J. Phys. Chem. 1985,89, 365-369
I
I
I
1
I O
10
30
ML
OF
I 40
hQUEOU5
1
I
50
60
PHASE
1175 M
* Konl
Figure 1. Plot of 2.30 mL of n-hexadecane/2.30 X lo-’ mol of stearic acfd/a variable volume of 0.375 M KOH titrated to clarity with I-pentan01 at 35 “e:intercept, 1.1353 mL of 1-pentanol; slope, 1.1561 x lo-* mL of 1-pentanol/mL of aqueous volume (0.375 M KOH).
be seen that the ratio of the number of moles of 1-pentanol to potassium stearate varies from 3.6:l to 6:l.From film penetration into monolayer experiments it is well-known that the presence of alcohols in the film produces low interfacial tension and an expanded monolayer.23 The mutual solubility of 1-pentanol and 0.375 N KOH was determined by the Hill and Malisoff method.24 At 27 OC,the mole fraction of 1-pentanol in 0.375 N KOH was found to be 8.73 X At this temperature, the mole fraction of 1-pentanol from the slope of the curve in ~i~~~~1 is approximately 2X Let us now assume that the difference [6.73X mol] between these two values corresponds to 1-pentanol associated
365
to 2.3 X mol of potassium stearate and is responsible for the formation of the dispersed oil phase. Interestingly, this corresponds to a 3:1 ratio of moles of 1-pentanol to mole of potassium stearate, comparable to the ratios tabulated in Table I. Entropy effects in microemulsion systems have been discussed fist by Ruckenstein and Chi2’ and Talman and PragerZ2and lately by De Gennes and Tauphzo From these studies a microemulsion system would result from the melting of a macrocrystalline structure. It is appropriate to mention that back in 1943 Schulman and M c R o b e r t ~pictured ~~ the role of the surfactant as increasing the disorder of the interfacial film necessary to produce high radii of curvatures of the microemulsion particles. Our results lead to the conclusion that spontaneous formation of n-hexadecane/potassium stearate/HzO/ 1-pentanol very fine dispersions depends on the way the various components are added. These facts account for the small AG values found. Moreover, these processes are entropy driven. Acknowledgment. We gratefully acknowledge the financial assistance of the Gillette carp., ~ ~MA in~supporting t this ~ work. Registry No. Hexadecane, 544-76-3;potassium stearate, 593-29-3; I-pentanoly 71-41-0. (20) de Gennes, P. G.; Taupin, C. J . Phys. Chem. 1982,86,2294-2304. (21) Ruckenstein, E.; Chi, J. C. J . Chem. Soc., Faraday Trans. 2 1975, 1690-1707. (22) Talmon, Y.;Prager, J. J . Chem. Phys. 1978,69,2984. McRoberts, M. Trans. Faraday Soc. 1946,42,165. (23) Schulman, J. H.; (24) Hill, A. E.; Malisoff, W. M. J . Am. Chem. SOC.1926,48,918-927. (25) Tadros, Th. F. In “Structure/Performance Relationships in Surfactants”; Rosen, M. J., Ed.; American Chemical Society: Washington, DC, 1984;ACS Symp. Ser. No. 253. (26) Scriven, L.E. In ‘Micellization, Solubilization, and Microemulsion“; Mittal, K. L., Ed.; Plenum Press: New York, 1977;p 277.
Voltammetric Study of CO and CO, Adsorption on Smooth and Platinized Platinum Electrodes Jerzy Sobkowski and Andrzej Czerwinski* Chemistry Department, The Warsaw University, 02-089 Warsaw, Poland (Received: April 13, 1984; In Final Form: July 9, 1984)
The adsorption of carbon monoxide and carbon dioxide on platinized and smooth platinum electrodes has been studied by the potentiodynamic technique. It is shown that the main product of CO and COz adsorption in the presence Hadon the Pt surface is similar, probably the COOHadradical. It has been found that there are some differences between CO and C 0 2 adsorption products on Pt/Pt and smooth platinum electrodes.
Introduction The phenomena of CO and C 0 2 adsorption on Pt electrodes from acid solutions have been known for more than 20 years.’“ About 60 papers*”4 on these problems have appeared, but the (1) S. Gilman, J . Phys. Chem., 66,2657 (1962). (2) S.Gilman, J . Phys. Chem., 67,78 (1963). (3) S.Gilman, J. Phys. Chem., 67, 1898 (1963). (4) S.Gilman, J . Phys. Chem., 68, 70 (1964). (5) J. Giner, Electrochim. Acta, 8,857 (1963). (6)J. Giner, Electrochim. Acta, 9,63 (1964). (7) W.Vielstich and V. Vogel, 2.Elektrochem., 68, 688 (1964). (8) P. R. Johnson and A. T. Kuhn, J . Electrochem. Soc., 122,599(1965). (9)T. B. Warner and S. Schuldiner, J. Electrochem. Soc., 111,992 (1964). (IO) B. J. Rersma, T. B. Warner, and S . Schuldiner, J. Electrochem. Soc., 113, 84 (1966). (11) S. B. Brummer and J. I. Ford, J . Phys. Chem., 69, 1355 (1965). (12) S.B. Brummer and M. J. Turner, J . Phys. Chem., 71,3494 (1967). (13) S.B. Brummer and K. Cahill, Discuss. Faraday Soc., 45,67 (1968). (14) S . B. Brummer and K. Cahill, J. Electroanal. Chem., 21,463(1969).
opinions of the authors are not consistent. They mostly agreed that the final product of COzadsorption (which is called “reduced (15) M. W.Breiter, J . Phys. Chem., 72, 1305 (1968). (16) M. W. Breiter, Electrochim. Acta, 12, 1213 (1967). (17) M. W.Breiter, J. Electroanal. Chem., 19, 131 (1968). (1 8) M. W. Breiter in “Proceedings of the Symposium on Electrocatalysis”, The Electrochemical Society, Princeton, NJ, 1974,p 115. (19) M. W. Breiter, J . Electroanal. Chem., 65,623,(1975). (20) M. W. Breiter in “Modern Aspects of Electrochemistry”, Vol. 10, Plenum Press, New York, 1975. (21) T. Biegler, J . Phys. Chem., 72, 1571 (1968). (22) B. I. Podlovchenko, W. F. Stenin, and A. A. Ekibaeva, Elektrokhimiya, 4, 1374 (1968). (23) M.W. Brieter, Z . Phys. Chem. (Frankfurt am Main), 98.23 (1975). (24) G. L. Padyukova, A. B. Fasman, and D. V. Sokolsky, Elektrokhimiya, 2, 885 (1966). (25) P. Stonehart, Electrochim. Acta, 18,63 (1973). (26) J. Bett, K. Kinoshita, K. Rontsis, and P. Stonehart, J. Catal., 29, 160 (1973). (27) P. Stonehart and G. Kohlmayer, Electrochim. Acta, 17, 369,(1972).
0022-3654/85/2089-0365$01.50/00 1985 American Chemical Society
~
,
366 The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 CO,” or “CO,”) is similar to one of the main products of C O adsorption on platinum electrode. In spite of that the problem of the structure of “COz” and “adsorbed CO” on platinum is still unresolved. The adsorption of C 0 2 on a platinum electrode was first observed by G i r ~ e r .It~ was shown that the adsorption was a result of a reaction between CO, and hydrogen previously adsorbed on the electrode. He found that the adsorbed “COz” molecule occupies one adsorption site and one electron is involved in its oxidation. He postulated a COOH radical as a main product of the reaction, the same as the product of HCOOH adsorption.6 Piersma et aLiOconcluded that there is no reaction between COz and Hadsbut that Hadsoxidation is inhibited by eo2.Brummer and Cahill14 and Kamath and Hira La134found that surface coverage (0) with “COz” is about 0.7 and the charge of oxidation of adsorbed product, i.e., “CO,”, per one adsorption site (eps) is approximately equal to one. They s u p p e d that the main product of C02adsorption on a platinum electrode was the COH radical. Brummer and CahillI3 showed that “adsorbed CO” and “COz” (28)G. Kohlmayer and P. Stonehart, Electrochim. Acta, 18,211 (1973). (29) M. Vogel, J. Lundquist, P. Ross, and P. Stonehart, Electrochim. Acta, 20, 79 (1975). (30) K. Kinoshita and P. Stonehart, Electrochim. Acta, 20, 101 (1975). (31) P.N.Ross,K. Kinoshita, A. J. Scarppellino, and P. Stonehart, J . Electroanal. Chem., 59, 177 (1975). (32) P. N. Ross, K. Kinoshita, A. J. Scarppellino, and P. Stonehart, J. Electroanal. Chem., 63, 177 (1975). (33) P. Stonehart, J . Electroanal. Chem., 77, 245 (1977). (34) V.N.Kamath and Hira Lal, J. Electroanal. Chem., 19, 137 (1968). (35) P. Sidheswaran, Electrochim. Acta, 18, 125 (1973). (36) W. E. Kazarinov, V. N. Andreev, and G. J. Tysiatchnaya, Elektrokhimiya, 8, 927 (1972). (37)V. N.Andreev, Yu. B. Vasilev, N. V. Osietrova, and T. N. Yastrebova, Elektrokhimiya, 19, 381 (1983). (38) Yu. B. Vasilev, V. N. Andreev, and N. V. Osietrova, Elektrokhimiya, 19, 414 (1983). (39) H. B. Urbach. L. G. Adams. and R. E. Smith. J . Electrochem. Soc.. 121,233 (1974). ’ (40) K. F. Burton and J. M. Sedlak, J . Electrochem. Soc., 121, 1315 (1974). (41) A. Czerwinski, J. Sobkowski, and A. Wieckowski, I n t . J . Appl. Radiat. Isotopes, 25, 295 (1974). (42) J . Sobkowski and A. Czerwinski, J . Electroanal. Chem., 55, 391 (1974). (43)A. Czerwinski and J. Sobkowski, J . Electroanal. Chem., 59, 41 (1975). (44) J. Sobkowski and A. Czerwinski, J. Electroanal. Chem., 65, 327 (1975). (45)A. Czerwinski and J. Sobkowski, J . Electroanal. Chem., 91, 47 (1978). (46) T.K. Gibbs, C. McCallum, and D. Pletcher, Electrochim. Acta, 22, 525 (1977). (47) C. McCallum and D. Pletcher, J. Electroanal. Chem., 70, 277 (1976). (48) L. Grambov and S. Bruckenstein, Electrochim. Acta, 22, 377 (1977). (49) N.V. Osietrova, Yu. B. Vasilev, and V. S. Bagotzky, Elektrokhimiya, 13, 512 (1977). (50)A. V. Zakkharian, N. V. Osietrova, and Yu. B. Vasilev, Elektrokhimiva. 13. 1011 (1977). (jl)A. V. Zakharian, N. V. Osietrova, and Yu. B. Vasilev, Elekfrokhimiya, 13, 1818 (1977). (52) A. A. Mikhailova, N. V. Osietrova, and Yu. B. Vasilev, Elektrokhimiya, 13, 1257 (1977). (53) S.A. Bilmes, N. R. de Tacconi, and A. J. Arvia, J. Electrochem. Soc., 127, 2164 (1980). (54) T . K. Gibbs, G. McCallum, and D. Pletcher, Electrochim. Acta, 22, 525 (1977). (55) J. W. Russel, J. Overend, K. Scanlon, M. Severson, and A. Bewick, J . Phys. Chem., 86, 3066 (1982). (56) J. W. Russell, M. Severson, K. Scanlon, J. Overed, and A. Bewick, J . Phys. Chem., 87, 293 (1983). (57) B. Beden, A. Bewick, K. Kunimatsu, and C. Lamy, J . Electroanal. Chem., 142, 345 (1982). (58) B. Beden, A. Bewick, M. Razaq, and J. Weber, J . Elecfroanal. Chem., 139, 203 (1982). (59) B. Beden, S Bilmes, C. Lamy, and J. M. Leger, J . Electroanal. Chem., 149, 295 (1983). (60) B. Beden, A. Bewick, and C. Lamy, J . Electroanal. Chem., 148, 147 (1983). (61) C. Lamy, J. M. Leger, J. Clavilier, and R.Parsons, J . Electroanal. Chem., 150, 71 (1983). (62) A. M. Baruzzi, E. P. M. Leiva, and M. C. Giordano, J . Electroanal. Chem., 158, 103 (1983). (63) M. W. Breiter, J. Electroanal. Chem., 101, 329 (1979). (64) 0.Wolter and J . Heitbaum, Ber. Bunsenges. Phys. Chem., 88, 6 (1984).
Sobkowski and Czerwinski were different species. Stonehart et studied the kinetics of “CO,” oxidation by potential sweep methods. From the value of na = 1.1 they suggested a two-electron oxidation process. It was shownz5that the rate constant of “CO,” oxidation was higher than that of “carbon monoxide hydrate” which was formed by utilizing two adjacent platinum sities, one covered with linear carbon monoxide and the other with a coadsorbed water molecule. They suggested that “adsorbed CO” and “COz” were not identical. Breiteri5-I9 has found that the charging curves of “CO,” and “adsorbed CO” coincide partially. On the basis of the experimental heat of activationi7 he ascertained that “reduced CO,” is identical with one of the products of CO adsorption on platinum. From gas-chromatographic measurements he found that two electrons are needed to oxidize the adsorbed molecule of “COz”. From the kinetic behavior of “CO,” electrooxidation on a smooth electrode Baruzzi et a1.6, postulated the existence of two intermediate species of similar chemical composition but different energy content. Beden et a1.57,58 have studied the structures of “CO,” and “adsorbed CO” on a smooth platinum electrode by IR spectroscopy (EMIRS technique). They conclude that the adsorbed species formed directly from the reduction of COz are similar but not identical with those of CO, the latter being almost exclusively a linear bounded adsorbed CO species. IR adsorption bands from COOHad, or (COO-)adsradicals should be in the range 1300 to 1650 cm-’; the asymmetric OCO stretch would be expected near 1640 cm-’ 65 (from surface selection rule, this band will not be observed in the EMIRS s p e c t r ~ mdue ~ ~to~ experimental ~~ constraints-detectors are not sensitive in this region) and the symmetric OCO stretch should be near 1340 cm-’. According to Beden et aL60 “EMIRS technique is more likely to detect the poisoning species rather than other possible intermediate. This could explain the present failure to detect species such as C O O W . Grambov and Bruckensteina using mass spectroscopic and electrochemical methods have found that after adsorption of CO at potentials below 0.4 V (vs. NHE) one adsorbed molecule of CO is bound with one atom of platinum. They found that for the oxidation of one adsorbed CO molecule (below 0.4V) 1.6 eare needed. The problem of CO and COz adsorption on platinum electrodes was studied by radiochemical methods36-39*41-45 using carbon oxides labeled with I4C. Urbach et al.39working with very low concentrations of CO, in acidic solutions concluded that the oxidation of the adsorption product is a two-electron process. Kazarinov et al.36found that “adsorbed CO” and “CO,” were identical with the COH radicals formed by the reaction of CO with water on the surface of platinum. They claimed that the oxidation of “COz”or ”adsorbed CO” was a three-electron process. Our previous on Pt/Pt electrodes suggest that oxidation of “CO,” is a one-electron process (eps and epm were equal to one) and a COOH radical is the main product of the reaction between COz and Had.The data for coverage, oxidation charge, and eps are similar to the results of Brummer et al.14 and Kamath et al.34 We found, from the surface concentrations and oxidation charges of adsorbed species,45 that the products of C O chemisorption consist of at least two or three species, one of them being the COOH radical formed by the reaction of CO with water. The other products are toad radicals bounded to Pt atoms in bridged and linear forms whose composition depends on the adsorption potential. Bilmes e t al.53 also suggested the possibility of the existence of various CO adsorbed species on a platinum surface. Likely, the differences between all these results are dependent on four factors: (1) the roughness of the platinum surface; (2) ageing of the electrode; (3) the electrochemical pretreatment of the electrode; (4) the time of carbon oxides adsorption on Pt electrode. (1) The experiments have been done over broad range of roughness factors: from smooth platinum electrodes (see, e.g., Beden et aL5**) to platinized platinum electrodes (e.g., Kazarinov et al.36, R = 800). (2) It has been shown by some a u t h o r ~ that ~ ~ ,results ~ ~ are different for fresh and aged electrode. (65)B. A. Sexton, Appl. Phys. A , 26, 1 (1981).
CO and COz Adsorption on Pt Electrodes (3) It has been proved by ESCA experiments66 that the properties of the platinum group metals surfaces and their structures depend on the number of polarization cycles. (4) The studied carbon oxides have been adsorbed over broad range of adsorption time (e.g., C O was adsorbed from 2 min64 to 1 h56). Breiteri5 found after only 12.5 min of adsorption of CO on Pt electrode from 1 M KOH that a second kind of adsorption product (oxidizable at lower potentials) was formed. The same situation true is for the C 0 2 adsorption. Baruzzi et a1.62 showed on cyclic voltammograms that the shape of oxidation peak of “COz” is dependent on the time of adsorption. From radiometric experiment^^^ it follows that the time needed for complete C02adsorption on Pt/Pt electrode is about 35-45 min (depending on potential of adsorption). For CO adsorption the time needed is about 25-35 m i d 7 (depending on adsorption potential). In this paper we present properties of smooth and rough surfaces of platinum electrodes with respect to CO and COz adsorption.
Experimental Section The experimental procedures are the same as described in our earlier communications on radiochemical studies of CO and C 0 2 adsorption on platinum e l e ~ t r o d e . 4 ’The ~ ~ working electrode was a polycrystalline platinum disk, about 16 mm in diameter and 0.5 mm thick. It was placed on the bottom of the three-compartment electrochemical cell (see Figure 1 ref 41). A smooth electrode was prepared by polishing and treatment in boiling concentrated H2SO4. A platinized platinum electrode was obtained by electroplating platinum from a 3% H2PtC16solution onto a platinum disk at constant current (about 10 mA cm-2 of geometric electrode surface). The real surface of the electrode was determined according to the procedure described by Capon and Parsons.71 The roughness factors for the smooth electrode and the platinized platinum electrode (Pt/Pt) were 3-5 and 100-200, respectively. The supporting electrolyte was 0.5 M H2S04 prepared from fourfold distilled water, the second destillation being performed from permanganate solution and concentrated sulfuric acid (AR BDH). Before running the experiments the solution (0.5 M H2S04) was preelectrolyzed for 24 h. All measurements were carried out at room temperature (ca. 20 “C). The potentials are referred to the hydrogen electrode potential in the same solution (0.5 M H2S04). Before the experimental runs the electrodes were cycled a few hundred times between 0.05 and 1.40 V (vs. NHE) to obtain highly reproducible voltammograms which corresponded to a clean Pt electrode s~rface.~~-’l A standard procedure was applied for the measurements. After removal of dissolved oxygen from the electrolyte by stirring with purified argon, the test electrode was kept at 0.05 and 0.4V during 30 min and voltammograms were taken to prove the absence of oxidizable impurities adsorbed on the electrode surface or contained in the electrolyte. Then the adsorption experiments were carried out with the procedure described earlier.41-45 To avoid the transport of CO or C 0 2 from the bulk of the solution to the electrode surface during the polarization the washing procedure followed. During this procedure the potential of the electrode was kept constant and air was excluded by bubbling argon through the solution. For voltammetric measurements a type EP-20 potentiostat and a GP-20 generator (both made in Poland) were used. The voltammograms were recorded with a BAK 5A instrument (made in Czechoslovakia). Results The voltammetric profile of the platinized Pt electrode in the supporting electrolyte (0.5 M H2S04) is shown in Figure 1. The two additional profiles are recorded after adsorption of CO from the solution saturated with CO and washing procedures. CO was adsorbed for 30 min at two different potentials, namely, 0.05 and (66) P. M. A. Sherwood, Inr. Lab., 42 (March, 1983) (67) A. Czerwinski, unpublished results. (68) D. A. Rand and R. Woods, J . Electroanal. Chem., 35,209 (1972). (69) T. Biegler, D. A. Rand, and R. Woods, J . Electroanal. Chem., 29, 269 (1971). (70) D. A. Rand and R. Woods, J . Electroanal. Chem., 31, 29 (1971). (71) A. Capon and R. Parsons, J . Elecrroanal. Chem., 65, 285 (1975).
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 361
111 I
i lpA.cm-21 x
!I i I
lo3
15 50
25
t
-25
-50
-75
F i p 1. Cyclic voltammogram for platinized electrode in 0.5 M H2S04 (1). Voltammetric oxidation of the CO adsorption products at Eads= 0.05 V (2) and at Eads= 0.40 V (3). Potential sweep 50 mVs-’.
B
\I v
Figure 2. Cyclic voltammogram for the smooth platinum electrode in 0.5 M H2S04(1). Voltammetric oxidation of the CO adsorption products at E a d J = 0.05 V (2) and at Ea&= 0.40 V (3). Potential sweeps 50 mV SS’.
0.40 V, and the corresponding cyclic voltammograms for the smooth and platinized platinum electrode are shown in Figures 1 and 2. After adsorption at 0.05 V two oxidation peaks can be observed.
Sobkowski and Czerwinski
368 The Journal of Physical Chemistry, Vol. 89, No. 2, 1985
15
5.0
25
0
Figure 4. Cyclic voltammogram for smooth platinum electrode in 0.5 M H2S04(1). Voltammetric oxidation of the adsorption products of CO at Ea& = 0.05 V (2) and C02 at Endads = 0.05 V. Sweep rate 50 mV s-l.
-2s
- 50 -15
Figure 3. Cyclic voltammogram for platinized platinum electrode in 0.5 M H2S04(1). Voltammetric oxidation of the adsorption products of CO at En&= 0.05 V (2) and C02 at Ea& = 0.05 V (3). Sweep rate 50 mV s-' . TABLE I: Data Concerning CO and COz Adsorption and Products of Adsorption Oxidation on Smooth and Platinized Platinum Electrodes
"adsorbed CO" Qox,
TO2"
0.40 0.05 0.40
0.87 0.92 0.91
0.199 0.250 0.240
P t - H t COP (bulk sol)
Pt-H.*.OH2
+
CO
-
1.10 1.30 0.71 1.26
0.158
1.06
In Figures 3 and 4 are shown the voltammograms of platinized and smooth platinum electrodes after adsorption of CO and COz at potentials where the surfaces of Pt electrodes are almost completely covered with adsorbed hydrogen ( C 0 2 was adsorbed for 40 min). For both types of platinum electrodes oxidation peaks of "COz" and "adsorbed CO" are situated at similar potentials. It has to be noted that for "C02" oxidation only one peak is observed. From comparison of the voltammograms for "C02" and 'adsorbed CO" one can see that the surface coverage of Pt electrode by adsorbed T O z " is lower than that by CO. In the former case hydrogen still exists on the electrode surface. In Table I the surface coverage (e), charges of adsorbed oxidation products (Qox), and number of electrons per single adsorption site (eps) are given. These parameter have been calculated by a standard pro~edure.'~ Discussion
As shown in Figure 1 and 2, there is a marked difference in the voltammograms for CO adsorbed at various potentials, though the data for 8, Q,,, and eps reported in Table I for a given type of electrode are similar. The main oxidation peak (denoted as 11) of CO adsorbed at 0.05 V is shifted toward cathodic potentials in comparison with the oxidation peak obtained after adsorption
(la)
ir Pt-H * . * C o p t Hz
1
Qox,
9 electrode E , V 6 mCcm-2 eps mC.cm-2 eps 1.12 0.73 0.185 1.20 smooth 0.05 0.87 0.208
Pt/Pt
of CO at 0.4 V. Moreover, a smaller but significant peak (denoted as I) is observed when CO is adsorbed in the hydrogen potential range. These effects are observed on smooth as well as platinized electrodes and indicate that a t least two different adsorption products of CO are formed on the electrode surface in the presence of Ha&.Padyukova et al.24 observed that during adsorption of CO on platinum electrode at 0.00 V (that is when the surface is completely covered with Had)in bulk solution significant amounts of C 0 2 and H2exist. This result shows that the conversion reaction of CO with water which is adsorbed on Pt-H (water is bonded with Ha&by hydrogen bond'9 takes place. The reaction of CO adsorption may be proposed as follows:
Pt-COOH
-
(Ib)
From kinetic data42the reaction Pt-H.-C02 COOH is fast. It has to be noted that COz was detected on Pt during CO adsorption by Beden et al.57 Wolter et al.64using a mass spectrometric technique demonstrated that COz is also produced during methanol oxidation at very low Pt potentials around 0.35 V (vs. NHE). The proposed CO adsorption mechanism (la, 1b) explains why the final product of C02 adsorption ("C02") is similar to one of the main products of C O adsorption on the platinum electrode. BreiterI5*l6found that when the platinum surface is covered with H atoms at 0.1 V during CO adsorption at open circuit the potential of electrode decreases with time, passes through a broad minimum, and becomes more positive. According to his suggestionI5 that adsorption leads to the production of hydrogen, we propose that apart from (la) and (1b) the following reactions take place. Pt-Hd 2Pt-H
+
+
CO CO
-
-
Pt-CO,,
t H
(2a)
Pt
C ' =O / Pt
t 2H
(2 b)
The reaction of CO in the double-layer range of potentials may be presented as follows: (72) A. Bewick and J. W. Russel, J. Electrounul. Chem., 132, 329 (1982). (73) 0. Wolter, Ch. Giordano, J. Heitbaum, and W. Vielstich, Electrochemical Society Spring Meeting, Minneapolis, May 1981, Abst. No. 515 p 1272.
The Journal of Physical Chemistry, Vol. 89, No. 2, 1985 369
C O and C 0 2 Adsorption on Pt Electrodes Pt-HZO
-+
,
Pt-COOH t H GO I Pt-C=O t H20
(3a) (3b)
The formation of linear form (Pt--C=O) or higher coordinated form (bridged bounded, P t - C ( 4 ) - P t ) appears to be a function of applied potential. According to the spectroscopic data of Beden et al.58 the linear C O dominates and is more strongly bound to the platinum surface than the higher coordinated form (2060 and 1865 cm-’, respectively). We found45that a t least two different adsorbed species exist on the Pt/Pt surface. The species adsorbed a t lower potential (0.1 V) are oxidized easier (see Figure 3 loc. cit.) and faster (Figure 2 loc. cit.) than those adsorbed at a more anodic potential (0.4 V). This result is consistent with the results of Breiter.15 Our former radiochemical and electrochemical data” suggest that the reaction 3b dominates when the potential of adsorption is increased and at a potential near 0.6 v only toad molecules exist on the platinum surface. The decrease in surface concentration of adsorbed CO with the increase of electrode potential at the constant values of Q,, suggests that the formation of a “bridged” toad species at the cost of the “ h e a r ” form may occur. This effect is also observed in infrared spectro~copy.~~ The question arises on what is the ratio of COOH radicals and C O adsorbed on the electrode surface. It was previously shown that statistically 1.4 and 1.6 e- are consumed per adsorbed species a t 0.1 and 0.4 V, respectively. It means that, at 0.1 V, the ratio of COOH,ds/COad, is 3:2 whereas at 0.4 V is only 2:3. In spite of the high concentrations of COOH,,,, radicals, they are not observed in the infrared because expected stretches are below 1650 cm-’. The changes of radicals composition on the surface can be observed by observing the changes of the CO absorption band peak areas and it was show# that the CO radical concentration increases with a rise of Pt electrode potentials. Adsorption of C 0 2 on platinum electrodes takes place merely in the hydrogen potential range. When the Pt electrode is covered with hydrogen and then C02is adsorbed the electrode potential increases owing to the consumption of hydrogen in the reaction with C02.424 Pt-H,,,
+ c02
+
Pt-COOH,,
(4)
Hence, the adsorption product of C 0 2 is the same as the product of CO adsorption (reactions Ib and 3a), although in the latter case other adsorption products also exist on the electrode surface. The voltammetric curves presented in Figure 3 confirm this conclusion, namely, that the peaks of CO,, (peak 11) and “COz” oxidation coincide strictly at the same potential.
The free energy of COOH,, oxidation is rather high because the oxidation peak appears at a relative high anodic potential. This may be explained by assuming the existence of hydrogen bonding between COOHad,species:
I
Pt
I
Pt
I
P1
The area of the platinum electrode which is occupied by one “C02” adsorbed species is larger compared to the area occupied by a CO molecule adsorbed linearly on Pt because of steric hindrance. It follows from the size of the COOH r a d i ~ a l which ~ ~ s ~occupies ~ a surface area equal to 11.0 A2 that the maximum surface concentration is 9.1 X 1014 molecule cm2. Since the number of platinum atoms per 1 cm2 is equal to 13.1 X 1014, the surface coverage should not exceed 69% of the area accessible to hydrogen atoms. The data of Table I (e = 0.7) confirm this estimation. The surface coverage by adsorbed CO is higher than that of “C02” (see Table I). This further proves the presence of a high concentration of Co,& radicals (linear or bridged form) after CO adsorption on the platinum electrode. The higher value of eps (Table I) and the differences in shapes and potentials of the main peaks in the voltammograms after adsorption of C 0 2 on smooth and platinized platinum electrodes (Figures 3 and 4) show the differences between the adsorption products on the two kinds of electrodes. In spite of numerous investigations dealing with the electrochemical reaction of CO and C02on noble metals the mechanisms of these processes are still not exactly known. Though spectroscopic methods are very helpful in recognizing the structures of adsorbed species, these methods, however, are limited. The problem of electrochemical conversion of C 0 2 into some organic products, which could be desorbed into the bulk of the solution, is very attractive and further work on this subject will be continued.
Acknowledgment. This work has been supported by the University of Lodz as a part of the M R 1.1 1 scientific project. We are grateful to Mrs. Z. Kawalkowska for her assistance with the experiments. Registry No. Pt, 7440-06-4; H2, 1333-74-0; CO, 630-08-0; C02, 124-38-9. (74) “Handbook of Chemistry and Physics”, 61th ed, CRC Press, Boca Raton, FL. (75) A. F. Wells, “Structural Inorganic Chemistry” 4th ed., Oxford, Clarendon Press, 1975.
