CATALYTIC EXCHANGE OF METHANE AND DEUTERIUM
481
Catalytic Exchange of Methane and Deuterium on Platinum, Ruthenium, and Platinum-Ruthenium Alloys
by D. W. McKee and F. J. Norton General Electric Research Laboratory, Schenectady, New York
(Received September 6 , 1069)
The exchange reaction between methane and deuterium has been studied in the temperature range 70 to 200' on unsupported platinum, ruthenium, and platinum-ruthenium alloy catalysts. Platinum-rich catalysts were found to be the most active and the order of initial appearance of the deuteriomethanes was CHSD > CH2D2> CHDI > CD4. Ruthenium-rich alloys gave CD4 as the most abundant initial product and the rates of formation of the different species were initially CD4 > CHID > CHD, > CH2D2. In all cases, the rates of the exchange reaction were approximately first order in methane pressure and negative order in deuterium pressure. I n addition, the concentration o,f chemisorbed hydrogen on the metal surface during the exchange reaction was followed by simultaneous measurements of the rate of formation of HD. The results indicate that the reaction takes place by different mechanisms with the two metals. With platinum, the dominant mechanism is stepwise exchange, whereas multiple exchange processes are more important in the case of ruthenium.
Introduction Considerable interest has recently arisen in the use of ruthenium as a hydrogenation catalyst, especially in reductions involving a carbonyl group, where it appears to show greater activity than other platinum group metals. * I n addition, it has been reported2 that ruthenium, when mixed with palladium or platinum, displays a synergistic effect in various liquid phase hydrogenation reactions. In spite of this unusual behavior, little attention has been given to simple vapor-phase hydrocarbon reactions on ruthenium catalysts, although in a recent study of alloy catalysts in isomerization reactions13it was observed that in the platinum-ruthenium series, a sharp maximum in benzene hydrogenation and n-pentane isomerization activity occurred a t compositions between 5 and 10% ruthenium. I n order to investigate the variation of catalytic activity in alloy systems, it is necessary to choose a reaction which is relatively simple mechanistically and is free from undesirable side reactions. I n addition, the products of reaction should not tend to poison the catalyst. The exchange reaction between methane and deuterium possesses several advantages for this
type of study. The reaction occurs readily on most metals in the convenient temperature range 25 t o 200°, the analysis of the products is rapidly and easily accomplished in a mass spectrometer, and the catalyst is readily regenerated after use. The mechanism of this reaction is relatively simple and has been discussed in detail in two papers by Kemba114~5;surprisingly, however, no attempt has been made to use this reaction in the study of alloy systems. This paper presents a preliminary study of the use of this technique in the investigation of the catalytic activity of platinum-ruthenium alloys. This system was chosen because of the lack of information on ruthenium catalysts in hydrocarbon reactions and also because these two metals form a continuous series of solid solutions in the range of 0 to 60% ruthenium.6 (1) G. Gilman and G. Cohn, Advan. Catalysis, 9 , 733 (1957). (2) A. Amano and G. Parravano, ibid., 9,716 (1957); P. N. Rylander and G. Cohn, Congr. Intern. Catalyse, Ze Paris, 1 , 977 (1960). (3) T.J. Gray, N . G. Masse, and H . G. Oswin, ibid., 2, 1697 (1960). (4) C. Kemball, Proc. Roy. SOC.(London), A207, 539 (1951). (5) C. Kemball, ibid., A217, 376 (1953). (6) W. A. Nemilow and A. A. Rudnizky, Izv. Akad. Nauk SSSR, 1 , 33 (1937).
Volume 68, Number 3 March, 1964
D. W. RICKEEA N D F. J. NORTON
482
In addition, homogeneous alloys of high surface area are conveniently made by reduction of mixed salt solutions a t low temperatures.
Experimental Preparation of Catalysts. The alloy catalysts were prepared by reduction of mixed salt solutions. Aqueous solutions containing appropriate concentrations of platinic chloride and ruthenium chloride were prepared from Fisher purified reagents. Reduction was carried out by adding a 5% solution of sodium borohydride dropwise to the stirred mixtures, the coagulated catalysts being washed thoroughly to remove traces of chlorides. The samples were dried in air a t looo, ground, and then reduced in flowing hydrogen a t 300’ for 3 hr. Pure platinum and ruthenium catalysts were prepared by an identical method starting from chloroplatinic acid and ruthenium chloride solutions, respectively. In some cases reduction was carried out using 85% hydrazine hydrate solution, but the reaction was more vigorous in this case and coagulation did not occur as easily as with sodium borohydride. However no differences in catalytic properties were observed between products prepared by the two methods. Surface areas of the reduced samples were measured in a Perkin-Elmer Shell “Sorptometer,” using flowing nitrogen a t -195’. The areas, which were accurate to within lo%, were calculated by the B.E.T. method and ranged from 3 to 12 m.2/g. X-Ray diffraction studies of the products showed the alloy preparations to be homogeneous and true solid solutions of ruthenium in platinum, up to about 50 wt. % Ru. Samples containing 58 and 78 wt. % Ru showed the presence of a small amount of free Ru and it is likely that these preparations were not completely homogeneous. Xemilow and RudnizkyG found that a continuous series of solid solutions was formed up to 68 wt. % Ru, this being the highest Ru content alloy investigated, whereas Ageev and Kuznetsov7 found the solid solution region to extend only to 51.9 wt. 7 0 Ru. In the light of this discrepancy, the phase diagram of this system remains somewhat uncertain. The particle size of the samples used in this work was in the range 300-500 A. and the Debye-Scherrer X-ray lines were quite diffuse. For this reason the lattice parameters of the alloys could be determined only to within *0.01 8. The composition of the alloys was determined by X-ray emission spectroscopy, using pressed wafers of the samples.* A calibration plot of counts/sec. RU Ka US. composition was obtained from standard mechanical mixtures of platinum and ruthenium blacks. The Journal of Physical Chemistry
The accuracy of this analysis was approximately f2% Ru . Materials. The methane used in this work was Phillips research grade, 99.57% pure, the main impurities being hydrogen and ethylene. The deuterium was obtained from General Dynamics Corp. and was 96% pure (3% HD, 1% ETz). Samples of CH3D, CH2D2, CHD3, and CD, used for calibration purposes were obtained from Merck, Sharp and Dohme (Canada), Ltd., and had a minimum purity of 98%. Procedure. The exchange experiments were carried out in a conventional vacuum system attached t o a General Electric mass spectrometer of the Xier sector type with 60” single focusing and 15.2-cm. radius of curvature. Catalyst samples weighing 1.0 g. were sealed into the adsorption cell, which had a total dead-space volume of 16.0 ml. Attachments to the cell included a mercury manometer and a gold foil trap to protect the catalyst from mercury vapor during the course of the reaction. The gas space was connected to a sampler section and molecular flow leak attached to the ionization chamber of the mass spectrometer. Slugs of gas, amounting to about 1% of the total present over the catalyst, were removed from the adsorption cell and analyzed a t regular intervals. An ionizing voltage of 44 e.v. and an accelerating voltage of 1150 v. were generally used, a complete scan of mass peaks 12 to 20 being made in approximately 2 min. Although the relatively high ionizing voltage used gave rise to appreciable amounts of mass peaks 12 to 14, due to fragmentation of the parent ions, this was not found to be a serious limitation as the product distributions were calculated directly from the fragmentation patterns for the individual deuteriomethanes determined under the same conditions. After correction for the trace amounts of heavier isotopic species present in the original methane, the concentrations of CD, and CHD, in the products were determined directly from the normalized yields of peaks 20 and 19. Concentrations of CH4, CIIsD, and CHZD2 were found by solving the appropriate equations obtained from the mass spectra of the pure deuteriomethanes normalized to unit pressure. This direct method of computation and calibration with pure deuteriomethanes is subject to less uncertainty than that involving purely statistical reasoning and assumptions concerning the relative ease of ionization of the deuterated me thane^.^ ~~~
~
(7) iS,W. Ageev and V. G. Kuznetsov, Zzv. A k a d . Nauk S S S R , 1, 753 (1937). (8) H. A. Liebhafsky, H. G. Pfeiffer, E. H. Winslow, and P. D. Zemany, “X-Ray Absorption and Emission in Analytical Chemistry,’’ John TViley and eons, Inc., New York, N. Y., 1960, p . 160.
CATALYTIC EXCHANGE OF METHANE AND DEUTERIUM
The catalyst samples were initially evacuated to 2 X mm., then treated with successive doses of deuterium until no hydrogen could be detected in the gas phase. After further evacuation, a measured pressure of deuterium was allowed to stand over the catalyst until the pressure reached a constant value; a measured pressure of methane was then added and the exchange reaction was followed as a function of time. Measurements of the rate of formation of each deuteriomethane were made on each catalyst as a function of time, temperature, and partial pressures of the reactants. In addition, measurements of mass peaks 2 , 3, and 4 allowed the rates of the simultaneous hydrogen-deuterium exchange reaction to be studied. In this case also the concentrations of Dz, HD, and HP in the gas products were computed with the aid of normalized mass spectra of the pure hydrogen isotopes, corrections being made for the small amounts of H D and H2 present in the original deuterium. Analyses were generally made a t 10-min. intervals, although the frequency of sampling depended somewhat on the rate of reaction. Measurements were usually continued for about 2 hr. or until the gas phase reached an approximately constant composition. Although in some cases of slow exchange, the composition of the gas phase was still changing after several days, the system usually reached equilibrium after about 24 hr. at the reaction temperature. As the initial stages of the exchange reaction were of most significance in this work, no attempt was made to establish ultimate equilibrium distributions between the deuterated methanes. It was found that the activity of the catalysts could be restored completely by treatment with deuterium a t the end of each measurement and there was no evidence of sintering of the catalysts during these experiments at relatively low temperatures.
Results and Discussion All the catalysts studied showed considerable activity in the exchange reaction a t temperatures above looo, the initial rate of formation of the deuteriomethanes on platinum-rich samples being generally CHID > CH2D2 > CHD, > CD,. Typical results for a Pt-8yo Ru alloy are shown in Fig. 1, in which the per cent concentrations of the deuteriomethanes are plotted as a function of time. The simultaneous exchange of the hydrogen isotopes is shown in Fig. 2 . Typical results for a Pt-58% Ru alloy are shown in Fig. 3. In this case the concentr.ations of the lower deuteriomethanes rose to maxima and then decreased as they were readsorbed and underwent further exchange on the metal. Pure ruthenium showed a much lower specific activity than all the catalysts containing platinum and it was
483
l q
I
I
gOL
I
1
I
I
Pt-87. RU 109.C 53 mn. Dt t 97mm. CH4
IS
TIME (MINS)
+
Figure 1. Exchapge of 53 mm. of DZ 97 mm. of CH4 on Pt-8c/, Ru a t 109”: 0 , CH4; A, CHBD; A, CHzDz; 0, CHD,; V, CDa.
necessary to carry out the measurements at temperatures above 150’ to obtain comparable rates to those found with platinum at 100’. In addition, the initial rate of formation of the deuteriomethanes on ruthenium were in the order CD, > CH,D > CHD, < CH2D2. Typical results on pure ruthenium are shown in Fig. 4. Following the treatment developed by Kemball,lo it was found possible to express the initial rates of the exchange reaction in terms of a function $J =
21
+ 2x2 + 3x3 + 4x4
where 2 1 is the percentage of the species CH,-,D, present a t time t. Assuming that all the hydrogen atoms in methane are equally liable to exchange, the course of the reaction should be given by the firstorder equation
or, in integrated form (9) G. C. Bond, “Catalysis by Metals,” Academic Press, Xew York, N. Y., 1962, p. 188. (10) C. Kemball, Adsan. Catalysis, 11, 228 (1959).
Volume 68, Number 3 March, 1964.
D. W. MCKEEA N D F. J. NORTON
484
I
I
I
I
1
(
PI-8% Ru 109.C 53 mm. D2 t 97mm.CH4
Pt-58% Ru 115.C 83 mm. D2 t 3 9 mn. CH,
1,
0
20
40
I
I
I
80 TIME IMINS.1
60
100
(
24 HRS.
+
+
0
IO
20
40 TIME IMINS.1
30
50
60
+
Figure 2. H Dz exchange during CHa DP exchange on Pt-8y0 Ru a t 109'; 53 mm. of I)P 97 mm. of CH4: 0 , 112; 0, HD; A, Hz.
38 mm. Figure 3. Exchange of 83 mm. of Dz of CH4 on Pt-58yO Ru at 115': 0, CH4; A, CHID; A, CHzD2; 0, CHD3; V, CD4.
+
10-25% ruthenium. This is illustrated in Fig. 6 where the values of log k a t 90" for a 80 mm. CH, 40 mm. D, mixture are plotted against alloy composition. In this case k is expressed in molecules of CH, disappearing/sec./m. of surface. A similar maximum in n-pentane isomerization activity was also observed recently3 for platinum-ruthenium preparations supported on alumina and the maximum appears to coincide with the absence of approximately one electron per atom in the d-band of the alloy. The effect of reactant partial pressure on the values of the rate constants k and k , and on the initial rates of formation of the individual deuteriomethanes is illustrated in Table I for a series of experiments a t 88" on a Pt-26y0 Ru alloy. At very low initial pressures of methane and deuterium, the kinetics of the reaction become unreliable, owing to rapid depletion of the reactants in the gas phase. By solving the appropriate simultaneous equations in thc form k 0: PcHI(IPD~~, the orders of reaction were found to be a = 0.8 and b = -1, with an accuracy of about k0.2 in each case. The actual values of a and b will depend on the relative strengths
+
where 40 and are the initial and equilibrium values of 4, respectively. The rate constant IC, represents the number of deuterium atoms entering 100 molecules of methane in unit time. A related first-order equation representing the disappearance of CHi is -log (z - zm) =
kt 2.303(100 -
-
log (100 - r m )
(2)
5),
where r and 2 , are the percentages of CH, present a t time t and infinity. The ratio of the two constants k,/k = M represents the average number of hydrogen atoms undergoing exchange in each molecule of methane. Both these rate equations were found to fit the data for the platinum-ruthenium alloy system quite well in the initial stages of reaction. Typical plots are illustrated in Fig. 5. The specific activity of the alloys showed a pronounced maximum a t a composition of approximately The Journal of Physical Chemistry
CATALYTIC EXCHANGE OF METHANE A N D DEUTERIUM
485
Table I : Exchange of Methane-Deuterium' on! Pt-26%' Ru Alloy, 89' P ~ C H ,m, m .
POD^, m m .
4 10 41 80 108
6 17 80 40 228
k, %/min.
kCHaD,
kCHzDz9
kCHDa,
kCD4.
Dv'CHI
%/rnin.
%/min.
%/min.
%/min.
%/min.
1.5 1.7 1.95 0.5 2.1
90 8.9 4.8 12.0 2.4
140 12.4 7.4 17.9 2.6
30 10 4.2 7.0 1.6
13 2.9 0.9 2.3 0.4
10 1 .O 0.3 1.1 0.2
kb,
13, 0.5 0.2 0.8, 0.15.
Table I1 : Distribution of Deuteriomethanes after 24-Hr. Contact with Catalysts Catalyst
Temp., OC.
POCH,
POD,
C€I4
CHsD
Pt-8% RU Pt-8Oj, RU Pt-8% RU Pt-87, R U 100% Ilu Pt-58% RU
84 80 96 99 154 90
107 83 59 65 40 78
200 45 120 139 78 41
1.3 7.5 4.1
15.1 16.0 7.2
... ...
...
I
7
I
I
I
CHDa
41.5 36.4 15.4 32.6 24.2 29.2
30.8 29.6 29.8 25.9 37.7 36.7
6.6 10.9
1.1
I
1
i
Final concentrations CHiDa
I
I
1
I
I
CD4
11.3 10.4 43.4 41.5 31.5 22.1
I
P I - 5 8 % Ru 90.E 80mm. D 2 t 36mm. CH,
2.4
..'
1.90
1.85
2.
m5t TIME IYINS.1
1
1.80
I
5
I
IO
I 15
I 20 TIME orllNS.1
I
25
I 30
Figure 5. Exchange of Pt-58% Ru a t 90' plotted according to eq. 1 and 2: 0, log (+ - @); 0 , log Cz - zm).
+
Figure 4. Exchange of 81 mm. of D2 39 mm. of CHd on lOO'j& Ru at 157': 0 , CHa; A , CHaD; A, C H ~ D I ;0, CHDI; V, CDI.
of adsorption of methane and deuterium and can be expected to vary with alloy composition. Although the inhibiting effect of a large D2/CH4ratio on the ex-
change kinetics was observed in other cases, detailed studies of the effect of initial reactant pressure on the kinetics were not made with every alloy system. In many cases increasing the initial ratio of D2/CH4 tended to increase the proportion of the more highly Volume 68, Number 3 March, 1964
486
D. W. MCKEEAND F. J. NORTON
17
rich samples and multiple exchange being most important with pure ruthenium, The rate of the stepwise process, R1, can be estimated by a direct measurement of the initial rate of CH3D formation, whereas the comparative rate of the multiple exchange process, Rz,can be found by plotting the sum of the percentages of CH2D2, CHD3, and CD,, against time, Exchange data for the different alloy systems over a wide range of temperature and reactant partial pressure are collected in Table 111. Lattice spacings of the samples are recorded in column 2, although owing to the broadening of the X-ray lines these are only accurate to =kO.Ol h. Column 3 gives the nitrogen surface area of each sample in m.2/g.; the reaction temperature and initial partial pressure of methane and deuterium are shown in columns 4-6. Column 7 gives the value of M discussed above and determined from the first-order plots (1) and (2). It is apparent that the value of M for all samples containing platinum lies in the range M = 1.0-1.5, indicating that stepwise exchange is predominant with these alloys. Although pure ruthenium gives values of M close to 3, the presence of as little as 20% platinum is sufficient to suppress completely the multiple exchange mechanism, in spite of the fact that a small amount of free ruthenium was apparently present in the 58 and 78% Ru samples. The relative rates of the two mechanisms are calculated in column 8, Table 111,from the relation
1
I2O
PI
20
40
60
BO
ALLOY COMPOSITION PERCENT
100
Ru
Figure 6. Variation of rate of exchange a t 90' (80 mm. of CHa 40 mm. of D2) log k with alloy composition.
+
deuterated methanes in the products as the probability of picking up a deuterium atom a t each residence of a methane molecule on the surface was thereby increased. Typical distributions of the deuteriomethanes after 24-hr. contact with the catalysts are shown in Table 11. The final composition of the gas phase shows wide variation with temperature and initial reactant pressure, but an excess of deuterium tends to favor the formation of the more highly deuterated species. As a result of his detailed studies of evaporated metal films, Kernballlo concluded that two basic mechanisms are involved in the exchange of methane with deuterium on metal surfaces. (1) Stepwise exchange reactions involve the replacement of one hydrogen atom of a methane molecule by one deuterium atom a t each residence on the metal surface. I n this case the initial product will be CH3D and formation of higher deuterated species will only take place by readsorption and further stepwise exchange. The value of M , as defined above, will approximate to unity for this type of process. (2) Multiple exchange reactions involve the replacement of more than one hydrogen a t a time by deuterium and are characterized by the initial appearance of species containing more than one deuterium atom per molecule and a value of d l greater than unity. The observed differences in the initial rates of formation of the various deuteriomethanes shown in Fig. 1-4 suggest that the relative importance of these two mechanisms varies along the Pt-Ru alloy series, the stepwise exchange process being predominant with PtThe Journal of Phvsical Chemistrv
3 = (d C E D ) o / (
+
+
d(ZCHzDz CHDa CD4) Rz dt >, The results indicate that multiple exchange becomes of increasing importance a t higher temperatures; this is to be expected if the further dissociation of chemisorbed methyl radicals to methylene or other multiply bonded radicals requires an activation energy. By utilizing the variations in rates a t different temperatures, it was possible to compare the average activation energies for the three basic processes, as shown in columns 9-11, Table 111. AEl is the activation energy for stepwise exchange, AEz that for multiple exchange, and AE3 that for the initial rate of disappearance of light methane. These values are uncertain to *2 kcal./mole but AEz is 6-8 kcal./mole greater than AEI in every case. Pt-rich alloys generally show substantially lower values of AE3 than pure ruthenium. Column 12, Table 111, shows values for the frequency factor log A3 associated with AE3 by the Arrhenius equation. These values are expressed in molecules of CH4/sec./m.2 of catalyst surface and may be compared with the value of 25.5 molecules/sec./100 cme2
CATALYTIC EXCHANGE OF METHANE A N D DEUTERIUM
487
Table 111 : Exchange of CHa-D2 on Pt-Ru Alloys Catalyst comp., wt. %
100 Pt Pt-870 RU
Lattice const.. ao,
A.
3.92 3.91
Surface area, m.*/g.
4.9 3.7
Pt-26% RU
3.90
5.6
Pt-2870 RU
3.89
6.0
Pt-7870 RU
100% R U
3.87 ( fR u )
12.1
2.70
8.5
Temp., O C .
83 139 73.5 80 97 109 130 89 75 81 98 90 90 115 29 70 79 117 154 157 157 203
POCH,. rnm.
72 74 49 83 59 97 60 80 79 79 83 78 36 39 81 80 83 77 40 75 39 36
P'Dn
rnm.
39 41 108 45 120 53 130 40 40 40 40 41 80 83 40 40 40 40 78 41 81 79
or 27.5 molecules/sec./m.2 obtained by Kernballs on evaporated films of platinum over the higher teniperature range of 159 t o 275'. The specific activity of the powdered catalysts used in this work was somewhat greater than that of Kernball's evaporated films. The values of AE3 and log A3 tend to increase together, indicating that these kinetic parameters exhibit a compensation effect. During the course of the exchange reaction, H D and later Hz are liberated into the gas phase. As these species can interfere with the exchange kinetics by dilution of the deuterium, only the initial rates of formation of the deuteriomethanes were used in the above computations. As the exchange of dissociated hydrogen atoms is known to be very rapid on most metals over the temperature range studied, l 1 it can be assumed that the rate of H D formation is determined by the rate of dissociation and exchange of methane on the metal. Typical data for the initial rate of formation of H D over a platinum-rich alloy and pure ruthenium are compared in Table IV. Column 5 gives the initial rate of disappearance of methane in %/min./g. of catalyst. The initial rate of formation of H D is shown in column 7 in %/min./g. The ratio of the initial rate of formation of H D and the rate of disappearance of Dz is close to unity in each case (column 8), indicating a firstorder reaction of the type
M
RdR2
1.3
6.1 1.9 15.0 4.6 3.2 3.1 1.9 1.7 1.6 5.5 3 .0 4.0 3.5 1.7
... 1.0 1.3 1.3 1.2 1.3 1.5 1.5 1.0 1.1 1.3 1.1 1.5 1.1 1.0 1.1 1.5 2.5 3.4 2.9 3.1
AEz, kcal./ mole
AEi, kcal./ mole
20
AEs,
kcal./ mole
log Aa molecules, sec./m.z
26
20.5
27.6
22
20
27. I
16
24
20.6
28.6
17
25
21
28.7
~ 1 6
24
21
28.2
27
35
27
31.0
23
30
27
29.1
-17
... 11.0 20 2.2 0.60 0.59 0.55 0.27
As all the hydrogen for the hydrogen-deuterium exchange is formed by dissociation of methane, then the ratio of the initial rate of H D formation to that of the disappearance of methane (column 9, Table IV) should be proportional to the mean number of hydrogen atoms displaced by deuterium in each molecule of methane i.e., M . This relation was found t o be followed approximately in most cases. Thus for metals on which one type of exchange mechanism predominates, a relative measure of the rate of exchange as a function of temperature or pressure can be made by comparing the initial rate of formation of H D in the gas. A similar technique whereby the total amount of hydrogen-deuterium exchange occurring after the adsorption of long-chain hydrocarbons on metallic catalysts is measured has recently been used to estimate the extent of dissociative adsorption of the hydrocarbon and the average composition of the residual surface radicals. l 2 (11) G. C. A. Schuit and L. L. van Reijen, Advan. Catalysis, 10, 286 (1958). (12) A. K. Galwey and C. Kernball, Trans. Faraday SOC.,55, 1959 (1959).
Volume 68, Number 9 M a r c h , 1964
D. W. MCKEEAND F. J. NORTON
488
+ Dz and C H I + Dz Exchange on Pt-Ru
Table I V : Simultaneous H
Temp.,
P°CH4,
PODS,
Catalyst
OC.
mm.
mm.
Pt-8% RU Pt-8% RU Pt-874 RU Pt-8Y0 Ru 100% RU lOO?o RU 100% RU
80 80 84 109 154 157 157
39 83 107 97 40 39 75
71 45 200 53 78 81 41
Although the two mechanisms of stepwise and multiple exchange can be clearly differentiated experimentally, it is not certain how the initial formation of multiply bonded radicals can arise as a result of methane chemisorption. Stepwise exchange certainly involves the formation of methyl radicals on the surface either by direct dissociation or by reaction of gas phase methane with deuterium atoms on the surface, the latter being more likely in the present case as the metals were initially covered with deuterium before admission of methane
Alloys
(3/ (3
K, %/min./g.
0.20 0.72 0.24 1.67 1.23 2.53 3.5
M
1.3 1.3 1.0 1.2 2.5 2.9 3.4
0.05 0.21 0.05 0.36 0.88 1,4 2.9
1.0 1.1 1.1 1.1 1.0 1 .o 1.2
0
0.25 0.29 0.21 0.22 0.71 0.56 0.83
M M
be avoided, however, if both processes occur simultaneously on different sites on the ruthenium surface, the formation of multiply-bonded radicals requiring a specific geometrical configuration of metal atoms, whereas dissociation t o methyl radicals and subsequent desorption as CH3D can occur a t more isolated metal sites. I t is interesting that the platinum and platinum-containing alloys used in this work were much less active in multiple exchange than the platinum films used by Kemball, and although the higher temperatures used in the latter work may account for this difference, it is also possible that the surface morphology of the metal has a considerable influence on the rates R1 and R2. The hexagonal close-packed structure of Ru does not appear to be solely responsible for multiple exchange as the metals Rh and Ir, which both have face-centered cubic lattices, also exhibit this type of behavior.16 There seems to be a rough correlation between the relative rates of the two mechanisms and the lattice parameters of the metals. Thus in Kemball's work, the metals with the highest ratios R1/R2 (at 200") were Pt, Pd, and W and these had substantially greater metallic radii (1.38, 1.37, 1.37 h., respectively) thao those metals, such as Xi and Rh (1.24, 1.34 A,)in which multiple exchange was predominant. In the present case, also, ruthenium has a considerably smaller atomic radius (1.32 A,)than does platinum and it seems feasible that the formation of highly dissociated species from methane is promoted by a small metal-metal atom spacing. In any case, it is also possible that the differences in the exchange behavior of platinum and ruthenium are related to differences in the relative strength of adsorption of hydrogen and methane on the t u o metals, a high affinity for hydrogen tending to dissociate further
However, in this case the rate of dissociation of methyl radicals would have to be very rapid to account for the observed rates of production of CD, and hence the appreciable amounts of CH3D which are also produced would be difficult to explain. This difficulty can
(13) A. K. Galwey, Proc. Roy. SOC.(London), A271, 218 (1963). (14) P. G. Wright, P. G . Ashrnore, and C. Kernball, Trans. Faraday Soc., 54, 1692 (1958). (15) J. R. Anderson, Reu. Pure A p p l . Chem., 7, 165 (1957). (16) D. W. McKee and F. J. Norton, to be published.
CH,(g)
+ D +CH, + HD(g) M
M
Recent results13 on the chemisorption of methane on nickel have shown that multiple dissociation occurs readily a t temperatures around 230'. I n addition, Wright, et al.,'4 have estimated that after 3 hr. the composition of the surface corresponded to CH2 at 150' and CH a t 200'. Multiple exchange, however, may take place as a result of spontaneous multiple dissociation of methane to give methylene, methine, or even carbon atoms CH4(g)
+ D D =+= I I
;\I ;\I
CH2
/I
AI
+ 2HD(g) 2-
CH etc.
/ I\
R4 M M
% 1
Alternatively, multiply bonded radicals may arise by progressive dissociation of methyl radicals formed Initially, as suggested by Anderson'j
D
I
M
+ CHZ +CH2 + HD(g) I
M
/I
The Journal of Phusical Chemistry
CATALYTIC EXCHANGE OF METHANE AND DEUTERIUM
the metal radicals formed initially. However, in the case of the exchange of ethane with deuterium over evaporated metal films, the importance of multiple exchange generally diminishes with increasing heat of adsorption of hydrogen on the metal. l7 Unfortunately, no information is available for methane and the only determinations of heats of adsorption of hydrogen on platinum and ruthenium under comparable conditions were made on silica-supported metals.I8 In this case, slightly higher initial isosteric heats were found for Pt (28 =t 1 kcal./mole) than for Ru (26 f 2 kcal./mole), but there is also evidence" that ruthenium is capable of sorbing appreciable amounts of hydrogen a t low temperatures. Calculations based on Pauling's atom electronegativities and the work functions of the metalslg give ruthenium the larger heat of adsorption of hydrogen (38.1 kcal./mole; 22.6 kcal./mole for Pt). In the light of this conflicting information it is not a t present possible to establish the reason for the observed differences in catalytic activity of these two metals. Reliable values for heats of adsorption of simple gases on metals in well defined and comparable states are urgently needed before the catalytic behavior of more complex systems can be completely understood.
Conclusions (1) The study of the kinetics of the methanedeuterium exchange reaction between 100 and 200' provides a convenient method of investigating variations in catalytic activity in alloy systems.
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(2) I n the platinum-ruthenium system, platinumrich alloys show greater activity than pure ruthenium, and the specific activity of the alloys reaches a maximum in the neighborhood of 10-25% ruthenium. ( 3 ) All platinum-containing alloys showed a predominance of the stepwise mechanism for the exchange reaction, whereas multiple exchange occurred more readily on pure ruthenium. It is likely that both processes occur simultaneously to some extent. The activation energy for multiple exchange is 6-8 kcal./ mole greater on these alloys than for stepwise exchange, leading to increasing contributions from the former mechanism a t elevated temperatures. (4) For a series of catalysts in which one type of mechanism predominates, an estimate of the rate of exchange can be obtained from the initial rate of formation of H D in the products.
Acknowledgments. This work was made possible by the support of the Advanced Research Projects Agency (Order Number 247-62) through the U. S. Army Engineer Research and Development Laboratories, Fort Belvoir, Virginia, under Contract No. DA-44-009ENG-4909. The authors also wish to thank Mr. I. B. Weinstock, who carried out the surface area determinations. (17) J. R. Anderson and C. Kemball, Proc. Roy. SOC.(London), A233, 361 (1954). (18) G. C. A. Schuit, N. H. de Boer, G. J. H. Dorgelo, and L. L. van Reijen in "Chemisorption," W. E. Garner, Ed., Butterworths, London, 1957, p. 39. (19) D. P. Stevenson, J . Chem. Phys., 23, 203 (1955).
Volume 68, Number S March, 1964
