J. Phys. Chem. 1996, 100, 17535-17538
17535
Molecular Orbital Investigation of Water Reactions with Tin Hydroxide Complexes in Association with Platinum Electrodes Alfred B. Anderson,* Seeyearl Seong, and E. Grantscharova† Chemistry Department, Case Western ReserVe UniVersity, CleVeland, Ohio 44106-7078 ReceiVed: April 8, 1996; In Final Form: July 22, 1996X
Calculations using the atom superposition and electron delocalization molecular orbital (ASED-MO) theory show that the redox reaction of Sn(OH)2 to Sn(OH)4 is likely to proceed by a mechanism of (i) collision of Sn(OH)2 with H2O, which will promote electron loss to the electrode when the potential is the oxidation potential, (ii) deprotonation of the water molecule and electron loss to yield strongly bound OH in Sn(OH)3+, (iii) immediate attack by H2O to form Sn(OH)3H2O+, and (iv) deprotonation to Sn(OH)4. The possible transfer of OH to Pt electrode surfaces is of interest in fuel cell catalysis. OH(ads) is capable of oxidizing the anode poison CO(ads). Calculations indicate that Sn(OH)3(ads) is the best candidate thermodynamically for transferring OH(ads), but it is not kinetically active. Coordinating H2O to Sn in this adsorbed complex does not activate OH(ads) transfer. Sn(OH)2 bound to a Pt surface step site does not activate H2O for OH(ads) formation. Neither does a Sn atom bound to a step site, or to a surface substitutional site, which was shown in a previous paper. It is concluded that tin does not promote the electrooxidation of CO adsorbed on Pt by a mechanism that involves OH(ads) as an oxidant.
Introduction Understanding the promoting effects of tin on the electrooxidation of water-soluble organic fuels has been a goal ever since the phenomenon was introduced to the literature by Cathro.1 The promotion centers on the oxidation of a strongly adsorbed poisoning species on the anode that is generated during oxidation of the fuel and its removal as CO2. This species, believed to be CO,2-8 participates in creating an overpotential for fuel oxidation. For example, methanol is oxidized on polycrystalline platinum at a potential about 600 mV positive of the thermodynamic potential. Over electrodes prepared by electrodeposition of platinum-tin mixtures the overpotential was reduced by ∼250 mV over the current range of 1-100 mA/cm2 compared to the potentials over platinum.1 This reduction translates directly into an increase in fuel cell voltage. Over the years a variety of mechanisms for the role of tin in reducing the overpotential have been proposed. The first was that Sn(OH)4 was oxidizing the surface blocking species in association with the Sn(OH)2/Sn(OH)4 couple.1 Later Janssen and Moolhuysen postulated that tin atoms alloyed into the surface might be weakening the adsorption bond of the blocking species, the so-called ligand effect, and also attracting H2O molecules involved in its oxidation.9 Quantum chemical calculations by Shiller and Anderson10 were supportive of the ligand effect, but recent calculations of Anderson et al.11 found that tin atoms present substitutionally in a platinum surface bind water only weakly. Specifically, ref 11 shows that substitutional surface Sn atoms are not likely to attract and activate H2O to form the oxidant OH(ads), a process that seems to be occurring when ruthenium is alloyed with platinum, according to experimental work of Gasteiger12-14 et al. and theoretical work of Anderson and Grantscharova.15 It was also suggested by Motoo and Watanabe that underpotential deposited tin, Snupd, in the form of adsorbed oxidized tin complexes might be providing oxygen atoms to oxidize the poisoning residue.16 BittinsCattaneo and Iwasita17 specifically suggested that active OH† On leave from the Institute of Physical Chemistry, Bulgarian Academy of Sciences, Sofia, Bulgaria 1113. X Abstract published in AdVance ACS Abstracts, October 15, 1996.
S0022-3654(96)01047-7 CCC: $12.00
(ads) forms on adsorbed Sn2+. It has been shown that SnII displaces a 14C-labeled residue formed by oxidation of methanol18 and that SnIV displaces preadsorbed CO.19 It has also been proposed that Snupd blocks sites where the poisoning residue would otherwise form.20,21 Finally, it has been proposed that changes in the oxidation states of surface platinum atoms caused by Snupd may be interfering with their binding to the poisoning species.22 Very recent experimental work is providing information relevant to the role of tin. Haner and Ross23 found that tin atoms on surfaces of the ordered Pt3Sn alloy block the methanol oxidation reaction and appear to have no catalytic effect. This is consistent with the recent theoretical study of ref 11. Haner and Ross found evidence for a catalytic effect of very dilute (1 µM) SnII over (111) and (100) platinum surfaces. Over disordered, sputtered, (110) Pt3Sn, Gasteiger et al. observed exceptional activity for CO oxidation with an onset 0.5 V lower than over Pt anodes.24 Leiva et al.25 observed a “weakly adsorbed” form of CO on high-area platinum for which the oxidation current begins about 300 mV cathodic of the main CO oxidation peak. This phase was further studied by Morimoto,26 who proposed weakly interacting H2O to be the oxygen source for oxidation. Morimoto electrochemically deposited tin on smooth and high-area platinum electrodes. On the smooth electrode a peak due to weakly adsorbed CO spanned 350-500 mV. There was no such peak for smooth platinum electrodes without tin. The main oxidation peak commenced at lower potential with tin present. For the high-area surfaces, tin shifts the oxidation peak for easily oxidized CO about 100 mV cathodic, compared to clean high-area platinum, to a position similar to that for the smooth platinum-tin surface. In all, these results may be similar to those of Gasteiger et al.24 In summary, then, roughened platinum surfaces appear to have two phases of chemisorbed CO in acid electrolytes, and it has been proposed that the easily oxidized phase is oxidized by interaction with weakly adsorbed H2O. There is theoretical basis for the possibility that H2O might be able to react directly with CO(ads) provided the structure of the surface site allows H2O to attack the C end of CO without becoming strongly bound to the surface. Shiller and Anderson27 calculated a low barrier © 1996 American Chemical Society
17536 J. Phys. Chem., Vol. 100, No. 44, 1996
Anderson et al.
TABLE 1: Parameters Used in the Calculations (See Ref 11): Principal Quantum Numbers, n, Diagonal Hamiltonian Matrix Elements, H (eV), Orbital Exponents, ζ (au), and Linear Coefficients, c, for Double-ζ d Orbitals s
p
d
atom
n
H
ζ
n
H
ζ
n
H
c1
ζ1
c2
ζ2
Pt Sn C O H
6 5 2 2 1
-10.5 -14.0 -15.09 -26.98 -12.1
2.554 2.226 1.6583 2.1460 1.2
6 5 2 2
-6.46 -9.34 -9.76 -12.12
2.25 1.9190 1.6180 2.1270
5
-11.1
0.65581
6.013
0.57150
2.396
TABLE 2: Structure of Sn(OH)n and Energy (eV) Required To Remove an OH structure energy
SnOH
Sn(OH)2
Sn(OH)3
Sn(OH)4
linear 4.10
bent 4.30
pyramidal 2.53
tetrahedral 4.96
for this process, and the barrier decreased as the platinum valence band was stabilized to model the effects of an increasing positive potential. Electrodeposited tin appears to activate this process, and it also moves the onset of the oxidation peak for the strongly bound majority adsorbed CO phase in the cathodic direction. This peak is believed to depend on the formation of OH(ads), which is the probable oxidant. In this study we examine theoretically the redox chemistry of tin hydroxide complexes and the proposal that they activate H2O and generate active OH(ads) at lower potential than appears to occur on pure platinum anodes. Method As in the above-mentioned studies of the formation of OH(ads) on platinum15 and platinum11,15 alloy surfaces, we used the atom superposition and electron delocalization molecular orbital (ASED-MO) theory with cluster models of the Pt(111) surface. In this present study we did not implement the theoretical electrode valence band shifting technique to represent different electrode potentials, but only used the band position corresponding to approximately 0 V. This is because not one of the mechanisms tried for OH transfer from tin hydroxide species to the platinum surface was calculated to have a low enough activation energy to warrant a study of potential dependency. The ASED-MO theory is based on integrating the electrostatic forces on the nuclei as atoms are brought together to form a molecule. The electron charge density distribution function is partitioned into free atom components, and the remainder, a delocalization function. The atom superposition energies are calculated using the free atom densities, and the delocalization energy is approximated using the molecular orbital energy from a modified extended Hu¨ckel Hamiltonian. The method was reviewed recently.28 Parameters used in this study, in Table 1 are the same as used in the study of H2O intersecting with substitutional tin.11 Sn(OH)n, n ) 1-4 In preparation for exploring mechanisms for OH transfer from tin species to the platinum surface, we calculated structures and energies for the tin hydroxide molecules. Table 2 gives the structures and OH binding energies to Sn(OH)n molecules, n ) 1-4. It is seen that the Sn II and IV oxidation states are especially stable against reduction, that is, the removal of an OH, Sn(OH)3 binds OH relatively weakly, and a tin atom binds OH strongly. Electronic structures for these complexes (Figure 1) provide a basis for explaining their relative stabilities. The tin atom has two electrons in the 5p orbitals that lie high on the energy scale, contributing over 2 eV of charge transfer stability
Figure 1. Electronic structure of Sn, OH, and Sn(OH)n, n ) 1-4.
as the first OH binds. A somewhat larger charge transfer stabilization awaits the binding of the second OH. The third OH binds weakly because the radical electron is promoted to a nonbonding tin 5p orbital. The fourth OH then binds strongly, benefiting from a large charge transfer stabilization. Mechanistic Proposals Concerning the SnII h SnIV + 2eEquilibrium The reaction associated with the change in tin oxidation state near the electrode surface but not adsorbed is, overall, proposed to be
Sn(OH)2 + 2H2O h Sn(OH)4 + 2H+(aq) + 2e-(electrode) (1) When concentrations are constant, the only variable is the potential of the electrode, which, when increased, moves the equilibrium to the right. We investigated a probable mechanism for this reaction, as presented next. Regarding the forward reaction first, our calculations show that H2O does not bind to the SnII in the Sn(OH)2 molecule, but binds weakly (0.01 eV) to Sn(OH)2+ and strongly (1.69 eV) to Sn(OH)22+. The sequential increase in binding energies results from the elimination of closed-shell repulsion involving the σ lone-pair orbitals of the two molecules. In other words, as tin is oxidized, it becomes a better acceptor for H2O lonepair donation bonding. When the oxygen end of an H2O molecule bumps into the tin end of Sn(OH)2, the closed-shell repulsion will destabilize the lone-pair orbital of Sn(OH)2, which begins to mix in an antibonding way with this lower lying H2O lone-pair orbital. This reduces the ionization potential of the Sn(OH)2(aq) system, and, when the Fermi level of the electrode is low enough, that is, when its potential is great enough, this invites oxidation by electron transfer to the surface, forming the weakly bound complex Sn(OH)2H2O+, which will deprotonate to Sn(OH)3. The temporal and structural details of the electron transfer and deprotonation are unknown, but it is possible that the oxidation and deprotonation steps are coupled.
Water Reactions with Tin Hydroxide Complexes
J. Phys. Chem., Vol. 100, No. 44, 1996 17537
Figure 2. Pt18 and Pt34 cluster models used to model the Pt(111) surface.
TABLE 3: Calculated Binding Energies, BE (eV) for Sn, OH, and Sn(OH)n, n ) 1-4, Series over Pt18 and Pt34 Cluster Models of Pt(111)a SnOH Pt18 Pt34
Sn(OH)2
Sn
OH
h
p
h
p
Sn(OH)3
Sn(OH)4
5.32 5.09
3.92 3.10
4.10 3.70
4.77 4.45
2.84 2.27
3.10 3.15
3.33 2.53
2.26 1.71
a For SnOH and Sn(OH)2, h stands for nearly horizontal adsorption and p for perpendicular adsorption through Sn; see Figure 3.
Figure 4. Initial and transition state structures for transfer of OH to the surface from Sn(OH)3(ads).
TABLE 4: Calculated Energy Changes (eV) for OH Loss from Sn(OH)n, Reaction 2, When Bound to the Pt18 and Pt34 Models of the (111) Surface When Reactants and Products Are in Their Most Stable Sites Pt18 Pt34
SnOH
Sn(OH)2
Sn(OH)3
Sn(OH)4
-0.37 0.34
-1.29 -0.12
-1.16 -1.21
-0.03 1.02
We calculated the energy change for OH loss on the surface according to the reaction
Sn(OH)n(ads) f OH(ads) + Sn(OH)n-1(ads)
Figure 3. Sn(OH)n, n ) 1-4, structures when bound to the Pt18 and Pt34 cluster models. Both horizontal (h) and perpendicular (p) orientations are shown for SnOH and Sn(OH)2.
When it loses an electron to become half-filled, the highest occupied orbital of Sn(OH)2 moves up from -10.94 to -9.95 eV. Coordination H2O to Sn(OH)2+ moves this level up to -7.61 eV, which suggests that the second oxidation may take place before Sn(OH)3 is fully formed. H2O binds strongly (1.30 eV) to Sn(OH)3+, forming Sn(OH)3H2O+, which should deprotonate spontaneously, yielding Sn(OH)4(aq). The reverse reaction should commence with protonation of Sn(OH)4 and loss of H2O. The resulting Sn(OH)3+ will pick up an electron from the electrode, and protonation of the Sn(OH)3 that is formed will lead to loss of a water molecule and the gaining of a second electron to yield Sn(OH)2. Sn(OH)n(ads) Surface Reactions Two sizes of clusters were used to model the Pt(111) surface, Pt18 with 8 unpaired electrons and Pt34 with 22 unpaired electrons, shown in Figure 2. In our model the electrons are unpaired at the top of the d band so that each d-like band orbital is occupied by at least one electron. Calculated binding energies of the tin hydroxide molecules and Sn and OH to these clusters (Table 3) are in all but one case smaller on the larger cluster because of the wider Pt filled band, resulting in occupation of more molecule-surface antibonding counterpart orbitals. For SnOH and Sn(OH)2 two orientations were tried: horizontal, with both Sn and OH touching surface Pt atoms, and perpendicular, with just tin touching. Structures are shown in Figure 3. The perpendicular orientation was favored.
(2)
and the results, given in Table 4, indicate that on the larger cluster model Sn(OH)3(ads) is least thermodynamically stable and Sn(OH)4(ads) is most thermodynamically stable. These results suggest that Sn(OH)3(ads) is a candidate for transferring OH to the surface but Sn(OH)4 is not, which might have been guessed from the gas phase results of Table 2. Given these findings, we next studied the process of Sn-OH bond scission on the Pt34 cluster. Given the exothermic reaction energy according to the calculations and considering the proximity of this Sn-OH bond to surface Pt atoms, it might appear that an oxidative insertion reaction to form OH(ads) + Sn(OH)2(ads) should proceed with a small activation energy barrier. However, the transition state comes with difficulty, the energy rising about 1.75 eV to a transition state when the bond is stretched about 0.7 Å, as shown in Figure 4. This activation barrier is high enough that a comprehensive study of its potential dependence is not warranted. A check with the Pt band shifted down 0.5 eV yielded an activation energy of 1.30 eV, still high. Water dissociation was calculated to occur with a much lower barrier over pure platinum at 0 and 1/2 V.15 A contributing factor to this high barrier is the lower stability of lying-down Sn(OH)2, which is the immediate product of OH loss, compared to upright Sn(OH)2. As was discussed above, Sn(OH)3+(aq) might be an intermediate in the potential dependent SnII h SnIV process, and so we have considered its reaction with the surface. Adsorbed Sn(OH)3 has an empty 5p acceptor orbital and therefore should attract a water molecule. Would coordination to H2O activate the loss of OH from Sn(OH)3(ads) to give H2OSn(OH)2(ads), which might deprotonate to regenerate Sn(OH)3(ads), setting up a catalytic cycle? Our calculations show that H2O does have a strong bond of 1.65 eV strength to Sn(OH)3(ads), but when it binds, the coordination structure changes to nearly tetrahedral. In this structure the Sn-OH bonds do not lie on the surface atoms. Sn is no longer bound to the surface, and the structure is no longer favorable for OH displacement by H2O.
17538 J. Phys. Chem., Vol. 100, No. 44, 1996
Figure 5. Energies for the process H2O(ads) f OH(ads) + H(ads) on various surface models. The lowest curve is for H2O bound to a 1-fold site on Pt(111) and transferring H to an adjacent central Pt. The Sn(sub) curve has water bound to a substitutional Sn atom, with H transferring to an adjacent Pt atom. These results are for Pt18 and Pt17Sn clusters and are from refs 15 and 11, respectively. The curve with the structure figure is for Sn(OH)2 and Pt bound to the Pt34 cluster of Figure 2, and the top curve is for Sn and Pt bound to Pt34 and H2O coordinating to the Sn atom.
Solvated Sn(OH)2 is likely to adsorb to the platinum surface, and according to Table 3, a perpendicular orientation bound through Sn to a bridging site is formed. This orientation has the Sn blocked for access by water molecules, but the lyingdown orientation, though less stable, is expected to attract H2O, as does Sn(OH)3(ads). An adsorption energy of 0.99 eV is calculated for H2O on lying-down Sn(OH)2, but this is only 0.17 eV with respect to the more stable upright structure. Thus, on this smooth surface we predict that Sn(OH)2(ads) will not bind water molecules, but how about a rough surface? We find that Sn(OH)2 is stable in a lying-down orientation adjacent to a Pt adatom in a 3-fold site though its adsorption energy is only about 2.7 eV. A water molecule is calculated to bind strongly, 1.48 eV to this Sn, though this is weaker than 1.73 eV to the adjacent Pt adatom. Nevertheless, this site is worthy of examination for activating OH formation, for it is possible to tilt an OH bond of the adsorbed water molecule into a bridging orientation between Sn and Pt, which is the structure that leads to bond scission. Since tin may also be reduced at low potentials, we studied the binding of a tin atom and found it was attracted by 0.18 eV to the 3-fold site next to the Pd adatom. This tin atom binds a water molecule by 1.26 eV. Energy profiles calculated for OH bond scission in H2O bound to Sn and Sn(OH)2 that are adjacent to the Pt adatom on the Pt34 cluster are shown in Figure 5, along with results for H2O bound to Pt(111) and to a substantial tin atom in Pt(111) from ref 11. It is seen that according to these calculations, Sn and Sn(OH)2 in the roughened surface model are even less active in forming OH(ads) by the oxidative insertion mechanism than a substitutional tin atom in the surface, which was previously11 judged to be inactive. Concluding Comments From our calculations we have been able to relate the tin hydroxide II T IV redox processes to (i) the coordination of H2O to Sn(OH)2, (ii) the effects of H2O coordination on the
Anderson et al. electronic structure, and (iii) electron transfer to the anode at the sufficiently anodic potential with concomitant proton loss, rapid coordination of a second water molecule to Sn(OH)3+, and its deprotonation. However, the complexes and tin atoms when bonded to Pt do not, according to our results, react with water molecules in such a way as to form OH(ads) in a form available to oxidize CO(ads). This means that tin, as an alloy in Pt, has some other mechanistic role in electrocatalytically oxidizing CO(ads). In an upcoming publication it will be shown that Sn atoms present substitutionally in the Pt surface activate the oxidation of easily oxidized CO by providing pathways for H2O to approach CO adsorbed to adjacent Pt without getting stuck to the surface.29 The mechanism for the effect of Sn in the case of CO(ads) formed during methanol oxidation remains, however, a mystery. Acknowledgment. This work was funded by ARPA through ONR Contract No. N0014-92-J-1848. References and Notes (1) Cathro, K. J. J. Electrochem. Soc. 1969, 116, 1608. (2) Lamy, C. Electrochim. Acta 1984, 28, 1589. (3) Kunimatsu, K. J. J. Electroanal. Chem. 1985, 145, 219. (4) Bedan, B.; Hahn, F.; Juanto, S.; Lamy, C.; Leger, J.-M. J. Electroanal. Chem. 1987, 225, 215. (5) Nichols, R. J.; Bewick, A. Electrochim. Acta 1988, 33, 1691. (6) Feddrix, F. H., Thesis, Ph.D. Case Western Reserve University, Cleveland, OH, 1989. (7) Leung, L.-W.; Weaver, M. J. Langmuir 1990, 6, 323. (8) Lin, S. A., Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1991. (9) Janssen, M. M. P.; Moolhuysen, J. Electrochim. Acta 1976, 21, 861. (10) Shiller, P.; Anderson, A. B. Surf. Sci. 1990, 236, 225. (11) Anderson, A. B.; Grantscharova, E.; Shiller, P. J. Electrochem. Soc. 1995, 142, 1880. (12) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1993, 97, 12020. (13) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Phys. Chem. 1994, 98, 617. (14) Gasteiger, H. A.; Markovic, N.; Ross, P. N., Jr.; Cairns, E. J. J. Electrochem. Soc. 1994, 141, 1795. (15) Anderson, A. B.; Grantscharova, E. J. Phys. Chem. 1995, 99, 9149. (16) Motoo, S.; Watanabe, M. J. Electroanal. Chem. 1976, 69, 429. (17) Bittins-Cattaneo, B.; Iwasita, T. J. Electroanal. Chem. 1987, 238, 150. (18) Sobkowski, J.; Franaszczak, K.; Piasecki, A. J. Electroanal. Chem. 1985, 196, 145. (19) Bae, I. T.; Takeshi, S.; Scherson, D. A. J. Electroanal. Chem. 1991, 297, 185. (20) Angestein-Kozlowska, H.; MacDougal, D.; Conway, B. E. J. Electrochem. Soc. 1973, 120, 756. (21) Bedan, B.; Kadirgan, F.; Lamy, C.; Leger, J.-M. J. Electroanal. Chem. 1981, 127, 75. (22) Katayama, A. J. Phys. Chem. 1980, 84, 376. (23) Haner, A. N.; Ross, P. N. J. Phys. Chem. 1991, 95, 3740. (24) Gasteiger, H. A.; Markovic, N. M.; Ross, P. N., Jr. J. Phys. Chem. 1995, 99, 8945. (25) Leiva, E. P. M.; Santos, E.; Iwasita, T. J. Electroanal. Chem. 1986, 215, 357. (26) Morimoto, Y., Ph.D. Thesis, Case Western Reserve University, Cleveland, OH, 1994. (27) Shiller, P.; Anderson, A. B. J. Electroanal. Chem. 1992, 339, 201. (28) Anderson, A. B. Int. J. Quantum Chem. 1994, 49, 581. (29) Anderson, A. B.; Grantscharova, E. Manuscript in preparation.
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