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Electrochemical reduction of carbon dioxide at a series of platinum single crystal electrodes
Electrochimica Acta 45 (2000) 4263– 4270 www.elsevier.nl/locate/electacta
Electrochemical reduction of carbon dioxide at a series of platinum single crystal electrodes Nagahiro Hoshi *, Yoshio Hori 1 Department of Applied Chemistry, Faculty of Engineering, Chiba Uni6ersity, 1 -33 Yayoi-cho, Inage-ku, Chiba, 263 -8522 Japan Received 7 November 1999; received in revised form 27 March 2000
Abstract Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single crystal electrodes examined were stepped surfaces (Pt(S)-[n(111)× (111)]), Pt(S)-[n(111) ×(100)], Pt(S)-[n(100)×(111)]), and kinked step surfaces (Pt(S)-[n(110)×(100)] and Pt(S)[n(100)× (110)]). Atomically flat surfaces, Pt(111) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111) BPt(100)B Pt(S)-[n(111)×(100)]B Pt(S)-[n(111)×(100)]B Pt(S)-[n(111)× (111)]B Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111)× (111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Reduction of CO2; Pt single crystal surfaces; Stepped surfaces; Kinked step surfaces; Voltammogram
1. Introduction We have been studying the electrochemical reduction of CO2 on various metal electrodes [1,2]. This reaction can be applied to a novel energy storage process for utilization of solar energy in the future. One of the major problems is whether a good electrocatalyst is developed to reduce the overpotential in this reaction. Platinum single crystal is an appropriate electrode for studying the structural effects on interfacial electrochemical reactions, since single crystals can be easily prepared according to Clavilier’s method [3]. Pt electrodes reduce CO2 to adsorbed CO with adsorbed hydrogen with low overpotential [4–6]: * Corresponding author. Tel./fax: +81-43-2903384. E-mail addresses: [email protected] (Y. Hori); [email protected] (N. Hoshi). 1 Also corresponding author. Tel./fax: + 81-43-2903382.
CO2 + 2Had COad + H2O The reaction kinetics was studied in detail on Pt polycrystal electrode using radiochemistry [7,8] and voltammetry [9]. The rate-determining step of this reaction is the hydrogen abstraction reaction [8]. So far no paper has appeared that studied the active sites for CO2 reduction using single crystal electrodes of Pt. Nikolic reported for the first time that the activity for CO2 reduction depends remarkably on the orientation of Pt low index planes: Pt(110) and Pt(100) reduce CO2 to adsorbed CO, whereas Pt(111) does not at all [10]. Some papers studied the time course of the reaction using low index planes [11– 13], but the origin of the orientation dependence was not discussed in detail. Rodes et al. [11,14,15] studied the CO2 reduction on stepped surfaces of Pt using IRRAS, and discussed the nature of the reduction product (adsorbed CO). However, no paper has appeared that systematically studies
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the structural effects on the rate of CO2 reduction using stepped surfaces of Pt. We attempt to reveal the atomic configuration of the active sites for CO2 reduction on Pt single crystal electrodes. We expect that this understanding will promote development of effective electrocatalysts for CO2 reduction for practical use in the future. This paper reviews our studies on the CO2 reduction on various stepped and kinked step surfaces of Pt electrodes, and proposes some hypotheses of the reaction mechanism.
2. Experimental Pt single crystal electrodes were prepared according to Clavilier [3], and the surface was treated by annealing about 1300°C and subsequent cooling in Ar + H2 stream. Blakely and Somorjai [16] reported that the annealing of Pt single crystals in oxygen containing atmosphere gives facets on the surfaces at 850°C. However, annealing at higher temperature and cooling in Ar+ H2 stream will give well-ordered surfaces, since the voltammograms depend reasonably on the step and terrace structures [17–19]. Our Pt single crystal electrodes employed in the present study gave voltammograms identical with those reported previously. The reduction of CO2 was conducted in 0.1 M HClO4 at a controlled electrode potential (Ead) for a
reaction time (tad). Then the potential was scanned positively to give the oxidation charge of adsorbed CO (QCO). QCO is equal to the amount of adsorbed CO reduced from CO2 for tad at the potential Ead. For the experimental details refer to our previous study [20].
3. Results and discussion
3.1. Low index planes Fig. 1 shows the voltammograms of the low index planes of Pt in 0.1 M HClO4. The voltammograms in Ar saturated solution are identical with those reported by Clavilier et al. [21– 23]. The oxidation peak of the adsorbed CO appears around 0.7 V in the positive scanning after holding the potential at 0.05 V in CO2 saturated solution. QCO grows with the holding time (tad). The velocity of QCO growth may be taken as an index of electrocatalytic activity, and the Pt(110) electrode gives the highest activity among three electrodes shown in Fig. 1. Pt(111) and Pt(100) show poor electrocatalytic activity in this reaction. The order of the activity is Pt(111) B Pt(100) Pt(110), compatible with other studies [11,12]. We also measured the rates of CO2 reduction on the low index planes of the other Pt group metals (Ir [24], Rh [25], and Pd [26]), and Ag on which CO2 is reduced to CO gas [27]. All these metals show the highest activity at the (110) surface, and the (100) and (111) surfaces give low activity. The morphology of Pt low index planes has been studied at the atomic scale using LEED and in-situ surface X-ray scattering. The (111) [28] and (100) [29] surfaces have flat structures and are not reconstructed up to the oxide film formation region. The (110) surface gives unreconstructed (1×1) and reconstructed (1×2) structures depending on the annealing and cooling conditions [30,31]. The (110) (1×1) surface of an fcc metal consist of 2 atomic rows of (111) terrace and monatomic (111) step (Pt(S)-[2(111)×(111)]), and (110) (1×2) is composed of 3 atomic rows of terrace and 2 atomic step (Pt(S)-[3(111)×2(111)]). Although it is not certain whether the present (110) electrode has (1×1) or (1×2) structure, both of them have step sites. Thus, the difference between the active (110) and inactive (111) surface is the presence of the step. Thus we presume that step site is active for CO2 reduction. We studied CO2 reduction on the stepped surfaces in order to verify the hypothesis.
3.2. Stepped surfaces 3.2.1. Pt(S) -[n(111)×(111)] surfaces Fig. 1. Voltammograms of the low index planes of Pt in 0.1 M HClO4. Left-hand side: stationary voltammograms in Ar saturated solution, right-hand side: voltammograms after holding the potentials at 0.05 V in CO2 saturated solution. Scanning rate: 0.050 V s − 1.
3.2.1.1. Voltammogram and initial rate of CO2 reduction (6t = 0). Well-defined Pt(S)-[n(111)×(111)] electrodes give voltammograms characteristic of the crystal orientation in 0.5 M H2SO4: the charge of the peak at 0.12 V is
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electrodes. Fig. 2 shows the stationary voltammograms of Pt(331)(n= 3) and Pt(997)(n= 9) in 0.1 M HClO4 saturated with Ar. Charges of the redox peaks at 0.12 V in Fig. 2 depend almost linearly on the step atom density. Former studies reported that the charge of the peak at 0.12 V in HClO4 is smaller than that in H2SO4, and attributed the excess charge in H2SO4 to the sulfuric acid adsorption [19]. But, our results and the recent report by Gomez et al. [39] show that the charge does not depend on the anion on the surfaces with n\ 4. The peak at 0.12 V may be due to the hydrogen adsorption rather than the anion adsorption on the surfaces with n\4. Voltammograms of Pt(S)-[n(111)×(111)] also gave CO oxidation peaks around 0.7 V after holding the potential at 0.05 V in CO2 saturated solution, as is the case of the low index planes (Fig. 1). QCO rose faster with the increase of the step atom density. The initial rate of CO2 reduction (6t = 0) was obtained from the tangential line of each time course at tad = 0 [37].
Fig. 2. Voltammograms of the Pt(S)-[n(111)× (111)] surfaces in 0.1 M HClO4 saturated with Ar [37].
proportional to the step atom density, and the charge of the other part in the adsorbed hydrogen region depends linearly on the terrace atom density [19,32–36]. The single crystal electrodes used in the present study give identical features. The electricity of all the electrodes examined nearly agreed with the values calculated on the assumption that hydrogen atoms occupy all the terrace and step atoms of the first layer of the (1 × 1) structure [37]. These facts confirm that the crystal surfaces are correctly oriented. The adsorption of sulfuric acid anion hinders the reduction of CO2 on Pt electrodes [38]; the reduction of CO2 in the present study was conducted in 0.1 M HClO4 in which no anion is strongly adsorbed on the Pt
Fig. 3. Potential dependeflnce of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(111)×(111)] electrodes in 0.1 M HClO4 [42].
3.2.1.2. Potential dependence of 6t = 0. Fig. 3 presents the potential dependence of 6t = 0 for various crystal orientations. The value of 6t = 0 gets higher at any potential for higher step atom density (i.e. lower n value). This fact indicates that the step is evidently connected with the active sites for CO2 reduction. The potential dependence in Fig. 3 may be rationalized on the basis of the following assumptions: (1) vacant step site activates CO2, (2) hydrogen adsorbed on the terrace adjacent to the step has high activity for CO2 reduction, and can reduce CO2 at the potential region where the step sites are fully occupied by Had. Iwasita et al. [40] reported that CO2 is adsorbed on Pt(111) and Pt(110) above 0.5 V as carbonate or bicarbonate anion in 0.1 M HClO4 saturated with CO2, but no report showed that CO2 is adsorbed on Pt electrodes in the adsorbed hydrogen region. Thus we do not take into account the adsorption of CO2 on Pt at present. At 0.05 V most surface sites including step will be occupied by adsorbed hydrogen atoms. We revealed that 6t = 0 is linearly related with the step atom density at this potential for all the crystal orientations [37]. This fact is compatible with the assumption that hydrogen atoms adsorbed on the terrace close to the step site favorably reduce CO2. The value of 6t = 0 reaches maximum at 0.2 V for the surfaces n 54 in Fig. 3. Since the amount of adsorbed hydrogen decreases at more positive potential, one would expect that 6t = 0 at 0.2 V would be lower than the value at 0.1 V or less. This apparently strange feature may be rationalized as follows. At 0.2 V the step sites will be vacant since the hydrogen atoms adsorbed at the step sites are oxidized at 0.12 V as shown in Fig. 2. The vacant step sites highly activate the reaction between CO2 and adsorbed hydrogen atoms according to assumption (1) above. Since the rate of the reproduction of the adsorbed
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Fig. 4. Voltammograms of the Pt(S)-[n(111)× (100)] surfaces in 0.1 M HClO4 saturated with Ar [42].
hydrogen is high [13,37] the reaction will proceed rapidly, although the amount of adsorbed hydrogen is small. Assumption (1) may be verified on the basis of the reaction conducted in 0.5 M H2SO4 electrolyte [38]. The value of 6t = 0 at 0.05 V in 0.5 M H2SO4, where most of the surface is occupied by adsorbed hydrogen, takes nearly identical values with those shown in 0.1 M HClO4 (Fig. 3). However, 6t = 0 in 0.5 M H2SO4 decreases monotonously with the increase of the potential without taking maximum at 0.2 V. We previously discussed this behavior in terms of the adsorption of sulfate anions, since specific adsorption of sulfate anions takes place at 0.15 V or more positive [38]. In 0.1 M HClO4, the step sites vacant at 0.2 V may be available for the activation of the CO2 reduction. In 0.5 M H2SO4, however, they are occupied 2− by sulfuric acid anions (HSO− 4 or SO4 ) and cannot promote the CO2 reduction effectively.
3.2.2. Pt(S) -[n(111)×(100)] surfaces The voltammograms of the present Pt(S)-[n(111)× (100)] electrodes gave sharp redox peaks at 0.27 V and
Fig. 5. Potential dependence of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(111)×(100)] electrodes in 0.1 M HClO4 [42].
broad redox peaks between 0.05 and 0.6 V in 0.5 M H2SO4. The charges of the former and the latter peaks were identical with those of the hydrogen desorption from the step and the terrace atoms of the (1 ×1) structure, respectively, as reported previously [17,32,34,41]. Fig. 4 shows the voltammograms in 0.1 M HClO4 saturated with Ar. The anodic currents get to minima at ca. 0.23 V. The occurrence of these minima suggests that all the hydrogen at terrace site is oxidized at 0.23 V. The charges of the sharp peaks between 0.25 and 0.4 V for all four electrodes of this series are almost proportional to the step atom density. This fact indicates that the adsorbed hydrogen atoms are present at the step sites between 0.25 and 0.40 V. Fig. 5 presents the potential dependence of 6t = 0 of Pt(S)-[n(111)× (100)] electrodes. The rate at 0.05– 0.15 V gives higher values in the order of the step atom density. All the surface sites will be occupied by adsorbed hydrogen atoms at 0.05 V, and the hydrogen atoms adsorbed adjacent to the step site will favorably react with CO2 on the Pt(S)-[n(111)×(100)] electrodes as well as on the Pt(S)-[n(111)× (111)] electrodes. The features above 0.15 V differ remarkably from those presented in Fig. 3. This feature, however, can be explained by the following assumption in addition to (1) and (2) mentioned in Section 3.2.1: (3) Adsorbed hydrogen at the step is strongly bound below 0.2 V and is inert for the reduction of CO2. Above 0.25 V where hydrogen adsorbed at the step is partly oxidized, the vacant sites become available for activating CO2 reduction. The value of 6t = 0 decreases with the increase of the potential or the decrease of adsorbed hydrogen, reaching a minimum near 0 at 0.2– 0.25 V, where hydrogen adsorbed at the terrace site disappears as mentioned above (Fig. 4). No hydrogen atoms will be supplied from the terrace to the sites adjacent to the step where CO2 reduction proceeds. In addition, hydrogen atoms occupy all the step sites and there is no vacant step site that activates CO2 at this potential. Thus 6t = 0 takes minimum near 0. Further increase of the potential enhances 6t = 0, leading to maxima at 0.3– 0.35 V. The magnitudes of the maxima are in the order of the step atom density. The maxima may be rationalized in terms of the evacuation of hydrogen atoms from the step sites similarly with Pt(S)[n(111)× (111)]. The reaction between CO2 and hydrogen atoms adsorbed at the step sites will be activated by the vacant step sites.
3.2.3. Pt(S) -[n(100)×(111)] electrodes The voltammograms of the present Pt(S)-[n(100)× (111)] surfaces gave sharp redox peaks at 0.27 and 0.37 V in 0.5 M H2SO4 [42], which originate from hydrogen adsorbed on the (100) terrace edge and wide (100) terrace, respectively. These voltammograms are identical with
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notation for the step structure. The potential dependence of 6t = 0 may be rationalized in a similar way with Pt(S)-[n(111)× (100)] electrodes as described above. Pt(100) shows low 6t = 0 between 0.05 and 0.40 V, and the activity rises above 0.40 V significantly. The study using surface X-ray scattering reported that Pt(100) gives an unreconstructed (1 ×1) structure up to the oxide film formation region in 0.1 M HClO4 [29]. However, according to a STM study [44] the surface of Pt(100) is reconstructed, forming an island structure at the positive potential. The STM study may rationalize our result, since the step sites at the edge of the island will enhance the activity for the CO2 reduction. Fig. 6. Voltammograms of the Pt(S)-[n(100)× (111)] electrodes in 0.1 M HClO4 saturated with Ar [42].
those reported previously [17,32,34,43]. The voltammetric charges due to the terrace edge and the terrace agreed with those calculated on the assumption that surfaces have (1 × 1) structures [42]. Voltammograms in 0.1 M HClO4 give peaks around 0.27 V that are similar to the peaks due to the (100) edge in 0.5 M H2SO4 (Fig. 6). Pt(S)-[n(100)×(111)] surfaces have the same notation for step as Pt(S)-[n(111) ×(111)]. Pt(S)-[n(111)×(111)] electrodes provide sharp peaks at 0.12 V due to the (111) step (Fig. 2), but Pt(S)-[n(100)× (111)] electrodes have no peak at 0.12 V (Fig. 6). The anodic currents give minima at 0.20–0.23 V as shown in the case of Pt(S)-[n(111) ×(100)]. The hydrogen at the terrace site is probably oxidized at this potential. Fig. 7 gives the potential dependence of 6t = 0 on Pt(S)-[n(100)× (111)] in 0.1 M HClO4. The features of Pt(S)-[n(100)×(111)] electrodes resemble those demonstrated for Pt(S)-[n(111)×(100)], differing significantly from Pt(S)-[n(111) ×(111)] in Fig. 3 in spite of the same
Fig. 7. Potential dependence of the initial rate of CO2 reduction (6t=0) on Pt(S)-[n(100)×(111)] electrodes in 0.1 M HClO4 [42].
3.2.4. Relationship between acti6ity and atomic arrangement on stepped surfaces We have demonstrated that the active sites for CO2 reduction are closely connected with the step sites. Here we discuss how the activity varies with the atomic configuration of the step structure. We tentatively take the highest value of 6t = 0 in the respective potential dependence (6t = 0(max)) as an index of the activity. Atomically flat surfaces, (111) and (100), are inert in the CO2 reduction, and the introduction of a step in the surfaces gives rise to a substantial increase in the activity. A comparison of 6t = 0(max) among the series of the electrodes is made with an equal number of n, and the order of the activity may be given as: Pt(S)-[n(111)× (100)]BPt(S)-[n(100)×(111)] BPt(S)-[n(111)×(111)]. The activity of Pt(S)-[n(111)×(111)] is much higher than that of Pt(S)-[n(100)× (111)]. Fig. 8 illustrates the electrode series of the stepped surfaces in the order of the activity. Significant difference of the activity between Pt(S)-[n(111)×(111)] and Pt(S)[n(100)× (111)] leads to a hypothesis that the active site for the CO2 reduction may be derived from the pseudo-4fold bridged sites sb in the step of Pt(S)-[n(111)× (111)]. It is impossible at present to reveal how the pseudo-4-fold bridged sites work in the enhancement of the activity; they may provide active hydrogen atoms adsorbed on the sites or activate the interaction between CO2 and adsorbed hydrogen. Among all the stepped surfaces, Pt(110) shows the highest activity in the reaction; over 200 times as high as Pt(111). Markovic´ and coworkers [30,31] reported that the annealing procedure can control the surface symmetry of the Pt(110), (1 × 1) or (1 × 2) structure. We annealed Pt(110) about 1300°C and immediately cooled it in Ar/H2 atmosphere. This procedure may provide (1× 1) rather than (1 × 2) according to Markovic´. In both cases, however, the atomic configuration is symmetric with regard to the pseudo-4-fold bridged sites. Such densely packed symmetric pseudo-4-fold bridged sites
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Fig. 10. Potential dependence of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(110) ×(100)] electrodes in 0.1 M HClO4 [20].
Fig. 8. Order of the activity for CO2 reduction shown by the hard sphere models of the stepped surfaces. Small solid circle sb represents the pseudo-4-fold bridged sites.
may provide an atomic configuration favorable for the reduction of CO2.
3.3. Kinked step surfaces 3.3.1. Pt(S) -[n(110)×(100)] surfaces We further studied CO2 reduction on Pt(980) (n=9) and Pt(210) (n=2) [20]. Introduction of a (100) step to the (110) surface gives rise to protruding atoms along the step lines, which is marked by solid spheres in a hard sphere model of Pt(980) (Fig. 9). We define a surface containing the protruding atoms along the step lines as a kinked step surface. The shapes of the voltammograms were identical with those reported by Furuya and coworkers [32,34]. It is difficult to relate voltammetric peaks with the adsorption sites of hydrogen on these electrodes; we confirm the crystal orientation on the basis of the desorption charge of the hydrogen in 0.1 M HClO4 saturated with Ar. The
Fig. 9. Hard sphere models of Pt(S)-[n(110)× (100)] surfaces. Solid spheres represent kink atoms.
desorption charges agree with the values calculated by assuming that one hydrogen atom adsorbed on one Pt atom of (1 ×1) structure [20]. The CO oxidation takes place around 0.7 V as measured by voltammetry. The values of 6t = 0 are plotted against the potential in Fig. 10. The value of 6t = 0 between 0.05 and 0.15 V gets higher with the increase of the (100) step density or the kink atom density. The presence of kink atoms further enhances the activity of Pt(110) which is the most active stepped surfaces in CO2 reduction.
3.3.2. Pt(S) -[n(100)×(110)] surfaces The hard sphere models of the Pt(S)-[n(100)× (110)] surfaces are illustrated in Fig. 11 in which kink atoms are shown as solid spheres. Pt(210) (n= 2) are also classified into this series. The anodic charges of hydrogen desorption in the voltammograms in 0.1 M HClO4 suggest that the surfaces have a (1 × 1) structure [20]. Fig. 12 presents the potential dependence of 6t = 0. The value of 6t = 0 decreases simply with the increase of the potential. The surfaces with higher kink atom density give higher values of 6t = 0. Hydrogen atoms will occupy all the kink atoms at 0.05 V, since the coverage of the adsorbed hydrogen is 1 at 0.05 V. The values of 6t = 0 at 0.05 V depend linearly on kink atom density at 0.05 V in Fig. 13, clearly demonstrating that the kink is the active site for CO2 reduction. The slope of the line represents the activity per kink atom. The slopes of the kinked step surfaces are more than twice as high as those of the stepped surfaces. This result definitely shows that the kink has higher activity for CO2 reduction than the step. The slope of Pt(S)[n(100)× (110)] surfaces is 65% higher than that of Pt(S)-[n(110)× (100)]. The kink atoms are packed sparsely along the step line on Pt(S)-[n(110)× (100)] (Fig. 9), whereas Pt(S)-[n(100)× (110)] surfaces have densely
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packed kink atoms along the step line (Fig. 11). This fact supports that densely packed kink atoms lead to high activity in CO2 reduction. The adsorbed hydrogen atoms adjacent to the kink site may have ex-
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tremely high activity for CO2 reduction. CO2 needs two hydrogen atoms to be reduced (CO2 + 2Had COad + H2O), thus active hydrogen atoms locating closely with each other will enhance the rate of CO2 reduction on the surfaces with densely packed kink atoms.
4. Electrocatalytic activity and broken bond density Interatomic bonds are cut when a surface is newly produced from a single crystal. The broken bond density per atom (dbb) depends on the crystal orientation, and geometrically determined for fcc metals as: dbb = (8h+ 4k)/(h 2 + k 2 + l 2)1/2, Fig. 11. Hard sphere models of Pt(S)-[n(100)× (110)] surfaces. Solid spheres represent kink atoms.
Fig. 12. Potential dependence of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(100)×(110)] electrodes in 0.1 M HClO4 [20].
Fig. 13. The initial rate of CO2 reduction (6t = 0) at 0.05 V (RHE) plotted against the kink atom density. The results from the stepped surfaces are also given as plotted against the step atom density for comparison [42].
where h, k, l denote Miller indices [45]. The broken bond density is closely connected with the surface energy of single crystals, and well correlated with the potential of zero charge of gold and silver [46,47]. Chang et al. [48] studied electrooxidation of CO on gold single crystal electrodes. They employed the electrodes Au(110), Au (210), Au (100), Au (221), Au (533), and Au (111). They obtained a correlation between the reaction rates and the broken bond density. The present 6t = 0 are correlated with the broken bond density calculated with the equation shown above. The correlation is shown in Fig. 14. No single line can correlate 6t = 0 and dbb for all the electrodes. A correlation straight line is obtained for each series of the electrodes, which has the identical atomic configuration of step or kink. For any series of the electrodes, high electrocatalytic activity appears at electrodes with high broken bond density. The high broken bond density may promote the activity of the adsorbed hydrogen adjacent to the step and the activity of the vacant step site.
Fig. 14. The values of the initial rate for CO2 reduction (6t = 0) at 0.1 V plotted against the density of broken bond (dbb).
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5. Conclusion (1) Flat surfaces, Pt(111) and Pt(100), have low activity for CO2 reduction. (2) The initial rate of the CO2 reduction gets higher with the increase of the step atom density. (3) The terrace and step structures affect the potential dependence of the initial rate remarkably. (4) The activity for CO2 reduction depends remarkably on the symmetry of the surface. Following activity series are obtained for stepped surfaces: Pt(111)BPt(100)BPt(S)-[n(111)×(100)] B Pt(S)-[n(100)×(111)]BPt(S)-[n(111)×(111)] B Pt(110). Pt(110) has the highest activity for CO2 reduction in the stepped surfaces. (5) Kinked step surfaces have higher activity for CO2 reduction than the stepped surfaces. (6) Pt(S)-[n(100) ×(110)] series, which have densely packed kink atoms along step lines, have the highest activity for CO2 reduction. Acknowledgements This study was supported by Grant-in-Aid for Scientific Research, 05740349, 04241106, 06226214, 0874 0440, 08232217,09450313 and 10131212. References [1] Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett. (1985) 1965. [2] Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc. Faraday Trans. 185 (1989) 2309 and references are cited therein. [3] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [4] J. Giner, Electrochim. Acta 8 (1963) 857. [5] M.W. Breiter, Electrochim. Acta 12 (1967) 1213. [6] B. Beden, A. Bewick, M. Razaq, J. Weber, J. Electroanal. Chem. 139 (1982) 203. [7] J. Sobkowski, A. Czerwinski, J. Electroanal. Chem. 55 (1974) 391. [8] J. Sobkowski, A. Czerwinski, J. Electroanal. Chem. 65 (1975) 327. [9] M.C. Arevalo, C. Gomis-Bas, F. Hahn, B. Beden, A. Arevalo, A.J. Arvia, Electrochim. Acta 39 (1994) 793 and references are cited therein. [10] B.Z. Nikolic, H. Huang, D. Gervasio, et al., J. Electroanal. Chem. 295 (1990) 415. [11] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 369 (1994) 183. [12] S. Taguchi, A. Aramata, Electrochim. Acta 39 (1994) 2533. [13] N. Hoshi, T. Mizumura, Y. Hori, Electrochim. Acta 40 (1995) 883. [14] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 373 (1994) 167.
[15] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 377 (1994) 215. [16] D.W. Blakely, G.A. Somorjai, Surf. Sci. 65 (1977) 419. [17] N.M. Markovic´, N.S. Marinkovic´, R.R. Azˇic´, J. Electroanal. Chem. 241 (1988) 309. [18] A. Rodes, K. El Achi, M.A. Zammakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245. [19] N.M. Markovic´, N.S. Marinkovic´, R.R. Azˇic´, J. Electroanal. Chem. 314 (1991) 289. [20] N. Hoshi, S. Kawatani, M. Kudo, Y. Hori, J. Electroanal. Chem. 467 (1999) 67. [21] J. Clavilier, D. Armand, B.L. Wu, J. Electroanal. Chem. 135 (1982) 159. [22] A. Rodes, M.A. Zamakhchari, K. El Achi, J. Clavilier, J. Electroanal. Chem. 305 (1991) 115. [23] R. Go´mez, J. Clavilier, J. Electroanal. Chem. 354 (1993) 189. [24] N. Hoshi, T. Uchida, T. Mizumura, Y. Hori, J. Electroanal. Chem. 381 (1995) 261. [25] N. Hoshi, H. Ito, T. Suzuki, Y. Hori, J. Electroanal. Chem. 395 (1995) 309. [26] N. Hoshi, M. Noma, T. Suzuki, Y. Hori, J. Electroanal. Chem. 421 (1997) 15. [27] N. Hoshi, M. Kato, Y. Hori, J. Electroanal. Chem. 440 (1997) 283. [28] F.H. Feddrix, E.B. Yeager, B.D. Cahan, J. Electroanal. Chem. 330 (1992) 419. [29] I.M. Tidswell, N.M. Markovic´, P.N. Ross, Phys. Rev. Lett. 71 (1993) 1601. [30] C.A. Lucas, N.M. Markovic´, P.N. Ross, Phys. Rev. Lett. 77 (1996) 4922. [31] N.M. Markovic´, B.N. Grgur, C.A. Lucas, P.N. Ross, Surf. Sci. 384 (1997) L805. [32] S. Motoo, N. Furuya, Ber. Bunsenges. Phys. Chem. 91 (1987) 457. [33] J. Clavilier, K. El Achi, A. Rodes, J. Electroanal. Chem. 272 (1989) 253. [34] N. Furuya, S. Koide, Surf. Sci. 220 (1989) 18. [35] P.N. Ross, J. Chim. Phys. 88 (1991) 1353. [36] J. Clavilier, A. Rodes, K. El Achi, M.A. Zamakhchari, J. Chim. Phys. 88 (1991) 1291. [37] N. Hoshi, T. Suzuki, Y. Hori, Electrochim. Acta 41 (1996) 1647. [38] N. Hoshi, T. Suzuki, Y. Hori, J. Electroanal. Chem. 416 (1996) 65. [39] R. Gomez, V. Climent, J.M. Feliu, M.J. Weaver, J. Phys. Chem. B 104 (2000) 597. [40] T. Iwasita, A. Rodes, E. Pastor, J. Electroanal. Chem. 383 (1995) 181. [41] A. Rodes, K. El Achi, M.A. Zamakhchari, J. Clavilier, J. Electroanal. Chem. 284 (1990) 245. [42] N. Hoshi, T. Suzuki, Y. Hori, J. Phys. Chem. B 101 (1997) 8520. [43] J.M. Feliu, A. Rodes, J.M. Orts, J. Clavilier, Pol. J. Phys. Chem. 68 (1994) 1575. [44] I. Villegas, M.J. Weaver, J. Electroanal. Chem. 373 (1994) 245. [45] J.K. Mackenzie, A.J.W. Moore, J.F. Nicholas, J. Phys. Chem. Solids 23 (1962) 185. [46] A. Hamelin, J. Lecoeur, Surf. Sci. 57 (1976) 771. [47] R. de Levie, J. Electroanal. Chem. 280 (1990) 179. [48] S.-C. Chang, A. Hamelin, M.J. Weaver, J. Phys. Chem. 95 (1991) 5560.
Electrochemical reduction of carbon dioxide at a series of platinum single crystal electrodes Nagahiro Hoshi *, Yoshio Hori 1 Department of Applied Chemistry, Faculty of Engineering, Chiba Uni6ersity, 1 -33 Yayoi-cho, Inage-ku, Chiba, 263 -8522 Japan Received 7 November 1999; received in revised form 27 March 2000
Abstract Electrocatalytic activity in the reduction of CO2 to adsorbed CO was studied systematically on a series of Pt single crystal electrodes using voltammetry. The single crystal electrodes examined were stepped surfaces (Pt(S)-[n(111)× (111)]), Pt(S)-[n(111) ×(100)], Pt(S)-[n(100)×(111)]), and kinked step surfaces (Pt(S)-[n(110)×(100)] and Pt(S)[n(100)× (110)]). Atomically flat surfaces, Pt(111) and Pt(100), show poor activity for CO2 reduction. Introduction of step sites to (111) or (100) surface significantly enhances the electrocatalytic activity in CO2 reduction. The rate increases proportionally with the step atom density. The order of the activity series is obtained for the stepped surfaces: Pt(111) BPt(100)B Pt(S)-[n(111)×(100)]B Pt(S)-[n(111)×(100)]B Pt(S)-[n(111)× (111)]B Pt(110). The most active site in the stepped surface is derived from the psudo-4-fold bridged site in Pt(S)-[n(111)× (111)]. Kinked step surfaces show higher activity than stepped surfaces: the reduction rate per kink atom is more than twice as high as the value per step atom. Densely packed kink atoms along the step line greatly promote the reduction of CO2. © 2000 Elsevier Science Ltd. All rights reserved. Keywords: Reduction of CO2; Pt single crystal surfaces; Stepped surfaces; Kinked step surfaces; Voltammogram
1. Introduction We have been studying the electrochemical reduction of CO2 on various metal electrodes [1,2]. This reaction can be applied to a novel energy storage process for utilization of solar energy in the future. One of the major problems is whether a good electrocatalyst is developed to reduce the overpotential in this reaction. Platinum single crystal is an appropriate electrode for studying the structural effects on interfacial electrochemical reactions, since single crystals can be easily prepared according to Clavilier’s method [3]. Pt electrodes reduce CO2 to adsorbed CO with adsorbed hydrogen with low overpotential [4–6]: * Corresponding author. Tel./fax: +81-43-2903384. E-mail addresses: [email protected] (Y. Hori); [email protected] (N. Hoshi). 1 Also corresponding author. Tel./fax: + 81-43-2903382.
CO2 + 2Had COad + H2O The reaction kinetics was studied in detail on Pt polycrystal electrode using radiochemistry [7,8] and voltammetry [9]. The rate-determining step of this reaction is the hydrogen abstraction reaction [8]. So far no paper has appeared that studied the active sites for CO2 reduction using single crystal electrodes of Pt. Nikolic reported for the first time that the activity for CO2 reduction depends remarkably on the orientation of Pt low index planes: Pt(110) and Pt(100) reduce CO2 to adsorbed CO, whereas Pt(111) does not at all [10]. Some papers studied the time course of the reaction using low index planes [11– 13], but the origin of the orientation dependence was not discussed in detail. Rodes et al. [11,14,15] studied the CO2 reduction on stepped surfaces of Pt using IRRAS, and discussed the nature of the reduction product (adsorbed CO). However, no paper has appeared that systematically studies
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the structural effects on the rate of CO2 reduction using stepped surfaces of Pt. We attempt to reveal the atomic configuration of the active sites for CO2 reduction on Pt single crystal electrodes. We expect that this understanding will promote development of effective electrocatalysts for CO2 reduction for practical use in the future. This paper reviews our studies on the CO2 reduction on various stepped and kinked step surfaces of Pt electrodes, and proposes some hypotheses of the reaction mechanism.
2. Experimental Pt single crystal electrodes were prepared according to Clavilier [3], and the surface was treated by annealing about 1300°C and subsequent cooling in Ar + H2 stream. Blakely and Somorjai [16] reported that the annealing of Pt single crystals in oxygen containing atmosphere gives facets on the surfaces at 850°C. However, annealing at higher temperature and cooling in Ar+ H2 stream will give well-ordered surfaces, since the voltammograms depend reasonably on the step and terrace structures [17–19]. Our Pt single crystal electrodes employed in the present study gave voltammograms identical with those reported previously. The reduction of CO2 was conducted in 0.1 M HClO4 at a controlled electrode potential (Ead) for a
reaction time (tad). Then the potential was scanned positively to give the oxidation charge of adsorbed CO (QCO). QCO is equal to the amount of adsorbed CO reduced from CO2 for tad at the potential Ead. For the experimental details refer to our previous study [20].
3. Results and discussion
3.1. Low index planes Fig. 1 shows the voltammograms of the low index planes of Pt in 0.1 M HClO4. The voltammograms in Ar saturated solution are identical with those reported by Clavilier et al. [21– 23]. The oxidation peak of the adsorbed CO appears around 0.7 V in the positive scanning after holding the potential at 0.05 V in CO2 saturated solution. QCO grows with the holding time (tad). The velocity of QCO growth may be taken as an index of electrocatalytic activity, and the Pt(110) electrode gives the highest activity among three electrodes shown in Fig. 1. Pt(111) and Pt(100) show poor electrocatalytic activity in this reaction. The order of the activity is Pt(111) B Pt(100) Pt(110), compatible with other studies [11,12]. We also measured the rates of CO2 reduction on the low index planes of the other Pt group metals (Ir [24], Rh [25], and Pd [26]), and Ag on which CO2 is reduced to CO gas [27]. All these metals show the highest activity at the (110) surface, and the (100) and (111) surfaces give low activity. The morphology of Pt low index planes has been studied at the atomic scale using LEED and in-situ surface X-ray scattering. The (111) [28] and (100) [29] surfaces have flat structures and are not reconstructed up to the oxide film formation region. The (110) surface gives unreconstructed (1×1) and reconstructed (1×2) structures depending on the annealing and cooling conditions [30,31]. The (110) (1×1) surface of an fcc metal consist of 2 atomic rows of (111) terrace and monatomic (111) step (Pt(S)-[2(111)×(111)]), and (110) (1×2) is composed of 3 atomic rows of terrace and 2 atomic step (Pt(S)-[3(111)×2(111)]). Although it is not certain whether the present (110) electrode has (1×1) or (1×2) structure, both of them have step sites. Thus, the difference between the active (110) and inactive (111) surface is the presence of the step. Thus we presume that step site is active for CO2 reduction. We studied CO2 reduction on the stepped surfaces in order to verify the hypothesis.
3.2. Stepped surfaces 3.2.1. Pt(S) -[n(111)×(111)] surfaces Fig. 1. Voltammograms of the low index planes of Pt in 0.1 M HClO4. Left-hand side: stationary voltammograms in Ar saturated solution, right-hand side: voltammograms after holding the potentials at 0.05 V in CO2 saturated solution. Scanning rate: 0.050 V s − 1.
3.2.1.1. Voltammogram and initial rate of CO2 reduction (6t = 0). Well-defined Pt(S)-[n(111)×(111)] electrodes give voltammograms characteristic of the crystal orientation in 0.5 M H2SO4: the charge of the peak at 0.12 V is
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electrodes. Fig. 2 shows the stationary voltammograms of Pt(331)(n= 3) and Pt(997)(n= 9) in 0.1 M HClO4 saturated with Ar. Charges of the redox peaks at 0.12 V in Fig. 2 depend almost linearly on the step atom density. Former studies reported that the charge of the peak at 0.12 V in HClO4 is smaller than that in H2SO4, and attributed the excess charge in H2SO4 to the sulfuric acid adsorption [19]. But, our results and the recent report by Gomez et al. [39] show that the charge does not depend on the anion on the surfaces with n\ 4. The peak at 0.12 V may be due to the hydrogen adsorption rather than the anion adsorption on the surfaces with n\4. Voltammograms of Pt(S)-[n(111)×(111)] also gave CO oxidation peaks around 0.7 V after holding the potential at 0.05 V in CO2 saturated solution, as is the case of the low index planes (Fig. 1). QCO rose faster with the increase of the step atom density. The initial rate of CO2 reduction (6t = 0) was obtained from the tangential line of each time course at tad = 0 [37].
Fig. 2. Voltammograms of the Pt(S)-[n(111)× (111)] surfaces in 0.1 M HClO4 saturated with Ar [37].
proportional to the step atom density, and the charge of the other part in the adsorbed hydrogen region depends linearly on the terrace atom density [19,32–36]. The single crystal electrodes used in the present study give identical features. The electricity of all the electrodes examined nearly agreed with the values calculated on the assumption that hydrogen atoms occupy all the terrace and step atoms of the first layer of the (1 × 1) structure [37]. These facts confirm that the crystal surfaces are correctly oriented. The adsorption of sulfuric acid anion hinders the reduction of CO2 on Pt electrodes [38]; the reduction of CO2 in the present study was conducted in 0.1 M HClO4 in which no anion is strongly adsorbed on the Pt
Fig. 3. Potential dependeflnce of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(111)×(111)] electrodes in 0.1 M HClO4 [42].
3.2.1.2. Potential dependence of 6t = 0. Fig. 3 presents the potential dependence of 6t = 0 for various crystal orientations. The value of 6t = 0 gets higher at any potential for higher step atom density (i.e. lower n value). This fact indicates that the step is evidently connected with the active sites for CO2 reduction. The potential dependence in Fig. 3 may be rationalized on the basis of the following assumptions: (1) vacant step site activates CO2, (2) hydrogen adsorbed on the terrace adjacent to the step has high activity for CO2 reduction, and can reduce CO2 at the potential region where the step sites are fully occupied by Had. Iwasita et al. [40] reported that CO2 is adsorbed on Pt(111) and Pt(110) above 0.5 V as carbonate or bicarbonate anion in 0.1 M HClO4 saturated with CO2, but no report showed that CO2 is adsorbed on Pt electrodes in the adsorbed hydrogen region. Thus we do not take into account the adsorption of CO2 on Pt at present. At 0.05 V most surface sites including step will be occupied by adsorbed hydrogen atoms. We revealed that 6t = 0 is linearly related with the step atom density at this potential for all the crystal orientations [37]. This fact is compatible with the assumption that hydrogen atoms adsorbed on the terrace close to the step site favorably reduce CO2. The value of 6t = 0 reaches maximum at 0.2 V for the surfaces n 54 in Fig. 3. Since the amount of adsorbed hydrogen decreases at more positive potential, one would expect that 6t = 0 at 0.2 V would be lower than the value at 0.1 V or less. This apparently strange feature may be rationalized as follows. At 0.2 V the step sites will be vacant since the hydrogen atoms adsorbed at the step sites are oxidized at 0.12 V as shown in Fig. 2. The vacant step sites highly activate the reaction between CO2 and adsorbed hydrogen atoms according to assumption (1) above. Since the rate of the reproduction of the adsorbed
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Fig. 4. Voltammograms of the Pt(S)-[n(111)× (100)] surfaces in 0.1 M HClO4 saturated with Ar [42].
hydrogen is high [13,37] the reaction will proceed rapidly, although the amount of adsorbed hydrogen is small. Assumption (1) may be verified on the basis of the reaction conducted in 0.5 M H2SO4 electrolyte [38]. The value of 6t = 0 at 0.05 V in 0.5 M H2SO4, where most of the surface is occupied by adsorbed hydrogen, takes nearly identical values with those shown in 0.1 M HClO4 (Fig. 3). However, 6t = 0 in 0.5 M H2SO4 decreases monotonously with the increase of the potential without taking maximum at 0.2 V. We previously discussed this behavior in terms of the adsorption of sulfate anions, since specific adsorption of sulfate anions takes place at 0.15 V or more positive [38]. In 0.1 M HClO4, the step sites vacant at 0.2 V may be available for the activation of the CO2 reduction. In 0.5 M H2SO4, however, they are occupied 2− by sulfuric acid anions (HSO− 4 or SO4 ) and cannot promote the CO2 reduction effectively.
3.2.2. Pt(S) -[n(111)×(100)] surfaces The voltammograms of the present Pt(S)-[n(111)× (100)] electrodes gave sharp redox peaks at 0.27 V and
Fig. 5. Potential dependence of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(111)×(100)] electrodes in 0.1 M HClO4 [42].
broad redox peaks between 0.05 and 0.6 V in 0.5 M H2SO4. The charges of the former and the latter peaks were identical with those of the hydrogen desorption from the step and the terrace atoms of the (1 ×1) structure, respectively, as reported previously [17,32,34,41]. Fig. 4 shows the voltammograms in 0.1 M HClO4 saturated with Ar. The anodic currents get to minima at ca. 0.23 V. The occurrence of these minima suggests that all the hydrogen at terrace site is oxidized at 0.23 V. The charges of the sharp peaks between 0.25 and 0.4 V for all four electrodes of this series are almost proportional to the step atom density. This fact indicates that the adsorbed hydrogen atoms are present at the step sites between 0.25 and 0.40 V. Fig. 5 presents the potential dependence of 6t = 0 of Pt(S)-[n(111)× (100)] electrodes. The rate at 0.05– 0.15 V gives higher values in the order of the step atom density. All the surface sites will be occupied by adsorbed hydrogen atoms at 0.05 V, and the hydrogen atoms adsorbed adjacent to the step site will favorably react with CO2 on the Pt(S)-[n(111)×(100)] electrodes as well as on the Pt(S)-[n(111)× (111)] electrodes. The features above 0.15 V differ remarkably from those presented in Fig. 3. This feature, however, can be explained by the following assumption in addition to (1) and (2) mentioned in Section 3.2.1: (3) Adsorbed hydrogen at the step is strongly bound below 0.2 V and is inert for the reduction of CO2. Above 0.25 V where hydrogen adsorbed at the step is partly oxidized, the vacant sites become available for activating CO2 reduction. The value of 6t = 0 decreases with the increase of the potential or the decrease of adsorbed hydrogen, reaching a minimum near 0 at 0.2– 0.25 V, where hydrogen adsorbed at the terrace site disappears as mentioned above (Fig. 4). No hydrogen atoms will be supplied from the terrace to the sites adjacent to the step where CO2 reduction proceeds. In addition, hydrogen atoms occupy all the step sites and there is no vacant step site that activates CO2 at this potential. Thus 6t = 0 takes minimum near 0. Further increase of the potential enhances 6t = 0, leading to maxima at 0.3– 0.35 V. The magnitudes of the maxima are in the order of the step atom density. The maxima may be rationalized in terms of the evacuation of hydrogen atoms from the step sites similarly with Pt(S)[n(111)× (111)]. The reaction between CO2 and hydrogen atoms adsorbed at the step sites will be activated by the vacant step sites.
3.2.3. Pt(S) -[n(100)×(111)] electrodes The voltammograms of the present Pt(S)-[n(100)× (111)] surfaces gave sharp redox peaks at 0.27 and 0.37 V in 0.5 M H2SO4 [42], which originate from hydrogen adsorbed on the (100) terrace edge and wide (100) terrace, respectively. These voltammograms are identical with
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notation for the step structure. The potential dependence of 6t = 0 may be rationalized in a similar way with Pt(S)-[n(111)× (100)] electrodes as described above. Pt(100) shows low 6t = 0 between 0.05 and 0.40 V, and the activity rises above 0.40 V significantly. The study using surface X-ray scattering reported that Pt(100) gives an unreconstructed (1 ×1) structure up to the oxide film formation region in 0.1 M HClO4 [29]. However, according to a STM study [44] the surface of Pt(100) is reconstructed, forming an island structure at the positive potential. The STM study may rationalize our result, since the step sites at the edge of the island will enhance the activity for the CO2 reduction. Fig. 6. Voltammograms of the Pt(S)-[n(100)× (111)] electrodes in 0.1 M HClO4 saturated with Ar [42].
those reported previously [17,32,34,43]. The voltammetric charges due to the terrace edge and the terrace agreed with those calculated on the assumption that surfaces have (1 × 1) structures [42]. Voltammograms in 0.1 M HClO4 give peaks around 0.27 V that are similar to the peaks due to the (100) edge in 0.5 M H2SO4 (Fig. 6). Pt(S)-[n(100)×(111)] surfaces have the same notation for step as Pt(S)-[n(111) ×(111)]. Pt(S)-[n(111)×(111)] electrodes provide sharp peaks at 0.12 V due to the (111) step (Fig. 2), but Pt(S)-[n(100)× (111)] electrodes have no peak at 0.12 V (Fig. 6). The anodic currents give minima at 0.20–0.23 V as shown in the case of Pt(S)-[n(111) ×(100)]. The hydrogen at the terrace site is probably oxidized at this potential. Fig. 7 gives the potential dependence of 6t = 0 on Pt(S)-[n(100)× (111)] in 0.1 M HClO4. The features of Pt(S)-[n(100)×(111)] electrodes resemble those demonstrated for Pt(S)-[n(111)×(100)], differing significantly from Pt(S)-[n(111) ×(111)] in Fig. 3 in spite of the same
Fig. 7. Potential dependence of the initial rate of CO2 reduction (6t=0) on Pt(S)-[n(100)×(111)] electrodes in 0.1 M HClO4 [42].
3.2.4. Relationship between acti6ity and atomic arrangement on stepped surfaces We have demonstrated that the active sites for CO2 reduction are closely connected with the step sites. Here we discuss how the activity varies with the atomic configuration of the step structure. We tentatively take the highest value of 6t = 0 in the respective potential dependence (6t = 0(max)) as an index of the activity. Atomically flat surfaces, (111) and (100), are inert in the CO2 reduction, and the introduction of a step in the surfaces gives rise to a substantial increase in the activity. A comparison of 6t = 0(max) among the series of the electrodes is made with an equal number of n, and the order of the activity may be given as: Pt(S)-[n(111)× (100)]BPt(S)-[n(100)×(111)] BPt(S)-[n(111)×(111)]. The activity of Pt(S)-[n(111)×(111)] is much higher than that of Pt(S)-[n(100)× (111)]. Fig. 8 illustrates the electrode series of the stepped surfaces in the order of the activity. Significant difference of the activity between Pt(S)-[n(111)×(111)] and Pt(S)[n(100)× (111)] leads to a hypothesis that the active site for the CO2 reduction may be derived from the pseudo-4fold bridged sites sb in the step of Pt(S)-[n(111)× (111)]. It is impossible at present to reveal how the pseudo-4-fold bridged sites work in the enhancement of the activity; they may provide active hydrogen atoms adsorbed on the sites or activate the interaction between CO2 and adsorbed hydrogen. Among all the stepped surfaces, Pt(110) shows the highest activity in the reaction; over 200 times as high as Pt(111). Markovic´ and coworkers [30,31] reported that the annealing procedure can control the surface symmetry of the Pt(110), (1 × 1) or (1 × 2) structure. We annealed Pt(110) about 1300°C and immediately cooled it in Ar/H2 atmosphere. This procedure may provide (1× 1) rather than (1 × 2) according to Markovic´. In both cases, however, the atomic configuration is symmetric with regard to the pseudo-4-fold bridged sites. Such densely packed symmetric pseudo-4-fold bridged sites
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Fig. 10. Potential dependence of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(110) ×(100)] electrodes in 0.1 M HClO4 [20].
Fig. 8. Order of the activity for CO2 reduction shown by the hard sphere models of the stepped surfaces. Small solid circle sb represents the pseudo-4-fold bridged sites.
may provide an atomic configuration favorable for the reduction of CO2.
3.3. Kinked step surfaces 3.3.1. Pt(S) -[n(110)×(100)] surfaces We further studied CO2 reduction on Pt(980) (n=9) and Pt(210) (n=2) [20]. Introduction of a (100) step to the (110) surface gives rise to protruding atoms along the step lines, which is marked by solid spheres in a hard sphere model of Pt(980) (Fig. 9). We define a surface containing the protruding atoms along the step lines as a kinked step surface. The shapes of the voltammograms were identical with those reported by Furuya and coworkers [32,34]. It is difficult to relate voltammetric peaks with the adsorption sites of hydrogen on these electrodes; we confirm the crystal orientation on the basis of the desorption charge of the hydrogen in 0.1 M HClO4 saturated with Ar. The
Fig. 9. Hard sphere models of Pt(S)-[n(110)× (100)] surfaces. Solid spheres represent kink atoms.
desorption charges agree with the values calculated by assuming that one hydrogen atom adsorbed on one Pt atom of (1 ×1) structure [20]. The CO oxidation takes place around 0.7 V as measured by voltammetry. The values of 6t = 0 are plotted against the potential in Fig. 10. The value of 6t = 0 between 0.05 and 0.15 V gets higher with the increase of the (100) step density or the kink atom density. The presence of kink atoms further enhances the activity of Pt(110) which is the most active stepped surfaces in CO2 reduction.
3.3.2. Pt(S) -[n(100)×(110)] surfaces The hard sphere models of the Pt(S)-[n(100)× (110)] surfaces are illustrated in Fig. 11 in which kink atoms are shown as solid spheres. Pt(210) (n= 2) are also classified into this series. The anodic charges of hydrogen desorption in the voltammograms in 0.1 M HClO4 suggest that the surfaces have a (1 × 1) structure [20]. Fig. 12 presents the potential dependence of 6t = 0. The value of 6t = 0 decreases simply with the increase of the potential. The surfaces with higher kink atom density give higher values of 6t = 0. Hydrogen atoms will occupy all the kink atoms at 0.05 V, since the coverage of the adsorbed hydrogen is 1 at 0.05 V. The values of 6t = 0 at 0.05 V depend linearly on kink atom density at 0.05 V in Fig. 13, clearly demonstrating that the kink is the active site for CO2 reduction. The slope of the line represents the activity per kink atom. The slopes of the kinked step surfaces are more than twice as high as those of the stepped surfaces. This result definitely shows that the kink has higher activity for CO2 reduction than the step. The slope of Pt(S)[n(100)× (110)] surfaces is 65% higher than that of Pt(S)-[n(110)× (100)]. The kink atoms are packed sparsely along the step line on Pt(S)-[n(110)× (100)] (Fig. 9), whereas Pt(S)-[n(100)× (110)] surfaces have densely
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packed kink atoms along the step line (Fig. 11). This fact supports that densely packed kink atoms lead to high activity in CO2 reduction. The adsorbed hydrogen atoms adjacent to the kink site may have ex-
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tremely high activity for CO2 reduction. CO2 needs two hydrogen atoms to be reduced (CO2 + 2Had COad + H2O), thus active hydrogen atoms locating closely with each other will enhance the rate of CO2 reduction on the surfaces with densely packed kink atoms.
4. Electrocatalytic activity and broken bond density Interatomic bonds are cut when a surface is newly produced from a single crystal. The broken bond density per atom (dbb) depends on the crystal orientation, and geometrically determined for fcc metals as: dbb = (8h+ 4k)/(h 2 + k 2 + l 2)1/2, Fig. 11. Hard sphere models of Pt(S)-[n(100)× (110)] surfaces. Solid spheres represent kink atoms.
Fig. 12. Potential dependence of the initial rate of CO2 reduction (6t = 0) on Pt(S)-[n(100)×(110)] electrodes in 0.1 M HClO4 [20].
Fig. 13. The initial rate of CO2 reduction (6t = 0) at 0.05 V (RHE) plotted against the kink atom density. The results from the stepped surfaces are also given as plotted against the step atom density for comparison [42].
where h, k, l denote Miller indices [45]. The broken bond density is closely connected with the surface energy of single crystals, and well correlated with the potential of zero charge of gold and silver [46,47]. Chang et al. [48] studied electrooxidation of CO on gold single crystal electrodes. They employed the electrodes Au(110), Au (210), Au (100), Au (221), Au (533), and Au (111). They obtained a correlation between the reaction rates and the broken bond density. The present 6t = 0 are correlated with the broken bond density calculated with the equation shown above. The correlation is shown in Fig. 14. No single line can correlate 6t = 0 and dbb for all the electrodes. A correlation straight line is obtained for each series of the electrodes, which has the identical atomic configuration of step or kink. For any series of the electrodes, high electrocatalytic activity appears at electrodes with high broken bond density. The high broken bond density may promote the activity of the adsorbed hydrogen adjacent to the step and the activity of the vacant step site.
Fig. 14. The values of the initial rate for CO2 reduction (6t = 0) at 0.1 V plotted against the density of broken bond (dbb).
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5. Conclusion (1) Flat surfaces, Pt(111) and Pt(100), have low activity for CO2 reduction. (2) The initial rate of the CO2 reduction gets higher with the increase of the step atom density. (3) The terrace and step structures affect the potential dependence of the initial rate remarkably. (4) The activity for CO2 reduction depends remarkably on the symmetry of the surface. Following activity series are obtained for stepped surfaces: Pt(111)BPt(100)BPt(S)-[n(111)×(100)] B Pt(S)-[n(100)×(111)]BPt(S)-[n(111)×(111)] B Pt(110). Pt(110) has the highest activity for CO2 reduction in the stepped surfaces. (5) Kinked step surfaces have higher activity for CO2 reduction than the stepped surfaces. (6) Pt(S)-[n(100) ×(110)] series, which have densely packed kink atoms along step lines, have the highest activity for CO2 reduction. Acknowledgements This study was supported by Grant-in-Aid for Scientific Research, 05740349, 04241106, 06226214, 0874 0440, 08232217,09450313 and 10131212. References [1] Y. Hori, K. Kikuchi and S. Suzuki, Chem. Lett. (1985) 1965. [2] Y. Hori, A. Murata, R. Takahashi, J. Chem. Soc. Faraday Trans. 185 (1989) 2309 and references are cited therein. [3] J. Clavilier, R. Faure, G. Guinet, R. Durand, J. Electroanal. Chem. 107 (1980) 205. [4] J. Giner, Electrochim. Acta 8 (1963) 857. [5] M.W. Breiter, Electrochim. Acta 12 (1967) 1213. [6] B. Beden, A. Bewick, M. Razaq, J. Weber, J. Electroanal. Chem. 139 (1982) 203. [7] J. Sobkowski, A. Czerwinski, J. Electroanal. Chem. 55 (1974) 391. [8] J. Sobkowski, A. Czerwinski, J. Electroanal. Chem. 65 (1975) 327. [9] M.C. Arevalo, C. Gomis-Bas, F. Hahn, B. Beden, A. Arevalo, A.J. Arvia, Electrochim. Acta 39 (1994) 793 and references are cited therein. [10] B.Z. Nikolic, H. Huang, D. Gervasio, et al., J. Electroanal. Chem. 295 (1990) 415. [11] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 369 (1994) 183. [12] S. Taguchi, A. Aramata, Electrochim. Acta 39 (1994) 2533. [13] N. Hoshi, T. Mizumura, Y. Hori, Electrochim. Acta 40 (1995) 883. [14] A. Rodes, E. Pastor, T. Iwasita, J. Electroanal. Chem. 373 (1994) 167.
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