Anodic Oxidation of Cyclic Hydrocarbons - ACS Publications

15 Anodic Oxidation of Cyclic Hydrocarbons M A X I N E L . SAVITZ and RITA L . CARRERAS 1

U . S. Army Mobility Equipment Research & Development Center, Fort Belvoir, Va. 22060 The adsorption and oxidation of cyclohexane and benzene on a smooth platinum electrode has been studied at 130°C. in 85% H PO using potentiostatic and galvanostatic pulse techniques. Adsorption occurs over the range 0.05 to 0.80 volts vs. R.H.E. with a maximum at 0.20-0.30 volts for both compounds. The composition of the finally adsorbed residues was investigated using cathodic and anodic desorption techniques. Much of the adsorbed species can be removed at 0.01 volt and the coverage varies with potential in the same way as the total adsorbate. The residue for both reactants appears to be in the same reduced state. The part of the adsorbate which is not cathodically desorbable is in a highly oxidized state releasing 2.0 ± 0.2 electrons per covered site on oxidation to CO .

Downloaded by UNIV OF LEEDS on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0090.ch015

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T n order to obtain efficient practical direct hydrocarbon fuel cells for •*· military use, it is necessary to oxidize the components of logistically available fuels. Military specifications (16) allow these fuels to contain as high as 2 5 % aromatics and 5 % olefins. Recently Luksha (15) found that a Niedrach-Alford Teflon bonded platinum electrode with phosphoric acid electrolyte at 150 °C. could tolerate a fuel containing up to 5 % olefins, 1 % aromatics, 5 % six-ringed naphthenes, 15% five-ringed naphthenes and the remainder saturated normal or isooctane with an increase of only 50 mv. i n polarization from that of pure octane. Previ ously we had reported (19) the adsorption characteristics of some of these representative compounds on bright platinum wire i n 8 5 % H3PO4 at 130°C. F o r a l l of the compounds studied, the rate of adsorption appeared to be diffusion controlled. Benzene adsorbed the most rapidly; cyclohexene and cyclopentene adsorbed the next fastest and both at the same rate; hexene-1 and hexene-2 also adsorbed at the same rate, but less rapidly than the unsaturated cyclic compounds; and cyclohexane and cyclopentane had the lowest rate of adsorption. F o r a l l compounds, 1

Present address: Department of Chemistry, Federal City College, Washington, D. C. 188 Baker; Fuel Cell Systems-II Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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maximum adsorption was between 0.2 and 0.4 volt. The highest steadystate coverage for a specific concentration was obtained for the most rapidly adsorbing species. The maximum amount of adsorption at each potential followed the same order as did the rate of adsorption—i.e., benzene adsorbed the most. In fuel cell tests (15), it has been found that cyclohexyl compounds affect octane performance more adversely than five C-ringed naphthenes and they do not behave like normal saturated hydrocarbons. Open circuit values indicated dehydrogenation to benzene was occurring for the six member ringed compounds; however, in the initial adsorption studies on a wire electrode, cyclohexane appeared to behave as a normal saturated hydrocarbon. To obtain more evidence as to whether there was dehy drogenation under our conditions, the type of electrochemically adsorbed species formed from benzene and cyclohexane was investigated using cathodic and anodic desorption techniques developed by Brummer (4). Experimental The experimental setup was similar to that which we have described previously (20). In brief, the electrochemical studies were performed on a flamed bright platinum wire of thermocouple grade platinum of geometric area 0.5 cm. maintained at 130°C. i n 85% H P 0 . The acid had been pretreated with hydrogen peroxide and was contained i n a standard three compartment electrochemical cell made of quartz. The reference electrode was the dynamic hydrogen reference electrode de scribed by Giner (11). The hydrocarbon was introduced into the working compartment by using an argon carrier gas through a glass tube containing organic compound maintained at a controlled temperature to give the desired partial pressure. For low partial pressures, the organic was con tained i n a tube cooled i n an ice-methanol bath. For higher partial pres sures, the tube containing the hydrocarbon was heated in an o i l bath to the desired temperature. Phillips research grade hydrocarbons of 99.6 mole % purity were used in all cases. In order to obtain a reproducible platinum surface for the electro chemical measurements, potentiostatic procedures similar to those of Brummer (5) and Gilman (9) were employed. Essentially, the electrode was held at 1.35 volts for 1 min., the last 30 sec. without stirring, at 0.05 volt for 10 msec, and then maintained at a potential of interest for varying times before examining the surface state of the electrode with an anodic or cathodic galvanostatic pulse (5). Potential sequences were obtained by switching of a series sequence of potentiometers with Hg-wetted relays. Time intervals at the potentials were controlled by Tektronix Type 162 waveform generators (6). The potentials were applied to the cell through the input of a Wenking potentiostat. A l l areas are based on real electrochemical areas with 210 μ coul. equal to 1 c m . of real area as obtained from deposition of a monolayer of hydrogen with a cathodic galvanostatic charging curve (2, 3). 2

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Baker; Fuel Cell Systems-II Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

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F U E L C E L L SYSTEMS

Results and

II

Discussion

From anodic galvanostatic charging curves, using current densities from 500 /*amp. to 150 ma./cm. , reliable estimates ( Q ) of oxidizable material on an electrode are obtained (5). The charge associated with the oxidation of the electrode and adsorption of H 3 P O 4 under argon is subtracted from the charge obtained with organic reactant. To obtain the fraction of the surface covered by organic material, cathodic galvano static pulses are used. The ratio of hydrogen deposition obtained i n the presence of organic with that obtained in an argon atmosphere gives 0 The fraction of the surface covered by organic material is 1 — 0 - The initial adsorption studies with benzene (19) indicated that at higher con centrations (200 mm. partial pressure) of benzene, 15% of the electrode was covered with material after adsorption at potential for 1 msec, the least amount of time of adsorption that could be reproducibly measured. In order to have a clean surface at initial adsorption, benzene at a partial pressure of 5 mm. was used in these studies. Since cyclohexane (19) adsorbed very slowly at the lower concentrations and at higher partial pressures behaved like normal saturated hydrocarbons (3, 7, 20), a higher concentration was used. This would also enable dehydrogenation to benzene to be detected more easily. 2

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H

Maximum

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Adsorption

The adsorption of benzene and cyclohexane appears to be initially diffusion controlled, the rate of adsorption then decreases and the surface concentration of adsorbed species reaches a constant steady state value (19). This value is reached within 30-60 seconds at all potentials except with the low concentration of cyclohexane where steady state coverage is not obtained until 300 seconds. Figure 1 shows the steady state amount of adsorption obtained from anodic galvanostatic pulses as a function of potential for benzene at 5 mm. pressure and cyclohexane at 200 mm. pressure. The electrode is pretreated as mentioned, held at the potential of interest for two minutes and then treated with an anodic galvanostatic pulse of 50 ma./cm. . Both compounds have appreciable adsorption from 0.05 to 0.80 volt, and benzene even has adsorption at 0.9 volt. Between 0.2 and 0.4 volt, maximum adsorption is observed. There is not, however, a sharp maximum at 0.2 volt as has been observed with n-parafBns (5, 7), but more of a bell shaped appearance for both of the compounds. Figure 2 is a plot of the fraction of the surface covered vs. potential as obtained after 2-minute adsorption with a cathodic galvanostatic pulse of 50 ma./cm. . These measurements indicate maximum coverage for both 2

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compounds at 0.2 and 0.3 volt. Both Figures 1 and 2 indicate that the steady state surface coverage does not differ too much for both com pounds though it must be kept i n mind that there is a much lower con centration of benzene. Figure 2 also shows steady-state coverage for 200 mm. pressure of benzene. Steady state coverage of adsorbed species for a specific compound might be expected to be independent of concen tration if long enough times of adsorption at potential are maintained, and little or no oxidation is taking place. Figure 2 indicates about 68% maximum coverage with benzene and 65% maximum coverage with cyclo hexane—numbers within experimental error of each other. Compounds of similar size and reactivity would be expected to occupy similar space on the electrode. The maximum charge to oxidize the adsorbate is slightly higher for the cyclic hydrocarbons than the n-parafBns (4, 5, 7, 8). X - BENZENE (5mm) O- CYCLOHEXANE (200mm) 1.35V

POTENTIAL (V vs. R.H.E.)

Figure 1.

Steady-state adsorption of benzene and cyclohexane as a function of potential

These results compare with 50% electrode coverage i n the region 0.3-0.5 volt vs. normal hydrogen electrode obtained by Bockris et ah (14) using radiotracer techniques for benzene adsorption i n H P 0 at 50 °C. Adsorption of benzene was not examined at 50 °C. with potentiostatic and galvanostatic techniques. Brummer and Turner (4, 8), however, have shown that the effect of temperature is small when propane is adsorbed from 8 0 ° - 1 4 0 ° C . 3

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Figure 2.

Fraction of surface of electrode covered for steady-state adsorption of benzene and cyclohexane as a function of potential

Type of Adsorbed Species Gilman ( 9 ) , Niedrach (17), and Brummer (4, 7, 8) have obtained evidence for several types of intermediates adsorbed on the electrode surface when normal saturated hydrocarbons or ethylene are reactants. Part of the adsorbate designated C H - α can be cathodically desorbed and is thought to contain only C and H . Another C H material not removed by cathodic treatment seems to be a combination of a C H polymer ( C H - β ) and a more highly oxidized material thought to contain at least one C - O bond ( O-type ). T o determine whether benzene and cyclohexane give the same type of adsorbed intermediates—specifically whether cyclo hexane dehydrogenates to benzene—the types of steady state adsorbed residues were investigated. Cathodic and anodic desorption techniques similar to those used b y Brummer (4) were employed. After the usual pretreatment, the electrode was held at a potential for 60 sec. at which point steady state coverage was obtained. Then the potential was lowered


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to 0.01 volt where little or no adsorption occurred (benzene had appre ciable adsorption at potentials higher than 0.01 volt, but not at 0.01 volt). A t 0.01 volt part of the adsorbate, C H - α , desorbs. The potential was held at 0.01 volt for various periods of time and then raised to 0.40 volt to oxidize any H adsorbed at 0.01 volt. In the time necessary to oxidize H off, there is no readsorption of organic material. The surface was then examined with a galvanostatic pulse. Q the charge required to oxidize 2

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Figure 3.

Desorption at 0.01 volt of caihodically desorbable material adsorbed at 0.40 volt

the remaining species (O-type, C H - β , and C H - α which has not yet desorbed) to C 0 and 0 r , the fraction of the surface occupied b y the remaining organic species, obtained from the cathodic stripping curve were followed as a function of time. The slope of Q vs. θ plot gives [e] the number of electrons released per covered site during oxidation of adsorbate. Changes i n [e] during the desorption of a given adsorbed species are taken as evidence for the presence of different adsorbed species ( 4 ) . Figure 3 is an example of Q vs. θ plot for cathodically desorbable material formed from cyclohexane on benzene for which steady-state coverage was obtained after adsorption for 60 sec. at 0.4 volt. 2

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Q and Θ in the figure refer to charge required to oxidize remaining species to C 0 after some C H - α has been removed and 0 r , the fraction of the surface covered by organic material after C H - α has been removed. Q and 0 are largest when potential is held at 0.01 volt for short times. As C H - α material is removed with time at 0.01 volt, these numbers decrease. For both cyclohexane and benzene, constant values of Q and θ are obtained after desorbing at 0.01 volt for five seconds; thus, all of the C H - α is desorbed. Both compounds give a slope corresponding to 5.5 electrons per covered site on the path to oxidation. A methylene group ( C H ) in cyclohexane would require six electrons to oxidize com pletely to C 0 . This result indicates that the cathodically desorbable material might be an equilibrium mixture of benzene and cyclohexane. There is a certain amount of scatter i n the plotted points; therefore, it is difficult to distinguish whether there are two separate slopes: one equal to five electrons and another equal to six. Thermodynamic equilibrium data for the gas phase dehydrogenation reaction: 2

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indicates that there would be an equimolar amount of the two compounds (18). There is substantial evidence that the C H - α material is a com posite of several species. Using a fuel cell electrode and propane as the reactant, Grubb (12) and Barger and Savitz (1 ) found that upon cathodic desorption, methane and ethane were observed with the gas chromatograph. Brummer has not examined the fine structure of C H - α for pro pane or hexane adsorption on a wire electrode. Using our experimental set-up, inconclusive results were obtained for the C H - α from propane adsorption, as the species occupied only 6% of the covered surface and C H - α had desorbed completely at 0.01 volt in 100 msec. Figure 3 also indicates a small part of the surface covered with a highly reduced species for the benzene adsorbed species. This could be a polymeric material although i n desorption experiments with propane at a fuel cell electrode, no species higher than propane was observed i n the gas chromatograph ( I ) . Figure 4 indicates that the 5.5 electron species appears to be relatively potential independent from 0.1 to 0.7 volt. A t 0.05 volt where there is most likely to be hydrogen evolved along with hydrocarbon adsorption, a lower number of electrons is obtained. The oxidation of hy drogen would account for 1 [e] and the average of the mixture would be lower. These results do not tell us whether the relative amount of the species which comprise C H - α is potential independent. Barger found the ratio of methane to ethane formed on cathodic desorption was very potential dependent ( J ) . Definite proof of whether there is an equilib-


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rium of cyclohexane and benzene and the relative amounts is currently being obtained with gas chromatographic procedures ( J ) . The value of 5.5 electrons is obtained for species which have been adsorbed at the potential of interest for 120 sec. to indicate steady state of adsorption had been obtained at 60 sec.

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Figure 4.

Electrons per covered site to oxidize residue to C0 as a function of potential 2

Ο—Cyclohexane (CH-a) X—Benzene (CH-a) Δ—Cyclohexane (Non Cathodically Desorbable) •—Benzene (Non Cathodically Desorbable) The total amount of Q C H - α is defined as Q

Anodic Oxidation of Cyclic Hydrocarbons - ACS Publications

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