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Mechanistic Understanding of Heterogenised Hydrogen Evolution Catalysts Through Vibrational Spectroelectrochemistry

Periodic Reporting for period 1 - VSHER (Mechanistic Understanding of Heterogenised Hydrogen Evolution Catalysts Through Vibrational Spectroelectrochemistry)

Reporting period: 2016-04-01 to 2018-03-31

Anthropological global warming and climate change pose a major threat to the future of our society. As internationally agreed on at the UN Climate Change Conference in Paris in 2015, it is therefore of central importance to significantly reduce net carbon dioxide (CO2) emissions. In this respect, transformation of atmospheric CO2 back into feedstock chemicals using chemical catalysis has emerged as a promising approach towards a carbon-neutral economy. To advance CO2 conversion catalysis towards full-scale applications, it requires highly effective catalysts that allow for driving the reaction with minimal energy losses and precisely controllable product selectivity. Up to now our understanding of how to rationally tune catalytic selectivity and activity remains poor hampering any advancement of the technology. Studying the catalytic mechanism of CO2 conversion catalyzed by small molecular complexes has become a favorable strategy towards understanding fundamental principle of the catalysis. Molecular metal complexes exhibit a simple and defined chemical framework for the catalytic reaction that also allows for tailored fine tuning of the ligand sphere to modulate the catalysis. Thus, they exhibit an ideal system to study structure-activity relationships. Understanding these is a major prerequisite for rational improvement of any CO2 catalytic conversion systems towards application.
In this project, we develop innovative spectroscopic approaches that allow for in situ investigations of the mechanism of molecular CO2 reduction catalysis. Specifically, powerful Fourier-transform infrared spectroscopy in the attenuated reflection mode coupled to electrochemistry is employed to investigate the interfacial catalytic reactions of manganese and rhenium transition metal complexes at a molecular level. Upon developing tailored immobilization strategies, we selectively bind the catalysts onto conductive electrode surfaces that enable controlled triggering of redox and catalytic reactions by applying potentials. Our studies aim at providing a detailed picture of the catalytic mechanism of these complexes on surfaces deriving general guidelines for rational enhancement of activity and selectivity for CO2 reduction catalysis.
We developed an approach to couple Fourier-transform infrared spectroscopy in the attenuated total reflection mode to electrochemistry. Using our advanced approach, we achieved collecting infrared signals of sample molecules on conductive supports in aqueous media in the concentration range of nanomols per cm². Our setup allows for applying potentials and simultaneously recording infrared spectra observing potential induced transformations at a molecular level.
Chemical synthesis was used to achieve highly catalytically active transition metal complexes bearing manganese and rhenium metal centers as active sites, respectively, as well as anchors at the ligand frame to allow for quantitative immobilization of the catalysts on highly conductive carbon nanotube film electrodes leading to improved activities that outcompete many molecular catalysts currently employed.
Investigations of the heterogenised molecular catalytic systems by means of the developed infrared spectroscopic approach led to a detailed understanding of redox induced reactions of the compounds. Infrared detection of redox intermediate species formed upon reduction of the immobilized catalysts enabled rationalization of activity and, importantly, selectivity of the compounds explaining the observed product distribution in electrocatalysis experiments conducted in parallel.
In its outcome, the project provided unprecedented information into reactions of immobilized molecular catalysts for CO2 reduction with major implications for rational improvement of the catalytic properties via organic and material chemistry. Product selectivity of these catalysts has been linked for the first time to appearance of surface intermediate states providing measures for fine tuning of selectivity towards desired catalytic products.
The developed infrared spectroscopic approach that enabled the detection of these species bears potential to investigate a range of interfacial reactions beyond molecular CO2 reduction catalysis. Its high sensitivity and the coupling to electrochemical control make this method also extremely powerful to study heterogeneous catalytic reactions such as CO2 reduction promoted by bulk surfaces as well as enzymatic transformations at conductive interfaces relevant for bioelectronical devices.
Graphical abstract of the project