Taro Uchida

The Role of Bridge‐Bonded Adsorbed Formate in the Electrocatalytic Oxidation of Formic Acid on Platinum

The oxidation of formic acid (HCOOH) on platinum electrodes has been extensively investigated as a model electrocatalytic reaction. It is generally accepted that HCOOH is oxidized to CO2 through a dual-pathway mechanism: one pathway (the main pathway) involves a fast reaction via a reactive intermediate and the second pathway includes a step in which a poisoning species is formed. This species, which is oxidized to CO2 at high potentials, has been identified as adsorbed CO, which is formed by dehydration of HCOOH. Adsorbed hydroxycarbonyl (COOHads) has long been assumed to be the reactive intermediate in the main pathway, but the spectroscopic detection of this species has not been reported to date. By using surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATRSEIRAS), Miki et al. observed that formate is adsorbed in a bridge-bonded configuration on Pt electrodes during HCOOH oxidation. On the basis of systematic time-resolved ATR-SEIRAS analysis of the oxidation dynamics, Samjesk et al. suggested that adsorbed formate (HCOOads) is a reactive intermediate in the main pathway and its decomposition to CO2 is the rate-determining step (rds). The adsorbed formate is in equilibrium with HCOOH in the bulk solution and the reaction pathway (formate pathway) can be represented by Equation (1)

Importance of Acid–Base Equilibrium in Electrocatalytic Oxidation of Formic Acid on Platinum

Electro-oxidation of formic acid on Pt in acid is one of the most fundamental model reactions in electrocatalysis. However, its reaction mechanism is still a matter of strong debate. Two different mechanisms, bridge-bonded adsorbed formate mechanism and direct HCOOH oxidation mechanism, have been proposed by assuming a priori that formic acid is the major reactant. Through systematic examination of the reaction over a wide pH range (0-12) by cyclic voltammetry and surface-enhanced infrared spectroscopy, we show that the formate ion is the major reactant over the whole pH range examined, even in strong acid. The performance of the reaction is maximal at a pH close to the pKa of formic acid. The experimental results are reasonably explained by a new mechanism in which formate ion is directly oxidized via a weakly adsorbed formate precursor. The reaction serves as a generic example illustrating the importance of pH variation in catalytic proton-coupled electron-transfer reactions.

Biomimetic Environment to Study E. coli Complex I through Surface-Enhanced IR Absorption Spectroscopy

In this study complex I was immobilized in a biomimetic environment on a gold layer deposited on an ATR-crystal in order to functionally probe the enzyme against substrates and inhibitors via surface-enhanced IR absorption spectroscopy (SEIRAS) and cyclic voltammetry (CV). To achieve this immobilization, two methods based on the generation of a high affinity self-assembled monolayer (SAM) were probed. The first made use of the affinity of Ni-NTA toward a hexahistidine tag that was genetically engineered onto complex I and the second exploited the affinity of the enzyme toward its natural substrate NADH. Experiments were also performed with complex I reconstituted in lipids. Both approaches have been found to be successful, and electrochemically induced IR difference spectra of complex I were obtained.

Speciation of Adsorbed Phosphate at Gold Electrodes: A Combined Surface-Enhanced Infrared Absorption Spectroscopy and DFT Study

Despite the significance of phosphate buffer solutions in (bio)electrochemistry, detailed adsorption properties of phosphate anions at metal surfaces remain poorly understood. Herein, phosphate adsorption at quasi-Au(111) surfaces prepared by a chemical deposition technique has been systematically investigated over a wide range of pH by surface-enhanced infrared absorption spectroscopy in the ATR configuration (ATR-SEIRAS). Two different pH-dependent states of adsorbed phosphate are spectroscopically detected. Together with DFT calculations, the present study reveals that pKa for adsorbed phosphate species at the interface is much lower than that for phosphate species in the bulk solution; the dominant phosphate anion, H2PO4(-) at 2 < pH < 7 or HPO4(2-) at 7 < pH < 12, undergoes deprotonation upon adsorption and transforms into the adsorbed HPO4 or PO4, respectively. This study leads to a conclusion different than earlier spectroscopic studies have reached, highlighting the capability of the ATR-SEIRAS technique at electrified metal-solution interfaces.