Ternary Ruthenium Complex Hydrides for Ammonia Synthesis

Pan, Jaysree; Guo, Jianping; Hansen, Heine Anton; Xie, Hua; Jiang, Ling; Liao, Hua; Li, Haiyang; Guan, Yuqing; Wang, Peikun; Gao, Wenbo; Lin, L.; Cao, Hunjun; Xiong, Zhitao; Vegge, Tejs; Chen, Ping

doi:10.26434/chemrxiv.13465760.v1

Cited by 6 publications

(16 citation statements)

References 48 publications

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“…The catalytically active ternary hydride surface with excess Li and hydrogen (consists of two additional LiH for every six [RuH6] centers, i.e., 4[RuH6]+2[RuH7]+2Li) 18 is energetically moderately selective towards N2 over H2 chemisorption. The model considered (110) plane of Li4RuH6, which is the most stable crystal face for this material (Figure 2A).…”

Section: Resultsmentioning

confidence: 99%

“…The extra two Li (from additional LiH) on the surface also create hindrances for N2/H2 adsorption on other neighbouring [RuH6] sites and cause partial deactivation. However, other [RuH6]/[RuH7] sites and extra Li on the surface are vital for the N2+H2 to NH3 conversion process 18 . The [RuH7]/[RuH6] on the Li4RuH6 active surface act as a reservoir of hydrides to reduce the activated N2 to NH3.…”

Section: Resultsmentioning

confidence: 99%

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Low-temperature ammonia synthesis on electron-rich [RuH6] catalytic centers

Pan¹,

Guo

Hansen³

et al. 2021

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Ammonia is a central vector in sustainable global growth, but the usage of fossil feedstocks and centralized Haber-Bosch synthesis conditions causes >1.4% of the global anthropogenic CO2 emissions. While nitrogenase enzymes convert atmospheric N2 to ammonia at ambient conditions, even the most active manmade inorganic catalysts fail due to low activity and parasitic hydrogen evolution at low temperatures. Here, we show the [RuH6] catalytic center in ternary ruthenium complex hydrides (Li4RuH6 and Ba2RuH6) activate N2 preferentially and avoid hydrogen over-saturation at low temperatures and near ambient pressure by delicately balancing H2 chemisorption and N2 activation. The active [RuH6] catalytic center is capable of achieving an unprecedented yield at low temperatures via a shift in the rate-determining reaction intermediates and transition states, where the reaction orders in hydrogen and ammonia change dramatically. Temperature-dependent atomic-scale understanding of this unique mechanism is obtained with synchronized experimental and density functional theory investigations.

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Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Low-temperature ammonia synthesis on electron-rich [RuH6] catalytic centers

Pan¹,

Guo

Hansen³

et al. 2021

Preprint

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“…a Reaction conditions: catalyst loading 30 mg, pressure 1 bar and temperature 573 K.As reported by our previous studies22 , a different reaction mechanism towards NH3 formation over the lithium-ruthenium ternary hydride catalyst was demonstrated, in which N2 undergoes non-dissociative hydrogenolysis over the electron-rich [hydride surface, which further affects its affinity towards N2, H2, NH3 and some NxHy intermediates and hence causes changes in the macrokinetic behaviors mentioned above. We speculated that, on Li-Ru or Ca-Ru complex hydride surface, Li or Ca cations prefer to stabilizing N2Hx (x=0, 1, 2, 3) species on [RuHm] center, thus reducing the barrier for N2 hydrogenation and facilitating NH3 formation at low temperatures; on K-Ru and Na-Ru complex hydride surface, K and Na cations have more benefits for destabilizing NHx (x=1, 2, 3) species on [RuHm] center, thus promoting the desorption of NH3 product and increasing the reaction rates at high temperatures or high ammonia concentration; while on Ba-Ru complex hydride surface, Ba cations help to maintain a balance between N2 hydrogenation and NH3 desorption over [RuHm] center, thus enabling better performance in the whole range of test temperature.…”

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confidence: 83%

“…BaH2 was obtained following the procedure described previously 32 . with a Ru content of 5.0 wt% were prepared according to the procedure described in our previous report 22 . Na4RuH6/MgO, K3RuH7/MgO and Ca2RuH6/MgO catalysts were prepared in a similar way, except impregnating the Ru/MgO (8.7 wt% Ru/MgO for Na4RuH6/MgO and K3RuH7/MgO, 15 wt% Ru/MgO for Ca2RuH6/MgO) in the sodium-ammonia solution with a molar ratio of Na:Ru=4:1, potassium-ammonia solution with a molar ratio of K:Ru=1:1, and calcium-ammonia solution with a molar ratio of Ca:Ru=6.5:1, respectively.…”

Section: Methodsmentioning

confidence: 99%

“…In situ synchrotron radiation powder X-ray diffraction (SR-PXD) experiments were performed at the diffraction beamline P.02.1, DESY (Hamburg, Germany). The experimental procedure in detail was described in our previous study 22 . The Ca2RuH6 sample was heated from 298 to 923 K with a ramping rate of 6 K min -1 in a steam of flow gas (N2:H2=1:3, 5 ml min -1 ).…”

Section: Preparation Of Ball Milledmentioning

confidence: 99%

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Ruthenium Complex Hydride Catalysts as a Platform for Ammonia Synthesis

Guo¹,

Chen²

2021

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Mild-condition ammonia synthesis from N2 and H2 is a long-sought-after scientific goal and a practical need, especially for the intensively pursued “Green Ammonia” production using renewable H2. Under this context, there have been growing interests in the development of new catalysts for effectively catalyzing N2+H2 to NH3. Particular attention has been given to Ru-based catalysts because they are well known to be more active at lower temperatures and pressures than non-noble-metal based catalysts. Here, we demonstrate that a series of Ru complex hydrides An[RuHm], where A is alkali or alkaline earth metal, n= 2, 3 or 4 and m = 6 or 7, exhibit universal and high catalytic activities that far exceed the benchmark Ru metal catalysts under mild conditions. Detailed investigations on the ternary Ru complex hydride catalytic system disclose that the kinetic behaviors depend strongly on the identity of alkali or alkaline earth metal cations. In clear contrast to the closed packed Ru metal catalyst, the unique configuration and synergized scenario of the Ru complex hydride center prefer a non-dissociative mechanism for N2 activation and hydrogenation, which provides a new platform for the design and development of efficient NH3 synthesis catalysts.

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Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions

Guo

Chen

2021

Acc. Chem. Res.

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Conspectus The fixation of dinitrogen to ammonia is critically important for the biogeochemical cycle on earth. Ammonia also holds promise as a sustainable energy carrier. Tremendous effort has been devoted to the development of green processes and advanced materials for ammonia synthesis and decomposition under milder conditions, and encouraging progress has been made. The reduction of dinitrogen to ammonia needs electrons and protons, which hydridic hydrogen H– could supply. Polarized, electron-rich N x H y intermediates, on the other hand, can be stabilized by alkali or alkaline earth metal cations to lower kinetic barriers in the transformation. The inherent properties of alkali/alkaline earth metal hydrides (denoted as AH) endow them with a unique function in ammonia synthesis. In this Account, recent efforts in the exploration of alkali or alkaline earth metal hydrides (denoted as AH), amides, and imides (denoted as ANH hereafter) for ammonia synthesis and decomposition reactions will be summarized and discussed. We begin with an introduction to the chemistry of A with N2, NH3, and H2, highlighting the interconversion between AH and ANH that has profound implications on the formation and decomposition of NH3. We then present our finding on the strong synergistic effect between ANH and transition metals (TM) in ammonia decomposition catalysis, which stimulated our subsequent research on AH for ammonia synthesis. We discuss the effect and function mechanism of AH in the thermocatalytic and chemical looping ammonia synthesis processes. In the thermocatalytic process, AH cooperates with both early and late TM forming either composite catalysts with two active centers or complex metal hydride catalysts with electron- and hydrogen-rich ionic centers facilitating ammonia synthesis with high activities at lower temperatures. Very interestingly, AH levels the catalytic performances of TMs and intervenes in the energy-scaling relations of TM-only catalysts. Moreover, ANH serves as a new type nitrogen carrier effectively mediating ammonia synthesis via a low-temperature chemical looping process, in which N2 is fixed by AH forming ANH. Subsequently, ANH is hydrogenated to ammonia and AH. Late TMs have a strong catalytic effect on the chemical looping process. The unique interplay of A, N, TM, and H– offers plenty of opportunities for achieving dinitrogen conversion under mild conditions, while further efforts are needed to address the challenges in the fundamental understanding and practical application.

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Ternary Ruthenium Complex Hydrides for Ammonia Synthesis

Cited by 6 publications

References 48 publications

Low-temperature ammonia synthesis on electron-rich [RuH6] catalytic centers

Low-temperature ammonia synthesis on electron-rich [RuH6] catalytic centers

Ruthenium Complex Hydride Catalysts as a Platform for Ammonia Synthesis

Interplay of Alkali, Transition Metals, Nitrogen, and Hydrogen in Ammonia Synthesis and Decomposition Reactions

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