Inorganic Chemistry
Article
Formate production increased steadily for the three
successive runs in Figure 8, and as such a saturation
concentration for this product is excluded but CO saturation
seems to limit both catalytic cycles. Another limitation is
evidently present because of the limited production of about 50
μmol per run. This may also be due to the increasing
concentration of BNA2 in the reactor. Alternatively, the limited
response may be a measure of solution pH, as 50 μmol of
formate corresponds to a 3 mM concentration in the water-
DMF mixture (though solvent expansion dilutes this some-
what), with the additional decrease in pH corresponding to the
dissolution of CO2 to carbonic acid. This would further explain
the decrease in formate production at higher CO2 concen-
trations, as undoubtedly the solution will have a lower pH. The
solution pH has been determined to be highly influential on
product formation in this catalytic system by earlier work,23 and
therefore must not be discounted from the explanations.
ASSOCIATED CONTENT
* Supporting Information
Further information on the high pressure setup, ion
chromatography procedure, choice of catalyst, electron donor
and proton carrier, and mathematical kinetic model of the
electron donor quenching mechanism. This material is available
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S
AUTHOR INFORMATION
Corresponding Author
Phone: +41 (0)21 693 31 51.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
P.V. would like to thank the Swiss National Science Foundation
(SNSF) for financial support. We also acknowledge financial
support from Poleswiss PSPB-035/2010.
CONCLUSIONS
■
The effects of increased CO2 pressure and imparting super-
critical conditions on the catalytic reaction of the well
characterized CO2 reduction catalyst [Ru(bpy)2(CO)(H)]+
have been presented. CO2 reduction was comprehensively
optimized with respect to proton carrier, cosolvent, and CO2
concentration/pressure, providing further insight into the
catalytic cycles, the relationship between catalyst, photo-
sensitizer, and electron donor, and demonstrating the
advantages of vastly increasing CO2 concentration on the
catalytic system.
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The two products identified in the analysis were carbon
monoxide and formate. The former was found to have a linear
relationship with pressure over a 10−150 bar range, such that
production was consistently doubled. Formate, however, was
independent of CO2 concentration in water/DMF solutions,
and decreased 25% in a TEOA/DMF medium between 10 and
150 bar. Increasing CO2 concentration therefore favors the CO
catalytic cycle and is rate limiting for the reaction, whereas
formate is independent of CO2 concentration because of
insertion later in the catalytic cycle. It was found that the
greatest CO yields were obtained from a biphasic water/DMF
supercritical CO2 system rather than a single phase of all
components. This was suggested to be due to the extraction of
CO from the liquid water phase where the catalyst was present
to the upper condensed/supercritical CO2 phase, thus
preventing the catalytic cycle from being poisoned. Nonethe-
less, a saturation concentration of about 200 μmol was
identified in subsequent experiments at optimum conditions.
TONs and TOFs for the optimized systems were
considerably greater than previously reported in the literatur-
e.5b,7b,8b,18 For a system irradiated for 2 h with 2 μmol of
catalyst, the TON was 120 with a TOFini of 174 h−1, but this
reached 1120 h−1 and 1600 h−1 when the catalyst was used at
its limit of 0.1 μmol. The rapid turnover and overall
productivity of the systems compared to the literature
demonstrates the enhancement offered by operating the
reaction at high-pressure over the range of 10−150 bar, and
the potential for obtaining much more product in a short time
frame. Work is underway to further elucidate the catalytic cycles
by electrochemistry and coupled mass spectrometry.
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