Theoretical Research On The Mechanism Of CH₄ Dissociation On Alloy Catalyst

Graphene,one of the most interesting nanomaterials in this century,have received significant attention owing to its exciting characteristics.However,its unique properties and diverse applications tightly depend on the quality and thickness of graphene.Therefore,it is urgent to achieve controllable synthesis with a high quality and large scale.Currently,the chemical vapor deposition(CVD)method is considered as the most popular method to synthesize graphene.However,the graphene growth mechanism and the effect of each controlled factor during the CVD processes are not clear.Therefore,the first principles method was used to investigate the mechanism of CH4 disociation and graphene nucleation on metal alloy surfaces.The obtained results are expected to provide important theoretical supports and guidance for the experimental development on controllable synthesis of graphene.The main contents are summarized as follows:1.CH4 dissociation in the early stage of graphene growth on Fe–Cu(100)surface:Theoretical insights.The mechanism of CH4 dissociation and carbon nucleation process on the Fe doped Cu(100)surface were investigated systematically by using density functionary theory(DFT)calculations and microkinetic model.The activity of the Cu(100)surface was improved by the doped Fe atom and the atomic Fe on the Fe-Cu(100)surface was the reaction center due to the synergistic effect.In the dissociation process of CH4,CH3?CH2+H was regarded as the rate-determining step.The results obtained from the microkinetic model showed that the coverage of CHx(x=1–3)was gradually decreased with the temperature increasing and CH3 was always the major intermediate at the broad range of the temperature(1035~1080 oC)and the ratio of H2/CH4(0~5).It is also found that the reaction rates were increased with the temperature increasing.However,the reaction rates were reduced(or increased)at the range of H2/CH4=0–0.2(or H2/CH4>0.2).It is noted that controlling the H2 partial pressure was an effective method to regulate the major intermediates and reaction rates of CH4 dissociation and further influence the growing process of graphene.2.Theoretical Study on CH4 dissociation on Ni–Cu(100)surface in the Early Stage of Graphene Growth.By using DFT calculation and microkinetic model we systematically studied the mechanism of CH4 dissociation and carbon nucleation process on Ni-Cu(100)surface.The doped Ni was not only regarded as the reaction center on the Ni-Cu(100)surface,but also improved the activity of Cu(100)surface due to the synergistic effectbetween Ni and Cu.The rate-determining step of the methane dissociation process was CH?C+H.The microkinetic model explained the effect of temperature(1035~1080 oC)and H2/CH4 ratio(0~5)on these reactions.We derived that CH was the major intermediate and the optimum conditions to prepare CH species was under 1035 oC and without or less H2 involving.The coverage of CHx(x=1-3)was reduced with the temperature increasing.On the contrary,the reaction rates were increased with the temperature increasing.The coverage of CHx(x=1-3)and reaction rates were varied with the increase of H2/CH4 ratios.It displays the tendency declining at beginning and rising up in the late.And the ratio of C/CH4 should be controlled less than 2.03×106 to keep the activity of the catalysts.Our current results showed that it is possible to control the CH4 dissociation rate and the growth of graphene on Ni-Cu(100)by changing the H2 partial pressure,temperature and the coverage of C.The bimetallic Ni-Cu(100)was predicted as a good catalyst for graphene growth.