Simulation Calculation Of Methane Adsorption And Decom Position On Fe And FeNi Matrix

In the petrochemical industry, the corrosion behavior of the material surface and internal coke, carburizing by the carbon gases getting worsening, it is necessary to clarify the mechanism of coke and its relationship with the surface material compositions, and thus provide guidance for the anti-coking and anti-carburizing.Generalized gradient approximation(GGA) of the density function theory(DFT) was applied to investigate the structure optimization of Fe(111),FeNi(111) and FeNi3(111) surfaces, calculate the methane and small molecules adsorption and decomposition processes. Many properties including band structure, density of states, population analysis, and electron density were calculated by CASTEP-module of Materials Studio software package.We Calculated the stable positions, the energy barrier for the decomposition process, and the rate-determining step, searched the decomposition transition state, and compared. It was shown that Fe(111) surface has apparent relaxation after adsorpted methane and methane molecules, the surface atoms has shifted compared to the initial positions. The d orbitals of Fe atoms on the surface didn’t bonded with CH4, CH4 weak interaction with the metal surface atoms,which in line with characteristics of physical adsorption. CH3 adsorption, the 2a1, 1e and 3a1 orbital of CH3 bonding with the surface atom of metal, 3a1 orbital move large displacement away from the Fermi level,which make a great contribution to the C-Fe bonding, 3a1 orbital peak has weakened relative to the state before absorption, CH bond length elongation. CH2 is similar to CH3. For CH, For CH, however,only 2p orbital of the C interact with Fe surface and then receive electrons, while there is no electron that transfers to theC–Hantibonding orbital. In the Fe(111) plane, CH decomposition is the rate-determining step in the process, it needs to overcome a high energy barrier, which is much less than the energy barrier of CH decomposition independently, thus,Fe(111) promote the decomposition of methane and contribute to the formation of coke face. εd on different catalyst surface has a significant linear relationship with CH, the strength of CH adsorbed on catalyst surface successively weakened, sort of Fe(111)> FeNi(111)> FeNi3(111). the rate-determining step of FeNi(111) and FeNi3(111) is CH = C + H, the energy barrier was 1.48 e V and 1.63 e V respectively. both of them are greater than energy barrier in Fe(111) surface, which indicating the catalytic performance of FeNi alloys surface is less than pure Fe surface, and with the increase of Ni alloys, catalytic performance weakened, reducing coke rate, but still far less than that of CH decomposition alone, therefore can not inhibit the formation of coke.