6780 J. Phys. Chem. A, Vol. 101, No. 36, 1997
Shebaro et al.
produce higher hydrocarbons in our experiments is consistent
2
5
with the pyrolysis of methane, which gives only a minuscule
yield of ethane and unsaturated products at short times.
A catalytic nozzle can produce large yields from association
reactions whenever precursor radicals and unsaturated bonds
are made available early in the expansion. Many other combi-
nations of metal surfaces and gaseous reactants invite study,
and pressure, temperature, and contact time within the nozzle
can be varied widely. Kindred studies of surface reactions with
flowing gas may also become feasible, by virtue of the recent
development of techniques capable of monitoring surface species
during reactions at high pressures and temperatures.26
Acknowledgment. We thank Benjamin Abbott, Theodore
Hong, Dr. Alkwin Slenczka, and Dr. Bretislav Friedrich for aid
with various aspects of the experiments. We have enjoyed
instructive discussions with Dr. Christopher Nagel (Molten
Metal Technology), with Dr. Anthony Dean (Exxon Corporate
Science), and with Professors Sylvia Ceyer (MIT), Cynthia
Friend (Harvard), Jack Lunsford (Texas A & M), and Gabor
Somerjai (Berkeley). Major support for this work was provided
by Molten Metal Technology; L.S. is grateful also to Boston
College for a Dissertation Fellowship, and D.H. to the Miller
Institute for Basic Science for a Visiting Professorship at
Berkeley.
Figure 10. Comparison of mass spectra of reactants and products for
(full curves) and for 1:1 CH /C mixture (dashed curves,
normalized to full for C2 peak) reacting in a Ni nozzle at 60 Torr and
045 °C.
C
2
H
6
4
2 6
H
1
ethane component, comparison with the Cn peak indicates that
methane contributes to the latter about as much as ethane does
for n ) 3, but considerably less for n ) 4 and practically not
at all for n ) 5. However, in the n ) 6 region (Figure 5), the
mixture displays a prominent mass peak at 79, and sizable peaks
at 80 and 81 showing substantial contributions from methane,
although the overall yield of benzene is less for the mixture
than for pure ethane.
References and Notes
(
(
(
(
1) Langmuir, I. J. Am. Chem. Soc. 1912, 34, 1310.
2) Lunsford, J. H. Langmuir 1989, 5, 12.
3) Fenn, J. B. Annu. ReV. Phys. Chem. 1996, 47, 1.
4) Leopold, K. R.; Fraser, G. T.; Novick, S. E.; Klemperer, W. Chem.
4
. Discussion
ReV. 1994, 94, 1807. See also: Faraday Discuss. Chem. Soc. 1994, 97.
(5) Castleman, A. W., Jr.; Wei, S. Annu. ReV. Phys. Chem. 1994, 45,
A few general aspects of the reaction mechanism can be
685.
(6) Larsen, R. A.; Neoh, S. K.; Herschbach, D. R. ReV. Sci. Instrum.
974, 45, 1511. Mariella, R. P., Jr.; Neoh, S. K.; Herschbach, D. R.;
Klemperer, W. J. Chem. Phys. 1977, 67, 2981.
7) Kim, S. K.; Lee, W.; Herschbach, D. J. Phys. Chem. 1996, 100,
7933.
inferred from our results, in the context of other work. The
catalytic metal surface clearly has an essential role, in supplying
precursor hydrocarbon radicals and hydrogen atoms. However,
at ∼800-1000 °C these would rapidly degrade to carbon and
hydrogen unless liberated into the gas phase. The desorption
is probably appreciably fostered by the strong flow of the
reactant gas as well as the surface temperature. Much or most
of the association reactions producing higher hydrocarbons may
occur during the supersonic expansion. The effectiveness of
the minimalist “button” version of the nozzle indeed suggests
it is sufficient for the catalytic process to occur only near the
exit aperture, thereby allowing the renascent products or reaction
intermediates to promptly undergo free jet expansion.
The key reactions forming the higher hydrocarbons involve
addition of radicals to unsaturated bonds. Recent model
calculations for association reactions in hydrocarbon pyrolysis
and flames have emphasized the role of chemically activated
association and isomerization in overcoming entropic inhibitions,
particularly for benzene formation.2 A supersonic expansion
can also promote association reactions, as in the formation of
fullerenes,24 since it greatly weakens entropic inhibition of
molecular combination by markedly lowering the translational
temperature for relative motion of the molecules.
1
(
(
8) Kohn, D. W.; Clauberg, H.; Chen, P. ReV. Sci Instrum. 1992, 63,
4
003.
(
9) See, for example: Weida, M. J.; Nesbitt, D. J. J. Chem. Phys. 1996,
1
05, 10310. Ohshima, Y.; Endo, Y. J. Mol. Spectrosc. 1989, 153, 627 and
references therein.
10) Kroto, H. W.; Heath, J. R.; O’Brien, S. C.; Curl, R. F.; Smalley,
R. E. Nature 1985, 318, 162 and references therein.
11) Shebaro, L.; Abbott, B.; Hong, T.; Slenczka, A.; Friedrich, B.;
Herschbach, D. Chem. Phys. Lett. 1997, 271, 73.
12) Benson, S. Thermochemical Kinetics, 2nd ed.; Wiley: New York,
(
(
(
1
976; pp 343-363.
(
(
13) Ceyer, S. T. Science 1990, 249, 133 and references therein.
14) Beebe, T. P., Jr.; Goodman, D. W.; Kay, B. D.; Yates, J. T. J.
Chem. Phys. 1987, 87, 2305. Sault, A. G.; Goodman, D. W. J. Chem. Phys.
988, 88, 7232.
15) Schmidt, L. D.; Huff, M. Catalytic Oxidation; Sheldon, R. A., Vam
1
(
Santen, R. A., Eds.; World Scientific: Singapore, 1995; pp 93-117.
(16) Stanton, J. F.; DePinto, J. T.; Seburg, R. A.; Hodges, J. A.;
McMahon, R. J. J. Am. Chem. Soc. 1997, 119, 429.
3
(17) McCarthy, M. C.; Travers, M. J.; Kovacs, A.; Chen, W.; Novick,
S. E.; Gottlieb, C. A.; Thaddeus, P. Science 1997, 275, 518 and references
therein.
(18) Kaiser, R. I.; Ochsenfeld, C.; Head-Gordan, M.; Lee, Y. T. Lee;
Suits, A. G. Science 1996, 274, 1508.
(19) Shebaro, L. Ph.D. Dissertation, Boston College, 1996.
The complexities of heterogeneous catalysis coupled with
supersonic flow inhibit discussion of specific chemical steps.
However, we note some aspects that seem akin to homogeneous
gas-phase pyrolysis of hydrocarbons (in the absence of a
catalyst). The resemblance we find between ethane and ethylene
in forming higher hydrocarbons is consistent with the classic
Rice-Herzfeld mechanism for pyrolysis of ethane, in which a
chain carried by methyl radicals and H atoms converts ethane
(
20) Morter, C. L.; Farhat, S. K.; Adamson, J. D.; Glass, G. P.; Curl, R.
F. J. Phys. Chem. 1994, 98, 7029.
(21) Cornu, A.; Massot, R. Compilation of Mass Spectral Data, 2nd
ed.; Heyden & Son: London, 1975; Vols. 1 and 2.
(22) Alkemade, U.; Homann, K. H. Z. Phys. Chem. (Munich) 1989, 161,
1
9.
(23) Westmoreland, P. R.; Dean, A. M.; Howard, J. B.; Longwell, J. P.
J. Phys. Chem. 1989, 93, 8171. Bozzelli, J. W.; Dean, A. M. J. Phys. Chem.
993, 97, 4427.
1
(
24) Goroff, N. S. Acc. Chem. Res. 1996, 29, 77.
25) Dean, A. M. J. Phys. Chem. 1990, 94, 1432.
1
2
to ethylene. This would also imply that ethylene should give
larger yields, as observed. The fact that pure methane fails to
(
(26) Somorjai, G. A. Z. Phys. Chem. 1996, 197, 1.