When electrocatalysis meets electron paramagnetic resonance

Ondertitel:gaining insight into electrochemical reactions

The interest in environmentally friendly production of chemical products has increased in recent years. Electrocatalysis provides a clean and inexpensive method for chemical synthesis, which can play its role in green chemistry if it is employed at optimum conditions. Decreasing the energy cost is the crucial challenge in performing electrosynthesis in large-scale industrial productions. To this end, an in-depth understanding of what exactly happens during the reaction is necessary, and it requires the identification of the short-lived intermediates. These intermediates are formed due to electron transfer in the electrochemical process, so they often are paramagnetic species such as organic radicals and transition-metal complexes in their specific oxidation states. Electron paramagnetic resonance (EPR) spectroscopy is used to detect paramagnetic species. Using different EPR techniques, one can identify the molecular structures. The combination of electrochemistry and EPR (SEC-EPR) can help to detect the paramagnetic intermediates and unravel the reaction mechanisms, which eventually leads to overcoming the energy cost challenge. Despite its benefits and long history, the SEC-EPR field has not received a lot of attention due to its challenges. As mentioned above, the intermediates are often short-lived, so the conventional ex-situ SEC-EPR experiments are not practical in many cases. Hence, in many cases, in-situ experiments need to be performed to get real-time information from the reaction. Performing in-situ experiments are challenging due to the lack of commercial SEC-EPR cells. Other methods for stabilising the intermediates, such as spin trapping and freeze-quenching, can be combined with in-situ and ex-situ experiments. This thesis focuses on both in-situ and ex-situ experiments, introducing novel SEC-EPR cells and the challenges of designing them and investigating the electrochemical reactions. Spin-trap and direct EPR techniques are employed depending on the requirements for each case study. Moreover, the freeze-quenching of an SEC-EPR cell to trap intermediates is tested. DFT computations completed the EPR data in the identification of paramagnetic species. This thesis is divided in four main parts: Part I (Introduction and methodology) consists of three chapters: Chapter 1 (General introduction) gives a short introduction to the main goals of this thesis. Chapter 2 (Experimental and Computational Methods) focuses on the theoretical background of the experimental and computational methods used in this thesis. Chapter 3 (State of art in EPR spectro-electrochemistry) gives explanations about the challenges of performing SEC-EPR methods and a review of prior works in this field. Part II (Pitfalls of spin-trap EPR) focuses on the non-innocent role of 5,5-dimethylpyrroline-N-oxide (DMPO) used for trapping radicals formed in a homogeneous copper-catalysed reaction. In Chapter 4 (The non-innocent role of spin traps), the reactivity of DMPO with Cu(II) complexes is investigated. Different EPR techniques are used to provide in-depth information about the copper ligands. DFT computations were performed for the identification of the intermediates. Part III (EPR-spectroelectrochemistry and spin trapping) contains four chapters focusing on combining spin trapping with SEC-EPR studies. In-situ experiments and DFT computations are used to unravel the reaction mechanism by detecting and identifying the radical intermediates for the electrochemical cyclisation of allyl 2-bromobenzyl in Chapter 5 (Reductive intramolecular cyclisation of allyl 2-bromobenzyl ether) and electrochemical aldol condensation of acetone in Chapter 6 (Re-evaluating the electrochemical self-condensation of acetone by EPR and DFT). Detailed information about the reaction mechanism and the structure of the trapped intermediates are given. In the same way, the ex-situ EPR experiments and DFT computations helped in ecstasy detection in Chapter 7 (How EPR can help in developing a screening strategy for ecstasy). Chapter 8 (Reactive oxygen species formation at Pt nanoparticles) is a small chapter in which the electrochemical formation of the oxygen radical species on Pt nanoparticles is studied by detection and quantification of radical species using different spin traps. Part IV (Direct EPR spectroelectrochemistry) focuses on the direct detection of paramagnetic species. In Chapter 9 (In-situ SEC-EPR cells, design and validation), I introduced two different in-situ setups designed by collaborators. The first setup, which is a static cell, focuses on the efficiency of ITO as a working electrode in the presence of different electrodeposited catalytic nanoparticles (Ag and NiO). The second cell is a flow cell that enables us to take advantage of controlling hydrodynamical flow and increase reproducibility, thus improving the efficiency of electrochemistry experiments. Both setups were validated by me for direct detection of electroreduction of methyl viologen and benzoquinone. Chapter 10 (Identifying reaction intermediates in carbon-halogen bond electroreduction) is an ongoing work which focuses on carbon-halogen electroreduction. In this chapter, electroreduction of 1- and 2-bromonaphthalene are investigated in which the reaction mechanisms have been already proposed previously on the basis of other techniques. The in-situ EPR data show the presence of radical intermediates. The DFT computations were employed to corroborate the interpretation of the EPR data. The results of the DFT computation of the radical intermediates proposed in the reaction mechanisms seem to contradict the EPR data. Chapter 11 (SEC-EPR study of the catalytic activity of Mo-Cu complexes in CO2 reduction) is the final research chapter that focuses on the EPR detection of Mo-Cu complexes employed as catalysts for CO2 reduction. The results of both ex-situ and quasi-in-situ methods are reported. In the ex-situ experiments, a freeze-quenching setup is designed for sample collection. A quasi-in-situ setup is developed and validated for performing electrochemistry inside an EPR tube. Different EPR techniques and DFT computations are used for data analysis.

Jaar van publicatie:2022

Trefwoorden:Doctoral thesis

Toegankelijkheid:Embargoed