Adsorption of Serine on Pt Single-Crystal Electrodes in Sulfuric Acid

Qiu Dai , Jane Frommer , David Berman , Kumar Virwani , Blake Davis , Joy Y. Cheng , and Alshakim Nelson. Langmuir 2013 29 (24), 7472-7477. Abstract |...
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Adsorption of Serine on Pt Single-Crystal Electrodes in Sulfuric Acid Solutions Yan-Juan Gu, Shi-Gang Sun,* Sheng-Pei Chen, Chun-Hua Zhen, and Zhi-You Zhou State Key Laboratory for Physical Chemistry of Solid Surfaces, Department of Chemistry, Xiamen University, Xiamen 361005, China Received May 3, 2003. In Final Form: August 21, 2003 Adsorption of the amino acid serine on Pt single-crystal electrodes in sulfuric acid solutions has been studied using cyclic voltammetry and in situ Fourier transform infrared (FTIR) reflection spectroscopy. The results demonstrated that, in the adsorption, serine has suffered an oxidative dissociation that leads to the production of different adsorbates. Through a comparative investigation of serine, ethanolamine, and glycine on a Pt(100) electrode and in situ FTIR studies, adsorbates derived from the adsorption of serine were determined as PtCOOH, PtnCO (PtCOL, Pt2COB), and PtCN species. The cyclic voltammetric results illustrated that the oxidation of the PtCOOH adsorbates occurs in a small current peak (jp1) near +0.25 V, while the oxidation of PtnCO and PtCN species gives rise to a large current peak (jp2) between +0.39 and +0.52 V, on the three low index planes and nine stepped surfaces lying on the [001], [011 h ], and [11 h 0] crystallographic zones. On the basis of the experimental results, a mechanism of adsorption and oxidative dissociation of serine was proposed, which includes the cleavage of the two C-C bonds of the serine molecule, the oxidation of PtH species, and the surface combination of PtCOOH with PtH to form PtnCO adsorbates. It has been revealed that the Pt single-crystal electrodes containing mainly surface sites of (110) and (111) symmetry exhibit a high efficiency for the surface combination of PtCOOH with PtH, while the efficiency the of surface consisting mainly of (100) symmetry elements is relatively low. The surface-structure effects of Pt single-crystal electrodes on the adsorption and oxidative dissociation of serine were clearly demonstrated through the quantitative data measured in cyclic voltammetric studies.

Introduction The electrochemical oxidation of small organic molecules is one of the key subjects in the fundamentals and applications of electrocatalysis. Amino acids are the building blocks of peptides and proteins. Studies concerning these species may provide simple models for a better understanding of the complex metal-protein interaction, as well as of processes associated with the use of these species in advanced applications such as biological material1,2 and biological sensors.3-5 An amino acid molecule possesses different functional groups such as -CHx, -NH2, and -COOH and is used often as a model reagent in surface electrochemistry and electrocatalysis for the purpose of investigating the interactions of different functional groups with electrocatalyst surfaces and the role of these functional groups in the electrocatalysis of small organic molecules.6,7 Horanyi and Rizmayer8,9 have shown, by the radiotracer technique, that the adsorption of glycine takes place on * Corresponding author. Fax: + 86 592 2183047. E-mail: sgsun@ xmu.edu.cn. (1) Rault-Beithelot, J.; Raoult, E.; Tahri-Hassani, J.; Deit, H. L.; Simonet, J. Electrochim. Acta 1999, 44, 3409-3419. (2) Sehnert, J.; Hess, A.; Metzlet-Nolte, N. J. Organomet. Chem. 2001, 637, 349-355. (3) Kwan, R. C. H.; Chan, C. Y.; Renneberg, R. Biotechnol. Lett. 2002, 24, 1203-1207. (4) Setford, S. J.; White, S. F.; Bolbot, J. A. Biosens. Bioelectron. 2002, 17, 79-86. (5) Dominquez, R.; Serra, B.; Reviejo, A. J.; Pingarron, J. M. Anal. Biochem. 2001, 298, 275-282. (6) Xiao, X. Y.; Sun, S. G.; Yao, J. L.; Wu, Q. H.; Tian, Z. Q. Langumir 2002, 18, 6274-6279. (7) Li, H. Q.; Chen, A. C.; Roscoe, S. G.; Lipkowski, J. J. Electroanal. Chem. 2001, 500, 299-310. (8) Horanyi, G.; Rizmayer, E. M. J. Electroanal. Chem. 1975, 64, 15-19. (9) Horanyi, G.; Rizmayer, E. M. J. Electroanal. Chem. 1986, 198, 393-400.

platinized platinum electrodes in both alkaline and acidic media. They illustrated that in acidic solutions a reversible weak adsorption was observed originating from the interaction between the -COOH group of glycine and the electrode surface, while in alkaline solutions a strong irreversible adsorption of glycine resulted from an interaction between the electrode surface and the -CH-NH2 group of glycine. However, Ogura and co-workers10-12 have proposed different adsorption modes that are in disagreement with those suggested by Horanyi et al. In the mode proposed by Ogura et al. the adsorption of amino acids onto Pt electrodes is achieved through the terminal COOgroup of fully unprotonated anions in alkaline solutions. Recently, Xiao et al.6 have investigated adsorption of simple amino acids on the Pt electrode in alkaline solutions by Raman spectroscopy. They have proved that, at a molecular level, all adsorbates proposed by Ogura et al. and Horanyi and Rizmayer may exist on Pt electrode surfaces in alkaline solutions. A series of studies of adsorption and oxidation of simple amino acids (glycine, alanine, and serine) on the three basal planes of Pt, in 0.1 M HClO4 solutions, were conducted by Huerta and coworkers13-16 using in situ Fourier transform infrared (FTIR) spectroscopy. Strongly adsorbed CO and cyanide (10) Ogura, K.; Kobayashi, M.; Nakayama, M.; Miho, Y. J. Electroanal. Chem. 1998, 449, 101-109. (11) Ogura, K.; Kobayashi, M.; Nakayama, M.; Miho, Y. J. Electroanal. Chem. 1999, 463, 218-223. (12) Ogura, K.; Nakayama, M.; Nakaoka, K.; Nishihata, Y. J. Electroanal. Chem. 2000, 482, 32-39. (13) Huerta, F.; Morallon, E.; Case, F.; Rodes, A.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1997, 421, 179-185. (14) Huerta, F.; Morallon, E.; Case, F.; Rodes, A.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1997, 431, 269-275. (15) Huerta, F.; Morallon, E.; Vazquez, J. L.; Perez, J. M.; Aldaz, A. J. Electroanal. Chem. 1998, 445, 155-164. (16) Huerta, F.; Morallon, E.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1999, 475, 38-45.

10.1021/la034758i CCC: $25.00 © 2003 American Chemical Society Published on Web 10/08/2003

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have been determined as the main adsorbates and amino acid anions were suggested to be weakly adsorbed on Pt surfaces by these authors. The mechanism of the amino acid reaction is nevertheless still far from well-understood. The aim of the present paper is to investigate further the electrochemical behavior of the interaction of serine with Pt single-crystal surfaces in 0.1 M H2SO4 solutions using cyclic voltammetry and in situ FTIR spectroscopy. New light has been thrown on the mechanism of adsorption and oxidative dissociation of serine on Pt single-crystal electrodes, as well as on the surface-structure effects of Pt single-crystal electrodes in the reaction. Experimental Section Platinum single-crystal electrodes were prepared in our group according to ref 17. The electrodes were treated before each measurement using Clavilier’s methods;18,19 that is, they were annealed in a hydrogen-oxygen flame, quenched with pure water, and then transferred into an electrochemical cell under the protection of a droplet of Milli-Q water. The irreversible adsorption of serine on Pt single-crystal electrodes was carried out in a 0.1 M serine solution. The electrode after flame treatment was immersed in the solution for 2 min and then transferred into the electrochemical cell at -0.20 V. In situ FTIR reflection spectroscopic measurement was conducted on a Nexus 870 FTIR apparatus (Nicolet) equipped with a MCT-A detector that is cooled by liquid nitrogen. The subtractively normalized interfacial FTIR (SNIFTIR) spectroscopy procedure was employed in collecting the spectra. The resulting spectrum is defined as a potential-difference spectrum, that is, ∆R/R ) [R(ES) - R(ER)]/R(ER), with R(ES) and R(ER) being the single-beam spectra collected respectively at a reference potential ER and a sample potential ES. In the SNIFTIR spectroscopy procedure, the alteration of the electrode potential between ER and ES was repeated 10 times and each time 500 interferograms were collected respectively at ER and ES. As a consequence, 5000 interferograms were co-added into the values of R(ER) and R(ES) that were used to calculate the resulting spectrum. The spectral resolution was 8 cm-1. All solutions were prepared from Millipore water (18.0 MΩ cm) provided by a Milli-Q Lab apparatus (Nippon, Millipore, Ltd.), superpure H2SO4 reagent, and serine of analytical grade. A saturated calomel electrode (SCE) served as a reference electrode. To avoid the interferences of ions such as K+ and Clfrom the SCE during the measurement, the SCE was separated from main electrochemical (IR) cell by a bridge made of a stopcock of two ends containing the study solution. The solution was deaerated by purging with pure N2 gas before each experiment. A flux of pure N2 gas was maintained over the solution during measurements to prevent possible interference of oxygen from the atmosphere. The potential scan rate used in all the cyclic voltammetric studies was 50 mV s-1. All the experiments were carried out at room temperature around 20 °C.

Results and Discussion Studies of Cyclic Voltammetry. Figure 1 shows cyclic voltammograms (CVs) of Pt(100) in a 0.1 M H2SO4 + 1 × 10-3 M serine solution. The Pt(100) electrode was cooled in air after flame annealing. The inset to this figure illustrates the first three cycles of potential cycling between -0.24 and +0.15 V, which demonstrates that the current for hydrogen adsorption-desorption in this potential region decreases continuously with the increase of the number of potential cyclings. A steady voltammogram was recorded after the electrode potential cycling of 10 min in this potential region. A substantial blockage of the Pt(17) Sun, S. G.; Chen, A. C.; Huang, T. S.; Li, J. B.; Tian, Z. W. J. Electroanal. Chem. 1992, 340, 213-226. (18) Clavilier, J., Faure, R., Gwinet, G.; Durand, R. J. Electroanal. Chem. 1980, 107, 205. (19) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 267-277.

Figure 1. CVs of the Pt(100) electrode in 0.1 M H2SO4 + 1 × 10-3 M serine solution after a potential cycling between -0.24 and +0.10 V for 10 min. The dashed line is the stable voltammogram. Inset: CVs of the first three cycles in the potential cycling between -0.24 and +0.15 V (solid line) and the voltammogram of Pt(100) (dotted line) recorded in 0.1 M H2SO4. Sweep rate: 50 mV s-1.

(100) surface is evident if the steady CV is compared with the CV of the Pt(100) recorded in a 0.1 M H2SO4 solution (shown as the dotted CV in the inset to this figure). It is evident that a certain quantity of adsorbate, derived from serine, accumulated on the Pt(100) surface during the electrode potential cycling. The oxidation of adsorbates in the first positive-going potential scan (PGPS) occurs in two current peaks at +0.29 V with an intensity of 28.15 µA cm-2 (jp1) and +0.48 V with an intensity of 130.7 µA cm-2 (jp2). The corresponding electric charges Qp1 of 33.3 µC cm-2 and Qp2 of 130.3 µC cm-2 have been measured by the integration of jp1 and jp2 peaks in the voltammogram. The ratio of Qp2 over Qp1, that is, Qp2/Qp1, is calculated to be 3.91. We can see that jp1 and jp2 have decreased in the second PGPS. In the stable voltammogram (dashed curve in Figure 1) recorded between -0.24 and +0.70 V, jp2 is of a small intensity and jp1 disappears nearly completely. The surface site occupancy by serine adsorbates (or the saturation coverage, θs) formed in the potential cycling of 10 min between -0.24 and +0.10 V may be calculated using the following equation,

θs )

QHs - QHser QHs

(1)

where QHs is hydrogen adsorption charge measured from the voltammogram of Pt(100) recorded in 0.1 M H2SO4 and QHser is the electric charge measured from the steady voltammogram recorded after the potential cycling of 10 min between -0.24 and +0.10 V. In the present experiment, QHs and QHser are respectively 236.3 µC cm-2 and 90.8 µC cm-2, yielding a θs value of 0.616. The adsorbates formed in serine adsorption can also be isolated on the Pt(100) electrode surface. The results of two different experiments are shown in Figure 2. In Figure 2a, the Pt(100) electrode, after flame treatment, was immersed initially in a 0.1 M serine solution for 2 min at open circuit. The Pt(100) electrode, together with a droplet of 0.1 M serine solution, was then transferred into a serinefree 0.1 M H2SO4 solution with the electrode potential maintained at -0.20 V for 10 min. The excess serine surrounding the Pt(100) electrode was finally dispersed by stirring the solution by bubbling pure N2 gas. In this way, only the adsorbed species were subjected to investigation. The electrode potential was scanned from -0.20

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important pieces of information: (1) The formation of serine adsorbates occurred efficiently when the electrode potential was scanned cyclically between -0.24 and +0.10 (or +0.20) V. This result may imply that an oxidation process will help the dissociation of the serine molecule. We can see from the inset to Figure 1 that the amplitude of the anodic current is always larger than that of the cathodic one in the same voltammogram recorded between -0.24 and +0.15 V in a 0.1 M H2SO4 + 1 × 10-3 M serine solution, which suggests that at least the oxidation of the hydrogen species derived from serine dissociation has taken place. On the basis of this point of view, we denote the process of producing serine adsorbates as oxidative dissociation and the adsorbates formed in this process as oxidative dissociation adsorbates (ODAs). (2) Three types of adsorbates may be generated in the oxidative dissociation of serine. The oxidations of the three kind adsorbates give rise to jp1 and jp2 (two peaks) in the voltammograms. From the quantitative data measured in Figure 2b, we can determine the average number of electrons transferred per surface hydrogen adsorption site (H site) in the oxidation current peaks jp1 and jp2, using the following two equations:

n1 )

Figure 2. CVs of the Pt(100) electrode in 0.1 M H2SO4 after an irreversible adsorption of serine in a 0.1 M serine solution at open circuit. (a) The CVs were recorded after a stay at -0.20 V for 10 min; (b) the CVs were recorded after a potential cycling between -0.24 and +0.20 V for 10 min. Sweep rate: 50 mV s-1.

to -0.24 V and then swept positively up to +0.70 V. Two small oxidation peaks appeared at approximately +0.44 and +0.49 V in the first PGPS (Figure 2a). The first anodic current peak decreased rapidly in the second PGPS between -0.24 and +0.70 V, while the second anodic peak decayed slowly. In Figure 2b, a potential cycling pretreatment, between -0.24 and +0.10 V for 10 min, was applied after the Pt(100) electrode was transferred into 0.1 M H2SO4. When the excess serine surrounding Pt(100) was dispersed by N2 bubbling, CVs with a fixed lower potential limit (EL) of -0.24 V and varying upper potential limits (EU) were recorded. We observed that the current for hydrogen adsorption-desorption was partially inhibited and was constant when EU was below +0.20 V. This indicates that serine adsorbates are stable on the Pt(100) electrode below +0.20 V. The electric charge of hydrogen adsorption measured from the voltammogram of EU ) +0.20 V is denoted as QH1 (159.2 µC cm-2). The second voltammogram was recorded with EU shifted up to +0.36 V, that is, the potential at which the oxidation current peak jp1 was completed. The oxidation charge of jp1 is measured to be 27.3 µC cm-2, and the charge for hydrogen adsorption increased to 179.8 µC cm-2 and is denoted as QH2. After the oxidation of the adsorbates that yield a current peak of jp1, the third voltammogram was recorded with EU increasing to +0.70 V. We can see that the jp1 completely disappeared, while the sharp current peak jp2 is still preserved at +0.45 V, 86.9 µA cm-2. The oxidation charge Qp2 is measured to be 119.1 µC cm-2. In the third voltammogram, the charge of hydrogen adsorption QH3 was augmented to 238.8 µC cm-2, that is, very close to QHs. Two small oxidation current peaks at around +0.45 and +0.50 V remained in the fourth voltammogram of EU ) 0.70 V similar to the features of the third voltammogram in Figure 2a. The results in Figure 2 provided two

n2 )

Qp1 2

QH - QH1 Qp2 3

QH - QH2

(2)

(3)

n1 and n2 have been calculated to be 1.33 and 2.02, respectively. It is interesting to take the quantitative data, Qp1, Qp2, and θS (Figure 1) measured for a saturation coverage of ODA on Pt(100) to estimate the coverage of θp1 and θp2 of ODA species that yield respectively jp1 and jp2. If we assume that that all adsorbates are linearly bonded on the Pt(100) surface, the surface site occupancy corresponding to adsorbates that yield respectively jp1 and jp2 may be calculated by

θp1 )

Qp1/n1 × θs Qp1/n1 + Qp2/n2

(4)

θp2 )

Qp2/n2 × θs Qp1/n1 + Qp2/n2

(5)

The results yielded θp1 and θp2 equal 0.172 and 0.444, respectively. Studies of in Situ FTIR Spectroscopy. The in situ FTIR spectroscopy studies were carried out to identify the ODA species at a molecule level. Prior to collecting in situ FTIR spectra, the Pt(100) electrode was cycled between -0.24 and +0.10 V for 30 min, which led to the formation of a saturation coverage of ODA on the Pt(100) surface. Figure 3 shows a series of SNIFTIR spectra collected with ER ) -0.20 V and ES varying from +0.0 to +0.45 V. We can observe from these spectra the following IR features: (1) A negative-going band near 2084 cm-1 appeared clearly in all the spectra of Figure 3. It can be assigned to C-N stretching of adsorbed cyanide species formed in the initial stage of serine oxidation.16,20 The center of this band is blue-shifted with the increase of ES,21,22 yielding (20) Huerta, F.; Morallon, E.; Perez, J. M.; Vazquez, J. L.; Aldaz, A. J. Electroanal. Chem. 1999, 469, 159-169. (21) Ashley, K.; Feldheim, D. L. J. Electroanal. Chem. 1994, 373, 201.

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Figure 3. SNIFTIR spectra of the Pt(100) electrode in 0.1 M H2SO4 + 0.02 M serine solutions. ER ) -0.20 V, ES is indicated for each spectrum.

Figure 4. Potential-dependent plots of νCN (a) and νCOL (b) on the Pt(100) electrode in 0.1 M H2SO4 + 0.02 M serine solutions.

a Stark tuning rate of 40 cm-1 V-1 (Figure 4a). It is interesting to notice that the intensity of this band remains constant in all the spectra. (2) Besides the cyanide band, two bipolar bands centered around 2015 and 1842 cm-1 appeared in the spectrum of ES ) 0.0 V. These two bipolar bands can be assigned to IR absorption at ER and ES of linear (COL) and bridge (COB) bonded CO species, respectively. The intensity of the negative peak of the COL band is much larger than that for the positive one; that is, this bipolar band is asymmetric. Nevertheless, the COB bipolar band is symmetric. When ES is increased from +0.0 to +0.20 V, (22) Huerta, F.; Morallon, E.; Vazquez, J. L.; Aldaz, A. Surf. Sci. 1998, 396, 400-410.

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the negative peak (IR absorption at ES) of the COL bipolar band is shifted linearly to a higher wavenumber, which produces a Stark tuning rate of 74 cm-1 V-1 as illustrated by Figure 4b. Such a large value of the Stark tuning rate may be caused by the coadsorption with CN species and the low coverage of COL species, because Chang et al.23 have demonstrated that the Stark tuning rate of COL species on Pt(100) electrode is increased from 36 cm-1 V-1 at saturation coverage of COad (θCOS ) 0.85) to 48 cm-1 V-1 at a θCO of 0.3. When ES reaches +0.25 V, a negative-going band located at 2345 cm-1 appears in the spectrum, which is assigned to the asymmetrical stretching of CO2 in solution. Meanwhile, the center of the negative peak of the COL bipolar band deviates from the linear variation (Figure 4a) and a loss of intensity of this band is obviously observed. It is evident that the COL species starts to be oxidized at this potential, that is, +0.25 V may be the onset potential of COL oxidation under constant potential conditions. After a further increase in ES from +0.25 to +0.35 V, the intensity of the COL bipolar band continuously decreases, while that for the COB bipolar band is augmenting in the spectra. At the same time, the CO2 band is also increased. This result suggests that the oxidation of COL species takes place at a lower potential than the COB species does, which confirms that the coverage of CO on Pt(100) is relatively low.24 As stated in Experimental Section, in the SNIFTIR spectroscopy procedure 500 interferograms were collected alternatively at ER and ES, and the alteration was repeated 10 times. It has been counted that collecting 500 interferograms requires about 3.5 min, and during that period serine can be dissociated slowly to produce a small quantity CO with the electrode potential at ER (-0.20 V), as illustrated by Figure 2a. However, COL can be oxidized while COB is stably adsorbed on the Pt(100) surface, when the electrode potential was stepped from ER to ES (ES > +0.20 V). As a consequence, in the whole SNIFTIR spectroscopy procedure COL was oxidized and COB was accumulated on Pt(100), which led to the increase of the COB band intensity and the decrease in the COL band intensity This phenomenon is seen in spectra of ES above +0.20 V. However, when ES is at +0.35 V the intensity of the COB band is also decreased, and both the COL and the COB bands disappeared in the spectra of ES above +0.40 V, confirming the complete oxidation of adsorbed CO species. (3) Only the CO2 and CNad bands remained in the spectra of ES at +0.40 and +0.45 V, which indicates that the CNad species is more stable than COad on the Pt(100) electrode under the present conditions. The above in situ FTIR results demonstrated that the adsorption and oxidative dissociation of serine yielded linearly, bridge bonded CO and cyanide. The linearly bonded CO can be oxidized at a potential as low as +0.25 V; however, the bridge bonded CO is more stable and its oxidation starts at +0.35 V. The adsorbed cyanide is even more stable and cannot be oxidized until +0.45 V. Figure 5 displays a spectrum that was recorded by applying a SPAFTIR procedure; that is, after the potential cycling between -0.24 and +0.20 V for 10 min to accumulate ODA species, the single-beam spectrum R(ER ) -0.20 V) was collected at first, and the electrode potential was stepped then to +0.60 V to collect the single(23) Chang, S. C.; Leung, L. W. H.; Weaver, M. J. J. Phys. Chem. 1989, 93, 5341-5345. (24) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 50955102.

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Figure 5. SPAFTIR spectra of the Pt(100) electrode in a 0.1 M H2SO4 + 0.02 M serine solution. ER ) -0.20 V, ES ) +0.60 V.

beam spectrum R(ES ) +0.60 V). The resulting spectrum was calculated in the same way, that is, ∆R/R ) [R(ES) - R(ER)]/R(ER). In this case, the ODA species are adsorbed on the Pt(100) surface only at ER while oxidized at ES. As a consequence, we observe positive-going adsorbates (CN, COL, and COB) bands, and a negative-going product (CO2) band in the spectrum. This SPAFTIR spectrum confirmed that all ODA species can be oxidized and removed from Pt(100) at +0.60 V. Studies of the Mechanism of Adsorption and Oxidative Dissociation of Serine. The formation of cyanide and CO species upon serine adsorption implies the cleavage of the two C-C bonds in the serine molecule. As shown in reaction 6, the molecule of serine can be divided into three parts, that is, -CH2OH, -CH-NH2, and -COOH.

We have also studied the irreversible adsorption of cyanide on the Pt(100) electrode. The experiments were carried out in the following way: (1) the adsorption of CN- was conducted at first in an alkaline solution containing CNions; (2) the Pt(100)/CN- was then transferred to a 0.1 M H2SO4 solution for electrochemical studies. The results demonstrated that the onset potential of cyanide oxidation was about +0.50 V, and no oxidation current, except that for double layer charging, was observed in the CV between +0.20 and +0.30 V. This is similar to the results reported by Huerta et al.22 On the basis of the CV and IR experimental results, jp2 in Figure 1 may be ascribed to the oxidation of CO and cyanide species. It may be worth noting that the measurement of the onset potentials for COL and COB oxidation respectively at +0.25 and +0.35 V in the SNFTIR spectroscopy studies was carried out under steady-state conditions; that is, the electrode potential was held at ER and ES respectively for 3.5 min. In the cyclic voltammetric studies, the onset potential of COad was measured at a dynamic condition; that is, the electrode potential was varied linearly at a rate of 50 mV s-1. The above analyses confirmed the assignment of jp2 to CO and cyanide oxidation, as reported in the literature.16,24 To clarify the assignment of jp1, we have studied comparatively the adsorption and oxidation behaviors of ethanolamine and glycine, which are similar to serine in molecule structure. As illustrated by reaction 6, the difference in molecule structures between serine and

Figure 6. CVs of the Pt(100) electrode recorded after a potential cycling between -0.24 and +0.10 V for 5 min, (a) in 0.1 M H2SO4 + 1 × 10-3 M ethanolamine and (b) in 0.1 M H2SO4 + 1 × 10-3 M glycine solutions. Sweep rate: 50 mV s-1.

ethanolamine is that the ethanolamine does not contain the -COOH functional group. Figure 6a shows the CVs of a Pt(100) electrode in a 0.1 M H2SO4 + 1 × 10-3 M ethanolamine solution under the same conditions as those in Figure 1. The ethanolamine can also dissociate on Pt(100) in potential cycling between -0.24 and +0.10 V for 5 min. The hydrogen adsorption is evidently inhibited partially by adsorbates in the steady-state voltammogram, recorded between -0.24 and +0.10 V. In the first PGPS, the oxidation of ethanolamine adsorbates occurs in a single anodic peak at +0.45 V, 145.2 µA cm-2. This sharp current peak decreases dramatically and turns into two small current peaks in the following potential cycle. It is evident that the current peak at around +0.45 V can be assigned to the oxidation of COad and cyanide species, indicating that the adsorption and oxidative dissociation of ethanolamine on Pt(100) also produces adsorbed CO and cyanide species. However, ethanolamine does not give any current peak in the potential range between +0.20 and +0.30 V. Figure 6b shows CVs of the Pt(100) electrode in a 0.1 M H2SO4 + 1 × 10-3 M glycine solution, recorded after applying the same potential cycling as that in Figure 6a. A small oxidation current peak at +0.28 V, 31.52 µA cm-2, and a weak oxidation peak near +0.46 V, 18.27 µA cm-2, were observed. It is obvious that the ODAs of glycine are mainly -COOH and cyanide species, and the current peak between +0.20 and +0.30 V is mostly due to the oxidation of -COOH species. The results of Figure 6 provided a strong argument for assigning the jp1 in Figure 1 to the oxidation of -COOH groups, resulting from the adsorption and oxidative dissociation of serine on Pt(100). This assignment of jp1 to Pt-COOH oxidation is supported also by following studies: (1) Sun and co-workers25 have demonstrated, via in situ FTIR spectroscopic studies, that the reactive intermediate involved in HCOOH oxidation on Pt single(25) Sun, S. G.; Clavilier, J.; Bewick, A. J. Electroanal. Chem. 1988, 240, 147-159.

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crystal electrodes is PtCOOH. (2) Yang and Sun26 have studied the oxidation of HCOOH on a Pt(100)/Sb surface. They illustrated that, when the dissociative adsorption of HCOOH was inhibited by Sb adatoms, HCOOH can be oxidized directly via the reactive intermediate PtCOOH at potentials between +0.20 and +0.30 V. (3) Huerta et al.15 have reported that cyanide is the sole detectable adsorbate by means of in situ FTIR spectroscopy in glycine dissociative adsorption. This implies that the -COOH functional group produced from the cleavage of C-C bonds in glycine does not dissociate further to form CO species as in the case of HCOOH on Pt(100), where adsorbed -COOH can react with adjacent Had to produce CO.27 From the above discussions, the adsorption and oxidative dissociation of serine may be described below: (1) The cleavage of the two C-C bonds in the serine molecule leads to the formation of PtnCO (with n ) 1 linear and n ) 2 bridge bonded CO), PtCN, PtCOOH, and PtH species,

(2) The oxidation of PtH species, +

PtH f Pt + H + e

-

(8)

(3) The surface combination of PtCOOH and PtH to form PtnCO,

PtH + PtCOOH f PtnCO + H2O

(9)

Figure 7. Comparison of j-E curves of different Pt singlecrystal electrodes recorded after a potential cycling between -0.24 and +0.10 V for 10 min in 0.1 M H2SO4 + 1 × 10-3 M serine solutions. Sweep rate: 50 mV s-1.

one-atom-high steps and (100) symmetry)

(511) S 3(100) × (111) (911) S 5(100) × (111)

The oxidation of PtH in the hydrogen potential region releases surface sites for further dissociation of serine on Pt(100), which also interprets the role of the potential cycling, between -0.24 and +0.10 V, to yield a saturation ODA layer on Pt(100). The combination of PtCOOH with PtH to form PtnCO is also a possible surface reaction in the adsorption and oxidative dissociation, but the efficiency of such a combination on Pt(100) is very low. As a consequence, in a dynamic potential scan, the oxidation of PtCOOH gives rise to jp1, and the oxidation of PtnCO and PtCN produces jp2. In our in situ SNIFTIR spectra, IR bands corresponding to the PtCOOH species were absent. This may be due to the surface concentration of PtCOOH being too low to determine, as illustrated by the small current peak jp1 and the low coverage (0.172) estimated for this species. Surface-Structure Effects of the Pt Single-Crystal Electrode. The above studies demonstrated that the adsorption and oxidative dissociation of serine is surfacestructure sensitive. To study systematically the surfacestructure effects on Pt single-crystal electrodes, three basal planes [(100), (111), (110)] and nine steppedsurface [(211), (511), (911), (610), (510), (210), (320) (331), (332)] Pt electrodes were prepared. The structure configuration of the stepped surfaces may be expressed as

(211) S 3(111) × (100) (i.e., terraces three atoms wide, and (111) symmetry, with (26) Yang, Y. Y.; Sun, S. G. J. Phys. Chem. B 2002, 106, 1249912507. (27) Sun, S. G.; Lin, Y.; Li, N. H.; Mu, J. Q. J. Electroanal. Chem. 1994, 370, 273-280.

(610) S 6(100) × (110) (510) S 5(100) × (110) (210) S 2(110) × (100) S 2(100) × (110) (320) S 3(110) × (100) (331) S 3(111) × (111) S 2(111) × (110) (332) S 6(111) × (111) S 5(111) × (110) According to the unit stereographic triangle,28 Pt(211), Pt(511), and Pt(911) belong to the [011 h ] crystallographic zone; Pt(610), Pt(510), Pt(210), and Pt(320) belong to the [001] crystallographic zone; and Pt(331) and Pt(332) belong to the [11 h 0] crystallographic zone. After applying the same potential cycling between -0.24 and +0.10 V for 10 min in a 0.1 M H2SO4 + 1 × 10-3 M serine solution as in Figure 1, j-E curves were recorded on different Pt single-crystal electrodes (Figure 7). As indicated previously, the potential cycling between -0.24 and +0.10 V ensures the achievement of a saturation adsorption of ODA species on Pt single-crystal surfaces. It can be observed that the oxidation of the ODA species depend also strongly on the orientation of the Pt single crystal. The property of each electrode in the adsorption and oxidative dissociation of serine, that is, the production of ODA species, may be inferred from the inhibition of the hydrogen adsorption current, and can be characterized quantitatively by evaluating the saturation coverage (θS) of ODA species. Table 1 lists the characteristic parameters (28) Blakely, D.; Somorjai, G. Surf. Sci. 1977, 65, 419.

Serine Adsorption on Pt Single-Crystal Electrodes

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Table 1. List of Characteristic Parameters of Formation and Oxidation of ODA Species Pt(hkl)

θS

Ep1, V

jp1, µA cm-2

Qp1, µC cm-2

Ep2, V

jp2, µA cm-2

Qp2, µC cm-2

Qp2/Qp1

Pt(110)

0.399

0.25

1.06

2.3

42.4

0.57

0.24

1.01

1.6

91.7

57.3

Pt(210) Pt(510) Pt(610) Pt(100) Pt(911) Pt(511) Pt(211) Pt(111) Pt(332) Pt(331)

0.504 0.428 0.444 0.616 0.487 0.395 0.596 0.647 0.598 0.525

0.28 0.28 0.28 0.29 0.28 0.28

7.185 11.52 10.49 28.15 18.72 11.51

2.8 14.4 9.3 33.3 25.3 12.8

0.28 0.22 0.24

15.08 16.53 1.02

12.4 17.1 1.9

50.98 54.72 38.69 44.62 48.61 47.87 54.02 130.7 84.48 47.46 31.56 78.34 34.95 43.38

97.6

Pt(320)

0.43 0.48 0.43 0.51 0.52 0.49 0.48 0.48 0.48 0.51 0.44 0.45 0.43 0.39

of formation and oxidation of ODA species, which clearly demonstrates the Pt electrode surface-structure effects: (1) For the three basal planes, the charges corresponding to the oxidation current peaks jp1 and jp2 (Qp1 and Qp2) stand in the maximum values 33.3 µC cm-2 and 130.3 µC cm-2 on the Pt(100) electrode, respectively. The ratio of Qp2/Qp1 is 3.91, which is nearly the smallest value in Table 1. Both Qp1 and Qp2 on Pt(111) are smaller than those on Pt(100), while the ratio of Qp2/Qp1 is increased to 10.3. However, on the Pt(110) electrode, except for a very weak jp1, two small peaks appear respectively near +0.43 and +0.48 V. Qp1 is only 2.3 µC cm-2, Qp2 is 66.2 µC cm-2 (including the two peaks), and the Qp2/Qp1 ratio is 42.4. As discussed previously, the surface combination of PtCOOH and PtH to form PtnCO, that is, reaction 7 introduced previously, is included in the adsorption and oxidative dissociation of serine. The Qp2/Qp1 ratio may be an indication of the efficiency of such a surface combination. The smallest Qp2/Qp1 value was for Pt(100), indicating that the efficiency of the PtCOOH and PtH combination is rather low. The largest value of Qp2/Qp1 measured on Pt(110) implies that reaction 7 occurred the most efficiently on this basal plane. The saturation coverage of ODA species on the Pt(110) electrode was the smallest (0.399), not only among the basal planes, but of all the Pt singlecrystal electrodes studied in the present paper. (2) Varying from Pt(100) to Pt(211) along the [011 h] crystallographic zone, the current peak jp1 decreases gradually and disappears at last on Pt(211). Accordingly, the jp2 also decreased progressively, becoming broad, with its center shifted to +0.44 V on Pt(211). The disappearance of the jp1 on Pt(211) signifies that the surface combination of PtCOOH and PtH is very fast in the adsorption and oxidative dissociation of serine on this surface. It is interesting to see that the θS values of stepped surfaces are all smaller than the θS values of Pt(100) and Pt(111), that is, the two basal planes situated at the two ends of this [011 h ] crystallographic zone. (3) For stepped surfaces situated in the [001] and [11 h 0] crystallographic zones, large values of the Qp2/Qp1 ratio were obtained on the planes that are close to Pt(110), that is, Pt(331) and Pt(320). In general, the values of the Qp2/ Qp1 ratio of stepped surfaces in these two zones are superior to those of Pt single-crystal electrodes in the [011h ] crystallographic zone, excepting that of Pt(211). This result indicates that the surface sites of (111) and (110) symmetry are more efficient than surface sites of (100) symmetry for the surface combination of PtCOOH and PtH. This point may be confirmed through the saturation coverage of ODA species; the data listed in Table 1 show that the θS in these two zones are generally larger than the θS in the [011 h ] zone. (4) The data listed in Table 1 and the j-E curves in Figure 7 illustrated also two interesting aspects: (a) Large

70.16 64.17 78.10 130.3 92.7 64.7 77.9 128.0 74.6 75.3

9.76 4.46 8.40 3.91 3.66 5.05 10.3 4.36 39.6

current peaks of ODA oxidation were observed on Pt singlecrystal planes that contain mainly (100) and (111) surface sites. When surfaces consist of mainly (110) sites, oxidation of the ODA yields small current peaks. (b) The oxidation of ODA gives rise to a sharp and narrow current peak jp2 on Pt single-crystal electrodes that contain mainly (100) and (111) symmetry. Nevertheless, on surfaces of mainly (110) symmetry the same oxidation current peak appears as a broad current peak with two maxima. Conclusions The goal of the present paper is to investigate the mechanism of adsorption and oxidative dissociation of serine and the surface-structure effects of Pt single-crystal electrodes in the reaction. Pt single-crystal electrodes of three basal planes [(100), (111), (110)] and nine stepped surfaces [(211), (511), (911), (610), (510), (210), (320) (331), (332)] of Pt the single crystal were prepared, and cyclic voltammetry and in situ FTIR reflection spectroscopy were employed in the study. The main conclusions that can be drawn from the present studies may be given as the following points. (1) Cyclic voltammetric studies illustrated that a potential cycling between -0.20 and +0.10 V for at least a few minutes is required to reach a saturation coverage of adsorbates derived from the adsorption and oxidative dissociation of serine on Pt single-crystal electrodes. The largest value of the saturation coverage of ODA (θS) is 0.647, which has been measured on Pt(111) electrode. The oxidation of the ODA species on all the Pt single-crystal electrodes except Pt(211) occurs in two current peaks. The first small peak appears between +0.22 and +0.29 V and is denoted as jp1, the second large peak is located in the potential region from +0.39 to +0.52 V and is named as jp2. From a comparative investigation in which the oxidation of oxidative dissociation adsorbates (ODA) derived from ethanolamine yielded solely the jp2 and the oxidation of ODA generated from glycine produced mainly jp1, the species in the ODA of serine that gave rise to jp1 has been ascribed to PtCOOH. (2) In situ FTIR reflection spectroscopy was employed to identify the chemical composition of serine ODA on Pt(100). The adsorbates have been determined as linearly and bridge bonded CO and cyanide, which give rise to IR absorption at around 2015, 1842, and 2084 cm-1, respectively. No IR bands concerning the IR absorption of PtCOOH species have been observed from in situ FTIR spectra, which may confirm that the surface concentration of PtCOOH is too low (θ ) 0.172) to be determined by in situ FTIR spectroscopy under the experimental conditions. With in situ FTIR spectroscopy results and referring to those reported in the literature, the jp2 in the voltammogram mentioned above is assigned obviously to the oxidation of COad and cyanide species.

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(3) On the basis of cyclic voltammetry and in situ FTIR spectroscopy studies, the mechanism of adsorption and oxidative dissociation of serine is proposed. The adsorption and oxidative dissociation of serine include the cleavage of the two C-C bonds of the serine molecule, the oxidation of PtH species, and the surface combination of PtCOOH with PtH to form PtnCO adsorbates. The process of surface combination of PtCOOH with PtH to form PtnCO may be illustrated by the Qp2/Qp1 ratio. A small value (3.91) of Qp2/Qp1 was measured on the Pt(100) electrode, implying that the efficiency of the surface combination of PtCOOH with PtH is relatively low on this surface. As a consequence, the intensity of jp1 stands as the largest value among those measured on all Pt single-crystal electrodes studied. (4) The surface-structure effects of the Pt single-crystal electrode on the adsorption and oxidative dissociation of serine have been demonstrated through quantitative data measured from cyclic voltammetric studies. The quantitative data concerning surface-structure effects are the θS, Ep1, Ep2, jp1, jp2, Qp1, Qp2, and Qp2/Qp1 ratio. It has been revealed that the surface sites of (111) and (110) symmetry

Gu et al.

are more efficient than the surface sites of (100) symmetry for the surface combination of PtCOOH with PtH in the adsorption and oxidative dissociation of serine and that the oxidation of the ODA species on Pt single-crystal planes containing mainly (100) and (111) surface sites yielded large current peaks; the oxidation on surfaces consisting of mainly (110) sites occurred in only small current peaks. Concerning the θS, large values were obtained on Pt(111) and Pt(100), and the smallest value was measured on Pt(110). For stepped surfaces, the θS is situated between those of Pt(111) and that of Pt(110). The present studies have thrown light upon understanding of the intrinsic interaction of amino acids with Pt single-crystal surfaces and revealing surface-structure effects in molecule adsorption. Acknowledgment. This work was supported by grants from National Natural Science Foundation of China (90206039, 20021002), 973 program (2002CB211804), and Education Ministry of China (01101). LA034758I