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J. Phys. Chem. B 2000, 104, 12292-12298
Adsorption and Decomposition of Dimethyl Methylphosphonate on TiO2 Camelia N. Rusu and John T. Yates, Jr.* Surface Science Center, Department of Chemistry, UniVersity of Pittsburgh, Pittsburgh, PennsylVania 15260 ReceiVed: July 19, 2000; In Final Form: October 21, 2000
By using Fourier transform infrared spectroscopy (FTIR) we have witnessed the sequential steps of surface diffusion, weak chemical bonding, and decomposition of dimethyl methylphosphonate (DMMP) on TiO2 powder. At temperature lower than ∼160 K, DMMP condenses as an ice layer on the outer surface of the TiO2 sample. In the temperature range 160- 200 K, diffusion into the TiO2 interior occurs and hydrogen bonding of DMMP to isolated TiOH groups is observed. In addition, bonding to Lewis acid sites occurs. Above 214 K, cleavage of P-OCH3 groups takes place, with the production of Ti-OCH3 surface species and this is accompanied by the consumption of surface hydroxyl groups. A reduction of P-O bond order then occurs as adsorbed phosphonate species are formed.
I. Introduction Some organophosphorus compounds are used as chemical agents (e.g., isopropyl methylphosphonofluoridate or Sarin) or pesticides (e.g., p-nitrophenyl diethylphosphonate or Paraoxon). It is important to find an efficient method for the decontamination of water and air containing organophosphorus compounds of this type. Dimethyl methylphosphonate (DMMP) is a widely used simulant for organophosphorus compounds. The DMMP molecule is an interesting molecule because it can interact with the substrate either through the PdO group, or through the methoxy groups. It is expected that the P-CH3 group will be less reactive. Templeton and Weinberg1,2 have used inelastic electron tunneling spectroscopy to examine the adsorption/reaction of three phosphonate esters on Al2O3 films. They found that at 200 K, DMMP is adsorbed molecularly whereas at surface temperatures between 295 and 473 K, DMMP adsorbs dissociatively to form the methyl methylphosphonate (MMP) adspecies. For temperatures above 573 K the decomposition of the adsorbed DMMP continues to methyl phosphonate (MP) by losing the second methoxy group. The dissociative adsorption sites consist of coordinatively unsaturated aluminum ions. Infrared spectroscopic studies of the adsorption of phosphonate esters on γ-Al2O3 suggest two forms of adsorption, molecular and dissociative.3 Adsorption of diisopropyl methyl phosphonfluoridate (Sarin) onto hydroxylated alumina powder shows that the molecule is strongly adsorbed via its phosphoryl oxygen. The decomposition of the compound liberated fluorine that is adsorbed on the substrate and the remaining phosphorusbearing species is bonded to the surface via the O-P-O group. The hydrolysis reaction is activated by basic surface sites. It was also found that the hydrolysis proceeds faster over MgO then Al2O3.3 White et al.4,5 have examined the adsorption of DMMP onto R-Fe2O3, SiO2, and Rh(100) by using Auger electron spectroscopy (AES), X-ray photoelectron spectroscopy (XPS), and temperature programmed desorption (TPD). They found that no significant decomposition occurred when DMMP was adsorbed on silica. However, on iron oxide, decomposition of DMMP was seen even when adsorption was carried out at 170 K followed by heating to 250 K. FTIR spectroscopy was used
by Aurian-Blajeni and Boucher6 to investigate the adsorption of DMMP on TiO2, ZnO, Al2O3, MgO, and WO3. In all cases they observed a decrease in the PdO stretching vibrational frequency, indicated that the surface species is bound through the PdO bond. Mitchell et al.7 used diffuse infrared reflectance spectroscopy to study the adsorption and decomposition of DMMP on four different oxides: Al2O3, MgO, La2O3, and Fe2O3. On the first three oxides, it was found that the decomposition of the DMMP involves the elimination of one methoxy group at approximately 373 K, followed by the elimination of a second methoxy group at approximately 573 K. The phosphorus-carbon bond was observed to remain intact, even upon heating to 673 K in oxygen after DMMP was adsorbed on Al2O3. Decomposition of DMMP on Fe2O3 does not show an obvious preference for eliminating the methoxy or the methyl groups attached to the phosphorus. Both types of methyl groups were observed to react as the temperature was increased, with complete elimination of all the carbon-containing fragments by 573 K. Our experiments with DMMP were carried out on powdered TiO2 surfaces. DMMP is molecularly adsorbed on the TiO2 surface at temperatures lower than 200 K. The DMMP molecule interacts through the electron-rich phosphoryl oxygen either with Lewis acid sites of the TiO2, donating lone pair electrons to the Tin+ (n ) 4, 3), or with hydroxyl groups by hydrogen-bond formation. On the other hand, for temperatures higher than 200K the adsorption of the DMMP on the TiO2 surface is dissociative. Transmission FTIR is shown to be a powerful technique, able to separate surface diffusion, adsorption, and dissociation processes for this molecule on TiO2. II. Experimental Section Experiments were carried out in a bakeable stainless steel IR cell capable of operating from 100 to 1500 K. The detailed description of the IR cell has been reported previously.8 In brief, the TiO2 powder was pressed into a tungsten grid which was contacted rigidly to 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 to measure the temperature of the sample. The sample temperature could be easily adjusted using liquid nitrogen and electrical heating of
10.1021/jp002560q CCC: $19.00 © 2000 American Chemical Society Published on Web 12/05/2000
Adsorption and Decomposition of DMMP
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Figure 1. FTIR spectra of adsorbed DMMP (condensed phase) on the reduced TiO2 at 160 K. In the upper part is shown the DMMP spectrum of the gas phase. In the lower part is the spectrum of DMMP on TiO2 for increasing amounts of DMMP admitted into cell, as follows: (a) 0.6 × 10+18 molec; (b) 1.7 × 10+18 molec; (c) 3.0 × 10+18 molec; (d) 4.3 × 10+18 molec; (e) 5.8 × 10+18 molec; (f) 7.4 × 10+18 molec; (g) 8.6 × 10+18 molec; (h) 1.2 × 10+19 molec.
the grid via a programmable controller.9 The TiO2 sample was pressed only on one-half of the tungsten grid while the other half of the grid had no sample so that the infrared spectra of the TiO2 surface and of the gaseous species through the empty half grid in the cell could be measured alternatively. The IR cell was connected to a stainless steel vacuum system pumped by both turbomolecular and ion pumps. 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 -103 Torr). A FTIR spectrometer (RS-1, Mattson) and a quadrupole mass spectrometer (M100M, Dycor Electronics Inc.) were used. The sample cell is mounted on a computer-controlled precision 2D-translation system ((1µm accuracy) built with components from the Newport Corporation. This system allows the sample to be moved reproducibly in and out of the IR beam. Infrared spectra were measured using the nitrogen-gas purged Fourier transform infrared spectrometer equipped with a wide band HgCdTe detector operating at 77 K. The sample spectra shown here were recorded with 4 cm-1 resolution (1000 scans) while the background spectra were recorded with 4 cm-1 resolution (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. Before any experiments, the sample was pretreated in an O2 atmosphere (6 Torr) at 673 K for 30 min to remove small amounts of organic contamination from the TiO2. In the experiments involving a reduced sample, the TiO2 was annealed at 900 K in a vacuum for 15 h. 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. III. Results 1. Low-Temperature Condensation of DMMP Ice on TiO2. The infrared spectra of the adsorbed DMMP on TiO2 have been broken up in two regions: the high-frequency region from approximately 3200-2700 cm-1 which contains the methyl group stretching vibrations and the low frequency region from approximately 1600-1000 cm-1 which contains the C-O and PdO stretching vibrations and the methyl deformation vibrations. Incident radiation below 1000 cm-1 is strongly absorbed by the TiO2. The high and low-frequency regions for the condensed ice phase of DMMP on TiO2 at 160 K are presented in comparison with the spectra obtained for gaseous DMMP in Figure 1. This experiment was performed on a reduced TiO2 sample with practically all the hydroxyl groups removed. In the upper part of the figure the IR spectrum of the DMMP in the gas phase is shown. In the lower part of the figure the spectrum of DMMP adsorbed on TiO2 for increasing coverages of DMMP in the condensed ice phase is shown. Interestingly, only certain vibrational modes of the DMMP are shifted from their gasphase values by condensation on TiO2. A comparison of the IR frequencies of DMMP for various phases is given in Table 1. The PdO stretching mode, which occurs at 1276 cm-1 for DMMP in the gas phase, is shifted to 1242 cm-1 upon condensation at 160 K. This is the largest shift (-34 cm-1) seen for any of the DMMP vibrational modes. This large shift of the PdO stretching mode between the gas phase and the condensed ice phase is known for other phosphonic esters and
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TABLE 1: Assignment of the Infrared Modes in DMMP Spectrum DMMP gas phase DMMP(ice)/TiO2 DMMP liquid phase frequency (cm-1) frequency (cm-1) frequency (cm-1) this work/lit10 this work lit10 assignment 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)
3014/3014 2962/2962 2924/2921 2860/2859 1467/1471 1421/1423 1314/1315 1276/1276 1188/1188 1073/1075 1049/1050
2992 2960 2924 2856 1463 1452sh 1424 1315 1242 1188 1052sh 1028
2995 2956 2927 2852 1466 1421 1314 1245 1186 1065 1038
TABLE 2: The PdO Frequency Dependence for Different Phases diisopropyl methylphosphonate (DIMP) DMMP a
gas phase
liquid phase
solid phase
1266 11
1245 11
1237 11
1276a
1245 10
124210
Measurement done in our laboratory (shown in Figure 1).
it is attributed to the environmental sensitivity of the phosphoryl (PdO) stretching mode11 (see Table 2). A similar strong mode at 1238 cm-1 was attributed to the PdO stretching mode for physisorbed DMMP on MgO.12 On TiO2 the symmetric and antisymmetric stretching modes of the C-O moiety are also shifted down in comparison with the gasphase DMMP, whereas the frequency of the modes characteristic of the P-CH3 portion of the molecule seems not to be affected by condensation into DMMP ice. This comment is related to observation of the δ(CH3P) (νδ ) 1315 cm-1) and νs(CH3P) (νs ) 2924 cm-1) modes for which the frequency can be exactly determined in both the condensed ice and gas phases. As seen in Figure 1, the ratio between the intensity of the stretching modes characteristic to the C-H bonds changes for DMMP conversion from the gas phase to the condensed ice phase. In the gas phase the νs(CH3P) and νa(CH3P) intensities are much smaller than the νs(CH3O) (2860 cm-1) and νa(CH3O) (2962 cm-1) modes, whereas in the condensed phase they are almost comparable in absorbance. 2. Interaction of DMMP with TiO2 for Temperatures Lower than 200 K. To monitor the diffusion and interaction of the DMMP with the TiO2, a layer of DMMP was frozen on the surface of TiO2 at 115 K. The TiO2 sample used in this experiment was only treated to remove the hydrocarbons impurities but was not reduced at high temperature. The purpose for this treatment was to maintain the hydroxyl groups on the surface and to study their interaction with the DMMP molecules. The temperature of the sample was raised to the desired value (dT/dt ) 0.5 K/s), kept it at this temperature for 5 min, and then immediately cooled back down to 115 K where the spectra were measured under vacuum. In Figure 2 are shown the spectra of DMMP on TiO2 for different final temperatures ranging from 115 to 200 K. The average increment in temperature for these experiments was ∼5 K, but only selected spectra are shown in Figure 2. The isolated hydroxyl groups on TiO2 exhibit characteristic modes at 3722, 3678, 3664, and 3610 cm-1. As shown in the first spectra (b-d) from Figure 2, the TiO2 spectrum in the hydroxyl-region is not significantly affected by the DMMP condensation process for temperatures lower than ∼165 K (spectra a-d). For temperatures between 165 to 200 K (spectra e-k), from the high-frequency region of the spectrum, it is clear that as
the temperature increases the isolated hydroxyl groups decrease in intensity whereas the associated hydroxyl groups (broadband centered near 3360 cm-1) increase in intensity. The isobestic point characteristic to this dramatic transformation is shown in the insert included in the left panel (high-frequency region) of Figure 2. The bands at 3728, 3722, 3678, 3664, 3610 cm-1 correspond to different types of isolated OH groups on the surface of TiO2, while the broad band located ∼3360 cm-1 corresponds to OH groups associated with DMMP on the surface. As the temperature increases also a new infrared band at ∼1210 cm-1 starts to develop. The PdO stretching vibration, which occurs at 1242 cm-1 for the condensed DMMP on TiO2 at 115 K, is shifted to 1210 cm-1 as the temperature increases. The isobestic point corresponding to this transformation is shown in the insert included in the right panel (low-frequency region) of Figure 2. The formation of a new peak at the expense of the 1242 cm-1 peak was seen also for the reduced, completely dehydroxylated surface. This observation indicates that DMMP chemisorbs to Lewis acid sites as well as to hydroxyl sites. In contrast, for the hydroxylated surface where the new peak is situated at 1210 cm-1 , for the dehydroxylated surface the new peak is situated at ∼1216 cm-1. The 1315 cm-1 band, which is the phosphorus-bound methyl group deformation mode, is consistently observed and undergoes essentially no change in frequency as DMMP diffusion occurs into the interior of the TiO2 powder. The frequencies of the C-O symmetric and asymmetric stretching modes (1028 and 1048 cm-1) also are almost not affected by the adsorption process on TiO2. Also, the frequencies of the C-H stretching modes do not undergo modification while a modification of the relative intensities for these peaks can be seen. The intensity of the νa(CH3P) mode (2992 cm-1) decreases with respect to the other C-H stretching modes as the temperature increases from 160 to 200 K. The shift to lower frequency of the PdO stretching mode is consistent with a mechanism involving an interaction between the phosphoric oxygen and the surface. The fact that in the range of 115 to 190 K none of the other modes shift, except ν(PdO) at 1242 cm-1, and also that the intensity increase of the associated hydroxyls occurs at the expense of the isolated hydroxyl intensity suggests that DMMP is hydrogen bonded on the surface through the PdO moiety. The behavior of DMMP as it diffuses into the interior of the TiO2 powder resembles the behavior observed for other organic molecules on Al2O3 as entry into the Al2O3 pores takes place by surface diffusion.13 3. Interaction of DMMP with TiO2 for Temperatures Higher than 200 K. As the temperature of the sample is increased (T > 214 K), significant spectral changes occur as shown in Figure 3. In the high-frequency range, associated surface hydroxyl groups start to decrease in intensity as the temperature of the sample increases. This decrease is not accompanied by a reappearance of the originally obserVed isolated hydroxyl groups. The ratio of the intensities of the C-H stretching modes in the region 3050 to 2800 cm-1 is modified in this temperature range. Simultaneously, a new IR band develops at 2827 cm-1. To determine whether the new mode at 2827 cm-1 could be due to the methyl stretching vibration of a surface bound methoxy group, an experiment was carried out exposing TiO2 to methanol and annealing in order to eliminate the molecularly adsorbed methanol molecules. It was found that the most intense band produced from methanol adsorbed on TiO2 was the symmetric stretch mode νs(CH3O)
Adsorption and Decomposition of DMMP
J. Phys. Chem. B, Vol. 104, No. 51, 2000 12295
Figure 2. FTIR spectra of adsorbed DMMP on TiO2 for different annealing temperatures [115 K e T e 200 K]. Lower spectrum (a) was obtained at 115 K for TiO2 before exposing to DMMP; all other spectra were obtained after the TiO2 was exposed to 1.4 × 10+19 molecules of DMMP and annealed to temperatures as follows: (b) 115 K, (c) 146 K, (d) 162 K, (e) 166 K, (f) 171 K, (g) 175 K, (h) 180 K, (i) 188 K, (j) 195 K, (k) 200 K. The insert in the left panel (high-frequency region) shows the isobestic point that characterizes the transformation of the isolated hydroxyl groups into associated hydroxyl groups.
at 2827 cm-1. The asymmetric stretch νa(CH3O) mode is located at 2928 cm-1. The vibrational mode at 2827 cm-1 can be clearly seen, whereas the 2928 cm-1 mode cannot be seen in the experiments for DMMP adsorbed on TiO2 since it is hidden by the strong CH stretching modes of DMMP. The new methyl stretching mode at 2827 cm-1 , specific to the methoxy groups, provides conclusive evidence for the decomposition of the DMMP to produce chemisorbed methoxy groups as the temperature is increased. Below 214 K, we observe only the pore diffusion of DMMP; above 214 K, chemical reaction and desorption take place. In the low-frequency range, the intensity of the PdO stretching mode decreases almost to zero for higher temperatures near 486 K. A new vibrational mode at 1118 cm-1 starts to develop in parallel with the development of the 2827 cm-1 CH3O-Ti mode as the temperature increases. The insert in Figure 3 shows the isobestic point associated with this transformation. The bands that correspond to the C-O symmetric and asymmetric stretching modes due to the CH3-O-P moiety are also significantly affected by heating above 214 K. In the range between 1100 and 1000 cm-1 a complex overlap of modes is observed. A band near 1100 cm-1 clearly develops on heating, and this band is assigned to an O-P-O moiety. A loss of the intensity of the CH3-O-P moiety occurs as the CH3O-Ti species is formed.
IV. Discussion 1. PdO Interaction with the Substrate. The band due to the PdO stretching vibration is strong and its frequency is commonly found in the 1350 to 1150 cm-1 region.14 Due to the size of the phosphorus atom, the frequency of the PdO stretching vibration is almost independent of the size of the alkyl and alkoxy substituents.14 However, there are studies showing that the PdO frequency is influenced by the number of electronegative substituents directly bonded to the P atom.14,15 Also the PdO frequency is very sensitive to association effects with the oxygen atom.14-16 Sequential processes are observed as the temperature is increased for TiO2 containing adsorbed DMMP. At temperatures lower than 166 K, the DMMP molecules self-associate with each other forming a condensed ice layer on the outer geometrical surface of the TiO2, and no changes were observed for the isolated Ti-OH species intensity. This is because the condensed DMMP molecules cannot diffuse into the pores where the majority of the hydroxyl groups exist. The environmentally sensitive phosphoryl (PdO) stretching mode of the DMMP molecule is found between 1200 and 1300 cm-1. As seen in Table 2, a phase change may cause a shift in the band of about 30 cm-1. At temperatures lower than 166 K, DMMP is entirely condensed on the TiO2 surface. The shift of the (PdO) stretching mode between the gas phase and the
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Figure 3. FTIR spectra of adsorbed DMMP on TiO2 for different annealing temperatures [214 K e T e 486 K]. The DMMP hydrolysis on TiO2 is shown by the development of the mode specific to the methoxy groups attached to the surface at 2827 cm-1. Annealing temperatures are as follows: (l) 214 K, (m) 229 K, (n) 250 K, (o) 288 K, (p) 314 K, (q) 352 K, (r) 400 K, (s) 441K, (t) 486 K.
condensed phase is consistent with the shift of the frequency found for other phosphonates in different phases11 (see Table 2). 2. Migration of DMMP into Pores of TiO2. When the temperature increases above 166 K, DMMP molecules diffuse into the porous TiO2 structure and associate with the isolated hydroxyl groups. At 200 K, essentially the entire surface layer of hydroxyl groups is consumed and transformed into associated hydroxyls. This is possible only if the DMMP molecules migrate into the pores and interact with the isolated hydroxyl groups found on the interior surface of the solid powder. The shift of the PdO stretching mode characteristic for the condensed phase (1242 cm-1) to the frequency characteristic for the adsorbed phase (1210 cm-1) is consistent with a mechanism involving an interaction between the phosphoric oxygen and the surface OH groups as the molecule diffuses from the ice overlayer into the interior of the porous TiO2 powder. As a consequence of the interaction with the substrate, the PdO moiety loses some of its double bond character and so the frequency decreases. It is interesting that almost the same magnitude of shift was seen comparing DMMP on diamond powder (1243 cm-1) to DMMP on Ca-montmorillonite (1210 cm-1).17 A similar PdO frequency shift was observed by Klabunde et al.12,18,19 as a consequence of the interaction of DMMP with the surface sites of magnesia. 3. Nondissociative Chemisorption of DMMP on TiO2. Two main adsorption sites exist on the TiO2 surface: isolated hydroxyl groups and Lewis acid sites. The Lewis acid sites can
be either Ti4+ ions characteristic of the fully oxidized TiO2, or Ti3+ or Ti 2+, depending on the degree of the reduction.20 By infrared spectroscopy in the OH stretching region, we can directly monitor the interaction of the DMMP with the hydroxyl groups. The fact that the development of the 1210 cm-1 PdO stretching mode is accompanied by a decrease in absorbance of the isolated hydroxyl groups and an increase in the absorbance of the associated hydroxyl groups is explained by hydrogen bonding of the DMMP molecule to the surface through the hydroxyl groups. During this heating process the characteristic modes of the CH3O groups in DMMP do not shift. This indicates that the major interaction between the DMMP and TiO2, in this temperature range, is through the phosphoric oxygen and involves nondissociated DMMP molecules. The adsorption complex formed involves a unidentate coordination of the DMMP molecule to the surface of titanium dioxide. There are three proposed structures for the DMMP adsorbed on the TiO2 surface as shown in Figure 4. The experimental results indicate that the PdO bond is significantly perturbed upon adsorption. However, the carbon-oxygen stretching mode νC-O is not perturbed, which suggests that adsorption does not involve the CH3-O- moiety. It seems most likely that initial adsorption simply involves the PdO bond, so structures as (I) and (II) in Figure 4 are most appropriate. In structure (I) the oxygen of the phosphoryl group is hydrogen bonded to the hydroxylated TiO2 surface. In the structure (II), the electronrich phosphoryl oxygen forms an adduct with a Lewis acid (Ti4+, Ti3+) site. From Figure 2, we see no selectivity toward hydrogen
Adsorption and Decomposition of DMMP
J. Phys. Chem. B, Vol. 104, No. 51, 2000 12297 TABLE 3: IR Frequencies for Adsorbed Methoxy Groups on TiO2 CH3O(a)/TiO2 (this work)
CH3O(a)/TiO2 In DMMP/TiO2 experiments
νa(CH3O)
2928
2936
νs(CH3O)
2827
2827
νa(C-O) νs(C-O)
1115 1058
1118 1052
assignment (cm-1)
Figure 4. Structures of the possible adsorption complexes of the DMMP on TiO2. It is most likely that structures I and II are formed.
bonding to a specific hydroxyl group. All the modes characteristic of the hydroxyl groups maintain the same relative ratio as they decrease in intensity as DMMP diffuses into the TiO2 interior. Our study of the development of the 1210 cm-1 band which occurs as the intensity of the 1242 cm-1 band decreases cannot determine whether other bands in this region are also produced because of strong overlap effects with neighboring bands. An adsorption process for the DMMP on TiO2 leading to structure (III) is unlikely. If structure (III) is formed, one expects to see a frequency shift in the C-O modes as the associated hydroxyls are formed. As the hydrogen bonds would form between the hydroxyl groups of the TiO2 and the oxygen of the methoxy groups, the force constant of the C-O stretching mode in the -OCH3 moieties would be modified so the frequency would be changed. The C-O stretching modes do not shift between 160 and 220 K, so that structure (III) is unlikely. Structures (I) and (II) portray two possible nondissociative chemisorption modes where Bronsted or Lewis acid sites are involved. Bonding of DMMP in the form of structures (I) and (II) was found by Templeton and Weinberg1,2 when they studied the adsorption of DMMP on Al2O3. They concluded that at 200 K DMMP adsorbs molecularly and that two different modes of interaction for DMMP with alumina were seen: one involving PdO interaction with coordinatively unsaturated Al3+ (Lewis acid) sites, and another one involving interaction of the phosphoryl group with surface OH (Bronsted acid) sites. 4. DMMP Decomposition on TiO2. For temperatures higher than 214 K, the hydrolysis of DMMP takes place on TiO2. The methyl stretching mode region provides clear evidence for the progress of this decomposition reaction as the temperature is increased. The formation of the adsorbed methoxy species is seen in this region as the temperature of the sample rises above 214 K. As a consequence of the adsorption process, the PdO moiety no longer exists with a double bond; it has an intermediate character between a single and a double bound. Because of this delocalization of the electrons and since the vibrations involving P-O, C-O and C-P bonds are all strongly coupled, it is expected that all these vibrations will be influenced as a consequence of the chemical interaction. This is actually what is seen. The absorption bands in this region of the spectrum strongly overlap and it is difficult to resolve the contributions of different vibrational modes.
CH3O(a)/TiO2 292221,22 293022,23 282321,22 283022,23 105521
Figure 5. Structure of the hydrolysis product of the DMMP on TiO2.
For temperatures higher than 214 K, the formation of the methoxy groups bonded to the surface was seen. The bands situated at 2827 and 1118 cm-1 are vibrations of the methoxy methyl groups bound directly to the TiO2 surface through the oxygen atom, based on our experiments with CH3OH adsorbed on the TiO2 suface and on results from the literature (Table 3). Previous studies by Taylor and Griffin23 showed that in methanol adsorption experiments on TiO2, the methyl stretching bands of the adsorbed methoxy group, CH3O (a), appear at 2930 and 2830 cm-1. Similar results were obtained by Chuang et al.22 The stretching modes of the methoxy methyl groups oxygenbonded to the TiO2 surface are distinct from the methoxy methyl group vibrations of the adsorbed DMMP at 2960 and 2856 cm-1 and also from the methyl phosphorus-bond CH3-P groups at 2992 and 2924 cm-1. Thus, it is easy to find out which are the groups that contribute to the formation of methoxy bonded to the surface. The measurement of the relative intensities in this region for increasing temperatures of annealing shows a preferential decrease of the CH3O-P group intensity compared to the CH3-P group intensity. Thus, the hydrolysis of the adsorbed DMMP on TiO2 affects only the methoxy groups and not the phosphorus-bound CH3 groups (CH3-P). The formation of the methoxy groups adsorbed on TiO2 occurs in parallel to the complete dissappearance of the PdO mode (1242 cm-1) by 486 K. This may be interpreted as a further reduction of the phosphoryl oxygen bond order as CH3O- ligands of the P atom are removed by hydrolysis. Therefore, we assign the band formed in the 1200-1000 cm-1 region, except those attributed to the C-O stretching modes, to the νa(O-P-O) and νs(O-P-O) vibrations from structure (IV) (Figure 5). The fate of the hydroxyl groups remains unclear to us. We believe that their disappearance is correlated with the liberation of gaseous CH3OH. The amount of gaseous CH3OH formed could be lower than the detection limit of the spectrometer for the gaseous species. The two equivalent P-O bonds in structure (IV) may be expected to produce vibrational modes roughly in the same spectral region as in ionic phosphonato compounds and in metal phosphonato complexes. The frequencies of the O-P-O bands depends on the substituents on the phosphorus atom,14,15 as well as the nature of the cation, but the bands are always found within the frequency range of 1300-1000 cm-1 (Table 4). A direct match between our data and those available in the literature is not possible. Deviations are expected and there are mainly two
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TABLE 4: Frequencies for the POO (νPOO) Moiety compound R(RO)PO2Ti(MMP)3 MMPdCH3O(CH3)PO2 Ti(DMP)3 DMPd(CH3O)2PO2
POO stretching vibration (cm-1)
literature source
1245-1150 (νa) 1110-1050 (νs) 1158 (νa) 1067 (νs) 1194 (νa) 1090 (νs)
14 24 24
causes for this: (a) the structure formed at the TiO2 surface is not the same as for a salt or a metal complex; and (b) the strong coupling between P-O and C-O in the adsorption complex. Thus, the O-P-O modes will be strongly overlapped by other intense modes falling in the same spectral region. The Ti-O bands in structure (IV) are expected to absorb at wavenumbers lower than 1000 cm-1 and hence cannot be observed. Annealing at temperatures higher than 486 K decomposes the molecule further so that by 800 K only the Ti-O-P-O-Ti bridging species are observed on the surface. It is obvious that the phosphorus retention on the surface will influence the further chemical activity of the substrate. Kuiper and co-workers3 report that ν(PdO) band splits into two frequencies when a bridging O-P-O group is formed on the surface and the new band is usually 90-100 cm-1 lower. On this basis we may attribute the vibration which develops at 1100 cm-1 to the O-P-O bridging species. A similar final structure was proposed by Templeton and Weinberg 1,2 for the dissociative adsorption of DMMP on Al2O3 for temperatures as low as 295 K. The observed FTIR data lead to an understanding of the adsorption and reaction of DMMP on the titanium dioxide surface. The initial step involves the adsorption of the DMMP molecules on titanium dioxide bound to the surface through the phosphoryl oxygen to surface hydroxyl groups and possibly to surface Lewis acid sites. The next step involves the loss of the methoxy group from the adsorbed DMMP molecule and its adsorption as a methoxy species on the TiO2 surface. In this step the hydroxyl groups on the TiO2 surface are consumed. This results in the formation of TiO2-bound phosphonate group possessing lower P-O bond order than in the parent DMMP molecule. V. Conclusions The adsorption of dimethyl methylphosphonate (DMMP) and its decomposition on a powdered TiO2 surface was investigated by transmission infrared spectroscopy for different temperature domains. The following conclusions have been obtained. (1) DMMP condenses at temperatures lower than 166 K on the outer surface of TiO2. (2) As the temperature increases between 166
to 200 K, DMMP diffuses into the TiO2 interior. (3) DMMP chemisorbs on the titanium dioxide through the phosphoryl oxygen to the surface hydroxyl groups and to surface Lewis acid sites. (4) For temperatures higher than 214 K, the dissociation of chemisorbed DMMP is seen. This decomposition reaction is initiated at the methoxy groups of the DMMP molecule and is accompanied by consumption of all of the isolated surface hydroxyl groups. As a consequence, adsorbed methoxy groups on titanium dioxide are produced and adsorbed phosphonate groups exhibiting lower P-O bond order are formed. Acknowledgment. We acknowledge with thanks, the full support of this work by the Army Research Office. References and Notes (1) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. Soc. 1985, 107, 97. (2) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. Soc. 1985, 107, 774. (3) Kuiper, A. E. T.; van Bokhoven, G. M.; Medema, J. J. Catal. 1976, 43, 154. (4) Hedge, R. I.; Greenlief, C. M.; White, J. M. J. Phys. Chem. 1985, 89, 2886. (5) Henderson, M. A.; Jin, T.; White, J. M. J. Phys. Chem. 1986, 90, 4607. (6) Aurian-Blajeni, B.; Boucher, M. M. Langmuir 1989, 5, 170. (7) Mitchell, M. B.; Sheinker, V. N.; Mintz, E. A. J. Phys. Chem. 1997, 101, 11192. (8) Basu, P.; Ballinger, T. H.; Yates, J. T., Jr. ReV. Sci. Instrum. 1988, 59, 1321. (9) Muha, R. J.; Gates, S. M.; Yates, J. T., Jr.; Basu, P. ReV. Sci. Instrum. 1985, 56, 613. (10) Bertilsson, L.; Engquist, I.; Liedberg, B. J. Phys. Chem. 1997, 101, 6021. (11) Crooks, R. M.; Yang, H. C.; McEllistrem, L. J.; Thomas, R. C.; Ricco, A. J. Faraday Discuss. 1997, 107, 285. (12) Li, Y, Schlup, J. R.; Klabunde, K. J. Langmuir 1991, 7, 1394. (13) Mawhinney, D. B.; Rossin, J. A.; Gerhart, K.; Yates, J. T. Jr. Langmuir 1999, 15, 4617. (14) Socrates, G., Infrared Characteristic Group Frequencies, 2nd ed.; Wiley & Sons: New York, 1994. (15) Thomas, L. C., The Identification of Functional Groups in Organophosphorus Compounds, Academic Press: New York, 1974. (16) Eaton, G.; Harris, L.; Patel, K.; Symons, M. C. R. J. Chem. Soc. Faraday Trans. 1992, 88, 3527. (17) Bowen, J. M.; Compton, S. V.; Blanche, M. S. Anal. Chem. 1989, 61, 2047. (18) Lin, Shaw-Tao, Klabunde, K. J. Langmuir 1985, 1, 600. (19) Li, Yong-Xi, Klabunde, K. Langmuir 1991, 7, 1388. (20) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Chem. Phys. 1994, 98, 11733. (21) Glisenti, J. Mol. Catal. A: Chemical 2000, 153, 169. (22) Chuang, C.-C.; Chen, C.-C.; Lin, J.-L. J. Phys. Chem. B 1999, 103, 2439. (23) Taylor, E. A.; Griffin, G. L. J. Phys. Chem. 1988, 92, 477. (24) Mikulski, C. M.; Karayannis, N. M.; Pytlewski, L. L. J. Inorg. Nucl. Chem. 1974, 36, 971. (25) Puri, D. M.; Parkash, A. J. Indian Chem. Soc. 1972, 49, 833.
