Article pubs.acs.org/JPCA
Acetate Induced Enhancement of Photocatalytic Hydrogen Peroxide Production from Oxalic Acid and Dioxygen Yusuke Yamada,† Akifumi Nomura,† Takamitsu Miyahigashi,† Kei Ohkubo,† and Shunichi Fukuzumi*,†,‡ †
Department of Material and Life Science, Graduate School of Engineering, Osaka University, ALCA, Japan Science and Technology Agency (JST), Suita, Osaka 565-0871, Japan ‡ Department of Bioinspired Science, Ewha Womans University, Seoul 120-750, Korea S Supporting Information *
ABSTRACT: The addition of acetate ion to an O2-saturated mixed solution of acetonitrile and water containing oxalic acid as a reductant and 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+−NA) as a photocatalyst dramatically enhanced the turnover number of hydrogen peroxide (H2O2) production. In this photocatalytic H2O2 production, a base is required to facilitate deprotonation of oxalic acid forming oxalate dianion, which acts as an actual electron donor, whereas a Brønsted acid is also necessary to protonate O2•− for production of H2O2 by disproportionation. The addition of acetate ion to a reaction solution facilitates both the deprotonation of oxalic acid and the protonation of O2•− owing to a pH buffer effect. The quantum yield of the photocatalytic H2O2 production under photoirradiation (λ = 334 nm) of an O2-saturated acetonitrile−water mixed solution containing acetate ion, oxalic acid and QuPh+−NA was determined to be as high as 0.34, which is more than double the quantum yield obtained by using oxalate salt as an electron donor without acetate ion (0.14). In addition, the turnover number of QuPh+−NA reached more than 340. The reaction mechanism and the effect of solvent composition on the photocatalytic H2O2 production were scrutinized by using nanosecond laser flash photolysis.
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(COO−)2 + 2H+ + O2 → 2CO2 + H 2O2
INTRODUCTION Utilization of natural energy to produce high energy chemicals, fuels, composed of earth abundant elements is the key technology for realizing sustainable society.1 Among natural energy, solar energy is the most exploitable energy source owing to its semipermanent and ubiquitous natures.2−5 Recently, hydrogen peroxide (H2O2) emerges as an attractive candidate to be an energy carrier for realizing sustainable society,6,7 because H2O2 can be produced by two-electron reduction of dioxygen (O2) in the presence of an appropriate reducing agent and used as a fuel of direct H2O2 fuel cells for electric power generation.8 The H2O2 fuel cells emit only O2 and water as byproducts.8−12 Thus, the production of H2O2 by reducing O2 with a renewable chemical utilizing solar energy enables H2O2 to be a promising energy carrier of the next generation.7,8,13 H2O2 currently supplied to industry is mainly produced by the anthraquinone process.14,15 In the process, hydrogen is used as an reducing reagent and large energy consumption is required for H2O2 extraction from an organic reaction medium. On the other hand, in natural systems, oxalic acid is used as a reductant for O2 reduction to produce H2O2 in the enzymatic active center of oxalate oxidase.16−19 Oxalate oxidase catalyzes the oxidation of oxalate and reduction of O2 to H2O2 to produce two moles of carbon dioxide (eq 1),16−19 although oxalate includes a C−C bond which is kinetically stable under ambient conditions.20 © 2013 American Chemical Society
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The produced H2O2 is utilized for decomposition of lignin in natural systems.21 This strong oxidizing nature of H2O2 has also been utilized in chemical oxidation processes, where H2O2 has been recognized not only as a clean oxidant but also as a selective oxidant for various oxidation processes.22 Oxalate ion used as the reductant is produced in a metabolic system during decomposition of glucose, which is produced from carbon dioxide and water by photosynthesis of plants. Thus, oxalic acid can be regarded as a carbon neutral reductant although CO2 is emitted after reaction. There have been many reports on photocatalytic H2O2 production via the O2 reduction by various electron donors such as alcohols and acetic acid using semiconductors23−34 or organic photosensitizers.35−38 Disodium oxalate was also used as a reductant in the photocatalytic production of H2O2 using 2-phenyl-4-(1-naphthyl)quinolinium ion (QuPh+−NA), which forms the long-lived electron-transfer state upon the photoexcitation, as a photocatalyst,39 because the photoinduced electron-transfer state of QuPh+−NA (QuPh•−NA•+) can oxidize oxalate dianion but not monoanion or acid form.13,40a In this reaction system, the basicity of the reaction solution Received: December 28, 2012 Revised: April 5, 2013 Published: April 30, 2013 3751
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Scheme 1. (a) Structure of QuPh+−NA and (b) the Overall Photocatalytic Cycles for H2O2 Production in the Presence of Acetate Ion
using a Hewlett-Packard 8453 diode array spectrometer with a quartz cuvette (path length 10 mm) at 298 K. Quantum Yield Determination. The quantum yields of H2O2 production were determined for the photocatalytic H2O2 production under the following conditions. A square quartz cuvette (light path length: 10 mm), which was filled with an O2-saturated mixed solution of MeCN and H2O containing oxalic acid, TBA acetate and QuPh+−NA (0.060 mM), was irradiated with monochromatized light of λ = 334 ± 10 nm from a Shimadzu RF-5300PC fluorescence spectrophotometer. The total number of incident photons was measured by a standard method using an actinometer (potassium ferrioxalate, K3[FeIII(C2O4)3]) in an aqueous solution at room temperature where photon flux was determined to be 1.15 × 10−8 einstein s−1.42 The produced H2O2 in the reaction solution was quantified by the colorimetric method using Ti−TPyP.41b Time-Resolved Absorption Spectral Measurements. Nanosecond laser flash photolysis measurements were executed for a mixed solution of MeCN and H2O containing QuPh+− NA excited by an Nd:YAG laser (Continuum, SLII-10, 4−6 ns fwhm) at λ = 355 nm with the power of 3.6 mJ per pulse. The transient absorption measurements in the visible and near-IR region were performed using a continuous Xe lamp (150 W) as a probe light and a photomultiplier (Hamamatsu R2949; 350− 800 nm) and an InGaAs-PIN photodiode (Hamamatsu G5125−10; 800−1200 nm) as a detector, respectively. The output from the photodiodes and a photomultiplier was recorded with a digitizing oscilloscope (Tektronix, TDS3032, 300 MHz).
increases by the formation of NaOH during the reaction (eq 2) to prohibit the O2 reduction to H2O2.40a (COONa)2 + O2 + 2H 2O → 2CO2 + H 2O2 + 2NaOH (2)
We report herein significant enhancement and increase in turnover numbers of photocatalytic H2O2 production by O2 reduction with oxalic acid as an electron donor using QuPh+− NA as a photocatalyst by the addition of acetate ion. Scheme 1 illustrates the chemical structure of QuPh+−NA and the photocatalytic cycles for H2O2 production with QuPh+−NA, acetate ion, and oxalic acid used as a photocatalyst, acid−base catalyst, and an electron donor, respectively. The effect of water content in the reaction solution was investigated for the photocatalytic H2O2 production to optimize the catalytic efficiency. Nanosecond laser flash photolysis and kinetic measurements were performed to reveal the detailed catalytic mechanism in Scheme 1 and the reason for the acceleration effect of acetate ion.
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EXPERIMENTAL METHOD Materials. All chemicals were obtained from chemical companies and used without further purification. Oxalic acid and perchloric acid (60%) were obtained from Wako Pure Chemical Industries. Acetonitrile (MeCN) and tetra-n-butyl ammonium (TBA) acetate were obtained from Nacalai tesque and Sigma−Aldrich, respectively. Oxo[5,10,15,20-tetra(4pyridyl)porphyrinato]titanium(IV) (Ti−TPyP) was obtained from Tokyo Chemical Industry. 2-Phenyl-4-(1-naphthyl)quinolinium perchlorate (QuPh+−NA) was synthesized by a reported method.39 Photocatalytic H2O2 Production. A typical procedure is as follows: a mixed solution (2.0 mL) of MeCN and H2O (water content: 10−50 vol%) containing QuPh+−NA (0.2 mM), oxalic acid (50 mM), and TBA acetate (100 mM) was flushed with O2 gas. The reaction vial was connected with a balloon filled with O2, which is supplied to the reaction solution, to maintain O2-saturated conditions. The concentration of O2 in a O2-saturated mixed solution was determined by colorimetric titration using 9,10-dihydro-10-methylacridine.41a The solution was then irradiated with a Xe lamp (Ushio Optical, Model X SX-UID 502XAM) through a color filter glass (Toshiba Glass UV−35) transmitting λ > 340 nm at room temperature. The produced H2O2 was reacted with the Ti−TPyP reagent to form a peroxide complex in an aqueous solution of perchloric acid.41b The absorbance of the corresponding peroxo complex was monitored at 434 nm
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RESULTS AND DISCUSSION
Photocatalytic H2O2 Production with Oxalic Acid as an Electron Donor in the Presence of Acetate Ion. The photocatalytic hydrogen peroxide (H2O2) production was examined by photoirradiation (λ > 340 nm) of an O2-saturated mixed solution of MeCN and H2O (20 vol%) containing QuPh+−NA (0.20 mM) and oxalic acid (50 mM) in the absence or presence of tetra-n-butyl ammonium (TBA) acetate (100 mM). The use of TBA salt of acetate assures the high solubility of corresponding acetate or oxalate salt to the mixed solutions with wide variety of MeCN and water compositions. Figure 1 depicts the time profiles of H2O2 production in each reaction system. In the absence of acetate ion (green squares), no H2O2 formation was observed by photoirradiation for 1 h. On the other hand, in the presence of acetate ion in the reaction solution, the concentration of H2O2 in the reaction solution linearly increased in proportion to photoirradiation 3752
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Figure 2. Time profiles of H2O2 production under photoirradiation (λ > 340 nm) of O2-saturated solutions (2.0 mL) of MeCN and H2O (20 vol%) containing QuPh+−NA (0.20 mM), oxalic acid (50 mM), and TBA acetate (50 mM, ●; 80 mM, ⧫; 100 mM, ▲; 150 mM, ■).
Figure 1. Time profiles of H2O2 production under photoirradiation (λ > 340 nm) of O2-saturated mixed solutions (2.0 mL) of MeCN and H2O (20 vol%) containing QuPh+−NA (0.20 mM) and oxalic acid (50 mM, ■, green), oxalic acid and TBA acetate (50 mM and 100 mM, ●, red) and di(TBA) oxalate (50 mM, ▲, blue).
became 4.0, indicating that the ratio among oxalate dianion, monoanion, and oxalic acid is 34:66:0. The addition of acetic acid (25 mM or 50 mM) to the reaction solution containing oxalic acid (50 mM) and 2 equiv of TBA acetate (100 mM) resulted in no significant change in H2O2 production rate as shown in Figure S2 in the Supporting Information (SI), indicating that the effect of acetic acid formed is negligible. Thus, more than equimolar concentration of TBA acetate relative to oxalic acid is necessary to produce a practical amount of oxalate dianion, which acts as an electron donor. The composition of the mixed solutions also affects the H2O2 production rate. As indicated in Figure 3a,b, the initial H2O2 production rate of 4.0 μmol h−1 observed in pure MeCN (for 3 h) increased by the addition of water to MeCN. When the water content was around 10 vol%, the H2O2 production rate increased to 22 μmol h−1. Further increase in the water content resulted in decrease in the H2O2 production rate to 7.2 μmol h−1 at the water content of 50%. In pure MeCN, acetic acid is expected to act as the sole proton donor to stabilize O2•−, however, the proton activity is significantly low because of lack of the proton hopping mechanism in the absence of water. The pKa value of acetic acid has been estimated to be 22.3 in MeCN,44 although the pKa value in pure water is 4.5−4.7.43 Thus, the water addition to MeCN increases the proton activity to enable proton hopping, resulting in increase in the H2O2 production rate. On the other hand, the high content of water in the reaction solution is less beneficial to a high quantum yield of QuPh+−NA to form the electron-transfer state of QuPh•−NA•+. The quantum yields for formation of the electron-transfer state were determined from the maximum absorbance change at 420 nm assignable to the QuPh• moiety obtained by nanosecond laser flash photolysis measurements (λex = 355 nm). The quantum yield of 83% in pure MeCN was gradually decreased to 34% by increasing the water content to 50% as indicated in Figure 3d.39 Furthermore, the maximum concentration of O2 in the mixed solution decreases with a higher water content. The maximum O2 concentrations in mixed solutions determined by the colorimetric titration with 9,10-dihydro-10-methylacridine were 10 mM, 8.5 mM, and 4.3 mM in the mixed solutions of MeCN and H2O (10, 20, and 50 vol%), respectively. The lower O2-saturation concentration in a mixed solution with a higher water content may also lead to a decrease in the H2O2 production rate.
time and reached more than 13 mM in 2 h (red circles). From the reaction solution, CO2 evolution has been detected as shown in Figure S1 in Supporting Information (SI), where the production rate of CO2 was nearly double of the H2O2 production rate. The use of TBA salt of oxalate dianion as the electron donor instead of oxalic acid also led to H2O2 formation; however, the concentration of H2O2 reached 3 mM in 30 min and there was no increase in the concentration at prolonged reaction time (blue triangles). This saturation behavior should be ascribed to the pH increase caused by formation of OH− in eq 2, where Na+ ions should be replaced by TBA+ ions. Therefore, the photocatalytic reaction without oxalate was investigated, because a previous report using ZnO as a photocatalyst for H2O2 production suggested that acetate ion acted as an electron donor.37 However, no H2O2 formation was observed from the reaction solution containing QuPh+− NA and TBA acetate without oxalic acid, indicating that acetate ion cannot act as an electron donor in the present photocatalytic system. The photocatalytic H2O2 production was checked with different concentrations of TBA acetate ranged from 50 mM to 150 mM in the presence of oxalic acid (50 mM) and QuPh+NA (0.20 mM) to examine the concentration effect of acetate ion (Figure 2). H2O2 production was observed from the reaction solution containing TBA acetate with the concentration higher than 80 mM. The initial rates of H2O 2 production were independent of the concentration of TBA acetate. On the other hand, significant decrease in the H2O2 production rate was observed when the concentration of TBA acetate was lower than 50 mM (black circles) which is one equivalent of oxalic acid, because the equimolar TBA acetate can hardly produce dianion species of oxalate. The monoanion formation was evidenced by the pH value of 2.2 for an aqueous solution containing oxalic acid (250 mM) and equimolar TBA acetate, which was mixed with four times volume of MeCN for the photocatalytic reaction, because the pH value of 2.2 is in between pKa1 (1.27) and pKa2 (4.27) values of oxalic acid.43 On the basis of the pH of the solution, the pKa1 and pKa2 values and pKa value of acetic acid, the ratio of oxalate dianion, monoanion, and oxalic acid can be estimated as 1:89:10. When the concentration of TBA acetate was increased to 1.6 equimolar in an aqueous solution, the pH of the solution 3753
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Figure 3. (a) Time profiles of H2O2 production under photoirradiation (λ > 340 nm) of an O2-saturated mixed solution (2.0 mL) of MeCN and H2O (10−50 vol%) containing QuPh+−NA (0.2 mM), oxalic acid (50 mM), and TBA acetate (100 mM). (b) H2O2 production rates as a function of water content of reaction solutions. (c) Time profiles of absorbance change at 420 nm due to QuPh• moiety detected by nanosecond laser excitation at 355 nm of a deaerated mixed solution of MeCN and H2O (10−40%) containing QuPh+−NA (0.056 mM) at 298 K. (d) Quantum yields of QuPh•−NA•+ formation determined by the maximum absorption change at 420 nm in panel c versus water contents.
The apparent quantum yield (Φ) of H2O2 production is defined by eq 3, where T = transmittance. Φ = {mole of H 2O2 produced}/{photon flux × (1 − T )} (3)
In previous reports, the Φ value of H2O2 production has been determined to be 14−20% by UV-light irradiation (313 nm) of an aqueous suspension containing ZnO (8 g L−1) and potassium oxalate (1−100 mM).36 Also, much lower value (Φ = 4.2%) has been reported using monochromatized light (λ = 340 nm) in the reaction system composed of 1 mM ZnO colloid and 1 mM oxalate dianion in an O2-saturated buffer (pH 7.5−8.0).37 The apparent quantum yield of H2O2 production by the photoirradiation of a mixed solution containing QuPh+− NA (0.060 mM), oxalic acid (50 mM), and TBA acetate (100 mM) was determined by using monochromatized light (λ = 334 nm) to be 14% in the mixed solution of MeCN and H2O (30 vol%) as shown in Figure 4 (green triangles). The Φ values increased to 27% and 34% by decreasing the water contents in the mixed solutions to 20 vol% (blue squares) and 10 vol% (red circles), respectively. Next, the effect of the water content of the mixed solutions on the durability of the photocatalytic system was examined by repetitive reactions. Figure 5 shows the time courses of H2O2 production under the photoirradiation of O2-saturated mixed solutions with different water contents ranging from 10% to
Figure 4. Time courses of H2O2 production under photoirradiation (λ = 334 nm) of an O2-saturated mixed solution (3.0 mL) of MeCN and H2O (10−30 vol%) containing oxalic acid (50 mM), TBA acetate (100 mM), and QuPh+−NA (0.060 mM).
30% containing oxalic acid (50 mM), TBA acetate (100 mM), and QuPh+-NA (0.20 mM). Oxalic acid (50 mM) was added to each reaction solution after the H2O2 production ceased. In all the reaction systems, the H2O2 concentration in the reaction solutions further increased by the addition of oxalic acid. The amount of H2O2 produced in the second cycle was virtually the 3754
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20 vol% of water. In total, the turnover number of the H2O2 production per the photocatalyst (QuPh+−NA) exceeded 340. Electron Transfer from Oxalate Dianion to NA•+ Moiety of Photogenerated QuPh•−NA•+. In order to clarify the photocatalytic reaction mechanism, nanosecond laser flash photolysis measurements of QuPh+−NA were conducted to detect the electron-transfer state (QuPh•−NA•+) under various conditions. Laser excitation at 355 nm of a deaerated mixed solution of MeCN and H2O (10%) containing QuPh+− NA (0.056 mM) results in the formation of the electrontransfer state (QuPh • −NA•+) with three characteristic absorption bands with λmax = 420, 690, and 1000 nm assigned to the QuPh• moiety, the NA•+ moiety, and the π-dimer radical cation, [(QuPh•−NA•+)(QuPh+−NA)], respectively.39,45,46,47 In Figure 6a, these bands were clearly observed at 6 μs after laser excitation and monotonously descended at 30 μs in the solution containing neither oxalic acid nor TBA acetate. A similar behavior of the absorption bands was observed for the solutions containing only oxalic acid (0.08 mM) (Figure 6b) and only TBA acetate (0.16 mM) (Figure 6c). In the presence of both oxalic acid and TBA acetate in the reaction solution, the absorption bands at λ = 690 and 1000 nm disappeared at 30 μs after the laser excitation, whereas the absorption band at λ = 420 nm remained (Figure 6d). This suggests that electron transfer from oxalate dianion to the NA•+ moiety of the πdimer radical cation occurs as predicted by the higher oneelectron reduction potential of the NA•+ moiety (Ered = 1.87 V
Figure 5. Time profiles of H2O2 production under photoirradiation (λ > 340 nm) of O2-saturated mixed solutions (2.0 mL) of MeCN and H2O (10−30 vol%) containing QuPh+−NA (0.20 mM), oxalic acid (50 mM), and TBA acetate (100 mM). Repetitive reactions were conducted by addition of oxalic acid (50 mM) to the solution after the initial reaction.
same as that of the first cycle in the reaction solution with the water content of 20 vol%, whereas slight and large decrease in the amounts of H2O2 produced in the second cycle was observed for the reaction solution with water content of 30 vol % and 10 vol%, respectively. The highest H2O2 concentration of 69 mM was achieved with the reaction solution containing
Figure 6. Transient absorption spectra of QuPh+−NA (0.056 mM) observed by nanosecond laser excitation at 355 nm (a) in the absence of oxalic acid and TBA acetate, (b) in the presence of oxalic acid (0.08 mM), (c) in the presence of TBA acetate (0.16 mM), and (d) in the presence of oxalic acid (0.08 mM) and TBA acetate (0.16 mM) in deaerated mixed solutions of MeCN and H2O (10 vol%). 3755
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Figure 7. (a) Decay time profiles of the absorption at 690 nm due to NA•+ moiety detected by nanosecond laser excitation at 355 nm of a deaerated mixed solution of MeCN and H2O (10 vol%) containing QuPh+−NA (0.056 mM) at 298 K with different concentrations of oxalic acid (0.05 mM, black; 0.10 mM, blue; 0.30 mM, green; 0.5 mM, red) and TBA acetate (two equimolar of oxalic acid). (b) Plot of the pseudo-first-order rate constant (kobs) for electron transfer from oxalic acid to QuPh•−NA•+ versus [oxalic acid]. (c) Decay time profiles of the absorption at 690 nm detected by nanosecond laser excitation of a deaerated mixed solution of MeCN and H2O (50 vol%) containing QuPh+−NA with different concentrations of oxalic acid (0.5 mM, black; 1.0 mM, blue; 3.0 mM, green; 4.0 mM, purple and 5.0 mM, red) and TBA acetate (two equimolar of oxalic acid). (d) Plot of the pseudo-first-order rate constant (kobs) for electron transfer from oxalic acid to QuPh•−NA•+ versus [oxalic acid].
vs SCE)39 than the one-electron oxidation potential of oxalate dianion [Eox = 0.80 V vs SCE in a deaerated mixed solution of MeCN and a phosphate buffer (pH 7.0) [1:1 (v/v)] and Eox = 0.3 V in MeCN].40 The rates of electron transfer from oxalate dianion to the NA•+ moiety of the π-dimer radical cation in mixed solutions with water contents of 10% and 50% were monitored by the decay curves of absorption at 690 nm with various concentrations of oxalate dianion as shown in Figure 7a,c, respectively. In both cases, the rate obeyed pseudo-first-order kinetics (Figure S3 in SI) and the pseudo-first-order rate constant (kobs) increased linearly with increasing concentration of oxalate dianion as shown in Figure 7b,d. The presence of an intercept in the plots of Figure 7b,d suggests that the NA·+ moiety can react with not only oxalate but also QuPh• moiety of another QuPh•−NA•+ molecule,39 because the intermolecular back electron-transfer process requires no oxalate anion to reduce NA•+. The second-order rate constants (kox) of electron transfer from oxalate dianion to the NA•+ moiety of the πdimer radical cation were determined from the slopes of linear plots in Figure 7b,d to be 1.6 × 108 M−1 s−1 and 7.7 × 106 M−1 s−1 in the mixed solutions with water content of 10% and 50%, respectively. The 20 times larger second-order rate constants in the mixed solution with water content of 10 vol% compared with that of 50 vol% may result from the destabilization of oxalate dianion with the decreased amount of water, which
makes it easier to oxidize oxalate dianion. This leads to the high quantum yield for H2O2 formation in the mixed solution with water content of 10%. In contrast to the decay of absorption at 690 nm due to the NA•+ moiety of the π-dimer radical cation by the electrontransfer reduction by oxalate dianon, the absorption at 420 nm due to the QuPh• moiety maintained a certain level depending on the concentrations of oxalic acid as shown in Figure 8. In the absence of oxalic acid or TBA acetate, the absorption at 420 nm decays because of the bimolecular back electron transfer to the ground state (black).39 In the presence of 0.50 mM oxalic acid with 1.0 mM TBA acetate, no significant absorption decay was observed (orange). Further increase in the concentration of oxalic acid and TBA acetate resulted in the rise of absorption. When the concentration of oxalic acid was increased higher than 2.0 mM (red), the absorbance at 420 nm assigned to QuPh• moiety reached 2 times larger than the maximum absorbance change observed in the absence of oxalic acid. These results suggest thermal formation of QuPh• after photochemical formation of QuPh•. The thermal reaction of QuPh+−NA with a reactive species, such as CO2•−, was derived from one-electron oxidation of oxalate dianion produces QuPh• by the electron-transfer reduction of QuPh+−NA by CO2•− in a deaerated solution. Electron Transfer from QuPh• to O2. The electron transfer from QuPh• to O2 in the mixed solutions of MeCN 3756
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kinetics (Figure S4 in the SI) and the pseudo-first-order rate constants (kobs) increased in proportion to the concentration of O2 as shown in Figure 9b,d. The second-order rate constants (kred(O2)) of electron transfer from QuPh• to O2 were determined from the slopes of linear plots in Figures 9b,d to be 7.9 × 108 M−1 s−1 and 8.2 × 108 M−1 s−1 in a mixed solution of MeCN and H2O (10% and 50%), respectively. The large rate constants are close to the diffusion-limited value.48 Under the photocatalytic reaction conditions, the concentration of oxalic acid (50 mM) did not exceed 5 times the concentration of O2 (10 mM), thus, the oxidation rate of oxalate dianion should be slower than the reduction of O2 by QuPh•−NA•+, because the second-order rate constant of electron transfer from oxalate dianion to the NA•+ moiety in a mixed solution of MeCN and H2O (10%) was 1.6 × 108 M−1 s−1, which is less than one-fifth of kred(O2) (8.2 × 108 M−1 s−1). During the reaction, O2 was continuously supplied to the reaction solution from an O2-gas balloon, thus, the O2 concentration was maintained at 10 mM; on the other hand, the concentration of oxalic acid decreased by the progress of the reaction. Thus, the O2 reduction by QuPh• occurs first and then, the reduction of NA•+ by oxalate dianion in the mixed solution of MeCN and H2O (10%). Overall Photocatalytic Reaction Pathway. The overall reaction pathway of H2O2 formation in the mixed solution of MeCN and H2O (10%) containing oxalic acid (
