Study of the Effect of Electrolyte on Electrochemical Hydrocarbon

18 Study of the Effect of Electrolyte on Electrochemical Hydrocarbon Oxidation

Downloaded by UNIV OF TENNESSEE KNOXVILLE on June 18, 2016 | http://pubs.acs.org Publication Date: June 1, 1969 | doi: 10.1021/ba-1969-0090.ch018

A R T H U R A. P I L L A , J O H N A. CHRISTOPULOS, and G A B R I E L J. D I MASI Power Sources Division, Electronic Components Laboratory, U S A E C O M , Fort Monmouth, N . J. 07703

The initial adsorption kinetics of propane on a platinum electrode

has been studied at 65°C. in 2F concentrations

of H3PO4,

H SO , 2

4

potentiodynamic techniques.

and

CF COOH

using the multipulse

3

and multi-step potentiostatic relaxation

The first method gave results concerning the

rate of propane adsorption indicating that the fastest rate occurred in

CF COOH. 3

An attempt to correlate this with

the degree of electrode-electrolyte

interaction was carried

out with the second method wherein the high capacitance of the electrode was determined.

frequency

This indicated

that the highest capacitance value shows the lowest degree of interaction and the highest propane adsorption rate. An added correlation was obtained by considering linear anodic sweeps on Pt in each acid showing that the point at which Pt surface oxidation occurs is more anodic for the acid in which propane adsorption is highest.

T

he electrochemical oxidation of hydrocarbons has received consider able attention i n recent years ( I , 2, 3, 4, 5, 6, 7, 8, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 22, 23). These studies have inspired the development of new electrochemical techniques and constitute a major application of relaxation methods to the study of reactions at solid electrodes. Major emphasis has been placed upon a study of the adsorption process involved in the anodic oxidation of these gaseous reactants. Little work has been reported concerning the effect of the nature of the acid electrolyte. A recent study (20) has indicated that the steady-state current obtained with various acids is considerably different. In addition, the effect of chloride ion (9) has been investigated, the major conclusion being that 231 Baker; Fuel Cell Systems-II Advances in Chemistry; American Chemical Society: Washington, DC, 1969.

232

F U E L

C E L L

S Y S T E M S

II

this ion blocks the most active sites otherwise available for the adsorption process. It is the purpose of the present work to investigate the rate of adsorption of hydrocarbons in acids of widely differing anion structure to determine if possible competition exists between the acid anion and the hydrocarbon molecule for adsorption at the electrode surface.


Experimental The technique chosen to study the rate of adsorption of propane at a smooth platinum electrode is a modified version of the multipulse potentiodynamic ( M P P ) technique. This was chosen i n order to compare results concerning the adsorption process given b y other workers who have used this technique and variations of it. One of the pulse sequences employed i n this work is shown i n Figure 1. This w i l l be referred to as Sequence 1. Here the electrode is main tained at 0.05 volt vs. a hydrogen electrode i n the same solution ( a l l further potentials w i l l be quoted vs. this reference) as shown i n Step A . D u r i n g Step Β the electrode is brought stepwise to 1.65 volts which, as is well known, oxidizes oxidizable impurities and forms an oxide layer at the electrode surface. The length of this step is 30 sec, during w h i c h the solution is stirred. A t Step C , the electrode is stepped back down to 0.05 volt and stirring is stopped to allow quiescent conditions to be obtained. Step C is maintained for 20 sec. Steps Β and C are pretreatment pulses which allow reproducibile surface conditions to be main tained on the platinum electrode. Step D is the study pulse during w h i c h adsorption of the hydrocarbon takes place. Its duration is from 1 to 300 sec. and the potential to w h i c h the electrode is stepped varies from 0.2 to 0.6 volt. During Step Ε a linear anodic voltage sweep of 10 volts/sec. is utilized to oxidize adsorbed hydrocarbon and/or other oxidizable species which are on the electrode surface as a result of Step D . 1.65V

.05V

.05V A

Β

c

D

30S

20S

I-300S

Ε |

Q

v

's Figure 1

Multipulse potentiodynamic pulse Sequence 1. Refer to text for details

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

18.

PILLA E T A L .

233

Effect of Electrolyte


The second pulse sequence employed i n this work is that shown i n Figure 2. This w i l l be referred to as Sequence 2. Here Steps A through D are identical to those i n Sequence 1. Step Ε is replaced b y a cathodic potential step of 1 to 5 msec, duration appropriate for an electrode of the surface area utilized i n this work. In this step the electrode is abruptly brought from the study pulse potential (0.2 to 0.6 volt) to the original rest potential of 0.05 volt. This cathodic pulse is for surface area deter mination. Sequence 2 may appropriately be termed the multistep potentiostatic relaxation method ( M S P ) . 1.65 V

.05 V

.05 V θ

30S

.05V

C 20S

D

Ε

I-300S

i-5ms

Figure 2. Multipulse potentiostatic relaxation Sequence 2. Step Ε is utilized for determination of double layer capacitance and electrode surface area The instrumentation employed i n this work is illustrated i n a block diagram shown i n Figure 3. The central point of the apparatus is the potentiostat which is the combination of a differential amplifier A , and a power amplifier PA shown enclosed i n the dotted lines. The potentiostat chosen for this work is the fast rise Tacussel M o d e l PIT-20-2A. It can conveniently accept pilot voltages of any form with rise times down to 10' sec. The pre- and study pulses are obtained from Tektronix 160 series pulse generators shown as P i , P , and P i n Figure 3. These have been especially modified to deliver pulses having up to 10 minutes duration, sufficient for this work. The function generator utilized for the linear anodic voltage sweep is the Tacussel M o d e l GSTP-2, chosen because of its versatility and convenience of use. A l l of the waveforms are mixed through a resistor adder network which is immediately followed b y diode D , Figure 3. This diode is utilized to clip the voltage obtained from any given waveform at +1.65 volts (near Ô evolution). This is merely a convenience feature to ensure that the slope of the linear anodic voltage sweep remains constant independent of the level from which the sweep initiates ehminating tedious adjustment of the function generator at each potential of the adsorption step (Step D , Figures 1 and 2 ) . Battery, Β (Figure 3) provides appropriate d.c. offset. 7

2

3

2



234

F U E L C E L L SYSTEMS

II

Figure 3. Block diagram of the instrumentation employed in this work. Of special interest is the possibility of simultaneously obtaining I(t) and Q(t). Refer to text for details The observed function for Sequence 1 is the current response to the linear anodic voltage sweep. For Sequence 2 the current response to the cathodic pulse is observed. Both the current, 1(f), and its integral, Q(t), may be simultaneously obtained. In order to perform the integration either the Tektronix Type 3A8 or Burr Brown M o d e l 1555 operational amplifiers ( A and A , Figure 3) are employed. Amplifier A is used to obtain the differential signal necessary for current measurement i n potentiostatic operation. The output of A is applied simultaneously to the gated integrator amplifier, A , and the input of the oscilloscope pre amplifier. A Tektronix Model 556 dual beam, dual time base oscilloscope is utilized for the observation of all transient signals. A type W plug-in preamplifier is employed for observation of the voltage signal. The type 1A4 plug-in unit which allows obtention of up to four chopped signals is utilized for the observation of I(t) and Q(t). A n example of the type of response which may be observed during a linear anodic sweep is shown in Figure 4. 2

3

2

2

3

The experimental cell consists of a single cylindrical borosilicate glass compartment (200 ml. capacity) with provision for passing gas through or over the electrolyte. The cell contains a central platinum wire working electrode of approximately, 0.1 c m . geometric area; an adjacent anodized tantalum reference, both encased in shrinkable Teflon tubing and an enclosing cylindrical platinum counter electrode (1^ inches X 2 inches, 52 mesh). A calomel reference is also introduced with a Luggin capillary for potential monitoring. 2

The acids employed in this work are 2F H S 0 , 2F H P 0 , and 2F C F C 0 H . A l l solutions are made with triply distilled water. Extensive purification is carried out, using constant potential preelectrolysis at large surface platinum electrodes (1.65 volts between electrodes), for at least 16 hours at 65 °C. prior to the experiment. The purified electrolyte is transferred to the study cell in situ to avoid contact with the atmosphere. 2

3

4

3

4

2


18.

PILLA E T A L .

235



H e l i u m is utilized to provide an inert atmosphere. This is passed through a gas train to eliminate oxygen (copper at 800°C.) and especially organic materials ( 1 3 χ molecular sieve at — 7 8 ° C ) . In addition, a l l gases are bubbled through a solution identical to that i n the study cell, prior to passage through this to maintain constant electrolyte composition. Both the temperature of the study cell and prebubbler are at 65 °C. Research grade propane (Phillips, 99.97% pure) is utilized for the present study.

Figure 4. Photo illustrating an actual oscillo scope sweep of the applied voltage signal (1 ), the current response (2) and its integral (3). The horizontal axis is 20 msec./div. and the vertical axis is 500 mv./div. for (1), 20 ma./div. for (2) and 100 mv./div. for (3) Results and

Discussion

The principal result sought for in this work is the rate at which propane adsorbs at a platinum electrode and this dependence upon the nature of the electrolyte utilized. In order to obtain this quantity the amount of electricity required completely to strip the electrode of oxidiz able material is experimentally determined. This is done by subjecting the electrode to Sequence 1 i n the presence, first of helium and then propane. The quantity of electricity obtained with helium, Q is sub tracted from that i n the presence of propane Q to obtain the desired result AQ which is proportional to the quantity of propane adsorbed at the electrode surface. Note that all blanks—i.e., all Que values, are obtained under exactly the same conditions as those with propane. Thus, Sequence 1 is applied with Step D of identical duration for both Q and Qp. This was done to ensure that all of the electricity utilized for propane stripping is actually obtained free from that which may be re quired for the oxidation of, for example, difficult-to-remove impurities. The results obtained using Ç and Qp to evaluate AQ indicates that this quantity is widely different for each of the acids. Thus, the H e

P

H e

H e


236


Π

initial rate of adsorption of the hydrocarbon on the surface of the plati num electrode is much greater i n the case of C F C 0 H than either H S 0 or H3PO4. This is shown i n Figure 5. A possible explanation for this may be given if the effect of the degree of interaction of the Pt electrode w i t h the various electrolytes upon hydrocarbon adsorption is considered. Inspection of sweep data from any potential utilized i n Step D for each of the acids indicates that the onset of platinum surface oxidation occurs at higher anodic potentials for C F C Q H , than either H S 0 or H P 0 . 3


3

0.16

2

2

2

2

4

4

3

4

n

0.12 -

° E 0.08 -

0.04 -

60

120

180

240

300

t (sec)

Figure 5.

Variation of 0(AQ) with time for the acids employed in this work

This is illustrated i n Figure 6, wherein the current response to a voltage sweep for each acid is given. The results shown appear to indicate that it is relatively more difficult to oxidize the platinum surface i n C F C 0 H or that the "reduced" state of the electrode is more stable. If the degree of positive charge character at the electrode surface plays a role i n hydro carbon adsorption and oxidation, then it would appear that the environ ment created i n the double layer region by C F C 0 H leaves more positive charge character—i.e., "unoxidized" surface states—available for hydrocarbon adsorption and that the hydrocarbon effectively competes with the electrolyte to occupy these surface states. T o further check this hypothesis a determination of the high frequency capacitance of the 3

3

2


2

18.

PILLA ET AL.

237



electrode is carried out. Thus, if it is considered that the electrolyte which interacts the least with Pt is that which allows a more rapid rate of hydrocarbon adsorption then the capacitance measured at times short enough to detect this interaction should provide more evidence for the proposed correlation.

CF C0 H 3

2

/ Figure 6. Illustration of the response to an anodic voltage sweep initiating from +0.2 volt for each of the studied acids. The horizontal axis is 20 msec./div. and the vertical axis is 20 ma./div. The high frequency capacitance is measured using the multistep potentiostatic relaxation method (Sequence 2) outlined above, both i n the presence and absence of propane. The actual procedure used is to pretreat the electrode (Steps A through C ) and upon the application of the study pulse ( Step D ) simultaneously apply a cathodic pulse ( Step Ε ). During this pulse the following phenomena occur: the double layer is charged and the interaction is modified. A t times of the order of the microsecond the majority of observed current is owing to double layer charging and the interaction modification. If double layer charging can be isolated, then this may serve as an indication of the degree of electrodeelectrolyte interaction. This is so since it may be expected that the greater the interaction the more electrode surface sites would be occupied to some extent resulting i n less total current required to charge the double layer. This results i n a higher double layer capacitance i n the case of least interaction. The above analysis may be made if it is considered that two parallel current paths are available upon the application of a potential step. It has been shown (21) that for any given electrochemical system there is a time (or frequency) range i n which the majority of current is used for double layer charging. If this condition is met, then the high frequency (or short time) equivalent circuit for the electrode is simply a resistor,


238


II

R , i n series with the double layer capacitance, C . T o obtain values for R and C the following differential equation must be solved: d

e

e

d

/( is very small compared with the Faradaic current (see Table I) then Q is directly proportional to the available electrode surface area. This quantity has been evaluated as a function of time for each acid at various potentials in the presence of propane. A n example of this variation is shown i n Figure 8. It may be seen that there is greater change in surface area per unit time for C F C 0 H than for either H S 0 or H P 0 . This follows directly from the results obtained using Sequence 1 for propane and further supports them. In addition, it may be seen that the surface area available for hydrogen deposition is different for each of the acids. This lends support to the supposition that either there are more sites or that the sites are more active depending upon the acid used. C

C

C

3

2

2

Summary and

4

3

4

Conclusion

In this work the relative rates of adsorption of propane on a platinum electrode i n C F C 0 H , H S 0 , and H P 0 have been studied. It was 3

2

2

4

3

4



240

FUEL CELL

SYSTEMS—Π

300

Figure 8. Variation of the electrode surface area with respect to avaihble sites for hydrogen deposi tion as a function of hydrocarbon adsorption time (step D) shown that steady-state conditions with respect to AQ are more rapidly attained i n C F C 0 H than for either of the other two acids. A possible explanation for this effect is proposed by considering the degree of inter action of the Pt electrode with a given acid electrolyte. The preliminary nature of this work does not allow the necessarily fine structure of this effect to be observed. Further work with emphasis upon double layer capacitance measurements w i l l , it is hoped, shed more light upon this problem. The general trend, however, of the relation of the "reduced" character of the electrode surface taking into account potential effects, to the adsorption rate of hydrocarbons has been demonstrated. The model is a simple one neglecting, for the first approximation, the possibility of specific adsorption of the respective acid anion at the potentials of interest in this study. It would appear that this effect would be more detrimental than beneficial to hydrocarbon adsorption if the positive charge character of the electrode surface is indeed an important factor i n this process. 3

2


18.

PILLA E T A L .


241

Acknowledgments The authors wish to thank D . Williams and J. Peck for their invalu able contributions to this work.


Literature

Cited

(1) Binder, H . , Kohling, Α., Sandstede, G., A D V A N . C H E M . SER. 47, 283 (1965). (2) Bockris, J. O'M., Wroblowa, H., Gileadi, E., Piersma, B. J., Trans. Fara day Soc. 515, 2531 (1965). (3) Bonnemay, M . , Bronoel, G., Levart, E., Pilla, Α. Α., "Hydrocarbon Fuel Cell Technology," p. 395, B. S. Baker, Ed., Academic Press, New York, Ν. Y., 1965. (4) Bonnemay, M . , Bronoel, G . , Levart, E., Pilla, Α. Α., Proc. 17th CITCE Meeting, Tokyo (1966). (5) Bravocos, J., B. Ch. Ε. Thesis, Paris (1966). (6) Brummer, S. B., Makrides, A. C., J. Phys. Chem. 68, 1448 (1964). (7) Brummer, S. B., J. Phys. Chem. 69, 562 (1965). (8) Ibid., 69, 1355 (1965). (9) Brummer, S. B., Turner, M . J., "Hydrocarbon Fuel Cell Technology," p. 409, B. S. Baker, Ed., Academic Press, New York, Ν. Y., 1965. (10) Gilman, S., Trans. Faraday Soc. 61, 2546 (1965). (11) Ibid., 61, 2561 (1965). (12) Ibid., 62,466 (1966). (13) Ibid., 62, 481 (1966). (14) Giner, J., Electrochim. Acta 8, 857 (1963). (15) Ibid., 9, 63 (1964). (16) Niedrach, L . W . , Gilman, S., Wienstock, I., J. Electrochem. Soc. 112, 1161 (1965). (17) Niedrach, L . W . , "Hydrocarbon Fuel Cell Technology," p. 377, B. S. Baker, Ed., Academic Press, New York, Ν. Y., 1965. (18) Niedrach, L . W., J. Electrochem. Soc. 113, 645 (1966). (19) Niedrach, L. W., Tochner, M., J. Electrochem. Soc. 114, 17 (1967). (20) Ibid., 114, 233 (1967). (21) Pilla, Α. Α., 6th Fuel Cell Status Rept., Power Sources Division, ECL, ECOM (1967). (22) Shropshire, J. Α., Electrochim. Acta 12, 253 (1967). (23) Shropshire, J. Α., Horowitz, Η. H., J. Electrochem. Soc. 113, 490 (1966). RECEIVED February 26,

1968.


Study of the Effect of Electrolyte on Electrochemical Hydrocarbon

Recommend Documents