Theoretical Studies Of Co And Ni Promoted Cu/γ-Al₂O₃ Catalyst For Methanol Steam Reforming

To achieve the target of carbon neutralization,methanol can be served as green hydrogen carrier by means of methanol reforming technology,which includes processes/operations such as hydrogen production,transportation and downstream utilization.Traditional supported Cu-based catalysts for methanol reforming reaction（MSR）have the issue of poor thermodynamic stability and deactivate easily.Currently,the research on high-efficiency MSR catalysts is mainly focused on enhancing the dispersion of Cu in Cu-based catalysts.The employment of metal additive enhances dispersion of Cu,specific surface area of Cu,and reduces the particle size of Cu microcrystals,resulting in the improvement of stability and catalytic activity of Cu-based catalysts realizably.However,it is lack of understanding on the thermodynamic basis and dynamic process of metal additives to improve the Cu/Al₂O₃catalyst during preparation,deactivation and MSR reaction.Aimed at the above scientific issues,the present thesis performed density functional theory calculations to investigate:（1）the doping mechanism of additives on carrier surface;（2）correlation between additives and thermodynamic stability of active copper;（3）thermodynamics and kinetics of the reactant water;（4）adsorption state of the reactant methanol;（5）effect of additives on the selectivity of by-product CO.（1）Simulate the doping process of metal additives.The substitutions of Co³⁺and Ni³⁺cations for Al³⁺cations ofγ-Al₂O₃（110）surface are thermodynamically favorable.The Co and Ni are preferentially substitution for the tetrahedral Al than the octahedral Al atom.Through the thermodynamics,it was found that both Co and Ni could form 100%substitution of top layer Al atoms inγ-Al₂O₃（110）surfaces.After all the top layer Al³⁺cations have been substituted,25%M(one Co³⁺/Ni³⁺)in tetrahedral position vs.75%M(three Co³⁺/Ni³⁺)in octahedral position.The band gap of doped surface become narrower,and the Co³⁺and Ni³⁺could be used as Lewis acid sites.（2）Expounding the effect of metal additives on the thermodynamic stability of activated copper.The results showed that the doped Co and Ni additives promoted the interaction of the active Cu with the carrier surface.The Bader charge indicates that there existed charge transfer between Cu andγ-Al₂O₃（110）surface,and the adsorbed Cu atoms lose 0.58 and 0.60 e,respectively.The adsorbed Cu is potential active site for MSR reaction.The structure,growth energy and aggregation energy of Cu₂clusters demonstrate that the doped Co and Ni additives contributes to the adsorption,dissociation and dispersion of Cu clusters on the catalyst surface,resulting in the preparation of Cu-based catalysts with better thermodynamic stability.（3）Investigation the adsorption state and activation mechanism of the reactant water molecules on the catalyst surface.The results showed that the doped Co additives slightly reduced the adsorption energy of H₂O molecule on active metal Cu;while the doped Ni additives increased its adsorption energy.On all catalysts surface,H₂O molecule was more favor to adsorbed on the Cu atom through dissociation adsorption.The doped Co and Ni additives stabilized the spatial position of Cu atoms on the surface,thus improved the thermodynamic stability of Cu.Kinetic calculation showed that the doped Ni additive promoted the dissociation of H₂O molecule on active metal Cu,vs,doped Co additive slightly inhibited.（4）Investigation the adsorption state of the reactant CH₃OH molecules on the catalyst surface.It was found that Cu and Co mutually inhibited the adsorption of CH₃OH molecule.Besides,Cu inhibited the adsorption of CH₃OH molecule on Ni,while Ni enhanced the adsorption of CH₃OH molecule on Cu.Comparing of the competing adsorption of H₂O and CH₃OH molecule,it was found that the low OH coverage on the（110）-4Ni surface caused to difficult MSR reactions and easy methanol decomposition reactions（MD reactions）.While the OH on the（110）-4Cu and（110）-4Ni-Cu surfaces contributes to the MSR reaction instead of MD reactions.This is helpful to improve CH₃OH conversion and reduce CO selectivity.（5）Investigation the adsorption and conversion of by-product CO on the catalyst surface.The results showed that the adsorption energies of single CO molecule on the（110）-4Co-Cu and（110）-4Ni-Cu surfaces were only-156 and-181 k J/mol.The doped Co and Ni reduced the toxicization and site coverage of the active metal Cu by CO.It was further found that the conversion process of CO to CO₂ on the（110）-Cu surface is thermodynamically unfavorable,while it is thermodynamically favorable on the（110）-4Co,（110）-4Ni,（110）-4Co-Cu and （110）-4Ni-Cu surfaces.It implies that the doped Co and Ni additives not only reduces the toxicity of the active metal Cu,but also the selectivity of CO.In summary,the results confirmed the formation mechanism of additive metals dope on carrier surface,correlation between additives and thermodynamic stability of active Cu,the adsorption mechanism of reactants,the adsorption and conversion mechanism of by-product CO,laying the foundation for the design,synthesis and application of the‘sustained release’spinel catalyst.