nanoparticulate TiO2. DMMP is a simulant for the nerve agent, Sarin (GB).4 Activation of lattice oxygen from semiconductor oxides is the key step in the mechanism that drives the thermal decomposition of a variety of organic adsorbates, including CWAs, in the absence of gas phase oxygen.2,5-9 This mechanism is also operative in photochemically initiated oxidation reactions on the surfaces of many semiconductors.3,10-14 The involvement of lattice oxygen in thermally driven catalytic reactions of organic compounds is well recognized as the Mars and van Krevelen oxidation mechanism.7,8,15-17 In addition, activation of lattice oxygen in the presence of supported nanoparticulate Au on reducible oxides has recently attracted the attention of researchers in efforts to develop effective catalysts for combustion of volatile organic compounds (VOCs).18,19 We have recently shown that TiO2 supported Au nanoparticles can promote the extraction of lattice oxygen to oxidize DMMP to carbonyl products, even at room temperature.20 Transmission5,20-34 and reflection35-41 absorption infrared spectroscopic techniques have been employed by a number of research groups to study the surface chemistry of DMMP on * Telephone: 540-231-2472. E-mail: [email protected].

different metal oxide materials, including TiO2, in both thermal and photochemical regimes. The utility of infrared (IR) probes stems from the nondestructive nature of the technique, which enables detailed in situ studies performed during a reaction between a CWA-simulant and monolithic or nanoparticulate material. IR studies of DMMP-TiO2 chemistry have revealed that the frequency of the phosphoryl group PdO stretching mode is red-shifted from 1276 to 1242 cm-1 as gas phase DMMP is condensed at 190 K.26 Molecular physisorption through bonding to hydroxyl groups further shifts the PdO stretching mode to ∼1210 cm-1.26,32,34,40 Bands at ∼2827 and ∼1118 cm-1, attributed to the vibrations of the methoxy methyl group bound directly to the oxygen atom from the TiO2 surface, provided further evidence for initial DMMP hydrolysis at a single methoxy site.26,32,40 The appearance of modes in the region of 1200-1000 cm-1, due to P-Ox groups, signals the decomposition of DMMP.26,27,32,34,40 Together, this work has been used to develop a sound understanding of many important aspects of the mechanism that govern uptake and reactivity of adsorbed organophosphonates on surfaces. Previous studies on thermal decomposition and photocatalytic oxidation of DMMP over TiO2 nanoparticulate surfaces have shown that surface hydroxyl groups (the product of background H2O adsorption) are the main reactive surface sites, where DMMP is adsorbed molecularly via hydrogen bonding through the PdO group.26,27,32,34,40 At low temperatures (165-200 K), hydrogen-bonding dominates the interaction of DMMP with TiO2.26 In addition, Lewis acid sites, either Ti4+ on oxidized or Ti3+ (Ti2+) on reduced TiO2, play an important role in adsorption.26,42 At temperatures above 214 K, the phosphoryl group is transformed into a variety of P-Ox species.26 At room temperature, hydrolysis of adsorbed DMMP by surface hydroxyl groups leads to production of methanol and methyl methylphosphonate (MMP).32 Simultaneously, surface-bonded methoxy groups (Ti-OCH3) are produced.26,32,40 The methyl group of DMMP is found to decompose only at high temperatures (>800

10.1021/jp9036233 CCC: $40.75  2009 American Chemical Society Published on Web 08/05/2009

Thermal Decomposition of DMMP on TiO2 K).5,26,34,43-45 However, Mitchell and co-workers have shown that P-CH3 bond scission via oxidation by lattice oxygen occurs at much lower temperatures (473-573 K) on the highly reducible oxides Fe2O35,6 and CeO2.6 We have recently shown that, on the surface of highly dehydroxylated oxidized TiO2, the initial uptake of DMMP occurs through both molecular and reactive adsorption.46 The molecular adsorption channel is driven by hydrogen bonding to the few isolated hydroxyl groups initially present on the surface of the particles. Reactive chemisorption appears to occur through interaction with Lewis acid sites and active oxygen species present on the initial TiO2 surface to produce carbonylcontaining moieties. However, the reactive sites are quickly poisoned by oxidation products that contain a nonvolatile phosphorus compound.46 In addition to directly probing vibrational modes of surface adsorbates during a reaction, a number of recent studies have shown that IR spectroscopy can be used to study the role of oxygen in the overall chemistry of semiconductor transition metal oxides.47 In the case of n-type semiconducting oxides, slight reduction by extraction of lattice oxygen (originating from the surface or deeper in the lattice) can lead to optical behavior that is attributed to the absorption of radiation by delocalized conduction band electrons. In the case of p-type semiconducting oxides, the reverse behavior has been observed.47 For n-type semiconducting TiO2, studies have shown that thermal activation2 and photoactivation3 cause lattice oxygen to interact with preadsorbed 2-CEES. The extracted lattice oxygen, which leaves behind an oxygen vacancy with two excess electrons, is usually attributed to 2-fold coordinated bridging sites.48-52 The two excess electrons may be transferred to the two neighboring Ti atoms to populate the Ti3d states, energetically located in the band gap. The energetics of electron trap states for a variety of TiO2 materials have been evaluated by several experimental methodologies.48-50,53,54 In nanoparticulate materials, the enhanced surface area and interconnected particles give rise to a large number of energy traps at the interface and grain boundaries.50,53-55 The broad energy distribution of trap states has been shown to be responsible for the large range of measured times for electron-hole (e-h) pair recombination under UV irradiation of titania nanoparticles.53,56-58 The chemical nature of a localized energy level just below the conduction band edge is well established as the Ti3+(3d) state48,56,59-63 and is observed by electron spin resonance (ESR) as paramagnetic TiIII centers.55,64-66 Ghosh et al.67 reported a large number of possible trapping sites for electrons or holes throughout the band gap region of single crystal rutile. They have shown that at least eight electron traps may exist at energies up to 0.87 eV below the conduction band edge for single crystal rutile. The origin of these states has been assigned to interstitial Ti3+ associated with oxygen vacancies in the bulk.52,54 At elevated temperatures, trapped electrons receive enough energy to be excited and delocalized into the conduction band.50 Once produced after high temperature (960 K) vacuum treatment of nanoparticulate TiO2, the delocalized conduction band electrons can be directly monitored by IR spectroscopy.68,69 A broad monotonic increase in the IR absorbance from 4000 to ∼1000 cm-1, which is indefinitely stable over time, has been observed at room temperature.68,69 The featureless absorption, which is due to mobile charge carriers or conduction-band electrons (CBEs), was first observed for silicon70,71 and germanium.72,73 The effect has been well studied for many types of semiconducting materials.2,3,47,57,58,66,68,69,74-82 According to Yamakata et al.,57,58 the broad IR absorbance originates from the population

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15685 of the conduction band by photogenerated electrons trapped in shallow midgap states via two routes: the intra-CB transition of free electrons thermally excited from the trap state, and the direct optical transition from the trap state to the CB. The contribution of each of the two possible transitions to the observed spectrum was difficult to identify in their work.57,58 However, these studies show the utility of infrared spectroscopy for studying electronic processes in TiO2. In this study, we employ transmission Fourier transform infrared (FTIR) spectroscopy to directly observe the thermal activation of lattice oxygen during oxidative degradation of DMMP. This is accomplished by monitoring the vibrational modes of adsorbed DMMP while, simultaneously, tracking the production of trapped electrons through observation of the rise in absorbance across the mid-infrared range of the spectra. These measurements are used to provide a more complete understanding of the key role that lattice oxygen plays in the degradation of adsorbates on the surface of nanoparticulate TiO2. Experimental Section The transmission FTIR experiments were performed in a stainless steel IR cell described previously.2,83 The IR cell was connected to a high vacuum system that consists of a Pfeiffer Vacuum 60 L s-1 turbomolecular pump and a Varian 20 L s-1 ion pump. The base pressure, following system bake-out, is ∼1 × 10-8 Torr. The TiO2 used in this work was Degussa P25 powder (∼70% anatase, 30% rutile, ∼50 m2 g-1).84 A flat photoetched tungsten grid (0.0508 mm thick, with 0.22 × 0.22 mm holes, obtained from Buckbee-Mears, St. Paul, MN) was used as the sample support. Approximately 6 mg of Degussa TiO2 was pressed (12 000 lb in.-2) onto the grid as a circular spot with a diameter of 7 mm. The temperature of the grid and the TiO2 sample spot was measured by welding a type-K thermocouple directly to the tungsten grid. The sample grid was then mounted in the vacuum IR cell on nickel clamps. These nickel clamps were connected to a regulated high current power supply. Cooling of the tungsten grid and nanoparticle sample was accomplished with either liquid nitrogen or a dry ice-acetone mixture held within a cryogenic trap attached to the vacuum cell. The thermocouple signal from the tungsten grid was used in concert with an electronic control unit to regulate the electric current supplied to the grid to maintain constant temperature to within (1 K. The dimethyl methylphosphonate (97%) used for this work was obtained from Sigma-Aldrich, Inc. It was stored in a glass bulb attached to the high vacuum system and was purified via five freeze-pump-thaw cycles. The oxygen used was obtained from Airgas Inc. and was ultrahigh purity grade (99.999% purity). A calibrated volume of the stainless steel gas line connected to a capacitance manometer (Baratron, 116A, MKS, range 10-3-103 Torr) was used for dosing of either DMMP or O2 into the IR vacuum cell. The transmission IR spectra were collected on an FTIR spectrometer (Mattson Research Series I) equipped with a mercury cadmium telluride (MCT) detector. All spectra in the region from 4000 to 500 cm-1 were scanned at 4 cm-1 resolution in the ratio mode using a vacant spot of the tungsten grid as a reference. Under adsorption regimes, the spectral changes were tracked with 200 scans, while 1000 scans were accumulated for each spectrum in the steady state regimes. Under dynamic temperature regimes, a heating rate of 12 K min-1 was used and 100 scans were collected for each spectrum in 10 K intervals.

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Figure 1. Difference IR spectra of TiO2 nanoparticles precovered with DMMP (0.6 Torr); the spectrum of the clean TiO2 obtained at room temperature before DMMP adsorption is subtracted from each spectrum. Spectrum a is obtained after 70 min of prolonged exposure to DMMP and evacuation; spectra b-c are collected during heating of the sample from 305 to 400 K at a rate of 12 K/min. For each spectrum, 100 scans averaged; the collection time per spectrum was 30 s.

A standard activation procedure for obtaining a clean, highly dehydroxylated TiO2 sample was employed.46 This procedure involves heating in vacuum to 675 K at a rate of 12 K min-1; vacuum treatment at 675 K for 4 h; oxidation with molecular O2 at 20 Torr pressure for 1 h; evacuation to 1 × 10-6 Torr; and readmission of 20 Torr O2 for 30 min. The sample was cooled to 475 K and evacuated to 1 × 10-6 Torr for 20 min, and then the sample was further cooled to room temperature in vacuum (1 × 10-6 Torr). The next stage of thermal activation was carried out in vacuum from 295 to 675 K at a heating rate of 12 K min-1 while the cryogenic trap was cooled to 200 K by a dry ice-acetone mixture. This cooling assured the condensation of remaining desorbing gases and thus the production of a highly dehydroxylated TiO2 surface.46 The reference spectra for the clean, highly dehydroxylated TiO2 sample and the sample saturated by DMMP (0.6 Torr) after 120 min of exposure were recorded at room temperature. During the adsorption of DMMP, the cryogenic trap was cooled to ∼200 K with a dry ice-acetone mixture to prevent condensable gas phase products, for example, water, from readsorbing on the TiO2 surface. The thermal decomposition studies of DMMP were carried out in vacuum at a heating rate of 12 K min-1. Results and Discussion We have recently shown that the room temperature uptake of DMMP on the highly dehydroxylated surface of TiO2 involves both molecular and reactive adsorption.46 Briefly, the molecular adsorption of DMMP occurs through hydrogen bonding to trace amounts of isolated surface hydroxyl groups, while the reactive channel likely occurs on coordinatively unsaturated (CUS) Ti4+ Lewis acid sites.46 Previous work suggests that highly reactive surface oxygen, that is present at locations adjacent to the CUS Ti4+ adsorption sites, oxidizes DMMP to produce carbonyl-containing surface adsorbates. The carbonyl-containing species are short-lived and rapidly convert to carboxylate or formate products. In the present study, we further explore the interaction of DMMP on nanoparticulate titania by simultaneously tracking the decomposition channels and monitoring the liberation of oxygen from the lattice during thermal cycling of the sample. Low-Temperature Annealing. Figure 1 shows the evolution of surface-adsorbed DMMP and associated products during

thermal annealing of the sample from room temperature to 413 K. The initial room-temperature spectrum, labeled (a) in Figure 1, represents that of DMMP-saturated TiO2 nanoparticles. In the room temperature spectrum, we find evidence for both molecularly and dissociatively adsorbed species.46 Molecular adsorption of DMMP is evidenced by the methoxy group stretching modes shown in Figure 1A, spectrum a: νa(CH3O) at 2960 cm-1 and νs(CH3O) at 2857 cm-1; the methyl group stretching modes νa(CH3P) at 3000 cm-1 and νs(CH3P) at 2923 cm-1. The corresponding deformation modes are shown in Figure 1B, spectrum a: δa(CH3O) at 1467 cm-1 and δs(CH3O) at 1454 cm-1 for the methoxy groups; δa(CH3P) at 1415 cm-1 and δs(CH3P) at 1314 cm-1 for the methyl group. The stretching vibration ν(PdO) appears at 1235 cm-1, and we previously assigned this mode to molecular DMMP physisorbed on the highly dehydroxylated surface of TiO2 via hydrogen bond PdO · · · HO formation.46 The mode at 1215 cm-1 is assigned to the phosphoryl group of molecular DMMP physisorbed on Lewis acid sites. The CO stretches of the methoxy groups appear at 1067 and 1038 cm-1.46 Reactive adsorption of DMMP is evidenced by several spectral features, including the νsTi-(OCH3) mode at 2820 cm-1 and the FTi-(OCH3) mode at 1120 cm-1 for methoxy groups bound to surface CUS Ti4+ Lewis acid sites.46 The strongly perturbed vibration of the phosphoryl group at 1190 cm-1 and a variety of modes assigned to single bonded P-Ox groups (broad band from 1170 to 1090 cm-1) also indicate decomposition upon adsorption.46 As the DMMP-saturated surface is heated under vacuum (Figure 1, spectra b-c), key changes in the IR spectra indicate that new reaction channels may open that rapidly degrade molecularly adsorbed DMMP and other phosphoryl-containing species. In the high-energy range, the most dramatic change arises from the disappearance of the hydrogen-bonded OH stretching modes at 3500-2500 cm-1. Accompanying the decrease of signal associated with surface-bound OH groups, the intensity of bands due to P-OCH3 groups at 2957 and 2857 cm-1 decline, while the intensity of the band at 2828 cm-1 (due to surface methoxy species) increases (Figure 1A). These changes in the ν(CHx) regions indicate that, in the temperature region 300-400 K, the surface bound DMMP transforms into methyl methylphosphonate (MMP) through loss of a methoxy group, as previously reported in the literature.24,26,34 In the low-

Thermal Decomposition of DMMP on TiO2

Figure 2. (A) Arrhenius plot of the kinetic data for the conversion of PdO groups into O-P-O groups in the temperature region 300-400 K (a) for the removal of ν(PdO) modes at 1235-1215 cm-1 and (b) for the emergence of the νa(O-P-O) mode at 1100-1097 cm-1. A standard Lorentzian fitting procedure was used to obtain the intensity of bands in the relevant regions. (B) Schematic diagram illustrating the proposed mechanism of DMMP adsorption through the phosphoryl group to a Lewis acid site and the consequent conversion of PdO groups into O-P-O groups.

energy region (Figure 1B), the stable intensity and energy of the δs(CH3P) mode at 1313 cm-1 shows that MMP and other phosphorus-containing fragments remain bound to the surface. The phosphoryl-containing species also appear to remain adsorbed on the surface, but they undergo extensive transformation: the ν(PdO) modes at 1235 and 1215 cm-1 (described above) are systematically converted into O-P-O groups with νa(O-P-O) and νs(O-P-O) modes at 1100-1097 and 1068 cm-1, respectively.24,26,32,40,43,85 The direct conversion of PdO moieties is evidenced by the isosbestic point at 1195 cm-1. Figure 2A summarizes the temperature dependence of the IR modes by showing an Arrhenius plot for the temperature region 300-400 K. An Arrhenius analysis of the change in the PdO mode during annealing indicates that the activation energy for the conversion of this group into an O-P-O species is 49 ( 3 kJ mol-1. A similar result is obtained by analyzing the rise in the νa(O-P-O) mode during heating, which provides an activation energy of 47 ( 2 kJ mol-1. The values for the activation energy of phosphoryl conversion agree well with the work of Templeton and Weinberg85 on similar systems. Their studies propose a mechanism for DMMP decomposition that involves bimolecular nucleophilic substitution at the phosphorus (SN2(P)), leading to P-O bond cleavage. This mechanism is analogous to that suggested for neutral and alkaline hydrolysis of phosphonate esters, where activation energies of 52-62 kJ mol-1 have been reported.85 Figure 2B illustrates a possible initial reaction mechanism by showing the conversion of the phosphoryl group into P-Ox adsorbates. In accord with the IR observations of Figure 1, the schematic shows how the conversion may coincide with the consumption of hydrogen-bonded OH groups. However, the further fate of hydroxyls is not clear. As suggested by Rusu and Yates, the disappearance of hydroxyls may be correlated with the liberation of gaseous CH3OH.26 Activation of Lattice Oxygen at Elevated Temperatures. As discussed above, heating the DMMP-saturated particles to 413 K simply transforms the adsorbed DMMP into MMP with a surface bound bridging O-P-O moiety; however, the chemistry changes dramatically as the temperature is elevated from 413 to 673 K (see Figure 3A). The extensive loss of

J. Phys. Chem. C, Vol. 113, No. 35, 2009 15687 ν(CH3O) stretching modes at 2957 and 2857 cm-1 indicate the removal of methoxy groups from the remaining MMP and conversion to surface-bound methyl phosphonate (MP).26 As shown in Figure S1 of the Supporting Information, the removal of a methoxy group from MMP is faster than that of a surface methoxy group, Ti-OCH3. At 675 K, the only distinct vibrational modes that remain are represented by bands attributed to νs(CH3P) at 2927 cm-1 and δs(CH3P) at 1309 cm-1. The most striking feature of the spectra in Figure 3A is the significant rise in absorbance across the entire mid-infrared region. It should be stressed that the spectra in Figure 3 are not artificially offset from one another; rather, we find that the absorbance systematically increases in the whole region from 500 to 4000 cm-1 during heating above ∼400 K. The comparison of Figure 3A to B clearly demonstrates that the large broad rise in absorbance above 1000 cm-1 does not occur to nearly the same extent for clean TiO2 particles. The small rise in the background for the clean TiO2 during annealing is due to the population of the conduction band by electrons via thermal excitation from shallow traps or ionized impurities originally present in the sample.69 The remarkable increase in background signal during heating of the MMP-covered TiO2 sample is most likely due to production of electrons released as oxygen is extracted from the lattice during the oxidation of surface adsorbates. Similar results have been reported in studies of the thermal decomposition of 2-CEES on TiO2 particles.2 Previous work suggests that the liberated bridging lattice oxygen leaves behind electron-hole pairs that are trapped within the band gap.2,69 Thermal activation causes neutral atomic oxygen to diffuse to the particle surface, leaving behind an O vacancy (VO) with two adjacent Ti3+ sites, that is, the charge remains associated with the pair of Ti3+ sites. The coordination number of these sites decreases from 6 to 5, and their formal electron charge increases by 1.49,86-88 These electrons become trapped in shallow donor states (Ti3+ (3d)1) that are located several hundreds of meV below the conduction band (CB) edge of TiO2.49,50,54 Excitation of shallow trapped electrons by IR radiation into a continuum of electronic states in the conduction band results in a featureless broad IR absorbance,2,3,66,68,69,74,75,77-81 as we observe and report in Figure 3. One of the most intriguing aspects of the process described above is that it enables one to use infrared spectroscopy to not only track the vibrational frequencies of adsorbates, but also follow the activation of lattice oxygen during the reaction to build a complete understanding of the overall chemistry. Figure 4A reveals a strong correlation between the population of the trapped electrons (as monitored by the rise in the IR background absorbance at 1800 cm-1 from Figure 3A) and the disappearance of P-OCH3 and Ti-OCH3 groups (as measured by the integrated intensity of stretching modes, at 3957 and 3827 cm-1). As shown in Figure 4A, the decomposition of surface Ti-OCH3 groups is directly correlated with the production of trapped electrons, that is, with the process of lattice oxygen extraction. As illustrated by the schematic in Figure 5, we propose that thermally activated bridging lattice oxygen leaves behind Ti3+-VO-Ti3+ donors and moves as a highly reactive O atom to the surface where it burns a methoxy group that is bound to a CUS Ti4+ Lewis acid site. Nucleophilic attack of the surface bond-methoxy species by lattice oxygen has been previously proposed by Templeton and Weinberg for DMMP decomposition on aluminum oxide.43,85 The IR observation of trapped electrons in our work appears to verify that this mechanism also occurs during heating of TiO2 nanoparticles.

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Figure 3. Development of broad band IR absorbance due to excitation of shallow-trapped electrons into a continuum of states in the conduction band. (A) Selected difference IR spectra of TiO2 nanoparticles precovered with MMP; the spectrum of clean TiO2 obtained at room temperature is subtracted from each spectrum: Spectra a-b are collected during heating of the sample from 413 to 675 K at a rate of 12 K min-1. For each spectrum, 100 scans were averaged; the collection time per spectrum was 30 s. (B) Selected difference IR spectra of clean TiO2 nanoparticles; spectra a-b were collected during heating of the sample from 363 to 675 K at a rate of 12 K min-1. For each spectrum, 100 scans were averaged. The collection time per spectrum was 30 s.

Figure 4. (A) Disappearance of P-OCH3 and Ti-OCH3 groups (as measured by the normalized integrated intensity of stretching modes, at 3957 and 3827 cm-1 using the spectra in Figure 3A) as a function of the population of trapped electrons (as monitored by the growth in the IR background absorbance at 1800 cm-1 from Figure 3A). To obtain the intensity of bands in the 3100-2700 cm-1 region, we subtracted a linear background and used a Lorentzian fitting procedure. (B) Arrhenius plot of the temperature dependence to the IR background growth due to excitation of trapped electrons for two cases: (a) the TiO2 sample covered with DMMP and (b) the clean TiO2 sample. The analysis was performed by using the spectra from Figure 3A and B, respectively.

The correlation we observe between the decomposition of surface methoxy species and the extraction of lattice oxygen is in general agreement with previously observed correlations between the decomposition of surface ethoxy and chloroethoxy species and the production of trapped electrons on anatase and rutile TiO2.2 We postulate here that, in the absence of gas phase oxygen, the decomposition of surface methoxy (or ethoxy) adsorbates via extraction of lattice oxygen is the main reaction channel for high temperature degradation of both CWAs: DMMP and 2-CEES.

Figure 4B provides further insight into the mechanistic details of thermal degradation of MMP by presenting an Arrhenius plot of the temperature dependence to the broad rise in IR signal for the two samples: (a) the TiO2 covered by MMP and (b) the clean TiO2 sample. For the adsorbate-covered particles, the analysis suggests that the chemistry is governed by two mechanisms that are active under different thermal regimes. The linear behavior of the data over the thermal range of 360-423 K suggests that the production of trapped (or conduction band) electrons requires an activation energy of 15 ( 1 kJ mol-1

Thermal Decomposition of DMMP on TiO2

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Figure 5. Schematic diagram illustrating thermally activated bridging lattice oxygen leaving behind Ti3+-VO-Ti3+ donors. The oxygen atom diffuses to the particle surface where it burns a methoxy group that is bound to a CUS Ti4+ Lewis acid site. Electrons trapped at shallow donor states are detected in our experiments when they are excited into the conduction band via absorption of IR radiation. The band gap energy of Degussa P25 TiO2 is taken from ref 92.

(∼0.16 eV), which is identical to the activation energy for electronic excitation in the clean particles, as determined by thermal analysis (360-673 K), also shown in Figure 4B. The activation energy for trapped electron production on both samples is very similar to the activation energy reported for electronic conductivity of a vacuum treated (673 K) rutile film (0.08 eV).89 Therefore, we attribute the activation energy measured over the low-temperature regime on the MMP-covered particles, and the entire thermal range on the clean particles, to thermal excitation of trapped electrons. At higher temperatures, the production of trapped electrons also exhibits Arrhenius behavior, but with a much greater activation energy on the MMP-covered sample. The analysis suggests an activation energy of 33 ( 1 kJ mol-1 for the population of trapped electrons. As discussed above, this activation energy is most likely due to the barrier for the extraction of lattice oxygen, the oxidant that drives high-temperature adsorbate decomposition. Interestingly, the slope of the curve for trapped electron production at the high-temperature end of curve a in Figure 4B returns to the original level (in the low-temperature end) after all the methoxy groups have been consumed. This implies that oxygen vacancy production strongly depends on the concentration of surface methoxy species. The experimentally determined activation barrier of 33 kJ mol-1 for the lattice oxygen extraction in this work is significantly lower than that obtained for a similar study into oxygen exchange between a TiO2(110) surface and a formic acid adlayer. Specifically, Henderson recently reported a 52 kJ mol-1 activation energy for the oxygen exchange reaction between an 18 O-enriched TiO2(110) surface and C16O at temperatures as low as 400 K.9 While the thermal onset of oxygen mobility is similar to that observed in our work (423 K), the activation energy is nearly 20 kJ mol-1 higher. Although the origin of the difference is unknown, isotope effects may account for some of the observations in ref 9. We also speculate that the highly defective surface of the small, ∼25 nm84 TiO2, particles employed here renders lattice oxygen near the surface more active than oxygen within the single-crystalline sample used in the work of Henderson.9 Further experiments are required to verify the origin of this difference.

Quenching of Trapped Electrons and Final Product Analysis. Following the rise in background IR absorbance and decomposition of surface bound MMP, the TiO2 sample appears to reach a stable state at 675 K. Spectrum a in Figure 6A is that of the MMP-covered sample held in vacuum at 675 K for 20 min. The modes observed at 2930 cm-1 (see the inset of Figure 6A) and at 1311 cm-1 are due to methyl phosphonate (MP), which remain unchanged on the surface under these conditions. More interestingly, the strong IR background absorbance does not diminish even after several minutes at 675 K. Furthermore, the broad infrared signal decreases only partially when the TiO2 sample is cooled to room temperature (Figure 6A, spectrum b). The initial spectral change is due to limited removal of trapped electrons by migration into deeper traps or by electron-hole (e-h) pair recombination as thermal energy is removed from the system. However, quenching is far from complete, even after extended periods of time at room temperature. Moreover, even exposure to high pressure oxygen (20 Torr), an electron scavenger,49,50,54,68,77,88,90,91 does not fully deplete population from these states, as shown by spectrum c in Figure 6A. The remarkable stability of trapped electrons in the presence of O2 may be evidence that adsorbed MP and other strongly bound products prevent not only direct e-h pair recombination, but also oxygen from approaching the particles closely enough to remove the trapped electrons. In an effort to completely quench the broad electronic absorbance, the sample was reheated in vacuum to 675 K and then re-exposed to 20 Torr oxygen. The high temperature oxygen treatment immediately decreased the IR signal in a process that likely involves the transfer of trapped electrons to impinging oxygen to form oxygen-derived surface species.49,50,54 Evidence for the formation of highly reactive surface oxygen comes from spectrum d in Figure 6B. The strongly bound methyl phosphonate is oxidized to gas phase CO and, over longer periods of time ∼ 60 min, MP further oxidizes to carboxylates, as indicated by the ν(COO) mode at 1349 cm-1. Furthermore, a fraction of the adsorbates fully oxidizes to gas phase CO2, which, along with the CO, desorbs upon evacuation (Figure 6B, spectrum e). Spectrum d in Figure 6B also shows that oxidation of MP by O2 to form CO2 and H2O produces isolated Ti-OH

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Figure 6. Quenching of trapped electrons and observation of final products for DMMP oxidative degradation. (A) Spectrum a: TiO2 sample covered with methyl phosphonate (recorded following the treatment reported in spectrum b of Figure 3A) after 20 min vacuum treatment at 673 K. Spectrum b: after cooling the sample to 295 K. Spectrum c: after admission of 20 Torr oxygen at 295 K. (B) Spectrum d: sample following vacuum treatment at 675 K and admission of 20 Torr oxygen at 675 K. Spectrum e: same sample following evacuation at 675 K and cooling down to 295 K.

moieties (the narrow band at 3654 cm-1) due to the dissociative adsorption of H2O on the surface. The most intense IR feature throughout the annealing study appears at ∼1095 cm-1. This feature remains unchanged even after O2 treatment at 675 K and prolonged evacuation at this temperature (Figure 6B, spectrum d). The 1095 cm-1 IR mode is due to both changes in the TiO2 lattice modes following thermal treatment and the formation of a highly stable Ti-O-P-O-Ti oxide layer that remains bound to the surface after full oxidation of DMMP.26,41,46 In a recent study, we have shown that this phosphorus-oxygen network does not prevent the molecular adsorption of DMMP on TiO2;46 however, the network appears to eliminate the dissociative adsorption channel due to poisoning of the active surface sites. Summary Transmission infrared spectroscopic studies of the thermal decomposition of dimethyl methylphosphonate (DMMP), a chemical warfare agent simulant, on high surface area commercial TiO2 nanoparticles (Degussa P25) revealed two key processes that appear to occur under different thermal regimes. At saturation coverage of DMMP at room temperature, both molecular adsorption to surface hydroxyl groups and reactive chemisorption through interaction with Lewis acid sites occur. In the low-temperature range, from 295 to ∼400 K, the main reaction channel includes nucleophilic attack of DMMP by neighboring oxygen to produce Ti-OCH3 and a variety of P-Ox surface bound groups. An activation energy of ∼48 kJ mol-1 is estimated for the conversion of surface phosphoryl-containing adsorbates into P-Ox groups in this temperature range. At elevated temperatures, an alternate reaction channel opens as activated lattice oxygen attacks Ti-OCH3 groups. The trapped electrons produced by lattice oxygen extraction are observed by their broad featureless IR absorbance as they are promoted into the continuum of states in the conduction band. An activation energy of ∼33 kJ mol-1 is estimated for the process of lattice oxygen extraction within the temperature range 423-580 K, while a much lower activation barrier of 15 kJ mol-1 regulates the thermal excitation of trapped electrons in

the clean TiO2 sample. Close agreement between the work described here for DMMP and previous studies into the thermal decomposition of 2-CEES on TiO2, strongly suggests that the degradation of surface adsorbates via extraction of lattice oxygen is the main reaction channel for the high temperature decomposition of both CWA simulants. Acknowledgment. Professor John T. Yates Jr. is gratefully acknowledged for providing significant support to this work in terms of a generous gift of equipment and several helpful discussions. This work is supported by the Army Research Office, W911NF-04-1-0195, and the Defense Threat Reduction Agency, W911NF-06-1-0111. Note Added after ASAP Publication. This paper was published on the Web on August 5, 2009, with an error to Figures 1 and 6. The corrected version was reposted on August 7, 2009. Supporting Information Available: Figure S1. Selected difference IR spectra of TiO2 nanoparticles precovered with MMP collected during heating of the sample from 413 to 675 K as shown in Figure 3A. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ekerdt, J. G.; Klabunde, K. J.; Shapley, J. R.; White, J. M.; Yates, J. T. J. Phys. Chem. 1988, 92, 6182. (2) Thompson, T. L.; Panayotov, D. A.; Yates, J. T., Jr. J. Phys. Chem. B 2004, 108, 16825. (3) Thompson, T. L.; Panayotov, D. A.; Yates, J. T., Jr.; Martyanov, I.; Klabunde, K. J. J. Phys. Chem. B 2004, 108, 17857. (4) Bermudez, V. M. J. Phys. Chem. C 2007, 111, 3719. (5) Mitchell, M. B.; Sheinker, V. N.; Mintz, E. A. J. Phys. Chem. B 1997, 101, 11192. (6) Mitchell, M. B.; Sheinker, V. N.; Tesfamichael, A. B.; Gatimu, E. N.; Nunley, M. J. Phys. Chem. B 2003, 107, 580. (7) Gellings, P. J.; Bouwmeester, H. J. M. Catal. Today 2000, 58, 1. (8) Wachs, I. E.; Jehng, J.-M.; Ueda, W. J. Phys. Chem. B 2005, 109, 2275. (9) Henderson, M. A. J. Phys. Chem. B 1997, 101, 221. (10) Muggli, D. S.; Falconer, J. L. J. Catal. 2000, 191, 318.

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