Influence of electrodeposited nanoparticles on the electrochemical halide reduction)

01/01/2018 - 31/12/2021

Fundamental advances in energy conversion and storage which are full of vigor in meeting outfaces of some environmental phenomena such as waste water pollution and impact of fossil fuels are held by electrosynthesis. In the past decade organic electrosynthesis has become an interesting, versatile and environmental friendly alternative compared to classical organic synthesis. A reaction of particular interest is the electrochemical reduction of organic halides (C-X) due to the efficient formation of carbon-carbon bonds and the degradation of organic halogenated waste.

The major bottle-neck problem of the electrochemical synthesis is the high overpotential which makes industrial applications unachievable due to the exuberant energy cost and low selectivity. Attempts to reduce the large overpotential are directed towards improving the catalytic activity of electrode materials within the field of electrocatalysis. The most important catalytic enhancement strategies are the influence of the morphology and the interaction with a second material. A general model for the prediction of the electrocatalytic activity is not yet possible, certainly with more complex electrocatalyst who are difficult to describe theoretically. A development of an efficient electrocatalyst will require an intertwined approach combining both the theoretical and experimental studies.

This work is devoted to gather experimental data concerning the use of electrocatalysis for C-X bond reduction. The aim is to investigate the parameters that influence the catalytic activity for the synthesis of an efficient, performant and stable catalyst towards the C-X reduction. To perform an in-depth study a combination of multiple techniques is used. Microscopical surfaces analysis, analytical- and electrochemical techniques will be simultaneous used in the unravelment of the electrocatalytic properties of the electrocatalyst.

Firstly, the influence of the morphology of different monometallic nanoparticles and their relationship with the electrocatalytic activity is investigated. A versatile double-pulse electrodeposition technique is used to obtain nanoparticles on a conductive substrate. By varying the nucleation and growth parameters it is possible to obtain tailor made nanoparticles regarding the morphology (size) and dispersion on the substrate. Electrodeposited Ag nanoparticles of ca 100 nm showed the most expressed electrocatalytic properties towards the benzyl halide reduction (benzyl bromide, benzyl chloride and benzyl iodide). All investigated nanoparticles (Ag, Au, Mn, Ni, Cu, Pb, Zn and Cd) showed an activity which almost coincides with their respective bulk material. Although a good similar activity is obtained for the nanoparticles in comparison with bulk materials no indication of an altered reaction mechanism is found.

Secondly, an enhancement strategy is examined by combining Ag, the most active electrocatalytic material with a second material. This is done by investigation of various support metals or creating an alloy structure. In the former Ag nanoparticles are electrodeposited on planar Glassy carbon, Ni, Au and Ti substrates. Results showed that the morphology and electrocatalytic properties are most expressed for Ag nanoparticles deposited on a Ni substrate. Following this finding a successful Ag/Ni electrocatalyst is synthesized with a combination of electrodeposition and galvanic replacement. The effect of the interplay between support or alloy is evaluated and the ratio primary (Ag)/secondary (Ni) metal adjusted for optimal catalyst performance.

In our search to develop performant electrocatalyst a third section of this work focusses on the stability. In this work it is shown that the predominant electrochemical degradation mechanisms of Ag nanoparticles for C-X bond reduction are agglomeration/clustering, dissolution and detachment. After identification of these degradation mechanism a successful particle confinement method with vertically oriented nanographite is proposed to increase the stability and avoid the degradation phenomena. Furthermore a regeneration approach with high cathodic polarization pulses is investigated to efficiently disperse clustered degraded nanoparticles evenly over a conductive support.

A last section of this work, shows the versatility of C-X bond reduction by investigating a more complex organic halide: the intramolecular cyclisation reaction of allyl 2-bromobenzyl ether. With a planar Ag electrode it is possible to alter to the reaction mechanism specific to dehalogenation or cyclisation.

In general, a variety of catalyst synthesis techniques are applied with the final goal being to induce the activity enhancement and obtain a stable electrocatalyst. Applying a step-by-step methodology where morphology, enhancement and stability are independently investigated, ensures adequate information is obtained to tweak the electrocatalyst towards our objectives.