Design,Preparation And Applications Of Heterogeneous Ammonia Synthesis Catalysts Over Metal Oxides By Li-Intercalation

Ammonia synthesis,as one of the most central catalytic reactions in contemporary chemistry,has greatly contributed to the development of the chemical industry and agriculture.Meanwhile,ammonia has received much attention as an important clean energy source and hydrogen storage carrier.The traditional Habor-Bosch method has been used for nearly 100 years for industrial ammonia synthesis,and its high energy consumption,harsh reaction conditions and large amount of greenhouse gas emissions do not meet the current demand for environmental protection and green catalysis,so the development of efficient ammonia preparation under mild conditions has obvious economic and scientific value.As the most stable diatomic molecule,the dissociative adsorption of N₂ on the catalyst surface is usually considered as the decisive speed step of the ammonia synthesis reaction.The ideal catalyst should have both strong nitrogen adsorption and weak intermediate binding ability,which can facilitate the rapid activation of nitrogen molecules while assisting the further hydrogenation and removal of nitrogen-containing intermediates.However,the activity of conventional transition metal catalytic materials is usually limited by the volcano curve,constrained by the inherent linear relationship that exists between the adsorption energies of the reacting species.Currently,the most active Ru-based catalysts are limited by high prices and hydrogen poisoning,which make it difficult to replace the traditional Fe-based catalysts in a short time.Considering the fact that almost all biological nitrogen fixation is done by molybdenum-based active centers in nitrogen fixing enzymes,molybdenum should be considered as another key element in catalyst design.However,for the metal Mo on the left side of the volcano diagram,although N₂activation occurs more readily at low temperatures,the subsequent NHx（x=0-2）intermediate produced is too strongly bound to the metal to recover the active site quickly,resulting in low NH₃ activity.In addition to metals,theoretical calculations also predict that transition metal oxides may provide more desirable scaling relationships than metals,but the design and preparation of new metal oxide-based ammonia synthesis catalysts with excellent catalytic activity remains a great challenge.In this paper,we adjust the electronic structure of the d-band centers and surface active sites of the oxides by intercalating Li⁺in the bulk phase of metal oxides,thus optimizing the inherent linear relationship between nitrogen adsorption and intermediate desorption.We construct a series of metal oxide-based ammonia synthesis catalysts with strong electron-feeding ability and surface oxygen defects to achieve efficient ammonia synthesis under milder conditions,while suppressing hydrogen poisoning of the catalysts.1.Preparation of Li-intercalation molybdenum dioxide bulk catalysts（MoO₂-x/Li）.The MoO₂ is a layered bulk material with a distorted rutile structure and unique Mo-Mo metal bonds,which shows great potential for energy storage and catalytic applications.Compared with most easily reducible metal oxides,MoO₂exhibits excellent structural and chemical stability under high temperature reducing conditions,making MoO₂ a reliable candidate for catalytic ammonia synthesis processes.In addition,as a promising intercalated lithium anode material in Li-ion batteries,MoO₂ is capable of achieving a large amount of Li⁺intercalation while accommodating an equal amount of electrons to balance the charge,which may generate a strong electron-giving capacity to promote N≡N bond breaking.We prepared MoO₂-x/Li using high-temperature solid-phase method with Li H as the Li source,and demonstrated the successful Li⁺intercalation in MoO₂ by the low-angle shift of diffraction peaks in XRD,lattice expansion in HRTEM photos,combined with XPS and ICP tests.Meanwhile,the ammonia catalysis results showed that,the performance of MoO₂-x/Li was improved by 24 times compared with pure-phase MoO₂,which was comparable to the performance of Ru@Cs-Mg O.We reveal through theoretical calculations that the intercalation of Li⁺into layered MoO₂ leads to a decrease in its d-band center,which increases the nitrogen adsorption energy from-2.5 e V to-0.9 e V.It is the key to improved catalytic performance by promoting the effective hydrogenation of nitrogen-containing intermediates and the desorption of NH3.These findings not only provide new means for the design and preparation of ammonia synthesis catalysts,but also open up new prospects for the application of Li-intercalation materials in heterogeneous catalysis.2.Preparation of Li-intercalation iron-modified molybdenum dioxide nanosheet catalysts（Fe@MoO₂-x/Li）.Based on the previous work,we found that the introduction of metallic Fe during the high-temperature solid-phase treatment can promote the decomposition of Li H（Fe+Li H《Fe H+Li⁺+e^-）,resulting in more Li⁺intercalated in MoO₂,leading to the exfoliation of layered MoO₂ bulk and the formation of a new two-dimensional Fe@MoO₂-x/Li ultrathin nanosheet material.The catalytic results showed that efficient and robust ammonia synthesis was achieved on Fe@MoO₂-x/Li nanosheets with Mo and Fe dual active sites,and the catalytic activity of Fe@MoO₂-x/Li was even higher than that of the ruthenium-based catalyst under the same conditions,and remained stable during the continuous 80 h test.More importantly,the ammonia synthesis onset temperature of this material is dramatically reduced by 100-150 K due to the embedding of Li⁺.Kinetic analysis reveals that the rate determining step of ammonia synthesis has changed from the conventional nitrogen activation to the formation of NHx intermediates.Further mechanistic studies and theoretical calculations show that the Fe-O-Mo units embedded in the backbone are able to reduce the energy barrier for the formation of NHx intermediates,resulting in Fe@MoO₂-x/Li catalysts with low apparent activation energy.The two-dimensional materials we prepared are not only limited to Fe loading,but also other transition metal loading can form two-dimensional materials,and these findings greatly expand the application of lithium-embedded materials in multiphase catalysis.3.Preparation of Li-intercalation ruthenium modified ceria catalysts（Ru-CeO₂/Li）.The CeO₂ can ensure a high degree of Ru dispersion with low oxygen vacancy formation energy with its unique electronic metal-support interaction.We treat ruthenium-loaded CeO₂ by high-temperature solid-phase method using Li H to construct Li-intercalation ruthenium modified CeO₂ materials with abundant surface oxygen defects.It was found that during ammonia synthesis,a large amount of electrons stored in the CeO₂ carriers could be effectively transferred to the surface-loaded metal Ru nanoparticles through the Ru-O-Ce groups at the interface,which promoted the cleavage of N≡N bonds.Meanwhile,the presence of oxygen vacancies on the CeO₂ surface not only promotes the heterolytic dissociation of H₂,but also can effectively accommodate the excess active hydrogen atoms on the catalyst surface,which suppresses the poisoning phenomenon of Ru to a certain extent.