Supercatalysis by superexchange
journal contributionposted on 02.12.2016, 11:44 by Stephen Fletcher, Nicholas J. Van Dijk
In modern transition state theory, the rate constant for an electron transfer reaction is expressed as the product of four factors: an exponential factor, a pre-exponential factor, an electronic transmission coefficient, and a nuclear transmission coefficient. The activation energy of the reaction manifests inside the exponential factor, and on the conventional view, catalysis occurs by decreasing this activation energy below its catalyst-free value. In the present work we report the discovery of an unusual counter-example in which catalysis occurs by increasing the electron transmission coefficient far above its catalyst-free value. The mechanism involves the formation of a superexchange bridge between an electron donor (a graphite cathode) and an electron acceptor (a pentasulfide ion). The bridge consists of a dz2 orbital inside a cobalt phthalocyanine molecule. The dramatic result is the acceleration of the reduction of pentasulfide ions by more than 5 orders of magnitude compared with the catalyst-free case.
This work was sponsored by the EPSRC (UK) Grant Number: EP/M009394/1, “Electrochemical Vehicle Advanced Technology” (ELEVATE). N.J.V.D. also thanks Regenesys Technologies Ltd. for a scholarship.