H. J. BARGER,JR.,AND A. J. COLEMAN
880
the -BH3 groups in both diborane and borane carbonyl are not planar. The BH3+ions formed by fragmentation of these molecules, therefore, are likely to have appreciable internal and translational energy. Consequently the value D(BH3-BHa) = 59 kcal/mol derived from appearance potentials31 must be taken as an upper limit. Mappes and Fehlner32 report a high absolute yield synthesis of borane from the carbonyl. The BH3 mass spectrum reported by them is in excellent agreement with those given in the present Table 11.
Conclusion The molecular beam mass spectrum of BHBCO believed to be free of pyrolysis products and other impurities has been obtained for the first time and is shown to have fragments of the type BH,C+ in addition to BH,CO+ (y = 0,1,2,3) and BH,+(x = 0,1,2); BHa* not formed in measurable amounts. Pyrolysis of BH&O under the specific conditions used in this investigation, low inlet pressure and short contact time for the reactor, gave a high enough concentration of BH3 that it could be identified clearly and its molecular beam mass spectrum obtained. This spectrum agrees with that obtained earlier from B2Hogand used in
interpreting results obtained for the high-temperature reaction of B4C with H2,29thus confirming the identification and quantitative estimates of BH3 concentration in the latterz9case. Destruction of BH3C0 to products other than BHs and BzHs takes place through the whole temperature range where the pyrolysis is studied, and is in disagreement with the assumptions of a previous kinetic study.8 The ion BHO+ as well as hydrogen and black deposits in the reactor also were observed. Porter reports33that evidence for BHzOH is observed when BH3C0 is decomposed by flowing through a hot quartz tube and the products are condensed in a rare gas matrix. Our observation of BHO+ is consistent with the previous observation of BH20H.84
Acknowledgments. The authors wish to thank R. E. Hollins for preparing the samples, and 8. 51. S c h i l d c r ~ u and t ~ ~ A. B. Baylis for valuable information from their preliminary study of this subject. Professor T. P. Fehlner gave us manuscripts16*S2 in advance of publication as did Professor R. F. Porter.a8 (32) G. W. htappes and T. P. Fehlner, private communication. (33) R. F. Porter, private oommunication. (34) R. F. Porter and 9.K.Gupta, J . Phys. Chem., 68,2732 (1964).
The Hydrogen-Deuterium Exchange of Benzene at a Fuel Cell Electrode by H. J. Barger, Jr., and A. J. Coleman Energy Conversion Research Division, U . S. Armu Mobility Equipment Research and Development Center, Fort Beluoir, Virginia 8~06'0 (Received June 11, 1969)
The hydrogen-deuterium exchange of benzene at a fuel cell electrode was studied at 0.30 V. After electrochemical pretreatment, the electrode was potentiostatically held at 0.30 V as benzene passed over the electrode surface. The amount of exchange increased rapidly for 1-2 min then abruptly diminished. The percentage of the various deuteriobenzenes ranging from CeHsDto CeDasuggests at least two types of exchange reactions, both of which occur concurrently,
Elucidation of the anodic oxidation reaction mechanism of hydrocarbons has been of interest for several years as a key step in the development of practical fuel cells which directly oxidize logistic fuels such as combat gasoline and kerosene. An important part in the overall mechanism is the initial adsorption and bonding of the hydrocarbon to the electrocatalyst. Recently hydrogen-deuterium exchange has been used to study the adsorption of propane on a fuel cell electrode.' Most of the material removed from the electrode by the cathodic pulse was completely deuterated. These I'he Journal of Physical Chemistry
results suggested that the intermediate species on the surface were highly mobile and that carbon-catalyst and carbon-hydrogen bonds could be made and broken very easily. I n this paper, the isotopic exchange of benzene a t a fuel cell electrode is reported. Benzene is a common component of many logistic fuels and has been shown to be extremely detrimental to the oxidation of alkanes on platinum electrodes.2 By studying (1) H. J. Barger, Jr. and A. J. Coleman, J . Phys. Chem., 7 2 , 2285 (1968).
HYDROGEN-DEUTERIUM EXCHANGE OF BENZENE the hydrogen-deuterium exchange of benzene under conditions similar t o those of an actual fuel cell, it was hoped that some information could be obtained as to how benzene adsorbed, what its structure on the surface was, and thus the reason why benzene acts as an electrocatalyst poison. I n addition, the many nonelectrochemical exchange studies of benzene on platinum could be compared to exchange at the electrode and perhaps aid in the Of these
Experimental Section Apparatus and Materials. The electrochemical cell and circuitry used in these experiments have been described p r e v i ~ u s l y . ~The working electrode was an American Cyanamid Type LAA 25 consisting of 25 mg/ cm2 of platinum pressed together with 25 wt % Teflon on a tantalum screen. A Teflon film was applied to the gas side of the electrode for wet-proofing. The helium carrier gas used in these experiments was 99.99% minimum purity and was used as received. The benzene, Fisher spectroquality, and the electrolyte, Brinkmann Instruments 85% perdeuteriophosphoric acid with a 99% deuterium content, were also used as received. The flow system diagrammed in Figure 1was connected to a CEC 21-130 mass spectrometer equipped with a CEC 5-124 oscillograph which allowed scans between mass 76 and 100 to be made in 2 sec or at the rate of 30 per min. By manipulation of valves 1, 2, and 3, helium could be passed over the working electrode or through a benzene saturator and the resulting mixture used as the reactant. Flow rates, 0.5 to 2.0 ml/sec, were controlled with a capillary flow meter and a precision needle valve, which were calibrated with a bubble flow meter. A benzene concentration of 2.8 X lo-' mol/ml was used most often since this represented the best compromise between good mass spectrometer sensitivity and electrode life. Reactant concentrations were determined by weighing the amount of benzene collected at liquid nitrogen temperature for 10 min at each flow and saturator temperature. The benzene saturator and cell temperatures were controlled to *0.1", and were monitored continuously with calibrated iron-constantan thermocouples connected to a Hewlett-Packard 7100 B dual channel strip chart recorder with a full scale range of 1mV. Experimental Procedure. Before conducting an experiment, a pretreatment procedure4 was followed to prepare a reproducible catalyst surface and to determine the electrochemical surface area.6 While sweeping with helium, the working electrode was held for 25 min at 1.35 V vs. the dynamic hydrogen electrode,6 dhe, in the same medium whereby the catalyst surface and species on the surface were oxidized. The potential was then lowered to 0.05 V for 3-5 min to reduce the platinum oxide. Next the potential was raised to 1.35 V for 5 min followed by cathodically pulsing the working electrode a t a constant current of 1.0 A. A Hewlett-
881
T
SPECTROMETER
Figure 1. Diagram of flow system: T, helium supply; M, mercury manometer; N, needle valve; F, capillary flow meter; S,benzene supply and constant-temperature bath; P, preheater; V, valves; C, electrochemical cell and oven; Tc,thermocouples.
Packard 7001A time based X-Y recorder was used to record the cathodic charging curve from which the surface area was calculated assuming 210 pcoul/real cm2for a monolayer of hydrogen.' Then the potential was held at 0.3 V, the potential of interest, until the deposited deuterium from the cathodic pulse had been oxidized (1-2 min). The adsorption potential, 0.30 V, was chosen for this study because it represented a point where benzene readily adsorbed but oxidized little.* An exchange experiment was started by opening valves 1and 3 while simultaneously closing valve 2 and starting the mass scan. A low ionization voltage (9.45 V) was used to minimize fragmentation of the hydrocarbons. After 2-5 min, the cell was flushed with helium until no peaks were seen on the oscillogram. The electrode was cathodically pulsed to remove any adsorbed hydrocarbons' after which the pretreatment could be started again.
Results and Discussion The measured values of the peaks comprising the mass spectra were treated with a Mathatron computercalculator to correct for naturally occurring isotopic contributions to the deuteriobenzenes, to calculate the percentage concentration of each deuteriobenzene with and without C6H6, and to calculate 41.~ No correction for differences in mass spectrometric sensitivities for the various deuteriobenzenes was made in these calculations.'O The quantity, 4, mentioned above equals 22% C ~ H B - ~and D ~reflects the number of hydrogendeuterium exchanges per 100 molecules of benzene, (2) (a) E.Luksha and E. Y. Weissman, J. Electrochem. Soc., 116,120 (1969): (b) J. F.Lennon, E. Luksha, and E. Y. Weissman, ibid., 116, 122 (1969). (3) H. J. Barger, Jr., and M. L. Savitz, ibid., 115,686 (1968). (4) (a) S. Gilrnan, J.Phy8. Chem., 67,78 (1963); (b) 5.E. Brummer and M. J. Turner, ibid., 71,2825 (1967). (5) B.E.Conway and D. Gilroy, Can. J. Chem., 46,875 (1968). (6) J. Giner, J.Electrochem. Soc., 111,376 (1964). (7)' 8. B. Brummer, J.Phys. Chem., 69,562(1965). ( 8 ) (a) W.Heiland, E. Gileadi, and J. O'M. Bockris, ibid., 70, 1207 (1966);(b) M.L.Savitz and A. L. Hubbard, J.Electrochem. Soc., 116, 714 (1969). (9) J. R.Anderson and C . Kemball, Advan. Catal., 9,51 (1957). (10) S. Meyerson, H. M. Grubb, and R. W. Van der Harr, J. Chem. Phys., 39,1445 (1963). Volume 74, Number 4 February 10,1970
H. J. BARGER, JR.,AND A. J. COLEMAN
882 -
310
FLOW RPTE = 1.00 ML/SEC CELL TEMP = 60°C
-
310
.
JW
*. PERCENT Cs H6
100-
no -
-Q
1 0
-
5o 5o
41
-
