ANODICMETHANOL OXIDATION ON PLATINUM-GOLD ALLOYS
3377
Reactivity and Surface Composition. Anodic Methanol Oxidation on Platinum-Gold Alloys
by M. W. Breiter General Electric Research Labmatmy, Schenectady, New York
(Received April 6, 1966)
The anodic oxidation of methanol in 0.5 M H2S04 was studied on heterogeneous platinumgold alloys whose surface composition is known from voltammetric current-potential curves measured in the absence of methanol. Periodic current-potential curves were taken potentiostatically in the potential range between hydrogen and oxygen evolution with 30 mv./sec. at methanol bulk concentrations between 0.004 and 1 M . The ohmic and capacitive component of the impedance was determined by voltammetry with superimposed alternating voltage. The rate of methanol oxidation which was characterized by different kinetic parameters depends in the same way upon the bulk composition of the alloys as the amount of the platinum-rich phase on the surface. As a first approximation the rate is proportional to the amount of the platinum-rich phase on the surface. Methanol oxidation occurs only on this phase. The results on the inhibition of methanol oxidation by oxygen layers lead to the same conclusion. There is no direct correlation between the reactivity and the d-band character of the platinum-gold alloys. Different mechanisms of the anodic oxidation of methanol on platinum or the platinum-rich phase are evaluated critically.
Introduction
It is known that systematic changes of the reactivity of a catalyst may be produced by alloying it with increasing amounts of another metal whose catalytic properties differ from that of the catalyst. If the influence of transport processes is negligible, the reactivity may be expressed by the rate constant of the rate-determining step of the same net reaction on the alloys or by equivalent kinetic parameters. Similar statements apply to electrochemical reactions in which the electrode affects the state of the reactants and intermediates.’ ‘Usually it is assumed in the interpretation of kinetic data of the above type that the siirface composition is equal to that of thebulk for each of the alloys. Indirect evidence like self-consistency of the results or independence of the reactivity of extended Pretreatment have been reported in support of this assumtion since its exDerimenta1 verification is difficult in most cases. To avoid these uncertainties it appeared desirable to the author to study an electrochemical reaction on a set of alloys for which the surface composition may be determined in au independent way.
Experimental results on the anodic methanol oxidation in acidic solutions will be reported and discussed for heterogeneous platinum-gold alloys in this communication. These alloys are composed2 of a platinumrich phase al and a gold-rich phase a 2 . It was found that the phase a1 behaves like platinum with respect to the electrochemical formation and removal of the oxygen layer3 and of the layer of adsorbed hydrogen atoms4while the phase a2has the electrochemicalproperties of gold. These conclusions were derived from the following results. Voltammetric current-potential curves may be constructed3 in a first approximation according to the equation
I(U)
= U’IPt(U)
+ b’I*Il(U)
(1)
from the respective c u ~ e son smooth and gold in acidic solution. The parameter ut agrees, well with another parameterc t for given alloy. (1)R. Parsons,
sei,,
2,
418 (1964).
(2) see A. S. Darling, PEatinum Metals Rev., 6, 60,106 (1962). (3) M, W. Breiter, J. phys. them,, 69,901 (1965). (4) M. w.Breiter, Trans. Faraday SOC., 61,749(1964).
Volume 69, Number 10 October 1966
M. W. BREITER
3378
c’ =
a&H/s&H,Pt
(2)
Here I designates the current density, U the potential, a’ and b’ are constants4 determined by a least-square fit, is the total amount of adsorbed hydrogen atoms on each of the alloys in millicoulombs per square centimeter, s & H , ~ that t on platinum as obtained by the integration of I-U curves in the so-called5 hydrogen region. The parameters a’ (or c’) and b’ represent a rehtive measure3t4of the amount of the phase a1 and a2,respectively, on the surface. It is a relative measure since the roughness factors are included in a’ and b’. This allows the kinetic data on methanol oxidation to be correlated t o those on the surface composition. Methanol oxidation was chosen as a suitable reaction for several reasons. As on the influence of transport processes is negligible at the foot and in the rising portion of the first wave during the anodic sweep in acidic solutions. Kinetic data are analyzed in this region. The rate of methanol oxidation on gold is much smaller* than that on platinum. The anodic methanol oxidation is inhibited by oxygen layers on platinum metals.6-8 Since the formation (or reduction) of the oxygen layers on the phases a1 and az, respectively, occurs in the same potential ranges on all the alloys, the inhibiting effect of the oxygen layers on methanol oxidation may be expected to be the same. Different mechanisms6-l5 have been suggested for the anodic oxidation of methanol on platinum. Earlier work is reviewed extensively in these papers. A critical evaluation of the recent mechanisms will be given under consideration of the new results in this paper.
Experimental Section The electrodes were prepared as described3 from the supply of platinum-gold alloys delivered in form of wires by the Sigmund Cohn Corp., New York, N. Y. The gold content of the alloys studied was 5, 10, 2070 atom yo. The measurements were carried out at 30” in a Pyrex vessel of conventional design in quiescent solutions. The solutions were made from A.R. sulfuric acid, double-distilled water, and A.R. methanol. Molecular oxygen was removed by stirring the solutions extensively with purified helium before the actual measurements. The electrode potential U was measured against a hydrogen electrode in the same solution as the test electrode. The current density I and the capacitive component l/wC, of the interfacial impedance in a series circuit were computed on thebasisof the geometric surface area of the electrodes. The assembly for the measurements of voltammetric I-U curves and of the ohmic and capacitive component The Journal of Physical Chemistry
of the impedance by voltammetry with superimposed alternating voltage was described previously.1e The sweep rate was 30 mv./sec., and the frequency for the impedance measurements was 1000 C.P.S. The measurements were always carried out on all the alloy electrodes in the same solution (0.5 M HzS04 X M CHaOH; X = 0, 0.004, 0.01, 0.1, 1). The procedure was the same for every electrode: cleaning in hot chromic acid solution, thorough rinsing in doubledistilled water, insertion in the solution, and removing of traces of molecular oxygen from the solution by He stirring with the electrode at open circuit. The recording of the I-U curves and of the l/wC,-U curves was started after the 20th cycle. It takes about 15 cycles before the curves approach a shape which changes only slightly with the number of subsequent cycles. The initial behavior is attributed as in the previous ~ t u d i e s in ~,~ 1 N H2S04 to the removal of impurities from the surface by the intermediate formation of the oxygen layers. The I-U curves and l/oC,-U curves were recorded on the Varian F80 X-Y recorder if a large resolution was required. Otherwise they were photographed from the screen of the Tektronix oscilloscope 502.
+
Results Different experimental curves are put together as an example for the measurements on the 30 atom Au alloy in Figure 1. The curves are reproductions of the original traces. Solid lines represent anodic sweeps; dashed lines correspond t o cathodic sweeps. Curves a and b were recorded in the absence of methanol, curves a’ and b’ in 0.5 M HzS04 0.1 M CHaOH. Curve a exhibits the three regions which are characteristic5 for platinum: hydrogen region (0 to 0.4 v.), double-layer region (0.4 t o 0.6 v.), and region of the oxygen layer (0.6 to 1.5 v.). The reduction waves of the oxygen layers on the phases a1 and a 2
+
(5) A. Slygin and A. Frumkm, A d a Physicochim. URSS, 3, 791 (1935). (6) M. W. Breiter and 5. Gilman, J . Electrochem. Soc., 109, 1099 (1962); 110, 449 (1963). (7) M. W. Breiter, ibid., 110, 1006 (1963). (8) M. W. Breiter, Electrochim. Acta, 8, 973 (1963). (9) R. P. Buck and L. R. GrifEth, J . Eledrochem. SOC.,109, 1006 (1962). (10) W. Vielstich, 2.Instrumntenk., 71, 29 (1963). (11) J. Giner, Eledrochim. Acta, 9, 63 (1964). (12) S. Gilman, J . Phys. Chem., 6 8 , 70 (1964). (13) V. S. Bagotzky and Yu. B. Vasilyev, Electrochim. Ada, 9,869 (1964). (14) J. E. Oxley, G. K. Johnson, and B. T. Buaalski, ibid., 9, 897 (1964). (15) C. Liang and T. C. Franklin, ibid., 9, 517 (1964). (16) M. W. Breiter, J . Eledroanul. Chem., 7, 38 (1964).
ANODICMETHANOL OXIDATION ON PLATINUM-GOLD ALLOYS
t
P
I-
b
b'
LI
0.4
0.8 Ulri
1.2
1.6
Figure 1. Periodic current-potential curves (a and a') and capacitive cornponent-potential curves (b and b') a t 1000 C.P.S. on the 30 atom % Au alloy a t 30 mv./sec.: a and b in 0.5 M H2S04; a' and b' in 0.5 M HzSOd 0.1 M CHsOH.
+
are marked. As to be expected on the basis of the common supply the I-U curve of each of the alloys coincided in the oxygen region with the previous3 curve of the same alloy. Thus the parameters a', b', c' of the preceding communication^^,^ may be used here to characterize the surface composition. The l/wC,-U curve b reflects as on platinum16 the same regions as the I-U curve a. The capacitive component has small values in the hydrogen region because of the large pseudo-capacity1' of adsorbed H atoms. The transition from the double-layer region to the potential region of the oxygen layer is less steep in curve b than on platinum.16 This is attributed to the presence of the cyL phase. The influence of the az phase on the shape of l/wC,-U curves is better discernible a t gold contents above 50 atom %. A relatively large amount of the azphase is required since the double-layer capacities of the two phases are in parallel and since the capacity of the phase a2 is smaller than that of the phase al. The Re-U curve which is not shown does not yield any additional information. Since the impedance is largely capacitive here, the 1/ wC,-U curves give more detailed information in general than the R,-U curves. Curve a' exhibits the methanol oxidation waves 1 and 2 during the anodic sweep and wave 3 during the cathodic sweep. It looks very similar to the curve on platinum.(? As on platinum6J3the peak current densities of the three waves are considerably smaller than are computed6 for a diffusion-controlled process at
3379
o C2 ~ 0.01 M under the present conditions. Thus the influence of transport processes is negligible. The comparison of curve a' with curve a shows that the rising portion of wave 1 below 0.7 v. is in the potential region of negligible oxygen coverage. The formation of the oxygen layer on the phase a2 starts at about 0.75 v. during the anodic sweep. Thus the peak of wave 1 is attributed as on platinum67l3to the inhibiting effect of the oxygen layer, here on the phase a2. Wave 2 is located in the potential region of large oxygen coverages and will not be discussed in detail for this reason. The two reduction waves of the oxygen layers on the phases a1 and az are just recognizable in curve a'. A methanol oxidation wave does not exist between 1.10 and 0.9 v. during the cathodic sweep although the phase a:!is free of its oxygen layer. Wave 3 appears when part of the oxygen layer on the phase a1has been reduced. Based on the analysis of the effect of the oxygen layers on methanol oxidation, it is concluded that methanol oxidation occurs at a noticeable rate only on the platinum-rich phase of the alloys. This conclusion will be confirmed subsequently by the strong evidence from the correlation between the rate of methanol oxidation and the composition of the alloys. Evidence for the adsorption of methanol on the alloy Au is presented by curve b' (see also with 30 atom Figure 2, curve b). The pseudo-capacity in the hydrogen region is smaller in curve b' than in curve b because methanol molecules occupy partly the sites for hydrogen adsorption. Methanol molecules are adsorbed in the double-layer region since the capacity is smaller in the presence than in the absence of methanol. This makes the double-layer region look broader in curve b' than curve b. The formation of the oxygen layer starts at about 0.75 v. during the anodic sweep as indicated by the rapid increase of the capacity between 0.75 and 0.85 v. in curves b and b'. The values of l/wCs do not differ much for curves b and b' in the potential region of the oxygen layer. Methanol adsorption is negligible there. These results correspond to those on platinume which were obtained by different techniques. The rate IM of methanol oxidation at o C = ~ 1 M is plotted as a function of potential in the rising portion of wave 1 for different alloys in Figure 3. I Mis practically equal to the current density at gold contents below 30 atom %; otherwise IM(u) I(u)- I R (3) The residual current density I E was determined as the minimal value of the current density between 0.2 and
(17) P. D o h and B. Ershler, Acta Physicochim. U R S S , 8, 747 (1940).
Volume 69, Number 10 October 1966
M. W. BREITER
3380
-
L/
L
1
0
" " " " 0.4
0.0
1.2
1.6
0.4
Ulvl
UI"1
1.2
1.6
U(U1
O
o
0.8
0.4
0.8 UI*I
1.2
1.6L
b
Figure 2. Capacitive component-potential curves in 0.5 M H&I4 1M CHsOH on different alloys: curve a, 10 atom % Au; curve b, 30 atom % Au; curve c, 50 atom % Au; curve d, 70 atom % Au.
+
'I-
0.05
1
-/
UIVI
Figure 3. Tafel plots of the rate of methanol oxidation in 0.5 M HzS04 1M CHaOH on different alloys in the rising portion of wave 1: 0, 5 atom % Au; 0, 10 atom % Au; A, 20 atom % Au; 0, 30 atom % Au; . , 40 atom % Au; A, 50 atom % Au.
+
0.5 v. during the anodic sweep. . The semilogarithmic plots show the existence of two different Tafel regions The Journal of Physical Chemistry
for each of the alloys. The rate I N does not differ much at a given potential for the alloys with gold contents up to 20 atom %. Then it decreases rapidly with the gold content above 30 atom %. Similar plots are obtained at the other methanol bulk concentrations. However, they are less reliable in the lower Tafel region since the corrective term I g becomes larger with decreasing OCM. Conclusions on the mechanism of methanol oxidation have been derived6Jafrom the rising portion of wave 1 for U > 0.65 v. The results in Figure 3 establish that the mechanism at low potentials (U < 0.65 v.) is different from that at potentials above 0.65 v. At U > 0.65 v. the slope b of the Tafel line U=a+blogI (4) is 68 mv. (a, = 0.85) for the alloys with 5 and 10 atom % Au, 82 mv. (a, = 0.71) for the 30 atom % Au alloy, and 106 mv. (a, = 0.55) for the 40 and 50 atom % Au alloys, where
2.3RT bF The b values of the alloys with gold contents between 5 and 30 atom yo Au are close to the value of 86 mv. (a, = 0.67) found6 on platinum. In the Tafel region below 0.65 v. the b value increases from 143 mv. (an - 0.4) for the alloys with 10 and 20 atom Yo Au to 161 mv. (a, = 0.36) for the alloys with 40 and 50 atom yo Au. The small decrease of OM with potential below 0.7 v. was neglected in the determination of the b values. This appears justified by the results on platinum6 at ,,CM= 1M . The change of the shape of the l/oC,-U curves with increasing gold content (10, 30, 50, 70 atom yo) is illustrated at OCM= 1 M in Figure 2. Curve a on the 10 atom % Au alloy is very similar to the curve on platinum.* Hydrogen adsorption has been largely replaced by adsorption of methanol. A considerable decrease of the double-layer capacity results from methanol adsorption in the double-layer region. During the anodic sweep the beginning of the formation of the oxygen layer is paralleled by the removal of the adsorbed methanol molecules. During the cathodic sweep methanol adsorption starts after the oxygen layer has been largely reduced. Small humps which are marked by arrows in curve a appear in the curves of the 5 and 10 atom Yo Au alloys as on platinums at ,,CM2 0.1 M . They are characteristic for the removal or the formation of an adsorbed layer of organic species as on mercury.l* These humps are not detectable any more an =
~
(18) A. N. Frumkin and V. I. Melik-Gaikazyan, DOH.Akad. Nauk SSSR, 77, 855 (1951).
3381
ANODICMETHANOL OXIDATION ON PLATINUM-GOLD ALLOYS
20 atom % Au the rate IM decreases rapidly with the gold content. It was found that the peak currents IP of the waves 1 and 3 depended in a characteristic way upon the gold content. As on platinumJ6 these peak currents decrease slightly with stirring on the platinum-rich alloys, indicating the influence of the diffusion of formic acid which is formed as an intermediate from the oxidation of CH30H to COz. The peak currents are plotted in Figure 5 for OCM = 1 M and o C =~ 0.01 M as a func-
10-
'
lo-'
Io-2
IO0
OC"
Figure 4. Rate of methanol oxidation a t 0.6 v. during the anodic sweep as a function of the bulk concentration of methanol: 0, 5 atom % Au; 0, 10 atom % Au; A, 20 atom % Au; 0, 30 atom % Au; A, 40 atom % Au; . , 50 atom % Au; V, 70 atom % Au.
at gold contents above 10 atom % at I1 M. The transition from the double-layer region to the region of the oxygen layer becomes less pronounced with increasing gold content and is not discernible above 40 atom % ' Au any longer. It is concluded from the small variation of the capacity between 0.4 and 1.0 v. during the anodic and cathodic sweep on the alloys with gold contents above 40 atom yo that methanol adsorption on the phase aZis negligibly small. This is in agreement with previous results*on gold. The rate IM of methanol oxidation at 0.6 v. during the anodic sweep is plotted as a function of the methanol bulk concentration for different alloys in Figure 4. IX was determined according to eq. 3. The corrective term IR is very small at OCM 2 0.01 M for the alloys with gold contents below 30 atom %. The above potential is located in the lower Tafel region of Figure 3. The shape of the upper three curves in Figure 4 is similar to that of adsorption isotherms. I M does not increase much with o C M and tends toward a limiting value which is nearly reached a t OCM = 1 M. Similar results are obtained at other potentials in the represents lower Tafel region. It is concluded that IM the rate of oxidation of adsorbed molecules. The variation of IM with the composition of the alloys is also illustrated by Figure 4. The alloys with 5 , 10, and 20 atom % Au possess nearly the same reactivity. Above
Figure 5. Rate of methanol oxidation a t the peaks of waves 1 and 3 as a function of bulk composition of the alloys: 0, wave 1, 1 M CH80H; 0, wave 1, 0.01 M CHaOH; W, wave 3, 1 M CH30H; 0, wave 3, 0.01 M CHsOH.
tion of the bulk composition of the alloys. The curves in Figure 5 have a parabolic shape. The variation of Ip with increasing gold content is small up to 20 atom Yo. Above this value the peak currents decrease rapidly with the gold content. It was demonstrated for platinum6 that the methanol coverage is negligibly small at the peak potentials during the respective sweeps. The same conclusion was derived here from the l/wC,-U curves for the platinum-gold alloys. Thus I p is representative of the inhibited rate of methanol oxidation on a surface with a small oxygen coverage. Volume 69,Number 10
October 1066
3382
M. W. BREITER
Discussion I o Reactivity and Surface Composition. The parameters a' and c' which are a measure for the amount of the platinum-rich phase on the surface are plotted in Figure 6 as a function of the bulk composition of the alloys. The resulting curve 1 has a parabolic shape. The increase of a' and c' between 0 and 20 atom % Au was attributed3 to an increase of the roughness factor of the phase al. This increase overcompensates the decrease of a' which is due to the gradual replacement of the phase al. Above 20 atom % ' Au the phase al is rapidly replaced by the phase az with increasing gold content. The results in Figures 4 and 5 ot, % Au were used to obtain curve 2. The ratio I M / I M , ~ * , was computed at ~ C M = 0.1 M for different composiFigure 6. Parameters a' and c', measuring the amount of the platinum-rich phase on the surface, and ratio I/I,= tioris from the data in Figure 4 with IM,max= 150 as functions of bulk composition: curve 1: 0, a'; 0,c'; pa./cm.2. From the data in Figure 5 the ratio I p / curve 2: A, IP/~P,, for wave 1 at 1 M IP,,,,, was determined for wave 1 as a function of comCH30H; 0, I M / I Y , , for ~ 0.6 v. a t 1 M CH30H. position at OCM = 1 M with I P , ~=~ 10 , ma./cm.2. Then the different scales of the ordinate for the curves character of the bulk of platinum-gold alloys. The 1and 2 were normalized as shown in Figure 5. d-band character was computed20121 under the asThe shape of curve 2 in Figure 6 is very similar to sumption that the d band of platinum is gradually that of curve 1. The specific reactivities I M and I P filled with increasing gold content. The filling of the depend in nearly the same way upon the bulk comd band is achieved20s21 between 30 and 40 atom % Au position as the amount of the platinum-rich phase on for homogeneous alloys. It occurs2oat even smaller the surface. I n a first approximation I M and Ip gold contents for heterogeneous alloys. Figure 6 are proportional to the amount of the phase a1 on the demonstrates that the reactivities decrease more slowly surface. This is direct evidence that methanol oxiwith gold content than it should be if the decrease dation occurs at a noticeable rate only on the phase were controlled by the d-band character. Thepresent al. Since each of these two rates is proportional to system exempliiies the oversimplification o n which the product of a rate constant and the electrochemically correlations between reactivity and d-band character active part of the geometric surface area, it is conare based. I n most cases the d-band character on the cluded that the respective rate constant remains nearly surface will not be equal to that of trhebulk and there independent of the amount of the phase az on the is no reliable method to determine the d-band character surface. However, there exist systematic deviations of the surface. from this rule at gold contents above 40 atom % as the Mechanism of Methanol Oxidation in the Upper Tafel comparison between curve 1 and 2 shows. These Region. It was demonstrated in the preceding section deviations are paralleled by the increase of the b that the platinum-rich phase has the electrochemical values found from results in Figure 3. properties of platinum with respect to methanol oxiThe fact that I M / I M , ~and = IP/IP," depend in dation in acidic solution. Therefore, the conclusions the same way upon the bulk composition implies that for platinum which were deriveds*'for the first time with the inhibition of methanol oxidation a t the peak of consideration of the coverage 8~ under working condiwave 1by the oxygen layer on the phase a1 is nearly the tions apply also to platinum-gold alloys. Both the same for all the alloys. A similar statement holds for the inhibition at the peak of wave 3 according to the CHaOH *CH30Had (5) results in Figure 5 . adsorption step and the subsequent discharge step (6) Jn recent years variations in the reactivity of the same electrochemical reaction have been correlatedlg (19) M.Oikawa, BuU. Chem. SOC.Japan, 28, 626 (1955); S. Schuldito the change of the d-band character of alloys. Usuner and J. P. Home, J . Phys. Chem., 61, 705 (1957); H.H.Uhlig, Z . Elektrochem., 62, 626,700 (1958). ally the d-band character on the surface is assumed (20) I(.A. Lapteva, I. R. Borissova, and M. G. Slinko, Zh. Fiz. Khim., equal to that of the bulk. It may be easily checked 30, 61 (1956). if such a correlation exists between the specific reactivi(21) R. J. Weiss and K. J. Tauber, Phys. Chem. Solids, 7, 249 ties IM and Ip of methanol oxidation and the d-band (1958). The J
O U T of ~
Physical ChemistTy
ANODICMETHANOL OXIDATION ON PLATINUM-GOLD ALLOYS
were established as rate-determining steps at ~ C M 2 0.01 M in 1 N HCIO, in the rising portion of wave 1 between about 0.68 and 0.78 v. duringthe'anodic sweep and in the falling portion of wave 3 between about 0.7 and 0.6 v. during the cathodic sweep. The results do not allow the specification of the configuration of the radical R beyond its net composition CH30. Similarly the single steps of eq. 6 are not known. It should be pointed out that the mechanism predicts satisfactorily the pH dependence observedgJa experimentally since the potential V, measured against a hydrogen electrode in the same solution as the test electrode, differs in a first approximation only by a constant from 7, the overvoltage of methanol oxidation, at methanol bulk concentrations between 0.01 and 1M . Mechanism of Methanol Oxidation in the Lower
3383
Tafel Region. I n general it is difficult to elucidate the mechanism of the anodic oxidation of fuels at low potentials. The current densities are small there. Side reactions whose contributions do not matter a t larger oxidation rates have to be considered. The subsequent conclusions on methanol oxidation in the lower Tafel region are tentative. The an values of the lower Tafel region in Figure 3 and the results in Figure 4 suggest that a reaction of the type 6 is rate-determining. The existence of two Tafel regions may be attributed t o different heats of adsorption of the species (CH30H,d or Rad) of reaction 6 in the respective regions. The exchange current densities which are obtained by extrapolation of the Tafel lines, for instance to U = 0, are larger in the lower Tafel region than in the upper one. Loosely bonded methanol molecules7are oxidized in the lower Tafel region. This may lead to a different configuration of R.
Volume 69,Ntimber 10 Odober 1966
