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Electrocatalysis on Model Interfaces

Final Report Summary - ELCAMI (Electrocatalysis on Model Interfaces)

Pt(hkl) single crystal electrodes have been examined by spectro-electrochemical techniques to explore relationships between the surface structure and its electrocatalytic reactivity towards the oxygen reduction reaction (ORR) in HClO4/NaClO4 and H2SO4/Na2SO4 solution at various pH (1, 7 and 10). These experiments revealed a distinct structure-sensitivity of the catalytic reactivity, in particular due to the existence of adsorbed species (reaction intermediates, anions and surface oxides) in the ORR. Reaction intermediates and adsorbed species, such as OH, solvent molecules, surface oxides and electrolyte anions, are key factors to understand the fundamental ORR mechanism. These key factors compete for active catalyst sites with molecular oxygen O2, critically affecting the overpotential and thus the overall kinetics of the multi-electron ORR. Despite the important insight obtained in the relevant literature, there has been no single report on a direct observation and monitoring of OH adsorbates under electrochemical reaction condition.
We performed in-situ Raman (Shelled-Insulated Nanoparticle Enhanced Raman Spectroscopy, SHINERS) and infrared (Surface Enhanced Infrared Reflection Absorption Spectroscopy, SEIRAS) studies of surface adsorbates on Au(111) and Pt(hkl) single crystal surfaces in dependence on the applied potentials and pH (pH 1, 7 and 10) in H2O and D2O solutions. Complementary spectroscopic in-situ methods provided direct access to local chemical information under potential control. These results were correlated with polarisation studies employing a rotating single crystal electrode in a hanging meniscus (HMRD) configuration in Ar- and O2- saturated electrolyte solutions. Combined electrochemical and in-situ spectroscopic studies were carried out to understand the role of reactive intermediates and spectators at the molecular-scale ORR mechanism.
The formation of OH-species on Au(111) and Pt(111) surfaces was found in neutral solutions at the potential range of the so-called “a butterfly peak”, which represents a major break-through in understanding reaction mechanisms of fundamental oxidation and reduction processes in electrocatalysis. We investigated in detail the replacement of OH species by surface oxides up to the irreversible place-exchange process of metal atoms/ions and oxygen species during the initial stages of surface oxidation at more positive potentials on single crystal gold and platinum electrodes. Isotopic shifts allowed to resolve OH-species. Surface oxide could be distinguished unambiguously by the isotopic shift of OH and OD species, as detected by in-situ Raman and infrared experiments in D2O on well-defined single crystal metal electrodes. These studies were extended towards exploring the role of OH and surface oxide on the ORR. Important correlations between surface structure and ORR-reactivity could be derived. We demonstrated the feasibility of structure-sensitive single crystal in-situ IR/Raman experiments and developed guidelines for fundamental electrocatalytic studies, such as the oxidation of CO and of short-chain alcohols.
In addition, several technical implementations have been accomplished within the ELCAMI project in close collaboration with members of the host group (1) development of a new polishing setup for the preparation of Au(hkl) and Pt(hkl) single crystal surfaces, (2) setting up a preparation stage for the inductive heating of mono- and multi-component single crystals in a controlled atmosphere, (3) implementation of the hanging meniscus rotating disk electrode configuration for kinetic studies on single crystal surfaces, (4) preparation and purification of shell-isolated nanoparticles for SHINERS studies and implementation of the methodology for a wide range of reactivity studies, including the ORR, on Au(hkl) and Pt(hkl) single crystals and alloy electrodes under a wide range of electrochemical conditions.