Proton-coupled electron transfer at modified electrodes by multiple pathways

Abstract

In single site water or hydrocarbon oxidation catalysis with polypyridyl Ru complexes such as [Ru^II(Mebimpy)(bpy)(H₂O)]²⁺ [where bpy is 2,2′-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog [Ru^II(Mebimpy)(4,4^′-bis-methlylenephosphonato-2,2^′-bipyridine)(OH₂)]²⁺ 2-PO₃H₂, accessing the reactive states, Ru^V = O³⁺/Ru^IV = O²⁺, at the electrode interface is typically rate limiting. The higher oxidation states are accessible by proton-coupled electron transfer oxidation of aqua precursors, but access at inert electrodes is kinetically inhibited. The inhibition arises from stepwise mechanisms which impose high energy barriers for 1e^- intermediates. Oxidation of the Ru^III-OH²⁺ or forms of 2-PO₃H₂ to Ru^IV = O²⁺ on planar fluoride-doped SnO₂ electrode and in nanostructured films of Sn(IV)-doped In₂O₃ and TiO₂ has been investigated with a focus on identifying microscopic phenomena. The results provide direct evidence for important roles for the nature of the electrode, temperature, surface coverage, added buffer base, pH, solvent, and solvent H₂O/D₂O isotope effects. In the nonaqueous solvent, propylene carbonate, there is evidence for a role for surface-bound phosphonate groups as proton acceptors.

Electron transfer reactions involving pH-dependent proton-coupled electron transfer (PCET) couples with a change in proton content between oxidation states, such as quinone/hydroquinone (Q/H₂Q) or M = O/M-OH/M-OH₂ oxo/hydroxo/aqua transition metal couples, are often slow at inert electrodes (1–5). For these couples, the change in protonation state and the requirement to add or lose protons adds to the normal kinetic barrier to electron transfer. The PCET effect arises from the fact that oxidation or reduction at the electrode occurs by electron transfer without a change in proton content. This effect restricts interfacial mechanisms to electron transfer followed by proton transfer (ET-PT) or proton transfer followed by electron transfer (PT-ET). Both involve high-energy intermediates in nonequilibrium protonation states.

As an example, oxidation of H₂Q, H₂Q - e^- → H₂Q^+•, occurs at E^o′ = 1.10 V vs. normal hydrogen electrode (NHE) independent of pH (E^o′ is the formal potential) (2). The thermodynamic potential for H₂Q oxidation at pH 7, H₂Q - e^- - H⁺ → HQ^•, is E^o′ = 0.63 V (2). An inert electrode at pH = 7 creates an overpotential of 0.47 V for the initial electron transfer in the ET-PT sequence. Following electron transfer, thermodynamic equilibrium is reached by proton loss from H₂Q^+• to the surrounding medium at the prevailing pH with ΔE^o′ = 1.10 V - 0.059{pH - (pK_a(H₂Q^+•)} with pK_a(H₂Q^+•) = -0.95. Oxidation can also occur by PT-ET with initial loss of a proton from H₂Q to give HQ^- followed by oxidation to HQ^• at E^o′(HQ^•/HQ^-) = 0.46 V. For this mechanism, an inhibition to rate arises from the pH dependence of the electroactive anion concentration with [HQ^-] = K_a[H₂Q]_T/([H⁺] + K_a) and [H₂Q]_T the total concentration of hydroquinone with pK_a(H₂Q) = 9.85 (2).

The role of PCET in electrochemical reactivity has been discussed in a series of papers (6–15). In a cyclic voltammetry (CV) study on the Os-based couples cis-[Os^IV(bpy)₂(py)(O)]²⁺/cis-[Os^III(bpy)₂(py)(OH)]²⁺ (where bpy is 2,2′-bipyridine) and cis-[Os^III(bpy)₂(py)(OH)]²⁺/cis-[Os^II(bpy)₂(py)(OH₂)]²⁺, Savéant and coworkers (7) concluded that oxidation of to Os^III-OH²⁺ is dominated by ET-PT below pH 7 and by PT-ET above pH 7. The Os^IV = O²⁺/Os^III-OH²⁺ couple is slower than the couple by approximately 10³ with a negligible H₂O/D₂O solvent kinetic isotope effect (KIE) in acidic solution. Rate accelerations were observed with added proton acceptor bases, the Britton–Robinson buffer (phosphate, citrate, borate, and acetate), accompanied by the appearance of a solvent KIE of 2–2.5. The latter was attributed to concerted electron-proton transfer (EPT) at the electrode with electron transfer to the electrode occurring in concert with proton transfer to the added base form of the buffer (7).

Although water as the solvent could, in principle, act as a proton acceptor or donor, its participation is limited by its acid-base properties with pK_a(H₃O⁺) = -1.74 and pK_a(H₂O) = 15.7. Concerted EPT pathways involving solvent are favorable only for exceedingly strong acids or bases. Where driving forces are comparable, ET is expected to be favored over EPT due to more complex reaction barriers and slower rates for the latter (2).

Electrocatalysis by deliberate introduction of PCET pathways at modified electrode surfaces has been reported (6–15). This study is important in extending solution reactivity, for example, toward water and hydrocarbon oxidation catalysis, to electrode interfaces in device configurations.

Oxidative activation of carbon electrodes introduces surface quinoidal functional groups which activate PCET couples by enabling concerted EPT pathways at the modified surface (10). For the surface-bound analog of [Ru^II(tpy)(bpy)(OH₂)]²⁺ (tpy is 2,2′:6′,2″-terpyridine) 1, [Ru^II(tpy)(4,4^′-(PO₃H₂CH₂)₂bpy)(OH₂)]²⁺ (4,4^′-(PO₃H₂CH₂)₂bpy is 4,4′-bis-methlylenephosphonato-2,2′-bipyridine) 1-PO₃H₂, on planar Sn(IV)-doped In₂O₃ (ITO) electrodes, a reversible Ru^IV = O²⁺/Ru^III-OH²⁺ wave appears but only at high surface coverages (11). This observation was attributed to the surface coverage-dependent, cross-surface disproportionation mechanism in Scheme 1. For ITO|1 - PO₃H₂, initial disproportionation on the surface occurs with ΔG^o′ = 0.09 eV. Disproportionation is presumably followed by the ET-PT sequence ; with ITO-Ru^III-OH²⁺ reentering the disproportionation cycle (11).

Scheme 1.

Cross-surface disproportionation pathway for accessing Ru^IV = O²⁺ at ITO|1-PO₃H₂ at pH 1 (0.1 M HClO₄).

PCET half reactions and EPT pathways play key roles in biological redox reactions (2, 16, 17) and, in a general way, in oxidation-reduction catalysis (2, 18–26). For example, in single site water oxidation catalysis based on polypyridyl Ru complexes such as [Ru^II(Mebimpy)(bpy)(H₂O)]²⁺ [Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog [Ru^II(Mebimpy)(4,4^′-(PO₃H₂CH₂)₂bpy)(OH₂)]²⁺ 2-PO₃H₂ (Scheme 2), oxidative activation occurs by sequential e^-/H⁺ transfer: ; Ru^III-OH²⁺ - e^- - H⁺ → Ru^IV = O²⁺ (18–21). PCET oxidation to Ru^IV = O²⁺ is followed by oxidation to Ru^V = O³⁺ (18–21). Either or both Ru^IV = O²⁺ and Ru^V = O³⁺ are active as oxidation catalysts. In catalytic cycles based on these oxidants, slow oxidation of Ru^III-OH²⁺ to Ru^IV = O²⁺ can contribute to or even dominate rate limiting behavior in electrocatalysis or in photoelectrochemical solar fuels applications.

Scheme 2.

Mechanism of water oxidation by 2-PO₃H₂ on metal oxide electrodes at pH 5 (17).

We report here the results of an investigation on the couple for surface-bound 2-PO₃H₂ on planar fluoride-doped SnO₂ (FTO), and nanostructured films of ITO (nanoITO) and TiO₂ (nanoTiO₂). The focus of the study was not on quantitation, but rather on uncovering microscopic phenomena associated with electrochemical interconversion of the two oxidation states. The goal of this research was to establish the mechanism by which surface oxidation and rereduction occur for the surface couple on metal oxide electrodes. Our results demonstrate the kinetic difficulties associated with this couple, identify multiple surface pathways by which it can occur, and assess the roles played by the electrode, temperature, surface coverage, added buffer base, pH, solvent, and solvent H₂O/D₂O isotope effect.

A Pourbaix (E_1/2 vs. pH) diagram for 2-PO₃H₂ on planar FTO electrodes is shown in SI Appendix, Fig. S1. It is slightly modified from a previous literature version (18). As a summary of the properties of the couples (22 °C), ; E_1/2(Ru^III/II-OH^2+/+) ∼ E^o′(Ru^III/II-OH^2+/+) = 0.29 V; ; ; ΔE^o′ =  ∼ 300 mV for the potential difference between Ru^IV = O²⁺/Ru^III-OH²⁺ and couples (18). In addition, there is evidence for a protonated form of Ru(IV), Ru^IV(OH)³⁺, with pK_a ∼ 3 (26).

Results and Discussion

Surface Electrochemistry.

Fig. 1 shows CVs of (A) FTO|2-PO₃H₂, (B) nanoITO|2-PO₃H₂, and (C) nanoTiO₂|2-PO₃H₂. All three were obtained at full surface coverages with Γ/Γ_o = 1 (note Table 1) at pH 5 (0.1 M HOAc/OAc^-) at 10 mV/s. The surface electrochemistry of the complex on planar ITO is similar to that on planar FTO and is not discussed here. CVs obtained at room temperature, 22 °C, are shown in blue and at 80 °C in red.

Fig. 1.

CVs of (A) FTO|2-PO₃H₂, (B) nanoITO|2-PO₃H₂, and (C) nanoTiO₂|2-PO₃H₂ with full surface coverages (Γ/Γ_o = 1, note Table 1) at pH 5 (0.1 M HOAc/OAc^-) at 22 °C (blue) and 80 °C (red). Scan rate, 10 mV/s.

Table 1.

Saturated surface coverages for 2-PO₃H₂ derivatized electrodes

Electrode	FTO|2-PO3H2	nanoITO|2-PO3H2	nanoTiO2|2-PO3H2
Geometric area (cm2)/filmThickness, µm	1.25/NA	1.25/approx 2.5	1.25/approx 10
Γo, mol/cm2	1.2 × 10-10*	1.7 × 10-8†	5.3 × 10-8†

^*Determined from the area under the CV wave for the Ru(III/II) couple at 0.68 V vs. NHE at pH 5.

^†Calculated from the film absorbance at λ_max = 494 nm with ε_max = 1.55 × 10⁴ M^-1 cm^-1 at pH 5 and the expression, Γ = A(λ)/(10³ × ε(λ)), with A(λ) and ε(λ) the absorbance and molar absorptivity at 494 nm.

Nature of the electrode.

At 22 °C, 1e^- waves appear for the couple at E_1/2 = 0.68 ± 0.01 V with ΔE_p( = E_p,a - E_p,c) = 15 mV (FTO), 40 mV (nanoITO), and 105 mV (nanoTiO₂) at a scan rate of 10 mV/s. A pH independent Ru^V = O³⁺/Ru^IV = O²⁺ wave (not shown) appears at approximately 1.65 V, as reported earlier in a study on water oxidation (18). Potential scans to this wave trigger catalytic water oxidation with only the onset shown in Fig. 1.

At FTO and nanoITO, oxidative peak currents (i_p,a) for the Ru(II → III) oxidation wave vary linearly with scan rate (υ) as expected for a surface couple (27) (SI Appendix, Figs. S2 and S3). By contrast, on nanoTiO₂, i_p,a varies with the square root of the scan rate (υ^1/2) (SI Appendix, Fig. S4). A related phenomenon was reported earlier for [Os(bpy)(4,4^′-(PO₃H₂)₂bpy)]²⁺ surface-bound to nanoTiO₂ and attributed to Os(II → III) oxidation by cross-surface, diffusional electron transfer (28). In this process, site-to-site electron transfer hopping and counter ion transport occur between surface couples providing a cross-surface electron transfer channel to the underlying electrode.

CV waveforms for Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation at this pH are highly dependent on the nature of the electrode (Fig. 1). At FTO|2-PO₃H₂ at 10 mV/s, a barely discernible wave or waves appear between 0.9 and 1.3 V (Fig. 1A). Reversal of the potential scan results in Ru(IV → III) rereduction at E_p,c =  ∼ 0.83 V. Kinetic inhibition to Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation is shown by the dependence of the current for the Ru(IV → III) rereduction wave on the time held past the Ru(III → IV) wave, either through scan rate variations (SI Appendix, Fig. S2) or by switching potential variations (SI Appendix, Fig. S5).

A dramatically different result is obtained for Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation at nanoITO|2-PO₃H₂ (Fig. 1B). At a scan rate of 10 mV/s, a nearly reversible oxidative wave is observed at E_p,a = 1.0 V but of integrated current only approximately 50% of that for the Ru(III/II) wave. For the reverse, Ru(IV → III) reduction, E_p,c = 0.92 V but with a waveform that is noticeably decreased in half-width. Integrated currents for oxidative and reductive waves are comparable.

Both the current and waveform for the Ru(III → IV) wave are strongly dependent on scan rate (SI Appendix, Fig. S3). At 2 mV/s, Ru(III → IV) oxidation is more nearly complete and integrated currents were nearly comparable for the Ru(III → IV) and Ru(II → III) waves. At higher scan rates (50 mV/s), a new, scan-rate-dependent Ru(III → IV) wave appears at E_p,a = 1.28 V at the expense of the wave at E_p,a = 1.0 V. The new wave dominates with further increases in scan rate up to 100 mV/s. In the reverse scan, the narrow waveform for Ru(IV → III) reduction is nearly independent of scan rate but E_p,c shifts negatively with increasing scan rate. The appearance of two Ru(III → IV) oxidation waves and the narrow waveform for Ru(IV → III) reduction are notable observations and will be discussed below.

As shown in Fig. 1C and SI Appendix, Fig. S4, at nanoTiO₂|2-PO₃H₂, kinetic inhibition is complete with no evidence for a wave or waves for Ru(III → IV) oxidation over a range of scan rates. This result is consistent with the slow cross-surface, site-to-site electron transfer hopping observed for the Ru(III/II) couple.

Temperature.

The three electrodes respond differently to an increase in temperature from 22 to 80 °C. At FTO|2-PO₃H₂ the overall waveform and peak current for the kinetically facile Ru(II → III) oxidation are relatively unaffected, but the peak current for the kinetically inhibited Ru(III → IV) oxidation increases from barely discernible to becoming comparable to the peak current for the Ru(II → III) wave. A similar effect was observed on nanoITO|2-PO₃H₂ with the Ru(II → III) wave relatively unaffected but the integrated current ratio for the Ru(III → IV) and Ru(II → III) waves increasing from approximately 50% at 22 °C to about 100% at 80 °C at 10 mV/s. At nanoTiO₂ the peak current for the Ru(III/II) wave at E_1/2 = 0.68 V is significantly enhanced and a kinetically inhibited wave for the Ru(IV/III) couple appears at E_1/2 =  ∼ 0.9 V. At all three electrodes, the increase in temperature from 22 to 80 °C causes an approximately -40 mV shift in E_1/2 for both couples (after accounting for the temperature dependence of the reference electrode). The negative temperature coefficients are consistent with a decrease in entropy for both couples, largely due to solvation of the released proton, e.g., (29).

Surface Loading Effects.

The data in Fig. 2A at pH 5 (0.1 M HOAc/OAc^-) at 22 °C reveal a surface loading effect on the Ru(III → IV) wave at nanoITO|2-PO₃H₂ while leaving the waveform for the Ru(III/II) couple unaffected. At low coverages, with the surface dilute in complex (green line, Γ/Γ_o = 0.1), a Ru(III → IV) oxidation wave appears at E_p,a =  ∼ 1.26 V with Ru(IV → III) rereduction occurring at E_p,c =  ∼ 0.83 V at 10 mV/s.

Fig. 2.

CVs of nanoITO|2-PO₃H₂ with (A) Γ/Γ_o = 0.03 (red), 0.1 (green), 0.25 (blue), 0.4 (cyan), 0.6 (magenta), 0.85 (yellow), 1 (dark yellow) at pH 5 (0.1 M HOAc/OAc^-), and (B) Γ/Γ_o = 0.1 (red), 0.4 (green), 1 (blue) at pH 1 (0.1 M HNO₃). Scan rate, 10 mV/s; temperature, 22 °C.

As surface loading is increased to Γ/Γ_o = 0.25 (blue line), the wave for Ru(III → IV) oxidation at E_p,a =  ∼ 1.0 V, observed on the fully loaded surface with Γ/Γ_o = 1 in Fig. 1, also appears but at the expense of the wave at E_p,a =  ∼ 1.26 V. The reverse Ru(IV → III) wave remains narrow in half-width with E_p,c shifting to more positive potentials as surface loading is increased. The switching potential result in SI Appendix, Fig. S6 shows that both oxidations result in the same wave for Ru(IV → III) rereduction at E_p,c = 0.92 V at 10 mV/s. On a fully covered surface, Γ/Γ_o = 1 (dark-yellow line), only the Ru(III → IV) wave at E_p,a = 1.0 V is observed.

Analysis of these results points to two pathways for Ru(III → IV) oxidation:

1. At low surface coverages and high scan rates, above the pK_a for , direct oxidation of Ru^III-OH²⁺ to Ru^IV(OH)³⁺ occurs at E_p,a ∼ 1.26 V by the ET-PT mechanism in [1]. There is no corresponding Ru^IV(OH)³⁺/Ru^III-OH²⁺ rereduction wave at this pH because once Ru^IV(OH)³⁺ is formed, it deprotonates rapidly ([1b]). As noted above, pK_a ∼ 3 for the equilibrium, Ru^IV(OH)³⁺↔Ru^IV = O²⁺ + H⁺. There is no additional characterization data for Ru^IV(OH)³⁺ and its structure is uncertain.

   [1a]

   [1b]

   This conclusion is reinforced by the surface coverage-independent results at pH 1 (0.1 M HNO₃) in Fig. 2B. Under these conditions, a kinetically distorted but chemically reversible Ru(IV/III) couple appears at E_1/2 = 1.27 V at 10 mV/s with the Ru(III/II) couple appearing at E_1/2 = 0.82 V. For the Ru(IV/III) wave, as for the Ru(III/II) wave, the current increases linearly with surface coverage with the waveform and peak potential nearly independent of surface coverage.

   From the E_1/2-pH diagram in SI Appendix, Fig. S1, for surface-bound 2-PO₃H₂. At pH 1, the Ru(III/II) couple is , and the Ru(IV/III) couple is .

   The electrode mechanism for Ru(III → IV) oxidation at pH 1 (0.1 M HNO₃) is presumably PT-ET as shown in [2]. In this mechanism, initial deprotonation of is followed by electron transfer oxidation of Ru^III-OH²⁺ to Ru^IV(OH)³⁺. The couple is kinetically inhibited as seen by the increase in ΔE_p( = E_p,a - E_p,c) = 140 mV as compared to 25 mV for the wave at 10 mV/s, perhaps indicative of a more significant structural change between Ru^IV(OH)³⁺ and Ru^III-OH²⁺ than simple proton transfer.

   [2a]

   [2b]
2. Facilitation of Ru(III → IV) oxidation at high surface coverages points to higher order involvement of surface-bound sites. A reasonable origin is oxidation of Ru^III-OH²⁺ to Ru^IV = O²⁺ by a disproportionation mechanism, [3], analogous to the mechanism shown in Scheme 1 (11). High loadings and close contact across the surface are required for the proton transfer part of EPT in [3a]. It is short range in nature due to the requirement for vibrational wave function overlap in order for proton tunneling to occur (2,30). Although there is no experimental evidence, the mediation of proton transfer by one or more water molecules may be involved.

   [3a]

   [3b]

   [3c]

   For the disproportionation step in [3a], ΔG^o′ = 0.3 eV for Ru^III-OH²⁺ on nanoITO|2-PO₃H₂ at pH 5 compared to 0.09 eV for Ru^III-OH²⁺ of 1-PO₃H₂. This increase contributes to the reaction barrier and decreased rate. The impact of slow kinetics is observed in the appearance of both direct oxidation and disproportionation pathways on fully loaded surfaces at relatively rapid scan rates (SI Appendix, Fig. S3). At these scan rates, oxidation by disproportionation at E_p,a = 1.0 V is incomplete at the thermodynamic potential for the couple. Sites that remain unoxidized during the scan undergo oxidation by direct electron transfer at the potential for the Ru^IV(OH)³⁺/Ru^III-OH²⁺ couple followed by proton loss to give Ru^IV = O²⁺, [1].

   Utilization of both pathways on less than fully loaded surfaces explains the simultaneous appearance of disproportionation and direct oxidation waves in Fig. 2A. Given the anticipated sensitivity to separation distance for disproportionation and the requirement to minimize proton tunneling distances on partly loaded surfaces, there is presumably a distribution of sites in appropriate orientations for disproportionation to occur. Oxidation of separated sites and sites with inappropriate orientations toward neighbors are limited to the direct oxidation pathway.

   At high surface coverages on FTO|2-PO₃H₂ at pH 5 (0.1 M HOAc/OAc^-), there is also (barely discernible) evidence for Ru(III → IV) oxidation waves at E_p,a ∼ 1.01 and 1.24 V at 10 mV/s (SI Appendix, Fig. S7A). This result points to contributions to Ru(III → IV) oxidation from both direct oxidation and cross-surface disproportionation oxidation pathways, [1] and [3], on planar FTO as well. However, the effect is far more prominent in the three-dimensional nanoITO structure, suggesting a role for the three-dimensional internal cavity structure in the latter. At pH 1 (0.1 M HNO₃), as on nanoITO|2-PO₃H₂, kinetic inhibition of the couple was also observed with an ill-defined Ru(III → IV) wave appearing at E_p,a =  ∼ 1.37 V and Ru(IV → III) rereduction at E_p,c = 1.15 V (SI Appendix, Fig. S7B).

Base Effect.

Significant rate accelerations for Ru(III → IV) oxidation are also observed with OAc^- added to the external solution as a proton acceptor base. As discussed below, this observation points to a third pathway for Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation, base-assisted, concerted EPT as reported by Savéant and coworkers in the oxidation of cis-[Os^III(bpy)₂(py)(OH)]²⁺ to cis-[Os^IV(bpy)₂(py)(O)]²⁺ (7). Illustrative data are shown in Fig. 3A for nanoITO|2-PO₃H₂ at a surface coverage of Γ/Γ_o = 0.35. The base effect is present at all surface coverages but is more pronounced at low surface coverages. In these experiments, the total concentration of buffer was increased with pH held constant at pH 5 at the buffer ratio, [OAc^-]/[HOAc] = 0.56. Addition of LiCF₃SO₃ to maintain the ionic strength gave equivalent results.

Fig. 3.

(A) CVs of nanoITO|2-PO₃H₂ (Γ/Γ_o = 0.35) at pH 5 with different concentrations of HOAc/NaOAc. (B) CVs of nanoITO|2-PO₃H₂ (Γ/Γ_o = 0.40) at pH 5 (0.1 M HOAc/OAc^-) with addition of increasing amounts of KNO₃. Scan rate, 10 mV/s; temperature, 22 °C.

The Ru(III/II) couple is largely unaffected by added buffer in this concentration range. However, increasing the concentration of OAc^- results in an increase in i_p,a for the Ru^III-OH²⁺ → Ru^IV = O²⁺ wave at E_p,a = 1.0 V at the expense of i_p,a for the Ru^III-OH²⁺ → Ru^IV(OH)³⁺ wave at E_p,a = 1.26 V. The peak current for Ru^IV = O²⁺ → Ru^III-OH²⁺ rereduction is unaffected by added buffer, but E_p,c shifts from 0.87 to 0.92 V as [HOAc/OAc^-] was increased from 0.1 to 1.0 M.

As shown in [4], HOAc/OAc^- may play a role in catalysis of the surface-bound Ru(IV/III) couple with preassociation of OAc^- to Ru^III-OH²⁺ occurring by a hydrogen bond/ion-pair interaction prior to oxidation (12). The wave for Ru(IV → III) rereduction remains skewed with a narrow half-width. The shift in E_p,c with added HOAc/OAc^- at fixed scan rate is consistent with a role for prior association between Ru^IV = O²⁺ and HOAc prior to reduction and the Ru(IV/III) couple, {Ru^IV = O²⁺⋯HOAc}²⁺/{Ru^III-OH²⁺⋯OAc^-}⁺ at E_1/2 = 0.96 V at pH 5 at 10 mV/s.

[4a]

[4b]

[4c]

The importance of preliminary ion pairing is shown by the influence of added KNO₃ (Fig. 3B). In these experiments, CVs of nanoITO|2-PO₃H₂ (Γ/Γ_o = 0.40) at pH 5 (0.1 M HOAc/OAc^-) with increasing amounts of added KNO₃ were obtained at 10 mV/s. Under these conditions, the ratio of peak currents for the two oxidative waves, i_p,a(1.0 V)/i_p,a(1.26 V), decreases as is increased from 0 to 1 M. The implied conversion in surface mechanism from EPT with OAc^- as acceptor base to direct oxidation is consistent with a generalized ion atmosphere effect and/or replacement of OAc^- by at the ion paired surface cation (7). E_p,c for Ru(IV → III) rereduction also shifts negatively with increasing because of competitive ion pairing with .

H₂O/D₂O Kinetic Isotope Effects.

At pH 5, notable H₂O/D₂O isotope effects appear for the Ru(III → IV) wave on nanoITO|2-PO₃H₂ (Γ/Γ_o = 1), whereas the Ru(II → III) wave is essentially unaffected (Fig. 4). In D₂O, i_p,a for Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation at E_p,a = 1.0 V is greatly decreased, whereas i_p,a for direct Ru^III-OH²⁺ → Ru^IV(OH)³⁺ oxidation at E_p,a = 1.26 V is increased.

Fig. 4.

(A) CVs of nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) at pH 5 (0.1 M HOAc/OAc^-) in H₂O (blue), 1∶1 H₂O/D₂O (green), and D₂O (red). (B) Dependence of i_p,a(X_D2O)/i_p,a(H₂O, X_D2O = 0) at 1.0 V (background subtracted) on X_D2O. Scan rate, 10 mV/s; temperature, 22 °C.

The isotopic discrimination between pathways is a reflection of surface mechanism. As shown by measurements at pH 1 in SI Appendix, Fig. S8, there is essentially no KIE for direct Ru^III-OH²⁺ → Ru^IV(OH)³⁺ oxidation with i_p,a(H₂O)/i_p,a(D₂O) =  ∼ 1 independent of surface coverage. By contrast, at pH 5 for the wave at E_p,a = 1.0 V in Fig. 4A, i_p,a(H₂O)/i_p,a(D₂O) =  ∼ 2.4 comparable to the KIE observed for oxidation of cis-[Os^III(bpy)₂(py)(OH)]²⁺ with added buffer bases (7).

At pH 5, there are two mechanistic contributors to the wave for Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation at E_p,a = 1.0 V. Cross-surface disproportionation, [3], is dominant with an additional contribution from the EPT pathway in [4]. Participation by two pathways appears in the nonlinearity of the plot of i_p,a vs. mole fraction of D₂O (X_D2O) (Fig. 4B). A linear relationship is predicted for a single pathway with a single proton transferred (2). At a lower surface coverage, Γ/Γ_o = 0.60, where EPT should play a greater role, KIE =  ∼ 3.6.

As noted above, at pH 1, electron transfer dominates with KIE =  ∼ 1. There is a slight but noticeable shift in E_1/2 for this wave from 1.27 V at X_D2O = 0 to 1.33 V at X_D2O = 1 (SI Appendix, Fig. S8). Given the proton content change for the couple at this pH, the shift in E_1/2, ΔE_1/2 is dominated by the proton equilibrium in [2a]. In this limit, . With (SI Appendix, Fig. S1), ) is estimated to be 1.3.

Rereduction of Ru^IV = O²⁺.

For nanoITO|2-PO₃H₂ or FTO|2-PO₃H₂ in acidic solution, a kinetically inhibited, but normal wave shape is observed for rereduction at E_1/2 = 1.27 V. Under these conditions, the mechanism for rereduction is ET-PT, the reverse of [2].

At higher pHs, with pH > pK_a(Ru^IV(OH)³⁺), the equilibrium concentration of Ru^IV(OH)³⁺ is reduced and rereduction by ET-PT, the reverse of [2], becomes insignificant. Under these conditions, Ru(IV → III) rereduction occurs by a skewed, narrow wave at lower potentials. Its E_p,c value is dependent on surface loading, scan rate, and base concentration.

Rereduction of Ru^IV = O²⁺ is constrained to occur by ET-PT with reduction to Ru^III-O⁺ followed by rapid protonation of Ru^III-O⁺, [5a] and [5b]. The thermodynamic potential for the Ru^IV = O²⁺/Ru^III-O⁺ couple can be estimated from Eq. 6 and the difference in pK_a values between Ru^IV(OH)³⁺ and Ru^III-OH²⁺. Given that, E^o′(Ru^IV(OH)³⁺/Ru^III-OH²⁺) = 1.27 V, and assuming pK_a(Ru^IV(OH)³⁺) ≈ 3 and pK_a(Ru^III-OH²⁺) > 14, E^o′(Ru^IV = O²⁺/Ru^III-O⁺) is estimated to be < 0.62 V. Based on this estimate for E^o′(Ru^IV = O²⁺/Ru^III-O⁺) and the data in Fig. 2A, reduction at E_p,c =  ∼ 0.83 V at Γ/Γ_o = 0.1 (green line) at 10 mV/s occurs with an underpotential of > 0.21 V.

[5a]

[5b]

[5c]

[5d]

[6]

From the surface loading dependence in Fig. 2A, there also appears to be an autocatalytic effect triggered by partial reduction arising from cross-surface comproportionation, [5c] and [5d]. With Ru^III-OH²⁺ formed by ET-PT reduction of Ru^IV = O²⁺, [5a] and [5b], a basis for autocatalysis exists by further reduction to and cross-surface comproportionation with the remaining Ru^IV = O²⁺ sites on the surface, [5c] and [5d]. As shown by the CV simulations in SI Appendix, Fig. S9 and Table S1, an analogous mechanism for the solution couples of 2 at pH 5 reproduces the narrow wave shape observed for the surface couple.

Propylene Carbonate as Solvent.

In propylene carbonate (PC) with added water (PC∶H₂O, water miscibility < 8%, vol∶vol), both 2 and its surface analog, nanoITO|2-PO₃H₂, are known water oxidation catalysts by the mechanism in Scheme 2 (31). There is evidence for continued coordination of H₂O based on the UV-visible spectra of 2 in solution (31) and for 2-PO₃H₂ on nanoITO (see below).

There is also evidence for surface PCET effects based on the surface-bound phosphonate groups with pK_a ∼ 1–2 for proton loss in water (12, 32). The procedure for surface attachment of 2-PO₃H₂, described in Materials and Methods (“as prepared”), involved soaking the complex for extended periods at pH 5 (0.1 M HOAc/OAc^-) followed by rinsing with distilled water, conditions under which deprotonation of the phosphonates occurs to give nanoITO-[{(-O)(O)₂PCH₂}₂bpy)Ru^II(Mebimpy)(OH₂)]⁰ on the surface, [7] (12, 32). In aqueous solutions, proton equilibration between the surface-bound phosphonate groups and the external solution occurs rapidly. In dry PC, the surface proton composition is fixed and, as shown below, impacts CVs for both Ru(IV/III) and Ru(III/II) couples:

[7]

A CV for an as-prepared nanoITO|2-PO₃H₂ slide is shown in Fig. 5A in 0.1 M LiClO₄/PC at 10 mV/s. In the first scan, a wave for a Ru(III/II) couple appears, but at E_1/2 = 0.78 V (ΔE_p = 90 mV) with a relatively small, irreversible Ru(III → IV) wave at E_p,a = 1.33 V. The potential difference between Ru(II → III) and Ru(III → IV) waves is ΔE_p,a ∼ 500 mV, which is comparable to ΔE_p,a ∼ 400 mV between and Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidations at pH 5, suggesting a common origin.

Fig. 5.

(A) Successive CVs (three cycles) of as-prepared nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) in 0.1 M LiClO₄/PC. The arrows indicate the current variations upon successive potential scans. Red line, potential scan reversal before E_p,a = 1.33 V. (B) Successive CVs (five cycles) of acid-treated nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) in 0.1 M LiClO₄/PC. Scan rate, 10 mV/s; temperature, 22 °C.

Following successive scans through the Ru^III-OH²⁺ → Ru^IV = O²⁺ wave accompanied by proton release, a new Ru(III/II) couple appears at E_1/2 = 1.06 V (ΔE_p = 40 mV). This wave arises from the couple (see below), with its E_1/2 value increased by 240 mV compared to E_1/2 = 0.82 V for the same couple in 0.1 M HNO₃ (Fig. 2B). Appearance of the couple is accompanied by decreases in peak currents for the couple at E_1/2 = 0.78 V and the Ru^III-OH²⁺ → Ru^IV = O²⁺ wave at E_p,a = 1.33 V.

A CV for an “acid treated” nanoITO|2-PO₃H₂ slide is shown in Fig. 5B. In the acid treatment, an as-prepared slide was soaked in a 0.1 M HClO₄ aqueous solution for 30 s followed by drying in a N₂ gas stream. In the resulting CV in 0.1 M LiClO₄/PC at 10 mV/s, a wave for the couple appears at E_1/2 = 1.06 V (ΔE_p = 40 mV) with no evidence for further oxidation to Ru^IV = O²⁺ through a series of five successive scans. Surface oxidation of to rather than Ru^III-OH²⁺ is the expected result in a nonaqueous solvent with a significant decrease in proton acidity for due to loss of hydration free energy for the proton (33).

An explanation is available for the difference in behaviors between as-prepared and acid-treated slides and the appearance of the proton-dependent and Ru^III-OH²⁺ → Ru^IV = O²⁺ waves based on PCET involvement by basic sites on the surface or, more likely, the surface-bound, phosphonate groups with pK_a ∼ 1–2. The deprotonated phosphonate groups on as-prepared slides presumably act as proton acceptors for oxidation with proton transfer to a surface phosphonate group (step 1 in Scheme 3). The couple is kinetically inhibited, presumably because of the requirements imposed on proton transfer by orientation and proton transfer distance. Kinetic inhibition is seen in the increase in ΔE_p from 40 mV for the same couple at pH 5 (0.1 M HOAc/OAc^-) in water compared to 90 mV in PC and by the scan rate dependence of the couple (SI Appendix, Fig. S10). At fast scan rates, oxidation is incomplete at the thermodynamic potential for the couple. Sites that remain unoxidized at E_p,a = 0.83 V undergo oxidation at E_p,a = 1.08 V.

Scheme 3.

Proposed interfacial proton transfer to surface phosphonate groups in the oxidation of to Ru^IV = O²⁺ in PC as solvent.

Given the proton stoichiometry in [7], an additional deprotonated phosphonate site is available at as-prepared slides for further oxidation and proton loss, enabling access to Ru^IV = O²⁺ (step 2 in Scheme 3), which would explain the appearance of the wave at E_p,a = 1.33 V as due to Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation. Oxidation occurs only through Ru^III-OH²⁺ and not through as shown by the absence of a Ru(III → IV) wave for the acid-treated slide in Fig. 5B. Surface-bound phosphonates may also play a role in the aqueous solution electrochemistry of the surface-bound Ru(IV/III) couple but with no evidence for it in our data.

The wave appears for as-prepared slides only on successive oxidative scans (Fig. 5A). It appears to be triggered by Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation and proton release. Presumably, partial protonation of surface phosphonate sites inhibits oxidation with unoxidized sites undergoing oxidation at E_p,a = 1.08 V.

The phenomena reported in Fig. 5 are independent of surface loading with closely related observations made at Γ/Γ_o = 0.2 (SI Appendix, Fig. S11). There is no evidence for cross-surface disproportionation in PC as solvent, presumably because the PCET-phosphonate pathways in Scheme 3 are kinetically more facile and dominate electron transfer reactivity of the surface Ru(IV/III) couple.

As shown by the switching potential variations in Fig. 5A, in PC, in the absence of a ready supply of protons, rereduction of Ru^IV = O²⁺ to Ru^III-O⁺ occurs at E_p,c = 0.34 V at 10 mV/s. This value is consistent with E^o′ < 0.62 V estimated for the Ru^IV = O²⁺/Ru^III-O⁺ couple in water by use of Eq. 6. As shown in SI Appendix, Fig. S12 and Table S2, CV simulations of 2 in PC based on a mechanism involving Ru^IV = O²⁺ → Ru^III-O⁺ reduction reproduces the Ru^IV = O²⁺ → Ru^III-O⁺ waveform.

Both potential and peak current for Ru^IV = O²⁺ → Ru^III-O⁺ rereduction are scan-rate dependent (SI Appendix, Fig. S10). As the scan rate is decreased, E_p,c shifts positively and the current ratio i_p,c(0.34 V)/i_p,a(1.33 V) decreases. These observations are qualitatively consistent with protonation of Ru^III-O⁺ by trace water or protonated surface phosphonate following reduction of Ru^IV = O²⁺ at slow scan rates.

The influence of added water on CVs of nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) in 0.1 M LiClO₄/PC is illustrated in Fig. 6. Increasing water from 0% to 6% (vol∶vol) causes a decrease in E_1/2 for the couple from 0.78 to 0.73 V, presumably due to a generalized solvation effect. The peak currents for the couple also grow at the expense of i_p for the couple, presumably due to the PCET couple being favored by enhanced solvation of the released proton.

Fig. 6.

CVs of nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) in 0.1 M LiClO₄/PC with addition of 0% (blue), 1% (green), and 6% (red) water. Scan rate, 10 mV/s; temperature, 22 °C.

Solvation also affects Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation with E_p,a = 1.33 V (0% H₂O) shifting to 1.28 V (6% H₂O). The influence of water is more profound on the Ru^IV = O²⁺ → Ru^III-O⁺ reduction wave because of rapid protonation of Ru^III-O⁺ once it is formed with E_p,c shifting to 0.46 V with 1% added water. At 6% added water, this wave is shifted more positively and masked by the reduction wave.

Comparisons with nanoITO|1-PO₃H₂.

CV measurements were extended to the Ru(IV/III) couple of 1-PO₃H₂ on nanoITO. As noted in the Introduction, ΔE^o′ = 0.09 V between the Ru(IV/III) and Ru(III/II) couples for 1-PO₃H₂ from pH 1 to pH 10. The form of 1-PO₃H₂ is more acidic than 2-PO₃H₂ with (11).

In Fig. 7 are shown CVs of nanoITO|1-PO₃H₂ at three different surface loadings in three different media. For the surfaces with Γ/Γ_o ≤ 0.4 in 1 M HClO₄ (red and green lines in Fig. 7A), a chemically reversible but kinetically distorted wave appears for the Ru(IV/III) couple at E_1/2 = 1.28 V. This wave appears at the expected E_1/2 value for the couple for 1-PO₃H₂ at this pH. On these dilute surfaces, a reasonable mechanism for oxidation is PT-ET, as proposed in [2] for 2-PO₃H₂. rereduction at 10 mV/s occurs at E_p,c = 1.17 V with a relatively normal wave shape, but the couple is kinetically inhibited with ΔE_p = 230 mV.

Fig. 7.

CVs of nanoITO|1-PO₃H₂ with (A) Γ/Γ_o = 0.1 (red), 0.4 (green), 1 (blue) in 1 M HClO₄, (B) Γ/Γ_o = 0.25 (red), 0.6 (green), 1 (blue) at pH 1 (0.1 M HClO₄), and (C) Γ/Γ_o = 0.1 (red), 0.6 (green), 1 (blue) at pH 5 (0.1 M HOAc/OAc^-). Scan rate, 10 mV/s; temperature, 22 °C.

On a completely loaded surface (Γ/Γ_o = 1) (blue line in Fig. 7A), there is evidence for dual pathways based on the appearance of a new wave at E_p,a =  ∼ 1.30 V at 10 mV/s. The surface loading dependence is consistent with the disproportionation mechanism in [3] and the appearance of dual pathways to a competition between disproportionation in [3] and direct oxidation in [2]. On the reverse scan, a narrow rereduction wave appears at E_p,a = 1.23 V (blue line in Fig. 7A). The wave shape is similar to that for rereduction of Ru^IV = O²⁺ for 2-PO₃H₂ at pH 5 (0.1 M HOAc/OAc^-) and the autocatalytic mechanism in [5].

At pH 1 (0.1 M HClO₄) at 10 mV/s, there is clearer evidence for surface loading-dependent, dual pathways for Ru(III → IV) oxidation with waves appearing at E_p,a = 1.36 and 1.19 V (Fig. 7B and SI Appendix, Fig. S13). For the wave at E_p,a = 1.19, i_p,a increases as surface coverage is increased, consistent with disproportionation in competition with direct oxidation. The disproportionation pathway dominates at Γ/Γ_o = 1. A single, narrow, surface loading-dependent rereduction wave appears with E_p,c = 1.16 V at Γ/Γ_o = 1, consistent with autocatalytic comproportionation, [5].

Related phenomena were observed at pH 5 (0.1 M HOAc/OAc^-) at 10 mV/s. A wave for direct Ru(III → IV) oxidation appears at E_p,a = 1.33 V (Γ/Γ_o ≤ 0.1) (the CV in red in Fig. 7C with a magnified view in SI Appendix, Fig. S14). At higher surface coverages, disproportionation dominates with Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation occurring at E_p,a = 0.98 V. Reverse, Ru(IV → III) rereduction also occurs by a single, narrow, surface loading-dependent wave with E_p,c = 0.90 V at Γ/Γ_o = 1. As noted above, disproportionation is expected to play a more important role than at nanoITO|2-PO₃H₂ because of the small driving force (ΔG = +0.09 eV) for Ru^III-OH²⁺ disproportionation.

As shown in SI Appendix, Fig. S15, CVs of nanoITO|2-PO₃H₂ in 0.1 M LiClO₄/PC have the same features as described for nanoITO|1-PO₃H₂, consistent with participation by the surface-bound phosphonate groups in PCET half reactions.

Real Time Spectrophotometric Study.

The use of optically transparent, conductive, high surface area nanoITO allows for the acquisition of UV-visible spectral data of electrochemically generated intermediates and for direct spectral monitoring of voltammograms (34, 35). Fig. 8 shows spectra acquired for nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) at a series of potentials during CV scans (see Fig. 2). CV scans at fixed wavelengths are shown in SI Appendix, Fig. S16.

Fig. 8.

UV-visible spectra of nanoITO|1-PO₃H₂ (Γ/Γ_o = 1) scanned to different potentials as indicated (F, forward scan; see Fig. 2). Solution, (A) pH 1 (0.1 M HNO₃), (B) pH 5 (0.1 M HOAc/OAc^-). Scan rate, 10 mV/s; temperature, 22 °C.

In 0.1 M HNO₃ (Fig. 8A), potential scans to 1.08 V, past E_1/2 = 0.82 V for Ru(II → III) oxidation, results in loss of the λ_max = 494 nm metal-to-ligand charge transfer (MLCT) absorption, a characteristic feature for surface-bound . A ligand-to-metal charge transfer (LMCT) absorption for appears at λ_max =  ∼ 650 nm with an increase in absorbance at 400 nm. A further increase in potential to 1.4 V, past E_1/2 = 1.27 V for Ru(III → IV) oxidation, results in a broad absorption at approximately 500 nm for Ru^IV(OH)³⁺ and the appearance of a shoulder at approximately 400 nm, which appear concomitantly with the disappearance of the absorption of . These spectral changes are consistent with those obtained for stepwise oxidation of [Ru^II(Mebimpy)(bpy)(OH₂)]²⁺ to and then to Ru^IV(OH)³⁺ by incremental addition of Ce(IV) in 0.1 M HNO₃ (20, 21). The spectra and spectral changes are reversible through multiple scans (SI Appendix, Fig. S16A).

In 0.1 M HOAc/OAc^- at pH 5 (Fig. 8B), scans past E_1/2 = 0.68 V for the Ru(III/II) couple result in loss of the absorption at λ_max = 494 nm, consistent with oxidation to Ru^III-OH²⁺. The latter is relatively featureless in the visible but increases in absorbance at approximately 400 nm. A further increase in potential to 1.4 V, past E_1/2 for the Ru(IV/III) couple, results in growth of the approximately 500 nm absorbance and approximately 400 nm shoulder for Ru^IV = O²⁺. Reversal of the potential scan from 1.4 to 0.1 V results in complete recovery of the original spectrum (SI Appendix, Fig. S16B).

Fig. 9 shows spectra at nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) obtained at fixed potentials during CV scans (see Fig. 5) in 0.1 M LiClO₄/PC. CV scans monitored at fixed wavelengths are shown in SI Appendix, Fig. S17. The spectral profiles obtained for and in PC are consistent with profiles in water but with λ_max blue shifted by approximately 2 nm because of a generalized solvent effect.

Fig. 9.

UV-visible spectra of (A) acid-treated and (B) as-prepared nanoITO|2-PO₃H₂ (Γ/Γ_o = 1) in 0.1 M LiClO₄/PC scanned to different potentials as indicated (F, forward scan; see Fig. 5). Scan rate, 10 mV/s; temperature, 22 °C.

At the acid-treated slide in Fig. 9A, with surface potential scanned to 1.3 V, past E_1/2 = 1.06 V for the Ru(III/II) couple in PC, the MLCT absorption for at λ_max = 492 nm decreases with an increase in LMCT absorption for at λ_max =  ∼ 648 nm, consistent with oxidation from to . There were no further spectral changes upon increasing the potential to 1.5 V, consistent with the inaccessibility of Ru^IV = O²⁺ on the acid-treated slide. Reversal of the potential scan from 1.5 to 0.1 V results in complete recovery of the original spectrum (SI Appendix, Fig. S17A).

The spectral evolution of an as-prepared slide in PC as solvent is complicated by participation by both the and couples and by Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation (Fig. 9B). With the surface potential scanned to 1.0 V, past E_1/2 =  ∼ 0.78 V for the couple, the MLCT absorption at λ_max = 492 nm decreases partially with no evidence for at λ_max =  ∼ 648 nm, consistent with oxidation to Ru^III-OH²⁺. Increasing the potential to 1.2 V, past E_1/2 =  ∼ 1.06 V for the couple, results in further loss of the MLCT absorption band at λ_max = 492 nm concomitant with appearance of the LMCT band at λ_max =  ∼ 648 nm for , consistent with oxidation of the remaining sites to . A further increase in potential to 1.5 V, past E_p,a =  ∼ 1.33 V for Ru^III-OH²⁺ → Ru^IV = O²⁺ oxidation results in a slight growth at λ =  ∼ 400–500 nm for Ru^IV = O²⁺ as observed in aqueous solution. In the additional oxidation to Ru^IV = O²⁺, the LMCT absorption at λ_max = 648 nm for is nearly unaffected, consistent with oxidation from Ru^III-OH²⁺ to Ru^IV = O²⁺, but not from . This result is consistent with the CV result described in the previous section. The fixed wavelength spectral evolution shown in SI Appendix, Fig. S17B reveals a series of spectral changes for the MLCT absorption band at λ_max = 492 nm, consistent with the sequence of electrochemical/chemical events suggested in the figure.

Conclusions

The overall water oxidation cycle in Scheme 2 consists of a linear sequence of reactions. In order to maximize rates, it is essential that activation barriers for the key rate limiting step or steps be minimized. For water oxidation in Scheme 2, a potential kinetic bottleneck appears in the oxidative activation sequence from to Ru^V = O³⁺ in the , Ru^III-OH²⁺ → Ru^IV(OH)³⁺, Ru^IV = O²⁺ stage. Its origin is PCET and the significant change in pK_a that exists between Ru^III-OH²⁺ and Ru^IV(OH)³⁺. In an overall catalytic scheme, slow rates for this step could compete with the O⋯O bond-forming step between Ru^V = O³⁺ and H₂O and limit catalytic rates and efficiencies. The results presented here highlight the origin of kinetic inhibitions and provide insights into how to overcome them.

1. Oxidative activation of Ru^III-OH²⁺ (in proton equilibrium with ) to Ru^IV(OH)³⁺ can occur by PT-ET. A pH-dependent overpotential exists for this pathway. It arises from the difference in E_1/2 values between the Ru^IV(OH)³⁺/Ru^III-OH²⁺ and Ru^IV = O²⁺/Ru^III-OH²⁺ couples.

2. On highly loaded surfaces, cross-surface disproportionation of Ru^III-OH²⁺ occurs to give Ru^IV = O²⁺. Disproportionation is followed by oxidation of on the surface and proton equilibration by . The kinetic facility of this pathway is dictated, in part, by the difference in E_1/2 values between the Ru^IV = O²⁺/Ru^III-OH²⁺ and couples.

3. Concerted EPT with added acid-base buffer pairs like HOAc/OAc^- can facilitate Ru^III-OH²⁺↔Ru^IV = O²⁺ interconversion by avoiding Ru^III-O⁺ and Ru^IV(OH)³⁺ as high-energy intermediates.

4. As shown by both CV and spectroelectrochemical results, in propylene carbonate, access to Ru^IV = O²⁺ depends on the initial protonation state of the phosphonate groups at the electrode surface. With these sites protonated, oxidation stops at and there is no access to Ru^IV = O²⁺. With deprotonated phosphonates at the surface, partial oxidation of to Ru^III-OH²⁺ occurs with proton transfer to a phosphonate. Oxidation to Ru^III-OH²⁺ is followed by further oxidation to Ru^IV = O²⁺ and proton transfer to a phosphonate. For a second fraction of surface sites, proton transfer is inhibited on the CV timescale and oxidation gives with no further oxidation to Ru^IV = O²⁺.

Ru^IV = O²⁺ rereduction presents related mechanistic challenges:

1. Reduction of Ru^IV(OH)³⁺ to can occur by ET-PT with Ru^IV(OH)³⁺ reduction to Ru^III-OH²⁺ followed by proton equilibration to . In water, this pathway only appears in relatively strongly acidic solutions where there is a kinetically significant concentration of Ru^IV(OH)³⁺.

2. Underpotential reduction of Ru^IV = O²⁺ occurs without initial protonation by narrow, kinetically skewed waves. At high surface coverages, autocatalytic reduction of Ru^IV = O²⁺ occurs by partial reduction to Ru^III-O⁺ followed by rapid protonation, further reduction to , and cross-surface comproportionation.

3. In PC with no added water, direct Ru^IV = O²⁺ → Ru^III-O⁺ reduction occurs at E_p,c = 0.34 V at 10 mV/s by a wave that shifts to positive potentials with added water.

Materials and Methods

Materials.

Nitric acid (70%, redistilled, trace metal grade), acetic acid (99.9%), sodium acetate (>  = 99%), deuterium oxide (D₂O, 99.9%), RuCl₃•3H₂O, and 2,2′-bipyridine (> 99%) were purchased from Sigma-Aldrich and used as received. Syntheses of salts of complex 2 and its phosphonated analog 2-PO₃H₂ were reported elsewhere (36). Complex 1 and its phosphonated analog 1-PO₃H₂ were prepared by similar methods but with [Ru(tpy)]Cl₃ as the precursor. All solutions were prepared with Milli-Q ultrapure water (> 18 MΩ).

FTO glass (Rs = 7–8 Ω) was obtained from Hartford Glass Company, Inc., and ITO glass (Rs = 4–8 Ω) was purchased from Delta Technologies. NanoITO powder (40-nm diameter) was obtained from Lihochem. Optically transparent, electrically conductive, high surface area nanoITO films were prepared as described previously (34, 35). The resulting films are light blue and approximately 2.5 µm in thickness with a resistance of Rs =  ∼ 200 Ω across 1 cm of the film by a two-point probe measurement on a borosilicate glass substrate. TiO₂ colloids (10- to 20-nm diameter) and semiconductive TiO₂-coated FTO slides (10 μm in film thickness) were prepared according to literature procedures (37).

Instrumentation.

UV-visible spectra were recorded on an Agilent Technologies Model 8453 diode-array spectrophotometer. Electrochemical measurements were performed with the model CHI660D electrochemical workstation (CH Instruments). The three-electrode system consisted of a glass slide (area 1.25 cm²) working electrode, a Pt mesh counter electrode, and an SCE reference electrode (approximately 0.244 V vs. NHE at 22 °C and approximately 0.20 V vs. NHE at 80 °C). (Temperature coefficient (∂E/∂T) = -0.76 mV/K was applied for correction.)

Procedures.

Stable phosphonate surface binding of 2-PO₃H₂ on planar FTO to give FTO|2-PO₃H₂, on nanoITO films to give nanoITO|2-PO₃H₂ or on nanoTiO₂ films to give nanoTiO₂|2-PO₃H₂ occurred following immersion of the slides in solutions 0.2 mM in phosphonated complex at pH 5 (0.1 M , HOAc/OAc^-). The slides were thoroughly rinsed with distilled water to remove physically adsorbed complex, followed by drying under a N₂ gas stream. The extent of surface loading was varied by varying the soaking time with complete coverage after 2 h for FTO and 4 h for nanoITO and nanoTiO₂. Table 1 lists saturated surface coverages, Γ_o in mole per square centimeter, for 2-PO₃H₂ derivatized FTO, nanoITO, and nanoTiO₂. Surface binding by 1-PO₃H₂ utilized the same procedure.