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CRC/TRR 247
Heterogeneous Oxidation Catalysis in the Liquid Phase

A09: Electrocatalysis: Triggering Nanocatalyst Transformations and Oxidation Reactions

Prof. Dr. Kristina Tschulik

Project A09

A heterogeneous catalytic reaction involves the adsorption, transformation and desorption of reactants, intermediates and products, respectively. For metal oxide catalysts in oxidation re­actions, this requires the intermittent oxidation of active sites during a catalytic cycle, which is accompanied by substantial chemical and structural transformations at surface and surface near regions. In the second funding period (FP2) we have revealed that metal oxide catalysts are particularly active when the change in oxidation state during the catalytic cycle is linked with minimal structural transformations. This can be achieved, e.g., by a frustrated phase transition that is a partial, dynamic transition to another phase under reaction conditions.[1]

In FP3 we will use electrochemical methods to form phases from Co3-xMxO4 (M = Ni, V) spinel nanoparticles that undergo these frustrated phase transitions readily and, thus, facilitate the electrochemical oxidation of alcohols. Structural characteristics of these phases will be linked to the achieved selectivity as determined by online Differential Electrochemical Mass Spectrometry (DEMS), Rotating Ring Disc Electrode analyses (RRDE) and High Performance Liquid Chromatography (HPLC). The phase transformation and interaction of the reactant/intermediates with the formed surface under electrocatalytic conditions will be probed by electrochemical Surface Enhanced IR Spectroscopy (EC-SEIRAS). Spectroscopic signatures of adsorbate-catalyst interactions at defect-engineered catalysts (C projects), together with product analysis, will be used to identify favorable catalyst-reactant-orientations jointly with A01, A08 and B03.

Since similar concepts have been identified during thermal and electrocatalysis in FP2, in A09 we will further try to bridge both by conducting electrocatalytic alcohol and cyclohexene oxidation in an autoclave. Designing and setting up this autoclave will benefit from the expertise of A01 and A02 gained during FP2 and will enable electrocatalysis at elevated temperatures (≤373 K) and pressures (≤30 bar). This will allow us to overcome existing limitations of Co oxide electrocatalysts, namely short catalyst lifetime and, hence, low reaction turnover for the oxidation of, for instance, ethylene glycol, cinnamyl alcohol, and to implement non-aqueous solvents to, e.g., oxidize cyclohexene. Product selectivity will be identified by gas chromatography (GC) and HPLC batch analysis and a generic concept of liquid phase oxidation catalysis at Co based oxides will be derived.

 

Figure: Schematic of SEIRAS setup used to collect the shown spectra at Co3O4 NPs in 0.1 M KOH and 0.1 M EG at 1.6 V vs. RHE, plotted as relative to open-circuit potential.


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

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.