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Effective electrocatalytic reduction of propene at a Pt electrode in strongly acidic solutions with Mo(VI) oxo-species
Electrochemistry Communications 24 (2012) 5–8
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Effective electrocatalytic reduction of propene at a Pt electrode in strongly acidic solutions with Mo(VI) oxo-species Maria Bełtowska-Brzezinska a,⁎, Tomasz Węsierski b, Teresa Łuczak a a b
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland The Main School of Fire Service, Słowackiego 52/54, 01-629 Warszawa, Poland
a r t i c l e
i n f o
Article history: Received 5 June 2012 Received in revised form 30 July 2012 Accepted 31 July 2012 Available online 10 August 2012 Keywords: Electrocatalysis Electroreduction Pt/molybdenum(VI) Propene system
a b s t r a c t An appreciable enhancement of the rate of propene electroreduction was discovered under CV conditions on a polycrystalline Pt electrode in a strongly acidified solution containing cationic Mo(VI) oxo-species. Simultaneously, the electroreduction of these species was found to be remarkably more effective in the presence of propene in comparison with that obtained using a supporting electrolyte without the alkene. Evidence was provided that the catalytic effect results from the continuous regeneration of the electroactive molybdenum moieties via a non-faradaic reaction between propene and the reduced forms produced at the electrode/solution interface from the parent Mo(VI) cationic species in the preceding one and three electron-transfer reactions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
A considerable increase in the reduction rate of alkenes and their hydroxyl derivatives on a polycrystalline Pt electrode has been achieved for the first time by us after the introduction of sodium molybdate into a strongly acidified electrolyte solution. In our previous paper [1], we reported on the kinetics and mechanism of electroreduction processes in allyl alcohol (AA)–Mo(VI) system at pH b 0. An important finding was that the side reaction consisting in the heterogeneous decomposition of AA and thus poisoning of the Pt surface with CO-like species was eliminated in the presence of Mo(VI) at the electrode/solution interface. The present work is devoted to the catalytic action of Mo(VI) oxo-species in the reduction of propene and vice versa on the same polycrystalline Pt electrode. A detailed recognition of the optimal conditions for reductive detection of propene and/or Mo(VI) is expected to provide the basis for development of appropriate voltammetric electrochemical sensors. Electrochemical characteristics of propene or Mo(VI) alone on Pt are reported for comparison. Earlier, the suitability of oxo-metalates as mediators for the electroreduction of some oxygen containing inorganic compounds (nitrate, nitrite, chlorate, bromate, hydrogen peroxide) has been demonstrated in a number of papers (references 16–34 in [1], [2]).
All cyclic-voltammograms (CVs) were obtained in a thermostated (at 295 K) glass cell with three compartments separated by glass frits and equipped with a Luggin capillary situated at a distance of 2 mm from the working electrode consisting of a polycrystalline Pt sheet of geometric area of 0.4 cm2. A large area Pt sheet was a counter electrode. A hydrogen electrode of Will's type [3], with 1 M HClO4 (NHE) was used as a reference. Prior to each experiment, the working electrode was activated in deaerated 1 M HClO4 solution by cycling within the potential range 0.025–1.6 V. Finally, the roughness factor of Pt (2.4 ± 0.2) was determined from the charge related to the hydrogen adsorption and/or desorption, according to the well-known procedure [3,4], assuming that a hydrogen monolayer requires 0.21 mC/cm 2. The apparatus included a computer-controlled Atlas 9431 potentiostat in connection with a 9515 signal generator and a Mescomp MC112-12 interface. Chemicals p.A. were used as received: propene 99.99%, argon 99.998 (Linde), HClO4 (Merck), Na2MoO4 (Aldrich), Millipore-MilliQ water.
⁎ Corresponding author. Fax: +48 618658008. E-mail address: [email protected] (M. Bełtowska-Brzezinska). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.07.028
3. Results and discussion 3.1. Electroreduction of propene and Mo(VI) Exemplary CVs presented in Fig. 1A for propene on a polycrystalline Pt electrode in solutions of different HClO4 concentrations (1 M to 4 M) at a constant scan rate (v) show that this alkene undergoes irreversible reduction at E b 0.25 V and begins to oxidize from E ≈ 0.6 V, although a large majority of the reactant converts to CO2 at E > 0.9 V when the Pt surface is substantially covered with the
6
M. Bełtowska-Brzezinska et al. / Electrochemistry Communications 24 (2012) 5–8
A
Pt / propene + x M HClO4
ENHE / mV
I / mA
0.0
-0.4
-0.8
1M
4M
200 2M
1M 150 0.4
0.1
4M 2M
- I / mA
0
800
1600
ENHE / mV
B
Pt / propene + 1M HClO4 -1
1. v = 0.005 V s -1 2. v = 0.0075 V s-1 3. v = 0.010 V s -1 4. v = 0.015 V s -1 5. v = 0.020 V s
1 2
-0.4
- Ipc / mA
I / mA
0.0
5
-0.8
1
0.0
v
1/2
-1 1/2
/ (V s )
0.6
200
400
ENHE / mV
C
9 0
peak I Mo(VI) reduction
2
-1
Q / mC
I / mA
1
1 stripping
peak II -2
9
0
ENHE / mV
0
400 600
ENHE / mV Fig. 1. CVs on a polycrystalline Pt electrode A) for propene at different HClO4 concentrations (1–4 M), v = 0.015 V/s; inset: Tafel plots for E b 0.25 V, B) for propene in 1 M HClO4 at various v; inset: Ipc versus v1/2; C) in 4 M HClO4 + 2 mM Na2MoO4 at various negative potential limits; inset: charge related to the electroreduction of Mo(VI) and stripping of the adsorbed Mo(VI)/Mo(V) species versus the potential of the scan reversal, v = 0.1 V/s. Black (solid) lines — CVs on Pt in the supporting electrolyte.
OHad/Oad species [4]. Strong suppression of the current related to the hydrogen adsorption–desorption reveals that the charge transfer in both these processes is preceded by irreversible adsorption of the reactant on the metal surface, similarly as reported for a porous Pt/PTFE electrode [4]. However the cathodic current peak corresponding to the propene reduction on the investigated polycrystalline Pt electrode is better developed and therefore it is useful in the determination of the kinetic parameters of this process. Taking into consideration the electrode coverage with the propene species after adsorption at Ead between 0.2 V and 0.5 V, we have established in experiments with electrolyte exchange that ca 5.75
electrons are transferred to each Pt surface site during the direct adsorbate oxidation to CO2, identified as the sole oxidation product of all organic adsorbates [4–7]. This result closely resembles that obtained on a Pt/PTFE electrode (neps ≈ 5.8) [4], indicating that the ad-layer formed in the potential range considered consists predominantly of the associatively (di − σ and π) bonded alkene moieties spread over three adjacent metal sites along with a small amount of partially dehydrogenated C3Hx residues. It is well known from DEMS studies [4] that such species undergo reduction to propane on a Pt/PTFE electrode. Therefore, it is reasonable to suppose that propane is also a dominant volatile product of propene reduction (at E b 0.25 V) on a polycrystalline Pt electrode investigated. As depicted in Fig. 1A, the I–E curves are shifted in the positive direction by about 0.060±0. 005 V over a tenfold increase in the acid concentration. Thus, it is clear that each electron transfer is accompanied by the addition of one proton to the reduced species according to the equation: C3H6 + 2H + + 2e − → C3H8. The slope of dE/dlogI = 0.045±0. 005 V/decade, irrespectively of the concentration of the supporting electrolyte between 1 M and 4 M HClO4 (inset in Fig.1A), points to the second reductive charge transfer reaction as the rate determining step of the overall reaction pathway in the Tafel range. However, the rate of propene consumption around the potential of the peak maximum (Epc) exceeded that of the diffusive transport of the reactant from the bulk solution to the electrode surface if v was below 0.03 V/s. This is evidenced in Fig. 1B by the linear plot of the current at the peak maximum (Ipc) versus the square root of v according to the Randles–Ševčik equation [8] and by the presence of cathodic current after reversal of the scan direction into the positive one as well as by the fact that stirring of the solution with propene results in the appearance of a cathodic wave with a distinct limiting current in the potential range negative to the peak maximum. On the other hand, a negative deviation in the Ipc–v 1/2 plot observed with increasing mass transport rate accompanying an increasing scan rate (inset in Fig. 1B) suggests that the Ipc value at v > 0.03 V/s is determined by the kinetics of the propene adsorption preceding its reductive hydrogenation. Comparison of CVs in Fig. 1A and B with that in Fig. 1C reveals that the electroreduction of propene occurs in the potential range close to that of the flat cathodic peak II (at E b 0.25 V) which appears on a Pt electrode in strongly acidified molybdate solutions after the preceding considerably lower cathodic peak I, in the potential range 0.5– 0.25 V. In our previous paper [1], we have shown that peak II corresponds to the irreversible electroreduction of singly and doubly charged mononuclear Mo(VI) cationic species (such as [MoVIO2(OH)(H2O)3]+, + [MoVIO2(H2O)4] 2 ) to the relevant protonated Mo(V)-species V ([Mo O(OH)2(H2O)3] +, [MoVO(OH)(H2O)4] 2+) and then to Mo(III) moieties ([MoIII(OH)2(H2O)4] +, [MoIII(OH)(H2O)5] 2+). The presence of the above mentioned cationic species in the molybdate solutions below pH= 0 was confirmed in earlier UV–VIS and Raman spectroscopy studies [9–11]. Analysis of CVs obtained at various acid and molybdate concentrations led to the conclusion that the amount of cationic Mo(VI) oxo-species being electroactive at E b 0.25 V, reaches a maximum in the 4 M HClO4 solution containing molybdate at a concentration near cMo(VI) ≈ 1–2 mM. In contrast, changes in the pH value of the electrolyte solution had no influence on the magnitude of the cathodic peak I, which, according to the XPS data [1], reflects the formation of a stable mixed-valent Mo(V)/Mo(VI) surface layer upon electroreduction of preadsorbed oligomeric Mo(VI) aggregates. Using XPS we have found that the latter process was continued in the potential range of peak II, in parallel with the several times faster electroreduction of cationic Mo(VI) oxo-species. Correspondingly, as depicted in Fig. 1C, when the potential scan direction was reversed at E b 0.25 V, the charge related to the actual reduction process was considerably higher than that involved in the oxidative desorption of the strongly bound Mo(V)/ Mo(VI) ad-layer, which was manifested in CVs by an anodic peak with a maximum at E ≈0.4–0.5 V. Therefore it was reasonable to suppose
M. Bełtowska-Brzezinska et al. / Electrochemistry Communications 24 (2012) 5–8
that the majority of cationic Mo(V) and Mo(III) moieties formed from the respective cationic Mo(VI) oxo-species moved from the Pt surface into the solution.
3.2. Electrocatalytic effect in the Pt/molybdenum(VI)–propene system When propene was introduced into a concentrated HClO4 solution containing Mo(VI) oxo-species and/or sodium molybdate was added to a strongly acidified solution saturated with propene, an appreciable
A
0
4
Pt / propene 1. 4 M HClO4+ 2.5 mM Mo(VI) 2. 2.5 M HClO4+ 2.5 mM Mo(VI) 3. 1 M HClO4+ 2.5 mM Mo(VI) 4. 1 M HClO4
3 -2
I / mA
2 -4 ENHE / mV
1 -6
220
Pt / propene
200 180
2.5 mM Mo(VI) 4M HClO4
160
0.1
0
100
- I / mA 1
200
300
ENHE / mV Pt/propene + 4 M HClO4 + x mM Mo(VI) = 2.5 mM 0.5 mM 1 mM 0.05 mM 0.25 mM
B 0
I / mA
-2 Mo(VI) propene
-4
propene + Mo(VI)
0
300 0
300 0
300 0
300 0
300
ENHE / mV Pt / propene+ 4M HClO4 + xM Mo(VI)
5.0
- Ipc / mA
- Ipc / mA
C
10
2.5
10
0
-1
10
-4
10
-3
cMo(VI) / M
0 0
2.5
5
cMo(VI) / mM Fig. 2. Fragment of CVs for propene on a polycrystalline Pt electrode A) in solutions 2.5 mM Na2MoO4 + HClO4 of 1) 4 M, 2) 2.5 M, and 3) 1 M. Curve 4: in 1 M HClO4 without Mo(VI), inset: Tafel plot of CV 1. B) In 4 M HClO4 with various cMo(VI) — olive lines. Propene alone — red lines. Mo(VI) alone — blue lines. C) Plot of the Ipc versus cMo(VI) in 4 M HClO4; inset: double logarithmic plot, v = 0.005 V/s.
7
rise in the reduction peak current on a polycrystalline Pt electrode, at E b 0.25 V, was noted (see Fig. 2A and B) in comparison with that characteristic of the reduction of propene (Fig. 1A and B) or molybdate (Fig. 1C) alone. However, the peak current corresponding to the electroreduction of propene in the presence of Mo(VI) was almost independent of the scan rate up to v ≈ 0.1 V/s, as predicted by the theory developed for the irreversible charge transfer reaction coupled with the irreversible non-faradaic catalytic reaction producing an initial reactant which can be re-reduced at the electrode/solution interface [12,13]. The representative CVs in Fig. 2A provide evidence that the extent of catalysis strongly depends on pH of the solution, although this parameter has no influence on the magnitude of the peak current corresponding to the electroreduction of propene in the absence of Mo(VI) oxo-species. The catalytic performance of the Pt/Mo(VI) electrode toward propene reduction achieves the highest level when using a molybdate solution with 4 M HClO4 as a supporting electrolyte. As mentioned above (Section 3.1), such H + concentration ensures the existence of a maximum amount of cationic Mo(VI) oxo-species, which undergo electroreduction to the cationic Mo(V) and Mo(III) moieties at the electrode potentials negative to 0.25 V. On the contrary, no catalytic effect was found in the molybdate solution of pH = 1 which contains solely dioxo‐ and trioxo moiVI VI eties of molybdic acid in equilibrium with oligomeric Mo12 to Mo36 aggregates [9–11]. Thus, it is obvious that the presence of cationic molybdenum oxo-species at the electrode/solution is essential for the enhanced reduction of propene at a Pt electrode. This conclusion is justified by the fact that no increase in the reaction rate was found on the Pt electrode modified with the electrochemically deposited mixed-valence Mo(V)/Mo(VI) ad-layer and transferred into the propene saturated 4 M HClO4 solution without Na2MoO4. Interestingly, the addition of propene into the acidified molybdate solution results in a considerable suppression of the current related to the formation of Mo(V)/Mo(VI) surface layer between 0.5 V and 0.25 V, i.e. prior to the beginning of the catalytic propene reduction (Fig. 2A and B). This implies that a relatively stronger competitive adsorption of propene prevents the approach of the oligomeric Mo(VI) species to the Pt surface and thus their electroreduction. Consequently, the current peak attributed to the oxidative stripping of the Mo(V)/Mo(VI) ad-layer disappears. Furthermore, we have found the same Tafel slope on the propene electroreduction (at E b 0.25 V) in the presence and in the absence of Mo(VI) oxo-species (inset in Figs. 1A and 2A). Fig. 2B illustrates the changes in the magnitude of the cathodic response related to the catalytic reduction process investigated upon increasing molybdate concentration in 4 M HClO4 solution saturated with propene. As shown in Fig. 2C, the rate of propene reduction at the potential of peak maximum (Ipc) increases linearly with increasing Mo(VI) concentration in 4 M HClO4 from cMo(VI) = 0.05 mM to 0.5 mM and approaches a limiting value at cMo(VI) ≈ 1–2 mM when the amount of cationic Mo(VI) oxo-species in the bulk solution reaches a maximum. It is evident that this Mo(VI) concentration is optimal for analytical application. The reduction rate is then by about one order of magnitude higher in comparison with that observed for propene or Mo(VI) alone. Considering the shape of the Ipc–cMo(VI) dependence, it is likely that the catalytic reduction process investigated involves the formation of an associative adduct between the cationic molybdenum species and propene. Similar regularities have been established by us also upon electroreduction of allyl alcohol of constant concentration in the presence of various amounts of Mo(VI) in 4 M HClO4 solution [1]. Unfortunately it was not possible to obtain CVs for various concentrations of propene dissolved in solution in order to establish the influence of changes in the concentration of this reactant on the catalytic current. Despite this, on the basis of the results described above, the observed catalytic effect can be explained by the continuous regeneration of the electroactive molybdenum species in a non-faradaic reaction between propene and the reduced Mo(V) and/or Mo(III) cationic species formed at the Pt/
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M. Bełtowska-Brzezinska et al. / Electrochemistry Communications 24 (2012) 5–8
solution interface in the consecutive one and three electron-transfer reactions from the parent Mo(VI) cationic species. This explains the appearance of cathodic current in CVs after reversal of the scan direction into the positive one (Fig. 2B). It is reasonable to expect that the hydrogen inserted into the structure of Mo(V) and Mo(III) cationic species upon electroreduction facilitates the hydrogenation of propene to propane. Since the exact structure of molybdenum species being active in this reaction is unknown at present, the general reaction sequence accounting for the catalytic phenomenon in the propene–Mo(VI) system is tentatively described below for one of their possible protonated forms (see Section 3.1). The first pathway (1) involves the cationic Mo(V) oxo-species, which
4. Conclusions For the first time a remarkable catalytic activity of cationic Mo(VI) oxo-species in the electroreduction of propene on a polycrystalline Pt electrode in strongly acidic solution was demonstrated under cyclic voltammetry conditions. This phenomenon ensures an increased sensitivity of propene detection in the presence of Mo(VI) and vice versa, giving the basis for analytical application of appropriate electrochemical sensors. Acknowledgments We appreciate financial support from the Ministry of Science and Higher Education, Poland. References
(1)
(2) in the reaction with propene, giving propane, are repeatedly re-oxidized to the parent Mo(VI) species. In the second pathway (2), the formation of propane from propene in a reaction with the cationic Mo(III) oxo-species is accompanied by a continuous regeneration of the appropriate Mo(V)-species and their re-reduction to Mo(III) moieties. Studies with in situ spectroelectrochemical techniques would be needed for further comprehensive mechanistic information.
[1] M. Bełtowska-Brzezinska, T. Węsierski, T. Łuczak, J. Camra, Electrochimica Acta 63 (2012) 89. [2] A. Molina, J. Gonzalez, E. Laborda, Y. Wang, R.G. Compton, Physical Chemistry Chemical Physics 13 (2011) 16748. [3] C.H. Hamann, W. Vielstich, Electrochemie II, Verlag Chemie, Weinheim, 1981. [4] M. Bełtowska-Brzezinska, T. Łuczak, H. Baltruschat, U. Mueller, The Journal of Physical Chemistry. B 107 (2003) 4793. [5] M. Bełtowska-Brzezinska, T. Łuczak, M. Mączka, H. Baltruschat, U. Mueller, Journal of Electroanalytical Chemistry 519 (2002) 101. [6] H. Baltruschat, Differential electrochemical mass spectrometry as a tool for interfacial studies, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, Basel, 1999, p. 577. [7] J.F.E. Gootzen, A.H. Wonders, W. Visscher, J.A.R. van Veen, Langmuir 13 (1997) 1659. [8] F. Scholz, Electroanalytical Methods, Springer, Berlin Heidelberg, 2010. [9] S. Himeno, H. Niiya, T. Ueda, Bulletin of the Chemical Society of Japan 70 (1997) 631. [10] J.J. Cruywagen, J.B. Heyns, Polyhedron 19 (2000) 907. [11] J.F. Ojo, R.S. Taylor, G. Sykes, Journal of the Chemical Society Dalton Transactions (1975) 500. [12] R.S. Nicholson, I. Shain, Analytical Chemistry 36 (1964) 706. [13] D.S. Polcyn, I. Shain, Analytical Chemistry 38 (1966) 376.
Contents lists available at SciVerse ScienceDirect
Electrochemistry Communications journal homepage: www.elsevier.com/locate/elecom
Effective electrocatalytic reduction of propene at a Pt electrode in strongly acidic solutions with Mo(VI) oxo-species Maria Bełtowska-Brzezinska a,⁎, Tomasz Węsierski b, Teresa Łuczak a a b
Faculty of Chemistry, A. Mickiewicz University, Grunwaldzka 6, 60-780 Poznań, Poland The Main School of Fire Service, Słowackiego 52/54, 01-629 Warszawa, Poland
a r t i c l e
i n f o
Article history: Received 5 June 2012 Received in revised form 30 July 2012 Accepted 31 July 2012 Available online 10 August 2012 Keywords: Electrocatalysis Electroreduction Pt/molybdenum(VI) Propene system
a b s t r a c t An appreciable enhancement of the rate of propene electroreduction was discovered under CV conditions on a polycrystalline Pt electrode in a strongly acidified solution containing cationic Mo(VI) oxo-species. Simultaneously, the electroreduction of these species was found to be remarkably more effective in the presence of propene in comparison with that obtained using a supporting electrolyte without the alkene. Evidence was provided that the catalytic effect results from the continuous regeneration of the electroactive molybdenum moieties via a non-faradaic reaction between propene and the reduced forms produced at the electrode/solution interface from the parent Mo(VI) cationic species in the preceding one and three electron-transfer reactions. © 2012 Elsevier B.V. All rights reserved.
1. Introduction
2. Experimental
A considerable increase in the reduction rate of alkenes and their hydroxyl derivatives on a polycrystalline Pt electrode has been achieved for the first time by us after the introduction of sodium molybdate into a strongly acidified electrolyte solution. In our previous paper [1], we reported on the kinetics and mechanism of electroreduction processes in allyl alcohol (AA)–Mo(VI) system at pH b 0. An important finding was that the side reaction consisting in the heterogeneous decomposition of AA and thus poisoning of the Pt surface with CO-like species was eliminated in the presence of Mo(VI) at the electrode/solution interface. The present work is devoted to the catalytic action of Mo(VI) oxo-species in the reduction of propene and vice versa on the same polycrystalline Pt electrode. A detailed recognition of the optimal conditions for reductive detection of propene and/or Mo(VI) is expected to provide the basis for development of appropriate voltammetric electrochemical sensors. Electrochemical characteristics of propene or Mo(VI) alone on Pt are reported for comparison. Earlier, the suitability of oxo-metalates as mediators for the electroreduction of some oxygen containing inorganic compounds (nitrate, nitrite, chlorate, bromate, hydrogen peroxide) has been demonstrated in a number of papers (references 16–34 in [1], [2]).
All cyclic-voltammograms (CVs) were obtained in a thermostated (at 295 K) glass cell with three compartments separated by glass frits and equipped with a Luggin capillary situated at a distance of 2 mm from the working electrode consisting of a polycrystalline Pt sheet of geometric area of 0.4 cm2. A large area Pt sheet was a counter electrode. A hydrogen electrode of Will's type [3], with 1 M HClO4 (NHE) was used as a reference. Prior to each experiment, the working electrode was activated in deaerated 1 M HClO4 solution by cycling within the potential range 0.025–1.6 V. Finally, the roughness factor of Pt (2.4 ± 0.2) was determined from the charge related to the hydrogen adsorption and/or desorption, according to the well-known procedure [3,4], assuming that a hydrogen monolayer requires 0.21 mC/cm 2. The apparatus included a computer-controlled Atlas 9431 potentiostat in connection with a 9515 signal generator and a Mescomp MC112-12 interface. Chemicals p.A. were used as received: propene 99.99%, argon 99.998 (Linde), HClO4 (Merck), Na2MoO4 (Aldrich), Millipore-MilliQ water.
⁎ Corresponding author. Fax: +48 618658008. E-mail address: [email protected] (M. Bełtowska-Brzezinska). 1388-2481/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.elecom.2012.07.028
3. Results and discussion 3.1. Electroreduction of propene and Mo(VI) Exemplary CVs presented in Fig. 1A for propene on a polycrystalline Pt electrode in solutions of different HClO4 concentrations (1 M to 4 M) at a constant scan rate (v) show that this alkene undergoes irreversible reduction at E b 0.25 V and begins to oxidize from E ≈ 0.6 V, although a large majority of the reactant converts to CO2 at E > 0.9 V when the Pt surface is substantially covered with the
6
M. Bełtowska-Brzezinska et al. / Electrochemistry Communications 24 (2012) 5–8
A
Pt / propene + x M HClO4
ENHE / mV
I / mA
0.0
-0.4
-0.8
1M
4M
200 2M
1M 150 0.4
0.1
4M 2M
- I / mA
0
800
1600
ENHE / mV
B
Pt / propene + 1M HClO4 -1
1. v = 0.005 V s -1 2. v = 0.0075 V s-1 3. v = 0.010 V s -1 4. v = 0.015 V s -1 5. v = 0.020 V s
1 2
-0.4
- Ipc / mA
I / mA
0.0
5
-0.8
1
0.0
v
1/2
-1 1/2
/ (V s )
0.6
200
400
ENHE / mV
C
9 0
peak I Mo(VI) reduction
2
-1
Q / mC
I / mA
1
1 stripping
peak II -2
9
0
ENHE / mV
0
400 600
ENHE / mV Fig. 1. CVs on a polycrystalline Pt electrode A) for propene at different HClO4 concentrations (1–4 M), v = 0.015 V/s; inset: Tafel plots for E b 0.25 V, B) for propene in 1 M HClO4 at various v; inset: Ipc versus v1/2; C) in 4 M HClO4 + 2 mM Na2MoO4 at various negative potential limits; inset: charge related to the electroreduction of Mo(VI) and stripping of the adsorbed Mo(VI)/Mo(V) species versus the potential of the scan reversal, v = 0.1 V/s. Black (solid) lines — CVs on Pt in the supporting electrolyte.
OHad/Oad species [4]. Strong suppression of the current related to the hydrogen adsorption–desorption reveals that the charge transfer in both these processes is preceded by irreversible adsorption of the reactant on the metal surface, similarly as reported for a porous Pt/PTFE electrode [4]. However the cathodic current peak corresponding to the propene reduction on the investigated polycrystalline Pt electrode is better developed and therefore it is useful in the determination of the kinetic parameters of this process. Taking into consideration the electrode coverage with the propene species after adsorption at Ead between 0.2 V and 0.5 V, we have established in experiments with electrolyte exchange that ca 5.75
electrons are transferred to each Pt surface site during the direct adsorbate oxidation to CO2, identified as the sole oxidation product of all organic adsorbates [4–7]. This result closely resembles that obtained on a Pt/PTFE electrode (neps ≈ 5.8) [4], indicating that the ad-layer formed in the potential range considered consists predominantly of the associatively (di − σ and π) bonded alkene moieties spread over three adjacent metal sites along with a small amount of partially dehydrogenated C3Hx residues. It is well known from DEMS studies [4] that such species undergo reduction to propane on a Pt/PTFE electrode. Therefore, it is reasonable to suppose that propane is also a dominant volatile product of propene reduction (at E b 0.25 V) on a polycrystalline Pt electrode investigated. As depicted in Fig. 1A, the I–E curves are shifted in the positive direction by about 0.060±0. 005 V over a tenfold increase in the acid concentration. Thus, it is clear that each electron transfer is accompanied by the addition of one proton to the reduced species according to the equation: C3H6 + 2H + + 2e − → C3H8. The slope of dE/dlogI = 0.045±0. 005 V/decade, irrespectively of the concentration of the supporting electrolyte between 1 M and 4 M HClO4 (inset in Fig.1A), points to the second reductive charge transfer reaction as the rate determining step of the overall reaction pathway in the Tafel range. However, the rate of propene consumption around the potential of the peak maximum (Epc) exceeded that of the diffusive transport of the reactant from the bulk solution to the electrode surface if v was below 0.03 V/s. This is evidenced in Fig. 1B by the linear plot of the current at the peak maximum (Ipc) versus the square root of v according to the Randles–Ševčik equation [8] and by the presence of cathodic current after reversal of the scan direction into the positive one as well as by the fact that stirring of the solution with propene results in the appearance of a cathodic wave with a distinct limiting current in the potential range negative to the peak maximum. On the other hand, a negative deviation in the Ipc–v 1/2 plot observed with increasing mass transport rate accompanying an increasing scan rate (inset in Fig. 1B) suggests that the Ipc value at v > 0.03 V/s is determined by the kinetics of the propene adsorption preceding its reductive hydrogenation. Comparison of CVs in Fig. 1A and B with that in Fig. 1C reveals that the electroreduction of propene occurs in the potential range close to that of the flat cathodic peak II (at E b 0.25 V) which appears on a Pt electrode in strongly acidified molybdate solutions after the preceding considerably lower cathodic peak I, in the potential range 0.5– 0.25 V. In our previous paper [1], we have shown that peak II corresponds to the irreversible electroreduction of singly and doubly charged mononuclear Mo(VI) cationic species (such as [MoVIO2(OH)(H2O)3]+, + [MoVIO2(H2O)4] 2 ) to the relevant protonated Mo(V)-species V ([Mo O(OH)2(H2O)3] +, [MoVO(OH)(H2O)4] 2+) and then to Mo(III) moieties ([MoIII(OH)2(H2O)4] +, [MoIII(OH)(H2O)5] 2+). The presence of the above mentioned cationic species in the molybdate solutions below pH= 0 was confirmed in earlier UV–VIS and Raman spectroscopy studies [9–11]. Analysis of CVs obtained at various acid and molybdate concentrations led to the conclusion that the amount of cationic Mo(VI) oxo-species being electroactive at E b 0.25 V, reaches a maximum in the 4 M HClO4 solution containing molybdate at a concentration near cMo(VI) ≈ 1–2 mM. In contrast, changes in the pH value of the electrolyte solution had no influence on the magnitude of the cathodic peak I, which, according to the XPS data [1], reflects the formation of a stable mixed-valent Mo(V)/Mo(VI) surface layer upon electroreduction of preadsorbed oligomeric Mo(VI) aggregates. Using XPS we have found that the latter process was continued in the potential range of peak II, in parallel with the several times faster electroreduction of cationic Mo(VI) oxo-species. Correspondingly, as depicted in Fig. 1C, when the potential scan direction was reversed at E b 0.25 V, the charge related to the actual reduction process was considerably higher than that involved in the oxidative desorption of the strongly bound Mo(V)/ Mo(VI) ad-layer, which was manifested in CVs by an anodic peak with a maximum at E ≈0.4–0.5 V. Therefore it was reasonable to suppose
M. Bełtowska-Brzezinska et al. / Electrochemistry Communications 24 (2012) 5–8
that the majority of cationic Mo(V) and Mo(III) moieties formed from the respective cationic Mo(VI) oxo-species moved from the Pt surface into the solution.
3.2. Electrocatalytic effect in the Pt/molybdenum(VI)–propene system When propene was introduced into a concentrated HClO4 solution containing Mo(VI) oxo-species and/or sodium molybdate was added to a strongly acidified solution saturated with propene, an appreciable
A
0
4
Pt / propene 1. 4 M HClO4+ 2.5 mM Mo(VI) 2. 2.5 M HClO4+ 2.5 mM Mo(VI) 3. 1 M HClO4+ 2.5 mM Mo(VI) 4. 1 M HClO4
3 -2
I / mA
2 -4 ENHE / mV
1 -6
220
Pt / propene
200 180
2.5 mM Mo(VI) 4M HClO4
160
0.1
0
100
- I / mA 1
200
300
ENHE / mV Pt/propene + 4 M HClO4 + x mM Mo(VI) = 2.5 mM 0.5 mM 1 mM 0.05 mM 0.25 mM
B 0
I / mA
-2 Mo(VI) propene
-4
propene + Mo(VI)
0
300 0
300 0
300 0
300 0
300
ENHE / mV Pt / propene+ 4M HClO4 + xM Mo(VI)
5.0
- Ipc / mA
- Ipc / mA
C
10
2.5
10
0
-1
10
-4
10
-3
cMo(VI) / M
0 0
2.5
5
cMo(VI) / mM Fig. 2. Fragment of CVs for propene on a polycrystalline Pt electrode A) in solutions 2.5 mM Na2MoO4 + HClO4 of 1) 4 M, 2) 2.5 M, and 3) 1 M. Curve 4: in 1 M HClO4 without Mo(VI), inset: Tafel plot of CV 1. B) In 4 M HClO4 with various cMo(VI) — olive lines. Propene alone — red lines. Mo(VI) alone — blue lines. C) Plot of the Ipc versus cMo(VI) in 4 M HClO4; inset: double logarithmic plot, v = 0.005 V/s.
7
rise in the reduction peak current on a polycrystalline Pt electrode, at E b 0.25 V, was noted (see Fig. 2A and B) in comparison with that characteristic of the reduction of propene (Fig. 1A and B) or molybdate (Fig. 1C) alone. However, the peak current corresponding to the electroreduction of propene in the presence of Mo(VI) was almost independent of the scan rate up to v ≈ 0.1 V/s, as predicted by the theory developed for the irreversible charge transfer reaction coupled with the irreversible non-faradaic catalytic reaction producing an initial reactant which can be re-reduced at the electrode/solution interface [12,13]. The representative CVs in Fig. 2A provide evidence that the extent of catalysis strongly depends on pH of the solution, although this parameter has no influence on the magnitude of the peak current corresponding to the electroreduction of propene in the absence of Mo(VI) oxo-species. The catalytic performance of the Pt/Mo(VI) electrode toward propene reduction achieves the highest level when using a molybdate solution with 4 M HClO4 as a supporting electrolyte. As mentioned above (Section 3.1), such H + concentration ensures the existence of a maximum amount of cationic Mo(VI) oxo-species, which undergo electroreduction to the cationic Mo(V) and Mo(III) moieties at the electrode potentials negative to 0.25 V. On the contrary, no catalytic effect was found in the molybdate solution of pH = 1 which contains solely dioxo‐ and trioxo moiVI VI eties of molybdic acid in equilibrium with oligomeric Mo12 to Mo36 aggregates [9–11]. Thus, it is obvious that the presence of cationic molybdenum oxo-species at the electrode/solution is essential for the enhanced reduction of propene at a Pt electrode. This conclusion is justified by the fact that no increase in the reaction rate was found on the Pt electrode modified with the electrochemically deposited mixed-valence Mo(V)/Mo(VI) ad-layer and transferred into the propene saturated 4 M HClO4 solution without Na2MoO4. Interestingly, the addition of propene into the acidified molybdate solution results in a considerable suppression of the current related to the formation of Mo(V)/Mo(VI) surface layer between 0.5 V and 0.25 V, i.e. prior to the beginning of the catalytic propene reduction (Fig. 2A and B). This implies that a relatively stronger competitive adsorption of propene prevents the approach of the oligomeric Mo(VI) species to the Pt surface and thus their electroreduction. Consequently, the current peak attributed to the oxidative stripping of the Mo(V)/Mo(VI) ad-layer disappears. Furthermore, we have found the same Tafel slope on the propene electroreduction (at E b 0.25 V) in the presence and in the absence of Mo(VI) oxo-species (inset in Figs. 1A and 2A). Fig. 2B illustrates the changes in the magnitude of the cathodic response related to the catalytic reduction process investigated upon increasing molybdate concentration in 4 M HClO4 solution saturated with propene. As shown in Fig. 2C, the rate of propene reduction at the potential of peak maximum (Ipc) increases linearly with increasing Mo(VI) concentration in 4 M HClO4 from cMo(VI) = 0.05 mM to 0.5 mM and approaches a limiting value at cMo(VI) ≈ 1–2 mM when the amount of cationic Mo(VI) oxo-species in the bulk solution reaches a maximum. It is evident that this Mo(VI) concentration is optimal for analytical application. The reduction rate is then by about one order of magnitude higher in comparison with that observed for propene or Mo(VI) alone. Considering the shape of the Ipc–cMo(VI) dependence, it is likely that the catalytic reduction process investigated involves the formation of an associative adduct between the cationic molybdenum species and propene. Similar regularities have been established by us also upon electroreduction of allyl alcohol of constant concentration in the presence of various amounts of Mo(VI) in 4 M HClO4 solution [1]. Unfortunately it was not possible to obtain CVs for various concentrations of propene dissolved in solution in order to establish the influence of changes in the concentration of this reactant on the catalytic current. Despite this, on the basis of the results described above, the observed catalytic effect can be explained by the continuous regeneration of the electroactive molybdenum species in a non-faradaic reaction between propene and the reduced Mo(V) and/or Mo(III) cationic species formed at the Pt/
8
M. Bełtowska-Brzezinska et al. / Electrochemistry Communications 24 (2012) 5–8
solution interface in the consecutive one and three electron-transfer reactions from the parent Mo(VI) cationic species. This explains the appearance of cathodic current in CVs after reversal of the scan direction into the positive one (Fig. 2B). It is reasonable to expect that the hydrogen inserted into the structure of Mo(V) and Mo(III) cationic species upon electroreduction facilitates the hydrogenation of propene to propane. Since the exact structure of molybdenum species being active in this reaction is unknown at present, the general reaction sequence accounting for the catalytic phenomenon in the propene–Mo(VI) system is tentatively described below for one of their possible protonated forms (see Section 3.1). The first pathway (1) involves the cationic Mo(V) oxo-species, which
4. Conclusions For the first time a remarkable catalytic activity of cationic Mo(VI) oxo-species in the electroreduction of propene on a polycrystalline Pt electrode in strongly acidic solution was demonstrated under cyclic voltammetry conditions. This phenomenon ensures an increased sensitivity of propene detection in the presence of Mo(VI) and vice versa, giving the basis for analytical application of appropriate electrochemical sensors. Acknowledgments We appreciate financial support from the Ministry of Science and Higher Education, Poland. References
(1)
(2) in the reaction with propene, giving propane, are repeatedly re-oxidized to the parent Mo(VI) species. In the second pathway (2), the formation of propane from propene in a reaction with the cationic Mo(III) oxo-species is accompanied by a continuous regeneration of the appropriate Mo(V)-species and their re-reduction to Mo(III) moieties. Studies with in situ spectroelectrochemical techniques would be needed for further comprehensive mechanistic information.
[1] M. Bełtowska-Brzezinska, T. Węsierski, T. Łuczak, J. Camra, Electrochimica Acta 63 (2012) 89. [2] A. Molina, J. Gonzalez, E. Laborda, Y. Wang, R.G. Compton, Physical Chemistry Chemical Physics 13 (2011) 16748. [3] C.H. Hamann, W. Vielstich, Electrochemie II, Verlag Chemie, Weinheim, 1981. [4] M. Bełtowska-Brzezinska, T. Łuczak, H. Baltruschat, U. Mueller, The Journal of Physical Chemistry. B 107 (2003) 4793. [5] M. Bełtowska-Brzezinska, T. Łuczak, M. Mączka, H. Baltruschat, U. Mueller, Journal of Electroanalytical Chemistry 519 (2002) 101. [6] H. Baltruschat, Differential electrochemical mass spectrometry as a tool for interfacial studies, in: A. Wieckowski (Ed.), Interfacial Electrochemistry, Marcel Dekker, New York, Basel, 1999, p. 577. [7] J.F.E. Gootzen, A.H. Wonders, W. Visscher, J.A.R. van Veen, Langmuir 13 (1997) 1659. [8] F. Scholz, Electroanalytical Methods, Springer, Berlin Heidelberg, 2010. [9] S. Himeno, H. Niiya, T. Ueda, Bulletin of the Chemical Society of Japan 70 (1997) 631. [10] J.J. Cruywagen, J.B. Heyns, Polyhedron 19 (2000) 907. [11] J.F. Ojo, R.S. Taylor, G. Sykes, Journal of the Chemical Society Dalton Transactions (1975) 500. [12] R.S. Nicholson, I. Shain, Analytical Chemistry 36 (1964) 706. [13] D.S. Polcyn, I. Shain, Analytical Chemistry 38 (1966) 376.