The Adsorption,Diffusion And Desorption Mechanism Of Hydrogen At The Surface/Interface Of Metal Based On Density Functional Theory

Atomic hydrogen species is of great significance because of its important adsorption,diffusion and desorption mechanisms in heterogeneous catalytic hydrogenation.In recent years,it has been paid much attention through studying the determined reaction rates in the hydrogenation.In general,there are two different atomic hydrogen species:surface hydrogen species and subsurface hydrogen species.The reaction mechanisms involving these two different atomic species especially subsurface hydrogen species have been widely investigated since the distinct reactivity and acitivation energy barriers in hydrogenation reaction steps.Consequently,an indepth understanding of the adsorption,diffusion and desorption of there two kinds of atomic hydrogen species on the surface/interface of different metals is very important,which is worth studying how to improve the reaction rates of hydrogenation reactions.Based on density functional theory,we have systematically studied the interactions of surface and subsurface hydrogen and six different metals.We mainly focused on the high reactivity and microkinetic mechanism of subsurface hydrogen.Six different metals are respectively Ni,Cu,Pd,Ag,Pt and Au.Three low Miller index crystal planes are（110）,（100）and（111）.Constructing some highly accurate ponteial energy surfaces,we have investigated the atomic adsorption,diffusion,and desorption on these surface/interface of six different metals.At the same time,the effects of temperature and hydrogen parital pressure on the H coverage was explored by using ab initio thermodynamics.In addition,in order to deeply investigate the source of H atoms and desorption mechanism,the molecular dissociation and desorption of H₂ on these metal surfaces were also discussed.The main conclusions are summarized as follows:（1）The stable hydrogen adsorption site on metal surfaces.For hydrogen on（110）surface,the most stable adsorption site are pseudo 3-fold hollow site（P3H）and Short-Bridge site;for hydrogen on（100）surface,the most stable adsorption sites on Ni,Cu,Pd and Ag are the Hollow sites,and the most stable adsorption sites for Pt and Au are Bridge sites;for hydrogen on（111）surface,the most stable adsorption sites on Ni,Cu,Pd,Ag and Au are the Fcc sites,and the most stable adsorption site on the Pt is the Top site.（2）The diffusion pathway on metal surface and permeation pathway into the subsurface:For hydrogen on the（110）surface,the lowest diffusion barrier pathway is following from the P3H site to the Short-Bridge site,and for hydrogen permeation into the subsurface,hydrogen passes from the Top site to the Short-Bridge site and then arrives at the surface’s P3H site,and finally permeats into the Short-Bridge site of sub-surface region.For hydrogen on（100）surface,the diffusion pathway is following from the Top site to the Hollow site,and the permwation pathway is that from the Hollow site via the Bridge site and finally permeating into the sub-Bridge site on the subsurface.For hydrogen on the（111）surface,the diffusion pathway is from the Top site diffusion onto the Fcc site,and the peameation pathway is from the Fcc site into the sub-Fcc site directly.（3）Thermodynamic equilibrium phase of hydrogen adsorption on metal surfaces.It is shown that,the equilibrium coverage of H atoms on the metal surfaces decreases as the increasing temperature and decreasing pressure.For hydrogen adsorption on the（110）surface,the highest hydrogen coverage on the Ag（110）surface is 6/9 ML,and the highest coverages on other five surfaces are 1.0 ML.For hydrogen adsorption on the（100）surface,the highest hydrogen coverages on the metal surfaces are 1.0 ML expect for Ag（100）being 0 ML.For hydrogen adsorption on the（111）surface,the highest hydrogen coverages on Ag（111）and Au（111）surfaces are 6/9 ML and 4/9 ML,respectively.The highest hydrogen coverages on other four surfaces are 1.0 ML.In addition,according to the Wulff Construction principle,it is found that the most stable metal surface is the（111）crystal plane under the normal reaction conditions.（4）The desorption mechanism of H₂ molecules on metal surfaces.Based on the results presented above,we have discussed the dissociation and desorption processes of H₂ molecules on metal surfaces.The dissociation energy barrier of H₂ molecules on Ni,Pd,Pt metal surface is lower than on Cu,Ag,Au metal surface.For desorption of H₂ molecules on the metal surfaces,it is found that,double peaks are observed on Ni（110）,Pd（110）,Pt（110）metal surfaces,and the lower peak is caused by the interaction of H and H.