Photochemical CO2 splitting by metal-to-metal charge-transfer excitation in mesoporous ZrCu(l)-MCM-41 silicate sieve

Article abstract of DOI:10.1021/ja040162l

Binuclear redox sites consisting of a Zr oxo-bridged to a Cu(I) center have been obtained on the pore surface of MCM-41 silicate sieve by a stepwise grafting procedure, along with isolated metal centers. The bimetallic site features a Zr(IV)-O-Cu(I) to Zr(III)-O-Cu(II) metal-to-metal charge-transfer (MMCT) absorption extending from the UV region to about 500 nm. The Zr-O-Cu linkage is revealed by a Cu(I)-O infrared stretch mode at 643 cm^-1. Irradiation of the MMCT chromophore of ZrCu(I)-MCM-41 loaded with 1 atm of CO₂ gas at room temperature resulted in growth of CO (2150 cm^-1) and H₂O (1600 cm^-1). Photolysis experiments using ¹³CO₂ and C¹⁸O₂ demonstrate that carbon monoxide and the oxygen atom of the water product originate from CO₂. This indicates splitting of the CO₂ by the excited MMCT moiety to CO and a surface OH radical, followed by trapping of the products at Cu(I) centers (OH is reduced to H₂O). This is the first observation of CO₂ photoreduction at a binuclear MMCT site at the gas-solid interface. Copyright

Full text of DOI:10.1021/ja040162l

Published on Web 01/20/2005
Photochemical CO2 Splitting by Metal-to-Metal Charge-Transfer Excitation in
Mesoporous ZrCu(I)-MCM-41 Silicate Sieve
Wenyong Lin and Heinz Frei*
Physical Biosciences DiVision, Mailstop CalVin Laboratory, Lawrence Berkeley National Laboratory,
UniVersity of California, Berkeley, California 94720
Received June 28, 2004; E-mail: HMFrei@lbl.gov
The photoreduction of CO2 by H2O is one of the key chemical
reactions for the generation of renewable fuels using light as an
energy source. It was first reported in heterogeneous systems using
excitation of large band gap semiconductor materials such as TiO2,
SrTiO3, ZnO, or SiC with UV photons.1,2 Methanol and methane
were detected upon prolonged irradiation along with varying
amounts of CO, formic acid, and O2 (recovery of the latter was
met with mixed success).3,4 Use of smaller band gap particles such
as ZnSe, CdS, CdSe, etc. allowed photoreduction of CO2 with
visible instead of UV light.5 However, organic donors or other
sacrificial reagents were required.6-8 Because of the need for
sacrificial reagents, a concept for visible light-induced CO2 reduc-
tion by H2O based on these results has thus far remained elusive.
Ligand-to-metal charge-transfer (LMCT) excitation of isolated
Ti centers of framework-substituted micro- or mesoporous silicates
by UV light was recently found by Anpo and co-workers to reduce
CO2 by H2O at substantially better efficiency than dense-phase TiO2
particles.9 Prolonged photolysis gave CH3OH and CH4 as the main
products. We have shown by in situ FT-IR and mass spectrometric
monitoring of the reaction (using Ti-substituted MCM-41 sieve)
that CO and O2 are the initial single-photon redox products.10 Use
of isolated tetrahedral Ti centers on nanoporous support is a
Figure 1. (a) UV-vis DRS of ZrCu(I)-MCM-41 and Cu(I)-MCM-41. (b)
promising development for CO2 photoreduction by H2O, but no
results have been reported so far that would allow us to utilize
visible instead of UV photons for accomplishing the reaction at
such metal centers. We have recently presented spectroscopic
evidence that Ti centers can be activated by visible light when they
are part of an oxo-bridged metal-to-metal charge-transfer (MMCT)
moiety covalently anchored on the pore surface of MCM-41 silicate
sieve (Ti-O-M, M ) Cu(I) or Sn(II)).11 In this approach, the
center M acts as electron donor. Here, we report the first visible
light-absorbing MMCT redox site that is capable of splitting CO2
to CO. The site consists of Zr(IV) oxo-bridged to Cu(I), covalently
anchored on the pore surface of MCM-41 sieve.
Assembly of covalently anchored ZrCu(I) moieties on the MCM-
41 pore surface was accomplished by first grafting tetrahedral Zr
centers using the established ZrCp2Cl2 precursor method.12-14
Subsequent removal of the remaining organic ligand by calcination
is known to result in tripodally anchored ZrOH groups (Si/Zr )
24 per ICP-AES analysis).12,15 This was followed by addition of
0.2 g of Zr-MCM-41 crystallites so produced to a solution of
CuI(NCCH3)4PF6 in 100 mL of CH2Cl2 (0.3 wt %) under a nitrogen
atmosphere. The mixture was stirred at 40 °C for 30 min, filtered,
and washed (Zr/Cu ) 1.3). Monometallic Cu(I)-MCM-41 was
prepared by the same procedure using the neat silicate as starting
material (Si/Cu ) 64). As-synthesized product was heated to 300
°C for 12 h under vacuum in order to remove residual CH3CN
ligands, which was verified by FT-IR spectroscopy.11 The diffuse
reflectance spectrum (DRS) of a pressed wafer of ZrCu(I)-MCM-
41 shows an optical absorption extending into the visible region,
as can be seen in Figure 1a. This new chromophore is more clearly
Difference DRS spectra recorded after and before exposure of ZrCu(I)-
MCM-41 to O2 gas (1 atm) at room temperature for 8 h. (c1) FT-IR spectrum
of ZrCu(I)-MCM-41 (referenced against neat MCM-41). (c2) Difference
FT-IR upon exposure to 1 atm of O2 gas for 24 h in the dark. (d) Difference
FT-IR spectra after photolysis (355 nm, 165 mW cm-1, 2.5 h) of the CO2-
loaded (1 atm) ZrCu(I)-MCM-41.
revealed by optical difference spectroscopy following exposure of
the ZrCu(I) sieve to 1 atm of O2 gas at room temperature, shown
in Figure 1b; the rise of the Cu(II) d-d band at 800 nm16 is
accompanied by absorbance loss in the 300-500 nm region. Since
neither Cu(I) nor Zr (IV) possesses ligand field transitions or a
LMCT band in this spectral region, the absorbance loss can only
originate from a MMCT transition Zr(IV)/Cu(I) f Zr (III)/Cu(II).
No such absorbance loss in the 300-500 nm region was detected
when monometallic Cu(I)-MCM-41 was exposed to an O2 atmo-
sphere; only the growth of the Cu(II) band was observed in that
case.13
The key information on the structure of the bimetallic site was
furnished by the low-frequency metal-oxygen stretching modes.
The FT-IR spectrum features a broad absorption at 930 cm-1 typical
for Si-O stretching vibrations of SiO bonds adjacent to a metal
center.11 More interestingly, a Cu(I)-O stretch absorption is
observed around 650 cm-1 (Figure 1c). When the Cu(I) precursor
was grafted onto a Zr-MCM-41 sample featuring Zr-18OH and
Si-18OH groups,17 the band showed a red shift of 30 cm-1
characteristic for a Cu-O mode. Upon exposure of the ZrCu(I)-
MCM-41 material to an O2 atmosphere, the ν(Cu(I)-O) absorption
decreased under concurrent growth of a band around 540 cm-1
(Figure 1c). Closer inspection of the depletion spectrum upon
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C O M M U N I C A T I O N S
TiMCM-41 sieve.10 This is the first observation of CO2 photore-
duction at a binuclear MMCT site at the gas-solid interface. If a
metal M can be identified that oxidizes H2O by visible light-induced
LMCT, thereby reducing the bimetallic moiety to its original state,
a Z-scheme for CO2 reduction by H2O under visible light can be
envisioned.
Acknowledgment. This work was supported by the Director,
Office of Science, Office of Basic Energy Sciences, Division of
Chemical, Geological and Biosciences of the U.S. Department of
Energy under Contract No. DE-AC03-76SF00098.
Supporting Information Available: UV-vis, FT-Raman, and
FT-IR spectra. This material is available free of charge via the Internet
at http://pubs.acs.org.
Figure 2. FT-IR spectra recorded before and after irradiation of ZrCu(I)-
MCM-41 loaded with 1 atm of CO2, 13CO2, or C18O2 at 355 nm (110 mW
cm-2) for 2.5 h.
References
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cm-1 (Figure 1d). Since MMCT-induced photolysis results only in
the oxidation of Cu(I) centers that are part of an MMCT site, this
indicates that the 643 cm-1 component of the ν(Cu(I)-O) profile
originates from a Cu(I)-O-Zr bridge. A similar Cu(I)-O band
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(13) Spectra shown in Supporting Information.
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period needed (5 h) resulted in 18O exchange of silanol groups as well, as
indicated by the Si-18OH stretch at 3735 cm-1
.
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characteristic absorptions of particles or clusters of Cu2O (190, 154 cm-1),
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sieve is engaged in Cu2O cluster formation.
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the pores until trapped and reduced to H2O at an isolated Cu(I)
site. Hence, the proposed mechanism is CO2 splitting at the excited
MMCT sites, consistent with the reaction path of single UV photon-
induced CO2 reduction by H2O to CO and O2 in framework
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