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The mechanism of tin-promoted electrochemical oxidation of organic substances on platinum
J. Electroanal. Chem., 97 (1979) 63--76
63
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
THE MECHANISM OF TIN-PROMOTED ELECTROCHEMICAL OXIDATION OF ORGANIC SUBSTANCES ON PLATINUM
Yu.B. VASSILIEV, V.S. BAGOTZKY, N.V. OSETROVA and A.A. MIKHAILOVA
Institute of Electrochemistry, U.S.S.R. Academy of Sciences, Moscow (U.S.S.R.) (Received 10th April 1978; in revised form 3rd July 1978)
ABSTRACT The promoting action of tin in the binary system platinum tin has been studied. A direct correlation of polarization and adsorption measurements has been carried out using an adsorption method of applying the promoter. The method of fast potentiodynamic pulses has been used to control the amount of tin and organic species on the surface. Studies have been made with smooth and platinized electrodes on the adsorption and oxidation of formic acid and methanol. A comparison of electro-oxidation rates at constant organic species coverage has made it possible to reveal the true catalytic effect as a result of introduction of tin. This effect has been found to be independent of both the nature of the substance under oxidation and the state of the electrode surface (smooth or rough). The promoting effect of tin is observed in the reactions where the limiting stage is represented by R + OHads; in the reactions where the limiting stage is represented by electron transfer or else the interaction R + Hads, the promoting effect of tin does not show up, which fact points to the selectivity of its catalytic action.
INTRODUCTION
Recent studies demonstrate that the best catalysts for the electro-oxidation of formic acid, formaldehyde, methanol and other simple organic substances .in acidic solutions are represented not by pure metals of the platinum group but by binary systems containing a metal of this group (normally platinum) and another component, namely a promoter. An enhanced catalytic activity with respect to electro-oxidation reactions is shown by mixed platinum ruthenium [1,2], palladium ruthenium [2], platinum tin [3--5], platinum rhenium [4,5], platinum lead [6,7] and platinum molybdenum [3] catalysts, as well as platinum promoted by sulphur and selenium [8]. At not very positive potentials the oxidation rate of methanol on platinum tin, platinum ruthenium and platinum rhenium catalysts is nearly two orders of magnitude higher than that on pure platinum. It has been shown in this case that the behaviour of deposited binary systems and adsorption layers of promoters on the platinum catalyst is essentially the same [ 9--13 ]. Only limited studies on the mechanism of these phenomena have been made up to the present time. In a number of works the explanations of the enhanced electrocatalytic activity of such systems are based on the ideas of surface redox couples [3,4,14]. But these ideas are useful in understanding the enhanced
64 activity of only some of the systems (platinum molybdenum, platinum rhenium), and they are incapable of explaining the promoting action of such additives as lead, ruthenium and, to some extent, tin, sulphur and selenium. Attempts have been made to find a correlation between the catalytic activity and physicochemical properties of the alloys [15--17]. Some authors believe that the p r o m o t e r reduces the poisoning effect of strongly adsorbed organic residues [7,12,18]. All these explanations do n o t take into account the mechanism of electro-oxidation of simple organic substances on platinum, i.e. participation at the limiting stage of t w o different species, viz. a chemisorbed carbon-containing species and an oxygen-containing radical OHads [19]. In most cases the promoting additive facilitates the adsorption of oxygen on the electrode-catalyst or renders it more reversible. Therefore it has been suggested in a number of papers [9,10] that the promoter on the electrode-catalyst surface causes the formation of centres where absorption of the oxygen active form (OH~ds) takes place and that its higher concentration results in an accelerated process of electro-oxidation. More definite conclusions as to the mechanism of electro-oxidation acceleration on platinum in the presence of promoters are only possible when qualitative consideration is given to the effect of promoters on the adsorption of organic substances and oxygen, and on the basis of a direct correlation of polarization and adsorption measurements. The results of such a study are given in the present paper. The system platinum tin was used as the model since it provides the highest acceleration of formic acid and methanol electro-oxidation. An adsorption method of applying the promoter (tin) to catalyst (platinum)was employed, which made it possible to effect a strict control of the amount of promoter on the catalyst surface. In order to check the selectivity of the promoter action, a parallel study was made of the effect of tin adsorption layers on platinum on the rate of some other electrochemical processes occurring by various mechanisms. EXPERIMENTAL We made our studies on smooth and weakly platinized platinum electrodes (roughness factor 300--500, E~ ep°s = 0.2 V, aged for 48 h) against a background of 0.5 M H2SO 4. The adsorption of tin ions, methanol and formic acid was studied by the method of fast potentiodynamic pulses as described earlier [20]. In order to determine the surface coverage with tin and the organic substance present simultaneously, use was made of the fact that these substances give oxidation waves on the anodic pulse in different potential regions. The electrode was pretreated in such a way that, in the presence of tin and organic substance in solution, it would be possible to obtain a reproducibly pure electrode surface, the holding time being 20 s at 0 V, 30 s at 1.2 V, after which a jump to the adsorption potential under study was made; the electrode was kept at this for some time Tads and then a measuring pulse was fed to it. The steady-state currents were measured after the surface had been covered with all the chemisorbed species to a stabilized level. All the potential values are given relative to the hydrogen electrode in the same solution, and all the
65 current values are related to the real unit area of the electrode as measured with respect to hydrogen adsorption. When measurements were made with platinized platinum electrodes, the need to allow for the variation of activity when passing from one electrode to another, required, prior to each test, the polarization curve to be taken in the absence of tin ions, and then at a potential of 1.2 V various additives were introduced into the solution and the polarization curve of the electro-oxidation organic substance in the presence of tin was measured. T H E BASIC PRINCIPLES OF TIN ION A D S O R P T I O N O N P L A T I N U M
The adsorption of tin ions was studied on a smooth platinum electrode in 0.5 M H2SO4 solution. The tin ion concentration in the solution was varied from 10 -7 M to 10 - 3 M and the adsorption potential from 0 V to 1.1 V. As can be seen from Fig. 1, in the application of a fast cathodic pulse the adsorption of tin ions results in a drop of hydrogen adsorption. This drop was used to determine the platinum surface coverage with tin. As tin adsorbs on platinum, oxidation peaks appear and grow on the anodic curve: the first peak at E = 0.55 V and the second peak at E = 0.75 V. As the tin ion adsorption increases, these peaks merge together. The first peak is probably linked with a partial oxidation of adsorbed tin (or with an adsorption thereon of the reversible form of oxygen OHads), as, following its, there is no decrease in the coverage. The second peak corresponds to the complete oxidation of tin and its transition into solution, as, following it, the surface turns o u t to be free from adsorbed tin. The steady-state coverage of the surface with tin as found through the drop in hydrogen adsorption shows a linear increase with the logarithm of bulk concentration (Fig. 2), i.e. it follows Temkin's isotherm: 0s, = as~ + (1/fsn)ln Cs,so4
(1)
The linearity remains unaltered as the adsorption potentials change from 0.2 V to 0.6 V. The factor fsn calculated from the isotherm slope has the value 11 for E = 0.2 V and 0.4 V and 23 for E = 0.6 V. The dependence of the steady state coverage of the platinum surface with tin ( 0 ~ ) on potential is shown in Fig. 3. In the potential region 0 V to 0.4 V the coverage remains practically unaffected, it showing a slight increase from 0.0 V to 0.2 V, and, starting with 0.4 V, a linear drop with E. At 0.7 V the platinum surface will be practically free from adsorbed tin despite the fact that tin is present in solution. The relation between the quantity of electricity required for the oxidation of chemisorbed tin and the drop in the hydrogen coverage in the presence of tin ions was useful in determining the number of electrons per single adsorption centre needed for the removal of chemisorbed tin: ne = Qox/Q'H. This number is different for different potentials: at E r ~ 0.2 V it is equal to 2 and at Er ~ 0.4 V it falls to 1. If follows from these data that adsorption of tin ions on platinum is accompanied by a partial charge transfer. In the potential region 0 V to 0.2 V tin on the platinum surface is practically present in the form of a neutral ad-atom Sn °. At E~ > 0.2 tin undergoes partial oxidation and at E~ = 0.5 V it
66
t/•A
cm"~
o
o~e
o,
,o
E,/~
Fig. 1. Cathodic and anodic i--E curves for s m o o t h p l a t i n u m at/~init. = 0.4 V in 0.5 M ~r H 2 S O 4 (--) and in the presence of 5 × 10 - 5 M SnSO 4 for various T: ( . . . . ) 10, (x) 20,
(o) 50, (o) 300 s. is formally present o n the surface as Sn ÷ (or SnOH). The higher f value when passing to E r = 0.6 V is due to a more p r o n o u n c e d ionic nature of chemisorbed species and a related growth of the forces of mutual repulsion between them as the potential shifts to the anode side.
06
04
02
O6
0
0.2 2
0,~
/
08 o
o
-7
-6
-5
(~gcd'/moL F'
Fig. 2. D e p e n d e n c e of the steady-state p l a t i n u m surface coverage with tin ions o n their bulk c o n c e n t r a t i o n in 0.5 M H2SO 4 at various adsorption potentials: (o) 0.2, (x) 0.4, (e) 0.6 V. Fig. 3. D e p e n d e n c e o n E r of the p l a t i n u m surface coverage with tin cations and the relation Qox/AQ~ for tin ions in 5 X 10 - 5 M SnSO 4 + 0.5 M H2SO 4.
67 T H E I N T E R F E R E N C E O F TIN IONS A N D O R G A N I C S U B S T A N C E S IN T H E I R ADSORPTION OF PLATINUM
As an organic substance and tin ions are simultaneously present in solution one can observe their interference on adsorption. The presence of the organic substance in solution leads to a decreased tin adsorption (R0's~) on the platinum electrode, which is, however, different at different potentials. With methanol, for example, at potentials below 0.2 V this decrease is smaller, whereas in the potential region for the m a x i m u m methanol adsorption (E r = 0.4 V) it is greater (Fig. 4). In the presence of methanol in solution, tin undergoes desorption from the surface of the platinum electrode at a less positive potential than in its absence. As the bulk concentration of tin ions increases, the coverage of the platinum electrode surface with tin in the presence of organic substances also increases, to a first approximation, along Temkin's isotherm (Fig. 5) but the constant a becomes smaller with increasing concentration of the organic substances. On the other hand, in the presence of tin in solution the coverage o f the platinum electrode with chemisorbed organic species decreases too. As a first approximation, the coverage of the surface with organic substances falls off linearly with the logarithm of the bulk concentration of tin ions (Fig. 6): Sa0tR = a2 - - ( 1 / / 2 ) i n Cs~so 4
(2)
or it falls off linearly with the coverage of the electrode surface with tin (Fig. 7): 1
O8
06 ®
04
0.2
\ xe~e
/j\ 02
0.4 o,
04
06
F,r/V
Fig. 4. Dependence of the platinum surface coverage with tin or CH3OH on E r in solutions: (x) eWR, 0.1 M CH30H + 0.5 M H2SO4;(®) Sn0'R, 0.1 M CHaOH + 5 X 10--5M SnSO 4 + 0.5 M H2S04; ($) 0'Sn , 5 X 10--5M SnSO 4 + 0.5 M H2S04; (®) ROtSn, 0.1 M CH3OH + 5 X 10 - 5 SnSO 4 + 0.5 M H2SO 4. Fig. 5. Dependence of the coverage o f smooth platinum with tin on the bulk concentration of SnSO 4 in solutions: 0.1 M HCOOH + 0.5 M H2SO 4 at 0.3 V (®) and 0.4 V (e); 10 - 3 M C 4 H 4 0 4 + 0.5 M H2SO 4 at 0.1 V (n); 5 × 10--4M C4H404 + 0.5 M H2SO 4 at 0.1 V (0). For platinized platinum solutions: 0.1 M CH3OH + 0.5 M H2SO 4 at 0.3 V (@) and at 0.4 V (A); 0.1 M HCOOH + 0.5 M H2SO 4 at 0.3 V (®) and at 0.4 V (x).
68
O6 06
04 04
02 02
-5
-/+
-3 h~r-s.'~olC /
r
02
r
I
04
06
R
0£!
Fig. 6. Dependence of the coverage of smooth platinum with organic species on the bulk concentration of SnSO4in 0.1 M HCOOH + 0.5 M H2SO4, (®) 0.3 V and (o) 0.4 V; 10 - 3 M C4H404 + 0.5 M H2S04, (0) 0.1 V; 5 X 10--4M C4H404 + 0.5 M H2S04, 0.1 V ((~); for platinized platinum: in 0.1 M CH30H + 0.5 M H2S04, 0.3 V (0) and 0.4 V (A); 0.1 M HCOOH + 0.5 M H2S04, 0.3 V (~) and 0.4 V (x). Fig. 7. Dependence of the platinum surface coverage with organic species on the coverage with tin ions from solutions containing various concentrations of SnSO4: for smooth platinum in 0.1 M HCOOH + 0.5 M H2SO4, (0) 0.3 V and (o) 0.4 V; for platinized platinum in 0.1 M CH3OH + 0.5 M H2SO4, (0) 0.3 V and (A) 0.4 V; in 0.1 M HCOOH + 0.5 M H2SO4, (®) 0.3 V and (x) 0.4 V; for platinized platinum at constant coverage with respect to tin, (A) 0.4 V, 0.1 M HCOOH + 0.5 M H2SO 4.
Sn0'R = A - - 7R0'sn
(3)
The values of fl, f2 and 7 for the different cases are shown in Table 1. It is interesting that in a number of cases 7 ---- 1 i.e. the total average of the surface with tin and the organic substance, as a first approximation, remains the same. A similar picture has been observed by the present authors for the adsorption of anions and methanol [21], methanol and butadiene [22], cations and maleic acid [23]. It follows from these data that the behaviour of the adsorption isotherm of TABLE 1 Solution
Electrode
Er/V
fl
f2
"Y
0.1 0.1 0.1 0.1 0.1 0.1
Smooth Pt Smooth'Pt Pt/Pt Pt/Pt Pt/Pt Pt/Pt
0.3 0.4 0.3 0.4 0.3 0.4
7.5 9.0 18.0 19.0 12.8 12.0
11.5 10.5 15.0 11.5 9.5 9.2
0.65 1.00 1.10 1.70
M M M M M M
HCOOH HCOOH HCOOH HCOOH CH3OH CH3OH
69
o
06
•
•
04 02 I
I
I
[
-3
-2
~t
0
I
l~c~/mol I.-t
Fig. 8. Dependence of the coverage of platinized platinum with organic species on the bulk concentration of HCOOH at constant surface coverage with tin ions. 0'sn values: (ll) 0.0; (0) 0.13; (®) 0.24; (®) 0.27; (m) 0.31; (A) 0.54. O, x, o, [a, A stand for the corresponding sums (O's~ + s~O'rO.
the organic substance in the presence of a constant a m o u n t o f tin on the surface remains similar to the adsorption isotherm on the pure platinum surface (Fig. 8), but in this case the coverage is smaller by the magnitude of the surface coverage with tin. THE EFFECT OF TIN IONS ON THE RATES OF ELECTROCHEMICAL PROCESSES ON PLATINUM ELECTRODES In ~ e presence of tin ions in solution at potentials which are m ore negative than 0.6 V one can observe a sharp acceleration of electrochemical oxi dat i on o f methanol and formic acid on s m o o t h and platinized platinum electrodes. As can be seen f r o m Fig. 9 the effect of tin ions decreases with increasing anodic potential. At potentials more positive than 0.6 V, polarization curves bot h in presence and the absence of tin in solution are practically the same. This suggests t h a t the process o f electro-oxidation is accelerated only when adsorbed tin ions are present on the surface of the platinum electrode. A study has been made of the effect of pre-adsorbed tin ions on the electro-oxidation of methanol on platinum in the absence of tin ions in solution. For this purpose a polarization curve of m e t ha nol electro-oxidation was measured in 1 M CH3OH + 0.5 M H2SO4, then the solution was p o u r e d off; the cell and the electrode were washed with a background solution (0.5 M H2SOa) in which the electrode was completely cleaned of chemisorbed species at E = 1.2 V. T hen the cell was filled with a 5 X 10 - 4 M SnSOa solution where, at E = 0.2 V, the tin ions u n d e r w e n t adsorption on the electrode f o r 10 min. At the same potential, washing was effected with the background solution (while this t o o k place the tin ions remained adsorbed on the electrode surface), and 1 M CH3OH + 0.5 M H2SO4 solution was i nt r oduc e d in to the cell, after which t he polarization measurements were repeated. These experiments have shown t hat the presence of preadsorbed tin ions results in the same acceleration o f the m e t h a n o l electro-oxidation process as the presence of tin ions in solution. In determining the surface coverage, the deposition of the tin and organic substance ions was affected by the following three methods: (1) in the process
70
2
t
0
-/
/
/
/.
-2
-2 I
,
I
I
I
i
i
02
04
06
aY
02
0.3
04
t__
05
R
!
OS.
Fig. 9. Polarization curves of methanol electro-oxidation on platinized platinum in 1 M H2SO 4 in the absence of tin (o) and after tin adsorption onto the surface from 5 X 10 - 4 M SnSO 4 (®). Fig. 10. Dependence of electro-oxidation rate on the surface coverage with tin ions for smooth platinum in 0.1 M HCOOH + 0.5 M H2SO 4 solution, at E = 0.4 V (A); for platinized platinum in 0.1 M CHsOH + 0.5 M H2SO 4 solution, at E = 0.4 V (®); 0.1 M HCOOH + 0.5 M H2SO4, at 0.3 V (®) and at 0.4 V (x); for platinized platinum at constant coverage with respect to tin, (@) 0.4 V, 0.1 M HCOOH + 0.5 M H2SO 4.
of their free competition out of the solution to the pure surface; (2) the deposition of the tin ions out of the solution with the carbon-containing species being present on the surface; (3) the deposition of the carbon-containing species out of the solution with the tin ions being present on the surface. It can be seen from Fig. 10 that at constant potential and concentration of the organic substance in solution the oxidation rate of formic acid and methanol, both on smooth and platinized platinum electrodes, increases exponentially with RO'S~ : i = K exp(k R.
ROsn)
(4)
In this case the rate increases despite the fact that the coverage of the surface with oxidizable organic species decreases. This shows that the accelerating catalytic effect of tin on electro-oxidation is far more pronounced than the inhibiting effect associated with a decreased adsorption of an oxidizable organic substance. The establishment of the true catalytic effect of tin on the electro-oxidation processes requires a comparison of the rates at constant surface coverage with organic species.
71
Figure 11 shows the dependence of the formic acid electro-oxidation rate on a platinized platinum electrode at various constant surface coverages with tin on the formic acid bulk concentration. This dependence is similar to that for the pure platinized platinum surface b u t is shifted with the same slope to the region of higher currents. Both in the absence of tin and its presence on the surface, the rate of formic acid electro-oxidation increases to a fractional power with the bulk concentration, i.e.
i=
k'0 d
(5)
I f data on the adsorption of formic acid on these electrodes are taken into consideration (Fig. 8) and i f use is made of the dependence of its electro-oxidation rate on a surface rather than on a bulk concentration (Fig. 12) then it will readily be seen that this rate increases exponentially with the coverage of the surface with chemisorbed species:
i = k0sa exp(/31f. Sn0~)
(6)
It should be noted that when changing over from a pure platinized electrode to those p r o m o t e d b y a varying amount of tin, the nature of the dependence remains unaltered. Figure 13 illustrates that an increase of the surface coverage with tin is accompanied b y an exponential increase of the constant in eqn. (6):
ko sn = k0 exp(~2f0s~)
(7)
where ~2f = 8.6. It is interesting that the relative catalytic effect as expressed
t~
2
I
-2
I
-/
I
1
0
¢
logc~/ f ~ l -~
/6
I
I
O.2
I
I
0.4
I
I
O6
~
I
O8
Fig. 11. Dependence of HCOOH electro-oxidation rate on platinized platinum at constant coverage with tin ions at E = 0.4 V on the bulk concentration of HCOOH. The values of 0t Sn-. 0.0 (O), 0.13 (x), 0.27 (~), 0.54 (A). Fig. 12. Dependence of HCOOH electro-oxidation current for platinized platinum at constant surface coverage with tin ions on the surface coverage with organic species. The values of ~t . Sn- (e) 0.0, (A) 0.13, (x) 0.24, (®) 0.31, (O) 0.54.
72
x
O
02
x
04
&
Fig. 13. D e p e n d e n c e of the o x i d a t i o n rate of organic substances on the surface coverage with tin ions at E = 0.4 V, at 0~tt = const for: s m o o t h p l a t i n u m f r o m 0.1 M H C O O H + 0.5 M H2SO 4 solution (x); platinized platinum: 0.1 M CH3OH + 0.5 M H2SO4 (o); 0.1 M H C O O H + 0.5 M H2SO 4 (@); platinized platinum at c o n s t a n t coverage with respect to tin ~ ( ~ ) , for chcmisorbed species f r o m H C O O H on s m o o t h p l a t i n u m electrodes: Er° x = 0.6 V (v), E °x = 0.65 V (0) and f r o m CH3OH o n platinized p l a t i n u m electrode: E ° x = 0.65 V after washing electrode (D).
b y eqn. (7) does n o t depend on either the nature of the oxidizable substance, or the electrode potential, or the state of the electrode surface (smooth or platinized platinum electrodes). Ad~id et al. [12] assumed that the promoting effect is due to a lowering in surface coverage of strongly b o n d e d chemisorbed particles inhibiting the oxidation rate. As can be seen from data on oxidation rates of chemisorbed species formed during adsorption of HCOOH, CH3OH and CO2 in the presence or in the absence of tin ions shown in Fig. 14a and b, tin ions enhance the oxidation of chemisorbed particles too. Thus the adsorbed tin accelerates n o t only the oxidation processes of an organic substance being present in solution b u t also the electro-oxidation process of chemisorbed organic species. The relative catalytic effect for oxidation of chemisorbed organic c o m p o u n d s in solution remains the same. Another study was concerned with the effect of adsorbed tin on some other electrochemical reactions on a smooth electrode. In the process of electrooxidation of anthrahydroquinone-2,6-disulphonic acid and the backward reaction of electroreduction of anthraquinone-2,6-disulphonic acid, i.e. reactions whose limiting stage is represented b y electron transfer [24], even a considerable concentration of tin ions does n o t result in a noticeable rate change. In the case of electroreduction of maleic acid, whose limiting stage is represented b y an interaction of a chemisorbed organic species with adsorbed hydrogen [25] (see Fig. 15); the reduction rate shows an exponential drop with increasing surface coverage with tin: i = io exp(--~lR0'sn)
(8)
where kl ~ 16. This drop is far above the magnitude that could be expected due
73
(~
(a)
i ~00. (b)
~00.
200.
'1 t
100-
50.
~o . . . .
2o . . . .
,~o ~/5
Fig. 14. O x i d a t i o n rate o f specms chemlsorbed f r o m s o l u h o n s of formm acid (1,1 ,1 ,2,2 ), t t t iw . . . I Ft t t I tr t m e t h a n o l ( 3 , 3 , 5 , 5 ) and CO 2 ( 4 , 4 , 4 ) , organm specms m presence (1 ,1 , 2 , 3 , 4 , 4 ,5 ) or absence of tin ions on s m o o t h (a) and platinized platinum (b) electrodes. (1) 1 M H C O O H , ads ox I r I . . ox r Er =0.3V, E r =0.6V, 0sn=0;(1)0Sn = 0 . 0 8 ; ( 1 ) 0 Sn = 0 . 1 5 ; ( 2 ) Er = 0 . 6 5 , 0 S n = t t ads ox p ~ t O ; ( 2 ) O s n = O . 2 0 ; ( 3 ) I M C H 3 O H , Er = 0.25 V, E r , = 0.6 V, ~Sn = 0; ( 3 ) 0 Sn = 0.4; (4) CO2, E~ ds = 0.1 V; E ° x = 0.6 V in 0.5 M H2SO4; ( 4 ) in 0.5 M H2SO 4 + 5 X 10 - 5 M SnSO4; ( 4 ) in 0.5 M H2SO 4 + 10 - 4 M SnSO4; (5) 1 M CH3OH , E ads = 0.30 V and Er° x = 0.65 V in the background solution after washing electrode: 0'Sn = 0; (5') 0'Sn = 0.27.
to a decreased coverage with organic chemisorbed species alone, it being connected with a simultaneous decrease in the coverage of the surface with the second reacting species, i.e. adsorbed hydrogen.
74
i
05
OI
02
03
04
0 ~
Fig. 15. Dependence o f C4H40 4 electroreduction rate at E = 0.1 V on the surface coverage
with tin or an adsorbed organic species in solutions: 5 X 10 - 4 M C4H404 + 0.5 M H2SO 4 and 10 - 3 M C4H404 + 0.5 M H2SO 4. (e) Sn0'R; (®) R~'sn"
RESULTS AND DISCUSSION
The experimental results obtained suggest that the nature of the effect of tin on the rate of some electrochemical processes on platinum is governed by the mechanism of these processes and by the influence of tin on the adsorption of the reacting species. The promoting effect of adsorbed tin ions is most pronounced for the electro-oxidation of methanol, formic acid and other simple organic substances in the potential region up to 0.5 V. In the potential region under study the limiting stage of the electro-oxidation of these substances is represented by an interaction of a chemisorbed organic species with a n O H a d s radical formed during a preceding rapid electrochemical stage of water molecule discharge [19]: Rad s + O H a d s ~ R - - O H
(9)
Platinum is a very good adsorbent with respect to methanol and formic acid, and it ensures a reasonably high coverage of the surface with oxidizable organic species but the coverage of the platinum surface with O H a d s radicals in this potential region is very low. The expression for the rate of reaction (9), considering that 0OH < 0.1 and 0.1 < O'R < 0.9 according to refs. 19 and 26, can be presented as i = k0ROOH e x p ( - - - ~ f 0 R )
where the reaction rate constant k includes an exponential term with the activation energy which, in turn, depends on the adsorption energy of the reacting species. For the dependence of the rate constant on the adsorption
(I0)
75
energy (--/kVoH) of the oxygen-containing species it can be assumed that
(11)
k = k ° exp(---~AGoH/RT )
Tin is prone to a ready and reversible oxygen adsorption in this potential region, therefore it can be assumed that the adsorbed tin atoms represent those centres where there will be an enhanced adsorption of OHad s participating in the process of oxidation, whereas the platinum atoms on the surface are contributors of organic species. Hence the number of OHad s radicals on the surface will shown a linear increase with the number of adsorbed tin atoms: 0'OH = nRO's~
(12)
Studying the adsorption of OH particles both on platinum and tin and considering that for the latter 0OH > 0.1, we shall have AGoH
= APtGoH
+ ASnG°H-
RTfOoH
(13)
or, with regard for (12), AGoH
=
A~GoH
+ ASnG°H --RTnfOsn
(14)
From (14) and (11) we shall obtain for the true catalytic effect a relation k = k °1 e x p ( ' y p n f O s n )
(15)
which at f12 = ~/P is in line with experimental dependence (7). The true promoting effect does not depend on either the nature of the oxidizable substance or the electrode potential but is governed by the surface coverage with the promoter (Fig. 13) and the OH adsorption energy on the promoter. Thus the participation at the limiting stage in the anodic oxidation of simple organic substances (methanol, formaldehyde and formic acid) of two different species, viz. a chemisorbed carbon-containing species and an oxygen-containing radical OHads, explains why the most active role is played not by a single metal catalysts but by mixed or promoted catalysts with one c o m p o n e n t being active with respect to chemisorption of organic substances (usually platinum group metals), the second providing for a high surface concentration of OH radicals (tin, rhenium, ruthenium, etc.). REFERENCES 1 J.O'M. Bockris and H. Wroblowa, J. Electroanal. Chem., 7 (1964) 428. 2 0 . A . Petty, Dokl. Akad. Nauk SSSR, 160 (1965) 871; V.S. Entina, O.A. Petry and V.T. Rysikova, Elektrokhimiya, 3 (1967) 758, 1238; V.S. Entina, O.A. Petry and I.V. Shelepin, Elektrokhimiya, 2 (1966) 457; O.A. Petty, B.I. Podlovchenko, A.N. Fnn-nkin and H. Lal, J. Electroanal. Chem., 10 (1965) 253. 3 J.A. Shropsh~re, J. Electrochem. Soc., 112 (1965) 465; 114 (1967) 773. 4 K.J. Cathro, Electrochem. Technol., 5 (1967) 441; J. Electrochem. Soc., 116 (1969) 1608. 5 D.F.A. Koch, Australian Pat. 46123, 1964. 6 E. Schwarzer and W. Vielstich in Troisi~mes Journ~es Internatinnales d ' E t u d e s des Piles ~i C o m b u s tibles, Bruxenes, Presses Acad. Europ~ennes, 1969, p. 220. 7 R.R. Ad~i~, D.N. Simi~, A.R. Despi~ and D.M. Dra~i~, J. Electroanal. Chem., 65 (1975) 587; 61 (1975) 117; 80 (1977) 81.
76 8 H. Binder, A. KShling and G. Sandstede, From Electrocatalysls to Fuel Cells, University of Washingt o n Press, Seattle, 1972, P. 43; Advanced Energy Conversion, 6 (1966) 135; 7 (1967) 77; Nature, 214 (1967) 268. 9 M. Watanabe, S. Motoo and T. Suzuki, Denkl Kagaku, 40 (1972) 205, 210; J. Electroanal. Chem., 69 (1976) 429. 10 D.F.A. Koch, D.A. Rand and R. Woods, J. Eleetroanal. Chem., 70 (1976) 73. 11 D.A.J. Rand and R. Woods, J. Electroanal. Chem., 36 (1972) 57; 44 (1973) 83. 12 R.R. Ad~ld, A.R. Despid and D.N. Slmi6, Workshop %n Electrocatalysis by Non-Metanic Surfaces, National Bureau of Standards, 1975, p. 191; A.R. Despid, R.R. Ad~id and A.V. Tripkov16, Elektrokh imiy a, 5 (1977) 660. 13 M.M. Janssen and J. Moolhuysen, Electrochim. Acta, 21 (1976) 8 6 1 , 8 6 9 . 14 M.W. Breiter, Z. Phys. Chem., 52 (1967) 73. 15 D.W. McKee and F.G. Norton, J. Catalysis, 3 (1964) 252. 16 G.C. Bond and D.E. Webster, Plat. Met. Rev., 10 (1964) 10. 17 J.O'M. Bockris and S. Srinivasen, J. Electroanal. Chem., 11 (1966) 350; Fuel Cells. Their Electrochemistry, McGraw-Hill, 1970. 18 M.R. Andrew, J.S. Drury and B.D. McNIcol, J. Appl. Electrochem., 6 (1976) 99. 19 V.S. Bagotzky and Yu.B. Vassiliyev, Electrochim. Acta, 12 (1967) 1323. 20 V.S. Bagotzky and Yu.B. Vassilyev, Electrochim. Acta, 11 (1966) 1439. 21 D.N. Pixtskhalava, Yu.B. Vassflyev and V.S. Bagotzly, E l e k t r o k h i m i y a , 6 (1970) 1144. 22 M.A. Khrizolitova, L.A. Mirkind, Yu.B. Vassilyev, M.Ya. Fioshin and V.S. Bagotzky, E l e k t r o k h i m i y a , 8 (1972) 548. 23 Sh.Sh, Khidirov, V.N. Andreyev, O.A. Khazova, V.E. Kazarinov,and Yu.B. Vassilyev, Elektrok h l m i y a , 13 (1977) 1275. 24 T.K. Zolotova, I.V. Shelepin and Yu.B. Vassfliyev, E l e k t r o k h i m i y a , 10 (1974) 1830. 25 V.V. Chernyl and Yu.B. Vassflyev, ElektrokhLmiya, 12 (1976) 872. 26 O.A. Khazova, Yu.B. Vassllyev and V.S. Bagotzky, E l e k t r o k h i m i y a , 10 (1974) 606.
63
© Elsevier Sequoia S.A., Lausanne -- Printed in The Netherlands
THE MECHANISM OF TIN-PROMOTED ELECTROCHEMICAL OXIDATION OF ORGANIC SUBSTANCES ON PLATINUM
Yu.B. VASSILIEV, V.S. BAGOTZKY, N.V. OSETROVA and A.A. MIKHAILOVA
Institute of Electrochemistry, U.S.S.R. Academy of Sciences, Moscow (U.S.S.R.) (Received 10th April 1978; in revised form 3rd July 1978)
ABSTRACT The promoting action of tin in the binary system platinum tin has been studied. A direct correlation of polarization and adsorption measurements has been carried out using an adsorption method of applying the promoter. The method of fast potentiodynamic pulses has been used to control the amount of tin and organic species on the surface. Studies have been made with smooth and platinized electrodes on the adsorption and oxidation of formic acid and methanol. A comparison of electro-oxidation rates at constant organic species coverage has made it possible to reveal the true catalytic effect as a result of introduction of tin. This effect has been found to be independent of both the nature of the substance under oxidation and the state of the electrode surface (smooth or rough). The promoting effect of tin is observed in the reactions where the limiting stage is represented by R + OHads; in the reactions where the limiting stage is represented by electron transfer or else the interaction R + Hads, the promoting effect of tin does not show up, which fact points to the selectivity of its catalytic action.
INTRODUCTION
Recent studies demonstrate that the best catalysts for the electro-oxidation of formic acid, formaldehyde, methanol and other simple organic substances .in acidic solutions are represented not by pure metals of the platinum group but by binary systems containing a metal of this group (normally platinum) and another component, namely a promoter. An enhanced catalytic activity with respect to electro-oxidation reactions is shown by mixed platinum ruthenium [1,2], palladium ruthenium [2], platinum tin [3--5], platinum rhenium [4,5], platinum lead [6,7] and platinum molybdenum [3] catalysts, as well as platinum promoted by sulphur and selenium [8]. At not very positive potentials the oxidation rate of methanol on platinum tin, platinum ruthenium and platinum rhenium catalysts is nearly two orders of magnitude higher than that on pure platinum. It has been shown in this case that the behaviour of deposited binary systems and adsorption layers of promoters on the platinum catalyst is essentially the same [ 9--13 ]. Only limited studies on the mechanism of these phenomena have been made up to the present time. In a number of works the explanations of the enhanced electrocatalytic activity of such systems are based on the ideas of surface redox couples [3,4,14]. But these ideas are useful in understanding the enhanced
64 activity of only some of the systems (platinum molybdenum, platinum rhenium), and they are incapable of explaining the promoting action of such additives as lead, ruthenium and, to some extent, tin, sulphur and selenium. Attempts have been made to find a correlation between the catalytic activity and physicochemical properties of the alloys [15--17]. Some authors believe that the p r o m o t e r reduces the poisoning effect of strongly adsorbed organic residues [7,12,18]. All these explanations do n o t take into account the mechanism of electro-oxidation of simple organic substances on platinum, i.e. participation at the limiting stage of t w o different species, viz. a chemisorbed carbon-containing species and an oxygen-containing radical OHads [19]. In most cases the promoting additive facilitates the adsorption of oxygen on the electrode-catalyst or renders it more reversible. Therefore it has been suggested in a number of papers [9,10] that the promoter on the electrode-catalyst surface causes the formation of centres where absorption of the oxygen active form (OH~ds) takes place and that its higher concentration results in an accelerated process of electro-oxidation. More definite conclusions as to the mechanism of electro-oxidation acceleration on platinum in the presence of promoters are only possible when qualitative consideration is given to the effect of promoters on the adsorption of organic substances and oxygen, and on the basis of a direct correlation of polarization and adsorption measurements. The results of such a study are given in the present paper. The system platinum tin was used as the model since it provides the highest acceleration of formic acid and methanol electro-oxidation. An adsorption method of applying the promoter (tin) to catalyst (platinum)was employed, which made it possible to effect a strict control of the amount of promoter on the catalyst surface. In order to check the selectivity of the promoter action, a parallel study was made of the effect of tin adsorption layers on platinum on the rate of some other electrochemical processes occurring by various mechanisms. EXPERIMENTAL We made our studies on smooth and weakly platinized platinum electrodes (roughness factor 300--500, E~ ep°s = 0.2 V, aged for 48 h) against a background of 0.5 M H2SO 4. The adsorption of tin ions, methanol and formic acid was studied by the method of fast potentiodynamic pulses as described earlier [20]. In order to determine the surface coverage with tin and the organic substance present simultaneously, use was made of the fact that these substances give oxidation waves on the anodic pulse in different potential regions. The electrode was pretreated in such a way that, in the presence of tin and organic substance in solution, it would be possible to obtain a reproducibly pure electrode surface, the holding time being 20 s at 0 V, 30 s at 1.2 V, after which a jump to the adsorption potential under study was made; the electrode was kept at this for some time Tads and then a measuring pulse was fed to it. The steady-state currents were measured after the surface had been covered with all the chemisorbed species to a stabilized level. All the potential values are given relative to the hydrogen electrode in the same solution, and all the
65 current values are related to the real unit area of the electrode as measured with respect to hydrogen adsorption. When measurements were made with platinized platinum electrodes, the need to allow for the variation of activity when passing from one electrode to another, required, prior to each test, the polarization curve to be taken in the absence of tin ions, and then at a potential of 1.2 V various additives were introduced into the solution and the polarization curve of the electro-oxidation organic substance in the presence of tin was measured. T H E BASIC PRINCIPLES OF TIN ION A D S O R P T I O N O N P L A T I N U M
The adsorption of tin ions was studied on a smooth platinum electrode in 0.5 M H2SO4 solution. The tin ion concentration in the solution was varied from 10 -7 M to 10 - 3 M and the adsorption potential from 0 V to 1.1 V. As can be seen from Fig. 1, in the application of a fast cathodic pulse the adsorption of tin ions results in a drop of hydrogen adsorption. This drop was used to determine the platinum surface coverage with tin. As tin adsorbs on platinum, oxidation peaks appear and grow on the anodic curve: the first peak at E = 0.55 V and the second peak at E = 0.75 V. As the tin ion adsorption increases, these peaks merge together. The first peak is probably linked with a partial oxidation of adsorbed tin (or with an adsorption thereon of the reversible form of oxygen OHads), as, following its, there is no decrease in the coverage. The second peak corresponds to the complete oxidation of tin and its transition into solution, as, following it, the surface turns o u t to be free from adsorbed tin. The steady-state coverage of the surface with tin as found through the drop in hydrogen adsorption shows a linear increase with the logarithm of bulk concentration (Fig. 2), i.e. it follows Temkin's isotherm: 0s, = as~ + (1/fsn)ln Cs,so4
(1)
The linearity remains unaltered as the adsorption potentials change from 0.2 V to 0.6 V. The factor fsn calculated from the isotherm slope has the value 11 for E = 0.2 V and 0.4 V and 23 for E = 0.6 V. The dependence of the steady state coverage of the platinum surface with tin ( 0 ~ ) on potential is shown in Fig. 3. In the potential region 0 V to 0.4 V the coverage remains practically unaffected, it showing a slight increase from 0.0 V to 0.2 V, and, starting with 0.4 V, a linear drop with E. At 0.7 V the platinum surface will be practically free from adsorbed tin despite the fact that tin is present in solution. The relation between the quantity of electricity required for the oxidation of chemisorbed tin and the drop in the hydrogen coverage in the presence of tin ions was useful in determining the number of electrons per single adsorption centre needed for the removal of chemisorbed tin: ne = Qox/Q'H. This number is different for different potentials: at E r ~ 0.2 V it is equal to 2 and at Er ~ 0.4 V it falls to 1. If follows from these data that adsorption of tin ions on platinum is accompanied by a partial charge transfer. In the potential region 0 V to 0.2 V tin on the platinum surface is practically present in the form of a neutral ad-atom Sn °. At E~ > 0.2 tin undergoes partial oxidation and at E~ = 0.5 V it
66
t/•A
cm"~
o
o~e
o,
,o
E,/~
Fig. 1. Cathodic and anodic i--E curves for s m o o t h p l a t i n u m at/~init. = 0.4 V in 0.5 M ~r H 2 S O 4 (--) and in the presence of 5 × 10 - 5 M SnSO 4 for various T: ( . . . . ) 10, (x) 20,
(o) 50, (o) 300 s. is formally present o n the surface as Sn ÷ (or SnOH). The higher f value when passing to E r = 0.6 V is due to a more p r o n o u n c e d ionic nature of chemisorbed species and a related growth of the forces of mutual repulsion between them as the potential shifts to the anode side.
06
04
02
O6
0
0.2 2
0,~
/
08 o
o
-7
-6
-5
(~gcd'/moL F'
Fig. 2. D e p e n d e n c e of the steady-state p l a t i n u m surface coverage with tin ions o n their bulk c o n c e n t r a t i o n in 0.5 M H2SO 4 at various adsorption potentials: (o) 0.2, (x) 0.4, (e) 0.6 V. Fig. 3. D e p e n d e n c e o n E r of the p l a t i n u m surface coverage with tin cations and the relation Qox/AQ~ for tin ions in 5 X 10 - 5 M SnSO 4 + 0.5 M H2SO 4.
67 T H E I N T E R F E R E N C E O F TIN IONS A N D O R G A N I C S U B S T A N C E S IN T H E I R ADSORPTION OF PLATINUM
As an organic substance and tin ions are simultaneously present in solution one can observe their interference on adsorption. The presence of the organic substance in solution leads to a decreased tin adsorption (R0's~) on the platinum electrode, which is, however, different at different potentials. With methanol, for example, at potentials below 0.2 V this decrease is smaller, whereas in the potential region for the m a x i m u m methanol adsorption (E r = 0.4 V) it is greater (Fig. 4). In the presence of methanol in solution, tin undergoes desorption from the surface of the platinum electrode at a less positive potential than in its absence. As the bulk concentration of tin ions increases, the coverage of the platinum electrode surface with tin in the presence of organic substances also increases, to a first approximation, along Temkin's isotherm (Fig. 5) but the constant a becomes smaller with increasing concentration of the organic substances. On the other hand, in the presence of tin in solution the coverage o f the platinum electrode with chemisorbed organic species decreases too. As a first approximation, the coverage of the surface with organic substances falls off linearly with the logarithm of the bulk concentration of tin ions (Fig. 6): Sa0tR = a2 - - ( 1 / / 2 ) i n Cs~so 4
(2)
or it falls off linearly with the coverage of the electrode surface with tin (Fig. 7): 1
O8
06 ®
04
0.2
\ xe~e
/j\ 02
0.4 o,
04
06
F,r/V
Fig. 4. Dependence of the platinum surface coverage with tin or CH3OH on E r in solutions: (x) eWR, 0.1 M CH30H + 0.5 M H2SO4;(®) Sn0'R, 0.1 M CHaOH + 5 X 10--5M SnSO 4 + 0.5 M H2S04; ($) 0'Sn , 5 X 10--5M SnSO 4 + 0.5 M H2S04; (®) ROtSn, 0.1 M CH3OH + 5 X 10 - 5 SnSO 4 + 0.5 M H2SO 4. Fig. 5. Dependence of the coverage o f smooth platinum with tin on the bulk concentration of SnSO 4 in solutions: 0.1 M HCOOH + 0.5 M H2SO 4 at 0.3 V (®) and 0.4 V (e); 10 - 3 M C 4 H 4 0 4 + 0.5 M H2SO 4 at 0.1 V (n); 5 × 10--4M C4H404 + 0.5 M H2SO 4 at 0.1 V (0). For platinized platinum solutions: 0.1 M CH3OH + 0.5 M H2SO 4 at 0.3 V (@) and at 0.4 V (A); 0.1 M HCOOH + 0.5 M H2SO 4 at 0.3 V (®) and at 0.4 V (x).
68
O6 06
04 04
02 02
-5
-/+
-3 h~r-s.'~olC /
r
02
r
I
04
06
R
0£!
Fig. 6. Dependence of the coverage of smooth platinum with organic species on the bulk concentration of SnSO4in 0.1 M HCOOH + 0.5 M H2SO4, (®) 0.3 V and (o) 0.4 V; 10 - 3 M C4H404 + 0.5 M H2S04, (0) 0.1 V; 5 X 10--4M C4H404 + 0.5 M H2S04, 0.1 V ((~); for platinized platinum: in 0.1 M CH30H + 0.5 M H2S04, 0.3 V (0) and 0.4 V (A); 0.1 M HCOOH + 0.5 M H2S04, 0.3 V (~) and 0.4 V (x). Fig. 7. Dependence of the platinum surface coverage with organic species on the coverage with tin ions from solutions containing various concentrations of SnSO4: for smooth platinum in 0.1 M HCOOH + 0.5 M H2SO4, (0) 0.3 V and (o) 0.4 V; for platinized platinum in 0.1 M CH3OH + 0.5 M H2SO4, (0) 0.3 V and (A) 0.4 V; in 0.1 M HCOOH + 0.5 M H2SO4, (®) 0.3 V and (x) 0.4 V; for platinized platinum at constant coverage with respect to tin, (A) 0.4 V, 0.1 M HCOOH + 0.5 M H2SO 4.
Sn0'R = A - - 7R0'sn
(3)
The values of fl, f2 and 7 for the different cases are shown in Table 1. It is interesting that in a number of cases 7 ---- 1 i.e. the total average of the surface with tin and the organic substance, as a first approximation, remains the same. A similar picture has been observed by the present authors for the adsorption of anions and methanol [21], methanol and butadiene [22], cations and maleic acid [23]. It follows from these data that the behaviour of the adsorption isotherm of TABLE 1 Solution
Electrode
Er/V
fl
f2
"Y
0.1 0.1 0.1 0.1 0.1 0.1
Smooth Pt Smooth'Pt Pt/Pt Pt/Pt Pt/Pt Pt/Pt
0.3 0.4 0.3 0.4 0.3 0.4
7.5 9.0 18.0 19.0 12.8 12.0
11.5 10.5 15.0 11.5 9.5 9.2
0.65 1.00 1.10 1.70
M M M M M M
HCOOH HCOOH HCOOH HCOOH CH3OH CH3OH
69
o
06
•
•
04 02 I
I
I
[
-3
-2
~t
0
I
l~c~/mol I.-t
Fig. 8. Dependence of the coverage of platinized platinum with organic species on the bulk concentration of HCOOH at constant surface coverage with tin ions. 0'sn values: (ll) 0.0; (0) 0.13; (®) 0.24; (®) 0.27; (m) 0.31; (A) 0.54. O, x, o, [a, A stand for the corresponding sums (O's~ + s~O'rO.
the organic substance in the presence of a constant a m o u n t o f tin on the surface remains similar to the adsorption isotherm on the pure platinum surface (Fig. 8), but in this case the coverage is smaller by the magnitude of the surface coverage with tin. THE EFFECT OF TIN IONS ON THE RATES OF ELECTROCHEMICAL PROCESSES ON PLATINUM ELECTRODES In ~ e presence of tin ions in solution at potentials which are m ore negative than 0.6 V one can observe a sharp acceleration of electrochemical oxi dat i on o f methanol and formic acid on s m o o t h and platinized platinum electrodes. As can be seen f r o m Fig. 9 the effect of tin ions decreases with increasing anodic potential. At potentials more positive than 0.6 V, polarization curves bot h in presence and the absence of tin in solution are practically the same. This suggests t h a t the process o f electro-oxidation is accelerated only when adsorbed tin ions are present on the surface of the platinum electrode. A study has been made of the effect of pre-adsorbed tin ions on the electro-oxidation of methanol on platinum in the absence of tin ions in solution. For this purpose a polarization curve of m e t ha nol electro-oxidation was measured in 1 M CH3OH + 0.5 M H2SO4, then the solution was p o u r e d off; the cell and the electrode were washed with a background solution (0.5 M H2SOa) in which the electrode was completely cleaned of chemisorbed species at E = 1.2 V. T hen the cell was filled with a 5 X 10 - 4 M SnSOa solution where, at E = 0.2 V, the tin ions u n d e r w e n t adsorption on the electrode f o r 10 min. At the same potential, washing was effected with the background solution (while this t o o k place the tin ions remained adsorbed on the electrode surface), and 1 M CH3OH + 0.5 M H2SO4 solution was i nt r oduc e d in to the cell, after which t he polarization measurements were repeated. These experiments have shown t hat the presence of preadsorbed tin ions results in the same acceleration o f the m e t h a n o l electro-oxidation process as the presence of tin ions in solution. In determining the surface coverage, the deposition of the tin and organic substance ions was affected by the following three methods: (1) in the process
70
2
t
0
-/
/
/
/.
-2
-2 I
,
I
I
I
i
i
02
04
06
aY
02
0.3
04
t__
05
R
!
OS.
Fig. 9. Polarization curves of methanol electro-oxidation on platinized platinum in 1 M H2SO 4 in the absence of tin (o) and after tin adsorption onto the surface from 5 X 10 - 4 M SnSO 4 (®). Fig. 10. Dependence of electro-oxidation rate on the surface coverage with tin ions for smooth platinum in 0.1 M HCOOH + 0.5 M H2SO 4 solution, at E = 0.4 V (A); for platinized platinum in 0.1 M CHsOH + 0.5 M H2SO 4 solution, at E = 0.4 V (®); 0.1 M HCOOH + 0.5 M H2SO4, at 0.3 V (®) and at 0.4 V (x); for platinized platinum at constant coverage with respect to tin, (@) 0.4 V, 0.1 M HCOOH + 0.5 M H2SO 4.
of their free competition out of the solution to the pure surface; (2) the deposition of the tin ions out of the solution with the carbon-containing species being present on the surface; (3) the deposition of the carbon-containing species out of the solution with the tin ions being present on the surface. It can be seen from Fig. 10 that at constant potential and concentration of the organic substance in solution the oxidation rate of formic acid and methanol, both on smooth and platinized platinum electrodes, increases exponentially with RO'S~ : i = K exp(k R.
ROsn)
(4)
In this case the rate increases despite the fact that the coverage of the surface with oxidizable organic species decreases. This shows that the accelerating catalytic effect of tin on electro-oxidation is far more pronounced than the inhibiting effect associated with a decreased adsorption of an oxidizable organic substance. The establishment of the true catalytic effect of tin on the electro-oxidation processes requires a comparison of the rates at constant surface coverage with organic species.
71
Figure 11 shows the dependence of the formic acid electro-oxidation rate on a platinized platinum electrode at various constant surface coverages with tin on the formic acid bulk concentration. This dependence is similar to that for the pure platinized platinum surface b u t is shifted with the same slope to the region of higher currents. Both in the absence of tin and its presence on the surface, the rate of formic acid electro-oxidation increases to a fractional power with the bulk concentration, i.e.
i=
k'0 d
(5)
I f data on the adsorption of formic acid on these electrodes are taken into consideration (Fig. 8) and i f use is made of the dependence of its electro-oxidation rate on a surface rather than on a bulk concentration (Fig. 12) then it will readily be seen that this rate increases exponentially with the coverage of the surface with chemisorbed species:
i = k0sa exp(/31f. Sn0~)
(6)
It should be noted that when changing over from a pure platinized electrode to those p r o m o t e d b y a varying amount of tin, the nature of the dependence remains unaltered. Figure 13 illustrates that an increase of the surface coverage with tin is accompanied b y an exponential increase of the constant in eqn. (6):
ko sn = k0 exp(~2f0s~)
(7)
where ~2f = 8.6. It is interesting that the relative catalytic effect as expressed
t~
2
I
-2
I
-/
I
1
0
¢
logc~/ f ~ l -~
/6
I
I
O.2
I
I
0.4
I
I
O6
~
I
O8
Fig. 11. Dependence of HCOOH electro-oxidation rate on platinized platinum at constant coverage with tin ions at E = 0.4 V on the bulk concentration of HCOOH. The values of 0t Sn-. 0.0 (O), 0.13 (x), 0.27 (~), 0.54 (A). Fig. 12. Dependence of HCOOH electro-oxidation current for platinized platinum at constant surface coverage with tin ions on the surface coverage with organic species. The values of ~t . Sn- (e) 0.0, (A) 0.13, (x) 0.24, (®) 0.31, (O) 0.54.
72
x
O
02
x
04
&
Fig. 13. D e p e n d e n c e of the o x i d a t i o n rate of organic substances on the surface coverage with tin ions at E = 0.4 V, at 0~tt = const for: s m o o t h p l a t i n u m f r o m 0.1 M H C O O H + 0.5 M H2SO 4 solution (x); platinized platinum: 0.1 M CH3OH + 0.5 M H2SO4 (o); 0.1 M H C O O H + 0.5 M H2SO 4 (@); platinized platinum at c o n s t a n t coverage with respect to tin ~ ( ~ ) , for chcmisorbed species f r o m H C O O H on s m o o t h p l a t i n u m electrodes: Er° x = 0.6 V (v), E °x = 0.65 V (0) and f r o m CH3OH o n platinized p l a t i n u m electrode: E ° x = 0.65 V after washing electrode (D).
b y eqn. (7) does n o t depend on either the nature of the oxidizable substance, or the electrode potential, or the state of the electrode surface (smooth or platinized platinum electrodes). Ad~id et al. [12] assumed that the promoting effect is due to a lowering in surface coverage of strongly b o n d e d chemisorbed particles inhibiting the oxidation rate. As can be seen from data on oxidation rates of chemisorbed species formed during adsorption of HCOOH, CH3OH and CO2 in the presence or in the absence of tin ions shown in Fig. 14a and b, tin ions enhance the oxidation of chemisorbed particles too. Thus the adsorbed tin accelerates n o t only the oxidation processes of an organic substance being present in solution b u t also the electro-oxidation process of chemisorbed organic species. The relative catalytic effect for oxidation of chemisorbed organic c o m p o u n d s in solution remains the same. Another study was concerned with the effect of adsorbed tin on some other electrochemical reactions on a smooth electrode. In the process of electrooxidation of anthrahydroquinone-2,6-disulphonic acid and the backward reaction of electroreduction of anthraquinone-2,6-disulphonic acid, i.e. reactions whose limiting stage is represented b y electron transfer [24], even a considerable concentration of tin ions does n o t result in a noticeable rate change. In the case of electroreduction of maleic acid, whose limiting stage is represented b y an interaction of a chemisorbed organic species with adsorbed hydrogen [25] (see Fig. 15); the reduction rate shows an exponential drop with increasing surface coverage with tin: i = io exp(--~lR0'sn)
(8)
where kl ~ 16. This drop is far above the magnitude that could be expected due
73
(~
(a)
i ~00. (b)
~00.
200.
'1 t
100-
50.
~o . . . .
2o . . . .
,~o ~/5
Fig. 14. O x i d a t i o n rate o f specms chemlsorbed f r o m s o l u h o n s of formm acid (1,1 ,1 ,2,2 ), t t t iw . . . I Ft t t I tr t m e t h a n o l ( 3 , 3 , 5 , 5 ) and CO 2 ( 4 , 4 , 4 ) , organm specms m presence (1 ,1 , 2 , 3 , 4 , 4 ,5 ) or absence of tin ions on s m o o t h (a) and platinized platinum (b) electrodes. (1) 1 M H C O O H , ads ox I r I . . ox r Er =0.3V, E r =0.6V, 0sn=0;(1)0Sn = 0 . 0 8 ; ( 1 ) 0 Sn = 0 . 1 5 ; ( 2 ) Er = 0 . 6 5 , 0 S n = t t ads ox p ~ t O ; ( 2 ) O s n = O . 2 0 ; ( 3 ) I M C H 3 O H , Er = 0.25 V, E r , = 0.6 V, ~Sn = 0; ( 3 ) 0 Sn = 0.4; (4) CO2, E~ ds = 0.1 V; E ° x = 0.6 V in 0.5 M H2SO4; ( 4 ) in 0.5 M H2SO 4 + 5 X 10 - 5 M SnSO4; ( 4 ) in 0.5 M H2SO 4 + 10 - 4 M SnSO4; (5) 1 M CH3OH , E ads = 0.30 V and Er° x = 0.65 V in the background solution after washing electrode: 0'Sn = 0; (5') 0'Sn = 0.27.
to a decreased coverage with organic chemisorbed species alone, it being connected with a simultaneous decrease in the coverage of the surface with the second reacting species, i.e. adsorbed hydrogen.
74
i
05
OI
02
03
04
0 ~
Fig. 15. Dependence o f C4H40 4 electroreduction rate at E = 0.1 V on the surface coverage
with tin or an adsorbed organic species in solutions: 5 X 10 - 4 M C4H404 + 0.5 M H2SO 4 and 10 - 3 M C4H404 + 0.5 M H2SO 4. (e) Sn0'R; (®) R~'sn"
RESULTS AND DISCUSSION
The experimental results obtained suggest that the nature of the effect of tin on the rate of some electrochemical processes on platinum is governed by the mechanism of these processes and by the influence of tin on the adsorption of the reacting species. The promoting effect of adsorbed tin ions is most pronounced for the electro-oxidation of methanol, formic acid and other simple organic substances in the potential region up to 0.5 V. In the potential region under study the limiting stage of the electro-oxidation of these substances is represented by an interaction of a chemisorbed organic species with a n O H a d s radical formed during a preceding rapid electrochemical stage of water molecule discharge [19]: Rad s + O H a d s ~ R - - O H
(9)
Platinum is a very good adsorbent with respect to methanol and formic acid, and it ensures a reasonably high coverage of the surface with oxidizable organic species but the coverage of the platinum surface with O H a d s radicals in this potential region is very low. The expression for the rate of reaction (9), considering that 0OH < 0.1 and 0.1 < O'R < 0.9 according to refs. 19 and 26, can be presented as i = k0ROOH e x p ( - - - ~ f 0 R )
where the reaction rate constant k includes an exponential term with the activation energy which, in turn, depends on the adsorption energy of the reacting species. For the dependence of the rate constant on the adsorption
(I0)
75
energy (--/kVoH) of the oxygen-containing species it can be assumed that
(11)
k = k ° exp(---~AGoH/RT )
Tin is prone to a ready and reversible oxygen adsorption in this potential region, therefore it can be assumed that the adsorbed tin atoms represent those centres where there will be an enhanced adsorption of OHad s participating in the process of oxidation, whereas the platinum atoms on the surface are contributors of organic species. Hence the number of OHad s radicals on the surface will shown a linear increase with the number of adsorbed tin atoms: 0'OH = nRO's~
(12)
Studying the adsorption of OH particles both on platinum and tin and considering that for the latter 0OH > 0.1, we shall have AGoH
= APtGoH
+ ASnG°H-
RTfOoH
(13)
or, with regard for (12), AGoH
=
A~GoH
+ ASnG°H --RTnfOsn
(14)
From (14) and (11) we shall obtain for the true catalytic effect a relation k = k °1 e x p ( ' y p n f O s n )
(15)
which at f12 = ~/P is in line with experimental dependence (7). The true promoting effect does not depend on either the nature of the oxidizable substance or the electrode potential but is governed by the surface coverage with the promoter (Fig. 13) and the OH adsorption energy on the promoter. Thus the participation at the limiting stage in the anodic oxidation of simple organic substances (methanol, formaldehyde and formic acid) of two different species, viz. a chemisorbed carbon-containing species and an oxygen-containing radical OHads, explains why the most active role is played not by a single metal catalysts but by mixed or promoted catalysts with one c o m p o n e n t being active with respect to chemisorption of organic substances (usually platinum group metals), the second providing for a high surface concentration of OH radicals (tin, rhenium, ruthenium, etc.). REFERENCES 1 J.O'M. Bockris and H. Wroblowa, J. Electroanal. Chem., 7 (1964) 428. 2 0 . A . Petty, Dokl. Akad. Nauk SSSR, 160 (1965) 871; V.S. Entina, O.A. Petry and V.T. Rysikova, Elektrokhimiya, 3 (1967) 758, 1238; V.S. Entina, O.A. Petry and I.V. Shelepin, Elektrokhimiya, 2 (1966) 457; O.A. Petty, B.I. Podlovchenko, A.N. Fnn-nkin and H. Lal, J. Electroanal. Chem., 10 (1965) 253. 3 J.A. Shropsh~re, J. Electrochem. Soc., 112 (1965) 465; 114 (1967) 773. 4 K.J. Cathro, Electrochem. Technol., 5 (1967) 441; J. Electrochem. Soc., 116 (1969) 1608. 5 D.F.A. Koch, Australian Pat. 46123, 1964. 6 E. Schwarzer and W. Vielstich in Troisi~mes Journ~es Internatinnales d ' E t u d e s des Piles ~i C o m b u s tibles, Bruxenes, Presses Acad. Europ~ennes, 1969, p. 220. 7 R.R. Ad~i~, D.N. Simi~, A.R. Despi~ and D.M. Dra~i~, J. Electroanal. Chem., 65 (1975) 587; 61 (1975) 117; 80 (1977) 81.
76 8 H. Binder, A. KShling and G. Sandstede, From Electrocatalysls to Fuel Cells, University of Washingt o n Press, Seattle, 1972, P. 43; Advanced Energy Conversion, 6 (1966) 135; 7 (1967) 77; Nature, 214 (1967) 268. 9 M. Watanabe, S. Motoo and T. Suzuki, Denkl Kagaku, 40 (1972) 205, 210; J. Electroanal. Chem., 69 (1976) 429. 10 D.F.A. Koch, D.A. Rand and R. Woods, J. Eleetroanal. Chem., 70 (1976) 73. 11 D.A.J. Rand and R. Woods, J. Electroanal. Chem., 36 (1972) 57; 44 (1973) 83. 12 R.R. Ad~ld, A.R. Despid and D.N. Slmi6, Workshop %n Electrocatalysis by Non-Metanic Surfaces, National Bureau of Standards, 1975, p. 191; A.R. Despid, R.R. Ad~id and A.V. Tripkov16, Elektrokh imiy a, 5 (1977) 660. 13 M.M. Janssen and J. Moolhuysen, Electrochim. Acta, 21 (1976) 8 6 1 , 8 6 9 . 14 M.W. Breiter, Z. Phys. Chem., 52 (1967) 73. 15 D.W. McKee and F.G. Norton, J. Catalysis, 3 (1964) 252. 16 G.C. Bond and D.E. Webster, Plat. Met. Rev., 10 (1964) 10. 17 J.O'M. Bockris and S. Srinivasen, J. Electroanal. Chem., 11 (1966) 350; Fuel Cells. Their Electrochemistry, McGraw-Hill, 1970. 18 M.R. Andrew, J.S. Drury and B.D. McNIcol, J. Appl. Electrochem., 6 (1976) 99. 19 V.S. Bagotzky and Yu.B. Vassiliyev, Electrochim. Acta, 12 (1967) 1323. 20 V.S. Bagotzky and Yu.B. Vassilyev, Electrochim. Acta, 11 (1966) 1439. 21 D.N. Pixtskhalava, Yu.B. Vassflyev and V.S. Bagotzly, E l e k t r o k h i m i y a , 6 (1970) 1144. 22 M.A. Khrizolitova, L.A. Mirkind, Yu.B. Vassilyev, M.Ya. Fioshin and V.S. Bagotzky, E l e k t r o k h i m i y a , 8 (1972) 548. 23 Sh.Sh, Khidirov, V.N. Andreyev, O.A. Khazova, V.E. Kazarinov,and Yu.B. Vassilyev, Elektrok h l m i y a , 13 (1977) 1275. 24 T.K. Zolotova, I.V. Shelepin and Yu.B. Vassfliyev, E l e k t r o k h i m i y a , 10 (1974) 1830. 25 V.V. Chernyl and Yu.B. Vassflyev, ElektrokhLmiya, 12 (1976) 872. 26 O.A. Khazova, Yu.B. Vassllyev and V.S. Bagotzky, E l e k t r o k h i m i y a , 10 (1974) 606.