A Nanohybrid Catalyst for Efficient Synthesis of Hydrogen Peroxide at

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C: Surfaces, Interfaces, Porous Materials, and Catalysis

A Nanohybrid Catalyst for Efficient Synthesis of Hydrogen Peroxide at Ambient Temperature and Pressure Miwako Teranishi, Shin-ichi Naya, and Hiroaki Tada J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00381 • Publication Date (Web): 28 Mar 2019 Downloaded from http://pubs.acs.org on March 28, 2019

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The Journal of Physical Chemistry

A Nanohybrid Catalyst for Efficient Synthesis of Hydrogen Peroxide at Ambient Temperature and Pressure Miwako Teranishi,a Shin-ichi Naya,a and Hiroaki Tada* a,b Environmental Research Laboratory, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Fax: +816-6721-2500; Tel:+81-6-6721-2332; E-mail: [email protected] a

Department of Applied Chemistry, Faculty of School of Science and Engineering, Kindai University, 3-4-1, Kowakae, Higashi-Osaka, Osaka 577-8502, Japan Fax: +81-6-6727-4301; Tel:+81-6-6721-2332; E-mail: [email protected] b

KEYWORDS: Hydrogen peroxide synthesis, Nanohybrid catalyst, Gold nanoparticle, Metal titanates, Admicelle ABSTRACT: The catalytic activity of gold nanoparticle-loaded metal oxides (Au/MOs) for the two electron-oxygen reduction reaction (2e--ORR) by formic acid increases with a rise in Fermi energy of the support. Among the MOs, metal titanates (MTiO3, M = Ca, Sr, Ba) possess high levels of Fermi energy locating in the range from -3.84 to -3.98 eV vs. vacuum. Nanohybrid catalysts consisting of Au/MTiO3 and adsorbed surfactant bilayer (or admicelle) progresses the 2e--ORR yielding hydrogen peroxide (H2O2) with turnover frequency (TOF) of (1.3~1.7) × 102 h-1 and initial selectivity > 99% at 298 K and 1 atm. This striking admicelle effect stems from the concentration of O2 into the reaction field due to the spontaneous transport from water phase to the hydrophobic nanospace in the admicelle and the removal of H2O2 from the reaction field due to the opposite directional transport.

1. INTRODUCTION

O2 + HCOOH → H2O2 + CO2 rG0 = -153.4 kJ mol-1

Hydrogen peroxide (H2O2) is of great use as a clean oxidant for organic synthesis,1-6 and a fuel for fuel cells.7,8 Presently, most H2O2 is industrially produced by the anthraquinone process via the multi-steps needing large amounts of input energy and hydrogen (H2). Direct synthesis of H2O2 from H2 and O2 has recently been achieved using Au-Pd nanoparticle (NP)-loaded carbon with H2 selectivity > 95%.9 However, most of H2 is acquired by the steam reforming of hydrocarbons at temperatures > 973 K. Also, generation of H2O2 from a mixture of CO, O2, and H2O has been reported using polyoxometalate-protected Au NP as a catalyst at 293 K.10 Further, photocatalytic synthesis of H2O2 by semiconductors at ambient temperature and pressure is currently being in rapid progress.11-22 On the other hand, formic acid (HCOOH) with hydrogen storage capacity of 4.3 wt% and normal boiling point of 374 K is regarded as a highly promising hydrogen storage material.23,24 HCOOH is one of major products from biomass,25 while it has recently been synthesized from H2 and CO2 using an iridium catalyst under mild conditions.26 Two electron-oxygen reduction reaction (2e-ORR) by HCOOH (eqn. 1) is exergonic. Thus, if we can develop a good catalyst for this reaction, an oxygen cycle is completed by combining with the natural photosynthesis system (Scheme 1). Major challenges in this case are to suppress the 4e--ORR to H2O (eqn. 2) more exergonic than the 2e--ORR and the decomposition of H2O2 once generated.

(1)

O2 + 2HCOOH → 2H2O + 2CO2 rG0 = -303.15 kJ mol-1 (2) where rG0 is the standard Gibbs energy of reaction.

Scheme 1. Oxygen cycle complemented by the natural photosynthesis system. "PS I and PS II" denote photosystem 1 and photosystem 2 in the natural photosynthesis, respectively.

Herein we show that a nanohybrid catalyst consisting of Au NP-loaded alkali earth metal titanates (Au/MTiO3, M = Ca, Sr, Ba) and surfactant admicelle exhibits a high level of thermocatalytic activity for 2e--ORR to yield H2O2 with initial selectivity > 99% at 298 K and 1 atm.

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solutions was regulated to 1.7 same as the reaction solution. The samples were transferred to a disposable folded capillary cell (DTS1060) for measurement. Electrophoretic mobility (u) measured at 25oC using the Malvern M3-PALS method were converted to -potential by the Henry’s equation of  = u/(0rf), where  is the viscosity of water, 0 and r are dielectric constants of vacuum and water, respectively, and f is Henry's coefficient. The conductivity of the samples was around 6 mS cm-1. For the determination of particle size, the samples were transferred to a disposable sizing cuvette. From the diffusion coefficient (D) of the particle measured at 25oC, the particle size (2r) was calculated using the Stokes-Einstein formula of r = kT/6D. The average count rate was between 200 and 500 kcps.

2. EXPERIMENTAL SECTION 2.1. Catalyst preparation and characterization. Au/MO was prepared by the deposition-precipitation method.27 An aqueous solution of HAuCl4 (4.86 mM, 100 mL) was neutralized by 1 M NaOH aq. to be pH 6. To the solution, MO (10 g) was added, and stirred at 70oC for 1 h. The resulting precipitate was collected by centrifugation, and washed with H2O ten times. The sample was dried in vacuo, and heated at 500oC for 4 h to obtain Au/MO. Pt/SrTiO3 was prepared by the photo-deposiiton method. SrTiO3 (4 g) was dispersed into an aqueous solution of H2PtCl6 (0.38 mM, 200 mL) with 10% EtOH. After the deaeration by Ar bubbling for 0.5 h, the suspention was illuminated by high pressure Hg lamp (the light intensity integrated from 290 to 390 nm was 3.0 mW cm-2) at 298 K for 3 h. The resulting suspension was washed with H2O, and dried in vacuo to obtain Pt/SrTiO3. Samples for transmission electron microscopy (TEM) characterization were prepared by dropping of a suspension of samples in ethanol onto a copper grid with carbon support film (grid-pitch 150 m, Okenshoji Co., Ltd., #10-1006). The observations were performed by JEOL JEM-2100F at an applied voltage of 200 kV. X-ray diffraction (XRD) measurements were performed on a Rigaku SmartLab X-ray diffractometer operating at 40 kV and 100 mA. The scans were collected in the range from 5 to 90 º (2) by the use of Cu Ka radiation ( = 1.545 Å). X-ray photoelectron spectroscopic (XPS) measurements were performed using a Kratos Axis Nova X-ray photoelectron spectrometer with a monochromated Al K X-ray source operated at 15 kV and 10 mA using C1s as the energy reference (284.6 eV). 2.2. Preparation of C18TAS aqueous solution. Ag2SO4 (0.5 mmol) was added to an aqueous solution of trimethylstearylammonium chloride (C18TAC, 10 mM, 100 mL), and stirred at 50oC for 3 h. After cooling to room temperature, the resulting precipitate was removed by centrifugation, and then the resulting solution was filtered with membrane filter to obtain an aqueous solution of trimethylstearylammonium sulphate (C18TAS, 10 mM). 2.3. Catalytic H2O2 formation. Au/MO (10 mg) was dispersed into an aqueous solution (10 mL) of HCOOH (1.06 M) with C18TAS (0 - 0.20 mM), and stirred at 25oC in the dark under aerobic conditions. The pH of every solution was controlled at 1.7. The concentrations of H2O2 generated were determined by iodometric titration.28 The amount of CO2 evolved was quantified by gas chromatography (GC-2010Plus with BID-detector, Shimadzu, column = Micropacked ST, Shinwa Chemical Industries, He flow rate = 10 mL min-1). To vary dissolved O2 concentration, the reaction solution was bubbled with Ar or O2 gas for 0.5 h. The O2 concentration was measured by a dissolved oxygen analyzer (S9 Seven2Go, Mettler Toledo). Also, the reaction temperature was varied from 278 K to 323 K in the dark under aerobic conditions. 2.4. H2O2 decomposition. A H2O2 solution (0.917 mM, 10 mL) with HCOOH (1.06 M) was bubbled with Ar for 0.5 h. Au/SrTiO3 and C18TAS (0 - 0.20 mM) was added to the solution, and stirred stirred at 25oC in the dark. The concentrations of H2O2 remained were determined by iodometric titration.28

3. RESULTS AND DISCUSSION 3.1. Catalyst characterization. Each metal oxide (MO) was identified by X-ray diffraction (XRD) measurement. Au NPs were loaded on various MOs by the deposition precipitation method. The mean sizes of MOs and Au NPs were determined by transmission electron microscopy (TEM) (Table S1, Figure S1 in Supporting Information). In every Au/MO, hemisphere-like Au NPs are highly dispersed on the MO surface. Au 4f-X-ray photoelectron (XP) spectra for Au/MOs confirmed that Au is metallic. The concentration of H2O2 generated was determined by the iodometoric titration,28 and the amount of CO2 evolved was quantified by gas chromatography. Detailed experimental procedures are described in Supporting Informaiton. 3.2. Au/MTiO3-catalyzed synthesis of hydrogen peroxide. The reaction was carried out in a closed reactor vessel to determine the stoichiometry at 298 K in the dark (Figure S2). Figure 1A compares time courses for the generation of H2O2 and CO2 from aerated 1.06 M HCOOH aqueous solution (pH 1.7) in the presence of Au/SrTiO3 or Pt/SrTiO3 (10 mg). The amount of CO2 generated at reaction time (tr) is shown by the subtraction of the amount at tr = 0 from the total amount at tr. In the Au/SrTiO3 system, the amounts of H2O2 and CO2 increase in proportion to tr, and the H2O2 concentration reaches ~2 mM at tr = 6 h. The plot of the mole number of H2O2 vs. that of CO2 provides a straight line with a slope of 0.71 at 0 < tr < 4 h (Figure S3). The H2O2 generated can further react with HCOOH to yield peroxyformic acid (HCOOOH) (Eq. 3). The equilibrium concentration of HCOOOH at the H2O2 concentration of 2 mM is estimated to be 8.8 × 10-3 mM using the equilibrium constant of 0.46 at 30°C.29 Then, even at the equilibrium, the concentration of HCOOOH is only 0.5% of the H2O2 concentration. Also, it takes longer than 2 h for reaction (3) to reach equilibrium because of the fairly large activation energy of 75.2 kJ mol-1 for the HCOOOH formation.29 Thus, the formation of HCOOOH could be neglected under the present conditions. HCOOH + H2O2 → HCOOOH + H2O

2.5. -Potential and particle size measurement. The Potential and particle size measurements were performed using a Zetasizer Nano ZS (Malvern Instruments). Samples were prepared by dispersing of Au/SrTiO3 (20 mg) into an aqueous solution (100 mL) of HCOOH (1.06 M) with C18TAS (0 - 0.20 mM), and ultrasonicated for 30 min. The pH of the

(3)

In the Pt/SrTiO3 system, H2O2 concentration has a maximum of 0.81 mM around tr = 2 h, gradually decreasing afterwards. This apparent stop of reaction does not result from the catalyst deactivation because the CO2 concentration continues to increase with a rate (6.54 mol h-1) larger than


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The Journal of Physical Chemistry that in the Au/SrTiO3 system (5.09 mol h-1). Also, TEM observation confirms that the Au and Pt particle sizes hardly change before and after the reaction (Figure S1). Figure 1B shows the initial rates of H2O2 generation (v) determined at tr = 1 h under controlled conditions. In the co-presence of O2 and HCOOH, the value for the Au/SrTiO3 system is 0.445 ± 0.032 mM h-1, while the v for the unmodified SrTiO3 system is only 0.031 mM h-1. In the former system, HCOOH is converted into CO2 without CO generation, whereas the amount of H2O2 generated is very small without O2 or HCOOH. The turnover frequency (TOF) defined as the rate of H2O2 molecule generation (h-1)/number of surface Au atoms was calculated to be 6.5 × 10 h-1 by assuming that the surface Au atoms are catalytically active sites and the shape of Au NP is hemisphere. In this case, O2 is continuously furnished from the air phase in the closed reaction vessel, and also, the dissolution of CO2 in water can be neglected at pH 1.7. Then, the selectivity in reaction (1) was defined as the mole ratio of the amount of H2O2 against that of CO2. The selectivity is 89.2% for the Au/SrTiO3 system and 73.6% for the Pt/SrTiO3 system (tr = 1 h). Further, to calculate the selectivity toward the HCOOH consumed, the reaction was carried out at the initial HCOOH concentration of 2 mM (Figure S4). In this case, the selectivity is 70.0% for the Au/SrTiO3 system (tr = 1 h). Pt is well known to have a high catalytic activity for the H2O2 decomposition.30 Also, the 2e--ORR to H2O2 preferentially occurs on the Au surface, while the 4e--ORR to H2O is favourable on the Pt surface.31,32 These facts explain the much higher selectivity of Au/SrTiO3 than Pt/SrTiO3. The effects of the O2 concentration and temperature on the catalytic activity were studied in the Au/SrTiO3 system. After the reaction solution was bubbled with Ar or O2 gas, the O2 concentration was measured by a dissolved oxygen analyzer. A perfect linear relation with a slope of 1.48 h-1 is observed A)

3.3. Additive effect of surfactant. Then, the additive effects of trimethylstearylammonium sulphate (C18TAS) on the v value and the catalytic activity of Au/SrTiO3 for the decomposition of H2O2 (vd) at an initial concentration ([H2O2]0) = 0.917 mM were examined as a function of C18TAS concentration (CTAS) at 298 K. Figure 2A shows that the activity steeply increases with increasing CTAS, going through a maximum of 0.92 mM h-1, whereas the vd decreases. Figure 2B shows -potential and particle size distribution of Au/SrTiO3 in water determined by dynamic light scattering measurements as a function of CTAS. Au/SrTiO3 has a potential of +10 mV without C18TAS, which steeply increases with the addition of C18TAS to reach +35 mV at CTAS = 0.05 mM. Also, the mean particle size in a dispersed state decreases from 1000 nm to ~80 nm. The size of the surfactant micelles was reported to be ~6.1 nm,33 and thus, the -potentials are the values for not the surfactant micelles but the admicelles. Clearly, a C18TAS bilayer (or admicelle) is formed on the surface of Au/SrTiO3 (Au/SrTiO3@C18TAS),34 and the dispersibility is dramatically improved by the electrostatic repulsion between the large surface charges. At pH 1.7, the electrostatic repulsion works between C18TAS molecules and the catalyst surface. However, the presence of counter anions weakens the electrostatic repulsion, and the attractive van der Waals interaction between C18TAS molecules would overwhelm it to yield the admielle on the surface. Nonpolar organic solvents dissolve much more O2 than water, e.g., the molar solubility of O2 calculated by Henry’s law at 298 K and 21 kPa in hexane (3.1 mM) is greater than that in water (0.26 mM) by a factor of 12. Thus, O2 molecules would be spontaneously incorporated into the hydrophobic nanospace in the admicelle. Previous our study on a Au/TiO2@C18TAC plasmonic photocatalyst showed that the addition of C18TAC enhances the catalytic and photocatalytic activities for the chemoselective oxidation of alcohols to carbonyl compounds by the addition of the surfactant.35 These results indicate that most catalytically active sites on the surface of Au NPs survive after the admicelle formation. This is also true for the present system, whereas the small decrease in the activity at CTAS > 0.05 mM may result from the blocking of the catalytically active sites by the surfactant molecules. Consequently, the remarkable increase in the activity by the C18TAS admicelle formation can be attributed to the O2concentration near the reaction sites on the Au NP surface and the increase in the effective reaction area with the improvement of dispersibility. 3.4. Support effect. When Au with the Fermi energy of EF(Au) is in contact with n-type MO semiconductor with

B)

Figure 1. (A) Time courses for the generation of H2O2 and CO2 in the Au/SrTiO3 (red) and Pt/SrTiO3 (blue) catalytic systems under the same conditions (pH 1.7). (B) Initial rates of H2O2 generation (v) determined at tr = 1 h under controlled conditions. Red bar, Au/SrTiO3 catalyst in aerated HCOOH solution. Blue bar, Au/SrTiO3 catalyst in deaerated HCOOH solution. Green bar, Au/SrTiO3 catalyst in aerated solution without HCOOH. Purple bar, SrTiO3 catalyst in aerated HCOOH solution.

A)

between the v and dissolved O2 concentration in the reaction solution (Figure S5A). Also, the v increases with increasing reaction temperature (Tr) to reach a maximum of 0.445 ± 0.032 mM h-1 at Tr ≈ 298 K (Figure S5B). As a result of the increase in Tr, the number of the reactant molecules exceeding the activation energy increases, whereas the solubility of O2 in the reaction solution decreases. The balance between them would determine the optimum Tr. Evidently, reaction (1) catalytically occurs in the Au/SrTiO3 system with initial selectivity of 89.2%, and the O2 concentration in the solution is the key factor for governing the reaction rate under the conditions.

B)

Figure 2. (A) Plots of the initial catalytic activities for H2O2 generation (v) and H2O2 decomposition (vd) at [H2O2]0 = 0.917 mM vs. CTAS. (B) Plots of the -potential (a) and particle size distribution (b) of Au/SrTiO3 determined by dynamic light scattering measurement as a function of CTAS.



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10-4 at 303 K.41 This spontaneous transport of H2O2 can

EF(MO), the interfacial electron transfer from MO to Au occurs till both the Fermi energies become equal (EF(Au/MO)). Figure 3A shows plots the Au 4f7/2-binding energy (EB) for Au/MOs determined by XP spectroscopy versus the conduction band minimum (ECBM) for the MO support previously reported.36-39 Clearly, the Au 4f-EB for Au/MOs decreases with rising ECBM for the MO support. Significant electronic interaction through the direct and intimate contact between n-type MO semiconductor and Au NPs (Figure S1) would induce the electron transfer from the former to the latter. In the Au/MOs, the EF(Au/MO) approaches EF(MO) since the density of states of Au is reduced by downsizing, i.e., the EF of Au NP can be tuned by the MO support. The support effect on the catalytic activity was studied without C18TAS. Figure 3B shows plots of the TOF for the Au/MO systems vs. ECBM of the MO support. A lucid trend is observed that the v increases with a rise in the ECBM or in the EF of Au NP on the MO support. Among the MOs, BaTiO3 affords a maximum TOF of 8.2 × 10 h-1. The MTiO3 supports (M = Ca, Sr, Ba) without Au NPs loading show no catalytic activity for H2O2 decomposition (Figure S6A). Also, the negligible amounts of CO2 were generated by the reaction of HCOOH with the MTiO3 supports (Figure S6B). The catalytic activity and selectivity for various Au/MOs and Au/MOs@C18TAS are summarized in Table 1. In every system, the admicelle formation significantly enhances the activity. The Au/CaTiO3@C18TAS and Au/BaTiO3@C18TAS systems exhibit the highest TOF of 1.7 × 102 h-1. In the system, the amount of H2O2 was also confirmed to increase with at tr < 6 h although the reaction rate somewhat decreases with increasing tr (Figure S7). After the catalytic reactions, Au particle size is maintained (Figure S1). Surprisingly, high initial selectivity > 99% is also achieved by the admicelle formation, and in this system, the initial selectivity towards the HCOOH consumed also reaches 98.0% (Figure S4). Although Au NP has low catalytic activity for H2O2 decomposition,40 hydrophilic H2O2 molecules generated in the hydrophobic reaction field in the admicelle would be transferred to water

A)

B)

Figure 3. (A) Relation between Au 4f-EB for Au/MO and ECBM of the MO support. (B) Plots of TOF vs. ECBM for the MO support. Table 1. Catalytic activity of Au/MOs and Au/MOs@C18TAS for H2O2 generation Catalyst

ECBM a

v / mM h-1 b

Au/SrTiO3

-3.98

Au/SrTiO3@C18TAS Au/CaTiO3

-3.88

Au/CaTiO3@C18TAS Au/BaTiO3

-3.84

Au/BaTiO3@C18TAS

TOF / h-1

Selectivity b

0.445 ± 0.033

64.6 ± 4.7

89.2 ± 6.5

0.927 ± 0.002

134.4 ± 0.3

> 99.9

0.567 ± 0.034

73.9 ± 4.4

94.8 ± 4.8

1.31 ± 0.018

170.3 ± 2.3

> 99.9

0.467 ± 0.025

81.8 ± 4.4

94.9 ± 2.6

0.950 ± 0.067

166.4 ± 11.7

> 99.9

Au/TiO2

-4.22

0.179 ± 0.046

49.5 ± 12.7

---

Au/BiVO4

-4.68

0.025 ± 0.008

19.9 ± 6.4

---

Au/WO3

-4.92

0.167 ± 0.009

29.2 ± 1.5

The conduction band minimum of the support. calculated at tr = 1 h. a

--b

Initial values

prevent its Au-catalyzed decomposition, giving rise to the extremely high selectivity. 3.5. Reaction mechanism. A plausible reaction mechanism is described below (Scheme 2). Upon addition of C18TAS to the aerated aqueous dispersion of Au/MTiO3, the

Scheme 2. A proposed mechanism on the ORR to H2O2 by the nanohybrid catalyst consisting of Au/MTiO3 and C18TAS admicelle.

phase e.g., the distribution coefficient of H2O2 in toluene is 1 ×

admicelle is formed on the surface. Nonpolar O2 molecules are


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The Journal of Physical Chemistry AUTHOR INFORMATION

incorporated into the hydrophobic nanospace in the admicelle. O2 can be adsorbed on active sites of the Au NP surface and/or dual perimeter sites of Au/MTiO342-44 with the partial electron transfer from Au NP to O2 to generate -O-O∙∙∙Au+/MTiO3 species. Proton-coupled electron transfer from Au NP yields H-O-O∙∙∙Au+/MTiO3 and HCOO-.45 The adsorbed H-O-O∙ radical abstracts the H atom of HCO2- ion to produce H-O-OH∙∙∙Au+/MTiO3 and COO∙- radical anion. Further electron transfer from CO2∙- with strong reducing ability46 to the Au+ produce H2O2 and CO2. Hydrophilic H2O2 molecules generated in the hydrophobic reaction field can be spontaneously transferred to the water phase. The regeneration of Au/MTiO3 and the continuous supply of O2 from the air completes the catalytic cycle. On the other hand, the decomposition of formic acid can afford H2. However, the selectivity for the Au NP-catalyzed synthesis of H2O2 from H2 and O2 is low,9 and thus, the possibility for H2 generated during the reaction to take part in the ORR seems to be low in this system. The red broken line in Figure 3B shows the electrode potential for the proton-coupled ORR ((E(O2/HOO∙) = -0.146 V vs. SHE at pH 1.7).47 At ECBM < -0.15 V, the catalytic activity remarkably increases with lowering ECBM, whereas it remains a low level above the value. This fact means that the reaction is enhanced with increasing electron density in Au NP through the electron transfer from the n-type MO support due to the significant electronic interaction. Thus, it is strongly suggested that the proton-coupled electron transfer to the reductively activated O2 by the adsorption on the Au NP surface of Au/SrTiO3 is the rate-determining step in this reaction. Further work is necessary for elucidating the detailed reaction mechanism.

Corresponding Author TEL: +81-6-6721-2332, FAX: +81-6-6727-2024 [email protected]

ACKNOWLEDGMENT This work was partially supported by a Grant-in-Aid for Scientific Research (C) No. 15K05654 and 18K05280, and MEXT-Supported Program for the Strategic Research Foundation at Private Universities.

REFERENCES (1) Sato, K.; Aoki M.; Noyori, R. A "Green" Route to Adipic Acid: Direct Oxidation of Cyclohexenes with 30 Percent Hydrogen Peroxide. Science, 1998, 281, 1646-1647. (2) Kamata, K.; Yonehara, K.; Sumida, Y.; Yamaguchi, K.; Hikichi S.; Mizuno, N. Efficient Epoxidation of Olefins with >99% Selectivity and Use of Hydrogen Peroxide. Science 2003, 300, 964-966. (3) Chen M. S.; White, M. C. Combined Effects on Selectivity in FeCatalyzed Methylene Oxidation. Science 2010, 327, 566-571. (4) Uyanik, M.; Okamoto, H.; Yasui, T.; Ishihara, K. Quaternary Ammonium (Hypo)iodite Catalysis for Enantioselective Oxidative Cycloetherification. Science 2010, 328, 1376-1379. (5) Prat, I.; Mathieson, J. S.; Güell, M.; Ribas, X.; Luis, J. M.; Cronin, L.; Costas, M. Observation of Fe(V)=O Using Variable Temperature Mass Spectrometry and its Enzymelike C-H and C=C Oxidation Reactions. Nat. Chem. 2011, 3, 788-793.

4. CONCLUSIONS

(6) Hallet-Tapley, G. L.; Silvero, M. J.; Bueno-Alejo, C. J.; GonzálezBéjar, M.; McTiemnan, C. D.; Grenier, M.; Netto-Ferreira, J.C.; Scaiano, J. C. Supported Gold Nanoparticles as Efficient Catalysts in the Solventless Plasmon Mediated Oxidation of sec-Phenethyl and Benzyl Alcohol. J. Phys. Chem. C 2013, 117, 12279-12288.

This study has shown that a nanohybrid catalyst consisting of Au/MTiO3 and C18TAS admicelle exhibits a TOF of (1.3~1.7) × 102 h-1 with initial selectivity > 99% for the 2e-ORR to H2O2 at 298 K and 1 atm due to the concerted effect of the components. We anticipate that further development of the science and technology for the synthesis of H2O2 from O2 leads to fruitful “green chemistry” in combination with the natural and artificial photosynthesis.

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Supporting Information The Supporting Information is available free of charge via the Internet at http://pubs.acs.org. Catalyst properties (Table S1) TEM images and Au or Pt particle size distributions before and after reaction (Figure S1) Reaction setup for H2O2 generation (Figure S2) Relation between CO2 and H2O2 generation (Figure S3) Selectivity on HCOOH conversion to H2O2 (Figure S4) The effect of dissolved O2 concentration and temperaturedependence (Figure S5) Time courses for H2O2 decomposition and time courses for the generation of CO2 by MO without Au loading (Figure S6) Time courses for H2O2 generation in the Au/BaTiO3@C18TAS catalytic system (Figure S7) (PDF)

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