Langmuir 1991, 7, 1388-1393
1388
Nanoscale Metal Oxide Particles as Chemical Reagents. Destructive Adsorption of a Chemical Agent Simulant, Dimethyl Methylphosphonate, on Heat-Treated Magnesium Oxide Yong-Xi Li and Kenneth J. Klabunde' Department of Chemistry, Kansas State University, Manhattan, Kansas 66506 Received October 22,1990. In Final Form: January 17,1991 Nanoscale particles of MgO were prepared as fine powders of varied surface areas and crystallite sizes. The capacities of these samples for dissociative (destructive)adsorption of dimethyl methylphosphonate (DMMP), a chemical agent simulant, were determined by using a pulsed microreactor. Surface stoichiometric reactions were encountered and large amounts of DMMP were destroyed, about one DMMP molecule to two surface MgO moieties. Volatile products were formic acid and methanol. Elemental analyses of the spent MgO samples coupled with Fourier transform infrared photoacousticspectroscopy Effects indicatedthat the phosphorus-containingmolecular fragment was immobilized as [CH~OPCHS]~. of contact time, temperature, and surface area were studied. A proposed reaction scheme for the decomposition is given, which involved loss of CH30followed by its oxidation by a second DMMP molecule.
Introduction The extremely high surface reactivity of heat-treated alkaline earth oxides is now well documented.lV2 Thus, fine powders of MgO and CaO exhibit strong surface basicity and moderate a ~ i d i t y .Adsorptive ~ capacities are also high.' And now that these reagents can be prepared in ultrahigh surface area form as nanoscale particles,s16 their adsorptive capacities and reactivities might be expected to be even higher, and perhaps useful for air purification purposes. As a potential application we have chosen to study the surface chemistry of organophosphorus compounds on a variety of samples of heat-treated metal pxide of varying surface areas and crystallite sizes. Many such organophosphorus compounds are toxic, some serving as a group of chemical warfare agents (chemical agents).' Usually charcoal filters are used to adsorb such toxic, air-borne substances. However better alternatives have been sought for some time, such as Cu-Cr-Ag impregnated charcoal,8 as well as other impregnated materials? and thermal and catalytic methods.lOJ1 In fact, the development of effective (1) Nievee, I.; Klabunde, K. J. Materials Chemistry and Physics; Tana b , K., Ed.; Elsevier, Amsterdam, 1988; Vol. 18, p 485. (2) (a) Klabunde, K. J.; Hoq,M. F.; Mouea, F.; Matsuhashi, H. Preparatioe Chemistry Using Supported Reagents; Laszlo, P., Ed.; Academic Preas: New York, 1987; p 35. (b) Klabunde, K. J.; Kaba, R. A.; Morris, R. Inorg. Chem. 1978,17,2684. (3) Tanabe, K. Solid Acids and Bases; Academic Press: New York, 1970. (4) Lin, S. T.; Klabunde, K . J. Langmuir 1988, 1, 600. (5) Teichner, S. J.; Nicolaon, G.A.; Vicarini, M. A.; Gardes, G.E. E. Ado. Colloid Interface Sci. 1976, 5, 245. (6) Utamapanya, S.; Klabunde, K. J.;Schlup, J. R. Chem. Mater. 1991, 3.176-181. (7) Ekerdt, J.; Klabunde, K. J.; Shapley, J. R.; White, J. M.; Yates, J. T., Jr. J. Phys. Chem. 1988,92,6182.
(8)Noyea, W. A., Jr. Science in World War II, Chemistry; Little and Brown Publishers: Boston, MA, 1948, p 296. (9) Smisek, M.; Carry, S. Active Carbon; Elsevier: Amsterdam, 1970; p 188.
(10) (a) Beier, R. W.; Weller, S. W. Ind. Erg. Chem. Process Design Deo. 1967,6,1,380. (b)Graven, W. M.; Weller, S. W.; Peters, D. L. Ind. Eng.Chem. Process Design Deo. 1966,5,183. (c) Graven, W. M.; Paton, J. D.; Weller, S.W. Ind. Eng. Chem.Prod. Res. Deo. 1966,5,34. (d)Tzou, T. 2.;Weller, S. W. Annual Report to Army CRDEC, 1988, and private communications. Theae authors have shown that some Pt A l a 0 8 catalysts opera$ for 50 hat 250 'C to completely air oxidize DMMibefore catalyst poisoning occurs.
catalysts would be best, but heteroatom-containing pollutants are severe poisons for catalytic sites.lM Thus, it is of interest to develop and understand the surface chemistry of solid reagents that simultaneously adsorb, immobilize, and destroy such pollutants. In this paper we explore the use of specially prepared and treated MgO as such a destructive adsorbent. Although many metal oxides and metal oxide mixtures, prepared and activated in the proper way, may be capable of such surface chemistry, MgO serves as a good model since it possesses a simple crystal structure and because it can be prepared with widely ranging surface areas. Additionally,MgO has exhibited high reactivity in catalytic processes,12and defect sites have been shown to be active sites. Therefore, high surface area material, having the most defect siteslunit area, should be of interest as destructive adsorbents. Regarding organophosphorus compounds, we have chosen dimethyl methylphosphonate (DMMP) as a model system. It serves as a relatively nontoxic simulant for chemical agents and possesses the types of bonds of interest.l0 And we have found that DMMP adsorbs quite strongly on MgO, as judged by heats of adsorption.13We deal herein particularly with the mode of decomposition, stoichiometric vs catalytic, adsorption/decomposition capacity, and general surface chemistry considerations.
DMMP
Experimental Section Materials. Magnesium oxide samples used in this study are listed in Table I. Two commercial products (samples A and B) were purchased from ROC/RIC Corp. and Fisher Co., respectively. Their surface areas are 10-60 m2/g. Sample C was prepared in our laboratory by using sample A (99.99%.purity) as a starting material. First, sample A was partially dissolved (11) (a) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. SOC.1985, 107,97. (b) Templeton, M. K.; Weinberg, W. H.J.Am. Chem. SOC.1986, 107, 774. (12) Hoq, M. F.; Nieves, I.; Klabunde, K. J. J. Catal. 1990,123,349. (13) Atteya, M.; Klabunde, K. J. Chem. Mater. 1991,3, 182.
0 1991 American Chemical Society
Nanoscale Metal Oxide Particles as Chemical Reagents
Langmuir, Vol. 7, No. 7, 1991 1389
Table I. MgO Samples surface density, crystallite' source and preparation area, mZ/g g/cm9 sizes by XRD, A commercial product from ROC/RIC Corp. 10 0.70 216 commercial product from Fisher Co. 60 0.23 191 sample A was dissolved in hot water and activated under 130 0.64 76 vacuum at 500 "C D Aerogel method in our lab., activation under vacuum at 500 "C 230 0.15 38 E Aerogel method in our lab., activation under vacuum at 500 "C 390 0.12 30 (slowly) Average particle sizes are generally larger than crystallite sizes due to severe crystal imperfections and cleavage planes.
sample A B C
Table 11. Amount of DMMP Decomposed on MgO Samples (0.1 g) under He Flow at 500 "C amount of DMMP decomposed MgO on no. of MgO on surface surface, mol % (0.1g), M x 1019 0.70 1.1 1.4 B 4.6 6.8 10 C 9.9 14.7 15 D 17.4 26.0 25 E 29.5 44.0 66 a Average value for at least three determinations. Reproducibility was generally *lo%.
sample A
surface area, m2/g 10 60 130 230 390
5
D W INlECTION
Figure 1. Diagram of our decomposition testing device: (1) valve; (2)flowmeter; (3 and 4) valves for flow rate control; (5) heating tape; (6)inlet; (7)GC; (8)liquid nitrogen trap; (9) recorder; (10)furnace; (11) thermocouple; (12)reactor; (13)temperature controller; (14)vacuum gauge; (15)vacuum pump. 35 I
ob
I
i
'100
' '200 '
'300 ' .4b0
Surface Area, mz/g Figure 2. Amount of DMMP decomposed vs surface area of MgO. in hot water (80 "C), the water decanted, and the slurry dried at 120 "C overnight. The resultant white powder was heat treated under vacuum (9 X 10-* Torr) at 500 "C for 12 h. Ita surface area stabilized at 130 m2/g. Samples D and E were prepared in our laboratory by an aerogel method.6 The starting material was magnesium metal, which was allowed to react with methanol and then hydrolyzed in an organic solvent solution, followed by hypercritical drying. Heat treatments for both samples were done under vacuum (9 X 10-8 Torr). However, preparation of sample D involved a relatively rapid heating rate (1h) to 500 "C, and the temperature was maintained there for 12 h. Preparation of sample E involved a slow heating rate (6 h) followed by maintenance at 500 "C for 7 h. These heat treatments were
molecules N X 101s 0.73 5.21 7.86 13.0 34.4
N/M (mol ratio) 0.66 0.77 0.53 0.50
0.78
carried out in the reactor tube (described below) and the samples were immediately ready for use. After the heat treatment, X-ray diffraction (XRD) patterns confirmed that pure MgO had been obtained. However, for samples D and E, the XRD lines were very broad due to their very small crystallite size. Surface areas were measured on a FlowSorb I12300 instrument by means of the adsorption of ultrapure nitrogen at -196 "C. XRD Analyses were carried out on a XDS-2000 instrument using Cu K a radiation (Scintag, Inc.). The tube voltage was 40 kV, the current was 45 mA, and scanning speed was 2.5 deg/min. DMMP was purchased from Strem Chemicals, Inc. Before it was used, it was freeze-thaw degassed for several cycles, and then stored under nitrogen over calcined 5-A zeolite. Elemental Analyses were mainly performed by Galbraith Laboratories, and, in a few cases, by our KSU Analytical Laboratory. Adsorption/Decomposition Studies. Decomposition reactions of DMMP over the MgO samples were carried out on a pulse microreactor-GCsystem. The system diagram is shown in Figure 1. An MgO sample (0.1g) was placed in a stainless steel reactor 6 mm in diameter. The reactor was connectedto a vacuum line for the in situ heat treatment under 9 X Torr vacuum at 500 "C overnight. The reactor was connected to a carrier gas line (He, Hz, or air). After the desired temperature and flow rate were attained, 2 pL of DMMP was injected into the carry gas through injector 6 (Figure l), which was heated to 200 "C in order to vaporizethe DMMP. The DMMP vapor was carried by the gas (usually helium at 15 mL/min) through the 0.1g of the MgO bed. The volatile products and unreacted DMMP were directly detected by an on-line GC. The column of the GC was an aluminum tube (6 m long and 6 mm in diameter) in which 10 wt % of OV-101on 80-100 mesh Chromosorb W-HP was packed. The column temperature was 250 "C. An improvedTDC detector was used for detecting the unreacted DMMP. When unreacted DMMP was detected, this was the signal that the MgO bed was nearly exhausted (i.e. 100% of decomposition could no longer be attained). At this point we noted the amount of DMMP that had been completelydecomposed and continued injections until the MgO bed was completelyexhausted. Volatile decomposition products were analyzed by the on-line GC and then trapped at -196 "C for further analysis. After the decomposition, the MgO sample was kept in a nitrogen-filled bottle for elemental analysis and Fourier transform infrared photoacoustic spectroscopy (FTIR-PAS) analysis. The FT-IR-PAS analyses were carried out on a FT-IR CYGNUS 100 instrument (Mattaon Instrument, Inc.). The photoacoustic cell was a MTEC Model 100.
Results Adsorption/Decompositon Capacities. (1) Effect of MgO Surface Area. In the absence of MgO, no
Li and Klabunde
1390 Langmuir, Vol. 7,No. 7, 1991
Table 111. Decomposition Capacity on 0.1 g of MgO Sample C under Different Contact Times between MgO and DMMP (at 500 OC under He Flow) amount of DMMP decomposed no. of MgO carry gas molecules on molecules, NIM velocity, mL/min contact time, s surface, M x lOz0 PL N x 1019 (mol ratio) 60 0.16 1.47 9 4.71 0.32 45 0.21 1.47 11.5 6.03 0.41 30 0.31 1.47 13 6.81 0.46 15 0.62 1.47 15 7.86 0.53 Table IV. Elemental Analyses for MgO Samdes after DecomDosition and Calculation of Atomic Ratios for Elements calculation MgO on elemental analysis, w t % wt % by difference atomic ratio sample surface, mol 7% PO C H Mg 0 PIMd PIC PIOb C/H A 0.9 0.45 0.40 0.12 59.04 39.99 111.6 112.4 1/23 113.6 B 5.2 2.42 2.33 0.58 54.66 40.01 111.5 112.5 112.9 113.1 C 11.4 5.10 3.68 0.93 48.36 41.93 111.4 111.9 113.7 113.0 D 19.7 7.90 5.29 1.33 42.81 42.67 111.4 1/13 113.5 113.0 8.23 2.51 34.48 E 33.6 11.21 43.57 111.3 112.0 113.6 113.6 From Table I1we can compare the amount of DMMP injected with the amount accounted for by elemental analysis (Table IV). The resulta are as follows: sample A, 0.0018g from elemental analysis and 0.0015 g from r L injected until breakthrough; sample B, 0.0098vs 0.011;sample C,0.021vs 0.016;sample D, 0.032 vs 0.027;sample E, 0.045 vs 0.071. In order to calculate P/Mg and PI0 ratios, we adjusted to observed values by using the percent surface MgO species from Table 11. In other words, we wanted the ratios of P, C, and H to surface Mg and 0. ~~
~~
~~~~~~
w w
&O Mecules and Unit Cell Figure 3. MgO molecules and unit cell.
25 20 15
10
5 0
200 300 400 500
Temper at ure ,
OC
Figure 4. Decomposition capacity va temperature: (A) MgO sample D; (B)MgO sample C (see Table I).
DMMP was decomposedwhen passed through the reactor tube a t reactor temperature. Table I1 and Figure 2 illustrate the amounts of DMMP decomposed over samples of varying surface area, using He as carrier gas. Also in Table I1 are shown, based on surface area, the approximate number of MgO moieties on the surface, which we needed to know in order to calculate the ratio of surface MgO to DMMP decomposed. These numbers were calculated based on the MgO(100) unit cell (Figure 3). The unit cell dimension was 4.21 A. Each MgO moiety, therefore, occupies 8.86 A2 (Le. 8.86 X m2).2b If, for example
~~
~
~~~~~~
~
~
~
~
the MgO surface area is 130 m2/g the number of surface MgO moieties (molecules)should be n = 130/(8.86 X 1 0 9 molecules/g. The capacities of decomposition of DMMP on MgO of varying surface area are reflected in Figure 2, and it is apparent that the adsorption/decomposition capacity of the MgO is roughly proportional to surface area. In fact, the calculations show that about two molecules of surface MgO can decompose one molecule of DMMP. (2) Effect of Contact Time. For a pulse microreactor the time that the reagent contacts the solid sample bed depends on carrier gas flow velocity. By varying the helium flow, we were able to study the effect of contact time on decomposition capacity. These data are collected in Table 111. It is clear that the decomposition capacity increased with contact time. However, for a relatively long contact time of 0.6 s or more, decomposition capacity began to level off. (3) Effect of MgO Temperature. Experiments were carried out a t 200, 300,400, and 500 O C with constant carrier gas flow. Figure 4 shows that higher temperatures were beneficial. However, a t 200 "Cthe GC peaks became so broad it was difficult to obtain quantitative results. In this case the DMMP was strongly adsorbed and products and DMMP itself very slowly desorbed from the MgO surface. Volatile Products. The decomposition products that desorbed from the surface of MgO was a mixture of formic acid (major product) and smaller amounts of methanol, dimethyl ether, and ethane. It should be noted that C02, CO, HzO, Hz, and H a 0 4 were not found. The products found were identified by GC retention times augumented by trapping the eluting material and analyzing each by MS, IR, and NMR. Nonvolatile Products. Our results clearly indicated that phosphorus-containing decomposition product(s) were retained on the MgO sample. These used MgO samples were analyzed by X-ray diffraction, FT-IR photoacoustic spectroscopy and elemental analyses. (1) Elemental Analyses. The used MgO samples were analyzed for P, C, H, and Mg, and the results for MgO samples A-E (after use) are given in Table IV. Oxygen analyses were not carried out and were estimated by difference from 100%. Also shown are the percent of surface MgO molecules. The atomic ratios of elements on
Langmuir, Vol. 7, No. 7, 1991 1391
Nanoscale Metal Oxide Particles as Chemical Reagents
.c fn
C
a,
c
(3
d n i7 Figure 5. XRD spectra for (A) MgO sample C, (B)MgO sample C after use, (C) MgO sample D, and (D) MgO sample D after use (see Table I).
,
2400
,
2000
,
,
1600
,
1200
8
Wavenumber
the surface (P/Mg, P/C, P/O, and C/H) have also been calculated. Only surface Mg2+and 0%were used in these calculations. Considering these results in this way, it is very interesting that for all the used MgO samples, the atomic ratios found fell into narrow ranges of P/Mg (surface), P/C, P/O (surface), and C/H of 1/(1.5-2), 1/2, 1/3, and 1/3, respectively. (2) XRD Patterns. For all the MgO samples (A-E) no XRD changes were found for fresh vs used samples. No new lines were found and only the MgO crystal structure was evident, although broad for the high surface area samples. Figure 5 shows examples of such XRD patterns. (3) FT-IRPhotoacoustic Spectroscopy (PAS).Figure 6 shows FT-IR-PAS spectra of fresh MgO samples A-E and the FT-IR spectrum of neat DMMP (spectrum F). The 1000-1280 cm-l region is very clear for the fresh MgO samples, and only small amounts of adsorbed water in the 1300-1650 cm-l region are evident. All assignments are based on our earlier study of room temperature adsorbed DMMP.14 FT-IR-PASspectra of used MgO samplesA-E are shown in Figure 7 and compared with neat DMMP. A broad peak, sometimes resolved into two and possibly due to three overlapping peaks appeared in the 1000-1200 cm-l region of interest. By estimation, by the appearance of shoulders on the broad peak, of where these three peaks are located, absorbances at 1188,1095,and 1035 cm-l are deduced. On the basis of the known positions for P=O stretch (1242 cm-1 in DMMP) and (P)-C-0 stretch (1035 cm-l in DMMP), and on previous work,l4J5 we assign the peaks at 1188 and 1095 cm-l to bidentate bridging phosphate (these two peaks usually have similar intensities). The 1035-cm-l absorption can be assigned to (PI-C-0 stretch. (14) Li,Y.-X.; Schlup, J. R.; Klabunde, K. J. Langmuir 1991,7,1394. (15) Kuiper,A. E. T.; VanBokhoven, J. J. G. M.; Medema, J. J. Catol. 1976,43,154.
Figure 6. IT-IR-PAS spectra for (A) M 0 sample A, (B) MgO sample B, (C) MgO sample C, (D) Mg8 sample D, (E) MgO sample E, and (F)pure DMMP (see Table I).
Three other peaks were also observed on the used MgO samples. The peak at 1315 cm-l could be assigned to a bending vibration of a CH3 group, while that at 905 cm-l could be assigned to rocking of a CH3 group. A third peak located a t 780 cm-l could be due to P-C stretching. The decomposition of DMMP on MgO sample C was carried out with different contact times, and FT-IR-PAS spectra are shown in Figure 8. Basically, the same results were obtained regardless of contact time.
Discussion Stoichiometricor Catalytic Process. It is clear that the decomposition of DMMP on MgO is surface area dependent, as demonstrated by Figure 2. And from the ratios in Table 11,basically we have found that two surface MgO moieties can decomposeabout one DMMP molecule, and this finding remains true for the different surface area MgO samples. The preparation of these MgO samples was different, and we might expect different types of active defect sites.' In this study, however, only surface area seems to be important. In addition, the XRD results showed that fresh MgO samples showed the same spectra as used MgO samples, although the used samplescontained large amounts of phosphorus, carbon, and hydrogen. These results show that the decomposition products are not crystalline and are spread out over the MgO surface. Thus, the decomposition products immobilized on the surface do not gather a t special reactive defect sites on the MgO surface. On the basis of on these observations we can conclude that the decomposition process is a stoichiometric process rather than catalytic. And since it is strictly surface dependent, the most effective solid reagents should be (and are) the highest surface area, smallest particle materials.
Li and Klabunde
1392 Langmuir, Vol. 7, No. 7,1991
I
2400 '
2000 ' 1600 ' 12'00
,
800
Wavenumber
Figure 7. FT-IR-PAS spectra after use for (A) MgO sample A, (B)MgO sample B, (C)MgO sample C, (D) MgO sample D, (E) MgO sample E, and (F) pure DMMP (see Table I).
Further considerations deal with material balance and whether all DMMP is decomposed. By comparing the data of Tables I1 and IV, we can determine the amount of phosphorus immobilized vs amount of DMMP injected until the MgO bed was exhausted. This material balance shown in Table IV is reasonably good. In addition, FTIR-PAS suggests that little intact DMMP remains on the MgO surface. These results imply that all or nearly all the DMMP injected is decomposed and can be accounted for by elemental analyses. DecompositionProducts. The main volatile product released upon decomposition of DMMP was formic acid. We did not detect the release of any phosphorus-containing compounds, again showing that phosphorus is immobilized on the MgO surface. As mentioned before, we know that about two surface MgO moieties are used up in the decomposition of one DMMP molecule. Since phosphorus remains on the surface, the atomic ratio of P/Mg in the used sample should be about 1/2. From elemental analyses (Table IV) the P/Mg ratio was found to be 1/(1.4-1.6), which is close to the expected value. Other ratios found are P/O (from DMMP) = 1/3 and C/H = 1/3. Therefore, the empirical formula is approximately C&P03(MgO)1.6-2.0. The FT-IR-PAS results seem to be consistent with the above formulation. All the spectra show absorptions in the 1000-1280 cm-1 region that are due to the nonvolatile decomposition product(& Compared with absorptions for DMMP, we can make probable assignments, as discussed in the results section. We note that bands at 780,905,1315, and 1035 are found in both pure DMMP and the decomposition product. However, the 1095- and 1188-cm-1 bands are shifted compared with the normal frequency of the P=O stretch.
2
10
'
20'00
'
16'00
'
1200
'
800
Wavenumber
Figure 8. FT-IR-PASspectra for MgO sample C (see Table I) after use under different contact times between DMMP and MgO: (A) 0.62 s; (B)0.31 s; (C)0.21 s;(D)0.16 s; (E)MgO sample C; (F) pure DMMP.
These results, coupled with knowledge of volatile products and elemental analyses of the used solid, suggest that a structure might be assigned to immobilized species as shown below (1). This corresponds to the empirical formula discussed earlier as CzH~POs(Mg0)1.5-2.0.
I
I I
If the surface-bound oxygens in 1 are assumed to come from the MgO, then the empirical formula is CzHaO compared with DMMP as C3HgP03 or a difference of CH302. The volatile products are mainly CH202 (formic acid) and some CHrO (methanol), which can account for the elements lost as volatile components. And finally, the FT-IR-PAS results show peaks that can be corroborated by structure 1. Effect of Contact Time and Temperature. Essentially no change in FT-IR-PAS spectra was observed when contact time was varied (Figure 8). This supports the concept of a stoichiometric solid surface reaction, since in a catalytic process it is expected that the surface-adsorbed species might change with contact time. Lower temperature decompositions were studied (Figure 41, and lower capacities were encountered. From the online gas chromatograms we observed that the evolution of volatile products was very slow, indicating that they were strongly adsorbed on the MgO surface. This explains why
Nanoscale Metal Oxide Particles as Chemical Reagents
OH
I
1:
0
I
+
O-m-O--Mg-O-Mg
-
H,CO-P--OCH3
-
Scheme I11
Scheme I
c\H3
Langmuir, Vol. 7, No. 7, 1991 1393 (CH,O)zPCH:,
4
Mg -0-Mg-O-Mg
CH3 \
P OCH,
H\
C ,H3
o&-Fo 0
--c
I
I
I
In this way an additional surface 0-H group would be produced, and available for the next cycle of DMMP decomposition. And with regard to the deoxygenated DMMP, (CHsO)zPCHs, this molecule could adsorb with the necessary oxygens coming from the MgO (Scheme111). Overall we must keep in mind that the MgO surface is not providing oxidation power, but simply coordination power, since the reaction stoichiometry is balanced in oxygen if we w u m e that surface Mg2+ ions become partially saturated with OH groups as would be expected for an equation balanced approximately as follows:
--"2 O, CH,
P o&-\O
I
I
0-Mg-0-Mg-0-Mg
H OCH~
I
\
Scheme I1
H
I
I
+
0-Mg-0-Mg-0-Mg
DMMP
6 surface
MgO
CH3OH
+
He0
+ 0
DMMP
H
H
0
II 3(CH30)2-P-CH3
C ,H3 O
3[CH30-P-CH&s
+
[OH]d
+
II
PHO-C-H
I
0-C-OH
I I A 0-Mg-0-Mg-0-Mg
+
H
0
II
HO-C-H
(CH30)2-PCH3
+
I
O-Mg-O-Mg-O-Mg
H
I
the capacities for DMMP adsorption dropped, since adsorbed volatile5 would block surface sites for further DMMP decomposition. It would seem that even at 200 OC DMMP is destructively adsorbed; however, release of volatile products must take place if the full capacity of the MgO is to be realized. F u r t h e r Discussion of Decomposition Process. On the basis of the reaction products (volatileand nonvolatile) the first steps given in Scheme I seem likely. These first steps call for the presence of a surface 0-H group, and if such a group was available, then CH3OH would be the volatile product released. However, we know that, at least initially, surface 0-H groups are present but not in abundance on 500 OC treated Mg0.12 Also, we have shown that formic acid is the main volatile product along with smaller amounts of methanol. It is possible that the initially formed methanol would dissociate on the surface (this is likely), and the adsorbed CH3O group could be oxidized by incoming DMMP (Scheme 11).
In this case the water formed could dissociate to provide the necessary surface 0-H groups for continued reaction. These equations suggest that the presence of small amounts of water in the DMMP feed might be beneficial for the decomposition process. Indeed our initial results support this, and detailed mechanistic studies will be presented at h later time. Summary (1)The decomposition of DMMP on nanoscale particles of MgO is a surface stoichiometric process. T w o surface MgO moieties are capable of destructively adsorbing one DMMP molecule. (2) Volatile products are formic acid and methanol. (3) The nonvolatile product is [ C H ~ O P C H S ]which ~, is completely immobilized. This fragment is probably bound to the MgO surface through two oxygen bridging atoms. In this way, MgO surface moieties are blocked from further participation in the reaction. Acknowledgment. We thank Professor S. W. Weller for helpful discussions. The support of the Army Research Office is acknowledged with gratitude. Partial support came from the U.S. EnvironmentalProtection Agency and the U.S. Department of Energy, through the Hazardous Substance Research Center headquartered at Kansas State University. The findings have not been subjected to EPA review and may not reflect the views of the Agency.
