direct and safe synthesis of h2o2 from o2 and h2 using

1. Introduction

Hydrogen peroxide is very important chemical in the present chemical industry1),2). The anthraquinone (AQ) process has been used for H2O2 production for a half century3), with a sound history but requires multi-step o p e r a t i o n s w i t h h i g h e n e r g y c o n s u m p t i o n . Consequently, a more efficient and direct production process that would reduce total energy consumption and production costs is very desirable. Pioneering investigations of the direct production of H2O2 using Pd catalysts were first undertaken about 30 years ago4),5), and attractive studies have used Pd and Pd_Au catalysts in methanol or acid aqueous solutions6)～10). Catalytic production of H2O2 has been a primary candidate to replace the AQ process; but the high explosive risk of a mixture of H2, O2 and H2O2 is a serious problem. Therefore, rapid progress in catalytic production is essential to achieve high concentration of H2O2 with a high selectivity. Electrochemical production of H2O2 is another possible method11)～29), and can be divided into two types, electroreduction of O2 in electrolyte using an external electrical power source and reduction of O2 with H2 through fuel cell reactions. Electro-reduction of O2 to H2O2 using cost-effective hydro-

electricity has been observed at the graphite cathodes in alkaline aqueous solutions14). However, the electrolysis method cannot be used for chemical processes because it is commercially not viable. In contrast, the fuel cell method can reduce O2 to H2O2 with H2 without an external source of electricity. The fuel cell method has several advantages for synthesis of H2O2

17)～19). The primary advantage is the lower explosion risk because O2 and H2 are separated by the electrolyte, and the cathode and anode membranes. New electrocatalysts and cell structures have been developed and reaction conditions improved to achieve efficient H2O2 forma-tion20)～29). Our studies of the direct synthesis of H2O2 using the fuel cell method are reviewed here.

2. Concept of H2O2 Synthesis Using a Fuel Cell Reactor

The H2/O2-fuel cell is a device intended to generate electricity by electrochemical reduction of O2 with H2 to water. The anode reaction is electrochemical oxida-tion of H2 to 2H＋ and 2e– (Eq. (1)). The cathode reac-tion is electrochemical reduction of O2 to water (Eq. (2))30),31). In this study, potentials are indicated using the standard hydrogen electrode (SHE), as a reference.

Anode: H H e V at pH2 2 2 0 0 0→ + =( )+ − . (1)

Cathode: 1 2 2 2 1 23 02 2O H e H O V at pH+ + → + =( )+ − . (2)

237Journal of the Japan Petroleum Institute, 57, (6), 237-250 (2014)

J. Jpn. Petrol. Inst., Vol. 57, No. 6, 2014

DOI: dx.doi.org/10.1627/jpi.57.237 ＊ E-mail: [email protected]

[Review Paper]

Direct and Safe Synthesis of H2O2 from O2 and H2 Using Fuel Cell Reactors

Ichiro YAMANAKA＊

Dept. of Chemistry and Materials Science, Graduate School of Science and Engineering, Tokyo Institute of Technology, 2-12-1-S1-16 Ookayama, Meguro-ku, Tokyo 152-8552, JAPAN

(Received April 24, 2014)

Direct synthesis of H2O2 aqueous solutions from O2 and H2 using a fuel cell reactor was reviewed. H2O2 can be catalytically produced by electrochemical reaction without external electrical supply. New electrocatalysts and four types of fuel cell reactors were designed and developed to achieve higher performance of H2O2 forma-tion, ex. an Au-mesh cathode and a type-1 reactor for H2O2/HCl aq. solutions, a heat-treated Mn_porphyrin/activated-carbon cathode and a type-2 reactor for H2O2/H2SO4 aq. solutions, a vapor-grown-carbon-fiber (VGCF) cathode and a type-3 reactor for H2O2/NaOH aq. solutions, and a heat-treated Co_porphyrin/VGCF cathode and a type-4 reactor for pure H2O2 aq. solutions. These studies indicated that synergy of selective electrocatalysis of the cathode and three-phase boundary of O2 (gas phase), electrocatalyst (solid phase) and electrolyte (liquid phase) were essential for the efficient reduction of O2 to H2O2. Therefore, direct synthesis of a pure H2O2 aq. solution of over 4.0 mol dm–3 was achieved with a good current efficiency of 42 %.

KeywordsHydrogen peroxide, Oxygen reduction, Electrocatalysis, Membrane electrode, Fuel cell reactor

Clearly, the fuel cell is a chemical reactor which can produce water without the direct mixing of O2 and H2. Selection of a suitable electrocatalyst for the cathode will allow reduction of O2 to H2O2 under fuel cell con-ditions (Eq. (3)). Therefore, several studies to achieve catalytic and direct synthesis of H2O2 from O2 and H2 using fuel cell reactions have been made. However, this is a very difficult reaction because H2O2 is a strong oxidant (Eq. (4)).

O H e H O V at pH 0223

22 2 0 68+ + → + =( )+ − k . (3)

2 2 2 2 1 78 02 24

2H O H e H O V at pH+ + → + =( )+ − k . (4)

2. 1. Structure of the Prototype Fuel Cell Reactor (Type-1)

The structure of the prototype fuel cell reactor is shown in Fig. 112). The cathode and anode compart-ments were divided by a Nafion-117 membrane. The Pt-, Pd-, Au-, Rh-, or Ru-black cathode (2.5 cm–2, ca. 10 mg cm–2) was attached to the surface of the Nafion-117 membrane by a chemical plate method using the specific precious-metal chloride solutions and NaBH4 solutions. The graphite cathode was attached by a hot-press method to the membrane using a mixture of graphite and poly-tetra-fluoro-ethylene (PTFE, Daikin Industries, Ltd.) powders. The Au-mesh cathode (55 mesh) was attached by physical pressure. The Pt-black anode (2.5 cm2, ca. 10 mg cm–2) was prepared on the other side of the Nafion-117 membrane by the same chemical plate method. An aqueous acid solution (0.1 mol dm–3 HCl, 40 cm3) was filled into the cathode compartment of Fig. 1. Pure O2 (10 cm3 min–1) and H2 (10 cm3 min–1) were introduced into the solution and the anode compartment, respectively. The fuel cell reactions were conducted by shorting the circuit at 298 K. O2

was reduced to H2O2 or water with electrons and pro-tons at the cathode. H2O2 and water were formed in the HCl solution at the cathode. The yield of H2O2 was determined by a KMnO4-titration method after the reaction. The current efficiency (CE) of H2O2 forma-tion was calculated as the two-electrons reaction versus the quantity of charge passed (Q) measured by a cou-lomb meter (Eq. (5)). The CE value corresponds to the H2O2 selectivity based on H2 consumed.

CE H O yield= × ( ) × × ( )2 96500 1002 2 Q % (5)

2. 2. H2O2 Synthesis Using the Prototype Fuel Cell Reactor

Figure 2(a) shows the time courses of H2O2 yield in HCl aq. solutions at the Au-mesh, graphite, Pt, Pd, Rh, and Au cathodes. The Au-mesh and the graphite cath-odes showed the best electrocatalytic activity for H2O2 formation (Eq. (2)) among the tested cathodes. The H2O2 yields increased with reaction time; but reached a plateau after 24 h at the Au-mesh cathode. The maxi-mum H2O2 yield was 2050 μmol (59 mmol dm–3) at 24 h12). The CEs for H2O2 formation are plotted in Fig. 2(b). The CE at the Au-mesh cathode was high at the beginning of the reaction and could be extrapolated to 100 % at 0 min, but decreased with increasing reac-tion time and accumulating H2O2 concentration in the

238


Fig. 1● Diagram of the Prototype Fuel Cell Reactor for H2O2 Synthesis

T＝298 K, catholyte; 0.1 mol dm–3, HCl 40 cm3, P(O2)＝101 kPa, P(H2)＝101 kPa.

Fig. 2● Time Courses for (a) Yield of H2O2 and (b) Current Efficiency for the Prototype Fuel Cell Reactor Using Various Cathodes and HCl Electrolytes

HCl cathode solution.These results indicate that a successive reduction of

H2O2 to water (Eq. (4)) accelerates with accumulating H2O2. The reaction rates of Eqs. (3) and (4) at the Au-mesh cathode reach equilibrium around a H2O2 concen-tration of 59 mmol dm–3. The H2O2 concentration of 59 mmol dm–3 was much higher than that of O2 (≈1 mmol dm–3) in HCl aq. solutions at 1 atm (1 atm＝101.325 kPa) of O2 and 298 K. This fact indicates that the ratio of rate constants of Eqs. (3) and (4), k3/k4, is about 60 at the Au-mesh cathode. Therefore, the unfa-vorable reaction of Eq. (4) was slow at the Au-mesh cathode. We have concluded that higher concentration of O2 is essential for electrocatalysis at the cathode for production of higher concentrations of H2O2.

3. H2O2 Synthesis by Improved Fuel Cell Reactor

Increasing the concentration of O2 is essential for the formation of higher concentrations of H2O2. Our idea was utilization of a three-phase-boundary consisting of a gas phase (O2), liquid phase (electrolyte solutions), and solid phase (cathode) for the O2 reduction.3. 1. Structure of Type-2 Reactor

The concept and structure of the new fuel cell reactor were investigated to achieve higher concentrations of H2O2

20). Figure 3 shows the cross section of the new fuel cell reactor. A gas-diffusion anode, Nafion-117, and a gas-diffusion cathode were symmetrically fabri-cated at the central part. A concentration of gaseous O2 of 1 atm corresponds to 40.9 mmol dm–3 at 298 K, which is 41 times higher than that in water. If O2 gas can be directly supplied to the active site on the cath-ode, the reduction of O2 to H2O2 will accelerate and the concentration of H2O2 will increase.

The gas-diffusion cathode and anode were prepared from electrocatalyst powder, vapor-grown-carbon-fiber powder (VGCF, Showa Denko K.K.), and PTFE pow-der by the hot-press method16)～19). VGCF powder was used as the carrier for the cathode because of its high

electrical conductivity and chemical stability. Acid electrolyte solution was placed in two baths (each volume is 2.1 cm3) between the electrodes and the Nafion-117 membrane. O2 and H2 were introduced to the cathode and anode compartments, respectively. The reaction started under short-circuit conditions at 298 K and electrochemical instrument were used to record data. 3. 2. H2O2 Synthesis Using the Type-2 FC Reactor

with Acid ElectrolyteScreening of cathodes prepared from various electro-

catalysts and VGCF poweder for H2O2 synthesis was conducted under short-circuit conditions. Pd-black, PdO, Pd0/VGCF, Au0/VGCF and activated carbon (AC) showed electrocatalytic activity for H2O2 formation. On the other hand, Pt, Ir, and Rh blacks were not effec-tive for the H2O2 formation despite their high current densities. Fairly good H2O2 yields of 179 mmol dm–3

and 153 mmol dm–3 were achieved using on the [Pd-black＋VGCF] and [AC＋VGCF] cathodes for 2 h, respectively. These concentrations were higher than the previous best result of 59 mmol dm–3 described in the section 2. 2. using the type-1 reactor12). Clearly the type-2 FC reactor was effective for the formation of H2O2 solutions20),21). The CE of 17 % at the [AC＋VGCF] cathode was higher than that of 9 % at the [Pd-black＋VGCF] cathode; so H2O2 formation on the [AC＋VGCF] cathode was studied.

The effects of the contents of AC and VGCF with the total weight of 80 mg and a 5 mg PTFE on H2O2 forma-tion were studied. The electrocatalytic activity of the AC cathode was as low as that of the VGCF cathode. Strong synergy of AC and VGCF was observed for O2 reduction and H2O2 formation21). High electrocatalytic activities for H2O2 formation were observed for a wide range of AC contents between 10 to 90 wt%, but a 35 wt% AC (58 wt% VGCF and 7 wt% PTFE) was optimum because of the good electrocatalytic activity and mechanical strength of the cathode. This optimum cathode is here called the [35AC＋58VGCF] cathode. Open circuit potentials of the [35AC＋58VGCF] cath-ode and the [Pt-black＋VGCF] anode were ＋0.62 V and 0.00 V (SHE), respectively, and the electromotive force (EMF) was 0.62 V. When the circuit was shorted, the cathode potential decreased from ＋0.62 to ＋0.14 V and the anode potential increased from 0.00 to ＋0.06 V. The EMF was 0.48 V for the cathode, 0.06 V for the anode reaction, and 0.08 V for the ohmic resistance of the electrolyte. The large overpotential of the cathode indicated that the cathode reaction controlled the over-all reaction rate. In fact, the H2O2 formation rate and CE increased with P(O2) and were constant without dependence of P(H2)＞10 kPa21). These findings also reflected rapid oxidation of H2 at the anode and slow reduction of O2 at the cathode.

The concentration of acid electrolyte strongly affected

239


Fig. 3● Diagram of the Improved Fuel Cell Reactor (type-2) for H2O2 Synthesis

the reduction of O2 to H2O2. Figure 4 shows the effects of normal concentrations of H2SO4 or HCl on the H2O2 formation using the [35AC＋58VGCF] cath-ode. Current densities increased exponentially with higher concentrations for the both acids. H2O2 forma-tion rate increased linearly up to 1.2 N (0.6 mmol dm–3) H2SO4 or 1.2 N HCl. High formation rates of H2O2 were obtained around 1.2-2.0 N for the both acids. The formation rates were lower using 3.0 N acids. Therefore, dilute acid was preferable for selective H2O2 synthesis and good CEs around 42 % were obtained at 0.1-0.2 N. Kinetic curves for H2O2 formation rate, concentration, and CE with the H2SO4 electrolyte were higher than those with the HCl electrolyte. The H2SO4 electrolyte seems to have a small advantage for the H2O2 synthesis compared to the HCl electrolyte. Counter anion species, HSO4

– and Cl–, may affect electro-catalysis at the [35AC＋58VGCF] cathode. When the fuel cell reaction was continued for 6 h using the [35AC＋58VGCF] cathode and 1.2 N H2SO4 electrolyte, a direct production of higher concentrations of H2O2 of 0.33 mmol dm–3 were accomplished but with lower CE of 21 % at 6 h.

CV and rotating-ring-disk-electrode (RRDE) studies were conducted to investigate the reaction mechanism of O2 reduction to H2O2. The conclusions are summa-rized as follows; (1) the active site for reduction of O2 is the AC surface and the VGCF functions as a lead wire (electron conductor), (2) O2 adsorbed on the AC surface is reduced to HO2• by one-electron reduction (Eq. (6)), (3) HO2• is immediately reduced to H2O2 on the VGCF (Eq. (7a)), or two molecules of HO2• dispro-portionate to H2O2 and O2 (Eq. (7b)) on the VGCF20),21), (4) H2O2 quickly diffuses into the bulk acid solutions and O2 is quickly supplied from the gas phase to the active site on the AC. Both fast diffusion rates of H2O2 and O2 and slow reduction rate of H2O2 to water are as essential for the accumulation of H2O2. In addi-tion, direct two-electron reduction of O2 to H2O2 is not a major mechanism on the [VGCF＋AC] cathode.

O e H HO V at pH2 2 0 05 0+ + → − =( )− + • . (6)

HO e H H O V at pH2 2 2 1 44 0• + + → + =( )− + . (7a)

2HO H O O2 2 2 2• → + (7b)

3. 3. NaHO2 (H2O2) Synthesis Using the Type-2 FC Reactor with Alkali Electrolyte

To confirm the functions of the type-2 FC reactor, H2O2 synthesis from O2 and H2 was studied using alkali electrolyte. Previously, selective electrosynthesis of H2O2 (NaHO2) has been reported using NaOH aqueous solution at a graphite block cathode13),14). VGCF was chosen as the cathode material because of the good graphite structure, high chemical stability and conduc-tivity. A NaOH aqueous solution of 2.0 mol dm–3 was filled into the type-2 FC reactor instead of H2SO4 aque-ous solutions (section 3. 2.). Reaction was started under short-circuit conditions at 298 K. When NaOH (alkali) electrolyte is used, the reaction equations at anode and cathode are different from those using H2SO4 (acid) electrolyte, as indicated in Eqs. (8)-(10). OH–

and water promotes the electrochemical reactions instead of H＋ and the charge carriers are OH– and Na＋. H2 is oxidized to water at the anode (Eq. (8)) and O2 is reduced to HO2

– (NaHO2) by two-electron reduction (Eq. (9)) or OH– (NaOH) by four-electron reduction at the cathode (Eq. (10)). Hydrogen peroxide forms as HO2

– (NaHO2) in the NaOH solution and water forms OH– (NaOH) form. When a cation-exchange mem-brane (Nafion-117) is used, Na＋ functions as a charge carrier.

Potentilas of Eqs. (8)-(10) were calculated at [OH–]＝2.0 mol dm–3 30),31).

Anode: H NaOH H O e NaV pH

2 22 2 2 20 85 14 3

+ → + +− =( )

− +

. . (8)

Cathode: O H O e Na HO Na NaOHV pH

2 2 22 20 24 14 3

+ + + → + +− =(

− + − +

. . )) (9)

240


Reaction conditions as in Fig. 4, except for the electrolyte.T＝298 K, react. time 2 h, electrolyte; 1.2 mol dm–3 HCl (2.1 cm3×2).Anode (1.3 cm2); Pt-black 20 mg, VGCF 70 mg, PTFE 7 mg. P(H2)＝101 kPa.Cathode (1.3 cm2); AC＋VGCF＝80 mg, PTFE 5 mg. P(O2)＝101 kPa.

Fig. 4● Effects of Normal Concentrations of H2SO4 (closed sym-bols) and HCl (open symbols) on (a) Formation Rate of H2O2 (●, ○), Concentration of H2O2 (▲, △), and (b) Current Density (■, □), Current Efficiency (▼, ▽) at the [30AC＋50VGCF] Cathode

O H O e Na NaOHV pH

2 22 4 4 40 22 14 3

+ + + →+ =( )

− +

. . (10)

Figure 5 shows the time courses of (a) current densi-ties and yields of H2O2 (NaHO2) and (b) concentrations and CEs using the type-2 FC reactor. A good yield of H2O2 over 2 mmol and a high concentration of 1.24 mol dm–3 were obtained with a high CE of 94 % for 2 h, but the current density decreased remarkably from 83 mA cm–2 at 15 min to 14 mA cm–2 at 2 h. The cause of the drastic decrease in current density was decreased catho-lyte volume and increased anolyte volume during the reaction. As described above, Na＋ was the charge carrier for the type-2 FC reactor using NaOH aq. elec-trolyte; so Na＋ conduction transported the coordination water from the anode to cathode compartments. The ratio of water and Na＋ was estimated as 6.5 (mol/mol) by volumetric experiments. Transportation of such a significant amount of electrolyte solution of water and Na＋ caused the changes in the catholyte and anolyte volumes.

Therefore, NaOH solution of 2.0 mol dm–3 was added at 1.5 cm3 h–1 using a micro-syringe pump to prevent the decrease in anolyte volume. Addition of this elec-trolyte flow system was considered to represent the type-3 FC reactor. Time courses of H2O2 yields, cur-rent densities and CEs are indicated in Figs. 5(a) and 5(b). The current density decreased gradually with

reaction time. The yield of H2O2 increased linearly for 4 h. The concentration of H2O2 increased rapidly and reached to 2.0 mol dm–3 with a CE higher than 85 %. The volume of the H2O2/NaOH solutions increased con-tinuously after 2 h. The H2O2/NaOH solutions over-flowed from the cathode compartment after 2 h. Continuous production of 1.8-2.0 mol dm–3 H2O2/NaOH solutions was possible using the type-3 FC reac-tor.

To accelerate the formation of H2O2/NaOH solutions using the type-3 FC reactor, the reaction system was improved as below. The overpotentials at the cathode and anode under the short-circuit conditions (the syn-thesis condition) were ＋0.52 V and ＋0.19 V, respec-tively, indicating that electrochemical reduction of O2 at the cathode was slow; so the VGCF cathode was modi-fied. To increase the area of the three-phase boundary in the hydrophobic VGCF cathode, hydrophilic carbon materials were added. Five carbon materials, AC (1300 m2 g–1), XC-72 (254), BP-2000 (1475), Seast-6 (119) and KB (800) were tested finding that the addi-tion of XC-72 was most effective to accelerate the H2O2 formation. The time course of H2O2 formation using the [VGCF＋XC-72] cathode is shown in Figs. 5(a) and 5(b). The current density in the early stage was over 140 mA cm–2 and decreased gradually with reac-tion time. The yield of H2O2 increased linearly and reached over 8 mmol at 4 h. The formation rate of H2O2 (2.0 mmol h–1 cm–2) at the [VGCF＋XC-72] cath-ode was 1.7 times larger than that (1.2 mmol h–1 cm–2)

at the VGCF cathode. The concentration of H2O2 increased rapidly with a high CE. A higher concentra-tion of 2.0 mol dm–3 and CE of 92 % were maintained at 4 h although the current density gradually de-creased22).3. 4. Improvement of Current Decrease with

Reaction TimeThe causes of the gradual decrease in current density

using the [VGCF＋XC-72] cathode and the type-3 FC reactor were investigated. Na＋ is the charge carrier for the FC reactor with NaOH aq. electrolyte. NaOH in the anolyte is converted to water and Na＋ (Eq. (8)). The flow rate of 1.5 mL h–1 of 2.0 mol dm–3 NaOH cor-responded to 80.4 mA (Na＋), which was lower than the measured current of 90 mA at 4 h using the type-3 FC reactor. This estimation indicates that a flow rate of 1.5 cm3 h–1 may be insufficient for a continuous reac-tion.

Figures 6(a) and 6(b) show time courses of the anode and cathode potentials and the currents under dif-ferent flow modes of the electrolyte. The both poten-tials fluctuated with reaction time at anolyte flow rate of 1.5 cm3 h–1 and the positive shift of the anode potential indicated lack of anolyte. Therefore, the flow rate of anolyte was increased to 12.0 cm3 h–1 corresponding to 640 mA (Na＋). Stability of the current in the early

241


T＝298 K, anode: Pt/XC-72-VGCF, 2.0 mol dm–3 NaOH aq. electro-lyte, type-2 reactor: divided cell using Nafion-117, type-3 reactor: di-vided cell with 1.5 cm3 h–1 flow of anolyte (2.0 mol dm–3).

Fig. 5● Effects of FC Reactors on Time Courses of (a) Current Density (Id) and Yield of H2O2, and (b) Concentration and Current Efficiency (CE)

stage was good but the current remarkably decreased and the cathode potential because negative in the later stage. The large increase in the cathode overpotential indicated the cause of decreased current density was associated with the cathode, so the NaOH aq. catholyte flow was mixed at 12.0 cm3 h–1. The steady current, and cathode and anode potentials were observed and a very stable and selective formation of H2O2/NaOH (95 % CE) was achieved.

The excellent effect of the catholyte flow on the steady reactions is considered below. Based on the observation that hydrogen peroxide (NaHO2) was pro-duced with 100 % CE and 6 equivalent water molecules per Na＋ ion were conducted from the anode to cathode compartments, a theoretical reaction equation cab be proposed as Eq. (11).

O e Na H O NaOH NaOH H O2 2 6 2 22 2 11+ + ( ) → + +− +

(11)

The calculated concentrations of hydrogen peroxide and NaOH were the same at 4.3 mol dm–3. The phase diagram of hydrogen peroxide and NaOH indicates that NaHO2 is not stable and converts to solid of Na2O2

13). The deposition of solid Na2O2 in the cathode should inhibit the electrochemical reduction of O2. In fact, a transparent crystal was observed in the cathode23). If the concentration of NaOH in the cathode compartment is controlled to about 2 mol dm–3 by addition of water

into the compartment, an efficient and continuous pro-duction of H2O2/NaOH solution at more than 2 mol dm–3 with 95 % CE can be achieved.

4. D e v e l o p m e n t o f N e w A c t i v e C a t h o d e Electrocatalyst with Acid Electrolyte

The type-3 FC reactor showed very good perfor-mance for the selective and continuous hydrogen perox-ide synthesis (NaHO2＞2 mol dm–3) from O2 and H2 with the NaOH aq. electrolyte, showing that the reac-tion zone of the three-phase boundary was very effec-tive for the catalysis of hydrogen peroxide synthesis. On the other hand, the maximum concentration of H2O2 was only 0.33 mol dm–3 with a low CE of 20 % at a suitable [AC＋VGCF] cathode using H2SO4 aq. elec-trolyte20),21). Clearly, electrocatalysis at the [AC＋VGCF] cathode was inferior in acid electrolyte. Therefore, a new electrocatalyst must be developed to achieve much higher concentration of H2O2 than 0.33 mol dm–3 with good CE in H2SO4 aq. electrolyte.4. 1. E l e c t ro c a t a l y s i s o f M e t a l – p o rp hy r i n

DerivativesElectrocatalysts prepared by pyrolysis of Fe_ and Co_

porphyrin or phthalocyanine supported on carbon black in inert gas act as none-precious metal electrocatalysts for four-electron reduction of O2 to water for PEFCs30)～34). We proposed that inactive electrocatalysts of metal_

porphyrin derivatives for the four-electron reduction of O2 could act as electrocatalysts for H2O2 formation. Electrocatalysts were prepared from metal_TPP/CH2Cl2 solutions and AC by the conventional impregnation method (TPP: 5,10,15,20-tetrakis(phenyl)-21H,23H-porphyrin). The metal(M)–porphyrin/AC powder was pyrolyzed in an Ar stream at 473-1073 K (T) for 2 h, here named M–porphyrin/AC(T).

Figure 7 shows the electrocatalytic activities of vari-ous M_TPP/AC(873) for the H2O2 formation at 298 K. The formation rate of H2O2 over Mn(TPP)Cl/AC(823) was double that over AC electrocatalyst24). The con-centration of H2O2 was 0.58 mol dm–3 at 2 h with 30 % CE. Fe(TPP)Cl/AC(823) and Co(TPP)Cl/AC(873) catalysts were active for reduction of O2 but not for H2O2 formation28). Other metal_TPP catalysts did not show s ign i fican t ac t iv i ty for reduc t ion of O2. Mn(TPP)Cl/AC(873) catalyst showed particular activity for reduction of O2 to H2O2. Various Mn-compound/AC(823) materials (Mn loading: 0.5 wt%) were exam-ined for the H2O2 synthesis. Electrocatalytic activities were Mn(OEP)Cl (356 µmol cm–2 h–1)＞Mn(TPP)Cl (290)＞n(TPPS)Cl (265)＞Mn(TMPyP)Cl (180)＞AC (116)＞Mn(salen) (110), Mn(Pc)Cl (99), Mn(acac)3 (87), where OEP is 2,3,7,8,12,13,17,18-octaethyl-21H ,23H-porphyrin, TPPS: 5,10,15,20-tetrakis(4- sulfaonate phenyl)-21H ,23H-porphyrin, TMPyP: 5,10,15,20-tetrakis(1-methyl-4-pyridinyl)-21H,23H-

242


T＝298 K, type-3 reactor: divided cell by Nafion-117 with flow modes of 2.0 mol dm–3 NaOH electrolyte, cathode: VGCF-XC-72, anode: Pt/XC-72-VGCF.

Fig. 6● Time Courses of (a) Current and (b) Potentials of the Cathode and Anode under Different Anolyte and Catholyte Flow Modes

porphyrin. The N4-ligand, porphyrin ring was effec-tive for the H2O2 synthesis but not the phthalocyanine ring. The Mn(OEP)Cl/AC(823) material was the most active electrocatalyst (178 TON h–1) for H2O2 synthesis using H2SO4 aq. electrolyte (0.5 mol dm–3), although the CE of 35 % should be improved.4. 2. O p t i m i z a t i o n o f M n – p o r p h y r i n / AC

ElectrocatalystFigure 8 shows the effect of pyrolysis temperatures

of Mn(OEP)Cl/AC on the activity for H2O2 formation at 25 ℃. Electrocatalytic activities of Mn(OEP)Cl/AC(＜423) materials were as low as that of only AC. The formation rate of H2O2 remarkably increased with higher pyrolysis temperature above 473 K. The maxi-mum formation rate of H2O2 (400 µmol cm–2 h–1) was obtained by pyrolysis at 723 K and the H2O2 concentra-tion was 0.78 mol dm–3 after 2 h. The CE showed a similar dependence on the pyrolysis temperature. Maximum CE was 47 % was obtained with the Mn(OEP)Cl/AC(723) electrocatalyst. Higher pyroly-sis temperatures were superior for the reduction of O2 to water but inferior for the H2O2 formation, with rates of only 20 µmol cm–2 h–1 at 1073 K. Electrochemical reduction paths of O2 to H2O2 (2e– reduction) and H2O (4e– reduction) were drastically changed by the pyrolysis temperatures24),25).

Effects of the Mn loading of Mn(OEP)Cl/AC(723) were studied on the H2O2 formation. The formation rate of H2O2 increased with Mn loading and the maxi-mum rate of 450 µmol cm–2 h–1 was obtained at 0.3 wt%

Mn loading, with the turnover number based on Mn reaching over 850 at 1 h. The CE of H2O2 formation jumped from 23 % (only AC) to 45-50 % at 0.1-0.6 wt% Mn loading of Mn(OEP)Cl/AC(723). Higher loadings reached both the formation rate and the CE. Longer reaction time for H2O2 formation using 0.3 wt% Mn(OEP)Cl/AC(723) electrocatalyst found that the concentration of H2O2 gradually increased from 0.85 mol dm–3 at 2 h up to an upper limit of 1.19 mol dm–3 after 4 h. The total yield of H2O2 linearly in-creased with reaction time and a constant current density of 45 mA cm–2 after 4 h. The upper limit of H2O2 con-centration was apparently due to increased catholyte volume during the fuel cell reaction, as similar to the situation using NaOH aq. electrolyte (the section 3. 3.). 4. 3. Characterization of the Mn–porphyrin/AC(T)

ElectrocatalystThe Mn(OEP)Cl/AC(T) electrocatalysts were charac-

terized to obtain information about the active site struc-ture using X-ray absorption fine structure (XAFS), Fourier transform infrared spectroscopy (FT-IR) and UV, but no clear conclusions could be obtained. CV studies were also not effective to obtain information about the electrochemical active site because the elec-tric double layer of AC was so large and no difference was found in the CV data of Mn(OEP)Cl/AC(T) and only AC25). However, significant information was obtained for the active site structure by temperature-programmed-desorption (TPD) spectra of Mn(OEP)Cl/AC. Desorption products from the Mn(OEP)Cl/AC

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T＝298 K, electrolyte: 0.6 mol dm–3 H2SO4 aq.Cathode: electrocatalyst(823)＋VGCF, O2 101 kPa, 20 cm3 min–1.Anode: Pt/CB＋VGCF, H2 101 kPa, 20 cm3 min–1.

Fig. 7● Electrocatalytic Activity of Metal_porphyrins Supported on AC for (a) Formation Rate of H2O2, Concentration, (b) Current Efficiency (CE) and Current Density in the Type-2 FC Reactor

Reaction conditions as in Fig. 7.

Fig. 8● Effects of Heat-treatment Temperature of Mn(OEP)Cl/AC on the Formation Rate of H2O2 (●), Concentration of H2O2 (△), and (b) Current Density (■), and Current Efficiency (▽) in the Type-2 FC Reactor

material were directly monitored by on-line mass spec-trometry, which detected CO (m/e＝28), CO2 (44), CH4 (16), H2 (2), HCl (36), and Cl (35). Differences in the TPD spectra of CO, CO2, CH4, and H2 were not observed for Mn(OEP)Cl/AC and AC. On the other hand, HCl and Cl were only observed for the Mn(OEP)Cl/AC from 523 K, peaked at 623 K, and stopped at 823 K. The dependences of HCl and Cl desorption on temperature (TPD) were very similar to the dependence of H2O2 formation rate on pyrolysis temperature in Fig. 8. This finding indicated that desorption of Cl from Mn(OEP)Cl/AC was essential for generation of the active site. We speculate that Mn(OEP) species with no Cl axial ligand would strongly interact with the AC surface and form an open site on the other face of Mn(OEP)25). At higher pyrolysis temperatures, the porphyrin ring structure of Mn(OEP)/AC decomposed and electrocatalysis at the Mn site changed from two-electron reduction to four-electron reduction of O2.

5. Synthesis of Pure H2O2 Aqueous Solution

As described in section 3., selective and efficient pro-duction of hydrogen-peroxide alkali solution (2.0 mol dm–3 NaHO2/NaOH aq.) was achieved by use of the [VGCF＋XC-72] cathode. On the other hand, efficient production of hydrogen-peroxide acid solution (1.2 mol dm–3 H2O2/H2SO4 aq.) was achieved by use of the [Mn(OEP)Cl/AC(723)＋VGCF] cathode. Alkali and acid hydrogen-peroxide aqueous solutions are useful in current industrial processes, but pure H2O2 aqueous solution containing no acid, base and salts is the most valuable, useful and flexible form. If a soluble elec-trolyte is used for H2O2 synthesis, the product H2O2 solutions must contain some electrolyte and pure H2O2 aq. solution cannot be obtained. Therefore, direct syn-thesis of a pure H2O2 aq. solution requires a new type of FC reactor utilizing a solid-polymer-electrolyte membrane. Nafion-117 (solid-polymer-electrolyte) membrane has been already applied to the type-1 FC reactor for the synthesis of H2O2 acid solutions in sec-tion 1. This fuel-cell reactor (type-4 FC reactor) is de-scribed in Fig. 9. A unit of the cathode/Nafion-117/anode was set up between two-compartment cells of type-4. Essential differences from the type-1 FC reac-tor were (i) exposure of the cathode in O2 stream by fill-ing deionized water to the half-full level and (ii) a new cathode electrocatalyst.5. 1. Screening of Electrocatalysts for Pure H2O2

Aqueous SolutionVarious metal-porphyrin derivatives on the AC and

VGCF supports were examined for the formation of pure H2O2 using the type-4 FC reactor (Fig. 9, mode-1) at 278 K, because Mn(OEP)Cl/AC(723) was active for the formation of H2O2/H2SO4 aq. solution. Pd and Au were also examined as electrocatalysts. Mn_TPP/

AC(823) showed very low electrocatalytic activity for formation of pure H2O2 aq. solution. Co_TPP, Ni–TPP, Pd and Au were also not efficient. These results suggested that AC was not a suitable support for the formation of pure H2O2 aq. solution26). Therefore, a VGCF support was used for screening studies. Co_

TPP/VGCF(823) and Co_OEP/VGCF(823) were found to catalyze the formation of pure H2O2 aq. solution. Co_phthalocyanine/VGCF(823) was active for water formation but inactive for H2O2 formation. Other Co-compound/VGCF(823) materials were inactive for O2 reduction. Pd and Au on VGCF were also inactive for H2O2 formation26).

The electrocatalytic activity of pyrolyzed Co_TPP/VGCF materials increased with pyrolysis temperature and the maximum formation rate of H2O2 was obtained at 1073 K. The effect of Co loadings on the formation of H2O2 was studied, finding the formation rates were almost constant below 0.30 wt% Co loading. Of course, zero Co loading was inactive. CE increased with lower Co loading from 0.30 to 0.05 wt%. These screening studies indicated that 0.05 wt% Co_TPP/VGCF(1073) was the optimum electrocatalyst for syn-thesis of pure H2O2 aq. solution and 0.30 mol dm–3 H2O2 was obtained with 8 % CE.

Electrochemical reduction of O2 should proceed at the three-phase boundary of Nafion-117 (liquid-like), cathode surface (solid), and O2 (gas). This mechanism would suggest that Co_TPP/VGCF in the zone of the three-phase boundary is just as effective for the forma-tion of H2O2 but Co_TPP/VGCF in the outer zone should suppress the accumulation of H2O2 by decompo-sition of H2O2. Therefore, fast diffusion of O2 (g) to the thin active zone, selective electrochemical reduction of O2 to H2O2 and fast diffusion of H2O2 to the outer zone would be essential for the accumulation of H2O2

26),27). Based on this hypothesis, a new cathode with a thin three-phase boundary was designed. Catalyst-ink was prepared from 0.05 wt% Co_TPP/VGCF(1073) powder, Nafion-117 solution and 2-propanol by ultrasonic irradiation. The Co_TPP/VGCF-ink was painted on one side of a VGCF base-electrode (Co_

TPP/VGCF-coating cathode). The formation rate of H2O2 at the Co_TPP/VGCF-coating cathode was three times faster than that at the Co_TPP/VGCF-mixed cath-ode, and H2O2 formation of 0.85 mol dm–3 with 13 % CE was remarkably improved. 5. 2. Effect of Reaction Modes on H2O2 Formation

As mentioned above, the Co_TPP/VGCF-coating cathode showed a fairly good activity for the formation of pure H2O2 aq. solution using the mode-1 condition (Fig. 9), but the CE was not adequate. To improve the CE, three different reaction modes were examined by changing the addition modes of deionized water as shown in Fig. 9; mode-2 with addition at both the anode and cathode, mode-3 no addition, and mode-4

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with addition at the anode. The results are indicated in Fig. 10. Current densities were very similar for the four modes, but the performances for H2O2 formation greatly differed. Mode-2 and mode-4 showed higher formation rates of H2O2 compared to mode-1. Mode-2 and mode-4 achieved 3.5 times higher CE compared to mode-1. The electrocatalytic activities of the Co_

TPP/VGCF-coating cathode in mode-2 and mode-4 were very similar, but the concentration of 3.09 mol dm–3 (0.61 cm3) in mode-4 was higher than 1.88 mol dm–3 (1.1 cm3) in mode-2, due to the initial water (0.5 cm3) in the cathode. As described above, mode-4 is a better reaction mode for H2O2 synthesis27). With deionized water added up to the top of the anode in mode-4, the current density decreased drastically to 1 mA. Exposure of the anode to H2 and the cathode to O2 is essential. Accumulation of water in the cathode compartment was observed due to transport of water coordinated to H＋ from the anode to cathode compart-

ments. The ratio of the quantity of transported water (mol) per transported H＋ (mol), water/H＋, was 3.2 for mode-427). The effect of addition of water to the half level of the anode may be washing away of H2O2 from the thin three-phase boundary at the cathode. Such quick desorption of H2O2 from the three-phase boundary will suppress successive reduction or decomposition of H2O2.

Time courses for the formation of H2O2 at the 0.05 wt% Co_TPP/VGCF(1073)-coating cathode were studied using the mode-4 condition. The current den-sity was stable and the H2O2 yield increased linearly with process time for 8 h. Pure 3.8 mol dm–3 H2O2 aq. solution dropped from the cathode and accumulated at the bottom of the cathode compartment with 42 % CE for 8 h. The turnover frequency of Co for H2O2 for-mation was 14 s–1 and the total turnover number was 4×105 for 8 h27). The effect of the amount of the elec-trocatalyst (0.05 wt% Co_TPP/VGCF) coated on the

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Area of electrodes: 2 cm2, electrolyte: Nafion-117 (Du Pont) membrane. Mode-1: 0.5 cm3 deionized water in the cathode. Mode-2: 0.5 cm3 deionized water in the both anode and cathode. Mode-3: no deionized water. Mode-4: 0.5 cm3 deionized water in the anode.

Fig. 9● Diagram of O2/H2 Fuel Cell Reactor (type-3) for Direct Synthesis of Pure H2O2 aq. Solution

VGCF base electrode on H2O2 formation was studied. The formation rate of H2O2 and the current density in-creased with higher amount of electrocatalyst. The maximum formation rate of 0.7 mmol h–1 cm–2 and con-centration of 4.0 mol dm–3 were obtained with 90 mA cm–2 and 42 % CE using 2 mg cm–2 of electrocatalyst.5. 3. Reaction Path for H2O2 Formation on Co–

TPP/VGCF(1073)Voltammetry of Co_TPP/VGCF(1073) was conducted

using RRDE in H2SO4 aq. solution28). The true pH at the active site in the three-phase boundary cannot be measured, but must be acid due to the properties of Nafion-117. Figure 11 indicates the RRDE results of Co_TPP/VGCF(1073) supported on a Glassy-Carbon disk electrode. The current at the disk electrode in-creased with rotation speed (Fig. 11(a)). Limiting currents were observed at lower potentials. The Koutecky-Levich (KL) equation (Eq. (12)) was applied to the reduction of O2 to determine the number of elec-trons.

1 1 1 0 620 2 3 1 6 1 2i i nFACD= + ( )−k . ν ω (12)

where n: number of electrons, C: concentration of O2 (1.03×10–3 mol dm–3), D: diffusion coefficient (2.1×10–5 cm2 s–1), ν: kinematic viscosity (1.07×10–2 cm2 s–1), ω: angular frequency of rotation (rad s–1), and ik: current in the absence of any mass-transfer effects.

The KL plots (1/iL (1/iD) versus ω1/2, Eq. (10)) at 0.0, －0.1, and －0.2 V showed good linearity. The num-bers of electrons in the reduction of O2 (n-value) were calculated from the slopes of the KL plots in Fig. 11(b). The actual cathode potential during H2O2 synthesis var-ied from 0.1 to 0 V, and n-values were 2.1 at －0.1 V and 2.0 at 0.0 V. These n-values indicated that selec-

tive reduction of O2 to H2O2 proceeded on the Co_TPP/VGCF(1073)/GC-disk electrode. Very similar yields of H2O2 produced at the disk electrode were detected at the Pt-ring electrode during the RRDE studies29).

The n-value can be converted to CE for H2O2 forma-tion and n＝2.1 equals 90 % selectivity. The selectivity for H2O2 of 90 % on the Co_TPP/VGCF(1073) electro-catalyst in the RRDE studies was high but the selectivity (CE) in the synthesis study was 40 %. To clarify this discrepancy in selectivity, decomposition of H2O2 was studied using Co_TPP/VGCF(1073) powder as a cata-lyst. Briefly, the decomposition rate of H2O2 by Co_

TPP/VGCF(1073) was not fast and the kinetic curve could be applied to a second-order equation (k＝0.009 mol dm–3 min–1 g-cat.–1)29).

H2O2 is stable on the Co_TPP/VGCF(1073) surface and the low H2O2 decomposition activity of Co_TPP/VGCF(1073) cannot explain the significant difference

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T＝278 K. Cathode: 0.05 wt% Co_TPP/VGCF(1073)-coating/VGCF; O2: 1 atm, 40 cm3 min–1. Anode: Pt/CB＋VGCF; H2: 1 atm, 40 cm3 min–1.

Fig. 10● Effects of Four Reaction Modes of the Fuel Cell System on the Formation of Neutral H2O2 Solution, (a) Formation Rate of H2O2 and Current Density, and of H2O2, and (b) Concentration of H2O2 and Current Efficiency (CE)

T＝298 K, potential scan rate: 20 mV s–1, Co loading: 0.1 mg, elec-trolyte: 0.6 mol dm–3 H2SO4 aq.

Fig. 11● RRDE Voltammetry of O2 Reduction on 0.3 wt% Co_TPP/VGCF(1073)/GC Disk Electrode, (a) Disk Current and (b) KL Plots

in CEs obtained from the RRDE and synthesis studies. The other possible reaction is electroreduction of H2O2, which should contribute to the decrease in CE in the synthesis reaction. Therefore, electrocatalytic activity of Co_TPP/VGCF(1073) for the electroreduction of H2O2 to water was studied using a one-compartment cell. Significant current and conversion of H2O2 was observed. The amounts of charge passed through the Co_TPP/VGCF was very close to the amounts of H2O2 consumption. These observations indicated that H2O2 was consumed by electroreduction and not catalytic de-composition under this experimental condition. The electroreduction rate of H2O2 on Co_TPP/VGCF(1073) was calculated to be 0.62 mmol min–1 g-cat.–1 29).

The decomposition rate of H2O2 of 0.002 mmol min–1 g-cat.–1 at 0.12 mol dm–3 H2O2 aq. solution and the elec-troreduction rate of H2O2 at 0.62 mmol min–1 g-cat.–1 were estimated on the Co_TPP/VGCF(1073) catalyst. The electroreduction rate was higher than the catalytic decomposition rate which indicates that successive electroreduction activity of H2O2 to water strongly affects the H2O2 yield.5. 4. Characterization of Active Site of Co–TPP/

VGCF(1073)X-ray photoelectron spectroscopy (XPS) of Co_TPP/

VGCF(1073) observed four peaks around 779 eV (Co 2p3/2), 399 eV (N 1s), 532 eV (O 1s), 284 eV (C 1s). The effects of treatment temperatures of Co_TPP/VGCF on XPS signals for Co around 779 and N around 399 eV were studied to clarify the chemical state and binding energy as shown in Fig. 12. Figure 12(a) shows Co 2p3/2 for Co_TPP/VGCF(323, 773, 973, 1023 and 1273). A Co_N bond assigned to the Co_N4 spe-cies (Co2＋) of Co_TPP was observed at 780.3 eV on the unheated sample (323 K). No XPS peak at 780.3 eV was observed for the sample heat-treated at 773 K and a new XPS peak at 780.0 eV did not shift after higher temperature treatments at 1023 K and 1273 K. The species at 780.0 eV was probably a Co_NX (Co2＋) com-pound on the carbon surface34)～37). Figure 12(b) shows a N_Co bond assigned to the Co_N4 species of Co_TPP was observed around 398.5 eV and a N_C bond in the pyrrole group of porphyrin was observed around 399.5 eV. An XPS peak assigned to a N_Co bond slightly shifted to 398.3 eV after heat treatment at 773 K. New XPS peaks around 400.5 eV after heat treatment at 1023 K (iii) and 1273 K (iv) may be a pyrrole or graphit ic N compound on the VGCF surface34)～37).

The heat-treated Co_TPP/VGCF samples were char-acterized by X-ray absorption fine structure (XAFS) analysis to obtain structural information about the active site. Figure 13(a) shows the Co K-edge XANES spectra of Co_TPP/VGCF(773, 1073, 1223) and refer-ence samples of nonheat-treated Co_TPP/VGCF, Co_

TPP (Co2＋) and Co foil (Co0). A pre-edge peak at

7713 eV was observed for the Co_TPP crystal sample, which corresponds to the electron transition from the 1s to 4pz orbitals36)～38). This reflects the tetrahedral structure of the Co_N4 compound (porphyrin structure). A broad and shoulder pre-edge peak at 7710 eV was observed for the Co foil sample. The shape of the pre-edge absorption of the nonheat-treated Co_TPP/VGCF was broader than that of the Co_TPP crystal and shifted to 7715 eV. The pre-edge peak at 7715 eV reflects the Co_N4 structure. This peak was observed for Co_

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Fig. 12● X P S o f 0.3 w t % C o _T P P / VG C F (323) , C o _T P P /VGCF(773), Co_TPP/VGCF(973), Co_TPP/VGCF(1023), Co_TPP/VGCF(1273) for Co 2p3/2 (a) and N 1s (b)

TPP/VGCF(773). The pre-edge peak at 7715 eV in Co_TPP/VGCF(1073) decreased considerably and a weak pre-edge peak appeared at 7710 eV that reflected the formation of Co metal. These XANES observa-tions indicated that the Co_N4 structure persisted o n VG C F w i t h h e a t t r e a t m e n t a r o u n d 773 K . Decomposition of the Co_N4 structure and formation of Co metal at 1073 K proceeded. After heat treatment at 1223 K, the Co_N4 structure disappeared and the for-mation of Co was evident.

Figure 13(b) shows the k3-weighted Co K-edge extended X-ray absorption fine structure (EXAFS) spectra for the three samples and the two reference samples. The peak at 2.3 Å (1 Å＝1×10–10 m) observed for the Co foil sample was assigned to the first neighboring

atom of Co. The same peak was observed for the Co_

TPP/VGCF(1073 and 1223) samples. The peak at 1.8 Å observed for the Co_TPP crystal was assigned to the first neighboring atom of N. This peak was observed for the Co_TPP/VGCF(non, 773 and 1073) samples.

Table 1 shows the coordination numbers and dis-tances of Co to the neighboring atoms calculated from the curve fitting results obtained by reverse Fourier transform of Co and N. The coordination number of N to Co was 3.89 for Co_TPP/VGCF(non). The coor-dination number of N to Co was 2.50 and that of Co to Co was nearly zero for Co_TPP/VGCF(773).

After heat treatment at 1073 K and 1223 K, the coor-dination numbers of N to Co were 1.67 and 1.12, respectively, whereas the coordination numbers of Co to Co were 2.38 and 4.15, respectively. The distances between N and Co atoms for 773, 1073 and 1223 K treatments were very close to 1.93, as for the nonheat treatment samples. Moreover, the distances between Co and Co atoms for 1073 K and 1223 K treatment were very close to 2.48 Å of the Co foil.

These observations suggested that Co_TPP was gently pyrolyzed and isolated on the VGCF surface by the 773 K treatment28),29),41),42). On average, approximately one C_N bond broke and the Co_N coordination number decreased to 2.5. Mild pyrolysis of Co_TPP on VGCF proceeded and about two Co_N bonds were broken by the 1073 K treatment. The Co_N coordination number was decreased to 1.7 and very small metal particles were generated. Severe pyrolysis of Co_TPP on VGCF proceeded and approximately three Co_N bonds were broken on average by the 1223 K treatment. The Co_N coordination number was decreased to 1.1 and large metal particles were generated. 5. 5. Reaction Scheme

The partial pyrolysis of Co_TPP on VGCF was essential for generation of the active sites for the reduc-tion of O2 to H2O2. The actual structure of the active site was estimated from XPS and XAFS studies. The CoN2 compound and small Co metal particles were formed simultaneously on the VGCF surface, as indi-cated in Fig. 14. To determine the contribution of the

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Co_TPP loading: 0.3 wt%, references: Co foil and Co_TPP crystal.

Fig. 13● (a) Co K-edge XAFS Spectra of Co_TPP/VGCF(323), Co_

TPP/VGCF(773) , Co_TPP/VGCF(1073) , Co_TPP/VGCF(1223), and Reference Samples (XANES spectra) and (b) k3-weighted Co K-edge EXAFS Spectra

Table 1● Coordination Number and Bond Length Obtained by EXAFS Spectra of Co_TPP/VGCF(T)

Co_TPP/VGCF(T)a)

T [K]

N Co

num dist [Å] num dist [Å]

1223 1.12 1.87 4.15 2.481073 1.67 1.92 2.38 2.47773 2.50 1.89 0.03 2.49none 3.89 1.93 1.44 3.00Co foilb) - - 12 2.48Co_TPP crystalb) 4.2 1.96 2.44 3.02

a) 0.3 wt％ Co_TPP/VGCF heat-treated at T K in He.b) Reference samples.

small Co metal particles to electroreduction of O2, 0.1, 0.3, and 0.5 wt% CoCl2/VGCF(1073) were prepared and the formation of small Co metal particles on VGCF was confirmed by X-ray absorption near edge structure (XANES). The CoCl2/VGCF(1073) materials were applied for the electroreduction of O2 using the type-4 FC reactor with the mode-4 condition, but these materi-als did not show any electrocatalytic activity. This ob-servation indicated that the small Co metal particles on VGCF were not the active sites for H2O2 formation. In addition, 0.3 wt% Co_TPP/VGCF(1073) washed with H2SO4 aq. and used for the H2O2 synthesis showed a very similar electrocatalytic activity to the unwashed sample for O2 reduction and H2O2 formation. Absence of the peak at 2.3 Å assigned to Co_Co and the signifi-cant peak at 1.8 Å assigned to Co_N were confirmed for the washed sample by EXAFS. Therefore, the CoN2 compound on carbon (CoN2Cx) structure is a candidate for the active site28),29).

O2 adsorbs on the CoN2Cx active site at the three phase boundary in the cathode. The pH value at the active site would be nearly zero and O2 is electrochemi-cally reduced to H2O2 through two-electron reduction (Eq. (3))30),31). H2O2 selectivity is higher than 90 % at the CoN2Cx active site. The catalytic activity of the CoN2Cx active site for decomposition of H2O2 is low, but the successive electrochemical reduction of H2O2 to water (Eq. (4)) cannot be neglected and reduces the final CE of H2O2 formation. Diffusion of H2O2 from the interface of the active site and Nafion membrane to the outer surface of the cathode is important. The pure H2O2 aq. solution was automatically separated from the cathode and accumulated at the bottom of type-4 FC reactor27),28). Suppression of the successive electro-chemical reduction of H2O2 to water in the cathode is essential for efficient H2O2 production with higher con-centration and CE.

6. Conclusions

Direct and effective syntheses of H2O2 solutions from H2 and O2 have been developed. Several FC systems using H2SO4 aq., NaOH aq. and solid (Nafion) electro-lytes could achieve fast and selective formation of H2O2. The optimum reaction system for H2O2 synthe-sis is the type-4 FC reactor with the mode-4 conditions using a [Co_TPP/VGCF(1073 K)＋VGCF] cathode which provides pure H2O2 aq. solution of 4.0 mol dm–3 with 42 % CE. The prototype type-1 reactor and the recent type-4 reactor are apparently similar, but essen-tial differences in the type-4 reactor are exposure condi-tions of the cathode and anode in the gas phase and new active and selective electrocatalysts. The type-4 reac-tor for H2O2 formation achieves 20 times higher forma-tion rate, 70 times higher concentration and 4 times higher CE than the type-1 reactor. If electrocatalysis of the active site can be improved to 100 % CE with water/H＋＝2.0, direct and continuous synthesis of pure 11 mol dm–3 H2O2 aq. solution can be achieved. To realize the ultimate synthesis of pure H2O2 solution, suppression of the successive electrochemical reduction of H2O2 to water and prevention of the permeation of H2O2 from the cathode to anode compartments are essential. The ultimate process will trigger advanta-geous changes in the chemical industry.

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Fig. 14● Mode l o f the Ac t ive S i t e Mode l o f the Co _TPP/VGCF(1073) Electrocatalyst

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要　　　旨

燃料電池型反応器を用いた酸素，水素からの過酸化水素の直接かつ安全な合成法

山中　一郎

東京工業大学大学院理工学研究科物質科学専攻，152-8552 東京都目黒区大岡山2-12-1-S1-16

燃料電池型反応器を用いた酸素，水素からの過酸化水素水の直接合成について解説した。過酸化水素は外部電力を用いずに触媒的に生成する。過酸化水素の高い生産性を達成するために，新規電極触媒と4種類の燃料電池反応器を設計開発した。具体的には金メッシュカソードとタイプ1反応器を用いた過酸化水素／塩酸水溶液，熱活性化マンガンポルフィリン担持活性炭カソードとタイプ2反応器を用いた過酸化水素／硫酸水溶液，気相成長カーボンファイバーカソードとタイプ3反応器を用いた過酸化水素／水酸化ナトリウム水溶液，熱活性化コバル

トポルフィリン担持気相成長カーボンファイバーカソードとタイプ4反応器を用いた純過酸化水素水合成である。これらの研究成果は，効率的な酸素の還元による過酸化水素の生成には選択カソード触媒と酸素（気相），電極触媒（固相）と電解質（液相）からなる三相界面の協奏効果が極めて重要であることを示していた。最近の研究成果では，4.0 mol dm–3 （13.5 wt%）の純過酸化水素水を42 %と良い電流効率で直接合成することに成功している。

direct and safe synthesis of h2o2 from o2 and h2 using

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