Walter, J. Willsau and J. Heitbaum, J. Electrochem. Sot., 132. (1985) 1635. ... H. Angerstein-Kozohvska, D. MacDougal and B.E. Conway, J. Electrochem. Sot.
Journal of Ekctroanalytical Chemistry, 362 (1993) 159-165
159
JEC 02865
Electrocatalytic oxidation of formic acid on Pt binary and ternary electrodes in H,PO, A.A. El-Shafei, H.M. Shabanah and M.N.H. Moussa Chemistry Department, Faculty of Science, El-Mansoura University, El-Mamoura
(Egypt)
(Received 4 November 1992; in revised form 15 March 1993)
The oxidation of formic acid on a Pt electrode is strongly catalysed by Pb, Tl, Cd and Ge adatoms. Under optimum conditions, the enhancement decreases in the order Pb > TI > Cd > Ge. The Pb adatoms enhance the height of the peak at 0.5 V/RHE by more than lo-fold. The &tt~itaneoua underpotential deposition of Pb + Ge, Pb + Cd, Tl + Ge and Tl + Cd on a ,Pt electrode greatly enhances the k-c activity over that of single metal adatoms. Enhancement by binary adatoms decreases in .the order Pb+Ge>Pb+Cd>Tl+,Ge>Tl+Cd.
1. IEm
Interest in formic acid oxidation has been reviewed as a result of its possible use as a fuel [l-3] and from a desire to study a simple oxidation reaction in. order to gain a deeper understanding of electrocatalyticozidation. The mechanisms of ozidation and poisoning have been studied by a number of authors. Most of this work has been reviewed by Capon and Parsons [4] on the basis of literature data and their own experiments [51. More recently, the mechanisms have been investigated using IR spectroscopy I6-81 and differential electrochemical mass spectroscopy (DECMS) together with isotope labelling [9] or X-ray photoelectron, spectroscopy @PSI [lo]. It was recognized that the electrolytic activity of the electrode materials for HCOOH oxidation may be greatly improved by using binary systems obtained either by alloying different catalytic metals [ll-131 or by modifying the catalytic surface (usually a Pt surface) with foreign metal adatoms [14271. However, very few ternary systems have been investigated: these include Pt + A&subs) and Au + Ptisubs) electrodes modified with Bi adatoms for HCOOH oxidation 1281,a Pt electrode modified by two 0022-0728/93/$6.00
adatoms (Cd + Tl, Cd + Pb) for .O, reduction 1291, ternary alloys (Pt + Pd + Bi or Pt + Pd + Pb) [30] and a Pt + Au alloy modified by Pb adatoms E311for ethylene glycol oxidation. A number of theories have been proposed to explain the catalytic effect of underpotential.deposited (UPD) metals: (1) the third-body effect which reduces the number of adsorption sites by geometrical hindrance [17,32]; (2). suppression of hydrogen adsorption on Pt which interferes with the formation of the strongly bound poison 120,331; (3) formation of surface domains ofqa size favourable for the direct electrooxidation of formic acid and unfavourable for the formation of the strongly bound poison by the presence of adatoms which occupy sites of different coordination number S, on the electrode surface [34,35]; (4) catalysis of the direct oxidation path or oxidation of the poison by oxygen species adsorbed to the metal adatoms [ 181; (5) the UPD layer itself catalyses the direct oxid& tion of formic acid to CO, [23,27]. In this work the eiectro-oxidation of HCOOH was studied at a Pt electrode modified by the presence of 0 1993 - Elsevier Sequoia S.A. All rights reserved
160
A. El-Shafei et al. / Oxihtion of formic acid on Pt electrodesin H,PO,
Pb,Tl,Cd,Ge,Pb+Ge,Pb+Cd,Tl+GeandTl+Cd in 10% H,PO,, which is one of the most attractive electrolytes for practical fuel cells. 2. ExperIlnental The effect of simultaneously adsorbed UPD metal adatoms on the oxidation of HCOOH at a Pt electrode in acidic medium has been examined by cyclic voltammetry, chronoamperometry and potential-step techniques. Measurements were carried out using equipment described previously [36]. A platinum counterelectrode and a reference hydrogen electrode 0XHE) [37] were used in the same solution. Underpotential deposition of the different metal adatoms was performed by adding the salts Pb(NO,), (BDH), CdSO, (8/3)H,O (Riedel-de Haen), GeO, (Fluka AG) and TlNO, (Prolabo) to the electrolyte solution at low concentrations (10-7-10-3 M). Solutions of the electrolyte (0.1 M HCOOH (Prolabo)) in 10% H,PO, (Merck) were prepared using Megapure water. H,PO, was purified using the procedure described by Perrier et al. ‘[38] and Huang et al. [39]. Nitrogen was bubbled through the cell to deoxygenate the solution before each experiment. 2.1. Preparation of a Pt electrode modi$ed by metal-adsorbed atoms (ad-electrode) A smooth Pt sheet modified with a Pb fii was prepared by applying the following potential programrne: Cl) the Pt eleotrode was activated between the onset of hydrogen and oxygen evolution in 0.1 M H,SO,; (2) the potential was held at 0.2 V and the solution was replaced by the metal-containing electrolyte (lo-’ M PMNO,)); (3) after 2 min the electrolyte was replaced by 0.1 M H,SO, to eliminate the bulk metal ions; (4) the electrode was held at various potentials in 0.1 M H,SO, for a given time to obtain the desired coverage, oxidizing some of the adsorbed adatoms; (5) the electrolyte was again replaced by 0.1 M H,S04 to eliminate the metal desorbed during the preceding potential treatment; (6) the volt ammogram was recorded in the pure supporting electrolyte (0.1 M H,SO,) between 0.05 and 0.6 V (Fig. 1). The coverage by the adatoms was determined from the decrease of hydrogen adsorption as follows [401: eE = (Q; - Q&&Q: = AQ:/Q: where Q,“,,, and QE are the charges for hydrogen adsorption in the presence and absence of. the adsorbed species.
ENC..
rhe
Fig. 1. Voltammograms of a Pt electrode modified by UPD Pb in 0.1 M H,S04 at 100 mV s-l and 25°C: -~fi,-__ e, = 0.5.
2.2. Cy& Voita#irme~ s (1) After recording the cyclic voltammogram in 10% H3PGd, the potential was stopped at 0.05 V during the negative sweep. (2) The electrolyte was replaced by 0.1 M HCGGH in 10% H,PO, solution containing the metal salt. (3) The potential scan was started in the negative direction (dE/dt = 100 mV s-l) and the voltammogram was recorded. 2.3. Potential-step ?neasureme?ats After step (2) above, two different experiments were performed. (1) The base electrolyte was replaced by 0.1 M HCOOH in 10% H,PO, containing two different metal Salk%
(2) The base electrolyte was replaced by 0.1 M HCGGH in 10% H,PO, containing one metal salt. This experiment was performed using an adelectrode prepared as described previously. In each experiment, the potential was stepped by 50 mV every 2 min and, for convenience, the oxidation current was measured at the end of each step. 2.4. Ckrorwa?npo??let~ measu~ts Current decay measurements were performed for HCGOH in 10% H,PO, in the presence of metal salts. In these experiments, the base electrolyte was replaced by 10% H,PO, containing HCGOH only or HCGOH and metal salts after determination of the Pt surface area. This replacement was carried out at 0.05 V and
161
A. El-Shafei et al. / oxidation of formic acid on Pt electrodes in H,PO,
the decrease in current with time was measured constant potential over a period of 60 min.
at
3.0
3. Results 2.5
3.1. Oxidation of HCOOH on Pt and Pt modified by foreign metal adatoms Figure 2 shows the oxidation of HCOOH on Pt in 60% and 10% H,PO,. The oxidation currents are much lower in 60% H,PO,. The lower activity of Pt as the concentration of H,PO, increases is due mainly to the decrease in H,O activity 1411 and the specific adsorption of phosphate ions. The cyclic yoltammetxy of HCOOH oxidation on bare Pt and on Pt surfaces modified by UPD submonolayers of Pb, Tl, Cd and Ge in 10% H,PO, are presented in Fig. 3. The presence of UPD Pb adlayers increases the electrocatalytic activity of the Pt electrode towards the oxidation of HCOOH. The peak height at 0.5 V in the positive sweep increased more than N&fold. Tl adatoms shift the beginning of the oxidation to a more negative potential giving a maximum catalytic factor of about 5 at 0.3 V. The catalytic factor decreases with increasing potential until it reaches about 3 at the peak potential. The presence of Cd or Ge has little effect on the voltammogram; the only change observed’is the increase in current density at peak potential (0.5 V) by a factor of 2. The maximum catalytic action appears at the following concentrations: 10m3 M Pb2+, 10e4 M Tl+, 10m4 M Cd2+ and 10m6 M Ge4+.
0. 16 -
r
5
u
0. 12 -
E
1
0.t 18 -
w 0.0
0.2
0.4
0.6 E/V
0.8
1.0
1.2
vs. rhe
2. Cunzot density vs. potential for 0.1 M HCOOH at 100 mV s-l and 25°C: 60% H,PO,; --10% H,PO+ Fig.
2.c
N ‘E ,” E 2
1.5
I.C
0.5
o,o’e 0.2
0.4
0.6 E/V
0.8
I.0
I.2
vs. rhe
Fig. 3. Anodic cyclic voltammograms o$ a Pt electrode in 10% no adatoms; --H,PO, containing 0.1 M HCFOH: ._._. lo-4 M TI+; . .__. . lo-6 M Ge’+; . . . . . . lo-4 f,4 @+; 1O-3 M Pb2+. 3.2. Undeptential deposition of two different metals The underpotential deposition of pairs of metal adatoms (Cd + Tl, Cd + Pb, Ge + Tl and Ge + Pb) was studied. The voltammograms obtained for Cd, Tl and Cd + Tl adatoms are shown in Fig. 4. The voltammogram obtained in the presence of two metals displays a rather complex pattern with several peaks which are characteristic of each metal, e.g. peak I for Cd adatom dissolution, peak II for Tl adatom oxidation and the peak couple III/III’ for the Tl++ T13+ redox reaction. The only difference observed was the decrease in the adsorption-desorption peak observed for each metal alone. This may be due to the competitive adsorption between these metals. All the other systems studied exhibited the same behaviour. 3.3. Formic acid oxidation at a ternary electrode Figure 5 shows the voltammograms for HCOOH oxidation in 10% H,PO, in the presence .of ,Pb, Pb + Ge and Pb + Cd. The curves resemble that for Pb with an increase in current density, particularly at the peak potential. fie maximum current density increaSed for 10T3 ‘M Pb + 5 x lo-’ M Ge and low3 M Pb + 5 x lO-‘j M Cd. The same result was obtained for Pb + Ge
A. El-Shafei et al / Odation of formic acid on Pt e.kctrodesin H,PO,
162
TABLE 1. Current density data obtained at Pt ekctmdes modified with Pb adatoms in the absence and presence of different metal satts in 0.1 M HCOOH + 10% H3P04
Tm2
E/V
i/d
(RI-E)
Pure Pt
e$ = 0.5
e&=0.5+
eg-0.5+
5x10-‘M Ge4+ 0.35 0.4 0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8
I
0.2
0.4
0.6
0.6 E/V
vs.
1.0
1.2
1.4
rhe
Fig. 4. Cyclic voltammograms for the UPD of different metals on Pt in 10% H,PG,: no adatoms; . . . . . . 10-4 M az+; ._._ _ 10-4
M
fl+;
---
1O-4 M Cd*+ + 1O-4 M Tl+.
and Pb + Cd in solution and in the presence of Ge or Cd at a Pb + Pt electrode with 19k = 0.5 using potential-step (Tables 1 and 2) and current decay measurements (Fig. 6). This can be attributed to the
4.6x
9 23 50 114 238 353 303 246 143 67
9 24 56 100 157 156 109 65 45
6 15 38 43 41 39
7 15 31 65 130 287 195 110 62 39
TABLE 2. Current density data obtained at Pt electrodes in the presence of single or double metal salts in 0.1 M HCOOH+ 10% H3PO4
E/V
j/PA
(FHE)
Pure Pt
0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8
,_ 6 15 38 43 41 39
cm-’ 10e3 M Pb2+ 15 42 106 219 270 181 85 48
10v3M Pb2+ +
1O-3 M Pb2+
15 54 155 285 334 224 130 61
11 31 126 240 2% 154 72 38
0
t/min E/V vs. rha
Fig. 5. Vohammetric curves of a Pt electrode in 10% HsPG, containing 0.1 M,HCGGH and lo-” M Pb2+ (-1, (*a. .*.) 10-’ M Pb2++5X10-6MCdZ+(---_)and10-3MPb2++5x10-7M Ge4+ (.-.-.). Sweep rate, 100 mV s-t.
Fig. 6. Influence of adatoms on the current-time transients on smoothPtfortheoxidationofformicacidatO.35V: bare pt, --lo-3 M pb2+; ._._. 10-3 M Pbz+ +5x lo+ M Cd2+; . .--. .--. . NV3 M Pb2++5xIW7 M Gc4+. Electrolyte: 0.1 M HCGOH+ 10% H3PG4 at 25°C.
A. El-Shafei et al. / Oxihtion of fmic
acid on Pt electrodesin Hj PO.,
163
formation of an oxygen layer at more negative potentials in the case of Ge [lg] and to the ionic character of Cd [42]. The same result is obtained when Ge or Cd is added to 0.1 M HCOOH in 10% H,PO, containing TlNO,. Enhancement by binary adatoms decreases in the order: Ge + Tl> Cd + Tl> Tl (Figs. 7 and 8 and Table 3). 4. Discussion
E/V
vs. rhe
Fig. 7. Vohammetric curves for a Pt electrode in 10% HsPO, containing 0.1 M HCGOH in the presence of lo-’ M Tl+ (-1, 1O-4 h4 Tl++lO+ M Cd’+ (;....*) and 1O-4 M Tl++5xlO-’ M Ge4+ (- - -_). Sweep rate, 100 mV s-r.
rE
0.120-
P 1
-\
t/min
Fig. 8. Influence of adatoms on the current-time transients on bare Pt; smooth Pt for the oxidation of formic acid at 0.45 V: --10-4 M fl+; _-_ 1()-4 M ‘lJ+ +10-e M C&+; . .__ . .--. . 10T4 M Tl+ +5X lo-’ M Ge4+. Electrolyte: 0.1 M HCOOH+ 10% H3P04 at 25°C.
TABLE 3. Current density data obtained at Pt electrodes in the presence of single- or double-metal salts in 0.1 M HCOOH+ 10% H3PO4
E/V
j/pAcm-*
mEH)
Pure Pt
10-4 MTl+
1O-4 M Tl+ +
1O-4 MTl++ 5x10-6 M Cd*+
6 15 38 43 41 39
2 7 96 161 195 150 101 71
3 9 111 181 230 170 110 85
4 10 104 190 22.6 170 120 80
0.45 0.5 0.55 0.6 0.65 0.7 0.75 0.8
According to the third-body effect, the catalytic oxidation of formic acid is enhanced by the inhibition of the formation of strongly adsorbed intermediates by geometric hindrance. However, this theory cannot explain the difference in catalytic activity obtained at a Pt electrode modified by Pb, Tl, Cd and Ge. Moreover, the additional catalytic activity observed in the presence of two different metal salts in solution cannot be accounted for either. The model proposed by Adzic and coworkers [20,41], in which the suppression of the adsorbate is caused by the suppression of the hydrogen layer, is less convincing because it cannot explain how the same coverage by two different adatoms (e.g. Pb and Tl) results in different catalytic activities. The theory proposed by Motoo and Shibata [43] cannot explain how Pb, Tl and Cd adatoms, which have the same surface coordination number, S, (S, = 2) [43&I], exhibit different catalytic activities. The alternative theory proposed by Motoo and Watanabe [18], which is based on the bifunctional theory of catalysis, cannot explain why Pb and Tl, which are non-oxygenadsorbing adatoms, exhibit higher cataiytic activity towards HCOOH oxidation than Ge which is considered to be an oxygen-adsorbing adatom. Also, it cannot explain why only Sn and Ru among the oxygen-adsorbing metal adatoms (Ge, As, Sn, Ru and Sb) have a catalytic effect for methanol oxidation [45]. The catalytic effect of Pb adatoms towards HCOOH oxidation differs considerably, ranging from a factor of 10 1461to a factor of 1000 [22] or even greater [25]. A substantial enhancement of more than lo-fold was obtained in the presence of Pb adatoms in this work. The catalytic effect observed for HCOOH oxidation in a solution containing formic acid and a Pb2+ salt is greater than that of pre-adsorbed Pb. adatoms in a solution containing only formic acid (Tables 1 and 2). These results agree with those reported by Adzic et al. [2Ol, Shibata and Motoo [24] and Fonseca et al. [47]. The catalytic effect of Tl adatoms is smaller than that of Pb. This agrees with the data obtained in 1 M HClO, [2Ol and in 85% H,PO, [41]. Ge and Cd show
164
A. El-Shafei et aL / Wdation of fovmic acid on Pt electrodesin H3PO,
only a small catalytic effect. This agrees with the result obtained by Motoo and Watanabe [18]. It is difficult to discuss the catalytic effect of UPD metals in terms of a single theory. We believe that the effect of submonolayer amounts of Pb, Tl, Ge and Cd on the electro-oxidation of formic acid on Pt can be described as follows. The four metal adatoms studied here can be classified into two groups. (1) Ge and Cd: these two metals can catalyse formic acid oxidation by the coadsorption of 0 or OH at more negative potentials than Pt. The adsorbed oxygen takes part in the oxidation of poisoning species through the following reactions: MOH + COH MOH+CO-
M + CO, + 2H++ 2eM+CO,+H++e-
where M is Ge or Cd. (2) Pb and Tl: these two metals can catalyse the direct oxidation of formic acid [231. In addition, Tl adatoms exhibit a small geometric blocking effect [27] towards poison formation compared with Pb adatoms. Furthermore, Pb adatoms are more strongly bound than l? to the Pt surface [18]. Therefore formic acid and its intermediates can affect the adsorption of Tl more than Pb. Partially charged Tl adatoms [481 can also induce the adsorption of phosphate ions which would decrease the rate of formic acid oxidation. The above classification is consistent with those reported by Castro Luna et al. [23], who assumed that the bulk oxidation of formic acid takes place not only at a site free from Pb, but also on a surface covered by a Pb monolayer, and by Hartung et al. [27], who suggested that the direct oxidation of formic acid is catalysed by a UPD layer of Tl on the Pt electrode. Thus the role of the bifunctional theory of catalysis cannot be excluded. This theory is more favourable in alkaline media [49,50]. J-Iowever, it may explain the additional catalytic activity obtained by the addition of Ge or Cd to the acidic solution of HCOOH containing Pb or Tl salts. I
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