Addition of weak Brönsted acids such as 1-propanol,
2-pyrrolidone, and CF3CH2OH triggers
a considerable
improvement of the catalysis of CO2 reduction by
iron(0) tetraphenylporphyrins. Both the catalytic currents
and the
life time of the catalyst increase without significant formation of
hydrogen. Unprecedented values of the turnover
numbers per hour can thus be reached. Carbon monoxide is the main
product, while formic acid is formed to a
lesser extent. The yield of formic acid counterintuitively
decreases as the acidity of the acid synergist increases,
becoming negligible with CF3CH2OH.
Analysis of the reaction kinetics suggests that the action of the
acid synergist
is to stabilize the initial
FeIICO2
2-
carbenoid complex by hydrogen bonding. The formation of a doubly
hydrogen-bonded complex opens the route to the cleavage of one of the two C−O
bonds resulting in the formation of CO
within the iron coordination sphere. The formation of formic acid
involves a reaction pathway where the
iron−CO2
interactions are weaker. The effect of the acid synergist is an
example of electrophilic assistance in a two-electron
push−pull mechanism where pulling the electron pair out of the
substrate by means of the synergist is as important
as pushing electrons from the catalyst into the substrate. With
CF3CH2OH, the production of CO is so
fast that it
commences to inhibit the catalytic reaction. This self-inhibition
phenomenon can be satisfactorily modeled under
the assumption that product adsorption on the electrode surface obeys a
Langmuir equilibrium and that the covered
portions of the surface are totally inactive toward reduction of the
catalyst.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.