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Sorption of oxygen from solution by noble metals
ELEcTROANAJ._YTICAL c HE3UISTRY ABID INTERFACIAL Eisevier Sequoia S-A., Lausanne - Printed in The Netherlands
SORPTION II. NITRIC
JAMES
OF OXYGEN
ACID-PASSIVATED
P. HQARE,
Eiecrrochernirtry 48090 (U.S.A.)
RAYMOND
Department,
FROM BRIGHT
THACKER
Research
ELJZCTROCHEh%ISTRY
SOLUTION
BY
NOBLE
15
METALS
PLATINUM
AND
Laboratories,
CHARLES General
R. WIESE Motors
Corporation,
Warren.
Michigan
(Received 22nd June 1970)
It was suggested by one of USHERthat the rest potential exhibited by a Pt electrode immersed in O,-saturated acid solution is a mixed potential in which the cathodic reaction of the local cell is the OJHzO reaction, 0, + 4H+ + 4ed2H,O, and the anodic reaction is the Pt/Pt-0 reaction, Pt +H,O+Pt-0+2H+ + 2e. As long as “bare” Pt sites are present, the local cell is established since Pt is not inert to oxygen. If the Pt surface were covered by an electronically conducting film inert to oxygen, only the 0JH20 reaction should take place, and the reversible oxygen potential (1.229
V) should-be observed_ Although the local cell is established such that a layer of Pt-0 is formed, a complete monolayer is not reached because oxygen is continuously dissolved in the metal. As a result, steady state is reached when the surface is covered with adsorbed oxygen to the extent of 25-30% (rest potential is 1060 mV)“. There is evidence4-6 that the adsorbed Iayers of oxygen on Pt are good eiectronic conductors; and when a complete layer of adsorbed oxygen is deposited on a Pt electrode surface by exposing the Pt to pure oxygen at elevated temperatures for
extended periods of time’ - lo_ or by exposing the Pt to concentrated HN03
for about
72 h”,
the reversible oxygen potential is observed_ This report describes the results of an investigation carried out to obtain evidence for the nature of the adsorbed fii present on nitric acid-treated Pt/O, electrodes_ EXPERIMENTAL
According to the methods described before’ 1, small (- 0.13 cm in diameter) Pt (99.99 + o/0 pure) beads were melted at the .end of Pt wires and treated in concentrated HNO, for about 72 h. After soaking the electrodes in triply distilled water in a Teflon celll, they were transferred to a second Teflon cell which contained the prepared electrolyte, as described elsewhere1 ‘. Area determinations and the use of cathodic stripping pulses td ascertain the amount of sorbed oxygen on f&e electrode were car-r-ied out by means of the procedures presented in Part I3 of this series. All potentials are recorded with respect to the’norma! hydrogen electrode (NFE) unle& noted otherwise, and the-temperature for z& of these experiments~ was 24O& IT. I .‘. . A series of experiments was carried out in a glove-box in an atmosphere.of 3. Electroank.
them.;
30 (1911)
.
15-23
16
3.-P- HOARE,
R. THACKER,
C. R. WYESE
argon to study the role of atmospheric oxygen in the HNO, treatment of Pt. The Pt beads were prepared in the box in argon-saturated HNOs and then washed with argon-saturated triply distilled water in the glove-box. The Teflon cell and the electrolyte were prepared outside the box, and after saturating the 2 N H,SO, solution with Nz, the cell was transferred to the glove-box. When the washed beads were placed in the 02-free H,S04 solution, the potential was recorded and a stripping pulse was applied to the test electrodes. -As discussed later on, rigorous control of the oxygen content of the glove-box atniosphere could not be maintained, so a series of experiments was carried out in the all-glass cell diagrammed h Fig. 1. The cell was pickIed in concentrated HNOs for RESERVOIR
ELECTRODE
EHJI3SLEfl
v/
STOk6CK
ELECTRODE
STOPC6CK Fig. 1. Diagram
" of the all-glass
cell.
4 days and leached in distilled water for 7 days. In general, the solutions were prepared
in the reservoir, after which they were passed to the cell through the connecting tube. The interior of the system was protected against contact with the external atmosphere by the use of water bubblers on the gas exit ports and a collar (filled with water) around the top of the cell. The stopcocks had Teflon cores, and the only lubrication used on the stopcocks and the ground glass tops was triply distilled water_ After the Pt electrodes were cleaned in hot HN03, they were sealed, with an oxygen micro torch, in glass tubes connected to the cell top. Platinum gauzes served as counter (large) and reference (small) electrodes. Purified argon was passed through the cell and reservoir continuously, and the oxygen content of the exit gases was monitored with a Beckman oxygen analyzer. In a typical run, the reservoir was fried with concentrated HNO,, and after the HN03 was saturated with argon as noted by the oxygen analyzer, the cell was filled with the oxygen-free acid. The Pt beads were allowed to soak in the MO3 for 3 days. The HNOs was removed, and the ceU was washed many times with oxygen-free, triplyJ. ‘Electroanal. Chem,
30 (1971) M-43
SORPTION
OF OXYGEN
BY BRIGHT
PLAl2NUh-l.
U
17
in the reservoir. Finally, the cell was filled from the reservoir with oxygen-free, 2 N H2S04 which had been preelectrolyzed in a Teflon cell for about 48 h before it had been placed in the reservoir. While in the Teflon cell, the 2 N H2SOa solution was saturated with Hz to remove any peroxides that might have been generated in the preelectrolysis process. Cathodic stripping curves were obtained on the Pt test electrodes in the O#ee system. Afterward, purified oxygen was injected into the system, and stripping curves were obtained in the presence of oxygen. As before, the area of the test electrodes was determined from anodic charging curves.
distilled water which had been prepared
RESULTS
Cathodic
AND
DISCtJ.S!SION
stripping
studies
Platinum electrodes which had been treated in concentrated HMO3 were placed in clean, O,-saturated 2 N H2S04 solution. After the potential had come to a value of 1225 +_5 mV (siu independent determinations), a cathodic stripping pulse was applied. A typical trace is shown in Fig. 2a. Similar to the curves obtained in Part 13, the trace of Fig. 2a contains a low overvoltage transition region (corresponding to surface adsorbed oxygen) and a high overvoltage region (corresponding to dermasorbed oxygen and adsorbed hydrogen). From the value of 420 PC cm- 2 for the quantity of charge equivalent to a monolayer of adsorbed oxygen3*’ 2*13, the monolayer equivalent, 0, for the surface adsorbed oxygen was calculated from the trace of Fig. 2a to be 1.03 and for dermasorbed oxygen, 0.62 (after correction for the adsorbed hydrogen). In the presence of oxygen, the HNO,-treatment enabled the Pt surface to be covered with a complete layer of Pt-0. The composition of the adsorbed film is considered to be Pt-0 because thin films of PtO, are not stable in the presence of Pt meta16*14in a potential region below 1SOOmV,because oxygen is adsorbed on Pt by adissociative adsorption mechanism’~ “, and because PtO, is made by anodization above 1.7 V3*‘*l 5 (these electrodes had never been anodized).
Fig. 2 Cathodic stripping curvesobtained on’ HNOJ-passivated pt electrodes. x-axis: (a) and (c) 5, (b) 20, y-axis: (a)-(c) 500, (d) 350 mV cm- 1 ; current: (a) and (c) 2.15, .(b) 1, (d) 5.35 mk (d)2mscm-‘;
J. Efectroann~. Gem., 30 (1971) 15-23
18
J. P.
HOAFE,
R. T-HACKER,
C.
R.
WXESE
After the trace in Fig. 2a was obtained. the electrode was puked again after standing in N,- saturated acid solution for 3 s (bottom curve of Fig. 2b), 15 s (middle curve), 500 s (top curve), and 1100 s (Fig. 2~). In as short a time as 15 s after the preceding pulse, the appearance of adsorbed oxygen can be detected on the trace. A value of 0.68 is found for 6 for adsorbed oxygen and 0.59 for 8 for derrnasorbed oxygen after 1100s. This behavior is to be contrasted with that of untreated Pt3 where the maximum amount of adsorbed oxygen was ever observed to exceed 30% of a monolayer. These high values of 8 occurred with correspondingly high values of the rest potential as shown by the plot (Fig. 3) of the amount of oxygen adsorbed, 8, on the
I
I
I
1.10 POTENTIAL
1.05
I
vs
1.15 N HE
I
(volt)
I
120
I
125
Fig.3.A pfot of the surface coverage in monolayer equivalents, 0, as a function of the rest potential with respect to the NHE_
Pt surface as a function of the rest potential of HNO,-treited Pt eIectrodes. It is seen that 0 increases linearly with the rest potential. At potentials close to the reversible oxygen potential, the surface coverage is about unity; at potentials close to the rest potential of untreated Pi, 6 has a value of about 0.25. Rao et aLi observed that the maximum amount of adsorbed oxygen on an untreated Pt electrode was about 25% of a monolayer, and the value of 6 for several other metals suggested that the maximum adsorption of oxygen is a function of the electronic structure of the metal. For a series of Pt-Rh alloys, 8 increased with the Rh content i 7, indicating that the adsorption of oxygen increased as the number of holes in the d-band in&eased. From X-ray studies of untreated and treated Pt electrodesl’, it was found that
oxygen was dissolved in treated Pt to such an extent that the Iattice was expanded_ The HNO,-treated platinum-oxygen system is considered5~‘1*fs to be an alloy of Pt and 0 atoms so that it is essentially a different electrode material from untreated Pt. The results reported here support this conclusion. On open circuit, a pt-0 alloy electrode can adsorb over three times as much oxygen
from O,-saturated
acid solution as can untreatecJ_R thus supporting the
contention that the electronic structure of the Pt has been modified by the dissolution J- E~ecmxmal.
Chem.,
30 (2971) 15-23
SORPTION
OF OXYGEN
BY BRIGHT
PLATINUM.
19
II
of oxygen in the Pt lattice. Apparently, the alloying of Pt with oxygen has the same effect on the adsorption of oxygen as does increasing the holes in the d-band by alloying Pt with Rh. How the dissolved oxygen modifies the band structure of Pt is not known at this time. Schuldiner and co-workers1 g pointed out that the presence of dermasorbed oxygen modified the catalytic activity of Pt for hydrogen and oxygen electrode processes. Dermasorbed oxygen is believed6 to be the agent responsible for the good reversible properties of preanodized Pt indicator electrodes for reduction processes20It has been found’l that the Pt-0 alloy is a superior catalyst for the reduction of oxygen. Since the catalytic activity of the electrode surface for certain electrode reactions may be related to the electronic structure of the electrode materia121- 23, these observations offer further evidence in favor of the Pt-0 alloy concept of HN03treated Pt. It is to be noted that the rate of change of oxygen coverage of the surface with potential is different for the HNO,-treated than for the untreated Pt. From the data of Fig. 3, the coverage changes with potential at a rate of 2.02 ,QC cmm2 rnV_l, whereas a rate of 0.44 I_LCcm-’ mV- 1 is obtained from the data plotted in Fig. 8 of ref. 3. Such results would be expected for electrodes composed of different materials_ If an unclean Pt electrode (not degreased) is treated with HN03, no oxygen is sorbed as demonstrated by the trace in Fig. 2d. Interaction of the Pt with the HNO, has been prevented by the film of grease. Duration of passivation studies In a series of experiments, clean Pt electrodes were passivated’in concentrated
HN03 for various periods of time after which they were removed from the acid and washed in triply distilled water for about 25 min. These electrodes were placed in O,-saturated 2 N H2S04 solution, and the oxygen coverage was determined from a cathodic stripping pulse. For a given run, the pulses obtained after 1,3,6, and 69 h of passivation in HN03 are displayed in Fig 4a-d_ respectively. The amount of oxygen adsorbed on the surface (circles) as well as the dermasorbed oxygen (triangles) are
Fig 4. Cathodic stripping c&es of time in contintrated HNO,;
obtied on a Pt electrode. which had been passivated~ for various lengths (a) 1, (b) 3, (c) 6; (d) 69 h; x-axis= 5 ms cm- ’ ; y-axis = 350 mV cm- * ;
current=222mk J. Electroanal.
Chem.,
30 (1971) 15-23
220 .,
J. P. HOAR&
R. THACKER,
6.
R.
T
-pl&d ik Fig. 5 in terms of monolayer equivalents, 8, as a function of the time of pa&iv&ion in the HN03. Included in Fig. 5 is the rest potential (inverted triangles) recorded
just
before
the stripping
pulse
was
applied_
I-
3-
1 3’
Fig. 5-A plot of the amount
sorbed by Pt, as surface adsorbed (circles) and dermasorbed (triangles) equivalents, 0, as a function of the time for which the Pt was @ssivated in cont. lHN03. Included is the rest pdtenrial (inverted triangles) VS.NHE as a function ofthe passivation time of oxygen
oxygen in terms of monolayer
The -sorption of oxygen rises exponentially with the time of passivation in ‘HN03 to a maximum value in about 30 h, after which further passivation (over a week) produces no further change in the parameters plotted in Fig. 5. In this timeinvariant- region, the surface is covered with a complete monolayer of Pt-0, the dermworbed layers are saturated with oxygen (equivalent of two layers is observed), and ‘the rest potential is the reversible potential. Stability. studies . It Was observed11*18 that after strong polarization OFa HNO,-passivated Pt .’ electrode, &e-reversible potential could not be regained on open-circuit conditions. However, .&e rest p&ntials (as well aS the catalytic activity toward’ O2 reduction) ~,tie@ @I much higher (1100-1150 mV) than on untreated Pt (1060 mV). It,.is .belieded’ lY1.8that the Pt-0 alloy stiucture is majntained but the complete monolayer of ads&b.&&3ti,xyieti is not, To obt&n evidZnce to ‘support such a viewpoint;-HNO,p&sivated eIectrddes.w,ere cathodized in %,-stirred HiSO solution well into the H2 ev&tioti r&i& (1OO.m.Acm-‘) for 20 min, after which they were allowed to ,re&+ ‘o%&p+-‘ci+it .iii-iV-stietid s~oli~tionfor. 1 h. Values ‘of 0.61 and- 1.29 were obtained ’ @n “the shyipping curve for. surface absorbed and de&nasor&d 0,.resp+ive!y (&st p~t~ti~~l~~aS_ iOg0: niv). @&~ah.ies’of 0 aqd the r+t potential +re.charactee&tic of tr+ated.:.Pt,~r#tilr_tha-fi&treated Pt3.-bpparently, any o*geti re@&& from t@e sur.fac&t&e c&-m&s~~b&d regi& was’
30 (167 1j_IFi23
_
SO-ON
OF OXYGEN
BY BRIGHT
PLATE’iiM.
II
21
plete layer of surface adsorbed oxygen is not present. When the beads were removed and heated to a red heat in a H, flame and then replaced in the N.-stirred HZS04 solution, a steady rest potential of about 1085 mV was recorded, and the surface was covered to the extent of about 47% with adsorbed oxygen. The 8 value for dermasorbed oxygen was still 1.29. Only after heating to a white heat did the beads behave as untreated Pt again. After such treatment, the rest potential in 02-saturated solution was 1055 mV, 0 for adsorbed oxygen was 0.35, and 8 for dermasorbed oxygen was 0.32. Apparently it requires melting of the Pt to drive out all of the dissolved oxygen and to convert the expanded lattice of the Pt-0 alloy structure to the untreated Pt structure. Extended periods (- 2 days) of extreme cathodization will also produce the conversion of the alloy structure to that of untreated Pt. Anodization
studies
From the data of Fig. 5, it is seen that the surface of a HNO,-treated Pt electrode is covered by a layer of adsorbed Pt-0 which does not grow beyond a monolayer by adsorption of oxygen from solution. If, however, such an electrode is anodized, the Pt-0 sites may be converted to PtO, sites. For example, a HNOs-treated Pt bead was anodized at 50 mA cmw2 for 30 min (potential - 2100 mV) in N2-stirred H,SO, solution_ After opening the circuit, the potential fell as observed in the untreated case3. When the stripping pulse was applied, the potential was 1468 mV, and a 8 value of 1.98 was determin ed for the adsorbed oxygen. As in the untreated case3, it was found that PtO, is unstable and decomposes to Pt-0. It was also observed that anodization does not destroy the Pt-0 alloy structure, although the complete layer of Pt-0 is impaired so that the reversible potential is no longer reached. Inert
atmosphere
studies
To study the role played by atmospheric oxygen in the HN03 passivation of Pt, a group of experiments was carried out in an inert atmosphere_ When an HN03treated Pt electrode was placed in N,-stirred acid solution and pulsed with a cathodic stripping puke, it was found that the surface was completely covered with a monolayer of Pt-0. In this case, the potential ranged between 1180 and 1210 mV in duplicate runs. Meibuhrz4 reported that Pt passivated in argon-saturated concentrated HNO, exhibited a potential of 1230 mV in O,-saturated HISO solutions. But in au these cases, the system had come in contact with oxygen at some point in the process. A series of runs was then carried out in a glove-box such that the Pt electrodes which were passivated in argon-saturated HN03 were placed directly in N,- or argon-saturated H,SO, solution_ Under these circumstances, the constant current stripping curve showed that the surface was covered with oxygen to the extent of only 4145°/0. The 8 for dermasorbed oxygen was 0.675. After7 min, the electrodes were pulsed again, and 8 values of 0.16 to 025 were found- for surface adsorbed oxygen- Absorbed oxygen diffusing from the metal interior had replaced the sorbed oxygen stripped by the first puke_ Since it was discovered that the gloveYbox could not be maintained oxygen-free by merely flushing the box with argon, a further series of experiments was made in the. all-glass cell of Fig.. 1 because it was suspected that the dermasorbed oxygen was not produced by the interaction of Pt with HNOs. In this case, the Pt heads were treated J. Electroanal. Chem, 36 (1971) 15-23
‘22
.,
J. P. HOARE,
R. THACKER,
C. R. WISE
I with argon-saturated HNO,, washed in argon-saturated, triply-distilled water, and studied in argon-saturated 2,.N H,SOi solution all in the same vessel. These experiments were done in triplicate. From an analysis of the cathodic stripping curves obtained, it was found that the ‘8 for surface adsorbed oxygen ranged in value from 0.10 to 0.11. It appears that so&e oxygen did enter the system probably through the polyethylene tubing which connected the argon tanks to, the cold traps and possibly through the Teff on stopcocks. If extreme precautions were taken for the exclusion of oxygen from the system in a manner similar to those techniques employed by Schuldiner and co-workers12*1g*25, ie.might be possible to prepare a HNO,-passivated Pt electrode whic& had sorbed no oxygen. As soon as oxygen was permitted to enter the system, the quantity of oxygen sorbed by the Pt treated in argon increased toward those values found for Pt treated in fhe presence of oxygen. From this series of experiments, it is concluded that the 13N03 does not inter.act with:the Pt in such a way that oxygen which might be liberated in the process would be dissolved in the Pt. In other words, the dissolved oxygen of the Pt-0 alloy does not cgme from the nitric acid. The role played by the HN03 seems to be the ability to render the Pt more accommodating to the dissolution of oxygen. The HN03-treated Pt becomes a sponge or a getter for oxygen present in the media surrounding the Pt. With .t_hedissolution of oxygen from the surroundings, the Pt-0 alloy is produced with catalytic and electrochemical properties different from those of untreated Pt. How this process takes place is not clear at this time, and the explanation will
have
to await
the results
of further
investigation.
SUhthaARY
The nature of the adsorbed films produced on platinum electrodes treated with concentrated nitric acid was studied by means of constant current stripping pulses as a fiinction of the time of passivation and the contact with atmospheric oxygen. It is concluded that the dissolved oxygen of the Pt-0 alloy does not come from the HNO,. Instead, the 13N03 treatment makes the Pt more accommodating to the sorption of .oxygen (in the manner of tin oxygen sponge), and the Pt then sorbs oxygen from the surrdutidings producing the Pt-0 alloy.,The presence of the oxygen in the Pt lattice .‘modifies the electronic structure of the Pt, generating an electrode material with different catalytic and electronic properties from untreated Pt. In the presence of oxygen, the HN03 treatment produces a Pt-0 alloy electrode with a complete monolayer of R-0 adsorbed on the surface. Such an electrode exhibits the reversible oxygen potential in O,-saturated, 2 lV H2S04 solution,.The rest potential is a linear function ._ of the.,d&gree of surface coverage. No, more than a monolay& is adsorbed by this procedure,. but the equivalknt of,two layers may be formed by anod&ng the Pt-0 : alloy e!&tiode.- Appar&tly the Pt-0 sites aire converted to PtO, sites which are ti&bie:at. potentials below 1’500 mV in the presence of Pt metal. The Pt-0 alloy : shcttire 6 father stable and is converted to,that of untreated Pt by heating the metal in-.the .‘&i&y of-its melt&g point. .:_ ._’
_.-
SORPTION
OF OXYGEN
BY BRIGNT
PLATINUM.
II
23
REFERENCES J. P. HOARE, J. Electrochem. Sot., 109 (1962) 858. J. P. HOARE, J. Elecfrochem. Sot., 112 (1965) 602. R. THACICEFC AND J. P. HOARE. J. Electroanai. Chem., 30 (1971) 1. W. BULD AND M. W. BREITER, Efecrrochim. Acta, 5 (1962) 145. J. P. HOARE, Nature, 204 (1964) 71. J. P. HOARE, J. Electroanai. Chem., 12 (1966)-260. J. O’M. E~OCKRIS AND A. K. M. S. HUQ, Proc. Roy. SOC. London, A237 (1956) 277. H. WROBLOWA, M. L. B. RAO. A_ DAMJANOVIC AND J. O’M. B~CKRIS. J. Eiecrroanak Chem., 15 (1967) 139. 9 N. WATANABE AND M. A. V. DEVANATHAN, J. E!ectrochem. Sot.. 111 (1964) 615. AND T. MUSSINI, E/ecrrochim. Acta, 10 (1965) 445. 10 G. BUNCHI 11 J. P. HOAE, J- Eiectrochem. Sot., 110 (1963) 1019; 112 (1965) 849. 12 S. SCHULDKNEFC AND R. M. ROE, J. Efecfrochem. SOC., 1 LO (1963) 332. 13 S. TRASAI-I-I. E!eclrochim. Merall., 2 (1967) 12. Sac., 113 (1966) 846. 14 J. P. HOARE, J. Efectrochem. 15 A. DAMJANOVIC, M. L. B. RAO AND M. GENSI-I&W, ASTIA Rept. No. AD405675, Nov. 1962. 16 M. L. B. RAO, A. DA~UANOVIC AND J. O’M. BOCKRIS, J. Phys. Chem.. 67 (0963) 2508. 17 J. P. HOARE, Eiectrochim. Acta, 14 (1969) 797. 18 J. P. HOARE, S. G. MEIBUHR AND R_ THACKER. J_ Eiecrrochem. Sot., 113 (1966) 1078. 19 S. SCHIJLDINER, B. J. Puxs~~ AND T. B. WARNER. J. Electrochem. SW., 112 (1965) 212; 113 (1966) 573. of Oxygen, Interscience Publishers. Inc., New York, 1968, p_ 171_ 20 J. P. HOARE, The Electrochemistry 21 M. OIKAWA. Bull. Chem. Sot. Japan, 28 (.1955) 626. J. P. HOARE, J. Phys. Chem.,61 (1957)705;62 (1958)229,504. 1608; J. Electrochem. 22 S. SCHULDINERAND Sot., 107 (1960) 820. 23 P. RUIXSCHI AND P. D EL_AHAY, J. Chem. Ph_v.s., 23 (1955) 195, 697. 24 S. G. Mnsvwn, J. Efectrochem. Sot.. 11.5 (1968) 725. AND T. B. WARMR, J. Electrochem. SOC., 112 (1965) 212. 25 S. SCHULDINER L J. Elecrroanal.
Chem,
30 (1971) 15-23
SORPTION II. NITRIC
JAMES
OF OXYGEN
ACID-PASSIVATED
P. HQARE,
Eiecrrochernirtry 48090 (U.S.A.)
RAYMOND
Department,
FROM BRIGHT
THACKER
Research
ELJZCTROCHEh%ISTRY
SOLUTION
BY
NOBLE
15
METALS
PLATINUM
AND
Laboratories,
CHARLES General
R. WIESE Motors
Corporation,
Warren.
Michigan
(Received 22nd June 1970)
It was suggested by one of USHERthat the rest potential exhibited by a Pt electrode immersed in O,-saturated acid solution is a mixed potential in which the cathodic reaction of the local cell is the OJHzO reaction, 0, + 4H+ + 4ed2H,O, and the anodic reaction is the Pt/Pt-0 reaction, Pt +H,O+Pt-0+2H+ + 2e. As long as “bare” Pt sites are present, the local cell is established since Pt is not inert to oxygen. If the Pt surface were covered by an electronically conducting film inert to oxygen, only the 0JH20 reaction should take place, and the reversible oxygen potential (1.229
V) should-be observed_ Although the local cell is established such that a layer of Pt-0 is formed, a complete monolayer is not reached because oxygen is continuously dissolved in the metal. As a result, steady state is reached when the surface is covered with adsorbed oxygen to the extent of 25-30% (rest potential is 1060 mV)“. There is evidence4-6 that the adsorbed Iayers of oxygen on Pt are good eiectronic conductors; and when a complete layer of adsorbed oxygen is deposited on a Pt electrode surface by exposing the Pt to pure oxygen at elevated temperatures for
extended periods of time’ - lo_ or by exposing the Pt to concentrated HN03
for about
72 h”,
the reversible oxygen potential is observed_ This report describes the results of an investigation carried out to obtain evidence for the nature of the adsorbed fii present on nitric acid-treated Pt/O, electrodes_ EXPERIMENTAL
According to the methods described before’ 1, small (- 0.13 cm in diameter) Pt (99.99 + o/0 pure) beads were melted at the .end of Pt wires and treated in concentrated HNO, for about 72 h. After soaking the electrodes in triply distilled water in a Teflon celll, they were transferred to a second Teflon cell which contained the prepared electrolyte, as described elsewhere1 ‘. Area determinations and the use of cathodic stripping pulses td ascertain the amount of sorbed oxygen on f&e electrode were car-r-ied out by means of the procedures presented in Part I3 of this series. All potentials are recorded with respect to the’norma! hydrogen electrode (NFE) unle& noted otherwise, and the-temperature for z& of these experiments~ was 24O& IT. I .‘. . A series of experiments was carried out in a glove-box in an atmosphere.of 3. Electroank.
them.;
30 (1911)
.
15-23
16
3.-P- HOARE,
R. THACKER,
C. R. WYESE
argon to study the role of atmospheric oxygen in the HNO, treatment of Pt. The Pt beads were prepared in the box in argon-saturated HNOs and then washed with argon-saturated triply distilled water in the glove-box. The Teflon cell and the electrolyte were prepared outside the box, and after saturating the 2 N H,SO, solution with Nz, the cell was transferred to the glove-box. When the washed beads were placed in the 02-free H,S04 solution, the potential was recorded and a stripping pulse was applied to the test electrodes. -As discussed later on, rigorous control of the oxygen content of the glove-box atniosphere could not be maintained, so a series of experiments was carried out in the all-glass cell diagrammed h Fig. 1. The cell was pickIed in concentrated HNOs for RESERVOIR
ELECTRODE
EHJI3SLEfl
v/
STOk6CK
ELECTRODE
STOPC6CK Fig. 1. Diagram
" of the all-glass
cell.
4 days and leached in distilled water for 7 days. In general, the solutions were prepared
in the reservoir, after which they were passed to the cell through the connecting tube. The interior of the system was protected against contact with the external atmosphere by the use of water bubblers on the gas exit ports and a collar (filled with water) around the top of the cell. The stopcocks had Teflon cores, and the only lubrication used on the stopcocks and the ground glass tops was triply distilled water_ After the Pt electrodes were cleaned in hot HN03, they were sealed, with an oxygen micro torch, in glass tubes connected to the cell top. Platinum gauzes served as counter (large) and reference (small) electrodes. Purified argon was passed through the cell and reservoir continuously, and the oxygen content of the exit gases was monitored with a Beckman oxygen analyzer. In a typical run, the reservoir was fried with concentrated HNO,, and after the HN03 was saturated with argon as noted by the oxygen analyzer, the cell was filled with the oxygen-free acid. The Pt beads were allowed to soak in the MO3 for 3 days. The HNOs was removed, and the ceU was washed many times with oxygen-free, triplyJ. ‘Electroanal. Chem,
30 (1971) M-43
SORPTION
OF OXYGEN
BY BRIGHT
PLAl2NUh-l.
U
17
in the reservoir. Finally, the cell was filled from the reservoir with oxygen-free, 2 N H2S04 which had been preelectrolyzed in a Teflon cell for about 48 h before it had been placed in the reservoir. While in the Teflon cell, the 2 N H2SOa solution was saturated with Hz to remove any peroxides that might have been generated in the preelectrolysis process. Cathodic stripping curves were obtained on the Pt test electrodes in the O#ee system. Afterward, purified oxygen was injected into the system, and stripping curves were obtained in the presence of oxygen. As before, the area of the test electrodes was determined from anodic charging curves.
distilled water which had been prepared
RESULTS
Cathodic
AND
DISCtJ.S!SION
stripping
studies
Platinum electrodes which had been treated in concentrated HMO3 were placed in clean, O,-saturated 2 N H2S04 solution. After the potential had come to a value of 1225 +_5 mV (siu independent determinations), a cathodic stripping pulse was applied. A typical trace is shown in Fig. 2a. Similar to the curves obtained in Part 13, the trace of Fig. 2a contains a low overvoltage transition region (corresponding to surface adsorbed oxygen) and a high overvoltage region (corresponding to dermasorbed oxygen and adsorbed hydrogen). From the value of 420 PC cm- 2 for the quantity of charge equivalent to a monolayer of adsorbed oxygen3*’ 2*13, the monolayer equivalent, 0, for the surface adsorbed oxygen was calculated from the trace of Fig. 2a to be 1.03 and for dermasorbed oxygen, 0.62 (after correction for the adsorbed hydrogen). In the presence of oxygen, the HNO,-treatment enabled the Pt surface to be covered with a complete layer of Pt-0. The composition of the adsorbed film is considered to be Pt-0 because thin films of PtO, are not stable in the presence of Pt meta16*14in a potential region below 1SOOmV,because oxygen is adsorbed on Pt by adissociative adsorption mechanism’~ “, and because PtO, is made by anodization above 1.7 V3*‘*l 5 (these electrodes had never been anodized).
Fig. 2 Cathodic stripping curvesobtained on’ HNOJ-passivated pt electrodes. x-axis: (a) and (c) 5, (b) 20, y-axis: (a)-(c) 500, (d) 350 mV cm- 1 ; current: (a) and (c) 2.15, .(b) 1, (d) 5.35 mk (d)2mscm-‘;
J. Efectroann~. Gem., 30 (1971) 15-23
18
J. P.
HOAFE,
R. T-HACKER,
C.
R.
WXESE
After the trace in Fig. 2a was obtained. the electrode was puked again after standing in N,- saturated acid solution for 3 s (bottom curve of Fig. 2b), 15 s (middle curve), 500 s (top curve), and 1100 s (Fig. 2~). In as short a time as 15 s after the preceding pulse, the appearance of adsorbed oxygen can be detected on the trace. A value of 0.68 is found for 6 for adsorbed oxygen and 0.59 for 8 for derrnasorbed oxygen after 1100s. This behavior is to be contrasted with that of untreated Pt3 where the maximum amount of adsorbed oxygen was ever observed to exceed 30% of a monolayer. These high values of 8 occurred with correspondingly high values of the rest potential as shown by the plot (Fig. 3) of the amount of oxygen adsorbed, 8, on the
I
I
I
1.10 POTENTIAL
1.05
I
vs
1.15 N HE
I
(volt)
I
120
I
125
Fig.3.A pfot of the surface coverage in monolayer equivalents, 0, as a function of the rest potential with respect to the NHE_
Pt surface as a function of the rest potential of HNO,-treited Pt eIectrodes. It is seen that 0 increases linearly with the rest potential. At potentials close to the reversible oxygen potential, the surface coverage is about unity; at potentials close to the rest potential of untreated Pi, 6 has a value of about 0.25. Rao et aLi observed that the maximum amount of adsorbed oxygen on an untreated Pt electrode was about 25% of a monolayer, and the value of 6 for several other metals suggested that the maximum adsorption of oxygen is a function of the electronic structure of the metal. For a series of Pt-Rh alloys, 8 increased with the Rh content i 7, indicating that the adsorption of oxygen increased as the number of holes in the d-band in&eased. From X-ray studies of untreated and treated Pt electrodesl’, it was found that
oxygen was dissolved in treated Pt to such an extent that the Iattice was expanded_ The HNO,-treated platinum-oxygen system is considered5~‘1*fs to be an alloy of Pt and 0 atoms so that it is essentially a different electrode material from untreated Pt. The results reported here support this conclusion. On open circuit, a pt-0 alloy electrode can adsorb over three times as much oxygen
from O,-saturated
acid solution as can untreatecJ_R thus supporting the
contention that the electronic structure of the Pt has been modified by the dissolution J- E~ecmxmal.
Chem.,
30 (2971) 15-23
SORPTION
OF OXYGEN
BY BRIGHT
PLATINUM.
19
II
of oxygen in the Pt lattice. Apparently, the alloying of Pt with oxygen has the same effect on the adsorption of oxygen as does increasing the holes in the d-band by alloying Pt with Rh. How the dissolved oxygen modifies the band structure of Pt is not known at this time. Schuldiner and co-workers1 g pointed out that the presence of dermasorbed oxygen modified the catalytic activity of Pt for hydrogen and oxygen electrode processes. Dermasorbed oxygen is believed6 to be the agent responsible for the good reversible properties of preanodized Pt indicator electrodes for reduction processes20It has been found’l that the Pt-0 alloy is a superior catalyst for the reduction of oxygen. Since the catalytic activity of the electrode surface for certain electrode reactions may be related to the electronic structure of the electrode materia121- 23, these observations offer further evidence in favor of the Pt-0 alloy concept of HN03treated Pt. It is to be noted that the rate of change of oxygen coverage of the surface with potential is different for the HNO,-treated than for the untreated Pt. From the data of Fig. 3, the coverage changes with potential at a rate of 2.02 ,QC cmm2 rnV_l, whereas a rate of 0.44 I_LCcm-’ mV- 1 is obtained from the data plotted in Fig. 8 of ref. 3. Such results would be expected for electrodes composed of different materials_ If an unclean Pt electrode (not degreased) is treated with HN03, no oxygen is sorbed as demonstrated by the trace in Fig. 2d. Interaction of the Pt with the HNO, has been prevented by the film of grease. Duration of passivation studies In a series of experiments, clean Pt electrodes were passivated’in concentrated
HN03 for various periods of time after which they were removed from the acid and washed in triply distilled water for about 25 min. These electrodes were placed in O,-saturated 2 N H2S04 solution, and the oxygen coverage was determined from a cathodic stripping pulse. For a given run, the pulses obtained after 1,3,6, and 69 h of passivation in HN03 are displayed in Fig 4a-d_ respectively. The amount of oxygen adsorbed on the surface (circles) as well as the dermasorbed oxygen (triangles) are
Fig 4. Cathodic stripping c&es of time in contintrated HNO,;
obtied on a Pt electrode. which had been passivated~ for various lengths (a) 1, (b) 3, (c) 6; (d) 69 h; x-axis= 5 ms cm- ’ ; y-axis = 350 mV cm- * ;
current=222mk J. Electroanal.
Chem.,
30 (1971) 15-23
220 .,
J. P. HOAR&
R. THACKER,
6.
R.
T
-pl&d ik Fig. 5 in terms of monolayer equivalents, 8, as a function of the time of pa&iv&ion in the HN03. Included in Fig. 5 is the rest potential (inverted triangles) recorded
just
before
the stripping
pulse
was
applied_
I-
3-
1 3’
Fig. 5-A plot of the amount
sorbed by Pt, as surface adsorbed (circles) and dermasorbed (triangles) equivalents, 0, as a function of the time for which the Pt was @ssivated in cont. lHN03. Included is the rest pdtenrial (inverted triangles) VS.NHE as a function ofthe passivation time of oxygen
oxygen in terms of monolayer
The -sorption of oxygen rises exponentially with the time of passivation in ‘HN03 to a maximum value in about 30 h, after which further passivation (over a week) produces no further change in the parameters plotted in Fig. 5. In this timeinvariant- region, the surface is covered with a complete monolayer of Pt-0, the dermworbed layers are saturated with oxygen (equivalent of two layers is observed), and ‘the rest potential is the reversible potential. Stability. studies . It Was observed11*18 that after strong polarization OFa HNO,-passivated Pt .’ electrode, &e-reversible potential could not be regained on open-circuit conditions. However, .&e rest p&ntials (as well aS the catalytic activity toward’ O2 reduction) ~,tie@ @I much higher (1100-1150 mV) than on untreated Pt (1060 mV). It,.is .belieded’ lY1.8that the Pt-0 alloy stiucture is majntained but the complete monolayer of ads&b.&&3ti,xyieti is not, To obt&n evidZnce to ‘support such a viewpoint;-HNO,p&sivated eIectrddes.w,ere cathodized in %,-stirred HiSO solution well into the H2 ev&tioti r&i& (1OO.m.Acm-‘) for 20 min, after which they were allowed to ,re&+ ‘o%&p+-‘ci+it .iii-iV-stietid s~oli~tionfor. 1 h. Values ‘of 0.61 and- 1.29 were obtained ’ @n “the shyipping curve for. surface absorbed and de&nasor&d 0,.resp+ive!y (&st p~t~ti~~l~~aS_ iOg0: niv). @&~ah.ies’of 0 aqd the r+t potential +re.charactee&tic of tr+ated.:.Pt,~r#tilr_tha-fi&treated Pt3.-bpparently, any o*geti re@&& from t@e sur.fac&t&e c&-m&s~~b&d regi& was’
30 (167 1j_IFi23
_
SO-ON
OF OXYGEN
BY BRIGHT
PLATE’iiM.
II
21
plete layer of surface adsorbed oxygen is not present. When the beads were removed and heated to a red heat in a H, flame and then replaced in the N.-stirred HZS04 solution, a steady rest potential of about 1085 mV was recorded, and the surface was covered to the extent of about 47% with adsorbed oxygen. The 8 value for dermasorbed oxygen was still 1.29. Only after heating to a white heat did the beads behave as untreated Pt again. After such treatment, the rest potential in 02-saturated solution was 1055 mV, 0 for adsorbed oxygen was 0.35, and 8 for dermasorbed oxygen was 0.32. Apparently it requires melting of the Pt to drive out all of the dissolved oxygen and to convert the expanded lattice of the Pt-0 alloy structure to the untreated Pt structure. Extended periods (- 2 days) of extreme cathodization will also produce the conversion of the alloy structure to that of untreated Pt. Anodization
studies
From the data of Fig. 5, it is seen that the surface of a HNO,-treated Pt electrode is covered by a layer of adsorbed Pt-0 which does not grow beyond a monolayer by adsorption of oxygen from solution. If, however, such an electrode is anodized, the Pt-0 sites may be converted to PtO, sites. For example, a HNOs-treated Pt bead was anodized at 50 mA cmw2 for 30 min (potential - 2100 mV) in N2-stirred H,SO, solution_ After opening the circuit, the potential fell as observed in the untreated case3. When the stripping pulse was applied, the potential was 1468 mV, and a 8 value of 1.98 was determin ed for the adsorbed oxygen. As in the untreated case3, it was found that PtO, is unstable and decomposes to Pt-0. It was also observed that anodization does not destroy the Pt-0 alloy structure, although the complete layer of Pt-0 is impaired so that the reversible potential is no longer reached. Inert
atmosphere
studies
To study the role played by atmospheric oxygen in the HN03 passivation of Pt, a group of experiments was carried out in an inert atmosphere_ When an HN03treated Pt electrode was placed in N,-stirred acid solution and pulsed with a cathodic stripping puke, it was found that the surface was completely covered with a monolayer of Pt-0. In this case, the potential ranged between 1180 and 1210 mV in duplicate runs. Meibuhrz4 reported that Pt passivated in argon-saturated concentrated HNO, exhibited a potential of 1230 mV in O,-saturated HISO solutions. But in au these cases, the system had come in contact with oxygen at some point in the process. A series of runs was then carried out in a glove-box such that the Pt electrodes which were passivated in argon-saturated HN03 were placed directly in N,- or argon-saturated H,SO, solution_ Under these circumstances, the constant current stripping curve showed that the surface was covered with oxygen to the extent of only 4145°/0. The 8 for dermasorbed oxygen was 0.675. After7 min, the electrodes were pulsed again, and 8 values of 0.16 to 025 were found- for surface adsorbed oxygen- Absorbed oxygen diffusing from the metal interior had replaced the sorbed oxygen stripped by the first puke_ Since it was discovered that the gloveYbox could not be maintained oxygen-free by merely flushing the box with argon, a further series of experiments was made in the. all-glass cell of Fig.. 1 because it was suspected that the dermasorbed oxygen was not produced by the interaction of Pt with HNOs. In this case, the Pt heads were treated J. Electroanal. Chem, 36 (1971) 15-23
‘22
.,
J. P. HOARE,
R. THACKER,
C. R. WISE
I with argon-saturated HNO,, washed in argon-saturated, triply-distilled water, and studied in argon-saturated 2,.N H,SOi solution all in the same vessel. These experiments were done in triplicate. From an analysis of the cathodic stripping curves obtained, it was found that the ‘8 for surface adsorbed oxygen ranged in value from 0.10 to 0.11. It appears that so&e oxygen did enter the system probably through the polyethylene tubing which connected the argon tanks to, the cold traps and possibly through the Teff on stopcocks. If extreme precautions were taken for the exclusion of oxygen from the system in a manner similar to those techniques employed by Schuldiner and co-workers12*1g*25, ie.might be possible to prepare a HNO,-passivated Pt electrode whic& had sorbed no oxygen. As soon as oxygen was permitted to enter the system, the quantity of oxygen sorbed by the Pt treated in argon increased toward those values found for Pt treated in fhe presence of oxygen. From this series of experiments, it is concluded that the 13N03 does not inter.act with:the Pt in such a way that oxygen which might be liberated in the process would be dissolved in the Pt. In other words, the dissolved oxygen of the Pt-0 alloy does not cgme from the nitric acid. The role played by the HN03 seems to be the ability to render the Pt more accommodating to the dissolution of oxygen. The HN03-treated Pt becomes a sponge or a getter for oxygen present in the media surrounding the Pt. With .t_hedissolution of oxygen from the surroundings, the Pt-0 alloy is produced with catalytic and electrochemical properties different from those of untreated Pt. How this process takes place is not clear at this time, and the explanation will
have
to await
the results
of further
investigation.
SUhthaARY
The nature of the adsorbed films produced on platinum electrodes treated with concentrated nitric acid was studied by means of constant current stripping pulses as a fiinction of the time of passivation and the contact with atmospheric oxygen. It is concluded that the dissolved oxygen of the Pt-0 alloy does not come from the HNO,. Instead, the 13N03 treatment makes the Pt more accommodating to the sorption of .oxygen (in the manner of tin oxygen sponge), and the Pt then sorbs oxygen from the surrdutidings producing the Pt-0 alloy.,The presence of the oxygen in the Pt lattice .‘modifies the electronic structure of the Pt, generating an electrode material with different catalytic and electronic properties from untreated Pt. In the presence of oxygen, the HN03 treatment produces a Pt-0 alloy electrode with a complete monolayer of R-0 adsorbed on the surface. Such an electrode exhibits the reversible oxygen potential in O,-saturated, 2 lV H2S04 solution,.The rest potential is a linear function ._ of the.,d&gree of surface coverage. No, more than a monolay& is adsorbed by this procedure,. but the equivalknt of,two layers may be formed by anod&ng the Pt-0 : alloy e!&tiode.- Appar&tly the Pt-0 sites aire converted to PtO, sites which are ti&bie:at. potentials below 1’500 mV in the presence of Pt metal. The Pt-0 alloy : shcttire 6 father stable and is converted to,that of untreated Pt by heating the metal in-.the .‘&i&y of-its melt&g point. .:_ ._’
_.-
SORPTION
OF OXYGEN
BY BRIGNT
PLATINUM.
II
23
REFERENCES J. P. HOARE, J. Electrochem. Sot., 109 (1962) 858. J. P. HOARE, J. Elecfrochem. Sot., 112 (1965) 602. R. THACICEFC AND J. P. HOARE. J. Electroanai. Chem., 30 (1971) 1. W. BULD AND M. W. BREITER, Efecrrochim. Acta, 5 (1962) 145. J. P. HOARE, Nature, 204 (1964) 71. J. P. HOARE, J. Electroanai. Chem., 12 (1966)-260. J. O’M. E~OCKRIS AND A. K. M. S. HUQ, Proc. Roy. SOC. London, A237 (1956) 277. H. WROBLOWA, M. L. B. RAO. A_ DAMJANOVIC AND J. O’M. B~CKRIS. J. Eiecrroanak Chem., 15 (1967) 139. 9 N. WATANABE AND M. A. V. DEVANATHAN, J. E!ectrochem. Sot.. 111 (1964) 615. AND T. MUSSINI, E/ecrrochim. Acta, 10 (1965) 445. 10 G. BUNCHI 11 J. P. HOAE, J- Eiectrochem. Sot., 110 (1963) 1019; 112 (1965) 849. 12 S. SCHULDKNEFC AND R. M. ROE, J. Efecfrochem. SOC., 1 LO (1963) 332. 13 S. TRASAI-I-I. E!eclrochim. Merall., 2 (1967) 12. Sac., 113 (1966) 846. 14 J. P. HOARE, J. Efectrochem. 15 A. DAMJANOVIC, M. L. B. RAO AND M. GENSI-I&W, ASTIA Rept. No. AD405675, Nov. 1962. 16 M. L. B. RAO, A. DA~UANOVIC AND J. O’M. BOCKRIS, J. Phys. Chem.. 67 (0963) 2508. 17 J. P. HOARE, Eiectrochim. Acta, 14 (1969) 797. 18 J. P. HOARE, S. G. MEIBUHR AND R_ THACKER. J_ Eiecrrochem. Sot., 113 (1966) 1078. 19 S. SCHIJLDINER, B. J. Puxs~~ AND T. B. WARNER. J. Electrochem. SW., 112 (1965) 212; 113 (1966) 573. of Oxygen, Interscience Publishers. Inc., New York, 1968, p_ 171_ 20 J. P. HOARE, The Electrochemistry 21 M. OIKAWA. Bull. Chem. Sot. Japan, 28 (.1955) 626. J. P. HOARE, J. Phys. Chem.,61 (1957)705;62 (1958)229,504. 1608; J. Electrochem. 22 S. SCHULDINERAND Sot., 107 (1960) 820. 23 P. RUIXSCHI AND P. D EL_AHAY, J. Chem. Ph_v.s., 23 (1955) 195, 697. 24 S. G. Mnsvwn, J. Efectrochem. Sot.. 11.5 (1968) 725. AND T. B. WARMR, J. Electrochem. SOC., 112 (1965) 212. 25 S. SCHULDINER L J. Elecrroanal.
Chem,
30 (1971) 15-23