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Essential Role of Hydride Ion in Ruthenium-Based Ammonia Synthesis Catalysts† Cite This: Chem

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Essential Role of Hydride Ion in Ruthenium-Based Ammonia Synthesis Catalysts† Cite This: Chem Chemical Science View Article Online EDGE ARTICLE View Journal | View Issue Essential role of hydride ion in ruthenium-based ammonia synthesis catalysts† Cite this: Chem. Sci.,2016,7,4036 Masaaki Kitano,a Yasunori Inoue,b Hiroki Ishikawa,b Kyosuke Yamagata,b Takuya Nakao,b Tomofumi Tada,a Satoru Matsuishi,a Toshiharu Yokoyama,ac Michikazu Hara*bcd and Hideo Hosono*abcd The efficient reduction of atmospheric nitrogen to ammonia under low pressure and temperature conditions has been a challenge in meeting the rapidly increasing demand for fertilizers and hydrogen À storage. Here, we report that Ca2N:e , a two-dimensional electride, combined with ruthenium À nanoparticles (Ru/Ca2N:e ) exhibits efficient and stable catalytic activity down to 200 C. This catalytic + À À performance is due to [Ca2N] $e1Àx Hx formed by a reversible reaction of an anionic electron with À + À À hydrogen (Ca2N:e + xH 4 [Ca2N] $e1Àx Hx ) during ammonia synthesis. The simplest hydride, CaH2, À with Ru also exhibits catalytic performance comparable to Ru/Ca2N:e . The resultant electrons in these Received 19th February 2016 hydrides have a low work function of 2.3 eV, which facilitates the cleavage of N2 molecules. The smooth Creative Commons Attribution 3.0 Unported Licence. Accepted 21st April 2016 reversible exchangeability between anionic electrons and HÀ ions in hydrides at low temperatures DOI: 10.1039/c6sc00767h suppresses hydrogen poisoning of the Ru surfaces. The present work demonstrates the high potential of www.rsc.org/chemicalscience metal hydrides as efficient promoters for low-temperature ammonia synthesis. Introduction practical application, and strong reducing agents and extra proton sources are required to afford NH3. Catalytic ammonia synthesis is vital for the production of In heterogeneous catalysts, it is widely recognized that synthetic fertilizers and serves as an active nitrogen source for ruthenium (Ru) catalysts work under milder conditions than This article is licensed under a – 8,9 important chemicals. Dinitrogen (N2) is a typically inert mole- iron-based catalysts for the Haber Bosch process. The activity À cule because of the strong N^N bond (945 kJ mol 1).1 There- of Ru catalysts is substantially enhanced by electron injection 8,10 fore, high temperature (400–600 C) and high pressure (20–40 from alkali or alkali earth metal oxide promoters. Although Open Access Article. Published on 21 April 2016. Downloaded 9/30/2021 11:35:21 AM. MPa) are required for industrial ammonia synthesis (Haber– these electronic promoters lower the energy barrier for N2 Bosch process),2 which results in high energy consumption. dissociation, the enthalpy of hydrogen adsorption on the Ru Recently, ammonia has also attracted much attention as catalyst is also increased, leading to high surface coverage by H 11 a hydrogen storage material due to its high capacity for atoms (hydrogen poisoning). Accordingly, the electronic ff hydrogen storage (17.6 wt%) and facile liquefaction under mild promotion e ect for N2 dissociation is retarded by the 3 conditions. Ammonia synthesis at low temperature is ther- competitive adsorption of H2. It is therefore highly desirable to modynamically favorable but still presents a major challenge. develop a new Ru catalyst that can promote N2 dissociation and Extensive studies on N2 activation with organometallic prevent hydrogen poisoning. It was demonstrated that the 4–7 $ À 12 complexes have been conducted over the last decade. 12CaO 7Al2O3 electride (C12A7:e ) -supported Ru catalyst Although NH3 has been successfully produced under ambient exhibits much higher activity for ammonia synthesis than 13,14 conditions, the rate of formation is still far from appropriate for alkali-promoted Ru catalysts. The intrinsically low work À 15 function (ca. 2.4 eV) of C12A7:e in this catalyst promotes N2 dissociation on Ru, which leads to a reduction in the activation À1 aMaterials Research Center for Element Strategy, Tokyo Institute of Technology, 4259 energy to half (ca. 55 kJ mol ) of that for conventional Ru Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. E-mail: [email protected] catalysts. A recent kinetic analysis revealed that the bottleneck bLaboratory for Materials and Structures, Tokyo Institute of Technology, 4259 for ammonia synthesis is shi ed from N2 dissociation to the Nagatsuta, Midori-ku, Yokohama 226-8503, Japan. E-mail: [email protected] – 16 formation of N Hn species. In addition, this catalyst has cACCEL, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan reversible exchangeability of electrons and hydride ions, and is dFrontier Research Center, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, almost immune to hydrogen poisoning of the Ru surface, which Yokohama 226-8503, Japan is a serious drawback for conventional Ru catalysts. These † Electronic supplementary information (ESI) available: Experimental details, results imply that both electrons and hydride ions play a crucial kinetic analyses, catalytic performance, detailed characterization, and DFT role in effective ammonia synthesis. However, the outstanding calculations. See DOI: 10.1039/c6sc00767h 4036 | Chem. Sci.,2016,7,4036–4043 This journal is © The Royal Society of Chemistry 2016 View Article Online Edge Article Chemical Science À activity of C12A7:e is diminished at low temperatures (<320 C), which is strongly correlated with the weak H2 desorption properties at low temperatures. The electron–hydride ion À exchange reaction in C12A7:e is accomplished by H desorption through a cage wall composed of a rigid monolayer of Ca–Al–O; À therefore, the exchange reaction in C12A7:e requires a rela- tively high temperature that is sufficient to excite thermal vibration and allow H to escape from the cage. Therefore, our design concept for a highly active low-temperature ammonia synthesis catalyst is embodied by inorganic electride materials with hydride ions exposed to the surface, i.e., metal hydrides. +$ À À Dicalcium nitride, [Ca2N] e (denoted as Ca2N:e ), was conrmed as a two-dimensional (2D) electride with a low work function (2.6 eV), in which anionic electrons conned between + 17–19 the [Ca2N] layers as counter anions can be partly exposed to the surfaces. In addition, this material can be converted into + À Ca2NH ([Ca2N] $H ) by the reaction between an anionic elec- À tron and a hydrogen, which is analogous to that for C12A7:e . Fig. 1 (a) Catalytic activity for ammonia synthesis over various Ru catalysts (2 wt%) as a function of reaction temperature (reaction Therefore, Ca2NH was selected as the rst test bed material to À1 À1 conditions: catalyst, 0.1 g; WHSV, 36 000 mL gcat h ; reaction verify our hypothetical design concept of metal hydrides for low- pressure, 0.1 MPa). (b) Reaction time profile for ammonia synthesis temperature ammonia synthesis. over Ru (5 wt%)/Ca2N at 340 C (reaction conditions: catalyst, 0.1 g; À1 À1 Here we report that metal hydride materials such as Ca2NH WHSV, 36 000 mL gcat h ; reaction pressure, 1.0 MPa). (c, d) Dependence of ammonia synthesis on the partial pressure of (c) N2 Creative Commons Attribution 3.0 Unported Licence. and CaH2, which are not intrinsically low work function mate- rials, strongly promote the cleavage of N to form NH on Ru and (d) H2 using various Ru catalysts (2 wt%) at 340 C under atmo- 2 3 spheric pressure. nanoparticles under low pressure and temperature conditions. The low work function (2.3 eV) is immediately realized by the formation of hydrogen vacancies in these Ru-loaded hydride functions as a stable catalyst for ammonia synthesis over long materials during ammonia synthesis, which in turn facilitates periods without degradation in activity. Fig. 1b shows that the N dissociation and prevents hydrogen poisoning of the Ru 2 initial ammonia synthesis rate was maintained even aer 54 h surface. under high pressure conditions (1.0 MPa), and the total amount This article is licensed under a of produced ammonia reached 27 mmol, which is more than 25 À Results and discussion times the total nitrogen content in Ca2N:e (1.06 mmol). This À Catalytic performance of Ru-loaded Ca2N:e result indicates that the ammonia produced is not derived from À the decomposition of the Ca2N:e support. Open Access Article. Published on 21 April 2016. Downloaded 9/30/2021 11:35:21 AM. Fig. 1 shows the temperature dependence for ammonia Fig. 1c and d show the dependence of the ammonia synthesis over various Ru catalysts and Table 1 summarizes the synthesis rate on the partial pressure of N and H , respectively. catalytic properties of various Ru catalysts. The Ru dispersion of 2 2 À – The reaction orders with respect to N2,H2, and NH3 over various Ru/Ca2N:e (3.1%) is much smaller than that of Ru(2%) Cs/ À Ru catalysts are also summarized in Table S1.† The reaction MgO (50.4%) because of its low surface area (1.5 m2 g 1), which À À order for N with conventional heterogeneous catalysts is 0.8– is similar to the results for Ru/C12A7:e . However, Ru/Ca N:e 2 2 1.0,11,20,21 where N dissociation is the rate-determining step for exhibits high catalytic activity (at 340 C) comparable to 2 ammonia synthesis. In contrast, the reaction order for N2 with Ru(2%)–Cs/MgO, which is one of the most active catalysts for À 20,21 Ru/Ca2N:e is almost one-half, which is attributed to a more ammonia synthesis reported to date. Accordingly, the turn- À À dense population of N adatoms on Ru/Ca N:e than on the over frequency (TOF) with Ru/Ca N:e is higher than that with 2 2 other catalysts. Two results were noted; one is that the Ru/ Ru–Cs/MgO by an order of magnitude.
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