Theoretical Study Of Gas Adsorption And Dissociation On Metal Surfaces

The interaction of atoms and molecules with metallic and alloy surfaces is an important step in process of heterogeneous catalysis. It is also the first elementary step in heterogeneous catalysis and the fundamental to understanding of catalytic mechanism. Nowadays it has becoming one of key subjects in the field of surface science and numerous studies have been carried out in experiments as well as theories. The first principles density function theory (DFT) calculation plays more and more important role in understanding of adsorption mechanics and explaining of experiment phenomenon at atomic scale.Chemisorption of H and O atom on Pd(111), Au(111) and PdAu(111) alloy surfaces were systematically studied using the density functional theory(DFT-GGA) method. The most favorable adsorption site, binding energy and geometry during adsorption were obtained with coverage at 0.25 ML. Results indicate that the preferred adsorption site is face centered cubic(fcc) site on pure metal (111) surfaces, the most stable adsorption site on various alloy (111) is different with differnent Au component. For H atom, the adsorption energy is significantly reuced when Au is present on the surface, and the activity of alloy surfaces is also decreased, making hydrogen atoms more difficult for adsorption, thus more easier involved in hydrogenation. For O atom, the order of the most stable adsorption energies for alloy (111) surfaces is Pd3(-4.83 eV)> Pd2Au(-4.16 eV)> PdAu2(-3.57 eV)> Au3(-3.0 eV). In addition, we focused on the d-band center of the electronic properties to explain the relationship between oxygen binding strength and the concentration of the alloy surfaces. Results indicate that the d-band center is further away from the fermi level, the chemisorption bond of atomic oxygen on the surface is weaker.The adsorption and dissociation of oxygen on Pd(111) and lAu/Pd(111) also have been studied using density functional theory(DFT-GGA) method. We also calculate the dissociate energy barrier and indicate the dissociate path. The dissociate energy for oxygen on Pd(111) and 1Au/Pd(111) are 1.06 eV and 1.50 eV, respectively. The result shows that present an Au atom on Pd(111) surface can obvious suppress oxygen dissociation, and thus avoide the synthesis of water by-product, which lead to the main product hydrogen peroxide.