New Oscillatory Electrocatalytic Oxidation of Amino Compounds on a

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J. Phys. Chem. C 2007, 111, 16902-16908

New Oscillatory Electrocatalytic Oxidation of Amino Compounds on a Nanoporous Film Electrode of Electrodeposited Nickel Hydroxide Nanoflakes Wei Huang,† Jufang Zheng,‡ and Zelin Li*,‡ Key Laboratory of Chemical Biology and Traditional Chinese Medicine Research (Ministry of Education of China), College of Chemistry and Chemical Engineering, Hunan Normal UniVersity, Changsha 410081, China, and Zhejiang Key Laboratory for ReactiVe Chemistry on Solid Surfaces, Institute of Physical Chemistry, Zhejiang Normal UniVersity, Jinhua 321004, China ReceiVed: May 20, 2007; In Final Form: July 31, 2007

Systematic investigations have been carried out on the electrocatalytic oxidation of several amino compounds in alkaline solutions on a nanoporous thin-film electrode of electrodeposited nickel hydroxide nanoflakes (NHNFs). The amino compounds studied here include two amines (ethylamine and propylamine), five amino acids (β-alanine, alanine, lysine, glycine, serine, and arginine), and one dipeptide (glycylglycine). Potentialdependent, time- and space-resolved in situ Raman spectra, together with electrochemical measurements, have been employed to reveal the electrocatalytic processes at the molecular level for the first time. Experimental results show that (i) the NHNFs act as an effective electron mediator with high electrocatalytic activity toward these amino compounds; (ii) the amino group is converted into a nitrile group, and decarboxylation occurs simultaneously for the R-amino acids; and (iii) the electrooxidation reaction rate of amino compounds is diffusion-controlled. Moreover, oscillations in both potential and current were observed for the first time during the electrocatalytic oxidation of these amino compounds on the film electrode of NHNFs. Periodic oxygen evolution plays a key role in the oscillations. It is initiated and terminated, respectively, when the surface concentration of amino compounds decreases to zero by diffusion-limited oxidation and is replenished by convection-enhanced flow from the gas release.

1. Introduction Amino compounds such as primary aliphatic amines, amino acids, and peptides are important precursors in chemical and biochemical synthesis. Electroadsorption and electrooxidation of these compounds have been studied mostly on single noble metals such as Pt,1-9 Au,10-12 and Ag12,13 and occasionally on their alloys such as Au-Ag12 and Pb-Ag.14,15 A variety of in situ techniques, such as radiotracer adsorption,4,5 Fourier transform infrared reflection-absorption spectroscopy (FTIRRAS),2,3,8-10 and electrochemical quartz crystal microbalance (EQCM) gravimetry,10 have been employed to study the electroadsorption and electrooxidation of various amino compounds under different experimental conditions. Because of the strong adsorption and accumulation of poisonous intermediates that deactivate the electrode, such as -CNad, Pt-based materials can hardly exhibit satisfactory electrocatalytic activity for the electrooxidation of amino compounds. As a widely used nonnoble-metal material, nickel has an effective electrocatalytic activitytowardsomeorganiccompoundscontaininghydroxyl,1,16-23 amido,16-18,20,23-26 and thiol23,27 groups. This property has been utilized in the electrocatalysis, electrosynthesis, and electroanalysis of many related organic compounds. In particular, a few studies have focused on the electrooxidation of primary amines and amino acids on electrodes of nickel, nickel-based alloys, and nickel (hydr)oxides.1,16-28 The amino group was most likely involved in the catalytic oxidation, whereas Ni(OH)2 acts * To whom correspondence should be addressed. Tel.: +86-57982283897. Fax: +86-579-82282595. E-mail: [email protected]. † Hunan Normal University. ‡ Zhejiang Normal University.

as an electron mediator.16,17,26,28 To the best of our knowledge, the electrocatalytic activity, being closely associated with the surface morphology, can be effectively increased when normal electrode materials are changed into nanostructures. To date, however, few works have reported the electrocatalytic oxidation processes of amino compounds on nanostructured electrode materials of nickel hydroxide. In this article, new findings are presented on the oscillatory electrocatalytic oxidation of different categories of amino compounds (Scheme 1) at a nanoporous film electrode of nickel hydroxide nanoflakes (NHNFs) in alkaline solutions. Among the compounds in Scheme 1, ethylamine (ETA) and propylamine (PA) have just one functional group of -NH2, whereas a second functional group of -COOis either connected to the carbon atom linking -NH2 as in the R-amino acids glycine (Gly), alanine (Ala), and serine (Ser) or separated from that carbon atom by methylene and acyl, respectively, as in β-alanine (β-Ala) and glycylglycine (Glygly). One additional -NH2 group is present in lysine (Lys) at the other end of the molecular chain, whereas arginine (Arg) is slightly more complicated than Lys in that a -C(NH)-Nsegment is inserted at that chain end. Comparison of the oxidation processes for these compounds can be thus made with different molecular structures. 2. Experimental Section Raman spectra were obtained with a Renishaw RM1000 microscopy confocal Raman spectrometer (Gloucestershire, U.K.). The exciting wavelength was 514.5 nm from an aircooled Ar+ laser with a power ca. 3.5 mW at the sample. A detailed description of Raman measurements with a spectroelectrochemical cell can be found elsewhere.29 Electrochemical

10.1021/jp073898o CCC: $37.00 © 2007 American Chemical Society Published on Web 10/16/2007

Oscillatory Electrocatalytic Oxidation of Amino Compounds

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SCHEME 1: Structures of Some Amino Compounds Studied in This Work

Figure 1. SEM images of the as-electrodeposited films of nickel hydroxide (a,b) before and (c,d) after pretreatment in 3 mol dm-3 NaOH by CV.

measurements were carried out with a CHI 660A Electrochemical Station (Chenhau, Shanghai, China) interfaced with a computer. A conventional H-type glass cell with three electrodes was used. Specifically, a nickel disk (1 mm in diameter, purity g99.99%), a platinum wire (1 mm in diameter and 8 mm in length), and a saturated mercurous sulfate electrode (SMSE) with a Luggin capillary were employed as the working, counter, and reference electrodes, respectively. All solutions were freshly prepared with Millipore water. All chemicals were of analytical or chemical grade and were used without further purification. The nanoporous thin film of Ni(OH)2 was cathodically deposited onto the nickel disk roughened with 4# metallographic paper under galvanostatic conditions, typically 600 µA cm-2 for 1200 s in 0.01 mol dm-3 Ni(NO3)2. The film electrode was then pretreated by potential cycling between -0.5 and 0.25 V in a background solution of 3 mol dm-3 NaOH until repeatable cyclic voltammograms (CVs) were obtained. The surface morphology of the deposits was characterized using a Hitachi S-4800 high-resolution scanning electron microscope (SEM). Prior to imaging, the samples were coated in gold. All experiments were performed at room temperature (∼20 °C) unless noted otherwise. 3. Results and Discussion 3.1. Nanostructures of the Deposited Nickel Hydroxide Film. Figure 1 shows the surface morphology of the electrodeposited Ni(OH)2 film with different magnifications. The images look almost the same before (a, b) and after (c, d) treatment by potential cycling in 3 mol dm-3 NaOH. It can be clearly seen that the surfaces are uniform (a, c) and composed of densely aggregated Ni(OH)2 nanoflakes (b, d) that form nanopores and nanochannels. Such a nanostructure exhibits a large specific surface area and good mass-transport capacity. In particular, a high electrocatalytic activity and specific nonlinear kinetics toward the oxidation of amino compounds were observed for the nanostructured electrode (see the details in sections 3.2 and 3.3). 3.2. Characterization of the Electrocatalytic Processes for the Oxidation of Amino Compounds on the NHNF Electrode. 3.2.1. Cyclic Voltammograms. Figure 2 (column 1) shows the CVs recorded on the film electrode of NHNFs in 3 mol dm-3 NaOH with (solid lines) or without (dashed lines) amino

Figure 2. (1) Cyclic voltammograms and (2) steady-state polarization curves on an NHNF electrode for (A) 1 mol dm-3 PA + 1 mol dm-3 NaOH, (B) 0.5 mol dm-3 β-Ala + 3 mol dm-3 NaOH, (C) 0.5 mol dm-3 Ala + 3 mol dm-3 NaOH, (D) 0.5 mol dm-3 Lys + 3 mol dm-3 NaOH, and (E) 0.1 mol dm-3 Glygly + 3 mol dm-3 NaOH. Dashed lines in column 1 are CVs for the NaOH background solutions. The insets in column 1 are the CVs for (B) 0.5 mol dm-3 CH3CH2COONa + 3 mol dm-3 NaOH on the NHNF electrode and (C) 0.5 mol dm-3 Ala + 3 mol dm-3 NaOH on the roughened nickel substrate. The inset in column 1, part E shows a local magnification of the corresponding CV.

compounds. A pair of distinct redox peaks in the background solutions (dashed lines), accompanying electrochromic phenomena,30 correspond to the transition between Ni(OH)2 and NiOOH. Electrocatalytic oxidation occurs when amino com-

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SCHEME 2: Representative Electrocatalytic Oxidation Reactions for Some Typical Amino Compounds (PA, β-Ala, Ala, and Lys)

pounds are present (solid lines), with the oxidation currents becoming larger for potentials beyond the peak for the formation of NiOOH and the peak currents for the reduction of NiOOH becoming smaller than those in the background solutions. Moreover, by comparing the oxidation peak currents of the inset and the solid CV in Figure 2C for the nickel substrate and the electrodeposited nanoporous film of NHNFs, respectively, it can be concluded that the contribution of the nickel substrate is so small (less that 1%) that it can be ignored. That is, the NHNF electrode exhibits much better electrocatalytic activity for the amino compounds than the normal nickel electrode does. However, the electrocatalytic oxidation processes also depend on molecular structures. Therefore, the large oxidation current peaks for PA in Figure 2A, column 1, arise from the electrocatalytic oxidation of its N-end segment, -CH2-NH2, into -CN (reaction 1 in Scheme 2), which was suggested by Fleischmann et al.16 and is further verified by in situ Raman spectroscopy in the next section. The same reaction process should occur at the N-end segment of β-Ala during its oxidation (reaction 2 in Scheme 2). To determine whether the carboxyl group, COO-, is also involved in the oxidation of β-Ala, the CV of propanoic acid (inset of Figure 2B) was recorded under the same conditions. Unlike the oxidation of β-Ala, there was no observable catalytic current in this case. Therefore, the carboxyl group in β-Ala is inactive under the present experimental conditions. As an isomer of β-Ala, Ala also exhibits an obvious catalytic current on the NHNF electrode (Figure 2C). However, its electrooxidation processes differ from those of β-Ala. Specifically, in addition to the oxidation of the -CH2-NH2 segment, decarboxylation can occur5 in the presence of an R-amino group (reaction 3 in Scheme 2). Moreover, the oxidation current of Ala is somewhat lower than that of β-Ala (Figure 2B) because of the steric inhibition of the neighboring carboxyl group, which is negatively charged and preferentially adsorbed on the electrode. As discussed above, the electrocatalytic oxidation of Lys (Figure 2D) probably involves three functional groups: a terminal amino group as for PA, a carboxyl group, and an R-amino group as for Ala. Apparently, the longer backbone chain of Lys leads to a relative low reactivity, as evidenced by a smaller oxidation current (Figure 2D) than for PA and Ala. The catalytic current for the electrooxidation of Glygly (Figure 2E) is even smaller. One main reason is again the chain length; others include the experimental conditions such as the smaller reactant concentration (0.1 mol dm-3) and lower temperature (7 °C). The NHNF electrode also showed good electrocatalytic activity for the oxidation of other amino compounds, and more examples including ETA, Gly, Ser, and Arg can be found in the Supporting Information (Figure S1).

Figure 3. Time-resolved Raman spectra from the NHNF electrode surface for NiOOH decay at open-circuit potential in a solution of 0.5 mol dm-3 Ala + 3 mol dm-3 NaOH as used in Figure 2C (column 1). The collection time for recording a single spectrum was 50 s.

3.2.2. Steady-State Voltammograms. The steady-state voltammograms for the electrooxidation of all amino compounds studied in this work are shown in the right columns of Figures 2 and S1. It can be seen that the electrooxidation processes of amino compounds are diffusion-controlled with limiting current plateaus because of the better electrocatalytic activity of the NHNF electrode. 3.2.3. In Situ Raman Spectroscopy for the Electrooxidation of Amino Compounds on the NHNF Electrode. In situ Raman spectroscopy is a powerful tool for tracking electrochemical processes at the molecular level. In this work, the transition of surface nickel species, the soluble oxidation products, and the concentration distribution in the diffusion layer during the electrocatalytic oxidation of amino compounds on the NHNF electrode were detected using the in situ Raman technique. 3.2.3.1. Time-Resolved Raman Spectroscopy. Time-resolved Raman spectra, e.g., during the electrooxidation of Ala, were acquired on the NHNF electrode surface at open-circuit potential after setting the potential at 0.05 V for 5 s to first transform some Ni(OH)2 into NiOOH species. Then, Raman spectra were recorded at different time. The bands in Figure 3 at 475 and 558 cm-1 are assigned to the Ni-O vibration of the produced NiOOH.31 The band intensity of the 475/558 cm-1 doublet gradually declined to 0 within 150 s, as the color changed from dark to light green. These facts indicate that the surface NiOOH was converted into Ni(OH)2 again by oxidizing the Ala. In contrast, both the Raman intensity and the dark color of the electrode surface remained unchanged when the electrode was in the background solution. Therefore, it can be easily concluded that (1) the redox pair Ni(OH)2/NiOOH is involved in the electrocatalytic oxidation of amino compounds and (2) the transition from Ni(OH)2 to NiOOH is faster than the reaction between NiOOH and the amino compounds, because the NiOOH produced in 5 s was consumed in 150 s. The reason is that the latter process is limited by the supply of reactant, as further confirmed in section 3.2.3.3 by space-resolved Raman spectroscopy. 3.2.3.2. Potential-Dependent Raman Spectroscopy. Figure 4 shows the in situ Raman spectra for the oxidation of PA on the NHNF electrode in alkaline solutions. At open-circuit potential, the bands at 1071 and 1450 cm-1 are assigned to the F(CH3) + ν(CH2-CH2) and νdef(CH3) vibrations of PA.32,33 With an increase in potential from -0.05 to 0.05 V, three new peaks appear at 1003, 1462, and 2252 cm-1. These new features correlate well with the νs(C-C), νs(CH3), and νs(CtN) bands,

Oscillatory Electrocatalytic Oxidation of Amino Compounds

Figure 4. Potential-dependent in situ Raman spectra about 100 µm from the NHNF electrode in a solution of 1 mol dm-3 PA + 1 mol dm-3 NaOH as used in Figure 2A. The collection time for recording a single spectrum was 100 s.

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Figure 6. Potential-dependent in situ Raman spectra 100 µm from the NHNF electrode in a solution of 0.5 mol dm-3 Lys + 3 mol dm-3 NaOH as used in Figure 2D. The collection time for recording a single spectrum was 100 s.

Figure 7. Space-resolved in situ Raman spectra obtained during PA oxidation at 0.15 V on the NHNF electrode. The exposure time of CCD is 100 s. The dashed curve indicates the concentration profile of PA in the diffusion layer. Figure 5. Potential-dependent in situ Raman spectra about 100 µm from the NHNF electrode in a solution of 0.5 mol dm-3 Ala + 3 mol dm-3 NaOH as used in Figure 2C. The collection time for recording a single spectrum was 100 s.

respectively, of propionitrile.34,35 Meanwhile, the peaks from the reactant, PA, decrease. Therefore, we can conclude that the main oxidation product of PA is propionitrile (reaction 1 in Scheme 2). Similarly to PA, ETA undergoes the same dehydrogenation process during oxidation. It can be seen that, at open-circuit potential, the Raman spectrum (Figure S2, Supporting Information) is dominated by three strong bands at 2880, 2933, and 2975 cm-1 that can be assigned to the νs(CH2), νs(CH2), and νs(CH3) vibrations, respectively, of ethylamine.36 New features appear as the potential increases from -0.05 to 0.15 V. The two bands at 2259 and 2297 cm-1 are attributed to νs(CtN) and the δ(CH3) + νs(C-N) Fermi resonance of acetonitrile,37,38 and the three bands at 924, 2945, and 1373 cm-1 are from the νs(C-N), νs(C-H), and νdef(CH3) vibrations of acetonitrile.34,35 In Figure 5, new Raman peaks appear at 1066, 1373, 2259, and 2297 cm-1 as the potential shifts from -0.05 to 0.05 V during the electrooxidation of Ala on the NHNF electrode. The band at 1066 cm-1 is assigned to νs(C-O)39 of CO32-, resulting from the decomposition of the carbonxyl group, whereas the new peaks at 1373, 2259, and 2297 cm-1 are attributed to νdef(CH3)35, νs(CtN), and the δ(CH3) + νs(C-N) Fermi resonance,37,38 respectively, of acetonitrile. Thus, the electrooxidation

of Ala on the NHNF electrode involves both decarbonxylation to carbonate and dehydrogenation to acetonitrile (reaction 3 in Scheme 2). Figure 6 shows the in situ Raman spectra recorded for the electrooxidation of Lys on the NHNF electrode. New peaks at 1068 and 2259 cm-1 are ascribed to carbonate39 and nitrile,34,35,37,38 respectively, according to the analysis above. In addition to decarbonxylation and dehydrogenation as for an R-amino acid such as Ala, the oxidation processes of Lys also include the dehydrogenation of one additional amino group at the other end of the chain (reaction 4 in Scheme 2). 3.2.3.3. Space-Resolved Raman Spectroscopy. As discussed in section 3.2.2, the occurrence of limiting current plateaus indicates that the electrooxidation of amino compounds is controlled by diffusion mass transfer. By using a space-resolved confocal Raman microscope, the concentration distribution of reactants and products in the diffuse layer during oxidation can be measured to provide further kinetic evidence. Figures 7 and 8 show such examples for the electrooxidation of PA, where the space-revolved Raman spectra were recorded at 0.15 V, a potential lying in the limiting current plateau of Figure 2A (column 2). When the laser focus is moved off the electrode surface into the bulk solution, the peak intensity at 1450 cm-1 increases from almost zero to a steady value, reflecting nearly complete depletion of the reactant PA32,33 in the diffusion layer (Figure 7). In contrast, the intensity at 2252 cm-1 decreases, showing a mirrorlike concentration profile for the product propionitrile34,35 (Figure 8). Similar concentration distribution curves in the diffusion layer were also observed in the

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Figure 8. Space-resolved in situ Raman spectra obtained during PA oxidation at 0.15 V on the NHNF electrode. The exposure time for CCD is 100 s. The dashed curve indicates the concentration profile of propioninitrile in the diffusion layer.

electrooxidation of ETA, as shown in Figures S3 and S4 of the Supporting Information. 3.3. Nonlinear Phenomena in the Electrooxidation of Amino Compounds on the NHNF Electrode. 3.3.1. Instability Analysis of the Electrode Processes. There are two ascending branches in the CVs recorded during the forward potential scans (solid lines in Figure 2 and red lines in Figure S1 of the Supporting Information) that represent two different reaction processes, namely, the electrooxidations of amino compounds without and with oxygen evolution. Note that oxygen evolution occurs in the second ascending branch. Between the two ascending banches, there is a descending branch that arises from the diffusion-limited depletion of amino compounds on the electrode surface. Limiting current plateaus (Figure 2A-E) appear instead under slower potential scans. From the viewpoint of stability, the two ascending branches represent the bistable states, and the descending branch represents the unstable state. Noticeably, crossing cycles occur in all CVs (solid lines in Figure 2 and red lines in Figure S1 of the Supporting Information), with the current value of the backward potential scan larger than that of the forward potential scan. A crossing cycle in a CV curve means that two opposite steps, i.e., positive and negative feedbacks, overlap within the bistable states. This is a universal characteristic for oscillatory electrochemical systems.40,41 Thus, oscillations can be expected to occur in these systems. In fact, current oscillations were already observed in the crossed CV of Figure 2A. The crossing cycles in CVs might originate from different electrode processes,40,41 so an understanding of the origin of crossed CVs is also helpful in understanding the oscillations. Recall that oxygen evolution occurs in the second ascending branch during the forward potential scan. The large current in the backward potential scan comes from the enhanced convection flow near the surface due to the oxygen evolution, because no crossing loop can be observed when the potential scan is reversed before oxygen evolution occurs (blue lines in Figure S1 of the Supporting Information). Therefore, the convection-enhanced replenishment and diffusion-limited depletion of amino compounds near the surface constitute the main positive and negative feedbacks in the crossing cycle. The crossing cycle in Figure 2E (column 1) is somewhat smaller than the others, which is mainly due to the lower reactivity of Glygly itself and also to the smaller concentration and lower experimental temperature. It might be worth noting that no crossing cycle appeared in the electrooxidation of Ala on the nickel substrate (inset in Figure 2C), and oscillations were also not observed. These facts indicate that the nanoporous films indeed exhibit some specific characteristics that dramatically change the reaction kinetics. It

Figure 9. Potential oscillations obtained by current sweeping on the NHNF electrode in (A) 1 mol dm-3 PA + 1 mol dm-3 NaOH, (B) 0.5 mol dm-3 β-Ala + 3 mol dm-3 NaOH, (C) 0.5 mol dm-3 Ala + 3 mol dm-3 NaOH, and (D) 0.5 mol dm-3 Lys + 3 mol dm-3 NaOH.

can also be concluded from these facts that diffusion mass transfer becomes the rate-determining step on porous films of NHNFs, unlike on the nickel substrate, where surface reactions are essential. 3.3.2. Electrochemical Oscillations. Potential oscillations under current scan during the electrooxidation of amino compounds on the NHNF electrode are shown for the first time in Figures 9, 11A and S5 (Supporting Information). It is interesting to note that (i) the potential oscillations appear only when the applied current is over the limiting current, (ii) the oscillatory amplitudes fall in the range of the limiting current plateau, (iii) periodic oxygen evolution occurs at the higherpotential side during the oscillations, and (iv) continuous stronger agitation (such as with a rotating electrode) can stop the oscillations and cause the system to stay at the lowerpotential side of the limiting current plateau. All of these facts strongly indicate that diffusion and convection mass transfer rather than surface processes play the key role in the oscillations. When the applied current is larger than the limiting current for the oxidation of amino compounds, their surface concentration soon depletes to zero because of the limited rate of supply by diffusion. Meanwhile, the potential moves to the higher-potential side of the plateau until oxygen evolution is triggered to maintain the applied current. Through growth, detachment, and movement, the oxygen bubbles act as an agitator and produce forced convection mass transfer. Oxygen evolution itself is thus terminated by the convection replenishment of the amino compounds, and the potential moves back to the lower side of the limiting current plateau. The cycling of these processes produces the potential oscillations. The current oscillations shown in Figures 10, 11B, and S6 (Supporting Information) can be explained in a similar way. The current oscillations also appear above the limiting current only when the applied potential is large enough. The current is higher at first, and it decreases because of the diffusion-limited depletion of the reactants by oxidation. The reaction of oxygen release is then initiated at the minimum current where the surface concentration of amino compounds depletes to zero and is terminated again by the replenishment of amino compounds with the enhanced convection flow near the surface, and the cycles repeat. The external series resistance (Re) used here can intensify the unstable nature of oscillatory systems,40 and sustained current oscillations can be obtained more easily. Current oscillations stop, too, when continuous strong agitation (as obtained with a rotating elec-

Oscillatory Electrocatalytic Oxidation of Amino Compounds

Figure 10. Current oscillations obtained by potential sweeping on the NHNF electrode in the same solutions as in Figure 2 with different external series resistances Re (Ω): (A) 300, (B) 500, (C) 2000, (D) 2000.

J. Phys. Chem. C, Vol. 111, No. 45, 2007 16907 reacting amino compounds as summarized in Scheme 2. Amines are oxidized into nitriles, and β-Ala also only undergoes dehydrogenation similarly to amines, retaining its carboxyl group. However, both dehydrogenation and decarboxylation occur in the electrooxidation of R-amino acids, producing nitrile and carbonate, respectively. In particular, during the oxidation of Lys, two amino groups are involved in the dehydrogenation, in addition to decarboxylation. Time-revolved Raman spectroscopy shows that the electrocatalytic oxidation of amino compounds is mediated by the redox pair Ni(OH)2/NiOOH. Owing to the better electrocatalytic activity of the nanoporous NHNF electrode, diffusion mass transport becomes the rate-determining step for the electrooxidation of amino compounds. Electrochemical reactions, i.e., electrocatalytic oxidation of amino compounds and periodic oxygen evolution, coupled with alternately predominant diffusion and convection mass transfer of amino compounds, are the main reasons for the oscillations (Scheme 3). Acknowledgment. We are grateful for the financial support of this research through the National Natural Science Foundation of China (20673013). Supporting Information Available: Voltammograms and electrochemical oscillations of some other amino compounds (ETA, Gly, Ser, and Arg) and potential-dependent and spaceresolved Raman spectra for ETA electrooxidation. These materials are available free of charge via the Internet at http:// pubs.acs.org. References and Notes

Figure 11. Current-potential curves obtained on the NHNF electrode in a solution of 0.1 mol dm-3 Glygly + 3 mol dm-3 NaOH by (A) current sweeping and (B) potential sweeping with an external resistance Re of 4000 Ω.

SCHEME 3: Schematic Illustration of the Oscillatory Electrocatalytic Oxidation Mechanism of Amino Compounds on the NHNF Electrode

trode) is applied and the system stays at the higher-current side because of unintermittent supply of reactant, which again confirms the oscillatory mechanism stated above. 4. Conclusions The electrocatalytic oxidation processes occurring at the NHNF electrode depend on the molecular structure of the

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