UC Berkeley

Abstract

Earth-abundant Transition Metal Complexes for Catalytic Proton and Carbon Dioxide Reduction

By Van Sara Thoi

Doctor of Philosophy in Chemistry

University of California, Berkeley

Professor Christopher J. Chang, Chair

The immediacy of global climate change and depletion of non-renewable resources have intensified efforts in sustainable energy technologies. In particular, the conversions of protons to hydrogen and carbon dioxide to value-added chemicals have garnered heavy interest in the search for sustainable alternative fuels. The present work will focus on the development of cheap and earth-abundant metal catalysts for proton and carbon dioxide reduction. Our group has published a previous report on the electrocatalytic generation of hydrogen from neutral water by a high-valent molybdenum-oxo complex supported by a pentadentate ligand, [(PY5Me₂)MoO]²⁺ (PY5Me₂ = 2,6-bis(1,1-di(pyridin-2-yl)ethyl)pyridine), on a mercury electrode. However, the use of a mercury electrode under aqueous conditions is not conducive for characterizing the active catalytic species. Thus, a study of this system in organic media with a carbon electrode is initiated to examine the catalytic mechanism. Voltammetry and controlled-potential electrolyses show that a total of three electrons are necessary to obtain the active catalyst and offer new insights into the mechanism for hydrogen generation. This work corroborates with a computational mechanistic study.

Moving towards first-row transition metal complexes for hydrogen evolution, a new series of Co complexes supported by a tetradentate pyridine platform has been developed for electrocatalytic and photocatalytic water reduction to understand the effects of open coordination sites. As direct tetradentate analogues to PY5Me₂, three new ligands of the form PY4Me_xH_y (2-(1,1-bis(2-pyridyl)ethyl)-6-(2-(2-pyridyl)alkyl)pyridine, x = 1-3 and y = 0-2) and their Co complexes are synthesized and characterized. In the solid-state structure, the Co complex supported by PY4Me₃ is a five-coordinate species, while [(PY4Me₂H)Co(CH₃CN)(OTf)]⁺ and [(PY4MeH₂)Co(CH₃CN)(OTf)]⁺ are six-coordinate complexes. The extra methyl group in PY4Me₃ platform impedes the binding of a sixth ligand in the equatorial site. Interestingly, this small difference leads to significant differences in the electrochemical and photochemical behaviors of all three Co complexes. Although [(PY4Me₃)Co(CH₃CN)]²⁺ has the lowest cathodic onset potential for electrocatalytic water reduction, it is the least active for photochemical hydrogen evolution. On the contrary, the six-coordinate [(PY4MeH₂)Co(CH₂CN)(OTf)]⁺, which has the highest catalytic onset potential, has the highest photocatalytic performance. At pH 7, [(PY4MeH₂)Co(CH₃CN)(OTf)]⁺ photochemically reduces water to hydrogen with turnover numbers of up to 1000 in 18 h. This series of complexes offer insights on structural tuning of the ligand platform to improve catalytic hydrogen evolution.

Another important reaction related to sustainable energy is the reduction of carbon dioxide. The abundance of carbon dioxide in the atmosphere offers tremendous opportunities for using CO₂ as a C₁ feedstock. However, carbon dioxide is a thermodynamic sink and its activation requires multiple electrons and a large amount of energy. Thus, the use of metal catalysts to serve as a multi-electron reservoir is highly desirable. In this vein, a novel motif that uses a nickel complex supported by bis(N-imidazolylpyridine)methane, [Ni(^Mebimpy)]²⁺, is discovered to be an active electrocatalyst for carbon dioxide reduction. Because of the large thermodynamic overpotential for carbon dioxide reduction, selectivity for CO₂ conversion over proton reduction is crucial. In this system, only the conversion of carbon dioxide to carbon monoxide is observed even in the presence of high proton concentration. Furthermore, the potential at which carbon dioxide is reduced can be tuned by elongating the alkyl bridge; the added flexibility of [Ni(^Etbimpy)]²⁺ and [Ni(^Prbimpy)]²⁺ decrease the potential by 100 mV and 200 mV, respectively, from the parent complex. The ligand platform can be further functionalized to tune the potential and increase the stability of the complexes. Thus, a new family of Ni catalysts supported by carbene-pyridine ancillary ligands has been synthesized and fully characterized.

Another approach towards sustainable long-term CO₂ utilization is solar-driven photochemical reduction of CO₂. To this end, a second generation of Ni catalysts has been developed that utilizes carbenes and isoquinolines as ligand donors. By tuning the  systems in the ligand platform, a Ni complex supported by bis(3-imidazoylisoquinolynl)propane ([Ni(^Prbimiq1)]²⁺) emerges as a promising electrocatalyst that reduces CO₂ to CO at a catalytic onset potential E_1/2 = −1.20 V vs. SCE. Moreover, photocatalytic CO₂₊ reduction has been accomplished that utilizes a molecular Ir photosensitizer and an amine sacrificial reductant in an acetonitrile solution. In the presence of the catalyst, a five-fold increase in CO production is observed above background reactions. Turnover numbers of up to 98,000 in 7 hours have been achieved and demonstrate the potential of solar-driven catalytic CO₂ reduction on a larger industrial scale.

Taken together, this body of work displays the potential of earth-abundant metal catalysts for proton and carbon dioxide reduction. Understanding the molecular basis for these reactions will further aid the development of robust and efficient catalysts in both homogeneous and heterogeneous systems.