Creation of Innovative Technologies to Control Carbon Dioxide Emissions
  Development of Carbon-Neutral Energy Cycle by Highly Selective Catalysis
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Research Outline

1. "Development of Highly Selective and Efficient Nanocatalysts for
  Realization of Carbon Neutral Energy Cycle"
 Production of rationally designed nano-alloy catalysts is a key target for achieving highly efficient and selective reactions. To this end, we will develop synthetic methods for a variety of metal and alloy catalysts with finite compositions and structures. 3d transition metals, such as Fe, Co, and Ni, are key target elements for nanocatalysts. From the viewpoint of an “Elemental strategy”, highly efficient base-metal catalysts are keenly desired. We have already succeeded in the precise synthesis of Fe-Co-Ni ternary and binary nano-alloys and simple nanometal catalysts with finite compositions and sizes as shown in Fig. 1. We will develop electrodes for an alkaline-type fuel cell by bottom-up construction with rationally designed nanomaterials. 
 Liquid fuels such as ethylene glycol and ammonia will be adopted to the CN cycle. These fuels will be regenerated by reduction of oxidized wastes, such as carboxylic acids and N2 (NO3-) with hydrogen. We have already demonstrated highly selective NH3 synthesis from NO3- using photocatalytically generated hydrogen and are developing regeneration systems for carboxylic acids using highly efficient catalytic reactions based on clean hydrogen.
 Developments in nanocatalysts for highly selective oxidation of liquid fuels and highly efficient reduction of waste enable us to move closer to realization of a CN cycle, which may fundamentally alter the present energy circulation systems and rescue humans from potential fuel and environmental crises.

 
 Fig.1  Newly Synthesized FeCoNi nano-alloy catalyst

2. "Selective Ethylene Glycol Oxidation Reaction for Carbon Neutral
  Energy Cycle System"
 In this study, ethylene glycol is used as the energy media. A direct ethylene glycol fuel cell has been developed to achieve selective oxidation of ethylene glycol. Ethylene glycol can be used for alkaline fuel cells, since C-H, C-OH, and C-C bonds can be activated under basic conditions. A direct ethylene-glycol fuel cell composed of an oxide electrolyte and anode alloy catalyst was developed.
 To construct the fuel cell, samples of Ruddlesden-Popper-type perovskites are synthesized by a solid-state reaction. The thus-obtained powder samples are reground in an agate mortar before pressing into a disk. The anode of the LaSr3Fe3O10 disk was painted with Pt/C, Pd/C, PtRu/C, or Fe-Co-Ni/C paste mixed with ethylene glycol. The metal loading was 1.0 mg/cm2. The LaSr3Fe3O10 disk was placed between two sheets of platinum mesh current collectors at the anode and carbon paper current collectors at the cathode. After construction, fuel-cell polarization curves were collected at 40oC. The current densities were measured under the flow of an aqueous solution of 10 wt% ethylene glycol and 10% KOH at the anode and under a flow of dry O2 at the cathode.

 Fig. 2 shows the overall energy diagram for the carbon-neutral energy-cycle system using ethylene glycol. Oxidation of ethylene glycol to oxalic acid can liberate 79% of the energy available from complete oxidation. Although photocatalytic reduction of CO2 is difficult, it may be possible to photocatalytically reduce oxalic acid to ethylene glycol. By using ethylene glycol as the energy media, it should be possible to realize a carbon-neutral energy-cycle system.

If the selective oxidation reaction proceeds ideally, the following reactions are expected.

Anode:  HOCH2CH2OH + 8 OH- →(COOH)2 + 6 H2O + 8 e- ,   E° = -0.690 V;

Cathode:  2 O2 + 4 H2O + 8 e- →8 OH- ,                         E° = +0.401 V;

Overall:  HOCH2CH2OH + 2 O2 →(COOH)2 + 2 H2O,             E  =  1.081 V.


 Although the cathode was catalyst-free, the open-circuit voltages are over 0.6 V as shown in Fig. 4, irrespective of the anode catalyst. The fuel cell shows a maximum open-circuit voltage (OCV) of 0.75 V and generates the highest output power density of 32 mW/cm2 at a current density of 80 mA/cm2. When Fe-Co-Ni is used, the fuel cell is completely free of noble metals. The fuel cell still generates a high output power density of 27 mW/cm2 at a current density of 90 mA/cm2. To clarify the reaction mechanism in detail, it will be necessary to perform in situ IR measurements during electrochemical oxidation and to analyze the products.
 For the selective oxidation of ethylene glycol, the anode catalyst must selectively activate the C-H bonds. To avoid the formation of CO2, it is important to prevent activation of the C-OH and C-C bonds. Therefore, the next target in this CREST project is to design the function of the Fe-Co-Ni anode catalyst to have high selectivity.

 Fig. 2 Carbon-neutral energy-cycle system using ethylene
  glycol.
Fig.3  Performance of direct ethylene glycol solid alkaline
inorganic fuel cell. Anode, Pt, Pd, Pt-Ru, or Fe-Co-Ni; electrolyte; RP perovskite; cathode: catalyst free; Anode: an aqueous solution of 10% wt% ethylene glycol and 10% KOH; Cathode gas; dry O2.


3. "Development of Fuel-Regeneration Processes with Highly Selective
  Photocatalyst Systems"
 The development of a clean and renewable energy carrier that does not utilize fossil fuels is a great technological challenge. One of the most attractive options is the large-scale utilization of hydrogen (H2) as a recyclable energy carrier. However, industrial H2 production consumes huge amounts of fossil fuels (e.g., natural gas), resulting in equally large CO2 emissions. Although the production of H2 gas from renewable energy sources as alternatives to fossil fuels has been extensively studied, such as photocatalytic generation of H2 from water utilizing solar radiation, the efficiencies of such systems are still insufficient for practical application. Furthermore, the large-scale utilization of H2 gas requires further technical development to allow for more convenient handling. In order to solve these problems, we propose a new concept of energy-cycling, in which a fuel-oxidation process produces clean electricity through an efficient alkaline-type fuel cell without CO2 release, and the oxidative products are regenerated to provide the original fuel by an efficient photocatalytic reduction processes that utilizes solar energy. In this section, we describe the development of efficient photocatalytic or photoelectrochemical systems that allow the oxidative products (e.g., carboxylic acids, nitrates, and nitrogen) to be reduced to the original fuels (e.g., alcohols and ammonia) under light irradiation.
The main subjects to be investigated are the following:
(1) Development of electrocatalyst materials for efficient and selective reduction of oxidative products
  • (carboxylic acids, nitrates, and nitrogen) in aqueous media.
(2) Development of p-type photocatalysts (or photoelectrodes) for the direct reduction of caboxylic acids.
(3) Development of efficient n-type photocatalysts (or photoelectrodes) for water oxidation, which will be
  combined with the electrocatalyst materials for reduction.


4. "Structure-Function Relationship Research of Energy Conversion
  Materials by Synchrotron Radiation"

 In order to develop a carbon-neutral energy cycle, it is important to reveal the underlying relationship between the crystal structures of the catalysts and their functioning in highly selective chemical reactions. Taking advantage of the third-generation synchrotron radiation facility of SPring-8, the RIKEN group aims to visualize the atomic and electronic structures of nano-alloys, oxide electrolytes and photo-catalysts which promote highly selective oxidation and deoxidation.
 The diffraction data obtained for high-performance catalysts are typically very poor and broad due to the nanometer particle size. Therefore, it is very difficult to investigate even the atomic-level structures with an in-house X-ray source. Using the highly brilliant and highly parallel X-ray generated from SPring-8, we visualize the atomic and electronic structures of solid-state nanocatalysts from the diffraction data with high counting statistics and high angular resolution.
 Another goal is to control the nanostructure of catalysts using external conditions such as temperature, photo and gas pressure to develop higher-performance catalysts. To achieve this purpose, we are developing an in-situ structure visualization system equipped with hybrid X-ray detectors, which will enable us to obtain time dependent data under various external conditions.

 The RIKEN group provides the Hokkaido University group with detailed guidelines for developing novel catalysts by comparison with theoretical data obtained from the Tohoku University group.

5. "Design of a Nano-Alloy Catalyst and Fuel Cell System via
  Computational Science"
 In order to support the search for a strategic catalyst and high-performance fuel cell system, we use computational approaches such as molecular dynamics methods and first-principles calculations to study the chemical reactions occurring in an alkaline fuel cell based on the ethylene glycol recycling system, and propose the design of high-activity nano-alloy catalyst materials and a ruggedized fuel cell system capable of high-efficiency power generation. In particular, we investigate the oxidation reaction processes and the origin of catalytic activity in oxidation of ethylene glycol in catalyst materials, such as nano-alloy particles and oxide catalysts, at the anode of the alkaline fuel cell. In addition to analyzing the oxidation reactions at the anode, we explore anion generation due to the reaction of water and oxygen and diffusion into the electrolyte in the cathode. Then, based on the results of computational simulations, we evaluate feasibility an alkaline fuel cell system with high-efficiency power generation and high-selectivity oxidization characteristics. Moreover, we evaluate the activity of the oxalic acid reduction reaction and its dependence on properties such as the most stable adsorption structure and electronic states of the nano-alloy and metal-oxide catalysts. In this way, we aim to contribute to the development of a high-efficiency and high-selectivity ethylene glycol recycling system.

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