Density Functional Study Of Catalytic Combustion Of Methane On Transition Metal Catalysts

Low concentration methane exists widely in coal bed gas.As one of the world’s most abundant low carbon alkanes,methane is the raw material for many industrial processes.Due to the high stability of methane,catalytic combustion is often used.However,the catalytic combustion of methane is a very complicated process,and the reaction mechanism remains to be further studied.Therefore,to explore the micro reaction mechanism of methane combustion on transition metal catalysts(Co,Ni and Cu)and bimetallic catalyst(NiNi,NiCo and NiCu)in molecular scale has important academic significance and engineering application value.Quantum chemical study is carried out in the dehydrogenation of CH4 and the oxidation of CH_x on transition metal catalysts(Co,Ni and Cu)and bimetallic catalyst(NiNi,NiCo and NiCu),based on the density functional theory,using B3 LYP / Genecp(LANL2DZ for Ni,Co and Cu,6-311 G for C,H and O)level,in order to optimize the structure of reactants,intermediates and products,and find the correct transition state.By comparing and analyzing the potential energy change of each elementary reaction,and combining with the thermodynamic and kinetic parameters,the reaction path of methane catalytic combustion was obtained.The results are as follows:(1)The dehydrogenation reaction of methane on transition metal catalysts Co,Ni and Cu is mainly divided into four steps,corresponding to the fracture of each C-H bond.The fourth C-H bond cleavage need to overcome a high energy barrier,and it is the rate-determining step in both reaction on catalyst Ni and Cu;Catalyst Ni has good activation behavior on the cleavage of the first C-H bond of methane.Sort the catalytic activity of the three catalysts for the dehydrogenation of methane from strong to weak is Co>Ni>Cu,according to activiton energy and reaction rate constant.That is,with the increase of atomic number,the catalytic performance of catalyst atoms to methane dehydrogenation is gradually weakened.(2)The dehydrogenation reaction path on bimetallic catalyst NiCo and NiCu is very similar.The reaction is centered on Ni atom,and the Co or Cu atom plays a supporting role in reaction.The dehydrogenation reaction path on catalyst NiNi requires more steps than the other two catalysts.Sort the catalytic activity of the three catalysts for the dehydrogenation of methane from strong to weak is NiCo> NiNi> NiCu,according to activiton energy and reaction rate constant.(3)The reaction energy barrier of COOHàCO2 process is higher in the oxidation reaction path of CH_x on all catalysts.COOH is difficult to translate into CO2.The activation energy of each elementary reaction on CHXOHàCHX-1OH+H is low,and if the CHXOH is generated,the dehydrogenation reaction will continue along this path to CO.(4)The main reaction paths of catalytic oxidation of methane on Ni,Co and Cu were the same: CH4àCH3àCH2OHàCHOHàCHOàCOàCO2.The direct dehydrogenation process of CH3 to CH2 has a high reaction energy barrier,so CH3 mainly reacts with O to produce CH2 OH.(5)The reaction path on NiCo and NiNi are the same to each other(CH4àCH3àCH2àCHàCHOàCOàCO2),but very different from that on NiCu(CH4àCH3àCH2àCH2OHàCHOHàCHOàCOàCOOHàCO).Besides,the three paths are different from those on single metal catalysts.The reaction path on Ni2 and NiCo is mainly based on O as oxidant,with a lower barrier.While the reaction path on NiCu is OH,with a higher barrier.This is mainly oweing to the low activity and hydrophilicity of Cu.The research results not only explain the reaction regularity of catalytic combustion of methane,but also give a complete reaction path of each catalyst.Which enriches the content of catalytic combustion mechanism of methane on transition metals,and plays an important role in guiding the preparation and improvement of catalysts.