The Surface/Interface Coordination Structures Of Catalyst And Their Effects On Catalytic Performance:A Theoretical Study

Currently,about 85%of the chemical reaction processes in the world are achieved through catalysis.However,the complex surface/interface structure of catalyst limits our understanding of the structure-activity relationship.How to accurately design and regulate the structure of the active site of the catalytic surface/interface,understand the reaction mechanisms from the atomic and molecular scale,and strengthen its directed catalytic function are the main problem in heterogeneous catalysis research.The controllable preparation,advanced experimental characterization,theoretical calculation and simulation not only help to clarify the structure-activity relationship of the catalytic surface/interface,but also provide theoretical support for constructing a new and efficient catalyst.In this thesis,the effects of surface modification,surface lattice modification and co-modification of surface and surface lattice on catalytic performance were systematically investigatived by using density functional theory(DFT).The main conclusions are as follows:1)The catalysts of Ru single atoms are commonly used to catalyze hydrogenation reaction,however,under hydrogenation conditions,the Ru single atoms are easily agglomerated by reduction which lead to the deactivation of catalysts.The stabilizing effect of surface Na+ modification on Ru3+/Al2O3 and the promoting catalytic activity for acetone hydrogenation were systematically investigated.Based on the spinel structure,the theoretical models of Ru single atoms which contained octahedral Al vacancies to satisfy the of stoichiometry Al2O3 were constructed.The models without and with the Na+ modification were referred to as Ru-Al2O3(110)and Ru(Na)-Al2O3(110),respectively.Comparing the heterolytic activation mechanism of H2 on both surfaces,it could be found that Na+ reduced the energy barriers of H2 dissociation through electrostatic interaction,but hindered the transfer of H+ from Ru3+to OH" or O2+,thereby inhibiting the reduction of Ru3+.The theoretical calculation also revealed the key role of the coordination H2O on Na+ in improving the activity of Ru1/Al2O3.The H of adsorbed H2O on Na+ participated in the second step of C=O hydrogenation which greatly reduced the energy barriers of the hydrogenation reaction,while maintained the valence of Ru throughout hydrogenation process.These findings explained why the surface Na+ modification simultaneously increased the stability and activity of Ru1/Al2O3.The above conclusions were confirmed by experiments.2)The direct epoxidation of propylene by molecular oxygen alone is one of the most challenging tasks in heterogeneous catalysis.Promoters,such as chlorine,were essential for selectivity and/or activity towards the direct epoxidation.However,in most cases,the location and state of doped chlorine would be inhomogeneous such that the promoting mechanisms were still unclear.Cl and 0 have similar atome radii,thus we proposed that embedding Cl into surface lattice of Cu2O(110)can not only stabilize the Cl in the catalyst,but also improve the activity and selectivity.Constructing the theoretical models of Cl-Cu2O(110)and Cu2O(110)with or without Cl in the lattice,respectively,we compared the reaction mechanism of propylene epoxidation by adosrpton O2 on both surfaces.It is found that Cl embedded in the lattice of Cu2O(110)facilitated the activation of O2 molecules by nearby Cu1,resulting in more electrophilic O2-species.The electrophilic O2-not only preferentially interacted with the C=C double bond to form propylene oxide(PO)and inhibited side reaction of a-H abstraction,but also reduced the energy barriers for the formation of PO.In addition,the theoretical calculations also proved that the activity and selectivity of adsorbed O2 were much higher than that of lattice oxygen in the mechanism of propylene epoxidation on the surfaces with or without Cl modification.Experimentally,the intergrowth method could be used,in which by NH2OH HCl was introduced as a reducing agent,to synthesize the Cu2O(110)catalyst with Cl embedded in the lattice.Experiments confirmed that the embedding Cl net only inproved the stability of the catalyst,but also simultaneously enhanced the the activity and selectivity of propylene epoxidation.3)Theoretical calculations have revealed the effect of thiol modification on the catalytic hydrogenation of alkyne by Pd-based catalysts.The theoretical calculations showed that HSPhF2 easily dissociated into S on Pd(111)and Pd(100)surfaces at room temperature,and the dissociation of S could penetrate into the bulk phase.Combined with experiments,it was confirmed that the Pd nanosheets after HSPhF2 modification formed the surface SPhF2 and the surface PdSx,and the selectivity of the catalyst to olefins was close to 100%.Theoretically,starting from the bulk structures of Pd4S and Pd3S,according to the principle of surface energy minimum,Pd4S(110)and Pd3S(100)were selected as theoretical models from 22 and 24 surface models,respectively.Comparing the hydrogenation of alkynes on Pd4S(110),Pd3S(100),and Pd(111)surfacs,it was found that the steric hindrance of SPhF2 on the surface not only prevented C=C from further hydrogenation,improving the catalyst’s selectivity to olefins,but also maked all the product of catalytic hydrogenation cis-olefins.Furthermore,the electronic/toxicity effect of surface SPhF2 and S reduced the interaction of adsorbed H with the surface so that it was much easier to desorb from the surface and bind to alkynes or intermediates during the hydrogenation process to accelerate the hydrogenation of C≡C to C=C than that of Pd(111).The experiment confirmed the above conclusions.Through deeply theoretical research,this thesis revealed the effects of surface modification,surface lattice modification and the co-modification of surface and surface lattice on the surface/interface structure,composition and function of catalysts,which can provide theoretical support for rational optimization of new catalysts.