ADSORPTION AND
OXIDATIOX 01;H Y D K O C B R B O S S ON S O B L E ;\IETAL
2397
ELECTRODES
The Adsorption and Oxidation of Hydrocarbons on Noble Metal Electrodes.
VIII.
Composition of Adsorbed CH, and Rate-Limiting Step
in t h e Overall CH, to CO, Reaction by A. H. Taylor and S. B. Brummer Tyco Laboratories, Inc., Waltham, ik?assachu%etts 0315.4 (Receised Januaru 3, 1060)
The oxidation of CHI on l’t electrodes at 130’ in 80% H,P04has been investigated with transient and steadystate electrochemical techniques. Transient currents observed during formation of the adsorbed layer when CH4reacts on a Pt electrode have been analyzed to obtain the Oxidation state of the layer. The integrated charge corresponding to this current transient’ has been compared with the charge to oxidize the adsorbed layer. The initial adsorption rate appears t o be limited b y CHd diffusion to the electrode; analysis of this regime at 0.30 V us. rhe yields a value of 2.95 i 0.15 electrons for the oxidation of the adsorbed species to COz. Integration of the total transient current to steady state yields 3.1 i 0.2 electrons for the oxidation of the adsorbed species to COS, without requiring the assumption that the adsorption rate is diffusion limited. This adsorbed species has the same oxidation state per site as other 0-type hydrocarbons; viz., ca.one electron per site is released on oxidation to COS. These data suggest a composition of 4C-OH for the adsorbed species. The rate of oxidation of this species has been compared with the over-all rate of CHI oxidation. At 0.30 V, the rate corresponds t o ca.three-eighths of the current for CHI oxidation. I n combination with t h e oxidation state measurements, this suggests that the reaction of CH, to COP proceeds via 0-type. The rate-limiting HzO -+ COz 3H+ 3e-. At higher potentials step is then the oxidation of 0-type to COz, viz. >C-OH (e.g., 0.40 V) there may be an alternative path for the reaction. Whether or not this is so, the slow step of the over-all process a t these higher potentials is the initial adsorption of CH,.
+
Introduction This paper is one of a series investigating the mechanism of the oxidation of saturated hydrocarbons on I’t electrodes at elevated temperatures in concentrated HB1’04. The work was initiated following reports that fuel cells could be operated with saturated hydrocarbon anodes at economically interesting rates.‘ It has been observed that although the coulombic efficiency (with respect to COz)is 100%,2-5the reaction rates are OW.^-^ This is despite stringent conditions of operation (85% H3P04 at 150°1s6). Detailed studies of the reaction mechanism seem appropriate, therefore, and we have explored the reactions of C3H8,7-9n-C6H14j1’ and COzl’ on Pt electrodes, mainly by examining the adsorbed species which accumulate. Three adsorbed residues have been distinguished on the electrode: CH-a, CH-p, and O - t y ~ e . The ~ last is the major species in terms of coverage, is similar for C3Hsand n-C6HI4,and is apparently identical” with reduced C02.’2 It is the easiest to oxidize at high POtentials.* l 3-16 The order of 0-type coverage with respect to gas pressure (approximately zero9.’O)is different from the order of the over-all reaction rate (approximately unity4s6). This discrepancy led us to the conclusion that formation of 0-type is highly undesirable. I n further work to test the consequences of this con~ l u s i o n ,we ’ ~ sought to examine the processes leading to 0-type formation, using CH, as adsorbate on smooth Pt I
9110
+
+
electrodes.’8 Data indicated that CH4 is not suited to a study of this type since we could find only the highly oxidized 0-type species on the electrode both during adsorption and in the steady state. I n no case could we find any CH-a material on the electrode sur(1) W. T. Grubh and L. W. Niedrach, J . Electrochem. Soc., 110,
1086 (1963). (2) W. T. Grubb and C. J. Michalski, ibid., 111, 1015 (1964). (3) H. Binder, A. Kahling, H. Krupp, K. Richter, and G. Sandstede, ibid., 112, 355 (1965). (4) E.Gileadi, G.Stoner, and J. O’M. Bockris, Unclassified Report No. AD 632-319, The University of Pennsylvania, Philadelphia, Pa., Apr 1966. (5) G. Stoner and J. O’M. Bockris, Unclassified Report No. AD 543-386, The University of Pennsylvania, Philadelphia, Pa., Oct 1966. (6) Report by General Electric Co. on Contract DA 44-009-ENG4909, Schenectady, N. Y., Dec 1963. (7) S. B. Brummer, J. I. Ford, and M. J. Turner, J . Phys. Chem., 69, 3424 (1965). (8) S. B. Brummer and M. J. Turner in “Hydrocarbon Fuel Cell Technology,” B. S. Baker, Ed., Academic Press, Inc., New York, N . Y., 1965, p 409. (9) S. B. Brummer and M. J. Turner, J . Phys. Chem., 71, 2825 (1967). (SO) S. B. Brummer and M. J. Turner, ibid., 71, 3494 (1967). (11) S. B. Brummer and M. J. Turner, ibid., 71, 3902 (1967). (12) J. Giner, Electrochem. Acta, 8 , 857 (1963). (13) S. Gilman, Trans. Faraday Soc., 61,2546 (1965). (14) L. W. Niedrach in “Hydrocarbon Fuel Cell Technology,” B. S. Baker, Ed., Academic Press, Inc., New York, N. Y., 1965, p 337. (15) L. W. Niedrach, J . Electrochem. Soc., 113, 645 (1966). (16) L. W. Niedrach and M. Tochner, ibid., 116, 17 (1967).
Volume ‘73,Number 7 July 1060
2398 face. The data indicated that C1species, once produced from higher hydrocarbons, would rapidly convert into O-type. Such an observation was not in conflict with our previous conclusion that formation of O-type is unde~irable.98~0 I n this paper we have sought to take advantage of our previous observations on the adsorption of CH4 to assess the role of O-type in hydrocarbon electrooxidation. I n particular, an important test of the role of 0type was made by comparing its oxidation rate with the rate of CH4 oxidation. The result is in disagreement with our previous conclusion that O-type formation is highly u n d e ~ i r a b l e . ~ It ~ ’ ~will appear that at low potentials the reaction proceeds via O-type and indeed is limited by its oxidation to COz.
A. H. TAYLOR A S D S. B. BRUMMER fore,’* anodic and cathodic galvanostatic pulses were employed to estimate oxidizable adsorbed CH4 and coverage of the electrode with adsorbed material, respectively. Potentials are reported against the reversible hydrogen electrode (rhe) in the test solution. Results and Discussion Oxidation State of O-Type CH4. The primary purpose of the present investigation was to compare the rates of O-type and CH4 oxidation. This would allow assessment of the role of O-type in the over-all reaction. An essential preliminary for the meaningful comparison of such rates (currents) is to know the oxidation state of O-type relative to CH4. I n addition, such information allows us to specify more closely the probable composition of O-type. As we have indicated, our previous resultsl8 have shown that when CH4 adsorbs it rapidly oxidizes to 0type on the electrode. This oxidation step is so fast that no CH-a or other material can be isolated on the electrode. This is true both during the early stages of adsorption and in the steady state. Electrochemical analysis reveals the presence of a single highly oxidized species releasing 1.1 k 0.1 electrons/site on oxidation to CO2. Having determined the number of electrons per site [e] to oxidize this species, we were interested in the present work in evaluating the number of electrons per carbon released when O-type is oxidized to COZ. This can be done by comparing the charge passed in forning O-type with the charge to oxidize the adsorbed layer. Correspondingly, the current transients during formation of the layer were explored. Results are shown in Figures 1-4. These measurements were made on a large (-20 r em2) smooth Pt electrode. The figures show currents during CH, adsorption in quiescent solution at POtentials from 0.25 to 0.40 V. These are corrected for background (N2),also shown on each figure. Measurements were made using a differential electrometer in conjunction with a two-channel recorder (lloseley Type 7100B with 17510A plug-ins), thus avoiding ground
Experimental Section The majority of the experimental procedures used are similar to those reported earlier.9-11,18 Some minor modifications have been incorporated here. Experiments were carried out as before in hot (130”), concentrated (80%) H3P04 solutions. These were prepurified by several recrystallizations, followed by stringent anodic and particularly adsorption electrolysis with a fuel cell e1ectr0de.l~ This followed observations that large cathodic background (Nz) currents were obtained at 0.25 V. Previously, a current maximum at potentials in the double-layer region on the potentiostatic curve of a platinum electrode in Hap04 had been attributed to ionization of strongly bound hydrogen‘g or to the occurrence of “spontaneous polymerization“ of phosphoric acid.20 Thacker21 demonstrated that the maximum in the “double-layer” region was due to oxidizable impurities, possibly organic, present in the concluded that the imphosphoric acid. Petrii, et purities were of an inorganic nature and might indeed be in part leached out of the glass cell at higher temperatures. They employed a cathodic treatment on a mercury electrode followed by adsorption on a platjnized platinum electrode. This treatment greatly improved the behavior but did not completely remove the impurities. Treatment of the HSP04 solution with H2Oz was not effective in their experiments. Here we have (17) This conclusion probably must be modified since we have found that a t appropriate potentials for comparison the oxidation of CsH8 found that the addition of HzOzleads to large and erratic has a zero or even negative order in gas pressure, See part V I of this background currents; hence, none was added. series. 8. B. Brummer, Preprints, Division of Fuel Chemistry, 154th National Meeting of the American Chemistry Society, Chicago. 111. For current transient measurements and charge Sept 1967, Vol. 11, No, 3, p 178. This work will be reported in more accumulation data we used a smooth Pt plate (99.98’%, detail in a forthcoming publication. Engelhard, roughness about 2 ) which was preannealed (18) A. H. Taylor and S. B. Brummer, J. Phys. Chem., 7 2 , 2866 (1968). in an oxidizing flame. I n some experiments a platinized (19) R. V. Vaucher and 0. Bloch, Compt. Rend., 2 5 8 , 6159 (1964). electrode was used. This mas prepared as beforel8 and (20) R. Jasinski, J. Huff, S. Tomter, and L. Swette, 2. EZektrochem., had a roughness factor of approximately 40 (surface 68,400 (1964). area N 80 r cmz). Measurements are related to the (21) R. Thacker, Electrochem. Technol., 3 , 312 (1965). measured “geometric area” (geom emz) and to the (22) 0. A. Petrii, R. V. Marvet, and Zh. N. Malysheva, Eleklrokhimiya, 3 , 861 (1967). real area (r cm2)-the latter being measured by cathodic Brummer, J. Phys. Chem., 69,562 (1965). hydrogen atom deposition on the clean e l e ~ t r o d e . ~ ~(23) ~ J S. ~ B. ~~ ~ A. N. Frumkin, “Advances in Electrochemistry and Electro(24) The area of the platinized electrode was measured by chemical Engineering,” Vol. 3, P. J. Delahay, Ed., John Wiley low current density oxide reduction.ls Again, as beSons, Inc., New York, N. Y., 1963. C%
The Journal of Physical Chemistry
ADSORPTION A N D OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES
1
I
01
IO
100 TIME Isec I
I
100 0
\O
_
e
IO
I 100
Figure 2. Current transient during methane adsorption on a smooth platinum electrode in quiescent solution (corrected for background).
I
=
04C"
103
100 0
Figure 4. Current transient during methane adsorption on a smooth platinum electrode i n quiescent solution (corrected for background).
I
I
,000
TlMEiiec I
1
040"
TlMElsecl
\
I
01
I0
01
' 0 ' 0
i
I
Figure 1. Current transient during methane adsorption on a smooth platinum electrode in quiescent solution (corrected for background).
I
2399
I
problems. J?%h an initial chart speed of 2 in./sec, accurate and reproducible data were obtained for the currents to within 1 sec of the start of adsorption. Prior to measurements, the electrode was cleaned at 1.35 V for several minutes, followed by oxide reduction at 0.075 V for 100 msec. Neasurements commenced on switching from 0.075 V to the adsorption potential. All these latter steps were carried out in quiescent solution. I n all cases background currents were cathodic. These varied from -2.50 to -0.15 pA/(r cmz) at 0.300.40 V. At 0.25 V, however, much larger background currents mere obtained, ranging from -5.0, initially, to -0.75 pA/(r cm2)after 30 sec. This is reflected in the CHa transient data at this potential. Hence, the figure shows that whereas at higher potentials (20.30 V) an initial sharp transient starting from -28.0 pA/ (r cm2)is obtained, the transient at 0.25 V is much more depressed with a maximum current of only 12.5 pA/(r cmz). Because of the large corrections from these background currents to be made to the 0.25-V data, we did not consider them further in this study. We had shown earlier that the initial stages of adsorption appear largely to be controlled by diffusion. l8 Correspondingly, in this time regime, we have attempted to analyze the current transients on the basis of diffusion theory. Assuming semiinfinite linear diffusion with no retarding effect on the adsorption due to blockage by adsorbed material2b dQada -dT1/'
-
~ O ~ C D ~ F ( ~ (C/(g ) ~ " Ccm2 sec'l'))
(1)
and I
01
I
10 0
IO
I
1000
TIMEIm)
Figure 3. Current transient during methane adsorption on a smooth platinum electrode in quiescent solution (corrected for background).
(25) H. A. Laitinen and I. M. Kolthoff, J . Amer. Chem. Soc., 61, 3344 (1939).
Volume 73, Number 7 July 1989
2400 from which (3) Here, Qads is the charge to oxidize the adsorbed layer, i is the current during adsorption, and l l l i i ~is the number of electrons released in oxidizing CH, to O-type. Similarly, oi?co2is the number of electrons between 0type and C01. D and C are the diffusion coefficient and the concentration, respectively, for CH, in SO% H3POr at 130". Experimentally, me measure the rate of accumulation of adsorbate and the transient current and, from eq 3, obtain the ratio of 0 ? ? c O 2 / ~ ~ ~ ?Then, 10. since the reaction CH, --+ O-type
4C 0 2
(4)
can occur, and involves eight electrons per CH4 molecule, me know that lll?l~
+mor
S
=
indicating pure diffusional control of :idsorption with every CH, molecule diffusing to the electrode :Idsorbing. Charge accumulntion is n little slower at 0.35 and 0.40 V. The likely reason is that the rates of thc over-all CH, + COZreaction are higher at these potentids (scc Figures 3 and 4). Heme, some of the adsorbable CH, molecules are oxidized to COz and do not appear in Qads.
Figure G shows the currents during the first 23 sec of adsorption plotted according to eq 2. Good linearity is observed, suggesting diffusion control, but a small systematic decrease in slope is found with increase in potential. It seems that at the higher potentials (0.33 and 0.40 V), the rate of adsorption is a little slower than diff usionally controlled (although still strongly affected by stirring). Note that this is a further reason for our observation that charge accumulation is a little slower at 0.35 and 0.40 V.
(5)
We can solve between eq 3 and 5 for 0?ico2. An important assumption is involved in this procedure. We assume that CH, is oxidized to COZ under the conditions of our experiment. There is little doubt that this is true.'* We do not assume that CH, normally does oxidize to COZvia O-type-although we mill show later that it does at these potentials-the reaction may proceed via another path. We do assume in this part of the analysis that the possible rate of CH, consumption by this other path is small compared with the rate of O-type formation. Also, in this analysis, we ignore effects from O-type oxidation during the early stages of adsorption (i.e., we would find too little Qads and too much i). This is not a large assumption since even if the over-all reaction does proceed via reaction 4, the eventual steady rate is much lower than the transient currents (Figures 1-4) Charge accumulation as a function of rl" is shown in Figure 5. Data at 0.25 and 0.30 V agree very well,
A
300mV
+
350 m V
0 400mV
20
0
01
I
I
I
02
03
04
I 05 (TIME$
I
I
I
06
07
08
I 0.9
I IO
m i ' / Z
Figure 6. Initial current transients during methane adsorption on smooth platinum in quiescent solution (corrected for background).
I
I
!
I
I
L
I
1
I
I
i
i
ZGGt
i I I
0
i I
I
I
I
I
I
I
I
I
I
2
4
6
8
10
12
14
16
18
20
( T I M E I '2
(I e c ) '2
/ ~ CHI adsorption Figure 5 . Variation of QF:4 with T ~ during in quiescent solution: X, 0.25 V; B, 0.30 V; +, 0.35 V; 0, 0.40 V.
The Journal of Physical Chemistry
1
The convergence of the charge accumulation data us. rl/' plots at low potentials (Figure 5 ) suggests that the 0.30-V current data may be used to calculate O ~ C O J ,lln~. Then we obtain a value of 2.95 f 0.15 electrons. At 0.35 and 0.40 V, we find 2.9 and 2.85 electrons, respectively, although for the reasons indicated above we put less credence in these results. There is an alternative method of computing 0ncoz which requires no assumption that the adsorption rate is limited by diffusion. Here, we integrate the charge under the current transients for a given time of adsorption and compare it with the charge to oxidize that adsorbate. For purposes of integrating the current, we assume that the steady-state rate of CH, oxidation (taken at 4 3 min) is the same as the rate of turnover of adsorbed CH, molecules during the approach to steady state. This is the opposite extreme from the assumption in the previous experiments where we assumed the turnover rate to be zero.
ADSORPTION AND OXIDATION OF HYDROCARBONS ON NOBLEMETALELECTRODES
32c
*
+
A
0--$-+-+-
+
*
n
24'
b
210
io
20
O
810
Id0
IlO TIMEliecl
la0
IQO
IiO
200
I
210
2 L '
Figure 7 . Number of electrons required to oxidize O-type material as a function of potential and time: 0.30 V; 0 , 0.35 V; A, 0.40 V.
+,
Figure 7 shows results for oncoz computed in this manner as a function of time of adsorption and potential. Data at 0.30 and 0.35 V are particularly of interest since here there are minimal difficulties with background currents (a problem at 0.25 V) and with correcting for the high steady-state rate of CH4 oxidation (as at 0.40 V). Values of 3.1 k 0.2 are found for oncoz, in good agreement with the results of the diffusion method. At 0.40V, the values of O n C O l were systematically lower (2.7 to 3.2, depending on adsorption time). This is because of the difficulty of allowing for the steady rate of CH4 oxidation at this potential. The evidence is that we are undercorrecting for the CHI turnover rate by assuming that during the adsorption process it is equal to the steady-state rate of CH, oxidation. Another word is appropriate about the integration method of determining onto,. As indicated, there are signs that at 0.40 V we are undercorrecting for the turnover rate of CH4. The evidence at say, 0.30 V is, on the contrary, that it does not matter whether or not we include the steady-state turnover rate in our calculations. We can put an extreme lower limit on the results of the integration procedure for O n C O z at 0.30 V by assuming that there is no turnover rate of CH, --t COZ during the current transient. If we do this, we obtain a value of 2.45 for oncoz. There is some evidence in the literature, which we will review later, that O ~ C O should be 2, but we see that the present data do not appear to allow such a low value. Our results suggests strongly that oncocis very close to 3. Reaction Path j o y C H I Oxidation on Pt. I n calculating onCoz, we assumed that the turnover rate of adsorbed CH, molecules during the transient was zero or equal to the steady-state CH, oxidation rate. However, another possibility exists; there may be a coupled parallel set of reactions, vix.
~
2401
If reaction 4 is the only one which occurs, oxidation of O-type is the rate-determining step in CH4oxidation. Our assumptions in calculating O n C O z would then be numerically exact. However, the reaction could proceed through some alternative route, reation 6, with 0type acting as a poison on the electrode surface for this reaction. That is to say, O-type, once formed, may remain immobile on the surface or oxidize to COz only at a very slow rate. The main reaction and consequently the steady-state current would then derive from reaction 6 . Transient currents would then arise not only from O-type formation directly but also from the fact that the current contribution from reaction 6 would be higher on the initially clean electrode. Reaction 6 would then be progressively inhibited by accumulation of O-type. This is similar to the mechanism proposed for HCOOH oxidation26-2s and would mean that the calculations of O n C O z are numerically incorrect. The effect would be that the true values of O n C O z would be higher than those observed. To test these possibilities, we have compared the rate of O-type oxidation with the steady current for CHI oxidation. The latter was measured on a platinized platinum electrode (surface area 'v 80 r cmz) at 0.30 V. After correction for background, a reproducible value of 0.59 pA/(r cmz) was obtained. If the reaction proceeds through reaction 4, we may tentatively take this to be the current from the reaction CH, 3 O-type COz. The results above suggest that five electrons are required to form O-type and three electrons to oxidize it. On this basis, the current from O-type oxidation should be three-eighths of the total current, namely 0.22 pA/(r cmz). The second part of the experiment was actually to measure the current corresponding to O-type oxidation. This was accomplished by low current density stripping of the steady-state layer (O-type) formed at 0.30 V. The basic experimental problem here is that during the stripping, CH, in solution may possibly diffuse to the surface and be oxidized. The magnitude of this problem is governed by two factors: the roughness of the electrode and the CH4 concentration in the electrolyte. Because of these factors, no meaningful data could be obtained on a smooth electrode. Consequently, a platinized electrode with a roughness of approximately 20 was used. I n this situation, and using the sequence outlined below, we were able to overcome diffusion problems during the stripping. The procedure was as follows. After cleaning the electrode we formed a steady-state layer by adsorbing CHd for 5 min with stirring and 1 min in quiescent solution. Then the potential was dropped to 0.25 V where the oxidation rate of the adsorbed layer is considerably less
very
CH4 +O-type COz unknown relatively fast CHI +intermediate -+ COz
(4) (6)
(26) S. B. Brummer and A. C. Makrides, J . Phys. Chem., 68, 1448 (1964). (27) S. B. Brummer, J . Electrochem. SOC.,113, 1041 (1966). (28) S. B. Brummer, Elektrokhimiya, 4, 243 (1968).
Volume 73, Number 7 July 1969
2402
A. H. TAYLOR AND S. B. BRUMMER
than at 0.30 V. K2 was then bubbled through the solution in the cell via a second presaturator containing only Nz-saturated water. It was found that in this situation the CH, concentration in the electrolyte was reduced sufficiently within 10 min that no diffusional problems arose even at the lowest current density subsequently employed. Experiment showed that insignificant oxidation of the adsorbed layer occurs during this 10-min period. Following this, the potential was again raised to 0.30 V in quiescent solution for 1 min. Varying low currents were then applied to the working electrode and the voltage-time curves recorded on a stripchart recorder. The curves (Figure 8) consist of a double-layer region followed by a potential plateau where oxidation of the adsorbed layer takes place. Above this potential, oxidation of the electrode occurs. The potential remains almost constant while most of the stripping occurs. This indicates the absence of any important coverage factor in the kinetics. The curves show that the total charge passed in the stripping is independent of the current density. Thus, very little of the observed charge comes from the CH4diffusing up to the electrode during the (slow) transients, and all the charge originates from the oxidation of the layer adsorbed before the application of the constant current pulse. 0.8
II
I
I
STRIPPING
C U R R E N T IN p A / r . c m '
A 2:
I
,
I
10
IO 0
CURRENTI,'A/,
I
1000
cm'l
Figure 9. Current us. potential relationship for anodic stripping a t low current density of the steady-state layer formed at 0.30 V on a platinhed platinum electrode.
well with that calculated from our oncon data (0.22 f 0.05 pA/(r cm2)). This excellent agreement between the predictions of reaction 4 and the observation of the relative rates of 0type and CHd oxidation has important consequences. We can conclude that at low potentials, e.g., 0.30 V, the oxidation of CH, proceeds exclusively via 0-type. Essentially none of the current proceeds via reaction 6. The rate-limiting step in the reaction muFt then be the oxidation of 0-type to COZ. This follows because of the relatively high coverage of the electrode with this species. Our previous concl~sion9~~O that 0-type poisons the main reaction (viz., reaction 6) is not correct. As indicated," some of the experimental evidence on reaction orders which was the basis of that conclusion is not relevant, referring to too high potentials (>0.40
V) *
07-
Y
i
r 06-
>
I
i
4 Y I
05-
200
150
100
I
,
50
0
1
CHARGE REMAINING(,UWUI / r c m z i
Figure 8. Anodic stripping curves of adsorbate formed at 0.30 V under CHI as a function of current density.
From these curves, me obtain the potential at which stripping commences. These are plotted in Figure 9 as stripping potential us. the logarithm of stripping current density. The relationship is linear over its entire range and by extrapolation of the data to 0.30 V, we obtain the current to oxidize 0-type at this potential (0.20 A 0.05 fiA/(r cm2)). This figure agrees very The Journal of Physical Chemistry
Our present conclusion that the oxidation of CH? is limited by the rate of oxidation of 0-type is to be contrasted with that of Marvet and Petrii.29 Their results (below 100') suggested that dehydrogenation of CH, is rate limiting. It is interesting to consider whether our reaction mechanism is operative at higher potentials. I t is clear that since the steady-state coverage of the electrode falls almost to zero at potentials above -0.4 V, the ratelimiting step cannot be the oxidation of 0-type. It must rather be the initial adsorption of CH?. Thus the 0-type oxidation step becomes faster than the adsorption step as the potential increases. It can also be seen from our data, however, that the initial adsorption rate probably is lower at 0.4 V than at lower potentials. Thus, in Figure 5 we see that the rate of accumulation of the CHI adsorbate at 0.40 V is slower than diffusion controlled (though it is still largely affected by stirring). This could be attributed to a greater rate of 0-type oxidation, giving smaller values of &XI. However, a consequence of this would be that the transient currents at 0.40 V should be higher than at the lower potentials. I n fact, the reverse is the case except in the approach to steady state (Figures 1-4, 6). (29) R. V.
Marvet and 0. A. Petrii, Elektrokhimiya, 3 , 153 (1967).
ABSENCE OF H ATOMPRODUCTION IN RADIOLYSIS OF SOLID HYDROCARBONS
Composition of 0-Type. We have discussed the literature concerning the likely composition of “reduced COz”-similar to O-type11,30-in recent publicat i o n ~ . ~The ~ , ~basic ~ controversy concerns whether that species is 33-35 or is nota1J6similar to adsorbed CO. The present data provide evidence on the structure of 0-type. Thus, we have found that OnCOl is 3 and, previously,ls that [e] ‘v 1. A species which would satisfy these findings is 3C-OH. This composition has been suggested to correspond to CH30Hads.37-39 The rate-limiting step in CHI oxidation at low potentials would then be +C-OH
+ HzO +C02 + 3H+ + 3e-
(7)
Summary and Conclusions (1) The species on Pt in presence of CH, (0-type) appears to lie ca. five-eighths of the way between CHI and COz. One electron per site is released when this species is oxidized. A composition of >C-OH fits these observations. (2) The oxidation current of this species at 0.30 V is ca. three-eighths of the oxidation current of CHI. This suggests that it lies in the path of the main CH, --t COz reaction. (3) The rate-limiting step of the over-all reactmionat
2403
low potentials (50.35 V) is the oxidation of this species,
viz. +C-OH
+ H 2 0 +COz + 3H+ + 3e-
(4) At higher potentials (20.40 V), the rate-limiting step is the rate of CH4adsorption.
Acknowledgments. We are pleased to acknowledge that this work was supported by the U. S. Army, Mobility Engineering Research and Development Center, Fort Belvoir, Virginia, on Contract DA-44-009AMC-l408(T). (30) J. Giner, Paper presented at 15th C.I.T.C.E. Meeting, London, 1964. (31) S. B. Brummer and K. Cahill, Discussions Faraday SOC.,45, 67 (1968). (32) 5. B. Brummer and K. Cahill, J . Electroanal. Chem., in press. (33) M. W. Breiter, Electrochim. Acta, 12, 1213 (1967). (34) T . Beigler and D. F. A. Koch, J . Electrochem. Soc., 114, 904 (1967). (35) T. Beigler, J . Phys. Chem., 72, 1571 (1968). (36) B. J. Piersma, T . B. Warner, and S. Schuldiner, J . Electrochem. Soc., 113, 841 (1966). (37) B. I. Podlovchenko and E. P. Gorgonova, Dokl. Akad. Nauk S S S R , 156, 673 (1964). (38) 0. A. Petrii, B. I. Podlovchenko, A. N. Frumkin, and H. Lal, J . Electroanal. Chem., 10, 253 (1965). (39) 0. A. Khnzova, Y . B. Vasiliev, and V. S. Bagotskii, Elektrokhimiya, 2, 267 (1966).
Absence of Hydrogen Atom Production in Radiolysis of Solid Hydrocarbons] by Dietrich Timm and John E. Willard Department of Chemistry, University of Wisconsin, Madison, Wisconsin
(Received January 7, 1969)
Radiolysis of ethane, n-hexane, 3-methylpentane, 3-methylpentane-dlc, or methylcyclohexane a t 4°K produces the expected esr spectra of trapped free radicals, but no evidence of trapped hydrogen atoms, although the H atom doublet is present with the CH, radical spectrum when CHI is irradiated. Radiolysis of 3-methylpentane-& a t temperatures in the range from 20 to 50°K produces no trapped D atoms, although it has been shown that D atoms from the photolysis of D I in this matrix can be trapped in this temperature range. The presence of 0.3 mol yoisobutene in CHI during irradiation a t 4°K does not reduce the trapped H signal. Ir’ CzHB in Ar a t 4°K produces little or no H atom yield compared t o that from 1 mol % radiation of 1 mol % CH, in Ar. These observations lead to the conclusion that the free radicals formed by radiolysis of solid hydrocarbons are not produced by elimination of H atoms. Ion-molecule reactions are, therefore, the probable mode of formation.
Introduction The assumption that hydrogen atoms are produced in the radiolysis of alkanes is consistent with extensive evidence.2 However, in each such case, an ionmolecule mechanism can be written which would produce the observed results. Only for CHI is there unambiguous evidence for H atom production in alkane
radiolysis, as far as we are aware. H atom esr lines have been observed f r o m liquid CH4 during steady(1) This work has been supported in part by U. S. Atomic Energy Commission Contract AT(l1-1)-1715 and by the W. F. Vilas Trust of the University of Wisconsin. (2) Examples and references are given by M. C. Sauer and I. Mani, J . phys. Chem., 72, 3856 (1968).
Volume 73, Number 7 J u l y 1069
