CO2 reduction with Re(i)-NHC compounds: driving selective catalysis with a silicon nanowire photoelectrode

Article abstract of DOI:10.1039/c6cc08240h

The CO₂-reduction activity of two Re(i)-NHC complexes is investigated employing a silicon nanowire photoelectrode to drive catalysis. Photovoltages greater than 440 mV are observed along with excellent selectivity towards CO over H₂ formation. The observed selectivity towards CO production correlates with strong adsorption of the catalysts on the photoelectrode surface.

Full text of DOI:10.1039/c6cc08240h

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CO2 reduction with Re(I)–NHC compounds:
driving selective catalysis with a silicon nanowire
Cite this:DOI: 10.1039/c6cc08240h
photoelectrode†
Tong Jin,‡a Da He,‡b Wei Li,b Charles J. Stanton III,c Sebastian A. Pantovich,a
George F. Majetich,c Henry F. Schaefer III,d Jay Agarwal,d Dunwei Wang*b and
Gonghu Li*a
Received 13th October 2016,
Accepted 17th November 2016
DOI: 10.1039/c6cc08240h
www.rsc.org/chemcomm
The CO2-reduction activity of two Re(I)–NHC complexes is investi-
gated employing a silicon nanowire photoelectrode to drive catalysis.
Photovoltages greater than 440 mV are observed along with excellent
selectivity towards CO over H2 formation. The observed selectivity
towards CO production correlates with strong adsorption of the
catalysts on the photoelectrode surface.
Fig. 1 Molecular structures of the Re(I)–NHC compounds under study:
ReCl(N-methyl-N0-2-pyridylbenzimidazol-2-ylidine)(CO)3 (1) and ReCl(N-
methyl-N0-2-pyrimidylbenzimidazol-2-ylidine)(CO)3 (2).
Many transition metal complexes have exhibited catalytic activity
for multi-electron CO2 reduction.1–7 Among these, diimine-
tricarbonyl rhenium(I) complexes, including fac-ReCl(bpy)(CO)3
where bpy = 2,20-bipyridine, have been extensively investigated in co-workers,30 NHC–pyridyl and NHC–pyrimidyl ligands were
both photocatalytic and electrocatalytic applications.8–20 Several employed, using a benzimidazole-based NHC. CO was observed
recent examples of Re(I) complexes have used N-heterocyclic as the primary electrolysis product (Faradaic efficiency Z 60%),
carbene (NHC)–aryl frameworks as redox-active ligands,21–25 along with production of H2 and HCOOH.
but few have been examined for catalysis.26–30 In this work,
In this present report, we compare the performance of the latter
we show that coupling with silicon nanowire (SiNW) photo- two Re(I)–NHC compounds (1 and 2 in Fig. 1) under photoelec-
electrodes significantly improves the selectivity of Re(I)–NHC trochemical conditions. We demonstrate that utilizing SiNWs
complexes in CO2 reduction.
significantly improves selectivity towards CO over H2 formation
Recent reports by Delcamp and co-workers28,29 and Agarwal during solar-assisted CO2 reduction, providing evidence that
and co-workers30 are among the few examples for CO2 reduction metal–NHC molecular catalysts may be successfully combined
using Re(I)–NHC compounds. In one of the studies by Delcamp with this nanostructured material. The photoelectrode surface
and co-workers,28 the efficacy of Re(I) complexes containing a after catalysis are examined with diffuse reflectance infrared
NHC–pyridyl motif was recorded under irradiation with and Fourier transform spectroscopy (DRIFTS) in order to explore
without a molecular Ir(III) photosensitizer. The authors noted interactions between the catalysts and SiNWs.
that the addition of an electron-deficient substituent on the NHC
The synthesis, characterization, and electrocatalytic efficiency
moiety improved photocatalytic performance.28 Conversely, of compounds 1 and 2 have been previously reported.30 As further
electron-deficient substituents on bpy often reduce the activity confirmation, cyclic voltammograms were recorded for each
of ReCl(bpy)(CO)3-type catalysts.31,32 In the study by Agarwal and Re(I)–NHC compound in acetonitrile with tetrabutylammonium
hexafluorophosphate (TBAPF6) as the supporting electrolyte. Aligned
with prior work, two irreversible reduction waves were observed
a Department of Chemistry, University of New Hampshire, Durham, NH 03824, USA.
under Ar saturation for each compound at 100 mV sꢀ1, with peak
E-mail: gonghu.li@unh.edu
b Department of Chemistry, Boston College, Chestnut Hill, MA 02467, USA.
currents at ꢀ2.06 V and ꢀ2.45 V for 1 (Fig. 2, potentials are versus
E-mail: dunwei.wang@bc.edu
Fc+/Fc), and at ꢀ1.91 and ꢀ2.30 V for 2 (see Fig. S1 in the ESI†).
Under an atmosphere of CO2, current enhancement was observed at
both the first and second reduction waves. The current enhance-
ment for 2 (Fig. S1, ESI†) is less significant than for 1 (Fig. 2).
Given the promising electrocatalytic activity exhibited by 1 and 2,
we sought to examine both compounds for photoelectrochemical
c Department of Chemistry, University of Georgia, Athens, GA 30602, USA
d Center for Computational Quantum Chemistry, University of Georgia, Athens,
GA 30602, USA
† Electronic supplementary information (ESI) available: Experimental proce-
dures, supplementary Table S1 and Fig. S1–S8. See DOI: 10.1039/c6cc08240h
‡ These authors contributed equally to this work.
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On the preparative scale at the potential of the first
reduction, 1 and 2 displayed modest Faradaic efficiencies for
CO production (FECO = 57% and 53%, respectively) in photo-
electrolysis on SiNWs (Table S1, ESI†). Likely, residual water in
the acetonitrile used in our study served as the proton source
for CO2 reduction. Complex 2 demonstrated excellent stability
over four hours, while the photocurrent decayed gradually over
time when 1 was used as the CO2-reduction catalyst (Fig. S3,
ESI†). This is likely due to the relatively low photostability of 1.
In recent studies by Vaughan and co-workers, photochemical
dissociation of CO was observed for a Re(I) NHC–pyridyl complex
(similar to 1) but not for a Re(I) NHC–pyrimidyl complex (similar
to 2).22,23 Nevertheless, more CO was produced by 1 in photo-
electrolysis than by 2 (Fig. S4(c and d), ESI†), which is consistent
with the more significant photocurrent enhancement observed
for 1 at the first reduction potential (Fig. 3 and Fig. S2, ESI†).
The differing photochemical stability of 1 and 2 was further
probed with surface analysis after photoelectrolysis. Specifi-
cally, the SiNWs were thoroughly washed with acetonitrile and
dried at room temperature after being used for catalysis. Then,
infrared spectra of the used SiNWs were collected on a DRIFTS
spectrometer (Fig. 4). Prominent absorptions corresponding to
surface-adsorbed Re(I) complexes are seen in the C–O stretching
region of the collected spectra. For 2, these transitions appear at
2021 and 1918 cmꢀ1 (Fig. 4b), and resemble the features of fac-
ReCl(bpy)(CO)3 grafted on silica surfaces found in our previous
work.49,50 Moreover, the similarity of these bands to peaks of 2
collected from infrared spectroelectrochemistry (IR-SEC) at
Fig. 2 Cyclic voltammograms of 1 under (a) Ar and (b) CO2 atmospheres
on a glassy carbon electrode in acetonitrile.
CO2 reduction, since harnessing sunlight to drive CO2 conversion
is a compelling solution to utilize CO2. In particular, the photo-
electrochemical approach eliminates the need for sacrificial
donors by combining well-defined molecular catalysts with a
photoactive electrode surface.33,34 Several molecular catalysts have
been coupled with photoelectrodes for solar fuel production via
CO2 reduction.35–46 We employ SiNW photoelectrodes, which
consist of vertically aligned arrays of SiNWs,46 as the electron
source for Re(I)-catalyzed CO2 reduction in this research. The
nanostructured morphology of SiNWs has been reported to be
well-suited for high-efficiency solar energy applications.47,48
Fig. 3 shows cyclic voltammograms of 1 recorded on a SiNW
photoelectrode in acetonitrile under light irradiation provided by
a Xe lamp. Under an atmosphere of Ar, the first reduction wave
of 1 displays a peak current at ꢀ1.58 V (Fig. 3a), representing a
photovoltage of 480 mV. A catalytic photocurrent is observed under
CO2 saturation (Fig. 3b), indicating that CO2 reduction persists
under photoelectrochemical conditions. The first reduction wave
for 2 appears at ꢀ1.47 V, yielding a slightly lower photovoltage
of 440 mV (Fig. S2, trace a, ESI†). A photocurrent enhancement
was also observed under CO2 (Fig. S2, trace b, ESI†), although
less pronounced than for 1, which matches the results under
electrocatalytic conditions.30
resting potential (2022, 1926, and 1899 cmꢀ1 30
would suggest
)
that the tricarbonyl moiety is retained after photoelectrolysis,
assuming coalescence of the 1926 and 1899 cmꢀ1 peaks in the
DRIFTS spectrum. In the infrared spectrum of SiNWs used with 1,
two carbonyl bands are present at 2011 and 1879 cmꢀ1 (Fig. 4a).
Compared to previous IR-SEC data, these transitions are red-
shifted with respect to the starting complex (2020, 1923, and
1894 cmꢀ1),30 which is consistent with structural changes
during photoelectrolysis. This presumed degradation aligns with
the aforementioned literature22 regarding the photostability
of Re(I)–NHC complexes. From these data, we can conclude
that 1 and 2 are strongly adsorbed to the SiNW surface, and that
Fig. 3 Cyclic voltammograms of 1 under (a) Ar and (b) CO2 atmospheres
on a SiNW photoelectrode in acetonitrile under light irradiation. The cyclic
voltammogram under Ar on a glassy carbon electrode is also plotted for
comparison (dotted trace, same as Fig. 2a).
Fig. 4 DRIFTS spectra of SiNWs used in photoelectrochemical CO2
reduction using (a) 1 and (b) 2 in acetonitrile for 4 h.
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(Table S1, ESI†). Using 2 as a catalyst led to a slightly higher
turnover number for CO production than using 1, but significant
H2 formation (H2/CO B 0.5) was observed for both catalysts.
Therefore, the SiNW photoelectrode appears to be a better energy
2+
source for the Re(I)–NHC catalysts than the Ru(dmb)3 photo-
sensitizer to selectively drive CO2 reduction over proton reduction.
We expect that CO2 reduction using the Re(I) catalysts on SiNWs
could be further improved by employing a two-compartment cell
which separates the SiNW photoelectrode from the counter
electrode, constantly stirring the reaction solution, and carrying
out photoelectrolysis for a longer period of time.
In summary, the presence of strongly adsorbed Re(I)–
NHC complexes on the SiNW photoelectrode surface is clearly
associated with selective CO2-reduction catalysis. We expect
that both the efficiency and selectivity in solar fuel generation
could be enhanced by tuning the interactions between mole-
cular catalysts and photoelectrode surfaces. Further studies are
Fig. 5 DRIFTS spectra of SiNWs used in photoelectrochemical CO2
reduction using (a) 1 and (b) 2 in an acetonitrile solution containing 5%
H2O (by volume).
1 undergoes significant structural changes under photoelectro- underway to test this hypothesis.
chemical conditions.
This work was supported by the U.S. National Science
The introduction of Brønsted acids into the electrolyte is a Foundation under grants CHE-1352437 (G. L.), DMR-1055762
popular technique to improve catalyst turnover.51 In our study, (D. W.), and CHE-1361178 (H. F. S. and J. A.). The work done at
the presence of 5% H2O (by volume) significantly improved the University of New Hampshire was partially supported by
CO2-to-CO conversion using 1 and 2 on SiNWs (Fig. S4(a and b), the James D. Morrison Early Career Award to G. L. G. F. M.
ESI†), and yielded a new photocurrent for proton reduction acknowledges the Donors of the American Chemical Society
(Fig. S5 and S6, ESI†). Consequently, the formation of H2 was Petroleum Research Fund for partial support (54228-ND1). We
confirmed with product analysis during photoelectrolysis thank Dr Chao Liu and Dr Pengcheng Dai for their assistance in
(Table S1 and Fig. S7, ESI†). For 1, the presence of 5% H2O experiments.
resulted in a lower Faradaic efficiency for CO production
(FECO = 20%) along with significant H2 formation (FEH = 54%).
Surface analysis of SiNWs after photoelectrolysis showed a negli-
gible amount of surface-adsorbed 1 (Fig. 5a). Control experiments
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