J . Phys. Chem. 1990, 94, 7069-7074
it)
7069
The general approach presented here is not restricted to molecular scattering; it has been used already for electron-atom scattering processes.' Similar procedures should be able to be used in conjunction with other variational approaches to molecular scattering as well. The method is attractive because of its simplicity. Because it uses the same procedures to obtain the contraction coefficients as to do the final calculations, the complexity for the programming is kept to a minimum. Contracted basis functions should be a powerful tool for exploring more complicated collision problems in a variety of contexts.2'
the bound degrees of freedom with various or to obtain completely arbitrary (cross-channel) contraction coefficients. Even for contraction of only the translational basis the techniques employed so far can be improved in many ways. Further studies will be performed to t s t whether it might be advantageous to use a different contracted basis set for every channel. This might allow for smaller basis set sizes, but the number of matrix elements and the amount of storage will increase. The success achieved here in contracting only the translational basis and in constraining the contraction coefficients to be the same for all channels in a given vibrational manifold bodes well for more general schemes. The accuracy of the contracted basis function method, as for other basis set methods, can be systematically improved by increasing the basis sizes. Therefore, one can achieve a required accuracy without losing the advantage of the contraction. This was shown in the examples throughout the paper. In these examples the case of total angutar momentum, J = 0, was considered, and one would expect more savings for cases with J > 0.
Acknowledgment. This work was supported in part by the National Science Foundation, NASA Ames Research Center, and the Minnesota Supercomputer Institute. (21) While this paper was being reviewed, another preprint concerning contraction of the translational basis for algebraic variational calculations of reactive scattering appeared: Manolopoulos, D. E.; DMello, M.; Wyatt, R. E. J . Chem. Phys., in press.
Multiphoton Ionization Detection of Methyl Radicals from Catalytic Oxidation of Methane Erol E. Gulcicek; Steven D. Colson,*.t~~ and Lisa D. Pfefferlet Department of Chemistry and Department of Chemical Engineering, Yale University, New Haven, Connecticut 0651 I (Received: October 26, 1989; In Final Form: April 17, 1990)
A catalytic microreactor coupled with a multiphoton ionization time-of-flight mass spectrometer (MPI TOF) is used to directly observe stable and unstable reaction products from oxidative coupling of methane. Production of gas-phase methyl radicals from 1O:l mole ratio methane/oxygen mixture over CaO/AI2O3and 1% Sr/La203catalysts as a function of temperature is studied. At low methane conversion, ethane production can be accounted for by gas-phase coupling of methyl radicals produced by the catalytic reaction. Apparent energies of activation for methyl radical production of 41.6 and 30.0 kcal/mol for CaO/A1203and 1% Sr/La203catalysts, respectively, were determined by Arrhenius analysis of relative methyl radical production rates measured at low methane conversion.
Introduction Due to the abundant natural gas reserves, there has been an increase of interest in converting methane gas, the major component of natural gas, to more useful or reactive C2 (CzH6,C2H4, C2H2)hydrocarbons and in turn to higher hydrocarbon chemicals such as liquid fuels. To overcome the large thermodynamic barriers of direct methane conversion which necessitates extreme temperature conditions, oxidative coupling of methane over a wide variety of transition metal, alkali metal, alkaline earth metal, and rare earth oxide catalysts has been the predominant method for converting methane with high selectivity to C2hydrocarbons. In these studies methane and oxygen were either mixed prior to being introduced into the reactor'-" or they were fed ~ y c l i c a l l y . ~In ~-~~ almost all of the above-cited catalytic oxidation experiments, the observed reaction products are the final stable molecules detected by conventional GC, MS, or similar type of analytical instruments. Consequently, most of the complicated reaction mechanisms are predicted from the initial and final gas product distributions and plausible thermodynamic considerations of intermediate freeradical and stable molecule reactions. The importance of methyl radicals in the mechanism for the oxidative coupling of methane over oxide catalysts has been widely accepted by many groups from the extensive available literature. 'Department of Chemistry. *Department of Chemical Engineering. 3 Present address: Molecular Science Research Center, Pacific Northwest Laboratory, Richland, WA 99352.
0022-3654/90/2094-7069$02.50/0 , I
,
Therefore, the direct detection of these radicals during the partial oxidation of methane is important to better understand the reaction ( 1 ) Aika, K.-I.; Moriyama, T.; Takasaki, N.; Iwamatsu, E. J . Chem. Soc., Chem. Commun. 1986, 1210. (2) Ali Emesh, I. T.; Amenomiya, Y. J. Phys. Chem. 1986, 90, 4785. (3) Asami, K.; Hashimoto, S.; Shikada, T.; Fujimoto, K.; Tominaga, H.-0. Chem. Lett. 1986, 1233. (4) Bytyn, W.; Baerns, M. Appl. Caral. 1986, 28, 199. (5) Campbell, K. D.; Zhang, H.; Lunsford, J. H. J . Phys. Chem. 1988,92, 750. (6) (7) (8) (9)
D e b y , J. M.; Hicks, R. F. J . Chem. Soc., Chem. Commun. 1988,982. DeBoy, J. M.; Hicks, R. F. J . Caral. 1988, 113, 517. DeBoy, J. M.; Hicks, R. F. Ind. Eng. Chem. Res. 1988, 27, 1577. Imai, H.; Tagawa, T. J . Chem. Soc., Chem. Commun. 1986, 52. (IO) Ito, T.; Lunsford, J. H. Nature 1985, 314, 721. ( 1 1) Iwamatsu, E.; Moriyama, T.; Takasaki, N.; Aika, K.-I. J. Chem. Soc., Chem. Commun. 1987, 19. (12) Lin, C.-H.; Ito, T.; Wang, J.-X.; Lunsford, J. H. J . Am. Chem. SOC. 1987, 109,4808.
(13) Matsuura, I.; Utsumi, Y.;Nakai, M.; Doi. T. Chem. Lett. 1986, 1981. (14) Moriyama, T.; Takasaki, N.; Iwamatsu, E.; Aika, K.-I. Chem. Lerr. 1986, 1165. (15) Otsuka, K.; Jinno, K.; Morikawa, A. Chem. Lerr. 1985, 499. (16) Otsuka, K.; Jinno, K.; Morikawa, A. J. Caral. 1986, 100, 353. (17) Otsuka, K.; Jinno, K. Inorg. Chim. Acra 1986, 121, 237. (18) Otsuka, K.; Komatsu, T. Chem. Lerr. 1986, 1955. (19) Otsuka, K.; Komatsu, T. Chem. Lerr. 1987, 483. (20) Otsuka, K.; Liu, Q.; Hatano, M.; Morikawa, A. Chem. Leu. 1986, 467. (21) Otsuka, K.; Liu, Q.; Hatano, M.; Morikawa, A. Chem. Lerr. 1986, 903.
0 1990 American Chemical Society
7070 The Journal of Physical Chemistry, Vol. 94, No. 18. 1990
Gulcicek et al.
MAIN CHAMBER
t
QTZ TUBE(8XlOmm)
LASER AXIS
IONIZATION REGK)N
.
1'1
u\
IONOPTICS
TOP VIEW
,
I
TALYST
17
TIME OF FLIGHT CHAMBER
I MICROCHANNELDETECTOR PLATES
Figure 1. A schematic view of the MPI TOF mass spectrometer.
mechanisms that lead to the observed final product distribution. Various techniques such as modulated beam time-of-flight mass spectroscopy,28matrix isolation infrared (MII),29matrix isolation laser-induced fluorescence electron spin resonance ( MIESR),30*31 (LI F),32*33and resonance-enhanced multiphoton ionization ( MPI)34v35have been used to directly detect gas-phase free radicals. Driscoll et al. in their review article mention some of these In addition, electron energy loss spectechniques in trometers (EELS) have been used to detect CH3' free radicals on the ~ u r f a c e . ~ ' *Although ~~ the detection and spectroscopy of many free radicals have been studied in detail with the above techniques for various catalytic processes, little work directly related to the observation of free radicals from catalytic oxidation of methane appears in the literature. The extensive work of Lunsford and his co-workers using MIESR5JW3is the exception.
(22) Otsuka, K.; Liu, Q.; Morikawa, A. Inorg. Chim. Acta 1986,118, L23. (23) Otsuka, K.; Liu, Q.; Morikawa, A. J. Chem. SOC.,Chem. Commun. 1986, 586. (24) Yamagata, N.; Tanaka, K.; Sasaki, S.; Okazaki, S. Chem. Lett. 1987, 81. (25) Keller, G. E.; Bhasin, M. M. J. Catal. 1982, 73, 9. (26) Sofranko, J. A.; Leonard, J. J.; Jones, C. A. J. Catal. 1987, 103,302. (27) Jones, C. A.; Leonard, J. J.; Sofranko, J. A. J. Catal. 1987, 103,311. (28) Amorebieta, V. T.; Colussi, A. J. J. Phys. Chem. 1982, 86, 2760. (29) Tevault, D. E.; Lin, M. C.; Umstead, M. E.; Smardzewski, R. R. In?. J . Chem. Kinet. 1979, 11, 445. (30) Martir, W.; Lunsford, J. H. J. Am. Chem. SOC.1981, 103, 3728. (31) Driscoll, D. J.; Lunsford, J. H. J. Phys. Chem. 1983, 87, 301. (32) Talley, L. D.; Tevault, D. E.; Lin, M. C. Chem. Phys. Lett. 1979,66, 584. (33) Talley, L. D.; Lin, M. C. Chem. Phys. 1981, 61, 249. (34) Smyth, K. C.; Taylor, P. H. Chem. Phys. Lett. 1985, 122, 518. (35) Squire, D. W.; Dulcey, C. S.; Lin, M. C. Chem. Phys. Lett. 1985, 116, 525. (36) Driscoll, D. J.; Campbell, K. D.; Lunsford, J. H. A h . Catal. 1987, 35, 139. (37) Lee, M. B.; Yang, Q. Y.; Tang, S. L.; Ceyer, S. T. J. Chem. Phys. 1986,85, 1693. (38) Lee, M. B.; Yang, Q. Y.; Ceyer, S. T. J. Chem. Phys. 1987,87,2724. (39) Campbell, K. D.; Lunsford, J. H. J. Phys. Chem. 1988, 92, 5792. (40) Campbell, K. D.; Morales, E.; Lunsford, J. H. J. Am. Chem. SOC. 1987, 109, 7900. (41) Driscoll, D. J.; Martir, W.; Wang, J.-X.; Lunsford, J. H. J. Am. Chem. SOC.1985, 107, 58.
Cooclffi WATER
/
HEATER
RADIATION SHIELD
Figure 2. A schematic view of the catalytic flow reactor.
In the past, resonance-enhanced multiphoton ionization (REMPI) time-of-flight mass spectroscopy has been a useful tool to detect and understand the spectroscopic properties of many stable and unstable atomic or molecular species, and this technique continues to provide insight for monitoring kinetics today. In this study we use REMPI-TOF mass spectrometer to monitor CH3' radicals directly from the catalytic oxidation of methane and deduce possible intermediate reaction pathways over the CaO/ A1203and 1% Sr/La203 catalysts. Experimental Section As Figure 1 illustrates, the MPI TOF mass spectrometer developed for these studies consists of two differentially pumped vacuum chambers. In the main chamber the reaction products from the catalytic microreactor are expanded supersonically into the ionization region where they are crossed with a laser beam. The ions generated in the interaction volume are repelled through a pinhole into the second 115 cm long time-of-flight chamber where they are mass separated and steered by the ion optics and detected by a dual multichannel plate detector. The main chamber is pumped by a freon baffled 10-in. diffusion pump which has an effective pumping speed of 1800 L/s. The pressure in the main chamber is usually kept at (2-3) X lo-" Torr when the experiments were in progress. Likewise, the T O F chamber is pumped with a freon baffled 6-in. diffusion pump which maintained a constant pressure of 10-5-1 0-6 Torr during the experiments. A mixture of 74.5% Ar, 23.3% CH4, and 2.2% O2gas is passed through an 8 X 10 mm quartz tube under 0.5-1 .O atm of pressure. As illustrated in Figure 2, the end of the quartz tube is drawn into a conical shape to create a small reaction volume where the catalyst is placed. A 2 X 3 mm round quartz tube is placed inside the larger tube to act both as a plug to keep the catalyst in place and to contain the thermocouple for temperature measurements. The reaction region is resistively heated from room temperature up to 1000 "C. The heater wire is suspended in vacuo (connections are not shown) to avoid contact with the quartz tube so uniform heating by radiation and longer heater life times can be achieved. The entire heating assembly is placed inside of a water cooled 1.5 X 2.0 in. cylindrical copper tube which contains three alternate layers of concentric 0.001 in. thick tantalum foil to reflect the radiation and 0.030 in. thick ceramic paper to maintain the spacing between the metal foils and to ensure proper insulation. As shown in Figure 2, the effective length of the catalyst is 4 mm and the heater coil is effectively 6-7 mm long to provide uniform heating of the catalyst region. Traversing the thermocouple probe through the reaction volume indicated an even temperture distribution throughout the reaction region. Samples of 0.03-0.05 g of 20-40 mesh alumina-supported CaO and 1 wt % Sr/La203catalysts were placed inside of the quartz tube and the gas mixture flow rate was around 50-90 cm3/min through a 50-100-pm pin hole. After accounting for the catalyst volume, the residence time was cal(42) Lin, C.-H.; Campbell, K. D.; Wang, J.-x.; Lunsford, J. H. J. Phys. Chem. 1986, 90, 534. (43) Tong, Y.; Rosynek, M. P.; Lunsford, J. H. J. Phys. Chem. 1989,93, 2896.
MPI Detection of Methyl Radicals
53
1
I
3p 'A2 2;
4
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I
( a ) 605" C
-I
1% SriLa 2 %
I. __
u 2
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-
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on resonance
-
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,
off resonance
- I .1
9 w
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318.5
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WAVELENGTH, nm Figure 3. R E M P I spectrum of methyl radicals ( m / r = 15) from 1% S r / L a 2 0 3 catalyst a t 500 OC.
culated to be approximately 2 ms. The methyl radicals were ionized by using REMPI and detected by TOF-MS. The second harmonic output (532 nm) of a Nd: YAG laser (Quanta Ray DCR-2) was used to pump a dye laser that was doubled to obtain wavelength scans from 310 to 330 nm (Quanta Ray PDL-I and WEX-I) and was focused into the ionization region. During the wavelength scan, the simultaneous absorption of two photons occurred when a resonant electronic excited state of the neutral methyl radical was reached. Consequently, an additional photon of the same wavelength was absorbed to ionize the molecule (2 + 1 MPI process), and the enhanced density of the mass ion signal in the m / z = 15 channel was observed on the T O F mass spectrum.
Results and Discussion Figure 3 shows a REMPI spectrum of methyl radical ions ( m / z = 15) as a function of the second harmonic of the dye laser wavelength taken at 500 OC with 1% Sr/La203 catalyst. As previously observed by Hudgens et a1.,44the two peaks observed in this spectrum are due to the CH3' ground state to Rydberg state 3p2A2 X2A," electronic transitions at 317.9 nm ( u , , 1 0) and 319.1 nm ( u z , 2 0). From the width of the observed bands, we can see that the gas has been cooled from 500 OC to room temperature or lower by the supersonic expansion into vacuum. This is an important point, indicating that we do not expect the ion fragmentation pattern to be a function of the reactor temperature. The ionization is performed after cooling the gases to temperatures low enough that the variations in their internal (thermal) energy are not expected to result in temperature-dependent fragmentation pattern. The time-of-flight mass spectra taken at two different temperatures and two different fixed wavelengths are shown on Figure 4. At each temperature the neutral methyl radical production from the catalytic oxidation reaction was obtained by subtracting the CH3' ion signal taken at an off-resonance wavelength (in this case 3 18.6 nm) from the CH3' ion signal taken at an on-resonance wavelength (319.1 nm). The subtraction of the off-resonance signal is required to account for the CH3' cation contribution from the photofragmentation of larger ions present in the system either as combustion products or as background gas that desorbs from the chamber walls. The few mass peaks appearing in the middle of the spectra around m l z = 24 and higher are also background photofragmentation signals that are due to fast multiphoton dissociation processes observed even when there is no gas flow into the chamber. An interesting observation in the high-temperature mass spectrum (Figure 4b) is that, at a CH3' off-resonance wavelength,
-
1 I
I
I
I
I
I
I
I
I
21 5
TIME-of-FLIGHT, microseconds
Figure 4. MPI TOF mass spectrum of catalytic combustion products at two different wavelengths: on resonance, 3 19.1 nm; off resonance, 3 18.6 nm. 1% S r / L a , 0 3 catalyst, p = 0.5 atm. 80 RUN1
70
-
(44) Hudgens, J. W.; DiGiuseppe, T.G.; Lin, M. C. J. Chem. Phys. 1983,
I
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on resonance
.
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I 300
I
I 400
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I
I 600
,
I 700
,
I 800
,
I 900
, 1(
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Figure 5. Plots of methyl radicals detected as a function of temperature: for 1% S r / L a 2 0 3catalyst run 1 p = 500 Torr, and run 2 p = 380 Torr; for C a O / A I 2 0 , catalyst run 1 and run 2 p = 500 Torr. Run 1 results are for the fresh catalyst. Run 2 results are for catalysts aged under reaction conditions for 6-8 h. The solid lines are simply intended to guide the eye connecting data from each run.
three ions at 64,66, and 68 mass units appear to be on resonance. A likely explanation is that at mass 68 an isomer of C5H8(most likely pentadiene) is on resonance at this wavelength, and a 2 1 MPI process produces not only the parent ion but also the fragmentation product ions which are observed at mass channels 66 and 64. This fragmentation pattern is not unexpected since MPI studies of butadiene have shown similar fragmentation characteri~tics.~~ As for butadiene, the multiphoton absorption cross sections for pentadiene or pentyne isomers are also expected to be large, and MPI signals observed in the mass spectra are likely to correspond to small amounts of these molecules that are formed as secondary products of the methane activation process. There is also the possibility that mass 66 signal could contain cyclopentadiene or one of its unsaturated straight chain isomers that has been previously observed in low yields as partial-oxidation products of methane. As will be discussed later, the important outcome from this observation is that the formation of these and
+
(45) Woodward, A. M . Ph.D. Thesis, 1984, pp 91-97 and references
therein.
7072
The Journal of Physical Chemistry, Vol. 94, No. 18, 1990
Gulcicek et al.
other large unsaturated molecules increases dramatically only at temperatures that correspond to the decreasing methyl radical intensities in the gas phase. The results for heterogeneous production of methyl radicals as a function of temperature for 1% S r / L a z 0 3and CaO/AI,O, catalysts are plotted on Figure 5. Two types of runs were made for each catalyst. Run I represents data obtained for the fresh catalyst, while run 2 represents a catalyst that is used after exposure to reaction products containing CO, for greater than 5-8 h at temperatures up to 800 "C. Since we found 1 % S r / L a 2 0 3 catalyst to be much more active and stable than the CaO catalyst, most of the experimental analysis will be discussed in terms of this catalyst. However, relevant comparisons between the two catalysts will be made. These results show an important new behavior of methyl radical production rate that has not been observed previously, namely that a maximum is observed with respect to increasing temperatures. The observed maximum and a sharp decline of methyl radical production was seen for both of the catalysts. While the location of the maximum for the 1 % Sr/La203 catalyst was around 600-700 "C. the maximum for CaO/A1,03 catalyst occurred at much higher temperatures. Runs without a catalyst showed similar maximum in methyl radical production rate with temperature around 800 "C, but the signal intensities from these experiments were near the detection limit of our apparatus. I t should be emphasized again that, prior to ionization, methyl radicals were cooled to the ground state upon expansion from the reactor and, therefore, changes in the reaction temperature should not affect the observed methyl radical signal. When compared to experiments with similar reaction conditions such as residence times, initial feed ratio, and catalyst loading that measure the final stable reaction product^,^^^^ methane conversion is estimated to vary from 5 to 15% as temperature increases from 600 to 850 "C for the aged I% Sr/Laz03catalyst (run 2 experiments). Methane conversion and O2conversion were measured in our reactor system by using newly developed laser-driven pulsed electron impact (LDEI) ionization technique.47 Preliminary results show that, for the 1 % S r / L a 2 0 3 catalyst after aging in the reaction environment (conditions of r u n 2 in Figure 5), the steep falloff in methyl radical concentration occurs at oxygen conversions of 95-96% and that 99R+ oxygen conversion is reached at temperatures only 50 "C higher. For the fresh 1% Sr/Laz03catalyst (run I , in Figure 5), however, methyl radical concentration falls off before the reaction is limited by oxygen transport to the surface. Arrhenius plots are shown for both runs of each catalyst in Figures 6 and 7. The data for relative methyl radical intensities is plotted for temperatures only up to the point where maximum net production rate was observed since gas-phase methyl radical coupling is the predominant pathway for methyl radical consumption at low methane conversion. I n Figure 6, both of the runs for the 1%' Sr/La203catalyst fall on the same line, indicting negligible poisoning of the catalyst for methyl radical production by adsorption of reaction products like CO,. The long period of activity for this catalyst was also found in other experiment^.^^^^,^^ Comparison with Figure 5 shows, however, that, although the initial slope for increase in net production of methyl radicals is constant for both aged and fresh 1% S r / L a 2 0 3catalyt runs, the peak in net methyl radical production is much broader for the aged catalyst. Methyl radical production falls off before the oxygen is depleted significantly for thefresh catalyst runs. This implies that some poisoning process is responsible for the increase in methyl radical selectivity at high temperatures and moderate oxygen conversions for the aged catalyst. Lanthanum oxides are known to adsorb CO, strongly, forming carbonates that are stable at our operating temperaturess Sr carbonates are known to be stable at temperatures up to approximately 850 "C' which is, interestingly, the temperature at which methyl radical production
rate drops precipitously over the aged 1% Sr/La203 catalyst. Strontium carbonate has been shown to be active for oxidative coupling of methanel but not deep oxidation and therefore is possibly the cause of the broadening in the methyl radical peak observed for the aged 1 % Sr/La,03 catalyst. The CaO/AI2O3 catalyst exhibits both different poisoning characteristics and a significantly higher activation energy for methyl radical production. Although Figure 7 shows no change in the slopes or activation energy between the fresh and aged catalyst runs, the maxima for the two experiments were shifted to higher temperatures (Figure 5 ) . This is evidence of considerable poisoning of the CaO catalyst in the second run, reflecting the degree of lost active sites for methane adsorption on the surface which is directly proportional to the collision frequency A* for the methyl radical formation. This points out an important difference in mechanism between the two catalysts: for the CaO catalyst poisoning by CO, blocks sites for methyl radical production, whereas for the 1 % Sr/La203catalyst poisoning blocks deep oxidation broadening the methyl radical peak and improving C, selectivity. The method used to obtain the apparent activation energies for methyl radical production was used by Campbell and L u n ~ f o r d ~ ~ for Li/MgO catalyst. It is based upon the reactions
(46) Barr, M. Amoco Research and Development Center, private communication. (47) Boyle. J.: Gulcicek, E.; Pfefferle. L. D.; Colson. S. Submitted to Rec. Sci. Instrum.
( 4 8 ) Wang, J.-X.; Lunsford, J. H. J . Phys. Chem. 1986, 90, 3890. (49) Ito, T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H. J . Am. Chem. SOC. 1985, 107. (50) Wang, J.-X.; Lunsford, J . H. J . Phys. Chem. 1986, 90, 5883.
CH,
-
+ 0-, or 02-s
kl
2CH3'
+M
CH,'
kz +
+ OH;
C2H6
+ M*
or OzH-,
(1)
(2)
where M is the third body required to absorb the energy relaxed by the formation of ethane. Using the steady-state approximation for methyl radical production and assuming E, 0 gives the slope of the Arrhenius plot as d In [CH,]
-Ea* =-=R
-El 2R
diSi and the activtion energy for methyl radical production over alumina-supported CaO and 1% Sr/La203catalysts, El, is found to be 41.6 kcal/mol (average of the two runs) and 30.0 kcal/mol, respectively. The activation energy for methyl radical production observed for the 1% Sr/Laz03 is slightly higher than the activation = 36.8 kcal/mol) energy observed by Deboy and Hicks7 (Eethane for oxidative coupling of methane to ethane over the same catalyst. There has been a wide consensus in the literature that most of the ethane formed through heterogeneous oxidation of methane comes from the dimerization of methyl radicals in the gas phase via three-body collisions. The proposed mechanism for gas-phase methyl radical formation is abstraction of a hydrogen atom from methane by reaction with surface oxygen followed by thermal desorption. For example, using MIESR spectroscopy Campbell and L ~ n s f o r dshowed ~ ~ that, using Na-promoted CaO catalyst at 675 "C, the square r a t of the relative C2 yield (conversion times selectivity) scales with the rate of methyl radical formation in gas phase at 700 "C. Similar MIESR studies up to 600 "C by Lin et al.42have shown that La203 is a very good methyl radical generator, and recently, Tong et al.43showed that this catalyst is a poor methyl scavenger with a reaction efficiency of 10% at 470 "C. Our experimental studies agree with these lower temperature findings. Thermally generated 0- and 02-ions have been p r o p o ~ e d ~ ~ , ~ ~ to be the species responsible for H abstraction from methane on La,03 and Li/MgO catalysts. Moreover, Wang and Lunsfordso have shown that the concentration of [Li'O-] active sites increases as a function of increasing temperatures on Li/MgO catalyst up to 650 O C . Also, low-pressure Li-doped MgO experiments between 800 and 1100 K by Amorebieta and ColussiS1showed that, at
The Journal of Physical Chemistry, Vol. 94, No. 18. 1990 7073
MPI Detection of Methyl Radicals
0
" ~ " " , " " 1 " " , " " 1
1 1 " I 1
1 2
13
1 6
1 5
1 4
1000/(T, K)
Figure 6. Arrhenius plots for methyl radical production for the 1% Sr/La203catalyst runs.
"i
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,
,
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1
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Figure 7. Arrhenius plots for methyl radicals production for the CaO/ A1203 catalyst runs.
higher temperatures, the chemisorption of O2on the surface with respect to C H 4 adsorption increases by at least 3 order of magnitude when compared to lower temperatures. Sr-doped and undoped La203 oxidative methane coupling experiments by Barr46 and DeBoy and Hicks* under very similar conditions but at different CHI to O2 ratios (5:l and 6:1, respectively) show that at lower temperatures the oxygen conversion is relatively low (ca. 40%) and the C , (CO + C 0 2 ) selectivity is at its highest (around 80-90%). When the temperature is increased beyond our observed maximum in the methyl radical production rate for the aged catalyst, the O2conversion is ca. 100% and the C , selectivity drops gradually to its lowest levels (around 40%), increasing the C2 selectivity to 60% at ca. 20% methane conversion. This is consistent with our recent LDEI measurements of CH4, CO, C 0 2 , and 0 2 , and our vacuum-UV ionization measurement of higher hydrocarbons. High-temperature noncatalytic studies by Lane and Wolfs2 show close to 8% O2conversion when a 6:l methane to oxygen gas feed is used with a flow rate of 50 cm3/min which corresponds to more than 8 s of residence time. Apparently the catalyst plays an important role in the consumption of the O2 molecules and, at lower temperatures, this is reflected in the formation of C O and C 0 2 on the catalyst surface since the activation energy observed ( 5 1 ) Amorebieta, V. T.; Colussi, A. J. J . Phys. Chem. 1988, 92, 4576. (52) Lane, G . S.; Wolf, E. E. J . Card. 1988, 113, 144, 5062.
is much smaller than for the gas-phase formation. All the experiments with La203 catalysts suggest that, at higher temperatures, the methane activtion process becomes oxygen limited. Under oxygen-limited reaction conditions it is likely that hydrocarbons and gas-phase methyl radicals react to form larger saturated and unsaturated hydrocarbons on the surface. We have performed experiments at the same reaction conditions described above using vacuum-ultraviolet (vacuum-UV) ionization T O F mass spectroscopy to detect higher hydrocarbons. These results show increasing quantities of C2H4 and C3H6as well as C4and C5 unsaturated hydrocarbons a t increasing temperatures, corresponding to decreasing methyl concentrations and oxygen conversion of greater than 90%. Under our reaction conditions (tempratures from 400 to 800 O C , 2 ms reaction time, and 74% Ar dilution) surface reactions were calculated to be the major contributor to the higher olefin production rates, although gasphase production rates became approximately equal at our highest temperatures. As noted above, many of the higher hydrocarbon formation reactions also proceed in the gas phase given long enough residence time. At 825 OC using a Mn-Mg oxide catalyst, Labinger and OttS3have shown that, when gas-phase free-radical reactions for production of >C2 hydrocarbons are included in their model, close agreement with their experiments was found. However, no O2 conversion was reported and the C2 selectivity decreased with increasing conversion. From our experiments there is evidence that the gas-phase methyl radical concentration is very sensitive to both increasing reaction temperatures and O2conversion. And, since the selectivity to C2 products i n c r e a ~ e sat ~ ,these ~ ~ temperatures, better understanding of the reaction pathways both in the gas phase and on the surface are needed to understand the role of the catalyst in determining selectivity.
Conclusions In this paper we have presented heterogeneous methyl radical yield as a function of temperature for 1% Sr/La203 and C a O catalysts. Activation energies for methyl radical production were determined for both catalysts (30 kcal/mol for 1% Sr/La203and 42 kcal/mol for CaO). A maximum is observed in the methyl radical yield as a function of temperature data for both catalysts. The dramatic decrease at higher temperatures for the aged 1% Sr/La203catalyst corresponds to O2depletion in the gas phase. The 1% S r / L a 2 0 3catalyst is active at lower temperatures and shows a significantly higher yield of methyl radicals than the CaO catalyst. For the fresh 1% Sr/La203 catalyst methyl yield decreased before O2 in the gas phase is substantially consumed. Probable C02poisoning was observed which resulted in a broader maximum in net methyl radical production for the 1% Sr/La203 catalysts but resulted in a reduction in sites available for methyl radical production for the CaO catalyst. Future work on these systems will build upon our preliminary experiments using vacuum-UV laser light source (10.5eV) as an ionization source which has shown the capabilities of observing methyl radicals, ethylene, propylene, ethanol, acetaldehyde, and many other stable and labile species. Also, preliminary experiments have also been carried out using a newly developed47pulsed electron ionization source for detecting molecules with higher C2H6, CO, C 0 2 , and H 2 0 . ionization potentials such as CH4, 02,. In addition, as demonstrated by previous studies, MPI detection of other free radicals such as HC0,S4CH30,55CH20HM,57 from the catalytic oxidation of methane should be possible with our current expeirmental setup. By using all these experimental techniques, we expect to develop kinetic tools that account for the (53) Labinger, J. A.; Ott, K. C. J . Phys. Chem. 1987, 91, 2682. (54) Tjossem, P. J. H.; Goodwin, P.M.; Cool,T. A. J . Chem. Phys. 1986, 84, 5334. (55) Long, G . R.;Johnson, r. D.; Hudgens, J . W. J . Phys. Chem. 1986, 90, 4901. (56) Bomse, D. S . ; Dougal, S.;Woodin, R. L. J . Phys. Chem. 1986, 90, 2640. (57) Dulcey, C. S.; Hudgens, J. W. J . Chem. Phys. 1986, 84, 5262.
7074
J . Phys. Chem. 1990, 94, 7074-7090
heterogeneous free-radical reactions as a function of t e m p e r a t u r e and o t h e r e x p e r i m e n t a l parameters.
valuable discussions and for making some of his e x p e r i m e n t a l results a v a i l a b l e t o us.
Acknowledgment. We thank Amoco Research and Developfinancial s u p p o r t and Mark Barr for
Registry No. CH,, 74-82-8; C2H,, 74-84-0; CaO, 1305-78-8; Sr, 7440-24-6; La2O,, 1312-81-8; CH,, 2229-07-4.
m e n t c e n t e r for providing
Effect of Rotational Excitation on State-testate Differential Cross Sections: D HD -I- H
+ H,
-
Meishan Zhao, Donald G.Truhlar,* Department of Chemistry, Chemical Physics Program, and Supercomputer Institute. University of Minnesota, Minneapolis, Minnesota 55455
David W. Schwenke, N A S A Ames Research Center, Mail Stop 230-3, Moffett Field, California 94035
and Donald J. Kouri Department of Chemistry and Department of Physics, University of Houston, Houston, Texas 77204 (Received: December 5, 1989: In Final Form: March 7 , 1990)
-
+
The differential cross sections for D H2(v = 0 , j = 0 or 1) HD(u',j? + H, where u and j are vibrational and rotational q u a n t u m n u m b e r s (without primes for precollision values a n d with primes for postcollision values), are calculated a t five total energies in t h e range 0.82-1.35 eV by variational quantum dynamics with t h e most accurate available potential energy surface. Results a r e compared to previous calculations on a similar potential energy surface for t h e ground initial state, a n d t h e effect of rotational excitation on converged differential cross sections is illustrated for t h e first time. T h e effect of rotational excitation on t h e angular distribution is substantial, and it is much larger than the effect of rotational excitation on integral cross sections or than the difference between results obtained for the two most accurate available potential energy surfaces.
Introduction
The reaction of D with H2 has been the subject of several recent experiments'-6 that should provide a stringent t e s t of state-oft h e - a r t dynamical t h e o r y . Converged quantum dynamical calculations have been reported for the two most accurate p o t e n t i a l energy surfaces a t b o t h low9 and high"'-I5 energies, (1) Gotting, R.; Mayne, H. R.; Toennies, J. P. J . Chem. Phys. 1986,85, 6396. Gotting, R.;Herrero, V.; Toennies, J. P.; Vodegel, M. Chem. Phys. Lett. 1987, 137, 524. Buchenau, H.; Herrero, V. J.; Toennies, J. P.; Vodegel, M. MOLEC VII: Abstracts of Invited Talks and Contributed Papers, Assissi (Perugia), Italy, Sept 5-9 1988; pp 191-192. (2) Buntin, S. A.; Giese, C. F.; Gentry, W. R. J . Chem. Phys. 1987, 87, 1443. Buntin, S. A. Ph.D. Thesis, University of Minnesota, Minneapolis, 1987. (3) Phillips, D. L.; Levene, H. B.; Valentini, J. J. J . Chem. Phys. 1989, 90, 1600. (4) Continetti, R. E.; Balko, B. A.; Lee, Y . T. Paper presented at the International Sympsoium on Near-Future Chemistry in Nuclear Energy Field, Ibarako-Ken, Japan, Feb 15-16, 1989, and to be published in proceedings [Lawrence Berkeley Laboratory Technical Report, University of California: Berkeley: February 1989, LBL-267621. ( 5 ) Michael, M. V.; Fisher, J. R. J . Phys. Chem. 1990, 94, 3318. (6) (a) Kliner, D. A. V.; Zare, R. N. J . Chem. Phys., in press. (b) Kliner. D. A . V.; Rinnen, K.-D.; Zare, R. N. To be published. (7) Liu,B. J . Chem. Phys. 1984,80, 581. Siegbahn, P.; Liu, B. J . Chem. Phys. 1978, 68. 2457. Truhlar, D. G.; Horowitz. C. J. J . Chem. Phys. 1978. 68. 2466; errata: 1979, 71, 1514. (8) Varandas, A. J . C.; Brown, F. B.; Mead, C. A,; Truhlar, D. G.; Blais, N . C. J . Chem. Phys. 1987, 86, 6258. (9) Schatz, G. C. In The Theory of Chemical Reaction Dynamics, Clary, D. C., Ed.; Reidel: Dordrecht, 1986; p 1. Garrett, B. C.; Truhlar, D. G.: Schatz, G . C. J . Am. Chem. SOC.1986, 108, 2876.
0022-3654/90/2094-1014.$02.5O f 0
although the calculations are still limited in terms of numbers of total angular momenta or initial rotational angular momenta that have been s t u d i e d . Nevertheless, it already appears that theory and experiments show surprisingly large differences, and differences b e t w e e n theory16-22 and experiment have also been found (IO) Haug, K.; Schwenke, D. W.; Shima, Y . ;Truhlar, D. G.; Zhang, J. Z. H.; Kouri, D. J. J . Phys. Chem. 1986, 90,6757. ( I I ) Zhang, J. 2.H.; Kouri, D. J.; Haug, K.; Schwenke, D. W.; Shima, Y . ;Truhlar, D. G. J . Chem. Phys. 1988,88, 2492. (12) Schwenke, D. W.; Mladenovic, M.; Zhao, M.; Truhlar, D. G.; Sun, Y . ;Kouri, D. J. In Supercomputer Algorithms for Reactivity, Dynamics, and Kinetics of Small Molecules; Lagan& A., Ed.; Kluwer: Dordrecth, 1989; p 131. .. .
(13) Zhao, M.; Truhlar, D. G.; Kouri, D. J.; Sun, Y . ;Schwenke, D. W. Chem. Phys. Lett. 1989, 156, 281. (14) Blais, N. C.; Zhao, M.; Mladenovic, M.; Truhlar, D. G.; Schwenke, D. W.; Sun, Y . ;Kouri, D. J. J . Chem. Phys. 1989, 91, 1038. ( 1 5 ) Zhang, J. Z. H.; Miller, W. H. J . Chem. Phys. 1989, 91, 1528. (16) Schatz, G. C.; Kuppermann, A. J . Chem. Phys. 1976, 65, 4668. (17) Schatz, G. C. Chem. Phys. Lett. 1983, 94, 183. Colton, M. C.; Schatz, G. C. Chem. Phys. Lett. 1986, 124, 256. Schatz, G. C. Annu. Rev. Phys. Chem. 1988, 39, 317. (18) (a) Mladenovic, M.; Zhao, M.; Truhlar, D. G.; Schwenke. D. W.; Sun, Y . ; Kouri, D. J. Chem. Phys. Lett. 1988, 146, 358. (b) Zhao, M.; Mladenovic, M.; Truhlar, D. G.; Schwenke, D. W.; Sun, Y . ; Kouri, D. J.; Blais, N . C. J . Am. Chem. Sot. 1989, 1 1 1 , 852. (19) Mladenovic, M.; Zhao, M.; Truhlar, D. G . ;Schwenke, D. W.; Sun. Y . ;Kouri, D. J. J . Phys. Chem. 1988, 92, 7035. (20) Zhang, J. Z. H.; Miller, W. H. Chem. Phys. Lett. 1988, 153, 465. (21) (a) Manolopoulos, D. E.; Wyatt, R. E. Chem. Phys. Lett 1989, 159, 123. (b) Manolopoulos, D. E.; Wyatt, R. E. J . Chem. Phys. 1990. 92, 810.
0 1990 American C h e m i c a l S o c i e t y
