Photocatalyst System for Hydrogen Production from Water - American

Jun 27, 2007 - the PVS radicals on the RuO2 catalysts. This study demonstrates an architecture for photoinitiated spatial separation of charge across ...
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J. Phys. Chem. C 2007, 111, 10575-10581

10575

An Integrated Zeolite Membrane/RuO2 Photocatalyst System for Hydrogen Production from Water Yanghee Kim and Prabir K. Dutta* Department of Chemistry, The Ohio State UniVersity, Columbus, Ohio 43210 ReceiVed: January 14, 2007; In Final Form: March 20, 2007

A zeolite Y membrane on a RuO2 containing alumina support for photochemical applications is described. Nanofingers of ruthenium oxide are assembled within the alumina support by thermal decomposition of ruthenium dodecacarbonyl followed by oxidation of the metal. Hydrogen evolution was monitored on the “dark” alumina side upon visible light illumination of a solution of sensitizer Ru(bpy)32+, electron acceptor N,N′-trimethylene-2,2′-bipyridinium (DQ2+), and sacrificial electron donor EDTA on the zeolite side of the membrane. The zeolite was ion-exchanged with DQ2+, and propylviologen sulfonate (PVS) was present in solution on the alumina side of the membrane. It is proposed that upon photoexcitation, DQ+• radical is formed in solution on the zeolite side, which can exchange charge with DQ2+ in the zeolite. Charge diffusion via self-exchange of electrons within the DQ2+/zeolite leads to charge migration onto the zeolite/alumina interface of the membrane and electron donation to PVS. Hydrogen is produced by the reduction of water by the PVS radicals on the RuO2 catalysts. This study demonstrates an architecture for photoinitiated spatial separation of charge across an ∼4 µm thick microporous membrane, along with utilization of the charge to produce H2 from water.

Introduction Fueled by the impending decrease in petroleum supplies and global weather changes due to greenhouse gases, there is considerable interest in harnessing solar energy. In particular, the use of solar energy to produce chemicals is one of the most important challenges.1 With plentiful sources of H2O, CO2, and N2, chemicals such as H2, CH3OH, and NH3 can in principle be obtained by the use of solar energy, H2 being the focus of much current research.2 There are significant obstacles to the use of solar energy to make chemicals. On the basis of our understanding of natural photosynthesis, issues that need to be addressed include choice of photosensitizers, electron acceptors, spatial separation of charge, and development of catalyst systems. Numerous studies have been done on the synthesis of photoactive molecules and fundamental electron-transfer dynamics.3,4 Only a few studies exist on coupling of photoactive molecules with an architecture that can sustain a solar-energydriven chemical process;5,6 however, electricity generation via dye-sensitized photovoltaic cells has met with success.7 Since most photochemical processes involve the transfer of a single electron with absorption of a photon, catalyst systems are required to do multielectron chemistry, for example, H2O to O2 involves loss of four electrons. Another critical aspect of the architecture is the spatial separation of the photochemically generated oxidized and reduced equivalents, which in photosynthesis occurs across a membrane. Many artificial photosynthetic models have focused on the formation of H2 from water.8 One of the most extensively studied system is Ru(bpy)32+ as sensitizer, methylviologen (MV2+) as electron acceptor, a sacrificial electron donor to regenerate the Ru(bpy)32+, and Pt as catalyst for the reduction of water.9,10 Since no attempt is made in these systems to * Author to whom correspondence should be addressed. E-mail: [email protected].

separate charges spatially, an integrated water-splitting system without a sacrificial electron donor would be inefficient. In order to address this problem, we have proposed the use of zeolite membranes and demonstrated that via charge propagation across the membrane, the oxidizing and reducing equivalents can be spatially separated.11-14 Zeolites are crystalline aluminosilicates with the general composition Mx/n(AlO2)x(SiO2)ywH2O.15 The cations Mn+ are necessary to balance the framework charge generated by the presence of aluminum. These cations can be readily ion-exchanged, and this property is exploited in the present study. Specifically, zeolite Y membranes with cages of ∼13 Å arranged in a three-dimensional, diamond-like, spatial topology is examined. Previously, we have shown that bipyridinium-exchanged zeolite membranes can serve as an electron transport medium by electron hopping via the electron self-exchange of the ionexchanged bipyridinium ions,11,13 similar to observations reported on bipyridinium-exchanged redox polymer systems.16-18 The electron can be injected into the bipyridinium-exchanged zeolite (BP-zeolite) by a photochemical reaction:11-14 hν

Ru(bpy)32+ (s) 98 Ru(bpy)32+* (s) Ru(bpy)32+* (s) + BP-zeolite f Ru(bpy)33+ + BP--zeolite hν

Ru(bpy)2Ln+-BP-zeolite 98 Ru(bpy)2(n+1)+L-BP--zeolite In the last reaction, Ru(bpy)2L is anchored on the surface of a BP-zeolite.12,14 However, in order to use the photogenerated redox species (for example, in an integrated water splitting system), catalysts have to be a part of the architecture. In this paper, we report on the incorporation of RuO2 as a H2-evolving catalyst onto the alumina substrate that acts as the support for the zeolite membrane. Using this assembly, we demonstrate that

10.1021/jp0703096 CCC: $37.00 © 2007 American Chemical Society Published on Web 06/27/2007

10576 J. Phys. Chem. C, Vol. 111, No. 28, 2007 upon photoexcitation of Ru(bpy)32+ and bipyridinium ion (N,N′trimethylene-2,2′-bipyridinium dibromide, DQ2+) in the presence of the sacrificial electron donor EDTA, charge can be transported across a DQ2+-exchanged zeolite membrane to the electron acceptor propylviologen sulfonate (PVS) within the alumina support, which reacts with water to produce H2 in the presence of the RuO2 catalyst. There are several reports on the use of RuO2 for H2 evaluation catalysts using reduced bipyridinium radical anions (e.g., methylviologen) as electron donors.19-21 The comparison of Pt and RuO2 catalysts has also been reported.19 With a membrane, we demonstrate that light can initiate an electron-transfer reaction on one side of the membrane, followed by a chemical reaction on the opposite “dark” side of the membrane; in this case, the reduction of water to make H2. For photolytic generation of both H2 and O2, the sacrificial electron donor will need to be replaced by a water oxidation catalyst that uses the photogenerated Ru(III), and this has yet to be accomplished with the zeolite membrane architecture, although O2 evolution has been demonstrated with RuO2/zeolite crystallites in a sacrificial electron acceptor system.22 Experimental Section Chemicals. Ru(bpy)32+ (Strem Chemicals), Na2EDTA (GFS Chemicals), H4EDTA (Aldrich), 1.0 M tetrapropylammonium hydroxide solution (Aldrich), 1,3-dibromopropane (Aldrich), 1,4-dibromobutane (Aldrich), 2,2′-bipyridine (Aldrich), 4,4′dipyridyl hydride (Aldrich), 1,3-propane sulfone (Aldrich), and triruthenium dodecacarbonyl (Aldrich) were purchased with the highest purity available. N,N′-Trimethylene-2,2′-bipyridinium dibromide and propylviologen sulfonate were synthesized following published procedures.23 Deionized water and acetate buffer solutions were used as solvents for the photolysis experiments. R-Al2O3 (CR-1) was obtained from Baikowski International Co. Membrane Synthesis. Nanocrystalline zeolite Y was synthesized from clear solutions of tetramethylammonium aluminate according to the literature and then used as seed crystals.24,25 Porous R-Al2O3 disks were prepared according to the literature and used as a support for zeolite membrane growth.26 The zeolite Y membranes were prepared by the secondary growth method detailed elsewhere.13 Briefly, a polished porous R-Al2O3 disk was prepared by polishing with SiC sand papers (Pace Tech., grit size 120 and 320) and placed with polished face down into a seeding solution, which was prepared by dispersing nanocrystalline zeolites Y in a pH 8.1 solution at a loading of 3 g/L. The triply seeded porous R-Al2O3 disk was placed into the synthesis solution with a molar composition of 0.037 Na2O, 1.0 Al2O3, 3.13 (TMA)2O, 4.29 SiO2, 497 H2O and heated at 98 °C for 7 days. X-ray powder diffraction (XRD) patterns of nanocrystalline zeolite Y and the zeolite Y membrane were recorded with a Rigaku Geigerflex diffractometer using Nifiltered Cu KR radiation (40 kV and 25 mA). Preparation of Catalyst. The catalysts were made by thermal decarbonylation of triruthenium dodecacarbonyl using methods described in the literature followed by heating in air, as follows.19,22 Zeolite-Based RuO2. The calcined nanocrystalline zeolite Y was dehydrated at 200 °C in vacuum (1 × 10-4 Torr) for 18 h and transferred to a glove box. Triruthenium dodecacarbonyl crystals were ground to a fine powder, a weighed amount was mixed with the zeolite, and the mixture was heated under vacuum at 70 °C for 1 h and then at 170 °C for another 5 h. After heat treatment, the gray solid was exposed to ambient

Kim and Dutta laboratory atmosphere at room temperature then heat-treated again in air at 200 °C for 24 h and is referred to as RuO2/Y200. Alumina-Based RuO2. The calcined zeolite Y membrane/ alumina support was dehydrated at 200 °C in vacuum (1 × 10-4 Torr) for 18 h and transferred to a glove box. A weighed amount of triruthenium dodecacarbonyl was mixed with acetone to form a suspension. The orange-yellow suspension was deposited on the alumina support, and the solvent was slowly evaporated at room temperature in vacuum (1.0 × 10-4 Torr) for 15 h and heated under vacuum at 70 °C for 1 h and then at 170 °C for another 5 h. After heat treatment, the membrane was exposed to ambient laboratory atmosphere at room temperature then further heat-treated in air at 170 °C for 24 h. Surface Analysis. Scanning electron microscopy (SEM) images of the catalyst were obtained with a Sirion scanning electron microscope equipped with a field emission electron gun. Transmission electron micrographs (TEM) were recorded with FEI Tecnai TF-20, which is a 200 kV (S)TEM with an XTwin objective lens and a Scherzer (point-to-point) resolution of 2.4 Å. Photolysis. A xenon arc lamp equipped with a water filter, a 420 nm cutoff filter, and a reflecting mirror (420-650 nm) was used as a light source. The power of the incident radiation on the membrane side of the cell was 250 mW/cm2 as measured by a Coherent 210 power meter. There were two types of photolysis cells that were used. For evaluation of H2 formation using reduced PVS as the electron donor, a dispersion of Ru(bpy)32+, EDTA, PVS, and RuO2/zeolite Y was stirred in a tube, and the H2 in the headspace was analyzed by GC. For the H2 evolution experiments using charge transport across zeolite/alumina membranes, the cell shown in Figure 6 was used. The characteristics of the cell are described in the Results Section. Chromatography. Headspace analysis of hydrogen was carried out using a Hewlett-Packard model 5890 Series II gas chromatograph equipped with a thermal conductivity detector and a 3 ft × 1/4 in. molecular sieve column (60/80 mesh, 5 Å, Supelco). Results and Discussion Membrane Design. The zeolite Y membrane was grown on an alumina support using procedures described previously.13 The growth strategy was to seed nanocrystalline zeolite Y on the alumina support, followed by hydrothermal secondary growth. The X-ray diffraction data (not shown) were consistent with previous studies indicating a well-grown zeolite Y membrane on the alumina support.13 Figure 1a shows the SEM of a cross section of a typical membrane. The zeolite membrane thickness is ∼4 µm, and the thickness of the alumina support is on the order of ∼1 mm, with the support diameter being on the order of 13 mm. Previous study has shown that the monolithic zeolite membranes are relatively pinhole-free, which prevents leakage of dye molecules from one side of the membrane to the other side.13 RuO2 Incorporation into the Zeolite/Alumina Membrane. The RuO2 was assembled in the pores of the Al2O3 support by thermal decomposition of Ru3(CO)12 to Ru metal, followed by oxidation in air at 170 °C to form RuO2. We have reported a similar procedure for synthesis of RuO2 on zeolite Y.19,22 Figure 2 compares the XRD of the RuO2/zeolite with zeolite/alumina/ RuO2 and a zeolite powder. The peaks due to RuO2 (marked with an asterisk) can be readily discerned in both samples, although the peaks are considerably weaker in the zeolite/ alumina membrane. In addition, SEM of the cross section of

Zeolite Membrane/RuO2 Photocatalyst System

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Figure 1. (a) SEM cross section image of a typical zeolite membrane on alumina support. (b) SEM cross section images of zeolite/alumina/RuO2 membrane with focus on the alumina support. Top to bottom are in the direction of the alumina support away from the zeolite (left-hand micrographs in (b), scale bar is 500 nm; right-hand micrographs, the scale bar is 200 nm).

Figure 2. (a) XRD pattern of zeolite/alumina/RuO2 membrane recorded from the alumina surface. (b) XRD patterns of zeolite and RuO2/zeolite powders. Peaks marked with an asterisk are from RuO2.

the alumina layer at two different magnifications in Figure 1b clearly shows the presence of RuO2 particles on the alumina. The RuO2 has a concentration gradient across the alumina membrane, with more particles toward the bottom edge of the support (away from the zeolite membrane). The TEM image in Figure 3 shows that the morphology of the RuO2 is rodlike with dimensions of 5 × 50 nm, and these rods protrude outward from the alumina grains into the pores.

Catalytic Activity of the Zeolite/Alumina/RuO2. The H2 evolution using the zeolite/alumina/RuO2 was investigated using PVS as the electron acceptor and EDTA as the sacrificial donor. Several reports in the literature have reported on the generation of H2 using MV2+ as the electron acceptor,10 but no studies are reported with PVS. Thus, using previously reported19,22 micrometer-sized RuO2/zeolite Y particles (5 mg) and 0.2 mM Ru(bpy)32+, 0.2 M EDTA, and 2 mM PVS, we examined the optimal pH for H2 evolution. As shown in Figure 4, the optimal pH with PVS is at pH ∼ 4 using an acetate buffer. With the optimized pH, the zeolite/alumina/RuO2 membrane was examined using 0.5 mM Ru(bpy)32+, 40 mM EDTA, and 25 mM PVS, all constituents in a single photochemical cell. Upon photolysis, in the presence of the sacrificial donor, PVS-• is generated in solution, as evident from the purple color. Hydrogen evolution is observed, increasing with time, as shown in Figure 5. The PVS-• radicals generated in solution must enter the Al2O3 pores and reduce water to H2 on the RuO2 catalyst. This data is confirmation that RuO2 within the alumina pores can serve as a catalyst for H2 evolution. H2 Evolution via Charge Propagation through a Zeolite Membrane. The eventual goal of this project is to set up an artificial photosynthetic system in which visible light excitation of a sensitizer molecule on the surface of the zeolite membrane leads to charge separation and charge transmission through the membrane and then utilization of both charge separated species in a chemical reaction. In this paper, we focus on only one-half of the process by coupling a photochemically driven sacrificial electron-donor-mediated charge separation process initiated on the zeolite side of the membrane with H2 evolution at the dark alumina side of the membrane. Figure 6 shows the design of the cell assembly that we have investigated with the zeolite/alumina/RuO2 membrane to examine H2 evolution. The membrane serves to separate the cell into two sections, which we have labeled cell A and cell B. The only communication between the two sides of the membrane has to proceed through the membrane. Three experiments with this cell form the basis of the study. In experiment I, cell A was filled with a solution of 0.5 mM Ru(bpy)32+, 40 mM EDTA, and 25 mM DQ2+. The zeolite/alumina/RuO2 membrane used was extensively ion-exchanged with DQ2+, so the super-

10578 J. Phys. Chem. C, Vol. 111, No. 28, 2007

Kim and Dutta

Figure 3. TEM micrograph of RuO2 within the alumina support, demonstrating the morphology.

Figure 4. H2 evolution from a 0.2 mM Ru(bpy)32+, 0.2 M EDTA, 2 mM PVS, RuO2/zeolite powder (5 mg) system as a function of pH (3.2-5) upon visible light illumination. Figure 6. Optical arrangement for photolysis to measure H2 evolution.

Figure 5. H2 evolution from a 0.5 mM Ru(bpy)32+, 40 mM EDTA, 25 mM PVS, zeolite/Al2O3/RuO2 membrane in pH ∼ 4 acetate buffer upon visible light illumination.

cages are occupied by DQ2+. Our previous study has shown that under these exchange conditions, 1.4 DQ2+ molecules are present per supercage.27 Because Na2EDTA is used as the sacrificial electron donor, Na+ can exchange DQ2+ out of the zeolite during photoillumination. In order to minimize this effect,

DQ2+ at a concentration of 25 mM was added to cell A. The solution in cuvette B was 141 mM PVS in acetate buffer at pH ∼ 4. Experiments II and III served as controls. In experiment II, there was no DQ2+ in the system (i.e., in neither cell A nor zeolite). For experiment III, DQ2+ was present in the solution in cell A at 25 mM concentration, but the zeolite membrane was not exchanged with DQ2+ prior to photoillumination. Figure 7 shows the H2 recovered from the headspace in cell B as a function of irradiation time. No H2 evolution is observed over a period of 300 min for experiment II, which had no DQ2+ in the system. This indicates that PVS is not leaking into cell A through the membrane, generating PVS-•, and leaking back into the alumina. For experiment I, with DQ2+ ion-exchanged zeolite, H2 evolution continues to increase with time. Two experiments with different DQ2+-exchanged membranes are shown in Figure 7 to indicate reproducibility, and comparable H2 evolution is observed for both of these samples. For experiment III using the zeolite membrane without any initial intrazeolitic DQ2+, H2 evolution begins after 120 min.

Zeolite Membrane/RuO2 Photocatalyst System

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Figure 7. H2 evolution using the setup in Figure 6 with zeolite/alumina/ RuO2 membrane assembly. Three experiments were performed. In experiment I (repeated with two separate membranes (b, O)), solution A has 0.5 mM Ru(bpy)32+, 40 mM EDTA, and 25 mM DQ2+, and solution B has 141 mM PVS, and the zeolite membrane is extensively exchanged with DQ2+ prior to cell assembly. Insert shows typical GC traces from experiment I. In experiment II (4), solution A has 0.5 mM Ru(bpy)32+ and 40 mM EDTA, and solution B has 141 mM PVS, and the zeolite membrane is Na+-exchanged. In experiment III (1),solution A has 0.5 mM Ru(bpy)32+, 40 mM EDTA, and 25 mM DQ2+, and solution B has 141 mM PVS, and the zeolite membrane is Na+exchanged.

The following photochemical scheme explains the observations in this study. Figure 8 is a schematic description of the proposed process. Zeolite membrane side: cell A solution: hV

Ru(bpy)32+ 98 Ru(bpy)32+*

(1)

Ru(bpy)32+* + DQ2+ f Ru(bpy)33+ + DQ+•

(2)

Ru(bpy)33+ + EDTA f Ru(bpy)32+ + EDTA decomposed products (3) solution/zeolite membrane interface:

DQ+• (S) + DQ2+ (Z) f DQ2+ (S) + DQ+• (Z)

(4)

Na+ (S) f Na+ (Z)

(5)

intrazeolitic chemistry:

DQ+• (Z) + DQ2+ (Z) f DQ2+ (Z) + DQ+• (Z)

(6)

alumina support side:

DQ+•(Z) + PVS (A) f DQ2+ (Z) + PVS-• (A)

(7)

Na+ (Z) f Na+ (A)

(8)

RuO2

2 PVS-• (A) + 2H+ (H2O, A) 98 2PVS (A) + H2 (A) (9) H2 (A) f H2 (solution) f H2 (gas phase)

(10)

S is solution, Z is zeolite, and A represents alumina. The experiment involved shining light on the zeolite side of the membrane and measuring H2 in the chamber on the dark alumina side of the membrane. Reactions 1-3 generate DQ+• in the solution surrounding the zeolite membrane upon light illumination, driven by the sacrificial electron donor. At the

Figure 8. A schematic description of the entire photochemical process.

solution/zeolite interface, the DQ+• can exchange electrons with the intrazeolite DQ2+ (reaction 4). The charge can then propagate through the zeolite membrane by charge hopping via the densely packed DQ2+ within the zeolite membrane (reaction 6). The electron transport through the zeolite membrane occurs by electron self-exchange reactions between DQ2+ molecules entrapped in the zeolite supercages. For such an electron transport mechanism, there is no need for the intrazeolitic DQ2+ ions to move, just that they be packed close enough for selfexchange, which is readily controlled by the extent of DQ2+ ion exchange within the zeolite. Extensive ion exchange of DQ2+ in zeolite Y leads to a loading level of 1.4 molecules per supercage.27 Charge hopping within the zeolite has been noted in previous studies. Yoon et al. noted that all of intrazeolitic methylviologen can be reduced within seconds by reducing agents that are too large to enter the zeolite pores, the reduction process occurring by self-exchange of the violgens.28 We and others have reported migration of charge within zeolite crystals to explain long-lived charge separation.29,30 Charge exchange on the zeolite surface has also been noted for polypridyl complexes of ruthenium adsorbed on the surface.31 Rapid charge transport has been noted for polymer films if the redox species are aligned properly and the packing densities of the redox species are high.16-18 Zeolite frameworks are crystalline microporous solids and because of their ion exchange characteristics provide opportunity for dense packing of charged guest species. With the occupancy of 1.4 DQ2+ per supercage, the concentration of the DQ2+ corresponds to a 1.2 M solution (using zeolite density as 1.92 g/cm3). In the polymer films, electron transport can occur by two pathways: an electron hopping between the redox species or the physical diffusion of the redox species. In the zeolite, only the electron hopping mechanism is likely, since physical diffusion of DQ2+ through the membrane is unlikely if supercage occupancy is greater than unity. In experiment III with Na+/zeolite Y membrane, the extra time required before H2 evolution is observed is the time necessary for ion exchange of DQ2+ present in cell A into the zeolite. In addition, if ion exchange of DQ2+ into the zeolite is occurring during the photolysis period, it is unlikely that all supercages are being occupied by DQ2+, unlike experiment I, in which the zeolite membrane was ion-exchanged with DQ2+ two times prior to the photolysis experiment. We have previously shown that ∼70% occupancy of supercages by DQ2+ will lead to electron propagation across the membrane.11 It was proposed that as long as a percolative pathway is set up, charge migration will occur.

10580 J. Phys. Chem. C, Vol. 111, No. 28, 2007 Reaction 5 involves overall charge balance by Na+ ion transport from solution to zeolite. There are eight to nine Na+ ions on average in the supercages, and we are replacing about three of these ions by DQ2+ at the loading level in this study. We have shown earlier that if a large cation such as tetrapropyl ammonium ion that cannot pass through the 7 Å windows of the supercage is used, then no charge transport is observed.14 PVS is a neutral zwitterionic compound and is not expected to replace the DQ2+ from the zeolite, so PVS can be present within the alumina pores and at the zeolite/alumina interface. At the zeolite/alumina interface, there is a driving force for directional electron transfer from DQ+• in the zeolite to PVS within the alumina pores, an observation made previously with zeolite particles.29,30 The driving force for this reaction is ∼140 mV (Eo (DQ2+) ) -0.55 V; Eo (PVS) ) -0.41 V vs SHE).32,33 The PVS-• generated within the alumina can interact with the RuO2 to form H2 (reaction 9), as we have shown in Figure 5. The H2 that makes it into the headspace is analyzed by gas chromatography. In order to judge the efficiency of the intrazeolitic charge transport process, the H2 evolution from two sets of experiments was compared. In one case, Ru(bpy)32+, EDTA, PVS (concentrations as in the experiment in Figure 7), and zeolite/alumina/ RuO2 were put into cell A and photolyzed, and the H2 formed in cell A was measured. The B side was blocked off with a Teflon piece. PVS-• is directly generated from Ru(bpy)32+* and interacts with the RuO2 in the alumina to produce hydrogen. After 300 min of photolysis, 90 nmol of H2 was formed (average of four experiments). This can be compared to the membrane setup in Figures 6 and 7, where PVS-• is generated through the intermediate DQ+• via intrazeolitic charge migration and electron transfer to PVS held in the alumina pores and resulted in 43 nmol of H2 after 300 min of photolysis. The membrane system is about 48% efficient as compared to the “homogeneous” system. Comparison of the steps in the two systems with regard to H2 evolution provides insight into the observed efficiency differences. For the Ru(bpy)32+, EDTA, PVS system, the key steps are electron-transfer quenching of Ru(bpy)32+* by PVS and the catalytic efficiency of PVS-•mediated H2 production. In the membrane assembly, the key steps are electron-transfer quenching of Ru(bpy)32+* by DQ2+, charge exchange between DQ+• and DQ2+ at the solution/zeolite interface, intrazeolitic charge transport through electron hopping, electron transfer to PVS at the zeolite/alumina interface, and PVS-•-mediated H2 production. Since the catalytic process is similar in both cases, it is reasonable to assume that it does not play a role in the different levels of H2 evolution. The electron-transfer quenching of Ru(bpy)32+* by DQ2+ has a lower driving force by ∼140 mV as compared to PVS. (Eo (Ru(bpy)32+*) ) -0.87 V;34 Eo (DQ2+) ) -0.55 V; Eo (PVS) ) -0.41 V vs SHE). The self-exchange reactions of bipyridinium ions are rapid: for methylviologen in polyethylene glycol, rate constants of 8.6 × 104 to 1.6 × 105 m-1 s-1 have been reported.35 Thus, the charge transport process through the zeolite will not limit the charge separation process. There is also a driving force of ∼140 mV for electron transfer from intrazeolitic DQ+• to PVS in the alumina membrane, so this reaction should not be limiting. The increased efficiency in the Ru(bpy)32+/PVS system must stem from the increased yield of the bipyridinium radical in the forward electron-transfer quenching reaction, as compared to the Ru(bpy)32+/DQ2+/zeolite membrane system.

Kim and Dutta Conclusions This study proposes a photochemical assembly in which spatial charge separation across a zeolite membrane is coupled with a heterogeneous catalyst system to produce H2 from water. The central feature of the assembly is a monolithic zeolite Y membrane synthesized on a porous alumina support. Nanofingers of RuO2 are assembled within the alumina support via thermal decarbonylation of Ru3(CO)12, followed by oxidation of the Ru metal. These RuO2 particles exhibited catalytic properties for H2 evolution using photochemically generated propylviologen sulfonate radicals as the electron donor. A glass cell was used to mount the zeolite/alumina/RuO2 membrane in such a way that two compartments were generated. On the zeolite side, a solution of Ru(bpy)32+, N,N′-trimethylene-2,2′bipyridinium (DQ2+), and EDTA was placed. Upon visible light illumination, charge transport through the bipyridiniumexchanged zeolite led to formation of H2, mediated by PVS-• radical within the alumina support and its interaction with RuO2. The efficiency of the membrane assembly was 48% as compared to a Ru(bpy)32+, PVS, EDTA, and zeolite/alumina/RuO2 catalyst system. The decreased efficiency is proposed to arise from a lower DQ+• yield via Ru(bpy)32+* quenching. Acknowledgment. We acknowledge funding from NASA and DOE (DE-FG02-06ER15776). References and Notes (1) LaVan, D. A.; Cha, J. N. Proc. Natl. Acad. Sci. 2006, 103, 52515255. (2) Nowotny, J.; Sorrel, C. C.; Sheppard, L. R.; Bak, T. Int. J. Hydrogen Energy 2005, 30, 521-544. (3) Alstrum-Acevedo, J. H.; Brennaman, M. K.; Meyer, T. J. Inorg. Chem. 2005, 44, 6802-6827. (4) Wasielewski, M. R. J. Org. Chem. 2006, 71, 5051-5066. (5) Hoertz, P. G.; Mallouk, T. E. Inorg. Chem. 2005, 44, 6828-6840. (6) Gust, D.; Moore, T; Moore, A. Acc. Chem. Res. 2001, 34, 40-48. (7) Gratzel, M. Nature 2001, 414, 338-344. (8) Serpone, N.; Pelizzetti, E.; Gra¨tzel, M. Coord. Chem. ReV. 1985, 64, 225-245. (9) Kiwi, J.; Gra¨tzel, M. Nature 1979, 281, 657. (10) Amouyal, E. Sol. Energy Mater. Sol. Cells 1995, 38, 249-276. (11) Lee, H.; Dutta, P. K. J. Phys. Chem. B 2002, 106, 11898-11904. (12) Kim, Y.; Lee, H.; Dutta, P. K. Inorg. Chem. 2003, 42, 4215-4222. (13) Kim, Y.; Dutta, P. K. Res. Chem. Intermediates 2004, 30, 147161. (14) Kim, Y.; Das, A.; Zhang, H.; Dutta, P. K. J. Phys. Chem. B 2005, 109 (15), 6929-6932. (15) Dutta, P. K. Zeolites: A Primer. In Handbook of Zeolites and Layered Materials; Auerbach, S. M., Carrado, K. A., Dutta, P. K., Eds. Marcel Dekker, Inc.: New York, 2003; pp 1-19. (16) Dalton, E. F; Murray, R. W. J. Phys. Chem. 1991, 95, 6383-6389. (17) Oyama, N.; Ohsaka, T.; Yamamoto, H.; Kaneko, M. J. Phys. Chem. 1986, 90, 3850-3856. (18) Hatozaki, O.; Ohsaka, T.; Oyama, N. J. Phys. Chem. 1992, 96, 10492-10497. (19) Dutta, P. K.; Vaidyalingam, A. S. Microporous Mesoporous Mater. 2003, 62, 107-120. (20) Keller, P.; Moradpour, A.; Amouyal, E. J. Chem. Soc. Faraday Trans. 1 1982, 78, 3331. (21) Kleijn, J. M.; Rouwendal, E.; Van Leeuwen, H. P.; Lyklema, J. J. Photochem. Photobiol. A 1988, 44, 29-50. (22) Das, S. K.; Dutta, P. K. Microporous Mesoporous Mater. 1998, 22, 475-483. (23) Homer, R. F.; Tomlinson, T. E. J. Chem. Soc. 1960, 2498. (24) Lassinantti, M.; Hedlund, J.; Stert, J. Microporous Mesoporous Mater. 2000, 38, 25-34. (25) Xomeritakis, G.; Nair, S.; Tsapatsis, M. Microporous Mesoporous Mater. 2000, 38, 61-73. (26) Pan, M; Lin, Y. S. Microporous Mesoporous Mater. 2001, 43, 319327. (27) Vitale, M.; Castagnola, N. B.; Ortins, N. J.: Brooke, J. A.; Vaidyalingam, A.; Dutta, P. K. J. Phys. Chem. B 1999, 103, 24082416.

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