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Sorption of oxygen from solution by noble metals
‘kLEe-l-RoANALk.rGu_
CJSEMISTRY AND INTERPAciAL ELEc-iRo CHErvnsTRY Elsevier Sequoia S.A., Lausanne - Prined in The Netherlands
-,
1
. .
.’
:.
SORPTION’
0;
.,
OXYGEN
L BRIGHT
PLATINUM
RAYMOND
THACKER
FROM
SOLUTION
BY NOBLE
METALS
AND JAMES P. HOARE
Electrochemistry Department, Research Laboratories, General Motors 48090 (U.S.A.)
Corborarion, Warren, Michigan
(Received 22nd June 1970)
INTRODUCTION
How oxygen interacts consideration-
in the
with the surface of a platinum investigation of the electrochemical
electrode properties
is an important of the oxygen
electrode, and in recent times answers to this problem have been sought by many workers. In general, there appear to be two schools of thought with regard to- the nature of the interaction between Pt atoms and 0, molecules. One group favors the concept that the adsorbed oxygen film is made up of hydrated oxide molecules, such as PtO or PtO, whereas the other group prefers to think of the fti as a hydrated layer of adsorbed oxygen atoms such as Pt-0. Unambiguous proof for either viewpoint does not exist. Although Anson and Lingane’ -presented chemical evidence for the presence of PtO and PtO, on an anodized Pt surface, Breiter and Weiuingeti were able to show that these data could be interpreted equally well by considering the adsorbed film to be one of Pt-O_ It was not possible to distinguish between the two types of bonding from the results of heats and entropy of adsorption experiments3. From potential sweep studies4s5, it appears that the surface of the platinum is covered with a film of adsorbed oxygen about a monolayer thick before molecular oxygen is evolved Under certain conditionsq Bagotskii and co-workers’*’ observed a shoulder on the oxygen reduction peak which indicated the existence of two kinds of adsorbed oxygen. The presence of two types of adsorbed oxygen can be seen in differ-. ,: entiated galvauostatid reduction curvesg. Schuldiner and Warner lo*l l have observed the presence of oxygen adsorb&l on the surface of.Ptas wellas that dissolved in the_surfa& layers (dermasorbed) of the Pt. They12 found that the’&pe of adsorption changed at a potential of about 1 V: Similar _ conclusions ,were reached from the results of ellipsometric studies on -Pt/02- elec:’ _.: .. . trodes l 3_ ,.’ When pt. is &mzd ik 0,saturated acid solution, a steady-state rest, pot&&l j of ab&t 1.04 V1? is’r&h,ed. This.potential is a mixed potentiai’h”,.but_there_~:dis-.,:. agreement about.& n&u.re of the anodiC r&&ion of the. lo&l Cell-The ‘oxygen:ad: ; sorbe$und& these conditions cannot be:dettitedwith ellipsdmetrtc [email protected];‘-. . ‘_-.=The irivestigation’of the’ Pt/c), electrode ‘sy&e+reported here ‘w~,tiudertaketi~ to,._obt%iiri &d&c& for the presenti, ofo-x&en dissolved in the Pt:.me~:!attice;:If:the~,_’ ; ,- _- : ., : _: _ ., ._’ ‘. :’ ..,,’ _ :. -_- -_ ,.:- _--‘,- I_ ._ __. .. .,
-.;.
-. ._
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. ....
..
.i.
.-.._
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‘J_L,Elec&a&i;_ ; :.
&&
&&&~l;i~
.-
:./2
*
R. THkKER,
J. P. HOARE
dissolved oxygen resides in the fust 1 or 2 atom layers (skin) of the metal, it is called dermasorbed oxygen, but ifit penetrates to deeper layers, it is called absorbed oxygen. EXPERBfENTAL
Most of the measurements were carried out on small Pt beads (-0.12 cm in diam) melted at the end of Pt (99.99 + o/o pure) wires mounted in one side of a dual Teflon celIZ4_An a-Pd reference electrode l 6 was placed in the other side of the cell. The Pt beads were cleaned by repeatedly heating them in a H,-Ilame followed by quenching in concentrated HNO,. A final heating in the flame concluded this pretreatment. Teflon spaghetti was used to cover the Pt lead wires so that only the bead was exposed to the solution. After heating the bead once more, it was sealed to the Teflon by pulling the hot head against the end of the Teflon spaghetti. The mounted beads were leached in triply distilled water for at least 24 h. IIIsome studies ofanodized Pt, measurements were made in an all-glass celll’ on Pt wires sealed in the ends of glass tubes; The Teflon cell was cleaned in aqua regia, concentrated HNOs, and fmally leached for l-2 weeks in water triply distilled from an all-quartz stiI114*18.After the ceII was filled with electrolyte and the electrodes mounted in the cell, the celI was sealed and the electrodes were preeIectrolyzed for 48 h against removable Pt wire cathodes. After opening the circuit by removing the Pt wires, both sides of the cell were saturated with purified Hz for several hours to remove any peroxides formed during the preelectrolysis procedure. During this time, the Pd bead was converted to an a-Pd reference electroder6. Finally, the solution was stirred (-300 cm3 min- ‘) with purified N2 or O7 depending on the experiment to be performed. A bubbler was placed on the gas exit from the cell to prevent back diffusion ofair into the cell_ The exit gases were monitored with a Beckman model 777 polarographic oxygen analyzer. The true area of the test electrodes was determined for each run by calculating the amount ofadsorbed oxygen deposited on the electrode during a constant current anodid pulse as described by Schuldiner and Roe lg . Before applying the anodic pulse, the Pt electrodes had been strongly anodized * followed by saturating the solution with H2 to remove any surface adsorbed oxygen. The H,-stirring was replaced with N,-stirring before the p&e was applied. In this way, the oxygen adsorbed during the anodic pulse remains on the surface since the dermasorbed layers are filled with oxygen diffusing from the interior of the metal. Correction for double Iayer charging was estimated graphically according to the method suggested by Trasatti20 and shown in Fig. la. By drawing the indicated tangents to the charging curves, one finds that the transition time, r& appropriate for determining the area is BC. The double layer . correctron, r& is the 1engthAB. Since poIycrystalIine Pt was used, a value of 1.31 x 1O1’ surface’sites per square centimeter4*1g.20 was chosen as the most reliable value*’ to calcttIate the charge associated with a complete monoIayer of adsorbed oxygen. Employmg this number of surface sites per square centimeter, the charge on the eIectrow 1.6 x.lO-~rg C, and the.assumption that one double valence oxygen atom adsorbs
* Thisprocedure Ioads theinterior of the metal with dissolved oxygen as pointed out in the se&jon;, .::. ._'. : ;'.:: .'-_. -'. .. ,' - ~. ',. ~., ~+Ek&i;moi. &m~.30(1971) 1'1.4 -' _ : .. ., '.;' -1 :_-
Discussion _
SORPTION
OF OXYGEN
BY BRIGHT
PLATINUM.
3
I
Fig. I. (a) Determination of anodic charge associated with adsorption of a monolayer of adsorbed oxygen molecules_ AC is the total transition time, r; AES is the correction for double layer charging, r,; BC is transition time used in area determio ations, rO. See text for details. (b) Determination of transition time. r,, for calculation of sorbed oxygen from stripping curve.
per site, one arrives at the value of 420 PC cme2 required to form a complete monolayer of Pt-0. Roughness factors based on the estimated geometric area ranged from 1.5 to 2.3. To determine the amount of adsorbed oxygen on a given test electrode, constant current stripping pulses were applied to the electrode by means of a pulsing circuit employing a mercury-wetted relay as described elsewhere2’. The cathodic transition, time, r,, was determined as shown in Fig. 1b. In general, the stripping pulses were applied to the electrode after bubbling N2 through the solution although there was very little difference in z, obtained in N2- or O,-saturated solutions, in agreement with the results reported by Wroblowa and co-workersl’. In studies of the dependence of the amount of sorbed oxygen on the potential, the potential was controlled with a 61R Wenking potentiostat. The charging curves were displayed on an oscilloscope and recorded photographically_ Potentials were read with a General Radio model 1230A electrometer and current with a Weston model 622 milliammeter. All experiments were carried out in 2 N H2S04 solution. The temperature of these experiments was 24% 1“C, and all potentials are recorded against the normal hydrogen electrode (NHE) unless stated otherwise. The degree of freedom of the system from adsorbable impurities can be determined from the anodic charging curves as suggested by Schuldiner and co-workers’ g. Ifa linear increase of potential with time is obtained in the oxygen adsorption region as seen in Fig. 2a, the system is f&c of adsorbable impurities1g*20. There is a fine structure in the hydrogen adsorption region (lower left-hand comer of Fig 2a) which corresponds to the two types of adsorbed hydrogen reported in the literature22-.24. By r&an.s of an electronic current interrupter, additional evidence for the freedom of the system from adsorbable impurities was obtained from the cathodic polari@tion of the Pt electrode in the solution when it was saturated with H,. The. very large pseudocapacitance for~the evolution of H2 on the Pt cathode, observed on the pulses J. EZecfrohnd
Chem, 30 (1971).i-14..
4
F% 2.
R. I-HACKER,
(a)Anodic charging curve taken .on a clean Pt ekcrrode, @) on an unclean R
as (bj after anodization;
x-axis= 1 ms cm-‘;
y-axis=350
mV cm-‘;
current=42
J. P. HOARE
electrode, m&
(c) same
fkom the interrupter, attested to the presence of a highly active Pt surface free of adsorbable impurities25. When an anodic pulse was applied to a Pt electrode prepared without special consideration for the control of impurities (no precleaning of electrodes, no leaching in triply distilled water for at least 48 h, no preelectrolysis), the curve in Fig. 2b was obtained Instead of a linear oxygen adsorption region, the rounded curve was obtained preceded by a very small hydrogen adsorption region_ After this bead had been anodized against a removable preelectrolysis cathode for about 2 h at a current density of about 70 mA cm-* (potential > 2000 mV), the pulse in Fig. 2c was recorded. Although the hydrogen adsorption region is extended, the oxygen adsorption region is still not very linear- Apparently, mere anodic pulsing such as that used in sequential pulse techniquesg*26 may not be enough preparation to ensure that one has a clean electrode surface. As a further check on the degree of freedom of the system from adsorbable impurities, the effect of preanodization on the hydrogen arrests is presented in the data of Fig. 3. After a precleaned Pt bead which had never been preanodized was placed in H,-stirred 2 N H2S04 solution for 5 min, it was held at 600 mV in N,-stirred solution for 30 s before applying the high-rate stripping pulse. The resulting trace is recorded in Fig. 3a showing a monolayer of adsorbed hydrogen (204 G cm-*). The second pulse was taken after the electrode was on open circuit in N,-stirred solution for 5 min. A monolayer of hydrogen (200 PC cmm2) is also detected here. This same electrode was anodized in N,-stirred solution (85 mA cmS2 at 2300 mV) for 2 min followed by cathodization (-80 mA cmB2 at - 148 mV) for 5 min to remove all dermasorbed oxygen. After the Pt bead was held at 600 mV in N,-stirred solution for 30 s, the cathodic pulse was applied. The resulting trace in Fig 3b also 3. Electroanal.
Chem.,
30 (1971) t-14
Fig_ 3. Cathodic x-axis=1
ms
stripping curves in H.-stirred solns. on Pt electrodes (a) not preanodized, (b) preanodized y-axis=350 mV cm-‘; current=4.1 mA; true area=0.083 cmz.
;
cm-‘;
exhibits a monolayer of adsorbed hydrogen (198 pC cmm2) and is virtually identical to the trace in Fig. 3a For a clean system, apparently, preanodization merely fills the R electrode with dermasorbed oxygen which is only removed by a strong reduction process. To check on the validity of our electrode area determinations, the true areas determined corn the hydrogen stripping and fi-om oxygen deposition on the trace of an anodic pulse (Fig 2a) are compared in Table 1. Comparison of the three areas obtained from the deposition of hydrogen (Fig. 3) and from the deposition of oxygen (Fig. 2a) are made in Table 2. In both cases the agreement is very good although the ratio Qo/2QH is closer to unity when deposition processes are compared. TABLE lRUE
I
AREA
DETEFSUN
ATION
FROM ::ANODIC i
PULSE
Geometric area/cm’
Time base /ms cm-’
Current /mA .L.
r/ms
0.0304 ,$0362 0.0435
2 2 2
4.1 :;. ,, 4.1 -: 4.1
6.52 8.10 10.3
0.0304
2
0.0362
2
0.0435
2
TABLE -lRU%AREAD
4.1 4.1 4.1
2.70 3.78 4.42
Qo/lrC
Q&o
True area/cm2
A (true)/ A (geometric)
Qolxhi
26.7 33.2 42.4
420 420 420
0.064 0.079 0.101
2.1 2.2
-
2.3
-
Qtr/N
&,
0.053 0.074 0.086
1.6 2.0 2.0
120 1.07 1.17
11.1 15.5 18.1
/pC
cm-’
/
PC cm- 2 210 210 210
2 E+ERMINATlON
FROM
CATHODIC
PULSE
Time base /mscnz-’
C&em /mA
r/n=
H, on
1
4.1
292
120
Cathodized Adsorb H2 on open circuit
1 1
4.1 4.1
2.97 3.97
122
210
16.3
210
Geometric area/ cm’
Treatment
0.0304
Adsorb
0.0304 0.0362
open circuit
Q&PC
Q.&m /pC
210
an-
2
True area/cm2
A (true)/ A (geometric)
~&/~QEz
0.057
1.9
1.11
0.058
1.9
1.09
0.078
2.2
1.01
J. Electroanal.
Chem,
30 (1971) l-14
6 RESULTS
R. THACKER, AND
3. P. HOARE
DI.!XU!SSION
Potentiostatic studies In the fit series of experiments, the Pt test bead was potentiostated at a given potential in N,-stirred 2 N H2S04 solution until a reasonably steady current was recorded, after which the potentiostat was disconnected by a mechanical switch and a galvanostatic cathodic pulse was applied to the bead. The series of traces obtained over the potential range from 450 to 2050 mV is presented in Fig_ 4.
Fig. 4. Cathodic stripping curves obtained on Pt electrodes which had been potentiostatted at (a) 450, (b) 750, (c) 950, (d) 1250, (e) 1550, (f) 2050 mV; x-axis: (c) 0.5. (a, b. d. e) I, (t) 2 ms cm- ’ ; y-axis: (a, b) 200, (c-f) 350 mV cm- 1 ; current = 42 mA_
In the potential range from 450 to 750 mV (Fig. 4a and b), only the arrests beginning at a potential of about 160 mV are detected for the deposition of a monolayer of hydrogen2’ - 2g. At a potential of 950 mV, a second transition region appearing at about 600 mV is recorded on the trace in Fig. 4~. As the potential is raised from 1250 mV (Fig. 4d) through 1550 mV (Fig. 4e) to 2050 mV (Fig. 4f), the low overvoltage arrest beginning at about 700 mV increases in length continuously. In addition, not only does the high overvoltage arrest increase in length with increasing potential between 1050 and 2050 mV, but the potential at which this arrest begins shifts to more noble potentials until a point is reached (Fig. 4f) where arrests can be detected at 300 mV and at 160 mV. Above 1 V, the amount of charge associated with the lower transition region becomes larger than that associated with a monolayer of adsorbed hydrogen atoms. J. Electroanal.
Chetn,
30 (1971)
l-14
SORPTION
OF OXYGEN
BY BRIGHT
PLATINUM.
7
I
By subtracting the charge for a monolayer of hydrogen atoms (210 PC cme2) from the charge measured from the traces of Fig. 4 for the lower arrest, the excess charge (Q - QH) may be plotted as a function of potential (triangles) in Fig. 5 in terms of monolayer equivalents of adsorbed oxygen (0). Also in Fig. 5 is plotted the charge associated with the upper transition region (circles)_ In both cases, the charge increases linearly with potential_ In a second series of experiments the potential behavior of the electrode was observed under open circuit conditions in N.-stirred solutions after being pulsed. In the experiment where the electrode had been held at potential values below 1 V, the open circuit potential after the stripping pulse had been applied remained in the hydrogen adsorption region and eventually with time drifted to potential values between 400 and 500 mV. With increasing values of the polarizing potential above 1 V, the open circuit potential returned to more noble potentials more rapidly until a point was reached in the vicinity of 1800 mV where the open circuit potential recovered to a value between 900 and 1000 mV within 20 s after the stripping pulse was applied. These data can be interpreted in terms of two types of sorbed oxygen ; namely, oxygen adsorbed on the metal surface and oxygen dermasorbed in the skin (first few atom layers) of the metal. It is reasonable to assume that oxygen sorbed on the surface would be removed more easily than that sorbed in the dermasorption layers. Consequently, the trace from a stripping pulse should show the surface oxygen first in a low overvoltage region followed by the dermasorbed oxygen in a high overvoltage region.
POTENTIAL
vs
NHE
(mv)
Fig. 5. The monolayer equivalent, 0, ofdermasorbed (A) and of surface adsorbed (0) OF potential at which the electrode was potentiostatted before stripping in N+tirred
oxygen as a Functicn 2 N H,SO,.
The charge determined in the low overvoltage region of the traces in Fig. 4 are associated with surface adsorbed oxygen. According to Fig. 5 and in agreement with the results of potential sweep studies4-‘, oxygen begins to be adsorbed at surface sites at a potential between 800 and 900 mV. As noted before in the literature5*30-32, Fig. 5 shows that the degree of coverage of the metal surface with adsorbed oxygen is a Iimear fun&ion of potential_ Above 1600 mV, where the evolution of oxygen beJ. Electroanal.
Chem,
30 (1971) I-14
8
R.
T-HACKER,
J. P_ HOARE
gins5*.r’*,the electrode surface is covered by a monolayer of oxygen (tI= 1) in agreement with other published observations (e.g. refs_45, 18,3O-32). At more noble potentials, 6 reaches values approaching 2. This phenomenon is interpreted in terms of the cotiversion of P&O sites to PtO, sites about which more will be said in a later section. Above I V, the quantity of charge determined fkom the high overvoitage traces in Fig. 4 in excess of that associated with a monolayer of adsorbed hydrogen atoms also increases linearly with the potential. Other researchers33-?T have published cathodic stripping curves which exhibit two transition regions tid James36 has critically discussed these results concluding that the number of coulombs of charge associated with the high.overvoltage transition region beginning at potentials between 200 and 300 mV is’too large to be accounted for by hydrogen atom adsorption alone. We propose that this excess charge is tobe associated. with dermasorbed oxygen. Since this type of sorbed .oxygen is so difficult to remove, the high overvoltage for its removal causes the arrest. for dermasorbed oxygen in the stripping curve traces to overlap with-the arrests for the deposition of hydrogen. In agreement with the reports of Schuldiner et aZ.l?, oxygen becomes dermasorbed at potentials above 1 V and the amount of oxygen dissolved in the Pt metal increases with the potential of anodization. At about 2000 mV (Fig. 4f), the concentration of dermasorbed oxygen is large enough to produce a separation of the arrests ; dermasorbed oxygen begins at about 300 mV whereas hydrogen adsorption begins at about 160 mV as shown in Fig 4a and b. These arrests are better defined on the trace in Fig. 6 obtained by stripping a preanodized Pt wire electrode with a small cathodic current (310 fi cm-‘) and recording on a strip recorder. In support of the viewpoint which identifies the excess charge in the low overvoltage region with dermasorbed oxygen, is the observation that the potential returns rapidly to a value over 900 mV under open circuit conditions after the stripping pulse had been applied. If after only 2 min under open circuit conditions in N,-stirred solution a second stripping pulse is applied to the electrode preanodized above 1800 mV, both surface and dermzkorbed oxygen can be detected on the recorded current traces in Fig. 7a. The upper trace is the first pulse (similar to Fig. 4e) and the two lower
Fi& 6_ Cathodic stipping curve obtained on a R wire eIectro& pre&odi& at 1450 mV by applying a const&tcathodj,ccut-rent of -3lO,~Acm-~ and recor&ng on a &rip &order. Lows indicate the posit& of arikts for surface-a&orbed (700 mV) and dermasorbed (310 mV) oxygen and tiydrogen adsdrption . ._ i (160.60 mv). J_‘ElecrTCwznuL them, 30 (1971) 1-14
!%GRlTKCJ
OF OXYGEN
BY BRIGHT
-:
:
PLATINUM.
L9
I
.-
Fig 7. Cathddic stripping pulses on Pt electrodes :‘(a) (upper &e<~otentiostatted in N1-stirred 2 N H2Sd4 soEn. at 2000 mV, (Iower trace)’ 2 min on ‘open circuit after first .pulse; (b) which had sorbed oxygen from oxygen-sat+ soln. under open circuit conditions for 17 h; (c) which had been anodized for 20 mti ‘at 74 mA cmm2 (-2100 mV) (upper trace), and re-pulsed after 100 s on open circuit after first pulse.(lower y-axis: (a, c) 35Q (b) 500 mV cm- l; current : (a; c) 4.5 (b) 0.98 mk trace); x-axis: (zi, b) 2, (c) 5 ms cm-‘;
traces were taken after 2-mm intervals under open circuit conditions. For kach trace, 22.& cm-= of surface oxygen and 54 ,uC cm- 2 of dermasorbed oxygen are determined With repeated pulses it was observed that the potential recovered to less noble values and the transition time of the high overvoltage arrest became shorter until a point was reached where traces similar to Fig_ 4a were obtained and the open’circuit potential remained below 500 mV in N.-stirred solution. At this point, all of the dissolved oxygen was removed from the metal. Kalish and Burshteinj!~ ‘reported that the equivalent of several monolayers of oxygen may be dissolved in Pt under certain conditions and Schuldiner and coworkersto-r2 concluded that oxygen whichthey called dermasorbed oxygen could be dissolved in the fit 2 or 3 atom layers (skin) of the Pt metal but continued anodic polarization’2 increased the amount of oxygen dissolved in the Pt. It was found from a study of thin Pt diaphragms 38*3g that oxygen may be made to dift%tsethrough a Pt foil by anodizing one side of the Pt foil diaphragm. When the oxygen was stripped from the surface and the skin of the Pt by the first pulse, oxygen dissolved in the deeper metal layers diffused to the surface, which raised the potential to very noble values and where it was stripped by the second pulse. The equivalent of several monolayers of dissolved oxygen can be measured with repeated pulses. Open circuit studies
We were interested in investigating the nature of the adsorbed oxygen layers on a Pt electrode which had sorbed oxygen from an Oz-satu_rated solution under open circuit conditions., A precleaned, oxygen-free (treated wrth Ha-stirring) Pt electrode was placed in an O,-saturated 2 IV HZS04 solution for 17 h after which a cathodic stripping pulse was applied_ The trace obtained is given in-Fig. 7b. Values of 121 PC cme2 (0=029) for the low overvoltage region and 246.& cme2 (36 PC crne2 of dermasorbed oxlggen) for the high overvoltage region are determined, At this point, the potential (1060 mV) is greater than 1 V and oxygen may be dermasorbed. From over 10 independent determinations, the coverage of the Pt surface with adsorbed J. Electroamd.
Chem,
30 (1971) 1-14
10
R. THACKER,
J. P. HOARE
oxygen at the steady-state rest potential (1060 mV) lies between O-23 >8 >0.30 in agreement with the findings of Rao et ~1.~~. To see how the amount of oxygen adsorbed on the surface of a Pt electrode under open circuit conditions changes with the time of exposure to 02, the following experiments were performed. After a Pt bead which had reached the steady-state potential (1060 mV) in O,-saturated solution was stripped of the surface adsorbed oxygen by a cathodic puke, it was pulsed again after various intervals of time for which it was on open circuit in the O,-saturated solution. The traces obtained after exposure to O2 of 1 min, 5 min, 13 min, and 4 h is recorded in Fig. 8a-d and the coverage
Fig_ 8. Cathodic strippingcurves obtained on Pt electrodes (with filled dermasorbed layers but free of surface adsorbed oxygen) which had been in contact with O,-saturated 2 N H,SO, soIn_ for (a) 1, (b) 5, (c) I3 min. (d) 4 h; x-axis=2 ms cm-l; y-axis=5OOmV cm-l; current=0.98 mA.
POTENTIAL
ua NHE
hnv)
Fig. 9. The coverage of a Pt electrode with adsorbed oxygen under open-circuit conditions as a function of the rest potenti& Steady state is reached at 1060 mV. At this point, 6 was never observed to exceed a value of 03 (ten determinations). J- Elec*oatzaL Chem, 30 (1971) l-14
SORPTION OF OXYGEN
BY BRIGHT
PLATINUM.
II
I
with surface adsorbed oxygen is plotted as a fknction of the rest potential in Fig. 9. There is a linear relationship between the rest potential and the coverage with surface adsorbed oxygen. Under steady-state open-circuit conditions, this coverage never exceeds 30% and the potential remains at about 1060 mV. This behavior of the Pt/O, electrode supports the contention14*18 that the rest potential of Pt in an oxygen-saturated acid solution is a mixed potential composed of the 0JH20 reaction, 0, + 4H+ +4e -*2Hz0, and the Pt/Pt-0 reaction, Pt + H 2O -+ Pt-0 + 2H + + 2e. The local cell is set up in the direction noted by the arrows ; consequently, a complete layer of Pt-0 should be formed. Since the reversible potential is not observed and since a 8 of only 0.3 is recorded, a complete layer of Pt-0 is never reached under open-circuit conditions because oxygen is continually sorbed into the Pt lattice thus setting up the steady-state coverage of the Pt surface with adsorbed oxygen. In the light of these results, the Pt/Pt-0 mixed potential mechanism is favored over the impurity-mixed potential mechanism suggested by Wroblowa et aL1’.
When adsorbable impurities are present, the steady-state potential of the Pt bead (Fig. 2b) was below 1 V, the cathodic stripping pulse showed virtually no surfaceadsorbed oxygen arrest at 700 mV, and the adsorbed hydrogen arrest was very narrow. Gahanostatic
studies
In one Teflon cell, the solution was prepared by preelectrolysis for 48 h against removable cathodes, followed by saturation of the solution with Hz to remove any peroxides present. The solution was then saturated with N,. In a second Teflon celi, the precleaned Pt bead electrodes were leached in triply distilled water and anodized at a constant current of 74 mA cmm2 for 2 h in 2 N HZS04solution (potential >2ooO mV). Finally, the anodized beads were transferred from the second cell to the N,-saturated 2 N HzS04 solution in the first cell. The electrode was pulsed giving the upper trace shown in Fig. 7c. When pulsed again after 100 s on open circuit, the lower trace of Fig. 7c was obtained_ The surface-adsorption and the dermasorption transition regions are substantially larger than those shown in Fig. 4. A value of 2.01 for the surface-adsorbed 0 was calculated as the average of six independent determinations, and similarly, 1.70 for the derrnasorbed 8. These data indicate that in the layer ofsurface adsorbed oxygen there is a ratio of 2 oxygen atoms to 1 Pt atom. It is possible that this strong anodic polarization has converted the monolayer of Pt-0 formed at lower potentials (Fig. 4e) to a monolayer of PtO, (Fig. 7~). A similar coneltiion was reached by Biegler and Woods4’. When the surface layer is stripped offwith a cathodic pulse in N.-saturated solution, a second pulse only 100 s later shows the presence of 81 PC cm-* of dermasorbed oxygen. It is evident from Fig. 7c that anodization greatly increases the amount of oxygen dissolved in the platinum as well as converting Pt-0 sites to PtO, sites. To investjgate the dependence of the amount of oxygen sorbed by Pt on the length of time oi anodization, experiments were carried out on thin wires, sealed in glass, in glass cells of conventional design_ As above, the wires were anodized in a separate cell and then transferred to the N,-saturated, peroxide-free acid solution in the test cell. The wire was anodized at constant current for various periods of time and was then pulsed in the cell after remaining in the N.-stirred solution for 15 min. Just J. Electroanal.
Chem,
30 (1971)l-14
: R_ THACKJZR,
TIME
J. P. HOARE
(min)
Fig. IO. A plot of 8 for adsorbed oxygen (circles) on an anodized Pt electrode as a fhnction of the time of anodization at 0.625 mA cm-* (0, A) and at 625 mA cm-” (0, A). The potential in N,-stirred acid solution (A} was recorded just before the stripping pulse was applied.
before the pulse was applied, the potential of the electrode was recorded. ‘. Figure 10 contains a plot of both the 8 for surface adsorbed oxygen (circles) and the &#e&al (t&x&s) as a function of the length of time the electrode was anodized at 0.625,mA cmm2. (open symbols) and 62.5 mA cme2 (filled symbols). At f&s&,both 8 ‘. tid_thq potential increase rapidly with the anodization time and then more slowly for &&onger periods of t&e.., Eventually, 8 reaches a value of about 2.
I
I
-’ a.’
I
I
I
0.90 I
.,
_
.: . Fic 11. Pl&df 8 for adsorbed oxygen (0) on-a ~rean~ Pt ekctrode (625 mA cm-*) as a fbnctidn’ of. -_, ..__’ ;.;. ~‘&&ti&e for &i& the ele&o&w~, .pett$d to haql in N2&irred a& sob before the St&ping pub! .I.-. 1’ -hi& ap’$ikd- ,T’he petehal’ .:. _. (A) lyas also recorded just befiiti+the .pulse was applied:
:
-:.,..
:_
_’ .,
:
I
.
SORPTION
OF OXYGEiN
BY BRIGHT
PLATINUM.
13
I
In another interesting experiment, the wires were anodized for 30 min at 62.5 mA cm-’ (potential > 2000 mV) and pulsed after resting in N.-stirred 2 N HzS04 solution for various periods of time. The 6. for surface-adsorbed oxygen along with the recorded potential are plotted in Fig_ 11 as a function of the resting time in N.-stirred acid solution. There is an exponential decay of both 8 and the potential. These results support the conclusion 42 that thin anodic films of PtO, are unstable in the presence of Pt at potentials below 1.55 V. Under: open-circuit conditions, the PtO, sites decompose to Pt-0 sites and to oxygen which may be dissolved in solution or in the metal. In conclusion, we submit-that the data presented in this report are consistent with an interpretation in terms of oxygen weakly adsorbed at surface sites, strongly sorbed at dermasorbed sites (in the first 1 or 2 atom layers of the metal), and dissolved. or absorbed in the bulk of the Pt metal. SUMMARY
Platinum electrodes in the form of beads or wires were permitted to sorb oxygen from O,-saturation H2SO4 solutions under steady-state conditions of constant current, constant potential, and open circuit. True areas were determined from constant current anodic p&es, and the amount of charge associated with the sorbed oxygen was determined from the measured transition times of potential-tune traces obtained with constant current, cathodic stripping pulses. Two kinds of adsorption sites are identified with respect to the difiuculty with which the sorbed oxygen is removed by a cathodic stripping pulse. The strong adsorption sites are located in the skin of the metal (first 1 or 2 atom layers of the metal) and the sorbed oxygen on these sites corresponds to the so-called dermasorbed oxygen. Weak adsorption sites are located on the metal surface, and the sorbed oxygen on these sites is called adsorbed oxygen. Oxygen which is dissolved in the interior of the metal beyond the dermasorb region is called absorbed oxygen. The amount of oxygen sorbed is a function of the potential. Below 800 mV, sorbed oxygen could not be detected. The first oxygen to be sorbed appears above 800 mV on the surface adsorption sites but dermasorbed oxygen is not detected until potentials greater than 1000 mV are reached. Under open-circuit conditions, the maximum amount of surface adsorbed oxygen present at steady state (potential = 1060 mV) &I O+aturated acid solution is 30% of a monolayer_ A complete layer of adsorbed oxygen (Pt-0) may be obtained with anodization at potentials of about 1600 mV_ At higher potentials ( > 2CKKlmV), the Pt-0 sites may be converted to PtO, sites until a maximum of a monolayer of PtO, is formed. Under open-circuit conditions, the PtO, sites in the presence of Pt decompose to Pt-0 sites. The equivalent of many complete layers of oxygen may be dissolved in the interior of the metal with anodic polarization, and this absorbed oxygen may replace any dermas orbed oxygen removed. REFERENCES
1 F. C. ANSON 2 M. W. Bm 3
AND J. J. LINGANE, J. Amer. Chem. Sec.. 79 (1957) 4901_
AND J. L. WEIMNGER, J. Electruchem:Sm.. R A_ FISHER, H. CHONAti J. G. Asro~, J.. Phys. Chem.,
109 (1962) 1135. 68 (1964) 3240 -
J. Elecqoqnal.
Chem,
30 (1971) J-14.
.a@ :;,-
..
‘. .:
_,’
::
RTHACKER,J.P.HO&:: ,’
.
‘. 4 F. G. ?‘rtL
A&
C. ~.KNORR,
z. ,?z~e&rochem., 64 (1960) 258.
..
.,I:,. ‘.<.
5’iV_‘EE&.&‘Mi--W_
z
B FtEmER&zec&*&i??z.’ Ac:u, 5 (1961) 145. _ 6, J.. Pi Ho&,. The Efectiochemistry of &yg&, Interscienaz Publishers,,Inc., New York, 1968, p. 29. 7. V. ~.:L.~‘YANCHEVA, Vr I. TIICHOMORIVA AND V. S. BAG~TSKJI, Elektrokhimiya, 1 (1965) 262.
8 V. I. TIKHO~AORWA, A. I. m V. S. BAGOTSKU AND V. I.LUK?YANYCHEVA, DokC. Akad. IVauk SSSR, 159 (1964) 644:. -, :--6 .D. GILXOY AFID B. El CCk&AY, &n. J. Chem.;46 (1968) 875. 10’s._sCI-IULDLNERAND T.’ B. WARNER, J. EIectrochem. Sot., 112 (1965) 212. .ll. T_.B. WARNER AND s. s CHULD~ J_ ~iectrochem. Sot, 112 (1965) 853. 12 s:s&uk m T: B. WARNER AND B. J. PIERSMA, J. Electroc.hem. Sot., 114 (1967) 343. 13 A. IC.N. REDDY, M_ G ENSHAW AND J. O’M, B~CKRIS, J. Electroanal. Chem.. 8 (1964) 408. 14 J:P. HOARE, L’Elec~ocheqz. So&, 109 (1962) 858. -15 H. WROBL~IWA, J. L. B.‘lUo, A. DAMJANOVIC AND J. 0-M. B~CKRIS, J. Electround. Chem., 15 (1967) 139. 16 D. J. G. IVES AND G. J. JANE, Reference Electrodes, Academic Press, New York, 2961, p. 112; J. P_ How Gen. &fot.- Eng. J., 9, No. 1 (1962). 14. 17 R. TH+KER, Hydrocarbon Fuel Cell Technology, Academic Press, New York, 1965, p. 525. 18. J. P. HOW 3. Electrochem. Sot., 112 (1965) 602. 19. s.s LINER AND R. M. ROE, J. Electrochem. Sot., 1IO (1963) 332; S. SCHULD INERANDT.B.WARNER, 1 Phys_ c&m., 68 (1964) 1223. 20’ S. TRASAI-I-I, Electrochim. MetaL, 2 (1967) 12. 21 J. P. HoARE, Electrochim. Ada. 9 (1964) 599. 22 T. C. FRANK&I AND S. L. Coon, J. Electrochem. Sot., 107 (1960) 556. 23 C. H. PRESSREV AND S. S CHULDINER, J. Eiectrochem. Sot., 108 (1961) 985. 24 -F. G. VI&’ J. Elecrrochem. Sot., 112 (1965) 451. AND J. P’ HOARE, J. Chem. Phys., 26 (1967) 1771. 25’5. S CHULDI%R 26 S.,Gu.k& EZectrochim. Acta, 9 (1964) 1025. 27 M. BECF& ALVD M. BREITER, 2. Elektrochem., 60 (1956) 1080, C. A. KNOW AND W. V~~LKL, Z. Elektrochem., 59 (i959) 681. 28 ._M, By and Electrochemical Engineering, Interscience Publishers, 29 _A:N. FRUMKIN, Advances in E2ectrochembtt-y Inc., Meti Yoik, Vol. 3, 1963, p. 314 et seq. 30 F. P. Bowom, Proc. Roy. Sot. London, Al25 (1929) 446. 31 J; k-V.-BUTL.ER AND G. ARMSTRONG, Proc. Roy. Sot. London, Al37 (1932) 604. 32-K. J. VEIXX AND D. BIZRNDT. Z Elektrochem.. 62 (1958) 378. 33.S. !h-m3hT& Bull. Chtmz. Sot. Japan, 36 (1963) 525. 34 k KOZAWA, J. EleciroanalChem., 8 (1964) 20. 3.5 H.‘A. LAITINm AND C. G. Em J. Electrochem. Sot., 107 (1960) 773. 36 S. D. Jm,J. Efecrrochem. Sot., 114 (1967) I 113. ‘37 I-. v* I(ALIsH ANX~R KH. Bv, Dokl. Akad iVauk SSSR, 81 (1951) 1093; 88 (1953) 863. 38 J. P_ HOARE. J. Electrochem. Sot., 116 (1969) 612. 39 J. P. HOARE. J_ Electrochem_ Sot., 116 (1969) 1390_ 4tJ M. L. B_ R+o, 4. DAMJANOVIC AND J. O’M. B~CKRIS, J. Phys. C-hem., 67 (1963) 2508. 41 -T. BIEGF kM, R WOGDS, J. Electroar& Chem., 20 (1969) 73. 42 J:P. HOARE, J. Electrounaf. Chem., 12 (1966) 260. J. EZ&rr&zal.
C&m.,
‘.
1’ : ‘. _. ,-.__’ L--. .
.-
30 (1971) l-14
‘,‘: _-
- ..’
CJSEMISTRY AND INTERPAciAL ELEc-iRo CHErvnsTRY Elsevier Sequoia S.A., Lausanne - Prined in The Netherlands
-,
1
. .
.’
:.
SORPTION’
0;
.,
OXYGEN
L BRIGHT
PLATINUM
RAYMOND
THACKER
FROM
SOLUTION
BY NOBLE
METALS
AND JAMES P. HOARE
Electrochemistry Department, Research Laboratories, General Motors 48090 (U.S.A.)
Corborarion, Warren, Michigan
(Received 22nd June 1970)
INTRODUCTION
How oxygen interacts consideration-
in the
with the surface of a platinum investigation of the electrochemical
electrode properties
is an important of the oxygen
electrode, and in recent times answers to this problem have been sought by many workers. In general, there appear to be two schools of thought with regard to- the nature of the interaction between Pt atoms and 0, molecules. One group favors the concept that the adsorbed oxygen film is made up of hydrated oxide molecules, such as PtO or PtO, whereas the other group prefers to think of the fti as a hydrated layer of adsorbed oxygen atoms such as Pt-0. Unambiguous proof for either viewpoint does not exist. Although Anson and Lingane’ -presented chemical evidence for the presence of PtO and PtO, on an anodized Pt surface, Breiter and Weiuingeti were able to show that these data could be interpreted equally well by considering the adsorbed film to be one of Pt-O_ It was not possible to distinguish between the two types of bonding from the results of heats and entropy of adsorption experiments3. From potential sweep studies4s5, it appears that the surface of the platinum is covered with a film of adsorbed oxygen about a monolayer thick before molecular oxygen is evolved Under certain conditionsq Bagotskii and co-workers’*’ observed a shoulder on the oxygen reduction peak which indicated the existence of two kinds of adsorbed oxygen. The presence of two types of adsorbed oxygen can be seen in differ-. ,: entiated galvauostatid reduction curvesg. Schuldiner and Warner lo*l l have observed the presence of oxygen adsorb&l on the surface of.Ptas wellas that dissolved in the_surfa& layers (dermasorbed) of the Pt. They12 found that the’&pe of adsorption changed at a potential of about 1 V: Similar _ conclusions ,were reached from the results of ellipsometric studies on -Pt/02- elec:’ _.: .. . trodes l 3_ ,.’ When pt. is &mzd ik 0,saturated acid solution, a steady-state rest, pot&&l j of ab&t 1.04 V1? is’r&h,ed. This.potential is a mixed potentiai’h”,.but_there_~:dis-.,:. agreement about.& n&u.re of the anodiC r&&ion of the. lo&l Cell-The ‘oxygen:ad: ; sorbe$und& these conditions cannot be:dettitedwith ellipsdmetrtc [email protected];‘-. . ‘_-.=The irivestigation’of the’ Pt/c), electrode ‘sy&e+reported here ‘w~,tiudertaketi~ to,._obt%iiri &d&c& for the presenti, ofo-x&en dissolved in the Pt:.me~:!attice;:If:the~,_’ ; ,- _- : ., : _: _ ., ._’ ‘. :’ ..,,’ _ :. -_- -_ ,.:- _--‘,- I_ ._ __. .. .,
-.;.
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&&
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.-
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*
R. THkKER,
J. P. HOARE
dissolved oxygen resides in the fust 1 or 2 atom layers (skin) of the metal, it is called dermasorbed oxygen, but ifit penetrates to deeper layers, it is called absorbed oxygen. EXPERBfENTAL
Most of the measurements were carried out on small Pt beads (-0.12 cm in diam) melted at the end of Pt (99.99 + o/o pure) wires mounted in one side of a dual Teflon celIZ4_An a-Pd reference electrode l 6 was placed in the other side of the cell. The Pt beads were cleaned by repeatedly heating them in a H,-Ilame followed by quenching in concentrated HNO,. A final heating in the flame concluded this pretreatment. Teflon spaghetti was used to cover the Pt lead wires so that only the bead was exposed to the solution. After heating the bead once more, it was sealed to the Teflon by pulling the hot head against the end of the Teflon spaghetti. The mounted beads were leached in triply distilled water for at least 24 h. IIIsome studies ofanodized Pt, measurements were made in an all-glass celll’ on Pt wires sealed in the ends of glass tubes; The Teflon cell was cleaned in aqua regia, concentrated HNOs, and fmally leached for l-2 weeks in water triply distilled from an all-quartz stiI114*18.After the ceII was filled with electrolyte and the electrodes mounted in the cell, the celI was sealed and the electrodes were preeIectrolyzed for 48 h against removable Pt wire cathodes. After opening the circuit by removing the Pt wires, both sides of the cell were saturated with purified Hz for several hours to remove any peroxides formed during the preelectrolysis procedure. During this time, the Pd bead was converted to an a-Pd reference electroder6. Finally, the solution was stirred (-300 cm3 min- ‘) with purified N2 or O7 depending on the experiment to be performed. A bubbler was placed on the gas exit from the cell to prevent back diffusion ofair into the cell_ The exit gases were monitored with a Beckman model 777 polarographic oxygen analyzer. The true area of the test electrodes was determined for each run by calculating the amount ofadsorbed oxygen deposited on the electrode during a constant current anodid pulse as described by Schuldiner and Roe lg . Before applying the anodic pulse, the Pt electrodes had been strongly anodized * followed by saturating the solution with H2 to remove any surface adsorbed oxygen. The H,-stirring was replaced with N,-stirring before the p&e was applied. In this way, the oxygen adsorbed during the anodic pulse remains on the surface since the dermasorbed layers are filled with oxygen diffusing from the interior of the metal. Correction for double Iayer charging was estimated graphically according to the method suggested by Trasatti20 and shown in Fig. la. By drawing the indicated tangents to the charging curves, one finds that the transition time, r& appropriate for determining the area is BC. The double layer . correctron, r& is the 1engthAB. Since poIycrystalIine Pt was used, a value of 1.31 x 1O1’ surface’sites per square centimeter4*1g.20 was chosen as the most reliable value*’ to calcttIate the charge associated with a complete monoIayer of adsorbed oxygen. Employmg this number of surface sites per square centimeter, the charge on the eIectrow 1.6 x.lO-~rg C, and the.assumption that one double valence oxygen atom adsorbs
* Thisprocedure Ioads theinterior of the metal with dissolved oxygen as pointed out in the se&jon;, .::. ._'. : ;'.:: .'-_. -'. .. ,' - ~. ',. ~., ~+Ek&i;moi. &m~.30(1971) 1'1.4 -' _ : .. ., '.;' -1 :_-
Discussion _
SORPTION
OF OXYGEN
BY BRIGHT
PLATINUM.
3
I
Fig. I. (a) Determination of anodic charge associated with adsorption of a monolayer of adsorbed oxygen molecules_ AC is the total transition time, r; AES is the correction for double layer charging, r,; BC is transition time used in area determio ations, rO. See text for details. (b) Determination of transition time. r,, for calculation of sorbed oxygen from stripping curve.
per site, one arrives at the value of 420 PC cme2 required to form a complete monolayer of Pt-0. Roughness factors based on the estimated geometric area ranged from 1.5 to 2.3. To determine the amount of adsorbed oxygen on a given test electrode, constant current stripping pulses were applied to the electrode by means of a pulsing circuit employing a mercury-wetted relay as described elsewhere2’. The cathodic transition, time, r,, was determined as shown in Fig. 1b. In general, the stripping pulses were applied to the electrode after bubbling N2 through the solution although there was very little difference in z, obtained in N2- or O,-saturated solutions, in agreement with the results reported by Wroblowa and co-workersl’. In studies of the dependence of the amount of sorbed oxygen on the potential, the potential was controlled with a 61R Wenking potentiostat. The charging curves were displayed on an oscilloscope and recorded photographically_ Potentials were read with a General Radio model 1230A electrometer and current with a Weston model 622 milliammeter. All experiments were carried out in 2 N H2S04 solution. The temperature of these experiments was 24% 1“C, and all potentials are recorded against the normal hydrogen electrode (NHE) unless stated otherwise. The degree of freedom of the system from adsorbable impurities can be determined from the anodic charging curves as suggested by Schuldiner and co-workers’ g. Ifa linear increase of potential with time is obtained in the oxygen adsorption region as seen in Fig. 2a, the system is f&c of adsorbable impurities1g*20. There is a fine structure in the hydrogen adsorption region (lower left-hand comer of Fig 2a) which corresponds to the two types of adsorbed hydrogen reported in the literature22-.24. By r&an.s of an electronic current interrupter, additional evidence for the freedom of the system from adsorbable impurities was obtained from the cathodic polari@tion of the Pt electrode in the solution when it was saturated with H,. The. very large pseudocapacitance for~the evolution of H2 on the Pt cathode, observed on the pulses J. EZecfrohnd
Chem, 30 (1971).i-14..
4
F% 2.
R. I-HACKER,
(a)Anodic charging curve taken .on a clean Pt ekcrrode, @) on an unclean R
as (bj after anodization;
x-axis= 1 ms cm-‘;
y-axis=350
mV cm-‘;
current=42
J. P. HOARE
electrode, m&
(c) same
fkom the interrupter, attested to the presence of a highly active Pt surface free of adsorbable impurities25. When an anodic pulse was applied to a Pt electrode prepared without special consideration for the control of impurities (no precleaning of electrodes, no leaching in triply distilled water for at least 48 h, no preelectrolysis), the curve in Fig. 2b was obtained Instead of a linear oxygen adsorption region, the rounded curve was obtained preceded by a very small hydrogen adsorption region_ After this bead had been anodized against a removable preelectrolysis cathode for about 2 h at a current density of about 70 mA cm-* (potential > 2000 mV), the pulse in Fig. 2c was recorded. Although the hydrogen adsorption region is extended, the oxygen adsorption region is still not very linear- Apparently, mere anodic pulsing such as that used in sequential pulse techniquesg*26 may not be enough preparation to ensure that one has a clean electrode surface. As a further check on the degree of freedom of the system from adsorbable impurities, the effect of preanodization on the hydrogen arrests is presented in the data of Fig. 3. After a precleaned Pt bead which had never been preanodized was placed in H,-stirred 2 N H2S04 solution for 5 min, it was held at 600 mV in N,-stirred solution for 30 s before applying the high-rate stripping pulse. The resulting trace is recorded in Fig. 3a showing a monolayer of adsorbed hydrogen (204 G cm-*). The second pulse was taken after the electrode was on open circuit in N,-stirred solution for 5 min. A monolayer of hydrogen (200 PC cmm2) is also detected here. This same electrode was anodized in N,-stirred solution (85 mA cmS2 at 2300 mV) for 2 min followed by cathodization (-80 mA cmB2 at - 148 mV) for 5 min to remove all dermasorbed oxygen. After the Pt bead was held at 600 mV in N,-stirred solution for 30 s, the cathodic pulse was applied. The resulting trace in Fig 3b also 3. Electroanal.
Chem.,
30 (1971) t-14
Fig_ 3. Cathodic x-axis=1
ms
stripping curves in H.-stirred solns. on Pt electrodes (a) not preanodized, (b) preanodized y-axis=350 mV cm-‘; current=4.1 mA; true area=0.083 cmz.
;
cm-‘;
exhibits a monolayer of adsorbed hydrogen (198 pC cmm2) and is virtually identical to the trace in Fig. 3a For a clean system, apparently, preanodization merely fills the R electrode with dermasorbed oxygen which is only removed by a strong reduction process. To check on the validity of our electrode area determinations, the true areas determined corn the hydrogen stripping and fi-om oxygen deposition on the trace of an anodic pulse (Fig 2a) are compared in Table 1. Comparison of the three areas obtained from the deposition of hydrogen (Fig. 3) and from the deposition of oxygen (Fig. 2a) are made in Table 2. In both cases the agreement is very good although the ratio Qo/2QH is closer to unity when deposition processes are compared. TABLE lRUE
I
AREA
DETEFSUN
ATION
FROM ::ANODIC i
PULSE
Geometric area/cm’
Time base /ms cm-’
Current /mA .L.
r/ms
0.0304 ,$0362 0.0435
2 2 2
4.1 :;. ,, 4.1 -: 4.1
6.52 8.10 10.3
0.0304
2
0.0362
2
0.0435
2
TABLE -lRU%AREAD
4.1 4.1 4.1
2.70 3.78 4.42
Qo/lrC
Q&o
True area/cm2
A (true)/ A (geometric)
Qolxhi
26.7 33.2 42.4
420 420 420
0.064 0.079 0.101
2.1 2.2
-
2.3
-
Qtr/N
&,
0.053 0.074 0.086
1.6 2.0 2.0
120 1.07 1.17
11.1 15.5 18.1
/pC
cm-’
/
PC cm- 2 210 210 210
2 E+ERMINATlON
FROM
CATHODIC
PULSE
Time base /mscnz-’
C&em /mA
r/n=
H, on
1
4.1
292
120
Cathodized Adsorb H2 on open circuit
1 1
4.1 4.1
2.97 3.97
122
210
16.3
210
Geometric area/ cm’
Treatment
0.0304
Adsorb
0.0304 0.0362
open circuit
Q&PC
Q.&m /pC
210
an-
2
True area/cm2
A (true)/ A (geometric)
~&/~QEz
0.057
1.9
1.11
0.058
1.9
1.09
0.078
2.2
1.01
J. Electroanal.
Chem,
30 (1971) l-14
6 RESULTS
R. THACKER, AND
3. P. HOARE
DI.!XU!SSION
Potentiostatic studies In the fit series of experiments, the Pt test bead was potentiostated at a given potential in N,-stirred 2 N H2S04 solution until a reasonably steady current was recorded, after which the potentiostat was disconnected by a mechanical switch and a galvanostatic cathodic pulse was applied to the bead. The series of traces obtained over the potential range from 450 to 2050 mV is presented in Fig_ 4.
Fig. 4. Cathodic stripping curves obtained on Pt electrodes which had been potentiostatted at (a) 450, (b) 750, (c) 950, (d) 1250, (e) 1550, (f) 2050 mV; x-axis: (c) 0.5. (a, b. d. e) I, (t) 2 ms cm- ’ ; y-axis: (a, b) 200, (c-f) 350 mV cm- 1 ; current = 42 mA_
In the potential range from 450 to 750 mV (Fig. 4a and b), only the arrests beginning at a potential of about 160 mV are detected for the deposition of a monolayer of hydrogen2’ - 2g. At a potential of 950 mV, a second transition region appearing at about 600 mV is recorded on the trace in Fig. 4~. As the potential is raised from 1250 mV (Fig. 4d) through 1550 mV (Fig. 4e) to 2050 mV (Fig. 4f), the low overvoltage arrest beginning at about 700 mV increases in length continuously. In addition, not only does the high overvoltage arrest increase in length with increasing potential between 1050 and 2050 mV, but the potential at which this arrest begins shifts to more noble potentials until a point is reached (Fig. 4f) where arrests can be detected at 300 mV and at 160 mV. Above 1 V, the amount of charge associated with the lower transition region becomes larger than that associated with a monolayer of adsorbed hydrogen atoms. J. Electroanal.
Chetn,
30 (1971)
l-14
SORPTION
OF OXYGEN
BY BRIGHT
PLATINUM.
7
I
By subtracting the charge for a monolayer of hydrogen atoms (210 PC cme2) from the charge measured from the traces of Fig. 4 for the lower arrest, the excess charge (Q - QH) may be plotted as a function of potential (triangles) in Fig. 5 in terms of monolayer equivalents of adsorbed oxygen (0). Also in Fig. 5 is plotted the charge associated with the upper transition region (circles)_ In both cases, the charge increases linearly with potential_ In a second series of experiments the potential behavior of the electrode was observed under open circuit conditions in N.-stirred solutions after being pulsed. In the experiment where the electrode had been held at potential values below 1 V, the open circuit potential after the stripping pulse had been applied remained in the hydrogen adsorption region and eventually with time drifted to potential values between 400 and 500 mV. With increasing values of the polarizing potential above 1 V, the open circuit potential returned to more noble potentials more rapidly until a point was reached in the vicinity of 1800 mV where the open circuit potential recovered to a value between 900 and 1000 mV within 20 s after the stripping pulse was applied. These data can be interpreted in terms of two types of sorbed oxygen ; namely, oxygen adsorbed on the metal surface and oxygen dermasorbed in the skin (first few atom layers) of the metal. It is reasonable to assume that oxygen sorbed on the surface would be removed more easily than that sorbed in the dermasorption layers. Consequently, the trace from a stripping pulse should show the surface oxygen first in a low overvoltage region followed by the dermasorbed oxygen in a high overvoltage region.
POTENTIAL
vs
NHE
(mv)
Fig. 5. The monolayer equivalent, 0, ofdermasorbed (A) and of surface adsorbed (0) OF potential at which the electrode was potentiostatted before stripping in N+tirred
oxygen as a Functicn 2 N H,SO,.
The charge determined in the low overvoltage region of the traces in Fig. 4 are associated with surface adsorbed oxygen. According to Fig. 5 and in agreement with the results of potential sweep studies4-‘, oxygen begins to be adsorbed at surface sites at a potential between 800 and 900 mV. As noted before in the literature5*30-32, Fig. 5 shows that the degree of coverage of the metal surface with adsorbed oxygen is a Iimear fun&ion of potential_ Above 1600 mV, where the evolution of oxygen beJ. Electroanal.
Chem,
30 (1971) I-14
8
R.
T-HACKER,
J. P_ HOARE
gins5*.r’*,the electrode surface is covered by a monolayer of oxygen (tI= 1) in agreement with other published observations (e.g. refs_45, 18,3O-32). At more noble potentials, 6 reaches values approaching 2. This phenomenon is interpreted in terms of the cotiversion of P&O sites to PtO, sites about which more will be said in a later section. Above I V, the quantity of charge determined fkom the high overvoitage traces in Fig. 4 in excess of that associated with a monolayer of adsorbed hydrogen atoms also increases linearly with the potential. Other researchers33-?T have published cathodic stripping curves which exhibit two transition regions tid James36 has critically discussed these results concluding that the number of coulombs of charge associated with the high.overvoltage transition region beginning at potentials between 200 and 300 mV is’too large to be accounted for by hydrogen atom adsorption alone. We propose that this excess charge is tobe associated. with dermasorbed oxygen. Since this type of sorbed .oxygen is so difficult to remove, the high overvoltage for its removal causes the arrest. for dermasorbed oxygen in the stripping curve traces to overlap with-the arrests for the deposition of hydrogen. In agreement with the reports of Schuldiner et aZ.l?, oxygen becomes dermasorbed at potentials above 1 V and the amount of oxygen dissolved in the Pt metal increases with the potential of anodization. At about 2000 mV (Fig. 4f), the concentration of dermasorbed oxygen is large enough to produce a separation of the arrests ; dermasorbed oxygen begins at about 300 mV whereas hydrogen adsorption begins at about 160 mV as shown in Fig 4a and b. These arrests are better defined on the trace in Fig. 6 obtained by stripping a preanodized Pt wire electrode with a small cathodic current (310 fi cm-‘) and recording on a strip recorder. In support of the viewpoint which identifies the excess charge in the low overvoltage region with dermasorbed oxygen, is the observation that the potential returns rapidly to a value over 900 mV under open circuit conditions after the stripping pulse had been applied. If after only 2 min under open circuit conditions in N,-stirred solution a second stripping pulse is applied to the electrode preanodized above 1800 mV, both surface and dermzkorbed oxygen can be detected on the recorded current traces in Fig. 7a. The upper trace is the first pulse (similar to Fig. 4e) and the two lower
Fi& 6_ Cathodic stipping curve obtained on a R wire eIectro& pre&odi& at 1450 mV by applying a const&tcathodj,ccut-rent of -3lO,~Acm-~ and recor&ng on a &rip &order. Lows indicate the posit& of arikts for surface-a&orbed (700 mV) and dermasorbed (310 mV) oxygen and tiydrogen adsdrption . ._ i (160.60 mv). J_‘ElecrTCwznuL them, 30 (1971) 1-14
!%GRlTKCJ
OF OXYGEN
BY BRIGHT
-:
:
PLATINUM.
L9
I
.-
Fig 7. Cathddic stripping pulses on Pt electrodes :‘(a) (upper &e<~otentiostatted in N1-stirred 2 N H2Sd4 soEn. at 2000 mV, (Iower trace)’ 2 min on ‘open circuit after first .pulse; (b) which had sorbed oxygen from oxygen-sat+ soln. under open circuit conditions for 17 h; (c) which had been anodized for 20 mti ‘at 74 mA cmm2 (-2100 mV) (upper trace), and re-pulsed after 100 s on open circuit after first pulse.(lower y-axis: (a, c) 35Q (b) 500 mV cm- l; current : (a; c) 4.5 (b) 0.98 mk trace); x-axis: (zi, b) 2, (c) 5 ms cm-‘;
traces were taken after 2-mm intervals under open circuit conditions. For kach trace, 22.& cm-= of surface oxygen and 54 ,uC cm- 2 of dermasorbed oxygen are determined With repeated pulses it was observed that the potential recovered to less noble values and the transition time of the high overvoltage arrest became shorter until a point was reached where traces similar to Fig_ 4a were obtained and the open’circuit potential remained below 500 mV in N.-stirred solution. At this point, all of the dissolved oxygen was removed from the metal. Kalish and Burshteinj!~ ‘reported that the equivalent of several monolayers of oxygen may be dissolved in Pt under certain conditions and Schuldiner and coworkersto-r2 concluded that oxygen whichthey called dermasorbed oxygen could be dissolved in the fit 2 or 3 atom layers (skin) of the Pt metal but continued anodic polarization’2 increased the amount of oxygen dissolved in the Pt. It was found from a study of thin Pt diaphragms 38*3g that oxygen may be made to dift%tsethrough a Pt foil by anodizing one side of the Pt foil diaphragm. When the oxygen was stripped from the surface and the skin of the Pt by the first pulse, oxygen dissolved in the deeper metal layers diffused to the surface, which raised the potential to very noble values and where it was stripped by the second pulse. The equivalent of several monolayers of dissolved oxygen can be measured with repeated pulses. Open circuit studies
We were interested in investigating the nature of the adsorbed oxygen layers on a Pt electrode which had sorbed oxygen from an Oz-satu_rated solution under open circuit conditions., A precleaned, oxygen-free (treated wrth Ha-stirring) Pt electrode was placed in an O,-saturated 2 IV HZS04 solution for 17 h after which a cathodic stripping pulse was applied_ The trace obtained is given in-Fig. 7b. Values of 121 PC cme2 (0=029) for the low overvoltage region and 246.& cme2 (36 PC crne2 of dermasorbed oxlggen) for the high overvoltage region are determined, At this point, the potential (1060 mV) is greater than 1 V and oxygen may be dermasorbed. From over 10 independent determinations, the coverage of the Pt surface with adsorbed J. Electroamd.
Chem,
30 (1971) 1-14
10
R. THACKER,
J. P. HOARE
oxygen at the steady-state rest potential (1060 mV) lies between O-23 >8 >0.30 in agreement with the findings of Rao et ~1.~~. To see how the amount of oxygen adsorbed on the surface of a Pt electrode under open circuit conditions changes with the time of exposure to 02, the following experiments were performed. After a Pt bead which had reached the steady-state potential (1060 mV) in O,-saturated solution was stripped of the surface adsorbed oxygen by a cathodic puke, it was pulsed again after various intervals of time for which it was on open circuit in the O,-saturated solution. The traces obtained after exposure to O2 of 1 min, 5 min, 13 min, and 4 h is recorded in Fig. 8a-d and the coverage
Fig_ 8. Cathodic strippingcurves obtained on Pt electrodes (with filled dermasorbed layers but free of surface adsorbed oxygen) which had been in contact with O,-saturated 2 N H,SO, soIn_ for (a) 1, (b) 5, (c) I3 min. (d) 4 h; x-axis=2 ms cm-l; y-axis=5OOmV cm-l; current=0.98 mA.
POTENTIAL
ua NHE
hnv)
Fig. 9. The coverage of a Pt electrode with adsorbed oxygen under open-circuit conditions as a function of the rest potenti& Steady state is reached at 1060 mV. At this point, 6 was never observed to exceed a value of 03 (ten determinations). J- Elec*oatzaL Chem, 30 (1971) l-14
SORPTION OF OXYGEN
BY BRIGHT
PLATINUM.
II
I
with surface adsorbed oxygen is plotted as a fknction of the rest potential in Fig. 9. There is a linear relationship between the rest potential and the coverage with surface adsorbed oxygen. Under steady-state open-circuit conditions, this coverage never exceeds 30% and the potential remains at about 1060 mV. This behavior of the Pt/O, electrode supports the contention14*18 that the rest potential of Pt in an oxygen-saturated acid solution is a mixed potential composed of the 0JH20 reaction, 0, + 4H+ +4e -*2Hz0, and the Pt/Pt-0 reaction, Pt + H 2O -+ Pt-0 + 2H + + 2e. The local cell is set up in the direction noted by the arrows ; consequently, a complete layer of Pt-0 should be formed. Since the reversible potential is not observed and since a 8 of only 0.3 is recorded, a complete layer of Pt-0 is never reached under open-circuit conditions because oxygen is continually sorbed into the Pt lattice thus setting up the steady-state coverage of the Pt surface with adsorbed oxygen. In the light of these results, the Pt/Pt-0 mixed potential mechanism is favored over the impurity-mixed potential mechanism suggested by Wroblowa et aL1’.
When adsorbable impurities are present, the steady-state potential of the Pt bead (Fig. 2b) was below 1 V, the cathodic stripping pulse showed virtually no surfaceadsorbed oxygen arrest at 700 mV, and the adsorbed hydrogen arrest was very narrow. Gahanostatic
studies
In one Teflon cell, the solution was prepared by preelectrolysis for 48 h against removable cathodes, followed by saturation of the solution with Hz to remove any peroxides present. The solution was then saturated with N,. In a second Teflon celi, the precleaned Pt bead electrodes were leached in triply distilled water and anodized at a constant current of 74 mA cmm2 for 2 h in 2 N HZS04solution (potential >2ooO mV). Finally, the anodized beads were transferred from the second cell to the N,-saturated 2 N HzS04 solution in the first cell. The electrode was pulsed giving the upper trace shown in Fig. 7c. When pulsed again after 100 s on open circuit, the lower trace of Fig. 7c was obtained_ The surface-adsorption and the dermasorption transition regions are substantially larger than those shown in Fig. 4. A value of 2.01 for the surface-adsorbed 0 was calculated as the average of six independent determinations, and similarly, 1.70 for the derrnasorbed 8. These data indicate that in the layer ofsurface adsorbed oxygen there is a ratio of 2 oxygen atoms to 1 Pt atom. It is possible that this strong anodic polarization has converted the monolayer of Pt-0 formed at lower potentials (Fig. 4e) to a monolayer of PtO, (Fig. 7~). A similar coneltiion was reached by Biegler and Woods4’. When the surface layer is stripped offwith a cathodic pulse in N.-saturated solution, a second pulse only 100 s later shows the presence of 81 PC cm-* of dermasorbed oxygen. It is evident from Fig. 7c that anodization greatly increases the amount of oxygen dissolved in the platinum as well as converting Pt-0 sites to PtO, sites. To investjgate the dependence of the amount of oxygen sorbed by Pt on the length of time oi anodization, experiments were carried out on thin wires, sealed in glass, in glass cells of conventional design_ As above, the wires were anodized in a separate cell and then transferred to the N,-saturated, peroxide-free acid solution in the test cell. The wire was anodized at constant current for various periods of time and was then pulsed in the cell after remaining in the N.-stirred solution for 15 min. Just J. Electroanal.
Chem,
30 (1971)l-14
: R_ THACKJZR,
TIME
J. P. HOARE
(min)
Fig. IO. A plot of 8 for adsorbed oxygen (circles) on an anodized Pt electrode as a fhnction of the time of anodization at 0.625 mA cm-* (0, A) and at 625 mA cm-” (0, A). The potential in N,-stirred acid solution (A} was recorded just before the stripping pulse was applied.
before the pulse was applied, the potential of the electrode was recorded. ‘. Figure 10 contains a plot of both the 8 for surface adsorbed oxygen (circles) and the &#e&al (t&x&s) as a function of the length of time the electrode was anodized at 0.625,mA cmm2. (open symbols) and 62.5 mA cme2 (filled symbols). At f&s&,both 8 ‘. tid_thq potential increase rapidly with the anodization time and then more slowly for &&onger periods of t&e.., Eventually, 8 reaches a value of about 2.
I
I
-’ a.’
I
I
I
0.90 I
.,
_
.: . Fic 11. Pl&df 8 for adsorbed oxygen (0) on-a ~rean~ Pt ekctrode (625 mA cm-*) as a fbnctidn’ of. -_, ..__’ ;.;. ~‘&&ti&e for &i& the ele&o&w~, .pett$d to haql in N2&irred a& sob before the St&ping pub! .I.-. 1’ -hi& ap’$ikd- ,T’he petehal’ .:. _. (A) lyas also recorded just befiiti+the .pulse was applied:
:
-:.,..
:_
_’ .,
:
I
.
SORPTION
OF OXYGEiN
BY BRIGHT
PLATINUM.
13
I
In another interesting experiment, the wires were anodized for 30 min at 62.5 mA cm-’ (potential > 2000 mV) and pulsed after resting in N.-stirred 2 N HzS04 solution for various periods of time. The 6. for surface-adsorbed oxygen along with the recorded potential are plotted in Fig_ 11 as a function of the resting time in N.-stirred acid solution. There is an exponential decay of both 8 and the potential. These results support the conclusion 42 that thin anodic films of PtO, are unstable in the presence of Pt at potentials below 1.55 V. Under: open-circuit conditions, the PtO, sites decompose to Pt-0 sites and to oxygen which may be dissolved in solution or in the metal. In conclusion, we submit-that the data presented in this report are consistent with an interpretation in terms of oxygen weakly adsorbed at surface sites, strongly sorbed at dermasorbed sites (in the first 1 or 2 atom layers of the metal), and dissolved. or absorbed in the bulk of the Pt metal. SUMMARY
Platinum electrodes in the form of beads or wires were permitted to sorb oxygen from O,-saturation H2SO4 solutions under steady-state conditions of constant current, constant potential, and open circuit. True areas were determined from constant current anodic p&es, and the amount of charge associated with the sorbed oxygen was determined from the measured transition times of potential-tune traces obtained with constant current, cathodic stripping pulses. Two kinds of adsorption sites are identified with respect to the difiuculty with which the sorbed oxygen is removed by a cathodic stripping pulse. The strong adsorption sites are located in the skin of the metal (first 1 or 2 atom layers of the metal) and the sorbed oxygen on these sites corresponds to the so-called dermasorbed oxygen. Weak adsorption sites are located on the metal surface, and the sorbed oxygen on these sites is called adsorbed oxygen. Oxygen which is dissolved in the interior of the metal beyond the dermasorb region is called absorbed oxygen. The amount of oxygen sorbed is a function of the potential. Below 800 mV, sorbed oxygen could not be detected. The first oxygen to be sorbed appears above 800 mV on the surface adsorption sites but dermasorbed oxygen is not detected until potentials greater than 1000 mV are reached. Under open-circuit conditions, the maximum amount of surface adsorbed oxygen present at steady state (potential = 1060 mV) &I O+aturated acid solution is 30% of a monolayer_ A complete layer of adsorbed oxygen (Pt-0) may be obtained with anodization at potentials of about 1600 mV_ At higher potentials ( > 2CKKlmV), the Pt-0 sites may be converted to PtO, sites until a maximum of a monolayer of PtO, is formed. Under open-circuit conditions, the PtO, sites in the presence of Pt decompose to Pt-0 sites. The equivalent of many complete layers of oxygen may be dissolved in the interior of the metal with anodic polarization, and this absorbed oxygen may replace any dermas orbed oxygen removed. REFERENCES
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30 (1971) l-14
‘,‘: _-
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