Visible-Light-Driven Photoproduction of Hydrogen Using Rhodium

Oct 14, 2014 - Jinheung Kim , Eswaran Rajkumar , Soojin Kim , Yu Mi Park ... Sol Ji Park , Soojin Kim , Tikum Florence Anjong , Sung Eun Lee , Jinheun...
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Visible-Light-Driven Photoproduction of Hydrogen Using Rhodium Catalysts and Platinum Nanoparticles with Formate Soojin Kim,† Ga Ye Lee,† Jin-Ook Baeg,‡ Youngmee Kim,† Sung-Jin Kim,† and Jinheung Kim*,† †

Department of Chemistry and Nano Science, Global Top5 Research Program, Ewha Womans University, Seoul 120-750, Korea Advanced Chemical Technology, Division, Korea Research Institute of Chemical Technology (KRICT), Daejon 305-600, Korea

‡

S Supporting Information *

ABSTRACT: Photochemical hydrogen production is carried out using molecular Rh complexes and sodium formate in the presence of platinum nanoparticles (PtNPs) in aqueous buffer solution. Visible-light-driven photocatalytic reactions for hydrogen production with and without nicotinamide adenine dinucleotide (NAD+) follow two different pathways. Complex [Cp*Rh(bpy)(OH2)]2+ selectively reduces NAD+ to generate NADH using formate as a proton and electron donor and the chemically generated NADH is sequentially used by PtNPs upon photoactivation of eosin Y to produce hydrogen. However, hydrogen is also produced in photoreactions of the Rh catalyst and PtNPs with formate in the absence of NAD+ and eosin Y. The second pathway for hydrogen production was performed under the conditions without NAD+ and eosin Y and derived from a direct electron transfer from in situ generated rhodium(III)-hydride species to photoexcited PtNPs. The direct electron transfer from the rhodium-hydride species to visible-light-driven PtNPs was first observed in this study. These two pathways for hydrogen production showed different rate-limiting steps based on a Hammett plot using Rh catalysts containing electron-donating and electron-withdrawing groups. Kinetic isotope effects as well as Hammett plot supported the rate-limiting step of the NADH generation for the first pathway of hydrogen production and the Rh−H formation for the second pathway.

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INTRODUCTION In recent years, the major greenhouse gas, carbon dioxide, has been proposed a practical hydrogen storage material.1 The selective photocatalytic reduction of CO2 to hydrogenated products containing a carbon has been the subject of considerable attention over the ages as a means for a carbon capture and light energy storage strategy. Various molecular metal complexes based on Ru, Rh, Ni, Fe, Mo, Ti, and Ir have demonstrated hydrogenation of carbon dioxide to formic acid.2,3 Most catalysts showing a high turnover frequency for CO2 reduction were used under supercritical conditions due to the solubility problem of carbon dioxide. Only a few molecular metal catalysts were used in water. Although most of transitionmetal-hydride species were known to be unstable, an isolated ruthenium-hydride species was reported for the reduction of CO2.4,5 In the study of the isolated ruthenium-hydride species, the Ru−H bond was inserted by CO2 to produce formate. On the other hand, the decomposition of formic acid/formate in the presence of homogeneous metal catalysts has also been studied to generate hydrogen and carbon dioxide.6 Although the decomposition of formic acid using molecular metal complexes has been poorly studied before, several reports in this area have been published recently. Most studies with molecular metal catalysts reported the non-photochemical decomposition of formic acid, and often use thermal energy to achieve high conversion rates and yields. However, examples of the photochemical system with such metal complexes are rare in the decomposition studies of formic acid/formate. More © 2014 American Chemical Society

examples of the photochemical system containing a molecular metal catalyst for the HCOOH decomposition are required to understand the photocatalytic mechanism of formic acid decomposition and develop an efficient catalytic system utilizing formate/formic acid as one of the major sources of hydrogen. Natural cofactor 1,4-dihydronicotinamide adenine dinucleotide (NADH) is regenerated in natural photosynthesis and used for many NADH-dependent enzymes to carry out various catalytic oxidation and reduction reactions in biological systems. Therefore, NADH regeneration is considered as another means for solar-to-chemical energy storage. Reduction of nicotinamide adenine dinucleotide (NAD+) to the enzymatically active NADH has been carried out using chemical, enzymatic, electrochemical, and photochemical methods. Enzymatic NADH regeneration methods have been studied extensively using formate dehydrogenase for various applications.7,8 Although platinum nanoparticles (PtNPs) and molecular cobalt complexes photochemically regenerated NADH,9,10 cyclopentadienyl-based Rh complexes have been frequently studied as catalysts to develop new electrochemical and photochemical systems for NADH regeneration and understand their reaction mechanisms. In particular, [Cp*Rh(bpy)(H2O)]2+ (1, Cp* = pentamethylcyclopentadienyl, bpy = Received: May 18, 2014 Revised: October 14, 2014 Published: October 14, 2014 25844

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A number of photocatalytic systems for hydrogen production using various molecular metal complexes have been developed over the past few decades.15−20 Metal-hydride species have been proposed as reactive intermediates in most proposed reaction mechanisms for hydrogen photoproduction. An organometallic Ir complex chemically reduces NAD+ to NADH by consuming H2, and an iridium-hydride intermediate was proposed as a reactive intermediate.21 In another study, a iridium-hydride intermediate, chemically generated by aliphatic alcohols, was proposed to react with a proton and NAD+ to generate hydrogen and NADH, respectively.22 Recently, a homogeneous system comprising [Rh III (dmbpy) 2 Cl 2 ]Cl (dmbpy = 4,4′-dimethyl-2,2′-bipyridine) and Ru(bpy)32+ was reported for a visible-light-driven hydrogen production, but a Rh−H intermediate was proposed to chemically react with a proton and produce hydrogen.17 Further studies are needed for a complete elucidation of the properties of metal-hydride species especially in photocatalytic hydrogen production systems. To the best of our knowledge, no reports are available regarding a catalytic hydrogen production by molecular Rh complexes and formate in the presence of Pt nanoparticles activated with visible-light energy. Recently, PtNPs have been reported to mediate photochemical NADH regeneration without any other photosensitizers upon visible-light irradiation.10 In the photocatalytic NADH regeneration with PtNPs, no hydrogen production was observed, showing the preference for NAD+ reduction. In other reports, Pt and Fe nanoparticles also mediated NADH decomposition in the presence of an organic chromophore as a photosensitizer to produce hydrogen.23 In the NADH oxidation, 9-mesityl-10-methylacridium (Acr-Mes) upon activation with visible light catalyzes the oxidation of NADH and mediates the electron transfer to PtNPs during hydrogen photoproduction. A photochemical study using molecular Co catalysts showed that the presence of colloidal Pt improves photocatalytic hydrogen production yield and rate.20 Thus, more research will be needed to completely elucidate the role of PtNPs, especially in the activation of metal-hydride species, for developing efficient hydrogen and NADH photoproduction systems. Here, we report the visible-light-driven photoproduction of hydrogen by molecular Rh complexes and formic acid/formate in the presence of PtNPs in aqueous solution. We first utilized in situ chemically produced NADH for photocatalytic hydrogen production. It is also provided that rhodium-hydride intermediates directly transferred electrons to visible-light-

2,2′-bipyridine; Figure 1) has been often used as an electrochemical and photochemical catalyst due to the relatively

Figure 1. Chemical structures of molecular Rh(III) complexes 1−3 and an ORTEP drawing of 2 (50% thermal ellipsoids; hydrogen atoms have been omitted for clarity; detailed crystal information in the Supporting Information Tables S1 and S2).

high efficiency and selectivity to regenerate NADH.10−12 Chemical regeneration of NADH was performed using 1 and sodium formate as a reducing agent.13 According to the proposed reaction mechanism, a 1−formate intermediate released carbon dioxide to form a hydridorhodium(III) intermediate which consecutively reacts with NAD+ to produce the enzymatically active 1,4-NADH (Scheme 1). However, the Scheme 1

cyclopentadienyl rhodium complex 1 was also used to chemically produce hydrogen using formic acid at low pH conditions14 but have been rarely studied for photocatalytic hydrogen production.

Table 1. Visible-Light-Driven Photoproduction of Hydrogen According to Reaction Conditionsa run

NAD+

1

formate

PtNPs

eosin

H2 (μmol)

rate (μmol/h)b

TNc

1 2 3 4 5 6 7 8

o o o o − − − −

o − o − o − o o

o o o − o o o o

o o o o o o o −

o o − o o o − −

20 ± 0.6 0.91 ± 0.08 3.0 ± 0.1 0 5.1 ± 0.2 0.34 ± 0.05 6.2 ± 0.2 0

9.1 ± 0.3 0.54 ± 0.05 2.1 ± 0.1 0 1.9 ± 0.1 0.21 ± 0.04 2.8 ± 0.1 0

78 2.3 12 0 20 1.3 24 0

a Reaction conditions: 0.13 mM 1, 1.0 mM NAD+, 0.4 M sodium formate, 20 nM PtNPs (5 nm), and 0.15 mM eosin Y in 50 mM phthalic acid (pH = 4.5) and CH3CN (v/v = 1/1). Each solution (2 mL) was exposed to visible light (420 nm cutoff) for 3 h at 25 °C to obtain the maximum hydrogen yields. The data points were obtained from the average of three independent measurements. bThe initial rates were obtained by fitting the data for the first 1 h. cThe turnover number (TN) of all reactions in Table 1 is calculated based on the amount of 1 (0.13 mM) for simple comparison, even though some reactions were carried out in the absence of 1.

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Scheme 2

experiments showed that the additional elimination of 1 afforded no significant formation of H2 under the same conditions (run 6). Such a significant difference in the hydrogen production in the absence (20 TN) and presence (78 TN) of NAD+ demonstrates that the hydride transfer reaction of RhIII−H to NAD+ (pathway I) was more favorable than to PtNPs (pathway II) under the run 1 conditions (Scheme 2). Pathway II in which the rhodium(III)-hydride species transferring electrons to the photoactivated PtNPs for hydrogen production was first observed in this study. Without any light, no hydrogen was observed in pathway II. In pathway II conditions (containing only 1, formate, and PtNPs), also no significant hydrogen was produced in the elimination of PtNPs. These data demonstrate that PtNPs were activated upon irradiation to abstract electrons from the RhIII−H species and the Rh−H was not directly activated by visible light for hydrogen production. Fukuzumi and co-workers reported the hydrogen production by 1 with HCOOH with no irradiation, but a high concentration of 1 and a continuous addition of HNO3 at a lower pH were used to produce hydrogen.14 When 1 and HCOOH were reacted in the dark under our reaction conditions, no hydrogen was observed over 8 h. The apparent quantum yields (QY) of 1.8 and 0.6% for hydrogen production were obtained in pathways I and II, respectively. Such low QY derived from the dependence of hydrogen production on NADH in pathway I and Rh−H in pathway II, which were generated during the catalytic reactions. The low QY due to the low stability of NAD+ and 1 during the catalytic cycles would be improved by developing more stable substitutes of them. The effects of eosin to pathways I and II were examined to understand the role of eosin in H2 production. After 3 h irradiation in the absence of eosin, runs 3 and 7 afforded 12 and 24 TN of hydrogen, respectively. In pathway I, the absence of eosin decreased the yield sharply (run 3), indicating that the NADH activation for hydrogen production on PtNPs required the photoexcitation of eosin, similar to other reports.23 To understand pathway I further, we separately carried out a photoreaction of NADH (Aldrich) in the presence of only PtNPs in phthalic acid buffer:CH 3 CN. No significant production of hydrogen was observed during photolysis of a solution of NADH and PtNPs, demonstrating that r4 is almost zero in the absence of eosin and that PtNPs alone had no photocatalytic activity for hydrogen production under the conditions. These data also hinted that the 12 TN hydrogen produced in run 3 mostly derived from pathway II, and NADH generation (r3) and hydrogen production (r4) competed for RhIII−H species. Based on these results, eosin and PtNPs worked cooperatively in pathway I to oxidize NADH and produce hydrogen upon visible-light activation. In contrast, H2 production in pathway II increased in the absence of eosin, demonstrating that eosin interfered with H2 production (run 7 vs run 5). Indeed, the H2 production rate

activated PtNPs to produce hydrogen. Two different reaction pathways starting from the rhodium(III)-hydride intermediates showed different rate-determining steps for hydrogen production, as evidenced by Hammett studies using three different Rh catalysts and kinetic isotope effects using DCOOH. A new pathway containing a proton-coupled electron transfer from the RhIII−H species to photoactivated PtNPs was first observed for hydrogen production in this study.

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RESULTS AND DISCUSSION The chemical regeneration of NADH was performed with 0.13 mM [Cp*Rh(bpy)(H2O)]2+ (1) and 0.4 M sodium formate, affording >95% based on initial NAD+ in pH 7.0 phosphate buffer, similar to results reported previously (Scheme 1).14 However, about 40% NADH was generated in 50 mM phthalic acid buffer (pH 4.5):CH3CN (1:1), which was the solvent condition used in the photoreaction for hydrogen production in this study (Supporting Information (SI) Figure S1). A solution containing 1, formate, NAD+, PtNPs (5 ± 1 nm, spherical; see the Supporting Information), and eosin Y (eosin) was irradiated with visible light (420 nm cutoff filter) in phthalic acid buffer/CH3CN to test the reactivity of PtNPs and eosin for visible-light-driven photoproduction of hydrogen using the chemically in situ generated NADH. Under these conditions, hydrogen corresponding to 78 turnover numbers (TN) of 1 was produced after 3 h of irradiation (Table 1, run 1). Assuming that one hydrogen molecule was produced from a molecule of NADH, the amount of hydrogen produced corresponded to 10 turnovers of NAD+, demonstrating that NADH was regenerated during photocatalytic hydrogen production. In the absence of 1 or 1/formate, H2 was barely observed due to no formation of RhIII−H (runs 2 and 4). These data also demonstrated that formate without the Rh complex was not activated in the presence of PtNPs and eosin to produce hydrogen. It was reported that a system containing only Acr-Mes and PtNPs afforded H2 using NADH under stoichiometric conditions upon visible-light irradiation.23 The H2 photoproduction resulted from the chemical regeneration of NADH and subsequent reduction of the protons on PtNPs utilizing visible-light-driven activation of eosin and the reducing power of NADH (Scheme 2, pathway I), similar to the NADH/ Acr-Mes/PtNPs reaction system.23 The photoreaction was performed in the absence of NAD+ to understand additional electron and proton sources for hydrogen production in this system. Interestingly, we observed 5.1 μmol of H2 (run 5), indicating that NADH is not the sole electron/proton source for H2 photoproduction. It was proposed that a rhodium(III)-hydride species was generated as a reactive intermediate in the reaction of 1 and formate.13 The preceding results demonstrate that the in situ generated RhIII−H species can transfer their electrons and proton to PtNPs upon photoactivation to produce H2. Indeed, control 25846

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above 0.15 mM, pathway I became much superior to pathway II. In pathway II with no eosin, the presence of 1.0 mM NAD+ reduced the amount of hydrogen almost by half (run 3), relative to that without NAD+ (run 7). In the presence of NAD+, NADH generation (r3) only competed with hydrogen production (r5), as shown in Scheme 1. Indeed, in the study of NAD+ concentration dependence on hydrogen production, the progressive decrease in H2 production rate was accompanied by a progressive NADH increase at 1 h with increasing NAD+ (Figure 3). Based on the reduction potentials of NAD+ and H+

decreased progressively with increasing eosin (Figure 2a). As eosin containing a carboxy functional group competes with

Figure 3. Photocatalytic H2 production rate (triangles) and NADH formation (red circles) in the presence of various concentrations of NAD+. Visible-light-driven (420 nm cutoff) H2 production initial rate vs NAD+ concentration in a homogeneous system with 0.13 mM 1, 0.4 M sodium formate, and 20 nM PtNPs in 50 mM phthalic acid(pH = 4.5)/CH3CN. The amount of NADH was obtained by the absorption band at 340 nm and 1 h.

under the conditions of 25 °C and pH 7.0, the hydride transfer from RhIII−H to NAD+ was expected to be more favorable than to a proton (Scheme 3).24 The NAD+ reduction was also faster

Figure 2. Dependence of hydrogen photoproduction (420 nm cutoff) on eosin Y concentration (a) by 1/formate/PtNPs (pathway II) and (b) NAD+/1/formate/PtNPs (pathway I) in 50 mM phthalic acid(pH = 4.5)/CH3CN (1:1). The concentrations of reagents are the same as those in Table 1.

Scheme 3

formate for binding to the Rh center in the r1 step, the RhIII−H formation rate slowed down in the presence of eosin. Indeed, the H2 production rate progressively decreased with eosin (SI Figure S2). In a control experiment, no hydrogen was observed in the absence of PtNPs (run 8), demonstrating that PtNPs were a main component for hydrogen production. It was also reported that PtNPs alone had a reducing power upon irradiation of visible light in NAD+ reduction.10 In pathway II, PtNPs were the only species to activate RhIII−H for electron transfer. These results also indicate that the hydrogen obtained in run 3 mostly derived from pathway II. Due to the different effects of eosin on pathways I and II under the run 1 conditions, the eosin concentration dependence on the H2 production rate showed that the rate increased with increasing eosin until 0.15 mM; then it started decreasing (Figure 2b). These results imply that pathways I and II contributed together to the H2 production rates until 0.15 mM eosin, but the contribution of pathway II decreases progressively with increasing eosin as shown in Figure 2a. When eosin was

than a proton reduction by cobalt(III)-hydride species generated upon visible-light activation of eosin.9 Therefore, the additional presence of NAD+ to the pathway II condition resulted in consuming the RhIII−H needed for the electron and proton transfer to PtNPs and, accordingly, decreased the H2 production rate. As shown earlier, the chemically generated rhodium(III) hydride was capable of reducing NAD+ as well as reducing H+ upon photoactivation of PtNPs. We compared hydrogen and NADH production by 1 to those by 2 and 3 which contain the electron-donating methoxy and electron-withdrawing carboxy substituents in bipyridine, respectively (Figure 1), to examine the effect of nucleophilicity of RhIII−H on hydrogen production rates and to understand the rate-determining steps in the two pathways. Under the run 1 conditions (pathway I), the electron-donating substituent resulted in a higher rate of H2 production (Figure 4a). The resulting negative ρ value (−0.64) demonstrated that the nucleophilic reaction of NAD+ reduction (r3) among r1−r4 was rate-determining because the r1 and r2 25847

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In order to clarify the reaction mechanisms of pathways I and II, the rates of r2 and r3 were compared by plotting the initial rates (kobs) vs formate (Figure 4b). The rate of r2 was obtained by observing the chemical NADH regeneration using 1, formate, and NAD+. The rate of r3 was obtained by observing the H2 production under the conditions of pathways II (run 7). These results showed that the rate of r3 was much slower than that of r2 and corresponded to the results of the Hammett study shown previously. Deuterium kinetic isotope effects were investigated with 1 and DCOOH in pathways I and II (Figure 5). In pathway I, the

Figure 4. (a) Linear correlation of the initial rate of H2 production according to pathway I (black squares) and II (red circles) reactions vs Hammett coefficients of the para substituents of 1−3. Pathway I uses the reaction conditions of run 1, and pathway II followed that of run 7, shown in Table 1. (b) Plots of pseudo-first-order rate constants (kobs) for r2 (red circles) and r3 (black squares) versus [formate]. Reaction conditions: for r2, 0.13 mM 1, 1.0 mM NAD+, 0.4 M sodium formate; for r3, 0.13 mM 1, 1.0 mM NAD+, 0.4 M sodium formate, 20 nM PtNPs and 0.15 mM eosin Y in 50 mM phthalic acid (pH = 4.5) and CH3CN (v/v = 1/1). Figure 5. Time course of hydrogen production in the photoreaction of 1 (128 mM) and HCOOH/HCOONa (black squares) or DCOOH/ DCOONa (red circles) (400 mM) under conditions of (a) pathway I (1 mM NAD+, 20 nM PtNPs, and 0.2 mM eosin Y) and (b) pathway II (20 nM PtNPs) in a deaerated solution of phthalic acid buffer solution (50 mM, pH 4.5)/CH3CN (1:1) at 293 K. 420 nm cutoff filter.

steps are electrophilic and r4 is independent of the properties of Rh catalysts. Furthermore, chemical reduction of NAD+ to NADH using 1 and formate in H2O/tetrahydrofuran showed the same rate-limiting step25 and the nucleophilic protonation step of Ir−H species was also rate-limiting in the chemical production of H2 using formic acid.26 We also carried out chemical reduction of NAD+ using 1−3 and formate. The rates obtained from the increase in the NADH absorption band at 340 nm showed a linear correlation of the log plot and also afforded a similar negative value (−0.64, SI Figure S3), consistent with the preceding conclusion. However, under the conditions of pathway II without NAD+ and eosin, introducing an electron-withdrawing substituent afforded a higher H2 production rate and a positive ρ value (0.66; Figure 4a). These results suggest that the r2 step of the RhIII−H formation was rate-limiting because the r5 step would be, in nucleophilic terms, similar to the reduction of NAD+. Such a positive ρ value in a Hammett plot also appeared in the Pd−H formation by the elimination of O2 from Pd-OOH species.27

rate of hydrogen production by DCOOH/DCOONa became much slower than that by HCOOH/HCOONa (Figure 5a). The kinetic isotope effect based on the initial rates turned out to be 4.7, similar to 4.3 obtained in the chemical NADH regeneration using 1 and DCOOH/DCOONa (SI Figure S4). These results also supported that the NADH generation step was rate-limiting in pathway I, as described previously. Pathway II was also influenced by DCOOH/DCOONa (Figure 5b). However, the kinetic isotope effect appeared to be 2.1 which derived from the difference in the generation step (r2) of RhIII− H and RhIII−D from RhIII−OC(O)H and RhIII−OC(O)D, respectively. These results agreed with that obtained in the 25848

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observations indicated that an electron was transferred from the photoexcited eosin to PtNPs and then NADH replenished an electron to eosin+.. However, through the overall process of pathway II, the rate-determining step turned out to be the formation of the Rh−H species from the rhodium-formate adduct based on Hammett and kinetic studies.

Hammett study shown previously. These normal kinetic isotope effects observed in pathways I and II corresponded to that obtained with [Cp*Rh(bpy)(H2O)]2+ and HCOOH/ DCOOH in the chemical hydrogen production14 but were in contrast to the inverse kinetic isotope effect observed with AcrMes/PtNPs and HCOOH/DCOOH in the absence of a Rh catalyst.23 The inverse kinetic isotope effect was interpreted to result from the zero-point energy of the Pt−H and Pt−D bonds on PtNPs which was generated by a proton-coupled electron transfer from the photoexcited Acr-Mes to PtNPs. If the hydrogen production from Pt−H generated by such a fast proton-coupled electron transfer step from Rh−H to PtNPs would have been rate-determining in pathway II, the similar inverse isotope effect should have been observed. To clarify the mechanism further, more research remains to be done. The Rh(III)/Rh(I) reduction potential of 1−3 changed systematically with the electronic nature of the substituents (Figure 6). The reduction peak potentials at pH 4.5 occurred at

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CONCLUSION Hydrogen was efficiently generated in a visible-light-driven photocatalytic system consisting of NAD+, Rh catalyst, eosin Y, sodium formate, and PtNPs in an aqueous solution. NADH chemically produced from the Rh catalyst 1 and sodium formate was consecutively consumed by photoactivated eosin Y and PtNPs to produce hydrogen using visible-light energy. In addition, the rhodium-hydride species generated in the reaction of 1 and formate was proposed to reduce a proton in the presence of photoactivated PtNPs. Two distinct pathways for hydrogen production were observed and divided by NAD+ and proton reduction including RhIII−H species. Interestingly, PtNPs should be activated by visible light to carry out hydrogen production from the rhodium(III)-hydride species. Based on kinetic studies, the NADH formation step was ratelimiting in the H2 production pathway through NADH (pathway I), whereas the rate-determining step in pathway II was the generation of RhIII−H. In pathway I, the photoexcited eosin Y shuttled electrons from NADH to PtNPs to produce hydrogen. In pathway II, the RhIII−H species directly transferred their electrons to photoexcited PtNPs upon irradiation. The different rate-limiting steps were evidenced by the Hammett plots using different Rh complexes and the kinetic isotope effect using DCOO−/DCOOH. This study has provided important insight into the mechanism of the photocatalytic hydrogen production from formate/formic acid using molecular metal complexes and Pt nanoparticles. The conversion of light energy into stored chemical energy, such as formic acid and NADH, is on an interesting subject for the development of artificial photocatalytic systems. The development of efficient photocatalytic systems for the hydrogen production from formic acid and NADH based on such a detailed mechanistic study will influence on practical applications for hydrogen storage and hydrogen production systems.

Figure 6. Cyclic voltammograms of 1 (black solid), 2 (red dot), and 3 (blue dash) in aqueous solution maintained at pH 4.5 with 0.1 M potassium phosphate buffer. The scan rate is 0.08 V/s, and the Ag/ AgCl electrode is used as the reference electrode.

−644, −722, and −632 mV vs Ag/AgCl for 1, 2, and 3, respectively. A close relationship existed between the reduction potentials and the electron-donating or -withdrawing characters of the substituents (SI Figure S5). The order of the reduction potentials was well associated with those of the hydrogen photoproduction reactivity of 1−3 in pathway I which included the nucleophilic rate-limiting step associated with Rh−H species. This conclusion well corresponded with what was shown in the Hammett study (Figure 4a). This indicates that the electron-donating methoxy group more readily allows for hydride transfer of RhIII−H to NAD+. Based on all data presented previously, the RhIII−H species was clearly the main component to produce NADH in pathway I and transfer electrons to PtNPs in pathway II. While the RhIII−H species chemically reduced NAD+, they produced hydrogen only upon irradiation of PtNPs. In pathway I for hydrogen production, the chemically regenerated NADH was oxidized by the photoexcited eosin finally to transfer a proton and electrons to PtNPs (Figure 7). On the other hand, in pathway II, PtNPs activated by visible-light energy abstracted electrons from RhIII−H. The fluorescence of eosin was not quenched by NADH, but quenched by PtNPs. These

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EXPERIMENTAL SECTION Materials and Instrumentation. All reagents purchased from Aldrich were used without further purification. Water was purified with a Milli-Q purification system. [Cp*Rh(bpy)Cl]2+ was prepared according to previously published methods.28,29 Transmission electron microscopy (TEM) images of Pt nanoparticles were recorded on a JEOL 2010FX electron microscope operating at 200 kV. The UV−vis absorption spectra were performed with a HP spectrophotometer. NMR spectra were recorded on a FT-NMR (300 MHz) spectrometer, Bruker Co. AVANCE III 3000. Preparation of PtNPs. PtNPs were prepared by poly(vinylpyrrolidone) (MW = 10k) reduction of potassium tetrachloroplatinate (K2PtCl4) as reported.30,31 A hot aqueous solution of K2PtCl4 (11.5 mM, 120 mL) was stirred with poly(vinylpyrrolidone) (3 g) for 4 h to turn black. The size and distribution of Pt nanoparticles were examined by TEM and dynamic light scattering (Malvern, Zetasizer Nano ZS). The concentration of PtNPs was measured by total reflection X-ray 25849

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Figure 7. Visible-light-driven hydrogen production describing pathway I and pathway II.

glass fiber. The CCD data were integrated and scaled using the Bruker-SAINT software package, and the structure was solved and refined using SHEXTL V6.12. All hydrogen atoms were placed in the calculated positions. The crystallographic data for 2 are listed in SI Table S1, and the selected bond distances are listed in SI Table S2. Structural information was deposited at the Cambridge Crystallographic Data Center (CCDC reference no. 918915). Chemical NADH Generation. NAD+ was chemically reduced using 1 and sodium formate in phosphate buffer. The yield of NADH was obtained based on absorbance at 340 nm (ε = 6220 cm−1 M−1). NADH was also generated in CH3CN: phthalic acid buffer (pH = 4.5) (1:1). To quantify NADH, the absorbance of authentic NADH was obtained at 338 nm in CH3CN:phthalic acid buffer and an extinction coefficient of NADH was 5560 cm−1 M−1. Measurements of Hydrogen Production. In a typical experiment of photocatalytic reactions, each sample was prepared in a 28 mL glass cell with a volume of 2 mL of phthalic acid buffer (pH 4.5, 50 mM) and MeCN (1:1 (v/v)) sample solution. The cell was sealed with a septum and degassed by bubbling argon through the solution for 7 min at room temperature. The samples were irradiated by a 450 W xenon arc light source instrument (Newport Co., Oriel product line M-69920) with a 420 nm cutoff filter. The amounts of hydrogen produced were determined by gas chromatography using a DS6200 gas chromatograph (Donam Instrument Inc., Seongnam, Korea) with a Carbosphere 80/100 mesh, 6 ft × 1/ 8 in. o.d. SS column (Alltech, Part No.5682PC). The QY was calculated from the ratio of the number of electrons used for hydrogen evolution to the number of incident photons according to the following equation.

fluorescence (TXRF) analysis (S2 PICOFOX, Bruker, Karlsruhe, Germany). Preparation of [Cp*Rh(4,4′-dimethoxy-2,2′bipyridine)(H2O)]2+ (2). Compound 2 was prepared according to a procedure similar to that for 1 except using 4,4′dimethoxy-2,2′-bipyridine instead of 2,2′-bipyridine. Pentamethylcyclopentadienylrhodium(III) chloride dimer (10 mM) and 4,4‘-dimethoxy-2,2‘-bipyridine (20 mM) were stirred in 60 mL of ethanol for 4 h. The initial red solution turned to orange after 2 h. Then, the solvent was rotary-evaporated to afford powder. The powder was recrystallized in ethanol/ether to afford rod-shaped light orange crystals suitable for X-ray crystallography. Yield: 14 mg (70%). Anal. Found for C22H27Cl2N2O2Rh: C, 50.37; H, 5.24; N, 5.31. Calcd: C, 50.30; H, 5.18; N, 5.33. MADI-TOF (M+) m/z. Calcd for C22H27Cl2N2O2 Rh: 489.82. Found: 489.91. 1H NMR spectrum of 2 in CDCl3 is in SI Figure S6a. Preparation of [Cp*R h(4,4′-dicarbo xy-2,2′ bipyridine)(H2O)]2+ (3). Compound 3 was prepared similarly using 4,4′-dicarboxy-2,2′-bipyridine. In a typical reaction, pentamethylcyclopentadienylrhodium(III) chloride dimer (10 mM) and 4,4′-dicarboxy-2,2′-bipyridine (20 mM) were stirred in 60 mL of ethanol for 4 h. The initial red solution turned to orange after 4 h. Then, the solvent was rotary-evaporated to afford powder. The powder was recrystallized in ethanol/ether. Yield: 6.8 mg (34%). Anal. Calcd For C22H23Cl2N2O4Rh: C, 47.76; H, 4.19; N, 5.06. Found: C, 47.65; H, 4.22; N, 5.15. MADI-TOF (M+) m/z. Calcd for C22H23Cl2N2O4Rh: 517.79. Found: 517.81. 1H NMR spectrum of 3 in D2O is in SI Figure S6b. X-ray Crystal Structure Determinations. The X-ray diffraction data for 2 were collected on a Bruker SMART APEX diffractometer equipped with a monochromator in the Mo Kα (λ = 0.71073 Å) incident beam. A crystal was mounted on a 25850

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2(no. of H 2 (or NADH) produced) × 100 (no. of incident photons)

Electrochemical Measurements. Cyclic voltammetry (CV) of complexes 1−3 was carried out using a 630C multipotentiostat (CH Instruments, Bee Cave, TX, USA). A Pt counter electrode and a Ag/AgCl reference electrode were used in all electrochemical measurements. A glassy carbon (GC) disk electrode, which served as the working electrode, was polished to a mirror finish using a 0.05 μm alumina slurry. After alumina polishing, the electrode was sonicated in a water bath for 1 min. CV was obtained in a 50 mM phosphate buffer solution (pH 4.5) prepared with triply distilled water. The solution was purged with Ar gas for 15 min before each measurement, and Ar was flowed over the solution during the experiments.

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ASSOCIATED CONTENT

* Supporting Information S

Tables listing crystallographic data, select bond lengths and angles, figures showing chemical generation of NADH, NMR spectra, and additional hydrogen production data, a Hammett plot from chemical regeneration rates, time courses of NADH generation, and reduction potentials of 1−3, and CIF data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: 82-2-3277-4453. Fax: 822-3277-3419. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This study was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MEST; Grant NRF-2013R1A2A2A03015101) and the KRICT-2020 Program.

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The Journal of Physical Chemistry C

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dx.doi.org/10.1021/jp504876w | J. Phys. Chem. C 2014, 118, 25844−25852