Theoretical Study on Dissolution and Reprecipitation Mechanism of Pt

Jan 27, 2011 - Theoretical Study on Solubility from Pt Electrocatalyst and Reactivity in Electrolyte Environment of Pt Complex in PEFC. Takayoshi Ishi...
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Theoretical Study on Dissolution and Reprecipitation Mechanism of Pt Complex in Pt Electrocatalyst Takayoshi Ishimoto,†,* Teppei Ogura,† Minoru Umeda,‡ and Michihisa Koyama†,§,* †

INAMORI Frontier Research Center, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan Department of Materials Science and Technology, Faculty of Engineering, Nagaoka University of Technology, 1603-1 Kamitomioka, Nagaoka, Niigata 940-2188, Japan § International Institute for Carbon-Neutral Energy Research, Kyushu University, 744 Motooka, Nishi-ku, Fukuoka 819-0395, Japan ‡

ABSTRACT: We systematically analyzed the formation energy and solvation free energy of four- and six-coordinate Pt(II) and Pt(IV) complexes with three types of ligands (H2O, OH-, and HSO4-) in concentrated sulfuric acid to rationalize the Pt electrocatalyst degradation and dissolution mechanisms as a model for a polymer electrolyte fuel cell. Because the formation energy and solvation free energy of Pt(IV) complexes are larger than those of Pt(II) complexes, the Pt(IV) complexes are more probable as the dissolved Pt species than Pt(II). The local relaxations about atomic charges and geometry by the substitution of OH- or HSO4- for H2O are one of the factors influencing the stability of dissolved Pt complex. We predict the [Pt(H2O)2(OH)4] and [Pt(OH)4]2- are important complexes for desorption from the Pt surface based on the desorption energy analysis of Pt complex from the Pt surface. The [Pt(H2O)4]2þ complex, which is formed by a reduction reaction from Pt(IV) and a proton addition reaction, shows the possibility of the final form before reprecipitation on the Pt surface. We theoretically estimated the Pt dissolution and reprecipitation mechanisms from an atomistic view.

1. INTRODUCTION Polymer electrolyte fuel cell (PEFC) is intensively studied as a promising power source for automobiles and cogeneration systems because PEFC has a high energy conversion efficiency. For the practical long-term operation of PEFC, the durability of the membrane electrode assembly (MEA) is one of the main issues to be tackled. Conventional PEFC catalysts in MEA typically consist of Pt nanoparticles on the order of 2-3 nm in diameter, which are dispersed on the surface of primary carbon particles of 20-50 nm. The degradation of Pt electrocatalyst is particularly significant for the overall performance of MEA although the Pt-based electrode is usually utilized as electrocatalyst due to the electrochemical stability over a wide potential region in various fields. Some important studies are pointed out the loss of electrochemical surface area by the Pt dissolution and reprecipitation phenomena during the operation.1-5 Iterative Pt dissolution and reprecipitation processes lead to the particle growth of Pt electrocatalyst called Ostwald ripening, causing the degradation of the electrocatalytic activity. Many experimental studies about Pt degradation are carried out using various experimental techniques.6-15 These experimental studies have found that Pt dissolution depends on the voltage, acidity, and temperature. For example, Wang et al. reported the difference of dissolved Pt concentration by the electrostatic potential.15 For a more detailed understanding, Mitsushima et al. analyzed the solubility and the dissolution mechanism of Pt in acidic media by the change of temperature and pH.10 They proposed a 4þ valence of the dissolved species of Pt and Pt(OH)3(H2O)3þ as the r 2011 American Chemical Society

most suitable conditions. Uchimoto et al. also observed a similar six-coordinate Pt(IV) complex using extended X-ray absorption fine structure (EXAFS).7,13 In addition, Umeda et al. found the Pt(OH)4 formation for the first step of Pt degradation in concentrated sulfuric acid using a rotating ring-disk electrode (RRDE) and a electrochemical quartz crystal microbalance (EQCM).8 They also proposed the two-step reduction process, Pt(IV) f Pt(II) f Pt(0), for Pt dissolution and reprecipitation. For a thorough understanding, it is meaningful to theoretically study what ligands stabilize the Pt(IV) complex in solution. Theoretically, oxygen reduction reaction (ORR) on the Pt surface has been intensively studied and has provided important insights from an atomistic view.16-19 Although electronic structure calculations enable us to analyze details of the ORR mechanism on the Pt surface, the elucidation of the Pt dissolution and precipitation mechanism still remains a challenging subject for quantum chemistry. However, the quantum chemical calculation is used as an effective approach for understanding chemical and physical phenomena of the stable geometry and reactivity of metal in solution.20-25 Bryantsev et al. reported that the solvation energy and coordination geometry of metal ions calculated by density functional theory and the conductor-like-screening model (COSMO) representing continuum solvent show good Received: August 10, 2010 Revised: December 27, 2010 Published: January 27, 2011 3136

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Figure 1. Optimized structures of four-coordinate Pt(II) complexes with three types of ligands.

agreement with the experimental ones.20 This result indicates that the solvation structures of the metal complex determined by theoretical calculation can be described with reasonable accuracy. In this study, we focused on the dissolved Pt species in concentrated sulfuric acid as a first step to tackle the Pt dissolution and precipitation phenomena. We theoretically analyzed the solvation structures and thermodynamic properties of Pt complex in aqueous solution. We also analyzed the desorption energy of Pt complex from the Pt surface. The reactivity between Pt complexes is finally discussed.

2. COMPUTATIONAL DETAILS All calculations were performed under the generalized gradient approximation with Becke-88 exchange and Lee-YangParr correlation functionals (BLYP) as implemented in the DMol3 package.26 Double numerical atomic basis sets augmented with polarization function (DNP) were used to describe the valence electrons, and the core electrons were represented by effective core potentials (ECP). To obtain atomic charges, the Hirshfeld approach27 was used. Full geometry optimization was performed for each system in the gas phase (isolated molecule) and also in solution. Solvent effects in water were estimated using the COSMO. The value of 100 was chosen for the relative dielectric constant ε of concentrated sulfuric acid. All thermodynamic data have been evaluated for a temperature T = 298 K to represent experimental condition.8 We used four- and six-coordinate Pt(II) and Pt(IV) complexes with three types of ligands (H2O, OH-, and HSO4-) as models of dissolved Pt species in concentrated sulfuric acid for analyzing the stability and formation energy. Note that we consider OH- as a ligand even in the acidic PEFC condition assuming that OH- formed on the Pt surface6,28-30 would play an important role in the formation of intermediate species for dissolution.

3. RESULTS AND DISCUSSION 3.1. Analysis of Dissolved Pt Complexes in Solution. We calculated the optimized geometry to estimate the formation energy (ΔE) and solvation free energy (ΔGsolv) for the four- and six-coordinate Pt(II) and Pt(IV) complexes with three ligands to model dissolved Pt species. The most stable structures of Pt(II) and Pt(IV) complexes with various combinations of ligands are shown in Figures 1 and 2, respectively. Four- and six-coordinate Pt(II) and Pt(IV) complexes were square and octahedral structures, respectively. The ΔE and ΔGsolv for Pt(II) and Pt(IV) complexes are also listed in Tables 1 and 2, respectively. The ΔE for Pt(II) and Pt(IV) complexes were evaluated by the following equations,

ΔE ¼ Eð½PtðH2 OÞm ðLÞn 2-n Þ-fEðPt2þ ÞþmEðH2 OÞ þnEðLÞgðmþn ¼ 4Þ

ð1Þ

ΔE ¼ Eð½PtðH2 OÞm ðLÞn 4-n Þ-fEðPt4þ ÞþmEðH2 OÞ þnEðLÞgðmþn ¼ 6Þ

ð2Þ

Here L stands for OH- and HSO4-.To express the stability of Pt complex in solution, the ΔGsolv was calculated as the difference between the free energy for the gas and the liquid phases for the optimized geometry of each system, ΔGsolv ¼ GðCOSMOÞ-GðGasÞ

ð3Þ

The ΔE and ΔGsolv of [Pt(H2O)4]2þ are -106.1 and -210.9 kcal/mol, respectively. We performed the geometry optimizations of [Pt(H2O)4]2þ with substitution of OH- or HSO4- for the H2O molecule to analyze the influence of the ligands on the ΔE. By the substitution of OH- for H2O, the ΔE of [Pt(H2O)3(OH)]þ became stable by about 70 kcal/mol in comparison with [Pt(H2O)4]2þ. The ΔE of [Pt(H2O)4-n(OH)n]2-n (n = 1-4) complex becomes stable as the number of OH- increases. 3137

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Figure 2. Optimized structures of six-coordinate Pt(IV) complexes with three types of ligands.

However, large stabilization was not observed when the H2O was substituted with HSO4-. We analyzed the possibility of SO42- as a ligand. To compare the monodentate [Pt(H2O)3(SO4)] and bidentate [Pt(H2O)2(SO4)] species, the [Pt(H2O)2(SO4)] is stable than the [Pt(H2O)3(SO4)] due to large ΔE and ΔGsolv. The [Pt(H2O)2(OH)(HSO4)] is also more stable than [Pt(H2O)3(SO4)] due to the proton transfer from H2O to SO42-. In this study, we focused on the HSO4- ligand because the HSO4is important species as a ligand rather than SO42- in concentrated sulfuric acid. Concerning the ΔGsolv, the solvation free energies of cationic and anionic Pt(II) complexes ((1) and (5)) were relatively large (-210.9 and -226.4 kcal/mol). The solvation free energies of neutral Pt(II) complexes ((3), (7), and (10)) were less than -25 kcal/mol. The ΔGsolv becomes large when the ionicity of complex is large. In the case of six-coordinate Pt(IV) complexes (Table 2), the ΔE (-431.2 kcal/mol) and ΔGsolv (-761.6 kcal/mol) of [Pt(H2O)6]4þ are much more stable than those of [Pt(H2O)4]2þ. The ΔE difference between [Pt(H2O)5(X)]3þ (X = OH- and HSO4-) and [Pt(H2O)6]4þ are about 100 and 30 kcal/mol,

respectively. As the number of OH- increases, the ΔE of [Pt(H2O)6-n(OH)n]4-n (n = 1-4) becomes more stable. The large stabilization was not observed in the case of [Pt(H2O)6-n(HSO4)n]4-n (n = 1-4). We analyzed the geometrical parameters and atomic charges of Pt(II) and Pt(IV) complexes to see more details on the interaction between Pt ions and ligands. The Pt 3 3 3 O distances for Pt(II) and Pt(IV) complexes are shown in Figure 3. The number of Pt(II) and Pt(IV) complexes in Figure 3 corresponds to the optimized structures in Figures 1 and 2. The Pt 3 3 3 O (H2O) distances in [Pt(H2O)6]4þ were shorter than those of [Pt(H2O)4]2þ due to the strong interaction energy. The Pt 3 3 3 O (OH-) distance was about 0.1 Å shorter than the Pt 3 3 3 O (H2O) because the interaction energy between Pt and OH- is larger than Pt and H2O. By forming the Pt 3 3 3 O (OH-) interaction, the Pt 3 3 3 O (H2O) distance became long in the case of [Pt(H2O)4-n(OH)n]2-n and [Pt(H2O)6-n(OH)n]4-n (n = 1-4). The change of Pt 3 3 3 O (HSO4-) distances shows tendency similar to that of Pt 3 3 3 O (H2O) distances because the interaction energies between Pt and H2O and HSO4- are almost same. 3138

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Table 1. Formation Energy (ΔE) and Solvation Free Energy (ΔGsolv) of Pt(II) Complexesa Pt(II) complex

ΔGsolv

-106.1

-210.9

(2) [Pt(H2O)3(OH)]þ (3) [Pt(H2O)2(OH)2]

-1740 -227.6

-71.8 -18.9

(4) [Pt(H2O)(OH)3]-

-268.6

-72.3

(5) [Pt(OH)4]2-

-301.5

-226.4

(6) [Pt(H2O)3(HSO4)]þ

-117.4

-76.4

(7) [Pt(H2O)2(HSO4)2]

-119.6

-23.8

(8) [Pt(H2O)3(SO4)]

-148.0

-33.0

(9) [Pt(H2O)2(SO4)]

-152.9

-35.1

½Pt4 ðOHÞn 2-n þðH2 OÞm fPt3 þ½PtðH2 OÞm ðOHÞn 2-n ðmþn ¼ 4Þ

(10) [Pt(H2O)2(OH)(HSO4)] (11) [Pt(H2O)2(OH)(SO4)]-

-176.2 -201.0

-24.7 -86.8

ð4Þ

(12) [Pt(H2O)(OH)2(SO4)]2-

-241.1

-191.4

½Pt4 ðOHÞn 4-n þðH2 OÞm fPt3 þ½PtðH2 OÞm ðOHÞn 4-n ðmþn ¼ 6Þ

2þ

(1) [Pt(H2O)4]

a

ΔE

Units are in kcal/mol.

ð5Þ

Table 2. Formation Energy (ΔE) and Solvation Free Energy (ΔGsolv) of Pt(IV) Complexesa Pt(IV) complex

ΔE

ΔGsolv

(1) [Pt(H2O)6]

-431.2

-761.6

(2) [Pt(H2O)5(OH)]3þ

-536.4

-442.7

(3) [Pt(H2O)4(OH)2]2þ (4) [Pt(H2O)3(OH)3]þ

-625.0 -700.0

-210.5 -71.9

4þ

a

That is, when the ΔE becomes large, ΔGsolv becomes small (for example (1), (2), (3), (4), and (5) or (6), (7), (8), and (9) series in Pt(IV) complexes). 3.2. Desorption Energy of Pt Complex from the Pt Surface. We next focused on the desorption energy of these Pt complexes from the Pt surface to estimate the initial dissolved species. To estimate desorption energy of Pt(II) and Pt(IV) complexes from the Pt surface, we used a simple Pt4 cluster, which is used for analysis of interactions between Pt and adsorbate species31 and analysis of the ORR mechanism on the Pt surface,16 as a Pt surface model. We assumed OH adsorption on the Pt surface, which is proposed as a precursor of dissolved Pt complexes.8

(5) [Pt(H2O)2(OH)4]

-756.6

-25.3

(6) [Pt(H2O)5(HSO4)]3þ

-460.7

-420.1

(7) [Pt(H2O)4(HSO4)2]2þ

-480.5

-200.6

(8) [Pt(H2O)3(HSO4)3]þ

-498.9

-76.6

(9) [Pt(H2O)2(HSO4)4]

-506.8

-28.2

(10) [Pt(H2O)5(SO4)]2þ

-517.3

-208.5

(11) [Pt(H2O)4(OH)(HSO4)]2þ (12) [Pt(H2O)3(OH)2(HSO4)]þ

-550.9 -309.0

-203.5 -75.9

(13) [Pt(H2O)2(OH)3(HSO4)]

-380.8

-26.0

(14) [Pt(H2O)3(OH)(HSO4)2]þ

-567.4

-73.2

(15) [Pt(H2O)2(OH)2(HSO4)2]

-327.8

-26.3

(16) [Pt(H2O)2(OH)(HSO4)3]

-572.2

-28.9

(17) [Pt(H2O)3(OH)2(SO4)]

-666.4

-27.2

(18) [Pt(H2O)3(HSO4)2(SO4)]

-539.9

-36.9

(19) [Pt(H2O)3(OH)(HSO4)(SO4)]

-603.2

-31.4

Units are in kcal/mol.

The atomic charges of Pt in [Pt(H2O)4]2þ and [Pt(H2O)6]4þ were 0.493 and 0.868, respectively. By the substitution of OHand HSO4- for H2O, the atomic charge of Pt became small. We show in Figure 4 the atomic charges of oxygen atom in ligands of Pt(II) and Pt(IV) complexes. When the number of OHincreased, the atomic charge of O in OH- took large negative value. In the case of [Pt(H2O)6-n(HSO4)n]4-n (n = 1-4), the remarkable tendency was not found when the number of HSO4increased. The change of atomic charge of O in ligands corresponds to the change of Pt 3 3 3 O distances. We expect that the soluble species have large ΔE and ΔGsolv. From this perspective, our result indicates that the Pt(IV) complexes are more favorable as dissolved species compared to Pt(II). The comparison of solubility in the Pt(IV) complex is, however, difficult, because the ΔE and ΔGsolv show ambivalent tendency.

Table 3 summarizes the desorption energy of Pt(II) and Pt(IV) complexes from the Pt surface model. The negative value represents that the desorption of thePt complex to the solvent from the Pt surface is favorable. The [Pt(OH)4]2- and [Pt(H2O)2(OH)4] prefer in solvent rather than the Pt surface because the desorption energies [Pt(OH)4]2- and [Pt(H2O)2(OH)4] are -20.7 and -14.3 kcal/mol, respectively. Other Pt complexes show positive desorption energies. This result indicates that the [Pt(OH)4]2- and [Pt(H2O)2(OH)4] might be a dissolved species from the Pt surface. To analyze desorption process in detail, we focused on the results for [Pt(OH)4]2- and [Pt(H2O)2(OH)4] complexes. The desorption energy, ΔE, and ΔGsolv of both [Pt(OH)4]2- and [Pt(H2O)2(OH)4] are all negative. The [Pt(H2O)2(OH)4] would play important role in the initial step of Pt dissolution mechanism by the large stabilization of ΔE, although the six-coordinate Pt(IV) complex will not be directly formed on the Pt surface by the steric hindrance. However, [Pt(OH)4]2- would also play important role as the dissolved species rather than [Pt(H2O)2(OH)4] due to the large ΔGsolv. This speculation is supported considering four-coordinate structure is sterically more probable than six-coordinate structure on the Pt surface. Further, [Pt(H2O)2(OH)4] and [Pt(OH)4]2complexes are experimentally estimated by Umeda et al. as the precursor and dissolved species in the Pt dissolution mechanism, respectively. It is worth while to analyze the reactivity between each Pt complex for an understanding of the Pt dissolution and reprecipitation phenomena. 3.3. Reactivity between Pt Complexes. We discuss the reactivity between Pt complexes after dissolution into electrolyte solution. The five reactions between Pt complexes are considered as shown by the following equations taking [Pt(H2O)2(OH)2] as an example. Proton addition reaction ½PtðH2 OÞ2 ðOHÞ2 þHþ f½PtðH2 OÞ3 ðOHÞþ

ð6Þ

H2O/OH- substitution reaction ½PtðH2 OÞ2 ðOHÞ2 þOH-f½PtðH2 OÞðOHÞ3 -þH2 O

ð7Þ

-

H2O/HSO4 substitution reaction ½PtðH2 OÞ2 ðOHÞ2 þHSO4-f½PtðH2 OÞðOHÞ2 ðHSO4 Þ-þH2 O

ð8Þ 3139

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Figure 3. Pt 3 3 3 O distance with three ligands for Pt(II) and Pt(IV) complexes.

Figure 4. Atomic charge of oxygen in ligands for Pt(II) and Pt(IV) complexes.

OH-/ HSO4- substitution reaction ½PtðH2 OÞ2 ðOHÞ2 þHSO4-f½PtðH2 OÞ2 ðOHÞðHSO4 ÞþOHð9Þ Reduction reaction from Pt(IV) to Pt(II) complex ½PtðH2 OÞ4 ðOHÞ2 2þ þ2e-T½PtðH2 OÞ2 ðOHÞ2 þ2H2 O ð10Þ In these cases, we can neglect eqs 7 and 9 involving OH- because the experimental condition is in strong acidic solution (concentrated sulfuric acid). In eq 6, another proton addition reaction via a water molecule shows reliable results,24 ½PtðH2 OÞ2 ðOHÞ2 þH3 Oþ f½PtðH2 OÞ3 ðOHÞþ þH2 O ð11Þ The proton addition reaction was estimated by eq 11 in this study. The reaction energies for eqs 8 and 11 of Pt(II) and Pt(IV)

complexes are listed in Tables 4 and 5, respectively. We started the analysis of reactivity from the [Pt(H2O)2(OH)4] complex. [Pt(H 2O)2(OH)4] was predicted as a dissolved Pt complex reacted with [Pt(H2O)3 (OH)3]þ and [Pt(H 2O)4(OH)2]2þ by the proton addition reaction (-24.3 and -5.8 kcal/ mol). The ΔGsolv of Pt(IV) complex also became large by the proton addition reaction (25.3, 71.9, and 210.5 kcal/mol). [Pt(H2O)3(OH)3]þ as a dissolved Pt complex was also experimentally suggested.10 A further proton addition reaction from [Pt(H2O)4(OH)2]2þ to [Pt(H2O)5(OH)]3þ and [Pt(H2O)6]4þ was not proceeded due to the positive reaction energies (7.7 and 24.4 kcal/mol) although [Pt(H2O)5(OH)]3þ and [Pt(H2O)6]4þ showed large ΔGsolv. However, the H2O/HSO4- substitution reactions from [Pt(H2O)3(OH)3]þ and [Pt(H2O)4(OH)2]2þ to [Pt(H2O)2(OH)3(HSO4)] and [Pt(H2O)3(OH)2(HSO4)]þ were proceeded instead of a proton addition reaction. We have found eight possible complexes ((3), (4), (5), (12), (13), and (15) 3140

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were focused on the Pt complexes with H2O and OH- ligands because the reduction reaction from Pt(IV) to Pt(II) with H2O and OH- ligands was experimentally proposed.8 The result of reduction reaction for eq 10 from Pt(IV) to Pt(II) complexes is listed in Table 6. The values in Table 6 represent as an electrode potential of a hydrogen standard electrode. The potential (2.92 and 2.11 V) of [Pt(H2O)6]4þ and [Pt(H2O)5(OH)]3þ, which are not formed by the proton addition reaction in the Pt(IV) complex, were relatively large. The most smallest potential (0.11 V) was the reaction of [Pt(H2O)2(OH)4]. The [Pt(OH)4]2- complex has the largest ΔE and ΔGsolv in Pt(II) complexes. This complex might be a intermediate structure proposed as the reduction reaction, Pt(OH)4 þ 2Hþ þ 2e- f Pt(OH)2 þ 2H2O.8 The proton addition reaction

in Pt(IV) complexes) as dissolved species by the proton addition and H2O/HSO4- substitution reactions. The reduction reactions Table 3. Desorption Energy (ΔE) from the Pt Surface for Pt(II) and Pt(IV) Complexesa ΔE

Pt(II) complex (1) [Pt(H2O)4]2þ

37.6

(1) [Pt(H2O)6]4þ

42.1

(2) [Pt(H2O)3(OH)] (3) [Pt(H2O)2(OH)2]

37.5 33.8

(2) [Pt(H2O)5(OH)]3þ (3) [Pt(H2O)4(OH)2]2þ

46.5 45.2

(4) [Pt(H2O)(OH)3]-

19.4

(4) [Pt(H2O)3(OH)3]þ

þ

2-

(5) [Pt(OH)4] a

ΔE

Pt(IV) complex

-20.7

(5) [Pt(H2O)2(OH)4]

34.4 -14.3

Units are in kcal/mol.

Table 4. Reaction Energies (ΔE) for Pt(II) Complexesa ΔE Proton Addition Reaction (2) [Pt(H2O)3(OH)]þ þ H3Oþ f (1) [Pt(H2O)4]2þ þ H2O

-13.0

(3) [Pt(H2O)2(OH)2] þ H3Oþ f (2) [Pt(H2O)3(OH)]þ þ H2O

-27.2

(4) [Pt(H2O)(OH)3]- þ H3Oþ f (3) [Pt(H2O)2(OH)2] þ H2O

-39.9

(5) [Pt(OH)4]2- þ H3Oþ f (4) [Pt(H2O)(OH)3]- þ H2O

-47.9

(10) [Pt(H2O)2(OH)(HSO4)] þ H3Oþ f (6) [Pt(H2O)3(HSO4)]þ þ H2O

-22.1

(8) [Pt(H2O)3(SO4)] þ H3Oþ f (6) [Pt(H2O)3(HSO4)]þ þ H2O

-15.4

(1) [Pt(H2O)4]2þ þ HSO4- f (6)

H2O/HSO4- Substitution [Pt(H2O)3(HSO4)]þ þ H2O

Reaction -11.3

(6) [Pt(H2O)3(HSO4)]þ þ HSO4- f (7) [Pt(H2O)2(HSO4)2] þ H2O (2) [Pt(H2O)3(OH)]þ þ HSO4- f (10) [Pt(H2O)2(OH)(HSO4)] a

-2.2 -2.2

Units are in kcal/mol.

Table 5. Reaction Energies (ΔE) for Pt(II) Complexesa ΔE Proton Addition Reaction (2) [Pt(H2O)5(OH)]3þ þ H3Oþ f (1) [Pt(H2O)6]4þ þ H2O

24.4

(3) [Pt(H2O)4(OH)2]2þ þ H3Oþ f (2) [Pt(H2O)5(OH)]3þ þ H2O (4) [Pt(H2O)3(OH)3]þ þ H3Oþ f (3) [Pt(H2O)4(OH)2]2þ þ H2O

7.7 -5.8

(5) [Pt(H2O)2(OH)4] þ H3Oþ f (4) [Pt(H2O)3(OH)3]þ þ H2O

-24.3

(11) [Pt(H2O)4(OH)(HSO4)]2þ þ H3Oþ f (6) [Pt(H2O)5(HSO4)]3þ þ H2O

9.3

(12) [Pt(H2O)3(OH)2(HSO4)]þ þ H3Oþ f (11) [Pt(H2O)4(OH)(HSO4)]2þ þ H2O

0.4

(13) [Pt(H2O)2(OH)3(HSO4)] þ H3Oþ f (12) [Pt(H2O)3(OH)2(HSO4)]þ þ H2O

-9.0

(14) [Pt(H2O)3(OH)(HSO4)2]þ þ H3Oþ f (7) [Pt(H2O)4(HSO4)2]2þ þ H2O

2.7

(15) [Pt(H2O)2(OH)2(HSO4)2] þ H3Oþ f (14) [Pt(H2O)3(OH)(HSO4)2]þ þ H2O

6.1

(16) [Pt(H2O)2(OH)(HSO4)3] þ H3Oþ f (8) [Pt(H2O)3(HSO4)3]þ þ H2O (10) [Pt(H2O)5(SO4)]2þ þ H3Oþ f (6) [Pt(H2O)5(HSO4)]3þ þ H2O

-7.5 -10.6

H2O/HSO4- Substitution Reaction (1) [Pt(H2O)6]4þ þ HSO4- f (6) [Pt(H2O)5(HSO4)]3þ þ H2O

-29.5

(6) [Pt(H2O)5(HSO4)]3þ þ HSO4- f (7) [Pt(H2O)4(HSO4)2]2þ þ H2O

-19.8

(7) [Pt(H2O)4(HSO4)2]2þ þ HSO4- f (8) [Pt(H2O)3(HSO4)3]þ þ H2O

-18.4

(8) [Pt(H2O)3(HSO4)3]þ þ HSO4- f (9) [Pt(H2O)2(HSO4)4] þ H2O

-14.4

(11) [Pt(H2O)4(OH)(HSO4)]2þ þ HSO4- f (14) [Pt(H2O)3(OH)(HSO4)2]þ þ H2O (14) [Pt(H2O)3(OH)(HSO4)2]þ þ HSO4- f (16) [Pt(H2O)2(OH)(HSO4)3] þ H2O

-16.5 -4.8

(3) [Pt(H2O)4(OH)2]2þ þ HSO4- f (12) [Pt(H2O)3(OH)2(HSO4)]þ þ H2O (12) [Pt(H2O)3(OH)2(HSO4)]þ þ HSO4- f (15) [Pt(H2O)2(OH)2(HSO4)2] þ H2O (4) [Pt(H2O)3(OH)3]þ þ HSO4- f (13) [Pt(H2O)2(OH)3(HSO4)] þ H2O a

-7.9

(2) [Pt(H2O)5(OH)]3þ þ HSO4- f (11) [Pt(H2O)4(OH)(HSO4)]2þ þ H2O

-7.2 -18.8 -4.0

Units are in kcal/mol. 3141

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The Journal of Physical Chemistry C

ARTICLE

Table 6. Reduction Energy from Pt(IV) to Pt(II) Complexa U (1) [Pt(H2O)6]4þ þ 2e- T (1) [Pt(H2O)4]2þ þ 2H2O (2) [Pt(H2O)5(OH)]3þ þ 2e- T (2) [Pt(H2O)3(OH)]þ þ 2H2O (3) [Pt(H2O)4(OH)2]2þ þ 2e- T (3) [Pt(H2O)2(OH)2] þ 2H2O (4) [Pt(H2O)3(OH)3]þ þ 2e- T (4) [Pt(H2O)(OH)3]- þ 2H2O (5) [Pt(H2O)2(OH)4] þ 2e- T (5) [Pt(OH)4]2- þ 2H2O a

2.92 2.11 1.36 0.62 0.11

Units are in V.

for [Pt(OH)4]2- complex proceeds to the [Pt(H2O)4]2þ complex via [Pt(H2O)3(OH)]þ, [Pt(H2O)2(OH)2], and [Pt(H2O)(OH)3]- (Table 4). This result indicates the possibility of [Pt(H2O)4]2þ as a final form before reprecipitation on the Pt surface because the desorption energies are positive values.

4. CONCLUSIONS In this study, we systematically analyzed the ΔE and ΔGsolv of four- and six-coordinate Pt(II) and Pt(IV) complexes with three types of ligands (H2O, OH-, and HSO4-) as a model of dissolved Pt species to rationalize the Pt electrocatalyst degradation and dissolution mechanisms. Because the ΔE and ΔGsolv of Pt(IV) complexes are larger than Pt(II) complexes, the Pt(IV) complexes are more probable as the dissolved Pt species than Pt(II). The stabilization of ΔE and ΔGsolv showed a countertrend. We also analyzed the geometrical parameters of Pt 3 3 3 O in ligands and the atomic charge of O in ligands. These local relaxations about atomic charges and geometry by the substitution of OH- or HSO4- for H2O are one of the factors influencing the stability of dissolved Pt complex. In addition, we evaluated the desorption energy of Pt complex from the Pt surface and reactivity of Pt complexes to understand the Pt dissolution and reprecipitation phenomena. We pointed out the two important complexes, [Pt(H2O)2(OH)4] and [Pt(OH)4]2-, to analyze the Pt dissolution mechanism. Two possible pathways, proton addition and H2O/HSO4- substitution reactions, are proposed in respective Pt(IV) and Pt(II) complexes. The [Pt(H2O)4]2þ complex, which is formed by a reduction reaction from Pt(IV) and a proton addition reaction, shows the possibility of a final form before reprecipitation on the Pt surface. These reaction mechanisms show good agreement with previous experimental results. We theoretically estimated the Pt dissolution and reprecipitation mechanisms from an atomistic view. ’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected], Tel: þ81-92-801-6969, Fax: þ81-92-801-6969 (T.I.). *E-mail: [email protected], Tel: þ81-92-801-6968, Fax: þ81-92-801-6968 (M.K.).

’ ACKNOWLEDGMENT This work was financially supported by the “Development of Technology on Basis and Common Issues” project under the “Strategic Development of PEFC Technologies for Practical Application” program from New Energy and Industrial Technology Development Organization (NEDO). We are grateful for the support of KYOCERA corporation.

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