Photooxidation of Dimethyl Methylphosphonate on TiO2 Powder - The

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J. Phys. Chem. B 2000, 104, 12299-12305

12299

Photooxidation of Dimethyl Methylphosphonate on TiO2 Powder Camelia N. Rusu and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: July 19, 2000

The photooxidation of dimethyl methylphosphonate (DMMP) on powdered TiO2 has been studied using UV irradiation in the range 2.1 eV to 5.0 eV. It has been found that the P-CH3 and the P-OCH3 moieties are destroyed together by photooxidation, producing adsorbed CO and CO2 as well as formate ions and water. By working at 200 K the photooxidation process has been studied alone, whereas at room temperature both hydrolysis and photooxidation will occur together. An adsorbed PO3 species is the final phosphorus-containing photooxidation product. High coverages of DMMP are not photooxidized due to site blockage for O2 adsorption on the TiO2 surface.

I. Introduction Understanding the destruction of organophosphorus compounds is of importance for devising new methods for the protection of persons exposed to chemical warfare agents (CW), pesticides, herbicides, and other chemically similar industrial compounds. The testing of chemical warfare agents in the laboratory is quite hazardous; therefore, studies are done typically using nontoxic simulants. Dimethyl methylphosphonate (DMMP) is a widely used model compound for this type of investigation because it possesses the appropriate molecular structure and elemental composition to simulate a number of environmentally undesirable organophosphorus compounds. There are different methods for decomposing the DMMP molecule. An important area of chemical warfare agent destruction is catalytic decomposition using metals and metal oxides. Early work has studied DMMP adsorption and interaction with surfaces of metals, including Mo (111),1 Pt (111),2 Pd (111),3 Ni (111),3 with metal oxides such as Al2O3,4-6 TiO2,7 SiO2,8 Fe2O3,6,8,9 MgO,6,10,11 La2O3,6 or with alumina-supported iron oxide.12 Briefly, the decomposition of DMMP and phosphonate esters is temperature dependent: for lower temperatures (T < 200 K) DMMP is adsorbed in a molecular state but as the temperature of the substrate increases, DMMP dissociates. On most substrates (but there are also exceptions such as Fe2O3), DMMP undergoes dealkylation to form methylphosphonate species on the surface. Because these experiments were carried out under vacuum, in the absence of the oxygen, no DMMP oxidation products were observed. Catalytic oxidation of DMMP was studied on Pt/Al2O3.13 In these studies, the major oxidation products were CO2 and H2O. Higher DMMP conversions were obtained at the beginning of the reactions. HPLC analysis of the liquid product coming from the reactor showed the presence of partial DMMP decomposition products such as dimethyl phosphate, monomethyl phosphate, methyl phosphonic acid and phosphoric acid.14 No P- containing products were observed in the gas phase.13,14 Catalytic deactivation was proposed to be due to the adsorbed phosphate species. The formation of P2O5 on the catalyst surface was proved using the scanning Auger microprobe (SAM) and X-ray photoelectron spectroscopy (XPS) for a monolithic Pt-TiO2 catalyst.14

Another very tempting method for CW agent destruction is photocatalytic oxidation. Oxidative photocatalysis on TiO2 has been shown to be effective for decomposing a wide range of organic compounds.15,16 The photooxidation process as a method to decompose the phosphonate compounds has the advantage not only that high temperatures required for the thermal degradation can be avoided, but also that solar energy may be used to degrade these compounds. A number of studies of the photocatalytic decomposition of organophosphorus compounds have been published;17-26 and of these a few have dealt with DMMP.24-26 DMMP has been photocatalytically decomposed in aqueous suspensions of TiO2. The major products were phosphoric acid and CO2. Intermediates such as methylphosphonic acid, formic acid, and formaldehyde were observed.24 The authors proposed that DMMP is degraded through a hydroxyl radical-mediated mechanism. Recently, the gas-phase photocatalytic decomposition of DMMP was studied over TiO2 films.25 The main reaction products identified were carbon dioxide and carbon monoxide in the gas phase, and methylphosphonic acid and PO43- on the catalyst. Deactivation of the catalyst was observed due to the build-up of surface phosphorus-containing species. DMMP photodecomposition at 313-343 K over manganese oxide produced CO2 and large amounts of methanol.26 It was shown that the CO2 produced comes from the photooxidation of methoxy groups and not from the oxidation of the P-CH3 moiety. In our paper, using FTIR spectroscopy, we found that the photooxidation of DMMP adsorbed on TiO2 leads to the formation of carbon dioxide, carbon monoxide, formate, and water. The photooxidation of the methyl groups seems to be nonselective. We did not see any preference, at 200 K, in the oxidation of the methyl of the methoxy CH3O- moiety comparing with the methyl group of the CH3-P- moiety. II. Experimental Section Experiments were carried out in a bakeable stainless steel IR cell capable of operating at sample temperatures from 100 to 1500 K. The detailed description of IR cell had been reported previously.27 In brief, the TiO2 powder was pressed into a tungsten grid which was held rigidly by a power/thermocouple feedthrough via a pair of nickel clamps. A K-type thermocouple was spot-welded on the top-central region of the tungsten grid

10.1021/jp002562a CCC: $19.00 © 2000 American Chemical Society Published on Web 12/05/2000

12300 J. Phys. Chem. B, Vol. 104, No. 51, 2000 to measure the temperature of the sample. The sample temperature could be easily adjusted using liquid nitrogen and electrical heating of the grid via a programmable controller.28 The TiO2 sample was pressed only on half of tungsten grid while the other half of the grid had no sample. The infrared spectra of the TiO2 surface and of the gaseous species through the empty half grid in the cell could be measured alternatively by translating the cell in the spectrometer. The IR cell was connected to a stainless steel vacuum system pumped by both turbomolecular and ion pumps and the base pressure was lower than 1 × 10-8 Torr. The system base pressure was measured by the ion current drawn by the ion pump (Varian, 921-0062), while reactant gas pressures were measured by a capacitance manometer (Baratron, 116A, MKS, range 10-3 to 103 Torr). A FTIR spectrometer and a quadrupole mass spectrometer (Dycor Electronics Inc.) were used in this system. The sample cell is mounted on a computer-controlled precision 2D-translation system ((1µm accuracy) from the Newport Corporation. This system allows the cell to move reproducibly as the sample or the gas phase are being measured. Infrared spectra were obtained with a nitrogen gas purged Mattson Fourier transform infrared spectrometer (Research Series 1) equipped with a wide band HgCdTe detector. The sample spectra shown here were recorded with 4 cm-1 resolution using 1000 scans while the background spectra were recorded with 4 cm-1 resolution using 2000 scans. The TiO2 used was Degussa titanium dioxide P25 which is reported to have 70% anatase and 30% rutile composition and a surface area of ∼50 m2/g at room temperature. Before any experiment, the TiO2 was pretreated in O2 at 6 Torr pressure at 673 K for 30 min to remove a small amount of organic contamination from the TiO2. In the experiments where a reduced sample was involved, the TiO2 was annealed in a vacuum at 900 K for 15 h. The UV source is a 350 W high-pressure mercury arc lamp (Oriel Corp.). The power received by the sample measured for the full Hg arc (2.1 eV-5 eV) was 660 mW/cm2. A manual shutter was installed to accurately control the UV exposure time. The TiO2 sample on the tungsten grid was positioned in such a way that both the IR beam from the FTIR spectrometer and the UV light from the mercury arc are focused on it at an angle of incidence of about 45° to the normal of the grid.29 The orthogonal arrangement of UV light with the IR beam makes IR measurements possible during photochemistry on the surface. The DMMP (99.4%) was obtained from Morton Thiokol, Inc. Impurities, as quoted by the supplier, included 0.4% trimethyl phosphonate, 0.004% trimethyl phosphonite and 0.2% water. The DMMP was further purified by performing five freezepump-thaw cycles. The oxygen gas was obtained from Matheson Gas Products with 99.998% purity. The DMMP photooxidation experiments were carried out on powdered TiO2 treated as follows: after oxidation of the TiO2 at 673K in O2(g) at 50 Torr for 30 min, the sample was heated in a vacuum to 900 K for 1 h and then for 13 h, in a vacuum, at 800 K in order to activate the surface. DMMP was then adsorbed at 200 K. After the cell was evacuated, oxygen was introduced into the cell. During the UV irradiation, the course of the photooxidation reaction was continuously monitored by FTIR measurements. To avoid possible false contributions from background, two types of control experiments were performed. In the experiments designed to check the contribution of wall reactions, the same number of DMMP and O2 molecules as that used in the photooxidation experiments were admitted into the cell that

Rusu and Yates contains only an empty grid (no TiO2). The system was irradiated with the full arc, as during the photooxidation experiments, and the gas phase was continuously monitored with transmission infrared spectroscopy. No changes were seen. A second type of control experiment was performed in order to exclude the possibility of the formation of CO and CO2 from other sources than DMMP. In the cell that contains the TiO2 sample, O2 was admitted to the same pressure as in the DMMP photooxidation experiments. The sample was also cooled to the temperature of the experiment (200 K). The UV irradiation started and the gas and the adsorbed phases were monitored. No production of CO and CO2 was seen in the gas phase or as adsorbed products on TiO2. III. Results 1. DMMP Interaction with the TiO2. DMMP forms a condensed ice layer on the TiO2 powder at temperatures lower than 160 K. The molecules diffuse into the pore structure, becoming chemisorbed at temperatures between 160 to 200 K. DMMP is hydrolyzed by the hydroxyl groups of the substrate for temperatures higher than 200 K, leading to the formation of methoxy groups adsorbed on the TiO2.7 Spectrum a from Figure 1 is characteristic to the DMMP condensed on TiO2 at 146 K. The hydroxyl groups of the titanium dioxide are unaffected by the condensation process as a consequence of the fact that DMMP is deposited as an ice only on the outer geometric surface of the powder and has not entered the interior of the powder where the majority of the surface sites exist. The disappearance of the isolated hydroxyl groups and the formation of the hydroxyl groups (∼3300 cm-1) which are associated with DMMP molecules, as seen in spectrum b, is direct evidence of the DMMP migration into the pores of TiO2 at 200 K. As a consequence of the DMMP adsorption on the TiO2 powder, the frequency of the ν(PdO) vibrational mode decreases. Spectrum c from Figure 1 shows the development of new vibrational modes at 2827 cm-1 upon heating to 314 K. This corresponds to the formation of methoxy groups on the titanium dioxide surface as a consequence of the DMMP hydrolysis, as was discussed in detail elsewhere.7 The spectrum of adsorbed DMMP on TiO2 is characterized by the ∼2992 cm-1 asymmetric νa(CH3P) stretching mode, 2960 cm-1 asymmetric νa(CH3O) stretching mode, 2924 cm-1 symmetric νs (CH3P) stretching mode, 2856 cm-1 symmetric νs(CH3O) stretching mode, 1463 cm-1 asymmetric δa(CH3O) bending mode, 1315 cm-1 δ(CH3P) bending mode, 1188 cm-1 F(CH3O) rocking mode, 1028 cm-1 symmetric νs(C-O) mode, and the environmentally sensitive ν(PdO) stretching mode at 1210 cm-1. The frequency of this vibrational mode is a direct fingerprint of the interaction between the DMMP and TiO2. For the condensed DMMP on the TiO2 the frequency of the vibrational mode is situated at 1242 cm-1 but for the adsorbed DMMP, as a consequence of the interaction with the substrate through the PdO moiety, the frequency of the vibrational mode decreases to 1210 cm-1. The assignment of all of the observed vibrational modes of the DMMP adsorbed on TiO2 is given in Table 1. 2. Lack of Reactivity of the Adsorbed DMMP and O2 in the Dark. The lack of reactivity at 200 K toward O2 when DMMP is adsorbed on TiO2 is shown in Figure 2 as a control experiment for subsequent photooxidation. Spectrum a is characteristic to the DMMP adsorbed on the reduced TiO2; spectrum b is obtained from the same sample of adsorbed DMMP but when 24 Torr O2 has been admitted into the cell. Spectrum c shows clearly that even if the time of contact

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Figure 1. FTIR Spectra of the DMMP for different stages of interaction with the substrate: (a) DMMP condensed as an ice layer on the TiO2 (146 K); (b) DMMP adsorbed on the TiO2 surface (200 K); (c) DMMP hydrolyzed by the hydroxyl groups from the TiO2 surface (314 K).

Figure 2. FTIR Spectra of the adsorbed DMMP on TiO2 in an atmosphere of oxygen showing the lack of an oxidation process at 200 K: (a) DMMP adsorbed on TiO2; (b) DMMP adsorbed on TiO2 when 24 Torr O2 was introduced into the cell; (c) DMMP adsorbed on TiO2 with 24 Torr O2 for 1 h.

between the adsorbed DMMP and O2 increases to 1 h, no change was seen. The experiment presented in Figure 2 was done at 200 K but the same result was obtained when the temperature of the substrate was 300 K. The gas phase was also monitored during this process and no reaction was seen. 3. DMMP Photooxidation at Small DMMP Coverages. FTIR spectra of the surface species formed during the photooxidation of DMMP adsorbed at small coverages on the reduced TiO2 with O2 (24 Torr) at 200 K are shown in Figure 3. The increase in the temperature of the TiO2 as a result of the UV

irradiation was electrically compensated when using liquid nitrogen coolant so that the temperature was kept constant at 200 K. It was very important to maintain a constant temperature during the photooxidation experiment since we wanted to avoid any thermally activated DMMP surface chemistry in these studies. A series of FTIR spectra of the surface species formed as the UV exposure time increases is presented in Figure 3. After irradiation important changes in the IR spectra of the surface species can be observed. In the ν(CH) stretching mode region

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Figure 3. FTIR spectra of the surface species formed during the photooxidation of DMMP adsorbed to small coverage on the reduced TiO2 with O2(24 Torr) at 190 K. In this experiment the amount of DMMP admitted into the cell is 9.3 × 1017 molec. The UV exposure periods are (a) 0 min; (b) 5 min; (c) 40 min; (d) 180 min; (e) 335 min; (f) 580 min.

TABLE 1: Assignment of the Infrared Modes in the Spectrum of DMMP Adsorbed on TiO27 assignment

DMMP/TiO2 frequency (cm-1)

νa(CH3P) νa(CH3O) νs(CH3P) νs(CH3O) δa(CH3O) δs(CH3O) δs(CH3P) δa(CH3P) ν(PdO) F(CH3P) νa(C-O) νs(C-O)

2992 2960 2924 2856 1463 1452sh 1424 1315 1210 1188 1052sh 1028

of Figure 3, the intensity of both the ν(CH3P) stretching modes and the ν(CH3O) stretching modes decreased as the UV irradiation time increased. Also a decrease in the δ(CH3O) and F(CH3O) bending mode intensities was seen. Dramatic changes as a consequence of the UV irradiation are observed in the 2400-2200 cm-1 region of the IR spectra. Two vibrational modes situated at ∼2360 and 2210 cm-1 develop respectively with the increase in the UV exposure time. An increase in the absorbance of the vibrational modes at 1580, 1376, and 1279 cm-1 was also seen during UV irradiation. The vibrational modes at 2360 and at 1320 cm-1 (see Figure 6) are assigned to the formation of adsorbed CO2 and the 2210 cm-1 vibrational mode is assigned to the formation of adsorbed CO. To determine whether the new absorption bands are due to CO2 and CO adsorbed on TiO2, an experiment was carried out exposing DMMP-covered TiO2 at 200 K to CO2 and CO. CO2 produces the broad bands at 2360 and at 1320 cm-1 and CO produces the band at 2210 cm-1. The assignment of the peak situated at 2210 cm-1 was done also in correlation with the literature values reported for the CO adsorption on TiO2 at low temperatures. The mode at 2210 cm-1 is characteristic of low coverages of CO adsorbed on TiO2.30-37

Figure 4. Kinetics plot of adsorbed CO2 and CO spectral intensities monitored during the photooxidation of DMMP adsorbed on TiO2.

The evolution of the CO2 and CO as function of the UV exposure is shown in Figure 4. The measurements of the integrated intensities correspond only to the surface species since no CO2 nor CO was detected in the gas phase during irradiation. In Figure 3, a band at 1586 cm-1 develops during DMMP photooxidation. This is assigned as the surface formate ions, and Table 2 shows the basis for the assignment of this species, based on the observation of the ν(COO-) and the δ(C-H) modes. Figure 5 shows the FTIR spectra in the ν(OH) region of the spectrum. The absence of the absorption feature in the region of the scissor mode (∼1620 cm-1) of the adsorbed water confirms that the broad -OH stretching mode is determined by the associated hydroxyl groups. The left panel shows the behavior of the isolated and associated ν(OH) stretching modes for different periods of photooxidation. The dotted line separates the region characteristic to the isolated hydroxyl groups from that characteristic to the associated hydroxyl groups. The difference spectra represented in the right panel of Figure 5

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Figure 5. FTIR Spectra in the ν(OH) stretching mode region of the surface species formed during the photooxidation of DMMP adsorbed on TiO2. Left panel shows the IR spectra monitored for different periods of UV irradiation: (a) 0 min; (b) 180 min:K; (c) 335 min; (d) 580 min. Left panel includes the correspondent difference spectra.

Figure 6. Difference FTIR spectra in the range 1400-1200 cm-1 of the surface species formed during the photooxidation of DMMP for different UV irradiation periods: (a) 180 min; (b) 335 min; (c) 580 min.

TABLE 2: Spectroscopic Assignments of the Vibrational Modes for Adsorbed Formate Ions system studied

νa(COO-)

δ(CH)

νs(COO-)

HCOO/ZnO38 HCOO/ZnO39 HCOO/Al2O338 HCOONa40 formate/TiO241 formate/TiO242

1580 1575 1595 1567 1590 1575 1560 1552 1537 1586

1382 1379 1395 1377 1385 1390 1380 1412 1386 1375

1365 1362 1380 1366 1365 1372 1360 1370 1359 1358

formate/TiO237 this work

clearly show that with increasing the UV irradiation time the intensity of the associated OH bands increases. As can easily be seen in the same figure the isolated hydroxyl groups are not monotonically affected by the increasing period of the UV irradiation.

The difference spectra in the range 1400-1200 cm-1 for different UV irradiation periods are shown in Figure 6. It is seen that as the irradiation time increases the absorbance of the vibrational modes situated at 1375, 1358, 1320, and 1275 cm-1 increases. The assignment of these absorption bands is explained in the Discussion section of the paper. 4. DMMP Photooxidation at Large DMMP Coverages. Figure 7 shows a series of FTIR spectra obtained during the photooxidation process when a large amount of DMMP was adsorbed on the TiO2 surface. As can be seen the FTIR spectrum of the adsorbed DMMP remains unchanged during UV irradiation. Also no CO2, CO or any other product was spectroscopically detected in the gas phase during this process. The pressure of the oxygen in the cell in the experiment presented in Figure 6 was 24 Torr, but the same result was obtained in a similar set of experiments at a pressure of O2 of 60 Torr. The large coverage of DMMP apparently shields all TiO2 sites for O2, causing the photooxidation process to be eliminated. IV. Discussion 1. DMMP Interaction with TiO2-Optimum Temperature to Study the DMMP Photooxidation Process. DMMP adsorbs nondissociatively on the TiO2 powder for temperatures lower than 200 K as shown in ref 7. For temperatures of the substrate higher than 200 K, the DMMP starts to interact dissociatively with the surface of the TiO2 leading to the formation of methoxy groups adsorbed on the surface. Our goal was to study the DMMP photooxidation on TiO2 and not of the methoxy groups formed on the surface as a consequence of the hydrolysis process. Therefore, the temperature for the photooxidation experiments was chosen to be 200 K. This temperature is high enough that most of the condensed DMMP (ice) was converted to chemisorbed DMMP, but at the same time is low enough that the hydrolysis reaction with the substrate can be avoided. Two papers that deal with the DMMP photooxidation process on TiO2 concerned the reaction at room

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Figure 7. FTIR Spectra of the surface species formed during the photooxidation of DMMP adsorbed to large coverage on the reduced TiO2 with O2(60 Torr) at 190 K. In this experiment the amount of DMMP admitted into the cell is 1.5 × 1019 molec. The UV exposure periods are (a) 0 min; (b) 35 min; (c) 65 min; (d) 120 min; (e) 200 min.

temperature.24,25 These papers therefore have reported a combination of photo- and thermally activated-surface chemistry. 2. Products of the DMMP Photooxidation on TiO2. Many organic molecules can be photooxidized on TiO2 with UV irradiation. As titanium dioxide is irradiated with photons of higher energy than the band gap (>3.2 eV), electron-hole pairs are produced and migrate toward the surface where they initiate redox reactions of adsorbates. Adsorbed O2 can trap electrons to form superoxide anion O2- and therefore to form reactive species for the oxidation process. Electron trapping by O2 also increases the lifetime of the hole states. The initiating mechanism of the photooxidation process of the molecules adsorbed on TiO2 is still under discussion. The photooxidation of the DMMP adsorbed on TiO2 leads mainly to the formation of CO2, CO, formates, and water, which produced associated OH groups on the surface as shown in Figure 5. This result is in agreement with the products obtained during the photodecomposition of organophosphonates in TiO2 suspensions24 and in TiO2 films.25 A. CO and CO2 Formation. The broad bands centered at 2210 cm-1 and at ∼2360 and 1320 cm-1 are due to the formation of CO and CO2 adsorbed on titanium dioxide. Their growth with the UV irradiation time is associated with the decrease in the intensities of the CH vibrational modes of DMMP. We cannot see any preference for the oxidation of a particular type of methyl group. The methyl groups of both the P-OCH3 and the P-CH3 moieties are photooxidized with almost equal probability. In contrast to these observations at 200 K, another study carried out at room temperature,25 has reported that the -OCH3 groups are preferentially photooxidized. It is likely that those studies were carried out under conditions where both DMMP hydrolysis and photooxidation took place simultaneously and that the -OCH3 groups are consumed by the hydrolysis reaction. B. Formate Ion Formation. Formate ion formation is observed through the development of asymmetric νa(-CO2-) and symmetric νs(-CO2-) stretching modes at 1586 and 1358 cm-1 respectively. The 1375 cm-1 vibrational mode is assigned to the formate deformation mode, δ(CH), and Table 2 shows the

correlation of our observation with the literature for adsorbed formate. The formate species formed by photooxidation are bidentate, because the carbonyl mode characteristic of monodentate formate (ν (CdO) ∼ 1670 cm-1) is absent. Furthermore, the frequency difference between the νa(-CO2-) asymmetric and νs(-CO2-) symmetric stretching modes is close to those of formate ions or formate salt, indicating that most of the formate groups on the TiO2 surface are adsorbed having a bridging bidentate coordination; i.e., the two oxygen atoms of the formate groups are bonded to two different Ti ions on the surface.40,43,45 C. The Symmetric Stretch Mode of Adsorbed CO2. Figure 6 shows the development of a band at 1320 cm-1 which is assigned to the symmetric stretch mode of adsorbed CO2. This normally IR inactive mode (in gas phase) becomes IR active upon adsorption on TiO2. The Raman frequency for νs(CO2) symmetric stretching mode is 1340 cm-1 in the gas phase.46 D. Water Formation. The TiO2 used in these experiments is highly dehydroxylated. Only a small coverage of isolated OH groups was initially present on the surface as it can be seen in Figure 5. In the (ν (O-H)) region of the surface spectra, an IR band at ∼3250 cm-1 was found to develop and increase in intensity with increasing UV exposure time. This IR feature is assigned to the stretching modes of associated OH groups. The isolated hydroxyl groups, located between 3720 and 3640 cm-1, are not significantly influenced during photooxidation. The formation of P-OH moieties was excluded since the (P)O-H stretching mode4,5,47 near 2450 cm-1 was not seen at any time during the UV photooxidation process. Therefore, the associated hydroxyl groups that evolve during photooxidation process cannot be caused by the association of the P-OH groups but originate from the association of Ti-OH groups. E. Formation of the Phosphonate Compounds. During the photooxidation process a new vibrational mode develops at 1275 cm-1. This band is assigned as the ν(PdO) stretching mode for a “free” PdO group. Thus, we conclude that the P is retained on the surface after the photooxidation process as a PdO group directed away from the surface. We designate this PdO group

Photooxidation of DMMP

Figure 8. Reaction products formed in the photooxidation of DMMP adsorbed on TiO2.

“free” in this case comparing with the case of the PdO group of the chemisorbed DMMP, where as a consequence of interaction with the TiO2 surface, the vibrational mode shifts from 1276 to 1210 cm-1. The frequency characteristic of the PdO stretching mode in DMMP gas is 1276 cm-1.7 3. DMMP Coverage Effect on the Performance of the Photooxidation Process. One method of activating the TiO2 is annealing in a vacuum. This process removes O2- anions, leaving behind anion vacancy defect sites which are formally associated with Ti3+ ion centers. It has been shown that the Ti3+ anion centers are directly responsible for the chemical and photochemical reactivity of the titanium dioxide.7,48,49 In these sets of experiments it was interesting to see that a large coverage of DMMP molecules adsorbed on the TiO2 surface quenches the photoactivity of the substrate. If a small coverage of DMMP molecules are adsorbed on the titanium dioxide there are still active centers left on the surface where the photooxidation process will take place via oxygen adsorption on these sites. A generalized schematic of the photooxidation reaction for adsorbed DMMP on TiO2 is shown in Figure 8. V. Conclusions The photooxidation of dimethyl methylphosphonate (DMMP) on a powdered TiO2 surface was investigated by transmission infrared spectroscopy for UV irradiation in the range hν ) 2.15.0 eV. . The following results have been found. (1) DMMP adsorbed on TiO2 is a stable molecule in the presence of molecular oxygen. (2) In the DMMP photooxidation on TiO2, both the methyl groups of the P-CH3 and P-O-CH3 moieties are destroyed at equal rates. (3) The photooxidation of DMMP adsorbed on TiO2 at 200 K produces mainly CO2, CO, and H2O (observed as associated OH groups). The formation of bidentate formate groups adsorbed on the surface was also detected. (4) Phosphorus is retained on the titanium dioxide surface as a species containing a free PdO moiety. (5) The photooxidation of DMMP adsorbed on TiO2 was seen only for small coverages as a result of the need for open sites on TiO2 for O2 adsorption and photoexcitation. Acknowledgment. We acknowledge, with thanks, the full support of this work by the Army Research Office. References and Notes (1) Smentkowski, V. S.; Hagans, P.; Yates, J. T., Jr. J. Phys. Chem. 1988, 92, 6351.

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