Binuclear ZrOCo Metal-to-Metal Charge-Transfer Unit in Mesoporous

Article pubs.acs.org/JPCC

Binuclear ZrOCo Metal-to-Metal Charge-Transfer Unit in Mesoporous Silica for Light-Driven CO2 Reduction to CO and Formate Marisa L. Macnaughtan, Han Sen Soo, and Heinz Frei* Physical Biosciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, California 94720, United States S Supporting Information *

ABSTRACT: Oxo-bridged heterobinuclear units of the type Zr(IV)OCo(II) covalently anchored on the pore surface of mesoporous silica SBA-15 have been synthesized with high selectivity. The unit exhibits a visible light absorbing metal-to-metal charge-transfer absorption (MMCT) extending to about 550 nm. The oxo-bridged structure of the binuclear moiety is manifested by spectral blue-shifts of the optical Co(II) spin−orbit bands due to reduced π-electron donating ability of the bridging oxygen caused by the electron-withdrawing Zr center. EXAFS measurements of the Zr and Co K-edges and curve fitting analysis revealed a Zr to Co distance of 3.4 Å. The coordination geometry of the Zr and Co metal centers in monometallic Zr and Co-SBA-15 samples is closely preserved in the ZrOCo unit. Illumination of the MMCT absorption at 420 nm and shorter wavelengths resulted in the reduction of CO2 to gas phase CO and HCO2−, the latter adsorbed on the silica pore surface. The branching between carbon monoxide and formate was found to be determined by the fate of the sacrificial donor (triethyl- or diethylamine), namely proton transfer versus H atom transfer to CO2 interacting with the transient Zr(III) center. The ZrOCo(II) unit on a silica surface constitutes the first example of an all-inorganic heterobinuclear unit for the photoinduced splitting of CO2 to free CO. Moreover, transient Co(III) formed upon MMCT excitation should possess sufficient oxidation potential for driving a catalyst for water oxidation, thereby opening up opportunities for replacing the sacrificial donor by water as electron source.

1. INTRODUCTION The reduction of carbon dioxide to a fuel using sunlight as the energy source is a formidable yet important challenge of renewable energy generation. Taking the approach of artificial photosynthesis in a single integrated system, a major scientific gap to bridge is the durability of the components. We are addressing this challenge by utilizing all-inorganic heterogeneous materials. Early work on photochemical CO2 reduction using inorganic photocatalysts focused on UV-driven conversion to methanol, methane, and varying amounts of CO or formic acid at TiO2 powders using water as electron source.1,2 While the efficiency remains low, the reaction using TiO2-based materials is viewed as of fundamental importance and continues to attract interest by exploiting doping with metal and nonmetal components,3,4 surface modification by quantum dots or plasmonic particles,5 and nanostructuring of the material.6−8 Recent mechanistic studies on TiO2 nanoparticles by EPR spectroscopic have been particularly insightful.9 Other large bandgap semiconductor materials capable of activating CO2 under UV light are SrTiO3, ZnO, and SiC.1 Carbon dioxide reduction with visible light is achieved with smaller bandgap semiconductor particles, which in early work involved materials such as ZnSe, CdS, CdSe, and more recently metal phosphides.7 Since the valence band edge potential is not sufficiently positive for oxidizing water molecules in these cases, organic donors or other sacrificial reagents are required. © 2014 American Chemical Society

Use of discrete transition metal centers anchored on an insulating surface like silica as charge transfer chromophore offers tight control as well as tunability of the redox potential for driving CO2 reduction. Excitation of the ligand-to-metal charge-transfer transition of isolated Ti centers in mesoporous silica frameworks afford CO2 activation under UV light, as demonstrated by Anpo10,11 and studied mechanistically by insitu FT-IR spectroscopy and mass spectrometric monitoring in our laboratory.12,13 By contrast, oxo-bridged heterobinuclear units such as ZrOCu(I) covalently anchored on silica surfaces afford CO2 reduction to CO by irradiation of a metal-to-metal charge-transfer (MMCT) chromophore, which typically extends deep into the visible spectral range.14 Heterobinuclear MMCT systems are attractive because the nearly free choice of transition metal, particularly of the donor center, allows us to match the redox potential of the chromophore with that of the catalyst. This ensures thermodynamic efficiency of the photon to chemical energy conversion process. To date, over a dozen different heterobinuclear units have been developed with photocatalytic activity demonstrated for most of them, including visible light driven water oxidation.15−25 However, ZrOCu(I) is thus far the only photocatalytic unit that affords Received: February 11, 2014 Revised: March 26, 2014 Published: March 26, 2014 7874

dx.doi.org/10.1021/jp5014994 | J. Phys. Chem. C 2014, 118, 7874−7885

The Journal of Physical Chemistry C

Article

model FRA-106/S module equipped with a Nd:YAG laser source and a liquid nitrogen cooled Ge detector. For 355 nm photolysis, a Nd:YAG laser (Continuum model Surelite III) was used. Photolysis at 420 nm was conducted with a Nd:YAG laser pumped tunable optical parametric oscillator (Continuum model Surelite II and Surelite OPO Plus). The laser pulse width was 8 ns with pulse energy in the range 20−40 mJ/cm2 at a repetition rate of 10 Hz. Powder X-ray diffraction (PXRD) measurements were taken on a Siemens model D500 Cu Kα. Inductively coupled plasma (ICP) analysis was run by Exova Inc., Santa Fe Springs, CA (Selected Metals by SOP 7040, Rev 10). For preparation of samples for ICP measurements, 0.05 g sample portions were mixed with 1.5 mL of hydrofluoric acid and 1.5 mL of nitric acid and then heated in closed vessels at 105 °C for 1 h. After cooling, 1 mL of 30% hydrogen peroxide was added and heating continued for 30 min, which dissolved the material completely. An internal standard solution was added upon cooling to RT, and dilution was made to 50 g. X-ray absorption spectroscopy (XAS) was performed at Beamline 7-3 (Zr and Co K-edge) at Stanford Synchrotron Radiation Lightsource (SSRL). The synchrotron ring SPEAR operates at 3.0 GeV with a beam current in the range of 100− 200 mA. The beamline is equipped with a Si(220) double crystal monochromator. The intensity of the incident X-rays was monitored by a N2-filled ion chamber (I0) in front of the sample. The samples were kept at 9 ± 1 K in a He atmosphere at ambient pressure using an Oxford CF-1208 continuous-flow liquid He cryostat. Data were recorded as fluorescence excitation spectra using a germanium 27-element energyresolving detector (Canberra Electronics). Energy was calibrated by the rising edge position of Co foil (7709.5 eV) for Co XAS, which was placed between two N2-filled ionization chambers (I1 and I2) after the sample.16 For Zr K-edge XAS, Zr foil (or ZrO2) was used with E0 = 17 998 eV corresponding to the first peak of the first derivative on the Zr edge.27,28 Data processing and analysis were carried out as reported before using SixPACK version 0.67 and IFEFFIT version 1.2.10a EXAFS analysis software package25 and are described in the Supporting Information. 2.3. Synthesis. 2.3.1. SBA-15. The material was synthesized based on literature reports.29−31 Briefly, pluronic P-123 [poly(ethylene glycol)−poly(propylene glycol)−poly(ethylene glycol)] (16.0 g) was dissolved in 600 mL of distilled water and 32 mL of concentrated hydrochloric acid in a 1 L Nalgene beaker. The solution was heated to 40 °C, and 35 g of tetraethyl orthosilicate was added while stirring. The solution was kept at this temperature and stirred for 24 h, after which it was split into three tightly capped 250 mL Nalgene bottles that were placed in an oven at 95 °C for 2 days. Solids were filtered and washed with 2 × 250 mL distilled water. The SBA-15 samples were calcined in air at 550 °C for 12 h; ramp rate was 2.5 °C/min. A small airflow into the oven was used to promote combustion of the template. Small-angle XRD, shown in Figure S1a (trace 1), confirmed the Bragg peaks characteristic for the mesoporous channel structure of SBA-15 materials.29−31 2.3.2. Zr-SBA-15 (1 mol %). The preparation of Zr-SBA-15 followed the same literature procedure as the preparation of ZrMCM-41.14,32,33 Calcined SBA-15 (1.082 g, 18.01 mmol, 1.00 equiv) was dried in a Schlenk tube under vacuum (10 mTorr) at 180 °C overnight. In a dry N2-filled glovebox, SBA-15 was suspended in 180 mL of dry methylene chloride and stirred for 1 h. Zirconocene dichloride (51.0 mg, 0.174 mmol, 0.0097 equiv) was dissolved in 20 mL of dry methylene chloride. The

photoreduction of CO2. In this system, CO generated inside the silica pores cannot escape from the silica host because it is trapped inside the pores by strong binding to Cu(I) sites. Therefore, it is an urgent task to explore new heterobinuclear units featuring a donor metal center that does not prevent desorption of the product into the gas phase. Motivated by this goal, we have explored in this work ZrOCo(II) units to overcome the limitation posed by the Cu(I) donor center. Furthermore, the oxidized donor, Co(III) in this case, should have sufficient potential for driving a water oxidation catalyst, thereby opening up the possibility of a single heterobinuclear unit capable of closing the cycle of CO2 reduction by H2O.

2. EXPERIMENTAL SECTION 2.1. Materials. Reagents used were ZrCp2Cl2 (Strem), triethylamine, diethylamine (EMD), dibenzylamine, anhydrous methylene chloride, acetonitrile (Honeywell, Burdick & Jackson), anhydrous cobalt chloride (Fluka), carbon dioxide (Airgas), and 13C-carbon dioxide (Aldrich Isotope, 99%13C), C18O2 carbon dioxide (Isotech, 98%), formic acid, 13C-formic acid (Aldrich Isotope, 99%13C), N,N-diethylformamide, tetraethylorthosilicate, Pluronic P-123, concentrated hydrochloric acid, and acetaldehyde (Alfa Aesar). Unless otherwise noted, reagents were purchased from Aldrich. All reagents were used as received unless noted below. Solvents were dehydrated with molecular sieves (activated at 170 °C under vacuum for 24 h). Acetonitrile was purged with nitrogen gas for 20 min, placed in a dry nitrogen glovebox, and dried over 3A molecular sieves for at least 72 h before use. Triethylamine was treated the same way and dried over 3A molecular sieves for at least 24 h before use. Diethylamine for CO2 reduction experiments was placed in a Schlenk flask and frozen in liquid nitrogen, evacuated, and allowed to thaw. This process was repeated three times to completely degas the diethylamine. Anhydrous methylene chloride was placed in a dry nitrogen glovebox, the Sure Seal cap was removed, and activated 4A molecular sieves were added to ensure dryness during use. Approximately 5 g of molecular sieves was used per 100 mL of solvent. Anhydrous cobalt chloride had previously been opened in air; therefore, the sample was heated at 120 °C under vacuum for 16 h to ensure complete dehydration. The ZrCp2Cl2 was kept in a dry nitrogen glovebox to avoid water contamination. 2.2. Spectroscopy. UV/visible diffuse reflectance spectra (DRS) were recorded with a Shimadzu UV-2450 spectrometer equipped with integrating sphere model ISR-2200. Barium sulfate was used as the reference. Samples of SBA-15, Zr-SBA15, Co(II)-SBA-15, or ZrCo(II)-SBA-15 were pressed at ≤1 ton in a KBr press into wafers with a diameter of 1.2 cm and were loaded into a home-built stainless steel optical vacuum cell with a quartz window under an N2 atmosphere. While mounted on the integrating sphere, the cell was connected to a vacuum manifold for evacuation. The diffuse reflectance spectra of all samples were ratioed against a reflectance spectrum of neat SBA-15 and converted to 1 − (REXP/RREF), denoted in figures as 1 − R for brevity. The same procedure for loading samples into a transmission infrared vacuum cell equipped with CaF2 windows was used.26 For recording spectra below 1000 cm−1, KBr windows were employed. The FT-IR spectrometer was a Bruker model IFS66 V instrument equipped with liquid nitrogen cooled MCT detectors, Kolmar model KMPV8-1-J2 with an 8 μm band gap, or Infrared Associates with a 25 μm band gap. FT-Raman spectra were recorded with a Bruker 7875



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nium chloride, slight leaching of the Co(II) into the solvent was observed. 2.4.2. Gas Phase Loading of Sacrificial Reducing Agent. Dried and deoxygenated triethylamine or diethylamine vapor (4−5 Torr) contained in a Schlenk tube was exposed for 30 min to an evacuated pellet of calcined ZrOCo(II)−SBA-15 mounted in the infrared vacuum cell. Carbon dioxide gas (740 Torr) was subsequently added before the start of photolysis. 2.4.3. Photochemical Reaction. The metal-incorporated SBA-15 sample was weighed (5−15 mg, 0.8−2.5 μmol of the metal) in a N2-filled glovebag and pressed with a KBr press for 5 s as described above. The final mass of the metal incorporated silica pellet was recorded. The pellet was placed in a stainless steel IR vacuum cell equipped with CaF2 windows and mounted in the FT-IR spectrometer.22 After evacuation for at least 30 min, a spectrum was recorded. Carbon dioxide (740− 760 Torr) was loaded into the infrared cell (volume 4.6 cm3), and FT-IR spectra were recorded immediately and again after 30 min. Laser photolysis (355 nm, 200 mW cm−2 or 420 nm, 200−500 mW cm−2) was conducted by starting with 15 or 30 min intervals followed by 1 or 2 h intervals. In order to capture any dark processes that might occur after turning off the photolysis beam, two spectra were recorded: one immediately after terminating laser illumination and a second spectrum 15 min later, before continuing the photolysis. At the end of the experiment, the pellet was evacuated and a final FT-IR spectrum taken.

zirconocene dichloride solution was added to the SBA-15 suspension, and the mixture was stirred for 1 h. Excess triethylamine (1.0 mL, 7.2 mmol, 0.40 equiv) was added to the mixture and stirred for 16 h. The mixture was filtered in air and washed with 3 × 100 mL of methylene chloride. Calcination of Zr-SBA-15 in air was conducted at 550 °C for 12 h; ramp rate was 2.5 °C/min. As shown by the XRD spectrum in Figure S1a (trace 2), the long-range order of the mesoscale channel structure remains intact after anchoring of the Zr centers. 2.3.3. ZrOCo(II)−SBA-15 (1 mol %). Synthesis of the binuclear unit employed the tetrahedral complex Co(NCCH3)2Cl2 in acetonitrile as the Co precursor, a method that we used previously for the preparation of TiOCo(II) chromophore in MCM-41.16,17 Calcined Zr-SBA-15 (1% Zr loading, 355 mg, 0.0582 mmol of Zr) was dried in a Schlenk tube under vacuum (10 mTorr) at 170 °C overnight. In a N2filled glovebox, anhydrous cobalt dichloride (7.7 mg, 0.059 mmol, 1.0 equiv with respect to Zr sites) was stirred in 20 mL of dry acetonitrile for 2 h until complete dissolution. Zr-SBA-15 was suspended and stirred in acetonitrile for 2 h, after which the cobalt chloride solution was added. The mixture was stirred for 2 h and triethylamine added (50 μL, 36.0 mg, 0.36 mmol, 6.1 equiv with respect to Co). After stirring the mixture overnight and filtering in the N2-filled glovebox, the resulting blue solid was washed with 3 × 25 mL of dry acetonitrile and allowed to dry in the glovebox to give the as-synthesized ZrOCo(II)−SBA-15. Half of the sample was calcined in air at 350 °C for 5 h; ramp rate was 1.5 °C/min. The products were kept in the dry N2 glovebox. ICP analysis indicated 1.3 ± 0.3 mol % Zr and 1.3 ± 0.3 mol % Co (Zr:Co = 1:1), in agreement with the amounts of Zr and Co precursor used for the synthesis. Figure S1a (trace 4) confirms the intact mesoporous structure of the functionalized SBA-15 material. 2.3.4. Co(II)-SBA-15 (1 mol %). Calcined SBA-15 (2.06 g, 34.3 mmol, 1.00 equiv) was dried in a Schlenk tube under vacuum at 180 °C overnight. In a N2-filled glovebox, anhydrous cobalt dichloride (49.3 mg, 0.380 mmol, 0.011 equiv) was stirred in 20 mL of dry acetonitrile for 2 h until dissolution. SBA-15 was suspended and stirred in acetonitrile for 2 h followed by addition of the cobalt chloride solution. The mixture was stirred for 2 h, triethylamine was added (1 mL, 726 mg, 7.17 mmol, 0.209 equiv), and stirring continued overnight. The filtered blue solid (N2 glovebox) was washed with 3 × 25 mL of dry acetonitrile and allowed to dry in the glovebox to give the as-synthesized Co(II)-SBA-15. Half of the sample was calcined in air at 350 °C for 5 h; ramp rate was 1.5 °C/min. ICP analysis indicated loading levels in the range of 1.4−2.0 mol % Co. XRD spectra confirmed that the mesoporous structure remains unaffected by anchoring of Co centers (Figure S1a (trace 3)).16,17 2.4. Photochemical Experiments. 2.4.1. Solution Loading of Sacrificial Reducing Agent. In a N2-filled glovebox, dried, calcined ZrOCo(II)−SBA-15 (50.0 mg, 0.832 mmol (8.32 μmol of ZrOCo centers, 1 equiv)) was suspended in 20 mL of dry methylene chloride, and diethylamine (59.4 mg, 0.812 mmol, 97.6 equiv) was added. The suspension was stirred overnight, filtered, and washed with 3 × 20 mL of dry methylene chloride and allowed to dry in the glovebox. Similarly, diethylammonium chloride, triethylamine, and dibenzylamine were added to dried, calcined Zr-SBA-15, Co(II)-SBA-15, SBA-15, or ZrOCo(II)−SBA-15 in a 1:100 molar ratio using the same method. With the diethylammo-

3. RESULTS 3.1. Structure of Anchored ZrOCo(II) Unit. 3.1.1. Spectroscopic Characterization of the ZrOCo(II) Unit Anchored on the Silica Nanopore. The optical diffuse reflectance spectrum (DRS) of ZrOCo(II)−SBA-15, trace 1 of Figure 1a,

Figure 1. (a) UV−vis diffuse reflectance spectra (DRS) of calcined ZrOCo(II)−SBA-15 (trace 1, black) and Co(II)−SBA-15 (trace 2, red). Inset: difference spectrum ZrOCo(II)−SBA-15 minus Co(II)− SBA-15. (b) Spectra on an expanded scale show the Co(II) d to d* transition of ZrOCo(II) (trace 1, black) and Co(II)−SBA-15 (trace 2, red).

shows the distinct spin−orbit triplet of the ligand field absorption 4A2(F) → 4T1(P) in the 500−700 nm region characteristic for tetrahedral Co(II) centers.17,34 The corresponding peaks of the ligand field absorption for isolated tetrahedral Co(II) of the monometallic Co(II)-SBA-15 sample are shown in trace 2 of Figure 1a. Spin orbit peaks are at 653, 579, and 513 nm for ZrOCo(II)−SBA-15, whereas Co(II)− SBA-15 has the corresponding maxima at 659, 593, and 528 nm. Subtraction of the Co(II) trace from the ZrOCo(II) spectrum, shown in the inset of Figure 1a for the case of thicker wafers with correspondingly more intense absorptions, indicates a tail extending from the near-UV region to approximately 600 nm. Because Zr(IV) has no bands at λ > 7876



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Table 1. Curve Fitting Analysis of EXAFS Measurements of Zr K-Edge and Co K-Edge sample

fit

Co K-edge Co-1-SBA ZrCo-1,1-SBA

1

ZrCo-1,1-SBA

2

Zr K-edge Zr-1-SBA

1

Zr-1-SBA

2

ZrCo-1,1-SBA

1

ZrCo-1,1-SBA

2

shell

r (Å)

Na

σ2 (×10−3 Å2)

E0 (eV)

R-factor (×10−3)

± ± ± ± ± ± ±

4.1 ± 0.2 1.5 ± 0.3 4.4 ± 0.3 1.4 ± 0.3 1.0 ± 0.2 4.3 ± 0.3 1.4 ± 0.3

8.5 5.5 9.3 5.0 20 9.1 4.6

−1.1

1.9

−0.69

4.4

0.81

−0.59

6.2

0.87

± ± ± ±

2.3 13 5.3 6.0

0.17

8.2

3.6

2.9 ± 1.2 4.2 ± 0.3 1.4 ± 0.2 5.1 ± 0.3 2.5 ± 0.3 1.2 ± 0.3 0.80 ± 0.17

1.0 16 4.7 17 1.2 5.3 6.1

0.79

15.9

4.4

2.1

5.0

5.6

5.1 ± 0.3 2.5 ± 0.3 1.1 ± 0.3

17 1.2 6.1

0.54

7.9

3.7

Co−O Co−Si Co−O Co−Si Co−Zr Co−O Co−Si

2.01 3.24 2.03 3.25 3.33 2.03 3.25

0.005 0.01 0.006 0.02 0.06 0.006 0.01

Zr−O Zr−O Zr−Si Zr−O−Sib Zr−O−Si Zr−O Zr−O Zr−Si Zr−O Zr−O Zr−Si Zr−Co Zr−O−Cod Zr−O Zr−O Zr−Si

2.02 ± 0.01 2.26 ± 0.04 3.35 ± 0.04 3.58 ± 0.04 123° c 2.02 ± 0.01 2.24 ± 0.05 3.30 ± 0.04 2.03 ± 0.04 2.22 ± 0.01 3.41 ± 0.09 3.49 ± 0.09 114°/106° 2.02 ± 0.03 2.21 ± 0.01 3.35 ± 0.05

3.7 4.2 1.4 1.4

1.3 0.3 0.2 0.2b

reduced χ2 (×102)

a For Co and Zr, S02 = 1. bZr−O−Si 3-leg multiple scattering (MS) path with the coordination number set to be identical as the coordination number as the Zr−Si single scattering path. cAngle derived by application of the trigonometric cosine rule, by using the MS path length to calculate the Si−O bond length and using the three known distances in the Zr−O−Si triangle to obtain the Zr−O−Si angle, using the shorter Zr−O bond length. dAngle derived by application of the trigonometric cosine rule, by using the two known Zr−O and the Co−O distances, and assuming an average of 3.4 Å for the Zr−Co distance in the Zr−O−Co triangle to obtain the two possible Zr−O−Co angles; the larger angle corresponds to the shorter Zr−O bond length.

distance of 2.24 Å (N = 4.2 ± 0.3), as presented in Figure 2b and fit 2 of Table 1. Subshell distances and coordination numbers are in agreement with an EXAFS study of Zr centers anchored on mesoporous silica MCM-48 and silicalite reported in the literature.35 For the second shell signal of the Zr K-edge FT plot, a substantially improved fit was obtained when the single scattering Zr−Si subshell at 3.35 Å was complemented by an associated 3-leg multiple scattering (MS) Zr−Si−O subshell at 3.58 Å, as can be seen from the improved R-factors and reduced χ2 in fit 1 of Table 1 and comparison of the FT-EXAFS component plots in Figure S2. The 3-leg MS path consists of both a Zr−O and an O−Si component but is constrained to the same coordination number as the Si subshell.36−49 The average Zr−O−Si angle derived from these distances is 123°. The principal result of EXAFS curve fitting of Zr and Co edge measurements of the binuclear ZrOCo(II) units is a common Zr−Co distance of 3.4 Å obtained for the Co edge (3.33 ± 0.06 Å, N = 1.0 ± 0.2) and for the Zr edge data (3.49 ± 0.09 Å, N = 0.80 ± 0.17).41 Accordingly, a bond angle for the oxo bridge of 114° is calculated, close to the 111 degree angle determined previously for the TiOMn(II) unit.25 It is interesting to note that the second shell fit of the Zr edge data (signal in the apparent distance range 2.7−3.5 Å, Figure 3b is substantially worse if Co is omitted (Figure S3); for the second shell fit of the Co edge measurements (signal spanning range 2.5−3.3 Å, Figure 3a, the omission of a Zr shell does not change the quality of the fit appreciably (Table 1 fits 1 and 2 of Co K-edge and Figure S4). Furthermore, the coordination

240 nm,14 the new absorption tail is assigned to a transition originating from electronic interaction of the Zr and the Co centers, the Zr(IV)OCo(II) → Zr(III)OCo(III) metal-to-metal charge-transfer transition (MMCT). The wave-like shape of the absorption contour between 450 and 600 nm is a consequence of the significant shift of the Co(II) spin−orbit peaks upon formation of the ZrOCo bridge, shown on an expanded scale in Figure 1b. As will be discussed in section 4, the blue-shift of the Co(II) spin−orbit maxima in ZrOCo(II)−SBA-15 relative to Co(II)−SBA-15 is caused by a perturbation of electron donation from the O-bridging ligand to the Co(II) center by Zr covalently bonded to the same shared O atom. Therefore, these spectral shifts are a direct spectroscopic manifestation of an oxo bridge between Zr and Co centers. Curve-fitting analysis of EXAFS measurements of both Zr Kedge and Co K-edge provided details of the geometry of the oxo-bridged unit. The FT of the Co K-edge EXAFS of Co-SBA15 (1 mol %) is best fit with a first-shell Co−O distance of 2.01 Å and an average coordination number of 4.1 ± 0.2, in agreement with the tetrahedral coordination geometry derived from the optical ligand field spectrum. The complete curve fitting results are summarized in Table 1. The second shell signal of the FT-EXAFS function spanning an apparent distance of 2.5−3.3 Å (Figure 2a) fits well a Si shell at 3.24 Å and a coordination number of 1.5 ± 0.3. The FT of the Zr K-edge of monometallic Zr−SBA-15 (1 mol %) is modeled well with two Zr−O subshells, namely one at 2.02 Å with a coordination number of 2.9 ± 1.2 and another subshell at a slightly larger 7877



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Figure 3. (a) Fit of k3-weighted Co K-edge EXAFS for ZrCo-1,1-SBA. Inset: Co K-edge fit up to k = 13.5 for ZrCo-1,1-SBA. (b) k3-weighted Zr K-edge EXAFS fit for ZrCo-1,1-SBA. Inset: fit for Zr K-edge up to k = 14.2 for ZrCo-1,1-SBA.

Figure 2. (a) Fit of k3-weighted Co K-edge EXAFS for Co-1-SBA. Inset: Co K-edge fit up to k = 13.5 for Co-1-SBA. (b) k3-weighted Zr K-edge EXAFS fit for Zr-1-SBA. Inset: fit for Zr K-edge up to k = 13.5 for Zr-1-SBA.

number of N = 1.0 ± 0.2 for Zr (Co edge measurement) and of N = 0.80 ± 0.17 for Co (Zr edge measurement) indicates that at least 80% of Co centers are engaged in a binuclear unit. The remarkably high yield of binuclear units can be attributed to preferential reaction of the Co(II) precursor complex with ZrOH groups because of the higher acidity, hence higher reactivity, of ZrOH compared to the many more SiOH groups on the nanopore surface.42 The higher acidity of the ZrOH group is indicated by a 100 cm−1 red-shift of the OH stretch mode relative to SiOH.32,33,43 Moreover, we anticipate that the Co−O−Zr moiety is thermodynamically more stable than a Co−O−Si group, favoring formation of binuclear units over isolated anchored Co centers. It is noteworthy that the Co as well as the Zr coordination geometry (Co−O, Co−Si; Zr−O, Zr−Si distances) remains remarkably unchanged for metal centers of the monometallic materials and centers engaged in the binuclear unit. Such structural preservation of the ligand environment of the individual metal centers of the binuclear units compared to the isolated centers was already encountered in the case of TiOMn(II) units.25 No evidence was found for the formation of Co oxide or Zr oxide clusters. In addition to the lack of peaks that might originate from metal oxide clusters in the wide angle XRD spectra of ZrOCo(II)−SBA-15 or the monometallic Zr and Co silica materials (Figure S1b), no Co−O mode of Co3O4 or other Co oxide phases were observed in FT-IR spectra of ZrOCo(II) or Co(II)-SBA-15, as evidenced by Figure 4a. Similarly, FT-Raman spectra of ZrOCo(II) and Zr-SBA-15 do

Figure 4. (a) FT-IR spectra of 5 nm particles of crystalline Co3O4 embedded in SBA-15 (trace 1) with peaks at 668 and 583 cm−1. FT-IR spectra of ZrOCo(II)−SBA-15 (trace 2) and Co(II)-SBA-15 (trace 3). (b) FT-Raman spectra of ZrO2 mechanically mixed with SBA-15 (trace 1), 1 mol % with peaks at 188 and 176 cm−1. ZrOCo(II)−SBA15 (trace 2) and Zr-SBA-15 (trace 3).

not show characteristic absorptions of ZrO2 modes (Figure 4b).44 Absence of Co and Zr oxide clusters is further supported by the lack of bands in the FT-EXAFS plots for distances greater than 4 Å (Figures 2 and 3).27,45−47 3.1.2. ZrOCo(II) Unit in the Presence of Triethylamine Donor. As will be described below, photochemical CO2 reduction at ZrOCo(II) sites requires the presence of an electron donor in the vicinity of the charge transfer chromophore. In most experiments, triethylamine (N(C2H5)3, abbreviated NEt3) was used because the molecule was already coloaded with the Co precursor to facilitate the synthesis of ZrOCo units. By omitting the calcination step, NEt3 resides in the SBA-15 nanopores and serves as an electron donor for 7878



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photochemical CO2 reduction (these samples are designated “as-synthesized”). Figure 5a shows the influence of the loaded

Figure 5. (a) UV−vis DRS of calcined ZrOCo(II)−SBA-15 (trace 1) and as-synthesized ZrOCo(II)−SBA-15 containing triethylamine (trace 2). Inset shows difference (2) − (1) with triethylamine absorption at 300 nm. Panel b shows the Co d−d* spin orbit peaks of the two samples on an expanded scale.

Figure 6. FT-IR difference spectra upon loading of 740 Torr of 12CO2 (trace 1) or 13CO2 (trace 2) into as-synthesized ZrOCo(II)−SBA-15. The main absorptions are the asymmetric stretch at 1636 cm−1 for 12 CO2 and 1600 cm−1 for 13CO2 loading. Arrows indicate the symmetric stretching modes of adsorbed 12CO2 and 13CO2 in traces 1 and 2, respectively. All other features are due to small intensity changes of triethylamine bands upon adsorption of carbon dioxide gas.

NEt3 on the optical spectrum. As-synthesized ZrOCo(II)− SBA-15 has a weak band between 280 and 350 nm attributed to a triethylamine absorption (trace 2).22 In addition, the d to d* spin−orbit peaks of Co(II) are red-shifted by the presence of the donor, shown in Figure 5b on an expanded scale for clarity. This shift, and the decrease in intensity of two of the peaks, is most likely caused by the change in geometry around the divalent cobalt from tetrahedral to 5-coordinate by the amine ligand(s). It is interesting to note that according to literature reports, impregnation of porous silica with Co(II) salts in the absence of heating does not lead to immediate Si−O−Co(II) binding.48 In this case, the d-to-d* transition indicates a tetrahedral Co(H2O)42+ species with peaks at 530, 575, and 611 nm. Heating above 200 °C results in the formation of anchored Co(OSi)x(H2O)4−x(2−x)+ as signaled by a substantial red-shift of the 611 nm peak to 660 nm.48 By contrast, our synthetic method results directly in binding of the Co(II) precursor as Co(OM)xLy (M = Zr, Si; L = triethylamine, acetonitrile and/or H2O), as indicated by the 659 nm peak in the as-synthesized sample (Figure 5). Evacuation of ZrOCo(II)−SBA-15 samples prior to loading of CO2 results in the complete removal of CH3CN, readily seen by the disappearance of the CN infrared modes of acetonitrile (at 2312 and 2285 cm−1 when coordinated to Co(II))17 and removal of most H2O molecules except those trapped inside the silica walls. Therefore, aside from Co−O−Si linkages and nonbonding interactions of Co with O lone pair electrons of SiOH, SiO−, or SiOSi groups, the only other ligand of Co(II) is NEt3. 3.2. Photochemical Reduction of Carbon Dioxide. 3.2.1. Adsorption of CO2 at ZrOCo(II) Site. Aside from silica bands, the infrared spectrum of an evacuated wafer of assynthesized ZrOCo(II)−SBA-15 shows NEt3 absorptions at 2982, 2947, 2911, 2888, 2845, 2501 (HNEt3+), 1659 (HNEt3+), 1491, 1478, 1459, 1451, 1433, 1393, and 1356 cm−1. Alternatively, HNEt2 was also used as donor, which has infrared absorptions at 2981, 2946, 2907, 2883, 2849, 2751, 2482 (H2NEt2+), 2384 (H2NEt2+), 1620 (H2NEt2+), 1486, 1471, 1458, 1437, 1391, 1362, and 1330 cm−1. Loading of 740 Torr of CO2 into as-synthesized ZrOCo(II)−SBA-15 results in the observation of its gas phase spectrum12 and a broad, intense band of an adsorbed CO2 species at 1636 cm−1, shown in Figure 6, trace 1. The corresponding peak of the adsorbed species when using 13CO2 gas is at 1600 cm−1 (Figure 6, trace

2). Carbon dioxide adsorbed on metal oxide surfaces49,50 or Tisubstituted microporous silicalite12 is known to form one or more types of adsorbed species with carboxylate, mono- or bidentate carbonate, bridging carbonate, or bicarbonate structure that can be removed more or less readily by evacuation or heating. A recent report indicates the formation of some of these species also on neat silica surfaces under very high CO2 pressure.51 For the 1636 cm−1 species observed in this work, evacuation for 5 min at room temperature resulted in complete removal from the silica mesopores. The band position and the 13C isotope frequency shift of 36 cm−1 indicate that the adsorbed species is a carboxylate (OC(−O−)M). Shoulders at 1680 cm−1 (12CO2) and 1650 cm−1 (13CO2) are attributed to a different site of the surface carboxylate, which might include H bonding or alternate metal bonding interactions. Sharp bands at 1381 cm−1 (CO2) and 1366 cm−1 (13CO2) are the symmetric stretch modes of the adsorbed CO2 species that are rendered infrared active by interaction with the surface. Aside from small intensity changes of the NEt3 bands upon adsorption of CO2 (Figure 6), no other infrared changes upon CO2 loading were noted in the absence of photolysis light. 3.2.2. Photochemical Reaction. Irradiation of carbon dioxide-loaded, as-synthesized ZrOCo(II)−SBA-15 with 355 or 420 nm light resulted in the growth of CO2 reduction products. Photons in this wavelength range excite predominantly the Zr(IV)OCo(II) → Zr(III)OCo(III) MMCT transition. The majority of experiments were conducted with isotopically labeled 13CO2 in order to unambiguously identify reduction products as originating from carbon dioxide. In the 2200−2000 cm−1 region, gas phase 13CO was readily detected by its characteristic ro-vibrational bands as shown in Figure 7a. According to Figure 7b, the growth of 13CO upon photolysis at 355 nm is linear over a period of several hours, indicating that the reacting carbon dioxide in the silica nanopores is continuously replenished from the gas phase. The yield of CO product after 4 h photolysis is 0.1 μmol, corresponding to approximately 10% of all ZrOCo sites present in the sample that participate in the photocatalysis. The corresponding photolysis experiments using C18O2 is shown in Figure S5. Significant albeit small product growth was detected upon illumination at 420 nm, reflecting at least in part the smaller fraction of light absorbed by the MMCT band at longer wavelengths (Figure 8, trace 1). Prolonged illumination at even 7879



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ZrOCo(II) unit with blue visible or UV light results in the splitting of CO2 to CO. Photochemically induced changes in the region below 2000 cm−1 reveal additional products. The difference infrared spectrum (1800−1400 cm−1 region) of ZrOCo(II)−SBA-15 loaded with NEt3 and 740 Torr of 13CO2 recorded immediately following 3 h of photolysis at 355 nm (220 mW cm−2), shown as the black trace in Figure 9, signals loss of absorbance at 1600

Figure 7. (a) Infrared difference spectra upon irradiation at 355 nm (220 mW cm−2) of ZrOCo(II)−SBA-15 loaded with 740 Torr of 13 CO2 and 4 Torr of NEt3. From bottom to top: 30 min, 1 h, 2 h, 3 h, and 4 h photolysis. (b) Growth kinetics of 13CO (2062.0 cm−1 band). The yield of CO product after 4 h photolysis is 0.1 μmol, corresponding to approximately 10% of all ZrOCo sites present in the sample that participate in the photocatalysis.

Figure 9. FT-IR spectra after 3 h photolysis at 355 nm (220 mW/cm2) of ZrOCo(II)−SBA-15 loaded with 740 Torr 13CO2. Black trace (1): immediately after photolysis is stopped. Blue trace (2): 10 min in the dark after photolysis was stopped. Green trace (3): deconvolution (Origin software) of the black spectrum yielding bands with maxima at 1660 (bleach), 1600 (bleach), and 1550 cm−1 (product).

longer wavelength of 459 nm (Ar ion laser, 400 mW cm−2, 8 h) did not show any growth of reaction product.

cm−1. The bleach coincides with the band of 13C surface carboxylate (compare with Figure 6), indicating that the adsorbed CO2 molecules with OC−O electronic structure are the reacting species. The depletion seen at 1660 cm−1 is due to consumption of protonated NEt3. The blue spectral trace of Figure 9 was recorded after keeping the sample in the dark for 10 min. The bleach at 1600 cm−1 recovers over this time period due to readsorption of gas phase 13CO2 into the silica nanopores, thus confirming our conclusion from the CO product growth behavior that the reactive species is continuously replenished. Furthermore, product absorption develops at 1709 cm−1 in the 13CO2 photolysis experiment and at this same frequency when using the parent 12CO2 reactant (Figure 10a, trace 1). On the other hand, no band grows in the region above 1700 cm−1 in the corresponding photolysis experiment using C18O2, as shown in Figure 11, trace 2. Here, the 18O counterpart of the 1709 cm−1 product band overlaps with the decreasing absorption of protonated NEt3 at 1660 cm−1. Spectral subtraction of NEt3 revealed that the maximum of the 18O shifted band is around 1675 cm−1, which implies an 18 O isotope shift of 34 cm−1 (represented as dotted trace in Figure 11, trace 2). The observed isotope frequency shifts indicate that the reaction product absorbing at 1709 cm−1 contains the split-off O atom of carbon dioxide while the carbon atom(s) originate from the sacrificial NEt3 donor. Comparison with the spectrum of an authentic sample of acetaldehyde loaded into ZrOCo-SBA-15, presented in Figure 10a,b, trace 3, shows good agreement of the most intense acetaldehyde band (CO stretch mode) with the 1709 cm−1 photoproduct. Furthermore, there is photochemical absorbance growth where the 1381 cm−1 mode of acetaldehyde absorbs (Figure 10). In fact, acetaldehyde is the known primary oxidation product of NEt3.52−54

Figure 8. FT-IR spectra of 13CO growth upon 420 nm photolysis (300 mW cm−2, 6 h) of 13CO2 (740 Torr) in ZrOCo(II)−SBA-15 containing HNEt2 as donor (loaded from the liquid phase) (trace 1). For comparison, trace 2 shows subsequent 13CO growth upon 355 nm photolysis (200 mW cm−2, 3 h).

No 13CO was formed in the absence of triethylamine or diethylamine donors. A comparison of irradiation of 13CO2loaded ZrOCo(II)−SBA-15 in the absence (calcined) and presence of NEt3 is shown Figure S6a, trace 1 and 2, confirming that an electron donor is required for the excited ZrOCo(II) unit to reduce CO2 molecules. We found that photochemical results were independent of how NEt3 was introduced into the nanoporous silica, i.e., whether we used the remaining NEt3 after assembly of the ZrOCo(II) units or whether the amine was added to calcined ZrCo(II)-SBA-15 by loading from the gas phase. For the photolysis periods used in this work, the CO product growth was unaffected by the NEt3 or HNEt2 added as long as the amount corresponded to at least 4 equiv relative to ZrOCo sites. Furthermore, no 13CO formation was observed when conducting 355 nm photolysis experiments with monometallic Zr-SBA-15 or Co-SBA-15 wafers loaded with 4 Torr of NEt3 and 740 Torr of 13CO2 (Figure S6b). We conclude that MMCT excitation of the heterobinuclear

CO2 + N(CH 2CH3)3 → CO + CH3CHO + HN(CH 2CH3)2 7880

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formate (HCO2−),56 most likely produced by the ZrOCo(II) MMCT induced photoreaction CO2 + N(CH 2CH3)3 → HCO2− + CH3CHN+(CH 2CH3)2

(2)

The assignment is confirmed by authentic spectra of HCO2− and H13CO2− obtained by loading of HCO2H or H13CO2H from the gas phase into ZrCo(II)-SAB-15, as shown in trace 2 of Figures 10a and 10b, respectively. The weak shoulder of the asymmetric CO stretch absorption in the 1550−1500 cm−1 region of all three formate isotopes is attributed to small amounts of mono- or bidentate carbonate, a possible alternate reaction product of the split-off oxygen. Furthermore, a product band at 1630 cm−1, most clearly seen after spectral subtraction of the intense formate band, does not exhibit an isotope shift when using 13CO2. The band agrees well with the expected CN mode of CH3CHN+(CH2CH3)2 coproduct.57 It is interesting to note that the photochemical product yield can be enhanced by the selection of a larger amine that acts as a more efficient electron donor. As shown in Figure S8, loading of dibenzylamine into ZrOCo(II)−SBA-15 and photolysis at 420 nm (485 mW cm−2) results in a 30-fold increase of the 13 CO yield compared to photolysis at 355 nm (220 mW cm−2) of ZrOCo(II)−SBA-15 in the presence of triethylamine. The 13 CO growth over a period of 3.5 h at 420 nm for ZrOCo(II)− SBA-15 loaded with dibenzylamine (100 equiv) corresponds to 10 catalytic turnovers per ZrOCo site (assuming all sites are active). While the longer wavelength optical absorption of the aromatic amine (dibenzylamine absorbs weakly at 420 nm) renders the mechanistic explanation less straightforward compared to the case of the aliphatic amines, photolysis control experiments with SBA-15 or monometallic Zr-SBA-15 samples loaded with dibenzylamine demonstrate that most of the photoproduct is generated by MMCT excitation of the ZrOCo site (Figures S9 and S10).

Figure 10. (a) Trace 1: FT-IR difference spectrum after 3 h photolysis at 355 nm (220 mW/cm2) of as-synthesized ZrOCo(II)−SBA-15 loaded with 740 Torr of 12CO2. Trace (2): authentic sample of 12Cformic acid loaded into as-synthesized ZrOCo(II)−SBA-15 from the gas phase. Trace (3): authentic sample of acetaldehyde loaded into assynthesized ZrOCo(II)−SBA-15 from the gas phase. (b) Trace 1: FTIR difference spectrum after 3 h of photolysis at 355 nm (220 mW cm−2) of as-synthesized ZrOCo(II)−SBA-15 loaded with 740 Torr of 13 CO2. Trace (2): authentic sample of 13C-formic acid loaded into assynthesized ZrOCo(II)−SBA-15 from the gas phase. The small product band at 1660 cm−1 is attributed to the formation of some additional HNEt3+ by protonation upon formic acid loading of ZrOCo(II)−SBA-15. Trace (3): authentic sample of acetaldehyde loaded into as-synthesized ZrOCo(II)−SBA-15 from the gas phase.

Figure 11. FT-IR difference spectra upon 355 nm (220 mW/cm2) photolysis of as synthesized ZrOCo(II)−SBA-15 loaded with 740 carbon dioxide gas. Comparison of isotope frequency shifts of infrared product bands upon photoreduction of 12CO2 (trace 1), C18O2 (trace 2), and 13CO2 (trace 3). Unassigned growth in the C18O2 photolysis spectra at 1609 (shoulder), 1442, and 1320 cm−1.

4. DISCUSSION The EXAFS curve-fitting analysis of the ZrOCo binuclear units confirms tetrahedral coordination for Co(II) by O ligands, which is consistent with the spin−orbit components observed in the optical spectrum. Furthermore, the coordination number for Zr of close to one derived from Co K-edge curve fitting and for Co derived from Zr K-edge curve fitting suggests that the majority of the Co centers are linked to a Zr via a covalent O bridge. The result shows that the room temperature synthesis method for assembling heterobinuclear units exhibits good selectivity. The Zr−Co distance is 3.4 A according to second shell fitting of both the Co and Zr edge data. The blue-shift of the maxima of the three spin−orbit components of the Co(II) 4A2(F) → 4T1(P) ligand field absorption in the 500−700 nm region34 provides independent confirmation of the oxo-bridged structure. Isolated Co(II) centers in Co-SBA-15 and Co(II) centers that are part of a ZrOCo unit in ZrOCo-SBA-15 are both in closely preserved, tetrahedral coordination with O ligands according to the EXAFS analysis. Therefore, the spectral shifts must originate from differences in the electronic properties of the ligands. Specifically, the interaction of the Co(II) with Zr (in a binuclear CoOZr unit) rather than a proton (as for an isolated CoOH group) causes a shift of the spin−orbit components that is expected for Zr due to its electron withdrawing properties. It is well established that σ- and π-electron donation of O ligands

Hence, these observations strongly suggest that the main reaction of the departing O of CO2 upon photoreduction is oxidation of NEt3 to acetaldehyde. Furthermore, the formation of acetaldehyde explains the observation of small amounts of gas phase 12CH4 and 12CO that grew in upon continued photolysis (355 nm) following a distinct induction period, independent of whether 12CO2 or 13CO2 was used (Figure S7). Methane and CO are formed upon photodissociation of acetaldehyde followed by reaction of the ensuing methyl and formyl radicals.55 Overlap of diethylamine infrared bands with those of triethylamine prevented detection of the NH(Et)2 coproduct. The most intense product band upon photoexcitation of ZrOCo units in the presence of 13CO2 is at 1550 cm−1 (Figures 9 and 10b, trace 1). The corresponding 12CO2 photoproduct absorbs at 1598 cm−1 (Figure 10a, trace 1), while for the C18O2 counterpart, the maximum is at 1565 cm−1 (Figure 11, trace 2; 12 CO2 and 13CO2 photolysis spectra are also displayed in the figure for convenience). These 13C and 18O isotopic shifts agree well with those of the asymmetric CO stretching mode of 7881



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Figure 12. Simplified diagram of the effect of the Co(II) (4A2(F) → 4T1(P)) transition on the ΔTd when π-donation from the oxygen is perturbed.

O− (most likely in the form of OH radical) subsequently reduced to H2O at another Cu(I) site upon diffusion through the mesopores.14 This implies that the reaction steps of CO2 reduction to CO do not necessitate the direct involvement of a sacrificial donor such as NEt3; here, the photon energy is stored in CO and the potential energy of Cu(II). We propose that the same mechanism applies for both ZrOCu(I) and ZrOCo(II), namely, electron transfer from transient Zr(III) to an adsorbed CO2 molecule that is H bonded to one of the many adjacent surface SiOH groups, yielding a HOCO radical intermediate in the initial step; the single electron CO2− reduction potential is far too negative60 relative to transient Zr(III) for the radical to be formed, whereas the potential of HOCO is around −1 V.61,62 Dissociation of HOCO to CO and OH is followed by diffusion through the silica pores and trapping at Cu(I) sites. The requirement of a sacrificial donor NEt3 for ZrOCo(II) but not for ZrOCu(I) is most likely determined by stronger competition between electron transfer from transient Zr(III) to CO2 and back electron transfer to Co(III) compared to the same competition for excited Zr(III)OCu(II). That is, the lifetime of the excited Zr(III)OCo(III) state may be too short and require instant reduction of Co(III) by the amine in order for electron transfer from Zr(III) to CO2 to take place. By contrast, direct participation of NEt3 (or HNEt2) is proposed in the steps, leading to formate (HCO2−). Upon reduction of transient Co(III) by NEt3, the radical cation [N(CH2CH3)3]+ so formed has available two reaction paths: One is elimination of a proton to form the CH3CHN(CH2CH3)2 radical, which could react with OH radical (splitting of HOCO) to give CH3CHO and HN(CH2CH3)2, as observed (eq 1). The eliminated H+ reprotonates the SiO− site that served as initial proton source. The second pathway for the radical cation [N(CH2CH3)3]+ is transfer of a H atom to CO2 under formation of the observed CH3CHN+(CH2CH3)2 (eq 2). The H atom transfer from [N(CH2CH3)3]+ and the concurrent electron transfer from transient Zr(III) to CO2 yield HCO2−. Because formate requires the direct involvement of NEt3, its appearance as a product requires the presence of the sacrificial donor, while CO does not. We conclude that the branching between CO and formate is controlled by the fate of NEt3 radical cation competition between [N(CH2CH3)3]+ deprotonation versus H atom transfer to CO2. The discovery of the ZrOCo(II) unit adds flexibility in the choice of donor metal centers for carbon dioxide activating binuclear units. Specifically, the donor of the ZrOCu unit in its oxidized form is Cu(II), which does not have sufficient potential for driving a water oxidation catalyst. This unit may be useful in a two-photon scheme (Z scheme) but is not suitable for single photon reduction of CO2 by H2O. In

to transition metal centers decrease the Δ splitting between e and t2 set of orbitals through increase of the e orbital energy.58 However, the resulting red-shift of the corresponding spin− orbit bands is diminished by electron withdrawing centers such as Zr if linked to an O atom shared with Co. As illustrated in Figure 12, the result is a reduced effect on the upward shift of the e orbital energy, causing a smaller shift due to weakened electron donation from the bridging O. This results in a net blue-shift of the spin−orbit components going from Co-SBA15 to ZrOCro-SBA-15, as observed. Consistent with this explanation, the blue-shift of the Co(II) spin−orbit components is significantly less (518, 588, and 657 nm for TiOCo(II)−MCM-41 compared to 528, 593, and 659 nm for Co(II)−MCM-41);17 the weaker electron withdrawing power of Ti reflects the reduced overlap of its smaller 3d orbitals compared to 4d orbitals of Zr. Precedents of the energy shift of Co(II) spin−orbit components caused by electron withdrawing ligands are Co complexes occluded in zeolites.48 We conclude that the energy shift of the Co(II) ligand field spin−orbit components is a direct manifestation of the oxo-bridged structure of the ZrOCo(II) MMCT charge transfer units. The production of gas phase CO by photoexcitation of the MMCT absorption of the ZrOCo(II) unit constitutes the first example of light-driven CO2 reduction to free CO by an allinorganic heterobinuclear site. In previous work,14 we have established CO2 photoreduction to CO at ZrOCu(I) sites (Zr(IV)OCu(I) → Zr(III)OCu(II)) in the mesopores of MCM-41. In that case, the emerging CO was trapped at Cu(I) sites inside the MCM-41 pores because of the familiar strong binding interaction of CO and Cu(I); hence, the CO product would not be readily available for further desired chemical transformations. With the discovery in the work reported here of Co(II) as an alternate donor center, this limitation has been removed. The finding of gas phase CO in steady state photolysis with no signs of residual CO remaining in the silica pores is in agreement with the desorption kinetics from silica mesopores measured previously by time-resolved FT-IR spectroscopy.59 By generating a pulse of carbon monoxide inside MCM-41 through photodissociation of a precursor with a nanosecond laser pulse, we were able to monitor the escape of CO molecules from the mesopores into the surrounding gas phase. The majority of CO desorbed with a fast time constant of 350 μs at room temperature. A further difference between the ZrOCu(I) and the ZrOCo(II) system for CO2 photoreduction is the fact that no sacrificial donor is required in the case of ZrOCu(I), which provides an important clue about the reaction mechanism. In the case of ZrOCu(I), the CO2 is split to CO under stoichiometric oxidation of Cu(I) to Cu(II), with the split-off 7882


The Journal of Physical Chemistry C contrast, the oxidized ZrOCo unit with the donor in the Co(III) oxidation state is expected to have sufficient positive potential for driving a water oxidation catalyst.

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REFERENCES

(1) Inoue, T.; Fujishima, A.; Konishi, S.; Honda, K. Photoelectrocatalytic Reduction of Carbon-Dioxide in Aqueous Suspensions of Semiconductor Powders. Nature 1979, 277, 637−638. (2) Hemminger, J. C.; Carr, R.; Somorjai, G. A. Photoassisted Reaction of Gaseous Water and Carbon-Dioxide Adsorbed on SrTiO3 (111) Crystal-Face to Form Methane. Chem. Phys. Lett. 1978, 57, 100−104. (3) Fujishima, A.; Zhang, X. T.; Tryk, D. A. TiO2 Photocatalysis and Related Surface Phenomena. Surf. Sci. Rep. 2008, 63, 515−582. (4) Tseng, I. H.; Chang, W.-C.; Wu, J. C. S. Photoreduction of CO2 Using Sol−Gel Derived Titania and Titania-Supported Copper Catalysts. Appl. Catal., B: Environ. 2002, 37, 37−48. (5) Navalon, S.; de Miguel, M.; Martin, R.; Alvaro, M.; Garcia, H. Enhancement of the Catalytic Activity of Supported Gold Nanoparticles for the Fenton Reaction by Light. J. Am. Chem. Soc. 2011, 133, 2218−2226. (6) Wang, W. N.; An, W. J.; Ramalingam, B.; Mukherjee, S.; Niedzwiedzki, D. M.; Gangopadhyay, S.; Biswas, P. Size and Structure Matter: Enhanced CO2 Photoreduction Efficiency by Size-Resolved Ultrafine Pt Nanoparticles on TiO2 Single Crystals. J. Am. Chem. Soc. 2012, 134, 11276−11281. (7) Habisreutinger, S. N.; Schmidt-Mende, L.; Stolarczyk, J. K. Photocatalytic Reduction of CO2 on TiO2 and Other Semiconductors. Angew. Chem., Int. Ed. 2013, 52, 7372−7408. (8) Dhakshinamoorthy, A.; Navalon, S.; Corma, A.; Garcia, H. Photocatalytic CO2 Reduction by TiO2 and Related Titanium Containing Solids. Energy Environ. Sci. 2012, 5, 9217−9233. (9) Shkrob, I. A.; Dimitrijevic, N. M.; Marin, T. W.; He, H. Y.; Zapol, P. Heteroatom-Transfer Coupled Photoreduction and Carbon Dioxide Fixation on Metal Oxides. J. Phys. Chem. C 2012, 116, 9461−9471. (10) Anpo, M.; Takeuchi, M. The Design and Development of Highly Reactive Titanium Oxide Photocatalysts Operating under Visible Light Irradiation. J. Catal. 2003, 216, 505−516. (11) Matsuoka, M.; Anpo, M. Local Structures, Excited States, and Photocatalytic Reactivities of Highly Dispersed Catalysts Constructed within Zeolites. J. Photochem. Photobiol., C: Photochem. Rev. 2003, 3, 225−252. (12) Ulagappan, N.; Frei, H. Mechanistic Study of CO2 Photoreduction in Ti Silicalite Molecular Sieve by FT-IR Spectroscopy. J. Phys. Chem. A 2000, 104, 7834−7839. (13) Lin, W. Y.; Han, H. X.; Frei, H. CO2 Splitting by H2O to CO and O2 under UV Light in TiMCM-41 Silicate Sieve. J. Phys. Chem. B 2004, 108, 18269−18273. (14) Lin, W. Y.; Frei, H. Photochemical CO2 Splitting by Metal-toMetal Charge-Transfer Excitation in Mesoporous ZrCu(I)-MCM-41 Silicate Sieve. J. Am. Chem. Soc. 2005, 127, 1610−1611. (15) Lin, W.; Frei, H. Anchored Metal-to-Metal Charge-Transfer Chromophores in a Mesoporous Silicate Sieve for Visible-Light Activation of Titanium Centers. J. Phys. Chem. B 2005, 109, 4929− 4935.

ASSOCIATED CONTENT

S Supporting Information *

Data processing and analysis of EXAFS measurements, additional FT-EXAFS data, XRD patterns, and additional photochemical data are displayed. This material is available free of charge via the Internet at http://pubs.acs.org.

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ACKNOWLEDGMENTS

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 DE-AC02-05CH11231. Portions of this research were carried out at the Stanford Synchrotron Radiation Lightsource, a national user facility operated by Stanford University on behalf of the U.S. Department of Energy, Office of Basic Energy Sciences. The SSRL Structural Molecular Biology Program is supported by the Department of Energy, Office of Biological and Environmental Research, and by the National Institutes of Health, National Center for Research Resources, Biomedical Technology Program.

5. CONCLUSIONS In summary, a binuclear ZrOCo(II) MMCT unit covalently anchored on a silica mesopore surface has been assembled and an oxo-bridged structure established by EXAFS curve fitting analysis. The coordination geometry of the Zr and Co metal centers of the ZrOCo moiety closely resembles that of the isolated centers in monometallic Zr and Co−SBA-15 samples. The oxo linkage of the two metal centers is further manifested by spectral shifts of the optical Co(II) spin−orbit bands, which is attributed to reduced π-electron donating ability of the bridging oxygen caused by the electron withdrawing Zr center. In the presence of a sacrificial electron donor (triethylamine or diethylamine), excitation of the MMCT chromophore at 420 nm and shorter wavelengths resulted in the reduction of gas phase CO2 to gas phase CO and HCO2−. The branching between carbon monoxide and formate is determined by the fate of NEt3 radical cation formed upon rereduction of transient Co(III) in the excited Zr(III)OCo(III) unit, namely the competition between deprotonation of [N(CH2CH3)3]+ versus H atom transfer to CO2. The ZrOCo(II) unit on a silica surface constitutes the first example of an all-inorganic heterobinuclear unit for the photoinduced splitting of CO2 to free CO. Adding a new heterobinuclear system to our previously reported ZrOCu(I) unit for photochemical CO2 splitting,14 the ZrOCo(II) system expands our ability to tailor the potential of the donor metal to the potential of the catalyst for improved solar conversion efficiency. Matching of the redox potential of light absorber, catalyst, and half-reactions is essential for achieving thermodynamic efficiency for carbon dioxide photoreduction. Importantly, transient Co(III) formed upon MMCT excitation should possess sufficient oxidation potential for driving a water oxidation catalyst and thereby utilize H2O as electron source. Work is in progress in our laboratory to replace the sacrificial donor with a multielectron catalyst for water oxidation and thereby close the photosynthetic cycle.

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected]; Ph 510 486 4325 (H.F.). Present Addresses

M.L.M.: The Clorox Company, 7200 Johnson Drive, Pleasanton, CA 94588. H.S.S.: Division of Chemistry and Biological Chemistry, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371. Notes

The authors declare no competing financial interest. 7883



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Microporous Zr-Silicalites Studied by EXAFS and ESR Techniques. J. Catal. 2001, 200, 250−258. (37) Imamura, S.; Yamashita, T.; Utani, K.; Kanai, H.; Hamada, K. The Structure of Zirconium Oxide in Silica Matrix and Its Epoxidation Activity. React. Kinet. Catal. Lett. 2001, 72, 11−20. (38) Newville, M.; Kas, J. J.; Rehr, J. J. Improvements in Modeling EXAFS with Many-Pole Self-Energy and FEFF8.5. J. Phys.: Conf. Ser. 2009, 190, 012023. (39) Poiarkova, A. V.; Rehr, J. J. Multiple-Scattering X-RayAbsorption Fine-Structure Debye-Waller Factor Calculations. Phys. Rev. B 1999, 59, 948−957. (40) Rehr, J. J.; Albers, R. C. Theoretical Approaches to X-Ray Absorption Fine Structure. Rev. Mod. Phys. 2000, 72, 621−654. (41) Seisenbaeva, G. A.; Gohil, S.; Kessler, V. G. Influence of Heteroligands on the Composition, Structure and Properties of Homo- and Heterometallic Zirconium Alkoxides. Decisive Role of Thermodynamic Factors in Their Self-Assembly. J. Mater. Chem. 2004, 14, 3177−3190. (42) Bouh, A. O.; Rice, G. L.; Scott, S. L. Mono- and Dinuclear SilicaSupported Titanium(IV) Complexes and the Effect of TiOTi Connectivity on Reactivity. J. Am. Chem. Soc. 1999, 121, 7201−7210. (43) Ferguson, J. D.; Yoder, A. R.; Weimer, A. W.; George, S. M. TiO2 Atomic Layer Deposition on ZrO2 Particles Using Alternating Exposures of TiCl4 and H2O. Appl. Surf. Sci. 2004, 226, 393−404. (44) Ishigame, M.; Sakurai, T. Temperature Dependence of the Raman Spectra of ZrO2. J. Am. Ceram. Soc. 1977, 60, 367−369. (45) Kanan, M. W.; Yano, J.; Surendranath, Y.; Dinca, M.; Yachandra, V. K.; Nocera, D. G. Structure and Valency of a Cobalt-Phosphate Water Oxidation Catalyst Determined by in Situ X-Ray Spectroscopy. J. Am. Chem. Soc. 2010, 132, 13692−13701. (46) Tohji, K.; Udagawa, Y.; Tanabe, S.; Ida, T.; Ueno, A. Catalyst Preparation Procedure Probed by EXAFS Spectroscopy. 2. Cobalt on Titania. J. Am. Chem. Soc. 1984, 106, 5172−5178. (47) Koyano, G.; Watanabe, H.; Okuhara, T.; Misono, M. Structure and Catalysis of Cobalt Oxide Overlayers Prepared on Zirconia by Low-Temperature-Plasma Oxidation. J. Chem. Soc., Faraday Trans. 1996, 92, 3425−3430. (48) Han, J. D.; Woo, S. I. UV Vis Diffuse Reflectance Spectroscopic (DRS) Study of Cobalt-Containing Y-Zeolites Dehydrated at Elevated-Temperatures. Korean J. Chem. Eng. 1991, 8, 235−239. (49) Baltrusaitis, J.; Schuttlefield, J.; Zeitler, E.; Grassian, V. H. Carbon Dioxide Adsorption on Oxide Nanoparticle Surfaces. Chem. Eng. J. 2011, 170, 471−481. (50) Busca, G.; Lorenzelli, V. Infrared Spectroscopic Identification of Species Arising from Reactive Adsorption of Carbon Oxides on MetalOxide Surfaces. Mater. Chem. 1982, 7, 89−126. (51) Santoro, M.; Gorelli, F.; Haines, J.; Cambon, O.; Levelut, C.; Garbarino, G. Silicon Carbonate Phase Formed from Carbon Dioxide and Silica under Pressure. Proc. Natl. Acad. Sci. U. S. A. 2011, 108, 7689−7692. (52) Wang, M.; Na, Y.; Gorlov, M.; Sun, L. C. Light-Driven Hydrogen Production Catalysed by Transition Metal Complexes in Homogeneous Systems. Dalton Trans. 2009, 6458−6467. (53) Carpenter, B. K. Computational Study of CO2 Reduction by Amines. J. Phys. Chem. A 2007, 111, 3719−3726. (54) De la Fuente, J. R.; Cañ ete, A.; Saitz, C.; Jullian, C. Photoreduction of 3-Phenylquinoxalin-2-Ones by Amines: TransientAbsorption and Semiempirical Quantum-Chemical Studies. J. Phys. Chem. A 2002, 106, 7113−7120. (55) Yeom, Y. H.; Frei, H. Step-Scan FT-IR Monitoring of Transient HCO Radicals in a Room Temperature Zeolite. J. Phys. Chem. B 2003, 107, 6286−6291. (56) Redington, R. L. Vibrational-Spectra and Normal Coordinate Analysis of Isotopically Labeled Formic-Acid Monomers. J. Mol. Spectrosc. 1977, 65, 171−189. (57) Bellamy, L. J. The Infrared Spectra of Complex Molecules, 3rd ed.; Chapman and Hall: London, 1975; Vol. 1, p 300. (58) Miessler, G. L.; Tarr, D. A. Inorganic Chemistry, 3rd ed.; Prentice Hall: Englewood Cliffs, NJ, 2004; p 367.

(16) Han, H. X.; Frei, H. Controlled Assembly of Hetero-Binuclear Sites on Mesoporous Silica: Visible Light Charge-Transfer Units with Selectable Redox Properties. J. Phys. Chem. C 2008, 112, 8391−8399. (17) Han, H. X.; Frei, H. Visible Light Absorption of Binuclear TiOCoII Charge-Transfer Unit Assembled in Mesoporous Silica. Microporous Mesoporous Mater. 2007, 103, 265−272. (18) Han, H. X.; Frei, H. In Situ Spectroscopy of Water Oxidation at Ir Oxide Nanocluster Driven by Visible TiOCr Charge-Transfer Chromophore in Mesoporous Silica. J. Phys. Chem. C 2008, 112, 16156−16159. (19) Nakamura, R.; Frei, H. Visible Light-Driven Water Oxidation by Ir Oxide Clusters Coupled to Single Cr Centers in Mesoporous Silica. J. Am. Chem. Soc. 2006, 128, 10668−10669. (20) Nakamura, R.; Okamoto, A.; Osawa, H.; Irie, H.; Hashimoto, K. Design of All-Inorganic Molecular-Based Photocatalysts Sensitive to Visible Light: Ti(IV)-O-Ce(III) Bimetallic Assemblies on Mesoporous Silica. J. Am. Chem. Soc. 2007, 129, 9596−9597. (21) Okamoto, A.; Nakamura, R.; Osawa, H.; Hashimoto, K. SiteSpecific Synthesis of Oxo-Bridged Mixed-Valence Binuclear Complexes on Mesoporous Silica. Langmuir 2008, 24, 7011−7017. (22) Wu, X. N.; Weare, W. W.; Frei, H. Binuclear TiOMn ChargeTransfer Chromophore in Mesoporous Silica. Dalton Trans. 2009, 10114−10121. (23) Frei, H. Polynuclear Photocatalysts in Nanoporous Silica for Artificial Photosynthesis. Chimia 2009, 63, 721−730. (24) Cuk, T.; Weare, W. W.; Frei, H. Unusually Long Lifetime of Excited Charge-Transfer State of All-Inorganic Binuclear TiOMnII Unit Anchored on Silica Nanopore Surface. J. Phys. Chem. C 2010, 114, 9167−9172. (25) Soo, H. S.; Macnaughtan, M. L.; Weare, W. W.; Yano, J.; Frei, H. EXAFS Spectroscopic Analysis of Heterobinuclear TiOMn ChargeTransfer Chromophore in Mesoporous Silica. J. Phys. Chem. C 2011, 115, 24893−24905. (26) Blatter, F.; Moreau, F.; Frei, H. Diffuse-Reflectance Spectroscopy of Visible Alkene-O2 Charge-Transfer Absorptions in Zeolite-Y and Determination of Photooxygenation Quantum Efficiencies. J. Phys. Chem. 1994, 98, 13403−13407. (27) Osuka, T.; Morikawa, H.; Marumo, F.; Tohji, K.; Udagawa, Y.; Yasumori, A.; Yamane, M. Changes in Zr Coordination-Number During the Pyrolysis of a SiO2-ZrO2 Gel. J. Non-Cryst. Solids 1986, 82, 154−159. (28) Li, P.; Chen, I. W.; Penner-Hahn, J. E. X-Ray-Absorption Studies of Zirconia Polymorphs. I. Characteristic Local Structures. Phys. Rev. B: Condens. Matter 1993, 48, 10063−10073. (29) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science 1998, 279, 548−552. (30) Zhao, D. Y.; Huo, Q. S.; Feng, J. L.; Chmelka, B. F.; Stucky, G. D. Nonionic Triblock and Star Diblock Copolymer and Oligomeric Surfactant Syntheses of Highly Ordered, Hydrothermally Stable, Mesoporous Silica Structures. J. Am. Chem. Soc. 1998, 120, 6024− 6036. (31) Jiao, F.; Frei, H. Nanostructured Cobalt Oxide Clusters in Mesoporous Silica as Efficient Oxygen-Evolving Catalysts. Angew. Chem., Int. Ed. 2009, 48, 1841−1844. (32) Kang, K. K.; Ahn, W. S. Physiochemical Properties of Transition Metal-Grafted MCM-48 Prepared Using Metallocene Precursors. J. Mol. Catal. A: Chem. 2000, 159, 403−410. (33) Maschmeyer, T.; Rey, F.; Sankar, G.; Thomas, J. M. Heterogeneous Catalysts Obtained by Grafting Metallocene Complexes onto Mesoporous Silica. Nature 1995, 378, 159−162. (34) Lever, A. B. P. Inorganic Electronic Spectroscopy, 2nd ed.; Elsevier: Amsterdam, 1984; p 496. (35) Morey, M. S.; Stucky, G. D.; Schwarz, S.; Froba, M. Isomorphic Substitution and Postsynthesis Incorporation of Zirconium into MCM-48 Mesoporous Silica. J. Phys. Chem. B 1999, 103, 2037−2041. (36) Ramaswamy, V.; Tripathi, B.; Srinivas, D.; Ramaswamy, A. V.; Cattaneo, R.; Prins, R. Structure and Redox Behavior of Zirconium in 7884



Article

(59) Andersen, L. K.; Frei, H. Dynamics of Co in Mesoporous Silica Monitored by Time-Resolved Step-Scan and Rapid-Scan FT-IR Spectroscopy. J. Phys. Chem. B 2006, 110, 22601−22607. (60) Koppenol, W. H.; Rush, J. D. Reduction Potential of the CO2/ CO-Couple - a Comparison with Other C1 Radicals. J. Phys. Chem. 1987, 91, 4429−4430. (61) Schwarz, H. A.; Dodson, R. W. Reduction Potentials of CO2 and the Alcohol Radicals. J. Phys. Chem. 1989, 93, 409−414. (62) Kim, E. H.; Bradforth, S. E.; Arnold, D. W.; Metz, R. B.; Neumark, D. M. Study of HCO2 and DCO2 by Negative-Ion Photoelectron-Spectroscopy. J. Chem. Phys. 1995, 103, 7801−7814.

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Binuclear ZrOCo Metal-to-Metal Charge-Transfer Unit in Mesoporous

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