Computational Study On The Hydrogenation Of CO₂ To Hydrocarbons Over Ni-Fe Bimetallic Catalysts

In this thesis,density functional theory（DFT）calculations were performed using the Vienna Ab-initio Simulation Package（VASP）software to investigate the effect of Ni doping on C₁ and C₂ hydrocarbons production from CO₂ hydrogenation on Fe catalyst.The adsorption and growth behaviors of Ni_n clusters on the Fe₅C₂（100）surface was also examined,which might impact the activity of iron carbide catalyst.On Fe（110）and Ni-Fe（110）surfaces,the CH*species was found to be the most favorable monomeric CH_x*species leading to both CH₄ and C₂H₄ formation.The plausible pathway went through CO₂?HCOO*?HCO*?CH*.Although CO*formation from direct dissociation of CO₂ is kinetically more favorable than CO₂ hydrogenation to HCOO*and COOH*intermediates on the two surfaces,the subsequent hydrogenation of CO*to HCO*was energetically detrimental,resulting in the reverse conversion of HCO*to CO*that is preferred.Therefore,CO*should not be the preferred key intermediate leading to CH₄ and C₂H₄ formation.Further conversion of CH*species leads to either CH₄ via several hydrogenation steps or C₂H₄through C-C coupling followed by two hydrogenation steps.On monometallic Fe（110）,the difference in ketic barrier associated with the selectivity-determining step is only 0.10 eV,suggesting similar selectivity to CH₄ and C₂H₄.In contrast,when Ni is doped onto Fe（110）,the selectivity difference becomes more pronounced,leading to an enhanced C₂H₄ production on Ni-Fe（110）.These results reveal that adding a small amount of Ni could promote CH*formation from CO₂ hydrogenation,C-C coupling of two CH*species and the further hydrogenation of C₂ intermediates to ethylene.The particle size and structure of Ni catalyst have potential influence on the catalytic activity.In the investigation on the adsorption and stability of Ni_n clusters on Fe,it was found that that the three-dimensional（3D）configurations are more stable for Ni_n clusters in the gas phase,whereas on the Fe₅C₂（100）surface,the size of Ni_n cluster and numbers of Ni atoms determin whether the preferable structure is 1D or 2D.The calculation results showed that when n≤6,the Ni_n clusters tends to be stabilized in 1D configurations on the Fe₅C₂（100）surface.While the 2D configurations become energetically more favorable when the numbers of Ni atoms were further increased to n>6 until the surface is fully covered by Ni atoms（n=16）.Such structural transformation and stability change indicatethat Ni atoms might shield active sites on the Fe₅C₂ surface even if the loading of Ni was low.Therefore,the Ni-doped Fe₅C₂ catalyst may exhibit similar catalytic activity to that of Ni nanoparticles,covering some active sites of Fe₅C₂ and inhibiting the formation of long-chain hydrocarbons.This study revealed the reaction mechanism of CO₂ hydrogenation to hydrocarbon products on Fe-Ni catalysts and uncovered the potential effect of the added second component Ni on the Fe carbide Fe₅C₂ catalyst.