Bio-inspired cooperative electrocatalyst design for sustainable CO2 utilisation
Final Report Abstract
During this fellowship, a simple and efficient synthetic strategy to peripherally functionalised bis(imino)pyiridine) complexes was developed. A series of functional nickel complexes were synthesised for the first time and structurally characterised. These compounds feature outersphere hydrogen bonding that significantly influences their solid-state structures. Despite previous reports in the literature on their electrocatalytic activity, these complexes were found unstable under electrochemical conditions. No catalytic activity towards CO2 reduction was observed on the timescale of electrolysis. Decomposition during electrolysis resulted in the formation of a deposit on the working electrode, that catalyses electrocatalytic proton reduction. When replacing bis(imino)pyridine ligands with terpyridine ligands of a similar coordination geometry, more stable nickel complexes were obtained that catalyse the electrochemical CO2 reduction. Formation of CO was observed using Ni(tpy)2(PF6)2 as the catalyst in organic solvents. During electrolysis, this catalyst is stable for several hours and achieves up to 30 turnovers with a high selectivity for CO over H2 production. Studies on this catalyst, however, were published by another group during the course of this fellowship. A derivative of this catalyst, Ni(tpySH)2(BF4)2 was synthesised and combined with CdS nanocrystals to drive CO2 reduction with sunlight rather than electricity. Under simulated sunlight, this hybrid system achieves selective photocatalytic reduction of CO2 to CO in aqueous solution, whereas the parent complex requires organic solvents to function. This system is currently under further investigation. Using N,N-dimethylformamide as the solvent gave rise to the observation that CdS nanocrystals can catalytically decompose formic acid. Ligand-free CdS nanocrystals (QD-BF4) show a high activity for photocatalytic dehydration of aqueous formic acid to CO. Activity is sustained for more than two weeks. Mechanistic studies suggest surface cadmium atoms as the catalytically active site. When the decomposition is performed using formic acid as the solvent, a selectivity switchover is observed with H2 as the main decomposition product. Ligand-capped nanocrystals (QD-MPA) show improved activity over QD-BF4, which can be further enhanced by a cobalt co-catalyst. MPA was found to stabilise the photocatalyst by preventing particle aggregation. Long-term activity, selectivity and quantum yield of this system exceeds all previously known catalysts for this purpose.
Publications
- “Photocatalytic Formic Acid Conversion on CdS Nanocrystals with Controllable Selectivity for H2 or CO”, Angewandte. Chemie International Edition 2015, 54, 9627–9631
M. F. Kuehnel, D. W. Wakerley, K. L. Orchard and E. Reisner
(See online at https://doi.org/10.1002/anie.201502773) - “Photocatalytic Formic Acid Decomposition on CdS Nanocrystals with Controllable Selectivity for H2 or CO”, ISACS17 Challenges in Chemical Renewable Energy, Rio de Janeiro, September 2015
M. F. Kuehnel
