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A combination of SERS and electrochemistry in Pt nanoparticle electrocatalysis: Promotion of formic acid oxidation by ethylidyne
Available online at www.sciencedirect.com
Electrochemistry Communications 10 (2008) 319–322 www.elsevier.com/locate/elecom
A combination of SERS and electrochemistry in Pt nanoparticle electrocatalysis: Promotion of formic acid oxidation by ethylidyne J. Solla-Gullo´n, R. Go´mez, A. Aldaz, J.M. Pe´rez * Departament de Quı´mica Fı´sica and Institut Universitari d’Electroquı´mica, Universitat d’Alacant, E-03080 Alacant, Spain Received 27 November 2007; accepted 11 December 2007 Available online 23 December 2007
Abstract In this paper, formic acid electrooxidation on ethylidyne modified Pt nanoparticles is reported. The formation as well as the stability electrochemical range of the ethylidyne adlayers was studied by surface enhanced Raman spectroscopy (SERS) and cyclic voltammetry. The presence of adsorbed ethylidyne on platinum nanoparticles improved their electrocatalytic activity towards formic acid oxidation, which could be attributed to an instabilization of the carbon monoxide poisonous species as evidenced by SERS. The use of in situ spectroscopic measurements with electrocatalysts similar to those applied in practice is highlighted. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pt nanoparticles; SERS; Electrocatalysis; Formic acid electrooxidation
1. Introduction The use of surface modifiers is one of the most interesting alternatives to increase the electrocatalytic activity of a metal substrate. Clear examples have been reported for formic acid and methanol electrooxidation on platinum electrodes using a wide variety of metal adsorbates [1–5] but also using adlayers of different nature such as carbon or ethylidyne [6]. Formic acid oxidation on platinum surfaces is considered a model reaction in electrocatalysis, which takes place through a widely accepted dual path mechanism in which both paths are structure sensitive [7,8]. One of them leads to the direct formation of CO2, whereas the other involves the formation of COads, which poisons the metal surface. Promotion of HCOOH and CO electrooxidation on ethylidyne modified Pt(1 1 1) electrodes has been previously reported [6]. The ethylidyne adlayers were obtained from adsorption of ethylene in vacuum forming a p(22) struc-
*
Corresponding author. E-mail address: [email protected] (J.M. Pe´rez).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.12.010
ture with a saturation coverage of 0.25 per Pt atom. On the other hand, Somorjai et al. studied the interaction, in this case in gas phase, between CO and ethylidyne when adsorbed on Pt(1 1 1) surfaces [9]. When CO was adsorbed on a ethylidyne pretreated Pt(1 1 1) surface a weakening of the CO bond was observed and explained as a d–p back donation from co-adsorbed ethylidyne molecules [9]. However, in these studies, well-defined Pt(1 1 1) surfaces have been employed. Thus, it would be particularly interesting to extend such studies to the systems similar to those employed in practical devices, such as Pt nanoparticulate electrodes. Recently, we have reported the formation of adsorbed ethylidyne during the ethylene electroreduction process by means of surface enhanced Raman spectroscopy (SERS), using the so-called nanoparticles-on-electrode approach [10]. Such an approach has been shown to be effective for the in situ detection and identification of poisons and intermediates [10–12], being this type of information of paramount importance in electrochemical kinetics and electrocatalysis. The aim of this work is to extend previous spectroelectrochemical studies as to show the effect of the presence of ethylidyne on the oxidation of formic acid.
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In our opinion, this contribution evidences how the use of in situ spectroscopic tools can guide the rational search for new electrocatalysts. 2. Experimental The platinum nanoparticles employed (diameter 4 nm) were synthesized by means of microemulsions [13–15]. For the electrocatalytic experiments a polycrystalline gold disc electrode, where nanoparticles were deposited, was used as a current collector. The counter electrode was a gold wire. Potentials were measured against a reversible hydrogen electrode (RHE). Ethylidyne adlayers were obtained by electroreduction at 0.15 V in a 0.1 M HClO4 ethylene-saturated solution at room temperature. Electrocatalytic measurements were performed in 0.1 M HClO4 + 0.1 M HCOOH solutions at room temperature. Chronoamperometric experiments were performed at 0.4 V. The electrode potential was controlled using a PGSTAT30 AUTOLAB system. SERS measurements were performed using the so-called nanoparticles-on-electrode approach [10–12]. A saturated Ag/AgCl electrode was used as a reference electrode. Raman spectra were obtained with a LabRam spectrometer (from Jobin-Yvon Horiba). The excitation line was provided by a 17 mW He–Ne laser at 632.8 nm. In this manuscript all the potentials are referred to the RHE. 3. Results and discussion Fig. 1 shows the spectroscopic behaviour of adsorbed ethylene as a function of the electrode potential, starting in the double layer region where no net reaction occurs and going into the reduction region. Representative bands for adsorbed molecular ethylene at 360 cm 1, assigned to the metal–adsorbate stretching of p-bound ethylene, and 1219 cm 1 and 1501 cm 1, assigned to ds[CH2] and m[C@C], respectively, can be observed [10]. In addition, and for electrode potentials lower than 0.26 V, bands originating from adsorbed ethylidyne and CO appear: m[Pt– CCH3] at 433 cm 1, ds[CH3] at 1337 cm 1, m[C–H] at 2888 cm 1, m[Pt–CO] at 489 cm 1 and m[CO] at 2029 cm 1. Their frequencies change (slightly) with the electrode potential but their intensities do it significantly, suggesting possible changes in their surface concentration. The maximum amount of both adsorbates, ethylidyne and CO, is detected at electrode potentials between 0.16 V and 0.11 V. Interestingly, their appearance and built-up on the electrode surface occurs simultaneously, which points to a common origin for both adsorbates. It is worth noting that differential electrochemical mass spectrometry (DEMS) studies on both polycrystalline and single crystal platinum electrodes have provided evidence about the formation of CO from the reduction of previously oxidized species coming from adsorbed ethene, with a tentative structure such as (H2C–C@O)ads [16]. Such an adsorbed intermediate could be at the origin of the simultaneous formation of both
Fig. 1. SER spectra for ethylene adsorption and reduction on Pt nanoparticles at different electrode potentials (indicated alongside) in contact with a C2H4-saturated 0.1 M HClO4 solution. Slit 200 nm and pinhole 600 nm. Adquisition time 120 s.
carbon monoxide and ethylidyne. According to the spectra in Fig. 1, both ethylidyne and CO SERS band intensities also seem to diminish in parallel as the electrode potential attains more negative values. Eventually, the surface is almost fully free of adsorbates below 0.01 V. On the other hand, the vibrational frequency of m[CO] is red-shifted by around 50 cm 1 in the presence of adsorbed ethylidyne with respect to the typical values on polycrystalline Pt [11], in agreement with Somorjai results [9]. Ethylidyne is known to be an electron donor. The local excess electron density on Pt surface in the vicinity of the ethylidyne moiety could weaken the co-adsorbed CO bond by a d–p back donation mechanism. In addition, the above quoted weakening of CO bond seems to be a prerequisite for the catalytic hydrogenation of CO. In an attempt to deepen on the study of the interaction between of the adsorbed ethylidyne and adsorbed CO, we have performed experiments in the absence of ethylene in solution. In this way, after contacting the nanostructured Pt electrode with the ethylene-saturated working solution, we have recorded a spectrum at 0.56 V. The spectrum obtained ((a) in Fig. 2) is similar to that reported in Fig. 1 at the same electrode potential and shows the representative bands for adsorbed molecular ethylene at 360 cm 1, 1219 cm 1 and 1501 cm 1. Next, the ethylene-saturated 0.1 M HClO4 solution was replaced by a 0.1 M HClO4 solu-
J. Solla-Gullo´n et al. / Electrochemistry Communications 10 (2008) 319–322
321
Fig. 3. (A) (1) Voltammetric profile of a Ptnano/Au electrode in a 0.1 M HClO4 solution. (2) Irreversibly adsorbed ethylene molecules reduction on a Ptnano/Au electrode in a C2H4-free 0.1 M HClO4 solution. Sweep rate 50 mV s 1. (B) Chronoamperometric measurements at 0.4 V for a Ptnano/ Au electrode, (1) clean surface, (2) ethylidyne modified surface (3) after reduction of the ethylidyne adlayer. Working solution: 0.1 M HClO4 + 0.1 M HCOOH.
Fig. 2. SER spectra for ethylene adsorption and reduction on Pt nanoparticles at different electrode potentials, after C2H4 adsorption, and in contact with C2H4-free 0.1 M HClO4 solution. Slit 200 nm and pinhole 600 nm. Adquisition time 120 s.
tion at open circuit potential, and a spectrum at 0.56 V was again recorded ((b) in Fig. 2). As it can be observed, the characteristic signals associated with adsorbed molecular ethylene are still observed, evidencing that ethylene is irreversibly adsorbed on the Pt nanoparticles. Subsequently the electrode potential was shifted to lower potentials. Again bands originating from adsorbed ethylidyne and CO appear for electrode potentials lower than 0.26 V. The spectra obtained at 0.21 V both in the presence (Fig. 1) and in the absence (Fig. 2) of ethylene are practically identical. However, the first consequence of the absence of ethylene in solution is the disappearance of the signal associated with adsorbed ethylene at 0.16 V whereas, in the presence of ethylene, their contributions were still observed at 0.06 V. Nevertheless, the most important observation in the absence of ethylene is that the removal of ethylidyne is accompanied by the removal of adsorbed CO. Thus, at 0 V both bands were absent whereas, in the presence of ethylene, the removal of CO was not evident in the same potential range and the CO contributions were observed even at 0.04 V. However, at this point there is no enough information to specify the exact nature of the reaction that takes place between both adsorbates. The formation of an irreversible ethylene adlayer, as well as its electrochemical stability was studied in parallel voltammetric experiments shown in Fig. 3A. Curve 1 shows the characteristic voltammetric profile of the gold-supported
Pt nanoparticles in 0.1 M HClO4. The voltammogram is virtually identical to that reported for polycrystalline platinum electrodes [17]. In addition, the definition and the symmetry of the adsorption states are indicative of surface cleanliness. Ethylene molecules were adsorbed in an ethylene-saturated working solution at 0.6 V. Subsequent removal of ethylene from solution was performed by bubbling Ar for 20 min at 0.6 V. Curve 2 shows a cyclic voltammogram corresponding to the reduction of irreversibly adsorbed ethylene molecules. The characteristic peak associated with the ethylene reduction on Pt at 0.11 V [10,16] appears, although with an intensity lower than that found when C2H4 was present in solution [10]. Furthermore, a broad oxidation wave peaking at 0.8 V appears which was previously attributed to the oxidation of ethene to finally yield CO2 [10,16]. As observed, the species resulting from ethylene adsorption are stable in the potential range between 0.2 V and 0.6 V. This opens up the possibility of exploring the electrocatalytic activity of Pt nanoparticles modified by adsorbed species coming from the reduction of C2H4, ethylidyne for instance. Fig. 3B shows the chronoamperometric results obtained before (curve 1) and after (curve 2) generation of ethylidyne moieties on the surface of the Pt nanoparticles. As observed, a clear enhancement on the electrooxidation current density is observed. After 600 s, a current density of 83 lA cm 2 is obtained with the ethylidyne modified Pt nanoparticles whereas a value of 49 lA cm 2 is obtained with the unmodified Pt nanoparticles. Thus, the presence of ethylidyne seems to favour the direct oxidation of formic acid to CO2 avoiding the formation of CO as poison intermediate. In such a way, higher current density can be obtained after the modification of the Pt surface. In addition, it is worth noting that once the adsorbed ethylidyne molecules are removed from the surface through their electrochemical reduction at potentials lower that 0.1 V,
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the Pt nanoparticles recover their intrinsic electrocatalytic properties (curve 3 in Fig. 3B (49 lA cm 2)). This fact again indicates that the presence of ethylidyne molecules does not alter in an irreversible way the surface structure of the Pt nanoparticles used as substrates. More work is needed to identify the promotion mechanism. In addition to the third body effect, the instabilization of adsorbed CO caused by the presence of co-adsorbed ethylidyne could also play a role. Such an instabilization is suggested by parallel SERS experiments. 4. Conclusions In summary, we have shown how in situ spectroscopic measurements with electrodes similar to those applicable in practice can guide the design of new electrocatalysts by surface modification (as in the present case). Thus, from the information gained in the SERS experiments, we have reported a novel and easy electrochemical procedure to modify the surface of Pt nanoparticles with ethylidyne. This electrochemical procedure avoids the use of UHV equipment as in previous papers. In addition, we have reported its potential application in the promotion of formic acid electrooxidation. The enhancement observed on the electrooxidation of formic acid on the ethylidyne modified Pt nanoparticles could be attributed to an instabilization of the carbon monoxide poisonous species as evidenced by SERS. Acknowledgments This work has been financially supported by MEC-FEDER Projects No. CTQ2006-04071/BQU and CTQ2006–09868. We are also grateful to the SS.TT.II of the University of Alicante.
References [1] J.M. Feliu, E. Herrero, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cells. Fundamentals Technology and Application, vol. 2, John Wiley & Sons Ltd., Chichester, UK, 2003, p. 625. [2] P. Waszczuk, A. Crown, S. Mitrovski, A. Wieckoski, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cells. Fundamentals Technology and Application, vol. 2, John Wiley & Sons Ltd., Chichester, UK, 2003, p. 635. [3] J. Clavilier, A. Ferna´ndez-Vega, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 258 (1989) 89. [4] J.M. Feliu, A. Ferna´ndez-Vega, J.M. Orts, A. Aldaz, J. Chim. Phys. 88 (1991) 1493. [5] M.D. Macia´, E. Herrero, J.M. Feliu, A. Aldaz, Electrochim. Acta 47 (2002) 3653. [6] D.E. Sauer, R.L. Borup, E.M. Stuve, ACS Sym. Ser. 656 (1997) 283. [7] A. Capon, R. Parsons, J. Electroanal. Chem. 44 (1973) 1. [8] A. Capon, R. Parsons, J. Electroanal. Chem. 45 (1973) 205. [9] P. Chen, S. Westerberg, K.Y. Kung, J. Grunes, G.A. Somorjai, Appl. Catal. A 229 (2002) 147. [10] R. Go´mez, J. Solla-Gullo´n, J.M. Pe´rez, A. Aldaz, ChemPhysChem 6 (2005) 2017. [11] R. Go´mez, J.M. Pe´rez, J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Phys. Chem. B 108 (2004) 9943. [12] R. Go´mez, J. Solla-Gullo´n, J.M. Pe´rez, A. Aldaz, J. Raman Spectrosc. 36 (2005) 613. [13] J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Clavilier, J. Electronal. Chem. 491 (2000) 69. [14] J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Clavilier, J. Electrochem. Soc. 150 (2003) E104. [15] J. Solla-Gullo´n, A. Rodes, V. Montiel, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 554 (2003) 273. [16] Th. Lo¨ffler, H. Baltruschat, J. Electronal. Chem. 554–555 (2003) 333. [17] R. Woods, in: Allen J. Bard (Ed.), Electroanalytical Chemistry, vol. 9, Marcel Dekker, New York, 1976, p. 1.
Electrochemistry Communications 10 (2008) 319–322 www.elsevier.com/locate/elecom
A combination of SERS and electrochemistry in Pt nanoparticle electrocatalysis: Promotion of formic acid oxidation by ethylidyne J. Solla-Gullo´n, R. Go´mez, A. Aldaz, J.M. Pe´rez * Departament de Quı´mica Fı´sica and Institut Universitari d’Electroquı´mica, Universitat d’Alacant, E-03080 Alacant, Spain Received 27 November 2007; accepted 11 December 2007 Available online 23 December 2007
Abstract In this paper, formic acid electrooxidation on ethylidyne modified Pt nanoparticles is reported. The formation as well as the stability electrochemical range of the ethylidyne adlayers was studied by surface enhanced Raman spectroscopy (SERS) and cyclic voltammetry. The presence of adsorbed ethylidyne on platinum nanoparticles improved their electrocatalytic activity towards formic acid oxidation, which could be attributed to an instabilization of the carbon monoxide poisonous species as evidenced by SERS. The use of in situ spectroscopic measurements with electrocatalysts similar to those applied in practice is highlighted. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Pt nanoparticles; SERS; Electrocatalysis; Formic acid electrooxidation
1. Introduction The use of surface modifiers is one of the most interesting alternatives to increase the electrocatalytic activity of a metal substrate. Clear examples have been reported for formic acid and methanol electrooxidation on platinum electrodes using a wide variety of metal adsorbates [1–5] but also using adlayers of different nature such as carbon or ethylidyne [6]. Formic acid oxidation on platinum surfaces is considered a model reaction in electrocatalysis, which takes place through a widely accepted dual path mechanism in which both paths are structure sensitive [7,8]. One of them leads to the direct formation of CO2, whereas the other involves the formation of COads, which poisons the metal surface. Promotion of HCOOH and CO electrooxidation on ethylidyne modified Pt(1 1 1) electrodes has been previously reported [6]. The ethylidyne adlayers were obtained from adsorption of ethylene in vacuum forming a p(22) struc-
*
Corresponding author. E-mail address: [email protected] (J.M. Pe´rez).
1388-2481/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.elecom.2007.12.010
ture with a saturation coverage of 0.25 per Pt atom. On the other hand, Somorjai et al. studied the interaction, in this case in gas phase, between CO and ethylidyne when adsorbed on Pt(1 1 1) surfaces [9]. When CO was adsorbed on a ethylidyne pretreated Pt(1 1 1) surface a weakening of the CO bond was observed and explained as a d–p back donation from co-adsorbed ethylidyne molecules [9]. However, in these studies, well-defined Pt(1 1 1) surfaces have been employed. Thus, it would be particularly interesting to extend such studies to the systems similar to those employed in practical devices, such as Pt nanoparticulate electrodes. Recently, we have reported the formation of adsorbed ethylidyne during the ethylene electroreduction process by means of surface enhanced Raman spectroscopy (SERS), using the so-called nanoparticles-on-electrode approach [10]. Such an approach has been shown to be effective for the in situ detection and identification of poisons and intermediates [10–12], being this type of information of paramount importance in electrochemical kinetics and electrocatalysis. The aim of this work is to extend previous spectroelectrochemical studies as to show the effect of the presence of ethylidyne on the oxidation of formic acid.
320
J. Solla-Gullo´n et al. / Electrochemistry Communications 10 (2008) 319–322
In our opinion, this contribution evidences how the use of in situ spectroscopic tools can guide the rational search for new electrocatalysts. 2. Experimental The platinum nanoparticles employed (diameter 4 nm) were synthesized by means of microemulsions [13–15]. For the electrocatalytic experiments a polycrystalline gold disc electrode, where nanoparticles were deposited, was used as a current collector. The counter electrode was a gold wire. Potentials were measured against a reversible hydrogen electrode (RHE). Ethylidyne adlayers were obtained by electroreduction at 0.15 V in a 0.1 M HClO4 ethylene-saturated solution at room temperature. Electrocatalytic measurements were performed in 0.1 M HClO4 + 0.1 M HCOOH solutions at room temperature. Chronoamperometric experiments were performed at 0.4 V. The electrode potential was controlled using a PGSTAT30 AUTOLAB system. SERS measurements were performed using the so-called nanoparticles-on-electrode approach [10–12]. A saturated Ag/AgCl electrode was used as a reference electrode. Raman spectra were obtained with a LabRam spectrometer (from Jobin-Yvon Horiba). The excitation line was provided by a 17 mW He–Ne laser at 632.8 nm. In this manuscript all the potentials are referred to the RHE. 3. Results and discussion Fig. 1 shows the spectroscopic behaviour of adsorbed ethylene as a function of the electrode potential, starting in the double layer region where no net reaction occurs and going into the reduction region. Representative bands for adsorbed molecular ethylene at 360 cm 1, assigned to the metal–adsorbate stretching of p-bound ethylene, and 1219 cm 1 and 1501 cm 1, assigned to ds[CH2] and m[C@C], respectively, can be observed [10]. In addition, and for electrode potentials lower than 0.26 V, bands originating from adsorbed ethylidyne and CO appear: m[Pt– CCH3] at 433 cm 1, ds[CH3] at 1337 cm 1, m[C–H] at 2888 cm 1, m[Pt–CO] at 489 cm 1 and m[CO] at 2029 cm 1. Their frequencies change (slightly) with the electrode potential but their intensities do it significantly, suggesting possible changes in their surface concentration. The maximum amount of both adsorbates, ethylidyne and CO, is detected at electrode potentials between 0.16 V and 0.11 V. Interestingly, their appearance and built-up on the electrode surface occurs simultaneously, which points to a common origin for both adsorbates. It is worth noting that differential electrochemical mass spectrometry (DEMS) studies on both polycrystalline and single crystal platinum electrodes have provided evidence about the formation of CO from the reduction of previously oxidized species coming from adsorbed ethene, with a tentative structure such as (H2C–C@O)ads [16]. Such an adsorbed intermediate could be at the origin of the simultaneous formation of both
Fig. 1. SER spectra for ethylene adsorption and reduction on Pt nanoparticles at different electrode potentials (indicated alongside) in contact with a C2H4-saturated 0.1 M HClO4 solution. Slit 200 nm and pinhole 600 nm. Adquisition time 120 s.
carbon monoxide and ethylidyne. According to the spectra in Fig. 1, both ethylidyne and CO SERS band intensities also seem to diminish in parallel as the electrode potential attains more negative values. Eventually, the surface is almost fully free of adsorbates below 0.01 V. On the other hand, the vibrational frequency of m[CO] is red-shifted by around 50 cm 1 in the presence of adsorbed ethylidyne with respect to the typical values on polycrystalline Pt [11], in agreement with Somorjai results [9]. Ethylidyne is known to be an electron donor. The local excess electron density on Pt surface in the vicinity of the ethylidyne moiety could weaken the co-adsorbed CO bond by a d–p back donation mechanism. In addition, the above quoted weakening of CO bond seems to be a prerequisite for the catalytic hydrogenation of CO. In an attempt to deepen on the study of the interaction between of the adsorbed ethylidyne and adsorbed CO, we have performed experiments in the absence of ethylene in solution. In this way, after contacting the nanostructured Pt electrode with the ethylene-saturated working solution, we have recorded a spectrum at 0.56 V. The spectrum obtained ((a) in Fig. 2) is similar to that reported in Fig. 1 at the same electrode potential and shows the representative bands for adsorbed molecular ethylene at 360 cm 1, 1219 cm 1 and 1501 cm 1. Next, the ethylene-saturated 0.1 M HClO4 solution was replaced by a 0.1 M HClO4 solu-
J. Solla-Gullo´n et al. / Electrochemistry Communications 10 (2008) 319–322
321
Fig. 3. (A) (1) Voltammetric profile of a Ptnano/Au electrode in a 0.1 M HClO4 solution. (2) Irreversibly adsorbed ethylene molecules reduction on a Ptnano/Au electrode in a C2H4-free 0.1 M HClO4 solution. Sweep rate 50 mV s 1. (B) Chronoamperometric measurements at 0.4 V for a Ptnano/ Au electrode, (1) clean surface, (2) ethylidyne modified surface (3) after reduction of the ethylidyne adlayer. Working solution: 0.1 M HClO4 + 0.1 M HCOOH.
Fig. 2. SER spectra for ethylene adsorption and reduction on Pt nanoparticles at different electrode potentials, after C2H4 adsorption, and in contact with C2H4-free 0.1 M HClO4 solution. Slit 200 nm and pinhole 600 nm. Adquisition time 120 s.
tion at open circuit potential, and a spectrum at 0.56 V was again recorded ((b) in Fig. 2). As it can be observed, the characteristic signals associated with adsorbed molecular ethylene are still observed, evidencing that ethylene is irreversibly adsorbed on the Pt nanoparticles. Subsequently the electrode potential was shifted to lower potentials. Again bands originating from adsorbed ethylidyne and CO appear for electrode potentials lower than 0.26 V. The spectra obtained at 0.21 V both in the presence (Fig. 1) and in the absence (Fig. 2) of ethylene are practically identical. However, the first consequence of the absence of ethylene in solution is the disappearance of the signal associated with adsorbed ethylene at 0.16 V whereas, in the presence of ethylene, their contributions were still observed at 0.06 V. Nevertheless, the most important observation in the absence of ethylene is that the removal of ethylidyne is accompanied by the removal of adsorbed CO. Thus, at 0 V both bands were absent whereas, in the presence of ethylene, the removal of CO was not evident in the same potential range and the CO contributions were observed even at 0.04 V. However, at this point there is no enough information to specify the exact nature of the reaction that takes place between both adsorbates. The formation of an irreversible ethylene adlayer, as well as its electrochemical stability was studied in parallel voltammetric experiments shown in Fig. 3A. Curve 1 shows the characteristic voltammetric profile of the gold-supported
Pt nanoparticles in 0.1 M HClO4. The voltammogram is virtually identical to that reported for polycrystalline platinum electrodes [17]. In addition, the definition and the symmetry of the adsorption states are indicative of surface cleanliness. Ethylene molecules were adsorbed in an ethylene-saturated working solution at 0.6 V. Subsequent removal of ethylene from solution was performed by bubbling Ar for 20 min at 0.6 V. Curve 2 shows a cyclic voltammogram corresponding to the reduction of irreversibly adsorbed ethylene molecules. The characteristic peak associated with the ethylene reduction on Pt at 0.11 V [10,16] appears, although with an intensity lower than that found when C2H4 was present in solution [10]. Furthermore, a broad oxidation wave peaking at 0.8 V appears which was previously attributed to the oxidation of ethene to finally yield CO2 [10,16]. As observed, the species resulting from ethylene adsorption are stable in the potential range between 0.2 V and 0.6 V. This opens up the possibility of exploring the electrocatalytic activity of Pt nanoparticles modified by adsorbed species coming from the reduction of C2H4, ethylidyne for instance. Fig. 3B shows the chronoamperometric results obtained before (curve 1) and after (curve 2) generation of ethylidyne moieties on the surface of the Pt nanoparticles. As observed, a clear enhancement on the electrooxidation current density is observed. After 600 s, a current density of 83 lA cm 2 is obtained with the ethylidyne modified Pt nanoparticles whereas a value of 49 lA cm 2 is obtained with the unmodified Pt nanoparticles. Thus, the presence of ethylidyne seems to favour the direct oxidation of formic acid to CO2 avoiding the formation of CO as poison intermediate. In such a way, higher current density can be obtained after the modification of the Pt surface. In addition, it is worth noting that once the adsorbed ethylidyne molecules are removed from the surface through their electrochemical reduction at potentials lower that 0.1 V,
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the Pt nanoparticles recover their intrinsic electrocatalytic properties (curve 3 in Fig. 3B (49 lA cm 2)). This fact again indicates that the presence of ethylidyne molecules does not alter in an irreversible way the surface structure of the Pt nanoparticles used as substrates. More work is needed to identify the promotion mechanism. In addition to the third body effect, the instabilization of adsorbed CO caused by the presence of co-adsorbed ethylidyne could also play a role. Such an instabilization is suggested by parallel SERS experiments. 4. Conclusions In summary, we have shown how in situ spectroscopic measurements with electrodes similar to those applicable in practice can guide the design of new electrocatalysts by surface modification (as in the present case). Thus, from the information gained in the SERS experiments, we have reported a novel and easy electrochemical procedure to modify the surface of Pt nanoparticles with ethylidyne. This electrochemical procedure avoids the use of UHV equipment as in previous papers. In addition, we have reported its potential application in the promotion of formic acid electrooxidation. The enhancement observed on the electrooxidation of formic acid on the ethylidyne modified Pt nanoparticles could be attributed to an instabilization of the carbon monoxide poisonous species as evidenced by SERS. Acknowledgments This work has been financially supported by MEC-FEDER Projects No. CTQ2006-04071/BQU and CTQ2006–09868. We are also grateful to the SS.TT.II of the University of Alicante.
References [1] J.M. Feliu, E. Herrero, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cells. Fundamentals Technology and Application, vol. 2, John Wiley & Sons Ltd., Chichester, UK, 2003, p. 625. [2] P. Waszczuk, A. Crown, S. Mitrovski, A. Wieckoski, in: W. Vielstich, A. Lamm, H.A. Gasteiger (Eds.), Handbook of Fuel Cells. Fundamentals Technology and Application, vol. 2, John Wiley & Sons Ltd., Chichester, UK, 2003, p. 635. [3] J. Clavilier, A. Ferna´ndez-Vega, J.M. Feliu, A. Aldaz, J. Electroanal. Chem. 258 (1989) 89. [4] J.M. Feliu, A. Ferna´ndez-Vega, J.M. Orts, A. Aldaz, J. Chim. Phys. 88 (1991) 1493. [5] M.D. Macia´, E. Herrero, J.M. Feliu, A. Aldaz, Electrochim. Acta 47 (2002) 3653. [6] D.E. Sauer, R.L. Borup, E.M. Stuve, ACS Sym. Ser. 656 (1997) 283. [7] A. Capon, R. Parsons, J. Electroanal. Chem. 44 (1973) 1. [8] A. Capon, R. Parsons, J. Electroanal. Chem. 45 (1973) 205. [9] P. Chen, S. Westerberg, K.Y. Kung, J. Grunes, G.A. Somorjai, Appl. Catal. A 229 (2002) 147. [10] R. Go´mez, J. Solla-Gullo´n, J.M. Pe´rez, A. Aldaz, ChemPhysChem 6 (2005) 2017. [11] R. Go´mez, J.M. Pe´rez, J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Phys. Chem. B 108 (2004) 9943. [12] R. Go´mez, J. Solla-Gullo´n, J.M. Pe´rez, A. Aldaz, J. Raman Spectrosc. 36 (2005) 613. [13] J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Clavilier, J. Electronal. Chem. 491 (2000) 69. [14] J. Solla-Gullo´n, V. Montiel, A. Aldaz, J. Clavilier, J. Electrochem. Soc. 150 (2003) E104. [15] J. Solla-Gullo´n, A. Rodes, V. Montiel, A. Aldaz, J. Clavilier, J. Electroanal. Chem. 554 (2003) 273. [16] Th. Lo¨ffler, H. Baltruschat, J. Electronal. Chem. 554–555 (2003) 333. [17] R. Woods, in: Allen J. Bard (Ed.), Electroanalytical Chemistry, vol. 9, Marcel Dekker, New York, 1976, p. 1.