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New Achievement by CCNU: Breakthrough in the Scaling Relationship of Ammonia Synthesis


Revision:Chen Jialong; Hu QiaoqiaoDate:2020/10/11


Catalyzing the ammonia synthesis reaction, which is significant to the growth of crops and the survival of humans, still confronts various problems such as harsh reaction condition (high-temperature and high-pressure), high-energy consumption, high CO2 discharge and so on. In the 21st century, the crises of energy and environment are increasingly serious. How to optimize catalyst to accomplish the ammonia synthesis in low-temperature and low-pressure conditions is a long-term objective in this area.

Energy barrier engineering is a crucial means to optimize catalyst. An ideal low-temperature, low-pressure and high-activity catalyst needs to have the low N2 activated energy barrier and low NHz hydrogenated energy barrier at the same time. The kinetic dilemma for the low-temperature Haber-Bosch ammonia synthesis originates from the “scaling relationship” of antagonistic activation energies between N2 dissociation (Ea(N-N)) and NHz intermediates (z = 0 to2) destabilization; catalysts that strongly activate N2 also unfavorably hinder the transformation of NHz intermediates, and vice-versa. Breaking this scaling relationship is key to improving the kinetics of ammonia synthesis and reaching the goal of less energy-intensive operating conditions.                      

       Figure 1. Breaking the “scaling-relation” in ammonia synthesis


Many theoretical investigations have attempted to understand how this scaling relationship could be broken, and one study has claimed experimental success. This experimental study exploited LiH in conjunction with strong N2 reduction-capability raised by Chen Ping research group from Dalian Institute of Chemical Physics, Chinese Academy of SciencesDICP. In this reaction scheme, dissociated N atoms diffuse from the transition metals to LiH, forming LiNHz. The LiNHzthen reacts with H2 to regenerate LiH while producing NH3. Easy N2 activation is achieved on strong N-bonding transition metals while NHz destabilized easily on weak N-bonding LiH. Considering the significance of breaking the scaling relationship, this work is raised wide concern when it came out in 2016. However, there are no other new mechanisms to break scaling relationship.

In contrast to this commonly accepted weak-strong N-bonding pair, Zhang Lizhi research group from College of Chemistry, CCNU describe a counter-intuitive approach of both strong N-bonding elements breaking the scaling relationship based on their recently discovered highly-reactive photocatalyst TiO2-xHy/Fe. In this hybrid catalyst, a Fe nanocrystal necklace (Fe-NL; bonding N strongly) is integrated with hydrogen-laden titanium oxide (TiO2-xHy; also bonding N strongly) nanoparticles featuring in cascade oxygen vacancies (OV-OV). During the catalytic process, easy N2 dissociation and easy NH3 assembly are triggered by Fe and hydrogen-laden oxygen vacancies (denoted OV-H; in TiO2-xHy), respectively, while the OV-H is recycled by a low-energy barrier hydrogen spillover from Fe via the cascade OV-OV pathway, thereby circumventing the kinetic dilemma, as illustrated in Scheme 1. TiO2-xHy-promoted Ammonia Synthesis on Fe.

Figure 2. The strong N-bonding TiO2-xHy counter-intuitively promotes another strong N-bonding element Fe in ammonia synthesis.


While the strong-strong N-bonding pair is projected to suffer from severe active sites blocking by strong N chemisorption, which should not have broken the scaling relationship in theory.

Oxygen Vacancies Trap Hydrogen, OV-H. The first task in understanding ammonia synthesis on TiO2-xHy/Fe NL was to identify the H-laden active sites in TiO2-xHy. This requires understanding the relationships between synthesis, structure and properties of TiO2-xHy.

Fortunately, EPR spectroscopy of TiO2-xHy, as a function of stoichiometry (OV concentration, x) and temperature (T) in conjunction with deuterium isotope labelling TiO2-xDy, can help to resolve this dilemma.

Figure 3. Schematic of OV-H formation in TiO2-xHy.


The EPR spectra of TiO2-xHy sample with lowest OV concentration (x ≤ 0.001) displayed typical axial line shapes for d1

Ti (III) sites with slightly differing axial g-tensors (gx = gy ≈ 1.975, gz ≈ 1.940). Coupled Fe and OV-H Hydrogen Cycle. As suggested by EPR measurements that both Fe and OV-H are active in ammonia synthesis in the initial TiO2-xHy/Fe-NL catalyst.

Figure 4. Schematic reduction process of TiO2-xHy/Fe-NL, as well as hydrogen transfer during H2-TPR measurement.


Figure 5. The DFT model of TiO2-xHy/Fe and schematic ammonia synthesis cycle, and (b) corresponding free energy diagram and (c) overall energy barriers


The related research results are published on JACS. The first author is Dr. Mao Chengliang. Prof. Zhang Lizhi, Prof. Geoffrey Ozin and Academician Zhao Jincai are instructors. The research was supported by The National Science Fund for Distinguished Young Scholars, National Key Research and Development Project and The National Natural Science Foundation of China.


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