Date of Award
Claudio N. Verani
The development of molecular electro/photocatalysts for water splitting is a promising method for hydrogen-based energy production. We focus on understanding the catalytic mechanisms of water-soluble copper-based electrocatalysts with polypyridine frameworks. We investigated two copper complexes on a planar tetradentate geometry with different electron donating substituents towards electro and photocatalytic hydrogen generation. By modulating the electron density on the metal center, possible formation of the CuIII-H- species was studied. However, based on experimental evidence, we found out that the catalysis does not stabilize the CuIII oxidation state or the formation of the CuIII-H- species. Instead, the plausible mechanism likely involves the formation of CuII, CuI, and Cu0. Moreover, the lack of stability of these catalysts mainly attributed to the lability of the reduced CuI species.
To address the lack of stability due to the lability of the CuI species, we increased the chelating effect by introducing another pyridine arm. We developed a copper complex based on a pentadentate polypyridine ligand framework and studied its catalytic activity towards water reduction at pH 2.5 and pH 7. This catalyst had an overpotential of 780 mV and yielded a turnover number (TON) of 1670 at pH 7 and a TON of 3900 at pH 2.5 after 3 h catalysis. Even with this complex, we observed a ligand radical formation during H2 generation. The formation of CuIII-H- intermediate is avoided by the pyridine ligand framework, instead, CuII-H- species formation has been proposed at both pHs. At pH 2.5, the protonation of the dangling pyridine takes place and acting as a proton relay. Although we introduced some stability to the complex using a pentadentate ligand framework, catalyst deactivation takes place with the ligand radical formation.
We were interested in studying a bimetallic copper system in which two metal centers are in close proximity to each other so that two metals can cooperatively generate hydrogen, thus avoiding the ligand involvement. Therefore, we developed a homobimetallic copper complex in which the two metal centers are bridged by a phenolate and a pyrazolate anion. The pyrazole moiety helps to keep the two Cu centers closer together. The complex showed two reversible reductions corresponding to CuII,II/CuII,I and CuII,I/CuI,I processes. Catalytic experiments yielded a TON of 1700 after 30 min catalysis. Experimental evidence revealed that the metal centers are involved in catalysis. A few more experiments will be required along with DFT calculations to provide insights about the cooperativity. Moreover, we observed the decomposition of the catalyst after 30 minutes, indicating that the pyrazole moiety might be falling apart. Studies for possible deactivation pathways are underway.
Ekanayake, Danushka Madushanka, "Insights And Challenges In The Development Of Molecular Copper-Based Electrocatalysts For Water Reduction" (2019). Wayne State University Dissertations. 2159.