Theoretical Study On The Cooperative Catalytic Cracking Of CO And CH₄ On Ni(111) Surface

Carbon source gas catalytic cracking technology has a good prospect on preparing hydrogen and high-quality carbon nanotubes.Previous studies have found that CO/CH4 bicomponent carbon source gas has a significant synergistic effect.Comparing with the one-component carbon source gas system,the two-component carbon source gas system which contains CO/CH4 has lower cracking onset temperature,higher CH4 conversion hydrogen production and CO conversion rate,Carbon nanotubes have higher yields,longer lengths and smooth walls during catalytic cracking.At present,the mechanism of the synergistic phenomenon is not yet clear.This paper has conducted a theoretical study on the synergy,calculated and analyzed the mechanism using the density functional theory.By constructing a periodic Ni(111)adsorption surface model,the adsorption and cleavage processes of CO and CH4 on the surface were calculated respectively.The adsorption sites,geometrical configurations and energy changes,charge transfer and cleavage paths were calculated.The results showed that CO is verticaliy adsorp at the hcp position,C atom adsorp at the hcp position,O atom adsorp at the fcc position,the reaction barrier is 3.76eV.According to the Mulliken layout analysis,0.42e electrons are transferred from the CO molecule to the Ni(111)surface when the CO is vertically adsorbed.C atoms lose 0.12e electrons,O lose 0.32e electrons,Ni(111)surface get 0.42e electrons.CH4 is physically adsorbed on the Ni(111)surface,and CH3,CH2,and CH are chemisorbed.The s and p orbitals of C overlap with the Ni,s,p,and d orbitals to form C-Ni bonds.The fast-control step in CH4 step-by-step cleavage is the last H atom detachment process,ie,CH?C+H,and the reaction energy barrier is 1.36 eV.Based on the study of CO and CH4 adsorption and cracking processes on Ni(111)surface,the synergistic effect of CO/CH4 catalytic cracking on Ni(111)surface was investigated.Firstly,investigating the interaction between intermediate product CHx which is producted by CH4 cleavage process and O atom generated after CO cleavage.Comparing the energy barrier of reaction between CH3O,CH2O and CHO the energy barrier of CH+O?CHO(1.33eV)is lower than the energy barrier of CH?C+H(1.36eV).However,other possible intermediates Energy barrier are higher than its direct cleavage reaction barrier.Therefore,the catalytic effect of CO on the catalytic cracking by Ni can be explained that the CO molecular cleavage product O is combined with the CH4 cleavage product CH to form CHO,then cleaved it to C and OH,lowering the energy barrier and increasing the CH4 conversion.The interaction between CO molecule and CHx(x=1-4)cleavage product H generates COH.The energy barrier(2.21eV)of CO+H?COH is lower than the energy barrier(3.76eV)of CO?C+O.The catalytic effect of CH4 on the catalytic CO cracking of Ni can be explained that CO is easily combined with CHx(x=1-4)to form COH,and COH is then cleaved to C and OH,which is lower than that of CO direct cracking,increasing the CO conversion rate.