J. Phys. Chem. 1993,97, 1445-1450
1445
Magnitude and Origin of the Deuterium Kinetic Isotope Effect during Methane Coupling and Related Reactions over Li/MgO Catalysts N. W. Cant' and E. M. Kennedy School of Chemistry, Macquarie University, NS W 2109, Australia
P. F. Nelson CSIRO Division of Coal and Energy Technology, PO Box 136, North Ryde NSW 2113, Australia Received: September 1 1 , 1992
The deuterium kinetic isotope effect during methane coupling over a Li/MgO catalyst at 746 OC has been determined as a function of methane concentration (10-80%) at two different oxygen concentrations (5 and 10%). In contrast to one earlier report, the overall rate ratio, CH4 versus CD4, appears constant (1.59 i 0.1) over this range of conditions. The isotope effect to ethane is slightly greater than the mean value. Those to ethylene and carbon monoxide are considerably greater still due to a second kinetic isotope effect in their production. The isotope effect to carbon dioxide is correspondingly less. Similar experiments have been carried out for the oxidation of ethylene at 660 OC. The overall isotope effect, C2H4 versus C2D4, is 1.45 f 0.12, and as with methane it is greater for carbon monoxide production than for carbon dioxide production. Methane oxidation has been modeled by coupling a simple literature model for surface steps involving 0, to a model for the subsequent gas-phase reactions of surface generated methyl radicals. The calculations show that a parameter set suggested for the surface processes in earlier work does not provide a good fit to the observed kinetic orders in methane or oxygen. The set also underestimates the extent of 1 6 0 2 / ' 8 0 2 mixing during methane coupling. Alternative parameter sets which are more consistent with the kinetic and exchange data predict a small dependence of the expected kinetic isotopeeffect on pressure which falls within the bounds of the experimental measurements. It is concluded that over the range of conditions used here the rate of methane oxidation over Li/MgO catalysts is largely governed by the rate of bond breaking in methane. However that rate is sufficiently close to the rate at which surface oxidation sites are being created that the latter could be rate influencing under substantially different conditions as for example when using N20 as the oxidant.
Introduction There is considerable evidence to show that the oxidative coupling of methane to C2 hydrocarbons over oxide catalysts proceeds with generation of gas-phase methyl radicals. With Li/MgO catalysts there is some evidence to suggest that the surface species detaching the hydrogen is 0;in which case the following reaction scheme has been suggested:'
-
CH4 + 0,- CH, 20H-
+
+ OH-
H 2 0 + 02-+
'/202 02-+ 0
-
20,-
(1) (2) (3)
Both (2) and (3) would involve several steps and could be reversible. Evidence concerning which, if any, of the above reactions is rate limiting is conflicting. .Since0,formed on some surfaces can react with alkanes at temperatures as low as -135 OC,it was concluded' that reaction 1 could not be rate limiting. However our experimentalmeasurements2Jshow that CH4 reacts faster than CD4 over Li/MgO. At 750 O C the kinetic isotope effect (KIE) is approximately 1.5 which is reasonably close to the maximum expected for a process in which the rate limiting step is hydrogen detachment. Experimentscomparing CH4 oxidation in the presence of H20 versus D2O showed similar rates which suggests that reaction 2 is not the origin of the isotope effect. It was therefore inferred that reaction 1 was indeed rate limiting contrary to the original conclusions of Ito et a1.I A KIE of similar value has been obtained in competitive experiments with CH4/ To whom correspondence should be addressed.
CD4 m i x t ~ r e and s ~ ~for ~ the oxidation of ethane.6 Other authors have observed the effect over different catalysts, with KIE values of 1.3-1.8, and also reached the conclusion that alkane activation was rate limiting.s,7-8 The interpretation of the KIE has been reopened by a recent claim9that the magnitude of the effect depends on the methane/ oxygen ratio. It was suggested that perhaps the isotope effect was not surface generated but arose in subsequent gas-phase processes, the contribution of which decreased with methane/ oxygen ratio. However, the conclusion arrived at after additional modelinglo was that the gas-phase contribution was probably insufficient to account for the full KIE. The purpose of the present work was largely to remeasure the KIE over our Li/MgO catalyst as was done before but employing a range of methane/oxygen ratios. We have also modeled the gas-phasecontribution in relation to reactions 1 and 3. A recent rather more detailed paper by Shi et al." concerns the same questions so that one might hope that a totally convincing picture would result. As it happens the conditions used for the two sets of experiments were not identical. Perhaps for this reason the results do not agree exactly although a reasonable reconciliation is possible on most points. Experimental Section The experiments were carried out in a similar manner to that described earlier2*'using Li/MgO samples from the same batch of catalyst. In essence the reactant mixtures were passed down flow at a total pressure of 1 atm through an alumina reactor of 4-mm i.d. The catalyst was supported in the center of the heated zone on a thin layer of a-alumina chips resting on the top of a 3-mm 0.d. alumina thermowell. A similar upper thermowell terminated a millimeter or so above the top of the catalyst bed 0 1993 American Chemical Society
Cant et al.
1446 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 . I . -
.-0e 3
1.0
2.00
-
0.8
1.75
-
h
d8
e
1.50 -
1.25
1.00
0
/
0.0 0
20
40
60
80
9% Methane in Feed
Figure 1. Dependence of rate of methane oxidation to all products on input methane concentration for reaction over Li/MgO at 746 OC: (a, and closed symbols) CHI; (b, and open symbols) CD4. The circles are for 10% input oxygen concentration, the squares for 5% input oxygen concentration.
thus minimizing the gas space. The flow rate/catalyst weight combination was changed somewhat from that used earlier in order to minimize CD4 consumption and to maintain oxygen conversions below 20% at all methane pressures. This was achieved using a catalyst bed of 25 mg and a combined methane, oxygen, and helium flow rate of 25 cm3/min (ambient). Each flow was controlled by an electronic mass flow controller (Brooks Div, Emerson Electric). The exit flow was monitored by a quadrupolemass spectrometer (Vacuum Generators SX200)with multiple ion monitoring and periodically sampled into a HewlettPackard HP5890chromatograph with both a thermal conductivity detector (TCD) and flame ionization detector (FID). Every effort was made to minimize systematic error during rate measurements. Comparisons between CH4 and CD4 were made by a standard bracketing procedure. Reaction characteristics were first determined for a CH4/02/He mixture then switched to CD4/02/He of the same chemical composition and finally back to CH4/02/Heagain. The progress of each change was followed on the mass spectrometer, and gas chromatographic analyses were made when all relevant signals were constant. The CH4 and CDI flows were directed by separate three-way valves which allowed the substitution of one methane by the other to be made without disturbance of the total flow, and the measurement of one flow on vent while the other was in use. The flow measurement was made with a soap bubble flowmetersince signals from the mass flow controller were slightly isotope dependent. Analyses for hydrocarbons were largely done on the gas chromatograph using the FID detector (since TCD and mass spectrometer signals also required isotope correction). The compositionsof feed streams were determined by bypassing the reactor. This was important for CD4 experimentssince unlike the high-purity CHI used (Mathieson UHP grade), the CD4 (Merck, Sharp and Dohme, Canada) did contain some ethane (0.08 1%) and ethylene (0.027%). This necessitated corrections of 5-8% for C2D6 analyses and 10-15% for C2D4 analyses depending on the fraction of CD4 in the feed and its conversion (2-5%). Sinceoxygenconversionswerealsolow( I 1 5%), reaction rates could be calculated using the differential reactor approximation knowing the conversion (calculated from the product analyses), together with thecomposition and flow rate of the feed stream. Results
The relationships between reaction rate (to all products) and methane concentration for the two isotopic methanes are shown
0
e
U
:
20
0
40
60
80
% Methane In Feed
Figure 2. Ratio of overall rate to all products with CH4 to corresponding rate with CD4 as a function of methane concentration for reaction at 746 OC. The closed circles are for 10% input oxygen concentration, the open circles for 5% input oxygen concentration.
TABLE I: Co a d CD, over concn, % methane oxygen 20
10
75
10
49
10
10
10
20
5
rison of Selectivities for Oxidation of CH4 at 750 oc methane used CH4 CD4 CH4 CD4 CH4 CD4 CH4 CD4 CH4 0
4
ethane 51.4 51.4 60.0 63.0 57.4 60.1 45.8 48.0 59.0 58.5
selectivity,% ethylene CO 12.9 8.0 16.3 9.9 15.4 9.4 10.9 6.6 13.2 8.0
2.5 1.4 2.0 1.2 2.0 1.4 2.7 1.5 1.3 1.0
CO2 33.5 39.1 21.7 25.9 25.3 29.1 40.6 43.8 26.3 32.5
in Figure 1. The rates are consistently lower for CD4 than for CH4, but the form of the dependence on concentration is the same. The apparent kinetic orders (as assessed from the slope of log-log plots) are 0.70 f 0.03 for CH4 and 0.68 f 0.09 for CD4 (the error being 2 standard deviations in the least-squares plot). The apparent kinetic order in oxygen estimated from the experiments using 20% methane, but with 5 and 10% oxygen, respectively, is about 0.3 using both CH4and CD4. These orders are similar to those which can be calculated from the methane conversion data in Table I of the paper of Shi et a1.I' They also correspond reasonably well to the values reported by Amorebieta and Colussi12for reaction at low pressures (first order in methane and half order in oxygen). They do differ from the behavior (first order in both reactants for pressuresbelow 1OOTorr) reported in the kinetic study of Roos et al.13 However those results are for reaction in the presence of added carbon dioxide, a known poison, which may have competed with oxygen for surface sites leading to a higher order in that reactant. The solid line and circled points in Figure 2 show the CH4/ CD4rate ratio (Le., the apparent KIE)as a function of methane concentration with inclusion of the correction for C2Db and C2D4 in the feed CD4. The closely adjacent pairs of points correspond torepeat determinationswithout changeinconditions. Theseveral pairs of points with 20%methane are the results fromexperiments on different days. The uppermost pair rely partly on a mass spectral analysis and may not be quite as reliable as the other determinations. The overall accuracy is believed to be rather better than fO.l. The mean KIE is 1.59, and unlike the data reported in the comparison paper" the value does not decrease as the methane-oxygen ratio is increased, whether this is done by increasing the methane concentration (solid circles) or by reducing the oxygen concentration (open circles). As shown in Figure 3 the KIEfor ethaneproduction issomewhat greater than that for carbon dioxide production. The value (1.6 independent of pressure) is a little lower than that estimated by
KIEs during Methane Coupling
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1447 I
0.08
. 3 *
1
1 e
t
Y
E
Y 3
d
a"
u
0.020.0 0.00
I .o
2.0
3.0
30
Time-on-stream, hours 0
20
40
60
80
9% Methane in Feed
Figure 3. Methane dependence of ratio of rate with CH4 to rate with CD4 for formation of different products over Li/MgO at 746 OC: (a) ethane; (b) carbon dioxide; (c) ethylene (D) and carbon monoxide (0).
Shi et ala1'and Mims et ala5by measurement of the hydrogendeuterium distribution in the ethanes by oxidation of CH4/CD4 mixtures. The KIE apparent for ethylene production is considerably greater still. This is expected if it results from two successiveeffects, one in the conversion of methane to ethane, the second in the conversion of ethane to ethylene (largely homogeneously). The overall effect (2.6 0.3) is the product of the primary o w . Within the same error limits the KIE measured for carbon monoxide production is the same as that for ethylene. While this agrees with the general belief that carbon monoxide is derived from ethylene by a gas-phase process the KIE is very different to that calculable from Table 1 of ref 11. That data indicate a KIE of 0.5 or less which is unreasonable. The difference in KIE for C2 production as compared to C 0 2 production in Figure 3 is in the same direction but less pronounced than that reported recently by Lehmann and BaernsI4J5 for a Na/CaO catalyst. With that system the KIE was approximately 2 for C2 production but absent altogether for C02 formation. Two different explanations are possible for the lower KIE for carbon dioxideas compared to ethane. One is simply that carbon dioxideis generated directly from methane by a different surface step to that which yields the methyl radicals which give rise to ethane. No equivalencein isotope effect is required. Alternatively it may be imagined that both ethane and carbon dioxide are derived from methyl radicals but that a lower absolute concentration of CD3 versus CH, radicals, due to the rate difference in their formation, decreasesthe probability of pairing to give ethane relative to further oxidation to carbon dioxide. In that case the ratio of CZproduction to carbon dioxide production should be lower for CD4 than for CH4 and the apparent isotope effects should differ in the way observed. With Na/CaO catalysts the difference in KIE is too large for this explanation to be tenable and only the former can hold.I4J5 As a consequence of the different kinetic isotope effects the selectivitiesto the various products are isotopedependent as shown in Table I. The double isotope discrimination results in a selectivity to ethylene which is about 1.6 times higher with CHI than CD4. As a consequencethe selectivitytoethane and ethylene combined is a few percentage points higher when using CHI even though the ethane selectivity is if anything slightly less. The ethane selectivity is expected to depend on the ethylene/ethane ratio in the product. When this ratio is low the kinetic isotope effect will be manifested largely in the ethane yield which should then be lower for CHI than CD4. That is observed during measurements at lower temperature.2J Selectivities to carbon monoxide were low (1-3% with CHI and 1-295 with CD4) the difference reflecting the large KIE to this product. The results of Shi et al." for CO selectivities show comparable values for
*
Figure 4. Rates and selectivity to carbon monoxide as a function of time on stream during oxidation of ethylene over Li/MgO at 660 OC: (a) rate using CzH4; (b) rate using C2D4; (c) selectivity with C2H4; (d) selectivity with C2D4.
CHI but much higher ones ( 3 4 % ) with CD4. There is noobvious reason for this rather large difference. Overall, the kinetic isotope measurements reported here for steady state catalysis differ in small but significant ways from those described in the comparison paper." Possible causes for the differences are discussed in the Appendix. Rate comparisons between C2H4 and C2D4 were carried out at 660 "C using ethylene and oxygen concentrations of 20% and lo%, respectively. The selectivity to carbon dioxide was 6268%, the remainder being predominantly carbon monoxide with traces of methane and acetylene. Results for the overall rate as a function of time-on-stream using a bracketing procedure are shown in Figure 4. The apparent kinetic isotope effect is 1.45 f 0.12. The error is higher than present in the measurements with methane primarily due to some unexplained fluctuations in carbon dioxide production when using C2H4 in particular. The apparent isotope effect for carbon monoxide production (1 -62 f 0.09)seemed higher than for carbon dioxide production (1.40 f 0.1 1) suggesting that, as in the case for methane coupling, they do not arise by a successiveprocesses. The similarityof theoverall kinetic isotope effect to that for methane, and also for ethane? means that there is every reason to believe that the origin of the effect is similar for all three oxidations.
Modeling of Methane Oxidation The reaction has been modeled by coupling the model of Shi et al." which represents the surface steps as solely
ki
CH, + 0,- CH, + OH,-
(1)
to the gas-phase reaction set used by MackieI6 which includes 147 elementary reactions. As in the early studies,'OJ1 reactions 1,4, and -4 were treated as though they were homogeneous, and the model was solved using the CHEMKIN d e l 7for a residence time which gave a methane conversion similar to that measured when the methane concentration was 20%. In our calculations parameters have been sought which best fit both the methane pressure dependence data of theaccompanying paper" (to which the data of Figure 1 here can be scaled) and independent oxygen isotope exchange data for the same reaction temperature.'* Shi et al.," choosing parameters which best fit the KIE variations they see, arrived at set A in Table 11. This set can also reproduce the kinetic data of Roos et ai.', for reaction in the presence of a large excess of carbon dioxide.'O The relationship calculated here for this set (curve A on Figure 5 ) is similar to that reported
Cant et al.
1448 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 0.6
TABLE II: Panmeter Sets for Surface Reaction Model and Their Predictions as to Oxygen Isotope Mixing during Oxidation at 700 OC Me& rate constants
param set
10'6kP
klb
A
5.2 3.0 2.6
1.4 4.2 26
B C
predicted IO"&-a"
1.o
3.0 19
' 6 0 ' 8 0 E (vel%)
0.009 0.17 1.4
In cm3 s-1. In s-l. For reaction using an input mixture of 90% CHI, 5% I6O2,and 5% I8O2with a methane conversion of 3.8% which corresponds to the experimental value in ref 18.
. P C
'E
Em
r,
--E
0.4
I
0.2
Q,
EE
2-
0.6 0.0 Oxygen, %
Figure 6. Dependence of the rate of methane oxidation over Li/MgO onoxygenconcentrationfor reactionat 700OC withmethaneconcentration of 25%: 0, values calculated from data of Table I of ref 1 I ; 0 , data of Figure 1 adjusted to 700 OC (using a multiplication factor of 0.5). Lincs are calculated values for the combined surface/gas phase model with parameter sets A, B, and C as noted.
0.4
0.2
0.0
Figure 5. Dependence of the rate of methane oxidation over Li/MgO on methaneconcentrationfor reactionat 700O C with oxygenconcentration of 7.6%: 0, values calculated from data of Table I of reference 11; 0, data of Figure 1 adjusted to 700 O C (using a multiplication factor of 0.4). Lines are calculated values for the combined surface/gas phase model with parameter sets A, B, and C as noted.
previously10 with little dependence of rate on methane concentrations when they are above 20%. This is in contrast to the experimental data under the conditions of the KIE experiments which exhibit a strong dependence at all concentrations. To improve the fit, it is necessary to increase k4 and k-4 relative to k l . Curve B calculated from set B with kq and k-4 each increased by a factor of three and kl reduced by 40% fits the observed dependence well. Other data are available to decide which is the better parameter set. On the basis of reaction 4, one expects 1 6 0 ~ /isotope 180~ mixing, i.e.
'bo2+ '80, * 160'80 Our earlier work's shows that this exchange reaction reaches equilibrium over Li/MgO at 700 "C in the absence of methane. In the presence of reacting methane, the exchange rate is much reduced, due to inhibition by product carbon dioxide, but still exceeds the rate of incorporation of oxygen into oxidation products. Oxygen isotope mixing occurring in parallel with methane oxidation has been modeled using the simplified surface model alone assuming that exchange and methane activation use the same sites. While there can be no certainty that this is so, it seems quite likely since both reactions are inhibited to similar extents by added carbon dioxide.Is The final column of Table I1 gives the amount of 160180 predicted by the parameter sets A and B for reaction of a 90% CHI, 5% 'boz,5% I8Ozmixture at 700 OC. The comparison is made for a residence time which produced a match between the calculated methane conversion and the measured one ( -3.8%).18 Under these conditions the experimental 160180 content was 1.4% (17% of the unreacted oxygen). As can be seen the amount predicted by parameter set A is low by more than 2 orders of magnitude. That predicted by parameter set B is still low but by a factor of 8. To obtain a closer match to the experimentalvalue, it is necessary to increase
k4 and k-4, and decrease kl,from those of set B. Set C in Table I1 fits the oxygen exchange data well. As shown in Figure 5 it does not provide as good a fit to the dependence of rate on methane concentration as set B does but the fit is still better than provided by set A. The parameter set can also be tested for reasonableness by comparison against the experimental determinations of the dependenceof rate on oxygen concentration. As shown in Figure 6 none predict the observed data very well but set C gives the best fit. On the basis of the above it is believed that parameter sets B and C are much more consistent with available data than are the values in set A favored by Shi et al." Set C may be the preferred one for the following reason. With the reactor configuration used here, the amount of carbon dioxide produced is probably sufficient to causesome inhibition in the latter part of thecatalyst bed. This will have the effect of reducing the apparent orders in methane or oxygen. At very low pressures, where inhibition is unlikely, the experimental order in methane is unity and that in oxygen is This is close to the behavior predicted by set C. The predictions of all three sets as to the expected dependence of the KIE on reactant concentration have been calculated. This has been done using the combined surface/gas model setting kl(CD4) equal to kl(CH4)/1.6 and likewise for all gas-phase steps involving the abstraction of an H (or D). The results of these calculationsare presented in Figures 7 and 8. For parameter set A thepredicteddependenceofthe KIEonoxygenconcentration is similar to that calculated by Shi et a1.I' for the same set but using a somewhat different gas-phase model. Parameter sets B and C predict a lesser dependence,which is rather moreconsistent with our twodata points but perhaps less so with the experimental data of Shi et al." The situation for the calculated dependence of the KIE on methane concentration is more exaggerated. Parameter set A predicts a strong dependence quite inconsistent with our experimental data. Set B predicts much smaller effects but still outside the limits of the experimentalerror. Set C predicts little change in KIE with either methane or oxygen pressure, in conformity with the present results, and is thus preferred. The significanceof the results in respect to the possibleexistence of a unique rate determining step will now be considered. With reference to the simple surface model represented by eqs 1,4, and -4 twoextremescan be recognized. If reaction 4 isstrictlylimiting then all 0,-will be consumed by reaction with methane and no oxygen isotope exchange would be possible. Clearly this is not the case. On the other hand, if (1) is limiting then 1 6 0 2 / 1 8 0 2
The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 1449
KIEs during Methane Coupling
greater than that reacting according to reaction 1. It should be noted that although this conclusion is based on calculations using a simple model, it does not really require that the model i w l f is fully correct. The conclusion is an automatic consequenceof the isotope exchange results which were obtained under conditions very similar to those in the first row of Table 111. The fact that reaction (4is) always faster than reaction (1) implies that bond breaking in methane is fairly close to, but not fully, rate limiting under the conditions of the table. Under substantially different conditions (4) could become limiting. The results of Shi et al." indicate that such is the case when N2O is the oxidant. One might expect the same to be true if the reaction were carried out in excess carbon dioxide since the kinetic order in oxygen is then unity.' 3
1.o
0.0
30.0
20.0
10.0
Oxygen, %
Figure 7. Predicted dependence of the KIE on oxygen concentration for methane oxidation over Li/MgO at 700 'C with methane concentration of 25% using the combined surface/gas phase model with parameter sets A, B, and C as noted. 1
I
1.6
-
1.4
-
9i
1.2
-
I
1.0
3
8
4
P .Y
P
*
0
20
40
60
80
Conclusions The oxidation of methane over Li/MgO catalysts exhibits a primary deuterium kinetic isotope effect the value of which changes only slightly, if at all, for methane concentrations from 10 to 80% and with oxygen concentrations of 5 or 10%. The reaction can be modeled by coupling a very simple model for the surface steps with a standard homogeneous model for the subsequent reactions of surface-generated methyl radicals. A parameter set chosen to fit 1 6 0 2 / ' 8 0 2 mixing data predicts that the kinetic isotope effect should vary little with reactant concentrations over the above range. Under the conditions used here the reaction is largely limited by bond breaking in methane, but the rate at which reactive surface oxygen sites are created could become limiting under rather different conditions.
Acknowledgment. This work has been supported by a grant from the Australian Research Council and a grant under the CSIRO-Macquarie University Scheme. We are indebted to Professors Lunsford and Rosynek for an advance copy of their paper and for extended discussions.
Methane, Yo
Figure 8. Predicted dependence of the KIE on methane concentration for methaneoxidationover Li/MgOat 700 'C withoxygenconcentration of 7.6% using the combined surface/gas model with parameter sets A, B, and C as noted.
TABLE HI: Calculated Relative Rates of 0,-Consumption by Reactions -4 a d 1 during Methane Oxidation over U/MgO at 700 O C Using Parameter Set C concn, % CHI 0 2 76 7.6
25 25
7.6 7.6
25 2.5
Dredicted re1 ratesb predicted" of loss of 0,160'80(o~t)/z02(in) reaction -4/reaction 1 -2 0.18 22 0.23 0.17 0.16
-
-14 -4
QFor 1:l 1 6 0 2 / ' 8 0 2 in feed with CH4 conversions corresponding approximately to those shown in Table I of ref 11 with the same methane and oxygen pressures. Calculated as 4X1h01X#02/XCH,FCH4 where the Fs are input molar flow rates, XCH,is the fractional conversionof methane and XIIQIXO is the predicted value of 160'80(out)/z02(in). The factor of 4 arises because each '60'80 molecule represents removal of two 0,and '6OI8Orepresents only one-half the total 0; returned to the gas phase. The remainder is returned as 1 6 0 2 and 1 8 0 2 .
mixing should be complete. This is not the case either, but it is closer to the truth. Parameter set C, which can reproduce the exchange data, can be used in conjunction with the surface model for 16O2/'802mixing to calculate the fraction of 0,which is returned to the gas phase as oxygen molecules relative to that which reacts with methane. Results are given in Table I11 for the range of conditions covered by the experiments reported here, and those of ref 11. The quantity of 0,-predicted to be removed from the catalyst surface by step (4) is always significantly
Appendix In comparison with the KIE results of Shi et al." in the comparisonpaper thevalues found here under conditionsof steadystate catalysis are slightly higher (near 1.6 versus 1.45 maximum) and do not show a fall in value as the methane/oxygen ratio is increased. The experiments of Shi et a1.I' were carried out at a slightly lower temperature (700 "C). However that is unlikely to be the cause of the differences since we have obtained similar values to that found here for 750 OC in experiments at 680 OC.' The value found for the oxidation of ethane at that temperature is also close to 1.6.6 It is, however, possible that other experimental factors are a partial cause of the difference. A correction applied in this work for the presence of C2D6 and C2D4 in the CD4 used accounts for about one-half the difference in KIE and in addition a small decrease in KIE with increasing methane/oxygen ratio would have been apparent if it wasomitted. Thecomction makes no provision for reaction of the impurity C2 compounds since their calculated conversions are < 15%even allowing for the fact they react several times as fast as methane. According to Shi et al.11 the CD4 they used was sufficiently pure that corrections were unnecessary. Differences in carbon monoxide selectivities may also be a small factor. The present data shows similar values for CH4and CD4 whereas Shi et al.1' report 3 times higher values with CD4 than CH4. This contributes -0.04 to the difference in KIE values. Finally the type of gas chromatograph detector could be factor. As far as could be determined the FID detector used here required no correction for a sensitivity difference between deuterated and undeuterated compounds. Shi et a1.I' used a thermal conductivitydetector which has significantlyhigher sensitivity for deuterated compounds. They applied appropriate corrections when measuring methane, and presumably with the
1450 The Journal of Physical Chemistry, Vol. 97, No. 7, 1993 C2 compounds as well. If no correction were applied, the calculated KIEvalue would be low by -0.04. Independent work will be required to establish the extent to which the above factors contribute to the observed differences in KIE behavior.
References and Notes (1) Ito,T.; Wang, J.-X.; Lin, C.-H.; Lunsford, J. H . J . Am. Chem. SOC. 1985, 107, 5062. (2) Cant. N. W.; Lukev, C. A.; Nelson, P. F.;Tyler, R. J. J . Chem. Soc., Chcm. Commun. 1988, 766. (3) Nelson, P. F.; Lukey, C. A.; Cant, N. W. J . Carol. 1989, 120, 216. (4) Nelson, P. F.; Lukey, C. A.; Cant, N. W. J . Phys. Chem. 1988, 92, 6176. (5) Mims, C. A,; Hall, R. 8.; Rose, K. D.; Myers,G. R. Catal. Len. 1989, 2, 361. (6) Nelson,P. F.;Kennedy,E. M.;Cant,N. W.ProceedingsoftheNatura1 Gas Conversion Symposium, Oslo,1990. Stud. Surf. Sci. Carol. 1991, 61, 89.
Cant et al. (7) Otsuka, K.; Inaida, M.; Wada, Y.;Komatsu,T.; Morikawa, A. Chem. Letr. 1990, 7 , 423. (8) Burch, R.; Tsang, S. C.; Mirodatos, C.; Sanchez, J. Caral. Len. 1990, 7. 423. (9) Lunsford, J. H. Proceedings of the Natural Gas Symposium, Oslo, 1990. Stud. Surf. Sci. Catal. 1991, 61, I . (10) Shi, C.; Hatano, M.; Lunsford, J. H . Caral. Today 1992, 13, 191. ( 1 1 ) Shi, C.; Xu, M.;Rosynek, M. P.; Lunsford, J. H. J . Phys. Chem. 1993, 97, 216. (12) Amorebieta, V. T.; Colussi, A. J . Phvs. Chem. 1988. 92, 4576. (13) Rws, J. A.; Korf, S.J.; Veehof, R. H. J.; van Ommen, J. G.; Ross, J. R. H. Appl. Caral. 1989, 52, 131. (14) Lehmann, L.; Baerns, M. Catal. Today 1992, 13, 265. (15) Lehmann, L.; Baerns, M.J . Carol. 1992, 135, 467. (16) Mackie, J . C. Catal. Rev. Sci. Eng. 1991, 33, 169. (17) Kee, R. J.; Miller, J. A.; Jefferson, T. H. CHEMKIN: A general
purpose, problem-independent, transportable Fortran chemical kinetics code package, S A N D 80-8003, Sandia National Laboratories, New Mexico, 1980. (18) Cant, N . W.; Lukey, C. A.; Nelson, P. F.J . Catal. 1990,124, 336.
