Adsorption and Photochemical Properties of a Molecular CO2

Letter pubs.acs.org/JPCL

Adsorption and Photochemical Properties of a Molecular CO2 Reduction Catalyst in Hierarchical Mesoporous ZSM-5: An In Situ FTIR Study Kevin D. Dubois,† Anton Petushkov,‡ Elizabeth Garcia Cardona,† Sarah C. Larsen,‡ and Gonghu Li*,† †

Department of Chemistry and Materials Science Program, University of New Hampshire, Durham, New Hampshire 03824, United States ‡ Department of Chemistry, University of Iowa, Iowa City, Iowa 52242, United States S Supporting Information *

ABSTRACT: As part of our recent effort to attach well-defined molecular photocatalysts to solid-state surfaces, this present study investigates adsorption and photochemical properties of a tricarbonyl rhenium(I) compound, Re(bpy)(CO)3Cl (bpy = 2,2′-bipyridine), in hierarchical mesoporous ZSM-5. The molecular Re(I) catalyst, a Ru(bpy)32+ photosensitizer, and an aminebased electron donor were coadsorbed in the mesopores of the hierarchical ZSM-5 through simple liquid-phase adsorption. The functionalized ZSM-5 was then characterized with infrared and UV−visible spectroscopies and was tested in CO2 reduction photocatalysis at the gas− surface interface. In the mesoporous ZSM-5, CO2 molecules were adsorbed on the amine electron-donor molecules as bicarbonate, which would release CO2 upon light irradiation to react with the Re(I) catalyst. The formation of important reaction intermediates, particularly a Recarboxylato species, was revealed with in situ Fourier transform infrared spectroscopy in combination with isotopic labeling. The experimental results indicate that hierarchical mesoporous zeolites are promising host materials for molecular photocatalysts and that zeolite mesopores are potential “reaction vessels” for CO2 reduction photocatalysis at the gas−solid interface. SECTION: Surfaces, Interfaces, Catalysis

C

in solar photocatalysis.19−24 Surface-immobilized molecular catalysts are much easier to recover than unbound homogeneous catalysts.25 More importantly, surface immobilization could significantly improve the efficiency and stability of molecular catalysts under photochemical conditions. Recently, Takeda and co-workers synthesized a tricarbonyl Re(I) complex covalently anchored in a light-absorbing periodic mesoporous organosilica.26 Upon UV light activation of the organosilica and subsequent resonance energy transfer, photocatalysis on the Re(I) complex led to enhanced CO2-to-CO conversion by a factor of 4.4 compared with direct excitation of the Re(I) complex.26 Furthermore, it was shown that the mesoporous structure of the organosilica protected the Re(I) complex against photochemical decomposition.26 Wang and coworkers incorporated Re(dcbpy)(CO)3Cl (dcbpy = 2,2′bipyridine-5,5′-dicarboxylic acid) in a highly stable and porous metal−organic framework.27 In photochemical CO2 reduction, the total turnover number using the supported Re(I) catalyst was three times higher than that of the corresponding homogeneous complex.27 Park and co-workers investigated the photochemical properties of tricarbonyl Re(I) catalysts encapsulated in NaY and Al-MCM-41 zeolites.28−30 Under light

arbon dioxide is a renewable C1 feedstock for the production of chemicals, materials, and fuels.1,2 Several recent review articles3−5 have discussed available approaches for direct chemical conversion of CO2, including hydrogenation6,7 and electrochemical8,9 and photochemical reduction.10−12 Photochemical and photoelectrochemical approaches are the preferable long-term solutions for CO2 reduction that employ sustainable energy input. Homogeneous transition-metal catalysts, such as complexes of Ru, Re, Co, and Ni, are capable of mediating efficient multielectron CO2 reduction.10,13 For instance, tricarbonyl rhenium(I) compounds,14,15 particularly Re(bpy)(CO)3Cl (1, bpy = 2,2′-bipyridine), have been extensively studied as homogeneous photocatalysts and electrocatalysts for CO2 reduction since the early report by Hawecker and co-workers.16 In photochemical CO2 reduction using homogeneous transition-metal catalysts, the coupling of light absorption and charge separation with dark catalytic reactions can be quite efficient. However, the photocatalysts are usually not stable enough to achieve high turnover numbers. This present study investigates adsorption and photochemical properties of compound 1 in hierarchical mesoporous ZSM-5 (denoted meso-ZSM-5)17 as part of our recent effort to develop robust CO2 reduction photocatalysts consisting of well-defined molecular catalysts attached to solid-state surfaces. A variety of molecular catalysts, including compound 1,18 have been immobilized on solid-state surfaces for potential use © 2012 American Chemical Society

Received: December 27, 2011 Accepted: January 30, 2012 Published: January 30, 2012 486

dx.doi.org/10.1021/jz201701y | J. Phys. Chem. Lett. 2012, 3, 486−492

The Journal of Physical Chemistry Letters

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distinct, well-resolved CO stretching bands at 2025, 1918, and 1902 cm−1 were seen in its infrared spectrum.37 The existence of multiple ν(CO) bands in the spectrum shown in Figure 1 is an indication of the heterogeneous nature of surface-adsorbed 1 on different sites in meso-ZSM-5. While a portion of 1 adsorbed on meso-ZSM-5 could be simply “sitting” in the mesopores, certain interactions between 1 and the surface of meso-ZSM-5 exist and can be probed with FTIR spectroscopy. For example, a potential surface adsorption site for 1 is silanol groups on meso-ZSM-5.17 In order to probe such a surface adsorption pattern, compound 1 adsorbed in meso-ZSM-5 was exposed to a flow of triethylamine (TEA) vapor. Figure 1b shows the spectrum of 1 adsorbed in meso-ZSM-5 after exposing to TEA. It can be seen from the spectra shown in Figure 1 that two carbonyl absorptions at 2039 and 1941 cm −1 quickly disappeared (or at least the peak intensities decreased significantly) upon the TEA flow, while other carbonyl peaks gained in intensity. Under the same experimental conditions, the disappearance of silanol groups upon exposing bare mesoZSM-5 to TEA was observed (not shown). Comparison between the spectra shown in Figure 1 suggests that a certain amount of 1 was adsorbed in meso-ZSM-5 via hydrogen bonding between the carbonyl ligands and the surface silanol groups (−SiOH···OC−Re) and that TEA interacts more strongly than the carbonyl groups with the silanol groups. It is worth noting that silanol groups are potential proton donors for photochemical CO2 reduction.38 As CO2 reduction photocatalysts, most tricarbonyl Re(I) compounds do not have significant absorption in the visible light region. Many studies used mixtures of molecular Re(I) catalysts and additional photosensitizers such as Ru(bpy)32+ (2) to achieve enhanced visible light photocatalysis. The catalysts and photosensitizers can also be covalently linked, forming supramolecular photocatalysts that have shown great promise for efficient CO2 reduction.39,40 Tertiary amines, including TEA and triethanolamine (TEOA), are common sacrificial electron donors that can quench the excited states of photosensitizers. In our study, the Re(I) catalyst (1), the Ru(bpy) 3 2+ photosensitizer (2), and TEOA were coadsorbed in mesoZSM-5 by simple mixing in TEA for use in photochemical studies. The FTIR spectrum of functionalized meso-ZSM-5 containing both 1 and 2 is almost identical to that of 1 in mesoZSM-5 in the spectral region of 2100−1300 cm−1 (Figure S3, Supporting Information). While the addition of 2 did not significantly alter the infrared spectrum of 1 in meso-ZSM-5, the photoresponse of meso-ZSM-5 in the visible light region was markedly enhanced in the presence of 2, which strongly absorbs light with wavelengths between 400 and 600 nm (Figure S4, Supporting Information). In our study, no covalent attachment exists between 1 and 2. Rather, the catalyst (1) and the photosensitizer (2) were coadsorbed in meso-ZSM-5. As will be discussed later in photochemical studies, energy transfer occurred between the photosensitizer and the catalyst in the mesopores. Functionalized meso-ZSM-5 samples were evaluated in photochemical CO2 reduction using a three-window reaction chamber equipped with infrared detection. The reaction chamber allows study of photochemical reactions at the gas− solid interface. Prior to photochemical studies, a catalyst sample in the reaction chamber was purged with argon at 60 °C for 1 h. The reaction chamber was then filled with CO2 at ambient pressure. Infrared spectra of the catalyst samples were recorded before and after exposing the sample to gaseous CO2. It can be

(λ > 350 nm) irradiation, CO2 was converted into CO and CO32− species on the encapsulated Re(I) catalysts without using organic solvents as reaction media.28 It was assumed that zeolite frameworks functioned as electron donors for CO2 reduction on the Re(I) catalysts encapsulated in the zeolites.28 This present study explores the advantages of using hierarchical mesoporous zeolites as host materials for molecular CO2 reduction photocatalysts. It is hypothesized that the confined environment of mesopores will allow facile adsorption of molecular catalysts and subsequent photochemical events relevant to CO2 reduction in the presence of a sacrificial electron donor. The mesopores would function as “reaction vessels”, eliminating the use of organic solvents and improving mass transfer of gaseous CO2. It is known that the solubility of CO2 in most solvents is low, and CO2 binding to molecular catalysts is often the rate-determining step for CO2 reduction in homogeneous systems.10 The use of mesoporous zeolites could further enhance adsorption of CO2 in the mesopores and subsequently facilitate CO2 uptake by the molecular catalysts. Mesoporous zeolites have been previously employed as host materials for highly active CO2 reduction photocatalysts, including single-site Ti oxides,31,32 which often require UV excitation and bimetallic redox sites33−35 that split CO2 upon visible light excitation. In our study, a tricarbonyl Re(I) catalyst (1) was adsorbed in meso-ZSM-5 by simply stirring in an organic solvent such as ethanol or triethylamine. A commercially available ZSM-5 zeolite with very limited mesoporosity was also used in the adsorption study in order to probe the importance of mesoporosity. Spectroscopic studies demonstrated that only a negligible amount of 1 was adsorbed in the commercially available ZSM-5, while a significant amount of 1 remained adsorbed in meso-ZSM-5 after thoroughly washing with ethanol (Figures S1 and S2, Supporting Information). Because micropores and channels of the ZSM-5 materials are not large enough to accommodate 1, it was concluded that compound 1 resides in the mesopores of meso-ZSM-5. The infrared spectrum of 1 adsorbed in meso-ZSM-5 shown in Figure 1a features multiple overlapping CO bands in the

Figure 1. FTIR spectra of 1 adsorbed in meso-ZSM-5 (a) before and (b) after exposing to triethylamine (TEA). The spectra were taken in the dark at room temperature.

spectral region of 2100−1800 cm−1, which are typical CO stretching bands of facial Re(I) tricarbonyl systems.36,37 For a homogeneous solution of compound 1 in acetonitrile, three 487



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Figure 2. FTIR spectra of (a) meso-ZSM-5 in Ar, (b) meso-ZSM-5 in the presence of CO2, (c) 1 + 2 in meso-ZSM-5 in Ar, and (d) 1 + 2 in meso-ZSM-5 in the presence of CO2. Spectra (a) and (c) are plotted with dotted lines. All samples contain TEOA.

Figure 3. Difference FTIR spectra of functionalized meso-ZSM-5 under different reaction conditions for 30 min, (a) 1 + 2 + CO2 in the dark at 45 °C, (b) 1 + 2 + Ar under light, (c) 1 + CO2 under light, and (d) 1 + 2 + CO2 under light. All samples contain TEOA.

seen from the spectra shown in Figure 2b and d (solid lines) that an intense absorption at 1650 cm−1 appeared upon CO2 adsorption. This absorption peak is associated with O−C−O stretching of bidentate bicarbonate species,41−44 which formed upon CO2 adsorption on TEOA in meso-ZSM-5. An isotopic shift of 50 cm−1 was observed for the O−C−O stretching band when exposing the samples to 13CO2 (Figure S5, Supporting Information), further confirming the assignment. The adsorption peak at 1650 cm−1 significantly decreased in intensity when gaseous CO2 in the reaction chamber was replaced with argon, suggesting that an adsorption/desorption equilibrium exists between gaseous CO2 and surface-adsorbed bicarbonate in the presence of TEOA and residual water (reaction 1).43

CO2 led to the appearance of new absorptions at 2017, 1911, and 1867 cm−1 in the difference spectrum (Figure 3d). These new absorptions are most likely associated with the formation of a Re-carboxylato species, Re(bpy)(CO)3(COOH), which was predicted to give ν(CO) frequencies at 2014, 1914, and 1864 cm−1.45 A negative peak at around 2030 cm−1 (not labeled) is also present in the difference spectrum (Figure 3d), suggesting the loss of ν(CO) of Re(bpy)(CO)3Cl species due to changes in the coordination sphere of the Re(I) center. Comparison between the spectra shown in Figure 3 (along with isotopic studies discussed later) clearly indicates that photochemical CO2 reduction occurred in Re(I)-functionalized mesoZSM-5. Furthermore, photochemical CO2 reduction did not occur to a great extent in the absence of the Ru(II) photosensitizer (Figure 3c). Because the Ru(II)-containing sample absorbs a greater portion of visible light than the Re(I)-only sample (Figure S4, Supporting Information), such enhancement in visible light photocatalysis implies that energy transfer from the Ru(II) photosensitizer to the Re(I) catalyst occurred in the mesopores of meso-ZSM-5. Another prominent feature in the difference spectrum of 1 + 2 in meso-ZSM-5 upon light irradiation is a negative peak at 1650 cm−1 associated with loss of the bicarbonate species (Figure 3d). In the presence of gaseous CO2, the loss of the carbonate species did not occur in the dark control experiment (Figure 3a) but was observed on functionalized meso-ZSM-5 containing only the Ru(II) photosensitizer (2) and TEOA. This observation suggests that the loss of bicarbonate species was largely mediated by 2 under visible light irradiation. Most likely, CO2 was released upon the loss of bicarbonate according to the adsorption/desorption equilibrium described in reaction 1. The CO2 molecules released from bicarbonate could not be detected by using FTIR or GC in our study because the amount of released CO2 was much too small in comparison to amount of the gaseous CO2 already present in the reactor. Prolonged visible light irradiation in the presence of CO2 led to the formation of additional Re(bpy)(CO)3(COOH) species on functionalized meso-ZSM-5 containing 1 + 2 and TEOA, as characterized by absorptions at 2017, 1911, and 1867 cm−1, which increased over time (Figure S6, Supporting Information). An absorption at 1923 cm−1 became more pronounced at

(C2H 4OH)3 N + CO2 + H2O ⇌ (C2H 4OH)3 NH+ + HCO3−

(1)

It should be noted that the band at 1650 cm−1 is also seen in the spectra (Figure 2a and c, dotted lines) prior to exposing the samples to CO2 in the reaction chamber. This is due to the fact that these functionalized samples were prepared under ambient conditions in the presence of atmospheric CO2, which adsorbed on TEOA in the form of bicarbonate. Because of the equilibrium shown in reaction 1, the surface-adsorbed bicarbonate species could desorb, releasing CO2 to participate in photochemical reduction. Formation of carbamate species, a possible product in the reaction between CO2 and TEOA, was not observed in our FTIR study in combination with 13C isotope labeling. In photochemical studies, infrared spectra were collected before and after visible light (λ > 425 nm) irradiation. Information on photochemical processes in the functionalized meso-ZSM-5 samples can be derived from difference FTIR spectra, which were obtained by subtracting spectra prior to light irradiation from corresponding spectra after irradiation. Figure 3 shows the difference FTIR spectra of functionalized meso-ZSM-5 samples after visible light irradiation for 30 min. The difference spectrum of a dark control experiment is also included in Figure 3. No appreciable change is seen in the difference spectrum of 1 + 2 in meso-ZSM-5 in the absence of either visible light irradiation (Figure 3a) or CO2 (Figure 3b). For the same sample, visible light irradiation in the presence of 488



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reaction chamber (not shown) contains an absorption at 1600 cm−1, characteristic of 13C-substitued bicarbonate (H13CO3−). The new sample was then subjected to visible light irradiation in the presence of gaseous 13CO2 prior to collecting the difference spectrum shown in Figure 4c. It can be seen from Figure 4c that the absorption at 2004 cm−1 and a broad band centered at 1855 cm−1 appeared upon visible light irradiation. In this study, these two peaks are assigned to 13C-containing Re-carboxylato species, Re(bpy)(CO)2(13CO)(COOH). This assignment suggests an isotopic shift of 12−13 cm−1 (2017 and 1867 cm−1 versus 2004 and 1855 cm−1, respectively). Such an isotopic shift for ν(CO) is reasonable for Re(bpy)(CO)3(COOH), which contains multiple CO ligands on the same metal center. For example, the ν(CO) frequencies for Rh(12CO)2, Rh(12CO)(13CO), and Rh(13CO)2 were observed at 2092, 2075, and 2043 cm−1, respectively.48 Formation of the Re-carboxylato and Re-formato species clearly indicates that photochemical CO2 reduction occurred on the molecular Re(I) complex adsorbed in meso-ZSM-5. Specifically, the generation of Re(bpy)(CO)2(13CO)(COOH) from Re(bpy)(CO)3Cl suggests that at least one 13CO2 molecule was reduced into a 13CO ligand on the Re(I) center (Figure 4, right). The original carbonyl ligand would be released as gaseous or surface-adsorbed CO. In this study, the formation of CO was detected by using gas chromatography when the functionalized meso-ZSM-5 was tested photochemically in a solution of acetonitrile. However, the formation of gaseous or surface-adsorbed CO was not observed in the diffuse reflectance reaction chamber after photochemical reactions. This indicates that the quantity of CO produced was too low to be detected by infrared spectroscopy in this study. The use of a nonvolatile electron donor (TEOA) represents a possible reason for the low CO yield when CO2 reduction photocatalysis was carried out at the gas−surface interface. In mesoZSM-5, most of the surface-adsorbed Re(I) catalyst, Ru(II) photosensitizer, and TEOA were spatially isolated in comparison to those in homogeneous solutions. Therefore, the yield of CO could be largely improved by using volatile electron donors such as TEA. In addition, the FTIR spectra shown in this study contain information of both the surface and the gas phase, with the former being dominant in intensity because of the diffuse reflectance mode adopted in this study. This fact made it even more difficult to detect gaseous CO under the experimental conditions. A transmission FTIR cell,49−51 which can separate information of the surface from that of the gas phase, is currently being used to investigate photochemical CO2 reduction on tricarbonyl Re(I) catalysts adsorbed in mesoporous materials. In our study, TEOA as a sacrificial electron donor was needed for photocatalysis to occur on the surface-adsorbed Re(I) centers. The use of such nonrenewable sacrificial agents represents a major drawback of current CO2 reduction photocatalytic systems. This challenge could be solved, at least partially, by using renewable amines52 or renewable hydrides.53 Water is the ideal electron donor for photocatalytic CO2 reduction and has been considered as the proton source in CO2 reduction photocatalysis.54,55 Furthermore, the proton source for CO2 reduction (CO2 + 2H+ + 2e− → CO + H2O) in our study is unclear based on available experimental results. Our recent theoretical study56 investigated triethylamine as both the hydrogen atom and electron donor for CO2 reduction and proposed a catalytic cycle to explain the production of formate on Re(bpy)(CO)3Cl based on prior work57,58 that

longer reaction times under visible light irradiation (Figure S6, Supporting Information). This new absorption might be associated with a Re-formato species, Re(bpy)(CO)3(OCHO).46 The Re-carboxylato and Re-formato species are known intermediates for the generation of CO and formic acid, respectively, in photochemical CO2 reduction.10 In our study, neither the OCO stretch (∼1572 cm−1)47 of the COOH group of the Re-carboxylato species nor the COO stretch (∼1630 cm−1)46 of the OCHO group of the Re-formato species was observed, largely due to significant changes of the band at 1650 cm−1 upon visible light irradiation (Figure 3d). Isotopic studies were conducted to confirm that photochemical CO2 reduction did occur on the molecular Re(I) catalyst adsorbed in meso-ZSM-5. Figure 4 shows the difference

Figure 4. Difference FTIR spectra of 1 + 2 in meso-ZSM-5 after visible light irradiation for 120 min in the presence of (a) H12CO3− and 12 CO2, (b) H12CO3− and 13CO2, and (c) H13CO3− and 13CO2. The yaxis of the individual spectrum was slightly adjusted for better comparison. Possible structures of Re-carboxylato species corresponding to the absorptions shown in (a) and (c) are shown on the right. All samples contain TEOA.

spectra of 1 + 2 in meso-ZSM-5 after visible light irradiation in the presence of 12CO2 or 13CO2. When Re(I)-functionalized meso-ZSM-5 was exposed to gaseous 13CO2, new absorption features, particularly a peak at 2004 cm−1 (Figure 4b), appeared together with the absorption bands at 2017, 1923, 1911, and 1867 cm−1, which were observed in the presence of gaseous 12 CO2 (Figure 4a). It should be noted that the sample used to collect the spectrum shown in Figure 4b was prepared under ambient conditions in the presence 12CO2. As a result, bicarbonate (H12CO3−; see Figure 2c) formed during sample preparation and remained in the functionalized meso-ZSM-5 samples even after exposing to gaseous 13CO2. As discussed earlier, the bicarbonate (H12CO3−) species would desorb according to reaction 1, releasing gaseous 12CO2. Therefore, the spectrum shown in Figure 4b was actually collected in the presence of gaseous 13CO2 and a small amount of gaseous 12 CO2. This explains the presence of the absorption bands at 2017, 1923, 1911, and 1867 cm−1, which would be generated in the presence of 12CO2, in the spectrum shown in Figure 4b. In order to minimize the content of H12CO3− (and 12CO2) in the isotopic studies, a new sample was prepared by functionalizing meso-ZSM-5 with 1 + 2 and TEOA in the presence of gaseous 13CO2. The infrared spectrum of the new sample collected without exposure to gaseous 13CO2 in the 489



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TEOA/Re = 140) in TEA for 24 h before being collected and dried at room temperature. UV−Visible and FTIR Spectroscopies. UV−visible spectra were obtained on a Cary 50 Bio spectrophotometer fitted with a Barrelino diffuse reflectance probe using BaSO4 as a standard and powder samples pressed on BaSO4 pellets. Infrared spectra were collected on a Nicolet 6700 FTIR spectrometer equipped with a Harrick Praying Mantis diffuse reflectance accessory, a three-window high-temperature photoreactor chamber, and a DTGS detector. Adsorption and Photochemical Studies. In adsorption studies, a powder sample in the photoreactor chamber was first purged with Ar at room temperature for 1 h. The sample was then cooled to room temperature under Ar prior to collecting spectra. In photochemical experiments, a mixture (∼10 mg) of KBr and functionalized ZSM-5 (mass ratio 4:1) in powder form was first loaded into the photoreactor chamber and was purged with Ar at 60 °C for 1 h. The sample was then cooled to room temperature and was exposed to a flow of CO2 (99.999%, Airgas) for 5 min. The photoreactor chamber full of CO2 was then closed and remained closed during photochemical studies. In isotopic studies, 13CO2 (>99% 13C, 425 nm) was provided using a Fiber-Lite series 180 illuminator.19 An optical fiber was used to guide light irradiation to reach the sample through a KBr window of the photoreactor chamber. The intensity of the visible light was measured to be 300 mW/cm2. Under light irradiation, the sample’s temperature was recorded to be between 30 and 35 °C.

does not involve transition-metal centers. Potential proton sources in this present study include TEOA, residual water, and surface silanol groups in the meso-ZSM-5. Experiments are underway to probe potential pathways for photochemical CO2 reduction on surface-immobilized Re(I) photocatalysts in mesoporous zeolites. Further studies also include a quantitative evaluation of the robustness of the surface-adsorbed Re(I) complex in the dark and under photochemical conditions. In a preliminary test, CO was detected by GC after photochemical CO2 reduction in a solution of acetonitrile using a meso-ZSM-5 sample functionalized with both 1 and 2 that was stored in the dark at room temperature for 6 months. In summary, we have investigated photochemical CO2 reduction at the gas−solid interface using a molecular Re(I) catalyst adsorbed in the mesopores of hierarchical ZSM-5 in the presence of TEOA as a sacrificial electron donor. It was observed that a bicarbonate species was produced upon CO2 adsorption on TEOA and that the bicarbonate species would be released as CO2 to participate in photochemical reduction upon visible light irradiation. Spectroscopic and isotopic studies demonstrated that photochemical CO2 reduction occurred on the surface-adsorbed Re(I) catalyst. The steady formation of a Re-carboxylato species, an important reaction intermediate for CO production, was observed under visible light irradiation. This supports our hypothesis that zeolite mesopores could function as “reaction vessels” for photochemical CO2 reduction at the gas−solid interface, eliminating the use of organic solvents and potentially improving mass transfer of CO2. Reasonable efforts are needed to tune the reaction system by optimizing interactions among catalytic centers, photosensitizers, and amine-based electron donors to achieve sustained CO2 reduction photocatalysis. Such studies would provide useful insight toward designing robust and efficient photocatalysts consisting of transition-metal complexes immobilized in mesoporous materials.

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ASSOCIATED CONTENT

S Supporting Information *

FTIR and UV−visible spectra of functionalized ZSM-5, FTIR spectra of meso-ZSM-5 containing TEOA in the presence of 12 CO2 and 13CO2, and difference FTIR spectra of 1 + 2 in mesoZSM-5 after visible light irradiation for 30, 60, 120, and 240 min in the presence of CO2 and TEOA. This material is available free of charge via the Internet at http://pubs.acs.org.

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EXPERIMENTAL METHODS Materials Synthesis. Synthesis and characterization of the hierarchical mesoporous ZSM-5 (meso-ZSM-5) was described in detail in a recent study.17 A commercially available ZSM-5 was purchased from Zeolyst and was used without further treatment. Mesopore volumes of meso-ZSM-5 and the commercial ZSM-5 were measured to be 0.359 and 0.040 cm3/g, respectively, on a Nova 1200 nitrogen adsorption instrument (Quantachrome). The tricarbonyl Re(I) compound (1) was synthesized following the method described by Smieja and co-workers.37 The 1H NMR spectrum of synthesized 1 in THF-d8 (δH = 7.65 (t), 8.15 (t), 8.50 (d), 9.05 ppm (d)) is in agreement with that in the literature.36 The ruthenium(II) photosensitizer (2), Ru(bpy)3Cl2·H2O, was purchased from Sigma-Aldrich and was used without further treatment. Functionalization of ZSM-5 Materials. For adsorption studies, compound 1 was mixed with ZSM-5 in ethanol by stirring at room temperature for 24 h. The mixture was then thoroughly washed with ethanol and dried at room temperature. For photochemical studies, the catalyst and photosensitizer were adsorbed in the zeolite materials by stirring a mixture of 10 mg of 1, 16 mg of 2 (molecular ratio Ru/Re = 1:1), 100 mg of ZSM-5, and 20 μL of triethanolamine (TEOA) in 5 mL of triethylamine (TEA) at room temperature for 24 h. After finishing stirring, the solid material settled, and the supernatant was decanted. The solid sample was allowed to dry in the air and then mixed with more TEOA (final molecular ratio

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by the National Science Foundation through the Nanoscale Science & Engineering Center for High-rate Nanomanufacturing (NSF EEC 0832785) and the Nanomanufacturing Center of Excellence at the University of Massachusetts, Lowell. We are grateful to Drs. Edward Wong, N. Dennis Chasteen, Roy Planalp, and Richard Johnson for helpful discussion and assistance in various aspects of experiments.

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REFERENCES

(1) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; et al. Catalysis Research of Relevance to Carbon Management: Progress, Challenges, and Opportunities. Chem. Rev. 2001, 101, 953− 996.

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