Article pubs.acs.org/JPCC
Modeling of Potential Oscillation during Galvanostatic Electrooxidation of Formic Acid at Platinum Electrode Dong Mei, Zheng-Da He, Dao Chuan Jiang, Jun Cai, and Yan-Xia Chen* Hefei National Laboratory for Physical Science at Microscale and Department of Chemical Physics, University of Science and Technology of China, Hefei, Anhui 230026, China S Supporting Information *
ABSTRACT: Oscillation during electrocatalytic oxidation of formic acid at Pt electrode under galvanostatic conditions in acidic electrolyte has been simulated based on a chemical model that involves three reaction pathways: (i) the indirect pathway via COad formation (formic acid dehydration) and oxidation; (ii) a formate pathway, involving adsorption− desorption of bridged adsorbed formate and its oxidation; and (iii) a direct pathway with successive cutting of O−H and C−H bonds. We found that only when the contribution of formate oxidation to the total formic acid oxidation current is negligible do the simulated results reproduce well the experimentally observed oscillatory patterns for electrode potential and the coverage of COad and bridge-bonded formate. It is found that the fast adsorption− desorption of both formate and OHad are responsible for the hidden negative impedance; the slow COad formation and its fast oxidation at higher potentials lead to a positive impedance. The simulated results further support the conclusion that bridge-bonded formate is not the reactive intermediate for the major pathway of formic acid oxidation at Pt electrode. oxidation current at both Pt film and Pt(111) electrodes,13,18 we concluded that HCOOb is not the reactive intermediate for the direct pathway in formic acid oxidation. Instead, a nonformate pathway can explain well the data for both FA oxidation obtained using conventional electrochemical and electrochemical in situ infrared (IR) spectroscopic techniques.11,13,19,20 From DFT calculation, Neurock et al. found that formate pathway has a large reaction barrier, as high as ca. 1.1 eV, which is more than two times higher than alternative direct pathway ( 0.4 V. In this time period, the decrease in the rate constant for FA oxidation is compensated by the increase in the numbers of active surface sites freed from water and formate desorption; both respond quickly to the potential change. During the galvanostatic oscillation, the evolution of formate and water coverage follows well with the time scale for potential oscillation; however, COad formation and its removal are much slower, as is consistent with the experiment result.25,30 This in turn indicates that formate and water adsorption−desorption (eqs 7 and 14) are very fast reactions, which are largely in equilibrium during such potential oscillation. Figure 7 gives the
Figure 5. Calculated time courses of (a) electrode potential E, (b) water coverage, (c) θCOL, (d) θformate, and (e) formic acid concentration at j = 0.20 mA cm−2. Parameters used for the simulation are given in Tables 1 and 2. θCOB is set at 0.1 ML.
Figure 6. Expanded time courses of (a) electrode potential E, (b) water (OHad) coverage, (c) θCOL, (d) θformate, and (e) formic acid concentration during galvanostatic potential oscillation at j = 0.20 mA cm−2. The parameters used for modeling are given in Tables 1 and 2.
During the ca. 70 s inductive period, the electrode potential value is low ( 0.3 V. On the basis of these facts, we conclude that negative regulation between E and j (dE/dj < 0) from formate adsorption and desorption is crucial for the oscillation, particularly in the potential regime from 0.6 V to Emin (ca. 0.35 V);such a picture is not in the models of the previous reports. 24,27,30 Both the OH ad adsorption and formate adsorption constitute the negative differential resistance (NDR) which is known to be a crucial prerequisite for an electrochemical instability. Figure 9 gives the mechanism of FA oscillation; the blue dashed lines in the figure indicate the negative influences, and
Figure 8. Calculated electrode potential as a function of reaction time during formic acid oxidation at Pt electrode under constant current density j of (a) 0.1, (b) 0.2, (c) 0.4, and (d) 0.5 mA cm−2. The parameters used for modeling are given in Tables 1 and 2. θCOB is set to 0.1 ML.
conditions. We found that the oscillation occurs within the current density range of 0.05 mA cm−2 < j < 0.58 mA cm−2 in which E oscillates between ca. 0.4 and 0.8 V. All the potential oscillating patterns display a certain inductive period in which the potential increases slowly from 0.42 to 0.48 V with reaction time; this is followed with a sharp increase from ca. 0.48 V to the maximum value (ca. 0.8 V) within about 2 s and a prompt drop from 0.8 V to the minimum value Emin between ca. 0.35 to 0.45 V. With an increase in FA oxidation current density, the inductive period for the potential oscillation decreases (ca. 200 s at 0.1 mA cm−2 to ca.70 s at 0.5 mA cm−2), while the minimum potential does show an obvious increase. All the calculated results are essentially in good accordance with the experimental results.24,27,30,31 It should be mentioned that although we found that the concentration of FA near electrode surface (CsHCOOH) oscillates synchronously with the potential oscillation, the amplitude is rather small (within 0.1 mM change). This indicates that in the experiment the formic acid concentration near the electrode surface almost equals its bulk concentration and the diffusion effect during the oscillations can be neglected. Because the concentration for formic acid (1 M) used in our modeling is higher than the concentration of formic acid (0.05 M) used in Strasser’s model,24 the formic acid concentration is not necessary taken as a variable in our case. Only when formic acid concentration is as low as 1 mM should the diffusion effect be considered, which conforms well to Strasser’s result.24,25 4.4. Mechanism of FA Oscillation. The minimal mechanistic requirements for formic acid oscillation are
Figure 9. Loop of formic acid oscillation at low potential and high potential. The blue dashed lines indicate negative influence, and the black solid line indicates positive influence.
the black solid line indicates the positive influence. During a period at the low potential, elongated reaction time will lead to the accumulation of COad at the electrode surface, and this in turn decreases the number of vacant sites. To maintain the constant current, the potential will increase. At high potentials, CO reacts with water (OHad), which leaves more vacant sites. To keep the current constant, the potential decreases. This is reflected in the figure 6; the CO coverage increases at the low potential and decreases at the high potential. In other words, the coexistence of a positive and a negative feedback loop in the system leads to oscillatory behavior of the galvanostatic oxidation of FA.
5. SUMMARY We have modeled oscillations during the electrochemical oxidation of formic acid at Pt electrode surface. The difference between the present model and earlier ones is that the bridgebonded formate is the site-blocking species instead of being the active intermediate and the oxidative adsorption of formic acid 6341
dx.doi.org/10.1021/jp500285j | J. Phys. Chem. C 2014, 118, 6335−6343
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Carbon Monoxide Adsorbed on Rh(111) Electrodes in Sulphuric Acid Solutions. J. Electroanal. Chem. 1997, 432, 1−5. (10) Samjeske, G.; Miki, A.; Ye, S.; Osawa, M. Mechanistic Study of Electrocatalytic Oxidation of Formic Acid at Platinum in Acidic Solution by Time-Resolved Surface-Enhanced Infrared Absorption Spectroscopy. J. Phys. Chem. B 2006, 110, 16559−16566. (11) Osawa, M.; Komatsu, K.-i.; Samjeske, G.; Uchida, T.; Ikeshoji, T.; Cuesta, A.; Gutierrez, C. The Role of Bridge-Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid on Platinum. Angew. Chem., Int. Ed. 2011, 50, 1159−1163. (12) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. J. Kinetics and Mechanism of the Electrooxidation of Formic AcidSpectroelectrochemical Studies in a Flow Cell. Angew. Chem., Int. Ed. 2006, 45, 981−985. (13) Chen, Y. X.; Heinen, M.; Jusys, Z.; Behm, R. J. Bridge-Bonded Formate: Active Intermediate or Spectator Species in Formic Acid Oxidation on a Pt Film Electrode? Langmuir 2006, 22, 10399−10408. (14) Chen, Y. X.; Ye, S.; Heinen, M.; Jusys, Z.; Osawa, M.; Behm, R. J. Application of In-Situ Attenuated Total Reflection-Fourier Transform Infrared Spectroscopy for the Understanding of Complex Reaction Mechanism and Kinetics: Formic Acid Oxidation on a Pt Film Electrode at Elevated Temperatures. J. Phys. Chem. B 2006, 110, 9534−9544. (15) Grozovski, V.; Vidal-Iglesias, F. J.; Herrero, E.; Feliu, J. M. Adsorption of Formate and Its Role as Intermediate in Formic Acid Oxidation on Platinum Electrodes. ChemPhysChem 2011, 12, 1641− 1644. (16) Grozovski, V.; Climent, V.; Herrero, E.; Feliu, J. M. Intrinsic Activity and Poisoning Rate for HCOOH Oxidation on Platinum Stepped Surfaces. Phys. Chem. Chem. Phys. 2010, 12, 8822−8831. (17) Joo, J.; Uchida, T.; Cuesta, A.; Koper, M. T.; Osawa, M. Importance of Acid−Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum. J. Am. Chem. Soc. 2013, 135, 9991−9994. (18) Xu, J.; Yuan, D.; Yang, F.; Mei, D.; Zhang, Z.; Chen, Y.-X. On the Mechanism of the Direct Pathway for Formic Acid Oxidation at a Pt(111) Electrode. Phys. Chem. Chem. Phys. 2013, 15, 4367−4376. (19) Cuesta, A.; Cabello, G.; Hartl, F. W.; Escudero-Escribano, M.; Vaz-Domínguez, C.; Kibler, L. A.; Osawa, M.; Gutiérrez, C. Electrooxidation of Formic Acid on Gold: An ATR-SEIRAS Study of the Role of Adsorbed Formate. Catal. Today 2013, 202, 79−86. (20) Cuesta, A.; Cabello, G.; Osawa, M.; Gutierrez, C. Mechanism of the Electrocatalytic Oxidation of Formic Acid on Metals. ACS Catal. 2012, 2, 728−738. (21) Neurock, M.; Janik, M.; Wieckowski, A. A First Principles Comparison of the Mechanism and Site Requirements for the Electrocatalytic Oxidation of Methanol and Formic Acid over Pt. Faraday Discuss. 2009, 140, 363−378. (22) Koper, M. T. M. Stability Study and Categorization of Electrochemical Oscillators by Impedance Spectroscopy. J. Electroanal. Chem. 1996, 409, 175−182. (23) Strasser, P.; Eiswirth, M.; Koper, M. T. M. Mechanistic Classification of Electrochemical OscillatorsAn Operational Experimental Strategy. J. Electroanal. Chem. 1999, 478, 50−66. (24) Strasser, P.; Eiswirth, M.; Ertl, G. Oscillatory Instabilities during Formic Acid Oxidation on Pt(100), Pt(110) and Pt(111) Under Potentiostatic Control. II. Model Calculations. J. Chem. Phys. 1997, 107, 991−1003. (25) Strasser, P.; Lubke, M.; Raspel, F.; Eiswirth, M.; Ertl, G. Oscillatory Instabilities during Formic Acid Oxidation on Pt(100), Pt(110) and Pt(111) Under Potentiostatic Control. I. Experimental. J. Chem. Phys. 1997, 107, 979−990. (26) Albahadily, F. N.; Schell, M. Observation of Several Different Temporal Patterns in the Oxidation of Formic Acid at a Rotating Platinum-Disk Electrode in an Acidic Medium. J. Electroanal. Chem. 1991, 308, 151−173. (27) Okamoto, H.; Tanaka, N.; Naito, M. Modelling Temporal Kinetic Oscillations for Electrochemical Oxidation of Formic Acid on Pt. Chem. Phys. Lett. 1996, 248, 289−295.
is the rate-determining step for the direct pathway. The interaction between the adsorbed carbon monoxide, water, and formate leads to the oscillation under galvanostatic conditions. The model reproduces well the experimentally observed phenomena, such as potential oscillation and the coverage oscillation of the bridge-bonded formate and COL. The kinetic parameters, such as rate constant and transfer coefficient derived from the simulation, are also very close to the values derived from experimental data. Our study further supports the conclusion that at Pt electrode FA is oxidized mainly through a nonformate pathway; the bridge-bonded formate acts as a poisoning species for both the direct and indirect pathway FA oxidation whose fast adsorption−desorption contributes greatly to the “hidden” negative impedance, which is indispensable for the occurrence of the oscillation.
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ASSOCIATED CONTENT
S Supporting Information *
Reaction rate and current for each reaction step and the calculation result of cases I and II. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Tel/Fax: +86-551-63600035. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors thank Prof. M. Koper and Prof. Z. H. Hou for invaluable discussion. This work was supported by 100 Talents’ Program of the Chinese Academy of Science, National Natural Science Foundation of China (NSFC) (Projects 20773116, 21273215), 973 program from the ministry of science and technology of China (Project 2010CB923302).
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REFERENCES
(1) Breiter, M. W. A Study of Intermediates Adsorbed on PlatinizedPlatinum during Steady-State Oxidation of Methanol Formic Acid and Formaldehyde. J. Electroanal. Chem. 1967, 14, 407−413. (2) Capon, A.; Parsons, R. Oxidation of Formic-Acid at Noble-Metal Electrodes 0.1. Review of Previous Work. J. Electroanal. Chem. 1973, 44, 1−7. (3) Beden, B.; Leger, J. M.; Lamy, C. In Modern Aspects of Electrochemistry; Bockris, J. O. M., Conway, B. E., White, R. E. , Eds.; Kluwer Adcademic/Plenum Publishers: New York, 1992, Vol. 22, 97− 263. (4) Sun, S.-G. In Studying Electrocatalytic Oxidation of Small Organic Molecules With In-Situ Infrared Spectroscopy; Lipkowski, J., Ross, P. N. (Eds.); Wiley-VCH: New York, 1998, 243−291. (5) Hamnett, A. In Interfacial Electrochemistry; Marcel Dekker Inc.: New York, 1999, 843−883. (6) Herrero, E.; Feliu, J. Encyclopedia of Electrochemistry, Volume 2, Interfacial Kinetics and Mass Transport. Wiley-VCH: Weinheim, Germany, 2003; Vol. 2, 443−465. (7) Waszczuk, P.; Crown, A.; Mitrovski, S.; Wieckowski, A. In Handbook of Fuel Cells; John Wiley & Sons: Chichester,U.K., 2003; Vol. 2, 635−651. (8) Capon, A.; Parsons, R. The Oxidation of Formic Acid on Noble Metal Electrodes: II. A Comparison of the Behaviour of Pure Electrodes. J. Electroanal. Chem. 1973, 44, 239−254. (9) Gomez, R.; Orts, J. M.; Feliu, J. M.; Clavilier, J.; Klein, L. H. The Role of Surface Crystalline Heterogeneities in the Electrooxidation of 6342
dx.doi.org/10.1021/jp500285j | J. Phys. Chem. C 2014, 118, 6335−6343
The Journal of Physical Chemistry C
Article
(28) Wolf, W.; Ye, J.; Purgand, M.; Eiswirth, M.; Doblhofer, K. Modelling the Oscillating Electrochemical Reduction of Peroxodisulfate. Ber. Bunsen-Ges. 1992, 96, 1797−1804. (29) Mukouyama, Y.; Kikuchi, M.; Samjeske, G.; Osawa, M.; Okamoto, H. Potential Oscillations in Galvanostatic Electrooxidation of Formic Acid on Platinum: A Mathematical Modeling and Simulation. J. Phys. Chem. B 2006, 110, 11912−11917. (30) Samjeske, G.; Miki, A.; Ye, S.; Yamakata, A.; Mukouyama, Y.; Okamoto, H.; Osawa, M. Potential Oscillations in Galvanostatic Electrooxidation of Formic Acid on Platinum: A Time-Resolved Surface-Enhanced Infrared Study. J. Phys. Chem. B 2005, 109, 23509− 23516. (31) Samjeske, G.; Osawa, M. Current Oscillations during Formic Acid Oxidation on a Pt Electrode: Insight into the Mechanism by Time-Resolved IR Spectroscopy. Angew. Chem., Int. Ed. 2005, 44, 5694−5698. (32) Koper, M. T. M.; Sluyters, J. H. Electrochemical Oscillators: Their Description through a Mathematical-Model. J. Electroanal. Chem. 1991, 303, 73−94. (33) Koper, M. T. M. Non-linear Phenomena in Electrochemical Systems. J. Chem. Soc. Faraday Trans. 1998, 94, 1369−1378. (34) Bard, A. J.; Faulkner, L. R. In Electrochemical Methods: Fundamentals and Applications; Wiley: New York, 1980. (35) Liao, L. W.; Liu, S. X.; Tao, Q. A.; Geng, B.; Zhang, P.; Wang, C. M.; Chen, Y. X.; Ye, S. A Method for Kinetic Study of Methanol Oxidation at Pt Electrodes by Electrochemical In Situ Infrared Spectroscopy. J. Electroanal. Chem. 2011, 650, 233−240. (36) Xu, J.; Lin, C.-H.; Mei, D.; Zhang, Z.-B.; Yuan, D.-F.; Chen, Y.X. Determination of Isotherm for Acetate and Formate Adsorption at Pt (111) Electrode by Fast Scan Voltammetry. Chin. J. Chem. Phys. 2013, 26, 191−197. (37) Grozovski, V.; Climent, V.; Herrero, E.; Feliu, J. M. Intrinsic Activity and Poisoning Rate for HCOOH Oxidation at Pt(100) and Vicinal Surfaces Containing Monoatomic (111) Steps. ChemPhysChem 2009, 10, 1922−1926. (38) Liao, L. W.; Li, M. F.; Kang, J.; Chen, D.; Chen, Y.-X.; Ye, S. Electrode Reaction Induced pH Change at the Pt Electrode/ Electrolyte Interface and Its Impact on Electrode Processes. J. Electroanal. Chem. 2013, 688, 207−215. (39) Li, M. F.; Liao, L. W.; Yuan, D. F.; Mei, D.; Chen, Y.-X. pH Effect on Oxygen Reduction Reaction at Pt(111) Electrode. Electrochim. Acta 2013, 110, 780−789. (40) Tripković, V.; Skúlason, E.; Siahrostami, S.; Nørskov, J. K.; Rossmeisl, J. The Oxygen Reduction Reaction Mechanism on Pt(111) From Density Functional Theory Calculations. Electrochim. Acta 2010, 55, 7975−7981. (41) Naito, M.; Okamoto, H.; Tanaka, N. Dynamics of Potential Oscillations in the Electrochemical Oxidation of Formic Acid on Pt. Phys. Chem. Chem. Phys. 2000, 2, 1193−1198. (42) Corrigan, D. S.; Weaver, M. J. Mechanisms of Formic Acid, Methanol, and Carbon Monoxide Electrooxidation at Platinum as Examined by Single Potential Alteration Infrared Spectroscopy. J. Electroanal. Chem. 1988, 241, 143−162. (43) Kunimatsu, K.; Kita, H. Infrared Spectroscopic Study of Methanol and Formic Acid Absorbates on a Platinum Electrode: Part II. Role of the linear CO(a) Derived from Methanol and Formic Acid in the Electrocatalytic Oxidation of CH3OH and HCOOH. J. Electroanal. Chem. Interfacial Electrochem. 1987, 218, 155−172. (44) Lu, G. Q.; Crown, A.; Wieckowski, A. Formic Acid Decomposition on Polycrystalline Platinum and Palladized Platinum Electrodes. J. Phys. Chem. B 1999, 103, 9700−9711. (45) Koper, M.; Jansen, A.; Van Santen, R.; Lukkien, J.; Hilbers, P. Monte Carlo Simulations of a Simple Model for the Electrocatalytic CO Oxidation on Platinum. J. Chem. Phys. 1998, 109, 6051−6062.
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