Surface Segregation Of Palladium Coinage Metal Alloys And Its Effect On Surface Properties

The surface composition and atomic arrangement of heterogeneous catalytic catalysts have a great influence on surface reactions,and they are also the key information for understanding surface catalytic reactions.Compared with monometallic catalysts,alloy catalysts are more complicated because the surface segregation pattern may change under different reaction atmospheres.In this study,we used the density functional theory based Monte Carlo method（DFT-MC）to study the surface composition and the atomic arrangement of 1:1 PdAg,PdAu and PdCu alloy catalysts under the vacuum conditions and the reaction conditions of semi-hydrogenation of acetylene to ethylene,in which we simulate the surface atmosphere of the catalytic reaction by co-adsorbing C₂H₂+H and C₂H₄+H on the three surfaces.We first studied the surface segregation of PdAg（111）,PdAu（111）and PdCu（111）under vacuum conditions.Our simulation results show that Ag and Au segregate to the（111）surface of PdAg and PdAu,while Pdsegregates to the（111）surface of PdCu.Analysis of the surface atomic arrangement revealed that there are isolated Pdmonomers on the segregated surfaces of PdAg and PdAu,while the segregated surface of PdCu tends to form Pdstripes.Combined with the analysis of the surface energy and lattice constant of the pure metals Pd（111）,Ag（111）,Au（111）and Cu（111）,we believe that the large difference in surface energy between the（111）surface of Pdand Ag（or Au）is the main driven force for segregation,and caused the surface segregation of Ag and Au that have lower surface energies,while the lattice mismatch of Pdand Cu is the main reason for the segregation of PdCu（111）because the difference in surface energy between Pdand Cu is small,and Pdthat is larger in radius segregates to surface.Different from PdAg and PdAu that have confirmed conclusion of segregation pattern,the theoretical and experimental study have different opinions on segregation element of PdCu.Our results agree with the latest experiment study and explain the phenomenon of Pdsegregation from the perspective of surface energy and lattice constant.Next,we studied the surface segregation of PdAg（111）,PdAu（111）and PdCu（111）when C₂H₂ and H were co-adsorbed on the surface.Under this situation,although the content of Pdon the surface increases compared with the vacuum condition,the content of Ag and Au on the surface of PdAg（111）and PdAu（111）is still higher than that of Pd.Moreover,due to the strong interaction between acetylene and Pdatoms,the Pdon the surface tends to move closer to the acetylene.The adsorption energies of the ideal and segregated surface also show that the interaction between the adsorbate and the segregated surface is stronger.Analysis of the surface shows that there is a linear relationship between the d-band center of the ideal surface and the segregated surface,which provides us with a method to predict the d-band center of the segregated surface based on that of the ideal surface.The comparision of the d-band center of the surface before and after adsorption is also included,and we found the d-band center of the absorbed surfaces are lower with respect to that of the clean surface.Then we studied the surface segregation of PdAg（111）,PdAu（111）and PdCu（111）when C₂H₄ and H were co-adsorbed on the surface.Under this situation,the Pdatoms on the segregated surface also tend to move closer to the ethylene.Due to the strong interaction between ethylene and Pdatoms,it is more difficult for ethylene molecules to desorb from the surface.The adsorption energies of the ideal and segregated surface also indicate that the interaction between the adsorbate and the segregated surface is stronger.This may result in further hydrogenation of ethylene to ethane,reducing selectivity to ethylene.Therefore,the catalyst selectivity obtained by using the ideal surface may be too optimistic.The analysis of the surfaces shows that there is also a linear relationship between the d-band center of the ideal surface and the segregated surface.This also provides us with a method for predicting the d-band center of the segregated surface based on the results of the ideal surface.The comparision of the d-band center of the surface before and after adsorption is also included,and we found the adsorbates decrease the d-band center of the surfaces.Finally,we took the PdAg system as an example to investigate the energy barrier of alloy segregation.According to the vacancy-assisted diffusion mechanism,our results show that the atomic exchange energy barrier that occurs on the surface is the lowest,followed by the atomic exchange energy barrier that occurs on the bulk phase,and the atomic exchange energy barrier when there are adsorbates on the surface is the highest（1.2 e V）.Most of the energy barriers are below 1.0 e V.Campared with other experimental and theoretical studies about the energy barriers of pure metals,it shows that the energy barrier we calculated is reasonable.In summary,this paper uses the DFT-MC method to study the surface atomic composition and arrangement of PdM（M=Cu,Ag,Au）under vacuum and reaction conditions,and investigates the electronic structure change caused by segregation and its effect on the selectivity of the hydrogenation of acetylene to ethylene.The results show that:（1）Au and Ag in PdAu and PdAg alloys segregate on the surface and easily form an isolated Pdmonomers on the surface,while palladium in PdCu tends to segregate on the surface to form a striped structure;（2）Under the reaction conditions,the interaction between the segregated surface and the adsorbate is stronger than that of the ideal surface,and there is a linear relationship between the adsorption energy of the segregated surface and the ideal surface;（3）There is also a linear relationship between the d-band center of the segregated surface and the ideal surface,and the d-band center of the segregated surface and adsorbed surface moves down.Finally,we studied the kinetics of segregation and found that the energy barrier of the segregation that directly interacting with the adsorbate is 1.2 e V,which is significantly higher than the energy barrier without the adsorbates.