S. GILMAN
2112
Studies of Anion Adsorption on Platinum by the Multipulse Potentiodynamic (M.p.p.) Method. 11.
Chloride and Phosphate Desorption and
Equilibrium Surface Concentrations at Constant Potential’
by S. Gilman General Electric Research Laboratory, Schenectady, h’ew Y o r k
(Received J a n u a r y I S , 1964)
When a platinum electrode is allowed to adsorb chloride ions a t 0.8 v. and the potential is then lowered, it is possible to follow changes in the “oxygen adsorption” trace as a function of desorption time a t the lower potential. The results indicate that desorption is rapid and terminates a t the equilibrium surface coverage for the lower potential. Since adsorption in the range 0 < U 6 0.8 v. is reversible, it is possible to measure equilibriurn adsorption of chloride and phosphate ions as a function of solution concentration and potential in this range. The results for chloride ions suggest a Temkin adsorption isotherm. Because of irreversible “oxygen adsorption” above 0.8 v., the surface coverage with anions a t any potential above 0.8 v. is a function of path and may vary from a maximuin (corresponding to the equilibrium value for 0.8 v.) to zero. Studies of nonequilibrium adsorption in the high potential range indicate that increasing surface coverage with “oxygen” results in decreasing surface coverage with the anion.
Introduction The preceding paper this series2 dealt with the developnient of an multipulse potentiodynaiiiic (n1.p.p.) method for the study of ion adsorption. This paper presents discussion of further adaptation of the 1ii.p.p. method to provide information on ion desorption and equilibrium surface coverage over a wide range of conditions.
Experimental E q u i p m e n t and Reagents.
Equipment, reagents, etc., have been described in the preceding paper.2 As previously, solutions of hydrochloric or phosphoric acid were prepared by addition of the concentrated acid to a stock solution of 1 N perchloric acid. The working electrode was the same used previously2 with Q s ~= 0.272 mcoulomb/cni.2. Procedures, Potential sequences applied to the working electrode are diagramed in Fig. 1 (tJiiiicdurations are not to scale). The procedures employed during each step of Fig. 1 are suiniiiarized in Table I. As previously, all iiieasuremeiits were made a t 30’ and all T h e Journal of Physical Chemistry
potentials are referred to a reversible hydrogen electrode immersed in 1 N perchloric acid.
Results I . Chloride Desorption Kinetzcs. Sequence I of Table I was employed in making these measurements. The solution studied was M HCI in 1 N perchloric acid. The procedure followed through steps A-D of the sequence was intended to provide a clean surface free of adsorbed chloride ion and with the concentration of dissolved hydrogen in the adjacent solution extremely small. This latter condition is required to eliminate (effectively) the hydrogen oxidation current during the ineasurenient of “oxygen adsorption” traces. Procedural steps 1-4 and 5a-7a result in obtaining a “blank” ( & - = 0) “oxygen adsorption” trace when sweep G is applied. Procedural steps 1-4 and 5b-7b result in the corresponding trace for the maxi(1) This work wa8 made possible by Research Projects Agency (Order No. Engineer Research and Development No. DA-44-009-EXG-4909. (2) S. Gilman, part I of this series, J
the support of the Advanced 247) through the C . S. Army Laboratories under Contract P h y s . Chem., 6 8 , 2098 (1964).
ANIONADSORPTION ON PLATINUM BY MULTIPULSE POTENTIODYNAMIC METHOD
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(u
E V
\ 0
E
-
IO sec.,TF: I msec.
ci
1.0
TG, msec.
TIME
2.0
Figure 2. “Oxygen adsorption” traces obtained during desorption of chloride ions a t U = 0.4 v. (sequence I of Table I; T Fis the desorption time).
-
10
Figure 1. Potential sequences used in the measurement of chloride and phosphat,e ion adsorption.
mum value of & I - obtainable a t 0.8 v., when T F is chosen equal to zero. Traces corresponding to these two different situations appear in Fig. 2 (traces 4 and 1, respectively; trace 1 actually is for a value of T F somewhat greater than zero). The area bounded by traces 1 and 4 of Fig. 2 and the dashed line definesa quantity AQ,’ as previously discussed.2 Under the conditions just described AQo‘ = A,&, and is proportional to the surface coverage with chloride ions.2 If, after inaximum chloride adsorption is allowed to occur a t 0.8 v., TF is made greater than zero, with U < 0.8 v., A@,’ is observed to decrease (Fig. 2 corresponds to U = 0.4 v.) signifying desorption of chloride a t potential U . Figure 3 is a plot of AQ,’ against, the log of desorption time, T F ,for desorption potentials, U , of 0.4 and 0.6 v. The value of AQo’ obtained ttfter 10 sec. of desorption is (within a few per cent) the same as the maximum values of AQ, obtained during adsorption a t the same values of U . The same observation may be made for U as low as 0.1 v., where the equilibrium value of AQo’ is zero (no adsorbed chloride). Hence, the adsorption of chloride iin the range 0 v. < U 0.8 v. is reversible, I n attempting to determine the kinetics of desorption froin our experiniental data, we must first make the following observations. (1) During previous studies2 of the adsorption of chloride a t constant potential, the experiment was begun with a low (equal to bulk) concentration of dis-
0.8 v.
F
(6a) TF = 1 msec.
(6a) To remove “oxygen” deposited on the surface during potential step E, without allowing time for appreciable adsorption of anions from solution (6b) To remove “oxygen” deposited during potential step E, while allowing a minimum amount of time for the desorption of anions and diffusion away from the surface
(6b) TF = 1 maec.
G
(7b) To obtain the “oxygen adsorption” trace corresponding to maximum (equilibrium) surface coverage with the anion a t potential U
(5a) To bring the electrode to potential U without allowing sufficient time for appreciable adsorption of anions from solution (5b) To allow maximum adsorptioii of anions a t potential U
(5b) T E = 10~iec.
a
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(7a) Apply linear anodic sweep of speed v = 2000 v./sec. and measure the current-time trace corresponding to steps 1-4 and 5a-6a (7b) Apply linear anodic m e e p and measure the current-time trace corresponding to steps 1-4 and 5b-6b
(7a) To obtain “oxygen adsorption” trace correspond ing t o zero surface coverage with anions
(7b) To obtain “oxygen adsorption” trace corresponding to maximum (but nonequilibrium) surface coverage with anions established after application of potential step 1; > 0.8 v.
“T” with the appropriate subscript indicates the duration of a particular step;
the electrode was further depleted. Throughout the experiment, therefore, there was no appreciable diffusion of chloride to the surface during the short linear anodic pulse used to measure the adsorption which preceded the pulse. Hence, AQ,’ was equal to AQo, which is proportional to chloride surface coverage. (2) During desorption of chloride, the concentration of dissolved chloride initially adjacent to the surface is equal to the small bulk value, but must soon increase greatly with time. Hence, AQ,‘ f AQ, and is not an accurate measure of chloride surface coverage until the dissolved chloride concentration adjacent to the surface diniiiiishes sufficiently by diffusion into the bulk of the solution. Consideration (2) leads us to the conelusion that the values of AQ,’ plotted iii Fig. 3 serve as a guide only to the upper limit of chloride surface coverage as a function of desorption time. A rather coinplicated analysis of the data would be required to obtain actual quantita-
e.g.,
T A is the duration of step A .
tive information. One extreme possibility is that there is instantaneous desorption of excess (compared to equilibrium) chloride and that AQ,’ is a measure of diffusion of the desorbed chloride into the bulk. I I . il4easurement of Equilibrium Chloride Sui:face Coverage in the Range 0.4 ,< U 0.8 u. Sequence I1 of Table I and a of solution of HCI in 1 N perchloric acid was used in making these measurements. The procedure was similar to that used previously in following adsorptioii as a function of time.2 Procedural steps 1-4 and 5a-Ga were used to obtain the “blank” “oxygen adsorption” trace. Steps 1-4 and 5b-6b were used to obtain the ‘(oxygen adsorption” trace c o r m sponding to inaxiniuin &I-. The choice of Z!E’ = 10 sec. with mechanical stirring was found to give maximum adsorption throughout the coilcentration range studied. For concentrations 3 Jf HCI, maxiilium surface coverage was achieved within 10 sec. without niechanical stirring, whereas mechanical stirring was
