Dynamically Confined Single Atom Catalysts And CO Catalytic Oxidation

In recent decades,the concepts of single-atom catalysis（SAC）and confined catalysis have been widely used in a wide range of chemical catalysis.But suppressing the aggregation of single-atom catalysts（SACs）during chemical reactions is a long-standing challenge in heterogeneous catalysis,mainly due to the ubiquitous design scheme of anchoring single-atom active centers on the substrate surface.Confined catalysis has been advocated as an intriguing rival concept by confining the active center in some 2D materials.In this thesis we establish a conceptually new design principle of SAC,termed dynamically confined SAC,by coherently integrating SAC and confined catalysis into a single heterogeneous catalytic platform.Based on the first-principles calculation method,the porous g-C₃N₄/MoS₂ is taken as a prototypal substrate to confine single-atom for CO oxidation.The results first show that many of the transition metals can be well stabilized as dispersive atoms within the porous centers.Next,oxygen molecules are found to readily win over CO in competing for adsorption onto the catalysts,followed by efficient CO oxidation.Intriguingly,the single-atom reactive sites adjust their vertical locations dynamically during the reaction cycle as electronic antennas,and return to their stable sandwiched homes after the reaction is completed.These findings may prove to be instrumental in discovering highly stable and efficient single-atom catalysts for practical applications based on two-dimensional materials.The details of this thesis are as the following:In this thesis we first introduce the geometric structure of g-C₃N₄/MoS₂,next,the deposition of transition metal（TM）atoms on the g-C₃N₄/MoS₂ heterojunction are investigated.Almost all 3d-,4d-and 5d-transition metal atoms are screened,and the corresponding adsorption configurations of different metal atoms on the substrate are classified.We confirm that the confined single atoms TM₁ are prohibited from lateral diffusion between neighboring sites on g-C₃N₄/MoS₂ substate,as manifested by the large diffusion barrier,indicating these metal atoms can be stably dispersed on the heterojunction as dynamically confined SAC candidates（TM₁@g-C₃N₄/MoS₂）.Based on the above selected single-atom catalyst system,we further investigate the kinetic process and the mechanism of CO oxidation.First,the adsorption of O₂ and CO molecules by the TM₁@g-C₃N₄/MoS₂ system is tested.Our results show that many systems（such as Nb,Hf,Cr,Cd,etc.）can readily activate O₂ molecules.Next,taking the Nb₁@g-C₃N₄/MoS₂ system as a typical example for CO catalytic oxidation,we find that the present designed single-atom catalyst system possess highly effiecient performance on CO oxidation.Intriguingly,the reactive TM atoms adjust their vertical locations,coordination numbers,and charge states as well dynamically during the reaction cycle as electronic antennas to modulate charge between the reactants and substrate,and finnaly return to their stable sandwiched homes after the reaction is completed.Furthermore,the underlying mechanism of the dynamic behavior of the active site TM single atoms in the reaction process is further analized,which proves that this dynamic behavior plays a key role in reduction of the catalytic reaction barrier.At the end of this thesis,we give a summary of the present study and prospects for future work.In this thesis,the dynamically confined SACs based on porous TM₁@-g-C₃N₄/MoS₂ system are predicted to be highly efficicent for CO oxidation.The kinetic process of CO oxidation by the single-atom catalyst system is explored,the charge transfer mechanism during the catalytic reaction is elucidated,and the dynamic behavior of TM active center in catalytic process and its underlying mechanism are also revealed.Therefore,it is hoped that the present theoretical findings can provide important guidance and information for future experiment in related fields.