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Hydrogen adsorption rate and discharge mechanism on palladium-carbon suspension electrode
Electroanalytical Chemistry and Interracial Electrochemistry, 44 (1973) 53-62
53
© ElsevierSequoia S.A., Lausanne - Printed in The Netherlands
H Y D R O G E N A D S O R P T I O N RATE A N D D I S C H A R G E M E C H A N I S M O N PALLADIUM-CARBON SUSPENSION ELECTRODE
E. KEREN* and A. SOFFER** Department of Chemistry, Nuclear Research Center-Negev, P.O.B. 9001, Beer-Sheva (Israel)
(Received 1st September 1972; in revised form 22nd November 1972)
INTRODUCTION The suspension electrode is a system which may have the unique property of separating adsorption and charge transfer of the electrode process into two stages which can be optimized separately: the adsorption and the non-charge transfer reactions of the active species are carried on rapidly within the whole volume of a stirred solution, while the charge transfer occurs on a collector electrode upon which the stirred particles are impinging. The performance of such an arrangement may be evaluated by comparison with the porous electrode which is highly efficient for low overvoltage reactions. The comparison of these two electrodes is particularly interesting in the case of the gas-discharge reaction. Here the three processes of gas dissolution, adsorption and charge transfer occur altogether within the three-phase contact region of the porous electrode, but are completely separated to three different interfaces in the case of the stirred suspension electrode. Moreover, mass transfer problems resulting from slow diffusion processes are greatly diminished for the second type of electrode, where the faster convection mass transfer prevails. It is, however, not obvious that the suspension electrode is simpler for quantitative treatment since new parameters which are not easily resolved are introduced 1. These are mainly the time of contact of the particle with the electrode, the collision frequency and the contact resistance. Suspension electrodes were not used in electrochemical research until the last twenty years, when charging curves of a stirred powder were obtained 2'3, and successful electrolytic oxidations of hydrogen and other fuels on suspended catalysts by means of an auxiliary inert electrode were performed 4-6. Held and Gerischer made a detailed study of the charge transfer mechanism between the particles and the collector electrode, and found linear relations between current and overpotential. F r o m oscillographic measurements of single collisions of nickel particles they found that the average collision time was ,,~ 10 -4 s, and only 1-2~o of the available charge is transferred. About 5" 10-2 s were needed for the relaxation of the total charge, measured by letting the particle adhere to the electrode without stirring. F r o m these observations they deduced that the current delivered by the suspension electrode is * Present address: Department of Chemical Physics, Weizmann Institute of Science,Rehovot, Israel. ** To whom enquiries may be addressed.
54
E. KEREN, A. SOFFEN
controlled by ohmic contact resistance. Lazorenko-Manevich and Ushakov 8 tried to fit their experimental data on nickel suspension electrodes to kinetic equations and got reasonable values for the exchange current of hydrogen on nickel. By means of a probe electrode situated near the collector, they found that the particles acquire the collector potential during the contact, from which they deduced that the particle actually transfers all its available charge in a single impact without any ohmic hindrance. The two orders of magnitude discrepancy from Gerischer's results was attributed to different hydrodynamic conditions which favoured a substantially larger contact time. However, no measurement of contact time were reported in ref. 8. In the present work we propose a mechanism for the charge transfer which explains the experimental results and settles the apparent disagreement between the above two measurements. An important theoretical work on contact properties of a particle with the collector electrode was published ~. Unlike refs. 6 to 8, clear discrimination was made between double layer capacity and pseudocapacity and a general solution of the problem of charge transfer was given, in which the contact resistance as a possible rate-determining step was considered. This work, however, is subject to some criticism. (a) The (ionic) resistance of the solution surrounding the impinging particle is not accounted for. The role of this resistance cannot be neglected since due to the electroneutrality condition, an amount of ionic charge must be transferred from the particle interface to the bulk of the solution, to compensate for the electronic charge transfer to the collector electrode; (b) The optimal effective contact distance of two water molecules between particle and collector surface seems arbitrary, while the proposed tunnelling of electrons is highly sensitive to such a distance. (c) The hydrodynamic calculation of contact time and collision frequency takes into account only bulk viscous forces and neglects the role of electrostatic forces between the double layers of the particle and the collector electrode 15. These forces should be operative within the small distances of two diameters of a water molecule. EXPERIMENTAL
The four-electrode cell design, preparation of the carbon-palladium and the electronic setup are described elsewhere 13. The main features of the electrolytic cell were a hermetically sealed, variable-speed mechanical stirrer and a closed gas system which enabled volumetric determination of the hydrogen adsorption on the suspension electrode. The electrode potential was measured against a reversible hydrogen electrode dipped into the working solution. The suspension (in 50 ml 2 M H2SO4, at room temperature) was initially stirred with oxygen and thence acquired, within a few minutes, a potential of 620-650 mV, which is nearly the oxidation potential of Pd in the same solution. This was always taken as the starting point prior to hydrogen adgorption. When the gaseous phase above the slurry was changed to hydrogen, without stirring, no detectable hydrogen-adsorption potential shift was found during several minutes. Fast hydrogen uptake started immediately after stirring was started and this instant was taken as the zero time. The minimal stirring speed which was taken into consideration was such that gas bubbles produced from the bottom of the vortex were distributed throughout the solution.
H O N Pdq2 S U S P E N S I O N E L E C T R O D E
55
RESULTS A N D D I S C U S S I O N
The rate of hydrogen adsorption The main purpose of these experiments was to measure separately the rate of the three consecutive processes mentioned in the introduction and to detect which process is the rate determining one. Fig. 1 shows that the rate of adsorption of hydrogen increases continuously with increased stirring speed. This indicates that, at the slurry concentration specified, either the dissolution of hydrogen or its diffusion towards the electrode particle and not the chemical adsorption step is rate-determining. It is also reasonable that the carbon-palladium powder contains an appreciable relative quantity of pure carbon particles which cannot adsorb hydrogen without making electrical contact with palladium particles 13. The frequency of collisions between the particles may then be rate determining and is obviously dependent on stirring speed.
0.1S
>~ 0.I0
0.05¸
aodo
2o6o
3obo
~obo
r e v rain-'
sobo
Fig. 1. Rate of hydrogen adsorption vs. stirring speed; . . . . . O O O O , 500 rag, in 50 cm 3 of 2 M H2SO 4.
50 mg C + I ~
Pd; × × x x ×, 250 mg;
In Fig. 2 we can see, however, that at a medium stirring velocity, the rate of hydrogen adsorption becomes essentially independent of the amount of powder electrode when this exceeds 0.1 g. Consequently, the hydrogen dissolution is the ratedetermining step and not the particle-particle collision frequency, nor the diffusion towards the particle. This is because the latter two steps are enhanced on increasing the amount and concentration of the slurry. This kinetic behaviour is also shown in Fig. 3 in a somewhat different fashion, from which the maximal adsorption rate is obtained, and is equivalent to 14.8 A g-1.
56
"i"
E. KEREN, A, SOFFEN
0.2
Cn
7
i,n
0.1
E
~.~ 0.05
0.02
0.01
0.01 o.o2
0.0~
0.1
0.2
0.~
m
2 w/g
~
zo
Fig. 2. Rate of hydrogen adsorption vs. amount of i% Pd/C in 50 cm 3 of 2 M H2SO 4. Stirring speed is 3300_+300 r.p.m.
2.
5"trl (3.27m Q'!
o~
E o.~j
°
'~
o
o
"~"~ 0.05
oo21 o.m I
0.01
,
092
,
,
,
,
i
,
0.05
0.1
0.2
0.5
1
,
,
2 w/g 5
°i
10
Fig. 3. Rate of hydrogen adsorption per unit suspension mass in 50 cm 3 of 2 M H2SO 4 at 3300_+ 300 r.p.m, stirring speed.
Polarization curves
After the adsorption step had been studied separately, the current-potential curve of the hydrogen-saturated suspension was obtained. This is given in Fig. 4 for different concentrations of the powder electrode. We attribute the peculiar currents which appear to be too high at low concentrations and overvoltages to catalytic powder constantly adhering to the collector electrode. This was illustrated by dipping the collector electrode into a stirred suspension, tranferring it to a pure, hydrogen-saturated 2 M H 2 S O 4 solution and measuring three subsequent polarization curves (Fig. 5). These curves became lower as time elapsed, indicating that the adhering powder was washed out from the electrode surface. We suppose that the adhering powder is continuously replenished in the suspension electrode system. The dotted line in Fig. 5 is the polarization curve of a clean collector electrode in pure H 2 S O 4 solution. One can easily observe that the current contributed in pure H 2 S O 4 solution. One can easily observe that the current contributed by the adhered powder is small and distorts the polarization curves of Fig. 4 only at low powder concentrations and at low overpotentials.
57
H O N Pd~C S U S P E N S I O N E L E C T R O D E
15
80"
4-
40"
220-
'
o
o:s
E/v
Fig. 4, Polarization curves for 5% Pd/C suspension electrode in 50 Cltl3 of 2 M H2SO 4. The n u m b e r s adjacent to curves indicate the a m o u n t of powder. Stirring speed is 3300 r.p.m. Fig. 5. Three subsequent polarization curves for 1% P d / C powder adhered to the collector electrodes. T h e d o t t e d curve stands for a clean collector electrode.
At exceedingly lower powder concentrations the major current may be that of the adhered powder, resulting in polarization curves which are independent of concentration. This should explain the behaviour obtained by Schwabe and Stasko t4. The current v e r s u s suspension concentration curves at different overvoltages are plotted in Fig. 6 and show linear dependence at sufficiently high currents. The too high currents at low concentrations are due to adhered particles, as mentioned above. From Fig. 6 one can easily deduce that the discharge currents are 3 to 4 orders of magnitude smaller than the equivalent current of 14.8 A which corresponds to the rate of hydrogen adsorption. Consequently, the rate-determining step in the overall process of hydrogen discharge on powder electrodes is the charge transfer from the particle to the collector electrode. We had not investigated the mechanism of this charge transfer experimentally. We shall, however, discuss this subject referring to some other works. In order to explain the contradictory results of refs. 6 and 8 we suggest a model which is valid for any reaction mechanism provided it has subsequent adsorption and ionization steps, such as the Volmer-Tafel mechanism9. An equivalent circuit for one particle of the suspension electrode is illustrated in Fig. 7. R c is the contact (electronic) resistance between a particle and the collector electrode, a n d / ~ is the (ionic) resistance of the solution surrounding the particle,
58
E. KEREN, A. SOFFEN
15
w/u Fig. 6. Polarization currents v s . amount of 1% Pd/C suspension in 50 c m 3 0 [ 2 M HzSO4 at constant overvoltages. The lower curve is at overvoltage of + 100 mV, increasing by 100 mV for each higher curve. S~.
R
R
-J-
R
-v
.S I
m
It8
Fig. 7. The equivalent electrical circuit for a metal catalyst particle colliding with a collector electrode. as mentioned in the introduction. R i and R a are, respectively, the impedances of the ionization and adsorption of the hydrogen molecules. Ca1 is an integral double layer capacitance per unit surface area and Cp is the reaction pseudocapacity. V is the Voltage between the collector electrode and the solution, fixed at will by the potentiostat. E represents the reservoir of hydrogen molecules in the solution (which are supplied by bubbling hydrogen through the suspension) and is related to the hydrogen gas pressure by the Nernst equation. The last two quantities are not measurable, but their difference, ~/= V - E is the measured overpotentiaL Usually,. the particle examined is not in contact with the collector. Therefore Sz is closed and S 1 is open except for periodic short contacts which represent the collisions of the particle with the collector. Relying on the experimental results of
H ON Pd~C SUSPENSION ELECTRODE
59
ref. 8, our main assumption is that the contact and solution resistances are small and, since Cd~ is two orders of magnitudes smaller than Cp9, we may write:
(Rs + Re)Cal'~ RiCp which means that the relaxation of the double layer is much faster than the chemical reaction. During the short contact Ca~ is discharged until the voltage between its plates equals V, thence the charge transferred in one collision is Ca~rl. The ionization reaction is too slow (Ri too large) for any appreciable charge to be transferred across R~ during the collision, so the voltage on Cp remains E. If the time between collisions is long enough, Ca1 is recharged through R~ and also through R a by adsorption of hydrogen molecules, so the cycle starts again. Provided the ionization reaction has not advanced to a considerable extent until the particles hit the probe, this electrode would show the potential of the adjacent collector electrode, a result which was in fact obtained. Thus the current delivered by the suspension electrode can be written in the form: i = f'S'Cdlt
I
where f is the average frequency of collisions and S is the average surface area of one powder particle. Because of the statistical nature of this equation the measured current fluctuates around an average value. Assuming constant integral double layer capacity, (which is reasonable because of the high concentration of electrolyte solutions of the order of 1 M 7' st, and constant f, the average current is proportional to the overvoltage, which is the usual experimental resqlt of Held and Gerischer 7. These authors also measured the total charge stored on a particle by stopping the stirrer and letting the particle adhere to the collector. This case is realized in our model when S x is closed indefinitely while S 2 is open since, after stirring is interrupted, the hydrogen supply is cut off. Now both capacitors are discharged to the voltage V and the charge is (C a ---I-Cdl)t/. The value of Cp for hydrogen on nickel is 840/~F cm-2 while Cdl can be roughly approximated 9 to 15 #F cm-:. We can thus find that the ratio of the charge transferred during a single collision to the total charge is:
Cdl/(Cp + Cal) = 0.019 which lies within the experimental results of Gerischer 7. The conclusion is that the experimental results of both Gerischer et al. and Lazorenko-Manevich and Ushakov 8 are explicable by one single mechanism, namely the particle does acquire the potential of the collector electrode in a single impact and transfers only about 2~o (the double-layer fraction) of its total charge. The apparent contradiction arises mainly because the difference between double-layer capacity and pseudocapacity had not been considered. The capacitative quantity appearing in the kinetic equations quoted in ref. 8 is, in fact, the double-layer capacity 10. The behaviour of the potential and the hydrogen surface coverage 0 during a discharge-regeneration cycle of a particle is schematically shown in Fig. 8. The line between the points 1 and 4 represents the steady state variation of 0, the adsorbed hydrogen atoms with the potential ~b. The linear behaviour corresponds
60
E. KEREN, A. SOFFEN I
I I 4
w l
3
a
I ,
0
21
o
~
~
~e
Fig. 8. The changes in hydrogen surface coverage and potential of a metal catalyst particle during a collision-regeneration cycle.
to an ideal Temkin isotherm ~a. The cycle begins at point 1 where the particle is at equilibrium (0= 1, q~--0 versus hydrogen electrode in the same solution), touches the collector electrode at potential q, discharges the double layer during a time (Rs + Re)Ca] (Fig. 7), and acquires the potential ~b= q. The contact time is short, hence 0 remains practically constant. At point 2 the particle leaves the electrode and the recharging of the double layer begins. From the equation: Haas+OH- ~ H 2 0 + e we see that both ~b and 0 decrease until the particle arrives at the steady-state curve at point 3. This step is represented in Fig. 7 by the charging of Ca~ by Cp via Ri. At point 3, 0 increases again as a result of adsorption (namely charging Cp by E via Ra as in Fig. 7), until it reaches equilibrium at point 1 and the cycle starts again. We assumed here that the ionization reaction is much slower than the relaxation of the double layer, and yet much faster than the adsorption. If these three steps are closer on their time scale, the triangular shape of the diagram will be distorted, but the general behaviour will still persist. If the time between successive collisions is not sufficiently long, the particle will not arrive at the electrode at point 1, but at the intermediate potential point 1', namely at a potential between 0 and t/. The cycle is now described by the triangle (1'; 2'; 3') and the charge transferred during a collision is smaller than Cdlq. This causes a deviation from linear i-r/relations, observed at high overpotentials and fast stirring 11. In the extreme cases a limiting current appears and is attributed to the rate of hydrogen adsorption on the powder 12 (when the limiting current is proportional to the concentration of the suspension), or to the rate of dissolution of hydrogen in the electrolyte (when it is independent of concentration). All these considerations can be put on a more quantitative basis by comparing the different time parameters appearing in the theory (Re + Rs) Cd~, RiCp, time of collision and time between collisions. CONCLUSION
1. According to the model proposed above, the particle double layer is
H ON Pd~C SUSPENSION ELECTRODE
61
discharged during a collision and only a small part of the available charge, that stored on the double layer capacitance, is delivered to the collector. Nevertheless, the particle has the collector potential on leaving it. Therefore the property determining the current delivered by one particle is the double-layer capacity of the powder, and not the contact resistance as Gerischer proposed 7. Consequently, when seeking a good catalyst for suspension electrodes one should look for the highest possible surface area rather than the best catalytic activity of the powder. 2. After having the best electrode powder, the collision frequency would still be the rate-determining step since the current is several orders of magnitude lower than the rate of adsorption. Therefore, one must increase the suspension concentration until a rather thick mass is obtained where the continuity of electrical contacts b e t w e e n the particles should play a new and important role. This situation, however, leads us to a sort of agitated porous electrode structure into which the threephase contact is tri-dimensional, hence more effective (per unit mass of electrode material) than the two dimensional one typical of solid porous electrodes. SUMMARY The activity of a palladium--carbon suspension electrode in hydrogen gas electro-oxidation was investigated in 2 M H2SO 4 solution. The main feature of the work was the attempt to separate experimentally the various kinetic steps. Thus the rate of hydrogen adsorption was studied by connecting the electrochemical cell to a volumetric system, while a four-electrode system was used for current-potential .relationships. It was possible to show that the rate of adsorption of hydrogen is determined by the dissolution of gas if the powder suspension concentration is higher than 0.5 weight percent. The charge transfer was 3 to 4 orders of magnitude lower than the rate of adsorption, indicating that the average collision frequency and the contact time of the particles with the collector electrode was the slowest step. A critical analysis of these two parameters is given, relying mainly on some other works. This showed that the double layer charge is completely transferred to the collector electrode during the short time of collision. Hence, electrode particles of high surface area, such as catalyst-impregnated carbon black, are preferable. It is concluded that in order to accelerate the slowest step of charge transfer, an extremely thick suspension electrode had to be used. This approximates the above electrode closely to the well-known porous electrode. REFERENCES 1 E. U. Muller and K. Schwabe, Kolloid Z., 52 (1930) 163. 2 Y. A. Podviazkin and A. I. Schlygin, Zh: Fiz. Khim., 31 (1957) 1305. 3 D. V. Sokolskii and Y. A. Skopin, Dokl. Akad. Nauk. SSSR., 126(2) (1959) 334. 4 P. Boutry, O. Bloch and J. C. Balaceanu, Compt. Rend., 254 (1962) 2583. 5 A. B. Fasman, D. V. Sokolskii and K. A. Shurov, Dokl. Phys. Chem. Sect., 153 (1963) 1030. 6 H. Gerischer, Ber. Bunsenges. Phys. Chem., 67 (1963) 164. 7 J. Held and H. Gerischer, Ber. Bunsenges. Phys. Chem., 67 (1963) 921. 8 R. M. Lazorenko-Manevich and A. V. Ushakov, Dokl. Phys. Chem. Sect., 161 (1965) 201. 9 M. A. Devanathan and M. Selvaratnam, Trans. Faraday Soc., 56 (1960) 1820.
62 10 11 12 13 14
E. KEREN, A. SOFFEN P. C. Milner, J. Electrochem. Soc., 107 (1960) 343. E. Gileadi (Editor), Electrosorption, Plenum Press, 1967, pp. 11, 15. J. Honz and M. Knize, Coll. Czech. Chem. Commun., 32 (1967) 2540. E. Keren and A. Softer, to be published. K. Schwabe and A. Stasko, J. Electroanal. Chem., 11 (1966) 308.
53
© ElsevierSequoia S.A., Lausanne - Printed in The Netherlands
H Y D R O G E N A D S O R P T I O N RATE A N D D I S C H A R G E M E C H A N I S M O N PALLADIUM-CARBON SUSPENSION ELECTRODE
E. KEREN* and A. SOFFER** Department of Chemistry, Nuclear Research Center-Negev, P.O.B. 9001, Beer-Sheva (Israel)
(Received 1st September 1972; in revised form 22nd November 1972)
INTRODUCTION The suspension electrode is a system which may have the unique property of separating adsorption and charge transfer of the electrode process into two stages which can be optimized separately: the adsorption and the non-charge transfer reactions of the active species are carried on rapidly within the whole volume of a stirred solution, while the charge transfer occurs on a collector electrode upon which the stirred particles are impinging. The performance of such an arrangement may be evaluated by comparison with the porous electrode which is highly efficient for low overvoltage reactions. The comparison of these two electrodes is particularly interesting in the case of the gas-discharge reaction. Here the three processes of gas dissolution, adsorption and charge transfer occur altogether within the three-phase contact region of the porous electrode, but are completely separated to three different interfaces in the case of the stirred suspension electrode. Moreover, mass transfer problems resulting from slow diffusion processes are greatly diminished for the second type of electrode, where the faster convection mass transfer prevails. It is, however, not obvious that the suspension electrode is simpler for quantitative treatment since new parameters which are not easily resolved are introduced 1. These are mainly the time of contact of the particle with the electrode, the collision frequency and the contact resistance. Suspension electrodes were not used in electrochemical research until the last twenty years, when charging curves of a stirred powder were obtained 2'3, and successful electrolytic oxidations of hydrogen and other fuels on suspended catalysts by means of an auxiliary inert electrode were performed 4-6. Held and Gerischer made a detailed study of the charge transfer mechanism between the particles and the collector electrode, and found linear relations between current and overpotential. F r o m oscillographic measurements of single collisions of nickel particles they found that the average collision time was ,,~ 10 -4 s, and only 1-2~o of the available charge is transferred. About 5" 10-2 s were needed for the relaxation of the total charge, measured by letting the particle adhere to the electrode without stirring. F r o m these observations they deduced that the current delivered by the suspension electrode is * Present address: Department of Chemical Physics, Weizmann Institute of Science,Rehovot, Israel. ** To whom enquiries may be addressed.
54
E. KEREN, A. SOFFEN
controlled by ohmic contact resistance. Lazorenko-Manevich and Ushakov 8 tried to fit their experimental data on nickel suspension electrodes to kinetic equations and got reasonable values for the exchange current of hydrogen on nickel. By means of a probe electrode situated near the collector, they found that the particles acquire the collector potential during the contact, from which they deduced that the particle actually transfers all its available charge in a single impact without any ohmic hindrance. The two orders of magnitude discrepancy from Gerischer's results was attributed to different hydrodynamic conditions which favoured a substantially larger contact time. However, no measurement of contact time were reported in ref. 8. In the present work we propose a mechanism for the charge transfer which explains the experimental results and settles the apparent disagreement between the above two measurements. An important theoretical work on contact properties of a particle with the collector electrode was published ~. Unlike refs. 6 to 8, clear discrimination was made between double layer capacity and pseudocapacity and a general solution of the problem of charge transfer was given, in which the contact resistance as a possible rate-determining step was considered. This work, however, is subject to some criticism. (a) The (ionic) resistance of the solution surrounding the impinging particle is not accounted for. The role of this resistance cannot be neglected since due to the electroneutrality condition, an amount of ionic charge must be transferred from the particle interface to the bulk of the solution, to compensate for the electronic charge transfer to the collector electrode; (b) The optimal effective contact distance of two water molecules between particle and collector surface seems arbitrary, while the proposed tunnelling of electrons is highly sensitive to such a distance. (c) The hydrodynamic calculation of contact time and collision frequency takes into account only bulk viscous forces and neglects the role of electrostatic forces between the double layers of the particle and the collector electrode 15. These forces should be operative within the small distances of two diameters of a water molecule. EXPERIMENTAL
The four-electrode cell design, preparation of the carbon-palladium and the electronic setup are described elsewhere 13. The main features of the electrolytic cell were a hermetically sealed, variable-speed mechanical stirrer and a closed gas system which enabled volumetric determination of the hydrogen adsorption on the suspension electrode. The electrode potential was measured against a reversible hydrogen electrode dipped into the working solution. The suspension (in 50 ml 2 M H2SO4, at room temperature) was initially stirred with oxygen and thence acquired, within a few minutes, a potential of 620-650 mV, which is nearly the oxidation potential of Pd in the same solution. This was always taken as the starting point prior to hydrogen adgorption. When the gaseous phase above the slurry was changed to hydrogen, without stirring, no detectable hydrogen-adsorption potential shift was found during several minutes. Fast hydrogen uptake started immediately after stirring was started and this instant was taken as the zero time. The minimal stirring speed which was taken into consideration was such that gas bubbles produced from the bottom of the vortex were distributed throughout the solution.
H O N Pdq2 S U S P E N S I O N E L E C T R O D E
55
RESULTS A N D D I S C U S S I O N
The rate of hydrogen adsorption The main purpose of these experiments was to measure separately the rate of the three consecutive processes mentioned in the introduction and to detect which process is the rate determining one. Fig. 1 shows that the rate of adsorption of hydrogen increases continuously with increased stirring speed. This indicates that, at the slurry concentration specified, either the dissolution of hydrogen or its diffusion towards the electrode particle and not the chemical adsorption step is rate-determining. It is also reasonable that the carbon-palladium powder contains an appreciable relative quantity of pure carbon particles which cannot adsorb hydrogen without making electrical contact with palladium particles 13. The frequency of collisions between the particles may then be rate determining and is obviously dependent on stirring speed.
0.1S
>~ 0.I0
0.05¸
aodo
2o6o
3obo
~obo
r e v rain-'
sobo
Fig. 1. Rate of hydrogen adsorption vs. stirring speed; . . . . . O O O O , 500 rag, in 50 cm 3 of 2 M H2SO 4.
50 mg C + I ~
Pd; × × x x ×, 250 mg;
In Fig. 2 we can see, however, that at a medium stirring velocity, the rate of hydrogen adsorption becomes essentially independent of the amount of powder electrode when this exceeds 0.1 g. Consequently, the hydrogen dissolution is the ratedetermining step and not the particle-particle collision frequency, nor the diffusion towards the particle. This is because the latter two steps are enhanced on increasing the amount and concentration of the slurry. This kinetic behaviour is also shown in Fig. 3 in a somewhat different fashion, from which the maximal adsorption rate is obtained, and is equivalent to 14.8 A g-1.
56
"i"
E. KEREN, A, SOFFEN
0.2
Cn
7
i,n
0.1
E
~.~ 0.05
0.02
0.01
0.01 o.o2
0.0~
0.1
0.2
0.~
m
2 w/g
~
zo
Fig. 2. Rate of hydrogen adsorption vs. amount of i% Pd/C in 50 cm 3 of 2 M H2SO 4. Stirring speed is 3300_+300 r.p.m.
2.
5"trl (3.27m Q'!
o~
E o.~j
°
'~
o
o
"~"~ 0.05
oo21 o.m I
0.01
,
092
,
,
,
,
i
,
0.05
0.1
0.2
0.5
1
,
,
2 w/g 5
°i
10
Fig. 3. Rate of hydrogen adsorption per unit suspension mass in 50 cm 3 of 2 M H2SO 4 at 3300_+ 300 r.p.m, stirring speed.
Polarization curves
After the adsorption step had been studied separately, the current-potential curve of the hydrogen-saturated suspension was obtained. This is given in Fig. 4 for different concentrations of the powder electrode. We attribute the peculiar currents which appear to be too high at low concentrations and overvoltages to catalytic powder constantly adhering to the collector electrode. This was illustrated by dipping the collector electrode into a stirred suspension, tranferring it to a pure, hydrogen-saturated 2 M H 2 S O 4 solution and measuring three subsequent polarization curves (Fig. 5). These curves became lower as time elapsed, indicating that the adhering powder was washed out from the electrode surface. We suppose that the adhering powder is continuously replenished in the suspension electrode system. The dotted line in Fig. 5 is the polarization curve of a clean collector electrode in pure H 2 S O 4 solution. One can easily observe that the current contributed in pure H 2 S O 4 solution. One can easily observe that the current contributed by the adhered powder is small and distorts the polarization curves of Fig. 4 only at low powder concentrations and at low overpotentials.
57
H O N Pd~C S U S P E N S I O N E L E C T R O D E
15
80"
4-
40"
220-
'
o
o:s
E/v
Fig. 4, Polarization curves for 5% Pd/C suspension electrode in 50 Cltl3 of 2 M H2SO 4. The n u m b e r s adjacent to curves indicate the a m o u n t of powder. Stirring speed is 3300 r.p.m. Fig. 5. Three subsequent polarization curves for 1% P d / C powder adhered to the collector electrodes. T h e d o t t e d curve stands for a clean collector electrode.
At exceedingly lower powder concentrations the major current may be that of the adhered powder, resulting in polarization curves which are independent of concentration. This should explain the behaviour obtained by Schwabe and Stasko t4. The current v e r s u s suspension concentration curves at different overvoltages are plotted in Fig. 6 and show linear dependence at sufficiently high currents. The too high currents at low concentrations are due to adhered particles, as mentioned above. From Fig. 6 one can easily deduce that the discharge currents are 3 to 4 orders of magnitude smaller than the equivalent current of 14.8 A which corresponds to the rate of hydrogen adsorption. Consequently, the rate-determining step in the overall process of hydrogen discharge on powder electrodes is the charge transfer from the particle to the collector electrode. We had not investigated the mechanism of this charge transfer experimentally. We shall, however, discuss this subject referring to some other works. In order to explain the contradictory results of refs. 6 and 8 we suggest a model which is valid for any reaction mechanism provided it has subsequent adsorption and ionization steps, such as the Volmer-Tafel mechanism9. An equivalent circuit for one particle of the suspension electrode is illustrated in Fig. 7. R c is the contact (electronic) resistance between a particle and the collector electrode, a n d / ~ is the (ionic) resistance of the solution surrounding the particle,
58
E. KEREN, A. SOFFEN
15
w/u Fig. 6. Polarization currents v s . amount of 1% Pd/C suspension in 50 c m 3 0 [ 2 M HzSO4 at constant overvoltages. The lower curve is at overvoltage of + 100 mV, increasing by 100 mV for each higher curve. S~.
R
R
-J-
R
-v
.S I
m
It8
Fig. 7. The equivalent electrical circuit for a metal catalyst particle colliding with a collector electrode. as mentioned in the introduction. R i and R a are, respectively, the impedances of the ionization and adsorption of the hydrogen molecules. Ca1 is an integral double layer capacitance per unit surface area and Cp is the reaction pseudocapacity. V is the Voltage between the collector electrode and the solution, fixed at will by the potentiostat. E represents the reservoir of hydrogen molecules in the solution (which are supplied by bubbling hydrogen through the suspension) and is related to the hydrogen gas pressure by the Nernst equation. The last two quantities are not measurable, but their difference, ~/= V - E is the measured overpotentiaL Usually,. the particle examined is not in contact with the collector. Therefore Sz is closed and S 1 is open except for periodic short contacts which represent the collisions of the particle with the collector. Relying on the experimental results of
H ON Pd~C SUSPENSION ELECTRODE
59
ref. 8, our main assumption is that the contact and solution resistances are small and, since Cd~ is two orders of magnitudes smaller than Cp9, we may write:
(Rs + Re)Cal'~ RiCp which means that the relaxation of the double layer is much faster than the chemical reaction. During the short contact Ca~ is discharged until the voltage between its plates equals V, thence the charge transferred in one collision is Ca~rl. The ionization reaction is too slow (Ri too large) for any appreciable charge to be transferred across R~ during the collision, so the voltage on Cp remains E. If the time between collisions is long enough, Ca1 is recharged through R~ and also through R a by adsorption of hydrogen molecules, so the cycle starts again. Provided the ionization reaction has not advanced to a considerable extent until the particles hit the probe, this electrode would show the potential of the adjacent collector electrode, a result which was in fact obtained. Thus the current delivered by the suspension electrode can be written in the form: i = f'S'Cdlt
I
where f is the average frequency of collisions and S is the average surface area of one powder particle. Because of the statistical nature of this equation the measured current fluctuates around an average value. Assuming constant integral double layer capacity, (which is reasonable because of the high concentration of electrolyte solutions of the order of 1 M 7' st, and constant f, the average current is proportional to the overvoltage, which is the usual experimental resqlt of Held and Gerischer 7. These authors also measured the total charge stored on a particle by stopping the stirrer and letting the particle adhere to the collector. This case is realized in our model when S x is closed indefinitely while S 2 is open since, after stirring is interrupted, the hydrogen supply is cut off. Now both capacitors are discharged to the voltage V and the charge is (C a ---I-Cdl)t/. The value of Cp for hydrogen on nickel is 840/~F cm-2 while Cdl can be roughly approximated 9 to 15 #F cm-:. We can thus find that the ratio of the charge transferred during a single collision to the total charge is:
Cdl/(Cp + Cal) = 0.019 which lies within the experimental results of Gerischer 7. The conclusion is that the experimental results of both Gerischer et al. and Lazorenko-Manevich and Ushakov 8 are explicable by one single mechanism, namely the particle does acquire the potential of the collector electrode in a single impact and transfers only about 2~o (the double-layer fraction) of its total charge. The apparent contradiction arises mainly because the difference between double-layer capacity and pseudocapacity had not been considered. The capacitative quantity appearing in the kinetic equations quoted in ref. 8 is, in fact, the double-layer capacity 10. The behaviour of the potential and the hydrogen surface coverage 0 during a discharge-regeneration cycle of a particle is schematically shown in Fig. 8. The line between the points 1 and 4 represents the steady state variation of 0, the adsorbed hydrogen atoms with the potential ~b. The linear behaviour corresponds
60
E. KEREN, A. SOFFEN I
I I 4
w l
3
a
I ,
0
21
o
~
~
~e
Fig. 8. The changes in hydrogen surface coverage and potential of a metal catalyst particle during a collision-regeneration cycle.
to an ideal Temkin isotherm ~a. The cycle begins at point 1 where the particle is at equilibrium (0= 1, q~--0 versus hydrogen electrode in the same solution), touches the collector electrode at potential q, discharges the double layer during a time (Rs + Re)Ca] (Fig. 7), and acquires the potential ~b= q. The contact time is short, hence 0 remains practically constant. At point 2 the particle leaves the electrode and the recharging of the double layer begins. From the equation: Haas+OH- ~ H 2 0 + e we see that both ~b and 0 decrease until the particle arrives at the steady-state curve at point 3. This step is represented in Fig. 7 by the charging of Ca~ by Cp via Ri. At point 3, 0 increases again as a result of adsorption (namely charging Cp by E via Ra as in Fig. 7), until it reaches equilibrium at point 1 and the cycle starts again. We assumed here that the ionization reaction is much slower than the relaxation of the double layer, and yet much faster than the adsorption. If these three steps are closer on their time scale, the triangular shape of the diagram will be distorted, but the general behaviour will still persist. If the time between successive collisions is not sufficiently long, the particle will not arrive at the electrode at point 1, but at the intermediate potential point 1', namely at a potential between 0 and t/. The cycle is now described by the triangle (1'; 2'; 3') and the charge transferred during a collision is smaller than Cdlq. This causes a deviation from linear i-r/relations, observed at high overpotentials and fast stirring 11. In the extreme cases a limiting current appears and is attributed to the rate of hydrogen adsorption on the powder 12 (when the limiting current is proportional to the concentration of the suspension), or to the rate of dissolution of hydrogen in the electrolyte (when it is independent of concentration). All these considerations can be put on a more quantitative basis by comparing the different time parameters appearing in the theory (Re + Rs) Cd~, RiCp, time of collision and time between collisions. CONCLUSION
1. According to the model proposed above, the particle double layer is
H ON Pd~C SUSPENSION ELECTRODE
61
discharged during a collision and only a small part of the available charge, that stored on the double layer capacitance, is delivered to the collector. Nevertheless, the particle has the collector potential on leaving it. Therefore the property determining the current delivered by one particle is the double-layer capacity of the powder, and not the contact resistance as Gerischer proposed 7. Consequently, when seeking a good catalyst for suspension electrodes one should look for the highest possible surface area rather than the best catalytic activity of the powder. 2. After having the best electrode powder, the collision frequency would still be the rate-determining step since the current is several orders of magnitude lower than the rate of adsorption. Therefore, one must increase the suspension concentration until a rather thick mass is obtained where the continuity of electrical contacts b e t w e e n the particles should play a new and important role. This situation, however, leads us to a sort of agitated porous electrode structure into which the threephase contact is tri-dimensional, hence more effective (per unit mass of electrode material) than the two dimensional one typical of solid porous electrodes. SUMMARY The activity of a palladium--carbon suspension electrode in hydrogen gas electro-oxidation was investigated in 2 M H2SO 4 solution. The main feature of the work was the attempt to separate experimentally the various kinetic steps. Thus the rate of hydrogen adsorption was studied by connecting the electrochemical cell to a volumetric system, while a four-electrode system was used for current-potential .relationships. It was possible to show that the rate of adsorption of hydrogen is determined by the dissolution of gas if the powder suspension concentration is higher than 0.5 weight percent. The charge transfer was 3 to 4 orders of magnitude lower than the rate of adsorption, indicating that the average collision frequency and the contact time of the particles with the collector electrode was the slowest step. A critical analysis of these two parameters is given, relying mainly on some other works. This showed that the double layer charge is completely transferred to the collector electrode during the short time of collision. Hence, electrode particles of high surface area, such as catalyst-impregnated carbon black, are preferable. It is concluded that in order to accelerate the slowest step of charge transfer, an extremely thick suspension electrode had to be used. This approximates the above electrode closely to the well-known porous electrode. REFERENCES 1 E. U. Muller and K. Schwabe, Kolloid Z., 52 (1930) 163. 2 Y. A. Podviazkin and A. I. Schlygin, Zh: Fiz. Khim., 31 (1957) 1305. 3 D. V. Sokolskii and Y. A. Skopin, Dokl. Akad. Nauk. SSSR., 126(2) (1959) 334. 4 P. Boutry, O. Bloch and J. C. Balaceanu, Compt. Rend., 254 (1962) 2583. 5 A. B. Fasman, D. V. Sokolskii and K. A. Shurov, Dokl. Phys. Chem. Sect., 153 (1963) 1030. 6 H. Gerischer, Ber. Bunsenges. Phys. Chem., 67 (1963) 164. 7 J. Held and H. Gerischer, Ber. Bunsenges. Phys. Chem., 67 (1963) 921. 8 R. M. Lazorenko-Manevich and A. V. Ushakov, Dokl. Phys. Chem. Sect., 161 (1965) 201. 9 M. A. Devanathan and M. Selvaratnam, Trans. Faraday Soc., 56 (1960) 1820.
62 10 11 12 13 14
E. KEREN, A. SOFFEN P. C. Milner, J. Electrochem. Soc., 107 (1960) 343. E. Gileadi (Editor), Electrosorption, Plenum Press, 1967, pp. 11, 15. J. Honz and M. Knize, Coll. Czech. Chem. Commun., 32 (1967) 2540. E. Keren and A. Softer, to be published. K. Schwabe and A. Stasko, J. Electroanal. Chem., 11 (1966) 308.