Molecular control of interfacial inner-sphere electron

Abstract

The interconversion of electrical and chemical energy relies on the intimate coupling of protons and electrons to activate small molecules including 02, C0 2, and H20. The mechanistic pathways governing the proton- and electron-transfer steps are key to the efficiency of catalysis because they determine the rate and selectivity of the reaction. Energy conversion devices commonly use metallic heterogeneous electrocatalysts, so a deep understanding of the factors governing the rate and thermochemistry of interfacial proton transfer (PT) and electron transfer (ET) steps at electrode surfaces is critical for improving the efficiency and selectivity of energy conversion.

This thesis focuses on developing a molecular-level understanding of interfacial inner-sphere PT and ET steps in two parts: (1) incorporation of molecular active sites into heterogeneous electrodes through graphite conjugation (Chapters 1-3), and (2) elucidation of the role of the proton donor in interfacial proton-coupled electron transfer steps through detailed kinetic studies (Chapters 4-6). Part 1 describes a class of catalysts that incorporate molecularly well-defined, highly-tunable active sites into heterogeneous graphite surfaces. These graphite-conjugated catalysts (GCCs) feature a unique conjugated linkage between a discrete molecular active site and the delocalized states of graphitic carbons. Electrochemical and spectroscopic investigations establish that GCCs exhibit strong electronic coupling to the electrode, leading to ET behavior that diverges fundamentally from that of solution phase or surface-tethered analogues.

We find that: (1) ET is not observed between the electrode and a redoxactive GCC moiety regardless of applied potential. (2) ET is observed at GCCs only if the interfacial reaction is ion-coupled. (3) Even when ET is observed, the oxidation state of a transition metal GCC site remains unchanged. From these observations, we construct a mechanistic model for GCC sites in which ET proceeds exclusively through inner-sphere mechanisms. We additionally demonstrate that the catalytic hydrogen evolution reaction (HER) at a Rh-based GCC proceeds via concerted electron transfer and substrate activation via elimination of the stepwise pathway. In these respects, GCC active sites behave like metallic solids, but with an unprecedented level of molecular control.

Using the GCC platform, we demonstrate that pKa is a useful thermodynamic descriptor for proton-coupled electron transfer (PCET) reactions at surface sites, just as it is in molecular sites, providing a framework with which to understand the thermochemistry of inner-sphere ET processes at electrode surfaces. Part 2 investigates the role of the explicit proton donor in interfacial PCET steps that result in metal-H bond formation by using the rate of HER on Au as a proxy for the rate of PCET to the Au surface. Detailed mechanistic studies revealed that in nonaqueous electrolytes, a trialkylammonium proton donor with a less bulky steric profile not only has overall faster HER kinetics than its bulkier counterpart, but also displays a greater transfer coefficient and a greater H/D kinetic isotope effect, demonstrating that even small changes to the identity of the proton donor can drastically affect the intrinsic kinetics of PCET at an interface.

In aqueous electrolytes, we also observed a strong dependence on the identity of the proton-donating species in solution and found that only proton donors that can pre-associate with acceptor sites on the electrode surface can accelerate the rate of interfacial CPET. Finally, we used an innocent mixed-buffer electrolyte to probe the pH-dependence of steady-state HER current on Au and found that, surprisingly, the loss in catalytic efficiency occurs between pH 1 and pH 4 rather than systematically decreasing across the entire pH range. These results highlight the crucial role of the proton donor in achieving selective and efficient electrochemical energy conversion across a range of solvent and pH conditions.