Langmuir 1992,8, 2473-2478
2473
Thermal Decomposition of 1,1,1-Trichloroethaneand 1,l -Dichloroethene over High Surface Area Alumina Todd H. Ballinger,? R. Scott Smith, and Steven D. Colson Molecular Science Research Center,t Pacific Northwest Laboratory, Richland, Washington 99352
John T . Yates, Jr.' Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received March 9,1992. In Final Form: July 24,1992
Mass spectrometry and X-ray photoelectron spectroscopy have been wed to study the decomposition of l,l,l-trichloroethane (CH3CCls) over high surface area y-alumina (A1203)in the temperature range 300-1000 K under flow conditions. The dehydrochlorination of CHaCC13(g)occurs in the temperature range 400-600 K to form 1,l-dichloroethene(g)(CHdClz(g))and HCl(g). Above 700 K the CH2=CC12(g) decomposes to form HCl(g),and carbon was deposited on the A1203 surface. No evidencefor the production of chloroacetylene or other major gas phase products from CH&C12 was found, indicating that CH3CC13 dehydrochlorination to adsorbed C and HCl(g) involves a single gas phase intermediate, CHz=CC12. Introduction The fate of chlorinated hydrocarbons that escape into the environment is a matter of great concern. Knowledge of the kinds of interactions, possible reactions, and kinetic lifetimes of chlorinated hydrocarbons in soil and groundwater is important for understanding depletion processes for these substances in our environment. Recently, the surface interactions and reactionsof l,l,ltrichloroethane (CH3CC13) with high surface area y-alumina (Al203) have been conducted.1 These studies found that CH3CCl3is reversibly adsorbedonto the A l a 0 3 surface at room temperature. However,at elevated temperatures, CHsCCl3decomposesvia an a,#?-HClelimination reaction to form 1,l-dichloroethene (CH2==CC12) and HC1. By blocking or removing chemical sites on the surface, it was found that Lewis acid sites (Al3+) are necessary for the reaction, while surface Al-OH groups are unimportant for the decomposition reaction.' The temperatures found to activate this reaction were in the range from 400 to 600 K, which are higher than usually found in natural environmentalconditions. However, it is reasonableto imagine the same reaction occurring over a longer period of time at ambient temperatures and, therefore, possibly being of environmental significance. Another set of studies has been conducted on the dehydrochlorination of CHsCC13 on AlC132and on A1203 surfaces chlorinated with CCL and COC12.3 These reactions proceed at room temperature to produce CHZ=CC~~ and HC1. This high reactivity may be due to the fact that AlC4 is a strong Lewis acid; the addition of chlorineto the AlzO3 surface is also known to increase Lewis acidity of
* Author to whom correspondence should be directed.
+ Visiting NORCUS graduate fellow from the Surface Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, PA 16260. t Pacific Northweeit Laboratory is operated for the U.S. Department of Energy by Battelle Memorial Institute under Contract DE-ACW76RLO 1830. (1) Ballinger, T. H.; Yatas, J. T., Jr. J. Phys. Chem. 1992, 96,1417.
(2)Thomeon, J.; Webb, G.; Winfield, J. M. J. Chem. SOC.,Chem. Commun. 1991, 323. (3) Thompson, J.; Webb, G.; Winfield, J. M. J. Mol. Catal. 1991,68, 347.
However, treatment of A1203 with anhydrous HC1 did not result in dehydrochlorination of CH3CC13 at room temperature; this may indicate that the surface treatment of A1203 with HC1 dnhances the Bronsted &03.4*5
In this study, we have extended our studies of CHsCCl3 decomposition on pure Al2O3I to investigate the decomposiiton pathwaysthat exist at higher temperatures. This investigationwas carried out under flowing-gasconditione using both laser and electron ionized mass spectrometry to measure the reaction products that form, in contrast to earlier static studies of CH3CC13decomposition. For CH3CCh, we have confirmedthat the HC1eliminationreaction occurs at temperatures greater than 400 K. At temperatures greater than 700 K the C H 4 C 1 2 produced undergoes further decomposition to form more HC1 and to deposit carbon on the A1203 surface.
Experimental Section The microreactor chambe$' used (schematically shown in Figure 1) consists of two differentially pumped vacuum chambers. The reactant gases are passed through a tapered quartz tube, and the reaction products are supersonically expanded into the main chamber and are detected by either of two maas spectrometers. The first type of detection is laser ionized mass spectrometry (LIMS), in which a laser beam crosses perpendicularly through the product gas stream from the top of the chamber in the ionization region (shown as an expanded drawing on the right side of Figure 1). The ion optics focus the generated ions through a pinhole into a second differentially pumped, 116 cm long, time-of-flight vacuum chamber containing a dual microchannel plate detector. The second type of detection is a quadrupole mass spectrometer (QMS). Product gases that flow directly across the main chamber enter a 2.5 cm stainless steel tube with a 0.4 cm hole which is connected to the QMS system that is pumped by a 60 L/s turbomolecular pump. The main chamber is pumped with a 10-in. diffusion pump that maintains (4) Melchor, A.; Garbowski, E.; Mathieu, M. V.;Primet, M. J. Chem. SOC.,Faraday Trans. 1 1986,82, 1893. (5) Baeset. J.: Firmeras.. F.:. Mathieu. M. V.:Prettre, M. J. Catal. 1970, 16,'53. (6) Peri, J. B. J. Phys. Chem. 1966, 70, 1482. (7) Tanaka, M.; Ogaaawara, S. J. Catal. 1970, 16, 157. (8) Gulcicek, E. E.; Colson,S. D.; Pfefferle, L. D. J. Phys. Chem. 1990, 94,7069.
0743-7463/92/2408-2473$03.00/0Q 1992 American Chemical Society
Ballinger et al.
2474 Langmuir, Vol. 8, No. 10, 1992 TOP VIEW Microreactor
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Figure 1. Schematic diagram of microreactor chamber used for decomposition experiments. The expanded regions show' the in the microreactor and the ion optics. a reaction pressure in the lo4 to Torr range, and the TOF chamber is pumped by a 6-in. diffusion pump that maintains a pressure of 10-6 Torr during experiments. The catalyst microreactor has been previously described8and is shown in an expanded drawing on the left side of Figure 1.It consists of an 8 mm (inner diameter) quartz tube that has the vacuum end drawn into a cone with a 100-pm pinhole. The catalyst is lightly packed into this tapered region, and a 2 mm (inner diameter) quartz tube is used both to hold the catalyst in place and to monitor catalyst temperature with a chromeValume1 thermocouple placed inside the tube. The catalyst and thermocouple are radiatively heated via a tungsten heater wire suspended in vacuum. The entire assembly is contained inside a water-cooled copper tube, which is not shown in Figure 1. Ionization for the LIMS studies was produced using vacuum ultraviolet (vacuum-UV)light. The vacuum-UV photons were generated from the frequency tripled output of a Nd:YAG laser (Molectron MY34) that was focused into a 25 cm long cell containing 24 Torr of Xe to produce the vacuum-UV (118 nm, 10.5 eV) harmonic. The X-ray photoelectron spectroscopic (XPS) studies of the AlzOa powder were performed in a separate vacuum system with aPerkin-ElmerPhysical Electronics560XPS/AES system,which had a double-pass cylindrical mirror analyzer (CMA). Mg Ka X-rays were used for sample excitation. The energy scale was calibrated using Cu 2~312and Cu 3p photolines at binding energies of -932.67 and -75.13 eV. Spectra were collected using a 100-eV pass energy. The transmission infrared (IR) spectroscopic studies were conductedin astainless steelcell,8containingKBr windows sealed with differentially pumped Viton O-rings. The AlzO3 is pressed into a tungsten grid that is held rigidly in the center of the cell.lo The IR cell is connected to a stainless steel vacuum system, interfaced with a Hewlett-Packard 5890 Series I1 gas chromatograph (GC). Infrared spectra were obtained on a Perkin-Elmer 580B grating infrared spectrometer, coupled with a 3600 Data Station for spectra storage and manipulation. Infrared spectra (9) Baeu, P.;Ballinger, T.H.; Yates, J. T.,Jr. Rev. Sci. Instrum. 1988, 59, 1321. (10) Ballinger, T.H.;Wong, J. C. S.; Yatea, J. T., Jr. Langmuir 1992, 8, 1676.
of the gases in the cell were obtained by signal averaging twice, collecting one data point per cm-1 and using a 2.3-cm-' spectral resolution. The spectra were corrected by subtraction of a background spectrum obtained in vacuum. The GC analysis was performed simultaneously with the IR experiment. Gas samples taken from the IR cell/vacuum syeteh were injected by a six-port valve into a JLW BD-624capillary column initially held at 50 OC for 1min before the temperature was increased at a rate of 5 deg/min. Detection of the separated compounds was achieved using a flame ionization detector coupled with a HP 3396 Series I1 integrator. The retention time of the C H 4 C 1 2 was obtained by injecting the pure gas into the GC. The yalumina (AlzO3) used was Degussa Aluminum Oxide C(101 m2/g). After the Alto3 was packed into the quartz microreactor or pressed into half of the tungsten grid (the other half was free of catalyst to measure gas phase species) in the IR cell, it was heated in vacuum to lo00 K for 30 min to remove hydroxyl groups on the ~urface.ll-'~ The l,l,l-trichlorethane and 1,l-dichloroethenewed in these experiments were purchased from Aldrich with 99+% and 99 5% purity, respectively. For the microreactor studies, the liquids were transferred under nitrogen into glass tubes, which were then sealed via an ultratorr fitting to a stainless steel valve connected to the catalyst sample through a stainless steel gas handling system. For the IR studies, the CHz-CClZ was transferred under nitrogen to a glass bulb and then connected via a stainless steel valve to the gas handling system. The liquids were further purified by several freeze-pump-thaw cycles.
Results and Discussion A. CHsCCls Decomposition in the Microreactor Chamber. The experiments were conducted by exposing CH3CC13 (100Torr) to A1203 in the high pressure side of the microreactor. Gases exiting through the pinhole of the microreactor into the chamber were then analyzed by ~
(11)Ballinger T.H.;Yatas, J. T.,Jr. Langmuir 1991, 7, 3041. (12) Knbinger, H.; F k & " y , P. Catol. Rev. Sei.Eng. 1978,17,31. (13) Peri, J. B.; Hannan, R. B. J . Phys. Chem. 1960,64, 1626.
Langmuir, Vol. 8, No.10, 1992
Thermal Decomposition of Chlorinated Hydrocarbons
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Figure 2. Plot of CH&C13(g) decomposition over yA1203, as measured by mass spectrometryin the microreactorchamber for AlzO3 heated from 300 to lo00 K. CH3CC13(g) is completely converted to CHdClZ(g) and HCl(g) at 600 K. The produced C H d C l z ( g )is also decomposed above 700 K, to form HCl(g). the two mass spectrometry methods. Figure 2 follows the reaction as a function of A1203 temperature. The solid lines, which have been drawn to guide the eye, show the results obtained using the QMS. The points represent the intensity of the largest mass peak in the fragmentation pattern of each of the molecules. The masses monitored were as follows: CH3CCl3,97 m u ; C H 4 C 1 2 , 6 1 amu; HC1, 36 m u . Appropriate fragmentation pattern corrections were made to relate the 61 m u intensity to the C H d C 1 2 partial pressure. The dashed curve plots the LIMS results for CHFCC~~only, since the ionization potential (IP) of C H d C 1 2 (9.79 eV14)makes it the only species ionizable with a single laser photon. No LIMS signals due to CH3CCl3 (IP = 11.25 eV16) or HC1 (IP = 12.7 eV14)were observed. The LIMS signal in Figure 2 was scaled such that the maximum signal equaled the maximum signal of CHyCC12 in the QMS. It was found that the flow of gas through the catalyst changed with catalyst temperature. This is a result of the change in number density of the gas molecules behind the pinhole orifice. To correct for this effect, pure Ar was run through an A1203 catalyst, and the Ar+ QMS signal was measured as a function of the temperature at a constant prepsure. The flow rate approximately followed at 1/T dependence due mainly to gas density change. All points in Figure 2 were then corrected for the actual mass flow change, which amounted to approximately 75% over the temperature range 300-1000 K. Figure 2, showing the reaction as a function of temperature, indicates that between 300 and 450 K the intensity of the mass 97 peak of CH3CCl3 remains fairly constant. Around 450 K and CH3CC13 signal begins to decrease, while some CHyCC12 and HC1 are produced. At 550 K the CH3CC13 intensity is near zero, and the CHpCClZ and HC1 have grown to local maximum intensities. The LIMS signal for the temperature dependence of the production of CHyCC12 is very similar to that obtained by the QMS. LIMS has the additional advantage of being able to detect reactive intermediates which would not be seen by the QMS.8 In this case, we find no significant generation of such species. (14)Ionization Potentials, Appearance Potentials, and Heats of Formation of Caseoua Positiue Ions; Franklin, J. L.; Dillard, J. G.; Rosanstock, H. M.; Herron, J. T.;Draxl, K., Field, F. H., Eds.; National Bureau of Standards: Washington, DC, 1969. (15) Ionization Potential and Appearance Potential Measurements; Levin,R.D.;Lias,S.G.,Eds.;NationalBureauofStandards:Washington. DC, 1982.
0.0
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Figure 3. Mass spectra obtained by the QMS during the CH3CCls decomposition reaction over AlzOs. The upper spectrum was obtained at 293 K and shows the fragmentation pattern of CH&Cls(g). The lower spectrum was obtained at 992 K and shows that HCl is the only gas-phase species present at this temperature.
These results are consistent with those found previously in the static gas cell measurements by IR spectroscopy.' The decomposition of CH&Cls(g) on A1203 occurs by the following overall reaction CH3CC13(g) CH2=CC12(g)+ HCl(g) This reaction was observed to occur above 400 K. Furthermore, no decomposition of CHyCC12 was observed at 600 K over a period of 1h.' The temperature for the onset of reaction is in good agreement for the static and flowing gas systems. When the A1203 is heated to temperatures above 650 K, the CHyCCl2 produced from CH3CCh begins to be depleted, as indicated by both the LIMS and the QMS in Figure 2. This is accompanied by a second increase in the steady-state rate of HC1production. This trend continues to lo00 K, until the mass spectral signal in the QMS and LIMS for C H d C 1 2 is no longer observed. The data in Figure 2 indicate that our measurement of the HC1 production is quantitatively in agreement with expectations. For the two plateau regions in HC1intensity, a ratio of 3.1:l (measured against the zero baseline) is found for the sequence CH3CC13 CH2=CC12 + HCl 2C(a) + 3HC1 The ratio of the C H d C 1 2 and CH3CCl3 plateau values should be approximately equivalent according to mass spectrometer calibrations made in this work; the lack of agreementwith this expectedratio is due to undetermined effects. The reaction stoichiometry for the depletion of CH3CCL3 and production of C H d C 1 2 was previously shown to be accurately 1:l under static conditions, using calibrated IR measurements.' The actual mass spectral patterns obtained by the QMS at the beginning and end of the reaction are shown in parts a and b of Figure 3, respectively. The fragmentation patterns displayed were recorded after subtraction of a background mass spectrum. Figure 3a shows the pattern taken between 1and 100 m u at an A1203 temperature of
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Ballinger et al.
2476 Langmuir, Vol. 8, No. 10, 1992
peaks remaining in the mass spectrum, at the bottom of Figure 3, are thoae due to HC1+(containingthe two isotopes of C1, and its C1+ fragmentation products). The A1203removed from the quartz tube after the reaction was black in color, indicating that carbon had been left on the surface. To confirm this, an XPS analysis was performed on this surface and compared to unreacted A1203 The resulting XPS spectra are plotted in Figure 4. The unreacted A1203 shows strong oxygen (1s) and aluminum (2s,2p) features, as expected, in Figure 4a. The broad peaks seen at approximately 21 eV higher binding energy than the labeled peaks are due to plasmon 10SSe8, while the small,sharp features at approximately 8 eV lower binding energy are X-ray satellite bands resulting from the family of Mg Kcr photons produced by the X-ray anode." Measured peak intensities and appropriate sensitivity factors18 were used to calculate the following atomicratio of elements in the depth of XPS samplingfor the A1203before reaction: oxygen, 60.9 % ; aluminum, 33.0%;carbon, 5.5%;chlorine,0.5%. The chlorine signal is probably a result of residual kc13 used in A1203 preparation and is in agreement with C1 levels measured in other studies.lg In Figure 4b,the reacted A1203 shows a large increase in carbon. Elemental analysis gives the following atomic ratios in the depth of XPS sampling: carbon, 85.3 % ;oxygen, 7.4%;aluminum, 6.5 5% ;chlorine, 0.8%. Carbon deposition on catalysts by the decomposition of hydrocarbons is a well-studied problem that has been
reviewed by Different types of coke can be formed on the surface by the same reactant, depending on the temperature of decomposition. The coke deposited at lower temperatures may be partially hydrogenated, while at higher temperatures successivefragmentationand dehydrogenationcan occur to produce a graphitic form of carbon. The coke formed at lower temperatures has been found to be more reactive toward H2, 02, and team.^^^^^ This was confirmed in our studies when air was accidentally leaked into the microreactor chamber during a C H d C 1 2 decompositionexperiment. CO2 was produced initially at 700 K (the onset of C H d C 1 2 decomposition)and reached a maximum productionrate at approximately825 K before the rate of production decreased. To compare the CH3CCl3 decomposition on A1203 to that due to pyrolysis in the 300-1000 K range, the A1203 was replaced with Pyrex wool in the microreactor. LIMS was used to monitor the production of CHpCC12 from CH3CC13 as the reactor was heated. The CHyCCl2 production curve was shifted 150 K higher than that seen for the A12O3. This is due to gas phase pyrolysis of CH3CC13which has been observed to occur in the temperature ranges of 636-707 K23and 587-685 K,24with the formation of C H d C 1 2 and HC1. Thus, the CH3CCb decomposition rate over AI203 has already reached a maximum rate before gas-phase pyrolysis would become significant. From the above results it is seen that the interaction between CH3CC13and A1203at temperatures ranging from 400 to 600 K results in an a,&HCl elimination reaction to form C H d C 1 2 . These results under gas flow conditions correlate closely with the IR results obtained previously under static gas conditions.' A t A1203 temperatures above 600 K, the CHpCC12 produced from CH3CC13 exhibits ita own decompositionpathway. The onset for C H d C h decompositionbegins around 650 K. Accordingto Figures 2, 3, and 4, the decomposition occurs by complete comsumption of the molecule to form HCl(g) and leaving carbon on the surface. B. C H d C 1 2 Decomposition in the Microreactor Chamber. In order to further understand the CHpCC12 decomposition process, pure C H d C 1 2 (220 Torr) was exposed to a new A1203 catalyst samplein the microreactor. Figure 5 plots the reaction as a function of the A1203 temperature. Both the QMS and LIMS signals for C H d C 1 2 remain constant (within experimental error) from 300 to 700 K. Above 700K the amount of C H 4 C 1 2 detected by the QMS and LIMS decreases, while that of HCl increases. This trend continuesup to lo00 K, at which temperature HC1 is again the only species detected in the QMS. After the reaction, the A1203 was once again black in color, as in the CH3CC13 experiment. The comparison of the behavior of CH3CCl3 and C H d C 1 2 over heated A1203shows clearly that the CHpCC12 is an intermediate species detected in the gas phase during the total decomposition of CH3CCl3to adsorbed C and HCl(g) over A1203. The decomposition of the C H d C 1 2 to ita observed products is somewhatsurprising since it could be expected that chloroacetylene (HCMC1) might be formed as a second gas-phase intermediate in the reaction. The standard Gibbs free energy change for each of the reaction steps was calculated to find out if chloroacetylene could
(16) Atlas of Mass Spectral Data; Stenhagen, S.; Abrahameaon, S.; McLafferty, F. W., Ma.; Interscience: New York, 1969; Vol. 1. (17) Woodruff, D. P.; Delchar, T. A. Modern Techniques of Surface Science; Cahn, R. W.; Davis, E. A.; Ward, I. M., Ma.;Cambridge: New York. 1986: Chaoter 3. (18)Wagner, 2.D. Practical Surface Analysis; Briggs, D.; Seah, M. P., Eds.; Wiley: New York, 1983. (19) Lavalley, J. C.;Benaisaa,M.; Busca, G.;Lorenzelli, V. Appl. Catal. 1986,24, 249.
(20) Karge, H. G.Introduction t o Zeolite Science and Practice; Van Bekkum, H.; Flanigen, E. M.; Jansen, J. C., Eds.; Elsevier: New York, 1991; Chapter 14. (21) Biswas, J.; Bickel, G. M.; Gray, P. G.; Do, D. D.; Barbier, J. Catal. Rev.-Sci. Eng. 1988, 30, 161. (22) Menon, P. G. J. Mol. Catal. 1990,59, 207. (23) Barton, D. H. R.; Onyon, P. F. J. Am. Chem. SOC.1980,72,988. (24) Huybrechts, G.;Hubin, Y.; Van Mele, B. Int. J. Chem. Kinet. 1989, 21, 575.
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293 K. The largest mass spectrometer peaks at 26,35,61, 63, 97, and 99 amu are in agreement with the known fragmentation pattern of CH3CC13.l6 At 992 K the only
Langmuir, Vol. 8, No. 10, 1992 2477
Thermal Decomposition of Chlorinated Hydrocarbons I
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Figure 6. Schematic energy diagram showing the relative standard Gibbs free energies for the various reaction pathways for CH3CCl3 decomposition. be expected as an intermediate. Free energy of formation values at 600 K from the for each of the compounds were used to calculate the free energy change of each reaction. For the CH&Cls(g) decomposition into CHdC12(g) and HCl(g),AGOWK = -9.98 kcal/mol. The subsequent decomposition of C H d C M g ) to graphite andHCl(g) involves AGOWK = -58.47kcal/mol. However, AGOWK = +8.23 kcal/mol for the decomposition of CHpCC12(g) to HC=CCl(g) and HCl(g). A schematic diagram of the reaction energies is shown in Figure 6, where the product states are plotted relative to the standard Gibbs free energy of formation from the elements, and the barriers to the reactions are schematicallyindicated with the dashed lines. The above calculations have been made for the intermediates in the gaseous state, while the decomposition reactions occur when species are adsorbed on the surface. The adsorbed species will have a lower free energy than their gaseous counterparts. The adsorbed species are schematicallyindicated in Figure 6. Therefore, C H d C 1 2 will be seen in the gas phase if a low desorption barrier exista, and the lack of observed HC-Cl in the gas phase could be due to a high barrier to desorption compared to the barrier to decomposition. (25) Chase, M. W., Jr.; Davies, C. A.; Downey, J. R., Jr.; Frurip, D. J.; McDonald, R. A.; Syverud, A. N. JANAF Thermochemical Tables, 3rd ed.; ACS: New York, 1985; part 1. (26) Hildenbrand, D. L.; McDonald, R. A.; Kramer, W. R.; Stull, D. R. J . Chem. Phys. 1969,30,930. (27) Chao, J.; Rodgera, A. S.; Wilhoit, R. C.; Zwolinski, B. J. J . Phys. Chem. Ref. Data 1974,3, 141.
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Figure 7. IR spectra obtained in selected IR regions at 320 K showing the decompositionof 220 Torr of CH&Cls(g) over Aln03 in the static gas cell. Spectrum a was taken through the gas phase before the AlzO3 was heated, and spectrum b was taken through the gas phase after the Alz03 had been heated to a maximum temperature of 950 K for -45 min.
C. CHdC12 Decomposition Studies in the Infrared Cell. Infrared studies were conducted on the decomposition of C H d C 1 2 on A1203 in the temperature range from 300 to lo00 K to compare with the resulta found above. It should be noted that the IR cell used is the static gas cell discussed earlier. To mimic the conditionsof the microreactor chamber,the same pressure (220Torr) of C H d C 1 2 was exposed to A1203 in the cell. The decomposition pathway is similar to that found in the microreactor chamber. Between 300 and 700 K,little, if any, decomposition occurs, as judged by the intensities of the absorption features through both the A1203 and the blank grid sides of the cell. Above 700 K the C H 4 C 1 2 features begin to decrease, but no IR absorption bands due to H C 4 C 1 were observed. Between 800 and 860 K, all IR transmission through the A1203 half of the grid is lost due to total absorption of IR radiation by the catalyst once carbon deposition on the surface causes the A1203 to turn black. IR spectra of the gas phase species in the cell show that from 700to lo00K,CHdC12(g) is extensively depleted and HCl(g) is produced. IR spectra of the gas-phasespecies at the beginning and end of the reaction are shown in Figure 7. Spectrum 7a shows the absorbancefeatures in a selected spectral region for 220 Torr CHdC12(g) at 320 K before reaction. The assignments for the absorption bands can be found e l s e ~ h e r e . ~The ~ >off-scale ~~ band at 1622 cm-l is due to the C = C stretch fundamental, while the other features are combination bands. Spectrum 7b shows the same IR regions after the catalyst has been heated to 960 K and then cooled to 320 K. The vibrational-rotational bands of HCl(g) can be clearly seen, while the strong bands due to CH.&.3C12(g) have been greatly reduced, but not eliminated. Carefulanalysisof the full spectrum indicates that several small, new features are present near 3340, 2113,1199,and 729 cm-l. Unfortunately, we are not able to assign these bands to a single species. The IR results do confirm, however, that the decomposition of CHyCC12 begins to occur around 800 K with HC1 being the predominant gas-phase species produced. A blank decomposition reaction carried out without A1203 in the cell showed that the contribution to the (28) Joyner, P.; Glockler, G.J. Chem. Phys. 1982,20,302. (29) Winther, F.; H u m e l , D. 0.Spectrochim. Acta 1967,37A, 1839.
Ballinger et al.
2478 Langmuir, Vol. 8, No. 10, 1992 Table I. Measured Gas Chromatograph Peaks during C H d C 1 2 Decomposition retention % area partial product pressure (Torr) time (min) ( % of total area) Gas Analysis before Reaction 1.890 0.002 2.017 0.001 CH+Xlz 220 2.895 99.997 Gas Analysis after Reaction 1.573 0.006 1.617 0.100 1.884 0.770 2.012 0.007 2.135 0.006 2.410 0.009 2.919 98.883 CHdC12 99 3.667 0.063 4.496 0.043 4.679 0.069 5.277 0.005 6.619 0.003 8.084 0.010 8.688 0.017 9.035 0.010 a In the parallel IR measurements, we estimate that the partial pressure of CHyCC12(g) is lower than 99 Torr near the A1203. This partial pressure difference is due to the slow rate of mixing of gases in the cell and its associated vacuum system connected to the gas chromatograph.
decompositionreactionof the heated W grid and Ni clamps is minor until lo00K. Thus, the decompositionthat occurs below lo00 K involves A1203 in the IR cell. In the previous study of CH3CCl3 decomposition,' IR results showed a small amount (4 %) of surface carboxylate species formed at 600 K. During the decomposition of CHfiC12 experiment, once again a very small amount of carboxylate was seen below 800 K, when the carbon deposition became large. Thus, the decomposition of C H H C 1 2 to carboxylatespecies is a very minor reaction pathway when compared to the decomposition pathway to adsorbed carbon. Simultaneouslywith the IR experiment, gas chromatographic analysis was performed on the gas mixture. The initial gas sample was taken from the cell at 320 K after the IR scan. Table I shows that CHyCC12 is eluted at 2.90 i 0.02 min, and that two other very small peaks (0.003%)are eluted before CHyCC12. These impurities have not been identified. After the reaction was run at maximum temperature of 950 K for approximately45 min,
and the cell was cooled to 320 K, a second GC sample was taken. This analysis shows that there are other species being produced that elute both before and after the remaining CH2=CC12. However, Table I shows that the combined integrated area of these peaks is less than 1.2% of the whole. Considering that the FID detector is only sensitive to C-H bonds and that the integrated area of CH2=CC12(g) has decreased to 45% of its original value, the amount of side products seen was very minor compared to the amount of HCl produced. Thus, it can be concluded that the major reaction pathway proceeds by the decomposition of CHyCC12 to HC1 and adsorbed carbon. Conclusions In summary, the results obtained for the decomposition of CH3CC13 over A1203 are as follows: 1. The primary decomposition pathway of CHsCCls over A1203 surfaces at temperatures above 400 K under gas flow conditions proceeds via the a,@-HC1elimination reaction
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CH,CCl,(g) CH,=CCl,(g) + HCl(g) 2. At temperatures greater than 700 K, the CHp=CC12 produced from CH3CC13also decomposes on A1203via the overall reaction
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CH2=CC12(g) 2HCl(g) + 2C(ads) 3. The decomposition of CHyCC12 was also studied by IR spectroscopyin a static gas cell. The decomposition occurs at temperatures greater than 700 K, and heavy carbon deposition on the A1203 occurs at 800 K in agreement with observations in the flow reactor. No chloroacetyleneproduct is observed at 600 K, indicating the presence of a desorptionkineticbarrier for this reaction. The combined yield of other gas phase products is 1%
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.
Acknowledgment. This research was supported by the Northwest College and University Association for Science (Washington State University) under Grant DEFG-06-89ER-75522 with the U S . Department of Energy. The authors thank Mark Englehard of the Pacific Northwest Laboratory for performing the XPS analyeis and Jason C. S. Wong at the Univeristy of Pittsburgh for assisting with the GC analysis. Registry No. CH3CC13, 71-55-6;Al203,75-35-4; CHdC12, 1344-28-1.
