Michael Busch

Switching on the Electrocatalytic Ethene Epoxidation on Nanocrystalline RuO2

Ruthenium-based oxides with rutile structure were examined regarding their properties in electrocatalytic ethene oxidation in acid media. A possible promoting effect of chloride ions toward oxirane formation was explored. Online differential electrochemical mass spectrometry combined with electrochemical polarization techniques were used to monitor the potential dependence of organic products resulting from ethene oxidation as well as the reaction solution decomposition products. Quantum chemical modeling by means of density functional theory was employed to study key reaction steps. The ethene oxidation in acid media led to CO(2), whereas oxirane was formed in the presence of 0.3 M Cl(-). In the Cl(-) promoted oxidation on RuO(2), oxirane and a small amount of CO(2) were the only detected electro-oxidation products at potentials below the onset of Cl(2) and O(2) evolution, resulting from Cl(-) and water oxidation. It is demonstrated here that the epoxidation is a surface-related electrocatalytic process that depends on the surface properties. Cl acts as the epoxidation promoter that switches off the combustion pathway toward CO(2) and enables the epoxidation reaction channel by surface reactive sites blocking. The proposed epoxidation mechanism implies binuclear (recombination) mechanism for O(2) evolution reaction on considered surfaces.

Hydroxide oxidation and peroxide formation at embedded binuclear transition metal sites; TM = Cr, Mn, Fe, Co

Key steps in electro-catalytic water oxidation on binuclear Transition Metal (TM) sites are addressed. These comprise (a) two one-electron oxidation steps of TM-OH moieties to form the corresponding two TM=O oxy-groups, and (b) a chemical step whereby the two oxy-species form a TM-O-O-TM peroxy-bridge. A test rig representing a generic low crystal field oxide support is described and employed. The energetics for homo-nuclear Cr(III-V), Mn(III-V), Fe(II-IV) and Co(II-IV) sites are compared. The uniqueness of the tyrosine/tyrosyl-radical (TyrOH/TyrO˙) reference potential for driving the oxidation steps is demonstrated. The oxidation of adsorbed TM-OH moieties on binuclear Mn and Co candidates requires an overpotential of approximately 0.5 V relative to the chosen reference potential. Correspondingly, the subsequent O-O bond formation becomes strongly exothermic, of the order of 1 eV. The hydroxide oxidation steps on binuclear CrCr and FeFe systems are, in total, exothermic by 1.21 and 0.61 eV, respectively, relative to the TyrOH/TyrO˙ reference potential. Consequently, the chemical step for transforming the TM=O moieties to the peroxo species is found to be endothermic by the order of 0.7 eV. Based on these findings, a catalyst containing one TM from each class is suggested. The validity of this concept is demonstrated for the FeCo binuclear site. The results are discussed in the context of experimental observations, which display a preference for mixed oxide systems.

Study of Hypochlorite Reduction Related to the Sodium Chlorate Process

AbstractReduction of hypochlorite is the most important side reaction in the sodium chlorate reactor leading to high energy losses. Today chromate is added to the reactor solution to minimize the hypochlorite reduction but a replacement is necessary due to health and environmental risks with chromate. In order to understand the effect of different substrates on the hypochlorite reduction, α-FeOOH, γ-FeOOH, Cr2O3 and CrOH3 were electrodeposited on titanium and subjected to electrochemical investigations. These substances are commonly found on cathodes in the chlorate process and can serve as model substances for the experimental investigation. The mechanism of hypochlorite reduction was also studied using DFT calculations in which the reaction at Fe(III) and Cr(III) surface sites were considered in order to single out the electrocatalytic properties. The experimental results clearly demonstrated that the chromium films completely block the reduction of hypochlorite, while for the iron oxyhydroxides the process can readily occur. Since the electrocatalytic properties per se were shown by the DFT calculations to be very similar for Fe(III) and Cr(III) sites in the oxide matrix, other explanations for the blocking ability of chromium films are addressed and discussed in the context of surface charging, reduction of anions and conduction in the deposited films. The main conclusion is that the combined effect of electronic properties and reduction of negatively charged ions can explain the reduction kinetics of hypochlorite and the effect of chromate in the chlorate process. Graphical AbstractTwo dimensional test rig for hypochlorite reduction on transition metal sites embedded in an oxy-hydroxide matrix. The adsorption of Cl is used as descriptor for the reaction.

On the Generality of Molecular Volcano Plots

In homogeneous catalysis, the structure and electronic configuration of the active catalysts can vary significantly. Changes in ligation, oxidation, and spin states have the potential to influence the catalytic cycle energetics strongly that, to a large degree, dictate the catalytic performance. With the increased use of computational screening strategies aimed towards the identification of new catalysts, ambiguity surrounding structure/electronic configurations can be problematic, as it is unclear which species should be computed to determine a catalysts properties. Here, we show that a single volcano plot constructed from linear free energy scaling relationships is able to account for variations in ligation, oxidation, and spin state. These linear scaling relationships can also be used to predict the free energies associated with a specific structure and electronic configuration of a catalyst based on a single descriptor. As a result, a single volcano plot can be used to screen prospective new catalysts rapidly.