BERNARD J. WOODAND HENRYWISE
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The Reaction Kinetics of Gaseous Hydrogen Atoms with Graphite] by Bernard J. Wood and Henry Wise Solid-state Catalysis Laboratory, Stanford Research Institute, iMenlo Park, California
94025
(Received September 1R , 1 9 6 8 )
The kinetics of reaction between solid graphite and gaseous atomic hydrogen was studied in the temperature range 450-1200°K. The products of reaction are molecular hydrogen and methane. The rate exhibits an activation energy of 5.55 kcal/mol and is a function of the concentration of both hydrogen atoms and hydrogen molecules. Kear 800°K the rate goes through a maximum value, probably because of the thermodynamic instability of methane. A mechanism for the reaction is proposed.
Introduction At temperatures below 1500"K, the rate of attack of graphite by gaseous molecular hydrogen to produce methane and other hydrocarbons is so slow as to be virtually unmeasurable. In one quantitative investigation,2 methane formation rates in the range 10-lato 10-l2mol g-l sec-l were reported for the temperature span 600-1 100°K. In contrast, graphite exposed to gaseous atomic hydrogen, such as is produced in a low-pressure electrical discharge, reacts relatively rapidly even at room temperature. Several investigatorsa-' have studied this reaction at room temperature and below and found the principal initial product to be methane. The effect of temperature on the reaction was investigated by King and Wise* by measuring the rate of change in the thickness of evaporated graphite films exposed to a stream of partially dissociated hydrogen. These investigators found that the reaction kinetics exhibited a dependence on the apparent degree of crystallinity of the film and on gas-phase hydrogen atom concentration raised to the one-half power, They reported activation energies for the rate of removal of carbon in the range 7-9 kcal/mol and pointed out that hydrogen atom recombination occurred concurrently on the surface at a rate several orders of magnitude faster than the formation of methane, but with an activation energy of only 2 kcal/mol. This information led us to inquire about the kinetics of the hydrogen atom-graphite reaction at higher temperatures. In addition to the question of a reaction mechanism, we were interested specifically in evaluating the effect on the rate of thermal accommodation between the solid and the reacting gas and in pursuing further the role of atom recombination in the surface chemical reaction. A further dimension of interest is added by the fact that such processes may be significant contributors to the ablation of solids exposed to high-velocity gas streams, such as a graphite nozzle in a rocket engine employing hydrogen as a working fluid. Experimental Section A 7.5-em diameter, cylindrical, low-pressure flow reactor mounted in a conventional vacuum system was employed (Figure 1) . The Journal of Physical Chemistry
Atomic hydrogen was supplied to the reactor by flowing prepurified grade molecular hydrogen at a reduced pressure through a radiofrequency electrical discharge region. The effluent from the discharge entered the reactor through a constriction with a diameter of 1.5 em to prevent back-diffusion of products. The gas velocity in the reactor was in the range 147-164 cm/sec at total pressures of 1-2.8 Torr. At the reactor outlet the gas passed through a liquid nitrogen cooled trap into a vacuum pump. The total pressure in the reactor, measured by a vacuum gaugeg mounted in the reactor, was adjusted by varying the mass flow through the inlet valve. The partial pressure of atomic hydrogen could be varied, within limits, independently of the total pressure, by adjusting the radiofrequency power input to the discharge. The actual concentration of atomic hydrogen was determined by titrating the atoms with nitric oxide admitted to the reactor through a small glass multiport nozzle which could be positioned a t any selected point on the axis of the reactor by means of an 0-ring-sealed gland on the reactor cap. The luminescence intensity produced by the gas-phase reaction between H and NO is proportional to the product of the respective concentrations of the two reactants.*O In our experiments the proportionality constant was evaluated by calibrating the light intensity against the recombination heat detected by a tungsten filament probell under fixed conditions of total pressure and radiofrequency dis(1) This work was sponsored by Project Squid, which is supporced by the OWce of Naval Research, Department of the Navy, under Contract No. N00014-67-A-0226-0005, NR-098-038, (2) P. Breisacher and P. 0.Marx, J . Amer. Chem. Soc., 85, 3518 (1963). (3) L. E. Avramenko, Z h . Fiz. Khim.. 20, 1299 (1946), (4) G . M.Harris and A. W. Tickner. Nature, 160, 871 (1947). (5) J. D. Blackwood and F. K. McTaggart, Australian J. Chem., 12, 533 (1969). (6) P. S. Gill, R. E. Toomey, and H. C. Moser, Carbon, 5, 43 (1967). (7) F. J. Vastola, P. L. Walker, and J. P. Wightman, ( b i d . , 1, 1 1 (1963). (8) A. B. King and H. Wise, J . Phys. Chem., 67, 1163 (1963). (9) An Autovac gauge, supplied by Consolidated Vacuum Corp., was employed. (10) M. A. A. Clyne and B. A. Thrush, Trans. Faradau SOC.,57, 1305 (1961). (11) B. J. Wood and H. Wise, J. Phys. Chem., 65, 1976, (1961); B. J. Wood J. S. Mills, and H. Wise, ibid., 67, 1462 (1963);Errata, i b l d . , 68, 3911 (1984).
THEREACTION KINETICSOF GASEOUSHYDROGEN ATOMSWITH GRAPHITE
PYREX INLET
FIXTURE
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"0" RING FLANGED JOINT CI
Cr7nlrn
1349
APH,= I.Otorr, T=785'K
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Figure 1. Schematic diagram of apparatus.
charge power. The atom concentration estimated from the probe was corrected for the concentration gradient between the free stream and the probe surface, which is a consequence of the rapid recombination of atoms on the tungsten filament."J* The specimen, a rod of graphiteIa 5 cm long and 0.05 em in diameter, was situated perpendicular to the reactor axis about 7 cm downstream from the constrie tion by means of a ceramic fixture in which the graphite rod was held a t its extremities by small tungsten-wire clips. These clips also provided electrical contact with the specimen, so that the rod could be electrically heated and its resistance measured. The electrical resistance of the rod was a function of the crosssectional area and of the temperature. Hence, when the rod was maintained at constant temperature, the rate of diminution of its mass could be measured with great sensitivity by recording its change in electrical resistance. This relationship, of course, was strictly applicable only when the rod retained the shape of a uniform cylmder along its entire length. As a consequence of the temperature gradient between the center portion and the ends of the specimen, the rod eroded preferentially near the center. Hence, the resistance measurements of reaction rate were employed only for small changes in rod mass ( 1200°K (Figure 3). The absolute values of rod temperature reported in Figure 3 represent the radiant temperature evaluated in the center portion of the specimen. Temperatureprofile measurements on the graphite rods showed that a central length of 2.75 ern had a uniform temperature, while the ends of the rod were cooler than the central portion (from 100 to 500'K, depending on the absolute magnitude of temperature). To assess the effect of this temperature profile on the observed temperature variation of reaction rate, the activation energy for the process was computed semiempirically by use of a model which, to a reasonable approximation, accounted for the temperature nonuniformity of the rod. This (12) H. Wise and C. M. Ablow. J . Chem. Phyr.. 20. 634 (1958). (13) Extruded rod. grade 580. obtained Imm SPBBF Carbon 0 0 . .
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1350
BERNARDJ. WOODA I ~ DHENRYWISE
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hydrocarbons in the product stream. The rate of carbon removal during the blank run was estimated to be -~lO-~mol/min, a negligibly small rate relative to those observed with atomic hydrogen. To determine whether variations in the reactant gas temperature relative to that of the graphite surface had any effect on reaction rate, the reactor was surrounded with a furnace. When the wall temperature was raised, however, the atom concentration diminished drastically presumably because of enhanced recombination rate on the wall.15 Hence, no evaluation could be made. A similar experiment was carried out reacting oxygen with graphite. In this case no variation in reaction rate was observed when the wall temperature was increased from ambient to 680°K while the rod was maintained at 1260°K. Reaction rates comparable t o those reported for a similar oxygen-graphite systeml6 were obtained.
Discussion
104
The occurrence of a maximum value of reaction rate with increasing temperature has been demonstrated for a number of systems where gaseous reactants combine with a hot solid phase to produce volatile product^.^^-'^ The temperature at which the maximum reaction rate appears varies with the chemical nature of the reactants and products. It seems likely, therefore, that it is associat'ed with processes governing the formation or dissociation of chemical bonds, rather than with simple energy accommodation between the hot solid surface and the cold, incident gas atoms. In the case of the hydrogen atom-graphite system, the principal product, methane, is thermodynamically unstable at high temperatures. For T > 830"K, the standard free energy of formation of methane turns positive,20which means that hydrogen and carbon become the favored constituents in chemical equilibrium with methane. Thus one would expect to obtain mainly carbon and hydrogen rather than methane from a surface wit,h a temperature greater than 4300°K. Recombination of hydrogen atoms on the graphite to form hydrogen molecules occurs at a rate greater than the maximum carbon removal rate by a factor of about 109. This suggests that recombination occurs by an entirely different route and on more generally available sites than does methane formation. Heterogeneous hydrogen atom recombination on graphite is first order with respect to gaseous atoms and thus most
1
0.8
1.0
1,2
1,4 I/T
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1.6
1.8
2.0
2.2
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Figure 3. Dependence of rate of carbon removal on temperature of graphite and HI pressure.
model considered the rod to be comprised of three sections. The center region with a surface area S,, possessed a uniform temperature 21' . The two identical end sections, with surface areas S,, were assumed to vary linearly in temperature between 2'1 and To, with a mean temperature T*. The reaction rate contribution of each section of the rod was assumed to be characterized by the product of the surface area and an Arrhenius term containing a frequency factor A , the mean temperature of the section, and the activation energy E. Hence, the total reaction rate Rt for the
rod was Rb = 2SJ exp( -E/RT*)
+ S,A exp( -E/RT*)
This equation, containing two unknowns, Le. , A and E, was solved by finding that value of E which fitted best the observed ratio of total rates Rt/Rtf a t the experimental temperatures TI and T", and TI' and T*', respectively. A value of E = 5.6 X lo3cal/mol over the temperature range (TI) 455-715°K was computed in this way. Hydrocarbon product analysis revealed that methane comprised 91% of the hydrocarbon content of the effluent stream under conditions of near maximum reaction rate. The remaining 9% consisted of unidentified fragments in the range Ca to Cs. A blank run, with molecular hydrogen but otherwise identical conditions, showed but a trace of methane and no other The Journal of Physical Chemistry
(15) B. J. Wood and H. Wise. J. Phys. Chem., 66, 1049 (1962). (16) D. E. Rosner and H. D. Allendorf, Carbon, 3 , 153 (1965); A.I.A.A. J.,3, 1522 (1965). (17) D. E. Rosner and H. D. Allendorf, J. Chem. Phys., 40, 3441 (1964). (18) D. E. Rosner and H. D. Allendorf, J. P h y s . Chem., 69, 4290 (1965). (19) P. 0. Schissel and 0. C . Trulson, J. Chem. Phys., 43, 737 (1965). (20) JANAF Thermochemical Data, Dow Chemical C o . , Midland, Mich.
THEREACTION KINETICSOF GASEOUSHYDROGEN ATOMSWITH GRAPHITE likely occurs by way of an Eley-Rideal mechanism. Such a process requires simply that the graphite surface be substantially covered with sorbed hydrogen atoms. Under the conditions of temperature, pressure, and atom concentration employed in our experiments, one would expect such a condition to be fulfilled. It seems likely, therefore, that although the atom recombination reaction is very fast and definitely concurrent with the methane formation reaction, it does not really compete with the latter. We suggest that a major rate-limiting factor in methane formation is the fraction of edge sites relative to the total sites on the graphite surface. Indeed, it has been demonstratedz1for the oxidation of graphite that the reaction of carbon atoms occupying sites at the edges of the basal planes occurs 20 times faster than the reaction of the atoms in the face of the plane. The observed one-half power dependence of the reaction rate on atomic hydrogen corresponds to that reported previously* for a lower temperature range 365 < T < 600"K, except in experiments where the graphite phase was considered to be mainly amorphous. In experiments of this type, one must be certain that the observed kinetic effects are not the result of diffusion-limited mass transport processes in the gas phase. An analysis of the relative magnitudes of reaction and
mass-transport kinetics for a system with characteristics and geometry comparable to oursz2indicates that the observed carbon removal rates are well within the kinetic-controlled region. Based on the observed kinetics, and on the demonstrated reactivity of edge carbon atoms in graphite121 we suggest the scheme outlined in Figure 4 for the mechanism of methane formation from hydrogen atoms and graphite. I n the presence of gaseous atomic and molecular hydrogen under the conditions of our experiments, the surface carbon atoms would be expected to be nearly saturated with sorbed hydrogen atoms (I). The approach of a hydrogen atom to any site is most likely to result in recombination by the Eley-Rideal mechanism, which has been demonstrated to be a highly favored process. However, the interaction of gaseous hydrogen atoms with exposed edge carbon atoms holding two sorbed hydrogen atoms may result in a CHa complex (11) which weakens the bond between adjacent carbon atoms. This intermediate I1 may decompose by loss of a molecule of hydrogen and restoration of the original surface structure (111) (molecular hydrogen from this source is but a small fraction of that formed by recombination by the Eley-Rideal route). Alternatively it may be encountered by a hydrogen molecule which produces a methane structure by completely breaking the weakened C-C bond in the intermediate 11, yielding gaseous methane and replacing the CH3 group by a surface hydrogen atom (IV) Cn-CHa
I1
/
Figure 4. Scheme for formation of methane from hydrogen and graphite.
1351
+ Hz + Cn-H + CHd IV
Such an elementary step has been postulated in a kinetic studyz$ of the formation of methane from molecular hydrogen and carbon char. In our scheme, the intermediate I1 depends on hydrogen atoms directly for its formation but can decompose (I1.--) 111) by a second-order surface process unless it encounters a hydrogen molecule (I1+ IV) . If the intermediate I1 is assumed to attain a steady-state concentration during reaction, the scheme predicts that the rate of formation of methane would be proportional to the pressure of molecular hydrogen and to the square root of the concentrations of hydrogen atoms and the reactive surface sites, in agreement with experimental observations. (21) G.R. Henntng, Proceedings of the Fifth Conference on Carbon, Buffalo, N.Y . , 1961,The Macmillan Co., New York. N . Y.. 1962,p 143. (22) R. A. Hartunian and S. W. Lin, P h y s . Fluids, 6, 349 (1963). (23) C. W. Zielke and E. Gorin, I n d . Eng. Chern.. 47, 820 (1955).
Volume 79,Number 6 May 1969
