First-principles Study Of Adsorption And Decomposition Of Small Molecules On TM-Au(111) Surfaces

Heterogeneous catalysis has become an indispensable part of the catalysis field with the needs of science,technology,and industrial development.The essential difference between heterogeneous catalysis and homogeneous catalysis is the different environments in which the reaction system is located.A heterogeneous reaction is a reaction between two or more phases,and the catalysts chosen are mostly solid.The heterogeneous catalytic process undergoes adsorption and transformation of reactants on the catalyst surface,i.e.,surface reaction and product desorption.Adsorption is the very first and critical step in the catalytic reaction.To understand the mechanism of catalytic reactions on the catalyst surface,the first thing to do is to study the adsorption of reactants on the catalyst surface.Bimetallic catalysts are an essential part of non-homogeneous catalysis.Au-based doping of a second metal to form gold-based bimetallic catalysts has received more and more attention in recent years due to its unique physical and chemical properties.H_2,CO,and O₂ are the most common gaseous small molecules in nature,and are also the most used in the field of multiphase catalysis at this stage and are common reactants or products in reactions.Therefore,the study of their adsorption on bimetallic surfaces is significant for both the selection and design of catalysts.In recent years,computational chemistry has been developing rapidly,and the first nature principle has been widely applied in the research of heterogeneous catalysis.In this thesis,the adsorption and decomposition of three standard gas small molecules,H₂,CO,and O₂ ,on the surface of the constructed gold-based bimetallic catalysts were investigated by constructing a model of the surface structure of the gold-based bimetallic with different surface structures based on density flooding theory and with the help of VASP software package.The specific work and main findings are as follows.（1）The adsorption and decomposition reactions of H₂molecules on the surface of TM-Au（111）（TM=Ni,Pd）bimetallic catalysts doped with three different structural units,monomer,dimer,and trimer,were investigated and the decomposition processes were analyzed together with the reaction kinetics.The effects of different structural units on the catalytic performance of gold-based bimetallic catalysts and metal catalyst surfaces were explored.The calculated results show that when Ni and Pd are doped as trimeric structural units on the Au（111）surface,the Ni_trimer-Au（111）and Pd_trimer-Au（111）bimetallic catalyst surfaces are more favorable for H₂adsorption and decomposition compared to the TM_dimer-Au（111）and TM_monomer-Au（111）surfaces.Therefore,we conclude that structural units can be designed and developed to dissociate H₂in the form of trimers of Ni and Pd.In addition,BEP（Br?nsted-Evans-Polanyi）relationships were established for the dissociation process of H₂on these six TM-Au（111）（TM=Ni,Pd）gold-based bimetallic surfaces with different surface structural units.（2）Density functional theory（DFT）was used to calculate the CO molecules on the six catalysts,Ni_monomer-Au（111）,Ni_dimer-Au（111）,Nit_rimer-Au（111）,Pt_monomer-Au（111）,Pt_dimer-Au（111）,Pt_trimer-Au（111）,and the adsorption process on the surface and its decomposition reaction on the surface of Pt_monomer-Au（111）,Pt_dimer-Au（111）,Pt_trimer-Au（111）.These mainly aim to explore the effect of different structural units on the surface of TM-Au（111）（TM=Ni,Pt）catalysts on the catalytic performance.The results of the computational analysis showed that the catalyst Pt_trimer-Au（111）surface obtained by doping Pt in the form of trimer on the Au（111）surface has a better performance in adsorbing CO.By searching the transition state and comparing the surface energy after decomposition,it was found that the aggregation of Pt promoted the decomposition of CO.It is concluded that the Pt_trimer-Au（111）surface is suitable for CO adsorption and the C-O bond is relatively weak after adsorption.（3）The adsorption processes of O₂ molecules on the surfaces of twelve gold-based bimetallic catalysts of TM_monomer-Au（111）,TM_dimer-Au（111）,and TM_trimer-Au（111）（TM=Ni,Rh,Pd,Pt）were studied and calculated.The analysis of O-O bond length,adsorption energy,DOS density of states,and Bader charge concluded that the TM_monomer-Au（111）（TM=Ni,Rh,Pd,Pt）catalysts obtained by doping of monomeric structural units are poor in both the activation degree of O₂ and the stability of the structure after adsorption;the trimeric structural units doped with TM_trimer-Au（111）（TM=Ni,Rh,Pd,Pt）catalysts are less stable than the dimer-doped TM_dimer-Au（111）catalysts after O₂ adsorption on the surface,but the bond length and charge transfer number of O₂ are relatively large.Analyzing all the data,it is concluded that the surface of Ni_trimer-Au（111）catalyst is more suitable for O₂ adsorption,while the activation of O₂ is the largest.The adsorption processes of O₂ molecules and O atoms on the surface of TM-Au（110）（TM=Ni,Ru,Rh,Os,Ir,Pt）catalysts,respectively,were studied and calculated.Different crystalline surfaces produce different surface structures,which can change surface properties.（110）crystalline surface is a low-index crystalline surface of Au,which has the highest surface energy,so it is also necessary to investigate the adsorption properties of Au（110）surface.From the analysis of the calculated data,it can be concluded that when O₂ is adsorbed on the Pt-Au（110）catalyst surface,the O-O bond is not broken and the O atom is not bonded to the doped metal.When the doped metal TM=Ni,Ru,Rh,Os,Ir,the O₂ molecule adsorbed on the surface of TM-Au（110）could not exist stably and decomposed into two O atoms and bonded to the adjacent TM and Au atoms,correspondingly the charge was transferred from the metal surface to the O atom.Compared with TMdimer-Au（111）,O₂ molecules cannot exist stably on the TM-Au（110）surface,and the adsorption energy on the surface after adsorption is much larger than that on the corresponding TM_dimer-Au（111）surface.