Density Functional Study Of Methane Partial Oxidation Reaction

It is difficult for experimental methods to examine the intermediates and transition states in the reaction of partial oxidation of methane for its short lifetime and low concentration. Quantum chemistry calculations are able to provide some insights of their structures and energies, and give some useful knowledge to the investigation of reaction mechanism. This dissertation presented a theoretical study of partial oxidation of methane to synthesis gas on ideal Ni surface (Ni (111) and Ni (100)) according to direct oxidation mechanism. Spin unrestricted Density Functional Theory (DFT) and period boundary condition were used to determine adsorption conformation, adsorption energy, electron transfer and relative stability of reactants,intermediates, transition states and products. Reaction energies and activation energies for the element reactions were also determined. The activation of methane on Ni surface was endothermal. In the process of methane activation the Ni surface bonded to both the H atom and hydrocarbon fragments and stablized them, which made the activation energy lowered. The activation energies of CH₄, CH₂, and CH in surface Ni (100) were lower than those in Ni (111), while for CH₃ the activation energy was lower in Ni (111). The formation of adsorbed H₂ was endothermal. It was easy for H2 to decompose at atop sites but to produce at bridge sites in surface Ni (111). In surface Ni (100) it is easy for H2 to decompose at both atop sites and bridge sites. The more stable C atom bonds to Ni surface, the larger tendency to form graphite, and the more difficult the formation of carbon monoxide. The avitivation energy to form CO on Ni (111) surface was lower than that on Ni (100), therefore it is more likely to form coke on Ni (100) rather than on Ni (111). The three element reactions were interrelated. On Ni (100) surface the existence of O atom weakened the interaction of C atom and surface Ni but increaced energy barrier of CH dissociation. The activation energy lowered when O atom directly interact with CH. Comparing different reaction pathways on Ni (100) surface, it is suggested that the formation of CO contributed to the oxidation of CH.