ADSORPTION A.ND OXIDATION OF CARBON MONOXIDE
1305
I n calculating the change in In Icq, differences in partial molar quantities are used; these, in turn, are calculated from differences (through graphical or analytical differentiation) between measured quantities, such as heats of mixing. The errors in the partial molar quantities, therefore, may vary from 10 to 20%. The agreement between observed and calculated changes in In I c d , then, is quite good. It is noted that Hudson and Loveday'l have previously observed specific solvation between alcohol and the transition state in nucleophilic substitutions involving alcohols and acid chlorides. The rates were found
to be approximately proportional to the concentration of the self-associated alcohol. More recently, Fletcher and Heller12 reported evidence for specific catalysis by the tetramer of octanol. (For data on the rate constants for the isomerization at 25 and 35" and an analysis of a typical kinetic run, see Tables I11 and IV, respectively.) (11) R. F. Hudson and G. W. Loveday, J . Chen. SOC.Sect. B, 766 (1966). (12)A. N. Fletcher and C. A. Heller, J . Phys. Chen., 71, 3742 (1967).
Adsorption and Oxidation of Carbon Monoxide on Platinized Platinum Electrodes by M. W. Breiter General Electric Reaearch and Development Center, Schenectady, New York 163001 (Received October d , 1967)
Carbon monoxide is adsorbed at open circuit on platinized platinum as two types that are oxidized in different potential regions in acid and alkaline electrolytes. The different mechanisms of the anodic oxidation of the two types of co&are discussed. The saturation coverage of H atoms decreases in a linear fashion with the C0,d coverage. The isotherms of hydrogen adsorption degenerate to Temkin-type isotherms with increasing coverage of cod. The C0,d coverage is equal to 1 between 0.1 and 0.4 V under potentiostatic or galvanostatic conditions in acid solutions stirred with carbon monoxide. The beginning of the rapid decrease of C0,d with potential depends upon the experimental conditions. The oxidation of dissolved carbon monoxide under steady-state conditions appears to involve the one type of COad as intermediate between 0.2 and 0.4 V and the other type above 0.4 V. Adsorption of the one type is the rate-determining step between 0.2 and 0.4 V. Diffusion of CO and adsorption of the other type are rate determining above 0.4 V.
Introduction I n recent years the adsorption of carbon monoxide has been extensively on smooth platinum electrodes in acid electrolytes. Carbon monoxide was allowed to interact with the platinum surface a t potentials between 0.2 and 0.4 V. Voltammetricl-4 and galvanostatic6J techniques were used to determine the adsorbed amount from the charge required for its anodic oxidation to COz. It was reported in two of the publications that the charge SQCO corresponding to is smaller than twice the saturation coverage with co&d charge SQH of a monolayer of H atoms on the same electrode ( ~ S & H / S & C O = 1.8 in ref 2, and 1.08 in ref 6). Gilman2 attributed this ratio to the presence of two forms of Coed in analogy to the interpretation' of infrared data of carbon monoxide adsorption on platinum in the gas phase. One molecule of the bridged or linear form occupies two or one platinum atoms, re-
spectively. Warner arid Schuldiner5 found that &&/ SQCO = 0.94. Physical adsorption of carbon monoxide on top of a chemisorbed monolayer was suggested5 as a reason that more than a monolayer was present. Codeposition of hydrogen atoms on a surface having the saturation coverage of G o a d led to the conclusion2 that carbon monoxide is only adsorbed on about SO$70 of the Pt atoms. I n contrast, a percentage of 98% was given in ref 6. The investigation^^^^ of carbon monoxide (1) S. Gilman, Phys. Chem., 66, 2657 (1962). (2) 8. Gilman, ibid., 67, 78 (1963). (3) S. Gilman, ibid., 67, 1898 (1963). (4) 8.Gilman, ibid., 68, 70 (1964). ( 5 ) T. B. Warner and S. Schuldiner, J . Electrochen. SOC.,111, 992 (1964). (6) S. B. Brummer and J. I. Ford, J . Phys. Chem., 69, 1355 (1965), (7) R. P.Eischens and W. Pliskin, Adwan. Catal., 10, 18 (1958). (8) A. B. Fasman, G. L. Padyukova, and D. V. Sokolskii, Dokl. Akad. Nauk SSSR, 150, 856 (1963). Volume 76,Number 4 April 1968
M. W. BREITER
1306 adsorption on platinized platinum are not detailed enough for the quantitative establishment of similar relations. Sokolskii and coworkers c o n ~ l u d e d *that ~~ physical adsorption occurs at about 10" in 0.5 M H2S04. Conversion to COz and H2 was postulated for temperatures between 20 and 30". Initial conversion and subsequent chemisorption were suggested between 50 and 70". Binder, Kohling, and Sandstedelo concluded from measurements on Raney platinum electrodes in phosphoric acid and sulfuric acid solutions that chemical conversion is the first step in the anodic oxidation of carbon monoxide a t temperatures between 90 and 150". It is the purpose of this article to report on quantitative studies of carbon monoxide adsorption at open cirsuit on platinized platinum in acid electrolytes 1". The charge Q c O for the and in 1 144' MOH at 25 oxidation of COad was measured simultaneously with the charge QH corresponding to a monolayer of H atoms on the Pt atoms remaining free of cod. The mechanisms of the oxidation of adsorbed ( c o a d ) and dissolved (CO) carbon monoxide in acid solutions will be discussed.
*
Experimental Section The experiments were made in a Pyrex vessel of standard design. The electrolytic solutions were prepared from reagent grade chemicals and double-distilled water. The apparent surface of the platinized Pt electrode was 50 cm2. The electrode has been aged by frequent use so that the gradual decrease1' of SQH due to a sintering effect was smaller than 5% during the course of the investigation. The electrode potential, U , was measured vs. a hydrogen electrode in the same electrolyte as the test electrode. The measurements of every set of runs were always started by a cathodic-charging curve with -50 mA from U = 1.3 V, which had been established by previous anodic charging. The cathodic-charging curve served to reduce the oxygen layer and to form the hydrogen layer12 up to 0.06 V in electrolytic solutions flushed extensively with purified helium. When U = 0.06 V was reached, the current was reversed to 50 mA and an anodic-charging curve was recorded up to 1.1 V. The charge sQH for the hydrogen arrest and the absence of an arrest in the double-layer region12 were used as a test for the absence of oxidizable species absorbed on the surface or dissolved in the electrolyte. Since the cathodic charging is slow compared to the time constant, about 0.67 sec, for the transition from instationary to steady-state diffusion, the equilibrium gradient of molecular hydrogen in the diffusion layer is practically present at any moment at potentials in the hydrogen region. The maximum contribution of Hz diffusing back to the surface from the solution during the anodic charging is of the order of 7 X lod3 mC/cm2. This value is considerably smaller than ~ Q H . The recent The Journal of Physical Chemistry
treatment of hydrogen diffusion by BenDaniel and Will13 is not applicable here because of different boundary conditions and the use of platinized platinum in the present work. After the electrode had been brought back from 1.1to 0.1 V with -50 mA, it was left at open circuit in the studies of adsorption as a function of time. The stirring with helium was replaced by stirring with a mixture of 80% Ar and 20% CO at about 2 cm3/sec. The gas mixture was Matheson CP grade. Interaction between carbon monoxide and the platinum surface was allowed for various times. The electrode potential decreasedB with time during the initial part of the stirring with CO. Then helium stirring was started and maintained for 1000 sec to remove the dissolved CO from the solution. It was verified by analysis of the CO content of the helium gas by gas chromatography that 1000 sec were sufficient to reduce the bulk concentration of CO to at least one hundredth of its initial value. Since a limiting diffusion current of about 10 mA was measured at large anodic potentials during the stirring with 80% Ar and 20% CO, the contribution to the arrest of the oxidation of toad or to the decrease of the length of the hydrogen arrest from the diffusion of traces of CO to the surface is not larger than 0.1 mA. The latter value is very small in comparison to the anodic charging rate of 50 mA. A second anodic charging curve was measured from 0.06 V with 50 mA. When the potential reached 1.3 V, the current was interrupted. The electrolyte was removed14 from th: vessel by gas pressure and subsequently replaced by fresh one saturated with He. The electrode potential was brought to about 0.5 V after the initial charging curve in the determinations of the carbon monoxide coverage as a function of U under potentiostatic conditions. Then the desired potential was applied by the Wenlring 6ITRS potentiostat. Stirring with CO (Illatheson C P grade) was started at 2 cm3/sec and maintained for 500 sec. A longer period of stirring did not lead to larger coverages with G o a d . Then the stirring was replaced by intensive He stirring. Simultaneously, the electrode was disconnected from the potentiostat. The potential decays to values between 0.2 and 0.3 V. The potential was adjusted galvanostatically to 0.06 V after 1000 sec of He stirring before taking the final U-t curve with 50 mA. The rest of the procedure is the same as for the studies of adsorption at open circuit. These measurements on (9) G. L. Padyukova, A. B. Fasman, and D. V. Sokolskii, Elektrokhimiya, 2 , 885 (1966). (10) H. Binder, A. Kohling, and G . Sandstede, Advan. Energy Conversion, 6 , 135 (1966). (11) J. Giner, Electrochim. Acta, 8 , 857 (1963). (12) A. Slygin and A. Frumkin, Acta Physicochim. URSS, 3 , 791 (1935). (13) D. J. BenDaniel and F. G. Will, J. Electrochem. SOC.,114, 909 (1967). (14) M. W. Breiter, Electrochim. A d a , 12, 1213 (1967).
ADSORPTION AND OXIDATION OF CARBON MONOXIDE
1307
platinized platinum allow a comparison with the studies of carbon monoxide adsorption on smooth platinum a t potentials between 0.2 and 0.4 V. Current and coverage with cod under steady-state conditions of galvanostatic measurements were obtained as follows. The solution was stirred for 500 sec with CO at 0.5 cm3/sec a t open circuit. Then the potential was determined 20 min after every adjustment of the current. The CO stirring was maintained. A cathodic charging curve was recorded by switching from the positive current to - 200 mA. Afterwards the anodic current was increased to its next of the value. The relative decrease 1 - (sQII'/SQH) hydrogen branch is a direct measure of the coverage with cod, as demonstrated later on. Here ~ Q H 'designates the charge of a monolayer of H atoms in the presence of cod.
charging curve and the amount of evolved COe by gas chromatography. The length of the arrest increases with the time of adsorption (curve b, 100 sec; c, 500 sec; d, 2000 sec). As noticed14 previously, a second short arrest appears at less anodic potentials, when the coverage is close to the saturation coverage. The length of the two arrests did not become larger with adsorption times greater than 2000 sec. Saturation coverage is practically achieved in 2000 sec under the given conditions. Adsorbed carbon monoxide that is oxidized in the upper arrest will be designated as type 1. The adsorbed species that are oxidized in the short arrest will be called type 2. The determination of different transition times is illustrated for curves a and d under consideration of the charging of the double layer. Curve a was extrapolated to U = 0 (dotted line). The product I s m represents the charge #QH corresponding to a monolayer of H atoms in the absence of COad. The transition time 700 is required for the oxidation of all the adsorbed molecules. The U-t curves are practically parallel at U 2 0.8 V. This demonstrates that the oxidation of toad is completed before the formation of the oxygen layer starts. A correction for the later process does not have to be applied on platinized platinum, in contrast to smooth platinum. The approximate determination of r1 of the upper arrest is shown in curve c. The same correction as for the double-layer charging in the absence of toad was used. This introduces only a slight error in n,since a large portion of the upper arrest is in a narrow potential range. The error due to the correction for double-layer charging is also small for 7 ~ 0 because , the surface is largely covered when the second arrest appears and because the double-layer capacity of platinum is considerably smaller5j6in the presence of COad than in the absence. The adsorption of carbon monoxide at open circuit was studied with equal adsorption times of 2000 sec in 0.5 M HzS04,0.1 M NazS04 0.05 M HzS04, and 0.1 M S a z S 0 4 0.005 M HzS04. The U-t curves in 0.5 M HzS04 and 0.1 M nTa2S04 0.05 M HzS04 coincide nearly in the hydrogen region if CO&dis absent (see Figure 2). The curve in 0.1 M nTa2SO43- 0.005 M HzS04 is located between 10 to 20 mV above the curves in the other two solutions. The measorements of the reaction
Experimental Results The charging curves in Figure 1 were taken in 0.5 M HzS04with 50 mA. Curve a is typical for the curve in the absence of toad. It displays the three characteristic regions :lZ hydrogen region, double-layer region, and initial part of the oxygen region. The adsorption of carbon monoxide occurs at 25" on platinized platinum,14 as on smooth platinum.1-6 This is demonstrated in Figure 1 by the decrease of the length of the hydrogen branch and the appearance of additional arrests at potentials in the double-layer region. The arrests between 0.55 and 0.65 V of curves b and c are due to the reaction
toad
+ HzO = COz
2H+
2e-
(1)
in acid solutions. This was confirmed14 recently by determining sirnultaneously the charge QCO from the 1.0-
os
-
-P6
I
_
3
0.4
-
+ +
+
Had = Hf 0.2-
Figure 1. Charging curves with 50 mA in 0.5 M HzSO4on platinized platinum (50 cm2 apparent surface) after different times, lad, of carbon monoxide adsorption a t open circuit: curve a, tad = 0; curve b, tad = 100 sec; curve c, tad = 500 sec; curve d, lad = 2000 sec.
+ e-
(2)
are nearly independent of pH, as to be expected, since a hydrogen electrode in the same electrolyte as the test electrode served as reference electrode. A large difference was found for the pH dependence of the processes occurring in the upper and lower arrest. The U-t curves are shifted by 40 to 50 mV against each other at a given time between 10 and 30 sec in the region of the lower arrest. I n contrast, they coincide nearly in the upper arrest. Volume 76,Number 4 April 1968
M. W. BREITER 1.0-
ID
-
0.5-
0.8
-
06-
0.6
-
-c
-
d
3
-
a
ab-
0.4
-
0.2-
0.2-
O
h
'
40 '
'
I
160
'
j , ,,
200
40
,
I
LlICCt
,
,
160
120
200
Figure 3. Charging curves after different adsorption times a t open circuit in 1 M KOH: a, t, = 0 sec; b, t, = 100 sec; C, tad 500 SeC; d, tad 2000 SeC.
A series of U-t curves, b, c, and d, measured after adsorption times of 100, 500, and 2000 sec in 1 M KOH is shown in Figure 3. The adsorption of carbon monoxide decreases the length of the hydrogen branch and causes the appearance of arrests at potentials in the double-layer region. Curves b and c demonstrate that the formation of type 1 carbon monoxide, that is subsequently oxidized at anodic potentials between 0.47 and 0.55 V, occurs first. Carbon monoxide that interacts with the surface after about 750 sec is oxidized at less anodic potentials, between 0.36 and 0.47 V (type 2). Limiting lengths corresponding to saturation coverage with C0,d are reached at 2000 sec. In contrast to acid solutions, the determination of the transition times is complicated in alkaline electrolytes by the overlapping of different electrode reactions. The slope of the U-t curve between 0.4 and 0.6 V cannot be used for the double-layer correction because the oxygen layer begins to formI2 there. An approximate determination of ~ T is H illustrated for curve a in Figure 3. A similar procedure was adopted to obtain T C O and TI for saturation coverage. A further evaluation of the curves in Figure 3 was not attempted because of the large uncertainty in the transition times. Charging curves that were measured with 50 mil after adsorption of carbon monoxide for 500 sec at different potentials in 0.5 M HzS04 are replotted in Figure 4. Curve a in Figure 4 appears similar to curve d in Figure 1. The arrest corresponding to the oxidation of type 2 species becomes shorter and appears at more positive potentials with increasing adsorption potential between 0.1 and 0.3 V. Simultaneously, an overshoot develops in this region. The width of the overshoot after adsorption at 0.4, 0.5, and 0.6 V is nearly equal to the length of the arrest of the oxidation of the type 2 species. The overshoot was also observed The Journal
of Physical
Chemistry
01
'
I
40
'
I
80
I
'
I20 t (recl
I
'
160
I
I
200
'
1 240
Figure 4. Charging curves with 50 mA after adsorption times of 500 sec a t different potentials in 0.5 iM HpS04: a, U = 0.1V; b, U = 0.3V; e, U = 0 . 5 V ; d, U = 0 . 7 V .
in studies6s6carried out under similar conditions on smooth platinum. Curve d indicates that the CO,a coverage decreases with U above 0.6 V. The ratio QCO/S&CO and the current after 500 sec of CO stirring are shown as a function of U in Figure 5. A limiting current of 50 mA was measured at 0.8 and 0.9 V at the stirring rate of 2 cm3/sec. The above technique for the determination of QCOis not reliable when QCO/S&CO < 0.8. Readsorption of CO on the bare surface cannot be eliminated completely, since the removal of dissolved CO from the electrolyte by vigorous stirring is not instantaneous. The value of QCO/SQCO at 0.7 V is probably too large. Current and coverage Q C O / ~ & C O under galvanostatic conditions are plotted as a function of U in Figure 6. The coverage is
1309
ADSORPTION A N D OXIDATION OF CARBON MONOXIDE
P
t
I
-i30
t
The potential d e c r e a s e ~ ~with ? ' ~ time, passes through a broad minimum, and becomes more positive afterwards at a slow rate. It was already suggestedl4 that stirring with CO leads to the production of molecular hydrogen under the given conditions
Had
0
I 0.2
0.4
u Ivl
0.6
Figure 5 . Current ( 0 ) and coverage (0)with C0,d as functions of potential under potentiostatic conditions.
+ Had f co = Hz + toad
Type 1 material is formed predominantly during the initial potential decrease, which is due to an increase in hydrogen pressure, P H ~ .The pressure remains nearly constant in the region of the broad minimum. The potential becomes more positive after the minimum, when the hydrogen pressure decreases because of the continued stirring with SO% Ar and 20% CO. The hydrogen coverage of the platinum atoms on which carbon monoxide has not yet been adsorbed is nearly 1 at U = 0.05V. Thereaction CO
0.q-
Figure 6. Current ( 0 ) and coverage (0)with C0,d as functions of potential for steady-state measurements under galvanostatic conditions.
equal to 1 between 0.2 and 0.4 V, in agreement with the results obtained under potentiostatic conditions. A rapid decrease of coverage with potential is observed between 0.5 and 0.6 V. Although the general shape of the I-U curve is reproducible, the potential at a given current may vary by as much as 0.1 V a t I > 10 mA in different runs. The drift of potential with time to more positive values is about 10 mV/hr at I 5 10 mA. It becomes larger at I > 10 mA. A steady-state potential could not be achieved at 30 mA anymore. A current of about 30 mA (0.6 mA/cm2 of apparent surface) represents the limiting diffusion current of dissolved CO as a computation with the numerical values for the diffusion coefficient and the solubility of CO in ref 1 and a diffusion-layer thickness of 0.08 cm at the small stirring intensity of 0.5 cm3/sec demonstrates.
Discussion Adsorption at Open Circuit. The platinum surface is covered with H atoms to about 90% at 0.1 V when the adsorption of carbon monoxide starts at open circuit.
(3)
+ HzO
=
CO,
+ Hz
(4) is considered a side reaction a t 25", in agreement with the gas analysis9 that only about 3% of CO is converted to COz and Hz at U = 0 and U = 0.5 V in 0.5 M HzS04 at 20 and 30". While the slope of the U-t curve b is practically the same as that of curve a between 0.35 and 0.5 V, the respective slope of curve c is smaller. The effect could be detected with certainty at Q c o / ~ Q c2~ 0.6. It is attributed to the oxidation of type 2 species. The net reaction for the oxidation of type 2 species was established by interrupting the charging curve when the upper arrest had been reached. The amount of Q C O ~ of evolved COz was determined14 together with the charge Q2 consumed in the lower arrest. The ratios QI/Qco~and Q2/Qco2agreed with each other within the error limits (20%) of the technique.13 The net reaction is represented in both cases by eq 1. Ratios of characteristic charges are put together in Table I for saturation coverage of the surface with CO,a. Since SQH is representative for the number of o to exposed Pt atoms, the ratio 2 s Q ~ / s Q ~corresponds the number of Pt atoms per adsorbed molecule in acid solutions. The average value of 2 s Q ~ / ~is &1.37 ~ ~in these solutions. It lies between the two values reported for smooth platinum in ref 2 and in ref 6. The ~~
Table I : Ratios of Characteristic Charges on Platinized Platinum 2sQa/sQco
0.05 M HzSO4 0 . 5 M HzSO4 0.1M NazSOd 0.05 M HzS04 0.1M NazSOa O.OOt5 M HzSOa 1 M KOH
+ +
sQi/sQco
1 .44f 0.14 1.27f0.13 1.37f 0.14
0.82f 0.08 0.76f 0.08 0.78f 0.08
1.40f0.14
0.80f 0.08
1.16 i- 0.12
0.77It 0.08
Volume 72, hiumber 4
April 1968
1310
M. W. BREITER
average value of 0.79 for SQ~/SQCO in acid solution demonstrates that about 21% of the adsorbed molecules are of type 2 . The latter value on platinized platinum is nearly the same as the value of 0.25 found for the ratio of the volumes of weakly and strongly adsorbed G o a d on platinum powder in the gas phase16a t room temperature. I n this case, the amount of weakly bonded C0,d desorbs by pumping alone. The value of ~ s & H / SQCO is somewhat smaller in 1 M KOH than the corresponding values in the acid solutions. The ratio ~Qr/sQcois about the same. Recently the adsorption of carbon monoxide was studiedlBon different samples of platinum-silica catalysts in the gas phase. The ratio of exposed Pt atoms to adsorbed molecules varied from about 1 to 2 with increasing platinum content. It was suggested that the coverage of Pt with toad depends upon the crystallite size. Linear bonding occurs predominantly on small, highly dispersed crystallites. Bridge bonding is favored when the uninterrupted area of platinum has attained a reasonable size. A distinction between was not made. weakly and strongly bonded co&d It is demonstrated in Figure 7 of the subsequent section that the parameter 1 - (SQH’/SQH) is equal to &CO/S&CO. The same number ~ S & H / S & C O of Pt atoms is covered on the average by one molecule of both types of cod for this reason. The result that the average number of exposed P t atoms per adsorbed molecule is larger than 1 may imply the presence of linearly and bridged bonded C0,d. A quantitative determination of the amount of these two forms is difficult because of the influence of the crystallite size16and because of the restriction that “packing” rules’’ apply to the linearly but not to the bridged COadon crystallites. bonded co&d, of sufficient size. The values of 2 s Q ~ / s Qand ~ ~ S Q ~ / S Q C O obtained in 1 M KOH indicate that the oxidation of toad is a two-electron process, as in acid solutions. The following scheme is suggested for alkaline electrolytes.
+ OH- = COOH,d + eCOOHad + 30H- = C03*- + 2H20 + eC0,d
(5) (6) Carbon dioxide was not detected in the He stirring gas by gas chromatography after the oxidation of co&din agreement with eq 5 and 6. E$ect of Adsorbed Caybon Monoxide o n the Hydrogen Coverage in Acid Electrolytes. The parameter 1 ( ~ Q H ’ / S & H ) was determined as a function of QCO/ S Q C O from the measurements in 0.5 M HzS04. The adsorbed amount of H atoms becomes very small when QCOapproaches SQCO, as evidenced by the curves d in Figure 1 and 3 and the curves in Figure 2 and 4. The results are plotted in Figure 8. A straight line may be drawn through the experimental points, as for smooth platinum.6 Thus the parameter 1 - (s&H’/s&H) is a direct measure of the coverage of platinized platinum with toad. The Journal of Physical Chemistry
1.0,-
0.0
t
1 I
I
I
0. I
0.2
0.3
?id
Figure 7. Isotherms for hydrogen adsorption in the presence of C0,d: 0, Qco/sQco = 0; 0, Qco/sQco = 0.41; A, Qco/~Qco= 0.25; V, Qco/sQco = 0.52.
d.
t
/I:
i 0.2
0.4
0.6
0.8
1.0
Qco/sQco Figure 8. Parameter, 1 function of Qco/sQco.
-
(sQH/sQH),
as a
The hydrogen coverage, 8~ = QH’/s&H’, was obtained as a function of hydrogen overvoltage, 7, in the presence of toad from the hydrogen branch of anodiccharging curves taken with 50 mA in 0.5 M HzS04. The procedure was outlined1* previously. The 8117 curves in Figure 7 coincide for 8~ > 0.2, as long as QCO/S&CO < 0.25. A decrease of hydrogen coverage is already detectable for 8 < 0.2 at small coverage with co&d.The 8 ~ - 7 curves are shifted to the left with increasing QCO/S&CO (>0.25). The free energy of hydrogen adsorption becomes less negative, as in the case19 of simultaneous adsorption of H atoms and specifically adsorbable anions. A distinction between (15) D. W. McKee, personal communication. See D. W. McKee, J . Catal., 8,240 (1967). (16) T.A. Dorling and R. L. Moss, ibid., 7, 378 (1967). (17) D.Brennan and F. H. Hayes, Phil. Trans. Roy. SOC.(London), B258, 347 (1965). (18) M.W.Breiter, Trans. Faraday SOC.,60, 1445 (1964). (19) M.W.Breiter, Ann. N . Y . Acad. Sci.,101,709 (1963).
1311
ADSORPTION AND OXIDATION OF CARBON MOXOXIDE weakly and strongly bonded hydrogen may still be ~ 0.41. The distinction is not made at Q c o / s & ~ = feasible at Q c ~ / ~ Q=c ~0.52 m y more. The 6-11 curve degenerates to a straight line (Temkin isotherm). An interaction between toad and Had is likely, as in the gas phase.20 A similar behavior was reportedz1 recently for hydrogen adsorption in the presence of adsorbed carbonaceous species formed previously by anodic polarization in 0.5 M H2SOeand 0.5 M CH30H. Coverage with toad under Potentiostatic Conditions in Acid Solutions. The U-t curves a, b, and c in Figure 4 demonstrate that the amount of type 2 species depends strongly upon potential. If the adsorption occurs at U < 0.1 V, the available 21% of sites are covered by that form. The coverage &2/sQ2 of the type 2 species decreases from 0.78 at 0.1 V to 0.65 a t 0.2 V and to 0.32 at 0.3 V. Simultaneously, the respective sites become covered again, probably by type 1 species. The decrease of the amount of type 2 species between 0.1 and 0.4 V is attributed to a kinetic effect, which will be discussed in more detail in the section concerning the oxidation mechanism of dissolved CO. The overshoot in curve c suggests that the hindrance of the oxidation of type 1 species is initially larger when the surface coverage with type 1 species is between 0.8 and 1. There are very few free sites for the formation of other adsorbed species like OHad. Since the adsorption of carbon monoxide has been studied2r6b6at U 2 0.2 V on smooth platinum and since relatively large current densities were employed5t6 during the subsequent oxidation of G o a d , the formation of type 2 species was not observed there. However, the existence of sites on which type 2 species may be formed is demonstrated by the overshoot of the U-t curves5r6on smooth platinum and by the desorption of C0,d a t 0.4 V after the stopping of the CO stirring. The ratio of the charge of the desorbed amount to the total charge in the presence of stirring is 0.22, according to the data in Figure 4 of ref 1. It was also noted6 that the initial 16% of the charge required for the oxidation of C0,d which had been adsorbed at 0.3 V possessed properties different from the residual 84%. The total coverage, as defined by &CO/S&CO, is practically 1 between 0.1 and 0.5 V (see Figure 5 ) . This is in agreement with the results on smooth platinum.1,2,6 The coverage decreases with U above 0.6 V. Measurements made under equivalent conditions have not been reported for smooth platinum. The observation of a sudden depletion of coverage at about 0.91 V during the anodic sweep of periodic voltammetric measurements with 40 mV/sec is considered characteristic for the voltammetric technique. I n general, the decrease of the coverage of type 1 species with potential started in the present studies when the current became larger than about 15 mA (compare Figure 6). Oxidation of Adsorbed Carbon Monoxide. The different pH dependence for the oxidation of type 1 and
type 2 species in Figure 2 is evidence for different oxidation mechanisms. The shift of the U-t curves to more positive potentials with increasing pH in the lower arrest corresponds roughly to the respective change of the potential of the hydrogen reference electrode. A reaction mechanism in which the electrode potential is not determined by an equilibrium between Had and H+ or between OHad and H 2 0 is consistent with these results. I n contrast, the pH dependence during the oxidation of type l species requiresz2the assumption of such an equilibrium. Charging curves were taken with currents between 10 and 200 mA after adsorption at open circuit in 0.5 M HzSO4to obtain more information on the mechanisms. The potentials at Q2/sQ2= 0.17, 0.33, 0.50, 0.67, and 0.83 were determined as a function of I in the lower arrest. Since the slope of the U-t curves is small in the upper arrest, the same determination was only carried out for Q1/sQ1 = 0.5. The results are presented in a semilogarithmic plot in Figure 9. Although the data obtained in the lower arrest scatter, parallel straight lines may be drawn through the points at the same coverage. The slope of these Tafel lines is 100 mV/decade of current, in contrast to a slope of 70 mV for type 1 species. Plots of U vs. Q2/2Qsat constant I were constructed with the aid of the Tafel lines in Figure 9. Parallel lines were obtained. The potential decreases approximately in a linear fashion with Q2/sQ2 between &z/ sQ2 = 0.17 and 0.83. Thus the current may be expressed
The parameters in eq 7 have the following values: IC2 = 1.3 X 10-4 mAp2 = 6.1, a2n2= 0.58. Equation 7 with an a2n2value smaller than 1 is consistent with the interpretation that the reaction
Goad
+ H20 = COOHad f H + + e-
(8)
is rate determining. The subsequent steps in the oxidation of type 1 species are less hindered and cannot be elucidated by the present method. The dependence of the rate upon coverage is of the Temkin type.23 A similar mechanism involving OH- instead of H20 is proposed for the oxidation of type 2 species in 1 M KOH. Two mechanisms have been suggested4J recently for the oxidation of type 1 species. Gilman4 postulated the “reactant-pair” mechanism, in which an adsorbed (20) H. Heyne and F. C. Tompkins, Trans. Faraday Soc., 63, 1274 (1967). (21) B. I . Podlovchenko and V. F. Stenin, Elektrokhimeya, 3 , 649 (1967). (22) A. N. Frumkin and B. I. Podlovohenko, Dokl. Akad. N a u k SSSR, 150,349 (1963). (23) M. Temkin, Zh. Fiz. Khim., 15, 296 (1941).
Volume 73, Number 4
April 1968
311. W. BREITER
1312
L
0.7
0/’LLULU-I--I
5
50
IO
100
500
I (mol
Figure 9. Current-potential relation for the oxidation of type 2 and type 1 species a t different coverage: 0, Ql/sQl = 0.5; 0, Qa/sQz = 0.66; 0, QZ/B&Z 0.17; m, Qz/sQz 0.83; A, &~/sQz= 0.50; V, Qz/sQ2 0.17.
carbon monoxide molecule reacts with a water molecule adsorbed at an adjacent site. The transfer of the first electron from the adsorbed complex is supposed rate determining. Warner and Schuldiner5 proposed a reaction between toad and adsorbed oxygen Oad. The formation of Oad from water molecules is assumed raie determining. Neither the reactant-pair mechanism4 nor the waterdischarge mechanism are consistent with the pH dependence found here for the oxidation of type l species. The pH dependence may be explained as suggested by Frumkin and PodlovchenkoZ2in their discussion of the mechanism of ethanol oxidation. A chemical reaction between adsorbed carbonaceous species and adsorbed OH radicals is supposed rate determining on a heterogeneous surface of the Temkin typez3 (9)
Here f (Q1/sQl) describes the dependence of the rate ~ the chemical potential of the upon toad and p o is adsorbed OH radicals p o = ~
FU
+ constant
(10)
The correctness of the latter interpretation depends in a critical way upon the validity of the exponential dependence of I upon OH in eq 9. Since this dependence cannot be verified in an independent way, owing to the small coverage with OHad, the preceding mechanism for the oxidation of type 1 species in acid electrolytes is considered tentative. The U-t curves c and d in Figure 3 consist of two parts in the potential region of the oxidation of type 1 material in 1 M KOH. The appearance of two arrests was also reported for the oxidation of adsorbed carbonaceous species that had been formed previously by anodic oxidation of methanol,24formic acid,26 or formaldehyde26 on platinized platinum in alkaline The Journal o j Physical Chemistry
electrolytes. It was s ~ g g e s t e d ~that ~ s ~two ~ forms of chemisorbed substance are present on the surface and that the second form results from the interaction between OH- ions with carbonaceous species, which are solely formed in acid solutions. It was noted in the present study that the length of the two parts in the upper potential region is about the same, independent of QI (see Figure 3). Therefore, it is proposed that the first arrest is due to reaction 5 . The further oxidation of COOH,d to c03’- occurs during the sccond arrest. A separation of the two one-electron steps may be observable in alkaline solutions, since three OH- ions are required in the second step. I n contrast, a hydrogen atom has only to be split up from COOH,d in acid solutions, making the second step a rapid process. The two arrests during the oxidation of adsorbed carbonaceous s p e ~ i e are s ~also ~ ~of~about ~ equal lengths in 0.1 M KOH and 1 M KOH (see Figure 7b in ref 24 and Figure 8 in ref 25). A two-electron process that is similar to the mechanism of the oxidation of toad is considered likely in agreement with results1aon the oxidation of these carbonaceous species in acid solutions. Oxidation of Dissolved Carbon Monoxide under SteadyState Conditions in Acid Solutions. The currentpotential data in Figure 6 represent steady-state values in the usual sense. The current was increased every 20 min. The change of potential after 20 min is small at I < 10 mA, although it is not zero. I n contrast, the I-U curve in Figure 5 was obtained by starting the CO stirring at a given potential, which was maintained potentiostatically, and by measuring the current when the change of current with time had become small after 500 sec. The two techniques do not yield the same 1-U curve. The potentiostatic I-U curve is shifted by about 0.2 V toward positive potentials with respect to the galvanostatic curve. Different interpretations of this effect are conceivable. (a) A truly steady state has not been achieved during the galvanostatic measurements. (b) The current distribution differs under potentiostatic and galvanostatic conditions. The local potential of some centers may be larger than the measured potential under galvanostatic conditions. It is considered likely that both effects contribute simultaneously. The poor reproducibility of the data in Figure 6 is probably due to factor a. An evaluation of the galvanostatic I-U curve on the basis of a Tafel plot was not attempted for the above reasons. The data in Figures 5 and 6 indicate, independent of the experimental conditions, that the decrease of Qco/ S&CO with U starts when the current becomes larger than 10 mA. The Tafel plots for type 2 species in Figure 9 show oxidation rates which are larger or comparable to the oxidation rates of dissolved CO under (24) 0. A. Petry, B. I. Podlovchenko, A. N. Frumkin, and H.Lal, J . Electroanal. Chern., 10, 253 (1965). (25) B. I. Podlovchenko, 0. A. Petry, A. N. Frumlcin, and H. Lal, ibid., 11, 12 (1966).
1313
THEBR$NSTEDa AND ISOTOPE EFFECTS FOR VINYLETHER HYDROLYSIS potentiostatic or galvanostatic conditions, respectively, a t U 5 4 V. It is suggested that the oxidation of dissolved CO involves the type 2 species as intermediates a t U < 0.4 V. This interpretation is substantiated by the shape of the potentiostatic I-U curve, which consists of an initial part with small oxidation rates between 0.1 and 0.4 V and of a second part with larger rates above 0.5 V. The decrease of Qz between 0.1 and 0.4 V implies that the rate of adsorption of type 2 species is insufficient to maintain full coverage with increasing oxidation rates. Adsorption of type 2 species is the rate-determining step at U 2 0.4 V. The type 1 species are considered the intermediates during the oxidation of dissolved carbon monoxide a t I > 10 mA. The oxidation rates of type 1 species in
The Brgnsted
a
Figure 9 are comparable to the rates of the oxidation of CO in Figure 5 and 6 . The coverage Q1/sQ1 decreases with potential at currents which are approaching the limit imposed by mass transport. Insufficient supply which leads to a smaller adsorption rate is the reason for the decrease of Q1/sQl. Partial mass-transport control was also concluded for the CO oxidation on smooth platinum free of Goad. I n contrast to the voltammetric measurements on smooth platinum,1~2e the decrease of Q 1 / ~ & 1 with U occurs here in a potential region in which the coverage with OHad and Oad is small.
(26) P. Stonehart, Electrochim. Acta, 12, 1186 (1967).
and Isotope Effects for Vinyl Ether Hydrolysis'.
by Maurice M. Kreevoy and Robert Eliasonlb Department of Chemistry, Unieersity of Minnesota, Minneapolis, Minnesota
66466 (Received October 8, 1967)
Like other reactions in which proton transfer is rate determining, ethyl vinyl ether hydrolysis is shown to obey the Br$nsted catalysis law with a variety of carboxylic acids, but acids of other structure give rates substantially varying from those predicted by the carboxylic acid correlation. The value of a for carboxylic acids is 0.66. The over-all solvent isotope effect, k ~ / k is~ 3.2 , f 0.1. The competitive isotope effect, H H / H D , is 7.0 f 0.1. If proton transfer is directly from M 8 0 +to the substrate, these lead to a primary isotope effect, ( k H / k D ) I , of 4.8 and a secondary solvent isotope effect, ( ~ E / ~ D ) I I o, f 0.66. From the latter an isotopic a, ai,of 0.56 is obtained. Competitive tritium isotope effects have been measured and the Swain-Schaad relations are obeyed. The reaction coordinate seems to be largely proton translation.
A number of papers have recently appeared establishing that vinyl ether hydrolysis is general-acid c a t a l y ~ e d ,shows ~ ~ ~ a solvent isotope effect, k ~ / k D , around 3 when catalyzed by mineral acid in water,2-6 and shows a substantially larger isotope effect when catalyzed by molecular formic acid.4 It has also been shown that the proton, once covalently affixed to the substrate, does not revert to the ~ o l v e n t . ~These observations convincingly show that vinyl ether hydrolysis proceeds through the mechanism given in eq 1 and 2. The mechanism is given for the specific case of ethyl vinyl ether hydrolysis, with which this paper is concerned. CH2=CHOCzH6 CH&H-OCzHs+
+ H + 3 CH&H-OCZHS+
+ HzO -+
(1)
series of ~
a
CH&H=O
~ -t s
~
+ CzH50H
p
~
(2)
The present paper provides a Brpinsted a for a series
of carboxylic acids and separates k ~ / into k ~ primary and secondary solvent isotope effects. From the latter, an isotopic a is obtained in general agreement A test of the Swain-Schaad with the Brpinsted a. relation is also described.' Some details of transitionstate structure can be surmised. (1) (a) Supported, in part, by the National Science Foundation through Grant No. GP-5088; (b) Hercules Corp. Summer Fellow, 1965; Du Pont Co. Summer Fellow, 1966; Ethyl Corp. Fellow, 19661967. (2) P. Salomaa, A. Kankaaperh, and M. Lajunen, Acta Chem. Scand., 20, 1790 (1966). (3) A. J. Kresge and Y . Chiang, J. Chem. SOC.,Sect. B , 53 (1967). (4) A. J. Kresge and Y . Chiang, ibid., Sect. B , 58 (1967). (5) (a) D. M. Jones and N. F. Wood, ibid., 5400 (1964); (b) A. Ledwith and H. J. Woods, ibid., Sect. B , 753 (1966). (6) M. S. Shostakovskii, A. 5. Atavin, B. V. Prokoljev, B. A. Trofimov, V. I. Lavrov, and N. M. Driglazov, Dokl. Akad. Nauk SSSR, 163, 1412 (1965). (7) C. G. Swain, E. C. Stivers, J. F. Reuwer, Jr., and L. J. Schaad, J . Amer. Chem. SOC.,80, 5885 (1958). Volume 78, Number 4 April 1988
