Dissociation Of Methylamine And Methane On Different Catalyst: A DFT Study

In this thesis, the decomposition mechanisms of methylamine and methane on some catalysts surfaces have been systematically investigated by DFT–GGA method and slab model. The reactivity difference of various catalysts surfaces has been studied by the calculation of activation energy. The main conclusions of this work are summarized as follow.(1) The adsorption of methylamine, methyl, and amino involved in the C–N bond cleavage of methylamine and the dissociation of C–N bond have been investigated on the clean Mo(100), C(N, O, P, Cl) atom modified Mo(100), Mo2C(100), MoN(100), and Pt(100) surface. For C–N bond cleavage of methylamine, compared with that on the clean Mo(100), the calculated results show that the activation energy on the Mo(100) surface modified with the C, N, O, or P atom is increased. That is, the clean surface is deactivated by these modification atoms. Whereas the barrier on the Cl atom modified Mo(100) surface is slightly decreased. For the C, N, and O atom listed in the same row in the element period table, when they are on the surface of Mo(100), the passivation effect induced by them on the Mo(100) is almost the same. However, the carbon atom on the subsurface increases the reactivity of Mo(100) dramatically. On the Mo2C(100) and MoN(100) surface, the activation energy of the C–N bond cleavage is higher than that on clean Mo(100). This indicates that the presence of carbon and nitrogen decreased the reactivity of Mo(100). The barriers for the C–N bond breaking on Mo2C(100) and MoN(100) surface are similar to that on Pt(100), suggesting that the properties of the transition metal carbides and nitrides for the C–N bond scission of methylamine might be very similar to the expensive Pt-group metals (Ru, Rh, Pd, Os, Ir, Pt).(2) As regards for methylamine decomposition on Ni(111), Ni(100), stepped Ni(111), and nitrogen atom modified Ni(100) (N–Ni(100)), the adsorption energies under the most stable configuration of the possible species and the reactivity difference of the catalysts to the C–H, N–H, and C–N bond breaking have been investigated. Through systematic calculations for the kinetics mechanism of methylamine decomposition on these surfaces, it is found that the reactivity of the four nickel surfaces decrease with the order of stepped Ni(111) > Ni(100) > Ni(111) > N–Ni(100) and the presence of nitrogen atom decreased the reactivity of Ni(100). The barrier for the C–N bond breaking is the highest on these four surfaces, the barrier for the C–H bond breaking is the lowest on Ni(111) and Ni(100), whereas the barrier for the N–H bond breaking is the lowest on stepped Ni(111) and N–Ni(100).(3) The cleavage of the C–H bond in methane has been studied on the palladium based catalysts. The results show that such a reaction is a structure sensitive reaction on clean palladium surfaces. For the reaction CH4+O→CH3+OH and CH4+O→CH3+H+O, compared with that occurred on clean palladium surface, the barrier is increased. And on the same palladium surface, for the reaction CH4+O→CH3+OH, the activation energy increased with coverage. From the calculation results of activation energy, we can see that the C–H bond breaking of methane forming methyl hydroxyl is structure insensitive on the oxygen atom modified palladium, PdO(100), and PdO(110) surfaces. The reaction CH4+O→CH3+OH is the limiting step of the methane catalytic combusition. So, the catalytic combustion reaction of methane on palladium surfaces is a structure insensitive reaction, which verifies the experiment. In addition, for the reaction CH4+O→CH3+OH, the presence of subsurface oxygen decreased the barrier on oxygen atom modified Pd(111).