J. Phys. Chem. 1988, 92, 6351-6357 active systems, it is still very useful for obtaining spectra, particularly when only the enhanced species is of interest,'@I2 or the added selectivity provided by resonance enhancement is necessary. This work demonstrates that the high spectral resolution of Raman spectroscopy allows selective monitoring of individual components and also allows structural inferences from the vibrational spectrum. By examining the reaction mechanism of dopamine oxidation with subsequent HCI or HBr addition, we have shown that kinetic information may be obtained from normal Raman scatterers produced by electrochemical generation on a time scale less than 100 ms. Given the ability to selectively monitor
6351
more than one species involved in the overall reaction, the reaction kinetics and mechanism may be deduced with better definition than possible with conventional electrochemical or UV-vis absorption spectroelectrochemical techniques.
Acknowledgment. This work was supported primarily by the Chemical Analysis Division of the National Science Foundation, with additional support from the Dow Chemical Co. Registry No. CPZ, 69-09-0; CPZ'+, 34468-21-8; H3DA, 5 1-61-6; HC1, 7647-01-0; HBr, 10035-10-6; 3-chloro-o-quinone,56961-34-3; 3bromo-0-quinone, 116596-77-1 ; o-dopaminoquinone, 50673-96-6.
Study of the Catalytic Destruction of Dimethyl Methylphosphonate: Oxidation over Mo(ll0) V. S. Smentkowski, P. Hagans,t and J. T. Yates, Jr.* Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received: January 26, 1988; In Final Form: June 17, 1988)
The catalytic oxidation of dimethyl methylphosphonate (DMMP) has been studied on a Mo( 110) single-crystal surface. Molecular beam dosing methods delivering a 1:2 DMMP/oxygen flux to the crystal were used, with mass spectrometer sampling of the desorbing products. Under steady-state conditions at 898 K, continuous catalytic production of CO and phosphorus oxide gas-phase species is observed as DMMP is oxidized on the Mo(l10) surface. Only surface oxygen is present at measurable coverage on the Mo( 110) surface during the catalytic oxidation process. Both CO and phosphorus oxide gas evolution rates are enhanced by heating to temperatures of the order of 1050 K, compared to the rates observed at 898 K. For DMMP decomposition at 898 K on Mo(l10) in the absence of oxygen, surface phosphorus is the dominant stable surface component present. This is the first report of the sustained catalytic destruction of compounds in this class, in which catalyst deactivation was not observed after long exposure.
I. Introduction Understanding the catalytic destruction of organophosphorus compounds is of importance for devising new methods for the protection of persons exposed to chemical warfare agents, pesticides, herbicides, and other chemically similar industrial compounds.' Dimethyl methylphosphonate (DMMP) is a widely used model compound for catalytic studies of this type, since it possesses the appropriate molecular structure and elemental composition to simulate a number of compounds which are environmentally undesirable.2 The structure of DMMP is OCH,
I
C Hg -P =O
I
OCH,
In this paper we present a new approach to the catalytic destruction (by catalytic oxidation) of DMMP, using a Mo( 110) catalyst surface. It has been found that for DMMP/oxygen mixtures, Mo( 110) provides sustained catalytic activity for the oxidation reaction at temperatures near 900 K. This is of importance, since to date no catalytic process has been reported for the sustained destruction of DMMP or related compounds at lower temperatures. Studies of other catalysts have not yet been carried to temperatures as high as 900 K.' Previous research on the surface chemistry of DMMP is briefly summarized below, and the reader is referred to a recent review article containing this information in greater detail.' A . DMMP Chemistry on Pt/A1203and Pt and Rh Single Crysrals. Studies of DMMP using supported Pt/A120, catalysts have shown that dealkylation, hydrolysis, and oxidation reactions +Permanent address: on sabbatical leave from Dow Chemical Company, Midland, MI 48674.
0022-3654/88/2092-635 1$01.50/0
It was shown that DMMP was incompletely converted to other products and that severe deactivation of the Pt surface occurred, probably by phosphorus poisoning. The poisoning led to a reduction in oxidation efficiency on the Pt surface and a conversion in the later stages of catalyst activity to hydrolysis and dealkylation chemistry, as catalyzed by the A1203 s ~ p p o r t . ~ , ~ Rh( 100) and Pt( 11 1) single crystals have been studied by surface science methods during interaction with DMMP.9*'o For a monolayer of DMMP on Rh(100), at 100 K decomposition occurs, leading to desorption of H2, CO, C 0 2 , H 2 0 , CH4, and C H 3 0 H , while C-, P-, and 0-containing species are left on the surface. On C-covered Rh(100), the activity for DMMP de(1) Ekerdt, J. G.; Klabunde, K. J.; Shapley, J. R.; White, J. M.; Yates, Jr., J. T. J . Phys. Chem., in this issue. (2) Bennett, S. R.; Bane, J. M.; Benford, P. J.; Pyatt, R. L. Enuironmental Hazards of Chemical Agent Stimulants; CRDC-TR-84055; Chemical Research and Development Center: Aberdeen Proving Ground, MD 21010, August 1984. West, A. R. Solid State Chemistry and Its Applications; Wiley: New York, 1984. (3) Baier, R. W.; Graven, W. M.; Linhart, H. D.; Okiver, R. C.; Peters, D. L.: Weller. S. W. Feasibilitv Studv of Catalvtic Methods of Air Purification (v); Contract DA-18-108-CML-6671(A){Final Report, August 1964. (4) Cheselke, F. J.; Wheeler, A,; Weller, S. W.; Baier; Dutch, P. H.; Weiler, F. B. Study of Catalytic and Thermal Decomposition of Toxic Agents; Contract DA18-0350AMC-279(A); Final Report, Book 1, CB-672378-10.1, October 1970. (5) Graven, W. M.; Weller, S. W.; Peters, D. L. Ind. Eng. Chem. Process Des. Dev. 1966, 5, 183. ( 6 ) Baier, R. W.; Weller, S. W. Ind. Eng. Chem. Process Des. Deu. 1967, 6, 380. (7) Stevens, R. R. Design, Fabrication, and Evaluation of First 400 CFM Catalytic Air Purifier Model; Contract DA-18-035-AMC-279(A); Final Report for Task No. 43, Book 1, March 1971. (8) Lester, G. R.; Marinangeli, R. E. Low Temperafure Air Purification Catalysts; Proceedings of the 1986 U S . Army CRDEC Scientific Conference on Chemical Defense Research, November 21, 1986; pp 193-8. (9) Hegde, R. I.; Greenlief, C. M.; White, J. M. J . Phys. Chem. 1985, 89, 2886. (10) Henderson, M. A,; White, J . M., submitted for publication in J . A m . Chem. SOC.
0 1988 American Chemical Society
6352 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 composition is severely r e d ~ c e d . For ~ Pt( 11 l ) , molecular adsorption of DMMP occurs with little decomposition below 300 K.Io Above this temperature, C O and H2 are desorbed. Negative ion SIMS and HREELS indicate that P-0 bonds are present in surface species on Pt( 11 1) up to at least 500 K.Io B. DMMP Chemistry on Oxides. The vibrational spectra of several organophosphonate molecules adsorbed on A1203have been measured by Weinberg and co-workers as a function of temperature (200-673 K) using inelastic electron tunneling spectroscopy (IETS).'].'* At temperatures below 273 K, and at low coverages, molecular adsorption of DMMP as a surface complex involving bonding to Lewis acid sites is proposed. Between 300 and 375 K, P-OR bond cleavage to produce a surface alkoxide occurs. At higher temperatures (475 K for DMMP), dealkylation to form a tridentate R-P(0)3 surface species is postulated to occur. I ] , I 2 In contrast to A1203,Si02does not promote bond cleavage in DMMP adsorbate molecules. This suggests that the relatively strong Lewis acid sites of A1203 are of i m p 0 r t a n ~ e . I ~ Recent studies of iron oxide (cu-Fe2O3) powders have shown that the oxide surface promotes decomposition of DMMP, leaving a phosphorus residue on the surface. Transport of P into the bulk is observed. Oxidation of the P-CH, bond is postulated to occur. Unfortunately, the Fe203is not a catalyst for the sustained decomposition of DMMP.I3 MgO powders have been shown to react stoichiometrically with molecules in the class RO(P03), but catalytic activity has not been observed. C. Rationale for This Work. It may be postulated from the studies summarized above that severe catalytic poisoning by phosphorus or carbon on late-transition-metal surfaces is likely to take place when DMMP decomposition and oxidation occurs. It is also likely that non-transition-metal oxide surfaces alone are not able to catalyze DMMP decomposition in a sustained fashion. Our approach in this work has been to move to an early transition metal (Mo) and to focus on the catalytic oxidation of DMMP using a DMMP/oxygen mixture. Comparative studies of the surface decomposition of DMMP on Mo( 110) in the absence of oxygen shows clearly that the surface behavior desired for sustained catalytic activity is controlled by the presence of surface oxygen on the Mo. For DMMP alone, a surface layer containing phosphorus forms at elevated temperatures. For DMMP and oxygen, surface phosphorus is not present above -900 K, and an 0 layer is present. 11. Experimental Section A . Description of UHV Chamber. The ultra-high-vacuum (UHV) system used in this work is shown in Figure 1. The
stainless steel chamber contains a Dycor MlOOM digitized and multiplexed quadrupole mass spectrometer (QMS) (mass range 1-140 amu), a Perkin Elmer single-pass CMA Auger spectrometer, a tungsten helix for electron bombardment of the crystal, an argon sputtering gun, and an off-axis microcapillary array collimated beam doser.I5 The chamber pressure was maintained at 1 X 1O-Io Torr through the use of a Leybold-Heraeus 150 1/s turbo pump, a Varian 270 l/s diode Vac Ion pump, and a Varian titanium sublimation pump. The chamber pressure was monitored using a Granville Phillips Series 27 1 ionization gauge. Gas could be delivered directly to the crystal via the microcapillary array doser or to the chamber through a leak valve. Pressure in the gas line was monitored using a MKS Type 221-A 0-100-Torr Baratron pressure gauge. For the Auger studies of surface layer formation, a pinhole aperture, nominally -2 pm in diameter (obtained from Buckbee Mears Co.), was used to (11) Templeton, M. K.; Weinberg, W. H. J . Am. Chem. Soc. 1985, 107, 774. (12) Templeton, M. K.; Weinberg, W. H. J. Am. Chem. Soc. 1985, 107, 91. (13) Henderson, M. A.; Jin, T.;White, J . M . J . Phys. Chem. 1986, 90, 4607. (14) Lin, S.-T.;Klabunde, K. J. Langmuir 1985, 1, 600. (15) Bozack, M. J.; Muehlhoff, L.; Russell, Jr., J. N.; Choyke, W. J.; Yates, Jr., J . T. J . Vat. Sci. Technol. 1987, 5, I .
Smentkowski et al. Micro-Capillary
,
Doser Pinhole ADerture
r
n
Ionization Gauge ~ M O ( I I OC)r q s t o l Mounting Holder
I
I
Figure 1. Top view of ultra-high-vacuumapparatus used for studies of molecular decomposition on single-crystal surfaces. The insert shows the crystal holder used for mounting the Mo(l10) single crystal.
control the effusion rate of the gas to the crystal through the doser. At the equilibrium DMMP vapor pressure behind the pinhole aperture, the flux of DMMP was measured to be 3.4 X lOI3 molecules/(cm2.min). For the mass spectroscopy experiments, a pinhole aperture nominally -6 pm in diameter was used to increase the flux of DMMP to 6.6 X 1013molecules/(cm2.min). These fluxes include a geometrical factor of 0.247, calculated from the doser-crystal geometry.I6 In addition, for long exposures using the larger pinhole aperture, the DMMP flux to the crystal drops by 5%/h due to depletion of DMMP pressure in the gas storage region. B. Crystal Preparation. The Mo( 110) single crystal was cut to give a disk 2 mm thick and 12 mm in diameter which then had 0.25-mm slots cut into the top and bottom for mounting. The crystal was prepared using 600-grit sandpaper, followed by 9- and 3-pm diamond polish on nylon. Final polishing was carried out with 0.05-wm alumina on a polishing cloth. This procedure resulted in a mirror finish with no visible scratches. The orientation of the crystal was determined to be within l o of the desired orientation by Laue back-reflection X-ray diffraction, The crystal was mounted and resistively heated with 0.25mm-diameter tungsten wire through the slots which were cut into the crystal (see insert of Figure 1). The 0.25-mm-tungsten support and heating wires were spot-welded to two 1.Cmm-diameter wire leads. Heating of the crystal to temperatures in excess of 1500 K was possible with this mounting configuration. The crystal temperature was measured using a 0.075-mm-diameter W-5% Re vs W-26% Re thermocouple spot-welded to the back of the crystal and is believed to represent the actual temperature of the reacting surface within f 2 K. LEED investigations have shown that the Mo( 110) surface does not undergo reconstruction upon annealing1' C. Materials and Spectroscopy Procedures. DMMP of 99.4% purity 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 freeze-pump-thaw cycles where continuous pumping was carried out on warming; after the fourth cycle, the DMMP vapor pressure remained constant at 0.074 Torr at 301.6 K. This value is in excellent agreement with the literature value (0.074 but is significantly less than the vapor pressure reported by White et aL9J3 (16) Campbell, C. T.; Valone, S. M. J . Vac. Sci. Technol. A 1985, 3, 408. (17) Haas, T. W.; Jackson, A. G. J . Chem. Phys. 1966, 4 4 , 2921. ( 1 8) Ohe, S. Computer Aided Data Book of Vapor Pressure; Data Book Publishing: Tokyo, 1976. (19) Kosolapoff, G.M. J . Chem. Sac. 1955, 2964.
Catalytic Destruction of Dimethyl Methylphosphonate
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6353
C
L
CH: 0
1161
PO,CH:
PO:
/Ofil61
C
F cn
1791
1941
I
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0
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.I
20
I
I 1
40
80
60
11091
CH30-F-OCH3 O'tI241 CH30-fi;cH3
I 100
120
amu
Figure 2. Pure DMMP mass spectrometer cracking pattern, obtained under steady-state conditions after long exposure of vacuum system to DMMP. The assignment of the 79-amu fragment to PO3+ may be questioned, and an alternate assignment to PO2CH4+is possible.26
When the freshly baked gas line was filled with DMMP, a reaction occurred on the stainless steel walls to give methanol (32 amu) and acetone (58 amu) as monitored by mass spectroscopy. The reaction is probably due to hydrolysis and leads to high apparent vapor pressures for the DMMP. This problem was eliminated by filling the gas line with DMMP, letting the system sit for a few hours, and then pumping out the gas line. This procedure was repeated until the pressure in the gas line remained constant upon addition of DMMP, and the mass spectra no longer showed impurities at 32 and 58 amu. It was found that conditioning of the microchannel plate capillary array with DMMP vapor was necessary to avoid hydrolysis products in the gas beam. The pure DMMP cracking pattern is shown in Figure 2. All mass spectra were obtained using the Dycor M100M quadrupole mass spectrometer equipped with an electron multiplier. The energy of the ionizing electrons was set to 70 eV, and the electron current from the filament was 1 mA. Because of background effects, H2 desorption measurements could not be reliably made in this work. The Auger spectra were obtained using a 3-kV electron beam which was 1 mm in diameter. The electron multiplier was set at 2.5 kV, the current at the crystal was kept constant at 2.5 pA, and the modulation amplitude was 3 Vpp. All Auger measurements were performed at the reaction temperature stated in the figures. D. Mo Cleaning Procedure. The Mo( 110) single crystal was initially found to contain carbon, oxygen, and sulfur impurities as determined by Auger spectroscopy. The first step in the cleaning procedure utilized electron bombardment to heat the crystal to temperatures above 2000 K in order to remove atomic oxygen from the surface. The electron bombardment procedure required that the back face of the crystal be located approximately 1.5-2 mm from the electron bombardment helix. A positive bias of 2.5 kV was then placed on the crystal, and a variable transformer was used to heat the electron bombardment helix until the desired crystal temperature was obtained. It was found that a current of 26 mA at 2.5 kV raised the crystal to 2020 K. An electrically isolated digital voltmeter connected to the thermocouple was used to measure the crystal temperatures during the electron bombardment procedure. While the electron bombardment procedure removed the oxygen, it also caused carbon to segregate to the surface. In order to remove the carbon, the crystal at 1100 K was treated with oxygen using a flux of 3.04 X 10l6molecules/(cm2.min) for 5-10 min. The Auger spectra obtained after a number of 5-min treatments showed that carbon was no longer present on the surface and that extended oxygen treatments had no further effect. The sulfur was easily removed from the surface by using cycles of Ar' sputtering (2 pA X 10 min, 1 kV, 300 K). Since the S(LMM) Auger transition occurs at the same energy as one of the Mo Auger transitions (151 eV), the crystal was considered to be free of sulfur when the Auger ratio of (Mo+S[l51 eV])/ Mo(187 eV) reached a steady value of 0.16. This is in excellent agreement with published Auger spectra of clean Mo.20-22
-
-
(20) KO,E. I.; Madix, R. J . Surf. Sci. 1981, 109, 221. (21) Pavlovska, A.; Bauer, E. Surf.Sci. 1986, 275, 369.
I 10
I
100
1
I
I
I
200 300 400 500 Electron Energy ( e V )
Figure 3. Auger spectrum of the clean Mo( 110) single crystal. Also shown are phosphorus, carbon, and oxygen peaks obtained at 300 K following the achievement of steady state after dosing with pure DMMP. The distance between the horizontal lines represents the intensities of the Auger transitions which were measured.
After the initial cleaning procedure, the crystal was routinely cleaned of impurities by using cleaning cycles which consisted of 5 min of electron bombardment at -2020 K, followed by 5 min of oxygen treatment at 1100 K and 10 min of Ar+ sputtering followed by a 1200 K anneal in vacuum. This sequence was performed until the Auger spectra were free of carbon and oxygen while the Mo(151 eV)/Mo(187 eV) ratio reached the steady-state value indicative of complete sulfur removal. The crystal was cleaned of phosphorus, carbon, and oxygen following exposure to DMMP by Ar+ sputtering at 1100 K ( 2 pA X approximately 10 min, 1 kV) followed by annealing at 1200 K. The crystal was considered to be free of phosphorus when the intensity ratio of the Auger transition at 120 eV (P Mo) to the Mo(187 eV) transition was 0.18. This value corresponds to the ratio obtained on the clean Mo( 110) crystal before any phosphorus was present in the chamber. This cleaning procedure was utilized between all experiments involving DMMP. A clean Mo(l10) Auger spectrum is shown in Figure 3.
+
111. Results and Discussion
A . Auger Spectroscopy Studies of Mo(l10) Following Surface Reactions. Auger spectroscopy studies of the surface layers produced by pure DMMP adsorption and thermal decomposition were made in this work. For a typical Auger spectral measurement, beam irradiation times of 3 min were used, and it is realized that ESD-induced chemical damage to the overlayer may occur. In order to understand the possible effect of the electron beam, control experiments were performed. It was found that slow electron beam degradation effects were observed for long bombardment of DMMP layers, producing a reduction of the order of 0.3% per minute in the Mo(187 eV) Auger intensity, with little (if any) effect on either the C, 0,or P Auger intensity. The exact reasons for this small effect are not understood at present. To avoid cumulative effects of electron beam damage in this work, the crystal was translated in 1-mm increments as a sequence of Auger spectra were measured. We believe that a semiquantitative measure of C, 0, and P surface behavior is obtained in this work. All Auger measurements were performed with the crystal maintained at the designated temperature. B . Auger Spectroscopy of DMMP Decomposition on Clean M o ( l l 0 ) . For this experiment, an Auger spectrum of the clean Mo( 110) crystal was obtained at 300 K. Next, the crystal was moved in front of the doser as shown in Figure 1, and a DMMP dose was performed. Following the dosing, an Auger spectrum (22) Walker, B. W.; Stair, P. C. Surf.Sci. 1981, 103, 315.
6354
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 3... 00K
1.0
701K
1100K
1008K
898K
Smentkowski et al.
I
Pl120eVl/Mo(187eVl A u g e r Ratios
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8
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D M M P Exposure ( x d 5 / c m 2 )
Figure 4. Surface layer development for pure DMMP on Mo( 110) by Auger spectroscopy at various crystal temperatures. The top row shows the P(120 eV)/Mo(187 eV) Auger ratio while the second and third rows show O(503 eV)/Mo(187 eV) and C(271 eV)/Mo(187 eV) ratios, respectively Each column corresponds to the temperature at which the Auger spectra were obtained.
was obtained. Cycles of dosing followed by measurements of Auger spectra were performed. For the first hour of the experiment, incremental DMMP dosing times were approximately 5 min; the dosing time was increased to about 30 min thereafter. The phosphorus (120 eV), carbon (271 eV), and oxygen (503 eV) peak-to-peak Auger intensities which result from DMMP exposure were measured as shown in Figure 3. Since the phosphorus (120 eV) Auger transition occurs at the same energy as one of the Mo transitions, the phosphorus signal was corrected for this Mo background effect. It is interesting to note that the P(LVV) Auger feature derived from DMMP adsorption on clean Mo(l10) at 300 K exhibits a sharp minimum at 120 eV, which suggests that it is due to a surface phosphide species rather than a phosphate feature which would give a strong negative feature at 110 eV.23 Plots of the ratio of the DMMP-derived Auger features to the Mo( I87 eV) Auger intensity vs exposure to DMMP at 300 K were then prepared (see the first column of Figure 4). Note that the phosphorus, carbon, and oxygen Auger ratios reach a steady value at 300 K at a DMMP exposure of -8 X 1OiS/cm2. Identical experiments were performed at temperatures of SO8 (data quantitatively similar to 300 K, not shown), 701, 898, 1008, and 1100 K (see columns 2, 3. 4, and 5 of Figure 4, respectively). At temperatures below 701 K, the Mo( 1 IO) crystal adsorbs the DMMP and an increase in phosphorus and oxygen Auger intensities occurs as exposure increases up to a saturation condition. I n the range above 508-898 K, a reaction occurs which causes the carbon Auger signal to decrease following an initial sharp increase. Since a parallel decrease is not observed for either surface P or surface 0 Auger intensity, it must be concluded that a hydrocarbon species is liberated from the surface leading to specific loss of surface C in the range -508-898 K. However, preliminary experiments did not detect CH, production at 16 amu. When the experiment is performed at even higher temperatures, the carbon and oxygen Auger signals disappear completely at higher DMMP exposures. and phosphorus appears to be the only surface species present. In this higher temperature region, CO desorption is observed from DMMP decomposition. At 1100 K , a carbon Auger signal reappears while the phosphorus signal reaches a somewhat lower value. The reason for this high-temperature carbon deposition process remains to be determined, but it is probably not due to simple carbon diffusion from the Mo bulk. Recall that when the crystal was annealed to 1200 K following Ar+ sputtering, the surface was found to be free of carbon. Upon inspection of Figure 4, it is noticed that a decrease in the carbon (23) Bernett. M. K.; Murday, J. S.; Turner, N.H. J . Electron Spectrosc. Relat. Phenon?. 1977. 1 2 , 3 7 5 .
(24) Carlson, T . A. Photoelectron and Auger Spectroscopy; Plenum: New York. 1975.
The Journal of Physical Chemistry, Vol. 92, No. 22, 1988 6355
Catalytic Destruction of Dimethyl Methylphosphonate I
i'
1037K
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O'O6k98K 'Ori""
i l
=
--
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20
40
60
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80
L e n g t h of Dose ( m i n ) Figure 6. Effect of temperature on the rate of CO evolution from the Mo(l10) single crystal using a 2:l oxygen/DMMP gas mixture incident. Irreversible behavior above 1050 K is suggested by the results.
is oxidized to CO while the surface phosphorus is believed to be oxidized to give volatile phosphorus oxide species. It is noted that both P4OlOand P406exhibit high vapor pressures at these temperatures.25 D . Gas-Phase Studies during Reaction. Figure 6 shows the effect of temperature on the rate of CO evolution from the Mo( I 10) crystal as measured by the flux of CO observed by the mass spectrometer when the crystal is in the reactant gas beam. For this experiment, the crystal was preexposed to the DMMP/oxygen gas mixture at 898 K for 4 h in order to be sure that a stable surface condition was achieved, along with a steady rate of CO production. The temperature was then increased, and a new steady value of C O production was reached. The crystal was heated to 950,1000, and 1050 K, followed by cooling to 1000,950, and 898 K. While increases in the rate of CO production are observed as the temperature is increased to 950 and 1000 K, it is observed that the rate of C O production increases by a very large increment as the temperature is increased from 1000 to 1050 K. Also notice that, after the crystal is cooled from 1050 K into the range 1000-898 K, the rate of C O production is significantly greater than it was at the same temperature before being heated to 1050 K. This observation suggests that irreversible effects occur above 1050 K on the Mo(l10) surface which lead to large changes in the catalytic reaction kinetics. In a similar experiment, as shown in Figure 7, PO3+, PO', and CO+ were monitored while the temperature was rapidly increased from 898 to 1037 K. The PO3+ and PO+ signals showed a significant increase after the temperature was raised. After 5 min the PO3+and PO+ signals reached a maximum value and after 12 min dropped to a steadystate value slightly above their initial 898 K levels (see Figure 7). The 5-min and 12-min time periods correspond to the arrival of -3 X lOI4 and 8 X IOI4 DMMP molecules/cm2, respectively, along with twice this fluence of 02.The reflected DMMP parent peak was observed to remain essentially constant throughout these experiments, and an increased yield of C O was observed when the crystal temperature was raised. While PO,+ and PO+ are present in the cracking pattern of DMMP, the increase in the signals of these two ions is not accompanied by an increase in the DMMP (1 24 amu) signal and is therefore believed to be the result of phosphorus oxide (P4O10, P406?)liberated from the surface. The loss of phosphorus from the surface by this process provides surface reaction sites which catalyze DMMP oxidation. This results in a higher steady-state production of CO and phosphorus oxide species at 1037 K as compared to 898 K as may be seen in Figure 7 . Based on the DMMP+, PO3+, and PO+ intensities obtained from the DMMP cracking pattern (Figure 2), the DMMP' signal would have increased by 0.021 X IO-" A if the increase in PO3+ and PO'
W
e
I
0.61
E
I
- - - - - - - -o OO
, I
0.5
22
1.5
Time ( h o u r s ) Figure 7. Effect of one-step increase in temperature from 898 K to 1037 K on DMMP', PO3+,PO', and CO' mass spectrometer signals. o,4 E
o 2
re-treatment
O,+ DMMP exposure
-+
-
(25) Cotton, F. A.; Wilkinson, G. Advanced Inorganic Chemistry: A Comprehensiue Text, 3rd ed.; Interscience: New York, 1972. (26) Batus, D. A.; Gallegos, E. J.; Kiser, R. W. J . Phys. Chem. 1966, 70, 26 14.
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2l.5 h urs I
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5
10'5rnolecules of DMMP/cm2)
Figure 8. Transient behavior of oxygen-pretreated Mo(l10) by Auger spectroscopy measurements taken at 898 K following the 13-min oxygen pretreatment at 898 K using a 2:l oxygen/DMMP gas mixture through the nominally 6 - ~ mpinhole aperture. Note that the time scale includes the 13-min oxygen pretreatment; therefore, in order to obtain the length of the experiment, 13 min must be subtracted from the time. The DMMP exposure scale is nonlinear.
signals were due to the evolution of DMMP and not to the evolution of phosphorus oxide species. Clearly, both transient and relatively constant phosphorus oxide evolution is observed here, indicative of catalytic behavior. After a routine bakeout of the ultra-high-vacuum chamber, it was found that if the crystal was positioned away from the doser and the doser was turned on to deliver a DMMP/oxygen mixture, the oxygen would reach the crystal while the DMMP was pumped
~
6356 The Journal of Physical Chemistry, Vol. 92, No. 22, 1988
Smentkowski et al. Initio1 Flu% D M M P z 6 . 6 x 10'3moleculer/cm2 min Initial Flux 0, 1.3 x 1 0 ' 4 m o l e c u l e r / c m 2 mm
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Time (hours) Figure 9. Transient behavior in the gas phase as monitored by mass spectroscopy using a 2:l oxygen/DMMP gas mixture at 898 K following a 13-min oxygen pretreatment. The DMMP', PO3', PO', and CO'
0
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0
1
,
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2
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I
3
,
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Time ( h o u r s ) Figure 10. Transient behavior in the gas phase as monitored by mass spectroscopy using a 2:l oxygen/DMMP gas mixture at 898 K on an initially clean Mo( 110) surface. The DMMP', PO,', PO', and CO'
mass spectrometer signals are shown.
mass spectrometer signals are shown. by the walls of the chamber and could not reach the crystal at measurable levels (as monitored by Auger spectroscopy; see the initial 13 min of Figure 8). The mass spectrometer measurements of Figure 9 confirm this observation. Thus, an effective and simple method for pretreating the crystal with oxygen prior to exposure to an adsorption of the DMMP/oxygen mixture was available. Figure 8 shows the behavior of the surface composition obtained by heating the crystal to 898 K, as determined by Auger spectroscopy. Note that during the initial 13-min oxygen pretreatment, the P/Mo ratio remained relatively constant near zero, while the O / M o ratio increased. After the 13-min pretreatment, the partially oxidized Mo( 110) crystal, containing approximately monolayer of chemisorbed 0 (as determined by separate O2adsorption experiments), was moved into the gas beam. As expected, the P / M o ratio increased initially, then started to decrease as in column 3 of Figure 5, while the O / M o signal increased and eventually reached a relatively stable value. The C / M o ratio is not shown since a carbon Auger signal was not observed. Mass spectroscopy was now used, in an identical experiment, to monitor DMMP', PO3+, PO', and CO' at 898 K. After obtaining base-line values for the four ionic species with the doser off, the crystal was exposed to a 13-min oxygen pretreatment. When the crystal was moved into the DMMP/oxygen gas beam, the reflected flux of DMMP initially increased sharply and then started to decrease to a relatively stable value over longer times of reaction (see Figure 9). After exposure of the crystal to the mixed gas for 25 rnin, rather stable desorption rates of DMMP and of the mass spectral cracking products PO3', PO', and CO+ were obtained. After 4 h, the doser was turned off as shown in Figure 9, and the four signals returned to their base-line values. In the next experiment, base lines for the four ionic species were obtained with the doser off at 898 K. The crystal was then moved in front of the doser, and the doser was turned on (Le., there was no oxygen pretreatment). The results are shown in Figure 10; notice that the sharp transient DMMP+, PO3+,and PO+ features
which were initially observed for the oxygen-pretreated surface in Figure 9 are not present although CO' shows transient behavior. Other experiments were performed in which the length of oxygen pretreatment was varied from 0 to 13 min at 898 K. The results clearly show that as the length of oxygen pretreatment is increased, an increase of the transient DMMP+, PO,+, and PO+ signals is observed when the oxygen-covered crystal is initially moved into the gas beam. It is also observed that the steady behavior of the gaseous species obtained after long exposures to the D M M P / 0 2 mixture is independent of the length of oxygen pretreatment. A possible explanation may be offered regarding the transient DMMP', PO3+, PO+, and COf features which were observed when the oxidized Mo(l10) crystal was moved into the beam of mixed gas (Figure 9). A fraction of the DMMP is initially reflected off of the oxidized crystal. As reaction occurs between the DMMP and the oxidized surface, surface Mo sites are exposed to some degree, increasing the rate of consumption of the DMMP and the rate of production of the oxidation product, CO. This leads to enhanced surface activity for the sustained catalytic decomposition of DMMP and causes the reflected DMMP signal to drop. Upon longer exposure to DMMP plus 02,the surface of the crystal is found to contain approximately 2 monolayers of oxygen and little phosphorus as monitored by Auger spectroscopy (Figure 8). It should be noted that as the reflected DMMP' signal passes through its maximum and decreases to a relatively stable value, the surface-phosphorus Auger signal (Figure 8) and the surface-oxygen Auger signal are changing dramatically. Phosphorus increases and then decreases to near zero; oxygen increases monotonically toward a limiting value. Thus, both mass spectrometry on the desorbing gas and Auger spectroscopy on the surface phosphorus indicate that phosphorus consumption occurs in the transient region of this experiment, leading to conditions in which surface phosphorus has been replaced by surface oxygen
J . Phys. Chem. 1988, 92. 6357-6359
through liberation of phosphorus oxide. Ratios of the PO3+and PO+ transient features from Figure 9 to the DMMP+ transient feature in Figure 9 were then measured and compared with the ratios obtained from the cracking pattern of DMMP (Figure 2). The ratio of P03+/DMMP+ from Figure 9 was 2.1 times greater than the ratio obtained from the DMMP mass spectral cracking pattern, while the PO+/DMMP+ ratio was 1.9 times greater than the ratio obtained from the DMMP mass spectral cracking patterns. Thus, the PO3+ and PO+ transient features must contain contributions from both the DMMP cracking pattern in the mass spectrometer and the phosphorus oxidation products. In the last experiment performed, steady conditions were obtained at 898 K with the 1:2 DMMP/oxygen reactant beam. Next, the temperature of the crystal was reduced to 279 K while the crystal was still being exposed to the DMMP/oxygen gas mixture. It was observed that the scattered DMMP signal increased to a steady value 2 times greater than that obtained when the temperature of the crystal was 898 K. This experiment confirms that activated oxidation of DMMP is being observed in the studies of the DMMP scattering from the crystal at elevated temperatures in the presence of oxygen.
IV. Summary and Conclusion The oxidation of DMMP by a Mo(l10) surface has been shown to occur catalytically, producing C O and phosphorus oxide gasphase species at -900 K. Auger spectroscopy indicates that an
6357
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oxide overlayer ( 2 monolayers) exists on the Mo( 110) surface under steady conditions at 900 K following exposure to a 1:2 DMMPloxygen gas mixture. Beam dosing studies of the DMMP scattering from the oxidized Mo( 110) surface suggest that maximum catalytic oxidative activity may be achieved on an oxide overlayer which contains little surface phosphorus and which possibly exposes Mo sites. In the absence of oxygen, DMMP decomposes on Mo( 110) above 700 K leaving an overlayer consisting mostly of phosphorus, possibly as a molybdenum phosphide. It is likely that catalytic oxidation of DMMP will occur on a number of transition metals and that a major factor leading to sustained catalytic chemistry is the desorption of surface carbon as C O and the desorption of surface phosphorus as phosphorus oxide(s).
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Acknowledgment. We thank Brett J. Stanley for preparing the crystal for mounting as well as for assisting in the construction of the UHV chamber and preliminary experiments. We thank L. Colaianni and J. G. Chen for assistance in studies of O2 adsorption on Mo(l10). We also thank Professor W. H. Weinberg for useful comments on the work. We also thank Dr. Noel H. Turner of the Naval Research Laboratory for helpful discussions regarding the P(LVV) Auger spectra. We gratefully thank the Army Research Office for support of this work through Contract NO. DAAL03-86-K-0005. Registry No. Mo, 7439-98-7; P,7723-14-0; DMMP, 756-79-6.
Probing Water Pools in Aerosol OT Reversed Micelles by the ESR Spin-Probe Method Yashige Kotake* and Edward G. Janzen* Guelph- Waterloo Centre for Graduate Work in Chemistry, Department of Chemistry and Biochemistry, University of Guelph, Guelph, Ontario, Canada N1 G 2 W l (Received: February 1 1 , 1988)
ESR spectra of Fremy's salt (sodium peroxylaminedisulfonate) have been obtained in the water pools of Aerosol OT reversed micelles in heptane. ESR line widths measured at various water pool sizes were analyzed to give rotational correlation times for the nitroxide. As the water pool size decreases, the radical experiences a more hindered environment for rotational diffusion. It is shown that the rotational correlation time of Fremy's salt is shorter than that reported for water in similar water pools.
Introduction Reversed micelles can form surfactant solubilized water pools in nonpolar solvents. The size of the water pool can be varied by changing the molar ratio of water and The properties of the water in the water pool have been investigated by various techniques. Direct observation of water by proton N M R in the water pool of sodium bis(2-ethylhexyl) sulfosuccinate (Aerosol OT or A0T)-heptane solutions has shown that water in small water pools has a drastically different character from that of the bulk water.4 The dynamic behavior of fluorescence probes or the hydrated electron in water pools as well as the N M R spectra of the AOT molecule itself suggests a considerable change in the property of water in small water p00ls.~3~9~ Recently water pools in reversed micelles have been shown to give favorable conditions for ENDOR s p e c t r o ~ c o p y . ~ ~ ~
Since the water pool in a reversed micelle has been used as a unique medium for reaction chemistry and for spectroscopy, it is of importance to study the state of solutes in the water pool. Probing techniques can be useful for this purpose. Previous ESR spin-probing studies using organic nitroxide spin probes have monitored both the organic and aqueous phases in a reversed micelle9-13because the probes are soluble in both. The hyperfine splitting (hfs) of the nitrogen nucleus in a nitroxide probes shows that when the water pool size decreases, the probe moves into the surfactant layer from the water pool. Thus the observed ESR spectrum is usually a dynamic average of probe spectra from both sites. In fact Yoshioka observed13the presence of two species when the temperature was lowered in a large water pool in AOTheptane and calculated the positional exchange rate. Fremy's salt (Na2(S03-)2NO)is a unique inorganic nitroxide which is apparently only soluble in aqueous solutions. Rotational
( I ) Menger, F. M.; Donohue, J. A.; Williams, R. A. J . A m . Chem. Sac. 1973, 95, 286.
(2) Fendler, J. H. Acc. Chem. Res. 1976, 9, 153. (3) Eicke, H.-F., Kvita, P. Reverse Micelles; Plenum: New York, 1984; p 21. (4) Wong, M.; Thomas, J. K.; Nowak, T. J . A m . Chem. SOC.1977, 99, 477n
