10621
J. Phys. Chem. 1994,98, 10621-10627
Infrared Study of the Oxidation of Hexafluoropropene on Ti02 Jingfu Fan and John T. Yates, Jr.* Suqace Science Center, Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 Received: May 11, 1994; In Final Form: August 6, 1994@
Transmission infrared spectroscopy is employed to study the oxidation of C3F6 on Ti02 particles. Ti02 alone, without dispersion of any transition metals, is capable of oxidizing C3F6 to completion in O2(g) at -600 K. CO2 and CO were found to be the two major products. The initial stage of the C3F6 oxidation on Ti02 follows the conventional redox mechanism: the Mars-van Krevelen cycle which involves both lattice and gaseous oxygen. CF3COO(a) was found to be a reaction intermediate on the Ti02 surface. Water was observed to reduce the oxidation rate of C3F6. A mechanism involving surface CF3COO(a) and FCOO(a) is proposed. Consumption of Ti occurs, producing TiF4(g) during the oxidation reaction.
I. Introduction There is growing interest in finding methods for the complete oxidation of halogenated hydrocarbons which are environmentally harmful. Heterogeneous catalytic oxidation has been studied extensively; however, most of the previous studies were mainly focused on partially oxidized products because of the commercial importance of these compounds. Since the reaction conditions for complete oxidation usually are very different from that of the partial oxidation process, the reaction mechanisms for deep oxidation and partial oxidation may be very different.' In addition, since catalytic oxidation reactions occur at the gassolid interface, the spectroscopic identification of intermediate surface species provides important information needed for understanding the general reaction mechanism. The oxidation of hexafluoropropene (c3F6) on the m i 0 2 catalytic surface is of importance in the catalytic approach to the destruction of chemical agents of the same general chemical composition as c3F62 and to the emission control of volatile fluorinated organic compounds such as those formed as byproducts in tetrafluoroethyleneproduction. Early reports on the oxidation of c3F6 show that C3F6 polymerizes at 600-1000 K when mixed with oxygen or ozone at atmospheric pressure, giving COF2 and CF3COF as major by product^.^.^ Recently, Kurosaki and O k a ~ a k istudied ~ . ~ the oxidation of c3F6 by oxygen over Pt group metals supported on carbon. They found that hexafluoroacetone was readily formed with high selectivity at temperatures of 400-440 K and C02 was formed at temperatures higher than 440 K. They believed that the Pt particles were the active sites for the reaction. A study of the oxidation of C3F6 on Pt/Al2O3 by Farris and co-authors8 indicates that deactivation of the catalyst occurs due to reaction between the alumina support and the hydrofluoric acid product. In this paper, C3F6 oxidation on the Ti02 alone is studied. The gas phase kinetic results as well as the spectral changes of the Ti02 surface during the oxidation reaction are presented. The role of Ti02, the possible mechanism, and effect of water in the C3F6 oxidation reaction are examined.
Thermocouple and power leads to temperature controller L-N1
T
icm
Thermocouple power feedthrough
Ni support leads for mesh Clamps to hold mesh . Tungsten mesh sample holder CaF2 window
'13
11. Experiment Experiments were carried out in a special high-temperature infrared cell shown in Figure 1. A detailed description of this cell can be found in another publi~ation.~In brief, the Ti02 particles were supported on a tungsten grid using a spraying @
Abstract published in Advance ACS Abstracts, September 15, 1994.
0022-3654/94/2098- 10621$04.50/0
Figure 1. Design of the high-temperature infrared cell with liquid nitrogen cooling capabilities for studies of the IR spectra of high area solids.
technique developed in this laboratory. The temperature of the Ti02 can be very easily adjusted between 150 and 1300 K using electrical heating of the grid and a programmable controller operating off of feedback from a thermocouple welded to the grid. The reaction cell was set up so that infrared spectra of the Ti02 surface and of the gaseous species inside the cell can be measured alternatively. Although CaF2 is shown in Figure 1 as the infrared window material, we can conveniently change to different window materials for specific applications. In this study, KBr single crystals were used as infrared windows. The concentric Viton O-ring seals on the windows were differentially pumped. The system is pumped by both turbo molecular and ion pumps which allow a base pressure better than 1 x Torr to be achieved routinely. The pressure measurements were made using a capacitance manometer (Baratron, 116A, MKS). The system we used in this study consists of a FTIR spectrometer (RS-1, Mattson), a gas chromatograph (5890 Series 11, 0 1994 American Chemical Society
10622 J. Phys. Chem., Vol. 98, No. 41, 1994 Hewlett Packard), and a quadrupole mass spectrometer (MlWM, Dycor Electronics Inc.). The C3F6 was purchased from PCR Inc. with a quoted minimum purity of 98.5% and was used without further purification. The oxygen was from Matheson Gas Products with a quoted purity of 99.999%. The oxygen-18-enriched oxygen gas is from ICON Services Inc. with a quoted isotopic purity of 96 atom %. The Ti02 powder (P25) was purchased from Degussa. Ultrapure H20 was prepared by ion exchange and distillation. The following procedure was followed to ensure a homogeneous mixture of C3F6 and oxygen inside the infrared reaction cell: C3F6 of the desired pressure was first introduced to the reaction cell and condensed on the Ti02 sample holder using liquid nitrogen. Oxygen was then introduced, and the C3F6 was allowed to evaporate back into the gas phase. In order to compare experimental results carried out at different conditions, we always use -4 Torr C3F6 initial pressure and then run the oxidation reaction only for 20 min at each temperature. In all experiments, 4 cm-l spectral resolution and signal averaging of 100 scans were used in the infrared measurements. Separate calibration experiments using pure substrates have shown that the infrared detection limits in our system are -0.01 Torr for C3F6, -0.05 Torr for Con, and -0.1 Torr for CO. In this paper, the integrated peak area of the C-C stretching mode of C3F6 centered at 1795 cm-' and the C=O stretching mode of CO2 centered at 2349 cm-' were used respectively to calculate the gaseous C3F6 and COz pressures inside the infrared cell. A good linear relationship exists between the corresponding integrated spectral peak areas and the gas phase pressures in the range 0-15 Torr. The P25 Ti02 is reported to have 70% anatase and 30% rutile structures and a surface area of -50 m2/g.10 It was pretreated at 473 K in vacuum overnight before any experiments. The amount of Ti02 used in the experiment was -30 mg sprayed over a grid of -5.2 cm2 area. The Ti02 weight can be visually estimated during the sample preparation9 and accurately determined after performing the oxidation reactions. Since surface hydroxyl groups of Ti02 were found to influence its chemistry, it is appropriate to first show the behavior of the surface hydroxyl groups at the temperatures of C3F6 oxidation. We measured the infrared spectra between 2500 and 3700 cm-' of Ti02 pretreated in vacuum at temperatures between 473 and 723 K, and the results are shown in Figure 2. After treatment at 473 K, molecular water was completely desorbed from the TiOz, leaving only hydroxyl groups on the surface. In Figure 2a, the bands at 3660-3720 and -3400 cm-' correspond to 0-H stretch modes of the hydroxyl groups for anatase and rutile Ti02, respectively.ll Other bands at -3000 cm-' are due to organic contaminants which were probably derived from acetone which was used in sample preparation. Figure 2b indicates that those organic contaminants can be removed by treating the Ti02 sample at 623 K in vacuum. The coverage of surface hydroxyl groups was reduced significantly after 623 K treatment and was further reduced following 723 K treatment, as indicated by the decrease in the infrared peak areas of the 0-H stretching modes. Figure 2d shows that these surface hydroxyl groups can be regenerated by adsorbing water onto the dehydroxylated Ti02 at room temperature and annealing to 473 K. Munuera and co-authors" reported that the organic contaminants on Ti02 can be removed through treatment under vacuum and then with oxygen at 673 K. Our results show that a simple thermal treatment at -700 K can remove the organic contaminants from Ti02 as judged by infrared spectra. This procedure was used to treat the fresh Ti02 samples used throughout this work.
Fan and Yates
I
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Wavenumber (cm ) Figure 2. Infrared spectra of Ti02 particles (P25) following heat treatment in vacuum. All spectra were obtained at room temperature following -2 h treatment at the indicated temperatures. Spectrum (d) was obtained after exposing ultrapure HzO to the dehydroxylated Ti02 at room temperature and annealing to 473 K.
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Figure 3. Gas phase infrared spectra for CzFs oxidation. All spectra were obtained at room temperature with 4 cm-l spectral resolution. The absorbance of the bottom spectra in both (a) and (b) are scaled to 10% of the original spectra.
111. Results
A. Gas Phase Kinetics. That complete oxidation of C3F6 on Ti02 occurs at 673 K is indicated in the infrared spectra shown in Figure 3. The bottom spectrum in Figure 3a was obtained after -4 Torr of C3F6 was mixed with -6 Torr of oxygen at room temperature. The spectrum is characteristic of gaseous C3F6. After the mixture was allowed to react on Ti02 particles for 20 min at 673 K, the top spectrum in Figure 3a was obtained. From Figure 3a, one sees that essentially
Oxidation of Hexafluoropropene on Ti02
J. Phys. Chem., Vol. 98, No. 41, 1994 10623
control
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(b) 573 K
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Time of Reaction (Min) Figure 4. Kinetics for C3F6 oxidation on TiO2. The reactant mixture was allowed to react sequentially at 473,573, and 673 K each for 20 min, and only the data for 573 and 673 K are plotted. For clarity, the gas phase concentrations are normalized to the initial (for C3F6) and
final (for/cOz) pressure in the infrared cell. complete reaction of C3F6 occurs and that COZand CO are the major gaseous products. In Figure 3b, a control experiment on a tungsten grid, raised to a very high temperature (1 173 K), indicates that only a small amount of conversion of C3F6 occurs even under these extreme temperature conditions, producing small quantities of COz and CO. A close examination of Figure 3a,b reveals that infrared bands between 2000 and 1800 cm-', which are characteristic of chemical species containing the carbonyl functional group such as COFz and CF3COF,'2 appear in (b) but not in (a). These products are characteristic of the homogeneous phase partial oxidation of C3F6.4q5 The fact that these products are not observed over Ti02 is another indication that C3F6 is oxidized to COz and CO on TiOz. On the basis of infrared absorbance calibrations, we found -60% of the C3F6 was converted to COz and the other -40% to CO. Mass spectral measurement of the final reaction mixture are in good agreement with the infrared results. Figure 4 shows kinetic results for the thermal oxidation of C3F6 over Ti02 at 573 and 673 K (spectra 4a and c, respectively). In both cases, the infrared spectra of the gas phase were recorded at -1 min intervals. Although the same initial C3F6 and oxygen pressure (-4 Torr of C3F6 and -6 Torr of oxygen) were used in all the four experiments, only the relative changes in the gas phase concentrations are plotted for simplicity. In all four experiments, the reactant mixtures were Fist allowed to react for 20 min at 473 K, followed by 20 min at 573 K, and then another 20 min at 673 K. As shown in Figure 4a,c in the presence of TiOz, the oxidation of c3F6 is much slower at 573 K than at 673 K. The oxidation of C3F6 essentially reached completion in -10 min at 673 K. The data also indicate that the production of COz shows a very good correlation with the consumption of C3F6. Control experiments at 573 and 673 K using only the tungsten grid were negative as shown in Figure 4, b and d, respectively. The gas phase kinetic results at various oxygen pressures are shown in Figure 5 . The initial pressure of c3F6 was -4 Torr in all three experiments, and all the reactions were canied out at 673 K. The number of gaseous molecules was calculated based upon a separately measured infrared peak area-pressure
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Reaction Time (Min) Figure 5. Kinetics for the oxidation of C3F6 over Ti02 at various oxygen pressures. The Ti02 sample used in the experiment with [02]/ [c&]= 1.1 was not the same sample as used for the other two
experiments. They were prepared following the same procedure.
relationship. Figure 5 indicates that -80% of the initial C3F6 was converted to CO2 and CO in the Fist 2 min and all the c3F6 was completely oxidized in -10 min when the initial [ 0 2 ] / [C3F6] ratios = 3.6, 1.1, and 0.56. The large variation in the initial oxygen pressure did not strongly affect the reaction kinetics in proportion to the 0 2 pressure, and the c3F6 was oxidized to completion even when the oxygen pressure was much less than that required for either of the following two stoichiometric oxidation reactions:
Incomplete chemical equations are given here for simplicity. In addition, we found that the large change in the initial oxygen pressure did not change the relative ratio of CO;! and CO in the gaseous products in any significant way (see the final C02(g) and CO(g) infrared spectra in Figures 3 and 9), indicating that the reaction product yields of CO2 and CO are independent of the oxygen pressure inside the reaction cell. In a separate experiment, C3F6 was allowed to react with Ti02 at 673 K in the absence of oxygen gas. We observed that the reaction was much slower, and species containing a carbonyl functional group were formed along with COz. B. Infrared Spectra of a Surface Intermediate. Figure 6 shows the spectra of Ti02 before and after partial oxidation of c3F6. Figure 6a shows the spectrum of the Ti02 surface after partial reaction, and Figure 6b shows the original Ti02 background spectrum before the reaction. The difference spectrum is shown in Figure 6c. Five pronounced infrared bands are labeled in the 1800-lo00 cm-' spectral region. These bands are assigned to a Ti-OOCCF3 surface species, based on the correlation and assignment given in the bottom portion of Figure 6 for the solid compound (CF3C00)3Al and CF3COOH.12313The C=O stretch frequency at 1734 cm-' is higher than expected for an inorganic salt but is consistent with that of acetate ions on oxide s ~ r f a c e s . ' ~Comparing J~ to gas phase fluorinated acetic acid, the 1734 cm-I C=O stretch frequency of the surface fluoroacetate that we observed was decreased by
Fan and Yates
10624 J. Phys. Chem., Vol. 98, No. 41, 1994 TiO,
- Surface Species
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Surface speciaa on TiO,
,(cFp*)$' (solid)
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~~
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Wavenumber (cm ) Figure 6. Development of infrared spectra of surface species for c3F6 oxidation on Ti02 at 573 K. Spectrum (c) was the result of a direct subtraction of (a) from (b) using the WinFirst program provided by Mattson. The spectrum for (CFsC02)A was obtained from a transmission spectrum reported in ref 12. There is some disagreement in the literature about the assignment of the two lower wavenumber carbonyl vibrational modes.13b
-80 cm-l.12 A similar magnitude of reduction in the CO stretch frequency was observed when acetic acid was chemically adsorbed onto a powdered Ti02 surface.14 This magnitude of the C - 0 stretching frequency shift corresponds to an unidentate bonding structure of fluoroacetate to Ti02.16 These facts suggest that the fluorinated acetate we observed is adsorbed on the Ti02 surface and that the formation of a salt compound with bulk Ti02 does not occur. The same surface species was observed when C3F6 directly reacts with Ti02 in the absence of 0 2 . Oxygen containing a large enrichment of l8O2 was used in an effort to confirm the assignment of surface fluoroacetate. Figure 7 shows the infrared spectra of the Ti02 surface obtained in the middle of the C3F6 oxidation reaction. The spectra obtained using both oxygen-16 and oxygen-18 are shown for comparison. Both the gas phase and the Ti02 surface spectra are plotted in Figure 7; the solid line shows the infrared spectra of the Ti02 surface containing the fluoroacetate species, while the dashed line shows the spectra of the unreacted c3F6 gas. Since the initial C3F6 and oxygen pressure and the reaction temperature were the same for both spectra (-4 Torr of C3F6 and -6 Torr of oxygen at 673 K), any shift in the infrared bands must be due to isotopic effects. The bands due to the C - 0 modes of surface acetate are observed at 1467, 1734 and 1451, 1716 cm-' for oxygen-16 and oxygen-18-labeled surface fluoroacetates, respectively. The shifts are 18 cm-' for v(C=O) and 16 cm-' for v ( C - 0 ) . The magnitude of these shifts is less than the -30 cm-' reported for oxygen-18 doubly labeled acetic acid CH3C1s0180H.17As will be discussed in section C for the case of C02, what we observed in the spectrum using oxygen-18 is actually an average spectrum of isotopic species consisting of normal oxygen-16, oxygen- 18 singly labeled, and oxygen- 18 double labeled fluoroacetates. After taking account of this, the isotopic shift of 16-18 cm-l for the carbonyl stretching frequencies is in the correct range.'* On the other hand, no appreciable broadening was found for v ( C - 0 ) and v(C-0) of oxygen-18-labeled fluoroacetates. The lack of
Wavenumber (cm
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Figure 7. Isotopic shift in the carbonyl frequencies of surface species on Ti02 during C3F6 oxidation in l6O2 or l802 at 673 K.
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Figure 8. Continued oxidation of surface species on Ti02 formed during oxidation of C3F6 at 673 K.
resolution of the three isotopic types of carboxylate expected here is probably due to the inherent width of the adsorbate IR bonds and to intensity sharing effects between surface species of comparable frequency. Figure 8 shows the infrared spectra of the surface fluoroacetate before and after reaction with oxygen gas. Figure 8a shows the spectrum obtained after evacuating the infrared cell following partial oxidation of C3F6. The surface fluoroacetate is indicated by the infrared bands at 1734,1470,1258, and 1124 cm-'. Figure 8b was obtained after the Ti02 containing surface fluoroacetate was allowed to react with -2 Torr oxygen for 2 min at 673 K. We observed that all the bands of fluoroacetate disappear and the C=O stretch of gaseous C02 at 2349 cm-' appears after the oxidation, suggesting that the surface fluoroacetate is readily oxidized by oxygen to form COz. In a separate experiment, we found that the surface fluoroacetate remained very stable in vacuum at 773 K. Therefore, the removal of fluoroacetate is through oxidation, not thermal decomposition.
J. Phys. Chem., Vol. 98,No. 41, 1994 10625
Oxidation of Hexafluoropropene on Ti02
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Reaction Time (Min) Figure 10. Kinetics for the oxidation of on Ti02 with and without added water. ,
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Figure 9. Infrared spectra of COZand CO isotopic products from c3F6 oxidation on Ti02 using l802(g) at 673 K. The missing branch assignments for COZare found in ref 34.
In Figure 8a, in addition to the labeled bands corresponding to surface fluoroacetate, another band centered at 1620 cm-' is also pronounced. This and other extra features in the spectra were later identified to be the result of deposition of reaction byproducts on the KBr windows of the infrared cell (see Figure 11 and section E for details). C. Isotopic Products. In the experiments using **O2gas, a significant amount of oxygen- 16 was found to be incorporated into the gaseous products. Figure 9 shows the gas phase infrared spectrum obtained after -4 Torr of C3F6 was completely oxidized in -14 Torr of lSO2 over Ti02 at 673 K. The missing Q branches centered at 2143 and 2091 cm-' are due to Cl60 and Cl80, respectively. Although the C02 stretching modes at -2332 cm-l do not clearly show any isotopic composition due to the superposition of multiple isotopic bands, the appearance of six well-resolved missing mode branches at 3714,3674,3647, 3612,3570,and 3524 cm-l indicate that Cl60:!, C160180,and Cl8O2were produced and C160180is the major component. The absorption bands in this region correspond to characteristic overtone and combination modes of gaseous CO:! fundamentals and are well documented in the literature.18-20 Because Ti02 is the only source of oxygen-16, the fact that Cl60(g) and Cl6O2(g) are observed clearly indicates that lattice oxygen of Ti02 was incorporated into the reaction products. D. Effect of Water. Figure 10 shows the gas phase kinetics of the oxidation of c3F6 with and without added water. In both (a) and (b) of Figure 10,an initial pressure of -4 Torr of c3F6 and -5.5 Torr of oxygen was used. The reactant mixture was allowed to react for 20 min at 573 K and continued for 20 min at 673 K and another 20 min at 773 K if necessary. In Figure 10a without water, 20% of the c3F6 was reacted at 573 K,and the rest was completely oxidized at 673 K in -10 min. In Figure lob, in the presence of more than 2 Torr of water, very little reaction was detected at 573 K,only about half of the C3F6 was destroyed at 673 K, and the remaining c3F6 was completely oxidized at 773 K. It is apparent from these results that water reduces the rate of C3F6 oxidation on TiO:!. We should point out that the experiment with water was a second oxidation cycle performed on a Ti02 sample which may have been slightly poisoned from the first oxidation cycle. But, in
I 3500
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Figure 11. Infrared spectrum of C3F6 oxidation products deposited on the KBr windows of the infrared cell.
other experiments in which batches of C3F6 were consecutively oxidized on the same Ti02 particles in the absence of water, no significant poisoning effect was observed in the second oxidation cycle (see Figure 5, curves b and c). E. Fate of the Fluorine. Although no fluorinated hydrocarbon gaseous products were observed in the C3F6 oxidation reaction on Ti02 based on infrared and mass spectral measurements, gaseous hydrogen fluoride was observed to be produced. The origin of the HF must be the residual surface OH groups on the Ti02. At 673 K, hydrogen fluoride reacts with Ti02 to form titanium fluoride which evaporates at the reaction temperature.21,22Figure 11 shows the infrared spectrum of deposits on the KBr windows alone after performing several C3F6 oxidation cycles. Analysis of the spectrum indicates that it is due to molecular complexes formed by TiF4 and The bands at -3370, -3200, and 1624 cm-' are due to the stretching and deformation of the hydroxyl groups of water.23 The bands at 732,614,and 480 cm-' are due to TiF4.24-26The closed cell procedure we used certainly promotes reaction between hydrogen fluoride product molecules and Ti02 particles; the behavior in a flow system may be very different.
IV. Discussion The gas phase kinetic results in Figures 3-5 indicate that C3F6 was rapidly and completely oxidized to COz and CO on Ti02 particles at 673 K. No other species containing the carbonyl functional group, such as COFz and CSCOF, which are normally found in the homogeneous phase thermal oxidation
Fan and Yates
10626 J. Phys. Chem., Vol. 98, No. 41, 1994
of C3F6, were found in the oxidation over Ti02. This suggests that all three carbon atoms in C3F6 are able to be oxidized to completion under relatively mild conditions. In the following discussion, we first examine the role of Ti02 in the oxidation reaction and then look at the possible reaction mechanism and the effect of water on the C3F6 oxidation reaction. A. Role of TiOz. The role of Ti02 in the oxidation of C3F6 can be one or a combination of the following three possibilities: (1) as a catalyst, (2) as a reactant, or (3) as a medium facilitating heat transfer to the gaseous reactants, promoting oxidation in the gas phase. In case 1, the role of Ti02 is to lower the activation energy of the oxidation reaction so it can proceed easily at lower temperatures. Although the final chemical equilibrium of the catalyzed reaction should not be affected by Ti02, the reaction pathway may be changed to avoid slow steps. Also, surface species may be found as intermediates since heterogeneous catalytic reactions occur at the gas-solid interface. Furthermore, lattice oxygen of the catalyst may take part in the oxidation reaction as is generally true for n-type semiconductor oxides such as T ~ O ZIn. case ~ ~ 2, the Ti02 serves only to supply reactant material to the oxidation process. In case 3, the role of Ti02 is only to provide large surface area to increase the thermal contact between the gaseous reactants and the heated surface. This increases the average gas temperature and hence the fraction of molecules with sufficient energy for the homogeneous phase reaction. In this case, because the reaction is only occurring between the gaseous molecules, no surface species and lattice oxygen involvement should be observed. All our experimental evidence strongly suggests that the oxidation of C3F6 on Ti02 is catalytic but also involves consumption of the Ti02 substrate, at least if surface hydroxyl groups are present. Although no exact activation energy data are available, the low temperature needed for oxidation over Ti02 (compared to that over the tungsten grid) suggests that the activation energy of the overall reaction of C3F6 oxidation is lowered significantly in the presence of TiOz. Figures 6 and 7 show that fluoroacetate was formed on the Ti02 surface during the oxidation of C3F6, and Figure 8 indicates that, as a reaction intermediate, the surface fluoroacetate was further oxidized to CO2 in the presence of oxygen. These observations, and the kinetic behavior that will be discussed in the next section, suggest that the oxidation of c3F6 on Ti02 is a surface process. The fact that no gas phase species containing the carbonyl functional group were formed in the TiOz-catalyzed reaction suggests that the catalytic oxidation reaction of C3F6 may follow a different reaction pathway from the homogeneous oxidation reactions studied by other^.^^^ The incorporation of the lattice oxygen of Ti02 into the CO COz reaction products indicates that lattice oxygen was actively participating in the reaction, since the reaction temperature is much too low for any isotopic exchange to take place between l 8 0 2 and Ti1602.z8 A mixture of oxygen- 16 and oxygen- 18-labeled fluoroacetates were also observed as intermediate surface species in the C3F6 oxidation reaction using l80z(g). This also clearly indicates that lattice oxygen from Ti02 as well as the oxygen from the gas phase participates in the oxidation reaction. Based upon our experimental findings, the overall reaction may be written as follows:
+
involving Ti02 as catalyst and reactant. Reaction 3 is thermodynamically favorable ( A W = --lo0 kcal/mol to our best
estimate) and approximates the carbon mass balance observed. We observed HF(g) production in the oxidation process, but eq 3 does not reflect this. We also have not proven that TiOFz(s) is a product. Our experiments have shown that the both reactants of 0 2 C3F6 and Ti02 f C3F6 are slow and produce species containing carbonyl functional group while the overall oxidation of C3F6 on Ti02 is fairly fast and complete in the presence of oxygen. In addition, Figure 8 clearly shows that gaseous oxygen oxidizes surface fluoroacetate into COz. Therefore, we believe that it is the catalytic function of Ti02 that drives the oxidation of C3F6 to completion at relative high reaction rate and mild conditions. Although Ti02 itself is also consumed, the reaction rate and the temperature dependence and the product distribution of the C3F6 oxidation reaction are very different from that of gas-solid stoichiometric reactions such as that between CaO and CC4.28b B. Reaction Mechanism. I. Involvement of Lattice Oxygen in the Reaction. The behavior of oxide catalysts in oxidation reactions is usually understood by redox mechanism^.^^ Specifically, a mechanism called the Mars-van Krevelen redox cycle is frequently used to explain the kinetic behavior of oxidation reactions over oxides such as v205.29930 According to this mechanism, both gaseous and lattice oxygen play active roles in hydrocarbon oxidation. The oxide catalyst is believed to be reduced first by hydrocarbon and then to be reoxidized later by gaseous oxygen. It was observed that the catalyst may continue to oxidize the reactant with the same selectivity even if the oxygen supply is dis~ontinued.~~ All of our experimental evidence here suggests that the oxidation of C3F6 on Ti02 fits well with the above redox mechanism. This should not be surprising given the fact that TiOz, like V205, is an n-type semiconductor oxide which usually operates by a similar mechanism in oxidation reaction^.^^ Based on the redox mechanism, the gas phase kinetics for the oxidation of C3F6 can be readily explained. In Figure 5 , C3F6 is oxidized to completion even under conditions where the gaseous oxygen is insufficient. This is possible because the oxidation of C3F6 on Ti02 is a surface reaction and Ti02 supplies the additional oxygen needed for the continuation of the oxidation reaction. It is not surprising that the 0 2 pressure does not seem to significantly affect the rate of the oxidation reaction as shown in Figure 5 . In their original paper, Mars and van Krevelen30 found that the reaction order of Oz(g) was always less than 1, and its value varied greatly in different reactions. These facts suggest that the conventional redox mechanism for the partial oxidation process applies to the early stage of the deep oxidation reaction of C3F6 on TiO2. 2. Mechanism for Production of the Fluoroacetate Surface intermediate. The spectroscopic observation of a surface fluoroacetate (CF3COO) intermediate in the C3F6 oxidation indicates that oxidation occurs first at the CFz=CF-moiety of CFz=CF-CF3. The CFz group of C3F6 and the CF3 group of the surface fluoroacetate intermediate would be oxidized in this case to either COz or CO by interaction with the Ti02 surface in the presence of oxygen from the gas phase. This could occur by a process involving an unstable intermediate FCOO(a) surface species. Surface formate species are known to decompose in two channels producing CO2 and C0.32,33The fact that the relative ratio of COz and CO products did not change significantly for a wide range of initial oxygen pressures suggests that surface carboxylate groups from fluoroacetate, CF3COO, and fluoroformate, FCOO, may be precursors to C02 and CO products. TiF4(g) formation is responsible for the consumption of titanium and may involve an HF intermediate produced from surface hydroxyl groups as well as F atoms from C-F
+
Oxidation of Hexafluoropropene on Ti02 bond scission on the surface. It is not possible in this study to determine whether both proposed reaction channels (FCOO and CF3COO) produce CO and COz in equal proportions. C. Effect of Water. It is observed in these experiments that water reduces the rate of the oxidation for C3F6. A similar effect has been reported by others for F " T i 0 2 in the oxidation of C3F,5.2 The onset temperature of the C3F6 oxidation reaction (-573 K, Figure 4) corresponds approximately to the temperature of removal of surface Ti-OH groups as seen in Figure 2. Thus, it would seem that surface Ti-0-Ti entities are deactivated by reaction with water to produce surface Ti-OH groups which do not participate in the oxidation of C3F6.
V. Summary of Results Transmission infrared spectroscopy has been employed as a tool to study the oxidation of C3F6, hexafluoropropene, over TiO2. The following conclusions were reached: 1. Ti02 is an effective catalyst for C3F6 oxidation and COz and CO are the major oxidation products forming above -573
K. 2. The oxidation of C3F6 over Ti02 involves lattice oxygen from the TiOz, replaced by oxygen gas in a Mars-van Krevelen cycle. 3. The fluoroacetate, CF3COO(a) species, is an observed intermediate in the reaction. The observation of this surface species suggests that the CF2-CF- moiety of CF2=CF--CF3 is the initial molecular site for oxidation. 4. It is possible that the CF2 moiety of C3F6 and the CF3 moiety of CF3COO(a) produces an intermediate FCOO(a) surface species, but this is a very reactive species and is not observed spectroscopically in the work. 5 . TiF4(g) species are produced in the oxidation process over Ti02. The involvement of HF(g) in a reaction with the Ti02 is postulated as one reaction channel liberating TiF4(g).
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