Journal of Advanced Research (2010) 1, 87–93
University of Cairo
Journal of Advanced Research
ORIGINAL ARTICLE
Electrocatalysis by nanoparticles: Oxidation of formic acid at manganese oxide nanorods-modified Pt planar and nanohole-arrays Mohamed S. El-Deab
*,1
Department of Chemistry, Faculty of Science, Cairo University, Cairo, Egypt
KEYWORDS Nanostructures; Nanohole-arrays; Manganese oxide nanorods; Modified surfaces; Electrocatalysis
Abstract The electro-oxidation of formic acid (an essential reaction in direct formic acid fuel cells) is a challenging process because of the deactivation of anodes by the adsorption of the poisoning intermediate carbon monoxide (CO). Pt electrodes in two geometries (planar and nanohole-array) were modified by the electrodeposition of manganese oxide nanorods (nano-MnOx). The modified Pt electrodes were then tested for their electrocatalytic activity through the electro-oxidation of formic acid in a solution of pH 3.45. Two oxidation peaks (Idp and Iind p ) were observed at 0.2 and 0.55 V, respectively; these were assigned to the direct and indirect oxidative pathways. A significant enhancement of the direct oxidation of formic acid to CO2 was observed at the modified electrodes, while the formation of the poisoning intermediate CO was suppressed. Idp increases with surface covd 1/2 erage (h) of nano-MnOx with a concurrent depression of Iind with p . An increase in the ratio Ip /m decreasing potential scan rate (m) indicates that the oxidation process proceeds via a catalytic mechanism. The modification of Pt anodes with manganese oxide nanorods results in a significant improvement of the electrocatalytic activity along with a higher tolerance to CO. Thus nano-MnOx plays a crucial role as a catalytic mediator which facilitates the charge transfer during the direct oxidation of formic acid to CO2. ª 2009 University of Cairo. All rights reserved.
* Tel.: +20 2 3567 6603. E-mail addresses:
[email protected],
[email protected] 1 Present address: Institute of Electrochemistry, University of Ulm, 89069 Ulm, Germany. Tel.: +49 731 50 25413; fax: +49 731 50 25409. 2090-1232 ª 2009 University of Cairo. All rights reserved. Peer review under responsibility of University of Cairo. Production and hosting by Elsevier
doi:10.1016/j.jare.2010.01.001
Introduction The development of durable and effective electrocatalysts is of prime importance for fuel cells. Among the various types of fuel cells, the direct formic acid fuel cell has several advantages over direct methanol fuel cell, including its high theoretical cell voltage as well as low fuel crossover [1,2]. A major drawback of such fuel cells however is the generation of poisonous reaction intermediates, particularly carbon monoxide (CO). The latter species get strongly adsorbed on the surface of the catalyst and lead eventually to its deactivation. It is well-known
88 that the electro-oxidation of formic acid on Pt proceeds via various pathways [3,4], including (i) direct oxidation to CO2 (i.e., dehydrogenation pathway) via a reactive intermediate (presumably the formate radical HCOOads [5]) and (ii) formation of a poisoning CO intermediate (i.e., dehydration pathway). In this context, several bimetallic Pt–M (M = Ru, Pd, Au, and Pb) catalysts have been studied extensively in order to enhance the activity of the direct oxidation of formic acid on the one hand and to reduce the Pt content on the other [6–9]. For instance, the deposition of Pt into Au nanorodsmodified carbon black electrodes has shown significantly higher activity towards the direct oxidation of formic acid [10]. Similarly, the modification of Pt electrodes with Fe-macrocycle compounds has been found to promote the direct oxidation pathway for formic acid [11]. Furthermore, bimetallic PtPb electrodes have been found to be catalytically more active than pure Pt towards the electro-oxidation of formic acid [12]. Over the last decade, catalysis and electrocatalysis at nanoparticles-based electrodes have attracted significant attention due to the unusual and fascinating properties of nanoparticles compared to bulk materials. These properties include high effective surface area, catalytic activity and quantum confinement [13,14]. The stimulus for the growing interest in nanoparticles can be traced to new and improved abilities to make, assemble, position, connect, image and measure the properties of nanometer-scale materials with controlled size, geometry, shape, composition, surface topography, charge and functionality for prospective use in the macroscopic real world [15]. In addition to the extraordinary catalytic activity with regards to oxygen reduction [16], Au nanoparticle-based substrates have been efficiently utilised for the hydrogenation of unsaturated organic compounds [17] as well as for the low temperature oxidation of CO [18]. The catalytic activity of nanoparticle-based electrodes is inherently related to particle size, shape, geometry, crystallographic orientation, nature of the support and the method of preparation [16,18]. These parameters are crucial for the kinetics of electrode reactions involving adsorption of intermediates [19]. Some metal oxide modified electrodes, such as Ni, Co and Mn oxide modified electrodes, have been reported to catalyse several electrochemical reactions e.g., oxygen evolution and reduction reactions. The extent of catalysis is based on the synthesis method of the oxide and the nature of the dopant such as Mo or W [20–22]. For instance, anodically deposited MnO2 has been utilised for the electrolytic evolution of oxygen gas from seawater [22]. Furthermore, MnOx modified Ru and Pt electrodes have been efficiently utilised to oxidise methanol [23,24]. The Pt nanohole-array is a typical example of a nano-scale electrode. This type of electrode brings several operational advantages over the ordinary planar electrode, including: (i) enhanced mass transport (due to the dominance of the radial diffusion), (ii) decreased charging currents and (iii) decreased deleterious effects of solution resistance. These properties render the Pt nanohole-array electrode a promising model catalyst for studying electrocatalytic reactions, such as formic acid oxidation. In this context, the present study addresses the modification of Pt surfaces (in planar and nanohole-array geometries) with manganese oxide nanorods (nano-MnOx) for obtaining high
M.S. El-Deab electrocatalytic activity towards formic acid oxidation. Accordingly, crystallographically oriented nano-MnOx (in the manganite phase, c-MnOOH [25]) was electrodeposited onto Pt substrates and were subsequently shown to enhance the direct oxidation of formic acid (to CO2) via facilitating the charge transfer, while suppressing the formation of the poisoning CO intermediate. Material and methods Pt electrodes of two geometries were used as working electrodes: (i) a planar Pt disk electrode sealed in a Teflon jacket (2.0 mm in diameter and having an exposed geometric surface area of 0.031 cm2) and (ii) a Pt nanohole-array supported on a glass substrate (4.0 mm in width, 5.0 mm in length and 0.5 mm in thickness). The nanohole-array Pt electrode had the following characteristics: an average of 200 Pt holes per lm2, each of which was 20–25 nm in diameter and 30 nm in depth. A saturated calomel electrode (SCE) and a spiral Pt wire were the reference and counter electrodes, respectively. The planar Pt electrode was mechanically polished with aqueous slurries of successively finer alumina powders (down to 0.05 lm), sonicated for 10 min in Milli-Q water, then electrochemically pretreated in 0.1 M H2SO4 solution by cycling in the potential range of 0.3 to 1.25 V vs. SCE at 50 mV s1 for 10 min or until a reproducible cyclic voltammogram (CV) characteristic for a clean Pt electrode was obtained (cf. curve a in Fig. 2A). The Pt nanohole-array electrode was subjected to the same procedure of the electrochemical treatment without mechanical polishing (cf. curve a in Fig. 2B). MnOx was electrodeposited on the surface of Pt (planar and nanohole-arrays) from an aqueous solution of 0.1 M Na2SO4 containing 0.1 M Mn(CH3COO)2 by cycling the potential at 20 mV s1 between 0.05 and 0.35 V vs. SCE [25,26]. The surface coverage h of the nano-MnOx on the Pt electrode was controlled by the number of potential cycles employed during the electrodeposition step. The values of h are listed in Table 1 for various numbers of cycles. Morphological characterisation of the prepared nanoMnOx was carried out by scanning electron microscopy (SEM) using a JSM-T220 (JEOL, Optical Laboratory, Japan) at an acceleration voltage of 12–30 kV and a working distance of 8 mm. The electrocatalytic activity of the nano-MnOx modified Pt electrodes towards formic acid oxidation was examined in a solution of 0.3 M formic acid of pH 3.45 adjusted by adding NaOH. The CVs were performed in a conventional three-electrode glass cell. All chemicals were of analytical grade and were used without further purification; all measurements were performed at room temperature; the solutions were de-oxygenated by N2 bubbling. Current densities were calculated on the basis of the geometric surface area of the Pt working electrodes. Results and discussion Characterisation of nano-MnOx/Pt electrodes Fig. 1 shows typical SEM micrographs obtained for (a) unmodified and (b) nano-MnOx modified (A) planar and (B) nanohole-array Pt electrodes. Inspection of the two images
Electrocatalysis by nanoparticles: Oxidation of formic acid
89
Table 1 Variation of Idp and Iind p for formic acid oxidation with surface coverage (h) of the nano-MnOx electrodeposited onto planar Pt electrode. The values of the real surface area (S) of the unmodified and modified Pt electrodes are also listed. No. of potential cycles employed for nano-MnOx deposition
Real surface area of Pt (S) a/cm2
Surface coverage (h)b/%
Idp /mA cm2
2 Iind p /mA cm
0 5 10 15 25
0.134 0.119 0.115 0.114 0.095
0 11 14 15 29
1.3 2.4 3.6 5.1 7.6
7.5 7.4 2.5 1.9 0.5
a
As estimated from the charge consumed during the reduction of the surface oxide monolayer (at ca. 0.4 V, Fig. 2A) using a reported value of 420 lC cm2 [28]. b The values of surface coverage (h = 1 – Smodified/Sunmodified) were calculated for the various nano-MnOx/Pt electrodes. Smodified and Sunmodified refer to the real surface area of the modified and the unmodified Pt electrodes, respectively.
Figure 1 SEM micrographs of (a) unmodified and (b) nano-MnOx modified (A) planar and (B) nanohole-array Pt electrodes. The nanoMnOx was electrodeposited as described in the experimental section by applying 50 potential cycles between 50 and 350 mV vs. SCE at 20 mV s1.
(marked b) reveals that MnOx was electrodeposited onto the Pt substrates in a porous texture composed of intersected nanorods with an average thickness of about 20 nm. This
porous texture enables the access of the solution species to the underlying substrate. XRD and the high resolution TEM patterns (data shown elsewhere [27]) show these nanorods were
90
M.S. El-Deab
identified to electrodeposit exclusively in the manganite phase (c-MnOOH). Fig. 2 shows CVs of (a) unmodified and (b) nano-MnOx modified (A) planar and (B) nanohole-array Pt electrodes in 0.1 M H2SO4. The decrease in the intensity of the cathodic peak current around 0.4 V indicates a decrease in the accessible surface area of the underlying Pt planar substrate as a result of the successful electrodeposition of the nano-MnOx on the Pt surface. The coverage of the electrodeposited nano-MnOx was estimated from the decrease of charge due to the reduction of the Pt-surface oxide using a reported value of 420 lC cm2 for a surface oxide monolayer [28]. Table 1 lists the real surface area (S) of the underlying Pt substrate and the corresponding surface coverage h of nano-MnOx electrodeposited by applying various numbers of potential cycles. Oxidation of formic acid at nano-MnOx/Pt electrodes The thus-prepared nano-MnOx/Pt electrodes were tested for their electrocatalytic activity for formic acid oxidation. Fig. 3A and B show typical CVs measured in 0.3 M formic
acid (pH 3.45) at (a) unmodified and (b) nano-MnOx modified (A) planar (h 30%). and (B) nanohole-array Pt electrodes (h 15%). Two oxidation peaks (marked as Idp and Iind p ) were observed at ca. 0.2 and 0.55 V in the forward (positive-going) scan for the unmodified Pt electrode (Fig. 3A, curve a). These two peaks were assigned to the direct oxidation of formic acid to CO2 and to the oxidation of the intermediate CO generated by the dissociative (non-faradaic) adsorption step [4]. The ratio of the two oxidation peaks (Idp /Iind p ) reflects the preferential oxidation pathway of formic acid at a particular electrode. The appearance of peak Ib in the reverse scan was attributed to the oxidative removal of the incompletely oxidised carbonaceous species formed during the forward scan. The modification of Pt (planar and nanohole-array) electrodes with nano-MnOx resulted in several significant changes (curves marked b in Fig. 3A and B): (i) Observation of a significant increase of the first peak, I dp , concurrently with a noticeable depression of the second oxidation peak, I ind p . This indicates that the direct oxidation of formic acid becomes more favourable, while less poisoning intermediate (CO) is produced.
A 0.2
A
a
Ib 15
b -2
I / mA cm
I / mA cm
-2
0 .
-0.2
10
Ipind a
5
-0.4
I pd 0
0
0.5 E / V vs. SCE
1
B
-2
0 . -20
I / mA cm
-2
0.4
0.8
1.2 1
20
I / mA cm
0
E / V vs. SCE
.
B
-0.4
b
0.8
Ib
0.6
b
0.4
b
0.2
-40
0.4 0.8 E / V vs. SCE
Ipind a
a 0
Ipd
0 -0.4
1.2
Figure 2 CVs for (a) unmodified and (b) nano-MnOx modified (A) planar (h 20%) and (B) nanohole-array (h 15%) Pt electrodes measured in 0.1 M H2SO4 at 50 mV s1.
-0.2
0 0.2 0.4 E / V vs. SCE
0.6
Figure 3 CVs for formic acid oxidation at (a) unmodified and (b) nano-MnOx modified (A) planar (h 30%) and (B) nanoholearray (h 15%) Pt electrodes at 50 mV s1 measured in 0.3 M HCOOH (pH 3.45).
Electrocatalysis by nanoparticles: Oxidation of formic acid
A 15
-2
Similar enhancement was observed at the Pt nanohole-array electrode following modification with a smaller coverage of nano-MnOx (h 15%). That is, the nano-MnOx modified Pt nanohole-array electrode (curve b, Fig. 3B) showed a significantly enhanced oxidation of formic acid at relatively lower anodic potential (ca. 0.2 V vs. SCE) with a 7-times higher peak current compared to the unmodified electrode (curve a, Fig. 3B). It is worth mentioning here that under the described experimental conditions the diffusion layers at individual Pt nanoholes overlap each other to form a linearly expanding diffusion region as can be expected by comparison of the diffup sion layer thickness (d = (pDt) 0.0336 cm; D is the diffusion coefficient of formic acid in aqueous medium (105 cm2 s1) and t is the electrolysis time (36 s)) and the average distance between Pt nanoholes (about 100 nm, see image a in Fig. 1B). As can be readily seen, d is much larger than the average distance between the Pt nanoholes and thus the CV pattern for formic acid oxidation (curve a, Fig. 3B) is similar to
that observed at the planar Pt electrode with the same geometric surface area. Table 1 shows the dependence of Idp and Iind p of formic acid on surface coverage h of nano-MnOx on planar Pt electrodes. It shows that Idp increases with h. Fig. 4 shows the variation of Idp /Iind p ratio with h. This shows that the rate of direct oxidation pathway increases strongly with h and reaches about 14-times higher than the poison formation pathway at h of about 30%. In other words, the amount of CO (produced as a poisonous reaction intermediate and remaining at the Pt active sites) is markedly reduced upon the deposition of nano-MnOx onto the Pt surface. This indicates that formic acid oxidation at the nano-MnOx/Pt electrode shifts towards the dehydrogenation pathway with increasing h. Long-term stability tests of nano-MnOx/Pt (planar and nanohole-array) electrodes were conducted by recording repetitive CVs up to 100 cycles. Reproducible CVs were obtained (data are shown in Fig. 5A and B). This demonstrates the high stability of the nano-MnOx/Pt and its high electrocatalytic performance towards the direct oxidation of formic acid. It demonstrates also that nano-MnOx/Pt has a high tolerance against CO formation.
I / mA cm
(ii) Nano-MnOx/Pt oxidises the adsorbed carbonaceous intermediates (formed during the forward scan) at lower anodic potentials (at ca. 0.13 V) than the unmodified Pt electrode (at ca. 0.3 V). This might indicate that the modified electrode maintains its high surface activity. In other words, the modified electrode has a high tolerance against the poisoning effect of CO. (iii) The I dp /Ib ratio probes the catalytic tolerance of the electrode against the formation of carbonaceous species formation [29]. A low I dp /Ib ratio indicates poor oxidation of formic acid to CO2 and excess accumulation of carbonaceous species at the electrode surface. A value of I dp /Ib of 0.85 was obtained at nano-MnOx/Pt (curve b, Fig. 3A), which is about 10 times higher than that observed at the unmodified Pt (curve a, Fig. 3A). This indicates that less intermediate carbonaceous species are produced in the forward scan at the modified Pt electrode surface than at the unmodified Pt surface. It also indicates a higher reversibility of the reaction at the modified electrode.
91
5
0 -0.5
B
20
10
0 E / V vs. SCE
0.5
1.2 1
.
10
I / mA cm
Ipd / Ipind
-2
0.8 .
0.6 0.4 0.2
0
0 -0.4
0
20
40
-0.2
0 0.2 0.4 E / V vs. SCE
0.6
Surface coverage θ / %
Figure 4 Variation of Idp /Iind ratio with surface coverage h of p nano-MnOx at planar Pt electrodes for formic acid oxidation in 0.3 M HCOOH (pH 3.45).
Figure 5 First (solid lines) and 100th (dashed lines) CV for formic acid oxidation at nano-MnOx modified (A) planar (h 30%) and (B) nanohole-array (h 15%) Pt electrodes measured in 0.3 M formic acid (pH 3.45).
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Mechanistic approach of the electrocatalytic enhancement Fig. 6A shows CVs recorded for formic acid oxidation at a nano-MnOx/Pt electrode at various potential scan rates (m). This figure shows that Idp increases with m in a rather non-linear fashion. Fig. 6B shows the variation of Idp /m1/2 ratio with m. As can be seen, Idp /m1/2 decreases with increasing m, which is a characteristic feature of catalytic reactions [30]. Thus the electrodeposited nano-MnOx plays a crucial catalytic role during the course of this reaction. A plausible involvement of the single crystalline MnOOH in the mechanistic pathway of formic acid oxidation might proceed in the following sequence. It has been reported that the non-catalysed oxidative pathway of formic acid proceeds at Pt surfaces according to [4]:
rectly formed by the oxidation of formate anions co-existing at this pH (i.e., HCOO fi HCOOads + e). The porous texture of nano-MnOx allows for Reactions (1)–(4) to proceed at the modified Pt surface. The presence of nano-MnOx would enhance the direct pathway (Eq. (3)) via a reversible oxidative transformation of MnOOH into MnO2 through a reversible proton removal process as [31]: MnOOH $ MnO2 þ Hþ þ e
ð5Þ
The produced MnO2 is involved in one (or more) of the above steps according to [32]: HCOOHads þ MnO2 ! HCOOads þ MnOOH
ð6Þ
HCOOads þ MnO2 ! MnOOH þ CO2
ð7Þ
HCOOH ! HCOOHads ðiÞ HCOOHads ! HCOOads þ Hþ þ e
ð1Þ ð2Þ
The sequential coupling of Reactions (6) and (7) results, effectively, in the generation of CO2. Thus the total reaction can be written as:
HCOOads ! CO2 þ Hþ þ e ðiiÞ HCOOHads ! H2 O þ COads
ð3Þ ð4Þ
HCOOHads þ 2MnO2 ! CO2 þ 2MnOOH
where the subscript ‘ads’ refers to the surface adsorbed species. Note that the adsorbed formate radicals (HCOOads) can be di-
A
40
30
a
Conclusion
I / mA cm
-2
b
.
d c
10
0
-0.5
0
0.5
E / V vs. SCE
B 1.5
Ipd / ν1/2
ð9Þ
This reaction shows that: (i) a re-activation of the Pt surface active sites is achieved by the oxidative removal of adsorbed CO and thus leads eventually to the high observed current (Idp ) at +0.2 V and (ii) the regeneration of MnOOH species, which are believed to participate in catalytic cycles through Reactions (5)–(9), facilitating the direct oxidation of formic acid.
e
20
i.e., the presence of manganese oxide favours the dehydrogenation pathway of formic acid oxidation. That is, one formic acid molecule requires two molecules of MnO2 to achieve a complete oxidation to CO2. This might account for the increase of Idp with increasing h (see Table 1). Another possibility for the catalytic role of manganese oxide nanorods might be assigned to the mediated oxidation of COads (produced in Reaction (4)) to CO2 with MnO2 as: COads þ MnO2 þ H2 O ! CO2 þ MnOOH þ Hþ þ e
f
ð8Þ
1
.
This paper addresses the development of a novel nano-MnOx modified Pt electrode for the electrocatalytic oxidation of formic acid with enhanced activity. Modification of Pt (planar and nanohole-array) electrodes with nano-MnOx resulted in a significant enhancement of the direct oxidation pathway for formic acid, at a rate up to 14-times higher than the indirect pathway (i.e., suppressing the generation of poisoning CO). The nano-MnOx (in the manganite phase, c-MnOOH) is believed to facilitate the oxidation of the reaction intermediates (formate radical and/or CO) into CO2 via a series of redox reactions.
0.5
Acknowledgement 0
0
100
200
300
400
Scan rate (ν) / mV s
500
-1
Figure 6 (A) CVs for formic acid oxidation at nano-MnOx/Pt planar electrode (h 30%) in 0.3 M formic acid (pH 3.45) at potential scan rate, m, of (a) 10, (b) 20, (c) 50, (d) 100, (e) 200, (f) 500 mV s1. (B) Variation of Idp /m1/2 with m.
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