Density Functional Theory Study Of Methane Adsorption On Nickel, Platinum, Palladium Metal Surfaces

Methane is the most plentiful component of nature gas. It can be instead of oil resources and become a kind of clean energy source. C-H bond in the structure of CH4 is very strong, so it is difficult to be activated under mild conditions. The widely used catalysts for catalytic conversion of methane are transition metal catalysts duo to their excellent catalytic activity, selectivity and chemical stability, in which the nickel, palladium, platinum metal catalysts are the more commonly used catalysts. Unfortunately, no effective and economic catalysts have been discovered. Therefore, whether theoretical study or not experimental study, it is a challenging topic for researchers to explore an efficient catalyst system.In this paper, all calculations are based on density functional theory as implemented in CASTEP module of Materials Studio software. The behavior of methane adsorption on Ni(110)、Pd(110)、Pt(110) and Pd-doped Ni(110) surfaces at different sites is investigated in detail, such as space structure, electronic structure and so on.Firstly, 24 kinds of geometries for CH4 adsorption on M(110)( M=Ni, Pd, Pt) surface were optimized. It was found that top-T4、top-T3、hollow-H2 were respectively most stable structures on Ni(110), Pd(110), Pt(110) surfaces, respectively. It indicated that the top atoms were the active adsorption site of the Ni(110) and Pd(110) surfaces, whereas the active adsorption site on the Pt(110) surface occurred on hollow position. These three kinds of adsorption are all physical adsorption. Their adsorption energies were 4.59, 13.05 and 22.93 kJ/mol, respectively. Comparing the adsorption energies, for M(110)( M=Ni, Pd, Pt) the order of the possibility and degree of adsorption is Pt(110)>Pd(110)>Ni(110). The Pd(110) surface is most for the number of lost electrons of 2s orbital in C atom and obtained electrons of 1s orbital in H atom near(110) surface, followed by the Ni(110) and Pt(110).We used density functional theory to examine two structures for different Pd doped positions(hollow or top sites) on Ni(110) surface, and research the effects for CH4 adsorption on Pd-doped Ni(110). The results showed that structure of the Pd instead of the top-Ni on Ni(110) surface was more stable structure, and the top-T2 was the most stable structure for the CH4 adsorption on Pd-doped Ni(110) surface. It indicated that the top Pd atom on Pd/Ni(110) surface was active position. The adsorption energy of CH4 on the Pd/Ni(110) surface was greater than on Ni(110) surface(8.20 kJ/mol>4.59 kJ/mol).This showed that the doping Pd at the top site could not only make the system more stable, but also improve the possibility and degree of adsorption. Compared with C-H bond length changes before and after methane adsorption, C-H2 bond were broken much easier. Mulliken population calculation for the CH4 adsorption on Pd-doped Ni(110) was carried out. The results showed that after the adsorption electrons from 5p orbital in Pd, 4s orbital in Ni12, 4p orbital in Ni22 and 2s, 2p orbitals in C were mainly transferred to the 1s orbital in H2 of CH4 relative to before adsorption.