Inhibition of Hydrogen Adsorption by Submonolayer Deposition of Metals on Platinum S. H. Cadle and Stanley Bruckenstein Chemistry Department, State University of New York at Buffalo,Buffalo, N. Y . 14214 Submonolayer deposition of copper, silver, lead, and gold inhibit the adsorption of hydrogen at a platinum electrode. At low surface coverage, copper and silver preferentially inhibit the sites occupied by weakly adsorbed hydrogen. l e a d and gold inhibit both weakly and strongly adsorbed hydrogen equally at all surface coverages. Copper(l1) and silver(1) react quantitatively with adsorbed hydrogen at open circuit to produce deposits of copper(0) and silver(0) which are not distinguishable from the underpotential deposited metals. Copper deposited at underpotential forms a discrete monolayer before the second layer deposits. On the other hand, silver deposited at underpotential forms films 1.4 monolayers thick before complete surface coverage is achieved. T H E PLATING AND STRIPPING Of Copper (I-3), Silver (4,and bismuth (5)have been studied previously at a rotating platinum ring-disk electrode in aqueous acid media. Two monolayers of copper and silver and 1.3 monolayers of bismuth are deposited at underpotential on platinum before the bulk deposition process can occur. The two monolayers of copper are deposited in discrete potential regions. The purpose of this study was t o investigate the relationship between underpotential deposition of a metal and the hydrogen adsorption process at platinum. Hydrogen adsorption is a better understood process than underpotential deposition of metals, and, hopefully, comparison of these two processes would be helpful in clarifying underpotential deposition processes. The adsorption of hydrogen on platinum in acid media has been extensively studied. It is assumed that a close 1 :1 correspondence exists between the number of platinum atoms at the surface of the electrode and the maximum number of hydrogen atoms that can be adsorbed-i.e., 210 pC/cm2 of hydrogen (6) are assumed to be adsorbed on a perfectly flat platinum electrode. Positive deviations from this number are attributed to surface roughness, and are expressed in terms of the roughness factor. Hydrogen adsorbs at three distinct potentials on platinum. The physical difference between the three states of adsorption has not been conclusively identified. Both Frumkin and Slygin ( 7 ) and Mignolet (8) have concluded that the initially adsorbed hydrogen is strongly bonded t o the platinum and negatively polarized while the more weakly adsorbed hydrogen is positively polarized. SuhrmanR et a/. (9), however,
( I ) G. W. Tindall and S. Bruckenstein, ANAL. C H E h i . , 40, 1051 ( 1968). (2) Ibid., p 1637. (3) G. W. Tindall and S. Bruckenstein. Electrochim. Acta, 16, 2, 245 (1971). (4) S. H. Cadle and S. Bruckenstein, ANAL.C H E M . . 43, 932 (1971). (5) S. H. Cadle, .I. Elecfroclirrii. Soc., 118, 2, 39C (1971). (6) A. Frumkin. “Advances in Electrochemistry.” Vol. 3, 1963, Interscience Publishers, New York, N. Y . (7) A. Frumkin and A. Slygin. Acta P/iysicoc/ii~n.URSS, 6 , 319 ( 1936). (8) J. Mignolet, J . Chim. Pliys., 54, 19 (1957). (9) R . Suhrmann, G. Wedler, and H. Gemlock, Z. Pliysik. Chrm., 17, 350 (1958). 1858
have concluded that both types of polarized hydrogen exist at low surface coverages. To account for the three types of hydrogen, both Eucken and Welkers (IO)and Breiter (11) have postulated that there are sites or areas at the electrode surface with different adsorption energies. Will (12) has studied hydrogen adsorption o n single crystal electrodes and has concluded that the different types of adsorption occur o n the different crystal faces. Adsorbed organic species and ions can displace the hydrogen adsorbed o n the platirrum surface. Also, when mercury (13) or arsenic (14) is adsorbed on platinum, the shape of the charging curves in the hydrogen region is affected. Recently, Bowles has reported that both copper (15) and bismuth (16) deposited at underpotential displace hydrogen. Furthermore, Zakumbaeva (17) has shown that the adsorption of thallium(I), cadmium(II), and zinc(I1) affects the shape of the current-potential curves for the adsorption of hydrogen as well as inhibiting the hydrogen adsorption process. Breiter has also reported the inhibition of hydrogen adsorption by underpotential deposited copper (18). He reports that one copper atom occupies one hydrogen adsorption site. EXPERIMENTAL
Chemicals and Solutions. All solutions were prepared with triply distilled water. Baker reagent grade acids were used to prepare supporting electrolytes which were either 0.2M sulfuric acid or 0.12M perchloric acid. Copper sulfate and silver sulfate solutions were prepared from Baker reagent grade chemicals. Mallinckrodt reagent grade lead nitrate and Fisher reagent grade gold chloride were used. Solution purity was determined from the shape of residual current-potential curves, noting how quickly adsorbed impurities inhibited the hydrogen adsorption process. Solutions were deoxygenated with nitrogen. Equipment. A rotating platinum ring-disk electrode was used in all experiments. The projected area of the disk was 0.462 cm2. The electrode parameters N and @ * I 3 were 0.374 and 0.992, respectively. The electrode was polished to a mirror finish with 0 . 0 5 ~alumina. The cell and the circuit for the independent potentiostatic control of the ring and the disk (19) have been described previously. The circuit for the constant current source is given by Bruckenstein and Miller (20). An EA1 X-Y-Y‘ recorder was used to record all data. (10) A. Eucken and B. Welkers. Z . Elektrorlim~..55, 114 (1951). (11) M. Breiter, elect rock in^. Aria, 7, 25 (1962). (12) F. Will, J . Electrocliem., 112, 451 (1965). (13) A. Slygin, A. Frumkin. and V. Medvedorsky, Acta Physiochim URSS, 4, 91 1 (1936). (14) B. Ershlev and A. Frumkin. Tram. Faraday Sor.. 35, 1 (1939). (151 B. J. Bowles. E/rcrrochiru. Acta, 15, 589 (1970). (16) Ibid., p 737. (17) G. D. Zakumbaeva, F. Taklabaeva, and D. V. Sokol’skli, Elertrokliimivu, 6, 777 (1970). (18) M. W. Breiter, Tra~ts.Furtrday Sac., 65, 2197 (1969). (19) D. T. Napp, D. C. Johnson, and S. Bruckenstein, ANAL. CHEW,39, 481 (1967). (20) S. Bruckenstein and B. Miller. J . Elrctrochrrn. Soc., 117, 8 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
-240
I
1
+ 0.8
I
f
I
+ 0.4
I
I
0.0
VOLTS Figure 1. Current-potential curve at a RPRDE Rotation speed = 900 rpm; scan rate X 200 mV/sec. CC"(II) = 5 X 10-6M and CEMO~ = 0.2M. The number of p C of Cu(0) deposited per cm2 at -0.25 V and the time of deposition. ( a ) 0 p C , 0 sec; (b) 52 p C , 27 sec; (c) 87 &, 45 sec; (d) 120 pC, 63 sec; (e) 153 pC, 83 sec; (f) 228 p C , 118 sec; (g)288 pC, 153 sec; (h) 351 pC, 180 sec Procedure. ELECTRODEPRETREATMENT. The electrode was pretreated in the following manner in supporting electrolyte. The electrode was polished with Buehler 0.05-11 Gamma alumina before each experiment, oxidized at +1.4 V for five minutes and then reduced at -0.20 V for five minutes, and scanned between these potential limits until a reproducible current-potential curve was obtained. POTENTIOSTATIC EXPERIMENTS. Experiments were then conducted by stepping the electrode potential from f 1 . 2 V to a potential in the region where underpotential deposition occurs. After a known time, the disk potential was scanned linearly with time toward anodic (or cathodic) potentials to determine how much metal had been deposited (or how much hydrogen could still be adsorbed). The amount of metal deposited was determined by integiating either the ring or the disk current using standard ring-disk procedures (16). The amount of hydrogen adsorbed was determined by averaging the charge consumed in the anodic and cathodic scans and subtracting the estimated charge due to double layer charging. The combination of a high scan rate and low bulk concentration served to minimize additional metal deposition during an experiment. After each such experiment, the electrode was oxidized at f 1 . 4 V in the supporting electrolyte solution of metal ion and then scanned between hydrogen and oxygen evolution to reattain a reproducible electrode surface. All potentials are reported us. the SCE. OPENCIRCUITSTUDIES. Open circuit experiments at the disk electrode were conducted to study the reaction: M"+csoiut,on,
+
nH(a 71 PC, (499 PC,( e ) 135 p C . Arrows indicate voltage scan direction
VOLTS plated at underpotential 1’s. the amount of hydrogen adsorbed. Deviations from linearity in curve B became apparent after approximately 30% surface coverage of the electrode. A maximum of 368 pC/cm2 Ag(0) was deposited at t-0.30 V before the hydrogen adsorption could be totally inhibited at our electrode. One monolayer of close packed silver atoms corresponds t o 268 PC/cm2 at our electrode. This value was calculated by multiplying the square of the ratio of atomic radii of silver and platinum atoms by the number of pcoulombs of hydrogen adsorbed per cm2. This approach is justified by the work of Bowles (21). Thus, approximately 1.4 monolayers of Ag(0) had t o be deposited on the electrode t o prevent hydrogen adsorption. Apparently, the second layer of Ag(0) begins t o deposit simultaneously with the first layer, but thick silver regions d o not form. Reaction of Ag(1) with Hadr. Deposition of Ag(0) by reaction between Ag(1) and adsorbed hydrogen occurs readily. The interpretation of this process is complicated by a n unknown process that deposits up t o 0.4 monolayers of Ag(0). This unknown process (22) occurs at a n opencircuited platinum electrode and can be detected by ring-disk techniques. It is not possible t o distinguish between Ag(0) deposited by Reaction 1 and the unknown process. The silver experiments were performed as follows. The disk potential was stepped t o the desired potential and the disk open-circuited for a known time. The Ag(0) deposited at open-circuit was collected at the ring on the subsequent anodic scan of the disk. The same quantity of Ag(0) (163 pCjcm2) was deposited o n the disk whether it was opencircuited at 0.0 V o r at +0.20 V. The quantity of Ag(0) deposited increases when the disk is open-circuited at potentials more negative than 0.0 V, and reaches a maximum of 360 pC/cm2 when open-circuited at -0.25 V. The latter quantity corresponds t o approximately 1.4 monolayers of Ag(0) at our electrode. Our results rule out the possibility of all the deposited silver being immobilized at the sites corresponding t o the original location of the reducing agent. Inhibition of Adsorbed Hydrogen by Pb(0) and Au(0). - -
~
~
(21) B. J . Bowles, Nuturr, London, 212, 1456 (1966). (22) D. Untereker and G . W. Tindall, Chemistry Dept., State University of New York at Buffalo, private communication, 1970.
Pb(0) was deposited at underpotential (-0.3 V 5 E 5 +0.5 V) on platinum from 0.12M perchloric acid. Figure 4 shows the effect of underpotential deposition of Pb(0) on the hydrogen adsorption process. Curve a is the residual curve. The increase in anodic current in the potential region (+0.1 V 5 E< +0.6 V) is due t o the oxidation of lead deposited at underpotential. These submonolayer deposits of lead inhibit the weakly and strongly bonded hydrogen equally at all surface coverages. Au(0) was deposited on platinum at 0.0 V from 0.2M sulfuric acid solution. Au(0) does not deposit on platinum at und-rpotential and is deposited simultaneously with the reduction of oxidized platinum. Comparison of the decrease in hydrogen adsorption with the amount of gold plated indicates gold does not deposit as a uniform monolayer. Figure 5 shows a series of current-potential curves obtained at a platinum electrode. Each curve corresponds t o a different quantity of deposited gold. The cathodic current peak at f0.9 V is due t o the reduction of gold oxide formed on the anodic scan. As increasing quantities of gold are deposited on the electrode, both the platinum oxidation peaks and the hydrogen adsorption peaks decrease. Both the weakly and strongly adsorbed hydrogen are inhibited equally at all fractional surface coverages. SUMMARY AND CONCLUSIONS
The deposition of submonolayer amounts of silver, copper, lead, gold, bismuth (9,and mercury (23)on platinum interfere with the adsorption of hydrogen. In the case of silver a n d copper, the weakly bonded hydrogen adsorption sites are inhibited preferentially in the early stages of underpotential deposition while the deposition of lead, gold, bismuth, and mercury inhibit both the weakly and strongly bonded hydrogen adsorption sites about the same amount. Bowles (21) has shown that the inhibition of hydrogen adsorption by a metal atom is directly proportional t o the square of the ratio of atomic radii of the metal and platinum. For copper, cadmium, tin, and thallium, he found a proportionality constant of unity and concluded that the platinum ~
~~
(23) M. Hassan, Chemistry Dept., State University of New York at Buffalo, private communication, 1971.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
1861
I I
I-
I
too Figure 5. Current-potential curves of Au on a RPRDE 0
C A ~ C =I ~2 - X 10-6M; CH2504 = 0.2M. Rotation speed = 2500 rpm, scan rate = 100 mV/sec. The number of pC of gold deposited (a) 11 pC, (b) 110 pC, (c) 238 p C , ( d ) 370 IC. Quantity of hydrogen adsorbed (a) 143 pC, (6) 110 pC, (c) 77pC, ( d ) 58
=\ I--
$0
a
3 0
PC
- 100 I .2
0.8
0.4 VOLTS
surface and the monolayer of metal atoms have the same geometrical structure. Our results on copper d o not agree with the work by Bowles. We find a 1 :1 relationship between the deposition of copper at underpotential and the inhibition of hydrogen adsorption. This result was also found by Breiter (18). Suppose we assume there is a random distribution of active sites o n the platinum surface and that the spacing between sites is equal t o the spacing of the platinum atoms. Then, it might be expected that large atoms such a s lead and bismuth, whose cross-sectional areas are 1.5 times that of a platinum atom, would not be able t o show site preference
0 .o during deposition. A smaller atom, however, could show site preference. In this regard, we find that only copper atoms, rcu = 0.92 rRt, and silver atoms rAg = 1.04 rRt, exhibit a preference for one of the hydrogen adsorption sites. All of the larger atoms simultaneously inhibit both the weakly and strongly adsorbed hydrogen sites equally. Curiously, gold, which is the same size as the silver atom, shows no site preference. RECEIVED for review April 23, 1971. Accepted July 23, 1971. Support of the US. Air Force Ofice of Scientific Research under Grant No. AFSOR 70-1832 is gratefully acknowledged.
Potassium Fluoride-A Reference Standard for Fluoride Ion Activity R. A. Robinson, Wayne C. Duer, and Roger G . Bates Department of Chemistry, Unioersity o j Florida, Gainesville, Fla. 32601
Potassium fluoride i s superior to sodium fluoride as a reference standard for the calibration of fluorideselective electrodes because of its higher solubility and i t s relative freedom f r o m ion association. Ion pairing i n sodium fluoride solutions is demonstrated by an analysis of activity coefficient data i n terms of t h e hydration theory developed by Stokes and Robinson. The fluoride ion has a n average hydration number of about 1.9, virtually identical with that f o r potassium ion. Accordingly, the activity coefficients of potassium and fluoride ions are equal in solutions of potassium fluoride. The ion pair dissociation constant for sodium fluoride i s 1.88 on the scale of molality a t 25 O C , and conductivity measurements are consistent with the conclusion that less ion pairing occurs in solutions of potassium fluoride than in solutions of t h e sodium salt. Standard values of pF (-log aF-) useful for the calibration of fluoride ion-selective electrodes over a wide range of fluoride activity are listed.
THEFLUORIDEION-SELECTIVE electrode developed by Frant and Ross (1) is characterized by a high reproducibility and selec~
~~
(1) M. Frant and J. W. Ross, Jr., Scieme, 154, 1553 (1966). 1862
tivity. These favorable properties have led to its extensive use for routine determinations of fluoride as well as for thermodynamic investigations of fluoride solutions (2). The determination of fluoride by other means is inconvenient and timeconsuming; hence, this electrode is a welcome addition t o the tools available to the analytical chemist. The fluoride electrode is one of the class of solid-state membrane electrodes. The membrane, which consists of a doped lanthanum fluoride crystal, is permeable to fluoride ion and capable of rejecting virtually all other ions with the exception of hydroxide. Several investigations have demonstrated (3-7) that the potential of the electrode changes with fluoride (2) J. N. Butler in “Ion-Selective Electrodes,” R. A. Durst, Ed., Chap. 5, NBS Special Publication 314, U. S. Government Printing
Office, Washington, D. C., 1969. (3) J. J. Lingane, ANAL.CHEM., 39, 881 (1967). (4) R . A. Durst and J. K . Taylor, ibid., p 1483. (5) R. Bock and S. Strecker, Z . Anal. Chem., 235, 322 (1968). (6) R . G. Bates and M. Alfenaar in “Ion-Selective Electrodes,” R. A. Durst, Ed., Chap. 6, NBS Special Publication 314, U. S. Government Printing Office, Washington, D. C., 1969. (7) G. Neumann, A r k . Kemi, 32,229 (1970).
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
