Heterogeneous Oxidation Catalysis in the Liquid Phase

A09: Electrocatalysis: Resolving Sub-Processes of Nanocatalyst Transformations and Oxidation Reactions

Dr. Julia Linnemann • Prof. Dr. Kristina Tschulik

A heterogenous catalytic reaction essentially involves the adsorption, transformation and desorption of reactants, intermediates and products, respectively. For metal oxide catalysts, the intermediate transformation of the catalysts themselves, i.e., (re-)changing oxidation states of active sites, is a necessary requirement during a catalytic cycle. However, in view of liquid phase electrooxidation reactions, such expected highly reversible catalyst transformations may be accompanied with substantial chemical and structural transformations at surface and sub-surface regions, i.a., prior to starting catalysis.

The central hypothesis of this project is that both, the defects present in the as-synthesized “pre-catalyst” and its electrochemical transformation prior and during the catalysis determine the type and number of active sites formed on the “working” catalyst. We expect that the relative importance of each of these two main factors depends on the experimental conditions (e.g., pH, potential, polarization method) prior to and during electrocatalysis. Besides varying conditions, two different electrocatalytic reactions will be investigated. While the harsh conditions (high potential and pH) needed for OER may usually cause profound transformation, comparably mild pH and low potentials required for oxidation of alcohols like ethylenglycol may decrease this contribution.

We will study the influence of the electrochemical parameters for nanocatalysts with well-defined properties and defects, synthesized in C area projects. We aim at identifying the resulting differences in reaction kinetics and mechanisms and elucidating the associated activation and degradation of active sites using electrochemical (cyclic and linear sweep voltammetry, chronoamperometry and rotating disc electrode experiments) and spectro-electrochemical techniques. Electrochemical impedance spectroscopy (EIS) will be utilized as an operando tool, studying electrochemical sub-processes at different conditions. The data will be analysed in conjunction with internal discharge measurements and differential electrochemical mass spectrometry (DEMS) analysis of product yield and selectivity. Further, adsorbed species are identified by surface-enhanced infrared absorption spectroscopy (SEIRAS). This framework will allow us to conclude reaction mechanisms and quantify the extent of catalyst transformation.

Measurements at nanocatalyst ensemble electrodes will be complemented by single particle electrochemistry to dissect intrinsic catalyst property-activity relations from ensemble effects possibly caused by inter-particle interaction and mass transport limitations. Obtained results will be rationalized in collaboration with theory projects and will be supported by analysis of the local structural and compositional changes within the consortium.

(Figure: Scheme of catalyst transformations (i) to (iv). An amorphized hydrous oxide interphase is formed at near-surface regions during electrochemical oxidation. Recovery is related to self-discharge of formed oxide phases and intermediate states corresponding to pseudocapacitance).