427
THEANODICOXIDATIONOF SATURATED HYDROCARBONS
The Anodic Oxidation of Saturated Hydrocarbons. A Mechanistic Study by J. O'M. Bockris, E. Gileadi, and G. E. Stoner The Electrochemistry Laboratory, University of Pennsylvania, Philadelphia, Pennsylvania
10104
(Received J u n e 12, 1068)
Determinations were made of the rate of electrochemical oxidation of propane, n-hexane, cyclohexane, and 2,2-dimethylbutane in concentrated phosphoric acid solutions at 80-150". Transient values were recorded at fixed potentials upon introducing the reactant to solution. Steady-state rates were measured as a function of propane pressure, the activity of water, and the electric field gradient applied across the surface on which the reaction occurred. Thr Tafel slope is F/RT in the low-rate section of the rate-potential relation. The order of reaction with respect to hydrocarbon is one; that for water is eero. The rate at constant field gradient as a function of time passes through a maximum at about t = 5 sec. At constant potential, the rate of oxidation to COz is propane > 2,Z-dimethylbutane > n-hexane > cyclohexane. A t potentials of more than 0.48 V on the hydrogen scale, the rate of oxidation at 80-130° decreases anomalously with increasing field. For propane, at 135-150°, the rate undergoes inhibition at 0.48 V but continues to increase with an increase in field strength. Some 15 partial-mechanism hypotheses (suggested sequences up to and including the rate-determining step) are used to predict the kinetic behavior for which data exist. Large numbers of the sequences are excluded by the coefficient b log (rate)/d (electric field strength). Two (partial) mechanisms are consistent with the observations. Of these, that which is most consistent with energy considerations of the bonding involved is the most likely. It is, for a saturated hydrocarbon RH: RH R a d s H+ e- and Rads2 organic radioals. The rate-determining step for the alternative possibility was also a chemical reaction. The hypothesis is applied to the interpretation of the i-t transient. It yields a value for &,~,(480 mV) of 0.3. The inhibition occurring above 0.48 f7is shown to be consistent with data published elsewhere for the adsorption of H*PO4-.
+ +
Introduction The electrooxidation of saturated hydrocarbons has received considerable attention in the past decade because of their connection with the development of electrochemical energy-conversion devices. The majority of the attention and, hence research effort, has been devoted either to the measurement of adsorption by various anodic and cathodic transient or to the technological aspects of actual fuel cell systems.'-'4 One factor contributing to lack of progress in the case of the latter is the absence of knowledge of the mechanism, in particular the rate-determining step, of these anodic hydrocarbon reactions under steadystate conditions. The present work is concerned with the determination and interpretation of criteria relevant to the mechanism for the anodic oxidation of a series of saturated hydrocarbons at platinum electrocatalysts. Experimental Section 1. The Cell. A special three-compartment cell was designed so as to allow its immersion in a thermostated bath which could be regulated from 80 to 150". Figure 1 is a diagram of the cell. The reference and auxiliary electrodes have been described elsewhere.'63 '6 The cap of the anode compartment, A, which contains the anode was replaced by a rotating-electrode assembly (described in another section) when hydrocarbon partial pressure measurements were made. 2. Reagents Used. The following reagents were used : Phosphoric Acid, Baker Analyzed reagent, 85
wt % H,PO,; Monsanto Phospholeum, concentrated, 105% (76 wt % P206)phosphoric acid; hydrogen peroxide, Baker Analyzed reagent, 30 wt %; propane, hlatheson CP grade, 99.5% minimum purity; n-hexane, Fisher Certified reagent, 99.94% minimum purity, bp 68.7" ; cyclohexane, Fisher Certified reagent, 99.92% minimum purity, bp 81.4" ; 2,2-dimethylbutane1 City Chemical Corp., New York, N. Y., reagent grade,
(1) S. Gilman, J . Phys. Chem., 67, 1898 (1963). (2) S. Gilman, ibid., 66, 2657 (1962). (3) S. Gilman, ibid., 67, 78 (1963). (4) S, B. Brummer, ibid., 69, 562 (1965). (5) 8. B. Brummer, ibid., 69, 1363 (1965). (6) 8. B. Brummer, J. I. Ford, and M. J. Turner, ibid., 69, 3424 (1965). (7) Progress Report No. 2, Contract KO.DA-44-009-AMC-897(T), ilmerican Cyanamid Co., 1965. (8) Progress Report No. 3, Contract No. DA-44-009-AMC-897(T), American Cyanamid Co., 1966. (9) Progress Report No. 4, Contract No. DA-44-009-AMC-897(T), American Cyanamid Co., 1967. (10) Progress Report No. 1, Contract No. DAAK02-67-C-0219, Engelhard Industries, 1967. (11) E. J. Cairns and D. I. iMacdonald, Electrochem. Technol., 2, 65 (1964). (12) W. T. Grubb, Nature, 201, 699 (1964). (13) W. T. Grubb and C. J. Micholske, ibid., 201, 287 (1964). (14) W. T. Grubb, J . Electrochem. Soc., 111, 1086 (1964). (15) Report No. 2, Contract No. DA-44-009-AMC-469(T), Electrochemistry Laboratory, University of Pennsylvania, Philadelphia, Pa., 1966. (16) Report No. 3, Contract No. DA-44-009-AMC-469(T), Electrochemistry Laboratory, University of Pennsylvania, Philadelphia, Pa., 1966. Volume 79, Number 2 February 1969
428
J. O’M. BOCKRIS,E. GILEADI, AND G. E. STONER
H2-
Mercury
C -F-Cooling coils
F
EPlatinum contact
-G-Glass
coo
F
Figure 1. Diagram of the cell for studies in HsPOa: A, working-electrode compartment; B, counterelectrode compartment; C, reference-electrode compartment; D, gas-outlet bubbler; E, working electrode; F, glass frits; G, gas inlet.
99.89% minimum purity; hydrogen, Matheson CP grade, 99.994% minimum purity. 3. Rotating-Electrode Assembly. For the determination of reaction orders with respect to hydrocarbon concentrations, a rotating-electrode assembly, shown in Figure 2, was used. The driving motor was a Bodine Electric Universal Motor Type KSE-12 (8800 rpm (maximum)). The speed of the motor was varied by a General Radio Variac speed control, Type 1701-AU. Electrical contact was made to the rotating electrode through a mercury seal in the cell cap. The mercury was in continuous contact with an inverted stainless steel cap which was Teflon coated on the side exposed to the atmosphere above the cell. This cap was connected with setscrews to the Teflon-coated rotating shaft which extended into the solution. The electrode was a platinum-plated 23 karat gold cylinder which was machine threaded to screw into the end of the stainless steel shaft, thus giving rigid physical and excellent electrical connections to the shaft. The rotating shaft could be raised from or lowered into the cell by three aluminum telescopic supports connecting the upper ball bearing housing and motor assembly to a solid-aluminum base plate which rested under the thermostated bath. 4. Electyical Accessories. The potentiostatic and galvanostatic circuitries have been described elsewhere. l8 6. Constant-Temperature Bath. The constant-temperature bath was a 1 ft X 1.5 f t X 0.75 in. porcelain tank containing 20 lb of A. H. Thomas No. 6407-J silicone fluid (bp 200’). The bath was heated by two A. H. Thomas KO. 6147-G3 immersion knife-type heaters (500 W (maximum)) and were controlled through Superior Electric Powerstat Type 110 voltage regulators. One heater-regulator was controlled by a Brooks Instrument Brookstat proportional control !
The Journal of Physical Chemistry
Figure 2. The rot’ating-electrode assembly.
thermoregulator, No. BS-1318, giving a bath-temperature control of 2=0.3”. 6. Apparatus f o r Pretreating Reagent. a. Phosphoric Acid. The phosphoric acid was refluxed a t 150” for 12 hr after addition of 5 vol % of a 30% solution of hydrogen peroxide to destroy any undesirable organic matter. The excess water was then distilled off until the original concentration of phosphoric acid (before addition of HzOz)was obtained. b. Hgdrocaybons. Propane was passed through a solution of 10% KlliIn04 in 6 M KaOH and then through concentrated sulfuric acid to remove any traces of unsaturated hydrocarbons which rnay have existed as impurities more active than propane. The liquid hydrocarbons (n-hexane, cyclohexane, and 2,2-butadiene) were passed, as liquids, over Union Carbide molecular sieves, Type 3-A, which will adsorb unsaturates, water, hydrogen, and oxygen-containing organic species. With the liquid hydrocarbons it was also necessary to vaporize them before passing them into the cell which was substantially above their boiling point, whereby the “hexanesJJJonce vaporized, were maintained as gases by wrapping the connecting tubing between the vaporization flask and the bath with Briscoe Rlanufacturing No. 61488 flexible heating tapes. c. Presaturation. All gases were passed through a series of presaturator liquid traps immersed in the bath containing phosphoric acid a t the same concentration as that in the cell. 7 . Experimental Procedure. The experimental pro(17) B. J. Piersma, Ph.D. Thesis, University of Pennsylvania, 1965. (18) J. O’M. Bockris, H. Wroblowa, E. Gileadi, and B. J. Piersina, Trans. Paraday Soc., 61, 2531 (1965).
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THEANODICOXIDATION OF SATURATED HYDROCARBONS 0 147'C X 129-C 0 104'C
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Figure 3. Current-potential behavior for the anodic oxidation of propane.
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Figure 5 . Ellipsometric data showing the potential of oxide formation on platinum in 85% &POa.
o Cyclohexane 700
