| Subsurface species have long attracted much research attention because of the novel reactivity and high selectivity in heterogeneous catalysis.In the process of catalytic hydrogenation,the adsorption,diffusion and permeation of the activated and dissociated hydrogen atoms on the metal interface greatly affect the reaction rate.Palladium,usually as active transition metal catalyst,may access remarkable properties in hydrogen storage,hydrogen purification,and heterogeneous catalysis.Consequently,A thorough framework for describing the hydrogen adsorption,diffusion,permeation and reaction behavior on covered Pd(100)surfaces would be very worthy of understanding the hydrogen diffusion dynamics in a real reaction.In this work,we present an in-depth study of the hydrogen adsorption,diffusion and permeation into different hydrogen-covered Pd(100)surfaces and the mechanism of methanation from syngas on the Pd(100)surface based on the Density functional theory.In order to investigate the effect of hydrogen coverage on hydrogen diffusion behavior,the formation rate and stability of subsurface hydrogen were considered.The reaction energy barrier for the methanation of syngas on the Pd(100)was obtained by searching the transition states.The kinetic results of the methanation were obtained by constructing a micro-kinetics model.The conclusions are as follows:(1)Stable adsorption sites of H atom on Pd(100)surface and sub-surface.The results show that the stable adsorption sites of H atom on the surface are Hollow and Bridge sites.Under low hydrogen coverage,the Bridge site nearest to the covered hydrogen cannot be stable adsorption site because of strong repulsion between hydrogen atoms.At high coverage,when the Hollow site is fully occupied,hydrogen atoms can be adsorbed at the Bridge site.The stable adsorption site on the subsurface is sub-Bridge site.Under low hydrogen coverage,the subsurface adsorption site is not affected by surface hydrogen coverage.Besides,under high coverage(surface monolayer saturation),the subsurface hydrogen adsorption is more stable than surface.(2)Surface diffusion and subsurface permeation pathways on covered surfaces.The surface diffusion path takes place between hollow and hollow through a Bridge site.For hydrogen permeation over a clean surface and low-coverage surfaces,the lowest energy reaction path follows hydrogen from the Bridge site to the Hollow site,where it penetrates into the subsurface and then stabilizes at the sub-Bridge site.The lowest energy reaction path follows the H-H synergetic permeation path,that is,the hydrogen atoms adsorbed on the Bridge push the neighboring atoms into the subBridge position,at the same time,the H atom from Bridge site occupies the empty Hollow position.(3)Hydrogen permeation promoted by C/O atom adsorption.When the surface is covered by carbon atom or oxygen atom,the diffusion of hydrogen atom is restrained and the permeation to subsurface is promoted.(4)The subsurface hydrogen has better hydrogenation activity.The DFT calculation of the step-by-step hydrogenation methanation of carbon atoms shows that the reaction energy barrier of the hydrogenation reaction can be greatly reduced by the high energy of the subsurface hydrogen.The same process takes place when oxygen atoms are hydrogenated to form water.(5)Reaction temperature and dissociation barrier greatly affect the total reaction rate.According to the DFT calculation results,the CO dissociation is taken as the ratedetermined step,and the turnover frequency is obtained by kinetic simulation.The kinetic results showed that increasing the reaction temperature greatly promoted the conversion frequency of the reaction.At the same temperature,the key to increase the total reaction rate is to promote the dissociation of CO and restrain the dissociation of hydrogen moderately.The kinetic simulation results show that the subsurface hydrogen still has significant catalytic hydrogenation activity when the reaction energy barrier of the above two reactions is changed. |
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