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Electrocatalytic oxidation of cyclopentanol on Pt and Pt surfaces modified by ad-atoms of heavy metals deposited at underpotentials
233
J. Electroanal. Chem., 271 (1989) 233-247 Elsevier Sequoia S.A., Lausanne - Printed m The Netherlands
Electrocatalytic oxidation of cyclopentanol on Pt and Pt surfaces modified by ad-atoms of heavy metals deposited at underpotentials G. Kokkinidis and A. Papoutsis Laboratory of Physical Chemistry, Department 54006 Thessalonrki (Greece)
of Chemistty,
Anstotle
Unwersity of Thessaloniki,
(Received 11 April 1989; in revised form 13 June 1989)
ABSTRACT The electro-oxidation of cyclopentanol was studied in aqueous acid solutions on platinum anodes modified by Pb, Cd and Ge ad-atoms deposited in the underpotential region. The ad-atoms caused a very pronounced electrocatalytic effect for cyclopentanol oxidation both in terms of the oxidation potential and the current density. Potentiodynamic methods were used to characterize the currents, and stationary methods to check the long-term stability of the modified electrodes and to produce a sufficient amount of product to be identified. It was proved that the oxidation yields cyclopentanone with a current efficiency of 100%. The catalytic effect - which, in fact, is not an enhancement but a suppression of poison formation reactions - was interpreted in terms of geometrical regulations of reaction sites by ad-atoms.
INTRODUCTION
The field of electrocatalysis has undergone extensive growth in recent years, and electrocatalytic concepts have been applied to some areas of basic and applied research and technology. In the initial period of this research, pure electrode materials - metals or semiconductors - were used as electrocatalysts, mainly for the hydrogen and oxygen electrocatalytic reactions and the complete oxidation of organic fuels, and occasionally for selective promotion of reaction paths for chemical processing or electro-organic synthesis. However, it was soon realized through the pursuit of better efficiencies that pure electrocatalysts are not sufficient. Owing to this situation, the modification of electrode surfaces by covalently bound or strongly adsorbed species to increase their catalytic activity or selectivity has become popular. A convenient method for modifying noble and transition metal electrodes is the formation of sub- and monolayers of heavy metal atoms by underpotential deposi0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
234
tion (upd). The upd ad-atoms (i.e. Pb, Tl, Bi, Ge, Cu, etc.) are dispersed homogeneously on the substrate surface, rather than condensed into islands. This is important for the geometrical regulation of the two-dimensional arrangement of the deposited metal atoms. Hence, the structure of the ad-atom layer depends on both the degree of coverage and the structure of the substrate metal. For details of upd see the reviews by Kolb [l], Jtittner and Lorenz [2] and the references cited therein. Upd ad-atom modified electrodes have been found to exhibit remarkable electrocatalytic activity for a number of technologically important electrochemical reactions, such as oxygen reduction, oxidation of organic fuels, reduction of nitro and nitroso compounds, and some quinone-type redox reactions. The results appearing in the literature have been reviewed by Adzic [3] and more recently by Kokkinidis [4]. A diversity of mechanisms has been proposed in each case to explain the catalytic role of the ad-atom layers. Particularly in the case of oxidation of organic fuels on Pt in acidic solutions, the catalytic effect is not in fact an enhancement, but the suppression of poison formation reactions. The increased activity was explained in terms of geometrical configurations (“third body” mechanism [5,6], “Sn” and “S ho,e” concepts [7,8]) or by assuming modification of the electronic properties of the substrate as a whole [9,10]. Thus, the catalytic activity of the modified electrodes does not decay as rapidly as that of Fdre platinum. As a result, higher oxidation currents are obtained, although bulk oxidation on these surfaces might occur at higher over-potentials than on Pt without ad-atoms. However, a question arose as to why the catalytic effect (based on both the current density and the potential) is not as important for methanol or ethylene glycol as it is, for instance, in the case of formic acid or hydrated formaldehyde. A reasonable explanation [ll] might be that the former fuels require oxygen donation for their complete oxidation to occur, and this can be accelerated only in the potential region where covering of the surface of the electrode with OH radicals commences. Since secondary alcohols are oxidized electrochemically on platinum to the corresponding carbonyl compounds [12-141 and this oxidation occurs without oxygen donation, it is worth investigating the upd effect on the oxidation of this class of alcohols. The aim was to gain more insight into the role of ad-atoms on the electrocatalytic processes. The present paper deals with the electrocatalytic oxidation of cyclopentanol on Pt and Pt surfaces modified by upd ad-atoms layers in aqueous HClO, solutions. EXPERIMENTAL
Cyclic voltammetric curves were recorded in the usual way under potentiodynamic control with a three-electrode cell (hydrogen reference electrode and Pt counter-electrode). Electrode potentials, E,, are given in the SHE scale. The cell was doublewalled and thermostated at 25 &-O.l” C, except where stated otherwise. The solutions were thoroughly de-oxygenated by purging the system with ultrapure nitrogen. The electronic set-up consisted of a Wenking potentioscan, type POS73, and a Houston 2000 X-Y recorder. A Pt sheet and a Pt gauge were used for the
235
voltammetric and electrolysis experiments, respectively. The electrodes were activated by applying a continuous sweep between hydrogen evolution and just before oxygen evolution in 0.5 M HClO, solution. Their real surface areas were estimated by integration of the hydrogen adsorption-desorption current-potential curves, assuming that 210 PC cm-’ of hydrogen is adsorbed on a perfectly smooth Pt electrode
WI-
The supporting electrolyte was prepared using triply-distilled water and HClO, (Merck, suprapure). Cyclopentanol, cyclohexanol “puriss p.a.” and cycloheptanol “purum” were from Fluka. The other reagents were oxides or carbonates of Pb, Cd and Ge (G.R. grade quality, Merck). Perchlorate salts were prepared by dissolving the stoichiometric quantity of oxides or carbonates in the base electrolyte. RESULTS
AND DISCUSSION
Voltammetric
behaviour of cyclopentanol
electro-oxidation
on bare Pt
Figure 1 shows a typical cyclic voltammogram for the oxidation of cyclopentanol (0.1 M) in 0.5 M HClO, on Pt, together with the cyclic voltammogram of Pt in the supporting electrolyte alone. The voltammogram was recorded after adsorption of cyclopentanol at 0.45 V. It should be noted that the potentiodynamic j(E) profile is almost independent of the adsorption time and potential scan rate. The currents during the positive sweep in the double layer (dl) and Pt-OH formation potential regions are very small and a normal oxidation peak (II) develops only in the Pt-0 region ((E,),, = 1.25 V at v = 50 mV s-i). This picture is different from that for most of the aliphatic primary and secondary alcohols which show two peaks [16,17].
2-
0
I
I
I
0.4
0.8
1.2
h/V
for cyclopentanol (0.1 M) oxidation Fig 1. Cyclic voltammogram on a Pt sheet in 0.5 M HClO,. to the voltammogram E .&,js= 0.45 v; C,& = 100 s. Scan rate: u = 50 mV s-l. The dashed line corresponds of Pt in 0.5 M HClO,. Inset: Plots of (J,,)~, and (j,),,, vs. I?‘*.
236
During the negative sweep, a higher oxidation peak (III) appears after the potential of Pt oxide reduction. The effect of variation of the potential scan rate on the peak currents is shown in the inset of Fig. 1. By varying the potential scan rate (in the range from 25 to 400 mV s-‘) one can see that (jr),, scales linearly with u112 while (jr),,, remains unaffected. This behaviour indicates that the oxidation of cyclopentanol in the Pt-0 region is controlled by diffusion, whilst in the dl region it is controlled by kinetic factors. More evidence for the diffusion character of the oxidation process in the Pt-0 region comes from the effect of the scan rate on the peak potential and the effect of stirring the solution on the peak current. The electro-oxidation of cyclopentanol is an irreversible process and, as expected for diffusion-controlled processes, a shift of ( Ep)II to positive potentials is observed with increasing scan rate. Also, stirring the solution causes an increase of the height of peak II of about 20% at all scan rates. It is worth noting that the diffusion-controlled currents in the Pt-0 region are much smaller than that predicted if all the Pt sites are considered to be available for cyclopentanol oxidation. This behaviour is characteristic for electrode processes occurring on partially covered surfaces with adsorbed species not participating in the process. Besides, the decay of the current as the potential is scanned to more positive potentials (E > 1.25 v) shows that the process is not limited solely by mass transport of cyclopentanol from the bulk solution to the electrode surface. The enhanced adsorption of 0 radicals restricts further the number of active sites for the anodic process. Oxidation of cyclopentanol can occur once again after the reduction of the Pt-0 film during the negative sweep. The process on the free Pt sites is now
ol
-5
’
I
I
-2
-1
I
1
-4
-3 bg(c/mol
Fig. 2. Adsorption isotherm E _js = 0.45 v; t,& = 100 s.
i’) of cyclopentanol
on a smooth
platinum
electrode
in 0.5 M HClO,.
237
governed by the kinetics of the adsorption of cyclopentanol itself, producing poisoning species and thus leading to deactivation of the electrocatalyst surface. Figure 2 shows the adsorption isotherm of cyclopentanol from a 0.5 M HC10, solution containing cyclopentanol in different concentrations. The surface coverages, B, were evaluated from the differences between the charges in pure HCIO, and those in the presence of cyclopentanol in the hydrogen potential range (8 = AQw’AQH mti ). By analogy to the adsorption of aliphatic secondary alcohols [16], we may w&e the following probable che~so~tion/dehydrogenation reaction for cyclopentanol: OH
OH i-2Pt Pt-H
-
rapid
=
CY-
Pt 4 Pt-H
(1) (2)
Pt f H”“+ e-
A systematic study of the adsorption of cyclopentanol on Pt which could eventually lead to a complete elucidation of the proposed mechanism - using, for instance, fast sweep techniques - is beyond the scope of this work. This will probably be the subject of another paper. Voltummetr~c electrodes
be~avio~r
of cycio~entano~
electrocatalytic
o~i~t~o~
on Pt/~~u~d~
The effect of lead ad-atoms obtained by upd from a solution containing Pb2* ions is shown in Fig. 3 for the oxidation of cyclopentanol. Cyclic voltammograms for cyclopentanol oxidation on bare Pt and for the upd of lead are also given in the same figure for the sake of comparison. As can be seen, the Pb(upd) ad-atom layer causes a remarkable enhancement in the electrocatalytic activity of platinum for cyclopentanol oxidation. Several points are worth noting. Firstly, the immense double peak obtained in the range between 0.6 and 0.9 V is found to be quite dependent on the initial cathodic polarization history of the electrode. Peak I, increases as the holding time at the starting potential 0.025 V increases from 0 to 6 min, while peak I, decreases. The cyclic voltammogram recorded after holding for 6 min at 0.025 V (the optimum holding time for the ma~mum catalytic effect) was compared with that obtained following another procedure. A complete lead surface coverage on Pt (8;; = 1) was established rapidly at 0.025 V by the upd of Pb from the base solution containing only 5 x 10m4 M Pb(ClO,),. Then, using a flow cell, this solution was replaced by a new one, into which cyclopentanol was additionally introduced. A cyclic voltammogram was recorded immediately. In both cases, similar voltammograms were obtained. This indicates that 6 min are probably required for the formation of a complete surface coverage of Pb(upd) at 0.025 V when cyclopentanol is present in the base solution. Secondly, there is a four-fold increase of the peak at 1.2 V in the presence of Pb2’ ions. This is in contrast to what is expected, since the presence of Pb2’ ions normally causes inhibition effects in the Pt-0 region for almost all organic fuels
238
I
0
I
I
I
0.4
0.8
12
hi
/
I
”
Fig. 3. Cyclic voltammograms for cyclopentanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the presence of 5 X10m4 M Pb(CIO.,),. ES,,,, = 0.025 V; thold/min: (1) 0: (2) 6. Scan rate u = 50 mV s-‘. The dashed line corresponds to the voltammogram of cyclopentanol wthout Pb*+ ions present. Insets: (A) Cyclic voltammogram for the upd of Pb on Pt in 0.5 M HClO,; u = 50 mV s-‘. (B) Plots of peak current densities vs. u”* for rho,,, = 6 min (I,, I,, II) and for th,,ld = 0 mm (IL).
log (Y/ v s-9 Fig. 4. Plots of E, vs. log u for cyclopentanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, presence of 5 x 1O-4 M Pb(ClO,),. Eads = 0.025 V; tad:,= 6 min.
in the
239
r
I
0
I
I
0.4
0.8 EH/
4
1.2
I
”
Fig. 5. Cyclic voltammograms for cyclopentanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the presence of 5 x 1O-4 M Cd(ClO,),. E,,,,, = 0.025 V; I hold/min: (1) 0; (2) 6. Scan rate o = 50 mV s-l. Insets: (A) Cyclic voltammogmm for the upd of Cd on Pt in 0.5 M HClO,; u = 50 mV s-‘. (B) Plots of peak current densities vs. u112 for thold = 6 min.
[6,17]. Thirdly, the diffusion-controlled character of peaks I, and II and the kinetic character of peak I,, may be seen from the plots of jr vs. ui/’ in inset (B) of Fig. 3. Fourthly, (E,),, and (E,),, vary linearly with log u (Fig. 4) yielding a slope of 44 mV for peak I, and 42 mV for peak II per decade, which would correspond to (in = 0.68 and 0.71, respectively (where (Y is the transfer coefficient and n is the number of electrons exchanged). In contrast, (E,),, remains practically constant at all scan rates, confirming the kinetic character of this peak. Figures 5 and 6 represent the catalytic effect of the upd of Cd and Ge on the oxidation of cyclopentanol on Pt. It is apparent that the catalytic activity of the Pt/Cd(upd) electrode presents similar characteristics to those of the Pt/Pb(upd) electrode, with the difference that the heights of peaks I, and I, are almost half those obtained on Pt/Pb(upd). In the case of the Pt/Ge(upd) electrode, only the peak at 0.8 V (Ib) appears, which, despite its height, exhibits kinetic character at scan rates u > 50 mV s-l. Finally, bismuth, copper and thallium ad-atoms were found to cause a smaller catalytic effect on cyclopentanol electro-oxidation. Effect of cyclopentanol concentration The effect of varying the concentration of cyclopentanol was studied on Pt/ Pb(upd), the most active electrocatalyst for the anodic process under examination. Figure 7 presents a double logarithmic plot of the peak current densities against the cyclopentanol concentration (log jr, vs. log c). Slopes equal to 0.85 for peak I, and 0.95 for peak II were found for c < 5 x lo-’ M, which are in reasonable agreement with the diffusion character of the oxidation peak currents. The deviation from linearity observed at c > 5 x 10e2 M indicates that mass transport of cyclopentanol to the electrode surface is no longer rate-determining and that the currents are controlled by the rate of the overall anodic process.
240
I
0
I
0
I
I
I
0.4
0.0
1.2
I
h/V
Fig. 6. Cyclic voltammograms for cyclopentanol(O.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the presence of 5 X 10e4 M Ge(ClO,),. E,,, = 0.025 V; th,ld/min: (1) 0; (2) 6. Scan rate u = 50 mV s-‘. Insets: (A) Cyclic voltammogram for the upd of Ge on Pt in 0.5 M HClO,; u = 50 mV s-l. (B) Plots of peak current densities vs. ul/* for thold = 6 min.
log(c/
mot
i’>
and log(Jp),, vs. log c of cyclopentanol Fig. 7. Plots of log(/,),, Pb(ClO,),. I?,,,, = 0.025 V; thold = 6 min.
in 0.5 M HClO,
and 5X10K4
M
241 TABLE
1
Experimental values of (jr),, s-i and jr values calculated c/m01
1-i
10-3 2.5x10-’ 10-2 10-l
for cyclopentanol oxidation on Pt/Pb(upd) in 0.5 M HClO, from eqn. (3) for an irreversible two-electron process
an
(jp)Ia (exp.) /mA cm-2
0.68 0.68 0.68 0.68
0.16 0.32 1.0 6.4
at u = 0.05 V
jr (talc.) /mA cmm2 0.33 0.83 3.3 33.5
It is important to compare the height of the peak at less positive potentials recorded on Pt/Pb(upd) with the values of the peak current densities calculated theoretically assuming an irreversible two-electron process occurring on an electrode surface with all the sites available for reaction. As we will see below, two electrons per molecule are exchanged during cyclopentanol oxidation in the potential range of peak I,. Theoretical jp values were determined by means of the equation jr = 3.01 X 105n(an)1’2D”2cu”2
(3)
using values of n = 2, (WZ) = 0.68 and D = 9.1 X 1O-6 cm2 s-r *. The data are given in Table 1 for various concentrations of cyclopentanol and a constant scan rate v = 0.05 v s-r. The jr values given in Table 1 imply that at low concentrations of cyclopentanol about 50% of the sites of the upd-modified surface are available for the anodic process. At higher concentrations, the number of effective sites is decreased further, most likely because of the acceleration of the adsorption rate of cyclopentanol itself on groups of vacant sites on the electrode surface when the ad-atoms are desorbed from the surface.
Effect of temperature The influence of temperature on the oxidation rate of cyclopentanol was studied in the range from 15 o C to 55 ’ C on both Pt and Pt/M(upd) electrocatalysts. Figure 8 shows plots of log jr, vs. l/T for two peaks exhibiting kinetic character (peak III on Pt and peak I, on Pt/Ge(upd)) and for two peaks with currents controlled by diffusion (peak II on Pt and peak I, on Pt/Pb(upd)). Straight lines were obtained. This allows estimation of the apparent activation energies for the overall process,
l This value was determined for cyclopentanone by means of the Einstein-Stokes equation using the value D = 5.3 x 10v6 cm2 s-’ calculated from polarographic data (rd/60.7m2’3t’/6c = 0.0046 cm se”*) in aqueous 90% C2H,0H 1181. It is assumed that cyclopentanol and cyclopentanone have equal D values.
242
-1.
?E :
Pt
II
(A)
-2.
E . .s H -3.1
b-t/-
lb (0)
-3 )
I
I
3.2
3.4
lo3
(l/T)
3E
/ K-'
Fig. 8. Plots of log jp vs. l/T for the oxidation of cyclopentanol (0.1 M) in 0.5 M HClO, with and without Pb** or Ge4+ ions present.
occurring in the range of each oxidation peak, by using the following Arrhenius-type equation: log jr, = constant + AH,*/RT
(4)
AH: values are given in Table 2. As expected, higher AH,* values were evaluated for the peaks where the process is kinetically controlled and lower AH3* values for the peaks where the process is governed mainly by diffusion. Current-time oxidation
plots and the long-term
catalytic effect of Pb ad-atoms
on cyclopentanol
The voltammetric behaviour certainly confirms that the Pt/Pb(upd) modified electrode is the most effective electrocatalyst for cyclopentanol oxidation in terms of
TABLE 2 Apparent activation energies for cyclopentanol electrocatalytic oxidation System
A H,*/kJ mol-’
Pt (peak II) Pt (peak III) Pt/Pb(upd) (peak Ia) Pt/Ge(upd) (peak Ib)
12.0 23.4 9.9 19.8
243
0
0
I
I
25
50
I
75 Time
100
125
lb
/ *
Fig. 9. Influence of upd lead on the current-time relation for the oxidation of cyclopentanol (0.05 M) m 0.5 M HClO, on Pt at 0.72 V. Inset: Long-term catalytic effect of upd lead on cyclopentanol oxidation when the coverage (by lead) was restored periodically by a short pulse at 0.025 V.
both the oxidation potential and the current density. The long-term stability of this electrocatalyst was tested by potentiostatic experiments using as the anode a platinum gauze of real surface area - 100 cm*. In Fig. 9 the current-time plots at E = 0.72 V, recorded on Pt with and without Pb(upd), are given. Contrary to our expectations, in view of the voltammetric behaviour a relatively high current density was observed also without lead ad-atoms. The upd of lead causes an increase in the rate of the reaction by a factor of four. This, of course, is much smaller than that observed under potentiodynamic conditions. At constant potential, the activity is lost after a few minutes. However, as can be seen from the j(t) response in the inset of Fig. 9, the activity can be maintained for several hours if the lead monolayer is renewed every few seconds by a short pulse to 0.025 V. This procedure was first applied by Pletcher and Solis [19] to assess the stability of the Pt/Pb(upd) electrocatalyst for formic acid oxidation. Following the above procedure, we performed long-time electrolysis of a solution of 0.05 M cyclopentanol in 0.5 M HClO,. After consumption of - 10% of the primary compound and neutralization of the solution with tetrabutylammonium hydroxide, a polarographic curve was recorded which showed the reduction wave of the cyclopentanone produced. Quantitative determination of the product, as its 2,4-dinitrophenylhydrazone, proved that the current efficiency of the oxidation is 100%. Hence the oxidation occurs by the overall reaction
o-
OH
-
o-
0 +2H++2e-
(5)
This behaviour is in line with previous findings reported in the literature for other secondary alcohols, such as iso-PrOH, set-BtOH and cyclohexanol [11,13].
244
0
0.4
0.8
1.2
h/”
Fig. 10. Cyclic voltammograms for cyclohexanol (0.1 M) oxidation on a Pt sheet in 0.5 M HCIO, in the = 0.025 absence (1) and presence (2) of 5 x 10K4 M Pb(ClO,),. (1) Eads = 0.45 V; fads= 100 s. (2) E start v; f ho,d=6min.Scanrateu=50mVs~’
Upd electrocatalytic
effect for cyclohexanol
and cycloheptanol
oxidation
on Pt
In a preliminary attempt to find a relation between the observed catalytic phenomena and the structural characteristics of cyclic alcohols, the influence of Pb(upd) ad-atoms on the oxidation of cyclohexanol and cycloheptanol was studied. Further considerations of the structural characteristics of cyclic diols or polyols are among our future aims. Figures 10 and 11 give the cyclic voltammograms of cyclohexanol and cycloheptanol on Pt with and without Pb(upd) present.
Fig. 11. Cyclic voltammograms for cycloheptanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the absence (1) and presence (2) of 5 x 10m4 M Pb(Cl0,) 2. (1) Eads = 0.45 V; tads= 100 s. (2) ES,,,, = 0.025 v; t ,,,,=6min.Scanrateu=50mVs-‘.
245
The potentiodynamic behaviour reveals the following features: (a) The cyclic voltammograms of cyclohexanol and cycloheptanol on bare Pt are, in general, similar to that of cyclopentanol. (b) The catalytic effect caused by upd of lead is less important compared to that observed for cyclopentanol. (c) The current densities on bare Pt decrease in the order cyclopentanol > cycloheptanol > cyclohexanol, while on Pt/Pb(upd) they decrease in the order cyclopentanol > cyclohexanol > cycloheptanol. These findings show clearly that catalysis of the oxidation of cyclic alcohols on Pt by upd ad-atoms is influenced by the structural characteristics of these molecules. However, further work is needed to draw any conclusion regarding the regularity of the catalytic behaviour. Mechanistic
features for the electrocatalytic process
The results described above show clearly that the electro-oxidation of cyclopentanol on Pt is catalysed significantly by the upd of Pb, Cd and Ge. The catalytic effect is comparable to that reported for formic acid and formaldehyde, and, to our knowledge, this is the first report of the remarkable catalysis of alcohol electrooxidation with ad-atoms in an acidic environment. The main features of the kinetic behaviour of the anodic process may be outlined as follows: (1) On bare Pt, the currents obtained in the region below the potential of Pt-0 formation (1.2 V) during the positive potential sweep are very small, most probably due to the self-inhibition of the anodic process from strongly bound intermediates. (2) On Pt/M(upd), the currents in this potential region are significantly higher. Under potentiodynamic conditions two peaks appear on Pt/Pb(upd) and Pt/ Cd(upd), the first at 0.67 V showing diffusion control characteristics, and the second with kinetic character at 0.8 V where covering of the electrode with OH radicals occurs. On Pt/Ge(upd), only the second peak is obtained. (3) Under potentiostatic conditions the catalytic effect appears to be less important. At constant E = 0.72 V, the current on Pt/Pb(upd) - the electrode with the highest catalytic activity - decays rapidly with time. Of course, this decay is not as rapid as on bare Pt. (4) Upon electrolysis, cyclopentanone is produced with 100% current efficiency, indicating a two-electron oxidation process. It has been reported in the literature that the catalytic effect with upd ad-atom layers on Pt for the oxidation of organic fuels is not in fact an enhancement, but the suppression of poison formation reactions. At complete coverages, each Pb, Cd or Ge ad-atom occupies two platinum sites [20]. Although geometrical space between the ad-atoms is available, this space is not sufficient for the chemisorption/ dehydrogenation reaction of cyclopentanol (reaction 1). Therefore no accumulation of poisoning species can take place on Pt/M(upd). This situation of the surface of the electrode should also decrease drastically the rate of bulk oxidation of cyclopentanol (reaction 5). Probably bulk oxidation of cyclopentanol at complete upd
246
coverages might occur at higher overpotentials, but this cannot be confirmed owing to the desorption of the upd monolayers. According to Motoo and co-workers [7,8], when an ad-atom is desorbed from a monolayer, an isolated group of vacant sites equal to that occupied by the ad-atom appears on the electrode surface. This is called a reactive domain. The number of reactive domains is controlled by the coverage of ad-atoms, but is actually a function of the potential. Assuming that desorption of ad-atoms from a monolayer takes place in an orderly way, the number of reactive domains takes its maximum value at fl: around 0.5 [7]. Depending on the sites occupied by an ad-atom and the lowest number of sites required for a molecule to react, bulk oxidation of an organic fuel without serious complications from poisoning intermediates might occur on reactive domains. So, the maximum catalytic effect appears at 0: - 0.5 for most of the systems studied. At lower f?; coverages, the number of sites in a reactive domain increases and a poison formation reaction can occur parallel to the bulk oxidation. The oxidation rate of cyclopentanol on reactive domains resulting from the desorption of Pb and Cd ad-atoms seems to be relatively high. Indeed, the rate of the anodic process on Pt partially covered with Pb and Cd at-atoms is determined by the mass transport of cyclopentanol in solution, as indicated by the diffusion character of the peak at 0.67 V. From a comparison of the theoretical and experimental jr, values given in Table 1, one can see that only at low concentrations of cyclopentanol is 50% of the real surface of the electrode available for bulk oxidation, which is in line with the view of the maximum number of isolated reactive domains at 62 - 0.5. At higher cyclopentanol concentrations, the fraction of the surface for bulk oxidation decreases further, probably because poison effects become more significant. Indeed, the rapid decay of the current with time at constant potential tells us that a poison formation reaction occurs and the poisoning species accumulate rapidly at E = 0.72 V where the Pb ad-atom coverage is less than 0.5 [19,21]. The currents of the second catalytic peak at 0.8 V exhibit a kinetic character. This can be interpreted by assuming that a surface reaction with participating OH radicals is the rate-determining step. Considering that the electro-oxidation of cyclopentanol to cyclopentanone does not require oxygen donation, the following reaction can be written:
o-
OH + Pt-OH
-
o-
-0
+Pt+HH,O
On Pt/Ge(upd), the oxidation proceeds mainly through this reaction. Ge adatoms are known as oxygen-adsorbing atoms since they adsorb oxygen at potentials less positive than Pt [20,21], while Pb and Cd ad-atoms do not adsorb oxygen species. In conclusion, the oxidation of cyclopentanol which, like formic acid, does not require oxygen donation to occur, is enhanced on Pt with upd ad-atoms in aqueous
241
HClO, solutions. The enhancement is related to the geometrical control of reaction sites by ad-atoms. REFERENCES 1 D.M. Kolb in H. Gerischer and C.W. Tobtas (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Wiley, New York, 1978, p. 125. 2 K. Jtittner and W.J. Lorenz, Z. Phys. Chem., N.F., 122 (1980) 163. 3 R.R. Adzic in H. Gerischer and C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, Wiley, New York, 1984, p. 159. 4 G. Kokkinidis, J. Electroanal. Chem., 201 (1986) 217. 5 H. Angerstein-Kozlowska, B. MacDougall and B.E. Conway, J. Electrochem. Sot., 120 (1973) 756. 6 R.R. Adzic, D.N. Simic, A.R. Despic and D.M. Drazic, J. Electroanal. Chem.. 65 (1975) 587; 80 (1977) 81. 7 S. Motoo and M. Shibata, J. Electroanal. Chem., 139 (1982) 119. 8 M. Shibata, 0. Takahashi and S. Motoo, J. Electroanal. Chem., 249 (1988) 253. 9 A. Castro Luna, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 196 (1985) 301. 10 G. Kokkmidis. J.-M. Leger and C. Lamy, J. Electroanal. Chem., 242 (1988) 221. 11 M. Beltowska-Brzezinska, J. Heitbaum and W. Vielstich, Electrochim. Acta, 30 (1985) 1465. 12 G. Horanyi. P. Koning and I. Telcs, Acta Chim. Acad. Sci. Hung., 72 (1972) 165. 13 G. Horanyi, G. Vertes and P. Kiining. Naturwissenschaften. 60 (1973) 519. 14 G. Horanyi, P. KBning and I. Telcs, Magy. Chem. Foly., 77 (1971) 432; Chem. Abstr. 75 (1971) 1365052. 15 T. Bieglier, D.A. Rand and R. Woods, J. Electroanal. Chem., 29 (1971) 269. 16 E. Sokolova, Electrochim. Acta, 20 (1975) 323. 17 G. Kokkmidis and D. Jannakoudakis, J. Electroanal. Chem., 153 (1983) 185. 18 P. Kabasakalian and J. McClotten. Anal. Chem., 31 (1959) 1091. 19 D. Pletcher and V. Solis, J. Electroanal. Chem., 131 (1982) 309. 20 N. Furuya and S. Motoo, J. Electroanal. Chem., 98 (1979) 189. 21 N. Furuya and S. Motoo, J. Electroanal. Chem., 98 (1979) 195.
J. Electroanal. Chem., 271 (1989) 233-247 Elsevier Sequoia S.A., Lausanne - Printed m The Netherlands
Electrocatalytic oxidation of cyclopentanol on Pt and Pt surfaces modified by ad-atoms of heavy metals deposited at underpotentials G. Kokkinidis and A. Papoutsis Laboratory of Physical Chemistry, Department 54006 Thessalonrki (Greece)
of Chemistty,
Anstotle
Unwersity of Thessaloniki,
(Received 11 April 1989; in revised form 13 June 1989)
ABSTRACT The electro-oxidation of cyclopentanol was studied in aqueous acid solutions on platinum anodes modified by Pb, Cd and Ge ad-atoms deposited in the underpotential region. The ad-atoms caused a very pronounced electrocatalytic effect for cyclopentanol oxidation both in terms of the oxidation potential and the current density. Potentiodynamic methods were used to characterize the currents, and stationary methods to check the long-term stability of the modified electrodes and to produce a sufficient amount of product to be identified. It was proved that the oxidation yields cyclopentanone with a current efficiency of 100%. The catalytic effect - which, in fact, is not an enhancement but a suppression of poison formation reactions - was interpreted in terms of geometrical regulations of reaction sites by ad-atoms.
INTRODUCTION
The field of electrocatalysis has undergone extensive growth in recent years, and electrocatalytic concepts have been applied to some areas of basic and applied research and technology. In the initial period of this research, pure electrode materials - metals or semiconductors - were used as electrocatalysts, mainly for the hydrogen and oxygen electrocatalytic reactions and the complete oxidation of organic fuels, and occasionally for selective promotion of reaction paths for chemical processing or electro-organic synthesis. However, it was soon realized through the pursuit of better efficiencies that pure electrocatalysts are not sufficient. Owing to this situation, the modification of electrode surfaces by covalently bound or strongly adsorbed species to increase their catalytic activity or selectivity has become popular. A convenient method for modifying noble and transition metal electrodes is the formation of sub- and monolayers of heavy metal atoms by underpotential deposi0022-0728/89/$03.50
0 1989 Elsevier Sequoia S.A.
234
tion (upd). The upd ad-atoms (i.e. Pb, Tl, Bi, Ge, Cu, etc.) are dispersed homogeneously on the substrate surface, rather than condensed into islands. This is important for the geometrical regulation of the two-dimensional arrangement of the deposited metal atoms. Hence, the structure of the ad-atom layer depends on both the degree of coverage and the structure of the substrate metal. For details of upd see the reviews by Kolb [l], Jtittner and Lorenz [2] and the references cited therein. Upd ad-atom modified electrodes have been found to exhibit remarkable electrocatalytic activity for a number of technologically important electrochemical reactions, such as oxygen reduction, oxidation of organic fuels, reduction of nitro and nitroso compounds, and some quinone-type redox reactions. The results appearing in the literature have been reviewed by Adzic [3] and more recently by Kokkinidis [4]. A diversity of mechanisms has been proposed in each case to explain the catalytic role of the ad-atom layers. Particularly in the case of oxidation of organic fuels on Pt in acidic solutions, the catalytic effect is not in fact an enhancement, but the suppression of poison formation reactions. The increased activity was explained in terms of geometrical configurations (“third body” mechanism [5,6], “Sn” and “S ho,e” concepts [7,8]) or by assuming modification of the electronic properties of the substrate as a whole [9,10]. Thus, the catalytic activity of the modified electrodes does not decay as rapidly as that of Fdre platinum. As a result, higher oxidation currents are obtained, although bulk oxidation on these surfaces might occur at higher over-potentials than on Pt without ad-atoms. However, a question arose as to why the catalytic effect (based on both the current density and the potential) is not as important for methanol or ethylene glycol as it is, for instance, in the case of formic acid or hydrated formaldehyde. A reasonable explanation [ll] might be that the former fuels require oxygen donation for their complete oxidation to occur, and this can be accelerated only in the potential region where covering of the surface of the electrode with OH radicals commences. Since secondary alcohols are oxidized electrochemically on platinum to the corresponding carbonyl compounds [12-141 and this oxidation occurs without oxygen donation, it is worth investigating the upd effect on the oxidation of this class of alcohols. The aim was to gain more insight into the role of ad-atoms on the electrocatalytic processes. The present paper deals with the electrocatalytic oxidation of cyclopentanol on Pt and Pt surfaces modified by upd ad-atoms layers in aqueous HClO, solutions. EXPERIMENTAL
Cyclic voltammetric curves were recorded in the usual way under potentiodynamic control with a three-electrode cell (hydrogen reference electrode and Pt counter-electrode). Electrode potentials, E,, are given in the SHE scale. The cell was doublewalled and thermostated at 25 &-O.l” C, except where stated otherwise. The solutions were thoroughly de-oxygenated by purging the system with ultrapure nitrogen. The electronic set-up consisted of a Wenking potentioscan, type POS73, and a Houston 2000 X-Y recorder. A Pt sheet and a Pt gauge were used for the
235
voltammetric and electrolysis experiments, respectively. The electrodes were activated by applying a continuous sweep between hydrogen evolution and just before oxygen evolution in 0.5 M HClO, solution. Their real surface areas were estimated by integration of the hydrogen adsorption-desorption current-potential curves, assuming that 210 PC cm-’ of hydrogen is adsorbed on a perfectly smooth Pt electrode
WI-
The supporting electrolyte was prepared using triply-distilled water and HClO, (Merck, suprapure). Cyclopentanol, cyclohexanol “puriss p.a.” and cycloheptanol “purum” were from Fluka. The other reagents were oxides or carbonates of Pb, Cd and Ge (G.R. grade quality, Merck). Perchlorate salts were prepared by dissolving the stoichiometric quantity of oxides or carbonates in the base electrolyte. RESULTS
AND DISCUSSION
Voltammetric
behaviour of cyclopentanol
electro-oxidation
on bare Pt
Figure 1 shows a typical cyclic voltammogram for the oxidation of cyclopentanol (0.1 M) in 0.5 M HClO, on Pt, together with the cyclic voltammogram of Pt in the supporting electrolyte alone. The voltammogram was recorded after adsorption of cyclopentanol at 0.45 V. It should be noted that the potentiodynamic j(E) profile is almost independent of the adsorption time and potential scan rate. The currents during the positive sweep in the double layer (dl) and Pt-OH formation potential regions are very small and a normal oxidation peak (II) develops only in the Pt-0 region ((E,),, = 1.25 V at v = 50 mV s-i). This picture is different from that for most of the aliphatic primary and secondary alcohols which show two peaks [16,17].
2-
0
I
I
I
0.4
0.8
1.2
h/V
for cyclopentanol (0.1 M) oxidation Fig 1. Cyclic voltammogram on a Pt sheet in 0.5 M HClO,. to the voltammogram E .&,js= 0.45 v; C,& = 100 s. Scan rate: u = 50 mV s-l. The dashed line corresponds of Pt in 0.5 M HClO,. Inset: Plots of (J,,)~, and (j,),,, vs. I?‘*.
236
During the negative sweep, a higher oxidation peak (III) appears after the potential of Pt oxide reduction. The effect of variation of the potential scan rate on the peak currents is shown in the inset of Fig. 1. By varying the potential scan rate (in the range from 25 to 400 mV s-‘) one can see that (jr),, scales linearly with u112 while (jr),,, remains unaffected. This behaviour indicates that the oxidation of cyclopentanol in the Pt-0 region is controlled by diffusion, whilst in the dl region it is controlled by kinetic factors. More evidence for the diffusion character of the oxidation process in the Pt-0 region comes from the effect of the scan rate on the peak potential and the effect of stirring the solution on the peak current. The electro-oxidation of cyclopentanol is an irreversible process and, as expected for diffusion-controlled processes, a shift of ( Ep)II to positive potentials is observed with increasing scan rate. Also, stirring the solution causes an increase of the height of peak II of about 20% at all scan rates. It is worth noting that the diffusion-controlled currents in the Pt-0 region are much smaller than that predicted if all the Pt sites are considered to be available for cyclopentanol oxidation. This behaviour is characteristic for electrode processes occurring on partially covered surfaces with adsorbed species not participating in the process. Besides, the decay of the current as the potential is scanned to more positive potentials (E > 1.25 v) shows that the process is not limited solely by mass transport of cyclopentanol from the bulk solution to the electrode surface. The enhanced adsorption of 0 radicals restricts further the number of active sites for the anodic process. Oxidation of cyclopentanol can occur once again after the reduction of the Pt-0 film during the negative sweep. The process on the free Pt sites is now
ol
-5
’
I
I
-2
-1
I
1
-4
-3 bg(c/mol
Fig. 2. Adsorption isotherm E _js = 0.45 v; t,& = 100 s.
i’) of cyclopentanol
on a smooth
platinum
electrode
in 0.5 M HClO,.
237
governed by the kinetics of the adsorption of cyclopentanol itself, producing poisoning species and thus leading to deactivation of the electrocatalyst surface. Figure 2 shows the adsorption isotherm of cyclopentanol from a 0.5 M HC10, solution containing cyclopentanol in different concentrations. The surface coverages, B, were evaluated from the differences between the charges in pure HCIO, and those in the presence of cyclopentanol in the hydrogen potential range (8 = AQw’AQH mti ). By analogy to the adsorption of aliphatic secondary alcohols [16], we may w&e the following probable che~so~tion/dehydrogenation reaction for cyclopentanol: OH
OH i-2Pt Pt-H
-
rapid
=
CY-
Pt 4 Pt-H
(1) (2)
Pt f H”“+ e-
A systematic study of the adsorption of cyclopentanol on Pt which could eventually lead to a complete elucidation of the proposed mechanism - using, for instance, fast sweep techniques - is beyond the scope of this work. This will probably be the subject of another paper. Voltummetr~c electrodes
be~avio~r
of cycio~entano~
electrocatalytic
o~i~t~o~
on Pt/~~u~d~
The effect of lead ad-atoms obtained by upd from a solution containing Pb2* ions is shown in Fig. 3 for the oxidation of cyclopentanol. Cyclic voltammograms for cyclopentanol oxidation on bare Pt and for the upd of lead are also given in the same figure for the sake of comparison. As can be seen, the Pb(upd) ad-atom layer causes a remarkable enhancement in the electrocatalytic activity of platinum for cyclopentanol oxidation. Several points are worth noting. Firstly, the immense double peak obtained in the range between 0.6 and 0.9 V is found to be quite dependent on the initial cathodic polarization history of the electrode. Peak I, increases as the holding time at the starting potential 0.025 V increases from 0 to 6 min, while peak I, decreases. The cyclic voltammogram recorded after holding for 6 min at 0.025 V (the optimum holding time for the ma~mum catalytic effect) was compared with that obtained following another procedure. A complete lead surface coverage on Pt (8;; = 1) was established rapidly at 0.025 V by the upd of Pb from the base solution containing only 5 x 10m4 M Pb(ClO,),. Then, using a flow cell, this solution was replaced by a new one, into which cyclopentanol was additionally introduced. A cyclic voltammogram was recorded immediately. In both cases, similar voltammograms were obtained. This indicates that 6 min are probably required for the formation of a complete surface coverage of Pb(upd) at 0.025 V when cyclopentanol is present in the base solution. Secondly, there is a four-fold increase of the peak at 1.2 V in the presence of Pb2’ ions. This is in contrast to what is expected, since the presence of Pb2’ ions normally causes inhibition effects in the Pt-0 region for almost all organic fuels
238
I
0
I
I
I
0.4
0.8
12
hi
/
I
”
Fig. 3. Cyclic voltammograms for cyclopentanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the presence of 5 X10m4 M Pb(CIO.,),. ES,,,, = 0.025 V; thold/min: (1) 0: (2) 6. Scan rate u = 50 mV s-‘. The dashed line corresponds to the voltammogram of cyclopentanol wthout Pb*+ ions present. Insets: (A) Cyclic voltammogram for the upd of Pb on Pt in 0.5 M HClO,; u = 50 mV s-‘. (B) Plots of peak current densities vs. u”* for rho,,, = 6 min (I,, I,, II) and for th,,ld = 0 mm (IL).
log (Y/ v s-9 Fig. 4. Plots of E, vs. log u for cyclopentanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, presence of 5 x 1O-4 M Pb(ClO,),. Eads = 0.025 V; tad:,= 6 min.
in the
239
r
I
0
I
I
0.4
0.8 EH/
4
1.2
I
”
Fig. 5. Cyclic voltammograms for cyclopentanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the presence of 5 x 1O-4 M Cd(ClO,),. E,,,,, = 0.025 V; I hold/min: (1) 0; (2) 6. Scan rate o = 50 mV s-l. Insets: (A) Cyclic voltammogmm for the upd of Cd on Pt in 0.5 M HClO,; u = 50 mV s-‘. (B) Plots of peak current densities vs. u112 for thold = 6 min.
[6,17]. Thirdly, the diffusion-controlled character of peaks I, and II and the kinetic character of peak I,, may be seen from the plots of jr vs. ui/’ in inset (B) of Fig. 3. Fourthly, (E,),, and (E,),, vary linearly with log u (Fig. 4) yielding a slope of 44 mV for peak I, and 42 mV for peak II per decade, which would correspond to (in = 0.68 and 0.71, respectively (where (Y is the transfer coefficient and n is the number of electrons exchanged). In contrast, (E,),, remains practically constant at all scan rates, confirming the kinetic character of this peak. Figures 5 and 6 represent the catalytic effect of the upd of Cd and Ge on the oxidation of cyclopentanol on Pt. It is apparent that the catalytic activity of the Pt/Cd(upd) electrode presents similar characteristics to those of the Pt/Pb(upd) electrode, with the difference that the heights of peaks I, and I, are almost half those obtained on Pt/Pb(upd). In the case of the Pt/Ge(upd) electrode, only the peak at 0.8 V (Ib) appears, which, despite its height, exhibits kinetic character at scan rates u > 50 mV s-l. Finally, bismuth, copper and thallium ad-atoms were found to cause a smaller catalytic effect on cyclopentanol electro-oxidation. Effect of cyclopentanol concentration The effect of varying the concentration of cyclopentanol was studied on Pt/ Pb(upd), the most active electrocatalyst for the anodic process under examination. Figure 7 presents a double logarithmic plot of the peak current densities against the cyclopentanol concentration (log jr, vs. log c). Slopes equal to 0.85 for peak I, and 0.95 for peak II were found for c < 5 x lo-’ M, which are in reasonable agreement with the diffusion character of the oxidation peak currents. The deviation from linearity observed at c > 5 x 10e2 M indicates that mass transport of cyclopentanol to the electrode surface is no longer rate-determining and that the currents are controlled by the rate of the overall anodic process.
240
I
0
I
0
I
I
I
0.4
0.0
1.2
I
h/V
Fig. 6. Cyclic voltammograms for cyclopentanol(O.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the presence of 5 X 10e4 M Ge(ClO,),. E,,, = 0.025 V; th,ld/min: (1) 0; (2) 6. Scan rate u = 50 mV s-‘. Insets: (A) Cyclic voltammogram for the upd of Ge on Pt in 0.5 M HClO,; u = 50 mV s-l. (B) Plots of peak current densities vs. ul/* for thold = 6 min.
log(c/
mot
i’>
and log(Jp),, vs. log c of cyclopentanol Fig. 7. Plots of log(/,),, Pb(ClO,),. I?,,,, = 0.025 V; thold = 6 min.
in 0.5 M HClO,
and 5X10K4
M
241 TABLE
1
Experimental values of (jr),, s-i and jr values calculated c/m01
1-i
10-3 2.5x10-’ 10-2 10-l
for cyclopentanol oxidation on Pt/Pb(upd) in 0.5 M HClO, from eqn. (3) for an irreversible two-electron process
an
(jp)Ia (exp.) /mA cm-2
0.68 0.68 0.68 0.68
0.16 0.32 1.0 6.4
at u = 0.05 V
jr (talc.) /mA cmm2 0.33 0.83 3.3 33.5
It is important to compare the height of the peak at less positive potentials recorded on Pt/Pb(upd) with the values of the peak current densities calculated theoretically assuming an irreversible two-electron process occurring on an electrode surface with all the sites available for reaction. As we will see below, two electrons per molecule are exchanged during cyclopentanol oxidation in the potential range of peak I,. Theoretical jp values were determined by means of the equation jr = 3.01 X 105n(an)1’2D”2cu”2
(3)
using values of n = 2, (WZ) = 0.68 and D = 9.1 X 1O-6 cm2 s-r *. The data are given in Table 1 for various concentrations of cyclopentanol and a constant scan rate v = 0.05 v s-r. The jr values given in Table 1 imply that at low concentrations of cyclopentanol about 50% of the sites of the upd-modified surface are available for the anodic process. At higher concentrations, the number of effective sites is decreased further, most likely because of the acceleration of the adsorption rate of cyclopentanol itself on groups of vacant sites on the electrode surface when the ad-atoms are desorbed from the surface.
Effect of temperature The influence of temperature on the oxidation rate of cyclopentanol was studied in the range from 15 o C to 55 ’ C on both Pt and Pt/M(upd) electrocatalysts. Figure 8 shows plots of log jr, vs. l/T for two peaks exhibiting kinetic character (peak III on Pt and peak I, on Pt/Ge(upd)) and for two peaks with currents controlled by diffusion (peak II on Pt and peak I, on Pt/Pb(upd)). Straight lines were obtained. This allows estimation of the apparent activation energies for the overall process,
l This value was determined for cyclopentanone by means of the Einstein-Stokes equation using the value D = 5.3 x 10v6 cm2 s-’ calculated from polarographic data (rd/60.7m2’3t’/6c = 0.0046 cm se”*) in aqueous 90% C2H,0H 1181. It is assumed that cyclopentanol and cyclopentanone have equal D values.
242
-1.
?E :
Pt
II
(A)
-2.
E . .s H -3.1
b-t/-
lb (0)
-3 )
I
I
3.2
3.4
lo3
(l/T)
3E
/ K-'
Fig. 8. Plots of log jp vs. l/T for the oxidation of cyclopentanol (0.1 M) in 0.5 M HClO, with and without Pb** or Ge4+ ions present.
occurring in the range of each oxidation peak, by using the following Arrhenius-type equation: log jr, = constant + AH,*/RT
(4)
AH: values are given in Table 2. As expected, higher AH,* values were evaluated for the peaks where the process is kinetically controlled and lower AH3* values for the peaks where the process is governed mainly by diffusion. Current-time oxidation
plots and the long-term
catalytic effect of Pb ad-atoms
on cyclopentanol
The voltammetric behaviour certainly confirms that the Pt/Pb(upd) modified electrode is the most effective electrocatalyst for cyclopentanol oxidation in terms of
TABLE 2 Apparent activation energies for cyclopentanol electrocatalytic oxidation System
A H,*/kJ mol-’
Pt (peak II) Pt (peak III) Pt/Pb(upd) (peak Ia) Pt/Ge(upd) (peak Ib)
12.0 23.4 9.9 19.8
243
0
0
I
I
25
50
I
75 Time
100
125
lb
/ *
Fig. 9. Influence of upd lead on the current-time relation for the oxidation of cyclopentanol (0.05 M) m 0.5 M HClO, on Pt at 0.72 V. Inset: Long-term catalytic effect of upd lead on cyclopentanol oxidation when the coverage (by lead) was restored periodically by a short pulse at 0.025 V.
both the oxidation potential and the current density. The long-term stability of this electrocatalyst was tested by potentiostatic experiments using as the anode a platinum gauze of real surface area - 100 cm*. In Fig. 9 the current-time plots at E = 0.72 V, recorded on Pt with and without Pb(upd), are given. Contrary to our expectations, in view of the voltammetric behaviour a relatively high current density was observed also without lead ad-atoms. The upd of lead causes an increase in the rate of the reaction by a factor of four. This, of course, is much smaller than that observed under potentiodynamic conditions. At constant potential, the activity is lost after a few minutes. However, as can be seen from the j(t) response in the inset of Fig. 9, the activity can be maintained for several hours if the lead monolayer is renewed every few seconds by a short pulse to 0.025 V. This procedure was first applied by Pletcher and Solis [19] to assess the stability of the Pt/Pb(upd) electrocatalyst for formic acid oxidation. Following the above procedure, we performed long-time electrolysis of a solution of 0.05 M cyclopentanol in 0.5 M HClO,. After consumption of - 10% of the primary compound and neutralization of the solution with tetrabutylammonium hydroxide, a polarographic curve was recorded which showed the reduction wave of the cyclopentanone produced. Quantitative determination of the product, as its 2,4-dinitrophenylhydrazone, proved that the current efficiency of the oxidation is 100%. Hence the oxidation occurs by the overall reaction
o-
OH
-
o-
0 +2H++2e-
(5)
This behaviour is in line with previous findings reported in the literature for other secondary alcohols, such as iso-PrOH, set-BtOH and cyclohexanol [11,13].
244
0
0.4
0.8
1.2
h/”
Fig. 10. Cyclic voltammograms for cyclohexanol (0.1 M) oxidation on a Pt sheet in 0.5 M HCIO, in the = 0.025 absence (1) and presence (2) of 5 x 10K4 M Pb(ClO,),. (1) Eads = 0.45 V; fads= 100 s. (2) E start v; f ho,d=6min.Scanrateu=50mVs~’
Upd electrocatalytic
effect for cyclohexanol
and cycloheptanol
oxidation
on Pt
In a preliminary attempt to find a relation between the observed catalytic phenomena and the structural characteristics of cyclic alcohols, the influence of Pb(upd) ad-atoms on the oxidation of cyclohexanol and cycloheptanol was studied. Further considerations of the structural characteristics of cyclic diols or polyols are among our future aims. Figures 10 and 11 give the cyclic voltammograms of cyclohexanol and cycloheptanol on Pt with and without Pb(upd) present.
Fig. 11. Cyclic voltammograms for cycloheptanol (0.1 M) oxidation on a Pt sheet in 0.5 M HClO, in the absence (1) and presence (2) of 5 x 10m4 M Pb(Cl0,) 2. (1) Eads = 0.45 V; tads= 100 s. (2) ES,,,, = 0.025 v; t ,,,,=6min.Scanrateu=50mVs-‘.
245
The potentiodynamic behaviour reveals the following features: (a) The cyclic voltammograms of cyclohexanol and cycloheptanol on bare Pt are, in general, similar to that of cyclopentanol. (b) The catalytic effect caused by upd of lead is less important compared to that observed for cyclopentanol. (c) The current densities on bare Pt decrease in the order cyclopentanol > cycloheptanol > cyclohexanol, while on Pt/Pb(upd) they decrease in the order cyclopentanol > cyclohexanol > cycloheptanol. These findings show clearly that catalysis of the oxidation of cyclic alcohols on Pt by upd ad-atoms is influenced by the structural characteristics of these molecules. However, further work is needed to draw any conclusion regarding the regularity of the catalytic behaviour. Mechanistic
features for the electrocatalytic process
The results described above show clearly that the electro-oxidation of cyclopentanol on Pt is catalysed significantly by the upd of Pb, Cd and Ge. The catalytic effect is comparable to that reported for formic acid and formaldehyde, and, to our knowledge, this is the first report of the remarkable catalysis of alcohol electrooxidation with ad-atoms in an acidic environment. The main features of the kinetic behaviour of the anodic process may be outlined as follows: (1) On bare Pt, the currents obtained in the region below the potential of Pt-0 formation (1.2 V) during the positive potential sweep are very small, most probably due to the self-inhibition of the anodic process from strongly bound intermediates. (2) On Pt/M(upd), the currents in this potential region are significantly higher. Under potentiodynamic conditions two peaks appear on Pt/Pb(upd) and Pt/ Cd(upd), the first at 0.67 V showing diffusion control characteristics, and the second with kinetic character at 0.8 V where covering of the electrode with OH radicals occurs. On Pt/Ge(upd), only the second peak is obtained. (3) Under potentiostatic conditions the catalytic effect appears to be less important. At constant E = 0.72 V, the current on Pt/Pb(upd) - the electrode with the highest catalytic activity - decays rapidly with time. Of course, this decay is not as rapid as on bare Pt. (4) Upon electrolysis, cyclopentanone is produced with 100% current efficiency, indicating a two-electron oxidation process. It has been reported in the literature that the catalytic effect with upd ad-atom layers on Pt for the oxidation of organic fuels is not in fact an enhancement, but the suppression of poison formation reactions. At complete coverages, each Pb, Cd or Ge ad-atom occupies two platinum sites [20]. Although geometrical space between the ad-atoms is available, this space is not sufficient for the chemisorption/ dehydrogenation reaction of cyclopentanol (reaction 1). Therefore no accumulation of poisoning species can take place on Pt/M(upd). This situation of the surface of the electrode should also decrease drastically the rate of bulk oxidation of cyclopentanol (reaction 5). Probably bulk oxidation of cyclopentanol at complete upd
246
coverages might occur at higher overpotentials, but this cannot be confirmed owing to the desorption of the upd monolayers. According to Motoo and co-workers [7,8], when an ad-atom is desorbed from a monolayer, an isolated group of vacant sites equal to that occupied by the ad-atom appears on the electrode surface. This is called a reactive domain. The number of reactive domains is controlled by the coverage of ad-atoms, but is actually a function of the potential. Assuming that desorption of ad-atoms from a monolayer takes place in an orderly way, the number of reactive domains takes its maximum value at fl: around 0.5 [7]. Depending on the sites occupied by an ad-atom and the lowest number of sites required for a molecule to react, bulk oxidation of an organic fuel without serious complications from poisoning intermediates might occur on reactive domains. So, the maximum catalytic effect appears at 0: - 0.5 for most of the systems studied. At lower f?; coverages, the number of sites in a reactive domain increases and a poison formation reaction can occur parallel to the bulk oxidation. The oxidation rate of cyclopentanol on reactive domains resulting from the desorption of Pb and Cd ad-atoms seems to be relatively high. Indeed, the rate of the anodic process on Pt partially covered with Pb and Cd at-atoms is determined by the mass transport of cyclopentanol in solution, as indicated by the diffusion character of the peak at 0.67 V. From a comparison of the theoretical and experimental jr, values given in Table 1, one can see that only at low concentrations of cyclopentanol is 50% of the real surface of the electrode available for bulk oxidation, which is in line with the view of the maximum number of isolated reactive domains at 62 - 0.5. At higher cyclopentanol concentrations, the fraction of the surface for bulk oxidation decreases further, probably because poison effects become more significant. Indeed, the rapid decay of the current with time at constant potential tells us that a poison formation reaction occurs and the poisoning species accumulate rapidly at E = 0.72 V where the Pb ad-atom coverage is less than 0.5 [19,21]. The currents of the second catalytic peak at 0.8 V exhibit a kinetic character. This can be interpreted by assuming that a surface reaction with participating OH radicals is the rate-determining step. Considering that the electro-oxidation of cyclopentanol to cyclopentanone does not require oxygen donation, the following reaction can be written:
o-
OH + Pt-OH
-
o-
-0
+Pt+HH,O
On Pt/Ge(upd), the oxidation proceeds mainly through this reaction. Ge adatoms are known as oxygen-adsorbing atoms since they adsorb oxygen at potentials less positive than Pt [20,21], while Pb and Cd ad-atoms do not adsorb oxygen species. In conclusion, the oxidation of cyclopentanol which, like formic acid, does not require oxygen donation to occur, is enhanced on Pt with upd ad-atoms in aqueous
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HClO, solutions. The enhancement is related to the geometrical control of reaction sites by ad-atoms. REFERENCES 1 D.M. Kolb in H. Gerischer and C.W. Tobtas (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 11, Wiley, New York, 1978, p. 125. 2 K. Jtittner and W.J. Lorenz, Z. Phys. Chem., N.F., 122 (1980) 163. 3 R.R. Adzic in H. Gerischer and C.W. Tobias (Eds.), Advances in Electrochemistry and Electrochemical Engineering, Vol. 13, Wiley, New York, 1984, p. 159. 4 G. Kokkinidis, J. Electroanal. Chem., 201 (1986) 217. 5 H. Angerstein-Kozlowska, B. MacDougall and B.E. Conway, J. Electrochem. Sot., 120 (1973) 756. 6 R.R. Adzic, D.N. Simic, A.R. Despic and D.M. Drazic, J. Electroanal. Chem.. 65 (1975) 587; 80 (1977) 81. 7 S. Motoo and M. Shibata, J. Electroanal. Chem., 139 (1982) 119. 8 M. Shibata, 0. Takahashi and S. Motoo, J. Electroanal. Chem., 249 (1988) 253. 9 A. Castro Luna, T. Iwasita and W. Vielstich, J. Electroanal. Chem., 196 (1985) 301. 10 G. Kokkmidis. J.-M. Leger and C. Lamy, J. Electroanal. Chem., 242 (1988) 221. 11 M. Beltowska-Brzezinska, J. Heitbaum and W. Vielstich, Electrochim. Acta, 30 (1985) 1465. 12 G. Horanyi. P. Koning and I. Telcs, Acta Chim. Acad. Sci. Hung., 72 (1972) 165. 13 G. Horanyi, G. Vertes and P. Kiining. Naturwissenschaften. 60 (1973) 519. 14 G. Horanyi, P. KBning and I. Telcs, Magy. Chem. Foly., 77 (1971) 432; Chem. Abstr. 75 (1971) 1365052. 15 T. Bieglier, D.A. Rand and R. Woods, J. Electroanal. Chem., 29 (1971) 269. 16 E. Sokolova, Electrochim. Acta, 20 (1975) 323. 17 G. Kokkmidis and D. Jannakoudakis, J. Electroanal. Chem., 153 (1983) 185. 18 P. Kabasakalian and J. McClotten. Anal. Chem., 31 (1959) 1091. 19 D. Pletcher and V. Solis, J. Electroanal. Chem., 131 (1982) 309. 20 N. Furuya and S. Motoo, J. Electroanal. Chem., 98 (1979) 189. 21 N. Furuya and S. Motoo, J. Electroanal. Chem., 98 (1979) 195.