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A Ru/La Pr O catalyst for low-temperature ammonia synthesis Yuta Ogura, Kotoko Tsujimaru, Katsutoshi Sato, Shin-ichiro Miyahara, Takaaki Toriyama, Tomokazu Yamamoto, Syo Matsumura, and Katsutoshi Nagaoka ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b04683 • Publication Date (Web): 31 Oct 2018 Downloaded from http://pubs.acs.org on November 7, 2018
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ACS Sustainable Chemistry & Engineering
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A Ru/La0.5Pr0.5O1.75 catalyst for low-temperature ammonia synthesis
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Yuta Ogura*,†, Kotoko Tsujimaru†, Katsutoshi Sato†,‡, Shin-ichiro Miyahara†, Takaaki
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Toriyama§, Tomokazu Yamamoto‖, Syo Matsumura§,‖, and Katsutoshi Nagaoka*†
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† Department of Integrated Science and Technology, Faculty of Science and Technology,
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Oita University, 700 Dannoharu, Oita 870-1192, Japan
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‡ Elements Strategy Initiative for Catalysts and Batteries, Kyoto University, 1-30 Goryo-
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Ohara, Nishikyo-ku, Kyoto 615-8245, Japan
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§ The Ultramicroscopy Research Center, Kyushu University, Motooka 744, Nishi-ku,
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Fukuoka 819-0395, Japan
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‖ Department of Applied Quantum Physics and Nuclear Engineering, Kyushu University,
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Motooka 744, Nishi-ku, Fukuoka 819-0395, Japan
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* Corresponding authors
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Email:
[email protected],
[email protected] 15
Tel: (+81)97-554-7896, Tel: (+81)97-554-7895
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Fax: (+81)97-554-7979
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Abstract
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To exploit the use of hydrogen as a source of sustainable energy, development of an efficient
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process for synthesizing an energy carrier such as ammonia under mild conditions will be
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necessary. Here, we show that Ru/La0.5Pr0.5O1.75 pre-reduced at an extraordinary high
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temperature of 650C catalyzes high NH3-synthesis rates under mild conditions. At 400 C
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under 1.0 MPa, the synthesis rate was comparable with that of most active oxide-supported
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Ru catalysts. Kinetic analysis revealed that hydrogen poisoning, which is a typical drawback
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for oxide-supported Ru catalysts such as Cs+/Ru/MgO, was effectively suppressed over
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Ru/La0.5Pr0.5O1.75. The high activity induced by high-temperature reduction was attributable
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to the good thermal stability of the support and a phase change of the La0.5Pr0.5O1.75 support
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during pre-reduction. Fourier transform–infrared spectroscopy measurements after N2
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adsorption on the catalyst revealed that electrons were efficiently donated from trigonal
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La0.5Pr0.5O1.5 to the antibonding π orbital of the N≡N bond of N2 via Ru atoms. Cleavage of
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the N≡N bond, the rate-determining step for ammonia synthesis, was thus accelerated. Our
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results expand the range of possibilities for developing more effective ammonia synthesis
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catalysts under mild conditions. Such catalysts will be needed to enable development of
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hydrogen-based sustainable energy resources.
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Keywords: NH3 synthesis, energy carrier, rare earth oxide, Ru catalyst, phase change
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Introduction
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Energy from hydrogen has recently attracted a great deal of interest as a form of sustainable
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energy.1 Burning clean hydrogen (H2) results in the emission of water vapor, which is
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harmless, and no emission of the greenhouse gas CO2. In addition, hydrogen can be produced
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via electrolysis2,3 or photo-catalytical decomposition4,5 of water using renewable sources of
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energy such as solar energy and wind power. One of the crucial obstacles to establishing a
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society based on hydrogen energy is the development of an efficient hydrogen storage and
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transportation system. On the one hand, the energy density of gaseous hydrogen is low (39.6
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kg-H2 m3 at 71 MPa). To store large amounts of H2, it is thus necessary to keep hydrogen
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under high pressure, and hence there is a risk of an explosion. On the other hand, a very low
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temperature is needed to liquify hydrogen, inconvenient pressure-resistant containers are
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needed to store the liquid hydrogen, and there is a tendency for the liquid hydrogen to boil off.
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To circumvent these problems, intensive studies have been carried out to identify methods to
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convert hydrogen to alternative chemicals, so-called hydrogen or energy carriers, that are
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easily transported and stored, and that release hydrogen easily.6 Ammonia,7-10 methyl-
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cyclohexane11 and other organic materials12 have been considered as energy carriers. Among
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these, ammonia has several advantages: easy liquefaction under 0.86 MPa at 20 C, low
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energy requirement for release of hydrogen (30.6 kJ mol–1 per hydrogen),13 and carbon-free
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character because ammonia does not contain carbon and thus does not emit COx during
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storage or upon release of hydrogen. Hydrogen produced from ammonia by catalytic
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decomposition is used in fuel cells, engines, and turbines.13 Furthermore, burning ammonia in
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a thermal power plant by mixing it with fossil fuels can considerably reduce the CO2 emitted
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per unit of calorific value generated.14 There is currently an active national project in Japan to
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create a hydrogen-based and low-carbon society using ammonia as a hydrogen carrier.15
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Ammonia is synthesized by the energy-intensive Haber–Bosch process, which is
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performed at very high temperatures (>450 °C) and high pressures (>20 MPa) using an Fe-
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based catalyst. However, such severe reaction conditions are incompatible with sustainable
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use of hydrogen because the supply of renewable energy varies locally and temporally.
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Therefore, Ru-based catalysts with high activity for ammonia production under mild
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conditions (
