포름산의 전기화학적 산화시 백금촉매에서의 전극계면 전위의 삼차원 형상연구

Theories and Applications of Chem. Eng., 2002, Vol. 8, No. 1

화학공학의 이론과 응용 제8권 제1호 2002년

이재영, 전희동

포항산업과학연구원, 환경에너지 연구센터, 수질환경연구팀

Spatiotemporal Pattern Formation of Interfacial Potential in the Electrocatalytic Oxidation of HCOOH on a Pt electrode

Jaeyoung Lee, Heedong Chun

Water Protection Research Team, Research Institute of Industrial Science & Technology

서론

The occurrence of temporally self-organised dynamical states immediately raises the question of spontaneously generated spatial inhomogeneities in the catalytic activity of the electrode. Obviously, studying the relation between the integral temporal behaviour and the corresponding spatial distribution of the interfacial potential, that is the local profile of the electrocatalytic activity, is important for the understanding of electrochemical instabilities. Thus, while earlier studies evidenced pattern formation of inhomogeneous interfacial potential in electrodissolution reactions, the research has been recently addressing pattern formations in electrocatalysis. Detailed theoretical descriptions of spatiotemporal electrochemical patterns provided valuable guidance for the understanding of spatiotemporal instability.

The electrocatalytic oxidation of HCOOH on Pt in the presence of foreign adatoms to be explained in this chapter is one of the most studied subjects experimentally and theoretically in electrocatalysis of organic oxidations. This reaction may be considered as a model to understand the oxidation of more complex organic species (e.g., methanol) on a Pt electrode, since it produces carbon dioxide and interfacial electrons that are of great interest in direct fuel cell applications.

In this study, after a short outline of the mechanistic origin of current oscillations in the electrocatalytic oxidation of formic acid (HCOOH) on a Pt electrode modified by Bi deposition, the investigation of spatiotemporal patterns in the interfacial potentials corresponding to temporal oscillatory behaviours will be followed. System parameters such as applied potential or mass-transport conditions could induce the primary interest of the distribution of interfacial potential along the ring-shaped media. We then explain experiments and computer simulations of an electrochemical system where the direction of a pulse can be reversed at will using an external perturbation [Lee et al., 2001].

실험

Figure 1 shows the schematic experimental setup for monitoring of the local potential distributions at electrochemical interfaces studied. The electrochemical cell body consisted of a glass cylinder capped with a Teflon lid holding all electrodes. A smooth polycrystalline Pt ring with inner diameter of 34.5 mm and with outer diameter of 40.5 mm (thickness of 0.1 mm) was used as working electrode (WE). The geometric area of the WE was 7 cm2. A concentric platinized Pt wire ring of 70 mm diameter (1 mm wire thickness) was used as counter electrode (CE). The tip of a Luggin-Haber capillary hosting a Hg/Hg2SO4, saturated K2SO4 reference electrode (RE) was placed in the centre of the ring WE.

A home-built potentiostat/scan generator (ELAB of Fritz-Haber-Institut) was used for all cyclic voltammetry (CV) and chronoamperometry (CA) experiments. Galvanostatic experiments were performed by means of a bi-potentiostat (EG&G, Model 366).

Figure 1. Schematic diagram of experimental setup for the measurements of HCOOH oxidation on a Pt ring electrode.

All solutions were prepared with ultrapure water (Millipore Milli(Q water, 18 M□ cm).  Prior to the experiment, the Pt ring WE was first subjected to cleaning with acetone and pure water in an ultrasonic bath followed by chemical cleaning in a mixture of conc. H2SO4 (Merck, suprapur) : 30 % H2O2 (1 : 1).

For all experiments involving formic acid oxidation, a deaerated solution mixture of 0.1 M HCOONa (Merck, p.a.) and 0.033 M H2SO4 (Merck, suprapur) was employed as the electrolyte. The formic acid buffer solution has a constant bulk pH value of 2.85 and a resistivity of 107 □ cm. After the experiments, the WE was treated in conc. HClO4 (Merck, suprapur) and conc. HNO3 (Merck, suprapur) in order to completely remove Bi complex residuals.

Bi deposition modifications of the Pt WE in this work was done with under potential deposition of Bi. In other words, Bi ions were present in the bulk electrolyte throughout the entire experiment. A 1×10-3 M Bi3+ containing solution was prepared by dissolution of high-purity Bi (III) oxide (Bi2O3, Strem Chemicals Inc., 99.9998 %) in 0.5 M HClO4 (Merck, suprapur). Appropriate amounts of the 1×10-3 M Bi3+ solution were added to the electrolyte to obtain final concentrations of Bi3+ ranging from 1×10-5 M to 1×10-7 M.

결과 및 토론

Oscillatory formic acid oxidation on Pt modified by Bi deposition

Figure 2 illustrates the effect of Bi adatoms on the formic acid oxidation on a Pt electrode. Figure 2(a) shows the typical cyclic voltammetry profile (dotted line) on an pure Pt electrode, while an about five times higher electro-oxidation current of HCOOH on the Bi-modified Pt electrode is observed (solid line). According to previous studies, this higher electrocatalytic activity of Bi/Pt for HCOOH oxidation can be due to two modification effects of Bi adatoms such as a third-body effect on a Pt (100) and an electronic effect on a Pt(111). In this work, polycrystalline Pt was used, and thus one can speculate that the enhanced current may be the result of both steric hindrance of the formation of poisoning species (CO) and intrinsic kinetic enhancement. The value of the oscillatory current related with HCOOH oxidation is significantly higher than that on pure Pt. This oscillation is observed over a broad potential region between +110mV to +280mV and it indicates periodic activating and passivating processes of the electrode. This result is in good agreement with previous studies that the current for HCOOH oxidation may increase by up to a factor of 50. The oscillatory cyclic voltammetry was found to be reproducible for many successive voltammetric cycles.

Figure 2. (a) Cyclic voltammetry of electro-oxidation of formic acid in the absence of Bi3+ (dotted line) and in the presence of Bi ions of 1×10-6 M Bi3+ (solid line). (b) IR correction of CV of Figure 2(a) in order to allow comparison with the potentials of Bi deposition/dissolution. Electrolyte is 0.1 M HCOONa/0.033 M H2SO4 and scan rate is 5 mV/s. The inset schematically shows that oscillatory systems can exhibit an advantage in the current yield. Extrapolation of the stationary current suggests that this may be the case in the present system.

The potential parameter dependence of the current oscillations exhibits the following interesting features. Current oscillations with high frequency and small amplitude abruptly set in on the left flank of the peak on the anodic scan. At the onset of oscillations, a stationary current-potential curve with positive slope (Figure 2(a)-I) is obtained, while at higher anodic overpotential, an current-potential profile with negative slope (negative differential resistance) shows higher amplitude and smaller frequency oscillation (Figure 2(a)-II). Analysis of Figure 4.9 suggests that the formation of Bi surface oxides leads to a relatively rapid decline in the high currents between +220 mV and +280 mV. On the other hand, a very high coverage of metallic Bi (below -200 mV) decreases the oxidation current, since Bi on the Pt electrode inhibits the adsorption of HCOOH. Interestingly, as sketched in Figure 2 (see inset), the average value (〈Iosc〉) of the oscillatory current during HCOOH oxidation on Bi/Pt electrode is higher than that of the (extrapolated) stationary operation points at the same applied potential. Accordingly, direct formic acid or methanol oxidation under autonomous oscillatory conditions and/or by applying external pulses can lead to an increased power output.

A correlation of oscillatory voltammetry in Figure 2(a) and Bi redox features in the solution without formic acid may offer valuable clues as to the chemical surface processes that occur under oscillatory conditions. However, a direct comparison of Figure 2(a) with CV profiles without formic acid on Bi-UPD Pt is not possible due to the uncompensated solution resistance. Thus, corrected for the ohmic potential drop (IRsolution) the curve plotted as solid line in Figure 2(b) refers to the true interfacial potential. In Figure 2(b) (1×10-6 M Bi3+), one recognises the current step to occur near the cathodic onset of the second plateau (II), which marks the beginning of Bi oxidation on the surface.

Pattern sequences on the cyclic voltammetry

Figure 3(a) shows the temporal signal of current oscillations during a cyclic voltammetry (CV) with a scan rate of 5 mV/s. The behaviour in the time domain reveals a transition from harmonic large-amplitude oscillations to a complex pattern of smaller peaks. The spatial patterns underlying the anodic scan of Figure 3(a) are reported in Figures 3(b)-3(d). At the beginning of the oscillations, the potential distribution in Figure 3(b) clearly represents standing waves (SW), while travelling pulses prevail at higher overpotentials (Figure 3(d)). When reversing the anodic scan within a pulse regime, the pulse velocity first became more and more inhomogeneous along the ring. Then, the spatiotemporal dynamics passed through the transition state, before it came back to SW. Oscillatory standing waves (SW) were recently described in the electrocatalytic oxidation of formic acid on unmodified Pt electrodes. It is characterised by the sustained periodic alternation of an active (high interfacial potentials) and a passive (low interfacial potentials) domain in space-time without spatial propagation. However, these standing waves do not seem to arise from the same bifurcation as in Figure 3; most likely, they are caused by propagation failure of pulses due to surface inhomogeneities.

화학공학의 이론과 응용 제8권 제1호 2002년

Figure 3. (a) Oscillatory current-potential profile during slow anodic scanning. The change in the waveform near t = 74.5 s indicates changes in the spatial dynamics. Figures 3(b) - 3(d). Spatiotemporal profiles of the interfacial potential corresponding to Figure 3(a). Figure 3(b): oscillatory standing wave (SW) regime, Figure 3(c): pattern transformation from SW to Pulse and Figure 3(d): travelling pulses. Scan rate is 5 mV/s and the electrolyte is 0.1 M HCOONa/0.033M H2SO4 with 5×10-7 M Bi3+.

참고문헌

Lee, J., "Electro-oxidation of small organic molecules," Ph. D. Thesis, Fritz-Haber-Institut der

Max-Planck-Gesellschaft and Freie Universit□t Berlin, 2001.

Lee, J., Christoph, J., Strasser, P., Eiswirth, M. and Ertl, G., "Spatio-temporal interfacial

potential patterns during the electrocatalyzed oxidation of formic acid on Bi-modified Pt,"

J. Chem. Phys., 115, 1485 (2001).

Lee, J., Christoph, J., Eiswirth, M. and Ertl, G., "Controlled Pulse Reversal on a Pt Ring

Electrode," Chem. Phys. Lett., 346, 246 (2001).