Langmuir 1986,2, 393-405
393
Adsorption from Solution on Platinum: An in Situ FTIR and Radiotracer Study P. Zelenay,t M. A. Habib, and J. O'M. Bockris* Department of Chemistry, Texas A&M University, College Station, Texas 77843 Received July 31, 1985. I n Final Form: December 10, 1985 FTIR and radiotracer methods have been used to examine the adsorption of H3P04and CF3S03-from aqueous solution onto Pt electrodes at various temperatures and concentrations. The FTIR measurements were calculated by assuming coincidence of the measurements with those of the radiotracer approach at the adsorption maximum on &potential plots. In the radiotracer method, three independent approaches were used to identify the potential region of negligible adsorption for CF3S03-. The time constant of the H3P04adsorption is in minutes, that of CF3S03- in seconds. IR peaks for CF3SO< are assigned. The is Bockris-Swinkels isotherm (modified for H3P04) is found consistent with the data. AHadBo(CF3S03-) -44 kJ mol-'; ma&"(H$%4) is -97 kJ mol-'. The ASah0values are respectively -97 and -220 J mor1 K-l. Error analysis shows the coverage data to be better than f17% (except for CF3SOc at extremes of the potential). Anomalous radiotracer results may arise as a result of decomposition reactions. The standard state in adsorption measurements from solution is discussed in extenso: a method for the comparison of AHo and AS" for different systems using different standard states is derived. The time dependence of adsorption was found to be consistent with control of adsorption by diffusion in the solution. A detailed examination of two models of adsorption for H3P04 leads to the conclusion that the parabolic potential dependence is influenced by both oxide layer formation and H20 adsorption. Corresponding analysis of CF3S03- adsorption shows this to be consistent with a single imaging model up to 0 = 0.25. An analysis of the entropy data indicates that CF3SOc adsorbs with two degrees of translational freedom, but for H3P04calculations are consistent only with a model for zero translational degrees of freedom on is with its -OH adsorption. The orientation of adsorbed CF3S03-is with its CF3- group and that of groups toward the metal.
Introduction Adsorption of molecular and ionic species at the mercury-liquid interface haa been studied for many years.1-8 However, studies at the solid-solution interface have been made infrequently.+l3 In recent times, the potentialities of such studies have been greatly increased by the advent of the IR spectroscopic technique^,'^-" a principal one of which is the use of an interferometric scanning technique to improve the signal-to-noise ratio of the information. In the present study, the FTIR method has been applied in combination with radiotracer measurements to the platinum-aqueous solution interface.
Experimental Section (1) Electrodes. A platinum electrode in the form of a disk 6 mm wide and 0.5 mm thick (Morton Thiokol)was used for FTIR
measurements. The electrode was mounted on a brass rod with a conductive silver epoxy and covered with a heat shrinkable Teflon sleeve so that only the front surface of the Pt disk remained in contact with the electrolyte. The platinum electrode was then carefully polished before each experiment with either &alumina or diamond paste (Metallurgid Supply) down to 0.05 pm in grain size until a mirrorlike surface was obtained. A more detailed description of the electrode preparation for FTIR work has recently been reported e l s e ~ h e r e . " ~ ~ ~ The platinum electrode for radiotracer measurements was prepared on the surfaceof a glass scintillator (NuclearEnterprises) of 30-mmdiameter and of 3 - m mthickness, basically in the same way as described by Wigckow~ki.'~Glass containing a material which reacts to radioactive emission to reemit visible radiation (Nuclear Enterprises) was first sputtered in vacuo with a thin (ca. 200 A) layer of Pt (99.9%, Morton Thiokol) on top of which a 3000-4000-Alayer of Au (99.99%, Morton Thiokol) was made by evaporation. The initial layer of Pt evaporated directly onto the surface of the glass was needed to provide good adhesion of Au onto the scintillator. A Pt black electrode was finally obtained by electrolyte deposition onto the Au/Pt covered scintillatorfrom *To whom all correspondence should be addressed. On leave of absence from the Department of Chemistry, Warsaw University, Zwirki i Wigury 101, 02-089 Warsaw, Poland.
0743-7463/86/2402-0393$01.50/0
3% aqueous solution of H,PtCI, for 10-15 min at a current density of 3 mA cm-2 (geometric). The Pt electrode was then subject to activation by the use of voltammetric sweeps for ca. 20 min at a rate of 50 mV s-l in the potential range from 0.05 to 1.45 V (NHE). As soon as the profiie of the cyclic voltammogram showed no further change with subsequent cycles, the electrode surface area was measured by means of hydrogen adsorption, assuming that 224 MC of charge was required to cover one real square centimeter of the Pt polycrystalline surface (see Discussion). After an activation treatment was completed, the electrode showed less than 10% change in surface area over 24 h.
(1) Grahame, D. C. Chem. Reu. 1947, 41, 441, (1947). (2) Mohilner, D. M. In Lectroanalytical Chemistry; Bard, A. J., Ed.; Marcel Dekker: New York, 1966; Vol. 1. (3)Barlow, C. A., Jr. In Physical Chemistry in Aduanced Treatise; Eyring, H., Ed.; Academic Press: New York, 1970. (4) Frumkin, A. N.; Petrii, 0. A.; Damaskin, B. B. In Comprehensiue Treatise in Electrochemistry; Bockris, J. OM., Conway, B. E., Yeager, E., Eds.; Plenum Press: New York, 1980; Vol. 1. (5) Payne, R. J. Electroanal. Chem. 1973, 41, 277. (6)Habib, M. A.; Bockris, J. OM. In Comprehensiue Treatise of Electrochemistry; Bockris, J. O'M., Conway, B. E., Yeager, E., Eds.; Plenum Press: New York, 1980; Vol. 1. (7) Damaskin, B. B.; Petrii, 0. A.; Batrakov, V. V. Adsorption of Organic Compounds on Electrodes; Plenum Press: New York, 1971. (8) Damaskin, B. B.; Kazarinov, V. E. In Comprehensiue Treatise of Electrochemistry; Bockris, J. OM., Conway, B. E., Yeager, E., Eds.; Plenum Press: New York, 1980; Vol. 1. (9) Horinyi, G. Electrochem. Acta 1980, 25, 43. (10)Woods, R. Electroanal, Chem. 1976, 9. (11) Parsons, R. Croat. Chem. Acta 1980,53, 133. (12) Parsons, R.J. Electroanal. Chem. 1981, 118, 3. (13) Laviron, E. Electroanal. Chem. 1982, 12. (14) Pons, S.J. Electroanal. Chem. 1983, 150, 495. (15) Habib, M. A.; Bockris, J. O'M. J. Electrochem. SOC.1983, 130, 2510. (16) Bewick, A.;Pons, S. In Aduances in Infrared and Raman Spectroscopy; Clar, R. J. H., Hester, R. E., Eds.; Wiley Heyden: New York, 1985; Vol. 12. (17) Habib, M.A,; Bockris, J. O'M. J . Electroanal. Chem. 1984,180, 287. (18) Habib, M. A.; Bockris, J. O'M. J. Electrochem. SOC.1985, 132, 108. (19) Wieckowski, J . Electrochem. SOC.1975, 122, 252.
0 1986 American Chemical Society
Zelenay et al.
394 Langmuir, Vol. 2,No. 4,1986 Counter electrodeswere made of Pt wire and Pt foil (99.99% Pt, Morton Thiokol) for FTIR and the radiotracer, respectively. In the latter case, the counter electrode of ca. 100-cm2surface area was separated from the working electrode with a glass frit. The working electrode potential was measured against a mercuryfmercuroussulfate reference electrode (EG&G Princeton) provided with a Vycor tip. All potentials reported in this paper are referred to the normal hydrogen electrode (NHE). (2) Solutions. Purification of phosphoric acid (MCB) was carried out as described before." Trifluorometbanesulfonicacid supplied by Aldrich Chemical was distilled under reduced pressure. A colorless intermediate fraction of distillate boiling at 45 "C was collected and then stored in a freezer for future use. Perchloricacid (Fisher Scientific),which served as a supporting electrolyte in a number of experiments, was taken without additional purification. AU solutions were prepared from fourfold distilled water with the third distillationb e i i made from an alkslineKMnO. solution. Both the third and the fourth distillationswere carried out under nitrogen. AU aqueous electrolyka underwent at least a 72h preelectrolysia before being used. Preelectrolysis was made in a glaas cell containing two large Ft' black auxillmy electrodes. The solution was stirred vigorously and purged with purified nitrogen. (3) Nitrogen Purification. Any traces of organic impurities being present in nitrogen were removed by oxidation to COzon a Pt on alumina catalyst (Morton Thiokol) at ca. IO0 'C. CO, l deoxygenation was then absorbed in 13X molecular sieves. N of nitrogen was achieved by passing the gas through a coppercontaining furnace at ca. 300 OC. Both molecular sieves and metallic copper were frequently activated by baking in an oven and reducing with hydrogen gas, respectively. (4) Preparation of the Radiometric Samples. Carbon-14 labeled trifluoromethanesulfonic acid (CF3S03H)supplied hy Pathfinder Laboratories was diluted with nonradioactive purified CF3S03H. The final specific activity used in experiments was 0.5 Ci mol-' as compared with 3.23 Ci mo1-l of the original sample. Similarly, the portion of phosphorus-32tagged H3P0, was diluted from 1.35 x lo4 Ci mol-' (carrier free radioisotope supplied hy ICN) to 0.2 Ci mol-'. Isotope dilution was carried out right after the samples had arrived in order to keep any radiolytic decomposition to a minimum. For the same reason, all radioactive solutions were stored in a freezer, whenever not being used. (5) Experimental Setup, Electronics. A description of the experimental setup, as well as an evaluation of experimental data for the FTIR technique, has been published recently."J8~M An EG&G Princeton potentiostat (Model 173) driven by a HI-TEK Instruments Waveform Generator (Model PPRI) and HewlettPackard 70468 X-Y recorder were used for controlling and monitoring the electrode potential in FTIR measurements of CF3SO< ion adsorption. The electrochemical setup for radiotracer measurements was a slightly mcdified version of the d constructed hy Wi~kowski,'g as shown in Figure 1. For the purpose of working at different temperatures,a nicbrome heater covered with glass was immersed in the solution,and an ORION ATC temperature probe was used. This was in tum connected to the digital meter (ORION Research 811Microprocessor Meter) which served as an accurate (b0.1 "C) temperature-monitoring device. The light pulse generated in a glass scintillator disk was converted into current impulse in the photomultiplier tube connected to a multichannel analyzer via a preamplifier or, alternatively,a preamplified signal was directed to the ORTEC NIM system (Figure 2). In the latter case, the number of counts was displayed digitally by the computer and printed out on a printer or plotted against time on an X-Y recorder. A Stonehart BC 1200 potentiostat, EG&G Parc Universal Programmer (Model 175), and Houston Model 200 X-Y recorder were used. HewletbPaekard 34E5A Digital Mdtimeters were used as the potential monitors in both FTIR and radiotracer measurements. (6) Procedure. After a careful electrode preparation, Le., polishing and voltammetric activation in the case of FTIR (20) ZPlenay, P.; Habib, M. A.; Bockris, J. O'M. 1984,132,2464
,,om P,Dt
~~
10 P * O T ( I U Y L l l ~ ~ L S
Figure 1. Radiotracer cell.
Figure 2. Schematic diagram of the experimental setup for radiotracer measurements. measurements or deposition and voltammetric activation in the case of radiometric experiments, the solution was expelled by nitrogen and replaced with a portion of fresh electrolyte and adsorption measured at potentioatatic conditions. The electrode was polarized in the range of 4.30 to 1.60 V (at 0.20-V intervals) and in the range O.(t1.50 V in FTIR measurements. In the latter caw, the use of extreme potentials was limited hy the formation of either hydrogen or oxygen bubbles in the space hetween the electrode and the zinc selenide window. A steady-state surface concentration of adsorbed CF3SOs- and H3P0, was calculated from the number of counts as measured radiometrically W i 3 and 4). The IR spectra for the adsorbed species were obtained by subtracting from spectra recorded at a series of potentials, at which adsorption was expected, the spectrum taken in the region of more cathodic potentials, where adsorption is expected to he absent (hut see Results,Potential Dependence of Adsorption, for evaluation of CF,SO< spectra). With respect to the construction of isotherms, and the determination of temperaturedependent data,only a single potential was studied. The potential chosen was 0.80 V, corresponding,
Langmuir, Vol. 2, No. 4, 1986 395
Adsorption from Solution on Platinum
t
40
1 ml of DMSO
t
.* 0
\
501
Background
X
-_--___-_-----Thermal Background
-~
0
I
10
20
t/min
0
Figure 3. Determination of the background counting rate for adsorption: c = 9 X M, T = 298 K. 1 mlofDMSO 0.40V
I6O
t_J
1
\ Background
40
t 0
4
Thermal Background - - -- - - - - - - -
2
4
6
8
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*
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Figure 4. Determination of the background counting rate for M, T = 298 K. CF3S03-adsorption: c = 2 X
roughly, to the maximum of adsorption for both substances. (7) Calculation of r from Radiotracer Measurements. The surface concentration of adsorbed species was calculated from the formula21
Here, I' is the surface concentration (mol cm-2),N z and NbaCk are the count rates in the adsorption situation and that in its absence, respectively (countss-l), c is solution concentration (mol ~ m - ~w )is, a linear absorption coefficient of the radiation emitted by the radioactive nuclide (cm-?), and R is a roughness factor. The value of w for /7particles irradiated from both 14Cand 32P were calculated from equations given by Libbyz2and were 314 and 8.3 cm-', respectively. Roughness factor, R, measured as a ratio of the real to geometric surface areas, varied from one (evaporated film) electrode to another but, due to applying the same procedure for each electrode preparation, was usually equal to 60 f 10. The values of surface coverage (0) were calculated from the radiotracer data by assuming the most likely orientation adsorbed species and using the following expression: B=NTP (la) where N is the Avogadro number and P (cm2molecule-') is the calculated area occupied by one molecule (ion). For CF3S03~ - f 1.0 A2 was used as the adsorption, the value of P ~ ~ f i=017.6 mean of two surface areas corresponding to two likely vertical orientations of the CF3S03-ion, i.e., those with either CF3or SO, groups toward the surface. A similar calculation for the virtually tetrahedral &PO4 molecule, adsorbed with its OH groups toward o ~Az. the electrode, gave P ~ ~ =p 19.4 (8) Determination of a Background Counting Rate. Method 1. The determination of the background counting rate
in the radiotracer approach to surface adsorption presents the (21) Wrbblowa, H.; Green, M. Electrochim. Acta 1970, 15, 1685. (22) Libby, W. F. Phys. Rev. 1956, 103, 1900.
2
4
6
8
1
0
1
2
-
t/min
Figure 5. Time dependence of H3P04adsorption: c = 2.1 X M, T = 298 K.
problem of the identification of a state in which adsorption in the interphase is negligible. For anion adsorption, for example, the potential (in the far cathodic range) negative to which no change in the counting rate with change of potential is observed is usually taken as the potential of no adsorption. The counting rate corresponding to this potential is noted as the background count. This method was found to give precise and repeatable as no adsorption values of background counting rate for occurred at 0.0 V (Figure 3). Method 2. Method 1 for background determination could not, however, be applied for CF3S03-adsorption as the counting rate in this case is still found to vary with potential in regions as negative as -0.30 V (Figure 4). Measurements at potentials cathodic to -0.30 V were found uncertain because of hydrogen evolution. Therefore, the following procedure, based on the known chemisorption of dimethyl sulfoxide (Me2SO)on platinum, was adopted. This approach relied upon observation made by various that Me2S0strongly chemisorbs on platinum. The range of maximum adsorption extends from 0.1 to ca. 0.6 with virtually no significant change in the r w value with potential.25 The assumption upon which the present measurements of background counting rate were made is that Me80 displaces both H3P04and CF3SO< from the electrode surface. Thus, when MezSO is added, the counting rate, equivalent to the sum of background and the adsorbed amount, is expected to decrease to zero due to Me2S0adsorption. The remaining counts correspond to that due to background. The relevance of method 2 of background determination to CF3S03- adsorption is shown in Figure 4 where it is seen that for MezSO added to the solution ( C M ~ ~ S= O 0.5 M) the count rate for CF3S03- present at a conM, decreases from 150 counts s-l (observed centration of 2 X at -0.30 V, method 1) to 110 counts s-l. The latter value is therefore taken as a background counting rate. Support of the usefulness of the method 2 is provided by an observation that the counting rate in the presence of Me2S0 is independent of electrode potential from 0.0 to 0.80 V, i.e., in the potential region where Me2S0 covers the Pt electrode surface. As might have been expected, for &Po4 adsorption, the background counting rate observed (Figure 3) after MezSO was M at 0.40 V is the introduced to the solution of 9 X same as the one obtained at 0.00 V without Me2S0 (ca. 75 X lo3 counts d).The latter result leads to the conclusion that no adsorption of H3P04takes place at 0.00 V. Method 3. A final verification of the amount of background count in CF3S03- adsorption was provided by measuring the number of counts from the radioactive solution of CF3SO
