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TRANSIENT SPECTROSCOPIC INVESTIGATIONS
OF INTERMEDIATES INVOLVED IN CO2
REDUCTION UNDER SUPERCRITICAL CO2
CONDITIONS
David C. Grills . * Hajime Kawanami, Z * Takayuki Ishizaka,2 Maya
Chatterjeel
1Chemistry Department, Brookhaven National Laboratory, Upton,
NY 11973-5000, USA
dcgrills@bnl.gov
2Research Center for Compact Chemical System, National Institute
of Advanced Science and Technology, 4-2-1 Nigatake, Miyagino,
Sendai 983-8551, Japan
h-kawanami aist go.jp
Introduction
Due to increasing concerns about global warming caused by the
burning of fossil fuels, the conversion of abundant molecules such as
CO2 and H20 into clean fuels and industrially important chemicals is
a key scientific challenge of this century. The use of sunlight to drive
these reactions in so-called artificial photosynthetic processes would
provide a sustainable pathway to carbon-neutral fuels due to the
abundance of available solar energy. Since CO2 is the end product of
combustion and therefore a very stable molecule, its conversion into
higher energy forms requires the use of catalysts. One family of
catalysts that has been shown to selectively reduce CO2 to CO (an
industrially important fuel precursor and intermediate) upon visible
light irradiation in the presence of a sacrificial electron donor has the
general formula, fac-[ReX(bpy')(CO)3]"+ (X = halide, phosphite, etc.;
bpy' = 2,2'-bipyridine and its derivatives; n = 0 or 1).1 Quantum
yields for CO production as high as 0.59 have been reported.2
However, the catalytic reactions are very slow with typical turnover
frequencies (TOFs) on the order of 1-12 h-'. In addition, the
formation of side products leads to rapid deactivation of the catalyst,
with typical turnover numbers (TONs) of only ~10-25 for the
mononuclear catalysts.
We have recently demonstrated a significant enhancement in
catalytic activity through the use of a high concentration of CO2
(15.5 M) in the form of supercritical CO2 (scCO2), mixed with N,N-
dimethylformamide (DMF) in a single-phase, high-pressure reaction
environment.3 We also found that the catalytic activity increases
linearly with [DMF]. Using fac-ReCl(bpy)(CO)3 as the catalyst, this
resulted in an optimized TON of 62 at 17.8 MPa CO2 / 60 C /
[DMF] = 6.3 M, and an initial TOF of 56 h-1, which is a significant
improvement over previous ambient pressure studies.
In order to more fully understand the enhanced catalytic activity
under scCO2 conditions, and the role of DMF in the reaction
mechanism, we have started using a combination of nano- to
millisecond UV-vis transient absorption and time-resolved infrared
(TRIR) spectroscopy to probe the individual reaction steps in the
catalytic cycle following light absorption. Such investigations are
cmcial since they will aid the design of new catalysts for use with
scCO2 with enhanced stability and activity. Preliminary results of
these transient spectroscopic investigations are presented here.
Results and Discussion
Fig. 1 shows a series of TRIR spectra recorded with step-scan
FTIR spectroscopy after 355 nm excitation offac-ReCl(bpy)(CO)3 in
DMF in the presence of 1 M triethylamine (TEA). Immediately after
excitation, two new v(CO) bands (labeled as MLCT) are observed,
corresponding to the metal-to-ligand charge transfer (MLCT) excited
state of the complex. These rapidly decay into three new bands at
1995, 1881 and 1862 cm1 (labeled as OER), which correspond to the
one-electron reduced complex, fac-[ReCl(bpy)(CO)3]'-. This is the
first step of the catalytic cycle for photocatalytic CO2 reduction with
this catalyst. The second step involves the ejection of the Cl- ligand
from the OER to generate a solvated species,
fac-Re(bpy)(CO)3(DMF).
S-
<
2100 2050 2000 1950
Wavenumbers (cm")
1900 1850
Fig. 1. (a) FTIR offac-ReCl(bpy)(CO)3 in DMF in the presence of 1
M TEA. (b) Time-resolved step-scan FTIR spectra obtained in the
first 50 ns after 355 nm laser flash photolysis of this solution.
We have used TRIR spectroscopy to monitor the kinetics of the
ejection of Cl- from the OER in DMF under both ambient pressure
conditions and scCO2 conditions. Fig. 2 shows quantum-cascade
laser TRIR kinetic traces recorded at 1863 cm1, corresponding to the
OER. The decay of these traces represents the Cl- ejection process
from the OER. These is a striking difference between the kinetic
trace recorded under ambient pressure (0.1 MPa N2) and those
recorded under scCO2 conditions (p = 8.9 - 30.5 MPa, T = 40 C). At
ambient pressure, the observed rate of decay is 0.15 s-, whereas
under high-pressure conditions the decay rate is much higher and is
observed to increase slightly with pressure (9.85 s-' @ 8.9 MPa to
16.2 s-i @ 30.5 MPa).
N
E
0
0
'C
0.00-
0.0
0.1 0.2
Time (s)
0.3 0.4
Fig. 2. Quantum-cascade laser TRIR kinetic traces recorded at 1863
cm after 355 nm excitation of fac-ReCl(bpy)(CO)3 in DMF in the
presence of 1 M TEA. The black trace was recorded under ambient
pressure of N2, which the colored traces were recorded in the
presence of high-pressure scCO2 at the indicated pressures.
Experimental
The apparatus for time-resolved step-scan FTIR spectroscopy
has been described previously.4 Quantum-cascade laser TRIR kinetic
traces were measured using a CW mode-hop-free external-cavity
(b) At=0-50 ns
OER MLCT OER
MLCT 1
(a) FTIR
- 0.1 MPa NZ 40 *C
89 MPa
- 18.5 MPa
+- 29.0 MPa
-30.5 MPa
