Manipulation of Electrocatalytic Reaction Pathways through Surface

catalytic reaction pathways involved in 1,3-butanediol (1,3-BD) oxidation through surface chemistry by modifying the surface of a Pt electrocatalyst w...
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J. Phys. Chem. B 2006, 110, 11383-11390

11383

Manipulation of Electrocatalytic Reaction Pathways through Surface Chemistry: In Situ Fourier Transform Infrared Spectroscopic Studies of 1,3-Butanediol Oxidation on a Pt Surface Modified with Sb and S Adatoms Qi-Hui Wu,†,‡ Nan-Hai Li,† and Shi-Gang Sun*,† State Key Laboratory of Physical Chemistry of Solid Surfaces, Department of Chemistry, College of Chemistry and Chemical Engineering, and Semiconductor Photonics Research Center, Department of Physics, Xiamen UniVersity, Xiamen 361005, China ReceiVed: February 12, 2006; In Final Form: April 18, 2006

Cyclic voltammetry and in situ Fourier transform infrared (FTIR) spectroscopy were employed to study the electrocatalytic properties of a Pt electrode modified with adatoms of antimony (Sb) or sulfur (S) for 1,3butanediol (1,3-BD) oxidation. The results demonstrated the possibility of manipulating the reaction pathways involved in 1,3-BD oxidation through chemical modification of the Pt electrode surface. Both Sb and S adatoms (Sbad and Sad) can inhibit the dissociative reaction of 1,3-BD into CO, which is the main source of selfpoisoning in electrocatalysis of small organic molecules. On Pt electrodes modified with a high coverage of Sbad (Pt/Sbad) the onset oxidation potential of 1,3-BD has been significantly decreased, which is attributed to the fact that the oxidation of Sbad occurs at lower potentials than that of the Pt surface. In situ FTIR results illustrated that, although at potentials below 0.5 V (vs a saturated calomel electrode), at which the Sbad is stable on the Pt electrode surface, both carbonyl and CO2 species have been observed, the principal oxidation products of 1,3-BD are carbonyl species. Such results indicate that the reaction is mainly the dehydrogenation of 1,3-BD molecules. However, at potentials above 0.5 V the proportion of CO2 species in the oxidation products increases quickly, implying that the reaction has turned to the breakage of C-C bonds in 1,3-BD molecules and the subsequent oxidation of the cleaved fragments. In contrast with the cases of 1,3-BD oxidation on Pt and Pt/Sbad electrodes, the reaction of 1,3-BD oxidation on a Pt electrode modified with S adatoms (Pt/Sad) is oriented completely to the production of carbonyl species when electrode potentials are below 0.9 V, though the reaction activity is relatively low. When the electrode potential is increased above 0.9 V, the intensity of the CO2 IR band in the FTIR spectra increases rapidly, corresponding to a fast oxidation of 1,3-BD on surface Pt sites recovered by the oxidation and desorption of Sad from the Pt surface.

1. Introduction The current paper demonstrates the manipulation of electrocatalytic reaction pathways involved in 1,3-butanediol (1,3-BD) oxidation through surface chemistry by modifying the surface of a Pt electrocatalyst with different adatoms. It is well-known that the convenient way to alter the electrocatalytic properties of an electrode is to modify its surface structure by using various foreign adatoms.1-3 For this purpose different techniques such as the electrochemical co-deposition,4 immersion method,5 underpotential deposition,6 and irreversible adsorption3 were employed in the literature. Depending on the electrode materials and adatoms, both geometric and electronic structures of the electrode surface may be altered, and enhancement or inhibition effects to different extents of the electrocatalytic activity toward the oxidation of small organic molecules have been reported.7,8 Janssen and Moolhuysen have investigated systematically, in an early study,9,10 the activity of platinum-based binary electrocatalysts for methanol oxidation. They classified the adatoms into four types (A, B, C, and D) according to their voltammogram features and found that the activity of these adatoms for methanol oxidation was in the order of type C (Sn, As, Sb, Bi, * Author to whom correspondence should be addressed. Phone: + 86592-2180181. Fax: +86-592-2183047. E-mail: [email protected]. † Department of Chemistry. ‡ Department of Physics.

Ru, Os, Dy) > type B (Ge, Pb, Re, U) > type A (Mo, W) ) type D (Ag, Hg, S, Se, Te, Zr, V, Nb). Motoo and co-workers later systematically examined the influence of adatoms on the oxidation of methanol, formaldehyde, formic acid, and 1,3dioxolane11-16 and suggested a few possible hypotheses on the effects of the adatoms. One of which consisted of the Shole control mechanism, i.e., different adatoms occupying a different number of platinum surface sites (SM ranged from 1 to 3). Only under conditions that the number of platinum sites unoccupied by adatoms (Shole) is greater than the number of reaction sites (SR) the oxidation could take place. The oxidation is enhanced by the inhibition of poison formation (such as CO species), since the process needs more Pt reaction sites. The other hypothesis was an oxygen-adsorbing mechanism. Oxygen atoms, which are required for the complete oxidation of most organic molecules,17 can adsorb on some kinds of adatoms such as Ge, Sn, As, Sb, etc. at relatively lower electrode potentials than that on a Pt surface, thus resulting in enhancement of the electrocatalytic activity for the oxidation at the low potential region. Besides, there are some other enhancement mechanisms proposed, such as the modification of apparent activation energies by adatoms,18 the blocking of hydrogen adsorption,7,19 the alteration of electronic properties,20 and the bifunctional effect of adatoms. 21 Among the adatoms mentioned above Sb and Bi were the most commonly used and studied.8, 22-29

10.1021/jp0609030 CCC: $33.50 © 2006 American Chemical Society Published on Web 05/18/2006

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The oxidation mechanism of 1,3-BD on a Pt electrode has been studied previously by using in situ Fourier transform infrared (FTIR) spectroscopy, and the reaction mechanism at the molecular level was illustrated.30 The present study has put an emphasis upon the management of electrocatalytic reaction pathways involved in 1,3-BD oxidation and has investigated the electrocatalytic properties of Pt electrodes modified by the irreversible adsorption of Sb and S adatoms (Pt/Sbad and Pt/ Sad). The results revealed, at a molecular level, the possibility of manipulating the reaction pathways in the electrocatalytic oxidation of small alcohol fuels such as 1,3-BD through surface chemistry. 2. Experimental Section A platinum disk of diameter ca. 6 mm was used as the working electrode, which was treated in flame before each measurement using Clavilier’s method;31 i.e., the Pt electrode was annealed in a hydrogen-oxygen flame, quenched by pure water, and transferred into an electrochemical cell under the protection of a droplet of pure water. The method with two electrochemical cells was employed in studies of irreversible adsorption of adatoms on a Pt electrode. The adsorption of adatoms was carried out in a beaker containing a solution of 10-3 mol L-1 S2- or Sb3+ ions. By immersion of a Pt electrode into the solutions for a few minutes a saturation coverage of irreversible adsorption of adatoms was formed. The electrode was then rinsed with superpure water three times to remove the solution sticking to the electrode surface, transferred into a cell containing 0.1 mol L-1 HClO4 solution, and characterized by cyclic voltammetry to measure the coverage of adatoms. The electrode of defined coverage of adatoms was finally transferred into the second cell that may be either the conventional cell or the in situ IR cell containing 0.1 mol L-1 1,3-BD + 0.1 M HClO4 solution to record the voltammograms of 1,3-BD oxidation or the corresponding in situ FTIR spectra. The coverage of adatoms was calculated from the comparison of the hydrogen adsorption charge before (QHs) and after (QHadatom) the adsorption of adatoms on the Pt electrode, i.e.,

Θadatom ) (QHs - QHadatom/QHs)

(1)

From the saturation coverage of adatoms, subsaturation coverage was obtained by partially stripping adatoms with a various number of potentials cycling between -0.25 and 1.20 V in pure HClO4 electrolyte solution. The in situ FTIR experiments were carried out in a Nexus 870 FTIR apparatus equipped with liquid-nitrogen-cooled MCT-A detector. The procedure of single potential alteration was applied to record in situ IR spectra, in which the potential was held first at E1 and n interferograms were collected, then it was switched to E2, and the same number of interferograms were acquired. The result spectrum is defined as the relative change in surface reflectivity and was obtained by Fourier transform of the coadded interferograms collected at each potential and calculated using the equation

(∆R/R) ) (R(E2) - R(E1))/R(E1)

(2)

where R(E1) and R(E2) represent the single beam spectra of IR reflections at E1 and E2, respectively. In general, negative bands appearing in the result spectrum indicate that more IR energy is absorbed at E2 than at E1 due to the formation of intermediate or product species, while positive bands denote a larger IR

Figure 1. Cyclic voltammogram of 1,3-butanediol oxidation on a Pt electrode in 0.1 M 1,3-BD + 0.1 M HClO4 solution: sweep rate 50 mV s-1.

absorption at E1 than that at E2 corresponding to the consumption of reactant or other species. All solutions were prepared with Milli-Q water provided by a Milli-Q Labo apparatus (Millipore Ltd., Japan). 1,3-Butanediol and perchloric acid were both superpure grade. Electrode potentials were refered to a saturated calomel electrode (SCE), and experiments were carried out at temperature around 20 ( 2 °C. 3. Results and Discussion 3.1. 1,3-BD Oxidation on a Pt Electrode. The cyclic voltammogram recorded in the first cycle of potential cycling for 1,3-BD oxidation on a Pt electrode is displayed in Figure 1. The oxidation of 1,3-BD yields very low current density in the positive going potential sweep (PGPS) below 0.4 V. Along with the increase of the potential, two 1,3-BD oxidation current peaks with densities of 0.24 and 0.55 mA cm-2 appear at around 0.55 and 1.0 V, respectively. In the negative going potential sweep (NGPS), a 1,3-BD oxidation current peak with a density of 0.28 mA cm-2 is found at 0.35 V right after the appearance of a small reduction peak at 0.55 V. The in situ FTIR spectra of 1,3-BD oxidation on a Pt polycrystalline electrode are shown in Figure 2. An IR band near 2048 cm-1 appears obviously in spectra of E2 bellow 0.4 V and is ascribed to the IR absorption of linearly adsorbed CO species (COL) that were derived from the dissociative adsorption of 1,3-BD on a Pt surface. Since COL is stable at low electrode potentials and occupies surface sites of the Pt electrode, it plays the role of a poisoning intermediate. When the electrode potential is increased to 0.2 V, an IR band near 2345 cm-1 attributed to the IR absorption of CO2 species in solution and an IR band at about 1722 cm-1 (corresponding to the CdO stretching) assigned to the IR absorption of carbonyl species are observed simultaneously with low intensity. The CO2 is probably generated from the oxidation of COL species on the Pt surface,32 since at this potential the oxidation current of 1,3BD is very weak as seen from the cyclic voltammogram displayed in Figure 1. With a further increase of the electrode potential, the COL band decreases gradually and disappears completely at 0.5 V, but the intensity of the CO2 band and that of the carbonyl band increase quickly. Moreover, new IR bands near 1270, 1306, 1367, 1396, 1455, 1499, and 1602 cm-1, which are ascribed, respectively, to OH deformation vibration, CH inplane deformation vibration, CH3 symmetrical bending, C-OH

FTIR Studies of 1,3-BD on Pt/Sbad and Pt/Sad

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Figure 3. Cyclic voltammograms of a Pt/Sbad electrode in 0.1 M HClO4 solution: ΘSb ) 0.84; sweep rate 50 mV.s-1.

Figure 2. In situ FTIR spectra of a Pt electrode in 0.1 M 1,3-butanediol + 0.1 M HClO4 solution: E1 ) -0.2 V; E2 varied from 0.0 to 1.2 V.

in-plane bending of COOH, CH3 asymmetrical bending, CH2 scissor vibration, and CdC stretch, appear in the fingerprint region. We observe also interference of water in the carbonyl band region. However, the inference of water is significant only at E2 below 0.2 V and can be neglected when the electrode potential is above 0.2 V, at which the carbonyl band can be clearly defined. These IR features can be attributed to IR absorption of reactive intermediate species involved in 1,3-BD oxidation such as CH3CHOHCH3, CH3CHdCH2, CH2dCHCH2COOH, CH3CHdCHCOOH, and CH3COCH2COOH. 33 A detailed reaction mechanism of 1,3-BD on a Pt electrode has been described previously in ref 33; here a simplified diagram is drawn as represented schematically by eq 3 to facilitate discussions in the following sections.

3.2. 1,3-BD Oxidation on Pt/Sbad Electrodes. 3.2.1. Characterization of Pt/Sbad Electrodes in 0.1 M HClO4 Solutions. The voltammogram of a Pt/Sbad electrode with a saturation coverage of Sb (ΘSb ) 0.86) recorded in HClO4 solution is shown in Figure 3. It can be observed that the presence of Sbad almost completely inhibits hydrogen adsorption-desorption. The oxidation-reduction of Sbad occurred in a pair of shoulder peaks near 0.16 V and a broad peak in the potential range between 0.25 and 0.45 V. When the upper limit potential of cyclic voltammetry is increased above 0.45 V, the Sbad will desorb gradually from the Pt surface, leading to a progressive decrease of ΘSb; the details have been reported in ref 34. 3.2.2. Cyclic Voltammetric Studies of 1,3-BD Oxidation on Pt/Sbad Electrodes. The influence of ΘSb on 1,3-BD oxidation is illustrated in Figure 4. When ΘSb is at 0.86 (i.e., the saturation coverage) and the electrode potential is below 0.2 V, the current density in the PGPS is near zero. With the increase of electrode

Figure 4. Influence of Sbad coverage on the oxidation of 1,3-BD in 0.1 M 1,3-BD + 0.1 M HClO4 solution: sweep rate 50 mV s-1; ΘSb ) 0.00 (s), 0.86 (sThinSpaces), 0.66 (- - -), 0.54 (- - - -), 0.39 (gray dashed line), 0.20 (gray solid line).

potential, the oxidation current density reaches a maximum value (∼160 µA cm-2) at about 0.39 V, then drops quickly and finally attains a plateau of a nearly stable value (90 µA cm-2) at potentials above 0.5 V. In the NGPS, the current density declines slowly at high potential, then goes up sharply and gives rise to an oxidation peak of about 238 µA cm-2 at 0.29 V. With the increase of the number of potential cycling, i.e., decreasing progressively the ΘSb,34 the potential of the oxidation current peak in the PGPS shifts to a more positive value, and the current density increases slightly. Meanwhile, the peak potential of 1,3BD oxidation in the NGPS becomes more and more positive; finally it approaches the same value as that measured in the voltammogram of 1,3-BD oxidation on a bare Pt electrode. It should be mentioned that the current density of 1,3-BD oxidation on Pt/Sbad electrodes with various coverages of Sbad is always smaller than that measured on a bare Pt electrode. In a comparison of the features observed in the voltammogram of the Pt/Sbad electrode at ΘSb ) 0.86 with those observed on the voltammogram of a bare Pt electrode, two interesting points may be remarked: (1) In the PGPS, the peak potential of 1,3-BD oxidation decreased (from 0.55 V on the Pt electrode to 0.39 V on the Pt/Sbad surface), while the current density decreased significantly (from 0.24 to 0.16 mA cm-2).

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Figure 6. Variation of the intensity of the COL band produced from 1,3-BD dissociation oxidation on a Pt electrode modified with different Sbad coverages in 0.1 M 1,3-BD + 0.1 M HClO4 solution: E1 ) -0.2 V; E2 ) 0.1 V.

Figure 5. In situ FTIR spectra of 1,3-BD oxidation on a Pt/Sbad electrode (ΘSb ≈ 0.86) in 0.1 M 1,3-BD + 0.1 M HClO4 solution: E1 ) -0.2 V; E2 is indicated for each spectrum and is varied from 0.0 to 1.2 V.

(2) In the NGPS, the peak potential of the oxidation current also decreases from 0.43 to 0.29 V, and the peak width became narrow. The above results may lead to the conclusion that the presence of Sbad has modified the reaction mechanism, which results in the lowering of the current peak potential of 1,3-BD oxidation in both the PGPS and the NGPS, and decreased at the same time the density of the peak current. Does this imply that different reaction processes are involved on the Pt and Pt/Sbad electrodes? To clarify this point, in situ FTIR spectroscopy was employed to investigate the reaction intermediate and product species implicated in different stages of 1,3-BD oxidation. 3.2.3. In Situ FTIR Spectroscopic Studies of 1,3-BD Oxidation on Pt/Sbad Electrodes. Figure 5 shows a series of in situ FTIR spectra obtained with a Pt/Sbad (ΘSb ) 0.86) electrode in 0.1 mol L-1 1,3-BD + 0.1 mol L-1 HClO4 solution, from which the main IR features of 1,3-BD oxidation can be summarized as the following: (1) There is no IR band appearing near 2045 cm-1 corresponding to the IR absorption of COL in all spectra displayed, which is different from that on the Pt electrode (Figure 2), implying that 1,3-BD cannot be dissociated into CO on a Pt/ Sbad electrode of high ΘSb. However, at low ΘSb (0.5 V), Sbad desorbed from the Pt surface gradually as pointed out previously,34 the CO2 band became more and more intense, and the carbonyl band increased merely slightly, indicating that the rate of increase of the CO2 species in the products of 1,3-oxidation is clearly faster than that of the carbonyl species. On the basis of the above results, the reaction mechanism of 1,3-BD oxidation on the Pt/Sbad electrodes may be briefly described as the following

3.3. 1,3-BD Oxidation on Pt/Sad Electrodes. 3.3.1. Characterization of Pt/Sad Electrodes in 0.1 mol L-1 HClO4 Solution. As seen in Figure 7, Sad can be oxidized at potentials above 0.5 V on a Pt/Sad electrode of saturation Sad coverage (ΘS ) 0.89). We observe, from the voltammogram of the first cycle, that the hydrogen adsorption-desorption was blocked completely

FTIR Studies of 1,3-BD on Pt/Sbad and Pt/Sad

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Figure 7. First five cycles of cyclic voltammetry curves recorded on a Pt/Sad electrode (the initial coverage of Sad is 0.89) in 0.1 M HClO4 solution; sweep rate 50 mV s-1.

by the presence of Sad at this saturation coverage. The current density is near zero when the electrode potential was below 0.5 V. The oxidation of Sad gives rise to a large current peak at 1.1 V in the PGPS. In the NGPS, the current drops slowly and remains a small reduction current when the electrode potential is below 0.63 V. Worth mentioning is that in the NGPS, for the saturation coverage of Sad, no reduction current peak is found at about 0.43 V, which is a characteristic feature of the reduction of Pt surface oxides formed at potentials above 1.0 V. This notification indicates primarily that the presence of a saturation coverage of Sad will inhibit the oxidation of the Pt surface at high potentials. When the electrode potential is scanned continuously to -0.14 V, a small reduction peak appears, which may be due to the reduction of the partially oxidized species of Sad formed during the oxidation in the PGPS.35 When the number of potential cyclings is increased, the peak current of hydrogen adsorption-desorption is augmented, while the oxidation current density of Sad decreases in the PGPS and the current peak shifts to negative potentials. A negative current peak at 0.43 V due to the reduction of the Pt surface oxide appears and becomes more and more pronounced. When the number of potential cyclings is larger than 4, the voltammograms recovered almost the same shape as those recorded on a bare Pt electrode. These results demonstrate that Sad can be removed from a Pt electrode progressively by potential cycling between -0.25 and 1.2 V. To study the product species of Sad oxidation, in situ FTIR spectroscopy was employed. In Figure 8 the spectrum of a Pt/ Sad electrode of saturation coverage of Sad in 0.1 mol L-1 HClO4 with E1 ) -0.2 and E2 ) 1.2 V is compared with the spectrum of a bare Pt electrode under the same conditions. The common IR feature of the two spectra consists of a negative going band around 1103 cm-1, which is assigned to the IR absorption of ClO4- ions near the electrode surface gathered at E2, since at a positive potential E2 the quantity of ClO4- species is certainly larger than that at a negative potential E1 caused by electrostatic attraction as a driving force. The large intensity of this band on a Pt/Sad electrode may be interpreted mainly by the low reflectivity of this surface at E1, since the single beam spectrum R(E1) served as the denominator in eq 2. The main difference between the two spectra consists of two small negative going bands that can be discerned at around 1198 and 1004 cm-1, respectively. These two bands can be assigned to the IR absorption of bisulfate and sulfate species36 appearing at 1.2 V. This result illustrates that Sad on a Pt surface will first capture

Figure 8. In situ FTIR spectra obtained in 0.1 M HClO4 solution on Pt and Pt/Sad electrodes: E1 ) -0.2 V; E2 ) 1.2 V.

Figure 9. Cyclic voltammograms of 1,3-BD oxidation on a Pt/Sad electrode with different ΘS in 0.1 M 1,3-BD + 0.1 M HClO4 solution: sweep rate 50 mV s-1; ΘS1 ) 0.89, ΘS2 ) 0.78, ΘS3 ) 0.64, ΘS4 ) 0.47, ΘS5 ) 0.33.

oxygen species in its oxidation at high potentials and the oxidation products of Sad are rather stable species (HSO4-, SO42-). Under this circumstance the oxidation of the Pt surface at the first cycle has been blocked completely by the presence of a saturation coverage of Sad; as a consequence we cannot observe the negative current peak near 0.43 V in the NGPS (Figure 7). 3.3.2. Cyclic Voltammetric Studies of 1,3-BD Oxidation on Pt/Sad Electrodes. Figure 9 displays voltammograms of 1,3-BD oxidation on a Pt electrode covered initially with Sad of ΘS ) 0.89. In the first cycle, the current density is near zero below 0.5 V, and above this potential the current density increases progressively and reaches a current peak of a density of 431 µA cm-2 at 1.13 V. Similar to the phenomena observed in Figure 7, there is no reduction current peak appearing in the NGPS close to 0.5 V in the first cycle. Moreover, the oxidation

11388 J. Phys. Chem. B, Vol. 110, No. 23, 2006 current cannot be observed at all below 0.6 V in the NGPS. The oxidation current peak in the PGPS shifts negatively to 1.0 V in the second cycle, due to the partial desorption of Sad in the first cycle, together with the appearance of a shoulder peak at 0.81 V. In the NGPS, a small reduction peak of the surface oxide of Pt appears near 0.51 V, and a small current peak of 1,3-BD oxidation near 0.35 V is observed. Starting from the third cycle we observe two current peaks of 1,3-BD oxidation, respectively, at 0.63 and 1.0 V in the PGPS and an oxidation current peak at 0.35 V in the NGPS. The amplitude of the current peak at 0.63 V in the PGPS decreases, while that at 0.35 V in the NGPS increases with the number of potential cyclings. It is worthwhile to mention that the larger the current density of the reduction peak at 0.51 V in the NGPS, the larger the current density of the oxidation peak at 0.35 V in the NGPS. The above observations may be interpreted as that the presence of saturation coverage of Sad on the Pt surface has inhibited completely the formation of Pt surface oxide species, which are necessarily involved in 1,3-BD oxidation. In the case of a Pt/Sad electrode with a saturation Sad coverage in HClO4 solution, the charge integrated from the cyclic voltammetry curve in the PGPS of the first cycle is about 2336 µC cm-2, which is even larger than the charge (2014 µC cm-2) acquired with the same electrode in a HClO4 solution containing 0.1 M 1,3-BD, implying that the oxidation current peak in the first cycle of the PGPS observed in Figure 9 is effectively due to the oxidation of Sad and that the existence of 1,3-BD in solution presents rather an inhibition effect for Sad oxidation. 3.3.3. In Situ FTIR Spectroscopic Studies of 1,3-BD Oxidation on Pt/Sad Electrodes. The in situ FTIR spectra recorded on a Pt/Sad electrode of ΘS ) 0.89 are shown in Figure 10. The IR features related to 1,3-BD oxidation may be summarized as following: (1) As predicted by the corresponding cyclic voltammetry data, there are no IR bands except that of ClO4- appearing in IR spectra when E2 is below 0.5 V, which confirms that the Pt/Sad electrode of a saturation Sad coverage is inactive for both oxidation and dissociation of 1,3-BD. (2) When the electrode potential is increased above 0.5 V, the Sad is partially oxidized and desorbed. The carbonyl band near 1720 cm-1 is clearly observed in the spectra. Together with other IR bands appearing in the fingerprint region (1000-2000 cm-1) and those seen in the C-H stretching region (25003000 cm-1), we can conclude that the reaction of 1,3-BD oxidation on the Pt/Sad electrode of ΘS ) 0.89 is oriented to produce carbonyl species; i.e., only dehydrogenation of 1,3BD has been taken place. We can observe also that the intensity of the carbonyl band increases slowly with the increase of E2. The CO2 band around 2345 cm-1 could not be observed significantly until E2 is raised as high as 0.9 V, and then its intensity increases quickly with the increase of E2. (3) The intensity of the carbonyl band is relatively small, in comparison with that in spectra recorded on a Pt electrode (Figure 2) as well as with that on a Pt/Sbad electrode (Figure 5). The in situ FTIR spectroscopic results demonstrated that the presence of a saturation coverage of Sad on a Pt electrode could inhibit both the dissociative adsorption and the oxidation of 1,3BD at low electrode potentials. When the electrode potential is increased above 0.5 V, the 1,3-BD oxidation was oriented toward the production of carbonyl species, i.e., occurring in the dehydrogenation. The production of CO2 species, which reflects the complete oxidation of fragments of 1,3-BD molecules, cannot be generated until the electrode potential reaches 0.9 V,

Wu et al. which is 0.4 V postponed in comparison with that on a Pt/Sbad electrode. When the electrode potential is more positive than 0.9 V, the oxidation product species are similar to those on a Pt electrode except that the proportion of different products is different (see discussion in a later section). Figure 11 displays in situ FTIR spectra of 1,3-BD oxidation on Pt/Sad electrodes of different Sad coverages recorded with E1 at -0.20 and E2 at 1.0 V. It is obvious that the intensity of both CO2 and carbonyl bands increases as the Sad coverage (ΘS) decreases. However, the intensity of the CO2 band increases faster than that of the carbonyl band. This result confirms once again that the Pt/Sad electrode of high Sad coverage leads the oxidation of 1,3-BD to yield mainly carbonyl species, while the Pt/Sad electrode of low Sad coverage orients the oxidation of 1,3-BD, producing more CO2 species. The ratio of the integral intensity of the CO2 band to that of the carbonyl band (ACO2/ ACdO) may represent the proportion of the CO2 and carbonyl species in the products of 1,3-BD oxidation and is plotted in Figure 12 as a function of Sad coverage. When the Sad coverage increases the ACO2/ACdO declines and reaches a minimum at ΘS ) 0.78. It is interesting to see that when ΘS further increases the ACO2/ACdO slightly increases. According to above results, the reaction mechanism of 1,3-BD oxidation on Pt/Sad electrodes can be represented schematically as the following

4. Discussion The in situ FTIR studies have demonstrated that the main products of 1,3-BD oxidation can be classified into two types of species, i.e., CO2 and carbonyl species, and the proportion of CO2 and carbonyl species in the products varies with surface modification of the Pt electrode by different adatoms of various coverages at a given electrode potential. To study further the effect of manipulating the reaction pathways involved in 1,3BD oxidation by surface chemistry through the modification of the surface structure of a Pt electrode, the ratios of integral intensity of the CO2 band versus the CdO band (i.e., A CO2/ ACdO) have been measured from spectra recorded on Pt, Pt/ Sbad(Θ ) 0.86), and Pt/Sad(Θ ) 0.89) electrodes and are compared in Figure 13 as a function of electrode potential. The values of ACO2/ACdO are almost independent of electrode potential and are slightly larger than 1.0 for 1,3-BD oxidation on a bare Pt surface. While on a Pt/Sbad electrode a small value of ACO2/ACdO is measured when the electrode potential is below 0.4 V, signifying that the main product of 1,3-BD oxidation is the carbonyl species. With the increase of electrode potential that leads to the decrease of the Sbad coverage by Sbad oxidation and desorption, ACO2/ACdO is increasing and reaches almost a plateau value around 0.85 between 0.7 and 1.0 V. ACO2/ACdO exceeds 1.0 when the electrode potential is increased above 1.0 V. In contrast, the value of ACO2/ACdO of the Pt/Sad electrodes is always the lowest among those of the three electrodes and smaller than 0.5 even at a potential as high as 1.2 V, indicating that the oxidation of 1,3-BD has been orientated to produce mainly carbonyl species on Sad-modified Pt electrocatalysts. It is known that each Sb adatom can block three surface sites of hydrogen adsorption (H-sites),34,37 while each S adatom occupies only 1 H-site on the Pt surface when the coverage of Sad is close to unity.38 According to Motoo and Shibata,11 the Pt surface sites can be divided into a large number of groups.

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Figure 12. Variation of ACO2/ACdO for 1,3-BD oxidation on a Pt/Sad electrode versus the Sad coverage. (All data are derived from Figure 11.)

Figure 10. In situ FTIR spectra of 1,3-BD oxidation on a Pt/Sad electrode (ΘS ≈ 0.89) in 0.1 M 1,3-BD + 0.1 M HClO4 solution: E1 ) -0.2 V; E2 is indicated for each spectrum and is varied from 0.0 to 1.2 V.

Figure 11. In situ FTIR spectra of 1,3-BD oxidation on a Pt/Sad electrode with different Sad coverages in 0.1 M 1,3-BD + 0.1 M HClO4 solution: (a) ΘS ) 0.89, (b) ΘS ) 0.78, (c) ΘS ) 0.64, (d) ΘS ) 0.47, and (e) ΘS ) 0.33; E1 ) -0.2 V; E2 ) 1.0 V.

Each group contains one to three H-sites. These groups of Pt surface sites are isolated from the other unoccupied Pt sites. Since the dissociation and breakage reaction of C-C bonds of 1,3-BD may need a certain number of continuous Pt sites (SR)

Figure 13. Variation of ACO2/ACdO for 1,3-BD oxidation on Pt (b), Pt/Sbad (2), and Pt/Sad (1) electrodes as a function of electrode potential (E2).

to assist the reaction, these reactions of 1,3-BD may take place only when the number of continuous Pt sites unoccupied by adatoms (Shole) is larger than SR. The decrease of continuous Pt sites by the increase of the coverage of adatoms obstructs the dissociation reaction and C-C breakage of 1,3-BD molecules. While the dehydrogenation reactions of 1,3-BD require fewer continuous Pt surface sites than those for the dissociation and C-C breakage, it could take place even with the presence of Sbad and Sad on the Pt surface. As a consequence it leads to the decrease in the oxidation peak current in cyclic voltammograms and the increase of ACO2/ACdO in IR spectra. From the above results, it can suggest that the presence of Sbad and Sad on the Pt electrode surface modifies the reactivity and selectivity of 1,3-BD oxidation via a geometric effect.39 Moreover, the modification of the Pt electrode by Sbad turns to enhance, to a large extent, the catalytic activity of the electrode by shifting negatively the onset potential of 1,3-BD oxidation. It is known from the literature that Sbad belongs to the group of oxygenadsorption adatoms;40 i.e., it can adsorb oxygen at potentials less positive than Pt does. Oxygen atoms adsorbed on Sb adatoms at low potentials will participate in the oxidation of 1,3-BD molecules or contribute to the oxidation of reactive intermediates adsorbed on Pt surface sites adjacent to the Sbad. As a consequence, the oxidation of 1,3-BD takes place at low electrode potentials. However, Sbad can be oxidized to SbO+ species at about 0.2 V on a Pt electrode.8 The positive electric charge of SbO+ would promote the charge transfer from 1,3BD to the electrode and then accelerate the oxidation reaction of 1,3-BD on Pt/Sbad. On the contrary to the oxygen-adsorbing species Sbad, Sad on a Pt surface captures oxygen species in its

11390 J. Phys. Chem. B, Vol. 110, No. 23, 2006 oxidation to bisulfate and sulfate species at high potentials. X-ray photoelectron spectroscopy analysis has proved that the Sad species on a Pt surface are partially charged negatively,35 which would hinder the electron transfer from 1,3-BD to the electrode and consequently slow the 1,3-BD oxidation. 5. Conclusions The present paper has placed an emphasis upon the manipulation of electrocatalytic pathways involved in 1,3-butanediaol oxidation through chemical modification of a Pt electrode surface. Two type adatoms, Sbad and Sad, were used to alter the surface structure of the Pt electrode. Results obtained from studies of cyclic voltammetry and in situ FTIR spectroscopy have demonstrated the following points. (1) The adsorption of the two kinds of foreign adatoms, i.e., Sbad and Sad, has altered the electrocatalytic activity of the Pt electrode in different ways. A negative shift of the onset potential of 1,3-BD oxidation was measured on Pt/Sbad surfaces (0.14 V for ΘSb ) 0.86), which implies an enhancement of electrocatalytic activity. However, a positive shift of the onset potential of 1,3-BD oxidation was found on Pt/Sad electrodes. (2) Chemical modification of a Pt surface by Sbad and Sad adatoms exhibited significant impacts on the manipulation of electrocatalytic reaction pathways. At a high (saturation) coverage of both Sbad and Sad adatoms, the dissociative adsorption of 1,3-BD into CO species has been completely inhibited, which leads to the consequence that the oxidation of 1,3-BD to CO2 via a poisoning intermediate is prevented. This effect is maintained even for low Sad coverages, while adsorbed CO species have been determined by in situ FTIR spectroscopy in the case of low Sbad coverages. At potentials below 0.5 V the oxidation of 1,3-BD on Pt/Sbad electrodes has been oriented to produce mainly carbonyl species, while at high electrode potentials the reaction is turned into increasing the proportion of CO2 species in the product. In contrast, the oxidation of 1,3BD on Pt/Sad electrodes produces mainly carbonyl species even at potentials as high as 0.9 V, at which the oxidation and desorption of Sad adatoms takes place. (3) The different impacts of chemical modification of Pt electrode surfaces by Sbad and Sad adatoms on the reaction pathways involved in 1,3-BD oxidation have been interpreted in terms of both geometric and electronic effects. It is known that one Sbad adatom blocks three surface sites for hydrogen adsorption on a Pt electrode, while one Sad adatom occupies only one surface site. In addition, positively charged Pt/Sbad surfaces and negatively charged Pt/Sad surfaces have been reported. These different properties resulted in different interactions between reactant 1,3-BD molecules with electrode surfaces. The results presented in this work illustrated the possibility of manipulation of reaction pathways involved in electrocatalytic oxidation of small organic molecules through surface chemistry and shed light upon the fundamental understanding of electrocatalysis as well as applications in fuel cells and electrosynthesis. Acknowledgment. This work was support by grants from the National Natural Science Foundation of China (Grant Nos. 20373059 and 90206039).

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