Applications Of Several Atomic Catalysts In Nitrogen Reduction Reactions:A First-Principles Study

Ammonia（NH₃）is not only an important chemical raw material,but is also identified as a potential hydrogen carrier because of its high hydrogen content,as well as holds the merits as cleanliness,safety and ease of transport.The industrial production of NH₃ is currently based on the Haber-Bosch method,which requires harsh reaction conditions and thus consumes abundant fossil fuels and contributes large carbon emissions.Electrochemical ammonia synthesis is a very promising emerging industry,taking which to partially replace the Haber-Bosch process could reduce the carbon emissions of the ammonia industry and enable the conversion to renewable energy sources.The cost of electrochemical ammonia synthesis mainly lies in the catalysts,thus improving the catalytic performance and reducing the cost of materials could facilitate its industrialization.Currently,the main challenges of catalysts are the activation of inert nitrogen（N₂）and the suppression of competing hydrogen evolution reaction（HER）.Atomic catalysts loaded on the substrates could effectively overcome above challenges,since the low coordination number leads a high catalytic activity,and the sole active site could effectively suppress competing reactions.Density functional theory（DFT）calculations are effective methods to study the electronic interactions and reveal the catalytic mechanisms.In this thesis,we theoretically design a series of atomic catalysts for electrocatalytic nitrogen reduction reaction（NRR）based on DFT calculations,which includes the following three parts:（1）A theoretical study of the catalytic performance on Cu-based single-atom alloy toward nitrogen reduction reactionsSingle-atom alloy（SAAs）catalysts assemble the merits of atomic catalysts and alloy catalysts,which were considered as ideal catalytic materials.Cu exhibits stable physicochemical properties with fully occupied d orbitals,and its abundant d electrons could further modulate electronic structure of the active site,thus it is very suitable as a substrate material for SAAs.Specifically,Cu performs high conductivity,good corrosion resistance and relatively low cost,and can effectively suppress competing HER.Herein,we constructed 108 Cu-based SAAs（with 18 elements doped on the 6 Cu facets）to evaluate their NRR performance by DFT calculations.We identified a distinct electronic structure in SAAs,where electrons can redistribute between suborbital of active sites to modulate the adsorption energy of N₂.In this case,we established a volcanic relationship between the electron number（N_e）of the active site and its NRR catalytic activity.Among all the SAAs,the best NRR was realized by Re-Cu（553）with the computed overpotential（η）of 0.17 V.Finally,the catalytic performances of the materials were directly bridged to their intrinsic properties by finding effective descriptors,which provided a deep understanding in the"structure-property"relationship of SAAs.This study not only designs efficient NRR catalyst systems,but also facilitates the prediction and development of novel NRR catalysts.（2）Single-atom doped copper twin boundaries as efficient catalysts for nitrogen reduction reactionsProducing defects into metallic nanostructures is a smart catalyst design strategy,which has introduced extra dangling bonds of catalytic sites for strengthening the adsorption ability.Twin boundary（TB）is recognized as one of the most stable defects,in whose vicinity the orbital overlapping works synergistically with lattice distortion to affect the catalytic performance.In order to further improve the catalytic activity of Cu-based SAAs,we doped different single atoms on the twin boundary edges（TBE）of Cu and explored their NRR catalytic behaviors.We have considered 40 thermodynamically stable TBE models and screened out the optimal catalyst as Re-Cu（111）-TBE,with the calculatedηvalue of 0.11 V.The overlapping orbitals are distinct in radial and tangential directions along the TBE,which induces unbalanced electron occupation states in the π^*-p_x,p_y orbital of the adsorbed N₂ and effectively activates the N≡N bond.Finally,we explored the catalytic selectivity and experimental feasibility of Re-Cu（111）-TBE and proposed a possible experimental preparation method.Overall,this study provides a comprehensive understanding of the catalytic mechanism of TB from an electronic structure point of view,and designs an efficient NRR catalyst,Re-Cu（111）-TBE.（3）Rational design of an Fe cluster catalyst for nitrogen reduction reactionsThe orbital matching both in energy and orientation are crucial for N₂ activation,where the more overlapping to the N₂-π^*orbitals lead a stronger the N₂ activation effect.Atomic cluster catalysts not only possess the advantages of single-atom catalysts,whose catalytic performance could be further improved by coordination effects.As well,the cluster catalyst with flexible active sites can realize a dynamic adsorption of the intermediates,for avoiding the limit of the scaling relationship.In this work,we designed a cluster catalyst of the Fe₄ cluster anchored to 2D GaS,which was denoted as Fe₄/GaS.Fe-based materials are well-established NRR catalysts with low cost and high reactivity,as well as effective suppression of the competing HER.One Fe atom of the Fe₄ cluster is inserted in the pore of 2D GaS,which contributes to the stability,while the other three Fe atoms serve as triatomic Hollow active sites.Compared with the single atomic active site,the Hollow site on Fe₄/GaS matches better to the electronic orbitals of N₂.Therefore,Fe₄/GaS exhibits remarkable catalytic performance for NRR,and the calculatedηvalue is only 0.08 V.Moreover,we propose a feasible preparation method for Fe₄/GaS,and then verify its stability by calculating thermodynamic binding energy and threshold potential.This study provides a deep insight into the complex catalytic mechanism of cluster catalysts,and provides a new idea for developing efficient NRR catalysts.