Theoretical Studies On The Adsorption And Hydrogenation To Methanol Of CO₂ On Ni₅Ga₃ Alloy Surfaces

With the continuous development of society and the rapid growth of the economy,people make heavy use of fossil fuels and other non-renewable energy resources,this behavior has caused various extent damage to the earth's environment and the survival of human race.Due to the burning of fossil fuels,the content of CO₂ in the atmosphere increase continuously,leading to global warming,sea level rising and a series of environmental problems.Therefore,it's particularly important to choose some cost-effective ways to store or convert CO₂.In addition,the reduction of non-renewable energy forces human to seek new clear energy that can replace fossil fuels.The process of hydrogenation of CO₂ to methanol can not only effectively capture CO₂ in the atmosphere,also be able to generate new sustainable clean energy,this process can be achieved by means of heterogeneous catalytic method.Ni₅Ga₃ alloy shows good catalytic activity in the process of CO₂ hydrogenation,so,it is of great theoretical and practical value to study the CO₂ hydrogenation to methanol on the surface of Ni₅Ga₃ alloy.In this paper,Ni₅Ga₃ alloy is used as the catalytic surface.We used a slab model and the density functional theory to explore CO₂ adsorption on the Ni₅Ga₃ alloy.We discussed the adsorption mechanism of CO₂ on the Ni₅Ga₃ surface by the most stable adsorption configuration,and systematically studied the reaction mechanism of CO₂ hydrogenation to methanol on the surface of Ni₅Ga₃?010?-Ni-rich.The main results are as follows:?1?We explored the adsorption of CO₂ molecules in the case that Ni₅Ga₃?100???010???001?three types on the Ga-rich and Ni-rich surface.The results show that when one of the metal atoms is relatively concentrated?whether it is Ga-rich or Ni-rich?on the Ni₅Ga₃ alloy surface,the surfaces all strengthen the adsorption of CO₂.Compared Ga-rich with Ni-rich,we found that Ni₅Ga₃?010?surface has a good stability to the adsorption of CO₂,and Ni₅Ga₃?001?surface is weaker,while Ni₅Ga₃?100?surface shows the weakest stability to CO₂ adsorption.In addition,the adsorption of CO₂ on the Ni-rich surface is much stronger than that on the Ga-rich surface in the Ni₅Ga₃ surface.Therefore,the adsorption of CO₂ on the metal Ni atoms is much stronger than that on the metal Ga atoms in the Ni₅Ga₃?010?surface.?2?In the adsorption structures we studied,the most stable adsorption configuration is the 2-C,O* configuration when CO₂ adsorbed on the surface,and the C atom and the O atom of the CO₂ molecule interact with the surface Ni atoms,respectively.Compared other adsorption configurations with 2-C,O*configuration,the latter shows good stability in the adsorption of CO₂.The electronic structure of CO₂ adsorption indicated that the 4?g?3?u?1?g?2?u orbitals of CO₂ interact with the dxz and dz2 orbitals of Ni atoms on the surface,respectively,when CO₂ adsorbs on the surface with the most stable adsorption configuration.The interaction between these orbitals is the dominant factor that CO₂ can adsorbs on the Ni₅Ga₃?010?-Ni-rich surface stably.?3?The possible pathways of CO₂ hydrogenation to methanol on the Ni₅Ga₃?010?-Ni-rich surface and the geometrical configuration of the intermediates were investigated,and determined the transition state of each elementary reaction.This article mainly explored the following four pathways:Pathway 1: CO₂ ? CO ? HCO ? H2 CO ? H₃ CO ? CH₃OHPathway 2: CO₂ ? CO ? HCO ? H2 CO ? H2 COH ? CH₃OHPathway 3: CO₂ ? CO ? HCO ? HCOH ? H2 COH ? CH₃OHPathway 4: CO₂ ? CO ? COH ? HCOH ? H2 COH ? CH₃OHIt was found that the reaction of CO₂ deoxidize to CO on Ni₅Ga₃?010?-Ni-rich surface is easy to proceed.Nevertheless,the elementary reaction of CO hydrogenation to COH is difficult to carry out due to the high activation energy?1.98 eV?,which is the rate determining step of pathway 4.CO + H ? CHO is the rate determining step of pathway 1?2 and 3,and the activation energy is 1.33 eV.Compared with four pathways,we found that each elementary reaction has a relatively low energy barrier in the pathway 3,so we speculated that the pathway 3 may be the most optimal pathway for CO₂ hydrogenation to methanol on Ni₅Ga₃?010?-Ni-rich surface.