Poison Formation upon the Dissociative Adsorption of Formic Acid on

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Poison Formation upon the Dissociative Adsorption of Formic Acid on Bismuth-Modified Stepped Platinum Electrodes Sean P. E. Smith, Karen F. Ben-Dor, and Hećtor D. Abrunã* Department of Chemistry, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301 Received June 24, 1999. In Final Form: September 22, 1999 The amount of poison formed and oxidatively desorbed upon the dissociative adsorption of formic acid on bismuth-modified Pt[n(111) × 100 and 110] electrodes has been investigated and found to decrease linearly with bismuth coverage on the step sites. For Pt(544) and Pt(554) (i.e., nine atom wide (111) terraces with (100) and (110) steps, respectively), the amount of charge to oxidatively desorb the poison decreases from 80 and 220 µC/cm2, respectively, to similar values of 25 and 30 µC/cm2, respectively, when the step sites are completely blocked with bismuth and the (111) terraces are bare. As the step site density of the platinum surface is increased, the amount of charge corresponding to the adsorbed poison, when the step sites are completely blocked with bismuth, decreases on the surfaces with (100) steps but remains constant or slightly increases on the surfaces with (110) steps. As bismuth is deposited on the (111) terraces, the amount of poison formed on both electrodes is greatly diminished, so that when bismuth coverages of ca. 0.09 are attained on the (111) terraces, little poison is formed. While both the (100) and (110) stepped surfaces exhibit this rapid diminution in poison formation with bismuth coverage on the (111) terraces, the (100) stepped surfaces reach the minimum at a faster rate, suggesting that the (110) steps influence the surface by making it more reactive to the poison formation reaction, even when they are blocked with bismuth. For formic acid oxidation, the (110) stepped Pt surfaces are less active than the (100) stepped surfaces when bismuth is only adsorbed on the step sites but become more active when bismuth is adsorbed on both the steps and partially on the (111) terraces.

Introduction It is now well established that the oxidation of formic acid on platinum occurs through a dual-path mechanism, in which one path leads to the final product through an active intermediate, while the other leads to the formation of a surface poison.1,2 This latter poison has been identified as being CO, which accumulates on the surface, thereby decreasing the activity of the catalyst.1-6 A great deal of emphasis, therefore, has been placed upon increasing the selectivity of platinum surfaces against the poison intermediate pathway in the development of commercially viable fuel cells.1,2 As with the oxidation rate of formic acid on platinum, the extent of poison formation upon the dissociative adsorption of formic acid on platinum surfaces is surface sensitive.7-14 Of the low index planes, the (110) surface exhibits the highest degree of poisoning, whereas the (111) plane exhibits the lowest degree of poisoning upon formic acid dissociative adsorption.9,15,16 (1) Parsons, R.; Vandernoot, T. J. Electroanal. Chem. 1988, 257, 9. (2) Jarvi, T. D.; Stuve, E. M. In Electrocatalysis; Lipkowski, J., Ross, P. N., Eds.; Wiley-VCH: New York, 1998; Chapter 3. (3) Clavilier, J.; Sun, S. G. J. Electroanal. Chem. 1987, 236, 95. (4) Iwasita, T.; Vogel, T. Electrochim. Acta 1988, 33, 557. (5) Chang, S. C.; Weaver, M. J. J. Phys. Chem. 1990, 94, 1034. (6) Kita, H.; Lei, H. J. Electroanal. Chem. 1995, 388, 167. (7) Clavilier, J.; Parsons, R.; Durand, R.; Lamy, C.; Le´ger, J. M. J. Electroanal. Chem. 1981, 124, 321. (8) Lamy, C.; Le´ger, J. M.; Clavilier, J.; Parsons, R. J. Electroanal. Chem. 1983, 150, 71. (9) Clavilier, J.; Sun, S. G. J. Electroanal. Chem. 1986, 199, 471. (10) Adzic, R. R.; Tripkovic, A. V.; Vesovic, V. B. J. Electroanal. Chem. 1986, 204, 329. (11) Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A.; Clavilier, J. J. Electroanal. Chem. 1989, 258, 101. (12) Tripkovic, A. V.; Popovic, K.; Adzic, R. R. J. Chim. Phys. 1991, 88, 1635. (13) Campbell, S. A.; Parsons, R.; J. Chem. Soc., Faraday Trans. 1992, 88, 833. (14) Iwasita, T.; Xia, X. H.; Herrero, E.; Leiss, H. D. Langmuir 1996, 12, 4260.

Foreign metal adatoms, especially those that irreversibly adsorb onto platinum, have been employed both to catalyze the oxidation of formic acid and to prevent the poison intermediate pathway. The role that the adatoms play has been found to be largely dependent upon the surface structure of the platinum substrate. On Pt(100), Sb,11,17 Bi,17,18 and Te19 have been found to prevent the poison formation pathway through a third-body type mechanism (i.e., the adatoms act to decrease the ensemble size of the surface), with the amount of poisoning of the surface decreasing monotonically with the adatom coverage until no poison remained at full coverage. On Pt(111), however, As,20 Sb,21 Bi,20 and Te22 have been found to prevent the poison formation pathway through an electronic effect, since the latter was prevented from occurring at very low adatom coverages. Se/Pt(111), however, prevents the poison intermediate pathway through a thirdbody mechanism.23 This is likely due to the higher work function of Se compared with Pt, whereas, the work functions of As, Sb, Bi, and Te are all lower than that of Pt.22 We have recently extended these studies of the surfacereactivity relationships of bismuth-modified platinum (15) Motoo, S.; Furuya, N. J. Electroanal. Chem. 1985, 184, 303. (16) Chang, S.-C.; Leung, L.-W. H.; Weaver, M. J. J. Phys. Chem. 1990, 94, 6013. (17) Herrero, E.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1994, 368, 101. (18) Clavilier, J.; Fernandez-Vega, A.; Feliu. J. M.; Aldaz, A. J. Electroanal. Chem. 1989, 261, 113. (19) Herrero, E.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 383, 145. (20) Herrero, E.; Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1993, 350, 73. (21) Climent, V.; Herrero, E.; Feliu, J. M. Electrochim. Acta 1998, 44, 1403. (22) Herrero, E.; Llorca, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1995, 394, 161. (23) Llorca, M. J.; Herrero, M. J.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1994, 373, 217.

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single-crystal electrodes toward the oxidation of formic acid by employing deliberately stepped platinum surfaces with (111) terraces and (110)24 and (100)25 steps. On both types of surfaces, it was found that there was an increase in catalytic activity for formic acid oxidation for surfaces with narrow (111) terraces, provided that the steps were modified with bismuth, which is opposite to the behavior exhibited by the bare (i.e., unmodified) surfaces.10,12,26,27 In the present work the extent of formation of the poisoning intermediate upon formic acid dissociative adsorption on bismuth-modified Pt[n(111) × 100] and Pt[n(111) × 110] electrodes has been studied as a function of bismuth coverage, step site geometry, and terrace width. Experimental Section Pt(111), Pt(544), Pt(554), Pt(755), Pt(332), Pt(211), Pt(221), and Pt(311) single-crystal electrodes were prepared according to Clavilier’s method.28 Prior to each experiment, the electrodes were flame annealed for 20 s, after which they were cooled and quenched in an atmosphere and 18 MΩ Millipore Milli-Q water that were saturated with a mixture of nitrogen and hydrogen (80:20). The electrochemical setup used to characterize these electrodes was as described earlier.29 All potentials are referenced against a saturated Ag|AgCl electrode. The Pt electrodes were modified with bismuth as reported earlier.25 When the modified electrodes on which bismuth is only adsorbed on the monatomic steps are compared, the bismuth coverage is reported as a fraction of a monolayer on the step sites, as determined from the decrease in the hydrogen adsorption/ desorption charge density for the (110) steps (at -0.15 V) or the (100) steps (at 0 V) following bismuth deposition on the steps. The total charge densities for hydrogen adsorption/desorption on the (110) steps were 32, 60, and 82 µC/cm2 for Pt(554), Pt(332), and Pt(221), respectively, while those on the (100) steps were 32, 45, 95, and 98 µC/cm2 for Pt(544), Pt(755), Pt(211), and Pt(311), respectively. When bismuth is adsorbed completely on the steps (i.e., bismuth coverage on the steps ) 1) and partially on the (111) terraces, the bismuth coverage is reported as the fraction of a monolayer on the terraces, as determined directly from the charge of the Bi/Pt(111) redox peak, taking the bismuth coverage in which each bismuth adatom blocks three platinum atoms (on the (111) surface) as the stable monolayer on Pt(111). When comparing among the various surfaces over the entire range of bismuth coverages (i.e., on the steps and terraces), the latter is reported as the percentage of Pt sites blocked, as determined by comparing the hydrogen adsorption/desorption charge (-0.2 to +0.3 V) prior to and after bismuth adsorption on each surface. While it has been shown that part of the charge in the hydrogen adsorption/desorption region on Pt(111) in sulfuric acid (-0.2 to +0.3 V) involves the desorption/adsorption, respectively, of sulfate/bisulfate anions,30,31 the relationship between the above measured charge and that associated with the irreversibly adsorbed bismuth redox process is linear.32 After the bismuth-modified electrode was characterized in 0.1 M H2SO4 within the potential range of -0.18 to +0.65 V, it was emersed at -0.18 V, and transferred to an electrochemical cell containing 0.25 M HCOOH/0.1 M H2SO4, where it was immersed at -0.18 V and kept for 1 min. The electrode was then emersed (at an applied potential of -0.18 V), rinsed with 0.1 M H2SO4 (at (24) Smith, S. P. E.; Abrunã, H. D. J. Electroanal. Chem. 1999, 467, 43. (25) Smith, S. P. E.; Ben-Dor, K. F.; Abrunã, H. D. Langmuir 1999, 15, 7325. (26) Motoo, S.; Furuya, N. Ber. Bunsen-Ges. Phys. Chem. 1987, 91, 457. (27) Palaikis, L.; Wieckowski, A. Catal. Lett. 1989, 3, 143. (28) Clavilier, J.; Armand, D.; Sun, S. G.; Petit, M. J. Electroanal. Chem. 1986, 205, 419. (29) Smith, S. P. E.; Abrunã, H. D. J. Phys. Chem. B 1998, 102, 3506. (30) Faguy, P. W.; Markovic, N.; Adzic, R. R.; Fierro, C. A.; Yeager, E. B. J. Electroanal. Chem. 1990, 289, 245. (31) Orts, J. M.; Gomez, R.; Feliu, J. M.; Aldaz, A.; Clavilier, J. Electrochim. Acta 1994, 39, 1519. (32) Feliu, J. M.; Fernandez-Vega, A.; Orts, J. M.; Aldaz, A. J. Chim. Phys. 1991, 88, 1493.

Figure 1. (a) Cyclic voltammograms of Bi/Pt(544) in 0.1 M sulfuric acid (1, solid line), Bi/Pt(544) blocked by the poison and stripping of the poison (2, solid line), and Bi/Pt(544) after the stripping of the poison (3, dashed line); bismuth coverage on the (100) steps, 0.48 (sweep rate, 25 mV/s). The inset shows the voltammetric profile of Pt(544) in 0.1 M sulfuric acid (sweep rate, 25 mV/s). (b) Cyclic voltammograms of Bi/Pt(554) in 0.1 M sulfuric acid (1, solid line), Bi/Pt(554) blocked by the poison and stripping of the poison (2, solid line), and Bi/Pt(554) after the stripping of the poison (3, dashed line); bismuth coverage on the (110) steps, 0.25 (sweep rate, 25 mV/s). The inset shows the voltammetric profile of Pt(554) in 0.1 M sulfuric acid (sweep rate, 25 mV/s). an applied potential of -0.18 V), and immersed back into 0.1 M H2SO4 (at an applied potential of -0.18 V), after which the oxidation of the poison (formed in the HCOOH solution) was achieved upon scanning the potential within the range noted above (i.e., -0.18 to +0.65 V). The extent of poisoning on each surface was determined by the magnitude of the charge density of the oxidation peak associated with the adsorbed poison at +0.43 V.

Results and Discussion Poisoning on the Bismuth-Modified Surfaces with Nine Atom Wide (111) Terraces and (100) and (110) Steps: Bi/Pt(544) and Bi/Pt(554). Figure 1 shows the voltammetric profiles in 0.1 M H2SO4 before (labeled 1, solid line), during (labeled 2, solid line), and after (labeled 3, dashed line) the dissociative adsorption and oxidation of the poison formed from formic acid on bismuth-modified Pt(544) and Pt(554), respectively. In both of these cases bismuth is deposited partially on the steps with the (111) terraces remaining unmodified. The bismuth coverage on the (100) steps of Pt(544) (Figure 1a) is 0.48, while that on Pt(554) (Figure 1b) is 0.25. The dotted line under the peak associated with the hydrogen desorption on the step

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Figure 2. Dependence of the poison oxidation charge density on the bismuth coverage on the step sites of Pt(544) (solid) and Pt(554) (outline).

sites in each figure shows the baseline and potential limits that were used to integrate the amount of hydrogen charge remaining upon depositing bismuth on the steps. The insets to Figure 1 show the voltammetric profiles of the bare Pt(544) and Pt(554) surfaces in 0.1 M H2SO4. As can be seen in the voltammetric profiles labeled 2, following the dissociative adsorption of formic acid on these surfaces the adsorbed poison causes a significant blockage of hydrogen adsorption and desorption on the remaining free Pt step and terrace sites (-0.20 to +0.30 V). After the oxidative desorption of the poison at +0.43 V (labeled 3), the hydrogen adsorption/desorption on the remaining free Pt step sites (0.0 V on Pt(544) and -0.16 V on Pt(554)) and terrace sites (-0.20 to +0.30) is largely recovered on both Pt(544) and Pt(554). Figure 2 shows the dependence of the poison oxidation charge density on the bismuth coverage on the (100) and (110) step sites for Pt(544) (solid) and Pt(554) (outline), respectively. It is immediately apparent that the Pt(554) surface with the (110) steps is initially poisoned to a much greater extent, with a poison oxidation charge of 218 µC/ cm2 vs 87 µC/cm2 for Pt(544), a phenomenon that has been reported previously for the bare Pt surfaces.7 Moreover, the deposition of bismuth on the step sites causes a linear decrease in the poison oxidation charge. This suggests that bismuth acts as a “third body” (i.e., no presence of long-range effects) by blocking the highly active (100) and (110) step sites, thus rendering the resulting modified stepped surfaces less prone to poison formation as bismuth is deposited on the step sites. Similar behavior has also been observed for bismuth modified Pt(100) electrodes and has been explained as arising from a “third body” type of mechanism.17,18 However, while the Bi/Pt(554) surface is initially more poisoned than the Bi/Pt(544) surface, the degree of poisoning on the former surface decreases more rapidly with bismuth coverage, so that when bismuth is deposited on all of the step sites (bismuth coverage ) 1) both surfaces appear to be poisoned to a similar extent: about 25 µC/cm2 and 30 µC/cm2 for Bi/ Pt(544) and Bi/Pt(554), respectively. Under similar conditions the magnitude of the poisoning oxidation charge on bare Pt(111) was about 90 µC/cm2, which shows the effects of reducing the ensemble size of the available surface area for poison formation (i.e., the (111) terrace width) to nine atoms wide on Pt(544) and Pt(554), provided that the reactive step sites are blocked with bismuth. Upon the complete deposition of bismuth on the step sites (bismuth coverage on the steps ) 1) and the partial deposition of bismuth onto the Pt(111) terraces, and the

Figure 3. (a) Cyclic voltammograms of Bi/Pt(544) in 0.1 M sulfuric acid (1, solid line), Bi/Pt(544) blocked by the poison and stripping of the poison (2, solid line), and Bi/Pt(544) after the stripping of the poison (3, dashed line); bismuth coverage on the (100) steps, 1, and on the (111) terraces, 0.03 (sweep rate, 25 mV/s). (b) Cyclic voltammograms of Bi/Pt(554) in 0.1 M sulfuric acid (1, solid line), Bi/Pt(554) blocked by the poison and stripping of the poison (2, solid line), and Bi/Pt(554) after the stripping of the poison (3, dashed line); bismuth coverage on the (110) steps, 1, and on the (111) terraces, 0.03 (sweep rate, 25 mV/s).

ensuing dissociative formation and oxidation of the poison from formic acid, the voltammetric profiles in 0.1 M H2SO4 in Figure 3 are obtained for Bi/Pt(544) and Bi/Pt(554), respectively, both at a fractional monolayer coverage of about 0.03 on the (111) terraces. The absence of hydrogen adsorption/desorption peaks on both the (100) and (110) steps indicates that these sites are completely modified with bismuth. The new redox feature at +0.37 V corresponds to processes associated with bismuth oxidation and reduction on the (111) terraces and occurs at a potential just negative of the oxidation of the adsorbed poison at +0.43 V. Figure 4 shows the dependence of the degree of poisoning on Bi/Pt(544) (solid) and Bi/Pt(554) (outline) on the bismuth coverage on the (111) terraces, with the step sites completely modified with bismuth. In both cases, and especially for the Pt(544) surface, the poison oxidation charge decreases rapidly with small changes in the bismuth coverage on the (111) terraces. That this behavior, when bismuth is deposited on the (111) terraces, is different to that when it is only deposited on the (100) or (110) steps suggests a different mechanism for the inhibition of the poison intermediate on the (111) surface. The behavior on the latter surface has been described as being due to “electronic” effects (i.e., those arising from work function differences between the substrate and adatom), likely due to the long-range effects that each bismuth adatom has on Pt(111)20 or due to the long-range Bi-O(H) network that has been suggested to form on Pt(111).29 While both Pt(544) and Pt(554) have equal (111) terrace widths with completely modified step sites, the surface with the (100) steps (Pt(544)) appears to be less

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Figure 4. Dependence of the poison oxidation charge density on the bismuth coverage on the (111) terraces of Pt(544) (solid) and Pt(554) (outline); bismuth coverage on the step sites, 1.

poisoned with increasing bismuth coverage, though the differences are not as great as in the case when bismuth is initially being deposited on the step sites alone. Differences in the reactivities of these surfaces with (111) terraces and (100) and (110) steps have been reported and related to the work function differences, with the (110) stepped surfaces having lower work functions than those with (100) steps.33,34 These findings suggest that the geometry of the step sites, though blocked by bismuth, affects, at least in part, the degree of poison formation on the (111) terraces likely via electronic effects arising from work function differences. Regardless of the step site geometry, though, the influences that each of the different substrate structural features modified with bismuth exert on the poison formation reaction from formic acid are readily apparent and identified. When bismuth is deposited on the (100) or (110) steps, a linear decrease in the poison formation reaction occurs, while when the steps are completely blocked and the bismuth begins to deposit on the (111) terraces, a much more rapid decrease in the poison formation reaction occurs. This would suggest that this effect is primarily third body in the former and electronic in the latter. Poisoning on the Bismuth-Modified Surfaces with Six and Five Atom Wide (111) Terraces and (100) and (110) Steps, Respectively: Bi/Pt(755) and Bi/ Pt(332). Figure 5 shows the dependence of the poison oxidation charge density on the bismuth fractional monolayer coverage on the step sites of Pt(755) (solid) and Pt(332) (outline). As with Pt(544) and Pt(554) initially the surface with the (110) steps, Pt(332), is poisoned to a much higher degree, with the bare surfaces exhibiting charge densities for the poison oxidation very similar to those reported above for Pt(544) and Pt(554). As bismuth is deposited on the step sites, both surfaces again exhibit an approximately linear decrease in the magnitude of the poison oxidation charge density. Though the poisoning on Pt(332) decreases more rapidly than that on Pt(755), it does not reach the value of the latter when bismuth is deposited on all step sites, as was the case for Bi/Pt(544) and Bi/Pt(554). Rather, the poison oxidation charge density decreases to about 28 µC/cm2 on Pt(332) with fully blocked step sites, which is similar to that reached on Pt(554) with the fully covered step sites, whereas that on Pt(755) decreases to 12 µC/cm2, which is about 50% of that reached (33) Michely, T.; Comsa, G. Surf. Sci. 1991, 256, 217. (34) Feibelman, P. J. Phys. Rev. B 1995, 52, 16845.

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on Pt(544). This suggests that the (110) step sites render the surface more reactive to the formation and/or adsorption of the poison from formic acid, and even when they are fully blocked with bismuth, they influence the bare (111) terraces, likewise, to become more reactive toward the formation and/or adsorption of the poison. The greater (110) step density of Pt(332) thus counterbalances the effects of the smaller ensemble size for poison formation on Pt(332) relative to the nine atom wide (111) surface, Pt(554). It appears, however, that the (100) steps influence the (111) terraces to a lesser extent, so that when they are completely blocked with bismuth, less poisoning occurs on Pt(755) than on Pt(544) because the smaller terrace width (and hence ensemble size) hinders the degree to which the poison can form from the dissociative adsorption of formic acid. As with the surfaces studied above, upon completely blocking the step sites (bismuth coverage on the step sites ) 1) and depositing bismuth on the (111) terraces (Figure 6), both Bi/Pt(755) and Bi/Pt(332) exhibit a rapid decrease in the amount of poison oxidatively desorbed from their surfaces, which is characteristic of behavior due to electronic effects. The (110) stepped surface again remains more highly poisoned than the (100) stepped surface; though when the bismuth coverage reaches 0.09, neither surface exhibits a significant oxidation charge of the adsorbed poison. Poisoning on the Bismuth-Modified Surfaces with Three Atom Wide (111) Terraces and (100) and (110) Steps: Bi/Pt(211) and Bi/Pt(221). Figure 7 shows the



dependence of the poison oxidation charge density on the bismuth coverage on the steps of Pt(211) (solid) and Pt(221) (outline). As before, the (110) stepped surface, Pt(221), is initially more highly poisoned with the amount of poison decreasing linearly with bismuth coverage on the (110) steps to a value of about 40 µC/cm2, which is slightly greater than the analogous cases on either Pt(554) or Pt(332). Since it would be expected that the three atom wide terrace would hinder, to a greater extent, the formation of the poison from formic acid than the surfaces with wider terraces, this is consistent with the above assertion that the (110) steps are enhancing the poisoning of the (111) terraces. However, on the (100) stepped surface, Pt(211), the amount of poisoning charge decreases to 7 µC/cm2 when the steps are completely blocked with bismuth, which is about 50% of that on Pt(755). This, again, is consistent with the weaker influence that the (100) steps have on the amount of poisoning formed on the (111) terraces. In this case, the decrease in the amount of poison formed on Pt(211) compared with Pt(755) and Pt(544) is likely due to the smaller ensemble size of the three atom wide (111) terrace, which hinders the formation of the poison from formic acid. In addition, on Pt(211) the dependence of the poison oxidation charge on the bismuth coverage on the (100) steps begins to lose its linear relationship, which has been noted before on Bi/Pt(100).17 This suggests that surfaces with (111) terraces and high densities of (100) steps exhibit behavior that represents a transition from a surface of primarily Pt(111) character to one that is Pt(100) in character, a phenomenon that we have previously observed with these electrodes on the oxidation of formic acid.25 The previously noted trends in the amount of poisoning on these surfaces is continued when the step sites are completely blocked (bismuth coverage on the step sites ) 1) and bismuth is deposited on the (111) terraces, as can be seen in Figure 8. Here the decrease in the poisoning on Pt(211) with bismuth coverage on the (111) terraces is less pronounced because of the low amount of poisoning when the steps are blocked and the terraces are bare. Poisoning on the Bismuth-Modified Surface with Two Atom Wide (111) Terraces and (100) steps: Bi/ Pt(311). Figure 9 shows the dependence on the magnitude of the poison oxidation charge density of the bismuth coverage on the (100) steps of Pt(311). The deviation from linearity is, again, readily apparent, and, again, suggests the transition to a surface exhibiting behavior primarily associated with Bi/Pt(100). More importantly, decreasing

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Figure 9. Dependence of the poison oxidation charge density on the bismuth coverage on the step sites of Pt(311).

the terrace width from three atoms wide (Pt(211)) to two atoms wide (Pt(311)) causes the amount of poisoning on the surface, when the steps are blocked with bismuth (bismuth coverage on the steps ) 1), to nearly vanish. This, once again, supports the requirement for an ensemble size (i.e., the (111) terrace width) of three to four atoms for the dissociative adsorption of formic acid to occur.15 Poisoning Effects on the Oxidation of Formic Acid on Bi/Pt[n(111) × 100 and 110]. Figures 10-12 show the dependence of the maximum current density obtained for the oxidation of formic acid on the percentage of Pt sites blocked by bismuth on the stepped surfaces with similar sized terraces, as we have reported earlier.24,25 It has been suggested that bismuth exhibits a true catalytic effect on the direct formic acid oxidation pathway on Pt(111), which is readily apparent by the dramatic increase in the formic acid oxidation current density in Figures 10-12 upon depositing bismuth on the (111) terraces (coverage values to the right of the dashed line).35,36 To compare the effects of both the terrace width and the step geometry among the surfaces, the current scale is reported as the absolute current density rather than the normalized current density as we had reported earlier. The data for the Bi/Pt[n(111) × 110] surfaces were modified to include those bismuth coverages in which only the step sites were modified with bismuth. (35) Clavilier, J.; Fernandez-Vega, A.; Feliu, J. M.; Aldaz, A. J. Electroanal. Chem. 1989, 258, 89. (36) Leiva, E.; Iwasita, T.; Herrero, E.; Feliu, J. M. Langmuir 1997, 13, 6287.

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Figure 10. Dependence of the maximum formic acid oxidation current density in the first anodic potential sweep on the percentage of Pt sites blocked by bismuth on Pt(544) (solid) and Pt(554) (outline). The dashed line corresponds to the bismuth coverage in which bismuth is adsorbed completely and only on the step sites.


The dashed lines in Figures 10-12 correspond to full coverages of the step sites by bismuth and clearly show the effects of the step geometry, density, and the bismuth coverage on the steps on the rate of formic acid oxidation. In each case it can be seen that, initially, the surfaces with the (110) steps exhibit a lower rate for the oxidation of formic acid than the (100) stepped surfaces with similar sized terraces, which correlates well with the above data showing the higher propensity of the (110) stepped surfaces to the formation and/or adsorption of the poison from formic acid. It can also be seen that for the bare surfaces the rate of formic acid oxidation decreases with increasing step density. While the poison oxidation charges reported above for these bare surfaces are rather similar, it has been reported that the blocking of the hydrogen adsorption reaction on these surfaces by the poison is more apparent on the highly stepped surfaces,7 likely giving rise to the decrease in the current densities with increasing step density. Finally, for the cases in which bismuth is completely deposited on the step sites, it can be seen that the rate of formic acid oxidation increases with increasing step density. This correlates well with the poison oxidation data above for the (100) stepped surfaces, in which the amount of poisoning at such a bismuth coverage decreased with increasing step density/decreasing terrace width. However, it is surprising that the formic acid oxidation rate increased on the (110) stepped surfaces, since at these bismuth coverages the amount of poison formed upon the adsorption of formic acid remained constant or slightly increased with increasing step density. This may suggest that while the (110) stepped surfaces are more reactive than the respective (100) stepped surfaces34 toward the formation and/or adsorption of the poison from formic acid,

they are also equally active in enhancing the rate of formic acid oxidation. Alternatively, at these bismuth coverages just prior to its deposition on the (111) terraces, the true catalytic effect of bismuth may be controlling the enhancement of formic acid oxidation to a greater extent than its blocking of the poison intermediate, since the surfaces with higher step densities would have correspondingly higher relative bismuth coverages. Behavior somewhat opposite to that above occurs on the oxidation of formic acid upon depositing bismuth on the (111) terrace sites, as can be seen at coverages greater than 20% for Pt(544) and Pt(554) (Figure 10), 33% for Pt(755) and Pt(332) (Figure 11), and 70% for Pt(211) and Pt(221) (Figure 12). In these cases the rate of formic acid oxidation becomes more enhanced on the surfaces with the (110) steps. Recall from Figures 4, 6, and 8 that the amount of poisoning at these coverages, and especially at the bismuth coverages in which the maximum current densities for formic acid oxidation are obtained, rapidly becomes negligible at low bismuth coverages on the (111) terraces. Thus, since poisoning is not a factor toward the formic acid oxidation current density, and comparisons are made between surfaces with equal step densities and bismuth coverages, this behavior toward the oxidation of formic acid supports the above suggestion that the (110) stepped surfaces are more reactive to the adsorption and oxidation of formic acid than the (100) stepped surfaces. In this case, though, since only a very small amount of the surface is poisoned at these bismuth coverages, the (110) stepped surfaces exhibit higher current densities for the oxidation of formic acid. Alternatively, in terms of the work function differences between the (110) stepped surfaces and the (100) stepped surfaces,34 it appears that the lower values of the former


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Conclusions


surfaces initially make them more active for the formation and adsorption of the poison at bismuth coverages where the steps remain partially unmodified, resulting in their lower oxidation rate of formic acid. When the amount of poisoning has been largely diminished by the adsorption of bismuth on the reactive step sites, however, the lower values of the work functions of the (110) stepped surfaces appear to enhance the adsorption and oxidation of formic acid on these surfaces, making them more reactive to this reaction than the (100) stepped surfaces. The above effects also seem to affect the maximum formic acid oxidation current densities obtained on each surface, which occur at bismuth coverages where no poisoning is detected. For the (100) stepped surfaces, the maximum current densities obtained decrease with increasing step density, though, as we reported earlier,25 the current densities normalized to the terrace width increased with increasing step density. On the other hand, for the (110) stepped surfaces the maximum current densities obtained remained around 25 mA/cm2 for each surface. Since poisoning was not a factor at these bismuth coverages, the behavior on the (100) stepped surfaces may be due to a lesser amount of formic acid oxidation occurring on the narrow (111) terraces, which appears to be the more significant factor affecting the formic acid oxidation than the decreasing work function with increasing step density. Due to the higher reactivity/lower work functions of the (110) stepped surfaces, however, the tendency for less formic acid to adsorb and oxidize on the narrow (111) terraces appears to be offset by the stronger reactivity effects of formic acid adsorption. In the latter case, the work function effects seem to be equally significant as, and opposite to, the terrace width effects on the rate of formic acid oxidation.

The poisoning of bismuth modified Pt[n(111) × 100] and Pt[n(111) × 110] electrodes through the dissociative adsorption of formic acid has been shown to be highly dependent upon the site location and magnitude of the bismuth coverage, the step density, and the step geometry. Depositing bismuth on the (100) and (110) step sites of these electrodes results in a linear decrease in the amount of poisoning formed, suggesting a “third body” mechanism. Upon completely modifying the step sites with bismuth and depositing bismuth on the (111) terrace sites, the amount of poisoning formed decreases dramatically, so that even at low bismuth coverages on the (111) terraces (i.e., by ca. 0.09) virtually no poison is observed. The latter behavior is indicative of electronic effects in the mechanism for poison suppression on the bismuth-modified (111) terraces. The differing poisoning behaviors with bismuth coverage on the two types of sites on these stepped surfaces allow for a clear distinction of the poison suppression mechanisms occurring on them and clearly show the structural sensitivity of the poison formation reaction from formic acid on platinum surfaces. The geometry of the step sites also appears to affect the magnitude of the poison formation on these surfaces. On the surfaces with bismuth-blocked (100) steps, the amount of poison formed upon the adsorption of formic acid decreases with decreasing (111) terrace width. This is likely due to the effect of the lower ensemble size (i.e., the (111) terrace width) of the available terrace acting to hinder the poison formation reaction. However, on the surfaces with bismuth-blocked (110) steps, the amount of poison formed remained constant or slightly increased with decreasing (111) terrace width. This is likely due to the greater electronic influence of the (110) steps on the bare (111) terraces, since it has been shown that these surfaces exhibit lower work functions than similarly stepped Pt surfaces with (100) steps.34 It would be expected, then, that the poison formation reaction would be favored on the (110) stepped surfaces, especially at higher step densities, since this effect seems to act opposite to the effects of the reduced ensemble size noted above for the (100) stepped surfaces. The same reasons that make the (110) stepped surfaces more reactive toward poisoning also make them more reactive to the oxidation of formic acid. However, this is only apparent when the majority of the poisoning has been diminished by depositing bismuth on all step sites and partially on the (111) terraces. Otherwise, the (110) stepped surfaces are less reactive toward the oxidation of formic acid. Moreover, whereas the maximum current density for formic acid oxidation decreases with decreasing terrace width on the bismuth-modified (100) stepped surfaces, it remains nearly constant on the bismuthmodified (110) stepped surfaces. Again, the behavior in the former case is likely due to the smaller ensemble sizes available for formic acid adsorption and oxidation. On the (110) stepped surfaces, however, the higher reactivities/ lower work functions of the more highly stepped surfaces, as compared to the analogous (100) stepped surfaces, appear to offset the effects of the smaller terrace width by enhancing the adsorption and oxidation of formic acid. These results show the combined effects of ensemble size (i.e., the (111) terrace width), step geometry and density, and adatom coverage on the formation of the poison through the dissociative adsorption of formic acid and the adsorption and oxidation of formic acid on bismuthmodified stepped platinum surfaces. It appears that while the (110) stepped surfaces are initially poisoned to a

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greater extent upon the adsorption of formic acid, their increased reactivity as compared to the (100) stepped surfaces make them more favorable catalysts toward the oxidation of formic acid when bismuth is deposited on the reactive step sites and partially on the (111) terraces, so that the poisoning becomes effectively blocked.

Smith et al.

Acknowledgment. This work is supported by the Office of Naval Research. S.P.E.S. acknowledges support by a fellowship from the National Defense Science and Engineering Graduate Fellowship Program. LA990816H

Poison Formation upon the Dissociative Adsorption of Formic Acid on

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