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 ½Ru II ðMebimpyÞðbpyÞðH 2 OÞ 2þ [where bpy is 2,2′-bipyridine, and Mebimpy is 2,6-bis(1-methylbenzimidazol-2-yl)pyridine] 2, or its surface-bound analog ½Ru II ðMebimpyÞ ð4,4 0 -bis-methlylenephosphonato-2,2 0 -bipyridineÞðOH 2 Þ 2þ 2-PO 3 H 2 , accessing the reactive states, Ru V ¼O 3þ ∕Ru IV ¼O 2þ , 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 -OH 2þ or Ru III -OH 2 3þ forms of 2-PO 3 H 2 to Ru IV ¼O 2þ on planar fluoride-doped SnO 2 electrode and in nanostructured films of Sn(IV)-doped In 2 O 3 and TiO 2 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 H 2 O∕D 2 O isotope effects. In the nonaqueous solvent, propylene carbonate, there is evidence for a role for surface-bound phosphonate groups as proton acceptors.concerted electron-proton transfer | metal oxide electrode | surface modification | electrocatalysis | spectroelectrochemistry E lectron transfer reactions involving pH-dependent protoncoupled electron transfer (PCET) couples with a change in proton content between oxidation states, such as quinone/hydroquinone (Q∕H 2 Q) or M¼O∕M-OH∕M-OH 2 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 H 2 Q, H 2 Q − e − → H 2 Q þ• , occurs at E o0 ¼ 1.10 V vs. normal hydrogen electrode (NHE) independent of pH (E o0 is the formal potential) (2). The thermodynamic potential for H 2 Q oxidation at pH 7,. An inert electrode at pH ¼ 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 H 2 Q þ• to the surrounding medium at the prevailing pH with ΔE o0 ¼ 1.10 V − 0.059fpH − ðpK a ðH 2 Q þ• Þg with pK a ðH 2 Q þ• Þ ¼ −0.95. Oxidation can also occur by PT-ET with initial loss of a proton from H 2 Q to give HQ − followed by oxidation to HQ• at E o0 ðHQ • ∕HQ − Þ ¼ 0.46 V. For this mechanism, an inhibition to rate arises from the pH depende...

Accumulation of Multiple Oxidative Equivalents at a Single Site by Cross-Surface Electron Transfer on TiO2

“…39,40,43 On conductive nanoITO electrodes, at pH 1, the −Ru IV OH 3+ /−Ru III OH 2 3+ couple was observed at E 1/2 = 1.26 V, and at pH 5 the −Ru IV O 2+ /−Ru III OH 2+ couple appeared at 1.0 V vs NHE (Table S1). 42,44,45 These results show that oxidation of −Ru II OH 2 2+ by −Ru III P 3+ on TiO 2 to give −Ru III OH 2 3+ , −Ru III OH 2+ , or −Ru IV O 2+ is thermodynamically favored. Spectral changes for oxidation of TiO 2 −Ru II P 2+ to TiO 2 − Ru III P 3+ , and TiO 2 −Ru II OH 2 2+ to TiO 2 −Ru III OH 2 3+ , and further to TiO 2 −Ru IV OH 3+ in 0.1 M HClO 4 are shown in Figure 2A.…”

“…With the dominant form of the catalyst on the surface as −Ru III OH 2+ at pH 5, E 1/2 = 1.1 V for the Ru IV OH 3+ / Ru III OH 2+ couple and E 1/2 = 1.0 V for the Ru IV O 2+ / Ru III OH 2+ couple(Table S1) 42. −Ru IV O 2+ is accessible by both sequential electron transfer-proton transfer (ET-PT, eq 9a), or, by simultaneous electron−proton transfer with an added base (EPT, eq 9b) 42. With −Ru III OH 2+ as the dominant form of the catalyst on the surface, cross-surface oxidative activation to −Ru IV O 2+ is in competition with rereduction of −Ru III OH 2+ to −Ru II OH + (eq 10a), which is less favored than reduction of −Ru III OH 2+ to…”

A bio-inspired coordination polymer as outstanding water oxidation catalyst via second coordination sphere engineering

“…When the water oxidation RDS of a catalyst is related to the proton transfer, the atom proton transfer (APT), with a Lewis base in solution as a proton acceptor, can decrease the barrier of the reaction. Hence, Lewis bases (such as phosphate) in solution usually influence the reaction kinetics [56][57][58] . The relationship between catalytic activity and the concentration of additional base (K 3 PO 4 ) were therefore studied.…”

“…When an electrode undergoes a extra base-dependent pathway for water oxidation, because of the linear relationship between the water oxidation reaction rate k cat and catalytic current density, it should be first order reactions for the concentration of phosphate (ρ phosphate ) (see Supplementary Note 4 for an explanation) 59,60 . The first order reactions of phosphate have been widely observed in catalyst-modified electrodes for OER, when the RDS for water oxidation is clearly related to proton transfer 56,61 . As shown in Fig.…”

Hydrogen Tunneling in Enzymes and Biomimetic Models

“…For example, photoinduced EPT reactions are initially in a nonequilibrium configuration due to the virtually instantaneous optical excitation, and the nonequilibrium dynamics of the solvent and solute during the relaxation process play a central role. 184 These types of processes can be simulated using the MDQT surface hopping algorithm 145–146 and other methods described in Section 3.1. For EPT processes, however, the classical nuclei evolve on electron-proton vibronic surfaces rather than purely electronic or vibrational surfaces.…”